Corrosion of steel in cracked concrete: Chloride microanalysis and service life predictions

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C

ORROSION OF STEEL IN CRACKED CONCRETE

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C

ORROSION OF STEEL IN CRACKED CONCRETE

CHLORIDE MICROANALYSIS AND SERVICE LIFE PREDICTIONS

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 9 juni 2015 om 12:30 uur

door

José PACHECO

FARÍAS

Civiele Techniek

aan Technische Universiteit Delft geboren te Monterrey, México.

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Dit proefschrift is goedgekeurd door de promotor: Prof. dr. R. B. Polder

Copromotor: Dr. O. Çopuro˘glu Samenstelling promotiecommissie:

Rector Magnificus, voorzitter

Prof. dr. R. B. Polder, Technische Universiteit Delft, promotor Dr. O. Çopuro˘glu, Technische Universiteit Delft, copromotor Prof. dr. ir. E. Schlangen Technische Universiteit Delft

Prof. dr. M. R. Geiker Norges teknisk-naturvitenskapelige universitet, Noorwegen Dr. M. C. Alonso Consejo Superior de Investigaciones Científicas, Spanje Prof. dr. R. D. Hooton University of Toronto, Canada

Prof. dr. L. L. Sutter Michigan Technological University, Verenigde Staten Prof. dr. ir. K. van Breugel, Technische Universiteit Delft, reservelid

The work reported in this Thesis is part of the STW project "Measuring, Modelling, and Monitoring Chloride ingress and the initiation in Cracked Concrete (M3C4)" (code No. 10978), which is part of an STW Perspectief Program "Integral Solutions for Sustainable Construction (IS2C)".

Cover design: K. Rosales

An electronic version of this dissertation is available at http://repository.tudelft.nl/.

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P

REFACE

This doctoral thesis is submitted to the Delft University of Technology for the degree of Philosophiae Doctor (PhD). The work contained herein was carried out at the Depart-ment of Materials and EnvironDepart-ment, Faculty of Civil Engineering ad Geosciences at Delft University of Technology in Delft, The Netherlands.

The main supervisor (promotor) has been Professor Dr. Rob B. Polder (Delft Univer-sity of Technology / TNO); and the co-supervisor has been Dr. O˘guzhan Çopuro˘glu (Delft University of Technology). This PhD project started in September 2010 and completed for publication in June 2015. The project was part of the program Integral Solutions for Sustainable Construction (IS2C) funded by the Dutch Technology Foundation (STW). This dissertation contributes to the project M3C4 (Measuring, Modelling and Monitor-ing Chloride Monitor-ingress and Corrosion initiation in Cracked Concrete), which is part of IS2C. The author, José Pacheco, declares that this thesis and the work presented in it are his own and have been generated by him as the result of original research while in candida-ture for the degree of Philosophiae Doctor at Delft University of Technology. The thesis contains no material that was previously submitted for a degree at this university or any other institution.

José Pacheco Farías Delft, June 2015

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L

IST OF

F

IGURES

1.1 Service life design of concrete infrastructure . . . 2

1.2 Service life modelling of reinforcement corrosion . . . 3

1.3 Construction of concrete bridges per year in The Netherlands, c. 2004. [5] 4 1.4 M3C4 project . . . 5

1.5 Thesis outline . . . 6

2.1 by Pore size distribution in hydrated cement paste [10] . . . 11

2.2 Pourbaix diagram for Fe [31] in aqueous solutions. . . 15

2.3 Chloride induced corrosion pits [9] . . . 16

2.4 Pit initiation and propagation. . . 17

2.5 Electrochemical open circuit potential measurement. . . 18

2.6 Relationship between electrochemical potential and corrosion current den-sity [9] . . . 20

2.7 Two Electrode Method for measuring concrete resistivity [9] . . . 21

2.8 Scanning electron microscope [49]. . . 23

2.9 Schematics of electron-solids interactions . . . 24

2.10 Imaging modes in SEM (FOV 255 x 173 µm). a) Secondary electron (SE) micrograph; b) Back-scattered electron (BSE) micrograph . . . 25

2.11 Cracking of concrete elements by the Concrete Society [83] . . . 27

2.12 Secondary (internal) cracking at the concrete-steel interface [84] . . . 28

2.13 Influence of cracking on chloride penetration . . . 30

2.14 Different specimen configurations for measuring corrosion of steel in cracked concrete . . . 32

2.15 Instrumented rebar by Pease [85]. . . 33

2.16 Corrosion products formed in concrete [72] . . . 35

3.1 Specimen geometries. . . 50

3.2 Schematics of sectioned samples for Cl penetration measurements . . . . 52

3.3 Pre-conditioning of the Round-Robin Series. a) AgNO3sprayed on sec-tioned concrete samples; b)Measuring the penetration depth. . . 52

3.4 Electrochemical potential of specimens in solution (SCE electrode was em-ployed) . . . 53

3.5 Sampling for chloride contents in PC and BFS series . . . 54

3.6 Sampling for chloride content analysis in RRT series. Dashed lines indicate sectioning. . . 54

3.7 Electrochemical potential of steel reinforcement embedded in PC speci-mens during exposure in 3.3% NaCl.. . . 55

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viii LIST OFFIGURES 3.8 Electrochemical potential of steel reinforcement embedded in BFS

speci-mens during exposure in 3.3% NaCl . . . 56

3.9 Electrochemical potential of steel reinforcement embedded in RRT speci-mens during exposure in 3.3% NaCl . . . 57

3.10 Chloride profiles of PC specimens after 31 - 195 days, see Table 3.5. . . 59

3.11 Chloride profiles of BFS specimens after 234 days of age.. . . 60

3.12 Chloride profiles of RRT specimens after 364 days of age, except RR-10 (631 days). . . 62

3.13 Corrosion pit found in specimen PC-2 after extraction.. . . 63

3.14 Corrosion pit found in specimen PC-4 after extraction.. . . 63

3.15 Defects observed in PC-9 due to improper compaction. PC-10 had similar defects. . . 64

3.16 Surface condition of steel in BFS specimens. No visible signs of pitting cor-rosion. Steel surface still had the as-received appearance . . . 65

3.17 Surface condition of steel in RRT specimens. Dark grey colour on steel sur-face after pre-rusting and subsequent embedding in concrete. . . 65

4.1 Monte Carlo simulatons of interaction volume. . . 77

4.2 EDS spectra of chlorine bearing minerals . . . 78

4.3 Wet chemical analysis of cement paste samples . . . 80

4.4 Cl quantification in cement paste samples by different standards. . . 82

4.5 Statistical summary of Cl quantification by EDS . . . 84

4.6 Normality test for Cl quantification in cement paste specimens with 20 analyses . . . 85

4.7 Normality test for Cl quantification in cement paste specimens with 50 analyses . . . 86

4.8 EDS spectra of cement paste (a) and cement paste with added chlorine (b) 87 5.1 Schematics of the HPHT set-up and the assembly inside the pressure plate 97 5.2 Stereomicrograph of HPHT-1 experiment (FOV 4548 x 3410 µm) . . . 100

