Corrosion prevention of reinforced concrete structures

S625.7(94)

AUS(ARR 332)

duet10n ot this report. p~opported by the National's s terest Service program

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Corrosion preventionof reinforced concrete

structures

Guangling SongAhmad Shayan

Research Report ARR 332

Corrosion prevention of reinforcedconcrete structures

Guangling SongAhmad Shayan

Producti_on of this report is supported by the .National I.nterest Service pro'gram

. NIS

TRANSPORT

ARRB Transport Research LtdResearch Report ARR 332 .

April 1999

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SONG G. and SHAYAN A. (1999). CORROSION PREVENTION or ~EINFORCED CO~JCRnE

STRUCTURES. ARRB Transport Research Ltd. Research Report No. 332. 112 pages including26 figures and 15 tables.

Corrosion prevention is a cost-effective approach to extend the service life of reinforcedconcrete structures subject to corrosion attacks. .

The investigation and application of corrosion prevention techniques in reinforced concrete hasbeen extensively addressed in the literature.

Much improvement and progress has been made in the understanding and application ofvarious corrosion prevention techniques, some of which have evolved into standard methodsand have been widely used in field structures. Others are still mainly at laboratory investigationand field trial stages.

This report aims to provide readers with In overall understanding of the current status ofcorrosion prevention techniques in reinforced concrete syst~ms .

Section 1 discusses the basic processes and mechanisms involved in reinforced concrete andavailable techniques that can retard the corrosion processes.

Sections 2 to 10 review widely used techniques in the corrosion prevention of reinforcedconcrete structures, including cathodic protection, re-alkalisation, electrochemical chlorideremoval, inhibitors, surface treatment and coatings on concrete, concrete quality control.reinforcing materials, and concrete repairs. The theoretical basis, current applications,advantages and difficulties of these techniques are the main subjects in each of the sections.

After comparison of these techniques in Section 11, some strategies for dealing with corrosionproblems are given in Section 12. Concluding remarks are made in Section 13, whererecommendations for further research and development are made.

ARR 332April 1999

ISBN 0 86910 789 5ISSN 0158-0728

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• Vermont South VIC 3133AUSTRALIA

About the authors

Guangling Song

Dr. Guangling Song, ResearchEngineer, is an expert in corrosionscience, protection engineering andelectrochemical testing. Heobtained his Ph.D in corrosionengineering from the University ofQueensland. He has been workingon the corrosion of metals invarious environments, corrosionmeasurement/monitoring/diagnosis,prevention, prediction, assessment,and electrochemical tests for manyyears. He has specialised incorrosion behaviour urreinforcement, passivation/transpassivation of steels, transienttechniques, AC electrochemicalimpedance spectroscopytechniques, corrosion of alloys, andunderground corrosion.

Ahmad Shayan

Dr Ahmad Shayan is aninternational expert in the field ofalkali-aggregate reactio~. He is

currently the Chairman of theStandards Australia Committee CE/12, on Aggregates and Rocks forEngineering purposes. He has 18years experience in research andconsulting on the various aspects ofAAR, and developed the accelerated21-day AAR test for Australianaggregates.Dr Shayan has successfully led inexcess of 90 consultancy projects inthe area of concrete durability andpublished more than 80 papers inthe international scientific media. Hehas undertaken numerous researchprojects on various aspects ofconcrete materials. His recentinterests have included theevaluation of the durability of epoxybonding systems (used to strengthenconcrete structures) under variousexposure conditions, and theevaluation of the interactionbetween cathodic protectioncurrents and AAR in concretescontaining potentially reactiveAustralian aggregates.

ARR 332Corrosion prevention of reinforced concrete structures

Contents

arObTransport

Research

EXECUTIVE SUMMARY v

. 1. Introduction . 11.1 Basic Processes in a Reinforced Concrete 1

1.1.1 Anodic reaction-The corrosion of reinforcement 31.1.2 Cathodic processes 41.1.3 Other important processes 51.1.4 Influences and interaction of the basic processes 5

1.2 Approaches to Retard Corrosion of Steel Reinforcement 71.2.1 Electrochemical approachs based on.cathodic polarisation 81.2.2 Techniques associated with the interface of steel/concrete 81.2.3 Techniques associated with the cover concrete 91.2.4 Summary of the approaches 9

2. Cathodic Protection2.1 Theory2.2 Practical Protection Criteria

2.2.1 Absolute potential (Eon) criterion2.2.2 Polarisation shift criterion (DEan)2.2.3 Instant switch-off (Eott) criterion2.2.4 Depolarisation (DEott) criterion2.2.5 Polarisation curve (E-logl) criterion2.2.6 .Compromise of criteria

2.3 Distribution and Variation of Protection Current Density2.4 Anode Systems

2.4.1 Impressed current anode2.4.2 Sacrificial anode2.4.3 Choice of anode systems

2.5 Monitoring Probes for Cathodic Protection2.6 Supply and Control of Cathodic protecting Current2.7 Some Practical Applications

3. Re-alkalisatiurJ3.1 Principle3.2 Anode System and Electrolyte3.3 Control of Operational Parameters3.4 Monitoring of the Alkalinity3.5 Effectiveness of realkalisation3.6 Some Practical Applications

4. Electrochemical Chloride Removal4.1 Principle4.2 Anode Systems for ECR4.3 Operational Parameters4.4 Practical'Applications4.5 ECR Monitoring4.6 Effectiveness of Electrochemical Chloride Removal

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Contents continued.

5. Side Effects of Electrochemical Treatments5.1 Hydrogen induced embrittlement5.2 Alkali-aggregate reaction5.3 Disbondment of steel/concrete5.4 Other possible effects-

6. Inhibitors- 6.1 Inhihition mechanisms

C.L Commonly Used Inhibitors6.3 Inhibitor Incorperation

4444454648

50505052

7. Surface Treatment and Coatings 54'. 7.1 Preparation for Coatings 54

7.2 General Requirement of Coatings for Concrete 547.3 Coating ~vpes and Their Functions 557.4 Surface Treatment and Coatings against Corrosion 57

7.4.1 Coatings against gas permeation (carbonation) 577.4.2 Coatings against liquid solution penetration (chloride) 57

7.5 Practical Application of Coatings 61

8. Control of Concrete Quality 638.1 Control of Water/Cement Ratio 638.2 Admixture Control 648.3 Design of Cover Concrete 658.4 Good Practice 65

9. Improvement of Reinforcing Materials 679.1 Corrosion Resistant Steel 679.2 Surface Treatments and Coatings on Reinforcing Steel 67

10. Repair of concrete 7010.1 Preparation for repair 7010.2 Repair Materials 7110.3 Patch Repair 7210.4 Overlay 73 .10.5 Injection 73

11. Comparison of Corrosion Prevention Techniques 74

12. Prevention Strategy 7612.1 Prevention Based on General Corrosion Degree and Environment 7612.2 Prevention Based on Corrosion Cause 7712.3 Prevention Based on Available Budget 8112.4Overall consideration . 82

13. Concluding Remarks

14. References

15. Appendix-Symbols

83

86

112

ARR 332,Corrosion prevention of reinforced concrete structures

Executive Summary

Corrosion prevention is a cost-effective approach to extend the service life of reinforcedconcrete structures subjected to corrosion attacks. The investigation and applicationof corrosion prevention techniques in reinforced concrete have been intensively andextensively addressed in the literature. Much improvement and progress has been,made in the past decades in the understanding and application of various corrosionprevention techniques, including cathodic protection, re-alkalisation, electrochemicalchloride removal, inhibitors, coating systems, concrete quality control, reinforcingmaterials, and repairing of damaged concrete. Some of these techniques have evolvedinto standard methods and have been widely used in field structures. Others are stillmainly at their laboratory investigation and field trial stages. There are still a numberof unsolved problems with each of the techniques, and some issues may continue tobe controversial in the near future. However, it is expected that new successfultechniques would eventuate once the controversies are resolved. There is no doubtthat corrosion prevention techniques will be playing very significant roles in the futurein extending the service life of reinforced concrete structures.

As research activities on the above-mentioned techniques are relatively intense, it isessential to have an overall understanding of the current situation of the corrosionprevention techniques in reinforced concrete systems, so that new developments inthe field of corrosion prevention of reinforced concrete could be followed moreeffectively. This state-of-the-art review is intended to serve this purpose. It is alsointended to be a useful and informative practical document for the corrosion engineersand the users who have a interest in solving corrosion problems. It is not intended todeal with detailed academic issues in this field, and we have tried to present a balancedreview in the area of science, technology and engineering application of corrosionprevention of reinforced concrete.

Our first report entitled"Corrosion of steel in concrete: causes, detection and prediction"was published in 1998 and provided background information on the understanding ofcorrosion of steel in concrete. This second state-of-the-art review report summarisecurrent techniques dealing with the corrosion problems of reinforced concrete.

Section 1 discusses the basic processes and mechanisms involved in a reinforcedconcrete and available techniques that can retard the corrosion processes.

Section 2 to 'I U review currently Widely used techniques in the corrosion prevention ofreinforced concrete structures, including cathodic protection (CP), re-alkalisation (RA),electrochemical chloride removal (ECR), inhibitors (INH), surface treatment (STC) andcoatings (CO) on concrete, concrete quality control (CQ), reinforcing materials (RM),and concrete repair!:; (RDC). The theoreticJI bJsis, current upplications, advantages anddifficulties of these techniques are the main subjects in each of the sections.

After comparison of these techniques in Section 11, some strategies for dealing withcorrosion problems are given in Section 12. Concluding remarks are made in Section13, where recommendation for further research and development on the preventiontechniques are also made.

It is hoped that this document, together with the previous review, will provide themost up to date collection of information in the field of corrosion prevention ofreinforced concrete, and will be of help to asset owners and engineers in understandingthe corrosion problems in reinforced concrete as well as the approaches to dealingwith such problems.

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1Rese.arch Report 332

1. Introduction

Preventing corrosion of reinforced concrete and extending the service life of reinforced concretestructures is the final goal of corrosion survey, testing and investigation.

Comparedwith corrosion mechanisms, the principles for protection are relatively simple. However,all the prevention techniques are developed based on the understanding of basic corrosion processesof reinforcement in concrete or the processes associated with the corrosion reaction.

A summary ·of current knowledge of corrosion In concrete and approaches to retarding corrosionprocesses will be presented in the introductory section of this report, which will be followed bysections on corrosion protection and prevention, and repair of concrete. It is hoped that this reportwill provide practitioners with adequate background knowledge to enable them to make sounddecisions on the adoption of appropriate strategies for the management of structures in their care.

1.1 Basic Processes in a Reinforced Concrete

Generally, corrosion can be defined as:

metal + aggressive medium ~ corrosion productssteel

.(1)

0'

X'

it : depolarisation reagent, 0' from surrounding enviroment,,penetrates into the surface layer of cover concrete and dissolves in pore solution,turning into dissolved depolarisation reagent 0

iz : depolarisation reagent, 0, transports deep into the cover concreteand arrives at the surface of reinforcement

i) or ic : cathodic electrochemical reaction, 0 + e--->R, occurs on cathodic zone (C)on the surface of reinforcement

i4 cathodic reaction product R departs from the cathodic zone (C)is cathodic reaction product R (reacts with some other species) and

turns into final corrosion products on the surface of reinforcementi6 anodic electrochemical reaction, Fe-->Fez+ + 2e, occurs on anodic zone (A)

on the surface of reinforcementi7 or ia : anodic reaction product Fez+ departs from the anodic zone (A)ig anodic reaction product Fez+ (reacts with some other species) and

turns into more stable corrosion products on the surface of reinforcementi9 the electron e produced from the anodic reaction transports through reinforcement

from anodic zone (A) to cathodic zone (C) to be consumed by the cathodicreaction

i10 or ij : an ionic current flows from anodic zone (A) to cathodic zone (C)through the cover COncrete by the migration of anions and cationsin the pore solution

ill.corrosion facilitating species from surrounding environment, X' ,penetrate into the surface layer of cover concrete, and become X

ilz corrosion facilitating species in the cover concrete, X, transport deeplyinto the cover concrete and arrive at the surface of reinforcement

i l3 or ip: external polarisation current from other area or counter electrode

Figure 1. Schematic representation of basic processes of corrosion of steel in concrete

ARRB Transport Research Ltd

2Research Report 332

This is only an overall expression for general corrosion process of a metal. As far as the corrosionof reinforcing steel in concrete is concerned, a large number of intermediate processes are involved.Figure 1 schematically represents these processes in a corroding reinforcement in concrete. This isageneralised case of corroding reinforcement in concrete. In a specific real corrosion case, not all the13 "processes are necessarily involved. For example, if the reinforcement is undergoing uniformcorrosion, then anodic and cathodic zones (A and C) are merged together," and there should be noprocesses i9 and i lO• If there is no external polarisation from remote area, then in (or ip) need not beconsidered.

Among the 13 basic processes shown in "Figure 1, there are two essential electrochemical reactions:anodic reaction ia (or i6) and cathodic reaction ic (or h). They are functions of potential, and theirpolarisation behaviours are summarised in Figure 2.

Ece

Ec

Ea

Ep

Eae

loglil

Bee: equilibrium potential for cathodic reaction process icBe: half-cell potential of cathodic zoneEa: half-cell potential of anodic zoneEp: polarisation potentialEae: equilibrium potential for anodic reaction of reinforcementReO: resistance of concrete between anodic and cathodic zonesia: anodic polarisation curve of reinforcementic

: cathodic polarisation curve for reinforcement in concreteiap

: anodic reaction rate of reinforcement at the applied potential Epicp

: cathodic reaction rate at the applied potential Epip

: externally applied current under the applied potential; ip = iap - iq)ig: galvanic current flowing between anodic and cathodic zones

Figure 2. Polarisation behaviour of anodic dissolution of reinforcing steeland cathodic reaction



1.1.1 Anodic reaction--The corrosion of reinforcement

The anodic dissolution (i6 or ia) is the most important process involved in the corrosion ofreinforcement. It is directly responsible for the corrosion damage of reinforcement, and can bebasically described by a simple overall reaction:

Fe =Fe2+ + 2e (2)

The intermediate corrosion product, Fe2+, could be further transformed into Fe(OH)2 by OR if the

pH value of the pore solution is high enough, or be further oxidised into Fe(OH)3 or Fe203 if there issufficient oxygen in the pore solution. The corrosion products (RP=Fe(OHh, Fe(OH)3 or Fe203 asrepresented in Figure 1) could either be accumulated at the surface of the steel rebar (is) (underoxidising conditions), or be dissolved into the pore solution and moved away from the steelreinforcement (i7) (under reducing conditions). The intermediate steps in the formation of rust areonly of academic interest. The accumulation of rust at the rebar surface causes the expansion andcracking associated with corrosion damage to the reinforced concrete.

Whether or not the anodic reaction (2) can occur easily, and what kind 'of corrosion products will beformed, depend on the electrochemical potential of the reinforcing steel and the chemistry of thepore solution of the concrete. The possibility or trend of electrochemical reactions of iron inaqueous media at different pH values have been summarised in a diagram known as the Pourbaixdiagram which is a potential versus pH relationship as shown in Figure 3.

potential(VISHE) 1:"'";----.r----------------,.

..L8,

1.0

- n....--....-----..lliiili:i:,;c

stable

- 1.4

- a 0 & a 1 9 Jl ,1$ IS pH

Figure J. Potential-pH diagram for steel in water

In the diagram, there are basically three different regions:

1) Immunity region: under lines c and d, Fe is stable and can not be anodically dissolved;

2) Corrosion region: above line c and left to curve e, Fe2+ and Fe3

+ are stable, but Fe is nolonger stable. Fe would spontaneously be dissolved or corroded, turning into Fe2

+ and Fe3+

through electrochemical reactions;


4

Research Report 332

3) Likely passive region: above lines d and f and right to curve e, corrosion products, .Fe(OH)2 , Fe(OHh, FeJ04' Fe20J, and other hydrated products can be formed on thesurface of steel. If the products are compact and can effectively retard further reaction ofthe steel with aqueous medium, then the steel will be at passive state, and there is nosignificant corrosion. However, if there is detrimental species, like cr, then the compactpassive film will become unstable. On the steel surface, instead of compact surface film,loose and non-protective surface film, such as a rust layer, would be formed. In this case,corrosion might still be very significant.

Normally, the concrete pore solution is rich in oxygen and has a high pH value (pH>12), i.e. steelreinforcement is always in region (3), the likely passive region. A thin passive film (RP = Fe(OHhor Fe(OHh) can be formed on the steel surface, which consequently retards procedure i6, so the steelcan be well protected in concrete, and there will be no detectable corrosion damage.

However, under some conditions the protective film may not be formed, or the formed passive filmmay break down. This applies, for example, to concrete that has been carbonated to a great extent,so that the pH value of its pore solution is low; or when a certain amount of aggressive chloride ionhas penetrated into the concrete and has reached the vicinity of the steel. In the first case, the steelis in the active corrosion region (Figure 3), and the passive film is not formed, or the formed passivefilm will be dissolved. The dissolved Fe2

+ in the pore solution tends to move away from the steelsurface. Hence, the cross section of reinforcement will keep reducing and finally the breakdown ofreinforcement results. in the second case where a certain amount ot chloride has reached thevicinity of the steel, even though the steel is still in the likely passive region, the passive film willbreak down due to pitting corrosion, or a non-protective rust layer will be formed on the surface ofreinforcing steel. In this case the corrosion proceeds rapidly.

However, if the pH is high and the chloride concentration is not above the critical threshold toinitiate pitting corrosion, the reinforcing steel should be in a passive state, and there is no COrrosionconcern. Nevertheless, in very rare cases that concrete is extremely oxygen-depleted, the steelsurface may cease to be passive and steel starts to dissolve in the form of HFe02' (the HFe02 regionin Figure 3) at a rate depending on the kinetics of oxygen diffusion through the concrete cover to thesteel [Wilkins (1983), Page (1987)]. This kind of dissolution of steel is usually characterised bycorrosion potential of the steel being more negative than -8501llVISCE [Page (1993)].

1.1.2 Cathodic processes

Besides the anodic reaction, cathodic process is also critical to the corrosion rate of reinforcing steelwhen there is no external polarisation (ip=O).

Commonly, the depolarisation agent is oxygen (02) from the atmosphere (i.e. in Figure 1,0'=02 inthe air. 0 is the dissolved oxygen in the pore solution, 0=02), At the steel surface in cathodic areaC, oxygen is reduced into hydroxyl via an electrochemical cathodic reaction i3 (or ic):

(3)

This is a common cathodic reaction invulved in Illusl corrosion cases (The cathodic reaction productR would be OK, R=OH' as shown in Figure 1).

However, in the environments where carbonation is the main cause of corrosion damage ofreinforcement, and the pH value of the pore solution is relatively low, the 0' (Figure 1) could becarbon dioxide gas (0'=C02). Correspondingly, the dissolved depolarisation agent 0 would beH2C03, or simply the proton W in the pore solution. So, besides reaction (3), the cathodic reactioni3 (ic) could still include:



211'" + 2e =H2(4)

In this case, the cathodic reaction product (Figure 1) would be hydrogen gas, Le., R=H2•

In some special cases, when the potential of reinforcement is considerably negative, the cathodicreaction may be hydrogen evolution, even though the pH value of the pore solution could berelatively high:

(5)

In this case, the corresponding 0' and 0 in Figure lare pore water, and R is OH'.

It should be stressed that, no matter which mechanism is operating (reaction (3), (4) or (5)), thecathodic reaction always produces OR or consumes H+ at the surface of reinforcing steel.Therefore, the overall cathodic reaction can always be simply written as:

O+e~R+OH(6)

which means that each consumed electron at the cathodic area would produce one OR anion there.As a result of the cathodic reaction, the pH value in the vicinity of the cathodic area (C) would beincreased.

1.1.3 Other important processes

In addition to the essential electrochemical reactions, there are some other processes that can affectthe corrosion reaction. For instance, in Figure 1, species X represents corrosion facilitating agents.Typical facilitating substances might be cr, CO2, H20, and other species, like sol that can causedeterioration of the cover concrete.

cr can accelerate the breakdown of the passive film on the reinforcement surface, and speed upprocess i6 (or ia), the anodic dissolution of steel. cr, mainly comes from the deicing salts orseawater. In Figure 1 X'= salt, and X=Cr.

CO2 comes from the environmental atmosphere. It penetrates into concrete and is dissolved in thepore solution, reducing the pH value of the pore solution. This also makes the passive film on thesteel surface unstable, and consequently, process i6 Cia) is accelerated. In this case, X'=C02, and X =H2C03 or H+ in Figure 1.

H20 exists in the cover concrete as the pore water remaining from the hydration and curing of theconcrete, or could come from outside of the concrete cover, such as rain, tidal splashing, andsurrounding water, etc. Water is essential for all the corrosion reactions, such that corrosion couldcompletely stop without water. In Figure 1 in this case, X'=X=H20.

1.1.4 Influences and interaction of the basic processes

According to their influences on corrosion rate, the 13 processes illustrated in Figure 1 can beclassified into three different groups:

Group 1: Processes that positively accelerate corrosion.



The rates of the processes in the first group are proportional to the corrosion rate, i.e., theseprocesses are coupling with the anodic dissolution of reinforcement. Slowing down anyone of theseprocesses will result in a lower corrosion rate. Most of the processes illustrated in Figure 1, such asi(, h, h, i6, i9, ilQ, i l1 , and i12, belong to the first group.

Group 2: Processes whose contribu.tion to corrosion (positive or negative) is uncertain.

This group consists of processes i13 , is and is. If i13 is an anodic current, i.e., it is flowing out fromthe steel in the studied area (including zones A and C), then corrosion of the reinforcement would beaccelerated. Otherwise, if it is flowing into the reinforcing steel in the selected area (including zonesA and C) as a cathodic current, the corrosion could ceaSe. is and is could lead to formation of acorrosion product RP on the steel surface. If the pH value in the vicinity of the steel surface is veryhigh and chloride contaminant is very low, then RP would be a protective passive film. In this case,increased levels of is and. is would speed up the formation of a passive film, and consequently reduceCthe·corr6sion~attack··onothecreinforcement··0n~the~othedlalld~ihheconcentration-of-eF~is~high~in~~~·

the vicinity of the steel bar, then a non-protective rust layer would be formed at the interfacebetween the steel bar and the cover concrete. The volume of the rust produced as a result ofcorrosion is usually 2-6 times greater than the volume of the metal consumed by the corrosionreaction. This means that an expansion stress will be induced by corrosion of steel at the interfacebetween steel and the surrounding concrete, which would eventually crack the cover concrete.

Group 3: Processes with insignificant influence on corrosion.

This group of processes, like i4 and i7, can not affect the corrosion of reinforcement, because theyhave no significant influence on the electrochemical processes.

When there is no external polarisation current (i 13 or ip) and the system is in steady state,

(7)

where ig is the galvanic current between anodic and cathodic areas. It causes the weight loss of steelor the cross-section reduction at area A. In this case, ig actually reflects the corrosion rate of thesteel bar. In addition, equation (7) indicates that h, i6, and i lO are coupling processes. Slowing downanyone process would retard all the processes. In this case the electrode potential at the cathodicarea, i.e. Ec, is higher than the electrode potential at anodic area, i.e. Ea, and there exists arelationship between them:

(8)

where ReO is the resistance of the cover concrete between the anodic and cathodic areas. Equation(8) indicates that a greater·gradient in the half-cell potential contour map corresponds to a highercorrosion rate. In this case, the measured half-cell potential of reinforcement would have a valuebetween those of Ec and Ea, depending on the location of the reference electrode used for thismeasurement. When the reference electrode is close to the cathodic area C (Figure 1), the measuredhalf-cell potential would be higher and closer to Ec. Otherwise, it would be lower and closer to Ea(Figure 2).

If the reinforcement is polarised by an external polarisation current (ip) or potential (Ep), both ia andic will be changed along their polarisation curves to values iap and icp as indicated in Figure 2, suchthat:



(9)

The more negative the external polarisation potential or the larger the cathodic polarisation current,the smaller the anodic dissolution rate of reinforcement. If the polarisation potential is verynegative, , e.g. more negative than the equilibrium potential (Eae) of the anodic dissolution of thesteel, i.e. Ep < Eae, or if the cathodic current is very strong, iap would be zero (Figure 2). Under thiscondition, the anodic dissolution of reinforcement could be completely suppressed.

When ip passes through concrete, some changes would occur in the concrete cover. In the concretepores, the convection process is negligible. In a high potential field, the diffusion contribution to thetransport of charged species in the pore solution is also relatively small and may be neglectedcompared with the electric charge migration process. So, ip is approximately equal to the sum ofionic charge flows Ok) when there is no convection and diffusion processes [Sa'id-Shawqi (1998)]:

(10)

where nk is the valency of ion k; F is the Faraday constant.

Correspondingly, any ion flow h can be related to ip:

(11 )

where

(12)

and tk (O<tk <1) is the transference number of ion k; Ck is the activity or concentration of ion k; andUk is the mobility of ion k which is a characteristic parameter reflecting the migration ability of theion. nj, Uj, and C j are the valency, mobility and concentration of any ion i in the electrolyte,including ion k. Therefore,

(13)

always holds.

For cations like Na+ and K+, Ca"2+ nk>O, so hand ip have the same direction of flow, i.e., cations flowin the same direction as the current. Whereas, anions, such as cr, CO/- and OR-, have negative nk(nk < 0), so they will flow against the current flow ip•

1.2 Approaches to Retard Corrosion of Steel Reinforcement

The corrosion processes are broadly driven by two factors, the properties and conditions of the steel,and the concrete medium. The interface between the steel and concrete and the reactions that takeplace at the interfacial zone are of considerable importance in the corrosion process. Corrosionprevention and protection techniques are based on the optimisation of these factors to achieveretardation of the corrosion processes. .



1.2.1 Electrochemical approachs based on cathodic polarisation

In prindple, the anodic dissolution process i6 or ia (Equation (2» is directly responsible forcorrosion damage of reinforcement. Therefore, directly reducing or suppressing the rate of i6 is themost effective method to protect reinforcement from corrosion attack.

Based on Figure 2 and Figure 3, the simplest way to achieve the above goal is to directly shift thepotential (Ep) of the reinforcement from its natural corrosion potential (Ecorr) to values below theEae value (Figure 2), or below lines c and d (Figure 3) into the immunity region. This would ensurethe reinforcement is not at risk of corrosion at all. Even if the polarisaton potential (Ep) was notdropped below Eae, the shift could still lower the anodic dissolution rate of the reinforcement tosome extent (Figure 2). This is how cathodic protection (CP) technique is used, in which thepolarisation potential is lowered to protect against anodic dissolution of steel.

