bester, 2015 - the influence of curing on restrained shrinkage cracking of bonded overlays

THE FACULTY OF ENGINEERING AND THE BUILT ENVIRONMENT The Department of Civil Engineering

The Influence of Curing on Restrained Shrinkage Cracking of

Bonded Overlays

Prepared By: Nick Bester

Supervisor: A/Prof. Hans Beushausen

Date of Submission: 30th January 2015

A dissertation submitted to the Department of Civil Engineering, University of Cape Town in partial fulfilment of the requirements for the degree of Master of Science in Civil Engineering.

Concrete Materials and Structural Integrity Research Unit.

The financial assistance of the National Research Foundation (DAAD-NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the author and are not necessarily to be attributed to the DAAD-NRF.

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Plagiarism Declaration 1. I know that plagiarism is wrong. Plagiarism is to use anothers work and to pretend that it is

ones own.

2. I have used the Harvard Convention for citation and referencing. Each significant contribution to and quotation in this report form the work or works of other people has been attributed and has been cited and referenced.

3. This dissertation is my own work.

4. I have not allowed and I will not allow anyone to copy my work with the intension of passing it as his or her own work.

Student Number: BSTNIC005 Surname: Bester

Date: 30/01/2015 Signature:________

Plagiarism Declaration

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Acknowledgements The author would like to thank and acknowledge with gratitude the following persons, institutions, and companies who made significant contributions towards the completion of this dissertation.

The supervisor, Associate Professor Hans Beushausen, for his invaluable guidance, encouragement, and continual academic and technical assistance throughout the duration of this dissertation.

The Directors of the Concrete Materials and Structural Integrity Research Unit (CoMSIRU), Professor Mark Alexander and Professor Pilate Moyo, for providing useful suggestions and constructive criticism.

Mr Steve Crosswell, on behalf of Portland Pretoria Cement Ltd (PPC), for their donation of cement used in this research.

Mr Kevin Kimbrey, on behalf of Sika South Africa (Pty) Ltd, for their donation of products used in this research.

Ms Rhuwayda Lalla, on behalf of Lafarge South Africa (Pty) Ltd, for their donation of readymix concrete used in this research.

The administration staff of the Department of Civil Engineering, especially Elly Yelverton, the Research Administrative Officer of CoMSIRU, for their assistance with administrative matters relating to this research.

Mr Nooredien Hassen, the Civil Engineering Concrete Laboratory Manager, for his assistance with the laboratory-related experimentation carried out.

Mr Charles Nicholas, the Civil Engineering Workshop Principal Technical Officer, for manufacturing the moulds and various experimentation-related items that were required for the experimentation carried out.

The Civil Engineering Concrete Laboratory staff, Mr Charlie May, Mr Leonard Adams, Mr Elvino Witbooi, and Mr Hector Mafungwa, for assisting in the laboratory with experimental work when needed.

The Postgraduate Students of CoMSIRU, especially Mr Gavin Golden, for an invaluable friendship and continual assistance with the experimentation conducted in this dissertation.

Family and friends for their continual encouragement, utmost support and unconditional love.

The author would also like to thanks and acknowledge, with gratitude, the financial support throughout the duration of the dissertation (2013-2014) received from the following institutions and companies.

The University of Cape Town (UCT) The Concrete Institute (TCI) The National Research Foundation

(DAAD-NRF) Sika South Africa (Pty) Ltd Pretoria Portland Cement Ltd (PPC)

AfriSam South Africa (Pty) Ltd Haw & Inglis Civil Engineering (Pty) Ltd The Tertiary Education Support

Programme (TESP) of ESKOM The Water Research Commission (WRC)

Acknowledgements

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Summary As more and more concrete structures are being constructed worldwide, the amount of infrastructure requiring repair and rehabilitation is increasing globally. This has resulted in an increasing need for effective concrete repair techniques. Bonded mortar or concrete overlays are a common type of concrete repair technique that involves casting of a new repair material, in the form of an overlay, in the place of the damaged or deteriorated portion of concrete. Bonded overlays are however prone to premature failure, due to differential volume changes between the newly cast overlay and the existing concrete substrate, which is primarily manifested as debonding or cracking.

The differential volume changes experienced by the overlay arise due to the inherent shrinkage of cementitious materials. The restraint provided by the substrate to the shrinkage of the overlay results in the induction of tensile stresses within the overlay. These stresses are proportional to the shrinkage strain and elastic modulus of the overlay but are partially reduced due to tensile relaxation. If the tensile stresses induced are greater than the tensile strength of the overlay then cracking of the overlay, termed restrained shrinkage cracking, will begin to develop. The time-dependent material properties of the overlay that govern the restrained shrinkage cracking are therefore the shrinkage, tensile strength, elastic modulus, and tensile relaxation of the overlay.

Despite the relatively sound understanding of the mechanisms of restrained shrinkage cracking and the numerous measures that can be taken to mitigate it, such as the use of a reduced paste content, increased aggregate content, admixtures, fibres, etc., restrained shrinkage cracking still often occurs. Another measure thought to increase an overlays resistance to restrained shrinkage cracking is adequate curing. However, the influence of curing on restrained shrinkage cracking has not yet been thoroughly investigated and so its influence is not fully known and understood.

In this study, the influence of curing on restrained shrinkage cracking of bonded overlays was investigated. The influence of a total of seven curing regimes which included air curing, sealed curing (plastic sheets) for 2 and 7 days, water curing (wet cloths) for 2 and 7 days, a protective coating and a curing compound was investigated for four mix types which included three laboratory-made mixes with water-binder (w/b) ratios of 0.45, 0.60 and 0.80, and one commercial repair mortar. The influence of the seven curing regimes on the material properties governing restrained shrinkage cracking (shrinkage, tensile strength, elastic modulus, and tensile relaxation) and on the age at cracking and crack area of ring tests and composite overlay-substrate specimens was investigated for the four mix types. Analytical modelling using a simple analytical model was carried out using the results of the individual material properties in order to predict the age at cracking. The results of the analytical modelling, in conjunction with the tests conducted on the material properties governing restrained shrinkage cracking, were used to explain the age at cracking results observed from the ring test and composite overlay-substrate specimens.

The results of the experimental testing showed that curing influences all of the material properties governing restrained shrinkage cracking. Prolonged and/or more effective curing that minimised moisture loss and promoted hydration was shown to either delay or reduce the rate of

Summary

iv shrinkage respectively (dependent on the curing method), increase the tensile strength and elastic modulus, and decrease the tensile relaxation. In general, prolonged and/or more effective curing was shown to have a positive influence on restrained shrinkage cracking by increasing the age and net age at cracking. Wet cloth curing was shown to outperform plastic sheet curing for all mix types. An increased curing duration was observed to increase the age at cracking, however only provided an increase in crack resistance for higher w/b ratio mixes as the net age at cracking was only observed to increase for higher w/b ratio mixes. A protective coating and curing compound were observed to significantly increase the resistance to cracking by increasing the age and net age at cracking, with the latter outperforming the former. The optimal curing regime, in terms of the age at cracking, was observed to be 7-day wet cloth curing for lower w/b ratio mixes and a curing compound for higher w/b ratio mixes. The optimal curing regime, in terms of increasing crack resistance, was observed to be a curing compound as it resulted in the greatest net age at cracking. Crack areas were shown not to be directly correlated to the curing regime but rather to the net age at cracking and the free shrinkage strain once cracking had occurred.

Additional findings not directly related to the influence of curing on restrained shrinkage cracking showed that an increasing w/b ratio was observed to increase the crack resistance under restrained shrinkage conditions; the age at cracking results of ring test specimens were observed to be correlated to composite overlay-substrate specimens, with the age at cracking results of the composite overlay-substrate specimens being, on average, 64 15% greater than the age at cracking of the ring tests; and that a degree of restraint of 55% 1% was observed for all composite overlay-substrate specimens which confirmed the typical degree of restraint of 60% measured in literature.

Summary

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Table of Contents Plagiarism Declaration i Acknowledgements ii Summary iii Table of Contents v List of Figures viii List of Tables xii Abbreviations xiv List of Variables xv 1. Introduction 1

1.1 Background to Research Problem 1 1.2 Problem to be Investigated 2 1.3 Hypothesis 2 1.4 Research Aim and Objectives 2 1.5 Key Research Questions 3 1.6 Scope and Limitations 3 1.7 Thesis Outline 4

2. Literature Review 6 2.1 Introduction 6 2.2 Fundamentals of Bonded Overlays 6

2.2.1 Definition 6 2.2.2 Applications 6 2.2.3 Design and Construction Considerations 7

2.3 Failure of Bonded Overlays 12 2.3.1 Overview 12 2.3.2 Debonding 13 2.3.3 Restrained Shrinkage Cracking 14

2.4 Restrained Shrinkage Cracking of Bonded Overlays 16 2.4.1 Overview 16 2.4.2 Shrinkage 17 2.4.3 Tensile Strength 26 2.4.4 Elastic Modulus 27 2.4.5 Tensile Relaxation 28 2.4.6 Factors Influencing Restrained Drying Shrinkage Cracking 29

2.5 Curing of Concrete 41 2.5.1 Fundamentals of Curing 41 2.5.2 The Need for Curing 44 2.5.3 Curing Methods 46 2.5.4 Specifying Curing 59 2.5.5 Influence of Curing on Properties of Concrete 61

