Concrete Arabian Repairs

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G u i d e\\to L \ lu of concrete structures in i

-

The Concrete Society in collaboration with The Bahrain Society of Enginee ~~ ~

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Founded in 1966, The Concrete Society brings together all with an interest in concrete to promote excellence in its design, construction and appearance, to encourage new ideas and innovations and to exchange knowledge and experience across all disciplines.

Technical development and advice - The Society is a centre of excellence for technical development of concrete, producing state-of-the-art reports, recommendations and practical guides, and collaborating with other organisations in technical development programmes. The Concrete Advisory Service provides prompt impartial technical advice to its members.

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Publications - Over 70 Concrete Society publications, covering concrete materials, design and construction, are available from the Concrete Bookshop at www.concretebookshop.com. Members are entitled to discounts and also receive a free copy of the annual Concrete Society Source Book.

Events - The Society organises conferences and exhibitions. Its regions and clubs run seminars, lectures and courses and provide social and sporting events for members to make valuable business and personal contacts.

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Awards - Awards for excellence in concrete are made annu- ally for buildings, civil engineering and mature structures.

Membership - The Society is the largest member-based concrete organisation in Europe. Members are entitled to discounts on publications, advertising and fees, and free entries in the Concrete Society Source Book and On-Line Directory.

The Concrete Society, Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS, UK

Tel: +44 (0) 1344 466007, Fax: +44 (0)1344 466008

Email: [email protected], Web: www.concrete,org.uk

Established in 1972, The Bahrain Society of Engineers aims to con- tribute to the industrial development and modernisation of Bahrain, and enhance standards of practice. It does this by promoting sci- entific and technical cooperation with engineers outside Bahrain, conducting and encouraging research through publications, seminars and conferences, and promoting training and professional development. It also provides arbitrators and expert witnesses in all matters related to engineering.

The Society has approximately 900 members from all engineering disciplines, the main ones being civil, architectural, mechanical, electrical, mining, and chemical engineering. It is the only engineering society in Bahrain and members benefit from participation in the Societys activities, conferences and seminars, and from working with other engineers in BSEs committees and learning from others experiences and knowledge. Over 20% of the members are students who especially benefit from this interaction with experienced engineers.

The main activities of the Society are:

organising conferences, exhibitions, seminars, workshops, lectures and technical visits publishing technical leaflets and conference proceedings, as well as AI Mohandis journal which covers engineering and technical issues.

employment services, training and development

public relations and information-related activities.

The Bahrain Society of Engineers has a high reputation, regionally and internationally, for organising successful engineering conferences. This includes the series of international conferences on deterioration and repair of reinforced concrete in the Arabian Gulf, arranged with The Concrete Society, and supported by the Bahrain Chapter of the American Concrete Institute.

Bahrain Society of Engineers, P.O. Box 835, Manama, Bahrain

Tel: (00973) 727 100, pax: (00973) 7298 I9

Email: [email protected], Web: www.mohandis.org

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Guide to evaluation and repair of concrete structures in the Arabian Peninsula

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Concrete Society Special Publication CS 137

Kay, T and Walker, M

0 2002 The Concrete Society

ISBN 0 946691 94 0

Keywords

Arabian Peninsula, concrete, contracts, corrosion, durability, environmental conditions, materials characteristics, materials selection, reinforcement corrosion, repair, site practice, specification, standards, testing.

Reader interest

Engineers and technologists involved in evaluating and identifying causes of deterioration of concrete structures in the Arabian Peninsula and similar hot climates and selecting appropriate repair techniques.

I

Classification Availability Unrestricted Content Guidance on concrete performance, testing and

repairs Status Committee guided User Civil, structural and materials engineers

Published by:

The Concrete Society, Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS, UK Tel: +44 (0) 1344 466007; Fax: +44 (0) 1344 466008; E-mail [email protected]; www.concrete.org.uk

in collaboration with: The Bahrain Society of Engineers, P.O. Box 835, Manama, Bahrain Tel: 00 973 727100; Fax: 00 973 729819; E-mail:[email protected]; www.mohandis.org

Design and production: Siriol Bowman and Jon Webb

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature.

The recommendations contained herein are intended only as a general guide and, before being used in connection with any report or specification, they should be reviewed with regard to the full circumstances of such use. Although every care has been taken in the preparation of this Guide, no liability for negligence or otherwise can be accepted by The Concrete Society, the Bahrain Society of Engineers, the members of its working parties, its servants or agents.

Concrete Society publications are subject to revision from time to time and readers should ensure that they are in possession of the latest version

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Prepared for a Joint Working Group of The Concrete Society and The Bahrain Society of Engineers

Ted Kay and Mike Walker

Published by The Concrete Society

in collaboration with the Bahrain Society of Engineers

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This document has been prepared by co-authors Ted Kay and Mike Walker with the support of the Joint Working Group, The Concrete Society and the Bahrain Society of Engineers.

Dr Ted Kay*

Mike Walker*

Dr Eng Habib M Zein AI Abideen

Miss Zahra Salman A1 Aboodi

Dr Jameel Al-Alawi

Dr Phi1 Bamforth

Nick Clarke*

Prof Peter Fookes

Dr Magdi M Khalifa

Dr Adil bin Abdul Aziz AI Kindy

Dr Abdulghafoor Qasemi

Adman Abdul Rehman Sharafi

Hisham Shehabi

Jim Sokolowski

*Editorial Group

Halcrow Group Ltd, and Visiting Professor, Queens University of Belfast

The Concrete Society

Deputy Minister for Public Works & Housing, Saudi Arabia

Ministry of Public Works & Housing, United Arab Emirates

Bahrain Society of Engineers

Taylor Woodrow Construction Ltd

The Concrete Society

Consulting Engineering Geologist

Ministry of Public Works & Housing, Saudi Arabia

Muscat Municipality, Sultanate of Oman

Dubai Municipality

Dubai Municipality

Bahrain Society of Engineers

Halcrow International Partnership, Dubai

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Grateful acknowledgements are extended to those many organisations particularly in the Arabian Peninsula, listed below, for their support and assistance. The expertise of their staff was important and the work could not have been undertaken without the financial assistance that they provided.

Authorities

Bahrain Ministry of Works and Agriculture

Bahrain Ministry of Housing, Municipal Affairs and Environment

Dubai Municipality

Muscat Municipality

Saudi Arabia, Ministry of Public Works

UAE, Ministry of Public Works and Housing

Operating Companies (see following page)

Carillion - AI-Futtaim Tarmac

Consolidated Contractors Company

Elkem Materials

Fosroc

Grace Construction Products

Halcrow International Partnership

RMC Group Services

In addition to The Concrete Society and the Bu-a in Society of Engineers, L..: following Engineering Societies an Organi- sations have supported this work:

The Kuwait Society of Engineers

The Forum of Qatari Engineers

The UAE Engineering Society

Contact was also established with the Saudi Arabia Engineering Committee and with colleagues in Muscat who are represen- tatives of the Oman engineering fraternity.

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C~U~OO~OOU - AO [F~tUsh UZIUODUZIU L ~ ~ O U & I J U ! ~ S D~V~S~OUU, is one of the largest providers of independent materials testing and consultancy services in the Middle East. Facilities exist for a wide range of tests on construction materials and the laboratory undertakes research projects, ground investigations and materials evaluation. Additionally environmental services are offered which include air-quality monitoring and ground-con- tamination investigations.

e-mail: [email protected]

C @ ~ ~ ~ d i d a U ~ d CUDGUU~XUCDUS C t ~ m p a ~ ~ y (CCC) has been established for 50 years and is based in Athens, Greece. CCC is the largest multi-disciplinary engineering and construction company in the Arabian Peninsula and is certified to IS0 9000. The company also operates extensively in the Eastern Mediter- ranean, through the whole of Africa and also the CIS States. Affiliated companies are located in the USA, England and Italy. CCC is placed at No. 1 in the Middle East, No. 7 in the IndustriaLPetroleum sector and No. 17 in the world in the 2001 ENR rankings of international contractors.

