Concrete Repairs: Performance in Service and Current Practice (EP 79)

It is estimated that around 50% of Europe’s annual construction budget is presently spent on the refurbishment and repair of existing structures. This report is the culmination of a wide-ranging survey into the performance of both current European concrete repair techniques and inspection practices, and current research projects. It assesses the case histories gathered from across the sector, including from owners of concrete structures, repairers and research institutes, and presents its findings using charts, graphs, tables and photographs. A review of the problems of concrete durability, current issues of sustainability, and the differing expectations of what concrete repairs should achieve, provide a practical introduction to the subject. The survey was part of the work carried out by the CONREPNET network, made up of European research and representative bodies sponsored by the European Commission.

Achieving durable repaired concrete structures EP 77, 2007 concrete structures in fire: performance, design and analysis BR 490, 2007

Conrepnet 1 coverv1.indd 1

Performance in service and current practice G P Tilly and J Jacobs

IHS bre press

IHS BRE Press, Willoughby Road Bracknell, Berkshire RG12 8FB www.ihsbrepress.com EP 79

Concrete Repairs

G P Tilly and J Jacobs

Related Titles from ihs bre press

Concrete repairs Performance in service and current practice

concrete repairs performance in service and current practice

12/11/2007 11:18:53

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CONREPNET Partners:

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CONREPNET Thematic network on performance-based remediation of reinforced concrete structures

Concrete repairs Performance in service and current practice

G P Tilly, Gifford & Partners Ltd J Jacobs, Belgian Building Research Institute

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Details of all publications are available from IHS BRE Press Website: www.ihsbrepress.com or IHS BRE Press Willoughby Road Bracknell RG12 8FB, Uk Tel: 01344 328038 Fax: 01344 328005 Email: [email protected]

Keywords Concrete structures, EN 1504, maintenance, management, performancebased intervention, protection, repair, remediation, sustainable assets, through-life care

Requests to copy any part of this publication should be made to the publisher: IHS BRE Press Garston, Watford WD25 9XX, UK Tel: 01923 664761 Fax: 01923 662477 Email: [email protected] EP 79 © CONREPNET 2007 First published 2007 ISBN: 978-1-86081-974-2

The companion publication Achieving durable repaired concrete structures is available through IHS BRE Press (order ref. EP77). Both publications can be purchased as a set (order ref. EP81).

For further details please visit www.ihsbrepress

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Contents

Executive summary CONREPNET partner organisations

vii viii

Acknowledgements

ix

Abbreviations

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Chapter 1

Introduction

1

Chapter 2

Expectations of repairs

3

Chapter 3 3.1 3.2 3.3 3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.6 3.4.7 3.4.8 3.5 3.5.1 3.5.2 3.5.3 3.5.4

Performance of repairs in practice Background Causes of deterioration Types of repair Performance of repairs Inspection Classification of performances Overall repair performances Overall repair performances (by repair type) Cathodic protection (CP) Types of deterioration Repairs in combination Modes of repair failure Causes of repair failures Incorrect diagnosis Incorrect design Poor workmanship Incorrect repair material

5 5 7 8 10 10 10 10 10 11 12 12 13 14 14 14 15 15

Chapter 4 4.1 3.2 4.3 4.4 4.5 4.5.1 4.5.2 4.5.3 4.6 4.6.1 4.6.2

Current repair practice Background Inspection Repair methods Quality control Comparison with earlier repair practice Relative use of the different methods of repair Types of patch repair Coatings Inspection strategy Methodology of inspection Post-tensioned structures

17 17 17 18 19 20 20 20 21 21 22 25

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Concrete repairs: Performance in service and current practice

Chapter 5 5.1 5.2 5.3 5.3.1 5.3.2

Current research Sizes of research projects Research topics Outcome of research projects Improved durability Performance-based repair

27 27 28 28 29 30

Chapter 6 6.1 6.2

European Standards The current position Application to performance-based repair

31 31 32

References

33

Appendices Appendix I Concrete Repair History Questionnaire Appendix II Concrete Repair Methods Questionnaire Appendix III Concrete Repair Evaluation Methods Questionnaire Appendix IV Concrete Repair Research Questionnaire Appendix V Related research projects

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Figures Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 3.5 Figure 3.6 Figure 3.7 Figure 3.8 Figure 3.9 Figure 3.10 Figure 3.11 Figure 3.12 Figure 3.13 Figure 3.14 Figure 3.15 Figure 3.16 Figure 3.17 Figure 3.18 Figure 4.1 Figure 4.2 Figure 4.3 Figure 4.4 Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9 Figure 4.10 Figure 4.11 Figure 4.12 Figure 4.13 Figure 4.14 Figure 5.1 Figure 5.2

Distribution of respondents Distribution of environments Distribution of case-histories by structure Ages of structures Primary causes of original deterioration Ages of structures when repaired Relative incidences of different types of repair Relative uses of different types of coatings Performances in relation to age of repairs Patch repair after five years, showing signs of incipient anode behaviour due to non-removal of chloride contaminated material Performances of patches Performances of patch repairs to corrosion Modes of failure – all types of repair Failure of polymer mortar patches and sprayed polymer coatings applied to an AAR affected bridge Failure of polymer mortar patches and polymer coating applied to a bridge affected by corrosion Reported causes of failures Aesthetic deterioration of cement-based coating repair Influence of environment on performance – all types of repair Use of repair techniques Acceptance ratings of repair products in the market Acceptance ratings of the repair companies Comparison between past and current usage of repair methods Comparative use of cement-based and polymer-based mortars in patches Comparative use of barrier and hydrophobic coatings Performances of repairs Levels of detectability of the deterioration of a repair Pull-off test to measure adhesion of coatings and patches Corrosion probes fitted to reinforcement prior to repair concrete being placed Location of reinforcement bars Measurement of crack widths Detection of carbonation Measurement of electrode potential Distribution of respondents to research questionnaire Distribution of research topics

6 6 6 7 8 8 9 9 10 10 12 13 13 14 14 15 15 16 18 19 19 20 21 21 22 22 24 24 25 25 25 25 27 28

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Concrete repairs: Performance in service and current practice

Tables Table 3.1 Table 4.1 Table 4.2 Table 4.3 Table 4.4 Table 5.1 Table 5.2 Table 5.3 Table 5.4 Table 5.5 Table 5.6 Table 6.1

Successful CP installations Relative usage of inspection techniques Use of the common methods of repair Examples of NDT to aid acceptance of repairs Continuous monitoring Number of participants in research projects Distribution of research topics and funding Distribution of research projects Research preferences of respondents Outcome of ongoing and completed research projects Research problems identified from case-histories European Standards related to concrete repair products and systems

12 17 18 24 24 27 28 28 29 29 29 31

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Executive summary

It has been estimated that some 50% of Europe’s annual construction budget is spent on refurbishment and repair of existing structures. This figure is expected to increase as the major population of concrete structures built in the 1960s and 1970s, which form a key part of Europe’s infrastructure, start to require further repair and refurbishment. However, limited available resources need to be applied with greater efficiency and owners of buildings and infrastructure now require greater certainty in the performance of their concrete structures in order to manage their assets more effectively. This has generated a requirement for industry to deliver more durable repairs to concrete structures. To help address these issues, a thematic network on the performance-based repair of reinforced concrete structures was established in 2003, known as CONREPNET. The object of this EU-funded network is to improve the durability of concrete repairs through performance-based rehabilitation. To this end, information about concrete durability and repair issues has been collected from industry and researchers. Problems and barriers to achieving durable concrete repairs have been identified and contemporary industry practices have been investigated. This report is concerned with sections of the project that deal with performances of repairs in practice, current practice and research. Performance in practice has been assessed through casehistories obtained from members of the network and others. Some 230 case-histories were obtained for concrete structures up to 150 years old but mostly 20 to 50 years old. The most common type of deterioration reported was corrosion of the steel reinforcement, which occurred in 55% of the cases. Performances of repairs are disappointing; 20% failed in five years, 55% failed in 10 years and 90% failed in 25 years. The longest repair life was 52 years. The most successful repairs were schemes involving restoration of strength and crack injection. Patches were applied in 60% of the repairs and were 30% successful when applied solo and 50% successful when applied in combination with a coating. Causes of repair

failures were ascribed to incorrect diagnosis, incorrect design of repair, poor workmanship, use of incorrect materials and other factors such as extreme weather conditions during the repair work. Most of the repair projects reported in the case histories were carried out in the period from 1960 to 1990, using practices current at the time. A survey of current repair practice (2003) indicated that there has been little change since the methods of repair continue to be broadly similar. Patching is becoming less common as electro-chemical techniques and polymer mortars become more prevalent. Inspection is regarded as critical to the repair process but some small repair works for private owners were reported as starting without any inspection. The most common methods of non-destructive testing were measurements of depth of cover, carbonation and chloride content. Around 25% of repair and inspection work is subcontracted. A total of 138 research projects were surveyed (66 obtained via questionnaires and 72 via the official website of the European Construction Research Network, www.e-core.org). Numbers of participants per project varied from one to 21 and budgets were from €5,500 to €5 million. The projects were concerned with the different aspects of concrete repair; durability, materials, inspection, maintenance and restoration of strength. It was found that only 60% of the research addressed problems identified from the case-histories. It is concluded that research to aid a performance-based approach to repairs should address performances under all weather conditions, and improved acceptance testing to provide assurance that repairs will be durable. Repair standards should be revised to have a more ‘performance friendly’ orientation.

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Concrete repairs: Performance in service and current practice

CONREPNET partner organisations Building Research Establishment, UK (BRE): network co-ordinator and overall catalyst for many technical aspects of the project, including the vision for future performance-based concepts (WP4) Belgian Building Research Institute (CSTC): leader of WP3 on current practices CT Koulutus Oy, Finland (CT Centre): leader of WP5 on dissemination, research and technical development exploitation, training and intellectual property rights (IPR) issues, led development of implementation of future performance-based concepts (WP4)

Freyssinet International, France: brought the repair specialist’s perspective to the project and to the development of WP4 concerning future performancebased concepts Gifford and Partners, UK: leader of WP2 on the performance of past repairs and interventions Institute of Construction Science ‘Eduardo Torroja’, Spain (IETcc): led development of one methodology for monitoring and assessment of performance of protection and repair interventions on concrete structures employed in WP4 on future performance-based concepts STÚ-K, Czech Republic: co-ordinated WP4 on future performance-based concepts

CONREPNET Member Organisations Autostrade per l'Italia

Hywel Davies Consultancy

Queens University, Belfast

British Nuclear Fuels plc

Ingenieurbuero Prof Schiessl

Rakennus Oy Wareco

Centrum Stavebniho Inzenyrstvi (CSI)

Ingenieurgemeinschaft Cossebaude GmbH

Red Nacional de Ferrocarriles Espanoles

City of Kotka

Karlomix Bohemia

Slovenian National Building and Civil Engineering Institute (ZAG)

City University, London

Kingston University

COWI

Konsultointi Jarvinen Oy

Swedish Cement and Concrete Research Institute (CBI)

Czech Roads and Motorways Directorate

Laboratoire de Recherche des Monuments Historiques

Swedish National Road Administration

Danish Technological Institute

Lund Institute of Technology

Team-Danielsson Oy

DYWIDAG Systems International

Metrostav

TNO Building and Construction Research

Entisointi Pulla Oy

Mott MacDonald Limited

Federal Institute for Materials Research and Testing

NCC Engineering

Union of the Czech and Moravian Housing Cooperatives

NECSO

University of Birmingham

Network Rail

University of Patras

Norut Teknologi AS

Vattenfall Utveckling AB

Norwegian Defence Estates Agency

Vilniaus Miestprojektas

Hellenic Cement Research Centre Highways Agency

Parish Union of Helsinki

FEREB FORCE Technology Glasgow City Council

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Acknowledgements

The authors acknowledge, with thanks, the support received from members of the project team: Dr Stuart Matthews and Dr John Morlidge, BRE; Ms Minna Sarkkinen, CT-Heikkinen Ltd; Jean-Phillipe Fuzier, Freyssinet International; Carmen Andrade, Institute of Construction Science ‘Eduardo Torroja; and Dr Vaclav Vimmr, STÚ-K. The reviewers, Dr Geir Horrigmore, Professor Paul Lambert and Mr David Sharp, made many helpful suggestions to improve this report. These are gratefully acknowledged. The authors and other members of the project team gratefully acknowledge the financial support provided by the European Commission for this work and for the interest and encouragement provided by the supervising scientific officers Dr Ir Georgios Katalagarianakis (September 2002 to March 2004), Mr Ir Christophe Lesniak (April 2004 to February 2006) and Dr Ir Dominique Planchon (February 2006 to September 2006). The work was carried out under GROWTH Project GTC1-2001-43067 ‘CONREPNET Thematic network on performance-based remediation of reinforced concrete structures’. The project partners also wish to acknowledge the essential contributions made to the project by the members of the CONREPNET Thematic Network in terms of data provided, experiences shared and contributions made, as well as in the review of project deliverables. The photographs 4.9 to 4.13 are the copyright of the Belgian Building Research Institute, 4.14 was supplied by Professor Lambert. Others were supplied by members of the project team.