2.6 Summary 63

Table of Contents

vi 3.Methodology 65 3.1 Introduction 65 3.2 Overview and Approach 65 3.3 Mortar Mixes 67

3.3.1 Laboratory-Made Mortars 67 3.3.2 Commercial Repair Mortar 69

3.4 Curing and Environmental Conditions 70 3.4.1 Curing Methods 70 3.4.2 Curing Duration 74 3.4.3 Environmental Conditions 75

3.5 Experimental Tests 77 3.5.1 Compressive Strength 77 3.5.2 Free Shrinkage 77 3.5.3 Tensile Strength 78 3.5.4 Elastic Modulus 80 3.5.5 Tensile Relaxation 81 3.5.6 Ring Tests 82 3.5.7 Composite Overlay-Substrate Specimens 85 3.5.8 Experimental Testing Summary 90

3.6 Analytical Modelling 91 3.6.1 Modelling Method 91 3.6.2 Modelling Assumptions 92

3.7 Summary 93 4. Results and Discussions 94

4.1 Introduction 94 4.2 Compressive Strength 94 4.3 Free Shrinkage 96

4.3.1 Influence of Curing Regime 96 4.3.2 Influence of Mix Type 99

4.4 Tensile Strength 101 4.5 Elastic Modulus 102 4.6 Tensile Relaxation 104 4.7 Ring Tests 106

4.7.1 Age at Cracking 106 4.7.2 Crack Area 109

4.8 Composite Overlay-Substrate Specimens 112 4.8.1 Age at Cracking 112 4.8.2 Restrained Shrinkage 115

4.9 Analytical Modelling 117 4.10 Comparison of Age and Net Age at Cracking 119

5. Conclusions and Recommendations 122 5.1 Introduction 122 5.2 Influence of the Curing Method on Restrained Shrinkage Cracking 122 5.3 Influence of the Curing Duration on Restrained Shrinkage Cracking 123

Table of Contents

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5.4 Additional Findings 124 5.5 Conclusions 125 5.6 Recommendations for Future Research 126

6. References 127 Appendix A: Detailed Experimentation Results 133

A1: Compressive Strength Results 133 A2: Free Shrinkage Results 136 A3: Tensile Strength Results 142 A4: Elastic Modulus Results 142 A5: Tensile Relaxation Results 143 A6: Ring Test Results 144 A7: Composite Overlay-Substrate Specimen Results 146

A7.1: Overlay Mortar Results 146 A7.2: Substrate Concrete Results 147

Appendix B: Detailed Analytical Modelling Results 149 B1: Regression Analysis Results 149 B2: Material Property Data 149

Appendix C: Environmental Recordings 155 Appendix D: Comparison of Ring Test Setups 156 Appendix E: Assessment of Ethics in Research Projects (EBE Faculty) 158

Table of Contents

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List of Figures Figure 1.1: Schematic overview of thesis outline. 5 Figure 2.1: Overview of design and construction considerations of bonded concrete overlays (Adapted

from Beushausen & Alexander, 2009). 7 Figure 2.2: Failure modes of bonded overlays (adapted from Crlsward, 2006). 12 Figure 2.3: Schematic overview of the time-dependent material properties governing restrained

shrinkage cracking of bonded overlays (adapted from Troxell et al., 1968). 15 Figure 2.4: Illustration of a simplified representation of (a) drying conditions, (b) free shrinkage

strains, (c) restrained stresses, and (d) cracking due to restrained shrinkage of bonded overlays (adapted from Pigeon & Bissonnette, 1999). 15

Figure 2.5: (a) Schematic representation of chemical and autogenous shrinkage volume changes of fresh and hardened cement paste (not drawn to scale) (adapted from Kosmatka et al., 2003), and (b) relationship between autogenous shrinkage and chemical shrinkage of cement paste at early ages (adapted from Hammer, 1999). 19

Figure 2.6: Schematic representation of (a) shrinkage-water loss relationships, and (b) shrinkage and internal relative humidity relationship during drying (adapted from Mindess et al., 2003). 21

Figure 2.7: Illustration of mechanisms of drying shrinkage; (a) capillary tension, (b) disjoining pressure, (c) surface tension (adapted from Mindess et al., 2003), and (d) movement of interlayer water (adapted from Wittmann, 1982, as cited in Kovler & Zhutovsky, 2006). 23

Figure 2.8: Basic characteristics of shrinkage when concrete is (a) dried from age t0 until age t and after which it is re-saturated, and (b), dried from age t0 until age t after which it is subjected to cycles of drying and wetting (adapted from Alexander & Beushausen, 2009). 24

Figure 2.9: The effect of relative humidity on drying shrinkage and carbonation shrinkage (adapted from Neville, 2004; Alexander & Beushausen, 2009). 26

Figure 2.10: (a) Typical stress-strain behaviour of individual phases of concrete (adapted from Mehta et al., 2006), and (b) different forms of elastic modulus (adapted from Alexander & Beushausen, 2009). 28

Figure 2.11: Characteristics of tensile relaxation. (a) Strain-time graph, and (b) stress-time graph as a result of tensile relaxation (adapted from Alexander & Beushasuen, 2009). 29

Figure 2.12: Influence of w/b ratio on (a) the shrinkage of cement pastes (adapted from Haller, 1940, as cited in Alexander & Beushausen, 2009), and (b) the tensile strength of mortar of various compositions (adapted from Graf et al., 1960, as cited in Reinhardt, 2013). 30

Figure 2.13: Influence of w/b ratio on (a) 28-day elastic modulus, and (b) 28-day tensile relaxation (data from Kizito, 2013). Note the scaling used to illustrate the trends. 31

Figure 2.14: Influence of w/b ratio on the (a) age at cracking, and (b) crack area of ring tests and bonded overlays (data from Chilwesa, 2012). 31

Figure 2.15: Influence of water content on (a) shrinkage for a variety of mix proportions (adapted from Hobbs, 1974, as cited in Alexander & Beushausen, 2009), and (b) influence tensile relaxation (data from Dittmer, 2013). Note the scaling used to illustrate the trends. 32

Figure 2.16: Influence of water content on (a) 28-day elastic modulus, and (b) age at cracking of ring test specimens (data from Dittmer, 2013). Note the scaling used to illustrate the trends. 33

Figure 2.17: Influence of aggregate content and size on (a) 56-day free shrinkage (50% RH), and (b) 28-day tensile strength (data from Dittmer, 2013). Note the scaling used to illustrate the trends. 35

Figure 2.18: Influence of aggregate content and size on (a) elastic modulus, and (b) tensile relaxation (data from Dittmer, 2013). Note the scaling used to illustrate the trends. 35

List of Figures and Tables

ix Figure 2.19: Influence of aggregate content and size on (a) age at cracking of ring test specimens, and

(b) crack area of ring test specimens (data from Dittmer, 2013). 36 Figure 2.20: Influence of shrinkage-reducing admixtures on (a) drying shrinkage (Adapted from Shah et

al., 1998), and (b) age at cracking of ring test specimens (data from Shah et al., 1998). 37 Figure 2.21: Influence of (a) steel fibre volume on age of visible cracking (adapted from Shah et al.,

2006), and (b) fibre type and volume on average crack width (adapted from Grzybowski & Shah, 1990). 38

Figure 2.22: Influence of (a) specimen size on the drying shrinkage of concrete (adapted from Hansen & Mattock, 1966), and (b) overlay thickness on the age at cracking (data from Weiss & Shah, 2002). 39

Figure 2.23: Influence of overlay thickness on crack area (adapted from Laurence et al., 2000). 39 Figure 2.24: Relation between shrinkage and time for concretes stored in different relative humidities

(adapted from Troxell et al., 1958). 40 Figure 2.25: (a) Unhydrated particles of Portland cement (magnification of 2000), and (b) partially

hydrated Portland cement (magnification of 4000), as observed through a scanning electron microscope (Sroos, 1994, as cited in ACI 308, 2001) 42

Figure 2.26: Influence of curing on (a) compressive strength of 150 300 mm cylinders, and (b) water permeability of concrete (adapted from Kosmatka et al., 2003). 43

Figure 2.27: Example of the variation of the internal relative humidity from the surface of concrete, at 7 days and 28 days, after exposure to a severe summer environment (adapted from Carrier, 1983). 44

Figure 2.28: Nomograph for estimating the rate of evaporation of water from a concrete surface (adapted from ACI 308, 2001). 46

Figure 2.29: Categorisation of methods of curing concrete (adapted from Kovler & Jensen, 2007). 47 Figure 2.30: (a) Water ponding of finished concrete (Online Civil Enigineering, 2010) and (b) immersion

of finished concrete in a water curing bath. 48 Figure 2.31: (a) Continuous fogging of a recently cast concrete slab (Infrastructure Research Institute,

2000), and (b) close-up photo of a fog nozzle atomising water into fog-like mist (Kosmatka et al., 2003). 49

Figure 2.32: (a) Water spraying of already set concrete (Specifier, 2013), and (b) concrete covered with a fabric covering (hessian) to prevent drying of the concrete during intermittent water sprinkling (New River Bridge, 2011). 50

Figure 2.33: Saturated/wet hessian coverings curing recently cast concrete (New River Bridge, 2011). 51 Figure 2.34: (a) White plastic sheets curing concrete (Kosmatka et al., 2003), and (b) concrete surface

temperature variations under clear, black and white plastic sheets due to solar radiation (adapted from Wojokowski, 1999). 52

