Concrete Materials: Problems and Solutions

Concrete Materials

Copyright 2003 by Taylor & Francis Group. All rights Reserved.

To Evelyn

Copyright 2003 by Taylor & Francis Group. All rights Reserved.

Concrete Materials Problems and solutions

M.Levitt PhD, FIQA, MICT

E & FN SPON An Imprint of Thomson Professional London • Weinheim • New York • Tokyo • Melbourne • Madras

Copyright 2003 by Taylor & Francis Group. All rights Reserved.

Published by E & FN Spon, an imprint of Thomson Professional, 2–6 Boundary Row, London SE1 8HN, UK Thomson Science & Professional, 2–6 Boundary Row, London SE1 8HN, UK Thomson Science & Professional, Pappelallee 3, 69469 Weinheim, Germany Thomson Science & Professional, 115 Fifth Avenue, New York, NY 10003, USA Thomson Science & Professional, ITP-Japan, Kyowa Building, 3F, 2–2–1 Hirakawacho, Chiyoda-ku, Tokyo 102, Japan Thomson Science & Professional, 102 Dodds Street, South Melbourne, Victoria 3205, Australia Thomson Science & Professional, R.Seshadri, 32 Second Main Road, CIT East, Madras 600 035, India First edition 1997 This edition published in the Taylor & Francis e-Library, 2003. © M.Levitt ISBN 0-203-47676-X Master e-book ISBN

ISBN 0-203-78500-2 (Adobe eReader Format) ISBN 0 419 21690 1 (Print Edition) Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library

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Contents

Preface 1 Concrete materials 1.1 OPC, strength gain and sulfate and frost resistance 1.2 OPC and curing 1.3 Aggregates and frost damage 1.4 Air-entraining agents and frost damage 1.5 Alkali-silica reaction 1.6 Calcium chloride 1.7 Aluminous cement 1.8 Steel reinforcement: additional requirements 1.9 Excess steel reinforcement 1.10 GRC and alkali-glass reaction 1.11 Fibre-reinforced sandwich panels 1.12 Delayed ettringite formation 2 Health and safety 2.1 Cement eczema 2.2 Cement burns 2.3 Pumping grout 3 Concrete on site 3.1 Covercrete d or k 3.2 Spacers for rebars 3.3 Tiling and moisture in floors 3.4 Flatness of floors 3.5 Flatness of formwork 3.6 Joints between precast paving and between kerbs 3.7 Tolerances 3.8 Rising damp and a chemical barrier 3.9 Cracking in the non-visual zone

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3.10 Large-area scaling of floors 3.11 Silanes 4 Specification problems 4.1 The CE mark 4.2 Durability 4.3 Concrete quality 4.4 Specifying strength 5 Precast concrete 5.1 Hydration staining 5.2 Lime bloom 5.3 Colour variations 5.4 Cracking and slenderness ratio 5.5 Thermal cracking in pipes 5.6 Tunnel segment impact damage 5.7 Tesserae detachment 6 Testing 6.1 Labcrete or realcrete 6.2 Design or performance 6.3 Camouflage testing 6.4 Repeatability and reproducibility 6.5 Changes in testing 6.6 Testing fixation 6.7 Testing accuracy Glossary

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Preface

In more than 40 years’ experience in the construction industry, I have spent a lot of time in what is commonly known as ‘troubleshooting’: on site with works in progress, on completed projects, and sometimes even on an occupied building or civil engineering project. The advice I gave to the party raising the problem was usually based on materials science. My main aims were to explain the mechanisms that I thought had caused the effect, and to suggest possible remedial measures. With the wide spectrum of problems that I have encountered, I felt that I could make a useful contribution to the concrete industry by setting these experiences down on paper. This book is the result. Each chapter deals with a particular problem area, organised into sections that are generally relevant to that chapter. While I was deciding how best to organise the material, my attention was drawn to the Building Research Establishment’s Defect Action Sheets. These DASs ceased issue in March 1990, but their format—illustrating a problem, dealing with its cause(s), and offering remedial proposals—was very useful. Following this vein, I have described each problem on the basis of personal experience, with sections discussing identification, remedial measures, and avoidance. In most sections these three items form the final paragraphs. However, in problem areas that generated secondary problems, the three items have sometimes been discussed with each of the subproblems. The book reflects personal experience, with a description of the best specific solutions that were found. The hardest part of problem solving is identification, and this is often aggravated by there being more than one damaging mechanism present. Therefore I make no guarantee that the recommendations in this book will always work. A significant degree of overlap between chapters was unavoidable. If a problem is described in a particular chapter, it does not mean that the problem could not or does not occur elsewhere. For example, lime bloom is discussed in Chapter 5 (Precast concrete), but it is also known to occur

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with in-situ concrete. However, it is not as much of a problem as it is for precast concrete products. A basic knowledge and appreciation of aspects of materials science is essential in dealing with these problems. Architectural preferences and engineering matters are not my discipline, and have been largely avoided. Even so, most problems that arose were probably due to a lack of mutual understanding between the various professions involved. The causes of most problems are to be found in design, workmanship or materials. The division between design and workmanship is not very distinct, but together these two account for over 80% of problems. It is unfortunate that materials have received, and continue to receive, too much attention. When I analysed troubleshooting problems recently, I found that materials accounted for only 18% of the total. More often than not, the construction industry has problems with concrete for the apparently simple reason that many procedures are undertaken in ignorance of the requirements of the other professions. In general, the solution is to identify the problem area and put the right questions to the right person(s). Although I have spent 20 years in the precast concrete industry, and a similar time in general construction, the views, recommendations and comments made in this book are mine alone. They do not necessarily reflect the views or policy of the precast concrete trade federations, nor of any companies within the Laing Group. I have used the minimum number of references necessary to support the text. The advice given should stand alone, as being based upon good faith and experience.

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Concrete materials

1

INTRODUCTION Although this chapter and Chapters 3 and 5 are devoted to materials, in nearly every case it was the deployment of these materials that gave rise to problems, and not the materials themselves. The introductions to the remaining chapters are much longer than this one. This chapter covers a large range of materials problems, and each section has its own specific introduction. The introductions to the remaining chapters have a somewhat different function, in that each has the role of colouring in the background to what follows in the individual sections. 1.1 OPC, STRENGTH GAIN AND SULFATE AND FROST RESISTANCE A Concrete Society technical report (Concrete Society, 1987a) compared cement property changes in the years 1960, 1974 and 1983. More recent data has not been published, as far as I know, and I have made some comments based upon individual cement test certificates that have come into my possession. The principal changes have been in the percentages of the two calcium silicates that are the main contributors to strength through their formation of calcium silicate hydrate (CSH) in the cement’s chemical reaction with water. These are tri-calcium silicate (C3S) and di-calcium silicate (C2S). Typical contents of these in the late 1940s were 40% and 35% respectively. Representative figures for 1990s production could be 60% and 15%. These percentages are intended to illustrate medians covering a range of cement plants and in-plant variations; I have a not-so typical cement test certificate in which the C3S and C2S contents are reported as 70% and 7% respectively. Because C3S hydrates much more rapidly than C2S, strength is built up much more quickly, but the 28-day strength is not significantly

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affected. In effect, the shape of the strength-time curve has changed. This more rapid strength gain has had most effect in the in-situ concrete industry, in that formwork can be stripped earlier, but the attraction has been that the usual stripping times have been maintained while using less cement than was the case before cements were so high in C3S. This is the main cause of the problems discussed in this section. The decision to decrease the cement content for an equivalent early strength behaviour ignores the detrimental effect that it can have on resistance to durability risks such as sulfate and frost attack. Decreased cement content promotes more voids in the concrete matrix that would otherwise have been filled up with cement paste. These voids result in increased space within the concrete or mortar for the ingress of sulfates, frost-prone water and/or de-icing salts/chemicals. Changes over the years in the equivalent Na O and alkali-silica reaction 2 are discussed more fully in section 1.5. Cement fineness also increased in the two decades reported from 1960 to 1983 (Concrete Society, 1987a), and this change also added to a more rapid hydration rate but with no significant effect upon the 28-day strength. The effect that this, coupled with the C3S and C2S changes, has had on thermal and moisture curing is outlined in section 1.2. A problem that disappeared with the cessation of precast concrete pipe manufacture by the spinning process was that too fine a cement resulted in too much cement separating at the inside of the bore. Although cement has a higher density than conventional aggregate as well as water for the very fine powders that are spun, segregation is more a function of particle size than of density. Lighting columns are commonly made by the spinning process, but the problem of segregation of cement has not been brought to my attention. The original problem with spinning pipes was solved by the manufacturer by purchasing a cement with a specific surface of about 260m2/kg and sold as ‘coarse-ground Portland cement’. A typical current OPC specific surface is about 330m2/kg. In the following sections, sulfate attack has been discussed singularly, but frost attack has been divided into two groups: direct frost attack with no other considerations, and attack accompanied by de-icing or anti-frost chemicals. Weathering due to the effects of wind and rain alone has also been included. 1.1.1 IDENTIFICATION (a) Sulfate attack A softening and loss of the surface layers of the concrete is due to the formation of calcium alumino-sulfate (ettringite) from the reaction of

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ground or air/rain-borne sulfate with the tri-calcium aluminate (C3A) component of Portland cement. The reaction is expansive, because the ettringite takes up more space than the C3A hydrate with which the sulfate reacted. Delayed ettringite formation is discussed in section 1.12. (b) Frost attack without de-icing salts or anti-frost chemicals The original international research (RILEM, 1977) explained the meaning of critical saturation and specified the relevant tests. In simple terms, when the water content in the water-accessible void/capillary structure reaches a critical level, if frost occurs damage can take place because of ice formation. For example, if the critical saturation of a specific concrete is 80% and its total water-accessible space is 15% by volume (approximately equal to 6% by mass), then frost damage is possible when the water saturation level reaches 0.8×15=12% by volume (about 5% by mass) or more. Frost damage usually manifests itself as surface spalling and/or softening, resulting in exposure of the aggregate. Less commonly, damage occurs not by any apparent surface loss but by a decrease in the elastic modulus, which for concrete used in structural applications could result in deflection beyond that designed. Attempts to look for frost damage in open-textured concrete, such as typical concrete blocks and no-fines concrete, are generally fruitless. This is probably because such concretes have plenty of time to drain out to below critical saturation level before frost occurs. In addition, in very severe winters problem areas of frost damage need be identified only when salts or chemicals have been used. This is because, for ordinary frost damage without salts or chemicals being involved, mild winters tend to cause more problems as more freeze-thaw cycles are involved than during very cold weather. The other area of frost damage identification that can safely be put to rest is that of water in blowholes being subject to freezing. No case of frost damage due to such action has ever been reported; ice in a blowhole has a free face out of which it can expand as water freezes. (c) Frost attack with de-icing salts or anti-frost chemicals Identification is generally in the form of severe surface spalling and a common ‘wet’ appearance. The use of salts or chemicals aggravates and accelerates frost damage for two main reasons and one subsidiary one. First, the use of salts (commonly salt as NaCl) and chemicals such as ethylene glycol or urea depresses the freezing point to below 0°C. The method of broadcasting these materials onto concrete results in concentration variations, which passing traffic can exacerbate. Additional freeze-thaw cycles can take place with no change in external temperature.

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Second, when common salt dissolves into solution the reaction is endothermic, which can cause concrete just above freezing point to freeze for a while as it cools the surface. Third, although ethylene glycol is not deliberately used to de-ice concrete (it is used for spraying onto aircraft) it is an additional risk. Not only can any chemical that drops onto the concrete cause a similar freezing point depression, but the glycol itself has a slow dissolution effect on the calcium salts in the hydrated cement. (d) Weathering This is a dusting and softening of a 0–2mm depth of the surface, which takes place over many years, and is noticeable on unsheltered surfaces. 1.1.2 REMEDIAL Remove all suspect and degraded concrete, and patch or full repair using (preferably) a polymer mortar (Concrete Society, 1984; Perkins, 1986). 1.1.3 AVOIDANCE The cementitious content should be kept in the range 375–450kg/m3. 1.2 OPC AND CURING Hydration of cement is an exothermic reaction: heat is produced, and the more rapid the reaction process the more rapid the heat production. The higher contents of C3S now common in OPC (as well as the form of the more finely ground rapid-hardening Portland cement), together with the higher fineness levels of modern cements, have caused concrete to emit heat more quickly than in previous years. These factors have resulted in two potential problem areas that have given increased cause for concern. The first of these is high thermal gradients, which can give rise to thermal cracking, particularly in thick sections of OPC concrete. This cracking is thought to be caused because the strain set up by the thermal gradient exceeds the strain capacity of the concrete in its hardening state (Harrison, 1992). Thermal cracking can also arise when concrete is cast against a cold substrate such as older concrete. The second risk area is too rapid a loss of moisture at the surface (Birt, 1985). This can give rise to drying shrinkage cracks, and can also result in surface softening, caused respectively by shrinkage from too high a rate of moisture loss and by there being insufficient water in the surface area to hydrate the cement effectively.

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It may be seen, therefore, that the subject of curing problems always needs to be qualified by reference to whether the curing is for thermal or moisture reasons or both. 1.2.1 IDENTIFICATION (a) Thermal cracking This is most noticeable when there is a maximum temperature gradient through the section, which for thick (over 0.5m) plain OPC concrete occurs typically at 40–50 hours. Crack apertures up to 2mm wide are not uncommon. However, as the temperature gradient becomes less steep, these cracks largely close down to apertures of about 0.5mm. These crack positions often coincide with the main rebars, and the cracks penetrate as far as the steel. However, in an in-depth examination of marine-exposed concrete I observed that the crack width tapered rapidly from the surface down to about 10mm deep, where it kept to a consistent value of about 0.1mm. Chloride penetration from exposure to sea water did not occur below this 10mm depth in the crack, and was marginal in all other areas. Thermal cracks occurring at the junction of new and old concrete interfaces (the old concrete being known as a ‘heat sink’) may generally be seen to occur at right angles to the interface and horizontal and parallel further up the ‘wall’. (b) Drying shrinkage cracking This has typical crack apertures in the range 0.1-0.5mm, tapering down to zero at depths of about 10mm. The cracking commonly manifests itself as a series of parallel lines about 100–200mm apart and at right angles to the longest axis of the section. For example, on a precast concrete column, the drying shrinkage cracks would be seen as a number of ‘bracelets’ along the unit. As with crazing in cast stone and other concretes, drying shrinkage cracks can seal themselves by autogenous healing, and this effect is illustrated in Figure 1.1 for the crazing on a cast stone pedestal. 1.2.2 REMEDIAL (a) Thermal cracking A low-viscosity resin can be used (Concrete Society, 1984) if the cracks are considered to be a risk. This can be assessed by estimating the crack depth by means of the ultrasonic pulse velocity (UPV) method (BS 1881 Part 203:1986). This method is practical only if the cracks are nominally free of water and the average crack width over all positions is at least

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Fig. 1.1 Autogenous healing of crazing on cast stone plinth.

0.2mm. The efficacy of the repair system used can be examined using the initial surface absorption test (BS 1881 Part 208:1996). Repaired and unrepaired areas can be compared to eliminate the effect of the absorption characteristics of uncracked concrete. (b) Drying shrinkage cracking These cracks are generally aesthetic defects rather than a corrosion hazard, so as a rule no repair is required. If the effect needs to be masked, then surface grinding followed by a flood application with a silicone (BS 6477:1992) is recommended. As far as the lifetime of this form of remedial measure is concerned, the longest in my experience was shown by cast stone lintels on a school, which were treated in 1958. The lintels were examined in 1996 and found still to be in good condition. 1.2.3 AVOIDANCE (a) Thermal cracking Use formwork with built-in thermal insulation and/or leave thermally insulating covers in place until both the concrete surface temperature and the temperature gradient have reached acceptable levels. As far as is known, no quantitative guidance is available on concrete surface temperature, because much depends on air temperature, wind speed,

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relative humidity and the possible risk of rain. Taking wind speed and wind chill into account, a difference of 10 deg C is probably the maximum acceptable. As far as temperature gradient acceptability in the concrete section is concerned, the recommendations suggested for precast concrete products should be similar to those for in-situ concrete (Levitt, 1982). Recommended targets are a maximum temperature gradient of 0.1 deg C/mm and a maximum cooling rate of 15 deg C/hr. These maxima could probably be relaxed (that is, increased) for lightweight aggregate or limestone aggregate concrete, but might need to be more severe for flint gravel or igneous rock aggregate concretes. As far as is known there is no published method for measuring the surface temperature of concrete. My experience has shown that an ordinary copper-constantan thermocouple spirally wound for its last 30–50mm and stapled to the face of a hand-held cork is suitable. In addition to or in place of using thermally insulating formwork and/ or covering the surface, it is worth considering procedures such as the inclusion in the concrete mix of additives such as PFA (BS 3892 Part 1:1993) or GGBS (BS 6699:1992), or the use of a retarding admixture, which will both modify the exotherm-time curve with less heat being produced in the critical period. (b) Drying shrinkage cracking Good practice in mix design and procedures to inhibit moisture loss (Birt, 1985; Neville, 1995) inhibit or prevent this form of cracking. Typical methods of moisture curing are by covering with soundly fixed polythene, or with hessian kept wet for at least the first 48 hours. These methods of curing should not be used for visual concrete such as cast stone and exposed-aggregate concrete, because they can promote hydration staining, lime bloom and colour variations (see sections 5.1, 5.2 and 5.3 respectively). Figure 1.2 illustrates typical locations in construction of both thermal and drying shrinkage cracking. The necessity for directing attention to curing being required for thermal or moisture reasons or both is repeated. 1.3 AGGREGATES AND FROST DAMAGE Aggregates such as flint gravels, and igneous rocks such as granite and basalt, are generally resistant to frost, but they have been observed to cause pop-outs when de-icing salt has been used (Fig. 1.3). In my experience the occurrence of these pop-outs is a function neither of concrete quality nor of the method of manufacture. It has been

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Fig. 1.2 Typical locations for thermal and drying shrinkage cracks (not to scale). (After C.J.Turton)

observed on in-situ concrete (both air-entrained and non-entrained) as well as on wet-cast vibrated and hydraulically pressed precast concrete units, and for both flint gravel and granite aggregates. The mechanism of frost damage and critical saturation has already been discussed in section 1.1, and it is possible that there are two processes that singly or jointly could exacerbate this mechanism. First, concrete that has been broadcast with de-icing salt will have become more hydrophilic (attracting water) because of the presence of salt in its voids and capillaries. Second, particles of aggregate of flint or granite and the like will have a higher specific heat than the surrounding mortar, and will subsequently respond more quickly to changes in temperature. This will result in these particles being colder nearer the surface of the concrete, and becoming wetter around their peripheries. These higher saturation levels would be due to the moisture in the surrounding area moving towards the particles,

Fig. 1.3 Example of frost-induced pop-out at piece of aggregate.

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which because of their lower temperature have a lower vapour pressure. (This is why, in a refrigerator, food will dry out unless it is encased, because moisture will migrate towards the colder condenser area.) 1.3.1 IDENTIFICATION These are aesthetic defects in the form of pop-outs typically 10–20mm in diameter and 1–3mm deep, with the aggregate particle visible at the bottom of the pop-out. In general the pop-outs occur only at the largest pieces of aggregate. For example, if the aggregate maximum size is 20mm, pop-outs would as a rule be observed at the near-surface 20mm particles. 1.3.2 REMEDIAL This is a difficult matter to address because, provided the presence of popouts can be accepted, no remedial action is necessary. Any attempt at remedial work in the form of a paint or mortar coating could exacerbate the occurrence of pop-outs, because moisture could become trapped or inhibited from evaporation. A possible way of dealing with the aesthetic problem of these pop-outs would be to feature the aggregate exposure rather than try and hide it. This could be achieved by exposing the aggregate over the whole of the visual face, possibly using a flame treatment or another suitable method. The use of de-icing salt can be minimised by ensuring well-maintained drainage. 1.3.3 AVOIDANCE Assuming that de-icing salts are likely to be used, and that efficient drainage (including its maintenance) is unlikely to be installed, it can be advantageous to select a coarse aggregate with a low specific heat, such as limestone or sandstone. An alternative or addition to this precaution would be to silicone (BS 6477:1992) the concrete on site, or to incorporate a waterrepellent admixture such as 1–2% m/m cement of stearic acid powder. If a water-repellent admixture is being considered and another admixture is also planned to be used, their compatibility needs to be assessed before concrete production (see sections 5.2 and 5.3). Some so-called limestones and sandstones are prone to frost attack because they have a significantly high water retention capacity of their own, and a study of their track history is recommended. Flint gravels with a thick cortex surface coating (white, 1–3mm thick typical layer) can be also prone to frost damage. Any such risk with these aggregates can be indicated by a simple test. Take exactly 100 pieces of the largest aggregate

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size to be used, soak them in tap water for about 24 hours, then drain off and place in a polythene bag. Seal the bag and place it in a deep freeze overnight; take the bag out the following morning, thaw, and recount. If there are more than 100 particles then pop-outs can be predicted, and the number above 100 gives an indication of the intensity of the pop-out frost damage. 1.4 AIR-ENTRAINING AGENTS AND FROST DAMAGE This section is solely concerned with the reliance placed upon the rather misconceived idea that having the specified amount of air in the fresh concrete always means that the concrete, in its hardened state, has good or improved frost resistance. I have been involved in several cases of troubleshooting where frost damage, in the form of severe surface spalling, has occurred for concretes with the correctly specified fresh concrete air content. It has been known for over 40 years (Powers and Helmuth, 1953) that the critical characteristics are optimum bubble sizes and spacing factors, and not the total amount of air in the compacted concrete. The fresh concrete air content test (BS 1881/106:1983) does not give any information on bubble size, nor on the bubble-spacing factor. An added disadvantage is that the BS test does not differentiate between entrapped air and entrained air; entrapped air contributes insignificantly to frost protection. These bubble geometries are usually verified by a microscopic method (ASTM, 1990) on slices taken from samples (usually drilled cores) of the hardened concrete. In all the troubleshooting cases that used this US standard to examine the air void structure, the total amount of air was found to comply with the specification, but the recommended bubble sizes and spacing factors did not obtain. Reference can be made to the British Standard (BS 5075 Part 2:1982), both for the recommended bubble geometries and for a freeze-thaw test on samples of ‘labcrete’. Typical diameters of the spherical air-entrained bubbles would lie in the range 0.5–0.05mm and the spacing factor in the range 0.2–0.3mm. The freeze-thaw test in this standard does reflect the bubble geometries, but the test is on a specified laboratory concrete— ‘labcrete’—and not on ‘realcrete’. A European standard (BS prEN 480–11:1996) is in course of preparation that specifies a microscopic test based upon the ASTM standard. This section is solely concerned with the form (geometries) of the entrained air, and is not concerned with workmanship factors. An example of misapplication is described in section 3.10, to which reference may be made when large-area scaling occurs without the occurrence of frost.

