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CFRP Wraps for Corrosion Repair of
Reinforced Concrete Columns and Corrosion Monitoring
Ioulia Milman
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Civil Engineering
University of Toronto
O Copyright by Ioulia Milman (200 1)
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ABSTRACT
CFRP Wraps for Corrosion Repair of
Reinforced Concrete Columns and Conwsion Monitoring
loulia Milman, M.A.Sc., 21 00 1
Department of Civil Engineering, Univaers@ of Toronto
This experimentai study is a part of an oagoing research project initiated by Lee (1998) and
continued by Khajehpour (2001). This research is aimed- at evaluating repair of large-scale
reinforced concrete columns using carbon fibre reinforceai polymer (CFRP) sheets, in terms
of establishing the effect of wraps on the corrosion of steen reinforcement.
This project involves the construction and testing of six reinforced concrete columns, testing
eleven colunuis remaining from the previous study. Four specimens were repaired with
CFRP wraps. Testing included accelerated corrosion of bmth new and repaired specimens and
natural corrosion of the reinforcement in five columns (including two repaired), and one
prism specimen.
It was found that corrosion activity temporarily increases after repair. After initial increase,
the rate of corrosion becomes the same as in unwrapped columns or slightly lower. Columns
subjected to natural corrosion expand less than before rewair due to restraining effect of the
wrap. Columns subjected to accelerated corrosion do notz expand for sornetime, after which
they expand gradually.
I would like to thank my supervisors, Dr- J.F. Bonacci and Dr. M.D.A. Thomas for providing
me with the opportunity to work on this project and for d l their invaluable support,
encouragement and patience during this work.
This research was supported by ISIS Canada and 1 would like to acknowledge their financial
suppoa of this project.
1 am very gratefd to P. Malvihill and G. Fishbein fiom Aerospace Engineering, University of
Toronto for providing help with fibre optic sensing technology.
1 am dso gratefùl to many people helping me to complete this project, especially to Nadine
Mobamed Ibrahim and Ali Debaj Ja£f?y for their invaluable help with my experirnents.
It would be impossible to complete this project without help of technical staff at the
University of Toronto. A special thanks to Joel Babbin, Giovanni Buueo, Mehmet Citak,
Peter Heliopoulos, Remo Basset, Alan McClenaghan and John MacDonald.
Findly, 1 wish to express rny gratitude to my husband and my parents for dl their love,
support, and understanding.
TABLE OF CONTENTS
ABSTRACT
ACKNOWEDGEMENT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF F I G W S
Chapter 1, INTRODUCTION
1.1 Background
1.2 Previous Research
1.3 Overview
Chapter 2, LITERATURE REVIEW
2.1 Corrosion of Steel in Concrete
2.1.1 Chloride-Induced Corrosion
2.1.2 Carbonation-Induced Corrosion
2.2 Mechanism of Concrete Confinement
2.3 RC and Steel Jacketing
2.4 Fibre Reinforced Polymers
2.4.1 Manufacture of Carbon Fibres
2.4.2 Extemally-Bonded FRP for Confinement
2.4.3 FRP for Corrosion Repair
2.5 Corrosion Inhibitors
2.5.1 Classification of Corrosion Inhibitors
2.5.2 Application of Corrosion lnhibitors
252.1 Tnhibitors in Concrete Mix
2.5.2.2 Penetrating Corrosion Inhibitors
2.6 Grout
2.7 Methods for Monitoring Corrosion
2.7.1 Non-Perturbative Methods
2.7.1.1 Potential Mapping
2.7.1.1.1 Interpretation of Results
2.7.1.1.2 Influences on Measured Potential
2.7.1.2 Electrochernical Noise
2.7.1.3 Electrical Resistance
2.7.2 Perturbative Methods
2.7.2.1 Linear Polarization Resistance
2.7.2.1.1 Tafiel Constants
2.7.2.1 -2 Galvanostatic and Po tentiostatic Approach
2.7.2.1.3 Interpretation of Results
2.7.2.2 AC Impedance
2.7.3 Probe Applications
2.8 Fibre Optic Sensors in Structure
2.8.1 Manufacture of FOS
2.8.2 Basic Principles of Application
2.8.3 Fibre Optic Bragg Gratings Sensor
2.9 Remarks on Literature Review
Chapter 3, EXPERIMENTAL PROGRAM
3.1 Laboratory Test Plan
3.1.1 Test Regimes
3.1.2 Farnilies of Column Specimens
3.1.2.1 Pilot Series
3.1.2.2 Natural Damage Series
3.1 -2.3 Moderate Damage Series
3.1 -2.4 High Damage Series
3.1.2.5 Expansive Grout Senes
3.1.2.6 Corrosion Inhibitor Series
3.1.2.7 One Layer Repair Series
3.1.2.8 Special Cases
3.2 Column Specîmen
3.2.1 Column Specimen Geometry
3 -2.2 Multi Elernent Probe (MEP)
3 22.1 Pnsm Specirnen
3 -2.2.2 Modified Multi-Element Probe
3 -2.3 Column Specirnen Fabrication
3.3 Matends
3 -3.1 Concrete Materials
3 -3.1.1 Mix Design
3 -3.1 -2 Mwng and Casting Procedures
3 -3-1 -3 Compressive Strength Test Results
3 -3 -2 Steel Reinforcement
3 -3 -3 Repair Materials
3 -4 Laboratory Test Procedures
3.4.1 Accelerated Corrosion Regirne
3 -4.1.1 Mechanical Expansion Collar
3.4.1.2 Fibre Optic Sensors
3 -4.2 Natural Corrosion Regime
3 -4.2.1 Linear PoIarization Resistance Technique
3 -4.2.2 Macro-Ce11 Measurements
3 -4.3 Repair Procedure
Chapter 4, DISCUSSION OF EXPERIMENTAL RESULTS
4.1 Natural Corrosion Testing
4.1.1 Linear Polarization Resistance Testing
4.1.1.1 Pilot Series (2,4)
4.1.1 -2 Linear Polarization Specimen (Prism)
4.1.1.3 Natural Darnage Series ( S 1 1, J 12)
4.1.1.3.1 Column SI1
4.1.1.3.2 Colunin .JI2
4.1.1.4 Moderate Darnage Series (S 10)
4.1.2 Macro-Ce11 Measurements
4.1.2-1 Pilot Series (2,4)
4.1.2.2 Linear Polarization Spechen (Prism)
4.1 -2.3 Natural Damage Series (S 1 1, J 12)
4.1 -2.4 Moderate Damage Series (S 10)
4.1.3 Circumferential Expansion (S 1.0)
4.2 AcceIerated Corrosion Testing
4.2.1 Corroded-Repaired-Accelerated Test Regime (S 1 and S2)
4.2.1-1 Steel Loss Rate
4.2.1.2 Circumferential Expansion
4.2.1.2.1 Mechanical Expansion Collar
4.2-1.2.2 Fibre Optic Sensors
4.2.2 High Damage, Expansive Grout, Corrosion Inhibitor Series
4.2.2.1 Applied Potential
4.2.2.2 Wetting and Drying Cycles
4.3 Summary of Experimentd Results
4.3.1 Natural Corrosion Testing
4.3 -2 Accelerated Corrosion Testing
Chapter 5, CONCLUSIONS AND RECOMMENDATIONS
5.1 Earlier Developments
5 -2 Conciusions
5 -3 Recomrnendations
Chapter 6, REFERENCES
APPENDM A. Procedure for Taking Readings Using FOS Equipment
APPENDIX B. Procedure for Linear Polarization Tests
LIST OF TABLES
Chapter 3
Table 3.1
Table 3.2
Table 3.3
Table 3.4
Table 3.5
Table 3 -6
Table 3 -7
Table 3.8
Table 3.9
Laboratory testing plan
History of colurnn specimens treatment
Electrode configurations used for LP tests of pnsm
Concrete mix design for pilot series
Concrete mix design for subsequent senes
Concrete properties
Mechanical properties of steel reinforcement
Mechanical properties of CFRP sheets
Electrode configurations used for LP tests of colurnns S 10 and S 1 1
Chapter 4
Table 4.1 The rate of steel loss for repaired column S 1
Table 4.2 The rate of steel loss for repaired column S3
Table 4.3 Accelerated corrosion history for S7, S8, S9
LIST OF FIGURES
Chapter 2
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2- 1 1
Fi,oure 2.12
Figure 2.13
Figure 2.14
Figure 2.15
Figure 2.16
Figure 2.17
Figure 2.18
Figure 2.19
Figure 2.20
Figure 2.21
Figure 2.22
Figure 2.23
Figure 2.24
Figure 2.25
Figure 2.26
Electrical curent loop during the corrosion process
The relative volume of iron and its corrosion reaction products
Stages of corrosion process
Chloride concentration at the steel surface in the region of crack
Schematic representation of carbonation process
Profile of pH range of carbonated and non-carbonated concrete
The degree of carbonation as a function of relative humidity
Confinement by (a) square hoops, (b) circdar spirals
Confining action of continuous sleeve
Stress-strain relationship for fibrous reinforcement and matrix
Typical behaviour for concrete confined with steel, CFRP, GFRP
Effect of adding (a) an anodic inhibitor, (b) a cathodic inhibitor,
(c) a mixed inhibitor
Effect of calcium nitrite on the corrosion of steel in rnortar
Moderating of inhibitors on corrosion process
Migration concentration curve for MCI@ 2000
Migration concentration cuve for Sika FerroGard-903
Corrosion monitoring techniques
Half-ce11 electrode measurements
Sarnple schematic of half-ce11 potential data
Linear polarization technique
Two types of polarization resistance probes
Simplified Randles' circuit for steel corrosion
Nyquist diagram
Bondable sensor
Bragg grating description
Schematic diagram of arbitrary gauge length, localized fibre optic
structural sensor based on 10 w coherence intefierometry
Figure 2.27 Application of the localized fibre optic structural sensor to the
measurement of hoop strain in concrete columns
Chapter 3
Figure 3.1 Geometry of concrete column specimen
Figure 3 -2 Commercial multi element probe
Figure 3.3 Geometry of the prism specimen
Figure 3.4 Set up for LP tests for prism
Figure 3.5 Modified multi element probe
Figure 3 -6
Figure 3.7
Figure 3 -8
Figure 3.9
Figure 3.10
Figure 3.1 1
Figure 3-12
Figure 3.13
Figure 3.14
Figure 3.15
Figure 3.1 6
Chapter 4
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
Reinforcement cage in the formwork
Alignment of probe details
MEP installation
Stress-strain characteristics of reinforcing steel
Corrosion ce11
Accelerated corrosion system
Mechanical expansion collar
Expansion measurements
Fibre optic sensor equipment
Experimental set up for LP testing
Repair procedure
Corrosion potential vs. time for columns 2 and 4
Corrosion current vs. time for columns 2 and 4
Corrosion potential vs. time for prism
Corrosion current vs. time for prism using probe
Corrosion current vs. t h e for prism using 10M bar
Current density vs. time for prism
Corrosion potential vs. time for column SI1
Corrosion current vs. time for column S 11 using probe
Corrosion current vs. time for column S 1 1 using 15M bar
Figure 4.10
Figure 4.1 1
Figure 4.12
Figure 4.13
Figure 4.14
Figure 4.1 5
Figure 4.16
Figure 4.17
Figure 4.18
Figure 4.1 9
Figure 4.20
Figure 4.2 1
Figure 4.22
Figure 4.23
Figure 4.24
Figure 4.25
Figure 4.26
Figure 4.27
Figure 4.28
Figure 4.29
Figure 4.30
Figure 4-3 1
Figure 4.32
Figure 4.33
Figure 4.34
Corrosion potential vs. time for column 512
Corrosion potential vs. time for naturall y corroding specimens
using probe
Corrosion potential vs. time for naturally corroding specimens
using rebar
Cross section of column 51 2 with modified MEP
Corrosion potential vs. tkne for column S IO
Corrosion current vs. time for column S 10 using Probe
Corrosion current vs. time for column S 10 using 15M bar
Macro-ce11 corrosion current vs. time for colurnns 2 and 4
Macro-ce11 corrosion current vs. t h e for pnsm specimen
Macro-ce11 corrosion current vs. t h e for column S 1 1 and J 12
Macro-ce11 corrosion current vs. time for column S 1 0
Expansion vs. time for column S 1 O
Current vs. time for repaired columns S 1 and S2
Steel loss vs. time for repaired columns S 1 and S2
Expansion vs. tirne for columns S 1 and S2 during
accelerated corrosion before and after repair
Expansion vs. time for repaired column S Z
Expansion vs. time for repaired column S 1
Cornparison of expansion data for column S 1
Applied potential vs. tirne for columns S7-S9
Current vs. time for columns S7-S9
Current vs. time for columns 513-17
Expansion vs. time for columns S7-S9, JI3417
Steel loss vs. time for columns S 7 4 9 and 513417
Expansion vs. steel loss for columns S7-S9,513-J17
Current vs. applied potential for S7-S9
C W T E R 1. INTRODUCTION
1.1 Background
Reinforced concrete has been successfully used over a numrber of decades. It possesses many
desirable performance properties including high strength,. durability, availability, and low
cost. Concrete normally provides a protective environment agaïnst corrosion of the
embedded steel. The thermodynamically unstable steel is protected fiom corrosion by the
high aikalinity of an appropriately thick and dense concrete cover (Muller and Henriksen,
1995). As a result, iron passivates and the passive film protects the steel against corrosion.
Nevertheless, once the passive layer is damaged (e.g. due bo the presence of chlorides at the
steel surface), the corrosion process may take place in reidorced concrete and cause severe
damage to the structure. Evidence of degradation of a reinforced concrete member due to
corrosion was found as earIy as 1907 (Chess et al, 1998). The darnage associated with
corrosion is cracking, spalling, and delamination of concrete as well as the loss of reinforcing
steel cross section, leading to the reduction of the ultirmate load bearing capacity of the
concrete member.
Inadequacies and errors in design of
susceptible to corrosion. Freeze-thaw
and ice removal) are major causes
concrete mixes produce structures that are relatively
cycles and exposure to de-icing salts (used for snow
of the deterioration in highway structures in many
countries. Many bridge columns in cold climate regions are suffering fkom reinforcement
corrosion and failing to meet their Me-expectancy requirements. While it is impossible to
immediately replace al1 of the corrosion-damaged concrete structures, appropriate and timely
rehabilitation can add years to the remaining service life.
As an alternative to the removal and replacement o f the damaged concrete cover,
circumferential reinforced concrete or steel jackets can be irnstalled around concrete columns.
Although these methods improve significantly the compressive, f l e d and shear strength of
the columns and increase ductility, they have some drawb~acks, such as increased weight of
the structure, and difficulties associated with hancilhg and installation. Moreover, since
repaired structures are still susceptible to corrosion, second and sometirnes third generation
repair are often necessary.
Advanced composite materials are now being used for the rehabilitation of structures around
the world. Fibre reinforced polymer composites have long been recognized for their high
strength, good fatigue life, light weight, ease of transportation and handling, low maintenance
cost and also good corrosion resistance.
The use of fibre-reinforced polymer (FRP) composites is growing significantly in
construction where durability under harsh environmental conditions is required. Although the
provision of external confinement is well established for strengthening reinforced concrete
(e-g. for seismic upgrade), the repair cornmunity does not yet generally accept the philosophy
of wrapping elements suffering f?om the corrosion of steel reinforcement. Such an approach
does not directly address the underlying cause of detenoration (Le. the presence of chlorides
in the vicinity of the steel). However, it is expected that the corrosion process may be
significantly retarded by the presence of the FRP, due to its action as an impermeable barrier
impeding the supply of oxygen and moisture required for corrosion, and perhaps also as a
result of the wrap providing physical restraint to the development of expansive corrosion
products. Furthemore, repaired columns appear to be able to tolerate continued corrosion of
the embedded steel reinforcement under concentric loading (Lee, 1998). These data are
prelirninary in nature and the need to properly monitor repairs of t h i s type in the laboratories
and in the field is emphasised.
1.2 Previous Research at University of Toronto
This study is a part of a larger project to develop a rehabilitation system for corroded
reinforced columns using carbon fibre reinforced polymer (CFRP) wraps. Lee (1998) and
Khajehpour (2001) conducted the previous studies. It is part of an ongoing research project at
the University of Toronto supported by ISIS Canada, a Canadian Network of Centres of
Excellence.
As a result of previous research, simulation of corrosion damage was achieved using
accelerated corrosion. Adding sodium chlonde to the concrete, applying a potential to the
reinforcement cage and subjecting specimens to cyclic wetting and drying accomplished the
accelerated corrosion. Structural tests performed by Lee (1998) and Khajehpour (2001) have
Midicated loss of strength and considerable loss of ductility due to corrosion as well as
extensive pitting corrosion of the spiral reinforcement. By structural testing of repaired
colurnns it was shown that CFRP wraps can restore the coIumn strength and ductirity lost due
to corrosion. Wrapped colurnns subjected to considerable post-repair corrosion still
outperform undarnaged control specimens. The performance of corroded and repaired
specimens has shown reasonable repeatability.
1.3 Overview
The present work sumrnarises the results of previous and ongoing research involving thirty-
one large-scale reinforced concrete columns, twenty-five of which were corroded using an
accelerated corrosion regime, twelve of which were then repaired using CFRP wraps. Six
non-repaired columns, six repaired and two control column specimens were structurally
tested. The rernainder of the colurnn specimens were left for long-term monitoring. The
detailed Iaboratory test plan is presented in Chapter 3.
The main objective of the overall project is to develop a "smart" wrap system for corrosion
damaged RC columns capable of restoring structural capacity, slowing down corrosion and
providing data that can be used to determine colurnn condition and estimate remaining
service life.
The following developrnents are required in order to rneet the above objective:
Provide realistic simulation of corrosion-damaged colurnns in the laboratory
environment.
Detennine the effect of confinement by FRP wraps on the corrosion of chloride-
contaminated, steel-reinforced concrete columns.
Develop a corrosion monitoring system for use with wmpped columns in the laboratory
and field.
Ongohg research is aimed at evaluating the repair technique using CFRP wraps, particdarly
in terms of establishing the effect of CFRP wraps on the corrosion process.
Many existing reinforced concrete structures in North Amer-ica show a high degree of
darnage and are not able to per£orm their original fùnctions. The cracking of concrete and
the use of de-icing salts in cold-climate regions lead to rapid corrosion of the reinforcing
steel in bridge columns, Recent studies (Emmons and Vaysburd, 1997) show that billions of
dollars are spent every year to repair existing structures. Therefore, there is a growing need
for cost-effective repair solutions- Rehabilitation of reinforced concrete columns using CFRP
wraps represents an alternative solution to the traditiod method of replacing the deteriorated
concrete.
This research is devoted to development of repair and monitoring technologies for corrosion
darnaged reinforced concrete infrastructure. This overall objective touches on a wide variety
of subjects, which are to be considered in the literature review that follows. These include:
aspects of corrosion in reinforced concrete, the effects of corrosion on structural
performance; aspects of repair (including steel or concrete jacketing and FRP wrapping for
confinement, corrosion inhibiting chemical products and grouting); and monitoring issues for
both corrosion and structural peI5orrnance-
This necessarily broad literature review is organized by these main aspects of the overall
problem- Section 2.1 discusses the mechanism of corrosion including both: chloride induced
corrosion and carbonation. Section 2.2 thrOugh Section 2.4 briefly discuss the mechanism of
concrete confinement and compares steel or concrete jacketing to FRP wrapping as a repair
strategy for corrosion damaged reinforced concrete columns. FRP wrapping with application
of penetrating corrosion inhibitors or grouting are discussed in Sections 2.5 and 2.6.
Furthermore, in order to understand the corrosion process in FRP wrapped colurnns,
laboratory monitoring is necessary. Thus some methods of corrosion monitoring are bnefly
descrïbed in Section 2-7. The introduction of advanced composite matenals into
rehabilitation and strengthening of existing structures could be accelerated by the integrated
monitoring capability provided by fibre optic structural sensing technology in the field. Fibre
optic sensing (FOS) technology is reviewed in Section 2.8.
2.1 Corrosion of Steel in Concrete
Corrosion of steel in concrete is an elec~ochernical process. It requires a cathode, an anode
and an electrolyte (solution that can conduct ions). Since concrete incorporates some water it
can act as an electrolyte. In new concrete, the pH of the pore solution is usually in excess of
13; and under such conditions steel is protected from corrosion by a passivating film of iron
oxide bonded to its surface. When steel becomes depassivated corrosion may occur. Many
factors can lead to the breakdoai of passivation of reinforcing steel in concrete. including
the ingress of aggressive chloride ions into the concrete and carbonation of concrete, which
are discussed in Sections 2.1.1 and 2-1 -2.
Corrosion can be descnbed by two electrochemical reactions:
Anodic reaction: Fe + ~ e ' - -+ 2 e -
Cathodic reaction: H 2 0 + - B 0 7 f 2 e - + 2 0 H -
where Fe - metallic iron (the main constituent of steel)
~ e ' - - dissolved iron in the electrolyte which carries positive elecû-ical
charges
Oz - oxygen dissolved in the electrolyte
O H - hydroxyl ion in the electrolyte. which carries negativs electricd charge
The anodic and cathodic reactions occur simultaneously, with the movement of the eIectric
charge in metal as electrons and through the electrolyte as ions. Electrons flow from the
anode to the cathode. Therefore, the loss of metai takes place on the anode since the product
of anodic reaction ~ e " dissolves in water around the steel. The anode and the cathode are
usually adjacent on the same bar. This cornrnon type of corrosion is called micro-ce11
corrosion.