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Abbreviations

AAR CEB CFRP CONREPNET CP ERS FIB HAC LVDT NDT QC RH RTD VWG

Alkali aggregate reaction Comité Euro-International du Béton, now part of FIB Carbon fibre reinforced plastic Concrete Repair Network Cathodic protection Electrical resistance strain gauge Fédération Internationale du Béton High alumina cement Linear variable differential transformer Non destructive testing Quality control Relative humidity Research and technical development Vibrating wire gauge

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1

Chapter 1

Introduction

Structural concrete in its modern form has been in use since the late 1800s and many early structures have continued in operational use for over 100 years. At the time they were constructed there were no design codes and little was known about durability. There was a general belief that concrete was a problem-free material requiring little or no maintenance. Indeed, concrete was used as a cladding on steel structures to protect them from corrosion and fire and for the most part it has been very successful in such applications. Performance of these early structures has been surprisingly good when it is considered that cover thicknesses over the steel reinforcement were very low. Moreover, the concrete was placed by hand with no vibration, invariably had cold construction joints, and voids were commonplace. The belief that concrete was problem-free continued until the late 1960s when various durability problems became apparent. These included alkali aggregate reaction (AAR), sulfate attack, reversion of concrete containing high alumina cement (HAC), and corrosion of the steel reinforcement and prestressing tendons. The maintenance problems were generally concerned with structures that were more than 10 years old and corrosion of the steel reinforcement was by far the most common occurrence. Recent concrete structures appear to have performed worse than the earlier ones, probably due to a number of reasons, listed as follows. ● The state-of-art designs became more ambitious with less material and higher operating stresses. ● Design and construction operations became more economical. ● New design details were introduced that turned out to be susceptible to corrosion, for example expansion joints that could not be made waterproof and permitted water to leak through to the underlying concrete. ● The increased emphasis on competitive tendering put pressures on the supply chain and, in some cases, caused suppliers to cut costs and provide a low value product. ● The application of whole life costing by economists, coupled with relative high discount rates, led to the philosophy of low cost construction.

●

● ●

●

Pressures to speed up construction encouraged the introduction of problematic materials, such as HAC, without sufficient knowledge of their performances. Likewise finely ground Portland cement enabled higher early strengths to be achieved but at the expense of having a concrete less tolerant of even mildly aggressive exposure conditions. Carbonation of the cover concrete. Most importantly, the introduction of de-icing salt during cold weather led to increased corrosion in highway structures, adjacent buildings, and multistorey car parks. This is probably the most common single cause of corrosion in reinforced concrete structures. Concrete came to be used in industrial buildings having aggressive environments.

In recognition of these durability issues, limit state design codes were introduced that had crack control and thickness of cover concrete as major requirements of the serviceability limit state. New materials were developed for repair work, such as polymer modified mortars, as well as new types of repair, such as injection of sealants (fine mortars and resins) into cracks. The concept of designing for an assessed life was introduced; the required lives varying according to the type of structure (the longest being 120 years given in BS 5400 for highway and rail bridges in the UK). However, service lives are not maintenance free and the structures require regular inspection and attention. Repair techniques have been continually improving and at different times there have been new materials and repair methods on the market that have been expected to result in better performances in the future. However, these expectations have turned out to be illusory because it has become apparent that performances of both new construction and repairs remain poor. While this is a generally accepted view supported by individual cases, it is based mainly on subjective judgements because apart from a few specific studies, there have been no comprehensive collections of performance data that include different environments and structures.

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2 In recent years the adoption of sustainability ideas and principles into construction has increased the pressures to maintain existing structures and minimise the consumption of natural resources required for repair and new construction. There are also pressures from heritage authorities to maintain an increasing stock of historic concrete structures using minimal intervention. Sustainable construction is an important global objective that involves not only minimising the consumption of resources in new construction but indirect resources such as demolition activities, transport of materials to site, additional traffic and congestion. Construction causes atmospheric pollution through exhaust products (such as carbon monoxide), dust and noise. Spent materials that cannot be re-cycled have to be placed in land-fill sites, or elsewhere, causing increased expense and damage to the environment. The achievement of durable concrete repairs is crucial to the sustainability of concrete structures. Activity in the field has steadily increased and it is estimated that maintenance repair work now takes 50 % of the European construction budget. In the US it is estimated that the annual expenditure due to damage by corrosion amounts to some US$8.3 billion. Moreover, about 27% of the 162,000 highway bridges surveyed in 2000 had become either structurally deficient or obsolete. Repair performance data from the US Corps of Engineers indicated that only 50% were classified as good, 25% failed, and 25% were fair or poor[1]. Materials and structural engineers charged with the development of durable repairs are faced with a variety of problems. ● Performances of new materials have to be demonstrated in the laboratory using accelerated and artificial representations of service conditions; this is less realistic than exposure to natural weathering in real time, but pressures for development are too great and it is not feasible to allocate five or 10 years to exposure testing. Potential users are reluctant to purchase repairs that have little or no track record and even more reluctant to be the first user. ● Potential users may be forced to purchase repair materials and techniques that are seen as best value for money; this invariably favours the cheapest proposal and gives little encouragement for suppliers to develop higher quality but more expensive solutions. ● Heritage authorities prefer the use of traditional materials and methods such as lime based mortar and locally sourced aggregate. Against this background the CONREPNET network has studied the possibilities of ‘performance based rehabilitation of reinforced concrete structures’ and the present report describes four elements of the work carried out.

Concrete repairs: Performance in service and current practice ● ● ● ●

Performance of repairs in practice in order to evaluate more accurately the durability of repairs over real time Current industry practice in relation to inspection, interpretation of results and methods of repair Current research including sizes of projects, levels of funding, research topics and outcomes Best practice, including the European Standard EN 1504 and use of national standards and other guidance documents.

The data in support of this work were collected through questionnaires sent to all sides of the repair industry, including owners of structures, repairers, materials suppliers, consultants, research institutes and universities. Achieving durable repaired concrete structures — Adopting a performance-based intervention strategy[2] is a companion book that addresses the evolution of performance based concepts to achieve durable repairs.

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3

Chapter 2

Expectations of repairs

Concrete structures are usually designed for an assessed or nominal life, taking into account the effects of loading actions during return periods defined by the assessed life. These include maximum occurrences of: ● Wind ● Temperature ● Traffic ● Crowds (on stadia, footbridges etc.) ● Wave action (for marine structures) ● Snow ● Numbers of repeated-load cycles

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●

Other factors that may have to be considered in less common circumstances include: ● Impact ● Explosion ● Aggressive industrial environment ● Vandalism Loading actions can influence durability in a number of ways; repeated-loading can, for example, cause initiation and propagation of fatigue cracks. Current design for durability is through prescriptive guidance and includes factors such as the disposition of reinforcement to control cracking and crack widths, thickness of concrete cover to reinforcement, quality of concrete, and management of water (effective drainage, waterproof membranes, leak proof expansion joints, etc). However, state-of-art solutions have not yet reached the stage when the processes of degradation can be expressed numerically to enable a structure to be designed in order to reach its required life, at which time, it is worn out and ready to be replaced. It follows that it is necessary to anticipate durability problems during the lifetime of a structure and, in consequence, to carry out maintenance and repair. Expectations of repairs vary according to the type of structure and requirements of its owner, as follows. ● Transport structures are long-life and represent very considerable national investments that have to be maintained in safe operational condition at reasonable cost. Although design codes refer to lives of 60 –

●

120 years, these are assessed lives in terms of fatigue and maximum occurrences of loading during the return period, as mentioned above, and require properly managed maintenance work. Repairs are generally expected to last for at least 25 years. Sensitive industrial structures such as nuclear power stations have shorter lives and have to be maintained in a safe operational condition at all costs. Any repairs that may be required are generally expected to last to the end of the operational life of the structure which may typically be 30 or 40 years. Some commercial structures have, as their main requirement, to remain operational. Only minimal time can be allowed for closure because of the high costs that can be incurred by losses in revenue. In consequence, it can be acceptable to have speedy repairs that are known to have a limited life. In some cases the repairs are required to enable the structure to remain safe and operational for a short time until it can be demolished and replaced with a new structure or subject to a more long term repair strategy.

The differing expectations are reflected in the guarantees required for the durability of the repairs. In many cases five year guarantees are provided, whereas in more prescriptive conditions the repairers are required to provide 10 year guarantees. The provision of guarantees requires insurance cover which is becoming increasingly expensive and generates additional costs that have to be passed on to the owners of structures. Furthermore, the pressures of potential litigation are causing insurance companies to become selective in the cover that can be offered and there is a reluctance to include construction work involving materials such as silica or asbestos that pose risks to the health of the repairers. Also, there is concern about the use of epoxy materials, which are now banned in some countries. The policy of some national authorities is to impose prescriptive requirements designed to ensure that repairs are durable and have an expected life of 25 years even though this may not be guaranteed. This approach leaves

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4 less scope for the introduction of alternative or innovative methods of repair. Owners of structures use different approaches to the management of repairs. Some supervise the work very closely using their own in-house experts. Others carry out acceptance testing of the repairs. The latter are a minority but in any case little guidance is available on methods of acceptance testing. When questioned about their expectations of repairs, owners of structures said that they required a better indication of the life of repairs. While this is a difficult parameter to predict with any accuracy, there is a need for a better understanding than the current sometimes optimistic figures often given. Owners also expressed a need for simplified explanatory guidance on repair processes. Currently available documents are seen as being written for experts and too esoteric for the average owner.

Concrete repairs: Performance in service and current practice

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5

Chapter 3

Performance of repairs in practice

3.1 Background If repairs are to be made more durable, it is necessary to have a better understanding of their performance in practice. The key performance data that are required include: ● Types and causes of the original deterioration of the concrete ● Types of repair carried out ● Success or otherwise of the repair ● Mode of failure of the repair ● Cause of failure of the repair ● Life of the repair. To this end data were collected for a range of case histories for structures, mainly in European countries. Case histories are especially valuable as they provide data on repairs designed and made under the pressures imposed by the realities of requirements and the rigours of site conditions (as opposed to the relative comfort of work in the laboratory). These pressures generate problems posed by: ● costs often having to be minimised to meet the demands of competitive tenders, ● available time for the repair to be carried out reduced by the need to minimise closure times and, on occasions, to keep structures operational at all costs, ● work carried out in all weathers, sometimes too hot or too cold for the repair materials being used, and ● the inevitable limitations of working from temporary access. The resulting repairs are subjected to varying combinations of weather and loading in real time and operational conditions that cannot be represented adequately in the laboratory. It follows that the quality and subsequent performance of repairs predicted from laboratory studies require calibration against practice. The case histories of repairs were obtained through questionnaires and searches of the literature. Bearing in mind that people, particularly busy engineers, are not enthusiastic about completing questionnaires, the

document was limited to one side of A4 paper, mostly made up of tick-boxes, and was non-attributable (the structures were treated as being anonymous as there was a general unease about issues of confidentiality). The main features of the questionnaire were type of structure, environment, key dates, type of deterioration, type of repair, performance of repair, and cause(s) of failure. This provided up to 40 data points per casehistory. The questionnaire was sent out with explanatory notes and an example is given in Appendix I. Some of the information requested was judgemental, such as causes of repair failure, and it was necessary to take the responses at face value. For the purposes of this investigation, repairs are considered as being works carried out to restore the structure to its original condition with regard to serviceability and ultimate strength. This involves a range of situations as follows. ● Deterioration due to progressive processes such as corrosion and AAR. ● Deterioration caused by weathering, mainly frost damage in Northern climates. ● Mechanical damage caused by actions such as impacts, vibrations, overloading and settlement. ● Wear caused by the action of water in spillways and repetitive mechanical actions. A total of 247 case histories were received but 17 could not be included as they were found to be irrelevant or having insufficient information on key points. In other cases it was evident that some of the data were not available, for example, date of construction and type of repair material. This left gaps so that some of the analyses involved less than 230 data points. In one of the responses it was noted that consultants may assess the problem and design the repair scheme but are rarely involved later and therefore have little or no opportunity to observe or record subsequent performances. Consequently there was a shortfall in case histories supplied by consultants. The case-histories were supplied by 24 respondents giving a success rate (number of productive replies related to total number of enquiries) of 45%.