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1.4.1 IDENTIFICATION This defect shows itself as surface spalling, typically to a depth of 10–20mm with exposure of the larger particles of aggregate. Sometimes the spalling is in the form of isolated areas, but at other times larger areas can spall. Confusion with the pop-outs discussed in the previous section is unlikely because frost spalling does not, as a rule, result in conical craters as illustrated in Fig. 1.3. Also, the large-area scaling discussed in section 3.10 is unlikely to be blamed, because that scaling is typically in the form of sheets 1–3mm thick and covering areas from 0.1m2 up to complete bay size. 1.4.2 REMEDIAL Remove all degraded material; wash the surface thoroughly, and reface with a polymer mortar or a frost-resistant concrete overlay. Consider complete concrete replacement only if there are doubts about the property of the apparently unaffected material. 1.4.3 AVOIDANCE If an air-entraining admixture is to be used it should comply with BS 5075 Part 2, and should be assessed on the actual concrete to be used in the works, taking into account all the variables that are likely to occur on site. The recommended method of assessment is by either microscopic or freezethaw tests on representative hardened concrete samples. The alternative to using an air-entraining agent is to use a well-mixed and compacted and cured (thermal and/or moisture) concrete with a cementitious content in the range 375–450kg/m3. The cost considerations of the selected path would need to be discussed. 1.5 ALKALI-SILICA REACTION Although the few recorded cases of alkali-silica reaction have generally resulted in severe cracking, leading to some loss of structural integrity, the problem discussed in this section is not this (chemical) reaction. It is, rather, the human reaction of a large cross-section of those involved in construction to the possibility of this chemically expansive reaction causing problems with their concrete. Alkali-silica reaction, or ASR as it is commonly known, is generally a long-term chemical reaction that takes place between the alkali in the cement and other ingredients in the aggregate (Concrete Society, 1987b; Swamy, 1991). Although virtually all siliceous aggregates contain some of this alkali-prone material, it is generally those containing strained quartz

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such as opaline that cause most problems. The reaction that takes place is the formation of sodium and potassium silicates by a slow and expansive reaction of the sodium and potassium hydroxides with the available silica. As there will always be some available silica, whichever aggregate is used, the thing that needs to concern those involved is the question: Is the ASR expansion that will occur going to be a problem? The longevity of so many constructions in the UK without any manifestation of such problems indicates that the general answer to this question is ‘No’. There is a requirement for updating on concrete performance in respect of ASR, because CSTR29 (Concrete Society, 1987a) referred to OPC property changes only by comparing data from three years: 1960, 1974 and 1983. The alkali equivalent of cement from those data, given as Na Oequiv., 2 remained constant at about 0.6%m/m. Not only did these data exclude the period since 1983, but also they did not take into account the properties of the (then and now) cements imported from Greece and Spain. A not untypical analysis of one of these imported cements gave a batch-to-batch Na Oequiv. of 1.2%. 2 Notwithstanding comparisons between UK-manufactured and imported cements in their Na Oequiv. contents, the few cases of damage 2 reported (Concrete Society, 1987a) indicate that ASR is a problem when specific aggregate location sources are likely to be used. Suppression of this reaction is relatively simple, and recommendations are made in section 1.5.3. The few cases reported seem to indicate that cements are not the main problem. 1.5.1 IDENTIFICATION On the few occasions that ASR damage has been observed it takes the form of generally random cracking, with crack apertures up to 10mm. However, the crack widths taper rapidly with depth. Secondary cracking often occurs in the form of map cracking with crack apertures typically up to 1mm. A gel-like exudation similar to ‘egg-glass’ (waterglass) may be seen at the apertures. 1.5.2 REMEDIAL If the problem is with the specification wording, which could be something like ‘Not more than 3kg Na Oequiv. per m3 concrete’, draw the attention of 2 the parties concerned to the low—or zero-risk situation if that concrete has already shown a good track record. If the problem is an actual risk, and damage has occurred, then either removal and replacement of the damaged concrete and/or a protective waterproofing render will generally inhibit further damaging reaction.

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1.5.3 AVOIDANCE If it is definitely known that there is a risk of damaging ASR then this can be inhibited by specifying an additive such as PFA, GGBS or MS in the concrete mix design. It is worth observing that, many decades ago, ships delivering cargo to London from the USA went back with Thames Valley aggregates and cements as ballast, from which much of the currently standing New York Harbour was built. These concretes are still performing well, even though they are subject to wetting and drying conditions and sea water effects. 1.6 CALCIUM CHLORIDE The use of calcium chloride is now generally banned in specifications. The main problem that used to be encountered was that corrosion of the reinforcement was accelerated if water and air penetrated to the rebars. Calcium-chloride-induced corrosion rarely occurs nowadays, but instances still come to light. Calcium chloride admixture accelerates both the setting and the hardening rates of Portland cement concrete. The main attraction of this was to the precast concrete industry, and the use of calcium chloride was concentrated in the 1950s and 1960s. It enabled manufacturers to achieve adequate demoulding strengths at earlier ages and/or a decrease in the cement content. This latter route was often chosen, and led to concrete that contained more voids than would otherwise have been the case, thus giving less protection to the reinforcement. Steel reinforcement has a passive oxide layer on its surface when it is surrounded by a highly alkaline environment such as uncarbonated cement. This high alkalinity can be reduced by a carbonation front reaching the steel or, much more rapidly, by chloride reaching the steel. Once this passive oxide layer breaks down, owing to either or both forms of ingress, the steel becomes subject to corrosion. CP 110, the old (and now superseded) code of practice for structural concrete, used to recommend a maximum of 1.5% anhydrous calcium chloride m/m cement. There also used to be a commercially available OPC into which this level of calcium chloride had been added. The material was sold as ‘417 Cement’. It is possible that if this admixture had been accompanied by a recommendation on the minimum cement content, and this had been adhered to, the status quo might be different. The structural concrete division of one large precast manufacturer used to use calcium chloride regularly in structural concrete units. As far is known, no case of corrosion of reinforcement in any of the company’s units has ever been brought to light. The reason for this probably lay in that company’s strict attention to quality control, and in its use of a high minimum cement content.

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Fig. 1.4 Chloride-accelerated corrosion in precast concrete cladding.

Although it is a dubious thought, because the addition of calcium chloride would accelerate the corrosion risk with mediocre or poorquality concrete, it could be a useful means of accelerating trouble that would otherwise have occurred rather late in the day. The facade shown in Fig. 1.4 was of white cement precast concrete architectural units, which became corrosion damaged at 2–3 years old. The word ‘quality’ glimpsed in the lower right corner of this photograph obviously applied to the cars, but it has been misused as an adjective (see section 4.3) instead of a noun. 1.6.1 IDENTIFICATION This defect is seen as spalling and/or cracking, with cracks commonly mirroring the rebar positions, together with rust staining marks at the crack apertures. When pieces of concrete are broken off to expose rebars, a slimy

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green coating may be seen on the steel surface. This is ferrous chloride, which, within some seconds’ exposure to air, turns into the brown ferric chloride. An additional test for chloride is to rub a finger over the steel and taste it, or even lick a piece of spalled concrete. This will generally give a salty taste. 1.6.2 REMEDIAL Following the Concrete Society’s recommendations (Concrete Society, 1984), effect a repair after removing all degraded and suspect concrete and applying a protective coating to the rebars. Even concrete where there is no visible damage needs to be assessed, as those areas may have corrosion occurring at the steel, and/or the quality of the covercrete might not be acceptable. 1.6.3 AVOIDANCE The answer is simple: do not use calcium chloride admixture. If excess chloride is likely to find its way into the mix, either by deliberate addition or by the use of contaminated materials, then the use of additives is recommended. Where this is to be applied as a remedial action then the cementitious content should lie in the range 375–450kg/m3 and have 30% PFA, 70% GGBS or about 7% MS cement replacement. 1.7 ALUMINOUS CEMENT This is the updated description for what used to be known as ‘high-alumina cement’ or HAC. The problem with this cement is that it is subject to what is known as ‘conversion’. This is an additional chemical change (following the initial hydrates formation) in the calcium alumino-hydrates, promoted under conditions of high humidity and temperature. These later changes generally cause a significant loss in strength (20–50%), and—possibly more importantly—this is coupled with an increase in the permeability. Virtually all aluminous cement concretes were produced as precast units by three main manufacturers specialising in the production of prestressed, pretensioned concrete products. This production was undertaken mainly in the years 1946–1975 but generally ceased at the time of the bans and part bans produced by the Government in the mid-1970s, following the partial collapse of two constructions. The popularity of this cement was probably due to its rapid build-up of strength: for example, 28-day equivalent OPC concrete strengths were obtained after 2–3 days. The setting times of aluminous cement concrete were (and are) not significantly different from those of OPC concretes; only the hardening times differed.

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Many organisations involved in consultancy or testing have carried out examinations and tests on buildings where aluminous cement concrete had been used. The prime purpose of these exercises was to assess the concrete from the structural viewpoint—that is, to assess the effect of any conversion on the strength of the concrete—rather than to study the permeability changes or any other design implications. Troubleshooting work has been undertaken in both the conversion and the permeability areas. In all, about 1000 constructions were examined (for this purpose I have categorised a number of dwellings on a housing estate as a single construction). Over the three decades referred to above about 15 000 000 precast units were manufactured, and these were built into 60000 constructions. The number of constructions would probably lie in the several hundreds of thousands if each dwelling on a housing estate had been separately counted. So a very small proportion of the total number of constructions was examined: about 1.5%. About four constructions out of these estimated 1000 were found wanting because of either loss of structural integrity or degradation of the concrete by chemical action. Four out of 60 000 is a very small proportion of the total number of units manufactured. Personal experience in site examination was directed towards examining samples of concrete in the laboratory, using differential thermal analysis to estimate the degrees of conversion. Most analyses gave a range of conversion levels of 70–90% with no noticeable distress due to structural causes. Webs of T-section precast concrete beams were examined using UPV but, again, problems were seldom found, and, when present, were not at significant levels. The problems found were in two areas: detailing at supports and the previously mentioned chemical attack. This risk has never previously been emphasised in any warnings as far as is known. As an illustration of the first problem area, visits to two schools with the precast units open to view (not hidden by a suspended ceiling or similar) showed that the beam ends sometimes rested on column haunch bed lengths of 1–2mm. This was observed with children and teachers in the class. The classrooms so concerned were immediately evacuated, and remedial work was put in hand. No other undue distress or deflexions of these precast elements were observed. The second problem area was highlighted in concentrating surveys in areas of construction where there were high humidities and warm conditions, such as kitchens, bathrooms and roof areas. It was in roof areas that several cases of chemical degradation were observed. In each case, woodwool slabs were used above the units, and condensation and/or leakage was taking place. It was ascertained that the wood fibres in woodwool slabs were commonly stabilised (to prevent organic growth) by treatment with calcium chloride. Effluent from condensate or leakage

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resulted in chloride attack on concrete made more prone to attack by conversion, causing increased permeability. The attack was mainly due to the formation of calcium alumino-chloride, and the surface had become softened to a depth of about 10mm. Although in some cases chloride was found to have reached the prestressing wires, no problems had occurred. Although, like OPC concrete, aluminous cement concrete is subject to carbonation (BRE, 1981a) this mechanism was not found to have caused a problem. 1.7.1 IDENTIFICATION Problems with aluminous cement can be identified by: (a) insufficient support bed lengths; (b) chloride chemical attack, resulting in surface softening. 1.7.2 REMEDIAL Where (a) applies, stainless steel or equivalent brackets should be fixed to the columns to extend the bedding lengths to an acceptable figure. Shims of stainless steel or similar would probably need to be placed in the gaps and grouted up to give continuity. Any shims used should not give rise to bimetallic corrosion reaction with the brackets. Where (b) applies, carry out remedial work to inhibit or preferably stop condensation and/or leakage. Units may have their degraded surface material removed if required and refaced with a polymer mortar. 1.7.3 AVOIDANCE The best way of avoiding problem (a) is probably to design adequate tolerances and to ensure in both precast and in-situ concrete work that these tolerances are achieved. Concerning (b), it is best to ensure that areas such as roof spaces, kitchens, bathrooms and similar risk zones have their precast units in an adequately ventilated and leak-and condensation-free environment. 1.8 STEEL REINFORCEMENT: ADDITIONAL REQUIREMENTS In addition to the normal structural need for reinforcing steel in concrete, two other problem areas were often met where steel was found to be necessary for other reasons. Although the two instances applied to precast concrete, there could well be cases when a similar need could arise for insitu concrete.

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The first additional requirement was for steel in the compression zone for handling purposes. The problem concerned precast concrete step units, as illustrated in Fig. 1.5. Structural reinforcement had been placed in the top of each step to counteract cantilever forces. What had not been considered was that, although the units were delivered to site in good condition, each unit was lifted manually from the truck and taken to the storage area. The two operatives involved in this lifting exercise caused the units to behave as end-supported beams, and each unit cracked under the bottom shoulder adjoining the boss. Although this particular saga continued in a manner unrelated to this section heading it shows how important it is to pay attention to handling. There were 14 of these units made for a spiral staircase to be prestressed to a ground anchor after installation. After making, scrapping and remaking (with additional bottom of step steel), the units were put in place and prestressed without any packing mortar between the moulded and trowelled faces. The spalling that occurred was the outcome. At the installation of the third remake units with 10mm thick mortar packing between the joints, only 13 could be installed up to the landing level,

Fig. 1.5 Compression-induced spalling in step units.

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leaving a gap in the spiral. As far as is known these units were dismantled and a wooden staircase was installed. The second problem was for a steel requirement in what was thought to be the compression zone. This need was probably because of differential moisture and thermal movement between the top and bottom of the section. The units in question were architecturally faced duct covers, each about 1m square in plan by 80mm thick. The top of each unit was faced with a cast stone finish of about 30mm nominal thickness, and the remainder was made up of an OPC concrete with 10mm maximum size aggregate. Figure 1.6 illustrates the placement of each duct cover over an environment of high humidity and a reasonably constant temperature, with the visual face exposed to the weather. Shrinkage cracking occurred on the top face of virtually all the units, and although these cracks did not approach anywhere near the interface between the two mixes, let alone the steel in the backing concrete, the units were rejected. They were remanufactured with a light stainless steel mesh at about mid-depth in the facing, and no further cracking was reported, as far as is known. 1.8.1 IDENTIFICATION The symptom is cracking in the design compression zone. This cracking tends to exhibit a rather random pattern, generally unrelated to the drying shrinkage and thermal cracking discussed in sections 1.1 and 1.2. 1.8.2 REMEDIAL If the crack apertures remain nominally static, and crack depths are not considered to be a corrosion risk, then consider a grinding to clean off detritus at the crack apertures, followed by a silicone treatment. If there are structural implications because of the cracking, seek the advice of a chartered engineer concerning such actions as replacement, stitching in extra steel, or other solutions.

Fig. 1.6 Visual-faced concrete duct cover unit (not to scale).

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1.8.3 AVOIDANCE When detailing reinforcement requirements, consider the need for additional steel to withstand either handling needs and/or differential thermal/moisture movements. 1.9 EXCESS STEEL REINFORCEMENT Reinforcement is generally put into concrete to cater for its relative weakness in tension compared with compression. The term ‘cross-sectional area’ (CSA) is used to refer to the area of the section under consideration, both for the concrete and for the steel. The ratio of the area of the steel to that of the concrete is the percentage of reinforcement, which for concrete sections such as slabs, beams or columns could typically be 3–5%. Many cases have been encountered where percentages of reinforcement of up to 25% have been used. These have led to problems on site, in the precast concrete factory and in mix design at the preliminary stage. One problem that was examined concerned precast columns in a building (Fig. 1.7). The columns were about 3.5m tall and 0.3m square section in plan. Four 40mm diameter bars, one at each corner with 40mm nominal cover, made up the main reinforcement. The main rebars were lapped by 10mm diameter stirrups at approximately 0.3m centres. The mix was a 20mm/10mm/5mm to dust limestone aggregate with a 450kg/m3 white Portland cement content and a total water/cement ratio of about 0.5 with a

Fig.1.7 Uncorroded rebar-induced spalling in new precast concrete column (not scale).

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slump of 100mm. About three months after installation on site, severe vertical cracking with no steel corrosion occurred in lines with accompanying spalling in lengths up to 0.5m long. This was diagnosed as probably being due to there being too much rebar restraining influence on a concrete with a high initial hydration shrinkage potential. Another example where excess reinforcement affected mix design concerned bifurcated in-situ white concrete columns, where congestion of reinforcing bars at the crossover resulted in there being about 25% steel of the CSA in plan at the throat. The original mix design, using a 20mm aggregate with a 75mm slump, had to be changed to a 10mm aggregate with collapse slump. Fortunately, a Portland limestone aggregate was used, and the 30–60 minute aggregate suction effect on the excess water content gave cube results that built up quite significantly after four days. The specified cube strength was obtained about a week later. 1.9.1 IDENTIFICATION The problems that occur from using excess reinforcement are numerous. The list below highlights those that have been commonly experienced: (a) pieces of tie wire and detritus on the soffit; (b) cracking mirroring the main rebars, without steel corrosion; (c) shrinkage cracking due mainly to the use of too much water and/or too dusty an aggregate in the mix; (d) honeycombing above the steel due to the close-packing of the rebars, allowing fine material sole passage. 1.9.2 REMEDIAL (a) Remove tie wires, detritus etc. from the face of the concrete as soon as possible, and reface cut-out areas with mortar of the same mix as the fine material in the substrate concrete. (b) On the assumption that by the time the problem is observed most of the hydration shrinkage considered responsible for the cracking will have taken place, remove all unsound concrete and repair (Concrete Society, 1984). (c) Repair shrinkage cracking, if considered necessary, as described in section 1.2.2(b). (d) Cut out and replace honeycombed zones with concrete or mortar to give a weathering match. 1.9.3 AVOIDANCE The general way to avoid most of the problems listed is either to ensure that no more reinforcement is put in than is needed and/or to distribute

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the placement of the main bars so as to spread stress shrinkage effects onto the steel more uniformly. In addition, allow as much room as possible for concrete to flow through the reinforcement, and use workability-promoting admixtures and/or additives where possible. Where high-workability mixes are necessary, consider the use of aggregates with a suction ‘Vacuum concrete’ effect. Not only can this assist in extracting some of the excess water, but the increased wetness of the aggregate can also assist in the moisture curing of the concrete. 1.10 GRC AND ALKALI-GLASS REACTION Publications on this subject (Swamy and Barr, 1989; Majumdar and Laws, 1991) have tended to concentrate on the design and use of GRC and composites rather than on materials science considerations. The problem encountered on several sites has been slight surface softening to a depth of less than a millimetre but with exposure of fibre ends, which exhibited a brittle nature. The detriment was solely aesthetic in nature. Concerning the chemistry of cement and glass, this form of degradation was predictable, but a little explanation at a basic level may be useful. When cement, mortar or concrete is splashed or otherwise brought into contact with window glass, etching occurs. This is because the alkali in cement attacks some of the silicates that are used in glass manufacture. The stock used in making glass fibres has better alkali resistance than window glass because zirconia is used as one of the constituents. In order to improve the alkali resistance further, a polymer coating is applied to the fibre as it is drawn from the melt and before it is chopped into strands or rolled into spools. The chopping process can take place at the manufacturing stage, or in the works, or on site during spray application from the spool of glass fibre. Whichever method of cutting is used, the cut ends as well as polymer-bruised fibre sides (bruised during the mixing and/or compaction stages) lose some of the coating protection. There is also a thermodynamic process concerning heats of solution of cement and glass. It means that more heat is required per unit weight to make cement than to make glass. The typical clinkering temperature of cement is about 1200°C, and the melt temperature for glass is about 800°C. When the two materials are put side by side, especially in an environment where chemical reaction can take place, there will be a transfer of heat energy from the cement to the glass in the form of a chemical reaction. It can therefore be predicted that unprotected GRC exposed to the weather, or to other conditions where water or other solutes can contact the surface, will have the fibre ends and ‘bruised’ areas attacked.

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1.10.1 IDENTIFICATION The symptom is exposure of fibre ends on the surface, with accompanying slight dusting to a fraction of a millimetre deep. With the assistance of a magnifying glass it might be also possible to observe some hollowing out of the fibre ends. This would be the residual thin skin of the polymer appearing as a ‘pipe’. 1.10.2 REMEDIAL Rub down with a fine-grade disc, carborundum stone or similar to remove all softened material. Apply a silicate-based or polymer-based paint system to give an approved finish. The silicate systems are preferred as they have a track record of good performance exceeding 100 years, and this thus avoids continuous maintenance apart, possibly, from cleaning. 1.10.3 AVOIDANCE If GRC is likely to be subject to the exposure risk mentioned above, the use of a protective silicate-based or polymer-based paint would greatly improve the performance. The alternative would be to apply a rich (e.g. 1/ 1) fine mortar 1–2mm thick to the risk face before laying up the GRC system. A protective coating, as described above, could also be considered as an additional precaution. 1.11 FIBRE-REINFORCED SANDWICH PANELS Although the problem met on several sites was with GRC sandwich panels the same situation would almost certainly have arisen if the fibres had been of fibrillated polypropylene, steel or carbon or other fibre. The sandwich panels in question were of half to full-storey height, and had inner and outer fibre-reinforced sheets about 8–12mm thick as well as side walls of the same material. The sandwich consisted of an expanded plastics or mineral wool with a thickness of about 100mm. The problem that occurred was of cracks of up to 2mm aperture at the visual face edges. Where moisture had gained ingress into the sandwich this cracking was sometimes accompanied by a lime leachate exudation at the crack apertures. As this cracking had not been reported from the returns or corners for cases of single-skin cladding, it seemed that temperature variations could well have been responsible. Not only was the outer skin exposed to the external temperature and general weathering conditions, but its response time was rapid compared with a monolithic solid typical concrete system.

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The webs and rear skins of the panel would have remained comparatively temperature static at most times, and there would therefore have been considerable thermal differential strain at the face-web junctions. 1.11.1 IDENTIFICATION The symptom is cracking at the face-web return junctions, sometimes accompanied by a lime or carbonated lime leachate from the cracks. Differential movement may also cause the panels to bow in a convex manner. This could be assessed with a straightedge, but should not be confused with any convexity that may have been there when the units were made. 1.11.2 REMEDIAL Although no remedial work was undertaken as far as is known on the sites visited, it is difficult to recommend anything that can cater for such cracking. If, as is likely, the cause is a thermal one, then the cracking is likely to be ‘live’. Therefore, if a crack-filling material is to be used, this should be a low-modulus material capable of moving with the changing crack apertures. The repair should be effected when the cracking is at the maximum opening so that the sealant used will generally be under compression. 1.11.3 AVOIDANCE Wherever possible, avoid the use of sandwich panels and consider using single-skin cladding. If sandwich panels have to be used they could be designed so that the web-face skin junction is hinged rather than rigid. 1.12 DELAYED ETTRINGITE FORMATION The problem encountered was more one of trying to find instances of DEF than of DEF causing distress. Time will tell whether DEF is a real problem; at the time of writing this book there is little or no indication of this. Ettringite is calcium alumino-sulfate, and in cement it is formed by the reaction of sulfate with the calcium aluminate in the cement. Ettringite is deliberately present as a reacted product in Portland cement concrete and mortar because gypsum (calcium sulfate) is added during the clinkergrinding process to control the setting and hardening properties. Without this addition, the speed of hydration would be too great for practical use. This ettringite is harmless, because it is probably in the form of welldispersed microcrystalline particles. For ettringite to cause distress it would

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have to be larger than microcrystalline in size, and would need to form in the hardened concrete. If such a reaction occurred solely at the surface of the concrete the effect would probably be that of surface softening, as discussed in section 1.1.1 (a). The only observed case that could possibly have been attributed to DBF was in the exposure of sections of hydraulically pressed kerbs containing high proportions of high-sulfate PFAs. This observation referred to a longterm laboratory study of units made in precast concrete works. The PFA loadings that caused trouble were 1.5–2.0 times the cement contents, and with sulfate/PFA levels up to 1.8% (Levitt, 1982, section 4.2). After about 10 years’ weathering, with no vehicular access, two of these kerb sections were found to have split cleanly through their mid-sections with no sign of impact. However, white crystalline deposits of up to 10mm in diameter were observed at the broken faces. These deposits were found to be rich in sulfate. The kerbs manufactured by hydraulic pressing were partnered by kerbs made by Kango hammer tamping. None of these hammer-compacted kerb sections exhibited distress on weathering. This could have been due to their weaker and more elastic properties, coupled with the relatively larger number of voids that could have accommodated expansion products. Manufacturers of hydraulically pressed products would be unlikely to use such high PFA loadings, because this would necessitate an unacceptable increase in the pressing time. 1.12.1 IDENTIFICATION If this problem is ever found it is likely to be in the form of severe cracking and/or spalling, with visible ettringite-rich white crystalline deposits at the broken face. This is a prediction based upon a single observation in laboratory samples and not from a site. 1.12.2 REMEDIAL Not enough experience is available to address this point. If damage were to occur on site in similar vein to that observed on the hydraulically pressed kerb samples (assuming that it was due to DBF) then it is likely that major remedial work or replacement would be necessary. 1.12.3 AVOIDANCE Until actual site problems become documented and the mechanism is understood and accepted it is not possible to take avoiding action. As far as is known at present, there does not appear to be anything to avoid.

