Cold Regions Pavement Engineering Guy Doré, Ph.D., Ing. Professor of Civil Engineering Laval University Quebec City, Quebec
Hannele K. Zubeck, Ph.D., P.E. Professor of Civil Engineering University of Alaska Anchorage Anchorage, Alaska
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Library of Congress Cataloging-in-Publication Data Doré, Guy. Cold regions pavement engineering / Guy Doré, Hannele K. Zubeck.— 1st ed. p. cm. Includes bibliographical references and index. ISBN 978-0-07-160088-0 (alk. paper) 1. Pavements—Cold regions—Design and construction. 2. Pavements— Cold regions—Maintenance and repair. I. Zubeck, Hannele K. II. Title. TE251.D67 2009 625.80911—dc22 2008037476 1 2 3 4 5 6 7 8 9 0 DOC/DOC 0 1 4 3 2 1 0 9 8 ISBN 978-0-07-160088-0 MHID 0-07-160088-4 Sponsoring Editor Larry S. Hager
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To Natalie, Brad, and our lovely daughters
About the Authors Guy Doré, Ph.D., Ing., is a professor of civil engineering at Laval University in Quebec City, Quebec. He earned B.S. and M.S. degrees in geological engineering and a Ph.D. in civil engineering at Laval University. Before joining academia, Dr. Doré worked at the Quebec Ministry of Transportation. During his time at MTQ, he joined the Canadian Strategic Highway Research Program (C-SHRP) in Ottawa and the U.S. Strategic Highway Research Program (SHRP) in Washington, D.C., as a visiting researcher. Dr. Doré has authored and co-authored numerous journal and conference papers in his specialty area of pavement performance under the effects of frost and thaw. He serves on several national and international committees on cold regions engineering and recently received the American Society of Civil Engineers’ Can-Am Award for his distinguished service in building relationships between engineers in Canada and the United States. He resides in Ste.-Catherine-de-la-Jacques-Cartier near Quebec City with his wife Natalie and daughters Léonie, Rosalie, and Flavie. Hannele K. Zubeck, Ph.D., P.E., is a professor of civil engineering at the University of Alaska Anchorage. She earned B.S. and M.S. degrees in civil engineering at Tampere University of Technology in Tampere, Finland, and a Ph.D. in civil engineering at Oregon State University. Prior to joining academia, Dr. Zubeck worked in her native Finland as a geotechnical engineer for engineering consulting companies and as a research engineer in bituminous pavement materials for Neste Oil. She later joined the SHRP research team at Oregon State University. Dr. Zubeck has authored numerous journal and conference papers in her specialty area of the behavior of bituminous pavements in cold regions. She currently chairs UAA’s arctic engineering online graduate program and serves on several national and international committees on cold regions engineering. She lives in Kenai, Alaska, with her husband Brad and daughters Maija and Elli.
Contents Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xiii xv
1
Cold Regions Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 Road Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2 Pavement Surface Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2-1 Hot Mix Asphalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2-2 Cold Mix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2-3 Surface Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2-4 Gravel Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2-5 Stabilized Bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3 Role of Pavements and Pavement Layers . . . . . . . . . . . . . . . . . . 1-3-1 Surfacing Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3-2 Base Course . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3-3 Subbase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3-4 Subgrade Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3-5 Special Pavement Layers . . . . . . . . . . . . . . . . . . . . . . . . . 1-3-6 Embankment Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 1 5 5 5 6 7 7 7 8 8 9 10 10 12 12 13 14
2
Pavement Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 Temperature Regime in Pavements . . . . . . . . . . . . . . . . . . . . . . . 2-1-1 Factors Inducing Heat in the Pavement System . . . . . . 2-1-2 Factors Contributing to Heat Extraction from the Pavement System . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1-3 Factors Contributing Either to Heat Induction or Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1-4 Thermal Balance and Thermal Cycles . . . . . . . . . . . . . . 2-2 Moisture Regime in Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2-1 Phases of Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2-2 Factors Contributing to Water Intake in the Pavement System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2-3 Factors Contributing to Moisture Extraction from the Pavement System . . . . . . . . . . . . . . . . . . . . . . . . 2-2-4 Moisture Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3 Stress Regime in Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3-1 Earth Pressure at Rest . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3-2 Static Stresses Induced by Traffic Loads . . . . . . . . . . . . 2-3-3 Stresses Related to Permanent Soil Movements . . . . . .
15 15 16 17 18 19 23 23 24 33 35 37 37 38 42
ix
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Contents
3
4
5
2-3-4 Moving Traffic Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3-5 Thermal Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3-6 Stresses Related to Frost Heave . . . . . . . . . . . . . . . . . . . . 2-3-7 Negative or Positive Pore Pressure . . . . . . . . . . . . . . . . . 2-4 Interaction with Geology and Morphology . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43 47 48 49 51 52 53
Cold Region Pavement Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 Thermal Cracking of Asphalt Concrete . . . . . . . . . . . . . . . . . . . . 3-2 Fatigue Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3 Crack Deterioration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4 Rutting of Asphalt Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4-1 Permanent Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4-2 Rutting Due to Studded Tire Wear . . . . . . . . . . . . . . . . . 3-5 Aging of Asphalt Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6 Pavement Disintegration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7 Potholes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8 Frost Heaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8-1 Differential Frost Action . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8-2 Frost Heave Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8-3 Frost Heaving in Granular Base Material . . . . . . . . . . . 3-9 Bearing Capacity Loss During Spring Thaw . . . . . . . . . . . . . . . . 3-10 Frost Deconstruction of Undisturbed Sensitive Clays in Seasonal Frost Conditions . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57 58 65 69 71 71 76 80 82 85 88 88 93 96 99 106 108 109
Investigation and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 Site Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1-1 General Site Investigation . . . . . . . . . . . . . . . . . . . . . . . . 4-2 Investigation of Existing Pavements . . . . . . . . . . . . . . . . . . . . . . 4-2-1 Evaluation of Pavement Structural Characteristics Using Falling Weight Deflectometer . . . . . . . . . . . . . . . 4-2-2 Evaluation of Pavement Functional Characteristics Using Longitudinal Profile Measurements . . . . . . . . . 4-2-3 Pavement Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . 4-3 Soils and Material Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3-1 Testing of Bituminous Pavement Materials . . . . . . . . . . 4-3-2 Soils and Unbound Materials . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
115 115 116 150
157 163 164 164 184 201 204
Calculation of Engineering Parameters . . . . . . . . . . . . . . . . . . . . . . . . 5-1 Air Temperature and Air Freezing and Thawing Indices . . . . . 5-2 Surface Temperature and Surface Freezing and Thawing Indices . . . 5-2-1 The n-Factor Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2-2 Radiation Index Approach . . . . . . . . . . . . . . . . . . . . . . . .
209 210 217 217 218
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Contents 5-3 5-4 5-5
Temperature in Asphalt Concrete . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Properties of Soils and Pavement Materials . . . . . . . . Freezing and Thawing Indices within the Pavement Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6 Frost and Thaw Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6-1 Transmitted Freezing Index Method . . . . . . . . . . . . . . . 5-7 Frost Heave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7-1 Konrad’s Method for Frost Heave Prediction . . . . . . . . 5-7-2 Saarelainen’s Method for Frost Heave Prediction . . . . 5-8 Thaw Settlement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9 Stresses and Strains in Pavements . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
220 223
6
Design Considerations and Approaches . . . . . . . . . . . . . . . . . . . . . . . 6-1 Lifetime Engineering Considerations . . . . . . . . . . . . . . . . . . . . . 6-2 Long-Term Procurement Methods . . . . . . . . . . . . . . . . . . . . . . . . 6-3 Life-Cycle Cost Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3-1 Factors of Life-Cycle Cost Analysis . . . . . . . . . . . . . . . . 6-3-2 Calculation of Life-Cycle Costs . . . . . . . . . . . . . . . . . . . . 6-4 Pavement Management Concepts . . . . . . . . . . . . . . . . . . . . . . . . 6-4-1 Network-Level PMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4-2 Project-Level PMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
247 247 248 251 252 255 258 258 261 262 262
7
Mix Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 Mix Design Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2 Hot Mix Asphalt Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2-1 Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2-2 Trial Aggregate Gradations . . . . . . . . . . . . . . . . . . . . . . . 7-2-3 Volumetric Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2-4 Performance Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3 Cold Mixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3-1 Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3-2 Selection of Optimum Asphalt Residue Content . . . . . 7-3-3 Cold Mix Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4 Stabilized Bases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 Asphalt Surface Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6 Gravel Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
265 265 266 269 278 281 286 292 293 296 298 302 305 306 307 309
8
Pavement Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 Current Practice in Pavement Design in Cold Climates . . . . . . 8-1-1 Pavement Design Approaches . . . . . . . . . . . . . . . . . . . . . 8-1-2 Synthesis of Design Methods Used by Highway Agencies in Cold Climates . . . . . . . . . . . . . . . . . . . . . . .
313 313 313
225 227 228 230 230 232 232 235 239 245
315
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Contents 8-2
Mechanistic-Empirical Pavement Design Procedure for Cold Region Pavements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 Selection and Design of Special Protective Features . . . . . . . . . 8-3-1 Control of Frost Heave . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3-2 Pavement Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . 8-4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
322 333 333 343 344 345 345
9
Maintenance and Rehabilitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-1 Routine Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2 Major Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-1 Surface Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-2 Overlays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-2-3 Maintenance of Drainage Systems . . . . . . . . . . . . . . . . . 9-2-4 Repair of Local Failures . . . . . . . . . . . . . . . . . . . . . . . . . . 9-3 Maintenance of Gravel Roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-4 Rehabilitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-5 Load Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
349 349 352 352 354 354 354 355 358 362 367 368
10
Pavements on Permafrost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-1 Causes of Instability and Problem Manifestation . . . . . . . . . . . 10-2 Climate Warming and Its Effect on Permafrost . . . . . . . . . . . . . 10-3 Management of Transportation Infrastructure Built over Thaw-Sensitive Permafrost . . . . . . . . . . . . . . . . . . . . . . . . 10-3-1 Identification of Thaw-Sensitive Areas . . . . . . . . . . . . . 10-3-2 Characterization of Thaw-Sensitive Soils . . . . . . . . . . . 10-3-3 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3-4 Technical and Economical Assessment of Applicable Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-3-5 Implementation of the Strategy . . . . . . . . . . . . . . . . . . . . 10-4 Embankment and Pavement Design over Permafrost . . . . . . . . 10-4-1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4-2 Protection Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4-3 Methods Based on Preventing Heat Intake Underneath the Embankment . . . . . . . . . . . . . . . . . . . . 10-4-4 Methods Based on Heat Extraction from the Embankment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4-5 Methods Based on Embankment Reinforcement . . . . . 10-4-6 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-4-7 Applicability and Relative Cost of Protection Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
369 369 374
Index
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374 375 375 375 376 376 376 377 380 380 385 391 394 396 397 398 403
Foreword
A
new book on cold regions pavement engineering is great news and addresses a real and growing need. Cold regions present unique challenges to engineers, and Guy Doré and Hannele Zubeck have the experience and expertise to meet these challenges. Cold regions cover not only a substantial geographic area that includes North America, northern Europe, the Nordic countries, Russia, and northern Asia, but cold regions are increasing in significance as oil and gas production, mining, and transportation links grow in importance. Pavement engineering and the supporting technologies must incorporate the effects of temperature extremes, materials behavior, snow and ice, variable soil conditions, long distances, limited financial resources, high costs, variable bearing capacity, and other special conditions in making planning, design, and construction decisions, as well as implementing them. In writing the first book of its kind, Doré and Zubeck focus on cold regions, but they have certainly not limited themselves. They clearly recognize these special conditions and effects, and they make a major contribution to the state of knowledge and practice. Practitioners and researchers alike in the public and private sectors as well as academia are the beneficiaries. The costs of constructing pavements in cold regions and preserving them through proper maintenance are enormous. It is vital that the required technologies are understood and applied. The timeliness and value of this book in providing a foundation for such understanding will be apparent to the reader. While the authors address the special influences and sensitivities of environment, subgrade, materials, construction, and maintenance, they place equal emphasis on basic principles. This book addresses a pressing need, but it is also noteworthy in its comprehensive treatment of the underlying fundamentals, its extensive coverage of the subject, its presentation of problem descriptions, assessments, and remedial solutions reinforced by examples, and its references to the original sources of technology development. Current reference citations are provided, of course, so that readers can pursue up-to-date details of design, maintenance, and rehabilitation. A chapter on permafrost adds to the value of this book. So what makes Doré and Zubeck so authoritative? Both authors enjoy international reputations and experience in pavement engineering, complemented by an impressive track record of research and professional accomplishments. Guy Doré, at Laval University in Quebec, is a transportation, geotechnical, and materials engineer who has worked on various parts of the U.S. Strategic Highway Research Program and spent a sabbatical
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Foreword leave in Alaska. He is certainly among North America’s key players in the pavement field. Hannele Zubeck, now with the University of Alaska Anchorage, builds upon extensive geotechnical background in her native Finland to add the complementary expertise and experience. Together, Doré and Zubeck have written a book that is a must-read for everyone in the field of pavement engineering and management. After many years of working in pavement engineering and management, as a teacher, researcher, and practitioner, and after two decades of teaching a course on northern engineering, I find this book by Doré and Zubeck a most welcome and timely contribution. It has been a pleasure to read the book, and it is an honor and a privilege to offer these few comments on what the book is about, why the subject is important, what is special about the book, and what is the authoritative background for the contribution. Ralph Haas, CM, FRSC, FCAE, FEIC, FCSCE, Ph.D., P.Eng. The Norman W. McLeod Engineering Professor and Distinguished Professor Emeritus University of Waterloo Waterloo, Ontario
Preface
O
ur goal was to author a book that will prepare engineers to make the right decisions in areas where freezing temperatures, unstable soils, snow and ice, sparse population, long road mileage, and often limited funds dictate design and maintenance actions on pavement structures. We aim for Cold Regions Pavement Engineering to be utilized by practicing civil engineers serving private consultants and by public agencies holding responsibility over roads in cold regions. We complemented the text with solved examples and problems that make the book also suitable as a textbook for graduate and upper-class civil engineering students. The book is divided into 10 chapters. Chapter 1 introduces readers to characteristics of cold regions pavements and pavement terminology. Road networks and their peculiarities in cold regions are explained in order to comprehend the special considerations required for pavement design and management. Pavement surface types used in cold regions are described as well as roles of pavements and pavement layers. Engineering challenges related to pavement design, materials, construction, and maintenance resulting from the aforementioned challenges in cold regions are introduced. Environment is the main cause of pavement engineering being different in cold regions than more temperate regions, and therefore a whole chapter is dedicated to it. Chapter 2 covers pavement environment, defined as a set of physical processes related to loading and climatic factors acting on a pavement in a given geological and geomorphologic context. Theories behind pavement temperature, stress regime, and moisture fluctuation including frost action are covered in detail. Chapter 3 leads the reader into the challenges in pavement performance in cold regions. Performance of asphalt pavements is explained regarding failure modes such as cracking, rutting, and disintegration of pavement surfaces. Pavement performance of the underlying structural layers and subbase including frost heaving, bearing capacity loss during spring thaw, and frost destructuration of clays is described. Problem assessment for each failure mode is explained with suggestions for appropriate mitigation techniques. Chapter 4 covers intensively investigation and testing of pavement materials, subgrade soils, and existing pavements. Focus is made on the test methods used mainly in cold regions. The test methods are described in detail including the analysis of the test results. Calculation of engineering parameters needed for pavement design is presented in Chap. 5. These parameters include air, ground, and pavement temperatures as well as estimation of frost depth, frost heave, and thaw settlement. Determination of stresses and strains at critical pavement interfaces needed for pavement structural design is also explained.
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Preface Design considerations and approaches are covered in Chap. 6. Pavements are longterm products and therefore lifetime engineering concepts and long-term procurement methods are introduced. Calculation of lifetime costs is covered in detail. Pavement management concepts including pavement condition assessment, prioritizing candidate sections, the impact of funding decisions, and the feedback process are discussed. Chapters 7 and 8 cover the design of pavements in cold regions. Chapter 7 focuses on mix design of bituminous pavement layers, surface treatments, stabilized bases, and gravel surfaces. Chapter 8 includes a synthesis of current pavement design practice, a description of the state-of-the-art methodology for mechanistic-empirical design of pavements, and guidelines for the selection and design of special features for the mitigation of cold region performance problems. Maintenance and rehabilitation of cold regions pavement have their own distinctiveness. Chapter 9 presents routine and major maintenance techniques including selection of appropriate rehabilitation techniques. Examples of winter maintenance quality standards are given as well as maintenance techniques. Rehabilitation techniques due to poor bearing capacity and widening of road embankments are proposed. Seasonal load restrictions unique to cold regions pavements are portrayed. Pavements in permafrost deserve their own chapter due to the fact that protection of permafrost from thawing is the most important part of the pavement design. Chapter 10 describes the causes and manifestation of thermal degradation, management considerations, design principles, and protection techniques for pavements on permafrost. Authoring this book would have not been possible without help from several supporters. We would especially like to acknowledge Dr. Ralf Haas, an internationally known expert in pavement engineering and management, who wrote the foreword for this book. Dr. Terhi Pellinen provided tremendous help in reviewing and providing materials for several sections, particularly the section on HMA mix design. Taina Rantanen coauthored the chapter on maintenance and rehabilitation of cold regions pavements, and Isabelle Beaulac synthesized most of the information used in Chap. 10. Several other individuals have provided their support and suggestions throughout the project. These persons include Ivar Horvli, Sven Knutsson, Kauko Kujala, Seppo Saarelainen, Steve Saboundjian, Safwat Said, and Ted Vinson. ASCE’s Technical Council on Cold Regions Engineering and its Committee on Transportation and Infrastructure viewed the book as important and provided their support and encouragement for making it available for cold regions engineers. Jean Parent perfected our drafts to create fabulous illustrations. Léon van Biljon translated our Frenglish and Finglish into English. Betsy Kulamer, the ASCE Press’ acquisitions editor, always had patience with us, accompanied by friendly words, when the schedule got stretched. Laval University and the University of Alaska Anchorage provided us with invaluable support throughout the authoring, especially in the form of sabbatical leave that made writing the book possible. The most faithful and encouraging supporters, however, have been our spouses Natalie and Brad, who endured with a smile on their faces the countless hours that we spent on researching and typing the manuscript. Our daughters, Léonie, Rosalie, Flavie, Maija, and Elli, deserve a big hug for their fond encouragement and understanding during the project. Guy Doré, Ph.D., Ing. Hannele K. Zubeck, Ph.D., P.E.
CHAPTER
1
Cold Regions Pavements
C
old regions pavements are pavement structures exposed to and affected by frost, ice, and snow for significant periods of time. They are located in seasonal or perennial frost areas, often connecting sparse populations spread out over hundreds of kilometers. Figure 1-1 illustrates the areas in the northern hemisphere where pavements are exposed to significant seasonal freezing and perennial frost conditions. The road alignment may cross regions with undesirable soils that are weak at all times, weak during the spring breakup, or experience heave due to frost, causing uneven driving surfaces. Ideal materials for the pavement structural layers may not be available, thus requiring that materials be brought from far away, local materials be modified, or performance expectations are lowered. Ground movements, thermal stresses and traffic loading, including the use of studded tires, cause pavements to rut and crack more severely in cold regions than in warm regions. Pavement funding in sparse population areas may not cover the capital and operating costs required for ideal pavement performance. Optimization is needed to use funds wisely so that roads are passable, safe, and meet desired performance levels. For these reasons, pavements in cold regions are considered from a different perspective than pavements in the warmer regions, where the traffic volume often dictates the design.
1-1
Road Networks To show the unique nature of the cold regions road networks, their densities are collected (in Table 1-1) from selected states and countries. The road density is defined as the ratio of the total length of roads to the total area of the state or country. The regions with the lowest road network density, namely, Alaska (United States), Yukon (Canada), and Mongolia are all cold regions. Alaska’s road network shown in Fig. 1-2 is 440 times less dense than the average road density in the United States. The road network density in Yukon is 10 times lower than the average density in Canada. The average road network density in Canada is about seven times lower than the average road network density in the United States and 20 times lower than that of France. The road network densities in Nordic countries (e.g., Iceland, Finland, Norway, and Sweden) are about five times lower than in the United Kingdom or France. Since populations in cold regions are lower, the total road length per person is not necessarily lower in cold regions than in warm regions. Low road density does not only mean few roads in a large area, but also long distances between settlements, maintenance stations, paving plants, and other resources.
1
FIGURE 1-1 Areas in the northern hemisphere where pavements experience significant seasonal and perennial frost conditions.
Total Length of Road Network, km Alaska∗
Land Area, km2
Road Density, km/100 km2
22,720
14,772,611
0.15
4,681
478,970
0.98
49,249
1,565,000
3.15
952,000
17,075,200
5.58
Canada
901,902
9,976,140
9.04
‡
12,955
103,000
12.58
§
78,161
338,145
23.11
‡
91,180
324,220
28.12
210,760
449,964
46.84
Yukon† Mongolia
‡
‡
Russia
‡
Iceland Finland
Norway
Sweden
‡
United States
‡
6,370,031
9,629,091
66.15
United Kingdom‡
371,603
244,820
151.79
France‡
892,900
547,030
163.23
1,152,207
377,835
304.95
Japan
‡
∗
http://www.dot.state.ak.us/stwdplng/highwaydata/pub/cprm/2002cprm.pdf [certified public road mileage in centerline km as of December 31, 2002: paved roads include asphalt surface treatments (AST)]. † http://www.gov.yk.ca/facts/#LAND (1990/2000 forecast for maintained road surfaces). ‡ The CIA’s World Fact Book, http://www.cia.gov/cia/publications/factbook/geos/rs.html. § http://www.tiehallinto.fi/pls/wwwedit/docs/17702.pdf (length of public roads in 2007).
TABLE 1-1
2
Road Network Densities Ranked in Order from Sparse to Dense for a Few Selected Areas
Cold Regions Pavements
FIGURE 1-2
Road network in Alaska.
Depending on government structure and politics, areas with low road densities may not receive adequate construction and operation funding. Since road construction and operation is more expensive per kilometer in cold regions than in warm regions, due to frost and other cold conditions, the funding limitations become magnified. Another unique feature of cold regions pavements, when compared to warm regions is the share of unpaved or undeveloped roads. Statistics of pavement types for selected states and countries are shown in Table 1-2. For example, 66 percent of Alaskan roads are not paved, whereas only 10 percent of the roads in the entire United States (including Alaska) are unpaved. The same applies for most of the cold regions (excluding Norway, Sweden, and Finland). The share of unpaved or undeveloped roads is significantly larger than paved roads. While the goal of public road agencies may be to increase the percentage of paved roads to minimize the maintenance costs for increased services to local populations and for dust control, it may not always be the goal of some local populations in quiet, remote areas. The beauty of many cold areas draws large numbers of tourists, which sometimes conflicts with the local lifestyle. Paving a road may change a quiet country road into a conduit for tourist buses, bringing hundreds of daily visitors and may change commercial traffic routes. Even if the funding for cold regions pavements were increased, there would probably still be unpaved roads. The proportion of unpaved roads and the unique effects of cold climate on pavement structures make it necessary for governments in cold regions to fund local research. For example, Superpave technology does not address all the conditions in cold climates adequately. Even if the research behind the Superpave was conducted by an outstanding research team, cold regions road authorities have to create their own design manuals and cannot rely purely on research conducted in warm regions.
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4
Chapter One Paved Roads, km HMA or PCC
Surface Treatment
Unpaved Roads, km Unpaved (Gravel)
Unknown or Undeveloped
Total, km (% Unpaved)
United States (1997)a
5,733,028
637,003
Alaska (2002)b
7,791
8,744
Canada (1999)a
318,371
583,531
901,902 (65%)
Yukon (2000)c
260
2,525
4,681 (54%)
Finland (2007)f
50,836g
27,325
78,161 (35%)
Sweden (1999)a
162,707
48,053
210,760 (23%)
Norway (1999)a
67,838
23,342
91,180 (26%)
Iceland (2003)a
3,863g
9,092
12,955 (70%)
Russia (1998)a
336,000
416,000h
200,000
952,000 (65%)
Mongolia (2000)a
1,563
1,824
45,862
49,249 (97%)
United Kingdom (1998)a
371,603
—
371,603 (NA)
Japan (1997)a
863,003
289,204
1,152,207 (25%)
France (1999)a
892,900
—
892,900 (NA)
1,897e
6,370,031 (10%) 6,185d
a
22,720 (66%)
The CIA’s World Fact Book, http://www.cia.gov/cia/publications/factbook/geos/rs.html. http://www.dot.state.ak.us/stwdplng/highwaydata/pub/cprm/2002cprm.pdf [certified public road mileage in centerline km as of December 31, 2002: paved roads include asphalt surface treatments (AST)]. c http://www.gov.yk.ca/facts/#LAND (1990/2000 forecast for maintained road surfaces). d Classified as unknown. e Bituminous surface treatment, BST. f http://www.tiehallinto.fi/pls/wwwedit/docs/17702.pdf (length of public roads in 2007). g Includes cold mixes. h All-weather gravel surface. b
TABLE 1-2
Road Networks Divided between Paved and Unpaved Roads
Cold Regions Pavements
1-2
Pavement Surface Types The most common pavement surface types in cold regions include traditional hot mix asphalt, cold mixes, surface treatments, and gravel surfaces. All of these pavement types cover thousands of kilometers of road as shown in Table 1-2. Which pavement surface is the most common depends on the region. Alaska and western Canada pave some of their roads with asphalt surface treatment (AST). Iceland and Finland have used cold mix (oil gravel), but currently use reduced temperature mixes containing soft asphalt cements. Portland cement concrete and cement treated materials are also used, to a limited extent, in seasonal frost areas. For instance, several states and provinces in north central United States and central Canada use these materials for road construction. Pavements built using cement-based materials, commonly referred to as rigid or semi-rigid pavements, can perform very well if built at the suitable locations using proper design methods, materials, construction, and maintenance techniques. Among other applications, rigid or semi-rigid pavements are known to perform well when subjected to intense heavy traffic loading. There are, however, some limitations to their use in cold climates which restricts their applicability. Their relatively high initial cost and their sensitivity to differential soil movements are two main reasons making these pavements unsuitable in cold areas where traffic volumes are low and soils are sensitive to frost action. Being rarely used in these areas, it also becomes difficult to maintain good expertise for design, construction, and maintenance of rigid or semi-rigid pavements. Without downplaying the importance of rigid or semi-rigid pavements in some specific contexts, the book will focus mainly on hot mix asphalt, on asphalt treated, and on gravel pavements which constitute the vast majority of pavements used in cold environments. The following sections describe pavement types, while selection criteria, mix design, and structural design are described in Chaps. 6, 7, and 8.
1-2-1
Hot Mix Asphalt
Hot mix asphalt (HMA) is the most common pavement type used for high traffic volume roads. It contains typically 94 to 96 percent aggregate by weight and 4 to 6 percent asphalt cement. Antistripping agents, polymer modifiers, and fillers are some of the additives occasionally used to address anticipated performance problems. HMA is produced in a centralized hot mix plant, transported to the site by trucks, spread out by pavers and compacted by rollers. The mixing temperature at the hot mix plant is based on equivalent asphalt cement viscosity of 1.75 Pa·s that allows complete coating of aggregate while preventing unnecessary heating. The mixing temperature is about 135 to 160°C for straight run asphalt cements (PANK 2000) and higher for polymermodified asphalt cements. The layer thickness varies between 50 and 150 mm. The mix design is based either on the Marshall, Hveem, Superpave (Asphalt Institute 1997; 2001) or a similar procedure.
1-2-2
Cold Mix
Cold mix differs from HMA by the mixing temperature. Cold mixes are mixed at the ambient temperature or slightly heated. Lower mixing temperatures are made possible by modifying the asphalt cement by either emulsification, addition of lighter oil components, or by using road oils or extremely soft asphalt cements. The layer thickness is typically 50 mm. The mix design is based on similar techniques as used in HMA mix design.
5
6
Chapter One
1-2-3
Surface Treatments
While chip seals and coatings are used widely as surface treatments in warm climates, bituminous surface treatment (BST) is used extensively in the Yukon and Canada as a low cost highway surface course (MacLeod 1989). Alaskans have been using the same type of treatment since 1987, calling it asphalt surface treatment (AST). The AST and BST consist of a thin layer of asphalt binder, typically high float asphalt emulsion, covered with well-graded aggregate. In comparison, single-size aggregates are used to cover the emulsion in a chip seal application. Figures 1-3 and 1-4 show AST surfaces in Alaska and Yukon.
FIGURE 1-3
Close-up of asphalt surface treatment on Taylor Highway, Alaska.
FIGURE 1-4
16 Year-old asphalt surface treatment section in Yukon.
Cold Regions Pavements
FIGURE 1-5
Gravel surface of Denali Highway, Alaska.
The advantages of surface treatment versus gravel road are dust control, improved drainage, improved driving surface and reduced maintenance. Furthermore, the dust free surface and improved driving surface, come without the costly capital outlays required for hot-mix pavements. In permafrost areas, BSTs can be re-profiled more easily than conventional hot-mix asphalt. They can be rehabilitated more often and still remain cost effective (MacLeod 2000).
1-2-4
Gravel Surface
Gravel surface is one of the most common pavement types in cold regions. While it has the lowest capital cost to construct, the maintenance costs are often higher than that for paved roads. Gravel roads need periodical grading and dust mitigation. A typical gravel surface for the Denali Highway in Alaska is shown in Figure 1-5. Gravel surface may be treated with dust control palliative (such as calcium chloride), asphalt emulsions or proprietary blends.
1-2-5
Stabilized Bases
The base course beneath the wearing course may be bound or unbound. Asphalt product stabilization is the most common binding method in cold regions due to its flexibility. The asphalt products used are asphalt cement, asphalt emulsion or high float asphalt emulsion. The asphalt content is typically lower for the base course than for the surface layer.
1-3
Role of Pavements and Pavement Layers Pavements are large-scale linear structures stretching out across various geomorphologic, geologic, and climatic environments. Pavements, acting as an interface between the traffic and the underlying soil, have a twofold role: from top-down, they distribute the load, and from bottom-up, they attenuate various geotechnical effects. The first
7
8
Chapter One
FIGURE 1-6
Pavement system and related terminology.
aspect of a pavement’s role is to distribute loads from heavy traffic, which is well known and widely documented. Each layer of the pavement system must sustain a large number of moving load repetitions and effectively transmit an acceptable level of stress to the underlying layer. The second aspect of a pavement’s role is predominant in cold climates, it is not as well known or formally taken into consideration in pavement and material design procedures. A pavement’s second role is to attenuate environment-related stresses and displacements, such as differential frost heave and postconsolidation, as well as temperature, moisture content, and negative pore pressure (matric suction) variations, in order to maintain a good structural and functional performance. In order to fulfill these essential roles, pavement systems are composed of several layers, each having a specific role to play within the structure. As illustrated in Fig. 1-6, from top-down, pavements systems are typically composed of the surfacing layer, the base course, the granular subbase and the subgrade soil. Other features of the pavement system also contribute to its structural and functional stability. The most important features are the embankment geometry and the drainage system.
1-3-1
Surfacing Layer
As discussed above, when surfacing layers are used on cold region pavements, they typically consist of hot mix asphalt, cold mix asphalt, or bituminous surface treatments. The surfacing layer plays a structural and a functional role in the pavement system. The surfacing layer is the stiffest layer and thus the most effective layer for load distribution (except for the BSTs, which are generally considered to have a negligible structural role in pavements). Surfacing layers play another structural role. By sealing the surface of the pavement, they contribute to keeping the underlying granular layers relatively dry. In doing so, they help maximizing stiffness of those layers. In that sense, BSTs will also contribute to the structural capacity of a pavement system. Surfacing layers also play an important functional role in the pavement for traveling vehicles by providing adequate adherence and ultimately (since the whole pavement system is contributing), a good drive quality. They also improve the overall appearance of the road embankment and support pavement markings.
1-3-2
Base Course
The base course is a relatively stiff layer playing, essentially, a structural role in the pavement structure. The ability of the base layer to distribute load depends mainly on the material density and moisture content. Aggregate gradation, roughness, and shape also contribute to the stiffness of the granular base material. When an open graded
Cold Regions Pavements drainage layer is used, it is also responsible for the rapid evacuation of infiltrated water at the top of the pavement system. Three primary types of material are used for the construction of base courses: dense graded granular material, stabilized granular material, and permeable granular material (Haas 1997). Dense-graded (or well-graded) gravel with grain size ranging up to 20 mm is the most common material used for the construction of base courses. Mechanical performances of these materials can be maximized by specifying a high percentage of crushed particles. Stabilized base courses can also be used to increase the stiffness of the base course or to reduce moisture and/or frost susceptibility of the material. Different products are used to stabilize the granular base material. Asphalt cement and emulsion are the most commonly used materials in cold regions, but Portland cement, flyash, hydrated lime, calcium chloride, and lignosulfonate are also used (Haas 1997). Permeable granular material can be used in conditions where rapid drainage of water infiltrating through cracks and gravel shoulders is needed. Free drainage is usually obtained by washing or sieving off fine particles from the graded granular material. The material then usually becomes vulnerable to segregation during construction and unstable under load. Thus permeable base materials are often stabilized using a low content of bituminous material or Portland cement.
1-3-3
Subbase
When pavements are constructed over moisture and/or frost susceptible soils in cold and humid environments, the granular subbase also becomes a very important layer in the system. Subbase layers are generally constituted of good quality pit run gravel. The only important requirements for subbase materials is the maximum particle size (generally around 100 mm) to allow for proper compaction and the maximum content in fine particles (generally less than 10 percent passing 0.075 mm) to limit frost and moisture susceptibility. The subbase layer, which is generally stiffer than the underlying subgrade soil, plays a role in load distribution. However, the main role of the subbase layer is to attenuate environmental effects. More specifically, the layer acts as • A drainage layer to minimize and regulate the moisture content of the pavement structure and, more specifically, the base layer. • A separation layer to intercept fine particles from the subgrade soil migrating up under hydraulic pressure during spring thaw and thereby to prevent contamination of the overlying base course. While achieving this role, part of the subbase layer will be lost over the life span of the pavement. Depending on the conditions, a specially designed granular filter layer or a geotextile might be required to help the subbase layer achieve this specific function. • A frost protection layer. The subbase will achieve this role in three different ways. Firstly, through accumulated heat, the layer will resist frost penetration and reduce the duration of frost action in the frost susceptible subgrade soil, thus reducing frost heave and thaw weakening of the pavement system. The thickness of the subbase should be designed to limit or prevent frost penetration in frost susceptible subgrade layers. Secondly, if frost heave occurs, the subbase layer will dampen any differential movement that may result from frost action. Thirdly, the subbase layer will contribute to some extent to load distribution and drainage while frost susceptible subgrade soils are thawing.
9
10
Chapter One
1-3-4
Subgrade Soil
Despite the fact that subgrade soil is not part of the pavement structure, it is an important layer of the pavement system. In fact, it is by far the most complex layer of the system. In most cases, subgrade soils are local materials ranging from barely modified natural soils to engineered filled material. In cut sections, subgrade soils are generally constituted of natural mineral soils graded and compacted in place. In fill sections, subgrade soils are generally constituted of mineral soils excavated elsewhere on the construction project and then compacted in terrain depressions in order to obtain the desired grade. Subgrade soils are often compressible as well as moisture and frost susceptible. Moreover, they are often heterogeneous which makes them prone to differential behavior. The investigation of subgrade soils and the identification of problems will be discussed in Chap. 4. When poor performance of the subgrade soil is anticipated, the design engineer needs to decide if subgrade soils should be improved or if the pavement structure should be adapted to withstand the expected problems. Subgrade improvement techniques, generally considered when granular materials are scarce and adaptation techniques are too expensive, include homogenization, stabilization, reinforcement, and replacement of the subgrade soil.
1-3-5
Special Pavement Layers
Special layers used in pavements to perform specific roles include drainage layers, separation layers, reinforcement layers, and insulation layers. Drainage layers are generally used when the normal pavement layers are not expected to effectively drain water out of the system. This situation might be caused by an expected excessive water infiltration in the pavement system or by the use of poor draining material in pavement layers. Drainage layers can consist of open graded gravel wrapped in geotextile or of a geocomposite material. As illustrated in Fig. 1-7a, typical use of drainage layers includes • Horizontal drainage layers placed underneath the surfacing layer when excessive moisture contents are expected due to seepage through cracks, joints or granular shoulders, or due to frost action. • Horizontal drainage layers placed underneath the pavement structure when excessive moisture contents are expected due to abundant precipitation or thawing of ice-rich subgrade soils. • Vertical drainage layers placed near the pavement edge in order to intercept and evacuate water infiltrating through gravel shoulders. • Horizontal capillary barriers placed below expected frost depth to reduce water migration toward frost susceptible soils. It is generally accepted that active drainage of pavement systems is beneficial for the seasonal and long-term pavement performance. However, the effectiveness and more specifically the cost effectiveness of some of these layers are often questioned. Separation layers are required when a coarse layer is placed in contact with a finegrained layer in the pavement system. Examples of incompatible materials include coarse granular subbases placed over fine-grained subgrade soils or dense-graded base
Cold Regions Pavements
FIGURE 1-7
Special layers in pavement structures.
courses placed over coarse-grained subbases. Hydraulic pressures (spring thaw, artesian pressure, or other) or gravity can lead to fine particle migration into the coarsegrained layer causing its contamination and reducing its structural capacity. Separation layers generally consist of a geotextile blanket interposed between the two incompatible layers. A granular material can also be used. In all cases, the separation material used must meet specific hydraulic, filtration, and constructability criteria in order to be effective. Reinforcement layers may be required when pavements are subjected to excessive stresses due to frost heave, spring thaw, compressible soils, or other environmental factors. Reinforcement layers typically consist of woven geotextiles, geogrids, or steel mesh. As illustrated in Fig. 1.7b, typical applications include: • Horizontal reinforcement layer at the base of pavement structures constructed over compressible soils
11
12
Chapter One • Horizontal reinforcement layer placed at the bottom of the base course to improve confinement of the granular material and bearing capacity during spring thaw • Horizontal reinforcement layer placed within or at the bottom of the bound layer to resist tensile stresses caused by temperature or frost heave The cost effectiveness of these applications still needs to be demonstrated. Insulation layers are often used in conditions where an excessive differential movement caused by frost action or excessive weakening during spring thaw is expected. As illustrated in Fig. 1.7c, the insulation layer consists of a horizontal layer of low thermal conductivity material. Extruded polystyrene is the most commonly used material for pavement insulation. Several other materials such as expanded polystyrene, sulfur foam, polyurethane foam, expanded clay, polystyrene-concrete mix’s, tire chips, sawdust, tree barks, and peat are used for pavement insulation material. The depth of the insulation layer should be carefully calculated considering the facts that the benefit of the insulation is maximized if the layer is near the surface (better protection of the pavement system) while the risk of poor mechanical performance and the risk of differential icing at the pavement surface decrease with depth. In addition, the cost of placing an insulation layer in an existing pavement increases considerably with depth. Pavement insulation is discussed further in Chap. 8.
1-3-6
Embankment Geometry
The road embankment also plays an important role in pavement performance. Granular materials constituting base and subbase layers generally have a stress dependant behavior under load resulting in a stiffness increase with confinement stress. Considering this typical behavior, embankments with narrow shoulders and steep slopes do not provide good confinement conditions to pavement structural layers resulting in larger deformations under loading and ultimately greater permanent deformation. Increasing embankment width and reducing slopes will result in a more stable embankment. In conclusion, despite the fact that pavements appear to be simple structures, they are complex multilayer systems where every layer plays an important role. Moreover, as discussed in the following chapter, pavements are in complex interaction with environmental factors among which traffic and climate are the most important.
1-4
Design Considerations Pavements are long term products, therefore lifetime engineering approaches starting with the investment planning and decision making should be applied. Lifetime engineering considerations in the integrated design, management, and maintenance planning, and in recovery and reuse of construction materials are all vital for wise use of resources. For a cold region pavement to technically perform according to the desired level of service, four criteria need to be considered: design, materials, construction, and maintenance. These components are as links in a chain. When the weakest link fails, the entire chain fails. Therefore, proper lifetime design considering available capital, desired performance, operation and maintenance funding, access to the site, local traffic, subgrade soils, available construction materials, construction equipment, skilled labor, and
Cold Regions Pavements weather conditions need to be applied. Due to changing conditions, cookie-cutter designs seldom work in cold regions and should not be used. Technologies transposed from warmer or more developed regions should not be implemented without validation through pilot projects. Availability of funding and funding sources affect pavement design. Ideally, unlimited funding would ensure long-lasting pavement structures with low maintenance needs. This situation, however, seldom occurs. Typically available funds are stretched over a multitude of projects. Pavement management methods are used to prioritize and optimize the road sections needing rehabilitation or reconstruction in relation to the costs of the individual projects. In cases where capital funding is more available than operation and maintenance funding, low maintenance pavements should be designed. On the other hand, if funding for capital projects is low, and there is more financial support for operation and maintenance, then roads with lower capital cost to perform under constant maintenance operations should be designed. After proper materials have been specified in the design phase, quality control and assurance should be carried out before and during construction to ensure that the materials do not become the weakest link. The materials need to resist not only the construction and traffic loading without breaking apart, but also environmental stresses such as freeze-thaw cycling, thermal stresses, extremely cold and sometimes hot temperatures. Construction in cold regions differs from warm regions due to the daylight, weather, short construction season, and a multitude of challenges due to long distances. Sometimes, no land or water access exists to the construction site. Excavation and compaction of frozen soils may be an issue. Paving, when the weather is cold or wet and when hauling distances are long may be a real challenge. Clearly written contract documents and end-result specifications with physically measurable values are recommended. As new materials or systems are introduced to market, special construction equipment or methods may be needed to meet the specifications. Cultural and ecological considerations are a vital part of the lifetime engineering methodology, and their importance increases in cold regions with indigenous populations and fragile ecosystems. For example, the selection of the wearing course is affected by sociological and ecological considerations. A paved surface would minimize road dust and its ecological impact, but also have an aesthetic impact changing the character of an area due to increased mobility and subsequent traffic.
Review Questions 1-1. Identify and briefly discuss three factors contributing to the complexity of designing and building roads in cold regions.
1-2. What is (a) the road network density and (b) percentage of paved roads in your area? 1-3. What are the main factors to consider for the selection of a pavement surface type? 1-4. What are the characteristics of a granular subbase and what are the main functions of the layer in a cold region context?
1-5. layer.
Identify three ways to reinforce pavement structures using a synthetic reinforcement
13
14
Chapter One
References Asphalt Institute (1997). MS-2 Mix Design Methods for Asphalt Concrete and Other Hot Mix Types, 6th ed. Asphalt Institute, Lexington, Ky. Asphalt Institute (2001). SP-2 Superpave Mix Design, 3d ed. Asphalt Institute, Lexington, Ky. Haas, R. (1997). Pavement Design and Management Guide. Transportation Association of Canada, Ottawa, Canada. MacLeod, D. R. (1989). A BST Management System for Yukon Highways. Public Works Canada, DIAND Technical Services, Government of Yukon Community & Transportation Services. MacLeod, D. R. (2000). “BST Management Systems in the Yukon Territory,” Technology for Alaskan Transportation, vol. 25, no. 4. PANK (2000). Finnish Asphalt Specifications. Finnish Pavement Technology Council, PANK, Helsinki, Finland.
CHAPTER
2
Pavement Environment
T
he terms environment and environmental factors have been used widely and rather loosely to describe pavement conditions. For the purpose of pavement engineering, pavement environment can be defined as a set of physical processes related to climatic factors acting on a pavement in a given geological and geomorphologic context. Thus, it involves the interaction of climatic factors, soils, and land morphology. Pavements have their own climate. There are several similarities and interactions between pavement climate and atmospheric climate. They both can be characterized by their temperature, their level of humidity, and by acting pressures. They are also affected by daily and seasonal variations of these parameters, by their spatial distribution, and their interactions with the system’s environment (Oliver 1973). As opposed to atmospheric climate characterized by physical processes active in a gaseous environment, pavement climate occurs in a porous mineral system governed by its own physical laws (Oliver 1973). Pavement climatic factors are in constant interaction with material properties and external loading on the system. Pavement climate can be characterized by its temperature, moisture, and pressure regimes. This chapter includes a summary description of important environmental effects on pavement systems. Basic equations describing relevant phenomena are presented in simple form to help the reader understand the role played by different contributing factors. Readers interested in the development and the use of these equations may need to refer to the references cited.
2-1 Temperature Regime in Pavements The temperature regime of a pavement-soil system is controlled by the boundary conditions of the system, but is also affected by energy available within the system. The temperature at the bottom of the system is practically constant throughout the year and roughly equal to the mean annual surface temperature; at that point, the temperature conditions correspond to the steady-state balance between the geothermal heat flux and the annual average heat loss in the atmosphere. Negligible temperature variations are usually observed at depths of about 10 m in a pavement-soil system. At the top of the system, temperature varies considerably between summer high and winter low temperatures. Temperature at the surface of the pavement-soil system is the result of a complex balance at the pavement-air interface. Figure 2-1 illustrates the main factors affecting pavement thermal regime.
15
16
Chapter Two
± 6
+
+ 7
4a
1
3
±
+
2
Factors contribution to heat intake Factors contributing to heat extraction Factors contributing either to heat induction or extraction
1. 2. 3. 4. 5. 6. 7.
4b
5
Solar radiation Geothermal heat Emitted radiations Convection and turbulence Latent heat of fusion Evaporation/condensation Heat exchange with precipitations
FIGURE 2-1 Summary of the factors affecting temperature regime in pavements (highlighted factors are considered most frequently in pavement engineering).
2-1-1
Factors Inducing Heat in the Pavement System
Solar Radiation Solar radiation is electromagnetic energy emitted by the sun and needs no support for its propagation. It is this parameter that has the largest effect on pavement surface temperature (Dysli et al. 1997). The amount of solar radiation reaching the earth’s surface is a function of the seasonal variations of the length of a day and the angle of incidence with the surface of the earth. The latter factor affects the distance traveled in the atmosphere and the resulting diffusion, reflection, and absorption by atmospheric particles. For a given amount of solar radiation reaching the pavement surface, part of it is absorbed and the rest is reflected back into the atmosphere. The ratio between the radiation reflected by a surface and the total radiation reaching the surface is termed the “albedo” of the surface. The albedo of a normal bituminous pavement surface is approximately 15 percent (10 to 18 percent) which means that 85 percent of solar radiation is absorbed by the surface. Furthermore, the albedo of a packed snow-covered pavement surface is around 55 percent (40 to 60 percent), thus reducing the absorbed radiation to 45 percent. The albedo of a pavement surface covered by fresh snow can reach up to 80 percent.
Geothermal Heat A large amount of heat, accumulated in the earth’s core and crust, flows toward Earth’s surface and is dissipated into the atmosphere. It is called geothermal heat. As a general
Pavement Environment
FIGURE 2-2 Thermal regime fields (trumpet and whiplash curves) in pavement systems for (a) seasonal frost and (b) permafrost conditions.
rule, whenever there is a thermal gradient, heat flow will occur. This mechanism is described by Fourier’s equation: qG = − kTG
(2-1)
According to Eq. (2-1), the heat flux in homogeneous soil, qG, is proportional to the thermal gradient, TG. The proportionality constant, k, is the thermal conductivity of the soil. The slope between the mean annual surface temperature (MAST) and the temperature at the center of the earth is the average geothermal gradient that governs the heat flux at the surface of the earth. For practical pavement engineering considerations, the area of interest is the depth where no significant temperature variation occurs. The average geothermal heat flux at the surface of the pavement system is proportional to the thermal gradient TG shown in Fig. 2-2. The average thermal gradient near the pavement surface increases considerably during winter, thus augmenting the geothermal heat flux at the surface of the cooling pavement system. However, during summer the geothermal gradient is nullified by a steeper opposed thermal gradient resulting from the warming of the pavement surface and as a result, the heat flux is inverted. Heat is thus accumulated in the pavement system until the following cooling cycle. This phenomenon is analogous to water flowing from a river affected by tidal forces into the sea. When the tide is low (low energy corresponding to a cold surface), the current is swift and the flow is high. When the tide is high (warm surface), the gradient and the current are inverted in the mouth of the river and water accumulates until the next low tide.
2-1-2
Factors Contributing to Heat Extraction from the Pavement System
Emitted Radiations All surfaces emit energy, qer, in the form of electromagnetic radiation. A perfect radiator emits radiation, the intensity of which is proportional to the fourth power of its temperature (T) as predicted by the Stefan-Boltzmann equation: qer = σ T 4
(2-2)
17
18
Chapter Two where s is the Stefan-Boltzman constant (5.67 × 10−8W/m2·K4). Pavement surfaces emit long-wave and infrared radiations mostly during the night when the pavement surface is often warmer than the air. Adapted to heat exchange between pavement surface and atmosphere, Eq. (2-2) can be written as
(
qer = σε s Ts 4 − Ta4
)
(2-3)
where es is the surface emissivity, which, for asphalt pavements is in the range of 0.90 to 0.95, and Ts and Ta are, respectively, the temperature of the surface and the atmosphere.
Convection and Turbulence Another important mechanism contributing to heat extraction from pavement surfaces is convection. Convection requires the support of moving fluid such as air. In order for heat to be extracted from the pavement surface, it must first be transferred to a thin film of air by conduction and radiation. The difference of temperature between the heated thin film of air and the rest of the air mass induces a difference of pressure, which will in turn induce a motion in the air mass. Heat will then move to another location and be dissipated. In the presence of wind-induced turbulence, the motion of the fluid and its dispersive capacity will be increased. As indicated by Eq. (2-4), heat extraction by convection, qc, is thus a function of the temperature difference between the surface, the fluid and wind speed. qc = hc (Ts − Ta )
(2-4)
where Ts and Ta are surface and air temperature and hc is the convection coefficient (W/m2·K). The coefficient hc is in turn a function of the surface drag coefficient, air density, specific heat of air, air Prandtl number and wind speed. Considering the difficulty of measuring or determining these parameters, McAdams (1954), cited by Zarling and Braley 1988, has proposed the following dimensional relationship to determine the convective heat transfer coefficient for a smooth surface as a function of wind speed: hc = 5.678 + 1.056U
(2-5)
where U is wind speed in km/h and hc is in W/m2·K. From Eq. (2-5), it can be inferred that the convective heat flux will double if wind speed goes from 0 to 5 km/h.
2-1-3
Factors Contributing Either to Heat Induction or Extraction
Latent Heat of Fusion One of the basic principles of chemistry is that a system always tends to oppose changes imposed to the system. Phase change of water is not an exception as heat is released during freezing and absorbed during thawing. The quantity of heat released or absorbed during the phase change is termed the latent heat of fusion. Latent heat of fusion, L, is constant at 334 kJ/kg or 334 MJ/m3 for freezing or thawing water. For soils or pavement materials, the latent heat of fusion, Ls, can be obtained from Ls =
ω ρd L 100 ρW
(2-6)
Pavement Environment where w is the gravimetric water content of soil or pavement material, rd and rw are the densities of dry soil and water, respectively.
Evaporation/Condensation Following the same principle for liquid-solid, vapor-liquid phase change will also absorb or release heat. Water accumulated in asphalt and gravel surface pores tends to evaporate in warm and dry conditions, absorbing heat and consequently impeding surface warming. Humidity present in air is likely to condense on cool pavement surfaces releasing heat and impeding surface cooling. Heat released or absorbed by the evaporation/condensation process, qe, can be approximated as qe = nw × h fg
(2-7)
where nw is the evaporative mass flux (kg/s·m2) and hfg is the heat of vaporization of water (2257 kJ/kg). The amount of water involved in the process is, however, generally small. For instance, Dysli (1991) has reported that phase change due to pavement deicing has modified surface temperature by less than 1°C for a period of about 20 min. Heat exchange resulting from the evaporation/condensation process is thus considered to be negligible for surfaced pavements, but can perhaps be significant for unsurfaced roads.
Heat Exchange with Precipitation Precipitation in the form of rain or snow is likely to affect pavement surface temperature, if different from the temperature of the precipitation. Heat will be transferred between the surface and the precipitation by conduction and the effectiveness of the process depends on the difference in temperature and quantity of precipitation. Heat exchange resulting from the contact between pavement surface and precipitation can be significant at the scale of an event, but can be considered negligible over a long period.
2-1-4 Thermal Balance and Thermal Cycles As illustrated in Fig. 2-1, absorbed solar radiation, Qsr, and geothermal heat, Qg, contribute to the heat intake at the air-pavement interface. The main contributors to heat extraction are emitted radiation, Qer, and air convection and turbulence, Qac. Latent heat of fusion, L, evaporation/condensation, Qe, and precipitations, Qp, can either contribute to induction or extraction of heat from the pavement surface depending on the prevailing thermal conditions. The thermal balance of the surface of the pavement can thus be obtained by Qsr + Q g − Qer − Qac ± Qe ± L ± Qp = 0
(2-8)
Qg, Qe, L, and Qp can have a significant contribution to short-term thermal variations of the pavement surface. It is however generally accepted (Dysli et al. 1997; Zarling and Braley 1988; Pavlov 1976) that for the establishment of long-term (seasonal) thermal balance these factors are not significant and can be neglected. For practical considerations, thermal balance equation can thus be written as Qsr − Qer − Qac = 0
(2-9)
19
20
Chapter Two
FIGURE 2-3 Schematic illustration of a diurnal thermal balance cycle at the pavement surface and resulting surface temperature.
All these factors undergo important temporal variations causing the balance to be different than 0. When the balance is positive, pavement surface tends to accumulate heat and its temperature increases. When the balance is negative, pavement surface tends to lose heat and its temperature decreases. As a result, pavement surface temperature follows two typical behavior cycles: diurnal and seasonal. Diurnal cycles: During the day, solar radiation is generally high and emitted radiation is relatively low. Surface temperature tends to increase until the end of daylight. Maximum surface temperature will be observed on clear sunny days without wind. At night, solar radiation becomes negligible and emitted radiation increases. The thermal balance then becomes negative and pavement surface cools down. A typical diurnal cycle is conceptually illustrated in Fig. 2-3. Seasonal cycles: During summer months, the high position of the sun in the sky maximizes the quantity of radiation reaching the surface of the earth. The overall diurnal thermal balance tends to be positive and heat builds up at the pavement surface. In contrast to summer conditions, the sun is low on the horizon during winter months. Consequently, absorption is low and diffusion of solar radiation in the atmosphere is maximized. As a result, the overall diurnal thermal balance is negative and the pavement surface temperature cools down. Figure 2-4 illustrates a typical seasonal cycle. Temperature regime in pavement systems will evolve between a stable bottom temperature and a continually changing surface temperature. The envelope of temperature conditions at any given depth in the pavement system is referred to as the “trumpet curve.” Figure 2-2 shows typical trumpet curves for seasonal frost conditions (Fig. 2-2a)
Pavement Environment
FIGURE 2-4 Schematic illustration of a seasonal thermal balance cycle at the pavement surface and resulting surface temperature.
and permafrost conditions (Fig. 2-2b). Maximum and minimum surface temperatures as well as frost and thaw depth can readily be obtained from these representations. Temperature variations at the surface of the pavement will cause the temperature regime curve to swing within the limits defined by the trumpet curve. These curves represent “snapshots” of temperature conditions at one point in time within the system and are often referred to as the “whiplash curves.” Typical spring and fall whiplash curves are illustrated in Fig. 2-2a. The shape of the curve depends on the boundary conditions at the surface and at the base of the pavement system. The shapes also depend on several factors specific to the physical properties of soils and pavement materials. These factors include: • Thermal conductivity, k: As defined in Eq. (2-1), thermal conductivity is the proportionality constant between heat flux in homogeneous soil, qG, and the thermal gradient, TG. Thermal conductivity represents the capacity of a material to transport heat by conduction. The thermal conductivity of soils and pavement materials increases as dry density increases and as degree of saturation increases. • Heat capacity: It represents the ability of soils or materials to accumulate heat. It is defined as the amount of heat required to raise the temperature of a unit quantity of soil or material by 1°C. Mineral particles and interstitial water contribute to the heat capacity of soils. When the unit quantity is a volume (1 m3), the property is termed the volumetric capacity (Cv) and when it is a mass (1 kg), it is termed the massic heat capacity (Cm). • Moisture affinity: Moisture in soils and pavement materials has an important influence on the thermal regime. The quantity of water present in soils directly affects its thermal conductivity and its heat capacity. It will also generate or absorb an important quantity of heat (latent heat of fusion, L) during phase change. Moisture regimes in pavement systems will be discussed in Sec. 2-2. Figure 2-5 is a schematic illustration of the effect of these factors on temperature regime within the pavement system. Figure 2-5a and b represent, respectively, a cooling
21
22
Chapter Two
FIGURE 2-5 Factors affecting the thermal regime in (a) cooling and (b) warming pavement system composed of asphalt concrete surfacing layer (1) underlain by granular soil (2).
and a warming bilayer system composed of asphalt concrete over granular soils. Temperatures at the top (Tt) and at the base (Tb) are imposed boundary conditions. Following the thermodynamic principle of energy conservation, energy entering a system plus energy generated by the system must be equal to energy leaving the system plus energy stored within the system. If no energy is generated or stored in the system, the heat flux in both layers will be equal and the temperature regime will be governed by the Fourier heat conduction law [Eq. (2-1)]. The permanent temperature regime illustrated by the short-dashed line will rapidly be reached and the thermal gradient will be inversely proportional to the thermal conductivity k of the layer. Soils and pavement materials have the capacity to generate and store heat. When the system is cooling (Fig. 2-5a), energy stored (C) will be progressively released and will impede cooling of the system. This is illustrated by a warmer temperature regime illustrated by the solid line in Fig. 2-5a. With heat being generated in Layer 2, the amount of heat leaving the layer will be greater than the amount of heat entering the layer, as illustrated by steeper gradients near the top of the layer than at the bottom of the layer. The same principle applies to a warming pavement system. Figure 2-5b illustrates that heat is stored in the system during the warming process. The result is a cooler temperature regime. Due to the heat loss within the system, the amount of heat leaving the system is less than the amount of heat entering the system. This is illustrated by a steeper gradient at the bottom of the system than at the top of the system. For soils and pavement materials with moisture available in pores or in ice lenses, energy is also available or storable in the form of latent heat of fusion. The process is illustrated in Fig. 2-5a and b through the phase change of a thin zone of high water content within Layer 2. As indicated by the long-dashed line, latent heat of fusion will increase the heat available in a cooling system, thus increasing internal temperature within and above the high moisture content zone. Here again, since the amount of heat leaving the system is greater than the amount of heat entering the system, the thermal gradients at the top of the system are steeper than at the bottom of the system. In a
Pavement Environment warming system, the latent heat of fusion is acting in the opposing direction absorbing some of the heat flowing through the system. Temperatures are thus reduced and thermal gradients at the bottom of the system are gentler. The latent heat exchange process is, however, limited to a short period of time during which phase change is occurring. Nevertheless, when moisture contents are high, latent heat of fusion is a major factor limiting frost/thaw penetration in pavement systems.
2-2
Moisture Regime in Pavements Moisture conditions in pavement systems ensue from the amount of water in soils and pavement materials, its movements, the form under which it occurs, and the phase under which it operates. Like moisture in the air, soil moisture can occur in gaseous, liquid, and solid forms, each significantly modifying the soils properties (Oliver 1973).
2-2-1
Phases of Water
The gaseous phase, water vapor, is always present in unsaturated soils. The humidity of the air in soil pores is always close to 100 percent (Oliver 1973). Vapor can play an important role in water transportation in unsaturated soils. The liquid phase is the most important phase of water for pavement engineering. Liquid water can be found under different forms in soils and pavement materials. These forms are free or gravitational water, capillary water, hygroscopic water, and chemically bound water. Free water is not subjected to any significant force resulting from the interaction between water and soil particles and can move without restraint in soil pores under acting forces such as gravitation or suction. Capillary water is the portion of water held by surface tension forces in continuous films around soil particles in capillary interstices. Surface tension exists at air-water or water-ice interfaces. It results from the fact that water molecules at the surface of a water body would be unstable if they were not submitted to a very high tensile pull along the surface of the liquid. The thin layer of water submitted to high tensile forces is termed the “contractile skin” (Fredlund and Rahardjo 1993) and controls the behavior of capillary water. Capillary water can move in soil pores under acting forces including capillary force. Hygroscopic water is a thin layer of water bound by chemical attraction to polarized clay particles. In the bound water layer, water molecule tends toward a pseudo-crystalline structure (Dysli 1991), which considerably reduces the mobility of water particles in a plane perpendicular to the surface of the particle. Hygroscopic water cannot move in soils under the forces typically acting in a pavement-soil environment. The only form of water that cannot be removed from soil at high temperature is water chemically bound to soil minerals such as hydrated oxides. In addition to binding forces acting on interstitial water, another important factor controlling water state and mobility in soils and pavement materials is the continuity of the mobile water film in an unsaturated soil matrix. As described by Dysli (1991), unsaturated soil can be classified in three different states: Lenticular water regime where water films are discontinuous, making the hydraulic conductivity null, but allowing for effective vapor transportation through communicating air voids.
23
24
Chapter Two
FIGURE 2-6
Water regime in soils.
Funicular water regime where water and air films are distinct and continuous, making possible water and vapor transportation through the soil matrix. Occluded air regime where vapor transport is not possible, but hydraulic conductivity is relatively effective as it tends toward saturated soil conditions. Solid phase of water begins to develop in soils and pavement materials when the temperature is cold enough for a period of time to allow phase change of interstitial water. Two types of ice are found in soils: interstitial ice and segregation ice. In the first case, water freezes in soil pores. In the case of saturated soils, phase change of the interstitial water can cause a volume change that can reach 1.09 times the soil porosity. For segregation ice, the freezing process involves transportation of water from the warm side of the freezing front toward the segregation front. Thus, it involves an increase in water content and a volume increase of the freezing soil. As for temperature, water regime in soils is governed by boundary conditions with many similarities. As shown in Fig. 2-6, at the bottom of the system, the conditions are relatively stable and correspond to the groundwater table where soils are saturated and water is at atmospheric pressure. At the top of the system, moisture conditions vary widely between saturated conditions and dry conditions as a result of surface characteristics and climatic events. In a typical pavement system, surface conditions may vary widely depending on the longitudinal and transverse position on the pavement. As indicated in Fig. 2-7, numerous water sources may affect water conditions and, consequently, moisture regime in pavement systems. On the other hand, other factors contribute to moisture removal from the pavement system. These factors are listed in Fig. 2-7 and described below.
2-2-2
Factors Contributing to Water Intake in the Pavement System
Capillary Rise Capillary forces will draw up water from the groundwater table zone into a zone called the capillary fringe. Capillary rise varies considerably depending on soil characteristics. The concept of capillary rise has often been demonstrated using an ideal
Pavement Environment
FIGURE 2-7
Factors affecting moisture regime in pavements.
capillary tube in which the rise hc is inversely proportional to the diameter of the tube according to hc =
4T cos α dγ w
(2-10)
where T is the surface tension at the interface of water and air, a is the angle of contact between the water meniscus and the capillary tube, d is the tube diameter, and gw is unit weight of water. Based on the principle that capillary rise is a function of the size of the openings, Hazen (1930) has proposed a formula to estimate capillary rise (hc, mm) in soils based on the effective particle diameter D10 (mm): hc =
C eD10
(2-11)
where C is a constant ranging between 10 and 50 mm2 and e is the void ratio.
Lateral Moisture Transfer Lateral moisture transfer is likely to occur when the groundwater table in the surrounding areas is higher than underneath the pavement. As illustrated in Fig. 2-8a, the resulting hydraulic gradient will induce a water flow underneath the pavement. In addition to the moisture regime, the water mass is likely to modify the temperature regime and to induce hydraulic pressures in the pavement system. A special case of lateral moisture transfer is illustrated in Fig. 2-8b and involves an artesian aquifer. In that specific case, hydraulic pressures and upward water flow can be generated underneath the pavement.
25
26
Chapter Two
FIGURE 2-8 Lateral moisture transfer in pavement systems from (a) adjacent high water table and (b) from artesian aquifer.
These phenomena are likely to reduce the bearing capacity and the stability of the highway embankment. Frost depth may be reduced by the latent heat of fusion accumulated in interstitial water. Frost heave is, however, likely to increase considerably if soils are frost susceptible, due to the more effective water movement toward the freezing front. It is possible to compute the flow rate, ql, into a pavement system from lateral moisture transfer on the basis of Darcy’s law: ql = k h i
(2-12)
where kh is the horizontal hydraulic conductivity of the soil and i is the hydraulic gradient. A comprehensive description of the method for computing lateral moisture flow can be found in Garber and Hoel (1997) and Moulton (1980).
Infiltration of Water from Precipitations Infiltration is one of the main sources of water in pavement systems above the capillary fringe. Water from liquid or solid precipitations (assuming it melts on the pavement surface due to the temperature of the pavement surface or action of deicing chemicals) is likely to seep into the pavement structure through cracks in the bound surfacing layer and through unprotected gravel surfaces such as shoulders and side slopes (case 3a on Fig. 2-7). The following empirical relationship is proposed by the Federal Highway Administration (FHWA) (Johnson and Chang 1984) to estimate the infiltration rate into cracked surfaced pavements: N Wc qic = I c c + + Kp W WCs
(2-13)
where qic is the design infiltration rate (m3/day·m2 of drainage layer), Ic is the crack infiltration rate = 0.22 m3/day·m linear of crack (recommended empirical value), Nc is the
Pavement Environment number of contributing longitudinal cracks or joints, Wc is the length of contributing transverse cracks (m), W is the width of granular base subjected to infiltration (m), Cs is the spacing of transverse cracks or joints (m), and Kp is the rate of infiltration (m3/day·m2) through uncracked pavements (can be assumed to be 0 for dense asphalt concrete). A significant amount of water will also seep into the pavement structure through granular shoulders and unprotected embankment slopes. From equations proposed in the literature (Johnson and Chang 1984) to estimate the runoff from rainfall, Eq. (2-14) can be derived to estimate the infiltration in pavement structures: qi = (1 − C)iA
(2-14)
where qi is the rate of infiltration (m3/s·m2), i is the average rainfall intensity (m/s), A is the infiltration surface (m2), and C is the runoff coefficient that varies from 0.4 to 0.6 for gravel surfaces or shoulders and from 0.5 to 0.7 for embankment slopes (Johnson and Chang 1984). The lower values should be used for flat slopes and permeable soils, while the higher values should be used for steep slopes and impermeable soils. Runoff water will reach the bottom of the ditch where more infiltration is likely to occur before the remaining water is evacuated from the pavement system. A special case of infiltration is likely to occur during winter and spring thaw periods. As illustrated by case 3b in Fig. 2-7, the presence of ice and snow accumulation on the shoulder may block surface drainage, causing water stagnation on gravel shoulders. This phenomenon combined with the possible presence of a frozen layer within the pavement structure may cause water to accumulate in the granular base near the pavement surface. This situation and the resulting problems it may cause to pavements will be discussed further in Chap. 3.
Frost Action Frost action is one of the important sources of excess water within the pavement system. It is widely accepted that three conditions are required for frost heave to occur in a pavement system: (1) the temperature must be sufficiently cold for a long enough period to allow for phase change of the interstitial water, (2) water must be available and allowed to flow freely to the freezing front and, (3) freezing soil must be frost susceptible. Cold region pavements are, to varying degrees, subjected to these three conditions and are likely to experience frost heaving during winter. 1. Temperature: In cold regions temperatures are low enough to induce freezing of the pavement system. Figure 1-1 illustrates areas where pavements are subjected to significant freezing. In North America, the northern half of the United States and most of Canadian territory are subjected to temperatures that are cold enough to induce frost penetration underneath pavement structures. In Eurasia, Russia, China, Mongolia, Finland, Sweden, Norway, and mountainous parts of central Europe are also subjected to substantial frost penetration. 2. Moisture: As indicated above, moisture is generally available in pavement systems through capillary rise from the water table, infiltration of moisture from precipitation and lateral moisture transfer. All these sources of water are likely to supply frost action in pavements.
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Chapter Two 3. Frost susceptibility: This is a soil property that describes the ability of water to flow toward a growing ice lens behind the freezing front. High negative pressures are generated at the base of the ice lens letting water flow in a partly frozen zone of soil between the freezing front and the ice lens. Fine-grained soils are generally more frost susceptible while coarse-grained soils are less. Frost susceptibility of soils will be further discussed in this section as well as in Chap. 4. During winter, frost penetrates pavement materials and subgrade soils. While progressing in the pavement structure, frost causes interstitial water to expand and can also cause some segregation ice to form in the unbound granular materials. Although these phenomena are not considered to cause excessive frost heave in pavement granular material, their significance comes from the fact that they cause the materials to loosen. Heaving of pavement surfaces reaching 10 mm has been observed on experimental test sites (Doré 2004) before the frost front reached the subgrade soil. When the frost front reaches the frost-susceptible subgrade soils, water is sucked toward the segregation front where ice lenses are formed. Heave of the pavement surface, resulting from the segregation ice formation can reach and even exceed 150 mm for climatic conditions prevailing in northern countries. The importance of this phenomenon for cold region pavements warrants a detailed description. Soils at rest with uniform temperature are in thermodynamic equilibrium, which means they are in thermal equilibrium, in chemical equilibrium and in mechanical equilibrium (Henry 2000). As discussed in Sec. 2-1, during the cooling process leading to pavement freezing, a thermal gradient is induced in the system, breaking the thermodynamic equilibrium and causing the system to react and try to regain equilibrium. Work by Taber, Beskow, Everett, Miller, Loch, and Gilpin (summarized in Henry 2000) has led to the current understanding of frost heave mechanism. The process is complex and involves the combined effect of thermal and chemical potential at particle/ice/ water interfaces acting against the mechanical contact between particles. The following paragraphs attempt to describe in a simple way the frost heave mechanism. As shown by experimental data in Fig. 2-9, when a cooling soil mass reaches freezing temperature (0°C for solute-free water), ice begins to form in pores and the unfrozen water content begins to decrease. Temperatures slightly below 0°C are required to force the initiation of ice crystal formation (nucleation) in the pores. The latent heat generated by the nucleation then raises the temperature near the freezing point before it starts to decrease again. Unfrozen water content then decreases progressively as temperature drops until it reaches a residual level of approximately 3.5 percent at about −2°C. At that point, most of the free water and capillary water is frozen and only hygroscopic water remains unfrozen in the soil matrix. The unfrozen water content then decreases very slowly with decreasing temperature. Transposed to a soil column subjected to freezing under a temperature gradient, this situation creates three distinct zones in the column. The lower zone is characterized by temperatures above the freezing point and interstitial water is completely unfrozen. The intermediate zone, comprised between the freezing temperature and the residual level, is characterized by partly frozen interstitial water. Free water and ice coexist in this zone, but their relative proportion changes with temperature. Finally, in the top part of the column, soil water is mostly frozen leaving only a relatively low proportion of hygroscopic water unfrozen. The presence of a zone where free water and ice coexist is the basis of most recent theories on frost heaving mechanisms.
Pavement Environment
FIGURE 2-9 Unfrozen water content as function of temperature: (a) experimental data (Doré et al. 2004) and (b) transposition to soil column subjected to freezing under thermal gradient.
The zone where free water and ice coexist can be several tens of centimeters thick in typical freezing pavement situations. It is a place of significant thermodynamic instability. As illustrated in Fig 2-10, the freezing front corresponds to the lowest temperature at which ice can form in the pores. As indicated by the phase diagram for water, at lower temperatures (i.e., higher in the freezing column), ice exerts more pressure on water
FIGURE 2-10
Partly frozen soil layer in freezing soils and acting pressures.
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Chapter Two and occupies more space in the pores thus creating smaller radii ice/water interfaces. The water film in contact with ice, referred to as the contractile skin (Fredlund and Rahardjo 1993), acts like a membrane in tension and is opposing ice pressure by exerting a tensile stress conferring a negative pressure to interstitial water. This situation has many similarities with water rise in a capillary tube. A narrow capillary will cause the air/water interface meniscus to have a short radius and will increase the negative pressure in the water film causing a high rise in the tube. Suction in frozen soils can be described and quantified by the thermodynamic theory and using the generalized Clausius-Clapeyron equation (Ladanyi and Shen 1989): pi pw − π ∆T − = −L Tf ρi ρw
(2-15)
where pi and pw are ice and hydrostatic pore water pressure, ri and rw are ice and water densities, p is the osmotic pressure associated with leachable solutes, L is the latent heat of fusion, Tf is the freezing temperature, and ∆T is the freezing point depression. Using appropriate values in Eq. (2-15) (i.e., ri = 916.8 kg/m3, rw = 1000 kg/m3, L = 334 kJ/kg, and Tf = 273.15 K), neglecting p for pure water and assuming constant ice pressure in the partly frozen zone, the differential water pressure (dpw/dT) can be estimated to be (Ladanyi and Shen 1989): dpw kPa = 1220 dT K
(2-16)
The differential pressure existing between the top and bottom of the partly frozen soil zone creates conditions favorable to water flow from the bottom of the zone toward the top. Two other conditions must exist for water to flow toward an eventual ice lens. The partly frozen zone must remain permeable and water must be removed at the end of the flow path. Similarly in the capillary tube, no flow will occur in the tube unless water is removed from the top of the water column. If, for example, it is removed through contact with blotting paper, water will flow up the tube to replace the extracted water. Permeability of the partly frozen zone of the freezing soil column varies from unfrozen soil permeability at the bottom to essentially no permeability at the top. Significant permeability is thus likely to be found in the bottom portion of the partly frozen zone of the soil column. The permeability of a partly frozen soil will remain significant, if a continuous film of free water continues to exist between the ice and hygroscopic water within soil pores. This is a soil characteristic that is essentially related to its frost susceptibility. Water removal in a sealed system (mostly frozen soil at the top of the column) needs to occur within the system. This occurs when the hydraulic pressure caused by the negative pressure gradient in unfrozen water exceeds the overburden pressure in the partly frozen zone, in other words, when the effective stress becomes null (Henry 2000). Soil grains are thus separated and an ice lens begins to grow, hence removing water from the underlying partly frozen soil. If all these conditions exist in the frozen zone and assuming saturated conditions, Darcy’s law of water flow in soils should apply: v = kf ×
dpw dx
(2-17)
Pavement Environment where v is the rate of flow in the partly frozen zone and kf is the effective hydraulic conductivity in the partly frozen soil. Consequently, the zone of interest in the partly frozen soil layer is a relatively thin layer of soil comprised between the growing ice lens (segregation temperature, Ts) and the freezing front (freezing temperature, Tf). This layer of soil has been termed the frozen fringe (Miller 1972). Note that the frozen fringe is a much thinner layer than the partly frozen zone described above. Experimental data has shown that the thickness of the frozen fringe was in the range of a few millimeters (Loch and Kay 1978, quoted by Ladanyi and Shen 1989). Within the frozen fringe most of the conditions described above exist and water migration to the growing ice lens is possible. Above the ice lens, the low hydraulic conductivity of the partly frozen soil and the lack of external source of water restrict water movement to a limited redistribution of water within the partly frozen zone. Another important aspect of the frost heave mechanism is that ice lens formation generates a large quantity of latent heat, which will oppose congelation. Ice lenses will continue to grow only if latent heat is effectively removed from the system. As discussed in Sec. 2-1, the effectiveness of the system to remove heat is described by Fourier’s law of heat transfer by conduction [Eq. (2-1)]. Rewording the three basic conditions for occurrence of frost heave given at the beginning of the section, it can be stated that (1) removal of heat (temperature), (2) removal of water through ice lensing (frost susceptibility), and (3) supply of water are required for an effective frost heave mechanism (Henry 2000). Frost heave in soils is, thus, the result of the combined action of heat and moisture transfer in freezing soils. For aforementioned reasons and as illustrated in Fig. 2-11, freezing soils in a pavement system are subjected to a thermal gradient. As a result the temperature differential induces a negative pore water pressure gradient, but also creates variable hydraulic conductivity conditions. Based on this understanding of the frozen fringe conditions and assuming that the validity of Fourier’s law for heat transfer, Clausius-Clapeyron’s equation for pressure conditions, and Darcy’s law for water flow all were valid in the frozen fringe
FIGURE 2-11 Thermodynamic conditions in frozen fringe (modified from Konrad and Morgenstern 1983, with permission of National Academies Press).
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Chapter Two conditions. Konrad and Morgenstern (1980) have developed the segregation potential concept to model one dimensional frost heave in soils. According to the model, the frost heave rate (v) is directly proportional to the thermal gradient (TG) in the frozen fringe following Eq. (2-18): v = SP × TG
(2-18)
where SP is the proportionality constant also termed the segregation potential. In this relationship, the term TG is related to pressure gradient through the Clausius-Clapeyron equation and is thus an expression of the driving force imposed by the pavement thermal regime. The term SP is a material- and site-condition-specific characteristic that translates the propensity to frost heaving. It can be seen as the hydraulic conductivity of the frozen fringe under a given thermal gradient. The segregation potential can thus be considered as a mechanistic frost susceptibility index. Considering a system subjected to freezing under fixed surface temperature (Tt) and bottom temperature (Tb) (Fig. 2-12) the segregation ice formation process can be summarized as follows: • A first ice lens will be initiated at a shallow depth. The temperature gradient in the frozen fringe is initially very steep, resulting in a very high rate of heat extraction. Despite the fact that the resulting negative pressure gradient is very high, the ice lens formation mechanism rapidly becomes ineffective due to the rapid cooling of the system, which reduces the hydraulic conductivity of the frozen fringe. • The freezing front progresses downward and a new ice lens is initiated at a location where the effective stress becomes null. The first ice lens might continue to grow but only with redistributed water available between the two ice lenses. • As the freezing front progresses downward, the thermal gradient is reduced and the net cooling of the frozen fringe is reduced. Thus, the growth of ice lenses is slower but lasts longer. Ice lenses become thicker and are more widely spaced due to the thicker frozen fringe. • Near steady-state conditions, the ice lens will grow as long as the system can effectively extract heat from the frozen fringe.
FIGURE 2-12 Schematic illustration of rhythmic ice lens formation (modified from Konrad and Morgenstern 1980, with permission from Canadian Geotechnical Journal).
Pavement Environment The last important consideration for moisture intake in the pavement system resulting from frost heave is that moisture is accumulated in solid form over a relatively long period of time and is released in a relatively short time during spring thaw. The large quantity of water released by melting ice lenses is likely to make pavements unstable during spring thaw. In this case, there is a coupled heat transfer-consolidation problem. The severity of the problem is related to three important factors: 1. The quantity of water accumulated in ice lenses per unit thickness of soil 2. The rate at which water is released (or the rate of progression of the thaw front) 3. The rate at which water is evacuated by the consolidation process The thaw-weakening problem will be discussed further in Chap. 3.
2-2-3 Factors Contributing to Moisture Extraction from the Pavement System Evaporation Evaporation is a rather important factor of moisture extraction from pavement systems. Obviously, the presence of an impervious surfacing layer will strongly reduce the amount of evaporation over a large proportion of the pavement surface. However, as for water infiltration in pavements, effective evaporation can still occur on gravel shoulder and embankment slopes as well as through pavement cracks. The rate of evaporation at the surface of a pavement system is a function of several factors. Two conditions are needed for effective evaporation: 1. Temperature must be sufficiently high to favor water vaporization and to supply the phase change reaction. 2. Vapor must be effectively transported away from the vaporization front. The second condition involves several factors including vapor pressure in the air, wind speed, and surface roughness. Wilson has formulated Eq. (2-19) to calculate the evaporation rate from unsaturated soil surfaces (Fredlund and Rahardjo 1993): Ev =
ΓQn + ηE Γ + ηA
(2-19)
where Ev is the vertical evaporative flux (mm/day), Γ is the slope of the vapor pressure versus temperature curve, Qn is the heat budget of all net radiations, h is a psychrometric constant, E = (0.35 + 0.051W)·uav (B − A), W is wind speed, uav is the vapor pressure of the air above the evaporating surface, B is the inverse of relative humidity in air, and A is the inverse of the relative humidity at the soil surface. Even if evaporation is relatively small in pavements due to the presence of the surfacing layer, the pavement system is significantly affected by evaporation and drying of adjacent soils. Moisture will be transferred laterally from the pavement system to dryer soils in an attempt to reach equilibrium.
Pavement Drainage The moisture regime of a system is, of course, strongly affected by all sources of moisture inputs. It is also strongly affected by the capacity and the effectiveness of the
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34
Chapter Two moisture outlet of the system. The drainage system of a pavement generally includes some or all of the following: • Surface drainage • Internal pavement drainage • Collectors and evacuation systems As discussed in Sec. 2-2, water from precipitation will either seep into the pavement system or flow at the surface toward the nearest water collector. An effective surface drainage involves an impervious surface and an appropriate cross-slope. Runoff surface water will eventually reach a collector and be evacuated. Infiltrated water will percolate vertically in the pavement structure until it reaches a low permeability surface, such as the interface between the subbase and fine grained subgrade soils or an internal drainage layer. In the first case, water will partly seep into the subgrade soil, but will also flow along the interface toward a collector (ditch or internal drain) if an adequate cross-slope has been prepared during pavement construction. If water is intercepted by a drainage layer (such as those illustrated in Fig. 1-7a), it will flow in the drainage layer following the effective drainage gradient. In the case of pavement drainage, the drainage gradient is not necessarily the slope between the location of the drop of water to be drained and the collector as would be the case for saturated soils. Pavement layers and subgrade soils immediately underneath the pavement are generally unsaturated and characterized by negative pore pressures. Thus gravity is not the only driving force acting on water. The driving potential or hydraulic head (hw) acting on the water phase of unsaturated soils is equal to (Fredlund and Rahardjo 1993): hw = y +
uw ρw g
(2-20)
where y = gravitational head and uw /rw g is the pressure head with uw = pore water pressure, rw = density of water, and g = gravitational acceleration. The hydraulic gradient, i, in the x direction for an unsaturated soil, thus, becomes i=
dhw dx
(2-21)
This can lead to situations where water tends to flow in different directions than expected, as illustrated in Fig. 2-13. Despite the fact that these systems are designed to extract moisture from pavements, drainage collectors and layers can sometimes feed moisture into unsaturated pavement systems. The presence of air in pores of unsaturated soils also affects the effective hydraulic conductivity of these soils. Permeability of the water phase, kw, in unsaturated soils decreases as the level of saturation decreases and as the matric suction increases. Using the following relationship, kw can be estimated (Fredlund and Rahardjo 1993): k w = k s Sδe
(2-22)
where ks is the permeability of the saturated soil, Se is the effective degree of saturation, d is an empirical constant = (2 + 3l)/l, l is the pore size distributions index that is the slope of the relationship between the effective degree of saturation (Se) and matric suction (ua − uw).
Pavement Environment
FIGURE 2-13 Unexpected flows in pavements due to unsaturated moisture regimes.
Given the above considerations, Darcy’s law for unsaturated soils becomes dhw (2-23) dx where vw is the flow rate of the water phase in unsaturated soils. The effectiveness of pavement drainage is also considerably affected by the effectiveness of the collector and evacuation systems. If these systems have an adequate capacity (dimension and slope), runoff water and water drained out of the pavement system will be effectively removed. If not, water will tend to stagnate within the system resulting in unfavorable moisture conditions. vw = − k w
2-2-4
Moisture Balance
All the factors described in this section will contribute to a constantly evolving moisture balance in the pavement system. Figure 2-14 illustrates a typical moisture balance at two levels in the pavement structure, based on experimental data collected from the Dickey
FIGURE 2-14 Moisture balance at two levels in pavement system (modified from Janoo and Greatorex 2002).
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Chapter Two Lake test site in Montana by Janoo and Greatorex (2002). The top half of Fig. 2-14 illustrates the moisture balance in the base material (depth of 292 mm), while the bottom half illustrates the moisture balance in the subgrade soil (depth of 584 mm). In both cases, the evolution of moisture balance with time can be divided into four distinct seasons: • During fall, heavier precipitations combined with lower evaporation at the surface of the pavement system cause moisture content to increase slightly during the months of October and November. • At the end of November, freezing temperatures cause interstitial water to freeze. For pavement engineering purposes, this can be considered as the beginning of winter. At the beginning of the winter season, unfrozen water contents of the base material and of the subgrade soils drop close to 0 percent. This process occurs slightly later and more progressively for the subgrade soil than for the granular base material. The trend observed is the result of the limitations of measuring instruments that typically measure unfrozen water content in soils instead of total water content. In reality, the total water content in soils and granular materials will typically increase during winter as a result of water and brine infiltration in pavements as well as the formation of segregation ice. The evolution of the total water content (in solid and liquid forms) will typically evolve from the highest content measured prior to freezing to the peak observed in early spring thaw as indicated by the dotted lines in Fig. 2-14. The pattern of water intake during winter for subgrade soils is well documented through several seasonal pavement monitoring projects (Palolahti et al. 1993; Imbs and Doré 2003; and several others). An increase of volumetric water content from about 8 to about 35 percent is observed in the silty sand subgrade of the Dickey Lake test site (Fig. 2-14) and is probably associated with segregation ice formation. The increase of volumetric moisture content from about 5 percent to more than 20 percent in the granular base material cannot be explained by the sole effect of water infiltration in the granular base layer. It is likely the result of a combined effect of frost heave, vapor transport, and water infiltration in the base material. The pattern of moisture intake could, thus, vary between patterns (a) and (b) in Fig. 2-14. More research is needed to fully understand the behavior of granular pavement materials submitted to frost action. • Spring is the season when moisture balance varies the most. During spring, water accumulated during winter is released over a relatively short period of time as indicated by the sharp increase in unfrozen water content in the base and subgrade soils. At the same time, precipitation and melting of snow accumulated on the shoulders and on the slopes of the embankment contribute to high moisture contents in the system through infiltration. Excess water in the system will progressively drain out of the system in the late spring and early summer seasons. • During summer, the residual excess moisture in the pavement system will continue to drain. Moreover, evaporation will become more and more effective as the embankment surface becomes warm and dry. Under these conditions, the moisture contents in the granular materials and subgrade soils will decrease progressively during the summer months. Intense precipitation may affect the moisture balance temporarily, as evident for the month of June in Fig. 2-14. The effect of that specific precipitation event is more apparent in the subgrade soil than in the pavement base.
Pavement Environment Moisture regime in pavements has an important effect on its response to temperature variations and to external stresses. Good pavement performance requires low moisture contents with minimum fluctuations thorough the year.
2-3
Stress Regime in Pavements Pavements are engineered structures designed to sustain stresses. Pavement engineering relies therefore on a good understanding of pavement stresses and on their quantification. Stresses acting on pavements can be classified as static or cyclic stresses. They can further be classified as load induced or environmentally induced stresses as shown in Table 2-1. Static stresses, described in Secs. 2-3-1 and 2-3-2, induced in the pavement system by loads, include the earth pressure at rest and the weight of equipment or vehicles standing on the pavement. The latter case should be considered in situations where vehicles are likely to be moving slowly or stopped for short or long periods. These include parking areas and intersections for roadways and aprons, taxiways and runway extremities for airport pavements. Cyclic stresses, covered in Secs. 2-3-4 to 2-3-7, induced in the pavement system include stresses induced by moving traffic loads and by seasonal environmental factors:
2-3-1
Earth Pressure at Rest
Geostatic stresses caused by the weight of soils and pavement materials are important to consider when assessing stresses in the pavement materials. They represent the initial stress state at a given point in the system prior to external loading. Since mechanical properties of soils and unbound pavement materials are stress dependent, the correct assessment of geostatic stresses is required for the mechanical analysis of the pavement system. As illustrated in Fig. 2-15, effective earth pressure at rest is the result of the summation of two factors: (1) stresses associated with the weight of soils and materials
Static stresses
Induced by loads
Earth pressure at rest Static traffic loads
Cyclic stresses
Induced by long-term environmental and soil effects
Stresses related to consolidation or other permanent soil movements
Induced by loads
Moving traffic loads
Induced by seasonal environmental factors
Thermal stresses Stresses related to frost heave Negative or positive pore pressure
Highlighted factors are considered most frequently in pavement engineering.
TABLE 2-1
Summary of Stresses Acting on Pavements
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Chapter Two
FIGURE 2-15
At rest earth pressures in pavements.
above the considered point in the pavement system and (2) pore water pressure. The first factor is usually expressed in terms of vertical stress sv:
σ v = ∑ γ i hi
(2-24)
in terms of horizontal stress (at rest) sh:
σ h = K 0 ∑ γ i hi
(2-25)
θ = (1 + 2K 0 )∑ γ i hi
(2-26)
or in term of total stress (at rest) q:
where gi is the unit weight of soil or pavement material in layer i, hi is the thickness of layer i, K0 is the coefficient of lateral earth pressure at rest (Das 2002 or other geotechnical engineering reference). Pore pressure, u, can be determined as u = γ w hw
(2-27)
where gw is the unit weight of water and hw is the difference between the depth of the point considered in the analysis and the surface of the water table. The effective stress s ′ can be obtained from
σ′ =σ −u
(2-28)
Note that the effect of negative pore pressure in unsaturated soils above the water level has not been considered in the previous equations. This specific aspect will be discussed in Sec. 2-3-7.
2-3-2
Static Stresses Induced by Traffic Loads
The evaluation of the stress distribution caused by traffic loads into the multilayer pavement systems is a complex problem. For practical reasons, pavement engineers
Pavement Environment
FIGURE 2-16
Stress-strain behavior of an elastic system.
have reduced the problem to the simplest possible form. Amongst the simplifications commonly used, the assumption of elastic behavior of pavement materials is very convenient. Since a pure elastic behavior is not time dependent, this assumption allows engineers to use a static load to compute stresses into a pavement structure. According to the basic law of the theory of elasticity, referred to as Hooke’s law, strain e in an elastic material is proportional to the applied stress s. The proportionality constant is termed the modulus of elasticity E and is a basic material property for material engineering. Hooke’s law can be formulated as follows:
σ = Eε
(2-29)
As opposed to the viscoelastic behavior also commonly used in pavement mechanics, strains will develop instantaneously in an elastic system subjected to a pulse load and will remain constant until the load is removed. The two behaviors are illustrated in Fig. 2-16. Static stress analysis, is thus, a practical way to assess stresses and strains in a pavement system under a stationary load as well as under a moving load considering the hypothesis of elastic behavior. The latter hypothesis has proven to be realistic in the context of pavements in “normal” operating conditions subjected to loads that are relatively small compared to the failure load (Brown 1993). Like most engineering materials, pavement materials and soils have the ability to distribute loads. Thickness and stiffness of the material will both contribute to load distribution. The simplest mathematical representation of load distribution with depth was given by Boussinesq in 1885 for a homogeneous half space. Considering a load uniformly applied on a flexible circular plate, the vertical stress sz and the radial (horizontal) stress sr at depth z, under the center of the plate of radius a can be obtained from (Ullidtz 1987): z3 σz = σ0 1 − 2 2 1.5 ( ) a + z
(2-30)
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Chapter Two and
σr =
σ0 2
z3 2 z(1 + µ ) (1 + 2 µ ) − (a 2 + z 2 )0.5 + (a 2 + z 2 )1.5
(2-31)
where s0 is the uniform pressure applied on the plate and m is Poisson’s coefficient. Despite the fact that the contact area of a truck tire has an oval-type shape, the hypothesis of a load transmitted by a circular flexible plate is generally considered to be reasonable for the analysis of load induced stresses in pavements (Peattie 1962). Boussinesq equations also allow calculating vertical and radial strains as well as vertical displacements in the homogeneous half space. These equations are developed further in Sec. 5-9. The resulting load distribution under the center of the plate in the homogeneous mass is illustrated in Fig. 2-17. A detailed description of the Boussinesq method and equations can be found in Ullidtz (1987). Pavement systems are obviously far from being homogeneous masses. The Boussinesq approach thus has limited applications, such as in the analysis of load distribution in multilayer pavement systems. A mathematician by the name of Burminster (1943a and b) formulated a complete theory on the calculation of stresses and strains in a multilayer elastic system based on the following principal assumptions: • Each layer is considered to be composed of homogeneous and isotropic material • The layers have a finite thickness, but are considered infinite in the horizontal direction • The multilayer system is resting on an infinite half space, that is, the last layer of the system has an infinite thickness • The layers are perfectly bound at their interfaces • The conditions for continuity at the interfaces are satisfied
FIGURE 2-17 Stress distribution in a homogeneous mass under a 150-mm radius circular plate for m = 0.35.
Pavement Environment The outcome of Burminster’s work is a complex set of mathematical functions allowing for the calculations of stresses and displacements at any point in space on the multilayer system subjected to loading. The vertical and the radial stress equations are given in Eqs. (2-32) and (2-33) as an example of Burminster’s theory.
σz =
∂ ∂ 2φ (2 − µ )∇ 2φ − 2 ∂z ∂z
(2-32)
∂ 2 ∂2φ µ∇ φ − 2 ∂z ∂z
(2-33)
σr =
where z is the depth, m is Poisson’s coefficient, ∇ is a Laplacian used in compatibility equations, and f is a Bessel stress equation computed for each layer of the system. Burminster’s publications also include equations for the calculation of radial, shear and bulk stress, vertical and radial strain as well as vertical displacement. Due to the complexity of the mathematic formulation of Burminster’s equations, several methods based on charts and tables have been published (Jones 1962; Peattie 1962; and others). Figure 2-18 illustrates Burminster’s solution for the vertical stress distribution under the center of a circular plate of radius a in a two-layer system for different elastic modulus ratios E1/E2. The slope of the lines in Fig. 2-18 illustrates the importance of the stiffness of the top layer and the effectiveness of load distribution on the bottom layer of the system. Figure 2-19 is inferred from Fig. 2-18 and illustrates how successive layers of decreasing stiffness distribute stresses in the pavement system. It illustrates how the load, initially applied over the contact area between the tire and the pavement, is distributed over much wider areas at pavement layer interfaces. Figure 2-19 illustrates the interfaces in the pavement structure at which the stresses or the strains are considered to be critical to pavement performance. These interfaces and
FIGURE 2-18 Distribution of vertical stresses in a two-layer system for different E1/E2 ratios (modified from Burminster 1958, quoted by Huang 2004; reprinted by permission of Pearson Education, Inc., Upper Saddle River, N.J.).
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Chapter Two
FIGURE 2-19 Stress distribution in pavement system and stresses at critical interfaces associated with resulting pavement performance problems.
the resulting performance problems associated with excessive stresses or strains are as follows: • Vertical stresses or strain near or at the surface of the pavement, associated with the development of rutting due to permanent deformation of the bituminous wearing course • Horizontal stresses or strains at the bottom of bound layers associated with the development of fatigue cracking • Vertical stresses or strains at the surface of the subgrade soil associated with the development of structural rutting Calculation of stresses and strains at these critical interfaces is the basis for mechanistic empirical pavement design and pavement analysis methods. These methods will be described in Chap. 8.
2-3-3
Stresses Related to Permanent Soil Movements
Important static stresses can be induced in pavement systems by permanent soil movements such as differential settlement, soil creep, and solifluction. In permafrost conditions, consolidation resulting from the degradation of ice-rich permafrost is a widespread soil movement problem. These types of soil movements typically induce large distortions of the pavement surface and subsequently, tensile stresses develop in bound pavement layers. If the movements are slow, the stresses may be relaxed by the viscoelastic behavior of the asphalt bound materials. If the movements are rapid or if the material is brittle due to cold temperature or aging, cracks that traverse to the surface of the pavement are likely to occur.
Pavement Environment
2-3-4
Moving Traffic Loads
The aforementioned practical considerations often lead to the assumption of static conditions for the analysis of traffic loads. It is, however, generally agreed that traffic loading is a complex phenomenon that would ideally require analysis using dynamic considerations. When applied to pavement loading, dynamic effects can be classified in three main categories: 1. The evolution of stresses in pavement structures subjected to loading by a moving wheel 2. Time-dependent material response 3. Stresses induced by an oscillating load
Evolution of Stresses Each element of the pavement is subjected to a stress history that evolves as a function of the position of loading wheel relative to the loaded element of soil or pavement material. As indicated in Fig. 2-20, as the load approaches the considered element, the principal stresses s1 and s3 are acting obliquely on the element. As a result (illustrated in Fig. 2-20b), a shear stress that is arbitrarily indicated as positive is generated. The shear stress increases as the load approaches a certain point where the effect of the oblique loading decreases and becomes null when the load is immediately above the element. The shear stress then becomes negative when the load moves away from the element. The negative shear stress increases as s1 becomes oblique and it later diminishes as the load moves away from the element. The horizontal and vertical stresses keep increasing until the load is above the element and decreases as it moves away from it. This phenomenon, referred to as the rotation of the principal stresses due to the
FIGURE 2-20 Stresses under a moving wheel (a) on a soil or unbound material element within the pavement system and (b) at the bottom of the bound surfacing layer.