Slowing down the anodic dissolution can also be achieved by forming a passive film on thereinforcement. To form a stable passive film, according. to Figure 3, high pH values (higher thancurve e in Figure 3) of the pore solution in the vicinity of the reinforcement is a necessary condition.This can be achieved through strong cathodic reactions (3), (4) or (5) as mentioned earlier. Thetechnique. used to produce high OR concentration and increases the pH value of the pore solution inthe vicinity of reinforcement, and to make the reinforcement passive through a strong cathodicpolarisation, is known as the re-alkalisation (RA) technique.

High pH values in the vicinity of the reinforcement alone is not enough in some cases to produce apassive film on the reinforcement, particularly when a considerable amount of chloride ions arepresent in that area. Therefore, if corrosion of reinforcement is caused by the presence of chloride inthe concrete cover, then removal of chloride is an effective method to reduce the corrosion rate ofthe reinforcement. As discussed in section 1.1, a cathodic current flowing into the reinforcementwould be able to repel the negatively charged chloride ions away [rum llt~ lein[orcement sUlface(equations (10), (11), and (12». This is called electrochemical chloride removal (ECR) technique(or desalination technique).

1.2.2 Techniques associated with the interface of steel/concrete

Corrosion damage is the result of reinforcing steel reaction with the surrounding concrete medium(Equation (1». Factors affecting the interface properties could be employed to control the corrosionrate.

The type of reinforcing material is one of the obvious factors that would significantly influence theinterface properties. A corrosion resistant material has lower dissolution rate, i.e. lower i6 or ia

(Figure 1), hence better corrosion performance. Choosing corrosion resistant materials (RM), suchas stainless steels, could be considered as an option to deal with the corrosion problems in thereinforced concrete.

According to (Equation (1», contact between metal and the environment can result in corrosion. Aspecial coating on the reinforcement surface to avoid the direct contact between the steelreinforcement and concrete medium is another way of reducing corrosion risk of the reinforcement.However, great care is needed to ensure that such coating remains intact before and after placementin concrete. Otherwise, this method may not be effective.

Small amounts of some special chemical species at the interface between the reinforcement andconcrete can' also impede the anodic dissolution process i6 or ia (Figure I) by changing the anodicdissolution mechanism, facilitating the formation of a passive film, or forming an adsorptive film bythe species itself on the reinforcement surface to block the anodic dissolution. Such species arecalled inhibitors. Addition of inhibitors (INH) is also an option for corrosion protection.



1.2.3 Techniques associated with the cover concrete

Technically, most of the processes (Figure 1) are closely related to one another as analysed above,especially the first and second groups which have sign!ficant influences on the anodic dissolution ofreinforcement. Therefore, in addition to the techniques directly dealing with the anodic process (in),corrosion protection could also be achieved through changing the reaction rates of most of the otherprocesses illustrated in Figure 1.

Within the concrete, to reduce i2, ilO, and il2 which are categorised in the first group, high qualityconcrete with low w/c ratio and incorporating mineral additives would be recommended. Oncecracking has occurred, impregnation by sealers could be considered. Furthermore, according toFigure 2 and equation (8), a smaller difference between the corrosion potentials in cathodic zone(Be) and anodic zone (Ea), and a higher concrete resistance (Rc) between these zones ~ould result ina lower ilO• Therefore, improving homogeneity and increasing resistivity of the cover concrete isalso a good option.

On the surface of the concrete, processes i, and ill belong to the first' group. Retarding theseprocesses would also lead to a low corrosion rate. In this case, surface treatment and coatingsystems for concrete would be good choices to decrease the rates of i l and ill.

1.2.4 Summary of the approaches

Figure 4 summarises the corrosion prevention measures that have been widely applied to reinforcedconcrete systems. They include cathodic protection (CP), re-alkalisation (RA), electrochemicalchloride removal (ECR), inhibitors (INH), surface treatment (STC) and coatings (CO) of concrete,concrete quality control (CQ), reinforcing materials (RM), and repair of damaged concrete (RDC).They will be reviewed and discussed in more detail in the following sections.


10

'Surface treatment (STC)and coating (CO):CG,C~

/'

I

I.\\

I II

I

Research Report 332

surface treatment (STS)and coat{ng (CR) .

. ~ resist nt steel (RS)

AD admixture controlCC cover concreteCG coatings against gas penneationCO coating technique for concreteCP cathodic protection techniqueCQ concrete quality controlCR coating on reinforcing steelCS coatings against solution penetrationECR electrochemical chloride removal techniqueINJ injectionINH . inhibitor techniqueOL overlayPAT patchPRA practiceRA realkalisation techniqueROC repair of damagedconcreteRS corrosion resistant steelSTS surface treatment for steelSTC surface tratment for concreteWC water/cement ratio control

Figure 4. Schematic illustration of corrosion prevention measures applicableto a reinforced concrete system



2. Cathodic Protection

Generally, cathodic protection (CP) is the technique used to cathodically polarise a the metallicobject to reduce its corrosion dissolution rate of the metal. The technique was first used in seawater[Davey (1824)]. Since the beginning of the 19th century, cathodic protection has been developedrapidly, and widely used in underground and offshore steel structures.

The earliest recorded applications of CP to steel in concrete were in buried pipeline systems [Unz(1960), Heuze (1965)]. In those cases where the material was surrounded by a conductiveelectrolyte, it was possible to use conventional discrete anode systems to distribute the current to thereinforcement. However, the application of CP to atmospherically exposed reinforced concrete ismorc difficult because of the need to ensure a reasonably uniform current distribution in a concretewith high resistivity.

In 1974, CP was reported to have had a successful application in a reinforced concrete bridge deck inthe USA [Stratfull (1974)]. Since then, substantial progress has been made, and CP started to beused extensively in the USA [Stratfull (1983), Schutt (1990)] and Canada [Manning (1990)] as asolution to corrosion problems in reinforced concrete contaminated with chloride salts. In the UKlarge scale applications of CP are also becoming more frequent because of the increased confidencein CP gained from field trials [Boam (1993)]. Since 1976 when successful use of CP on bridgedecks was reported [Federal Highway Administration (1976)], more than 150 bridge decks have hadCP installed, and some have performed satisfactorily [Boam (1993)]. Particularly, from the early1990s, various state of the art reports, codes of practice and recommended practices have beenpublished [British Standard Institute (1991), NACE RP0187-90 (1990), NACE RP0390-90 (1990a),NACE RP0290-90 (1990b), Institute of Corrosion/Corrosion Society (1991), Wyatt (1993a)], andthe application of CP in reinforced concrete structures has become widespread. It was estimated in1981 [Scheffey (1981)] that up to 50 billion dollars in repair costs could be saved over the following30 years by the use of cathodic protection.

It is particularly worth mentioning that around 1993, the Strategic Highway Research Program,National Research Council, released a series of SHRP reports onCP techniques [Bennett (1993d),Clear (1993), Bartholomew (1993), Sagues (1994a), Eltech Research Corporation (1993), Weyers(1993)], in which the application of CP in reinforced concrete was well addressed. The publicationof these documents might be considered a significant milestone in the development of CP inreinforced concrete structures.

Currently, cathodic protection (CP) techniques are not only used to retard the corrosion rates of theexisting reinforced concrete structures, but they are also employed as electrochemical means aimingat improving the corrosion resistance of new reinforced concrete structures subjected to chloridepenetration [Pedeferri (1992), Bcrtolini (1993)].

Besides preventing reinforcement from corrosion, cathodic protection was claimed to have severaladditional beneficial effects on the protected structures [Pedeferri (1995)]. In the laboratory, it wasfound [Hassanein (1996)] that a relatively small amount of charge passed enabled a significantquantity of chloride to be removed from the concrete close to the reinforcement (cathode), andoxygen-starved conditions were readily achieved in the concrete. If a rebar which has beenpreviously corroded is cathodically protected, it could be repassivated when the applied potential islower than -800mV/SCE [Joiret (1996)]. Based on the effects of cathodic current on a reinforcedconcrete, Yokota et al (1992) proposed an electrodeposition concept for reinforced concretestructures in seawater. Chemical compounds such as CaC03 and Mg(OHh were expected toprecipitate on reinforcing steel surface in concrete under cathodic protection in sea water, and reduce



corrosion rate of the reinforcement. Their experiments inQicated that electruuepusits tellueu toprecipitate in and around cracks and other places of higher conductivity; the pore structure ofconcrete with electrodeposited materials was denser than that before electrodeposition; and theadhesion strength was not significantly affected by the electrodeposition.

These findings would undoubtedly lead to more extensive application of CP in reinforced concretestructures.

2.1 Theory

The principle of cathodic protection (CP) is relatively simple and consists of lowering the potentialof the reinforcement in order to reduce the corrosion rate (Section 1'.1). The anodic dissolution ofsteel involves the generation of electrons, i.e., a corrosion current (equation (2». These electrons areconsumed at the cathode in the cathodic reaction (equation (2». By applying a cathodic current andsupplying the electrons needed for the .cathodic reaction, the anodic reactions, i.e., dissolution ofsteel is suppressed.

The polarisation curves for a corroding system was shown earlier in Figure 2. The purpose ofcathodic protection would be to lower the magnitude ofEp below that of Eae. This situation sirepresented in Figure 5. In Figure 5, when Ep < Eae, then no anodic dissolution current exists, i.e.,iap =O. In this case, the applied CP current (ip) would be equal to the cathodic current (icp) under thatprotection current or potential according to equation (9), i.e., ip= -icp.

The effectiveness of a CP system is influenced by many factors, such as the properties of counteranode, resistivity of concrete, voltage provided by external power (rectifier), etc.. Generallyspeaking, high effectiveness would easily be achieved if:

1) the counter anode has a highly negative open-circuit potential and very small Tafel slope(i.e. difficult to be polarised);

·2) the resistivity of concrete is very low;

3) the Voltage provided is high enough.

The detailed reason for these are as follows:

In Figure 5, Epanod is the potential of the counter anode used to apply a CP current (ip ) to thereinforcement (at this potential, ip is flowing out from the counter anode to the reinforcement); andEp is the potential of reinforcement after the CP is applied.



E

loglil

i anoda

I, 1-' anodIp- lap

Ii I

I, anodaO

Eae

Ep

Ea

Ec

I

Eaanod 'c - - -0 :• ~~ I baanOd

I &.-=- I

Epanod W-~ - ---- --- ~ -- -Ea'ano _T_ - -

la:Ie:

I, anod.a .

V powe:Rco:

Rc:

lap:iep:I p:

I, anod,aO •

I· anod.ap .

Ece: equilibrium potential for cathodic reaction process ieEc: half-cell potential of cathodic zoneEa: half-cell potential of anodic zoneEp: cathodic protection potential for reinforcementEae: equilibrium potential for anodic reaction of reinforcementEaanod: open-circuit potential (without polarisation) of counter anode aloneEa'anod: open-circuit potential (without polarisation) of counter anode after driven

by an external power by V Ea'anod =Eaanod_Vpower, powerEpanod: polarisation potential of counter electrode at which

anodic current iapanod is provided by the counter anode to reinforcementvoltage provided by external power (rectifier)resistance of concrete between anodic and cathodic zonesresistance of concrete between reinforcement and counter anodeanodic polarisation curve of reinforcementcathodic polarisation curve for reinforcement in concreteanodic polarisation curve of counter anodeanodic reaction rate of reinforcement at the applied potential Epcathodic reaction rate at the applied potential Epexternally applied current under the applied potentialself-dissolution rate of counter anode at open-circuit potentialanodic current flowing from counter anode (at potential Epanod )into reinforcement· i anod =Ii I, ap p

Ig: galvanic current flowing between anodic and cathodic zonesbaanod : Tafel slope of anodic reaction of counter anode (the slope of curve i:nod)

Figure 5. Diagram for cathodic protection principle


14

Research Report 332

In order to completely protect the reinforcement from corrosion, the anodic dissolution rate of thereinforcement should be zero, i.e. iap =0, which requires:

Eae cEp; or I · I . anod > . Ilp = lap - -lcp Eae (14)

This means that the value of the applied cathodic protection current lipl should be high enough.

According to Figure 5 and the above equations, we have:

Ep = Epanod + lipl Rc=Ed!,!(Id - v~ower + btl/nod lug lipl _lx/mod log Iiaoanodl + lipl Rc

E anod b anod, I' F anod I I' I R· 'T= a + a 08 '",laO + lp r: - Vpower

(15)

Equation (15) suggests that the CP potential Ep is determined by the following parameters: Eaanod,Vpower, baanod , liaoanodl , and Rc: Lower Eaanod, baanod , and Rc, and larger Vpower would lead to a morenegative Ep, which makes the CP criterion easier to be achieved.

Among these parameters, the most easily adjusted one is Vpower. Regardless of the values of theother parameters, as long as Vpower is adjusted to be negative enough, Ep would be brought down tothe desired value to satisfy the CP criterion; However, in some cases that Eaanod is negative enough,even there is no external power voltage (Vpower =0), Ep could have been very negative to achieve theCP protection.

Therefore, there are two ways to shift the potential of the reinforcement to more negative values:

1) Impressed current cathodic protection (ICCP): a cathodic current is applied from a counteranode to the reinforcement [Crerar (1996)]. ipwill be directly controlled by a powersupply through the counter anode.

2) Sacrificial anode cathodic protection (SACP): a counter anode, whose natural potentialEanod is even lower than Eae, is coupled (short-circuited) to the reinforcement.

In the first case, the control of the impressed current or Vpower is the main issue, as the current andvoltage can be easily adjusted, ICCP can be flexibly operated in the field. In the second case, Epanodand ip are mainly dependent on the Tafel slope (baanOd), open-circuit potential (Eaanod) and selfdissolution rate (iaoanod) of the sacrificial counter anode. In other words, besides the concreteresistance Rc, the electrochemical polarisation behaviour for the sacrificial counter anode also has acritical influence on the effectiveness of cathodic protection. Sufficjently negative values of Eaanod

and baanod, and large value of iaoanod would be the basic requirements of a sacrificial anode.However, these parameters have their upper and lower limits for a given anode, and according toEquation (15) it is quite possible that effective cathodic protection may not be achieved if Rc is toohigh. Currently sacrificial anode cathodic protection is still mainly· experimental [Broomfield(1997)].

Impressed current cathodic protection (ICCP) has been demonstrated to be able to effectively retardthe corrosion of reinforcing steel in chloride-contaminated concrete [Bartholomew (1993), Perenchio(1985)]. It is more operational than the sacrificial anode cathodic protection even though the latteris more economical.

ARR.B Transport Research Ltd


2.2 Practical Protection Criteria

Theoretically, a fully protected reinforced concrete system would have the criteria of Ep~ae and ip=-icp. However, meeting these criteria has been proved to be unnecessarily costly in practical use.So, the practically adopted criteria in reinforced concrete systems are more or less different from thetheoretical criteria. This is mainly due to the following considerations:

1) High ipmeans high energy consumption, and requires high quality anode, but results inshortened anode service life.

2) At high ip, the decrease of corrosion rate becomes less significant as ip increases.

3) Very high ipcould have side effects un the prutected steel, such as hydrogen inducedembrittlement, alkali-aggregate reaction in concrete, and disbonding ofreinforcement/concrete.

In practice, a less negative potential (Ep>Eae) or protection current (Iipklicpl) is often used. Actually,the service life of reinforced concrete can considerably be extended by achieving a potential lessnegative than Eae. It has been proven that a negative shift in potential (Ep) of 100 to 150mVreduces the corrosion rate by at least an order of magnitude [Bennett (1994), Bartholomew (1993),Bennett (1989), Funahashi (1991), Funishashi (1992a), Chaix (1995)].

