Concrete Repairs Performance in Service and Current Practice

ConCrete repairsperformance in service and current practice

G p tilly and J Jacobs

ConCrete repairs performanCe in serviCe and Current praCtiCeIt 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.

related titles from ihs bre press

aChievinG durable repaired ConCrete struCturesEP 77, 2007

ConCrete struCtures in fire: performanCe, desiGn and analysisBR 490, 2007

IHS BRE Press, Willoughby RoadBracknell, Berkshire RG12 8FB

www.ihsbrepress.comEP 79

ConCrete repairs performance in service and current practice

G p tilly and J Jacobsihs bre press

Conrepnet 1 coverv1.indd 1 12/11/2007 11:18:53

Concrete repairs

Performance in service andcurrent practice

CONREPNET

Thematic network on performance-basedremediation of reinforced concrete structures

G P Tilly, Gifford & Partners Ltd

J Jacobs, Belgian Building Research Institute

CONREPNETPartners:

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Details of all publications are availablefrom IHS BRE PressWebsite: www.ihsbrepress.comorIHS BRE PressWilloughby RoadBracknell RG12 8FB, UkTel: 01344 328038Fax: 01344 328005Email: [email protected]

Requests to copy any part of thispublication should be made to thepublisher: IHS BRE Press Garston, Watford WD25 9XX, UKTel: 01923 664761Fax: 01923 662477Email: [email protected]

EP 79

© CONREPNET 2007First published 2007ISBN: 978-1-86081-974-2

Keywords

Concrete structures, EN 1504, maintenance, management, performance-based intervention, protection, repair, remediation, sustainable assets,

through-life care

The companion publication Achieving durable repaired concrete structures isavailable 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

Prelims 23/11/07 3:20 PM Page ii

iii

Executive summary viiCONREPNET partner organisations viii

Acknowledgements ix

Abbreviations x

Chapter 1 Introduction 1

Chapter 2 Expectations of repairs 3

Chapter 3 Performance of repairs in practice 53.1 Background 53.2 Causes of deterioration 73.3 Types of repair 83.4 Performance of repairs 103.4.1 Inspection 103.4.2 Classification of performances 103.4.3 Overall repair performances 103.4.4 Overall repair performances (by repair type) 103.4.5 Cathodic protection (CP) 113.4.6 Types of deterioration 123.4.7 Repairs in combination 123.4.8 Modes of repair failure 133.5 Causes of repair failures 143.5.1 Incorrect diagnosis 143.5.2 Incorrect design 143.5.3 Poor workmanship 153.5.4 Incorrect repair material 15

Chapter 4 Current repair practice 174.1 Background 173.2 Inspection 174.3 Repair methods 184.4 Quality control 194.5 Comparison with earlier repair practice 204.5.1 Relative use of the different methods of repair 204.5.2 Types of patch repair 204.5.3 Coatings 214.6 Inspection strategy 214.6.1 Methodology of inspection 224.6.2 Post-tensioned structures 25

Contents

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iv Concrete repairs: Performance in service and current practice

Chapter 5 Current research 275.1 Sizes of research projects 275.2 Research topics 285.3 Outcome of research projects 285.3.1 Improved durability 295.3.2 Performance-based repair 30

Chapter 6 European Standards 316.1 The current position 316.2 Application to performance-based repair 32

References 33

Appendices 35Appendix I Concrete Repair History Questionnaire 36Appendix II Concrete Repair Methods Questionnaire 38Appendix III Concrete Repair Evaluation Methods Questionnaire 40Appendix IV Concrete Repair Research Questionnaire 42Appendix V Related research projects 44

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Figures

Figure 3.1 Distribution of respondents 6Figure 3.2 Distribution of environments 6Figure 3.3 Distribution of case-histories by structure 6Figure 3.4 Ages of structures 7Figure 3.5 Primary causes of original deterioration 8Figure 3.6 Ages of structures when repaired 8Figure 3.7 Relative incidences of different types of repair 9Figure 3.8 Relative uses of different types of coatings 9Figure 3.9 Performances in relation to age of repairs 10Figure 3.10 Patch repair after five years, showing signs of incipient anode behaviour due

to non-removal of chloride contaminated material 10Figure 3.11 Performances of patches 12Figure 3.12 Performances of patch repairs to corrosion 13Figure 3.13 Modes of failure – all types of repair 13Figure 3.14 Failure of polymer mortar patches and sprayed polymer coatings applied

to an AAR affected bridge 14Figure 3.15 Failure of polymer mortar patches and polymer coating applied

to a bridge affected by corrosion 14Figure 3.16 Reported causes of failures 15Figure 3.17 Aesthetic deterioration of cement-based coating repair 15Figure 3.18 Influence of environment on performance – all types of repair 16Figure 4.1 Use of repair techniques 18Figure 4.2 Acceptance ratings of repair products in the market 19Figure 4.3 Acceptance ratings of the repair companies 19Figure 4.4 Comparison between past and current usage of repair methods 20Figure 4.5 Comparative use of cement-based and polymer-based mortars in patches 21Figure 4.6 Comparative use of barrier and hydrophobic coatings 21Figure 4.7 Performances of repairs 22Figure 4.8 Levels of detectability of the deterioration of a repair 22Figure 4.9 Pull-off test to measure adhesion of coatings and patches 24Figure 4.10 Corrosion probes fitted to reinforcement prior to repair concrete being placed 24Figure 4.11 Location of reinforcement bars 25Figure 4.12 Measurement of crack widths 25Figure 4.13 Detection of carbonation 25Figure 4.14 Measurement of electrode potential 25Figure 5.1 Distribution of respondents to research questionnaire 27Figure 5.2 Distribution of research topics 28

Contents v

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Tables

Table 3.1 Successful CP installations 12Table 4.1 Relative usage of inspection techniques 17Table 4.2 Use of the common methods of repair 18Table 4.3 Examples of NDT to aid acceptance of repairs 24Table 4.4 Continuous monitoring 24Table 5.1 Number of participants in research projects 27Table 5.2 Distribution of research topics and funding 28Table 5.3 Distribution of research projects 28Table 5.4 Research preferences of respondents 29Table 5.5 Outcome of ongoing and completed research projects 29Table 5.6 Research problems identified from case-histories 29Table 6.1 European Standards related to concrete repair products and systems 31

vi Concrete repairs: Performance in service and current practice

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It has been estimated that some 50% of Europe’s annualconstruction budget is spent on refurbishment and repairof existing structures. This figure is expected to increaseas the major population of concrete structures built in the1960s and 1970s, which form a key part of Europe’sinfrastructure, start to require further repair andrefurbishment. However, limited available resources needto be applied with greater efficiency and owners ofbuildings and infrastructure now require greater certaintyin the performance of their concrete structures in order tomanage their assets more effectively. This has generateda requirement for industry to deliver more durable repairsto concrete structures.

To help address these issues, a thematic network on theperformance-based repair of reinforced concretestructures was established in 2003, known asCONREPNET. The object of this EU-funded network is toimprove the durability of concrete repairs throughperformance-based rehabilitation. To this end, informationabout concrete durability and repair issues has beencollected from industry and researchers. Problems andbarriers to achieving durable concrete repairs have beenidentified and contemporary industry practices have beeninvestigated.

This report is concerned with sections of the project thatdeal with performances of repairs in practice, currentpractice and research.

Performance in practice has been assessed through case-histories obtained from members of the network andothers. Some 230 case-histories were obtained forconcrete structures up to 150 years old but mostly 20 to50 years old. The most common type of deteriorationreported was corrosion of the steel reinforcement, whichoccurred in 55% of the cases. Performances of repairsare disappointing; 20% failed in five years, 55% failed in 10years and 90% failed in 25 years. The longest repair lifewas 52 years. The most successful repairs were schemesinvolving restoration of strength and crack injection.Patches were applied in 60% of the repairs and were 30%successful when applied solo and 50% successful whenapplied in combination with a coating. Causes of repair

failures were ascribed to incorrect diagnosis, incorrectdesign of repair, poor workmanship, use of incorrectmaterials and other factors such as extreme weatherconditions during the repair work.

Most of the repair projects reported in the case historieswere carried out in the period from 1960 to 1990, usingpractices current at the time. A survey of current repairpractice (2003) indicated that there has been little changesince the methods of repair continue to be broadly similar.Patching is becoming less common as electro-chemicaltechniques and polymer mortars become more prevalent.Inspection is regarded as critical to the repair process butsome small repair works for private owners were reportedas starting without any inspection. The most commonmethods of non-destructive testing were measurements ofdepth of cover, carbonation and chloride content. Around25% of repair and inspection work is subcontracted.