43
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Chapter Two . passage of a wheel, is described by several authors (Blazejowski et al. 1996; Lekarp and Dawson 1998; Barksdale et al. 1998). It is believed to have an important effect on the mechanical performance of soils and unbound layers in pavements. Stresses in the bound layer(s) also evolve considerably as the loading wheel passes above a given point on the pavement. As indicated in Fig. 2-20a, the asphalt-bound layer bends under the wheel load, inducing tensile stresses at the bottom of the layer. On each side of the wheel, the bound layer is subjected to inverse bending, inducing compressive stresses at the bottom of the layer. As a result, a wheel approaching a given element at the bottom of the asphalt-bound layer will first induce compressive stresses (represented by positive values in Fig. 2-20a) followed by a sharp increase in tensile stress. The maximum value of tensile stress occurs when the load is at the vertical of the considered element. The pattern described will be inversely reproduced as the wheel moves away from the loaded element.
Time-Dependent Material Response Assumption that materials have an elastic behavior makes them insensitive to loading time. Thus, the elastic theory fails to explain permanent deformation in pavement structures. It also fails to explain the effect of wheel speed on strains illustrated in Fig. 2-21. According to literature data compiled by Coulombe (2002), strains under a slowly moving or stopped vehicle can be more than twice those under a vehicle moving at 70 km/h. More complex rheological models have to be used to explain material responses under a moving load. Figure 2-22 illustrates two models used in pavement engineering to represent the rheological behavior of pavement materials. Figure 2-22a illustrates Burger’s viscoelastic model commonly used to model the response of materials such as asphalt bound materials. The elastic element (E) of the model will cause the material to deform instantly under a loading pulse. The viscoelastic (VE) elements will induce a delayed elastic response characterized by a decreasing rate of strain with time. The viscous (V) element will in turn accumulate strain linearly with time as long as the load is maintained on the element. When load is removed from the viscoelastic material, the
FIGURE 2-21 Effect of vehicle speed on (a) horizontal strains at the bottom of the asphalt layer and (b) vertical strains at the top of subgrade soils (modified from Coulombe 2002).
Pavement Environment
FIGURE 2-22 Rheological models used in pavement engineering: (a) Burger’s viscoelastic model and (b) Rowe’s visco-elasto-plastic model.
elastic response is immediate and the viscoelastic response is delayed in time. The viscoelastic element will restore all the accumulated strain if unloading time permits, while the displacement accumulated by the viscous element is irrecoverable allowing for permanent deformation to accumulate in the system with load repetitions. The equation of the Burger’s model is the following (Huang 2004):
ε=
σ E0
t t σ 1 + T + E 1 − exp − T 0 1 1
(2-34)
where e is total strain, s is applied stress, E0 and E1 are the elastic modulus of the elastic element (E) and the elastic component of the viscoelastic element (VE), respectively, T0 and T1 are relaxation times T = l/E, where l is viscosity and t is time. Figure 2-22b illustrates a rheological representation of a visco-elasto-plastic model presented by Rowe (Rowe et al. 1995, cited by Brown 1997). The model was used by Rowe to compute dissipated energy related to fatigue cracking and permanent deformation in dense bitumen macadam. The model is also likely to adequately represent the behavior of unbound granular materials. The elastic component of the visco-elastoplastic model will cause an important instantaneous deformation upon loading of the system. The visco-plastic (VP) element of the system will then linearly accumulate
45
Chapter Two deformation as long as the load acts on the element. The elastic deformation will be recovered instantaneously when load is removed leaving a residual visco-plastic permanent strain. Climatic conditions have an important effect on the rheological behavior of pavement materials. The elastic component will dominate the viscoelastic behavior of asphalt bound materials at low temperatures while the viscous component will have a significant effect on material response at high temperatures. Similarly, for relatively dry soils or unbound granular materials, elastic and plastic behaviors are dominant, while viscous behavior will prevail for soils or materials at or approaching saturation.
Stresses Induced by Oscillating Load The third aspect of dynamic loading of pavement structures is related to stresses induced by an oscillating load. If pavement surfaces were perfectly uniform, the load applied by a moving wheel would remain constant with time or distance on the pavement. In reality, pavements are not uniform. As illustrated in Fig. 2-23, irregularities in longitudinal profiles induce oscillations of the suspended masses of a vehicle, causing variations in the loads transmitted to the pavement structure. Dynamic loads caused by oscillating vehicle mass can vary substantially depending on surface roughness, speed, and characteristics of the vehicle, such as suspension type. Equation (2-35) (OECD 1988) represents the load equivalence law adapted for uneven pavements: P Ni = α s Ns Pi
γ
(2-35)
where Ni is the number of applications of a given load Pi causing pavement failure, Ns is the number of applications of a standard load Ps causing pavement failure, g is the “aggressiveness” factor generally considered as 4 for flexible pavements, and a is the dynamic load coefficient equal to Wd /Ws (see Fig. 2-23).
Dynamic load Wd Wheel load
46
FIGURE 2-23
Constant (static) load Ws
Dynamic load induced by an oscillating mass.
Pavement Environment Experimental studies in Belgium (OECD 1988) have shown that typical values for the coefficient a are 1.06 for fairly even surfaces, 1.24 for average surface conditions, and 1.54 for uneven road surfaces. Bad surface conditions can, thus, lead to a reduction of allowable load applications on a pavement reaching 50 percent. Results from an experimental study at the CAPTIF facility in New Zealand (Steven et al. 1999) showed that the dynamic load coefficient for parabolic spring suspension doubles when speed increases from 20 to 45 km/h, while it is not affected by speed for multileaf spring suspensions. Uneven loading resulting from vehicle oscillation is also an important cause of differential permanent deformation causing pavement roughness (Steven et al. 1999; Ullidtz 2002).
2-3-5 Thermal Stresses Thermal stresses are environmental stresses caused by diurnal and shorter-term temperature variations in the bound pavement layer that is restricted from contracting. As the temperature gets colder, a thermal stress starts to develop gradually. Figure 2-24 illustrates the development of thermal stress under constant cooling rate. The stress does not increase linearly with temperature at warm temperatures (close to 0°C) due to asphalt cement’s viscoelastic behavior that allows partial relaxation of stresses. At a certain transition temperature depending mainly on the binder properties, the asphalt concrete starts to behave as a pure elastic material and the thermal stress increases linearly with the temperature. When the thermal stress reaches pavement tensile strength, cracking occurs. The stress at which cracking occurs in the field is termed “cracking strength” and the corresponding temperature “cracking temperature.” The development of the thermal stress can be modeled with Eq. (2-36): T2
σ thermal = α ∫ SdT
(2-36)
T1
FIGURE 2-24 Typical stress versus temperature relationship (modified from Jung and Vinson 1994; reproduced with permission of TRB, from Transportation Research Record: Journal of the Transportation Research Board, No. 1417, Transportation Research Board of the National Academies, Washington, D.C., 1993, Figure 1, p. 13).
47
48
Chapter Two where a is the linear thermal contraction coefficient, S is temperature and loading time dependent stiffness of the asphalt concrete, T1 is the initial temperature, and T2 the final temperature. The material properties, a and S, in Eq. (2-36) can be measured or estimated using values reported in the literature. Asphalt concrete has two distinct contraction coefficients depending on the temperature range. At cold temperatures where the thermal stresses develop, a typical value for the contraction coefficient is 2.93 × 10−5/°C ( Jones et al. 1968). Zeng and Vinson (1998) have measured values for the linear contraction coefficient varying from 1.89 to 3.33 × 10−5/°C. The mixture stiffness is traditionally determined using binder stiffness and volumetric mixture properties (Christensen et al. 2003; Bonnaure et al. 1977; Heukelom and Klomp 1964). The binder stiffness can be predicted using loading time, temperature step, and binder properties (Van der Poel 1954; McLeod 1976). Currently, the binder stiffness is measured using the bending beam rheometer (BBR) and used to predict the pavement thermal stress through complex algorithms performed by specific software (AASHTO Provisional Standards 2003). The thermal stress restrained specimen test (TSRST) described in Chap. 4 can be used to directly measure the thermal stress as a function of temperature. In the TSRST a thermal stress is developed as illustrated in Fig. 2-24 by keeping an asphalt aggregate mixture specimen at a constant length while cooling it down at a standard rate. The experiment captures the mixture’s fracture strength and the fracture temperature that are generally representative of field conditions. As the thermal stress develops in a pavement slab, a resisting stress develops simultaneously that opposes the contraction of the pavement slab. The maximum resisting stress that can be developed is the shear strength at the interface of the contracting slab and the underlying layer and can be represented as s = sv tanf + c
(2-37)
where sv is the vertical stress [Eq. (2-24)], f is the friction angle, and c is the cohesion at the interface (Zubeck and Vinson 2007). When the thermal stress exceeds the maximum mobilized restraint stress, or restraint strength, the pavement slab is able to contract and the thermal stresses are relaxed as further explained in Chap. 3.
2-3-6
Stresses Related to Frost Heave
As described in Sec. 2-3-3, important stresses can be induced in pavement systems by soil movements. The phenomena described in Sec. 2-3-3 are long-term irreversible soil movements. Similar movements can occur as a result of cyclic phenomena such as frost heave and thaw consolidation. Frost heave is rarely uniform and significant differential movements can be induced in pavements. Two main mechanisms, identified by Doré (2002), can act on pavements. The first one, referred to as random differential heaving, is mainly associated with variations of soil properties along the highway corridor. The resulting distortions tend to increase pavement roughness during winter and, by forcing pavements to bend upward, can also cause pavement cracking. The second type of differential frost heaving occurs along the transverse axis of the pavement and is the result of the variation of pavement geometry and snow accumulation on pavement sides. Both mechanisms are likely to force pavements to bend upward. The following
Pavement Environment model has been proposed by Doré et al. (1999) to estimate strains caused by upward bending of pavements caused by differential frost action:
εf =
χ tan θ L + (∆h tan θ)
(2-38)
where ef is the flexural strain caused by differential heaving, ∆h is the difference of heaving between two points separated by a distance L on the pavement. c and q are parameters describing the geometry of the flexion in the pavement structure (more details are given in Chap. 3). Assuming an elastic behavior of the asphalt concrete, stresses induced by the bending action can be calculated using Eq. (2-29), but if viscoelastic behavior is expected then Eq. (2-34) should be used. Differential frost heaving will be further discussed in Chap. 3.
2-3-7
Negative or Positive Pore Pressure
Water can be a major source of problems in pavements. Therefore, drainage is considered to be an essential precaution in pavement engineering. As a result of good design and drainage practice, pavement systems are mostly constituted of unsaturated soils and materials. The relationship between the degree of saturation and pore pressure is shown in Fig. 2-25, where pore pressure is represented by the balance between air pressure ua and water pressure uw or matric suction (ua − uw) in the pore. Matric suction is one of the two components of the total suction in unsaturated soil pores (the second component, osmotic suction, is not discussed in this book). The relationship is soil specific and is referred to as the soil-water characteristic curve. From the saturation state at atmospheric pressure (Point 1 in Fig. 2-25), a certain level of pressure is needed to force air penetration into soil pores (Point 2). This pressure is referred to as the air-entry pressure and is denoted (ua − uw)e. As air occupies more space in the pores (Point 3), pore water resists the intrusion of air by opposing an increasing tensile force through the contractile skin at the air-water interface. This phenomenon is similar to the one described
FIGURE 2-25
Relationship between the degree of saturation and matric suction in soils.
49
50
Chapter Two earlier for ice intrusion into soil pores (Sec. 2-2). Assuming that pore air is at atmospheric pressure, pore water pressure is a negative value (or matric suction) that tends to increase with decreasing level of saturation. Increasing pore air volume forces water to retreat in exiguous spaces in the pores (Point 4) reducing considerably the radius of curvature of the air-water interface and, thus, increasing the negative pressure in pore water as described by Eq. (2-39) (Fredlund and Rahardjo 1993). (ua − uw ) =
2Ts Rs
(2-39)
where Ts is the surface tension at the air-water interface and Rs is the radius of curvature of the air-water interface. Note that in Fig. 2-25, the path followed by a drying soil is not the same as the path followed by a wetting soil. The hysteresis in the soil-water characteristic curves is mainly due to nonuniform pore size diameter and distribution, the different contact angles between an advancing and a receding interface, as well as the presence of entrapped air in a wetting soil (Fredlund and Rahardjo 1993). The soil-water characteristic curves of two soils are transposed to a typical pavement situation in Fig. 2-26. In the illustrated case, the two unbound granular layers of the pavement are assumed to have the same water retention characteristics. Point 1 of Fig. 2-26 corresponds to the water table in the subgrade soil. At that point, soil is saturated and water is in balance with atmospheric pressure. Point 2 corresponds to the top of the capillary fringe. The soil is still saturated, but is affected by negative pore pressures corresponding to the air-entry pressure. At Point 3, fine-grained soils are in contact with pavement granular materials. The suction at the interface is in balance, meaning that
FIGURE 2-26 Saturation and matric suction regimes in a pavement system in dry (solid line) and damp (dashed line) conditions.
Pavement Environment the two types of soils need to coexist at different levels of saturation. At Point 4 in the pavement system, granular materials are at a relatively low level of saturation, which induces considerable matric suction in pavement materials. Values of matric suction ranging from 10 to 150 kPa have been measured in pavement granular bases by Perera et al. (2004). Matric suction has an important effect on stress state in soils and pavement materials. Bishop in 1959 (cited by Fredlund and Rahardjo 1993), has proposed the following expression of effective stress s′ in unsaturated soils:
σ ′ = (σ − ua ) + χ (ua − uw )
(2-40)
where s is the total normal stress, ua and uw are air and water pore pressures, c is a parameter varying between 0 and 1 and related to the degree of saturation. From Eq. (2-40), it is easy to see the importance of matric suction in the stress state of a soil element in the pavement system. For example, assuming a matric suction of 70 kPa, c = 0.7 and ua = 0, the second term of Eq. (2-40) would have a dominant effect on the effective stress (49 kPa) as compared to the first term for a soil element located 1.0 m below the surface (s ≈ 20 kPa). Since rigidity of soils and pavement materials is stress dependent, this situation translates into an increased resilient modulus. Doucet and Doré (2004) have proposed the following empirical relationship to relate saturated resilient modulus of granular base materials to unsaturated resilient modulus: ∆Mr sat = −8, 700(ua − uw ) − 17 , 000
(2-41)
where ∆Mrsat is the increase in resilient modulus from the saturated state to a given unsaturated state (kPa), (ua − uw) is matric suction (kPa). Based on Eq. (2-41), the resilient modulus of a saturated base material would increase by 157,000 kPa for a 20-kPa matric suction, which is typically measured for granular materials at levels of saturation around 20 percent. Obviously, climatic conditions and pavement characteristics are going to have an important effect on matric suction fluctuations in the pavement system. As indicated by the dashed line on Fig. 2-26, heavy precipitation and spring thaw conditions can cause near-saturation conditions in pavements. Matric suction is, thus, likely to be strongly reduced in those conditions. Positive pore pressures are also likely to occur for soils or materials decompacted by frost heave. The effective stresses and rigidity of pavement materials are consequently strongly reduced.
2-4
Interaction with Geology and Morphology As discussed throughout this chapter, temperature, moisture, and stresses are closely interrelated and constitute the environment in which pavement structures must perform. Pavement environment is also affected by several other factors, among which the most important are the geology and morphology of the surrounding areas. The morphology of the terrain in which the pavement is built has an important influence on the pavement environment. In high terrain, the water table is likely to be low and drainage is usually effective. In those conditions, water content within the pavement system tends to remain relatively low and constant through the seasons. Consequently, frost action is reduced by the reduction of the effective flow gradient
51
Chapter Two toward the ice lens. Moreover, effective stresses are high due to high suction levels in pavement materials. Pavements built in high lands are also more exposed to wind, increasing heat extraction by convection and are also likely to be more exposed to solar radiations. Pavements built in low land areas are likely to be affected by poor drainage conditions and high water tables. As a result, frost action is expected to be more severe and effective stresses in pavement materials are expected to be reduced. They are less affected by wind, but shading effects can reduce solar radiations on the pavement surface. These pavements can also be exposed to extreme low air temperature conditions due to the effects of temperature inversions (Dysli 1991). Pavements built in sloped areas are generally easy to drain, but they are more likely to be affected by seepage and artesian flow conditions. These conditions can lead to severe frost or stability problems. The orientation of the slope can considerably affect pavement surface temperature and its variation along the road. The geology of the surrounding terrain is another important factor affecting pavement environment. Soil interaction with pavement environment has been discussed throughout this chapter. The main characteristics acting on temperature, moisture, and stress regimes in pavements are permeability, stiffness, frost susceptibility, and moisture sensitivity of subgrade soils. The other factors of major concern for pavement engineers are homogeneity and uniformity of soils. Most of the problems found for pavements in cold climates are related to the lack of homogeneity or uniformity along the road corridor. Poor homogeneity within a soil deposit will lead to differential frost penetration and frost heaving. It also leads to uneven mechanical properties and behavior of the entire pavement structure. Lack of uniformity is a widespread characteristic of geological formations and deposits. Contacts between different soil deposits or between bedrock and soil deposits as well as soil and rock stratification are often the cause of localized differential mechanical, thermal, and hydric behavior. These interactions cannot be overemphasized. Proper characterization of soil properties and conditions prior to pavement construction or rehabilitation will significantly reduce the risk of poor performance of the pavement structure in any given environment. Pavement performance problems related to soil characteristics will be further discussed in Chap. 3. Soil investigation and characterization approach and techniques will be described in Chap. 4.
Review Questions
2m
2-1. For the conditions given below, determine the vertical effective stress at point A.
2m
γd = 14.5 kN/m3
3m
γd = 15.3 kN/m3 γsat = 17.7 kN/m3
2.5 m
52
γsat = 16.9 kN/m3 A
Pavement Environment 2-2. A load of 1100 kPa is applied uniformly on a circular plate with a diameter of 3.5 m. Find the vertical and horizontal stress at a depth of 3 m in a homogenous soil for which Poisson’s coefficient is 0.35. Use Boussinesq’s stress distribution theory.
2-3. Based on experimental results, a dynamic load of 20 kN circulating on an uneven flexible pavement causes failure when it is applied 30,400 times. If the load is reduced to 6 kN, how many applications will cause a failure?
2-4. A saturated subgrade soil has a resilient modulus of 600 MPa. A matric suction of 17 kPa is measured in the same soil at an unknown unsaturated state. What is the resilient modulus in these conditions?
References AASHTO Provisional Standards (2003). June 2003 Edition, American Association of State Highway and Transportation Officials. Barksdale, R. D., Alba, J., Khosla, N. P., Kim, R., Lambe, P. C., and Rahman, M. S. (1998). “Laboratory Determination of Resilient Modulus for Flexible Pavement Design,” NCHRP Web Doc 14, TRB. Bishop, A. W. (1959). “The Principle of Effective Stress,” lecture delivered in Oslo, Norway, in 1955, published in Technisk Ukeblad, vol. 106, no. 39, pp. 859–863. . Blazejowski, K., Nilsson, R., Hopman, P., and Sybilski, D. (1996). “Visco-Elastic Analysis of Typical Polish Flexible Pavements Using VEROAD,” Proceedings of the Second International Conference for Durable and Safe Road Pavements, Kielce, Poland. Bonnaure, F., Gest, G., Gravois, A., and Uge, P. (1977). “A New Method of Predicting Stiffness of Asphalt Paving Mixtures,” Proceedings of the Association of Asphalt Pavement Technologists, White Bear Lake, Minn., vol. 46. Brown, S. F. (1993). Structural Analysis of Pavements, Ehrola and Turumen (eds.), University of Oulu, Publications of road and transport laboratory, 22 Oulu, Finland, pp. 259–273. Brown S. F. (1997). “Achievements and Challenges in Asphalt Pavement Engineering,” Proceedings of the Eighth International Conference on Asphalt Pavements, International Society of Asphalt Pavements, White Bear Lake, Minn. Burminster, D. M. (1943a). “The Theory of Stress and Displacements in Layered Systems and Applications to the Design of Airport Runways,” Proceedings of the 23rd Annual Meeting of Transportation Research Board of the National Academies, Washington, D.C., vol. 23, pp. 126–144. Burminster, D. M. (1943b). “The General Theory of Stresses and Displacements in Layered Soils Systems,” Journal of Applied Physics, vol. 16, no. 2, pp. 89–96. Burminster, D. M. (1958). “Evaluation of Pavement Systems of the WASHO Road Test by Layered Systems Method,” Bulletin 177, Transportation Research Board of the National Academies, Washington, D.C., pp. 26–54. Christensen, D. W., Pellinen, T., and Bonaquist, R. (2003). “Hirsch Model for Estimating the Modulus of Asphalt Concrete,” Journal of the Association of the Asphalt Paving Technologists, vol. 72, pp. 97–121. Coulombe, C. (2002). “Effet de la vitesse des véhicules sur les paramètres de conception des chaussées souples en milieu municipal (Effect of vehicle speed on flexible pavement design parameters in municipal context),” Essai de Maîtrise, département de génie civil, Université Laval. (in French)
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Chapter Two Das, B. M. (2002). Principles of Geotechnical Engineering, Brooks/Cole, Pacific Grove, Calif. Doré, G. (2002). “Cold Region Pavements,” Journal of Glaciology and Geocryology, vol. 24, no. 5, pp. 593–600. Doré, G. (2004). “Development and Validation of the Thaw Weakening Index,” International Journal of Pavement Engineering, vol. 5, no. 4, pp. 185–192. Doré, G., Konrad, J. -M., and Roy, M. (1999). “Deterioration Model for Pavements in Frost Conditions,” Transportation Research Record 1655, Transportation Research Board of the National Academies, Washington, D.C., pp. 110–117. Doré, G., Pierre, P., Bilodeau, J. P., and Abdelrazik Idriss, A. (2004). “Développement d’un essai simple et rapide pour l’estimation du potentiel de ségrégation (Development of a simple and rapid test for the estimation of the segregation potential),” Rapport GCT-2004-12. (in French) Doucet, F., and Doré, G. (2004). “Module des matériaux granulaires c-ltpp (Resilient modulus of C-LTPP granular materials),” Proceedings of the 57th Canadian Geotechnical Conference, Canadian Geotechnical Society, Quebec City, October 24–27. Dysli, M. (1991). Le gel et son action sur les sols et les fondations. Presses Polytechniques et Universitaires Romandes, Lausanne. Dysli, M., Lunardi, V., and Stenberg, L. (1997). “Related Effects on Frost Action: Freezing and Solar Radiation Indices,” Ground Freezing 1997— Frost Action in Soils, Knutsson (ed.), Proceedings of an International Symposium, 15–17 April 1997, A. A. Balkema, Rotterdam. Fredlund, D. G., and Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils, Wiley InterSciences, New York, New York. Garber, N.J., and Hoel, L. A. (1997). Traffic and Highway Engineering, PWS Publishing Company, Boston, Mass. Hazen, A. (1930). “Water Supply,” American Civil Engineering Handbook, Wiley, New York. Henry, K. (2000). “A Review of the Thermodynamics of Frost Heave,” ERDC/CRREL report TR-00-16, U.S. Army Corps of Engineers. Heukelom, W., and Klomp, A. J. G. (1964). “Road Design and Dynamic Loading,” Proceedings of the Association of Asphalt Pavement Technologists, White Bear Lake, Minn., vol. 33. Huang, Y. H. (2004). Pavement Analysis and Design, 2d ed., Prentice-Hall, Upper Saddle River, N.J. Imbs, C., and Doré, G. (2003). “Méthode d’évaluation des effets du dégel et de son évolution sur les différentes couches d’une structure routière (A method for the evaluation of the effect of thawing and it’s evolution on pavement layers),” Research report GCT03-01, Laval University, Civil Engineering Department, Quebec City, Canada. Janoo, V., and Greatorex, A. (2002). “Performance of Montana Highway Pavements during Spring Thaw,” Report FHWA/MT-02-006/8155, Federal Highway Administration and Montana Department of Transportation. Johnson, F. L., and Chang, F. F. M. (1984). “Drainage of Highway Pavements,” Federal Highway Administration, Publication No. FHWA-TS-84-202. Washington, D.C. Jones, A. (1962). “Tables of Stresses in Three-Layer Elastic Systems,” Bulletin 342, Transportation Research Board of the National Academies, Washington, D.C., pp. 76–214. Jones, G. M., Darter, M. I., and Littlefield, G. (1968). “Thermal Expansion-Contraction of Asphaltic Concrete,” Proceedings of Association of Asphalt Pavement Technologists, White Bear Lake, Minn., vol. 37.
Pavement Environment Jung, D., and Vinson, T. (1994). “Thermal Stress Restrained Specimen Test to Evaluate Low-Temperature Cracking of Asphalt-Aggregate Mixtures,” Transportation Research Record 1417, Transportation Research Board of the National Academies, Washington, D.C. Konrad, J. -M., and Morgenstern, N. R. (1980). “A Mechanistic Theory of Ice Formation in Fined-Grained Soils,” Canadian Geotechnical Journal, no. 17, pp. 473–486. Konrad, J. -M., and Morgenstern, N. R. (1983). “Frost Susceptibility of Soils in Terms of Their Segregation Potential,” Proceedings Permafrost: Fourth International Conference, National Academy Press, Washington, D.C., pp. 660–665. Ladanyi, B., and Shen, M. (1989). “Mechanics of Freezing and Thawing in Soils,” Proceedings of FROST ’89, Technical Research Center of Finland, Espoo, Finland. Lekarp, F., and Dawson, A. (1998). “Modeling Permanent Deformation Behaviour of Unbound Granular Materials,” Construction and Building Materials, Elsevier, vol. 12, no. 1, pp. 9–18. Loch, J. P. G., and Kay, B. D. (1978). Water Redistribution in Partially Frozen, Saturated Silt under Several Temperature Gradients and Overburden Loads, J. Soil Sci. Soc. of Amer., 43, 3, pp. 400–406. McAdams, W. C. (1954). Heat Transmission 3d ed., McGraw-Hill, New York. McLeod, N. W. (1976). “Asphalt Cements: Pen-Vis Number and Its Applications of Moduli of Stiffness,” ASTM Journal of Testing and Evaluation, vol. 4, no. 4. Miller, R. D. (1972). “Freezing and Heaving of Saturated and Unsaturated Soils,” Highway Research Records, No. 393, Transportation Research Board of the National Academies, Washington, D.C., pp. 1–11. Moulton, L. K. (1980). “Highway Subsurface Drainage,” Federal Highway Administration, Publication No. FHWA-TS-80-224, Washington, D.C. OECD (1988). “Heavy Trucks, Climate and Pavement Damage,” Organisation for Economic Co-operation and Development, Paris. Oliver, J. E. (1973). Climate and Man’s Environment: An Introduction to Applied Climatology, John Wiley & Sons, Hoboken, N.J. Palolahti, A., Slunga, E., Saarelainen, S., and Orama, R. (1993). “Sulavan Maan Kantavuus (Elastic Stiffness of Thawing Soils),” Helsinki University of Technology, Faculty of Civil Engineering and Surveying, p. 99. Pavlov, A. V. (1976). “Heat Transfer of the Soil and the Atmosphere at Northern and Temperate Latitudes,” CRREL Draft Translation 511, U.S. Army Corps of Engineers. Peattie, K. R. (1962). “A Fundamental Approach to the Design of Flexible Pavements,” Proceedings of International Conference on the Structural Design of Asphalt Pavements, University of Michigan, Ann Arbor, Mich., pp. 403–411. Perera, Y. Y., Zapata, C. E., Houston, W. N., and Houston, S. L. (2004). “Moisture Equilibria beneath Highway Pavements,” Transportation Research Board 2004 Annual Meeting CD-ROM. Rowe, G. M., Brown, S. F., Sharrock, M. J., and Bouldin, M. G., 1995, “Visco-elastic analysis of hot mix asphalt pavement structures,” Transp. Res. Record No. 1482, Transp. Res. Board, Washington, D.C., pp. 44–51. Steven, B. D., de Pont, J. J., Pidwerbesky, B. D., and Arnold, G. (1999). “Accelerated Dynamic Loading of Flexible Pavements at CAPTIF,” Proceedings of International Conference on Accelerated Pavement Testing, Paper GS2-3, Reno, Nevada. Ullidtz, P. (1987). Pavement Analysis, Elsevier Science, New York.
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Chapter Two Ullidtz, P. (2002). “Analytical Tools for Design of Flexible Pavements,” Keynote address at the Ninth International Conference on Asphalt Pavements, International Association of Asphalt Pavements, White Bear Lake, Minn., http://www.asphalt.org/ (July 2, 2007). Van der Poel, C. (1954). “A General System Describing the Viscoelastic Properties of Bitumens and Its Relation to Routine Test Data,” Journal of Applied Chemistry, May 1954. Zarling, J. P., and Braley, W. A. (1988). “Embankment Design and Construction in Cold Regions,” section 3, Geotechnical Thermal Analysis, E. G. Johnson, A. Phukan, and W. H. Haas (eds.), ASCE, Reston, Va. Zeng, H. Y., and Vinson, T. S. (1998). “Thermal Contraction of an Asphalt Concrete Mixture,” Proceedings of the Ninth International Conference on Cold Regions Engineering, ASCE, Reston, Va. Zubeck, H. K., and Vinson, T. S. (1996). “Prediction of Low Temperature Cracking of Asphalt Concrete Mixtures with Thermal Stress Restrained Specimen Test Results,” TRR No. 1545, Transportation Research Board, National Research Council, Washington, D.C. Zubeck, H. K., and Vinson, T. S. (2007). “Prediction of HMA Low Temperature Crack Spacing Using TSRST Results,” Proceedings of the Eighth International Symposium on Cold Regions Development, Finnish Association of Civil Engineers, Helsinki, Finland.
CHAPTER
3
Cold Region Pavement Performance
P
avements are built to provide a safe and comfortable ride for road users. Fulfilling this vital role, referred to as the functional role of the pavement, involves that the surface must be smooth and skid resistant. The ability of a pavement to play its functional role is often referred to as the serviceability. Distortions of the pavement surface reduce pavement smoothness. These distortions can be caused by differential movements, crack deterioration, and raveling of the pavement surface. The loss of skid resistance is generally the result of the wear of the surface texture or of the presence of distortions, which can affect vehicle dynamics and cause water accumulation at the pavement surface. Pavements also need to be cost-effective assets, which implies that they must perform or, in other words, provide an adequate level of service over a reasonable period of time. Performance is directly linked to the structural adequacy of the pavement structure. Traffic action will cause fatigue cracking and permanent deformation on weak pavements. Other factors related to climate will intensify damage caused by traffic or cause other damages specific to mechanisms triggered by climatic factors. The mechanisms involved in pavement deterioration in cold climates needs to be fully understood in order to apply good engineering principles when selecting pavement materials, designing pavement structures or analyzing an existing structure needing maintenance or rehabilitation. Cold region pavements are subjected to intense loading by climatic and environmental factors which are, in good part, responsible for the seasonal and the longterm loss of structural and functional capacity of the pavement. These factors also intensify the damaging effect of heavy loads acting on the pavement structure. The main deterioration mechanisms for pavements in cold climates can be grouped in those that are acting in the asphalt-bound materials and those that are acting in the unbound layers and subgrade soils. The factors acting on the asphalt-bound materials include: • Thermal contraction or fracture (Sec. 3-1) • Fatigue (Sec. 3-2) • Crack deterioration (Sec. 3-3) • Rutting due to lack of stability or wear (Sec. 3-4) • Aging (Sec. 3-5)
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58
Chapter Three • Pavement disintegration caused by the action of water, salt, and frost within the asphalt-bound layer (Sec. 3-6) • Potholes (Sec. 3-7) The factors acting on unbound layers and subgrade soil are • Volume change and more specifically differential volume change caused by frost heave (Sec. 3-8) • Bearing capacity loss during spring thaw (Sec. 3-9) • Frost destructuration of sensitive clays (Sec. 3-10) • Thaw consolidation of frozen soils in permafrost regions (this problem is described in Chap. 10) These deterioration modes are described in the following sections. A first subsection, labeled “problem description,” includes general descriptions of the problems including, when available, a mechanistic explanation in order to facilitate identification and quantification of the action of contributing factors. A second subsection describes available techniques to assess the problem when applicable. Finally, a third subsection includes a brief discussion on mitigation techniques applicable in cold environment contexts.
3-1 Thermal Cracking of Asphalt Concrete Problem Description Thermal cracking of asphalt pavements displays itself as fairly straight cracks perpendicular to the direction of the road (Fig. 3-1). In some cases the cracking progresses with time to the extent that the crack spacing becomes smaller than the width of the road.
FIGURE 3-1
Thermal transverse cracking.
Cold Region Pavement Performance
FIGURE 3-2
Thermal block cracking.
Cracks then start to form parallel to the direction of the road and form blocks with the transverse cracks as shown in Fig. 3-2. Low-temperature cracking are generally initiated in the asphalt bound layer, but can also be initiated in the underlying frozen pavement layers or subgrade possessing tensile strength due to the binding effect of pore ice. Figure 3-3 shows a crack initiated below the asphalt concrete layer that expands over the sidewalk, highway, bicycle path, and in-between green area.
FIGURE 3-3
Thermal cracking initiated below the HMA layer.
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60
Chapter Three The cracks decrease the riding quality of the pavement and allow water and deicing agents to penetrate the pavement structure causing frost-related problems, which shortens the pavement service life. Annual crack sealing is necessary as the cracks reopen during winters due to pavement contraction at the vicinity of the crack. It is difficult to prevent the cracks from reflecting through a new overlay. Thermal cracking is often divided into low-temperature cracking and thermal fatigue cracking. Low-temperature cracking occurs when temperature drops fast below −16 to −35°C and the thermal stress in the pavement exceeds its tensile strength. The development of thermal stresses is described in Sec. 2-3. Thermal cracking is also observed in climates where these kinds of cold temperatures never occur. In these cases, the thermal cracking is typically assumed to be thermal fatigue cracking, which occurs due to the diurnal temperature cycling. This section describes low-temperature cracking that is the most typical form of thermal cracking in cold regions.
Problem Assessment Cooling temperature causes thermal stress to develop in hot-mix asphalt (HMA), but only if the pavement slab is constrained from contracting. The two conditions that have to be satisfied for low-temperature cracking to occur for a given HMA could be termed “cold” and “constraint.” If one of these does not exist, there is no low-temperature cracking. The thermal stress given in Eq. (2-36) with the associated cracking temperature (see Fig. 2-26) could be considered representing the “cold.” It is the on/off switch of the initial low-temperature cracking. If the temperature change in the field is not sufficient in magnitude and rate to cause the thermal stress to reach the tensile strength of the pavement (assuming adequate restraint) no cracks occur. This means that the pavement properties affecting its cracking temperature are the most important factors influencing low-temperature cracking for a given climate. Once cracking occurs, i.e. the two conditions are satisfied, the “constraint” controls the crack spacing. The spacing is mainly affected by the pavement’s tensile strength (temperature and loading time dependent), and the shear strength at the pavementbase interface (Zubeck and Vinson 2007). The best estimate for the tensile strength at the cracking temperature is obtained from the thermal stress restrained specimen test (TSRST, see Chap. 4). The cooling rate in the TSRST should equal the maximum cooling rate in the field. A standardized test method to determine the constraint conditions in a routine mix design is yet to be developed. Factors influencing lowtemperature cracking of asphalt pavements are listed by Vinson et al. (1996). These factors and their effect in cracking through either “cold” or “constraint” are summarized in Table 3-1. Binder properties are often used to predict the low-temperature cracking resistance of a pavement mixture, as they dictate the cracking temperature. The property that is crucial for low-temperature cracking is binder’s stiffness at low temperatures. Soft binders have better cracking resistance than hard binders. Penetration at a low temperature, from 4 to 15°C (ASTM D5), Fraass Breaking Point (DIN 52012), bending beam rheometer test and direct tension tester (AASHTO T 313 and T 314) can be used to assess the binder consistency at low temperatures. The degree of aging of the binder also affects the cracking temperature as the binder becomes stiffer with time. Different binders age at different rates, and therefore, an aging rate needs to be considered. Aging of binder during the hot mix production and in service can be simulated in the laboratory using the thin film oven test, TFOT, (ASTM D 1754), rolling thin film oven test, RTFOT, (AASHTO T 240), or pressure aging vessel, PAV, (AASHTO R-28).
Cold Region Pavement Performance
Factor
Effects through
Change in Factor
Effect of Change on Cracking
Material factors
Asphalt cement
Cold
Consistency increases
Increases
Aggregate type
Constraint
Durability decreases, absorptiveness increases
Increases
Environmental factors
Temperature
Cold
Minimum air temperature decreases
Increases
Rate of cooling
Cold
Increases
Increases
Pavement age
Cold/ constraint
Increases
Increases
HMA thickness
Cold/ constraint
Increases
Decreases
Shear strength between the HMA layer and base course
Constraint
Increases
Increases
Subgrade type
Cold/ constraint
Indirect effect through associated microclimates and ground thermal cracking
Increases/ decreases
Cold/ constraint
Increases
Increases
Pavement structure geometry
Construction flaws
TABLE 3-1
Factors Affecting Low-Temperature Cracking
Two approaches have been proposed to assess or predict low-temperature cracking. The first one is based on the cracking temperature alone; the lower the cracking temperature is, the better the cracking resistance. The methods based on cracking temperature evaluate the cracking tendency of a mixture without assessing crack spacing or cracking frequency. Hills and Brien (1966), reported by Vinson et al. 1996, estimate the cracking temperature by first calculating the thermal stress using Eq. (2-36) with measured values of thermal contraction and stiffness. The cracking temperature is obtained at the intersection of the thermal stress curve and the curve for measured tensile strength of the mixture as illustrated in Fig. 3-4. The AASHTO PP42-02 (AASHTO 2003) uses the same principle. Bending beam rheometer test results are used to calculate a relaxation modulus master curve and subsequently the thermal stress. The calculated thermal stress is then compared to the failure stress from the direct tension test to determine the critical cracking temperature. Simulation methods, such as TSRST, can be used to directly determine the cracking temperature of an asphalt mixture. The second approach predicts the crack spacing either by statistically derived predictive models (Hajek 1971; Ehrola 1986; Haas et al. 1987) or by a mechanistic approach (Zubeck and Vinson 1996; Timm and Voller 2003; Konrad and Shen 1997).
61
62
Chapter Three
FIGURE 3-4 Estimating the fracture temperature of asphalt concrete (after Hills and Brien 1966, reported by Vinson et al. 1996).
Hajek (1971) developed a statistical model for a cracking index based on 42 observations from pavements in Ontario and Manitoba, Canada. The cracking index is the number of cracks per 152 m section of two-lane roadway. The model in its current version (Huang 2004) is given in Eq. (3-1): I = 30.3974 + (6.7966 – 0.8741 · h + 1.3388 · a) log (0.1Sbit) – 2.1516 · d – 1.2496 · m + 0.06026 · Sbit log (d)
(3-1)
where I = cracking index, Sbit = stiffness modulus of the original asphalt cement in kg/cm2 as determined by McLeod’s method for loading time of 20,000 s and for winter design temperature, a = age of pavement in years, m = winter design temperature in −°C (use only positive values), d = dimensionless subgrade code 5-sand, 3-loam, and 2-clay, h = combined thickness of bituminous layers in inches (see Example 3-1). Example 3-1 Predict the crack spacing after 2 and 10 years assuming that all transverse cracks are full cracks (extending across the entire two-lane pavement) for a 51-mm-thick pavement with the following design parameters: stiffness of the original asphalt cement 310 kg/cm2, winter design temperature −25°C, sand subgrade.
Cold Region Pavement Performance Solution With Sbit = 310 kg/cm2, a = 2 years, m = 25°C, d = 5 and h = 2 in, from Eq. (3-1) I = 30.3974 + (6.7966 – 0.8741 · 2 + 1.3388 · 2) log(31.0) – 2.1516 · 5 – 1.2496 · 25 + 0.06026 · 310 · log(5) = 10.48 Spacing = 152.4 m/10.48 = 14.54 m. Answer: 15 m. Similarly for a = 10 years, Eq. (3-1) yields I = 20.45, and spacing = 5.76 m. Answer: 6 m.
In order to study the crack spacing with a mechanistic model, Zubeck and Vinson (1996, 2007) suggest performing an equilibrium analysis that considers the forces affecting an asphalt concrete slab (see Fig. 3-5a). For the analysis, stresses given in Eqs. (2-36) and (2-37) are transferred into forces. A thermal force for a pavement slab with width, W, and thickness, D, becomes Fthermal = sthermal WD
(3-2)
The opposing force when the shear strength is mobilized can be calculated as Fresisting = (sv tanf + c)Wx
(3-3)
FIGURE 3-5 (a) Forces affecting pavement slab, (b) stresses in pavement slab before and after cracking (after Zubeck and Vinson 2007).
63
64
Chapter Three where sv = g D, g is the unit weight of the HMA, f = friction angle and c is the cohesion at the interface (see Fig. 3-5a), W = width of the slab and x is the distance from the slab edge toward to the center of the slab (≤0 x ≤ ½ slab length). The thermal force that is associated with the cracking of the pavement can be determined as Ffailure = (sthermal) failure WD = Pts(T, t)WD
(3-4)
where (sthermal)failure is the thermal stress that equals the temperature and loading time dependent pavement tensile strength Pts(T, t) obtained from the TSRST (the cracking strength at the cracking temperature). For cracking to occur Fthermal = Fresisting = Ffailure
(3-5)
Then, using Eqs. (3-3) and (3-4) Pts(T, t) WD = (g D tan f + c)Wx
(3-6)
The crack spacing can be evaluated with the scenarios illustrated in Fig. 3-5b. The temperature drops and the thermal stress together with the opposing stress develops. At the vicinity of the slab edges, where the maximum possible opposing stress is small, the slab is able to contract and consequently the thermal stress will be released. When the temperature gets cold enough for the thermal stress to equal the tensile strength of the pavement, cracking will occur within the fully restrained area (see Fig. 3-5b). The minimum possible crack spacing occurs, when the initial crack forms at the point closest to the slab edge where the thermal stress equals the strength of the pavement. The minimum spacing can be solved from Ffailure = Pts(T, t)WD = (g D tanf + c)W Spacingmin Spacing min =
Pts(T, t) ⋅ D γ D tan φ + c
(3-7) (3-8)
The maximum possible spacing occurs, when the initial crack occurs at a location where the residual thermal stress after cracking equals the strength of the pavement. The maximum spacing is Spacingmax = 2 Spacingmin
(3-9)
The crack spacing of the pavement will stay between these two values depending on the location of the initial cracks. Additional cracking will not occur, as the entire new slab is now able to contract at cold enough temperatures, and the thermal stress will not reach the pavement’s tensile strength. However, as the pavement ages, it becomes more brittle. Consequently, the tensile strength with temperature is affected leading to the possibility of future cracking (Zubeck and Vinson 2007). The determination of crack spacing using the TSRST results is demonstrated in Example 3-2. Example 3-2 Laboratory tests were conducted for an asphalt-aggregate mixture. The following results were obtained: Unit weight was 24 kN/m3. The TSRST gave fracture strength of 4.6 MPa at a fracture temperature of −28°C. A direct shear test was conducted for a 50-mm-thick asphalt concrete slab on 20-mm maximum aggregate size dense graded base course to be used on the paving project. The test results indicated no cohesion and a friction coefficient (tan f) of 2.4 at the asphalt concrete and base
Cold Region Pavement Performance course interface. What is the anticipated range of cracking for a 50-mm pavement layer if the predicted coldest pavement temperature is (a) −32°C and (b) −25°C? Solution (a) Since the TSRST fracture temperature of −28°C is warmer than the predicted pavement temperature of −32°C the pavement will crack. The minimum crack spacing from Eq. (3-8) is Spacingmin = Pts(T, t)/(g ⋅ tanf) Spacingmin = 4600 kPa/(24 kN/m3 ⋅ 2.4) = 80 m The maximum spacing from Eq. (3-9) is Spacingmax = 2 Spacingmin = 2 ⋅ 80 m = 160 m The cracking will occur at intervals from 80 to 160 m. The average crack spacing will approach 80 m as the pavement ages. (b) Since the TSRST fracture temperature of −28°C is colder than the predicted pavement temperature of −25°C, the thermal stress does not reach the tensile strength of the pavement. Consequently no cracking will occur until the pavement has aged significantly.
Remedial Solutions The best remedy to avoid low-temperature cracking is the use of mixtures with low TSRST fracture temperatures and high fracture strengths. Use of soft asphalt cements may lead to rutting in the summer time due to permanent deformation, especially on roads with a high volume of traffic. Therefore, binder selection is often a compromise that depends on peak temperatures (during winter and summer) and the traffic volume. Polymer-modified asphalts can be developed to have both good rutting and low-temperature cracking resistance and are recommended for high-traffic roads. For low-traffic roads, soft asphalt cement is recommended; its grade should be selected based on the climate zone (see Chap. 7). Lowtemperature cracking can also be reduced by increasing the thickness of the HMA layer.
3-2
Fatigue Cracking Problem Description Fatigue cracking is often called alligator cracking because its closely spaced pattern is similar to the pattern of an alligator’s skin (Fig. 3-6). It is a fracture phenomenon caused by a repeated application of tensile strains that are less than the strength of the material. In a fatigue process, microscopic flaws in a material grow in size under repeated loading, becoming more densely concentrated until visible flaws or cracks develop. The visible cracks then propagate through the material. Fatigue cracking is made worse by inadequate pavement drainage. The HMA layers experience high strains when the underlying layers are weakened by excess moisture and consequently fail prematurely in fatigue (Roberts et al. 1996). This is an important factor in cold regions, where pavements become saturated regularly during the spring thaw. Doré and Savard (1998) report that most of the fatigue cracking in Quebec, Canada, occurs during spring. Not only are the deflections larger, but also the HMA layer is still cold and consequently more brittle. In thin pavements, cracking starts at the bottom of the asphalt layers and propagates upward. In thick pavements, bending of pavement layers is reduced eventually to the level that crack initiation is restrained and no bottom up fatigue cracking occurs.
65
66
Chapter Three
FIGURE 3-6
Fatigue cracking.
In recent years a term, perpetual pavements, has been introduced to describe these pavements (Newcomb et al. 2001). However, thick pavements may suffer top-down cracking, that is, cracking which starts from surface layer and propagates downward. At what HMA thickness and traffic levels does the bottom-up fatigue cracking transform to top-down cracking in cold regions is yet to be determined. Fatigue cracking is one of the common forms of pavement distress and is incorporated in the pavement design process. The following sections describe the assessment and remedial solutions for bottom-up cracking that is the most prominent fatigue cracking type in cold regions.
Problem Assessment The fatigue characteristics of asphalt mixture are usually expressed as relationships between the initial stress or strain and the number of load applications to failure. They are determined using repeated flexure (beam fatigue test), direct tension or diametral tests, performed at several stress or strain levels (Tayebali et al. 1992). The fatigue behavior of a specific mixture can be characterized by the slope and relative level of the stress versus the number of load repetitions to failure and can be defined by a relationship given in Eq. (3-10) (Huang 2004; Monismith et al. 1985): f2
1 1 N f = f1 ε t E1
f3
(3-10)
where Nf is the allowable number of load repetitions to prevent fatigue cracking, et is the tensile strain at the bottom of the asphalt layer, E1 is the elastic modulus of asphalt layer, f1, f2, and f3 are constants determined from laboratory fatigue tests with f1 modified
Cold Region Pavement Performance to correlate with field performance observations. Several values are suggested for f1, f2, and f3. An example of the fatigue failure criterion [Eq. (3-10)] is the method given by the Asphalt Institute [Eq. (3-11)] (Huang 2004). This stress-controlled model was developed using the beam fatigue test for a standard mixture having 5 percent air-void content and 11 percent effective binder volume. N f = A ⋅ 0.00432 ⋅ C ⋅ 796ε t−3.291 E∗
−0.854
(3-11)
where A = 18.4 (factor that accounts for differences between laboratory and field conditions), C is a volumetric correction factor [see Eqs. (3-12) and (3-13)], et is the initial tensile strain and |E∗|is the dynamic modulus. C = 10 M
(3-12)
Vbeff M = 4.84 − 0.69 Va + Vbeff
(3-13)
where Vbeff is effective binder volume, %, and Va is air-void content, % (see Example 3-3). Example 3-3 Fatigue tests were performed using 400 × 100 × 50 mm3 beams resting on a flexible polymer base by two-point loading at 15°C. The test results are given in Table 3-2. Develop an equation relating the number of repetitions to failure similar to Eq. (3-11). Plot the measured and predicted Nf values versus the measured strain. Use log-log scale on axis.
Strain, µm/m
N
Force, kN
E, MPa
1030
1933
1.5
3000
785
2846
1.1
3100
720
11783
1.2
3500
700
5776
1.3
3900
680
16400
1
3200
620
28491
0.9
3100
610
28863
0.8
2800
520
71244
0.85
3800
340
443660
0.75
5000
270
1923705
0.65
5000
1130
307
1.7
3200
Source: Spoof 1992.
TABLE 3-2
Fatigue Test Results for Example 3-3
67
68
Chapter Three
FIGURE 3-7 The relationship between strain and number of repetitions to failure for Example 3-3.
Solution Write out Eq. (3-10) as Eq. (3-14) and run a regression analysis in which Y = log(Nf), X1 = log(et), and X2 = log(E). log( N f ) = log( f1 ) + f 2 log(ε t ) + f 3 log(E)
(3-14)
The results of a linear regression analysis by using the “least squares” method gives the following values: the intercept log(f1) = −8.926, the least square estimators f2 = −6.525 for log(et) and f3 = −2.173 for log(E). Solving for f1 = 10−8.926 gives a value of 1.185 × 10−9. The equation then becomes N f = 1.185 × 10−9 εt−6.525 (E)−2.173 where the unit for strain, et, is mm/m and the unit for modulus, E, is MPa. The relationship between strain and number of repetitions to failure is given in Fig. 3-7.
Fatigue test can be conducted using constant stress or strain. In the constant stress test, the strain is increased with the number of repetitions in order to keep the stress constant. In the constant strain test, the stress or load is decreased with the number of repetitions to keep the strain constant. There is not a complete agreement as to whether real pavement systems should be designed using stress- or strain-controlled fatigue relationships. It is usually assumed that the constant stress test is applicable to thick pavements, where the HMA layer is more than 150 mm thick and is the main loadcarrying component. For thin pavements (<50 mm), the strain in the asphalt layer is governed by the underlying layers and is not affected by the decrease in stiffness of HMA, and therefore constant strain test is applicable. For intermediate thicknesses, a combination of constant stress and strain exists (Huang 2004). In addition to the mode of loading, fatigue response of asphalt mixtures is affected by mixture variables and temperature (Rao Tangella et al. 1990). Air-void content is an important factor and should be as small as possible (permanent deformation considerations require air-void content >3 percent). Asphalt content also has a significant effect on fatigue response. It should be as high as possible without risking rutting resistance of the mixture. Stiff asphalt cements and dense graded mixtures should be used for thick pavements and soft asphalt cements and more open-graded mixtures for thin pavements. The temperature affects the fatigue life through mixture stiffness that decreases with increasing temperature. As a consequence, the fatigue life of thick pavements decreases and the fatigue life of thin pavements increases with increasing temperature (Rao Tangella et al. 1990).
Cold Region Pavement Performance
Remedial Solutions Because of the aforementioned stress/strain conditions for thick and thin pavements, it is better to have stiff HMA layer for thick pavements, and soft HMA layer for thin pavements to minimize fatigue cracking. The mix designer needs to know the thickness of the HMA layer in order to optimize the composition of the mixture to withstand rutting, thermal and fatigue cracking (Pellinen 2001). Drainage of the roadway becomes imperative as moisture reduces the strength of the pavement layers and consequently increases the risk for cracking. Proper quality control during the construction, especially for pavement thickness, is also important in reducing the cracking tendency (Roberts et al. 1996).
3-3
Crack Deterioration Once cracks are initiated by thermal contraction or traffic action, pavement deterioration is accelerated. The reduction of the layer stiffness in the vicinity of the crack combined with weakened base material caused by water infiltration amplifies pavement damage caused by truck traffic. As a result, secondary cracks are initiated and the main crack tends to become faulted and depressed.
Problem Description As illustrated in Fig. 3-8a, an uncracked asphalt concrete layer is very effective in distributing the load to the underlying base layer. Once a crack appears in the layer (Fig. 3-8b), the stress distribution pattern is strongly affected by the ineffective load transfer between the two faces of the crack. In the presence of a load near the crack and in the complete absence of friction between the two faces of the crack the load distribution effectiveness of the asphalt layer is reduced by a factor of almost two. The problem is worsened by the presence of water seeping into the granular base through the crack (see Sec. 2-2). The deflection of the surface is thus excessive in the vicinity of the crack leading, under the repetitive action of wheel loads, to the development of spalling of
FIGURE 3-8
The process of crack deterioration.
69
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Chapter Three
FIGURE 3-9
Deteriorated cracks.
the crack faces, the formation of secondary cracks and to the accumulation of differential permanent deformation (Fig. 3-8c, d and Fig. 3-9). Crack deterioration contributes to increased pavement roughness and to reduced pavement structural capacity.
Remedial Solutions Two categories of actions can be taken to mitigate the effect of pavement cracking. In all cases, the action must be taken before significant crack deterioration occurs, involving the loss of structural capacity in the vicinity of the crack (Fig. 3-8c and d). The first type of action is a preventive strategy and it involves sealing the crack early after crack initiation. Crack sealing prevents the intrusion of water and incompressible materials into the crack; thus, reducing further deterioration of the crack. Crack openings should be wide enough to allow penetration of the sealant, but should also be small enough to allow good friction and load transfer between the two faces of the crack. The National Center for Asphalt Technology recommends openings between 6 and 12 mm (Roberts et al. 1996). Two approaches can be taken for crack sealing. The first involves routing the top part of the crack to create a reservoir to contain the sealant (Masson 2001). The second involves bridging the crack with sealant poured onto the pavement surface forming a 3- to 4-mm-thick and 40- to 50-mm-wide band (Pouliot 2003). Both techniques appear to give good results (Roberts et al. 1996; Pouliot 2003; Masson 2001). The second category of action is applicable when the crack has reached a certain level of deterioration without important structural damage to the pavement. It involves localized repair of the pavement in the vicinity of the crack. This is generally done by partial or complete removal of the asphalt concrete in the damaged area and filling of the gap with hot mix asphalt. If the repair is followed by an overlay, special attention needs to be given to prevent crack reflection in the new asphalt layer. Several approaches can be used to control reflective cracking. They include acting on the cause of crack reflection, using techniques or materials, which will reduce stress concentration above the existing crack. For example, stress absorbing membranes, bitumen-rich asphalt concretes, and granular layers have been used with variable success as stress absorption layers between existing and new asphalt concrete layers.
Cold Region Pavement Performance
3-4
Rutting of Asphalt Concrete Rutting manifests itself as depressions of the wheel paths as a result of traffic load. Except for intersections, it does not increase the longitudinal roughness of the road significantly, but may still become a safety hazard due to its effect on lateral maneuverability of vehicles and possible risk of hydroplaning on ponding water. Ruts decrease the structural capacity of the pavement due to decreased layer thickness and changed properties. When the rut depth exceeds a level where the serviceability of the road starts to decrease, the road needs rehabilitation. Rutting of roads in cold regions has several sources. The rutting may be limited in the asphalt layer, where it is caused either by permanent deformation or wear by studded tires. The rutting may also result from permanent deformation in the unbound structural layers or in the subgrade. This kind of rutting occurs mostly due to bearing capacity loss during the spring thaw. It is included in the pavement structural design and is described in Chap. 8. The following sections describe rutting in the bound HMA layer due to permanent deformation and wear by studded tires.
3-4-1
Permanent Deformation
Problem Description Permanent deformation is a result of initial densification and subsequent plastic deformation of the HMA with an increased number of load applications. The volume of the HMA decreases during densification due to reduced air voids in the mixture. After the air voids drop below a mixture specific limit (e.g., from 1 to 2 percent), plastic flow starts to occur. During plastic flow volume does not change anymore (assuming incompressible material); instead rutting forms when the mixture flows from the wheel paths to the small upheavals beside the wheel paths (Fig. 3-10). The relationship of
FIGURE 3-10
Plastic deformation at a bus stop.
71
72
Chapter Three
FIGURE 3-11 Division of permanent deformation into the initial densification and plastic deformation (Saarela et al. 1993).
the densification to total deformation for a laboratory tested sample is illustrated in Fig. 3-11 (Saarela et al. 1993). As the stiffness of the mixture decreases with increasing temperature and loading time, the permanent deformation occurs during the summer months especially in areas of slow or standing traffic, such as intersections and loading areas. Deformation can be visually differentiated from wear-related rutting by smooth dark asphalt concrete surface and by the presence of upheavals between wheel paths. Permanent deformation is a severe problem in almost all cold regions, as the pavement temperature often rises higher than the air temperature.
Problem Assessment The permanent deformation of HMA mixes is affected by material properties, mix design, and in-service conditions. The factors and their effects on rutting resistance summarized in Table 3-3 are by and large universally accepted tendencies. The only controversial effects are the aggregate gradation and maximum size. Gap-graded and stone matrix asphalt (SMA) mixtures are reported to have both higher (Sousa et al. 1991) and lower (Saarela 1993) plastic deformation rates than dense-graded mixtures. The mechanistic or mechanistic-empirical pavement design procedures limit rutting of asphalt pavement system to a tolerable level by controlling the maximum vertical strain or stress at the surface of the subgrade. These methods do not prevent rutting by permanent deformation in the HMA layer, and a separate analysis may be needed. Two analytical procedures have evolved to predict the amount of rutting in the HMA layer, namely, layer-strain predictive methodology and closed-form viscoelastic analysis. In addition, statistically derived models exist to predict the permanent strain (Sousa et al. 1991). The layer-strain method predicts rut depth using laboratory test results and linear or nonlinear elastic theory. Each layer of the pavement structure is divided into sublayers, i. The stress state is calculated at the center of each sublayer directly beneath the wheel load using elastic analysis. With the average stress state, the corresponding axial plastic strain can be determined from laboratory test results. The total rut depth for a
Cold Region Pavement Performance
Factor Aggregate
Change in Factor
Effect of Change in Factor on Rutting Tendency
Surface texture
Smooth to rough
Decreases
Gradation
Gap graded to continuous
Increases/ decreases
Shape
Rounded to angular
Decreases
Size
Increase in maximum size
Increases/ decreases
Binder
Stiffness*
Increases
Decreases
Mixture
Binder content
Test/field conditions
Increases
Increases
†
Air-void content
Increases
Increases
Voids in mineral aggregate (VMA)
Increases
Increases‡
Method of compaction
§
§
Temperature
Increases
Increases
State of stress/ strain
Increase in tire contact pressure
Increases
Load repetitions
Increase
Increases
Water
Dry to wet
Increase if mix is water sensitive
∗
Refers to stiffness at temperature at which rutting propensity is being determined. Modifiers may be utilized to increase stiffness at critical temperatures, thereby reducing rutting potential. † When air contents are less than about 3 percent, increase in air voids reduces the rutting potential. ‡ It is argued that very low VMAs (e.g., <10 percent) should be avoided. § The method of compaction, either laboratory or field, may influence the structure of the system and therefore the propensity of rutting. Source: Adapted from Sousa et al. 1991.
TABLE 3-3
Factors Affecting Permanent Deformation of HMA Mixtures
given number of load applications is obtained from Eq. (3-15) by summing the products of the average plastic strain occurring at the center of each sublayer and the corresponding sublayer thickness: n
∆ p = ∑ (ε ip )(∆zi )
(3-15)
i =1
where ∆ p = total rut depth, eip = average plastic strain in the ith sublayer, ∆ zi = thickness of the ith sublayer and n = total number of sublayers. The layer-strain method is considered a simplified engineering approach for predicting rut depth (Sousa et al. 1991). The deformation predictions depend on the testing
73
74
Chapter Three method and sample preparation procedures. These variations together with uncertainty in traffic and environmental conditions make the prediction of the rutting extremely difficult. Therefore, use of simplified methods is acceptable (see Example 3-4). Example 3-4 Estimate permanent deformation of a 150-mm-thick HMA layer at 30°C with a unit weight of 24.0 kN/m3 using layer strain method and repeated load test. Solution The pavement layer is divided in three layers: from 0 to 50 mm, from 50 to 100 mm, and from 100 to 150 mm. The vertical stress sz and the radial stress sr are estimated at the sublayer boundaries using Eqs. (2-30) and (2-31). The mid-height stresses of each sublayer are estimated as average stresses at the boundaries. Overburden pressure is calculated at the mid height as a product of the depth and the unit weight of the HMA. Laboratory samples made out of the actual materials and mix design to be used in the HMA layer are tested in repeated load test. The testing is conducted at 30°C using the average vertical stress as the repeated deviator stress and the sum of the average radial stress and the overburden pressure is used as the confining pressure. The calculation of the stresses is shown in Table 3-4. The test results in the form of permanent strain, the thickness of each sublayer, deformation of each sublayer and the total deformation from Eq. (3-15) are shown in Table 3-5. The estimated deformation for the HMA layer is approximately 4 mm. The more complex viscoelastic methodology considers moving wheel loads in conjunction with time-dependent material properties to define the states of stress and strain at particular points in the pavement structure. The material properties are defined either in terms of models consisting of finite numbers of Maxwell and/or Kelvin elements in various arrangements or in terms of generalized compliance relationships. The viscoelastic method is theoretically more appealing than the layerstrain method. However, its complexity and the relatively poor agreement between measured and predicted values indicate that it does not present a significant advantage over the layer-strain method (Sousa et al. 1991).
An example of statistically derived predictive models for separation of permanent and resilient strain is given in Eq. (3-16). The log-linear model developed originally by Leahy and further modified by Ayres (summarized by Pellinen 2001) is based on 2860 data points in unconfined repeated permanent deformation test. εp log = −4.80661 + 2.58155 ⋅ log T + 0.4295611 ⋅ log N εr
z, mm
rz, kPa
0
522
25
520
50
505
75
475
100
433
125
385
150
337
Average rz, kPa
rr, kPa
(3-16)
Average rr, kPa
Overburden Pressure = f z, kPa
rr + f z, kPa
336
0.6
337
163
1.8
165
68
3.0
71
444 514
329 229
469
152 97
385
61 38
TABLE 3-4 Determination of Test Conditions for Example 3-4
Next Page
Cold Region Pavement Performance
z, mm
Permanent Strain, %
Thickness, mm
Deformation, mm
2.82
50
1.41
2.75
50
1.37
2.37
50
1.18
0 25 50 75 100 125 150 Total 3.96 TABLE 3-5
Test Results and Calculation of Permanent Deformation for Example 3-4
where ep = plastic strain, er = resilient strain, N = number of equivalent load cycles and T = temperature in °F. This approach is being further modified and may become the preferred approach in permanent deformation predictions (Pellinen 2001).
Remedial Solutions Solutions to avoid rutting of HMA mixtures due to permanent deformation reflect on factors and their effect on rutting resistance as listed in Table 3-3. Selection of rough, angular aggregate and proper volumetric mix design reduces the rutting potential. The aforementioned controversy about the effect of gradation on rutting resistance still exists. Saarela et al. (1993) recommends that only experienced pavement engineers should design gap-graded and SMA mixtures. If the mix design is not conducted with extreme care, the resulted rutting may be even worse than in the case of dense graded mixtures. Controversy also exists on the importance of the effect of binder over the other factors. All other factors being the same, the stiffer the asphalt cement is at the high in-service pavement temperatures, the better the rutting resistance. However, stiff binders are often prone to cracking, and therefore, the selection of the asphalt cement needs to consider both, rutting and cracking resistance. Well-designed polymer-modified asphalts characteristically have excellent rutting resistance without compromising the cracking resistance. The asphalt cement selection is covered in Chap. 7. Rutting resistance can also be increased with other additives than polymers, such as naturally occurring asphalt cements (Saarela et al. 1993). An increase in lane width together with lack of obstructions on the roadside decreases rut depth. This is due to the fact that the lateral distribution of traffic (wander) increases and consequently wheel paths become wider. High level of service (in terms of highway capacity) and high free flow speed reduce plastic deformation due to shorter loading time.
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Chapter Three
3-4-2
Rutting Due to Studded Tire Wear
Problem Description Rutting due to studded tire wear occurs as the studs remove particles from the pavement surface. When a stud hits the cold pavement surface, it often fractures the exposed coarse aggregates or the mastic (mixture of aggregate fines and asphalt binder). When the stud leaves the pavement surface, the scratching effect detaches the fractured particle. A rut starts to form after a large number of stud passages. Wear-related rutting may also occur due to the combined action of regular tires and abrasives used for winter maintenance (Tervahattu et al. 2004). Rutting related to wear can be visually differentiated from plastic deformation by rough asphalt concrete surfaces with exposed aggregate and by a single narrow depression in each wheel path without adjacent upheavals (Figs. 3-12 and 3-13). Studded tires have been used in cold regions since the 1960s to provide enhanced vehicle traction under winter driving conditions with snowy and icy roads. The leading countries using the studded tires are the Nordic countries, especially Finland and Sweden. In Finland, for example, the studded tire usage has varied between 89 and 96 percent during the winter months (Fig. 3-14). The advantages of studded tire use (that come with the enhanced winter traction) are reduced accidents and savings in travel time. The disadvantages include the generated pavement wear and increased vehicle operation costs. The Nordic countries and U.S. states that allow stud usage, report spending millions of dollars annually on repairing the pavement damage due to the studded tires. However, the Nordic countries have reduced the rutting due to studded tires significantly from what it used to be in the 1970s and 1980s (Gustafson 1997; Jacobson 1997; Unhola 1997). These reductions result mainly from regulations in studded tire usage (stud types, frequency in the tire and
FIGURE 3-12
Asphalt-aggregate specimen tested for studded tire wear in Prall tester.
Cold Region Pavement Performance
FIGURE 3-13
Pavement rutting primarily due to studded tire wear.
seasonal use), constructing wear resistant pavement surfaces, and decreasing winter driving speeds. Figure 3-15 shows the change in typical road wear in Finland by one passenger car from 11 kg per 100 travel kilometers in 1960 to 2 kg per 100 travel kilometers in 2000 (Unhola 2004). As the pavement wear is the most significant adverse effect of studded tire usage, natural response from a pavement engineer is to advocate for a ban on studs. However,
FIGURE 3-14 Stud usage in Finland (Unhola 2004).
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Chapter Three
FIGURE 3-15 Typical road wear by one passenger car driving with studded tires at highway speeds (Unhola 2004).
the use of studded tires needs to be considered at a broader level, as its socio-economic effects influence different groups, such as vehicle owners, insurers, and government, at different degrees. On the basis of the recent studies by Elvik (1999), Öberg and Wikström (2004), Unhola (2004), and Zubeck et al. (2004), it is possible that the usage of studded tires causes a positive economic effect for northern areas with icy road conditions. Pavement repair costs would indeed be reduced by banning stud use, but costs of accidents as well as the increased requirement of surface applications to improve surface traction (e.g., sand, salt) result in an overall increased financial burden at the state level. These studies have led to legislation that continues the use of studded tires during winter months, but limits that use to lightweight studs to minimize adverse effects.
Problem Assessment A large number of studies exist for factors affecting pavement wear (Sistonen and Alkio 1986; Lampinen 1993; Brunette and Lundy 1997; Gustafson 1997; Jacobson 1997; Kavussi and Edgar 1997; Jacobson and Wågberg 1998, 2004; Malik 2000; Unhola 2004). The results of these studies are summarized in the following sections. Site specific models for the severity of the wear rutting can be found in Lampinen (1993), Jacobson and Wågberg (1998, 2004), and Kurki (1998). Pavement wear is affected by factors relating to traffic, road geometry, pavement characteristics, and environment. Some of these factors have larger effect than others, and the magnitude of the effect may be site specific. Table 3-6 lists these factors and their effect on pavement wear. The rutting is directly related to the traffic volume and the percentage of vehicles using studs. Stud mass and vehicle speed have great effect on wearing of roads, as well as pavement surface conditions (wet, dry, or snow covered). Pavement temperature has a smaller effect. The wear by studded tires is reported to be higher where the vehicles are accelerating or decelerating. These areas include curves, uphills, downhills, and intersections. The
Traffic
Environment
Road geometry
Mixture characteristics
Factor
Change in Factor
Effect of Change in Factor on Wearing Tendency
Volume
Increases
Increases
Percent of vehicles using studs
Increases
Increases
Stud mass
Increases
Increases
Speed
Increases
Increases
Enforcement of seasonal regulations
Increases
Decreases
Pavement temperature*
Decreases
Increases
Duration of wet surface
Increases
Increases
Duration of traction sanding
Increases
Increases
Amount of deicing applications
Increases
Increases
Lane width
Increases
Decreases
Distance of roadside obstructions from the traffic lane
Increases
Decreases
Vertical grade
Increases
Increases
Density of intersections
Increases
Increases
Horizontal curvature
Increases
Increases
Crown slope†
Increases
Decreases
Abrasion resistance of aggregate
Increases
Decreases
Use of stone matrix asphalt mixtures
Increases
Decreases
Coarse aggregate content
Increases
Decreases
Use of antistripping agents Use of polymer modified asphalt cement
Needs to be tested Increases
Decreases‡
∗
For temperatures <0°C. Indirect effect; steeper crown accelerates drainage. ‡ With use of low quality aggregate. †
TABLE 3-6 Factors Affecting Wearing Resistance of HMA Mixtures
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Chapter Three
FIGURE 3-16 Relative importance of pavement factors to wearing by studded tires (Saarela 1993).
lane width and existence of obstructions on the roadside (e.g., guard rails, bridge abutments, and tunnel walls) affect the rut depth due to a channeling effect of traffic. Pavement materials and mixture types are reported to be one of the main factors affecting road wear by studded tires. Aggregate is stated to be the most important material factor. The abrasion resistance and the content of the coarse aggregates are the significant aggregate characteristics. The nominal maximum size has a significant influence in some cases (larger size reduces wearing). The next most important pavement factor after aggregate is the mix design. The stone matrix asphalt (SMA) mixtures are reported to be more wear resistant than dense-graded asphalt mixtures. The asphalt cement has a significantly smaller effect on pavement wear than the aggregate and mix design, which has made it difficult to determine the specific effect of asphalt cement on rutting resistance. It has reported that in some cases, especially with poor aggregates, the use of polymer-modified asphalts reduces pavement wear. The effect of each asphalt-aggregate mixture component on the wear resistance according to Saarela (1993) is given in Fig. 3-16.
Remedial Solutions The solution in reducing rutting by studded tire wear reflects on factors affecting rutting listed in Table 3-6. The results experienced by the Nordic countries that proved to be effective in reducing the wear rutting, included the use of less aggressive studs, strictly enforcing seasonal tire usage, and building wear resistant pavements. The biggest improvement that contributed to wear resistant pavements entailed use of abrasion resistant aggregates and SMA mixtures.
3-5 Aging of Asphalt Concrete Problem Description As explained in Secs. 3-1 to 3-4 asphalt concrete deteriorates with time due to traffic and environmental loading. This deterioration may be accelerated or decelerated by aging of the asphalt cement mainly due to loss of volatiles during the hot-plant mixing and oxidation in the field, but also due to other aging phenomena. Aging hardens the asphalt concrete and makes it more brittle. Therefore aging improves rutting resistance due to plastic deformation. However, its negative effects of increased tendency toward cracking, moisture damage, pothole formation, and wearing by studded tires outweigh the positive effect of improved rutting resistance.
Cold Region Pavement Performance
Problem Assessment The four principal mechanisms of aging are oxidation, evaporation, exudation, and physical hardening (Johansson 1998). Oxidative aging is an irreversible chemical reaction between components of asphalt cement and atmospheric oxygen. It can occur during mixing, laying, and while in service. Oxidation is affected by asphalt cement chemical composition, temperature, and other complex factors not fully understood. Evaporation, or loss of volatile components, occurs predominantly during mixing, when the binder is in the form of thin films, and large surface areas are exposed to high temperatures. Exudation takes place especially with porous aggregate, when the oily components exude from asphalt cements into the aggregate. It results in irreversible compositional changes in the asphalt cement that may affect binder properties and other forms of aging. Physical hardening is a reversible process, which does not alter the chemical composition of the binder, but can produce significant changes in the binder’s rheological properties. It is attributed to molecular structuring or reorganization of binder’s microstructural systems to approach an optimum thermodynamic state under a specific set of conditions. It can be reversed by heating or agitation. An example of the effect of aging on the asphalt cement physical properties is given in Fig. 3-17. A significant part of the aging occurs during the hot-plant mixing. Another significant aging period is the first few months in service, after which the hardening of the binder slows down. The main factors affecting aging besides the binder properties are the air-void content of the mixture, HMA layer thickness and the climate at the pavement site. The HMA
FIGURE 3-17 Viscosity at 60°C versus time in months (Kandhal et al. 1973, with permission from ASTM International).
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Chapter Three hardens faster with increasing air-void content, decreasing layer thickness and increasing mean annual air temperature (Mirza and Witczak 1995). The effect of aging on the pavement performance is considered in mix design by conducting various laboratoryaging procedures either on the asphalt cement, mixture, or both before performance testing. These aging methods are described in Chap. 4.
Remedial Solutions Solutions to minimize the effects of aging on the pavement performance start with the selection of the asphalt cement and aggregate. The performance-based asphalt specifications and most of the traditional asphalt specifications consider aging in their procedures. If porous aggregate is used and/or open-graded mixtures, the aging of the HMA mixture should be tested. Aging, during the construction phase, can be minimized by controlling the mixing temperatures in the hot mix plant and can also be controlled by using batch plants or counter flow drum plants. In these types of plants, the chamber temperature through which the binder is sprayed is typically lower than in the traditional drum mix plant. As most of the age hardening occurs during the mixing phase, this is the most important factor in prevention of unnecessary binder aging. In cold regions, long hauling distances combined with cold air temperatures create a temptation to raise the hot-plant mixing temperature higher than recommended for rheological considerations. As this accelerates the aging of the mixture, it should be avoided. Instead, the truck beds should be covered or slightly heated to retain the mixture temperature high enough for adequate compaction. In-service aging can be minimized by using dense-graded mixtures, high asphalt cement contents and by applying surface dressings as the pavement ages. These suggestions need to be verified with the desired pavement performance and economic considerations.
3-6
Pavement Disintegration Problem Description Disintegration is the breakup of the pavement structure. It manifests itself as loss of individual pieces of HMA from each other. Disintegration takes the form of raveling, stripping, and potholes (Roberts et al. 1996). It may be accelerated by wear due to the usage of studded tires or chains. Effects of raveling include loose debris on the highway which reduces skid resistance and may be picked up by tires and thrown at other vehicles or passengers. If depressions are formed, ponding water may saturate the HMA and cause stripping, and may also be a safety hazard for traffic. A pavement with surface raveling is shown in Fig. 3-18. Moisture-related stripping accelerates cracking and rutting of pavements, and may cause disintegration of the entire asphalt layer (Terrel and Shute 1989). Short paving season, cold paving weather, precipitation, and freeze-thaw cycles make this problem even more severe in cold regions than in warm regions.
Problem Assessment Raveling is disintegration of the HMA from the top downward, as aggregate particles become loose due to loss of bond between the asphalt binder and aggregate. There are several conditions that can lead to raveling (Roberts et al. 1996):
Cold Region Pavement Performance
FIGURE 3-18 Severe surface disintegration of HMA pavement.
1. A coating of fine dust on the aggregate surface, thick enough to cause the asphalt to stick to the dust rather than to the aggregate. Surface traction forces abrade the asphalt film and the aggregate gets loosened. 2. Insufficient amount of fine aggregate matrix to hold coarse aggregates together. This is especially prevalent on segregated spots in the surface layer where most of the fines are missing. In these areas, only a few contact points bind the aggregate together that could be broken by traction forces after the bond has been weakened by aging or stripping. 3. Low in-place density in the asphalt surface course. As in the case of segregation, fewer contact points that result in inadequate compaction may be broken by traction forces. 4. Deficient asphalt content. If the asphalt content is too low, there is not enough binder to cover all the aggregates, and the resulting cohesive bond will be weak. 5. Excessively aged asphalt binder. Asphalt binder may be excessively aged during mixing at the hot plant if the mixing temperature has been excessively above the recommended equivalent viscosity mixing temperature. The binder may also age in service especially if the air-void content is high and the asphalt films are thin. Stripping is defined as the weakening or eventual loss of the adhesive bond in the presence of moisture between the aggregate surface and the asphalt cement in an HMA mixture (Roberts et al. 1996). Stripping typically begins at the bottom of the HMA layer and progresses upward. The strength of the mixture is derived from cohesion provided by the binder and friction provided by the aggregate. The cohesion is only fully developed if a good adhesive bond exists between the binder and the aggregate. If the bond
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Chapter Three Mechanism
Description
Caused by
Detachment
Separation of an asphalt film from aggregate surface without a break in the film
Thin film of water existing at the asphalt-aggregate interface
Displacement
Displacement of bitumen at the aggregate surface through a break in the bitumen film
Chemical incompatibility of the asphalt and the aggregate that leads in water-soluble compound
Spontaneous emulsification
Invert emulsion of water droplets in asphalt cement that breaks the adhesive bond
Water immersion
Pore pressure
Excessive pore water pressure disrupts the asphalt film from the aggregate surface or grows microcracks in the mastic
Entrapped water and traffic load
Hydraulic scouring
Stripping results from the action of tires on a saturated surface. Osmosis and pullback are suggested mechanisms
Surface water and traffic load
TABLE 3-7 Mechanisms of Stripping Process
is poor or becomes poor, failure occurs at the binder-aggregate interface, which leads to premature failure of the mixture and the HMA pavement. Suggested mechanisms for stripping process, their descriptions and causes summarized by Little and Jones (2003) are listed in Table 3-7. Moisture damage is often a result of multiple mechanisms rather than a single cause. Moisture or moisture vapor are the only widely claimed causes for stripping. All other factors are debatable. In the presence of water, however, stripping tendency of a pavement is considered to depend on aggregate properties, type and use of mixture, asphalt cement properties, environment, traffic, construction practices, and the use of antistripping agents.
Remedial Solutions To prevent moisture damage from occurring, the main thing is to keep the water and moisture out of the pavement. Kandhal and Rickards (2001) emphasize this by stating that the three vital elements to eliminate moisture damage are drainage, drainage, and drainage. In the presence of water and moisture, the following issues should be considered. To prevent moisture damage due to incompatible materials, the aggregate from the source for a project needs to be laboratory tested together with the asphalt product to be used. Additives, such as liquid antistripping agents and lime, may be used to improve the asphalt-aggregate bond. However, if an incompatible antistripping agent is used, the end result may be even worse than without it (Tunnicliff 1997). Therefore, the mix design including tests for moisture sensitivity needs to be conducted with the considered antistripping agent. To prevent the moisture damage resulting from the construction practices, the production rate at the hot plant need to be adjusted, so that all moisture from the aggregate
Cold Region Pavement Performance has been evaporated when the ideal mixing temperature with the asphalt has been reached. Aggregate may also be preheated to dry it out. Other pretreatments, such as weathering, washing to remove surface coatings, and crushing have been shown to improve the adhesion between asphalt and aggregate (Hunter and Ksaibati 2002). The asphalt-aggregate mixture should not be placed on a wet surface due to generated steam pressure that may force the moisture between the binder and the aggregate. Proper field compaction and avoidance of segregated areas reduce the tendency for moisture-related damage. When the pavement “in-service” is exposed to traffic and environmental stresses, maintenance is an important factor in keeping the moisture damage from occurring. Rut filling, crack sealing, surface treatments, and immediate pothole filling are some of the maintenance procedures to be considered. Water migration from underneath the pavement has to be eliminated. Overlays on pavements that are damaged by water-related distress are not recommended. In many reported cases, the water in the existing stripped layers starts to strip the new layer from the bottom upward, immediately after the placement (Kandhal and Rickards 2001). To prevent non-moisture-related raveling from occurring, precautions need to be taken during the mix design and construction phase. Optimum amount of fines and asphalt content play an important role. During construction, excessive asphalt aging, inadequate compaction, and mixture segregation need to be avoided.
3-7
Potholes Potholes are circular or elongated cavities resulting from a localized disintegration of the pavement surface. Potholes are not a type of distress per se, but an extreme manifestation of pavement distresses such as cracking, raveling or deterioration of pavement joints. They can be the result of an important construction fault or material defect or they can be an indication of a pavement, which has exceeded its design life.
Problem Description Three conditions are required for potholes to occur: • A breach in the pavement surface • Water • Traffic action Pothole development is a disintegration process, which requires a break in the pavement surface in order to be initiated. Pavement cracking (see Secs. 3-1, 3-2, and 3-3), pavement disintegration (see Sec. 3-6), and deteriorated construction joints are the main types of distresses leading to the formation of potholes. Figure 3-19 illustrates potholes initiation in a construction joint (a), along a longitudinal crack (b), and in a raveling thin surface (c). Even if potholes can sometimes occur in dry conditions, water usually plays a major role in the process. As described in Sec. 3-3, cracked pavements allow water and brine to seep into the granular base. Higher strains from pavement loading and frost heave
85
FIGURE 3-19
Pothole initiation in pavements.
Cold Region Pavement Performance
FIGURE 3-20 Development of a pothole.
occurring in the vicinity of the crack lead to further deterioration and partial disintegration of the surfacing layer. Water is also an important factor in stress redistribution under moving traffic, which can lead to drastic degeneration of the disintegration process. As illustrated in Fig. 3-20, when a moving wheel enters the sensitive saturated zone, the load can induce intense hydrostatic pressures under the weakened asphalt concrete layer, forcing an upward movement in the adjacent unloaded pavement (Fig. 3-20b). In worst cases, the pressure is sufficient to remove broken pieces of asphalt concrete (Fig. 3-20c). When a hole is open in the pavement structure, the tire can apply pressure directly to water forcing it against the wall of the pothole and causing active erosion of pavement materials (Fig. 3-20d).
Remedial Solutions In properly managed pavement networks, potholes should not occur. In order to prevent pothole occurrences, pavement engineers have to act on one of the three main factors responsible for pothole development. Considering that pavement’s main role is to carry traffic, removal of traffic loads on the pavement can only be seen as an exceptional solution. Alternative options are to act on pavement drainage in order to effectively remove excess water from pavement surface and pavement structure and/or act on the breach before disintegration occurs. The first solution can only be implemented at the construction or the rehabilitation stages. The second involves effective maintenance strategies including crack sealing and resurfacing of pavements shortly after distress manifestation. If potholes occur on pavements, they need to be patched rapidly in order to avoid creating hazardous driving conditions and further deterioration of the affected area. Pothole patching involves the following operations: • Cutting the asphalt concrete around the affected area • Removal and replacement of contaminated materials in the hole • Compaction of the bottom of the hole • Coating the walls of the asphalt concrete hole with an appropriate tack coat • Filling the hole with good quality materials
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Chapter Three Since potholes tend to occur mainly in thawing or wet conditions, pothole-filling operations might have to be done in poor conditions. In such cases, temporary filling with asphalt cold mix should be considered, but not as a permanent repair, which should be scheduled and performed as soon as weather conditions permit.
3-8
Frost Heaving As described in Sec. 2-2, during cold winter months, frost penetrates pavement materials and subgrade soils. While progressing in the pavement structure, frost causes interstitial water to expand and can also cause segregation ice to form in unbound granular materials. Notwithstanding the fact that the latter phenomenon is generally considered insignificant in pavement granular materials, it causes the materials to loosen. Heaving of pavement surfaces reaching 10 to 15 mm are systematically being observed on experimental test sites before the frost front reaches the subgrade soil. This specific problem will be discussed in Sec. 3-8-3. When the frost front reaches frost susceptible subgrade soils, water is sucked toward the frozen fringe where ice lenses are formed. Heave of the pavement surface resulting from these phenomena can reach and even exceed 150 mm for climatic conditions prevailing in cold regions. If frost heaving were uniform, it would not be damaging for pavements. However, frost action is generally uneven because of the variability of subgrade soil characteristics and because of the embankment geometry. Differential frost heaving is therefore a major factor affecting winter roughness of roads built in cold environments.
3-8-1
Differential Frost Action
Problem Description The problem with frost heave results mainly from the fact that the phenomenon is rarely uniform. As a result, pavement becomes distorted during winter causing increased roughness and cracking of the pavement surface. Differential frost heaving can be attributed to four major causes. The first cause, described by Peterson and Krantz (1998), is associated with the instability of the one-dimensional freezing process. The onedimensional frost heave has the propensity to evolve into multidimensional differential frost heave as a function of soil properties and environmental conditions. The other causes of differential heaving are the variability in the frost susceptibility characteristics of the subgrade soil (including moisture availability), the variability of the thermal regime and the topography of the surface and/or the geometric characteristics of an earth structure (Doré 1997). Figure 3-21 shows a good example of differential heaving associated with the variability of soil characteristics obtained during a study on differential frost heaving done by Doré et al. (2001). The pseudo profiles (profiles measured using an inertial profiler) were measured during summer and winter for a pavement section built on glacial till north of Quebec City, Canada. The average frost heave of 40 mm, estimated from segregation potential tests, was added to the winter profile to compensate the absence of absolute elevation reference for the profile measurements. The winter profile for pavement section follows a close resemblance to the summer profile. However, it is characterized by a succession of 10-m long humps. The summer profiles were subtracted from the winter profile in order to bring out the characteristics of differential frost
Cold Region Pavement Performance
FIGURE 3-21 Longitudinal profiles measured in the summer and in the winter on a pavement built on glacial till (Doré et al. 2001, Reproduced with permission of TRB, from Transportation Research Record: Journal of the Transportation Research Board, No. 1755, Transportation Research Board of the National Academies, Washington, D.C., 2001, Figure 1, p. 91).
heaving in different geological contexts. Figure 3-22 shows the characteristics of differential heaving specific to three different geological contexts: Section SA-2 is characterized by 5 to 10-m long humps, which can be associated with the intrinsic variability of the underlying silty till deposit. Section SP-2 is affected by large deformations, which are the result of the alternating soil layers with different frost susceptibility under the pavement section. Finally, Section SH-1 built on sandy soils, shows regularly spaced peaks, which are probably associated with heaving around transverse cracks. The later phenomena will be discussed in Sec. 3-8-3.
FIGURE 3-22 Differential heaving profiles for pavements built in three different geological contexts: Fine-grained lacustrine deposits (SP-2), silty glacial tills (SA-2) and sandy alluviums with silt interlayers (SH-1) (Doré et al. 2001, Reproduced with permission of TRB, from Transportation Research Record: Journal of the Transportation Research Board, No. 1755, Transportation Research Board of the National Academies, Washington, D.C., 2001, Figure 2, p. 92).
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Chapter Three Based on the analysis of the heaving profiles of the test sections, it appears that distortions caused by differential heaving have typical wavelengths in the range of 5 to 20 m when the pavement is underlain by a variable soil deposit such as glacial till. Distortions can be characterized by much larger wavelengths when the pavement is intersecting several soil layers with variable frost susceptibility. Wavelengths ranging from 10 to 80 m have been observed for those conditions. Shorter wavelength distortions (1 to 3 m) can also be found on heaving profiles for pavements experiencing frost heave around cracks.
Problem Assessment For pavement design and analysis, within an area where similar soil characteristics are observed during soil investigation, soils are generally considered homogeneous. This assumption is convenient for pavement engineers, but rarely reflects the geological reality. Soil deposition is a very complex phenomenon, which sometimes induces important variations in soil characteristics and properties. In order to assess differential movements induced by subgrade soils, the variability needs to be quantified. Not much has been done in the area of differential frost heave prediction. Doré et al. (2001) have developed a framework for the prediction of differential frost heave and applied it to a limited number of test sections in order to develop a prediction model for roughness increase during winter. Frost heave can either be measured directly on the pavement during a typical winter or it can be estimated based on subgrade soil properties. In the latter case, segregation potential (frost susceptibility) of the subgrade soil needs to be measured in the laboratory or estimated from soil characteristics. Different approaches to measure or estimate the segregation potential of soils are described in Chap. 4. To accurately estimate the variability of a variable, a large number of measurements are required. Thus, estimating differential frost heave based on freezing test results is not practical. Two approaches are thus proposed: assessment of differential frost susceptibility of subgrade soils based on a gradation index and measurement using differential heaving based on the frost heave profiles. Figure 3-23 illustrates the procedure proposed to assess differential frost heave based on subgrade soil properties. The geostatistical tool called the “semivariogram” is used to characterize subgrade soil variability and to quantify a soil variation index, denoted g4, which represent the variability of subgrade soil at a 4-m distance. The distance of 4 m has been selected as a
FIGURE 3-23 Differential frost heaving in relationship with the variation of subgrade soil properties (Doré et al. 2001).
Cold Region Pavement Performance critical distance for differential soil behavior assessment, because of its incidence on vehicle dynamics.
γ ( D) =
2 1 [ g(x) − g( x + D)] 2n ∑
(3-17)
where g = semivariogram, D = distance between the samples (4 m), n = number of pairs of samples considered in the analysis, and, g(x) = measured characteristic at location x. The semivariogram is the statistical equivalent of a variance. The square root of the semivariogram is therefore the equivalent of a standard deviation. By dividing the square root of g4 by the average value of soil particles passing 2 mm for all samples, a coefficient of variation (COV) of that characteristic for 4-m spaced samples is obtained. Saarelainen (1996) has shown that it is fair to assume that, for equal frost penetration, frost heave is proportional to the percentage of soil particles smaller than 2 mm. Thus, by multiplying the average frost heave of a pavement section by the COV, we obtain an estimate of the typical difference in frost heave at 4-m distance. The following equation is the definition of the longitudinal variation coefficient, VL, used to estimate differential frost heave at 4-m distance based on a frost susceptibility index measured on soil samples:
γ4 (3-18) h ≈ ∆ h4 x where VL is the longitudinal variation coefficient, which is an estimation of typical differential heaving at a 4-m distance (∆h4), h is the average frost heave for a pavement section, x is the average value of the frost susceptibility indicator (passing 2 mm) for all samples. VL is a complex variable that takes into account most factors contributing to differential frost heaving. The term “h” can be estimated or measured. It takes into consideration climatic factors, moisture availability, and frost susceptibility of subgrade soils. When combined, the g4 and the x terms are used to characterize the variability in subgrade soil characteristics at the critical 4-m distance. A similar geostatistical approach was used to measure differential frost heave directly from longitudinal profiles collected during winter and summer. In order to achieve this, the two profiles had to be perfectly superposed. Then, the semivariograms were obtained using the following equation: VL =
2γ =
2 1 [(E − ES( x + h) ) − (EW ( x ) − ES( x ) )] n ∑ W ( x+ h)
(3-19)
where EW = elevation measured during winter, ES = elevation measured during summer, h = distance between measurements, n = number of samples. In this specific case, typical differential frost heaving at 4-m (denoted ∆h4) distance can readily be obtained by calculating the square root of g4. A performance model has also been developed by Doré et al. (2001) to predict winter deterioration of roughness caused by differential frost heave. The purpose of the model is to allow for the prediction of poor winter pavement performance in terms of roughness. If excessive winter roughness is expected, then remedial actions can be taken during design and construction to mitigate the problem. It has also been observed (Doré and Savard 1998) that rough winter pavements tend to have a faster long-term deterioration rate. Reducing winter roughness will, thus, improve long-term performance of pavements subjected to differential frost heaving.
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Chapter Three The model given in Eq. (3-20) relates the longitudinal variation coefficient VL to the ratio of winter to summer roughness in terms of international roughness index (IRI). IRI w = 0.446 ln(VL ) + 1.333 IRI s
(3-20)
The model shows a relatively good fit, but is based on a limited quantity of observations. The use of the model in pavement engineering practice is, therefore, not recommended unless it has been properly validated and calibrated to local conditions. However, it shows that a relationship between winter roughness and the variability of frost action in soils exists. See Example 3-5. Example 3-5 Predict the differential frost heave and the winter roughness (IRI) for a pavement on which the summer roughness was measured to be 2.0 m/km (IRI). A typical frost heave of 50 mm is expected on the pavement and, based on gradation testing and geostatistical analysis of the subgrade soil, the average content in fines (<0.075 mm) is 25 percent and the g4 value is estimated to be 20. Solution Using Eq. (3-18), VL =
γ4 x
h=
20 50 = 8.9 mm ≈ ∆h4 25
And based on Eq. (3-20), IRI w = 0.446 ln(8.9) + 1.333 = 2.3 IRI s Thus, IRI w = 2.3 × 2 = 4.6 (m/km)
Remedial Solutions Two conditions must exist for the occurrence of problems related with differential frost heaving. First, frost heave has to be significant. Frost heaving becomes more important with increasing frost depth, frost susceptibility of subgrade soil, and moisture availability. Second, frost heaving has to be uneven. Spatial variability of soil characteristics is the major factor contributing to the unevenness of frost heaving. If only one of these conditions is present in the field, differential frost heaving should not be a major problem. By applying the proposed model [Eq. (3-20)] during preliminary studies of a road construction project, it should be possible to reliably predict poor winter performance of the pavement. It would thus be possible to modify pavement design or construction practices in order to mitigate the inconvenience caused by differential heaving. If variability seems to be the dominant problem (high g4 values), subgrade homogenization techniques can be used to reduce the differential effects of frost heaving. Moreover, if frost heave is expected to be the dominant problem (high predicted or observed h values), frost penetration could be reduced by increasing the thickness of a granular subbase or by placing an insulation layer in the pavement structure. In the longer term, increased construction costs associated with these techniques would probably be compensated by a significant reduction in maintenance costs. They would also improve the winter and the long-term ride quality of the road, resulting in reduced vehicle maintenance and operation costs.
Cold Region Pavement Performance
3-8-2
Frost Heave Cracking
Problem Description Another important differential heaving problem affecting pavement structures is the result of the important variations of thermal regime along the transversal section of the pavement. As illustrated in Fig. 3-24, snow accumulation on the pavement sides impedes heat extraction at that location. As a result, frost penetration at the center of the pavement is much greater than at the edge of the pavement. Frost heave being proportional to frost penetration, a transverse differential heaving is initiated in the pavement. The movement can be assimilated to a bending moment (Nordal and Refsdal 1984; Scher 1996) imposed to the pavement transverse section. It is likely to generate excessive tensile stresses and initiate longitudinal or meandering cracks. Figure 3-25 shows a severe case of longitudinal cracking resulting from the transverse differential heaving problem. Road users are not directly affected by the phenomena, but the resulting cracks can be highly detrimental to the structural performance of the pavement because they intercept running water at the surface, which then infiltrates the pavement structure.
Problem Assessment Longitudinal cracking will occur when tensile strain, eF, induced in pavement by the bending action exceeds the low-temperature failure tensile strain of the asphalt layer, eR. Tensile strain at the pavement surface can be estimated using a simplified multilayer bending system as illustrated in Fig. 3-26 (Doré et al. 1999). A few simplifications are, however, required to effectively compute the stresses and the strains induced in the asphalt layer. The following assumptions are made: 1. The transverse section of the pavement is assumed to be a beam of uniform inertia with a rectangular section having a unitary width. This approach was proposed by Scher (1996) to analyze the formation of longitudinal cracks caused by frost action in pavements. 2. Pavement materials and layers are assumed to be bound by frozen pore water. 3. The bending of the beam is assumed to be circular and uniform.
FIGURE 3-24 Stresses and cracking induced by transverse differential heaving.