www. ccc . gr

EkSrrnO MZI%eUiaOS markets Elkem Microsilica, an ultrafine powder created in ferrosilicon and silicon production. Microsilica is used throughout the world, wherever concrete durability is the prime concern. Elkem Materials is part of Elkem ASA and was set up in 1982 to promote and market microsilica via sales offices, agents and distributors in most regions of the globe. With its advanced research and development facilities, the company is committed to improvements through product technology and developing new solutions and better products for its customers. This is part of Elkem Materials' role in protecting the environment, both in production and where its products are used.

www.concrete.elkem.com

IFOSk'OC is one of the world's foremost manufacturers of advanced technology products for construction, and recognised market leader in concrete repair and protection. In the Middle and Near East it has eight manufacturing plants and over 30 offices. The company has a strong reputation in many product fields, and its success is built on a combination of market knowledge, technical resource, manufacturing expertise and problem-solving capabilities. Fosroc's commitment to total quality and environmental management systems is reflected in its IS0 9002 and 14001 certification. Fosroc develops close working partnerships with international contractors, specifiers

and operators, with a strong emphasis on after-sales service. Its products help maximise the use of local materials and improve productivity on site, while meeting safety standards and the demands of new construction and operational techniques.

www.fosroc.com

Grace CapUUSfhUd~OUU PUOdUdS, a core business of W R Grace and Co., is a world leader in the construction industry. Grace's concrete admixtures, cement additives, and speciality building materials, which include waterproofing and fire- proofing products, strengthen and protect the world's most important structures. For more than 40 years Grace Construction Products has improved the quality of reinforced concrete by developing value-added admixtures recognised for their strength, durability enhancement and chloride resistance. Grace is committed to raising standards through development of inno- vative products and practices to meet the needs and demands for high-quality concrete. Through its affiliate in the Arabian Gulf, Emirates Chemicals LLC, Grace has contributed and supported the publication of this Guide.

www.graceconstruction.com

HdCUOW is an independent provider of infrastructure-based business solutions. Specialising in the transport, water and property sectors, it offers professional consultancy resources for the planning, design and supervision of development on a global basis. The company regularly provides consulting engineering services to government departments, public sector authorities and utilities, industrial and commercial companies, international funding agencies, financial institutions and private individuals. Halcrow has been at the forefront of research and development of durable concrete structures in the Middle East for over a quarter of a century.

www. halcrow.com

RMC GuapUg se[IpIkeS is the representative office of the RMC Group in the UAE. Its operations include RMC Topmix, supplying ready-mixed concrete to the Dubai and Sharjah markets with five batch plants, over 60 transit mixers and 10 pumps. All products meet strict quality standards and are IS0 9001 accredited. RMC Supermix operates three batching plants in Abu Dhabi, supplying numerous prestigious projects. Falcon Cement markets Duracem ground granulated blastfurnace slag, produced at the purpose-built facility at Jebel Ali. Gulf Quarries in the Hajjar mountains uses state-of-the-art technology to produce high-quality aggregate for use in Dubai Municipality.

www.rmctopmix.com

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Members of the Joint Working Group Acknowledgements Sponsoring Organisations List of Tables List of Figures

Sketch map ~ Preface

iv 2.15 V 2.16 v i 2.17 X

X

xi xii

3.1

101 ONUWODMCUOON . . . . . . . . . . . . page 1 3.2 1.1 Background 1.2 Purpose and scope

3.3 1

2. I Introduction

UWE PROBLEMS. . . . . . . . . . . . page3 3 3.4

I 2.2 Factors affecting the type and rate of I deterioration 4

2.3 Timescale for appearance of defects 4

I

2.4 Relationship between damage and deterioration 4

2.5 Corrosion of reinforcement 4 2.5.1 Introduction 4 2.5.2 Characteristics of carbonation-induced

and chloride-induced corrosion 6 2.6 Sulfate attack 7 2.7 Physical salt weathering 8 3.5

' I

2.8 Deterioration related to aggregate properties 2.8.1 Introduction 2.8.2 Aggregate shrinkage and swelling 2.8.3 Aggregate softening 2.8.4 Alkali-silica reaction

2.9 Abrasion, erosion and cavitation 2.10 Cracking occumng during or soon after

construction 2.10.1 Introduction 2.10.2 Plastic shrinkage cracking 2.10.3 Plastic settlement cracking 2.10.4 Early thermal contraction cracking 2.10.5 Crazing Long-term drying shrinkage cracking Cracks induced by temperature changes Other features related to construction

2.1 1 2.12 2.13 2.14 Weathering of structures

8

8 m 8

4.1 4.2

8 8 9 4.3

4.4

9 9 9

10 I1 4.5 I1 12 12 12 14

Bacteriological attack Staining Lime leaching

14

14 14

ONVESUOGAUOONS . . . . . . . . . . . page 15

Introduction Preliminary survey - planning and preparations Preliminary inspection 3.3.1 Introduction 3.3.2 Inspection 3.3.3 Results Additional information and initial diagnosis of causes of deterioration 3.4.1 Introduction 3.4.2 Existing records 3.4.3 Design 3.4.4 Materials 3.4.5 Construction 3.4.6 History of the structure 3.4.7 3.4.8

Main investigation

Current and future use of the structure Initial diagnosis of causes of deterioration

15

16 17 17 17 18

20 20

20 20 20 21

21 21

21 21

UESUUNG . . . . . . . . . . . . . . . . page 22 Introduction Selection of sampling and test locations Access Sampling and testing 4.4.1 Types of test 4.4.2 Test locations 4.4.3 Samples 4.4.4 Reinstatement of test sites In-situ testing 4.5.1 Introduction 4.5.2 Surface hardness 4.5.3 Ultrasonic pulse velocity 4.5.4 Near-surface strength 4.5.5 Reinforcement depth and position

22 22 23 23 23

23 24 26 26

26 26 26 27 27

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4.6

H 5.1 5.2

5.3 5.4

6.1 6.2

6.3

6.4

4.5.6 Carbonation depth 4.5.7 Half-cell potential 4.5.8 Resistivity 4.5.9 Surface absorption Laboratory testing 4.6.1 Introduction 4.6.2 Core testing for strength 4.6.3 Dust samples 4.6.4 Petrographic examination 4.6.5 Water permeability

28 28 29 29 30 30 30 30 30 30

. . . . . . . . . . . . . . page31 Objectives Interpretation of non-destructive test results 5.2.1 General 5.2.2 Carbonation depth 5.2.3 Chloride profiles 5.2.4 Concrete strength and density 5.2.5 Ultrasonic pulse velocity 5.2.6 Initial surface absorption 5.2.7 Electrical potential mapping 5.2.8 Resistivity Use of results Predicting the future behaviour of a structure

31 31 31 31 31 32 33 33 33 34 34 35

Introduction Reinstatement with concrete or mortar 6.2.1 Introduction 6.2.2 6.2.3 Breaking out 6.2.4 Treatment of reinforcement 6.2.5 Bonding aids 6.2.6 Mortar repairs 6.2.7 Repairs in shutters 6.2.8 Sprayed concrete 6.2.9 Incorporation of sacrificial anodes 6.2.10 Quality control and testing Coatings for concrete 6.3.1 6.3.2 Materials 6.3.3 Application 6.3.4 Quality control and testing Crack injection 6.4.1 6.4.2 Materials

Areas of application and limitations



36 36 36 37 37 38 40 40 42 44 45 46 46 46 46 47 48 48 48 48

6.5

6.6

6.7

6.8

7.1 7.2 7.3

7.4 7.5

8.1 8.2 8.3

6.4.3 Method of injection 6.4.4 Leaking cracks 6.4.5 Quality control and testing Corrosion inhibitors 6.5.1 Corrosion-inhibiting admixtures 6.5.2 Migrating corrosion inhibitors 6.5.3

6.5.4 6.5.5

Cathodic protection 6.6.1 Introduction 6.6.2 Sacrificial anode systems 6.6.3 Impressed current systems 6.6.4 Quality control and testing Re-alkalisation 6.7.1 Introduction 6.7.2 Quality control and testing Chloride extraction (desalination) 6.8.1 Introduction 6.8.2 Quality control and testing

Areas of application and limitations of MCIs Method of application of MCIs Quality control and testing of MCIs

48 49 49 49 50

50 ~

50 50

50 50 50 50 51 51 51 51 52 52 52 52

and protection systems Electrochemical systems Comparing costs and performance

NS . . . . . . . . . . . . . . . . .page 53 Expected performance 53 Temporary works 53 I Performance of non-electrochemical repair I

. .