A total of 138 research projects were surveyed (66obtained via questionnaires and 72 via the official websiteof the European Construction Research Network, www.e-core.org). Numbers of participants per projectvaried from one to 21 and budgets were from €5,500 to€5 million. The projects were concerned with the differentaspects of concrete repair; durability, materials,inspection, maintenance and restoration of strength. It wasfound that only 60% of the research addressed problemsidentified from the case-histories. It is concluded thatresearch to aid a performance-based approach to repairsshould address performances under all weatherconditions, and improved acceptance testing to provideassurance that repairs will be durable. Repair standardsshould be revised to have a more ‘performance friendly’orientation.

vii

Executive summary

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CONREPNET partner organisationsBuilding Research Establishment, UK (BRE): networkco-ordinator and overall catalyst for many technicalaspects of the project, including the vision for futureperformance-based concepts (WP4)

Belgian Building Research Institute (CSTC): leader ofWP3 on current practices

CT Koulutus Oy, Finland (CT Centre): leader of WP5 ondissemination, research and technical developmentexploitation, training and intellectual property rights (IPR)issues, led development of implementation of futureperformance-based concepts (WP4)

Freyssinet International, France: brought the repairspecialist’s perspective to the project and to thedevelopment of WP4 concerning future performance-based concepts

Gifford and Partners, UK: leader of WP2 on theperformance of past repairs and interventions

Institute of Construction Science ‘Eduardo Torroja’,Spain (IETcc): led development of one methodology formonitoring and assessment of performance of protectionand repair interventions on concrete structures employedin WP4 on future performance-based concepts

STÚ-K, Czech Republic: co-ordinated WP4 on futureperformance-based concepts

viii Concrete repairs: Performance in service and current practice

Autostrade per l'Italia

British Nuclear Fuels plc

Centrum Stavebniho Inzenyrstvi(CSI)

City of Kotka

City University, London

COWI

Czech Roads and MotorwaysDirectorate

Danish Technological Institute

DYWIDAG Systems International

Entisointi Pulla Oy

Federal Institute for MaterialsResearch and Testing

FEREB

FORCE Technology

Glasgow City Council

Hellenic Cement Research Centre

Highways Agency

Hywel Davies Consultancy

Ingenieurbuero Prof Schiessl

IngenieurgemeinschaftCossebaude GmbH

Karlomix Bohemia

Kingston University

Konsultointi Jarvinen Oy

Laboratoire de Recherche desMonuments Historiques

Lund Institute of Technology

Metrostav

Mott MacDonald Limited

NCC Engineering

NECSO

Network Rail

Norut Teknologi AS

Norwegian Defence EstatesAgency

Parish Union of Helsinki

Queens University, Belfast

Rakennus Oy Wareco

Red Nacional de FerrocarrilesEspanoles

Slovenian National Building and CivilEngineering Institute (ZAG)

Swedish Cement and ConcreteResearch Institute (CBI)

Swedish National RoadAdministration

Team-Danielsson Oy

TNO Building and ConstructionResearch

Union of the Czech and MoravianHousing Cooperatives

University of Birmingham

University of Patras

Vattenfall Utveckling AB

Vilniaus Miestprojektas

CONREPNET Member Organisations

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The authors acknowledge, with thanks, the supportreceived from members of the project team: Dr StuartMatthews and Dr John Morlidge, BRE; Ms Minna Sarkkinen,CT-Heikkinen Ltd; Jean-Phillipe Fuzier, FreyssinetInternational; Carmen Andrade, Institute of ConstructionScience ‘Eduardo Torroja; and Dr Vaclav Vimmr, STÚ-K.

The reviewers, Dr Geir Horrigmore, Professor PaulLambert and Mr David Sharp, made many helpfulsuggestions to improve this report. These are gratefullyacknowledged.

The authors and other members of the project teamgratefully acknowledge the financial support provided bythe European Commission for this work and for the interestand encouragement provided by the supervising scientificofficers Dr Ir Georgios Katalagarianakis (September 2002to March 2004), Mr Ir Christophe Lesniak (April 2004 toFebruary 2006) and Dr Ir Dominique Planchon (February2006 to September 2006). The work was carried outunder GROWTH Project GTC1-2001-43067 ‘CONREPNET -Thematic network on performance-based remediation ofreinforced concrete structures’.

The project partners also wish to acknowledge theessential contributions made to the project by themembers of the CONREPNET Thematic Network in termsof data provided, experiences shared and contributionsmade, as well as in the review of project deliverables.

The photographs 4.9 to 4.13 are the copyright of theBelgian Building Research Institute, 4.14 was supplied byProfessor Lambert. Others were supplied by members ofthe project team.

ix

Acknowledgements

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AAR Alkali aggregate reactionCEB Comité Euro-International du Béton, now part of FIBCFRP Carbon fibre reinforced plasticCONREPNET Concrete Repair NetworkCP Cathodic protectionERS Electrical resistance strain gaugeFIB Fédération Internationale du BétonHAC High alumina cementLVDT Linear variable differential transformerNDT Non destructive testingQC Quality controlRH Relative humidityRTD Research and technical developmentVWG Vibrating wire gauge

x x

Abbreviations

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Structural concrete in its modern form has been in usesince the late 1800s and many early structures havecontinued in operational use for over 100 years. At thetime they were constructed there were no design codesand little was known about durability. There was a generalbelief that concrete was a problem-free material requiringlittle or no maintenance. Indeed, concrete was used as acladding on steel structures to protect them fromcorrosion and fire and for the most part it has been verysuccessful in such applications. Performance of theseearly structures has been surprisingly good when it isconsidered that cover thicknesses over the steelreinforcement were very low. Moreover, the concrete wasplaced by hand with no vibration, invariably had coldconstruction joints, and voids were commonplace.

The belief that concrete was problem-free continued untilthe late 1960s when various durability problems becameapparent. These included alkali aggregate reaction (AAR),sulfate attack, reversion of concrete containing highalumina cement (HAC), and corrosion of the steelreinforcement and prestressing tendons. Themaintenance problems were generally concerned withstructures that were more than 10 years old andcorrosion of the steel reinforcement was by far the mostcommon occurrence. Recent concrete structures appearto have performed worse than the earlier ones, probablydue 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 expansionjoints that could not be made waterproof andpermitted water to leak through to the underlyingconcrete.

● The increased emphasis on competitive tendering putpressures on the supply chain and, in some cases,caused suppliers to cut costs and provide a low valueproduct.

● The application of whole life costing by economists,coupled with relative high discount rates, led to thephilosophy of low cost construction.

● Pressures to speed up construction encouraged theintroduction of problematic materials, such as HAC,without sufficient knowledge of their performances.Likewise finely ground Portland cement enabled higherearly strengths to be achieved but at the expense ofhaving a concrete less tolerant of even mildlyaggressive exposure conditions.

● Carbonation of the cover concrete.● Most importantly, the introduction of de-icing salt

during cold weather led to increased corrosion inhighway structures, adjacent buildings, and multi-storey car parks. This is probably the most commonsingle cause of corrosion in reinforced concretestructures.

● Concrete came to be used in industrial buildingshaving aggressive environments.

In recognition of these durability issues, limit state designcodes were introduced that had crack control andthickness of cover concrete as major requirements of theserviceability limit state. New materials were developedfor repair work, such as polymer modified mortars, aswell as new types of repair, such as injection of sealants(fine mortars and resins) into cracks. The concept ofdesigning for an assessed life was introduced; therequired lives varying according to the type of structure(the longest being 120 years given in BS 5400 forhighway and rail bridges in the UK). However, servicelives are not maintenance free and the structures requireregular inspection and attention.

Repair techniques have been continually improving and atdifferent times there have been new materials and repairmethods on the market that have been expected to resultin better performances in the future. However, theseexpectations have turned out to be illusory because it hasbecome apparent that performances of both newconstruction and repairs remain poor. While this is agenerally accepted view supported by individual cases, itis based mainly on subjective judgements because apartfrom a few specific studies, there have been nocomprehensive collections of performance data thatinclude different environments and structures.

1

Chapter 1

Introduction

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In recent years the adoption of sustainability ideas andprinciples into construction has increased the pressuresto maintain existing structures and minimise theconsumption of natural resources required for repair andnew construction. There are also pressures fromheritage authorities to maintain an increasing stock ofhistoric concrete structures using minimal intervention.

Sustainable construction is an important global objectivethat involves not only minimising the consumption ofresources in new construction but indirect resourcessuch as demolition activities, transport of materials tosite, additional traffic and congestion. Constructioncauses atmospheric pollution through exhaust products(such as carbon monoxide), dust and noise. Spentmaterials that cannot be re-cycled have to be placed inland-fill sites, or elsewhere, causing increased expenseand damage to the environment.

The achievement of durable concrete repairs is crucial tothe sustainability of concrete structures. Activity in thefield has steadily increased and it is estimated thatmaintenance repair work now takes 50 % of the Europeanconstruction budget. In the US it is estimated that theannual expenditure due to damage by corrosion amountsto some US$8.3 billion. Moreover, about 27% of the162,000 highway bridges surveyed in 2000 had becomeeither structurally deficient or obsolete. Repairperformance data from the US Corps of Engineersindicated that only 50% were classified as good, 25%failed, and 25% were fair or poor[1].

Materials and structural engineers charged with thedevelopment of durable repairs are faced with a variety ofproblems.● Performances of new materials have to be

demonstrated in the laboratory using accelerated andartificial representations of service conditions; this isless realistic than exposure to natural weathering inreal time, but pressures for development are too greatand it is not feasible to allocate five or 10 years toexposure testing. Potential users are reluctant topurchase repairs that have little or no track record andeven more reluctant to be the first user.

● Potential users may be forced to purchase repairmaterials and techniques that are seen as best valuefor money; this invariably favours the cheapestproposal and gives little encouragement for suppliersto develop higher quality but more expensivesolutions.

● Heritage authorities prefer the use of traditionalmaterials and methods such as lime based mortar andlocally sourced aggregate.

Against this background the CONREPNET network hasstudied the possibilities of ‘performance basedrehabilitation of reinforced concrete structures’ and thepresent report describes four elements of the workcarried out.

● Performance of repairs in practice in order to evaluatemore 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 offunding, research topics and outcomes

● Best practice, including the European Standard EN 1504 and use of national standards and otherguidance documents.

The data in support of this work were collected throughquestionnaires sent to all sides of the repair industry,including owners of structures, repairers, materialssuppliers, consultants, research institutes anduniversities.

Achieving durable repaired concrete structures —Adopting a performance-based intervention strategy[2] isa companion book that addresses the evolution ofperformance based concepts to achieve durable repairs.

2 Concrete repairs: Performance in service and current practice

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Concrete structures are usually designed for anassessed or nominal life, taking into account the effectsof loading actions during return periods defined by theassessed 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

Other factors that may have to be considered in lesscommon circumstances include: ● Impact● Explosion● Aggressive industrial environment● Vandalism

Loading actions can influence durability in a number ofways; repeated-loading can, for example, cause initiationand propagation of fatigue cracks.

Current design for durability is through prescriptiveguidance and includes factors such as the disposition ofreinforcement to control cracking and crack widths,thickness of concrete cover to reinforcement, quality ofconcrete, and management of water (effective drainage,waterproof membranes, leak proof expansion joints, etc).However, state-of-art solutions have not yet reached thestage when the processes of degradation can beexpressed numerically to enable a structure to bedesigned in order to reach its required life, at which time,it is worn out and ready to be replaced. It follows that it isnecessary to anticipate durability problems during thelifetime of a structure and, in consequence, to carry outmaintenance and repair.