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Concrete repairs: Performance in service and current practice

Industrial (10)

Consultant (23)

Coastal (27) Repairer (64)

Urban (84)

Academe (83)

Highway (54)

Owner (60)

Rural (54)

Figure 3.1 Distribution of respondents (Numbers in brackets denote case histories supplied. Sizes of segments of the chart represent numbers who responded)

Figure 3.2 Distribution of environments (Numbers in brackets denote case histories supplied. Sizes of segments of the chart represent numbers who responded)

Comments made on the questionnaires by respondents, which relate to case histories, are included in italics on the following pages.

Most of the responses were prepared in 2003 and times such as age of repair are therefore related to this date

The respondents were from all sides of the industry; academe, owners, repairers and consultants, their numbers being represented in Figure 3.1. Academe represents all those engaged in research, including research institutions and universities. Owners are the organisations who ultimately pay for the upkeep of the structures and in many instances are responsible to the public for their operation and safety. Repairers are defined as suppliers of materials and contractors (in Section 4 of this book, suppliers and contractors are considered as separate groups). Contractors are the organisations who are responsible for carrying out the repair works and are sometimes given other responsibilities such as recommending the actions to be taken. The assignment of responsibilities varies according to the policy of the owner, but consultants are usually responsible for the preliminary investigation, assessment and design of repair. The case histories were from countries having a wide range of climates and conditions: Finland, Denmark, Sweden, Czech Republic, Germany, France, Belgium, Netherlands, Spain, Greece and the UK. The distribution of environments, illustrated in Figure 3.2, shows that there were rather low numbers of case histories for coastal and industrial sites albeit the relative distribution is not unrepresentative of reality. Highways are treated separately since they relate almost exclusively to bridges and are well known as being generally one of the most aggressive environments, particularly in Northern Europe, where de-icing salt is used on highways during cold periods. Bridges are rarely sheltered by the terrain or by other structures so that they tend to experience extremes of weather.

The main types of structure reported were: buildings, bridges, dams, power stations and car parks (see Figure 3.3). Other less common structures included: piles, water towers, tunnels, hangers and industrial structures (a vertical shaft, an inland jetty and a silo). The bridges were mainly, but not exclusively, highway bridges. Some were in coastal regions and could experience chloride contamination from the environment as well as de-icing treatment. Dates of construction varied from a church, composed of masonry and concrete and built in 1852, to office buildings built in 1998. Most of the structures reported are between 20 and 50 years old and are mainly of precast and prestressed construction. There are 41 reinforced in situ structures that are more than 60 years old. The distribution of ages (related to 2003) is shown in Figure 3.4.

Other (22) Buildings (77)

Dams (36)

Power stations (12) Car parks (8)

Bridges (75)

Figure 3.3 Distribution of case-histories by structure Numbers of structures in brackets

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Performance of repairs in practice

60

50

40

Number

Text

30

20 10

0 0-10

11-20

21-30

31-40

41-50

51-60

61-70

71-80

81-151

Years (to 2003) Figure 3.4 Ages of structures

Key points

The distribution of the primary causes of deterioration is shown in Figure 3.5.

● The earliest structure to be repaired was 151 years old ● Most (about 60%) of the structures were 20 to 50 years old ● Consultants advising on repairs are rarely involved

subsequently and are often not aware of the repair performance

3.2 Causes of deterioration The main causes of the deterioration of the original concrete were ascribed to: corrosion, frost action, cracking, alkali aggregate reaction (AAR) and faulty construction. Corrosion was almost exclusively concerned with reinforcing steel but there were nine instances of corroded pre-stressing steel and one of an anchorage plate. Not surprisingly, corrosion was the most common process of deterioration, being responsible for 55% of the problems. Frost relates to freeze-thaw action accentuated by poor quality concrete and leakages, usually at construction joints or expansion joints. Cracking was mainly associated with corrosion but it was not always clear whether it was a contributory cause or a consequence. There were also incidences of cracking due to structural actions such as loss of prestress. Faulty construction included inadequate thickness of cover concrete and incorrectly placed concrete resulting in voids, honeycombing and cracking. Some 40% of the cases of faulty construction were repaired immediately but the rest were not identified until many years later when problems had developed.

Other less common types of deterioration included: inadequate strength (usually due to factors such as losses of section caused by corrosion or inadequate design in the first place), ● scour, leaking (of dams and water containing structures), ● leaching (of spillways), impact damage, damage from overloading, and ● sagging due to prestress loss, and structural movement. ●

Inadequate thickness of cover concrete and carbonation were only reported in a few of the case histories but this probably represents a failure to identify the mechanism since they are usually found to be among the more common causes of deterioration. The respondents commonly reported more than one type of deterioration. For example, there were several instances of corrosion, frost damage and cracking being cited as having occurred simultaneously on the one structure. However, some may be considered to be consequences rather than original causes. In order to deal with this potential confusion, the data were interpreted so that the primary cause could be assigned. Ages of the structures at the time of repair, as opposed to the date of construction, were in the range 0 to 100 years, most being in the range 10 to 40 years, nine were over 70 years and the oldest was 100 years (see Figure 3.6).

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Concrete repairs: Performance in service and current practice

60 50

Per cent

40 30 20 10 0 Frost

Corrosion

AAR

Cracks

Faulty Construction

Figure 3.5 Primary causes of original deterioration

60 50 40 Number

Text

30 20 10 0 0-10

11-20

21-30

31-40

41-50

51-60

61-70

71-100

Age (Years) Figure 3.6 Ages of structures when repaired

3.3 Types of repair With only one exception, the repairs were designed and carried out with the intention of achieving as long a life as possible. The exception was a ‘holding repair’ which was required to last a relatively short time until the structure could be replaced. Although only one holding repair was identified, one respondent pointed out that they are fairly common and may be carried out for visits by public figures, for public relations purposes, when budgets are tight, or to calm public alarm. The most common types of repair were: patching; coating; crack injection; restoration of strength; sprayed concrete; electro-chemical methods (mainly cathodic protection) and added prestress. Numbers of repair types are shown in Figure 3.7. Two or three methods were commonly applied per repair, for example, patching was often accompanied by coating or crack injection. The data in Figure 3.7 are for all incidences of repair

types and the total therefore exceeds the number of case histories. Less common repairs included application of corrosion inhibitors, wrapping with carbon fibre reinforced plastic (CFRP), re-alkalisation, added cover concrete and added prestress. Associated measures included added thermal insulation, repaired expansion joints, re-tiling (of facades) and waterproofing. It should be noted that although these different methods have been considered in a single group, they could be subdivided as follows. ● Protection to maintain the existing condition by exclusion of moisture, chlorides, carbon dioxide etc. ● Repair to halt the deterioration process and restore durability. ● Strengthening to restore the load carrying capacity.

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9

Performance of repairs in practice

160 140

Number

120 100 80 60 40 20 0 Patch

Coating

Crack Injection

Restoration of strength

Sprayed concrete

Electrochemical

Added prestress

Figure 3.7 Relative incidences of different types of repair (Numbers of repair types exceed numbers of case histories because more than one repair type was often applied)

50 45 40 35 Number

Text

30 25 20 15 10 5 0 Barrier

Hydrophobic

Aesthetic

Other

Figure 3.8 Relative uses of different types of coatings

Patching was applied in 60% of the case histories, mostly in cases where corrosion had occurred it was necessary to remove defective concrete and clean or replace corroded reinforcement. The material used for the patching was ‘cementitious’* in 60% of the patches and polymer modified mortar in 30%. Other types of patching material included mortar containing steel fibres and polymer modified mortar containing polyethylene fibres. Coatings were applied in 35% of the repairs, the main types being barrier and hydrophobic (numbers are given * ‘Cementitious’ refers to mortar having no more than simple admixtures added to improve flow or curing characteristics as an addition to the mix. ‘Polymer modified’ refers to mortars composed of cement plus polymeric materials added to improve the physical properties of the mortar.

in Figure 3.8). Other types were anti-carbonation, aesthetic and several that were not specified, probably representing situations where more detailed information was not available to the respondent. Coatings were usually applied in combination with other repairs such as patches and crack injection, with only 30% being solo. Restoration of strength was carried out in 17% of the repairs. The most common scheme was for reinforcement bars that had corroded and lost so much material that it was considered necessary to replace them with new bars. This has the added value that new bars are less likely to corrode than corroded bars, which are difficult to clean properly and likely to harbour residual chlorides. Other methods of restoring strength

Text

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Page 10

10 included additional reinforcement, added concrete, added anchors, added prestress and bonded plating (steel or carbon fibre reinforced polymer). Sprayed concrete was applied in 13% of the repairs.

Concrete repairs: Performance in service and current practice ●

●

3.4 Performance of repairs 3.4.1 Inspection

There are three stages in the repair process when inspection should be carried out: ● Prior to repair. Inspection of the deteriorated concrete to determine the extent and nature of the problem ● After completion of the repair. Inspection to determine whether the work has been carried out properly and is acceptable ● Routine inspection. As part of the maintenance schedule and to determine whether the repair remains in a satisfactory condition. The inspections reported in the case-histories were mainly routine although inspections for acceptance appear to be rarely carried out in any depth. Only 15% of the inspections were reported as having utilised non-destructive testing (NDT), the majority of inspectors apparently being satisfied with visual examinations. In cases where NDT was carried out the tests included: ● measurements of electrode potentials to determine the likelihood of active corrosion being present, ● thickness of concrete cover to determine the extent of protective concrete present, although surprisingly this was apparently not carried out on a routine basis as part of the investigation before the repair was designed, ● pull-off strength to determine the adhesion of patches to the substrate, ● carbonation depth to determine the progress of deterioration, ● chloride gradients to determine the likelihood of corrosion developing, and ● impact-echo testing to determine whether delamination or debonding had occurred. Removal of cores for strength tests and petrographic studies were reported as being occasionally carried out. While petrographic studies can be invaluable in helping to identify deterioration processes, strength tests on cores appear to be carried out as a matter of tradition and it is more useful to identify properties of the concrete such as stiffness to enable repairs to be made with material that is fully compatible with the substrate.

exhibiting early evidence of failure, considered to be unsatisfactory and eventually requiring further action, for example, minor cracking suspected to be associated with corrosion, and identified as failed and requiring immediate attention, for example, continued corrosion.

There are some situations when classification of repair performance can be, to some extent, a subjective judgement. For example, a repair may be accepted as being structurally successful but failing due to aesthetics; some coatings and patches can become discoloured and unacceptable to owners despite meeting all other requirements (an example is given in Section 3.5). Needless to say these situations can lead to disputes but ultimately repairs have to be carried out to the satisfaction of the owner of the structure and the repairer should provide information about likely outcomes beforehand. 3.4.3 Overall repair performances

For all types of repair, 50% were reported as being successful at the last inspection, 25% exhibited evidence of failure, and 25% failed. However, it is more informative to consider types of repair, their performances and explanations given for causes of failures. Repair performance is shown in Figure 3.9 and it is evident that most failures occur in the first 10 years after repair. Significant numbers also occur beyond this age; the longest reported life to failure was 52 years. In the subsequent analysis of performances in relation to time, successful short duration repairs have been successively deleted. Thus, when calculating percentage failures in, say, 25 years, successful repairs of less than 25 years duration have been discounted. On this basis it was found that 20% of repairs failed in five years, 55% failed in 10 years and 90% failed in 25 years. In this analysis, failure is defined as exhibiting early evidence of failure or having failed altogether. These performances are expressed in relation to the common guarantee times and expectations quoted in Chapter 2.

