Corrosion Of Metals In Concrete.pdf

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CORROSION OF METALS 222R-1

This committee report has been prepared to reect the stateof the art of the corrosion of metals, and especially steel, inconcrete. Separate chapters are devoted to the mechanismsof the corrosion of metals in concrete, protective measures

for new concrete construction, procedures for identifyingcorrosive environments and active corrosion in concrete,and remedial measures. A selected list of references isincluded with each chapter.

Keywords : admixtures; aggregates; blended cements; bridge decks; calci-um chlorides; carbonation; cathodic protection; cement pastes; chemicalanalysis; chlorides; coatings; concrete durability; corrosion ; corrosion re-sistance ; cracking (fracturing); deicers; deterioration; durability; marine at-mospheres; parking structures; plastics, polymers and resins; portlandcements; prestressing steels ; protective coatings; reinforced concrete ; re-inforcing steels ; repairs; resurfacing; spalling; waterproof coatings.

CONTENTSChapter 1Introduction, p. 222R-11.1Background

1.2Scope1.3References

Chapter 2Mechanism of corrosion of steel inconcrete, p. 222R-3

2.1Introduction2.2Principles of corrosion2.3Effects of the concrete environment on corrosion2.4References

Chapter 3Protection against corrosion in new

construction, p. 222R-113.1Introduction3.2Design and construction practices3.3Methods of excluding external sources of chloride

ion from concrete3.4Methods of protecting reinforcing steel from chlo-

ride ion

ACI 222R-96

Corrosion of Metals in Concrete

Reported by ACI Committee 222

ACI 222R-96 replaces ACI 222R-89 and became effective May 23, 1996.Copyright (c) 1997, American Concrete Institute. All rights reserved including

rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed orwritten or oral, or recording for sound or visual reproduction or for use in any knowl-edge of retrieval system or device, unless permission in writing is obtained from thecopyright proprietors.

ACI Committee Reports, Guides, Standard Practices, andCommentaries are intended for guidance in designing, plan-ning, executing, or inspecting construction and in preparingspecifications. Reference to these documents shall not bemade in the Project Documents. If items found in these doc-uments are desired to be part of the Project Documents, theyshould be phrased in mandatory language and incorporatedin the Project Documents.

222R-1

ACI COMMITTEE 222Corrosion of Metals in Concrete

Brian B. HopeChairman

Keith A. PashinaSecretary

John P. Broomeld Bret James Robert E. PriceKenneth C. Clear Thomas D. Joseph D. V. ReddyJames R. Clifton David G. Manning William T. ScannellIsrael Cornet Walter J. McCoy David C. Stark Marwan Daye Theodore L. Neff Wayne J. SwiatBernard Erlin Charles K. Nmai Thomas G. WeilJohn K. Grant William F. Perenchio Richard E. WeyersKenneth C. Hover Randall W. Poston David A. Whiting

Associate MembersStephen L. Amey Odd E. Gjorv Mohamad A. NagiSteven F. Dailey Clayford T. Grimm Morris Schupack Stephen D. Disch Alan K. C. Ip Ephraim SenbettaHamad Farzam Andrew Kaminker Robert E. ShewmakerPer Fidjestol Mohammad S. Khan Bruce A. SuprenantRodney R. Gerard Philip J. Leclaire William F. Van SiserenMichael P. Gillen Joseph A. Lehmann Michael C. Wallrap


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222R-2 ACI MANUAL OF CONCRETE PRACTICE

3.5Corrosion control methods3.6References

Chapter 4Procedures for identifying corrosionenvironments and active corrosion in concrete,p. 222R-21

4.1Introduction4.2Methods of evaluation4.3References

Chapter 5Remedial measures, p. 222R-235.1Introduction5.2General5.3Applicability5.4The remedies and their limitations5.5Summary5.6References

Chapter 6References to documents of standard-producing organizations, p. 222R-28

Appendix AStandard Test Method for Water-Soluble Chloride Available for Corrosion ofEmbedded Steel in Mortar and Concrete Using theSoxhlet Extractor, p. 222R-28

CHAPTER 1INTRODUCTION1.1Background

Concrete normally provides reinforcing steel with excel-lent corrosion protection. The high alkaline environment inconcrete results in the formation of a tightly adhering filmwhich passivates the steel and protects it from corrosion. Inaddition, concrete can be proportioned to have a low perme-ability which minimizes the penetration of corrosion-inducingsubstances. Low permeability also increases the electricalresistivity of concrete which impedes the flow of electro-chemical corrosion currents. Because of these inherent pro-tective attributes, corrosion of steel does not occur in themajority of concrete elements or structures. Corrosion of steel, however, can occur if the concrete is not of adequatequality, the structure was not properly designed for the ser-vice environment, or the environment was not as anticipatedor changes during the service life of the concrete.

The corrosion of metals, especially steel, in concrete hasreceived increasing attention in recent years because of itswidespread occurrence in certain types of structures and thehigh cost of repairs. The corrosion of steel reinforcementwas first observed in marine structures and chemical manu-facturing plants. 1.1,1.2,1.3 More recently, numerous reports of its occurrence in bridge decks, parking structures, and otherstructures exposed to chlorides has made the problem partic-ularly prominent. The consequent extensive research on fac-tors contributing to steel corrosion has increased ourunderstanding of corrosion, especially concerning the role of chloride ions. It is anticipated that the application of the find-ings of this research will result in fewer instances of corrosionin new concrete structures and improved methods of repair-ing corrosion-induced damage in existing structures.

For these improvements to occur, the information must bedisseminated to individuals responsible for the design,

construction, and maintenance of concrete structures. Themain purpose of this report is to present the state of the art.While several metals may corrode under certain conditionswhen embedded in concrete, the corrosion of steel reinforce-ment is of the greatest concern, and, therefore, is the primarysubject of the report.

Chloride ions are considered to be the major cause of pre-

mature corrosion of steel reinforcement. Corrosion can oc-cur in some circumstances in the absences of chloride ions,however. For example, carbonation of concrete results in re-duction of its alkalinity, thereby permitting corrosion of em-bedded steel. Carbonation, however, is usually a slowprocess in concrete which has a low water-cement ratio andcarbonation-induced corrosion is not as common as corro-sion induced by chloride ions. Chloride ions are common innature and small amounts are usually unintentionally con-tained in the mix ingredients of concrete.

Chloride ions also may be intentionally added, most often asa constituent of accelerating admixtures. Dissolved chlorideions also may penetrate unprotected hardened concrete in struc-

tures exposed to marine environments or to deicing salts.The rate of corrosion of steel reinforcement embedded in

concrete is strongly influenced by environmental factors. Bothoxygen and moisture must be present if electrochemical cor-rosion is to occur. Reinforced concrete with significant gradi-ents in chloride ion content is vulnerable to macrocellcorrosion, especially if subjected to cycles of wetting and dry-ing. Other factors that affect the rate and level of corrosion areheterogeneities in the concrete and the steel, pH of the con-crete pore water, carbonation of the portland cement paste,cracks in the concrete, stray currents, and galvanic effects dueto contact between dissimilar metals. Design features alsoplay an important role in the corrosion of embedded steel. Mixproportions, depth of cover over the steel, crack control mea-sures, and implementation of measures designed specificallyfor corrosion protection are some of the factors that control theonset and rate of corrosion.