Different forms of criteria that can indicate the effectiveness of the cathodic protection by measuringpotential of reinforcement have been widely recommended. They are summarised in Figure 6. Theparameters defined on the diagram are used in establishing criteria for the effectiveness of a CPsystem. The term IR drop refers to the potential difference across the concrete between thereinforcing steel and the reference electrode for potential measurement.

~~~----------------------------~ .Eon

~c Ecorr~oc.. IR drop

ip swith-on

IR drop

ip switch-off time

Figure 6. Schematic illustration of potential changes of the reinforcement when the CP systemis switc.hed on and off,

2.2. 1 Absolute potential (Eon) criterion

This criterion is based on the thermodynamic behaviour of steel in concrete. The potential (Eon) ofreinforcement relative to a standard half cell is monitored while cathodic protection is operating(Figure 6). For steel in concrete containing chloride, this criterion requires that the potential belowered into stable passive region [Lourenco (1992)].

The potentials commonly used are either -700mV/CSE or -850mV/CSE [Vrable (1977), Jurach(1981)]. Experimental work has indicated that steel embedded in concrete which is polarised topotentials more negative than Eon=-770mV/CSE would be satisfactorily protected even in chloridecontaminated concrete [Schell (1985)]. In addition to these values, different values have also been



proposed. Hausman (1969) suggested that the corrosion of sled iu t:um;rele t:au ut: prevented if thesteel is polarised to a minimum of -500mV/CSE. Vrable (1976) believed that corrosion could bestopped at -770mV/CSE. For prestressing steel, -900mV-Ag/AgCl/0.5M KCI is recommended as alower potential limit in the European Draft Standard (1996).

The most outstanding advantage of the Eon criterion is its ease of operation. Eon can be easilyobtained any time while the reinforced concrete structure is under cathodic protection. However,the reliability of this criterion is threatened by the influence of IR drop [Lourenco (1992)]. Thiscriterion does not consider the influence of the IR drop between the reinforcement and the referenceelectrode used to measure Eon on Eon value (Figure 6). In a reinforced concrete whose resistivity isalways very high, the IR value is very significant when a cathodic protection current is applied. As aresult, the measured Eon may not reflect the real Ep of the reinforcement. When the measured valueof Eon ~ -770mV/SCE. the value of Ep might be much more positive than this measured value.Therefore, the Eon criterion is not an ideal indicator of the cathodic protection effectiveness for steelin concrete lHausmann (1969), Broomfield (1997)].

2.2.2 Polarisation shift criterion (J1Eon)

One of the alternatives to the absolute potential criterion is the polarisation shift (LlEon) from the asfound potential (actually natural potential or Ecorr) [Hartt (1996), John (1993)] (Figure 6). Variousvalues for LlEon, from 300 to 100 mV have been proposed [Lourenco (1992)]. Recently, mostresearchers tend to adopt a 300mV shift as a necessary criterion for the assessment of CPperformance. In the operation of this criterion, it is required that the potentials of the reinforcementbe obtained at various locations and the standard deviation of the potentials determined; and that themost negative (anodic) site be shifted cathodically by a potential at least equivalent to the standarddeviation of potentials previously observed [NACE RP0290-90 (1990a,1990b), John (1993)].Published work [McKenzie (1988)] has shown that this criterion does lead to protection of steel inconcrete for both active and non-active areas without excessive current demands.

However, this criterion also takes no account of the IR drop between the steel reinforcement and·reference electrode (standard half-cell electrode) in concrete, and has similar shortcomings as the Eoncriterion. Furthermore, this criterion can not be readily used in the routine assessment of CPperformance. For this purpose, the system would have to be completely depolarised (often requiringseveral days), and then charged to determine LlEon. In addition, this criterion fails to recognise thechange in the corrosion potential of reinforcement over time after CP has been applied [Cherry(1993)].

2.2.3 Instant switch-off (Eoff) criterion

This criterion is based on the instant residual polarisation on switching-off the cathodic protectioncurrent which is defined as Eoff as shown in Figure 6~ In practice, Eoff is determined by the potentialmeasured in not less than 0.1 second and not more than 1.0 second following interruption of DCcathodic current [Corrosion Engineering Association/Concrete Society (1989), Institute of Corrosion/Concrete Society (1991)]. This would eliminate the problem of IR drop between the referenceeletrode and the reinforcement, which is the main attractive feature of this criterion. Eoff valuesmore negative than -720mV with respect to Ag/AgCI/0.5M KCI electrode was recommended forcathodic protection practices. Sometimes. a more negative instant switch off value (EofF-900 to 950mV/CSE) has been used in the field [Godson (1998)].

However, after long periods of cathodic protection, the environment in the vicinity of thereinforcement could be dramatically changed due to the electrochemical polarisation. In this case,the equilibrium potential (Eae) of the anodic dissolution of steel reinforcement could be shifted,hence the requirement of sufficient protection could be altered. Under this condition, a previouslysatisfactory switching-off potential does not mean that the reinforcement could always be



sufficiently protected as time elapses. Furthermore, the measurement of Eoff requires to switch offthe CP operation, and record the instant potential change in a very short time. Such recording issometimes difficult in the field.

2.2.4 Depolarisation (.1Eo") criterion

Considering the variation of Eae during cathodic protection, a more practical criterion,depolarisation potential (.:iEoff), is recommended and used in the field. The potential shift (AEoff asshown in Figure 6) is the difference between the instant-switching-off potential (Eoff) and thepotential after a certain period of time, e.g. 4 hours or 24 hours.

The depolarisation criterion of 100mV has been accepted by NACE Standard RecommendedPractice [NACE RP0290-90 (1990a,1990b)]. However, Laird (1991) has suggested that 100 mVdepolarisation was too high, and could result in over polarisation of the reinforcement.Nevertheless, Bennett et al (1989) reconullended a .:iEoff value uf 150mV. Bartholomew et al. (1993)also reported that ISUmV depolarisation was a reasonably accurate criterion for newly appliedcathodic protection system, but it is likely to result in over-protection over time. In addition, higherdepolarisation values, ranging from 155 to 240mV (.:iEofFI55-240mV) have also been suggested,depending on the chloride content in the concrete [Funishashi (1989), Funishashi (1992)].

Increasing practical evidences have indicated that reinforcing steel in concrete is protected [Slater(1979), Wyatt (1988)] provided that this criterion (.:iEoff =IOOmV) is met. This is also the leastargued criterion used in reinforced concrete.

In practice, 100mV shift in potential is often measured from instant switch-off to a period, typically4 hours later. However, whether or not the 4 hours is an appropriate time at which to infer the levelof polarisation by measuring the depolarisation is still open to argument. It was believed that therate of depolarisation was substantially slower than IOOmV/4h in good quality concrete whencathodic protection had been applied effectively for a long time [Kendall (1989)), whereas in poorquality concretes the depolarisation is likely to be very much quicker [Ashworth (1993)]. The timenecessary for depolarisation is assumed to be dependent of the availability of oxygen at the steelconcrete interface [Funahashi (1991)]. In some structures where oxygen access has been limited orwhich have been well polarised for a number of years, only small observed potential decay can beachieved after 4-6 hours, but over 24 hours there could be upward of 200mV decay [John (1993)].Bennett et al. (1990b) recommended a period of 24-48 hours if the structure depolarises slowly.Funahashi et al (1992a) reported that when a depolarisation criterion is used, the corrosivity of theconcrete surrounding the reinforcing steel due to chlorides and temperature should be considered.The effects of the oxygen and moisture contents in concrete should also be investigated. Thedepolarisation shifts of the reinforcing steel needed to reach a non-corroding state in chloridecontaminated concrete were 200 to 240mV for lkglm3 of chloride content, 175-215mV for 3kglm3

of chloride content, and 155-177 mV for 9kglm3 of choride concrete. Therefore, it was suspectedthat the 100mV value used as the polarisation shift criterion might not be adequate for all exposedconditions of steel in concrele; if environmental temperature and moisture did not changesubstantially during the monitoring time, the 4-hour depolarisation time might still need to beextended.

2.2.5 Polarisation curve (E·logl) criterion

In addition to the above criteria, a method known as the break-in-the-curve, originally used inunderground cathodic protection, can also be adapted for use in concrete [Stratfull (1983), Swiat(1989), Bennett (1993d)]. This criterion requires that the amount of current passing to the reinforcedconcrete system is increased in logarithmic steps and the potential, compensated for IR drop,recorded. On the E-logI curve, the break (between the 2nd linear region and curved region as shownin Figure 7), starting from where the curve deviates from its linear region,could be found



graphically [NACE RP0290-90 (l990a, 1990b)j. The cathodic protection current requirement isgiven by the current associated with this break point [Gummow (1986), Bennett (l993d)] (Figure 7).

43

. 2nd Unear Regien

1

E-ColT

1 E-Protect

t at Unear Region__ I·

-!:!~=:=:;==:::::=':::==~~==:=;=~~~===~'l2

Log of Current (rnA)

Figure 7. Schematic illustration of E-logI criterion [Bennett (1993d)]

660

640

620

600

580

:> 560E 540~

iii 520'0::l:III 500..0Q.

480

460

440

420

400

3800

Theoretically, this "break point" corresponds to the Eae potential of reinforcement, so under thiscriterion, reinforcement should be fully protected. However, this criterion has severaldisadvantages. There are too many factors in the field that could distort the curve, so on a measuredE-logI curve, it is hard to determine the break point. Also, to obtain a complete E-logI curve whichcan clearly display the "break point", a current density much higher than the required cathodicprotection current density is needed. This would be difficult for a portable power supply to achieve.Furthermore, this technique can not be applied to an operating CP system, and it is alos very labourintensive and time consuming.

2.2.6 Compromise of criteria

Which criterion can best reflect the perfomiance of an operating CP system depends on the concretesituation and our understanding of the field structure. In a survey [Broomfield (1991)], 223

. structures were controlled using IOOmV depolarisation criterion; 146 structures were with E-logIcriterion; 58 structures were monitored by -850mV/CSE criterion; and a smaller number ofstructures were using other criteria.

In some cases, it was found that some of the criteria were not met in a real structure, so modificationof the criteria would be needed, and different criteria should simultaneously be used to ensure aneffective protection [Gedge (1996)].

Mter comparing the advantages and disadvantages of these different criteria, it has beenrecommended [Corrosion Engineering Association/Concrete Society (1989), John (1993)] that: forthe structure on commissioning, the potential of reinforcing steel at representative points should benegatively shifted from "as found" state by at least 300mV (~on) or to a most negative potential of llOOmV/CSE (Eon) at anyone point whichever occurs first; the protection criterion should be aminimum of IOOmV potential decay (~off) over all representative points within the area of the



structure being protected, subject to a most negative limit of -1l50mV/CSE instantaneous offpotential (Eoff) with a typical decay period of 4 hours. John et al (1993) recommended acompromised period between 4 and 24 hours. If 100 mV is not achieved after 4 hours, then aslightly higher decay limit (for example 120mV) should be required after 24 hours. Solomon et al.(1998) employed the following protection criteria in a structure. In the atmospheric zones,LlEoff~100mV within -72 hours. They believed that the higher moisture levels in the concreteelements would result in a lower rate of depolarisation. In submerged zones, Eoff < -850mV/CSEwas required. The most negative measured potentials should not be more negative than11OOmV/CSE (Eoff ~ -11OOmV/CSE), otherwise, the structure might be over-protected.

2.3 Distribution and Variation of Protection Current Density

It should be noted that the applied protection current ip discussed above is the total current from thecounter anode to the reinforcement. For a large structure, non-uniform distribution of ip along thereinforcement would be unavoidable, because the polarisation resistance of reinforcing steel and theresistance (Rc) between the counter anode and the reinforcement could vary from place to place inthe structure. In addition to the geometric arrangement of the reinforcing steel bars, the existence ofactive/passive regions on the reinforcement, and the heterogeneity of the cover concrete, the depthof-cover, all have significant effects on the distribution of ip [Bartholomew (1993)].

The non-uniform distribution of ip has detrimental effects on the effectiveness of the cathodicprotection system, and makes the choice of a reasonable ip much harder. At a given ip, some areas ofthe reinforcement may receive high cathodic protection current densities and become over protected,whereas oth~r surfaces of the reinforcement may have insufficient cathodic protection currentdensities and still undergo serious corrosion.

Further increasing the magnitude of ip is not a good option, as this would cause over-protection andcould have side effects on the reinforced concrete, such as hydrogen induced embrittlement ofreinforcement, AAR in concrete, and loss of the bond strength at steeVconcrete interface. Apractical solution is to install the anode system in the concrete in the most effective locations suchthat it generates a more uniform distribution of ip. This is actually the most important activity in the

. application of a cathodic protection system to a reinforced concrete structure:

A reasonable installation of anode system and a correct selection of ip rely on accurate prediction ofthe distribution of cathodic protection current density in the cover concrete. For simple and regularshapes of objects, the calculation of resistance and distribution of cathodic protection currentdensity has been established [Morgan (1993)]. Unfortunately, reinforced concrete structures aremore complicated than simple shapes. Overcoming the difficulty in obtaining an even spread ofprotection current in concrete is an academic issue worth comprehensive investigation.

Another unavoidable factor that can affect the effectiveness of cathodic protection is the dependenceof the cathodic protection current ip on time. This makes ip unpredictable in the field. Raharinaivo(1996) found that the reinforcing steel potential under cathodic protection was not constant if it wasnot controlled. In the short term, the main factor causing the change of potential was temperature.The temperature was found to int1uence the relative humidity around the concrete, inducing oxygenmovement, and the saturation value of oxygen in the concrete. Over a longer term, the curing (age)and degradation of th~ cover cuncrete, and the development of corrosion on the reinforcing steel, allhave significant influences on the distribution of ip [Morgan (1993)]. Therefore, in CP design andpractice, the value of ip must be carefully chosen to tolerate such variations in the ip requirement.

2.4 Anode Systems

The core part of a cathodic protection system is its anode system. An anode system comprises anodematerials, fixings, connections, and in many cases overlay materials.


20

Research Report 332

An anode system for reinforced concrete should fulfil some or all of the following requirements[Wyatt (1993)]:

1) adher~ to or be capable of being fixed to concrete;

2) be capable of being installed'on some or all orientations of surfaces of reinforced concretestructures;

3) be compatible with the mechanical and environmental features of the structures on whichthey are installed;

4) be cost effective, i.e., have a long service life and low installation cost.

Usually, counter anodes can not be directly placed in concrete. They need to be placed in aconducting backfill [Morgan (1993)] in the concrete.

Anode systems have been developed in various forms, such as conductive coatings, metalsembedded in concrete overlays, conductive concrete overlays and probes drilled into the concrete,etc. However, depending on the form of current application, there are two kinds of anode systems:impressed current anode (lCA) and sacrificial anode (SA) systems.

2.4.1 Impressed current anode

Impressed current anode (ICA) is the anode used to impose cathodic current on the reinforcement.Its function is to spread the current to all areas to be protected. Usually, it has a very slow orcontrolled consumption rate.

The basic requirement for an ICA includes:

1) long durability (service life);

2) known and acceptable polarisation characteristics and low polarisation resistance;

3) capability of being formed by simple and economical processes into shapes which mayeasily be used, and adequate mechanical strength to retain the desired form throughout itsdesign'life;

4) commercially aviable and not expensive to users.

So far, commonly used ICA anodes are steel, high silicon cast iron, magnetite, aluminium alloys,graphite, platinum and platinised metals, metal oxides, and conductive ceramic materials. Theircosts have been summarised and compared [Moreland (1993)].

Titanium Anodes: Titanium is a very successful anode commercially available as mesh strips andother forms [Berkeley (1990), Broomfield (1997), Crerar (1996), Mudd (1988a), Pastore (1990),Bennett (1998)]. Metals covered with an activated metal oxide have similar functions as titaniumanode. Some manufacturers use conductive formulations including oxides on Ti (titanium), Ta(tantalum), Ir (iridium), and Ru (ruthenium) [Hock (1988)]. The anode is usually fixed onto theconcrete 'surface with plastic fixings and a cementitious overlay whose quality is critical to theservice life of the anode system.

The titanium mesh anode systems are flexible, and the different grades of material available canprovide concrete surface area current densities typically from 15 to 20 rnNm2

• They can be used onhorizontal and vertical surfaces of concrete elements. Inappropriate distribution of current caninduce damage on the anode when the anodic current density exceeds 100rnNm2 of the activatedtitanium net or 20-50rnNm2 for the other type of anodes [Kendel (1986), Hayfield (1988)]. It iswidely accepted that activated titanium mesh anodes have an expected service life in the range of 20to 100 years or even longer [Martin (1987), Mudd (1988)].



A typical configuration i~ schematically shown in Figure 8 [Wyatt (1993)].

Transformerrectifier

l50mmtypical

o' 0

100 mmtypical1---'

tJ ··tllt OO •.(t-, ~ n·d~'(\··.·oO()o·DOOQ·.O.00'" 0. 0 D·O' -0. O. '01.00'

•• ·00·.·.·.·:°.• '0· _-:° 0 •. 0 •.. '0. 0

o 0 c' 0 I. 0'0. 0 °0 0', 0 1,), (J: 0 00

• ~ . ,>0

• • 0 00 •. o. '.". • .• 0·····"() • .'0' t ..•. 0 . 0 0 '. Q 0 0 ' •• D.

• II 0

Wire meshanode

INon-metallic fixings at500 mm grid spacings (max)

Figure 8. Titanium mesh anode [Wyatt (1993)]

Platinised titanium wire can also be placed into holes (Figure 9) drilled in the concrete or intocontinuous slots. The anode should be backfilled· with an alkaline mixture which may consist ofplaster with lime [Morgan (1993)].

backfill

Figure 9. Plati.nised titanium rod anode installed in concrete [Morgan (1993)]


22

. Research Report 332

Overlay Anodes: In the case of corrosion ofbiidge decks' in sea water spraying environment, orconcrete attacked by deicing salts, it is pussible lO use a conducting asphalt [Morgan (1993),Broomfield (1997), Wyatt (1993)] and conductive polymer concrete overlay [Fontana (1990)]. Forthis purpose flat plate anodes of silicon iron have been developed. The conductive asphalt is laidabout two to three inches thick with an overlay of road asphalt. 'The anodes are placed in theconducting asphalt and protection current is carried by this asphalt to protect the reinforcing bars inthe deck (Figure 10). This kind of system was installed in the US during the period 1973-1980, andmany have been reported to be in satisfactory working order [Wyatt (1986), Stratfull (1985),Anderson (1985)]. They are cost effective for those applications that can withstand the weight, butthey have limitations because of their bulk weights, and are only applicable to bridge decks.

, wearin,g andwaterproofoverlay

counter anode

Figure 10. Illustration of conductive asphalt overlay anode system[Morgan (1993), Broomfield (1997), Wyatt (1993), Bennett (1993d)]

Polymer concrete overlays have also been used on concrete bridge decks to prevent chlorides frompenetrating into the concrete [Fontana (1990)]. This system also provides an excellent skid- andabrasion- resistant surface and is very resistant to freezing and thawing.

Similarly, a conductive rubber anode system has been used on the piles of Howard Frankland Bridgein the USA [Bennett (1998)]. The anodes consisted of flat sheets of carbon-loaded ethylenepropylene diene monomer conductive rubber. Anode sheets were held in place on the flat concretesurfaces by fibreglass panels. Permanent stainless steel straps were used to press the jackets againstthe concrete surface. The electrical connection to the titanium bars in the sheets was made with abutt splice. Connections were covere~ by nonconductive epoxy.

Recently, carbon fibre- and latex-containing mortar and concrete overlays have also been reportedto be very effective in reducing the driving voltage required for cathodic protection [Hou (1997)].

Slotted anodes: A conductive polymer grout (carbon-loaded resin) is put into parallel slots cut intothe concrete surface [Broomfield (1997), Halverson, (1985)]. This system requires a good concretecover and good bond between the polymer and the concrete. Therefore, coated titanium ribbons are

, usually cast in the slots wilh cementitious grout (Figure 11). Some slotted anode systems have bee,ninstalled with 'noneonduetive wearing sUlface overlays of concrcte [Halverson, (1985)].



+cementitiousgrout

Figure 11. Slotted anodes installed using a eementitious grout[Bennett (1993d), Broomfield (1997)] .

Similarly, a new anode system has also been developed which uses a conductive fibre concrete [dePeuter (1993)]. It is a combination of primary and secondary anodes, and can directly be applied tothe concrete surface. .

Conductive Coating Anodes: Conductive coating anodes have achieved considerable commercialsuccess in practice [Eskola (1997), Broomfield (1997)]. This system is illustrated in Figure 12. Athinner layer of conducting paint is used in the place of the asphalt [Morgan (1993), Broomfield(1997)]. The organic conductive coatings contain graphite dispersed in acrylic .or chlorinated rubberbinders. They have a range of effective service lives of 5-15 years when properly applied andoperated at current densities of up to 20mNm2 in appropriate environments [Wyatt (1993)]. Theirelectric connection to the power supply is through a series of wires of platinised titanium, platinisedniobium copper, carbon fibres or other ine!1 materials (primary anode), running through the coatingat a separation of O.5:-1m. ,Sometimes, carbon fibre anodes are also suggested to provide a dispersed'connection to the paint film.

pr'mary connector conducting coating

Figure 12. Conductive coating ano~e system [Bennett (1993d), Broomfield (1997)]

Although such thin coatings are not as durable as the titanium mesh with overlay, they are cheaperand easier to apply, repair and maintain. The advantage of using this conducting paint is that it canbe applied to· all surfaces including vertical surfaces, the underneath of bridge decks, and complexshapes in any orientation, and presents no problems of weight or dimensional limitations. Inaddition, conductive coating systems canbe installed onto suitable structures at low cost. However,they are likely to encounter the problems of disbondment which could occur with cementitiousoverlay systems.

Conductive coating anode systems have bee!1 successfully applied to bridges, car parks, andbuildings [Lehmann (1985), Eskola (1997)]. In a trial application on a bridge [Eskola (1997)], awater-based acrylat~styren-copolymer with a conductive binder doped with graphite was used assecondary anode, and ribbons of pure silver protected between 2 layers of synthetic material asprimary anode.

ARRB Transport Researcfi Ltd


However, conductive coating anodes do not withstand conditions of continuous wetting or abrasionand undergo gradual degradation. They usually have some problems like disbondment, blisteringand flaking related to degradation by anodic acidity and oxidation of the organic binder. Two mainmechanisms are involved in these failures [Berkely (1990)]. The first one is that the conductingmaterials in the coatings, such as carbon, can react with oxygen, which leads to chemical breakdownof the conducing materials, resulting in the paint flaking off the structure. The second mechanism isthe chemical reaction at the interface between the concrete and the paint, leading to loss of adhesionand disbonding. It has been' reported [Bennett (1995a)] that carbon-based conductive coating anodein a bridge was showing signs of early disbondment after only a few weeks of operation.

Sprayed zinc anodes: An alternative to. the conductive coating system is sprayed zinc applied bymeans of electric arc or flame [Broomfield (1997), Page (1997), Bennett (1998)]. The electricalconnection is made through a metal plate fixed to the concrete surface and the zinc is sprayed on theconcrete. The system has a high conductivity, and needs very few electrical connections. It also hasa greater tolerance than the organic conductive coating system to the moisture at the time ofapplication or during service. It has been reported to have had some success in north Americantrials [Apostolos (1987)]. Two problems are noted with this system. The first is that the coatingitself is not very corrosion resistant, and corrosion products may form between the coating and theconcrete surface which would decrease the electric conductivity and therefore the protection effect[Weale (1992)]. The second concern is the toxicity of zinc. So there are environmental restrictionson the application of this coating system.

In fact, other metals can also be sprayed onto the concrete surface to act as anode. For example, arcsprayed titanium anode has been tried [Bennett (1995)], and it seems to hold promise.

Polymer wire anodes: The need for distributed anodes that are suitable for vertical and soffitsurfaces and that can be easily installed has 'resulted in the development of conductive polymer wiremesh anode system [Wyatt (1993)]. The typical arrangements of the system is shown in Figure 13.

Conductivepolymerwire mesh<!nonp.

•• .• "0 ·0 -0. 000'0' 0'Q' ~ •.•••• 0 0" • ". '. 0. II 00 • • . ~ 0' 0

" •• ••• .. 0 •• 00. 0 . o· : .0.: 0.0 ',,00'· .°. 0 ,

Transformerrectifier

T250 mm_typical

Non-metalliccombined fixing

/ and spacer

1-500 mm I .. ' I~tyPical -s

Conductivecleat

Figure 13. Conductive polymer wire mesh anode [Wyatt (1993)]

Ceramic anodes: Ceramic anode system, e.g. conductive titanium oxide ceramic, has also beendeveloped [Berkeley (1990), Wyatt (1993)], which has the following special features:


25Research Report332

1) the gas generated at the anode/concrete interface is dissipated by permeation through theinner porous ceramic tube of the anode;

2) there is no need for maintenance and 'replenishment of this material, and the anodes can beplaced deep down holes where access will subsequently be restricted;

3) anodes are simply grouted into place using a pumpable grout;

4) the anodes can be assembled in a string to address differing protection requirements atvarious depths of concrete;

5) the anode can be readily cast into concrete;

G) high resislanllo al:ius fOllllt:U al llteauuut:;

7) mechanical properties compatible with concrete;

8) can operate at high current density; and

9) no need for an intimate concrete/anode interface.

Such an anode system has been adopted in real structures [Solomon (1998)].

Simple clamping anodes: The simplest way to install an anode system would be clamping on thesurface of structure [Broomfield (1997)] or mounting on the concrete surface [Crerar (1996)]).However, such "clamp on" system is still under trial for their effectiveness.

In addition to the above systems, the use of a remote anode system has also been reported whichrequires to be embedded in an electrolyte along with the cathode but placed at a distance far wayfrom the cathode. It has been designed and installed to protect reinforced concrete tunnel unitsimmersed in the sea in South-east Asia [Green (1992)].

Different anode systems have different technical features. Table 1 summaries some of the typicalanode systems [Berkeley (1990), Eltech Research Corporation (1993»).

Table 1. Comparison of impressed current anode systems[B k I (1990) Eit h R h C f (1993)]er e ey I, ec esearc orpora .on

anode system current Thickness weight cost life anode suitabl•density on (Jlm) addition (year) dissipator efor

anode to surfacl: spacing wetsurface (kglm2

) (m) surface(mA/m2

)

conducting 2-20 400 0.5 $54/m:l 5-15 3 Nographite paint

zinc spray 200 $9/m:l 10 9 Notitanium based 200 . 1000 0.1-0.2 30-50 Yes

meshslotted anode $657/m >15

2

overlayed $97/mol 20-35anode

coke-asphalt $65/m2 >20deck systemconductive 80 7000- 1 >30 Yes

polymer rode 8000ceramic tiles 50 3.5-4 >50 Yes


26

Research Report 332

2.4.2 Sacrificial anode

In the past, sacrificial anode (SA) CP has been used with success on buried structures and on thesubstructures of reinforced concrete bridges in warm marine environments, for example, on theconcrete members underwater [Hejji (1989)]. Generally, a sacrificial anode should have thefollowing properties [Crundewell (1993)]:

1) the material must have a natural potential more negative than Eae;

2) the driving potential of the anode must be stable with ~espect to time and have known andacceptable polarisation characteristics;

3) the material must have high ampere hours capacity per consumed kilogram;

4) the material must be capable of being formed by simple and economical processes intoshapes, and have adequate mechanical strength to retain the desired form throughout itsdesign life;

5) the material must be commercially aviable and not expensive to users.

The sacrificial anodes should better not be embedded directly in the concrete as the corrosion of theanodes is most likely to cause the failure of concrete by spalling. In addition, low resistivity ofconcrete and low contact resistance at the interface between anode and concrete are essential for aneffective sacrificial anode cathodic protection system. The contact resistance can become very highby the formation of oxides on the sacrificial anode as it co~odes.

Normally, it is relatively difficult to achieve protection using sacrificial anodes unless they can beplaced in recesses which are filled with a conducting electrolyte [Morgan (1993)]. However, afterappropriate design and careful installation, sacrificial anode cathodic protection can also providesatisfactory protection to reinforced concrete, especially when the concrete resistivity is not too high.Currently, the sacrificial anode systems are mainly limited to the periodically wetted seawater splashand tidal zone where concrete is moist and conductive [Bennett (1996)].

Although sacrificial anode cathodic protection (SACP) systems have lif!1ited application due to thelimited driving voltage compa~ed with the impressed current CP system, they have some advantages[Funahashi (1997)]. They do not need a power supply, so they are cheaper. Also they are easy toapply and need only minimal monitoring and no external wiring.

Alloys of Mg, Al and Zn are the widely used commercial sacrificial anodes [Funahashi (1997)].Their relevant properties are listed in Table 2. In very few cases, pure Zn, AI, and Mg are used asanodes, although their alloys are more suitable for practical uses.

tt (1996)]t . I [Bd"fi • Ift·T bl 2 Pa e . roper les 0 sacrl ICla ano e rna erla s enneanode theoretical practical current thickness material cost working

energy ennergy efficiency consumed ($/ft2) potential

efficiency efficiency (%) (mil/yr), (based on costs in (mV/SCE)(Ahr/lb) (Ahr/lb) (based on a chemical marketing

current density of reporter, March

1.5 mNft2)

(992)

Zinc 372 353 95 1.0 0.18 -1030Aluminium 1352 1285 95 0.7 0.06 -1030Magnesium 1000 500 50 2.9 0.38 -1530

Broomfield (1996) tested AI, Zn, and Mg sacrificial anodes in 'simulated pore solution in sand, andconcluded that Mg was an effective anode at low resistivity; in concrete slabs, aluminium anodeswere more effective than zinc. Zinc and aluminium alloys have been used as sacrificial anodes inexperimental SACP system [Whiting (1995)]. Laboratory evaluation also showed that aluminiumand zinc have promise as sacrificial anodes in reinforced concrete bridge decks [Whiting (1996)].



However, the zinc alloys and most of the aluminium alloys were ineffective in producing sufficientcurrent to protect the steel embedded in the concrete at low temperatures and low relative humidityconditions [Funahashi (1997)].

In the practical application of sprayed zinc anode, the electrical contact between the reinforcing steelan the zinc anode can be achieved either by a connecting wire, or more conveniently, by directspraying of zinc on the exposed reinforcement [Sagues (1994a)]. The latter procedure is particularlysuited to the common morphology of damage, where some reinforcement is exposed.

Bennett et al (1995a) reported that thermally sprayed zinc anodes operating in sacrificial mode werenot capable of providing sufficient current on a long-term basis to meet cathodic protection criteria.Sagues et al (1994a) reported that the arc-sprayed zinc which was deposited on the external concretesurface, after sandblasting, as sacrificial anode, was a low-cost method based on laboratoryexperiments and field installations at bridg~s in the Florida Keys. The field test showed that theanodes retained physical integrity over at least 1.5 ycars in a subtropical environment. 100mVdepolarisation criterion could routmely be met. Laboratory experiments indicated that in marinesubstructure, concrete resistivity did not represent a main limiting factor in the performance of thesacrificial anode. However, absence of direct wetting of the anode surface could result in long-termloss of adequate current delivery, even when the concrete was in contact with air of 85% relativehumidity. This means that periodic wetting is necessary for long-term anode performance.

In 1977, two types of SACP systems were installed and tested in the USA [Saner (1977), Whiting(1981)]. In one type, zinc ribbon was placed in saw-cut slots in a bridge deck. In the other oneperforated Zn sheet was placed on a concrete deck surface and covered with an open graded asphalt.These CP systems were not very successful because of the poor distribution of CP current to therebars. This limited further development in sacrificial anode cathodic protection systems. Later. intrial bridge deck applications, the sprayed sacrificial anodes were also proven to have insufficientdriving voltage to achieve adequate protection [Wyatt (1993)]. However, Apostolos (1984), andSagues (1994a) reported that sacrificial anodes in the form of thermally sprayed zinc coatingsinstalled in the splash zone appeared to perform well because of the high moisture and lowerresistance of concrete.

Metallised zinc has been used as sacrificial anode in Florida on eight structures over 14,800 m2 ofconcrete surface area [Flynn (1"991), Kestler (1990)]. The delaminated and loose concrete wasremoved, and the zinc anode was directly metallised over the exposed rebar and surroundingconcrete. Corresponding results [Powers (1994)] supported the use of a metallised sacrificial anodeon splash and tidal zone of marine bridges; but the results also demonstrated that the metallised zincanode was not capable of maintaining an adequate flow of galvanic current without periodic wettingof the surface.

In addition to the sprayed zinc, perforated zinc sheet anodes have also been applied to the surface ofsquare concrete piles in the splash and tidal zone [Kestler (1991)]. The anodes were held by woodplastic supports which also helped to retain water, and the interim results were promising. In thatcase, the periodic wetting was also an important factor for maintaining the effectiveness of the SACPsystem.

Zinc sacrificial anodes have been utilised to protect underground reinforced concrete pipeline[Gourley (1978)]. The pipeline consisted of precast concrete pipes wound by prestressed wire andcovered with a layer of gunite. Zn anodes were used in that project, and delivered an averagecathodic protection current density between 0.1 and 1.8mNm2

•

Aluminium seems to be the most attractive candidate in terms of its availability, ease of handling andlow cost. Aluminium sacrificial anodes have been used in the CP system on pilings in LakeMaracaibo, Venezuela [DeRincon (1992)]. The system was protecting the steel reinforcement after



30 years without cracking of the mortar overlay. Recently, aluminium-zinc-indium alloy anode wasdeveloped and examined in USA [Funahashi (1997, 1998)]. This anode could provide higher currentthan the zinc anode, but its current output fluctuated with relative humidity, temperature and wetnessof the concrete. It had higher levels of cathode depolarisation than the zinc anode and top-coating ofthe anode was not necessary. The pH of the concrete environment played a critical role in thecurrent output and driving potential of the alloys tested. As the pH dropped below 12, the currentdecreased and the potential became more positive.

As for the magnesium anode, the early use on a bridge deck indicated that magnesium was also apromising material for use as sacrificial anode [Vrable (1977)].

2.4.3 Choice of anode systems

In selecting an anode system, some important factors must be considered [Funahashi (1997)]:

1) material composition;

2) potential;

3) current output;

4) anode efficiency;

5) change of polarisation characteristics with time;

6) surface area of steel needed to be protected;

7) cover concrete characteristics (chemical composition, temperature, resistivity, pH);

8) electrical contact of the anode with the concrete.

fi Id (1997)][BdfT bl 3 Ca e. . ompanson 0 ano e systems room Ieanode systems applied areas estimated characteristic

durability(years)

Ti mesh in overlay all >20 durable, but with an overlayASIJltalL deck >20 durable, but freeze-thaw risk

Ti ribbon in slots deck >20Conductive all 10-20 needs wearing course for traffic

concreteGrout in slot deck 1O? poor durabilityPaint coating substructure >10 easily repaired, but low wearing and

wetting resistanceSprayed Zn substructure 1O? stand wetting and can be used as

sacrificial anode,but with environmental risk.

Sprayed Ti substructure >20 lower environmental impact and moredurable than sprayed Zn

clamp on substructure 10?embedded rod substructure 1O? careful design and installation

or soffit

To achieve an effective cathodic protection, a correct choice of anode system is vital. Differentanode systems have their different advantages and drawbacks, and are suitable for uses in differentenvironments. Features of some of the anode systems have been summarised by Broomfield[Broomfield (1997)] (see Table 3). Detailed comparisons of properties, service life, and costs of theanode systems have been given by Bennett et al (1993d). Clear et al (1993) also provided asummary on the costs of various cathodic protection systems based on field installation in the USA.



Generally, in the non-immersion environments, an impressed current counter anode should be firstconsidered, If the surface is subjected to wear, then the use of titanium anode in an overlay or slotscould be the first choice, and the coating anode systems the last [Broomfield (1997)]. If an elementto be protected is in dry zone, then a coating anode system is often considered, but in regularlywetted environments, sprayed zinc systems have attractive advantages, and could be used instead ofpaint type coatings [Broomfield (1997)].

Sacrificial anode systems could be used in immersion zones where the resistivity is relatively low[Broomfield (1997)]. Arc sprayed zinc seems to be successful on marine bridge substructures. In atrial application of sacrificial and impressed current anodes to bridge columns, Andrews-Phaedonaset al (1992) concluded that sacrificial anode systems were cost-effective, only requiring minimalmaintenance. They could provide periodic "full" protection when operating, and still providepossible protection with residual polarisation after they fail to operate.

2.5 Monitoring Probes for Cathodic Protection

Half-cell electrodes or other small probes are sometimes embedded in concrete to monitor cathodicprotection effectiveness, or to feed back the information to adjust the cathodic protectionrequirement.

Usually the probe embedded should be located near the most actively corroding area of thereinforcement, and the placement of the probe should not disturb the concrete around the corrodingsteel or block the cathodic protection current flowing to the reinforcing steel.

Portable reference electrodes can also be used on the concrete surface for this purpose. They aresimpler, but more time consuming than the embedded ones [Green (1992)], and errors in measuredpotentials due to junction potential can result [Berkeley (1990)]. The sodium chloride inducedjunction potential sometimes could be as high as 200mV [Eltech Research Corporation (1993),Bennett (1990b)] which is significant and could completely distort the measured data.

The electrodes commonly used as probes are [Eltech Research Corporation (1993)]:

1) zinc/zinc sulphate;

2) copper/copper sulphate;

3) silver/silver chloride; .

4) molybdenUm/molybdenum oxide;

5) graphite;

6) lead.

The potentials of some of these references relative to the standard hydrogen electrode are comparedin Figure 14 [Elteeh Research Corporation (1993)].


30Rese·arch Report 332

:hfl (--- CoppDr Sulfll.tc Elecll"odl!+300 - . (Snturo.t.",ct)

+200

mV

+100

242 ( Calomel Electrode (Sat.urated)

~~~ ( •• , I . )SilVer-Silver Chloride Ele(;Lrode11/// (a=l,KCI)II! ! II I ! I / i \. Reported Range or t; r61'1lile/;1 /Ii ( I!:leclrode In Concret.eJ ,I' l

},I/I/I)-116 ( .

o -r- 0 < .. - .- Normal Hydrogen F.lectrode

Figure 14. Relative potentials of reference electrodes [Eltech Research Corporation (1993) ]

Among these references, the most popular half-cell probe is copper/copper sulphate, silver/silverchloride, and mercury/mercury oxide. Carbon, coated titllfiium and lead are the widely used probeswhich are commercially available [Broomfield (1997)].