Figure 2.35: Spray application of a curing compound with (a) hand-operated (Lambert Chemicals, 2014), and (b) power-driven (Gomaco, 2014) spray equipment to cure concrete. 54

Figure 2.36: A (a) pumice stone particle used as a water-saturated light-weight aggregate, and (b) dry, collapsed and saturate, swollen super-absorbent polymers used to internally cure concrete (Kovler & Jensen, 2007). 57

Figure 2.37: (a) Influence of sealed curing on direct tensile strength of concrete (adapted from Heilmann et al., 1969, as cited in Reinhardt, 2013), and (b) influence of plastic sheet curing on direct tensile strength of mortar (data from Chilwesa, 2012). 62

Figure 2.38: Influence of plastic sheet curing on elastic modulus (compression) of mortar (data from Chilwesa, 2012). 63

Figure 3.1: Schematic flowchart of the methodology. 66


x Figure 3.2: Grading curve of fine aggregate (Klipheuwel sand) used. 68 Figure 3.3: Photographs of (a) a ring test specimen, and (b) parameter experimentation specimens

cured with plastic sheets. 71 Figure 3.4: Photographs of (a) a ring test specimen, and (b) parameter experimentation specimens

cured with wet cloths (covered plastic sheets). 72 Figure 3.5: Photographs of (a) a ring test specimen, and (b) parameter experimentation specimens

cured with the protective coating. 73 Figure 3.6: Photographs of (a) a ring test specimen, and (b) parameter experimentation specimens

cured with the curing compound. 74 Figure 3.7: Schematic representation of the curing regimes considered. 75 Figure 3.8: Photographs of (a) the large laboratory room in which the specimens were placed, and (b)

the environmental data logger used to monitor the environmental conditions. 76 Figure 3.9: Free shrinkage strain prismatic specimen geometry. 77 Figure 3.10: Photographs of the (a) free shrinkage specimens placed next to their corresponding overlay

of the composite overlay-substrate specimens in the curing environment, and (b) the strain extensometer used to take shrinkage strain readings. 78

Figure 3.11: (a) Tensile strength notched dog-bone specimen geometry, and (b) notch detail. 79 Figure 3.12: Photographs of the (a) general UTM setup for tensile strength testing, and (b) a failed

tensile strength specimen showing failure path through the notched cross-section. 79 Figure 3.13: (a) Elastic modulus specimen geometry, and (b) a photograph of the elastic modulus

specimens drying in the appropriate environment. 80 Figure 3.14: Photographs of the (a) general UTM setup for elastic modulus tests, and (b) measurement

of longitudinal strains of the elastic modulus specimens under load. 81 Figure 3.15: Tensile relaxation dog-bone specimen geometry. 81 Figure 3.16: Photographs of (a) a tensile relaxation specimen sealed with paraffin wax, and (b) the

general UTM setup for tensile relaxation tests. 82 Figure 3.17: (a) Top view illustration (with dimensions labelled), and (b) section A-A of full ring test

setup. Not drawn to scale. 83 Figure 3.18: (a) A typical photograph of a cracked ring test specimen showing crack measuring

locations, and (b) a screenshot of the crack-length algorithm used to calculate the crack length. 85

Figure 3.19: 3D illustration of composite overlay-substrate specimens. 87 Figure 3.20: Photographs of the substrate surface (a) after casting before surface preparation, and (b)

after 24 hours and after surface preparation. 87 Figure 3.21: 3D illustration showing the location of strain targets used to monitor the substrate slab

shrinkage. 88 Figure 3.22: Photographs showing (a) the sides of the overlays sealed with paraffin wax, and (b) the

curing regimes of a set of overlays and the free shrinkage specimens placed next to their overlay counterparts. 89

Figure 3.23: 3D illustration showing the location of strain targets used to monitor the overlay shrinkage. 90 Figure 4.1: 28-Day compressive strength development of the various overlay mortar mixes. 95 Figure 4.2: (a-d) 28-Day free shrinkage strain development, and (e-h) 56-day free shrinkage strain

development from the completion of curing of the w/b 0.60 mix for the various curing regimes (see Appendix A2 for full-size figures). 97

Figure 4.3: 56-day free shrinkage strain development for the various curing regimes (see Appendix A2 for full-size figures). 100


xi Figure 4.4: 7- and 28-day direct tensile strength results of the moderate strength mix (w/b 0.60) with

various curing regimes. 101 Figure 4.5: 7- and 28-day elastic modulus results of the moderate strength mix (w/b 0.60) cured with

various curing regimes. 103 Figure 4.6: 7- and 28-day tensile relaxation results of the moderate strength mix (w/b 0.60) cured with

various curing regimes. 104 Figure 4.7: Overview of the age at cracking of ring test specimens made from various mortar overlay

mixes cured under various curing regimes. 106 Figure 4.8: Influence of the curing regime on the age at cracking of ring test specimens. 107 Figure 4.9: Influence of the curing regime on the net age at cracking of ring test specimens. 107 Figure 4.10: Influence of the mix type on the age at cracking of ring test specimens. 108 Figure 4.11: Influence of the curing regime on the 14-day crack area of ring test specimens. 109 Figure 4.12: Influence of the mix type on the 14-day crack area of ring test specimens. 110 Figure 4.13: Correlation between 14-day crack area and (a) free shrinkage strain at the time of crack

measurement, and (b) net age at cracking. 111 Figure 4.14: Correlation between stress rate and net age at cracking (adapted from See et al., 2004) 111 Figure 4.15: Overview of the age at cracking of composite overlay-substrate specimens made from

various mortar overlay mixes cured under various curing regimes. 112 Figure 4.16: Influence of the curing regime on the age at cracking of overlay-substrate specimens. 113 Figure 4.17: Influence of the curing regime on the net age at cracking of overlay-substrate specimens. 113 Figure 4.18: Influence of the mix type on the age at cracking of overlay-substrate specimens. 114 Figure 4.19: 28-day free and restrained shrinkage strain development of the moderate strength mix (w/b

0.60) for the various curing regimes. 115 Figure 4.20: Analytical modelling results of the moderate strength mix (w/b 0.60) for the various curing

regimes. 117 Figure 4.21: Influence of the curing regime on the (a) age at cracking, and (b) net age at cracking

predicted from analytical modelling. 118 Figure 4.22: Correlation between ring test and composite overlay-substrate specimen (a) age at

cracking, and (b) net age at cracking. 119 Figure 4.23: Comparison of the age at cracking of the moderate strength mix (w/b 0.60). 120 Figure 4.24: Comparison of the net age at cracking of the moderate strength mix (w/b 0.60). 120 Figure A1: 90-Day compressive strength development of the various overlay mortar mixes. 135 Figure A2: 28-day free shrinkage strain development of the (a) w/b 0.45, and (b) w/b 0.60 laboratory-

made mortars cured using various curing regimes. 136 Figure A3: 28-day free shrinkage strain development of the (a) w/b 0.80 laboratory-made mortar, and

(b) commercial repair mortar cured using various curing regimes. 137 Figure A4: 56-day free shrinkage strain development of the (a) w/b 0.45, and (b) w/b 0.60 laboratory-

made mortars cured using various curing regimes. 138 Figure A5: 56-day free shrinkage strain development of the (a) w/b 0.80 laboratory-made mortar, and

(b) commercial repair mortar cured using various curing regimes. 139 Figure A6: 56-day free shrinkage strain development from the completion of curing of the (a) w/b 0.45,

and (b) w/b 0.60 laboratory-made mortars cured using various curing regimes. 140 Figure A7: 56-day free shrinkage strain development from the completion of curing of the (a) w/b 0.80

laboratory-made mortar, and (b) commercial repair mortar cured using various curing regimes. 141

Figure A8: 90-Day compressive strength development of the concrete substrate. 147 Figure A9: Shrinkage strain measurements of the concrete substrate. 148


xii Figure A10: Rate of shrinkage of the top of the concrete substrate slab. 148 Figure C1: Environmental recordings of the temperature and relative humidity of the environment

within which the various specimens were allowed to cure. 155 Figure D1: (a) Top view illustration (with dimensions and induced stresses labelled) and (b) section A-

