Faculty of Engineering and Physical Sciences Department of Civil and Environmental Engineering

321 Pages·2015·10.08 MB·English

Faculty of Engineering and Physical Sciences
Department of Civil and Environmental Engineering

Patterns of corroded rebar
surfaces and their impact on
tensile mechanical properties

A thesis submitted for the degree of Doctor of Philosophy

By

Leticia Llano Trueba

August 2015
Acknowledgments

I would like to express my most sincere gratitude to my main supervisor, Professor Marios
Chryssanthopoulos, for his assistance throughout my research. His support, specifically
through hours of meetings where he would help with useful advice and expert knowledge,
constant encouragement throughout, pragmatic checking of written work and wise vital
counsels assisted me along my journey to this submission of my PhD thesis. I would also
want to thank my co-supervisor, Dr Alex Hagen-Zanker, who, although joining half-way
through this study, has always provided valuable suggestions and advice.
Many thanks must be given for the financial support to the Marie Curie Initial Training
Network ‘SmartEN Project’ founded by the European Commision under the FP7 Framework
and to the University of Surrey for providing me with the necessary facilities for undertaking
this research. I also take this opportunity to acknowledge the British Spanish Society and
Ferrovial Aeropuertos for providing financial assistance in the form of a joint scholarship that
enabled me to finish the writing of this thesis.
My gratitude is extended to research colleagues and senior researchers in the University of
Surrey including Alex, Helder, Nurul, Hooi and Imran and others for providing with valuable
advice and a very enjoyable working environment. Also, I would like to thank fellow PhD
researchers and senior researchers at SmartEN Project for all the interesting knowledge
exchange and the very pleasant project meetings.
To my friends in Santander, who I have known for so long and have always been there for
me and to the great people I have met in the course of this study in Guildford with whom I
have shared so many adventures. Thank you.
And finally, my deepest gratitude goes to my family. To my parents, Merche and Felipe,
thank you for your love and support throughout my life, I owe you everything. To my
grandmothers, Asun and Maria, thank you for your love and for being my inspiration to work
hard to achieve my goals. And to Marc, thank you for your encouragement, patience and for
always making me smile through the ups and downs of this work. ¡Muchas gracias a todos!

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Abstract

Chloride-induced corrosion is regarded as one of the most important sources of deterioration
in reinforced concrete (RC) structures, whose maintenance during their service life is of
foremost importance in order to avoid unnecessary human risks and economic losses. The
availability of effective mechanisms for quantifying the condition and performance of these
structures is, therefore, indispensable. The search for improved methods to assess this type of
corrosion and its impact on tensile mechanical properties is the main objective of this
research.
Time variant non-spatial models are currently the methods of choice for the assessment of the
effect of corrosion on the mechanical properties of reinforcing bars. Although these models,
based on the relationship between mechanical properties and critical points in the geometry,
give fairly good predictions, they still leave room for improvement. The consideration of the
spatial component of corrosion has barely been addressed in relation to reinforcing bars
embedded in concrete. Thus, the present study focuses on the spatial structure of chloride-
induced corrosion on steel reinforcing bars and its effects on mechanical properties using a
variety of approaches. The use of innovative techniques, originating from different disciplines
and applications, has offered new possibilities in tackling this problem.
First, based on the application of anodic current to steel reinforcing bars embedded in concrete
from an external power source, a set of artificially corroded bars, at different degrees of
severity, was produced. The use of a three-dimensional (3D) computerized imaging
methodology was utilised to characterise these rebars in terms of a grid of corrosion depth
measurements.
After the acquisition of these measurements, use was made of a variety of surface metrology
and image analysis techniques, through which a number of intensity, texture and shape
corrosion quantifiers have been proposed for the spatial characterisation of corrosion patterns.
Surface-metrological based parameters and image analysis-based features were found to yield
useful metrics to investigate the corrosion structure of corroded rebars. The lack of an
objective definition for a pit could be overcome with the characterisation of corrosion defects
in terms of their depth and size using image segmentation.

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All the corroded bars were subjected to a uniaxial tension test and the relevant tensile
mechanical properties throughout the strain-stress response were recorded. It was confirmed
that traditional non-spatial corrosion models had limitations in terms of identifying and
utilising a single corrosion quantifier, and, as a result, the introduction of different possible
spatial corrosion quantifiers was investigated, in order to improve the model performance.
It was concluded that the addition of spatial quantifiers as predictors, resulted in improved
predictions of mechanical properties, compared to the currently used non-spatial models.
However, for the range of corrosion levels examined in this study, the improvement in the
prediction was relatively modest (circa 10% on the coefficient of determination), confirming
the maximum corrosion depth or the minimum cross-sectional area as the pre-eminent
quantifiers. The range of techniques developed in this thesis can be implemented in other
applications where spatial corrosion characteristics need to be explored.