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Health and safety

2

INTRODUCTION The three examples in this short chapter reflect personal experience in the industry coupled with involvement in litigation work. Health and safety matters such as protection from falling objects, safety clothing, and the use of mechanical and electrical machines are well documented in company procedures, and are not covered here. The three risk areas discussed here do not seem to have attracted the attention that they should. They should interest not only safety officers but also site managers, operatives and personnel officers. Because the first two sections refer to cement chemistry, it may be helpful to put cement into perspective by comparing it with another chemical that is commonly found on site and in the precast works: concentrated hydrochloric acid. Not only is hydrochloric acid a simple two-element chemical, but it is one of the few acids that is a reducing agent and not an oxidising agent like nitric or sulfuric acid. Hydrochloric acid’s fuming property, its pungent smell and (usually) delivery in glass carboys, coupled with its ability to etch concrete with accompanying bubbling, cause it to be respected. However, because human skin is generally acidic (except for the eyes), and because that acid is dilute hydrochloric, then provided the skin has no lesions, spillage of fuming hydrochloric acid onto the skin causes little harm. It is not a caustic oxidising acid: it does not attack flesh. Compare cement, which arrives as a dry powder, and commands very little respect. However, it should be treated with a level of safety consciousness that makes hydrochloric acid pale into insignificance. There are two reasons for this, and they are both based upon the fact that water is to be added to cement, which is a multicomponent chemical. First, one of these components is water-soluble hexavalent chromium. Second, caustic alkali is released. These two risk areas are discussed in the first two sections.

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2.1 CEMENT ECZEMA Chromium (chemical symbol Cr) is a minor ingredient in OPC. It is present at a typical concentration of about 6mg/kg or 6ppm (parts per million). As far as is known, this figure represents the total Cr and not the water-soluble part. I have encountered only a few cases of skin problems, and so it is reasonable to assume that UK cements have low contents of soluble chromate. Chromium does not occur in the form of a heavy metal but as a chromate salt. When this salt is present in its hexavalent Cr form it is watersoluble, and it is in this form that—for some personnel-eczema, dermatitis or, more rarely, skin cancer can occur. There is nothing new in the knowledge of this risk factor; it was first reported over 30 years ago (Burrows and Calnan, 1965). In Denmark, research has been undertaken more recently into ways of inhibiting this risk (Aunstorp, 1989a, b, c). Ferrous sulfate was added to Danish cement, and the effects on operatives before and after this addition were reported. Danish legislation then followed, with a limit on the maximum amount of soluble chromate salt permitted in Portland cements, expressed as ppm of Cr. Aunstorp found that the allergic reaction to the chromate in the cement was a much more significant factor than attrition brought about, for instance, by fresh concrete or mortar being rubbed onto the skin. The Danish research was based upon a real situation, and was not a laboratory assessment. Interestingly, the study found inter alia that the use of protective creams before starting work with concrete did not ease the irritant effect of contact with wet cement. The wearing of protective gloves had only a marginal effect in counteracting any allergy or in inhibiting irritation. Irritation, in all cases, would be the first sign of a skin reaction. The last examination of the Danish cement specification revealed that the maximum limit for the soluble chromate salt, as Cr, was 2ppm. Publications by the Health and Safety Commission/Executive (HSC, 1988a, 1988b; HSE, 1988) drew, seemingly, rather marginal attention to these risks. However, at the time of writing it seems that an updated document is due for publication. 2.1.1 IDENTIFICATION The symptoms of cement eczema are irritation of the contact area, probably accompanied by discoloration and a blotchy appearance. If the effect of cement burns is occurring at the same time (see section 2.2), skin irritation is unlikely to be felt because of the damage to nerve ends. 2.1.2 REMEDIAL Wash affected areas immediately with copious quantities of water, and remove cement-stained clothing. Seek medical advice as soon as possible

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after this, and ensure that there will be no future work likely to result in contact with fresh concrete, mortar or grout. 2.1.3 AVOIDANCE Most people are relatively insensitive to the risk of cement eczema, and so it is advisable to question all personnel at the pre-employment or deployment stages. Has the person and/or any member of that person’s family ever been prone to allergic skin reactions or complaints? The 1965 study reported earlier indicated that hereditary factors can play a role in a person’s proneness to a reaction. For adequate documentation, the posing of these questions and the answers received need to be accurately recorded. As far as protective handwear is concerned, only waterproof gloves known to be unreactive to chromates should be used. Handcreams only prevent the skin from drying out; they do not offer resistance to the chromate salt. 2.2 CEMENT BURNS Throughout this section the term ‘cement burns’ has been used. In USA litigation reports and published articles the term ‘concrete burns’ is generally used. It is the chemistry of the wet cement that causes burns, and so ‘cement burns’ is probably the more explicit term. Abrasion by the aggregate and/or cement on the skin exacerbates the caustic chemical mechanism involving necrosis that is considered to be responsible. The UK problems I have encountered have generally been where operatives have been kneeling and carrying out floor-topping work. Other cases, such as concrete getting inside a Wellington boot, have been illustrated in the UK press, and these are identified later. Severe injury was suffered in all cases, resulting in the inability of the persons concerned to carry out further manual work. Continuous, permanent pain and unsightly skin grafts were also common. The incidence of cement burns was recorded nearly six decades ago in an American medical journal (Meherin and Schomaker, 1939), but there appears to have been a dearth of reports in the literature for more than two decades thereafter. Rowe (1962) described cement burns as ‘unusual’. Many cases of litigation took place in the USA, with Erlin, Hime Associates, a consultancy practice in Illinois, not only acting for claimants but also building up a useful dossier of case histories and references to relevant US legislation. At the beginning of my involvement as an expert witness acting for claimants in a county court case, a close liaison was set up with Erlin, Hime Associates. Their earlier 1980s publication (Hime and Erlin, 1982) had a significant impact upon both US and UK litigation. This US

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publication did not pursue a detailed mechanism of the form that is proposed later. It will be argued that there is a simple theory capable of explaining the cement burn mechanism and of showing why the risk is far more serious than previously considered. Although section 2.1 refers to the possible issue of an updated Health and Safety Executive publication, the two information sheets published nearly a decade ago appear at present to be the sole UK guidance (HSC, 1988a, 1988b). The references therein, together with warnings in documents such as delivery notes and safety instructions, will be highlighted later in the proposed theory as not being detailed enough. One of these publications contains a photograph of a cement burn to the knee of an operative. Three further publications appeared a few years later (Anon, 1993a, b, c). No reference can be made to personal case history experience here as one of the cases was settled out of court and another one is current as at early 1997. It is not known how many cases of cement burns have occurred in the UK without being reported in the press. The pressures being placed upon contractors and subcontractors could have resulted in precautions being sacrificed for the sake of speed and/or economy. However, the apparently low incidence of cement burns in the UK implies that a high quality of care is being exercised on site, in the works and plants. In 1988 Hime and Erlin reported a study where research by others was referenced, without much detail. This stated that trouser material acts as a chemical buffer and increases the alkalinity at the skin, with an increase in pH from about 12.8 to nearly 14. It was not understood how material fabric could cause this, and I have conducted laboratory tests using fresh cement mortar on one side of various fabrics such as worsted, linen, denim and nylon. In all cases the pH was found to be about 12.5 on both sides of all materials tested, with no significant gradients. It is therefore logical to turn to what is known about personnel suffering cement burns on site. Two factors seem to be necessary. First, the spillage needs to be static and in contact with the skin for at least half an hour. Second, the skin where the spillage has taken place needs to be on a relatively warm part of the body. Cement burns do not appear to occur when the operative’s hands are in and out of concrete. These two conditions need to be correlated with the known cause of cement burns, which is necrosis, or in simple terms a caustic burning effect on skin, nerves and muscle. The most critical of these three effects is the destruction of nerve ends, because this dulls any feeling of burning or irritation. This is probably why operatives carry on working after spillage, not knowing that flesh burning is continuing. The 1993 Construction News article (Anon, 1993c) describing spillage into a Wellington boot is an example of this. The presence of alkali in the forms of sodium and potassium hydroxides was mentioned in section 1.5 and expressed as Na Oequiv. A typical level 2

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for OPC would be about 0.6%. This is equivalent to approximately 0.4% as sodium hydroxide. This is known as caustic (i.e. it burns flesh) soda. Both the alkaline lime and the caustic soda contribute to the alkalinity of cement paste. What is important is that lime is alkaline without being an alkali, but caustic soda is both. Furthermore, lime is only slightly soluble in water, whereas the alkalis are very soluble. Consider now a realistic scenario: an operative kneeling on fresh concrete so that the chemicals in 1kg of concrete can access the skin. If the cement content is say 350kg/m3, then that 1kg of concrete will contain about 150g of cement, assuming a fresh wet concrete density of 2350kg/ m3. At a 0.4% equivalent caustic soda level the operative is in contact with 0.6g of one of the most caustic chemicals known. Its uniform distribution throughout the contact area explains why body heat and time are jointly required. As body heat causes the water in the concrete to evaporate, the effective concentrations of both lime and the alkalis increase. Lime, being only fractionally soluble in the moisture on the skin, will not cause much distress. The approximate half gram of caustic alkali would be responsible for the necrosis that occurs. This explains why the combined conditions of both warmth and time are necessary, and why a continuous replenishment and/or renewal of fresh concrete with the skin does not apparently cause the same distress. The alkalis do not have the chance to become concentrated by evaporation of the water. In addition, it may be predicted that the abrasion effect of contact with aggregate will have an exacerbating effect on flesh already undergoing necrosis. 2.2.1 IDENTIFICATION The symptoms are soreness experienced after some hours, followed, usually the following day, by severe pain and ulceration, with a flesh colour varying from green to purple and, generally, an accompanying discharge. 2.2.2 REMEDIAL It is possible to ameliorate the situation slightly by immediate washing of the affected area with copious quantities of water, coupled by the removal of all affected clothing. This clothing should be washed or discarded. The US articles referred to earlier advise not to cover the area nor to apply any form of dressing. Immediate hospitalisation is advised with, preferably, a department qualified to deal with cement burns. The patient’s contact with cement and caustic alkalis needs to be mentioned to the hospital staff.

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2.2.3 AVOIDANCE Protective waterproof (the latter word being commonly omitted in safety guidance) clothing should always be worn. If operatives have to kneel on fresh cement then special knee covers or string-held cut-outs of car tyres should be used. Any cement ingress behind these should be treated as in section 2.2.2. 2.3 PUMPING GROUT Typical uses of grout pumps are for operations such as filling the annuli between prestressing post-tensioning strands/wires and the duct tubes, and for filling gaps under large machine baseplates. It was in the former application that an operative suffered injury to an eye that nearly resulted in blindness of that eye. A blockage occurred in the tube line to the prestressed unit, and a coupling was loosened to open up the tube. Admittedly the person concerned should have been wearing eye protection, but he assumed that because the machine had been turned into the recirculation mode, pressure in the feed line had been relieved. He did not appreciate that, with the particular machine in use, the pressure had not been taken off the feed line. The pump supplier, when advised of this mishap, informed the project manager that there was a third position for the control tap. In addition to the ‘12 o’clock’ recirculation and the ‘3 o’clock’ line pressure positions, there was another one between 1 and 2 o’clock that took the pressure off the feed line. It was admitted by the pump supplier that this omission in the instructions for use was an error, and that it would be rectified. Figure 2.1 shows a simple schematic outline of the system.

Fig. 2.1 Grout pump system (not to scale).

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2.3.1 IDENTIFICATION This potential problem can be identified as non-existent or inadequate supplier’s guidance on how to deal with blockages in the system. 2.3.2 REMEDIAL Ask the supplier for written instructions on how to deal with pressure line blockages. If a coupling in the pressure line has to be undone because of a blockage, and no instructions are available, run the pump with the tap in approximately the midway position between the recirculation and pressurising positions. Ensure that the operatives wear full eye and hand protection at all times, and that there is a convenient optical douche nearby. 2.3.3 AVOIDANCE Use grout pumps only where there are distinct instructions on how to relieve pressure in the feed line should a blockage occur. Wear only approved eye and hand protection, and ensure that there is a convenient optical douche station.

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Concrete on site

3

INTRODUCTION Most of the main problems encountered with in-situ concrete are grouped in this chapter, but inevitably there is some overlap with Chapters 1 and 5. It will be seen in Chapter 5 that the many instances of troubleshooting involving precast concrete products could have also applied to in-situ concrete. However, materials faults were seldom involved; the problems generally related to design and/or workmanship. It is easy to blame the material, because it is usually covered by a stringent specification. Design and workmanship are dealt with by codes of practice, which are state-of-the-art documents, and can be interpreted in different ways. They also relate to a service rather than a product or material, and are therefore difficult to define. When using materials on site there is a resulting tendency to concentrate unfairly on the standardised material ‘shall’ wording, and to use or sometimes misuse the guidance Codes ‘should’ clauses. As referred to in the Preface, less than 18% of all troubleshooting cases encountered were attributable solely to the material. This is not only my experience, but also applies to many decades of Laing library problem analyses. It leads inevitably to the conclusion that there is a misdirection of effort in construction quality control. The main focus of testing should be on the construction rather than on the material or product. Why is there a concentration on specifications to resolve less than 18% of the problems? Presumably because it is fairly easy to specify materials, but difficult to specify or control design and workmanship. Whether this situation will change in the future remains to be seen. 3.1 COVERCRETE d OR k Although covercrete d and k are defined in the Glossary, some qualification may be useful. Concentration on the depth of cover, d, is universal; there is

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little or no interest in the quality, k, of the concrete in that cover. These variables are generally recorded as d, the depth of cover in mm, and the lesser-known k, the permeability, in cm/s. The problem encountered on site was due to non-compliance with a d specification. This had secondary repercussions. First, it was not clear whether the specification requirements were for ‘minimum’, ‘nominal’ or ‘actual’ cover. Second, where cover to the main steel was of concern, little account seemed to have been taken of cover to the stirrups. As may be seen in Fig. 3.1, this cover is reduced by at least that of the stirrup steel diameter. In general, no consideration was given to the quality of the allegedly reduced cover area(s). Where this quality was considered to be good, attempts were made (sometimes with success) to achieve a deemed-tosatisfy situation with no other work being carried out. Let us consider each of these variables, d and k, separately. 3.1.1 COVERCRETE d Why have a d in the specification? The simplest answer might be ‘to protect the steel from corrosion’. Unfortunately, this reasoning seems tacitly to admit an inability to qualify the answer. What is not apparently designed is whether d maintains its protective property over the planned lifetime, or whether it gradually loses it. From site experience, it seems that the underlying philosophy of a d specification is ‘defer the fatal date’. In effect, we could well assume that d should be maximised, because the cover will lose its protectiveness with time. However, there are drawbacks to having too much cover: • •

fewer cracks, but each with an enlarged crack aperture; less restraint against shrinkage cracks (see section 1.8).

Fig. 3.1 Difference between strirpup and main rebar cover (all dimesions in mm).

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The loss of protectiveness of d is generally thought to be due to the advance of a carbonation front. If chlorides and/or aggressive de-icing or anti-frost chemicals are present, this loss of protection may be far more rapid. Carbonation is a reaction between carbon dioxide in the atmosphere and the alkaline hydroxides of hydrated cement. The pH value of the matrix in concrete unaffected by external elements is of the order of 12.5, and at any pH value above approximately 9 the rebars will retain a protective passive iron oxide film on their surfaces. This protection is lost at slightly alkaline pH values of 7–9, and much more rapidly under acidic conditions and/or in the presence of chlorides; corrosion can ensue. Note the word ‘can’: corrosion of steel in or at the carbonated zone front needs both moisture and air to be present. The modern design philosophy is possibly to specify d as though effectiveness would be lost according to a rule of thumb such as 1mm gives 1 year. However, there does appear to be an underlying assumption that the cementitious part of the specification will ensure that adequate effective d is left at the end of the planned lifetime. This implies that considerable reliance is actually being placed on k. However, k is still generally placed in a secondary role to d, and it could be argued that, for corrosion durability of reinforced and prestressed concrete, these roles should be reversed. It can be misleading to predict a linear relationship between the speed of advance of carbonation and time. For instance, if 40mm becomes carbonated after 40 years it would be useful to say that 50mm would fare likewise at 50 years. However, in general, the rate of advance of the carbonation front follows the square root of time (BRE, 1981b). If this is applied to the above example then 50mm would become carbonated within 60 and not 50 years. Another problem encountered on site was a one-off, but it is a risk area that is easy to overlook. For carbonation of cement to take place, moisture and air need to be present. If the void/pore structure of the concrete is too dry then the reaction cannot proceed. At the other extreme, if the void structure contains enough water to inhibit or stop the passage of the carbondioxide-containing air then, similarly, carbonation is inhibited or cannot occur. Carbonation tends to proceed most rapidly when the relative humidity of the void structure is about 75% (BRE, 1981b). This could well mean that the reinforcement at the back of concrete exposed to a cavity situation could be more at risk than the rebars nearer the visual face. This was found to be the case when the rear faces of precast panels were exposed for examination by removal of the insulation and partition boards. It is not known what guidance is given in codes concerning this, but the risk factor has been taken into account in the latest revision of the cast stone standard (BS 1217:1997), which gives mandatory requirements for

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cover to all exposed faces. The term ‘exposed’ is defined as unprotected by any mortar or similar material, and so applies to visual and cavity faces. It is probable that the only instance when d is of importance is in fire resistance, where the purpose of having cover is to keep the steel sufficiently ‘cool’ to avoid loss of strength caused by a phase change in the metal. 3.1.2 COVERCRETE k It is commonplace to specify minimum cement or cementitious contents; there seems to be a growing tendency to control k with little knowledge of what it actually is. At the same time, it could well be an admission that there will be variations in the actual steel covers being achieved, but that even when these are at their minima the concrete left as covercrete will still give good performance over the planned lifetime. The main problem with k is that knowledge already exists on how to measure permeability, but it takes too long (at least 3 months) before useful test data emerge. It can be argued that d can be easily confirmed by covermeter tests or similar, but there are errors involved in such testing (as with any testing). More importantly, if the concrete has hardened, what action can be taken to deal with non-complying areas? A strict mix design specification, which can be easily verified by supervision, can generally give compliance with a k requirement. This applies whether the k design is for general weathering durability requirements or for protection when chlorides are present. Long-term research can establish how the permeability characteristics of concrete can be controlled under a variety of durability hazards. It thus follows that a contractually viable solution to producing the optimum k for any risk situation lies in the area of mix design. 3.1.3 IDENTIFICATION The problem is manifested as non-compliance with minimum cover requirements (assuming that the parties concerned are agreed on what is meant by ‘minimum cover’) coupled with the covercrete being potentially good enough to give adequate performance: in effect, a low d but accompanied by a low k. 3.1.4 REMEDIAL Illustrate by such means as the history performance of similar concrete and/or by agreed testing of the covercrete that a deemed-to-satisfy situation exists. Proposed action, such as protective coatings or other surface treatments, may have an apparently remedial effect, but it is almost certain to be a waste of resources if the k value is too low.

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If the quality of the covercrete is mediocre or poor, but that concrete has to be retained, remedial measures such as polymer-mortar-based rendering or siliconing might need to be considered. 3.1.5 AVOIDANCE For carbonation, chloride attack or fire resistance a logical appreciation of both d and k would seem to be essential. The carbonation advance with the square root of time, mentioned above, is a general rule, which appears to apply to concretes whose cementitious content is below about 375kg/m3. Above this content, carbonation is generally a few millimetres deep after many years. Therefore, if carbonation is the sole risk, the solution seems to be a minimum cementitious content of the order of 375kg/m3. When chlorides and other corrosion-promoting chemicals are additional (to carbonation) risks, then the mix to be used needs strict specification and control. For example, in marine or de-icing salt exposure, the suggested 375kg/m3 minimum cementitious content needs to be supplemented by specifying that the cementitious content have 30% PFA, 70% GGBS or 7% MS cement replacement. If fire resistance is the only requirement then d is almost certainly the only variable that needs to be specified. Design factors such as secondary meshtype ‘holding’ reinforcement in the covercrete zone may also need to be considered. The most difficult recommendation to make for carbonation and corrosion-promoting chemical hazards is what the minimum value of d and the maximum for k should be. For the present, assuming that the minimum cementitious content has been achieved and that it has the right ingredients, then a few millimetres of cover seem to be all that is required. However, in order to allow for workmanship and the risk of ingress of aggressive material at the interfaces between mortar and aggregate facets, the minimum cover should logically be twice the maximum aggregate size. This would cater for there being a rebar-aggregate-aggregate path to the exposed face. The definition of ‘minimum cover’ still remains to be established. This possibly needs to be resolved at Eurocode level, but for a single meaning the wording ‘actual minimum cover to any steel’ might suffice. This form of wording does not have any obvious interpretative modes. 3.2 SPACERS FOR REBARS Six problems have been experienced with both in-situ and precast concrete; these have been identified separately below as (a)–(f). None of these has been found to predominate over the other five. Only example (d) referred to spacers made from mortar; all the others were experienced with the commonly used plastics spacers.

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(a) The use of trestle-type spacers on vertical and other non-horizontal rebars instead of wheel-type spacers. Figure 3.2 illustrates the difference between these two main types of spacer. It can be easily seen that, under the action of filling with concrete and/or compaction, a trestle-type spacer placed onto a vertical bar can rotate and lose some (or, when dislodged, all) of the intended cover design. (b) Because of overloading, spacer feet either punched into relatively soft formwork/mould material or, when the concrete was formed against steel or similar surfaces, distorted spacer ‘feet’ sprung on stripping. In both cases, spacer feet protruded from the surface of the concrete, but in the latter case this was sometimes accompanied by minute spalling. In the former case (relatively soft formwork), the holes left in this read as ‘pimples’ in subsequent form uses unless the holes were made good before each reuse. (c) Following aggregate exposure processes, such as grit-blasting, spacer feet were observed to protrude; the blasting had little or no effect on the plastics material. This resistance was predictable, because plastics, and polythene in particular (the common base material for spacers), have good energy-absorption characteristics. Thus concrete will wear away more quickly than polythene, because it does not possess good energy absorption. (d) Mortar spacers either too impermeable or too permeable. In the first case (too impermeable), poor bonding occurred between the fresh concrete and the spacer mortar. This resulted in a gap at the interface, allowing moisture and air to enter and corrosion to take place. In the second case (too permeable) the bond was generally found to be good, but the spacer itself allowed moisture and air penetration, with ensuing rebar corrosion. (e) Because of an inadequately pierced cross-section, and under the action of fire, the plastics material was easily degraded, and the rebar at the position where the spacer had been became too hot.

Fig. 3.2 Trestle-type and wheel-type plastics rebar spacers.

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(f) As (e) but, under the action of normal weathering, spalling occurred at spacer positions with no steel corrosion but with exposure of the rebar to the elements. This problem was commonly observed during the first few months of site exposure in hot weather. The mechanism considered to be responsible was the differential in thermal expansions and responses between the polythene in the spacers and the surrounding concrete. Polythene has about 16 times the thermal expansion coefficient of a typical concrete. A well-pierced spacer section would have had interwoven concrete, causing it to act compositely instead of individually. All these examples have been described at length (Levitt and Herbert, 1970), and Concrete Society (1989) touches on (b) and matters concerned more with design than with troubleshooting. Further references together with photographs are given in the author’s book on precast concrete (Levitt, 1982). 3.2.1 IDENTIFICATION (a) Reinforcement cage dislodged with the spacer in the wrong configuration or lying loose on the soffit. Detection of this problem could be by a simple visual inspection or, more commonly, by means of a covermeter survey. (b) Spacer feet protruding from the concrete’s surface, with holes in relatively soft formwork and minute spalling possible for concrete formed against steel or similar hard, unyielding material. (c) Unsightly protrusion of spacer feet or bases in abrasion-blasted finishes. (d) Corrosion rusting, either in lines at the interface between spacer and concrete boundaries or fairly uniformly over the spacer base. (e) A blackened hole of easily removed carbon, giving direct access to the steel. If the steel has suffered a phase change because of the heat, additional deflection may also have occurred. (f) An approximately conical-shaped spall, exposing the spacer base and, generally, the rebar; the rebar has no corrosion on it at the time that spalling took place. 3.2.2 REMEDIAL (a) Treat as in section 3.1.4 in attempting to achieve a reduced-cover deemed-to-satisfy agreement, or undertake remedial work, such as applying extra cover (Concrete Society, 1984). (b) Knife-cut protruding spacer feet and make good holes in the formwork. The use of heat to remove these protrusions is not recommended because carbon staining could occur, and it is also possible that

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(c) (d) (e)

(f)

aggregate being calcined could spit out of the surface, constituting a safety risk. Remedy as (b). Cut out mortar spacer down to the rebar and repair with suitable polymer mortar (Concrete Society, 1984). Suspected structural damage due to fire having gained direct access to the steel through the passage where the spacer used to be should be referred to a chartered civil engineer for advice before any decision is made concerning remedial work. Remove as much of the plastics spacer as possible, as well as spalled and unsound concrete, and repair (Concrete Society, 1984).