A of full ring test setup. Not drawn to scale. 156

List of Tables Table 2.1: Simplified summary of factors influencing restrained shrinkage cracking. 41 Table 2.2: Recommended curing regime for bonded concrete overlays 60 Table 2.3: Recommended curing durations for repair materials (adapted from ICRI, 1996) 61 Table 3.1: Mix proportions and key mix properties of the laboratory-made mortars. 69 Table 3.2: Mix proportions and key mix properties of the commercial repair mortar. 70 Table 3.3: Key mix properties of the substrate concrete. 86 Table 3.4: Summary of the experimental tests conducted. 91 Table 4.1: Mix and curing regime notations used in the results presented. 94 Table 4.2: 28-Day average degree of restraint for the moderate strength overlays (w/b 0.60) cured

using the respective curing regime. 116 Table 4.3: Comparison of the age at cracking results of the moderate strength mix (w/b 0.60). 120 Table 4.4: Comparison of the net age at cracking results of the moderate strength mix (w/b 0.60). 120 Table A1: Compressive strength results of w/b 0.45 mix specimens. 133 Table A2: Compressive strength results of w/b 0.60 mix specimens. 134 Table A3: Compressive strength results of w/b 0.80 mix specimens. 134 Table A4: Compressive strength results of commercial repair mortar (CRM) specimens. 135 Table A5: 7-Day tensile strength results for the w/b 0.60 mix specimens using the respective curing

regime. 142 Table A6: 28-Day tensile strength results for the w/b 0.60 mix specimens cured using the respective

curing regime. 142 Table A7: 7-Day elastic modulus results of the w/b 0.60 mix specimens cured using the respective

curing regimes. 142 Table A8: 28-Day elastic modulus results of the w/b 0.60 mix specimens cured using the respective

curing regimes. 143 Table A9: 7-Day 24-hour tensile relaxation results for the w/b 0.60 mix specimens cured using the

respective curing regime. 143 Table A10: 28-Day 24-hour tensile relaxation results for the w/b 0.60 mix specimens cured using the

respective curing regime. 143 Table A11: Age at cracking and net age at cracking results of ring test specimens. 144 Table A12: 3-day and 14-day crack areas of ring test specimens. 145 Table A13: Age at cracking and net age at cracking results of the composite overlay-substrate

specimens. 146 Table A14: Compressive strength results of the concrete substrate (SUB). 147 Table B1: Regression coefficients obtained from regression analysis. 149 Table B2: Material property data for the air cured (AC) w/b 0.60 mix. 149 Table B3: Material property data for the 2-day plastic sheets cured (PS (2D)) w/b 0.60 mix. 150 Table B4: Material property data for the 2-day wet cloth cured (WC (2D)) w/b 0.60 mix. 151 Table B5: Material property data for the 7-day plastic sheets cured (PS (7D)) w/b 0.60 mix. 151 Table B6: Material property data for the 7-day wet cloth cured (WC (7D)) w/b 0.60 mix. 152


xiii Table B7: Material property data for the w/b 0.60 mix cured with a protective coating (PC). 153 Table B8: Material property data for the w/b 0.60 mix cured with a curing compound (CC). 154 Table D1: Comparison of ASTM C1581-09a (2009) ring test apparatus dimensions to those used in

the experimentation. 156 Table D2: Comparison of ASTM C1581-09a (2009) ring tests induced stresses to those used in the

experimentation. 157


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Abbreviations 2D/7D 2-Day/7-Day Curing Duration AASHTO American Association of State Highway and Transport Officials AC Air Curing ACI American Concrete Institute ASTM American Society for Testing and Materials C-S-H Calcium Silicate Hydrate CC Curing Compound CCAA Cement Concrete & Aggregates Australia CoV Coefficient of Variation CRA Crack-Reducing Admixture CRM Commercial Repair Mortar CSF Condensed Silica Fume FA Fly Ash GGBS Ground Granulated Blastfurnace Slag GGCS Ground Granulated Corex Slag ICRI International Concrete Repair Institute ITZ Interfacial Transition Zone LDPE Low-Density Polyethylene LWA Light-Weight Aggregate MATLAB Matrix Laboratory NWA Normal-Weight Aggregate OPC Ordinary Portland Cement OPI Oxygen Permeability Index PS Plastic Sheets PC Protective Coating PVA Polyvinyl Acetate RH Relative Humidity SANS South African National Standard SAP Super-Absorbent Polymer SRA Shrinkage Reducing Admixture SSD Saturated Surface Dry UTM Universal Testing Machine WC Wet Cloth WSI Water Sorptivity Index w/b Water-Binder Ratio w/c Water-Cement Ratio

Abbreviations

xv

List of Variables , Free shrinkage strain regression coefficients (unitless) , Tensile strength regression coefficients (unitless) , Elastic modulus regression coefficients (unitless) , Tensile relaxation regression coefficients (unitless) Inner diameter of mortar annulus (mm) Outer diameter of mortar annulus (mm) Mean elastic modulus in the interval from 1 to (GPa) Tensile elastic modulus (GPa) Tensile strength (MPa) , Tensile strength at (MPa) Height of steel ring (mm) Disjoining pressure (kPa) Capillary pressure (kPa) Surface free energy (J/g) Arbitrary point in time (any unit of time) Time i (days) Initial point in time (any unit of time) Thickness of inner steel ring (mm) Thickness of mortar annulus (mm) Thickness of outer steel ring (mm) Thickness of wooden board (mm) Degree of restraint (unitless) Mean tensile relaxation in the interval from 1 to (%) Free shrinkage strain ( 106) , Change in free shrinkage strain from 1 to ( 106) Tensile stress (MPa) , Tensile stress at (MPa) , Exterior circumferential stress (MPa) , Interior circumferential stress (MPa) , Exterior radial stress (MPa) , Interior radial stress (MPa) Poissons ratio (unitless) Tensile relaxation factor (unitless) Mean tensile relaxation factor in the interval from 1 to (unitless)

List of Variables

1

1. Introduction 1.1 Background to Research Problem As more and more concrete structures are being constructed worldwide, the amount of concrete infrastructure requiring repair and rehabilitation is increasing. This is due to the fact that over time concrete structures deteriorate, often prematurely, due to numerous factors, resulting in structures not reaching their intended service lives. In order to restore safety and/or serviceability to the deteriorated concrete structures they must be repaired or rehabilitated. The costs associated with these concrete repairs may be significant and can have a large influence on the feasibility of maintaining structures, particularly large infrastructure (Beushausen & Alexander, 2009). In the United States of America alone, the American Society of Civil Engineering reported that in 2006 the annual cost of concrete repair and rehabilitation was estimated to be $18 to $21 billion (Emmons, 2006; Raupach, 2006). While the costs associated with repair of concrete structures can be substantial, the costs of poorly design or executed concrete repairs may be even higher (Beushausen & Alexander, 2009). This has resulted in an increasing need for effective concrete repair techniques.

Bonded mortar or concrete overlays, henceforth referred to as bonded overlays, are a common type of concrete repair technique that involves the casting of a new repair material, in the form of an overlay, in the place of the damaged or deteriorated portion of concrete. Bonded overlays are however prone to premature failure due to differential volume changes between the newly cast overlay and the existing substrate. Failure of bonded overlays is primarily manifested as debonding or cracking (Beushausen, 2005; Bissonnette et al., 2011). Debonding can be relatively easily prevented by ensuring a high bond strength between the overlay and substrate by thorough substrate preparation and cleaning, overlay compaction, and curing. Failure due to restrained shrinkage cracking on the other hand has proven significantly more difficult to prevent (Beushausen & Alexander, 2009; Bissonnette et al., 2011).

The volume changes experienced by the overlay that lead to its failure arise due to the inherent shrinkage of cementitious materials. The restraint provided by the substrate to the shrinkage of the overlay results in the development of tensile stresses within the overlay. These stresses are proportional to the shrinkage strain and elastic modulus of the overlay but are partially reduced due to tensile relaxation. If the tensile stresses induced are greater than the tensile strength of the overlay then cracking of the overlay, termed restrained shrinkage cracking, will begin to develop. Not only is cracking aesthetically unpleasing, but it also hinders the primary function of bonded overlays - to improve durability (Banthia & Gupta, 2009).

Despite the relatively sound understanding of the mechanisms of restrained shrinkage cracking and the numerous measures that can be taken to mitigate it, such as the use of an increased water-binder (w/b) ratio, reduced paste content, appropriate cement extenders, increased aggregate content, admixtures, fibres, etc., restrained shrinkage cracking still often occurs. Another measure thought to increase an overlays resistance to restrained shrinkage cracking is curing. However, the

Chapter 1: Introduction

2 influence of curing on restrained shrinkage cracking has not yet been thoroughly investigated, and so its influence is not fully known and understood.

1.2 Problem to be Investigated Bonded overlays often fail prematurely due to restrained shrinkage cracking. In an attempt to remedy this problem, it is suggested that the use of curing may help to prevent or delay premature failure due to restrained shrinkage cracking. However, the influence of curing on restrained shrinkage cracking has not yet been thoroughly investigated and so its influence is not fully known and understood. The problem to be investigated, therefore, is to determine the influence of different curing regimes (method and duration) on restrained shrinkage cracking of bonded overlays, and the relevant time-dependent properties of concrete (shrinkage, tensile strength, elastic modulus, and tensile relaxation), using the latter to provide an explanation of the influence of curing in terms of the time-dependent material properties governing restrained shrinkage cracking.

1.3 Hypothesis This research proposes that different curing methods have different influences on the age at which restrained shrinkage cracking is observed and corresponding crack area measured, and that increasing the curing duration of bonded overlays increases the age at which restrained shrinkage cracking occurs. Furthermore, it is proposed that there exists an optimal curing regime for a particular overlay mix, and that this optimal curing regime is governed by the interrelation of the time-dependent material properties of the overlay.

1.4 Research Aim and Objectives The aim of the proposed research is it to determine the influence of different curing regimes on the cracking of non-load-bearing bonded overlays due to restrained shrinkage. The aforementioned aim will be achieved by following the four objectives of the research:

Experimentally determine the influence of curing on the time-dependent material properties (shrinkage, tensile strength, elastic modulus, and tensile relaxation), of specimens of a moderate strength mortar subjected to various curing regimes.

Experimentally determine the age at cracking and crack area due to restrained shrinkage on specimens of various mortar mixes subjected to various curing regimes.

Analytically model, using the experimentally determined time-dependent material properties, the age at cracking due to restrained shrinkage cracking of a moderate strength mortar subjected to various curing regimes.