Deterioration of concrete due to corrosion results becausethe products of corrosion (rust) occupy a greater volume thanthe steel and exert substantial stresses on the surroundingconcrete. The outward manifestations of the rusting includestaining, cracking, and spalling of the concrete. Concurrent-ly, the cross section of the steel is reduced. With time, structuraldistress may occur either by loss of bond between the steeland concrete due to cracking and spalling or as a result of thereduced steel cross-sectional area. This latter effect can be of special concern in structures containing high strength pre-stressing steel in which a small amount of metal loss couldpossibly induce tendon failure.

The research on corrosion to date has not produced a steelor other type of reinforcement that will not corrode when usedin concrete and that is both economical and technically feasible.However, research has pointed to the need for quality con-crete, careful design, good construction practices, and reason-able limits on the amount of chloride in the concrete mixingredients. Other measures that are being investigated in-clude the use of corrosion inhibitors, protective coatings onthe steel, and cathodic protection. There has been some


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success in each of these areas though problems resulting fromcorrosion of embedded metals are far from eliminated.

1.2 ScopeThis report discusses the factors that cause and control

corrosion of steel in concrete, measures for protecting em-bedded metals in new construction, techniques for detectingcorrosion in structures in service, and remedial procedures.Consideration of these factors, and application of the dis-cussed measures, techniques, and procedures should assist inreducing the occurrence of corrosion and result, in most in-stances, in the satisfactory performance of reinforced andprestressed concrete elements.

1.3 References1.1. Tremper, Bailey; Beaton, John L; and Stratfull, R. F. Causes and

Repair of Deterioration to a California Bridge Due to Corrosion of Rein-forcing Steel in a Marine Environment II: Fundamental Factors CausingCorrosion, Bulletin No. 182, Highway Research Board, Washington, D.C.,1958, pp. 18-41.

1.2. Evans. U. R., The Corrosion and Oxidation of Metals: Scientic Prin-ciples and Practical Applications , Edward Arnold, London, 1960, 303 pp.

1.3. Biczk, Imre, Concrete Corrosion and Concrete Protection , 3rdEdition, Acadmiai Kiad, Budapest, 1964.

CHAPTER 2MECHANISM OF CORROSION OFSTEEL IN CONCRETE

2.1 IntroductionThis chapter is divided into two sections. In the first, em-

phasis is placed on the processes responsible for corrosion of steel reinforcement in concrete. The corrosion mechanism isgenerally accepted to be electrochemical in nature. Some of the major causes of corrosion of steel in concrete are exam-ined and related phenomena are discussed.

In the second part, a discussion is given on the concrete

variables that influence corrosion, including concrete mixproportions, quality, and cover, and the effects of corrosioninhibiting admixtures and carbonation.

2.2Principles of corrosion2.2.1 Corrosion An electrochemical process Al-

though iron can corrode by chemical attack, the most com-mon form of corrosion in an aqueous medium iselectrochemical. 2.1 The corrosion process is similar to theaction which takes place in a flashlight battery. An anode,where electrochemical oxidation takes place, a cathode,where electrochemical reduction occurs, an electrical con-ductor, and an aqueous medium must be present. Any metal

surface on which corrosion is taking place is a composite of anodes and cathodes electrically connected through the bodyof the metal itself. Reactions at the anodes and cathodes arebroadly referred to as half-cell reactions.

At the anode, which is the negative pole, iron is oxidizedto ferrous ions.

Fe Fe++ + 2e-

E = - 0.440 Standard Redox Potential (2-1)

The Standard Redox Potential is the potential generatedwhen the metal is connected to a hydrogen electrode and is

one method of expressing electromotive forces. The Fe ++ inEq. (2-1) is subsequently changed to oxides of iron by anumber of complex reactions. The volume of the reactionproducts is several times the volume of the iron.

At the cathode, reduction takes place. In an acid medium,the reaction taking place at the cathode is the reduction of hydrogen ions to hydrogen. However, as will be shown later,

concrete is highly basic and usually has an adequate supplyof oxygen, so the cathodic reaction is

1 / 2 H2O +1 / 4 O2 + e

- OH -

E = 0.401 Standard Redox Potential (2-2)

The corroding iron piece has an open circuit potential,also called a rest potential, related to the Standard RedoxPotentials of the reactions in Eq. (2-1) and (2-2), to the com-position of the aqueous medium, the temperature, and to thepolarizations of these half-cells. The rate of corrosion isrelated to the polarizations as will be discussed later.

2.2.2 Availability of oxygen in concrete Although theavailability of oxygen is one of the main controlling factorsfor corrosion of steel, there appears to be little quantitativeinformation on its effect in concrete. Some information isshown, however, in Fig. 2.1 , where the rate of oxygen diffusionthrough water-saturated concrete of varying quality andthickness is illustrated. 2.2 The rate of oxygen diffusionthrough concrete is also significantly affected by the extentto which the concrete is saturated with water. A number of investigations indicate that if the steel passivity is destroyed,conditions will be conducive to the corrosion of steel rein-forcement in those parts of a concrete structure that are ex-

posed to periods of intermittent wetting and drying.Investigations also indicate that, although chlorides arepresent, the rate of steel corrosion will be very slow if theconcrete is continuously water-saturated. 2.3 In wet concrete,dissolved oxygen will primarily be diffusing in solution,while in a partly dry concrete the diffusion of gaseous oxy-gen is much faster. For oxygen to be consumed in a cathodicreaction, however, the oxygen has to be in a dissolved state.Since it is the concentration of dissolved oxygen that is im-portant, all factors affecting the solubility of oxygen shouldalso affect its availability. The effect of salt on the corrosionrate has been demonstrated by Griffin and Henry 2.4 (seeFig. 2.2 ). Corrosion increased as the sodium chloride con-

centration increased until a maximum was reached. Beyondthis, the rate of corrosion decreased despite the increasedchloride ion concentration. This change in relationship be-tween corrosion and sodium chloride concentration is attrib-uted to the reduced solubility and diffusivity of oxygen, and,therefore, the availability of oxygen to sustain the corrosionprocess. This represents corrosion in a salt solution; howev-er, the availability of oxygen in wet concrete may be different.

Problems due to corrosion of embedded steel have seldombeen observed in concrete structures that are continuouslysubmerged, even as in seawater.

2.2.3 The importance of chloride ions As will be discussedlater, concrete can form an efficient corrosion-preventive


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environment for embedded steel. However, it is welldocumented 2.5-2.7 that the intrusion of chloride ions in rein-forced concrete can cause steel corrosion if oxygen andmoisture are also available to sustain the reaction. No othercommon contaminant is documented as extensively in theliterature as a cause of corrosion of metals in concrete. Chlo-ride ions may be introduced into concrete in a variety of ways. Some are intentional inclusion as an accelerating ad-mixture; accidental inclusion as contaminants on aggregates;or penetration by deicing salts, industrial brines, marinespray, fog, or mist.

2.2.3.1 Incorporation in concrete during mixing. One of the best known accelerators of the hydration of portland ce-ment is calcium chloride. Generally, up to 2 percent solidcalcium chloride dihydrate based on the weight of cement is

added. Chlorides may be contained in other admixtures suchas some water-reducing admixtures where small amounts of chloride are sometimes added to offset the set-retarding ef-fect of the water reducer.

In some cases, where potable water is not available, seawa-ter or water with a high chloride content is used as the mixingwater. In some areas of the world, aggregates exposed to sea-

water (or that were soaked in seawater at one time) can containa considerable quantity of chloride salts. Aggregates that areporous can contain larger amounts of chloride.

