Steel Corrosion in Concrete: Fundamentals and civil engineering practice: 1

208 Pages·1997·28.75 MB·English

by  Arnon Bentur, Neal Berke, Sidney Diamond

Steel Corrosion in Concrete:
Fundamentals and Civil Engineering
Practice
Arnon Bentur
National Building Research Institute-Faculty of Civil Engineering,
Technion, Israel Institute of Technology, Haifa, Israel
Sidney Diamond
School of Civil Engineering, Purdue University,
W. Lafayette, Indiana, USA
Neal S. Berke
W.R. Grace & Co. -Conn., Grace Construction Products,
Cambridge, Massachusetts, USA
Spon Press
Taylor & Francis Group
First edition 1997
Transferred to Digital Printing 2005
© 1997 Arnon Bentur, Sidney Diamond and Neal Berke
Typeset in 10/12pt Palatino by Saxon Graphics Ltd, Derby
ISBN 0 419 22530 7
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A catalogue record for this book is available from the British Library
Contents
1 Introduction 1
2 Mechanisms of Steel Corrosion 7
2.1 Corrosion Reactions 7
2.2 Likelihood of Corrosion Occurring 10
2.3 Corrosion Rates 12
References 19
3 Relationships between Corrosion and the Structure 20
and Properties of Concrete
3.1 Structure of Concrete and of Cement Paste 20
3.2 Chemistry of Concrete 22
References 23
4 Corrosion of Steel in Concrete 24
4.1 Introduction 24
4.2 The Corrosion Initiation or Depassivation Stage 26
4.2.1 Depassivation by Carbonation 26
4.2.2 Depassivation by the Effect of Chloride Ions 27
4.3 Concrete Cover: Properties and Thickness 33
4.3.1 Uncracked Concrete Cover 34
4.3.1.1 Diffusion Processes and Diffusion 34
Coefficients
4.3.1.2 Chloride Ion Diffusion through 38
Uncracked Concrete Cover
4.3.1.3 Diffusion of C0 and 0 through 41
2 2
U ncracked Concrete
4.3.1.4 Carbonation through Uncracked 43
Concrete
4.3.1.5 Electrical Resistivity of Uncracked 45
Concrete
4.3.1.6 Corrosion Rates in Uncracked 46
Concrete Cover
4.3.2 Cracked Concrete Cover 49
4.3.3 Defects in Concrete Cover other than Cracks 52
References 55
iv Contents
5 Corrosion Damage 59
5.1 Damage in Conventionally Reinforced Concrete 59
5.2 Damage in Prestressed Concrete 62
5.2.1 Ductile Versus Brittle Failure Modes 63
5.2.2 Stress Corrosion Cracking 64
5.2.3 Hydrogen Embrittlement 65
References 66
6 Corrosion Measurements 68
6.1 Theory 68
6.2 Laboratory Corrosion Measurements 73
6.2.1 Visual Techniques and Mass Loss 74
6.2.2 Electrochemical Techniques 76
6.2.2.1 Corrosion Potential 76
6.2.2.2 Polarization Resistance 76
6.2.2.3 Electrochemical Impedance 78
Spectroscopy (EIS)
6.2.2.4 Cyclic Polarization 80
6.2.2.5 Macrocell Techniques 82
6.2.2.6 Potentiostatic/Galvanostatic 83
Measurements
6.2.2.7 Conductivity Measurements 84
6.3 Field Measurements 87
6.3.1 Visual Inspection and Delamination Survey 87
6.3.2 Potential Mapping 88
6.3.3 Confirmation of Potential Mapping 89
6.3.3.1 Detailed Visual Inspection 89
6.3.3.2 Chloride and Carbonation Analysis 89
6.3.3.3 Corrosion Rate Measurements 90
6.3.3.4 Analysis of Cores 92
6.3.3.5 Assessment of Epoxy-Coated 92
Reinforcing Bars
References 92
7 Corrosion Control 94
7.1 Introduction 94
7.2 Control of Carbonation 95
7.2.1 Influences of Environmental Conditions 95
7.2.2 Concrete Composition 96
7.2.3 Curing 100
7.3 Control of Chlorides 104