93
FIGURE 3-25 Severe case of longitudinal cracking on a local road.
FIGURE 3-26 Simplified bending model for pavements subjected to transverse differential heaving (Doré et al. 1999, Reproduced with permission of TRB, from Transportation Research Record: Journal of the Transportation Research Board, No. 1655, Transportation Research Board of the National Academies, Washington, D.C., 1999, Figure 2, p. 111).
94
Cold Region Pavement Performance Based on this approach, the strain at the surface of the pavement can be computed as a function of two parameters: differential heaving between the center and the edge of the pavement (at a distance L of the center) and the depth of the neutral fiber in the bending pavement section. The first parameter is a function of the climatic and geological conditions at the site. It also reflects the geometry of the pavement section and the winter maintenance practices. The depth to the neutral axis is, in turn, a function of the characteristics of the pavement structure and the materials mechanical properties. The ratio eF /eR is defined as the solicitation index and it has been used as a mechanistic variable to predict the extent of frost heave cracking on pavements affected by frost action. In order to adequately represent the stresses imposed on the asphalt layer by the transverse differential heaving phenomena, the thermal contraction stress acting in the transverse direction also needs to be considered. The solicitation index is therefore written Rε =
ε F + εT εR
(3-21)
where eF = strain induced by the bending of the pavement section, eT = restrained strain induced by the thermal contraction, eR = low-temperature failure strain of the asphalt concrete as measured from laboratory tests. Several pavement test sections have been used to relate the mechanical action of transverse differential heaving to the deterioration of the pavement surface by longitudinal cracking (Doré et al. 1999). The following relationship was obtained: ∆Ff = 1.38 Rε 2 − 0.23 Rε
(3-22)
where ∆ Ff is the rate of progression of frost heave cracking in linear meters per year and Re is the solicitation index defined in Eq. (3-21). This model shows good predictive capacity (R2 = 0.94; standard error of the estimate = 7.9 m), but has limited robustness due to a limited number of observations. The use of the model in pavement engineering practice is, therefore, not recommended unless it has been properly validated and calibrated to local conditions.
Remedial Solutions For pavement engineers, there are different ways to control the transverse differential frost heave problem. The first approach is to reduce the transverse differential heave. By reducing frost penetration in the frost susceptible subgrade, frost heave and, consequently, the differential frost heave are going to be reduced. The reduction in frost penetration will affect the position of the neutral fiber, while the reduction of the differential frost heave will affect the bending strain. This can be achieved by increasing the thickness of non-frost-susceptible granular material or by introducing an insulation layer (polystyrene or other) into the pavement. Another possible solution, which is likely to give similar results, is to reduce the segregation potential of the frost susceptible soil. This can be achieved by lowering the water table or by chemically treating the subgrade soil. Another possible approach is to cope with the differential heaving by strengthening the pavement. This can be achieved by increasing the failure strain of the asphalt concrete [eR in Eq. (3-21)]. In the later case, the use of a soft asphalt binder should be considered. The use of a steel mesh or a geogrid would also yield significant benefits by increasing the tensile strength of the asphalt concrete.
95
96
Chapter Three The last mitigation approach is a maintenance strategy. As a matter of fact, increasing the width of the lane cleared of snow can significantly reduce the transverse differential heave problem. Removing snow off the pavement shoulder will cause a deeper frost penetration at that location, thus, reducing transverse differential frost penetration and related differential frost heave.
3-8-3
Frost Heaving in Granular Base Material
Problem Description It is generally considered that frost-induced deterioration originates from the frostsusceptible subgrade soil and that frost action in standard granular layers is not a significant factor of pavement deterioration. Frost-related phenomena have, however, been observed within the granular pavement structures. The manifestation of these phenomena is a volume increase within the granular structure. Figure 2-14 shows variation in moisture content within a granular base material as measured in a pavement test site in Montana (Janoo and Greatorex 2002). A volumetric water content exceeding 20 percent was measured (during springtime) in a granular material classified A-1-a and having between 10 and 13 percent passing the 0.080-mm sieve. Figure 3-27 illustrates other experimental data collected on a test road in Quebec, Canada. In that case, despite the absence of cracks in the pavement surface, a frost heave reaching 8 mm is measured in the pavement granular base layer subjected to one freeze-thaw cycle. In that case, the granular material has between 6.5 and 9.5 percent of particles smaller than 0.080 mm. Assuming full saturation and approximately 20 percent porosity for the 300-mmthick base material, frost heave due to expansion of freezing pore water should not exceed 5.5 mm. The problem seems to be much more severe near cracks where frost heaves in the pavement materials exceeding 45 mm have been measured (Martel et al. 2000) and moisture contents reaching 35 percent by weight have been measured (Lupien et al. 1994). The mechanisms involved are not well understood. Several theories are proposed in the literature to explain frost heave in the granular base materials:
FIGURE 3-27 Total frost heave and frost heave in the granular base layer as measured during winter 2003 on an experimental road in Quebec, Canada (DeBlois 2005).
Cold Region Pavement Performance Formation of segregation ice in the granular base material: Formation of segregation ice during the progression of the freezing front in the granular base is likely to explain a certain amount of frost heave in the granular base. The pavement base can be exposed to steep thermal gradients at the beginning of the freezing season and generally freezes rapidly. Despite the fact that granular base materials are generally considered nonfrost susceptible, their segregation potential can be significant. Experience has shown that temperature gradients in the range of 0.020 to 0.060°C/mm can be expected in early freezing season. Under these conditions, pavement base would normally freeze in less than 2 days. Assuming the steepest gradient and a segregation potential of 20 mm2/°C·day, the resulting frost heave calculated using Eq. (5-33) should not exceed 2 mm. Thus, it is unlikely that segregation freezing during the progression of the freezing front can explain alone the type of behavior illustrated in Fig. 3-27. Contamination of the granular base and/or subbase materials by fine particles is also considered to be a possible factor contributing to the formation of segregation ice in the pavement structure. Fine particles can be transported from the subgrade soil by interstitial water migrating upward under pore pressure gradients occurring mainly during spring thaw. Dust present on the pavement surface can also be transported by runoff water, seeping into the pavement structure through cracks. In both cases, the segregation potential of pavement granular material is likely to increase with time. Thus, explaining some of the frost heave occurring within the pavement structure. Vapor migration toward the pavement surface and condensation underneath the asphalt concrete layer: Some authors have reported observations about the accumulation of a thin layer of ice immediately underneath the asphalt concrete. It has been hypothesized (Eigenbrod and Kennepohl 1996) that vapor could be transported toward the pavement surface and condensate at the contact between the cold asphalt concrete surface and the base material. The hypothesis is supported by measurements of water contents exceeding considerably the saturation water content immediately at the interface between the asphalt concrete and the base material, a dirty granular material containing between 15 and 20 percent of particles smaller than 0.080 mm. More research needs to be conducted to study how vapor transportation occurs in dirty granular materials and observe the role played by segregation freezing in the moisture buildup. Formation of segregation ice caused by the contamination of granular base materials near cracks: A frost heave phenomenon occurring in the granular base material contaminated by deicing salt has been observed by several authors (Lupien et al. 1994; Doré et al. 1998; Kestler et al. 1998; Martel et al. 2000). In this specific case, frost heave occurs in the vicinity of discontinuities such as cracks or pavement edge. Short wavelength surface deformation resulting from frost heaving along transverse cracks seriously affects driving conditions at the end of winter. Several cases are reported and studied each winter in Canada. Profile measurements along a line perpendicular to the crack indicate that frost heave tends to be at a maximum at a short distance (approximately 300 mm) on both sides of the crack which is generally slightly depressed. The measured difference in elevation of the crack area compared with the surrounding uncracked pavement surface has been found to be as much as 90 mm (Lupien et al. 1994). Recent studies (Doré et al. 1998, Martel et al. 2000) have shown that ice segregation can occur in pavement granular layers near cracks and that deicing salt plays a major role in the process. It appears
97
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Chapter Three
FIGURE 3-28 materials.
Ice enrichment process caused by salt contamination of pavement granular
that salt concentration gradient and the resulting gradient in freezing temperature can effectively replace the temperature gradient in the ice segregation process in salt-free frost susceptible soil. Moreover, it seems that the saline gradient can contribute to increase the frost susceptibility of granular materials. Figure 3-28 is a schematic illustration of the ice enrichment process and of the resulting frost heave. Figure 3-29 illustrates a crack-heaving problem on an access ramp to an urban freeway. The phenomenon can cause severe differential behavior during winter. Roughness data collected in summer and winter on a pavement section affected by a severe crack-heaving problem in the province of Quebec, Canada, showed an increase in IRI from 2.5 m/km in summer to 8 m/km in the winter.
FIGURE 3-29 Crack heaving caused by frost action in pavement granular materials contaminated by deicing salt.
Cold Region Pavement Performance Considering that 1 m/km is a very good ride quality and that for most pavements, an IRI between 4 and 6 warrants rehabilitation, the prejudice of crack heaving to the traveling public is considerable. Except for the crack-heaving problem, frost-heaving occurring in the granular pavement material can be considered insignificant with respect to winter pavement behavior. However, it has an important impact on pavement behavior during spring thaw. This aspect is going to be discussed in Sec. 3-9.
Remedial Solutions The problem of volume expansion in granular pavement materials is still not fully understood. It is difficult to identify the factors causing the problem, and thus difficult to identify effective remedial actions. Three principles should, however, be followed to assure stability in the pavement granular pavement materials during freeze-thaw cycles. They are • Waterproofness: As indicated in Sec. 2-2, water can infiltrate the granular layers through unpaved surfaces (shoulders) and through cracks. Effective sealing of pavement surfaces will minimize water ingress and reduce associated heaving problems. • Low sensitivity to water and frost action: Most of the problems described above occurred in granular materials with high contents of fine particles. These materials are generally water sensitive (high-equilibrium moisture contents) and frost susceptible. The use of granular materials with low fine contents (<10 percent) will contribute to a low sensitivity to water and frost action. Opengraded base layers are generally considered inert to water and frost action and are likely to mitigate the problems described above (Eigenbrod and Kennepohl 1996). However, these materials might require stabilization with asphalt or Portland cement in order to make them resistant to traffic loading. • Homogeneity: Contamination of granular materials by deicing chemical and fine-grained abrasives near pavement cracks is likely to create heterogeneous conditions in the pavement structure which can be detrimental to pavement performance during loading and freeze-thaw cycles. Therefore, it is important when rehabilitating a contaminated pavement to consider homogenization of the pavement materials prior to resurfacing.
3-9
Bearing Capacity Loss During Spring Thaw Problem Description During spring, thaw penetrates into the pavement structure and releases the water accumulated in the interstitial and segregation ice. High water contents combined with lower densities are essentially responsible for the weakening of pavement materials and subgrade soils. The strength is then progressively recovered as soils and materials consolidate (drain) over time. Many authors consider spring thaw as being the most important damage factor for pavement subjected to frost action (White and Coree 1990; Berg 1988; OCDE 1988; Janoo and Berg 1990). Several studies have been conducted to quantify the loss of bearing
99
100
Chapter Three
Soil Type
Range of Bearing Capacity Loss*
Typical Value
Clean gravel and sand
0–15%
10%
Silty-clayey gravel and sand
20–50%
35%
Silt
30–70%
50%
Clay
40–60%
50%
∗
Difference between summer and spring modulus divided by summer modulus (%). Sources: Compiled from Palolahti et al. 1993 (back-calculated modulus), Saarelainen and Gustavsson 2001 (back-calculated modulus and FEM analysis), Janoo 2002 (Laboratory CBR tests), Doré 2004 (back-calculated modulus) and Ovik et al. 2000 (back-calculated modulus).
TABLE 3-8 Bearing Capacity Loss during Spring Thaw according to Various Authors
capacity during spring thaw. Based on studies conducted using California Bearing Ratio (CBR) tests, plate bearing tests, and Falling Weight Deflectometer (FWD) tests, the loss of bearing capacity of the subgrade soil is ranging from 20 to 60 percent depending on soil type and other factors. Most recent studies are based on seasonal monitoring programs using falling weight deflectometers to assess variations in soil stiffness. Table 3-8 summarizes findings of various studies. Weakening during spring thaw also translates into important reduction of pavement performance. White and Coree (1990) have reported that 60 percent of pavement failures during the AASHO road test occurred during spring. Based on a fatigue criterion and measurements done on several test sections in Quebec, St-Laurent and Roy (1995) have established that the relative damage caused by a given load during springtime is between 1.5 and 3 times higher than the average annual damage. Based on observations from two test roads, Doré and Savard (1998) have reported that more than 90 percent of the fatigue damage to these pavements occurred during partial winter thaw or spring thaw periods. Full size testing in the Danish Road Testing Machine has indicated that between 60 and 75 percent of the permanent deformation in the tested pavement structures occurred during the thawloading periods (Zhang and Macdonald 2000).
Problem Assessment There seems to be two critical periods for thaw weakening: • During partial winter thaw events and in early spring thaw affecting the granular base material • At the end of spring when the ice rich subgrade soil is thawing In the early stage of thaw weakening, the frost front progresses rapidly in the granular base. As illustrated in Fig. 3-30a, because of the presence of accumulated snow on the pavement side and of the dark pavement surface absorbing solar radiations, the thaw front tends to be deeper at the center of the pavement as compared to the pavement sides. During this period, water accumulated in the granular material (according to the mechanisms described in Sec. 3-8-3) is rapidly released and cannot be drained
Cold Region Pavement Performance
FIGURE 3-30 (a) Early stage of pavement thawing involving partly thawed and nearly saturated pavement material and (b) late stage of thawing process involving ice frost susceptible soils.
effectively because of the underlying frozen layer. Moreover, water at the surface of the pavement is often trapped by the snow and ice accumulated on the pavement sides which tends to seep into the pavement granular material. As shown on Fig. 3-31, the bearing capacity of the base layer can be considerably reduced during that period and poorly supported bound surfacing layers tend to experience excessive fatigue damage.
FIGURE 3-31 Severe case of pavement deterioration caused by thaw weakening of pavement granular materials.
101
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Chapter Three
FIGURE 3-32 Normalized vertical strains (relative to fall measurements) in the granular base of a flexible pavement structure as measured using a multidepth deflectometer under a moving reference load (DeBlois 2005).
Experimental measurements of vertical strains in the granular base of a pavement section are shown in Fig. 3-32. There is an important increase of vertical strains in the base layer immediately after the beginning of thawing. Measurements were made using a multidepth deflectometer under a standard axle circulating on the pavement at 50 km/h. In the specific case shown in Fig. 3-32, vertical strains in the granular base peaked at values approaching 250 percent of the fall measurements. When the thaw front reaches the ice-rich subgrade soil (Fig. 3-30b), the soil undergoes a consolidation process similar to the one described by Therzaghi’s theory of consolidation with the difference that water is unevenly distributed in the soil mass and that it is progressively released as the thaw front progresses in the soil. Figure 3-33 illustrates the frost heave and thaw consolidation processes. Frost heave can be divided in four distinct phases. Assuming initial density and water content near optimal conditions, frost penetration generates high suction at the segregation front. Available water is then forced to flow toward the freezing front increasing the water content of the freezing soils (phase A). During the ice segregation process, water is pumped through the frozen fringe causing an important volume increase (reduction of the soil dry density) near the segregation front (phase B). During the second phase, the soil structure is essentially supported by frozen water. When thaw progresses in the soil layer, water is gradually released from interstitial and segregation ice. This phase, denoted “C” on Fig. 3-33, is considered as the critical period during the thawing process. At the scale of a very small soil element located at the thaw front, the effective soil strength tends toward zero due to high pore pressures prevailing at that location. If the amount of ice in the soil element and the thaw rate tend to be low while the drainage capacity and resulting consolidation rate are high, the effect of the strength loss of the soil element is going to be short and limited to a small area. Therefore, the weakening occurring at the thaw front is going to have a limited effect on the bearing capacity of the soil layer. Increasing ice content and/or thaw rate combined with decreasing drainage capacity will cause the thickness of the weakened area and the duration of the weakening to increase. The effect on the bearing capacity and permanent deformation of the layer can then be considerable.
Cold Region Pavement Performance
FIGURE 3-33 Frost heave and thaw consolidation illustrated in (a) heave-time, (b) dry density water content, and (c) void ratio-effective stress spaces (Doré 2004).
After pore pressures are dissipated in the thawed soil, drainage of interstitial water will continue until equilibrium is reached between the flow gradient and soil suction. During that phase (D), additional settlement is caused by increasing matric suction. Thaw weakening is, thus, a complex process, which is essentially a function of three major factors: 1. The amount of frost heave occurring in the considered layer 2. The rate at which the layer is thawing 3. The rate at which the layer consolidates
103
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Chapter Three Several authors have indicated the importance of those factors on the behavior of pavements during spring thaw. Dysli (1991a) has identified the three factors as being directly related to thaw weakening. Experimental work by Dysli (1991a and b) has demonstrated the importance of the thaw rate on pavement damage during thawing. Simonsen and Isacsson (1999) have identified the amount of frost heave per unit of frozen soil as an important factor of loss of shear strength. High pore pressures that result from segregation ice thawing and poor drainage are identified as key factors. The rate of thawing is also identified as an important factor of strength reduction. Nixon and Morgenstern (1971) have proposed the concept of the thaw-consolidation ratio to help predict consolidation behavior of thawing permafrost. The ratio (R) is defined as follows: R=
α x& = &S 2 c v
(3-23)
where x& = progression rate of the thaw front, S& = consolidation rate, a = coefficient of √t in the modified Berggren equation: a = (2ku · Tt/L)0.5, ku = thermal conductivity of the unfrozen soil, L = volumetric latent heat of fusion, Tt = surface temperature, and cv = coefficient of consolidation. Many authors (Rydén 1985; Rydén and Axelsson 1988; Saarelainen 1997) have used the thaw-consolidation theory to better explain pore pressure buildup and assess effective stress in thawing soils. This approach provides a good basis for the theoretical analysis of thawing soils, but is difficult to apply in pavement engineering. Doré and Imbs (2002) and Doré (2004) have proposed an index, referred to as the thaw-weakening index (TWin) that combines the weakening potential represented by the total heave normalized by the thickness of the considered layer with the thaw-consolidation ratio as indicated in Eq. (3-24): TWin =
h x& × D S&
(3-24)
where h is the total heave resulting from frost action in the subgrade soil, and D is the thickness of subgrade soil affected by frost action. The dimensionless index, therefore, incorporates most of the factors contributing to thaw-weakening behavior of a given material in a specific environment. Frost heave represents the weakening potential accumulated by frost action. The rate of thawing is in turn a function of the climatic conditions during spring (heat transmitted to the pavement system) and the resulting thermal response (heat absorbed) of the material. Finally, the rate of consolidation is the result of the effectiveness of the drainage of the excess moisture in the pavement system. The thaw-weakening index has been validated using data from test sites in Finland and Canada and results show a good correlation between the index and the loss of bearing capacity obtained from deflection testing. Figure 3-34 illustrate the relationship which translates into the following equation: ∆M = 0.25 ln(TWin) + 0.33 Ms
(3-25)
where ∆M is the difference between summer and spring modulus, Ms is the summer elastic modulus of the subgrade soil, and TWin is defined in Eq. (3-24) (see Example 3-6).
Cold Region Pavement Performance
FIGURE 3-34 Relationship between the thaw-weakening index and the loss of bearing capacity of the subgrade soil during spring thaw obtained from back-calculated modulus from fallingweight deflection data and from multidepth deflectometers (Doré 2004; DeBlois 2005). Example 3-6 Data collected in a pavement section during previous years have indicated that the summer stiffness of the subgrade soil is 75 MPa and that the typical settlement rate of the thawing subgrade soil is around 1 mm/day. Frost penetration in the subgrade soil has reached 600 mm during the current winter inducing 60 mm of frost heave. You are asked to estimate the loss in bearing capacity for the coming spring considering a thaw penetration rate ranging from 9 to 12 mm/day estimated from historical climatic data. Solution Using Eq. (3-24), the range of the TWin can be obtained TWin =
60 9 × = 0.9 600 1
TWin =
60 12 × = 1.2 600 1
And based on Eq. (3-25), the bearing capacity loss should range between ∆M = (0.25 ln(0.9) + 0.33) × 75 = 22.8 MPa ∆M = (0.25 ln(1.2) + 0.33) × 75 = 28.2 MPa The spring modulus of the subgrade soil should thus range between 46.8 and 52.2 MPa.
Remedial Solutions Based on the information provided above, the thaw-weakening process appears to be controlled by three main factors: the amount of frost heave, the rate of thawing, and the rate of consolidation. Based on Eq. (3-24), a reduction of the thaw rate should help mitigate the problem of bearing capacity loss during spring thaw. The rate of thawing is controlled by climatic variables and soil thermal properties. Considering that climatic factors are generally difficult to control, the rate of thawing can only be influenced by modifying the thermal properties of soils and pavement materials. To our knowledge, no attempt has been made by researchers and practitioners to act on those properties in order to reduce thaw weakening. The use of low thermal conductivity materials and the accumulation of latent heat in hydrophilic materials are two techniques that could act positively on the thaw rate as well as on frost heave. The effectiveness of such techniques in reducing thaw weakening remains to be investigated and demonstrated.
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Chapter Three Most of the known strategies to mitigate thaw-weakening act on one of the other two factors, for example, frost heave or consolidation rate. By reducing frost heave [h term in Eq. (3-24)], water released by thawing and related soil weakening is reduced. It is important to note that even in cases where frost heave is minimal, significant thaw weakening tends to occur during spring thaw. For instance, Doré (2004) has reported nearly 20 percent loss of stiffness for a subgrade soil where less than 20 mm of frost heave has occurred. Another approach to mitigate thaw-weakening effects in pavements is to improve drainage within the pavement system. An effective drainage system is likely to dissipate rapidly excess pore-water pressures thus allowing an effective consolidation of the thawing soil [maximizing the S term in Eq. (3-25)]. Drainage layers are typically placed in the pavement base layer and at the interface between the pavement and the subgrade soil. In the first case, the objective is to remove water trapped between the frozen layer and the asphalt concrete layer and to avoid pore pressure buildup under moving loads (Kestler 1996; Janoo 2002). The drainage layer placed at the interface between the pavement subbase layer and the subgrade soil is meant to reduce the drainage path for water released from the melting ice lenses in frost susceptible soils (Kestler 1996; Dysli 1993). The concept is appealing and is likely to somewhat improve spring thaw behavior of pavements. The effectiveness of drainage layers is, however, questioned by Dysli (1993) whose experimental work suggests that most of the water released by the thawing process in frost susceptible subgrade is reabsorbed by the overlying thawed soil. The most common approach used to deal with weakening of thawing pavements is to structurally adapt the pavement structure to withstand mechanical damage resulting from the weakening of thawing pavement layers and subgrade soils. This is typically achieved by taking into account variations in layer stiffness while computing pavement mechanical damage on a seasonal basis. The thickness and/or the stiffness of the pavement layers are increased until the damage computed over the life of the pavement is reduced to an acceptable level. This approach is going to be further discussed in Chap. 8.
3-10
Frost Destructuration of Undisturbed Sensitive Clays in Seasonal Frost Conditions When affected by frost action, frost-susceptible subgrade soils are subjected to an intense stress caused by the suction at the segregation front also termed cryosuction (see Sec. 2-2). As a result, undisturbed sensitive clays subjected to frost action are likely to be mechanically altered.
Problem Description Several studies have been conducted on the behavior of undisturbed sensitive clays (Yong et al. 1985; Vähäaho 1988; Leroueil et al. 1991; Roy et al. 1992; Konrad and Seto 1994; Konrad et al. 1995; Roy et al. 1995). As illustrated in Fig. 3-35, the behavior of sensitive clays differs considerably depending if a whole soil column or a discrete layer is considered. Freeze tests conducted on undisturbed sensitive clays have indicated that these soils display high segregation potential values. As a result, a soil column will experience significant frost heaving as illustrated on the left side of Fig. 3-35. Konrad et al. (1995) have reported frost heave reaching 200 mm during the first
Cold Region Pavement Performance
FIGURE 3-35
Frost action in undisturbed sensitive clay.
freezing season on a highway constructed over sensitive clays that have never been exposed to frost action. It has also been observed that sensitive clays exposed to frost action undergo important volume and moisture content reduction at the end of the first few freeze-thaw cycles. It appears that the important stresses induced by suction at the segregation front breaks the cementation between clay particles and brings them closer together. The stress level resulting from the cryosuction can be such that it exceeds the over consolidation stress and causes additional settlement following the virgin consolidation curve. As indicated on the right side of Fig. 3-35, for a distinct soil layer between two ice lenses, frost action will cause a volume reduction, which will not be recovered upon thawing. The main mechanism involved appears to be particle aggregation. As a result, the liquid limit, specific surface area, undrained shear strength, and segregation potential will decrease. Roy et al. (1995) have reported volume and moisture content reduction in the range of 30 percent occurring during the first freezing cycle affecting undisturbed sensitive clays. Approximately 6 percent of additional volume reduction will occur during the subsequent few freeze-thaw cycles. Vähäaho (1988) has reported settlements reaching 250 mm on a pavement in Finland. He suggests that thaw settlement in intact sensitive clays can be estimated as follows: A% =
w% 3
(3-26)
where A% is the thaw settlement occurring in the clay layer affected by frost action, and w% is the initial water content. After the first freeze-thaw cycle, reduced moisture content in the newly frozen layer will facilitate frost penetration during the following freezing season exposing a new layer of intact clay to frost action. Additional settlement will thus occur during five or six subsequent freeze-thaw cycles before reaching an equilibrium state.
Remedial Solutions The results of several studies conducted on the destructuration of sensitive Champlain Sea clay has led the Quebec transportation department to require that special attention be given to clays characterized by liquid limits equal or exceeding 0.9. It is recommended
107
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Chapter Three that the properties of these clays be assessed through freeze-thaw tests in order to estimate frost heave and settlements to be expected and that full frost protection for sensitive soils be considered.
Review Questions 3-1. A two-lane roadway has 3 transverse cracks on a 38-m-long section (assume all transverse cracks are full cracks). The pavement is 55 mm thick, on a loam subgrade. Assuming the winter design temperature is −30°C and the stiffness modulus of the original asphalt cement is 325 kg/cm2, what is the age of the pavement according to Eq. (3-1)? If the winter design temperature would have been −25°C, how many cracks would have been observed in the section?
3-2. A pavement slab is 5 m wide, 6 m long, and 0.15 m thick. The thermal failure stress is 3.2 MPa. What is the possible minimum crack spacing? The unit weight of the hot mix asphalt is 18 kN/m3, the friction coefficient at the interface (tan f) is 1.8 and there is no cohesion.
3-3. An asphalt-aggregate mixture has a unit weight of 23.5 kN/m3. Fracture strength of 5.2 MPa was obtained with the TSRST, at −25°C. Assuming a friction coefficient tan f of 2.7 and no cohesion, what are the predicted minimum and maximum crack spacings if the pavement temperature is (a) −20°C (b) −30°C? 3-4. Considering that the method given by the Asphalt Institute was used to evaluate the fatigue failure criterion and the following results came out, what is the dynamic modulus of the material tested? Strain, l m/m
N
980
1989
850
3254
829
3368
822
3474
809
3602
819
3503
841
3269
850
3257
866
2951
886
2749
907
2569
Permanent deformation tests were made on an HMA mixture at a temperature of 20°C. According to Eq. (3-16), what is the ratio of plastic strain to resilient strain at 20,000 load cycles?
3-5.
3-6. In order to evaluate typical frost heave differential for a pavement section, the following elevation measurements were obtained: Distance, m
0
4
8
12
16
Elevation (summer), m
100
100.1
100.05
100
100.15
Elevation (winter), m
100.05
100.30
100.15
100.35
100.20
Cold Region Pavement Performance The average fine content is 15 percent and the typical frost heave is 150 mm. Estimate the typical differential frost heave for the section considered.
3-7. The winter roughness (IRI) of a pavement was measured as 5 m/km. If the average frost heave is 100 mm, the fine content is 20 percent and g4 is estimated to be 22, what is the summer roughness?
3-8. Tests were run on a section of a pavement and the following results were obtained: The summer elastic modulus of the subgrade soil is 90 MPa and the spring elastic modulus is 65 MPa. The total frost heave is 75 mm on a 500-mm deep subgrade soil layer affected by frost action. It is known that the thaw penetration rate is 10 mm/day. Determine the consolidation rate of the thawing subgrade.
References AASHTO (2003). Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 23d ed., American Association of State Highway and Transportation Officials, Washington, D.C. Berg, R. L. (1988). “Pavement Design for Frost Conditions-Current and Future Methods,” Proceedings of the Fourth Annual Airport Engineering/Management Conference, FAA, Great Lakes Region, Des Plaines, ill. Brunette, B., and Lundy, J. (1997). “The Use and Effects of Studded Tires on Oregon Pavement,” Proceedings of the Fifth International Symposium on Cold Region Development, Anchorage, AK, May 4–10, 1997, pp. 593–596. DeBlois, K. (2005). “Analyses du comportement saisonnier de planches expérimentales et validation du nouvel indice d’affaiblissement au dégel (Analysis of seasonal behaviour of experimental sections and validation of the thaw weakening index),” Mémoire de maîtrise présenté à la faculté des études supérieures de l’Université Laval, Québec, Canada (in French). Doré, G. (1997). “Détérioration des chaussées en conditions de gel; une nouvelle approche prévisionnelle (Pavement deterioration in frost conditions; a new predictive approach),” Thèse de doctorat, Faculté des études supérieures, Université Laval, Québec, Canada (in French). Doré, G. (2004). “Development and Validation of the Thaw-Weakening Index,” International Journal of Pavement Engineering, vol. 5, no. 4, http://www.informaworld. com, pp. 185-192. Doré, G. Flamand, M., and Tighe, S. (2001). “Prediction of Winter Roughness Based on the Analysis of Subgrade Soil Variability,” Transportation Research Record: Journal of the Transportation Research Board, No. 1755, Transportation Research Board of the National Academies, National Academy Press, Washington, D.C. pp. 90–96. Doré, G., and Imbs, C. (2002). “Development of a New Mechanistic Index to Predict Pavement Performance during Spring Thaw,” Proceedings of the 11th International Conference on, Cold Regions Engineering, Kelly S. Merrill (ed.), American Society of Civil Engineers, Reston, Va, pp. 348–359. Doré, G., Konrad, J. -M., and Roy, M. (1998). “The Role of Deicing Salt in Pavement Deterioration by Frost Action,” Transportation Research Record: Journal of the Transportation Research Board, No. 1596, Transportation Research Board of the National Academies, National Academy Press, Washington, D.C., pp. 70–75. Doré, G., Konrad, J. -M., and Roy, M. (1999). “A Deterioration Model for Pavement in Frost Conditions,” Transportation Research Record: Journal of the Transportation Research
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Chapter Three Board, No. 1655, Transportation Research Board of the National Academies, National Academy Press, Washington, D.C., pp. 110–117. Doré, G., and Savard, Y., (1998). “Analysis of Seasonal Pavement Deterioration,” Transportation Research Board, preprint no. 981046, Transportation Research Board of the National Academies, Washington, D.C. Dysli, M. (1991a). “Le gel et son action sur les sols et les fondations (Frost an its effects on soils and foundations),” Presses Polytechniques et Universitaires Romandes, Lausanne, Switzerland p. 250 (in French). Dysli, M. (1991b). “Resilient Modulus of Freeze-Thaw or Resilient Frost Heave,” Ground Freezing 91, Yu and Wang (eds.), A. A. Balkema Publishers, Rotterdam, pp. 225–229. Dysli, M. (1993). “Where Does the Water Go during Ice Lense Thaw?” Frost in Geotechnical Engineering, Phukan (ed.), A. A. Balkema Publishers, Rotterdam, pp. 45–50. Ehrola, E. (1986). “Transverse Cracking of Asphalt Pavement in Low Temperatures and Factors Affecting It,” University of Oulu, Road and Traffic Laboratory, Publication 4, Oulu, Finland (in Finnish). Eigenbrod, K. D., and Kennepohl, J. A. (1996). “Moisture Accumulation and Pore Water Pressures at Base of Pavements,” Transportation Research Record: Journal of the Transportation Research Board, No. 1546, Transportation Research Board of the National Academies, National Academy Press, Washington, D.C., pp. 151–161. Elvik, R. (1999). “The Effects on Accidents of Studded Tires and Laws Banning Their Use: A Meta-analysis of Evaluation Studies,” Accident Analysis and Prevention, vol. 31, The Association of Asphalt Pavement Technologists, White Bear Lake, Minn., pp. 125–134. Gustafson, K. (1997). “Pavement Wear from Studded Tyres—the Swedish Solution,” Proceedings of the Fifth International Symposium on Cold Region Development, Anchorage, Alaska, May 4–10. Haas, R., Meyer, F., Assaf, G., and Lee, H. (1987). “A Comprehensive Study of Cold Climate Airfield Pavement Cracking,” Journal of the Association of Asphalt Pavement Technologists, vol. 56. The Association of Asphalt Pavement Technologists, White Bear Lake, Minn. Hajek, J. (1971). “A Comprehensive System for Estimation of Low-Temperature Cracking Frequency of Flexible Pavements,” Technical Report, The Transport Group, Dept. of Civil Engineering, University of Waterloo, Waterloo Ontario, Canada. Hills, J. F., and Brien, D. (1966). “The Fracture of Bitumens and Asphalt Mixes by Temperature Induced Stress,” Journal of the Association of Asphalt Pavement Technologists, vol. 35. The Association of Asphalt Pavement Technologists, White Bear Lake, Minn. Huang, Y. (2004). Pavement Analysis and Design, 2d ed., Pearson Prentice Hall, Upper Saddle River, NJ. Hunter, E., and Ksaibati, K. (2002). “Evaluating Moisture Susceptibility of Asphalt Mixes,” Department of Civil and Architectural Engineering, University of Wyoming, P.O. Box 3295, Laramie, W.Y. 82071-3295, http://www.ndsu.nodak.edu/ndsu/ugpti/ MPC_Pubs/html/MPC02-138/pg2.html#tmfms2 (March 30, 2004). Jacobson, T. (1997). “The Wear Resistance of Bituminous Mixes to Studded Tires—the Swedish Experience,” Proceedings of the Fifth International Symposium on Cold Region Development, Anchorage, Alaska, May 4–10. Jacobson, T., and Wågberg, L-G. (1998). “Development of Prediction Model for Pavement Wear, Wear Profile and Annual Cost,” VTI report 76A-1998, Swedish Road and Transportation Research Institute, Linköping, Sweden.
Cold Region Pavement Performance Jacobson, T., and Wågberg, L-G. (2004). “Prediction Models for Pavement Wear and Associated Costs,” Proceedings of the Winter Cities 2004 Conference, Anchorage, Alaska, February, 18–22, 2004, Municipality of Anchorage, Alaska. Janoo, V. C., and Berg, R. L. (1990). “Thaw Weakening in Seasonal Frost Areas,” Transportation Research Records: Journal of the Transportation Research Board, no. 1246, Transportation Research Board of the National Academies, Washington, D.C., pp. 217–233. Janoo, V. C. (2002). “Performance of Base/Subbase Materials under Frost Action,” Proceedings of the Eleventh International Conference on Cold Regions Engineering (ASCE, Merrill ed.), Anchorage, Alaska, pp. 878–889. Janoo V. C., and Greatorex, A. (2002). “Performance of Montana Highway Pavements During Spring Thaw,” Report FHWA/MT-02-006/8155, Federal Highway Administration and Montana Department of Transportation. Johansson, L. (1998). “Aging of Road Bitumens—State of the Art,” TRITA-IP FR 98-36, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Kandhal, P., and Rickards, I. (2001). “Premature Failure of Asphalt Overlays from Stripping: Case Histories,” Journal of AAPT, vol. 70, pp. 301–344. Kandhal, P., Sandvik, L., Koehler W., and Wenger, M. (1973). Asphalt Viscosity—Related Properties of In-Service Pavements in Pennsylvania, ASTM, Special Technical Publication No. 532. ASTM International, West Conshohocken, Pa. Kavussi, A., and Edgar, R. (1997). “Characterization of Pavement Aggregates for Studded Tire Resistance Purposes,” Proceedings of the Fifth International Symposium on Cold Region Development, Anchorage, Alaska, May 4–10. Kestler, M. (1996). “An Open Graded Base to Reduce Thaw Weakening,” Proceedings of the Eighth International Conference on Cold Regions Engineering (ASCE, Carlson ed.), Fairbanks, Alaska, pp. 299–310. Kestler, M. A., Krat, A. S., and Roberts, G. (1998). “Winter Tenting of Highway Pavements,” Proceedings of the Ninth International Conference on Cold Region Engineering (ASCE, Newcomb ed.), Duluth, Minn. pp. 501–513. Konrad, J. -M., and Seto, J. (1994). “Frost Heave Characteristics of Undisturbed Sensitive Champlain Sea Clay,” Canadian Geotechnical Journal, vol. 31, pp. 285–298. Konrad, J. -M., and Shen, M. (1997). “Prediction of the Spacing between Thermal Contraction Cracks in Asphalt Pavements,” Canadian Journal of Civil Engineering, NRC Research Press, vol. 24, pp. 288–302. Konrad, J. -M., Bergeron, G., Roy, M., La Rochelle, P., and Leroueil, S. (1995). “Field Observations of Frost Action in Intact and Weathered Champlain Sea Clay,” Canadian Geotechnical Journal, vol. 32, pp. 689–700. Kurki, T. (1998). “Asfalttipäällysteiden urautumisen mallintaminen (Modeling of rut propagation of asphalt pavements),” FinnRoad Reports; Tiehallinnon selvityksiä 13-1998 (in Finnish). Lampinen, A. (1993). “Kestopäällysteen urautuminen (Rutting of Pavements),” Technical Research Centre of Finland, VTT Publication 781 (in Finnish). Leroueil, S., Tardif, J., Roy, M., La Rochelle, P., and Konrad, J. -M. (1991). “Effects of Frost on the Mechanical Behaviour of Champlain Sea Clays,” Canadian Geotechnical Journal, vol. 28, pp. 690–697. Little, D. N., and Jones, D. R. (2003). “Chemical and Mechanical Processed of Moisture Damage in Hot-Mix Asphalt Pavements,” TRB Miscellaneous Report on Moisture Sensitivity of Asphalt Pavements, National Seminar, February 4–6, San Diego, Calif.
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Chapter Three Lupien, C., Aitcin, P. C., Roireau, M. (1994). Importance de l’étanchéité des chaussées; Exposé des communications du 29e congrès annuel de l’Association Québécoise des Routes et Transports (AQTR), 10 au 12 avril, Salaberry de Valleyfield, Quebec, Tome 1, pp. 283–293. Malik, M. G. (2000). “Studded Tires in Oregon: Analysis of Pavement Wear and Cost of Mitigation,” Oregon Department of Transportation, Salem, Oreg. Martel, N., Doré, G., and Fortier, R. (2000). “Comprendre et contrer le soulèvement au gel des fissures transversales en présence de sels de déglaçage (Understanding and controlling frost heaving of transverse cracks in presence of deicing salt),” Compte rendu du 35 congrès annuel de l’association Québécoise du transport et des routes (AQTR) Avril 2000, Quebec, Canada. Masson, J. F. (2001). “Sealing Cracks in Asphalt Concrete,” Construction technology update no. 49, National Research Council Canada. Mirza, M. V., and Witczak, M. V. (1995). “Development of a Global Aging System for Short and Long Term Aging of Asphalt Cements,” Journal of the Association of Asphalt Pavement Technologists, The Association of Asphalt Pavement Technologists, White Bear Lake, Minn. vol. 64, p. 393. Monismith, C., Epps, J., Finn, F. (1985). “Improved Asphalt Mix Design,” Journal of the Association of Asphalt Pavement Technologists, vol. 54, pp. 347–406. Newcomb, D., Buncher, M., and Huddleston, I. (2001). “Concepts of Perpetual Pavements,” Perpetual Bituminous Pavements, TRB Circular No. 503, pp. 4–11. Nixon, J., and Morgenstern, N. R. (1971). “One Dimensional Consolidation of Thawing Soils,” Canadian Geotechnical Journal, vol. 8, no. 4, pp. 558–565. Nordal, R. S., and Refsdal, G. (1984). “Frost Protection in Design and Construction,” VTT Symposium 94, Frost in Geotechnical Engineering, vol. 1, Technical Research Centre of Finland, Espoo, Finland, pp. 127–163. Öberg, G., Wikström, M. (2004). “Winter Tires—Traffic Safety,” Proceedings of the Winter Cities 2004 Conference, Anchorage, Alaska, February, 18–22. OCDE (1988). “Véhicules lourds, climat et dégradation des chaussées (Heavy vehicles, climate and pavement deterioration),” Organisation for Economic Co-operation and Development, Paris, France. Ovik, J. M., Birgisson, B., and Newcomb, D. E. (2000). “Characterizing Seasonal Variations in Pavement Material Properties for Use in a Mechanistic-Empirical Design Procedure,” Missesota Department of Transportation Report MN/RC - 2000-35, p. 191. Palolahti, A., Slunga, E., Saarelainen, S., and Orama, R. (1993). “Sulavan maan kantavuus (The elastic stiffness of thawing subgrade soils),” Research report, Helsinki University of Technology, Faculty of civil engineering and surveying, p. 99 (in Finnish). Pellinen, T. (2001). “Investigation of the Use of Dynamic Modulus as an Indicator of Hot-Mix Asphalt Performance,” Ph.D. Dissertation, Arizona State University, Tempe, Ariz. Peterson, R. A., and Krantz, W. B. (1998). “A Linear Stability Analysis for the Inception of Differential Frost Heave,” Proceedings of the Seventh International Conference on Permafrost, Éditions Nordicana, Laval University, Quebec, Canada. Pouliot, N. (2003). “Où et quand sceller les fissures,” Info DLC, Bulletin d’information technique, vol. 8 no. 8., Ministère des transports du Québec, Canada (in French). Rao Tangella, S., Craus, J., Deacon, J., and Monismith, C. (1990). “Summary Report on Fatigue Response of Asphalt Mixtures,” SHRP-A/IR-90-011, Strategic Highway Research Program, National Research Council, Washington, D.C.
Cold Region Pavement Performance Roberts, F. L., Kandhal, P. S., Brown, E. R., Lee, D-Y., Kennedy, T. W. (1996). Hot Mix Asphalt Materials, Mixture, Design and Construction,” National Asphalt Pavement Association Research and Education Foundation, 2d ed., Lanham, Md. Roy, M., Bergeron, G., La Rochelle, P., Leroueil S., Konrad J. -M. (1995). “Effets de cycles de gel-dégel sur les propriétés d’une argile sensible,” Canadian Geotechnical Journal, vol. 32, pp. 725–740 (in French). Roy, M., Tardif, J., Leroueil, S., Larose, G., and La Rochelle, P. (1992). “Effets du gel sur les infrastructures routières argileuses au Québec,” Canadian Geotechnical Journal, vol. 29, pp. 131–142 (in French). Rydén, C. G. (1985). “Pore Pressures in Thawing Soils,” Proceedings of the Fourth International Symposium on Ground Freezing, Kinosita and Fukuda (eds.), Balkema, Rotterdam, Netherlands pp. 223–226. Rydén C. G., and Axelsson, K. (1988). “Laboratory Determination of Pore Pressure during Thawing of Three Different Types of Soils,” Fifth International Symposium on Ground Freezing, Jones and Holden (eds.), A. A. Balkema, Rotterdam, Netherlands, pp. 213–217. Saarela, A. (1993). “Asfalttipäällysteet; Suunnittelu, (Design of Asphalt Pavements),” Research Program for Asphalt Pavements, PANK Ry, Research Centre of Finland (in Finnish). Saarela, A., Kurki, T., Halttunen, K., Kollanen, T., Saarinen, L., Spoof, H., Vuorinen, J., Laitinen, V., Peltonen, P., and Jämsä, H. (1993). ASTO 1987–1992 Final Report– Loppuraportti,” Technical Research Centre of Finland, VTT , PANK ry (in Finnish). Saarelainen, S. (1996). “Routakestävyysmallien arviointi (Evaluation of frost resistance models),” VTT, Interim Report E6, Project E312, Espoo, Finland. Saarelainen, S. (1997). “Field and Laboratory Methods for Determining Properties in Thawing Soils,” Proceedings of the Ground Freezing 97, Knutsson (ed.), A. A. Balkema, Rotterdam, Netherlands, pp. 53–61. Saarelainen, S., and Gustavsson, H. (2001). “Thaw Weakening of Subgrades in Finland,” Fifteenth International Conference on Soil Mechanics and Geotechnical Engineering, XV ICSMGE, A. A. Balkema, Rotterdam, Netherlands, pp. 2183–2186. Scher, R. L. (1996). “Environmental Induced Longitudinal Cracking in Cold Region Pavements,” Proceedings of the Eighth International Conference on Cold Region Engineering, ASCE, Fairbanks, Alaska, pp. 899–911. Simonsen, E., and Isacsson, U. (1999). “Thaw-Weakening of Pavement Structures in Cold Regions,” Cold Region Sciences and Technology, vol. 29, pp.135–151. Sistonen, M., and Alkio, R. (1986). “Nastan painon vaikutus tien päällysteen kulumiseen (Effect of stud mass on pavement wear),” VTI Research Report 583, Espoo, Finland. Sousa, J., Craus, J., and Monismith, C. (1991). Summary Report on Permanent Deformation in Asphalt Concrete. SHRP-A/IR-91-104, Strategic Highway Research Program, National Research Council, Washington, D.C. Spoof, H. (1992). “Asfaltin väsyminen (Fatigue of asphalt mixtures),” ASTO 1987–1992 Report TR4/2, Technical Research Centre of Finland, VTT, PANK ry (in Finnish). St-Laurent, D., and Roy, M. (1995). “Évaluation structurale des chausses souples dans un contexte climatique nordique: une étude avec le FWD (Structural evaluation of flexible pavements in a northern context: A study using the FWD),” Proceedings of the 30th Annual Conference of AQTR, Association Québécoise du Transport et des Routes, Quebec, Canada. Tayebali, A., Rowe, G., and Sousa, J. (1992). “Fatigue Response of Asphalt-Aggregate Mixtures,” Journal of the Association of Asphalt Pavement Technologists, The Association of Asphalt Pavement Technologists, White Bear Lake, Minn., vol. 61.
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Chapter Three Terrel, R., and Shute, J. (1989). Summary Report on Water Sensitivity. SHRP-A/IR-89-003, Strategic Highway Research Program, National Research Council, Washington, D.C. Tervahattu, H., Kupiainen, K., Räisänen, M., Mäkelä, T., Aurela, M., and Hillamo, R. (2004). “The Influence of Studded Tires and Traction Sanding on Dust Emissions from Road Abrasion,” Proceedings of the Winter Cities 2004 Conference, Anchorage, Alaska, February, 18–22, Municipality of Anchorage, Alaska. Timm, D., and Voller, V. (2003). “Field Validation and Parametric Study of a Thermal Crack Spacing Model,” Journal of the Association of Asphalt Pavement Technologists, The Association of Asphalt Pavement Technologists, White Bear Lake, Minn., vol. 72. Tunnicliff, D. (1997). “Performance of Antistripping Additives,” Journal of the Association of Asphalt Pavement Technologists, The Association of Asphalt Pavement Technologists, White Bear Lake, Minn., vol. 66, p. 344. Unhola, T. (1997). “Studded Tires the Finnish Way,” Proceedings of the Fifth International Symposium on Cold Region Development, Anchorage, Alaska, pp. 609–612, Zubeck, H., Woolard, C., White, D. and Vinson, T. (eds.), US Army Cold Regions Research and Engineering Laboratory, Hanover, NH. Unhola, T. (2004). “Studded Tires—Finnish Update,” Proceedings of the Winter Cities 2004 Conference, Anchorage, Alaska, February, 18–22, 2004, Municipality of Anchorage, Alaska. Vähäaho, I. T. (1988). “Soil Freezing and Thaw Consolidation Results for a Major Project in Finland,” Proceedings of the Ground Freezing 88, A. A. Balkema, Rotterdam, Netherlands, vol. 1, pp. 219–223. Vinson, T., Hicks, G., and Janoo, V. (1996). “Low Temperature Cracking and Rutting in Asphalt Concrete Pavements,” Roads and Airfields in Cold Regions, ASCE Technical Council on Cold Region Engineering Monograph, American Society of Civil Engineers, Reston, Va, pp. 203–248. White, T. D., and Coree, B. J. (1990). “Treshold Pavement Thickness to Survive Spring Thaw,” Proceedings of the Third International Conference on Bearing Capacity of Roads and Airfields, Trondheim, Norway, Norwegian University of Science and Technology, Trondheim, Norway, pp. 41–51. Yong, R. N., Boonsinsuk, P., and Yin, C. W. P. (1985). “Alteration of Soil Behavior after Cyclic Freezing and Thawing,” Fourth International Symposium on Ground Freezing, Sapporo, Japan, A. A. Balkema, Rotterdam, Netherlands, pp. 187–195. Zhang, W., and Macdonald, R. A. (2000). “Response and Performance of a Test Pavement to Freeze-Thaw Cycles in the Danish Road Testing Machine,” Proceedings of the Fifth International Conference on Unbound Aggregates in Roads, Nottingham, U.K., Dawson (ed.), Balkema, Rotterdam, Netherlands, pp. 77–86. Zubeck, H. K., Larson, E., Aleshire, L., Harvey, S., and Porhola, S. (2004). “Socio-Economic Effects of Studded Tire Use on Alaska,” Proceedings of the Winter Cities 2004 Conference, Anchorage, Alaska, February, 18–22, Municipality of Anchorage, Alaska. Zubeck, H. K., and Vinson, T. S. (1996). “Prediction of Low Temperature Cracking of Asphalt Concrete Mixtures with Thermal Stress Restrained Specimen Test Results,” TRR No. 1545, Transportation Research Board, National Research Council, Washington, D.C. Zubeck, H. K., and Vinson, T. S. (2007). “Prediction of HMA Low Temperature Crack Spacing Using TSRST Results,” Proceedings of the Eighth International Symposium on Cold Regions Development, Finnish Association of Civil Engineers, Helsinki, Finland.
CHAPTER
4
Investigation and Testing
T
he success of a road construction project depends heavily on the quality of the information available on soils and pavement materials. This is particularly true in cold climates where soils and material properties are very sensitive to climatic and environmental factors. For road construction projects in difficult terrains, earthwork can represent up to 80 percent of the total construction costs. Site investigation is therefore a very important activity which allows adjusting the final alignment and grade of the road, planning drainage facilities, geohazard mitigation, and so on, and minimizing the construction costs. Information provided by the site investigation activities will support pavement structural design, allow for the identification of available construction material, help selecting proper construction techniques, and facilitate quality control. For existing roads, site investigation and characterization is an essential step to the selection of an appropriate rehabilitation technique. Existing pavements have lots to tell and pavement engineers have to take advantage of the information accumulated throughout the years in old pavements. The condition of pavement materials, their reuse potential in the rehabilitation process and factors that have caused pavement to fail are critical information to be gathered during the investigation process. For new road alignments, technical studies typically include a general site investigation and specialized studies. For existing pavements, activities include pavement inspection, pavement monitoring, and pavement investigation. Figure 4-1 illustrates the general site investigation process for new or existing pavements. In addition to standard procedures for the characterization of soils and pavement materials, special investigation and testing procedures should be adopted for the assessment of properties, as well as their seasonal and spatial variation. This chapter includes best practices and recommendations for site investigation prior to the construction of a new pavement section, investigation of existing pavement structures, as well as soils and materials characterizations for cold region pavement engineering purposes.
4-1
Site Investigation Site investigation normally includes two levels of studies. The first level is a general site investigation which allows for the identification of soil and rock units and the identification of potential technical problems. The second level of site investigation includes a series of specialized technical studies focused on the expected problems. This section of the chapter includes a general description of investigation techniques with emphasis on the detection of potential cold temperature related problems.
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FIGURE 4-1
4-1-1
Site investigation process for new and existing pavements.
General Site Investigation
General site investigation comprises a series of technical activities aiming at the following objectives: • Establish the nature, the distribution, and the properties of the rock and soil units along the proposed alignment • Provide the information necessary for the establishment of the final alignment, grade, and right of way • Provide information required for pavement design • Specify conditions for soil reuse in the road corridor • Identify specific problems requiring a specialized study • Identify and propose solutions to specific construction problems Earthwork is the construction activity that involves the highest risks of unexpected problems. The information collected during the general site investigation activities reduces uncertainty, and helps to obtain realistic cost estimates of the construction project. It also helps in the selection of adequate construction equipment and techniques. The information gathered can also help quality control operation and can support future studies on the performance of the pavement. Good site investigation requires information on the preliminary alignment of the road. Horizontal and vertical alignments, as well as information on embankment
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-2
Preliminary alignment and grade of a road project.
geometry, are essential to estimate adequate depths of cuts and thicknesses of fills. They also help to locate the horizontal and vertical positions of embankment slope toes, tops of cut slope, and ditches. Figure 4-2 illustrates typical road alignment information and related transverse sections. Soil characterization must cover adequately the area affected by cuts and fills. Assessment of geotechnical, hydrological, and hydrogeological risks associated with road construction must include relevant influence area beyond the road right of way. In all cases, good site investigation practice should start with the analysis of general, easy to obtain information and progress toward site specific, more complex, and more expensive studies. The following procedure is generally recommended when conducting a general site investigation project: 1. Analysis of topographical maps 2. Photo-interpretation and landform analysis 3. Analysis of relevant geological and technical reports 4. Visual reconnaissance the project site 5. Geophysical surveys 6. Drilling and sampling 7. In situ soil testing
Topographical Maps Topographical maps include large quantities of information that can be compiled and used for site investigation. Two categories of information can be gathered: information on site accessibility for investigation activities and information on potential technical problems associated with road construction at the suggested location. Table 4-1 summarizes the information available from topographical maps and its possible use for site
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Chapter Four Significance for Feature
Site Investigation Activities
Road Construction and Performance
Relief of site and surrounding areas
Ease of access, selection of adapted investigation equipment
Preliminary assessment of potential problems related to slopes such as instability, erosion, falling rocks, avalanches, and so on
Forested/ deforested areas
Ease of access, selection of adapted investigation equipment
Irregular forested surfaces in agricultural or urban areas are often indicators of poor soil conditions (glacial tills or rock) or difficult terrain conditions (steep and/or unstable slopes)
Hydrographic network and streams
Access problems and likelihood of exposed soil and rock formations
The shape and the size of the hydrographic network in which the investigated site is located can provide information on potential problems related to water flow along the road (erosion, water stagnation, and the like). It can also provide information on the type of soil deposits around the investigation site
Swamps, bogs, and poorly drained areas
Access problems
Direct implications for the drainage and the stability of the proposed road project
Existing roads and trails
Very useful for planning access for site investigation and testing activities. Possibility of exposed soil and rock formations in cut sections of existing roads and trails
Presence of public utility services
Can facilitate access to the site. Special attention must be given, however, to the presence of aerial or buried utility lines when planning and accomplishing geophysical surveys and boring activities
TABLE 4-1
Must account for utilities in design to avoid conflicts and damage to utilities
Information from Topographical Maps
investigation. Figure 4-3 gives an example of information that can be gathered on topographical maps.
Geomorphology and Photo-Interpretation of Landforms Natural transport and deposition processes are responsible for the formation of soil deposits. These processes can sort soil grains and structure soil deposits. The analysis of landforms can thus provide valuable information on soil nature characteristics and properties.
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-3 Topographical map and useful information for site survey.
Soil deposits can be classified in three main types: • Deep basin deposits • Fluvial deposits • Glacial and fluvio-glacial deposits Deep basin deposition typically occurs in lakes and marine environments. When a water stream enters a deep basin, it rapidly loses its transport energy. Large grains are deposited near the basin shores forming deltas and coastal (littoral) deposits. Fine particles remain longer in suspension and will deposit further inside the basin. Except for littoral deposits which can take linear ribbed shapes, deep basin deposits are generally flat. Figure 4-4 illustrates the main types of landforms associated with deep basin deposits.
FIGURE 4-4
Deep basin deposits and landforms.
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Alluvial plain Terrace
Gully Abandoned meanders
FIGURE 4-5
Fluvial landforms.
Fluvial deposits occur in the influence zone of a stream. Stream with high gradients have high transportation energy and will effectively transport fine to medium size particles. Soil deposits will, therefore, be essentially constituted of coarse-grained soils such as coarse sand and gravel. Stream flowing on flat terrains have considerably less transportation energy and fine-grained soils are likely to be deposited in pools and meanders along the stream. As indicated in Fig. 4-5, landforms associated with streams include terraces and alluvial plains. The presence of straight and braided streams in alluvial plains indicates high transportation energy and is usually associated with the presence of thick deposits of coarse-grained soils. Furthermore, the presence of active or abandoned meanders is a clear indication of a much lower transportation energy, which often translates into the presence of silt interlayers. In a seasonal frost environment, silt interlayers can cause severe differential heaving and need to be carefully identified and characterized. Fluvial deposits are generally good foundations and good construction materials for road projects when deposited in a high transportation energy environment. They can however be erosion susceptible. Special attention should be given to fluvial deposits associated with flat terrains and meandering water streams. These are likely to include silt interlayers making them susceptible to frost and moisture action. Glacial invasions that have occurred in the recent geological ages have strongly influenced the landscape of most northern countries. Most of the features and deposits of the last glacial advance are still intact and dominate northern landscapes. Landforms associated with recent glaciations can be divided in two major categories: • Glacial landforms include features and materials directly deposited and shaped by glaciers • Fluvio-glacial landforms include features and materials transported, sorted, and deposited by glacial streams Materials eroded, transported, and deposited by glaciers are heterogeneous materials referred to as “glacial tills” and are deposited in rugged landforms called moraines. During glacier advances, glacial till is laid down on the ground and remolded by the moving glacier forming ground moraine and drumlins. For glaciers flowing in valleys, some glacial till is also deposited along the valley wall forming uneven terraces called lateral moraines. During glacier recession, glacial till accumulates at glacier front during stagnation periods forming frontal or terminal moraines. The latter type of moraine indicates the farthest advancement of the glacier. These major glacial landforms are illustrated in Fig. 4-6. Glacial tills are stiff soils and good foundation materials for road construction. They can, however, cause severe construction problems due to the presence of large boulders in the materials. They can also be moisture and frost sensitive due to high content in fine particles.
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-6
Glacial and fluvio-glacial landforms.
Fluvio-glacial landforms are illustrated in Fig. 4-6. Streams fed by melting glaciers transport, sort, and deposit glacial materials forming fluvial forms associated with the presence of a glacier. Some sorted materials are accumulated by streams flowing underneath the glacier or between the glacier and the valley wall. Associated respective landforms are eskers and kame terraces and are essentially constituted of sand and gravel. Water from the melting glacier is often accumulated behind a frontal moraine. This water will eventually spill over the moraine eroding the accumulated material. The sorted material will be deposited in outwash plains below the moraine. These fluvioglacial landforms are often associated with depressions (kettles) left by abandoned ice blocks, around which the sorted material has been deposited. Finally, sorted material accumulated in depressions and crevasses in the glacier will eventually be deposited as little granular mounds referred to as “kames.” Fluvio-glacial deposits are generally good foundations and good construction materials for road projects. They can, however, be erosion susceptible. Interpretation of aerial photographs is one of the most powerful and cost-effective analysis techniques for preliminary study of road alignments. In addition to the possibility to observe most significant features of the land surface, aerial photographs generally allow stereoscopic analysis of landforms. Several types of information can thus, be gathered on aerial photographs. The quality of information, however, depends on the knowledge and the experience of the interpreter. Key information includes: • Landforms • Shape and characteristics of hydrographic networks • Land use • Signs of soil and rock instability • Presence of swamps and poorly drained areas • Presence and characteristics of exposed rock formations • Site access and restrictions Based on the observations, useful information can be interpreted on the nature and the extent of soil deposits, as well as the general characteristics, and properties of soils and rock formations. Table 4-2 provides summary information on soil types and
121
122 Environment
Landform
Characteristics
Deep basin (marine and lacustrine)
Basin beds
Flat surfaces in lowlands, dark colored soils
• Fine-grained soils • Compressible, weak and frost susceptible subgrades • Risks of unstable slopes and embankment failures • Limited possibilities to reuse soils in embankments
Coastal and littoral deposits
Slightly sloped surfaces, dark colored
• Fine- to coarse-grained soils • Compressible, weak and frost susceptible subgrades • Risks of embankment failures • Limited possibilities to reuse soils in embankments
Clear bands along sloped basin edges
• Fine- to coarse-grained soils • Fair bearing capacity and drainage • Risks of differential frost heave problems
Flood plain produced by the filling of a valley by alluvium
• Coarse-grained soils • Good bearing capacity and drainage • Abundant granular material for pavement construction
Fluvial and fluvioglacial
Alluvial plain (sloped plain, high transport energy)
Alluvial plain (Flat plain, low transport energy)
Drainage Pattern
Significance for Road Construction
• Fine to medium-grained soils • Fair to good bearing capacity and drainage • Risk of differential frost heave problems (silt patches) • Risk of localized compressible organic soils
123
TABLE 4-2
Terrace
Flat horizontal or gently inclined surfaces bounded by steeper descending slope on one side
• • • •
Alluvial fan
Cone-shaped deposit of alluvium made by a stream running into a level plain or into a slower stream
• Medium to coarse-grained soils • Good bearing capacity and drainage • Abundant source of good granular pavement materials • Risks of gully erosion
Kame
Conical hill or short irregular ridge of gravel or sand deposited by a melted glacier
Kame terrace
Terrace-like body of stratified drift deposited between a glacier and an adjacent valley wall
• • • •
Kettle
Depression in drifts (outwash plains) made by the wasting away of a detached mass of glacier that has been partly or entirely buried in the drifts
• Risk of localized compressible organic soils • Risk of differential soil behavior • Fair to poor drainage
General Information from Aerial Photographs
Medium to coarse-grained soils Good linear support for road construction Good bearing capacity and drainage Good source of granular pavement materials • Risks of gully erosion
No specific pattern
• Possible source of good granular material for pavement construction
Medium to coarse-grained soils Good linear support for road construction Good bearing capacity and drainage Good source of granular pavement materials • Risks of gully erosion
124 Environment
Glacial
Landform
Characteristics
Drainage Pattern
Significance for Road Construction
Esker
Long tortuous ridges, mounds and hummocks composed of stratified accumulations of sand and gravel in the general direction of drainage by stream flowing under a glacier
No specific pattern
• • • •
Outwash plain
Plain composed of material washed out from glaciers and moraine deposits. Flat surface with irregular texture and color
• Medium to coarse-grained soils • Fair to good bearing capacity and drainage • Possible source of good granular material for pavement construction
Basal moraine
Drift deposited by glacial action. Irregular surface, texture and color. Presence of boulders and blocks
• • • • •
Frontal moraine
Elongated accumulation of drifts at the front of the glacier. Irregularly shaped ridges and mounds. Sometime have an arc shape
No specific surface drainage
Lateral moraine
Elongated accumulation of drifts between the side of the glacier and the valley wall
No specific surface drainage
Medium to coarse-grained soils Good linear support for road construction Good bearing capacity and drainage Abundant source of good granular pavement materials • Risks of gully erosion
Fine- to coarse-grained soils Good bearing capacity Risk of differential frost action Fair to poor drainage Possible source of granular material for pavement construction • Possible excavation problems due to the presence of blocks and boulders • Risks of problems with slope stability
Drumlin
Streamlined hill or ridge of glacial drift with long axis paralleling direction of flow of former glacier
No specific surface drainage
Dunes
Mounds and ridges of wind-blown sand
Absence of surface drainage
Loess
Homogeneous nonstratified deposit of silt
Organic
Swamp or bog
Wetland covered with peat supporting low vegetation, e.g., sedges, mosses and shrubby plants. Clear color (mainly peat) alternating with dark spots (water ponds)
No specific surface drainage pattern
• • • •
Permafrost
Hummocks
A small mound (generally 1 m high or less) rising out of the surrounding terrain. Often have exposed mineral soils at the top. Can be isolated, but generally cover large surfaces (hummocky terrains)
No specific surface drainage pattern
• Silty or clayey soils • Possibility of ice-rich permafrost
Eolian
125
TABLE 4-2
(Continued)
• Possible source of good granular material for pavement construction • Frost susceptible subgrades
Highly compressible soils Poor bearing capacity and drainage Risks of embankment failures If possible, should be avoided for road construction • If excavated, can be used for revegetation of slopes
126 Environment
Landform
Characteristics
Drainage Pattern
Significance for Road Construction
Palsa
Circular or elongated earth mound (generally less than 10 m high) covered by a peat layer. Found alone or in group in muskegs
No specific surface drainage pattern
• Fine grained soils underlying a peat layer • Ice rich core (segregation ice)
Thermokarst
Steep wall depression caused by settling or carving of the ground due to melting of ground ice. Often filled with water (thermokarstic ponds or lakes).
• Indication of ice rich permafrost • Generally in fine grained soils
Ice wedges Ice-wedge polygons
Vertical wedge-shaped vein of ground ice On aerial photos, ice wedges may appear in linear scars, ridges or depressions often organized in polygonal patterns
• Indication of presence of massive ground ice in linear structures that can reach 3 m wide and 10 m deep.
Source: Information is compiled from Avery 1977, Lo 1976, Gagnon 1974, Allard et al. 1991 in Ladanyi 1996, Brown 1974, American Geological Institute 1976, and Conseil National de Recherche Canada 1988.
TABLE 4-2
(Continued)
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-7 Aerial photograph and useful information for site investigation. (Reproduced with the permission of Natural Resources Canada 2008, courtesy of the National Air Photo Library.)
associated landforms and drainage networks. Figure 4-7 illustrates an example of information that can be gathered from an aerial photograph. Detailed information on geomorphology and aerial photo-interpretation can be found in Avery (1977), Gagnon (1974), or Mollard and Janes (1985). Photo-interpretation is a very good tool to identify potential problems related to road construction and performance. The major problems are generally associated with the presence of blocks or boulders and bedrock near or at the subgrade line in the projected pavement structure, steep and irregular slopes in the projected alignment, low poorly drained lands and ice rich soils in permafrost areas. The presence of rock is the most important source of unexpected problems and cost variations in a road construction project. The presence of outcrops (clear colored, shiny surfaces, absence of vegetation) is a clear indication of the presence of bedrock at or near the surface. The dominance of landforms related to rock formation over landforms related to soil formation is also a good indication of shallow bedrock. The presence of boulders or large blocks at the surface has also a considerable effect on roadway construction operations and need to be noted. Steep and irregular slopes can be identified by the topography of the surface and by the presence of irregular forested areas. These areas involve cut and fill construction and are likely to induce important differential behavior in the road embankment. The presence of ancient landslide scars, seepage faces, exposed rock and soil are indications of potential slope activity and special attention should be given to those features. The risk of slope instability, landslides, gully erosion, and rock or debris falls should be properly assessed by qualified experts.
127
128
Chapter Four Low lands with flat surfaces are also potential problem areas. These areas are difficult to drain and can be subjected to flooding. Water ponding, dark colored wet soils, and wetland vegetation (moss, alders, and the like) are good indicators of poor drainage conditions. Soils are often fine grained and are likely to have low shear strengths and to be compressible and frost susceptible. When in contact with coarser soils, severe differential frost and consolidation behavior may occur. Ice-rich soils are a major source of concerns for pavements built in permafrost conditions. The presence of permafrost features such as hummocks, frost-boils, palsas, and ice wedges is a good indication of potential performance problems for pavements built over these soils. Permafrost degradation may lead to excessive settlement and/or cracking of the pavement surface. A more detailed treatment of photo-interpretation for road site analysis can be found in Avery (1977), Gagnon (1974), and Allard et al. (1991).
Geological and Technical Report Useful information can also be found in existing geological and technical reports. Geological reports typically contain regional information on rock and/or soil formations. The information significant to site investigation and road constructions includes the nature, stratigraphy, and structure of rock units in the area of the road project. Preliminary assessment of excavation techniques and cut-face stability can be inferred. Indications of the presence of outcrops or shallow bedrock can also help identify areas where rock excavation might be required. Information on the nature of soil units and properties should also be carefully referenced and documented to facilitate investigation and reconnaissance activities. When available, technical reports should be consulted in order to extract useful information. The most frequent documents available are geotechnical studies for bridge, building, power line and pipeline construction, and hydrogeological reports for domestic, industrial, or municipal wells, or for aquifer contamination studies. These studies are generally localized, but can provide useful information such as • Depth of the bedrock • Nature and characteristics of soils and rock units • Depth to the groundwater table • Bearing capacity and stiffness of soils (standard penetration test) • Soil compressibility and shear strength • Presence of ice-rich soils in permafrost conditions
Visual Reconnaissance of the Site Visual reconnaissance of a projected road site is the final step of the preliminary analysis of site information. It allows verifying some of the information gathered from the topographical maps, the aerial photographs, and the technical reports. It is also the occasion to finalize the planning of the site investigation activities. Site reconnaissance should allow collecting the following information: • Description of exposed soils and rock formations • Presence and characteristics of blocks and boulders • Verification and description of stream beds and banks
I n v e s t i g a t i o n a n d Te s t i n g • Presence and characteristics of soft and compressible soils • Identification of erosion and instability signs • Indications of seepage • Identification of flood zones and potential drainage problems • Identification of fragile natural habitats and environments Special attention should be given to areas where important cuts or fills are intended. The site reconnaissance should also be the occasion to verify access conditions for investigation, drilling and testing equipment. All information collected during site reconnaissance should be properly referenced with respect to the road alignment.
Surface Geophysical Investigations Geophysical investigations are often cost effective techniques to obtain useful and sometimes essential information on soil and rock formations along the highway alignment. Some of the techniques are well suited to obtain information on soil and rock units at great depths. Most geophysical techniques allow for close measurements of soil and rock units’ characteristics and properties making it possible to analyze and represent them in quasi-continuous profiles. Geophysical methods are, however, based on the interpretation of a signal picked-up at the surface. The level of accuracy of the investigations is, thus, relatively low and the quality of the information obtained depends heavily on the knowledge and the experience of the interpreter. Maximum benefit is generally obtained by using geophysical methods to interpolate or extrapolate information observed at the surface or in boreholes. Three methods are typically used for road site investigation purposes (LCPC 1982; Sylwester and Dugan 2002): • Electrical resistivity • Seismic refraction • Ground penetrating radar
Electrical Resistivity Electrical resistivity surveys are based on the observation of electric potential and current distribution at the earth surface intended to detect subsurface variations in resistivity typically related to soil and rock characteristics. Resistivity is the resistance to an electrical current of a three-dimensional, unbound, and multiphase medium. It is an intrinsic property of materials that varies as a function of mineralogy and water content. In the electrical resistivity method, a direct current is circulated between two electrodes (A and B in Fig. 4-8). The potential difference is measured between two electrodes M and N located inside the electrodes A and B. Figure 4-8 illustrates the configuration of the electrodes and the resulting electrical current and equipotentials in soils. The resistivity measured is the weighted average of the resistivity of all materials affected by the electrical field and is termed the apparent resistivity. The electrode configurations commonly used in electrical resistivity surveys are • The Wenner configuration: AM = MN = NB = AB/3 = 2y
ρa =
2π (2 y )∆ V I
(4-1)
129
130
Chapter Four
FIGURE 4-8 Configuration of the electrodes in the resistivity method and schematic distribution of electrical current in soils.
• The Schlumberger configuration: MN or 2y << AB or 2x (AB should be at least 5 times MN)
ρa =
π x2 ∆V 2 Iy
(4-2)
where x, y = configuration parameters (Fig. 4-8), I = electric current intensity circulated between A and B, DV = potential difference measured between M and N (see Example 4-1). Example 4-1 A potential difference of 1.5 V is measured between the two inner electrodes during a resistivity measurement in homogeneous soil using a Wenner configuration with a 4-m spacing between the electrodes. A 0.2-A current is circulated between the two outer electrodes. What is the apparent resistivity of the soil? Solution Using Eq. (4-1), one obtains:
ρa =
2π × 4 m × 1 . 5 V = 188 Ω ⋅ m 0.2 A
According to Fig. 4-11, the soil type is likely to be gravel or fractured rock.
Two techniques are generally used for site investigation using electrical resistivity. The first technique is used to establish a vertical soil sounding. The second technique is used to establish a horizontal resistivity profile of the soil units. Vertical soil sounding: The vertical sounding is achieved by increasing the distance 2x between the two outer electrodes of a Schlumberger (preferred) configuration in order to progressively increase the thickness of soil affected by the electrical field. The distance 2y between the inner electrodes remains constant during the tests. For short electrode spacing, the measurements are only influenced by the top layer of soil and the apparent resistivity obtained is representative of that layer. As indicated in Fig. 4-9, as the electrode spacing reaches a distance roughly corresponding to the depth of an interface with a second layer with contrasting resistivity, the measurements
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-9
Interpretation of resistivity sounding for a two-layer soil system.
will begin to be affected by the resistivity of the second layer. If the second layer has an infinite thickness, the apparent resistivity of the soil mass will tend toward the resistivity of the second layer as the electrode spacing increases. Horizontal resistivity profiles: Horizontal profiles are done by taking several measurements along a line using the Wenner (preferred) configuration keeping the distance between electrodes constant. Electrode spacing is adjusted according to the depth of interest. As indicated schematically in Fig. 4-10, the technique allows identifying variations in soil layer thicknesses or characteristics and contacts between layers. Typical values of resistivity for common unfrozen soils and rock formations are provided in Fig. 4-11. Figure 4-12 illustrates the effect of below-freezing temperatures on the resistivity of some soils and rock types. The reduction of unfrozen water content in soil pores with decreasing temperature has a strong influence on soil’s resistivity. Seismic refraction is a geophysical investigation method using the refraction of a generated seismic wave to characterize soil layers. The method is based on measurements of time required for the seismic waves to reach geophones placed along a line at the surface of the soil investigated. The system only records the time required for the first wave to reach the geophone. Knowing the distance between geophones, the speed of the seismic wave can thus, be obtained providing information on the nature and the properties of the layer in which the wave travels. As illustrated in Fig. 4-13a, when the seismic wave reaches a stiffer layer, it travels faster in the new layer and reemits refracted waves toward the surface. Since the wave travels faster, the refracted wave eventually reaches the surface before the wave traveling in the softer layer. From that point, the geophone starts to record the speed of the refracted wave providing information on the thickness of the first layer
Seismic Refraction
131
132
Chapter Four
FIGURE 4-10 Interpretation of a horizontal resistivity profile.
FIGURE 4-11 Typical resistivity ranges for different materials (compiled from Todd 1980 and McCarty 1998).
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-12 Effect of freezing temperature on the resistivity of different materials. (Hoekstra and McNeill 1973; reproduced with permission of National Academies Press.)
FIGURE 4-13 Principle (a) and interpretation (b) of a seismic refraction survey.
133
134
Chapter Four and on the nature of the second layer. Figure 4-13b illustrates a typical output of a seismic survey in a two-layer system. The slope of the curve corresponds to the seismic speed V in each layer. The distance X of the break point between two adjacent slopes can be used to compute the thickness of the layer characterized by the slope on the left side of X using Eq. (4-3): H1 =
X1 2
V2 − V1 V2 + V1
(4-3)
And, when a third layer is present H 2 = 0 . 85 H 1 +
X2 2
V3 − V2 V3 + V2
(4-4)
Example 4-2 illustrates the interpretation of the seismic refraction data. Example 4-2 A seismic refraction survey has been given in Table 4-3. What is the stratigraphy of the soil investigated? Solution 1. From the data, the distance-time chart is plotted in Fig. 4-14. It can be observed that two layers of soil are present. The following parameters can be derived from the data: V1 =
60 m = 600 m s 0.1 s
V2 =
60 m = 3000 m s 0 . 02 s
X = 60 m 2. From Eq. (4-3), the depth to the interface can be obtained: H1 =
60 2
3000 − 600 = 30 0 . 666 = 24 . 5 m 3000 + 600
3. Based on Fig. 4-15, the top layer could be either gravel or silt underlain by shale or limestone.
Geophone Number
Distance from Impact Point, m
Time Required for the Wave to Reach the Geophone, s
1
15
0.025
2
30
0.05
3
60
0.10
4
90
0.11
5
120
0.12
TABLE 4-3
Seismic Refraction Survey Data for Example 4-2
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-14 Distance-time chart for data in Example 4-2.
FIGURE 4-15 Typical seismic wave speed ranges for different materials (compiled from McCarthy 1998 and LCPC 1982).
One of the main limitations of the seismic refraction method is the fact that a stiff layer overlying a soft layer will mask the presence of the soft layer. The seismic wave being faster in the stiff layer, the wave will always reach the geophones before the wave refracted by the soft layer making it impossible to establish the depth to the contact between the two layers and the characteristics of the softer layer. This is, namely, the case when a layer of frozen soil is present over a layer of unfrozen soil. The slope of the contact between two layers can also be assessed by inducing a seismic wave at each end of the geophone line (forward shot and backward shot). The measurement of the dynamic modulus of soils and rock can be calculated if the velocities of both the compressional wave and the shear wave are measured. Typical values of seismic wave speeds for different soils are provided in Fig. 4-15.
Ground Penetrating Radar The ground penetrating (or probing) radar (GPR) provides information on material properties and on the position of interfaces between materials
135
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Chapter Four
FIGURE 4-16 Schematic illustration of soil profiling using GPR signals.
with contrasting electrical properties. The GPR technique uses an electromagnetic wave pulse which is emitted into the soil by an antenna located above or on the ground. A second antenna records the waves reflected on interfaces between units with contrasting dielectric constants and electrical conductivities. The quality of the reflected signal depends heavily on the sharpness of the contrast between two adjacent soils or objects. As illustrated in Fig. 4-16, the amplitude (a) of the reflected signal is a function of the quality of the reflector or, in other words, of the contrast between the dielectric properties between the adjacent layers. The time (t) between two echoes is in turn a function of the distance traveled between two reflectors. Different results can be obtained by varying the frequency of the transmitted signal. High frequency waves will provide good resolution at shallow depth, while low frequency waves will allow reaching greater depth with limited resolution. Sounding depth varies considerably depending on the conditions, but the practical reach of the technique is approximately 10 m. The short time required to record information at one point makes it possible to obtain detailed soil profiles by moving the antennas along a traverse. GPR is very effective in coarse-grained soils, ice, and frozen ground. However, it has limited effectiveness in fine-grained soils (silt or clay) or conductive soils (salt water, wet clay, and the like) because of the high attenuation of the electromagnetic signal (Sylwester and Dugan 2002). Table 4-4 provides a summary of information on the applicability of different geophysical methods for soil investigation for road projects.
Boring and Sampling A boring campaign is generally required to complete site characterization activities. Soil boring and sampling are needed to validate and complete the information gathered using photo-interpretation, site reconnaissance, and geophysical techniques. The general objective of boring and sampling operations is to characterize the nature and properties of soils that are within the range of influence of a roadway embankment.
Planning
Geophysical Survey Method
Applications
Advantages
Limitations
Electrical resistivity
• • • • •
Estimating depth to the water table Subsurface stratigraphic profiling Groundwater resource evaluation Contaminated groundwater studies Depth to and thickness of frozen ground
• • • •
• Susceptibility to natural and artificial interferences • Limited utility in urban areas • Interpretations that assume a layered subsurface • Lateral heterogeneity not easily accounted for • May require predrilling to install electrodes in frozen surface soils
Seismic refraction
• • • • • • •
Variations in bedrock properties Depth to bedrock Rippability of bedrock Dynamic moduli of soils and bedrock Depth to the water table Depth to frozen ground Slope stability
• Relatively easy accessibility (clearing generally not required) • Rapid area coverage • Results can be approximated in the field • High depth of penetration • Not susceptible to electrical interferences from power lines
• Resolution is reliant on contrasts in physical properties • Susceptibility to noise from construction or urban development • Cannot detect soft layer beneath hard layer (velocity inversion): limits analysis of permafrost thickness
Ground penetrating radar
• Delineation of sand, gravel and boulder horizons • Profiling bedrock surface and detecting fractures and fault zones • Mapping water depth, muskeg, ice thickness, and permafrost • Delineation of hydrocarbon contamination • Locating buried debris and utilities
• • • • •
• Limited depth of penetration on conductive soils (saline or clayey) because signal is absorbed • Interpretation of data is qualitative • Limited on very rough surfaces • Requires adequate clearing for antennae corridor • Susceptible to interference from nearby steep slopes • Susceptible to interference from power lines
Rapid area coverage High mobility, even on rough terrain Little to no clearing required Results can be approximated in the field
Good portability Little or no environmental impact Rapid area coverage High resolutions Visual picture of data
137
Source: Sylwester and Dugan 2002.
TABLE 4-4
Summary of Applications, Advantages, and Limitations of Different Geophysical Investigation Methods
138
Chapter Four Situation
Maximum Spacing
Minimum Depth
GENERAL Soil conditions: Uniform Variable Highly variable
100–300 m 60–100 m 20–40 m
1.5 m
CUTS Uniform Variable Highly variable
100–200 m 40–80 m 10–20 m
FILLS H<2m H≥2m
100–200 m 50–100 m
1.5 m H or refusal
ORGANIC SOILS
50–100 m
1.0 m under the organic soil deposit
PERMAFROST Not thaw-sensitive
100–200 m
Thaw-sensitive
40–60 m
The deepest of: 2.0 m under the final grade of the pavement surface or ditch grade
The deepest of: 1.0 m under the bottom of the active layer or 1.5 m under subgrade line 3.0 m under the bottom of the active layer
Source: A synthesis of information compiled from practices of several highway administrations (Quebec, British Columbia, Ontario, Manitoba, Saskatchewan, Vermont, Minnesota, Washington, Connecticut, and the U.S. Federal Highway Administration) and from selected references (Atkins 2003; Haas 1997; and Morin 1994).
TABLE 4-5 General Recommendations on Boring Spacing and Depth
In permafrost conditions, the thickness of the active layer and the ice content of the top part of permafrost are essential information required in addition to normal soil characterization. Moreover, soil borings often help locate and characterize the bedrock. Boring techniques generally allow for soil sampling for laboratory characterization. Boring and sampling are labor intensive and relatively expensive activities and need to be carefully planned. As a general rule, boring should be done according to spacing and depth specified in Table 4-5. Specific location of the drill holes should, however, be adapted to the terrain in order to cover adequately embankment toes, bottom of ditches, and cut edges as suggested in Fig. 4-17. Representative samples must be collected for each soil type found along the road alignment. Bulk samples of granular soils and undisturbed samples of cohesive soils should be collected for each soil strata in each borehole. General guidelines for material sampling are provided in Table 4-6.
Boring Techniques Several techniques are available for boring and sounding soils for earthwork and pavement design purposes. The techniques available and frequently used for pavement engineering can be classified in two categories: • Manual sounding techniques • Mechanical boring and sounding techniques
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-17 Locations requiring specific attention when planning boring and sampling for a road construction project.
Soil Type
Minimum Sampling Frequency
Cohesive soils
One undisturbed sample every meter in first 3 m and every 3 m afterward
Granular soils
One sample per soil strata
Permafrost
One sample per soil strata in the active layer One undisturbed sample every meter under the active layer (thaw-sensitive permafrost)
Source: A synthesis of information compiled from practices of several Highway administrations (Quebec, British Columbia, Ontario, Manitoba, Saskatchewan, Vermont, Minnesota, Washington, Connecticut, and the U.S. Federal Highway Administration) and from selected references (Haas 1997; Morin 1994; and Atkins 2003).
TABLE 4-6 General Recommendations for Material Sampling
Manual sounding techniques are a good practical approach to obtain basic information on soils at shallow depths. Among others, manual shovel as well as shell augers and continuous flight augers (see Fig. 4-18) are the most commonly used tools for manual sounding. These techniques are inexpensive and easy to use. They generally allow soil investigation in difficult terrains. In good conditions, some manual techniques can reach 2 m deep and allow for limited soil sampling. Manual boring techniques, however, have limited effectiveness in soils with pebbles, blocks, and boulders. Mechanical boring and sounding techniques are required to gather information at large depth, to characterize coarse granular materials as well as to confirm contact with the bedrock, and to gather information on its properties. Mechanical boring and sounding are generally effective in most type of soils up to large depths. Most techniques allow for the collection of good quality undisturbed or bulk samples. Mechanical boring and sounding is, however, an expensive operation which also requires, depending on the equipment used, good access conditions. Excavating test pits with a backhoe or a tracked excavator is a simple and relatively inexpensive technique for effective
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Chapter Four
FIGURE 4-18 Commonly used manual sounding techniques.
sounding of soils up to a depth of approximately 4 m. This technique allows for the direct observation of soil structure and characteristics. It also allows collection of good quality bulk samples. For deeper soil investigation, boring equipment is generally required. The most commonly used tools for soil boring are hollow-stem auger drills and churn drills (Fig. 4-19). Both techniques are effective for boring and sampling most types of soils. Table 4-7 provides a summary of sounding and boring techniques with their advantages and limitations.
Boring and Sampling Frozen Soils Several authors discuss how to carry out boring and sampling in permafrost. Obtaining undisturbed frozen samples is possible with various boring and coring techniques. However, it is difficult to collect samples that stay frozen throughout the boring or sampling process. Collection of good quality samples of frozen soils is essential for the determination of frozen soil properties. For example, thaw consolidation tests require good quality undisturbed frozen samples.
FIGURE 4-19 Commonly used mechanical boring and sampling techniques.
Method
Applications
Practical Reach
Advantages
Limitations
Manual Methods • Shovel
• All types of soils • All types of terrains
0.5 m
• Low cost • Good identification of soils • Possible to collect bulk samples
• Limited reach • Limited use in soils with boulders
• Shell auger
• Sandy soils
1.5 m
• Low cost • Good identification of soils
• Limited sampling capability • Low effectiveness in coarse granular soils
• Continuous flight auger
• Cohesive soils
2.0 m
• Low cost • Good identification of soils
• Limited sampling capability • Low effectiveness in coarse granular soils
• Backhoe or excavator (test pit)
• All types of soils
4.0 m
• Good identification of soils type and structure • Possible to collect bulk samples • High productivity
• Moderate cost • Limited access to forested or difficult terrain
• Auger drill rig
• All types of soils
10.0–15.0 m
• Good identification of soils • Possible to collect bulk and undisturbed samples (when stem auger is used) • High productivity
• High operation cost • Limited access to wooded or difficult terrain • Soil contamination along auger limits reliability of soil identification and sampling • Poor reliability for locating interface between soil strata
• Churn drill
• Cohesive soils and granular soils without large cobbles or blocks
10.0 m +
• Good reliability for soil identification and location of interfaces • Possible to collect bulk samples
• High operation cost • Limited access to forested or difficult terrain
Mechanical Methods
141
TABLE 4-7
Summary of Applications, Advantages and Limitations of Different Sounding and Boring Techniques
142
Chapter Four According to Riddle and Hardcastle (1991), frozen soil boring operations are best performed during winter. Winter conditions will facilitate site access and minimize damage to soil and vegetation. Cold temperatures also facilitate cooling of boring bits, which reduces damage to frozen soil samples. However, several operational restrictions may affect the effectiveness of winter boring operations. Frozen soil samples obtained in the field should be kept at their in situ temperature during shipment to the laboratory. Tables 4-8 and 4-9 summarize the application field, the advantages and the disadvantages of available methods for boring and sampling frozen soils. Boring Methods Highway auger
Soil Coarse
Advantages Availability of the equipment Small crew size No drilling fluid required
Continuous flight solid stem auger Continuous flight hollow stem auger
Coarse
Low cost
Coarse
Can be used in different soil conditions
Percussive hammer
Coarse
Continuous sampling Casing prevents material contamination
Vibratory drill
Coarse
Ability to break large boulders Continuous sampling High rate of production
Rotary drill
Coarse
Jet-drive drill
Coarse
Blade bit with air
Fine
High rate of production Light weight rigs needed Light and portable equipment High rate of production Easy to get inexpensive data from difficult access areas High rate of production
Disadvantages Possible breakdown of the coarser material Contamination by groundwater Caving Poor recovery of ice Poor recovery of ice Need relatively large rig Poor recovery of ice (ice melts) Difficulty to penetrate large cobbles, boulders, and dense frozen materials Difficulty to break large cobbles Poor recovery of ice Large and heavy carrier required System plugged with fine-grained soil Fracturing ice Degradation of the sample Have to pull a string to retrieve the sample Caving if groundwater is present Poor ice recovery Sample disturbance Difficulty to penetrate large cobbles Need a drilling fluid Need experienced personnel Material can block the hole Difficulty to recover the samples sometimes
Source: after Riddle and Hardcastle, 1991, with permission from Society of Petroleum Engineers ©.
TABLE 4-8 Synthesis of Available Methods for Boring Frozen Soils
I n v e s t i g a t i o n a n d Te s t i n g Sampling Methods Hand
Soil Coarse
Split-spoon
Coarse
Refrigerated coring
Coarse
Fluids
Air
Do not contaminate the sample Environmentally acceptable
Water
Environmentally acceptable
Mud and brine
Environmentally acceptable
Glycol
Environmentally acceptable Excellent recovery of ice Coring can be performed even in summer Know the degree of temperature sensitive ice-soil bonding within the material Excellent recovery of ice High quality undisturbed frozen samples
Petroleum fluid
Split-spoon
Fine
Modified Shelby tube Continuous hollow-stem auger
Fine Fine
Advantages Simple, inexpensive, large number of samples Large samples can be obtained
Disadvantages Poor ice recovery
Difficult to get samples in gravel and bedrock Penetration is hard Poor ice recovery Equipment damaged by coarsegrained soils If temperature >0°C, air tends to melt the ice Silt can plug the bit area Weight of the equipment Have to defrost the compressor Poor ice recovery Wash out the fines from samples Water freezes in winter Poor ice recovery Corrosion of the equipment Poor ice recovery Environmentally unacceptable Possibility of fire
Obtain only disturbed samples Poor ice recovery
Long process Not all the drilling rigs are able to use the equipment to obtain the samples
Source: after Riddle and Hardcastle, 1991, with permission from Society of Petroleum Engineers ©.
TABLE 4-9
Synthesis of Available Methods for Sampling Frozen Soils
143
144
Chapter Four
In Situ Testing of Soils Useful information can be obtained through simple field tests during the soil investigation. Information relevant to pavement engineering include mechanical properties of soils and soil sensitivity to frost and thaw action. Unfortunately, there is no known in situ test that can easily be used to assess soil sensitivity to freeze-thaw cycles. However, basic information on mechanical properties of soils can be gathered including resilient properties, strength, and compressibility of soils. The standard penetration test (SPT) has a long history in geotechnical and foundation engineering. The test consists of using a 64-kg mass falling free from a height of 760 mm in order to drive a standard split-spoon (barrel) sampler (illustrated on Fig. 4-19) a distance of 450 mm into the soil. The standard penetration number is defined as being the number of blows (N) required to drive the sampler 300 mm (150 mm plus 150 mm). The standard penetration test is a rough and poorly standardized test. However, the fact that it is easy to perform and routinely used during soil boring operations makes it a very useful tool for preliminary indication of soils’ mechanical properties including density, bearing capacity and shear resistance. It can also provide good indications on soil profile and depth of the bedrock. The standard penetration number needs to be corrected to compensate for energy loss which increases with depth. Typical soil characteristics and related correlations are provided in Table 4-10.
Standard Penetration Test
Shear Strength Test The in situ shear strength test or “vane test” is a widely used technique for preliminary estimation of shear resistance and bearing capacity of soft cohesive soils. It consists of a cross-shape arrangement of four metal blades welded to the end of a rod. The vane is pushed to the desired depth at the bottom of a drill hole or
Granular soils
Fine-grained soils
Ncorr
Soil State
g (kN/m3)
f′(°)
0–3
Very loose
11–13
26–28
3–9
Loose
14–16
29–34
9–25
Medium
17–19
35–40
25–45
Dense
20–21
38–45
>45
Very dense
>21
>45
0–2
Very soft
<10
3–5
Soft
10–25
6–9
Medium
25–50
10–15
Stiff
50–100
15–30
Very stiff
100–200
>30
Extremely stiff
>200
cu (kPa)
Source: Budhu 2000 (copyright 2000 John Wiley & Sons, Inc.; reprinted with permission of John Wiley & Sons, Inc.).
TABLE 4-10 Interpretation of Data from the Standard Penetration Test
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-20
Field measurement of shear strength.
directly into the soil. A torque is then applied to the vane at a slow rate (6°/min) until shear failure occurs around the blades. Figure 4-20 illustrates the equipment and the procedure used for simple vane tests. The torque required to induce shear failure (maximum torque measured during a test) is recorded and used to estimate shear strength, cu, using the following relationship: cu =
2T πd h + 1 3 d 3
(
)
(4-5)
where cu = undrained shear resistance (Pa), T = maximum torque measured during the test (N·m), h = length of the blade set (m), and d = diameter of the blade set (m). The information can be used for preliminary estimation of the bearing capacity of the soil using the following relationship: H=
6c u γF
(4-6)
where H = maximum thickness of the embankment (m), cu = shear strength of the soil (kPa), g = unit weight of the embankment material (kN/m3), F = selected safety factor (1.5) (see Example 4-3). Example 4-3 A vane test has been conducted at 1.0, 2.0, and 3.0 m in a soft clay deposit. The lowest value of maximum torque was 3 N·m and recorded at 2.0-m depth with a 50-mm diameter and 100-mmlong blade. What is the maximum thickness of the highway embankment that the soil can support considering that the unit weight of the embankment material is 20 kN/m3?
145
146
Chapter Four Solution From Eq. (4-5) cu =
2×3
(
π × 0 . 05 0 . 1 3
0 . 05
+ 1
3
)
=
6 N = 6548 2 = 6 . 55 kPa 9 . 16 × 10− 4 m
Allowable embankment thickness can be obtained from Eq. (4-6): h=
6 × 6 . 55 m = 1 . 31 m 20 × 1 . 5
The dynamic cone penetrometer (DCP) is also a widely used tool in pavement engineering used to estimate mechanical properties of pavement materials or subgrade soils at shallow depths. Its simplicity, portability, and low cost make it an attractive tool for site investigation. The test involves driving a cone-shaped tip of a rod in soils using the impact of a falling mass. As illustrated in Fig. 4-21a, the standard DCP used in pavement engineering uses a 20-mm diameter and 60° angle cone driven by the impact of an 8-kg hammer falling from a 575-mm height. The most widely used DCP is the manual version (Fig. 4-21b), while several automated models are now available on the market (Fig. 4-21c). The dynamic penetration index (DPI), defined as the penetration of the cone for each drop of the mass (mm/blow) can be correlated with many engineering parameters such as the California Bearing Ratio (CBR), the resilient modulus, and the shear strength. Correlations between the DPI and shear strength and elastic modulus are given in Tables 4-11 and 4-12. These correlations, developed by Boutet (2007), include basic levels (Table 4-11) for situations where minimal information is available on soils tested, and more accurate advanced correlations (Table 4-12) for situations where basic soil
Dynamic Cone Penetrometer
FIGURE 4-21 (a) Standard dynamic cone penetrometer used in pavement engineering, (b) portable version, and (c) automated version.
I n v e s t i g a t i o n a n d Te s t i n g Basic Correlation All soils
log(Cu) = −0.0042 × DPI + 2.10
R2 = 0.78; RMSE = 26
Advanced correlations CH
log(Cu) = −0.20 × (IP0.5 × rd0.25) × log(DPI) + 3.90
R2 = 0.99; RMSE = 4
CL and ML-CL
log(Cu) = −0.10 × (IP0.5 × rd0.25) × log(DPI) + 2.39
R2 = 0.84; RMSE = 9
DPI: dynamic penetration index (mm/blow); Cu: undrained shear strength (kPa); IP: plasticity index (%); rd: dry density (Mg/m3); R2: coefficient of determination; RMSE: root mean squared error of the prediction. Source: Boutet 2007.