3.2.3 AVOIDANCE (a) Use only trestle-type spacers on horizontal bars in contact with the base of the form or mould. Wheel-type spacers can be used in all applications, bearing in mind that some types are not strong enough for use on the bottom steel. Wheel-type spacers can be prevented from slipping down vertical and other non-horizontal rebars by holding each spacer in place with an elastic band, which is then left in the concrete. (b) Ensure that the load distribution on spacers is such that the tendency to punch into formwork is minimised. At the same time ensure that not too many spacers are used, because they can encourage planes of possible weakness in the concrete. (c) Avoid using plastics spacers for exposed-aggregate finishes. Mortar spacers are preferred with spacers having aggregate exposure by the same process planned for the concrete. (d) Mortar spacers should be made with a ‘sand’/cement ratio between 1.5 and 2.0, preferably of the same fine aggregate (‘sand’) planned for the concrete. Curing of the spacers under damp conditions for at least the first 48 hours is recommended. (e), (f) Ensure that the seating area of both trestle-type and wheel-type spacers is pierced to at least 25% of its section. This permits concrete, under the action of compaction, to interweave with the plastics and cause each spacer to act compositely rather than as an individual piece of material. This caters both for the thermal expansion differential and for degradation under the action of fire. 3.3 TILING AND MOISTURE IN FLOORS Concrete construction processes involve the use of water, and as a rule more water is used than is necessary to hydrate the cement. The result of concrete’s losing or trying to lose this excess water has been problems in

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the application of floor coverings such as PVC tiles and sheeting, rubber tiles and synthetic-backed carpet tiles. The problem has been generated from an advisory clause in a particular code of practice (BS 8203:1996). This recommends that, when the surface relative humidity (RH) is measured by a specified means, tiling should not be laid until the RH drops below 75%. Putting aside the validity or otherwise of this recommendation, this 75% is commonly invoked as a contractual mandatory clause. The problem arises because the waiting time necessary to reach this maximum is generally impracticable on site. On the rare occasions when such delay can be countenanced, adhesion and/or floor covering failures are rare. However, it is not definitive that 75% is the right number, nor that it is being measured the correct way, nor that the property of RH is the right one to address, nor that efforts should be directed towards the suitable adhesive for different conditions of ‘dampness’ of the floor. Another area of difficulty was that of the actual failures, and the five or more causes that could have been responsible, either singly or jointly: (a) (b) (c) (d) (e)

moisture in the concrete softening the adhesive; an alkaline concrete-adhesive reaction; moisture pressure gradient pushing the coverings upwards; inability of the adhesive to stick to the concrete; temperature gradient across the thickness of the slab, causing mois ture to be driven to the colder lower vapour pressure face, and pushing the covering off of the surface.

The mechanism of (e) is shown in Fig. 3.3. The main problem with the 75% RH recommendation is contractual, and lies within the remits of the main and specialist flooring subcontractors. Because they have to work to stringent time requirements, usually derived

Fig. 3.3 Tiling on ground and supended floors.

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from the site schedule, they tend to withhold any form of guarantee because it is virtually impossible to achieve that maximum. The rule of thumb commonly applied to predict how long a typical concrete slab will take to dry to a moisture condition complying with the 75% maximum is to wait one week for every millimetre thickness of slab. This implies that a wait of two years would be required for a 100mm thick slab. Contractually, this is unacceptable. Clearly, a host of problems arise from this code and the way it is applied, not least the five listed above. It can also be argued that, irrespective of the value of any specification number, there is a tendency to cling to that number just because there is a number there. This is discussed in depth in section 6.6. As far as the causes listed under (a)–(e) above are concerned, a project scheduled to last about 30 months began at the end of 1996. It is jointly sponsored by the Department of the Environment under its Partners in Technology (PIT) Scheme and by the Concrete Society, which acts as the main contractor. It is the aim that a technical report (a CSTR) will be published by the Society in 1999, and that the findings will be aimed at making clauses in BS 8203 and other relevant codes more realistic. The author is the convenor of the working party that is controlling the work. Feedback is essential, and the Society will possibly have one or more launches of the CSTR in a preliminary draft form for discussion at seminars and workshops. The project working party reports to a more widely constituted steering group; no matter how wide these representations are, there will possibly be points to report that have not been covered. Some of these points will now be discussed in terms of experience by others; this may well trigger additional feedback. The role of chemical reactions as the cause of adhesion failures has been mentioned earlier, but some reactions also lead to the emission of rather unpleasant gases. Reports on this subject would be useful to the Concrete Society, together with any related research work by organic chemists. Another matter of interest is the growing use of plasticisers and superplasticisers, both in the concrete and in the screeding and levelling materials laid on top. The main advantage of these admixtures is that, for a given workability, they permit large reductions in the total water/cement ratio. This in turn reduces the excess water waiting to dry out, so that the 75% should be reached more quickly. In general, the use of these admixtures in concrete does not seem to have caused any problems. However, the admixtures for some screeds and underlayment levelling compounds are polymer based. Although these will have very low water contents, and hence a lower initial RH, they can also have low permeabilities to the passage of moisture, with a consequently reduced loss rate. Other than by using a vacuum concrete process, methods of trying to eliminate excess water have not proven fruitful. The use of hot air blowers

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has been found to have no beneficial effect on drying rates. The process drives the water from the surface into the body of the slab but, within as short a period as one day after turning the heating off, the original RH is reinstated as the temperature becomes reasonably uniform in the slab. Moisture moves towards the colder face because of the vapour pressure gradient. Knowledge of this mechanism could be useful both in laying floor coverings and in heating buildings. The aim is to keep moisture as far away as possible from the face to be covered, so consideration could be given to tiling and heating the top floor of a building first and working down to the ground floor as the last to be covered. 3.3.1 IDENTIFICATION This is the discovery that a 75% RH limit has been specified for laying floor coverings, but that this cannot be met within the site limits as laid down in a bar chart. 3.3.2 REMEDIAL The only possible avenues that may be worth pursuing are either to get the specifier to accept the risk situation, or possibly to allow a variation resulting in the use of a more expensive adhesive that can tolerate high levels of water in the floor. 3.3.3 AVOIDANCE The use of plasticising or superplasticising admixtures in the concrete is worth examining as an interim measure and, possibly, as a long-term one pending the findings of the CSTR in 1999. Specifiers should have their attention drawn to the problems generated by the 75% specification, and should be told inter alia that there is no cheap way of getting over the problem. 3.4 FLATNESS OF FLOORS How do we define and specify ‘flatness’? This question has created a host of site problems, not least in measuring whether compliance has been achieved. Well-known publications (Concrete Society, 1988; Perkins, 1988) have discussed many of these facets at length, but a simpler approach has been adopted in this section, based on dividing the problem into three main groups identified by increasing dimensions, with a fourth group containing ramps and relatively large and severely exposed floor areas such as multistorey car parks:

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(a) (b) (c) (d)

short-distance flatness, 0–100mm; middle-distance flatness, 0.1–5.0m; large-distance flatness, >5.0m; multi-storey car parks.

‘Flatness’ here refers to achieving the target of having any three points on the surface lying on a single straight line. In (d) the overall floor design is a slope towards drainage, but the term ‘flatness’ still applies. 3.4.1 SHORT-DISTANCE FLATNESS, 0–100MM This problem shows up as sinkings commonly known as ‘elephant footprints’, caused by poorly compacted and/or mixed areas of screed underneath PVC tiles or sheeting. PVC is light reflective, which makes these fairly easy to observe, and the edges of the sinkings are liable to attrition damage. The ‘footprints’ are generally 1–3mm deep and of random pattern. The problem has been commonly encountered in hospitals, where this form of construction has been used for corridors, operating theatres and preparation rooms. The general quality of husbandry in these buildings does not seem to have helped the situation, with the movement of vehicles, concentrated loads from operating trolleys, and anaesthetists sitting on three-legged stools not necessarily with all three legs in contact with the ground. Figure 3.4 illustrates the effect of an ‘elephant footprint’. Remedial work can be carried out, preferably before applying the smoothing compound and/or the tiling, by cutting out defective patches of screed material and (avoiding any feather-edging) making good with new screed material and, most importantly, ensuring full compaction. It is common to have this defect brought to one’s attention after the tiling. In such cases, when the tiling/sheeting have been removed, defective compaction in the screed has been found, and the screed has to be

Fig. 3.4 ‘Elephant footprint’ on PVC tiles’.

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repaired and the surface retiled (see section 3.3) when ready to receive covering. The building’s maintenance department should keep a supply of new floor coverings to cater for the likelihood of repair. These coverings should be identical to the original, to avoid replacements being made with PVC tiling that does not match in colour or shade. The short-distance flatness problem can be avoided by strict supervision of screed laying and compaction. The screeds generally used where this problem has occurred are known as ‘semi-dry’, and good workmanship in laying them is essential. Self-levelling screeds are becoming increasingly popular, as they are not so sensitive to the standard of workmanship. Screeds used to be commonly laid ‘wet’, and the use of superplasticisers seems to have been instrumental in bringing about a return to this practice. 3.4.2 MIDDLE-DISTANCE FLATNESS, 0.1–5.0M This problem has been identified on site with in-bay and bay-to-bay flatnesses, where a flatness-measuring device is used. Typical wording in a specification might be ‘a deviation of not more than 3mm under a 3m straightedge’. Straightedges are commonly made from a hardened steel, and their straightness can be certified by companies registered with the United Kingdom Accreditation Service. On site, a reasonably accurate method of checking the straightness of a straightedge is simply to look along the edge. It has been found quite easy to record a failure with the 3mm requirement when adjoining bays have been cast to very good individual flatnesses, but they are not horizontal. Figure 3.5 illustrates this, where there is an inverted V at the joint. Failure can also be indicated where the adjoining bays have the same sloping directions, because there is a step at the joint. Problems at stepped joints due to wear and tear are common, and therefore it is sensible to check flatness tolerances both within bays and from bay to bay. Remedial work has been found to be best directed at removal of excesses rather than addition to the deficient zones. This can be achieved by grinding or other suitable means, accepting the aesthetic defect of a terrazzo-like appearance at the ground-off areas. Thin mortar applications to cater for small negative tolerances have generally been found to have mediocre to poor performance. A longer-lasting mortar repair can

Fig. 3.5 Planeness at joint.

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be achieved when at least 10mm of the surface is removed and a repair of at least that thickness can be effected without feather-edging. Many of these middle-distance flatness problems can be avoided by accurate setting-up of stop-ends, rails etc. to which bays have been designed to be cast. Optical setting-out both from site datum levels and from bay-to-bay levellings is recommended. The use of laser alignment on one site to align the floor of a tunnel led to problems because temperature variations along the tunnel caused the laser beam to refract. Forced air circulation had to be used to create a more uniform temperature distribution. 3.4.3 LARGE-DISTANCE FLATNESS, >5M This problem has been identified in buildings such as warehouses and cold stores, where a lack of (mainly) bay-to-bay flatness leads to stepping at joints and interference in the mechanics of mobile racking systems. Storage areas such as these are subject to relatively heavy wear and tear. Even a 3mm step, which could well be permitted under the example specification in section 3.4.2 (not more than 3mm under a 3m straightedge), would be at risk of spalling. Any out-of-flatnesses, especially at joints, will be at risk under the action of moving vehicles, sliding racks and similar. Remedial work would probably be best undertaken following the recommendations listed in section 3.4.2. Severer structural requirements may be under consideration than for smaller floor areas. If so, then fulldepth replacement of all or part of a bay may be necessary. 3.4.4 MULTI-STOREY CAR PARKS Identification of the problem has commonly taken the form of puddling, frost damage and/or reinforcement corrosion where the flatness datum has not been to a slope that is effective for drainage. Although any roof with a slope of less than 10° is defined as ‘flat’, roofs of car parks as well as other buildings are commonly designed to a ‘flat slope’: 1:70 is typical in a specification. Roofs examined in troubleshooting exercises with a slope of less than 1:60 have commonly been observed to suffer degradation due to inadequate drainage. Clearly, the target for such roofs must be a combination of ‘slope’ and flatness. Carriage in of water and de-icing salts by vehicles is common, and if drainage is poor, collection of water can promote rebar corrosion. Another observation made on site concerned control joints that should have been placed at high or intermediate parts of a slope or slopes. Sealantfilled joints, not all of which were performing well, had been placed in the bottom of drainage gullies. Ingress into the construction had occurred.

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Remedial work is difficult to recommend, bearing in mind that the common aim of such work would be to improve drainage by, for instance, cutting drainage grooves in the slope. Care would needed in such cutting work, in view of the likely effect of this on the covercrete d. The problem can be avoided by paying special attention to drainage details: not only the slope and flatness of bays, but bay-to-bay slope uniformity, drainage and positioning of control joints. Manual car-washing facilities have been introduced in some car parks. Special drainage is important in these areas to ensure that the effluent from washing operations is removed. Collection of water, especially when cars are being washed in cold weather, can lead to personnel risk if the surfaces become slippery because of water containing detergent or because of icing. 3.5 FLATNESS OF FORMWORK The problem encountered on site concerned the use of untreated birch-faced plywood formwork for in-situ concrete wall construction in concreting sections about 3.5m high by 4.0m wide. A concreting subcontractor was assembling the panels of formwork, then cleaning and reassembling them for four or five reuses in an unprotected enclosure on the site. Each panel of formwork consisted of about eight separate pieces of plywood. Significant movement of the birch was observed in the form of bowing and dishing of individual sheets and lack of alignment at the butt joints in the formwork panels. Figures 3.6 and 3.7 illustrate respectively the problems with the concrete and the formwork that caused them. A Class B finish to BS 8110 Structural use of concrete had been specified, and although the walls were for internal parts of a building and were to be painted, the stepping was unacceptable to the architect. A limit of 2mm maximum stepping was eventually agreed; the wall areas that did not comply with this limit were ground down at these joints to remove positive tolerance. Observations on site indicated that there was minimal supervision of the subcontractor by the main contractor. It is debatable whether any of the problems would have arisen if the main contractor had realised that responsibility for the subcontract work was in the contractor’s remit, as the expertise should have been there for this sort of work. Perhaps if solar reflectors, escalators or other specialist subcontract work had been undertaken the involvement of the main contractor might have been expected to have been more restricted. 3.5.1 IDENTIFICATION The problem is identified by stepping or lipping at formwork joints, as well as convexity or concavity, discernible by the naked eye, by a

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Fig. 3.6 Steps in fromwork.

Fig. 3.7 Steps mirrored onot concrete.

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straightedge or by a vertical spirit level. Both types of defect have been found easier to observe in reflected light. 3.5.2 REMEDIAL Grind down lips and other unacceptable high areas. If the appearance of the ground areas is a problem, then grinding the whole area could be considered, because a patchy terrazzo-like effect would result from grinding solely at the joints. 3.5.3 AVOIDANCE Use good-quality timber formwork with known relatively stable moisture and temperature behaviour. Protect formwork from the effects of the weather as much as possible, as well as from direct timber contact with concrete and mould-release agents. For the latter in-use protection, seek the formwork supplier’s views on the suitability of painting. If no information can be obtained on a suitable paint, a pigmented paint (Levitt, 1982) could be applied to both the inside and the outside of the forms. The type of paint selected would depend upon how many reuses are planned. Whichever active ingredient base is used in the paint, performance has been found to be more a function of the presence of a pigment than of the base. Consideration could also well be given to improved setting-out of panels of formwork. It is reasonable to assume that with each use there will be some movement. Therefore jig-setting before each use, with the necessary adjustment of soldiers, whalings and shims, could well promote an improved planeness of finish. 3.6 JOINTS BETWEEN PRECAST PAVING AND BETWEEN KERBS Three problems have been encountered on site with the use of both these types of precast concrete product: • • •

uneven bedding of paving flags; joints between vertical faces of flags or kerbs; jointing kerbs on a hill.

As in section 3.4, the identification, remedial and avoidance comments have been discussed separately with each of the problems. Some of the recommendations repeat and qualify those published in the mid-1970s (Concrete Society, 1974). For some reason, no reference was made to this useful technical report in the concrete industry’s well-known annual reference book, the Concrete Year Book (1997).

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The three problems listed above are often invoked to cast aspersions on the properties of the precast concrete units. Any troubleshooting problem needs to be examined carefully, and these cases have been found to cause considerable aggravation in this respect. It is not common for the single causes of the three problems in this section to be identified individually. 3.6.1 UNEVEN BEDDING OF PAVING FLAGS Identification of this problem is simple, in general unevenness and stepping at joints (Fig. 3.8). Cost rather than cost-effectiveness considerations have generally been found to be the reason for this problem, in that the base and sub-base were low-cost materials, and a minimum of labour was used in compaction. In addition, the practice has been observed of placing a dab of mortar at each corner of a prepared seating area, placing a paving flag on top, and applying a bolster at the centre to settle the unit in place. In effect, an impact flexural test is being conducted: breakage of the paving flag is the common result. The danger to pedestrians from tripping cannot be overemphasised. Although it is not known how many cases are brought against local authorities for personal injury, my experience with some authorities indicates that the departments dealing with installation, maintenance and legal matters seem to operate in isolation from each other. Information brought to my attention in the mid-1990s indicated that one local authority took maintenance action only when the stepping at paving flag joints exceeded 25mm. The recommended remedial work is to remove all affected slabs, base and sub-base material and reinstate with well-compacted Type 2 sub-base material followed by a minimum of 50mm of well-compacted dry lean concrete. When this has hardened sufficiently, apply a full bed of a sand/ cement 4/1–5/1 mortar and re-bed flags, leaving vertical joints unfilled and the thickness of a trowel’s blade apart.

Fig.3.8 Paving slabs on unsuitable base.

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The problem is probably best avoided by having a sub-base, base and bedding specification, as suggested in the previous paragraph. Although this is probably the most expensive method of laying paving flags, the reduced cost of maintenance work and the potential costs for personal injury should also be taken into account. 3.6.2 JOINTS BETWEEN VERTICAL FACES OF FLAGS OR KERBS This problem is commonly found in the form of wedge-shaped spalls or compression cracking at joints. If the products butt, or a stone becomes trapped in the nose, or a line of mortar has been forced into the nose, then point stresses are set up (Fig. 3.9). Paving flags or kerbs laid during cold weather tend to exhibit this spalling more than products laid in warm weather. Even though kerbs are commonly bedded on a backing haunch of in-situ concrete there will still be a weather-exposed face responsive to solar-induced thermal movement. Remedial work is fairly straightforward but, as with most work, the cause has to be removed or inhibited from acting further. All butting joints should be sawn down as far as possible, targeting full product depth, with as thin a blade as possible (2mm, for example). Remove spalled areas and feather-edging, and repair with an SBR or similarly approved mortar. Apart from the possible use of an antifoliant, leave the joint untreated. Spall-repair joints with an aperture wider than 2mm as above, and fill the joint (cut wider if necessary to 3mm or more) completely with a 3/1 to 4/1 sand/cement mortar having an aggregate maximum size of 2mm. The object of leaving a joint of 2mm aperture or narrower unfilled is to promote air-borne dust forming a natural control joint. The filling of wider joints with mortar is to prevent joint-butting or stone-trapping from causing stress-raisers.

Fig. 3.9 Spalling at kerb noses.

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The problem is probably best avoided by specifying that all products be laid to an unfilled trowel’s-thickness joint, or that joints be deliberately designed to a nominal 5–10mm width and completely filled with mortar during or after the laying sequence. Although not directly related to this problem, control joints placed through the haunch concrete that backs a kerb should be continued through a kerb-kerb joint and be visible on the face. Control joints in road construction work where precast concrete units are being used are probably best designed to cater for separate and individual movements in bespoke sections of the road. 3.6.3 JOINTING KERBS ON A HILL This problem manifested itself as kerbs moving slowly down the hill because there was little or no restraint. Kerb joints opened up along the incline and dislodged at radius kerbs or similar at the bottom of the hill and/or at corner junctions. The problem appeared to be confined to relatively steep hills: on the few occasions that it has been observed the hills had inclines of 1 in 20 or steeper. Remedial work was not undertaken in any of these cases as far as is known. Common sense suggests that one answer could be to replace or realign dislodged kerbs, coupled with some form of restraint. The restraint could be effected by concreting in a vertical butting unit. This could be a small column, or even a kerb on its end, so that runs of kerb could butt against these restraints. The problem could be avoided by taking this principle of ‘column’ restraints into the design. An engineering appraisal might be needed to calculate at which kerb centres these restraints would need to be placed. I have had only hearsay evidence of problems with precast interlocking paving blocks, and am therefore unable to include any personal experience here. Most reports have been of these blocks sinking below datum level. Therefore much of what is recommended in section 3.6.1 would appear to be the way in which the main problem of units sinking could have been avoided. 3.7 TOLERANCES The main problems encountered on site with tolerances seem to have been associated with a general lack of appreciation that, although building loci are never at exact and predictable points in space, their variations and catering for these variations are both within the realms of the designer, builder and/or manufacturer. In general, the problems investigated fell into two broad categories: • •

positive and negative tolerances; matching tolerances.

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When a dimension is put into a drawing, tolerances need to be put alongside, because what has been asked for has to be both achievable and buildable. As far as tolerances of achievability are concerned, if what has been dimensioned is something like a window, for example, then it is not logical either to omit tolerances or to specify tolerances that cannot be achieved by the manufacturer. Concerning buildability, and using the same window as the example, if there is too much positive tolerance for the window and/or too much negative tolerance for the opening designed to receive it then it is very likely not to fit. If the case under consideration was a precast concrete cladding unit, then in addition to the geometry of the opening and the dimensions of the cladding unit, there would be the tolerances (for both support and restraint) of the fixings. There is a useful general rule: tolerance is an easy thing to find on a construction site but a difficult thing to lose. More difficulty has been found in assembling parts of a building because they were too large than because they were too small. There are a variety of gap-filling materials available to cater for undersizing, but mechanical or similar removal is all that is generally available to deal with too much positive tolerance. There are notable but rather singular exceptions to this guidance: the aluminous cement beams resting on column haunches described in section 1.7 are an example where significant positive tolerance was critical. I have been involved in many cases where troubleshooting revolved around the subject of tolerances. Of the three examples that follow, the first two relate to positive and negative tolerances and the third to matching tolerances. Example 1 A prestige building had a canopy of single-skin GRC cladding panels fixed to a stainless steel subframe. It had apparently been assumed that the specialistmanufactured subframe would be made to strict tolerances because, possibly, of the use of an expensive alloy. Unfortunately, this did not turn out to be the case; and, probably worse than that, the whole frame was under-dimensioned, and great difficulty was experienced in fixing the GRC panels in their designated positions. Much of the excess negative tolerance was countered by the use of large numbers of stainless steel shims at the fixings. In some cases as many as 12 shims, 3mm thick, were used. The only thing found ‘right’ on this contract was that, in the other two space dimensions, the fixing points were found to be within an acceptable tolerance. Example 2 The problem concerned rain penetration around the window details of an office block gable end constructed of cast stone ashlar cladding, sill and

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lintel units. On leaning out of one of the top-storey windows where leakage had been occurring, it was found that both the sill and ashlar unit below could be easily rocked by hand. Arrangements were made to have these taken off the building before a second visit. It was found during the second visit that there were no fixings to the insitu wall relative to the removed stones, nor, with the aid of a torch, could any be seen to the adjacent stones. The rain ingress points at the sill were easily discernible; a direct mortar path allowed passage under the sill. When all 10 storeys of gable end stonework were removed, it was found that less than 10% of the total number of units had been secured to the insitu concrete wall. The main reason for this lack of fixings appeared to be that the in-situ wall was so far out of vertical that the Abbey Dovetail tails were unable to reach the slots cast into the wall. Units were reinstated with countersunk stainless steel expansion bolts both securing the cast stone units and fixing to the in-situ concrete. Example 3 This was an instance of troubleshooting at the design stage, and concerned matching tolerances. Precast concrete units were to be fixed using proprietary cast-in stainless steel slots in the units to locate with floor bolts cast into the in-situ concrete. The problem was that the site, being a refurbishment in a busy metropolis, had no storage room and could not tolerate any delay. Fixing had to be direct from the delivery vehicle onto the facade, and had to be timed for when the tower crane was available. It was agreed that, in order to correlate the precast unit dimensions and the tolerances for the positions of both the site and unit fixing points, the opening (the gap to receive the unit) centre lines rather than column centres would be used for location. Precast units were dimensioned from the centre vertical and horizontal lines, both for their overall sizes and for the cast-in fixing slots. Few problems were encountered on site. Figure 3.10 illustrates how this system was used. 3.7.1 IDENTIFICATION The problem shows itself as difficulty or inability in putting parts of a construction together. 3.7.2 REMEDIAL This is difficult to address, because each case probably needs to be considered individually. Fortuitously, the 1980s and 1990s have seen the advent of many new types of proprietary fixing, which can be used, for instance, as single or double locking or expansion devices. In addition,

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Fig. 3.10 Tolerances for a slot fixing in a precast concret cladding unit; x and y each ±2mm (not to scale).

polymer adhesive systems have some applications where injection or gravity feed are possible. Endoprobes can be used to inspect the efficacy of many of these remedial actions. Sometimes difficulties are known to be likely because of poor alignment, and on-site welding of stainless steel fixings or similar might be a viable remedial measure. 3.7.3 AVOIDANCE Most of the problem areas mentioned above can be avoided by applying realistic detailing and dimensioning, always bearing in mind that tolerances are important all the way from material to product to completed construction. Buildability could possibly be improved by aiming to keep positive tolerances as small as possible and allowing variations to be catered for in the negative tolerance range.