2.2.3.2 Diffusion into mature concrete. Chlorides canpermeate through sound concrete, i.e., cracks are not neces-sary for chlorides to enter the concrete. 2.8 Approximatedeterminations of the diffusion coefficients for chloride inconcrete have been published, 2.9 but largely the literature ne-glects the interaction between concrete and chloride.

2.2.3.3 Electrochemical role of free chloride ions. Thereare three modern theories to explain the effects of chlorideions on steel corrosion:

(a) The Oxide Film Theory Some investigators believethat an oxide film on a metal surface is responsible for pas-sivity and thus protection against corrosion. This theory pos-tulates that chloride ions penetrate the oxide film on steelthrough pores or defects in the film easier than do other ions(e.g., SO 4

-). Alternatively, the chloride ions may colloidallydisperse the oxide film, thereby making it easier to penetrate.

(b) The Adsorption Theory Chloride ions are adsorbed onthe metal surface in competition with dissolved O 2 or hydrox-yl ions. The chloride ion promotes the hydration of the metalions and thus facilitates the dissolution of the metal ions.

(c) The Transitory Complex Theory According to thistheory, chloride ions compete with hydroxyl ions for the

Fig. 2.2Effect of concentration of sodium chlorideon corrosion rate 2.4

Fig. 2.1Effect of water-cement ratio and thickness on diffusion of oxygen through mor-tar and concrete 2.2


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ferrous ions produced by corrosion. A soluble complex of iron chloride forms. 2.9 This complex can diffuse away fromthe anode destroying the protective layer of Fe(OH) 2, per-mitting corrosion to continue. Some distance from the elec-trode, the complex breaks down, iron hydroxide precipitatesand the chloride ion is free to transport more ferrous ionsfrom the anode. Evidence for this process can be observed

when concrete with active corrosion is broken open. A lightgreen semisolid reaction product is often found near the steelwhich, on exposure to air, turns black and subsequently rustred in color.

Since corrosion is not stifled, more iron ions continue tomigrate into the concrete away from the corrosion site and toreact with oxygen to form higher oxides that result in a four-fold volume increase. It is the expansion of iron oxides asthey are transformed to higher oxidation states that produceinternal stress, which eventually cracks the concrete. Formationof iron chloride complexes may also lead to disruptive forces.

2.2.4 Corrosion rate and pH As illustrated in Fig. 2.3 , thecorrosion rate of iron is reduced as the pH increases. Sinceconcrete has a pH higher than 12.5, it is usually an excellentmedium for protecting steel from corrosion. Only under con-ditions where salts are present or the concrete cover has car-bonated does the steel become vulnerable to corrosion.

2.2.5 Pourbaix diagrams Pourbaix 2.10 devised a com-pact summary of thermodynamic data in the form of electri-cal potential versus pH diagrams, which includes theelectrochemical and corrosion behavior of iron in an aqueousmedium. Fig. 2.4 indicates that iron is in a passive state at apH in the range of 8 to 13. This accounts for the protectiveproperties of concrete and the absence of steel corrosion inconcrete when no chloride ions, or only small amounts, arepresent. A careful inspection of Fig. 2.4 indicates, however,that corrosion may begin if the pH of the system is raised toabove about 13. In this case, a soluble ferrite, HFeO 2

-, forms.Any mechanism where iron dissolves at an appreciable ratein concrete can be considered serious corrosion. Thus, theaddition of materials that further increase the pH of concretemay be detrimental; however, the occurrence of this phe-nomenon in concrete has not been confirmed.

2.2.6 High-strength steels and other metals There is lit-tle information in the literature suggesting that the newerhigh-strength steels are either more or less subject to corrosionthan the previous standard and lower strength steels. Al-

though information is lacking, it is expected that whateverdifferences might exist would be secondary to the major fac-tors influencing corrodibility of the steel.

Studies have been made on the corrosion of prestressedsteel in concrete 2.11 that demonstrate the hazards of addingchlorides to concrete containing prestressed steel.

Aluminum corrodes in concrete and the rate of corrosionis higher if the aluminum is in contact with steel and chlo-rides or if alkalies are present. 2.12 Lead and tin (e.g., tin sol-der) can corrode similarly.

At pH about 12.5, zinc reacts rapidly to form solublezincates and hydrogen gas is liberated

Zn + OH-

+ H2O -> HZnO 2-

+ H2 (2-3)If galvanized steel is to be used in concrete, appropriate

measures should be taken to prevent hydrogen evolution inthe fresh concrete. A small amount of a chromate salt is gen-erally used. 2.13

In submerged concrete structures having freely exposedsteel components in metallic connection with the reinforcingsteel, galvanic cells may develop with the freely exposedsteel forming the anode and the embedded steel the cathode.This may cause an increased corrosion rate on the freely ex-posed steel.

Fig. 2.3Effect of pH on corrosion of iron in aerated soft water at room temperature 2.1

Fig. 2.4Pourbaix diagram for iron 2.10


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Copper, chromium, nickel, and silver generally do notcorrode in concrete. However, they are somewhat suscep-tible to corrosion when the concrete is located in a marineenvironment. 2.14

2.2.7 Polarization of half-cells The equilibrium of ahalf-cell is characterized by its reversible potential whichdepends on the Standard Redox Potential, on the concen-

trations (activities) of the species participating in the half-cellreaction (e.g., OH -, Fe++, etc.), and on the temperature.

When a current flows through the half-cell, there is a shiftof its measured potential away from the reversible potential.This shift is called polarization. For a given current, the po-larization is large for half-cell reactions that are irreversibleor nearly so. For instance, the oxygen half-cell reaction[Eq. (2-2) ] is almost irreversible and can have a relativelyhigh degree of polarization.

Some corrosion processes, although thermodynamicallymore favored than others as indicated by the reversible po-tentials of the half-cells, are actually slower in practice be-cause of polarization effects.

There are three general kinds of polarization that applyto the anode as well as to the cathode. These are concen-tration polarization, ohmic polarization, and activationpolarization. These three kinds of polarization can bepresent simultaneously.

(a) Concentration polarization occurs when the concen-tration of the electrolyte changes in the vicinity of the elec-trode. An example of this would be depletion of oxygenconcentration at the cathode.

(b) Ohmic polarization occurs because of the ohmic resis-tance of the electrolyte (e.g., moist concrete) and of anyfilms on the electrode surface. This produces an ohmic po-

tential drop in accordance with Ohms law (IR drop).(c) Activation polarization occurs due to kinetic hindrance

of the rate controlling step of the electrode reaction.Tafel 2.1 has shown experimentally that for large currents

in the absence of concentration and ohmic polarization, themeasured polarization , which is the activation polarization, isdirectly proportional to the logarithm of the current density i

= a + blog i

where a and b are the so-called Tafel intercept and Tafelslope parameters, respectively. These parameters can be ob-tained by plotting versus i on semilogarithmic paper. The

Tafel intercept parameter a is related to the exchange currentdensity io, which is the equilibrium current flowing back andforth through the electrode electrolyte interface at equilibri-um and a measure of the reversibility of the reaction.

The Tafel slope parameter b, on the other hand, gives aninsight according to modern electrode reaction theory 2.15

into the mechanism of the electrode reaction.

2.2.8 Passivity and transpassivity Specific to the anodichalf-cell in the corrosion process are the concepts of passiv-ity and transpassivity. Passivity of a metal is generally char-acterized by a thin and tightly adherent oxide film on themetal surface, which tends to protect the metal against fur-ther corrosion. The exact composition of the thin and nor-mally invisible film has been difficult to determine. It seemsclear, however, that it is made up of chemical combinationsof oxygen and is simply called an oxide film.