Expectations of repairs vary according to the type ofstructure and requirements of its owner, as follows.● Transport structures are long-life and represent very

considerable national investments that have to bemaintained in safe operational condition at reasonablecost. Although design codes refer to lives of 60 –

120 years, these are assessed lives in terms of fatigueand maximum occurrences of loading during the returnperiod, as mentioned above, and require properlymanaged maintenance work. Repairs are generallyexpected to last for at least 25 years.

● Sensitive industrial structures such as nuclear powerstations have shorter lives and have to be maintainedin a safe operational condition at all costs. Any repairsthat may be required are generally expected to last tothe end of the operational life of the structure whichmay typically be 30 or 40 years.

● Some commercial structures have, as their mainrequirement, to remain operational. Only minimal timecan be allowed for closure because of the high coststhat can be incurred by losses in revenue. Inconsequence, it can be acceptable to have speedyrepairs that are known to have a limited life.

● In some cases the repairs are required to enable thestructure to remain safe and operational for a shorttime until it can be demolished and replaced with a newstructure or subject to a more long term repairstrategy.

The differing expectations are reflected in the guaranteesrequired for the durability of the repairs. In many casesfive year guarantees are provided, whereas in moreprescriptive conditions the repairers are required toprovide 10 year guarantees. The provision of guaranteesrequires insurance cover which is becoming increasinglyexpensive and generates additional costs that have to bepassed on to the owners of structures. Furthermore, thepressures of potential litigation are causing insurancecompanies to become selective in the cover that can beoffered and there is a reluctance to include constructionwork involving materials such as silica or asbestos thatpose 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 imposeprescriptive requirements designed to ensure that repairsare durable and have an expected life of 25 years eventhough this may not be guaranteed. This approach leaves

3

Chapter 2

Expectations of repairs

Text 12/11/07 12:29 PM Page 3

less scope for the introduction of alternative or innovativemethods of repair.

Owners of structures use different approaches to themanagement of repairs. Some supervise the work veryclosely using their own in-house experts. Others carry outacceptance testing of the repairs. The latter are aminority but in any case little guidance is available onmethods of acceptance testing.

When questioned about their expectations of repairs,owners of structures said that they required a betterindication of the life of repairs. While this is a difficultparameter to predict with any accuracy, there is a needfor a better understanding than the current sometimesoptimistic figures often given.

Owners also expressed a need for simplified explanatoryguidance on repair processes. Currently availabledocuments are seen as being written for experts and tooesoteric for the average owner.


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3.1 Background

If repairs are to be made more durable, it is necessary tohave a better understanding of their performance inpractice. The key performance data that are requiredinclude:● 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 casehistories for structures, mainly in European countries.

Case histories are especially valuable as they providedata on repairs designed and made under the pressuresimposed by the realities of requirements and the rigoursof site conditions (as opposed to the relative comfort ofwork in the laboratory). These pressures generateproblems 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, onoccasions, to keep structures operational at all costs,

● work carried out in all weathers, sometimes too hot ortoo cold for the repair materials being used, and

● the inevitable limitations of working from temporaryaccess.

The resulting repairs are subjected to varyingcombinations of weather and loading in real time andoperational conditions that cannot be representedadequately in the laboratory. It follows that the quality andsubsequent performance of repairs predicted fromlaboratory studies require calibration against practice.

The case histories of repairs were obtained throughquestionnaires and searches of the literature. Bearing inmind that people, particularly busy engineers, are notenthusiastic about completing questionnaires, the

5

document was limited to one side of A4 paper, mostlymade up of tick-boxes, and was non-attributable (thestructures were treated as being anonymous as therewas a general unease about issues of confidentiality).The main features of the questionnaire were type ofstructure, environment, key dates, type of deterioration,type of repair, performance of repair, and cause(s) offailure. This provided up to 40 data points per case-history. The questionnaire was sent out with explanatorynotes 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 totake the responses at face value. For the purposes of thisinvestigation, repairs are considered as being workscarried out to restore the structure to its originalcondition with regard to serviceability and ultimatestrength. 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 couldnot be included as they were found to be irrelevant orhaving insufficient information on key points. In othercases it was evident that some of the data were notavailable, for example, date of construction and type ofrepair material. This left gaps so that some of theanalyses involved less than 230 data points. In one of theresponses it was noted that consultants may assess theproblem and design the repair scheme but are rarelyinvolved later and therefore have little or no opportunityto observe or record subsequent performances.Consequently there was a shortfall in case historiessupplied by consultants.

The case-histories were supplied by 24 respondentsgiving a success rate (number of productive repliesrelated to total number of enquiries) of 45%.

Chapter 3

Performance of repairs in practice

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

The respondents were from all sides of the industry;academe, owners, repairers and consultants, theirnumbers being represented in Figure 3.1. Academerepresents all those engaged in research, includingresearch institutions and universities. Owners are theorganisations who ultimately pay for the upkeep of thestructures and in many instances are responsible to thepublic for their operation and safety. Repairers aredefined as suppliers of materials and contractors (inSection 4 of this book, suppliers and contractors areconsidered as separate groups). Contractors are theorganisations who are responsible for carrying out therepair works and are sometimes given otherresponsibilities such as recommending the actions to betaken. The assignment of responsibilities variesaccording to the policy of the owner, but consultants areusually responsible for the preliminary investigation,assessment and design of repair.

The case histories were from countries having a widerange 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 casehistories for coastal and industrial sites albeit the relativedistribution is not unrepresentative of reality. Highways aretreated separately since they relate almost exclusively tobridges and are well known as being generally one of themost aggressive environments, particularly in NorthernEurope, where de-icing salt is used on highways duringcold periods. Bridges are rarely sheltered by the terrain orby other structures so that they tend to experienceextremes of weather.

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

The main types of structure reported were: buildings,bridges, dams, power stations and car parks (see Figure3.3). Other less common structures included: piles,water towers, tunnels, hangers and industrial structures(a vertical shaft, an inland jetty and a silo). The bridgeswere mainly, but not exclusively, highway bridges. Somewere in coastal regions and could experience chloridecontamination from the environment as well as de-icingtreatment.

Dates of construction varied from a church, composed ofmasonry and concrete and built in 1852, to office buildingsbuilt in 1998. Most of the structures reported are between20 and 50 years old and are mainly of precast andprestressed construction. There are 41 reinforced in situ structures that are more than 60 years old. The distributionof ages (related to 2003) is shown in Figure 3.4.


Figure 3.1 Distribution of respondents (Numbers in bracketsdenote case histories supplied. Sizes of segments of the chartrepresent numbers who responded)

Figure 3.2 Distribution of environments (Numbers in bracketsdenote case histories supplied. Sizes of segments of the chartrepresent numbers who responded)

Figure 3.3 Distribution of case-histories by structureNumbers of structures in brackets

Rural (54)

Urban (84)

Industrial (10)

Coastal (27)

Highway (54)

Consultant (23)

Academe (83)Repairer (64)

Buildings (77)

Other (22)

Dams (36)

Power stations (12)

Car parks (8)

Bridges (75)

Owner (60)

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3.2 Causes of deterioration

The main causes of the deterioration of the originalconcrete were ascribed to: corrosion, frost action,cracking, alkali aggregate reaction (AAR) and faultyconstruction. Corrosion was almost exclusivelyconcerned with reinforcing steel but there were nineinstances of corroded pre-stressing steel and one of ananchorage plate. Not surprisingly, corrosion was themost common process of deterioration, beingresponsible for 55% of the problems. Frost relates tofreeze-thaw action accentuated by poor quality concreteand leakages, usually at construction joints or expansionjoints.

Cracking was mainly associated with corrosion but it wasnot always clear whether it was a contributory cause or aconsequence. There were also incidences of crackingdue to structural actions such as loss of prestress.

Faulty construction included inadequate thickness ofcover concrete and incorrectly placed concrete resultingin voids, honeycombing and cracking. Some 40% of thecases of faulty construction were repaired immediatelybut the rest were not identified until many years laterwhen problems had developed.

The distribution of the primary causes of deterioration isshown in Figure 3.5.

Other less common types of deterioration included: ● inadequate strength (usually due to factors such as

losses of section caused by corrosion or inadequatedesign in the first place),

● scour, leaking (of dams and water containingstructures),

● leaching (of spillways), impact damage, damage fromoverloading, and

● sagging due to prestress loss, and structuralmovement.

Inadequate thickness of cover concrete and carbonationwere only reported in a few of the case histories but thisprobably represents a failure to identify the mechanismsince they are usually found to be among the morecommon causes of deterioration.

The respondents commonly reported more than one typeof deterioration. For example, there were severalinstances of corrosion, frost damage and cracking beingcited as having occurred simultaneously on the onestructure. However, some may be considered to beconsequences rather than original causes. In order todeal with this potential confusion, the data wereinterpreted so that the primary cause could be assigned.

Ages of the structures at the time of repair, as opposedto the date of construction, were in the range 0 to 100years, most being in the range 10 to 40 years, nine wereover 70 years and the oldest was 100 years (see Figure 3.6).

Performance of repairs in practice 7

Key points

● 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 repairperformance

Figure 3.4 Ages of structures

Years (to 2003)

0-10 11-20 21-30 31-40 41-50 51-60 61-70 81-15171-80

0

10

50

40

60

Num

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30

20

Text 12/11/07 12:29 PM Page 7

3.3 Types of repair

With only one exception, the repairs were designed andcarried out with the intention of achieving as long a life aspossible. The exception was a ‘holding repair’ which wasrequired to last a relatively short time until the structurecould be replaced. Although only one holding repair wasidentified, one respondent pointed out that they are fairlycommon and may be carried out for visits by publicfigures, for public relations purposes, when budgets aretight, or to calm public alarm.

The most common types of repair were: patching;coating; crack injection; restoration of strength; sprayedconcrete; electro-chemical methods (mainly cathodicprotection) and added prestress. Numbers of repairtypes are shown in Figure 3.7. Two or three methodswere commonly applied per repair, for example, patchingwas 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 casehistories.