TABLE 4-11 Correlations between DPI and Shear Strength (Cu) for Cohesive Soils
Basic Correlation All cohesive soils
log(EFWD) = −0.45 × log(DPI) + 2.52
R2 = 0.33; RMSE = 54
All granular soils
log(EFWD) = −0.62 × log(DPI) + 2.56
R2 = 0.42; RMSE = 45
Advanced correlations CH
log(EFWD) = −1.17 × (IL0.25 × rd) × log(DPI) + 3.90
R2 = 0.98; RMSE = 7
CL and ML-CL
log(EFWD) = −2.78 × log(DPI)/(IP × rd)0.5 + 2.82
R2 = 0.93; RMSE = 14
SP and GP
log(EFWD) = −0.013 × w × log(DPI) + 1.76
R2 = 0.79; RMSE = 5
SM, SC, SM-SC, and GM
log(EFWD) = −4.04 × log(DPI)/w0.016 + 6.65
R2 = 0.84; RMSE = 13
DPI: dynamic penetration index (mm/blow); EFWD: elastic modulus (MPa); IP: plasticity index (%); IL: liquidity index (%); w: water content (%); rd: dry density (Mg/m3); R2: coefficient of determination; RMSE: root mean squared error of the prediction. Source: Boutet 2007.
TABLE 4-12 Correlations between DPI and Elastic Modulus (EFWD) Obtained from Portable Falling Weight Deflectometer Measurements
characteristics are known. The test also allows drawing vertical profiles of soil properties. Vertical profiles can help identify soil stratigraphy when sufficient contrast exists between adjacent layers (Fig. 4-22). The inaccuracy of the tool is compensated by the ease of collecting information on mechanical properties specific to the soil or material considered in the pavement project. Example 4-4 Using DCP information illustrated in Fig. 4-22, estimate the shear strength and the elastic modulus of the subgrade soil (DPI = 12.7 mm/blow). Solution Using basic correlations, the following estimates can be obtained: log(cu ) = (− 0 . 0042 × DPI) + 2 . 10
147
148
Chapter Four
FIGURE 4-22
Example of a DCP profile for a road built on sandy clay (CL).
log(cu ) = (− 0 . 0042 × 12 . 7 ) + 2 . 10 log(cu ) = 2 . 05 cu = 11 1 1 . 3 kPa log(EFWD ) = (− 0 . 45 × log(DPI)) + 2 . 52 log(EFWD ) = (− 0 . 45 × log(12 . 7 )) + 2 . 52 log(EFWD ) = 2 . 02 EFWD = 105 . 5 MPa Knowing that the soil has the following characteristics: IP = 8 percent; rd = 2.1 Mg/m3, the advanced correlations can be used for more accuracy: log(cu) = [−0.10(80.5 × 2.10.25) × log(12.7)] + 2.39 = 2.01 cu = 103.3 kPa
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-23
Soil testing with the light weight deflectometer.
log(EFWD ) =
− 2 . 78 × log(12 . 7 ) + 2 . 82 = 2 . 0 7 (8 ⋅ 2 . 1)0 .5
EFWD = 117.9 MPa
Light Weight (Portable) Deflectometer The light weight deflectometer (LWD) is a portable version of the trailer-mounted falling weight deflectometer (FWD). As shown in Fig. 4-23, it uses the impact of a falling 10-kg mass on a rubber buffer to induce a mechanical response (deflection) at the surface of a pavement or soil layer. The applied stress, transmitted through a circular plate having a radius of 50, 100, or 150 mm, can reach 150 kPa for a load pulse duration of 20 ms. The deflection is measured at the center of the plate and is used to estimate the elastic modulus of the underlying material using the Boussinesq equation [Eq. (4-7)] assuming that the measurement is made on a semi-infinite homogeneous space: E=
Aσ 0 a(1 − µ 2 ) d0
(4-7)
where A = plate stiffness factor equal to p/2 for a rigid plate, s0 = applied stress measured by the stress cell of the LWD, a = radius of the loading plate (mm), m = Poisson’s ratio, and d0 = deflection measured at the center of the plate (mm). Most commercially available equipment include hardware and software to allow direct computation and reporting of the soil elastic modulus. When using deflection measured at the center of the loading plate, LWD measurements might include the influence of several layers. According to Lambert et al. (2006) the depth of significant stress is expected to be approximately 1.5 to 2 times, the bearing plate diameter. The elastic moduli estimated from these measurements are thus composite moduli (Ecomp) including the effect of all materials within the depth of influence of the deflectometer.
149
150
Chapter Four
4-2
Investigation of Existing Pavements Good management of existing highways depends upon the quality of information available on their structural and functional condition. The structural condition of a pavement is its ability to sustain loading without excessive damage over a given performance period. The structural condition can be assessed using deflection testing, dynamic cone penetration testing, and/or through detailed characterization of thickness and condition of pavement layers through boring and sampling. The functional condition of a pavement is defined as its aptitude to provide a safe and comfortable ride to highway users. The three main characteristics contributing to the functional condition of the highway are surface roughness (or smoothness), rutting, and skid resistance. With respect to the structural condition, dynamic cone penetration testing and boring/ sampling operations have been described in Sec. 4-1. This section will focus on the use of deflection testing for structural characterization of highway pavements. Functional condition surface distresses including cracking and rutting are described in Chap. 3. This section will describe mainly roughness and longitudinal profile measurements and their analysis. The general use of these tools will be summarized with an emphasis on their application for the characterization of seasonal behavior of pavements in cold regions.
4-2-1 Evaluation of Pavement Structural Characteristics Using Falling Weight Deflectometer Deflection testing is used to measure the mechanical response of the pavement structure under a test load. The falling weight deflectometer (FWD) is a widely used and accepted tool to measure pavement response under dynamic loading. As illustrated in Fig. 4-24, modern FWD are mounted on trailers or in test vehicles. The FWD includes two essential components: a mechanical loading system composing of the falling weight and related control and handling equipments and a measurement system consisting of sensors and related data acquisition systems. The loading system is designed to induce load pulses adjusted to mimic the vertical loading effect of a heavy wheel traveling at normal highway speed on the pavement. The load pulse is induced by a weight falling on a rubber buffer connected to a loading plate. The resulting pulse has a haversine shape lasting approximately 0.030 s. The load
FIGURE 4-24
Falling weight deflectometer and its main components.
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-25
Falling weight deflectometer loading and related pavement response.
pulse is illustrated in Fig. 4-25a. Deflection of the pavement surface is recorded by a set of geophones in contact with the pavement at increasing distance from the loading plate. Figure 4-25b illustrates the deflection signals recorded by the sensors. For most types of analysis done with the deflections, only the maximum deflection for each sensor is considered and used to reproduce a deflection basin as illustrated in Fig. 4-25c. Two types of analysis can be done with the deflection data. One type of analysis is based on deflection basin-shaped indicators. The other type of analysis involves backcalculation of pavement layer moduli.
Deflection Basin Indices The first level of analysis relies on simple deflection basin indices providing an indication of the mechanical behavior of the pavement structure. These indices are illustrated in Fig. 4-26 and described in Table 4-13 (see also Example 4-5).
FIGURE 4-26
Deflection basin indices commonly used in pavement engineering.
151
152 Deflection Basin Index
Description
Maximum deflection (d0 or dmax)
Maximum deflection recorded. Represents the global response of the pavement system subjected to the applied load
Surface curvature index (SCI) (Horak 1987) Base curvature index (BCI)
Radius of curvature of the center of the basin (Jung 1988) Tensile strain at the bottom of the asphalt bound layer (Jung 1988) Basin area A and normalized basin area Anorm
SCI = d0 − d300
(4-8)
BCI = d1200 − d1500
(4-9)
R=
εt =
(d0 − d x )2 + a2 a2 ~ − 2(d0 − d x ) 2(d0 − d x )
h1 2R
d d d Anorm = 6 1 + 2 300 + 2 600 + 900 d0 d0 d0
(4-10)
(4-11)
(4-12)
Curvature of the inner portion of the basin. Provides an indication of the stiffness of the top part of the pavement (0–200 mm) Curvature of the outer portion of the basin. Provides an indication of the stiffness of the bottom part of the pavement and/or of the top part of subgrade soil (800–1000 mm) Where “dx” is the deflection (mm) measured at the sensor located just outside the loading plate and a is the radius of the loading plate (mm) Where h1 is the thickness of the asphalt bound layer (mm). Tensile strain can be used to verify structural adequacy of the pavement structure based on a fatigue cracking criteria Considered as a good indicator of overall pavement strength during spring thaw (Several authors, quoted by St-Laurent 1995)
A = Anorm × d0 Subgrade strength index (SSI)
SSI =
d900t d900 s
(4-13)
Source: modified from St-Laurent, 1995.
TABLE 4-13 Deflection Basin Indices Commonly Used in Pavement Engineering
Where d900t is the deflection measured at 900 mm offset during thaw and d900s is the deflection measured at the same sensor after thaw recovery. SSI is considered a good indicator of subgrade soil bearing capacity loss during spring time (Janoo and Berg 1990)
I n v e s t i g a t i o n a n d Te s t i n g Example 4-5 Given the following deflection basin, determine the basin indices. Distance
0 mm
200 mm
300 mm
450 mm
600 mm
750 mm
900 mm
1200 mm
1500 mm
Deflection (mm)
213
176
156
128
104
86
74
58
46
Solution Considering a loading plate radius of 150 mm and a 180-mm-thick asphalt concrete layer: dmax = 213 mm or 0.213 mm SCI = d0 − d300 = 213 − 176 = 37 µ m BCI = d1200 − d1500 = 58 − 4 6 = 12 µ m R=
a2 1502 = = 304, 054 mm 2(d0 − dx ) 2(0 . 213 − 0 . 176)
Note that d200 has been used for the calculation as it is the measurement from the geophone, which is the closest to the 150-mm-radius loading plate. Deflections in millimeter are used in order to be consistent with the radius measurements.
εt =
180 = 296 × 10 −6 2 × 304054
Tensile strain can then be used in a fatigue damage model to estimate the residual life of the pavement in terms of allowable number of equivalent single axle loads (ESALs).
Indices such as dmax, SCI, and BCI are good indicators for a preliminary assessment of weak zones along a highway section. Figure 4-27, adapted by the Quebec Ministry of Transportation from Loudon and Partners (1995), provides a reference for a rapid analysis of dmax and SCI given the total number of ESALs that pavement should withstand during its service life.
FIGURE 4-27 Allowable values of dmax and SCI as a function of traffic volume (ESAL) (adapted by the Quebec Ministry of Transportation from Loudon and Partners 1995).
153
154
Chapter Four
FIGURE 4-28
Example of deflection index profiles along a four-lane divided highway.
An example of deflection index profiles on a four-lane divided highway in Canada is provided in Fig. 4-28. Given an expected volume of traffic of 4 million ESAL over the service life of the highway, the pavement is experiencing excessive dmax and SCI over most of the investigation length. In addition, it is very interesting to note that up to 7500 m, excessive pavement deflection follows the variation pattern of SCI while beyond that point, excessive deflection appear to correlate mainly with BCI. This observation suggests that pavement weaknesses along the investigation section seem to be associated mainly to bearing capacity problems in the upper part of the pavement structure up to 7500 m and in the bottom part of the structure (or subgrade soil) afterward.
Assessment of Layer Moduli Using Deflection Data Two approaches are used to determine resilient modulus of unbound pavement layers and subgrade soils. One of them uses empirical models relating deflection parameters with the resilient modulus of subgrade soil. Several models are proposed in the literature. Rhode (1994) has proposed two models to estimate the structural number of the pavement structure and the resilient modulus of the subgrade soils. The models are based on a rational analysis of the mechanical behavior of the pavement structure subjected to FWD loading. The model proposed for the estimation of the resilient modulus of the subgrade soil is the following: ESG = 1010,655 SIS −1254 Hp −2453
(4-14)
where SIS = structural index of the subgrade soil, defined as SIS = d1.5Hp – d1.5Hp+450, Hp = total pavement thickness, mm, d1.5Hp, d1.5Hp+450 = deflections (mm) obtained from the basin for distances from the loading points corresponding to 1.5 × Hp and 1.5 × Hp + 450 mm.
I n v e s t i g a t i o n a n d Te s t i n g The other approach for the determination of resilient modulus is a widely used mechanistic based technique for the interpretation of FWD data referred to as “backcalculation of pavement layer moduli.” The analysis is generally based on multilayer elastic theory and models which make it possible to compute stresses, strains, and deflections at any point of a pavement system. Based on the thickness, modulus and Poisson’s coefficient of each layer of the system, linear elastic models allow computation of deflections at the pavement surface at various distances of the loading plate. Knowing layer thicknesses and assuming reasonable Poisson’s ratios for pavement materials and soils, it is thus possible to find a set of moduli that would theoretically reproduce the deflection basin observed during FWD testing. The process involves the following steps: 1. The user enters position of deflection sensors, available pavement information (layer thicknesses and depth of the rigid layer), load data, and deflection data. 2. The user defines ranges of reasonable modulus values for each pavement material and tolerance (error) for deflection basin fitting. 3. For each set of modulus values, the algorithm computes deflections for each deflection sensor and computes error between calculated deflections and measured deflections. The goodness-of-fit between the calculated basin and the measured basin is assessed using the root mean square error (RMSE) for all sensors. 4. The process is repeated using different modulus values until all errors are within the specified tolerances and RMSE is minimized. 5. The set of elastic modulus corresponding to the best fit between calculated and measured deflection basins are the output of the process. Backcalculation methods use iterative approaches in attempting to converge toward a theoretical solution matching field observations. The level of sophistication of these approaches varies between methods. Some methods are based on simple theories such as the equivalent thickness method developed by Odemark in 1949 (e.g., ELMOD and BOUSDEF). Some other methods use more sophisticated optimization approaches to estimate values to be used in the next iteration based on partial derivative of functions relating deflection and modulus (e.g., MODULUS or WESDEF) or least square method (e.g., MODCOMP and EVERCALC) (Von Quintus and Simpson 2002, St-Laurent 1995). Backcalculated modulus can then be used to compute stresses and strains in the pavement structure and to assess reinforcement requirements. Several backcalculation programs are available commercially or as freeware on the Web. All backcalculation methods are based on a number of assumptions. The most important is the linear elastic behavior of pavement layers and soils. This simplification of the problem reduces considerably the computation effort required to find a satisfying solution to the problem. However, it tends to cause convergence problems or induce errors in backcalculated moduli. This is particularly true when pavement materials tend to exhibit significant nonlinear or nonelastic (time dependent) behaviors. For example, the viscous behavior of asphalt-bound materials tend to become important in hot summer conditions, while the visco-plastic behavior of unbound materials and soils becomes significant in saturated (thawing) conditions. The use of emerging dynamic analysis methods dealing explicitly with time-dependent material behavior is likely to improve significantly the interpretation of deflection data, more specifically in conditions where material behavior tends to drift away from assumed idealized behavior (Grenier 2007).
155
156
Chapter Four
Analysis of Seasonal Variation of Deflection Parameters The FWD is a very useful tool to assess seasonal variations in soil and pavement material properties required to support seasonal damage assessments in pavement design and analysis. Deflection basin indices or backcalculated elastic moduli can be used to describe pavement response as a function of seasonal factors such as temperature, moisture content, and state (frozen or unfrozen). One way of using seasonal deflection parameters is to infer seasonal “modular ratios” used to modify material properties for each season considered in the analysis. Figure 4-29 illustrates an example of the seasonal response of pavement layers as measured using deflection testing. The top part of Fig. 4-29a illustrates the pavement
FIGURE 4-29 Seasonal response of pavement layers as measured using deflection testing (a) pavement structure and progression of frost and thaw depths, (b) variation of the modulus of the asphalt-bound layer, and (c) variations in modulus of unbound pavement layers and subgrade soil (redrawn from St-Laurent 1995).
I n v e s t i g a t i o n a n d Te s t i n g structure as well as progression of frost and thaw depths during the monitoring season 1994 on Highway 352 in Quebec, Canada (St-Laurent, 1995). Figure 4-29b illustrates variation of the modulus of the asphalt-bound layer, which essentially follows the air temperature variations (not shown). Figure 4-29c illustrates variations in modulus of unbound pavement layers and subgrade soil (clay). The intensive monitoring program makes it possible to assess the reduction of elastic modulus of unbound materials and subgrade soils during spring thaw. Based on the example given in Fig. 4-29, the loss of stiffness (∆E) relative to summer modulus is equal to 36, 30, and 54 percent for the granular base, subbase and subgrade soil, respectively. Also, recovery periods extend over approximately 3 months for the base, 4 months for the subbase and nearly 5 months for the clay subgrade. This type of information is extremely valuable for seasonal pavement damage analysis described in Chap. 8. Monitoring pavement response and assessing elastic modulus of pavement materials and soils on a seasonal basis is a challenging task. Table 4-14 summarizes operational and analysis difficulties related to seasonal monitoring of pavements using the FWD and provides general recommendations to help prepare for that type of activity.
4-2-2 Evaluation of Pavement Functional Characteristics Using Longitudinal Profile Measurements Road surface roughness (smoothness) has a major influence on the comfort and safety of road users as well as on the cost of vehicle operation. It is therefore, an important indicator of road condition or functional service level. The main factors influencing roughness development are differential movements caused by heavy traffic and environmental factors. According to the American Society of Testing and Materials (ASTM E867) roughness can be defined as “the deviation of a pavement surface from a true planar surface with characteristic dimension that affects vehicle dynamics, ride quality, and dynamic loads.” This definition implies that roughness ensues from surface distortions with amplitude and length that affect vehicle dynamics and ride quality. Roughness is thus a function of the characteristics of pavement distortions but also of the characteristics of vehicle suspensions and the sensitivity of human body to vibrations. Two general approaches are used to measure pavement roughness. The first approach relies on the use of “response-type systems” to measure directly the reaction of a reference vehicle to road surface condition. Figure 4-30 illustrates the principle of a “ride meter” designed to measure the cumulative movement of the suspended mass (in this case, the chassis of a test trailer) relative to the axle. Other response-type systems use acceleration of the suspended mass or of the axle as main input for roughness assessment. These systems report pavement roughness in terms of a device specific index. The main advantage of response-type systems is their accessibility. Their main disadvantage is the fact that the response of the system is highly dependent on many factors including vehicle wear and temperature. It is, thus, necessary to frequently calibrate these systems to assure their reliability. The other approach for roughness assessment is the use of road profiling. In this approach, two steps are required to assess pavement roughness. The first step involves a detailed measurement of the longitudinal profile of the pavement surface (Fig. 4-31a). Generally, longitudinal profiles are recorded in both wheel paths for all traveling lanes
157
158 Summer and fall
Season
Specific Feature
Associated Difficulties
Recommendations
High and variable temperature of asphalt-bound materials
Analysis • Important variations in elastic modulus associated with temperature • Increasing influence of nonelastic behavior with temperature can make interpretation of deflection data more difficult • Significant variation of resilient modulus associated with moisture content of unbound materials
Testing should be done several times (3–5) after thaw recovery to capture variations in material properties during summer and fall
Operational • Signal processor and data acquisition equipment should be kept in operating conditions as specified by the equipment manufacturer • Hydraulic fluid should be replaced by light-weight synthetic fluid • Test should be conducted on ice and snow-free surfaces • FWD electromechanical components should be protected from brine sprays (where deicing chemical are used) and abrasives Analysis • Low deflection levels and large relative errors can cause convergence problems • Frost depth needs to be known to facilitate interpretation
At least one test should be done during winter conditions; ideally just before beginning of thawing (maximum frost depth and no thawed layer)
Variable water content in soils and unbound materials Cold and stiff asphalt-bound materials
Winter
Frozen soils and unbound pavement materials
Spring
Moderately cold asphaltbound layers
Saturated thawed unbound layer(s) possibly overlying a frozen layer
Operational • Same difficulties as in the winter can be encountered in early spring time Analysis • Separation between layers and sublayers with important difference in behavior: frozen, thawed saturated, and thawed drained • Nonelastic behavior of unbound materials and soils in thawed saturated (or nearly saturated) state
Monitoring of thaw depth is required to facilitate data interpretation At least one test per week should be done during the first month of thawing. More than one test a week should be considered when a detailed assessment is required or when thaw is progressing rapidly. One test every second week should be done during the recovery period Backcalculation models need to be able to handle several layers (≥6). Software assuming decreasing stiffness with depth should not be used
Source: compiled from St-Laurent 1995, Van Deusen 1996, and Schmalzer 2006.
TABLE 4-14
Guidelines for Seasonal Monitoring of Pavement Mechanical Parameters Using the Falling Weight Deflectometer
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FIGURE 4-30 Schematic illustration of a “ride meter.”
FIGURE 4-31 Principle of pavement profiling: (a) longitudinal profile measurement, (b) quarter-car simulation to assess roughness (IRI), and (c) detailed profile analysis.
of the highway. For practical pavement engineering applications, this is usually achieved using a “walking profilometer” (Fig. 4-32a) or an “inertial profilometer” (Fig. 4-32b). Measurements of profile should be made in accordance with ASTM E950. As illustrated in Fig. 4-31b, the second step involves the use of a mechanical model to compute a “standard vehicle” response index from the longitudinal profile. The mechanical model, referred to as the “quarter-car model” is designed to reproduce the reaction of a car traveling at 80 km/h on the pavement surface. The international roughness index (IRI) is a widely accepted index used to estimate the amount of roughness in a measured longitudinal profile. The IRI is computed from a single longitudinal profile using a quarter-car simulation as described by Sayers (1995). The IRI represents cumulative vertical displacement (m) of the sprung mass relative to the axle over the traveled distance (km). IRI is thus expressed in units of m/km or mm/m. Theoretically, it varies from 0 to more than 10. Experience shows that an IRI between 0.8 and 1.2 is usually achieved on new pavements. Several
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-32 Different types of profilometers available in the market: (a) the “walking profilometer” for project evaluation and (b) the “inertial profilometer” for network evaluation.
highway administrations have introduced IRI acceptance criteria for quality control of new pavements. Terminal roughness conditions for roads generally range between 3 m/km for freeways and 5 m/km for local roads. Most pavement management systems use trigger values based on IRI or other roughness indices to identify pavements requiring maintenance or rehabilitation treatments. Roads with IRI exceeding 6 m/km can be considered as very rough and only accessible at reduced speed. IRI provides an overall appreciation of the quality of the road surface over a given distance (usually 100 m). Detailed analysis of pavement conditions is, thus, difficult without going back to raw profile information. As illustrated in Fig. 4-31c, longitudinal profiles can provide extremely valuable information for identification of local anomalies and diagnostic of pavement condition. Figure 3-21 illustrates a good example of information that can be gathered from raw profiles. In this specific case, a 150-m pavement section has been surveyed during summer and winter. It should be noted that the two profiles have been separated by the average amount of frost heave observed at the site to facilitate analysis. A specific anomaly can be observed on the summer profile around the 25 m mark. This anomaly is considerably amplified during winter causing a 50-mm-high (from valley to crest) and approximately 5-m-long bump. This problem is likely the result of frost heave acting around a culvert. It can also be the result of a poor transition between different soil units, or pavement structures between the very smooth first 20 m and the rest of the section. Figure 3-21 also illustrates typical distortions caused by differential frost heaving. Several 10-m-long bumps can be observed on the winter profile indicating high sensitivity of the pavement system to frost action. This clearly demonstrates the usefulness of profile and roughness measurements to identify frost-sensitive pavements and quantify the level of frost action. The use of profiling is a relatively new approach for the identification of frostsensitive pavement sections and very little information is available in the literature on this approach. The Quebec Ministry of Transportation (MTQ) in Canada has been monitoring winter roughness on the provincial pavement network for about 10 years. Observations made by MTQ suggest that pavements exhibiting an IRI increase during winter exceeding 1.0 m/km are likely to have a poor long-term performance. Based on MTQ’s experience, ∆IRI values are specified as indicators (Table 4-15) for flagging frostsensitive pavement sections.
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Frost Sensitivity
Freeways, 100 km/h
Major Highways, 90 km/h
Local Roads, 50–80 km/h
Moderate
>1.1
>1.25
>1.5
High
>1.75
>2.00
>2.25
Source: MTQ, 2007, unpublished pavement engineering course material.
TABLE 4-15
∆IRI Values (m/km) Used by Quebec Ministry of Transportation as Indicator of Frost
Susceptibility
Advanced analysis of longitudinal profiles can lead to valuable information to support research and advance analysis of pavement condition. As described in more details in Sayers and Karamihas (1998), pavement profiles are signals that can be decomposed in sinusoids of different wavelengths. The signal can thus be filtered to highlight the effect of distortions of specific wavelengths. Public domain software is available for advanced analysis of pavement profiles. Among other, “ROADROUGH” (University of Michigan Transportation Research Institute) and PROVAL (Federal Highway Administration) offer several profile viewing, filtering, and analysis functions. Analysis of pavement profiles based on wavelength content can provide insight on causes and consequences of roughness deterioration. As a general rule, short wavelength distortions (<3 m) tend to be caused by phenomena occurring at shallow depth in the pavement system and can have an effect of user safety (OECD 1984). Longer wavelength distortions (>3 m) are typically the results of problems occurring at greater depths and tend to affect mainly user comfort. The 10-m-long distortions observed on winter profile illustrated in Fig. 3-21 is a good example of the manifestation of a problem (frost action) occurring at depth between 1.5 and 2.0 m in the pavement system. When used to study the effects of frost action in pavements, advanced analysis of profile and roughness data can provide interesting insight on the development and the cause of winter roughness. Figure 4-33 illustrates the result of research work done on two sections of Highway 367 in Quebec, Canada (Fradette et al. 2005). In this study, profile filtering has been used to highlight the effects of two different frost heave mechanisms acting on pavements. The study is based on profile measurements made every second week during winter and every week during spring on several test sections. As a first step, the raw profile is used to compute IRI (nonfiltered). The raw profile is then filtered to remove wavelengths smaller than 3 m (smoothed) and longer than 3 m (antismoothed). IRI is then recalculated using smoothed and antismoothed profiles. Nonfiltered and filtered IRI values are plotted as a function of time in conjunction with frost and thaw penetration which were also monitored during the study. It can be observed on Fig. 4-33a that nonfiltered IRI follows closely the pattern of antismoothed IRI. Moreover, IRI increase occurs very early during winter, and, in a large proportion, before frost depth reaches the frost susceptible subgrade soil. Similarly, IRI decreases rapidly while thaw progresses through the granular pavement structure. These observations suggest that the pavement is strongly affected by crack heaving problems as described in Sec. 3-8-3. Crack heaving typically cause short wavelength distortions that can cause an important increase in roughness during winter as indicated by
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-33 Using advanced profile analysis techniques in studying the development of winter roughness on two test sections in Canada. [Fradette et al. 2005 (Figure 4, p. 140, and Figure 5, p. 141) with permission from the Transportation Research Board.]
the observed ∆IRI of more than 2 m/km. Figure 4-33b illustrates a totally different frost heave pattern. In this case, non-filtered IRI follows closely the pattern of smoothed IRI and is somewhat independent of the evolution of anti-smoothed IRI. The dominant effect of long wavelengths in roughness development suggest that differential frost heaving from frost-susceptible subgrade soil (as described in Sec. 3-7-1) is the dominant mechanism on that pavement section. This hypothesis is supported by the fact that winter roughness develops mostly at the end of winter, when frost penetration reaches the frost-susceptible soil (ML).
4-2-3
Pavement Instrumentation
Pavement instrumentation can be considered as the “ultimate” way to gather information on pavement condition and response to load and climate induced stresses. Pavement instrumentation involves installation of sensors at strategic positions in the pavement during pavement construction or through boreholes in existing pavements. It also involves intensive data collection, treatment, management, and analysis. The type of instruments that can be used to collect information in pavements can be divided in two categories: load response measuring sensors and climate response sensors. Figure 4-34 illustrates the type of sensors that can be used in pavements and Table 4-16 summarizes information relevant to installation and the use of these sensors.
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FIGURE 4-34 Load response sensors and climate response sensors commonly used in pavement engineering (see Table 4-16).
4-3
Soils and Material Testing Materials used in cold region pavement structures generally include different types of unbound granular materials, asphalt concrete, and asphalt-stabilized granular materials. Together with the characteristics of traffic and the properties of a road foundation, the properties of pavement materials and the characteristics of the pavement layers are key parameters for pavement design. Mechanistic-empirical pavement design and analysis methods use pavement material properties to compute strains transmitted to the pavement foundation and to assess pavement damage in terms of fatigue, rutting, and roughness. This section will describe laboratory testing techniques with emphasis on characterization techniques related to cold region performance of soils and pavement materials. The following subsections will be included: • Testing of asphalt concrete (thermal cracking, stripping, fatigue, rutting, and durability tests) • Testing of pavement materials (Stiffness, durability and stability) • Soil testing (Stiffness and stability) General description of the investigation/testing technique with proper references to detailed descriptions and standards are included, as well as useful information for the interpretation and engineering use of the properties, allowable values, and links with relevant design methods.
4-3-1 Testing of Bituminous Pavement Materials Testing of bituminous pavement materials is needed in order to avoid deterioration of the bound pavement layer due to rutting, cracking, or disintegration as described in Chap. 3. The tests aid in material selection, volumetric mix design, and verification of the performance of pavement mixtures. The use of the tests is further explained in Chap. 7, where mix design of asphalt-aggregate mixtures is discussed.
Type of Sensor, Principle, and Application 1.
Horizontal (tensile) strain gauges Measurement of tensile strain using variable electric resistance or fibre optic based technologies, at the bottom of asphalt bound layers
2.
Vertical (compressive) strain gauges Measurement of compressive strain in unbound pavement layers and/or subgrade soil. Typically uses technologies based on induction loops or displacement sensors
3.
Multidepth deflectometer Measurement of displacements and vertical strains at selected levels. Typically uses technologies based on displacement sensors. If design of system allows, can be used to monitor frost heave
4.
Pressure gauges Measurement of vertical stress at selected level in unbound layers or in subgrade soil. Typically uses pressure induced by a fluid in a flat cell
Installation
Data Collection
• Generally installed on top of the granular base prior to paving • Gauges installed prior to paving often experience excessive stress due to high temperatures and compaction operations • Some gauge models can be retrofitted in existing asphalt layers
• Data should be collected under a moving reference vehicle (known characteristics) • Data collected during short periods at high sampling rate (≥500 Hz) to capture peak strains. This involves the use of advanced signal conditioner and data acquisition technology • The speed of the vehicle and the position of the wheel on the sensor need to be carefully controlled and measured as they have an important effect on sensor output • For seasonal variation assessment, the data collection should be repeated several times with the same reference vehicle (see recommendation for seasonal FWD testing in Table 4-13)
• Can be installed after placement and compaction of the layer for a new construction • Can possibly be retrofitted through a borehole if installation depth allows adequate installation • Precautions should be taken to assure proper compaction around the gauge • Installed through a borehole in existing pavement structures • Adequate reconstitution of pavement structure and material density between anchor plates is critical • For systems anchored on the wall of the borehole, good contact needs to be establish and precautions need to be taken to avoid disintegration of the wall • Generally installed at the desired level prior to placement of the overlying layer during pavement construction
165
TABLE 4-16 Synthesis of Information on Sensors Commonly Used in Pavement Engineering (Continued)
166 Type of Sensor, Principle, and Application 5.
Frost tube Measurement of frost and thaw depth. Uses a methylene blue solution to track phase change through a change in color of the solution
6.
Resistivity probes Measurement of levels of phase change using resistivity variations between copper rings installed along a rod. Resistivity of soil and unbound materials changes drastically with phase change of pore water
7.
Thermistor strings Measurement of temperature using temperature sensitive resistor technology. Thermistor strings are composed of several thermistors mounted on low thermal conductivity rod to allow for the measurement of thermal regimes and phase change in pavements
Installation
Data Collection
• Installed through a borehole in existing pavement structures • Requires a protective cover for manual access to the tube • Should ideally be installed at the center of the road to capture maximum frost depth • The casing of the tube needs to be carefully anchored underneath the maximum expected frost depth to avoid frost jacking • The casing of the tube needs to be sealed to avoid freezing of water inside the casing
• Manual reading only • Frequent reading needs to be done to capture frost and thaw evolution. Reading semi weekly during winter and weekly during spring thaw is adequate for most applications • Requires adequate signing and often lane closure for safe reading of the tube
• Installed through a borehole in existing pavement structures • The probe needs to be carefully anchored underneath the maximum expected frost depth to avoid frost jacking
• Manual reading most of the time, but can also me automated • Reading can be done from a junction box located on the side of the road • Frequent reading needs to be done to capture frost and thaw evolution. Reading biweekly during winter and weekly during spring thaw is adequate for most applications • Readings can be influenced by infiltration of brine from deicing chemicals through pavement cracks • Can be read manually but ideal for data logging • Several readings a day (>6) are recommended to capture daily variations of surface temperatures • Using 0°C as phase change indicator might be misleading if pavement materials and soil are contaminated by deicing chemicals
8.
TDR antennas Assessment of volumetric moisture content through measurements of dielectric constant of unbound materials and soils based using “time domain reflectometry” technology
9.
Suction probes Measurement of negative pore pressure (suction) in unsaturated pavement materials and soils. Can be related to moisture content through soil moisture retention characteristic curve
10.
Piezometer Measurement of water table level and of positive pore water pressure
TABLE 4-16 (Continued)
• Installed during construction of a new pavement structure or retrofitted in an existing pavement through a borehole • When installed in a borehole, the main difficulty is to properly assess dry density of the material surrounding the antenna. Errors in dry density estimation can lead to important errors in moisture content readings
• Manual or automated reading. • Measurements on a daily basis during spring thaw and on a weekly basis during the rest of the year are recommended • Contamination by deicing chemical can affect Tonnage distribution roster (TDR) readings
• Installed during construction of a new pavement structure or retrofitted in an existing pavement through a borehole
• Manual or automated reading • Measurements on a daily basis during spring thaw and on a weekly basis during the rest of the year are recommended
• Installed through a borehole in existing pavement structures • The tube needs to be carefully anchored underneath the maximum expected frost depth to avoid frost jacking
• Manual (observation of water level) or automated (water pressure) reading • If monitored manually, requires adequate signing and often lane closure for safe reading • Monitoring not possible when frost is present in pavement
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Tests for Material Selection Selection of the binder for hot mix asphalt (HMA) mixtures can be conducted using results from empirical tests described in Table 4-17 or from tests that measure fundamental properties used in performance based grading systems described in Table 4-18. Tests that aid in selection of aggregates for HMA are described in Table 4-19. Tests specific for cold regions that allow use of winter traction devices include aggregate toughness testing that simulates wear by studded tires in wet conditions. The Nordic abrasion test described in Table 4-19 is widely used in the Nordic countries and Alaska. Figure 4-35 shows the test equipment for the Nordic abrasion test.
Compacted Mixture Sample Preparation and Conditioning Gyratory compactor is used to prepare mixture samples for determining the mechanical and volumetric properties of asphalt-aggregate mixtures. The prepared samples simulate the density, aggregate orientation, and structural characteristics obtained in actual roadway when proper paving mix placement procedure is used (AASHTO T312). Adequate weight of aggregate is mixed with the binder at appropriate binder content and mixing temperature or adequate weight of mixture is collected from a plant to produce a sample of a desired size. The mixture is prepared typically at a temperature at which the kinematic viscosity of the unaged binder is 170 mm2/s. The temperature can be determined from the bitumen test data chart shown in Fig. 7-4. In the same way, compaction temperature is selected using 280 mm2/s equivalent viscosity. Before compaction the mixture is conditioned in an oven according to the mix design requirements. AASHTO R30 requires conditioning of 2 h in a forced-draft oven at the mixture’s compaction temperature for volumetric mix design test samples, and conditioning of 4 h at 135°C for test samples prepared for mechanical property testing. After the conditioning, the mixture is placed in a 150-mm-diameter mold that is positioned in the gyratory compactor at an angle (see Fig. 4-36 for a schematic picture of the test setup). A loading ram actuates a compaction pressure that is maintained during the test, while the bottom plate is rotated at a constant rate. The resulting kneading action is continued until a predetermined number of gyrations is achieved. The height of the specimen is recorded for each gyration and used in calculation of the specimen volume and uncorrected specimen bulk unit weight. After compaction the sample is extruded from the mold and its bulk specific gravity is determined. Using the determined mixture bulk (Gmb) and maximum specific gravities (Gmm) the relative density of the sample (Gmmx) at any gyration (x) can be determined from Eq. (4-15): %Gmmx =
Gmb hm × 100 % Gmm hx
(4-15)
where hm = height of the extruded specimen and hx = height of the specimen after x gyrations. The compacted mixture samples may then undergo long-term mixture conditioning. AASHTO R30 calls for five days conditioning in a forced-draft oven at 85°C.
Tests for Volumetric Mix Design The HMA volumetric mix design is covered in Chap. 7. In order to determine the volumetric mix design parameters, such as air voids in the mixture, specific gravities of the materials and the mixture need to be measured. The required specific gravities and their test methods are described in Table 4-20.
Test
Illustration
Description
Application
Penetration at 5°C and 25°C, measured in 0.1 mm ASTM D5
A needle with a weight is released and let penetrate 5 s into the asphalt sample at the test temperature
Consistency, stiffness, penetration grade classification
Softening Point, °C ASTM D36
Two discs of bitumen cast in brass rings are heated in a bath at 5°C/min while supporting a steel ball. The softening point is the temperature at which the two discs soften enough to allow each ball to fall 25 mm.
Drainability, plastic deformation, transition from viscoelastic to viscous behavior
Viscosity, 60°C, Pa·s ASTM D2171 AASHTO T202
A fixed volume of binder is placed in a viscometer that is placed in bath at the test temperature. The binder is let flow and the time is measured for the binder to flow past the two marks in the viscometer. The kinematic viscosity is then calculated using the time and a conversion factor. The dynamic viscosity at 60°C is determined similarly, but the binder is drawn up by means of vacuum
Plastic deformation, AC grade classification
Viscosity, 135°C, mm2/s ASTM D2170 AASHTO T201
169 TABLE 4-17
Binder Tests for Penetration Grade and AC Grade Specifications (Continued)
Constructability
170 Test
Illustration
Description
Application
Fraass breaking point, °C DIN 52012
A 0.5-mm-thick binder film is spread on a steel plate. The plate is cooled at 1°C/min and bent and straightened once a minute. The temperature at which the sample breaks is the Fraass breaking point in °C
Flexibility/brittleness at low temperatures
Thin film oven test ASTM D 1754 AASHTO T179
A 3-mm-thick film of asphalt cement is placed on a pan that is slowly rotated. The horizontal rotating does not cause the asphalt film to flow exposing more surface area. The oven has intake and outlet vents for natural air exchange. After the tests, mass change is reported
Aging simulation, work safety, smoking. The residue from these tests is used for additional rheometric tests (see Tables 7-2 and 7-3)
Rolling thin film oven test (RTFOT) ASTM D 2872 AASHTO T240
35 g of asphalt cement flows along the walls of a horizontally placed cylinder that is attached into a vertically rotating frame. The flowing asphalt exposes new surface continuously. Air is blown once into to the cylinders during each rotation. After the tests, mass change is reported
Flash point, °C ASTM D92 AASHTO T48
70 mL of binder is poured into a test cup. The temperature of the specimen is increased, while at specified intervals a test flame is passed across the cup. The flash point is the lowest liquid temperature at which the test flame causes vapors of the test specimen to ignite
Safety
Solubility in trichloroethylene ASTM D2042
A 2-g sample is dissolved in trichloroethylene and filtered. The insoluble material is washed, dried, and weighted. The percentage of soluble matter is calculated
Purity
TABLE 4-17
(Continued)
171
172 Test
Illustration
Description
Application
Viscosity measured with rotational viscometer, Pa·s ASTM D4402 AASHTO T316
A tube of asphalt cement is heated at the test temperature in a cylindrical chamber shown. A specified spindle is submerged into the sample and rotated. The torque required to keep the spindle spinning with the specified rate is measured, and the viscosity is calculated with the torque and the spindle dimensions
Constructability
Dynamic Shear (DSR), kPa AASHTO T315
The standard test procedure measures the complex shear modulus (G*) and phase angle (d) of asphalt binders with parallel plate test geometry. Binder sample is cast between parallel metal plates. One of the plates is oscillated with respect to the other at preselected frequencies and rotational deformation amplitudes. The complex shear modulus is the ratio of total shear stress to total shear strain. The phase angle, d, is related to the time lag between the shear and responding strain
Fatigue, plastic deformation
Bending beam rheometer (BBR) AASHTO T313
A small creep load is applied to a binder at a specified temperature and the deformation as a function of time is measured. The creep stiffness is calculated with the applied load and the beam dimensions
Direct tension tester AASHTO T314
A small “dog bone”-shaped asphalt cement sample is pulled at a slow, constant rate until it fails. The elongation at failure is used to calculate the failure strain, which indicates if a binder behaves in a brittle or ductile manner at low temperatures
Pressure aging vessel test (PAV) AASHTO R28
RTFOT residue (see Table 4-17) is exposed to high pressure and temperature for 20 h. The pressure aging apparatus consists of the pressure aging vessel shown and a forced draft oven. Three 50-g PAV samples are prepared on a pan for each binder tested and aged under 2070 kPa pressure either at 90, 100, or 110°C
1
Low temperature cracking1
Long-term in-service aging simulation
173
The critical cracking temperature method given in the AASHTO MP 1a and PP42 is obtained by first estimating the development of thermal stress using the BBR test results and then comparing the stress curve with binder’s tensile strength obtained from the DDT.
TABLE 4-18
Binder Tests for Performance Based Specifications (AASHTO MP 1a, 2003)
174
Chapter Four Test Method
Description
Application
Soundness ASTM C88 AASHTO T104
Aggregate sample is repeatedly immersed in saturated solutions of sodium or magnesium sulfate followed by oven dr ying. During the dr ying phase, the salt precipitates in the permeable pores of the aggregate. On reimmersion, the salt rehydrates and exer ts internal expansive forces that simulate the expansive forces due to freezing water. The test result is total percent loss over various sieve inter vals for a specified number of cycles (e.g., 5 cycles)
Durability— disintegration due to in-service weathering
Nordic abrasion EN 1097-9 (CEN 2006) ATM 312 (AKDOT&PF, 2005)
An aggregate sample (passing 16.0 mm sieve and retained on 11.2 mm sieve) is rotated in a standard mill (see Fig. 4-35) with 7 kg of 15-mmdiameter steel balls and 2 L of water for 1 h at 90 rpm. After the test the sample is sieved through a 2-mm sieve, and the ball mill value is defined as the percent passing the 2-mm sieve
Toughness— disintegration due to mechanical degradation during construction and in service
Los Angeles abrasion method ASTM C131 or 535 AASHTO T96
The test measures degradation of coarse aggregates (>2.36 mm) resulting from abrasion, impact and grinding in a rotating steel drum. The drum contains a specified number of steel spheres and shelf plates that pick up and drop the sample and the spheres. After the specified number of revolutions, the aggregate is sieved to measure the degradation as percent loss of material
Percentage of fractured faces ASTM D5821
The mass percent of coarse aggregate (>4.75 mm) with one or more fractured faces
Uncompacted void content of fine aggregate AASHTO T304
A sample of washed and dried fine aggregate (<2.36 mm) is poured into a small calibrated cylinder of known volume through a standard funnel. The void content of the fine aggregate is then calculated with the weight retained in the cylinder and aggregate’s bulk specific gravity
Flat and elongated particles ASTM D4791
The mass percent of coarse aggregate particles that have maximum to minimum dimension ratio >5. The particles are evaluated in a proportional caliber that divides the particles pass/no pass of the ratio requirement
TABLE 4-19 Test Methods Used in HMA Aggregate Evaluation
Angularity
Shape
Strength of asphalt concrete
I n v e s t i g a t i o n a n d Te s t i n g Test Method
Description
Application
Clay content (sand equivalent test) ASTM D2419 AASHTO T176
A sample of fine aggregate is mixed with a flocculation solution and agitated to loosen clayey fines. After a settling period, the cylinder height of suspended clay and settled sand is measured. The sand equivalent value is computed as the ratio of the sand to clay height, expressed as a percentage
Purity
Deleterious materials ASTM C142 AASHTO T112
The test measures the mass-% of contaminants, such as clay lumps, shale, wood, mica and coal in the blended aggregate. An aggregate sample is wet-sieved over specified sieves. The mass percent of material loss is reported as the percent of clay lumps and friable particles
TABLE 4-19 (Continued)
FIGURE 4-35 Nordic abrasion test equipment.
FIGURE 4-36 Schematic of gyratory compaction device.
175
176
Chapter Four Test Method
Description
Specific gravity of coarse aggregate ASTM C127 AASHTO T85
A sample of aggregate is immersed in water for 15 h, removed, surface dried, and weighed. The sample is subsequently weighed while submerged. Finally the sample is oven dried and weighed. The specific gravities are calculated as illustrated in Fig. 4-37
Bulk and apparent specific gravity of fine aggregate ASTM D128 AASHTO T84
Approximately 1 kg of fine aggregate is oven dried, covered with water and let stand 15–19 h. The sample is then dried on a flat sur face under a current of warm air and frequent stirring. A cone test is conducted periodically to determine when sur face-dr y condition is reached: a standard cone mold with its large diameter down in loosely filled with the fine aggregate and then lightly tamped with 25 tamper drops. The mold is lifted ver tically. When the aggregate ceases holding its molded shape, it is sur face dr y. At this point 500 g of aggregate is placed in a pycnometer with water and agitated to remove all air bubbles. The pycnometer is filled with water to its capacity, and the specific gravities are determined as illustrated in Fig. 4-38
Specific gravity of asphalt cement ASTM D70 AASHTO T228
The asphalt cement sample is placed in a pycnometer and weighed. The volume of the sample is obtained by filling the container level full of water and weighing in the air. The specific gravity of the asphalt cement is obtained with the same principle as illustrated in Fig. 4-38
Specific gravity of mineral filler ASTM D854 AASHTO T100
A sample containing natural moisture or oven dried is placed in a pycnometer and covered with distilled water. Entrapped air is removed either by applying a vacuum of 13.33 kPa or by gently boiling for at least 10 min while occasionally rolling the pycnometer. The pycnometer is filled to its capacity with distilled water, and the specific gravity is obtained as illustrated in Fig. 4-38 using the equation for apparent specific gravity. If a sample with natural moisture was used, the oven-dr y weight of the sample is determined at the end of the test
Theoretical maximum specific gravity of loose pavement mixture ASTM D2041 AASHTO T209
A sample of oven-dry and loose paving mixture is placed in a vacuum vessel and covered with water. Vacuum is applied for 15 min to gradually reach a suction pressure of 3.7 kPa and then gradually released. The volume of the sample is determined by filling the container full of water and weighing in the air, and the maximum specific gravity is obtained as illustrated in Fig. 4-38 using the equations for apparent specific gravity
Bulk specific gravity of compacted asphalt mixture ASTM D1188 / D2726 AASHTO T166
A sample of compacted mixture is oven dried and weighed. The sample is subsequently weighed while submerged. Finally the sample is surface dried and weighed. The specific gravities are calculated as illustrated in Fig. 4-37
TABLE 4-20
Specific Gravity Tests for Volumetric Mix Design
FIGURE 4-37 Principle of determination of specific gravity of paving materials with immersion method.
FIGURE 4-38 Principle of determination of specific gravities of paving materials with pycnometer method.
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Chapter Four
Tests for Moisture Damage Moisture damage in pavements is difficult to predict, although important in cold regions. Several test methods exist to evaluate pavements’ resistance against moisture damage, but none of them has obtained vast popularity. The tests could be divided in tests using loose asphalt-aggregate mixture and in tests using compacted asphaltaggregate mixture specimens. Common test methods are described in Table 4-21. The advantages of the tests on loose mixtures are simple equipment and procedures. However, the test results are qualitative and subject to interpretation. The test results also ignore the effects of traffic, climate, and mixture properties (Solaimanian et al. 2003).
Description
Static immersion test AASHTO T182
100 g of oven-dry aggregate (6.3–9.5 mm) is coated with bitumen at a specified mixing temperature and cured for 2 h at 60°C. After curing, the sample is mixed with spatula until it has cooled to room temperature. The aggregate is then immersed in distilled water at 25°C. After 16–18 h, the bitumen-aggregate mixture is evaluated under water. The total area of retained bituminous film is estimated visually being below or above 95%; below 95% denoting “failure” and above 95% denoting “passing”
Boiling test ASTM D3625
Rolling bottle
Loose mixture—screening tests
Test Method
Modified Lottman AASHTO T283
Immersion compression ASTM D1075 AASHTO T165
Marshall immersion test
TABLE 4-21
Compacted specimen
178
250 g or coated aggregate is immersed in boiling water. The water is brought back to boiling and maintained for 10 min. The water is decanted after cooling and the aggregate sample is spread on white paper towel. The amount of stripping is determined by visual inspection Aggregate chips are coated with binder and placed in glass bottles filled with water at 20°C. The bottles are then slowly rotated. The coverage is visually estimated after 5, 24, 48, and 72 h. An example of test results is shown in Fig. 4-39 Six samples are prepared to a target air void ratio of 7%. Three of the samples are tested as dry, and the other three after conditioning that exposes them to the effects of moisture. The conditioning consists of partial vacuum saturation followed by a freeze cycle and a 24-h thaw cycle in warm water at 60°C. The moisture sensitivity is the ratio of the average tensile strengths of the conditioned subset divided by the average tensile strengths of the control subset Eight samples are prepared to a target air void ratio of 6%. Four of the samples are tested as dry, and the other four after conditioning that exposes them to moisture. The conditioned samples are immersed in water at 49°C for 4 days or at 60°C for 24 h. The samples are then moved to a water bath at 25°C for 2 h and tested for compressive strength (deformation rate of 1.27 mm/min per 25 mm of height). The index of retained strength is reported The conditioning phase of the test is identical to the one used for immersion compression test. However, Marshall stability is used instead of compressive strength
Moisture Sensitivity Tests (Continued)
I n v e s t i g a t i o n a n d Te s t i n g Test Method
Description
Environmental conditioning system ECS
A specimen (102 mm in diameter and in height with 7.5% target air voids) is subjected to static immersion saturation for 5 min, enclosed within a membrane and placed in a resilient modulus (MR) test setup. Water at room temperature is circulated through the specimen for 1 h, after which the vacuum is released and the reference MR is measured. The specimen is then conditioned for 6 or 18 h by allowing water at 60°C flow through the sample while a compressive cyclic load is applied to the specimen. After 6 h, the circumference of the sample is measured. The process is stopped if the circumference has increased more than 2%, and the material is considered moisture susceptible. Otherwise the conditioning is continued for the remaining 12 h after which the specimen is let cool and the MR is measured again. If the MR ratio is, e.g., ≥0.8, the mixture is considered well-performing (Solaimanian et al. 2003)
Wheel testers
For example, Hamburg wheel tracking device: Two cylindrical gyratory compacted samples are immersed in water at 50°C. The device then applies rolling steel wheel passes on the cylinders until 20 mm of deformation is reached or a maximum of 20,000 passes. The test results are illustrated in Fig. 4-40. Research suggests that the stripping inflection point is higher than 10,000 for pavements that are moisture resistant (Solaimanian et al. 2003)
TABLE 4-21
(Continued)
FIGURE 4-39 An example of rolling bottle test results (courtesy of Per Redelius, AB Nynäs Petroleum).
179
180
Chapter Four
FIGURE 4-40 Example of Hamburg wheel tracking test results [Solaimanian et al. 2003 (Figure 7, p. 96); reproduced with permission of TRB].
Tests for compacted asphalt-aggregate mixtures comprise of preparation of two sets of samples. One set is conditioned dry to the testing temperature. The other set undergoes either immersion or more severe handling, such as vacuum saturation and freeze-thaw cycling. The Superpave mix design (a mix design method adapted recently by most of the states in the United States and also by other road agencies; see Chap. 7 for more information) evaluates the moisture sensitivity of compacted mixtures using indirect tensile strength test at 25°C (AASHTO T283). Use of Marshall stability ratio has been used in the past and is again regaining approval (Mostafa et al. 2006). Other methods that test the moisture susceptibility of compacted asphalt mixtures include different types of wheel tracking devices, where conditioned samples are exposed to a rolling wheel.
Mixture Performance Tests Pavement performance tests include tests for resistance against plastic deformation and fatigue cracking. These tests are described in Table 4-22 (see Chap. 7 for more information on performance testing). Performance tests that predict the pavement behavior in cold climates include tests for low-temperature cracking resistance and tests for resistance against wear by studded tires. Monismith et al. (1965) suggested that in order to predict the low-temperature cracking resistance of pavement mixtures, the thermally induced stress, strength, and temperature at failure could be measured in a laboratory test that simulates the conditions to which a pavement slab was subjected in the field. The basic requirement for the test system is that it maintains the test specimen at constant length during cooling. The thermal stress restrained specimen test (TSRST) specified, for example, by the AASHTO TP10-93 is the most recent version of this system (Jung and Vinson, 1994). The TSRST is shown in Fig. 4-41. A beam or cylindrical specimen is mounted in the load frame that is enclosed by the cooling cabinet. The chamber and specimen are cooled with vaporized liquid nitrogen. As the specimen contracts, linear variable differential transducers (LVDTs) sense the movement and a signal is sent to the computer that in turn causes the screw jack to stretch the specimen back to its original length. This closed-loop process continues
Test
Description
Application
Dynamic modulus ASTM D3497, variations are being developed
Cylindrical specimens are loaded with a uniaxial haversine stress pattern. Resulted strains are measured with LVDTs attached to the sides of the sample to calculate the dynamic complex modulus |E*| and the phase angle, f
Fatigue, plastic deformation, mix design, MEPD software
Wheel track1
A wheel moves back and forth (or one way) in an environmental chamber
Fatigue, rutting, research
Road simulator
The road simulators consist of a circular track loaded by a rotating wheel(s) with actual vehicle tire and tire pressure. The systems are enclosed in an environmental room with temperature and moisture control systems
181
TABLE 4-22
Illustration
Pavement Performance Tests (Continued)
182 Test
1
Illustration
Description
Application
Beam fatigue
A piston rod applies upward haversine load cycles with a rest period to an HMA beam. Downward load, approximately 10% of the upward load, is applied to force the beam back to its horizontal position and stay there during the rest period. The dynamic deflection of the beam at a midspan is measured with an LVDT. A range of stresses is used to establish the fatigue relationship at various test temperatures
Fatigue, research
IDT creep compliance and strength1
A static load is applied along the diametral axis of a specimen for a fixed duration of time. The vertical and horizontal deformations are measured near the center of the specimen and used to calculate a tensile compliance at a particular duration of time. The strength test is conducted immediately after the creep compliance test. Without releasing the creep load, a constant rate of vertical deformation is applied to the specimen until it fails
Thermal cracking in MEPD software (see Chap. 7)
Photo courtesy of U.S. Department of Transportation, Federal Highway Administration.
TABLE 4-22
(Continued)
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-41 (a) Schematic of TSRST system and (b) TSRST specimen after testing [after Jung and Vinson 1994 (Figure 1, p. 13); reproduced with permission of Transportation Research Board].
as the specimen is cooled and ultimately fails. Measurements of elapsed time, temperature, deformation, and tensile load are recorded with a data acquisition system. The thermally induced stress gradually increases as temperature decreases until the specimen fractures (see Fig. 2-24). At the break point, the stress reaches its maximum value, which is called the cracking strength, with a corresponding cracking temperature (see Fig. 2-24). Two laboratory tests exist to evaluate compacted asphalt-aggregate mixtures’ suitability for road use under-studded tire traffic: the Prall test and the PWR test. In the Prall test (EN 12697-16 Method A, CEN 2006), a cylindrical specimen (Fig. 3-12) having a diameter of 100 mm and a height of 30 mm is conditioned at 5°C and then hammered for 15 min with forty bouncing steel spheres. The steel spheres are bounced using a rotating counter force at 950 rpm (see Fig. 4-42). Water is circulated continuously at 5°C, which rinses the worn pavement particles out of the testing chamber. The loss of volume in cm3 is the Prall or abrasion value. It is defined as the ratio of the mass difference
FIGURE 4-42 Schematic of Prall device.
183
184
Chapter Four
FIGURE 4-43 PWR testing equipment. (Photograph courtesy of SR Consulting Ltd.)
of surface-dry water-stored specimens weighed in the air before and after the test to the bulk density of the specimen. In the PWR method (EN 12697-16 Method B, CEN 2006), three miniature-studded tires are rotating around a wet 100-mm Marshall mix design sample at 5°C for 2 h (see Fig. 4-43). The test result, an abrasion value, is the volume of lost mixture in cm3 during the test.
4-3-2
Soils and Unbound Materials
From the mechanistic point of view, the only property of soils and unbound pavement materials that matters is stiffness. Indeed, most mechanistic-empirical pavement design and pavement analysis models are based on elastic properties (resilient modulus and Poisson ratio) of soil and pavement materials. Although they can be improved using different types of treatments, soils are generally considered as a “given” in pavement engineering. Pavement materials are selected and modified to meet specific requirements. In addition to stiffness required to ensure proper load distribution, other properties are generally sought to ensure seasonal stability and long-term durability of mechanical
Property
Test (Standard) Purpose
Stiffness Direct measurement or estimation from simple test (CBR or stabilometer)
Resilient modulus and poison ratio (AASHTO T307) • Determination of the resilient modulus of soils and unbound granular materials based on cyclic triaxial testing for different stress states California Bearing Ratio (CBR) (AASHTO T193; ASTM D1883) • Determination of a material stiffness/strength index based on the load required to force penetration at constant speed of a piston in the sample Stabilometer or R-Value (AASHTO T190; ASTM D2844) • Determination of a material stiffness index based on the resistance of a confined compacted sample to induced lateral deformation using a pressurized fluid
TABLE 4-23
Tests for Soils and Unbound Materials (Continued)
185
186 Properties contributing to stiffness
Property
Test (Standard) Purpose Particle size distribution
Particle-Size Distribution (AASHTO T88, T27; ASTM D422, C136) • Determination of proportion of soil mass in each particle-size class. Soil sample is washed through a set of standard sieves and dry mass retained in each sieve is recorded
Density
Laboratory Compaction Characteristics (AASHTO T99, T180; ASTM D698, D1557) • Determination of the relationship between water content and dry density of compacted soil or material. Compaction tests are conducted for various water content using standard compaction energy and protocol. Dry unit weight is recorded as a function of water content
Particle shape Particle surface texture
Amount of flat and elongated particles • Determination of the massic proportion of particles meeting the flat and/or elongated particle criteria for the specified particle-size class Amount of crushed particles • Determination of the massic proportion of particles meeting the fractured face criterion for the specified particle-size class
Durability (Long term) Hardness
Los Angeles abrasion test (AASHTO T96; ASTM C131) • Determination of the massic proportion of the sample crunched to a specified particle-size class after rotation in a steel drum with a mixing blade in presence of steel balls
Micro-Deval abrasion test (AASHTO T327) • Determination of the massic proportion of the sample reduced by attrition to a specified particle-size class after rotation in a smooth steel drum in presence of steel balls and water
Durability
Soundness test (AASHTO T104; ASTM C88) • Determination of the massic proportion of the sample reduced by weathering to a specified particle-size class after cycles of exposure to magnesium sulfate crystallization and drying in an oven
Stability (Seasonal) Permeability and water retention
Constant head permeability test (AASHTO T215; ASTM D2434, D1557) • Determination of the permeability of pavement materials based on the measurement of a water flow induced through a soil sample (Permeability can be estimated from particle-size distribution) Suction test • Determination of water retention characteristics of soils and pavement materials
Frost susceptibility (segregation potential)
187
TABLE 4-23
(Continued)
Frost heave test • Measurement of frost heave rate or total frost heave resulting from a thermal gradient induced in a soil sample placed in a freezing cell (Frost susceptibility can be qualified based on particle-size distribution)
188
Chapter Four properties of these materials. Table 4-23 summarizes soil and material testing generally required for pavement engineering purposes.
Assessment of Soil and Material Stiffness Stiffness is a fundamental property of soils and unbound pavement materials for mechanistic design and analysis. Resilient modulus is now widely accepted as the best available parameter to characterize mechanical properties of unbound pavement materials and subgrade soils. It is one of the main parameters required to compute mechanical response (stresses, strains, and displacements) of pavements subjected to loading. Resilient modulus is essentially an elastic modulus measured in conditions representative of stress state and history experienced by unbound pavement materials and soils in a pavement system. It can be measured directly on an intact or reconstituted sample subjected to cyclic loading in a triaxial cell. It can also be estimated from the CBR test or from physical properties of soils and aggregates. By improving the quality of grain contacts, high densities increase the stiffness of unbound materials and the load distribution effectiveness. Particle-size distribution can be used to maximize the density of a given material while the density/water-content relationship obtained through the laboratory compaction test provides a reference for achievable field density. Particle shape and surface texture have a significant influence on the level of density of pavement materials and on the quality of grain contact. Flat and elongated particles tend to reduce density and stiffness of granular materials. They are also more prone to fragmentation, and therefore, their content should be limited. Crushed particles tend to increase internal friction in granular material. As a consequence, stiffness and strength tend to increase with increasing crushed particle content. However, they also resist compaction and, for equal compaction energy, density tends to be reduced with increasing crushed particle content.
Factors Contributing to Stiffness
Laboratory Measurement of Resilient Modulus The resilient modulus test method is designed to accurately represent the loading conditions of soils and materials while remaining simple and manageable. Resilient modulus testing is done in a triaxial cell using computer-controlled cyclic loading. As illustrated in Fig. 4-44, the compacted sample is placed between two loading platens and wrapped by a latex membrane. Water can flow freely to the sample base and head through porous stones attached to the platens. Confinement pressure is applied in the triaxial chamber using a pressurized fluid (water, oil, or air). Cyclic axial stress is applied to the sample using a loading piston attached to the top platen. The cyclic load pulse transmitted to the sample has a haversine shape with a 0.1-s loading period and a 0.9-s rest period (Fig. 4-45). After 500 to 1000 cycles of sample conditioning at the stress level specified in Table 4-24, the sample is submitted to a series of 100 load cycles for each stress level specified in Table 4-24. Axial strain is recorded in the center portion of the sample using two or three displacement transducers attached to the wall of the sample. The average of the five recovered strains, er is used to compute the resilient modulus MR according to Eq. (4-16): MR =
σd εr
(4-16)
where sd is the deviator stress = s1 − s3 (see Fig. 4-44). Resilient modulus is a mechanical property of soils and unbound pavement materials which strongly depends on the level of stress applied to the specimen. Resilient
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-44 Schematic illustration of the triaxial cyclic loading apparatus used to determine the resilient modulus of soils and unbound pavement materials.
FIGURE 4-45 (a) Loading conditions and (b) parameters used for the determination of the resilient modulus.
modulus test results are thus, usually reported as a function of stress state. The generalized constitutive equation proposed as part of the Mechanistic-Empirical Pavement Design Guide (M-E PDG; NCHRP 1-37A, ARA 2004) is as follows: k
θ 2 τ MR = k1 pa oct + 1 pa pa
k3
(4-17)
where MR = resilient modulus, MPa, q = bulk stress (s1 + 2s3), toct = octahedral shear stress = 31 (σ 1 − σ 2 )2 + (σ 1 − σ 3 )2 + (σ 2 − σ 3 )2 , pa = normalizing stress (atmospheric pressure), k1, k2, k3 = regression constants obtained by fitting resilient modulus test data to Eq. (4-17).
189
190
Chapter Four Granular materials Stresses, kPa
Soils Stresses, kPa Step
s3
sd
s3
sd
Conditioning
101.4
27.6
103.4
103.4
1
41.4
13.8
20.7
20.7
2
41.4
27.6
20.7
41.4
3
41.4
41.4
20.7
62.1
4
41.4
55.2
34.5
34.5
5
41.4
68.9
34.5
68.9
6
27.6
13.8
34.5
103.4
7
27.6
27.6
68.9
68.9
8
27.6
41.4
68.9
137.9
9
27.6
55.2
68.9
206.8
10
27.6
68.9
103.4
68.9
11
13.8
13.8
103.4
103.4
12
13.8
27.6
103.4
206.8
13
13.8
41.4
137.9
103.4
14
13.8
55.2
137.9
137.9
15
13.8
68.9
137.9
275.8
These loading conditions are specified in AASHTO T307 procedure for the determination of resilient modulus of soils and unbound pavement materials.
TABLE 4-24
Loading Conditions Specified in AASHTO T307 (from Tables 1 and 2 from T307 in Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 2003, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission.)
More simple constitutive relationships are also used to report resilient modulus testing results. Among others, the simple k-θ model is expressed as follows (Huang 2004): MR = k1θ k2
(4-18)
As described in Sec. 2-3 (Chap. 2), in addition to overburden stress and transient load stress, moisture content is also likely to induce internal negative pressure or matric suction, which also has an important influence on the resilient modulus. Figure 4-46 illustrates the effect of the degree of saturation on the resilient modulus of granular base materials as measured on some 20 materials sampled and tested at three levels of saturation as part of the Canadian Strategic Highway Research Program (C-SHRP) (Doucet and Doré 2004). Level of saturation is one of the important parameters related to seasonal change in material conditions. Resilient modulus should, thus, be seasonally adjusted by testing with various water contents going from saturation (spring conditions) to low saturation
I n v e s t i g a t i o n a n d Te s t i n g
800
200 Initial Saturated Drained
100
600 400 200 0 0
Initial Drained
150
M R – M R sat (MPa)
Resilient modulus, M R (MPa)
1000
50 0
–50 200
400 600 Bulk stress, (kPa)
800
–100
0
20
(a)
40 60 80 Degree of saturation (%)
100
(b)
FIGURE 4-46 (a) Influence of the level of saturation on the resilient modulus of a granular base material sample and (b) relationship between the level of saturation and the change in resilient modulus (from saturated conditions) for C-SHRP samples (Doucet and Doré, 2004).
levels (summer conditions). It is important to consider that saturation level will not fully explain seasonal changes in material behavior, which can also be affected by changes in density through freeze-thaw cycles as explained in Sec. 3-8-3 (Chap. 3).
Prediction of Resilient Modulus Resilient modulus can also be estimated from simple tests or physical properties of soil and unbound materials. The most commonly used approach is to estimate MR from CBR test results. Several correlations are proposed to link the CBR to MR. The following model is proposed for level 2 MR assessment in the M-E PDG (ARA 2004): MR (MPa) = 17 . 6 × CBR 0. 64
(4-19)
CBR testing is relatively simple and, being based on the measurement of a stress-strain relationship, it is somewhat related to resilient modulus. CBR value is the load required to force penetration of a 50-mm-diameter piston at a constant speed in a compacted sample expressed as a proportion (percent) of a reference load. The highest value between the load measured at 2.5 mm and the load measured at 5.0 mm is recorded at the CBR value. Despite several imperfections, CBR has the merit of being a measurement made specifically on the soil or material considered for construction. Resilient modulus can also be estimated based on stabilometer R-value based on the following model: MR(MPa) = 8.0 + 3.8·R
(4-20)
The stabilometer test is also a relatively simple test in which a relationship between applied stress and sample deformation is measured. The force is applied by increasing lateral confinement between 35 and 700 kPa for a sample subjected to a vertical confinement of 1120 kPa. The quantity of fluid required to increase the confinement pressure is used to measure deformation of the sample. Resilient modulus can also be estimated from soil or unbound material properties. Among several models proposed in the literature, the following models were proposed by Rahim and George (2005):
191
192
Chapter Four For fine-grained soils σd MR = k1 Pa 1 + 1 + σ c
k2
(4-21)
with LL k1 = 1 . 12(γ dr )1 .996 ω
0 . 639
and LL k2 = − 0 . 27(γ dr )1. 04 (ω cr )1. 46 P80 µm
0 . 47
and, for coarse grained soils θ MR = k1 Pa 1 + 1 + σ d
k2
(4-22)
with k2 = 0.12 + 0.90(gdr) – 0.53(wcr) – 0.017 (P80mm) + 0.314(logcu) and k2 = 0 . 226(γ dr ⋅ ω cr )
1 . 2385
P80 µm log(c ) u
0 . 1 24
where Pa = atmospheric pressure, sd = deviator stress, sc = confining stress, q = bulk stress, gdr = gd/gdopt (ratio of dry density to maximum proctor dry density), LL = liquid limit, w = water content, wcr = w/wopt (ratio of water content to optimum proctor water content), P80mm = passing 0.080 mm and cu = uniformity coefficient.
Assessment of Material Durability Long-term pavement performance requires that the integrity of granular particles remain over extended periods of time. Construction operations (handling, placement, and compaction) tend to provoke fragmentation of particles. Figure 4-47a illustrates an example of the evolution of the grain-size distribution during construction of a pavement with a schistose aggregate. From the aggregate production to final compaction in the pavement, a 40 percent increase in the passing 5 mm and a 100 percent increase in the passing 80 mm were observed. Repeated loading of the pavement structure by construction vehicles and heavy traffic during the life of the pavement induces wear of the aggregates. The Los Angeles abrasion test is a good indicator of the resistance of mineral particles to fragmentation, while the Micro-Deval abrasion test is an indicator of long-term resistance of particles to wear.
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-47 Evolution of the grain-size distribution of schistose granular materials exposed to (a) construction operations and to (b) weathering.
Long-term exposure to weathering cycles such as wetting-drying, freezing-thawing, and warming-cooling is also likely to induce stresses in aggregates and ultimately to break them. Figure 4-47b illustrates the effect of exposure to these cyclic events for a schistose embankment material. Embankment materials were sampled near a surface exposed to weathering for 20 years, from another surface exposed recently and from an unexposed layer of the embankment. It can be observed that the evolution of coarse particle size occurs very rapidly during the first months of exposure. The proportion passing 5 mm has doubled in the first 4 months and do not appear to evolve significantly thereafter. Small particles tend to evolve more progressively as shown by the 50 percent increase in passing 80 mm during the first 4 months and an additional 50 percent increase afterward. Despite some problems with the reliability of the soundness test, it can provide some valuable insight on the durability of pavement aggregates exposed to weathering cycles.
Assessment of the Seasonal Stability of Soil and Material As previously discussed in Chaps. 2, 3, and 4, the mechanical behavior of soils and unbound pavement materials is highly dependent on the material density and level of saturation. Seasonal stability of the mechanical behavior of these materials is essentially a function of the materials’ ability to remain at a low level of saturation and at a high level of density. Seasonal stability is thus a function of three properties: • Permeability • Water retention characteristics • Frost susceptibility (segregation potential) Permeability and water retention characteristics will control the ability of the material to effectively drain excess pore water and to rapidly reach a low and stable saturation level. Permeability can be measured using the constant head permeability test (AASHTO T215; ASTM D2434, D1557) or estimated from grain-size distribution. Water retention characteristics can be assessed using the pressure-plate test (Fig. 4-48). In this test, a soil sample is subjected to increasing pressure forcing water to drain out of the sample. The relationship between applied pressure and water content can thus be
193
Chapter Four
FIGURE 4-48 The pressure-plate test for measurement of water retention characteristics of a soil.
established to construct a water retention characteristic curve such as the one illustrated in Fig. 2-25. Matric suction in unbound pavement material can have an important effect on the stiffness of these materials. Figure 4-49 illustrates the results of suction measurements and resilient modulus tests on some 20 samples of granular base materials sampled on the Canadian Strategic Highway Research Program (C-SHRP) test sites. The study shows that negative pressures exceeding 20 kPa are generated in samples with low saturation levels and the resulting increase in resilient modulus from saturated conditions can exceed 100 MPa. This type of variation can be found in cracked or unpaved pavements between spring-thaw conditions and dry summer conditions. Segregation potential can be measured directly on an intact or reconstituted sample subjected to step freezing in a freezing cell. It can also be estimated from simple tests or from physical properties of soils. 200
100 80 60 40
Initial Saturated Drained
0.1 kPa; 88 %
100
– 2.4 kPa; 52 % – 10 kPa; 28 %
20 0 – 25
Initial Drained
150
M R – M R sat (MPa)
Degree of saturation (%)
194
50 0
– 50 – 20
–5 – 15 – 10 Matric suction (kPa) (a)
0
5
– 100 – 25
– 20
–5 – 15 – 10 Matric suction (kPa) (b)
FIGURE 4-49 Effect of saturation level on (a) matric suction and (b) on resilient modulus of granular base materials (Doucet and Doré 2004).
0
I n v e s t i g a t i o n a n d Te s t i n g Laboratory Determination of the Segregation Potential In the laboratory, the segregation potential of pavement subgrade materials is usually measured by freezing tests using step-freezing conditions, which simulate closely the freezing conditions of pavement subgrade soils. Under these conditions, frost penetrates at the selected rate and tends to stabilize at a certain level in the soil. Depending on the soil type, the soil is placed into the freezing cell using either the modified proctor procedure or soil consolidation. Undisturbed samples can also be tested with appropriate equipment. Several thermistors are inserted into a freezing cell wall and are in contact with the soil sample inside the cell. They are used to determine the temperatures regime in the sample throughout the test. The cell is fixed to a base plate in which thermal liquid can flow. A top plate is used to cover the specimen and thermal liquid can also flow in this plate. Porous stone and filter paper are placed at each end of the sample. Thermal liquid circulates through thermal baths in order to control the temperature at the base and the top of the sample. Temperatures of −4°C at the top of the sample and 2°C at the base of the sample are common for subgrade soils. A linear variable displacement transducer is fixed to the top plate shaft and installed on a fixed reference. To measure the segregation potential of pavement subgrade soils, an overburden pressure of approximately 20 kPa can be applied onto the sample in order to simulate the stress applied by a pavement structure of approximately 1 m. The sample is saturated using low hydraulic gradients to prevent fines migration in the sample. To perform the test, this burette is equipped with a mariotte to allow free flow of the water in the sample. Throughout the test, the temperatures at different depths within the sample and the frost heave are recorded at fixed intervals. A schematic illustration of frost cell is presented in Fig. 4-50. Typical results of a segregation potential test for St-Alban silty clay are presented in Fig. 4-51. To determine the segregation potential, the relationship between the frost front penetration in the sample and time must be plotted and the time to obtain steady-state
FIGURE 4-50 Frost heave cell.
195
196
Chapter Four
FIGURE 4-51 Typical results of a frost heave test on the St-Alban silty clay; (a) frost front depth versus time, (b) frost heaving curve, and (c) temperature gradients.
conditions must be determined as shown in Fig. 4-51a. Then, the heaving rate dh/dt must be determined at the time steady-state conditions are reached as shown in Fig. 4-51b. This is done by drawing the tangent line to the plot at that time. To determine the temperature gradient, the temperatures in the sample are plotted for different times. The temperature gradient is determined by drawing a tangent line at 0°C at the time of the steady-state conditions. The segregation potential of this sample from St-Alban is 141 mm2/°C·day.
Estimation of the Segregation Potential from Simple Tests or Soil Physical Properties Several authors have developed methods for estimation of the segregation potential from simple tests or physical soil properties. The following sections describe the methods developed by Rieke et al. (1983), Kujala (1991), Doré et al. (2004), and Konrad (2005). Rieke et al. (1983) performed a study on various combinations of sand, silt, and different types of clay in order to develop an empirical parameter that includes variables related to the soil fines fraction to estimate the segregation potential of soils. The fines percentages tested were 5, 10, and 20 percent and these fines were blends of silt, montmorillonite, and two types of kaolinite. Strong correlations were observed between the segregation potential and the specific surface area of the fines. In addition, a good correlation was also observed between the liquid limit of the fines and the specific surface area of the fines. The observations suggested that the segregation potential is dependent on the clay mineralogy of the soil. The segregation potential increases as the percentage of fines increases and decreases as the activity (defined as the liquid limit of the fines fraction divided by the % clay sizes in fine fraction) increases. The fines factor parameter Rf was suggested to estimate the segregation potential. Rf is defined by the
I n v e s t i g a t i o n a n d Te s t i n g
FIGURE 4-52 Estimation of the segregation potential with Rf.
equation given in Fig. 4-52, where % fines is the particles percentage with particle diameter < 75 mm and LLff is the liquid limit of the fine fraction. This relationship is presented in Fig. 4-52, where the segregation potential is plotted versus the fines factor, Rf . Kujala (1991) developed a predictive model for the segregation potential using two independent variables. Those are the volumetric water content wvol and the unfrozen water content a(T=−2.5°C). The model is presented in Fig. 4-53. Using these two independent variables, Kujala obtained regression coefficients ranging from 0.76 to 0.80. It was found that the unfrozen water content is the most influent variable on the segregation potential, since water flows through the partly frozen layer on the colder side of the zero isotherm. Doré et al. (2004) developed a method to estimate the segregation potential of various soils using simple tests including unfrozen water content measurements and the methylene blue test. The effect of unfrozen water content on the frost susceptibility is widely described in the literature (Kujala 1991). The unfrozen water is composed from adsorbed water to soil grains and capillary water. Thus, the segregation potential is related to adsorbed water content, which is related to soils specific surface, and to capillary unfrozen water. To perform an unfrozen water content test, Doré et al. (2004) use a 200-mm-high cylindrical sample (diameter = 101.4 mm). This sample is placed in a plastic mould using either dynamic modified proctor compaction or consolidation depending
197
198
Chapter Four
FIGURE 4-53 Prediction of segregation potential based on unfrozen water content (Kujala 1991).
on the soil type. The compacted soils are saturated from bottom up afterward. A porous plate and a filter paper are placed under the sample to ensure good water distribution and to prevent fines migration. The mould is entirely surrounded with circulating conduits connected to a programmable liquid bath and a thermistor is inserted into the sample, as shown in Fig. 4-54. A temperature conditioning of 1°C is applied to the sample until it has reached thermal stability. The sample is then cooled down at a rate of 0.01667°C/min. The unfrozen water content (UWC) is measured using ThetaProbes. UWC and temperature were recorded every 5 h. ThetaProbes measure the dielectric constant, which can be converted to the volumetric unfrozen water content of the sample
I n v e s t i g a t i o n a n d Te s t i n g
14
Unfrozen water content (Vallée-Jonction) DC +1
Dielectric constant
12 10 8 6 4 2 0 –6
–5
–4
–3 –2 –1 0 Temperature (°C) (b) Typical test results
(a) experimental setup
1
2
40
45
200
2
R = 0.7102
(
BV 0.6
–2.8273
(
160
SP (mm2 /°C*d)
SP = 56 894
x DC+ 1
120 80 40 0 0
5
10
15
20
25
30
35
0.6
* DC +1 /BV (c) Segregation potential
FIGURE 4-54 (a) Experimental setup for the unfrozen water content test, (b) typical results, and (c) correlation of test parameters with the segregation potential.
using appropriate calibration. Dielectric constant was, however, used directly in the test in order to avoid inducing errors through the calibration function. Typical results of dielectric constant as a function of temperature are presented in Fig. 4-54. Using unfrozen water content tests and methylene blue tests, Doré et al. (2004) developed the relationship given in Fig. 4-54 to estimate the segregation potential. In the relationship, SP(mm2/°C·day) is the segregation potential, g is the ratio of the dielectric constant measured at −2°C divided by the dielectric constant measured at 1°C, DC+1 is the dielectric constant measured at 1°C, and BV(cm3/g) is the blue value measured with the methylene blue test. All factors selected to build the relationship represent parameters that are physically linked to the segregation freezing process. DC+1 represents the total water volume available for freezing in the sample pores, while g represents the unfrozen water proportion in the frozen sample. Those two parameters are proportional to SP, since the available unfrozen water is the main path for water flowing in freezing soil. On the other hand, the BV value is inversely proportional to the segregation potential since it is related to the soil specific surface. A higher BV leads to a decreasing water flow channels, since water adsorbed by particles is fixed and cannot contribute significantly to water flowing. The standard error of the estimate of the relationship is 24.5 mm2/°C·day. This relationship is based on 21 measurements of unfrozen water content, and methylene blue tests on several subgrade soils sampled in various Quebec geologic conditions and ranging from silty clay to sand and gravel.
199
200
Chapter Four Konrad (2005) developed a methodology to estimate the segregation potential using the frost heave response of two reference soils. This methodology is based on Konrad’s (1999) demonstration that the segregation potential can be assessed from soil index properties that considers the grain-size distribution and the fines content, the clay mineralogy, the soil fabric, and the overburden pressure. According to his work, the segregation potential with no surcharge SP0 is related to the fines fraction (< 75mm), d50 of the fine fraction (FF), the specific surface of the fines fraction SS and the ratio of the material’s water content to its liquid limit, w/wL. These properties are related to water movements in capillary channels. It is suggested to use two reference soils used in the study performed by Rieke et al. (1983), which are sand-silt-kaolinite mixture with fines content of 20 percent. Using these soils, it can be observed that both SS and d50(FF) increase with increasing clay mineral content. Using the two reference soils, the following relationships suggest the reference characteristics: (1) For d50(FF) <1mm, SS ref = 25 . 95 SP0 ref = 489
m2 g
(4-23)
mm 2 ° C × day
(4-24)
(2) For d50(FF) >1mm, SS− ref = 25 . 95 − 11 . 78 × log(d 50 (FF))
(4-25)
SP0− ref = 489 − 232 × log(d50 (FF))
(4-26)
in which d50(FF) is expressed in mm. Good relationships were observed for w/wL ratios of 0.7 and >0.8. The following equations are proposed to characterize the frost heave response of fine-grained soils: (1) For SS/SS-ref <1, SP0/SP0 ref = SS/SS ref
(if w/wL = 0.7 ± 0.1)
SP0/SP0 ref = 0 . 08 + 1 . 42SS/SS ref
(4-27)
(if w/wL > 0.8)
(4-28)
(if w/wL = 0.7 ± 0.1)
(4-29)
(2) For SS/SS-ref >1, SP0/SP0 ref = (SS/SS ref )− 0. 85
SP0/SP0 ref = 1 . 5(SS/SS ref )−0 . 55
(w/wL > 0.8)
(4-30)
As a reference, Table 4-25 provides typical values of SP as a function of frost susceptibility classification.
I n v e s t i g a t i o n a n d Te s t i n g Frost Susceptibility Class
Segregation Potential, mm2/°C·day
Negligible
<12
Low
12–96
Moderate
96–192
High
>192
Source: Saarelainen 1996.
TABLE 4-25
Segregation Potential of Soils
Review Questions 4-1. (a) What is the apparent resistivity of the soil, if during a resistivity measurement 0.4 A circulates between electrodes (A and B, Fig. 4-8) at 10 m apart and a potential difference of 2 V is measured between inner electrodes (M and N) 2 m apart? What type of soil could it be? (b) The person performing the test misread at first the distance AB and used 100 m instead of 10 m for a previous test on the same soil. If the resistivity measured was 1900 Ω, what could one conclude? 4-2. A horizontal resistivity profile needs to be done. The current intensity used is 15 A and the inner electrodes are 6 m apart. For the first four tests, distanced of 15 m, the potential difference measured is 1, 4, 1.3, and 3.8 V, respectively. What are the corresponding electrical resistivities and what could one conclude about the soil type and configuration?
4-3. A seismic refraction survey has given the following results: Geophone Number
Distance from Impact Point, m
Time Required for the Waves to Reach the Geophones, s
1
20
0.150
2
40
0.300
3
60
0.322
4
80
0.345
5
100
0.366
6
120
0.385
What is the stratigraphy of the soil investigated and what types of soil are present?
4-4. A seismic refraction survey has given the following results: Geophone Number
Distance from Impact Point, m
Time Required for the Waves to Reach the Geophones, s
1
20
0.133
2
40
0.266
3
60
0.287
4
80
0.307
5
100
0.310
6
120
0.313
201
202
Chapter Four What is the stratigraphy of the soil investigated and what types of soil are present? Would the same results have been obtained if the topmost layer had been frozen?
4-5. A vane test has been conducted to estimate the bearing capacity of a soft soil. If a blade set of 120 mm of length and 60 mm of diameter is used and the minimum torque observed during the test was 12 N·m, what is the maximum thickness of the road embankment that can be supported by the soil? Consider the embankment material’s unit weight to be 22 kN/m3. 4-6. A light weight deflectometer was used to estimate the elastic modulus of a homogenous soil. The following results were obtained with a rigid plate of 75 mm of radius. If Poisson’s coefficient is 0.2, what is the elastic modulus of the soil?
so, kPa
d, mm
13
0.021
27
0.041
53
0.086
68
0.103
96
0.160
4-7. A falling weight deflectometer has been used to evaluate the pavement response to dynamic loading. The following data were acquired. Evaluate the surface curvature index, the base curvature index, the radius of curvature of the center of the basin, and the tensile strain if the asphalt-bound layer is 120-mm thick and the loading plate has a radius of 150 mm.
Distance, mm
0
150
300
450
600
750
Deflection, mm
225
212
201
185
153
120
Distance, mm
900
1050
1200
1350
1500
93
75
63
55
49
Deflection, mm
4-8. Estimate the resilient modulus of a subgrade soil, using data from Question 4-6, if the pavement total thickness is 600 mm. 4-9. An oven-dry sample of coarse aggregate has a mass of 2.4 kg. If the apparent specific gravity is 1.72 and the bulk specific gravity of the dry sample is 1.68, what is the bulk specific gravity of the saturated-surface-dry sample?
4-10. A pycnometer was used to evaluate specific gravities. The apparent specific gravity of the sample is 1.85. The weight of the sample and vessel full of water is 12.8 N. The pycnometer has a mass of 500 g and can contain 500 mL of water. What is the apparent volume of the sample? 4-11. The following results obtained from a resilient modulus test done in a triaxial cell.
I n v e s t i g a t i o n a n d Te s t i n g s1, kPa
s3, kPa
er
58
43
8.571E-05
73
43
1.604E-04
120
43
3.348E-04
40
25
1.351E-04
69
25
3.077E-04
135
25
5.116E-04
25
12
1.711E-04
52
12
3.960E-04
80
12
5.271E-04
Find the coefficients k1, k2 of the kq model [Eq. (4-18)] for the soil tested.
4-12. Considering the following grain-size distribution of a coarse-grained soil and s1 and s3 data from Question 4-11, find the resilient modulus of the soil, using Eq. (4-21). Use gd/gopt = 90 percent and w/wopt = 75 percent. Sieve Opening, mm
Passing, %
31.5
100
20
96
14
83
10
69.6
5
50
2.5
39.2
1.25
30.4
0.63
21
0.315
13.8
0.16
9.8
0.08
5
4-13. The 50 percent passing diameter of the fine fraction of a soil is 5 mm. Considering a specific surface of 0.015 km2/kg and a ratio w/wL to be 0.85, in what frost susceptibility class is the soil? What would be the frost heave rate if the thermal gradient was 1°C/m? If the ratio was 0.8 instead of 0.85, would it be in the same class?
4-14. The segregation potential of a soil has been estimated to be 12 mm2/°C·day. If the ratio of the material’s water content to its liquid limit is 0.82 and the 50 percent passing diameter of the fine fraction is 0.8 mm, what is the specific surface of the fine fraction?
203
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Chapter Four
References AASHTO (2003). June 2003 Edition of AASHTO Provisional Standards, American Association of State Highway and Transportation Officials, Washington, D.C. AKDOT&PF (2005). Alaska Test Methods Manual. Alaska Department of Transportation and Public Facilities. Juneau, Alaska. Allard, M., Lévesque, R., Séguin, M. -K., and Pilon, J. A. (1991). “Les caractéristiques du pergélisol et les études préliminaires aux travaux de génie au Québec nordique (Permafrost characteristics and preliminary studies for engineering work in northern Quebec),” Report for Quebec Ministry of Transportation, Centre d’études nordiques, Université Laval, p. 94 (in French). American Geological Institute (1976). Dictionary of Geological Terms, Anchor Books, New York, New York, p. 471. ARA Inc. (2004). “Guide for the Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures,” Final report, NCHRP 1-37A. Transportation Research Board of the National Academies, Washington, D.C. Atkins, H. N. (2003). Highway Materials, Soils, and Concretes, Prentice Hall, United States, p. 383. Avery, T. E. (1977). Interpretation of Aerial Photographs, 3d ed., Burgess Publishing Company, Minneapolis, Minn., p. 392. Boutet, M. (2007). “Élaboration de modèles mathématiques pour l’interprétation des données obtenues avec le pénétromètre à cône dynamique (Development of mathematical models for the interpretation of dynamic cone penetrometer data),” Master thesis, Civil Engineering Department, Laval University, Quebec City, Canada (in French). Brown, R. J. E. (1974). “Some Aspects of Airphoto Interpretation of Permafrost in Canada,” Technical paper no. 409 of the Division of Building Research, National Research Council of Canada, Ottawa, Canada. Budhu, M. (2000). Soil Mechanics and Foundations, John Wiley & Sons, Hoboken, New Jersey, p. 586. CEN (2006). Standards and Drafts, European Committee for Standardization, http:// www.cenorm.be/cenorm/index.htm (December 14, 2006). Conseil National de Recherche Canada. (1988). “La terminologie du pergélisol et notions connexes (Permafrost terminology and related notions),” Note de service No. 142, Ottawa, Canada. Doré, G., Pierre, P., Abdelwahab, A. I., Juneau, S., and Bilodeau, J. P. (2004). “Développement d’un essai simple et rapide pour l’estimation du potentiel de segregation (Development of a simple and rapid test for the estimation of the segregation potential),” Research report GCT-2004-14, Civil Engineering Department, Laval University, Quebec City (in French). Doucet, F., and Doré, G. (2004). “Module réversible et coeffcient de poisson réversible des matériaux granulaires C-LTPP (Resilient modulus and resilient poison coefficient of the C-LTPP granular materials),” Proceedings of the Annual Conference of the Canadian Geotechnical Society, Canadian Geotechniocal Society, Quebec City, Canada (in French). Fradette, N., Doré, G., Pierre, P., and Hébert, S. (2005). “Evolution of the Pavement Winter Roughness,” Transportation Research Record: Journal of the Transportation Research Board, No. 1913, Transportation Research Board of the National Academies, Washington, D.C., pp. 137–147.
I n v e s t i g a t i o n a n d Te s t i n g Fredlund, D. G., and Rahardjo, H. (1993). Soil Mechanics for Unsaturated Soils, Wiley InterSciences. Hoboken, New Jersey. Gagnon, H. (1974). La photo aérienne (Aerial photography), Les éditions HRW, Montréal (in French). Grenier, S. (2007). “Analyse dynamique du déflectomètre à masse tombante (Dynamic analysis of the falling weight deflectometer),” Ph.D. thesis, Laval University, Civil Engineering Department. Quebec City, Canada (in French). Haas, R. (1997). Pavement Design and Management Guide, Transportation Association of Canada, Ottawa, Canada, p. 389. Hoekstra and McNeill (1973). “Electromagnetic Probing of Permafrost,” Permafrost: North American Contribution to the Second International Conference, National Academy of Sciences, Washington, D.C., pp. 517–527. Horak, E. (1987). “The Use of Surface Deflection Basin Measurements in the Mechanistic Analysis of Flexible Pavements,” Proceedings of the Sixth International Conference on the Structural Design of Asphalt Pavements, vol. 1, International Society for Asphalt Pavements, White Bear Lake, Minn., pp. 990–1001. Huang, Y. H. (2004). Pavement Analysis and Design. 2d ed. Pearson Prentice Hall, Upper Saddle River, New Jersey. Janoo, V. C., and Berg, R. L. (1990). “Thaw Weakening of Pavement Structures in Seasonal Frost Areas,” Transportation Research Record: Journal of the Transportation Research Board, No. 1286, Transportation Research Board of the National Academies, Washington, D.C., pp. 217–233. Jung, D., and Vinson, T. (1994). “Thermal Stress Restrained Specimen Test to Evaluate Low-Temperature Cracking of Asphalt-Aggregate Mixtures,” Transportation Research Record: Journal of the Transportation Research Board, No. 1417, Transportation Research Board of the National Academies, Washington, D.C., p. 13. Jung, F. W. (1988). “Direct Calculation of Maximum Curvature and Strain in Asphalt Concrete Layers of Pavements from Load Deflection Basin Measurements,” Transportation Research Record: Journal of the Transportation Research Board, No. 1196, Transportation Research Board of the National Academies, Washington, D.C., pp. 125–132. Knutsson, S., Domaschuk, L., and Chankler, N. (1985). Analysis of large scale laboratory and in situ frost heave tests, Fourth International Symposium on Ground Freezing, Sapporo, Japan, Kinosita, S. and Fukuda, M. (eds.), pp. 65–70. Konrad, J. -M., and Morgenstern, N. R. (1982). Prediction of frost heave in the laboratory during transient freezing, Canadian Geotechnical Journal, vol. 19, no. 3, pp. 250–259. Konrad, J. -M., and Morgenstern, N. R. (1983). Frost susceptibility of soils in terms of their segregation potential Prediction of frost heave in the laboratory during transient freezing, Proceedings of the Fourth International Conference on Permafrost, National Academies Press, Washington, D.C. Konrad, J. -M. (1999). “Frost Susceptibility Related to Soil Index Properties,” Canadian Geotechnical Journal, vol. 36, pp. 403–417. Konrad, J. -M. (2005). “Estimation of the Segregation Potential of Fine-Grained Soils Using the Frost Heave Response of Two Reference Soils,” Canadian Geotechnical Journal, vol. 42, no. 1, pp. 38–50. Kujala, K. (1991). “Factors Affecting Frost Susceptibility and Heaving Pressure in Soils,” Acta Universitatis Ouluensis, Series C 58, Oulu, Finland.
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Chapter Four Ladanyi, B. (1996). “La conception et la réhabilitation des infrastructures de transport en régions nordiques (Design and rehabilitation of transportation infrastructure in northern regions),” Études et recherches en transports, RTQ-94-07, Gouvernement du Québec, Ministère des Transports, p. 123 (in French). Lambert, J. P., Fleming, P. R., and Frost, M. W. (2006). “Laboratory Assessment of Coarse Granular Road Foundation Materials,” TRB 2006 Annual Meeting CD-ROM. Transportation Research Board of the National Academies, Washington, D.C. LCPC (1982). “Reconnaissance géologique et géotechnique des tracés de routes et autoroutes (Geological and geotechnical survey of highway alignments),” Note d’information technique, Ministère de l’urbanisme et du logement—Ministère des Transports, Paris, France, p. 111 (in French). Lo, C. P. (1976). Geographical Application of Aerial Photography, Crane, Russak & Company, N.Y., David & Charles, London, p. 330. Loudon, A. A., and Partners (1995). Cold Deep in Place Recycling: Technical Recommendation and Application Specifications, South Africa. McCarthy, D. F. (1998). Essentials of Soil Mechanics and Foundations Basic Geotechnics, Fifth edition, Prentice Hall, New Jersey. Mollard, J. D., and Janes, J. R. (1985). “Airphoto Interpretation and the Canadian Landscape,” Dept. of Energy Mines and Resources, Canada, p. 415. Monismith, C., Secor, G., and Secor, K. (1965). “Temperature Induced Stresses and Deformations in Asphalt Concrete,” Journal of the Association of Asphalt Pavement Technologists, vol. 34., White Bear Lake, Minn. Morin, P. (1994). Manuel canadien d’ingénierie des foundations (Canadian Foundation Engineering Manual), Seconde édition, Société canadienne de géotechnique, Richmond, Canada, p. 558 (in French; also available in English). Mostafa, A., Abd El Halim, A. O., Easa, S., and Niazi, Y. (2006). “Suitable Test Method for Predicting Effect of Stripping on Mechanical Properties of Canadian Pavements” Proceedings of the Tenth International Conference on Asphalt Pavements, International Society for Asphalt Pavements, White Bear Lake, Minn., vol. 2, pp. 562–571. OECD (1984). “Surface Characteristics of Pavement Surfaces, Their Interaction and Their Optimization,” Research in Pavements and Transportation, Organisation for Economic Co-operation and Development, Paris. Paré, J. J., Lavallée, J. G., and Rosenberg, P. (1978). Frost penetration studies in glacial till on the James Bay hydroelectric complex, Canadian Geotechnical Journal, vol. 15, no. 4, November, pp. 473–493. Penner, E., and Ueda, T. (1977). The dependence of frost heaving on load application, Proceedings of Frost action in soils 1, University of Lulea, Sweden. Phukan, A. (1985). Frozen Ground Engineering, Prentice-Hall International Series on Civil Engineering and Engineering Mechanics, Upper Saddle River, New Jersey, p. 336. Rahim A. M., and George K. P. (2005). Models to estimate subgrade resilient modulus for pavement design, The International Journal of Pavement Engineering, Taylor & Francis, Oxfordshire, U.K., vol. 6, no. 2, pp. 89–96. Riddle, C. H., and Hardcastle, P. K. (1991). “Drilling & Sampling of Permafrost for Site Investigation Purposes: A review,” AIME International Arctic Technology Conference, Anchorage, Alaska, 29-31 May, Journal of Society of Petroleum Engineers, pp. 611–620. Rieke, R., Vinson, T., and et Mageau, D. (1983). “The Role of Specific Surface Area and Related Index Properties in the Frost Heave Susceptibility of Soils,” Proceedings of the Fourth International Conference on Permafrost, National Academies Press, Washington, D.C., pp. 1066–1071.