7.3.1 Introduction 104
7.3.2 Estimating Chloride Ingress 105
7.3.3 Influence of Environmental Conditions 106
7.3.4 Composition of Concrete 108
7.3.5 Curing 111
Contents v
7.4 Special Protection Measures for Severe Chloride 113
Corrosion Environments
7.4.1 High-Performance Concretes 114
7.4.2 Corrosion Inhibitors 118
7.4.2.1 Anodic Inhibitors 121
7.4.2.2 Cathodic Inhibitors 122
7.4.3 Sealers and Membranes 123
7.4.3.1 Materials 123
7.4.3.2 Application 125
7.4.3.3 Carbonation Control 125
7.4.3.4 Resistance to Chloride Diffusion and 129
Moisture Penetration
7.4.4 Coatings of Reinforcing Bars 130
7.4.4.1 Galvanized Steel Reinforcement 131
7.4.4.2 Epoxy-Coated Steel 136
7.4.5 Cathodic Protection 139
References 142
8 Specifications and Design 146
8.1 Specifications and Codes 146
8.1.1 Maximum Chloride Levels Permitted in 147
Concrete
8.1.2 Thickness and Composition of Concrete Cover 147
8.1.2.1 British Codes 147
8.1.2.2 American Codes 148
8.1.2.3 European Codes 148
8.1.2.4 Specifications for Special Structures 152
and Severe Exposure Conditions
8.1.3 Maximum Crack Width 153
8.2 Design Considerations for Carbonation-Induced 155
Corrosion
8.3 Design Considerations for Chloride Exposures 160
8.3.1 Introduction 160
8.3.2 Effective Diffusion Coefficient, Dell' for 161
Chloride Ingress
8.3.3 Effective Chloride Concentration at the 164
Concrete Surface
8.3.4 Threshold Chloride Concentrations 165
8.3.5 Service Life Calculations according to the 166
Model
Appendix 8.A Suggested Revisions of the European 168
Prestandard EN 206
References 173
9 Repair and Rehabilitation 175
9.1 Introduction 175
Contents
Vl
9.2 Field Assessment to Quantify Damage 175
9.3 Principles of Repair 176
9.4 Patch Preparation: Removal of Damaged Concrete 179
and Cleaning of Steel
9.5 Treatment of the Steel 182
9.6 Repair Mortar/Concrete 183
9.6.1 Materials Composition 183
9.6.1.1 Polymer Mortars 183
9.6.1.2 Cementitious Materials 184
9.6.2 Primer Coating 184
9.6.3 Application Methods 185
9.7 Electrochemical Protection 187
9.8 Sealers and Membranes 187
References 188
10 Life-Cycle Cost Analysis 190
10.1 Basic Principles 190
10.1.1 Net Present Value 190
10.1.2 Time to Repair 190
10.2 Service-Life Cost Example for a Bridge Deck in a 191
De-icing Salt Environment
10.2.1 Time-to-Corrosion Initiation 192
10.2.2 Time-to-Damage after Corrosion Initiation 193
10.2.3 Service-Life Cost Analysis 193
References 196
Index 197
CHAPTER 1
Introduction
Reinforced concrete is a relatively new construction material which has
been developed and applied extensively only in the 20th century. It has
always been stated that the combination of concrete and reinforcing
steel is an optimal one not only because of the mechanical performance
but also from the point of view of long-term performance. Concrete is a
durable material, much more than steel, and the encasement of steel in
it provides the steel with a protective environment and allows it to
function effectively as a reinforcement. Theoretically, this combination
should be highly durable, as the concrete cover over the steel provides
a chemical and physical protection barrier to the steel, and can poten
tially eliminate steel corrosion problems which occur readily in bare
steel structures.