Less common repairs included application of corrosioninhibitors, wrapping with carbon fibre reinforced plastic(CFRP), re-alkalisation, added cover concrete and addedprestress. Associated measures included added thermalinsulation, repaired expansion joints, re-tiling (of facades)and waterproofing.

It should be noted that although these different methodshave been considered in a single group, they could besubdivided 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.


Per c

ent

Figure 3.5 Primary causes of original deterioration

0Corrosion AARCracksFrost Faulty Construction

10

20

30

40

50

60

Figure 3.6 Ages of structures when repaired

Num

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0-10 11-20 21-30 31-40 41-50 51-60 71-10061-70

Age (Years)

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Patching was applied in 60% of the case histories, mostlyin cases where corrosion had occurred it was necessaryto remove defective concrete and clean or replacecorroded reinforcement. The material used for thepatching was ‘cementitious’* in 60% of the patches andpolymer modified mortar in 30%. Other types of patchingmaterial included mortar containing steel fibres andpolymer modified mortar containing polyethylene fibres.

Coatings were applied in 35% of the repairs, the maintypes being barrier and hydrophobic (numbers are given

in Figure 3.8). Other types were anti-carbonation,aesthetic and several that were not specified, probablyrepresenting situations where more detailed informationwas not available to the respondent. Coatings wereusually applied in combination with other repairs such aspatches and crack injection, with only 30% being solo.

Restoration of strength was carried out in 17% of therepairs. The most common scheme was forreinforcement bars that had corroded and lost so muchmaterial that it was considered necessary to replacethem with new bars. This has the added value that newbars are less likely to corrode than corroded bars, whichare difficult to clean properly and likely to harbourresidual chlorides. Other methods of restoring strength


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

Figure 3.8 Relative uses of different types of coatings

120

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Num

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Patch Coating Crack Restoration Sprayed Electro- AddedInjection of strength concrete chemical prestress

Barrier Hydrophobic Aesthetic Other

50

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30

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10

5

0

* ‘Cementitious’ refers to mortar having no more than simple admixtures added toimprove flow or curing characteristics as an addition to the mix. ‘Polymer modified’refers to mortars composed of cement plus polymeric materials added to improvethe physical properties of the mortar.

Num

ber

160

140

Text 12/11/07 12:30 PM Page 9

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

3.4 Performance of repairs

3.4.1 InspectionThere are three stages in the repair process wheninspection should be carried out:● Prior to repair. Inspection of the deteriorated

concrete to determine the extent and nature of theproblem

● After completion of the repair. Inspection todetermine whether the work has been carried outproperly and is acceptable

● Routine inspection. As part of the maintenanceschedule and to determine whether the repair remainsin a satisfactory condition.

The inspections reported in the case-histories weremainly routine although inspections for acceptanceappear to be rarely carried out in any depth.

Only 15% of the inspections were reported as havingutilised non-destructive testing (NDT), the majority ofinspectors apparently being satisfied with visualexaminations. In cases where NDT was carried out thetests 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 thiswas apparently not carried out on a routine basis aspart of the investigation before the repair wasdesigned,

● pull-off strength to determine the adhesion of patchesto the substrate,

● carbonation depth to determine the progress ofdeterioration,

● chloride gradients to determine the likelihood ofcorrosion developing, and

● impact-echo testing to determine whether delaminationor debonding had occurred.

Removal of cores for strength tests and petrographicstudies were reported as being occasionally carried out.While petrographic studies can be invaluable in helping toidentify deterioration processes, strength tests on coresappear to be carried out as a matter of tradition and it ismore useful to identify properties of the concrete such asstiffness to enable repairs to be made with material thatis fully compatible with the substrate.

3.4.2 Classification of performancesPerformances of the repairs were classified as:● successful, as identified at the most recent inspection

and not requiring attention for the time being,

● exhibiting early evidence of failure, considered to beunsatisfactory and eventually requiring further action,for example, minor cracking suspected to beassociated with corrosion, and

● identified as failed and requiring immediate attention,for example, continued corrosion.

There are some situations when classification of repairperformance can be, to some extent, a subjectivejudgement. For example, a repair may be accepted asbeing structurally successful but failing due toaesthetics; some coatings and patches can becomediscoloured and unacceptable to owners despite meetingall other requirements (an example is given in Section3.5). Needless to say these situations can lead todisputes but ultimately repairs have to be carried out tothe satisfaction of the owner of the structure and therepairer should provide information about likelyoutcomes beforehand.

3.4.3 Overall repair performances For all types of repair, 50% were reported as beingsuccessful at the last inspection, 25% exhibited evidenceof failure, and 25% failed. However, it is more informativeto consider types of repair, their performances andexplanations given for causes of failures.

Repair performance is shown in Figure 3.9 and it isevident that most failures occur in the first 10 years afterrepair. Significant numbers also occur beyond this age;the longest reported life to failure was 52 years. In thesubsequent analysis of performances in relation to time,successful short duration repairs have been successivelydeleted. Thus, when calculating percentage failures in,say, 25 years, successful repairs of less than 25 yearsduration have been discounted. On this basis it was foundthat 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 orhaving failed altogether. These performances areexpressed in relation to the common guarantee timesand expectations quoted in Chapter 2.

3.4.4 Overall repair performances (by repair type)Success rates for the different types of repair within thisstudy were reported as follows:


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

Text 12/11/07 12:30 PM Page 10

● 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.

These figures are indicative of repair performances butcannot be considered to be statistically rigorous forcertain types of repair that have relatively small numbersof case-histories. In any case the numbers are likely to beinfluenced by the reporting organisations, somerespondents being reluctant to report failures of repairsand others possibly over-reacting to minor defects.Nevertheless, the data are credible and there are nosignificant surprises.

3.4.5 Cathodic protection (CP) The success rate reported for CP (35%) is poor but thenumber of case histories (12) is low so that this resultmay be atypical. However, an additional 62 abbreviatedcase-histories were obtained for installations in the UK.The performances of all 74 installations (including the 12full 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.


Time since repair (Years)

Failed Evidence of deterioration

Figure 3.9 Performances in relation to age of repairs

Figure 3.10 Patch repair after five years, showing signs ofincipient anode behaviour due to non-removal of chloridecontaminated material

0-5 6-10 11-15 16-20 21-25 26-52

25

20

15

10

5

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Num

ber

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

Text 12/11/07 12:30 PM Page 11

It can be argued that few of the installations were reallyfailures since the action of the CP would remain effectivefor some time into the future and repairs could usually becarried out quickly and economically.

It was reported that 17 of the installations weremonitored but it is believed that there were many more.The effective lives of CP installations are not yetestablished but one supplier commented on theperformance 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 CPinstallations having data supplied are summarised inTable 3.1.

A detailed description of the processes of corrosion andcathodic protection is given by Broomfield in Corrosion ofsteel in concrete[ 3].

3.4.6 Types of deterioration ● 50% of repairs to corrosion were successful. The

types of repair reported and their success rates werecrack injection (70%), and patching (40%). The highsuccess rate for crack injection is probably anindication that corrosion was less advanced than in thecases where it was considered necessary to gothrough the steps of removing all the affected

concrete, replacing, or cleaning, the reinforcementand putting in place patching material. Moreover,crack injection could be regarded as being aprotective measure rather than a repair.

● 65% of repairs to corroded prestressing steel weresuccessful. Corrosion of prestressing steel presentsspecial problems as it can occur without any visibleevidence externally; there have been several caseswhen it has led to structural collapse. This is discussedin more detail in Section 4.6.

● 20% of repairs to AAR were successful. The types ofrepair reported were: patch plus coating, coatingalone and patch alone. There were insufficient data forsuccess rates of these individual repair methods to bemeaningful but the overall value of three successes for14 cases is indicative of the difficulties in making aneffective and lasting repair of AAR affected concrete.

● 25% of repairs to frost-damaged concrete weresuccessful. This is especially low and is an indicationthat the failed concrete was repaired but the rootcause of the problem was not tackled.

● 90% of repairs to poor construction were successful.This high success rate is probably due to the defectsbeing identified before processes of deteriorationsuch as corrosion had commenced, in fact 40% of thecases were repaired immediately.

● 65% of repairs to cracking were successful.

3.4.7 Repairs in combinationIn the previous sections, types of repair have beenconsidered irrespective of whether they were appliedsolo or in combination. In 60% of the case-histories, morethan one type of repair was applied.

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


Table 3.1 Successful CP installations

Age (years) Number

0 – 4 95 – 9 510 – 19 620+ 6

Figure 3.11 Performances of patches

Type of PatchSuccessful Evidence of failure Failure

Cementitious Polymer Modified Other

0

10

15

20

25

30

35

45

5

Num

ber

Text 12/11/07 12:30 PM Page 12

3.4.8 Modes of repair failureThe common failure modes for all types of repair werereported as cracking, continued corrosion, de-bonding,continued AAR and leakage (Figure 3.13). Other lesscommon 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 continuedcorrosion 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 toother modes. An example of a failed coating applied toAAR affected concrete is given in Figure 3.14.

● For sprayed concrete, failures modes were mainlycracking, de-bonding and continued corrosion.

● CP failure modes are listed in section 3.4.5. Insummary, there were failures of anodes, electricalconnections, installations accidentally switched off,and a variety of other causes that could easily berectified. There was only one case where there wascontinued corrosion and the CP was reported to havebeen ineffective.


Figure 3.12 Performances of patch repairs to corrosion

Successful Evidence of failure Fail

Num

ber

Patch Solo Patch plus Coating

Figure 3.13 Modes of failure – all types of repair

35

30

25

20

15

10

5

0Corrosion Cracking Debonding Continued AAR Continued Other

leakage

Num

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25

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3.5 Causes of repair failuresIn the case-histories reported, failures of repairs wereattributed 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.

Problems with repair materials could be regarded as asub-set of incorrect design since they are mainly a matterof incorrect specification. They are listed separatelybecause they represent a specialist element, and in anycase there are other factors such as whether advice wasprovided or obtained from the materials suppliers.

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

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

In four cases it was admitted that the cause of failure wasunknown. Numbers of the different causes of failure areshown in Figure 3.16.