Besides the half-cell probes, macro-cell galvanic effect is sometimes employed to detect theeffectiveness of the cathodic protection. A steel probe (Rebar probe or corrosion null probe, seeFigure 15) is embedded in an excessively salty patch with electrical connection to the reinforcement,so that a macro-cell galvanic current flows from the probe to the reinforcement [Bartholumew(1993), Bennett (l993h), Broomfield (1997)]. After cathodic protection is applied, if the galvaniccurrent is reduced and reversed, this would indicate that the most anodic area has been cathodicallyprotected, and therefore the reinforcement which is less anodic than the embedded steel probe wouldbe satisfactorily protected. However, this method has the risk that the salt in the patch may diffuse~awayoan(:rmaktnhe~probeolessoactive·afLera°(;ertaiwperiod~oftime~·~~~..~=~.~~.~~~·o·~.~~~ .._- ..~~ .. - •. - ..

Patch Excavation withCI - Free Concrete

To Anode(1IIOt ohllW'll)

PowerSupply

H

Rebar Probe(IS B... .nth lSI ca-IT4' """""ate)

Voltm.eter

I.!"

Figure 15. Rebar probe construction [Bartholomew (1993)]



2.6 Supply and Control· of Cathodic Protecting Current

Several points must be borne in mind when a CP system is applied to reinforced concrete:

1) The high resistivity characteristic of concrete makes uniform distribution of cathodicprotection current difficult; .

2) The heterogeneity of the cover concrete over large areas makes the current distributionunpredictable;

3) The outer (top) reinforcing bars could screen the cathodic protection current to a greatextent, so the reinforcement under (beyond) the top layer of steel bars would be hard tocathodically protect. However, most reinforced concretes degrade from the outer surfaceand corrosion of reinforcement mainly takes place there. Therefore, only these zones ofsteel reinforcement require the most protection [Morgan (1993)];

4) The current density required to protect the steel in concrete depends on the age of theconcrete and on its alkalinity [Morgan (1993)]. This requires that the applied ip beadjustable with time to meet the full protection criteria;

5) The current density required to protect reinforced concrete can change with temperature,relative humidity around the concrete, and the oxygen saturation of concrete [Raharinaivo(1996)]. Therefore, ip must be carefully chosen to tolerate the variations in the iprcquirement.

Only the impressed current cathodic protection requires a power supply. Most such systems aredesigned for a current density of about 10-20 rnA per square metre of steel surface [Broomfield(1997), Concrete Society (1989), Corrosion Engineering Association/Corrosion SoCiety (1989),Institute of Corrosion/Corrosion Society (1991)], and 16mA/m2 of steel surface is regarded as arule-of-thumb [Eltech Research Corporation (1993)]. After the initial energisation, the systemwould be expected to operate at 1/3 to 1/10 of the initial current density. For buried structures, thesteel current densities of 5mA/m2 or less are generally considered sufficient [Cherry (1986)]. Themaximum voltage, usually around 12-24 V DC, is restricted to ensure negligible risk to humans oranimals from electric shock.

For a sacrificial anode system, Gourley (1978) suggested that an appropriate galvanic current densityis of the order of 2 mA/m2 for the initial polarisation which may decrease subsequently to as low as0.5mNm2

•

However, the demanded current level is dependent of many factors, such as the alkalinity andchloride concentration in the vicinity of reinforcing steel. Bennett et al (1994) established a roughrelationship between the demanded cathodic protecting current density and the chloride content atrebar depth. Higher chloride levels require higher protecting current densities. However, a modestpolarisation has been reported to be able to achieve perfect passivity and obviated the risk ofhydrogen embrittlement [Bertolini (1993)].

Most impressed current cathodic protection systems are run under constant current or constantvoltage control.

Research by Glass (1994) indicated that the protection afforded by a CP system would last for asignificant period after the current was interrupted. Thus the protective effects of the CP current canbe divided into immediate and long term effects. The long term and persistent protective effects ofCP are associated with the repassivation of the steel due to desalination [West (1985), Broomfield(1992)], re-alkalisation [Ross (1972), Glass (1986)] and consumption of oxygen [Glass (1991)].Such phenomena suggest that it is not necessary to keep a cathodic protection current all the time,and an intermittent CP might be more cost-effective. A study on intermittent cathodic protection'was reported by Bartholomew et al (1993). It was suggested to be an attractive alternative to provide


32

Research Reporl332

protection to the structure in certain circumstances where power might only b~ available al aparticular time of day, not be readily available, or be cost prohibitive to install. This system caneven be powered by wind or solar radiation [Weber (1976)]. The experim~nts by Barlhululll~w d al(1993) also indicated that such a system was feasible under certain conditions, but further field workis still needed to verify their laboratory results.

2.7 Some Practical Applications

CP is the most extensively used prevention method in reinforced concrete structures, and has beenapplied to many field structures [Crerar (1996), Muller (1996), Broomfield (1992), AndrewsPhaedonas (1992), Andrews-Phaedonas (1996), Polder (1992b), Funahashi (1997)].

Due to the complexity ot' the protected structures in the field, various criteria and controlledparameters have been adopted in practice Crable 4).

"rh d'T bl 4 Sa e . orne cat o IC protection aJ ,plication exampI esprotected method anode controlling monituring references

object parameterswater tank ICCP active Ti IC=2-5 mAlm.l of [Muller (1996)]

mesh concrete surfacepiers of ICCP active Ti IC=3...;5mAlm.l of Eon, Eoff [Crerar (1996)]bridge mesh concrete surfaceBridge ICCP 1.3-3.3 mAlm2 of [Eskola (1997)]

reinforcementSupporting ICCP mesh anode Initial ~off=100mV [Lourenco

wall of with overlay IC=15mA1m2 of over 24 hours (1992)]bridge steel surface

Trident ICCP Ti mesh and Eoff, ~off [Cheaitanibuilding slotted (1998)]

anodesbeams and ICCP Zn, Ti E-IgI [Bennett (1998)]

piles of mesh, and Eoff, ~offbridge conductive

rubberpilings, SACP Zn/hydrogel Initially [Bennett (1996)]

caps, and O.67-2.8mAlfebeams of reinforcement

surface

Most standards state that cathodic protection should not be applied to prestressed concretestructures, because there is a risk that hydrogen embrittlement could be induced in the stressedreinforcement, which may result in catastrophic failure. However, in practice, CP has been appliedto stressed elements such as anchorages and tendons in bridges [Baldo (1991)]. In thoseapplications, very modest currents and potentials were used. In USA, sacrificial anode cathodicprotection was also tried on prestressed bridge piles [Kestler (1995)]. In that case, it was believedthat the concrete resistivity was low due to the marine exposure conditions [Hartt (1994)] and therisk of hydrogen embrittlement of the prestressing was negligible.



3. Re-alkalisation

Re-alkalisation (RA) is a relatively new technique compared with CP, but it has been patented[Vennesland (1992)]. It was developed in the late 1980s for concrete structures where carbonationhas advanced beyond the depth of the cover concrete. Since the first realkalisation project wascarried out in 1987, over 350 projects have been completed with such techniques worldwide [Decter(1998)].

The loss of alkalinity of the pore solution due to carbonation is one of the main causes of corrosionof reinforcement. According to the analysis in Section 1.1, re-passivation is an effective techniqueto reduce the corrosion rate of reinforcement if there is no considerable chloride ingress. Highalkalinity of the pore solution is an essential condition for the reinforcement to remain in passivecondition or to become re-passivated. Therefore, restoring the alkalinity of concrete would be anecessary and effective option. However, realkalisation (RA) is a temporary treatment which onlyre-establishes high alkalinity around the steel reinforcement by promoting the production ofhydroxyl ions at the steel cathode.

3.1 Principle

The principle of realkalisation is to increase the alkalinity of carbonated concrete in the vicinity ofreinforcement by generating alkaline conditions in the concrete pore solution by means of cathodicpolarisation of the reinforcement, so that passivity of the reinforcing steel can be re-established.

According to equation (6), the cathodic reaction (i3 or ie) at the surface of reinforcing steel cangenerate certain amounts of alkali. In principle, the rate of OH- generation can be represented by i3

or ie• For example, a cathodic current density of INm2 ofsteel surface produces nearly 0.9 moles ofhydroxide in 24 hours at the cathode [Polder (1992)]. When the applied cathodic current ip is highenough, according to equation (9), we have:

(16)

i.e. the rate of OR" generation on the steel surface is equal to lip!.

On the other hand, the generated OH- can migrate away from the steel surface along the ionic currentpath according to equation (11). The departure rate of OH- (i4 as illustrated in Figure 1) can beexpressed as follows:

(17)

where COH is OH- concentration, UOH is the mobility of OH- ion, and tOH is the transference numberof OR" ion.

For example, in a pure sodium hydroxide solution, hydroxide ions have a transference number ofabout 0.8 and sodium ions of about 0.2 [Polder (1992)]. So only 20% of the amount of hydroxidegenerated by the current passed remains at the steel surface to increase the alkalinity there.

According to the processes described in' Figure 1, OH- could also react with other species andbecome deposited in another form. This type of contribution to the concentration of OH- at the



vicinity of the steel surface is assumed to be represented by is (Figure 1), and it is obviouslydependent of the concentration of OH- (CUH)'

Therefore, the change of OH- concentration at the vicinity of the steel bars can be described as:

dCoHldt = ( li3 1-li4 1- lis I)IF =[Iipl -tOH lipl - lisl]IF= {Iipl [l-toH]- lisl}IF(18)

Normally the deposition of species in concrete is limited, so after a certain period of application ofcathodic current, lisl becomes relatively small. Since tOH<I, we have

dCOHldt:::: lipl [l-toH ]IF >0 (19)

which means that the concentration of OR, i.e. pH, of the pore solution, in the vicinity of steel barswould increase with time. The higher the applied ip" the faster the pH will be increased.

Obviously, the alkalisation effect is much more significant when pH is low. In that case, tOH islower, so dCoH/dt has a higher value according to Equation (19). So, RA is very effective inincreasing the pH value of the concrete which has been carbonated and has a relatively low pHvalue. However, the effectiveness of the re-alkalisation decreases after long term treatments,because tOH becomes larger with the increase in pH of the cover concrete.

3.2 Anode System and Electrolyte

Usually, coated titanium and mild steel mesh are used as anude. III sUllie l:ases, sprayed cellulose ortank anode systems are also used [Vennesland (1992)].

Sodium carbonate solution is the first anolyte to consider. The typical concentration of Na2C03

used.is 1M [Polder (1992)] with a pH of 11.5, (which could protect the concrete aga~nst furtheringress of carbon dioxide);~Itis expected that the alkalinity of thee cover concrete can be furtherenhanced by absorption and diffusion of the alkaline electrolyte [Mietz (1994)].

In some cases, tap water is also used as the electrolyte. In laboratory, 2 M LiOH was also tried[Sergi (1996)], and was found to be successful in re-alkalising fully carbonated mortar specimens.Its effectiveness was comparable to that of 2M Na2C03, but the penetrated concentration of Li+ washigher than that of Na+ and K+, which was likely to reduce the risk of AAR reaction in concreteswith susceptible aggregates.

3.3 Control of Operational Parameters

There is no fixed current density for the realkalisation treatment [Broomfield (1997)]. For example,in one case a current density of O.3-0.5A/m2 was applied to a building for 3-5 days; in another case10-22 V potential was applied on a bridge tower to give a current density of 0.4-1.5 A/m2 for 12days; and for other structures, 1-2A/m2 was used for 9 days.

3.4 Monitoring of the Alkalinity

A very simple measurement of carbonation depth with an appropriate pH' indicator can revealwhether re-alkalisation has effectively restored the alkalinity of the concrete [Broomfield (1997),Sergi (1996)]. This can be done by drilling small cores out of the concrete, and spraying theindicator on them [Mattila (1996)].



Polarisatiun technique has also been employed to assess the degree of realkalisation of carbonatedconcrete [Mietz (1994), Mietz (1994a)]. This technique involves anodic polarisation of a knownarea of the steel embedded in the concrete (either potentiaostaically or galvanostatically), andobtaining the relationship between anodic current or potential versus time. The resultant shape ofthis curve indicates the corrosion activity of the steel at the time and the stability of any protectivefilm that may have formed on the surface.

Sergi et al (1996) proposed a two-stage method to determine the effectiveness of the realkalisationprocess, which was actually a combination of the use of acidlbase indicators and galvanostaticpolarisation technique. It was stated that in the case of realkalisation, the half-cell potential may notbe able to give reliable indication of the corrosion activity because the large polarisation currentemployed in the RA could completely change the surface state of the reinforcing steel [Sergi(1996)]. .

3.5 Effectiveness of realkalisation

As a relatively new method, RA has no long track record as far as the durability of the treatment isconcerned. However, short term effects of the method have been examined in the laboratory.

Polder (1992) demonstrated different pH conditions in different areas of concrete around thereinforcement, as a result of realkalisation (Figure 16). The area behind the steel bar had nosignificant change, whereas the alkalinity in the vicinity of the steel bar was greatly increased to pHvalues higher than 14 after the realkalisation treatment.

FIRST PHASE

carbonation front

:-l

Jnot carbonated

10<pH<13·

~JI:~~~~~=J=i==~=---------- -

C c.d. 10<pH<14----- --- ------

L- L--lL---1-_-3 pH< 10Concrete surface

concrete

LATER PHASE

carbonation front

i: i i! II:

J..

Tnot carbonated concrete10<pH<13

====-== ======r.:.. ---- .-.~,A~~~~~~ -_U_. --l--------J pH>14

!!:':!::!, . C c.d. 10<pH<14I!!! i!!; I!! I.

Concrete surface

Figure 16. Development of alkalinity around the steel bar due to realkalisation [polder (1992)]



3.6 Some Practical Applications

In the application of realkalisation, the following steps were recommended by Polder et al (1992):

i) Clean the concrete surface to remove dust, fouling and coatings;

2) Determine the steel continuity to assure metallic conduction throughout the area to betreated;

3) Carry out potential and resistance mapping. Shoit-circuiting between reinforcement andthe surface to install anode system must be avoided and corrected if necessary;

4) Install electrical contacts to the reinforcement;

5) Repair damage such as spalled concrete, honeycombs and cracks which may provide a lowresistance path;,

6) Install anode system and supply current;

7) Remove the anode system and clean the concrete after treatment;

, 8) Apply a coating or spray a concrete layer o'n the treated surface.

The treatment on the buildings at Trondheim University, Norway [Deeter (1998)] is an example ofthe application of realkalisation in the field. More than 500m2 of concrete on three buildings wasrealkalised~ Several years after realkalisation of the first building, the condition was still good 'andno further damage was observed due to continued corrosion of the reinforcing steel.



4. Electrochemical Chloride Removal

The electrochemical chloride removal (ECR) is also a newly developed technique, and is on trial in .the field [Bennett (1993a)]. Similar to the RA pro'cess, ECR is also a temporary system, which isremoved after the chloride concentration around the reinforcement has been reduced to an acceptablelevel. .

The possibility of removing chloride from concrete by an electrochemical process was first reportedin the USA in the 1970s [Lankard (1975), Slater (i976), Morrison (1976)], and appeared to bepromising. In the early stage of ECR development, very high DC· voltages, up to 220V, were used.The ECR method was patented in 1986 with an anode that had good adherence to the concretesurface and with water retention [European Patent (1986)], and used low voltages, preferably lessthan 30V. In the 1990s, ECR attracted more interes, and a large number of reports were publishedon this method [Anon (1990), Miller (1990), Liu (1991), Polder (1992), Polder (1992a), Polder(1994a), Elseher (1993), Tritthart' (1993), Hansson (1993), Bennett (1993a), Bennett· (19936),Bennett (1993e), Bennett (1993f), Bennett (1993g), Page (1994), Page (1995), Chatterji (1994),Clemena (1996»). Through those studies, ECR has been greatly improved and has been patented[US Patent 4832803] as a commercially available repair technique.

4.1 Principle

An electrochemical chloride removal system basically consists of an external counter anode systemand a power supply (Figure 17).

nter anode

Figure 17; Schematic illustration of electrochemical chloride remuval[Polder (1992), Andrade (1995)]

As mentioned in Section 1.1 ,ions can migrate under the influence of an electrical field. When acathodic current is applied to the reinforcing steel in concrete, anions will be expelled from the steelsurface and will flow against the current flow to the counter anode. Therefore, if there are chlorideions in the cover concrete, they will migrate away from the reinforcement against th~ currentpassage.

According to equations (11) and (12), , the rate of cr flow or chloride removal from thereinforcement surface OCl), without considering the effects of diffusion and convection, can beapproximately written [Elsener (1993), Polder (1994)] as:


38Re~earch Report 332

JCI = ~ tCI i/F =-IlCI CCI i'/[L(lni1ui C )F](20)

where tCl, UCl, and CCI are the transference number, mobility and concentration of chloride ions,respectively.

According to equation (20), the desalination rate depends on the applied cathodic current ip as wellas the transference number of chloride which is actually determined by the concentrations Ofchloride (Cel), and all the ion's (Cj) in the pore solution and their mobilities (UCI and Uj). The mainions in the pore solution are OH-, Na+, and K+, as well as small amounts of Ca2

+. It is well knownthat only small amounts of chloride in the pore solution could induce serious corrosion to reinforcingsteel. Therefore, compared with the main ions, the chloride content in a structure cannot be too·high. In order to obtain a fast extraction rate of chloride from concrete, imposing a large cathodic ipwould be the most convenient option.

However, as the chloride concentration decreases during the removal (and the hydroxylconcentration increases), the chloride transference number decreases and consequently the chlorideremoval efficiency decreases [Polder (1992)]. Therefore, at the later stage, the removal efficiency isvery low.

4.2 Anode Systems for ECR

Various anode systems have been investigated and tried, including graphite/Ca(OH)2, [Collins(1983)], platinised titaniurnlCa(OHh and ion exchange resin [Slater (1976a)], copper/water[Morrison (1976)], and steel mesh/alkaline cellulose [Miller (1989)]. Except for the last one, theother systems were only suitable for application to horizontal surfaces, and the treatment tended tobe localised to discrete anode locations [Collins (1992)].

Recently, it was reported that a wet "papier mache" has been applied in Norway [Polder (1992),Broomfield (1997»). The wet "papier mache" was put on the concrete surface, then anode (steelmesh) was placed on it, and finally covered with another layer of "papier mache". Bennett et al(1993f), based on their evaluation of ECR treatment, believed that the wet cellulose fibre mat couldprovide an effective electrolyte for the ECR treatment.

In addition, electrolyte solution circulating "blanket" anode systems [Bennett (1993a)]; water bath[Broomfield (1997)]; tank anode system [McFarland (1995)]; fibreglass cassette shutter system[Armstrong (1996)] with mild steel fabric anode and calcium hydroxide anolyte; and steel meshencased in an electrolyte of cellulose fibre moistened with lime water [Manning (1991)] have alsobeen used or tried on various bridges.

The most popular anode is the coated titanium mesh that is also used in cathodic protection[Broomfield (1997)]. The titanium wire mesh anode must stay in contact with an electrolyte toensure an electro-conductive contact between the anode and cathode [Tritthart (1996)].

As to the anolytes used for ECR, water is the usual electrolyte [Elsener (1993), Broomfield (1997)].Sometimes, lime-water is also used [Manning (1994), Polder (1992), Manning (1990)]. In othercases, the choice of external electrolyte varies, with solutions of NaOH [Bennett (1990a)], andNa2C03 [Liu (1991), Collins (1992)] commonly used. Polder et al. [Polder (1995)] reported thatlime was more effective than soda as an electrolyte for ECR.

Some additives are sometimes added into the water to keep the water from becoming too acidic, tostop chlorine gas evolution, or to retard AAR [Broomfield (1997)]. A high pH value of the anolytecan prevent chlorine evolution due to formation of hypochloride, so sufficient lime is usually addedin the anolyte [Polder (1992)]. The addition of Mg2+also seems to be successful in the prevention of



chlorine gas evolution [Bennett (1990a)]. In some cases, corrosion in'hibitors were also added[Asaro (1990)].

4.3 'Operational Parameters

The ECR treatment is expected to be completed in a short time. However, a higher applied currentdensity would be required for a shorter treatment. Unfortunately, very high cathodic currents are notpractical in application, and can incur unexpected damage.

The applied current density of ECR is around 100 times higher than that of CP, i.e. about 1-5Nm2 ofconcrete surface area. It should not exceed 5Nm2 of concrete surface, and the period of treatment isusually 2-10 weeks with the total charge typically varying from 500 to 1500 Ahr/m2 of concretesl:lrface [Bennett (1990a), Bennett (l990b), Tritthart (1996), Nustad (1997)].

These proposed parameters are not fixed, and can vary from case to case. For example, it has beenreported that some harmful effects could be induced if the applied current exceeded 2 Nm2 of steelsurface area [Bennett (1993b)]. Therefore, in practice, the current levels have been suggested inthe range of 0.5-1Nm2 of steel surface area. Moreover, long periods of ECR treatment can alsoinduce harmful effects in the concrete. The total applied charge proposed by SHRP is 600-1500Ahr/rri2 [Bennett (1993c)]. In a trial application of ECR, a total charge of 200Ah/m2 of concrete areawas recommended to be applied to a bridge [Armstrong (1996)] to attain a level of chloride of 0.2%by mass of cement at the steel depth. Different amounts of charge were used on different parts of thebridge (Table 5).

(1996)]td' ECR T tIn t f b 'd [AT bl 5 T tal ha e . 0 c ar2e passe III rea en 0 a n lee rms ron2location total char2e passed area treatedSoffits 187 hr x lA/m2 of concrete surface 24m2

Columns G3, F3 , 293 hr x INm.l of concrete surface 17m.l

Columns D4, E4 748 hr x INm:l of concrete surface 13m:l

Different parameters have also been applied in other practical trials of ECR [Armstrong (1996),Cheaiani (1998a), Collions (1983), Collins (1992), Morrison (1976), Manning (1991), Manning(1994), Slater (1976)].

4.4 Practical Applications

In practical application, the same steps as those recommended for the realkalisation treatment(Section 3.6) are necessary [Polder (1992)].

Currently, ECR is still at the trial stage, and as yet there are no standards to follow in the field. Fromthe examples of the application of ECR on real structures given below, it can be seen that theparameters and practices chosen in various projects are quite different.

In the repair of Victoria pier, St. Helier, Jersey, the trial system comprised a steel anode insertedinside a shallow cassette shutter filled with electrolyte [Armstrong (1996)]. The cassette shuttercontained a compressible gasket around the perimeter to provide a seal.

On a pier of a multi-span bridge, Burlington Bay Skyway [Manning (1991), Manning (1994),Manning (1991a)], an ilJitial current density of 0.77 Nm2 of concrete surface was used, which was

, 2dropped to 0.26 Nm at the end of the eight week treatment. The total charge passed was 610Ahr/m2 of concrete surface. The anode consisted of steel mesh encased in an electrolyte ofcellulose fibre moistened with lime water.


40

Research Report 332

A similar treatment was applied to colulnlls of a four-span bridge overpass [Manning (1994)].Wooden strips were fastened vertically to the concrete, and cellulose fibre, soaked with lime waterwas sprayed on the concrete to the same thickfless as the wooden strips. A steel mesh was attachedto the wooden strips. and an additional layer of the cellulose fibre was applied to cover the mesh.The current density was 0.85 A1m2 of concrete surface initially and was decreased to 0.35 A1m2 atthe end of the 8-week treatment, with the total charge passed being 630Ah/m2

.

In another ECR treatment on the abutment of a river bridge [Manning (1994)], wooden boards weremounted on the concrete at the perimeter of each zone and at the comer of each abutment. The anodeblanket consisted of a sandwich of 4 materials: a thick layer of highly absorbent material in contactwith the concrete, a geotextile material which allowed the acid formed at the anode to drain to theelectrolyte tank, a titanium mesh anode, and a high strength geotextile materials to hold the othermaterials in position. A 0.2 M lithium borate buffer solution was used as the electrolyte. Thetreatment took place over 22-25 days, with an average current density of 1.4-2.05 A1m2 and a totaicharge passed ranging from 700 to 1100 Ahr/m2varied from zone to zone.

ECR was also used on a bridge substructure affected by wind-borne salt attack in Australia[Cheaiani (1998a)]. Three extraction systems were installed, being steel mesh sprayed withcellulose fibre; titanium anode with industrial felt; and reservoir panel system with built-in titaniumanode mesh. The ECR treatment was satisfactorily completed after nearly 10 weeks of operation.The level of chloride in nearly all test locations was reduced to values at which chloride-inducedcorrosion of reinforcement became unlikely.

Some successful field trials of ECR were also reported by Bennett et al (1993a) on an Ohio bridgedeck, a Florida marine column substructure, a New York land column substructure, and an Ontariobridge abutment. .

Recently, ECR was tried on decks of a pilot concrete bridge in Virginia [Clemena (1996), Clemena(1997)]. A temporary electrolyte-soaked anode system of inert catalysed titanium mesh, sandwichedbetween two layers of felt, was used on the deck surface. The application of total charges variedbetween 741 and 1077 Ahr/m2 in 57-58 days. About 72-82% of the initial chloride ions wereremoved from the concrete in various depths. Lithium was used in the anolyte and was observed tomigrate readily into the concrete.

4.5 ECR Monitoring

As chloride ions are removed from the concrete, tel becomes smaller (equations (12) and (13».Therefore, as the ECR treatment continues, chloride removal becomes less effective according toequation (20). Hence, it is impossible to remove all the chloride ions from concrete by ECR in alimited time, and consequently ECR is usually used to lower the chloride concentration in the coverconcrete to an acceptable level.