Introduction European standards Specifications 8.3.1 Introduction 8.3.2 General matters 8.3.3 Access 8.3.4 Surface cleaning 8.3.5 8.3.6 Concrete removal 8.3.7 Reinforcement 8.3.8 Reinstatement 8.3.9 Materials 8.3.10 Trial repairs 8.3.11 Testing

Survey and location of defects

53 54 54

I

. page 55 ~

55 55 55 55 55 56 56 57 57 57 57 58 58 58

1

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Con tents

8.3.12 Surface preparation for protective treatment 59

8.3.13 Application of protective treatment 59 8.3.14 Resin injection 59 8.3.15 Cathodic protection 60 8.3.16 Electrochemical chloride extraction

and re-alkalisation 60 8.4 Bills of quantities and measurement 60

APPENDIX U Less common testing Uechniqnnes. . . . . A 1.1 Introduction Al.2 In-situ testing

A I .2.1 Crack movement Al.2.2 Ground-probing radar A1.2.3 Corrosion rate A1.2.4 Moisture A1.2.5 Radiography A 1.2.6 Thermography A1.2.7 Fluid transport properties

A1.3 Laboratory testing

page 63 63 63 63 63 63 64 64 64 65 65

A 1.3.1 Water absorption 65 A1.3.2 Gas permeability 65 A1.3.3 Gas diffusion 66 A1.3.4 Chloride diffusion 66 A1.3.5 Bulk chloride diffusion 66 A1.3.6 The value of measuring transport

properties 66 A 1.3.7 Concrete porosity 66

APPENDIX 2

of siUane and s~loxane . . . . . . . . . . . . page 68 DQdQlrM!liMliUUg depth Of pQIIIQdradiQn

A2.1 Dye penetration test 68 A2.2 Capillary rise test 68

REFERENCES. . . . . . . . . . . . . . . . . .page 73

DMBflECU INDEX . . . . . . . . . . . . . . . page 77

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Guide to evaluation and repair of concrete structures in the Arabian Peninsula I

Table 2. I

Table 2.2

Table 2.3 Table 2.4 Table 3. I

Table 3.2

Table 4.1

Table 4.2: Table 4.3:

Table 5.1 :

Table 5.2: Table 5.3:

Table 5.4:

Common forms of deterioration in concrete structures.

Typical times of appearance of various types of defect (after Fookes, 1976). page 5 Damage related to cover and bar diameter: pnge 6 Some construction defects. page 13 Examples of classification of defects (after RlLEM TC 104). page 18 Strength classes for normal-weight concretes, from BS EN 206. page 20 Relative numbers of readings necessary for various tests (from Bungey and Millard, 1996 and BS 1881: Part 207, 1992). page 22 Principal test methods. page 24 Tests and techniques for various types of structure.

Chloride contents (% by weight of cement) at 3 years.

Interpretation of ISAT results. page 33 Relationship between potential and risk of corrosion. page 34 Relationship between resistivity and corrosion rate.

page 5

page 25

page 33

page 34

Table 6. I : Table 6.2:

Features of methods of breaking out concrete. page 38

Considerations in the choice between bonding aids and soaking the substrate for different repair types.

Advantages and disadvantages of sprayed concrete.

page 4 I

page 44 Table 6.3:

Table AI . 1 : Values of water permeability. page 65

Table A I .2: Relationship between sorptivity and concrete quality.

Table A I .3: Values of gas permeability. page 66

Table A I .4: Values of gas diffusion coeficients. page 66

Table A I .5: Diffusion coeficients. page 66

Table A I .6: Classi3cation of porosity. page 67

Table A3. I : European Standards dealing with protection and repair methods and products. page 69

Table A3.2: Protection and repairprinciples from BS EN 1504: Part 9. page 70

Table A3.3: Performance testing requirements for structural and non-structural repair mortars or concretes from BS EN 1504: Part 3. pnge 70

Table A3.4: Requirements for hand-applied mortar from BS EN 1504: Part 10. page 71

Table A3.5: European CEN Standards for test methods and repair materials. puge 72

page 65

Figure 2.1 :

Figure 2.2:

Figure 2.3:

Figure 2.4:

Figure 2.5:

Figure 2.6:

Figure 3.1 :

Figure 3.2:

Figure 3.3:

Figure 4.1 :

Figure 4.2:

Figure 4.3:

Figure 4.4:

Figure 4.5:

Figure 4.6:

Figure 4.7:

Figure 5.1 :

Moisture movement. page 4

Examples of intrinsic cracks in hypothetical structure (from Concrete Society TR22, based on Fookes, 1976). page 10

Formation of plastic shrinkage cracks. page 10

Formation of plastic settlement cracks. pnge I I Formation of plastic shrinkage cracks in columns.

Drying shrinkage cracks on wall (vertical lines indicate the position of reinforcement). page 12

Flow chart - from routine inspection to solution. page 15

The whole assessment process. page 16

An example of a preliminary inspection summary form. page 19

Method of obtaining dust samples. page 25

Applications of UPV measurement. (T = transmitter; R = receiver) page 27

Covermeter being used on vertical surface. page 27

Half- cell potential equipment (courtesy of Wexham Developments). page 28

Half-cell potential measurements. page 28

Typical map of equal potential contours (values in mV). page 29

Typical layout of resistivity test array. page 29

Typical chloride profiles. page 32

/.'age II

Figure 5.2: Bestfit to measured values. page 33

Figure 5.3: Interpretation of indirect UPV measurement. p q e 33

Figure 5.4: The effect of moisture and chloride content on corrosion rate (after Browne, 1982). page 34

Figure 6.1 : Undercutting of repair edges. page 37 Figure 6.2: Rounding of repair corners to facilitate compaction.

puge 38

Figure 6.3: Method of injecting resin into anchor holes. page 39

Figure 6.4: Ways in which bonding aids may affect passage of cathodic protection current. page 4 I

Figure 6.5: Some features of shutters for repairs. page 43

Figure 6.6: Funnel and hose used for flowing concrete. page 43 Figure 6.7: Flow through test for flowing concrete. page 44 Figure 6.8: Method of spraying for vertical repairs. From ACI

506R-90, reapproved 1995. Guide to shotcrete. page 45

Figure 6.9: Method of injection using packers. page 49

Figure 6.10: Schematic of cathodic protection system. pccge 5 I Figure 6.1 I : Schematic of the re-alkalisation process. page 51 Figure 6.12: Schematic of the chloride extraction process. page 52

Figure AI. 1 : Demec gauge with digital read-out (courtesy of

Figure A I .2: Typical ground-probing radar equipment (courtesy

Figure A I .3: Layout of typical radiography system for concrete.

Figure A I .4 Equipment for in-situ permeability testing (courtesy

Wexham Developments). page 63

of Aperio). page 63

page 64

of Wexham Developments). page 65

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The Concrete Society and the Bahrain Society of Engineers have enjoyed a close collaboration for many years, particularly their joint involvement with the International Con- ferences on the Repair and Maintenance of Reinforced Concrete in the Arabian Gulf. Following the fifth conference in 1997 it was agreed that a review document should be prepared outlining the basic steps for evaluating the condition of a concrete structure and giving details of available, and workable, repair systems and methods.

For this purpose two authors, Ted Kay and Mike Walker, both with close working knowledge of structural performance as well as repair and maintenance in the Arabian Peninsula, have prepared this Guide in collaboration with a Joint Working Group centred on The Concrete Society and the Bahrain Society of Engineers.

A Draft for Comment was issued to delegates to the Sixth International Conference in November 2000 and to the sup-

porting authorities and organisations acknowledged previously. Comments on the draft were considered by the Joint Working Group and incorporated into this final document.

Following an introduction, the Guide is in eight main chapters. The first chapter covers the problems encountered in the Region, and deterioration processes. The following three chapters deal with inspection, tests and the evaluation of the present and future behaviour of a structure. Chapter six contains guidance on repair techniques, their appropriateness and application. The two final chapters deal with selection of repair options, and with contract documents for repair works.

The Guide also has three appendices, the first providing more information on some of the tests described in the main text, particularly less common testing techniques. The second outlines the tests for silane penetration, and, for information on current developments, the third summarises developing European standards for concrete repair.