3.5.1 Incorrect diagnosis

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

Examples of original problems being incorrectlydiagnosed, or not identified as needing attention,included porous or honeycombed concrete; the presenceof deleterious materials such as calcium chloride or highalumina cement in the original concrete; and failure toidentify the root cause of cracking

3.5.2 Incorrect design

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


Figure 3.14 (top and bottom) Failure of polymer mortarpatches and sprayed polymer coatings applied to an AAR affectedbridge. This failure was considered to be partly due to incorrectdiagnosis of the original problem and partly to incorrect design ofthe repair

Figure 3.15 (top and bottom) Failure of polymer mortarpatches and polymer coating applied to a bridge affected bycorrosion. This failure was considered to be due partly to incorrectdesign of the repair and partly to incorrect application of anti-corrosion treatment to corroded reinforcement

Text 12/11/07 12:30 PM Page 14

Where the repair was considered to have been incorrectlydesigned, typical errors included: insufficient defectiveconcrete removed before patching; low cover at the timeof construction which was left uncorrected; cosmetictreatment instead of a properly designed repair; andinadequate arrangements made for drainage. In caseswhere there was continued corrosion, it was generallydue to insufficient defective concrete having beenremoved and incipient anodes becoming dominant; thiswas a common shortfall in design of repairs.

3.5.3 Poor workmanshipPoor workmanship was a general problem. In one case acorrespondent noted on the questionnaire that:

‘The work had been a textbook example of how not tocarry out a repair’.

In another case:

‘Coatings were incorrectly applied despite clearinstructions being given’,

And in yet another,

‘Poor workmanship; too thin coating [againstinstructions]’.

There were also other instances where coatings wereapplied too thick or too thin. An example of ‘aestheticdeterioration’ is given in Figure 3.17.

3.5.4 Incorrect repair material

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

cause of failure, it was rarely suggested that the materialwas inadequate per se. One exception was cathodicprotection where some early anode materials proved tohave inadequate durability. More commonly, failed repairmaterials were found to be incompatible with the originalconcrete due to differing strength or absorption rates. In one case, coating material intended only for internaluse was used externally and, not surprisingly, failed.

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


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

Figure 3.16 Reported causes of failures

Incorrect design Incorrect material Poor workmanship Wrong diagnosis Other

45

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0

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Text 12/11/07 12:30 PM Page 15

conditions during the work followed by damp and cold. Itis likely that poor workmanship also contributed to theproblem. This is only one example of many similar cases.Appearance of efflorescence is a typical problem when using cement-based coating systems in northerncountries. Aesthetic appearance is considered veryimportant especially concerning repairs of buildingfaçades. Recoating and cleaning due to efflorescencehave caused extra costs and angry customers. On theother hand the durability properties and technicalfunctionality of cement-based coatings when applied tofacades are much better than organic coatings andpaints.

Influence of environmentThe performances of concrete and repairs to concretestructures located in different environments are shown inFigure 3.18. The data follow the normal trends withrepairs in coastal and industrial environments exhibitinghigh failure rates. However, the number of industrial casehistories was relatively small.


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

Urban Rural Highway Coastal Industrial

80

70

60

50

40

30

20

10

0

90

Key points

● The performance of 230 concrete repairs have been collectedand analysed

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

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

workmanship and wrong diagnosis were the most commoncauses of repair failure

Per c

ent s

ucce

ssfu

l

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The use of inspection techniques can be summarised asfollows.● The majority of respondents reported that they often

used visual inspection but surprisingly, fourrespondents only sometimes carried out visualinspections.

● Depth of cover, carbonation depth, chloride contentand 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 manuallyand about 50% used computer aided methods.

The popularity of electrode potential measurements,corrosion current measurements and monitoring wasseen to be increasing.

The organisation selected to carry out the inspectionprior 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.

17

4.1 Background

Most of the repairs reported and analysed in Chapter 3,were carried out during the period 1960 to 1990 andinvolved the state-of-the-art methods contemporary tothat time. In subsequent years materials and techniques,as well as an understanding of the processes, have beenimproved and it is appropriate to examine how currentpractice has been developed in relation to inspection andmethods of repairing deteriorated concrete. In thiscontext current practice is related to 2003.

Questionnaires about current practice were designed tobe complementary to the one for case-histories, (seeAppendix II). Responses were received from 55organisations in 16 countries, giving a balancedrepresentation of views. The organisations employedsome 64,500 people in total and about 6,000 wereinvolved in concrete repair. For some of theorganisations, concrete repair represented less than 1%of their business, for others it was 100%.

4.2 Inspection

Respondent (consultant) comment: ‘Inspection priorto the repair is critical to the whole process’.

From the responses received, the relative usage of themore common methods of inspection (expressed aspercentages) have been summarised in Table 4.1.

Surprisingly, there were respondents who apparently hadnot heard of common tests such as location ofreinforcement, electrode potential measurement,corrosion current measurement, monitoring andpetrographic analysis (this may, however, have been due tomisunderstanding of a questionnaire written in English).

Chapter 4

Current repair practice

Table 4.1 Relative usage of inspection techniques

Technique Used sometimes Used commonly

Visual inspection 4 96Thickness of 14 86cover concreteDepth of carbonation 13 87Core tests 21 79Chloride content 24 76Electrode potential 43 57Petrographic analysis 66 34Monitoring 69 31Corrosion rate 76 24Loading tests 77 23

Key point

● Current practice data were provided by 55 organisationsemploying some 6,000 people on repair work

Text 12/11/07 12:30 PM Page 17


4.3 Repair methods

Respondent (contractor) comment: ‘[We] sometimeshad to apply a repair method specified by an owner orconsultant while [we] would have proposed and used amore appropriate method if the choice had been up to[us]…’.

The relative uses of the most common types of repairmethod are shown in Table 4.2. Other methods listed inTable 4.2 include restoration of strength (bondedplating), electro-chemical techniques (cathodicprotection, re-alkalisation and chloride removal) andcorrosion inhibitors. Respondents reported increasedinterest in cathodic protection and hydrophobic coatings,and decreased interest in sprayed concrete and crackinjection. The reported decrease in interest in sprayedconcrete is in contradiction to the increased number ofapplications that can be seen on the market. However,the small number of respondents may make this anincorrect interpretation.

Some 25% of the respondents reported that the repairsare subcontracted to specialist repairers.

Those owners who carry out inspections themselvesinvariably decide what methods of repair will be usedand, in general, they make the decision in 30% of allrepair projects. Contractors select the repair methods inabout 50% of projects. On occasions when specialmethods such as CP or strengthening, are proposed, itwas reported that the contractor invariably decideswhether it is appropriate or not. The relative uses of thecommon repair techniques are shown in Figure 4.1.Acceptance ratings, representing the popularity of thedifferent methods of repair in the market, are shown inFigure 4.2. Acceptance ratings, representing thepreferences of the repair industry are shown in Figure 4.3.

Comparison of Figures 4.2 and 4.3 show that the views ofthe repair industry are generally in accord with the preferences of the market place. The exceptions to thisare the sprayed concrete and crack injection methods,

Key points

● Visual methods are used in the majority of inspections● 20 to 30 % of inspection work is subcontracted● The most commonly used methods of NDT are measurements

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

inspection

Table 4.2 Use of the common methods of repair

Repair Used sometimes Used commonly

Patches 14 86Coatings 37 63Crack injection 29 71Sprayed concrete 36 64Electro-chemical 63 37methods‘Other’ methods 9 91

Figure 4.1 Use of repair techniques

Never/rarely Sometimes Often

Patch Coating Crack Sprayed Electro- Otherinjection concrete chemical

100

80

60

40

20

0

Per c

ent

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4.4 Quality control (QC)

It was found that about 90% of repair projects aresubjected to QC. The type and number of QC tests dependin many cases on the available budget. There is nogenerally accepted procedure for quality control and in anattempt to regularise the situation one consultant felt itnecessary to prepare a document outlining a more logicalapproach to the question.

Current repair practice 19

which the companies rate as being equally important asother repair methods (Figure 4.3) compared to very lowacceptance in the market for these methods (Figure 4.2).This may be aggravated by the fact that these techniquesrequire specialised workmanship. Only repairers that oftenapply these techniques, and rate them highly, are able toprovide the necessary quality and deliver durable repairs.However, the position is influenced by the limited numberof repairers offering these products.

Figure 4.2 Acceptance ratings of repair products in the market

Figure 4.3 Acceptance ratings of the repair companies

Cem

ent-

base

d m

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Various quality assurance and quality control tests werementioned 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, acoustictests, pull-off tests, laboratory tests on cores and insome cases, structural loading tests.

● Special tests to verify the correct functioning oftechniques such as chloride removal and cathodicprotection.

● Thickness measurements of applied coatings.

Visual inspection and checks should be made withintouching distance of the structure.

4.5 Comparison with earlier repair practice

In this section comparison is made between currentpractice and the earlier repair practice described inChapter 3 (mainly for the period 1960 to 1990) toidentify the developments and changes that haveoccurred as materials and techniques have beenimproved and experiences of the various repair methodshave been gained.

4.5.1 Relative use of the different methods of repairComparative data for the most common methods ofrepair, shown in Figure 4.4, suggest that patching isapparently becoming less popular. This is surprising

since reinforcement corrosion remains the mostcommon problem and repairs invariably require removalof contaminated concrete and cleaning of thereinforcement, which must be followed by patching to fillthe excavation and provide good protective cover to thereinforcement.

On the other hand, sprayed concrete and electro-chemical methods have become more popular. It issuggested that the most credible of these trends is forelectro-chemical methods – and cathodic protection (CP)in particular – to become more popular as engineers gainconfidence from experiences in the US and UK wherelarge numbers of CP installations have been in use formany years.

Repair methods that are occasionally used (‘other’methods), reported in both past and present responses,included restoration of strength and use of corrosioninhibitors.

Overall, current uses of the different types of repairmethod are broadly similar to past practice.