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Table of Contents

Acknowledgments .............................................................................................................. 2
Abstract .............................................................................................................................. 3
Table of contents ................................................................................................................ 5
Symbols and Abbreviations .............................................................................................. 10
List of figures ................................................................................................................... 14
List of tables ..................................................................................................................... 22

1. Introduction ............................................................................................................... 24
1.1 Preamble ............................................................................................................ 24
1.2 Problem description ............................................................................................ 25
1.3 Scope and Objectives .......................................................................................... 27
1.4 Thesis organisation ............................................................................................. 28

2. Background and Literature Review ............................................................................ 30
2.1 Introduction ........................................................................................................ 30
2.2 Corrosion Process in Reinforced Concrete Structures ......................................... 30
2.2.1 Main causes of corrosion ............................................................................. 31
2.2.1.1 Chloride attack ..................................................................................... 31
2.2.1.2 Carbonation .......................................................................................... 33
2.2.2 Transport mechanisms leading to corrosion ................................................. 33
2.2.2.1 Chloride transport ................................................................................. 33
2.2.2.2 Carbonation .......................................................................................... 34
2.2.3 The process of reinforcement corrosion ....................................................... 35
2.2.4 Local and uniform corrosion and their effect on performance of RC structures
……………………………………………………………………………… 38
2.3 Modelling of corrosion deterioration ................................................................... 40
2.3.1 Initiation models .......................................................................................... 41
2.3.2 Propagation models ..................................................................................... 43
2.3.2.1 Uniform corrosion ................................................................................ 44
2.3.2.2 Localized corrosion .............................................................................. 46
2.3.2.3 Spatial characteristics ........................................................................... 50
2.3.3 Corrosion uncertainty .................................................................................. 54
2.3.3.1 Pit depth parameters ............................................................................. 54

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2.3.3.2 Area loss parameters ............................................................................. 59
2.3.3.3 Spatial variability ................................................................................. 60
2.4 Corrosion measuring techniques ......................................................................... 62
2.5 Summary and conclusions .................................................................................. 64

3. Experimental Work.................................................................................................... 66
3.1 Introduction ........................................................................................................ 66
3.2 Laboratory-based corrosion testing ..................................................................... 67
3.2.1 Choice of accelerated corrosion technique ................................................... 67
3.2.1.1 Admixed chlorides ............................................................................... 68
3.2.1.2 Impressed voltage/current ..................................................................... 68
3.2.1.3 Wet and drying cycles .......................................................................... 71
3.2.1.4 Selected method ................................................................................... 72
3.2.2 Geometry of the reinforced concrete specimens ........................................... 72
3.2.2.1 Type of rebar ........................................................................................ 75
3.3 Casting of Corrosion Specimens ......................................................................... 76
3.4 Experimental set-up ............................................................................................ 78
3.5 Test results ......................................................................................................... 81
3.5.1.1 Scanning of the rebars .......................................................................... 85
3.5.1.2 Grid size selection ................................................................................ 87
3.5.1.3 Measurement Accuracy ........................................................................ 89
3.5.1.3.1 Accuracy test using artificial pits ....................................................... 89
3.5.1.3.2 Accuracy test using real pits .............................................................. 90
3.5.1.4 Comparison between Mass Loss and Average Corrosion Depth ............ 90
3.6 Non-spatial analysis of corrosion depths ............................................................. 91
3.7 Representation and visualization of the data ........................................................ 95
3.7.1 Pictorial ....................................................................................................... 96
3.7.2 Graphical ..................................................................................................... 97
3.7.2.1 Two dimensional plots .......................................................................... 97
3.7.2.2 Three dimensional plots ........................................................................ 98
3.8 Summary and conclusions .................................................................................. 98

4. Corrosion Characterisation based on Intensity and Texture Quantifiers: Surface
Metrological Parameters and Image Analysis .................................................................. 100
4.1 Introduction ...................................................................................................... 100
4.2 Surface metrology ............................................................................................ 101

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4.2.1 Applicability to corroded surfaces ............................................................. 101
4.2.2 Overview and selection of surface metrological parameters ....................... 103
4.2.3 Application to corrosion surfaces ............................................................... 110
4.3 Image analysis .................................................................................................. 116
4.3.1 Image features ........................................................................................... 118
4.3.2 Texture features ......................................................................................... 119
4.3.3 Wavelet-based method for texture characterisation .................................... 121
4.3.4 Theoretical background ............................................................................. 121
4.3.4.1 Continuous Wavelet Transform .......................................................... 126
4.3.4.2 Discrete Wavelet Transform ............................................................... 127
4.3.4.3 Two-dimensional Wavelet Analysis.................................................... 128
4.3.4.4 Review of Wavelet-based Methods for Corrosion Images ................... 130
4.3.4.5 Selection of the mother wavelet .......................................................... 134
4.3.4.6 Description of corrosion image decomposition ................................... 135
4.3.4.7 Application of wavelet analysis to corrosion surfaces ......................... 137
4.3.5 Analysis of spatially continuous data ......................................................... 142
4.3.5.1 Kernel estimation ............................................................................... 142
4.3.5.2 Spatial Autocorrelation ....................................................................... 147
4.4 Summary and conclusions ................................................................................ 150