5.3 Montage of BSE images of HPHT-1 experiment . . . 101

5.4 Increased magnification on phases in HPHT-1 (FOV: 107 x 54 µm each) . . 102

5.5 EDS spectra of crystals and matrix in HPHT-1 . . . 103

5.6 Stereomicrograph of HPHT-2 experiment (FOV 4548 x 3410 µm) . . . 106

5.7 Montage of BSE images of HPHT-2. . . 107

5.8 Increased magnification on phases in HPHT-2 FOV: 215 x 180 µm . . . 108

5.9 HPHT-2 – 1100±_{C - 1 GPa - 30 min.} _{. . . 109}

5.13 HPHT-3 1200±_{C - 1 GPa - 30 min} _{. . . 114}

5.17 EDS spectra of HPHT-4 1500±_{C - 1 GPa - 30 min}_{. . . 119}

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LIST OFFIGURES ix

5.20 Increased magnification n HPHT-5 FOV: 215 x 180 µm . . . 123

5.21 EDS spectra of 100 analyses in HPHT-5 . . . 123

5.22 FT-IR of glass. . . 124

5.23 Cl content by EDS of paste samples in Chapter 5 . . . 127

5.24 Chloride profiles of PC specimens by titration and microanalysis. . . 128

6.1 Reinforced concrete specimens subject to cracking . . . 134

6.2 COD-Load curve for PC specimens . . . 137

6.3 COD-Load curve for BFS specimens . . . 138

6.4 COD-Electrical resistance of PC specimens. . . 139

6.5 COD-Electrical resistance of BFS specimens . . . 140

6.6 Relationship between COD and relative increase in electrical resistance of PC and BFS specimens . . . 141

6.7 Relationship between COD and relative increase in electrical resistance from the lattice model . . . 142

6.9 Lattice simulations of the influence of cracks on electrical potentials . . . 144

6.10 Crack geometry of PC specimens. . . 146

6.11 Crack geometry of PC specimens. . . 147

6.12 Relationship between COD and estimated crack volume. . . 149

6.13 Relationship between relative increase in electrical resistance and estimated crack volume . . . 150

7.1 Service life modelling of reinforcement corrosion . . . 156

7.2 Influence of cracks on chloride penetration and reinforcement corrosion. 157 7.3 Concrete-steel interface for microanalysis studies. . . 161

7.4 Sample preparation for microanalysis studies on the concrete-steel interface161 7.5 Electrochemical potential of steel reinforcement in PC specimens . . . 163

7.6 Electrochemical potential of steel reinforcement in BFS specimens . . . . 163

7.7 Corrosion rate of steel reinforcement in PC specimens . . . 164

7.8 Corrosion rate of steel reinforcement in BFS specimens . . . 165

7.9 Electrical resistance of PC specimens . . . 166

7.10 Electrical resistance of BFS specimens. . . 166

7.11 Surface condition of bars extracted from PC specimens . . . 168

7.12 Surface condition of bars embedded in BFS specimens. . . 170

7.13 Different corrosion pit geometries found during visual inspection of cracked specimens . . . 171

7.14 Estimation of pit area and pit volume in PC specimens . . . 172

7.15 Estimation of pit area and pit volume in BFS specimens . . . 173

7.16 Chloride concentrations at the concrete-steel interface opposite to corro-sion pits . . . 174

7.17 Corrosion control mechanisms, [27] . . . 177

7.18 Estimated evolution of pit area at early stage . . . 179

8.1 Relationship between chloride ingress and time to corrosion initiation,[19] 185 8.2 Conceptual model for chloride ingress in sound and cracked concrete. . . 186

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x LIST OFFIGURES 8.3 Influence of cracks in concrete cover on chloride transport, i.e. t = 10 years. 187 8.4 Chloride ingress in sound and cracked concrete expressed as the concept

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L

IST OF

T

ABLES

1.1 Highlighted topics related to this Thesis and their relation to service life of

concrete structures . . . 6

2.1 Cement types according to EN 197-1 [4]. . . 10

2.2 Chloride limits for fresh concrete, wt. % of cement . . . 14

2.3 Standard potentials of reference electrodes in concrete [39] . . . 18

2.4 Probability of reinforcement corrosion based on OCP measurements on the concrete surface [42] . . . 19

2.5 Classification of reinforcement corrosion by corrosion rate measurements 20 2.6 Concrete resistivity (≠m) of existing concrete structures > 10 years, [45]. . 22

2.7 Backscattering coefficient (¥) in cementitious materials and other phases, after [57] . . . 26

2.8 Maximum allowed crack width for reinforced concrete structures exposed to chloride contaminated environment . . . 28

3.1 Characteristic values of critical chloride content Ccr i t according to Du-raCrete [3], by wt. % binder . . . 47

3.2 Parameters required for estimating the time to corrosion initiation accord-ing to DuraCrete. . . 48

3.3 Cement composition, wt. % of cement . . . 49

3.4 Concrete composition . . . 51

3.5 Chloride content by mass of cement (%) and diffusion coefficient (m2_s°1₎ in PC specimens exposed at an age of 28 days . . . 58

3.6 Chloride content by mass of cement (%) and diffusion coefficient (m2_s°1₎ in BFS specimens, exposed after 28 days . . . 60

3.7 Chloride content by mass of cement (%) and diffusion coefficient (m2s°1₎ in RRT specimens, exposed after 35 days . . . 61

3.8 Summary of results . . . 67

4.1 Cement composition, wt. % oxide . . . 75

4.2 Mineral standards for EDS analysis in cement paste (ASTIMEX MINM25-53) 79 4.3 Theoretical compositions of mineral standards and Cl concentrations for EDS analysis . . . 79

4.4 Mineral composition of standards for Cl quantification based on XRF anal-ysis, wt.%. . . 79

4.5 Statistical summary of Cl quantification by EDS . . . 83

4.6 Examples of experimental parameters in literature on the use of quantita-tive EDS analysis in cement and concrete research . . . 88

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xii LIST OFTABLES

5.1 IR bands of hydrated cement [13] . . . 95

5.2 HPHT experimental conditions. . . 96

5.3 Mineral standards employed in WDS . . . 98

5.4 Chemical composition of matrix and crystals in HPHT-1 by WDS, wt. % oxides . . . 104

5.7 Chemical composition of matrix and pore inclusions in HPHT-4 by WDS, wt. % oxides . . . 115

5.8 Chemical composition of HPHT-5 by WDS . . . 120

5.9 Cross-referencing of minerals and the synthesised glass, Cl wt. % interac-tion volume (relative error %) . . . 125

5.10 Mean Atomic number ¯Z and backscattering coefficient ¥ comparison be-tween C-S-H and minerals. . . 126

5.11 Relative error (%) between C-S-H and minerals for ¯Z and ¥ . . . 126

5.12 Dc(x10°12m2s°1) and Ccr (% wt. cement / int. volume) at rebar level of PC specimens obtained by titration and EDS . . . 128

6.1 Concrete composition . . . 135

6.2 Estimations of crack volume in PC and BFS specimens . . . 148

7.1 Concrete mix design . . . 159

7.2 Area and volume estimations of corrosion pits in PC specimens . . . 169

7.3 Area and volume estimations of corrosion pits in BFS specimens. . . 172

8.1 Values of DRC M in uncracked PC and BFS concrete according to [1] and used in the present study. . . 190

8.2 Recommended DRC M x 10°12m2s°1values for 100 years design service life of a reinforced concrete structure with 60 mm of concrete cover. Bold values are higher than the maximum values recommended in the guideline.190 8.3 Influence of initial DRC M x 10°12 m2s°1 on tolerable wk/wli mi t ratios. Bold numbers indicate values of Dcr,RC Mthat are higher than the recom-mended value for the corresponding exposure class. . . 191

8.4 Parameters for predictions of time to corrosion initiation in cracked concrete192 8.5 Predicted time to corrosion initiation (years) for PC and BFS concrete mix-tures including cracking.. . . 193

8.6 Effective concrete cover xe f f (mm) in cracked concrete compared to the original cover x0for different wk/ wli mi t ratios and their corresponding predicted ti. . . 194

8.7 Parameters for estimations of the time to corrosion initiation in cracked concrete according to error function model described by Eq. 8.1.. . . 195

8.8 Observed time to corrosion initiation of cracked concrete reported in Chap-ter 7 and predicted time to corrosion initiation with Eq 8.3. . . 196