It is important to know when the acceptable level has been attained. To monitor the chloride level inconcrete under ECR treatment, samples can be taken directly from the concrete and the chloridecontent measured. An indirect method is to measure the chloride level in the anolyte in to which thechloride is extracted. Polder et al. (1995) claimed that the measurement of chloride accumulated inthe external electrolyte offered a simple and effective method of monitoring the progress of ECR incases where the electrolyte maintained a high pH value throughout the process.

Bennett et al. (1990b) reported that the best indicator of corrosion rate, following ECR treatment ontest slabs, was the macro-cell corrosion current between top and bottom mats of reinforcing steelbars. In addition, half-cell potential and linear polarisation resistance can also provide some relevantinformation on the effectiveness of ECR [Bennett (I990b)].



4.6 Effectiveness of Electrochemical Chloride Removal

As mentioned earlier, the practical application of ECR is still on a trial basis, and its long termeffects need to be further verified for its general acceptance.

It has been calculated that the ECR treatment removed about 42%-87% of the available chlorideions, and the highest values corresponded to areas with the greatest contamination [Manning(1994)]. Bennett et al (1990b) and Gautefal (1996) reported that the total amount of chlorideremoval from the structure by the treatment was typically 40-59% of that originally present. Thecurrent efficiency during the first four weeks was about 11-13%, but after eight weeks of treatmentthe total current efficiency of chloride had been reduced to 7% [Gautefal (1996)]. Elsener et al(1993) reported that 50% of the initial chloride content was removed within 8 weeks in a 2-year fieldstudy. Stoop et al (1996)] also reported that ECR removed 1/3-2/3 of the chloride present. It hasalso been found that the corrosion rate was dramatically reduced after the ERC treatment[Broomfield (1995), Broomfield (1997)]. Approximately 80-90% reduction in corrosion activity,resulting from ECR treatment, was maintained during the three years following the treatment[Manning (1994)].

Chloride analysis on concrete prisms which had experienced one year of outdoor exposure after ECRtreatment [Stoop (1996)], indicated that the potentials of the treated specimens could remain at quitepositive values. It was indicated by field data [Broomfield (1997)] that the effectiveness of thetreatment could last at least 5 years, but it is uncertain how long it would continue.

It was found [Elsener (1993)] that a second treatment several months after the first chloride removalcould also remove chlorides as efficiently as the first treatment. This might be due to the slowchemical equilibrium between the bound and free chlorides. Hence, for severe and inhomogeneouschloride contamination, local zones with active rebars, and high chloride contents may remain afterthe treatment, and may require a second ECR treatment.

Since ip is the main factor governing the ECR process according to Equation (20), the distribution ofip in concrete would crucially affect the effectiveness of the desalination and the final distribution ofcr in the concrete after ECR treatment. In principle, ip would be mainly concentrated on the steelbars of the top layer. The current density would be very high at and above the top layer ofreinforcement, relatively low between the bars, and only a little current can penetrate down to thesecond and further layers of steel bars. This is due to the screening effects of the top layer of bars.Therefore, the removal of chloride in concrete can not be uniform, and can only be effective in thecover concrete.

A low chloride content was reported [Stoop (1996), Gautefal (1996)] at the reinforcing bars afterECR, whereas near the surface the chloride level was much higher, meaning that the ECR treatmentsuccessfully removed chloride ions from the reinforcement/concrete interfacial region andrepassivated the reinforcing steel [Green (1993)].

The potential gradient is lower within zones between the reinforcing bars or remote from the anode.It was found that [Manning (1994)], in a structure whosesteel concentration is relatively low, the crremoval above the steel bars is very high, but it is very low between the bars. About 78-87% of thechloride was removed from the zone directly above the bars and only 42-77% of the chlorideremoved from the zone between the bars.

Varying outcomes have been reported on the efficiency of the ECR system. Slater (1976) carried outa laboratory study to examine the performance of anode systems, electrical requirements, duration oftreatment and influence of initial chloride content on the effectiveness of treatment. He found thatwith platinised titaniurnlCa(OHh and ion exchange resin/anolyte systems, chloride content as low as0.02% by total weight of concrete was achievable within 24-48 hours. Chloride removal was


42

R€3Carch Report 332

greatest from the concrete surface layer underlying the anode and the effectiveness of removal wasfound to diminish with distance away from the anode. Similar r~su1Ls were reported by otherresearchers [Morrison (1976), Collins (1983)] who applied a very high current density. Theelectrical requirements could be lowered with similar chloride removal efficiency, but treatmenttimes needed to last a few weeks [Collins (1992)].

It has also been confirmed by experiments [Tritthart (1996), Tritthart (1997)] that even though aconsiderable amount of chloride can be extracted by ECR treatment from the cover concrete, thechloride that has penetrated behind the reinforcement can only be removed to a limited extent. ECRis most efficient in the area of most severe chloride contamination close to the concrete surface. Inother words, if large amounts of chloride have penetrated beyond the first layer of reinforcing steelbars, then ECR may remove much of the chloride in the cover concrete, but much of the chloridebeyond the lop layer of steel bars remains un-extracted. This chloride then diffuses back to the areaaround the top layer of steel bars. This would make the ECR ineffective in such situations. So, in apractical trial on a bridge, Amstrong (1996) reported only limited removal of chloride ions from thecolumns.

The total amount of chloride removed is also affected by the distribution of reinforcement in theconcrete. The effectiveness of ECR treatment is proportional to the density of reinforcement[Amstrong (1996)], so that closely spaced reinforcement gives better chloride removal than morewidely spaced bars.

The transference number (tCl) of chloride in concrete determines the efficiency of ECR as describedby equations(12), (13) and (20). A higher transference number means a more efficient ECR, while alower transference numberleads to a slower ECR treatment. Banfill (1994) has suggested a value of0.2 for tel. whereas Andrade (1993) suggested 0.338. It is impossible that the transference numberwould remain constant with time, because the composition of the pore solution varies with timeduring the ECR treatment. As the ECR treatment continues, the chloride concentration in the poresolution C9 becomes smaller, and consequently the transference number (tel) of chloride in concretealso gets lower, so JCI gets smaller. Therefore, the chloride in concrete can not be completelyremoved by ECR treatment, unless the treatment lasts forever.

Bennett (1993b) reported that the efficiency of chloride ion removal was relatively high, about30-40%, at the beginning of the treatment, but decreased to very low values near the end of thetreatment. The overall efficiency for chloride removal for the entire treatment was typically10-20%. Experiments [Tritthart (1996), Tritthart 91997)] proved that the ECR became less efficientat greater depths with time due to the continuous rise in OR concentration at the reinforcement andthe corresponding drop in the chloride transference number.

In Andrade's review [Andrade (1998)], a regression equation was used to describe the dependenceof transference number of chloride on the applied electric charge (Q) in Coulomb:

log tel =7.62-0.63 log (Q/m2) (21)

Since the effectiveness becomes very low as the ECR treatment continues, the ECR treatment will beimpractical at the later stage. In many cases, the chloride content drops to "safe" levels before theOR concentration of the pore solution rises to values that make chloride removal inefficient. So,the extension of ECR treatment beyond this time does not seem to be desirable, because thecontinual decline in the efficiency of the chloride removal with time.

It is interesting that carbonation also affects the effectiveness of ECR. Higher ECR effectiveness wasobserved in non-carbonated concrete than in carbonated concrete blocks [Ihekwaba (1996)]. It waspredicted that chloride contaminated concrete with a considerably carbonated cover would likely



show an inefficient ECR performance. Broomfield et al (1997a) claimed that ECR could increasethe electrical resistivity of concrete and its ability to resist the ingress of water, oxygen, and ions.This might be due to a redistribution of calcium hydroxide, and should make the concrete moredurable and more resistant to future chloride ingress and corrosion. Different findings were reportedby Polder et al (1995). In their study of ECR treatment on cores taken from a reinforced concretecoastal structure, they claimed that there were complex changes in the pore solution chemistry and amajor decrease in electrical resistance of concrete when soda was used as the anolyte.


44Research neport 332

50 Side Effects of Electrochemical Treatments:

The principles of the electrochemical techniques (CP, ECR, and RA) are similar. All thesetreatments require a cathodic current applied to the reinforcement in the concrete, and this couldhave some side effects on the reinforced concrete system. The principal concerns are: hydrogenembrittlement of the steel (HIE), degradation of the steel/concrete bond, and alkali-aggregatereaction (AAR) in concrete close to the steel/concrete interfacial region [Page (1992)].

These side effects have been extensively investigated in recent years, because of their implicationsfor the application of the widely used CP technique. Theoretically, the re-alkalisation treatment usesa higher current density than CP, and would have higher risks of causing the side effects. Also, asthe current density employed in ECR is much higher than in CP (about 10-100 times higher), -andthe total charge used is also much higher than that used in re-alkalisation, ECR is more likely toproduce stronger side effects on the concrete and reinforcing steel.

5.1 Hydrogen induced embrittlement

The phenomenon of hydrogen embrittlement is associated with intense cathodic polarisation whenthe potential of steel becomes more negative than the reversible potential for the hydrogen dischargereaction (equation (5». The primary product of this reaction, adsorbed atomic hydrogen (Hads), mayeither be released as gaseous molecular hydrogen (Hz) or become dissolved in the metal latticewhere it leads to embrittlement [Page (1992), Hartt (1996)].

The main concerns for CP application are hydrogen embrittlement and loss of bond, particularly forprestressed concrete elements [Klisowski (1996a)], and researchers have tried to quantifying thelikelihood of this effect.

The risk of significant embrittlement depends on many factors, which have been the subject ofseveral studies [Hartt (1993), Scannell (1987), Parkins (1982), Galvez (1985), Wagner, Wagner(1993)]. Most authors have agreed that hydrogen embrittlement and brittle fracture are likely tooccur if the thermodynamic potential for hydrogen evolution is exceeded [Wagner (1993), Hartt(1989), Hope (1990)]. The potential dependence of the fracture stress of notched prestressingtendons in saturated Ca(OH)z solution showed that significant reduction in tensile strength started tobecome apparent at potentials more negative than about -lOOOmV/SCE, which is consistent with thereversible potential for the hydrogen electrode in the test solution. On this basis, it has beensuggested that a lower limiting potential of -900mV/SCE may be appropriate for cathodic protectionapplied to prestressed concrete [Hartt (1990)]. Therefore, the risk appears to be acceptably low ifthe potential is maintained at a level less negative than -900mV/SCE [Klisowski (l996a)]. Thissuggested limitation is similar to the lower potential limit of -900mV with respect to theAglAgCl/0.5M KCl electrode, proposed for prestressing steel in European Standard [European DraftSandaI'd (1996)]. Ishill et al. (1992) also reported that the susceptibility to hydrogen embrittlementtended to increase at more negative potentials ($-lOOOmV/SCE) for prestressed concrete. Eventhough it has rarely been suggested that potentials less negative than -900mV/SCE may result inhydrogen embrittlement of prestressing steel, Hartt (1993) repurleu a (eduction in fracture load atpotentials less negative than -900mV/SCE, at a pH of 12.5 for notched specimens uf prestressedsteel. This indicated that -900mV/SCE might not be an appropriate lower limit for cathodicprotection of prestressing steel. Based on their experiments and analyses, it was confirmed [Hartt(1996)] that -900mV/SCE limit was safe for non-microalloyed prestressing steel; but not suitable formicroalloyed tendon specimens. They recommended that prestressed concrete members with microalloyed tendon material not be catholically protected. Recently, in order to avoid hydrogenembrittlement in prestressed concrete elements, a set of criteria were formulated for quantifying



corrosion-damaged prestressed concrete members for cathodic protection, based on the prestressinglevel and the extent of corrosion damage [Hartt (1998)]. Wagner et al (1993) suggested that the IRfree potential of any prestressed steel member should not be permitted to become more negative than-800mVICSE (-126mVISCE) . ..

However, other experiments have shown [Bertolini (1996)] that CP can be regarded as a safetechnique as far as hydrogen embrittlement of high strength steel is concerned, because the steelpotential is higher than -300mV/SCE for current densities up to 2 mA/m2 of reinforcement. Bennettet al (1998) also claimed that CP was safe when used on prestressed components if current andvoltage levels were carefully monitored. Effective operation of CP on prestressed structures wasbest accomplished in constant voltage mode, and the voltage should be selected to be more positivethan the potentials required to generate hydrogen at the prestessed steel surface.

For the RA and ECR treatments, there would be a higher risk of HIE, particularly if they are appliedto prestressed concrete elements. Special car~ is r~quired for ERC applications to prestressing steelto ensure that potentials are more positive than -1100 mV/CSE [Collins (1992)], and there are onlyvery rare cases of the application of RA to restore alkalinity of concrete structure with prestressed,high strength steel reinforcement.

5.2 Alkali-aggregate reaction

Alkali-aggregate reaction (AAR) is a process in which certain siliceous aggregates react with thealkali hydroxide present in the concrete pore solution to form an expansive alkali-silica gel withinand/or around the reacted aggr~gat~ particles. The gel is very hygroscopic and has a much largervolume compared with the materials consumed in the reaction. This causes expansion force to buildup around the reacted aggregate particles, resulting in cracking damage to concrete structures.

The AAR reaction requires reactive aggregates, alkali hydroxide, and water. The cathodic protectionprocess can produce alkali hydroxide ions according to Equation (6), and attract sodium andpotassium cations as described by Equation (11) in the vicinity of the reinforcing steel. Hence inprinciple, CP could cause AAR damage, or at least there is an AAR risk in the reinforced concreteunder cathodic protection.

It has been experimentally verified [Chaussadent (1996)] that alkali-silica gel was formed near theribs of the reinforcements at the steel/concrete interfacial zones. It was also reported byBartholomew et al. (1993) that cathodic protection currents could initiate or slightly acceleratealkali-silica reactivity in structures containing alkali sensitive aggregates.

The effects of CP on AAR have been investigated in the laboratory, and it has been shown that CPdoes accelerate the AAR process under certain conditions [Torii et al (1996), Kuroda et al (1996)].A detailed study by Shayan et al (1998) using. commercially available slowly reactive Australianaggregates in concrete showed that practical cathodic current levels qid not have an appreciableinfluence on the AAR expansion of the concrete, although in highly reactive aggregates could bemore sensitive to the CP currents. It appeared from the above study that the enhanced risk ofexpansive AAR developing in cathodic regions of reinforced concrete containing slowly reactiveaggregates would probably be insignificant. Sergi (1994) had also reached a similar conclusion andstated that the effects of CP on AAR could be insignificant provided that the cathodic current densityis uniformly and consistently maintained at a low level «20mA/m2 of reinforcement). No case ofAAR acceleration by cathodic protection has been reported for field structures. Although Torii(1996) concluded that CP enhanced the AAR expansion of beams subjected to a 50 mAlm2 CPcurrent density, it has been noted that AAR was not accelerated in afield structure under impressed

. current cathodic protection at practical levels [Bennett (1993)].



However, for re-alkalisation, the risk of AAR could be increased, because large amounts of hydroxylions are produced in the concrete during the treatment and the pH of the concrete rises to highvalues. As ARR requires the simultaneous presence of suitable quantities of alkali-reactiveaggregate, moisture and high alkalinity, it seems more likely that AAR will be a potential problem inchloride contaminated concrete than in carbonated (lower pH) concrete, although the realkalisationwould restore high alkalinity of the concrete. However, some short term experiments by AlKadhimi (1996) showed no detrimental effects of re-alkalisation

The ECR treatment also poses an increased risk of AAR. As discussed above regarding thealkalisation and accumulation of OH-, Na+ and K+ in the vicinity of reinforcing steel bars, the'AARrisk could be very high at the current levels employed by ECR. Thi~ effect has been experimentallydemonstrated [Page (1992), Bennett (1993b), Page (1994), Page (1995)]. However, in theapplication of ECR on a real structure with a potentially reactive aggregate, no AAR developmentwas observed. It was also reported that adding lithium ion into the electrolyte could suppress theAAR reactivity and no petrographic evidence of AAR could be seen LBennett (l993e)]. The lithiumions could migrate towards the reinforcing steel at the same time as the chloride ions were beingremoved [Manning (1994)], and suppress AAR expansion.

5.3 Disbandment of steel/concrete

The possible effect of cathodic current on bond strength of steel in concrete has also receivedattention for many years. The degradation of the steel/concrete bond, associated with softening ofthe cement matrix in contact with the metal, has been reported in several previous studies whichinvolved the passage of high cathodic current densities for prolonged periods [Vrable (1977), Locke(1983), Rasheeduzzafar (1993), Bartholomew (1993)]. The bond strength decreased as the potentialbecame more negative [Hartt (1993), Klisowski (1996a)]. The mechanism might be somewhatsimilar to the detrimental effect of NaOH on cement paste which was believed to have been relatedto the partial substitution of Na for Ca in CSH gel [Shayan (1989)], and the steel/concrete interfacecould be weakened by softening of the concrete immediately around the steel, due to the productionof hydroxyl ions [Bennett (1990)].

However, at lower current densities, more typical of those used in practical CP of reinforcedconcrete, the effect does not appear to be of major concern [Sergi (1992), Vrable (1977)]. Weyers etal (1984) calculated from the test results by Varable (1977) that it would take about 138 years toobtain a 25% loss of bond strength at a cathodic protection current density of 2.82 mAlft2• There isno convincing field evidence to show that disbonding has occurred as a result of the cathodicprotection current levels employed. Even for prestressed concrete members, no significant negativeeffect of cathodic protection has been detected on tendon bond strength, after long service periods[Hartt (1996), Hartt (1998), Bennett (1998)].

Theoretically, during the ECR and RA treatments, ions in the concrete pore solution, like sodium,potassium, and hydroxyl ions, which move in the electrical field, also affect the bonding betweensteel and concrete [Collins (1992)], as the increase of sodium and potassium ions and the productionof hydroxyl ion on the reinforcement surface could result in the softening of the concrete in thevicinity of the reinforcement [Weyers (1984)]. So, some researchers [Vrable (1977), Locke (1983)]raised the concern that excessive polarisation of steel «-1100mV/CSE) could lead to loss of thebond strength of steel to concrele. In the laboratory, it was found that the bond strength could bedramatically reduced using a high current densily when a large amount of chargc had passed throughthe reinforcement [Buenfeld (1994)].

In the SHRP program [Bennett (1993b)], the bond strength of concrete to rebar was evaluated bymeasuring the ultimate bond stress. It was found that the ultimate bond stress was not affected byany treatment processes, up to a maximum current density of 50Alm2 and a charge of 2000Ah/m

2of

steel. However, the loaded-end slip was reduced by about 40% for the specimen subjected to both



..the highest current and charge. Free-end slip was also reduced significantly for specimens subjectedto either the highest current density or the highest charge. Less than 5 Nm2 and about 1500 Ah/m2

could be expected to have little or no effect on the bond strength. Gautefal (1996) found asignificant reduction in the bond stress of steel rebar within the charge range of 600-5000Ah/m2

,

and there was a sharp increase in the bond stress at higher charges.

It appears that different findings have been obtained by different investigators. Nustad (1997)summarised the findings from some independent investigation programs related to the ERetreatment and bond strength studies (Table 6). It is hard to draw a definite conclusion based on thesefindings. Differences in some concrete properties may have influenced the results.

Table 6. Findings of various investigations related to changes in the steel to concreteb d t th It f d r f [N t d (1997)]on s ren~l as a resu 0 esa IDa Ion us a

type of Chloride content current density total charge per change in pull-rebars (% of cement per steel surface steel surface out load (%)

mass) (A/m2) (Ah/m2

)

ribbed 2 0.8 1340 -30ribbed 1.4 0.2 215 -23

1.4 0.2 538 -121.4 1.8 538 -111.4 1.8 2153 -11.4 53.8 538 +1.1.4 53.8 2153 -12

smooth 2 1.6 269 +352 1.6 538 -432 1.6 998 -42 1.6 2150 -302 4.0 672 -432 4.0 1344 -522 4.0 2496 -262 4.0 5376 -482 8.0 1344 -43

.2 8.0 2688 -42 8.0 4992 -132 8.0 10752 +261

smooth 1.5 0.75 252 . +131.5 0.75 504 +51.5 0.75 756 +91.5 0.75 1008 +51.5 0.75 1260 +101.5 0.75 1512 +51.5 0.75 1760 +141.5 0.75 2016 +25

smooth 0 7.6 4956 -720 7.6 9277 +270 7.6 13388 +1030 15.3 10279 +1410 15.3 17866 +1520 15.3 21269 +136

0.4 7.6 4956 -600.4 7.6 8680 +58·0.4 7.6 13224' +2810.4 15.3 10279 +1650.4 15.3 15671 +211



0.4 15.3 18145 +2001.0 7.6 4956 -581.0 7.6 7350 -161.0 7.6 11457 +2461.0 15.3 10279 +2091.0 15.3 14187 +1631.0 15.3 18061 +1832.0 7.6 4956 -572.0 7.6 7052 +82.0 7.6 11767 ·+1982.0 15.3 .10279 +1632.0 15.3 17598 +1422.0 15.3 20577 1169

c

.....,..-"•.-=.:0..--.-smooth high 1.7 9.5 12768 -27slr~nglh sleel 1.7 28.6 38438 -58

3 9.5 12768 -253 28.6 38438 -45

smooth 3.1 4 3840 -31smooth 3.1 12 11520 +28ribbed 3.1 4 3840 +Rribbed 3.1 12 11520 -to

5.4 Other possible effects

Besides the above influences, there may still be some other effects when high l:alltuuil: l:unenldensities are passed through the concrete.

It has been claimed [Miller (1994), Polder (1993)] that carbonate ions can also penetrate towards thesteel bar, and that electro-osmosis (the water molecules drag the carbonate ions towards the steelbar) can also occur. This 'would increase the moisture in the vicinity of the steel bar and is likely tofacilitate the corrosion of the bar.

In addition to the water accumulation, possible chloride evolution could occur when pH<7 duringthe ECR treatment [Bennett (1993b)].

Also, after the re-alkalisation, the concrete properties change in a manner consistent with thedeposition of materials in the pores of the concrete [AI-Kadhimi et al (1996)]. The pore sizedistribution changes in the direction of smaller pores; the total water absorption, capillary absorptionand initial surface absorption decrease; and also the compressive strength, flexural strength, pulloutstrength, dynamic modulus of elasticity and ultrasonic pulse velocity increase.

Moreover, resistive heating might also be a problem ·with ECR if the applied current density is toohigh [Collins (1983), Slater (1976)].. If an electric current at densities of about 1 A/m2 of concretesurface (corresponding to 1-5A/m2 of steel surface) is passed through the concrete,· the heatingeffect could possibly cause cracks, adversely affecting the integrity of the concrete [AI-Kadhimi el al(1996)].

If the OR" concentration is raised to levels that never occur in normal concrete, another element ofuncertainty, with other unknown detrimental influences will be added [Tritthart (1996)]. Forexample, the aggregates that have been considered harmless so far may become reactive under suchextreme conditions.


49.ResearC?h Report 332

In sununary, it seems that the risks uf HIE, AAR, and disbondment are not very high for cathodicallyprotected conventional reinforced concrete stuctures, because the cathodic current densitiesemployed in CP are relatively low. Even for prestressed components, CP can be effectively used tocontrol the corrosion of prestressing steel with careful monitoring of the applied current and voltage[Bennett (1998)]. However, if the distribution of cathodic protection current is not reasonablyuniform; the risks at some locations could be greatly increased [Page (1997)]. It was reported[Bennett (1998)] that for structures with very non~homogeneous concrete resistance, it was difficultto achieve CP criteria at sites where resistivity was high, while at the same time precluding hydrogengeneration at sites where resistance was low; the latter being conducive to HIE damage.

However, for the RAand ECR treatments, the risk of HIE, AAR, and disbondment of steel/concretewould be higher than for CPo In addition, there may be some other effects on the reinforcedconcrete system due to the high current densities applied. These need to be further investigated.



6. Inhibitors

A corrosion inhibitor (INH) is an agent that can greatly reduce the corrosion rate of metal with asmall addition to the medium to which the metal is directly exposed. The use of corrosion inhibitorsis also one of the effective approaches to prevent or reduce the corrosion risk of reinforcement inconcrete.

Corrosion inhibitors are generally classified into organic and inorganic. For example, calcium nitriteis a typical inorganic inhibitor, and most amine derivatives are organic inhibitors. Most of thecorrosion inhibitors for steel are used in acidic or neutral conditions. However, the reinforcing steelis always in a basic environment in concrete. Even in carbonated concrete, the pH value. of the poresolution would still be above 8.5. So, very few of those inhibitors that are widely used in aqueousmedia can be directly used in reinforced concrete systems with success. In fact, the need to ensureadequate protection without altering the physical and mechanical properties of concrete considerablyrestricts the choices of effective inhibitors for reinforced concrete.

In the past decades, the use of corrosion-inhibiting concrete admixtures has emerged as a promisingmethod of delaying the onset of corrosion [Virmaini (1997»). Inhibitors are also employed withpermeability-reducing pozzolanic additives such as fly ash or silica fume in some situations. Theyincrease the chloride concentration threshold necessary for corrosion initiation , thereby increasingthe resistance to corrosion.

6.1 Inhibition mechanisms

The role of inhibitors is to slow down process i6 (or ia) (Figure 1). This is usually realised throughforming a three-dimensioned inter-phase film (passive film) or a two-dimensioned interface film(adsorptive film) on the steel surface, particularly at the active anodic or cathodic sites. Such filmscan effectively retard the electrochemical reactions on the steel surface, hence dramatically reducethe corrosion rate of the steel.

For example, calcium nitrite is a typical inhibitor that can form a film of ferric oxide on reinforcingsteel surface [Rosenberg (1979)]:

(22)

The Fe203 film formed on the steel surface is very stable in contact with the alkaline pore solution,and it separates the steel substrate from the pore solution, resulting in very slow dissolution of the.steel.

In the case of chloride-induced corrosion, the increased chloride concentration threshold mentionedearlier is also related to the film formed by the inhibitors, which suppresses the adsorption of crdirectly on the steel surface, and increases its corrosion resitance.

6.2 Commonly Used Inhibitors

There is a wide range of inhibitors available for protection of steel in reinforced concrete [Graig(1970), Berke (1986), Griffin (1975), Slater (1983), Berke (1989b), Alonsao (1983), Berke (1990),El-Jazairi (1990)].



Calcium nitrite is the most commonly used inorganic inhibitor for reinforced concrete [Rosenberg(1977), Rosenberg (1979), Berke (1988), Berke (1991)], and has been commercially available since1978. In the USA, over 200 parking structures, 100 marine structures and over 230,000 cubic metersof precast/prestressed bridge girders have been constructed with concrete containing calcium nitrite[Berke (1994a)]. Substantial research has been conducted demonstrating the effectiveness ofcalcium nitrite as a corrosion inhibitor, and Berke (1989, 1992), and Tomosawa (1990) et al havesummaris~d the available data.