Figure 2.1 illustrates the current flow, in which electrons travel £iom the anode to the cathode
inside the metal. There is also a flow of ionic cment through the electrolyte (concrete pore
solution). ~ e ~ ' a n d (OH) react to form Fe (OH) 2 which will react with additional (OH) and
oxygen to form rust FeO, which may expand to occupy £kom two to seven times the volume
of the original steel, as shown on Figure 2.2. (Rosenberg et al, 1989). This imposes intemal
tension, which can Iead to cracking and spalling as well as loss of bond between steel and
concrete (Chess and Gronvold, 1996). Cracking of concrete may accelerate corrosion since
the steel is open to the environment, providing an ample supply of moisture and oxygen.
Figure 2.1. Electrical Current Loop during the Corrosion Process
(Bentur et al, 1997)
Volume -cms
Figure 2.2. The Relative Volume of Iron and its Corrosion Reaction Products
(Rosenberg et al, 1989)
As corrosion proceeds, the reduction in the cross-section area of the reidorcement steel takes
place, reducing the strength and ductility of the RC concrete member. Fi-aure 2-3 summarizes
two major concerns associated with corrosion of reinforcement in concrete; speiling of
concrete and reduction of cross-section area of the reinforcing bar.
t Soluble uon ions
Hydraxyl
Imn hydroxide
Red . u t (haema-1
Figure 2.3. Stages of Corrosion Process (Chess and Gronvold, 1996)
2.1.1 Chloride-Induced Corrosion
As rnentioned above, one of the causes of steel depassivation is due to the presence of
aggressive chloride ions. Chlondes may be introduced to the concrete as set accelerators
(such as calcium chloride) or constituents of some aggegates or the mixing water. It is now
standard practice to limit chloride content in concrete mixes.
If a concrete structure is exposed to seawater or de-icing salts, chlorides rnay penetrate into
the concrete surface. This process rnay take some tirne depending on the amount of chlorides
coming into concrete, the arnount of moisture present, pH of the concrete, permeability of the
concrete, presence of cracks, and the depth of the concrete cover. Repeated wetting and
drying accelerate the ingress of chlorides. Drying to a greater depth aIlows subsequent
wetting to cany the chlorides well into concrete, thus speeding up the ingress of chloride ions
(Neville, 1997). As descnbed by Rosenberg et al, (1989), chloride ions becorne uicorporated
in the passive film, replacing some of the oxygen and increasing its conductivity and
solubility causing the fiLm to lose its protective character.
Corrosion due to chloride penetration is often localized and takes the f o m of pitting. In
addition to overall chloride concentration and the availability of moisture and oxygen, other
factors such as the cathodehode area ratio and the elecûical resistivity of the concrete affect
the rate of dissolution of iron (Rosenberg et al, 1989). If a concrete structure is subjected to
alternative wening and drying in combination with chlonde salts, the critical chlonde content
at the crack tip will be reached often within years. The concentration profile dong the
reinforcement will have the form depicted in Figure 2.4. This Spe of corrosion is
charactenzed by galvanic action between relatively large areas of passive steel acting as a
cathode and small anodic pits where the local environment develops a high chlonde
concentration
CRACK
/ y / / / / / / O '
2-1.2 Carbonation Induced Corrosion
Another cause of steel passive layer breakdown is a decrease in the pH value of the aqueous
solution in the concrete pores due to reaction of the cernent paste with CO2 from the
Amounts o f 4 chloride at the steel surface - c - - - - Crifical anoun?
Figure 2.4. Chloride Concentration at the Steel Surface in the Region of Crack
(Bakker, 1988)
atmosphere. Such a reaction is called carbonation. Schematic representation of the
carbonation process is illustrated in Figure 2-5.
MOOEL Chernial rearhn wifir free - 9 m w m : 1 Ca(0HI2 + CO2 - CaC03 + HP.
-Sm I - t
Figure 2.5. Schematic Representation of Carbonation Process
(Bakker, 1 98 8)
By difision, carbon dioxide penetrates into concrete and reacts with calcium hydroxide
dissolved in water.
As a result of this reaction, pH of the pore solution reduces. As carbonation proceeds, the pH
varies accorduigly in different regions, as shown on Figure 2.6.
d'istance from concme surface
Figure 2.6. Pronle of pH range of Carbonated and Non-Carbonated Concrete
(Chandra and Ohama, 1 994)
The rate of carbonation highly depends on the relative humidity of the environment around
the concrete structure. If a concrete sû-ucture is exposed to air, carbonation proceeds faster
than if the concrete structure is exposed to water because the rate of gas diffusion is 4 tirnes
higher than the liquid penetration (Torok, 1995). As can be seen fiom Figure 2.7, carbonation
does not occw at relative humidity of O or 100%. In a completely dry concrete, the CO2
c a ~ o t react due to the lack of water in the pores for the dissolution. If a concrete structure is
submerged in water (Le. relative humidity is 100%) carbonation will not occur since the
pores are so full that there is no room for carbon dioxide to enter and disperse (Chandra and
Ohama, 1994). Therefore, when the pores contain some moisture but are not completely
saturated, the CO2 rapidly reaches the area of the pore walls and has enough moisture for
reaction (Rosenberg et al, 1989).
Figure 2.7. The Degree of Carbonation as a Function of Relative Humidity
(Rosenberg et al, 1989)
Phenolphthalein indicator can be used to indicate the change in the pH value by changing
colour at pH of approximately 9.5. However, it is not the measure of carbonation depth.
When the carbonation reaction is in progress (pH between 9.5 and 12.5, as shown on Figure
2.6), i.e. beyond die boundary shown by the colour change, it cannot be detected by a
phenolphthalein indicator (Chandra and Oharna, 1994). Depth of the carbonation can also be
established by core sampling. In some cases, when the exposure conditions are steady, the
depth of carbonation (D in mm) is proportional to the square root of t h e ( r in years):
D = i2-41
where K= carbonation coefficient in ~ n m / y r ~ - ~ , which is approximately 3 or 4 mm/yr0.5 for
low-strength concrete (Neville, 1997).
2.2 Mechanism of Concrete Confinement
The pnnciple of concrete confinement consists of restraining lateral expansion that occurs
under axial compression. The compressive strength, and especially ductility of the concrete
can be increased by confinement, Circular sections are easiest to confine efficiently since
confuiing pressure puts the concrete in a compressive state of stress in al1 directions when
axial compression is applied and because the connning shell is in tension only and thus
provides a contuiuous connning pressure around the circumference. In case of square hoops,
the confining pressure varies from a maximum at the corners to a minimum in between the
edges (Figure 2.8) thus the considerable portion of the concrete cross-section may be
unconfined (Park and Paulay, 1975)
Unwnfined
(a) (6
Figure 2.8. Confinement by (a) Square Hoops, (b) Circular Spirals (Park and Paulay, 1975)
Confinement effectiveness is defined as f / f ,,, where f, is the strength of confined
concrete, fi0 is the strength of unconfined concrete. Richart et al (1928) suggested the
following relationship between confinement effectiveness and confinement ratio Cf, /&),
where f, is confinement pressure:
f cc /fco = If k r r r / f CU)
where kl is effectiveness coefficient = 4.1, depends on lateral pressure
Since confinement is not as effective at high levels of lateral pressure, Newman suid Newman
(1 97 1) suggested the following relationship:
k, = 3.7( fI/fca) do-14 [2.6]
Saatcioglu and Rami (1992) suggested the foUowing non-linear relationship based on the test
data fiom Richart et al (1 928): -0.17 kl = 6-75 12-71
The confinement provided by transverse reinforcement or an extenial jacket depends on two
factors: the tendency of concrete to diIate and the radial sti£fiiess of the confinhg member to
restrain the dilation. In modelling confinement effects, the following conditions are assumed
to be satisfied:
1. Strain compatibility between the core and the shell
2. Equilibriuni of forces in the free-body diagram for any sector of the confined section, as
illustrated in Figure 2-9.
Figure 2.9. Confining Action of Continuous Sleeve (Mirmiran and Shahawy, 1997)
Assuming the case of confinement providing by a cylindricd shell, the following relationship
can be drawn fiom equilibrium and geometry of Figure 2.8:
5 = 2L 5 / D
where f,- confinement pressure
A- hoop stress in the confining element
Mirmiran and Shahawy (1997) estabiished the following relationship for FRP-encased
concrete:
fcc =fco +d-2695 0.587
~2.91
2.3 RC and Steel Jacketing
Until recently, the most common method of strengthening was to install circumferential
reinforced concrete or steel jackets around concrete column, It is an effective method of
column strengthening. However, installation of steel jacketing is a labour intensive and time-
consuming method that requires heavy equipment. Individual plates are heavy and they need
on-site welding md/or bolting to form a continuous jacket.
Steel plate jackets c m o t be constnicted to fit in direct contact with the existing concrete
shape. Therefore, in order to provide confinement, the annular space between the steeI plate
jacket and the concrete must be filled with a structural concrete or epoxy-type resin that is
capable of transferring structural loads. This process is labour intensive and it is generally
difficult to completely fiII the annular space (Ballinger et al, 1993).
Moreover, installation requirements rather than confinement requirements determine steel
jacket thickness and weight. Each jacket has to be extremely heavy and strong to prevent it
fiom buckling under its own weight during lifting, placing and grouting, which makes this
retrofit method expensive (Cercone and Korff, 1997). Increased weight and cross-sectional
area of the structure may influence seismic response and/or clearance (Bdlinger, 1997).
As an alternative to steel and RC concrete jackets, fibre reinforced polymer composites can
be used as wraps for retrofitting and rehabilitation of existing concrete columns. The use of
externally-bonded fibre reinforced polymer materials is thought to elirninate many of
problems associated with application of steel and RC concrete jackets for concrete repair.
2.4 Fibre Reinforced Polymers (FRP)
"Fibre reuiforced composite materials have long been recognized for their high strength,
good fatigue life, light weight, ease of transportation and handling, low maintenance cost and
good corrosion resistance" (Levergne and Labossiere, 1997). They have been used
extensively in the aerospace, aeronautic, automotive and other fields.
The FRP materials are made of fibre such as glass, ararnid, or carbon embedded in a resin
rnatrix. As iilustrated in Figure 2.10, the fibres are stronger than the rnatrix. Therefore, the
fibres are the primary load-carrying part of the composite. The ma& binds fibres together,
bonds them to the structure (in case of extemally bonded FRP), and protects them fiom
environmentai attack.
Figure 2.10. Stress-S train Relations hip for Fibrous Reinforcement and Matrix
(ISIS Canada, 2000)
In this thesis, mostly extemaily-bonded carbon fibres are discussed. Although, carbon fibres
are more expensive, they have higher strength and higher elastic modulus than other fibres.
For example, the effective strength of g las fibres is less thao 2/3 that of carbon fibres and the
elastic modulus is about 1/5 of that of carbon fibres (Ballinger, 1997).
Carbon fibre reinforced polyrner (CFRP) requires little if any heavy equipment, hardens
rapidly and needs no additional treatment after hardening. Therefore, less installation time is
required than with other conventional strengthening methods. While carbon fibre reinforced
composite materials are relatively expensive, experiments have shown that the labour costs
for CFRP strengthening can be 20 to 30% less than for conventional strengthening work
(Emmons et al, 1998). The advaotages of carbon fibres also include ultraviolet resistance and
the fact that carbon fibres are not reactive with concrete, and do not corrode.
2.4.1 Manufacture of Carbon Fibres
Carbon fibres are manufactured by extrusion of a polymer into a continuous nlament. The
filament undergoes a stabilization treatment in air at 200 to 350°C, after which it is heated-
treated (carbonized) at temperatures between 350 and 1600°C in a . inert gas atmosphere to
remove H, O, N and other contaminating elements.
The mechanical properties of the resulting fibres can be modified by a subsequent heat
treatment at temperatures between 1300 and 3000°C. Commercial carbon fibres with elastic
modulus of about 230 GPa are known as high-strength or low-modulus fibres. If the elastic
modulus is in the range of 480 to 700 GPa, this is referred to as high-modulus fibres.
Theoretically, the modulus of perfectly aligned fibres would be about 1000 GPa (Meier,
1992).
2.4.2 Externally-Bonded FRlP for Confinement
External confinement of concrete by means of high-strength fibre composites can
significantly enhance its strength and ductility as well as result in a large energy absorption
capacity (Mk-mkan et al, 1997).
An FRP jacket, as opposed to a steel one, has an elastic behaviour up to failure and therefore
exerts a continuously increasing connning action (Spoelstra and Monti, 1999). Figure 2.1 1
represents the stress-strah relation for CFRP, GFRP and steel-confined concrete. The Fm-
confined concrete shows an ever-increasing branch, as opposed to steel-confined concrete,
which after reaching the peak strength decays with a sofiening branch.
Parameters that affect ductility of FRP-confined concrete include unconfined concrete
strength, types of fibres and resin, fibre volume fraction and fibre orientation, thickness of
wrap, and interface bond between the member and FRP. Also, shape of the column as well as
column length-to-diameter ratio can a e c t the effectiveness of FRP rehabilitation methods.
The effect of some of these parameters on FRP-confhed concrete performance is discussed
below.
CFRP
nonnalized axial strain
Figure 2.11. Typical Behaviour for Concrete Confined with Steel, CFRP, and GFRP
(Spoelstra and Monti, 1999)
Orientation of Fibres
The orientation of fibres is very important for rehabilitation of columns. Fibres can be
oriented al1 in one direction, but for the optimum structural performance they may be
oriented in different directions. The layers with fibres in the axial direction enhance the
flexural strength of a colurnn, while the layers with fibres in the circumferential direction
provide confinement. When the confinement layers cover only a lirnited region of the
column, shear resistance does not increase, but ductility under axial compression is expected
to irnprove. Experiments conducted by Levergne and Labossiere (1997) have shown that
application of one layer of carbon fibres oriented in the axial direction in the potential plastic
hinge region did not enhance signîficantly the flexural strength of a column, although, it
inhibited the development of transverse cracking in the composite straps until failure. When a
column was repaired with 6 layers of carbon fibre oriented in the circumferential direction on
top of one layer oriented in the longitudinal direction, its initial strength was exceeded.
A retrofit concept developed in Japan involves application of CFRP in the longitudinal
direction of a column and CFRP sheets or strands in the transverse direction, to provide the
necessary flexural and shear strength and ductility to meet new seisrnic design critena. This
rehabilitation technique has shown good results as far as flexural and shear strength and
ductility is concerned. This methodoIogy was also suggested for repair of damaged colurnns
(Baiiinger, 1 997)-
Thickness of Wrap
Experiments carried out at the uni ver si^ of Toronto involved 150 x 300 mm cylinders
containing polypropylene and micro fibres. Cylinders were wrapped with zero degree angles
of orientation (i. e. perpendicular to the loading direction) CFRP sheets. The results have
indicated that the increase in the nurnber of wrapping layers increases signifïcantly the
maximum strength of concrete (Zanganeh, 1997).
Experiments carried out at the University of Sherbrooke have shown that the stren-& of 150
x 300 mm cylinders increased with the increase of number of layers of ararnid/epoxy
composite wrap (Demers et al, 1996).
Mirmiran et al. (1998) reported the results of a study to determine the effect of the thickness
of E-glas fibre on the confinement of circular and square section colurnns. The results have
indicated that while wrap thickness greatly affected the response of circular section, for
square section this effect was minimal.
Bond
Extemal FRP requires a sound bond to sound concrete substrate to remain intact throughout
its service lifte. The bond strength and integrïty depend on the physical and chernical
characteristks o f the components, which include the fibres, the concrete substrate and the
epoxy that acts as a binding agent. The interface bond affects the state of stress in the
concrete, and also affects the capacity of the member. The bond must be capable of
withstanding stresses and the processes of degradation normally irnposed on the concrete
structure. EssentiaIly, FRP provides exterior non-corroding reinforcement to the concrete.
Unfortunately, the bond surface, the skin of existing concrete, is the weakest part of the
concrete cross Section (Emmons et al, 1998). Factors detennuiing the strength and durability
of FRP bond sumniarized by Emmons et al, (1998) include:
Tensile strength of the concrete skin
S trength of the epoxy adhesive system
Cleanliness of the concrete and FRP surface
Uniformity and thickness of the adhesive layer
Geometry of exterior strengthening
Types of loading
Environmental conditions at the time of application and curing
Workmanship and quaiity control.
Colunui Shape
Experiments performed by Mirmiran et al, (1998) have shown that confinement of square
sections using E-glas fibre jackets is less effective than their circdar counterparts. It can be
explained by the distribution of confining pressure in circular and square sections. While for
a circular section the confinhg pressure is uniform, for a square section the connnuig
pressure varies fiom a maximum at the corners tu a minimum in between the edges.
2.4.3 FRP for Corrosion Repair
FRP wraps c m also be used for repair of corroded reinforced concrete mernbers. Post-repair
corrosion c m be altered by a combination of two mechanisms. First the FRP wrap works as a
diffusion barrier that isolates the column fkom the environment and lirnits the ingress of
oxygen and moisture to the reinforcing steel. Aiso the wrap can develop conf ing pressure
to resist the expansive tendencies that are generated by corrosion products. (Sheikh et al,
1997).
CFRP, because it is composed of inert substances, is ideal as a strengthener in corrosive
environments because it does not react with the majonty of corrosive agents. Carbon
composites are unafEected by water and alkalis, and do not corrode (Cercone and KorfT:
1997).
Experiments on the tensile performance of FRP composite sheets subjected to wet/dry and
fkeezehhaw cycles performed by Toutanji and El-Korchi (1999) have shown that glass-fibre-
wrapped specimens experienced a significant reduction in strength due to environmental
exposures, as opposed to carbon-fibre-wrapped ones. Therefore, carbon-fibre-reinforced
polymer is superior to g las when exposed to harsh environmental conditions.
Experiments conducted at the University of Toronto involved the repair of large-scale
corroded RC columns. The specimens 1524 mm long and 406 mm in diarneter were repaired
with the TYFO Fibnvrap System. The material is a woven fabric 1.7mm thick, containing
glass fibres in the primary direction and orthogonally oriented aramid fibres. A strength gain
in the range of 11 to 25% was observed when the columns were wrapped with 2 Iayers with
various expansive grouts between the wrap and the original column surface (Sheikh et al,
1997). Additionally the ductïlity of the repaired column was significantly higher than that of
the undamaged column.
Experiments carried out by Lee (1998) and Khajehpour (2001) at the University of Toronto
as a part of the overall project involved large-scale RC columns. The corroded specimens
10 16 mm high and 305 mm in diameter were repaired with 2 layers of pre-impregnated
Replark Type 30 CFRP sheets. Corrosion damage reduced the load-carrying capacity of the
specimens by 7 to 16% (dependhg on the level of damage). As a result of repair, Ioad-
carrying capacity of the corroded specimens increased by up to 32%. Moreover, repair
signifïcantly uicreased ductility of the column.
2.5 Corrosion Inhibitors
As defined by Bradford (1993), an inhibitor is a chernical substance added to the corrosive
environment to reduce the corrosion rate. In the majority of cases inhibition is achieved
through the reaction between the corrosion inhibitor and the metal surface, resdting in the
formation of an inhibitive surface film. In other cases the chemistry of the environment may
be modified to render it less corrosive, e.g. by chemically scavenging dissolved oxygen;
chemically neutralizing dissolved acidic gases; or by adjusting pH to promote stable
passivation (Harrop, 1990). Inhibitors have a critical concentration that rnust be reached or
exceeded for them to be effective, and in some cases to prevent them fiorn making corrosion
worse (Bradford, 1993).
2.5.1 Classification of Corrosion Inhibitors
Corrosion inhibitors can either affect the anodic partial reaction, cathodic partial reaction or
both. Thus, there are two major types of inhibitors: anodic and cathodic. This classincation
depends on inhibitor's reaction at the metal surface and how the potential of the metal is
affected. in reducing the corrosion current, the corrosion potential is shifted positively for an
anodic inhibitor (Figure 2.12a) and negatively for a cathodic inhibitor (Figure 2.12b). In the
case where the inhibitor affects both partial reactions, the shift in corrosion potential will
depend on which is the more dominant effect (Figure 2.12~) (Bradford, 1993).
Figure 2.12. Effect of Adding (a) an Anodic Inhibitor, (b) a Cathodic Inhibitor, (c) a Mixed
Inhibitor. Broken lines show inhibitor addition (Bradford, 1993)
2.5.2 Application of Corrosion Inhibitors
Corrosion inhibitors have been used successfully for a number of years in order to control
corrosion of steel in aggressive environments. There are a large variety of corrosion
inhibitors available on the market that can be used for specific applications, such as water
treatment, packaging, and petroleurn refining (Montani, 1996).