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3.8 RISING DAMP AND A CHEMICAL BARRIER The problem experienced on site related to concrete block masonry walls exhibiting apparent rising damp and how to rectify the situation. There was little point in addressing a rising damp problem unless it could be proved beyond reasonable doubt that the dampness was due to water being drawn up from the ground by capillary attraction. Dampness can be caused by one or more of the following: (a) (b) (c) (d)

rising damp from the ground; water penetration through the wall from outside; water penetration through the roof; condensation.

Attention was drawn to a British Standards publication (DD 205:1991) that suggests specific tests for identifying (a) with respect to the other three causes. However, rising damp tends to exhibit a common behaviour, whether it is on brickwork, blockwork or rendered faces: the dampness rises to only 1.0–1.5m from ground level, and the damp area is generally covered with black mildew. On the site visited it was found that the damp-proof membrane had been omitted. The dampness was diagnosed as rising damp, and it was recommended that a chemical damp-proof course be installed following the guide lines in the code of practice (BS 6576:1985). Apparently, an inexperienced company was chosen to undertake the injection work, and a site re-visit was requested to comment on the possible reasons for the apparent lack of success of the remedial work. Most of the advice given in BS 6576 had apparently been ignored, and the errors that were observed on site included the following: • •

• •

The silicone used (an aluminium stearate might also have been suitable) had a low solids content (about 5% is a typical content). The internal rendering had been reinstated too soon after the injection. The chemical should have been left to become fully effective. This normally takes a year or a little longer. The rendering reinstatement should have been undertaken using a water-repellent mortar, stopping this at the injection line. Where party wall work was being undertaken the neighbour had not been advised of the smell of solvent permeating the wall, nor of the need for ventilation.

3.8.1 IDENTIFICATION The problem is characterised by dampness and mildew growth to a height of 1.0–1.5m above ground, and chemical injection remedial work is of little apparent benefit.

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3.8.2 REMEDIAL Using a contractor proficient in the application of the British Standard recommendations, step drill and silicone inject the masonry wall as shown in Fig. 3.11, and reinstate mortar and decorations after a period of at least a year. 3.8.3 AVOIDANCE Take special care when installing mechanical damp-proof courses and trays. Many problems found on site have been associated with damaged membranes, as well as with mortar bridging dpm lines. The masonry code of practice (BS 5628 Part 3:1985) recommends that something like 5mm of membrane should be left protruding from the face of the masonry. This is not as common as it probably ought to be, and membranes tend to get bridged by mortar, allowing a path for rising damp. A non-concrete case of dealing with rising damp was studied at the installation date and again just over a year later. It concerned a building that was 150 years old, with a single skin of clay brickwork 320mm thick.

Fig. 3.11 Chemical injection steps for a damp-proof membrance.

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The system studied used an electrical method by placing a direct curren field across the base of the wall just above ground level. The system allegedly relied on the electrical properties of the rising damp, and put an opposing field into the wall. Little or no improvement was observed a the second visit. A possible lesson to be learnt from this is that processes should be examined and understood as far as possible. The best test assessment is probably track history of performance if knowledge of the mechanisms is limited. 3.9 CRACKING IN THE NON-VISUAL ZONE The Glossary defines both ‘crack width’ and ‘crack aperture’; the problems found on site all related to crack aperture, as it was this that was visible. The crack aperture was generally assumed to reflect both the crack-width and the number of cracks beneath the surface in the concrete. For example, it was generally thought that if a crack of 0.5mm aperture was present then the crack width was constant at 0.5mm right down to the steel, and that there was just that one crack. However, the various types of crack geometry shown in Fig. 3.12 demonstrate that this assumption can be misleading. Once a crack has been seen, one or more of the following possibilities arise: (a) Water and air can access the rebar and initiate corrosion. (b) Dirt can collect at the apertures, giving the surface an unsightly appearance. (c) Structural distress may be occurring. (d) The crack may be static or dynamic.

Fig. 3.12 Various crack geometries surface crack widths in mm (not to scale).

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In general, the problem manifests itself on site as (a). Once a crack is seen it is commonly assumed that there is a corrosion risk, and that something needs to be done about it. Guidance in the British Standard (BS 8110 Part 1:1985) refers to a limiting crack aperture of 0.2mm as acceptable from the point of view of corrosion. The code of practice for water-retaining structures (BS 8007:1987) suggests a limiting crack width of 0.1mm. However, little or no attention appears to have been paid in these guidance documents to the crack geometry. The reference to crack apertures alone, with no reference to what the crack does as it goes into the concrete, might apply to one or two crack-forming mechanisms but cannot cover all causes. Section 1.2 referred to the observed geometry of thermal cracks that, although traversing down to the steel at a constant width of about 0.1mm, had not promoted carbonation, nor chloride-induced corrosion (it was a case of marine exposure), nor additional chloride ingress. There are many publications that refer to crack width geometries as well as to crack aperture, but it probably serves little purpose on the troubleshooting trail to list these. Cracking in concrete is common, and there are very few cases where either macro or micro-cracking is not present; yet most concrete carries on doing the task for which it was intended without exhibiting distress. Reinforced concrete is designed to transfer its weakness in tension to the rebars placed in the tension zone. The steel controls the extent of the concrete yield; it does not stop cracks. As pointed out in a paper published in the 1980s (Richardson, 1986): ‘If it isn’t cracked, it isn’t working.’ The aim must be to identify whether or not a crack aperture is a risk. The indirect method of the application of UPV at the surface (BS 1881 Part 203:1986) can indicate both depth and direction fairly accurately if the crack is not bridged by sound-conducting material such as water or detritus. However, one needs to know the risk of water getting to the rebars via the crack only if there is a reasonable indication that the crack is singular and might be as deep as the rebars. The initial surface absorption of concrete (BS 1881 Part 208:1996) has been used on site quite successfully to assess both cracked and uncracked areas. Because of the point-to-point variability in surface permeability it has not yet been possible to suggest a comparison limit for acceptance or rejection. The small amount of site data collected indicates that, if the crack could allow water ingress, significantly higher results would be obtained from cracked zones than would be expected for general surface variabilities. Crack aperture has generally been found to be irrelevant to the dirt retention risk referred to in (b). The mechanism observed was that capillary attraction of rain at the crack aperture resulted in water being drawn in, and the dirt from the water was left on the surface (Fig. 3.13). What this amounts to is that all crack apertures appear larger than they really are. Rubbing one’s finger across the crack to remove the dirt reveals a much narrower defect.

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Fig. 3.13 Dirt retention at crack aperture (not to scale).

The structural and crack mobility factors referred to in (c) and (d) fall within the province of an engineering assessment rather than being a material problem. There is little point in dealing with cracks from any of the points of view of identification, remedy or avoidance without a reasonably clear picture of the mechanism(s) that caused the cracking, and of the crack geometry. The two common types of crack-measuring device used on site are transparent plastics cards with different thicknesses of lines for laying alongside cracks, and the crack microscope. Both of these measure crack aperture and not crack width. The crack microscope is possibly too precise a tool, because the need to measure crack aperture beyond a 0.05mm accuracy (which can fairly easily be judged with the human eye against a reference line on the card) is debatable. Furthermore, site conditions and ergonomics need to be considered. There are attractions to using a cheap card that is pocket sized (100mm×40mm, say), sufficiently accurate, and virtually unbreakable. However, whichever measuring device is used, the information it provides has been shown to have little value. 3.9.1 IDENTIFICATION The problem is identifiable in one of two forms: either there is some wording in the specification limiting crack apertures, or visual cracks are observed whose position, crack apertures or times of occurrence are not as expected. 3.9.2 REMEDIAL For static cracks that are not considered to be a corrosion risk, rub down to remove all dirt, and apply a flood coat of silicone water-repellent to the recommendations of BS 6477:1992. Static cracks that are considered to be a corrosion risk may be amenable to repair by injecting an epoxide resin. The

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injection equipment should be connected to a pressure gauge, because it is quite easy, using ordinary hand-pump actuation, to make the damage worse. Cracks exhibiting dynamic behaviour (that is, ‘live’ cracks) need individual consideration depending upon the site conditions. If the crack aperture is in a narrow range (0.05–0.25mm), a copolymer capillary introduction may be successful. Wider crack apertures may be amenable to repair using a silicone or similar sealant. 3.9.3 AVOIDANCE Although good design and workmanship minimise the occurrence of cracks, it is essential—both at specification stage and during the work-not to raise problems when the risks are insignificant or unfounded. The best approach is probably for the parties to agree at a preliminary stage which crack apertures necessitate further investigation and possible action, as well as which cracks do not need attention. 3.10 LARGE-AREA SCALING OF FLOORS The problem occurred with an industrial in-situ concrete floor where a superplasticiser complying with BS 5075 Part 3:1985 had been specified. The admixture was based on a chemical that was added into the truck mixer in the specification range 0.7–1.2% m/m cement at the delivery point on site. It was added at a concentration of about 1.0% m/m cement for this particular site. As is common with truck-mixed concrete, the admixture was stored in a tank above the mixer drum, and was added and truckmixed on site just before the concrete was discharged. Because of an error at the ready-mix plant, the admixture tank was inadvertently charged with the same volume (about 20L) of an airentraining agent based upon a neutralised vinsol resin. This type of admixture is normally added at one tenth of the concentration of a superplasticiser: 0.1% m/m cement is typical. More importantly, it is recommended to be added into the mixing water when the truck mixer is charged at the plant. The day after casting the bays with the 6m3 of concrete from this truck load, large-area scaling of sheets of surface material occurred down to a thickness of 1–3mm. The scaled material was darker than the remaining concrete, and at the interface there was a concentration of coagulated bubbles with accompanying delamination parallel to the surface. The remaining concrete was uniformly air-entrained, but the total air content in the hardened concrete was about 12%. Of this percentage, about 1.0% consisted of irregularly shaped entrapped air voids; the remainder consisted of the common spherically shaped air-entrained voids in the

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diameter range 0.05–0.50mm. The coagulated bubbles at the interface were mostly in the diameter range 0.05–0.20mm. The bays affected were replaced with concrete complying with the specification. Although the causes of this scaling were not discussed at the time of the discovery, a comparison of the mechanisms of the action of superplasticisers and air-entraining agents could yield a relevant clue. The plasticising mechanism of a superplasticiser is generally accepted as electrostatic action in setting up repulsive forces between aggregate and cement particles. Thus underdosing or overdosing such admixtures would tend to cause less or more film-over-particle formation respectively. Airentraining agents generally rely on the mixing action’s trapping bubbles that are electrostatically bonded between cement particles and the surrounding aggregate-rich matrix. So an overdosing of air-entraining agent could result in a concentration of bubbles, which might join together under the same forces and form a pseudo-entrained/entrapped system of plates of bubbles. These plates might have only enough buoyancy under the action of site compaction processes to rise to just below the surface. If this mechanism is relevant, then if there were cement-rich areas in the ready-mix truck due, perhaps, to insufficient mixing, similar large-area scaling would be expected. However, the scaled areas would be in patches, because if cement-rich areas were responsible they would be unlikely to be uniform in the truck mixer. Furthermore, the amount of air-entraining agent overdosing would not need to be particularly high, because a bubbletrapping mechanism would tend to operate at the cement-rich areas. 3.10.1 IDENTIFICATION The problem would cause large areas of scaling up to 3mm deep of surface layer, exhibiting little air entrainment but with a weak interface of coagulated laminar bubble plates also showing laminar cracking. The remaining substrate concrete from the same batch of concrete would tend to exhibit fairly uniform air bubble distribution. The surface planarity would not change significantly over the scaled areas. The scaling observed, and illustrated in Fig. 3.14, could not have been misdiagnosed as the familiar blistering or small-area scaling, about which much has been published. 3.10.2 REMEDIAL Depending upon the degree of unsound material, a decision may be taken to cut out the top layer to a depth of, say, 10–20mm and resurface with new concrete if the underlying substrate is acceptable. If there are more serious doubts (as there were with the case mentioned) then complete replacement may have to be considered.

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Fig. 3.14 Large-area scaling on a slab.

3.10.3 AVOIDANCE All admixtures should be used in accordance with known and acceptable good practice. Those added at the plant or works should be fed in with the mixing water. Superplasticising admixtures added on site should be fed in only after the concrete in the drum has been uniformly mixed and is as close as possible to the required discharge position. 3.11 SILANES The problem encountered on site was with large concrete units precast on site for placement in and above tidal water. The specification required that virtually all concrete be treated with silane according to government recommendations (DoT, 1990). The nature of the problem was twofold. First, the concrete was too impermeable to absorb more than a marginal amount of the two ‘coat’ specified silane applications. Second, what little managed to get into the concrete penetrated only about 1mm deep. As far as was known at that time (and probably at the time of preparation of this book), silanes had little or no track history in the UK. In contrast, the long-established silicones have had a proven performance over at least 40 years. It is of interest to note that the superseded 1984 edition of the current standard on masonry water repellents (BS 6477:1992) replaced a standard (BS 3826:1969) that referred to silicone only in its title. The aim of these types of treatment is to impregnate (not coat) concrete with a chemical that has or endows the concrete with water-repellent properties. With silanes, one has to wait for a significant amount of time to obtain this property. Therefore the concrete or other material being treated must be capable of being impregnated to a depth that will give an acceptable performance: that is, not so shallow as to be at risk of losing its efficacy too quickly because of wear or weathering. In addition, what impregnates the concrete must be designed to act as a water repellent and not be subject to nor designed for water under more than a nominal pressure. Only silicones, usually based in a volatile solvent such as white spirit, act immediately as water repellents by lining the sides of voids and capillaries with a hydrophobic layer. Silanes have much lower viscosities

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than silicones in solvent, and should penetrate further if viscosity is the sole criterion. However, silanes are not water repellents, and have to be hydrolysed (chemical transformation by reaction with moisture in the concrete or air) to form silicone. It would be expected that water-repellent properties would only become apparent with time. This was observed on site, where the thin layer of penetration was observed to be water repellent only after several months’ weathering. An additional factor to bear in mind is based on the physics of fluid penetration. Viscosity controls the speed at which fluids pass through a capillary system, but there has to be pressure driving that fluid. The ‘pressure’ for fluids in contact with permeable materials such as concrete arises from the capillary attraction force, which is mainly a function of the surface tension of the fluid and of the capillary radii of the permeable material. There is considerable knowledge of capillary sizes in concrete and of the viscosity of silane and silicones, but nothing on the surface tension of either silanes or silicones. Another potential disadvantage of silane is that because hydrolysis is the mechanism that converts the chemical to the water-repellent silicone form, moisture cannot get past the hydrolysed material to hydrolyse the underlying silane. Silanes are virtually solvent-free systems, and have very low boiling points, which can, at 35°C, sometimes be lower than the temperature of the concrete. Not only does this make application troublesome in warm weather, but it may also be necessary to spray the concrete with water to promote hydrolysis if its own free moisture availability is low. Another consideration is what to do with the unabsorbable run-off material. A typical cost of silane is about £3 per litre; from the two specified applications of 300mL/m2 each, it would not be unusual to have 500mL/ m2 or more being lost. In the case in question, gutters, filters and collection tanks were installed, and the silane saved was reused. 3.11.1 IDENTIFICATION The problem is flagged by a silane application requirement in the specification. 3.11.2 REMEDIAL Advise the specifier of the following: •

The concrete will have to be of mediocre or poor quality for the silane to be able to penetrate to a significant depth.

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• • • • •

A wait of some months will be required for the silane to hydrolyse into the form of a silicone water-repellent. It may not be possible to achieve the specified coverage rates with little or no rejection of the silane. Problems are likely to be encountered in hot weather. Silane is expensive. Silicone in a solvent may be a better proposition.

3.11.3 AVOIDANCE Pre-tender discussion is advised. The contractual aims may well be achievable by better-known and tried means.

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Specification problems

4

INTRODUCTION Before going into the details of the four problem areas discussed in this chapter, it may be helpful to describe some of the technical and materials aspects that have coloured the current activity. Every effort has been made to keep to materials problems, although it is difficult to avoid the contractual aspects of construction. Hopefully, what will emerge is the need for an interdisciplinary appreciation of all the interlocking features of the construction programme. It is much more effective to get involved in problem solving and troubleshooting at the preliminary stages rather than during the work on site or after it has been completed. Some of the general problems encountered tend to have common features related to the technical requirements of the contract: • • • • • • •

a tendency to use wording that is either vague or subject to debatable interpretation; general reference to standards without detailing the relevant parts or clauses; a misguided concentration on initial cost, often at the expense of scientific or technical needs; contradictions between the specification and other submitted documents in matters such as references and drawings; reluctance by the party receiving tender documents to comment on technical and materials preferences or buildability factors; reluctance by the party preparing tender documents to consult the relevant disciplines on these matters before submission for pricing; reticence in the application of scientific and technical state-of-the-art knowledge to construction.

There are possibly a few more factors that could be invoked. However, it is hoped that optimism for the future of the industry will result in a sensible

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appreciation of the roles that the technical and scientific parties should play in construction. 4.1 THE CE MARK The problem related to what the reader understood when the CE mark was seen written on goods, literature, invoices or delivery notes. All too often the interpretation was that what was being offered or supplied was high-quality, reliable articles. This may well have been so, but it is essential to understand what the claim means. I gained considerable experience in this by representing the Chartered Institute of Building on the Department of the Environment’s Joint Advisory Committee. The basis of the CE mark is the safety requirements contained in a Harmonized Standard, a Standard or an Agremént Certificate (the latter two documents do not have a European equivalent). The Joint Advisory Committee, with members constituting trade associations, professional institutions and the like, advised the UK government’s representatives on the European Standing Committee concerning the policy of Directives. For the construction industry the main Directive of relevance is the Construction Products Directive, which was issued as a Statutory Instrument (HMSO, 1990). There are other Statutory Instruments based upon other Directives, such as Low Voltage Equipment or Gas Appliances, which also apply—sometimes in contradiction to the CPDbut the CPD predominates for construction. The CE mark for construction products relates to six Essential Requirements: mechanical, fire, health, noise, energy and safety. These refer to the behaviour of the product in the works, and not to the product itself. How these six features apply to the European Standards Technical Committees who receive their instructions from the Standing Committee has been subject to a great deal of interpretation. Eight Interpretative Documents have been published (fire has three: structural, noxious emission and extinguishers), which are an order of magnitude larger than the original CPD. The Harmonized Standard on which a CE claim is based is a normative Annex derived from the CPD’s Essential Requirements. Therefore goods claimed to be CE marked need to be studied for qualification as to the basis of that claim. The following points merit investigation: • •

•

Against what Harmonized or equivalent Standard is the claim made? Do the clauses in that Standard meet both the technical requirements and the health and safety needs of the product when installed in the works? Is it appreciated that the CE mark applies only to the Essential Requirements, and that if there is another contract specification, such as an architectural one, this is unlikely to be covered?

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•

•

Is it appreciated that the CE mark refers to the suitability of the product in the works, but contains no reference to the construction processes directly? Is it appreciated that the main basis of a CE mark claim is that the product complies when assessed under the clauses relating to health and safety in the Harmonized Standard (or equivalent document)? This is irrespective of whether or not those clauses meet the end-user’s ‘essential requirements’.

In effect, the CE mark is a ‘safe to use’ claim based upon safety defined by others; it is not a ‘fit for purpose’ claim. Although a product manufacturer placing the CE mark on goods generally has to have external assessment, the goods do not, in my opinion, generate the same degree of confidence that the British Standard Kitemark gives. The Kitemark specifies the standard to which the product is being assessed, and how regularly it is being examined. 4.1.1 IDENTIFICATION The problem arises when the CE mark appears in the contract documentation without the parties concerned knowing to which Harmonized Standard or equivalent specification the product is specified, or how all this relates to the required performance of the product in the works. 4.1.2 REMEDIAL Advise the party or parties concerned of the limitations of having only part of the knowledge necessary for the construction. 4.1.3 AVOIDANCE CE marked products should be specified only when it is clear which are the supporting documents, and what their relevance is to the job requirements. The better-tried route of invoking relevant codes of practice coupled with supporting standards and adequate supervision on site might be more cost-effective. 4.2 DURABILITY The problem is to define the word ‘durability’. It has been used for many years in relation to concrete and concrete structures, but all too often it can mean any of the following: • •

strength, usually with respect to the cube results; sulfate resistance, usually for concrete placed in the ground;

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• •

freeze/thaw resistance, for pavement-quality concrete; a generally vague reference to how long it ‘lasts’.

The best way of tackling the problem could well be to pose the questions that seem relevant, and propose how these questions may be answered. This has manifested itself in four main ways on site: • • • •

What is meant by ‘durable concrete’? What is/are the durability risk(s) and effect(s)? How is durability to be assessed? For how long is the concrete required to be durable?

4.2.1 DEFINITION OF DURABILITY The simple dictionary definition of ‘durable’—‘strong and long-lasting’— is possibly what has resulted in the use of concrete strength to define concrete durability. However, the relationship between concrete strength and any particular durability risk is, at best, tenuous. As far as is known the word ‘durability’ has not been defined in the Standard (BS 6100 Part 6.2:1986), and the following might be suitable and relevant: The ability of concrete to perform in the manner expected under the defined conditions of use for the time expected. This definition is subject to interpretation with respect to such phrases as ‘manner expected’ and ‘time expected’. However, if it is accepted that these aspects of durability are relevant then they need to be described as accurately as possible. The phrase ‘defined conditions’ is probably the part of the definition that is simplest to understand. For example, if concrete is damaged by a durability hazard arising from an unexpected change of use, then the concrete was and still is durable by its basic definition. The following sections should clarify the meaning of ‘manner expected’ and ‘time expected’. 4.2.2 RISKS AND EFFECTS OF DURABILITY HAZARDS There are at least a dozen durability risk factors that, either singly or in combination, could be relevant to any particular assessment: • • • • • • • •

Sulfate attack: weakens surface or can cause splitting. Chloride ingress: promotes rapid unprotected rebar corrosion. Carbonation ingress: permits rebar corrosion. Freeze-thaw (F/T) without de-icers: degrades surface slightly. F/T with de-icers: severe surface degradation. F/T with anti-frost chemicals: surface softening and dissolution. Inorganic chemicals: dissolution in depth. Organic chemicals: generally a more rapid dissolution than inorganic chemicals. (Note: The calcium salts resulting from some organic acid

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• • • •

•

reactions can form an acid-insoluble protective layer during the reaction. Citric acid tends to behave in this manner; the initial rapid reaction tends to stop within a minute or two.) Attrition: loss of surface and dusting. Impact: spalling, cracking or shattering. Fire: integrity loss, spalling or cracking. Weathering: change in appearance. (Note: Architectural durability should be considered alongside the other durability factors. An appendix in the cast stone standard (BS 1217:1997) tabulates these considerations: this is possibly the first time that this matter has been addressed in a BS.) ASR: severe cracking and loss of integrity when the reaction is unacceptably expansive.