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Guide to evaluation and repair of concrete structures in the Arabian Peninsula ~

SKETCH MAP

ed Sea \, t I -,B Height range (approximately)

2000to4000m Gulf of Aden

Fcv3

Some principal cities not shown. Boundaries shown are not meant to be dejnitive.

E5- kalla

Arabian Sea

Health and safcty

Construction activities, particularly on construction sites, have significant health and safety implications, either as the result of the activities themselves, or from the nature of the materials and chemicals used. This Guide does not endeavour to cover health and safety issues relevant to the construction of reinforced concrete in the Arabian Peninsula comprehensively, although specific points to note are mentioned where appropriate in the text. Readers should consult other specific published guidance relating to health and safety in construction.

I Quarries and pits and associated processing areas can be dangerous. Visitors should take appropriate safety precautions and be escorted by I quarry staff.

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Construction of large civil engineering projects and major buildings in the Arabian Peninsula region has generally taken place only in the last 30 to 40 years. During that time, many huge projects, including harbours, airports, sewage treatment works, power and desalination plants and industrial facilities, and whole cities of modern buildings with their associated infrastructure have been constructed. This work posed a substantial challenge to the construction industry and, like any undertaking of this magnitude, has not been without problems. These have been extensively documented elsewhere and the causes can be summarised as follows:

there was no previous experience anywhere in the world of concrete construction at such high temperatures often in a salty environment and therefore no established technology

there were few local resources in terms of a concrete industry, concrete materials supply or labour force; local quarries and borrow pits were opened up but some of these sources were only marginally suitable for high-quality concrete production

in some regions there was little potable water for making or curing concrete

many aspects of the local environment are not conducive to the production of high-quality concrete and are aggressive to concrete structures in service; key factors in terms of aggressivity are a saline water table close to the ground surface and the widespread occurrence of salty soils.

As a result of these factors, in combination and with others, many concrete structures have not been as durable as expected. This has left a legacy of repair and protection of concrete structures, which has been required for some time, needs to be undertaken now, and which will continue into the foreseeable future.

All concrete structures deteriorate with time, though the rate of deterioration is affected by many factors. The result is a change in the performance of the structure, which may affect its behaviour under normal working conditions or its structural safety.

The starting point in most cases is that deterioration is evident because of visible signs of damage, such as cracking or excessive deflections; these signs may be identified during routine inspection of the structure, during maintenance or during day-to-day operation. Detailed inspections of structures are often carried out when ownership or use changes, e.g. when an hotel or apartment block is changed into offices. Sometimes the deterioration may be too severe to justify

retaining the affected structure or component, but generally repair and protection can add many useful years to its life. Repair and protection measures must be selected to meet the needs identified as part of a structured process that involves inspection and testing, determining the causes of deterioration and assessing the probable future behaviour of the structure. The cost of repair and protection must also be assessed in relation to the value of the structure over its remaining useful life and the costs of the alternative of demolition and replacement. Carrying out a detailed investigation to identify the underlying causes of defects is always necessary so that a clear remedial strategy can be determined.

A sound understanding of the underlying causes of the deterioration or lack of durability of a structure is necessary before setting out on a protection and repair programme. The changes that constitute deterioration can be the result of several factors, including the design of the structure, the standard of workmanship during construction, the materials used, the action of the environment during construction and in service, and the loads acting upon the structure. Fortu- nately, many deterioration processes lead to characteristic visible features such as distinctive crack patterns. This means that visual inspection often plays an important part when starting an investigation. An early visual inspection can be extremely useful both in setting the scene and in providing information to help in developing a full investigation programme. Other useful pointers are the time at which a particular type of defect became apparent and the fact that some structural types or locations are likely to lead to a particular form of deterioration. Typical timescales for the various deterioration processes and the types of structure that are most at risk are outlined in this Guide.

This report has been written as a guide to the whole process of concrete repair from the first realisation that there may be a problem through to the mechanics of various repair and protection options. Information is provided on each step on the way. The emphasis is inevitably on repair of structures undergoing corrosion of reinforcement because this is by far the commonest cause of deterioration in the Region.

This Guide complements the Guide to the construction of reinforced concrete in the Arabian Peninsula (The Concrete Society/ClRIA, 2002), which is published simultaneously.

This Guide describes the major causes and processes of deterioration likely to be encountered in the Arabian Peninsula Region. These include:

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Guide to evaluation and repair of concrete structures in the Arabiun Peninsula I

in-built problems such as unsatisfactory design, poor workmanship, the use of unsuitable aggregates or cement, or inappropriate concrete mix design features that appear during construction or soon afterwards the action of the local environment on the completed structure.

Deterioration may have a single cause but usually several factors are involved.

Damage caused by fire or structural problems such as over- loading or impact damage are not covered specifically but some repair techniques described are equally applicable to these situations. Concrete Society Technical Report 33 (The Concrete Society, 1990) is a useful guide in the case of fire- damaged concrete structures.

Three stages in the assessment process are essential for a successful repair and protection contract:

Investigation of the present condition of the structure Analysis of the results of the investigation, leading to a clear understanding of the causes of deterioration and an assessment of future performance Consideration of the available options, including repair and protection, partial or full demolition and replacement, and selection of the most appropriate solution.

The methods described in this Guide are suitable for cast in- situ and precast concrete structures that are unreinforced, reinforced or prestressed. Where one of these forms has been found to be susceptible to a particular deterioration process, this is noted in the text. The durability of a structure depends on the influence of the local environment and reference is made to the common exposure zones encountered in the Region. The micro-environment can also have a significant effect on performance of individual elements of a structure, for example where local wetting occurs in an otherwise dry situation. This needs to be taken into account when planning and assessing the results of investigations and considering repair and protection options.

The local environment in the Arabian Peninsula Region is extremely aggressive to concrete and its reinforcement. Any- one dealing with repair and protection of a concrete structure needs to understand how this environment is acting on the structure under consideration and why this action has resulted in the deterioration that can be seen. Only then can rational and informed decisions be taken on how to restore the structure to a satisfactory condition and protect it against deterioration in the future. This Guide therefore starts by describing the common forms of deterioration encountered in the Region and how the environment influences these deterioration processes.

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The difficulties that surround the concrete construction process in the Arabian Peninsula and the destructive effects of the aggressive environment on concrete structures in service have been well documented. See, for example, Guide to the construction of reinforced concrete in the Arabian Peninsula (The Concrete Society/CIRIA, 2002), and the Proceedings of the series of conferences on Deterioration and repair of reinforced concrete in the Arabian Gulfheld regularly since 1985 and published by the Bahrain Society of Engineers. However, it is worth summarising the principal features as they will be relevant to many structures under investigation.

The rapid and widespread development in the Region during the late 1960s and 1970s it presented an unusual situation to designers and materials engineers. The average, ambient and maximum temperatures were higher than in most other regions where concrete construction was undertaken, the soils and groundwater contained much higher salt concentrations, there was no high volume aggregate industry, no local cement production, in some areas there were only limited supplies of potable water, no local concrete supply industry and above all, little time to develop a relevant local concrete technology. This being the case, designers adopted design standards from Europe and America which were not necessarily appropriate in the local situation; contractors had to rely on cement and reinforcement imported by sea, some-times as deck cargo and sometimes with prolonged waiting periods offshore because of congestion in the ports; marginal local sources of aggregate had to be used; contractors had to import mainly unskilled labourers and train them in the art of concrete production and construction on the job; little or no water was available for curing concrete.

It is not surprising, therefore, that a survey of reinforced concrete marine structures in the late 1970s predicted a life of between 15 and 20 years. Sadly, this has turned out to be an accurate prediction in many cases. The whole concrete construction scene in the Arabian Peninsula is now vastly different to that in the 1960s and 1970s but the changes have been gradual and the requirement to repair older concrete structures will continue into the future. Modem concrete specifications should result in a product that is much more durable than in earlier projects. However, the local environment is still highly aggressive to concrete and reinforcement and life expectancy cannot be as great as for similar structures in Europe, America or even the Far East. Structure owners need to be vigilant, to undertake routine inspections, to maintain protective systems, and to undertake timely repairs.