Critically evaluate the performance of the various mortar mixes subjected to various curing regimes, using the results of the influence of curing on the time-dependent material properties and the associated analytical modelling to provide fundamental insight.


3

1.5 Key Research Questions The key research questions of the research are listed herein.

What effect do different curing methods have on the governing time-dependent material properties, the age at cracking, and crack area, of restrained shrinkage cracking of bonded overlays?

What effect does the curing duration have on the governing time-dependent material properties, the age at cracking, and crack area, of restrained shrinkage cracking of bonded overlays?

Are there preferential curing regimes for different overlay mixes? If so, which curing regime is preferential for each overlay mix and to what degree can restrained shrinkage cracking be reduced with the respective curing regimes?

1.6 Scope and Limitations There are numerous factors that influence the performance of bonded overlays with regard to restrained shrinkage cracking and so it is necessary to define the scope of research and to identify limitations as it is not possible to include all the influencing factors in a limited research project.

The scope of the research is to determine the influence of a select number of curing regimes, exposed to a single environmental condition, on the time-dependent material properties governing restrained shrinkage cracking (shrinkage, tensile strength, elastic modulus, and tensile relaxation) and on restrained shrinkage cracking directly (age at cracking and crack area) of a limited number of overlay mixes. The results of the material property tests are to be used in an analytical model which in turn, is used to explain the trends observed from the results of the direct restrained shrinkage tests, namely the age at cracking.

The limitations of the research are listed herein.

The number of overlay mixes used in experimentation was limited to four due to the large number of tests and the extensive amount of time required for the full range of tests that needed to be conducted on each mix. Furthermore, these mixes were limited to laboratory-made, cementitious mortars and a commercial polymer-modified cementitious mortar. No ordinary, high-performance, self-compacting or fibre reinforced concretes, or resin-based overlays were considered.

Only five curing methods and a total of two curing durations of 2 and 7 days, making a total of seven curing regimes, were used due to similar reasons outlined in the previous point.

Material property tests (shrinkage, tensile strength, elastic modulus and tensile relaxation) were limited to two test ages per mix. This was due to similar reasons outlined in the first point.

Restrained shrinkage cracking tests were limited to the ring test and composite overlay-substrate specimens.

The analytical modelling was limited to a simple material property analytical model.


4

1.7 Thesis Outline The first chapter (Chapter 1) provides an introduction to the research by establishing the background to the research problem, listing the aims and objectives as well as the key research questions. The scope of the research is also defined and the limitations are noted. This is followed by a review of relevant literature (Chapter 2). The literature is reviewed by first providing a brief overview of the fundamentals of bonded overlays, including applications and design and construction considerations. An overview of the failure of bonded overlays is then presented after which failure due to debonding and restrained shrinkage cracking is discussed respectively. Following this, restrained shrinkage cracking of bonded overlays, particularly the material properties governing it and the factors influencing it, is reviewed in further detail. An extensive review of curing of concrete is then provided. General aspects of curing are presented first followed by a review of the various curing methods available after which more specific aspects of curing relating to bonded overlays are then investigated. The experimental approach taken, curing regimes considered, and details of the tests conducted and analytical modelling carried out are then discussed in Chapter 3. The results obtained from the experimentation and analytical modelling are then presented and discussed in Chapter 4. Conclusions and recommendations made based on the results presented are provided in Chapter 5. A list of references used is provided in Chapter 6. Relevant detailed experimental results, detailed analytical modelling results, environmental recordings, and a comparison of ring tests setups are included in Appendix A to D respectively. A schematic representation of the thesis outline is provided in Figure 1.1.


5

Figure 1.1: Schematic overview of thesis outline.

Introduction

Fundamentals of Bonded Overlays

Chapter 2: Literature Review


Chapter 3: Methodology

Chapter 4: Results and Discussions

Chapter 5: Conclusions and

Recommendations

Restrained Shrinkage Cracking of Bonded Overlays

Curing of Concrete

Summary

Restrained Shrinkage Cracking

Time-Dependent Material Properties

Methodology

Results and Discussions

Conclusions and Recommendations

Explanation of Influence of Curing on Restrained Shrinkage Cracking


Time-Dependent Material Properties

Experimental Tests

Curing Regimes

Failure of Bonded Overlays

Analytical Modelling

Analytical Modelling

Overview and Approach


6

2. Literature Review 2.1 Introduction This chapter covers the fundamentals of aspects relating to the performance of bonded overlays, with particular emphasis on those that influence cracking due to restrained drying shrinkage of non-load-bearing bonded overlays. A brief overview on the fundamentals of bonded overlays is provided first in Section 2.2, which includes applications and design and construction considerations. Failure of bonded overlays, with particular emphasis on debonding and restrained shrinkage cracking, is then covered in Section 2.3. This is followed by a review of the restrained shrinkage cracking of bonded overlays in Section 2.4, focusing on the material properties governing it and the factors that influence it. A detailed review of curing of concrete is then provided in Section 2.5, in which general aspects of curing are presented first, followed by a review of various curing methods after which more specific aspects of curing relating to bonded overlays are then investigated. Lastly, a summary of the literature review is presented in Section 2.6.

2.2 Fundamentals of Bonded Overlays 2.2.1 Definition A bonded overlay is a layer of mortar or concrete typically 25 to 150 mm in thickness which is placed on top of a pre-existing concrete surface, known as the substrate, to which a bond develops (Tayabji et al., 2009). The primary use of a bonded overlay is to extend the service life of a structure by restoring or improving the structural integrity, serviceability and/or durability. Bonded overlays used for repairs that are relatively small in surface area are commonly termed patch repairs (Beushausen & Alexander, 2009).

2.2.2 Applications Over time concrete structures start to deteriorate which leads to a decrease in the structural performance, reduction in the durability of the structure and may render the concrete aesthetically unpleasing. Repair of the concrete is required in order to reinstate the structure to its original specifications in order to prevent further deterioration of the member or structure. Among the possible choices for repair or rehabilitation, bonded overlays are considered the most economical option for concrete structures (Bissonnette et al., 2011). They are therefore widely used for repairing, as well as strengthening concrete structures. Successful implementation of bonded overlays can result in a number of advantages, these include:

Restoration of structural strength of the repaired member Restoration of structural integrity (strength, abrasion resistance, etc.) of the repaired member Restoration or improvement of the appearance of the repaired member Restoration of the durability performance of the repaired member Improvement of the serviceability of the repaired member

Applications of bonded overlays, as previously mentioned, include that of thin surface repairs and/or strengthening of concrete that has delaminated, spalled and/or cracked. They are also


7 particularly suited for members with large surface areas (slabs on grade, pavements and tunnel linings, etc.) where the concrete can be poured or sprayed (Granju et al., 2004). Additionally, they are used on precast members which receive an in-situ topping (Beushausen, 2005). The research conducted in this dissertation largely relates to the use of non-load-bearing bonded overlays for concrete repair, and so the use of the term bonded overlay will henceforth refer to this type of overlay.

2.2.3 Design and Construction Considerations In order to achieve successful implementation of bonded overlays, there are various design and construction factors that must be considered to ensure sufficient bond strength and crack resistance. The former being largely linked to substrate surface preparation and properties whilst the latter being linked to overlay material properties (tensile strength, shrinkage, elastic modulus, and relaxation). An overview of these factors are shown diagrammatically in Figure 2.1 and are elaborated on in Sections 2.2.3.1 to 2.2.3.4. Further details of the design and construction of bonded overlays are given in standards and specifications (ICRI ACI, 2013) and in recommendations (Bissonnette et al., 2011). However, standards and specifications for design of bonded overlays generally lack scope and detail, and so it is often left to the engineer to specify appropriate materials and construction procedures (Beushausen, 2005). This is largely due to the fact that each repair situation is unique and so needs to be treated as such.

Figure 2.1: Overview of design and construction considerations of bonded concrete

overlays (Adapted from Beushausen & Alexander, 2009).

2.2.3.1 Substrate Preparation and Properties Correct substrate preparation is generally considered to be the most crucial steps in a concrete patch repair as it significantly influences the bond strength of the substrate. A poorly prepared substrate will always limit the performance of an overlay, no matter how good the repair material or how well it was applied. The substrate should therefore be prepared by removing deteriorated concrete and creating a good surface interface. Substrate preparation consists of two distinct processes that are both equally important substrate surface removal and substrate surface cleaning (Beushausen & Alexander, 2009). These two processes, as well as substrate moisture condition and the use of bonding agents, are discussed further in the following four subsections.

ENVIRONMENT Overlay material properties

(fresh and hardened properties) Curing procedures

Edge conditioning

Substrate preparation Roughness Cleanliness Bonding agent

Rebar preparation: Cleaning from rust Protective coating

Undercutting of corroded rebars

Substrate Removal of

contaminated Prevention of


8 Substrate Surface Removal Substrate surface removal involves the bulk removal of poor-quality (including the laitance layer) and deteriorated substrate concrete, and any previously applied coatings. This should be carried out until the substrate is sound and has sufficient surface texture to form a strong bond. Edges of removed concrete should be straight and normal to free edges (as shown in Figure 2.1) to prevent feather edges which result in localised overlay failure (Beushausen & Alexander, 2009). Additionally, any corroded steel must be undercut and all corrosion products should be removed.