The use of inhibitors for corrosion control of reinforcing steel in concrete is a relatively new
development. In 1957 Moskvin and Alexeev found that barium nitrite, sodium nitrite,
potassium chromate and potassium dichromate added to the concrete mix, in arnount varying
fkom 0.5 to 3.0% based on the weight of cernent, improve corrosion protection, with sodium
nitrite offering the best protection fkom chloride-induced corrosion (Ehrlich and Rosenberg,
199 1). In the 1960s, calcium nitrite became available for use as a corrosion inhibitor. It was
found to be very effective as far as corrosion resistance is concerned (Figure 2.1 3), relatively
inexpensive, and it does not lower the strength of concrete. However, calcium nitrite is a set
accelerator, so care must be taken that flash setting does not occur @roomfield, 1997). In the
1980s amino-aicohol-based inhibitors were discovered and have s h o w good results
(Montani, 1996).
I I I I t *
- -
OAtA OETE RUINE0 fN THE PRESEUCE O F 013% SODIUM CHLORIDE BASE0 ON WElGt fT O F SAWO ot -400 mv 4
d
-
Figure 2.13. Effect of Calcium Nitrite on the Corrosion of Steel in Mortar
(US. Pat. No 092 109) (Ehrlich and Rosenberg, 199 1)
As illustrated in Figure 2.14, corrosion inhibitors may have two moderating effects on the
corrosion process in the reinforced concrete; increase the time for corrosion initiation and
reduce the rate of corrosion (Montani, 1996).
Time to and rate of corrosion Limit of service life
To Tz TL! +-. f r*
-- - without inhibitor with inhibitor
Figure 2.14. Moderating Effect of InhibZtors on Corrosion Process (Montani, 1996)
The method of application varies: in new structures inhibitors are admixed in suficiently
high concentrations to the fiesh concrete; for repair work inhibitors c m be present in paints
for the reinforcement or in repair mortars; on existing structures, where the onset of corrosion
has to be prevented, inhibitors are applied at the concrete surface (Elsener et al, 1997).
2.5.2.1 Inbibitors in Concrete Mix
In order to increase the tïme for corrosion initiation in newly cast concrete, inhibitors can be
added to the concrete mix dong with such preventive measures as low permeability, concrete
cover and epoxy-coated re-bars (Montani, 1996). However, some inhibitors when added to
concrete may influence other concrete properties; e.g. lower the ultimate strength, retarded
setting time of the cernent, and the development of efflorescence. Therefore, it is very
important to test the inhibitor under the conditions intended for use to insure that it does not
impair the performance of the concrete (Ehrlich and Rosenberg, 1991).
Inhibitors also can be incorporated in large-scale concrete repairs. When the carbonated or
chloride-contaminated concrete is replaced with a new low permeability material, a liquid
inhibitor can be sprayed on the concrete prior to repair to protect the reinforcement below the
surface andor an inhibitor can be included in the overlay material as an admixture. However,
it is very important to consider any synergistic effects of inhibitors and other admixtures on
concrete properties in order to avoid alteration of setting time, strength, slump, and air
content (Montani, 1996).
2.5.2.2 Penetrating Corrosion Inhibitors
The usual approach of locdized repairs includes the replacement of the unsound concrete
with low permeability materid. However, indications of future corrosion ofien occur shortly
d e r repair, particdarly at the interface of the replaced material; such areas are called
incipient anodes. These areas corrode at an even façter rate then the original anode areas. Ln
order to prevent corrosion on the incipient anodes, penetrating corrosion inhibitors c m be
included in the repairing material (Montani, 1996).
The future corrosion activity in larger areas surrounding repairs is cucrently addressed by
applying protective coatings to stop COa and/or chlorides fiom penetrating. However,
additional surface treatment with penetrating corrosion inhibitors prior to the application of
protective coating may reduce the effect of active corrosion present at the steel (Montani,
1996).
Penetration of corrosion inhibitor occurs via both liquid and vapour diffusion. The rate of
penetration depends on the density and permeability of the concrete and is relatively
independent of the rnoisture content The migration of the MCI@ 2000 admixture is
documented to be about 7Scm within 7 days of initial application (Figure 2.15, CORTEC
Corporation - 2, 1996). Corrosion inhibitor Sika FerroGard-903 was proven to achieve
penetration rated of 2-20mm per day, as stated in the manufacturer brochure (Figure 2.16,
SIKA Ferrogard @, 1996).
Figure 2.15. Migration Concentration Curve for MCI@ 2000
(CORTEC Corporation - 2, 1 996)
Figure 2.16. Migration Concentration C u v e for Sika FerroGard-903
(SIKA Ferrogard @, 1996)
2.6 Grout
Grout is a mixture of cernent (or binder of cernent plus supplementary cementing materials)
with water and adrnixtures. However, it also may include inert filler (i.e. a fine aggregate).
Cementitious grouts are used for an increasing number of existing structurai purposes. Any
type of cernent may be used though care should be taken if rapid-hardening Portland cernent
is used in large quaatities due to its uicreased heat of hydration Thus, the use of grout in
structures is generally restricted to smaü ùiickness sections. Grout needs to be sufficiently
fluid to be efficiently pumped and injected to fiIl al l the voids. (McLeish, 1994).
When considering the use of cementitious grout for structural repair purposes attention must
be paid to the inherent properties of the grout and in particuiar the problems that may be
encountered. Compared with concrete, grout typically has a nurnber of disadvantages when
used as a structural materiai (McLeish, 1994):
It is expensive when used in large quantities
It sufférs fkom a high drymg shrinkage
It has a high heat of hydration and coefficient of thermal expansion, often resulting in
temperature-induced cracking when used in thick sections.
The inclusion of small-size aggregate overcomes to a large extent many of these problems.
The mixture consisting of grout and small aggregate can be designed to have the same
properties as grout. It can be pumped to flow into position around reinforcernent without
vibration (McLeish, 1994).
Expansive Grouts
Expansive cements are inorganic hydraulic binders which, when hydrated, generate stresses
in the set matrix leading to an overail expansion of the concrete mass (Odler, 199 1). Cements
with high sulphate content exhibit their expansive properties due to formation of ettringite
(Cusick and KesIer, 1980).
It has long been recognized that there is a problern of cracking of concrete afler hardening
and that this problem is contrïbuted to by many different factors such as cooling and drying.
Cooling causes contraction; drying causes shrinkage; both contraction and shrinkage cause
tension, which if in excess of the tensile strength, causes cracking (Mather, 1980). Therefore,
the basic purpose of expansive cements is to eliminate the drying shrinkage inherent in
conventiond concrete by an initial expansion, and thus minimize cracking (Li and
Ramakrishnan, 1980). Thus, this cernent may be referred to as shrinkage compensating. The
reduction or elimination of cracking will reduce or prevent the rapid corrosion of the
reinforcement that can occur at a crack (Cusick and KesIer, 1980).
The use of expansive grouts in conjunction with FRP wraps for repair of corroded concrete
columns with circula cross-section was investigated at University of Toronto (Sheikh et al,
1997). The specimens were 1524 mm long and 406 mm in diameter. Expansive grout was
introduced in order to establish initial prestressing forces in the wrap, and thus enhance initial
amount of confinement provided by the wrap. The Level of confinement stress applied to the
member by the connoing repair system depends on the amount of deformation induced in the
composite wrap by concrete expansion. The composition of the grout used in these
experiments was 60% (by weight) Portland cement Type 10,25% high a l d a cement, 12%
plaster of Paris, 3% hydrated lime, superplasticizer (0.3% by weight of Portland cernent), and
wlc of 0.4. This type of grout develops 2.540% unrestrained expansion, and it is known to
develop significant confining stress under restrained conditions. The use of expansive cernent
with two layers of FRP wrap resdted in excellent behaviour of the specimens (Sheikh et al,
1997).
2.7 Methods for Monitoring Corrosion
The need for early detection and diagnosis of corrosion related deterioration in reinforced
concrete structures is widely acknowledged. Many types of corrosion monitoring techniques
have been developed. McKenzie (1986) classified corrosion monitoring techniques into two
broad groups, as shown in Figure 2.17. The Grst group represents non-perturbative
monitoring methods, whic h measure the signal generated b y an electrode (reinforcement)
without the application of any extemal signals. On the contrary, the other group represents
perturbative methods, which measure the response of the reinforced concrete structure to
extemally applied stimulants (electrical signais).
Corrosion monitoring of steel in concrete
resisrance
Potentiallcurrent W P ~ ~ Q noise
resis tance I
Figure 2.17. Corrosion Monitoring Techniques (McKenzie, 1986)
2.7.1 Non-Perturbative Methods
Non-perturbative techniques include half-ce11 potentiai (potential mapping), electrical
resistance, and potential noise measurements. These are discussed in the following sections.
2.7.1.1 Potential Mapping
The potential mapping technique is the most widely used on-site electrochemical method.
This method has the advantage of being a non-destructive and easy to perform technique for
determinhg the probability of active corrosion. Theoretically this method can be used to
cdculate the rate of corrosion, although Chess et al, (1998) noted that in practice s a c i e n t
accuracy couldn't be achieved to provide anythmg but general indication of the corrosion
rate.
Half-ce11 potential method is described in ASTM C876-91. As corrosion proceeds, some
areas depassivate and becorne anodic, while others remain passive, thus cathodic. The
objective of the half-ceil potential test is to mesure the areas of different potential on the
surface of the concrete with respect to a stable reference cell in order to identis, and map
anodic and cathodic areas. Many types of electrodes can be used as a stable reference half-
cell:
Silver/silver chloride (AdAgCi)
Mercury/mercury chloride (Hg/HgzCl2)
Copper/copper sulphate (Cu/CuS04)
Zirdseawater (Zdseawater)
Manganese dioxide (Mi10z)
Generally, copper/copper sulphate cells are used since they are robust, relatively cheap and
easy to use, although silver/silver chloride ceils can be more stable, and have faster response
time (Baker, 1986).
The values of reinforcement potential c m be found using the following equation (Rosenberg
et al, 1989):
Eco, = Eo + (RT/F) Zn K
where Eco, - metal potential at equilibrium
Eo - standard electrode potential for the reaction
R - the gas constant
T - the absolute temperature
F - Faraday's constant
K - equilibrium constant for the ions present in the solution
Therefore, E,, c m be measured with respect to the standard reference electrode (Figure
2.18).
l High impedance voltmeter
Reference electrode Sintered glass eg. CulCu S O ~ membrane plug
High conductivity sponge
- Reinforcing
1 Concrete
Figure 2.18. HaE-Cell Electrode Measurements (McKenzie, 1986)
2-7.1.1.1 Interpretation of Results
Van Deveer found the following relationship between the calcdated potential and the
probability of corrosion (Chess et al, 1998).
E,,,, (vs Cu/CuS04), volts
More negative than -0 -3 5
More positive than 4.20
Between -0.20 and 4.35
Probabiiity of Corrosion, %
95
5
uncertain (probably b e ~ e e n 5 and 95%)
This method was initialIy used to monitor corrosion of steel pipelines. Later, the use of the
half-ce11 potential method was developed to investigate the corrosion process in concrete
bridge decks; therefore, the results should be interpreted very carefully in structures other
than bridge decks.
Half-ce11 potential results can be presented as equipotential contour maps (Figure 2.19).
Potential mapping survey gives difference in potential, which can identiS the anodic zones
(rnacro-cells), and their paaicular location. The measurements of the potential gradients
created by the corrosion process allow areas of possible corrosion to be identified. It was
found that the pattern of these contours provides more valuable information about the status
of the corrosion in concrete structures than the precise values of the potentials (Baker, 1986).
Figure 2.19. Sample Schematic of Half-Cet1 Potential Data
(Chess, Gronvold & Karnov, 1998)
Spacîng between plotted points on the contour maps should satisS the type of investigation.
For example, when chloride-induced pitthg corrosion is under investigation, relatively small
spacing should be used in order not to overlook localized pits (Baker, 1986).
2.7.1.1.2 Influences on Measured Potential
Potential measured on the surface of the concrete can be influenced by a wide variety of
factors. Baker (1 986) surnmarized these factors as follows:
Equi~ment and Technique
There could be up to about lOOmV difference due to voltmeter stability, electrode variability,
electrode position, and electrolyte concentration in the reference electrode.
Surface Skid Resistance Effect
Carbonation of surface, quality of surface, contact electrolyte between electrode and
concrete, could give up to approximately 500mV difference.
Concrete Properties
Quality of concrete (Le. depth of concrete cover, moisture content of concrete, salt
concentration effects, concrete degradation due to some destructive mechanisms, electrical
potential of steel in concrete) effects electrical resistivity and could give up to 400mV effect.
It must be pointed out that greater depth of cover will result in shallower potential gradients
and corrosion in lower layers of reinforcement wilI be dBïcult to detect. In structures with
complicated reinforcement layout or with a concrete of low resistance it might be hard to
i d e n t e which layer of reinforcement is corroded (Chess and Gronvold, 1996). Also, if the
concrete structure is submerged in the water or soil, local corrosion occumbg in the
submerged parts of the structure cannot be dehed. Half-ce11 potential mapping can give
precise information about the local corrosion damage on structures that do not have several
layers of reinforcement (Chess and Gronvold, 1996). However, the analysis of the results can
be rnisleading and a qualified specialist must interpret the data in order to identie the
influence of the circurnstances, such as variations in hurnidity and other inhomogeneties,
which may lead to wrong conclusions.
2.7.1.2 EIectrochemical Noise
The technique of electrochemical potentid noise monitoring of steel in concrete measures the
noise fluctuations in the free corrosion potential of the reinforcernent (E,,,) by applying a
high input irnpedance voltmeter against a stable reference electrode, as for potential mapping.
It is believed that the noise represents the activity of the passive oxide film on the surface of
the electrode (McKenzie, 1986). When reinforcement is corroded, broken or damaged
passive oxide film generates curent.
When noise data are collected they should be conditioned in order to remove trends. The
relationship between the standard deviation of the amplitude of noise tirne record and a
corrosion rate determined by polarization resistance could be established (McKenzie, 1986).
2.7.1.3 Electrical Resistance
The electrical resistance method for monitoring corrosion is based on the fact that when
metal corrodes, electricd resistance changes. In case of uaiform corrosion, measured
electrical resistance is proportional to the growth of corrosion; however, in case of pitting
corrosion this method rnay give misleading resdts (McKenzie, 1986).
The embedded corrosion probe adjacent to the reinforcement consists of two components of
an AC bridge. While one element is exposed, the other one is covered to compensate for
fluctuations in resistance due to temperature changes. The electrical resistance monitoring
method measures the instantaneous value of corrosion rate. Therefore, a few readings should
be taken in order to find an average value of corrosion rate (McKenzie, 1986).
2.7.2 Perturbative Methods
This group includes the linear polarization resistance technique and the AC impedance
method, which are explained in the following sections.
2.7.2.1 Linear Polarization Resistance
Linear polarization has been used extensively for corrosion rate measurements and dates
back to Tafel's original work in 1905 (Cameron and Chiu, 1986). The polarization resistance
method involves the displacement of the voltage of an electrode relative to the solution or
concrete in contact with this electrode, and consequently some modifications to the rates o f
cathodic and anodic reactions (Lawrence, 1990). The potential - current density plot is
approximately linear in the region within lOmV of the corrosion potential (Figure 2.20). The
slope of this plot in terms of potential divided by curent density has the units of resistance-
area and is often called the polarisation resistance (Dean, 1986). The polarization resistance
is related to corrosion current density by the Stem-Geary relationship (McKenzie, 1986):
r,, = (p, *pj/2.3(--+ pj *m =BIR, [2.111
where l$, = polarization resistance
B = Stem-Geary constant, dependent on contributions fiom cathodic and anodic
reactions
Pa = anodic Tafel constant
p, = cathodic Tafel constant
AI = applied current
AV= change in potentiai
I ,, = corrosion current
Figure 2.20. Linear Polarization Technique (Ehrlich and Rosenberg, 1991)
Linear polarization method is very attractive both in the laboratory and in the field, because it
is relatively quick and easy to perform. The luiear polarization techniques determine the
"instautaneous7' value for corrosion of steel in concrete (McKeruie, 1986). Because the
extent of polarization of the specimen is small, the measurement technique does not
significantly damage the specimen, and so the measurement approach can be used for a long
penod of t h e as long as the general corrosion rate is not too high.
2.7.2.1.1 Tafel Constants
In order to use linear polarization technique it is necessary to know the Tafel constants. In
many cases reasonable estimates can be made as to what these Tafel constants should be, and
in other cases, the method can be calibrated using mass loss data. The Stern-Geary constant
(B) is a function of the environment and alloy under study, and seldom known with accuracy.
The Stem-Geary constant can be related to TaféI constants using the relationship:
As suggested by Corrosion Senrice Co. Ltd. (1998), Stem-Geary constant in many cases can
be approxirnated with 20 rr,V for active state (corrosion current >1 to 2 jA) and 52 mV for
passive state (corrosion current 1 pA).
2.7.2.1.2 Galvanostatic and Potentiostatic Approach
Chess et al, (1998) describe two approaches for measuring corrosion rate by linear
po larization methods.
The first one is called the galvanostaric approach. Two similar electrodes are cast into
concrete at srnall spacing (Figure 2.2Ia). After measurine, the corrosion potential (Eco,), a
few fixed smdl levels of direct current (DC) are prissed fiom the auxiliary electrode to the
reinforcement. Then the corresponding change in potential is measured. in this case, it is
assurned that the potential drop divides equally between the cathode and anode.
BOTH ARE TEST
pELECTROOES * REFERENCE
AUXlLlARY ELECfRODE + ELECTFtODE - __ _1 M
TWO ELECTRODE SYSTEM THREE ELECTRODE SVSTEM
Figure 2.21. Two Types of Polarization Resistance Probes; (a) Two-Electrode System,
(b) Three-Electrode System (Dean, 1986)
The second method is cdled the potentios~aîic qproach. The system used for this method
employs three electrodes (Figure 2.21b). One of them is the working electrode, while the
second one is the auxiliary electrode and the third one is the reference electrode. The
potential between the working and reference electrode is monitored. In order to mesure the
corrosion rate, a small voltage signal (10-20 mV) is applied both anodically and cathodically
to the test sample using a potentiostat and separate counter electrode (Ehrlich and Rosenberg,
1991). The current required for this potential perturbation is recorded and the potential is
plotted as a function of current (Figure 2.20). In conductive electrolytes, the slope of this
curve is referred to as polarization resistance (in ohms). The corrosion rate can be determined
using the Stem - Geary relationship (equation 2.1 1).
2.7.2.1.3 Interpretation of Results
The original method, proposed by Stem and Geary denves the corrosion current from
polarization resistance measurement under the assurnptions, sumrnarized by Giuliaai (1 986):
The potential at which rneasurement is taken (i.e. the corrosion potential) is sufnciently
far from equilibrium potentials that the cathodic deposition of the metal and the anodic
oxidation of the oxidant do not affèct it.
The potential shift induced during the measurement is sufficiently small to allow a series
expansion for the exponential function linking potential and partial current and to limit
such series to the second tenn.
Both anodic and cathodic reactions exhibit Tafel slopes in a voltage vs. log of current plot
at the corrosion potential or the cathodic reaction is under diffusion control and therefore
has an infinite slope.
Only the anodic reaction occurs at the corrosion potential, Le., the entire anodic current is
due to metal dissolution.
Corrosion system is in a steady state, Le., that potential variation over time is negligible,
at least during the measurement time.
Practical expenence indicates that generally reasonable agreement exists between resdts
obtained using linear polarization technique and measured weight losses for a large number
of corroding systems (Giuiiani 1986).
There are some cases, however, for which polarization resistance technique may lead to large
errors, therefore care needs to be taken while interpreting the results. Tests performed in low-
conductivity electrolytes, Iike concrete, c m lead to an underestimation of the corrosion rate if
the electrolyte resistance is not subtracted out Çom the measured resistance. The measured
resistance (the slope of the cuve on Figure 2.20) is actuaily a sum of the electrolyte
resistance and the polarization resistance and only the polarkation resistance is inversely
proportional to the corrosion rate as can be seen fiom Stern-Geary relationship (equation
2.1 1). Since the electrolyte resistance in concrete is so hi& (1000 - 30000 ohm-cm), it
should be accounted for when using polarization resistance technique to measure corrosion
rates (Ehrlich and Rosenberg, 199 1). At high corrosion rates, readings c m be misleading in
case of localized (pitting) corrosion. Moreover, it is usually hard to accurately deterrnine the
area of polarized steel (Chess et al, 1998).
2.7.2.2 AC Impedance
Another non-destructive curent-measuring technique is AC impedance. The AC impedance
method involves application of a small amplitude (t 10 to 20 mV) AC signal to an embedded
electrode and cornparison of the initial perturbation with the response of the embedded
electrode (McKemie, 1986). In order to measure current required to cause voltage
perturbation, the srnall voltage signals should be applied over a range of fiequencies usually
O.SmHz to 100 mHz for concrete (Ehrlich and Rosenberg, 1991). The impedance method is
based on the assumption that the corroding electrode can be represented by an equivalent
electric circuit, combining resistors, capacitors, and indicators to represent the corrosion
process. Most corroding interfaces can be modeled by a simplified Randles' circuit illustrated
on Figure 2.22; where & represents the polarization resistance, Rn is the ohrnic (or
electrolyte) resistance, and C is the double-layer capacitance which exists as a result of
charge separation at the metaVelectrolyte surface (Ehrlich and Rosenberg, 1991).