When a potential is applied to an iron electrode, the rate of current flow depends on the state of passivity. Iron in con-crete is generally passive, such that little current flows whena potential is progressively increased, eventually the currentwill flow. This is because, at this point, oxygen is evolvedand the electrode reaction involves the electrolysis of water(see Curve I, Fig. 2.5 ).

2H2O -> 4H+ + O2 + 4e

- (2-4)

This phenomenon is called transpassivity. The use of acorrosion testing procedure involving an impressed potentialhas been reported. 2.17 Grimes et al. 2.18 found that when ahigh anodic voltage was applied to steel in concrete, the pHaround the steel was changed to values ranging between al-most 0 and 4. He also found that a liquid pool formed quickly

around the reinforcing steel. Neither steel nor concrete is du-rable in a low pH (acidic) environment.

2.2.9 Types of corrosion-controlling mechanisms Aspreviously mentioned, it is necessary to have both a cathodicand an anodic reaction for a corrosion process to occur.However, different corrosion situations can be visualized byplotting, on the same graph, the log of the absolute current(I) versus potential (E) curves which would be obtained bypolarizing each half-cell with an auxiliary counter electrodein absence of the other half-cell. For this discussion, we as-sume (a) no IR drop between anodic and cathodic areas and(b) simple algebraic addition of the anodic and cathodic cur-rents. Under these conditions, the rest potential or open cir-cuit potential of the corroding sample will be that at whichthe two i versus E curves intercept. At this point no net, ex-ternal current flows, and the absolute value of current at theintercept is equal to the corrosion current.

If the cathodic process is the slower process (the one withthe larger polarization), the corrosion rate is considered to becathodically controlled (Fig. 2.6 ). Conversely, if the anodicprocess is slower, the corrosion rate is said to be anodicallycontrolled (Fig. 2.7 ).

In concrete, one or two types of corrosion-rate-controllingmechanisms normally dominate. One is cathodic diffusion,where the rate of oxygen diffusion through the concrete

Fig. 2.5Iron electrode in concrete 2.16


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determines the rate of corrosion. In Fig. 2.8 , cathodic diffusioncontrol is shown for two different rates of oxygen diffusion.

The other type of controlling mechanism involves the de-velopment of a high resistance path. When steel corrodes inconcrete, anodic and cathodic areas may be as much as severalfeet apart; therefore, the resistance of the concrete may be of great importance. Fig. 2.9 illustrates two cases of resistancecontrol where the potential available for corrosion is the dif-ference in the potential between anode and cathode minusthe relevant IR loss.

2.2.10 Stray current corrosion Stray electric currents arethose that follow paths other than the intended circuit. Theycan greatly accelerate the corrosion of reinforcing steel. Themost common sources of these are electric railways, electro-plating plants, and cathodic protection systems. Kondo etal.2.9 reported corrosion of reinforcing steel embedded inconcrete used in an electric railroad.

2.2.11 Cathodic protection The principle of cathodicprotection is to change the potential of a metal to reduce thecurrent flow and thereby the rate of corrosion. This is accom-plished by the application of a protective current at a higher

voltage than that of the anodic surface. The current thenflows to the original anodic surface resulting in cathodic re-actions occurring there. The difficulties in using this method,however, are to determine the correct potential to apply tothe system and to make sure that it is applied uniformly.

2.2.12 Effects of other salt ions It has been reported thatsulfate2.20 and carbonate 2.21 salts can also cause reinforcingsteel to corrode. However, this has not been well document-

ed. Although the concrete may have cracked in certain casesand the exposed steel may have appeared rusted, these saltsmay not have been the primary cause. Their low solubility ina high calcium ion environment would reduce their availabil-ity. However, certain soluble salts, such as perchlorates, ac-etates, and salts of halogens other than chlorine may becorrosive to steel embedded in concrete. Hydrogen sulfidehas also been cited as a cause of corrosion. 2.22

2.2.13 Stress corrosion cracking Stress corrosion 2.23 isdefined as the process in which the damage caused by stressand corrosion acting together greatly exceeds that producedwhen they act separately.

Fig. 2.8Cathodic diffusion control

Fig. 2.9Resistance controlFig. 2.7Anodic control

Fig. 2.6Cathodic control


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In stressed steel, a small imperfection caused by corrosioncan lead to a serious loss in tensile strength as the corrosioncontinues at the initial anode area.

Another form of corrosion that is related to stress corrosioncracking is intergranular corrosion. In this case, a gas, usual-ly hydrogen, is absorbed in the iron, causing a loss of ductil-ity and cracking. Hydrogen cracking in connection withcathodic protection will be discussed in Chapter 5 . Other ma-terials that may cause intergranular corrosion are hydrogensulfide and high concentrations of ammonia and nitrate salts.The mechanism of how this type of corrosion proceeds is not

fully understood; however, it is believed that it involves thereduction in the cohesive strength of the iron. Documentedexamples of stress corrosion cracking of steel in concretehave not been found in the literature.

2.3 Effects of the concrete environment oncorrosion

2.3.1 Portland cement When portland cement hydrates,the silicates react with water to produce calcium silicate hy-drate and calcium hydroxide. The following simplifiedequations give the main reactions of portland cement with water

2(3CaO SiO2) + 6H 2O 3CaO 2SiO2 3H20 +

3Ca(OH) 2 (2-5)2(2CaO SiO2) + 4H 2O 3CaO 2SiO2 3H2O +

Ca(OH) 2 (2-6)

As previously mentioned, the high alkalinity of the chem-ical environment normally present in concrete protects theembedded steel because of the formation of a protective ox-ide film on the steel. The integrity and protective quality of this film depends on the alkalinity (pH) of the environment.

Differences in the types of cement are a result of variationin composition or fineness or both, and as such, not all typesof cement have the same ability to provide protection of

embedded steel. According to Pressler et al. 2.24 a well-hy-drated portland cement may contain from 15 to 30 percentcalcium hydroxide by weight of the original cement. This isusually sufficient to maintain a solution at a pH about 13 inthe concrete independent of moisture content.

Rosenqvist 2.25 has described an example of exceedinglyrapid steel corrosion in a tropical concrete wharf where a

pozzolan was added to the portland cement. Corrosion of thereinforcement was observed shortly after construction wascompleted. Measurements of pH in the extract from the con-crete adjacent to the steel gave values from 5.7 to 8.5. Whenthe pozzolan was omitted, an increase in pH was obtainedand no damage was observed.

Other research 2.26 showed that a pozzolan did not adverse-ly affect the performance of steel in concrete that was partial-ly immersed in a saturated salt solution. In this latter study,the pozzolan studied was a calcined volcanic tuff that wasadded to the concrete at dosages of 16 and 32 percent byweight of the cement. It may be that the protective qualitiesof the pozzolan are related to its source or generic type.

The use of blended cements might, under certain circum-stances, be detrimental because of a reduction in alkalinity.However, blended cements can give a substantial reduction inpermeability and also an increase in electrical resistivity espe-cially where a reduction in the water-cement ratio is made pos-sible. Also, such blended cements may give concrete as muchas two to five times higher resistance to chloride penetrationthan concrete made with portland cements. 2.27 The effectswould be beneficial as far as corrosion is concerned and insome circumstances the benefits associated with blended ce-ments more than offset the adverse effects.

Even for cements with the highest reserve basicity, the al-

kalinity may be reduced in a number of ways. Reduction of alkalinity by leaching of soluble alkaline salts with water isan obvious process. Partial neutralization by reaction withcarbon dioxide (carbonation), as present either in air or dis-solved in water, is another common process.