Mechanical surface removal techniques such as jackhammering, milling, scarifying, or hand-chipping are very efficient in removing poor concrete however are likely to cause microcracking of the substrate. Microcracks, commonly referred to as bruising, result in zones of weakness that are prone to bond failure. The extent of bruising is dependent of the surface removal method and the quality of the concrete, however the bruised layer is typically of the order of 3 mm thick (Bissonnette et al., 2013). Water-jetting (hydromilling) and sandblasting are alternative surface preparation techniques that have been shown to be efficient in removing concrete and produce substrate surfaces with sufficient surface texture, without causing microcracking (Silfwerbrand, 1990). It has been shown that mechanical removal, which is sometimes necessary to remove deteriorated concrete, followed by water-jetting can achieve good bond strength (Silfwerbrand & Peterson, 1993).

Substrate Surface Cleaning Substrate surface cleaning refers to the removal of loose pieces of concrete/debris and any contaminants such as dust, oil, grease, etc. that may inhibit bond strength. It is essential not only that the substrate has been cleaned but that it is clean at time of overlay casting, as the substrate may have been contaminated by pollutants (from construction and/or the environment) in the interim.

The use of water-jetting as a surface removal technique provides a very clean substrate which is dust-free and also removes corrosion products. It should however be followed by thorough removal of loose concrete particles to prevent unhydrated cement particles from bonding to the saturated substrate surface (Bissonnette et al., 2013).

Substrate Moisture Condition Not only must a substrate be free of poor-quality and deteriorated concrete, and be clean of any loose particles and contaminates, but it should also be in an appropriate moisture condition. A substrate with a surface that is too wet will cause an increase in the w/b ratio of a thin layer of the overlay in contact with the substrate lowering its strength. Additionally, the water may block open pores causing a reduction in mechanical interlocking and hence lower bond strength. A dry substrate will absorb water, resulting in a harsh overlay mix that may not interlock sufficiently with the substrate, and may lead to less water than that required for full cement hydration. Substrates are therefore commonly specified to be in a saturated surface dry (SSD) condition (Beushausen & Alexander, 2009).


9 Bonding Agents Bonding agents, consisting of commercial products or site-made cement slurries, are specified by many engineers to improve bond strength. Extensive research has been carried out on the use of bonding agents which has resulted in mixed opinions. It is however generally agreed upon that bonding agents cannot make up for poor substrate surface preparation. They have been shown to improve bond strength when using overlays of low workability which cannot easily flow into the pores and rough surface texture of the substrate however when used to bond a highly workable overlays they have no beneficial influence on bond strength (Beushausen, 2010). Bonding agents may cause failure if used inappropriately, whereby they are allowed to dry/set/cure prior to overlay placement causing them to act as a bond breaker, and so should be used with care.

2.2.3.2 Overlay Material Types and Properties The type of overlay material used and its associated properties in both the fresh and hardened state are of importance as they strongly influence the bond strength and durability. Additionally, the hardened material properties also govern the long-term overlay performance, as they collectively determine the crack resistance of the overlay (Beushausen & Alexander, 2009). Commonly used repair materials are discussed and desired properties of overlays in the fresh and hardened state are provided in the three subsections that follow.

Material Types There are a large number of repair materials that may be used for bonded overlays. These materials can be divided into cementitious repair materials which include concrete or mortar made from pure or blended Portland cements, and polymeric repair materials which include polymer or polymer-modified Portland cement concrete or mortar (Fowler, 2009). In addition, these repair materials may be fibre-reinforced.

Cementitious repair materials that are Portland cement-based are commonly used for concrete repairs as they are widely available, are low in cost compared to many other repair materials, and are relatively easy to place and finish (Fowler, 2009). Blended Portland cements may be used to reduce cost or provide improved durability of the repair. Cementitious repair materials are however limited due to shrinkage which may result in cracking of the overlay (see Section 2.3.3).

Polymer concrete or mortar differ from polymer-modified concrete or mortar in that the former uses a polymer binder (as opposed to a cementitious binder) whilst the later uses cement as a binder with a polymer as an admixture. Polymer and polymer-modified concretes or mortars have some improved mechanical and durability properties however are more expensive cementitious repair materials as the polymers that are used are the most expensive component (Fowler, 2009).

Concretes (cementitious or polymeric), due to the inclusion of aggregates, are generally used for thicker repairs. It should be noted that mortars are more susceptible to cracking as they exhibit considerably higher shrinkage (as opposed to concretes) due to their higher water and cement (paste) content and lower aggregate content (Fowler, 2009).

The choice of repair material used is based on many factors including construction, performance, and durability requirements (Fowler, 2009). More detailed information on the various repair material types and the selection thereof are given by the American Concrete Institute (2006).


10 Fresh Properties The workability of the fresh overlay influences its ability to fill open cavities/voids of the substrate surface and therefore determines the effective contact area of the overlay-substrate bond. Highly workable mixes (used for horizontal or vertical repairs using formwork), promote capillary suction from the substrate resulting in a stronger bond through increased anchorage with the substrate surface. The converse is true for stiff mortar overlays (commonly used for small patch repairs) which have much lower capillary suction and corresponding bond strength. Such overlays may require the use of a bonding agent to improve adhesion, as discussed in Section 2.2.3.1. In general, concrete or mortar of conventional workability is sufficient.

Hardened Properties Hardened properties of the overlay that must be considered are load-bearing (compressive strength, tensile strength, elastic modulus, etc.), dimensional stability (shrinkage, creep and relaxation, and thermal deformation) and durability (permeability, absorption, etc.) properties.

Overlays are not required to have a high compressive strength and elastic modulus, particularly those used in non-load-bearing applications. Higher strength concretes with corresponding high elastic moduli actually have lower resistance to cracking. This is due to the fact the overlays are deformation governed, and not force controlled. Their high stiffness leads to higher stresses, than that of low strength overlays which are considerably less stiff and lead to lower stresses. However, a high tensile strength will increase the overlay materials resistance to cracking.

In general, the substrate undergoes significantly less shrinkage than that of a newly cast overlay and so it is beneficial to lower the rate and minimise the magnitude of shrinkage of overlay materials. This minimises differential deformations and thus the induced stresses that may cause cracking of the overlay. High levels of relaxation of the overlay which reduce the stresses induced by shrinkage and low thermal deformations which limit differential deformation as a result of temperature changes are also advantageous in this regard (Beushausen & Alexander, 2009).

2.2.3.3 Overlay Application Methods There are various application methods for bonded overlays used for concrete repair, each application method being suitable for different repair applications. The application method should be chosen ensuring that the repair material fully encapsulates the exposed reinforcing steel, achieves satisfactory bond with the substrate, and fills the prepared cavity without segregating. Failure of the application method to achieve this will result in a poorly performing repair, irrespective of the quality of the repair material. Selection of the appropriate application method is therefore crucial for a successful repair. Brief descriptions and common repair application methods are provided in the subsections that follow. Further information on these (and other) application methods is detailed by Emmons (1994) and the International Concrete Repair Institute (1996).

Hand-/Trowel-Applied In the hand-/trowel-applied application method, the repair material is mixed into a troweable, non-sag consistency. The repair material is pressed into the substrate pore structure to develop good contact, without the formation of voids. The repair material is designed to hang in place until subsequent layers are added. Each layer should be roughened to promote bond to the next layer. This application method is typically used for small surface restoration repairs on vertical and


11 overhead locations when reinforcing steel is not present. The presence of reinforcing steel makes it difficult to consolidate the repair material and provide complete encapsulation of the reinforcing steel. Problems associated with this technique typically include poor bond between layers and voids around embedded reinforcing steel (Emmons, 1994).

Form and Cast-in-Place For the form and cast-in-place application method, formwork is used to confine the prepared cavity within which the repair material is cast. It is important that the repair material is of adequate flowability and is consolidated by rodding or conventional vibration to aid in the formation of a good bond with the concrete substrate. This application method is one of the most common methods repair of vertical (columns, walls and slab edges), and in some cases, overhead locations (slab soffits). It can be used for partial- or full-depth repairs. An advantage of this application method is that curing can easily be employed by means of formwork retention. Problems associated with this technique are that formed surfaces make the placement of bonding agents difficult and that in some cases complete filling of the cavity may be difficult (Emmons, 1994).

Dry or Wet Spraying Dry or wet spraying, also known as dry-mix or wet-mix shotcrete, involves spraying (at high velocity) of the repair material onto the repair area using compressed air. For dry spraying, the repair material is placed dry or slightly damp into the pan mixer and is then transported via the hose to the exit nozzle where water and admixtures (if any) are added to the mix. In contrary, for wet spraying, the pre-mixed repair material is placed pump and is then transported via the hose to the nozzle where compressed air and admixtures (if any) are added to the mix. Dry-spray repair materials should have well-graded aggregates and well-proportioned mixes to compensate for rebound losses whilst wet-spray repair materials should be pumpable and should not sag. The spray application methods are economical for large repair areas and are therefore typically used for large vertical or overhead repairs where no reinforcing bars or thin reinforcing bars are present with minimal congestion. Problems associated with this technique include sloughing off of the concrete or mortar due to excessively thick layer placement and the formation of voids behind reinforcement (Emmons, 1994).