The main durability problems encountered in the Region are cracking and spalling due to reinforcement corrosion: the key factors causing these are outlined below.

Climate

The very high temperatures accelerate deterioration reactions. They also make concrete production and placing dif- ficult, not only because of loss of water from the mix by evaporation but also because at higher temperatures mixes are less workable as hydration proceeds faster and mixes tend to stiffen more quickly. There may be a temptation to add water to mixes but this will reduce their durability and their ability to protect the reinforcement. High temperatures in association with high humidity make working outdoors extremely uncomfortable and exhausting. Workers and super- visors tire and lose concentration, and less attention is then paid to the detail needed to produce high-quality concrete. Good workmanship, particularly in achieving the design cover to reinforcement and well-compacted and cured concrete, can be a major factor in durability. High rates of evaporation during the day and hot dry winds mean that concrete dries out quickly and is susceptible to cracking. Curing systems suffer rapid evaporation and curing may become ineffective for long periods. The cover concrete, which is most affected by curing and which gives direct protection to the reinforcement, may be the most permeable concrete in an element. Large variations in temperature from day to night and the increases in temperature that result from intense solar radiation can cause cracking on exposed surfaces.

Materials

Cement and reinforcement of good quality are produced in the Arabian Peninsula and are now widely available throughout the Region. However, cement and clinker are still imported into some parts of the Region from various sources. Major variations in their characteristics can be experienced.

High-quality aggregates are available in particular locations and are exported to other countries in the Region. However, because of high transport costs or expediency, aggregates from sources local to the project may be used. Some local sources may be contaminated with salts, leading to corrosion of reinforcement when used in concrete, or be mineralogi- cally unsound, leading to deterioration of the concrete itself. Some rock types may form poorly shaped aggregate, and rock may be processed inappropriately to produce flaky or elongated particles. Aggregate of poor shape requires more water to produce workable concrete and hence the concrete is of lower quality and less durable.

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Environment and exposure conditions

Much of the development in the Region has been concentrated in the coastal areas. These areas usually have a highly saline water table close to the surface which can extend tens of kilometres inland. Capillary rise in granular soils brings moisture from the water table to the surface. The moisture evaporates leaving salts behind. The ground surface can be heavily contaminated by salt which is taken up into the atmosphere by wind as dust. This salty dust is then deposited on buildings and other structures. This dust, in combination with moisture from condensation, leaks or discharge from air-conditioning units, can permeate into concrete and can attack it.

Structures that are founded in salty soils and the capillary rise zone are particularly vulnerable to salt. If the concrete below ground is not properly tanked, moisture rises through the concrete, carrying salts with it. Once above ground, the moisture moves towards the warm outer surface and evaporates, leaving behind the salt in concentrated form. The process is shown in Figure 2.1. Many problems with concrete structures are first observed at or just above ground level. Similar moisture movements can occur in water-retaining structures and other structures where one face is in contact with water.

2.2 FACTORS AFFECnNC THE TYPE AND RATE OF DETERIORATION

Most concrete deterioration processes depend on the presence of water, the moisture state within the concrete being more important than that of the surrounding atmosphere. Concrete takes in water from the environment more rapidly than it loses it and so the average internal humidity in the concrete is generally higher than the average external humidity.

All chemical reactions are accelerated by increases in temperature. In general terms, an increase in temperature of 10C causes a doubling in the rate of reaction. Putting this another way, the time taken to reach a particular condition is halved by a rise in temperature of 10C, all other factors being equal. It is understandable therefore, that deterioration of concrete

Evaporation

Figure 2.1 : Moisture movement.

structures can be two or three times faster than in most parts of Europe.

The types of deterioration that occur in a concrete structure are often related to its environment. The deterioration processes likely to occur in various types of structure are summarised in Table 2.1. The list only indicates the deterioration that may occur; as outlined above there may be many other contributory factors.

2.3 TIMESCALE FOR APPEARANCE OF DEFECTS

The time of appearance of a defect is one indicator of its cause. Table 2.2 indicates the likely times for various types of defect.

2.4 RELATIONSHIP BETWEEN DAMAGE AND DETERIORATION

Many processes can cause concrete structures to deteriorate and can lead to defects. Anyone involved in determining the causes of defects has to have knowledge of these underlying processes. The necessary background information on the principal types of deterioration is given in this Chapter.

An investigation should lead to the identification of various types of fault and mechanisms of deterioration in the structure. It is important to distinguish between those due to an earlier event (e.g. a construction defect) and those symp- tomatic of an on-going problem (e.g. cracking of the concrete due to reinforcement corrosion). Some faults may not be detrimental to the structure at the time of inspection but may cause problems in the future (e.g. cracks developing during construction may be a path by which chlorides can reach the reinforcement and lead to corrosion). In addition, the signifi- cance of a particular type of deterioration will depend on the type of structure and the location of the affected element within the structure.

As noted earlier, many forms of deterioration give rise to characteristic defects and the visual appearance of the defect can be extremely helpfil in the diagnosis process. In the Sections covering the individual processes that follow, the characteristic defects are described, in italics, as an aid to diagnosis, particularly at the preliminary stage.

2.5 CORROSION OF REINFORCEMENT

2.5.1 Introduction

Reinforcement in concrete does not naturally rust as it is shielded from the external environment by the thickness of the cover concrete. It is also protected by the alkaline environment provided by the hydrated cement. The large amount of calcium hydroxide in the pore solution of concrete gives a pH of about 12.5 and the small amounts of sodium and potassium present in the cement push this to a higher value.

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Marine structures

Table 2. I: Common forms of deterioration in concrete structures.

Chlorides

Sulfates

Corrosion of reinforcement particularly in the splash zone

Salt weatheringnoss of concrete section particularly in splash zone

Possible cause of deterioration Type of structure

Bridges and highway structures

Deterioration

Marine creatures

Chlorides from environment (dust and condensation)

Leaching/mechanical damage below water level

Corrosion of reinforcement particularly on upper deck surfaces and substructures where water leaks from above

Section

2.5

~~

Buildings

Corrosion of reinforcement on facades. Interior concrete carbonates but is unlikely to lead to corrosion unless there is a source of moisture

May be deposited in dust on facades. Can then be carried into concrete by condensation or leakage from air-conditioning units

Carbonation

Chlorides

2.6,2.7

2.7,2.15

Buried structures or structures in contact with the ground

Sdfates Loss of concrete section

Chlorides

Salt weathering

Corrosion of reinforcement, particularly just above ground level

Loss of concrete surface and later loss of section

2.5

~~ ~

Ground slabs Chlorides

2.5

~

Industrial plants (including sewage treatment works)

2.5

Chlorides

2.6

Chlorides

Sulfates Tunnels

Trafficked areas Abrasion

Corrosion of reinforcement - will occur first near leaking joints

Loss of concrete section

Loss of concrete surface

2.5

2.7

Corrosion of reinforcement in top mat because chlorides are drawn through from the ground or chloride-contaminated soil or dust is blown onto them.

2.5

Corrosion of reinforcement anywhere that saline water comes into contact with reinforced concrete; corrosion unlikely below water level where there is permanent contact with water and concrete remains saturated

2.5

2.5

2.6,2.15

2.9

2.15 Sewer pipes Sulfates Loss of section by acid attack after sulfates converted to sulfuric acid by bacteria

Table 2.2: Typical times of uppearunce of various types of defect (after Fookes, 1976).

Typical time of appearance Type of defect

Plastic settlement cracks*

Plastic shrinkage cracks*

Construction defects*

Crazing

Early thermal contraction cracks*

Long-term drying shrinkage cracks

Chemical attack (including sulfate attack)

Damage due to temperature movements (seasonal)

Alkali-silica reaction

Reinforcement corrosion

Section

Ten minutes to three hours

Thirty minutes to six hours

On removal of formwork

One to seven days - sometimes much later

One day to two or three weeks

Several weeks or months

Few months up to several years depending on nature of the materials

Probably up to a year, but may be longer

Several years

Several years, but may be much shorter

2.10.3

2.10.2

2.13

2.10.5

2.10.4

2.1 I

2.6, 2.7

2.12

2.8.4

2.5

In these conditions a protective oxide layer is formed and maintained on the surface of the steel. This is generally referred to as a passive layer. Loss of this passive condition can occur as a result of: carbonation of the concrete, excessive concentrations of chlorides present in the concrete materials during construction, ingress of chlorides into finished concrete (commonly sodium chloride from seawater, groundwater or soil), or a combination of carbonation and

I chlorides.