The silicates are the major components in portland cementimparting strength to the matrix. No reactions have been de-tected between chloride ions and silicates. Calcium chlorideaccelerates the hydration of the silicates when at least 1 per-cent by weight is added. 2.28 Calcium chloride seems to act asan accelerator in the hydration of tricalcium silicate as wellas to promote the corrosion of steel.

Also present in portland cement are C 3A and an alumino-ferrite phase reported as C 4AF. The C 3A reacts rapidly in thecement system to cause flash set unless it is retarded. Calci-um sulfate is used as the retarder. Calcium sulfate forms acoating of ettringite (C 3A 3CaSO4 32H2O) around the alu-minate grains thereby retarding their reactivity (Fig. 2.10 ).

Calcium chloride also forms insoluble reaction products withthe aluminates in cement (see Fig. 2.11 ). The most commonlynoted complex is C 3A CaCl2 xH2O, Friedels salt (Table 2.1 ).The rate of formation of this material is slower than that of ettringite (compare Fig. 2.10 with Fig. 2.11 ). The chloride-aluminate complex forms after ettringite and prevents furtherreactions of sulfate with the remaining aluminates. 2.29

Fig. 2.10Rate of reaction of gypsum with tricalcium alu-minate in portland cement 2.29


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The chemical combining of C 3A with chlorides is fre-quently referred to as a beneficial effect in that it will reducethe rate of chloride penetration into concrete. According toMehta,2.30 a chemical binding of penetrating chlorides can-

not be expected unless the C 3A content is higher than about8 percent. However, recent experimental observations haveshown that as much as 8.6 percent C 3A does not effectivelyreduce chloride penetration when concrete is immersed inseawater 2.31 and other studies have found more corrosion-induced distress associated with high C 3A contents.

2.32

2.3.2 Aggregate The aggregate generally has little effecton the corrosion of steel in concrete. There are exceptions.The most serious problems arise when the aggregates con-tain chloride salts. This can happen when sand is dredgedfrom the sea or taken from seaside or arid locations. Porousaggregates can absorb considerable quantities of salt.

Care should be exercised when using admixtures contain-ing chloride in combination with lightweight aggregates.Helms and Bowman 2.33 found that reinforcing steel in light-weight concrete was particularly susceptible to corrosionwhen exposed to moisture and changes in temperature.Lightweight aggregate containing sulfides can be damagingto high-strength steel under stress.

2.3.3 Water The effect of moisture content or degree of water saturation on the rate of oxygen diffusion into concretehas already been discussed. A high moisture content willalso substantially reduce the rate of diffusion of carbon diox-ide and hence the rate of carbonation of the concrete.

An important effect of the moisture content of concrete isits effect on the electrical resistivity of the concrete. Progres-sive drying of initially water-saturated concrete results inthe electrical resistivity increasing from about 7 103ohm cm to about 6000 103 ohm cm (Fig. 2.12 ).2.34 Field ob-servations indicate that when the resistivity exceeds a lev-el of 50 to 70 103 ohm cm, steel corrosion would benegligible. 2.35 Other authors quote values of resistivity of 10 x 10 3 ohm cm 2.36 and 12 103 ohm cm 2.37 abovewhich corrosion induced damage is unlikely even in thepresence of chloride ions, oxygen, and moisture.

2.3.4 Corrosion inhibiting admixtures Numerous chemi-cal admixtures, both organic and inorganic, have been sug-gested as specific inhibitors of steel corrosion. 2.13 Some of the

admixtures, however, may retard time of setting of the cementor be detrimental at later ages. Many would be subject toleaching and hence less effective in concrete that has lost sol-uble material by leaching. Among those compounds reportedas inorganic inhibitors are potassium dichromate, stannouschloride, zinc and lead chromates, calcium hypophosphite,

Fig. 2.11Rate of reaction of calcium chloride with portland cement 2.29

Table 2.1Comparison of producers analysis ofeight different portland cements with total amount ofcalcium chloride that reacted with each cement 2.29

1 / 3 SO3 content* C3A + C4AF -

1 / 3SO3Amount of calcium

chloride reacted0.20 1.43 1.190.05 1.42 1.37

0.30 1.10 1.180.07 1.62 1.450.72 0.69 0.630.25 1.03 1.080.26 1.05 1.070.23 1.15 1.05

Average 1.18 1.13*Data expressed in molar equivalents rather than usual percent for comparison pur-

poses. Wide differences in SO 3 content obtained intentionally.

Fig. 2.12Effect of water saturation on the resistivityof concrete 2.34


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sodium nitrite, and calcium nitrite. Organic inhibitors suggest-ed have included sodium benzoate, ethyl aniline, and mercap-tobenzothiazole. With some inhibitors, inhibition occurs onlyat addition rates sufficiently high to counteract the effects of chlorides. Sodium nitrite has been used with apparent effec-tiveness in Europe. 2.38 Calcium nitrite 2.39 is being studied inthe United States since several side effects of the sodium saltwould be avoided. Some of the side effects are low strength,erratic times of setting, efflorescence, and enhanced suscepti-bility to the alkali-aggregate reaction. 2.40

2.3.5 Concrete quality Concrete will offer more pro-tection against corrosion of embedded steel if it is of a high

quality. A low water-cement ratio will slow the diffusion of chlorides, carbon dioxide, and oxygen and also the increase instrength of the concrete may extend the time before corrosion-induced stresses cause cracking of the concrete. The pore vol-ume and permeability can be reduced by lowering the water-cement ratio. The type of cement or use of superplasticizingand mineral admixtures may also be an important factor incontrolling the permeability and the ingress of chlorides. 2.31

2.3.6 Thickness of concrete cover over steel The amountof concrete cover over the steel should be as large as possible,consistent with good structural design, the severity of the ser-vice environment, and cost. The effect of cover thickness ismore than a simple linear relationship. Considering the normaldiffusion of an electrolyte into a porous solid without chemi-cal reaction, a relationship such as shown in Fig. 2.13 wouldbe anticipated. However, in the case of cement paste, the dif-fusion of chloride ions into the paste is accompanied by bothphysical adsorption and chemical binding. These effects re-duce the concentration of chloride ion at any particular siteand hence the tendency for inward diffusion is further re-duced. This is also illustrated in Fig. 2.13 .

2.3.7 Carbonation Carbonation occurs when the con-crete reacts with carbon dioxide from the air or water and re-duces the pH to about 8.5. At this low pH the steel is nolonger passive and corrosion may occur. For high-quality

concrete, in situations where the rate of carbonation is ex-tremely slow, carbonation is normally not a problem unlesscracking of the concrete has occurred or the concrete coveris defective or very thin. Carbonation is not a problem invery dry concrete or in water-saturated concrete. Maximumcarbonation rates are observed at about 50 percent water sat-uration. A more complete discussion of carbonation and the

corrosion of steel in carbonated concrete is given in Refer-ences 2.17 and 2.21.

2.4References2.1. Uhlig, Herbert H., Corrosion and Corrosion Control , 2nd Edition,

John Wiley & Sons, New York, 1971, 419 pp.2.2. Gjrv, O. E.; Vennesland, ; and El-Busaidy, A. H. S., Diffusion of

Dissolved Oxygen through Concrete, Paper No. 17, NACE Corrosion 76,National Association of Corrosion Engineers, Houston, Mar. 1976, 13 pp.