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C

ONTENTS

List of Figures vii

List of Tables xi

1 Introduction 1

1.1 Durability of concrete structures . . . 2

1.1.1 Motivation for research . . . 3

1.1.2 Objective of the thesis . . . 4

1.2 M3C4: Measuring, modelling and monitoring chloride ingress and corro-sion initiation in cracked concrete . . . 5

1.2.1 Outline of the thesis . . . 5

References. . . 8

2 Literature study 9 2.1 Concrete . . . 10

2.1.1 Portland cement and other types . . . 10

2.1.2 Cement hydration and pore structure . . . 10

2.1.3 Transport properties of hardened concrete . . . 12

2.1.4 Critical chloride concentration. . . 13

2.1.5 Critical chloride content and service life. . . 14

2.2 Corrosion of steel in concrete. . . 14

2.2.1 Initiation Period . . . 14

2.2.2 Propagation Period . . . 17

2.2.3 Electrochemical open circuit potential. . . 17

2.2.4 Corrosion rate . . . 19

2.2.5 Concrete resistivity. . . 20

2.3 Scanning electron microscopy of cementitious materials. . . 22

2.3.1 Electron-solid interactions. . . 22

2.3.2 Microanalysis . . . 24

2.4 Cracking of concrete . . . 27

2.4.1 Chloride transport in cracked concrete . . . 28

2.4.2 Corrosion of reinforcement in cracked concrete. . . 29

2.4.3 Cracks and their influence on concrete durability . . . 31

2.5 Summary and conclusions . . . 35

References. . . 36

3 Corrosion initiating chloride concentrations in concrete 43 3.1 Introduction . . . 44

3.1.1 Ccr i tin the literature. . . 45

3.1.2 Ccr i tand service life modelling . . . 46

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xiv CONTENTS

3.2 Experimental program . . . 48

3.2.1 Materials and specimen preparation. . . 48

3.2.2 Chloride content analysis . . . 53

3.3 Results . . . 55

3.3.1 Electrochemical potential . . . 55

3.3.2 Chloride concentrations. . . 57

3.3.3 Visual inspection of concrete-steel interface. . . 62

3.4 Discussion . . . 66

3.4.1 Electrochemical potentials. . . 66

3.4.2 Chloride concentrations. . . 66

3.4.3 Influence of pre-rusting or pre-conditioning, RRT series. . . 66

3.4.4 Influence of measuring technique . . . 67

3.5 Summary. . . 68

3.6 Conclusions. . . 69

References. . . 70

4 Quantitative Energy-Dispersive X-ray microanalysis of chlorine in cement paste 73 4.1 Introduction . . . 74

4.2 Experimental procedure . . . 75

4.2.1 Materials. . . 75

4.2.2 Fabrication of specimens . . . 75

4.2.3 Wet chemical analysis . . . 76

4.2.4 EDS analysis. . . 76

4.3 Results . . . 80

4.3.1 Wet chemical analysis . . . 80

4.3.2 Cl quantification by EDS. . . 80

4.3.3 Precision and accuracy. . . 81

4.4.1 ZAF: Matrix effects between minerals and cement paste. . . 84

4.4.2 Influence of experimental parameters on EDS microanalysis . . . . 86

4.4.3 Porosity of hydrated cement. . . 87

4.5 Summary. . . 88

References. . . 90

5 A synthesised glass standard for microanalysis of cementitious materials 93 5.1 Introduction . . . 94

5.2 Experimental details . . . 95

5.2.1 Materials. . . 95

5.2.2 High-pressure/High-temperature experiments . . . 96

5.2.3 Sample preparation for microanalysis studies . . . 98

5.2.4 Microanalysis studies on chlorine quantification . . . 98

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CONTENTS xv

5.3 Results . . . 99

5.3.1 HPHT experiments. . . 99

5.3.2 Presence of hydrated matrix: FTIR. . . 124

5.4.1 Cross-checking of minerals . . . 125

5.4.2 Performance of the standard vs minerals. . . 126

5.4.3 Microanalysis studies on chloride profiles. . . 127

5.5 Summary. . . 129

References. . . 130

6 Investigation of cracking of concrete by means of electrical resistance and image analysis 131 6.1 Introduction . . . 132

6.2.1 Materials and specimen fabrication . . . 133

6.2.2 Cracking of specimens. . . 135

6.2.3 Lattice model for cementitious composites . . . 135

6.2.4 Estimation of crack volume by image analysis. . . 136

6.3 Results . . . 137

6.3.1 Cracking of reinforced specimens . . . 137

6.3.2 Monitoring of electrical resistance during cracking . . . 138

6.3.3 Lattice model simulations . . . 140

6.3.4 Crack geometry assessment . . . 145

6.3.5 Estimation of crack volume . . . 145

6.5 Summary. . . 151

References. . . 152

7 Influence of cracking of the concrete cover on corrosion initiation of steel reinforcement 155 7.1 Introduction . . . 156

7.2.1 Materials and specimen fabrication . . . 159

7.2.2 Cracking of specimens. . . 160

7.2.3 Exposure conditions and monitoring . . . 160

7.2.4 Visual assessment and estimations of pit volume . . . 160

7.2.5 Sample preparation for microanalysis studies . . . 161

7.2.6 Microanalysis studies on chlorine quantification . . . 162

7.3 Results . . . 162

7.3.1 Electrochemical potential . . . 162

7.3.2 Corrosion rate . . . 164

7.3.3 Electrical resistance . . . 165

7.3.4 Visual assessment of bars and estimation of pit volume . . . 167

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xvi CONTENTS

7.3.6 Microanalysis studies on concrete subject to reinforcement

corro-sion . . . 173

7.4.1 Electrochemical potential and corrosion rate . . . 175

7.4.2 Electrical resistance . . . 176

7.4.3 Visual inspection of reinforcing steel. . . 177

7.4.4 Pit dimension estimations. . . 178

7.4.5 Microanalysis studies of the concrete-steel interface. . . 179

7.5 Summary. . . 180

References. . . 181

8 Service life predictions of cracked concrete structures 183 8.1 Introduction . . . 184

8.2 Incorporating cracks in chloride transport in concrete . . . 185

8.3 Service life design of cracked concrete . . . 188

8.4 Model Validation . . . 190

8.4.1 Influence of cracks on the diffusion coefficient . . . 190

8.4.2 Incorporating cracks in the design service life of concrete structures 192 8.5 Validation of model with experimental results from Chapter 7 . . . 195

8.6 Summary. . . 196 8.7 Conclusions. . . 197 References. . . 198 9 Conclusions 201 9.1 Summary. . . 202 9.2 Conclusions. . . 205

9.3 Recommendations for further research. . . 207

Summary 209

Samenvatting 213

Resumen 217

Curriculum Vitæ 221

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1

I

NTRODUCTION

The durability of reinforced concrete structures is a concern in the coming decades. In Western countries, concrete infrastructure is expected to reach their intended service life in the coming decades. Aside from degradation mechanisms, cracking in concrete structures could be a determining factor when estimating the service life of concrete structures. In this chapter, the basic concepts of concrete durability and service life are given. The motivation for conducting this research stems from the influence of cracks in service life predictions.

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1

2 1.INTRODUCTION

1.1. D

URABILITY OF CONCRETE STRUCTURES

Reinforced concrete has been used as the main construction material since the early 1900’s. The reasons promoting the use of reinforced concrete include its lower costs compared to other materials, a.o. steel, and its resistance to weathering. Initially, it was believed that the concrete-steel system would endure environmental actions while maintaining its mechanical performance intact. At the beginning, the resistance of con-crete to environmental action was tough to be permanent. It was later found that this re-sistance was not permanent. The concept of concrete durability was therefore conceived [1–3,6,7,9,13]. Reinforced concrete durability is, therefore, related to the resistance of concrete to degradation mechanisms that reduce its performance over time. The pe-riod of time in which concrete infrastructure performance complies with the reliability theory traditionally used in structural design is known as service life (see Figure1.1).

Probabilistic service life modelling considers both the resistance of concrete struc-tures R(t)and the loads S(t)applied on them as probabilistic distributions that change

over time. New structures can be designed on the basis of durability by considering prob-abilistic distributions of resistance, loads and the environmental action. During the de-sign of new structures, both R(t) and S(t) distributions are apart from each other due

to the application of safety factors [1]. The effect of environmental action S(t)increase

continuously due to weathering whilst a decrease in the resistance due to different dete-rioration mechanisms occurs during the same period. Over time, the distribution of R(t)

and S(t)reach a point where the target probability of failure, as described during the

de-sign process, is achieved [2]. This is also known as limit state. In the utmost case, leaving structures untreated can lead to collapse [3].