Carbonation is the reaction of carbon dioxide in the environment with calcium hydroxide in the cement. The results of the reaction are calcium carbonate and a lowering of the pH to about 9. Carbonation starts at the concrete surface which is in contact with the atmosphere. As carbon dioxide pene- trates into the concrete a carbonated layer develops at the surface. The 'layer' within the concrete at which concrete changes from carbonated to uncarbonated is known as the carbonation front. Over time the carbonation moves inwards

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and eventually reaches the reinforcement. As noted above, the pH drops from 13 to about 9 over a few millimetres. It is possible to distinguish between carbonated and uncarbonated concrete on a freshly exposed surface using a simple test, such as the phenolphthalein indicator described in Section 4.5.6. This enables the risk of corrosion of reinforcement to be assessed. At a pH at or below 9, the protective oxide layer on the surface of the reinforcement cannot be maintained and corrosion of the reinforcing bar becomes possible if moisture and oxygen can gain access.

The reaction of carbon dioxide and calcium hydroxide requires moisture, so in very dry concrete carbonation will be slow. In saturated concrete the moisture presents a barrier to the penetration of carbon dioxide and again carbonation rates will be low. The most severe condition for carbonation is when there is sufficient moisture for the reaction to occur, but not enough to act as a barrier. This usually occurs when the concrete is exposed to atmospheric relative humidity in the range 50-70% or when the conditions vary from wet to dry. Wetting and drying alternately allows ready ingress of water vapour and oxygen, providing all the conditions required for corrosion. Relative humidities within the critical range occur daily throughout the year in coastal locations and from November to March even at sites that are a considerable distance from the coast.

The passive layer on reinforcement is not static but is in dynamic equilibrium. The passive layer is continually broken down and re-established in the highly alkaline conditions within uncarbonated concrete. Chloride ingress does not cause a reduction in background pH, but inhibits the mechanism by which the protective oxide layer is maintained. Chlorides penetrate concrete in solution and are able to penetrate most rapidly in dry environments that are infrequently wetted. Examples are the splash zone above high tide level on marine structures, dry dock floors and walls, areas which are infrequently washed down with saline water, and areas in process plant where saline water occasionally leaks. Alterna- tively, chloride may be present in the concrete from the start, having been inadvertently added to the mix as a contaminant of one of the mix constituents, for example, in beach sand or from stockpiles that have been contaminated by groundwater or wind-blown salty dust.

Corrosion of metal is an electro-chemical process, with different reactions occurring at anodic and cathodic sites. A ready supply of moisture and oxygen to the cathodes is required to fuel the process. Reinforced concrete can deteriorate if the steel reinforcement corrodes. The corrosion products (rust) form at the anodes and usually occupy a much larger volume than the uncorroded steel. This expansion exerts bursting stresses on the concrete and can cause cracking and spalling of the cover. In very rare and special circumstances in which the supply of oxygen is restricted, black rust forms which does not produce expansion. This anaerobic corrosion causes little or no disruption to the concrete.

A very small amount of corrosion of the reinforcement - a layer less than 0.1 mm thick on the surface of the bar - can cause the concrete to crack. This amount of corrosion rep-

resents only about 2.5% of the cross-sectional area of a typical 16 mm bar. Hence evidence of corrosion will be visible long before the loss of steel cross-section is likely to affect structural integrity. However, there may still be a risk of injury or damage caused by loose concrete spalling from the structure.

Characteristic defects

Damage caused by reinforcement corrosion is ojien characterised by cracks running parallel to the steel, which may cause spalling, particularly at external corners of members such as beams and columns. Corrosion of reinforcement can also result in delamination of the surface ofslabs or walls. If the bars are widely spaced and close to the surface (in relation to their diameter), cracking will be the most likely form of damage. Deeper bars, and bars close together; tend to cause the concrete to delaminate as the cracks can combine before they reach the surface. The nature of damage is thus a function of the bar size and spacing, and the cover As a guide, the type of damage can be related to the ratio ofcover to bar diameter; as indicated in Table 2.3. When corrosion is very far advanced the whole of the cover concrete may become severely cracked and almost disintegrated.

2.5.2 Characteristics of carbonation-induced and chloride-induced corrosion

The characteristics of corrosion resulting from carbonation and chloride ingress differ in one principal respect. Carbona- tion-induced corrosion tends to be general, though isolated bars with low cover can lead to local problems. Chloride- induced corrosion is characterised by local, rapidly corroding areas of bars. There may be areas of intense localised corrosion on bars while only a few centimetres away the bar surface is in perfect condition. Acting within the concrete, chlorides (in combination with water, oxygen and the reinforcing steel) form and drive an electrical corrosion cell with anode and cathode sites at different positions on the reinforcement. The anodes corrode whilst the cathodes are unaf- fected. Once delamination has occurred, oxygen and moisture can gain access to the reinforcement along the plane of delamination and hence the whole mat may take on a more or less uniformly corroded appearance.

An essential difference between chloride-induced and carbonation-induced corrosion is the propagation period, that is, the time between corrosion starting and damage occurring. Carbonation occurs most rapidly in moderate humidity environments (relative humidity 50-70%) but at this humidity the corrosion will occur very slowly (for example inside buildings) and will be insignificant in relation to the life of the

Table 2.3: Damage related to cover and bar diamete,:

Cover bar diameter Likely type of damage on a flat surface

I 1 I Cracks and possible local spalling I 2 I Larger cracks and risk of delamination I 3 I Delamination

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structure. However, cycles of wetting and drying can give rise to a situation where carbonation can result in corrosion. Under exposure to chlorides, however, the conditions that lead to the most rapid chloride penetration, i.e. wetting and drying or high humidity (90-95%), are also the conditions leading to the most rapid corrosion. While there may be a long period of corrosion propagation in carbonated concrete, if chlorides are present the propagation period is likely to be relatively short. An exception is when chlorides are included in the concrete mix and the concrete is subsequently exposed to a low humidity environment. Corrosion begins on the externally exposed (wetted) surfaces but not on the internal (dry) surfaces.

The relationship between corrosion and chlorides is complex. In uncontaminated concrete the passive layer on the surface of the reinforcement is continually breaking down and being rein- stated. If chlorides enter the concrete from outside, the chloride concentration at the reinforcement increases over time. Chlorides disturb the equilibrium of the process under way at the surface of the reinforcement. They interrupt the process to an extent that is dependent on the amount of chloride present. It is generally believed that the ratio of chloride to hydroxyl ions in the pore water is critical in relation to depassivation, but for practical reasons chloride concentrations are most commonly measured and reported as a percentage by weight of binder in the concrete. At low chloride concentrations the process of reinstatement of the passive layer is slowed down and eventually a concentration is reached at which the rate of reinstatement falls below the rate of breakdown. At this stage complete depassivation occurs.

An additional difficulty in assessing the corrosion risk associated with chlorides is that macro-cells (i.e. corrosion cells with the anode and cathode separated by several metres) can be formed by variations in chloride content. Steel in concrete with a chloride content higher than the assumed threshold at which corrosion can be initiated may be protected locally if an anode has developed at a nearby site where the chloride content is even higher. The reinforcement corrodes preferen- tially at this anodic site and the neighbouring steel is protected.


Deterioration of concrete due to sulfate attack takes two forms: 9 Expansive formation of ettringite and/or gypsum in the

hardened concrete causes cracking and exfoliation. Hydrated cementing compounds soften and disintegrate to a crumbly mass with substantial loss of surface concrete. This is due to direct attack on the cement compounds by sulfates or by their decomposition when calcium hydroxide is removed by its reaction with the sulfates.

,

Either or both of these mechanisms can occur; depending on the temperature, types and concentrations of sulfate in solution and the composition of the concrete.

Most sulfates are potentially harmful to concrete. Sulfates occur naturally in soils, rocks and groundwater and in many parts of the Arabian Peninsula there are very high concentrations of sulfates in the groundwater and the surface soils, particularly in coastal regions and the adjoining sabkhas. Gypsum (calcium sulfate) may be present as a contaminant in some aggregates.