Figure 2.22. Simplified Randles' Circuit for Steel Corrosion
(Ehrlich and Rosenberg, 199 1)
At low fiequencies, the current going through the circuit can flow only through Rp and Rn.
Therefore, the measured impedance is & + Rn. While at high fiequencies current flows
through the capacitor instead of R,,, thus measured impedance is Rn. Consequently,
polarization resistance is the difference between the low fiequency and high fiequency
ùnpedances (Ehrlich and Rosenberg, 199 1).
Theoretically, the imaginary part of the measured impedance shouid descnbe a semicircle
when plotted against fiequency (Nyquist diagram, Figure 2.23). The two intercepts on the x-
axis are Rn or Rs and Rs + % values. The drawback of this sirnplified impedance method is
the difficulty in finding a second intercept at very low fiequencies (Guiliani, 1986).
I
nrru RE8iSTAHCL ohm 1
Figure 2.23. Nyquist Diagram (Dean, 1986)
2.7.3 Probe Applications
Embedded corrosion probes adjacent to the reinforcernent are beneficial for measurements of
corrosion rate. The major advantage occurbg fiom embedding corrosion probes is that the
corrosion monitoring techniques c m be promptly applied to determine corrosion rates. Some
of the disadvantages sumrnarized by McKenzie (1986) include:
The probe only yields corrosion data confined to its Iocal environment
The probe vdl not be in the same electrochemical state as the resorcement
If embedded subsequent to construction, the probe will necessarily see a different
environment to that of the reinforcement bar
Unknown working area of the working electrode/reinforcement.
There are different types of commercial probes now available. In the case of resistance
probes, the electrical comection to the reinforcement pennits the probe to adopt a potential
and corrosion state similar to, although not exactly the same, as the reinforcement. An
alternative to a resistance probe would be to embed a haE-ce11 reference electrode, working
electrode and cornter electrode close to the reinforcement. This three-electrode arrangement
would permit ha-cell potential, DC polarization and AC impedance measurements to be
conducted (McKenzie, 1986). There are different embeddable haif-ceil electrodes available.
Ag/AgCl cells have shown a stable performance and a very fast response tirne (Baker, 1986).
However, they give potentids that are a function of chioride concentration in them
(Broomfield, 1997). In 1986 an embeddable MnOz reference electrode was developed.
Results compare favourably with published data for other embeddable elecirodes and show
that the MnOz electrode has found wide acceptance, rnainly because of its very good long-
term stability (Amp et al, 1997).
2.8 Fibre Optic Sensors (FOS) in Structures
Until adequate repairs are made, monitoring of deterioration and progressive decay, and the
ability to wam against impending faiIure are essential for saving human life. Recent
developments in the field of fibre optic sensors offer advantages that can essentially
eliminate such deficiencies and enable the early warning of impending failure.
The introduction of advanced composite materials into rehabilitation and strengthening of
existing structures could be accelerated by the integrated monitoring capability provided by
fibre optic structural sensing technology. Installation of FRP wraps and patches provides the
opportunity to integrate fibre optic sensors with the composite materials to monitor
subsequent behaviour of the structure.
Fibre optic structural sensors are h u n e to electromagnetic interference making them more
suitable than conventional sensors for remote settings within large structures that are subject
to lightning or manmade sources of electrical interference. Due to their small size, linear
geometry and dielectric nature, optical fibres are embeddable in advanced composite
materials used for rehabilitation of concrete structures. Besides ruggedness, flexibility, and
small size, the most attractive feature of fibre optic sensors are their inherent ability to serve
as both the sensing element and the signal transmission medium, allowing the electronic
instrumentation to be remotely located fiom the measurement site (Ansari, 1 997).
2.8.1 Manufacture of FOS
As shown in Figure 2.24, optical fibres manufactured at the University of Toronto Institute
for Aerospace Studies (1998) incorporate severai protective layers of material. A 0.007 mm-
thick optical core, at the centre of the fibre, carries the optical signai. Enclosing the core is an
amorphous solid hsed silica g las shell about 0.125 mm in thickness. The optical ciadding is
contained within a 0.75-mm thick coating of material such as acrylate or polymid buffer. The
various coatings surrounding the fibre optic core protect the g las fibre surface fiom abrasion
during handling and installation; fkom moisture, which weakens the fibre and can contribute
to the growth of micro cracks; and fiom the alkaline environment in concrete, which is
corrosive to conventional glass fibre. An outer jacket, either tightly bound to the fibre or in
the form of a loose tube, is surrounded by Kevlar-reinforcing fibres embedded in a polyrner
such as PVC plastic.
Keviar (Tight or toose) Fibre Buffer Reinforcing Fibres
Figure 2.24. Bondable Sensor (U of T, Institute for Aerospace Studies, 1998)
2.8.2 Basic Principlesof Application
Fibre optic sensing technology involves the installation of optical fibre sensors, which
measure strain and temperature, in or on concrete or steel structures and advanced composite
material patches. Basic principles described by U of T Institute of Aerospace Studies (1998)
are presented below.
Due to strain in the structure and temperature variations, fibre optic sensors expand or
contract by small amounts. A light signal is sent down the fibre to the sensor and is
moduiated according to the amount of expansion or contraction (the change in length of the
sensor). The sensor reflects back an optical signal to an analytical device, which translates
the reflected light into numencal measurements of the change in sensor length. These
measurements indicate the precise amount of strain in the structure, and compensatory
calculations are made to eliminate the eEect of temperature change on the strain
measurements.
2.8.3 Fibre Optic Bragg Grating Sensor
A fibre Bragg grathg (FBG) is a segment in optical fibre having an axial periodic variation
in its core index of refraction, as illustrated in Figure 2.25.
Optical Fibre Grating Sensor
Figure 2.25. Bragg Grating Description (U of T, lnstitute for Aerospace Studies, 1998)
A fibre Bragg grating sensor consists of a continuous fibre core surrounded by germanium-
doped silica The grating portion consists of modulation in the index of lefiaction dong a
short length of that continuous fibre core. A change in length of the grating is due to
mechanicd strain or thermal effects in the matenal in which it is ernbedded (U of T, lnstitute
for Aerospace Studies, 1998). Fibre optic Bragg grating sensors rneasure strain through a
spectral shift of wavelength, which provides good isolation fiom noise sources such as
intensity fluctuation caused by light source or bending loss in the fibre. Typical grating
lengths range fiom 1.5 to 15 mm, with longer gratings giving a narrower spectral bandwidth
(Nawy and Chen, 1994). The grating forms a narrow band reflechon at wa-velength, which
matches the Bragg condition:
where RB = Bragg wavelength
n = refiaction index of fibre
A = Penod of Bragg grating in the fibre
The narrowness o f this band gives good resolution and strains less then 10 miicrostrain can be
detected. A theoretical relationship has been established between fiactieonal change in
wavelength and induced strain (Nawy and Chen, 1994):
where k is a constant of strain coefficient for grating. A typical value of k is about 1.3.
For certain applications it is desirable to have a gauge length that is longer t&an practical for
a single Bragg grating. In this case it is possible to use either one short brmadband grating
and mirrored tip of the optical fibre. or two short broadband gratings. The sëeparation of the
reflection elements now defines the gauge Ien-@h of the localized sensor. The arbitrary bauge
length structural sensor is illustrated in Figure 2.26 (Measures, 1997). T h i s type of sensor
makes an ideal hoop-strain sensor for use directly ullth concrete coa1urnns or with
rehabilitation and strengthening wraps for corroded or earthquake darnaged cmncrete columns
as shown in Figure 2-37.
Figure 2.26. Schematic Diagram of Arbitrary Gauge Length, Localized Fibre
Optic Structural Sensor Based on Low Coherence uiterferometry
(Measures, 1997)
Corrvded Cuncrete Colum
Jxketed Opticai Fi
Demodulation System
Cornpositc Material ~ehabilitation and Strenglhening Wrap
Figure 2.27. Application of the Localized Fibre Optic Structural Sensor to the
Measurement of Hoop Strain in Concrete Columns (Measures, 1997)
The performance of these kinds of sensors has been recently tested on site: a concrete column
under Highway 401 in Toronto (Measures, 1997). It is anticipated that these sensors will be
able to monitor pressure built-up within the composite repair wraps and relate this to
continuhg corrosion of the steel within the concrete column.
2.9 Remarks on Literature Review
The outlined literature review represents the main background for the effective experimental
design. The future research will focus on the development of restorative and monitoring
techniques for corrosion damaged CO ncrete infrastnrcture repaired with FRP wrapping
material.
The deterioration of the reinforcing steel embedded into concrete due to electrochemical
process has been discussed and appreciated. While mechanism of corrosion is well
understood, several interrelated factors determine the nature and severity of the corrosion.
These factors Vary fiom case to case and will be addressed specifically in the research.
The mechanism of concrete confinement restraining the Iateral expansion has proven to be
extremely effective and will be utilised in the research. FRP wrapping rather than reinforced
concrete or steel jacketing will be chosen as a repair strategy for extemal confinement of
corrosion damaged columns. Fibre reinforced polyrnets will be chosen because of the high
strength, ease of handling, low maintenance cost, corrosion resistance and impermeability to
chlorides, moisture and oxygen. The proper orientation of fibres, thickness of wrapping
sheets, and colurnn shape will be considered to optimize the structural pedormance and to
enhance the energy absorption capacity. The restorative techniques of the steel reinforced
corrosion damaged colurnns may include the application of the expansive grouts. The usage
of appropriate corrosion inhibitors of the proper concentration will be also thoroughly
considered to facilitate the effective research.
Since the corrosion of reinforcement is one of the most important causes of the inadequate
performance of the reinforced concrete stnictures, great attention will be provided to the
effective monitoring of this process in FRP wrapped columns. Various monitoring techniques
have been discussed and acknowledged in this literature review. These techniques are more
or less advantageous depending if used to monitor corrosion in the laboratory or in the field
environment. Half-cell potential mapping can yield good laboratory results in cases of pitting
corrosion on structures with uncomplicated reinforcement layouts. However, this technique is
mostly used to i d e n w the existence of corrosion rather than quantity. It gives little
information regarding the severity of a corrosion process. Electrical resistance as well as the
electrochemical noise method have shown satisfactory laboratory performance in the case of
unifôrm corrosion, however, in the case of pitting corrosion, results can be rnisleading.
Linear polarisation method has a good record of being used in the field. However, iinear
polarisation method detects the instantaneous corrosion rate, which can change with
temperature and relative humidity. To address the mentioned advantages and to generate
highly reliable monitoring results the half-ce11 potential mapping and the linear polarisation
rnethods will be used in the current research.
The monitoring of the lateral expansion of columns due to corrosion of reinforcing steel will
be conducted using mechanical collars in conjunction with the effective application of the
fibre optic stnictural sensing technology. The unique benefits associated with fibre optic
sensors include immunity to electromagnetic fiequency interference, small dimensions and
light weight, low disturbance to the structure, capability of intermittent readings with
reconnection between readings, long-tem stability, and corrosion resistance. Thus, the usage
of fibre optic sensors will insure the high monitoring standards in the investigation of the
FRP repaired columns.
CHAPTER 3. EXPERIMENTAL PROGRAM
This work is part of a larger study, initiated by Lee (1998) and continued by Khajehpour
(2001), involving the development of a repair system consisting of advanced composite
materials for rehabilitation and remote monitoring of corrosion-darnaged reinforced concrete
columns. A detailed laboratory testing plan for a total of thirty-one columns in the ove rd
study is outlined in Table 3.1.
Specific objectives of this experimental work include:
1. Establishing an optirnai laboratory procedure for accelerating corrosion activity.
2. Monitoring naturai corrosion behaviour in reinforced concrete columns.
3. Long-term monitoring of the corrosion process in repaired columns.
4. Obtaining the necessary data to compare the corrosion process in repaired and non-
repaired reinforced concrete columns.
3.1 Laboratory Test Plan
The previous laboratory studies conducted by Lee (1998) and Khajehpour (2001) consisted
of casting twenty-five coluwi specimens, and conducting various experiments on them,
which are discussed in the earlier theses.
This experimental study consisted of:
casting 6 new column specimens identical in form to earlier specimens (305 mm
diameter, 10 16 mm height), as detailed in Section 3.2.1.
subjecting 10 column specimens to accelerated corrosion regime (5 newly cast columns,
3 specimens cast by Khajehpour (200 1) and 2 repaired columns)
mod-g the accelerated corrosion regirne to maintain the consistency of the degree of
damage caused by corrosion process in al1 new corroding columns
m o d e i n g the commercial multi-element probe for corrosion monitoring
monitoring corrosion in 5 column specimens and one pnsm
repairing 4 coliimns with carbon fibre reinforced polyrners
Corroded-Tested Corroded-Repaired-Tested
Corrodcd-Rcpaired- Accelerated Corrodcd-Repaircd-Natural
P Mdti-Element Probe MP Modified Multi Element Probe li< Expansion Grout and Corrosion Inhibitor - High Darnagc ** Specimen trcatcd with corrosion inhibitor should not be subjected to accelerated corrosion after repair.
Table 3.1. Laboratory Testing Plan
3.1.1 Test Regimes
Specimens summarized in Table 3.1, are compared by test regimes (horizontally) and by
series (vertically). There are seven different test regimes- The main purpose of the first three
regimes is to assess the structural performance of corroded specimens confined with CFRP
wraps with the performance of unconti~ned column specimens. First row (control) represents
specimens, which afier casting were tested to failure in compression in order to evaluate the
capacity, strength and ductility of non-corroded spirally reinforced RC columns. Second test
regirne, designated corroded-tested, corresponds to the column specimens that are subjected
to an accelerated corrosion, as described in Section 3.4.1. until they reach targeted Ievel of
damage (moderate or hi&). Moderate damage is consigned to 6.5% of steel loss, while hi&
damage is assigned to 13% of steel loss. Afier reaching the targeted level of damage,
specirnens are to be tested to failure in corr~pressien to compare the capacity. ductility and
strength of corroded unconfined col~mns with the performance of control specimens. The
third test regime is denoted corroded-repaired-rested. Afier being corroded to a targeted
leveI of darnage, colurnn specimens from this row are to be wrapped with CFRP sheets prior
to structural testing. The repair procedure is discussed in details in Section 3.4.3. The
structura1 performance of corroded confined columns is to be compared with the performance
of corroded unconfined and control column specimens.
TweIve specimens assigned to the above-described test regimes and shown in shaded cells of
the Table 3.1 have already been structurally tested and results are described in earlier theses
(Lee, 1998; Khajehpour, 200 1). Remaining specimens are currently subjected to accelerated
corrosion and will be repaired and tested afier they will reach targeted level of darnage, For
that reason, this thesis is concentrating only on corrosion monitoring rather then structural
performance of the columns.
The prime purpose of the next two rows of specirnens is post repair corrosion monitoring.
Specimens subjected to the test regime denoted corroded-repaired-accelerated (corrosion),
are corroded to a targeted level of damage (moderate or high), repaired with CFRP wraps and
subjected to post-repair induced corrosion in order to evaluate the amount of expansion and
the rate of steel loss under accelerated corrosion. and to compare the performance of
specimens before and after repair. Column specimens assigned corroded-repaired-natural
(corrosion) test regime are rneant for long term monitoring to assess the performance of
CFRP wrapped column specimens subjected to "natural corrosion", as specifically deçled
for the purpose of this study. The rate of steel loss and the amount of expansion is compared
to that before repair. Natural corrosion was achieved by cyclic variation of wetting in 3%
NaCl solution and dryïng in laboratory air (1 day wetting and 2.5 days drying) without
applying an electrical potential. Natutal corrosion was monitored using multi element probe,
a commercial device for corrosion monitoring described in Section 3.2.2.
Specimens in the next row (natural corrosion) are subjected to natural corrosion regime
since the time of casting. These specimens were also cast with multi-element probe for
corrosion monitoring- The purpose of this test regime is to establish natural corrosion rate
and to evaluate the performance of multi element probe. The 1 s t row, named natural
corrosion-moisture barrier consists of one column, which was cast with modified rnulti
elernent probe discussed in detail in Section 3.2.2.2. Since the time of casting, thîs column is
subjected to naturd corrosion regime. It will be repaired with a membrane, which replicates
the moisture barrier effect of the CFRP wrap without the mechanical restraining effect.
3.1.2 Families of Column Specimens
Prirnary variables of the test plan (Table 3-1) are organized in columns for the various
families of specimens. The main purpose of the pilor series was to establish an acceptable
accelerated corrosion regime. Furthermore, the repeatabiIity of structural test results was
evaluated. The function of the nutural damage series was to establish natural corrosion rate
and to evaluate performance of the commercial and modified multi element probes described
in Section 3.2.2. Specimens assigned to the moderate damage series are to be subjected to an
accelerated corrosion as discussed in Section 3.4.1 until they reach a moderate level of
damage (estimated as 6.5% of steel loss). Consequently, specimens fkom the high damage
series are supposed to reach high level of damage (estimated as 13% of steel loss) as a result
of an accelerated corrosion.
As opposed to the previous three families of columns, which corresponded to the level of
damage, the next three characterize different repair techniques. After being corroded,
specimens fiom the expansive grout series are to be repaired using expansive grout. This
kind of repair involves the removal of delaminated concrete. In order to make this procedure
easier, columns should be subjected to an accelerated corrosion until they reach a high level
of damage (13% steel 1oss)- Afier removal of concrete cover and application of expansive
grout, colurnn shoüld be wrapped with two layers of CFRP sheet. Accordingly, since the
repair of colurnn fiom corrosion inhibitor series involves the removd of delarninated
concrete, this coIumn should be corroded to high level of darnage (13% steel loss). M e r
removal of cover and application of penetrating corrosion inhibitor. column should be
confined with two layers of CFRP wrap. These repair techniques are then compared to the
approach of wrapping column without "fixing" it first. Colurnn specimens fiom one Zayer
repair series are corroded until they reach a high level of darnage and repaired using only
one layer of CFRP wrap in order to evaluate the effect of number of CFRP layers as far as
confinement is concerned. Furthemore, the amount of expansion and the rate of steel loss of
one and two layer wrapped columns are compared.
Individual column specirnens f?om Table 3.1 are discussed in the remainder of this section.
Table 3.2 surnmarizes the time of casting, beginnuig and end of accelerated corrosion as well
as variations of wetting and drying cycles in attempt of promoting corrosion process; and
tirne of repair of individual column specimens including:
2 and 4 fiom pilot series cast by Lee (1998), the long term monitoring of which has been
continued as a part of the current study;
SI-SI1 cast by Khajehpour (2001) and remained for monitoring as a part of this
experimental study ;
5 12-5 17 cast and monitored as a part of the current study.
Beginning of macro-ceIl measurements and linear polarkation (LP) testing, discussed in
detail in Section 3.4.2, are also presented in Table 3.2.
Specimen Cast Acc. Corrosion Repair Macm-Cell LP CyclesofWetthgandDrying 2 JuVAug-96 No No 12-Dec-97 11 -Jun-99 Wrapped in burlap and plastic 17-Dec-96
Wet/drv 2,511 30-Am--98
1 22-Dec-99 - present S2 24-Apr-98 10-Sept48 - 8-Jul-99 1 -Dec-99 No No
22-Dec-99 - present S4 1-May-98 10-Sept-98 - 1-Jul-99 1-Dec-99 No No
~ d f tiry/wet 2 . ~ 1 11 -srp-ga WeVdry 2.511 7-Dec-98 Half drylwd 2,511 1 1 -Sep48 Wet/drv 2.5/1 7-Dec-98 Hal f dry/w et 2.5/ 1 1 1 -Sep-98
Half dry1wd 2 3 1 1 1 -Sep-9E Wetldrv 2.91 7-Dec-98
1 S7 1 10 Jul-98 1 22-Dec-99 - presmt 1 No 1 No 1 IWeVdry 2.511 22-Dec-95 S8 S9
SI0
S l l 512
1 517 11 l -NOV-991 22-Dec-99 - present 1 No 1 No 1 lwe~dry 2.5/1 22-~ec-991
J 13 514 J 15 J 16
Table 3.2. History of Column Specimms Treatrnent
10-Jul-98 15-Jul-98 15-Jul-98
3-Dec-98 10-Nov-99 1 1 -Nov-99 Il-Nov-99 10-Nov-99 1 1 -Nov-99
22-Dec-99 - présent 22-Dec-99 - present
10-Sept-98 - 18-Dec-98
No No
22-Dec-99 - present 22-Dec-99 - present 22-Dec-99 - present 22-Dec-99 - present
No No
1 -Dec-99
No No No No No No
No Na
18-May-99
7-Sep-99 29-Dec-99
No No No No
,
18-Aug-98
12-Jan-99 22-Dec-99
Wetldry 2.50 22-Dec-99 Wet/âry 2.511 22-Dec-99 Wetldry 2 .W 22-Dec-99 Wetldry 2.511 22-Dec-99
WeVdry 2.5/1 22-Dec-99 Wet/dry 2,511 22-Dec-99 Wetldry 2.511 16-Jul-98 Half ârylwet 2,511 1 1 -Sv-98 WeUdry 2,511 7-Dec-98 Weüdry 2.511 12-Jan-99 WeUdry 2,511 22-Dec-99
3.1.2.1 Pilot Series (2,4)
Column specimens fiom this series were cast, rnonitored, repaired and tested by Lee (1 998)
and Khajehpour (200 l), and discussed in greater detail in their theses.