Mean Service life

Time [years]

Service life of concrete infrastructure

R S R(t) S(t) [1] [2] [3] S(t): Mechanical Environmental - Chlorides R(t): Strength Transport - Microstructure Design Assessment Maintenance / Repair

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1.1.DURABILITY OF CONCRETE STRUCTURES

1

3

Service life of concrete structures can be divided into two different concepts. Design Service life predicted from initial material properties, execution aspects and environ-mental conditions that follow a probabilistic approach; and remaining service life which determines the future performance of the structure derived from its current conditions. Tuutti published the first conceptual model for service life of concrete infrastruc-ture based on reinforcement corrosion in 1982[12]. Later the fib model updated Tuutti’s model by incorporating intermediate limit states within the propagation period. Both models are given in Figure1.2

Initiation Propagation Collapse Level of deterioration Corrosion onset Corrosion deterioration

Service life of concrete infrastructure based on corrosion deterioration

Time [years] (a) by Tuutti [12] Cracking Spalling Collapse Initiation Propagation Level of deterioration Corrosion onset

Service life of concrete infrastructure based on corrosion deterioration

Time [years]

(b) by fib [4]

Figure 1.2: Service life modelling of reinforcement corrosion

Tuutti’s model considers that the service life of concrete due to reinforcement corro-sion is composed of two periods. The initiation period, which includes the period of time in which the conditions or agents involved in the corrosion mechanism are penetrating or reacting. The second period called propagation, is the period where the steel degrada-tion is actively occurring with potential damage to the concrete. The fib model revised Tuutti’s model by incorporating intermediate steps within the propagation period. In both models, the initiation period ends with the onset of reinforcement corrosion.

1.1.1. M

OTIVATION FOR RESEARCH

In Western European countries, most of the existing concrete bridges were built during the second half of the last century. In The Netherlands, the majority of concrete bridges were built between the 1960’s and the 1980’s (see Figure1.3, [5]). Evidently, most of the bridges in The Netherlands were designed and built before the concept of concrete dura-bility was conceived in the early 1980’s and later developed into reliadura-bility analysis and design for concrete durability [10,11]. Gaal studied the occurrence of reinforcement corrosion in concrete bridges in the Netherlands [5] and found that 5% of bridges had chloride induced reinforcement corrosion when in service for 40 years. In the case of bridges with 70 or more years of service, the percentage of structures affected by corro-sion was 50%. It is therefore anticipated that during the coming decades, a significant

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1

4 1.INTRODUCTION

number of concrete bridges will present signs of degradation, particularly due to rein-forcement corrosion [8]. Apparently, the time to corrosion initiation and its subsequent propagation exhibit a wide variation across a given population of bridges. In this sense, accurate predictions of the remaining service life of existing concrete structures will be necessary.

Figure 1.3: Construction of concrete bridges per year in The Netherlands, c. 2004. [5]

1.1.2. O

BJECTIVE OF THE THESIS

This thesis aims to contribute on two major topics. First, research on the quantification of chlorides in cementitious materials with particular interest in the concentration of chlorides that initiate reinforcement corrosion (Ccr i t) was performed. Investigations on

chloride concentrations have been performed with bulk (wet chemical analysis) and mi-croanalytic techniques (scanning electron microscope). This Thesis discusses the impli-cations on differences in accuracy of bulk and microanalytic determination of corrosion initiating chloride concentrations.

The second part of the Thesis is focused on the influence of cracking of the concrete cover on reinforcement corrosion. Normally, structural codes control allowable crack width in concrete infrastructure depending on exposure conditions. However, when de-termining the time to corrosion initiation, service life models consider concrete to be ’sound’ and crack free. This part of the Thesis studies the shape of cracks occurring when reinforced concrete is subject to tensile stress; followed by monitoring the behaviour of reinforcement corrosion in cracked concrete specimens; and finally, the consequences of incorporating cracks in service life modelling.

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1.2.M3C4: MEASURING,MODELLING AND MONITORING CHLORIDE INGRESS AND CORROSION INITIATION IN CRACKED CONCRETE

1

5

1.2. M3C4: M

EASURING

,

MODELLING AND MONITORING CHLO

-RIDE INGRESS AND CORROSION INITIATION IN CRACKED

CONCRETE

This Thesis contributes to a STW (Dutch Foundation for Applied Research) research campaign on Integral Solutions for Sutainable Construction, IS2C. The project entitled M3C4: Measuring, modelling and monitoring chloride ingress and corrosion initiation in cracked concrete is composed of three PhD projects as shown in Figure1.4.

Figure 1.4: M3C4 project

Parts of this PhD thesis have been carried out in collaboration with PhD 1 chloride ingress in cracked concrete. The dissertation of Branko Šavija entitled Experimental and Numerical Investigation of Chloride Ingress in Cracked Concrete [14] contributes to the influence of cracks on chloride ingress in concrete. The second PhD project by Jinping Han is focused on the transport of chlorides and moisture by non-destructive techniques such as Nuclear Magnetic Resonance (NMR) [15]. The dissertation derived from the lat-ter will be presented in the near future.

1.2.1. O

UTLINE OF THE THESIS

This thesis is composed of several Chapters organised as in Figure1.5. Highlighted topics in this thesis can be found in Table1.1. The Introduction chapter describes briefly the concept of service life design of concrete infrastructure and reinforcement corrosion that will be covered in the thesis.

Chapter 2 presents a literature survey and fundamentals of reinforcement corrosion. It also includes the current state of the art regarding chloride ingress and corrosion of steel in cracked concrete, followed by a brief introduction to microanalysis. The chapter concludes with the available literature on cracking and its influence on service life of concrete infrastructure.

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1

6 1.INTRODUCTION

SERVICE LIFE OF CRACKED CONCRETE

INITIATION PERIOD PROPAGATION PERIOD

Chapter 3 RILEM CTC Ccrit

Chapter 6 Electrical resistance of cracks and vis. analysis

Chapter 7 Cracked concrete corrosion behaviour Chapter 4 Microanalysis of Cl in cement paste Chapter 5 HPHT experiments Synthetic ref. mineral

Chapter 8 Service life modelling

of cracked concrete

Figure 1.5: Thesis outline

Chapter 3 presents the test method devised by the RILEM Technical Committee 235-CTC. Using this test, three different series of reinforced concrete specimens were subject to chloride penetration. During chloride ingress, the electrochemical potential of em-bedded reinforcement was monitored. The critical chloride concentration (Ccr i t) was

determined after the corrosion onset. The Chapter concludes by comparing the three different series of specimens and presenting the influence of Ccr i ton estimations of

ser-vice life.

Chapter 4 reviews the application of quantitative microanalysis of chloride in cemen-titious materials. Cement paste samples with five different chloride concentrations were prepared. The quantification of chlorine in the cement pastes was performed by wet chemical analysis and microanalysis. Five different chlorine bearing rock-forming min-erals containing chlorine were used as microanalysis standard for analysing X-ray spec-tra. The Chapter concludes with discussing the influence of selecting of rock-forming Table 1.1: Highlighted topics related to this Thesis and their relation to service life of concrete structures

Durability parameter Corrosion initiation Corrosion propagation Chapter

Chloride penetration Yes No 3,5

Critical chloride content Yes No 3,8

Chloride quantification Yes No 3,4,5

Electrochemical potential Yes Yes 3,7

Corrosion rate No Yes 7, 8

Electrical resistance/resistivity Yes Yes 3,4,7,8

Pit dimensions No Yes 7

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1.2.M3C4: MEASURING,MODELLING AND MONITORING CHLORIDE INGRESS AND CORROSION INITIATION IN CRACKED CONCRETE

1

7 minerals as reference standards on quantitative microanalysis. Also, it provides an overview on the application of microanalysis in cement and concrete research.