4.5.2 Types of patch repairIt is evident from Figure 4.5 that whereas in the past,cementitious mortars were used on almost twice asmany occasions as polymer modified mortars in patchrepairs, there is little difference in current practice. Thisis probably a consequence of improvements in thepolymer modified materials over those used in the pastand the knowledge of laboratory testing, which indicatesthat polymer modified mortars have advantages overcementitious materials. However, this is not whollysupported by the evidence from past performances,which indicates that there is little difference in respectivedurability:● 55% of cement based mortar patches failed● 50% of polymer based mortar patches failed

Figure 4.4 Comparison between past and current usage of repair methods

Past practice

Presentpractice

Patching Coating Crack Sprayed Electro- OtherInjection concrete chemical

40

35

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20

15

10

5

0

Per c

ent

Key point

● There is no generally accepted procedure for quality controland in an attempt to regularise the situation one consultant feltit necessary to prepare a document outlining a more logicalapproach to the question.

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4.5.3 CoatingsThe relative use of barrier coatings and hydrophobiccoatings has remained virtually unchanged; barriercoatings continue to be applied twice as often, as shownin Figure 4.6. It is suspected that financial pressures anda desire to cover unsightly patching with a coating ofuniform appearance, make barrier coatings moreattractive to owners of structures. Evidence from pastperformances suggests that there is little difference inthe performances of the coatings in practice.● 50% of barrier coatings failed● 45% of hydrophobic coatings failed

4.6 Inspection strategy

From the responses it is apparent that too little attentionhas been given to inspection at the different stages of therepair process and, in consequence, many of the earlyfailures of repairs have been due to inadequate control ofthe work. As part of a comprehensive repair strategy,deterioration curves shown in Figure 4.7 delineate failureenvelopes in relation to the common requirements ofrepairs.● Guaranteed repair lives of five years or 10 years● Life expectation of 25 years.

The performance in practice, as determined from thecase-histories analysed in Chapter 3, are shown withinthese envelopes; these are 20% of repairs fail in fiveyears, 55% in 10 years and 90% in 25 years. It followsfrom these performances that although the normal timingof principal inspections, commonly at intervals of five orsix years for the better managed structures, may beappropriate to the general maintenance of those havinglong lives, in the first 10 years it would be better to


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

Figure 4.6 Comparative use of barrier and hydrophobic coatings

Past practice

Presentpractice

Past practice

Presentpractice

Per c

ent

Cement-based Polymer-based

Barrier Hydrophobic

70

60

50

40

30

20

10

0

80

70

60

50

40

30

20

10

0

Per c

ent

Key points

● About 90% of repair projects are subjected to quality control ofone sort or another but there is no generally acceptedprocedure

● Patching has apparently become less prevalent in currentpractice

● Electro-chemical techniques and sprayed concrete havebecome 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

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

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inspect repairs at closer intervals, of say one year. Inaddition, NDT should be used in support of visualobservations bearing in mind that 35% of repairs failed atbetween five and 10 years.

4.6.1 Methodology of inspectionThere are three stages in the development ofdeterioration which can be exemplified by the processesof corrosion caused by ingress of chlorides, as shown inFigure 4.8:1 Incubation when processes of corrosion are

developing but not yet active, for example chloridesdiffusing from the surface of the concrete and movingtowards the steel reinforcement. The presence ofthese chlorides is detectable.

2 The onset of corrosion when the local passivity hasbeen destroyed and the steel reinforcement is nolonger fully protected. The electro-chemical reaction isactively progressing and can be detected bytechniques such as measurement of electrode-potential.


3 Damage, for example cracking and rust staining on thesurface of the concrete can be seen with the nakedeye and is evidence of significant corrosion of the steelreinforcement buried beneath the surface.

However, it is evident from the case-histories that mostinspectors rely on visual evidence so that by thencorrosion will have reached an advanced stage andopportunities to take timely defensive action will havebeen lost.

Data can be added to Figures 4.7 and 4.8 to provide abroad picture of performances and enable inspection andrepair work to be planned and carried out more effectively.The model in Figure 4.8 for corrosion, can be applied toother processes of deterioration, for example frost andAAR, albeit there may be fewer stages.

It is also important to recognise that inspection andmonitoring should be carried out as a step-by-stepprocess and different tests and approaches are requiredto aid diagnosis, acceptance, and assessment of thesubsequent performances of repairs. In the followingsection, four stages of inspection are identified.

Stage 1. Identification of the underlying cause ofdeterioration

In the case-histories discussed in Chapter 3, 50% offailures were ascribed to either incorrect diagnosis of the

Figure 4.7 Performances of repairs

Key point

● In the 10 years after repair, inspections should be at relativelyclose intervals

Figure 4.8 Levels of detectability of the deterioration of a repair

Det

erio

ratio

n

Time - Years

Time - Years

Det

erio

ratio

n

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underlying problem or incorrect design of the repair.Many of the cases of incorrect diagnosis were due topriority being given to identifying the process ofdeterioration. This could result in failure to recognise thatproblems 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 case-histories were related to corrosion of steelreinforcement. One of the underlying causes is often theprovision of inadequate protection against chloridesthrough a low thickness of cover to the reinforcement,the presence of permeable concrete, cracking, or localleakage (commonly by failing expansion joints) thatpermits water and chlorides to diffuse into the concreteand initiate the process of corrosion. Identification of theunderlying cause is the first step in diagnosis and is asimportant as identification of the deterioration process.

Cracking, which occurred in 22% of the reported failures,is a product of various underlying causes. It can becaused by mechanisms as disparate as loss of pre-stress, structural movement, overloading, impact,corrosion, freeze-thaw action and AAR. The mechanism inquestion needs to be identified using a combination ofobservation, NDT, and numerical analysis. Numericalanalysis is not normally considered as being a form ofNDT but nevertheless it is an invaluable tool that issometimes overlooked.

Stage 2. Identification of the deterioration process

The data in Chapter 3 indicate that only 15% of theresponses related to investigations using non-destructivetesting. It follows from the rather disappointingperformances of repairs that there is a clear need forNDT to be used more generally and targeted moreeffectively in order to improve the quality of differentstages of the repair process, i.e. as an aid to diagnosis,acceptance of repairs, and their subsequentperformance. Visual inspection is important but must becarried out properly by experienced inspectors able todetect and interpret early evidence of problems.Moreover, it should be carried out within touchingdistance of the concrete unless the nature of thestructure makes this impossible.

Common processes of deterioration, as reported in thecase-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 processesdetectable by specific tests but many repairs failed due toincorrect diagnosis or failure to identify the full extent ofthe affected concrete. The classic example is where apatch repair is made, but insufficient affected concrete is

removed from the vicinity of the corrosion and incipientanodes develop in the region around the patch.

Stage 3. Acceptance testing of repairs

In practice, the likelihood of acceptance testing ofrepairs being carried out is rather variable. Someauthorities prefer to rely on close supervision of the workand feel that testing would be an unnecessary additionalactivity (and expense). Nevertheless acceptance testingshould 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 beaccepted.

Moreover, access can be the most expensive element intesting so it is therefore more economic to carry outtests immediately after the work when access is stillavailable. Later testing, in the absence of easy access, islikely to be restricted and less comprehensive.

The acceptance tests should be selected to suit the typeof repair and likely modes of failure.

The most commonly reported modes of repair failure inthe short term include cracking, debonding, and spalling.In the longer term, continued corrosion and AAR areprevalent. It follows that acceptance tests should betargeted at these problems.

Although there have been numerous publicationsdescribing tests and methods of inspection, none havedealt adequately with the specific requirements ofrepairs. Some of the methods of NDT that can be used toaid diagnosis include tests less commonly used on site.Although there are numerous reports and papers dealingwith the different methods of NDT, most addressconcrete in general or properties of the repair material,for example Table A1 in EN 1504-9.

Some examples of appropriate methods of NDT to aidacceptance are listed but not considered in any detail inTable 4.3

Stage 4. Subsequent inspection of mature repairs

Subsequent inspections of mature repairs are usuallyperiodic and carried out at the same time as normalmaintenance. This procedure is rather unsatisfactorybecause methods used are the same as for normalinspections and repairs are rarely treated any differently.

For most types of structure there are recommendedmaintenance schedules that comprise generalinspections, which should be carried out annually, areusually visual and can only identify defects in a fairlyadvanced stage of development. Principal inspections


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are carried out at intervals of five or six years dependingon the requirements of the owner and the type ofstructure but as a significant proportion of repairs werereported to have failed or exhibited evidence ofdeterioration in the first five years, the first inspectionafter repair work is important and requires specialattention to assess the repairs.

In certain cases it may be deemed necessary to carry outcontinuous 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 themeantime it remains in a safe condition.

● It is considered necessary to demonstrate that therepair has been effective and remains effective.

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

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


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

Table 4.4 Continuous monitoring

Requirement Measurement Method

Ensure structural Crack growth LVDT, VWGintegrityLoading data - Traffic - Weigh-in-motion

- Crowds* - Autocounting- Wind speed & direction - Anemometers- Temperature - Thermo-couples

Vibration data Amplitudes and Accelerometersfrequencies

Fatigue data - Stress cycles - ERS plus analysis- Crack growth - Acoustic emission- local fractures - Acoustic emission

Corrosion Electro-potential Buried probes (Figure 4.10)

* on footbridges and in stadia

KeyLDVT: Linear variable differential transformerVWG: Vibrating wire gaugeERS: Electrical resistance strain gauge

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

Table 4.3 Examples of NDT to aid acceptance of repairs

Repair Common NDT defects

Patch - Built-in defects - Impact-echo- Fine cracking - Permeability- Poor adhesion - Pull-off strength

(Figure 4.9)Coatings, - Pin holes, fine cracks - Permeabilityall types - Poor adhesion - Pull-off resistance

- Incorrect thickness - Thicknessmeasurement

Coatings, hydrophobic Failure to protect Water absorptionCrack injection Failure to seal PermeabilityCathodic protection - Inadequate electrical - Electrical

continuity continuity- Failed electrical control - Depolarisation

(aftercommissioning)

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4.6.2 Post-tensioned structuresThere can be situations where conventional methods ofinspection are inadequate and it is necessary to take adifferent approach. Inspection of the prestressingstrands in grouted post-tensioned structures is a relevantexample as it presents problems and conventionalmethods of inspection are inadequate. Moreover, visualinspection of the structure is rarely helpful as the strandscan corrode to an advanced stage and fractures canoccur without any superficial evidence on the surface ofthe external concrete. It is a particularly sensitive issuebecause corrosion can lead to wire fractures, structuralcollapse, and on occasions, loss of life. It has thereforebeen necessary to identify effective non-standardmethods of inspection[5].