Two coliimns were left for long-tenn natural corrosion monitoring. These columns, denoted
column 2 and column 4, were cast in July 1996 (Table 3.2). Column 2, subjected to naturaz
corrosion testing was under natural corrosion (i.e. no applied electrical potential) since the
thne of casting. Until May 1998 it was covered with wet budap and plastic to achieve 100%
relative humidity as shown in Table 3.2. However, it was f o w d more efficient, in t ems of
promohng corrosion, to use cyclic wetting and drying (lday wetting in 3% NaCl solution and
2.5 days drying in laboratory air). This regime was then used for al1 other column specimens
that required natural corrosion testing.
Column 4, assigned corroded-repaired-namal test regime was under accelerated corrosion
for 343 days until it reached an estimated steel Ioss of 7.8% by mass and a circumferentid
expansion of 0.179% (Lee, 1998). Then it was repaired with two layers of CFW sheets and
since has been subjected to a natural corrosion cycles of 1 day wetting in 3% NaCl solution
and 2.5 days drying in laboratory air.
Column specimens 2 and 4 were cast without a multi-elernent probe. Nevertheless, linear
polarization tests were conducted using the procedure outlined in Section 3.4.2.1. Based on
the results of corrosion monitoring, the performance of naturally corroding column 2 was
compared to the performance of corroded and repaired col- 4.
3.1.2.2 Natural Damage Series (S11, J12)
This series consists of two columns, denoted SI 1 and J12, which undergo natural corrosion
since the tirne of casting. Column S11 was cast in December 1998 with multi-element probe
for corrosion monitoring in order to evaluate the long-term stability of the probe in the
aggressive environment provided by cyclic variation of wetting in 3%NaCl solution and
drymg in laboratory air. Another purpose of this column was to establish a natural corrosion
rate. Column 512 allocated to nuîural corrosion-rnoisture barrier test regime was cast in
November 1999. It contains a modified multi-element probe for corrosion monitoring
described in Section 3.2.2.2. Column 512 is currently monitored under natural corrosion
regime. M e r the corrosion process is active and stable, column 512 will be wrapped with a
membrane to inhibit moisture penetration into the column and to compare its performance
with carbon fibre reinforced polymer wraps.
3.1.2.3 Moderate Damage Series (SI, S10)
The moderate damage series consists of four colunins that underwent accelerated corrosion
regime until they reached a targeted steel loss of 6.5 %. Two of these columns (S3 and S5)
were tested by Khajehpour (ZOO 1) and resdts are discussed in his thesis. The two remaining
columns, denoted S 1 and S 10, are discussed in this section.
Column S1 subjected to corroded-repaired-accelerated (corrosion) test regkne was cast in
April 1998. It was under an accelerated corrosion for 105 days until it reached a steel loss of
6.75% and circumferential expansion of 0.175% (Khajehpour, 2001). It was repaired with
two Iayers of CFRP sheet in Januafy 1999, after which it took a year for the corrosion
activity to stabilize before it was subjected to accelerated corrosion testing again in order to
evaluate the performance of CFRP wrap under these conditions and to compare it with
performance of other columns subjected to corroded-repaired-accelerated (corrosion) test
regime (see Table 3.1).
Column SI0 subjected to the corroded-repaired-natural (corrosion) test regime was cast in
July 1998 with multi element probe for corrosion monitoring. It was under an accelerated
corrosion for 99 days until it reached a steel loss of 6.93% and circumferential expansion of
0.248% (Khajehpour, 2001). It took about a year for corrosion activity to stabilize, d e r
which column S10 was repaired with two Iayers of CFRP wrap. It is currently subjected to
wetldry cycles without applied potential in order to evaluate the effect of the CFRP wrap on
post repair natural corrosion and to compare its performance with other specimens
undergoing corroded-repaired-natural test regime (see Table 3.1).
3.1.2.4 Eigh Damage Series (S7-S9,J14)
The high damage senes consists of four columns. Three of which, denoted S7, S8 and S9,
were cast in July 1998 by Khajehpour (200 l), while column J14 was cast in November 1999
as a part of this research program (Table 3.2). All four columns are currently subjected to
accelerated corrosion until they reach a targeted steel loss of 13%.
After column S7, assigned to the cowoded-tested regime, will reach a steel loss of 13%, it
will be tested to failure in compression and its strength and ductility will be compared to that
of columns with different levels of damage subjected to the same testing regime (see Table
3 -1). To evaluate the extent of damage achieved by accelerated corrosion, the performance of
column S7 will also be compared to the performance of control specimens.
After column specimen S8, allocated to the cowoded-repaired-tested regime, will reach a
steel loss of 13%, it will be repaired with two layers of CFRP wrap and tested to failure in
compression. The performance of column 58 will then be compared to the performance of
other column specimens subjected to the same testing procedure but with different levels of
damage and different types of repair. In order to assess the effect of confinement on the
stnictural performance of corroded specimens, the capacity, strength and ductility of column
S8 will also be compared to that of unconfined corroded columns subjected to corroded-
tested regirne (see Table 3.1).
Column specimen S9 subjected to corroded-repaired-accelerated (cornsion) test regime
will be repaired with two layers of CFRP wrap subject to achieving a high level of damage
(13% steel loss). After repair, it will be connected to accelerated corrosion regime again in
order to evaluate the performance of CFW wrap under M e r accelerated corrosion testing.
The amount of expansion and the rate of steel loss before and after repair will be compared to
evaluate the effect of wrap on specimen's corrosion activity. Furthemore, the performance
of the CFRP wrapped column under accelerated corrosion will be compared with the
performance of other columns subjected to corrosion-repaired-accelerafed testing regime to
evaluate the repeatability of results (see Table 3.1).
Column specimen 514 wiil also be repaired with two layers of CFRP wraps after reaching a
targeted steel loss of 13%. This column, assigned to corroded-repaired-natd (corrosion)
regime was cast with a multi-element probe in order to evaluate natural corrosion activity in
CFRP wrapped column and to compare the natural corrosion rate before and after repair.
Moreover, post repair natural corrosion activity will be compared to that of specimens
repaired using expansive grout or penetrating corrosion inhibitor prior to wrapping.
3.1.2.5 Expansive Grout Series (J13,515,516)
The expansive grout series consists of three colurnns denoted 51 3-51 6, and J15, which were
cast in November 1999 as a part of the current experimental program. They are currently
subjected to an accelerated corrosion regime until they reach a high Level of darnage (target
steel loss of 13%). The reason for these columns to be highly damaged before repair is that
cover could be removed easily to be replaced with an expansive grout to induce active
confinement from the wrap. M e r that, these columns will be wrapped with 2 layers of CFRP
sheets.
Column 513 will be tested to failure in compression in order to evduate the effect of active
confinement by comparing it with the performance of passively confined columns and
unconfined specimens. Columns 515 that contains multi element probe for corrosion
m o n i t o ~ g and column 51 6 will remain for long-term corrosion mûnitoring in order to assess
the repair technique that involves replacing delamuiated concrete with an expansive grout by
comparing the corrosion activity of columns 515 and 51 6 with that of columns repaired only
with CFRP wraps subjected to the same testing regime (see Table 3.1).
3.1.2.6 Corrosion Inhibitor Series (J17)
The corrosion inhibitor series consists of one column that was cast in November 1999. It is
currently subjected to an accelerated corrosion regirne until it will reach a target steel loss of
13%. Concrete cover will be rernoved and penetrating corrosion inhibitor will be applied
before placement of a new concrete to reduce the rate of corrosion after repair. Then column
will be wrapped with two layers of CFRP sheet and will remain for long term monitoring.
The effect of penetrating corrosion inhibitor on corrosion activity will be assessed by
comparing the natural corrosion rate of JI7 with that of other repaired specimens undergoing
natural corrosion testing (see Tabie 3- 1).
3.1.2.7 One Layer Repair Series (S2, S6)
The one layer repair series consists of two columns cast in May 1998, which underwent
accelerated corrosion- Column S2 reached 1 3 -52% steel loss and circumferential expansion
of 0.348% while column S6 reached 13.53% steel loss and circumferential expansion of
0.752% (Khajehpour, 1 999). M e r half a year of stabilizing they were repaired with one layer
of CFRP sheets. Colunin 6 is left for structural testing which will be conducted together with
specimens 57, S8 and 513 &er these columns will reach targeted steel loss in order to
maintain similar testing set up and thus minimize test variables.
M e r being repaired, column S2 is currently subjected to an accelerated corrosion regime in
order to evaluate the performance of one layer repair under induced corrosion regime and to
compare it with the performance of column specimens repaired with two layers of CFRP
wraps under induced corrosion (see Table 3.1).
3.1.2.8 Special Cases
There are two special specimens rnentioned in Table 3.1. First one, denoted column 1
belongs to the pilot series. The purpose of this specimen was to investigate cycles of wetting
and drying as an alternative accelerated corrosion technique. However, it was found that
corrosion could not be initiated rapidly enough without applying an electricd potential.
Column 1 discussed in detail in earlier thesis (Lee, 1998).
Column specimen designated 54 was connected to accelerated corrosion regime incorrectly,
i-e. rebar cage became a cathode (Khajehpour, 2001). After 50 days, specimen S4 did not
demonstrate any sign of corrosion as opposed to al1 other specimens connected to an
accelerated corrosion regime. The mistake was discovered and connections were corrected on
day 50. Four days later c o h m S4 demonstrated visual signs of corrosion activity and was
corroding rapidly thereafter. The average rate of steel loss for column S4 was four times of
that for al1 other corroding columns. Therefore, it was proposed to further investigate this
accelerated corrosion technique in terms of becoming an alternative to the procedure used for
d l other columns of the overall research project described in Section 3.4.1, After reaching
44.16% of steel loss and stabilizing corrosion activity, column S4 was repaired with two
layers of CFRP wrap. To evaluate structural performance of this highly corroded specimen
confined with two layers of CFRP wrap, column S4 will be structurally tested together with
columns S 6 , S7, S8 and 513 to maintain similar testing set up and thus minimize test
variables,
3.2 Column Specimens
Specimens used in the overall study were designed to mode1 aging reinforced concrete bridge
columns. In order to simdate chloride contamination of the cover by de-icing salts, two types
of concrete were placed simultaneously in each specimen. To initiate corrosion rapidly, the
cover concrete was cast using chloride-contaminated concrete; while the inner core was cast
using essentidy the same concrete mix but without chlorides (Figure 3.1). To facilitate the
ingress of oxygen and moisture through the cover, a high water-to-cement ratio was used.
3.2.1 Column Specimen Geometry
A total of thirty-one large-scale reinforced concrete columns have been constructed to date.
The overall dimensions and cross-sectional details are shown in Figure 3.1. Al1 specimens
were 305 mm in diarneter and 1016 mm in height. The clear cover to the spiral steel was
20 mm. Each column was reinforced with six 15M longitudinal reinforced bars and D5 spiral
at 44-mm spacing, providing reinforcement ratios of 1.7% (by area) in the longitudinal
direction and 1 .l% @y volume) in the transverse direction. Longitudinal bars were pre-
drilled and tapped at one end in order to accommodate an electrical comection for
accelerated corrosion and macro-ceII measurements. For the pilot series, a seventh 15M bar
was placed longitudinally in the centre of the colurnn to serve as an intemal cathode for the
accelerated corrosion set up. For the rest of the specirnens, a perforated hollow 15-mm
diameter stainless steel pipe was used as an interna1 cathode in order to maintain oxygen
supply at the cathode and to accommodate the ingress of hydroxyl ions to the concrete.
chloride-fiee concrete 6 - 15M bars
mm D5 spiral @ 44 mm
15M bar intemal cathode chIoride concrete
SECTION
ELEVATION
Figure 3-1. Geornetry of Concrete Column Specimen (Lee, 1998)
A 61 0 mm length of the column at nid-height constituted the effective test region. Ln order to
eliminate end effects, corrosion of steel outside the test region was prevented by painting the
upper and lower 203-mm long sections of the reinforcement cage with epoxy (CP 1047B
supplied by Niagara Paint Inc.) and using uncontamlliated concrete (Le. no chlorides) for
both the cover and the core.
3.2-2 Multi Element Probe W P )
To monitor the corrosion process, six coiumns were cast with embedded commercial multi-
eiement probes: two columns fiom the previous study, and four columns from this portion of
the experimentai study.
The multi element probe (MEP) for corrosion monitoring consisted of a carbon-steel working
electrode, a manganese-dioxide reference electrode and a stainless-steel counter electrode as
s h o w on Figure 3.2. Electrode connections are epoxy encapsulated in a PVC housing and
connected to a multi-wire monitoring cable (Corrosion Service Co. Ltd, 1998).
1. - PVC Probe Houçing 2. - Stainless steel counter electrode 3. - Mn02 reference electrode 4. - Carbon steel working electrode 5. - Rebar connection wke 6. - Multiwire connection cabte
Figure 3.2. Commercial Multi Element Probe (Corrosion Service Co. Ltd, 1998)
The MEP has following specifications, as stated in the manufacturer's Literature (Corrosion
Service Co. Ltd, 1998):
Probe Dimensions: (L=l6Omm) x (H=100mm) x (W40m.m)
Weight: Probe head ~0.5 kg (2.5 kg c/w 1 Om cable)
Sensors: S tainless steel counter electrode
Carbon steel working electrode
ERE 20 MnOî reference electrode
Comection cable: 6c-Anixter SO HA- 1806 (10m = Typical).
The multi element probe enables measurement of:
* Corrosion potential of rebar, working and counter electrodes
Corrosion rate via linear polarization resistance
Concrete resistivity with qualitative indication of chlondehumidity ingress
Macro-ce11 corrosion curent.
For this experimental study, the nahirai corrosion rate (Le. corrosion rate in the absence of an
applied electrical potential) was estimated using the linear polarization resistance test (see
Section 3 .W. 1).
3.2.2.1 Prism Specimen
To evaluate the performance of the MEP in tems of stabiIity of the ernbedded Mi102 half-
ce11 in the aggressive environment, one of the probes was embedded into a prism specimen.
The prisrn specimen was constnicted by Khajehpour (2002) and was 280 mm long, 165 mm
hi&, and 165 mm wide. The geometry of the prism is shown in Figure 3.3.
Lonrritudinal Section Cross-Section
Figure 3.3. Geometry of the Pnsm Specimen (Khajehpour, 2001)
The prism specimen was subjected to natural corrosion achieved by cyclic variation of
wetting and drying (2 days wet / 5 days dry) using a 3% NaC1 solution ponded on the top
surface of the specimen. Linear polarization was performed every two weeks at the end of
wetting cycle using four different electrode configurations as described in Table 3.3. The
performance of embedded Mn02 was compared by assessing the stability of its output versus
that of an externally mounted Cu/CuS04 half-ce11 illustrated in Figure 3 -3.
Table 3.3. Electrode Configurations used for LP tests of P r i m
Working Electrode
Probe
10M re-bar 1 stainless steel re-bar 1
Counter Electrode
Probe
Ab breviation
Mn-Pr-cPr
Mn- 1 OM-CS S Cu-Pr-cPr
CU- 1 OM-CS S
probe
Reference HaKCell
Mn02
Mn02
CdCuSO4
C ~ C U S O ~
probe
10M re-bar 1 stainless steel re-bar 1
The regdar set up for h e a r polarization is illustrated in Figure 3.4. The results of linear
polarization tests performed on the prism can be found in Section 4.1.1.2.
Figure 3.4. Set up for LP Tests for Prism
3.2.2.2 Modified Multi-Element Probe (MMEP)
The working electrode of the multi element probe had clean, rust-free shiny appearance as
opposed to the regular rebars fiom the reinforcement cage. Thus, the concern associated with
the probe was that the probe working electrode was most likely made of a higher quality steel
than the rebar cage, therefore, it might be corroding slower. As a result, the multi element
probe was modified in order to compare linear polarization readings measured using the
carbon steel working electrode of the probe with the linear polarization readings using a
piece of black steel bar as the working electrode. A piece of regular 10M black steel re-bar
was attached so that the distance between the 10M working electrode and the Mn02 reference
half-cell would be equal to the distance between the embedded probe working electrode and
MnOz reference half-cell. The geometry of the probe with 10M working electrode is shown
on Figure 3.5. This modified multi-element probe (MMEP) was installed in columns 512,
514, J15, and 51 7. The installation of the probe in the specirnens is discussed in the next
section.
2. - MnOz reference elec'trode 3- - Stainless steel counter electrode 4. - 1 OM rebar working elecîrode
Figure 3.5. Modifïed Multi-Element Probe
3.2.3 Column Specimens Fabrication
Specimens were cast in the inverted position. The reuiforcement cage and the intemal
cathode were fixed in predrilled holes in the formwork in order to have access to them fioni
the top of the specimen after casting. All columns were fabricated in 305-mm diameter
sonotubes. Figure 3.6 shows a photograph of six cylinder forms; one sonotube forrn has been
removed to allow the positioning of the steel cage to be observed.
Figure 3.6. Reinforcement Cage in the Formwork
64
The probe was attached with plastic fasteners to the spiral, on the inner side of the
reîdorcing cage, between two longitudinai bars near mid-height of the column as
schematicdy illustrated in Figure 3.7. In order to prevent electrical continuity with the
spiral, a plastic chair was placed between the spiral and the probe. The Mn02 reference hdf-
cell cover was removed prior to casting. This cover was instdled by manufacturer in order to
protect the ha-cel l against my possible damage. The photo of the installed MEP is shown in
Figure 3.8.
P% , Spiral
Plastic chair
Figure 3.7. Alignment of Probe Details
bars
Figure 3.8. MEP Installation
Columns were cast in the following sequence. The f i s t layer of chloride-fiee concrete was
placed to a depth of 203 mm and vibrated. In order to keep chloride-fiee and chloride
contaminated concrete separate in the test region, a thui aluminium sleeve (178 mm in
diameter, 4 mm thick) was then placed concentricaily inside the column. Two sleeves have
been used during casting: one was rounded for specimens without the modifïed probe, and
the shape of another similar sleeve was adjusted for the shape of the modified probe. Then
chlonde-fiee concrete was placed inside the sleeve, while chloride-contamuiated concrete
was placed outside the sleeve. The rate of placing was controlled to maintain approxirnately
the same depth inside and outside the sleeve. This procedure was used to place al1 of the
concrete inside the test region. After placing this concrete, the sleeve was removed. Prior to
the removal of the sleeve, both types of concrete were vibrated. Finally, the upper portion of
the sonotube (approximately 203 mm) was filled with chloride-fkee concrete, vibrated and
finished. In order to keep consistency, this procedure was identical while casting control
specimens with no chloride-contaminated concrete (Lee, 1 998; Khajehpour, 200 1).
3.3 Materials
Materials used in fabrication and repair of the specimens consisted of cernent, fine and coarse
aggregate, potable water, sodium chloride, admixtures (air entrainer Micro Air - 1, and water
reducer 25 - XL), reinforcing steel and carbon fibre reinforced plastic sheets.
3.3.1 Concrete Materials
Type 10 (CSA) Portland cernent supplied by St. Lawrence was used in all the concrete
mixes.
The fine aggregate used in the concrete mixes consisted of natural sand supplied by Dunlop
Sand Inc. The coarse aggregate used in the concrete mixes consisted of crushed h e s t o n e
supplied by Dufferin. The maximum particle size of the coarse aggregate used was nominally
10 mm.
The water used in concrete mixes w s reguiar city water, which was clean and fiee of
deleterious substances in accordance witb OPSS 1302. A water-to-cernent ratio of 0.62 was
used for al1 concrete mixes.
The air-entraining admixture, Micro Air - 1, was used at a dose selected to achieve a target
air content of 7% for al1 mixes. Furtherrnore, a water reducer 25 - XL was used in al1 mixes
to reduce the amount of water required to achieve the desired workability. Master Builders
Inc. supplied both admixtures.
3.3.1.1 Mix Design
Table 3.4 illustrates the concrete mix design for the pilot series of colurnns- Table 3.5 shows
concrete mix designs for all other concrete specirnens.
Table 3.4. Concrete Mix Design for Pilot Series
Table 3.5. Concrete M x Design for Subsequent Series
Constituents (kg/m3) Cernent Water
Coarse Aggregate Fine Aggregate
Water Reducer (ml) Air Entrainer (ml)
NaCl Water-to-Cernent Ratio
Chloride Mix 300.0 186.0 882.7 869.6 700.0 80.0 9.9
Regular Mix 250.0 155.0 870.0 1040.0 630.0 80.0
- 0.62
Constituen ts (kg/m3) Cernent Water
Coarse Aggregate Fine Aggregate
Water Reducer (ml) Air Entrainer (ml)
NaCl 1
Water-to-Cernent Ratio 0.62 0.62 1
Chloride Mix 250.0 155.0 870.0 1 040 .O 630.0 80.0 8.25 0.62
-
Replar Mix 300.0 186.0 882.7 869.6 700.0 80.0
-
This mix design was modined as needed to account for variations in the aggregate water
absorption. The total aggregate volume and the water content were adjusted in order to
maintain the same water-to-cernent ratio and the mUc volume.