Calcium nitrite has been shown to be able to provide paSSIVity at relatively high chlorideconcentrations, and to bring about considerable resistance to chloride induced corrosion [Berke(1987), Virmani (1988)]. Some researchers [Rosenberg (1979), Berke (l994a)] have examinedseveral old structures, and concluded that all the structures with calcium nitrite were performingwell, whereas corrosion was in progress on adjacent structures that were not protected with calciumnitrite. The calcium nitrite was stable in concrete and remained at the surface of reinforcing bars.Diffusion of chloride was not increased in the concretes with calcium nitrite, and there was evidenceof a reduction in chloride penetration in some cases.

It appears that the inhibition effectiveness of calcium nitrite is dependent on the chloride content inconcrete (Table 7) [Berke (1997)], and that the nitrite and chloride ions seem" to be involved incompeting reactions [Rosenberg (1979), Berke (1989a)]. Table 7 shows that larger doesages ofcalcium nitrite in concrete result in larger tolerances to chlroide before corrosion is initiated.

[B rke (1997)]hi 'd t It't 't da e , aClUmmrle osa2e ra es vs C on e 0 erance e30% solution calcium nitrite (Llm3

) chloride ion (kwm3)

10 3.615 5.920 7.725 8.930 9.5

T bl 7 C I '

It has been found that at a mass ratio of chloride to nitrite close to 1, the calcium nitrite appears toreduce the corrosion rate by an order of magnitude [Virmani (1983), Virmani (1988)]. Tomosawa etal (1992a) also reported that nitrite nearly completely prevented chloride-induced corrosion when itexisted at a nitrite-to-chloride ratio above 1.0 in concrete.

Although an anodic inhibitor, nitrite could accelerate the corrosion rate of reinforcing steel[Phanasgaonkar (1997), ] when its concentration is too low to have enough anodising ability to bringthe steel into the passive region. However, a number of reports have indicated that such risks couldbe ruled out [Tomasawa (1987), Lundquist (1977), Berke (1989c)]. Gonzalez (1998) believed thatan inadequate dose of nitrite posed no special danger, as the inhibitor did not act as a passivator butrather prevented local breakdown of the passive film.

It seems to be well established that nitrite can counter the action of chlorides at concentrations whichwould result in no significant loss of mechanical resistance [Andrade (1973)]. Gonzalez et al (1998)studied the ability of nitrite to counter the corrosive effects of chloride on reinforcement barsembedded in mortar mixed with artificial sea water. They found that the addition of 2% NaN02 by "mass of cement to the mortar effectively countered the risk of corrosion of the reinforcement.However, under immersed conditions, some nitrite was leached out, so reinforced concrete structuresimmersed in salty waters might not be permanently protected. The length of time for which theprotection remains effective would depend on the quality of the concrete.

In addition to nitrite, some other inorganic inhibitors are also effective in inhibiting the corrosion ofreinforcement in concrete. After screening various inhibitors, including sodium nitrite (NaN02),sodium molybdate (Na2Mo04), sodium dihydrogenphosphate (NaH2P04), sodium



monofluorophosphate (Na2P03F), sodium tetraborate (NazH40 7), as well as some commercialinhibitors, Dillard et al (1993) found that sodium nitrite and sodium monofluorophosphateperformed well in the screening test. Their observation indicated that the nitrite reacted with steeland formed an iron oxide surface layer on the steel, but the inhibitor itself was not adsorbed asnitrite; borate al~o interacted with substrate steel but was undetectable by surface-sensitive analyticalmeasurements; other inhibitors interacted via adsoption on reinforcing steel and lead to enhancementof surface functionalities. Dillard et al (1991, 1993) found that the migration of inhibitor varied withthe square root of time; the transport of sodium tetraborate through a concrete disk resulted in nochemical change in the inhibitor; while migration of sodium monofluorophosphate led to thehydrolysis of the salt. Dillard et al (1993) also tried to develop feasible corrosion inhibitor andchloride scavenging treatments: Calcium nitrite, sodium monofluorophosphate, sodium tetraborate,zinc boratesilicone, and some organic and commercial agents were tried. However, the resultsseemed to be unsatisfactory.

Gonzales et al (1998) believed that none of the alternative inhibitors they studied, includingresorcinol, phloroglucinol, urotropin, sodium phosphate, potassium chromate, zinc oxide, sodiumgluconate, calcium gluconate, etc., was comparable to nitrite in terms of protective efficiency.Some other inorganic compounds, such as stannous chloride, zinc and lead chromates, potassiumdichromate, and calcium hypophosphite [Boyd (1968)] have also been suggested as inhibitors.However, according to Craig et at. (1970), the additiol) of three effective inhibitors for steelcorrosion, potassium chromate, sodium benzoate, and sodium nitrite, iri a variety of media,substantially decreased the compressive strength of concrete. Other inhibitors, including zinc oxideand phosphate can also reduce corrosion rate [Jin (1991)], but they delay the setting of concrete.

Organic-based inhibitors have recently appeared on the market are believed to contain amides andesters. They are relatively new to the market, and very little performance data is available.However, they have been tried in reinforced concrete. For example, it has been suggested thatamines- based inhibitors can be applied as remedial agents on the concrete surface to inhibit furthercorrosion of the reinforcement [Nmai (1992)], by diffusing through the cover concrete to thereinforcing steel [Bjegovic (1993)]. They can also be used as admixtures in which case, the amountsrequired to achieve the desired inhibition were reported to be lower than that of inorganic inhibitors[Bjegovic (1993)].

Recently, a comparison among several amine-based inhibitors· was carried out in the laboratory[Phanasgaonkar (1997)]. It was found that dicyclohexylamine nitrite and commercially availablemigratory inhibitor (aromatic amine based) gave excellent inhibition for extended periods, whereasdimethylethanolamine had only moderate and short term protection. Other recommended organicinhibitors include sodium benzoate, ethyl aniline, and mercaptobenzothiazole [Boyd (1968)]. Thereare still other commercial inhibitors containing amino tris (methylene phosphonic acid), hydroxyethylidene diphosphonic acid, and hexapotassium hexamethylene diarrune (methylenetetraphosphonate), etc. [Dillard (1993)].

In addition to the inhibitors, . oxygen scavengers, such as sodium sulfite and hydrazine, cantheoretically also be used to prevent corrosion by limiting the oxygen supply through reacting withdissolved oxygen to form products inert to the corrosion process. Unfortunately, in a reinforcedconcrete system, this leads to little promise due to the unlimited supply of oxygen [Dillard (1993)].

6.3 Inhibitor Incorperation

As already mentioned above, inhibitors can be directly added as admixtures [Nmai (1992), Griffin(1975), Dillard (1993)], or added by surface impregnation on the corroding structures [Bjegovic(1993), Mader (1995), Bleibler (1998), Dillard (1993), Asaro (1990)]. For example, the use ofcalcium nitrite as an admixture has become fairly widespread, and concrete made with addedcalcium nitrite has considerable resistance to chloride-induced corrosion [Berke (1987), Virmani



(1988)]. Sodium monofluorophosphate has be~n used as <in additive of the road slat [Dressman(1991)] so that it can diffuse into concrete after the salt spraying. The nitrite ion has also been foundto be able to diffuse downward into concrete from a. bonded concrete overlay doped with calciumnitrite [Jayaprakash (1982)].

Furthermore, some inhibitors can also be applied in coatings on the surface or onto the exposed steelthrough patch repairs. They can be incorporated into the patch repairs, applied in grooves or drilledholes in the. concrete cover or incorporated into concrete overlays [Broomfield (1997), BIeibler(1998)].

It needs to be particularly mentioned that there is still a special catergory of inhibitors suitable forelectrical injection into concrete (migration toward reinforcement under a strong electrical field) forpreventing steel corrosion. Asaro et al (1990) synthesised four synergistic corrosion inhibitorcandidates: tetramethylphosphonium nitrite (TMPNOz), tetraethylphosphonium nitrite (TEPNOz),tctrabutylphosphonium nihilt: (TBPNOz), and tetraphenylphosphonium nitrite ('I'PPNOz). All theseshowed inhibition efficiencies in the range 63%-85%. In the treatment, the cations such as TEP+and TBP+ were electrically injected into mortar and concrete matrices. The best one among theseinhibitors appeared to be TEPNOz in terms of its transference number and ease of injection, andcould provide adequate corrosion protection of corroding steel rebars. Effective injection can occurunder a potential gradient of 5-lOV/cmat 0.45-1.24mNcm2 within a period of 10 to 15 days. Animportant advantage of this technique is that the equipment required is similar to that used incathodic protection, but only needs a temporary installation for a few days. At the same time, itmay be possible to inject lithium ions into the concrete. if it contains ASR products, to reduce itsexpansion potential.

The choice for the mode of application would depend on the corrosion situation of the element, theenvironmental around the element to be treated. and the budget.


54Research Report 33~

7. Surface Treatment and Coatings

According to the processes analysed in Section 1.1 (Figure 1), the corrosion rate of reinforcing steeldepends on the supplies of reactants and facilitating species. Stopping the supplies of thesesubstances from the environment would finally hinder the corrosion of reinforcement in concrete.So, processes, il and ilion the concrete surface, which are related to the corrosion reaction, arecritical to the corrosion rate. This means that, any surface treatment (STC) and coatings (CO) on thesurface of concrete that can stop or slow down the penetration of 0' and X' would be effective inpreventing the corrosion of reinforcement.

Asa barrier on the concrete surface, the coating should have good durability, which is dependent onvarious factors, such as surface preparation, adhesion, alkali-resistance, moisture-resislam;e,abrasion-resistance, resilience, and chemical-resistance. In addition, in order to provide an effectivecorrosion protection,the coating should be particularly resistant to the penetration of X' and 0'.

In terms of concrete durability ~nd reinforcement protection, most of the studies referred ·in thissection are focused on coating materials with respect to their abilities to resist the penetration ofaggr~ssivespecies into the concrete [Browne (1987)].

7.1 Preparation for Coatings

Surface preparation is necessary before concrete is coated. Up to 70% of premature coating failuresare due to incorrect or inadequate surface preparation. A properly prepared surface with a poorquality paint may last longer in many environments than a poorly prepared surface with a highquality coating.

It is important to ensure that the concrete is dried before the coating is applied. For a newlyconstructed structure, curing time is particularly important to moisture sensitive coatings (e.g.urethanes, vinyl esters or polyester). Some epoxies are more moisture tolerant and can be appliedover less cured concrete.

In order to obtain a good quality coating on concrete, the defects of the concrete surface should becarefully treated. These defects include: laitance, efflorescence, pits, voids beneath the surface,rough projections, chemical additives, dust, oil and grease, etc. For example, rough spots andprojections should be smoothed out by grinding, and large voids and pits should be filled withmortar or grout; oily contaminants should be removed by detergent water cleaning or steam cleaning.

A concrete surface can be prepared by mechanical abrasion or abrasive blasting. In practicalapplication, sand blasting is normally used to clean the concrete surface [Vennesland (1996)]. Highpressure water blasting is also· a method which can be used on poured concrete surfaces whereabrasive blasting is not allowed. This is effective on contaminated, eroded or weak surfaces. Acidetching can also be used in some cases, and it is a quick, easy, dust-free and inexpensive method.

7.2 General Requirement of Coatings for Concrete

Compared with steel, there are far fewer coatings which can be applied to concrete surfaces becauseof the special chemical and physical characteristics of concrete which influence the compatibility ofmany coatings. A number of properties are essential for a coating to be effective on a concretesurface:



1) Adhesion is the primary property required for concrete coatings [Munger (1984)]. Thecoatings should adhere strongly to the concrete, and retain its adhesion when the concreteis dry or subjected to moisture vapour or even liquid water;

2) Penetration of the coating material into the concrete surface is extremely important inproviding a strong adhesion and for ensuring a long service life of the coating system[Munger (1984)]. The coating should have good penetrating ability and should retain thepenetration even in the presence of moisture within the concrete. Coatings which penetrateconcrete have much improved adhesion and increased long-time performance as comparedwith those that remain on the concrete surface relying on adhesion alone;

3) Concrete surfaces are highly alkaline and can react with certain coatings, such as alkylsand other oil-based paints. Any coating applied to concrete must be able to resist thealkaline nature of concrete [Munger (1984)]. The two best known alkali-resistant coatingsare those based on vinyl and epoxy resins, and both make good coatings for concrete[Munger (1984)];

4)· Concrete surfaces inevitably contain some moisture. Coating systems should be tolerant tothe moisture and have certain breathability, allowing the moisture trapped within theconcrete to be released. In other words, the coatings should be permeable to water vapour,but resistant against capillary suction of liquid water or water condensed from theatmosphere [Schiessl (1997)];

5) The porosity of concrete can cause bubbling and pinholing of some coatings. These canresult when air or solvent trapped in the voids cannot escape before the coating dries. Toothick or viscous films are most at risk, particularly with fast drying. Hence less viscous,and penetrating coatings should be preferentially considered in the coating application;

6) On the other hand, coating should have a substantial thiCkness over the surface to coverany imperfections and irregularities of the concrete [Munger (1984)]. An appreciablethickness also helps in masking the minor shrinkage cracks. Coatings in 10-20 milcategory are capable of bridging hair cracks, particularly cracks present in the concretesurface before the coating is applied. Appreciable thickness is also required to resist theback pressure of the water where water pressure can develop within the concrete andunderneath the coating, which can result in blistering of the coatings on concrete;

7) Concrete can crack, expand and shrink due to factors such as thermal expansion andsettlement. Application of rigid coatings would result in coating rupture, so coatings forconcrete need to have some resilience, flexibility, and extensibility [Munger (1984)] toaccommodate reasonable levels of shrinkage or expansion, and resist the impact to whichconcrete is frequently subjected. Crack bridging ability would be ideally expected of thecoating material [Schiessl (1997)];

8) Abrasion resistance is also required for coatings on concrete in some situations [Munger(1984)]. The coating systems that have the greatest abrasion resistance are liquid epoxytype toppings, or a polyurethane coating system;

9) Coating systems should have good durability under ultra-violet light and chemical attack.Certain pigments can be added to the coating to provide resistance to ultra-violet light.

Besides the above basic requirements for a concrete surface coating system, high resistance toingress of CO2 and lor chloride is a specific and also essential requirement [Schiessl (1997)] from acorrosion prevention point of view.

7.3 Coating Types and Their Functions

Various types of coatings have been used on the concrete surface for corrosion prevention purposes.There are different criteria for classifying the commonly used coatings [Munger (1984), Jacobsen


56Research Report 33?

. (1997)]. According to the compositions of the coating rrtateti<lls, rhf:':y r.:.ln ht>: r.I:.lssifip,d intosilane/siloxane, vinyl cster, polyurethane resins, epoxy resins, acrylic resins, coal tar epoxies,chlorinated rubber coatings, polymer modified cementitous materials, and bituminous cutbacks, etc.Based on their functions, they are classified as pore-lining penetrants (impregnants), surface sealers,film-forming coatings, and thick overlays. In terms of their roles, they can be categorised intohydrophobic, gas barrier, and combined coating systems.

There are certain relationships among the coating compositions, functions, and concrete parameters,which are summarised in Figure 18 and Table 8 [jacobsen (1997)]. It can been seen that normallythe substances used to deal with firie pores in concrete, like gel or capillary pores, are hydrophobicand termed as pore linings or impregnants. To deal with large defects in the concrete, such as airvoids and fine cracks, film forming coatings should be used, and the required thickness of thecoatings increases with the. defect size.

hydrophobic pore fiDing/fiIm forming thick coating film

term impregnation.. thin coating thick coating

pore lining pore filling

.-r-5 10

~ film thickness---1

concrete pore sizeF gel p>~s ~--- capillary pon:s ==j== .irvoids I large cavities

Figure 18. Coating terminology-film thickness-concrete pore size [Jacobsen (1997)]

(1997)][J bft. I fT bl 8 Ga e . ernenc matena s 0 concre e coa milS aco senHydro.pho.bic pore fillin~ and film formin2; thi«:.k coatin!! films

silicates/silicones, silicon epoxies (eg. silicon/silicates or cementresins, silanes, siloxanes, bisphenollpolyamide), based with various additives,oligomeric, alkoxysilanes, polymetylmetacrylates, mainly organic: latices (eg.

meta-acrylates, oils polyurethanes, polyacryl, polymethyl meta-acrylate, -polyacrylates, rubbers, PVC, vinyl acetate, -styrene, -

polyesters butadiene) epoxies

In terms of the composition, epoxy based coatings have been successful in corrosion prevention ofreinforced concrete, with a well proven service [Munger (1984)]. They can offer good abrasion,penetration, and chemical resistance, and have been extensively used in many industries for floors,tank linings, pump bases. and chemical process areas. Particularly, some liquid epoxies havesufficient·ly low molecular weights and excellent concrete wetting characteristics [Munger (1984)],both of which are important factors for impregnating the concrete surface, and making the treatedarea much stronger than the untreated concrete. The conventional, extensively used amine-cured orpolyamide-type epoxies, have the necessary alkali resistance and good adhesion.

Vinyl coatings have also been used for many years on concrete of all types [Munger. (1984)]. Theyhave excellent chemical resistance to both acids and alkalies, and are reliable and flexible. Inapplication, vinyl coatings usually dry rapidly. A small depth of penetration can be obtained byapplying a diluted vinyl primer prior to the application of the coating. Vinyl coatings have broader


57Research Report 33~

weather ,and general chemical resistance than epoxies, while epoxies have the properties ofpenetration and wetting which give them maximum adhesion.

Bituminous cutback coatings are solvent solutions of coal tar or asphait [Munger (984)]. A thincoat is generally applied as a primer. Heavy and rather thick-bodies of coal tar cutbacks d<?' anexcellent job in protecting concrete from chemical attack and water absorption. Some bituminouscutbacks are also used as concrete penetrants.

Coal tar epoxies combine the useful properties ofboth the coal tar and epoxy for concrete surfaces.They are chemical-resistant as well as abrasion-resistant [Munger (1984)]. '

Acrylic resins or polyurethanes have been found to offer excellent carbonation barrier properties.

Chlorinated rubber coatings have water and chemical resistance, including the important alkaliresistance [Munger (1984)], as well as the adhesion required for concrete, coatings. They can dryrelatively rapidly, and perform well under conditions of high humidity, but they are not resist.anf; to,vegetable oils and greases.

7.4 Surface Treatment and Coatings 'against Corrosion

For the purpose of corrosion prevention of reinforcement, especially under aggr~ssive environments,the coating used on the concrete surface should have a high resistance to the penetration of water,gas (C02), or cr, etc. Appropriate coatings for'these purposes include impregnants, paints andwaterpoofing membranes. In terms of cost, impregnants are the cheapest and membranes the mostexpensive.

7.4.1 Coatings against gas permeation (carbonation)

The principle of anti-carbonation coatings (CG) is that they are porous enough to let water vapourmove in and out of the concrete, but the pores are too small for the large carbon dioxide molecule topass through [Broomfield (1997)]. A simple and practical approach has been developed forassessing coatings as carbon dioxide barriers [Ho (1988), Ho (1990)]. ' This method is based on afactor which relates the depth of carbonation of treated and untreated ~urfaces.

Acrylic coatings generally have good carbonation resistance. It has been shown [Seneviratne(1996)] that coating systems such as acrylic modified siloxane primer/water based elastomericacrylic coating; water based acrylic primer/water based acrylic elastomeric coating; water basedepoxy primer/water based acrylic elastomeric coating; and oligomeric alkyl alkoxy siloxanetreatment appear to be able to reduce the corrosion activity of reinforcing steel in carbonatedconcrete structures exposed to natural weathering. However, the same treatments were not alwayssuccessful in maintaining a crack-free coating on chloride-contaminated concrete elements whichhad suffered from significant corrosion prior to the treatments.

7.4.2 Coatings against liquid solution penetration (chloride)

Usually, ingress of chloride is associated with liquid solution percolation into concrete. Therefore,using hydrophobic agents to reduce the absorption of water by concrete surfaces is an effectiveapproach to prevent chloride induced corrosion. It has been reported that- the ingress of chloridesinto structural concrete can successfully be prevented over many years by hydrophobic poreliningimpregnation and concrete coatings [Lunk (1998)].

In general, the surface treatments and coatings against solution penetration or chloride ingress (CS)include hydrophobic sealing treatments, surface coatings, and water-proof membrane.


concrete(pore wall)


7.4.2. 1 HvdrophobiC sealing

Earlier, various hydrophobic agents, such as paraffin, wax, mctallic soaps and silicones, were llsedagainst corrosion. Currently, reactive silicon organic compounds, such as silanes, siloxanes, andsiloxysilanes,etc., are more frequently used.

Hydrophobic treatment of concrete can reduce the water absorption by 70-90% and can alsosignificantly reduce chloride penetration particularly under wetting/drying conditions [Polder(1996)]. The best results are obtained with hydrophobic agents based on silanes and oligomericsiloxanes, and applying hydrophobic coatings does not adversely affect the adhesion of coatings orasphalt to concrete. It is interesting that hydrophobised concrete can reach its equilibrium moisturecontent more quickly than untreated concrete. The performance of hydrophobic agents is differentfor concretes of different cement types and formworksurfaces [Polder (1996)].

Specifically, among the various hydrophobic sealing treatment materials, sHanes have SUllie

advantages [Carter (1994)]:

1) good penetration;

2) good durability due to strong chemical bonds;

3) excellent breathablility to allow escape of internal moisture; and

4) a transparent appearance with no change in the colour or texture of the treated surface.

As silane-like penetrating sealers form a hydrophobic layer inside the concrete pores, there is noimpact on the design or performance of the structures. I~ is also cheaper than epoxy coated steel barsor CPo However, silanes do not prevent percolation of water into submerged concrete elements,although they are effective in shedding water from vertical surface.

Silanes and siloxanes are two of the most important silicone compounds for concrete [Polder(1996)]. Silanes are small molecules having one silicon atom, and siloxanes are short chains of afew silicon atoms. Their molecules contain organic alkoxy groups linked to the silicon atoms, whichcan react with the silicates in the concrete to form a stable bond. In addition, silanes and siloxanes

contain organic alkyl groups (CH3 - ) which have a fatty character. When they are applied UII

concrete, hydrolysis occurs. Water is consumed and alcohol is produced [Bonsak (1997), Bartlett(1998)]. After the reaction, the alkoxy or alkyl groups protrude from the pore walls into the pores(Figure 19). Therefore, water molecules will be repelled and will no longer be able to wet thesurface of the pores. In commercial hydrophobic products, silanes are dissolved in alcohol orhydrocarbons (10 to 40% silane) or consist of 100% silane. Siloxanes are often dissolved in alcoholor hydrocarbon solvents (about 10 to 20 siloxane).

water

~organic groups R R

I Isilicon atoms - Si - 0 - Si -

I Io 0

-------~ ----1- --- Si - - Si -

I IFigure 19. Chemical bond between the hydrophobic agent and concrete

[Polder (1996), Bonsak (1997)]



There are many types and variations among silane sealers (Table 9), and their performances oftendiffer.

Table 9. Common silane types [Carter (1994)]Chemical name Abbreviated name formula

iso-octyl-tri-methoxy silane IOTMOS CH3

\0/

CgH J7 - Si - OCH3

\0/

CH3

iso-butyl-tri-mehtoxy silane mTMOS CH3

\0/

CJ19 - Si - OCH3, \

0/

CH3

iso-butyl-tri-ethoxy silane mTEOS C2Hs\ ..

0/

CJ19 - Si - OC2Hs\0/

C2Hsn-octyl-tri-ethoxy silane NOTEOS C2Hs

\0/

CgH J7 - Si - OC2Hs\0/

C2Hs

Different silane/siloxane water repellents have different penetration ability in concrete [Bartlett(1998)]. Table 10 lists some typical water repellent silane-based materials and their applications.


60.Research Report 332

Table 10. Properties of some typical commercial silalle/siloxane water repellents

fBartiett (1998)] .-.repellclIl penetration applications

depth (mm)

isobutyl trimethoxy 5 maximum protection in marine or coastal conditions, butand triethoxy sil~mes not very suitable for application in hot or windy

conditions

silanes or 3-4 general purpose use or in climates where hot, windysilane/siloxane conditions can not be avoided

mixture in organicsolvent

oligomeric siloxane in 1-3 for less critical areas and non-alkaline substrates; lessorganic solvent influenced by hot, windy conditions.

Cater (1994) evaluated the damp-proofing performance and effective penetration depth of silanesealers in concrete. His data indicated that the permeability of modern high quality concretes couldbe significantly reduced by silane sealers, whereas porous, high water/cement ratio concretes mightbe more effectively sealed by barrier coatings that seal the surface of porous concrete but do notpenetrate. As permeable void content and the waler/cement ratio of concrete increase, the dampproofing effectiveness of silane sealers decreases. Increasing the concentration of the active silanein a penetrating sealer improved both the effective penetration depth and the overall damp-proofingperformance on modern, good quality concrete. Neat silanes were more effective for damp-proofingthan 40% silnncs. Periodic re-sealing of the concrete with lower concentrations of silane resulted ina similar effect to that of high concentration silane.

Bartlett (1998) also reported that monomeric and oligomeric silicon-based water repellents were themost effective products for increasing the water impermeability of concrete and other comentitiousbuilding materials.·

Silanes are not the only sealers available and they may not be the best choice in certain situationswith graffiti-covered, or porous, low quality concretes, or concretes subjected to prolongedsubmerged conditions or freezing in the presence of de-icing salts [Litvan (1992)].

7.4.2.2 Surface 'coatings

Thick coatings are also effective in preventing water containing chloride ions from penetrating intoconcrete.' They can vary from inorganic materials (cement, silicate) to organic polymermodifications using latex and epoxy [Jacobsen (1997)]. Latex modifications normally consist of anemulsion of polymer particles that is mixed with cement and water [Jacobsen (1997]. Epoxy ismuch less polymerised than latex before being mixed with cement and water. In the hardenedproduct the polymers forin large molecules bonded with the cement hydrates [Justnes (1994)].

7.4.2.:3 Waterpoofing membrances

Waterproofing membranes are also routinely used to stop water penetration into concrete. Theyhave two types [Broomfield (1997)]: one is aliquid that solidifies after application, and the other asticky sheet. They are usually overlain by asphalt, but they are not compatible with CP, RA, orECR.


r . 61Research Report 332

Due to their excellent mechanical properties, waterproofing membranes have become verysuccessful in many applications in th,e building industry.