I n v e s t i g a t i o n a n d Te s t i n g Rhode, G. T. (1994). Determining a Pavement Structural Number from FWD Testing, TRB 73rd Annual Meeting, Preprint no. 940351, Transportation Research Board, Washington, D.C. Saarelainen, S. (1996). “Pavement Design Applying Allowable Frost Heave,” Proceedings of the Eighth International Conference on Cold Regions Engineering, ASCE Press, Reston, Va. Sayers, M. W. (1995). “On the Calculation of IRI from Longitudinal Road Profile,” Preprint TRB 74th Annual Meeting, Washington, D.C., January 1995. Sayers, M. W., and Karamihas, M. (1998). The Little Book of Profiling, University of Michigan, Transportation Research Institute, http://www.umtri.umich.edu/ content/LittleBook98R.pdf (July 25, 2007). Schmalzer, P. N. (2006). LTPP Manual for Falling Weight Deflectometer Measurements, V 4.1, FHWA-HRT-06-132, Federal Highway Administration, Office of Infrastructure Research and Development, McLean, Va. Solaimanian, M., Harvey, J., Tahmoressi, M., and Tandon, V. (2003). “Test Methods to Predict Moisture Sensitivity of Hot-Mix Asphalt Pavements,” Proceedings of Moisture Sensitivity of Asphalt Pavements, A National Seminar, San Diego, California, TRB, February 4–6, p. 96. St-Laurent, D. (1995). “Évaluation structurale de chausses souples dans un contexte climatique nordique (Structural evaluation of flexible pavements in cold climate),” Rapport GCS-85-05, Civil Engineering Department, Laval University, Quebec City, Canada (in French). Sylwester, R. E., and Dugan, B. (2002). “Evaluation of Geophysical Methods, Field Program,” Report No, FHWA-AK-RD-02-07, Alaska Department of Transportation and Public Facilities, Juneau, Alaska. Todd, D. K. (1980). Ground Water Hydrology, John Wiley & Sons, Inc., Hoboken, New Jersey. Van Deusen, D. A. (1996). Selection of Flexible Backcalculation Software for the Minnesota Road Research Project, Final report, Minnesota Department of Transportation, St Paul, Minn. Von Quintus, H. L., and Simpson, A. L. (2002). “Back Calculation of Layer Parameters for LTPP Test Sections, Volume II: Layered Elastic Analysis for Flexible and Rigid Pavements,” Report FHWA-RD-01-113. Federal Highway Administration, Washington, D.C.
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CHAPTER
5
Calculation of Engineering Parameters
D
esigning a structure that extends over large distances across several geologic and climatic environments is a major challenge for pavement engineers. Prior to pavement construction, natural soils are in balance with their environment. The construction of the pavement structure will unavoidably disrupt this balance by modifying temperatures, moisture, and stress regimes in the natural soil. The following principles are proposed as general guidelines when undertaking a pavement project in cold climates: 1. Minimize disruptions of natural soil conditions. 2. Always strive for more stable conditions. 3. Minimize spatial and temporal variations in soil and material properties. Important environmental effects on pavement systems have been described in the previous chapters. It is important to take into consideration all of those factors while preparing a pavement project in cold regions. However, not all of the factors need to be implicitly taken into account in engineering calculations. This section proposes a list of basic parameters that need to be obtained or calculated in the design process. They include: • Air temperature, freezing and thawing indices (Sec. 5-1) • Surface temperature, freezing and thawing indices (Sec. 5-2) • Representative temperature in hot mix asphalt (HMA) layer (Sec. 5-3) • Thermal properties of soils and pavement materials (Sec. 5-4) • Freezing and thawing indices within the pavement structure (Sec. 5-5) • Frost and thaw depth (Sec. 5-6) • Frost heave (Sec. 5-7) • Thaw settlement for pavements in permafrost areas (Sec. 5-8) • Stresses and strains at critical pavement interfaces (Sec. 5-9) With modern computer capacity, most of these parameters can be calculated using sophisticated computerized methods. This section proposes simple calculations methods that can be used manually for estimation and verification purposes.
209
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Chapter Five When available, computer programs allowing for detailed calculation of the parameters will be identified.
5-1 Air Temperature and Air Freezing and Thawing Indices Most pavement engineering applications require the surface temperature as a boundary condition parameter. However, information on surface temperature is rarely available and the parameter has to be estimated from a widely available climatic parameter: air temperature. As described in Sec. 2-1, excluding changes due to alternating climatic systems, air temperature follows daily and seasonal temperature cycles. Figure 5-1 illustrates temperature data taken in Quebec City between February 1st and 16th, 2004. The figure illustrates hourly temperature data, daily averages, and medium term trend. A nice daily pattern can be observed on February 5th: the minimum temperature was −16.6°C at 8:00 hours and the maximum temperature of −8.9°C was reached at 15:00 hours. The average daily temperature during that day was −12.9°C. When to use each of these parameters? The relevant air temperature parameters to be considered for each application are given in Table 5-1. Mean air temperature: Mean air temperature for a given period can be obtained by averaging mean daily temperatures over the period considered: MATt =
n
1 MDAT n∑
(5-1)
t=1
where MDAT is the mean daily air temperature and n is the number of days in the period considered (see Example 5-1).
FIGURE 5-1
Air temperature data for 15 days in February 2004 in Quebec City (Canada).
Calculation of Engineering Parameters Parameter
Application
Mean air temperature (MAT) Mean annual air temperature (MAAT)
• Estimation of the yearly temperature variation • Estimation of the boundary condition at the bottom of the pavement system (Tb ≈ MAAT) • Indication of presence of permafrost (if MAAT <0°C) • Indication of the risk of permafrost degradation (if MAAT >0°C)
Maximum air temperature
• Selection of asphalt cement grade • Prediction of asphalt concrete rutting tendency
Minimum air temperature
• Selection of asphalt cement grade • Prediction of asphalt concrete cracking tendency
Maximum daily cooling rate
• Prediction of asphalt concrete thermal cracking performance
Freezing index
• Prediction of frost depth and frost heave
Thawing index
• Prediction of thaw depth and thaw consolidation in permafrost conditions • Prediction of thaw depth in seasonal frost conditions
TABLE 5-1
Air Temperature Parameters Used in Pavement Engineering
Example 5-1 Considering the data shown in Fig. 5-1, the mean air temperature for the 15 day period is MAT15 =
1 15 MDAT 15 ∑ 1
MAT15 =
− 147 . 92 °C = − 9 . 86 °C 15
Mean annual air temperature: It is a climatic parameter which can usually be found in climatic databases. For many applications, the MAAT should be averaged over a number of years in order to obtain a representative value. When several years of data are used to assess MAAT, a probabilistic approach using probability functions and standard deviation of MAAT can be used to assess the probability of extreme values of MAAT over a given analysis period. By definition, MAAT is an historical value that can lead to errors when estimating future climatic conditions in a context of climate change. The use of historical temperature data for design without consideration for local warming trend can lead to unconservative designs. A probabilistic assessment of the evolution of climatic parameters over the design life of transportation facilities is now a must in cold region pavement engineering. Cooling rate: The air cooling rate is an important factor to take into consideration when analyzing thermal contraction and related stress development in asphalt concrete. The cooling rate can be readily obtained from hourly air temperature data by subtracting each hourly temperature by the previous hourly temperature. The result can be plotted as shown in the top part of Fig. 5-1. It can be seen from the plot that the maximum cooling rate was obtained at the end of the day on February 4th and exceeded 3°C/h. It can also be noted that a less intense, but more persistent cooling occurred on the 14th leading to the lowest temperature during the 15 day period. Air freezing and thawing indices from daily temperature data: Air freezing index (FIa) and air thawing index (TIa) are two widely used climatic parameters for the quantification
211
212
Chapter Five
FIGURE 5-2 Schematic illustration of the freezing and thawing indices.
of the “severity” of a winter or of a summer with respect to freezing or thawing effects on pavements. The indices can be defined as the area between the MDAT curve and the 0°C line over a given period of time (usually one year). Figure 5-2 is a schematic illustration of the freezing and the thawing indices. The freezing and thawing indices can be defined mathematically as follows: t
FI a = ∫ −T− dt 0
(5-2)
or t
FI a = ∑ − MDAT− 0 t
TI a = ∫ T+ dt 0
(2-3)
or t
TI a = ∑ MDAT+ 0
where T− and T+ are temperatures below and above 0°C, respectively, MDAT and MDAT are mean daily air temperatures below and above 0°C respectively, and t is the period of time considered (see Example 5-2). Example 5-2
Considering the data shown in Fig. 5-1, the freezing index for the 15 day period is t
FI a = ∑ −MDAT− = 148 °C ⋅ days 0
The freezing index corresponds to the area between the 0°C line and the MDAT line in Fig. 5-1. It also roughly corresponds to the area between the trend line and the 0°C line.
Calculation of Engineering Parameters
FIGURE 5-3 Sinusoidal representation of the temperature relationship with time.
Air freezing and thawing indices from annual summary temperature data: When daily temperature statistics are not available for a specific site, air freezing and thawing indices can be estimated using the assumption that air temperature is following a sinusoidal relationship with time. Two parameters are used to estimate the sinusoidal curve: the mean annual air temperature (MAAT) and the maximum amplitude of the sinusoid A0 (see Fig. 5-3). Based on the assumption, the air temperature “Ta” at any point “t” in time can be obtained from (Zarling and Braley 1988): 2π t 2πφ Ta = MAAT − A0 cos − p p
(5-4)
where p is the period of time considered (365 days) and f is the phase lag (note that the cosine and sine arguments in Eqs. (5-4) to (5-8) are expressed in radians). The beginning of the thawing, t1, and the freezing, t2, seasons can readily be obtained from Eq. (5-4) by setting Ta = Tf (freezing temperature) in the equation (Zarling and Braley 1988): MAAT − T f p cos −1 +φ 2π A0
(5-5)
MAAT − T f p 2π − cos −1 + φ 2π A0
(5-6)
t1 =
t2 =
It should be noted that the phase lag factor, f, can be used to adjust the timing of t1 and t2 as well as the occurrence of the maximum and minimum temperature during the
213
214
Chapter Five year. Having estimated the air temperature function, freezing, and thawing indices can be obtained from (Zarling and Braley 1988): FI a =
365+ t1
∫
t2
2π t 2πφ T f − MAAT + A0 cos p − p dt
or
(5-7)
pA0 FI a = (T f − MAAT)( p + t1 − t2 ) + 2π
2π ( p + t1 − φ ) 2π (t2 − φ ) − sin sin p p
and t
2 2π t 2πφ TI a = ∫ MAAT − A0 cos − − T f d t p p t1
or
(5-8)
TI a = (MAAT − T f )(t2 − t1 ) −
pA0 2π
2π (t2 − φ ) 2π (t1 − φ ) − sin sin p p
Example 5-3 illustrates the determination of freezing and thawing indices using Eqs. (5-7) and (5-8). Example 5-3 Considering a mean annual air temperature of −2°C, an amplitude of the seasonal air temperature variation of 25°C and a phase lag of 30 days, compute the air temperature function and the freezing and the thawing indices. Solution Step 1: The air temperature function can be computed using Eq. (5-4) in a spreadsheet. The resulting function is given in Fig. 5-4: Step 2: Compute season change times (t1 and t2) from Eqs. (5-5) and (5-6): t1 =
365 days − 2 − 0 cos − 1 + 30 = 125 . 9 days 25 2π
t2 =
365 days − 2 − 0 2π − cos −1 + 30 = 2 9 9 . 1 days 2π 25
Step 3: Compute air freezing and air thawing indices in °C·day from Eqs. (5-7) and (5-8):
FI a = (0 − (− 2))(365 + 125 . 9 − 299 . 1) +
2π (299 . 1 − 30) 365 × 25 2 π (365 + 125 . 9 − 30) sin − sin 2π 365 365
FI a = 3278 . 9 TI a = (− 2 − 0)(299 . 1 − 125 . 9) − TI a = 2548 . 9
2 π (125 . 9 − 30) 365 × 25 2 π (299 . 1 − 30) sin − sin 2 π 365 365
Calculation of Engineering Parameters
FIGURE 5-4
Air temperature function computed for Example 5-3.
Creating a virtual weather station for site specific evaluation: When using climatic data for pavement evaluation purpose, it is recommended to make best possible use of information from weather stations located in the vicinity of the considered site. A “virtual” weather station can be created by combining the information from the nearby weather stations using a 1/r2 weighting scheme (Wu et al. 2000). Virtual weather data can be interpolated using the following equation: k
V=
V
∑ r 2i i =1 i k
1 ∑ r2 i =1 i
(5-9)
where V = climatic value to be estimated, Vi = climatic value measured at weather station i, ri = distance between weather station i and the pavement site, and k = number of stations considered for the estimation (see Example 5-4). Weather stations selected for the development of the virtual weather station should be within a 30 km radius of the considered pavement site and the difference of elevation should not exceed 500 m (Wu et al. 2000). Example 5-4 Considering the situation illustrated in Fig. 5-5, compute the freezing index for a virtual weather station located at the indicated pavement site. All sites are within a 500 m difference of elevation. Solution 1210 1250 1275 + + 2 22 2 302 = 22 . 9 0 6 = 1218 . 4 FI = 8 0 . 0188 1 1 1 + + 82 22 2 302
215
216
Chapter Five
FIGURE 5-5 Construction of a virtual weather station (Example 5-4).
Dealing with multiple freeze-thaw cycles: It is common to have periods of freezing temperatures alternating with periods of warm air temperatures. Though these alternating sequences can occur any time during the year, they are more commonly observed at the beginning of winter and at the beginning of spring. Corté et al. (1995) and Dysli et al. (1997) have proposed the concept of “significant freezing index.” The calculation of the significant freezing index is based on the following three logical rules: 1. (|FI(i)| > 25°C·day) and (TI(i+1) < 15°C·day) 2. |FI(i)| > TI(i+1) 3. TI(i+1) < |FI(i+2)| The freezing and thawing indices for n successive periods are accumulated if the three conditions are verified. Example 5-5 clarifies this concept.
FIGURE 5-6
Accumulation of freezing and thawing indices (Example 5-5).
Calculation of Engineering Parameters Example 5-5 Based on the data provided in Fig. 5-6, compute the significant freezing index. Solution First freezing event: Rule 1. (|FI(1)| = 20°C·day < 25°C·day) ⇒ rule not verified. The first event will thus, not be accumulated in the significant freezing index. Second freezing event: Rule 1. (|FI(3)| = 30°C·day > 25°C·day) and (TI(4) = 12°C·day < 15°C·day) ⇒ rule verified; Rule 2. |FI(3)| > TI(4) ⇒ rule verified; Rule 3. TI(4) < |FI(5)| = 300°C·day ⇒ rule verified. The second freezing event and the following thawing event will be accumulated in the significant freezing index. Thus FI = 30 – 12 + 300 = 318°C·day
5-2
Surface Temperature and Surface Freezing and Thawing Indices Surface temperature is the most important temperature data in pavement engineering as it represents the boundary condition at the surface of the pavement system. As explained in Sec. 2-1, surface temperature and air temperature are in complex interaction. A complete surface energy balance analysis [Eq. (2-8)] should ideally be done to obtain the true surface temperature. Such an approach is, however, impractical due to the difficulty to obtain site specific information for the calculation of the radiation balance, the convective heat exchange coefficients and other factors necessary to compute the surface energy balance (Shur and Slavin-Borowskly 1993). The current state of the practice in pavement engineering is, thus, to use empirical approaches to convert air freezing and thawing indices into surface freezing and thawing indices. Two approaches are proposed in the literature for the conversion: The first one uses a coefficient referred to as the “n-factor” which is a multiplier used to correct the air freezing and thawing indices. The second method consists of adding a correction term named the “radiation index” directly to the freezing index.
5-2-1 The n-Factor Approach The n-factor has widely been used for soil and pavement temperature analysis since the 1950s. Amongst others, it has been documented by Carlson (1952), Brown (1963), Lunardini (1978), Shur and Slavin-Borowskly (1993), and Andersland and Ladanyi (2004). The n-factor can be defined as follows: nf =
FI s FI a
(5-10)
TI nt = s TI a where nf , nt = the freezing and the thawing n-factors, FIa, FIs = air and surface freezing indices, and TIa, TIs = air and surface thawing indices. Notwithstanding the fact that the n-factor approach is highly practical, the variability of the factor makes it an unreliable tool. The n-factor is known to vary considerably with • Surface characteristics (albedo, latent heat of fusion/evaporation, thermal conductivity, and thermal capacity of soil) • Radiation balance on the pavement (latitude, season, cloud cover, slope, and direction of the slope, presence of shading obstacles)
217
218
Chapter Five nf
nt
Material
Range
Suggested Practical Range
Range
Asphalt concrete
0.25–2.50
0.8–0.95
1.60–3.00
Gravel
0.60–1.50
0.9–1.0
1.10–2.00
Trees and bushes, moss, and peat soil
0.25–0.50
0.30–0.35
0.37–0.80
Snow
1.00
Source: compiled from Dysli et al. (1997), Zarling and Braley (1988), Lunardini (1978), Ladanyi (1996), Andersland and Ladanyi (2004).
TABLE 5-2
Typical Values of n-Factors for Pavement Engineering Purposes
• Convective heat transfer (difference in temperature between air and surface, wind speed) • Damping effects by large water bodies Typical values for n-factor reported in the literature for pavement conditions are listed in Table 5-2. As a general rule, higher values of the practical range should be used where surfaces are exposed to intense sun radiation, while lower values of the range should be used in conditions where surfaces are protected from the sun radiation (clouds, shading obstacles, or low sun angle) or exposed to high winds. Note that typical values are difficult to find in the literature for nt as it varies considerably with site condition. Additional information can be found in the literature. For example, Shur and SlavinBorovskly (1993) propose a map of nt values for Russia (values for bare silty soils) showing variation of nt-factor with distance from a large water body, latitude, and period during the year. The U.S. Department of the Army (1966), cited by Andersland and Ladanyi 2004, also proposes a chart relating nt to wind speed for different pavement surface types. Lunardini (1978) proposes a rationale method to obtain n-factor values for specific site conditions. The method, however, requires detailed information on surface thermal characteristics and surface heat transfer characteristics and is, thus, difficult to apply in practice. More information in Lunardini’s approach can be found in Dysli et al. (1997) and Lunardini and Ibrahim (1990). The n-factor is sometimes used to directly convert air temperatures into surface temperatures. This approach assumes that the days with an average temperature below (or above) 0°C is the same for the air and surface temperatures. Prediction of an asphalt concrete surface temperature is given in Sec. 5-3.
5-2-2
Radiation Index Approach
Dysli (1991) and Dysli et al. (1997) have proposed an interesting alternative to the n-factor approach. The proposed approach is based on the premises that solar radiation is the main contributor to the energy balance at the surface of pavements and that the energy balance is generally obtained by summing the effect of various contributing factors including solar radiation. According to Dysli, the freezing and thawing indices at
Calculation of Engineering Parameters
FIGURE 5-7 The radiation index as a function of quantity of sunshine or total radiation, GH [redrawn from Dysli et al. 1997 (Fig. 4.5, p. 22) with permission from Taylor & Francis Books UK].
the surface of the pavement can be obtained by subtracting a correction factor termed the radiation index (RI) from the air freezing index as follows: FI s = FI a − RI
(5-11)
The radiation index can be obtained from Fig. 5-7 based on total radiation data from weather stations, on the average daily number of hours of sunshine during the considered period (also available from weather stations) or on a simple qualitative assessment of exposure to sunlight. The radiation index relationship proposed in Fig. 5-7 was derived from data gathered in Switzerland and is considered valid between the 40th and the 55th degrees of latitude north. Validation of the relationship should be done before applying it to other contexts. Example 5-6 demonstrates the calculation of surface freezing indices using the n-factor and RI approaches. Example 5-6 Considering a pavement asphalt concrete surface exposed to an average of 3 h of daily exposure to sunshine in moderately windy conditions, compute the surface freezing index using the n-factor and the RI approaches. The air freezing index obtained from a nearby weather station has been established as 1200°C·day. Solution The n-factor, based on the given site conditions; a n-value of 0.9 is selected from Table 5-2. FIs = 0.9 × 1200°C·day = 1080°C·day RI, from Fig. 5-7, RI can be estimated to be about 105°C·day FIs = 1200°C·day – 105°C·day = 1095°C·day
219
220
Chapter Five
5-3 Temperature in Asphalt Concrete Representative temperature in asphalt concrete is needed for pavement deflection analysis, selection of appropriate asphalt cement, and low-temperature cracking and rutting prediction testing. One single temperature analysis is not always adequate, as the deflection analysis needs the actual pavement temperature at the moment of the deflection measurement, whereas the selection of the asphalt cement and performance prediction testing need design maximum and minimum pavement temperatures. Determination of the maximum and minimum design pavement temperatures are discussed further in Chap. 7. This section describes the determination of representative pavement temperature for deflection or another type of analysis, when real time pavement temperature is needed. When the surface temperature is measured as a part of the analysis, such as deflection analysis, the representative pavement temperature can be estimated using the air temperature, measured pavement surface temperature, depth, and seasonal and diurnal temperature variations. The estimation could be conducted using mathematical models, such as finite difference approximation or finite element method (e.g., Hermansson 2002). Other methods are based on regression analysis where measured pavement temperatures have been fitted with sinusoidal seasonal and diurnal temperature variations. A model by Lukanen et al. (2000) is based on the long-term pavement performance (LTPP) data from 41 sites in North America including several sites in cold regions. The models given in Eqs. (5-12) and (5-13) apply for HMA layer thickness from 46 to 305 mm and surface temperature range from 0 to 40°C. Equation (5-12) is used with deflection measurements according to LTPP testing protocol, where the surface temperature is measured from a pavement that has been in shade for 6 min. Equation (5-13) used with deflection measurements according to routine testing methods, where the surface temperature is measured from a pavement that has been in shade for 15 to 30 s (Lukanen et al. 2000). 2π (h1 − 15 . 5) Td = 2 . 78 + 0 . 912 ⋅ IR + (log(d) − 1 . 25) − 0 . 428 ⋅ IR + 0 . 5 5 3 (1 − day) + 2 . 63 ⋅ sin 18 2π ( h2 − 13 . 5) + 0 . 027 ⋅ IR ⋅ sin 18
(5-12)
2π ( h1 − 15 . 5) Td = 0 . 95 + 0 . 892 ⋅ IR + (log(d) − 1 . 25) − 0 . 448 ⋅ IR + 0 . 6 2 1 ⋅ (1 − day) + 1 . 83 ⋅ sin 18 2π ( h2 − 13 . 5) + 0 . 042 ⋅ IR ⋅ sin 18
(5-13)
where Td = pavement temperature at depth d(°C), IR = infrared surface temperature (°C), d = depth at which pavement temperature is to be predicted (mm), 1 − day = average air temperature the day before testing (°C), sin = sine function on an 18-h clock system (2p rad equal to one 18-h cycle), h1, h2 = time of day in 24-h clock system, but calculated using an 18-h asphalt concrete temperature rise- and fall-time cycle (rules given in Table 5-3). The hours are used as decimals, for example, 13:15 hours = 13.25. The 18-h sine function is assumed to have a flat −1.0 segment between 05:00 and 11:00 hours for the
Calculation of Engineering Parameters Time of the Day, h
h1
h2 Actual decimal hour + 24.00
0–3:00
Actual decimal hour + 24.00
3:00–5:00 5:00–9:00
9.00 11.00
9:00–11:00 11:00–24:00
Actual decimal hour Actual decimal hour
Source: based on Lukanen et al. 2000.
TABLE 5-3 Values for the Variables h1 and h2 in Eqs. (5-12) and (5-13)
FIGURE 5-8
Eighteen-hour cycle sine functions (adapted from Lukanen et al. 2000).
(h − 15.5) term, and between 3:00 and 9:00 hours for the (h − 13.5) term as shown in Fig. 5-8. Example 5-7 clarifies the determination of the h variables. Example 5-7 Pavement surface temperatures and deflections were measured from 8:00 to 16:00 hours using an infrared temperature gauge and a falling weight deflectometer. Some of the test results and testing times are given in Table 5-4. The surface was shaded for about 20 s during the routine type testing, and the average air temperature the day before was 14.0°C. Determine (a) values for variables h1 and h2 for the given testing times and (b) the pavement temperature for the measurement taken at 9:15 hours at the depth of 25 mm. Solution
(a) Determination of the values for the variables h1 and h2 using Table 5-3:
Time of the Day, h
h1
h2
4:12
4 + 12/60 + 24 = 28.2
9.0
8:00
11.0
9.0
9:15
11.0
9 + 15/60 = 9.25
13:21
13 + 21/60 = 13.35
13 + 21/60 = 13.35
221
222
Chapter Five Time of the Day, h
Measured Surface Temperature, °C
4:12 8:00
15.2
9:15
17.8
13:21
26.3
TABLE 5-4 Testing Times and Surface Temperatures
(b) Because the testing area was shaded for only 20 s, estimate the pavement temperature using Eq. (5-13):
T25mm
− 0 . 448 × 1 7 . 8 + 0 . 621 × 14 + . + . × . + (log( ) − . ) 0 95 0 892 17 8 25 1 25 2π (11 . 0 − 15 . 5) 1 . 83 ⋅ sin ° C = 15 . 9 ° C = 18 2π (9 . 25 − 13 . 5) + 0 . 042 × 17 . 8 ⋅ sin 18
For pavement temperature predictions, were the surface temperature cannot be measured, models developed with pavement temperature data from Temmes, Finland, could be used (Ryynänen 2000, Savolainen et al. 2001). Temmes’ latitude is approximately 65°C and the instrumented test sections were located in both sunny and shady areas. The maximum and minimum recorded air temperatures were +31.1 and −36.6°C, respectively. The pavement surface temperature for warm and cold season can be estimated with Eqs. (5-14) and (5-15) using the seasonal and diurnal temperature variation and average air temperature (Ryynänen 2000). 2π ( h − 10) 2π (day − 97 ) Tsw = 1 . 981 sin + 6 . 655 sin + 0 . 702Tair + 2 . 49 24 365
(5-14)
2π (day − 97 ) Tsc = 6 . 655 sin + 0 . 702Tair + 2 . 4 9 365
(5-15)
where Tsw, Tsc = surface temperatures for warm and cold season, respectively, h = hour of the day (from 0 to 24), day = sequel number of the days in a year from 0 to 365 and Tair = air temperature. The warm season is defined as time between March 31 and October 15, and the cold season is defined as time between October 16 and March 30. The average pavement temperature for the top 100 mm of asphalt concrete can then be estimated using Eq. (5-16) (Savolainen et al. 2001): 2π (4 . 681 − h) 2π (108 . 6 − day) Tave = 0 . 470 sin − 1 . 212 sin + 0 . 917Tsurface + 0 . 308 365 24 (5-16) where Tsurface is Tsw [Eq. (5-14)] for warm season and Tsc [Eq. (5-15)] for cold season. Example 5-8 illustrates the calculation of pavement surface and average temperatures using air temperature data.
Calculation of Engineering Parameters Example 5-8 Determine the average pavement temperature for the top 100 mm for a pavement with surface temperature of 17.2°C taken at 9:15 hours on July 3rd. Use an air temperature of 14.0°C. Solution July 3rd is the184th day of the year, so day = 184, and h = 9 + 15/60 = 9.25. Use Eqs. (5-14) and (5-16) to calculate the pavement temperature:
Tsw
2π (9 . 25 − 10) 2π (1 84 − 97 ) + 6 . 655 sin 1 . 981 sin °C = 18 . 57 °C 24 365 = + 0 . 702 × 14 . 0 + 2 . 49
Tave
2 π ( 4 . 681 − 9 . 25) 2 π (108 . 6 − 184) − 1 . 212 sin 0 . 470 sin °C = 18 . 1 °C 24 365 = + 0 . 917 × 1 8 . 57 + 0 . 308
5-4 Thermal Properties of Soils and Pavement Materials The estimation of thermal conditions within the pavement system requires the use of basic thermal properties of soils and pavement materials. These properties include: • Thermal conductivity (k) • Heat capacity (c) • Latent heat of fusion (L) • Thermal diffusivity (a) Thermal conductivity: For most practical applications, it is estimated based on relevant physical properties of soils and pavement materials. The most commonly used reference for thermal conductivity of soils is characterization work done by Kersten in 1949. Fine-grained soils and coarse-grained soils were characterized for different dry densities and water content and results were reported in charts and equations. The results of Kersten’s work are summarized in the following four equations (converted to metric by Farouki 1981): Clay and silt: Unfrozen: k u = 0 . 1442(0 . 9 log ω + 0 . 2) × 100 . 6243 ρd
(5-17)
k f = 0 . 001442(10)1 . 373 ρd + 0 . 01226ω(10)0 . 4994 ρd
(5-18)
k u = 0 . 1442(0 . 7 log ω + 0 . 4) × 10 0 . 6243 ρd
(5-19)
Frozen:
Sand and gravel: Unfrozen:
223
224
Chapter Five Frozen: k f = 0 . 01096(10)0 . 8116 ρd + 0 . 00461ω (10)0 . 9115 ρd
(5-20)
where ku and kf are unfrozen and frozen thermal conductivities (W/m·°C) and w is gravimetric moisture content (%) and rd is dry density (Mg/m3) Heat capacity of soils is as well a function of density and moisture content of the soil. It can be estimated using the following empirical relationship (Ladanyi 1996): ρ c vf = 4 . 187 d (0 . 17 + ω u + 0 . 5ω f ) ρω
(5-21)
where cvf = volumetric heat capacity of frozen soils (MJ /m3·°C), rd = dry density of soil (kg/m3), rw = density of water (1000 kg/m3), wu = unfrozen gravimetric water content (decimal), and wf = frozen gravimetric water content (decimal). For unfrozen soils, Eq. (5-21) becomes ρ c vu = 4 . 187 d (0 . 17 + ω) ρω
(5-22)
The corresponding mass heat capacities are obtained from relationship cm =
cv cv = ρ ρ d (1 + ω )
(5-23)
where r is the wet density of the soil. Latent heat of fusion of soil is the energy released by freezing water or absorbed by melting ice present in the pores of 1 m3 of soil. Latent heat of fusion of water being 0.334 MJ/kg, latent heat of fusion of soil (Ls) can be estimated using the following formula: Ls = ρd ⋅ ω ⋅ 334
(5-24)
where Ls = latent heat of fusion of the soil volume (kJ/m3), rd = dry density of soil (kg/m3), and w = water content (decimal). Thermal diffusivity of soil is defined as being the ratio of thermal conductivity on heat capacity:
α=
k cv
(5-25)
where a = thermal diffusivity (m2/s), k = thermal conductivity (W/m°C = J/s·m·°C), and cv = volumetric heat capacity (J/m3·°C). Table 5-5 gives typical values for different types of soils and common pavement materials.
Calculation of Engineering Parameters
Soil or Material
Thermal Conductivity, k (W/m°C)
Volumetric Heat Capacity, cv (MJ/m3°C)
Fresh snow
0.06–0.10
0.21
Compacted snow
0.3–0.6
0.42–1.05
Asphalt concrete
1.50
2.0–2.5
Granular material
1.3–1.7
2.0
Polystyrene insulation
0.03–0.06
0.04–0.06
Peat
0.6
3.0
Sand-gravel
1.2–3.0
2.4–3.0
Silt
1.2–2.4
2.5–3.1
Clay
0.9–1.8
2.6–3.4
Sources: Ladanyi 1996; Pufahl 1996; Andersland and Ladanyi 2004.
TABLE 5-5
Thermal Properties of Various Soils and Pavement Materials
It should be noted that for most thermodynamic calculations, a multilayer system can be replaced by an equivalent homogeneous volume of soil having thermal properties equal to the weighted average of the thermal properties of all layers included in the volume according to the following equation: n
Pv =
∑ Pi × Di i=1
(5-26)
n
∑ Di i=1
where Pv = equivalent value of the thermal property for a volume of thickness, Di = thicknesses of layer i included in the volume, and Pi = value of the property for layer i having a thickness Di.
5-5
Freezing and Thawing Indices within the Pavement Structure Without the use of numerical analysis or similar computing techniques, it is impractical to calculate actual temperatures within the pavement structure. Instead, the severity of the temperature variations within the pavement system is estimated using freezing and thawing indices. The indices can be predicted based on the thermal diffusivity “a ” of soils and pavement materials. Assuming that the annual temperature variation follows a sinusoidal trend, the temperature at any depth in the pavement system will follow a similar sinusoidal trend with a reduced amplitude Ax and a time lag ∆tx. Figure 5-9 illustrates the relationship between surface temperature and temperature at depth x in the pavement system. Ax and ∆tx can be estimated using
Ax = A0 e − x
π
p α
(5-27)
225
226
Chapter Five
FIGURE 5-9
Temperature variations at a depth x in the pavement system.
and ∆tx =
x p πα 2
(5-28)
where A0 = amplitude of the sinusoidal temperature wave at the pavement surface (°C), x = depth in the pavement system (m), p = period considered (365 days), and a = computed using Eq. (5-25). The temperature at any point in time, the length of the freezing period, and the freezing index transmitted can thus, be estimated at depth x in the system by replacing A0 by Ax and f by (f + ∆tx) in Eqs. (5-4) to (5-8) (see Example 5-9). Example 5-9 Given the characteristics of the pavement provided in Table 5-6, estimate the freezing period at the surface of the subgrade soil and the freezing index transmitted at that level. A sinusoidal variation of surface temperatures, with an amplitude of 15.5°C, f = 30 days and a mean annual surface temperature of 5°C is assumed for the pavement. The resulting surface freezing index computed from Eq. (5-7) is 982°C·day. Solution Step 1: Compute the equivalent thermal properties for the pavement structure using Eqs. (5-26) and (5-25): k=
(1 . 5 × 0 . 15) + (1 . 3 × 0 . 65) J = 1 . 34( W m°C) = 1 . 34 s ⋅ m ⋅ °C 0 . 15 + 0 . 65
cv =
MJ (2 . 2 × 0 . 15) + (2 . 4 × 0 . 65) = 2 . 36 3 m ⋅ °C 0 . 15 + 0 . 65
α=
1 . 34 × 10− 6 = 0 . 57(m 2 s × 10− 6 ) = 0 . 049(m 2 day) 2 . 36
Calculation of Engineering Parameters Layer
Thickness, m
k, W/m°C
cv, MJ/m3
Asphalt concrete
0.15
1.5
2.2
Granular base
0.65
1.3
2.4
Subgrade: Clayey silt: rd = 1600 kg/m3, w = 0.20, segregation potential (SP) = 100 mm2/°C·day
TABLE 5-6 Pavement Properties for Examples 5-9 to 5-12
Step 2: Compute the modified parameters of the sinusoidal curve at depth x using Eqs. (5-27) and (5-28):
Ax = 15 . 5e − 0. 8 ∆ tx =
π
365⋅0 . 049
= 11 . 08(°C)
0 . 8 365 = 19 . 48(day) π × 0 . 049 2
Step 3: Compute freezing period and freezing index at surface of subgrade soil using Ax and f + ∆tx in Eqs. (5-5), (5-6), and (5-7):
t1 =
5−0 365 cos −1 y) + 49 . 48 = 113 . 53(day 11 . 08 2π
t2 =
5 − 0 365 2 π − cos−1 + 49 . 4 8 = 350 . 43(day) 11 . 08 2π
The duration of the freezing period in the subgrade soil is, thus t fs = 113 . 53 + (365 − 350 . 43) = 128 . 10(day) And the transmitted freezing index is FIt = (0 − 5)(365 + 113 . 53 − 350 . 43) +
2π (350 . 43 − 49 . 48) 365 × 11 . 08 2π (365 + 113 . 53 − 49 . 48) sin − sin 2π 365 365
FIt = 508 . 2(°C ⋅ day)
5-6
Frost and Thaw Depth Frost penetration in soils and pavement system is a result of the heat extraction process described in Sec. 2-1. As illustrated in Fig. 5-10, when surface temperature (Ts) is below the freezing temperature, the resulting thermal gradient will induce a heat flow toward the pavement surface. If the resulting heat flow is larger than the geothermal heat flow,
227
228
Chapter Five Ts
Tb
0°C T G1 ∆X
Frost depth
TG2
Heat flux
FIGURE 5-10
Heat extraction and progression of the frost front in the pavement system.
the system is unbalanced and attempts to regain balance by releasing heat. A sustained cold temperature at the surface of the system will consume the heat stored in the pavement system, and will eventually fall below the freezing temperature. The freezing front will initially progress rapidly in the pavement because the temperature gradient is steeper at shallow depth, TG1, and because the pavement materials in the top portion of the pavement are drier and have thus less heat accumulated (heat capacity and latent heat of fusion). When reaching lower layers of the pavement system (frost penetration ∆X) and eventually the subgrade soil, the frost front progresses more slowly because of the gentler thermal gradient, TG2, and the larger quantity of moisture available in the subgrade soil. It is, thus, easy to understand the role of an insulation layer that will impede heat flow and consequently reduce pavement cooling. Frost penetration is therefore a function of the thermal properties of soils and pavement materials and is a square root function of the “quantity of below-freezing temperature” (or freezing index) to which the system is exposed. A simple method is proposed to estimate the frost (thaw) depth in pavement systems. The method is a two step approach based on (1) the estimation of the freezing (thawing) index transmitted at the base of the pavement structure and (2) the calculation of frost (thaw) depth in the subgrade soil, assumed to be homogeneous. The method is based on the Stefan’s equation extended to include the effect of the segregation potential in soils. The modified Berggren equation, also known as the Aldrich-Berggren method (Aldrich 1956; Ladanyi 1996) can also be used to perform the frost depth calculation, but is not described in this book. Detailed explanation on the use of the modified Berggren equation to compute frost depth in a multilayer system can be found in Andersland and Ladanyi (2004). Frost and thaw depth can alternatively be calculated using free or commercial software.
5-6-1 Transmitted Freezing Index Method The transmitted freezing index method is based on the aforementioned estimation of the freezing index transmitted to the frost susceptible subgrade soil. An empirical alternative method for the estimation of the transmitted freezing index through the pavement
Calculation of Engineering Parameters structure is proposed by Corté et al. (1995). The transmitted freezing index can be estimated based on the following empirical relationship: FI − b h s e FIt = 1 + ah
2
(5-29)
where FIt = freezing index transmitted at the surface of the subgrade soil, FIs = freezing index at the surface of the pavement, h = total pavement thickness (cm), a, be = coefficients depending on the nature of pavement materials. If the pavement structure is constituted of a single material, then be is equal to b in Table 5-7. If the pavement structure is constituted of several layers, then the effective coefficient be can be obtained as follows: n
be =
∑ bi hi
(5-30)
i =1 n
∑ hi i =1
where a and bi are coefficients for materials in layer i obtained from Table 5-7 and hi is the thickness of layer i (cm). The method proposed by Corté et al. (1995) is simple and appealing. It is, however, based on the conditions in France and tends to give too low FIt values in cold climates. The method should be calibrated to local conditions before being used in frost depth predictions. The frost penetration into the subgrade soil can then be determined using the Stefan equation modified to include the effect of segregation on freezing through the incorporation of the segregation potential of the freezing soil: Xss =
2(k f − (SP × L)) Ls
× FIt
(5-31)
where Xss = depth reached by the frost front in the subgrade soil (m), k = thermal conductivity of the frozen soil (W/m·°C or J/s·m·°C), L = latent heat of fusion of water (334 MJ/m3), Ls = latent heat of fusion of the freezing soil and can be obtained from Eq. (5-24), and SP = segregation potential of the freezing soil, m 2/s·°C (see Example 5-10).
Material
a, cm−1
b, (°C·day)0.5/cm
Asphalt concrete Asphalt stabilized base
0.008
0.06
Granular material
0.008
0.10
Source: Corté et al. 1995.
TABLE 5-7 Experience.
Coefficients for Calculation of Transmitted Freezing Index Based on the French
229
230
Chapter Five Example 5-10 Given the characteristics of the pavement structure and the climatic conditions described in Example 5-9, estimate frost depth using the transmitted freezing index method. Solution The transmitted freezing index is equal to 508 . 2(°C ⋅ day) = 43 . 91 × 106 (°C ⋅ s) Step 1: Compute thermal characteristics of subgrade soil: J k f = 0 . 001442 × 101. 373×1. 6 + 0 . 01226 × 20 × 100. 4994×1 . 6 = 1 . 77 s ⋅ m ⋅ °C Ls = 1600 × 0 . 2 × 334 = 106, 8 80(kJ m 3 ) = 106 . 88 × 106 ( J m 3 ) SP = 100(mm 2 °C ⋅ day) = 100 × 10− 6 (m 2 °C ⋅ day) = 116 × 10− 11 (m 2 °C ⋅ s) Step 2: Compute frost depth. Based on the information provided and on soil thermal characteristics, frost depth can be obtained from Xss =
2(1 . 77 − (116 × 10−11 × 334 × 106 )) × 4 3 . 91 × 106 = 1 . 07(m) 106 . 88 × 106
The total frost depth from the pavement surface can be readily obtained by adding the total thickness of the pavement to Xss: X = (1 . 07 + 0 . 80)m = 1 . 93 m
5-7
Frost Heave Frost heave is the most important consequence of frost penetration in the pavement system. Frost heave causes stresses and distortions at the pavement surface. Water accumulated by the frost heave process is one of the important factors of bearing capacity loss during spring thaw. Several models are available to estimate frost heave in soils based on different physical theories. Methods based on the segregation potential theory (Konrad and Morgenstern 1980) remain the only practical methods for the prediction of frost heave based on relatively simple test methods. Two simple methods are described to estimate frost heave from the segregation potential of the freezing soil.
5-7-1
Konrad’s Method for Frost Heave Prediction
The method proposed by Konrad (2001) requires the following information: • The segregation potential of the subgrade soil • The average temperature gradient in the freezing soil • The duration of the freezing period in the subgrade soil The segregation potential can be obtained using one of the procedures described in Chap. 4. The average temperature gradient can be estimated using the simple procedure illustrated in Fig. 5-11. The temperature gradient, TG, is thus equal to TG =
Txf X−D
(5-32)
Calculation of Engineering Parameters
FIGURE 5-11
Determination of average thermal gradient [see Eq. (5-32)].
where T xf is the average temperature at the surface of the subgrade soil during the freezing period that can be obtained by dividing the freezing index transmitted to the subgrade soil by the duration of the freezing period at the subgrade soil level, X is the maximum frost depth, and D is the total thickness of the pavement structure. The duration of the freezing period in the subgrade soil, tfs, can be estimated based on Eqs. (5.5) and (5.6) using the procedure described in Example 5-5. The total amount of the frost heave, h, can then be estimated using the following equation: h = 1 . 09 ⋅ SP ⋅ TG ⋅ t fs
(5-33)
where SP = segregation potential, TG = thermal gradient is the estimated thermal gradient in the subgrade soil, and tfs = duration of the freezing period in the subgrade soil. Example 5-11 Given the information provided in Examples 5-9 and 5-10, estimate the frost heave in the pavement system. Solution Step 1: Compute relevant parameters: SP = 100(mm 2 °C ⋅ day) = 100 × 10− 6 (m 2 °C ⋅ day) Txf =
FIt 508 . 2 = = 3 . 97(°C) 128 . 1 t fs
TG =
3 . 97 = 3 . 7 (°C m) 1 . 87 − 0 . 80
Step 2: Compute frost heave: h = 1 . 09 × 100 × 10− 6 × 3 . 7 × 128 . 1 = 0 . 052(m)
231
232
Chapter Five
5-7-2
Saarelainen’s Method for Frost Heave Prediction
Saarelainen (1996) has observed that the ratio of frost heave on the thickness of the frozen soil is constant for a given soil and is proportional to the segregation potential of the soil following the relationship: h=
2 × SP(X − D) X FI s
(5-34)
2
where h = average frost heave (m), SP = segregation potential (m2/°C·day), X = maximum frost depth (m), D = thickness of the pavement structure (m), and FIs = surface freezing index (°C·day) (see Example 5-12). Example 5-12 Given the information provided in Examples 5-9 and 5-10, estimate the frost heave using the Saarelainen method. Solution h=
2 × 100 × 10− 6 (1 . 87 − 0 . 80) 1 . 93 982
2
= 0 . 056 m
5-8 Thaw Settlement Pavement engineers are confronted to two important questions in relation to pavement behavior when subjected to thawing. For seasonally frozen pavements, it is generally assumed that heaved pavements will settle back to their original elevation. The important question in those conditions is the duration of the consolidation period or time required for full recovery of pavement strength. In permafrost conditions, time is less of a concern but the amount of settlement that will occur as a result of thawing permafrost is a major concern for engineers. Ice-rich permafrost may become unstable as a result of changes in thermal regime caused by the construction of a pavement structure or caused by climatic changes. As a result, the pavement system will be subjected to progressive settlement which will take place over a period of several years. As illustrated in Fig. 5-12, a modification in the thermal balance at the surface of the pavement will cause thaw to penetrate deeper into the permafrost (light-grey zones in the thaw penetration bars). The resulting settlement of ice-rich permafrost will reduce the heat capacity and the latent heat of fusion of the newly thawed layer of soil increasing the thaw penetration in the subsequent year. The process will continue until a new equilibrium is reached. In conditions where surface conditions keep changing with time, thaw consolidation will keep evolving with time until the climatic trend changes or until a remedial action is applied to the pavement. Two methods can be used to estimate thaw settlement in pavements constructed on unstable permafrost. The first one uses the result of laboratory thaw-consolidation tests and the second one is based on measurements of soil density (Ladanyi 1996).
Calculation of Engineering Parameters
FIGURE 5-12 Change in thaw depth resulting from (a) a sudden warming of surface temperature and (b) a progressive warming of surface temperature.
The most accurate method to predict thaw settlement of ice-rich permafrost is to use results of thaw-consolidation tests done on undisturbed samples of frozen soil immediately beneath the active layer. As shown on Fig. 5-13a, most of the consolidation occurs during thawing and subsequent consolidation will occur as the effective stress increases in the thawed sample. The total settlement (sx) that will be experienced by the sample at
FIGURE 5-13 Typical results of a thaw-consolidation test of an ice-rich soil sample in (a) a void ratio-stress space and in (b) a settlement-stress space.
233
234
Chapter Five a given effective stress is the sum of the settlement caused by thawing (st) and of the subsequent consolidation (sc). A typical result of a thaw-consolidation test is shown on Fig. 5-13b. The relative settlement caused by soil thawing is A0 =
e f − et
(5-35)
1+ ef
where ef and et are frozen and thawed void ratios. The settlement related to thawing can be obtained by st = A0D f
(5-36)
And the settlement related to subsequent consolidation by sc = mvσ 'x D f
(5-37)
where mv is the coefficient of volume compressibility obtained from the consolidation test and Df is the thickness of the soil layer subjected to thawing on which an effective stress s¢x is acting. A quick evaluation of the potential thaw settlement of all soils within the thaw depth without t he need for thaw-consolidation tests can be obtained using soil densities (Ladanyi 1996): ρdf s = 1 − D f ρd ,th
(5-38)
where s is the thaw settlement, rdf and rd,th are the frozen and thawed dry densities of the soil, respectively (see Example 5-13). Example 5-13 Estimate the thaw settlement using Eq. (5-38) for a site with a thaw depth of 5.0 m in frozen silty clay with the specific gravity of soil solids of 2.70. The total water content is 58 percent with ice inclusions forming about one-third of the frozen volume and the unforzen water content is estimated to be 15 percent. Soil between the ice lenses had a saturated water content of 30 percent (adapted from Andersland and Ladanyi 2004). Solution Step 1: Compute the thawed dry density of soil between ice inclusions:
ρ d ,th =
ρw 1 + wsat Gs
=
1000 1 + 0 . 30 2.7
kg = 1492 3 m
Step 2: Compute the frozen dry density:
ρ d , fr =
kg ρw 1000 = = 1011 3 m 1 1 + 1 . 09(w − wu ) + wu + 1 . 09(0 . 588 − 0 . 15) + 0 . 15 2.7 Gs
Step 3: Compute the thaw settlement using Eq. (5-38): 1011 s = 1 − 5 . 0 = 1 . 6(m) 1492
Calculation of Engineering Parameters
5-9
Stresses and Strains in Pavements A simple way to calculate stresses, strains, and deflections in a soil mass is to use a set of equations developed in 1885 by French mathematician, Boussinesq. The equations, initially valid for a point load, were later refined and adapted for loads applied on flexible circular plates by Foster and Ahlvin (1954) and Ahlvin and Ulery (1962). Boussinesq’s solution is based on the following assumptions: • The soil volume is a semi-infinite space, it is infinite in the horizontal plane and in depth from the surface • The soil volume is constituted of elastic material characterized by an elastic modulus E (Young’s modulus) and Poisson’s coefficient m • The material properties are homogeneous and isotropic Based on these assumptions, vertical and radial stresses induced at any depth in the soil mass underneath the center of the circular loading plate can be obtained by: z3 σ z = σ 0 1 − 2 2 1.5 (a + z )
σr =
σ0 z3 2(1 + µ )z + 2 1 + 2µ − 2 2 0 5 . 2 (a + z ) ( a + z 2 )1. 5
(5-39)
(5-40)
where s0 = stress uniformly applied on a plate of radius a at the surface of the soil mass, z = depth considered, m = Poisson’s coefficient that typically varies from 0.15 to 0.45. Table 5-8 gives typical values for the coefficient for several soils and pavement materials. Vertical and radial strains can be determined using Eqs. (5-41) and (5-42):
εz =
z3 (1 + µ )q 2µ z − 2 1 − 2µ + 2 2 0 . 5 2 1 . 5 E (a + z ) (a + z )
(5-41)
εr =
z3 (1 + µ )q 2(1 − µ )z + 1 − 2µ − 2 2E (a + z 2 )0 . 5 (a 2 + z 2 )1 . 5
(5-42)
where E is the Young’s modulus and q is the vertical load on the ground surface. And finally, the vertical deflection can be determined as d=
(1 + µ )qa 1 − 2µ 2 a + [(a + z 2 )0 . 5 − z] 2 2 0.5 E a ( a + z )
(5-43)
More detailed information on the use of these equations, including graphical solutions, can be found in Ullidtz (1987). Solutions at any points in the soil mass have also been developed by Foster and Ahlvin (1954) and Ahlvin and Ulery (1962). These solutions and detailed information on stress and strain analysis in pavements can be found in Huang (2004).
235
236
Chapter Five Soil or Material
Range
Typical Value
Asphalt concrete
0.30–0.40
0.35
Portland cement concrete
0.15–0.20
0.15
Dense graded aggregates
0.30–0.40
0.35
Dense sand
0.30–0.45
0.35
Loose sand
0.20–0.40
0.30
Fine grained soils
0.30–0.50
0.40
Saturated clays
0.40–0.50
0.45
Source: Huang 2004 (reprinted by permission of Pearson Education).
TABLE 5-8
Typical Values of Poisson’s Coefficient for Different Soils and Pavement Materials
The main limitation of the Boussinesq model is that it is limited to a homogeneous soil mass infinite in depth and in the horizontal plane. Thus, it limits considerably the possibility to use the model in a multilayer system such as pavements. Burmister (1943a and b) was the first to propose a complete analytical solution for the calculation of stresses and displacement in a multilayer elastic system. The Burmister solution is, however, complex and can only be resolved through a series of calculation charts or using a computer. Most of the software codes currently available are based on the Burmister solution. In parallel to Burmister’s work a Swedish mathematician, Odemark, has developed a simple method to convert a layer of elastic material into an equivalent thickness of another material. The method is based on the principle that a material of low rigidity has less load distribution capacity than a more rigid material. As illustrated in Fig. 5-14, more thickness of material 1 will be required to have the same load distribution capacity than the material 2. The Odemark method, also known as the equivalent thickness method, is much easier to use than the Burmister solution. It is, however, more restrictive and based on additional assumptions: • The layer must have decreasing rigidity with depth with a minimum modulus ratio between two adjacent layers equal to two • The layers must have at least a thickness equal to the radius of the loading plate. A correction factor is proposed for cases where the thickness of the surfacing layer is smaller than the plate radius [see Eq. (5-44)] • All layers above the subgrade soil are considered to have a pure flexural behaviour and to have perfect friction at interfaces The method proposed by Odemark is the following: 1. When the analysis is conducted at a depth less than the depth of the first interface, stresses and strains can be computed with Eqs. (5-39) to (5-42) using the characteristics of the first layer.
Calculation of Engineering Parameters
FIGURE 5-14 Schematic illustration of the Odemark principle.
2. When the analysis is conducted at the first interface or between the first and the second interface, the first layer is transformed in an equivalent thickness, he, of the material in the second layer:
( (
E 1 − µ22 he = f h1 1 × E2 1 − µ12
) )
1/3
(5-44)
where h1 is the original thickness of layer 1 having a modulus E1 and Poisson’s coefficient m1, E2, and m2 are the elastic properties of layer 2, f is a correction factor: f = 1.0 for the first interface f = 1.1 for the first interface if the radius of the plate a > h1 f = 0.8 for all other cases 1. Stresses, strains, and deflections can be computer at the new depth “z” considering the equivalent thickness using the Boussinesq equations. 2. The same principle can be applied to other underlying layers. When two adjacent layers have the same Poisson’s coefficient, Eq. (5-44) can be written using the following simplified form: E he = f h1 1 E2
1/3
(5-45)
The Odemark/Boussinesq method is simple and accessible, yet it is accurate enough for most pavement engineering applications requiring an estimation of stresses and strains under a circular load. When the conditions of the method are respected, estimations done using the Odemark/Boussinesq method are generally in close agreement
237
238
Chapter Five with the results obtained using sophisticated analytical or numerical models (Ullidtz 2002). Moreover, the method can easily be programmed in a spreadsheet. Example 5-14 illustrates the use of the method. Example 5-14 Given the characteristics of the pavement described below, compute the horizontal strain at the bottom of the asphalt layer and the vertical strain at the top of the subgrade soil under a wheel applying a 560-kPa pressure on a circular area of 0.15 m radius.
E = 5,000 MPa, m = 0.35, D = 0.15 m E = 250 MPa, m = 0.35, D = 0.50 m E = 60 MPa, m = 0.35, D = ∞
Solution Step 1: Transformation of layer 1. Horizontal strain needs to be calculated at interface 1. According to Odemark’s method, the first layer needs to be converted in an equivalent thickness of material 2 using Eq. (5-45) (since m1 = m2). Being at the first interface, a correction factor of 1.0 is selected. 1/3
5000 he = 1 . 0 × 0.15 250
m = 0.407 m
Step 2: Computation of horizontal strain. A depth z of 0.407 m is used in the computation of strain using Eq. (5-42).
εr =
2(1 − 0 . 35)0 . 407 (1 + 0 . 35)0 . 56 0 . 407 3 + 1 − 0 . 70 − mm/mm 2 2 0 . 5 2 2(250) (0 . 15 + 0 . 407 ) (0 . 15 + 0 . 407 2 )1 . 5
ε r = − 141 . 7 × 10− 6 mm/mm The negative sign indicates a tensile strain. Step 3: Transformation of the pavement in equivalent thickness of subgrade soil. The calculation of vertical strain at the second interface requires transforming the pavement layers (already transformed into equivalent thicknesses of material 2) into an equivalent thickness of material 3 using Eq. (5-45). Since the original pavement system is a multilayer system, a correction factor of 0.8 is used. 1/3
250 he = 0 . 8(0 . 50 + 0 . 407 ) 60
m = 1 . 167 m
Step 4: Compute the vertical strain at the depth corresponding to the equivalent thickness of the transformed pavement structure using Eq. (5-41):
εz =
2 × 0 . 35 × 1 . 167 (1 + 0 . 35)0 . 56 1 . 167 3 mm − 1 − 0 . 7 + mm/m 2 2 0.5 2 2 1.5 60 (0 . 15 + 1 . 167 ) (0 . 15 + 1 . 167 )
ε z = 234 . 2 × 10− 6 mm/mm
Calculation of Engineering Parameters
Review Questions 5-1. Determine the maximum cooling rate of air temperature for the following hourly temperature data set. Month
Day
Hour
Temperature, °C
3
15
0
−12.2
3
15
1
−13.3
3
15
2
−14.4
3
15
3
−15.5
3
15
4
−16.4
3
15
5
−17.3
3
15
6
−18.2
3
15
7
−18.3
3
15
8
−18.5
3
15
9
−18.6
3
15
10
−17.6
3
15
11
−16.6
3
15
12
−15.6
3
15
13
−14.9
3
15
14
−14.2
3
15
15
−13.5
3
15
16
−12.4
3
15
17
−11.4
3
15
18
−10.3
3
15
19
−10.0
3
15
20
−9.6
3
15
21
−9.3
3
15
22
−10.2
3
15
23
−11.1
3
15
24
−12.0
239
Chapter Five 5-2. Determine the air freezing index and the air thawing index of the following mean daily air temperature records.
Day 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0
–1 –3
Temperature (°C)
–5
–4 –5
–6
–7
–10
–8
–9 –10 –12 –13 –13 –15
–10 –11 –12 –14
–15
–11 –13 –15
–14
–17
–20
–20 –22 –22 –23
–21 –25
–25–26
–30 (a)
20 17 15 Temperature (°C)
240
10
10 8 6 5
6
5
7
6
5
4
3 2
2
1 2 0
0 –2
–2 –5
7
–4
4 2 2
2
1 1
2
1
–3 –3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Day (b)
Calculation of Engineering Parameters 5-3. The air temperature can be estimated from a sinusoidal curve as shown in the figure below, for which parameters in Eq. (5-4) can be found. Using Eqs. (5-7) and (5-8), estimate the freezing and the thawing indices for the illustrated curve.
25 20
Temperature (°C)
15 10 5 0 –5 –10 –15 0
50
100
150
200 Day
250
300
350
400
5-4. Climatic measurements are made at points 1, 2, 3, and 4 (elevations are denoted by z for each point). Determine the freezing index at point 0 with the following mean daily temperatures:
z4 = 2020 m z1 = 1503 m .4
km
.7
16
37
km zo = 2030 m 22.
29 .3
km
8k
z2 = 1980 m
m
z3 = 2000 m
241
Chapter Five Temperature, °C
Days
Point 1
Point 2
Point 3
Point 4
1
−5.0
−8.0
−12.0
−10.0
2
−4.8
−7.3
−11.6
−9.5
3
−4.5
−7.0
−11.5
−8.8
4
−3.8
−6.9
−10.6
−8.6
5
−3.7
−6.1
−10.5
−8.1
6
−3.3
−5.5
−9.8
−7.2
7
−3.1
−5.1
−9.0
−6.2
8
−2.7
−5.1
−8.6
−5.8
9
−2.3
−4.3
−8.2
−5.7
10
−1.4
−3.8
−7.7
−5.5
11
−1.2
−3.3
−7.0
−5.5
12
−1.0
−3.1
−6.5
−4.8
13
−1.0
−3.1
−6.1
−4.0
14
−0.5
−2.6
−6.0
−3.3
15
0.0
−2.5
−5.2
−2.3
5-5. Determine the significant freezing index of the following data:
10 8
8
7 7 6
6
5
5
4
4 Temperature (°C)
242
2
2
2
2
2
0 –1
–2 –4 –6 –8
–2
–2 –3
–3 –3 –4
–4 –5
–5 –6 –7 –8
–3 –4
–5 –5 –6
–7 –8
–10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Day
Calculation of Engineering Parameters 5-6. A site characterized by an air freezing index of 1000°C·day is exposed to sunshine during an average of 2.5 h/day. Considering a pavement with an asphalt concrete surface, evaluate the surface freezing index at the surface of the pavement using the radiation index and the n-factor approaches.
5-7. Pavement surface temperatures and deflections were measured from 8:00 to 14:00 hours using an infrared temperature gauge and a falling weight deflectometer. The pavement (asphalt concrete layer thickness = 69 mm) has been in shade for 6 min during the whole test. The average air temperature the day before testing was 8°C. Determine the pavement temperature at 8:00 hours, at a depth of 48 mm.
Time, h
Measured Surface Temperature, °C
8:00
6.2
10:00
8.2
12:00
14.6
14:00
12.4
16:00
10.3
5-8. Estimate the average temperature of the top 100 mm of asphalt concrete on October 26th at noon. The air temperature is 5.2°C.
5-9. Evaluate the thermal conductivity of the following multilayer system: Compacted snow 0.18
0.05 0.10
Asphalt concrete Frozen layer
1.05
Granular material
r = 1.65 Mg/m w = 10%
Sand
r = 2.65 Mg/m w = 40%
0.78
Clay
r = 2.10 Mg/m w = 45%
0.57
5-10. A pavement has as the following structure: • 0.15 m of asphalt concrete • 1.00 m of granular base • 2.00 m of silt Will the top of the silt layer be below freezing (<0°C) on April 2nd if the amplitude of the sinusoidal wave representing the temperature at the pavement surface is 20°C? The mean annual air temperature is 3°C and the phase lag of the air temperature sinusoidal curve [in Eq. (5-4)] is 20 days.
243
Chapter Five 5-11. Given the characteristics of Question 5-10, what is the length of the freezing period? What is the freezing index at the pavement surface?
5-12. Mean daily air temperature data collected for the month of January are shown below. Consider a pavement structure that is composed of 0.10 m of asphalt concrete over a layer of 0.75 m of granular material and a silt subgrade (r = 1.56 Mg/m3, w = 20 percent, SP = 120 mm2/°C·day). Compute frost penetration in the subgrade.
0 –5 Temperature (°C)
244
–10 –15
Day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0 –1 –2 –4 –4 –5 –7 –8 –9 –9 –9 –10 –11 –11 –12 –12 –13 –13 –13 –14 –14 –14 –17
–20
–20 –21 –22 –22
–21 –25
–25 –26
–30
5-13. Using data for Question 5-12, evaluate the total amount of frost heave. Assuming that frost action in the subgrade soil has lasted 10 days during the month, compare results obtained using methods proposed by Konrad and Saarelainen.
5-14. Evaluate the thaw settlement potential of a frozen sand deposit of which the top 2.5 m is expected to thaw. The specific gravity of the soil solids is 2.65. Soil between the ice lenses has a saturated water content of 45 percent. The total water content is 75 percent with ice inclusions forming about a quarter of the frozen volume.
5-15. Using Odemark’s principle, estimate the horizontal strain 0.10 m under the top of the subgrade soil made of fine-grained soil. The pavement is a multilayer pavement system made of 0.25 m of asphalt concrete over a 1.2-m deep layer of dense-graded aggregate. Consider Young’s modulus of 4000 MPa for the asphalt concrete, 320 MPa for the aggregate, and 75 MPa for the subgrade soil. A load of 100 kN is applied on a circular plate that has a radius of 0.2 m.
5-16. For the pavement surface temperature data listed below, determine the pavement temperatures at the depth of 50 mm. The temperatures were measured for LTPP program deflection measurements. Given Data Time of Day, h
Surface Temperature, °C
Day before Temperature, °C
6:05
12
25
10:23
24
18
16:00
28
19
Calculation of Engineering Parameters
References Ahlvin, R. G., and Ulery, H. H. (1962). “Tabulated Values for Determining the Complete Pattern of Stresses, Strains, and Deflections beneath a Uniform Circular Load on a Homogeneous Half-Space,” Highway Research Bulletin 342, Transportation Research Board, National Research Council, Washington, D.C., pp. 1–13. Aldrich, H. P., Jr. (1956). “Frost Penetration Below Highway and Airfield Pavements,” Highway Research Bulletin 135, Transportation Research Board, National Academies Press, Washington, D.C., pp. 124–149. Andersland, O. B., and Ladanyi, B. (2004). Frozen Ground Engineering, 2d ed., John Wiley & Sons, Inc. Hoboken, New Jersey, published in cooperation with ASCE Press, Reston, Va. Brown, R. J. E., (1963), “Relation between Mean Annual Air Temperature and Ground Temperature in the Permafrost Regions of Canada,” Proceedings of the International Conference on Permafrost, Publ. 1287, National Academy of Sciences, Lafayette, Ind., pp. 241–247. Burmister, D. M. (1943a). “The Theory of Stresses and Displacements in Layered Systems and Applications to the Design of Airport Runways,” Proceedings of the 23rd Annual Meeting of the Highway Research Board (currently: Transportation Research Board of the National Academies), Washington, D.C., vol. 23, pp. 126–144. Burmister, D. M. (1943b). “The General Theory of Stresses and Displacements in Layered Soils Systems,” Journal of Applied Physics, American Institute of Physics, Melville, New York, vol. 16, no. 2, pp. 89–96. Carlson, H. (1952). “Calculation of Depth of Thaw in Frozen Ground,” Frost Action in Soil, Highway Research Board Spec. Rep. 2, National Research Council, Washington, D.C., pp. 192–223. Corté, J. -F., Odéon, H., and Boutonnet, M. (1995). “Vérification au gel des structures de chaussée,” Bulletin de liaison des ponts et chaussées, Laboratoire Central des ponts et chaussées, Paris, France, no. 198, pp. 13–27. Department of the Army. (1966). Arctic and Subarctic Construction: Calculations Methods for Determination of Depths of Freeze and Thaw in Soils. Technical Manual TM 5-852-6. Government Printing Office, Washington, D.C. Dysli, M. (1991). Le gel et son action sur les sols et les fondations. Presses Polytechniques et Universitaires Romandes, Lausanne, Switzerland. Dysli, M., Lunardini, V., and Stenberg, L. (1997). “Related Effects on Frost Action: Freezing and Solar Radiation Indices,” Ground Freezing 97, Knutsson, S. (ed.), A. A. Balkema, Rotterdam. Farouki, O. T. (1981). Thermal Properties of Soils, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, Monograph 81-1. Foster C. R., and Ahlvin, R. G. (1954). “Stresses and Deflections Induced by a Uniform Circular Load,” Proceedings of the Highway Research Board (currently: Transportation Research Board of the National Academies), Washington, D.C., vol. 33, pp. 467–470. Hermansson, Å. (2002). “Calculation of Asphalt Concrete Layer Temperatures for Falling Weight Deflectometer Analysis,” CD-ROM, TRB 81st Annual Meeting, January 13–17, Washington, D.C. Huang, Y. H. (2004). Pavement Analysis and Design, 2d ed., Prentice-Hall, Upper Saddle River, New Jersey. Huang, Y. H. (2004). Pavement Analysis and Design, 2d ed., 329, Pearson Education, Inc., Upper Saddle River, New Jersey. Kersten, M. S. (1949). Laboratory Research for the Determination of the Thermal Properties of Soils, Arctic Construction and Frost Effects Laboratory (currently U.S. Army Cold
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Chapter Five Regions Research and Engineering Laboratory, Hanover, New Hampshire) Technical Report 23, AD71256. Konrad, J. -M., and Morgenstern, N. R. (1980). “A Mechanistic Theory of Ice Formation in Fined-Grained Soils,” Canadian Geotechnical Journal, vol. 17, pp. 473–486. Konrad, J. -M., (2001). “Cold Region Engineering,” chap. 20, Geotechnical and Geoenvironmental Engineering Handbook, Rowe, K. (ed.), Kluwer Academic Publisher, Springer, New York, pp. 593–613. Ladanyi, B. (1996). “La conception et la rehabilitation des infrastructures de transport en regions Nordiques (Design and rehabilitation of transportation facilities in northern regions),” Ministère des Transports du Québec, Canada, RTQ-94-07, p. 126 (in French). Lukanen, E., Stubstad, R., and Briggs, R. (2000). “Temperature Predictions and Adjustment Factors for Asphalt Pavement,” FHWA, Publication No. FHWA-RD-98-085, Federal Highway Administration, Washington, D.C. Lunardini, V. J. (1978). “Theory of n-Factors and Correlation of Data,” Proceedings of the Third International Conference on Permafrost, National Research Council of Canada, Ottawa, Canada, vol. 1, pp. 41–46. Lunardini, V. J., and Ibrahim, H. (1990). “Surface Energy Balance and Surface Temperature in Cold Regions,” Proceedings of the Polartech ’90, Danish Hydraulic Institute, Copenhagen, Denmark, pp. 101–110. Pufahl, D. (1996). “Frost Action,” Roads and Airfields in Cold Regions, Vinson, T., Rooney, J. M., and Haas, W. (eds.), ASCE Press, Reston, Va. pp. 57–85. Ryynänen, T. (2000). “Instrumentoidun tierakenteen mittaukset—Ympäristötekijät 1998–1999 (Measurements of instrumented road structure—Environmental factors 1998–1999),” Finnish Road Administration, Finnra reports 42/2000, Helsinki, Finland (in Finnish). Saarelainen, S. (1996). “Pavement Design Applying Allowable Frost Heave,” Proceedings of the Eighth International Conference on Cold Regions Engineering, ASCE Press, Reston, Va., pp. 890–898. Savolainen, M., Ryynänen, T., Belt, J., and Ehrola, E. (2001). “Temmeksen koetien tutkimukset 1996–2001: Yhteenvetoraportti (Temmes test road project 1996–2001: Summary report),” Finnish Road Administration, Finnra reports 69/2001 (in Finnish). Shur, Y. L., and Slavin-Borovskly, V. B. (1993). “N-Factors Maps of Russian Permafrost Regions,” Proceedings of the Sixth International Conference on Permafrost, South China University of Technology Press, Wushan Guangzhou, China, vol. 1, pp. 564–568. Ullidtz, P. (1987). Pavement Analysis, Elsevier Science, New York, p. 318. Ullidtz, P. (2002). “Analytical Tools for Design of Flexible Pavements—Keynote address at the Ninth International Conference on Asphalt Pavements,” Copenhagen, Denmark, International Society for Asphalt Pavements, White Bear Lake, Minn., http://www. asphalt.org/ (April 5, 2007). Wu, C. -L., Rada, G. R., Lopez, A., and Fang, Y. (2000). “Accuracy of Weather Data in Long-Term Pavement Performance Database,” Transportation Research Record: Journal of the Transportation Research Board, No. 1699, Transportation Research Board of the National Academies, Washington, D.C., pp. 151–159. Zarling, J. P., and Braley, W. A. (1988). “Section 3, Geotechnical Thermal Analysis,” Embankment Design and Construction in Cold Regions, Johnson, E. G., Phukan, A., and Haas, W. H. (eds.), ASCE Press, Reston, Va., pp. 35–92.
CHAPTER
6
Design Considerations and Approaches
A
gencies are moving toward lifetime engineering in pavement design. Longterm procurement methods are used increasingly especially in northern Europe to accomplish the objectives of lifetime engineering. In order to make informed decisions in selecting a specific pavement structure, human, economic, socio-cultural, and ecological aspects need to be considered over the entire lifetime of the structure. This chapter introduces the lifetime engineering concept, long-term procurement methods used for pavements, calculation of the life-cycle costs, and pavement management concepts. These principles will be utilized in Chaps. 8 to 10, where pavement design and management are described.
6-1
Lifetime Engineering Considerations The integrated lifetime engineering methodology involves the development and use of technical performance parameters to optimize and guarantee the lifetime quality of the structures in relation to the requirements arising from several considerations listed in Table 6-1. The lifetime quality is the capability of the whole network or an object to fulfill the requirements of users, owners, and society over its entire life (Sarja 2003). The components that help in the organization of the tasks in design, management, and maintenance of pavements in the lifetime engineering method are described in Table 6-2. Example 6-1 Briefly describe factors that need to be considered in lifetime engineering methodology between alternatives of gravel surface and asphalt surface treatment (AST). Solution The following considerations are organized according to the categories described in Table 6-1. The human considerations favor AST due to smoother ride and considerably less dust generation when compared to gravel surface. The benefits include savings due to reduced travel time, accidents, vehicle operating costs, and improved human health. When economics are considered, construction and rehabilitation cost of AST is higher than gravel surface, but it requires less maintenance. Several factors affect the economy of demolition, recovery, and reuse or recycling of the surfacing materials. Gravel surface can be graded and reused on site, which is economically efficient. However, its quality for recycling for other uses is typically marginal due to the fines content that makes it weak and frost-susceptible material. The AST can be left underneath a new surfacing layer and reused in providing an impermeable layer, or it can be recycled in other pavement layers. If the surfacing materials need to be disposed off and placed in a land fill, gravel surfacing could be used as daily cover material, whereas asphalt surface treatment needs special handling in some jurisdictions.
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Chapter Six Human considerations
• • • • •
Mobility Level of service Safety Health Driving comfort
Economic considerations
• Investment economy • Construction economy • Lifetime economy in operation, maintenance, repair, rehabilitation, reconstruction, demolition, recovery and reuse, recycling of materials
Cultural considerations
• • • • •
Traditions Life style Business culture Aesthetics Architectural styles and trends
Ecological considerations
• • • • •
Raw materials economy Energy economy Environmental burdens economy Waste economy Biodiversity
Source: modified from Sarja 2003.
TABLE 6-1
Lifetime Engineering Aspects for Pavements
Socio-cultural considerations depend on the setting of the project. Gravel surfaces are preferred in cases where the local residents do not want to improve mobility and access to the area, but instead want to maintain their traditional lifestyle and keep, for example, tourists away from their pristine milieu. Aesthetics and architectural styles and trends may favor either gravel surface or AST. Business culture is tied to the transportation costs; how far is economic to bring AST equipment if the local construction business does not support it? Ecological consideration relates to the fact that the AST contains bituminous binder. Bitumen reduces its raw material economy, energy economy (use of energy to produce the binder, its alternative use as fuel and energy required for transportation), and waste economy. The benefit of the AST is the protection of the environment from the dust effects when compared with gravel surface. A life-cycle cost analysis needs to be conducted to select the proper surfacing material for the project (see Sec. 6-3). Although not easy, it should consider all the listed factors; that is, monetary values need to be assigned for each consideration.
6-2
Long-Term Procurement Methods When inspecting the components in Table 6-2, it becomes apparent that each responsible party for the project needs incentives to become involved in the lifetime engineering concept. One way to accomplish this is to use alternatives for the traditional procurement method, where investment costs steer the selection of pavement structure, and the facility is transferred to the owner at the end of the construction or short-term
Design Considerations and Approaches Lifetime investment planning and decision making
Multiattribute optimization and decision making methodology is used to rationally organize and measure various distinctive requirements listed in Table 6-1 throughout the design—construction—maintenance— reuse/recycle-disposal sequence. Availability or lack of funding for capital building projects versus funding for operation and maintenance is considered.
Integrated lifetime design
Involves analysis of the requirements listed in Table 6-1 and their interpretation into technical performance specifications, creation of additional design solutions, life-cycle analysis and optimization of the design options, and finally the detailed design of the selected structural system and its components.
Integrated lifetime construction
Involves analysis of the requirements listed in Table 6-1 and their interpretation into construction methods, identification of other methods, life-cycle analysis and optimization of the methods, and finally the detailed planning of the selected construction method and sequence. Cold regions constraints include short construction season, marginal weather conditions, frozen aggregates and soils, access to the site, labor and equipment, long transportation and hauling distances for construction materials and recycled materials, and sensitive terrain especially in permafrost areas.
Integrated lifetime management and maintenance planning
The economic and performance models developed in the design stage are used as the first estimates for the life-cycle maintenance planning. In course of use, the results of periodic condition assessments are used for updating the forecasting models (still considering requirements listed in Table 6-1).
Reuse, recycling, and disposal
High priority should be given to reusing pavement demolition materials (e.g., crushed concrete and asphalt concrete or excavated granular material) for a pavement construction or rehabilitation project. When feasible and appropriate, the use of industrial by-products (e.g., fly ash and blast furnace slag) and other construction and demolition waste materials (masonry, roofing materials) should be considered for reducing material consumption with appropriate performance specifications reflecting the requirements listed Table 6-1. The recycling possibilities of new materials and components shall be considered during the design phase (e.g., the ability to separate the structural layers during demolition).
Source: modified from Sarja 2003.
TABLE 6-2
Components of Lifetime Engineering Suitable for Cold Regions Pavements
warranty period. Some agencies are using Design-Operate-Transfer procurement concept with its variations to aid in keeping the long-term cost minimal, serving the customers needs and allowing innovations in each step of the process (Levy 1996; Andersen et al. 2000). The Finnish Road Administration (Finnra) has used Design-Build-Finance-Operate (DBFO) method for their large-scale road projects (Jokela 2002; Kalliokoski and Kasari 2003). The goal is to achieve the aforementioned objectives by creating a long-term service
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Chapter Six
FIGURE 6-1
Relative life-cycle costs for road projects (Jokela 2002).
concept that is based on lifetime responsibility. The selection of an alternative is based on life-cycle cost (LCC) that considers the lifetime quality criteria set up by the owner agency. The LCC here includes the cost of investment and residual value, cost of operation and maintenance, and cost of funding including risks during the analysis period. Figure 6-1 shows the rough relationship between the cost categories over a lifetime of a road project. The procurement model aims at a win-win situation by considering the expectations of each involved party (see Fig. 6-2) including the contractor. Reports from Finnra’s pilot DBFO project of Lahti motorway indicated that the procurement method was beneficial (Salmela et al. 2003): Users benefited by accessing the facility faster than anticipated and realized savings due to avoided accidents and reduced travel time. The desired benefits for the society were realized without compromising the quality of the service. The road enterprise is profitable while the owner receives adequate service at lower cost. There is a great incentive to minimize the life-cycle costs when one service producer takes the responsibility over the entire lifetime of the project (up to 30 years).
FIGURE 6-2
Expectations of parties involved (Jokela 2002).
Design Considerations and Approaches Temptation to reduce the quality of the design, the construction or the maintenance level quickly disappears as the service provider considers the long-term performance consequences. Innovations are encouraged throughout the lifetime of the project from planning to the end of the service life of the structure in order to maximize the benefit of the procurement.
6-3
Life-Cycle Cost Analysis The life-cycle cost analysis (LCCA) is a technique that helps evaluating the long-term economic efficiency between competing alternative investments (Walls and Smith 1998). It incorporates initial and discounted future costs for the owner agency, users, and other effected parties over the lifetime of the alternatives. The goal of the LCCA is to identify the best value, that is, the lowest long-term investment expenditures that satisfy the lifetime quality requirements. Each consideration in the human, socio-cultural, and ecological category can be assigned a monetary value and included in the LCCA. The costeffectiveness of different maintenance strategies can also be optimized using the LCCA. For example, the strategy of preventive maintenance can be compared to the rehabilitation strategy (see Fig. 6-3). Calculation of the lifetime cost is based on the comparison of costs of several investment scenarios over a relatively long analysis period. This can be done by comparing annual costs or the summation of the discounted present values for each cost. Only differential costs between the alternatives are considered (common cost to all alternatives cancel out). The costs typically included are the road owner costs and user costs. The road owner costs consist of the investment costs, routine maintenance costs, major maintenance operations, and rehabilitation. The road user bears costs during normal operation of the roadway and during construction, maintenance, and rehabilitation activities. The user costs during normal operation derive from differences in pavement roughness dependent on the maintenance strategies (see Fig. 6-3). The costs at work zone originate from reduced road capacity (mainly free flow versus forced flow) and work-related road roughness. Table 6-3 lists the costs for the owner agency and road users.
FIGURE 6-3 Comparison of preventive maintenance and rehabilitation strategies (Haas et al. 1994).
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Chapter Six Agency costs (including associated administrative costs and costs related to tort liability)
User costs
Investment
• Engineering (including environmental clearance) • Contract administration • Construction supervision • Construction
Routine maintenance
• Operations that involve local repairs at district level, e.g., patching and crack sealing
Major maintenance
• Operations that involve more comprehensive measures at district level, e.g., overlays
Rehabilitation
• Operations to address severe pavement problems, e.g., restoration or reconstruction
Normal operation
• Driving discomfort • Vehicle operation • Accidents
Work zone
• • • •
Time delay Driving discomfort Vehicle operation Accidents
Source: modified from Haas et al. 1994 and TAC 1997.
TABLE 6-3 Typical Costs Considered in Pavement Design Life-Cycle Cost Analysis
An example of the flow of an LCCA is given in Fig. 6-4. The analysis is based on a predetermined interest (discount) rate, maintenance strategy, criteria for pavement distress, and analysis period. The cost analysis is conducted once a year, after which the pavement structure is aged by another year. The effect of the environmental and estimated traffic load on the pavement’s distress level is predicted for the year in question using relevant performance models. The modeled pavement condition is then checked against the performance criteria, such as maximum rut depth, roughness, severity of cracking, and so on. When one or more pavement distress types equal or exceed the criteria, also called trigger value, specific maintenance treatments are considered with the associated agency and user costs. These costs together with the annual routine maintenance and user costs are calculated for each year within the analysis period.
6-3-1
Factors of Life-Cycle Cost Analysis
In addition to the costs listed above, the following factors have an important effect on the outcome of the analysis.
Interest Rate The interest rate is based on the minimum attractive rate of return (MARR) that the owner agency will accept. The lowest acceptable MARR should not be less than the interest rate at which the money could be borrowed.
Design Considerations and Approaches
FIGURE 6-4 Flowchart example for life-cycle cost calculations (modified from Petäjä and Spoof 2001).
Analysis Period The selected analysis period depends on the maintenance strategies and the alternatives to be compared. The longer the period is, the less effect future costs of maintenance operations and the residual value have on the decisions. One school of thought is to select an analysis period long enough to make the differences in the residual values of each alternative insignificant, and omit them from the calculations of the annual costs (Tammirinne 2002). The U.S. Federal Highway Agency’s policy recommends an analysis period of a minimum of 35 years (Walls and Smith 1998).
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Chapter Six
Residual Value The residual value of a pavement structure depends on the pavement’s condition at the end of the analysis period. If it is considered in the LCCA, it could be estimated as the difference between the construction of a new road with similar characteristics and the rehabilitation of the existing road to its original condition. For example, if construction of a new road in a similar context was $250,000 per kilometer and rehabilitation or reconstruction was $150,000, then the residual value would be $100,000. Significant differences in the residual values may occur when substantially different alternatives are compared [e.g., Portland cement concrete (PCC) and an asphalt concrete surface layer], in which case the residual value should be included into the LCCA.
Selection of Pavement Alternatives Selection of pavement type alternatives is constricted by several variables such as traffic, climate, subgrade, condition of existing structure if applicable, availability of construction materials, and availability of expertise (AASHTO 1993; Tammirinne 2002). All lifetime engineering principles and considerations listed in Table 6-1 should be kept in mind in the selection of the alternatives. Figure 6-5 shows an example of a selection process that is based on the Road Structures Research Program carried out in 1994–2001 by the Finnish Road Administration, Finnra (Tammirinne et al. 2002). The selection of the pavement type typically starts with the selection of the surfacing layer. Preliminary selection of the surfacing type can be conducted using average daily traffic (ADT). Table 6-4 shows an example of preliminary pavement type selection used
FIGURE 6-5 Design system for a road structure (modified from Tammirinne 2002).
Design Considerations and Approaches Traffic Volume (ADT vehicles/day)
Pavement Type
<300
Gravel surface or surface treatment
200–1500
Soft asphalt concrete [viscosity grade of the binder (at 60°C, mm2/s) 1000–4000]
500–2500
Soft asphalt concrete (penetration grade of the binder 330–900)
1000–6000
Dense-graded hot mix asphalt (HMA) concrete
>3000
Stone matrix asphalt (SMA)
Source: Pulkkanen et al. 1999.
TABLE 6-4 Example Guidelines for Preliminary Selection of Pavement Surfacing Based on Traffic Volume
by Finnra (Pulkkanen et al. 1999). Selection guidelines for an HMA mixture type based on traffic level are provided, for example, in Garcia and Hansen (2001).
Activity Timing One of the most challenging phases of the LCCA is to predict the activity timing (the point in time when the pavement needs treatment) and the appropriate maintenance operations. The performance models required to perform the prediction are developed using pavement management data and historical experience. In the simplest form, the pavement life and activity timing is based on the collective experience of the agency’s senior engineers (Walls and Smith 1998). The mechanistic-empirical pavement condition prediction model explained in Chap. 8 can be used in the predictions, if related mechanistic explanatory variables are available.
6-3-2
Calculation of Life-Cycle Costs
The calculations of the LCCs are conducted as follows: The estimated income and benefits (positive) and expenditures and costs (negative) associated with the alternative are converted to the annual net equivalent value (ANEV) of that alternative. The one with the highest ANEV is selected (or in case of independent decisions, such as “which pavements should be repaved this year?” all pavements with a positive ANEV are selected). The ANEV can be calculated using Eqs. (6-1) to (6-3) (Potter 1990): ANEV = −( A/P)in P − A + ( A/F)in F
(6-1)
where ( A/P)in =
i(1 + i)n (1 + i)n − 1
(6-2)
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Chapter Six is the annual value for a present amount (P), ( A/F)in =
i (1 + i)n − 1
(6-3)
is the annual value for a future amount (F), where A is annual amount, i is interest rate (MARR), and n is number of periods. Alternatively, the alternatives can be compared on the basis of present net equivalent value (PNEV). If the alternatives are mutually exclusive, the one with the highest PNEV is selected. If the alternatives are independent, all alternatives with positive PNEV are selected. The PNEV can be calculated using Eqs. (6-4) to (6-6): PNEV = − P − (P/A)in A + (P/F)in F
(6-4)
where (P/A)in =
(1 + i)n − 1 i(1 + i)n
(6-5)
is the present value for an annual amount (A), (P/F)in =
1 (1 + i)n
(6-6)
is the present value for a future amount (F), where P is present amount, i is interest rate (MARR), and n is number of periods. The PNEV method requires that all alternatives are evaluated over the same analysis period. If the lives of the alternatives are not equal, the lowest common multiple of the lives is used, with each alternative repeated to fill the analysis period. Example 6-2 Cost estimates for two pavement alternatives are listed in Table 6-5. MARR is 10 percent. Select the best alternative by comparing the annual costs. Solution Alternative A: % ( A/P)10 = 20
% = ( A/F )10 20
0 . 1(1 + 0 . 1)20 = 0 . 1175 (1 + 0 . 1)20 − 1 0 . 10 = 0 . 0175 (1 + 0 . 10)20 − 1
Alternative
Investment Cost
Annual Maintenance Cost
Salvage Value
Life (years)
A
11,500
1,200
1,000
20
B
8,000
1,600
0
10
TABLE 6-5
Relative Costs for Pavement Alternatives in Example 6-2
Design Considerations and Approaches ANEV = −0.1175 × 11,500 − 1200 + 0.0175 × 1000 = −1351 − 1200 + 17 = −2534 Alternative B: 0 . 1(1 + 0 . 1)15 = 0 . 1627 (1 + 0 . 1)15 − 1
% = ( A/P)10 10
ANEV = −0.1627 × 8000 − 1600 = −1302 − 1600 = −2902 Alternative A has lower annual cost than Alternative B and is selected as the pavement structure to be built. Example 6-3 Consider the scenario given in Example 6-2, except that Alternative A needs an additional maintenance procedure in year 10 that costs 1500. Solution % = ( A/F )10 10
0 . 10 = 0 . 0627 (1 + 0 . 10)10 − 1
ANEV = −2534 − 0.0627 × 1500 = −2637 Alternative A has still lower annual cost than Alternative B and is the chosen pavement structure. Example 6-4 Repeat Example 6-2 by comparing the alternatives with the present net equivalent value concept. Solution Note that the alternatives have different life spans. The lowest common multiple of the lives is 20 years, which means that Alternative B is repeated twice. Alternative A:
% = (P/A)10 20
(1 + 0 . 10)20 − 1 = 8 . 513 6 0 . 10(1 + 0 . 10)20
% = (P/F )10 20
1 = 0 . 1486 (1 + 0 . 10)20
PNEV = −11,500 − 8.5136 × 1200 + 0.1486 × 1000 = −11,500 − 10,216 + 149 = −21,568 Alternative B: % = (P/F )10 10
1 = 0 . 3855 (1 + 0 . 10)10
PNEV = −8000 − 0.3855 × 8000 − 8.5136 × 1600 = −8000 − 3084 − 13,622 = −24,706 Alternative A has a larger PNEV than Alternative B and is selected. Note that for both alternatives % PNEV = (P/A)10 ANEV 20
(6-7)
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Chapter Six
6-4
Pavement Management Concepts The pavement maintenance management is the process of coordinating and controlling a comprehensive set of activities in order to maintain pavements, so as to make the best possible use of resources available, that is, maximize the benefits for society (OECD 1987; Haas et al. 1994; TAC 1997). Pavement management system (PMS) is a set of tools used to assist decision makers at all levels in making rational and well-informed decisions. Pavement management is generally divided in two levels: network level and project level. (AASHTO 2001; Haas et al. 1994). The outcome of the network-level analysis include maintenance and rehabilitation needs, forecasted future impacts for various funding options considered, and prioritized listings of candidate projects needing repair for the evaluated options. These results can be used to provide support to legislative, administrative, and technical users. At the project level, the purpose is to provide maintenance rehabilitation or reconstruction strategy within available funds that most effectively meet the desired lifetime engineering requirements.
6-4-1
Network-Level PMS
The basic elements of a network-level PMS include an inventory, a condition assessment, determination of needs, prioritization of projects needing maintenance and rehabilitation, a method to determine the impact of funding decisions, and a feedback process. Inventory: The inventory includes information about the location, limits, size, connectivity to other sections, number of traffic lanes, route designations, and functional classification for each management section. The data are normally entered into the database only once, after which they are changed when necessary. Pavement condition assessment: It begins with data collection that includes observations for the type, amount and severity of surface distress, structural integrity, ride quality, and surface friction of the pavement (see Table 6-6). Some of these techniques are described in chapter 4, section 2. Some agencies also include other items in their data, such as rut depth. The condition of the pavements is then ranked based on the separate measures listed in Table 6-6, or the data are combined into an index that describes the pavement condition in one single number. Determination of needs: Once the pavement network has been defined with its associated condition, the work needed and funds to complete the work are identified. This is conducted over an analysis period to bring the pavement network to a desired level of service. Established trigger values are used for the condition index, or indices, to determine which sections or management units need work. These trigger values depend on the surface type, functional classification, speed, and traffic loading of the road. Figure 6-6 shows the different treatment categories with the associated pavement condition and trigger values. For example, at the scale from 0 to 100, the trigger values for rural collector highway could be 70, 50, and 25, whereas for urban interstate highway the corresponding trigger values could be set at 80, 65, and 40 (AASHTO 2001). Agencies that use several condition indicators establish a trigger value for each condition indicator. Table 6-7 gives an example of trigger values for a subnetwork used by the Finnish road administration, Finnra. A certain percent of pavement sections that do not meet these threshold values are accepted, though watched continuously (Pulkkanen et al. 1999). Models that project the condition of the pavements without treatments with time, the impact of treatments on the pavement condition, and the condition of the pavements with certain treatment with time are needed in order to predict when the trigger values are reached. The feedback from the pavement management data and programs such as the long-term pavement performance (LPPT) program in the United States are used to create and update deterioration models.
Description
Method
Outcome
Use
Surface distress
Damage to the pavement surface: type, severity, and quantity of observable distress
Visual observation or semi-automated condition survey systems
Pavement condition index (PCI) or distress index
• Identify timing of maintenance and rehabilitation • Identify fund needs
The maximum load and number of repetitions a pavement can carry
Nondestructive deflection testing
Current pavement load-carrying capacity
Comparison with the capacity needed to accommodate projected traffic
• A measure of pavement surface distortion • Ability of the pavement to provide a comfortable ride
• Profilometers • Response-type ride meters
• International roughness index (IRI) • Serviceability or performance indices
• For pavements with high-speed limits • Pavement condition indicator for the using public • Calculation of user costs, e.g., vehicleoperating costs
Ability of the pavement surface to provide sufficient friction to avoid skid-related problems
• Locked wheel trailer procedure (ASTM E274) • Stopping distance using a passenger vehicle (ASTM E445)
• Skid number (SN) • Stopping distance number (SDN)
• For pavements with high-speed limits • Determination of need for remedial maintenance to address safety
Structural capacity
Structural characteristics
Factor
Surface friction
Functional characteristics
Roughness
Source: Collected from AASHTO 2001, Huang 2004, Haas et al. 1994, and TAC 1997.
TABLE 6-6 Factors Used in Pavement Condition Assessment
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Chapter Six
FIGURE 6-6
Example of trigger values in pavement management.
Prioritizing candidate sections: This is needed when the available funds are less than needed funding or when the funds need to be allocated over a number of years. The goal of the prioritizing is to provide the best possible pavement network condition for the funds expended. Ranking the worst sections first is the simplest, but it can also be the least cost-effective prioritizing method (AASHTO 2001). Optimization tools used to determine the optimal allocation of funds include linear programming, integer programming, Markov decision analysis, and dynamic programming. However, due to the difficulties in the true optimization tools, such as quantifying all the lifetime engineering benefits and dramatic changes in selected sections with small funding changes, some agencies use other prioritizing tools. For example, marginal cost-effectiveness in a multiyear prioritization gives “near optimal” solution and is more stable than the true
Surface Condition
Structural Condition
Distress Index, m2
Relative Bearing Capacity†, %
Allowed Percentage of Road Length‡ below the Trigger Value, %
ADT*
IRI, mm/m
Rut Depth, mm
<350
5.5
NA
140
70
18
350–1500
4.1
20
80
70
14
1500–6000
3.5
20
60
70
7
>6000
2.5
20
30
70
7
∗
Highways with ADT <1500: use values for ADT 1500–6000. Target bearing capacity depends on ADT. ‡ Per each ADT-class. Source: Pulkkanen et al. 1999. †
TABLE 6-7
Examples for Trigger Values for Pavement Condition Indicators at Network-Level Maintenance Analysis
Design Considerations and Approaches optimizing tools (Haas et al. 1994). After the optimum pavement sections to be treated are selected, sections may still be manually removed or added based on other consideration that are not included into the prioritization software or the used data set. Impact of funding decisions: Keeping the condition of the pavement network at a certain level or improving it requires adequate level of funding from the network administration. When applying for the funds, agencies can show the health of the pavement network by projecting its condition over a period of time with various levels of funding. For example, the length of the network that will be in poor condition (does not meet the assigned network-level trigger values) can be predicted with different funding levels as shown in Fig. 6-7. Feedback process: Feedback process provides information on how well past estimates have matched observed values and provides information to improve future estimates (Haas et al. 1994; AASHTO 2001). The feedback system may or may not be part of PMS software, and most often the feedback data is not old enough for long-term analysis. The feedback system is a very useful tool in improving the pavement performance models. However, the feedback process cannot be the only way to accomplish this, as new materials and structural solutions cannot be considered. Accelerated testing using, for example, a heavy vehicle simulator on new pavement structure candidates is conducted in order to assess their feasibility as long-term performers.
6-4-2
Project-Level PMS
Project-level pavement management involves technical and the economic analysis of solutions applicable to pavements prioritized in the network-level management process. It includes the detailed analysis of the existing structure and the design of applicable solutions to determine material types, thicknesses, and possible inclusion of special layers, such as reinforcement or board insulation, for a pavement structure. Project-level management addresses new design, but also maintenance, rehabilitation design, and reconstruction design. The purpose of the project-level management is to find the solution with the best lifetime quality for a section of pavement selected for work. The design concepts are described in Chaps. 7 and 8, and the maintenance and rehabilitation in Chap. 9.
FIGURE 6-7 Effect of funding on pavement length below target condition (Pulkkanen et al. 1999).