3.3.1.2 Mixing and Casting Procedures
Prior to mixing, al1 materials were weighed and stored in covered buckets. In order to allow
complete dissolution, NaCl powder was added to the water of chloride-contamhated
concrete m.ïx 24 hours prior to casting. Both chloride-f?ee and chlonde-contaminated
concrete were mixed in the University of Toronto laboratory using the medium-size pan
mixer (GE 2036E3)- The slump and air-content for each mix were measured and results are
siimmarized in Table 3 -6.
In order to prevent penetration of cernent paste into the perforated stainless steel pipe (used
as a cathode), a transparent rubber tube was lubricated and inserted into the pipe prior to
casting. Then the pipe was placed into the predrilled hole at the centre of the forrnwork The
rubber tube was removed f i e r the concrete had hardened. Concrete was cured under wet
burlap and plastic sheets for seven days.
3.3.1.3 Compressive Strength Test Results
Twelve standard 100 x 200-mm cylinders were cast for each concrete mix for compressive
strength testing. Concrete was placed in two different layers. Each of the layers was rodded
fïfteen times before placing the next one. Cylinders were cured for twenty-four hours under
wet burlap and plastic. m e r that they were stored in buckets filled with water in the
Iaboratory until test.
The 28-day compressive strength test involved six specimens per concrete mix (three f?om
regdar concrete and three fiom chloride-contaminated concrete). The diameter of cylinders
was measured in five different places at the top and bottom by micrometer. The ends of al1
cylinder specimens were ground to a smooth surface prior to testing. The standard rate of
loading of 1.9 kN/s was used. The average value of 28-day compressive strength of each mix
is presented in Table 3.6. The large discrepancy in concrete properties of different batches is
due to poor quality control during specimen fabrication and testing.
Table 3.6. Concrete Properties
Chloride 180
Batch
1
1 Chloride 210 6 1 9,12 1 Reguiar 1 220
Chloride 200
Specimens
Cl
- -
Regular 191 Chloride 191 Regular 220 Chloride 215 Regular 175 Chloride 185 Regular 175 Chloride 192 Regular 210 Chloride 210
Concrete
iype Remilar
8 9
10
11
-
18 Prisrn 1 Regular ( 1 60
Slump (mm)
65
C2 SI, S2
S3, S4
S5, S6
Air (%) 8.0
f c (at 28 days) W a )
25 .O
Regular Regular Chloride Regular Chloride Regular Chioride
180 250 240 205 230 254 228
33.2 Steel Reinforcement
1SM longitudinal bars and DS spiral transverse reinforcement were used in each of the
twenty-four columns. Mechanical properties are summarized in Table 3.7. The stress-strain
curves for the reinforcement are illustrated in Figure 3.9.
Table 3.7. Mechanicd Properties of Steel Reinforcement (Lee, 1998)
. -
700 - 800 -. I 1
œ
100
a 1 O 0.05 O. 1 0.1 5 0.2 0.25
Strain (mmlmm)
(i) 15M Longitudinal Bars
Properfy Yield Strength, MPa
Ultimate Strength, MPa Modulus of Elasticity, MPa
0.00 0.Oi 0.02 0.03 0.04 0.05 0.08 0.07 O. 08
Strain (mmmim)
(ii) D5 Spiral Rcinforcmient
Figure 3.9. Stress-strain Characteristics of Reinforcing Steel (Lee, 1 998)
15M Bar 44 1 626
198 500
D5 Spiral 529 699
192 500
3.3.3 Repair Materials
Carbon fibre reinforced polymers (CFRP) were used in this project for repair of the corroded
concrete specimens. Unidirectional carbon filaments oriented in the longitudinal direction
were Replark Type 30 sheets manufactured by Kasei Composites Ltd. (Japan) and supplied
by Mitsubishi Canada Ltd. Mechanical properties of the CFRP sheets as stated in the
manufacturer's product manual are summarized in Table 3 -8.
Table 3.8. Mechanical Properties of CFRP Sheets
Fibre Weight, g/mZ Fibre DensiS.,
Design Thickness, mm Tensile Strength, MPa Tensile Modulus, MPa
Elongation, %
Replark, Type 30
300
1.8 0.167 2 936
227 828 1 -4
Two more components were used in the repair of column specimens: primer and epoxy, both
provided by Mitsubishi Chernical Inc. Epoxy consisted of two components: L700S-main
agent BPA epoxy resin and L700S-modified aliphatic polyamine hardener.
3.4 Laboratory Test Procedures
Various laboratory tests were performed on col= specimens and the pnsm specimen.
Accelerated corrosion testing, natural corrosion monitoring as well as repair procedures are
discussed in following subsections.
3.4.1 Accelerated Corrosion Regime
For this experimental study, ten columns were subjected to accelerated corrosion testing, 2
repaired columns (SI and S2), 3 specimens cast by Khajehpour (2001) (S7 - S9), and 6 new-
cast columns (513 - 517) and. Ln order to be consistent with the previous study, the same
accelerated corrosion set up was used. To simulate field conditions, all column specimens
were subjected to wetting and drying cycles (1 day wetting in 3 % NaCl solution and 2.5
days drying in laboratory air) while under accelerated corrosion. Accelerated corrosion was
created using a galvano-static approach. A fixed 12-V potentid was applied across the
reinforcing cage and the intemal cathode. The voltage was supplied by a power source with
fifteen independently regulated channels. A schematic drawing of the corrosion ce11 is
illustrated in Figure 3.10, where V, represents the applied voltage and Vi is the voltage drop
across the 1-i2 resistor.
1
Figure 3.10. Corrosion Cell
Figure 3.1 1 shows a photograph of corroding columns, the accelerated corrosion system and
power supply dong with the data acquisition system.
Figure 3.11. Accelerated Corrosion System
The percentage of steel loss was estimated fiorn the measured value of the electric current by
measuring the voltage drop across 1-0 resistor as shown in Figure 3.10. Assuming uniform
corrosion and that al1 of the measured current is associated with the corrosion process, the
estimated amount of steel loss can be calculated using the equation:
where Aw - the incremental steel loss
W, - the atomic weight in gram (for reinforcing steel, 55.85 g/mol)
I - average current in Amperes
At - time increment in seconds
z - metal vaiency (the number of electrons transferred per atom,
z = 2 for ferrous metal)
F - Faraday's constant (96500 Amps s)
The voltage &op, Vi (Figure 3.10) was recorded by a personal computer every 6 hours on
average ushg a Heliotronic Data Acquisition System. This was converted to the current, 1,
and the total steel loss was determined fiom estimating the area under the corrosion current
versus t h e curve by integration:
While specimens were subjected to an accelerated corrosion testing, the circumferential
expansion was monitored in order to determine the extent of damage caused by corrosion.
Circumferential expansion was measured using mechanical expansion collars. The repaired
column S 1 was also instrumented with two fibre optic senson; one of them was placed inside
the wrap at mid-height, while the other one was mounted at the sarne height on top of the
CFRP wrap. A cornparison of readings taken by mechanical expansion collars and by fibre
optic sensors made for column S 1 is presented in Section 4.2.1.2.2.
3-4.1.1 Mechanical Expansion Coiiar
A mechanical collar, shown in Figure 3.12, was made of a fl2-76-mm wide and 0.85-mm
thick stainless steel. The collar had a stainless steel head at each end. The gap between the
heads was held closed by two stainless steel springs, attached to the top and bottom of the
heads with stainless steel screws. The collar was wrapped acound the colurnn at the mid-
height. In the case of coIumn SI, the collar was placed 50 mm below the mid-height, in order
to avoid interference with the fibre optic sensor.
Figure 3.12. Mecfianical Expansion Collar
Circumferential expansion readings were taken twice a week immediately pnor to the
wetting period; the measurements were made using a micrometer (accurate to 0.01 mm) to
measure the change in the size of the gap between the heads, as iilustrated in Figure 3.13.
Figure 3-13. Expansion Measurements
3.4.1.2 Fibre Optic Sensors
Fibre optic Bragg pting long-gage strain sensors were wrapped around column specirnen
Si at the mid-height (Khajehpour, 2001).
A one circumference of the column (0.974 rn) sensor was wrapped between the first and
second layer of the CFRP wrap. Prior to the repair procedure, the fibre optic sensor (FOS)
was secured on the d e r side of the CFRP sheet with adhesive tape. The distance between
twc adjacent pieces of tape was 0.15 m. Care was taken in order to prevent placing of the
tape on the &or of FOS. A six circderence of the column (2.875 m) sensor was wrapped
around the repaired column after allowing the epoxy to cure for 7 days. Adhesive tape and an
additional layer of epoxy were used to secure the sensor.
Demodulation system for long-gauge FOS consisted of a mechanical actuator, a motion
controller and a digital oscilloscope as shown in Figure 3.14. Readings were taken every two
weeks in order to compare results with the expansion measured by the mechanical collar. To
account for temperature and relative humidity, readings were also taken for the reference
FOS. A detailed procedure for taking readings using the equipment described above cm be
found in Appendk A.
Figure 3.14. Fibre Optic Sensor Equipment
75
3.4.2 Natural Corrosion Regïme
NaturaI corrosion was achieved by subjecting column specimens to cyclic variation of
wetting in 3% NaCl solution and drying in air without any applied potential. As found Çom a
previous study by Lee (1998), the most favourable regirne was 1 day wetting and 2.5 days
drying*
Two different methods have been used for monitoring natural corrosion in column specimens
(as well as in the prisrn specimen): a linear polarization resistance technique and macro-ce11
rneasuements.
3.4.2.1 Linear Polarization Resistance Technique
Naturd corrosion in two columns fiom the pilot series (naturdly corroding column 2 and
corroded-repaired column 4, two columns fiom the low-damage senes (S 11 and 512) and one
repaired column fiom the moderate damage series (S10) were monitored using a linear
polarization resistance technique.
Column specimens fiom the pilot series were cast without a rnulti element probe, therefore
linear po larïzation tests were performed using:
Cu/CuS04 extemal reference half-ce11 located in 3% NaCl solution dong the
reinforcement cage as a reference electrode (see Figure 3.1 5 )
15M bar located at the centre of the column (cathode) as a counter electrode
One of the 15M reinfurcing bars (rebar) as a working electrode
The natural corrosion behaviour of column S 1 1 fkom the natural damage senes and S 10 fiom
the moderate damage series was monitored using an embedded multi-element probe. In order
to evaluate the long-term stability of the embedded MnOz reference haif-cell, readings were
also taken using an extemal Cu/CuS04 reference half-ce11 placed as shown in Figure 3.16.
Furthemore, LP readings were taken using one of the 15M bars from the reinforcing cage as
a working electrode (bar closest to the embedded MEP). Al1 three different electrode
configurations are summarized in Table 3.9.
cathode
Figure 3.15. Experimental Setup for LP Testing
Table 3.9. Electrode Configurations used for LP tests of Column S 10 and SI 1
1 C U - ~ r - c ~ r 1 Cu/CuS04 1 probe I probe I
Abbreviation
Mn-Pr-cPr
1 Mn-15M-cPr 1 &O2 15M rebar probe 1
Column 512 from the naturai damage senes was cast with a modified multi-element probe
(MMEP), therefore, readings were taken using both the embedded probe working electrode
as w e l as the attached piece of 1 OM rebar as a working electrode.
Reference Half-CeH
- 0 2
Linear polarization tests were performed every week after 24 hours of wetting while the
column was subjected to the natural corrosion regime (i.e. no electricd potential was applied
when LP readings were made). n i e equipment for potentio-dynamic linear polarization tests
and the accompanying cornputer software (CMS 100) was supplied by Gamry Instruments.
The general procedure for linear polarization tests is described in Appendbc B.
Working: Electrode
probe
Counter Electrode
probe
3-4.2.2 Macro-Cell Measurements
A macro-cell was created by installing a resistor between the intemal cathode and the
reinforcing steel. A 100042 resistor was installed on columns 2 and 4 fkom the pilot series,
512 fiom the natural damage series, as well as on column SI0 fkom the moderate damage
series. A 10042 resistor was installed on column SI1 and the pnsm specimen between the
working electrode and the bottom layer of reinforcing steel (cathode).
The voltage drop was measured across the resistor using a high impedance d i - m e t e r
acquisition system. The electncal current was calcdated fiom Ohm's law:
Where 1- electrical current, Amps
V- measured potential, Volts
R - electrical resistance, Ohms
3-43 Repair Procedure
In this experimental study, four column specimens were repaired using pre-impregnated
carbon fibre reinforced polymer sheets; column SI0 fiom the rnoderate damage series and
special column specimen S4 were repaired using two layers of CFRP while columns S 2 and
S6 from one layer repair series were repaired with one layer of CFRP sheet. Al1 columns
were instnunented with 0.974-m fibre optic sensors during repair.
A continuous layer of CFRP sheet was used; for one layer repair the length of the sheet was
one circumference of the column (0.96 m) with 0.153-m overlap, while the length of the
sheet for two layer repair was two column circumferences (1.92 m) with 0.153-m overlap.
Since the CFRP sheets were 0.254-m wide, it was necessary to use three sheets applied edge
to edge in order to cover the test region. Fibre optic sensors were installed in the middle layer
of the CFRP sheet as was described in Section 3.4.1.2. The repair procedure illustrated in
Figure 3.26 is s m a r i z e d betow.
Figure 3.16. Repair Procedure (Lee, 1998)
Surface Preparation
1 Al1 fixtures and sensors were removed fiom the coliimn.
2. The column surface was sanded with a sanding d i x attached to a rotary Ml; a metal
brush was used to remove rust fkom the cracks.
3. The column sudace was washed with a wet mg to remove al1 dust and the surface was
then left to dry overnight.
Primer Application
4. A two-component primer supplied by Mitsubishi Chernical Inc. was mixed using a resin
to hardener ratio = 2: 1 by m a s
5. The primer was applied on the entire column surface using a hairless paint roller.
Care was taken to ensure that al1 voids are filled.
6. The colurnn was left to dry overnight.
CFRP Wraminq
7. The required length of CFRP sheets was cut (1.1 13 m for one-layer repair, and 2.073 m
for two-layer repair.
8. The epoxy was prepared by mixing resin and hardener in the proportion 2: 1 by mass.
9. The first layer of epoxy was applied on the concrete surface using a hairless paint roller.
10. The CFRP sheet was applied around one circumference, pressing firmly with a rag in
order to remove air bubbles and insure good bond between concrete and CFRP wrap.
11. The protective layer of paper was removed fiom the CFRP wrap. In the case of the two-
layer repair an overcoat was applied on the first layer to serve as an undercoat for a
second layer.
12. Upon installation of al1 CFRP sheets, an additional layer of epoxy was applied to provide
some protection to the wrap.
13. The wrapped column was left to cure for 7 days, as suggested by manufacturer.
CILAPTER 4. DISCUSSION OF EXPERIMENTAL RESULTS
The results fiom the laboratory experiments are presented and discussed in this chapter. A
cornparison is made between these results and the results obtained fiom the previous studies
conducted by Lee (1998) and Khajehpour (2001). The discussion of natural corrosion
monitoring in column specirnens is presented including issues associated with multi element
probe (MEP) for corrosion monitoring such as: stability of the MnOz half-cell, evaluated on
the basis of its performance in the pnsm specimen, as well as the performance of modified
MEP. The discussion of results of repaired and non-repaired specimens' response to an
accelerated corrosion testing is also provided.
4.1. Natural Corrosion Testing
In order to evaluate the natural corrosion activity and the effect of wrapping on the corrosion
process, linear polarization tests were performed on five column specimens as well as the
pnsm specimen. Macro-ce11 readings were also made for these specimens to estimate the
corrosion current, which is an indication of the corrosion activity.
4.1.1 Linear Polarization
Linear polarization tests were performed weekly immediately before the drying period on
column specimens 2 and 4 fiom the pilot series, on specimens SI1 and 512 from the natural
damage senes, and on specimen S 10 fkom the moderate damage series. Linear polarization
tests were also performed bi-weekly on the prism specimen. Details of the procedure used are
given in Appendix B. The results of these tests are discussed in this section.
4.1.1.1 Pilot Series (2,4)
Columns 2 and 4 fiom the pilot series were cast by Lee (1998). A detailed description of the
treatment of these columns is presented in Section 3.1.2.1. In order to have a better
understanding of the effect of the CFRP wrap on natural corrosion activity, linear
polarization tests were conducted once a week on both the naturally corroding specimen 2
and the corroded-repaired specimen 4. Both column spechens were cast without a multi-
element probe; therefore LP tests were performed using the electrode configuration described
in Section 3.4.2.1.
This approach has a limitation Since the spirais electrically connect rebars from the steel
cage, the effective anodic area is not known. Considering this limitation, the following
obsewations can be made comparing the corrosion behaviour of the naturally corroding
column 2 and corroded - repaired specimen 4 (Figures 4.1,4.2). It should be noted that Iinear
polarization tests started 1.5 years after the repair of column 4, which corresponds to the time
zero on both graphs- A potential of -350 mV measured against the Cu/CuS04 hdf-cell
indicates a "high nsk" of corrosion activity.
time O - 1.5 years after repair of column 4 +4 - Corroded-RepairebNahraI -700 -, 1 8 t
O 100 200 300 400 500 600
Time, days
Figure 4.1. Corrosion Potential vs. T h e for Columns 2 and 4
As can be seen from Figure 4.1, the corrosion potential between the reinforcing steel and the
Cu/CuS04 reference half-ce11 shows that the corrosion is active in both specimens (threshold
for CulCuS04 is -350 mV).
The potential is more negative in repaired column 4, which is an indication of a higher risk of
corrosion. The most likely explanation for this behaviour would be a higher chlonde
concentration around the reinforcing cage resulting fiom the applied potential used during
accelerated corrosion testing which would act to draw chlondes towards the steel cage
(anode).
+ 2 - Natural time O - 1.5 years aRer repair of column 4 -4 - Corroded-Repaired-Natural
, 1 I I I
O 100 200 300 400 500 600
Time, days
Figure 4.2. Corrosion Current vs. Time for Columns 2 and 4
Even though the corrosion potential is more negative in the repaired column, the corrosion
current is about two times higher in the naturally corroding colurnn 2 (Figure 4.2). One of the
reasons for this is a higher electrical resistivity of the corroded and repaired column 4
perhaps as a resuit of lower intemal moisture content.
4.1.1.2 Linear Polarization Specimen (Prism)
Linear polarization readings for the next three column specimens were taken using the rnulti
element probe (commercial device for corrosion monitoring described in Section 3 2.2). One
of the concems associated with the probe was the long-term stability of the embedded Mn02
reference half-ce11 in the aggressive environment created by wetting the specimen in 3%
NaCl solution. This concern rose fiom the fact that the Mn@ reference half-cell cannot be
monitored d e r it is cast in the specimen; therefore its performance was compared with the
extemal Cu/CuS04 reference half-ce11 which can be monitored on the regular basis. P r i m
specimen was cast to evduate the stability of t h e probe. Linear polarization tests were made
every two weeks just before the drying cycle using four different eIectrode configurations as
descnbed in Seciion 3.2.2.1.
The approach of conducting LP readings usinig 10M rebar as a working electrode has a
limitation discussed in the previous section. Considering the limitation, the following
observations c m be made f?om the examinations of LP results as represented in Figures 4.3 - 4.6. It should be noted that time zero on dl graphs represents the time of casting. A potential
of -455 mV measure against the MnOz half-ceKi is equivalent to -350 rnV rneasured against
the Cu/CuS04 half-cell. These values indicate a "hi& risk" of corrosion activity.
O 1 O0 200 300 400 500 600 700 800 900 Time,:, days
-700 -
-800
Figure 4.3. Corrosion Potential vs. Time for Prism
Tirne O - casting June 1998 8 n 8 1 t t
As c m be seen from corrosion potential graph, it took about a year for readings taken using
the probe working electrode to show that corrosion is active, as opposed to readings taken
using 10M bar working electrode, which shoowed active corrosion within 40 days after
beginning the test. Such a behaviour provided s~ome proof of the concem outlined in Section
3.2.2.2, that the difference in the steel quality oof the probe working electrode and the 1 SM
rebar is believed to be a key factor for such behmviour.
The potential between the probe working electrode and the embedded Mn02 reference half-
cell differed fiom the potential between the probe working electrode and the external
Cu/CuSOs reference half-cel by about lOOmV during the overail testing period. This
ciifference is expected due to dserent potentials of the standard haE-cell: and it indicates that
the embedded MnOz haif-ce11 is stable.
Tme O - casting June 1998 I
Time, days
Figure 4.4. Corrosion Current vs. Time for Pnsm using Probe
, Tme O - casting June 1998
O 100 200 300 400 500 600 700 800 900
Time, days
Figure 4.5. Corrosion Current vs. Tirne for Prism using 10M bar
O 100 200 300 400 500 600 700 800 900
Time, days
Figure 4.6. Current Density vs, T h e for Pnsm
The corrosion current rneasured using the probe working electrode increased after about 400
days (Figure 4.4), which corresponds to the decrease in corrosion potential and indicates the
increase in corrosion activity
The corrosion current measured using the 10M rebar as a working electrode (Figure 4.5) is
higher than the current measured using probe working electrode due to the increased anodic
area of the rebar compared to the probe working electrode.