Key points ● The longest reported repair life was 52 years ● 50% of the repairs reported had failed ● After exclusion of repairs that were successful but of shorter

duration ● 20% failed in five years ● 55% failed in 10 years ● 90% failed in 25 years

3.4.2 Classification of performances

Performances of the repairs were classified as: ● successful, as identified at the most recent inspection and not requiring attention for the time being,

3.4.4 Overall repair performances (by repair type)

Success rates for the different types of repair within this study were reported as follows:

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Performance of repairs in practice

25

20

Number

Text

15

10

5

0 0-5

6-10

11-15

16-20

21-25

26-52

Time since repair (Years) Failed

Evidence of deterioration

Figure 3.9 Performances in relation to age of repairs

●

●

● ● ● ●

Patching was 50% successful (all types) ● Cementitious patches were 45% successful ● Polymer modified materials were 50% successful (see Figures 3.10 and 3.11) Coatings were 50% successful (all types) ● Barrier coatings were 50% successful ● Hydrophobic coatings were 55% successful ● Other coatings were 25% successful Sprayed concrete was 30% successful Cathodic protection was 35% successful, although see section below Schemes involving restoration of strength were 75% successful Schemes involving crack injection were 70% successful.

Figure 3.10 Patch repair after five years, showing signs of incipient anode behaviour due to non-removal of chloride contaminated material

These figures are indicative of repair performances but cannot be considered to be statistically rigorous for certain types of repair that have relatively small numbers of case-histories. In any case the numbers are likely to be influenced by the reporting organisations, some respondents being reluctant to report failures of repairs and others possibly over-reacting to minor defects. Nevertheless, the data are credible and there are no significant surprises. 3.4.5 Cathodic protection (CP)

The success rate reported for CP (35%) is poor but the number of case histories (12) is low so that this result may be atypical. However, an additional 62 abbreviated case-histories were obtained for installations in the UK. The performances of all 74 installations (including the 12 full case-histories) were as follows: ● Wholly successful: 46 (62%) ● Attention needed: 13 (18%), due to: ● Transmission problems ● The installation accidentally switched off ● ‘Failures’: 15 (20%), due to: ● Phone lines vandalised ● Control box overheated or fire ● An overlay (the anode) becoming debonded* ● Short circuit ● Anode failure ● Control failure ● Operational failure ● Unsuitable application.

* It was reported that although the overlay became de-bonded, the system continued to function.

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Concrete repairs: Performance in service and current practice

45 35 30

Number

Text

25 20 15 10 5 0 Cementitious

Polymer Modified Successful

Type of Patch Evidence of failure

Other Failure

Figure 3.11 Performances of patches

It can be argued that few of the installations were really failures since the action of the CP would remain effective for some time into the future and repairs could usually be carried out quickly and economically. ●

It was reported that 17 of the installations were monitored but it is believed that there were many more. The effective lives of CP installations are not yet established but one supplier commented on the performance of a 20 year old installation, that: ●

‘……natural deterioration of conductive coating anode [had occurred] as expected’ Numbers and ages of 26 of the successful CP installations having data supplied are summarised in Table 3.1. ●

A detailed description of the processes of corrosion and cathodic protection is given by Broomfield in Corrosion of steel in concrete[ 3]. ●

Table 3.1 Successful CP installations Age (years)

Number

0–4

9

5–9

5

●

concrete, replacing, or cleaning, the reinforcement and putting in place patching material. Moreover, crack injection could be regarded as being a protective measure rather than a repair. 65% of repairs to corroded prestressing steel were successful. Corrosion of prestressing steel presents special problems as it can occur without any visible evidence externally; there have been several cases when it has led to structural collapse. This is discussed in more detail in Section 4.6. 20% of repairs to AAR were successful. The types of repair reported were: patch plus coating, coating alone and patch alone. There were insufficient data for success rates of these individual repair methods to be meaningful but the overall value of three successes for 14 cases is indicative of the difficulties in making an effective and lasting repair of AAR affected concrete. 25% of repairs to frost-damaged concrete were successful. This is especially low and is an indication that the failed concrete was repaired but the root cause of the problem was not tackled. 90% of repairs to poor construction were successful. This high success rate is probably due to the defects being identified before processes of deterioration such as corrosion had commenced, in fact 40% of the cases were repaired immediately. 65% of repairs to cracking were successful.

10 – 19

6

3.4.7 Repairs in combination

20+

6

In the previous sections, types of repair have been considered irrespective of whether they were applied solo or in combination. In 60% of the case-histories, more than one type of repair was applied.

3.4.6 Types of deterioration ●

50% of repairs to corrosion were successful. The types of repair reported and their success rates were crack injection (70%), and patching (40%). The high success rate for crack injection is probably an indication that corrosion was less advanced than in the cases where it was considered necessary to go through the steps of removing all the affected

The added value of combining repair methods can be seen for patching; when applied solo, patches were 30% successful compared to 50% when coated, as shown in Figure 3.12.

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13

Performance of repairs in practice

50 45 40 35

Number

30 25 20 15 10 5 0 Patch plus Coating

Patch Solo Successful

Evidence of failure

Fail

Figure 3.12 Performances of patch repairs to corrosion

3.4.8 Modes of repair failure

The common failure modes for all types of repair were reported as cracking, continued corrosion, de-bonding, continued AAR and leakage (Figure 3.13). Other less common modes included deteriorated concrete, deteriorated coatings and spalling. ●

●

For patches, 30% of failures were due to cracking, 25% due to de-bonding, 25% due to continued corrosion and 20% due to other modes. For coatings, 25% of failures were due to cracking, 25% due to de-bonding, 20% due to continued

● ●

corrosion, 10% due to continued AAR and 20% due to other modes. An example of a failed coating applied to AAR affected concrete is given in Figure 3.14. For sprayed concrete, failures modes were mainly cracking, de-bonding and continued corrosion. CP failure modes are listed in section 3.4.5. In summary, there were failures of anodes, electrical connections, installations accidentally switched off, and a variety of other causes that could easily be rectified. There was only one case where there was continued corrosion and the CP was reported to have been ineffective.

35 30 25

Number

Text

20 15 10 5 0 Corrosion

Cracking

Figure 3.13 Modes of failure – all types of repair

Debonding

Continued AAR

Continued leakage

Other

Text

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Concrete repairs: Performance in service and current practice

Figure 3.14 (top and bottom) Failure of polymer mortar patches and sprayed polymer coatings applied to an AAR affected bridge. This failure was considered to be partly due to incorrect diagnosis of the original problem and partly to incorrect design of the repair

Figure 3.15 (top and bottom) Failure of polymer mortar patches and polymer coating applied to a bridge affected by corrosion. This failure was considered to be due partly to incorrect design of the repair and partly to incorrect application of anticorrosion treatment to corroded reinforcement

3.5 Causes of repair failures

‘When construction is on the basis of competitive tenders, the cheapest one is chosen’ .

In the case-histories reported, failures of repairs were attributed mainly to: ● incorrect diagnosis of the underlying problem, ● incorrect design of repair (an example of failure due to incorrect design is shown in Figure 3.15), ● poor workmanship, ● use of incorrect repair materials ● failure to follow manufacturer’s instructions on the use of repair materials, and ● other factors.

In four cases it was admitted that the cause of failure was unknown. Numbers of the different causes of failure are shown in Figure 3.16. 3.5.1 Incorrect diagnosis

Respondent comment: ‘[It was] the wrong diagnosis to propose patch repair works’

Problems with repair materials could be regarded as a sub-set of incorrect design since they are mainly a matter of incorrect specification. They are listed separately because they represent a specialist element, and in any case there are other factors such as whether advice was provided or obtained from the materials suppliers.

Examples of original problems being incorrectly diagnosed, or not identified as needing attention, included porous or honeycombed concrete; the presence of deleterious materials such as calcium chloride or high alumina cement in the original concrete; and failure to identify the root cause of cracking

Other factors that caused failure were extremes of weather during repair work, subsequent overloading, vandalism and low expenditure (too little was spent on the repair). This last point is closely related to the system of competitive tenders; as one respondent summed up:

3.5.2 Incorrect design

Respondent comment: ‘Partially wrong design of repair, partially wrong option of repair material, partially wrong design of concrete surface’.

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15

Performance of repairs in practice

45 40 35 30

Number

Text

25 20 15 10 5 0 Incorrect design

Incorrect material

Poor workmanship

Wrong diagnosis

Other

Figure 3.16 Reported causes of failures

Where the repair was considered to have been incorrectly designed, typical errors included: insufficient defective concrete removed before patching; low cover at the time of construction which was left uncorrected; cosmetic treatment instead of a properly designed repair; and inadequate arrangements made for drainage. In cases where there was continued corrosion, it was generally due to insufficient defective concrete having been removed and incipient anodes becoming dominant; this was a common shortfall in design of repairs. 3.5.3 Poor workmanship

Poor workmanship was a general problem. In one case a correspondent noted on the questionnaire that: ‘The work had been a textbook example of how not to carry out a repair’. In another case: ‘Coatings were incorrectly applied despite clear instructions being given’,

Figure 3.17 Aesthetic deterioration of cement-based coating repair. The photo is of a test area, where appearance of efflorescence has been activitated on purpose

And in yet another, ‘Poor workmanship; too thin coating [against instructions]’. There were also other instances where coatings were applied too thick or too thin. An example of ‘aesthetic deterioration’ is given in Figure 3.17.

cause of failure, it was rarely suggested that the material was inadequate per se. One exception was cathodic protection where some early anode materials proved to have inadequate durability. More commonly, failed repair materials were found to be incompatible with the original concrete due to differing strength or absorption rates. In one case, coating material intended only for internal use was used externally and, not surprisingly, failed.

3.5.4 Incorrect repair material

Respondent comment: ‘The strength of the repair material was considerably greater than the substrate’ In cases where the repair material was reported to be the

Figure 3.17 shows an example of strong efflorescence, which is not a technical problem (durability or bond of coating are not weakened), but has an unpleasant appearance. It has been caused by difficult weather

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16

Concrete repairs: Performance in service and current practice 90 80 70

Per cent successful

Text

60 50 40 30 20 10 0

Urban

Rural

Highway

Coastal

Industrial

Figure 3.18 Influence of environment on performance – all types of repair

conditions during the work followed by damp and cold. It is likely that poor workmanship also contributed to the problem. This is only one example of many similar cases. Appearance of efflorescence is a typical problem when using cement-based coating systems in northern countries. Aesthetic appearance is considered very important especially concerning repairs of building façades. Recoating and cleaning due to efflorescence have caused extra costs and angry customers. On the other hand the durability properties and technical functionality of cement-based coatings when applied to facades are much better than organic coatings and paints.

Key points ● The performance of 230 concrete repairs have been collected

and analysed ● Ages of structures when repaired were mainly in the range 10

to 40 years, the oldest was 100 years ● The most common problem to be repaired was corrosion ● 60% of repairs involved patching ● Cracking, debonding and continued corrosion were the most

common modes of repair failure ● In a number of cases corrosion was so far advanced that it was

considered necessary to replace wasted bars with new ones. ● The owner of a structure is the ultimate judge of whether a

Influence of environment The performances of concrete and repairs to concrete structures located in different environments are shown in Figure 3.18. The data follow the normal trends with repairs in coastal and industrial environments exhibiting high failure rates. However, the number of industrial case histories was relatively small.

repair has been successful ● Incorrect design of the repair, use of incorrect material, poor

workmanship and wrong diagnosis were the most common causes of repair failure

Text

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17

Chapter 4

Current repair practice

4.1 Background Most of the repairs reported and analysed in Chapter 3, were carried out during the period 1960 to 1990 and involved the state-of-the-art methods contemporary to that time. In subsequent years materials and techniques, as well as an understanding of the processes, have been improved and it is appropriate to examine how current practice has been developed in relation to inspection and methods of repairing deteriorated concrete. In this context current practice is related to 2003. Questionnaires about current practice were designed to be complementary to the one for case-histories, (see Appendix II). Responses were received from 55 organisations in 16 countries, giving a balanced representation of views. The organisations employed some 64,500 people in total and about 6,000 were involved in concrete repair. For some of the organisations, concrete repair represented less than 1% of their business, for others it was 100%.