The tendency of bare steel to corrode is well known, and we are all
aware of the sophisticated technologies developed to protect it, such as
paint systems and active electrochemical methods (e.g. cathodic protec
tion). Even with these protective means tedious maintenance procedures
can not be avoided and their costs can be quite high. Encasement of steel
in concrete can theoretically provide durable and maintenance-free
construction material. Experience with many structures has demon
strated that this can be indeed the case, except perhaps in some severe
environmental conditions.
In spite of the theory and favourable performance record in many
structures, corrosion of steel in concrete has become in the past three
decades a considerable durability problem in mild as well as in severe
climatic conditions. Whereas in the past we were mainly concerned with
the performance of the concrete itself, e.g. resistance of concrete to
sulphate attack typical of marine structures, it seems that, at present, the
most common durability problem is corrosion of steel in concrete.
Performance in marine structures is an example of the 'turning around'
in durability concerns: although many of the text books and specifica
tions give considerable attention to the concrete itself in these environ
mental conditions and specify the use of sulphate-resistant cement, the
major durability problem in this environment is the corrosion of the steel
in concrete. This corrosion usually occurs before any noticeable sulphate
2 Introduction
attack, and it is the 'bottle-neck' in the durability performance of rein
forced concrete structures in such environment.
The increased incidence of durability problems involving steel corro
sion in reinforced concrete structures is the result of several changes
which have taken place in concrete technology and in the environmental
conditions in which concrete is being increasingly used. Some of these
will be outlined here:
• Since the performance of concrete is specified in terms of 28-days'
strength, the modern cements are designed to achieve higher strength
levels and obtain most of their strength potential within 28 days (i.e.
higher C S content and higher fineness), leading to the use of
3
concretes with higher w/c ratio and less reserve for post 28 days'
hydration. These two factors reduce the effective long-term protection
of the concrete cover.
• The need to optimize the structure has led to a tendency to reduce the
concrete cover depth, which when coupled with less attention to
proper workmanship can result in effective reduction in the protection
offered by the concrete cover.
• Many environments have become more severe compared with those
for which the structure was originally designed. De-icing by chloride
salts is carried out more frequently to meet the needs of the heavier
winter traffic in cold climates, leading to an effectively higher
chloride ingress in bridge decks and parking garages (chloride is a
major agent inducing corrosion of steel). Another example is the
increasing corrosive nature of the environment in industrially
polluted zones.
• Increase use of architectural concrete in which the reinforced concrete
component is not protected with an additional mortar rendering.
• Increased use of concrete in marine structures and in urban structures
along the sea shore where the concentration of chloride in the
environment is higher than usual.
The higher incidence of corrosion of steel in concrete is well docu
mented. The damage is usually observed first as rust stains and minute
cracking over the concrete surface (Fig. 1.1) and frequently the cracks run
in straight lines parallel to the underlying reinforcement (Fig. 1.2). This
type of damage is the result of the increase in volume associated with the
formation of the corrosion products (i.e. rust). Such damage can be
observed usually in 'critical locations' such as parts of the structure where
humidity is more readily maintained, or at the base of columns in contact
with the soil where there is a greater tendency for accumulation of salts
due to capillary rise. If repair means are not taken at this early stage, the
corrosion of the steel will proceed further, causing severe damage
through delamination and spalling (Figs 1.2 and 1.3), as well as exposure
of the steel and reduction of its cross-section to an extent which may
Introduction 3
Fig. 1.1. (A) Local corrosion damage on exposed concrete showing up as
'swelling' and local cracking. (B) The damaged concrete can be readily removed,
exposing beneath it the corroded reinforcing bar. (Reproduced courtesy of
C. Jaegerman.)
Fig. 1.2. Corrosion damage in a column which shows up as longitudinal cracks
parallel to the main reinforcement located at the corner of the column (bottom
part of the column). The damaged concrete can be readily removed, exposing
beneath it the corroded reinforcing bar at the corner of the column (upper part of
the column). (Reproduced courtesy of C. Jaegerman.)