As can be seen f?om current density graph (Figure 4.6), afler about a year of natural
corrosion current density measured using probe working electrode is approximately the same
as current density measured using the 10M rebar working electrode. The smdl difference in
current density c m be attributed to the fact that effective anodic area of the 10M rebar
working electrode is an estimated value of the epoxy-fiee area of the bar.
4.1.1.3 NaturaI Damage Series
In order to evaluate the natural corrosion activity and to address the probe issues, two
specirnens fiom natural damage series were monitored: column SI1 and column 512. Column
SI1 was cast in December 1998 by Khajehpour (2001). Specimen 512 was cast in November
1999 as a part of the curent study. A detailed description of the treatrnent of these two
colunin specimens is presented in Section 3.1.2.2.
4.1.1-3.1 Column SI1
The naturai corrosion behaviour of column SI1 was monitored fkom the t h e of casting using
an embedded rnulti-element probe (MEP) as described in Section 3.4.2.1. In order to assess
the long-term stability of the embedded Mn02 reference half-cell, readings were also taken
using extemai Cu/CuS04 reference half-cell, placed dong the steel cage, as a reference
electrode. Furthemore, LP readings were taken using one of the 15M bars fiom the
reinforcing cage as a working electrode (the rebar ciosest to the embedded MEP). All three
diEferent electrode configurations are described in Table 3.10,
The approach of conducting LP readings using 15M rebar as a working electrode has a
limitation discussed in Section 4.1.1.1. Considering the limitation, the following obseniations
can be made fiom the examinations of LP resdts as represented in Figures 4.7 - 4.9. It should
be noted that time zero on al1 graphs represents the t h e of casting.
-+- Mn-Pr-cPr Threshold -a- CwPr-cPr
CuCuS04 -350mV Mn02 4 5 5 mV a Mn-15m-cPr
J tirne O - castrng December 1998
-800 , t 1 1 1 8 1
O 100 200 300 400 500 600 700 800
T ime, days
Figure 4.7. Corrosion Potential vs. Time for Column S 1 1
The potenîial between the probe working electrode and the MnOz reference half-ce11
indicated active corrosion approxuriately after 1 year (Figure 4.7). The data are consistent
with the results using the extemai Cu/CuS04 half-cell.
It took about a year for the probe working electrode to show the active corrosion state as
opposed to the 15M rebar working electrode which appeared to be actively corroding from
the first measurement made at 160 days. These results reflect back on the observation made
in the previous section for the difference in the qualit. of steel of the probe working electrode
and the 15M rebar.
The potential between the probe working electrode and the embedded MnOz reference half-
cell differed fiom the potential between the probe working electrode and the extemal
Cu/CuS04 reference half-cell by about 100 mV during the overall testing period. This
difference is expected, and it confirms the previous finding that the embedded MnOz
reference half-ceIl is stable in the aggressive environment created by wetting column in 3%
NaCl solution.
time O - casting December l998
O 100 200 300 400 500 600 700 800
Time, days
Figure 4.8. Corrosion Current vs. Time for Column S 1 1 using Probe
time O - casting December 1998
L t Mn-:SM-cPr 0.00 * Ci, * i & I I
O 1 O0 200 300 400 500 600 700 800
Time, days
Figure 4.9. Corrosion Current vs, Tirne for CoIumn S 1 1 using 15M bar
As can be seen fiom Figure 4.8, the corrosion current measured using the probe working
electrode increased after about a year. The timing of the observed increase in current
coincides with the timing of the decrease in the measured corrosion potentiai of the probe.
Because of the increased effective anodic area of the rebar compared to the small working
electrode of the probe the corrosion current measured using the 15M-rebar was much higher
than the current measured using the probe working electrode (Figure 4.9).
4.1.1.3.2 Column 512
As can be seen from the results of pnsm and column S 11 monitoring, it took longer for the
probe working electrode to indicate active corrosion than for the 15M rebar working
elecbode. Therefore, another concern associated with the probe was the difference in steel
quality of the probe working electrode and the reinforcement cage. To address this concern,
coiumn JI2 fiom the natural darnage series was cast with a rnodified multi-element probe
(described in Section 3.2.2.2). A detailed treatment of the column is presented in
Section 3.1.2.2.
Linear polarkation readings were taken using both the probe working electrode (called
"probe" on the graphs) as well as attached piece of the 10M rebar as a working electrode
(called "rebar" on the graphs). Figure 4.10 illustrates the potential vs. time graph. The
potential between the probe working-electrode and the reference half-ce11 as well as
the potentiai between the 10M rebar and the MnOz reference half-ce11 indicates that corrosion
is in active state.
O time O - casting Threshold November 1999 Mn@ -455 mV
-t probe ++ rebar
O 50 1 00 1 50 200 250 300 350 400
lime, days
Figure 4.10. Corrosion Potential vs. Time for Column J12
Figure 4.1 1 represents the cornparison of readings taken using multi element probe for al1 3
naturally corroding specimens (S 1 1, 51 2 and prism). Time zero on this graph represents the
tirne of commencement of LP tests for each specimen. Potential cuves are similar for al1
three specimens. The drop in potential occurred for colurnn SI 1 and pnsm specimen
indicated active corrosion. Since column 512 was monitored for 300 days only, the potential
drop carmot yet be codhned. Therefore, M e r monitoring is necessary.
Figure 4.12 represents the change of corrosion potential with t h e for d l 3 naturally
corroding specimens. Readings of corrosion potential represented on the graph were made
using rebar cage for column S 1 1, 10M rebar for pnsm specimen, and a piece of 10M rebar
mounted on the probe for column 112 as a working electrode. For all 3 specimens Mn02
reference haif-cell and probe counter electrode were used. Time zero on this graph represents
the tirne of the commencement of LP tests.
Threshaid -100 1 Mn02 -455 mV
+SI1 probe -+-JI2 probe -+- Prism probe
O 100 200 300 400 500 600 700 8M3
Time, days
Figure 4.11. Corrosion Potential vs. Time for Naturally Corroding Specimens using Probe
O 1 00 200 300 4I30 500 600 700
Tirne, days
O
Figure 4.12. Corrosion Potential vs. Time for Naturally Corroding Specimens using Rebar
-1 -
-200 -
Threshoid + Sl 1 15M rebar cage Mn02 -455 mV -e- J i 2 piece of 10M rebar
-- - Prism 10M rebar
Potentîd for SI1 and prism specimen indicated active corrosion since the beginning.
However, potentiai of 512 was less negative, thus indicated Iower corrosion activity.
Figure 4- 13 shows the cross section area of the column 512. An aluminium sleeve, adjusted
for the specîmens with modified MEP, was used during casting as described in Section 3.2.3.
During casting, chloride contaminated concrete was placed outside the sleeve, while chloride
fiee concrete was placed inside the sleeve. As a result, attached 10M rebar working electrode
happened to be located on the border of the two concrete mixes, while the re~orcement cage
is located in the chloride-contaminated area of the specimen. Therefore, the concentration of
chlorides around the 10M rebar working electrode is most likely lower compared to rebar
cage. This can result in lower corrosion activity of column JI2 according to the potential
readings taken using 1OM rebar as a working electrode.
Longitudinal bars
Figure 4.13. Cross Section of column J12 with Modified MEP
4.1.1.4 Moderate Damage Series (SPO)
Column SI0 fiom the moderate damage series was cast with an embedded MEP by
Khajehpour (2001). A detailed description of the specimen treatment is presented in
Section 3 -3.3.
Column SlO was subjected to natural corrosion testing during the first month and LP tests
were conducted weekly. M e r that, the column was under accelerated corrosion for 99 days
during which LP tests could not be conducted- Upon achieving the moderate damage, the
specimen was discomected fiom the accelerated corrosion regime and LP readings
continued.
The corrosion behaviour of this column was monitored using the embedded (non-modified)
multi-element probe (MEP). LP tests were conducted before and after the specimen was
subjected to the accelerated corrosion testing. It was not possible to perform LP readings
during accelerated corrosion regime due to the high potential applied to the reinforcement
cage. The acceIerated corrosion regime appeared to have changed the electrochemical
environment of the column since the potential between the probe working electrode and the
MnOz reference half-ce11 became less negative after the accelerated corrosion was completed
(Figure 4.14), which indicates a decrease in corrosion activity. Therefore it was decided to
allow corrosion activity to stabilize prior to repairing the column.
To assess the stability of the embedded MnOz reference half-cell, readings were also taken
using extemal Cu/CuS04 reference half-cell. Furthemore, LP readings were taken using one
of the bars from the reinforcing cage as a working electrode. Al1 three electrode
configurations are described in TabIe 3.9.
The following observations can be made from the examinations of LP results obtained after
accelerated corrosion was completed as represented in Fiagres 4.14 - 4.16. It shouId be noted
that time zero on al1 graphs represents the time of casting.
time O - casting July 1998
Threshold CuCuS04 -350mV
Mn02 -455 mV
O 1 00 200 300 400 500 600 2 0 0 800 900
Time, days
Figure 4.14. Corrosion Potential vs. Time for Column S 110
Prior to accelerated corrosion testing the potential of the proabe was between
-500 and -600 mV indicating an active state of corrosion. M e r t h e specimen was
disconnected fiom the accelerated corrosion regime the potential between- the probe working
electrode and the M n 0 2 reference half-cell indicates a passive state. The potential between
the 15M rebar working electrode and Mn02 reference half-ce11 however indicates that
corrosion is active (Figure 4.14). It is possible that the applied potemtial resulted in an
increase of chlonde concentration in the vicinity of the steel and ; a decrease in the
surrounding area of the probe.
M e r removing the applied potential, the working electrode of the probe became steadily
more electronegative with time such that after about 3 months the potential readings
indicated an active state of corrosion (i.e. potential is < -455 mV). Fuahier decreases in the
potentiai of the probe working electrode were observed during the next 3 months until a
stable value in the region of -700 mV was reached.
The potential between the probe working electrode and the Cu/CuS04 reference half-cell was
100 mV less negative than that between the probe working electrode and the Mn02 reference
half-ce11 before repaÏr- This again confirms the long-term stability of the embedded MnOz
reference half-ce11 in the aggressive environment.
After the specimen was repaired with 2 Iayers of CFRP wrap the corrosion activiq increased,
however, the increase in corrosion activïty was temporary, and after about 2 months, the
corrosion potential graph showed a decreasing trend, which indicated a decrease in corrosion
activity. The following explanation for such behaviour can be provided. When specimen is
unwrapped it has access to moisime and oxygen fluctuation with wetldry cycles. When
column is wrapped, the arnount of moisture and oxygen get trapped inside, representing
temporarïly a higher than average supply for corrosion reaction. After a while corrosion
reaction depletes the moisture and oxygen and since wrapped column is insensitive to
wet/dry cycles, corrosion activity decreases.
The corrosion current had an increasing tendency right after repair (Figure 4.15). However,
d e r approximately 2 months, the corrosion current measured using the probe working
electrode had a decreasing trend, which supports the previous observation and can be
explained by the sealing effect of the wrap. lncrease in corrosion current measured using
probe working electrode corresponded to tirne of decrease in corrosion potential, thus
indicating increase in corrosion activity. Corrosion current measured using 15M bar working
electrode (Figure 4.16) is higher than current rneasured using the probe due to the Iarger
anodic area of the cage as compared to the probe working electrode.
tirne O - casting July 1998
acc elerated
repaired r
I , natural corrosion
<
0.000 I 1 1 I 1 1' 1 1 I
O 1 00 200 300 400 500 600 700 800 9013
Time, days
Figure 4.15. Corrosion Current vs. Time for Column SI0 using Probe
Time, days
Figure 4.16. Corrosion Current vs. Time for Column S10 using 15M bar
4.1.2 Macro-CeU Measurements
A macro-cell was created by installing a resistor between the intemal cathode and the
reinforcing steel on colurnns 2 and 4 from the pilot series, as weil as on columns S11 and JI2
fiom the low-damage series and on column SI0 fkom the moderate-damage series.
Furthemore, a resistor was installed on the prism specimen between the working electrode
and the bottom layer of reinforcing steel (cathode). Macro-ce11 measurements were
performed weekly just pnor to the drying period, Corrosion current can be calculated fiom
macro-ce11 measurements as discussed in Section 3.4.2.2.
4.1.2.1 Pilot Series (2 and 4)
Columns 2 and 4 from the pilot series were cast and tleated by Lee (1998). Initially column 2
was covered with wet burlap and plastic to achieve a hi& humidity environment, and after
that it was subjected to wetldry cycles in 3% NaCl solution in an attempt to promote
increased corrosion activity. Column 4 was subjected to accelerated corrosion, repaired and,
since then, it was subjected to cyclic wetting and drying. A detailed description of the
laboratory treatment of these columns is presented in Section 3.1.2.1. In order to have a
better understanding of the effect of CFRP wrap on the natural corrosion activity and to
compare results fi-orn lhear poiarization resistance techniques, macro-ce11 measurements
initiated by Khajehpour (2001) were continued during this experimental study. A resistor was
installed 1.5 years after casting on column 2, and 4 rnonths after repair on coiumn 4. Figure
4.17 illustrates the rnacro-ceil corrosion current vs. tirne. T h e zero on the graph represents 6
months after repair of column 4. It also corresponds to 1.5 years since casting of both
columns.
Gradua1 increase in current c m be observed for naturally corrodîng column 2, indicating
continuhg increase in corrosion rate due to the increased chloride content at the
resorcement cage. As can be seen fiom corrosion current graph for repaired column 4,
CFRP wrap prevents the corrosion rate from increasing.
0.12
0.1 u E G 008 C
006 O
0134
O 02 Time O - 6 months + 2 - Natural
O O 100 200 300 400 500 600 700 800 900 1000
Time, days
Figure 4.17. Macro-CeLl Corrosion Current vs. Time for Columns 2 and 4
4.1.2.2 Linear Polarization Specimen (Prism)
A small pnsm specimen was constructed by Khajehpour (2001) in order to evaluate the long-
term performance of the probe as discussed in Section 3.2.2.1. A resistor was installed
between the working electrode and bottom layer of reuiforcing steel (cathode) of the prism
specimen. Figure 4.18 represents current vs. t h e graph. Time zero is the tirne of casting the
pnsm specimen. After 200 days, macro-ce11 corrosion current has a gradually increasing
trend, which indicates the continuous increase in corrosion activity of naturally corroding
prism specimen subjected to wetldry cycles without applied potential.
O 100 MO 200 4M 500 600 700 rn 900
Tirne, days
Figure 4.18. Macro-Ce11 Corrosion Current vs. Time for Pnsm Specimen
4.1.2.3 Natural Damage Series
Macro-cell current of columns SI1 and 512 fiom the natural damage series was measured
using a resistor installed between the cathode and the reinforcement cage. To initiate macro-
cell corrosion current readings, resistor was installed 49 days d e r casting of colurnn 512 and
278 days after casting of column S 1 1. The results of macro-ce11 measurements are presented
on Figure 4.19.
From cornparison of macro-ce11 corrosion current of two natumlly corroding columns SI1
and 512, it can be detected that current of column S I 1 is about 10 times larger than current of
column 512. This merence is due to the differences in concrete strength for columns S 11
and 112. As summarized in Table 3.6, the concrete strength for column 512 is higher than
strength of column S 1 1. Higher strength generdl y correlates with lower permeability, and
since resistivity is inversely proportional to permeability, thus, resistivity of column 512 is
expected to be higher than the resistivity of column S 1 1. Therefore the dif5erences in macro-
ceU corrosion current of two naturally corroding specimens SI1 and J12 can be attributed to
the differences in concrete strength descnbed in Section 3.3.1 -3.
resistor installed -&-SI1
278 days afier -+-JI2 casting S1 7
resistor installed 49 days afier casting J I 2 ?
i- 1 1 I I r
O 100 2W 300 400 500 600 700 800
Time, days
Figure 1.19. Macro-CeIl Corrosion Current vs. Time for Column S 1 1 and JI2
4.1.2.4 Moderate Damage Series (S10)
Column SI0 fiom the moderate-damage series was cast by Khajehpour (2001). A detailed
description of the treatrnent of column S 10 is presented in Section 3.1.2.3. A resistor was
instaIled 307 days after casting and 188 days before repair of column S10, as indicated in
Figure 4.20. The overall trend of the graph indicates that macro-ceIl corrosion current is
indifferent to the changes in the environment of the column (Le. corrosion current did not
change after repair with CFRP wrap).
0.25
307 days afier casting
Figure 4.20. Macro-Ce11 Corrosion Current vs. T h e for Colurnn S 10
0.05 -
0.m
4.1.3 Circumferential Expansion (S10)
-4- before repair +after repair
I 1 1 I 1
Circumferential expansion of column S10 fiom the moderate darnage series was measured
before and after repair using mechanical collar described in Section 3.4.1.1. Figure 4.21
shows the expansion readings taken twice a week immediately pnor to the wetting period.
307 407 507 607 707 807
Time, days
It c m be observed that column expands rapidly while cracking when subjected to an
acceIerated corrosion. As cracks started to accommodate corrosion products, the rate of
expansion reduced. Column S 10 continued to expand under natural corrosion regime without
applied potential. After repair wîth 2 Iayers of CFRP wrap, the rate of expansion reduced, as
indicated by the dopes of the graph 100 days before and 100 days &er repair. The decrease
in expansion rate was presumably due to restraining effect of the wrap.
Natural Corrosion
* before repair
+ afier repair
O 100 200 300 400 500 600 700 800 900
Time, days
Figure 4.21. Expansion vs. Time for Column S 10
4.2 Accelerated Corrosion Testing
A total of ten column specimens were subjected to accelerated corrosion testing as discussed
in Section 3.4.1. Two of them (S 1 and S2) assigned to the corroded-repaired-accelerated
testing regime (see Table 3.1) were previously damaged and repaired specimens (Le.
specimens subjected to a second stage of accelerated corrosion a e r the damage fiom the
initial phase had been repaired by CFRP wraps). Eight columns: S7, S8, S9, 514 fYom the
high damage senes, J13,115, J16 fiorn the expansive grout senes, and 317 fiom the corrosion
inhibitor series were subjected to initial stage of accelerated corrosion in order to proceed
with assigned testing regimes (see Table 3.1).
4.2.1 Corroded-Repaired-Accelerated Test Regime (SI and S2)
Colurnns S1 and S2 were subjected to accelerated corrosion before they were repaired.
Column S1 had a steel loss of 6.75% and a circumferential expansion of 0.18% after the
applied potential was removed. As a result of accelerated corrosion, the electrochemical
environment of the column was probably changed and thus afler column S 1 was repaired
with two layers of CFRP wrap, it was left to stabilize for about a year. During this period the
column continued to expand and the total expansion at the time of post repair accelerated
corrosion initiation was 0.22%. Column S2 had a steel ioss of 13.52% and a circumferential
expansion of 0.35% after the applied potential was removed. During stabilization period
column continued to expand so that the total amount of expansion at the time of repair with
one layer of CFRP wrap was 0.42%. After being repaired, colurnns S 1 and S2 were subjected
to accelerated corrosion again in order to assess the rate of steel loss and the amount of
expansion of CFRP wrapped column under accelerated corrosion by comparing it to that
before repair.
4.2.1.1 Steel Loss Rate
As c m be seen £kom the current vs. time graph (Figure 4.22), columns S1 and S2 initially
experienced a high current and thus hi& steel loss rate f i e r being repaired and subjected to
the same accelerated corrosion regime as before repair. Repaired columns initially experience
high current presumably because as a result of repair, some moisture and oxygen get trapped
inside the wrap and for some tirne it may represent an excessive supply for the corrosion
reaction. This hypothesis is supported by the fact that column S2 experienced higher initial
current than column S l perhaps because c o b S2 underwent wet/dry cycles up to the tirne
of repair as opposed to column S 1, which was lefi to dry out for a month before wrapping. It
is believed that after a while the corrosion reaction consumes moisture m d oxygen inside the
wrap and, since a wrapped column is less sensitive to wet/dry cycles than unwrapped, the
current drops significantly during the first month and continued to decrease as rust is formhg
on the reinforcement cage. As a result of repair, the corrosion current dropped significantly
after the initial increase.
As c m be seen fkom Table 4.1 and 4.2, after the columns were repaired with CFRP sheets,
the average rate of steel loss reduced for both columns.
-- -- S i disconnected afier SI and S2 have the
same trend of
S2 disconnected afier reaching
Accelerated Corrosion
--
Accelerated Corrosion before Repair
1
Time, days
Figure 4.22. Current vs. Time for Repaired columns S1 and S2
Table 4.1. The Rate of Steel Loss for Repaired Column S 1
m S p e e ï Z e n 1 Time 1 Steel Loss 1 Steel Loss Rate 1 Sl
before repair
Table 4.2. The Rate of Steel Loss for Repaired Column S2
after repair To ta1
days 1 O5
Figure 4.23 supports the previous observation by showing that the gradient of the steel toss
320 425
curves alter repair is slightly less, than the gradient before repair.