The most promising methods of NDT listed in guides andstandards (including X-ray, radar, impact-echo andultrasonics) have been developed and applied to post-tensioned concrete with only limited success. In theevent it has been necessary to turn to relatively lowtechnology and the most reliable method has been foundto be intrusive drilling into the duct and directly observingthe condition of the strands by experienced inspectors. If

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

The process of inspection of prestressing strands is bestcarried out in stages as recommended in the HighwaysAgency document BA50/93[6], which was prepared todeal 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 andit is practical to await events, acoustic emission can beused to monitor the situation and record occurrences ofany wire fractures. This has been done successfully onpost-tensioned floor slabs in office blocks and parkingstructures, especially when the tendons are unbonded.

Repair of damaged post-tensioning has also presentedspecial problems. When corrosion has not yet developedor is considered not excessive, voids in the grout can befilled so that the steel strands are protected from furtherdamage. It is important that the grout remains stable


Figure 4.11 Location of reinforcement bars Figure 4.12 Measurement of crack widths

Figure 4.14 Measurement of electrode potentialFigure 4.13 Detection of carbonation

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when injected and does not bleed because this candevelop further voids. Stability of the grout can beassessed using the BRITE Euram test[7].

Small voids can be filled with an epoxy grout. In work onautostrada bridges in Italy it was recommended thatepoxy used to fill the voids should have the followingproperties: ● 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 isempty, a cementitious grout is generally the best option.The grout can be injected under pressure or the processcan be aided by vacuum.

When corrosion is advanced to the extent that thestructure is assessed as being at risk, it may benecessary to replace the element or take steps torestore the strength. This can be achieved by variousmethods, 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 ofexternal prestress can be an effective way to restorestrength and is often used. It is, however, necessary toensure that the concrete is not over-compressed locallyor globally. Also, if the prestressing tendons are externalto the structure and exposed to weathering, they requireadequate corrosion protection. In all cases, repairsshould be accompanied by measures to ensure there areno routes for water or de-icing salts to penetrate into theconcrete or tendons.

There have been cases when corrosion has been found tobe too advanced for the structure to be repaired and ithas been necessary for it to be demolished and replaced.


Key points

● Inspection of grouted post-tensioning strand presents specialproblems because corrosion and fractures can lead tocollapse without the appearance of any external evidence andcontemporary 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 directobservation of the strands by experienced personnel.

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This chapter is concerned with current research onconcrete 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

The data were obtained from questionnaires sent torelevant organisations.

Responses to enquiries produced data for 66 differentprojects from 28 respondents representing some 308organisations. The distribution of respondents is shownin Figure 5.1. About 40% of the projects were led byconsultants, 31% by academe and 21% by repairers. Thisis considered to be an appropriate balance betweenindustry concerned with the need to address practicalissues and professional researchers concerned withacademic rigour. The two are not always mutuallycompatible.

The longest running project, started in 1995, lasted foreight years. There were 44 projects reported to becompleted and 22 on-going. Data from thequestionnaires were augmented by information obtainedfrom 72 other research projects related to concreterepair and identified from the official website of E-core(The Thematic Network E-CORE, European ConstructionResearch Network)[8].

5.1 Sizes of research projectsNumbers of participants per project are summarised inTable 5.1

It is notable that about 20% of the projects were carriedout by one organisation alone. At the other extreme, oneproject was reported as having 21 participants. Financial information was provided for 49 of the 66projects. The total budget for these projects was €37.5million at an average per project of €0.8 million. Thesmallest 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 in-kind, often in the form of materials and time, which arenot always acknowledged as equivalent financialcontributions. The total budget for the 66 projects,including this ‘in kind’ contribution, could amount to some€50 million.

Numbers of projects funded by the different agenciesand the distribution of funding across the research topicsare shown in Table 5.2.

27

Chapter 5

Current research

Figure 5.1 Distribution of respondents to research questionnaire

Key point

● Data for 66 research projects were obtained from 28respondents and supplemented by 72 projects fromwww.e-core.org

Table 5.1 Numbers of participants in research projects

Number of Participants Number of Projects

1 122 – 4 325 – 10 12More than 10 10

Academe (24%)

Repairer (36%)Consultant (32%)

Owner (8%)

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The international projects listed here are mainly fundedby the European Commission’s RTD programme andinclude thematic networks on concrete repair andrehabilitation such as CONFIBRECRETE[9], ONTECVET[10]

and REHABCON[11] (see Appendix V).

5.2 Research topics

The research projects are listed under the followinggeneral topics.● Concrete durability. Research aimed at

understanding the performance of concrete, itsdeterioration mechanisms, and the impact these haveon the expected service life of the structure. This classincludes performance and behaviour of repairedstructures, the preparation of the substrate and anyaction taken at the time of construction to enhanceservice life. The latter is only relevant when it can beapplied to repair work.

● Materials. Research on repair materials and methodsincluding the performance of impregnations andcathodic protection.

● Inspection. Research related to inspection ofconcrete structures and evaluation of the data,monitoring techniques prior to repair and choice ofrepair methods.

● Maintenance. Research to improve service life by anappropriate management strategy, long termmonitoring and development of databases andsoftware to aid an understanding of the aging ofstructures.

● Strengthening. In relation to concrete repair, relevantstrengthening is concerned with restoration to theoriginal condition rather than increased strength to alevel above the original design requirement.

The distribution of these projects is shown in Figure 5.2.

Most of the research projects reported to CONREPNETare concerned with materials (38%) followed by durability(21%) and strengthening (20%). This distribution maysimply relate to the perceived potential for a successfuloutcome and commercial advantages to theorganisations involved. However, if the research projectslisted on www.e-core.org are also taken into account(resulting in a total of 138 projects), there is lessemphasis on materials and more on inspection andmaintenance (see Table 5.3).

The preferences expressed by the different types oforganisation towards the research topics are shown inTable 5.4

● Consultants, contractors, owners and suppliers allexpressed greatest interest in repair materials andmethods

● Members of academe expressed most interest indurability

● Owners apparently had no interest in eithermaintenance or strengthening but this is notconsidered to be representative of all owners.

● Preferences for research on inspection andmaintenance are at a low level but otherwise broadly inline with the numbers of projects on each topic.

5.3 Outcome of research projects

There were 45 completed research projects and 21ongoing projects reported. General information about theoutcome of the projects at the time of reporting is givenin Table 5.5. Of the 21 projects still running, articles andpresentations have been given for six and the progress offive can be followed on the web site .

Further research has been identified as being necessaryfor 75% of the projects, including the ongoing ones. Theneed for further research can lead to contradictoryinterpretations; an industrial sponsor requires a practicaland usable end product and would regard a projectneeding more work as being not entirely successful. Onthe other hand a researcher might feel differently as themotivation for follow-up research projects that lead to amore comprehensive outcome would be welcomed.

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


Table 5.2 Distribution of research topics and funding

Research Private National International BudgetTopics (€ million)

Durability 1 10 3 8.7Materials 3 16 6 6.6Inspection 0 2 3 2.9Maintenance 0 3 5 12.7Strengthening 4 7 2 6.7

Table 5.3 Distribution of research projects

Research Percentage (%) projects

Materials 31Durability 18Inspection 18Maintenance 18Strengthening 15

Durability

Materials

Maintenance

Inspection

Strengthening

Figure 5.2 Distribution of research topics (66 projects)

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completed projects. This is a high success rate but is notentirely compatible with 51 of the 66 projects claiming torequire further research.

5.3.1 Improved durabilitySummaries of research problems have been identifiedfrom the performances of repairs in practice, given inChapter 3, and are listed in Table 5.6. Researchrequirements identified from these data are outlined inthe following sections.

DurabilityRepairs to concrete affected by corrosion and AAR havelow success rates and despite the high volume of pastresearch there are aspects of these topics that stillrequire attention.

Patches and sprayed concrete are repair techniques thatare reported to have high failure rates and merit moreresearch under site conditions.

It is notable that none of these topics were mentioned inthe responses describing current projects.

Repair materials and methodsThere have been cases in practice where failures ofrepair materials were attributed to incorrect selection orhostile weather during application but there was nosuggestion that the materials were inadequate.Nevertheless there would be benefit from researchcarried out with the objective of developing materials thatare more forgiving and tolerant to misuse and extremesof weather. This is an area where materials suppliers arebest placed to take a lead.

Inspection and assessmentIn practice 85% of inspections rely on visual means andonly 15% used NDT. While there is a continued need forrobust and reliable methods of inspection, the mainrequirement is to produce convincing arguments anddata that will persuade clients of the long term value ofperiodic in-depth inspections. It is also necessary to havemethods that are not seen as being unduly expensive interms of equipment or operational time.

Current research 29

Table 5.4 Research preferences of respondents

Respondents Durability (%) Materials (%) Inspection (%) Maintenance (%) Strengthening (%)

Consultant 16 32 11 21 21Contractor 0 53 7 0 40Owner 33 50 17 0 0Academe 47 7 13 13 20Supplier 18 55 9 18 0Number 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 33There has been dissemination 6 31The results are used 9 36Further research is required 15 36Further research is planned 11 23

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 incorrectand methods use of materials but there is no criticism

of the materials per seInspection and Only 15% of inspections use NDT.assessment Incorrect assessment is blamed for many

of the failuresMaintenance The processes of corrosion and AAR

continue to be difficult to manageStrengthening Strengthening projects have a high

success rate and no problems are identified

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MaintenanceIt is evident that guidance notes and manuals are notbeing used sufficiently by practitioners in the field,possibly because they are seen as voluminousdocuments that are rather complex and time consumingto use. Moreover, clients, as opposed to practitioners,have indicated that they would like to havestraightforward, clearly written, jargon-free guidelinesthat generalists will find easy to understand.