The importance of solvent- and plasticiser-free sprayable polyurethane elastomers for waterproofingpurposes in field concrete structures has recently increased considerably [Recker (1994)].

A large number of water-proofing membranes have been developed and used in the field. Thesemembranes have various characteristics, some of which are listed in Table 11 [Pullar-Strecker(1987)].

k (lQS7)][P II ShfiT billSa e • . orne waterproo ID~ mem ranes II ar-.. tr~c. erMembranes Characteristics

bitumen high efficiency, abk to span cracks; service life over 50 years ifcovered by backfill, probaly 20 years or more if exposed

bitumen on inorganic fabric very high efficiency if properly bonded, able to span cracks;.-

base alone or in combination service life over 20 years if covered by backfill,with other materials up to 20 years if exposed

bitumen on organic paper high efficiency if properly bonded to the concrete, able to spancrack; short service life

PVC, Polyurethane, Butyl service life between 5-20 or more yearsrubber, chlor-sulphonated

polyethylene, polychloroprenelow-density polyethylene 10 year service life or more if not exposed to UV light, but very

short service life if exposed to UVpitch or coal-tar epoxy high efficiency if free from pinholes, not able to span active

(250 J..lm thick dry coating) cracks, should be applied in more than one coat;up to 20 year service life

epoxy resin, two-pack good efficiency, but hard to recoat because of poor inter~coat

polyurethane coatings adhesion, not able to span active cracks, should be applied in two(250 J..lm thick) coats; service life more than 10 years

solvent-based acrylic good efficiency, combine protective and decorative qualities,methacrylate, styrene-acrylic, multi-coat system essential to reduce incidence of pinholes, not

one-pack polyurethane, able to span active cracks; up to 10 year service lifechlorinated rubber

emulsion-based acrylic or fair efficiency, not able to span active cracks;styrene-butadiene polymer up to 10 year service life.and co-polymers with orwithout other materials

It needs to be stressed that these membranes might be effective in retarding the ingress of chloride into concrete. However, once chloride-induced corrosion has been initiated, these coatings areunlikely to stop it. There is no evidence either that the water content in concrete can be sufficientlyreduced by coatings, in case of chloride-induced corrosion, to limit the progress of the corrosion[Schiessl (1977)].

1,5 Practical Application of Coatings

In practical applications, coatings are usually expected" to perform a multi-role in the reduction ofcorrosion rate, including retarding the transport of CO2 and O2 gases, and preventing the penetrationof water and chloride ions. Therefore, the coating systems used in the field usually consist of morethan one or two layers. For example, the coating system on traffic decks is composed of primer,water proofing membrane, wearing course, and tie coat [Mailvaganam (1992)] (Figure 20).


62Rc::;carch Report 332

Wearing Course -----

WaterproofingMembrane :.....----

Primer or SeaJer-

Concrete Deck - :

F'igure 20. Typical traffic deck sy~tems

In another case, a three-layered epoxy coating was applied to a bridge, with a glass fibrereinforcement between the second and third layers [Vennesland (1996)]. It was found that O2

reduction rates were reduced with time.

In repairing an underground car park [Cronin (1992)], after the concrete was removed and reinstatedby using a polymer modified mortar, the external surface of the roof was coated with apolymer/cement waterproofing system. At the final stage, an acrylic anti-carbonation coating wasapplied to the entire internal surface of the car park. A similar repair system was also applied to aviaduct structure [Cronin (1992)].

Recently, Kristiansen (1997) reported that polymer modified cementltlous slurry, containing aprimer, had good technical qualities, and its performance in coastal climates was also satisfactory.

Another noticeable report [Mattila (1996)] was that the application of cement based coatings couldinduce re~alkalisation. When coating a carbonated concrete surface with a cement based coating ofcertain thickness and permeability, the carbonated concrete would be re-alkalised under favourablemoisture conditions. This may open a new easy way for the re-alkalisation of carbonated fieldstructure by a simple overlay with new concrete.

Surface coatings can also affect alkali-silica reaction (ASR) [Kobayashi (1989), Taki (1989),Takeyoshi (1989), Abe (1992)]. The water-impermeable type of coating, like epoxy resin, wasfound to promote ASR, whereas the water vapour-permeable type of coating, such as water repellentsilane and flexible polymer-modified cement mortar, were found to inhibit ASR. This is ascribed tothe water trapped by the coating in the inner part of concrete, which may slowly promote ASR. TheASR cracking exposes the interior of the concrete to the aggressive agents, and renders the appliedcoating ineffective for its intended purpose. This situation may accelerate the corrosion of thereinforcement in the concrete.



8. Control of Concrete Quality

A vast literature exists on the durability of concrete, most of which is focussed on the influence ofvarious parameters on improving the quality of concrete to resist corrosion of the steelreinforcement. The major aim to be achieved is to reduce the permeability of concrete and tosuppress the gas, moisture and ionic transport mechanism into and within the concrete. A completereview of the literature in this area is not within the scope of this document, and only the main issueswill be briefly covered.

All the processes iJ, i2, i4, itO, ill, i12, and i7, depicted in Figure 1, are directly related to the propertiesand quality of the cover concrete.

First of all, the permeability of the concrete is responsible for the easiness of processes iJ, h, ill, andi 12, i.e. the ingress of H20, O2, CO2, cr, etc. On the other hand, the permeability of concrete isdetermined by its microstructure, particularly the porosity and pore size distribution, which arefunctions of the water/cement (w/c) ratio of the concrete and curing conditions, and its ingredientsfoumulation.

Secondly, procedure itO is governed by the resistivity of concrete. The latter is not only dependenton the microstructure of the concrete but also greatly influenced by the physical-chemical propertiesof the pore solution and cement gel, which in turn are also dependent on the type and amount ofadmixtures used in the concrete, as well as the w/c ratio.

Furthermore, concrete placement and compaction praCtices a:re also very important to the aboveproperties. Bad practices could induce considerable defects in concrete, forming short-cuts for allthe ingress processes iJ, i2, i4, ill, and i12•

Therefore, any factors, like w/c ratio, cover thickness, admixtures, and workmanship, which canchange the microst~cture of concrete, would have influences on the processes ii, i2, itO, i4, ill, andi12, consequently affecting the corrosion rate of the reinforcing steel. If these factors could be wellcontrolled, the corrosion performance of reinforced structures would be much improved. Indeed,the importance of concrete quality control (CQ) for durability purposes has been well recognised.Fergestad (1997) states that significant improvements can be seen in the design and construction ofstructures if those built in the 1990s are compared with those built in the 1970s.

8.1 Control of Water/Cement Ratio

It has been known for several decades that the ratio of water/cement (w/c) has a very significantinfluence on the porosity of concrete. A high w/c ratio produces a high porosity concrete which iseasily penetrakd by aggressive species, and steel in such a COncrete is more easily corroded.Reducing the w/c ratio will result in a reduction in concrete porosity and the number ofinterconnected pores, and consequently retard the ingress of aggressive species by both the diffusionand capillary suction mechanisms. Cook (1951) reported that the permeability of concrete couldvary by as much as two orders of magnitude as w/c increased from 0.4 to above 0.7. Particularlywhen w/c>0.55, the permeability of concrete increases more sharply with the increase in w/c ratio[Dick (1954)]. Therefore it is understandable that w/c ratio generally has a marked effect on ionicdiffusion [Page (1981), Lambert (1984)]. This has been attributed to the influence of the w/c on thepore structure of the hydrated cement, and implies that controlling the w/c ratio in concrete is aneffective approach to mitigate the corrosion rate of steel in concrete. Hawkins et al (1996) havereported greater corrosion current densities at higher water-cement ratios.


64Research Report :332

These days, much improvement has been achieved in concrete technology, and low permeabilityconcrete with increased strength properties have bet:1I Jeveloped throughout the world. Bindersincorporating supplementary cementitious materials, such as slag, fly ash, and silica fume, areincreasingly used with water reducers and superplasticisers to enable a low waterlbinder (wlb) to beachieved for manufacturing dense, impermeable concretes, with much refined microstructure andimproved strength. For example, the w/c ratio of so-called OSP concrete is only 0.15 [Aarup(1996)]. The materials used in constructing piles, columns, crossheads, abutments and prestressedbeams of a new bridge [Andrews-Phaedonos (1998)] had a w/c ratio of only about 0.36. Ifconcretes with these improved formulations are placed and cured correctly, then the chance ofbuilding a durable structure is considerably increased.

8.2 Admixture Control

It is a common knowledge in the field of concrete technology that additives amI ct:lnent replacementmaterials, such as fly ash, blast furnace slag, and silica fume, react with the Ca(OH)2 produced as aresult of cement hydration, to produce additional CSH and other hydrates in the concrete, whichwould reduce the pore size and even block pores. This would result in an improved microstructure,low permeablility and increased durability, in addition to improved strength properties.

Blended cement concretes containing blast furnace slag, fly ash, and silica fume had improveddurability in marine environment, and their service lives were estimated to be several times longerthan the service life of plain portland cement concrete [Polder (1996a), Bijen (1993), Sagues (1997),Berke (1994), Chang (1997)]. For Example, incorporation of silica fume in concrete reduces waterabsorption and permeability, thus water penetration, chloride and carbon dioxide diffusion becomemore difficult [Loland (1981)], which is beneficial to the corrosion resistance of steel in suchconcretes.

In general, high resistance to the ingress of chloride ions, and against AAR and sulfate attack can beachieved with these materials. In a potential monitoring experiment, blended cement concrete,compared with portland cement concrete of equal strength, took longer to reach the critical corrosionpotential criterion for severe corrosion [Baweja (1994), Baweja (1995)].

It was reported [Hussain (1994)] that corrosion initiation time for steel reinforcement in blendedcement concrete containing 30% fly ash was about twice longer than that in the corresponding plaincement concrete, even though the OH' concentration in pore solution of the fly ash blended cementwas lower than that in the corresponding plain cement. This might be due to the following facts:Firstly, the unbound chloride ions in pore solution were decreased with the partial replacement ofcement by fly ash. Secondly, 30% fly ash blending refined the distribution of pore size, and theaverage pore radius was reduced from 240 to 166 A, so the permeability of concrete was reduced,and the chloride diffusivity was also reduced by about five fold. Thirdly, the electrical resistivity ofconcrete was found to be increased nearly 2.2 times with the fly ash addition.

Similar effects of fly ash on the binding capacity of chloride ions was also reported by Kayyali et al(1992) who found that the oxygen diffusion in a silica fume concrete is about the same as in anordinary portland cement based concrete, but the chloride diffusion is greatly reduced when silica isadded.

Studies by Ohir et al (1992,1993,1994) have revealed that the inclusion of a pulverised fuel ash inconcrete could also reduce the chloride diffusion rate. Ngala et al (1995) found that addition of PFAdecreased the rates of diffusion of chloride ion and dissolved oxygen. These might be due tochanges of concrete microstructure, chloride binding capacity, and pore solution chemistry.

Ground granulated blastfurnace slag (GGBS) has a high chloride binding capacity, and is used toimprove the resistance of concrete against chloride ingress. This property arises from the high



aluminate content of GGBS, which reduces the quantities of Friedel's salt [Dhir (1996)] that mayform in the concrete. Dehghanian et al (1997) studied the influence of blended cement concrete onchloride diffusion rate; they concluded that blended cement concrete containing up to 30% slag andwith a water-cement ratio of 0.45 did not affect the chloride diffusion rate at an early age. However,the diffusion rate was reduced with concrete age..

It should be noted that some chemical additives have detrimental effects on the corrosion of steel inconcrete. In particular, the additives containing chloride can act as catalysts and accelerate thelocalised corrosion on reinforcing steel. For example, the accelerator CaCh is well known andexperimentally proven for its potential adverse side effect on the corrosion of steel reinforcement.These: sllhstanr.es shQuld be avoided or reduced in practice.

8.3 Design of Cover Concrete

Increasing the thickness of the cover concrete is another simple measure for corrosion prevention[Vassie (1996a)]. The thickness of the cover concrete determines the time for aggressive species toreach the steel rebar in concrete. The penetration of chloride or carbon dioxide is to a large extentdiffusion controlled, and roughly follows a square root of time function. If the cover concrete wasincreased from 40mm to 100mm, the time taken for carbonation or chlorides to reach the level of thereinforcement would increase by a factor of 6 fold [Arya (1996)]. Therefore, the service life ofreinforced concrete structures can be extended greatly simply by increasing the thickness of thecover concrete [British Standard Institute (1992)].

The normal thickness for most structures is around 50 mm [AS 3600 -1994]. Unfortunately, not allparts of structures strictly follow the designed cover thickness. Field studies [Morgan (1982),Marosszeky (1987)] showed that encroachments on specified cover did occur and were widespread;62% of the buildings surveyed had cover thicknesses less than specified. This is an obvious cause ofcorrosion problem with structure. If improvement could be made in this respect, i.e., compliancewith specified cover thickness, then the corrosion problems would be much less in the future.

A well compacted and defect-free concrete tends to be resistant against corrosion of steelreinforcement [Aarup (1996)]. In fact, there are no defect-free materials, and almost all theinternational codes for reinforced concrete design are concerned with permissible crack width. Thedetrimental effect of cracks in concrete on corrosion of the embedded reinforcement is obvious.Cracks in concrete enhance the corrosion susceptibility of reinforcement compared with anuncracked concrete [Ohno (1996)]. Anodic dissolution of steel can also be facilitated in crackedconcrete. Recently, Arya et al (1996) reported a newly designed supercover concrete which couldsignificantly reduce or eliminate the incidence of reinforcement corrosion in concrete structures.Glass fibre reinforced plastic (GFRP) rebars was used at nominal cover depths to control surfacecrack widths while conventional steel was used at deep cover depths, providing tensile strengthwithout being subjected to corrosion. It has been demonstrated that the GFRP is effective inreducing surface crack widths, but not effective in enhancing structural strength.

8.4 Good Practice

The microstructural defects like capillary pores and cracks in concrete, are to il greilt extentdependent on the casting practices and curing process. The hydration process of cementsignificantly influences its porosity and permeability. It was found [Burchler (1996)] that theporosity for a hardened cement paste changed from 29% at age 40 days to 25.8% at age 296 days.

Cracks in concrete could be produced due to bleeding effects, rapid drying of the exposed surface offresh concrete, temperature difference in the core and surface of a freshly cast concrete element,shrinkage of hardened concrete, freeze/thaw cycles and external seasonal temperature variation, etc.These defects can be significantly reduced by good practices.



Various measures" are usually recommended lo" be employed together. For example, in the. recommendation of a strategy for neWcOhsttuction for Florida Deparltllelll uf Ttansportation,Sagues et al (1997) recommended that the formulations for highly aggressive environments specify atotal cementitious material content of 444 kg/m3

, requiring between 18% and 22% class F fly ash.cement replacement, and a maximum w/b ratio of 0.41. For concrete cover thickness, a minimum of100mm was recommended, and for each high strength application (for example" in driven piles), anadditional 8% microsilica cement replacement was specified. The addition of 22 11m3 of calciumnitrite corrosion inhibitor (30% by weight solution) was specified for above-water marinesubstructure members that cannot meet the 100mm cover requirement.


67 .

Research Report 332

9. Improvement of Reinforcing Materials·

As can be seen in Figure 1, the most import reactions i6 and i3 occur directly on the surface ofreinforcement. This means that the properties and surface state of the reinforcement would havedecisive effects on its corrosion rate. The use of a good quality material with a high corrosionresistance (e.g., stainless steeQ is undoubtedly the safest and most reliable way to ensure a lowcorrosion rate at all times. Although this is an expensive option in terms of initial cost, it could bemore cost effective conside:rine the: lif~-cyde cost of the structure. The use of coated steel bars isanother option. However, coating only increases the corrosion resistance on the surface ofrcinforcement so long as it remains intact. Once the coating breaks and the substrate is exposed,corrosion attack may be concentrated on the broken areas if the coating is an anodic barrier (thecoating has more positive pote~tial than the substrate steel). In some cases, the concentrated attackmight be much more severe than on an uncoated reinforcement. Therefore, care should he exercisedon the choice of the steel rebar and the coatings on the rebar, and consideration should be given tothe life-cycle cost-effectiveness.

9.1 Corrosion Resistant Steel

Stainless steels are well known for their high corrosion resistance. Stainless steel and stainless steelclad have been used in Europe for many years as corrosion resistant reinforcement in concrete.Laboratory examinations and field tests carried out world wide have shown that many kinds ofstainless steels were much more resistant against corrosion than plain steel [McDonald (1995)]. Thecorrosion rate of stainless steel is a few orders of magnitude less than that of plain steel, and chloridetolerance of stainless steel is several times higher than that of carbon steel rebar in concrete[Sorensen (1990), Rasheeduzzafar (1992)]. Austenitic steel has the best performance, with almostno corrosion even in relatively high chloride environments, and after 22 years of exposure of 302,315 and 316 stainless steel reinforcing bars at an industrial site in east London, no corrosion wasobserved in concretes with up to 3.2% chloride by mass of cement [Cox (1996)].

However, the initial cost of using stainless steel is much higher than plain steel. In most concretestructures, plain steel can last long enough to satisfy the designed service life, so it is not worth usingthe expensive stainless steels. Only in some very important reinforced elements, for which nocorrosion risk is permitted, would stainless steel be considered. In this case, the long-term costwould not increase significantly due to fewer repairs and less maintenance costs. Cutler et al (1998)has reported that for concrete highway bridges the selective use of stainless rebar increases overallproject costs by only 0.5% to 20% compared to conventional steel, depending on the size andcomplexity of the bridge. However, the final life cycle cost of a structure using stainless steel ismuch lower than that using carbon steel.

It is not a good idea to mix mild steel and stainless steel together in a structure to lower the overallcost, because this could induce galvanic corrosion. If different kinds of metallic materials have tobe used together, then they should be carefully insulated from each other.

In addition to Stainless steels, carbon fibre LClarke (lYY3)J and glass fibre lNanni(l993)] are also.non-corrodible reinforcement materials for concrete. Carbon fibre is more expensive and glass fibreless expensive than 316 stainless steel.

9.2 Surface Treatments and Coatings on Reinforcing Steel

Surface treatment:



Surface roughness produced by a slight layer of rust which is well adhered to the underlying steelbefore it is used helps the bond between the steel amlwlK;rele [Maslehuddin (1990)]. Treatmcnt ofthe steel surface with water before incorporating it in concrete also increases the bond strength [Fu(1996)]. Although slight surface corrosion increases the bond strength, severe corrosion is verydetrimental to the bond strength of steellconcrete[Fu (1997)].

Zinc coating: '

Galvanised steel is one of the most widely used coated reinforcements, and is of relatively low costwhen compared to other protection systems. The attractive advantage of the galvanised steel is thatdefects in the coatings are not critical to the corrosion resistance of reinforcement [Broomfield(1997)]. This is because the zinc coating acts as sacrificial anode and provides cathodic protectionto the reinforcement if the steel is exposed in damaged areas.

The corrosion behaviour of galvanised steel in concrete has been studied extensively in thelaboratory and in the field [Arup (1979), Cornet (1968), Cornet (1981), Wilkins (1980),Rengaswamy (1984), Andrade (1985), Sergi (1985), Shimada (1985), Maahn (1986)]. Successfuluse of galvanised ,rebars in structures has also been reported in the literature [Slater (1979a), Stark(1980), Treadaway (1980a)]. It has been shown that galvanised steel can tolerate a higher chlorideconcentration than conventional steel [Surbramanian (1988)]. However, in laboratory concretescontaining substantial concentrations of cfilbiide, the behaviour of ga1vanised reinforcement hasbeen somewhat variable [Arup (1979), Treadaway (1988)]. The reason might be that the simulationin the laboratory does not reflect the actual cbnditions [Swamy (1990)]. Thangavel el al (1995)investigated the performance of electrogalvalised steel in concrete under immersion and weatheringconditions, and found that zinc performed better in atmospheric exposure. Comparatively speaking,galvanised bar is more resistant to carbonation-induced corrosion than to the chloride inducedcorrosion [Broomfield (1997)]. High corrosion rates tend to occur in concrete with high alkalicontent which might be due to the fact that Zn becomes unstable in highly alkaline media.

In comparison with severe attacks on uncoated steel under aggressive conditions, zinc coated steelexhibits delayed corrosion initiation, and this delay was considerable in the case of uncrackedconcrete [Fratesi (1996)]. Morgan (1993) believed that the use of zinc coated reinforcementprovided a considerable degree of protection, to the extent that it was considered adequate withoutcathodic protection. For the zinc coated reinforcement, only 10% of the cathodic protection currentrequired by bare steel, is enough to reduce the dissolution of the reinforcement down to 1-'3% of itslevel for uncoated steel [Morgan (1993)].

Views on the influence of galvanisation on the bond strength between rebar and concrete seem to becontroversial [Bird (1964), Rehm (1970)]. The observed loss of adhesion is usually ascribed tohydrogen evolution on the rebar surface resulting from the attack on the zinc coating by the hydroxylions in the pore solution [Fratesi (1996)].

Epoxy coatings:

Epoxy is another popular coating on reinforcement. The epoxy coatings are normally applied toreinforcing steel bars before they are cast in concrete. Usually a fine epoxy powder is sprayed andfused onto heated bars. then cured and quenched for use. The advantage of epoxy coating is itsresistance against chloride penetration and its excellent adhesion on the steel. However, there mayexist some pinholes and/or other defects in the coating which would reduce its resistance againstcorrosion. Moreover, further damage may also occur on the coating as a result of handling andplacement practices. These would be the defects that would initiate corrosion in the future, inaggressive environement.



Based on earlier research, work [Clifton (1974), Clifton (1975), Clear (1983)], epoxy coatedreinforcement (ECR) has been used in North America as a corrosion prevention method againstchloride-induced corrosion of reinforced concrete structures exposed to deicing salts or marineenvironments. The coating is expected to prevent moisture and chlorides from reaching thereinforcing steel surface [Virmain (1997)], thereby reducing the risk of corrosion of thereinforcement. Since the introduction of epoxy-coated steel in concrete systems in the late 1970s,more than 100,000 structures have been built in the USA and Canada using epoxy-~oated rebar[Broomfield (1997)].

However, several cases of corrosion 'damage to structures containing epoxy-coated rebar have beenreported in the 1990s, which have involved litigations. Sauges (1997) detected corrosion occurringunderneath the epoxy coating on the reinforcement. The corrosion damage may have resulted fromimperfections in the coating or damage to it during placement. Some disbondment may take placeduring fabrication, and the defects may be multiplied during handling. Furth'er disbondent andbreaks in the coating develop during exposure to the construction yard environment. Additionaldamage to coating may occur during the erection of the reinforcement frame as well as concretingprocesses such as vibration and other -mechanical action. Chloride ions can penetrate these defectsprior to the time of incorporation of the reinforcement in concrte. Chloride ions may also penetratethe concrete and stay in the disbonded crevices. The occluded cell corrosion will exhibit the classicsymptoms of local acidification and chloride accumulation, with autocatalytic propagation (crevicecorrosion). Galvanic macro-cell effect created by other cathodic ,areas in the substructure, can befacilitated by the low concrete resistivity, and is also an aggravating factor [Sagues (1994)].Morgan (1993) has stated that epoxy coated steel bars can dramatically reduce the cathodicprotecting current requirement. Sagues et al (1997) also believed that sacrificial anode cathodicprotection would be successful as it cost-effective maintenance alternative for corroding epoxycoated reinforcement.



10. Repair of concrete

Concrete repair (RDC) covers a wide spectrum of restoration ranging from a simple patching orcoating to mechanical strengthening and. replacement of some structural components. For thedamage caused by corrosion of reinforcement, the repair of concrete is also a method to prevent orretard further corrosion. This may involve the installation of corrosion protection and monitoringsystems as well as repair of the concrete itself.

In this section, we will discuss the more commonly applied repair methods for structures sufferingfrom corrosion damage, which include patch repair, overlay and epoxy injection. Regardless of therepair option chosen for a particular case, issues related to preparation work and selection of reparimaterials should also be examined carefully.

10.1 Preparation for repair

Before repair is undertaken, it is important to identify the extent of the problem and remove thedamaged concrete'. There are several methods for removing concrete [Vorster (1992)], such aspneumatic hammer; hydrojetting, and milling, etc. The choice depends on the location, forms andextent of damage and available budget.

Generally, there are a few import points in relation to achieving a good repair outcome:

1) The damaged concrete (unsound, cracked, carbonated, or cr polluted, etc.) around the steelbar should be completely removed;

2) The steel bars should be cleaned if they have been exposed after removal of the coverconcrete;

3) The repair materials should be compatible with the original matrix.

Whether the concrete at the damaged areas has been completely removed is sometimes critical to theeffectiveness of the repair. Insufficient removal of chloride contaminated or ·carbonated concretecould result in the macro-cell corrosion of steel bars [Schiessil (1996)]. This has frequently beenobserved in the laboratory and in the field.

Vassie (1989) found that the durability of conventional repairs was very sensitive to the steel surfacecondition, i.e., to the extent of cleaning of the corroded steel. If chloride contaminated corrosionproducts are not removed during the repair operation, it is possible that the reinforcement willcontinue to corrode, and none of repair methods applied in this situation would be effective inpreventing further corrosion.

The steel/mortar interface after repair is extremely important to the durability of the repair system[Vaysburd (1993)]. The protection effect expected from the mortar is dependent on the bufferingaction of the lime-rich layer at the steel/mortar interface and the bond between the steel and therepair mortar. A dense steel/mortar interface with good bond will block the access of aggressiveagents to the steel and prevent the formation of corrosion products. When voids are presenl al lheinterface, all protective mechanisms provided by the mortar are weakened and corrosion is promoted[Pedersen (1997)].



10.2 Repair Materials

Various repair materials, such _as grouting materials, polymer-modified mortars, polymericcomposite materials, etc., have been developed and used for the rehabilitation of reinforced concretestructures.

Demura et al (1998) reviewed the materials used for the rehabilitation of reinforced concretestructures in Japan, and proposed a selection procedure for repair materials and systems (Figure 21).