2.3. Shalon, R., and Raphael, M., Inuence of Sea Water on Corrosionof Reinforcement, ACI J OURNAL , Proceedings V. 55, No. 8, Feb. 1959, pp.1251-1268.

2.4. Grifn, Donald F., and Henry, Robert L., Effect of Salt in Concreteon Compressive Strength, Water Vapor Transmission, and Corrosion of Reinforcing Steel, Proceedings , ASTM, V. 63, 1963, pp. 1046-1078.

2.5. Stratfull, R. F.; Jurkovich, W. J.; and Spellman, D. L., CorrosionTesting of Bridge Decks, Transportation Research Record No. 539, Trans-portation Research Board, 1975, pp. 50-59.

2.6. Browne, Roger D., Mechanisms of Corrosion of Steel in Concretein Relation to Design, Inspection, and Repair of Offshore and CoastalStructures, Performance of Concrete in Marine Environment , SP-65,American Concrete Institute, Detroit, 1980, pp. 169-204.

2.7. Aziz, M. A., and Mansur, M. A., Deterioration of Marine ConcreteStructures with Special Emphasis on Corrosion of Steel and Its Remedies,Corrosion of Reinforcement in Concrete Construction , Ellis Horwood Lim-ited, Chichester, 1983, pp. 91-99.

2.8. Gjrv, Odd E., Durability of Concrete Structures in the OceanEnvironment, Proceedings, FIP Symposium on Concrete Sea Structures(Tbilisi, Sept. 1972), Federation Internationale de la Precontrainte, Lon-don, 1973, pp. 141-145.

2.9. Foley, R. T., Complex Ions and Corrosion, Journal , Electrochemi-cal Society, V. 122, No. 11, 1975, pp. 1493-1549.

2.10. Pourbaix, M., Atlas of Electrochemical Equilibrium in AqueousSolutions , Pergamon Press Limited, London, 1976.

2.11. Monfore, G. E., and Verbeck, G. J., Corrosion of Prestressed Wire inConcrete, ACI J OURNAL, Proceedings V. 57, No. 5, Nov. 1960, pp. 491-516.

2.12. Monfore, G. E., and Ost, Borje, Corrosion of Aluminum Conduitin Concrete, Journal , PCA Research and Development Laboratories, V. 7,No. 1, Jan. 1967, pp. 10-22.

2.13. Boyd, W. K., and Tripler, A. B., Corrosion of Reinforcing SteelBars in Concrete, Materials Protection , V. 7, No. 10, 1968, pp. 40-47.

2.14. Baker, E. A.; Money, K. L.; and Sanborn, C. B., Marine CorrosionBehavior of Bare and Metallic-Coated Steel Reinforcing Rods in Con-crete, Chloride Corrosion of Steel in Concrete , STP-629, ASTM, Phila-delphia, 1977, pp. 30-50.

2.15. Stern, M., and Geary, A. L., Electrochemical Polarization No. 1Theoretical Analysis of the Shape of Polarization Curves, Journal , Elec-trochemical Society, V. 104, No. 1, 1957, pp. 56-63.

2.16. Rosenberg, A. M., and Gaidis, J. M., The Mechanism of NitriteInhibition of Chloride Attack on Reinforcing Steel in Alkaline AqueousEnvironments, Materials Performance , V. 18, No. 11, 1979, pp. 45-48.

2.17. Corrosion of Reinforcement and Prestressing Tendons: A State of the Art Report, Materials and Structures/Research and Testin g (RILEM,Paris), V. 9, No. 51, May-June 1976, pp. 187-206.

2.18. Grimes, W. D.; Hartt, W. H.; and Turner, D. H., Cracking of Con-crete in Sea Water Due to Embedded Metal Corrosion, Corrosion , V. 35,No. 7, 1979, pp. 309-316.

2.19. Kondo, Yasuo; Takeda, Akihiko; and Hideshima, Setsuji, Effectof Admixtures on Electrolytic Corrosion of Steel Bars in Reinforced Con-crete, ACI J OURNAL , Proceedings V. 56, No. 4, Oct. 1959, pp. 299-312.

Fig. 2.13Gradient of total chloride concentration depthdepends on whether chemical reaction occurs with cement 2.41


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2.20. Eickemeyer, J., Stress Corrosion Cracking of a High-StrengthSteel in Saturated Ca(OH) 2 Solutions Caused by Cl and SO

4 Additions,Corrosion Science , V. 18, No. 4, 1978, pp. 397-400.

2.21. Hamada, M., Neutralization (Carbonation) of Concrete andCorrosion of Reinforcing Steel, Proceedings , 5th International Sym-posium on the Chemistry of Cement (Tokyo, 1968), Cement Associa-tion of Japan, Tokyo, 1969, V. 3, pp. 343-360.

2.22. Treadaway, K. W. J., Corrosion of Prestressed Steel Wire inConcrete, Brit ish C orrosion Journal (London), V. 6, 1971, pp. 66-72.

2.23. Sluijter, W. L., and Kreijger, P. C., Potentio Dynamic Polar-ization Curves and Steel Corrosion, Heron (Delft), V. 22, No. 1, 1977,pp. 13-27.

2.24. Weise, C. H., Determination of the Free Calcium HydroxideContents of Hydrated Portland Cements and Calcium Silicates, Analy t-ical Chemistry , V. 33, No. 7, June 1961, pp. 877-882. Also, Research

Department Bulle tin No. 127, Portland Cement Association.2.25. Rosenqvist, I. T., Korrosjon av Armeringsstal i Betong,

Teknisk Ukeblad (Oslo), 1961, pp. 793-795.2.26. Spellman, D. L., and Stratfull, R. F., Concrete Variables and

Corrosion Testing, Highway Research Record No. 423, HighwayResearch Board, 1973, pp. 27-45.

2.27. Page, C. L.; Short, N. R.; and El Tarras, A., Diffusion of Chlo-ride Ions in Hardened Cement Pastes, Cement and Concrete Research ,V. 11, No. 3, May 1981, pp. 395-406.

2.28. Ramachandran, V. S., Calcium Chloride in Concrete , AppliedScience Publishers, London, 1976, 216 pp.2.29. Rosenberg, Arnold M., Study of the Mechanism through

Which Calcium Chloride Accelerates the Set of Portland Cement, ACIJOURNAL , Proceedings V. 61, No. 10, Oct. 1964, pp. 1261-1270.

2.30. Mehta, P. K., Effect of Cement Composition on Corrosion of Reinforcing Steel in Concrete, Chloride Corrosion of Steel in Con-crete, STP-629, ASTM, Philadelphia, 1977, pp. 12-19.

2.31. Gjrv, O. E., and Vennesland, ., Diffusion of Chloride Ionsfrom Seawater into Concrete, Cement and Concrete Research , V. 9,No. 2, Mar. 1979, pp. 229-238.

2.32. Stratfull, R. F., Discussion of Long-Time Study of CementPerformance. Chapter 12Concrete Exposed to Sea Water and FreshWater, by I. L. Tyler, ACI J OURNAL , Proceedings V. 56, Part 2, Sept.1960, pp. 1455-1458.

2.33. Helms, S. B., and Bowman, A. L., Corrosion of Steel in Light-

weight Concrete Specimens, ACI J OURNAL , Proceeding s V. 65, No.12, Dec. 1968, pp. 1011-1016.

2.34. Gj rv, O. E.; Vennesland, .; and El-Busaidy, A. H. S., Elec-trical Resistivity of Concrete in the Oceans, OTC Paper No. 2803, 9thAnnual Offshore Technology Conference, Houston, May 1977, pp.581-588.