Key point ● Current practice data were provided by 55 organisations

employing some 6,000 people on repair work

4.2 Inspection Respondent (consultant) comment: ‘Inspection prior to the repair is critical to the whole process’. From the responses received, the relative usage of the more common methods of inspection (expressed as percentages) have been summarised in Table 4.1. Surprisingly, there were respondents who apparently had not heard of common tests such as location of reinforcement, electrode potential measurement, corrosion current measurement, monitoring and petrographic analysis (this may, however, have been due to misunderstanding of a questionnaire written in English).

Table 4.1 Relative usage of inspection techniques Technique Visual inspection

Used sometimes

Used commonly

4

96

14

86

Depth of carbonation

13

87

Core tests

21

79

Chloride content

24

76

Thickness of cover concrete

Electrode potential

43

57

Petrographic analysis

66

34

Monitoring

69

31

Corrosion rate

76

24

Loading tests

77

23

The use of inspection techniques can be summarised as follows. ● The majority of respondents reported that they often used visual inspection but surprisingly, four respondents only sometimes carried out visual inspections. ● Depth of cover, carbonation depth, chloride content and core tests were popular. ● Measurement of corrosion rate, petrographic analysis, monitoring and loading tests were rarely used. ● 20 – 30% of the work was subcontracted. Almost all respondents evaluated inspections manually and about 50% used computer aided methods. The popularity of electrode potential measurements, corrosion current measurements and monitoring was seen to be increasing. The organisation selected to carry out the inspection prior to repair varies widely. ● Consultants usually carry out inspections in the projects they control. ● Owners carry out inspections in 65% of their projects. ● Contractors claim to do inspections prior to repair in 45% of their projects. ● Some small concrete repair works for private owners may start without any inspection.

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Concrete repairs: Performance in service and current practice Table 4.2 Use of the common methods of repair

Key points Repair

Used sometimes

Used commonly

● Visual methods are used in the majority of inspections ● 20 to 30 % of inspection work is subcontracted

Patches

14

86

● The most commonly used methods of NDT are measurements

Coatings

37

63

Crack injection

29

71

Sprayed concrete

36

64

Electro-chemical

63

37

9

91

of depth of cover, carbonation, core tests and chloride content ● Some small repair works may start without any proper

inspection

methods ‘Other’ methods

4.3 Repair methods Respondent (contractor) comment: ‘[We] sometimes had to apply a repair method specified by an owner or consultant while [we] would have proposed and used a more appropriate method if the choice had been up to [us]…’. The relative uses of the most common types of repair method are shown in Table 4.2. Other methods listed in Table 4.2 include restoration of strength (bonded plating), electro-chemical techniques (cathodic protection, re-alkalisation and chloride removal) and corrosion inhibitors. Respondents reported increased interest in cathodic protection and hydrophobic coatings, and decreased interest in sprayed concrete and crack injection. The reported decrease in interest in sprayed concrete is in contradiction to the increased number of applications that can be seen on the market. However, the small number of respondents may make this an incorrect interpretation.

Some 25% of the respondents reported that the repairs are subcontracted to specialist repairers. Those owners who carry out inspections themselves invariably decide what methods of repair will be used and, in general, they make the decision in 30% of all repair projects. Contractors select the repair methods in about 50% of projects. On occasions when special methods such as CP or strengthening, are proposed, it was reported that the contractor invariably decides whether it is appropriate or not. The relative uses of the common repair techniques are shown in Figure 4.1. Acceptance ratings, representing the popularity of the different methods of repair in the market, are shown in Figure 4.2. Acceptance ratings, representing the preferences of the repair industry are shown in Figure 4.3. Comparison of Figures 4.2 and 4.3 show that the views of the repair industry are generally in accord with the preferences of the market place. The exceptions to this are the sprayed concrete and crack injection methods,

100

80

Per cent

Text

60

40

20

0

Patch

Coating

Crack injection Never/rarely

Figure 4.1 Use of repair techniques

Sprayed concrete Sometimes

Electrochemical Often

Other

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19

Current repair practice

5

4

Rating

3

2

1

CP (sacrificial anodes)

CP (impressed current)

Chloride removal

Re-alkalisation

Sprayed concrete

Crack injection

Impregnation

Resin-based coatings

Cement-based coatings

Polymer-based mortar

Cement-based mortars

0

Figure 4.2 Acceptance ratings of repair products in the market

5

4

Rating

3

2

1

CP (sacrificial anodes)

CP (impressed current)

Chloride removal

Re-alkalisation

Sprayed concrete

Crack injection

Impregnation

Resin-based coatings

Cement-based coatings

Polymer-based mortar

0 Cement-based mortars

Text

Figure 4.3 Acceptance ratings of the repair companies

which the companies rate as being equally important as other repair methods (Figure 4.3) compared to very low acceptance in the market for these methods (Figure 4.2). This may be aggravated by the fact that these techniques require specialised workmanship. Only repairers that often apply these techniques, and rate them highly, are able to provide the necessary quality and deliver durable repairs. However, the position is influenced by the limited number of repairers offering these products.

4.4 Quality control (QC) It was found that about 90% of repair projects are subjected to QC. The type and number of QC tests depend in many cases on the available budget. There is no generally accepted procedure for quality control and in an attempt to regularise the situation one consultant felt it necessary to prepare a document outlining a more logical approach to the question.

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Concrete repairs: Performance in service and current practice

Various quality assurance and quality control tests were mentioned by the respondents. ● Trial repairs carried out beforehand to determine whether the proposed method of repair is practical ● On-site checking of repair materials to ensure that they meet the claimed specifications. ● The common site tests which are known and practiced by most repairers such as visual inspection, acoustic tests, pull-off tests, laboratory tests on cores and in some cases, structural loading tests. ● Special tests to verify the correct functioning of techniques such as chloride removal and cathodic protection. ● Thickness measurements of applied coatings. Visual inspection and checks should be made within touching distance of the structure.

Key point ● There is no generally accepted procedure for quality control

and in an attempt to regularise the situation one consultant felt

since reinforcement corrosion remains the most common problem and repairs invariably require removal of contaminated concrete and cleaning of the reinforcement, which must be followed by patching to fill the excavation and provide good protective cover to the reinforcement. On the other hand, sprayed concrete and electrochemical methods have become more popular. It is suggested that the most credible of these trends is for electro-chemical methods – and cathodic protection (CP) in particular – to become more popular as engineers gain confidence from experiences in the US and UK where large numbers of CP installations have been in use for many years. Repair methods that are occasionally used (‘other’ methods), reported in both past and present responses, included restoration of strength and use of corrosion inhibitors. Overall, current uses of the different types of repair method are broadly similar to past practice.

it necessary to prepare a document outlining a more logical

4.5.2 Types of patch repair

approach to the question.

4.5 Comparison with earlier repair practice In this section comparison is made between current practice and the earlier repair practice described in Chapter 3 (mainly for the period 1960 to 1990) to identify the developments and changes that have occurred as materials and techniques have been improved and experiences of the various repair methods have been gained. 4.5.1 Relative use of the different methods of repair

Comparative data for the most common methods of repair, shown in Figure 4.4, suggest that patching is apparently becoming less popular. This is surprising

It is evident from Figure 4.5 that whereas in the past, cementitious mortars were used on almost twice as many occasions as polymer modified mortars in patch repairs, there is little difference in current practice. This is probably a consequence of improvements in the polymer modified materials over those used in the past and the knowledge of laboratory testing, which indicates that polymer modified mortars have advantages over cementitious materials. However, this is not wholly supported by the evidence from past performances, which indicates that there is little difference in respective durability: ● 55% of cement based mortar patches failed ● 50% of polymer based mortar patches failed

40 35 Past practice

30 Per cent

Text

25

Present practice

20 15 10 5 0 Patching

Coating

Crack Injection

Sprayed concrete

Figure 4.4 Comparison between past and current usage of repair methods

Electrochemical

Other

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21

Current repair practice 70

Past practice

60

Present practice

Per cent

50 40 30 20 10 0 Cement-based

Polymer-based

Figure 4.5 Comparative use of cement-based and polymer-based mortars in patches

80

Past practice

70

Present practice

60 Per cent

Text

50 40 30 20 10 0 Barrier

Hydrophobic

Figure 4.6 Comparative use of barrier and hydrophobic coatings

4.5.3 Coatings

The relative use of barrier coatings and hydrophobic coatings has remained virtually unchanged; barrier coatings continue to be applied twice as often, as shown in Figure 4.6. It is suspected that financial pressures and a desire to cover unsightly patching with a coating of uniform appearance, make barrier coatings more attractive to owners of structures. Evidence from past performances suggests that there is little difference in the performances of the coatings in practice. ● 50% of barrier coatings failed ● 45% of hydrophobic coatings failed

Key points ● About 90% of repair projects are subjected to quality control of

one sort or another but there is no generally accepted procedure ● Patching has apparently become less prevalent in current

practice ● Electro-chemical techniques and sprayed concrete have

become more prevalent in current practice ● Use of polymer modified mortars has increased ● Barrier coatings continue to be used twice as much as

hydrophobic coatings ● Overall, uses of the different methods of repair are broadly

4.6 Inspection strategy From the responses it is apparent that too little attention has been given to inspection at the different stages of the repair process and, in consequence, many of the early failures of repairs have been due to inadequate control of the work. As part of a comprehensive repair strategy, deterioration curves shown in Figure 4.7 delineate failure envelopes in relation to the common requirements of repairs. ● Guaranteed repair lives of five years or 10 years ● Life expectation of 25 years.

similar to past practice ● 25% of repair work is subcontracted

The performance in practice, as determined from the case-histories analysed in Chapter 3, are shown within these envelopes; these are 20% of repairs fail in five years, 55% in 10 years and 90% in 25 years. It follows from these performances that although the normal timing of principal inspections, commonly at intervals of five or six years for the better managed structures, may be appropriate to the general maintenance of those having long lives, in the first 10 years it would be better to

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Concrete repairs: Performance in service and current practice

Time - Years Figure 4.7 Performances of repairs

inspect repairs at closer intervals, of say one year. In addition, NDT should be used in support of visual observations bearing in mind that 35% of repairs failed at between five and 10 years.

Key point ● In the 10 years after repair, inspections should be at relatively

3 Damage, for example cracking and rust staining on the surface of the concrete can be seen with the naked eye and is evidence of significant corrosion of the steel reinforcement buried beneath the surface. However, it is evident from the case-histories that most inspectors rely on visual evidence so that by then corrosion will have reached an advanced stage and opportunities to take timely defensive action will have been lost.

close intervals

4.6.1 Methodology of inspection

There are three stages in the development of deterioration which can be exemplified by the processes of corrosion caused by ingress of chlorides, as shown in Figure 4.8: 1 Incubation when processes of corrosion are developing but not yet active, for example chlorides diffusing from the surface of the concrete and moving towards the steel reinforcement. The presence of these chlorides is detectable. 2 The onset of corrosion when the local passivity has been destroyed and the steel reinforcement is no longer fully protected. The electro-chemical reaction is actively progressing and can be detected by techniques such as measurement of electrodepotential.