The literature on sulfate attack is complex and confusing and there is no consensus on some of the mechanisms involved.

In the Arabian Peninsula, probably the commonest form of attack is from calcium sulfate in the groundwater or soil, which attacks concrete to form ettringite in the hardened cement paste. This formation generates stresses in the cement paste and, as a result, cracks develop in the concrete until the outer surface disintegrates. This disintegration exposes fresh areas to attack. There are such high concentrations of sulfate in the soil and groundwater that there is still plenty available to further attack the affected area. Disintegration can be very rapid.

The form of attack varies, depending on how long it has been taking place, the moisture conditions, temperature, concrete type and so on. In many cases, spectacular deterioration and disintegration of the concrete that is in a terminal condition destroys the evidence of the earlier phases of the attack. Examination of concrete below ground presents special problems because removal of the soil inevitably disrupts many of the areas that have been attacked and special care is needed to gain an accurate picture of the attack.

The external appearance of sulfate attack varies considerably: Sulfate attack may lead to heaving and cracking in foun- dations and ground floors on or in fill or soil containing sulfates. Heave is due to the formation of expansive sulfates within the concrete.

During the early stages of attack by sodium sulfate and calcium sulfate, the physical effect is that the outer layers of the concrete exfoliate. This is accompanied by corrosion of the reinforcement and a change of the hardened paste around aggregates to a soft condition, with depo- sition of salt on surfaces and in exfoliation cracks.

Concrete attacked by sodium sulfate, and in the long term by calcium sulfate, is eventually reduced to a soft crumbly material; but when magnesium sulfate is the main agent, the concrete remains hard but becomes expanded and cracked. Magnesium sulfate is present in seawater, but seawater is not generally regarded as being aggressive to concrete because, in the presence of sodium chloride, there is an inhibition of, or retarding action on, the expansive reaction.

Another much rarer form of attack is through the formation of thaumasite as a result of the reaction between calcium sili- cates in the cement, calcium carbonate from limestone aggregates or fillers, and sulfates, usually from external sources. The conditions generally considered necessary for its formation (Hartshorn and Sims, 1998) are:

Low temperature (less than 15OC)

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Consistently high relative humidity

The thaumasite form of sulfate attack is unlikely in the Region because of the cold conditions required for thaumasite formation. However, it could theoretically occur in cold stores and similar environments with continuous low temperatures.

Supplies of calcium, silicate, sulfate and carbonate

Initial reactive alumina (0.4 to 1 .O%).


Initially, physical salt weathering causes flakes of cement paste and possibly $ne aggregate to become detached from the surface. npically, this form of attack occurs from the ground surface upwards fo r a f ew tens of centimetres, depending on the chemistry of the soil, the concrete and the capillary rise conditions. Eventually the surface may be eroded to a depth of 20 or 30 mm, leaving the aggregate standing proud.

I f the aggregate is more susceptible to this form of weathering than the cement paste (e.g. porous and friable materials like some weaker limestones and weathered gabbro) small discs of paste between the aggregate pieces and the outside surface may be blown off due to the expansive action within the aggregate. Such features are called pop-outs. Eventually, the fragmentation of the aggregate itself may lead to its complete disintegration, leaving a void in the surface of the concrete.

Attack by salt weathering is designated in the literature as physical salt weathering to distinguish it from chemical weathering. The mechanism is similar to freeze-thaw disintegration of concrete in that crystals (usually salts of sulfates and possibly chlorides) develop in the pores of the concrete close to its surface. The crystal growth exerts pressure in the pores leading to local tensile stresses in the cement paste.

When capillary rise occurs, salt from groundwater or damp soil is transported in solution vertically up through the concrete member. Above ground level, the moisture is drawn to the surface and evaporates, leaving crystals of salt growing in the near-surface pores and resulting in significant surface erosion. This form of attack is common just above ground level and may also occur in marine structures and in process plants where seawater is used for cooling or where parts of the structure are frequently splashed by salty water.

2.8.1 Introduction

Most rocks used as aggregates are not affected by the surrounding cement paste, but a few exhibit changes that can affect the performance of the concrete. Some of these changes may occur very rapidly while others may continue

for several decades. Some of the significant changes that may take place within the concrete due to the materials used are considered in the following sections.

2.8.2 Aggregate shrinkage and swelling


Aggregate swelling will lead to pop-outs on the surface and the formation of three-legged cracks radiating from a point (also known as Isle of Man cracks, as their appearance resembles the symbol of the island on the west coast of Britain). Eventually individual aggregate pieces may disintegrate leaving pock marks on the surface. /f most of the aggregate in a concrete member is subject to swelling, the tensile forces generated may crack the paste and lead to general breakdown of the rnembev.

Shrinkage of clay-rich particles is very common although these do not usually form a large proportion of an aggregate. Some clays are particularly prone to expansion on wetting and shrinkage on drying. Clay can be a contaminant of the aggregate or a natural part of it. For example, instances have been reported of shrinkage of natural aggregates that initially appear to be strong.

2.8.3 Aggregate softening


Aggregate softening will lead to pop-outs on the surface and the formation of three-legged Isle of Man cracks. Individual aggregate particles may disintegrate leaving pock marks on the surface

Softening can take place when an aggregate contains weak or porous particles. In addition, aggregates composed of, or containing large proportions of, clay minerals are also soft and porous. When such rocks are present in hardened concrete, the concrete will exhibit greater volume changes on wetting and drying than similar concrete containing non- swelling aggregate. Softening also facilitates the ingress of aggressive substances such as chlorides.

2.8.4 Alkali-silica reaction

It should be noted that alkali-silica reaction (ASR) is rare in the Arabian Peninsula.


The main external evidence fo r damage to concrete due to alkali-silica reaction is cracking. In the early stages, cracks are centred on individual reacting aggregate pieces near the surface and may give rise to three-legged Isle of Man crack patterns. In unrestrained concrete the cracks develop with a random distribution, often referred to as map cracking, where a network offine cracks is bounded by a few larger cracks. When the expansive forces are restrained, f o r example by reinforcement or by applied loads in a column, the pattern of cracking is modified. Cracks tend to run parallel to the direction of restraint, i.e. parallel to the reinforcement or vertically in a column. The cracks may exude gel, which shows up as white deposits on the surface. There

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may be pop-outs where individual pieces of aggregate have expanded and disintegrated just beneath the surface. This pattern of cracking should not be confused with plastic shrinkage cracking which may appear early in the life of the structure or drying shrinkage cracks which may appear after some months or years. The earliest time at which cracking due to ASR has been observed in structures in the UK has been about Jive years after casting. With some aggregates, cracking may not appear until a much longer time has elapsed. There may be evidence of expansion of whole members such as the closing of expansion joints.

Very f ew cases of alkali reactivity have been confirmed in the Region, nonetheless the possibility of ASR should be considered when undertaking condition surveys.

Alkali-silica reaction occurs when the alkaline pore fluid and the minerals in some aggregates react together to form a calcium alkali-silicate gel. This gel takes up water, producing a volume expansion that can disrupt the concrete, see Digest 330 (Building Research Establishment, 1999). ASR will only cause damage to concrete when all three of the following factors are present:

sufficiently high alkalinity

sufficient moisture.

If any one of these factors is absent, then damage from ASR will not occur.

Much work has been carried out on the structural effects of ASR-induced expansion. In general it has been found that there is little effect on structural performance. Indeed it has been suggested that structural performance can be enhanced in some situations; where the reaction causes some com- pression in the concrete, this can offset some of the tensile stresses caused by loading. However, the structural signifi- cance of the expansion will depend on the type of structure, the effectiveness of the reinforcement detailing and the exact location within the structure. More importantly, cracking induced by ASR can reduce the long-term durability of com- ponents, particularly through corrosion of reinforcement.

A working party of the Institution of Structural Engineers has made recommendations on assessing structures damaged by ASR (Institution of Structural Engineers, 1992).

a critical amount of reactive silica in the aggregate


Abrasion results in localised or general depressions in a concrete surface. The abraded area usually has a rougher surface texture.

Abrasion is the wearing away of the surface of the concrete. Localised abrasion of floors may result from foot or vehicular traffic. A common problem is the abrasion caused by the wheels of fork-lift trucks. Abrasion may occur if

traffic is permitted to move on concrete slabs too early before they have gained sufficient maturity.