261
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Chapter Six
Review Questions 6-1. Cost estimates for three pavement alternatives are given in Table 6-8. The minimum attractive rate of return is eight percent. Select the best alternative by comparing the annual cost. Annual Maintenance Cost
Periodic Maintenance Cost (Year)
Alternative
Investment Cost
A
20,000
700
1,000 (5)
B
10,000
900
0
C
7,000
1,100
Salvage Value
Life (Year)
500
10
−500
8
0
6
2,000 (3)
TABLE 6-8 Relative Costs for Pavement Alternatives for Question 6-1 6-2. Repeat Question 6-1 by comparing the alternatives with the present net equivalent value concept.
6-3. Explain briefly why it may be difficult to obtain funds for preventive maintenance. 6-4. Rehabilitation of a road through a residential area includes drainage improvements, additional structural layers (that raise the road centerline) and a new surfacing. Which lifetime engineering aspects listed in Table 6-1 need to be considered? List issues under each aspect.
References AASHTO. (2001). Pavement Management Guide, American Association of State Highway and Transportation Officials, Washington, D.C. AASHTO. (1993). Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials, Washington, D.C. Andersen, A., and Enterprise LSE. (2000). “Value for Money Drivers in the Private Finance Initiative,” U.K. Office of Government Commerce, http://www.ogc.gov. uk/sdtoolkit/reference/ogc_library/PFI/series_1/andersen/7tech_contents.html (March 6, 2007). Garcia, J., and Hansen, K. (2001). HMA Pavement Mix Type Selection Guide, National Asphalt Pavement Association and Federal Highways Administration Information Series 128. National Asphalt Pavement Association, Lanham, Md. Haas, R., Hudson, W. R., and Zaniewski, J. (1994). Modern Pavement Management, Krieger Publishing Company, Malabar, Fla. Huang, Y. (2004). Pavement Analysis and Design, 2d ed., Pearson, Upper Saddle River, NJ. Jokela, P. (2002). “Life-Cycle Model,” Finnish Road Administration, Finnra Report 54/2002, Helsinki, Finland (in Finnish). Kalliokoski, A., and Kasari, T. (2003). “Life Cycle of Road Structures in Road Construction in Practice,” Proceedings of the Intergrated Lifetime Engineering of Buildings and Civil Infrastructures, ILDCES 2003, Finnish Association of Civil Engineers RIL, Helsinki, Finland, pp. 1001–1006. Levy, S. M. (1996). Build Operate Transfer, Paving the Way for Tomorrow’s Infrastructure, Wiley, NY.
Design Considerations and Approaches OECD. (1987). Pavement Management Systems, Organization for Economic Cooperation and Development, Paris, France. Petäjä, S., and Spoof, H. (2001). “Päällysterakenteen elinkaarikustannusanalyysi (Lifecycle cost analysis for pavement structure),” Method description TPPT 20, Technical Research Centre of Finland and Finnish Road Administration, Helsinki, Finland (in Finnish). Potter, M. C. (1990). Fundamentals of Engineering, Great Lakes Press, Okemos, MI. Pulkkanen, J., Toivonen, T., and Männistö, V. (1999). “Päällystettyjen teiden ylläpidon toimintalinjat ja ohjaus (Principles and directions for pavement management),” Finnish Road Administration, Finnra, Helsinki, Finland (in Finnish). Salmela, V., Saltevo, A., Tolvanen, R., Kuorikoski, P., and Numminen, P. (2003). “Lahti Freeway DBFO—interim evaluation—Lahdentien SRRK—väliarviointi,” Finnish Road Administration, Finnra Internal Report 9/2003, Helsinki, Finland (in Finnish). Sarja, A. (2003). “System-Based Vision for Strategic and Creative Design,” Proceedings of the Second International Structural Engineering and Construction Conference ISEC02, A. A. Balkema Publishers, Netherlands, pp. 1703–1708. TAC. (1997). Pavement Design and Management Guide, Association des Transports du Canada, Ottawa. Tammirinne, M., Valkeisenmäki, A., and Ehrola, E. (2002). ”Road structures research Programme 1994-2001, summary report,” Finnish Road Administration, Finnra report 37/2002, Helsinki, Finland. Tammirinne, M. (2002). “Tierankenteen suunnittelu ja mitoitus; TPPT-suunnittelujärjestelmän kuvaus (Road pavement structure design; description of TPPT design system),” Finnish Road Administration, Finnra report 7/2002, Helsinki, Finland (in Finnish). Walls III, J., and Smith, M. (1998). “Life-Cycle Cost Analysis in Pavement Design,” FHWA-SA-98-079, Federal Highway Administration, Washington, D.C.
263
CHAPTER
7
Mix Design 7-1
Mix Design Approaches The purpose of a mix design is to produce a mixture that meets preset requirements for pavement performance and constructability at a specific geographical location. The designed mixture must resist environmental and traffic loads, it has to be relatively easy to place, and it must provide a safe and smooth driving surface. Mix design is always an optimization process, which is imperative in cold regions due to the extreme temperature regime at which the pavement must perform. Due to the low albedo of the dark-colored asphalt pavement, the surface temperature rises high in the summer time, whereas in the winter time the pavement is subjected to extremely low temperatures (see Chap. 2). Therefore, one must design a pavement that is flexible enough at low temperatures to resist thermal cracking, but stiff enough at high temperatures to resist permanent deformation resulting from traffic loads. The mixture must also be able to resist thermal and load related fatigue cracking, excessive binder aging, and moisture-related damage. For the design of wearing course mixtures, additional requirements may include resistance against ruts caused by wear of studded tires and optimum surface roughness that provides adequate skid resistance while trying to minimize pavement noise and rolling resistance. Mix design may at times be constrained by contract documents of the paving job. Table 7-1 lists the different types of specifications, their descriptions, and the party that typically takes the risk for a possible pavement failure. Traditionally, owner agencies specify in detail how the mix design is conducted, and consequently buys the job if the specified mixture properties and other specified factors are met. A new trend in pavement procurement is to pass the responsibility of the pavement performance to the contractor (see Chap. 6). In this case, the contractor selects the appropriate level of effort for the mix design to meet an end result specification, for example, the maximum rut depth at the end of the contract period. The selection of bound or unbound pavement surface type, that is, hot mix asphalt concrete (HMA), cold mixes, surface treatments, or gravel surface is based on lifetime engineering considerations (see Chap. 6). The main considerations are traffic volume and percentage of heavy vehicles and passenger cars using studded tires. In permafrost areas, the type of subgrade (thaw stable or unstable) tends to dictate the selection (see Chap. 10). Table 6-4 gives an example of the preliminary selection of surfacing type for areas in seasonal frost areas. The following sections describe mix design procedures for HMA, cold mixes, stabilized bases, surface treatments, and gravel surfaces.
265
266
Chapter Seven
Description
Formula/recipe
Contains requirements for aggregate, binder, target air voids, and mixture density
Owner agency
Method specification
Contains requirements for test results from specified mix design procedures and test equipment, e.g., Superpave mix design (AASHTO PP28 2003b)
Owner agency
End result specification
Determines pavement condition at the end of the construction, e.g., thickness
Contractor
Performance specification
Determines field performance for a given warranty period, e.g., rut depth
Contractor
Combinations
Contain an end result specification and a formula or a method specification
Unknown; should be avoided
TABLE 7-1
7-2
Risk of Pavement Failure Is Typically Taken By
Type
Types of Pavement Mixture Specifications in Contract Documents
Hot Mix Asphalt Concrete Conventionally, the mix design for hot mix asphalt-aggregate mixtures have been designed using Marshall mix design procedure (ASTM D1559). It consists of volumetric mix design procedure and a mixture stability test at a single critical test temperature and loading rate. While this method has served adequately in the past, today engineers feel that the sample preparation with Marshall hammer compaction is inadequate to effectively evaluate the present traffic loads. The Marshall stability test conducted at 60°C gives an empirical index value of mixture stability against permanent deformation that cannot be used as a material parameter in the structural pavement design process. In addition, the test does not address cracking characteristics of a mixture. For these reasons, the paving industry is moving toward mix design methods that address the compaction issues and include laboratory tests to obtain fundamental mechanical material properties. Figure 7-1 illustrates the main steps of a mix design procedure for HMA mixtures. The design starts with a selection of an appropriate HMA type [e.g., dense-graded, stone matrix asphalt (SMA) or open-graded friction course] and a maximum aggregate size. The selection is based on the end use of the mixture and lifetime engineering concerns. Factors considered include traffic volume, ratio of heavy vehicles and passenger cars using studded tires, and traffic speed (highway speed versus standing traffic). In some cases, other factors, such as noise reduction and moisture permeability dictate the selection of the mixture type. Figure 7-2 shows aggregate gradations for different mixture types. Garcia and Hansen (2001) provide guidelines for selection of HMA pavement mix types, but alas, cold climate is not specifically considered. The aforementioned Table 6-4, applicable for regions where studded tires are used, shows
Mix Design
FIGURE 7-1
Principles of HMA mix design procedure.
FIGURE 7-2
Example aggregate gradations for different mixture types.
an example of selection between soft asphalt concrete mix, dense-graded traditional asphalt concrete mix, and SMA. A comprehensive HMA mixture type selection guide for cold regions is yet to be developed and awaits data from long-term pavement studies. The volumetric mix design includes selections of suitable raw materials, formulation of trial mixtures, testing, and analyzing compacted samples until a desirable mixture is achieved (see Fig. 7-3). This basic mix design procedure is enhanced with performance
267
268
Chapter Seven
FIGURE 7-3
Example of a flowchart for volumetric mix design.
testing for critical pavements with high levels of traffic and for mixtures with new and unknown raw materials. Performance testing assures that the pavement will perform well in the field, resisting distresses caused by traffic and climate. Performance testing may also be used to produce fundamental material properties that can be used in the pavement structural design. In this way, the mix design and pavement structural design are connected to produce the best possible pavement. Mixture field verification should be considered as a part of the mix design process, as specimen preparation in the laboratory cannot be assumed to accurately simulate
Mix Design construction practices. Each parameter considered important for the pavement performance should be verified in the field. The following sections describe volumetric and performance mix design procedures in further detail including the special considerations due to cold climates.
7-2-1
Material Selection
Asphalt Cement The selection of asphalt cement type and grade is an essential part of the mix design process as the viscoelasticity of the mixture is mainly provided by the binder. Stiffer mixtures with stiffer binder tend to have brittle failure modes, while the softer mixtures tend to fail in a more ductile manner. For the HMA, the selected binder type is either neat or conventional asphalt cement, or modified asphalt cement (European term for asphalt cement is bitumen). While many modifiers exist, only the most typical type of modification, polymer modification, is discussed in the following sections. The asphalt cement grade refers to its classification specified, either by its consistency at a certain temperature (penetration or viscosity) or behavior across a temperature range (performance-based grading systems). The main factors affecting asphalt cement selection are the anticipated pavement temperatures and traffic load. In cold regions, the asphalt cement used should be the softest possible that can carry the traffic loads without risking rutting by permanent deformation. Straight run asphalt cements (a refining term for an asphalt cement manufactured by distillation) are classically graded using either penetration grade or viscosity grade specifications. Examples of these are given in Tables 7-2 and 7-3. In the penetration grading system, the asphalt cements that have similar penetration at 25°C have the same grade. The higher the penetration grade the softer the asphalt cement. Likewise, in the viscosity grading system, the asphalt cements that have similar viscosity at 60°C have the same grade. The higher the viscosity grade, the harder the asphalt cement. Straight run asphalt cements may also be graded using newer performancebased specifications (see an example in Table 7-4). In recent years, many road agencies have moved from the traditional penetration and viscosity grading to performance grading systems. Polymer modification is used in challenging conditions to enhance asphalt cements’ properties (especially the rutting and fatigue cracking resistance) if warranted by lifetime engineering considerations. Polymer-modified binders are specified by performance-based specifications (see Table 7-4) and sometimes by specifications based on conventional test methods (see Table 7-5). Many polymer types exist to modify asphalt cements. The most popular modifiers are thermoplastic elastomers, such as styrene-butadiene-styrene (SBS). The thermoplastic elastomers derive their strength and elasticity from a physical cross-linking of the molecules into a three-dimensional network. The styrene end-blocks impart strength to the polymer (and permanent deformation resistance to the pavement) and the mid-block, butadiene, gives the binder its elasticity (Read and Whiteoak 2003). Special to cold regions is the use of extremely soft asphalt cements for the lowvolume roads, where the environmental stresses will dictate the pavement service life. Table 6-4 shows criteria for selecting soft asphalt concrete on the basis of the daily traffic. Table 7-6 gives examples for soft asphalt cement specifications.
269
270 Penetration Grade Test
B20/30
B35/50
B50/70
B70/100
B100/150
B160/220
Penetration at 25°C, 1/10 mm
20–30
35–50
50–70
70–100
100–150
160–220
Softening point, °C
55.0–63.0
50.0–58.0
46.0–54.0
43.0–51.0
39.0–47.0
35.0–43.0
2
Viscosity, 135°C, mm /s
≥530
Fraass breaking point, °C
≥370
≥295
≥230
≥175
≥135
≤−5
≤−8
≤−10
≤−12
≤−15
Flash point, °C
≥240
≥240
≥230
≥230
≥230
≥220
Solubility in toluene, mass-%
≥99.0
≥99.0
≥99.0
≥99.0
≥99.0
≥99.0
Tests on Residue from Rolling Thin Film Oven Test Mass loss, %
≤0.5
≤0.5
≤0.5
≤0.8
≤0.8
≤1.0
Retained penetration, %
≥55
≥53
≥50
≥46
≥43
≥37
Softening point, °C
≥57.0
≥52.0
≥48.0
≥45.0
≥41.0
≥37.0
Source: PANK 2000. Reprinted by permission.
TABLE 7-2
Example of Penetration Grade Specifications
Mix Design Viscosity Grade Test
AC-2.5
AC-5
AC-10
AC-20
AC-40
Viscosity at 60°C, Pa·s
25 ± 5
50 ± 10
100 ± 20
200 ± 40
400 ± 80
Viscosity at 135°C, mm2/s
≥80
≥100
≥150
≥210
≥300
Penetration at 25°C, 100 g, 5 s
≥200
≥120
≥70
≥40
≥20
Flash point, COC, °C
≥163
≥177
≥219
≥232
≥232
Solubility in trichloroethylene, %
≥99.0
≥99.0
≥99.0
≥99.0
≥99.0
Tests on Residue from Thin Film Oven Test Viscosity at 60°C, Pa·s
≤100
≤200
≤400
≤800
≤1,600
Ductility at 25°C, 5 cm/min, cm
≥100∗
≥100
≥50
≥20
≥10
∗If ductility is less than 100, material will be accepted if ductility at 15.6°C is 100 minimum. Source: Table 1 from AASHTO 2003a, by the American Association of State Highway and Transportation Officials. Used by permission.
TABLE 7-3
Example of AC Grade Specifications
Bitumen Test Data Chart The consistency of asphalt cements and the effect of polymer modification on their properties can be demonstrated on a bitumen test data chart (BTDC) shown in Fig. 7-4. The BTDC was originally developed by Heukelom (1969) to allow Fraass breaking point, penetration, softening point, and viscosity to be plotted as a function of temperature on one chart. Figure 7-4 plots test data of two penetration grade asphalt cements and a polymermodified binder. Out of the two penetration grade asphalt cements, the softest B160/220 has better resistance against the low-temperature cracking than the B70/100 (lower Fraass temperature), but poorer resistance against permanent deformation (lower softening point). The polymer-modified asphalt (PMB85) has better cracking and permanent deformation resistance than the two neat penetration grade asphalt cements as it has lower Fraass temperature and higher softening point. In this way, the BTDC can be used to assist in the binder selection when only conventional test results are available.
Superpave Binder Grade Selection The Superpave performance-graded binders are selected based on climate, traffic conditions, and desirable reliability factors under which the pavement must serve. The physical property requirements are constant among all performance grades (see Table 7-4). The grade selected is the temperature interval at which the specified requirements must be met. For example, an asphalt cement classified as PG 58-34 is expected to perform adequately at a temperature range of −34 to +58°C. The grade is first selected based on the historical temperature data, and then adjusted for traffic conditions. The hottest 7-day periods have been identified for 7000 weather stations in North America, and mean maximum air temperature and
271
272 Performance Grade
PG 46 34
Average 7-day max. pavement design temp. (°C)a
<46
Min. pavement design temperature (°C)
> −34
40
PG 52 46
10
16
22
28
PG 58 34
40
46
<52
> −40
> −46
> −10
16
22
28
PG 64 34
40
<58
>
>
>
>
>
−16
−22
−28
−34
−40
> −46
> −16
10
230
Viscosity, T316:b Max. 3 Pa, test temp (°C)
135
Dynamic shear, T315:c G∗/sinδ, min.
46
> −22
> −28
>
−34
> −40
>
−10
52
58
64
52
58
64
1.00 kPa, test temp @ 10 rad/s, (°C) Rolling thin-film oven test residue (T240) Mass changee, max., %
1.0
Dynamic shear, T315: G∗/sinδd, min.
46
2.20 kPa, test temp @ 10 rad/s, (°C)
22
28
34
40
> −16
>
>
−22
−28
> −34
−40
<64
Original binder Flash point temp, T48, °C
16
>
Pressure aging vessel residue (R 28) PAV aging temp., °Cf
90
Dynamic shear, T315:
10
7
4
25
22
19
16
13
10
7
25
22
19
16
13
31
28
25
22
19
16
−24
−30
−36
0
−6
−12
−18
−24
−30
−36
−6
−12
−18
−24
−30
0
−6
−12
−18
−24
−30
90
100
100
G*sinδd, max. 5000 kPa, test temp @ 10 rad/s, (°C) Critical low cracking temp., PP 42:g Determine critical cracking temp. as described in PP42, test temp, (°C)
PG 70 Performance grade
10
Average 7-day maximum pavement design temperature, °Ca
<70
Minimum pavement design temperature, °C
> −10
16
230
Viscosity, T316:b Maximum 3 Pa·s, test temp, °C
135
TABLE 7-4
28
34
40
10
16
22
PG 82 28
34
<76 > −16
Original binder Flash point temp, T48, °C
22
PG 76
Example of Performance-Based Specifications
> −22
> −28
> −34
> −40
> −10
10
16
22
28
34
> −16
> −22
> −28
> −34
< 82 > −16
> −22
> −28
> −34
> −10
273
274 PG 70 Performance grade
10
Dynamic shear, T315:c G∗/sinδ, minimum 1.00 kPa, test temp @ 10 rad/s, °C
70
16
22
28
PG 76 34
40
10
16
22
PG 82 28
34
10
76
82
76
82
16
22
28
34
Rolling thin-film oven test residue (T240) Mass changee, maximum, %
1.0
Dynamic shear, T315: G∗/sinδd, minimum 2.20 kPa, test temp @ 10 rad/s, °C
70
Pressure aging vessel residue (R 28) PAV aging temperature, °Cf
100 (110)
Dynamic shear, T315: G∗sinδd, maximum 5000 kPa, test temp @ 10 rad/s, °C
34
31
28
25
22
19
37
34
31
28
25
Critical low cracking temperature, PP 42:g Determine critical cracking temp as described in PP42, test temp, °C
0
–6
–12
–18
–24
–30
0
–6
–12
–18
–24
100 (110)
100 (110) 40
0
37
34
31
28
–6
–12
–18
–24
a Pavement temperatures are estimated from air temperature using an algorithm contained in the LTPP Bind program, may be provided by the specifying agency, or by following the procedures as outlined in MP 2 and PP 28. b This requirement may be waived at the discretion of the specifying agency if the supplier warrants that the asphalt binder can be adequately pumped and mixed at temperatures that meet all applicable safety standards. c For quality control of unmodified asphalt binder production, measurement of the viscosity of the original asphalt cement may be used to supplement dynamic shear measurements of G∗/sinδ at test temperatures where the asphalt is a newtonian fluid. d ∗ G /sinδ = high temperature stiffness and G∗sinδ = intermediate temperature stiffness. e The mass change shall be less than 1.0 percent for either a positive (mass gain) or a negative (mass loss) mass change. f The PAV aging temperature is based on simulated climatic conditions and is one of three temperatures 90, 100, or 110°C. Normally the PAV aging temperature is 100°C for PG 58-xx and above. However, in desert climates, the PAV aging temperature for PG 70-xx and above may be specified as 110°C. g For verification of grade, perform T 313 at the test temperature and the test temperature minus 6°C and T 314 at the test temperature. Compare the failure stress from T 314 to the calculated induced thermal stress as per PP 42. If the failure stress exceeds the induced thermal stress, the asphalt binder is deemed a “PASS” at the specification temperature. Source: Table 1 from MP 1a in AASHTO 2003a, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission.
TABLE 7-4
Example of Performance-Based Specifications (Continued)
Mix Design Grade Test
PMB65
PMB75
PMB85
Penetration at 25°C, 1/10 mm
70–150
70–150
50–100
Softening point, °C
≥65.0
≥70.0
≥75.0
Flash point, °C
≥235
≥220
≥220
Elastic recovery at 10°C
≥75
≥75
≥75
Softening point difference top-bottom, °C
≤5.0∗
≤5.0∗
≤5.0∗
∗If the softening point difference is greater than 5°C, the binder supplier must provide recommendations on the handling necessary to prevent separation. Source: PANK 2000. Reprinted by permission.
TABLE 7-5
Example of Polymer-Modified Binder Specifications
Grade Test
B250/330
B330/430
B500/650
B650/900
Penetration at 15°C, 1/10 mm
70–130
90–170
140–260
180–360
Viscosity, 60°C, Pas
≥18
≥12
≥7.0
≥4.5
Viscosity, 60°C, mm2/s
V1500
V3000
1,000–2,000
2,000–4,000
Viscosity, 135°C, mm2/s
≥100
≥85
≥65
≥50
Fraass Breaking Point, °C
≤−16
≤−18
≤−20
≤−20
Flash Point, °C
≥180
≥180
≥180
≥180
≥160
≥160
Solubility in toluene, mass-%
≥99.0
≥99.0
≥99.0
≥99.0
≥99.0
≥99.0
Tests on residue from:
Rolling Thin Film Oven Test
Thin Film Oven Test
Mass loss, %
≤1.0
≤1.0
≤1.5
≤1.5
≤2.0
≤1.7
Viscosity ratio, 60°C
≤4.0
≤4.0
≤4.0
≤4.0
≤3.0
≤3.0
Source: PANK 2000. Reprinted by permission.
TABLE 7-6 Example of Specifications for Soft Asphalt Binders
its standard deviation is calculated for each station including data for all the years in operation. In the same way, a 1-day minimum air temperature of each year was identified, and the mean and standard deviations were calculated (Asphalt Institute 2001). The air temperatures are converted to pavement temperatures using Eqs. (7-1) and (7-2). Equation (7-1) is based on theoretical analyses of actual conditions performed with models for net heat flow and energy balance, and assuming typical values for solar absorption (0.9), radiation transmission through air (0.81),
275
276
Chapter Seven
FIGURE 7-4 Bitumen test data chart comparing two penetration grade asphalt cements and polymer-modified binder.
atmospheric radiation (0.7), and wind speed (4.5 m/s). It gives the high 7-day pavement design temperature at a depth of 20 mm below the pavement surface, T20mm (Asphalt Institute 2003): T20mm = (Tair – 0.00618 Lat2 + 0.2289 Lat + 42.2)0.9545°C – 17.78°C
(7-1)
where Tair is 7-day average high air temperature, °C and Lat is the geographical latitude of the project. Several methods have been proposed to convert the minimum air temperature to minimum pavement design temperature (Raad et al. 1997; Asphalt Institute 2003). For cold regions, it is generally agreed that the pavement temperature is warmer than the air temperature during a cooling trend when the low-temperature cracking occurs. Equation (7-2) developed by Canadian Strategic Highway Research Program (SHRP) researchers is used as a recommended conversion model for the minimum pavement design temperature, Tmin (Asphalt Institute 2001): Tmin = 0.859 Tair + 1.7°C
(7-2)
where Tair is minimum air temperature in average year, °C. The Superpave system allows the engineers to use reliability concepts to determine the degree of risk to the high and low pavement temperatures. The reliability is the percent probability in a single year that the actual pavement temperature will not exceed the design temperatures. This concept is illustrated in Example 7-1. Example 7-1 The mean high 7-day pavement design temperature is calculated to be 40°C with a standard deviation of 2°C. The mean low 1-day pavement design temperature is calculated to be −27°C with a standard deviation of 3°C. Select the performance grade using (a) 50 percent reliability and (b) 98 percent reliability.
Mix Design Solution (a) Select the appropriate grade from Table 7-4. The average temperatures of 40°C and −27°C represent the 50 percent reliability. However, these specific values are not listed in Table 7-4. In this case, the grade will be given using the principle of 6°C increments between the grades. Select PG 46-28. (b) Assume normal temperature distribution. The 98 percent confidence limits can then be found at the mean temperature plus 2 times the standard deviation: high temperature: (40 + 2⋅2)°C = 44°C; low temperature: (−27 − 2⋅3)°C = −33°C. Select PG 46-34.
The PG grades estimated by temperature considerations apply for typical highway loading conditions. If the traffic is standing, slow, or has extremely high volume, the high temperature grade needs to be increased to avoid permanent deformation (AASHTO MP2, 2003b): • Standing traffic; average speed <20 km/h: increase by 12°C (optional if design equivalent single axel load [ESAL] <300,000) • Slow traffic; average speed 20–70 km/h: increase by 6°C (not necessary if design ESAL <300,000) • Standard traffic; average speed >70 km/h: increase by 6°C if design ESAL ≥30 million (optional for 10 million < design ESAL <30 million) Note that recent research suggests that the Superpave performance grading may not be as effective for the modified binders as it is for the neat asphalt cements. The grading system is based on the theory of linear viscoelasticity, while some modified binders may exhibit nonlinear viscoelastic behavior. Software (LTPPBIND, FHWA 2005) is developed in aiding the binder selection procedure for U.S. and Canadian users. The Long Term Pavement Performance Program (LTPP) pavement temperature algorithms used in the software are still being improved and are not necessarily those given in Eqs. (7-1) and (7-2).
Aggregates Aggregates used in hot mix asphalt concrete are typically processed aggregates that have been quarried, crushed, and separated into fractions. A certain amount of uncrushed aggregate is allowed, the amount decreasing with increasing traffic volume. For heavy traffic loading, it is essential that all the aggregate used are crushed aggregate. While the binder provides cohesion, the aggregate’s function is to provide the frictional component of the asphalt concrete strength. To properly do this for the service life of the pavement, the aggregate particles need to be durable and have appropriate angularity and shape. The aggregate should be free of clay, organic, and other materials detrimental to the strength of the asphalt-aggregate mixture. Soft minerals, such as mica, should be avoided. Most of these requirements are included in aggregate specifications (see example specifications in Table 7-7). Specific requirements for aggregates are needed in regions with studded tire traffic (see Chap. 3). An example of aggregate specifications for wear resistant mixtures is given in Table 7-8.
Additives Additives (in addition to aforementioned polymer modifiers) are used to improve mixture properties and to ease construction. Properly used additives improve moisture susceptibility and overall durability of mixtures. They prevent binder draining (SMA mixtures), improve workability, and prevent segregation. These benefits are especially
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Fractured Faces, Coarse Aggregate, % min
Uncompacted Void Content of Fine Aggregate, % min
Design ESALs∗ (million)
>100 mm
Sand Equivalent, % min
Flat and Elongated‡, % max
≤100 mm
>100 mm
≤100 mm
<0.3
55/–
–/–
–
–
40
–
0.3 to <3
75/–
50/–
40
40
40
10
60/–
45
40
45
10
Depth from Surface
3 to <10
85/80
†
Depth from Surface
10 to <30
95/90
80/75
45
40
45
10
≥30
100/100
100/100
45
45
50
10
∗The anticipated project traffic level expected on the design lane over a 20-year period. Regardless of the actual design life of the roadway, determine the design ESALs for 20 years. † 85/80 denotes that 85 percent of the coarse aggregate has one fractured face and 80 percent has two or more fractured faces. ‡ This criterion does not apply to 4.75-mm nominal maximum size mixtures. Note: If less than 25 percent of construction lift is within 100 mm of the surface, the lift may be considered to be below 100 mm for mixture design purposes. Source: Table 5 from MP2 in AASHTO 2003a, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission.
TABLE 7-7 Example of Aggregate Specifications; Superpave Aggregate Consensus Property Requirements
AADT Speed Limit >60 km/h
Speed Limit <60 km/h
Class
Nordic Abrasion Value
>5000
>10,000
I
≤7.0
2500–5000
5000–10,000
II
≤10.0
1500–2500
2500–5000
III
≤14.0
500–1500
500–2500
IV
≤19.0
Source: adapted from Alkio et al. 2001.
TABLE 7-8
Example for Nordic Abrasion Test Specification for Wear Resistant Mixtures
important in cold regions due to challenges caused by unforgiving climate and short construction season. The most common additives, their application, and the appropriate mix design phase are given in Table 7-9.
7-2-2 Trial Aggregate Gradations The selected mixture type and maximum aggregate size constrain the use of different aggregate fractions in the mixture. Quarried aggregates are crushed into stranded-size fractions specified in, for example, AASHTO M43 (2003a). Selected fractions are then combined in various percentages to create trial aggregate blends that meet the
Mix Design Additive Type
Examples
Effect
Mix Design Phase
Antistripping agents
Fatty-acid-based amines
Reduce moisture susceptibility
Added to binder
Fillers
Lime, fly ash
Reduce moisture susceptibility, increase stiffness of mixture
Added to aggregate blend
Fibers
Cellulose
Increase binder film thickness (SMA and porous mixtures), prevent draining or bleeding during construction, increase stiffness of mixture
Added to aggregate blend
Natural asphalt
Trinidad Lake natural asphalt, Gilsonite
Improve resistance against permanent deformation by increasing mixture stiffness
Added to mixture
Crumb rubber
Recycled tire rubber
Depends on the case, typically improve resistance against permanent deformation
Added to binder or mixture; note gradation has to be open graded
Others
Recycled plastic, pigments, other chemicals
Depends on the case, typically improve resistance against permanent deformation, slow aging rate or provide color
Added to binder or mixture
TABLE 7-9
HMA Additives and Their Application
broadband gradation requirements for a given mixture type. The trial blends may also contain varying types or contents of additives listed in Table 7-9. If bag house fines are planned to be incorporated into the mixture during construction, it is recommended to add 1 to 2 percent of bag house fines into the trial aggregate blends to account for their effect on the mixture properties. Before mixture specimens are produced and tested, the aggregate gradation is evaluated by plotting the percent passing curve with its maximum density line on a chart with sieve sizes raised to 0.45 power. Gradation that plots directly on the maximum density line may not provide sufficient voids in the aggregate, and therefore the gradation curve should be a few percentage points above or below the line (Roberts et al. 1996). Gradation curves that intersect the maximum density line between the 0.3 and 2.36 mm sieve sizes may create so called tender mixtures that are difficult to compact in the field. Superpave mix design restricts gradation curves by using control points through which gradation must pass. The percentages of the control points depend on the nominal maximum aggregate size and can be found in AASHTO MP2 (2003b). Example 7-2 illustrates the use of the 0.45 power chart. The trial gradations can be further evaluated during the gyratory compaction procedure (see AASHTO PP28 2003b).
279
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Chapter Seven Given
Sieve Size, mm
Solution
Passing, %
(Sieve Size, mm)0.45
Control Points (AASHTO 2003b) Min.
Max.
0.075
7.5
0.31
2
8
0.15
9
0.43
–
–
0.3
12
0.58
–
–
0.6
15
0.79
–
–
1.18
21
1.08
–
–
2.36
31
1.47
23
49
4.75
44
2.02
–
–
9.5
60
2.75
–
–
12.5
70
3.12
–
90
19
95
3.76
90
100
25
100
4.26
100
–
TABLE 7-10 Data for Example 7-2 Example 7-2 Plot the gradation given in Table 7-10 on the 0.45 aggregate gradation chart, and analyze its suitability for a job mix formula. Solution The maximum aggregate size (smallest sieve size that 100 percent of the aggregate particles pass) is 25 mm, and the nominal maximum aggregate size (largest sieve size that retains ≤10 percent of aggregate particles) is 25 mm. Obtain the appropriate control points from specifications, for example, AASHTO MP2 (2003b) for a 19-mm nominal maximum size gradation. Plot the passing percentages against sieve sizes raised to 0.45 power, and draw the maximum density line from zero to 100 percent at 25 mm (see Fig. 7-5). The gradation could be suitable for a job mix formula, because it does not plot directly on the maximum density curve and is inside the control points.
FIGURE 7-5
“0.45 aggregate gradation chart” for Example 7-2.
Mix Design
7-2-3 Volumetric Parameters Volumetric parameters are obtained from the compacted mixture samples to determine the optimum amount of asphalt and aggregate in the HMA. The important parameters for achieving the desired mixture performance and constructability are the air voids in the mixture, also called voids in total mix (VTM), voids in mineral aggregate (VMA) and voids filled with asphalt (VFA). These parameters together with other specified parameters, such as unit weight of the mixture, are calculated and plotted as a function of asphalt content. The engineer then selects the optimum asphalt content based on specified optimum or limiting values. In the Superpave mix design method, the asphalt content that gives 4 percent air-void content is selected, if all other criteria for volumetric parameters at specific levels of compaction are met. In the Marshall method, the optimum asphalt content is selected from values that give air voids ranging between 3 and 5 percent considering and optimizing all criteria for volumetric and stability parameters. HMA mixture is a three-phase system consisting of mineral aggregates, asphalt binder, and air. To solve the aforementioned volumetric parameters, VTM, VMA, and VFA, the three phases (i.e., solid, liquid, and gas) need to be separated from each other as shown in Fig. 7-6. As almost all aggregates absorb asphalt cement, the liquid phase
FIGURE 7-6
Elements of HMA mixture volume.
281
282
Chapter Seven
FIGURE 7-7
Illustration of absorbed asphalt cement in aggregate particle.
is separated into “effective” asphalt cement and “absorbed” asphalt cement. The existence of the absorbed asphalt cement in an aggregate particle is illustrated in Fig. 7-7. As shown, three different aggregate volumes can be defined, and consequently three different specific gravities (an apparent, effective, and bulk specific gravity). Both bulk and effective specific gravities of aggregate are needed in order to solve the amount of absorbed asphalt cement and other parameters. The apparent specific gravity of aggregate is normally used for the mineral filler, since its bulk specific gravity is very difficult to obtain (Roberts et al. 1996). To calculate the VTM and VFA, the specific gravities of the mineral aggregate fractions, asphalt cement, loose mixture without air (maximum theoretical specific gravity) and compacted mixture with air needs to be measured in the laboratory (see Table 7-11). Table 7-12 lists the weight-volume relationships and equations for each volumetric parameter. The effective asphalt volume (Vfa in Fig. 7-6), instead of the total asphalt volume, is the amount of asphalt that coats the aggregate particles and, therefore, affects the performance of the HMA mixture. The Vfa can be used on its own in comparing mixture alternatives or used in the determination of asphalt cement film thickness (see, e.g., Roberts et al. 1996). The effective asphalt content by weight (defined in Table 7-12) is used in the dust ratio specifications (dust ratio = fine content/effective asphalt content).
Parameter Description
Test Designation
Weight-Volume Relationships as Illustrated in Fig. 7-6
Bulk specific gravity of compacted asphalt mixture
AASHTO T166 ASTM D1188/D2726
Gmb =
Theoretical maximum specific gravity of loose asphalt mixture
AASHTO T209 ASTM D2041
Gmm =
Specific gravity of asphalt cement∗
AASHTO T228 ASTM D70
Gb =
∗Provided typically by binder supplier.
TABLE 7-11 Specific Gravities Measured in the Laboratory
Wm−dry Vmbγ w Wm−dry Vmmγ w
Wb Vbγ w
Mix Design
Parameter Description
Weight-Volume Relationships as Illustrated in Fig. 7-6
Asphalt content, % by total weight of mixture
Pb =
Aggregate content, % by total weight of mixture
Ps =
Wb × 100% Wm−dry Ws−dry Wm−dry
Bulk specific gravity of combined aggregate*
Gsb =
Effective specific gravity of aggregate
Gse =
Calculated maximum theoretical specific gravity of mixture with different asphalt contents Asphalt absorption, weight-% by weight of aggregate Effective asphalt content of mixture, weight-% of total weight of mixture Voids in mineral aggregate, volume% of total volume of mixture Air voids in compacted mixture, volume-% of total volume of mixture Voids in mineral aggregate filled with asphalt
Equation
× 100%
or 100 − Pb
Ws−dry Vsbγ w
Ws−dry Vseγ w
Gmm−calc =
Wm−dry Vmmγ w
G
sb
=
G
se
=
G
P1 + P2 + ⋅ ⋅ ⋅ + Pn P1 P2 P + + ⋅⋅⋅ + n G1 G2 Gn 100 − Pb 100 Pb − Gmm Gb
mm−calc
=
100 Ps P + b Gse Gb
(7-3)
(7-4)
(7-5)
Gse − Gsb G × 100 GsbGse b
(7-6)
Pba × Ps 100
(7-7)
Pba =
Wba × 100 Ws−dry
Pba =
Pbe =
Wfa × 100 Wm−dry
Pbe = Pb −
VMA =
Vmb − Vsb × 100 Vmb
VMA = 100 −
Gmb × Ps Gsb
(7-8)
VTM =
Vair × 100 Vmb
VTM = 100 ×
Gmm − Gmb Gmm
(7-9)
VFA =
Vma − Vair × 100 Vma
VFA =
VMA − VTM × 100 VMA
(7-10)
∗Substitute or G for the each fraction specific gravity, G , G , ….G ; P , P , P = weight-% of each fraction. sb 1 2 n 1 2 n
TABLE 7-12
Equations to Calculate Volumetric Mixture Parameters
283
284
Chapter Seven These three parameters, the effective asphalt volume, the film thickness, and the dust ratio are used to assess pavement durability. The VMA of the mixture reflects the compactability of the selected aggregate blend relative to the selected compaction level (number of gyrations); a low VMA with high compaction level indicates a tightly packing aggregate structure, unable to hold enough binder which leads to a low film thickness and a brittle mixture, while the high VFA approaching 100 percent suggests an unstable mixture as the aggregate particles are pushed apart by the expanding binder volume when the pavement temperature increases. An example of the volumetric mixture design requirements is given in Table 7-13. After the laboratory phase of the mix design is completed, it is important to conduct verification tests in the field, preferably from mixture collected from the pavement mat behind the paver using plate samples. Example 7-3 Derive Eq. (7-4) for the effective specific gravity of aggregate. Solution Gse can be written using the definitions for the specific gravity and the effective unit weight of the aggregate: Gse =
Ws − dry γ se = γ w Vse × γ w
(a)
Now, use the definition of Pb to solve Wb and Ws-SSD: Pb = and Pb =
Pb × Wm− dry Wb × 100 ⇒ Wb = Wm− dry 100 Wm− dry − Ws -dry Wm− dry
P × 100 ⇒ Ws-dry = Wm -dry 1 − b 100
(b)
(c)
Solve Vse from Vse = Vmm − Vb =
Wm −dry
γ mm
−
Wb γb
(d)
and by substituting γmm = Gmmγw, γb = Gbγw and (b) to (d) Vse =
Wm − dry Gmm × γ w
−
Pb × Wm− dry 100 × Gb × γ w
Equation (a) can now be written using (c) and (e) as
Gse =
P Wm − dry 1 − b 100 Wm − dry Pb × Wm −dry − γ G × γ 100 × Gb × γ w w w mm
The Wm-dry and γw cancel. Multiply by 100 to obtain Gse =
100 − Pb P 100 − b Gmm Gb
(e)
VMA, min. % Design ESALs,∗,† million
Nini
<0.3
6
≤91.5
50
0.3–3
7
≤90.0
75
115
65-78
3–10
7 or 8
≤89.0
75 or 100
100 or 160
65-75
10–30
8
100
160
≥30
9
125
205
Required Density at N Gyrations, % of Gmm Ndes
Nmax 96
75
≤98.0
Nominal Maximum Aggregate Size, mm 37.5
25.0
19.0
12.5
9.5
11.0
12.0
13.0
14.0
15.0
VFA, %
Dust to binder ratio
70-80
0.6-1.2
Nini = initial number of gyrations, Ndes = design number of gyrations, Nmax = maximum number of gyrations ∗ESALs on the design lane over a 20-year period. † When top of the design layer is ≥100 mm from the pavement surface and the estimated traffic level ≥0.3 million ESALs, the estimated traffic level may be decreased by one class, unless the mixture will be exposed to significant construction traffic prior being overlaid. If <25 percent of a construction lift is within 100 mm of the surface, the lift may be considered to be below 100 mm for mixture design purposes. Source: Table 6 from MP2 and Table 1 from PP 28 in AASHTO 2003b, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission.
TABLE 7-13 Superpave HMA Design Requirements
285
286
Chapter Seven
Materials Used
Measured Specific Gravity
AASHTO Test Method
Trial asphalt cement
Gb 1.02
T228
Coarse aggregate
Gsb 2.642
T85
Fine aggregate
Gsb 2.616
Mixture
Gmm 2.439
Mass-% by Mass of Total Mixture
Mass-% by Mass of Total Aggregate
6.0 40.4
43
T84
53.6
57
T209
100.0
Measured bulk specific gravity of mixture at other asphalt contents, AASHTO T166 Asphalt content, %
Bulk specific gravity, Gmb
Asphalt content, %
Bulk specific gravity Gmb
4.5
2.328
6.0
2.361
5.0
2.342
6.5
2.368
5.5
2.354
TABLE 7-14
Data for Example 7-4
Example 7-4 Plot unit weight, VMA, Va, and VFA with the asphalt content from 4.5 to 6.5 percent for the test results given in Table 7-14. Solution Use Eq. (7-3) to solve bulk specific gravity of the combined aggregate: Gsb =
P1 + P2 43 + 57 = = 2.627 43 57 P1 P2 + + 2 . 642 2 . 616 G1 G2
Determine the effective specific gravity with Eq. (7-4): Gse =
100 − Pb 100 − 6 . 0 = = 2 . 677 100 6.0 100 Pb − − 2 . 439 1 .00 2 Gmm Gb
Asphalt absorption, mass-% by mass of aggregate from Eq. (7-6): Pba =
Gse − Gsb 2 . 677 − 2 . 627 Gb × 100 = × 102 = 0 . 72 Gsb Gse 2 . 627 × 2 .66 77
The rest of the calculations depend on the asphalt content and are given in Table 7-15 and plotted in Fig. 7-8.
7-2-4
Performance Tests
For pavements of great importance or mixtures with new raw materials or innovative production techniques, the volumetric mix design is confirmed by subjecting mixture to appropriate performance tests. The objective of the testing is to give further assurance that the pavement will not fail prematurely due to stripping, raveling, cracking, and/or rutting. In addition, performance testing may be conducted to obtain fundamental
Mix Design
Pb
Ps
Gmb
Gmm Eq. (7-5)
Pbe Eq. (7-7)
VMA Eq. (7-8)
Va Eq. (7-9)
VFA Eq. (7-10)
4.5
95.5
2.328
2.495
3.8
15.4
6.7
56.5
5.0
95.0
2.342
2.476
4.3
15.3
5.4
64.7
5.5
94.5
2.354
2.457
4.8
15.3
4.2
72.5
6.0
94.0
2.361
2.439
5.3
15.5
3.2
79.3
6.5
93.5
2.368
2.421
5.8
15.7
2.2
86.0
TABLE 7-15
Calculation Results for Example 7-4
FIGURE 7-8
Plots for Example 7-4.
mixture properties for mechanistic-empirical pavement design. The testing can be approached from two perspectives depending on the concerns of a specific site. The approach presented in Fig. 7-9 is based on the anticipated critical pavement distresses followed by the selection of the suitable performance tests to verify the mixture performance. This approach can be used when the selected tests have performance criteria developed and it is appropriate to assume that the criteria are applicable. The dynamic modulus testing is conducted if an accurate stiffness value for the bound layer is regarded as an important factor in the mechanistic-empirical pavement design. Mixture stiffness can also be obtained using predictive stiffness equations, although with less accuracy (Andrei et al. 1999; Christensen et al. 2003). An alternative approach for the performance testing is to first conduct performance tests required by the new mechanistic-empirical pavement design guide software
287
288
Chapter Seven
FIGURE 7-9
Flowchart for performance-based mix design in cold regions.
(MEPDG) (NCHRP 2006), and then run some trial pavement designs to evaluate potential pavement failure modes (see Fig. 7-10). This approach does not rely on predetermined performance criteria for the specific performance tests, as the overall pavement performance is determined by the empirical distress prediction models and model calibration factors embedded into the design software. However, engineers must consider how well these models and predetermined calibration factors will apply to the pavements located in cold regions. For example, the dynamic modulus testing yields a stiffness master curve that describes the mixture behavior over a large spectrum of temperatures and loading times. The master curve is then used in predicting permanent deformation and fatigue cracking considering seasonal and daily pavement temperature variations by the MEPDG software. In order to predict thermal cracking using the MEPDG software, the creep compliance and strength of the mixture must be measured in the laboratory. Wear by studded tires needs to be considered and tested individually, as it is not included in the MEPDG software. In all tests, field aging of mixture and physical hardening of asphalt binder should be considered in the test sample preparation and conditioning. The short-term conditioning
Mix Design
FIGURE 7-10
Alternative approach to performance-based mix design.
(STC) or long-term conditioning (LTC) procedures can be conducted before the actual performance testing. The following sections provide more details for possible performance requirements. The referenced test methods are discussed in Chap. 4.
Moisture Sensitivity Test This is the most common performance test conducted in addition to the volumetric mix design. In fact, the moisture sensitivity testing is often considered as a part of the volumetric mix design (see Fig. 7-3). As indicated in Chap. 4, moisture damage in pavements is difficult to predict and out of several test methods none has obtained vast popularity. The specified parameter in tests using loose asphalt-aggregate mixture is typically a minimum bitumen coverage percent, for example, >70 percent. In tests using compacted asphalt-aggregate mixture samples the specified parameter is a strength ratio between the cured and uncured samples. In cold regions, where water and freezethaw cycling affects pavement performance, strength ratio test, that is, indirect tensile strength or Marshall stability, should be included in the mix design. A minimum value should be specified (e.g., 80 percent).
Low-Temperature Cracking This is the most distinctive design criterion in the cold climates compared to the design of pavements in the warmer climates. The only design criterion is often the selection of
289
290
Chapter Seven a proper binder stiffness grade, as the binder stiffness affects the thermal cracking more than any other mixture property. Further testing of binder or mixture is seldom required by road agencies. However, for pavements expected to resist low-temperature cracking in extreme conditions, Temperature Stress Restrained Specimen Test (TSRST) test criteria should be specified; that is, the maximum allowable fracture temperature and the minimum allowable fracture strength. The TSRST fracture temperature should be kept lower than the anticipated lowest pavement temperature to avoid cracking. The fracture strength could be specified to 3500 kPa for nonaged specimens and 3000 kPa for STC treated specimens to keep the initial crack spacing greater than ~ 50 m. If MEPDG software is to be used in the analysis, indirect tensile creep compliance and strength must be measured according to test protocols required by the software.
Abrasion Resistance Abrasion resistance is extremely important mix design criterion for cold climates and should be included in mix design for roads that are exposed to traffic using traction devices such as studded tires and chains. Table 7-16 shows an example of mix design specifications used in Finland for PWR and Prall test results. The engineer specifies the test method and a proper wear class. The wear of gyratory compacted mix design samples needs to stay below the given maximum value.
Permanent Deformation Resistance Permanent deformation resistance of HMA can be evaluated using various laboratory tests such as dynamic modulus test, repeated load permanent deformation test, and Marshall stability test, or the mixture performance can be simulated using the wheel track test or full scale test tracks. The dynamic modulus test gives a fundamental material property (stiffness) that can be used in the performance analysis and also in the mechanistic-empirical pavement design software. If the dynamic modulus is used as the simple performance test, performance criteria must be adopted/developed to evaluate test results. If the dynamic modulus test results are used in the MEPDG software, the performance models embedded in the software must be calibrated to the geographical location where the pavement must perform. Table 7-17 gives an example of the proposed dynamic modulus performance criteria that can be used in the mix design verification (Pellinen 2007). Minimum mixture stiffness is given for three traffic levels at four temperature regimes expressed as effective temperatures, Teff. The required minimum mixture stiffness values are selected so that AADT Class
PWR Wet Wear at 5°C, cm3
Prall∗ Wet Wear at 5°C, cm3
I
≤28
≤22
5,000–10,000
II
≤37
≤30
1,500–2,500
2,500–5,000
III
≤46
≤38
500–1,500
500–2,500
IV
≤55
≤46
Speed Limit >60 km/h
Speed Limit <60 km/h
>5,000
>10,000
2,500–5,000
∗Prall method is not used for polymer-modified asphalt cements. Source: adapted from PANK 2000.
TABLE 7-16
Abrasion Resistance Specifications for HMA Mixtures
Mix Design Min. |E∗| at 40°C and 10 Hz, MPa ESALs (106)
Teff 20°C (MAAT −10°C)
<3
250
Teff 30°C (MAAT 1°C)
Teff 40°C (MAAT 10°C)
Teff 50°C (MAAT 26°C)
550
1000
1600
3–30
300
750
1300
2000
>30
450
1100
2000
3100
1. Applicable for traffic speed of 70 km/h. For slow moving traffic, multiply |E∗| with 1.8. 2. Applicable for gyratory compacted STC aged mix design samples with 4 percent air-void content. Source: Pellinen 2007.
TABLE 7-17
Proposed Stiffness Performance Criteria for Dynamic Modulus
the rut depth stays below 12 mm for 10 years of service. The effective temperature is defined as, the single test temperature at which the amount of permanent deformation within a given pavement system would be equivalent to that which would be estimated by considering the seasonal fluctuation of temperature and cumulative damage principles throughout the year (Witczak 1992). The effective temperature Teff for permanent deformation is calculated using Eq. (7-11). Teff = 30 . 8 − 0 . 12 × Zcr + 0 . 92 × MAATdesign
(7-11)
where Teff is effective temperature for permanent deformation, °C, Zcr is critical depth within the mix layer on question, mm, and MAATdesign is evaluated from Eq. (7-12) MAATdesign = MAAT + KasMAAT
(7-12)
where MAAT is mean annual air temperature computed from historical data, °C, sMAAT is standard deviation of the distribution of mean annual air temperature for the geographical location, and Kα is value computed from normal probability tables related to the designer’s selection of appropriate reliability level desired for the project. Table 7-18 gives some examples of the MAAT and corresponding effective temperatures at various geographical locations. The difference in effective temperatures
Geographical Location
MAAT, °C
sMAAT, °C
Ka
Zcr, mm
Teff, °C
Kapuskasing, Ontario
1.0
0.5
1.65
25
29.5
Anchorage, Alaska
3.6
1.0
1.65
25
32.6
Helsinki, Finland
5.6
1.0
1.65
25
34.5
Chicago, Illinois
9.4
0.7
1.65
25
37.5
Baltimore, Maryland
12.8
0.5
1.65
25
40.3
Richmond, Virginia
14.4
0.4
1.65
25
41.7
Houston, Texas
20.2
0.5
1.65
25
47.1
TABLE 7-18 Some Examples of MAAT and Corresponding Effective Temperatures for 90% Reliability
291
292
Chapter Seven between, for instance Houston, Texas (52.4°C) and Kapuskasing, Ontario (29.5°C) demonstrates that at cold regions with low traffic volumes the stiffness requirements can be considerably relaxed when compared to the warmer climates. This allows using softer binders, which in turn reduces thermal and fatigue cracking of pavements. Road simulators and test tracks are simulation tests that would reveal possible problems in the mix design. However, performance criteria for each simulation test must be calibrated for the local conditions and engineering judgment must be exercised, if criteria developed for some other environmental and traffic conditions is to be adopted.
Fatigue Cracking Resistance Fatigue cracking resistance of HMA can be measured using beam fatigue testing or mixture performance can be simulated using accelerated or full scale pavement testing. In addition to mixture properties, the pavement fatigue resistance is dependent on HMA layer thickness and also underlying layers. Therefore, the mix design engineer and structural designer need to communicate and select a mixture that yields the most economical end result considering the lifetime of the project. If road simulators or test tracks are used, threshold criteria in terms of minimum load applications should be developed and specified. As laboratory or road simulator fatigue tests are somewhat variable and extremely time consuming, they are very seldom used in HMA mix design. Instead, empirical equations are used, such as the Asphalt Institute’s fatigue model given in Eq. (3-11).
7-3
Cold Mixes Cold mixes are used for low-volume road surface layers (see, e.g., Table 6-4) and for bound base or subbase layers in any traffic conditions. Incentives to use cold mixes comes from lower pavement capital costs when compared to the construction of HMA pavements. Factors that lower the cost include the use of low binder contents, light construction equipment requirements, faster production rate and extended workability of the mix. As the name indicates, cold mixes are produced at cold temperatures; at the ambient temperature or slightly heated. This is possible by decreasing the viscosity of the binder by either emulsifying the base asphalt with an emulsifying agent and water or mixing the base asphalt with petroleum solvents (cutback asphalt). Road oil (a special type of cutback asphalt) is produced by adding slowly evaporating petroleum solvent into soft asphalt cement. Traditionally, cold mixes are composed of aggregates and cutback asphalts, especially road oils in cold regions. Emulsified and soft asphalt cements are replacing cutback asphalts due to environmental concerns resulting from evaporation of the cutbacks’ light oil components into the atmosphere. Several methods exist for mix design of cold mixes, although none of them is standardized. The Asphalt Institute (Asphalt Institute 1989 and Asphalt Institute MS-19) has given a comprehensive description of the methods, one of them being the Marshall method. The Marshall method for emulsified asphalt-aggregate cold mixture design is described in further detail to show the particularities of cold mixes and their design. The mix design consists of a selection of aggregate and emulsified asphalt, estimating the approximate amount of emulsified asphalt, determining the water content at mixing and compaction, and variation and selection of the optimum residual asphalt
Mix Design content. The goal of the mix design is to develop a mixture that withholds traffic loads without permanent deformation and fatigue cracking while being insensitive to moisture effects.
7-3-1
Material Selection
Materials in cold mixes include aggregate, emulsified asphalt, and possible additives. Aggregate properties determine many of the choices made during developing an optimum mixture. Therefore, aggregates should be tested thoroughly. A wide range of aggregates is suitable for cold mixes, including crushed and pit run materials. The selection of the aggregate depends on the end use of the cold mix. For example, the wearing course requires the highest aggregate quality. Tables 7-19 to 7-21 show typical gradations for dense-graded, open-graded, and sand emulsion mixes. Compatibility of the aggregate with the asphalt emulsion and need for imported aggregates or additives should always be determined using trial mixtures prepared in the laboratory. The selection of a proper emulsion type depends on the electronic charge of the asphalt globules that are floating within a mixture of water and an emulsifying agent. Anionic emulsions have electro-negatively charged asphalt globules and cationic emulsions have electro-positively charged asphalt globules. Depending on the surcharge of the used aggregate, either anionic or cationic emulsion will be selected.
Sieve Size, mm
Semi-Processed Crusher, Pit, or Bank Run
50
Processed Dense-Graded Asphalt Mixtures, % Passing by Weight 100
37.5
100
25
80–90
19
90–100
100 90–100
60–80
12.5
100 90–100
60–80
9.5
100 90–100
60–80
4.75
25–85
100 90–100
20–55
25–60
35–65
45–70
60–80
2.36
10–40
15–45
20–50
25–55
35–65
0.3
2–16
3–18
3–20
5–20
6–25
0.075
3–15
0–5
1–7
2–8
2–9
2–10
Sand equivalent, min. %
30
35
35
35
35
35
Los Angeles @ 500 revolutions, max.
40
40
40
40
40
Crushed faces, min. %
65
65
65
65
65
Source: Asphalt Institute MS-19. Table courtesy of the Asphalt Institute, Inc.
TABLE 7-19
Aggregates for Dense-Graded Emulsion Mixtures
293
294
Chapter Seven Base Sieve Size, mm
Coarse
37.5
100
25
95–100
19
Medium
Fine
Surface Course
100 90–100
12.5
25–60
9.5
100 20–55
4.75
0–10
0–10
2.36
0–5
0–5
85–100
100 30–50
0–10
1.18
5–15
0–5
0.075
0–2
0–2
0–2
0–2
Los Angeles @ 500 revolutions, max.
40
40
40
40
Crushed faces, min. %
65
65
65
65
Source: Asphalt Institute MS-19. Table courtesy of the Asphalt Institute, Inc.
TABLE 7-20 Aggregates for Open-Graded Emulsion Mixtures
Sieve Size, mm
Poorly Graded
Well Graded
Silty Sands
12.5
100
100
100
4.75
75–100
75–100
75–100
0.3
–
15–30
–
0.15
–
–
15–65
0.075
0–12
5–12
12–20
Sand equivalent, max. %
40
40
40
Plasticity index, min.
65
65
65
Source: Asphalt Institute MS-19. Table courtesy of the Asphalt Institute, Inc.
TABLE 7-21 Aggregates for Sand Emulsion Mixtures
The selection also depends on the setting time and consistency of the emulsion, and the desired properties of the asphalt residue that is left behind when the emulsion breaks and the water separates. Table 7-22 shows suitability of emulsified asphalts based on construction technique. Letters M and S in the emulsion labels refer to medium setting and slow setting emulsions, respectively. The slower the setting time, the longer the workability time before the emulsion breaks. The numeral relates to the relative viscosity of the emulsion, “2” being more viscous than “1”.
Mix Design
√
CSS-1h
CMS-2h
√
CSS-1
CMS-2
Cationic AASHTO M208 ASTM D2397 SS-1h
√
SS-1
√
HFMS-2s
MS-2h / HFMS-2h
Type of Construction
MS-2 / HFMS-2
Anionic AASHTO M140 ASTM D977
Cold-Laid Plant Mix Open-graded aggregate Dense-graded aggregate
√
√
√
√
√
Sand
√
√
√
√
√
√
√
Mixed-in-Place (Road Mix) Open-graded aggregate Dense-graded aggregate
√
√
√ √
√
√
√
Sand
√
√
√
√
√
Sandy soil
√
√
√
√
√
Source: adapted from Asphalt Institute MS-19. Table courtesy of the Asphalt Institute, Inc.
TABLE 7-22
Uses of Emulsified Asphalts in Cold Mixes
High-float (HF) emulsions differ from conventional medium setting emulsions by existence of a gel structure in the asphalt residue that prevents them from flowing at high temperatures during summer (Asphalt Institute M-19). The lower case letters “h = hard” and “s = soft” relate to the consistency of the residue from a distillation test. According to the AASHTO and ASTM specification designations shown in Table 7-22, emulsions without “soft/hard” designations break into residues having 25°C penetration at a range of 100 to 200 1 10 mm (100 to 250 for cationic emulsions). Hard residues have penetration between 40 and 90, whereas soft residues have a minimum penetration of 200 1 10 mm. In general, medium setting emulsions are used for open-graded mixtures, densegraded mixtures with low fine contents and when stockpiling is needed. Slow setting emulsions are used for dense-graded mixtures with high fine contents and when stockpiling is not needed (Asphalt Institute MS-19). More than one emulsion type is often acceptable for a given aggregate, and the final selection should be conducted on the basis of comparative mix designs. Additional factors that should be accounted for during construction include anticipated weather, type of mixing process, and construction equipment and field procedures. Additives used in cold mixes include Portland cement and hydrated lime. They are used to obtain higher early strength and to reduce moisture susceptibility of emulsion mixes, particularly those produced with sand and sand-gravel aggregates. Laboratory testing is required to determine if the additives provide sufficient benefit to justify the increased cost (Asphalt Institute MS-19).
295
296
Chapter Seven
7-3-2
Selection of Optimum Asphalt Residue Content
The selection of optimum asphalt residue content differs for dense-graded and opengraded emulsion-aggregate mixtures. For dense-graded mixtures, the mix design includes determination of trial emulsion content, coating and adhesion testing, and stability and flow testing. Starting point for the emulsion content can be evaluated using simple formulas. Equation (7-13) applies for base mixtures and Eq. (7-14) for surface mixtures (Asphalt Institute MS-19): Base mixtures: % emulsion =
[(0 . 06 ⋅ B) + (0 . 06 ⋅ C)] × 100 A
(7-13)
% emulsion =
[(0 . 07 ⋅ B) + (0 . 03 ⋅ C)] × 100 A
(7-14)
Surface mixtures:
where % emulsion is estimated initial percent of asphalt emulsion by dry weight of aggregate, A is percent residue of emulsion by distillation (ASTM D244), B is percent of dry aggregate passing 4.75 mm sieve, C is 100 – B = dry aggregate retained on 4.75 mm sieve. Coating and adhesion tests are used for preliminary evaluation of asphalt emulsions. Damp job aggregate is mixed with the emulsion at the content determined with Eqs. (7-13) or (7-14), placed on a flat surface and the degree of coating is visually estimated either satisfactory or unsatisfactory. Aggregate coating in excess of 50 percent could be considered acceptable. A portion of the mixture can be totally submerged in water, drained, and then visually evaluated on a flat surface. Surface mixtures require a much greater degree of coating than base mixtures. If balling of the asphalt with fines is observed during mixing, an increase of water content should be evaluated. Once a mixture passes the coating test, an additional adhesion test can be conducted. A small portion of the sample (100 g) is cured in a forced draft oven at 60° for 24 h. The cured sample is then placed in boiling distilled water stirred at one revolution per second for 3 min. The sample is then drained, dried on white absorbent paper, and visually evaluated for the degree of coating (Asphalt Institute M-19). Mixture properties are closely related to the density of the mixture, thus it is important to optimize the water content at compaction. The optimum water content at compaction can be evaluated using similar techniques as the Proctor test for soils and aggregates. Instead of the Proctor mold and compaction technique, the Marshall mold and compaction hammer are used. More detailed method can be found at Asphalt Institute (1989). To determine the optimum residual asphalt content, six specimens are prepared at a minimum of three different emulsion contents. The samples are compacted by 50 hammer blows per side using a technique described in the Marshall mix design method (AASHTO T245 or ASTM D1559). If the Marshall hammer bounces and/or liquid exudes from the specimen, the total liquid volume exceeds the VMA and proper compaction cannot be achieved. When this happens, the remainder of the mixture needs to be dried to reduce the water content, before new specimens are compacted. The compacted specimens are then cured at elevated temperature for two days. The volumetric parameters can be determined as described in Sec. 7-2-3. In determination of the theoretical maximum density, the loose mixture needs to be free of moisture.
Mix Design Asphalt Institute (1989) describes how to obtain the volumetric parameters using an additional test of weighing oven dried specimens set aside from stability testing, and thereby avoiding the need for theoretical maximum density. Marshall stability and flow can be determined following the procedures of ASTM D1559, except that the compacted specimens are conditioned at a low temperature of 22°C. Three of the six specimens are subjected to vacuum saturation and immersion. A stability value of 2224 N or greater and stability loss of less than 50 percent have been found to be satisfactory for most pavements with low to moderate traffic volumes (Asphalt Institute MS-19). The following parameters are plotted as a function of the residual asphalt content: dry and soaked stability, percent stability loss (dry stability minus soaked stability divided by dry stability), dry bulk density, percent moisture absorbed, percent total voids (air + moisture). The optimum residual asphalt content is selected at the maximum soaked stability (see Fig. 7-11), and adjusted depending on moisture absorption,
FIGURE 7-11 Schematic emulsion-aggregate mix design plot (Asphalt Institute 1989) (Figure courtesy of the Asphalt Institute, Inc.)
297
298
Chapter Seven
FIGURE 7-12 Selection of optimum emulsion content for open-graded emulsion-aggregate mixtures (Asphalt Institute MS-19). (Figure courtesy of the Asphalt Institute, Inc.)
percent loss of stability, total voids, and coating of aggregates. If one or more criteria for the stability, volumetric parameters and coating cannot be met, the mix design should be rejected. Open-graded emulsion aggregate mix design for aggregate gradations given in Table 7-20 is based on an evaluation of asphalt runoff (Asphalt Institute MS-19). Damp aggregate at 2 percent moisture content is hand-mixed for 2 min with the selected emulsion preheated to 60°C at varying emulsion contents and are observed for workability (stiff, satisfactory, or sloppy). The mixtures are transferred immediately after the preparation onto a 2.36-mm sieve placed on the top of a standard pan. After a 30 min draining period, the mixtures are spread on white paper for coating observation. The pans with the drained emulsion are oven dried to a constant mass and the asphalt residue runoffs are determined in grams. The emulsion content by weight of aggregate is then plotted versus the asphalt residue runoff. The optimum emulsion content is found as the emulsion content that gives an asphalt residue runoff of 10 g (see Fig. 7-12). At the optimum emulsion content, the mixture must have satisfactory workability and coating close to 100 percent. Surface courses with coating above 85 percent and base courses with coating above 60 percent can be considered suitable.
7-3-3
Cold Mix Recycling
The main benefit of recycling is to reuse the aggregate and asphalt from deteriorated pavements, thereby reducing the need for new materials. Adding binder in to the existing unbound base course increases pavement structural strength and makes it less frost susceptible, which is another important feature in cold regions. The recycling procedure often includes addition of new binder and sometimes new aggregate to the reclaimed asphalt pavement (RAP) or to the reclaimed aggregate material (RAM).
Mix Design Asphalt emulsion is the prominent binder used for cold mix recycling in cold regions (Roadex 2001). In order to determine the material’s suitability for recycling and to conduct a mix design, representative samples from the asphalt pavement to be reclaimed need to be collected. After extracting the binder from the samples, the aggregate gradation, aggregate purity, and the emulsion content are determined. For the reclaimed aggregate to be suitable for recycling, its plasticity index multiplied by its fine content (% passing 0.075 mm sieve) should be less than 72 (Asphalt Institute 1983). Another evaluation method recommended by Asphalt Institute (1983) is the sand equivalent value of the aggregate (AASHTO T176, ASTM D 2419). Aggregates with sand equivalents over 30 can be recycled successfully, whereas values from 20 to 30 need additional testing (e.g., tube suction test) to make sure that the binder is able to bind the aggregate fines and make the mixture water proof. Aggregates with sand equivalents <20 are not considered suitable for recycling. Guidelines for suitable aggregate gradations for open-graded mixtures are given in Table 7-20 and for dense-graded mixtures in Table 7-23. The ability of the reclaimed material to resist stripping should be tested, as described earlier. The material should be mixed with the emulsion at a water content that is estimated to prevail during the recycling. If additional water needs to be added to the test samples, the water needs to be taken from the source to be used in situ. In case the gradation of the reclaimed aggregate is not suitable for recycling or if the layer thickness needs to be increased, new aggregates are added into the mixture. The gradation of the new aggregate is selected so that the gradation of the combined aggregate meets one of the aforementioned requirements for suitability. Either medium or slow setting asphalt emulsions can be used as an added binder during the recycling. Table 7-22 lists general guidelines for suitable emulsions based on the aggregate gradation. Medium setting emulsions (MS and CMS grades) are appropriate for open-graded aggregates. High-float medium setting emulsions (HFMS) are recommended for extreme temperature conditions for either dense- or open-graded mixtures. Slow setting emulsions (SS and CSS grades) are used with dense-graded aggregates with high fine contents. The final selection of the asphalt emulsion should
Passing, % Sieve Size, mm
A
37.5
100
25
80–100
12.5 4.75
25–85
B
C
100
100
75–100
75–100
0.30
15–30
0.15 0.075
3–15
0–12
5–12
Source: adapted from Asphalt Institute 1983, courtesy of the Asphalt Institute, Inc.
TABLE 7-23
Guidelines for Dense-Graded Recycled Cold Mixes
299
300
Chapter Seven be based on laboratory tests that evaluate the compatibility of the emulsion and aggregate to be used and the time the mixture remains workable. The demand for the emulsion content for the combined aggregate gradation can be calculated empirically with Eq. (7-15) (Asphalt Institute 1983): Pc =
0 . 035 ⋅ a + 0 . 045 ⋅ b + K ⋅ c + F % R
(7-15)
where Pc is percent of asphalt emulsion by weight of total mix, a is percent of aggregate retained on 2.36 mm sieve, b is percent of aggregate passing 2.36 mm sieve and retained on 0.075 mm sieve, c is percent of aggregate passing 0.075 mm sieve, F is 0…2 percent based on absorption of the binder into the aggregate (use value 0.7 to 1.0 percent if no data is available), R is fraction of the residual asphalt cement in the emulsion (typically 0.60 to 0.65). K depends on the fine content (% passing 0.075 mm sieve) and is 0.15 for fine content of 11 to 15, 0.18 for fine content of 6 to 10 and 0.20 for fine content of ≤5. The quantity of new emulsion to be added to the recycled mixture, Pr equals Pc minus the asphalt content in the reclaimed pavement [see Eq. (7-16), Asphalt Institute 1983]. Pr = Pc −
Pa ⋅ Pp R
(7-16)
where Pa is percent of asphalt in the reclaimed asphalt pavement, and Pp is fraction of reclaimed asphalt pavement in the recycled mix. Proportion of new emulsion per weight of aggregate (for in situ recycling) can be obtained from Eq. (7-17): Pd =
100 % × Pr 100 % − Pr
(7-17)
The obtained emulsion content needs to be verified in the field during the recycling operation. Example 7-5 illustrates how to determine the emulsion content for a recycling project combining bound and unbound pavement layers. Example 7-5 A rural paved road is sampled and found to consist of a 50-mm asphalt pavement over 200 mm of unbound granular base. The dry densities of the materials were found to be 2320 and 1600 kg/m3, respectively. The pavement is to be recycled to a depth of 150 mm. Extraction tests indicate that the asphalt content of the reclaimed asphalt pavement is 7.0%. The aggregate gradations can be found in Table 7-24. Determine if the reclaimed aggregate blend is suitable for recycling and the amount of new emulsion needed. (Source: adapted from Asphalt Institute 1983, courtesy of the Asphalt Institute, Inc.) Solution Determine the mass of aggregate per square meter of asphalt pavement corrected for the 7 percent binder content: [2320 – (0.07 × 2320)] kg/m3 × 0.05 m × 1.0 m2 = 107.9 kg Mass of aggregate of granular base per square meter: 1600 kg/m3 × 0.10 m × 1.0 m2 = 160.0 kg Ratio of blending of aggregates: Asphalt pavement layer: Granular base layer:
107.9/(107.9 + 160.0) = 0.40 160.0/(107.9 + 160.0) = 0.60
Mix Design Passing, % Sieve Size, mm
Asphalt Pavement
Granular Base
19.0
100
100
12.5
95
90
9.5
70
78
4.75
50
65
2.36
30
40
0.30
10
15
8
12
0.075
Source: adapted from Asphalt Institute 1983, courtesy of the Asphalt Institute, Inc.
TABLE 7-24
Gradation of Reclaimed Materials for Example 7-5
Passing, % Sieve Size, mm
Asphalt Pavement∗
Granular Base†
Blend
19.0
40
60
100
12.5
38
54
92
9.5
28
46.8
74.8
4.75
20
39
59
2.36
12
24
36
0.30
4
9
13
0.075
3.2
7.2
10.4
∗(% passing in Table 7-24) × 0.40 † (% passing in Table 7-24) × 0.60 Source: adapted from Asphalt Institute 1983 courtesy of the Asphalt Institute, Inc.
TABLE 7-25
Combined Aggregate Gradation for Example 7-5
The combined aggregate gradation for the blend is given in Table 7-25. The blended aggregate gradation meets the requirements for gradation A in Table 7-23, and is, therefore, considered suitable for recycling. Either high-float medium setting or slow setting emulsion can be selected for the blend. The estimated emulsion requirement is obtained using Eq. (7-15): Pc = =
0 . 035 ⋅ a + 0 . 045 ⋅ b + K ⋅ c + F % R 0 . 035 ⋅ (100 − 36) + 0 . 045 ⋅ (36 − 10 . 4) + 0 . 18 ⋅ 10 . 4 + 1 . 0 % = 9 . 64 % 0 . 65
301
302
Chapter Seven Percent of new emulsion per total weight of the mix, Pr, is obtained from Eq. (7-16) and per weight of the aggregate, Pd, from Eq. (7-17): Pr = Pc − Pd =
7-4
Pa × Pp R
= 9 . 64 % −
7 . 0 % × 0 . 40 = 5.3% 0 . 65
100 % × Pr 100 % × 5 . 3 % = = 5.6% 100 % − Pr 100 % − 5 . 3 %
Stabilized Bases Portland cement and hydrated lime are the most commonly used stabilizers in road structures. They are used mainly to increase the base course stiffness in order to reduce the vertical stresses on the subgrade. However, while the base course’s stiffness increases, its cracking tendency also increases. Therefore Portland cement and lime have not gained vast popularity in cold regions, where frost action related movements and shrinkage due to thermal stresses call for more flexible bases (Kolisoja and Vuorimies 2005). Instead, stabilization with asphalt products, mainly asphalt emulsions or foamed asphalt cement has been the choice of the road authorities in the cold regions. Improvements from bitumen stabilization when compared with unbound bases built from the same aggregate include better deformation resistance, and reduced frost susceptibility and moisture absorption. Stabilization can be conducted in situ or at a central mixing plant. In situ stabilization can be performed for new base courses or as a rehabilitation procedure, often called a remix, for existing pavement structures. The residual asphalt content is typically from 3 to 4 percent and the air-void content is typically higher for stabilized bases than for the surface mixtures. Specialized remix equipment performs the stabilization procedure during one pass. The old bound pavement layer is heated and milled together with the top part of the existing base course. The milled materials are then mixed with additional asphalt emulsion and aggregate if needed. The “remixer” then lays the mixture for compaction by rollers. The mix design is similar to that of cold mixes given in Sec. 7-3 involving an analysis of the existing bound and unbound materials, selection of virgin materials, determination of the combined aggregate gradation, residual asphalt content, and optimum water content during compaction. Tube suction test is used in determining the minimum required binder content to assure that the base will be nonfrost susceptible and does not retain water. The gradation and plasticity of the reclaimed materials is first determined and compared with the suitable range for stabilization. Coarse or fine aggregate may be added in order to correct the existing gradation and plasticity. Finnra (2002) recommends a maximum fine content of 8 percent for mixes stabilized with asphalt emulsion. Depending on the aggregate gradation and economics (including technical considerations and availability of suitable equipment) an appropriate binder is selected (see Table 7-22). The evaluation of the residual binder content depends on the road classification and the desired function of the improved base. Techniques described in Sec. 7-3 can be used. However, the selection of the final residual binder content is often based purely on volumetric considerations (Finnra 2002). If the primary purpose of the stabilization is to mitigate degradation due to frost action, the binder content is selected as the minimum content that makes the base nonfrost susceptible. Tube suction test can be used in the evaluation (Roadex 2001). The Finnish road authority, Finnra (Apilo and Eskola 1998; Finnra 2002), uses semianalytical mix design method in determining the emulsion content of the stabilized bases. It calculates the mixture VMA using a method originally proposed by Hudson and Davis (1964). The aggregate gradation and VMA of the fines are determined in the
Mix Design laboratory and used together with experimental reduction factors to determine the VMA of the combined aggregate. The desired VFA is then used to select the binder content (that is further checked with the tube suction test). To calculate the VMA using the reduction factors, the aggregate gradation is expressed as cumulative volume percentages passing through each sieve having a fixed ratio of sizes (e.g., the opening increases by a factor of two between the consecutive sieves). Weight percentages can be used if the specific gravities of the aggregate fractions are close to each other. The VMA of the fines (<0.075 mm) are determined with the Rigden filler compaction apparatus (see, e.g., NAPA 1999). The VMA of the combined aggregate is determined with Eq. (7-18). VMAn = VMA0.075 · F(Pn/Pn-1) · F(Pn-1/Pn-2) · . . . · F(P0.15/P0.075)
(7-18)
where VMAn is the smallest sieve size that 100 percent of the aggregate passes through, VMA0.075 is the VMA of the fines determined in the laboratory, Pn/Pn-1 is the ratio obtained by dividing the % passing the sieve “n” by the % passing the next smaller sieve, “n-1”, and F(Pn/Pn-1) is the reduction factor for this ratio obtained from Table 7-26. Example 7-6 demonstrates the use of this method. Pn/Pn-1
Fr
Fa
Pn/Pn-1
Fr
Fa
1
1
1
1.8
0.94
0.955
1.05
0.9805
0.985
1.85
0.9465
0.963
1.1
0.9583
0.97
1.9
0.9528
0.97
1.15
0.9325
0.951
1.95
0.9589
0.978
1.2
0.9098
0.935
2
0.9647
0.985
1.25
0.9015
0.924
2.05
0.9703
0.993
1.3
0.8945
0.92
2.1
0.9757
1
1.35
0.8908
0.919
2.15
0.9805
1.4
0.8908
0.919
2.2
0.9856
1.45
0.8926
0.92
2.25
0.9905
1.5
0.8971
0.921
2.3
0.9953
1.55
0.9032
0.924
2.35
1
1.6
0.9107
0.926
2.4
1.0045
1.65
0.9193
0.931
2.45
1.009
1.7
0.926
0.938
2.5
1.0133
1.75
0.9332
0.947
Pn/Pn-1 = the ratio obtained by dividing the % passing the sieve by the % passing the next smaller sieve. Fr = reduction factor for rounded aggregate, such as natural sand or gravel or cubical crushed stone. Fa = approximate factors for angular aggregate, such as manufactured sand, screenings for elongated or flat stone. Source: Hudson and Davis 1964, with permission from Association of Asphalt Paving Technologists.
TABLE 7-26
Reduction Factors for Eq. (7-18)
303
304
Chapter Seven Sieve Size, mm
% Passing
Sieve Size, mm
% Passing
19
100
0.6
15 11
9.5
65
0.355
4.75
46
0.15
6
2.36
28
0.075
2.5
1.18
21
Gsb = 2.630
TABLE 7-27
Data for Example 7-6
Example 7-6 An aggregate gradation for cubic rock is given in Table 7-27. The VMA of the fines (<0.075 mm) was determined using the Rigden filler compaction apparatus and found to be 32 percent. (a) Determine the VMA of the combined aggregate using the Hudson and Davis (1964) method. (b) Determine the required emulsion content if the target air-void content is 5 percent. The emulsion candidate has a 65 percent residual asphalt content (weight-%) and a specific gravity of 1.015. The residual asphalt cement’s specific gravity is 1.020. Solution (a) See Table 7-28. List the sieves from the smallest to the largest and designate a number “n” to each sieve. Calculate the ratios of % passing between the adjacent sieves and round up the ratio to the nearest 0.05. Find the corresponding reduction factor F from Table 7-26. Starting with a VMA of the fines, successfully compute the VMA after each addition of coarser material by multiplying the previous VMA by the reduction factor associated with the size of the coarser material added. Note that adding the material retained on 0.15 mm sieve increases the VMA, while all the successive additions decrease the VMA. The VMA of the combined aggregate is 15.90 percent. (b) If the VMA is 15.9 percent, the volumetric residual asphalt content, Pb−vol, is 15.9% − 5.0% = 10.9%. The residual asphalt content in weight-% of the weight of aggregate, Pb, can be calculated from Eq. (7-19): Pb ' =
Gb P Gsb b −vol
(7-19)
1 . 020 10 . 9 % = 4 . 23 % Pb ' = 2 . 630
Sieve Size, mm
n
% Passing, P
Pn/Pn−1
Reduction Factor
VMA, %
0.075
1
2.5
–
–
32.00
0.15
2
6
2.40
1.004
32.13
0.355
3
11
1.85
0.946
30.39
0.6
4
15
1.35
0.891
27.08
1.18
5
21
1.40
0.891
24.13
2.36
6
28
1.35
0.891
21.50
4.75
7
46
1.65
0.919
19.76
9.5
8
65
1.40
0.891
17.60
9
100
1.55
0.903
15.90
19
TABLE 7-28 VMA Calculations for Example 7-6
Mix Design The residual asphalt content in weight-% of the total mix, Pb, can be solved using Eq. (7-20): Pb =
100 % × Pb ' 100 % + Pb '
(7-20)
100 × 4 . 23 Pb = % = 4.06% 100 + 4 . 23 Determine the required emulsion content, Pc, of the total weight of the mix: Pc =
Pb 4 . 06 % = = 6 . 24 % R 0 . 65
Calculate the volumetric-% of the emulsion, Pc−vol, to ensure that it fits into the VMA. In order to do this without the bulk specific gravity of the mix, the volumetric-% of the emulsion of the weight of the aggregate needs to be determined first: Pc − vol ' =
Gsb 2 . 630 P = 6 . 24 % = 16 . 17 % Ge c 1 . 015
Then, Pc − vol =
100 ⋅ Pc − vol ' 100 × 16 . 17 % = 13 . 92 % %= 100 + Pc − vol ' 100 + 16 . 17
The volumetric emulsion content of 13.9% means that the air-void content is about 2 percent during the recycling operation.
7-5 Asphalt Surface Treatment Bituminous surface treatments (BST) and asphalt surface treatments (AST) are used as driving surfaces on otherwise unpaved roads especially on permafrost areas of Yukon and Alaska. The treatment layer thickness equals the maximum aggregate size. They are constructed by first applying a specified amount of high-float emulsion to the surface of base course and then covering the emulsion with a specific amount of dense-graded aggregate. The aggregate is compacted and pressed into the emulsion that rises up and fills the aggregate voids. The mix design consists of selecting appropriate materials and optimum coverage rates. Some typical emulsion grades and application rates are collected in Table 7-29. An example of aggregate specifications is given in Table 7-30.
BST∗ †
AST
High-Float Emulsion
Application Rate
Aggregate Application Rate
HF-250S
2.8 L/m2
45 kg/m2
2
41 kg/m2
HFMS-2, MFMS2s
3.4 L/m
∗Janz and MacLeod 2002. † McHattie 2005.
TABLE 7-29
Typical Application Rates for BST and AST
305
306
Chapter Seven
Sieve Size, mm
Passing, %
Test
Test Designation
Required Value
19
100
% of wear
AASHTO T 96
50 max.
12.5
63–89
Degradation value
ATM T-13
30 min.
9.5
54–76
Sodium sulfate loss
AASHTO T 104
9 max.
4.75
36–56
% fracture
WAQTC TM-1
70 min.
2.36
18–38
Thin-elongate pieces
ATM T-9
8 max.
1.18
12–30
Plasticity index
AASHTO T-90
3 max.
0.30
4–18
0.075
0–5
0.005
0–3
Source: McHattie 2003.
TABLE 7-30 Example of Aggregate Specifications for AST
The application rates given in Table 7-29 are based on experience form repetitive use of familiar aggregate sources. McHattie (2005) suggests determining the emulsion content for dense-graded ASTs using volumetric calculations that account for the moisture content of the aggregate. The aggregate application rate, Ragg, is calculated from Eq. (7-21): Ragg = 1.15 · rdry · t
(7-21)
where rdry is dry compacted aggregate density (determined in the laboratory) and t is the design AST thickness (maximum aggregate particle size). This aggregate application rate includes an additional 15 percent sacrificial aggregate layer that aids in the compaction of the aggregate and the curing of the AST. The emulsion application rate, Remulsion, is determined by solving the volume of voids in the aggregate that is not occupied by aggregate moisture. Equation (7-22) gives the emulsion application rate in L/m2: ρdry Remulsion = t × 1 − ρw
3 × 1000 L/m sb
1 w + G
(7-22)
where rw is density of water, w is aggregate moisture content (as a decimal number), and Gsb is bulk specific gravity of aggregate.
7-6
Gravel Surfaces Gravel surface needs to provide a smooth and safe ride, withstand traffic, provide a fairly impermeable layer to protect the underlying structure from moisture penetration and stay bound. Suitable materials are crushed rock, crushed gravel, and glacial till. Fines are needed to add cohesion to the surface. Typical ranges vary from 4 to 20 percent
Mix Design Sieve Size, mm
Passing, %
Sieve Size, mm
Passing, %
19
100
0.425
13–35
4.75
50–78
0.075
4–15
2.36
37–67
Plasticity Index: 4–12
Source: modified from Skorseth and Selim 2000.
TABLE 7-31 Example of Gradation Requirements and Plasticity for Gravel Surfacing
passing 0.075 mm sieve. The clay size fraction (<0.002 mm) should be at least 25 percent of the fines, and best surfaces have over 50 percent clay size fraction (Havu et al. 1995). The gradation should be dense graded and the maximum aggregate size should be smaller than 16 to 19 mm. Table 7-31 shows an example of gradation and plasticity requirements for gravel surfacing.
Review Questions 7-1. (a) Use Eqs. (7-1) and (7-2) to calculate T20mm and Tmin for a northern city located at the latitude of 64°. The 7-day average high air temperature is 28°C and the minimum air temperature in average year is −29°C. (b) Select the appropriate performance grade using 50 percent reliability.
7-2. Plot the aggregate gradation given in Table 7-32 in a chart with the sieve size raised to 0.45 power. Is the proposed gradation dense or open graded? Is it suitable for HMA? Prove that for two aggregates with weight-% of each fraction of P 1 and P 2 and specific gravities of G 1 and G 2, the bulk specific gravity of the combined aggregate, G sb, is
7-3.
Gsb =
P1 + P2 . P1 P + 2 G1 G2
7-4. Derive Eq. (7-6) for the asphalt absorption from Pba =
Wba × 100 . Ws −dry
7-5. (a) Plot unit weight, VMA, Va, and VFA with the asphalt content from 4.5 to 6.5 percent for the test results given in Table 7-33. (b) If the fine content is 7.6 percent, what is the effective dust ratio at asphalt content of 5.7 percent?
Sieve Size, mm
Passing, %
Sieve Size, mm
Passing, %
0.075
7.5
4.75
49
0.15
9
9.5
65
0.3
13
12.5
75
0.6
19
19
91
1.18
25
25
100
2.36
35
–
TABLE 7-32 Aggregate Gradation for Review Question 7-2
–
307
308
Chapter Seven
Materials Used
Measured Specific Gravity
AASHTO Test Method
Trial asphalt cement
Gb 1.01
T228
5.7
Coarse aggregate
Gsb 2.630
T85
49.0
Fine aggregate
Gsb 2.601
T84
45.3
Mixture
Gmm 2.457
T209
100.0
Mass-% by Mass of Total Mixture
Mass-% by Mass of Total Aggregate – 52.0 48.0 –
Measured bulk specific gravity of mixture at other asphalt contents, AASHTO T166 Asphalt content, %
Bulk specific gravity, Gmb
Asphalt content, %
Bulk specific gravity, Gmb
4.5
2.328
6.0
2.361
5.0
2.342
6.5
2.368
5.5
2.354
–
–
TABLE 7-33
Data for Question 7-5
7-6. Determine the minimum required dynamic modulus for a northern location where the mean annual air temperature is −2.5°C with a standard deviation of 1.5°C. The road in question has a design ESAL of 2 × 106 and design speed of 50 km/h. Use the critical depth of 50 mm and 95 percent reliability. 7-7. Determine trial emulsion content for a dense-graded surface cold mix using the aggregate gradation given in Table 7-34 and Eq.( 7-14). The percent residue of emulsion is 65 percent.
7-8. A paved road is sampled and found to consist of a 50-mm asphalt pavement over 250 mm of unbound granular base. The dry densities of the materials were found to be 2400 and 2200 kg/m3, respectively. The pavement is to be recycled to a depth of 150 mm. Extraction tests indicate that the asphalt content of the reclaimed asphalt pavement is 4.0 percent. The aggregate gradations can be found in Table 7-35. Determine if the reclaimed aggregate blend is suitable for recycling and the amount of new emulsion needed. There is 65 percent of asphalt cement in the emulsion.
7-9. An aggregate gradation for rounded aggregate is given in Table 7-36. The VMA of the fines (<0.075 mm) was determined using the Rigden filler compaction apparatus and found to be 34.0 percent. (a) Determine the VMA of the combined aggregate using the Hudson and Davis (1964) method. (b) Determine the required emulsion content if the target air-void content is 4 percent. The emulsion candidate has a 65 percent residual asphalt content (weight-%) and a specific gravity of 1.010. The residual asphalt cement’s specific gravity is 1.013.
Sieve Size, mm
Passing, %
Sieve Size, mm
Passing, %
37.5
100
2.36
30.0
25.0
95.0
0.30
9.0
12.5
70.0
0.075
4.0
42.5
–
4.75
TABLE 7-34 Aggregate Gradation for Question 7-7
–
Mix Design Passing, % Sieve Size, mm
Asphalt Pavement
Granular Base
25
–
100
19.0
100
82
12.5
80
50
9.5
68
42
4.75
50
24
2.36
34
16
0.30
11
6
0.075
4
3
TABLE 7-35 Gradation of Reclaimed Materials for Question 7-8
Sieve Size, mm
Passing, %
37.5
Sieve Size, mm
Passing, %
100
1.18
32
25
78
0.6
25
19
71
0.355
19
9.5
53
0.15
11
4.75
45
0.075
5
2.36
38
Gsb = 2.70
TABLE 7-36
Data for Question 7-9
7-10. Determine aggregate and emulsion application rate for AST using aggregate with maximum particle size of 12.5 mm and laboratory compacted dry density of 2100 kg/m3. The stockpile moisture content is 4.0 percent and the specific gravity of the aggregate is 2.7.
References AASHTO (2003a). Standard Specifications for Transportation Materials and Methods for Sampling and Testing, 23d ed. American Association of State Highway and Transportation Officials, Washington, D.C. AASHTO (2003b). June 2003 Edition of AASHTO Provisional Standards, American Association of State Highway and Transportation Officials, Washington, D.C. Alkio, R., Juvankoski, M., Korkiala-Tanttu, L., Laaksonen, R., Laukkanen, K., Petäjä, S., Pihlajamäki, J., and Spoof, H. (2001). “Tien rakennekerroksen materiaalit (Materials for road’s structural layers),” Finnra Report 66-2001, Finnish Road Administration, Helsinki, Finland (in Finnish). Andrei, D., Witczak, M. W., and Mirza, M. W. (1999). “Development of Revised Predictive Model for the Dynamic (Complex) Modulus of Asphalt Mixtures.” Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures,
309
310
Chapter Seven NCHRP 1-37A. Interim Team Technical Report. Department of Civil Engineering, University of Maryland, College Park, Md. Apilo, L., and Eskola, K. (1998). “Design of Soft Asphalt Pavements,” Finnra Report 3200497, Finnish Road Administration, Helsinki, Finland (in Finnish). Asphalt Institute. (1983). Asphalt Cold Mix Recycling, MS-21, 1st ed., Lexington, Ky. Asphalt Institute. (1989). Asphalt Cold Mix Manual, MS-14, 3d ed., Asphalt Institute, Lexington, Ky. Asphalt Institute. (2001). Superpave Mix Design, Asphalt Institute Superpave series no. 2, Lexington, Ky. Asphalt Institute. (MS-19). Asphalt Emulsion: A Basic Asphalt Emulsion Manual, MS-19, 3d ed., Asphalt Institute, Lexington, Ky. Asphalt Institute. (2003). Superpave Asphalt Binder Specifications, 3d ed. Asphalt Institute Superpave series no. 1, Lexington, Ky. Christensen, D. W., Pellinen, T., and Bonaquist, R. (2003). “Hirsch Model for Estimating the Modulus of Asphalt Concrete.” Journal of the Association of the Asphalt Paving Technologists, vol. 72. FHWA (2005). LTTP Bind—Windows-Based Software Program, Federal Highway Administration, http://www.fhwa.dot.gov/pavement/ltpp/ltppbind.cfm (January 29, 2008). Finnra (2002). “Stabilointiohje (Stabilization guidelines),” Finnra TIEH 2100009-2, Finnish Road Administration, Helsinki, Finland (in Finnish). Garcia, M., and Hansen, K. (2001). “HMA Pavement Mix Type Selection Guide,” NAPA and FHWA, Information series 128. Havu, K., Hörkkö, R., Kosonen, O., Olkkonen, N., Pernu, H., Pöyhönen, A., and Vuorinen, M. (1995). ” Sorateiden hoito ja kunnostus (Maintenance and rehabilitation of gravel roads),” TIEL 2230013, Finnish Road Administration, Helsinki, Finland (in Finnish). Heukelom, W. (1969). “A Bitumen Test Data Chart Showing the Effect of Temperature on the Mechanical Behaviour of Asphaltic Bitumens,” Journal Institute of Petroleum, Institute of Petroleum, London, vol. 55, pp. 404–417. Hudson, S., and Davis., R., (1964). “Relationship of Aggregate Voidage to Gradation,” Journal of AAPT, vol. 34. Janz, V., and MacLeod, D. R. (2002). “Yukon BST Management System,” 2002 Condition Report, Yukon Department of Highways and Public Works, http://www.hpw.gov. yk.ca/pdf/YTBSTReport2002.pdf (accessed November 22, 2006). Kolisoja, P., and Vuorimies, N. (2005). “Material Treatment,” Report on Task 2.2, Roadex II Northern Periphery Project, The Highland Council, Transport, Environmental & Community Service, Glasgow, Scotland. McHattie, R. (2003). Asphalt Surface Treatment Guide, Alaska Department of Transportation and Public Facilities, FHWA-AK-RD-01-03. McHattie, R. (2005). Material Application Rates for Dense-Graded Asphalt Surface Treatments Using High Float Emulsion, Alaska Department of Transportation and Public Facilities, FHWA-AK-RD-05-03, Juneau, Alaska. NAPA (1999). “Evaluation of Baghouse Fines for Hot Mix Asphalt,” National Asphalt Pavement Association Information Series 127., National Asphalt Pavement Association, Lanham, Md. NCHRP (2006). Mechanistic-Empirical Pavement Design Guide Software, National Cooperative Highway Research Program, Washington, D.C., http://www.trb.org/mepdg/(January 30, 2008). PANK (2000). Finnish Asphalt Specifications, Finnish Pavement Technology Advisory Council, Helsinki, Finland.
Mix Design Pellinen, T. (2007). “Asphalt Mixture Stiffness Performance for Cold Regions,” Proceedings of the Eighth International Symposium on Cold Region Development, Finnish Association of Civil Engineers RIL, Helsinki, Finland. Raad, L., Saboundjian, S., Sebaaly, P., Epps, J., Camilli, B., and Bush, D. (1997). Low Temperature Cracking of Modified AC Mixtures in Alaska, Institute of Northern Engineering INE/TRC 97.05, University of Alaska, Fairbanks, http://www.dot.state. ak.us/stwddes/research/assets/pdf/ine_trc_97_05.pdf (January 29, 2008). Read, J., and Whiteoak, D. (2003). The Shell Bitumen Handbook, 5th ed., Thomas Telford Publishing, London. Roadex. (2001). “Creating Effective Technical Exchange & Co-operation between Road Districts in the NP Region,” EU Northern Periphery Roadex Project 1998–2001, CDROM. Roberts, F. L., Kandhal, P. S., Brown, E. R., Lee, D-Y., and Kennedy, T. W. (1996). Hot Mix Asphalt Materials, Mixture, Design and Construction, 2d ed., National Asphalt Pavement Association Research and Education Foundation, Lanham, Md. Skorseth, K., and Selim, A. (2000). “Gravel Roads Maintenance and Design Manual,” South Dakota Local Transportation Assistance Program (SD LTAP), FHWA, November 2000, http://www.ltapt2.org/gravel/gravelroads.htm (April 20, 2006). Witczak, M. W. (1992). “Effective Temperature Analysis for Permanent Deformation of Asphaltic Mixtures,” A-001 MIDAS Study, Department of Civil Engineering, University of Maryland, College Park, Md.
311
CHAPTER
8
Pavement Design
T
he design of a structure involves adjusting the thickness of each structural component in order to avoid failure during the selected design life. Failure of a structure takes different meanings depending on the structure type and the consequences associated with the failure. In pavement engineering, failure involves exceeding limit values of structural and functional condition. Therefore, as opposed to other civil engineering structures for which failure can have catastrophic consequences, the result of poor design is premature deterioration of pavement condition and excessive maintenance costs. The general goal of pavement design is thus, to maximize pavement performance in a given environment, subject to budgetary and technical constraints. This chapter includes a synthesis of current practice with respect to pavement design in cold regions, a description of the state-of-the-art methodology for mechanistic-empirical design of pavements in cold regions, as well as guidelines for the selection and the design of special features for the mitigation of cold region performance problems.