Data can be added to Figures 4.7 and 4.8 to provide a broad picture of performances and enable inspection and repair work to be planned and carried out more effectively. The model in Figure 4.8 for corrosion, can be applied to other processes of deterioration, for example frost and AAR, albeit there may be fewer stages. It is also important to recognise that inspection and monitoring should be carried out as a step-by-step process and different tests and approaches are required to aid diagnosis, acceptance, and assessment of the subsequent performances of repairs. In the following section, four stages of inspection are identified. Stage 1. Identification of the underlying cause of deterioration In the case-histories discussed in Chapter 3, 50% of failures were ascribed to either incorrect diagnosis of the

Deterioration

Text

Time - Years Figure 4.8 Levels of detectability of the deterioration of a repair

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underlying problem or incorrect design of the repair. Many of the cases of incorrect diagnosis were due to priority being given to identifying the process of deterioration. This could result in failure to recognise that problems are invariably initiated by an underlying cause, usually poor design or inadequate water management, which permit deterioration processes to develop. The most common problems reported in the casehistories were related to corrosion of steel reinforcement. One of the underlying causes is often the provision of inadequate protection against chlorides through a low thickness of cover to the reinforcement, the presence of permeable concrete, cracking, or local leakage (commonly by failing expansion joints) that permits water and chlorides to diffuse into the concrete and initiate the process of corrosion. Identification of the underlying cause is the first step in diagnosis and is as important as identification of the deterioration process. Cracking, which occurred in 22% of the reported failures, is a product of various underlying causes. It can be caused by mechanisms as disparate as loss of prestress, structural movement, overloading, impact, corrosion, freeze-thaw action and AAR. The mechanism in question needs to be identified using a combination of observation, NDT, and numerical analysis. Numerical analysis is not normally considered as being a form of NDT but nevertheless it is an invaluable tool that is sometimes overlooked. Stage 2. Identification of the deterioration process The data in Chapter 3 indicate that only 15% of the responses related to investigations using non-destructive testing. It follows from the rather disappointing performances of repairs that there is a clear need for NDT to be used more generally and targeted more effectively in order to improve the quality of different stages of the repair process, i.e. as an aid to diagnosis, acceptance of repairs, and their subsequent performance. Visual inspection is important but must be carried out properly by experienced inspectors able to detect and interpret early evidence of problems. Moreover, it should be carried out within touching distance of the concrete unless the nature of the structure makes this impossible.

removed from the vicinity of the corrosion and incipient anodes develop in the region around the patch. Stage 3. Acceptance testing of repairs In practice, the likelihood of acceptance testing of repairs being carried out is rather variable. Some authorities prefer to rely on close supervision of the work and feel that testing would be an unnecessary additional activity (and expense). Nevertheless acceptance testing should be carried out in order to: ● identify inadequate repairs in time for corrective actions to be taken, and ● provide data to satisfy owners of structures that the work has been carried out properly and can be accepted. Moreover, access can be the most expensive element in testing so it is therefore more economic to carry out tests immediately after the work when access is still available. Later testing, in the absence of easy access, is likely to be restricted and less comprehensive. The acceptance tests should be selected to suit the type of repair and likely modes of failure. The most commonly reported modes of repair failure in the short term include cracking, debonding, and spalling. In the longer term, continued corrosion and AAR are prevalent. It follows that acceptance tests should be targeted at these problems. Although there have been numerous publications describing tests and methods of inspection, none have dealt adequately with the specific requirements of repairs. Some of the methods of NDT that can be used to aid diagnosis include tests less commonly used on site. Although there are numerous reports and papers dealing with the different methods of NDT, most address concrete in general or properties of the repair material, for example Table A1 in EN 1504-9. Some examples of appropriate methods of NDT to aid acceptance are listed but not considered in any detail in Table 4.3 Stage 4. Subsequent inspection of mature repairs

Common processes of deterioration, as reported in the case-histories, were: ● Corrosion, 55% of identified occurrences ● Freeze-thaw action, 10% of identified occurrences ● AAR, 5% of identified occurrences Corrosion, frost and AAR are fundamental processes detectable by specific tests but many repairs failed due to incorrect diagnosis or failure to identify the full extent of the affected concrete. The classic example is where a patch repair is made, but insufficient affected concrete is

Subsequent inspections of mature repairs are usually periodic and carried out at the same time as normal maintenance. This procedure is rather unsatisfactory because methods used are the same as for normal inspections and repairs are rarely treated any differently. For most types of structure there are recommended maintenance schedules that comprise general inspections, which should be carried out annually, are usually visual and can only identify defects in a fairly advanced stage of development. Principal inspections

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Table 4.3 Examples of NDT to aid acceptance of repairs Repair

Common

It is considered necessary to demonstrate that the repair has been effective and remains effective.

NDT

defects Patch

●

- Built-in defects

- Impact-echo

- Fine cracking

- Permeability

- Poor adhesion

- Pull-off strength

Coatings,

- Pin holes, fine cracks

- Permeability

all types

- Poor adhesion

- Pull-off resistance

- Incorrect thickness

- Thickness

Coatings, hydrophobic

Failure to protect

Water absorption

Crack injection

Failure to seal

Permeability

Cathodic protection

- Inadequate electrical

- Electrical

continuity

continuity

(Figure 4.9)

Continuous monitoring can be expensive and there has to be good reason to do so. Some of the methods that can be used for continuous monitoring are given in Table 4.4. General rules for the inspection and assessment of concrete structures can be found in the CEB Bulletin 243 Methods of testing and assessment of concrete structures[4]. This approach can be used prior to repair, but also for evaluating the behaviour of repaired structures.

measurement

- Failed electrical control - Depolarisation (after commissioning)

Examples of some of the common methods of NDT are illustrated in Figures 4.11 to 4.14

Table 4.4 Continuous monitoring Requirement Ensure structural

Measurement Crack growth

Method LVDT, VWG

integrity Loading data

- Traffic

- Weigh-in-motion

- Crowds*

- Autocounting

- Wind speed & direction - Anemometers Vibration data

- Temperature

- Thermo-couples

Amplitudes and

Accelerometers

frequencies Fatigue data

Corrosion

- Stress cycles

- ERS plus analysis

- Crack growth

- Acoustic emission

- local fractures

- Acoustic emission

Electro-potential

Buried probes (Figure 4.10)

* on footbridges and in stadia

Figure 4.9 Pull-off test to measure adhesion of coatings and patches (see Table 4.3)

are carried out at intervals of five or six years depending on the requirements of the owner and the type of structure but as a significant proportion of repairs were reported to have failed or exhibited evidence of deterioration in the first five years, the first inspection after repair work is important and requires special attention to assess the repairs. In certain cases it may be deemed necessary to carry out continuous monitoring before or after repair work. Typical reasons for continuous monitoring are as follows: ● Definitive measurements of deterioration are required before a repair can be designed ● There is evidence of deterioration but it is not clear whether it has stabilised or is continuing ● Loading data (traffic, thermal, wind) are required over a period of time to aid design of an effective repair ● The structure has deteriorated to the extent that it is to be replaced and it is necessary to confirm that in the meantime it remains in a safe condition.

Key LDVT:

Linear variable differential transformer

VWG:

Vibrating wire gauge

ERS:

Electrical resistance strain gauge

Figure 4.10 Corrosion probes fitted to reinforcement prior to repair concrete being placed (see to Table 4.4)

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Figure 4.11 Location of reinforcement bars

Figure 4.12 Measurement of crack widths

Figure 4.13 Detection of carbonation

Figure 4.14 Measurement of electrode potential

4.6.2 Post-tensioned structures

a void is discovered, its size can be assessed and an endoscope inserted to enable the exposed strands to be examined for evidence of corrosion along their length.

There can be situations where conventional methods of inspection are inadequate and it is necessary to take a different approach. Inspection of the prestressing strands in grouted post-tensioned structures is a relevant example as it presents problems and conventional methods of inspection are inadequate. Moreover, visual inspection of the structure is rarely helpful as the strands can corrode to an advanced stage and fractures can occur without any superficial evidence on the surface of the external concrete. It is a particularly sensitive issue because corrosion can lead to wire fractures, structural collapse, and on occasions, loss of life. It has therefore been necessary to identify effective non-standard methods of inspection[5]. The most promising methods of NDT listed in guides and standards (including X-ray, radar, impact-echo and ultrasonics) have been developed and applied to posttensioned concrete with only limited success. In the event it has been necessary to turn to relatively low technology and the most reliable method has been found to be intrusive drilling into the duct and directly observing the condition of the strands by experienced inspectors. If

The process of inspection of prestressing strands is best carried out in stages as recommended in the Highways Agency document BA50/93[6], which was prepared to deal with these problems. The stages are: ● desk study, ● preliminary inspection and, if necessary, ● detailed site investigation. In situations when corrosion is believed to be present and it is practical to await events, acoustic emission can be used to monitor the situation and record occurrences of any wire fractures. This has been done successfully on post-tensioned floor slabs in office blocks and parking structures, especially when the tendons are unbonded. Repair of damaged post-tensioning has also presented special problems. When corrosion has not yet developed or is considered not excessive, voids in the grout can be filled so that the steel strands are protected from further damage. It is important that the grout remains stable

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when injected and does not bleed because this can develop further voids. Stability of the grout can be assessed using the BRITE Euram test[7]. Small voids can be filled with an epoxy grout. In work on autostrada bridges in Italy it was recommended that epoxy used to fill the voids should have the following properties: ● Maximum viscosity 0.2 Pa at 65% RH and 20°C ● Minimum life of 150 minutes at 65% RH and 20°C ● A value of pH between 10.5 and 12.5. On occasions when there are large voids, or the duct is empty, a cementitious grout is generally the best option. The grout can be injected under pressure or the process can be aided by vacuum. When corrosion is advanced to the extent that the structure is assessed as being at risk, it may be necessary to replace the element or take steps to restore the strength. This can be achieved by various methods, for example: ● Additional prestress by external post-tensioning ● Replacement of the damaged strands ● Repairs to the damaged strands ● Strengthening by addition of bonded steel or CFRP ● Propping the structure with elastic supports. For grouted internal post-tensioning, the addition of external prestress can be an effective way to restore strength and is often used. It is, however, necessary to ensure that the concrete is not over-compressed locally or globally. Also, if the prestressing tendons are external to the structure and exposed to weathering, they require adequate corrosion protection. In all cases, repairs should be accompanied by measures to ensure there are no routes for water or de-icing salts to penetrate into the concrete or tendons. There have been cases when corrosion has been found to be too advanced for the structure to be repaired and it has been necessary for it to be demolished and replaced.

Key points ● Inspection of grouted post-tensioning strand presents special

problems because corrosion and fractures can lead to collapse without the appearance of any external evidence and contemporary codes and methods of inspection offer no help. ● The most reliable method of inspection of grouted post-

tensioning strand is by intrusive drilling into the duct and direct observation of the strands by experienced personnel.

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Chapter 5

Current research

This chapter is concerned with current research on concrete repairs in relation to: ● recent and on-going projects ● relevance of current research to durable repairs of concrete ● extent of adoption of research results by the repair industry ● requirements for further research

Key point ● Data for 66 research projects were obtained from 28

respondents and supplemented by 72 projects from www.e-core.org

5.1 Sizes of research projects

The data were obtained from questionnaires sent to relevant organisations.

Numbers of participants per project are summarised in Table 5.1

Responses to enquiries produced data for 66 different projects from 28 respondents representing some 308 organisations. The distribution of respondents is shown in Figure 5.1. About 40% of the projects were led by consultants, 31% by academe and 21% by repairers. This is considered to be an appropriate balance between industry concerned with the need to address practical issues and professional researchers concerned with academic rigour. The two are not always mutually compatible.

It is notable that about 20% of the projects were carried out by one organisation alone. At the other extreme, one project was reported as having 21 participants. Financial information was provided for 49 of the 66 projects. The total budget for these projects was €37.5 million at an average per project of €0.8 million. The smallest project had one participant and a budget of €5,539, the largest had 16 participants and a budget of €5.1 million. Many funding agencies require a 50% contribution from industry and many were provided inkind, often in the form of materials and time, which are not always acknowledged as equivalent financial contributions. The total budget for the 66 projects, including this ‘in kind’ contribution, could amount to some €50 million.

The longest running project, started in 1995, lasted for eight years. There were 44 projects reported to be completed and 22 on-going. Data from the questionnaires were augmented by information obtained from 72 other research projects related to concrete repair and identified from the official website of E-core (The Thematic Network E-CORE, European Construction Research Network)[8].

Repairer (36%)

Numbers of projects funded by the different agencies and the distribution of funding across the research topics are shown in Table 5.2.