Chapter 5 describes the development of a reference standard for microanalysis for chloride in cementitious materials. High-pressure and high-temperature experiments were carried out in collaboration with VU Amsterdam in order to synthesise a cement-like amorphous matrix. Five different HPHT experiments were carried out at different temperatures. Products from each experiment were then subject to stereo-microscopic and microanalysis (EDS and WDS) studies, followed by qualitative detection of bound water in the amorphous matrix. The chapter concludes in comparing the product of one HPHT experiment with the rock-forming minerals presented in Chapter 4.

Chapter 6 is focused on the assessment of cracks in concrete by means of electrical resistance and image analysis. Reinforced concrete specimens were subject to cracking. During the cracking process, the electrical resistance across the crack was monitored. Subsequently, the cracks were impregnated with a fluorescent resin and the crack ’area’ and crack ’volume’ were estimated by image analysis.

Chapter 7 studies the influence of cracks on reinforcement corrosion and their re-lationship with corrosion propagation. Following the test set-up presented in Chapter 6, reinforced concrete specimens were subject to cracking and the to cyclic exposure to chloride penetration. During the cyclic exposure, the behaviour of reinforcement cor-rosion in embedded bars was monitored. At the end of exposure, the condition of the concrete-steel interface was assessed by destructive analysis. The accumulated corroded ’area’ and ’volume’ is then related to the crack width.

Chapter 8 overviews the results obtained in this Thesis and in Šavija’s Thesis. First, it studies the influence of using different Ccr i t values and transport properties on

pre-dicted time of corrosion initiation according to DuraCrete [3]. This is performed by us-ing the results obtained in Chapter 3. Also, results from Šavija’s rapid chloride migra-tion tests (RCM) contained in Chapter 3 are included. Finally, this Chapter concludes by proposing a way to incorporate cracking of concrete in service life modelling based also in DuraCrete. The outcome of the proposed model is compared to the results obtained in Chapter 7.

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1

8 REFERENCES

R

EFERENCES

[1] ACI 201. Guide to Durable Concrete, September 2000.

[2] L. Bertolini, B. Elsener, P. Pedeferri, E. Redaelli, and R. B. Polder. Corrosion of steel in concrete:

prevention, diagnosis, repair. John Wiley & Sons,

2013.

[3] DuraCrete. DuraCrete - General Guidelines for Durability Design and Redesign. Technical re-port, The European Union - Brite EuRam III BE95-1347/R17, May 2000.

[4] fib. Model Code for Service Life Design. Techni-cal Report 34, FIB, April 2006.

[5] G. C. M. Gaal. Prediction of Deterioration of

Con-crete Bridges. PhD thesis, Delft University of

Technology, 2004.

[6] P.J. Hovde and K. Moser. Performance Based Methods for Service Life Prediction. Technical report, CIB W080 / RILEM 175-SLM Service Life Methodologies, March 2004.

[7] S. Lay, P Schießl, and J. Cairns. Lifecon deliv-erable d 3.2 - Service life models. Technical re-port, Technical University of Munich, Munich, Germany, 2003.

[8] R.B. Polder, W.H.A. Peelen, and W.M.G. Courage. Non-traditional assessment and maintenance methods for aging concrete structures – techni-cal and non-technitechni-cal issues. Materials and

Cor-rosion, 2012.

[9] S Rostam. Service life design of concrete structures-a challenge to designers as well as to owners. Asian Journal Civil Engineering,

6(5):423–445, 2005.

[10] A.J.M. Siemes and A. van den Beukel. Durability of Buildings: Reliability Analysis. HERON, 30(3), 1985.

[11] A.J.M Siemes, R.B. Polder, and H. de Vries. Design of concrete structures for durability.

HERON, 43(4):227–244, 1998.

[12] K. Tuutti. Corrosion of Steel in Concrete. Tech-nical report, CIB, July 1982.

[13] G. van der Wegen, R.B. Polder, and K. van Breugel. Guideline for service life design of structural concrete -A performance based ap-proach with regard to chloride induced corro-sion. HERON, 57(3):1–16, August 2013.

[14] B Šavija. Experimental and numerical

investiga-tion of chloride ingress in cracked concrete. PhD

thesis, Delft University of Technology, 2014. [15] J Han. Non-destructive real-time transport of

Cl, Na and H2O by Nuclear Magnetic Resonance

(NMR). PhD thesis, Eindhoven University of

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2

L

ITERATURE STUDY

This Chapter describes the basic concepts behind chloride transport and reinforcement corrosion in concrete. First, the characteristics of Portland cement a hydrated cement paste are presented, follow by transport mechanisms in cement paste and concrete. Af-ter the concrete-steel system is described, corrosion inducing mechanism are presented, in particular chloride induced reinforcement corrosion. The most important concepts of reinforcement corrosion and corrosion monitoring are presented an discussed, followed by a general introduction to electron microscopy and microanalysis. Finally, the current State-of-the-Art regarding concrete cracking and its influence on concrete durability are presented. The Chapter concludes with a summary and highlights that will be reviewed in the following Chapters.

Parts of this chapter have been published in Pacheco and Polder, Advances in Modeling Concrete Service Life, 2010 [1]

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2

10 2.LITERATURE STUDY

2.1. C

ONCRETE

2.1.1. P

ORTLAND CEMENT AND OTHER TYPES

Portland cement is composed of four main phases: Tricalcium silicate (C3S), Dicalcium

silicate (C2S), Tricalcium aluminate (C3A), and Ferroaluminate (C4AF). Due to

techno-logical reasons, gypsum (C¯S) is added to complete the main components of Portland cement [2,3]1_{. Other types of cement are based on the addition of supplementary}

ce-mentitious materials including fly ash (FA), ground granulated blast furnace slag (BFS), natural pozzolans and/or silica fume. According to EN 197-1 [4], five types of cement are available in Europe as shown in Table2.1. In The Netherlands, cement with a high content of ground granulated blast furnace slag (around 70%) is commonly used [5,6]. Environmental and durability performance characteristics of blended cements justify their use in the construction industry.

Table 2.1: Cement types according to EN 197-1 [4]

Type Description Composition

CEM I Portland cement >95% clinker

CEM II Portland composite cement >65% clinker

CEM III Blast furnace cement 36 - 95% slag

CEM IV Pozzolanic cement 11 - 55% pozzolans

CEM V Composite cement 18 - 50% slag + pozzolans

2.1.2. C

EMENT HYDRATION AND PORE STRUCTURE

Hydration of Portland cement occurs when cement is brought into contact with wa-ter. Cement chemistry and cement hydration involve very complex phenomena that have been studied extensively [2,3]. The hydration of cement particles and volumet-ric changes during this process have been studied both experimentally [2,3] and mod-elled [7,8]. Cement hydration begins when aluminates (C3A and C4AF) react with water

producing an exothermic reaction that results in the setting of hydrated cement paste. The incorporation of gypsum (C¯S) controls the early rate of hydration and the hydra-tion temperature. The hardening, i.e. strengthening of the matrix, occurs when the sili-cates (C3S and C2S) are hydrated. During this process, C-S-H (calcium silicate hydrates)

compounds are formed. C-S-H has an amorphous structure that contains pores in the range of up to 10 nm [2,3]. A schematic representation of the size distribution of cement phases, pores and air voids, is shown in Figure2.1[9].

Hydrating cement particles that are close to each other combine in order to create the cement matrix. This matrix is composed of capillary pores of up to 1 µm. C-S-H is the most abundant product of cement hydration, occupying 50 to 60% of the hydrated paste volume. Another result of cement hydration is the precipitation of crystalline cal-cium hydroxide (Ca(OH)2) also known as ’portlandite’ [3]. Hexagonal Ca(OH)2) crystals

represent 20 to 25% of the hydrated paste volume. Contrary to C-S-H, Ca(OH)2does not 1_{In cement chemistry notation: C = CaO; Si = SiO}

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2.1.CONCRETE

2

11

contribute to the mechanical properties of the hydrated paste; however, in combination with KOH and NaOH produce a high alkalinity of the pore solution.