2.2.3.4 Environmental Conditions and Curing The environmental conditions, more specifically the temperature and relative humidity, significantly influence the development of the mechanical and durability properties of concrete and mortar. In cold, wet environments the cold temperature slows down the rate of hydration and so the development of mechanical and durability properties are slowed. In hot, dry environments the high temperature and low relative humidity increase the rate of drying from the concrete surface and therefore the rate at which moisture is lost from the concrete (Banthia & Gupta, 2006). This adversely effects hydration and so the mechanical and durability properties may not develop sufficiently resulting in failure of the overlay or more protection of the underlying reinforcement (Hassan et al., 2000). It has been suggested that these adverse environmental effects can be partially mitigated through curing (Beushausen & Alexander, 2009; Bissonnette et al, 2013). This is the focus of the research at hand and shall be explored in more detail throughout the remainder of this dissertation.


12

2.3 Failure of Bonded Overlays The failure of bonded overlays is of discussed herein. An overview of the failure of bonded overlays, including the typical causes of failure and common failure modes, is provided in Section 2.3.1. The failure mechanisms and factors influencing the predominant failure modes, debonding and cracking, are then discussed in further detail in Sections 2.3.2 and 2.3.3 respectively.

2.3.1 Overview Bonded overlays are a widely used, economical repair solution for concrete structures that afford many advantages such as the restoration of structural integrity, durability, and the improvement of aesthetics of a concrete member, as previously stated in Section 2.2.2. Despite these advantages, the performance of bonded overlays is often adversely affected by premature failure of the overlay which is predominantly manifested as debonding and/or cracking (Beushausen, 2005; Beushausen & Alexander, 2006b; Crlsward, 2006; Bissonnette et al., 2011; Shin & Lange, 2012).

These failure modes, debonding and cracking, occur as a result of the shear and/or tensile stresses induced by the differential deformation between the overlay and substrate. Stresses develop within the overlay as the contracting movement (shrinkage) of the newly cast overlay is to some extent restrained by the substrate and/or the periphery in the case of an enclosed overlay. If the resulting shear and tensile interface stresses induced are sufficiently large, they will cause debonding at the interface between the overlay and substrate whilst if the resulting tensile stresses induced within the overlay are sufficiently large, they will cause cracking of the overlay. In some cases, edge lifting (also known as curling) may occur due to development of a stress field within the overlay near the free edges which tends to lift the overlay edges vertically (Crlsward, 2006). These failure modes are illustrated in Figure 2.2.

Figure 2.2: Failure modes of bonded overlays (adapted from Crlsward, 2006).

The differential deformations which induce the stresses that cause failure of bonded overlays due to debonding or cracking largely originate from plastic, autogenous, and drying shrinkage, of the overlay, as well temperature variations, caused by internal hydration and external environmental conditions (Shin & Lange 2012). These deformations typically occur within the first few hours to weeks of casting of a new overlay (Beushausen & Alexander, 2006b, Shin & Lange, 2012) and thus failure of overlays generally occurs during this time period. These deformations may also occur due to substrate settlement or externally applied loads (Denari & Silfwerbrand, 2004). It is however generally accepted that differential deformation due to shrinkage, termed differential shrinkage, is

Overlay

Concrete Substrate

Bond between overlay and substrate

Edge Lifting (Curling)

Debonding

Edge Lifting (Curling) Humidity gradient Cracks


13 the primary cause of failure of bonded overlays (Beushausen, 2005; Beushausen & Alexander, 2006b; Bissonnette et al., 2011; Shin & Lange 2012).

Shrinkage of the overlay occurs as moisture is lost to the environment or to the substrate through absorption if the substrate is dry relative to the overlay, or a combination thereof. This is termed drying shrinkage and results in the occurrence of a hygral (humidity) gradient in the overlay due to differential drying conditions (see Figure 2.2). The gradient induces non-uniform stresses within the thickness of the overlay that encourage edge lifting (curling). It should be noted that shrinkage may also occur as a result of moisture lost during cement hydration and/or other shrinkage mechanisms (see Section 2.4.2), however drying shrinkage is generally considered to be the most critical type of shrinkage with regards to bonded overlays (Pigeon & Bissonnette, 1999).

2.3.2 Debonding Debonding is the separation of the overlay from the substrate to which it was bonded, and is characterised by delamination and/or spalling (Beushausen & Alexander, 2006b). It is directly related to the strength of the bond between the overlay and the substrate, and can therefore be easily minimised, relative to that of cracking, by ensuring appropriate bond strength between the overlay and the substrate.

2.3.2.1 Debonding Mechanism Debonding arises due to the shear and tensile stresses that are induced at the overlay-substrate interface as a result of the differential shrinkage between the overlay and substrate, as discussed in Section 2.3.1. It my however also occur due to the breakdown of the overlay-substrate bond caused by the penetration of aggressive chemicals through failure cracks of the overlay. The induced interface stresses occur in a mixed form of shear and tension and are commonly believed to have maximum values at the boundaries of the overlay due to the lack of continuity of stress transfer within the overlay. As such, debonding is usually initiated at the overlay boundaries such as edges, joints, and cracks (full-depth separations) when the interface stresses exceed the bond strength (Carter et al., 2002; Granju et al., 2004). It has however been argued that the main reason why debonding commonly occurs at the overlay boundaries is as a result of overlay slip due to the lack of adjacent restraint (Beushausen, 2005). The initiation of debonding at overlay boundaries suggests that debonding is closely related to cracking (Carter et al., 2002; Beushausen et al., 2006b). It should however be noted that debonding may also occur in areas within the centre of the overlay however this is usually as a result of poor surface preparation, improper overlay placement, or weak substrate concrete (Knutson, 1990; Carter et al., 2002).

2.3.2.2 Factors Influencing Debonding The bond strength is one of the main parameters governing debonding (Bissonnette et al., 2011) as debonding occurs when the interface stresses exceed the bond strength, as discussed in Section 2.3.2.1. The bond strength, and therefore the likelihood of debonding, is dependent on a large number of factors. However, it is commonly agreed upon that substrate preparation and cleaning (absence of micro-cracks, removal of the laitance layer, and cleanliness of the substrate surface,), overlay compaction, and curing are the most important factors to achieve good mechanical adhesion and bond (Beushausen & Alexander, 2009; Bissonnette et al., 2011). These factors, and


14 therefore good bond strength, are clearly all directly related to the quality of workmanship/construction. Factors of lessor importance include the overlay mechanical properties (fresh and hardened properties) and the moisture condition of the substrate (Beushausen & Alexander, 2009). The environment, substrate mechanical properties, interface roughness, overlay placement procedures, and bonding agents are of significantly less importance in terms of bond strength (Beushausen & Alexander, 2009). The majority of aforementioned factors were discussed in further detail in Section 2.2.3.

2.3.3 Restrained Shrinkage Cracking Cracking of overlays as a result of the tensile stresses induced by restrained shrinkage deformations is termed restrained shrinkage cracking. Restrained shrinkage cracking is related to the interaction between the time-dependent overlay material properties including shrinkage, tensile strength, elastic modulus, and tensile relaxation. It has therefore proven to be difficult to control, relative to that of debonding, and commonly occurs in practice (Beushausen & Alexander, 2006a).

2.3.3.1 Restrained Shrinkage Cracking Mechanism Restrained shrinkage cracking arises due to the tensile stresses induced within the overlay as a result of the differential shrinkage between the overlay and substrate, as discussed in Section 2.3.1. These tensile stresses () develop when the free shrinkage of the overlay () is restrained to a certain degree () by the substrate. The tensile stresses induced by the restrained shrinkage of the overlay are proportional to the stiffness (elastic modulus) () of the overlay, with stiffer (higher elastic modulus) overlays inducing greater tensile stresses. A portion of the induced tensile stresses are however alleviated by tensile relaxation (), a stress-relieving viscoelastic property of the overlay. The magnitude of the tensile stresses induced by restrained shrinkage is therefore:

= (2.1)

where is the restrained shrinkage stress within the overlay, is the degree of restraint, is the free shrinkage strain of the overlay, is the tensile modulus of elasticity of the overlay, and is the tensile relaxation factor (see Section 3.6.1) of the overlay. It should be noted that this is a simplified representation of the induced tensile stress as only shrinkage is considered and the shrinkage considered is assumed to be constant throughout the overlay thickness, as discussed later in this section. If the resulting tensile stresses after tensile relaxation are less than or equal to the tensile strength of the overlay then the overlay will remain crack-free. However, should the resulting tensile stresses after tensile relaxation be greater than the tensile strength of the overlay then the overlay will crack. The failure criterion for restrained shrinkage cracking is therefore:

= if if >

(2.2)

It should be noted that, in general, the shrinkage-induced stresses within common overlay repair materials are often much greater that the tensile strength of the overlay and so cracking commonly occurs (Pigeon et al., 1999). The interaction of the time-dependent material properties and the failure criterion for cracking are illustrated in Figure 2.3.


15

Figure 2.3: Schematic overview of the time-dependent material properties governing restrained shrinkage cracking of bonded overlays (adapted from Troxell et al., 1968).

It should be noted that drying and hence moisture loss predominantly occurs from the surface of the overlay exposed to the environment, as discussed in Section 2.3.1 and shown in Figure 2.4a. This causes a hygral gradient to develop throughout the thickness of the overlay which results in non-uniform drying shrinkage throughout the thickness of the overlay with the greatest free shrinkage strains at the exposed surface of the overlay, as illustrated in Figure 2.4b. The corresponding restrained shrinkage stresses are therefore also non-uniform throughout the thickness of the overlay with the maximum stresses occurring at the exposed surface, as illustrated in Figure 2.4c. Cracking therefore develops from the surface of the overlay down to the depth at which the restrained stresses are equal to the tensile strength of the overlay, as illustrated in Figure 2.4d.