Consultant (32%) Table 5.1 Numbers of participants in research projects Number of Participants

Owner (8%) Academe (24%)

Figure 5.1 Distribution of respondents to research questionnaire

Number of Projects

1

12

2–4

32

5 – 10

12

More than 10

10

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Table 5.2 Distribution of research topics and funding Research

Private

National

International

Topics

Table 5.3 Distribution of research projects

Budget

Research

(€ million)

projects

Percentage (%)

Durability

1

10

3

8.7

Materials

Materials

3

16

6

6.6

Durability

18

Inspection

0

2

3

2.9

Inspection

18

Maintenance

0

3

5

12.7

Maintenance

18

Strengthening

4

7

2

6.7

Strengthening

15

The international projects listed here are mainly funded by the European Commission’s RTD programme and include thematic networks on concrete repair and rehabilitation such as CONFIBRECRETE[9], ONTECVET[10] and REHABCON[11] (see Appendix V).

5.2 Research topics The research projects are listed under the following general topics. ● Concrete durability. Research aimed at understanding the performance of concrete, its deterioration mechanisms, and the impact these have on the expected service life of the structure. This class includes performance and behaviour of repaired structures, the preparation of the substrate and any action taken at the time of construction to enhance service life. The latter is only relevant when it can be applied to repair work. ● Materials. Research on repair materials and methods including the performance of impregnations and cathodic protection. ● Inspection. Research related to inspection of concrete structures and evaluation of the data, monitoring techniques prior to repair and choice of repair methods. ● Maintenance. Research to improve service life by an appropriate management strategy, long term monitoring and development of databases and software to aid an understanding of the aging of structures. ● Strengthening. In relation to concrete repair, relevant strengthening is concerned with restoration to the original condition rather than increased strength to a level above the original design requirement. The distribution of these projects is shown in Figure 5.2. Strengthening Materials Inspection

Maintenance

31

Most of the research projects reported to CONREPNET are concerned with materials (38%) followed by durability (21%) and strengthening (20%). This distribution may simply relate to the perceived potential for a successful outcome and commercial advantages to the organisations involved. However, if the research projects listed on www.e-core.org are also taken into account (resulting in a total of 138 projects), there is less emphasis on materials and more on inspection and maintenance (see Table 5.3). The preferences expressed by the different types of organisation towards the research topics are shown in Table 5.4 ●

● ●

●

Consultants, contractors, owners and suppliers all expressed greatest interest in repair materials and methods Members of academe expressed most interest in durability Owners apparently had no interest in either maintenance or strengthening but this is not considered to be representative of all owners. Preferences for research on inspection and maintenance are at a low level but otherwise broadly in line with the numbers of projects on each topic.

5.3 Outcome of research projects There were 45 completed research projects and 21 ongoing projects reported. General information about the outcome of the projects at the time of reporting is given in Table 5.5. Of the 21 projects still running, articles and presentations have been given for six and the progress of five can be followed on the web site . Further research has been identified as being necessary for 75% of the projects, including the ongoing ones. The need for further research can lead to contradictory interpretations; an industrial sponsor requires a practical and usable end product and would regard a project needing more work as being not entirely successful. On the other hand a researcher might feel differently as the motivation for follow-up research projects that lead to a more comprehensive outcome would be welcomed.

Durability Figure 5.2 Distribution of research topics (66 projects)

Most importantly, it is claimed that results are used in nine (40%) of the ongoing projects and 36 (80%) of the

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Current research Table 5.4 Research preferences of respondents Respondents

Durability (%)

Materials (%)

Inspection (%)

Maintenance (%)

Strengthening (%)

Consultant

16

32

11

21

21

Contractor

0

53

7

0

40

Owner

33

50

17

0

0

Academe

47

7

13

13

20

Supplier

18

55

9

18

0

Number of projects

14

24

7

8

13

Table 5.5 Outcome of ongoing and completed research projects Outcome

Ongoing (21)

Completed (45)

The results are generally available

5

33

There has been dissemination

6

31

The results are used

9

36

Further research is required

15

36

Further research is planned

11

23

completed projects. This is a high success rate but is not entirely compatible with 51 of the 66 projects claiming to require further research. 5.3.1 Improved durability

Summaries of research problems have been identified from the performances of repairs in practice, given in Chapter 3, and are listed in Table 5.6. Research requirements identified from these data are outlined in the following sections. Table 5.6 Research problems identified from case histories Research topics

Problems identified

Durability

The poor performance of repairs is attributed to a variety of causes including incorrect diagnosis, incorrect design and poor workmanship

Repair materials

Many failures were attributed to incorrect

and methods

use of materials but there is no criticism

Inspection and

Only 15% of inspections use NDT.

assessment

Incorrect assessment is blamed for many

Maintenance

The processes of corrosion and AAR

of the materials per se

of the failures continue to be difficult to manage Strengthening

Strengthening projects have a high success rate and no problems are identified

Durability Repairs to concrete affected by corrosion and AAR have low success rates and despite the high volume of past research there are aspects of these topics that still require attention. Patches and sprayed concrete are repair techniques that are reported to have high failure rates and merit more research under site conditions. It is notable that none of these topics were mentioned in the responses describing current projects. Repair materials and methods There have been cases in practice where failures of repair materials were attributed to incorrect selection or hostile weather during application but there was no suggestion that the materials were inadequate. Nevertheless there would be benefit from research carried out with the objective of developing materials that are more forgiving and tolerant to misuse and extremes of weather. This is an area where materials suppliers are best placed to take a lead. Inspection and assessment In practice 85% of inspections rely on visual means and only 15% used NDT. While there is a continued need for robust and reliable methods of inspection, the main requirement is to produce convincing arguments and data that will persuade clients of the long term value of periodic in-depth inspections. It is also necessary to have methods that are not seen as being unduly expensive in terms of equipment or operational time.

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30 Maintenance It is evident that guidance notes and manuals are not being used sufficiently by practitioners in the field, possibly because they are seen as voluminous documents that are rather complex and time consuming to use. Moreover, clients, as opposed to practitioners, have indicated that they would like to have straightforward, clearly written, jargon-free guidelines that generalists will find easy to understand. Strengthening In the reported case-histories, the most common methods to restore strength were by replacement of corroded reinforcement with new bars, addition of posttensioning where prestress had been lost for one reason or another, and externally bonded plating. There were no reported problems and the success rate of 75% was high compared with 50% for repairs in general.

Concrete repairs: Performance in service and current practice

Research carried out under service conditions and involving exposure to all weather conditions may therefore be required to provide sufficiently convincing evidence of performances. Improved methods of acceptance testing may be required to provide assurance of the potential durability of repairs. Performance-based repair is discussed in the companion book Achieving durable repaired concrete structures[2], which is also an output of CONREPNET.

Key points ● Research to improve the durability of repairs should address

the following topics

In the current projects, a disproportionate amount of work is being carried out on strengthening and the use of fibre reinforced materials than is justified by the evident research requirements in the field, listed in Table 5.6.

● Patching is a common repair method but gives poor

Research needs It was found that only 60% of the research projects tackled problems identifiable in past performances (Chapter 3) as requiring attention.

● Repair materials are required to be more forgiving and

performance, particularly with chloride contaminated concrete, and requires further work to improve durability ● Corrosion and AAR are processes of deterioration that are

difficult to stop and require further research tolerant of misuse ● Guidance notes are required that are simple and easy for

generalists to understand and use ● Research to aid a performance-based approach to repairs

Analysis of past performance indicates that the chain of events in practice is not represented in laboratory-based research and there is room for more site orientated work and long term exposure. Laboratory testing cannot replicate site conditions and results can be misleading. This is exemplified by the work on repair mortars where research indicated that polymer modified materials performed much better than cementitious. In practice there was little difference since polymer modified patches were reported as being 50% successful whereas cementitious patches were 45% successful. However, cement-based materials are generally popular because they are cheaper. 5.3.2 Performance-based repair

Performance-based repair is seen as a promising method of achieving improved performances because it enables novel techniques to be introduced. However, it is anticipated that clients will continue to require persuasion to accept new repair methods for the following reasons: ● There is a general reluctance among many clients to be ‘first in the field’ when a new method is proposed ● Clients invariably require evidence that a new method has already been used successfully elsewhere ● It is common to require convincing evidence of good performance under service conditions.

should address the following topics ● Performance under all-weather conditions ● Relevant acceptance testing to provide assurance of

improved durability

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Chapter 6

European Standards

6.1 The current position It is current practice for concrete repair to be carried out to the specifications of local or national standards and guidelines. The characteristics and the performances required may vary, as well as the approval systems that have been developed in most countries. With the introduction of European Standards, these differences should be minimised. The EN 1504 Standard, ‘Products and systems for the protection and repair of concrete structures’, is in 10 parts. Part 1 gives the definitions and terms used in all the parts [12-21] (see Table 6.1). Parts 2 to 7 address different products, relevant performance characteristics and performance requirements. Part 8 deals with the quality control of the products, Part 9 provides guidance on the choice of the possible solutions and Part 10 deals with the quality control of the work on site. These standards do not, however, apply to all types of concrete damage. Repair of concrete structures damaged by fire or the repair of defects in existing post-tensioned systems are not covered.

In addition to the EN 1504 series, some standards are drafted or are being prepared that cover re-alkalisation, chloride extraction, cathodic protection and sprayed concrete for repair. Concrete repair works are covered by ENV 1504-9. A new version updated and adapted to the other standards of the EN 1504 series is due to be published as soon as all the standards are finalised. This standard deals with all phases of a repair project, starting at the awareness of a problem up to the maintenance and inspection after the repair work is done. According to ENV 1504-9, safety before, during and after the works is to be ensured. Also an assessment is considered necessary to reveal all defects and identify their extent and their causes. The standard also helps in the choice of the most appropriate option to the identified problem. This option can always be found somewhere between doing nothing and demolition of the structure. Factors that may influence the choice are divided into four main categories: general, health, structural and environmental. More information on these categories can be found in the Standard.

Table 6.1 European Standards related to concrete repair products and systems Number

Year

Title

EN 1504-1

1998

Part 1: Definitions

EN 1504-2

2004

Part 2: Surface protection systems for concrete

EN 1504-3

2006

Part 3: Structural and non-structural repair

EN 1504-4

2004

Part 4: Structural bonding

EN 1504-5

2004

Part 5: Concrete injection

EN 1504-6

2007

Part 6: Anchoring of reinforcing steel bar

EN 1504-7

2007

Part 7: Reinforcement corrosion protection

EN 1504-8

2004

Part 8: Quality control and evaluation of conformity

ENV 1504-9

1997

Part 9 : General principles for the use of products and systems

EN 1504-10

2003

Part 10: Site application of products and systems and quality control of the works

CEN/TS 14038-1

2004

Electrochemical re-alkalization and chloride extraction treatments for reinforced concrete – Part 1: Re-alkalization

EN 12696

2000

Cathodic protection of steel in concrete

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32 All this information should assist the project leader (owner, consultant or repair specialist) to make the choice of action based on the actual condition of the structure (extent and causes of defects and exposure) and the future conditions (this can include modified use of the construction). The selected protection or repair option also has to be taken into account. Standard ENV 1504-9 mentions 11 repair principles and their related repair methods. ENV 1504-9 states clearly that after repair, a report of the repair work must be delivered. This report should document all relevant information that may help future users to understand the choice of the solution applied. The report also includes suggestions for inspection and maintenance, necessary to reach the intended lifetime of the construction. The publication of Parts 2 – 7 of the EN 1504 Standard will result in the appearance of the CE-mark for concrete repair materials. This CE-mark will be required from January 2009 onwards.

Concrete repairs: Performance in service and current practice

Actual practice in most of the European countries follows the available guidance documents which are based on the European Standards of the EN 1504 series. However, like EN 1504, they rarely, if ever, take account of the behaviour of the repaired structure. A true performance-based approach to repair will in the future require revised ‘performance friendly’ standards. New criteria and methods to measure them will need to be introduced. Models to evaluate the aging of concrete repairs must be developed to allow the estimation of the remaining service life of the repaired construction. In addition to the EN 1504 series, some standards are drafted or are being prepared that cover re-alkalisation, chloride extraction, cathodic protection and sprayed concrete for repair.