Figure 2.1: by Pore size distribution in hydrated cement paste [10]

Within the gel pores and the capillary pores , an interstitial solution containing vari-ous ions is present. The chemical composition of the pore solution is mainly dependent on the cement type; although it can be modified by the action of carbonation or the transport of other substances through the pore system. In general, the concentration of hydroxyl ions (OH°_{) in young, non-carbonated and chloride-free concrete is between}

0.1 to 0.9 M [9] for pH values of 13 or higher. The presence of Na+_{and K}+_{ions is}

pre-dominant in all cement types but Ca2+_{and SO}2°

4 can also be found albeit in much lower

concentrations. The use of blended cements usually results in a decrease in the ionic concentration of the aforementioned ions in the pore solution, rendering a slightly lower pH [2,9].

After setting, the pore structure of the hydrated matrix is continuously changing for a period of 28 days, and beyond in the case of blended cements. At the end of this period, the hydrated matrix contains capillary pores and air voids. Generally, the pore volume can be between 20 to 30% of the cement paste volume [2,3,9]. Aside from capillary pores, air voids of dimensions up to the millimetre scale can be found in hydrated paste if the fabrication conditions were unfavourable for the development of the hydrated ma-trix, or air-entraining admixtures were employed. Within hydrated cement paste, water can be found in three different conditions [9]:

a. Free water - contained in wide pores and air voids. The pore solution is highly alkaline due to the presence of dissolved and mobile ions [Na+_{, K}+_{, Ca}2+

, OH°_,

Cl°].

b. Physically bound water - adsorbed to the inner surface of capillary pores in the form of a very thin layer. Its contribution to transport properties is negligible. However, when the relative humidity of concrete is lower than 30% the removal of the adsorbed water can result in shrinkage (and possible cracking). Water can

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2

also be found in between the layers of the C-S-H matrix. Although gel pores are too small to contribute to transport properties, the loss of interlayer water can result in shrinkage and creep effects when the relative humidity is lower than 11%. c. Chemically bound water - is integrally combined into the C-S-H matrix. It can only

be released when the hydrated paste is heated at high temperatures. The presence of chemically bound water is of significance for analysing the composition of paste containing chlorides.

The development of the pore structure depends on several parameters such as the water-to-binder ratio, degree of hydration, curing and temperature effects. Curing, a process in which the paste is kept in high moisture conditions, represents the most im-portant post-casting process that can influence the hydrated paste [2,9]. Deficiencies in curing can potentially lead to a more permeable or an immature pore structure [2]. Transport properties of concrete can be influenced by the use of blended cements. Per-formance of blended cement concrete has been studied extensively [5,11–18]. In gen-eral, blended cements can produce a denser C-S-H matrix due to hydration of fly ash, pozzolans or slag particles [2,9]. In the case of silica fume, finer particle packing is also responsible for reducing the permeability of concrete.

2.1.3. T

RANSPORT PROPERTIES OF HARDENED CONCRETE

The concrete pore network constitutes a permeable medium in which fluids can be trans-ported. Transport of multiple species occurs simultaneously, depending on the envi-ronmental conditions affecting the structure. The transport mechanisms of matter in concrete can be summarised as follows:

1. Capillary suction - transport of chlorides occurs by moisture gradients. In non-saturated conditions, chloride-rich solution is transported to zones with lower moisture content due to surface tension in capillary pores.

2. Diffusion - a concentration gradient is responsible for transporting, e.g. chlorides, to zones with lower concentrations.

3. Permeation - hydraulic pressure is responsible for transport of ions.

4. Migration - where ionic transport occurs due to the presence of electrical fields. In polarised systems, Cl°_{are transported to zones with positive potentials. This}

mechanism of transport has been adopted in NT Build 492 [19].

Regarding chloride ingress , capillary suction and diffusion are the predominant trans-port mechanisms in practice. Under aggressive environmental conditions, both capil-lary suction and diffusion processes occur, a.o. marine splash or tidal zones. Chloride ingress in such structures is typically modelled as a diffusion process. Chloride diffusion may be under one of two regimes: steady state or constant regime, and non-steady state. In steady state conditions, such as deeply immersed structures, the rate of transfer per unit area of section J is given by Fick’s First Law of diffusion :

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2.1.CONCRETE

2

13

where J is the flux in kg/m2_{s, D is the diffusion coefficient in m}2_{/s and dC/dx is the}

concentration gradient in kg/m4_{. When the concentration gradient and the diffusion}

coefficient are constant, the flux of ions through concrete also becomes constant. This constant flow is called steady state. The characteristics of D in concrete are a function of the cement type, w/b ratio and curing [2,9].

However, in most cases diffusion in concrete is not constant so a non steady-state regime controls the ingress of chlorides. The transport of chlorides in non-steady state conditions can be modelled by Fick’s Second Law of diffusion :

@C @x = D µ @2C @x2 ∂ (2.2) There are several solutions to Eq 2.2 derived from applying different boundary con-ditions. For a semi-infinite medium, with the following initial and boundary conditions:

C = 0 at x > 0 at time t = 0 (initial) C = Csat x = 0 at time t > 0 (boundary)

a solution to Eq 2.2 for chloride transport in concrete is [20]:

C (x, t) = Cs ∑ er f c µ 1 2pDt ∂∏ (2.3) where:

Csis the chloride surface concentration at x = 0, wt. % of cement,

D is the diffusion coefficient, m2_s°1_,

t time, s,

erfc is the complementary Gaussian error function.

The nature of the pore structure reduces the effective ionic transport since ions must find a way through connected pores filled with pore solution. This tortuous network re-sults in a reduced effective diffusion of ions by 3-4 orders of magnitude when compared to aqueous solutions [2,9]. During their transport, chlorides can be bound to hydrated phases such as C-S-H or aluminates [21–23] . The concepts of binding are not part of this Thesis but can be found elsewhere [24].

2.1.4. C

RITICAL CHLORIDE CONCENTRATION

When chlorides are present in sufficient concentration, known as chloride threshold or critical chloride content (Ccr i t) , the passive layer on the surface of steel bars is destroyed

(see Section 2.2.1). This condition results in localised deterioration on the surface of steel known as pitting corrosion . Many factors like cement type, water-binder ratio, relative humidity and temperature have a significant influence on the value of Ccr i t. There are

two different approaches in the definition of Ccr i t[25]:

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2

• Engineering: As the chloride content related to visible or acceptable deterioration of steel

In the literature, a significant amount of studies have been reposted over the past decades. An extensive overview of Ccr i trelated research was performed by Angst [25]. In

this review, the implications of precise quantification of Ccr i tin concrete was not only

demonstrated, but the challenges related to such endeavour were pointed out. From an electrochemical point of view, a strong change in the potential is the determining factor when considering if corrosion has started (see below).

2.1.5. C

RITICAL CHLORIDE CONTENT AND SERVICE LIFE

Determining the service life of concrete infrastructure is a very important aspect in terms of durability. A common misconception occurs when comparing values found in struc-tural codes and considering those values as useful for service life modelling. Strucstruc-tural codes in Europe and North America aim to control the ’maximum’ chloride concentra-tion allowed in concrete infrastructure . Table2.2shows the maximum allowed chloride concentrations for fresh concrete. These values, however, should not be used for estima-tions of the service life of concrete infrastructure.

Table 2.2: Chloride limits for fresh concrete, wt. % of cement

Standard wt. % cement Conditions

ACI 222 [26] 0.2 reinforced concrete in dry conditions

0.1 reinforced concrete in wet conditions

0.08 prestressed concrete

BS8110 [27] 0.35 reinforced concrete

Eurocode [28] 0.2 - 0.4 reinforced concrete

0.1 - 0.2 prestressed concrete

In Chapter 3, a test method for determining the chloride threshold concentrations for concrete in laboratory conditions is presented. This method has been proposed by the RILEM Technical Committee 235 CTC . In Chapter 8, the influence of Ccr i ton service

life modelling is presented.