O h
6.75
Steel Loss Rate %/day 0.0449 0.03 04 0.0753
%/day 0.0643
9.9 1 16.66
Steel Loss '!!.'O
13 -52 9.73 23.25
Specirnen S2
before repair after repair
Total
0.03 1 O 0.0953
Time days 301 320 62 1
1 . -./ S2 disconnected after ,/
*' reaching 13.52% of steel loss ,,,--
SI disconnected after 4 reaching 6.75% of steel loss
,/I Accelerated Corrosion after Repair
Accelerated Corrosian - SI bef ore Repair - 52
I I I l I I 1
O 1 O0 200 300 400 500 600 700 800 900
Time, days;
Figure 4.23, Steel Loss vs. Time for Repaired columns S 1 and S2
4.2.1.2 Circumferential Expansion
Expansion readings were performed twice a week just before the wetting penod using a
mechanicd coilar (Section 3 -4.1.1 ). Column S 1 was alsor instnunented with two fibre optic
sensors to compare the readings taken by mechanical expansion collars and by fibre optic
sensors made for column S 1 (Section 3.4.1.2).
4.2.1.2.1 Mechanical Expansion Coilar
Figure 4.24 - 4.26 illustrate the change in expansion with time for columns S 1 and S 2 before
and after repair. It should be noted that time zero on al1 three graphs represents the time of
the commencement of accelerated corrosion (5 rnontths afler casting). The following
observation can be made fiom analysis of these figures.
The change of expansion with time for column SI befo-re repair is airnost identical to the
expansion of column S2 before repair (Figure 4.24). B-oth columns S1 and S2 expanded
rapidly during the first 50 days of accelerated corrosion, which corresponds to the time of
extensive cracking. This expansion initiation is followed by a 70-day period of 1ittIe or no
expansion, because of the corrosion products accommodation by the cracks.
4
Beginning of Post Repair Accelerated Corrosion
SI disconnected - after reaching
6.75% of steel -
52 disconnected afkr reaching 13.52% of
- f steel loss +SI before repair -2- S2 before repair 6 -+-SI after repair = S2 aRer repair
O 1 O 0 200 300 400 500 600 700 800 900
Time, days
Figure 4.24. Expansion vs. Time for columns S 1 and S2 during Accelerated Corrosion
Before and Afier Repair
After column S1 was corroded to moderate damage (estimated at 6.75% of steeI loss),
accelerated corrosion was removed. Column S 1 was allowed to dry for a month pnor to its
repair with 2 layers of CFRP wrap. Specirnen S1 continued to expand slowly under the
natural corrosion regime (Figure 4.25).
After being subjected to the sarne accelerated corrosion regirne as before repair, column S 1
did not expand significantly during the first 80 days. By comparing corrosion current of
corroded repaired specimen subjected to post repair natural corrosion with corrosion current
of specimen subjected to accelerated corrosion after repair, it c m be approximated that 80
days of accelerated corrosion correspond to about 220 years of natural corrosion. However, it
should be taken into account that accelerated corrosion produces more steel loss and less
damage than natural corrosion. This dormant period is followed by a significant amount of
expansion. Perhaps this dormant period represents the existing cracks being filled with
corrosion products. Because of the wrap no m e r cracks can be formed and m e r
corrosion results in expansion.
1 +- before repair + after repair
Accelerated Corrosion Natural Corrosion Mer Repair
O 1 00 200 300 400 500 600 700 800 900
Time, days
Figure 4.25. Expansion vs. Time for Repaired column S 1
Column specimen S2 was highly damaged (estimated at 13 -52% of steel loss) when the
accelerated corrosion was rernoved. It was expanding slowly under natural corrosion regime
(Figure 4.26). The sudden hcrease in the expansion at about 380 days can be associated with
sliding of the mechanical collar fiom its original position, since the slope of the curve before
and &ter this jump is the same. CoIumn 52 was subjected to wet/dry cycles up to the t h e of
repair. M e r being repaired with 1 layer of CFRP wrap, specirnen S2 was subjected to the
same accelerated corrosion regime as before repair. Slope of the expansion curve afier repair
is smaller than before repair. This suggests that CFRP wrap somewhat reduced the rate of
expansion presumably due to its restraining effect, however, the wrap does not stop the
expansion of the c o l m .
1 -- before repair
O 1 O0 200 300 400 500 600 700 800 900 lime, days
Figure 4.26. Expansion vs. Tirne for Repaired colurnn S 1
Accelerated Corrosion after Repair
I 0.1 4
a Accelerated Corrosion rsr. ~1 before Repair
4.2.1.2.2 Fibre Optic Sensors
During the repair procedure, column S 1 was instmented with two fibre optic senson (FOS)
as discussed in Section 3.4.1.2. FOS readings were conducted every two weeks before the
wetting penod. Figure 4.27 illustrates the cornparison of expansion measured using the
mechanical collar with the expansion measured using fibre optic sensors. T h e zero on the
graph corresponds to the beginning of the post-repair accelerated corrosion regime (464 days
total). It should also be noted that 0% expansion on this graph corresponds to the actual
expansion of 0.22% prior to the column being subjected to the accelerated corrosion regime
after repair.
Natural Corrosion
As c m be seen fiom the graph, values of expansion measured using two FOS and a single
mechanical collar were similar during the first 80 days of accelerated corrosion testing.
Akhough good agreement was observed between readings taken using the colla and the
2.875 m long sensor (6 column circumferences, or 6 PD) up to 80 days, a large discrepancy
can be seen between these values thereafter. Since readings taken using 0.974 m long (IpD)
FOS showed sirnilar rates of expansion as the mechanical collar through the accelerated
corrosion-testing penod, it is suggested that the observed discrepancy is due to the 6 pD
I I l b L 0.0 -' I I
sensor slip fkom its original position. As mentioned in Section 3.4.1.2, the 1pD FOS was
h e d on the inner side of the wrap as opposed to the 6 pD sensor, which was mounted on the
surface of CFRP wrap and secured with an additional layer of epoxy. However, this rnay not
be sufftcient to prevent the sensor fiom sliding when the column expands. Moreover, the
1 pD sensor may be considered to be better protected fiom any type of damage than the 6
pD sensor.
0.2
Beginning of Post-Repair Accelerated Corrosion
+i PiD -6 PiD 4 Collar
O 1 1 1
O 20 40 60 80 100 1 20 140 160
Time, days
Figure 4.27. Cornparison of Expansion Data for Coiumn S 1
4.2.2 High Damage, Expansive Grout, Corrosion Inhibitor Series
Eight newly cast column specimens S7-S9 and 513-517 were subjected to accelerated
corrosion and will be kept under these conditions until they achieve a hi& level of damage
with a target steel loss of 13.5%. Figures 4.28-4.33 represent the results of steel loss and
expansion monitoring for dl eight specimen subjected to accelerated corrosion. These results
are discussed in the rernainder of this section.
for S7, S8. S9 reduced to 6V
+ S7. S8. S9 have the same trend of voltage
Applied Voitage for S7. S8. S9 reduced ta 3V -
Applied Voltage for ST, S8. S9--
reduced to 1 S V
1 50 200
Time, days
Figure 4.28. Applied Potential vs. Time for columns S7-S9
Applied Voltage for 57. S8. S9 reduced to 6V
Voltage SE. S9
!d tu 3V
S7. S8. S9 have similar trend of corrosion
current
Applied Voitage for S7. S8. S9
reduced to 1 S V
50 100 1 50 200 250
Time, days
Figure 4.29. Current vs. T h e for Columns S7-S9
OTT
0-0
1'0
2'0
F s-O
3 E* O
P'O "a oc
s'O
9'0
L'O
N p i i e d AppEed Voltage for 57, 58. S9 for S7. S8. S9 reduced to SV
reduced to 1.5V
O 50 100 1 50 200 250 300 350
Time, days
Figure 4.32. Steel Loss vs. Time for Columns S7-S9 and 513-517
O 2 4 6 8 10 12 14
Steel Loss, %
Figure 4.33. Expansion vs. Steel Loss for Columns S7-S9, JI3417
Figure 4.31 shows how the expansion changes with time; Figure 4.32 illustrates the rate of
steel loss; Figure 4.33 represents the relationship between the expansion and steel Loss for dl
columns subjected to an accelerated corrosion testing.
Two different patterns can be observed fiom these graphs. The J-series columns (J13-JI 7)
expand gradually. The S-senes columns (S7-S9) initially showed the same expansion as the
J-senes specimens, however, after 50 days the expansion virtually stopped. Evidence of the
initiation of steel loss, such as rust-coloured salt solution and deposits on the surface of the
columns, were observed within a few days for column specimens S7-S9. It took longer to
observe mst leaching on columns 513-517. Withùi a month, it was evident that columns S7-
S9 were losing steel much faster than columns J13-J17, while expanding more slowly. The
reason for this is the fact that columns S7-S9 had a significantly higher water-to-cernent ratio
than J-series of columns because of the poor quality conkol during specimen fabrication.
A few modifications were made in order to accelerate the expansion and to reduce the steel
loss rate of three columo specirnens S7, S8, and S9 in order to be consistent with other
corroding specimens. Oxygen was introduced into the hollow cathode through a small
diameter plastic tube; this was done in an effort to increase the rate of corrosion by increasing
the availability of oxygen at the cathode. Also, an attempt was made to increase the
expansion by reducing the wetting time. In order to slow down the rate of steel loss, the
applied potential for column specimens S7, S8 and S9 was reduced £kst down to 6V, then
down to 3V and afler that down to 0.5V. These modifications as well as the time of their
irnplementations are summarized in Table 4.3, and discussed in detail in the following
subsections.
Table 4.3. Accelerated Corrosion History for S7, S8, S9
S pecimen Change in Regime Accelerated Corrosion
Insert Bubbler in the Cathode Dry out the Cathode 1 day wet / 6 day dry
hsert Bubbler in the Cathode Dry out the Cathode
1 day wet / 2.5 day dry Switch Potential to 6V Switch Potential to 3V
Switch Potential to 1.5 V Accelerated Corrosion
Insert Bubbler in the Cathode Dry out the Cathode 1 day wet / 6 day dry
Insert Bubbler in the Cathode Dry out the Cathode
1 day wet / 2.5 day dry Switch Potential to 6V Switch Potential to 3V
Left dry for 14 days 1 day wet / 2.5 day dry
Switch Potential to 1.5 V Accelerated Corrosion 1 day wet / 6 day dry
1 day wet / 2-5 day dry Switch Potential to 6V Switch Potential to 3V
Switch Potentiai to 1.5 V
Date 22-Dec-99 13-Jan-O0 4-Feb-O0 7-Feb-O0 14-Feb-00 20-Feb-00 2 1 -Feb-00 24-Feb-00 I O-Mar-O0 24-May-00 22-Dec-99 22-Jan-O0 4-Feb-O0 7-Feb-O0 14-Feb-00 20-Feb-00 2 1-Feb-00 24-Feb-00 10-Mar-00 13 -Apr-O0 27-Apr-00 24-May-00 22-Dec-99 7-Feb-O0
21-Feb-00 24-Feb-00 t 0-Mar-00 24-May-00
4.2.2.1 Applied Potential
An initially constant 12V potential was applied to al1 ten specimens. However, due to the
unexpectedly high rate of steel loss of three column specimens (S7, S8 and S9) compared to
J-series specimens, it was decided to reduce the potential applied to these three c o l m s . A
plot of the applied voltage is s h o w on Figure 4.28. First, the applied potential was reduced
to 6V; however, the decrease in current (and thus the steel loss rate) was negligible.
Therefore, it was decided to M e r reduce the voltage to 3V. Later, the voltage was brought
down to 1 . 3 7 in order to allow specimens to expand without losing a large amount of steel.
As can be seen fkom the current vs. h e graph for columns S7-S9 (Figure 4-29), the current
dropped fiom 0.35A to 0.20A on average as a result of decreasing applied potential fkom
12V to 6V. However, this effect was omly temporary, since the current tended to rise back up
such that after two weeks the current was 0.30A. The decision was made to M e r decrease
the potential applied to columns S7, S8, and S9 to 3V. As a resdt, the current initidy
dropped to 0.1 A and this t h e it did not rise significantly over time (0.013A increase over 75
days). The voltage was decreased to 1.5V (Iowest possible) in order to vimially stop the steel
loss and to allow specimens to expand without loshg a significant amount of steel. The
change in slope of the steel Ioss vs, time curves can be observed in Figure 4.32. As
illustrated in Figure 4.34, the relations3ip between corrosion current and applied potential is
not linear. Therefore there is an o p t i m m value of applied potential to cause higher current,
after which current might not increase significantly.
O 2 4 6 8 10 12 14
Applied Potential, V
Figure 4.34. Current vs. Applied Potentid for S7-S9
4.2.2.2 Wetting and Drying Cycles
While Linder accelerated corrosion testing, ail specimens were subjected to cyclic wetting and
drying. The optimal wetting and drying time determined by Lee (1998) was found to be 1
day of wetring and 2.5 days of drying. An attempt was made to increase the expansion of
columns S7, S8 and S9, as shown in Table 4.3, by increasing the dryïng period to 6 days.
Furthemore, column S8 was left to dry for two weeks. However, no significant change in
expansion was observed.
Results fkorn a previous experimental study conducted by Khajehpour (2001) have shown
that pumping air through the perforated hollow cathode can increase the rate expansion. Air
was purnped through the hollow cathode of column S7 and S8; however, the increase in
expansion rate was only temporary.
4.3 Summary of Experimental Results
This summary compares and contrasts the experimental results and observations for the
columns involved in the research project. It also attempts to explain the observed corrosion
process, the similarities and differences in the columris' performance.
4.3.1 Natural Corrosion
As can be seen from corrosion potential vs. time (Figure 4.1, 4.3, 4.7, 4.10, and 4-14),
corrosion is in the active state for al1 column specimens and the prism. Measurements
performed using the probe incorporated in columns SlO, S1 1 and the prism, have indicated
that it takes about a year for the probe working electrode to show the active corrosion state as
opposed to the 15M rebar working electrode, which appeared to be actively corroding within
one month. This is most likely due to dBerences in steel quality used for fabrication of the
probe working electrode and reinforcement cage. In order to investigate this difference, a
modified probe, incorporating of the piece of 10M rebar as a working electrode, was installed
in column 512. Due to the lower chloride concentration around the 10M rebar working
electrode than around the reinforcement cage, the potential between the M n 9 reference half-
ce11 and the 10M rebar working electrode of column 512 is lower than potential between
Mn02 reference half-ce11 and 15M rebar (reinforcement cage) working electrode of column
Sll (Figure 4-12)
The potential between the probe working electrode and embedded MnOz reference half-ceil
M e r e d fiom the potential between probe working electrode and extemal CuICuS04
reference half-ce11 by about lOOmV for columns S11, SI0 and the prism. This difference was
expected and indicates that the embedded MnOz reference half-ce11 is stable in aggressive
environment created by wetting the specimen in 3% NaCl solution.
M e r repair of column specimen S10, the rate of expansion decreased, as indicated in
Figure 4.21, presumably due to restraining effect of CFRP wrap.
4.3.2 Accelerated Corrosion
Columns SI and S2 experienced a high current thus hi& steel loss rate immediately after
repair (Figure 4.22). This was especially evident with column S2 because of its continued
wet/dry cycles up to the tirne of repair. Column S1 was left to dry for a month before
wrapping. As a result of repair, some moisture and oxygen was most likely trapped inside the
wmp and initially it represented an excessive supply for the corrosion reaction. Once the
oxygedmoisture was consumed in the corrosion reaction and since the impermeable wrap
out the column off from the effect of wetldry cycling, the current dropped significantly
during the fïrst month and continued to decrease as the rust was fomiing on the
reùiforcernent cage.
After repair of columns SI and S2 a dormant period of expansion can be observed under
induced corrosion regime (fïrst 100 days as indicated in Figure 4.24). M e r this period
columns continue to expand presumably because the top and the bottom of the c o l a are
not completely isolated fiom moisture and oxygen and corrosion process continues.
The alteration in the applied voltage of columns S7-S9, fiom 12V to 6V resulted in a
temporary decrease in the corrosion current (Figure 4.29). This decrease may be amibuted to
the polarkation effect. The M e r decrease in applied voltage from 6V to 3V resulted in a
permanent decrease in the corrosion current. The effect of polarization is probably less
significant at lower range of electrical current.
The rate of steel loss was lower for the newly cast 513 - 517 column specimens as opposed to
the S7 - S9 specimens constructed by Khajehpour (2001) due to the better q d t y concrete as
a resdt of poor qudity control of the specimen fabrication.
CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS
This is an ongoing project at the University of Toronto aimed at developing a "smart?' wrap
system for corrosion darnaged reinforced concrete columns.
In order to assess stnictural performance of corroded CFRF' confined columns, the previous
studies conducted by Lee (1998) and Khajehpour (2001) involved stnictural tests. The
developments on the earlier parts of this research project are surnrnarized below.
This part of the overall research project was concentrated mainly on corrosion monitoring in
wrapped columns, to evaluate post repair corrosion activity. Conclusions on this part of the
overall project are also provided in this chapter followed by recommendations.
5.1. Earlier Developments
1. Structural tests have shown loss of strength and considerable loss of ductility due to
corrosion.
2. Extensive-pitting corrosion occurs in spiral at overall steel loss on the order of 10%.
3. Wraps restore colurnn strength and ductility lost due to corrosion.
4. Wrapped columns subjected to considerable post-repair corrosion still outperform
control specimen.
5.2. Conclusions
1. CFRP wraps do not stop corrosion activity in the colunin.
2. Natural corrosion current sliphtly increases right after repair. This penod of higher
corrosion activity is temporary. Afier initial increase corrosion current becomes the
same as before repair or slightly less.
3. Column subjected to post repair natural corrosion regime (without applied potential)
expanded less than before repair.
4. Colurnns subjected to post repair accelerated corrosion (12V applied potential) did
not expand during first 80 days of accelerated corrosion. After this dormant period,
repaired columns expand gradually.
5.3 Recommendations
1. Fuaher monitoring is necessary to have a better understanding of corrosion after
repair and to check a repeatability of results
2. In order to improve natural corrosion monitoring it is suggested to take macro-ce11
readings during both wet and dry cycles.
3. While conducting linear polarization tests with external CdCuS04 reference half-cell
it is recommended to place the half-ceii at different locations dong the column to
have more data.
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Appendix A
Procedure for Taking Readings Using FOS Equipment
1. Motion ControiIer
To clear the signal press CLEAR, then REV; when see the red signal flash press
CLEAR and CLEAR.
Should see O on the screen of the Motion Controller
In order to have slow velocity tuni the handle VELOCITYall the way ccw.
2. Osciiioscope
Adjust to channel 1
There are 2 knobs on the upper right corner called REFERENCE and DIFFERENCE-
Adjust REFERENCE (horizontal solid line on the screen of the oscilloscope) to O.
Should be able to see it in the middle of the screen. Adjust DIFFERENCE (dotted
line) at 1 division below the solid line.
Adjust Volts/Division to 5, using little knob called VOLTS/DrVISION on top of the
bigger one on the left side of the oscilloscope (channel 1).
Adjust Time/Division to 0.5 s, using knob TIMEDIVlS..ON on the right second fiom
the top.
Check that apparatus at DC level (a button on the lower left).
Check that mode is normal (a button on the right side). If the mode is Normal, line is
moving on the screen).
Check source - channel 1
Check coupling - AC
3. Taking Readings
Oscilloscope:
Plug in mirror and reference (do not screw too much in the upper one)
SetDiFFERENCEon 10Volts
Touch the sensor so it is fitted with the horizontal dotted line (between solid line in
the midde and second dotted line)
Set to AC mode (button on the lower left)
Using BIG VOLTS/DMSION knob set it to 50 mV
Motion Controller:
Press M D
Should be able to see the signal at about 2500 for reference
Once can see big pattern press FWD again to stop, Write down the value and press
REV. Get that big pattern again and write down the value. Take an average of the two
values-
If signal is off the screen (too large) instead of 50mV get to 0.1 V/div or 0.2 V/div
using; BIG VOLTS/DMSION knob.
Appendix B
Procedure for Linear Polarization Tests
The hardware installed and set up by Gamry Instruments included voltmeter and ammeter. In
order to perform the linear polarization tests, the proper connections between the computer
and specimen were established. The general procedure for linear polarization tests is
described below.
The cables were comected to the electrodes
CMS 100 program was open.
The relevant data (output file name, area of the steel subjected to LP test, etc.) entered
into the dialog box titled "Polarization Resistance".
Tafel constants were entered according to the corrosion conditions: 0.052 VDec if
corrosion is passive, 0.026 VDec if corrosion is active. The threshold used for
Cu/CuS04 reference half-ce11 was -350mV, for MnOz reference half-cell was
45SmV.
Afier open circuit potential was measured, the working electrode was tested as the
potentio-stat polarized fiom -20mV below the initial open circuit potential and
increased in small steps to reach the final working electrode potential, which was
+20mV above the initiai open circuit potential.
While the CMSlOO program was running, electncai current was recorded for each
potential step and plot of log of current vs. potential was displayed.
Upon completion of polarization, data analyses were performed using CMSIOS
program, which run under CMSlOO and uses MS Excel program for drawing graphs
and fitting numerical curve.
The Line fit to the LP cuve was generated f?om least squares fit between -IOmV and
t 1 OmV with respect to initial open circuit potential.
The slope of the fitted line indicated the polarization resistance in Ohms cm2
Rp = AE/N
where dE - change in potential
AI- change in current density.