5. Corrosion Characterization using Shape Features: Segmentation Analysis ............... 151
5.1 Introduction ...................................................................................................... 151
5.2 Segmentation applied to corrosion images ........................................................ 152
5.2.1 What is segmentation? ............................................................................... 152
5.2.2 Segmentation of corrosion images ............................................................. 154
5.3 Global threshold-based segmentation ................................................................ 155
5.3.1 Effect of threshold level in pit definition .................................................... 157
5.3.2 Probability distribution function for the surface area of pits ....................... 162
5.3.3 Probability distribution function for the volume of pits .............................. 166
5.4 Watershed-based segmentation ......................................................................... 168
5.4.1 Watershed algorithm applied to corrosion images ...................................... 171
5.4.2 Probability distribution function for the surface area of pits ....................... 177
5.4.3 Probability distribution function for the volume of pits .............................. 180
5.5 Multi-spectral thresholding ............................................................................... 182
5.5.1 Multi-spectral thresholding for corrosion images ....................................... 183
5.6 Summary and conclusions ................................................................................ 188

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6. Mechanical Properties of Corroded Rebars .............................................................. 189
6.1 Introduction ...................................................................................................... 189
6.2 Effect of corrosion on behaviour of RC beams and slabs ................................... 189
6.2.1 Effect of corrosion on bond ....................................................................... 189
6.2.2 Corrosion-induced cracking ....................................................................... 190
6.2.3 Effect of corrosion on flexural strength ...................................................... 191
6.2.4 Effect of corrosion on post-ultimate behaviour .......................................... 192
6.3 Effect of corrosion on tensile mechanical properties of steel rebars ................... 193
6.3.1 Overview of available studies .................................................................... 193
6.3.1.1 Classification of previous studies ........................................................ 193
6.3.1.2 Discussion of previous studies ............................................................ 197
6.3.2 Inter-group comparison of available studies ............................................... 200
6.3.3 Mechanical models .................................................................................... 206
6.4 Experimental Set up .......................................................................................... 209
6.5 Experimental results ......................................................................................... 213
6.5.1 Elastic modulus ......................................................................................... 214
6.5.2 Yield and ultimate strengths ...................................................................... 215
6.5.3 Yield plateau ............................................................................................. 219
6.5.4 Ductility .................................................................................................... 220
6.5.5 Study of critical points ............................................................................... 224
6.5.6 Conclusions ............................................................................................... 234

7. Influence of Surface Pattern on Mechanical Properties............................................. 237
7.1 Introduction ...................................................................................................... 237
7.2 Relationship between intensity features and tensile properties ........................... 238
7.2.1 Strength properties..................................................................................... 238
7.2.2 Strain properties ........................................................................................ 242
7.3 Relationship between texture features and tensile properties ............................. 245
7.4 Relationship between spatially continuous metrics and tensile properties .......... 248
7.4.1 Relationship between kernel estimation-based metrics and tensile properties
……………………………………………………………………………. 248
7.4.2 Relationship between spatial autocorrelation-based metrics and tensile
properties ................................................................................................................. 251
7.5 Relationship between shape features and tensile properties ............................... 252
7.6 Effect of corrosion pattern on tensile mechanical properties .............................. 253
7.7 Summary and conclusions ................................................................................ 257

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8. Conclusions and Future work ................................................................................... 259
8.1 Introduction ...................................................................................................... 259
8.2 Conclusions ...................................................................................................... 261
8.3 Recommendations for future work .................................................................... 262

Appendix A.1 : Corrosion digital images of all rebars .................................................... 264
Appendix A.2 : Corrosion photographic images of representative rebars ......................... 283
Appendix B : Methods for analysis of statistical relationships of variables ...................... 297
References ...................................................................................................................... 300

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Symbols and Abbreviations
Abbreviations
RC Reinforced concrete
Rebars Reinforcing bars
1D One-dimensional
2D Two-dimensional
3D Three-dimensional
CTL Chloride threshold level
REDOX Reduction-oxidation
FT Fourier Transform
FFT Fast Fourier Transform
PSD Power Spectral Density
PC Portland Cement
CAD Computer Assisted Design
ACF Autocorrelation function
LISA Local Indicator of Spatial Association
LVDT Linear variable differential transformer

Symbols (in order of appearance)
Chapter 2
𝑝𝐻 Measure of the acidity or basicity of an aqueous solution (chem.)
𝐶 Chloride concentration
𝐶𝑠 Surface chloride concentration
𝐷 Chloride diffusion coefficient
𝑀𝑡ℎ Theoretical mass of rust per unit of surface area
𝑊 Equivalent weight of steel
𝐼𝑐𝑜𝑟𝑟 Corrosion rate or current density
t Time since corrosion initiation

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