2.2. C

ORROSION OF STEEL IN CONCRETE

2.2.1. I

NITIATION

P

ERIOD

PASSIVATION OF STEEL REINFORCEMENT IN CONCRETE

Before discussing chloride-induced reinforcement corrosion, a description of the concrete-steel system is necessary. Thermodynamical conditions of pH and alkalinity favour the formation of the passive layer , an atomically thin layer composed of ∞-Fe2O3[9,29,30].

Under these conditions, the passive layer reduces the rate of corrosion to negligible lev-els. The thermodynamics of steel in aqueous solutions with different pH values was first studied by Pourbaix [31], who developed the following diagram as depicted in Figure2.2.

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2.2.CORROSION OF STEEL IN CONCRETE

2

15 Fe Fe³⁺ Fe₂O₃•nH₂O FeO₄²⁻ Fe²⁺ Fe₃O₄ Fe(OH)₂ HFeO₂ pH ɛ₀ [V] 0 2 4 6 8 10 12 14 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2.0

Figure 2.2: Pourbaix diagram for Fe [31] in aqueous solutions

As long as these thermodynamical conditions are preserved, passivity can last for decades. Unfortunately, concrete pores allow the transport of aggressive substances that can disrupt the thermodynamic conditions of the steel-concrete interface. Two main mechanisms are responsible for initiating reinforcement corrosion:

• Carbonation when carbon dioxide CO2penetrates through the pore network

re-acting with calcium hydroxide (Ca(OH)2), reducing the concrete alkalinity to pH<10.

At neutral conditions, the passive film becomes thermodynamically unstable and therefore destroyed over the complete steel surface. Carbonation induced corro-sion has the form of uniform corrocorro-sion.

• Chlorides chlorides (Cl°_{) contained in either sea water or de-icing salts can}

pene-trate through concrete pores by capillary suction or diffusion. During their trans-port, they can accumulate at certain locations of the steel reinforcement resulting in a local breakdown of the passive film. Chloride induced corrosion has the shape of pitting corrosion.

The electrochemical process of corrosion of steel in concrete is composed by the oxidation half-reaction (anode) cell at a certain location on steel surface according to:

Fe ! Fe2++ 2e° (2.4)

and the reduction half-reaction (cathode) occurs elsewhere on the steel surface:

O2+ 2H2O + 4e°! 4OH° (2.5)

when OH°_{ions are transported through the electrolyte, they can react with Fe}2° to pro-duce ferrous hydroxide [Fe(OH)2] or ferric hydroxide [Fe(OH)3]:

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2

Fe2++ 2OH°! Fe(OH)2 (2.6)

4Fe(OH)2+ 2H2O +O2! 4Fe(OH)3 (2.7)

The transfer of electrons is driven through the steel bar to the cathodic zone on the adjacent non-corroding steel surface (cathode), while the ionic transport occurs in the electrolyte (pore solution). Because the ratio of anodic/cathodic area is often low, the corrosion inside the pit is intense. Figure2.3shows a schematic of the effect of chlorides in corrosion pits .

Corrosion pit

Cl⁻, OH⁻ _{O₂ + 2H₂O + 4e⁻ 4OH⁻}

γ-Fe₂O₃ Passive film Steel Concrete pH > 12 Fe Fe²⁺ + 2e⁻ e⁻

Figure 2.3: Chloride induced corrosion pits [9]

Inside the corrosion pit, reaction between iron and chloride ions with water results in the formation of hydrochloric acid which is responsible for acidity inside the pit:

Fe2++ 2Cl°! FeCl2 (2.8)

Fe2++ 2Cl°+ 2H2O ! Fe (OH)2+ 2HCl (2.9)

When hydrochloric acid is separated again into hydrogen and chloride ions, the pro-cess takes again place becoming auto-catalytic. The chloride concentration in corrosion pits has a significant influence in the rate of iron dissolution. The formation and growth of corrosion pits has been described by the stages of pit nucleation, metastable pitting and stable pit growth for stainless steel [32,33]. Evidence of pitting corrosion followed by ’neutralisation’ of those pits in a short period of time, suggest that pit growth in em-bedded reinforcement is determined not only by the breakdown of the passive film, but also by the available thermodynamical condition and concentration of moisture, Cl°_{, O}₂

and OH°_[₃₄_,₃₅_{]. The influence of chloride ions on corrosion kinetics of steel in concrete}

has been hypothesysed. A simplified view is that as chloride ions break down the pas-sive film whereas the influx of hydroxyl ions may reduce the activity in the corrosion pit if the alkalinity of concrete remains high [34]. Dissolved iron ions and a low pH in a pit attract both chloride and hydroxyl ions in, proportionally to their availability and mobil-ity. A minimum concentration of chloride ions in the pore solution is necessary to keep the process running. Both availability of chloride and low resistivity (representing easy

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17

ion transport) are favourable to sustain such an incipient corrosion pit and to develop a high corrosion rate in a short time. At some point, the corrosion rate is high enough that repassivation is no longer possible. If the resistivity is high, transport of chlorides may be too slow to sustain corrosion and allow the pit to grow. A schematic representation of the dynamics between pit initiation and pit growth is given in Figure2.4[9] .

Figure 2.4: Pit initiation and propagation

2.2.2. P

ROPAGATION

P

ERIOD

Several international research committees have been active on research related to rein-forcement corrosion. COST Action 521 [36] provided detailed scientific and technical recommendations on the design, assessment and repair methods with regard to rein-forcement corrosion in concrete structures in both laboratory and on-site conditions. Other recommendations focused on the study of reinforcement corrosion in concrete include ACI 365 [37] and RILEM TC 130-CSL [38].

As corrosion propagates , the rate of deterioration is determined by the speed at which steel is dissolved. The structural capacity of the structure is therefore degraded when a reduction of the steel cross section is followed by loss of bond between concrete and reinforcement. The use of electrochemical techniques to assess the condition of embedded reinforcement was developed extensively during the last quarter of the pre-vious century. Two techniques have been the most prominent for the determination of corrosion onset and propagation: electrochemical potential and linear polarisation re-sistance. These two techniques will be expected in the following section.

2.2.3. E

LECTROCHEMICAL OPEN CIRCUIT POTENTIAL

The electrochemical open circuit potential (OCP or Ecor r) of steel in concrete is a

use-ful parameter when identification and monitoring of corrosion of steel is required. The potential of steel in concrete is indicative of the corrosion state of steel. In practice, mea-surements are performed by employing either embedded reference electrodes, (i.e. Ti+_,

Mn/MnO2) or external reference electrodes (i.e. saturated calomel electrodes [SCE] or

silver chloride [Ag/AgCl]. Electrical connection with the reinforcement must be made and, in the case of an external reference electrode, electrolytic connection must be ob-tained with a water-soaked sponge. Also, a high impedance voltmeter (>10 M≠) is

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re-2

quired for a stable measurement. Figure2.5shows a schematic representation of mea-surements of electrochemical potential of embedded reinforcement.

-+

Reference electrode SCE, Ag/AgCl, Cu/CuSO₄

Steel

Concrete

Figure 2.5: Electrochemical open circuit potential measurement

The absolute value of the reading depends on the reference electrode that is used during the measurement. Each electrode has a different potential difference with regard to standard hydrogen electrode (SHE). Table2.3shows values of electrochemical poten-tial for reference electrodes used in concrete [39].