Note: Refer to section 1.12 for the reason why DEF is omitted from this list. 4.2.3 DURABILITY ASSESSMENT Although it seems desirable to assess the concrete’s degree of durability by a test, the apparent logic of this does not always prevail. An example that arose from a recent site experience related to concrete in an estuarine environment, with chloride ingress being the main risk. Section 3.1 discussed the diffusion characteristics and the symbol k, where test data took at least 6 months to yield indicative data. Contractually, it is much more acceptable to use a strictly supervised mix design regime with its known track record of good site performance. The same considerations largely apply to freeze-thaw testing and ASR assessments. It is not the purpose of this section to discuss how to deal with each of the 13 risks listed above, but simply to suggest that although testing has its attractions it is not always the most effective or acceptable. There are wellestablished track records of specific recipes of concrete, and with the need to ensure good mixing, compaction and curing procedures, it is attractive to specify and control mix design. If the parties concerned with the assessment of durability agree that these control aspects are those that prevail, then the incorporation of supervisory items in the contract probably merits further discussion. 4.2.4 DURABLE LIFETIME This difficult subject is possibly best tackled from the point of view of track record, coupled with on-site monitoring to check predictions. This may be achieved by concrete mix/concreting specification, or testing, or both. A reasonable prediction can be made of the onset of something unacceptable, or of the need for maintenance or remedial work; then a reasonable

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assurance of a 10-, 25-, 50- or 100-year life will result. Concrete has been in use for well over 100 years, and most of it is documented as performing satisfactorily. There is enough information available to explain why things go wrong when they do. The main problem with concrete is that it cannot be designed like an electric lamp, for example, to give 1000, 5000 or 10 000 hours’ burning time. Most of my troubleshooting experiences have related to concrete under 20 years old, with most of the problems becoming manifest within the first five years. The inference from all this is that concrete should be designed to last as long as possible. It seems too risky to design concrete deliberately to show durability distress within a limited time, such as 10–20 years. When it comes to proving the point that the planned durability has prevailed over the years, there are, in addition to visual examination, a large number of monitoring methods. For example, carbonation depth, chloride gradient, half-cell potential and the initial surface absorption test (ISAT) have all been found to be useful on-site tools. Of particular note was an article referring to the use of the ISAT in limit-state design for the time of the onset of rebar corrosion (Levitt, 1985). 4.2.5 IDENTIFICATION Look out for the words ‘durable’ or ‘durability’ appearing in documents with no qualification, risk description or assessment/control. 4.2.6 REMEDIAL Probably the best way to deal with this is to ask for the qualification. When it comes to dealing with durability as defined in 4.2.1, there appear to be two camps. The first is the ‘cater’ camp, which accepts that something is going to go wrong and caters for it either by repair or by replacement. The second is the apparently rarer ‘cure’ camp, which ensures that the concrete needs no maintenance apart, possibly, from occasional cleaning. 4.2.7 AVOIDANCE Pre-contract liaison between the members of the construction team seems to be the best way to promote an understanding of the relevant durability considerations, and of the best way to deal with them in each specific case. 4.3 CONCRETE QUALITY The problem is concerned specifically with the word ‘quality’, and with what much of the specifying and purchasing communities think the word means. Quality, and all that the word engenders, has resulted in a growth

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industry in both the manufacturing and servicing sectors. The subject is complex, and a simplified step-by-step approach may help to throw some light into a grey area. Many users of the identical EN 29000 or ISO 9000 series of standards are as likely to be misled by the inference of the word ‘quality’ as by the deliberately badly worded title of this section. Quality is defined (BS 4778 Part 3:1991) as The totality of features and characteristics of a product or service that bear on its ability to satisfy stated or implied needs. In my opinion this definition could be substantially improved, but first note that the word ‘quality’ is a noun. It is generally misinterpreted as an adjective, to mean that the thing it refers to is ‘good’, ‘better’ or ‘excellent’ in some way. This initiates a discussion on the interpretations of the definition of the word, which will lead naturally into a description of the pit-falls awaiting the unwary. Most people seem to accept that the word is always a guarantee of something good. 4.3.1 QUALITY: THE NOUN As a noun ‘quality’ needs an adjective to describe the quality on offer, such as ‘good’, ‘mediocre’ or ‘poor’. It does not matter whether what is being considered is a product or a service. The current common use of the word ‘quality’ as an adjective before either of these words—or any other word— is either meaningless or misleading. It gives no information as to the sort of quality on offer, and it implies, without supporting information, that what it refers to is superior or good. 4.3.2 PRODUCT AND/OR SERVICE The ‘and/’ part of the title of this section does not occur in the definition. Usually this will have no effect on a product manufacturer, nor on the designer or specifier. However, when it comes to a contractor or a subcontractor the definition fails, because what is under consideration is a mixture of both product and service. The simple solution would be to change the definition to include ‘and/ or’. A more complicated approach might be to restrict the contractor or subcontractor’s work to quality assurance of the work undertaken as a service. This would leave any product involved subject to separate assurance. 4.3.3 IMPLIED NEED The words ‘or implied’ in the definition introduce a grey area of interpretation. It would be far less complicated if these two words had been omitted from the definition.

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4.3.4 QUALITY PROBLEM MANIFESTATIONS Great store is currently being set by the optimism that seems to be engendered by specifying quality in some form or the other in products or services. However, when a product or service is ‘quality assured’, all that is known is that there is some independently assessed paperwork confirming that a claimed level of quality is being achieved. Thus, if a quality-assured product or service is specified or offered, the parties concerned have a reasonable assurance of consistency, and an implied expectation of something that is good or very good. The impression created by a framed ISO 9000 certificate in a foyer or a claim in trade literature seems to be out of proportion to what that certificate or claim actually means. The information has to be accompanied by one or more appendix pages stating what quality is on offer, and how the person making the offer substantiates the claim. It is only for a service organisation such as an architectural practice that the lead certificate could carry words such as ‘Complete range of architectural services’ and provide sufficient information on a single piece of paper. As an example of the complications manifested by the subject of quality, consider a specifier asking for a product from an organisation with ISO 9000 registration. The interest is solely in the product, not in the way it is to be used. If the logical assessment of the suitability of that product is in its properties, then surely a BSI Kitemark is sufficient? A second example that might be more relevant to the construction industry would be a ‘manufacture and install’ requirement. The first ‘head’ of this two-headed specification could probably be dealt with by a Kitemark. The second ‘head’ needs to be a service-oriented control by the ISO 9000 route, or by a known track record, or by adequate site control and supervision. Hence there needs to be a clear definition in a series of specification clauses that define, preferably with little or no ambiguity, what is required to be purchased. Where a product and/or service is on offer, in the same light, that product and/or service requires a fully qualified and unambiguous description. There are also grounds for discussion outside the subject matter of this section for the inclusion of the control of quality as specific cost items in the bill of quantities. Although it can be expensive to install a quality system, a well-organised and well-run system can soon be shown to be cost-effective, and can pay for itself in the long run. The most dangerous trap awaiting the unwary is probably tied up with the word ‘complacency’. In-place quality systems generate a considerable amount of paperwork, the need for some of which is questionable. An ISO 9000 claim or statement of compliance with an established quality procedure conveys an aura of well-being to the uninitiated recipient. In

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spite of all this, it is still incumbent on all associated with the quality scene to understand what is being asked for and what is supplied. Products and services are fit for purpose only if the parties concerned are fully aware of what it is they are buying or selling, and realise that fitness remains within the constancy of that claimed and specified throughout the contract. The author, with many years’ experience as a test house quality manager (how one ‘manages’ quality could well be subject to another and longer discussion) and a Fellow of the Institute of Quality Assurance, condones the application of quality principles in any industry or service. Nevertheless, the 1980s and 1990s have seen a tendency to over-react to the assumed benefits of registration. A recent publication in the Institute’s journal (Seddon, 1994) discussed ISO 9000 in general terms. Seddon stated that registration was related to competitiveness in a tendering position, and that the results of a survey by Vanguard Consulting painted a picture of a standard that was not making a strong positive contribution to quality in UK Ltd. Before ending with the usual three sections, there is another popular phrase that deserves criticism: ‘quality improvement’. Great care is necessary before embarking on an all-round application of this principle, because dangers can lurk. It may be relevant in the training of operatives to carry out specific processes, but for a product there could well be hazards. Consider the case of a paint that has a proven track record of at least 20 years’ satisfactory use on site. Should the manufacturer change to a ‘quality-improved’ product? There could be two arguments against this: • Twenty years was good enough in any case. • Do we wait for more than 20 years to see if it is improved? There is nothing against improving something when an improvement is necessary, but nothing for it simply for the sole aim of improvement. Many cases of the unfortunate loss of track records have probably been caused by unnecessary improvements. 4.3.5 IDENTIFICATION Look out for the use of the word ‘quality’ in any context without the parties concerned knowing in unambiguous terms what the word implies in terms of product and/or service. 4.3.6 REMEDIAL Ensure that what is under consideration is qualified by a full description. If this is not forthcoming, explain why a problem area still exists, and how this may affect the work.

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4.3.7 AVOIDANCE Four points are listed below as suggested constructive criticisms in tack ling the problem at the pre-tender or tender stages. However, overriding all this discussion is the possibility that quality assurance is the wrong peg on which to hang one’s hat. Complaints are the indicators that there is something wrong with quality. The way to avoid complaints or keep them minimal is to ensure that quality is controlled (not assured) at all stages: that is, total quality control. The installation of such a system, coupled with its supervision would seem to have a lot in its favour without many of the disadvantages associated with quality assurance. It is difficult to summarise the avoidance of the problems described earlier. The following points could form the basis for future discussion as avoidance targets: •

• • •

The specifier or purchaser needs to state what is wanted as exactly as possible, by test data, drawings, specification wording, description and dimensions. The specifier or purchaser should neither specify nor describe how to achieve what is wanted. The supplier’s suitability needs to be confirmed by a strict form of control. The way this control is achieved needs to be documented and available for audit by the other contract parties.

4.4 SPECIFYING STRENGTH Concrete strength in a contractual supply situation is commonly specified by the term ‘characteristic strength’. The term applies only to cubes or cylinders made from that concrete, and not to the concrete in the structure. The term is statistical (BS 8110 Part 1:1985); it means that not more than 10% of all cube results would be expected to fail the specification level. For example, if 40 cubes of a C40-specified concrete are tested at 28 days, and four of these cubes give results below 40MPa, that concrete can still comply. The relative ease of using this form of specification, coupled with the apparent relative simplicity of the manufacturing and testing regimes, has generated serious problems on site, mainly with in-situ concrete. Problems of this nature do not usually arise with precast concrete products because, apart from beams, columns, cladding panels, cast stone and similar, most precast products are specified by tests on the product. This is the fundamental difference between type testing (a test on the concrete supplied) and proof testing (a test on the actual finished concrete). The terms are not to be confused with words ‘realcrete’ and ‘labcrete’

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(defined in the Glossary). Proof testing of precast concrete products such as masonry units, kerbs and pipes is on realcrete. As cubes or cylinders for strength testing are usually made on site and then tested in a laboratory, they do not fall into either the realcrete or the labcrete category. 4.4.1 VALUE OF CUBE/CYLINDER STRENGTH From the foregoing discussion, it may be concluded that the value of a test result is solely in the potential compressive strength. The test cannot cater for realcrete factors such as: (a) (b) (c) (d)

how well the concrete has been compacted; how well the concrete has been cured (thermal and/or moisture); the effect of segregation, honeycombing and similar; the potential difficulty in applying the term ‘characteristic’ to highstrength specifications; (e) the potential problem of the ‘10% below’ all being in a single and critical place on site; (f) the likelihood of specifying strength when strength is an unnecessary specification requirement. An in-depth analysis of the pros and cons of the subject matter could well extend this list, but the discussion here is restricted to my own experience. For convenience (a), (b) and (c) have been integrated into this section, (d) and (e) into 4.4.2, and (f), by itself, in 4.4.3. My experience in the real world of site, plant and works concrete has been coloured by the observation that people rarely follow specified procedures to the letter of the specification. Happily, where deviations have occurred, these have usually had a marginal or insignificant effect on the result. Identification of the problem area is in two parts: the laboratory data, and the strength that the concrete has in the construction. The first part manifests itself in questions about the validity of the data. For example, confidence can be undermined when a cube test certificate gives two decimal places of strength or, for that matter, comments on a pass-or-fail situation. The second part of the problem is much more common, and is encountered when a site dispute arises over aspects such as compaction, curing or materials in general. This form of dispute can be triggered from the cube or cylinder strength data, from a site inspection of the concrete, or perhaps from some form of behaviour at a later date giving grounds for suspicion. It is remarkable how frequently data from cube or cylinder testing are emphasised with little concentration on any part of the manufacture, cure

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or testing of these items. Remedial measures seem to be tied up with sensible and total quality control all the way down the line. If as much attention was paid to the manufacture and testing of test items as to the results that the tests produce, then many of the problems would disappear. Remedying queries relating to the realcrete on site is a different matter, and one that should probably be included at the tender stage, to avoid or inhibit the likelihood of disputes. There are a number of non-destructive and semi-non-destructive tests that could be agreed as criteria. For example, cores could be tested (BS 1881 Part 120:1983), or the concrete could be examined for surface hardness (BS 1881 Part 202:1986). The latter test would need to be supported by correlation data, and the former test would need to be subject to a pre-agreed interpretation, both with respect to the specified strength. The way to avoid the problem is to treat all aspects of testing, from sampling of the concrete right through to reporting, as part of a strictly controlled regime. The primary documentation needs to spell out the method for dealing with disputes on site relating to the quality of the concrete in the construction. If all parties agree a detailed procedure for tackling subsequent problems then much late and unnecessary argument can be avoided. The parties probably also need to accept that a multicomponent material emanating from a number of different disciplines is always likely to create problems. 4.4.2 THE DISADVANTAGES OF SPECIFYING ‘CHARACTERISTIC’ Two problems have been met in connection with this statistically based specification. They are both easily identified. The first problem concerns high-strength specifications such as C50 and higher when the coarse aggregate used or specified has an ultimate yield compressive strength not much higher than 50MPa. In order to meet the specification, a manufacturer or supplier of concrete would, for example, strive to achieve an average strength of at least SOMPa plus two standard deviations. It is assumed that all cube results would sit within a bell-shaped Gaussian (normal) distribution, and that not more than 10% of the total data would fall below SOMPa. Thus with a standard deviation of, say, 3MPa an average strength of 56MPa would be targeted. Most importantly, the statistical concept would assume a mirror reflection of data on either side of the average. Because 80% of the data would be expected to lie within the range 50–62MPa (there would be 10% ‘failing’ the upper limit), any aggregate strength below 62MPa would annul the characteristic concept. The new distribution of data would be skew and possibly of a Poisson type. The second problem concerns the potential risk that all of a 10% failure batch might find its way into a single and possibly critical part of the

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structure. As an example, a part-load of 2m3 of ready-mixed concrete or an even smaller load of site-mixed concrete could be used in concrete piles. If the total amount of concrete used in the piles on the contract was 20m3 and the cubes from the 2m3 were the only failures then the concrete could be said to have complied. A chartered engineer would need to advise on how critical the 2m3 pile positions were and on their relation to the cube strength data. A remedy for the first problem—the inapplicability of the Gaussian distribution to high-strength concrete—is suggested by a current case, where the solution was to vary the requirement of BS 8100 from ‘not more than 10%’ to ‘not more than 5%’. As far as the second problem is concerned, if the cube data relative to some pile positions are suspect although the specification is met, then an engineer’s appreciation of that concrete is required. The non-destructive or semi-non-destructive tests referred to in section 4.4.1 might be of help. The avoidance of both types of problem is easier to address. If the grade of strength required means that the Gaussian distribution has a significant overlap with the ultimate aggregate strength, then either a Poisson statistical basis of specification needs to be introduced, or the specification could be based on minimum strength as the criterion or on some other acceptable scheme. For the second form of the problem, it is possible that a minimum rather than a characteristic strength specification would address the risk. Remember that the ‘10% below’ refers to the number of results and not to the level of each result. Because it is a habit to specify concrete cube or cylinder strength on the characteristic basis it does not have to be mandatory. 4.4.3 THE NEED FOR A STRENGTH SPECIFICATION This problem has generally concerned contractual situations where the concrete is expected to perform as a function of cube or cylinder test data. The performance requirement has been either unrelated or only tenuously related to strength. The problem arising from the reliance on test sample strength data was aggravated by having to wait for several days for the numbers to arrive on site, by which time the construction would have probably advanced by several stages. The following are examples of the part or full inapplicability of strength tests: (a) situations such as chloride resistance where the selection of mix ingredients is the most important aspect; (b) situations where concrete is likely to be subject to deformation, and its elasticity is more important; (c) concrete subject to the hazards of fire, impact, explosion and impact sound insulation;

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(d) situations where the concrete is subject to an aesthetic weathering durability risk, and its water and dirt resistance are more important than strength. The only possible remedy would be to introduce an assessment related to the performance requirement, or to have the strength specification modified. As far as waiting for strength data to arrive on site is concerned, the use of a non-destructive test such as the rebound hammer (BS 1881 Part 202:1983) could be considered. However, at the remedial stage, it is unlikely that better than a 15% accuracy can be placed upon strength prediction, because a calibration relationship for the specific concrete is unlikely to be available. As in section 4.4.2, the problem is fairly easy to avoid, because action can usually be taken at tendering or pre-contract stage. The parties concerned with the technical requirements need to agree what performance characteristics are relevant, how these are to be assessed, and with what limits. The specification of mix design as a priority over the properties of the hardened concrete could also yield fruitful results, provided that this was coupled with a realcrete assessment to cater for such things as curing and compaction.

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Precast concrete

5

INTRODUCTION In this chapter I have taken the opportunity to update several items from my first book, Levitt (1982), as well as to add knowledge accumulated both in the time spent with the Laing organisation and since then. There are no clear dividing lines between many of the problems encountered with precast concrete products and those encountered with in-situ concrete. This chapter describes problems that have been observed with precast products, but where the same problems are met with in-situ concrete there is no reason to think that the same approaches to identification, remedy and avoidance cannot be taken, as with aggregates and frost damage discussed in section 1.3, for example. As in Chapters 1 and 3, the actual material being used is rarely the reason for the problem. Matters of design or workmanship nearly always lie at the root of the problem. 5.1 HYDRATION STAINING Levitt (1982) described this problem at length, but this virtually irremovable and most unsightly of aesthetic defects continues to occur. It has been observed in both in-situ and precast concrete finishes, although the complaints recorded have referred mainly to precast concrete products. This is probably because the surface finish requirements and expectations for precast products are more stringent than those for in-situ concrete. Also, precast products are commonly stacked at an early age. Mould smoothness or polish and the effect of stacker packs have both been found to be significant factors in causing this problem, and so it was considered best to place this section in this chapter. Before listing the factors associated with hydration staining, it is worth restating the underlying mechanism suggested in Levitt (1982) to be the

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cause of this problem. The words ‘macro’ and ‘micro’ have been used at various places in this book, and will be found in many treatises dealing with concrete and other materials. About four orders of magnitude below the micrometre (µm), atomic forces are encountered at the ångstrom level of length measurement (1Å=10-10m). Strong levels of atomic attraction due to van der Waals’ forces are known to operate at atomic distances. A reasonably smooth surface of a mould or a stacker piece is likely to have areas of atomically smooth surface. Therefore fresh or fairly young concrete in contact with such a surface will tend to adopt the same macro, micro and atomic smoothness as that of the surface with which it is in contact. A relevant manifestation of the strength of these forces may have been experienced when mounting 35mm photographic slides. In addition to the two plastics locking frames for each slide there is a stack of tissue-wrapped smooth glass plates, of which two are used to encase each transparency. Each piece of glass can generally be removed only by a sideways sliding action. It is almost impossible to pull a piece of glass off vertically along the same axis as the stack of glass plates. Another less well-known example of these forces occurs when two pieces of pure copper have their ends polished to a mirror smoothness in a nitrogen box (to avoid oxidation), and these polished faces are brought together under light hand pressure. A bond as strong as a brazed or soldered joint is obtained, and the two pieces of copper cannot be parted. The main factors involved in hydration staining are as follows: • •

• • • •

•

•

Concrete shrinks during the hydration process of the cement. The shrinkage away from the mould or formwork leaves a small gap, which is nevertheless large enough for air with its small carbon dioxide content to enter. For typical OPC concrete this results in a light grey colour because of slight surface carbonation. Such concrete cured under air-free conditions exhibits a colour between dark grey and black. White Portland cement concrete and cast stone exhibit a light bluegrey colour. Smooth polish-finish, gloss-painted moulds or formwork will promote hydration staining, because atomic smoothness can be obtained over quite large areas. Smooth-faced packing pieces or stacking blocks placed against visual faces before the concrete is about 2 days old will promote the same problem. Hydration staining has been found to bear no relation to the type or amount of mould release agent or oil used.

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• • •

The glossier the finish of the mould or formwork the worse the problem. The strong attraction forces between the concrete and the mould or formwork make demoulding or stripping difficult. Weathering does not ameliorate the effect; if anything, the concrete surrounding the stained area tends to lighten in colour while the stain remains virtually unaffected. This makes the stain stand out more in contrast.

Figure 5.1 illustrates an example of hydration staining on Portland-finish cast stone units, caused by stacking planks. This twin-line staining was first observed in 1991, and when last inspected in 1996 showed no sign of improvement. 5.1.1 IDENTIFICATION The symptom is dark glossy area(s) on the surface, usually accompanied by difficulty in demoulding or stripping. This staining typically penetrates 10–30mm into the concrete. Weathering, if anything, tends to emphasise the contrast between stained and unstained areas. Hydration staining has been found to have no similarity to the staining caused by leaves—planks of timber used for stacking after the concrete is a few days old—where saponification by the lime in cement generally causes such stains to fade or disappear. 5.1.2 REMEDIAL Once hydration staining has occurred there is no direct remedial action that can be taken. The areas affected can be cut out and ‘made good’, but

Fig. 5.1 Hydration staining on a case stone unit.

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the aesthetics of the made-good concrete would need to be discussed. The alternative indirect treatment would be to paint the concrete with a lowmaintenance paint such as a silicate or acrylic-based system, ensuring that the surface can still breathe: that is, can allow the passage of water in vapour form. 5.1.3 AVOIDANCE Avoid the use of gloss-finish moulds or formwork. Where paints are used they should be matt finish and pigmented, with the pigment colour changed if more than one coat is to be applied. The pigment in paint is more significant in promoting wear resistance than the type of paint used (Levitt, 1982), and changing the colours of subsequent coats facilitates the assessment of paint wear rates. Where the mould or formwork has a high gloss finish, two or three consecutive daily mortar applications and removals help to ‘weather’ the surface into a consistent use behaviour. The alternative is to sandblast the surface lightly so as to produce a matt finish. Where stacking pieces are to be used they should not be deployed until the concrete is 48 hours old. Even after this time, for visual faces, the stacking items used should preferably allow part-air ingress. For example, expanded polystyrene blocks would be preferable to polythene-wrapped pieces of timber. The associated experience of the concrete’s sticking to the mould or formwork can often be alleviated by incorporating a valve into which compressed air is blown during demoulding or stripping. The sticking would be due to the van der Waals’ attraction forces, which at a simulated total vacuum suction level could reach 0.1MPa. Over an area of concrete of say, 0.1m2, the force required to release this area of adhesion would be 1 tonne. 5.2 LIME BLOOM The most abundant alkaline material in hardened concrete is calcium hydroxide, often referred to as lime. It is slightly soluble in water, and can move through the void structure. When it reaches the surface it quickly reacts with the carbon dioxide in the air to form calcium carbonate. This is white in colour, and is known as ‘lime bloom’. It should not be confused with efflorescence, which is normally in the form of a white crystalline deposit of calcium sulfate, and is a problem (outside the scope of this book) that is associated with some clay bricks. In general, the simple difference between calcium carbonate as lime bloom and calcium sulfate as efflorescence is that when each is placed in turn in dilute hydrochloric acid, lime bloom bubbles whereas efflorescence does not.