8-1
Current Practice in Pavement Design in Cold Climates Highway pavements can be seen as an interface between the subgrade soil and vehicles traveling at high speed at the surface. From top-down, the pavement structure plays a structural role by distributing effectively heavy vehicle loads to the subgrade soil in order to avoid excessive deformations at that level and by regulating environmental effects on the bearing capacity. From bottom-up, the pavement plays a functional role by attenuating differential movements in order to maintain comfortable and safe surface conditions at the surface of the pavement. Finally, the pavement needs to be designed in order to maintain a reasonable level of structural and functional conditions over the design life of the structure.
8-1-1
Pavement Design Approaches
Two general approaches are commonly used to design pavement structures. The first approach is referred to as the empirical pavement design approach. It is based on the systematic observation of the performance of in-service pavements and on the establishment of statistical relationships between pavement performance and a number of factors related to structural characteristics and site conditions. A good example of the empirical approach is the widely used AASHTO method (AASHTO 1993), which uses a relationship between performance, (defined as the number of axle repetitions causing a selected serviceability loss) the factors quantifying pavement characteristics and site conditions. The relationship is based on the observation of the performance of several hundreds of test sections submitted to accelerated loading during the AASHO road test held in Illinois in the early 1960s.
313
Chapter Eight
Condition
Design objectives [design life (Nd ) and minimum service level (Ct )]
Empirical model
Material and site characteristics
Ct Nd Number of load applications
Layer thicknesses
FIGURE 8-1
Principle of empirical pavement design.
Empirical pavement design approaches are generally easy to use and are well calibrated to “true” pavement performance. Their applicability for contexts different from those used for the development of the models is not warranted, at least without calibration. Figure 8-1 illustrates the principle of empirical design of pavement structures. The second approach to pavement design is referred to as the mechanistic-empirical pavement design. It is based on the theoretical calculation of pavement response under idealized loading conditions coupled with the statistical relationship between response parameters and pavement performance. As illustrated conceptually in Fig. 8-2, for new pavement structures, the design process involves the following two steps: 1. Calculation of allowable strains at critical levels in the pavement structure as a function of expected number of load repetitions during pavement life and given an acceptable level of deterioration. This is usually accomplished using distress specific empirical transfer functions calibrated using pavement performance data. Design objectives [design life (Nd) for a given minimum service level]
Strain
314
Empirical transfer function adm
Nd Number of load applications (log)
Allowable strains (
adm )
Mechanical computation model adm adm
Layer thicknesses
FIGURE 8-2
Principle of mechanistic-empirical pavement design.
Material and site characteristics
Pavement Design 2. Calculation of required pavement layer thicknesses to meet the allowable strain criteria computed in step 1. The calculation is done using multilayer elastic analysis or finite element modeling based on the mechanical properties of soils and pavement materials subjected to idealized loading conditions. Since the approach is based on mechanistic principles, it can be used for designing pavement structures in conditions and with materials that differ from those used for the development of the models. As an example, the Asphalt Institute pavement design method (Asphalt Institute 1991) is a widely used method following the principles of mechanistic-empirical pavement design. The method uses multilayer linear elastic analysis coupled with empirical damage functions calibrated using AASHO road test data. In most cases in the two general approaches described, the basic assumption is that there is no uncertainty in the design variable or in the model output. The pavement design approach is then considered “deterministic.” In some cases, the design approach includes the possibility to incorporate probability distribution for some or all variables. The results of the design models are then described in terms of probability distribution or in terms of reliability of the results. The design approach is then considered to be “probabilistic.” Figure 8-3 is a schematic illustration of the probabilistic approach used in the “AASHTO-93” empirical pavement design procedure. In this approach, the standard deviation of design input, S0, is used to predict the probability distribution of the model output. The designer can thus select a reliability factor, R, that will ensure that the design life, Ndes, is equal or greater than the predicted design life, Np, with a level of confidence, which is generally selected to reflect the importance of the highway. In this procedure, the design life is defined as the number of load applications required to reduce the level of serviceability between the initial serviceability index, PSIi, and the terminal serviceability index, PSIt.
8-1-2 Synthesis of Design Methods Used by Highway Agencies in Cold Climates Several methods used by highway agencies managing pavement networks in cold environments have been reviewed and are summarized in Table 8-1. For each method, a brief description of empirical, mechanistic, and probabilistic components are provided as well as descriptions of necessary procedures when considering frost heave and thaw weakening.
FIGURE 8-3 Illustration of probabilistic design approach used in AASHTO-93 pavement design method.
315
316 Design Method
Mechanistic Components
Empirical Components
Probabilistic Components
Consideration for Frost Heave
Consideration for Thaw Weakening
AASHTO93a United States
Soil and material characterization using resilient (MR) and dynamic moduli (Converted into a stiffness index a)
Structural index (Structural number) combining material stiffness and layer thickness. Equivalent axle load index (ESAL) combining axle weight and configuration. Performance indicator (PSI) based on user perception of level of serviceability (also correlated to roughness, rutting and cracking). Drainage quality index (m). Performance model relating the number of axle loading to a terminal serviceability condition and pavement characteristics (SN, m, MR and ∆PSI)
Standard deviation of design inputs (S0) Reliability of design (R%) based on estimated variability of predicted performance
A procedure for the estimation of the serviceability loss due to frost heave (and soil swelling)
A procedure for the calculation of the effective annual modulus of subgrade soil based on damage related to seasonal variations on soil properties
Pavement design method widely used in North America. Essentially uses empirical procedures. Computer software (DarWIN) available from AASHTO
NCHRPMEPDGb
Soil and material characterization using resilient (MR) and dynamic moduli. Computation of pavement structural response under a complete load spectra considering daily, seasonal and longterm variations in material properties
Distress specific performance models including fatigue (topdown and bottom-up), permanent deformation, thermal cracking and roughness
Variability of design inputs and performance models are considered in the analysis
Prediction of frost heave by enhanced integrated climatic model (but frost heave information is not used in pavement design procedure)
Detailed procedure to predict temperature and moisture content in pavement layers as a function of climatic data and to assess variation in material properties to be used in damage calculations
Design method being implemented by several highway agencies in NorthAmerica. Supported by MS Windows based computer software. Procedure for calibration to local conditions provided. Data intensive method
Comments
CRRELc, United States
NA
Relationship between frost penetration and pavement thickness Relationship between strength reduction during spring thaw and effective CBR estimated from soil frost susceptibility
NA
The “Limited Subgrade Frost Penetration” (LSFP) procedure is based on an empirical equation or on a design chart allowing for the calculation of required thickness of granular pavement layers to control frost penetration in the subgrade soil to the desired level
The “Reduced Subgrade Strength” (RSS) procedure
AKPAVEd Alaska, United States
Uses computer code ELSYM5 to compute stresses and strains at critical locations in pavement structures
Uses the Asphalt Institute empirical equation to relate horizontal strain at the bottom of the asphalt layer to the number of allowable load repetitions to fatigue failure. Uses Ullidtz’equation to relate the vertical stress on lightly bound or unbound layers to the number of allowable load applications to rutting failure of the pavement
NA
NA
Damage computed on a seasonal basis for summer/fall, winter and spring conditions
TABLE 8-1
Summary of Pavement Design Methods Used by Highway Agencies in Cold Climates
Method described in report FHWA-AKRD-03-01. Method supported by MS Windows based design software “AKPAVE”
317
318 Design Method
Mechanistic Components
Empirical Components
Probabilistic Components
Consideration for Frost Heave
Consideration for Thaw Weakening
MnPAVEe Minnesota, United States
In MnPAVE, critical strains are computed using a linear elastic analysis (LEA) model and used to determine damage and reliability. The critical strains are the tensile strain at the bottom of the asphalt layer and the compressive strain at the top of the subgrade
Fatigue: use a version of the “Illinois” equation calibrated using data from the Minnesota Road Research Project (Mn/ROAD). Rutting Use a rutting transfer function calibrated for Mn/ROAD
MnPAVE uses Monte Carlo simulation to calculate the reliability (probability of success) of a given pavement design. The Monte Carlo Method works by randomly selecting input values from known distributions, and generating an output distribution from which probabilities can be determined. Variability is expressed as a coefficient of variation (CV)
NA
Use a resilient modulus multiplier to take into consideration variations in material properties for seasonal damage computation. Five seasons are used in the analysis: fall, winter early spring, late spring and summer
Comments Practical method well adapted to cold region environment. Climatic information limited to Minnesota. Method described in report “Pavement Designer’s Guide Mn/DOT Flexible Pavement Design MnPAVE Beta Version 5.1” available at: http://www.mrr. dot.state.mn.us/ research/mnpave/ files/MnPAVE_ Design_Guide.pdf Method supported by MS Windows based design software “MnPAVE.” Available on WEB at: http://www. mrr.dot.state. mn.us/research/ mnpave/mnpave. asp
OPACf Ontario, Canada
Computation of subgrade deflection using the Odemark model
Performance indicator (RCI) representing ride comfort on a 1-10 scale. Performance models based on Brampton and AASHTO road tests
NA
NA
One of the performance models estimates the loss of serviceability caused by environmental factors. The model is based on an estimation of increased subgrade deflection during spring thaw
Chausséesg Quebec, Canada
Material, structure and traffic characterization using AASHTO-93 procedures
Serviceability and performance prediction using AASHTO-93 procedures
Reliability based design using AASHTO-93 procedures
Mechanistic prediction of frost depth and frost heave in pavement using the Finnish SSR model. Total pavement thickness adjusted to meet allowable heave criteria (50 mm). Possibility to incorporate an insulation layer in the analysis
Effective annual modulus computed using AASHTO-93 procedure
MS Windows based design software (CHAUSSÉES) available on WEB (in French) at www. mtq.gouv.qc.ca
Finlandh
Computation of surface modulus using Odemark model or use of multilayer elastic analysis (MLEA) to optimize economical pavement structure
Transfer functions to estimate fatigue resistance of asphalt mixtures and rutting of subgrade layer
NA
Total thickness of pavement structure is adjusted based on frost heave estimation FHcalc. Calculated frost heave must be less than allowed value obtained from in-situ measurements
Subgrade modulus used in the design is based on plate loading test measurements during spring thaw period and laboratory tests of materials
Odemark method is used typically for thin structures and MLEA for thick structures
319
TABLE 8-1
(Continued)
320 Design Method
Mechanistic Components
Empirical Components
Probabilistic Components
Consideration for Frost Heave
Consideration for Thaw Weakening
Comments
FHcalc = (S–a0R2–a2R2 – …)·t/100 where S is frost penetration depth, Ri is layer thickness, ai is insulation coefficient for layered materials and t is frost swelling of subgrade Norwayi
NA
Pavement design catalog based on road class
Consideration for the probability of a severe winter in frost protection procedure. The return period for the average freezing index is adapted to the importance of the highway: A 2 year return period is selected for local roads while a return period of 10 and even 100 years is selected for major highways
The total thickness of the pavement structure is adjusted according to the following rule: d = f · X, where d is total pavement thickness, X is frost penetration computed with the freezing index for the selected return period, f is a frost protection factor (0 < f < 1) predetermined for different regions in Norway
NA
The use of the return period for the reliabilitybased design of frost protection is interesting in the Norwegian method
Francej
A pavement design catalog has been established based on mechanisticempirical simulations for 4 traffic classes (T0-T3), three classes of bearing capacity of the subbase (Pf1-Pf3) and five classes of mechanical properties for the subgrade soils (S0-S4)
a
NA
AASHTO (1993). NCHRP 1-37A. c Berg, R. L. (1988). d Alaska Department of Transportation and Public Facilities. (ADoTPF) (2004). e Chadbourn, B. et al. (2002). f Ontario Ministry of Transportation (1990). g St-Laurent, D. (2006). h Dysli, M. (1991), Jämsä and Orama (1990), communication with Terhi Pellinen of HUT, (2007). i Dysli, M., (1991). j Peyronne and Caroff (1991). b
TABLE 8-1
(Continued)
The thickness of the pavement structure is adjusted to meet an “allowable freezing index transmitted to the subgrade soil” criterion, which is a function of the frost susceptibility class of the soil obtained from a freezing test. The freezing index transmitted to the subgrade soil is estimated using an empirical model
NA
321
322
Chapter Eight When available, information on available documents and software are also provided. Detailed information on design procedures described in Table 8-1 are available from the references listed.
8-2
Mechanistic-Empirical Pavement Design Procedure for Cold Region Pavements Several approaches used by agencies managing road networks in cold climates have been described in Sec. 8-1. This section proposes a simple step-by-step procedure for the design of new or rehabilitated pavements to be constructed in cold climate conditions. The procedure is based on state-of-the-art mechanistic-empirical (M-E) methods adapted to cold environments and based on the following principles: • The pavement structure needs to be able to sustain repeated loading across the seasons and over the design life of the pavement without significant loss of structural capacity of the pavement • The pavement structure needs to be able to sustain frost action over the design life of the pavement without significant loss of functional capacity of the pavement The procedure is similar in concept to some of the methods described above (MEPDG, MnPAVE, and the like) and is not meant to replace these methods. It is presented to help understand the principles of mechanistic-empirical design of pavements in cold environments. Figure 8-4 illustrates the proposed procedure.
Step 1: Description of Site Conditions The essential information to gather as part of Step 1 includes climatic data as well as soil and pavement material characteristics. Table 8-2 summarizes the minimal information required for the proposed pavement design procedure.
FIGURE 8-4 climates.
Schematic illustration of mechanistic-empirical design procedure adapted for cold
Pavement Design Data Type
Required Data Element
Reference
Climatic data
Surface freezing index
Chap. 5, Sec. 2
Soil data
Seasonal resilient modulus
Chap. 4, Sec. 3-2
Poisson’s coefficient
Chap. 4, Sec. 3-2
Segregation potential
Chap. 4, Sec. 3.2
Thermal conductivity (frozen)
Chap. 5, Sec. 4
Seasonal resilient modulus
Chap. 4, Sec. 3-2
Poisson’s coefficient
Chap. 4, Sec. 3-2
Thermal conductivity (frozen)
Chap. 5, Sec. 4
Pavement material data
TABLE 8-2
Data Elements Required for Pavement Design in Cold Regions
Mechanical properties of soils and pavement materials should be selected to represent conditions prevailing during a season. Figure 8-5 illustrates the principle of selecting representative mechanical properties for each distinct season throughout the year. Table 8-3 is a data template provided to help assemble the soil and pavement material information required for the design procedure. The number of seasons and their duration need to be selected carefully to adequately represent typical variations of material behavior without increasing complexity too much. Three to five seasons are generally considered when performing seasonal pavement damage analysis.
FIGURE 8-5 Example of selection of mechanical properties representative of seasonal variations for asphalt bound materials, unbound materials, and subgrade soils.
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Chapter Eight Season Layer
Data Element
Unit
Asphalt concrete
Resilient modulus
MPa
Poisson’s coefficient
Granular base
Granular subbase
Subgrade soil
TABLE 8-3
1
2
3
4
5
–
Thermal conductivity
W/m·°C
Resilient modulus
MPa
Poisson’s coefficient
–
Thermal conductivity (frozen)
W/m·°C
Resilient modulus
MPa
Poisson’s coefficient
–
Thermal conductivity (frozen)
W/m·°C
Resilient modulus
MPa
Poisson’s coefficient
–
Thermal conductivity (frozen)
W/m·°C
Segregation potential
mm2/°C·day
Data Template for Soil and Pavement Materials
Step 2: Establish Performance Objectives Performance objectives involve selecting a design period during which the structural and the functional conditions of the pavement will remain above a selected level of terminal condition. The selection of performance objectives, thus, involves two key parameters: the expected design life and the minimum level of service to be provided during this period. The design life can be expressed in terms of years and/or in terms of number of load applications. Typical design life used for flexible pavements varies between 15 and 25 years depending on road classification. The design life can be translated into an expected number of load applications, usually expressed in terms of equivalent single axle loads (ESAL), using the following relationship: N (ESAL) = AADT × D × L × T × TF × d × A
(8-1)
where AADT is the average daily traffic number (vehicle/day in both directions), D is a direction distribution factor (typically equal to 0.5 per direction), L is a lane distribution factor (equal to 1 for a road with one lane in each direction; can be estimated using Table 8-4 for roads with more than one lane per direction), T is the proportion of commercial vehicles (decimal), TF is the truck factor, that is, the average number of ESAL for all truck circulating on the road (see Table 8-5), d is the number of days of commercial traffic per year (typically around 300), A is a factor including the design life of the
Pavement Design Proportion of Commercial Vehicles in Design Lane AADT (both directions)
2 lanes
3 lanes
4,000
0.94
0.82
8,000
0.88
0.76
12,000
0.85
0.72
16,000
0.82
0.70
20,000
0.81
0.68
30,000
0.77
0.65
40,000
0.75
0.63
50,000
0.73
0.61
60,000
0.72
0.59
70,000
0.70
0.58
80,000
0.69
0.57
100,000
0.67
0.55
Source: Darter et al. 1985, with permission from the Transportation Research Board.
TABLE 8-4
Lane Distribution Factor for Roads with 2 or 3 Lanes in One
Direction
pavement (years) corrected for the expected growth of traffic volume over the design life and is equal to: A=
(1 + g)n − 1 g
(8-2)
where g is the growth factor and n is the design life in years. Example 8-1 illustrates the calculation of N(ESAL). Example 8-1 Compute the number of load applications for a four-lane principal road with the following traffic forecast over the 20-year design life: AADT = 20,000 vehicle per day % of commercial vehicles = 15 percent Expected growth = 4 percent Solution Assuming equal volume of traffic in both directions, D can be set to 0.5. According to Table 8-4, for 20,000 vehicles per day and for two lanes in one direction, L = 0.81. Since no specific information is available on the type of truck circulating on the road, the “truck factor” is set to 0.38 for a principal road according to Table 8-5. The number of days of commercial traffic is assumed to be 300. Given the 20-year design life and the 4 percent growth factor, the “A” factor can be computed as follows: A=
(1 + 0 . 04)20 − 1 = 29 . 8 0 . 04
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326
Chapter Eight
Vehicle Type Single Unit 2-axles 3-axles Tractor-Semi Trailers 4-axles or less 5-axles 6-axles or more All trucks
Interstate
Principal
Minor Arterial
Major Collector
Minor Collector
0.21 0.61
0.25 0.86
0.28 1.06
0.41 1.26
0.19 0.45
0.62 1.09 1.23
0.92 1.25 1.54
0.62 1.05 1.04
0.37 1.67 2.21
0.91 1.11 1.35
0.52
0.38
0.21
0.30
0.12
Source: Asphalt Institute 1991, with permission from Asphalt Institute.
TABLE 8-5
Truck Factors for Different Classes of Highways and Vehicles in the United States
According to Eq. (8-1), the number of load applications is thus equal to: N(ESAL) = 20000 × 0 . 5 × 0 . 81 × 0 . 15 × 0 . 38 × 300 × 29 . 8 = 4.. 13 M
Performance objectives also involve the selection of structural and functional indicators that represent the minimum acceptable level of condition beyond which irremediable damage will affect the pavement and unacceptable service is provided to road users. In most cases, the terminal service level is built in the empirical models used to predict performance. The performance model should thus, be selected to reflect local preferences with respect to the terminal service level. A detailed description of pavement deterioration mechanisms in cold climates is provided in Chap. 3. Table 8-6 includes proposed performance indicators for pavements in cold climates.
Step 3: Select a Trial Pavement Structure Despite the fact that it is possible to compute required thicknesses using a mechanisticempirical (M-E) design approach, most computer-based procedures will use M-E calculation tools to validate a given pavement structure and iterate until an optimal solution is reached. Initial selection of a pavement structure is generally based on past experience, which usually takes the form of a more or less detailed design catalog.
Step 4: Analyze Pavement Structure The selected initial trial pavement structure is analyzed to verify response to traffic and climate “loading” and to predict related performance. In cold climates, the verification should include the following analysis: • Analysis of cumulative seasonal fatigue damage • Analysis of cumulative seasonal permanent deformation • Validation of hot mix asphalt (HMA) thickness for thermal cracking • Validation of total pavement thickness for allowable frost heave or roughness
Pavement Design Type of Performance Structural
Functional
TABLE 8-6
Related Pavement Response
Recommended Terminal Condition
Comments
Fatigue cracking
Tensile strain at the bottom of asphalt bound layers
20% of wheel path area cracked (Shell, Asphalt Institute)
Essential information to assess structural performance
Permanent deformation
Compressive strain (or stress) at the top of the subgrade soil
12 mm (Shell, Asphalt Institute) 10 mm (TRRL, BRRC)
Essential information to assess structural performance
Thermal cracking
Can be related to the thermal contraction properties of the asphalt concrete mixture and to the low temperature tensile strength in a given climatic context
No information available on recommended terminal conditions
Useful information to assess performance of HMA. Primary use is for mix design but can help validate thickness of HMA layer
Roughness
Can be related to compressive strain on subgrade soil and variability of soil characteristics (Ullidtz, 1998). Can be related to variability of soil frost susceptibility (Doré, 1997)
Between 2.0 and 3.5 m/km based on road classification (based on survey of Canadian agency practices)
Essential information to assess functional performance of pavement. Models generally related to frost heave and permanent deformation of pavement structure. Few reliable roughness models exist
Indicator
Performance Indicators for Pavement Design in Cold Regions
In this first analysis, the allowable number of load repetitions is estimated based on the strain induced at the bottom of the asphalt layer by an idealized load corresponding to a standard axle load. The following calculations should be done:
Analysis of Cumulative Seasonal Fatigue Damage
(a) For each season, compute the tensile strain at the interface of the HMA layer in a multilayer elastic analysis software program by imputing soil and pavement material characteristics specific to each season and layer thicknesses as selected for the trial design. The calculation method described in Sec. 5-9 can alternatively be used. (b) For each season, compute the allowable number of load applications using a fatigue cracking model such as given in Eq. (3-11).
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328
Chapter Eight (c) Compute damage for each season based on Miner’s assumption of linear damage accumulation. According to Miner’s hypothesis (Miner 1945), damage D is a linear function of the ratio of the number of load application n accumulated or expected on the pavement during the considered season to the allowable number of load applications N obtained in step (b) for the season as indicated in Eq. (8-3): D=
n N
(8-3)
(d) Compute total pavement damage DT by adding the damage computed for each season Dn as indicated in Eq. (8-4): i
DT = ∑ Di
(8-4)
1
where i is the number of seasons considered. (e) Verify if total pavement damage is acceptable. DT should always be less than one. If total damage is much less than one (<0.7 to 0.8), the pavement structure is considered over designed and a thickness reduction should be considered.
Analysis of Cumulative Seasonal Permanent Deformation Damage In this analysis, the allowable number of load repetition is estimated based on the vertical compressive strain induced at the top of the subgrade layer. The computation steps for this analysis are similar to the one described for fatigue damage using a damage function specific to permanent deformation damage. The transfer function proposed by the Asphalt Institute can be written as follows (Huang 1993): N p = 1 . 365 × 10− 9 (ε c )− 4 . 477
(8-5)
where Np is the allowable number of load repetitions and ec is the vertical compressive strain on the subgrade soil.
Validation of HMAC thickness for Thermal Cracking An empirical procedure for estimating crack spacing is described in Sec. 3-1. According to Eq. (3-1), a thermal cracking index I representing the average crack spacing at a pavement age a can be estimated as a function of the stiffness modulus of the original asphalt cement in kg/cm2 as determined by McLeod’s method for loading time of 20,000 s (Sbit), the winter design temperature in −°C (m) (use only positive values), a dimensionless subgrade code (d = 5-sand, 3-loam and 2-clay) and the combined thickness of bituminous layers (h in inches). It is, therefore, possible to validate hot mix asphalt concrete (HMAC) thickness h with respect to a selected crack spacing criterion at the end of the pavement design life (a). An alternative procedure based on a mechanistic assessment of crack spacing is also provided in Sec. 3-1.
Validation of Total Pavement Thickness for Allowable Frost Heave Frost heave is the direct response of the pavement structure to the stress induced by frost action on frost susceptible subgrade soils. Experience in Finland and Canada has shown that pavements with high frost heave tend to deteriorate faster with respect to roughness and frost heave cracking. The Finnish Road Administration and the Quebec Ministry of Transportation
Pavement Design Allowable Frost Heave, mm Road Classification
Finnra*
MTQ†
Freeways
30
50
Main highways
50
55
Regional roads Local roads
60 100
70–80
*
Tammirinne et al. 2002. St-Laurent 2006.
†
TABLE 8-7
Allowable Frost Heave Criteria Used in Finland and Canada
(MTQ, Canada) have established allowable levels of frost heave for different classes of roads. These criteria are given in Table 8-7. These allowable frost heave values are supported by observations made on highway sections in Quebec, Canada, as illustrated in Fig. 8-6. The scatter of the data in the figure can be explained by the fact that frost heave is not the only factor causing roughness deterioration. Among other important factors, subgrade and material variability, traffic action and transverse cracking play an important role in this type of deterioration. Pavement deterioration mechanisms are described in Chap. 3. According to Fig. 8-6, average frost heave ranging from 40 to 80 mm will lead to ∆IRI per year ranging from 0.1 to 0.15, which yield typical terminal conditions reported in Table 8-6 after design lives ranging between 15 and 20 years. The total thickness of the trial pavement structure can be validated by computing expected frost heave in given climatic conditions using the procedure described in Chap. 5. A frost heave prediction model or software can alternatively be used to compute frost heave.
FIGURE 8-6 Relationship between frost heave and roughness evolution on several road sections in Quebec Province (Doré 1997, 2002).
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330
Chapter Eight Using these criteria or locally developed criteria, the total pavement thickness will be adjusted to meet the allowable frost heave value. For new pavements, the structure is usually adjusted by changing the thickness of the subbase layer, which is generally the least expensive layer per unit of thickness. For existing pavements, thickness adjustments can be done by increasing the thickness of the granular base layer. If excessive thickness of granular material is required to meet the criteria, pavement insulation or other frost heave reduction techniques should be considered. The damage analysis procedure is illustrated in Example 8-2. Example 8-2 Considering three seasons in a year, compute pavement damage for the following pavement structure subjected to an expected traffic volume of 10 MESAL over a 15-year design life. Pavement structure:
Layer
Thickness
HMA Granular base Granular subbase
150 mm 200 mm 450 mm
Solution The strain analysis is done using the linear elastic analysis software “WinJulea” (PCASE 2003) for selected seasonal material properties. Results of the analysis are reported in line 1 of Table 8-8. Computation of fatigue damage The number of allowable load repetitions is computed using the “Asphalt Institute” fatigue model [Eq. (3-11)] as follows: N f = 0 . 0796 ε t− 3 . 291 (145 × E*)− 0 . 854 For conditions prevailing in season 1 (line 2 of Table 8-8) N f = 0 . 0796 × ( 43 × 10 − 6 )− 3 . 291 (145 × 10000)− 0 . 854 N f = 102 . 2 MESAL Damage can then be computed as follows (line 3 of Table 8-8): Df 1 =
n1 3 . 08 = = 0 . 03 N 1 102 . 2
Total fatigue damage is thus equal to the sum of the three seasonal damages as indicated in line 4 of Table 8-8: n
D fT = ∑ Dn = 0 . 03 + 0 . 73 + 2 . 13 = 2 . 89 1
Computation of permanent deformation damage The number of allowable load repetitions to permanent deformation failure can be computed using Eq. (8-5): N p = 1 . 365 × 10− 9 (ε c )− 4 . 477 For season 2, the following answer is obtained (line 5 of Table 8-8): N p = 1 . 365 × 10− 9 (349 × 10− 6 )− 4 . 477 = 4 . 10 MESALs
Pavement Design Season Layer
Data element
Unit
Duration (week) 6
Weighted volume of traffic (ESALs × 10 ) Asphalt concrete
1
2
3
16
6
30
3.08
1.15
5.77
Resilient modulus
MPa
10000
4000
3000
Poisson’s coefficient
–
0.35
0.35
0.35
Resilient modulus
MPa
1000
120
225
Poisson’s coefficient
–
0.35
0.35
0.35
Resilient modulus
MPa
800
80
120
Poisson’s coefficient
–
0.35
0.35
0.35
Resilient modulus
MPa
500
25
50
Poisson’s coefficient
–
0.45
0.45
0.45
43
194
177
2. Allowable number of load applications (×10 )
102.2
1.57
2.71
3. Seasonal fatigue damage
0.03
0.73
2.13
4. Total fatigue damage
2.89
5. Vertical compressive strain (×10-6)
30
349
220
6. Seasonal permanent deformation damage
0.00001
0.28
0.18
7. Total permanent deformation damage
0.46
Granular base
Granular subbase
Subgrade soil −6
1. Tensile strain (×10 ) 6
TABLE 8-8
Analysis Results for Example 8-2
For season 2, the damage is thus equal to (line 6 of Table 8-8): DP2 =
n2 1 . 15 = = 0 . 28 4 . 10 N2
And total permanent deformation damage is equal to (line 7 of Table 8-8): n
DPT = ∑ Dpn = 0 . 000013 + 0 . 28 + 0 . 18 = 0 . 46 1
The damage analysis shows that the trial pavement structure will suffer 289 percent of its fatigue capacity during its design life. It also shows that the permanent deformation damage is only 46 percent of the pavement capacity. The proposed pavement structure is thus adequate with respect to permanent deformation but much too weak to support the expected traffic without excessive fatigue damage. As suggested in Table 8-7, the structure should thus be improved by increasing the thickness and/or the stiffness of the top layer(s). The pavement was also analyzed with respect to its response to cold temperature and frost action. First, the thickness of the HMAC layer is validated using a 5-m minimum crack spacing at the end of the 15-year design life. Considering the following pavement and climate characteristics:
331
332
Chapter Eight Stiffness of the original asphalt cement (Sbit): 300 kg/cm2 Winter design temperature (m) −25°C Clay subgrade (d = 2) 150-mm-thick HMAC layer (h) Computation of thermal cracking damage According to Eq. 3-1: I = 30.3974 + (6.7966 – 0.8741·h + 1.3388·a) log(0.1Sbit) – 2.1516·d – 1.2496·m + 0.06026·Sbitlog(d) I = 30.3974 + [6.7966 – (0.8741 × 6) + (1.3388 × 15)] log(0.1 × 300) – (2.1516 × 2) – (1.2496 × 25) + (0.06026 × 300 × log(2)) I = 30.3974 + 31.95 – 4.3032 – 31.24 + 5.44 I = 32.24 Spacing = 152.4/I = 152.4/32.24 = 4.72 m, which is slightly below the criterion selected for the analysis. Increasing the thickness of the HMAC layer to 190.5 mm (7.5 in) or the selection of a softer type of bitumen would allow one to meet the criterion. Validation of pavement structure for frost heave The validation of total pavement thickness with respect to an allowable frost heave criterion has also been conducted by computing the frost heave using a computer-based model. The information given in Table 8-9 was used in the analysis. Using a freezing index of 1236°C·day and the thermal characteristics listed above in the SSR model (St-Laurent 2006), the predicted frost heave of the pavement structure is estimated to be 118 mm. Considering an allowable frost heave criterion of 60 mm, the thickness of the granular subbase layer would have to be increased to 1200 mm to meet the criterion. In such a case, pavement insulation should be considered.
Step 5: Improve Pavement A pavement structure is considered to be optimal when computed damage over the analysis period is close to 1 without exceeding it for all damage criteria. When damage exceeds 1 or is much less than 1 (<0.70 to 0.80), the pavement structure needs to be modified. Table 8-10 provides some guidelines for possible modifications to the structure to be considered by the designer.
Step 6: Reiterate Pavement Analysis Repeat Steps 4 to 6 until a satisfactory pavement structure is obtained.
Layer
Dry Density, t/m3
Surface
2.35
0
0
1.48
Granular base
2.2
4.0
0
1.89
Granular subbase
2.05
8.0
0
2.98
Subgrade soil (clay)
1.3
40.0
120
2.29
TABLE 8-9
Water Content, %
Segregation Potential, mm2/°C·day
Input Variables for Computer-Based Model Used in Example 8-2
Frozen Thermal Conductivity, W/m·°C
Pavement Design Damage Indicator
Difference
Modification
Fatigue cracking
>1
Increase thickness or stiffness of HMA layer. Increase stiffness of granular base layer.
<<1
Reduce thickness or stiffness of HMA layer.
>1
Increase thickness or stiffness of granular base.
<<1
Reduce thickness or stiffness of granular base.
Thermal cracking
> AV*
Increase HMA layer thickness
Frost heave
> AV*
Increase thickness of granular subbase
Permanent deformation
Modify mix design
Use insulation layer ∗Recommended allowable value.
TABLE 8-10
Suggested Adjustments to Pavement Structure When Performance Objectives Are
Not Satisfied
8-3
Selection and Design of Special Protective Features Two main strategies are available for the protection of pavements subjected to frost action. The first strategy consists of reducing the effects of frost heave in order to control excessive seasonal and long-term roughness development. Limiting frost heave also helps reducing frost heave cracking and helps control thaw weakening. The second strategy is to reinforce the pavement structure in order to make it more resistant to frost heave forces and/or to thaw weakening.
8-3-1
Control of Frost Heave
The two main structural approaches used to control frost heave effects on pavements are the use of longitudinal transitions and the use of pavement insulation.
Longitudinal Transitions The construction of longitudinal transitions is one of the most commonly used techniques to mitigate performance problems related to differential frost heave in areas where the road crosses abrupt contacts between low and high frost-susceptibility soil units. Figure 8-7 illustrates a typical transition wedge used in pavement construction. The main principles used to design a transition wedge are the following (Dysli 1991; Gouvernement du Québec, 1994): • The maximum thickness of the wedge should be such that the total thickness of granular protection (D) at the contact between the two soil units is equal or greater than the maximum expected frost penetration in granular soils • The length of the transition should allow adequate attenuation of differential frost heave between the soil units. The length of the transition (L) should take into account traffic speed and expected differences in frost heave between two
333
334
Chapter Eight
FIGURE 8-7 Longitudinal transition wedge used to mitigate differential behavior of a road crossing two soil units with contrasting frost susceptibility.
adjacent soil units. Slopes (S) of 20H: 1V and minimum length (L) of 20 m are typically recommended for transition wedges • The transition wedge must be effectively drained Transition wedges are cost effective when severe differential frost heave is expected between adjacent soil units and when geological contacts between contrasting behavior units are not too frequent.
Pavement Insulation Pavement insulation is used in cases where widespread severe frost heave and differential frost heave is expected. Contrary to common belief, the insulation layer does not block the progression of the cold front in the pavement system. In fact, it impedes heat flow, reducing considerable heat loss from the pavement system when pavement surface is exposed to cold air. By doing so, soil temperature is likely to remain above freezing temperature throughout winter. As a result, frost penetration, frost heave, and differential frost heave are minimized if not eliminated. Pavement insulation is also an effective technique to protect permafrost. This application is discussed in Chap. 10. Insulation layers are however mechanically weak materials and overlying pavement layers need to be designed with special care. Different types of insulation materials have been used for road construction. Most pavement insulation materials use air trapped in micro pores to reduce thermal conductivity and provide effective insulation. This is the case for expanded and extruded polystyrene as well as for urethane-based materials. Other materials take advantage of their water absorption capacity to store large quantities of latent heat, which will oppose the progression of the frost front when it reaches the insulation layer. Among other materials, wood residues and peat have been used as pavement insulation materials. The most frequently used material for pavement insulation is polystyrene boards. The very low thermal conductivity, the high compressive strength, and the long-term stability of this material in the pavement environment make it an ideal insulation material. Extruded polystyrene is the most commonly used and the best performing material. Expanded polystyrene is also used as a low-cost alternative to extruded polystyrene. It is, however, more sensitive to water intake which reduces its insulation properties. This problem needs to be compensated by increasing the thickness of the polystyrene by a factor of 1.2 (St-Laurent 2006). The major problem with the use of polystyrene boards is
Pavement Design the manpower required for the placement of the insulation boards. Special care must be given to joints between boards during placement and while covering the boards with granular materials. The use of two board layers allows for overlaps at joints, thus minimizing the risk of heat loss at the joint location. Several other insulation products are being manufactured and can be used for pavement insulation purpose. Among other products, lightweight concrete and lightweight aggregates can be used to produce a relatively stiff layer with a low thermal conductivity. The materials are also easy to lay down with standard construction equipment and it is possible to gradually reduce the thickness of the layer in order to construct effective transitions. Lightweight aggregates such as expanded clays and foam glass are becoming interesting insulating materials. They must, however, be protected against contamination using a separation geotextile. All these materials tend to be sensitive to moisture intake and their long-term mechanical and thermal performance has not yet been assessed. Several types of recuperated and/or recycled materials have also been used for pavement insulation. Laboratory and field tests have been done with wood residues (bark, sawdust, and wood chips), peat, crumb rubber, and plastic chips. Most of these materials offer good thermal resistance, but typically have poor mechanical properties. These materials become interesting for pavement insulation when they are available in large quantities and at low cost. They are typically used in the construction or the rehabilitation of low-volume local roads. Table 8-11 provides a summary of available insulation materials and related thermal characteristics. Four important aspects need to be considered when designing an insulation layer: • The structural implications of inserting a weak layer in the pavement system • The thermal design of the insulation layer • Considerations for the risk of differential icing at the pavement surface • Considerations for differential behavior at the end of insulated areas
Structural Implications These include considerations for stresses transmitted to the insulation layer and modifications to the mechanical behavior of the pavement structure. In the first case, it is important to make sure that the level of stress transmitted to the insulation layer does not induce excessive deformation of the insulation material, which would result in mechanical and thermal degradation of the layer. The equivalent thickness of granular layer required to protect the insulation layer can be determined using Eq. (8-6) (Nixon 1979) or using the design chart illustrated in Fig. 8-8. It is important to note that part of this granular thickness needs to be converted in asphalt concrete using Odemark’s equation (see Sec. 5-9). ω πp Z= − 2/3 σ a − 1 1 − p
1/2
(8-6)
where Z is the thickness of material (equivalent granular material) required above the insulation layer (m), w is the applied load (kN) (typical value: 40kN), p is the tire pressure (kPa) (typical value: 700 kPa), σa is the allowable stress on the insulation layer.
335
336 Insulation Material
Thermal Conductivity, W/m·°C
Heat Capacity, kJ/m3·°C
Typical Water Content (%)
Compressive Strength, kPa
Comment
Extruded polystyrene
0.03–0.04
~ 40
0.3–5.0
200–700
Widely used and documented
Expanded polystyrene
0.03–0.05
~ 40
1.0–10.0
150–400
Low-cost alternative to extruded polystyrene. Risk of higher water intake in insulation material must be compensated by an increased layer thickness
Lightweight concrete
0.5–0.7
1500–1700
1.0–5.0
2000–4000
High stiffness material; easy to lay down
Expanded clay
0.15–0.25
350–500
5.0–20.0
Foam glass aggregate
0.15–0.25
300–360
5.0–20.0
Sawdust and wood bark
0.15–0.5
110–130
Separation geotextile needs to be used to protect the insulation layer against contamination E modulus between 5 and 10 MPa
Structural design of pavement layers above the insulation layer is critical
Information synthesized from: Dysli 1991, Andersland and Ladanyi 2004, Doré et al. 1998, ∅iseth and Refsdal 2006, ∅iseth et al. 2006, Juneau et al. 2005.
TABLE 8-11
Insulation Materials and Their Thermal Properties
Pavement Design
FIGURE 8-8 Design chart for estimation of layer thickness (given in equivalent thickness of granular base material) required to protect insulation layer in flexible pavement against loading by heavy traffic for three levels of compressible strength of the insulation material (275, 415, and 690 kPa).
sa = 0.50 Rc, if number of load applications N = 103 6
= 0.12 Rc, if N = 10
= 0.10 Rc, if N = 108 Rc is the compressive strength of the insulation material as specified by the manufacturer. The other structural consideration to take into account when insulating a pavement structure is the long-term mechanical performance of the pavement structure. The use of an insulation layer may prove to be an effective way to control frost-related deterioration of pavements. It is, however, likely that the use of a relatively soft layer in the pavement structure will have significant mechanical implications. In order to justify using insulation layers in pavement structures, the “thermal” benefit has to overcome the detrimental mechanical effects and the costs related to the placement of the layer. The analysis of data collected at a test road in Quebec, Canada, (Doré et al. 1998; Juneau et al. 2004; Juneau et al. 2005) shows that when compared to a reference section, insulated pavement sections are characterized by higher maximum deflections, steeper deflection basins, and higher surface curvature indices (SCI), base curvature indices (BCI), and strains at the bottom of the asphalt layer. For pavements insulated with soft insulation materials (sawdust and tire chips/sand mix) these responses are much higher than those measured on the noninsulated pavements, while the section insulated with polystyrene exhibits moderately higher responses. However, insulated pavement sections are less sensitive to structural weakening during spring thaw. The long-term performance observations at the test site suggest that for the polystyrene section, the benefit associated with a better mechanical behavior during spring thaw seems to
337
338
Chapter Eight overcome the detrimental effects associated with the presence of the soft insulation layer in the pavement system. For softer insulation material, it is essential to carefully design the layers above the insulation material in order to minimize the effect of traffic on the fatigue performance of the HMA layer. The thickness of the insulation layer is calculated based on thermal properties of the pavement components including the insulation layer and on climatic conditions, generally represented by the surface freezing index. In order to adequately protect the pavement structures against frost action, the pavement designer needs to have clear performance objectives. As described in Sec. 8-2 and Sec. 3-7 of this book, frost heave is related to winter roughness as well as long-term roughness evolution of pavements and is thus a good indicator of pavement performance in cold climates. Table 8-7 and Fig. 8-6 provides some indications on allowable frost heave criteria that can be used as reference. However, the use of conservative heave criteria should be considered when taking into consideration the high cost of pavement insulation. Thermal design of the insulation layer can be done using a multilayer thermal model. It can alternatively be estimated using the design charts illustrated in Fig. 8-9. Charts were developed using the Finnish SSR model incorporated in the “CHAUSSÉE II” software developed by the Quebec Ministry of Transportation (StLaurent 2006). Charts are valid for a pavement including 150 mm of HMA over 450 mm of granular base material (see “Considerations for the Risk of Differential Icing” below) above the polystyrene insulation layer and 300 mm of granular subbase material underneath. Three cases are presented: 0, 40, and 80 mm allowable frost heave. Severe freezing index conditions should be used for thermal insulation purpose. Severe winter conditions can be obtained by averaging the three coldest freezing indices in 30 years. Note that polystyrene insulation can be replaced with an alternative insulation material provided that the thickness of the layer is adjusted to provide equal thermal resistance. The equivalent thickness can be obtained as follows:
Thermal Design of the Insulation Layer
xp kp
=R=
xa ka
(8-7)
where R is the thermal resistance, xp and kp are the thickness and the thermal conductivity of polystyrene insulation, xa and ka are the thickness and the thermal conductivity of an alternative insulation material. From Eq. (8-7), the thickness of the insulation layer of alternative material providing equal thermal resistance is equal to (see Example 8-3): xa = x p ×
ka kp
(8-8)
Example 8-3 What is the required thickness of expanded clay aggregate to provide the same thermal resistance as 50 mm of polystyrene? Solution From Table 8-11, kp = 0.035 and kec = 0.2. According to Eq. (8-8): xec = x p ×
k ec 0.2 = 50 × = 286 mm kp 0 . 035
Pavement Design HMAC 150 mm Granular base 450 mm Polystyrene insulation (variable thickness) Granular subbase 300 mm
Freezing index (°C·day)
Freezing index (°C·day)
Freezing index (°C·day)
Subgrade soil (variable SP) 2500 Allowable frost heave = 0 mm
38 mm 50 mm 63 mm
2000 1500 1000 500
0
20
40 60 SP (mm 2 /°C·day)
2500
80
100
Allowable frost heave = 40 mm
2000 1500 1000 500 50 3000
25 mm 38 mm 50 mm 63 mm 100
150 SP (mm 2 /°C· day)
200
250
250
300
Allowable frost heave = 80 mm
2500 2000 1500 1000 100
25 mm 38 mm 50 mm 150
200 SP (mm 2 /°C·day)
FIGURE 8-9 Thermal design of pavement insulation layer (thicknesses varying between 25 and 63 mm) as a function of surface freezing index (freezing indices representing severe winter conditions should be used) and segregation potential of subgrade soil. Note that these charts provide a first approximation of the thickness required to meet a specific design objective. Frost heave may vary significantly due to the variable thermal response of different types of soils. Specific calculation using local material and soils properties is strongly recommended.
As described earlier in this section, pavement insulation impedes ground heat flow toward the surface. It is thus likely to cause a reduction of surface temperature when the air temperature is low. Figure 8-10 illustrates a situation that can possibly occur during late fall, when the soil temperature is relatively warm and the air temperature is cold. In the case of a noninsulated pavement, ground heat is likely to maintain surface temperature above the freezing point while surface temperature can possibly drop below the freezing point for insulated pavement
Considerations for the Risk of Differential Icing
339
340
Chapter Eight
FIGURE 8-10 Changes in pavement thermal regime caused by the presence of an insulation layer (right) during cold fall periods.
sections. If air moisture is available, hoarfrost or black ice can thus, form at the surface of the insulated pavement section while water will remain in an unfrozen state at the surface of adjacent uninsulated pavement sections. This phenomenon, referred to as differential icing, is illustrated in Fig. 8-11. It is very difficult to anticipate and can be extremely hazardous for people traveling on the road. In order to minimize the risk of differential icing on insulated pavement sections, highway agencies usually require a minimum thickness of granular material on top of the insulation layer. The heat contained in aggregates and in interstitial moisture of the granular layer will partly compensate heat loss at the pavement surface, thus reducing the risk of surface icing. Most agencies using pavement insulation specify a minimum thickness based on experience and empirical rules based on experimental work done in Scandinavia (Refsdal 1973; Gustafson 1982; Gandahl 1987). The recommended thickness of granular material above an insulation layer varies typically between 300 and 600 mm.
FIGURE 8-11 Differential icing at the joint of insulated and noninsulated pavement sections (in front of the pickup truck) on a road in Alaska. (Photo courtesy of Alaska Department of Transportation and Public Facilities.)
Pavement Design Considerations for physical phenomena have been included in recent studies in order to develop rational procedures for the design of the granular protection layer above insulation materials (Gustavsson et al. 2002; Coté and Konrad 2006). Coté and Konrad have proposed design charts incorporating four main factors related to the formation of differential icing: the mean annual temperature, the thermal conductivity of mineral particles of the granular material as well as the moisture content, and thickness of the granular material above the insulation layer. The design charts proposed by Coté and Konrad for the climatic context of Quebec Province, Canada, are reproduced in Fig. 8-12. Application of these charts to other environments requires the development of site-specific parameters following the procedure described by Coté and Konrad. Total frost protection using pavement insulation is generally used in areas where excessive frost heave or differential frost heave is expected. If adjacent pavement sections are underlain by soils moderately sensitive to frost action, pavement insulation will probably not be needed and differential frost heave behavior is likely to occur between the insulated (no frost heave) and the noninsulated sections (moderate frost heave). In order to (minimize) the differential behavior problem, transitions between insulated and noninsulated pavement sections are generally recommended. Transitions are required transversely as well as longitudinally. Transverse transitions help minimize the risk of edge cracking in the bound surfacing layer and reduce
Considerations for Differential Behavior at the End of Insulated Areas
h GP = minimum thickness of the granular protection h INS = thickness of the insulation layer MAAT = mean annual air temperature k s = thermal conductivity of the solid particles in the granular protection w GP = water content by weight in the granular protection 800
a) k s = 2.0 W/m °C, w GP = 3.5 %
h GP (mm)
700 600
h INS = 100
500
75 50 25 (mm)
h INS = 100
75 50 25 (mm)
400 300
c) k s = 3.0 W/m °C, w GP = 3.5 %
200 800
b) ks = 2.5 W/m °C, w GP = 3.5 %
700 h GP (mm)
Working example MAAT = 4.5 °C h IN = 50 mm h GP = 450 mm
d) ks = 3.5 W/m °C, w GP = 3.5 %
600
h INS = 100
75 50 25 (mm)
500 h INS = 100
75 50 25 (mm)
400 300 200 0
1
2
3 4 MAAT (°C)
5
60
1
2
3 4 MAAT (°C)
5
6
FIGURE 8-12 Design charts for thickness of granular material above insulation layer required to minimize the risk of differential icing for climatic context of Quebec Province (redrawn from Coté and Konrad 2006, with permission from the Canadian Geotechnical Journal).
341
342
Chapter Eight
FIGURE 8-13 Transverse section showing lateral transition and plan view showing longitudinal and lateral transition for insulated pavements.
the risk of heaved shoulder obstructing surface drainage. As illustrated in Fig. 8-13 extension of the insulation layer beyond the paved surface and gradual reduction of insulation thickness are the two principles used to reduce the risk of differential behavior in the transverse direction. Differential behavior in the longitudinal direction can reduce ride quality and has therefore, more effect on the traveling public. The problem can be reduced by longitudinal transitions over distances ranging between 10 and 30 m at each end of the insulated section. The length of the transition zone depends on traffic speed, thickness of the insulation layer, and expected differential frost heave. As illustrated in Fig. 8-13, longitudinal transitions are usually achieved by reducing the thickness of the insulation layer and, in the case of insulation boards, by progressively spacing the boards to gradually reduce their insulation effectiveness. Lateral transitions underneath the gravel shoulder are also recommended. Example 8-4 illustrates design of insulated pavement structure. Example 8-4 structure:
Considering the following conditions, design an appropriate insulated pavement
Surface freezing index: Traffic volume:
1200°C·day
1 × 106 ESAL (typical wheel load = 40 kN and tire pressure = 700 kPa)
Compressive strength of the insulation layer: SP of the subgrade soil: Allowable frost heave:
415 kPa
2
150 mm /°C·day 40 mm
Solution According to Eq. (8-6):
σ a = 0 . 12 × 415 kPa = 49 . 8 ≈ 50 kPa 40 π 700 Z= −2 / 3 − 1 1 − 50 700
(
)
1/2
0 . 018 = 0 . 050
1/2
= 0 . 59 m
Pavement Design This value is confirmed by the design chart (Fig. 8-8). It is important to note that part of this granular thickness needs to be converted in asphalt concrete using Odemark’s equation (see Sec. 5-9). The thickness of the insulation layer for 40 mm frost heave, SP = 150 mm2/°C·day and FI = 1200°C·day is equal to 25 mm according to Fig. 8-9. From Fig. 8-12, assuming that the thermal conductivity of the mineral particles in the granular material is 3.0 W/m·°C and that the moisture content is 3 percent, for a mean annual temperature of 4°C, the thickness of granular material required to limit the risk of differential icing would be 450 mm. The pavement structure should thus, include a 25-mm layer of polystyrene insulation overlain by 450 mm of granular material and a thickness of asphalt concrete corresponding to a minimum of 140 mm of equivalent granular material.
8-3-2
Pavement Reinforcement
Pavement reinforcement can be a good strategy to mitigate some of the effects caused by frost heave and spring thaw. Two types of reinforcement can be considered: • Stabilization of granular base material to increase the stiffness of the top part of the structure and to minimize sensitivity to moisture • Reinforcement of the structure using geogrids, reinforcement geosynthetics, or steel nets
Stabilization of Granular Base Material Stabilization of granular base material using asphalt cement, Portland cement, or lime is likely to improve pavement behavior and performance during spring thaw. Stiffer layers in the upper part of the pavement will contribute to a more effective load distribution within the pavement structure and reduce vertical strain and associated permanent deformation in the weakened thawing subgrade soil. Stabilization of granular base material also reduces significantly the sensitivity of these materials to moisture and frost action. The stiffness of bound materials is consequently more stable across seasons, thus reducing pavement damage during spring thaw. It is, however, important to take into consideration that a reduced pavement thickness resulting from an increased stiffness of the base layer might lead to a reduced longterm performance of the pavement, if subjected to frost heave action. As a matter of fact, a stiffer pavement structure is more likely to crack if subjected to differential frost heave than a flexible structure.
Reinforcement Using Steel Nets Steel nets, or wire fabrics, have been used in Finland for mitigation of longitudinal cracks both in rehabilitation projects and in new construction. Nets are typically embedded at a minimum depth of 200 mm from the top of the unbound layer. The principal reinforcement is the transverse steel that is continuous throughout the width of the pavement. The transverse steel controls longitudinal cracks that form in the pavement due to differential movement resulting, for example, from frost heave, and as a result restrain the lateral movement of the embankment. Currently, no design procedure exists to determine the amount of transverse reinforcement required. Recommendations for the wire diameters and spacing based on the experience in Finland are given in Table 8-12. Installation recommendations given in Fig. 8-14 are a result of maintenance considerations rather than functional considerations. The steel net needs to be adequately covered from all sides so that maintenance operations do not expose the net.
343
344
Chapter Eight Steel Yield Strength 500 MPa Transverse Wire
Longitudinal Wire
Embankment Type
Spacing, mm
Diameter, mm
Spacing, mm
Diameter, mm
Road, width ≥10 m
200
8
150
6
Road, width <10 m
200
7
150
5
Bike trail
150
6
100
5
Source: adapted from Lehtonen et al. 2005.
TABLE 8-12 Recommendations for Steel Nets Restraining Embankment Lateral Movement
FIGURE 8-14 Installation of steel nets in road embankment as part of rehabilitation project (modified from Lehtonen et al. 2005).
The optimum net length (in the transverse direction) depends on the embedment depth and the grade of the side slope and becomes the width of the road surface plus 200 to 500 mm (see Fig. 8-14). Nets are placed side by side without fastening or overlapping. Other considerations include the following (Lehtonen et al. 2005): • If the pavement structure widens due to a bus stop or a turning lane, the steel nets need to expand over the widened area as cracks tend to form at the edge of the net. The dimensions of the nets need to be individually determined at these locations • In cases that there is no need to remove the existing bound pavement layer, steel nets can be installed on the top of it, if two conditions are met: (1) the existing layer is at least 80 mm thick, and (2) the overlay thickness is at least 60 mm • The use of steel nets is not recommended for roads with a history of large rocks jacking into the surface. The jacking rocks tend to lift the net with them and expose it at the surface
8-4
Conclusion A good understanding of pavement interaction with traffic and climatic variables is required to master pavement performance in cold climates. Pavement structural design
Pavement Design is a key aspect in performance achievement. When considering seasonal variation in material properties, pavement design allows controlling structural performance with structural rutting and fatigue cracking used as main performance criteria. Proper consideration for temperature and frost action is also needed to control the functional performance of the pavement with thermal cracking, frost heave cracking, and roughness development as being the main performance criteria to consider.
Review Questions 8-1. The annual average daily traffic on a four-lane divided principal highway is expected to be 10,000 including 12 percent of commercial vehicles with an annual growth of 3 percent. What will be the number of standard load applications on this road during its 20-year design life?
8-2. Considering an expected number of load applications of 5 MESALs, compute the allowable tensile strain at the bottom of the HMA layer and the allowable compressive strain on top of the subgrade soil.
8-3. The designer of a pavement structure wants to replace 60 mm of extruded polystyrene insulation by lightweight concrete. What thickness should he use to obtain equivalent thermal effectiveness?
8-4. 50 kN loads will be applied on an insulated gravel pavement designed to carry 1 MESALs. Assuming tire pressure to be in the vicinity of 600 kPa and knowing that the strength of the insulation boards is 400 kPa, at what depth should the insulation layer be placed?
8-5. Considering a severe freezing index of 1800°C·day and a segregation potential of 150 mm2/ °C·day, what should be the thickness of the extruded polystyrene insulation layer?
References AASHTO (1993). AASHTO Guide for the Design of Pavement Structures, American Association of State Highway Transportation Officials, Washington, D.C. Alaska Department of Transportation and Public Facilities (2004). Alaska Flexible Pavement Design Manual, DOT&PF Research Report FHWA-AK-RD-03-01. Andersland, O. B., and Ladanyi, B. (2004). Frozen Ground Engineering, 2d ed., John Wiley & Sons and ASCE Press. Hoboken, N.J. Asphalt Institute (1991). Thickness Design—Asphalt Pavements for Highway and Streets, Manual series no. 1, Asphalt Institute, Lexington, Ky. Berg R. L. (1988). “Pavement Design for Seasonal Frost Conditions—Current and Future Methods,” Fourth Annual Airport Engineering/Management Conference, FAA, GreatLakes Region. Chadbourn, B., Dai, S., Davich, P., Siekmeier, J., and VanDeusen, D. (2002). “Pavement Designer’s Guide Mn/DOT Flexible Pavement Design MnPAVE Beta Version 5.1.” Minnesota Department of Transportation, Office of Materials and Road Research. Coté, J., and Konrad, J. -M. (2006). “Granular Protection Design to Minimize Differential Icing on Insulated Pavements,” Canadian Geotechnical Journal, vol. 43, pp. 260–272. Darter, M. I., Becker, J. -M., Sydney, M. B., and Smith, R. E. (1985). “Portland Cement Concrete Pavement Evaluation System (COPES),” NCHRP Report 277, Transportation Research Board, National Academies Press, Washington, D.C.
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Chapter Eight Doré, G. (1997). “Détérioration des chausses en conditions de gel; une nouvelle approche prévisionnelle (Pavement deterioration in frost condition; a new predictive approach),” Ph.D. Thesis, Laval University, Quebec City, Canada (in French). Doré, G. (2002). “Validation des modèles de détérioration de la chaussée soumise au soulèvement par le gel (Validation of performance models for pavements subjected to frost heave),” Research report GCT-02-07, Civil Engineering Department, Laval University, Quebec City, Canada (in French). Doré, G., Konrad, J. M., and Roy, M. (1998). “Mechanical Implications of Using Insulation Layers in Pavements,” Proceedings of the Ninth International Conference on Cold Region Engineering, ASCE Press, Reston, Va. Dysli, M. (1991). “Le gel et son action sur les sols et les fondations (Frost and its effects on soils and foundations),” Presses Polytechniques et Universitaires Romandes, Lausanne, Switzerland (in French). Gandhal, R. (1987). “Polystyrene Foam as Frost Protection Measure on National Roads in Sweden,” Transportation Research Record 1146, National Academies Press, Washington, D.C., pp. 1–9. Gouvernement du Québec (1994), Normes – Ouvrages routiers : Construction routière, Tome 2, Les publications du Québec, Canada. Gustafson, K. (1982). “Icing Conditions on Different Pavement Structures,” Transportation Research Record 860, National Academies Press, Washington, D.C. pp. 21–28. Gustavsson, H., Ravaska, O., and Hermansson, A. (2002). “The Effect of Insulation on Road Surface Icing,” Proceedings of the Eleventh International Conference on Cold Region Engineering, ASCE Press, Reston, Va. Huang, Y. H. (1993). Pavement Analysis and Design, Pearson Prentice Hall, Upper Saddle River. Jämsä, H., and Orama, R. (1990). “The Measurement of Seasonal Subgrade Properties under Existing Pavements and the Finnish Pavement Design Method,” Preprint no. 890619, TRB 69th Annual Meeting, Washington, D.C. Juneau, S., Doré, G., Pierre, P., et Savard, Y. (2004). “Long Term Performance and CostEffectiveness of Insulated Pavements,” 12th International Conference on Cold Region Engineering, ASCE Press, Reston, Va. Juneau, S., Doré, G., and Pierre, P. (2005). “Mechanical Behavior of Insulated Pavements,” Proceedings of BCRA 05, Trondheim, Norway (CD-ROM). Lehtonen, K., Salo, P., Kallionpää, T., Rantanen, T., and Junnila, A. (2005). “Rakenteen parantamisen suunnittelu; suunnitteluvaiheen ohjaus (Planning of road rehabilitation projects),” TIEH 2100035-05, Finnish Road Administration, Helsinki (in Finnish). Miner, M. A. (1945). “Cumulative Damage in Fatigue,” Transactions of the ASME, vol. 67, New York, pp. A159–A164. NCHRP. (2006). “Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures,” NCHRP 1-37A, Transportation Research Board, Washington, D.C., http://www.trb.org/mepdg/guide.htm (December 12, 2006). Nixon, J. F. (1979). “Some Aspects of Road and Airstrip Pad Design in Permafrost Areas,” Canadian Geotechnical Journal, vol. 16, no. 1. ∅iseth, E., Aaboe, R., and Hoff, I. (2006). “Field Test Comparing Frost Insulation Materials in Road Construction,” 13th International Conference on Cold Region Engineering, ASCE Press, Reston, Va. ∅iseth, E., and Refsdal, G. (2006). “Lightweight Aggregates as Frost Insulation in Roads—Design Charts,” 13th International Conference on Cold Region Engineering, ASCE Press, Reston, Va.
Pavement Design Ontario Ministry of Transportation (1990). Pavement Design and Rehabilitation Manual, Survey and Design Office, SDO-90-01. PCASE (2003). “WINJULEA, Layered Elastic Analysis Software,” available through the Pavement-Transportation Computer-Assisted Structural Engineering (PCASE), http://www.pcase.com (January 3, 2007). Peyronne, C., and Caroff, G. (1991). Cours de route, dimensionnement des chaussees (Highway course, pavement design), 2e édition, Presse de l’Ecole Nationale des Ponts des Chaussées, Paris, p. 244 (in French). Refsdal, G., (1973). “The Use of Thermal Insulating Materials in Highway Engineering: Results from the Norwegian Test Roads,” Frost I Jord 9, pp. 27–39. St-Laurent, D. (2006). “CHAUSSÉE 2 : Logiciel de dimensionnement des chaussées souples Guide de l’Utilisateur (Flexible pavement design software, user guide),” Ministère des transports du Québec, Direction du laboratoire des chaussées (in French). Tammirinne, M., Valkeisenmäki, A., and Ehrola, E. (2002). “Road Structures Research Program 1994–2001” Summary report, Finnra Reports 37/2002. Finnish Road Administration, Helsinki, Finland. Ullidtz P. (1998). Modelling Pavement Response and Performance, Polyteknisk Forlag, Lyngby, Denmark.
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CHAPTER
9
Maintenance and Rehabilitation
T
aking care of a road investment involves routine and major maintenance operations. When these fairly local operations are no longer feasible due to their extent and frequency along the roadway, rehabilitation is conducted that further extends the pavement’s service life. Finally, as the pavement’s service life is exhausted, including the extension provided by the rehabilitation, reconstruction of the entire pavement structure may become more reasonable alternative than another rehabilitation procedure. Reconstruction includes typically demolishing the existing pavement structure, improving subgrade when necessary, and rebuilding a new pavement structure. Life time engineering aspects and pavement management concepts (see Chap. 6) are considered when selecting between maintenance strategies, rehabilitation, and reconstruction alternatives. Descriptions of each of maintenance category with applicable cold region concerns are collected in Table 9-1. Routine maintenance includes daily operations, and periodical small scale corrective actions, such as patching and crack sealing. Major maintenance operations include local drainage improvements, repair of local failures, surface treatments, and overlays. Rehabilitation projects involve substantial procedures that typically address corrective actions of the surface, subsurface, and drainage systems. The following sections describe maintenance and rehabilitation issues specific for paved roads in cold regions. Maintenance of gravel roads differ in its frequency and techniques significantly from that of paved roads, and is therefore covered separately in Sec. 9-3. For some road sections, load restrictions are imposed during spring thaw to protect the pavement structure from damage. Section 9-5 explains the load restriction policies and their logic.
9-1
Routine Maintenance Daily maintenance operations specific for cold regions include snow removal, deicing, and traction sanding. Jurisdictions publish winter maintenance policies that set guidelines for daily maintenance operations to guarantee traffic conditions that allow society to function efficiently in winter driving conditions. The factors considered in making the policy include customized levels of service for a road network and its components, environmental impacts, user costs resulting from road conditions and operational costs. Table 9-2 lists main principles of winter maintenance policy used in Finland. The policy assigns each road a maintenance class and quality standards stating acceptable surface
349
Routine maintenance
Chapter Nine
Technique
Cold Region Concerns
Daily operations: maintain level of service by surface deicing, traction sanding, snow removal, dust control, sweeping, and so on
Mitigate snowy and icy driving conditions
Patching: filling holes with cold mix
Prevent problems related to frost action
Crack sealing Deicing drainage systems
Major maintenance
Surface treatments Overlays Local drainage improvements Repair of local failures
Preventive maintenance, prevent problems related to frost action Mitigate failures due to frost action
Addition of structural layers: unbound layers, bound layers or reinforcements In-place recycling: cold milling with or without addition of aggregate and stabilizations agents Rehabilitation
350
Cross-section improvements: repairing too steep side slopes or increase of pavement width
Roads that are not engineered or constructed to any standards
Drainage improvements: installing drains and culverts or repairing failed systems
TABLE 9-1
Description of Maintenance Categories for Paved Roads
characteristics for stable and changing weather conditions. The road classification is based on the administrative classification and average daily traffic of the roads (Finnra 1995). The aforementioned daily maintenance procedures may have indirect effects that need to be considered: As explained in Chap. 3, the transverse differential heave can be significantly reduced by increasing the width of the lane cleared of snow. Therefore, the snow removal should always extend to the road shoulders, and the fore slopes should be kept clear of snow when possible. Deicing and sanding used to increase winter traction and traffic safety have potential adverse effects on pavement performance, vehicles using the roads, public health, and environment, and therefore should be used sparingly. Routine maintenance techniques for paved roads in cold regions are similar to those used in temperate regions and include patching, crack sealing, and keeping drainage systems functional. The importance of these procedures, however, is magnified in cold regions, as most of the pavement distresses are related to water infiltration. The materials and techniques used need to address challenges resulting from cold temperatures, snow and ice. For example, patching may have to be conducted during wintertime in undesirable temperatures and in wet conditions. In such cases, temporary filling with asphalt cold mix should be considered, but permanent repair should be scheduled and
Winter Maintenance Class† Is
I
Ib
II
III
0.3 (0.25 when surface temp. < −6°C)
0.3 (0.25 when surface temp. < −4°C)
0.25
Rough, compacted snow
Rough, compacted snow
Max. surface roughness, mm
NA
10
20
30
30
Time of day quality standards are imposed
Always
5:00–22:00
5:00–22:00
6:00–22:00
6:00–22:00
Maximum snow depth, mm (powder/slush)
40/20
40/20
40/20
80
100
Max. time elapsed from end of snowfall to completion of snow removal, hours
2.5
3
3
4
6
Max. time elapsed from going below friction coefficient requirement to completion of deicing, hours
2
2
Salt: 3 Sand: 4
6
12
Max. time elapsed from snow fall to completion of surface leveling, days
1
1
1
3
5
Changing weather conditions*
Stable weather conditions*
Min. friction coefficient
*
More details and exceptions can be found at Finnra 1993 and Finnra 1995. Explanations for winter maintenance classes: Is Roads are kept free of snow and ice using salt when necessarily. During long freezing spells, when salting does not have an effect, sporadic ice is allowed. I Roads are kept free of snow and ice except in the middle of the night. Thin layer of compacted snow between the lanes and on the shoulders is allowed. Ib In the middle of winter a uniform thin layer of ice or compacted snow providing sufficient traction is allowed. Other seasons road surface is kept bare. II Layer of compacted snow is allowed. Road is sanded at steep hills, curves and intersections. III Driving conditions are kept at a satisfactory level. Skid prevention is applied only at critical locations resulting in varying levels of service. Source: modified from Finnra 1995.
†
351
TABLE 9-2 Principles of Winter Maintenance Quality Standards in Finland
352
Chapter Nine performed as soon as weather conditions permits. Proper pothole patching involves the following operations: • Cutting the asphalt concrete around the affected area • Removal and replacement of contaminated materials in the hole • Compaction of the bottom of the hole • Coating the walls of the asphalt concrete hole with an appropriate tack coat • Filling the hole with good quality materials Special consideration in crack sealing comes from the fact that cracks widen up during cooling trend due to thermal contraction and narrow down again during warming trend. This leads to possible cracking of the sealant and consequent need for frequent repairs. Elastic crack sealants and backer rods produced from polymer-modified asphalts can be used to address this problem. Those sealants that meet AASHTO M173 or ASTM D1190, 3405, or 5078 are suitable for permanent crack sealing. Proper crack sealing technique involves (Asphalt Institute, 1997): • Cleaning the crack using, for example, hot compressed-air lance • Using backer rods in cracks deeper than 20 mm • Applying sealant from the bottom to 3 to 6 mm below the top of the crack Keeping drainage systems functional includes keeping them free of blocked ice. This is important especially just before the thaw period when the released water needs to be kept out off the pavement structure. The blocked ice in culverts should be melted using, for example, steam generators (Finnra 1993).
9-2
Major Maintenance Major maintenance techniques include preventive and corrective maintenance procedures. Preventive procedures are conducted before pavement commences deterioration and include surface treatments. Corrective measures include some surface treatments, pavement overlays, repair of drainage systems, and local failures. In order to select a proper maintenance technique, the current situation needs to be analyzed using visual observations, bearing capacity measurements, or ground penetrating radar scans (see Chap. 4). After all data is collected and analyzed, engineering judgment needs to be applied in order to determine the cause for the observed problems. The following sections describe major maintenance techniques and their applicability.
9-2-1
Surface Treatments
Surface treatments are used in maintenance of surface distress (small cracks or raveling) or preventive maintenance. Treatments consist of a single application of asphalt material that may or may not be covered or combined with aggregate (Asphalt Institute, 1997). Table 9-3 lists different types of surface treatments and when they should be used. More information on coverage rates and recommended aggregate gradations can be found in Sec. 7-4.
Surface Treatment
Description
Asphalt Product
Aggregate
Applications
Fog seal
Light application of emulsified asphalt
SS-1, SS-1h, CSS-1, CSS-1h diluted with water
NA
• Renews dry asphalt surfaces • Seals small cracks • Coats surface aggregate
Slurry seal
Creamy textured slurry applied in thickness of 3–6 mm
CQS-1h, CSS-1h, QS1h, SS-1h
Fine aggregate, mineral filler
• • • •
Micro-surfacing
High performance resurfacing similar to slurry seal
Polymer modified CSS-1h
Mineral aggregate, mineral filler, additives
• Same as for slurry seal • Provides additional resistance to abrasion
Chip seal
Sprayed asphalt emulsion immediately covered with aggregate
CRS-2, RS-2
One-size mineral aggregate
• Same as for slurry seal • Low-cost maintenance method
Asphalt surface treatment
Sprayed asphalt emulsion immediately covered with aggregate
HFMS-2, HFMS-2s, CRS-2 (McHattie 2003 and 2005)
Dense-graded mineral aggregate
• Same as for slurry seal • Provides thicker wearing course than slurry seal
Source: adapted from Asphalt Institute, MS-16 and MS 19.
TABLE 9-3 Surface Treatments Used in Cold Regions Pavement Maintenance
Seals surface cracks Stops raveling Creates an impermeable surface Improves skid resistance
353
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Chapter Nine
9-2-2
Overlays
Overlays are used to extend the service life of a pavement in cases that surface treatments do not adequately address the pavement problems, such as rutting and cracking. Before applying an overlay, the cause of the distress needs to be carefully analyzed. Overlay does not work in situations, where rutting initiates from the pavement subgrade. Quite the opposite, the rutting will become more severe with time due to the additional load from the weight of the overlay applied on the subgrade. Overlay is suitable for situations where the rutting results from densification or permanent deformation of the bound or unbound structural layers or from wear due to studded tire use. When cracking is considered, an overlay addresses fatigue cracking by providing additional structural stability. The overlay thickness design is conducted using principles described in Chap. 8. Overlays are not successful in treating thermal and longitudinal cracking. These types of cracks typically reappear in a form of reflective cracks after the first few winters. Overlays are not recommended in cases that water-related distress, such as stripping, has deteriorated the existing asphalt pavement. Waterrelated distress will damage the new overlay in a short period of time. Before an overlay is applied, the pavement needs to be milled at the base level of the existing ruts in order to prevent them from reappearing. The reclaimed material can be recycled either in-place or during hot-plant mixing. Need for binder rejuvenators and virgin aggregates is assessed during the mix design procedure.
9-2-3
Maintenance of Drainage Systems
As explained in Chaps. 2, 3, and 8, water and moisture have a significant influence on pavement performance. Water and moisture-related pavement distress results from reduced bearing capacity, frost action, and asphalt pavement deterioration. It is essential to keep the road structure and surface as dry as possible. This entails not only welldesigned and installed drainage systems, but also their appropriate maintenance. Drainage systems are composed of elements, such as permeable pavement layers, drains, culverts, and ditches that form a network for water and moisture removal from the pavement. This network works as well as its weakest component, that is, if one component gets clogged, the whole system fails. Therefore, the maintenance of the system should focus on keeping each of its components functioning at their designed capacity (Rantanen et al. 2005). A simple systematic approach is needed to evaluate the network condition as shown in Table 9-4. While going through the analysis, a list of specific maintenance operations is simultaneously created. This approach prevents unnecessary deepening of ditches, which is the most often applied drainage maintenance operation in some regions. In fact, deepening of ditches should be avoided, as it may lead to exceedingly steeper slopes, poor bearing capacity of the pavement shoulders while yet not necessarily taking care of the drainage problems as demonstrated in Fig. 9-1.
9-2-4
Repair of Local Failures
Local failures may result from differential frost heave, settlement, embankment erosion or local embankment spreading, and subsequent longitudinal cracking. Before the maintenance operation is selected, cause of the problem needs to be identified to avoid reoccurrence of the problem. Drainage is often the culprit and needs to be brought back to its design capacity (see Sec. 9-2-3). In some cases, the problem lies with the subgrade or pavement’s structural materials that may have become contaminated and need to be
Maintenance and Rehabilitation
Drainage Element
Checklist
Cross section
Does the water have free access from road surface to ditches? Are permeable pavement layers clogged or dammed up (see Fig. 9-1)?
Condition of side ditches
Are the slope of the ditch and the discharge capacity sufficient? Are there any rocks, cobbles, or other items blocking the water from flowing freely? Are the front and back slopes eroded?
Condition of culverts
Are the ends of the culverts open and clean? Are the culvert capacity and length sufficient? Is the elevation of the culvert correct? Is the discharge area eroded?
Condition of collector ditches
Are the slope of the ditch and the discharge capacity sufficient? Are there any rocks, cobbles, or other items blocking the water from flowing freely? Are the front and back slopes eroded?
Source: modified from Rantanen et al. 2005.
TABLE 9-4 Evaluation of Drainage System Condition
FIGURE 9-1 Entrapped water in road structure needing specific remediation technique (Lehtonen et al. 2005).
replaced. Installation of special protective features, such as geosynthetics, steel nets, insulation, or erosion control products (natural or geosynthetics), can be considered as alternative maintenance techniques for local failures (see Sec. 8-3).
9-3
Maintenance of Gravel Roads Maintenance of gravel roads differs from that of paved roads in the maintenance techniques and their frequency. Because the surface of gravel roads is not bound, it needs regular maintenance and rehabilitation. In cold regions, the maintenance is also affected by frost action, and is therefore time sensitive. In spring time, distress due to frost action,
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Chapter Nine Condition Concern (4 = significant effect, 0 = no effect) Cross Section
Bound Surface
Dust Control
Optimum Application Season
Major reshaping and mixing in dust control agent
4
1
0
Spring
0
2
4
Routine blading
3
1
0
Summer
Leveling
1
0
0
Spring and fall
Surface application of additional dust control agent
0
2
4
Summer
Pothole filling
3
0
0
As needed
Technique
Source: modified from Havu et al. 1995.
TABLE 9-5 Gravel Road Maintenance Techniques in Cold Regions
such as rutting, gravel loss, and spreading of the embankment, needs to be treated. In summer time, the road shape and surface condition is maintained using routine blading, and in fall time, the surface is leveled for easy winter operation (Havu et al. 1995). Table 9-5 summarizes maintenance techniques, their effect on road condition concerns, and gives an optimum treatment season that is applicable for cold regions. Major reshaping is needed whenever the road needs reshaping of its cross section and when new virgin materials are added. In springtime, major reshaping is conducted right after the thaw-weakening period. The goals or major reshaping are to return the cross section back to the designed shape, remove potholes, washer boards or other irregularities, and construct a wearing surface with homogenous materials and uniform thickness. This is accomplished by cutting the existing surface preferably using a blade with carbide bits at the base level of the holes, windrowing the loosened material at the road edges, mixing the material by replacing the windrow at the centerline, and finally spreading the windrowed material on the both lanes. The base is cut at the desired angle, typically 4 to 5 percent, to establish a proper crown. In case that no additional materials are added, the edge windrows can be directly spread on the lanes (Havu et al. 1995). Dust control agents (dust palliatives) are spread on the top of the centerline windrow. After the windrow is spread out and leveled, watering is recommended to bind the salt with the surface course. Calcium chloride (CaCl2) is the most common dust control agent used in cold regions. Its benefits are based on its abilities to absorb water from the air, to increase surface tension of water film between particles, to slow evaporation, and further tighten the compacted soil as drying progresses. Its use has a few limitations though, including decreasing performance with increasing duration of a dry spell, corrosive properties, leaching out by rainwater, and potential for surface becoming slippery when wet (Bolander and Yamada 1999). Therefore, the application rate should be optimized to adequately address the dust problem while minimizing the adverse effects. The amount of dust palliative depends on several factors listed in Table 9-6 and
Maintenance and Rehabilitation
Factor
Change in Factor
Change in the Amount of Dust Palliative
Traffic volume
ADT increases
Increases
Land use
Road passes through residential area or crops
Increases
Subgrade
Capillarity increases
Decreases
Shading
Duration that the road section is shaded (e.g., by forest) increases
Decreases
Surface material
Fines content increases
Decreases
Source: modified from Havu et al. 1995.
TABLE 9-6 Factors Affecting Amount of Dust Controlling Agent
needs to be customized for each road section with different characteristics. Havu et al. (1995) recommend 150 g/m2 of CaCl2 (77 percent purity) to be mixed into the road surface during the spring reshaping for ADTs lower than 100 vehicles per day and increasing the amount up to 330 g/m2 for ADTs higher than 500 vehicles per day. These quantities can be further modified using guidelines given in Table 9-6. After the mixing and leveling is completed, it is important to compact the treated surface. This can be accomplished by grader or truck-mounted pneumatic rollers ideally at the optimum moisture content of the surface material. Routine blading is needed periodically to remove corrugation and small holes. It can be accomplished with a self-propelled grader or a drag-type device. The minimum cut depth should be 1.5 times the maximum aggregate size or the base level of the irregularities. The maximum cut depth needs to be kept shallower than the depth of the surface course to avoid contamination with undesirable aggregate sizes. Leveling with a drag-type device or a bottom blade installed under a construction truck is possible during spring thaw or late fall when the surface course is soft due to moisture. The treated depth varies between 5 and 10 mm. Bottom blades are typically used for road sections with areas of limited surface roughness. If the road is in good condition except for isolated potholes or isolated clusters of potholes, hole filling is more efficient maintenance operation than grading. The fill material should be moist, include dust palliative and conform to the grading of the surface layer. Surface application of additional dust control agent may be required during the summer months. The application rates are typically smaller than the amount mixed in during the spring reshaping. The application rate should be sufficient to eliminate need for further salting operations for the summer. The treatment should be conducted immediately after rain to avoid unnecessary watering of the road. Surface salting is not recommended for roads with high traffic. Instead, the salt should be mixed into the surface layer to avoid it becoming airborne. Regraveling becomes necessary periodically as the surface material is lost as dust, flung to the ditches or mixed with the base course. The material from the fore slopes of the ditches can be reused as a surface material thereby reshaping the slope while reducing need for virgin materials. Gradation of the reclaimed slope material is analyzed and
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Chapter Nine the need for additional aggregate fractions is determined to obtain a suitable surface layer gradation (see Sec. 7-6). The operation is conducted using preferably two graders. The first grader reclaims material from the fore slope and windrows it at the edge of the road. The second grader picks up the edge windrow while simultaneously cutting the existing surface layer and builds another windrow at the centerline of the road. The first grader then makes another pass and reclaims more material from the fore slope, ditch, and back slope, and leaves it again at the road edge. The second grader makes also another pass and finishes cutting the existing surface layer and places the loosened materials at the centerline. Rocks, peat, and other impurities are picked up with a screen attachment. Materials are then mixed by grading them over to the road edge and back to the centerline. Potential virgin materials are added at this phase on the top of the windrow. The windrow is then spread over the lane, leveled and compacted. If fines are needed, they should be added to the virgin aggregate when it is crushed. For example, water with dispersed clay can be sprayed over the crushed aggregate right at the crusher. Other performance problems on gravel roads requiring major maintenance operations result from poor drainage, weak subgrade, or poor bearing capacity due to contaminated or otherwise inadequate structural layers. In order to select a proper maintenance technique, the current situation needs to be analyzed using visual observations, bearing capacity measurements, or ground penetrating radar scans in a similar manner than for paved roads. Again, it is essential to differentiate between causes and consequences. For example, is the poor bearing capacity due to nonexistent ditches, or are the ditches nonexistent due to spreading of embankment? In the first case, drainage is the cause of the problem and should be fixed, whereas in the latter case, drainage improvements would not necessarily solve the problem. Instead, causes for embankment spreading need to be further analyzed, and the problem solved either by improving the drainage or adding more structural layers. In some cases, stabilization, reinforcement, or insulation can be used to address the problem. Whatever the selected maintenance operation is, the widened embankment’s cross section needs first to be brought back to its original shape. Maintenance of a wide gravel road is difficult and traffic safety is reduced due to poor bearing capacity at the road edges. Increasing bearing capacity of a gravel road by adding crushed aggregate remains the most typical major maintenance procedure. Separation geotextile can be installed between the existing and new material in order to avoid further contamination. Design of the required layer thickness is covered in Chap. 8. Drainage maintenance is described in detail in Sec. 9-2-3.
9-4
Rehabilitation The objective of rehabilitation is to bring the pavement’s level of service back to its designed level to and increase the pavement’s service life significantly (see Fig. 6-6). An appropriate timing for a road to be rehabilitated is determined by pavement management techniques described in Sec. 6-4. The optimum timing derives from the chosen trigger values for the pavement conditions, rehabilitation needs of other roads in the road network, and available funding. When a certain road rehabilitation project lands on the engineer’s desk, its planning starts with identifying the pavement distress modes and their causes (see Chap. 3) in a similar manner than described for the major maintenance techniques. Test methods used in collecting data for the analysis are described in Sec. 4-2. When a pavement needs rehabilitation, the problems typically reside in pavement subsurface
Maintenance and Rehabilitation and include distresses that an overlay or localized maintenance operations cannot correct. The symptoms that are observed typically include poor bearing capacity, subgrade rutting, transverse and longitudinal cracking. The goal of the selection of appropriate rehabilitation procedures is to eliminate the causes of these problems, which is the only way to significantly extend the design life of the road. If alternative techniques exist, the final selection should be based on the life time engineering principles described in Chap. 6. Table 9-7 describes a selection of rehabilitation technique to address poor bearing capacity. Rehabilitation Technique
Insufficient structural layer thickness (materials meet specifications)
•
•
Sandy bearing course
•
•
Other techniques*
Replacing open ditches with drains
Drainage Improvements
Extending pavement width
Building shallower side slopes
Stabilization agent
Cross Section Improvements
• •
Bearing course with excess fines Impermeable layer close to surface otherwise acceptable materials
•
•
•
•
•
Steep side slopes; poor bearing capacity on shoulders High water level in ditches
Virgin aggregates
No addition
Reinforcement
Bound
Presumed Cause for Poor Bearing Capacity
Unbound
Addition of Structural Layers
In-Place Recycling— Cold Milling and Mixing With:
• •
•
•
•
*
See Sec. 9-2-3. With other modifications to improve the drainage capacity. Source: adapted from Lehtonen et al. 2005.
†
TABLE 9-7 Recommended Rehabilitation Techniques to Address Poor Bearing Capacity
• •†
•
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360
Chapter Nine Addition of structural layers Can be accomplished by placing unbound materials, bound materials, reinforcements, or their combination on the existing road surface. The existing bound layer needs to be removed if the new layer thickness is less than 400 mm, and always at the low points of the vertical alignment to prevent water from pooling in the pavement structure. Use of bound materials, that is, an overlay, is recommended if the existing bound layer is at least 150 mm thick or if the pavement deformations are small. Reinforcements Such as steel nets, composite nets, and some stiff geogrids, are used primarily to prevent lateral spreading of embankments due to the frost action and the subsequent longitudinal cracking. Contribution of the reinforcement to the bearing capacity is typically not taken into consideration for permanent roads, as the tensile strength of the nets is not mobilized in the vertical direction unless the ruts are substantial. However, steel and composite nets are found to reduce heavy-load-related rutting rate for narrow roads with steep side slopes due to the lateral support at the pavement edges (Lehtonen et al. 2005). In-place recycling Includes milling of the existing road surface, mixing the loosened material with possible size reduction and compaction. The milled material may consist of bound layers, unbound layers, or their combination. The goal of in-place recycling is to remove the damaged pavement and create a level surface for paving. Virgin aggregates can be added during mixing in order to fix the gradation of the existing material. Stabilization That is, mixing in a stabilization agent such as asphalt emulsion during in-place recycling, is recommended if the elevation of the road cannot be raised either due to the existing steep side slopes or other constraints. Virgin aggregates can be mixed in during the stabilization procedure in case the gradation of the milled material is not suitable for stabilization, the required bearing capacity cannot be achieved with addition of a stabilization agent alone, or the bearing capacity of the existing road is less than 90 to 100 MN/m2 during the stabilization operation (Lehtonen et al. 2005). Chapter 8 explains the procedure for design of required layer thicknesses and selection of appropriate reinforcements. Several acceptable alternatives can be designed and the selection of the rehabilitation technique is based on the life time engineering principles described in Chap. 6. Improvement of the cross section Include building shallower side slopes, and in some special cases, extending the pavement width. Especially for narrow roads, settlements at the road shoulders due to the poor lateral support of steep side slopes accelerate the pavement deterioration. The deterioration rate can be decreased by building shallower side slopes. The need for road widening comes typically from traffic-related issues rather than structural needs. However, if the widening is not done properly, problems will arise in the form of differential frost heave. To decrease the risk for differential frost heave, the new structure should be similar to the old structure, that is, similar layer thicknesses and materials. A new bearing course is built on the top of the entire road width (see Fig. 9-2a). A transition zone can be constructed to mitigate differential movements as illustrated in Fig. 9-2b. In cases where it seems that differential frost heave is bound to occur and also for roads that are not engineered or built to any standards, the old pavement should be spread out and used as a subbase for the entire widened road width. In localized cases, steel nets can be used to mitigate the differential frost heave (Lehtonen et al. 2005). If the drainage is diagnosed to be the cause for the poor bearing capacity, it needs to be improved using the method described in Sec. 9-2-3. Use of edge drains instead of open ditches can be used to build shallower side slopes and to benefit from the aforementioned reduced pavement deterioration rate.
Maintenance and Rehabilitation
FIGURE 9-2 Widening techniques for existing road embankment (modified from Lehtonen et al. 2005).
Emergency Repairs In case of low volume roads it is very likely that poor bearing capacity is caused by spring thaw weakening. This problem is magnified by the fact that many low-volume roads in cold regions are not engineered or constructed by using any standards. The structural layers of the pavement may be insufficient, which results in excessive vertical loads applied to the subgrade. These excessive loads combined with a pumping effect caused by heavy vehicles during the thaw-weakening period enable infiltration of fines in the subgrade soil into the pavement structural layers. The structural layers become gradually contaminated and weak resulting in ruts and other roughness on the road surface. In many cases, the preferable alternative for road sections with contaminated structural layers is reconstruction. However, if insufficient funding does not allow reconstruction, building additional structural layers should be considered. Further contamination being still a possibility, a sacrificial aggregate layer or a separation geotextile should be placed below the new structural layer. An emergency repair may be required to improve the mobility and driving comfort of a road before it can be rehabilitated. In this case, in-place recycling of the road surface and base course could be implemented. Asphalt emulsion and virgin aggregates are mixed in depending on the desired level of service and service life. Eliminating reflective cracking Causes behind longitudinal cracking are often frost action related. Remediation techniques include eliminating one of the following: freezing temperatures, frost-susceptible soil, or water source. Eliminating freezing temperatures can be achieved by increasing the thickness of non-frost-susceptible granular material or by installing an insulation layer into the pavement structure. Water source can be eliminated by lowering the water table by drainage improvements. Frost-susceptible soil can be treated chemically or replaced if it appears in limited quantities. Another possible approach to cope with longitudinal cracking is using reinforcements, such as steel nets or geogrids. Transverse cracks caused by thermal cracking may initiate in the bound surface layer or the underlying layers, including the subgrade as described in Chap. 3. To eliminate thermal cracking from initiating in the bound surface layer, it needs to be considered in the mix design phase (see Chap. 7). To eliminate thermal cracking from initiating in the sublayers, water needs to be kept out of the structural layers by immediate crack and pothole repairs, and cohesive subgrade needs to be kept warm by adequate aggregate or other insulation layers.
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9-5
Load Restrictions As described in Chap. 3, many cold region roads have significantly lower bearing capacity during spring thaw period than during the rest of the year. A few heavy vehicles during this period can cause similar road damage to the road structures as thousands of load applications at any other time. This is especially true for unengineered pavement structures that are typically not part of the main road networks. Rehabilitation of these road sections may not be feasible in a timely manner due to lack of funds and rehabilitation needs of the other roads in the network. Therefore, the road authorities try to protect the structural integrity of the roads subjected to spring damage by imposing spring load restrictions (SLR). Restrictions that allow axle loads of 50 to 90 percent of the maximum allowable axle load or a total shut down are applied for the critical periods (see Fig. 9-3). The restrictions are typically enforced using permanent or portable weigh stations. The roads with SLR are often low volume roads. However, they still serve an important role in providing access to rural communities and natural resources. The load restrictions cause financial losses for industry and public that depend on timely transport of essential commodities and equipment. Road agencies collect and analyze data to determine the times for posting and removing the load restrictions as promptly as possible in order to protect the road structures while at the same time causing as little disruption to the road transportation system as possible. The data collected and its analysis vary widely between the road agencies and can be divided to direct and indirect methods. The data collection devices for direct measurements include frost tubes, deflectometers and instrumented frost stations. The indirect methods include the use of historical databases, weather forecasts, prediction models using the aforementioned data and/or expert judgment (C-SHRP 2000).
FIGURE 9-3
Load restriction on a local road in Nikiski, Alaska.
Maintenance and Rehabilitation The frost tubes are thin transparent tubes containing liquid used to measure the depth of frost and thaw in the pavement structure. Deflection testing is typically conducted by a falling weight deflectometer (FWD). Back calculated pavement modulus of the pavement structure is then used to evaluate the timing for imposing the SLR. The instrumented frost stations measure the road structure’s dielectric value, electrical conductivity and temperature with depth. Models to predict the weakening period are developed using previous physical observations and climate data. This historical data together with weather forecasting is then used in determination of the dates to impose and lift the SLR. These prediction methods often utilize the use of freezing and thawing indices that yield the accumulated degree days below or above a reference temperature. For example, State of Minnesota uses a modified thawing index, TIm, given in Eq. (9-1) that changes over time: TIm = ∑ (ADT − RT)
(9-1)
where ADT is average daily temperature and RT is the reference temperature that decreases linearly 0.56°C per week from −1.67°C on February 1 to −4.44°C on March 15. The start of the load restriction period is determined for each zone using measured and forecasted daily temperatures for several cities within the zone. The load restrictions will be placed when the TIm for a zone exceeds 14°C-days based on the 3-day weather forecast. The use of the 3-day advance forecast temperatures provides 3 days of notice to the public (MnDOT 2004). There are about as many policies for SLR as there are jurisdictions. Isotalo (1993) and a report by C-SHRP (2000) give overviews of different policies around the world’s cold regions. However, the policies are in flux and change frequently. Table 9-8 lists updated policies for a few selected regions around the northern hemisphere. Figure 9-4 shows a flowchart for annual procedure used by the Finnish Road Authority, Finnra, in selection of road sections that will have imposed SLR each year. First, each road section is classified based on the risk for seasonal load restrictions (SLR classification). This classification is based on the road section’s historical tendency for thaw weakening and the resulting damage. The SLR classification is verified each fall by taking into account the classification upgrades due to past year’s repairs and rehabilitation operations. Each November, the Finnra publishes maps showing the road sections that will likely have load restrictions the next spring. In each February, a forecast for the severity of the spring thaw weakening will be evaluated using the following factors: percentage of road section length with historical thaw weakening, duration of the frost formation (days to increase freezing index from 2500 to 6000°C·h), and the deviation of the ground water table from the median ground water table at the moment when the freezing index is 2500°C·h. Using the SLR classification and the forecast for the thaw weakening, Finnra creates preliminary lists of road sections with SLR and forwards them to the regional road authorities. The posting and lifting of the load restrictions is conducted by the regional authorities, who consider the local conditions and recommendations from the maintenance contractors. The local conditions that may cause changes to the preliminary list of the SLR road sections include the depth of the thaw penetration, traffic demands, and current weather. The load restriction for a road section is imposed when the thaw penetration is approximately 150 mm. The traffic demand affects the SLR list in the following way: Road sections may be dropped from the list if they carry only essential traffic (such as dairy and food transports, school buses, and commuting traffic). On the other
363
364 Jurisdiction
Load Restriction Imposed
Load Restriction Lifted
Testing
State of Alaskaa
Expert opinion; 85% of legal axle load generally when thaw penetration reaches the bottom of the bound pavement layer; based on measured pavement temperatures, visual observations and 3-day weather forecast. SLRs may be reduced to 75% or 50% based on visual observations and expert opinion
Expert opinion. Generally when thaw penetration reaches 1.2 m
Thermistors, visual observations
Minnesotab
When the TIm [Eq. (9-1)] for a zone exceeds 14°C-days based on the 3-day weather forecast
SLR is removed after 8 weeks, or 2 weeks after complete thawing (min. duration 4 weeks)
Air temperature
Yukonc
100% restrictions (i.e., no overload permits) are imposed when surface thawing is present and roadway surfaces show signs of distress On nonstructural highways, 75% of legal axle load are recommended when thaw is 0.3 m the below road surface
100% SLR is removed when thaw penetration is 1.0 m
Thermistors, visual observations
Quebecd
Axle load limits and gross vehicle weights are given for normal period and thaw period. SLR starts when thaw penetration is 300 mm
Five weeks after thaw penetration reaches 900 mm
Swedene
SLRs are imposed when 1% of the road length (3% for oil-gravel and BST paved roads) shows marks of rutting, cracking, or upward water flow through the pavement cracks. Gravel roads: SLRs are imposed when more than 5% of the road length requires grading due to rutting
Finlandf
Maximum vehicle weight is reduced to 12 tons when thaw penetration is 150 mm (from 36 to 60 t depending on the number of axlesg)
All SLRs are removed when the thaw penetration is 1.4 m Frost probes
Visual observations
Thaw penetration is 0.80 to 1.00 m and the road surface has dried up to form a good bearing surface.