Erosion may be caused by air or waterbome particles, such as the erosion of coastal structures by the action of sand and pebbles carried by the waves. Erosion will only occur if the abrading medium is harder than the concrete.

In structures such as spillways and in tunnels and pipes with a high flow rate, pitting of the surface due to cavitation may occur.

2.10.1 Introduction

Cracks and crack patterns have different characteristics depending on the underlying cause. For example, plastic shrinkage cracks (see Section 2.10.2) tend to have an irregular pattern over the structure while cracks due to corrosion (see Section 2.5) will follow the lines of the reinforcement.

The presence of cracks can influence the behaviour and durability of a concrete member. They can reduce the shear capacity of a section or provide a path by which moisture, oxygen, carbon dioxide, chlorides etc can penetrate into the concrete surrounding the reinforcement; in time this may result in reinforcement corrosion. These aspects are covered in more detail in Section 2.5.

Concrete Society Technical Report 22 Non-structural cracks in concrete (The Concrete Society, 1994) provides information on the most common forms of intrinsic cracks in concrete. Figure 2.2 illustrates most of the types of crack likely to be experienced in the lifetime of a concrete structure. Different types of crack may occur at different times in the life of a concrete member. So as well as crack patterns, a knowledge of the time of first appearance of cracks is helpful in diagnosing causes of cracking. Crack types and the time at which they are most likely to develop are listed in Table 2.2.

2.10.2 Plastic shrinkage cracking

In the period just after it has been placed, the cement paste in a concrete mix is still plastic and has little strength, and water is able to move relatively freely in what is still a mixture. Water, which is the least dense component of the mixture, and tends to move upwards towards the surface as heavier materials move down under gravity during compaction. This upward movement of water is known as bleeding. Evapo- ration of water occurs at the surface, becoming more rapid at high temperatures andor low humidity, particularly in windy conditions, which can occur at any time of the year in the Arabian Peninsula. If evaporation occurs faster than bleeding, there is a net loss of water from the surface layer of concrete leading to a reduction in volume. The surface layer of concrete tries to shrink but is restrained by underlying layers that are not subject to the same reduction in volume.

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The result of the restraint is that tensile stresses are developed in a zone near the surface. As the concrete is still in a plastic state and of very low strength, irregular tension cracks develop. The process is illustrated in Figure 2.3, the upper part showing initiation and the lower the condition after a few hours. (a) Initiation Characteristic defects

Plastic shrinkage cracks tend to be I to 2 mm wide and are

Evaporation

t 1 W 1 5 1 5

typically 300 to 500 mm long and 20 to 50 mm deep, though, in some extreme circumstances, they may extend through the full depth of a member The pattern of cracks is random but may be influenced by the direction in which finishing operations have been carried out. As the cracks form in concrete when the paste is still in a plastic state, they run through the paste rather than through pieces of aggregate. These cracks can form in both unreinforced and reinforced concrete.

2.10.3 Plastic settlement cracking

(b) A,,er a few hours

The upward movement of water described in Section 2.10.2 can also lead to plastic settlement cracks. As the water moves upwards, the denser constituents of the mixture move downward. This downward movement may be obstructed by the top layer of reinforcement or by the shuttering. The plastic concrete may arch over the top of individual reinforcing bars, bringing the surface into tension. Regular cracks over the bars often occur in conjunction with voids under the bars, as shown in Figure 2.4; the upper part shows initiation and the lower the condition after a few hours. If the top layer of bars is closely spaced, the whole surface layer of the concrete may hang up on the reinforcement while the concrete below settles. This can lead to a horizontal discontinuity beneath the bars, resulting in loss of bond and a loss of the layer of concrete that would protect the bars against corrosion.


The patterns of cracks associated with plastic settlement depend on what is obstructing the downward movement. Most

Figure 2.3: Formation of plastic shrinkage cracks.

commonly, movement is restrained by reinforcement. The cracks occur on the top surface and usually follow the line of the uppermost bars, giving a series of parallel cracks: there may also be shorter cracks over the bars running transversely. Cracks are typically 1 mm wide and can run from the sui$ace to the bars, see Figure 2.4. The settlement may also result in visible undulations in the su$ace with high points over the top reinforcing bars. The concrete can also be supported by the shuttering, causing restraint to the concrete in connected members. This typically happens at mushroom heads on columns, as illustrated in Figure 2.5, but can also occur at other locations, such as under spacer blocks. Cracks at mushroom heads of columns are generally horizontal, I mm wide and can cross the full section. As these cracks form at very early age they pass through the cement paste and not through aggregate particles.

sion bending cracks Cracks at kicker join

Type of cracklng

Plastic settlement

Plastic shrinkage

Early thermal contraction

Long-term drying shrinkage

Crazing

Corrosion of reinforcement

Alkali-silica reaction

Figure 2.2: Examples of intrinsic cracks in hypothetical structure (from Concrete Society TR22, based on Fookes, 1976).

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(a) Initiation

Crack

(b) After a few hours

figure 2.4: Formation of plastic settlement cracks.

2.10.4 Early thermal contraction cracking

The hydration reaction between cement and water that takes place in concrete generates heat. The amount of heat generated and the rate at which it is generated depend on the amount and type of cement and its fineness. The peak temperature reached is dependent upon the cement type and content, initial temperature of the concrete, ambient conditions, geometry of the member and type of formwork. The high ambient and concrete temperatures encountered in the Region speed up the reaction, resulting in a more rapid temperature rise. Slabs have a large exposed surface area through which the concrete can lose heat. Members with large cross- sectional areas can develop higher internal temperatures than those with smaller section, as the loss of heat through the top and side surfaces has greater effect. Timber formwork provides more insulation than steel and so higher peak temperatures may be reached when timber formwork is used.

As concrete heats up it expands. If there is any restraint to this expansion, for example from previous pours, such as when a wall stem is concreted on a base slab or foundation, com- pressive stresses will be generated in the young concrete. These stresses are low, due to the low elastic modulus of the young concrete, and are generally relieved by creep. Once the peak temperature has been reached, at say 12 to 18 hours after

Figure 2.5: Formation of plastic shrinkage cracks in columns.

placing (much later for very thick members), the concrete starts to cool and reduce in volume. Restraint to this thermal contraction will result in the development of tensile stresses. At this stage, the concrete is more mature and has less capacity for relief of strain by creep. The Young's modulus is greater and hence the stresses generated are higher. The concrete is still relatively weak in tension and the stresses caused by restraint to thermal contraction can cause cracking.

Cracks may also be caused by differential temperatures in thick members. When the surface layer cools, movement is restrained by the core of the member, which is still at a higher temperature, and cracks may form in the surface. When the temperature through the member eventually becomes uniform, the surface cracks usually close. In large members, there will tend to be a series of cracks across the short direction in plan and elevation and possibly a series of com- plimentary cracks in the long direction. Cracks will tend to be wider near to edges and corners where heat is lost from two or more faces.

Differential thermal contraction cracks are inevitable in some situations, such as when a member is cast onto a previously poured and hardened foundation. These cracks are antici- pated and reinforcement is provided to control them and limit their width. Even so, some cracks wider than the nominal design width can occur. They do not necessarily mean that there are shortcomings in the design or workmanship. Cracks of this type often occur in the walls of water-retaining structures and may cause initial leakage. The leakage will tend to reduce with time because of autogenous healing of the cracks.


A classic case of early thermal contraction cracking is that occurring in some walls poured on strip footings that were cast several days earliez The stiflstrip footings restrain the thermal movements in the wall. Cracks may form in the wall, starting at the base and running approximately vertically. They will usually pass right through the wall section. Cracks near the end of bays may be inclined at approximately 45". Crack spacing and width will depend on the amount of reinforcement provided. Because these cracks form a fer hardening but before full strength is achieved, they generally run entirely through the paste and not through the aggregate.

2.10.5 Crazing

Crazing can occur both on exposed surfaces and on surfaces in contact with formwork. It occurs either when there is a change in properties close to the surface or a high moisture content gradient. The type of formwork is also important, as it can affect the permeability of the formed concrete surface. Steel and plastic formwork faces

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