Key points ● When EN 1504 is fully introduced, conflicting National

6.2 Application to performance based repair The approach presented in EN 1504 allows a performance-based specification to be used for concrete repair but has a prescriptive element that would sometimes have to be overridden. However, the criteria presented in the constituent parts of the EN 1504 Series do not cover the behaviour of the repaired structures or the performances of the repair products after aging. This is not helpful to the specification of performance-based repairs and it may be more practical to work out prescriptive-based repair projects that are afterwards subjected to a performance-based maintenance programme. Some techniques are amenable to being specified using the performance-based approach. For example, when applying electro-chemical chloride removal techniques, the acceptable remaining chloride content in the concrete can be specified beforehand. Unfortunately, when this approach has been used, practitioners have experienced difficulties in estimating the costs and have become reluctant to tender for such specifications. Although it is possible for electro-chemical techniques to have precise acceptance criteria, there is no certainty that the repaired structure will be durable unless an appropriate level of competence is maintained throughout the inspection, design, repair and operation phases of the work. Cathodic protection is a repair method that requires periodic monitoring and adjustment, initially quarterly but annually in established systems. Regular measurements are required to control the correct performance of the system.

Standards will be withdrawn ● A true performance-based approach to repair of concrete

structures will in the future require ‘performance friendly’ standards

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References

[1] Emmons A. Vision 2020: A strategic plan for improvements to the concrete repair industry. Unpublished presentation, BRE 2006 [2] Matthews S, Sarkkinen M & Morlidge J (Eds). Achieving durable repaired concrete structures. Adopting a performancebased intervention strategy. EP 77 Bracknell, IHS BRE Press, 2007 [3] Broomfield JP. Corrosion of steel in concrete, 2nd edition, London, Spon Press, 2007 [4] Comité Euro-International du Béton, (fib). Strategies for testing and assessment and testing of concrete structures by reinforcement corrosion. Lausanne CEB Bulletin 243, 1998 [5] Tilly G P. Performance and management of post-tensioned structures. Proc, ICE Structures & Buildings 2002: 152 (Feb): 3 – 16 [6] Highways Agency. Post-tensioned concrete bridges. Planning, organisation and methods for carrying out special inspections. BA50/93. London, The Stationery Office, 1993. [7] Tilly GP, de Cuyper J & Stouffs A. Assessing the stability of grout. Concrete: 1999 (July/August ): 35 – 37. [8] E-core. (The Thematic Network E-CORE, European Construction Research Network) www.e-core.org [9] CONFIBRECRETE (Training and Mobility of Researches) http://encore.ci.group.shef.ac.uk/confibrecrete [10] CONTECEVET (Eduardo Torroja Institute for Construction Science) A Validated Users Manual for Assessing the Residual Service Life of Concrete Structures www.ietcc.csic.es/ [11] REHABCON (Strategy for maintenance and rehabilitation in concrete structures) www.cbi.se/rehabcon/index.htm [12] Committee for Standardization (CEN) EN1504 -1: 2005 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 1: Definitions. Available through the CEN online catalogue, www.cen.eu/esearch/

[13] Committee for Standardization (CEN) EN1504 -2: 2004 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 2: Surface protection systems for concrete. Available through the CEN online catalogue, www.cen.eu/esearch/ [14] Committee for Standardization (CEN) (CEN) EN1504 -3: 2005 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 3: Structural and non-structural repair Available through the CEN online catalogue, www.cen.eu/esearch/ [15] Committee for Standardization (CEN) EN1504 -4: 2004 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 4: Structural bonding. Available through the CEN online catalogue, www.cen.eu/esearch/ [16] Committee for Standardization (CEN) EN1504 -5: 2004 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 5: Concrete injection. Available through the CEN online catalogue, www.cen.eu/esearch/ [17] Committee for Standardization (CEN) EN1504 -6: 2006 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 6: Anchoring of reinforcing steel bar. Available through the CEN online catalogue, www.cen.eu/esearch/ [18] Committee for Standardization (CEN) EN1504 -7: 2006 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 7: Reinforcement corrosion protection. Available through the CEN online catalogue, www.cen.eu/esearch/

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34 [19] Committee for Standardization (CEN) EN1504 -8: 2004 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 8: Quality control and evaluation of conformity. Available through the CEN online catalogue, www.cen.eu/esearch/ [20] Committee for Standardization (CEN) ENV1504 -9: 1997 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 9: General principles for the use of products and systems. Available through the CEN online catalogue, www.cen.eu/esearch/ [21] Committee for Standardization (CEN) EN1504 -10: 2003 Products and systems for the protection and repair of concrete structures – Definitions, requirements, quality control and evaluation of conformity. Part 10: Site application of products and systems and quality control of the works. Available through the CEN online catalogue, www.cen.eu/esearch/

Concrete repairs: Performance in service and current practice

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Appendices

The following appendices are examples of the questionnaires that were sent to members of the industry to gain the information contained in this report.

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Appendix I Concrete Repair Case History Questionnaire Concrete Repair Case-history

A blank questionnaire

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Appendix 1

Concrete Repair Case history

An example of a completed questionnaire

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Appendix II Concrete Repair Methods Questionnaire Concrete Repair Methods

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Appendix II

Please also add a list of relevant standards, recommendations, working rules etc., applied in your company x = used technique

> increasing importance

< decreasing importance

Guidance on to complete the questionnaire This questionnaire should provide sufficient information to obtain an idea on the repair methods actually used. Please do not forget to indicate the type of construction (building, bridge, …). If the type of construction influences the repair method, please inform us on the differences. Eventually fill in two separate questionnaires. Also different types of damage may influence the choice of the repair method. Please inform us on differences. Generally numbers refer to the number of projects carried out by the company. Whenever necessary, add other used methods. In addition to the above information, provide an inventory of guides and standards you use now : Reference Title Field of application (national, some owners,…) Abstract Acceptance degree (general, selected number of companies) Eventually: same information for coming guides, standards + estimated date May we insist on trying to be as complete as possible? Your information is absolutely indispensable to obtain a complete image of concrete repair in Europe.

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Appendix III Concrete Repair Evaluation Methods Questionnaire

Concrete Repair Evaluation Methods

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Appendix III

Guidance on to complete the questionnaire. This questionnaire should provide sufficient information to obtain an idea how the inspection prior to a repair job, is performed. Please do not forget to indicate the type of construction (building, bridge, …). If the type of construction influences the inspection method, please inform us of the differences. Generally numbers refer to the number of projects carried out by the company. Whenever necessary, add other used methods. In addition to the above information, provide an inventory of guides and standards you use now : Reference Title Field of application (national, some owners,…) Abstract Acceptance degree (general, selected number of companies) Eventually : same information for coming guides, standards + estimated date May we insist on trying to be as complete as possible? Your information is absolutely indispensable to obtain a complete image of concrete repair in Europe. Additional remarks on the evaluation methods enquiry Giving respondents the opportunity to mention other actions related to the inspection of concrete structures resulted in the following list of remarks : ● Selective removal of the bituminous surfacing and waterproofing membrane.

The above procedures are used on bridges on provincial highways (major roads and freeways). The procedures are described in the Ministry’s Structure Rehabilitation Manual. ● Estimation of concrete strength by non-destructive tests, as _Rebound Hammer _Nail pull off (HILTI) and concrete condition by ultrasonic

measurements ● Regular checks of all the repairs done in the past ● Specific NDT methods like ultrasonic pulse echo, SASW, impact echo, radar and digital radiography are used for determination of concrete

integrity, and also location and conditions of pre-stressed cables ● We use other specialized tests occasionally to determine the cause of defects where the above tests do not provide sufficient information –

particularly where sulfate attack, thaumasite attack, ASR or there are structural problems etc. are suspected. Other tests may help with the development of an ongoing management strategy, or use of a particular remedial technique. The test information will be supplemented by structural assessment information, and review of design constraints, and individual inspection data in order to allow decisions to be made ●

Performance check with time

● We have revisited completed projects some 20+ years after repair to assess both condition of refurbishment system and the integrity of the

background concrete ● Assessment of absorption factors to determine feasibility of applying and monitoring corrosion inhibitors ● Car Parks are becoming a focus for repair, where the investigation is more robust and the monitoring is more common ● Design and detailing of repairs, engineering supervision of repair work ● Assessment of voids and sign of corrosion in ducts of prestressing tendons ● Our company sometimes acts as owner (BOT projects etc.), sometimes as consultant (company-internal) and sometimes as contractor. Out of

our total turnover the contracting part is largest and I have tried to answer the questions from the contractor divisions part of view. ● Technical advice concerning the concrete repair methods. Supervising the execution of the concrete repair ● Beside of rebar localization, also the depth, size and the function in the structure are sometimes registered

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Appendix IV Concrete Repair Research Questionnaire

Concrete Repair Research

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Appendix IV

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The purpose of this questionnaire is to obtain information about recent research activities in the field of concrete repair. In this as well methods for the evaluation of the condition of the concrete as well as repair methods and repair products may be subject of the research program. Many research items may or may even not be considered as dealing with concrete repair initially. A main criterion for this enquiry is that the method must by applicable on existing concrete elements, even if its initial application field is new constructions. As subjects of research programs generally are to be kept secret until results are available, we understand that it may not be possible to comment on all actual running projects. We sincerely hope you will provide sufficient information, allowing us to report a rather complete review on research regarding to concrete repair. Wherever possible questions are asked in such a way that you only have to strike out what’s not appropriate. In order to obtain a correct impression on the research activities, it is important to know the following items : Who is the prime partner in the research project? What is the correct name of the project? When was it carried out (start and end date)? If you are not able to answer one of the other questions, just mention ‘no answer’. A filled in questionnaire is added as an example. Please fill in 1 questionnaire for each relevant research project you or your company is involved in. May we insist on trying to be as complete as possible? Your information is absolutely indispensable to obtain a complete image of concrete repair in Europe.

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Appendix V Related research projects CONFIBRECRETE ConFibreCrete — Training and Mobility of Researchers (TMR) network ConFibreCrete was a European Union TMR network aiming to develop guidelines for the design of concrete structures, reinforced, prestressed or strengthened with advanced composites. The network comprised 10 teams from eight different European counties collaborating over a period of five years, starting December 1997. The total research effort that was contributed was around 800 man-months. The work of the network was closely linked to the work of the FIB Task Group 9.3. ConFibreCrete guidelines can be downloaded at: http://encore.ci.group.shef.ac.uk/confibrecrete/ CONTECEVET A Validated Users Manual for Assessing the Residual Service Life of Concrete Structures. The manual resulting from this project can be downloaded at: http://www.ietcc.csic.es/fileadmin/Ficheros_IETcc/Web /EventosPublicaciones/PublicacionesElectronicas/manu al_ingles.pdf REHABCON Strategy for maintenance and rehabilitation in concrete structures, started in 2001 and completed in June 2004. Information about this project and its manual, can be found at : http://www.cbi.se/rehabcon/index.htm.

UI#CRETE REPAIRS PERFORMANCE IN SEMCE AND CURRENT PRACTICE

n

It is estimated that around 5096 of Europe’s annual construction budget is presently spent on the refurbishment and repair of existing structures. This repmt is the culmination of a wide-ranging survey into the performanceof both current European concrete repair techniques and inspection practices, and c u m t mearch projeds. It assessesthe case histories gathered from across the sector, includingfrom owners of concrete structures, repairers and research ins&utes, and presents its findings using charts, graphs, tables and photographs. A review of the problems of concrete durability, current issues of sustainability, and the differingexpectations of what concrete repairs should achieve, provide a practical introduction to the subject.

The suwey was part of the work carried out by the CONREPNET network, made up of European research and representative bodies sponsored by the European Commission.

ACHIEVING DURABLE REWIRED CONCRETE STRUCTURES EP 77,2007 I CONCRETE STRUCTURES IN FIE ff RFORMANCE. DESIGN AND ANALYSIS BR 490,2007

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IHS BRE Press, Willoughby Road Bracknell, Berkshire RG12 8FB www. ihsbrepress.com EP79

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