2.35. Tremper, Bailey; Beaton, John L.; and Stratfull, R. F., Causesand Repair of Deterioration to a California Bridge Due to Corrosion of Reinforcing Steel in a Marine Environment II: Fundamental FactorsCausing Corrosion, Bulle tin No. 182, Highway Research Board, Wash-ington, D.C., 1958, pp. 18-41.

2.36. Browne, R. D., Design Prediction of the Life for ReinforcedConcrete in Marine and Other Chloride Environments, Durabil ity of

Build ing Ma teria ls (Amsterdam), V. 1, 1982, pp. 113-125.2.37. Cavalier, P. G., and Vassie, P. R., Investigation and Repair of

Reinforcement Corrosion in a Bridge Deck, Proceedings , Institution of Civil Engineers (London), Part 1, V. 70, 1981, pp. 461-480.

2.38. Woods, Hubert, Durabil ity of Concrete Const ruction , ACIMonograph No. 4, American Concrete Institute/Iowa State UniversityPress, Detroit, 1968, p. 102.

2.39. Rosenberg, A. M.; Gaidis, J. M.; Kossivas, T. G.; and Previte,R. W., A Corrosion Inhibitor Formulated with Calcium Nitrite for Usein Reinforced Concrete, Chloride Corrosion of Steel in Concrete , STP-629, ASTM, Philadelphia, 1977, pp. 89-99.

2.40. Craig, R. J., and Wood, L. E., Effectiveness of CorrosionInhibitors and Their Inuence on the Physical Properties of PortlandCement Mortars, Highway Resea rch Record No. 328, HighwayResearch Board, 1970, pp. 77-88.

2.41. Verbeck, George J., Mechanisms of Corrosion of Steel in Con-crete, Corrosion of Metals in Concrete , SP-49, American ConcreteInstitute, Detroit, 1975, pp. 21-38.

CHAPTER 3PROTECTION AGAINSTCORROSION IN NEW CONSTRUCTION

3.1IntroductionMeasures which can be taken in reinforced concrete con-

struction to protect the steel against corrosion can be dividedinto three categories:

(a) Design and construction practices that maximize theprotection afforded by the portland cement concrete.

(b) Treatments that penetrate or are applied on the surfaceof the reinforced concrete member to exclude chloride ionfrom the concrete.

(c) Techniques that prevent corrosion of the reinforce-ment directly.

In the last case, two approaches are possible: to usecorrosion-resistant reinforcing steel or to nullify the effectsof chloride ions on unprotected reinforcement.

3.2Design and construction practicesThrough careful design and good construction practices,

the protection provided by portland cement concrete to em-

bedded reinforcing steel can be optimized. It is not the so-phistication of the structural design that determines thedurability of a concrete member in a corrosive environmentbut the detailing practices. 3.1 The provision of adequatedrainage and a method of removing drainage water from thestructure are particularly important.

In reinforced concrete members exposed to chlorides andsubjected to intermittent wetting, the degree of protectionagainst corrosion is determined primarily by the depth of cover to the reinforcing steel and the permeability of the con-crete.3.2-3.6 Estimates of the increase in corrosion protectionprovided by an increase in cover have ranged between slight-ly more than a linear relationship 3.3,3.7 to as much as thesquare of the cover. 3.8 The time to spalling is a function of the ratio of cover to bar diameter, 3.8 the reinforcement spac-ing, and the concrete strength. Although conventional port-land cement concrete is not impermeable, concrete with avery low permeability can be made through the use of goodquality materials, a minimum water-cement ratio consistentwith placing requirements, good consolidation and finishingpractices, and proper curing.

In concrete that is continuously submerged, the rate of cor-rosion is controlled by the rate of oxygen diffusion that is notsignificantly affected by the concrete quality or the thicknessof cover. 3.9 However, as noted in Chapter 2 , corrosion of embedded steel is a rare occurrence in continuously sub-merged concrete structures.

Placing limits on the allowable amounts of chloride ion inconcrete is an issue still under active debate. On the one sideare the purists who would like to see essentially no chloridesin concrete. On the other are the practitioners, includingthose who must produce concrete under cold weather con-ditions, precast concrete manufacturers who wish to mini-mize curing times, producers of chloride-bearing aggregates,and some admixture companies, who would prefer the leastrestrictive limit possible. Since chlorides are present natural-ly in most concrete-making materials, specifying a zerochloride content for any of the mix ingredients is unrealistic.


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222R-12 ACI COMMITTEE REPORT

However, it is also known that wherever chloride is presentin concrete, the risk of corrosion increases as the chloridecontent increases. When the chloride content exceeds a cer-tain value (termed the chloride corrosion threshold), unac-ceptable corrosion may occur provided that other necessaryconditions, chiefly the presence of oxygen and moisture, ex-ist to support the corrosion reactions, It is a difficult task to

establish a chloride content below which the risk of corrosion isnegligible, which is appropriate for all mix ingredients andunder all exposure conditions, and which can be measuredby a standard test.

Three different analytical values have been used to desig-nate the chloride content of fresh concrete, hardened con-crete, or any of the concrete mixture ingredients: (a) total,(b) acid-soluble, and (c) water-soluble.

The total chloride content of concrete is measured by thetotal amount of chlorine. Special analytical methods arenecessary to determine it, and acid-soluble chloride is oftenmistakenly called total chloride. The acid-soluble methodis the test method in common use and measures that chlo-

ride that is soluble in nitric acid. Water-soluble chloride ischloride extractable in water under defined conditions. Theresult obtained is a function of the analytical test procedure,particularly with respect to particle size, extraction time,and temperature, and to the age and environmental expo-sure of the concrete.

It is also important to distinguish clearly between chloridecontent, sodium chloride content, calcium chloride content,or any other chloride salt content. In this report, all referenc-es to chloride content pertain to the amount of chloride ion(Cl-) present. Chloride contents are expressed in terms of themass of cement unless stated otherwise.

Lewis 3.10reported that, on the basis of polarization tests of steel in saturated calcium hydroxide solution and water ex-tracts of hydrated cement samples, corrosion occurred whenthe chloride content was 0.33 percent acid-soluble chloride,or 0.16 percent water-soluble chloride based on a 2 hr ex-traction in water. However, it has been shown that the pore-water in many typical portland cement concretes madeusing relatively high-alkali cements is a strong solution of sodium and potassium hydroxides with a pH approaching14. Since the passivity of embedded steel is determined bythe ratio of the hydroxyl concentration to the chloride con-centration,3.11 the amounts of chloride that can be toleratedin concrete are higher than those that will cause pitting cor-rosion in a saturated solution of calcium hydroxide. 3.12

Work at the Federal Highway Administrationlaboratories 3.5 showed that for hardened concrete subject toexternally applied chlorides, the corrosion threshold was0.20 percent acid-soluble chlorides. The average content of water-soluble chloride in concrete was found to be 75 to 80percent of the content of acid-soluble chloride in the sameconcrete. This corrosion threshold value was subsequentlyconfirmed by field studies of bridge decks including those inCalifornia3.13 and New York. 3.14 These investigations showthat, under some conditions, a chloride content of as little as0.15 percent water-soluble chloride (or 0.20 percent acid-soluble chloride) is sufficient to initiate corrosion of embedded

steel in concrete exposed to chlorides in service. However, indetermining a limit on the chloride content of the mix ingre-dients, several other factors need to be considered.