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However, the occurrence of lime bloom on the face of concrete is a serious problem, which all too often encourages the use of an apparently cheap cleaning agent such as hydrochloric acid. The drawbacks of this are discussed later. The lime bloom forms a patchy appearance, which for darkcoloured backgrounds looks very unsightly if aesthetics is a consideration. Calcium carbonate is almost insoluble in water, although it is very slightly soluble in carbonic acid (water containing carbon dioxide). In practice, the rate at which lime reaches the surface tends to decrease with time, because hydration products and carbonated lime tend to obstruct the voids. Therefore a long wait, of the order of several months or even a year or more depending upon the site exposure conditions, is necessary before the bloom lessens or disappears. Lime bloom is most likely to occur in mid-spring, commonly at the end of April and in early May. At that time of the year the UK experiences its lowest relative humidities; I have measured levels as low as 30% with a whirling hygrometer. Associated with these low humidities are low temperatures, which tend to be below the temperature of the body of the concrete. The dry, low-temperature surface zone will have a low saturated vapour pressure compared with the concrete or cast stone below, and so the calcium hydroxide in solution will be drawn towards the surface in an attempt to reach a pressure equilibrium. This mechanism, which is virtually the same as that described in section 3.3, exacerbates the formation of lime bloom. In summer the average relative humidity is high, and it would only be at night time—when the surface is colder than the body of the concretethat lime bloom would be promoted. Acting against this process could well be the nightly comparatively high relative humidity, promoting surface dampness, and a slow diffusion process would have to take place between the calcium hydroxide in solution in the body of the matrix and the relatively fresh and new water in the surface layers. 5.2.1 IDENTIFICATION Lime bloom consists of a white powdery deposit on the surface, which is insoluble in water but soluble in dilute hydrochloric acid with the formation of bubbles. (Note: The person carrying out this test would not be expected to identify the bubbles as being carbon dioxide.) 5.2.2 REMEDIAL Lime bloom can either be removed by dry brushing, or left to the slow dissolution process described previously. Although brushing will remove most of the bloom there is still an abundant source of lime from the cement hydrates waiting to go into a very dilute lime solution and replace the

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bloom on the surface. Once the lime bloom has been brushed off, a silicone solution complying with BS 6477 is very effective as a more permanent remedy. The use of hydrochloric acid to remove lime bloom is not advised, even though it appears to be very effective. Even if the concrete or cast stone is thoroughly wetted both before and after the acid application (in order to inhibit acid penetration) the treatment will leave the reaction product calcium chloride in the void structure. Calcium chloride is very soluble in water, but it is also very hydrophilic, and promotes surface dampness. This in turn will allow calcium hydroxide to get to the surface and, if anything, will exacerbate lime bloom formation. 5.2.3 AVOIDANCE Use a water-repellent admixture such as stearic acid in wet-cast concrete, or calcium or aluminium stearate in earth-moist products such as most cast stone production. Do not use this type of admixture with any other admixture unless representative tests have shown that the two admixtures are compatible in all properties. The alternative would be to use a silicone as in section 5.2.2 as soon as possible after manufacture. In this case care would be needed to avoid spillage onto bedding faces. Provided other parts of the construction are adequately protected, site application can have its attractions, most obviously the fact that mortar joints would be siliconed at the same time. 5.3 COLOUR VARIATIONS The word ‘colour’ is used here to embrace the many terms invoked architecturally to describe the visual effect that the surface of concrete has on the human eye. There is considerable subjectivity in this assessment, because factors associated with the surface, such as texture, tone and reflectance, all play parts. Superimposed on all this are the conditions when viewing takes place, such as: • • • • •

direct sunlight on the surface; sunlight on a surface adjoining the one being assessed; wetness, dampness or dryness of the surface; the appearance under cloudy, misty or foggy conditions; the effect of adjoining coloured material, such as a window surround.

It may be seen that the view likely to be reached is a function of two groups of variables: those that seem to lie with the producer, and those that are subject to the viewing conditions. The problem of colour variations is mainly concerned with the first of these, because the viewing conditions should be controllable on site or during storage.

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The problem has arisen because concrete, including cast stone, is a multiingredient material subject to a wide variety of circumstances before it is used on site. No matter how competent the producer is, colour variations will occur from product to product as well as within a product. This also applies within small areas of the face of a section of visual insitu concrete. Consider a few of the relevant variables that affect colour, tone and texture, and which the most realistic of control systems cannot control to give a uniform ‘sameness’ to the surface appearance: • • • • • •

slight changes in the fine aggregate grading; slight changes in the colour of the fine aggregate; small variations in the cement content; small changes across the concrete mix in water/cement ratios; slight changes in the surface curing conditions; small variations in cement colour.

The words ‘small’ and ‘slight’ apply solely to the region of ultra-fine control, which is not within the practical control that the strictest of manufacturers can exercise. As far as the relationship between manufacturer/producer and purchaser/user is concerned, what this amounts to is an acceptance that there will be control but not at a level that would be impossible to obtain. Therefore any agreement on what colour variations are acceptable needs to be based upon samples that represent both the limits within which the manufacturer can operate and any effects that the geometry of the units in question may have. To cover the first aspect, samples ideally need to be unit-sized and replicated for both works and site comparison. Each sample, as well as a comparison of one sample with another, should demonstrate the variabilities within which the full production can work. In dealing with the second aspect, two of the many factors that have to be considered are the inclusion of insulation, causing differential curing rates, and the differing aggregate facets shown by vertically cast and horizontally cast faces of exposed-aggregate concrete. The subject of colour leads on to concrete deliberately coloured by the inclusion of pigments in the mix. These materials are generally fine powders of iron oxide for the blacks, reds, yellows and browns, but also include green chromic oxide and blue cobalt oxide. All these have the same order of fineness as Portland cement, and are nominally spherical particles. The exception is yellow pigment, which has dendrite-shaped particles (shaped, microscopically, like the branch of a fir tree). Yellow iron oxide pigment (actually an iron oxide/iron hydroxide complex) needs extra care in dispensing and mixing. Carbon black is available as a pigment but has two general disadvantages. First, it is commonly an order of magnitude or more finer than the metallic oxide

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pigments, and the low dosage needed is difficult to control, which often results in large colour variations. Second, and more important, carbon black pigment is not only irregular in shape but has a sponge-like porous matrix, which occludes any lime bloom that forms much more than the impervious and generally spherical metallic oxide pigments. This has resulted in carbon-pigmented concrete being wrongly labelled with a colour fade description. Although carbon can slowly oxidise at weathering temperatures it does not fade in the short term; it is masked by lime bloom. Figure 5.2 shows a dwelling with tiles pigmented with both carbon and iron oxide. The carbon-pigmented tiles are mainly those further from the camera. Recommendations for avoiding colour variation are listed at the end of this section. However, it has always seemed incongruous to me that a material costing 5–500 times the price of cement should be put into a concrete mix with little or no plan in mind to promote colour retention during the planned aesthetic durability period. The most expensive pigment is blue cobalt oxide, for which a daily price would probably have to be obtained as it is based on a semi-precious metal. A blue organic copper complex pigment is available, but its colour tends to change to green as the concrete carbonates. The natural blue mineral ultramarine is unstable when used as a pigment, and fades completely after only a few weeks’ exposure. Concerning applied colours in the form of paints, no problems have been found with the inorganic silicate-based systems. They not only require nominally zero maintenance (apart from washdown needs), but have a good performance track record of more than a century. Maintenance has been found necessary with organic chemical-based paints, mainly because of ultra-violet effects.

Fig. 5.2 Lime bloom on tiles pigmented with iron oxide and with carbon.

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Concerning the second group of variables relating to site conditions, an optimum method of recording colour and its change with time is provided by standardised night-time flash photography during a dry moonless night, using the same film, camera and position each time. 5.3.1 IDENTIFICATION There is an apparent lightening in colour with age, and a variation in colour, texture, shade and reflectance, both within distances as small as a few centimetres within a unit and from location to location and unit to unit. The lightening with age tends to promote an eventual uniformity, which can occur after only a few months on a south or west unprotected face, and after several months or a year or two on other faces. 5.3.2 REMEDIAL Either apply a silicone to BS 6477 after any remedial work, or do nothing. Any other form of surface treatment, including acid etching, might seem to be beneficial in the short term, but nearly always makes matters worse. 5.3.3 AVOIDANCE In addition to exercising the best manufacturing control, any concrete (pigmented or otherwise) will generally benefit from the use of stearic acid in wet-cast mixes, or aluminium or calcium stearate in earth-moist mixes. Both these admixtures endow the concrete with water-repellent properties, and therefore, if control is mediocre or poor, there will be minimal lightening due to weathering, and defects in colour variation will be visible for a long time. Consideration could also be given to using pigments in pre-blended form, where the pigment and the water-repellent admixture are mixed with the cement in a powder disperser, as illustrated in Fig. 5.3. The advantages of this quickly offset the cost of the blender, because not only can the pigment content be reduced by up to 50% for the same staining power as if it had been added to the mixer, but the blend containing the water repellent becomes a hydrophobic cement. During storage it would not be subject to the typical caking or sack-hardening associated with untreated cement. Mixes containing water-repellent admixtures need to be mixed by directaction as in pan-type mixers. Drum mixers generally do not have enough energy to mix effectively when stearine-type admixtures are used. 5.4 CRACKING AND SLENDERNESS RATIO The problem of cracking has been encountered mainly with cast stone sills and lintels, either during handling and storage or when built into the

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Fig. 5.3 Elutriation blending of powders inot cement.

structure. The cause was almost certainly the specification of units that had a large z length dimension compared with the x and y section dimensions. This is also related to the false expectation that a product having a cube strength in the range 30–40MPa is as ‘structural’ as a 40– 60MPa wet-cast concrete unit. As far as the product is concerned, cast stone, especially the common moist-mix design products, is generally a mortar mix and not a typical concrete mix. The bending strength of concrete is about 10–14% of the compressive strength. For the mortar-mix cast stone this ratio has been found to concentrate at the 10% end of the range. Therefore, if a cast stone with 40MPa compressive cube strength is being made, the highest bending strength will be about 4MPa. However, this assumes that the compaction and strength of the unit are the same as the cube, but this is rarely likely to be the case. The safety factor that can be ostensibly applied has not been specified, and it is only in the standard for cast stone (BS 1217:1997) that maximum limits for z have been specified compared with the x and y dimensions. This part of the cast stone specification is based upon x and y being represented by an inscribed or superscribed circle in the xy section, depending both on shape and on whether x or y is vertical when the product is handled. As far as handling, transport and storage are concerned, it is obvious that if a long unit is being lifted at its ends and x is larger than y, then it is

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best moved with the x axis vertical. However, when lifted by two operatives, such a unit is more comfortable to hold with the shorter y axis vertical. This will result in a larger bending moment, and therefore the recommendations in BS 1217 need to be accompanied by strict supervision. Specifications for cast stone sills and lintels have perhaps been influenced by the use of prestressed units, which not only can be manufactured and installed virtually crack-free with high slenderness ratios, but also can support large loads of superimposed masonry. The problem at the manufacturing stage for cast stone or similar products (other than prestressed) is that producers have been loath to tell the specifier that there is a cracking risk due to the unit’s being too slender. This problem will remain as both a specifier and manufacturer remit unless the specifier receives a documented risk warning. Figure 5.4 shows a lintel of cast stone that has a high slenderness ratio. On site, in addition to picking up a unit with the shorter of the x and y dimensions in the vertical direction, there is the extra bending moment brought about by the manual or cranage acceleration in changing from the rest position to a moving one. As far as I know no data have been published on the effect of this, but consider the effect of this acceleration on the lintel in Fig. 5.4. Take the following reasonable assumptions: • • • • • •

a cube compressive strength of 40MPa; an idealised bending strength (equivalent prism) of 4MPa; a realcrete bending strength of 2MPa; a cast stone density of 2100kg/m3; acceleration at the first lift that is double the static bending moment; unit weight=95kg.

Fig. 5.4 Cast stone lintel with slenderness ratio (not to scale).

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The calculations give the following approximate data: • • •

Dynamic (during lifting) bending moment=0.50×106Nmm; Moment of inertia divided by distance of neutral axis=0.3×106mm3 Flexural stress applied at lifting stage=2MPa approx.

This is the same as the assumed realcrete bending strength, and so the unit is likely to crack. An additional problem with cast stone on many sites is that such products are treated with little respect. Not only have storage and handling conditions been commonly found to be poor, but the installation is generally carried out by bricklayers. If natural stone was being installed it would be handled by masons. Although cast stone is generally cheaper than natural stone it is made (by its BS 1217 definition) to be used in place of natural stone. It therefore seems illogical to use price when comparing installation processes. 5.4.1 IDENTIFICATION Look for one or more cracks near the centre of the unit, tending to narrow or disappear towards the top of the section. End-lifting a sill or lintel in its upside position rather than its design installation position would result in severer cracking, as there would probably not be any reinforcement in the tensile zone created during the lifting. 5.4.2 REMEDIAL A polymer-based system is often suitable, provided that an engineer has advised on the suitability for repair rather than rejection. The selection depends upon whether or not the cracks are dead or live. Whichever type of repair is selected, it is an aesthetic advantage to cut a trench along the crack about 10mm wide and deep, and to make this good after attending to the crack. The epoxide-based resins are suitable for dead cracks in many cases, and the copolymer emulsions for live cracks. The trench repair can be undertaken for the two types of repair material with a polymer matching (after weathering) mortar or a sealant respectively. These are broad recommendations, but each case has to be treated individually. There are many geometrical variations with concrete and cast stone sills and lintels, which make it difficult to generalise. 5.4.3 AVOIDANCE Units should be designed and manufactured with limiting slenderness ratios. Cast stone units need to be designed to the specified clause of BS 1217, and the conditions laid down therein could well be good starting

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points for other types of precast concrete unit. On site, contractors need to be aware that the product being handled, especially if it is cast stone, needs careful consideration at all stages. When the products are cast stone, masons should be employed for the installation. 5.5 THERMAL CRACKING IN PIPES The problem related to concrete pipes that exhibited cracks at their inverts, with crack apertures up to 0.5mm. As far as was known, in all cases the pipes were of well-designed, manufactured and cured concretes, and the incidence was not found to be a function of the method of manufacture. However, the cracking took place only during the hot summer months, which indicated that a thermal effect was responsible. Of more and, as it turned out, relevant interest was the opinion of one or two manufacturers that storing pipes with their axes in the north-south direction seemed to cause more cracking. To investigate this possible link, the author tested works-manufactured pipe rings of about 1m diameter by 0.5m long in the laboratory. Heat was applied from an arc battery of infrared lamps, which could be rotated to simulated solar radiation with both north-south and east-west axis orientations. The movement caused by this was measured by mechanical strain gauges round the periphery of each pipe. Surface temperatures were also measured, at locations referring to the centre of each strain gauge length. In the north-south simulated orientation there was a significant tendency for the heated side of the pipe ring to become ellipsoidal in shape; in some cases cracking was induced at the inverts similar to that observed in the works. For pipes tested in a simulated east-west storage direction there was a slight tendency towards an ellipsoidal shape, but the movements observed were not significant. Figure 5.5 illustrates the pipe ring positions during the test. It was also of interest to observe in the measurements of surface temperature that, in a laboratory running at about 20°C, concrete surface temperatures up to 60°C were recorded. The response of concrete to solar (or other thermal) radiation is fairly well known. On a hot summer’s day, dark-coloured or black concrete will feel warmer than grey or lightercoloured concrete. At night-time, the converse is observed. A body that is 100% efficient in absorbing (and consequently in emitting) radiation is known as a ‘black body radiator’. The author calculated the radiation coefficient for the grey-coloured concrete rings and found it to be about 0.8. This meant that ordinary grey concrete was almost as efficient in absorbing or emitting heat as a perfect black body. From this small apparent difference between grey and a theoretical black-coloured concrete it can be predicted that white concrete would not

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Fig. 5.5 Thermal cracking in concrete pipe.

be all that much cooler on a summer’s day than grey concrete. It would therefore follow, if research supported this prediction, that thermal protection covers or enclosures/containers should be silver-coloured rather than white or light in colour. 5.5.1 IDENTIFICATION The problem shows up as a single crack at one or both of the inverts, with crack apertures in the range 0.2–0.5mm. The cracks are common in the top row of pipes and in those stored with their main axes in a north-south direction. 5.5.2 REMEDIAL Subject to an engineer’s acceptance of remedial action, repair the cracks with an epoxide or polyester resin system, and re-store pipes with their axes in an east-west orientation. If the pipes cannot be stored in this way, protect them from direct sunlight with a suitable covering. 5.5.3 AVOIDANCE Store pipes with their main axes in an east-west orientation. If this is not possible, protect them from direct sunlight with a suitable covering.

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5.6 TUNNEL SEGMENT IMPACT DAMAGE The problem was observed on a site where contra-shaped trapeziumsectioned concrete pipe ring units were being hydraulically rammed into the forming of a tunnel lining. The precision required in the trapezium shape made it necessary for the manufacturer to choose concrete moulds because of their stability. The concrete in the precast units was specified to have an average cube strength of not less than 45MPa. In all other respects the concrete was observed to have been well designed, compacted and cured. The problem manifested itself during the ramming as edge-spalling and cracking, with most of the damage taking place with the final installed unit, namely the top dead centre locking piece. To counter the problem, which was apparently considered to be due to lack of adequate strength, the minimum average cube strength specification was increased from 45 to 50MPa. This resulted in a significant increase in the incidence of spalling and cracking. A subsequent discussion on the energy-absorption characteristics gave some credence to the attraction of using a more elastically behaving weaker concrete. It was thought that this property would apply to both slow and fast (impact) strain induction. The latter applied on this particular site. Figure 5.6 illustrates how weak concrete compares with strong concrete; the area under the curves is a function of the energy absorption.

Fig. 5.6 Stress versus strain and energy absorption.

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As a site trial, a batch of segments was manufactured with the specified minimum average strength decreased from 50 to 35MPa. It was found that the spalling and cracking problems virtually disappeared. From then on, all concrete segments were made to this weaker specification. A similar mechanism was discussed in section 3.2, where, under the impact of sandblasting, the face of the concrete receded but the elastic energy-absorptive plastics spacers were left proud of the surface. In section 4.2 impact was listed as one of the durability hazards. These observations all seem to imply that impact durability is inversely related to resistance. 5.6.1 IDENTIFICATION The problem shows up as spalling and cracking occurring during handling or installation, especially when high-strength concrete is being used. 5.6.2 REMEDIAL Subject to an engineer’s consideration, repairs can be undertaken following the general principles of CSTR26 (Concrete Society, 1984). 5.6.3 AVOIDANCE Where concrete, precast or in situ, is at impact risk and has not been fibre’reinforced’ for that risk, characteristic strengths are probably best kept within the range 30–40MPa. 5.7 TESSERAE DETACHMENT A tessera (plural tesserae) is generally a square or rectangular-shaped piece of ceramic or glass that is used to form a multi-tessera mosaic on the face of concrete or other materials. The problem met has been the detachment of these and/or hollowness, and has been commonly observed in precast concrete units, but has also occurred with in-situ concrete. It is common for mosaic to be used in sheets (e.g. 300mm square), with paper stuck to the visual face. For precast products, these sheets are normally laid paper-face down at the bottom of the mould, which then has a mortar applied, followed by concrete. The paper is washed or scrubbed off after demoulding, as the glue on the paper is water soluble. For in-situ concrete facades, the sheets are generally pressed onto an applied polymer mortar; when the mortar has hardened sufficiently, the paper is removed similarly. Both processes are very sensitive to the quality of workmanship, and this, together with specific design considerations, has been found to affect the bond detrimentally in the following ways:

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Fig. 5.7 Mosaic subject to edge weathering.

Fig. 5.8 Mosaic subject to compression at a joint.

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(a) inadequate surface preparation; (b) adhesive mortar allowed to stiffen or harden before applying the individual tesserae or sheets of mosaic; (c) the use of an adhesive mortar with inadequate adhesion; (d) weathering at an exposed mosaic edge return, as shown in Fig. 5.7; (e) tessera compression failure across a joint, as shown in Fig. 5.8. Note: A failure due to (c) alone would be a design failure, not a mater ial failure. 5.7.1 IDENTIFICATION The symptom is detached tesserae found on the pavement, ground or subroofs. Hollowness may also be present, and can be detected by tapping. This form of testing is best concentrated at the edges or joints of precast units and at random areas on in-situ concrete with, possibly, concentration at discontinuities such as windows, doors and corners. 5.7.2 REMEDIAL With reference to the above five reasons for probable bond failure, the following remedies are generally effective: (a)–(e) Remove tesserae from suspect or hollow areas. The use of heat in the form of a propane gas torch is very effective over large areas. The heat debonds the mosaic much more quickly than mechanical methods do. Take precautions to protect personnel and the building from falling hot tesserae. (a), (b) and (c) For (b) and (c) remove all mortar, and for all three treat the substrate mechanically, by grit-blasting, mechanical tooling or by heat calcination of the aggregate, to produce an exposed aggregate finish. At this stage or before aggregate exposure the substrate concrete could usefully be surveyed for the depth of cover of the steel, depth of carbonation and other properties. This would confirm that the concrete was suitable to receive remedial mosaic work without the possible later risk of failure due to rebar corrosion or other mechanisms. Once the surface has been prepared, apply an SBR mortar in areas no bigger than can be mosaic-applied without the mortar becoming dry or stiffening. All these activities should be undertaken by specialists. (d) and (e) Remove affected tesserae and mortar and prepare the substrate surface, preferably by mechanical means, to produce an exposed-aggregate finish. For (e) cut the joint to give an opening of 5–10mm and point up with a suitable sealant. For both (d) and

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(e) replace the tesserae to within about 50mm of the edge or joint, and finish to the edge with the same mortar as used for bedding the tesserae to form a margin. Note: For buildings constructed before about 1970, the common square tessera size was 0.75 inches. Replacement square tesserae are now 20mm in size, and filling in patches can be a problem, because a metric tessera is about 0.5mm larger than the imperial one it replaces. 5.7.3 AVOIDANCE This can be usefully set out as a number of do’s and don’ts in a format similar to a code. Do not: (a) design or apply mosaic right up to a joint or edge; (b) allow tesserae to butt across a joint. Do: (c) have a mortar margin about 50mm wide beside a joint or edge; (d) for mosaic to be applied to a hardened concrete face, expose the aggregate either by the use of a suitable retarder or by mechanical or thermal methods; (e) use polymer mortars in preference to plain mortars for sticking or casting against mosaics (the SBR-based systems have a good track record); (f) for glass mosaic apply an epoxide resin followed by a sand blinding to the adhesion face, and allow to harden before sticking onto the concrete as in (e); (g) in the rare case of using net-backed (as distinct from paper-faced) mosaic, follow the same procedure as in (f), ensuring compatibility between the epoxide and the glue used in the net. Note: Mosaics have a good track record, going back over two millennia. These ancient structures (the mosaic floor in the Roman villa at Bignor near Bognor Regis is a good example) indicate that good workmanship is the significant factor.

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Testing

6

INTRODUCTION Materials form the sole subject matter of this book, and so it is not surprising that testing so often comes into the picture. So often, problems have arisen in testing when the focus has been on the numbers or results that a test produces, with little or no attention paid to such matters as: • • • • •

the relevance of the test to the performance required; the value of the test result as affected by the tester; misleading interpretation of test data based on selective sampling of results; sanguine acceptance of complying results when there may be hazards unaccounted for; omission of relevant test requirements.