Table 2.3: Standard potentials of reference electrodes in concrete [39]

Electrode Abbreviation potential mVN HE

Saturated calomel electrode SCE +244

Silver/Silver chloride SSE, Ag/AgCl +199

Copper/Copper sulphate CSE, Cu/CuSO4 +316

Manganese oxide MnO2 +365

Activated titanium Ti+ ₊₁₅₀

Stainless steel ° +150

Lead ° -450

Measured values of corrosion potential are related to different conditions in the cor-rosion state of steel. For example, more noble values (0 to -200 mVSC E) are commonly

associated to a passive state of steel, independent of the environmental conditions. Less noble values between -300 and -500 mVSC Eare found in general corrosion of steel (due

to carbonation), however, there are also reports of values in this corrosion state between -450 and -600 mVSC E[36,40,41]. Pitting corrosion is commonly found when values of

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19

corrosion potential are between -400 and -700 mVSC E. However, it is not possible to

as-sure which mechanism is deteriorating the steel only by measuring corrosion potential. In this sense, values of probability of corrosion according to the electrochemical poten-tial measured on the concrete surface (as shown in Figure2.5) are shown in Table2.4 [42].

Table 2.4: Probability of reinforcement corrosion based on OCP measurements on the concrete surface [42]

Measured potential E, mVC SE Probability of corrosion

E > -200 <10%

-200 < E < -350 Uncertain

E < -350 >90%

2.2.4. C

ORROSION RATE

Linear polarization resistance (LPR) is a technique which estimates indirectly the cor-rosion current density icor r from a metal. Corrosion degradation may be expressed in

terms of current density (mA/m2_{) or corrosion rate (µm/year). This technique is based}

on the application of a potential increase and interpretation of current variations for such potential change in both anodic and cathodic regions. A current-potential plot is generated that represents a quasi-linear relationship between icor rand Ecor r. The slope

of this curve is known as polarization resistance. The application of the technique is derived from the anodic and cathodic curves proposed by Stern and Geary [43]. The equation of Stern – Geary is defined as:

icor r= ØÆØc

2.3°ØÆØc¢

1

Rp (2.10)

Where icor ris the corrosion rate, ØÆis the anodic slope of the polarisation curve and Øcis the cathodic slope. Because ØÆand Øcare constant, Stern-Geary’s equation may be

redefined as:

icor r= B § 1

Rp (2.11)

The value of B is related to the slopes of both ØÆand Øc and is generally found in

the range of 13 to 52 mV depending on the system [44]. For embedded steel in concrete, values of B for a passive state and an active corrosion state are 52 mV and 26 mV, respec-tively. The value of B increases from active to passive state in which a range of values can be found. A proper way to determine which state is most probable is described in [44] =. Nevertheless, a value of B = 26 mV is generally used. In general terms, corrosion rates above of 1.0 mA/m2_{indicate active corrosion [}₃₆_,₄₄_{]. Table}_2.5_{shows criteria for}

evaluation of corrosion rate in concrete structures.

The relationship between electrochemical potential and corrosion current density can be observed in Figure2.6[9]. The broad range of electrochemical potential values

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2

Table 2.5: Classification of reinforcement corrosion by corrosion rate measurements Corrosion current density mA/m2 _{Classification}

< 1.0 to 2.0 Negligible

2.0 to 5.0 Low

5.0 to 10.0 Moderate

> 10.0 High

at which the corrosion current density is low (near vertical left curve) demonstrates that the concrete-steel system is thermodynamically stable. However, the loss of alkalinity or an increase in chloride content produces a shift in the anodic reaction curve (lower right) that represents active reinforcement corrosion.

Figure 2.6: Relationship between electrochemical potential and corrosion current den-sity [9]

2.2.5. C

ONCRETE RESISTIVITY

Concrete resistivity is a geometry-independent property that describes the electrical re-sistance, which is the ratio between the voltage applied to and the resulting current in a unit cell [45,45,46]. In concrete, electrical current is carried by ions in the pore so-lution and its transport is dependent on moisture and temperature. Higher moisture content (wet concrete) as well as more and larger pores with a high degree of connec-tivity and a low tortuosity (high water-binder ratio) cause a lower resistance to ionic flow. For a homogeneous moisture content in stationary conditions, electrical resistiv-ity is increased by a lower water-binder ratio, extended curing time or the addition of supplementary cementitious materials e.g. fly ash, blast furnace slag or silica fume [5]. When temperature is lower, the mobility of ions is reduced. Therefore, ionic flow is de-creased. This behaviour is opposite when temperature is high. Variations in electrical

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resistivity are generally observed in concrete elements between laboratory and field ex-posure condition. This variation is caused temperature or moisture (wetting and drying) gradients[45,45,46]

Other influencing parameters that result in an effect on electrical resistivity include pore size distribution and pore connectivity [5,45]. Blended cements result in refined pore structures that result in increased resistance to electrical current flow [47].

An extensive literature research performed by Hornbostel et al. [46] showed that the variation of electrical resistivity is related to differences in composition of concrete el-ements in both laboratory and on-site conditions. Recently, Reichling et al. [48] devel-oped a technique that allows to simulate the behaviour of electrical equipotential lines in a reinforced concrete element. For this, a Wenner array with different configurations was employed. This technique would allow to detect the presence of layers within the concrete with different resistance properties (e.g. different saturation levels or the pres-ence of a carbonation front). Figure2.7shows a Two-Electrode Method (TEM) set-up for measuring concrete resistivity [9].

Figure 2.7: Two Electrode Method for measuring concrete resistivity [9] The resistivity is calculated by using the formula [36]):

Ω= R ·C = R ·A_L (2.12)

With Ω as the concrete resistivity (≠m), C is a cell constant which depends on the ge-ometry of the specimen (m) and R is the electrical resistance measured with a voltmeter in AC to avoid electrode polarisation (≠). The cell constant in this set-up is determined by the ratio between the cross sectional area (A, in m2) and the length of the specimen (L, in m) Concrete resistivity values according to the environment and cement type are shown in Table 2.6[45].

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2

Table 2.6: Concrete resistivity (≠m) of existing concrete structures > 10 years, [45]

Environment CEM I CEM III / CEM II / CEM IV

Submerged, splash zone, fog room 50 -200 300 - 1000

Outisde, exposed 100 - 400 500 - 2000

Outside, sheltered 200 - 500 1000 - 4000

Carbonated 1000 or higher 2000 - 6000 or higher

Indoor, 20 C 50% RH 3000 and higher 4000 - 10000 and higher

2.3. S

CANNING ELECTRON MICROSCOPY OF CEMENTITIOUS MA

-TERIALS

The use of scanning electron microscopy (SEM) studies on the microstructure of cement-based materials increased substantially in the past three decades. In principle, the most basic objective of conducting SEM studies is topographic and/or compositional analysis of materials. One of the main advantages of SEM over polarised light microscopy, for in-stance, is the availability of employing high magnifications while still having a high depth of field. In SEM, an electron beam is projected to the sample surface causing complex interactions between the beam and the atoms at the sample surface. At the same time, several types of radiation are produced by the excitation of the atoms. The basic working principles of SEM can be found in Goldstein et al. [49], Newbury [50] and Joy [51]. The application of SEM microscopy in geological studies can be found in Reed [52] and in concrete by Winter [53]. Figure2.8shows a schematic view of the main components of a SEM [54].

2.3.1. E

LECTRON

-

SOLID INTERACTIONS

The volume of the sample that is being excited by the beam is known as ’interaction volume’ [50,52,54]. Figure2.9shows the X-rays released during the excitation of the sample surface under the electron beam. The characteristic X-rays being emitted are depending on the associated energy level of the elements present in the material [49]. Other emissions include the release of auger electrons, secondary electrons, backscat-tered electrons and cathodoluminescence. Secondary and backscatbackscat-tered electrons are usually used for imaging, whilst characteristic X-rays are primarily used for chemical analysis (microanalysis). Auger electrons have very low energy such that they require an ultra high vacuum system and specialised equipment for effective use.

When the electron beam excites the sample within the SEM, electron-sample inter-action releases energy that is dissipated by a series of scattering events. For cementitious materials, the dimensions of the interaction volume has been estimated to be in the mi-crometer scale [55–57] and the volume of material analysed by the electron probe is ap-proximately 1-2 µm3[58]. The volume of material contributing to any emitted signal is known as the sampling volume; for X-rays, this is of the same order as the interaction volume [59].