Frost monitoring stations, visual observations, crowbar penetration
Norwayh
No SLR on main road network since 1995. Secondary roads: Expert opinion based on temperatures and visual observations
a
After 2 to 3 weeks based on expert opinion
Visual observations
Personal communication with Clark Milne and Dan Breeden of Alaska Department of Transportation and Public Facilities, February 23, 2007. http://www.dot.state.mn.us/tecsup/tmemo/active/tm04/20mat03.pdf (2/15/2006). c http://www.gov.yk.ca/transportation/highways/weightrestrictions.html (2/5/2007). d http://www.mtq.gouv.qc.ca/en/camionnage/charges/degel.asp (2/5/2007). e New guidelines to handle seasonal load restriction—Nya riktlinjer för handläggning av tjälrestriktioner.pdf Swedish Road Administration Decision on Augut 30, 2005. f http://kelirikko.tiehallinto.fi/kelirikko/index.jsp (2/5/2007). g http://www.tiehallinto.fi/pls/wwwedit/docs/7723.PDF, (2/5/2007). h Personal communication with Ivar Horvli of NTNU, Norway, February 23, 2007. b
TABLE 9-8 Implementation of Seasonal Load Restrictions for Selected Jurisdictions
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Chapter Nine
FIGURE 9-4
Flowchart for SLR procedure used by Finnra (adapted from Finnra 2007).
hand, road sections may be added to the list, if unexpected heavy timber or earth moving traffic emerges. The current weather affects the list in the similar way: The SLR list may become shorter if the current spring weather is sunny and windy, or it may become longer in the case of wet spring weather. The maintenance contractor monitors the depth of the thaw penetration and gives weekly (or daily during the critical season) recommendations for imposing and lifting the load restrictions to the regional road authority. The thaw penetration is evaluated using either daily air temperatures or using permanent frost monitoring stations. The 150-mm thaw penetration can also be accurately observed by striking a crowbar into the softened road surface. The SRL is lifted when the thaw front has penetrated into the depth of 0.80 to 1.00 m and when the road surface has dried up to form a good bearing surface. The SLR can also be lifted temporarily, if a freezing period has stiffened the road structure. Temporary lifts are used especially in cases of early imposing dates and long SLR periods. Another way to address the low bearing capacity of roads in general, and also during thaw-weakening period, is the use of central tire inflation (CTI) system. The CTI is a mechanical system for heavy vehicles that allows the operator to adjust the inflation pressure of tires while the vehicle is in motion (Stuart et al. 1987). The use of this system allows tire pressure to be varied so that low-strength, low-speed roads could be driven with low tire pressures and thereby reducing the stress and subsequent damage due to cumulative strain on the pavement surface (see Fig. 9-5). The driver can inflate the tires at a higher pressure, when the vehicle reaches a higherstrength, higher-speed road. The CTIs are popular especially in the transportation of logs from the forest over unpaved low-volume roads. Several field trials have been
Maintenance and Rehabilitation
FIGURE 9-5 Effects of reduced tire pressure [modified from Kestler et al. 2005 (Figure 1b, p. 127) with permission of TRB].
conducted to study whether the SLR period can be shortened for vehicles with CTI. Based on the success of these trials, the British Columbia Ministry of Transportation introduced a program in 2004 that exempts trucks operating with CTI from SLR on approved roads (Bradley 2006). Kestler et al. (2005) report that based on the mechanistic pavement design and evaluation model for seasonal frost areas developed by the U.S. Army Corps of Engineers, reduction of tire pressures reduces the cumulative pavement damage significantly. Their findings indicate that trucking could resume approximately 2 to 4 weeks before the anticipated end of the standard load restriction period (for an assumed 2-month SLR period) using reduced tire pressures. At the start of thaw, the cumulative damage curve is potentially steep, and therefore the SLRs should be enforced without exemptions for vehicles with CTI. Some agencies use winter load increases in order to encourage a shift in the spring loads into the winter, thus reducing the overall anticipated damage to pavements. For example, the state of Minnesota allows 10 percent increase of the maximum axle weight when a cumulative freezing index is greater than 155°C-days based on the 3-day weather forecast (MnDOT 2004).
Review Questions 9-1. Briefly describe challenges in daily winter maintenance. 9-2. What are the benefits of pavement preservation? 9-3. Describe situations where overlay should not be used as a maintenance operation. 9-4. Should gravel roads be graded always using the same procedure? If not, how should the grading be adjusted?
9-5. How does salting of gravel roads work in dust control? 9-6. In what kind of conditions are steel nets effective as a reinforcement method? 9-7. Describe possible problems caused by widening of road embankments. 9-8. What kind of seasonal load restrictions are used in your area? 9-9. Describe difficulties in determining when to post and lift seasonal load restrictions.
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References Asphalt Institute (1997). Asphalt in Pavement Maintenance, MS-16, 3d ed. Asphalt Institute, Lexington, Ky. Asphalt Institute, A Basic Asphalt Emulsion Manual, MS-19, 3d ed. Asphalt Institute, Lexington, Ky. Bolander, P., and Yamada, A. (1999). “Dust Palliative Selection and Application Guide,” Project Report, 9977-1207-SDTDC, U.S. Department of Agriculture, Forest Service, San Dimas Technology and Development Center San Dimas, Calif. Bradley, A. (2006). “Hauling with Full Axle Weights and Reduced Tire Pressures on Weight-Restricted Roads in British Columbia,” Journal of the Transportation Research Board, no. 1967. C-SHRP (2000). “Seasonal Load Restrictions in Canada and Around the World,” C-SHRP Technical Brief, No. 21, http://www.cshrp.org/products/brief-21.pdf (January 5, 2007). Finnra (1993). “Winter Road Maintenance Methods in Finland,” TIEL 2230006E, Finnish Road Administration, Helsinki, Finland. Finnra (1995). “Winter Maintenance Policy in Finland 1996,” TIEL 2230014E, Finnish Road Administration, Helsinki, Finland. Finnra (2007). “Kelirikkoteiden liikenteen rajoittaminen (Traffic restrictions on roads during spring thaw weakening),” TIEH 2200047-v-06. Finnish Road Administration, Helsinki, Finland (in Finnish). Havu, K., Hörkkö, R., Kosonen, O., Olkkonen, N., Pernu, H., Pöyhönen, A., and Vuorinen, M. (1995). “Sorateiden hoito ja kunnostus (Maintenance and rehabilitation of gravel roads),” TIEL 2230013, Finnish Road Administration, Helsinki, Finland (in Finnish). Isotalo, J. (1993). “Seasonal Truck-Load Restrictions and Road Maintenance in Countries with Cold Climate,” Infrastructure Notes, Transportation, Water and Urban Development Department, the World Bank, March 1993, Transport No. Rd-14. Kestler, M., Berg, R., and Haddock, J. (2005). “Can Spring Load Restrictions on LowVolume Roads Be Shortened without Increasing Road Damage?” Journal of the Transportation Research Board, No. 1913, pp. 126–136. Lehtonen, K., Salo, P., Kallionpää, T., Rantanen, T., and Junnila, A. (2005). “Rakenteen parantamisen suunnittelu; suunnitteluvaiheen ohjaus (Planning of road rehabilitation projects),” TIEH 2100035-05, Finnish Road Administration, Helsinki, Finland (in Finnish). McHattie, R. (2003). “Asphalt Surface Treatment Guide,” Alaska Department of Transportation and Public Facilities, FHWA-AK-RD-01-03. McHattie, R. (2005). “Materials Application Rates for Dense-Graded Asphalt Surface Treatment Using High Float Emulsion,” Alaska Department of Transportation and Public Facilities, FHWA-AK-RD-05-03. MnDOT. (2004). “Guidelines for Seasonal Load Limit Starting and Ending Dates,” http:// www.dot.state.mn.us/tecsup/tmemo/active/tm04/20mat03.pdf (January 3, 2007). Rantanen, T., Turunen, J., and Nousiainen, A. (2005). “Vähäliikenteisten teiden kuivatus, ominaispiirteet ja kunnostaminen (Drainage of low-volume roads, typical features and maintenance),” TIEH 3200979, Finnish Road Administration, Helsinki, Finland (in Finnish). Stuart III, E., Gililland, E., and Della-Moretta, L. (1987). “The Use of Central Tire Inflation Systems on Low-Volume Roads.” Transportation Research Record: Journal of the Transportation Research Board, No. 1106, Transportation Research Board of the National Academies, Washington, D.C.
CHAPTER
10
Pavements on Permafrost
P
avement structures in permafrost areas require additional design and management considerations compared to pavements in seasonal frost areas. These considerations result from the fact that construction of pavements unavoidably affects the thermal regime of frozen soils leading to possibility for thermal degradation of the permafrost. If the underlying permafrost is thaw sensitive, consequences of thermal degradation include pavement settlement, pavement cracking, formation of thermokarsts, and slope instability in cuts. Creep of ice-rich permafrost is increased due to warming ground temperatures, which leads to increased creep settlement and embankment instability. This chapter describes the causes and manifestation of thermal degradation, management considerations, and protection techniques for pavements on permafrost.
10-1
Causes of Instability and Problem Manifestation The temperature regime of permafrost depends on the same factors as those explained for pavements in Sec. 2-1. Under stable climate and surface conditions, the temperature stays within the trumpet curves (see Fig. 2-2). The depth of the active layer and the average temperature of the permafrost stay constant. If the mean annual surface temperature (MAST) gets warmer either due to warming climate or changing surface conditions, the trumpet curve shifts toward warmer temperatures. As a consequence, the depth of the active layer and the permafrost temperature increase. Thus, the permafrost degrades. Under sufficient warming trend, permafrost may thaw entirely. Out of the factors listed in Fig. 2-1, the most significant factors affecting the thermal regime of the newly constructed pavement structure are the changes in absorbed solar radiation and latent heat of fusion. The pavement surface may be darker than the original soil surface and the reduction in albedo increases the heat intake. Removing vegetation eliminates the shading the vegetation provided, which increases solar radiation of the ground or road surface. The aggregate in the pavement structure has low fine content to prevent frost action, and therefore retains less water than often mossy and peaty permafrost surface layer. Reduction in water content results in low latent heat of fusion in the pavement structure, which allows fast penetration of the thaw front through the system. To illustrate the effect of changing surface conditions on permafrost degradation, Linell (1973) observed three 61-m2 test sections near Fairbanks, Alaska. One section remained in natural condition, another had trees removed, and the last section had all surface vegetation removed and the soil was stripped to a depth of about 0.4 m (test summarized by Andersland and Ladanyi 2004). The permafrost degradation during 26-year period is shown in Fig. 10-1. Removal of all vegetation led to summer thawing
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FIGURE 10-1 Permafrost degradation under different surface conditions (reproduced from Linell 1973, with permission of National Academies Press).
of permafrost from 1 to 6.7 m in 26 years. Similar effect would occur from construction of a pavement structure, if measures protecting permafrost are not taken. The main principle for retaining the integrity of the pavement structures on thawsensitive permafrost is to keep the underlying permafrost frozen. The conservative approach to embankment design over permafrost is to increase the thickness of the subbase layer if the thaw penetration exceeds the total thickness of the pavement structure. While this approach is straight forward and technically sound, problems arise at the embankment edges, where the embankment thickness gradually reduces and where snow tends to accumulate during winter (see Fig. 10-2). Exposed mineral surface and thin granular protection at toe slope facilitate thaw penetration during summer. In addition, accumulation of thick snow cover at that location during winter prevents heat loss and permafrost cooling during that period. As a result of those two effects, the thaw front penetrates into the “warm” thaw-sensitive permafrost at the slope toes, which causes thaw settlement under the embankment shoulder and subsequent embankment spreading and longitudinal cracking (see Fig. 10-3). Precipitation water is now able to penetrate through the longitudinal cracks, and water tends to pond along
FIGURE 10-2
Problems in keeping permafrost from thawing.
Pavements on Permafrost
FIGURE 10-3 Longitudinal cracking in the Glenn Highway, Alaska.
the embankment toe which brings more heat into the system. These effects may not be uniform throughout the pavement length, which causes differential thaw settlement. This in turn causes uneven riding surface and may cause water accumulation in the drainage systems. Water starts to accumulate in the ditches and culverts bringing extra heat to the pavement structure. This vicious cycle is presented in a flowchart form in Fig. 10-4.
Build pavement to thickness that keeps thaw front within thaw stable materials. Time passes Thaw penetrates thawsensitive materials at slope toes.
Water accumulation
Snow accumulation stores heat in winter time.
Climate warming
Thawing
Thaw settlement, embankment spreading and longitudinal cracking
Increasingly rougher driving surface until thermal equilibrium is achieved.
FIGURE 10-4 Flowchart for effects of permafrost degradation under road embankment.
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FIGURE 10-5 Thermokarst underneath the Dalton Highway, Alaska.
Thermokarsts form when massive ground ice thaws. Figure 10-5 illustrates the astonishing end result of the formation of a thermokarst underneath a section of the Dalton Highway in Alaska. In the case illustrated, intensive maintenance has kept the surface of the road level. It is, however, obvious that the drainage system is no longer in operating condition. Massive ice may go unnoticed by a traditional field investigation of boreholes even when drilled at frequent distances. Thawing massive ice under a road embankment causes tremendous hardship to the entity in charge of road maintenance. Slope failures due to degrading permafrost occur at the cut sections of the road alignment. The photo in Fig. 10-6 illustrates degradation of a slope, after the protecting ground cover is removed. Thawing of the slope leads to unstable slope conditions and slope failures. After the initial slope failure, a new frozen slope surface is exposed and the thawing progresses to another slope failure. The process repeats itself until the newly thawed surface is inclined at an angle that provides stable slope conditions. Figure 10-7 shows a healed slope. Ditches need to be cleaned at the end of the process to restore proper drainage of the pavement structure. Decreasing mechanical properties of frozen soils with warming temperature are described in detail by Andersland and Ladanyi (2004). As the frozen ground warms up, its strength decreases and creep rate increases. Practical considerations for pavement engineering relate to the decreased bearing capacity and possible slope failures of the permafrost subgrade. Creep settlement of ice-rich permafrost may also become significant. Considering the mechanical properties and the aforementioned serious problems with thawing permafrost, it becomes evident that the best practice in design of pavement structures on thaw-sensitive permafrost is to keep the thermal regime of the permafrost subgrade unchanged. The significance of this principle becomes more
Pavements on Permafrost
FIGURE 10-6
Actively thawing road cut (photo courtesy of David Esch).
important considering the warming climate the cold regions are currently experiencing. Pavements that appear presently stable will start to degrade if protection measures are not taken. After a short discussion on climate warming, the next sections will address design and management of infrastructure built over thaw-sensitive permafrost. Several permafrost protection techniques will also be described.
FIGURE 10-7
Healed road cut on the Glenn Highway, Alaska.
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10-2
Climate Warming and Its Effect on Permafrost Since the last glacial period (Wisconsinien in the North American continent, 10,000 years ago), a postglaciation warming is now prevailing as a part of the glaciation cycles. However, many experts have detected a drastic warming trend in the last three decades, which does not seem to follow the postglaciation trend. Is the recent climate warming a normal cycle in the postglacial period or is it a climatic warming caused by anthropogenic activities? The question now attracts much research attention. Whatever the cause, cold regions engineers need to be prepared for climate warming and its impacts on permafrost thermal regime. Climate warming can have an important influence on the existence of permafrost especially in the discontinuous permafrost zone. Air temperatures and snow cover especially influence the temperature of permafrost. If temperature remains constant and snow cover increases, heat loss during winter is reduced and as a consequence permafrost temperature increases. Burn (1998) concluded that increases in permafrost temperatures are not necessarily associated with the increase of mean annual air temperature (MAAT) but can also be attributed to increase in precipitation. According to Geological Survey of Canada (2006), general circulation models predict that if atmospheric concentrations of carbon dioxide are doubled due to anthropogenic sources, MAAT may rise by several degrees over most of the Arctic territory. In the discontinuous permafrost region, where the ground temperatures are within 1 to 2 degrees of melting temperature, permafrost will ultimately disappear as a result of ground thermal changes associated with climate warming. According to the Alaska Regional Assessment Group (1999), a warming of approximately 1.5 to 3°C is projected by 2030 with 5 to 10°C warming by 2100 in Alaska and Canada. Recent observations and simulations based on climate warming scenarios have led several authors to predict important changes to permafrost thickness and limits in the next decades (Smith and Burgess 1999; Yongjian 1998; Pavlov 1997, cited in An & Devyatkin, 1998; Sharkhuu 2003). According to Burn (1998), field evidence from observations in continuous (colder) permafrost indicates that the considerable persistence of permafrost following climate change is due to the huge latent heat contained in ground ice. Global warming impacts may increase the active layer thickness and the permafrost temperature, but the most rapid response will still be changes in surface conditions. Many experts throughout the world (Osterkamp 2003; Khrustalev 2000 and 2001, cited in Instanes 2003) have already found indication of climate changes such as increased active layer, warming permafrost, increased mass movement, thawing of ground ice, coastal erosion, and damages to infrastructure.
10-3
Management of Transportation Infrastructure Built over Thaw-Sensitive Permafrost Effective management of transportation infrastructure built over permafrost requires adapted tools and decision-making processes taking into consideration design, construction, maintenance, and rehabilitation of transportation facilities. As described in Chap. 6, a management strategy should include a set of well coordinated activities allowing for the identification of adequate design, construction and maintenance strategies for highway sections and networks. In the case of a highway planned or built in permafrost conditions, the management strategy involves the identification of thaw-sensitive areas, the
Pavements on Permafrost characterization of thaw sensitive soils in these areas, and the selection of an adequate design or rehabilitation strategy. The information presented in this section is complementary to the design and management principles described in Chaps. 4, 6, 8, and 9. The activities that should be included in a management process are summarized in the following sections.
10-3-1 Identification of Thaw-Sensitive Areas New Roads Road alignment and pavement design require detailed information on the extent of thaw-sensitive soils. The detection of these soils remains a major challenge in pavement engineering. Several site investigation techniques, such as photo-interpretation, geophysics, and boring/sounding need to be combined in order to identify sensitive areas. These techniques are described in Chap. 4.
Existing Roads The performance of existing roads provides valuable information on thaw-sensitive subgrade areas. Roughness, as measured using a profilometer or a ride meter (see Sec. 4-2-2) is an excellent indicator of the occurrence of differential thaw settlement in thaw-sensitive permafrost. If roughness measurements are not available, subjective roughness assessment and visual inventory of pavement surface distresses, including cracking (edge and pavement) and settlements, are likely to help assessing the extent of thaw-sensitive subgrades along a given road section or road network.
10-3-2
Characterization of Thaw-Sensitive Soils
New Roads The key information to obtain on soils immediately below the subgrade line of a new pavement is their thaw-consolidation properties as well as their bearing capacity during the consolidation process. The information on thaw settlement and the mechanical properties of soils during the thaw-consolidation process can be obtained from a thaw-consolidation test on an undisturbed soil sample as described in Chap. 4. Thaw settlement can alternatively be estimated using moisture content of samples recovered from the top part of permafrost. Thaw settlement assessment also requires an estimation of the thickness of permafrost subjected to thawing after construction using thermal modeling.
Existing Roads Road sections showing signs of instability should be monitored in order to assess the level of sensitivity of the subgrade soils. Characterization activities should include measurements of the rate of evolution of settlements (mm/year), roughness (IRI, m/ km·year) and cracking (longitudinal and edge, m/year). Complete assessment of the sensitivity of these road sections also requires information about the ice content of permafrost and the thickness of the ice-rich layer(s).
10-3-3
Design Considerations
Embankment and pavement design in permafrost conditions requires specific considerations with respect to structural adequacy of the pavement structure, long-term thermal stability, drainage, and slope protection in cut areas. These considerations are described in Sec. 10-4-1.
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10-3-4 Technical and Economical Assessment of Applicable Solutions In the case of the construction of a new road as well as of the rehabilitation of an existing road, the technical feasibility and the economical implications of implementing a protection technique should be assessed in comparison with the alternative strategy, which involves intensive maintenance of the road section exposed to thaw-sensitive subgrade soils. In extreme cases, relocation of the road alignment should also be considered as an alternative. This activity, thus, involves the identification of applicable solutions considering the specific context of the project. Some of these solutions are described and discussed in Section 10.4. They can be grouped in three main categories: • Regular and/or intensive maintenance • Application of one or more stabilization technique adapted to site context • Relocation of the road alignment if the above strategies do not allow for a safe and reliable operation of the infrastructure The technical feasibility and effectiveness of these solutions should first be evaluated on the basis of past experience. Information available within the agency or documented in the literature can help in assessing the likelihood of good performance of these solutions in a given context. In many instances, the performance of the techniques can also be evaluated on a theoretical basis using thermal and/or mechanical modeling. The applicable solutions should also be assessed on the basis of their economical implications over the life cycle of the road. The analysis should include parameters such as the costs of construction or treatment application and cost of related maintenance (as often as required during the analysis period, which should be longer than the design life of the facility). It should also include an evaluation of the benefits resulting with the application of the solution. Benefits can be expressed in terms of durability of the solution, as well as associated level of service and inconvenience to users. The concept of life-cycle cost analysis is summarized in Chap. 6. More detailed information on the subject can be found in TAC (1997) or Haas et al. (1994).
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Implementation of the Strategy
The last step of the management process is the development and implementation of a multiyear plan including design, construction, maintenance, and alignment improvement activities for a road section or a road network providing safe, comfortable, and reliable driving conditions for road users. The strategy, thus, involves the effective management of information on the condition and the evolution of unstable areas or road sections, a technical and economical evaluation of applicable solutions or strategies, and the implementation of a plan.
10-4
Embankment and Pavement Design over Permafrost Pavement and embankment design approaches used for seasonally frozen ground cannot be applied to permafrost conditions without considering a few basic principles. These principles as well as mitigation techniques adapted specifically for permafrost protection are described in this section.
Pavements on Permafrost
10-4-1
General Considerations
Embankment and pavement design for roads, airfields, and railways in permafrost conditions needs to be adapted in order to take into consideration permafrost distinctive features. Six basic rules are proposed to help design high-quality embankment in thawsensitive permafrost conditions. 1. Geometric design: When possible, avoid alignments requiring cuts in thawsensitive permafrost. Cuts in thaw-sensitive permafrost are likely to result in thermal degradation and eventually thawing of permafrost. As a result, higher maintenance costs and lower levels of service are to be expected. In cases of highly sensitive permafrost, rapid degradation can even lead to hazardous driving conditions. Cuts in thaw-sensitive soils should thus be avoided unless no other economical solution is available. In addition to minimizing the risk of thermal degradation, keeping embankments above natural ground surface also facilitates winter maintenance of the pavement surface. 2. Thermal design: Long-term stability of an embankment built on thaw-sensitive permafrost relies heavily on favorable ground thermal conditions. Long-term thermal stability of pavement systems is the prevailing consideration for embankment and structural design of pavements in permafrost conditions. Considering the high risk of disruption of the ground thermal regime resulting from the construction of the embankment and expected long-term changes in climate conditions, adequate thermal design is required to guarantee a strong and stable foundation for the embankment over the life of the pavement. As illustrated in Fig. 10-8, construction of an embankment with a gravel or hot mix asphalt (HMA) surface exposed year round to solar radiation and cold winter air will increase the amplitude of the temperature swing at the surface compared to the original soil surface. As a result, as indicated in Fig. 10-8a, thaw depth will increase without an adequate thickness of granular material to protect the permafrost. Thermal design of the embankment involves calculation of the required thickness of embankment material to make sure the permafrost table will stay above its original position during the entire life of the embankment (Fig. 10-8b). This can be done by calculating the depth of thaw penetration in the pavement structure using one of the methods described in Sec. 5-6. It can also be done using a
FIGURE 10-8 Effect of embankment construction on the ground thermal regime for (a) a thermally underdesigned embankment and (b) an embankment with adequate thermal design.
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Chapter Ten one-dimensional or bidimensional numerical thermal analysis using commercially available software. Proper considerations should be given to • Embankment geometry: If one-dimensional thermal analysis is used, the analysis should, as a minimum, take into consideration thermal regime at the center of the embankment, underneath the shoulder and at the toe of the slope. • Probability of unusually warm summers: The analysis should be based on the probability of an unusual warm year. The average warm year occurring every 10 years can be used for thermal analysis in the case of paved roads, airfields, or railways. The average warm year occurring every five years can be used for gravel roads. These can be obtained by averaging respectively the three highest and the six highest thawing indices over a period of 30 years. • Climate warming trend: It is important to consider that climatic data typically used for thermal analysis are based on historical records and should be corrected to take into consideration the climate warming trend over the design life of the structure. Several climate change models are available and should be used to estimate expected changes in relevant climatic parameters in a specific region. 3. Embankment design: Embankment geometry should be adapted to minimize risk of permafrost degradation along the embankment slope. As described in Sec. 10-1, permafrost degradation underneath an embankment is often initiated at the toe of the slope and eventually progresses toward the center of the embankment. Several modifications to conventional embankments have been used to reduce edge effects by reducing snow accumulation on the slope and/ or by providing a lateral buffer, which can retard the progression of permafrost degradation toward the center of the embankment. Some of the techniques used are illustrated in Fig. 10-9 and include gentle slopes (Fig. 10-9a), widened gravel shoulders (Fig. 10-9b), and berms (Fig. 10-9c). 4. Pavement design: Pavement structural design(s) need to take into consideration the mechanical properties of the active layer. Pavement design principles described in Chap. 8 can be applied to permafrost conditions considering the fact that the subgrade soil is frozen for approximately half of the year and is in thawing state for the rest of the time. Under these conditions, the performance analysis
FIGURE 10-9 Possible modifications to the embankment geometry to reduce the risk of permafrost degradation: (a) embankment with gentle slopes, (b) embankment with widened gravel shoulders, and (c) embankment with berms.
Pavements on Permafrost should take into consideration two “design seasons” including frozen material properties during winter and thawing material properties for the rest of the year. If the pavement structure is resting on a thick active layer, a third season characterized by improved mechanical properties in the top part of the active layer can be considered in the analysis. 5. Design of the drainage system: Avoid water accumulation in the vicinity of the embankment. Water flow concentrations in ditches along the embankment and in culverts across the embankment are important source of heat likely to induce thermal degradation along or underneath the embankment. Two important rules below should be followed to reduce the risk to thermal degradation of thaw-sensitive permafrost and thermal erosion of soils near or underneath the embankment: • As indicated in Fig. 10-10a, combining several surface water sources into one stream should be avoided to minimize the risk of localized thermal degradation due to heat concentration by running surface water. It is generally recommended to use culverts wherever water is likely to accumulate. • Ditches along the toe of embankment slopes should be avoided in thawsensitive permafrost areas. If required, they should be placed at a large distance (~10 m) from the embankment toe to minimize the risk of thermal degradation at that critical location. Figure 10-10b illustrates adequate drainage design. 6. Stability of newly exposed ice rich soils in cut slopes: As described in Sec. 10-1, newly exposed ice-rich permafrost in cut slopes are prone to long-term instability. Unstable cut slopes require intensive maintenance to maintain integrity of drainage systems and embankments. Several stabilization techniques have been proposed in the literature (amongst other, Berg and Smith 1976; Smith 1986; Macleod 2007). The design proposed in Fig. 10-11 has been successfully used for several years in northern Canada (Macleod 2007).
FIGURE 10-10 Possible modifications to the drainage configuration to reduce the risk of permafrost degradation: (a) direct drainage of surface water and (b) modified ditch configuration.
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FIGURE 10-11 Slope stabilization technique used in northern Canada.
10-4-2
Protection Techniques
As explained in Sec. 10-1, one of the important impacts caused by highway embankment on permafrost is the disruption of the thermal balance between the atmosphere and the permafrost. The mean annual temperature at the surface of a paved embankment is greater than 0°C in many areas of the discontinuous permafrost zone (Nidowicz and Shur 1998). As a result, the depth of seasonal thaw is greater than the depth of seasonal freezing, and residual thaw zones called taliks tend to form beneath the roadway and the runway under side slopes. This process accelerates if temperature increases over the years. Taliks will continue to grow and more settlement will take place. They form particularly at the toe of embankment slopes where permafrost is vulnerable due to the aforementioned low thickness of embankment material and thick snow accumulation during winter. Moreover, taliks are likely to draw surface and underground water underneath the embankment bringing, thus, additional heat and permafrost degradation at that location causing additional settlement. When seeking a solution for permafrost degradation problems, it is important to identify the contribution related to conductive versus convective heat transport processes (Rooney and Vinson 1996). Some other thermal effects have to be understood and taken into account to protect adequately embankments from permafrost thaw degradation, such as heat intake from underground and surface flowing water, net surface warming, side-slope warming, massive subsurface ice, and inadequate thermal resistance (Esch 1988). Since the 1960s, many methods have been proposed and tested to stabilize embankments built over degrading permafrost. These methods can be classified into three categories: • Methods based on limiting heat intake underneath the embankment • Methods based on heat extraction from the embankment • Methods based on the reinforcement of the embankment in order to resist permafrost degradation problems Several methods given in the literature are described in the following paragraphs. The applicability of each method discussed is based on different factors such as the permafrost context, cost, material, and equipment availability as well as safety issues. It is important to note that each one of these protection techniques must be properly designed in order to ensure their long-term thermal and mechanical stability.
10-4-3 Methods Based on Preventing Heat Intake Underneath the Embankment The main source of heat causing permafrost degradation is solar radiation acting on the embankment surface. The following methods are either attempting to intercept solar
Pavements on Permafrost radiation or to impede heat from flowing toward the underlying permafrost reducing thus the risk of degradation.
Increasing Embankment Thickness In cold permafrost area, the most common method used to protect embankments against thaw settlement is to use a gravel-fill layer that is thick enough to contain the active layer (Zarling et al. 1988). The required thickness is calculated using a thermal model as described in Chap. 5. When the mean annual surface temperature is around the freezing point, the use of fill is becoming uneconomical because the thickness of gravel required to keep the soil in frozen state is too large. It is often recommended to place non-frost-susceptible (NFS) material on top of the vegetation layer to retain some of the insulating effect and some of the latent heat retention of the near-surface organics (Crory 1988). During fill construction, special precautions should be taken in order to avoid vegetation and permafrost disturbances, particularly along embankment shoulders. The thickness of gravel embankments over permafrost normally ranges from 1.5 to 2.0 m.
Embankment Insulation The purpose of the embankment insulation is to prevent temperature increase, thus, inducing thawing of the upper permafrost. This can be achieved using an insulation layer in areas of continuous permafrost zone where mean annual temperature beneath embankment is below 0°C. Some authors (Nidowizc and Shur 1998) have suggested that embankment insulation might be useless in warm permafrost areas. The role of the insulation layer within a road embankment is to impede conductive heat flow toward the active layer. Permafrost temperatures during summer will thus remain relatively low and permafrost degradation is likely to be reduced. Embankment insulation has, however, the same effectiveness in impeding heat flow during winter. During this period, insulation does not allow evacuation of ground heat and permafrost to cool. If the mean annual surface temperature is and remains below 0°C, embankment insulation is likely to help preserving permafrost degradation, particularly if the insulation layer is installed during winter, when permafrost temperature is low. On the other hand, if the mean annual surface temperature is or evolves toward positive values, insulation alone is not efficient and some other methods have to be integrated in the design. Pavement insulation is illustrated in Fig. 10-12. Insulation is often used with a thick granular pad or in combination with a heat extraction technique to maximize its effectiveness.
FIGURE 10-12 Effect of an insulation layer in an embankment.
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Chapter Ten In embankments, two kinds of polystyrene can be used: expanded polystyrene and extruded polystyrene. Expanded polystyrene is a rigid type of insulation formed by expanding polystyrene beads in a mould. Expanded polystyrene can deteriorate due to water absorption and to freeze-thaw cycles. Extruded polystyrene is formed using an extrusion process. It has a good R-value per unit of thickness although it has a high cost per R-value. Based on field observations, Esch (1994, 1996) reported that extruded polystyrene typically absorbs less than 2 percent water by volume, while expanded polystyrene typically absorbs around 5 percent water. In spite of a slightly increased tendency toward water long-term absorption when compared to extruded polystyrene, expanded polystyrene has been proven suitable for soil burial under less severe moisture conditions. The use of polystyrene in roads is well documented in the literature. Insulation will be most effective if placed as close to the surface as possible, but the depth of cover above has to be thick enough to prevent crushing of the insulation from cyclic wheel loadings (Esch 1996). Moreover, the insulation should not be placed too close to the surface in order to prevent the formation of differential icing at the pavement surface. If insulation is used at the right place and is well designed, thaw settlement can be negligible and fill material be reduced (Johnston 1981). Other aspects of polystyrene insulation are discussed in Chap. 8 (pavement design).
Polystyrene Insulation
Polyurethane Insulation Polyurethane is a type of insulation that can be foamed in place. The biggest advantage to use polyurethane is the reduction of the shipping costs. Only the drums of reactants have to be shipped and mixing and foaming can be done on site (Esch 1996). Problems encountered with polyurethane are water absorption and compression with time of the insulation foamed in place. In some conditions, peat can be used as insulation. Embankments can be built directly on the peat layer where trees were removed with precaution to make sure surface vegetation will remain and will keep its isolating effects. The peat layer can be prethawed and consolidated prior construction (Esch 1996). In other cases, a peat layer of 1.2 to 1.5 m thick can be added after excavation on permafrost as a thermal underlayer where peat is available (McHattie and Esch 1983). Using peat has a huge advantage. Its frozen thermal conductivity may be twice as much as its thawed thermal conductivity (McHattie and Esch 1983 cited in Esch 1988). The presence of peat, thus, allows heat extraction during winter and impedes effectively heat flow into the ground during summer. The difference of peat thermal conductivity in frozen and unfrozen states and its high latent heat, reduce the thaw depth and cause the mean annual temperature of the underlying permafrost to be significantly lower than that of the overlying road surface (Esch 1988). As a result, the presence of peat has caused a net long-term heat removal and has prevented talik development under some embankments. Peat has reduced thaw settlement beneath pavement of 1 m and has lowered the permafrost temperature at a depth by 0.5°C compared to an uninsulated road (Reckard et al. 1988 cited in Esch 1996). However, peat has not eliminated thaw under the side slopes and the ditches. For maximum effects, peat has to be installed into the active layer. Because of the low-peat elastic modulus and in order to carry wheel loadings, minimum thickness of 1 m of granular-fill material has to be placed over the peat layer (Esch 1996).
Peat Insulation
Pavements on Permafrost
FIGURE 10-13 Schematic illustration of the principle of a reflective surface (right side) and its associated low level of heat absorption from solar radiation as compared to a regular paved surface (left side).
Reflective Surfaces The warming of the soil beneath paved roads can be attributed to several causes such as removal of vegetation, reduction in evaporation due to the presence of pavement, loss of shading, and reduction of albedo caused by darker surfaces (ADOTPF 1985). The effect of these factors is often integrated in the n-factor (described in Section 5.2). Figure 10-13 illustrates the principle of reflective surfaces, while Fig. 10-14 gives an example of the effectiveness of the protection technique. In Fig. 10-14, white paint has been applied to a test section at the U.S. Army Corps of Engineers experimental site (Farmer’s Loop Road) in Fairbanks. More than 30 years after application, the painted surface is still in good condition, while the unprotected reference section (top part of the photo) has settled and is now a swamp. To reduce the n-factor by increasing albedo, ADOTPF has applied white paint on roads to reduce thaw settlement problems. For example, on Peger Road in Fairbanks, paint application has reduced average pavement temperatures by 1°C (Esch 1996). In Svalbard Airport, in Norway (Molmann et al. 1998), the use of white-painted surface has reduced thaw depth by 0.4 m. ADOTPF, however, concludes that this solution would only slow permafrost degradation. Light-colored aggregates have also been used in the asphalt composition to reduce the n-factor. Many problems with white-painted surfaces were observed. These include: • High costs associated with the application of paint • Localized frost formation causing the roads and runways to be slippery • Slippery surface especially after rain in curves, intersections, and braking zones • Dazzling surface causing hazardous driving or landing conditions • Rapid wearing of the surface requiring annual repainting (Esch 1988) • Loss of effectiveness near road shoulders due to heat input from the unpainted embankment slopes (ADOTPF 1985) • Loss of effectiveness for narrow embankments
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FIGURE 10-14 Long-term performance of a white-painted surface at the Farmer’s Loop U.S. Army Corps of Engineers experimental site (Fairbanks, AK) compared to an adjacent reference unprotected section (behind the sign).
Some experiments at Thule, Greenland, were performed in 1959. It was possible to observe that new white paint application absorbed about 16 percent of the incoming solar radiation. The absorptivity rose to about 42 percent after 1 year due to weathering and traffic (ADOTPF 1985). After a certain amount of time, the use of pale surfaces do not prevent heat intake underneath the embankment because the road is becoming darker due to dust and to tire passage. In a recent experiment at the Kangerlussuaq Airport, Greenland, a reduction of thaw depth of approximately 1.0 m (maximum thaw depth approximately 4 m in adjacent areas) was observed underneath a white-painted area using a ground penetrating radar (Stuhr-Jorgensen et al. 2007). New products with high friction characteristics offer new perspective for the use of white or light-colored surfaces for permafrost protection. These products include lightcolored asphalt products, cement grouts, latex polymers, and epoxy compounds. In the industry, these products are used for road marking, ceramic installation, and crack sealing. Several products were evaluated in recent studies and appear to have good potential for large scale application (Doré et al. 2007).
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Pavements on Permafrost
FIGURE 10-15 Effects of sunsheds (summer) and snowsheds (winter) used for embankment protection in permafrost conditions.
Sunsheds or Snowsheds Sunsheds or snowsheds play a double role in embankment protection. In the winter, they act as snowsheds and they permit cold air circulation along the embankment slopes allowing for maximum heat extraction during winter. In summer, they act as sunsheds and eliminate direct solar radiation on the embankment slopes. Figure 10-15 is a schematic illustration of the effect of sunsheds and snowsheds. At Bonanza Creek, Alaska, an experimental embankment was built with snowsheds along the slopes. The mean annual slope surface temperatures were reduced from a normal slope value of 3.9°C to a value of – 2.3°C beneath the sheds (Esch 1988). Figure 10-16 illustrates sunsheds used in Alaska and China. In spite of good performance, the use of sheds was not considered practical in Alaska for safety reasons. Guard rails are needed along the embankment to prevent hazards for vehicles accidentally leaving the road (Esch 1996). Also, sheds have a highmaintenance cost and lack of durability, but are a low-cost deployable system compared to some other methods.
10-4-4
Methods Based on Heat Extraction from the Embankment
The following methods are based on active removal of heat from underneath the embankment in order to preserve stable permafrost conditions. Most of the methods within this category use natural heat exchange phenomena in order to maximize heat loss. Some attempts have also been made to use cooling systems to extract heat from the soil and to use recuperated heat to help heat buildings.
Air Ducts The air duct cooling is a system that allows heat extraction beneath the embankment by natural convection. As a result, air ducts can cool the embankment or side slope areas during winter. This system has to be disabled during summer and protection has to be installed at the pipe outlets to prevent warm air intrusion and to avoid snow blockage in winter (Esch 1996). There are two types of duct systems. The first one consists of open-ended ducts in the pad or roadbed oriented in the direction of the prevailing wind (Nixon 1978 cited by
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(a)
(b) FIGURE 10-16 (a) Snowsheds used on the Parks Highway, Bonanza Creek, AK (courtesy of John Zarling) and (b) on the Qinghai-Tibet Roadway (courtesy of Professor Ma, Lanzhou, China).
Niu et al. 2003). The second system is based on the chimney effect to induce air movement in horizontal ducts placed under the embankment (Tobiasson 1973 cited by Niu et al. 2003). Figure 10-17 illustrates the principle of the two types of systems. Water ponding and ice formation within the ducts can be a real problem and compromise their performance. They will impede air flow and reduce heat extraction. Culverts are placed through the embankment from side to side and should be spaced at longitudinal intervals of 8 to 12 m (Esch 1996). To have the best chance of long-term performance without water ponding or snow plugging risks, they should be placed high in the embankment and slightly sloped.
Pavements on Permafrost
FIGURE 10-17
Air ducts used for permafrost protection.
Air ducts are not used by ADOTPF at this time, although their use is being studied in view of their application in Qinghai-Tibet railway embankment (Niu et al. 2003). Because of the presence ice-rich permafrost in the Qinghai-Tibet plateau, ventilated ducts have been chosen to extract heat from the ambient soil in order to stabilize the permafrost table. In the Qinghai-Tibet Roadway, the ventilated embankment technology has already been used (Fig. 10-18).
Thermosyphons Thermosyphons make full use of heat transfer principles including conduction, condensation, evaporation, and convection. The purpose of the devices is to extract heat beneath embankments in order to keep the soil frozen. A thermosyphon is composed of a pipe which includes a refrigeration gas, such as ammonia, carbon dioxide, or propane in liquid and gas phases. When the air is colder than the ground, heat from the ground causes the liquid to vaporize. As illustrated in Fig. 10-19, vapor flows upward from the evaporator section (below ground) to the condenser section (above ground) where it condensates back to a liquid because of cooling by ambient air. The condensate flows downward by gravity on the internal wall surface of the thermosyphons where it absorbs heat from the ground and is re-evaporated to continue the process (Heuer et al. 1985; Sorensen et al. 2003). When air temperature is warmer than the saturation temperature of the liquid, the thermosyphon is dormant. In practice, thermosyphons involve pipes buried in parallel trenches near the base of embankment and inclined upward toward the radiator section. An inclination greater than 6 percent is generally necessary for proper functioning because the condensate must flow down from above the ground (radiator and condenser) to the end of the pipe buried into ground (evaporator) (Esch 1996). Thermosyphons are placed in a compacted sand or gravel pad above the top of the permafrost table. Thermosyphons effectively reduce the temperature of the material surrounding the evaporator during winter reducing considerably the risk of thawing during the subsequent summer. The use of an insulation layer above the evaporator increases the effectiveness of the thermosyphons.
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(a)
(b) FIGURE 10-18 Ventilated embankment using culverts on (a) the Alaska Highway and (b) on the Qinghai-Tibet Roadway (courtesy of Professor Ma, Lanzhou, China).
Pavements on Permafrost
FIGURE 10-19
Principle of the thermosyphon.
“Flat loop” horizontal evaporators can also be used. When a liquid-filled loop is subjected to different temperatures at the opposite end of the pipe, the fluid will begin to move around the loop (DenHartog 1988; Yarmack and Long, 2002). Flat thermosyphons are easier to install on roads because they need less fill material to bury them. The main problem with thermosyphons is their relatively high cost. For this reason, thermosyphons are typically used in severe permafrost degradation areas. Another problem is the presence of vertical condensers along roads. The condensers are exposed to vandalism and impacts by vehicles. They also constitute a risk for traveling vehicles. A solution to avoid problems is to bury the condensers underneath the surface of the pavement (hairpin thermosyphons). Other problems such as formation of internal ice, icing of the finned radiator (Kuzmin 1998) and accumulation of noncondensable gases, a by-product of corrosion or chemical dissociation of anhydrous ammonia (Sorensen et al. 2003), can occur in thermosyphons. Figure 10-20 illustrates an application of the thermosyphons technology on the Chena Hot Springs Road in Alaska.
Air Convection Embankments Air convection embankment (ACE) technique is based on the formation of convective cells in embankment using large poorly-graded porous rocks with a low fine content. Difference in temperature between the surface and the base of the embankment during cold winter period creates convective cells if the voids in a rock embankment are large and interconnected. In winter, the air present in voids is cooled at the top of rock layer. Dense cool air moves downward pushing warm air upward. The induction of convective cells, such as illustrated in Fig. 10-21, speeds the cooling and the refreezing of the permafrost underneath (Esch 1996). In summer, with warm air at the top and cold air at the bottom, convection heat transfer does not occur and the system becomes dormant. The rock layer provides greater heat exchange in winter than in summer. Convection can transfer heat upward out of the embankment at a rate that may be more than an order of magnitude larger than conduction, resulting in an effective winter cooling (Goering 2003). As a result, ACE will increase wintertime cooling rates and will decrease summertime warming rates (Esch 1996).
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FIGURE 10-20
Thermosyphons on Chena Hot Springs Road, Alaska.
FIGURE 10-21
Air convective embankment.
Pavements on Permafrost Goering (1998) found after two years of monitoring at a test site near Fairbanks, AK, that the mean annual temperature at the surface of the embankment was about 2°C compared to –1.2 to –3.6°C at the subgrade surfaces. According to thermal modeling, the mean annual permafrost temperature could decrease by up to 5°C (Esch 1996). Further monitoring of the performance of ACE is currently underway in Alaska and China (Fig. 10-22). Because of the interlock effect, angular rocks rather than round rocks should be used to stabilize the embankment. The required height of embankment is around 1.5 to 2.5 m (Esch 1996). In order to protect the ACE from infiltration of fine particles and from fine filling in voids, a geotextile should be placed on top of the embankment. Finally, to prevent thawing of the underlying foundation soils during construction, ACE should be built during winter by placing the core material in a frozen state (Goering 2001). The main problem with ACE is to find good competent coarse rocks that are big enough to allow the creation of convective cells. ACE is an alternative that can become costly because of the crushing and sieving operations required.
Heat Drain Heat drains allow for heat extraction from the embankment during winter taking advantage of a convective air flow occurring in a highly permeable geocomposite drainage layer. As illustrated in Fig. 10-23, the drain can be placed in the shoulder or as a layer in the embankment. An air intake is installed at the foot of the embankment and an outlet is installed at the top to facilitate air circulation into the drain when snow or ice is present on the slope of the embankment. The heat drain is designed to work during winter when air temperature is colder than the subgrade soil. Heat is transported from the soil to the air in the drainage layer by conduction. The warm air generates an upward movement in the drain forcing cold air to enter the system through the air intake. Heat drains have been tested using small-scale models in cold rooms and in the field (Beaulac and Doré 2006; Doré et al. 2007) and appear to give good results. Figure 10-24 illustrates the installation of a heat drain in the shoulder of the Tasiujaq airstrip in Nunavik (Canada).
10-4-5
Methods Based on Embankment Reinforcement
In some conditions where preventing permafrost degradation is not practical and where moderate distortions are expected, embankment reinforcement may be used to resist faulting, spreading, and localized subsidence. The following methods have been documented in the literature.
Geosynthetics Geosynthetics are synthetic polymer materials that are specifically manufactured to be used in geotechnical and geoenvironmental applications. They are used to enhance, augment, and make possible cost-effective environmental, transportation, and geotechnical engineering construction projects. Geosynthetics provide one or more of the following functions: separation, reinforcement, filtration, drainage, liquid, or vapor barrier, protection. Instanes et al. (1998) report that in permafrost embankment applications the basic roles of a geosynthetic are separation, reinforcement, and to a lesser extent, filtration and drainage. Typical types of geosynthetics used in permafrost road embankments include geotextiles, geogrids, geocells, geofoam (insulation), and geocomposites (drainage, heat drains).
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(a)
(b) FIGURE 10-22 (a) Air convection embankments tested in Alaska (University of Alaska Fairbanks access road) and (b) in China (Tibetan Plateau Highway, courtesy of Professor Ma, Lanzhou, China).
Pavements on Permafrost
FIGURE 10-23 Schematic illustration of heat drains installed across the embankment and within the embankment shoulder.
FIGURE 10-24 (Canada).
Installation of heat drain in the shoulder of the Tasiujaq airstrip, Nunavik
A method described in the literature involves building embankments with geotextile reinforced core as illustrated in Fig. 10-25. The embankment is constructed with multiple layers of geotextile in order to form two geosynthetic walls tied together through the embankment (Rooney and Johnson 1988). This method is called a pillow embankment method. The edges of the reinforced core are located in a way that they will not be affected by the thermal degradation of permafrost under slopes (Rooney and Johnson 1988).
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FIGURE 10-25 Core embankment reinforced with geotextiles (pillow) (redrawn from Rooney and Johnson 1988).
As geotextiles, geogrids can be used to reinforce the embankment. In Russia, geocells have been used to reinforce the embankment in order to stabilize the fill material and the cut side slopes (Grechishchev et al. 2003).
Berms and Gentle Slopes The use of berms and gentle slopes are other solutions that are likely to improve the thermal and the mechanical behavior of embankments built over thaw-sensitive permafrost (Esch 1996; Hoeve et al. 2004). By reducing snow accumulation along the embankment and by providing a protection against temperature variations, these techniques are likely to improve the thermal behavior of the core of the embankment. According to Hoeve et al. (2004), a side slope of 6H: 1V is required to prevent snow accumulation on the slope and provides a cost-effective protection for embankments built over thaw-sensitive permafrost. Figure 10-26 is a schematic illustration of the effect of gentle slopes for embankment protection. Berms and gentle slopes also provide lateral support to the core of the embankment improving thus their mechanical performance. Figure 10-27 is an example of the use of berms for thermal and mechanical protection of highway embankments in Yukon.
10-4-6
Other Methods
A few other methods are described in the literature. They include techniques applied at the construction stage such as prethawing and excavation/replacement of ice-rich
FIGURE 10-26
Effect of gentle slopes for thermal and mechanical protection of embankments.
Pavements on Permafrost
FIGURE 10-27
Rock berm for embankment protection on the Alaska Highway, Yukon, Canada.
permafrost. Still other techniques involve adapted maintenance strategies, such as intensive maintenance or snow removal on the embankment slopes.
Prethawing Prethawing can be used to achieve thawing and consolidation prior construction for unstable permafrost layers. Performing prethawing prior construction during one or two thawing seasons can reduce significantly thaw settlements. Prethawing can be performed in shallow ice-rich permafrost layers, but has to be avoided when deeper ice layers are present (Esch 1988). Different methods can be used to accelerate prethawing (Esch 1996): • Vegetation can be stripped to expose the soil to the sun in summer • A clear plastic cover can be placed on the ground to create a greenhouse effect • A dark material (gravel of plastic) can be used to absorb solar radiation • A layer of hot asphalt can be applied on a thin gravel pad Vegetation stripping can be very effective (Nidowicz et al. 1999). Prethawing does not involve high costs, but requires flexibility and time. It is also difficult to accurately anticipate the final result. Persistent thaw of permafrost could result in settlement and refreezing at the boundaries could produce heave.
Excavation and Replacement Excavation and replacement can be used to replace shallow ice-rich soils by nonfrostsusceptible soils when time is not available to proceed to prethawing. This method can be expensive, but is generally considered to be effective, namely, in cases of localized buried ice bodies, such as ice wedges.
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Build and Maintain The most frequently used method is to build a structurally adequate, but thermally inadequate embankment (Esch 1988). Designers simply accept that excessive embankment movements are going to occur and that intensive maintenance and rehabilitation will be required when problems occur.
Snow Removal on Embankment Slopes The removal of snow on the embankment slopes can be an effective method to facilitate heat extraction from embankment slopes during winter (Esch 1988). This method is, however, expensive and requires skilled operators.
Gravel-Surfaced Roads Another way to reduce problems with permafrost is to leave roads with gravel surfaces. Gravel roads reduce absorption of sun radiation compared to black asphalt-paved roads. Unpaved runways and roadways can easily be graded to correct surface irregularities. However, they require more maintenance and do not offer a good level of service to users.
10-4-7 Applicability and Relative Cost of Protection Techniques Many methods described above only slow permafrost degradation and are not longterm solutions. Settlement, cracking, and lateral spreading still occur over the years. For many reasons such as effectiveness, cost, safety, and high maintenance, few of these methods are widely used. Table 10-1 summarizes the applicability and provides some indications of the relative cost involved with the application of each method. It is important to note that costs can vary considerably depending on the location of the site and available resources and materials in the vicinity.
Continuous (Cold) Permafrost
Discontinuous (Warm) Permafrost
Sporadic Permafrost
Embankment thickening
$
$$
$$$
Insulation
$$$
$$$*
*More effective if used in combination with heat extraction methods
Sunshed/ snowshed
$$$
$$$
Safety concerns and high level of maintenance
Reflective surface
$$$
$$$
Safety concerns and high level of maintenance
Air ducts
$$$$
$$$$
Appealing solution if well designed to avoid water ponding
Comments Applicability depends on material availability
TABLE 10-1 Applicability and Relative Cost of Protection Techniques (Beaulac et al., 2004)
Pavements on Permafrost Continuous (Cold) Permafrost
Discontinuous (Warm) Permafrost
Thermosyphons
$$$$$
$$$$$
More suitable for severe localized problems
ACE
$$$$
$$$$
Promising technique. Requires competent rock near construction site
Heat drain
$$$$
$$$$
Promising technique. No field experimentation done to date
Geotextile and geogrid
$$$$
$$$$
Berms
$$$
$$$*
Prethawing
$$$
$$$
$$$
Appealing solution if time permits
Excavation/ replacement
$$
$$$
$$$$
Depends on availability of granular material
Build and maintain
$$
$$$
$$$
Low level of service. High level of maintenance required
Sporadic Permafrost
$$$$
Likely to reduce settlement and cracking problems *More effective if used in combination with heat extraction methods. Granular material needs to be available
Snow removal Unpaved surfaces
Comments
Not practical $$
$$
$$
Low level of service. High level of maintenance required
Suggested application Application possible but not optimal
TABLE 10-1 (Continued)
Review Questions 10-1. Describe briefly why warming climate threatens pavement structures on permafrost. 10-2. How is degrading permafrost manifested? 10-3. Why are each of the six rules described in Section 10-4-1 important in design of pavements on permafrost? Could few of them be neglected? If so, which ones?
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Chapter Ten 10-4. Consider a cooling climate trend following a warming climate trend. Would the damage caused by the warming trend and subsequent permafrost degradation be automatically repaired? Explain.
10-5. Which permafrost protection techniques have been tried in your area? Which ones have had the most success?
References ADOTPF (1985). “White Paint for Highway Thaw Settlement Control” ADOTPF, Report No. FHWA-AK-RD-85-16, Fairbanks, Alaska, pp. 1–7. Alaska Regional Assessment Group. (1999). “The Potential Consequences of Climatic Variability and Change,” Report of the Alaska Regional Assessment Group, published by Center for Global Change and Arctic System Research, University of Alaska, http:// www.besis.uaf.edu/regional-report/regional-report.html (March 3, 2008). An, V. V., and Devyatkin, V. N. (1998). “The Influence of Climatic, Geodynamic and Anthropogenic Factors on Permafrost Conditions in Western Siberia,” Proceedings of the Seventh International Conference on Permafrost, A. Lewkowicz and M. Allard (eds.), Collection Nordicana, Yellowknife, Canada, pp. 13–17. Andersland, O., and Ladanyi, B. (2004). Frozen Ground Engineering, 2d ed., John Wiley & Sons, ASCE Press, Reston, Va. Beaulac, I., and Doré, G. (2006). “Development of a New Heat Extraction Method to Reduce Permafrost Degradation under Highway Embankments,” Proceedings of the 13th International Conference on Cold Regions Engineering (CD-ROM), ASCE Press, Reston, Va. Beaulac, I., Doré, G., Shur, Y., and Allard, M. (2004). “Permafrost Thawing Impacts on Road and Airfields in Nunavik (Northern Quebec, Canada); Problem Assessment and Review of Possible Solutions,” Proceedings of the Twelfth International Conference on Cold Regions Engineering (CD-ROM), ASCE Press, Reston, Va. Berg, R., and Smith, N. (1976). “Observations along the Pipeline Haul Road between Livengood and the Yukon River,” U.S. Army CRREL Special Report 76-11, p. 83. Burn, C. R. (1998). “Field Investigations of Permafrost and Climatic Change in Northwest America,” Proceedings of the Seventh International Conference on Permafrost, A. Lewkowicz and M. Allard (eds.), Collection Nordicana, Yellowknife, Canada, pp. 107–120. Crory, F. E. (1988). “Airfields in Arctic Alaska,” Proceedings of the Fifth International Permafrost Conference, K. Senneset (ed.), Tapis Publishers, Trondheim, Norway, vol. 1, pp. 49–55. DenHartog, S. L. (1988). “A Thermosyphon for Horizontal Applications,” Cold Regions Science and Technology, Elsevier Science Publishers B.V., Amsterdam, Netherlands, vol. 15, pp. 319–321. Doré, G., Beaulac, I., and Voyer, É. (2007). “Adaptation of Nunavik’s Roads and Airfields to Climate Warming,” Proceedings of the International Conference Arctic Roads, The arctic technology center, Depart, Technical university of Denmark, Sisimiut, Greenland, pp. 62–69. Esch, D. (1988a). “Roadway Embankments on Warm Permafrost Problems and Remedial Treatments, Alaska Department of Transportation, USA,” Proceedings of the Fifth International Permafrost Conference, K. Senneset, (ed.), Tapis Publishers, Trondheim, Norway, vol. 2, pp. 1223–1228. Esch, D. (1988b). “Embankment Case Histories on Permafrost,” Embankment Design and Construction in Cold Regions, Johnson, E. G. (ed.), ASCE Press, Reston, Va. pp. 127–159.
Pavements on Permafrost Esch, D. (1994). “Long Term Evaluation of Insulated Roads and Airfields in Alaska,” Report No. FHWA-AK-RD-94-18, Alaska Department of Transportation and Public Facilities, Fairbanks, Alaska, pp. 1–16. Esch, D. (1996). “Road and Airfield Design for Permafrost Conditions,” Roads and Airfields in Cold Regions, Vinson, T. (ed.), ASCE Press, Reston, Va. pp. 121–149. Geological Survey of Canada. (2006). “Permafrost,” http://sts.gsc.nrcan.gc.ca/permafrost/ (December 12, 2006). Goering, D. J. (1998). “Experimental Investigation of Air Convection Embankments for Permafrost-Resistant Roadway Design,” Proceedings of the Seventh International Permafrost Conference, Collection Nordicana, No. 55, Whitehorse, Canada, pp. 319–326. Goering, D. J. (2001). “ACE and Thermosyphons Design Features Loftus Road Extension Project,” Report No. FHWA-AK-RD-02-01, Alaska Department of Transportation and Public Facilities, Fairbanks, Alaska, pp. 1–58. Goering, D. J. (2003). “Thermal Response of Air Convection Embankments to Ambient Temperature Fluctuations,” Proceedings of the Eighth International Permafrost Conference, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger B. V. (Lisse), Zurich, Switzerland, pp. 291–296. Grechishchev, S. E., Kazarnovsky, V. D., Pshenichnikova, Y. S., and Sheshin, Y. B. (2003). “Experimental Road Structures for Permafrost Regions,” Proceedings of the Eighth International Permafrost Conference, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger B. V. (Lisse), Zurich, Switzerland, pp. 309–311. Haas, R., Hudson, W. R., and Zaniewski, J. (1994). Modern Pavement Management, Krieger Publishing Company, Malabar, Fla. Heuer, C. E., Long, E. L. and Zarling, J. P. (1985). “Passive Techniques for Ground Temperature Control,” Thermal Design Considerations in Frozen Ground Engineering, Krzewinski, T., and Tart, R. (eds.), ASCE Press, Reston, Va. pp. 72–154. Hoeve, T. E., Seto, J. T. C., and Hayley, D. W. (2004). “Permafrost Response Following Reconstruction of the Yellowknife Highway,” Proceedings of the Twelfth International Conference on Cold Regions Engineering, (CD-ROM), ASCE Press, Reston, Va. Instanes, A. (2003). “Climate Change and Possible Impact on Arctic Infrastructures,” Proceedings of the Eighth International Conference on Permafrost, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger B. V. (Lisse), Zurich, Switzerland, pp. 461–466. Instanes, A., Fannin, R. J., and Haldorsen, K. (1998). “Mechanical and Thermal Stabilisation of Fill Materials for Road Embankment Construction on Discontinuous Permafrost in Northwest Russia,” Proceedings of the Seventh International Permafrost Conference, Collection Nordicana, No. 55, Whitehorse, Canada, pp. 495–500. Johnston, G. H. (1981). Permafrost, Engineering, Design and Construction. National Research Council, Canada, John Wiley & Sons, New York. Khrustalev, L. N. (2000). Allowance for climate change in designing foundations on permafrost grounds. In Proceedings of the International Workshop on Permafrost Engineering, Longyearbyen, Svalbard, Norway, 18–21 June, 2000: 25–36. Tapir Publishers, Trondheim, Norway. Khrustalev, L. N. (2001). Problems of permafrost engineering as related to global climate warming. In Paepe, R., and Melnikov, V. (eds.), Permafrost Response on Economic Development, Environmental Security and Natural Resources, Dordrecht, Holland, pp. 407–423. Kuzmin, G. P. (1998). “Experimental Studies of the Process of Ice Formation and Evaporation in Air Thermosyphons,” Proceedings of the Seventh International Permafrost Conference, Collection Nordicana, No. 55, Whitehorse, Canada, pp. 617–621.
399
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Chapter Ten Linell, K. (1973). “Long-term Effects of Vegetative Cover on Permafrost Stability in and Area of Discontinuous Permafrost,” Second International Conference on Permafrost, National Academy of Sciences, Washington, D.C., pp. 688–693. Macleod, D. (2007). “Construction on Permafrost,” International Conference on Arctic Roads, Sisimiut, Greenland, www.arktiskcenter.gl (December 8, 2007). McHattie, R. L., and Esch, D. C. (1983). Benefits of a Peat Underlay Used in Road Construction on Permafrost–Permafrost. Fourth International Conference, Proceedings, National Academy Press, Washington, D.C., pp. 826–831. Molmann, T., Bergheim, B., and Valeriote, M. (1998). “Svalbard Airport Geptechnical Study: Engineering Methodology and Results,” Proceedings of the Seventh International Permafrost Conference, Collection Nordicana, No. 55, Whitehorse, Canada, pp. 745–755. Nidowicz, B., Osterkamp, T. E. and Shur, Y. (1999). “Permafrost prethawing in Farming, Mining, and Civil Engineering.” Proceedings of the Tenth International Conference on Cold Regions Engineering, ASCE Press, Reston, Va, pp. 243–254. Nidowicz, B., and Shur, Y. (1998). “Pavement Thermal Impact on Discontinuous Permafrost,” Proceedings of the Ninth International Conference on Cold Regions, ASCE Press, Reston, Va., pp. 34–45. Niu, F. J., Cheng, G. D., and Lai, Y. M. (2003). “Laboratory Study on a Duct-Ventilated Roadbed of the Qinghai-Tibet Railway,” Proceedings of the Eighth International Permafrost Conference, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger B. V. (Lisse), Zurich, Switzerland, pp. 815–820. Nixon, J. F. (1978). Geothermal aspects of ventilated pad design. Proceedings of the Third International Conference on Permafrost: National Research Council of Canada, Ottawa, pp. 841–846. Osterkamp, T. E. (2003). “A Thermal History of Permafrost in Alaska,” Proceedings of the Eighth International Conference on Permafrost, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger B. V. (Lisse), Zurich, Switzerland, pp. 863–868. Pavlov, A.V. (1997). Permafrost –climatic monitoring of Russia: Methodology, Results of Observation and Forecast. Earth Cryosphere, vol. 1, no. 1, pp. 47–58 (in Russian). Reckard, M., Esch, D. C. and McHattie, R. (1988). Peat Used as Roadway Insulation over Permafrost. Report AK-RD-88-11, Alaska Department of Transportation, Juneau, Alaska. Rooney, J. W., and Johnson, E. G. (1988). “Embankment Stabilization Techniques,” Embankment Design and Construction in Cold Regions, Johnson, E. G. (ed.), ASCE Press, Reston, Va., pp. 13–34. Rooney J. W., and Vinson, T. S. (1996). “Road and Airfield Development in the Subarctic and Arctic, Alaska and Northwest Canada,” Roads and Airfields in Cold Regions, Vinson, T. S. (ed.), ASCE Press, Reston, Va., pp. 1–22. Sharkhuu, N. (2003). “Recent Changes in the Permafrost of Mongolia,” Proceedings of the Eighth International Conference on Permafrost, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger (Lisse), Zurich, Switzerland, pp. 1029–1034. Smith, D. W. (1986). Cold Climate Utility Manual, Canadian Society for Civil Engineering, Montreal, Quebec, Canada. Smith, S. L., and Burgess M. M. (1999). Mapping the sensitivity of Canadian permafrost to climate warming. Interactions Between the Cryosphere, Climate and Greenhouse Gases. Proceedings of International Union of Geodesy and Geophysics (IUGG) 99 Symposium HS2, Birmingham, July 1999 [Tranter, M., R. Armstrong, E. Brun, G. Jones, M. Sharp, and M. Williams (eds.)]. International Association of Hydrological Sciences (IAHS), Birmingham, United Kingdom, vol. 256, pp. 71–80.
Pavements on Permafrost Sorensen, S., Smith, J., and Zarling, J. (2003). “Thermal Performance of TAPS Heat Pipes with Non-Condensable Gas Blockage,” Proceedings of the Eighth International Permafrost Conference, Phillips, M., Springman, S., and Arenson, L. (eds.), Swets & Zetlinger B. V. (Lisse), Zurich, Switzerland, pp. 1097–1102. Stuhr-Jorgensen, A., Ingeman-Nielsen, T., and Brock, N. (2007). “Annual Variation of Frost Table in Kangerlussuaq Airport, Western Greenland,” Proceedings of the International Conference Arctic Roads, Sisimiut, Greenland, pp. 62–69. TAC (1997). Pavement Design and Management Guide, Association des Transports du Canada, Ottawa. Tobiasson, W. (1973). Performance of the Thule hangar soil cooling systems. North American Contribution. Second International Permafrost Conference. National Academies Press, Washington, D.C., pp. 752–758. Yarmack, E., and Long, E. L. (2002). “Recent Developments in Thermosyphon Technology,” Proceedings of the Eleventh International Conference on Cold Regions Engineering, Kelly Merril (ed.), ASCE, Reston, Va. pp. 656–662. Yongjian, D. (1998). “Recent Degradation of Permafrost in China and the Response to Climatic Change,” Proceedings of the Seventh International Permafrost Conference, Lewkowicz, A., and Allard, M. (eds.), Collection Nordicana, no. 57, Bibliothèque Nationale du Québec, Canada, pp. 225–230. Zarling, J. P., Braley, W. A., and Esch, D. (1988). “Thaw Stabilization of Roadway Embankments,” Proceedings of the Fifth International Permafrost Conference, Senneset, K. (ed.), Tapis Publishers, Trondheim, Norway, vol. 2, pp. 1352–1357.
401
INDEX Note: Page numbers referencing figures are italicized and followed by an “f”; page numbers referencing tables are italicized and followed by a “t”.
Index Terms
Links
A AASHTO-93 design method
313
abrasion resistance test
290
absorbed asphalt cement
282
ACE (air convection embankments)
389
activity timing
255
additives, hot mix asphalt concrete
277
ADT (average daily traffic)
255t
agency costs
252t
315
316t
392f
397t
324
325t
392f
397t
aggregates hot mix asphalt concrete
277
pavement insulation and
335
road wear by studded tires and
80
Superpave aggregate consensus property requirements aging, asphalt concrete
278t 80
air convection embankments (ACE)
389
air cooling rate
211
air ducts
385
396t
air freezing index (FIa) from annual summary temperature data
213
from daily temperature data
211
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
air temperature converting to pavement temperature
275
freezing and thawing indices and
210
air thawing index (TIa) from annual summary temperature data
213
from daily temperature data
211
air-entry pressure
49
air-void content
68
AKPAVE design method Alaska’s road network albedo
317t 1
2f
16
Aldrich-Berggren method
228
alluvial fan
123t
alluvial plain
122t
α (thermal diffusivity)
224
analysis period
253
ANEV (annual net equivalent value)
255
anionic emulsions
293
annual net equivalent value (ANEV)
255
antistripping agents
279t
asphalt cement
269
295t
asphalt concrete aging of
80
coefficients for calculation of transmitted freezing index data template estimating fracture temperature for Poisson’s coefficient for
229t 324t 62f 236t
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
asphalt concrete (Cont.) rutting of due to studded tire wear
76
overview
71
permanent deformation
71
temperature in
220
thermal conductivity and heat capacity of
225t
227t
thermal cracking of overview
58
problem assessment
60
problem description
58
remedial solutions
65
asphalt runoff
298
asphalt stabilized base
229t
asphalt surface treatments (AST)
5
305
353t
average daily traffic (ADT)
255t
324
325t
average thermal gradient
231f
B backcalculation methods
155
backer rods
352
basal moraine
124t
base course layer
8
base curvature index
152t
basin beds landform
122t
BBR (bending beam rheometer)
48
173t
beam fatigue test
66
182t
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
bearing capacity loss during spring thaw overview
99
problem assessment
100
problem description
99
remedial solutions bending beam rheometer (BBR)
105 48
Berggren equation
228
berms
394
173t
395f
397t
binders asphalt cement and properties stiffness
269 60 290
bitumen. See asphalt cement bitumen test data chart (BTDC)
271
bituminous pavement materials compacted mixture sample preparation and conditioning
168
mixture performance tests
180
overview
164
tests for material selection
168
tests for moisture damage
178
tests for volumetric mix design
168
bituminous surface treatments (BST)
6
black ice
340
blade bit with air
142t
blocked ice
352
bogs
125t
boiling test
178t
305
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
boring and sampling frozen soils
140
planning
136
techniques
138
bottom-up cracking
66
Boussinesq equations
40
235
6
305
BST (bituminous surface treatments) BTDC (bitumen test data chart)
271
build and maintain method
397t
Burger’s viscoelastic model
45f
Burminster’s theory
41
C c (heat capacity)
21
224
calcium chloride (CaCl2)
356
California Bearing Ratio (CBR)
100
146
190
194
225t
Canadian Strategic Highway Research Program (C-SHRP) capillary rise
24
capillary water
23
cationic emulsions cement, cement treated materials
293 5
central tire inflation (CTI) system
366
Champlain Sea clay
107
Chaussées design method
319t
chemically bound water
23
chimneys, ducts with
386
chip seal
353t
Clausius-Clapeyron equation clay, thermal conductivity and heat capacity of
295t
30
387f
32
225t
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
climate response sensors
164f
climate warming
374
coastal deposits
122t
coating and adhesion tests
296
cohesive soils
139t
cold-laid plant mix
295t
cold mixes defined
5
material selection
293
overview
292
recycling
298
selection of optimum asphalt residue content
296
cold region pavements aging of asphalt concrete base course
80 8
bearing capacity loss during spring thaw overview
99
problem assessment
100
problem description
99
remedial solutions
105
crack deterioration
69
design considerations
12
embankment geometry
12
fatigue cracking
65
frost destructuration of undisturbed sensitive clays
106
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
cold region pavements (Cont.) frost heaving cracking
93
differential frost action
88
in granular base material
96
overview
88
overview
1
pavement disintegration
82
performance overview
57
7
potholes overview
85
problem description
85
remedial solutions
87
road networks
1
rutting of asphalt concrete overview
71
permanent deformation
71
rutting due to studded tire wear
76
special pavement layers subbase subgrade soil
10 9 10
surface types cold mix
5
gravel surface
7
hot mix asphalt (HMA)
5
overview
5
stabilized bases
7
surface treatments
6
surfacing layer
8
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
cold region pavements (Cont.) thermal cracking of asphalt concrete overview
58
problem assessment
60
problem description
58
remedial solutions
65
compacted snow
225t
compressive strain gauges
165t
concrete condensation
5 19
continuous flight hollow stem auger
142t
continuous flight solid stem auger
142t
continuous hollow-stem auger
143t
contractile skin
30
convection
18
convective heat transfer
218
cooling rate
211
costs life-cycle cost analysis
255
of protection techniques
396
crack deterioration
69
crack heaving
98f
crack sealing
60
crack spacing
64
70
352
cracking frost heaves
93
thick pavement
65
thin pavement
65
top-down
66
transverse
361
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
cracking strength
47
cracking temperature
47
CRREL design method
317t
crumb rubber
279t
cryosuction
107
C-SHRP (Canadian Strategic Highway Research Program)
190
CTI (central tire inflation) system
366
cultural considerations
248t
cumulative seasonal fatigue damage
327
194
cumulative seasonal permanent deformation damage
328
cutback asphalt
292
cyclic stresses
37t
D Danish Road Testing Machine Darcy’s law
100 26
30
DBFO (Design-Build-Finance-Operate) method
249
DCP (dynamic cone penetrometer)
146
deep basin deposits
119f
deflection analysis
220
deflection basin indices
151
deflection data, assessing layer moduli using
154
deflection parameters
156
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
deflectometers falling weight
100
363
analysis of seasonal variation of deflection parameters
156
assessment of layer moduli using deflection data deflection basin indices
154 151
seasonal monitoring of pavement parameters using
158tt
light weight
149
multidepth
165t
deicing
350
Denali Highway, Alaska
7f
dense graded aggregates
236t
dense sand
236t
dense-graded emulsion mixtures
293t
dense-graded gravel
295t
9
design. See also embankment and pavement design; mix design; pavement design drainage system
379
life-cycle cost analysis activity timing
255
analysis period
253
cost calculation
255
interest rate
252
overview
251
pavement alternatives
254
residual value
254
lifetime engineering methodology
247
long-term procurement methods
248
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
design. See also embankment and pavement (Cont.) overview
247
pavement management concepts
258
design life
324
Design-Build-Finance-Operate (DBFO) method
249
Design-Operate-Transfer procurement concept
249
deteriorated cracks
70f
deterministic design approach
315
Dickey Lake test site in Montana
35
differential frost action
88
differential frost heaving
48
differential water pressure
30
direct tension tester diurnal cycles DPI (dynamic penetration index)
173t 20 146
drainage design
379
layers
9
10
106
352
354
355t
290
291t
maintenance pavement
33
drumlin
125t
dunes
125t
dust control agent
356
dust control palliative
360
7
dynamic cone penetrometer (DCP)
146
dynamic load
46f
dynamic modulus ASTM D3497
181t
dynamic modulus test
287
This page has been reformatted by Knovel to provide easier navigation.
Index Terms dynamic penetration index (DPI)
Links 146
E E (elastic element)
44
earth pressure at rest
37
earthwork
116
ecological considerations
248t
economic considerations
248t
effective asphalt cement
282
elastic crack sealants
352
elastic element (E)
44
elastic system, stress-strain behavior of
39f
elastic theory
44
electrical resistivity
129
137t
embankment and pavement design building and maintaining
396
embankment reinforcement methods berms and gentle slopes
394
geosynthetics
391
overview
391
excavation and replacement
395
gravel-surfaced roads
396
heat extraction from embankment air convection embankments
389
air ducts
385
heat drain
391
overview
385
thermosyphons
387
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
embankment and pavement design (Cont.) pavements on permafrost general considerations
377
overview
376
protection techniques
380
prethawing
395
preventing heat intake methods embankment insulation
381
increasing embankment thickness
381
reflective surfaces
383
sunsheds or snowsheds
385
snow removal on embankment slopes embankment geometry embankment thickening emitted radiations
396 12
378
396t 17
empirical pavement design approach
313
emulsions
293
295t
296
engineering parameters freezing and thawing indices air temperature and
210
within pavement structure
225
surface temperature and
217
frost and thaw depth overview
227
transmitted freezing index method
228
frost heaves Konrad’s method
230
Saarelainen’s method
232
overview
209
stresses and strains in pavements
235
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
engineering parameters (Cont.) temperature in asphalt concrete
220
thaw settlement
232
thermal properties of soils and pavement materials
223
environmental conditioning system ECS
179t
esker landform
124t
evaporation
19
33
81
excavation and replacement method costs
397t
embankment and pavement design
395
existing roads characterization of thaw-sensitive soils
375
identification of thaw-sensitive areas
375
site investigation process
116f
expanded polystyrene
334
336t
382
extruded polystyrene
334
336t
382
exudation
81
F failure
313
fall
36
falling weight deflectometers (FWD) analysis of seasonal variation of deflection parameters
156
assessment of layer moduli using deflection data
154
bearing capacity loss
100
deflection basin indices
151
load restriction and
363
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
falling weight deflectometers (FWD) (Cont.) seasonal monitoring of pavement parameters using
158t
fatigue cracking
65
feedback process
261
292
333t
FIa. See air freezing index fibers and fillers
279t
fine grained soils
236t
Finland, studded tire use in Finland design method
77f 319tt
flat loop horizontal evaporators
389
fluvial landforms
120f
fluvio-glacial landforms
120
fog seal
353t
Fourier’s equation
17
fracture temperature
62f
France design method
321t
free water
23
freeze tests
106
freezing and thawing indices air temperature and
210
within pavement structure
225
surface temperature and n-factor approach
217
radiation index approach
218
fresh snow, thermal conductivity and heat capacity of
225t
frontal moraine
124t
frost and thaw depth
227
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
frost destructuration of undisturbed sensitive clays in seasonal frost conditions frost heave cell
106 195f
frost heaves controlling longitudinal transitions
333
pavement insulation
334
cracking overview
93
problem assessment
93
problem description
93
remedial solutions
95
differential frost action overview
88
problem assessment
90
problem description
88
remedial solutions
92
in granular base material overview
96
problem description
96
remedial solutions
99
Konrad’s method overview Saarelainen’s method stresses related to
230 88 232 48
thaw consolidation and
103f
validation of total pavement thickness for
328
frost penetration frost protection layer frost tubes
228 9 166t
363
This page has been reformatted by Knovel to provide easier navigation.
Index Terms frozen fringe
Links 31
frozen soils, boring and sampling
140
functional performance
327t
funding decisions
261
funicular water regime
24
FWD. See falling weight deflectometers
G gaseous phase
23
gentle slopes
394
geogrids
397t
geological and technical report
128
geology, pavement stress and
51
geometric design
377
geomorphology and photo-interpretation of landforms geophysical investigation methods
118 137t
geostatic stresses
37t
geosynthetics
391
geotextiles
393
geothermal heat
16
glacial landforms
120
“glacial tills”
120
GPR (ground penetrating radar)
135
397t
137t
granular base material coefficients for calculation of transmitted freezing index data template
229t 324t
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
granular base material (Cont.) frost heaving in overview
96
problem description
96
remedial solutions
99
sampling
139t
stabilization of
343
thermal conductivity and heat capacity of
225t
gravel road maintenance gravel surfaces
227t
355 7
gravity tests, volumetric mix design
176t
ground penetrating radar (GPR)
135
gyratory compaction device
175f
306
396
137t
H hairpin thermosyphons
389
hand sampling
143t
heat capacity (c) heat drains heat exchange with precipitation
21
224
225t
391
393f
397t
19
heat extraction from embankments air convection embankments
389
air ducts
385
heat drain
391
thermosyphons
387
factors contributing to in pavements
18
heat flow
227
heat intake, preventing under embankments
380
HF (high-float) emulsions
295
334
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
HFMS (high-float medium setting emulsions)
299
high terrain pavement environment
51
high-float (HF) emulsions
295
high-float medium setting emulsions (HFMS)
299
highway agency design methods
315
highway auger
142t
HMA. See hot mix asphalt concrete HMA aggregate evaluation test methods
174t
hoarfrost
340
homogeneity
99
Hooke’s law
39
horizontal capillary barriers
10
horizontal drainage layers
10
horizontal reinforcement layers
11
horizontal resistivity profile
130
horizontal strain gauges
165t
horizontal stresses
132f
42f
hot mix asphalt (HMA) concrete material selection additives
277
aggregates
277
asphalt cement
269
bitumen test data chart
271
Superpave binder grade
271
overview
5
266
performance tests abrasion resistance
290
fatigue cracking resistance
292
low-temperature cracking
289
moisture sensitivity test
289
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
hot mix asphalt (HMA) concrete performance tests (Cont.) overview
286
permanent deformation resistance
290
trial aggregate gradations
278
volumetric parameters
281
human considerations
248t
hummocks
125t
hydrated lime
302
hygroscopic water
23
I ice enrichment process
98f
ice lenses
32
ice wedges
126t
ice-wedge polygons
126t
IDT creep compliance and strength test
182t
immersion compression test
178t
in situ testing of soils dynamic cone penetrometer
146
light weight deflectometer
149
shear strength test
144
standard penetration test
144
inertial profilometer
160
infiltrated water
34
infiltration of water from precipitations
26
in-place recycling
360
insulation costs
396t
embankment
381
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
insulation (Cont.) layers
12
95
pavement differential behavior at end of insulated areas
341
differential icing
339
structural implications
335
thermal design of insulation layer
338
peat
382
polystyrene
225t
integrated lifetime construction
249t
integrated lifetime design
249t
integrated lifetime management and maintenance planning
249t
interest rate, life-cycle cost analysis
252
international roughness index (IRI)
92
inventory
160
258
investigation. See also site investigation of existing pavements falling weight deflectometer
150
longitudinal profile measurements
157
pavement instrumentation
163
surface geophysical electrical resistivity
129
ground penetrating radar
135
seismic refraction
131
IRI (international roughness index) irregular slopes
92
160
127
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
J jet-drive drill
142t
K k (thermal conductivity)
21
kame terrace
123t
kames
121
kettles
123t
Konrad’s method
230
223
225t
123t
L landforms
118
latent heat exchange process
22
latent heat of fusion
18
224
lateral moisture transfer
25
26f
lateral moraine
124t
lateral transitions
341
layer moduli, assessing using deflection data
154
layer-strain predictive methodology LCC (life-cycle cost)
369
72 250
253f
LCCA. See life-cycle cost analysis lenticular water regime leveling
23 356t
357
life-cycle cost analysis (LCCA) activity timing
255
analysis period
253
cost calculation
255
interest rate
252
overview
251
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
life-cycle cost analysis (LCCA) (Cont.) pavement alternatives
254
residual value
254
life-cycle cost (LCC)
250
lifetime engineering methodology
247
253f
lifetime investment planning and decision making
249t
light weight deflectometer
149
lightweight aggregates
335
liquid phase
23
littoral deposits
122t
load response sensors
164f
load restrictions
362
loading conditions, AASHTO T-307
190t
loess
125t
longitudinal cracking
93
94f
longitudinal profile measurements
89f
157
longitudinal transitions
333
341
long-term conditioning (LTC)
289
long-term pavement performance (LTPP) data
220
long-term procurement methods
248
loose sand
236t
Los Angeles abrasion test
192
low road density low-temperature cracking
361
370
1 59
LTC (long-term conditioning)
289
LTPP (long-term pavement performance) data
220
61t
289
This page has been reformatted by Knovel to provide easier navigation.
371f
Index Terms
Links
M MAAT (mean annual air temperature)
211
213
291t
374
drainage systems
352
354
355t
360
embankment and pavement design
396
gravel roads
355
load restrictions
362
291t
374
maintenance
minimizing frost heaves
96
overlays
354
overview
349
preventive
251f
repair of local failures
354
routine
349
surface treatments
352
manual boring techniques
139
MARR (minimum attractive rate of return)
252
Marshall immersion test
178t
Marshall mix design procedure
266
296
17
369
MAT (mean air temperature)
210
211t
matric suction
49f
MAST (mean annual surface temperature)
256
maximum air temperature
211t
maximum daily cooling rate
211t
maximum deflection
152t
MDAT (mean daily air temperature)
210
212
mean air temperature (MAT)
210
211t
mean annual air temperature (MAAT)
211
213
17
369
210
212
mean annual surface temperature (MAST) mean daily air temperature (MDAT)
This page has been reformatted by Knovel to provide easier navigation.
Index Terms mechanical boring techniques
Links 139
mechanistic-empirical pavement condition prediction model
255
mechanistic-empirical pavement design guide (MEPDG)
287
290
mechanistic-empirical pavement design procedure analyzing pavement structure analysis of cumulative seasonal fatigue damage
327
analysis of cumulative seasonal permanent deformation damage overview
328 326
validation of HMAC thickness for thermal cracking
328
validation of total pavement thickness for allowable frost heave
328
description of site conditions
322
improving pavement
332
overview
314
performance objectives
324
reiterating pavement analysis
332
trial pavement structure
326
MEPDG (mechanistic-empirical pavement design guide software) micro-surfacing
287
290
353t
Miner’s assumption of linear damage accumulation
328
minimum air temperature
211t
minimum attractive rate of return (MARR)
252
256
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
mix design asphalt surface treatment
305
cold mixes cold mix recycling
298
material selection
293
overview
292
selection of optimum asphalt residue content gravel surfaces
296 306
hot mix asphalt concrete material selection
269
overview
266
performance tests
286
trial aggregate gradations
278
volumetric parameters
281
overview
265
stabilized bases
302
mixture field verification
268
mixture performance tests
180
MnPAVE design method
318t
modified Lottman test
178t
moisture balance
35
factors contributing to extraction
33
factors contributing to intake capillary rise
24
frost action
27
infiltration of water from precipitations
26
lateral moisture transfer
25
overview
24
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
moisture (Cont.) overview
23
phases of water
23
tests for damage
178
moisture affinity moisture sensitivity tests morphology multidepth deflectometer
21 178t
289
51 165t
N National Center for Asphalt Technology
70
natural asphalt
279t
NCHRP-MEPDG design method
316t
negative pore pressure network level PMS
49 258
new roads characterization of thaw-sensitive soils
375
identification of thaw-sensitive areas
375
site investigation process
116f
n-factor approach
217
non-frost-susceptible (NFS) material
381
Nordic abrasion test equipment
175f
Nordic abrasion test specification
278t
383
Nordic countries road networks
1
studded tire use
76
Norway design method nucleation
320t 28
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
O occluded air regime
24
Odemark method
236
OPAC design method
319t
open-ended ducts
385
open-graded aggregate
295t
open-graded emulsion mixtures
293t
oscillating load stresses
46
outwash plain
124t
overlays
354
oxidative ageing
387f
81
P palsa
126t
partly frozen soil layer
29f
patching
350
PAV (pressure aging vessel) test
60
paved roads
4t
173t
pavement design current practice in cold climates approaches
313
methods used by highway agencies
315
mechanistic-empirical pavement design procedure analyzing pavement structure
326
description of site conditions
322
improving pavement
332
performance objectives
324
reiterating pavement analysis
332
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Index Terms
Links
pavement design mechanistic-empirical pavement design procedure (Cont.) trial pavement structure
326
protective features control of frost heave
333
pavement reinforcement
343
pavement disintegration
82
pavement instrumentation
163
pavement management concepts
258
pavement reinforcement
343
pavements. See also cold regions pavements condition assessment
258
forces affecting and stresses in slab
63f
insulation
259t
336t
interaction with geology and morphology
51
moisture regime in factors contributing to moisture extraction
33
factors contributing to water intake
24
moisture balance
35
overview
23
phases of water
23
profiling principle
160f
selection of alternatives
254
stress regime in earth pressure at rest
37
moving traffic loads
43
negative or positive pore pressure
49
overview
37
static stresses induced by traffic loads
38
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
pavements. See also cold regions pavements stress regime in (Cont.) stresses related to frost heave
48
stresses related to permanent soil movements thermal stresses
42 47
surface types cold mix
5
gravel surface
7
hot mix asphalt
5
overview
5
stabilized bases
7
surface treatments
6
temperature regime in factors contributing to heat extraction
17
factors contributing to heat induction
18
factors inducing heat
15
thawing thermal balance and thermal cycles
101f 19
peat insulation
382
thermal conductivity and heat capacity of
225t
penetration grading system
269
percussive hammer
142t
performance based specifications
272t
270t
271
performance tests abrasion resistance
290
fatigue cracking resistance
292
low-temperature cracking
289
moisture sensitivity
289
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Index Terms
Links
performance tests (Cont.) overview
286
permanent deformation resistance
290
permafrost climate warming and
374
embankment and pavement design applicability and relative cost of protection techniques
396
build and maintain
396
embankment reinforcement methods
391
excavation and replacement
395
general considerations
377
gravel-surfaced roads
396
heat extraction from embankment
385
overview
376
prethawing
395
preventing heat intake methods
380
protection techniques
380
snow removal on embankment slopes
396
material sampling
139t
overview
369
pavement instability and problem manifestation
369
thaw settlement
232
transportation infrastructure
374
permanent deformation of asphalt concrete modifications for permanent deformation resistance test permanent soil movements
71 333t 290 42
This page has been reformatted by Knovel to provide easier navigation.
Index Terms permeability permeable granular material physical hardening
Links 30
81 167t
pillow embankment method
393
394f
71
plate bearing tests
100
PNEV (present net equivalent value)
256
Poisson’s coefficient
40
polymer modification
269
236
polymer-modified asphalts
65
polymer-modified binders
275t
polystyrene boards
334
336t
polystyrene insulation
225t
382
polyurethane insulation
382
poor bearing capacity
359t
portable deflectometer
149
Portland cement positive pore pressures
193
9
piezometers
plastic flow
34
5
236t
302
49
potholes filling
356t
overview
85
problem description
85
remedial solutions
87
Prall device
183f
present net equivalent value (PNEV)
256
pressure aging vessel (PAV) test pressure gauges
60
357
173t
165t
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
prethawing costs
397t
embankment and pavement design
395
preventive maintenance
251f
prioritizing candidate sections
260
probabilistic design approach
315
project level PMS
258
261
protective features control of frost heave
333
pavement reinforcement
343
pseudo profiles PWR testing equipment
88 184f
R radial strains
235
radiation balance
217
radiation index (RI) approach
218
radius of curvature of center of basin index
152t
RAM (reclaimed aggregate material)
298
random differential heaving RAP (reclaimed asphalt pavement) raveling
48 298 82
reclaimed aggregate material (RAM)
298
reclaimed asphalt pavement (RAP)
298
reflective surfaces
383
refrigerated coring
143t
regraveling
357
rehabilitation
251f
396t
350t
358
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Index Terms
Links
reinforcements embankments pavements
360 11
repair of local failures
354
reshaping
356
residual binder content
302
residual value
254
resilient modulus laboratory measurement of
188
prediction of
191
resistivity effect of freezing temperature on
133f
probes
166t
ranges
132f
resistivity sounding
131f
response-type systems
157
reuse, recycling, and disposal component
249t
rhythmic ice lens formation
32f
RI (radiation index) approach
218
risk of pavement failure
266t
RMSE (root mean square error)
155
road mix
295t
road networks
1
road simulator
181t
road widening
360
361f
roads. See existing roads; new roads rolling bottle test rolling thin film oven test (RTFOT)
178t 60
root mean square error (RMSE)
155
rotary drill
142t
170t
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Index Terms
Links
roughness, (definition)
157
routine blading
356t
routine maintenance
350t
Rowe’s visco-elasto-plastic model
45f
RTFOT (rolling thin film oven test)
60
runoff surface water
34
357
170t
rutting due to studded tire wear
76
overview
71
permanent deformation
71
S Saarelainen’s method
232
sand
299
sand emulsion mixtures
294t
sand-gravel
225t
sanding
350
saturated clays
236t
saturation
49f
Schlumberger configuration
130
SCI (surface curvature index)
152t
seasonal cycles
20
seasonal thermal balance cycle
21f
segregation ice
97
segregation potential
32
230
seismic refraction
131
137t
seismic wave speed ranges
135f
semivariogram sensors, pavement engineering separation layers
90 165t 9
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Index Terms
Links
shear strength test
144
shear stresses
43
short-term conditioning (STC)
288
significant freezing index
216
silt, thermal conductivity and heat capacity of
225t
simplified bending model
94f
simulation tests
292
site investigation boring and sampling frozen soils
140
planning
136
techniques
138
geological and technical report
128
geomorphology and photo-interpretation of landforms overview
118 115
in situ testing of soils Dynamic Cone Penetrometer
146
light weight deflectometer
149
Shear Strength Test
144
Standard Penetration Test
144
surface geophysical investigations electrical resistivity
129
ground penetrating radar
135
seismic refraction
131
topographical maps
117
visual reconnaissance
128
slope failures
372
373f
SLR (spring load restrictions)
362
364t
slurry seal
353t
366
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Index Terms
Links
SMA (stone matrix asphalt)
72
snapshots, temperature
21
255t
snow removal costs
397t
embankment and pavement design
396
maintenance
350
snowsheds
385
soft asphalt binders
275t
soft asphalt cement
65
soil movements, permanent
42
396t
269
soils and material testing assessment of material durability
192
assessment of stiffness
188
assessment of seasonal stability estimation of segregation potential
196
laboratory determination of segregation potential overview
195 193
bituminous pavement materials compacted mixture sample preparation and conditioning
168
mixture performance tests
180
overview
164
tests for material selection
168
tests for moisture damage
178
tests for volumetric mix design
168
overview
184
solar radiation
16
solid phase
24
sounding techniques
141t
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Index Terms
Links
specific gravity
282
split-spoon sampling
143t
spring bearing capacity loss during thaw
99
moisture and
36
spring load restrictions (SLR)
362
364t
SPT (standard penetration test)
143t
144
stabilization
360
379
stabilized base courses
9
stabilized bases
7
stabilometer test
191
standard penetration test (SPT)
143t
static immersion test
178t
static stresses
37
STC (short-term conditioning)
288
steel nets
343
steep slopes
127
Stefan equation
229
Stefan-Boltzmann equation
17
stiff asphalt cements
68
stone matrix asphalt (SMA)
72
straight run asphalt cement
269
366
302
144
38
stress regime earth pressure at rest
37
moving traffic loads
43
negative or positive pore pressure
49
overview
37
in pavements
235
static stresses induced by traffic loads
38
stresses related to frost heave
48
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Index Terms
Links
stress regime (Cont.) stresses related to permanent soil movements thermal stresses stripping structural performance studded tire wear
42 47 83 327t 76
subbase layer
9
subgrade soil
10
subgrade strength index
152t
suction probes
167t
summer
36
sunsheds
385
Superpave binder grade
271
Superpave HMA design requirements
285t
Superpave technology surface curvature index (SCI)
396t
2 152t
surface drainage
34
surface emissivity
18
surface energy balance analysis
324t
217
surface geophysical investigations electrical resistivity
129
ground penetrating radar
135
seismic refraction
131
surface temperature boundary condition parameter and
210
freezing and thawing indices and
217
surface treatments
6
surfacing layer
8
swamps
125t
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Index Terms
Links
T taliks
380
TDR antennas
167t
temperature in asphalt concrete
220
factors contributing to heat extraction
17
factors inducing heat in pavements
16
versus stress relationship
47f
tensile strain at bottom of asphalt bound layer
152t
tensile strain gauges
165t
tensile stresses terrace landform
44 123t
testing. See also soils and material testing beam fatigue
66
CBR
100
FWD
100
RTFOT site investigation
60 144
TFOT
60
TSRST
48
60
324
326t
60
170t
TF (truck factors) TFOT (thin film oven test) thaw consolidation
103f
settlement
232
weakening of pavement granular materials
101f
thaw-consolidation tests
233
thawing index
211t
thaw-sensitive areas
375
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Index Terms
Links
thaw-sensitive soils
375
thaw-weakening index (TWin)
104
thermal block cracking
59f
thermal conductivity (k)
21
223
225t
60
180
thermal cracking of asphalt concrete overview
58
problem assessment
60
problem description
58
remedial solutions
65
modifications for
333t
validation of HMAC thickness for
328
thermal design
377
thermal diffusivity (α)
224
thermal fatigue cracking
60
thermal properties of soils and pavement materials thermal regime fields
223 17f
thermal stress restrained specimen test (TSRST)
48
thermal stresses
47
thermal transverse cracking
58f
thermistor strings thermodynamic equilibrium
183f
166t 28
thermokarsts
126t
thermoplastic elastomers
269
thermosyphons
387
Therzaghi’s theory
102
thick pavement cracking
65
thin film oven test (TFOT)
60
371f
390f
397t
170t
This page has been reformatted by Knovel to provide easier navigation.
290
Index Terms thin pavement cracking
Links 65
TIa. See air thawing index time-dependent material response
44
top-down cracking
66
topographical maps
117
torque
145
traffic loads
43
transition wedges
333
transmitted freezing index method
228
transportation infrastructure
374
transverse cracking
361
transverse differential heaving
93f
trial aggregate gradations
278
truck factors (TF)
324
trumpet curves
326t
20
TSRST (thermal stress restrained specimen test) tube suction test turbulence TWin (thaw-weakening index)
48
60
180
183f
302 18 104
U undisturbed sensitive clay unfrozen water content (UWC) unpaved surfaces user costs UWC (unfrozen water content)
107f 29f
199f
4t
397t
252t 29f
199f
This page has been reformatted by Knovel to provide easier navigation.
290
Index Terms
Links
V V (viscous) element vane test
44 144
vapor migration
97
vapor-liquid phase change
19
VE (viscoelastic) elements
44
vehicle speed effects
44f
ventilated embankment technology
385
vertical drainage layers vertical soil sounding
388f
10 130
vertical strain determining
235
gauges
165t
in granular base of flexible pavement structure
102f
vertical stresses
42f
VFA (voids filled with asphalt)
281
vibratory drill
142t
virtual weather stations
215
viscoelastic (VE) elements
44
viscous (V) element
44
284
216f
visual reconnaissance
128
voids filled with asphalt (VFA)
281
284
voids in mineral aggregate (VMA)
281
284
voids in total mix (VTM)
281
volumetric mix design
168
volumetric parameters
281
VTM (voids in total mix)
281
302
304t
267
This page has been reformatted by Knovel to provide easier navigation.
Index Terms
Links
W walking profilometer
160
water intake capillary rise
24
frost action
27
infiltration of water from precipitations
26
lateral moisture transfer
25
overview
24
water retention
193
waterproofness
99
wearing resistance of HMA mixtures
79t
well-graded gravel
9
Wenner configuration
129
wheel testers
179t
wheel track test
181t
whiplash curves
21
white-painted surfaces
383
winter maintenance
350
moisture and
36
wire fabrics
343
Y Yukon road network
1
This page has been reformatted by Knovel to provide easier navigation.