StrengtheningIn the reported case-histories, the most commonmethods to restore strength were by replacement ofcorroded reinforcement with new bars, addition of post-tensioning where prestress had been lost for one reasonor another, and externally bonded plating. There were noreported problems and the success rate of 75% was highcompared with 50% for repairs in general.

In the current projects, a disproportionate amount ofwork is being carried out on strengthening and the use offibre reinforced materials than is justified by the evidentresearch requirements in the field, listed in Table 5.6.

Research needsIt was found that only 60% of the research projectstackled problems identifiable in past performances(Chapter 3) as requiring attention.

Analysis of past performance indicates that the chain ofevents in practice is not represented in laboratory-basedresearch and there is room for more site orientated workand long term exposure. Laboratory testing cannotreplicate site conditions and results can be misleading.This is exemplified by the work on repair mortars whereresearch indicated that polymer modified materialsperformed much better than cementitious. In practicethere was little difference since polymer modifiedpatches were reported as being 50% successful whereascementitious patches were 45% successful. However,cement-based materials are generally popular becausethey are cheaper.

5.3.2 Performance-based repairPerformance-based repair is seen as a promising methodof achieving improved performances because it enablesnovel techniques to be introduced. However, it isanticipated that clients will continue to require persuasionto 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.

Research carried out under service conditions andinvolving exposure to all weather conditions maytherefore be required to provide sufficiently convincingevidence of performances.

Improved methods of acceptance testing may berequired to provide assurance of the potential durabilityof repairs.

Performance-based repair is discussed in the companionbook Achieving durable repaired concrete structures[2],which is also an output of CONREPNET.


Key points

● Research to improve the durability of repairs should addressthe following topics● Patching is a common repair method but gives poor

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

● Repair materials are required to be more forgiving and 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 repairsshould address the following topics● Performance under all-weather conditions● Relevant acceptance testing to provide assurance of

improved durability

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6.1 The current positionIt is current practice for concrete repair to be carried outto the specifications of local or national standards andguidelines. The characteristics and the performancesrequired may vary, as well as the approval systems thathave been developed in most countries. With theintroduction of European Standards, these differencesshould be minimised.

The EN 1504 Standard, ‘Products and systems for theprotection and repair of concrete structures’, is in 10parts. Part 1 gives the definitions and terms used in allthe parts [12-21] (see Table 6.1). Parts 2 to 7 addressdifferent products, relevant performance characteristicsand performance requirements. Part 8 deals with thequality control of the products, Part 9 provides guidanceon the choice of the possible solutions and Part 10 dealswith the quality control of the work on site. Thesestandards do not, however, apply to all types of concretedamage. Repair of concrete structures damaged by fireor the repair of defects in existing post-tensionedsystems are not covered.

In addition to the EN 1504 series, some standards aredrafted or are being prepared that cover re-alkalisation,chloride extraction, cathodic protection and sprayedconcrete for repair.

Concrete repair works are covered by ENV 1504-9. Anew version updated and adapted to the other standardsof the EN 1504 series is due to be published as soon asall the standards are finalised. This standard deals with allphases of a repair project, starting at the awareness of aproblem up to the maintenance and inspection after therepair work is done.

According to ENV 1504-9, safety before, during and afterthe works is to be ensured. Also an assessment isconsidered necessary to reveal all defects and identifytheir extent and their causes. The standard also helps inthe choice of the most appropriate option to the identifiedproblem. This option can always be found somewherebetween doing nothing and demolition of the structure.Factors that may influence the choice are divided intofour main categories: general, health, structural andenvironmental. More information on these categories canbe found in the Standard.

31

Chapter 6

European Standards

Table 6.1 European Standards related to concrete repair products and systems

Number Year Title

EN 1504-1 1998 Part 1: DefinitionsEN 1504-2 2004 Part 2: Surface protection systems for concreteEN 1504-3 2006 Part 3: Structural and non-structural repairEN 1504-4 2004 Part 4: Structural bondingEN 1504-5 2004 Part 5: Concrete injectionEN 1504-6 2007 Part 6: Anchoring of reinforcing steel barEN 1504-7 2007 Part 7: Reinforcement corrosion protectionEN 1504-8 2004 Part 8: Quality control and evaluation of conformityENV 1504-9 1997 Part 9 : General principles for the use of products and systemsEN 1504-10 2003 Part 10: Site application of products and systems and quality control of the worksCEN/TS 14038-1 2004 Electrochemical re-alkalization and chloride extraction treatments for reinforced

concrete – Part 1: Re-alkalizationEN 12696 2000 Cathodic protection of steel in concrete

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All this information should assist the project leader(owner, consultant or repair specialist) to make thechoice of action based on the actual condition of thestructure (extent and causes of defects and exposure)and the future conditions (this can include modified use ofthe construction). The selected protection or repairoption also has to be taken into account. Standard ENV 1504-9 mentions 11 repair principles and theirrelated repair methods.

ENV 1504-9 states clearly that after repair, a report ofthe repair work must be delivered. This report shoulddocument all relevant information that may help futureusers to understand the choice of the solution applied.The report also includes suggestions for inspection andmaintenance, necessary to reach the intended lifetime ofthe construction.

The publication of Parts 2 – 7 of the EN 1504 Standardwill result in the appearance of the CE-mark for concreterepair materials. This CE-mark will be required fromJanuary 2009 onwards.

6.2 Application to performance basedrepair

The approach presented in EN 1504 allows aperformance-based specification to be used for concreterepair but has a prescriptive element that wouldsometimes have to be overridden. However, the criteriapresented in the constituent parts of the EN 1504 Seriesdo not cover the behaviour of the repaired structures orthe performances of the repair products after aging. Thisis not helpful to the specification of performance-basedrepairs and it may be more practical to work outprescriptive-based repair projects that are afterwardssubjected to a performance-based maintenanceprogramme.

Some techniques are amenable to being specified usingthe performance-based approach. For example, whenapplying electro-chemical chloride removal techniques,the acceptable remaining chloride content in theconcrete can be specified beforehand. Unfortunately,when this approach has been used, practitioners haveexperienced difficulties in estimating the costs and havebecome reluctant to tender for such specifications.

Although it is possible for electro-chemical techniques tohave precise acceptance criteria, there is no certaintythat the repaired structure will be durable unless anappropriate level of competence is maintainedthroughout the inspection, design, repair and operationphases of the work. Cathodic protection is a repairmethod that requires periodic monitoring andadjustment, initially quarterly but annually in establishedsystems. Regular measurements are required to controlthe correct performance of the system.

Actual practice in most of the European countries followsthe available guidance documents which are based onthe European Standards of the EN 1504 series. However,like EN 1504, they rarely, if ever, take account of thebehaviour of the repaired structure.

A true performance-based approach to repair will in thefuture require revised ‘performance friendly’ standards.New criteria and methods to measure them will need tobe introduced. Models to evaluate the aging of concreterepairs must be developed to allow the estimation of theremaining service life of the repaired construction.In addition to the EN 1504 series, some standards aredrafted or are being prepared that cover re-alkalisation,chloride extraction, cathodic protection and sprayedconcrete for repair.


Key points

● When EN 1504 is fully introduced, conflicting NationalStandards will be withdrawn

● A true performance-based approach to repair of concretestructures will in the future require ‘performance friendly’standards

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[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 performance-based 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 Structureswww.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 repairAvailable 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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References

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[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/


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35

Appendices

The following appendices are examples of the questionnaires that were sent to members of the industry to gain theinformation 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 37

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 39

Please also add a list of relevant standards, recommendations, working rules etc., applied in your companyx = 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 actuallyused. Please do not forget to indicate the type of construction (building, bridge, …). If the type of construction influences the repair method, pleaseinform 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 : ReferenceTitleField of application (national, some owners,…)AbstractAcceptance 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 inEurope.

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Appendix III

Concrete Repair Evaluation Methods Questionnaire

Concrete Repair Evaluation Methods

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Appendix III 41

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 thedifferences.

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 : ReferenceTitleField of application (national, some owners,…)AbstractAcceptance 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 inEurope.

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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42

Appendix IV

Concrete Repair Research Questionnaire

Concrete Repair Research

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Appendix IV 43

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 theevaluation of the condition of the concrete as well as repair methods and repair products may be subject of the research program. Many research items mayor 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 existingconcrete 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 actualrunning projects. We sincerely hope you will provide sufficient information, allowing us to report a rather complete review on research regarding toconcrete 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 theresearch 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 1questionnaire 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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CONFIBRECRETEConFibreCrete — Training and Mobility of Researchers(TMR) network

ConFibreCrete was a European Union TMR networkaiming to develop guidelines for the design of concretestructures, reinforced, prestressed or strengthened withadvanced composites. The network comprised 10 teamsfrom eight different European counties collaborating overa period of five years, starting December 1997. The totalresearch effort that was contributed was around 800man-months.

The work of the network was closely linked to the work ofthe FIB Task Group 9.3.

ConFibreCrete guidelines can be downloaded at:http://encore.ci.group.shef.ac.uk/confibrecrete/

CONTECEVETA Validated Users Manual for Assessing the ResidualService Life of Concrete Structures.

The manual resulting from this project can bedownloaded at:http://www.ietcc.csic.es/fileadmin/Ficheros_IETcc/Web/EventosPublicaciones/PublicacionesElectronicas/manual_ingles.pdf

REHABCONStrategy for maintenance and rehabilitation in concretestructures, started in 2001 and completed in June 2004.

Information about this project and its manual, can befound at : http://www.cbi.se/rehabcon/index.htm.

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Appendix V

Related research projects

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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 performance of both current European concrete repair techniques and inspection practices, and c u m t mearch projeds. It assesses the case histories gathered from across the sector, including from 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 differing expectations 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

I A

IHS BRE Press, Willoughby Road Bracknell, Berkshire RG12 8FB

www. i hsbrepress.com

ISBN 978-1-86081-974-2

EP79 ll I bre press 9 1860 a1974

Concrete Repairs Performance in Service and Current Practice

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