The reader has probably experienced other problems, but the following sections refer to my own ‘hands-on’ problem encounters. My hope is that these discussions will encourage the construction team members to question, discuss and offer suggestions before or at the tender stage. In addition, rather than look upon testing as a built-in item overhead, its importance and relevance to performance in practice might be better served by the incorporation of specific bill items. 6.1 LABCRETE OR REALCRETE The term ‘labcrete’ is often used to define concrete or mortar that is made and tested under strict laboratory conditions, whereas ‘realcrete’ applies to concrete made on site or in the works and used on site. The grey area here is that of concrete samples (such as cubes) made on site and then tested in a laboratory. The problems described here, with both laboratory-

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made and tested concrete and with site-produced samples, were in the validity and applicability of the results obtained. Consider, first, laboratory-made and tested concrete, taking admixtures as an example. The standard for plasticising admixtures (BS 5075 Part 1:1982) specifies inter alia that nominated concrete ingredients shall be used in a certain way to produce test data that confirm or deny that the admixture complies with the standard. The appendix of that standard (like those of the other admixtures standards) warns the reader of the need for site trials to assess the suitability of that admixture for the conditions on site or in the works. However, site or works conditions will not emulate the BS specification, and so compliance of any admixture with the standard does not necessarily mean that it will produce the target performance in realcrete. If samples are made on site and tested in the laboratory—a mixture of labcrete and realcrete—the results obtained are likely to be more meaningful than those from labcrete alone. However, the attention paid to the manufacture of the simple and small size of a cube or prism compared with, say, that of a column, is likely to give rise to doubts. Probably the best way to describe these two cases is to say that the labcrete admixtures standards give assurance of classification, coupled with potential for use, whereas the realcrete-labcrete hybrid indicates the maximum potential compressive strength of the realcrete. The specifier has a choice: •

•

Use data from labcrete or labcrete-realcrete (site-made cubes, for example) as a be-all and end-all, with or without the application of safety factors. Use realcrete data alone.

The second choice applies to a minority of concrete made: dimensionally coordinated precast concrete products, where the product itself is tested. (I prefer the phrase ‘dimensionally coordinated’ to ‘standardised’ because there could be 100 diagrams in a standard deemed to comply, but possibly only a few could be used with each other.) In-situ concrete and bespoke precast units such as cladding and cast stone are generally assessed for strength in a specification by a cube test. The latest standard for cast stone (BS 1217:1997) accepts this, and has a division between type tests (labcreterealcrete) and proof tests (realcrete). The main points to be addressed are the validity and applicability of realcrete and labcrete information, and this has to include the common realcrete-labcrete hybrid, generally known as a cube. To discuss the materials science and technical requirements of this problem, the test needs can usefully be listed under three headings: • •

The test must be meaningful. The test must be accepted by all parties.

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•

The test data must be accepted as final, with minimum or no interpretation.

6.1.1 MEANINGFUL It would be logical to assume that the purpose of carrying out a test is to produce data that relate either directly or, in a constant manner, indirectly to a necessary or desirable performance characteristic. If strength (compressive, tensile or shear) is in question, then it would be simple to assume that a cube or prism result is sufficient. This is difficult to accept, because a labcrete-realcrete test usually gives maximum potential strength and little else. The use of the cube or prism density figures (specified to be calculated and reported) generally gives misleading information. This is because nominal cubes are tested, and these are not necessarily geometrically true cubes: up to 1% deviations are permitted on all dimensions. Thus nominal cubes, all from the same concrete and virtually equally compacted, with a true density of 2350kg/m3, can have nominal densities in the range 2280–2420kg/m3. Therefore, apart from an indication of the maximum potential strength, there is a risk of sacrificing the target of ‘meaningful’ on the altar of traditionalism and the attractive cheapness of the cube test. The codes of practice, such as BS 8110 Part 1:1985, generally apply safety factors to the cube data to cater for structural design purposes. It could be argued, with hindsight, that if the rebound hammer had been invented before the crushing machine this problem would not exist. This leads to the interim conclusion that, wherever possible, preference should be given to realcrete testing, if there is any way in which it can be shown to be of use. If realcrete testing is the preference for producing meaningful data, then the next question is: which of the durability hazards listed in section 4.2 are relevant to the concrete being tested? It follows that the parties concerned with the test regime as well as the testers need to set up a matrix of properties versus tests so that a sensible application of the available tests to the concrete can be made. 6.1.2 ACCEPTABILITY Scientific and technical development of labcrete and realcrete in the construction industry will proceed only when three factors are addressed: (a) The test methods (included as costed bill items) are agreed in the specification. (b) Test limits or ranges are agreed. If interpretation is likely, this wording should also be agreed at a preliminary stage.

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(c) Preserving anonymity of the information source, data are fed back to the BSI Committee Secretary so that the revision of standards may make full use of the state of the art. The current example of the tardiness of acceptability is the BS 1881/200 series (non-destructive testing), all of which are currently ‘recommendations’, and not mandatory. An example of where (b) above has caused arguments is in the interpretation of site-drilled, laboratory-tested cores. In an attempt to deal with the interpretation of cores tested to BS 1881 Part 120, the Concrete Society produced guidance in the form of a report (Concrete Society, 1976; Addendum, 1987). The Society has accepted that this report (CSTR11) needs updating, and is currently carrying out research with this aim in mind. CSTR11 suggests various interpretative approaches in trying, amongst other exercises, to relate core strengths to the strength of cubes that would have been made from that concrete. However, the suggested operating factors are based on a small quantity of data, and definitive dogmatism should be avoided. The other factor relating to acceptability is that no matter how relevant any test procedure is to the property in question, there is the contractual matter of timing to consider. For example, if it takes six months to produce data relating to resistance to chloride ingress, and the track record shows that this resistance can be achieved by the use of PFA, GGBS or MS additives, then it makes sense for a testing specification to concede to a mix design specification. 6.1.3 FINALITY OF DATA Arguments often arise over the finality of data; many of these are based on lack of knowledge of the test criteria, including the status of the testing facility. If the interpretation aspect discussed in section 6.1.2 is brought into the picture, then it is possible that the wording was somewhat loose. Therefore it is probably best to aim at a test regime that has a minimum of or, preferably, no interpretative clauses. This reflects the discussion in section 4.3, and implies that total quality control at all stages—from specification to handover—would present the fewest obstacles to agreement on the finality of the data. The finality would naturally be based upon the three steps of the test regime’s being meaningful, acceptable and final. 6.1.4 IDENTIFICATION The problem reveals itself in the form of contract data invoking labcrete, labcrete-realcrete and/or realcrete tests that have little or no relation to the

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properties required and/or are capable of misinterpretation and/or do not carry bill items to cater for testing. 6.1.5 REMEDIAL No remedy appears to be possible; the situation would be a contemporaneous one, and not one that occurs at the tender or pre-tender stage. A contract review could perhaps be undertaken, in order to deal with possible problems to come, but this is in the contractual field and hence outside the remit of this book. 6.1.6 AVOIDANCE The main thing to avoid is a contractual dispute over any of the items in sections 6.1.1–6.1.3. One way to achieve this might be for the tendered parties to adopt a more proactive role, coupled with strong liaison between all members of the construction team. The setting up of a properties-versustests matrix, mentioned earlier, could well have much to commend it. 6.2 DESIGN OR PERFORMANCE I have commented in several sections on the testing specification being design based or performance based. Where the problem in this subject has reared its head is in a tendency to ignore the factors relating to this choice and to concentrate—wrongly—on performance testing. It is only partly logical to conclude that if concrete is required to perform in a specific manner then a performance test should apply. This conclusion ignores the many scientific, technical, architectural, engineering and contractual requirements that also apply. Compounding all this is the slackness that is sometimes met in the format of those parts of the contractual documents relating to the materials: slackness in • • • •

the words used; the intended meaning of those words; the interpretation of the words by the receiving party; whether or not the words addressed the property requirement.

It is highly unlikely that concrete would be needed in the construction with only one property requirement. Thus each property needs to be discussed in the light of the boundary conditions that pertain. There is growing pressure from the European standards organisations to concentrate on performance testing, with an apparent disregard of other considerations. This could create future problems. This pressure should be resisted; performance-based specifications should be supported only when they have minimum interference with buildability, and they relate to sensible

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property targets. This may mean that a specification has to have a mixture of design-based and performance-based clauses, but if the five requirements listed below are considered, this will form the basis for a logical materials approach: (a) (b) (c) (d) (e)

funder—price, speed and financial return; specifier—unambiguous, relevant and sensible clauses; specialists—appreciation of services involved; contractor and subcontractor—buildability; tester—timing of data returns and meaningful tests.

Other factors may also be relevant, but it is these five that, singly or in combination, have led to problems and discussions. None of these requirements is an independent variable; altering one of them will almost certainly affect one or more of the others. Chloride diffusion control by design rather than as a performance-based specification is an example: input at (b) involves (d) and (e). Another example commonly met in troubleshooting is the use of air entrainment to produce frost-resistant concrete (see also section 1.4). A typical specification for a concrete with 20mm maximum size aggregate would be 3.5–7.5% total air in the fresh concrete (BS 1881 Part 106:1983). This specification is design based, but has the aura of a performance test. It possibly comes into the category of the next section. Consider how this specification relates to (a)–(e) on the reasonable assumption that the construction team members wish to have a frost-resistant concrete (putting aside other property targets such as strength, flatness and appearance): (a) The funder is unlikely to be affected by the price or speed of putting the admixture into the concrete. Where the funder may be concerned is with costs arising after handover or completion due to (b)-oriented problems. (b) The specifier will not know whether compliance with the air content requirement means that the air bubbles are present in the optimum sizes and geometrical distribution. (c) Data from the specialist would probably emanate from the admixture manufacturer and are likely to be misapplied because we are dealing with labcrete, not realcrete or a hybrid. (d) The contractor’s buildability is unlikely to be affected unless (b) applies, in which case remedial or replacement work might be required. (e) On the basis of (b) and (c) the testing is not likely to be meaningful, but a delayed timing of data—to wait for petrographic results—will probably need to be considered if pre-works data have not been obtained. There are dangers of ‘tunnel vision’, in concentrating on design at the expense of performance testing (or vice versa), as well as in considering

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only one of the five requirements listed and discussed above and overlooking the others. The choice of design testing, performance testing, or both, depends upon what the concrete has to achieve in cost-effective performance terms. 6.2.1 IDENTIFICATION Look for any documentation in which design testing should have been specified instead of performance testing, or vice versa, as well as a lack of consideration of any one or more of (a)–(e) listed above. 6.2.2 REMEDIAL The only possible remedy for a current situation is to try and obtain a contractual variation or instruction to cater for the obstructing matters. 6.2.3 AVOIDANCE Pre-contract discussions or comments at tender stage seem to be the way to address specific cases. In general, the properties versus materials matrix proposed in section 6.1 could be used and qualified by method statements and test data limits. The benefits of having standard contract clauses addressing each of the construction targets could also be discussed. 6.3 CAMOUFLAGE TESTING This was one of the types of testing listed in Levitt (1985), which dealt with the philosophy of testing. Camouflage testing may be defined as any test requirements or procedures that are completely irrelevant to reasonable and sensible materials property targets. The problem with camouflage testing is that it is largely irrelevant, misleading, dishonest, and defies logic. There are a number of bases for camouflage: (a) trying to impress others by having a test clause; (b) copying something that has been done before without checking its relevance; (c) catering for a problem by invoking a test that has little or no relevance to that problem; (d) promotion of a test facility; (e) promotion of a proprietary product. An example of (a) was an instance where one of the construction party’s aims was to set out before another member of the building team a considerable amount of test data in order to impress by the amount of paperwork.

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Probably (b) is the most insidious form of camouflage testing, because it reflects strongly on the traditionalism that pervades the construction industry. The reader may be able to pick out examples in the previous text, but the author has had to be careful not to mention specifics. In (c) the case was where a client experiencing a problem with the concrete was ‘satisfied’ with additional but irrelevant testing. Persons becoming unwillingly involved in such a situation should record and document their views to the relevant party. Both (d) and (e) need no qualification, and examples can be found in the earlier text. 6.3.1 IDENTIFICATION The problem reveals itself in the inclusion of a test requirement (method and/or limits) that is completely irrelevant to a property that should be under consideration. 6.3.2 REMEDIAL Unless the requirement is deleted or altered, no remedy is possible. 6.3.3 AVOIDANCE As with so many of the other problems described earlier, a sensible discussion between the construction team members at pre-tender or tender stage is suggested. 6.4 REPEATABILITY AND REPRODUCIBILITY There is a growing trend to include data on these two properties in both British and American standards. Briefly, the meanings of these two words are as follows: Repeatability refers to the production of data by a specific centre, either by the repetition of testing on the same sample (non-destructive tests, for example) or by replicate tests on subsamples from the one master sample. These data can be produced by more than one operative working in that centre. Repeatability is commonly described in statistical terms such as variance, standard deviation or range. Reproducibility refers to the production of data on nominally identical subsamples or samples tested at more than one centre, and compares the results within the group of centres. Again, as with repeatability, a statistical method is generally used to compare numbers.

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The term often used in standards and other documents to describe repeatable and reproducible data is ‘precision data’. Concrete is a multi-component (sand, coarse aggregate, water, cement, admixtures, additives), multi-variable (mixing, compaction, curing) material. The problems that this causes are twofold. First, whatever test is being considered there will be variations, and the demand for stringency in repeatability limits has to be realistic. Second, although statisticians prefer relatively large numbers of centres to be involved for reproducibility studies, there have been instances when inferences or ‘conclusions’ have been drawn from as few as six cooperating laboratories. In my view, the number should be at least 12. Therefore, as far as repeatability is concerned, it is possible for a single centre to produce enough data for a statistical analysis to be meaningful (assuming that the test being examined is one for which that centre can produce the required quantity of data with acceptable interference on its other commitments). However, the test has to be of a common genre and common to a large number of centres. So, for concrete testing laboratories, it follows that a study of repeatability and reproducibility would be feasible for data such as cube strengths, and aggregate specific gravities, but restrictions could well be encountered for petrographic tests, oxygen diffusion tests and the like. Caution is necessary when using statistics, because it is an applied and not a pure form of mathematics. Because assumptions are made in the mathematical treatment of data, any results produced are not definitive; statistics does not ‘prove’ or ‘show’ anything. The results can only indicate likelihood, comparison, relationship or trend. Section 4.4 discussed the inadequacy of the normal or Gaussian distribution in catering for cube or cylinder strength when the target strength is close to the aggregate crushing strength. For UK aggregates, ultimate strengths in the range 60–100MPa could be assumed as typical, and so C50 and higher specifications for concrete strength might well require a different approach for both specifying and drawing inferences. This application of data would apply to repeatability tests inter alia. Example 1 This example concerned the use of the Brinel hardness pistol to assess the strength of prestressed concrete units in a factory. The pistol used to be in common use as a hand-held test tool for hardness testing of metals and alloys. Its principle was to impact a hardened steel ball against the surface under test; the hardness of the metal was assessed by the diameter of the spherical impression. (The same principle is now used for metal testing, but a diamond with strict geometry to its facets is used. All modern test equipment is in the form of a composite machine.)

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In the precast concrete works, the manufacturer’s target strength was 45MPa at 28 days. Strict total quality control was exercised, and the cube strengths obtained lay in the range 40–50MPa. Each time the units were tested with the pistol an average impression diameter of 3mm was recorded, with a range from about 2.7 to 3.3mm. As these data referred to a specific strength-repetitive concrete, a range of concrete cubes were made in another laboratory with cube strength targets ranging from 15 to 60MPa at 28 days old. Just before crushing, each cube was tested with 10 pistol impressions, and the average of these was compared with each cube result. It was found that, irrespective of the strength, the impression diameter was always about 3mm. Two points arise out of this. First, consistency of data can be misleading. Second, as discussed in section 5.6, the weaker concrete could have been predicted to have improved resistance, because its energy absorption characteristic would have been better than that of the stronger concrete. As an aside, this leads to an apparent anomaly, in that the rebound hammer generally gives a positive relationship between rebound and strength; the rebound numbers increase with increasing strength. The reason for this may be the difference between the relatively large area of impact of the hammer and the 6mm diameter steel ball in the old Brinel hardness pistol. Example 2 The problem relates to the recently issued recommendation for nondestructive testing of concrete using initial surface absorption (BS 1881 Part 208:1986). The standard refers to the omission of precision data, as there was not enough information to hand when the standard was prepared. The ISAT, by its nature, generally measures only the surface voidage property, and at a relatively short interval from the start of the test. Observation of a typical concrete surface drying out after rain would reveal a patchy appearance over distances as small as a few millimetres, caused by variations in the absorption properties. The sensitivity of the ISAT would be expected to reflect this variation, and experience has shown this to be so. ISAT units are specified to be recorded in units of mL/m2.s, and the apparatus has minimum and maximum range limits of 0.01 and 3.0 of these units respectively. At the lower end, the result can be read to an accuracy of 0.01, and at the higher end to 0.2. In practice it has been found that, taking readings at 10 minutes as examples, concrete averaging 0.01 will vary from zero to 0.03. The more permeable example would vary from 2.6 to ‘too fast to measure’. This, in my opinion, indicates that precision data will be difficult to obtain for the ISAT, and that it is unrealistic to expect ‘ideal’ repeatability and

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reproducibility. An additional problem found on site with the low (say 0.01) results is that if the reading is taken as the sun starts to shine on the equipment, a liquid expansion occurs in the cap and a ‘negative’ absorption can be recorded. 6.4.1 IDENTIFICATION There may be pressure to ask for precision data when they are either not justified or irrelevant, as well as the use or tabulation of data based upon a small number of results. 6.4.2 REMEDIAL In a current situation there would appear to be no remedy apart, possibly, from a review of or amendment to the conditions of application. 6.4.3 AVOIDANCE The recipients of precision documentation in standards, specifications and regulations preparation should take a proactive role. A defensive, reactive response to the receipt of such data is not constructive. 6.5 CHANGES IN TESTING The problem is, quite simply, tradition. This takes the general form of strong resistance to anything new. I neither condone nor condemn this attitude, but I cannot agree with a generalisation either way. If a test has been established for a long time this neither means that it is the right test (see 6.1–6.4) nor that there is necessarily a better test that could take its place. The problem is probably exacerbated by the lack of use of the currently available mechanisms to correct the problem. It is logical for members of the construction team to accept that testing needs to have a nominated position in the control of material properties. If testing is a weak link in the chain joining performance to materials, design and workmanship then science and technology will have little or nothing to contribute to construction. The best way to tackle this problem is to study each test requirement on the basis of the matrix suggested earlier in this chapter, and then do one of the following: (a) (b) (c) (d) (e)

Confirm and/or reinforce that test. Replace it with a different test. Run a new test alongside the existing test: that is, (a)+(b). Remove the test requirement completely. Introduce a test where there was no test before.

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It is in respect of (c) that the future appears to be the most attractive. Both British and European standards have a tendency for a specific test to be the ‘reference’, with other tests being subsidiary. For several British Standards, where an alternative test to the reference test is included, users are requested to submit data (but obviously not contract details) to the BSI. This is a good idea, but lacks the power to change things. It might be better to make both the reference and the alternative tests mandatory, with all results to be sent to the BSI. (The BSI would be the secretariat for national as well as European and international standards). The complete removal of a test, as listed under (d), can form a large discussion platform. Over the years many revised standards have omitted earlier test specifications. Reasons for the omission of a test would be given in the revised standard. There is no reason to conclude that this process is complete; there are still some tests that have no reason for their presence other than tradition. By the same token, under (e), there is no reason to conclude that every test necessary to define a property or performance need is present in every Standard. If the matrix approach suggested in sections 6.1–6.3 is acceptable, a method of dealing with the problem and its spin-offs could result. 6.5.1 IDENTIFICATION The problem reveals itself as a reliance on inappropriate or misplaced tests, often coupled with a resistance to consider or accept anything new or different. 6.5.2 REMEDIAL Apart from discussing the possibility of variations to the test requirements there seems to be little that can be done to remedy a current problem. 6.5.3 AVOIDANCE Refer to section 6.4.3 for a nominally identical approach. BSI publications such as BSI News provide a monthly update on the progress of British, European and international standards. In addition, any person can purchase a draft at the public comment stage and submit their opinion to the relevant secretariat. 6.6 TESTING FIXATION Although this title implies that the problem is a mixture of the earlier discussion in sections 6.3 and 6.5, there is in fact a completely different

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facet that warrants exposure. The problem encountered was a dogmatic insistence that wherever or whenever a property or requirement was under consideration there had to be a test accompanying that part of the specification. This insistence was often found to generate a spin-off problem in the form of a reversal, which commonly manifested itself as an insistence on some form of property requirement so that a test could be proposed to accompany it. An example of test insistence that in my opinion was (and still is) largely unjustified was described in reference to aluminous cement in section 1.7. The test was differential thermal analysis on drilled powder samples taken from precast pretensioned concrete beams, made of high alumina cement (as it was then called and is still known), in order to ascertain the degree of conversion. Virtually every construction examined in my experience showed 70–90% conversion, with stable performance of the precast units and the construction. An example of testing that was not really sensible was described in section 3.1 in reference to chloride diffusion, where track records have shown that good performance has been achieved by the mix design route. Testing would have not furnished data of significance for about 6 months, and such a potential contract delay to await test results would have been unacceptable to most parties in the construction team. It is debatable whether either the alkali-silica reaction (section 1.5) or delayed ettringite formation (section 1.12) comes into the spin-off category referred to above. In my experience damage has been almost certainly due to ASR on only three constructions. As far as DEF is concerned, apart from the possibility of its having been the cause of the splitting observed in experimental kerbs described in section 1.12, no case on site has been experienced. 6.6.1 IDENTIFICATION Someone will insist on the presence of a test and/or call up a property, whether relevant or not, so as to have a test to address that property. 6.6.2 REMEDIAL If discussion is possible, and logic can be applied, a change in the wording to the testing or property documentation should be attempted. 6.6.3 AVOIDANCE The most fruitful approach would seem to be full discussion at committee, institution or authority levels before requirements are put into formal documents.

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6.7 TESTING ACCURACY The problem refers to the data produced from a test initiated at the specifying stage by the demand for an impossible or unreasonable accuracy, and at the reporting stage by a form of impressionism. Examples are given below. 6.7.1 PROBLEMS AT SPECIFYING A common example of this family of problems is in typical specification wording such as ‘The concrete cube strength shall be SOMPa at 28 days old’. There are two virtual impossibilities here. First, even under the strictest form of production, it is impossible to get each cube to reach the exact strength of 30MPa at that age. Second, cube strengths are specified to be reported to the nearest 0.5MPa, so the 30MPa (if it had been possible to achieve) should be 30.0MPa. The omission of that SOMPa being specified to be a minimum, maximum or average could also be called into question. In other instances, dimensions have been specified to an accuracy of 1mm, and tolerances have been completely omitted from the drawings. In the former case, the contractor or producer was being asked to work to the unachievable; in the latter case, no tolerance would seem to be permissible in the work. 6.7.2 PROBLEMS AT REPORTING An example of this is with a typical cube-testing machine that ‘locks in’ the failing load reading to the nearest 1kN. Thus it would appear that a 100mm (nominal) cube failing at 424kN load could be reported to have had a 42.4MPa strength. Putting aside the specification requirement of reporting to a 0.5MPa accuracy, this report ignores the machine accuracy. At the best this would be no better than 1% under a Class 1 machine certification. It also ignores the cube’s being only nominal in size, with a 1% allowance on all dimensions. Therefore the 42.4 could be anywhere between 42.0 and 42.8 on the machine accuracy. The crushing area of the cube could be up to 2% larger or smaller than the specified nominal size calculation. Taking the largest negative and positive cube area sizes on the final load range, the actual cube strength (ignoring other testing variables) could lie anywhere in the range 41.2–43.6MPa. So although the specified reporting accuracy for this cube gives a strength of 42.5MPa, it is still subject to an error of about 1MPa. Another example of a reporting problem is with a water absorption test on, say, an approximately ‘cubic’ sample of 100mm ‘side’ cut from concrete. It, and its weight changes from oven drying to wetting, can usually be measured to an accuracy of 1g. For a sample weight of about 2kg, this

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represents an accuracy of 0.05%. If an operative carried out 30 minute absorption tests on three subsamples and obtained readings of 3.50%, 3.50% and 3.55%, an average of 3.52% could be reported. The lesson here is that reporting accuracy should not be based upon unnecessary mathematics, which can give answers indicating a form of superiority. 6.7.3 IDENTIFICATION Look for an unreasonable or impossible accuracy specified or an unjustified accuracy in the data reported. 6.7.4 REMEDIAL In the first example, the specifier should be advised of the impossibility or inapplicability of the requirement; in the second example, the report should be returned to the issuing activity. The corrected replacement report should have the same report reference number as the superseded one but be marked ‘Rev’ or ‘Superseding Report No......’ or similar, and the superseded report should be marked as such. 6.7.5 AVOIDANCE Both specifiers and testers should be aware of the problems that can be generated, and should take appropriate steps to avoid them. Other members of the construction team should also draw the attention of the specifier or the testing authority to any cases that come into their remits.

Copyright 2003 by Taylor & Francis Group. All rights Reserved.