Figure 2.4: Illustration of a simplified representation of (a) drying conditions, (b) free shrinkage strains, (c) restrained stresses, and (d) cracking due to restrained shrinkage

of bonded overlays (adapted from Pigeon & Bissonnette, 1999).

2.3.3.2 Factors Influencing Restrained Shrinkage Cracking Restrained shrinkage cracking occurs when the restrained shrinkage stresses, which arise due to the shrinkage, elastic modulus, and tensile relaxation of the overlay repair material, exceed the tensile

Casting

Stre

ss o

r St

reng

th

Age Age at loading

Age at which cracking develops

( > )

Curing ()

Tensile strength

Cracking starts to develop ( > )

()

Elastic tensile stress after relaxation

(, )

Tensile relaxation

Elastic tensile stress

Overlay

Substrate

Extension Contraction Compression

()

Tensile Strength

Drying Free Shrinkage Strain

Restrained Shrinkage Stress


Interface

Tension

()

Free Shrinkage Strain

()

Restrained Shrinkage

Stress

> Cracked

Uncracked

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


16 strength of the overlay repair material, as discussed in Section 2.3.3.1. Restrained shrinkage cracking is therefore governed by the shrinkage, tensile strength, elastic modulus, and tensile relaxation of the overlay repair material, all of which are time-dependent material properties. As a result, factors that influence these material properties are likely to have an influence on restrained shrinkage cracking. Factors that influence restrained shrinkage cracking therefore include the w/b ratio, water and cement (paste) content, cement composition and fineness, cement extenders, aggregate content and properties, admixtures, fibres, member geometry, environmental conditions, and curing. The time-dependent material properties influencing restrained shrinkage cracking and the influence of the aforementioned factors on restrained shrinkage cracking is discussed in further detail in Section 2.4.

2.4 Restrained Shrinkage Cracking of Bonded Overlays Restrained shrinkage cracking of bonded overlays is discussed herein. A brief overview of restrained shrinkage cracking is provided in Section 2.4.1. The material properties that govern restrained shrinkage cracking, namely shrinkage, tensile strength, elastic modulus, and tensile relaxation, are then discussed briefly in Sections 2.4.2 to 2.4.5 respectively, after which the factors influencing restrained drying shrinkage cracking are discussed in the subsections of Section 2.4.6.

2.4.1 Overview Bonded overlays are subjected to differential deformations that can cause cracking of the overlay, as discussed in detail in Section 2.3. The stresses develop within the overlay as a result of the contracting movement (shrinkage) of the newly cast overlay which is to some extent restrained by the substrate and/or periphery in the case of an enclosed overlay. The induced stresses are therefore due to the inherent shrinkage of the overlay and are proportional to the elastic modulus of the overlay material. A portion of the induced tensile stresses are however alleviated by tensile relaxation, a stress-relieving viscoelastic property of the overlay. If the resultant tensile stresses induced within the overlay are greater than the tensile strength of the overlay then cracking of overlay, termed restrained shrinkage cracking, will start to develop.

As previously noted, restrained shrinkage cracking occurs due to shrinkage which induces tensile stresses in the overlay that are proportional to the elastic modulus overlay however they are the somewhat reduced by tensile relaxation of the overlay. Shrinkage of concrete is therefore the driving force behind the cracking. There are a variety of different types of shrinkage (see Section 2.4.2), and as a result, restrained shrinkage cracking may be attributed to different types of shrinkage. Restrained shrinkage cracking commonly occurs due to either plastic shrinkage which results in restrained plastic shrinkage cracking, or drying shrinkage which results in restrained drying shrinkage cracking. These two types of restrained shrinkage cracking are briefly discussed respectively in the paragraph that follows.

Restrained plastic shrinkage cracking occurs within a few hours of placing concrete when the concrete is still in its plastic state. It is largely as a result of plastic shrinkage which occurs when rapid evaporation of bleed water which leads to drying of the concrete surface, causing menisci to form between solid particles and capillary tension forces to be set up resulting in contraction


17 (shrinkage) of the cement paste (see Section 2.4.2.1). Restrained drying shrinkage cracking may start to occur after approximately 24 hours of the placement of the concrete when the concrete is in its hardened state. It is largely as a result of drying shrinkage which occurs when water is lost to the environment resulting in capillary tension, disjoining pressure, surface tension and movement of interlayer water, all of which lead to the contraction (shrinkage) of the cement paste (see Section 2.4.2.4). Attention will largely be paid to restrained drying shrinkage cracking in the sections that follow.

2.4.2 Shrinkage The inherent shrinkage of concrete is the key deformation that leads to failure of bonded overlays (cracking or debonding) (see Section 2.3) (Pigeon & Bissonnette, 1999; Beushausen & Alexander, 2006). This occurs when the shrinkage of bonded overlays is restrained and stresses that are sufficiently large to cause cracking develop within the overlay, as discussed in Section 2.3.3.1. The shrinkage of a repair material is therefore an important material property with regards to restrained shrinkage cracking as it directly influences the magnitude of the stresses induced within the overlay.

Shrinkage is a non-load induced, time-dependent decrease in the volume of concrete, during its fresh and hardened state, due to the movement of moisture within or out of the concrete. The total volume decrease of concrete as a result of moisture movement or loss is known as the total shrinkage. It comprises of five broad types of shrinkage, namely plastic, chemical, autogenous, drying, and carbonation shrinkage. The two main contributors to the total shrinkage of bonded concrete overlays, however, are plastic and drying shrinkage which are both due to moisture diffusion (Alexander & Beushausen, 2009). Furthermore, the focus of this research is related to restrained shrinkage cracking due to drying shrinkage. For these reasons, drying shrinkage is discussed in more detail.

The various types of shrinkage (plastic, chemical, autogenous, drying, and carbonation shrinkage) are discussed in Sections 2.4.2.1 through 2.4.2.5 and are presented in the order in which they typically occur in hardened concrete. For each type of shrinkage, the associated mechanisms and the influencing factors are noted.

2.4.2.1 Plastic Shrinkage Plastic shrinkage, also known as early-age or capillary shrinkage, is the decrease in volume of concrete due to the rapid removal of water from concrete during early ages when the concrete is still in its plastic state. It therefore typically occurs within the first few hours of placement but may occur up to around 24 hours after placement of the concrete (Banthia & Gupta, 2009; Theiner & Hofstetter, 2012). Plastic shrinkage is often accompanied by surface cracking, known as plastic shrinkage cracking.

In fresh concrete, the solid particles (particles and aggregates) are covered by a layer of bleed water and the spaces between the solid particles form a system of interconnected pores that are completely filled with water (Mindess et al., 2003; Schmidt & Slowik, 2013). The layer of bleed water may be removed by evaporation, absorbed by dry substrates and/or lost to absorbent


18 formwork. If it is removed faster than it can be replaced by bleeding water (i.e. when the rate of evaporation is greater than the rate of bleeding), it will begin to decrease in thickness until it eventually diminishes. When the solid particles are no longer covered by the layer of bleed water, menisci are formed between solid particles at the surface of the concrete. The curvature of the water surface as a result of the menisci formation causes negative capillary pressures to develop. The pressure consolidates the solid particles and forces pore water to the surface of the concrete, resulting in a volume reduction of the fresh concrete known as plastic shrinkage (Mindess et al., 2003; Schmidt & Slowik, 2013).

The rate of water evaporation of the bleed water, and hence the magnitude of plastic shrinkage, is strongly dependent on the environmental conditions, increasing with higher concrete temperatures, higher ambient temperatures, lower relative humidities, and higher wind velocities (see Section 2.5.2). It should however be noted that there is still a risk of plastic shrinkage in cool weather if the wind velocities are high (Kellerman & Crosswell, 2009). To minimise the likelihood and magnitude of plastic shrinkage, it is generally agreed upon that evaporation rates should not exceed 1.0 kg/m2/h (Neville, 2004). It should however be noted that the relationship between the rate of water loss and the magnitude of plastic shrinkage is not straightforward as the magnitude of plastic shrinkage is not only dependent on the rate of water loss but also the rigidity of the concrete mix in its plastic state (Neville, 2004). Set retarding admixtures therefore have the effect of allowing more bleeding by delaying the setting time however result in increased plastic shrinkage as the concrete is in a plastic state for longer (Neville, 2004). In general, plastic shrinkage is increased for concrete mixes with higher water or cement contents due to the increased paste content in which the shrinkage occurs (Neville, 2004). Additionally, concrete mixes that have lower w/b ratios and/or contain fine fillers would be expected to bleed less and experience a greater magnitude of plastic shrinkage (Neville, 2004).

2.4.2.2 Chemical Shrinkage Chemical shrinkage, also known as hydration or endogenous shrinkage, is the absolute volume reduction of concrete due to cement hydration. It occurs as a result of the reactions between cement and water, known as cement hydration, which results in a volume reduction. The basic reactions of Portland cement clinker involve four reactions of tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium alumina (C3A), and tetracalcium alumina ferrite (C4AF). Each of these reactions requires water (H2O), is exothermic and is associated with a decrease in volume (Holt, 2

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