DesalinationCathodic protection

Decision of Strengtheningrehabilitation

+methodsCrocldilling I ISurfoce cooting I I Potching l0ther methodsI

I, J, J, J, J,*

Basic material ~ Grouting,I Isurface coatingI I . I Other I IType ofdesign materials ,materials Patching materials structure

I I I

+Decision of Factors for basic material r1flsjgn ,

requirements II Environ~ental II Execution II Lifetime of I Specific values(materials and

condition condition , structure of properties ofexecution)

1= repair materials

t ,Determination of I==- Required 08rlormanc8 of rePair materials

required perform- Basi~ ; II Protecti~n p~rformance II Workability II Durability It--anca of materials

1= properties to detenoratlon factors

t ,ISelection of type of repair materials I1= Determination

Selection of,

of items of testapplicable repair ISelection of commercial repair materials I and analysis of

materials j properties ofmaterials

1= II Determination of repair system materials and systems

X~amination 1 Test & analytical evaluation of repair materials & systems for required~

performance, or comparison of test results with specified values

I~amination 2 Evaluation of financial aspects of repair materials and systems

fDetermination of repair material or system l,Re-examination a8 rehabilitation method l ,

Figure 21. Repair materials and systems for reinforced concrete structures [Demura (1998)]

Polymer-modified mortars are amongst the most popuiar repair materials, because of their superiorprotective properties with respect to carbonation and chloride ion pen~tration [Ohama (1988),Karbhari (1998)].'



f'olymer-modified mortars and rust inhibitors have alsu been used together for repair pruposcs[Ohama (1991), Prowell (lY93)). Demura et al (1992) reported that tItel:UlTusiuJl-iJlIJititionperformance of polymer-modified mortars was improved by the addition of Ca(N02h- Fieldapplication and short-term corrosion performance of six trial installations of two inhibitor-modifiedconcrete systems were reported by Prowell et al (1993). The trial installations were on the deck andsubstructure components in a range of environments. The results showed that the inhibitor-modifiedconcrete systems could be successfully applied by construction and maintenance personnel with aminimum of technical supervision.

Fibre-reinforced polymer matrix composites are also increasingly being considered for use in therehabilitation and renewal of concrete [Karbhari (1998)] due to their good corrosion resistance andoverall durability.

10.3 Patch Repair

Patch repair (PAT) is one of the most commonly chosen repair techniques [Cronin (1992)]. Theproperties of the selected repair materials and their compatibility with thosc of the substrate concreteis very important to the success of the repair. Low early-age-shrinkage is an important requiredperformance characteristic of the repair mortars [Blankvoll (1997)]. Scerri et al (1997) haveinvestigated the performance criteria of repair mortars and recommended evaluation methods.

The main concern in patch repair is the introduction of "incipient anode" [Gulikers (1992),Broomfield (1997)], which is caused by the electrochemical difference in the patched zone and thesurrounding areas (Figure 22). The steel bar section in the patched zone acts as cathode and thesections in the surrounding areas become anodes, termed "incipient anodes". Hence, the steel bar inthe surrounding areas will be subjected to new corrosion threats. Alternate drying and wetting canencourage the formation of such macro-galvanic corrosion cells, and intensify the corrosion attack[Gulikers (1992)]. .

Figure 22. Incipient anode caused by patch repair [Broomfield (1997)]

Laboratory experiments [Schiessil (1996)] have indicated that macro-cell corrosion can occur in thevicinity of a repaired reinforcement. The experiments also suggested that in the case of chlorideinduced macro-cell corrosion, all areas in which the critical chloride content is exceeded must beremoved, irrespective of whether damage was visible. Blankvoll (1997) recommended that theboundary of the repair area be cut at 45° rather than 90°, to provide a larger contact surface area withthe substrate.

Natural exposure tests of repaired specimens in the UK [Hollinshead (1996)] has shown that theinterface between the repair material and concrete was vulnerable to carbonation and chlorideingress, which then spread along the reinforcement, causing corrosion. Pedersen (1997) also clearlydemonstrated the potential weaknesses of transition zones between repaired and non-repaired areas.These weaknesses can be regarded as potential causes of macro-cell corrosion, particularly for thehand-applied patches which contain unfavourable amounts of air voids.



10.4 Overlay

Overlay (OL) is another repair method to prevent further corrosion in a damaged concrete, and haswidely been used in the field [Cronin (1992)]. In this technique a new layer of concrete is placed onthe existing concrete. Overlays may absorb some of the chlorides in the substrate concrete, reducingthe chloride level at the concrete. Overlays can be polymer modified concretes, low slump denseconcrete, or micro silica concretes.

The first latex modified overlay was used on a concrete bridge deck in 1957, [Cardone (1957)], andthe first use of low-slump dense concrete was reported in 1977 [O'Conner (1977)], to reduce the rateof chloride ingress, It was also expected that these overlays could reduce the movement of oxygenand moisture, hence'the corrosion rate of reinforcement. By 1989, about 37 states in America hadused such treatments on their concrete structures [Chamberlin (1989)].

The use uf thin, high performance concrete overlays to rehabilitate corrosion damaged concretebridge decks in the US and Canada has been very successful over the last 20 years [Chamberlin(1994)]. Experience suggests that these treatments have the potential for extending the service lifeof the wearing surface. These treatments are particularly effective, and service life extensions of 30to 50 years is likely, if the removal criteria for the substrate concrete are based on half-cell potentialrather than present damage, and the removal of chloride contaminated concrete is extended beyondthe rebar and the substrate is sandblasted to remove micro-cracking [Chamberlin (1994)].

10.5 Injection

This is a relatively low-cost quick solution to stabilise the structure and to extend its service life.However, this method may not be effective with actively corroding reinforced concrete, and needs tobe used in combination with other measures, such as inhibitors and coatings.

Injection (INJ) of epoxy resin into cracks can bond the delaminated concrete and seal the cracks,thus restricting further ingress of water and chlorides [Cronin (1992)]. Epoxy injection is often usedfor rebonding cracked concrete. If cracking is due to delamination, resulting from corrodedreinforcing steel, this method not only fails to eliminate the cause of the problem, it also hinders theuse of CPo However, epoxy injection into cracks perpendicular to the concrete surface is compatiblewith CP [Eltech Research Corporation (1993)].

Corrosion 'inhibitors can also be injected into cracked concrete to prevent steel corrosion. Lowpressure, injection [Tomosawa (1992)] can enable nitrite corrosion inhibitors to rapidly seep intolarge areas of concrete. This is an effective way of corrosion prevention of rebars in concrete inwhich minute cracks are occurring on the surface of the concrete. However, if cracking damage istoo serious, replacement would be a cost effective option [Cronin (1992)].



11. Comparison of Corrosion Prevention Techniques

Each corrosion prevention technique has its .. own features, including 'advantages and limitations.They are compared in Table 12. In practical application, it is important to make use of theiradvantages and avoid their shortcomings.

fi Id (1997)][Bf t h .fT bl 12 Ca e . omparIson 0 corrosIon preven Ion ec mques room Ie

technique advantages limitations

Cathodic • non-destructive • limited on prestressed, poorprotection • treat large area of structure conductivity, poor electrical

• cheaper than conventional repair continuity, coated, or AAR sensitive

• long lasting concrete structures

• no vibration or noise • require maintenance

re-alkalisation • non-destructive • can not be used on prestressed, poor•. treat large area of structure conductivity, poor electrical

• cheaper than conventional repair continuily, coaled, or AAR sensitive

• treatment lasts 2-4 weeks, then concrete structures

no maintenance required • no available standards or

• no vibration or noise specifications.

• long-term effectiveness unclear

• some risks associated with cathodicpolarisation, such as AAR, Hill, anddisbonding of steel

• unknown problemselectrochemical • non-destructive • can not be used on prestressed, poorchloride removal • treat large area of structure conductivity, poor electrical

• cheaper than conventional repair continuity, coated, or AAR sensitive

• treatment lasts 6-8 weeks then no concrete structuresmaintenance required • no available standards or

• no vibration or noise specifications.

• long-term effectiveness unclear

• some risks associated with cathodicpolarisation, such as AAR, Hill, anddisbonding of steel

• unknown problemsinhibitor • no significant influences on the • long term effectiveness unclear

properties of concrete .. unknown length of time for

• no maintenance after addition inhibitors to reach reinforcement if

• compatible with other prevention not added in admixturemeasures, such as admixture, • effectiveness dependent on thecoatings, and concrete repairs quantities of inhibitors; inadequate

quantities could accelerate corrosion

• unknown effective amount requiredcoatings on • non-destructive • low wear resistance

concrete • low maintenance • durability uncertain

• cheap • not very successful for corroding

• on selected areas systems



concrete quality • no maintenance .. only on structures under constructioncontrol • permanent effects • affecting workability

reinforcing • low cost for coatings on steel • expensive to use stainless steelmaterials • no maintenance reinforcement

• very low corrosion risk for • bond strength may be affectedstainless steel reinforcement • only in structures under construction

concrete repair • can be used on prestressed, AAR .. compatibility between repairs andsensitive, poor conductivity,. or the original matrixpoor continuity concrete II "recipient anode" corrosioncomponents • relatively expensive

• selected area

• able to rehabilitate severelydamaged structures

As different techniques have different advantages and limitations, only one of them alone may notmeet the desired protection requirements of a real structure. Application of more than two methodswhose drawbacks can be well compensated for would be a good option. In fact, there are only veryfew cases of single prevention techniques being applied on a structure in the field. In most cases, amulti-prevention-measure is used to ensure the durability of reinforcement in concrete. For example,in the construction of a new bridge exposed to coastal environment in Australia, apart from usinggood quality concrete materials and enforcing strict practice ensuring no chloride to be introducedinlo the structure, impregnation sealer and top coatings were ·also simultaneously applied on thesurface [Andrews-Phaedonos (1998)]. In fact, in many other corrosion damaged structures, optionssuch as, local concrete repair, local concrete repair plus coatings; cathodic protection;electrochemical chloride removal plus coatings have simultaneously been considered [Gedge(1996), Armstrong (1996), Cilason (1998)].



12. Prevention Strategy

Corrosion prevention is a complicated engineering problem [Purvis (1994), Rilem (1994), Weyers(1993)], and concerns not only technical methodology, but also economical and social issues. Thereis no universal methodology or strategy for corrosion prevention in reinforced concrete structures[Pullar-Strecker (1987)], and depending on the purpose of prevention, the approach taken couldvary considerably.

Development of a prevention strategy for a particular structure should consider the mechanismscausing the particular problem, the condition of the structure, the exposure environment, theexpected service life, and a costlbenefit analysis of the prevention approach. Some approaches arebriefly outlined below.

12.1 Prevention Based on General Corrosion Degree and Environment

Generally, the prevention methodology depends on the. corrosion state of the structure and theenvironment to which the structure is exposed, and in many cases, several different protection.options are available for a structure (Table 13).

Table 13. General corrosion prevention techniques suitable for reinforced concrete structures, h d f 'd . d'n tWIt variOUS eerees 0 corrosIOn amaeem I eren enVironmentsr:s:: Under no detectable slight corrosion severe corrosion

construction corrosion damage damageEnvironmen

damage .. -~ "-_. -~.

underground CQ, RS, CR INH, CP,CS,OL CP,ECR,CS, CP,ECR,OL,CS,CP OL,PAT CS,INJ

submerged CQ, RS,CR, CP, CS, OL, INH CP, ECR, PAT, CP, ECR, PAT,INH,CS,CP OL,INH,CS, OL, INJ, CS,

INHatmosphere CQ,INH,RS, CG,OL,CP RA,PAT,OL, RA,PAT,OL,

CR,CG,CP INJ, CG, CP, INJ, CG, CP,INH INH

wetting-drying CQ,INH,RS, CG,CS,OL, ECR, RA, PAT, ECR, RA, pAT,cycles CR, CG, CS, CP INH,CP OL, INJ, CG, CS, OL, INJ, CG, CS,

CP,INH CP,INHCG coatings against gas permeationCP cathodic protection techniqueCQ concrete quality controlCR coating on reinforcing steelCS coatings against solution penetrationECR electrochemical chloride removal techniqueINJ injectionINI-I inhibitor techniqueOL overlayPAT patchRA realkalisation techniqueRS corrosion resistant steel



Different damage intensItIes would require different prevention strategies. Seriously damagedstructures need to be rehabilitated, then protected from further corrosion attacks, whereas limiteddamage may only require simple, localised repair, followed by corrosion protection, and for intacr ornew structures corrosion protection measures could be enough. Sprinkel (1993) summarised somerapid protection, repair and rehabilitation methods for concrete bridge decks. In that report, polymeroverlays, sealers, high-early-strength hydraulic cement concrete overlays; and patch repair werecompared from the standpoint of performance characteristics and service life. (Table 14).

Table 14. Some rapid treatments for bridge deck protection, repair, and rehabilitation[S . k I (1993)]iprm e

Purpose Methodology and materiall'equin:d

protection 1. asphalt overlays on membranes2. polymer overlays3. sealers

repair, 1. asphalt overlays2. crack repair and sealing3. High-early-strength hydraulic cement concrete overlays4. joint repair5. patching with high-carly-stength hydraulic cement concrete6. patching with polymer concrete7. patching with steel plate over conventional concrete "

8. polymer overlays

rehabilitation 1. asphalt overlays on membranes and patches2. high-early-strength hydraulic cement concrete overlays3. polymer overlays'

12.2 .Prevention Based on Corrosion Cause

The effectiveness of corrosion prevention techniques would depend on the mechanisms causing aparticular corrosion problem. Hence, it is essential to detect and understand the main cause ormechanisms of the corrosion damage before embarking on a protection or prevention strategy.

Broomfield (1997) has provided a very good summary in this regard (see Figure 23 and Figure24).



Designsystem

Install

·Chooseanode

MaintainCP

system

( ~\

(Test and \

\ commission )

'" /

no

./' ..........Rebar>

cont~nuity

no

yes

noConsider

eleetrochemidii .'?

no yes

Mainselectricityavailable

?

CIRem CPAny

reasonfor choosing

CPorCIremoval?

CIRemCP

Comparecosts

"-

\1I

yes

Start

A/ Short ~

1--...""""" life?

yes no

CO:v~d~r.....

? .,

~yes

IsCI Remeconomic?

Treat withchlorideremoval '

'----_//

no

no

....// "".4 \/ Patch yes

.and

-).~seal

no

ConsiderCIRem

?

(

Figure 23. Prevention methodology for chloride induced corrosion [Broomfield (1997)]



Start

no

"

Rebafycontinuity? .

Iscarbonationadvanced

?

/' Isrealkalization.

economic?

yes yes

'"-----../~..life?

no

no

yes

no

Check costbut likelyoutcome

is

Patch andapply

anticarbonationcoating

Treat withrealkalization

--_._------

Figure 24. Prevention methodology for carbonation induced corrosion [Broomfield (1997)]

In practice, the selection of prevention methods may vary from case to case. For example, thestrategy illustrated in Figure 25 was adopted in the repair work for concrete jetties deteriorated bysalt attack [Fukute (1998)]. Schiessl (1997) also presented an approach to stop corrosion from anelectrochemical point of view (Figure 26). He believed that there were no generally proven methods


80

Research Report 332

to stop the cathodic process. Olh~r siuular strategies havc also bcen proposed by others regardingprevention of detenoration caused by chloride indu(;~d wlTusion .[Miyagawa (1998)].

stanCase3

C

IDeterioration is I IDeterioration I sInot observed observed

IExanination of chloridelconcentration in concrete

Evaluation of Surface chlorideconcentaration(CO) and apparentdiffusion coefficient(D)_. .......

T

Si IIU Iat ion of ChlQride concentratiOn inconcrete by Fick' s diffusion law

ase1 • + Case2Chloride Ion around Chloride ion aroundrebars is slla II er than rebars is larger thanthreshold Values during threshold Values duringthe life the life

~3j~ulation of Chloride ,onc.ntrlti~n in concrete Iby Finite differential nethods

l ...Chloride ion around rebars Chloride Ion around

rebars is large r thanissll8ller thin threshold threshold values evenva Iues when the chloride though the chloridesupply froll the surface is supply frOIl the surflceterllinated Is 'terllinated

~ •Repair i s (l)Surface coatings (2) Sect ion recoveryunnecessary (3) Catodic protection

,j. ~.... 4 .. "W. •

Study on cOllbination of relledial treatllents fron perforllances and costs

Figure 25. Strategy for selection of rehabilitation methods

for structure attacked by chloride [Fukute (1998)]



Exampl•• or Technlqu..I· Bille Principle. J

I~M.pleclng lb. conllmlnated

concrlle by Ilkllllne repllr morblr

I I RStop thll R..lkallaatlon In the ea,e of

enodlc procil. I Rlp•••lntlon of the

~C1rbl?natlon Induced co"ollon

reinforcement

~\l' Chloride ••tractlon

\ ,~ C COltlngln tl'1e a,... of local "palr

\~Co.tlng of (hit If repDlrmortlr clnnot pfuwld.I'8lnlDrcement durable protection

,,;\ cia~ ......__ .

" Impr....d current 8yehiiiis" C.thodlc protlctloll I

""""""

..-----..J;Stop the

electrolytic prOC'1I11

w

Reduction ofthemollture content

of t .... concre'e

Provl.lon 01 m.mbrlln.. Dr clAdding,to ••perate the concrell IU"lce. from the outdoor envlronm.nt

Figure 26. Selection of prevention methodology based on electrochemical principles[Sc.hiessl (1997)]

12.3 Prevention Based on Available Budget

In most cases, budget limitations decisively affect the adoption of prevention techniques. Differenttechniques have different costs, and when there are several techniques available that are able toachieve the same goal with similar reliability, the cheapest one would always be considered first.

fi Id (1997)]. 1992 $USI 2 [8T hi 15 Ca e . omparatlve costin2s ID m room Iemethod on D or S repaired areaD=deck; S=substructure 0-500m2 500-1000ml ca.5000m2

~JOOOOm2

CP (D and S) 13-30 9-21 6-14 5-13ECR (D and S) 13-30 9-21 6-14 5-13epoxy coatings (S) 8-80 8-50 8-40 8other coatings (S) 8-32 1-25 1-20 1-12penetrating sealer (D and S) 4-44 4-34 2-36 4-6polvmethylmethacr. sealer (0 and S) 11-52 7-19 7-10 7inhibitors (D and S) 220-2000 120730 70-500 70-500latex mod. cement ov~r1av (part n) 250-1200 250-350 250350 250-350polymer mod. cement overlay (part 0) 300-1300 300-500 300-500 300-500LMCIPMC (full D) 25-175 25-65 30-55 30waterproof + asphalt (0) 7-45 6-25 5-12 10thin polymer overlay (0) 45-103 45-103 42-55 42-50microsilica overlay (D) 25-108 25-80 25-50 25shallow patch OPC (D) 110-1200 110-800 110-400 100deep patch OPC (D) 100-5000 100-800 100-700 100shotcrete (S) 100-9400 100-3000 100 100



However;the cost of each technique could vary significantly within a very wide range, depending onlocal conditions. Nonelhdess, Broomfield (1997) has provided comparative costings of sometechniques for bridge repair, as presented in (Table 15).

The above are· only the direct costs of the prevention methods. In fact, the calculation of indirectcosts would be more difficult if future maintenance costs, the effect of the protection on the servicelife ofthe structures and the associated social aspects are considered [Vassie (1996)].

12.4 Overall consideration

A comprehensive corrosion prevention project includes many aspects. Technically, the selectedmethods must be able to offer effective protection against corrosion. Economically, the costs shouldbe within the budget. Sodally, lhe irnpacts of the project on the environment, safety, psychology,ae.sthetics, etc, 'need to be carefully alJalysed. Therefore, a corrosion prevention project is actually acomplicated system, and an overall evaluation might be needed in the selection of the mostreasonable approach.



13. Concluding Remarks

Compared with other concrete deterioration problems, the corrosion of steel reinforcement concretewas highlighted relatively late, and the application of designed corrosion prevention measures is apractice only a few decades old. However, the development of various corrosion preventiontechniques that can mitigate the intensity and extent of corrosion of reinforced concrete systems hasbeen so rapid that a large number of reviews and monographs have been published in this field. It isencouraging to see that yesterday's laboratory experiment has become today's practical pilot trial inthe field, and this could quite probably be one of tomorrow's reliable techniques to deal with thecorrosion problems in reinforced concrete structures.

Cathodic protection might be the earliest technique consciously used in reinforced concrete toprevent the reinforcement from corrosion attacks. The core issue of this technique is a reasonabledistribution of the cathodic current to the reinforcements that require protection. The fundamentalaspect. of this technique is its anode system, which determines the distribution of the cathodicprotection current.

In the past few decades, the most noticeable development in CP has been its anode system. Variousanode systems have been developed which are flexible, feasible, and effective in concrete systems,and have performed very well. Particularly, the impressed current CP system has been proven to besuitable for use in most cases and capable of achieving the designed criteria. It is certain that in thefuture, more effective, long-lasting, low maintenance, and easily-applied anode systems will beemerging to meet various field requirements.

The second aspect worth mentioning in the CP development is the establishment of protectioncriteria which mark the maturity of this technique in field application. However, controversial issueswill remainin relation to the application of CP criteria. For example, whether the recommendedpotential shift is adequate and whether the depolarisation time (4-hour or 24-hour) is long enough in

. various circumstances. It is expected that further detail will emerge regarding the criteria, accordingto variations in field circumstances.

With regard to the possible side effects of CP, it seems that the risks of problems, such as HIE, AAR,and disbonding of rebars, are unlikely for conventional reinforced concrete, if reasonable protectioncriteria have been strictly set up and followed. However, great care is still needed when CP isapplied to a prestressed element, as uneven distribution of current density can still cause overprotection in some areas in the prestressed elements, leading to HIE of the prestressed steel.

Further development is expected to take place in computer modelling of CP for reinforced concretestructures. This has been intensively investigated for steel structures in marine environments, butthe modelling is much more difficult for concrete structures because of complicated reinforcementsystems. In the future, it may be possible to provide a reasonable distribution of anode systemstogether with automatic adjustment of power parameters through computer modelling and input ofsome essential concrete structure parameters as well as continually measured CP parameters. This isto say that a "intelligent" power supply system, which can automatically adjust the cathodic currentdensity with time, based on some feedback, will be realistic equipment in the future.

Technically, re-alkalisation and electrochemicai chloride removal are similar to CP, except for themuch higher cathodic current densities employed in RA and ECR.

In the past decade, a great deal of research work has been done in the laboratory, as well as some insitu pilot studies carried out in the field, regarding the feasibility, effectiveness and possible side


84neseareh Report 332

effects of ECR and RA techniques. Al:cunJillg to the results of short-tcrm laboratory tests, thereseems to be no doubt as to the effectiveness of these two tel:hni4Ues ill l,;uulItering the carbonationand chloride induced corrosion in reinforced culII.:rete. FUithermore, most of thc short-term fieldtests have shown somewhat promising results for the application of RA and ECR to real structures.

However, it is still too early to say that RE and ECR can now be widely used in the field, becausethere are still some uncertainty in the following aspects:

1) The long-term effectiveness of RA and ECR is still unclear, and there are still insufficientdata collection concerning their long-term performance. Therefore, some issues are stillunresolved, such as, the length of the beneficial effect after completion of the treatmentand whether repeated treatments are feasible and acceptable if carbonation or chloride"pollution" resumes;

2) The possible sidc cffeets of RA and ECR are still controversial issues. Theoretically, therisks of HIE, AAR and disbondment of steel should be high due to the high level of therequired cathodic current densities, particularly on prestressed concrete elements.However, different practical phenomena are usually observed, which indicate that the sideeffects are not always as serious as theoretically described. This also means that thecurrent understanding of those side effects are far from being adequate;

3) Since high current densities are employed in RA and ECR, there may be some otherunknown problems associated with sudl high current densities and this issue requiresfurther investigation.

Hence, in the future, research emphases may preferentially be placed on the above issues in order todevelop a better understanding of possible side effects. Consequently, based on sufficient field trialsand long-term data collection, reasonable standards regarding the treatment operations should bepossible to establish.

In view of the low efficiency of RA and ECR systems at the later stages of the treatments,development of more efficient methods will also be part of the future research in this area.

Calcium nitrite has been shown by a number of researchers to be the most successful inhibitorsuitable for use as an admixture. Other inhibitors, ranging from inorganic to organic compounds,have also been extensively investigated, and different methods of incorporation of the inhibitors inthe various systems have been tried. However, it seems that caleium nitrite is still the mostsuccessful inhibitor in reinforced concrete in terms of its inhibition efficiency and lack ofdetrimental effects on concrete properties.

Various questions still remain unanswered on the use of corrosion inhibitors in concrete, such ashow to ensure that the inhibitor will reach the rebar in a reasonable time? What concentration of theinhibitor is needed and can be ;'lchieved at the interface of steel and concrete? Are there anypotentially deleterious effects of inhibitors on the structures? Is surface application of someinhibitors acceptable, and does it produce a uniform and sufficient concentration at the rebar level?

Future efforts should be directed to aspects such as development of new efficient inhibitors for usewith existing damaged concrete systems; specification of the application of inhibitors in concrete inorder to achieve the best efficiency in the cUlIl:l'ete; and development of ncw inhibitor applicationmethods (such as in combination with electric fields) to make the inhibitor penetration more efficientin concrete systems. May be the development and use of chloride scavenging agents in the concretewould be an interesting aspect in the future.

As very important corrosion prevention measures, coating systems have been widely studied in termsof their abilities to retard the ingress of water, chloride and carbon dioxide. Pore-lining impregnants,surface sealers, thick coatings, and waterproofing membranes have been developed and used on field



structures. The lIlulLi-layer waling systems, which combine the advantages of all the layers togetherto achieve a more effective corrosion prevention, have received considerable attention, and beenused in field structures.

In the future, coatings will continue to be among the most important corrosion prevention measuresfor reinforced concrete structures. It can be expected that more effective coating systems compatiblewith concrete surface conditions will be emerging in the market. In addition to the good resistanceagainst ingress of aggressive substances, coating systems with special functions are. likely to bedeveloped.

Also, in the future, a successful strategy for corrosion prevention would essentially depend on theselection of appropriate materials and techniques prior to the construction of a new structure, ratherthan On sulving corrosion problems encountered as a result of inadequate attention to these issues.The concept will be widely accepted by then that it is more effective to apply corrosion preventionmeasures on a new structure even at higher costs than on an existing damaged structure aftercorrosion attack has become a threat to its service life.



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Buzzard

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admixture controlcover concretecoatings against gas permeationcoating techniquecathodic protection techniqueconcrete quality controlcoating on reinforcing steelcoatings against solution· penetrationelectrochemical chloride removal techniqueimpresseu cune.it anodeimpressed current cathodic protectioninjectioninhibitor techniqueoverlaypatchpracticerealkaltsahon techniquereinforcement materialrepair of concretecorrosion resistant steelsacrificial anodesacrificial anode cathodic protectionsurface treatment for concretesurface treatment for steelwater/cement ratio control

15. Appendix--SymbolsADCCCGCOCPCQCRCSECRrCArccpINJINHOLPATPRARARMRDCRSSASACPSTCSTSWC


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