As noted in the figures already given, the water-solublechloride content is not a constant proportion of the acid-soluble chloride content. It varies with the amount of chlo-ride in the concrete, 3.10 the mix ingredients, and the test meth-

od. All the materials used in concrete contain some chlorides,and, in the case of cement, the chloride content in the hardenedconcrete varies with cement composition. Although aggre-gates do not usually contain significant amounts of chlo-ride,3.15 there are exceptions. There are reports of aggregateswith an acid-soluble chloride content of more than 0.1 percentof which less than one-third is water-soluble when the aggre-gate is pulverized. 3.16 The chloride is not soluble when the ag-gregate is placed in water over an extended period and there isno difference in the corrosion performance of structures insouthern Ontario made from this aggregate when compared toother chloride-free aggregates in the region. However, this isnot always the case. Some aggregates, particularly those from

arid areas or dredged from the sea, may contribute sufficientchloride to the concrete to initiate corrosion. A limit of 0.06percent acid-soluble chloride ion in the combined fine andcoarse aggregate (by mass of the aggregate) has been suggest-ed with a further proviso that the concrete should not containmore than 0.4 percent chloride (by mass of the cement) de-rived from the aggregate. 3.17

There is thought to be a difference in the chloride corrosionthreshold value depending on whether the chloride is presentin the mix ingredients or penetrates the hardened concretefrom external sources. When chloride is added to the mix,some will chemically combine with the hydrating cementpaste. The amount of chloride forming chloroaluminates is afunction of the C 3A content of the cement. 3.18 Chloride add-ed to the mix will also tend to be distributed relatively uni-formly and, therefore, has less tendency to cause the creationof concentration cells.

Conversely, when chloride permeates from the surface of hardened concrete, uniform chloride contents will not existaround the steel because of differences in the concentrationof chlorides on the concrete surface resulting from poordrainage, for example, local differences in permeability, andvariations in the depth of cover to the steel. All these factorspromote differences in the environment (oxygen, moisture,and chloride content) along a given piece of reinforcement.Furthermore, most structures contain reinforcement at dif-ferent depths, and, because of the procedures used to fix thesteel, the steel is electrically connected. Thus, when chloridepenetrates the concrete, some of the steel is in contact withchloride-contaminated concrete while other steel is inchloride-free concrete. This creates a macroscopic corrosioncell that can possess a large driving voltage and a large cath-ode to small anode ratio which accelerates the rate of corrosion.In seawater, it has been suggested that the permeability of the concrete to chloride penetration is reduced by the precip-itation of magnesium hydroxide. 3.19

In laboratory studies 3.20 in which sodium chloride was addedto the mix ingredients, it was found that a substantial increase in


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corrosion rate occurred between 0.4 and 0.8 percent chloride,although the moisture conditions of the test specimens werenot clearly defined. Other researchers have suggested 3.21

that the critical level of chloride in the mix ingredients toinitiate corrosion is 0.3 percent and that this has a similareffect to 0.4 percent chloride penetrating the hardened con-crete from external sources. In studies in which calciumchloride was added to portland cement concrete, it wasfound3.22 that the chloride ion concentration in the pore so-lution remained high during the first day of hydration. It de-clined gradually, but it appeared that substantialconcentrations of chloride ion remained in solution indefinitely.

Chloride limits in national codes vary widely. ACI 318 al-lows a maximum water-soluble chloride ion content of 0.06percent in prestressed concrete, 0.15 percent for reinforcedconcrete exposed to chloride in service, 1.00 percent for re-inforced concrete that will be dry or protected from moisturein service, and 0.30 percent for all other reinforced concreteconstruction. The British Code, CP 110, allows an acid-sol-uble chloride ion content of 0.35 percent for 95 percent of the

test results with no result greater than 0.50 percent. Thesevalues are largely based on an examination of several struc-tures in which it was found there was a low risk of corrosionup to 0.4 percent chloride added to the mixture. 3.23 However,corrosion has occurred at values less than 0.4 percent, 3.24

particularly where the chloride content was not uniform. TheNorwegian Code, NS 3474, allows an acid-soluble chloridecontent of 0.6 percent for reinforced concrete made with nor-mal portland cement but only 0.002 percent chloride ion forprestressed concrete. Both these codes are under revision.Other codes have different limits, though the rationale forthese limits is not well established.

Corrosion of prestressing steel is generally of greaterconcern than corrosion of conventional reinforcement be-cause of the possibility that corrosion may cause a local re-duction in cross section and failure of the steel. The highstresses in the steel also render it more vulnerable to stress-corrosion cracking and, where the loading is cyclic, to cor-rosion fatigue. However, most examples of failure of pre-stressing steel that have been reported 3.24-3.26 have resultedfrom macrocell corrosion reducing the load carrying areaof the steel. Nevertheless, because of the greater vulnera-bility and the consequences of corrosion of prestressingsteel, chloride limits in the mix ingredients are lower thanfor conventional concrete. Based on the present state of knowledge, the following chloride limits in concrete usedin new construction, expressed as a percentage by weight of portland cement, are recommended to minimize the risk of chloride-induced corrosion.

Normally concrete materials are tested for chloride con-tent using either the acid-soluble test described in ASTM C1152, Acid-Soluble Chloride in Mortar and Concrete, orwater-soluble test described in ASTM C 1218, Water-Soluble Chloride in Mortar and Concrete. If the materialsmeet the requirements given in either of the relevant col-umns in the previous table they are acceptable. If they meetneither of the relevant limits given in the table then they maybe tested using the Soxhlet test method. Some aggregates,such as those discussed previously, 3.16 contain a consider-able amount of chloride that is sufficiently bound that it doesnot initiate or contribute towards corrosion. The Soxhlet testappears to measure only those chlorides that contribute to thecorrosion process, thus permitting the use of some aggre-gates that would not be allowed if only the ASTM C 1152 orASTM C 1218 tests were used. If the materials fail theSoxhlet test, then they are not suitable.

For prestressed and reinforced concrete that will be exposedto chlorides in service, it is advisable to maintain the lowest pos-sible chloride levels in the mix to maximize the service life of

the concrete before the critical chloride content is reached and ahigh risk of corrosion develops. Consequently, chlorides shouldnot be intentionally added to the mix ingredients even if thechloride content in the materials is less than the stated limits. Inmany exposure conditions, such as highway and parking struc-tures, marine environments, and industrial plants where chlo-rides are present, additional protection against corrosion of embedded steel is necessary.

Since moisture and oxygen are always necessary for elec-trochemical corrosion, there are some exposure conditionswhere the chloride levels may exceed the recommended val-ues and corrosion will not occur. Concrete which is continu-ously submerged in seawater rarely exhibits corrosion-induced distress because there is insufficient oxygen present.Similarly, where concrete is continuously dry, such as the in-terior of a building, there is little risk of corrosion from chlo-ride ions present in the hardened concrete. However, interiorlocations that are wetted occasionally, such as kitchens orlaundry rooms or buildings constructed with lightweight ag-gregate that is subsequently sealed (e.g., with tiles) beforethe concrete dries out, are susceptible to corrosion damage.Whereas the designer has little control over the change in useof a building or the service environment, the chloride contentof the mix ingredients can be controlled. Estimates or judg-ments of outdoor dry environments can be misleading.

Stratfull3.27

has reported the case of approximately 20 bridgedecks containing 2 percent calcium chloride built by the Cal-ifornia Department of Transportation. The bridges were lo-cated in an arid area where the annual rainfall was about 5in., most of which fell at one time. Within 5 years of con-struction, many were showing signs of corrosion-inducedspalling and most were removed from service within 10years. For these reasons, the committee believes a conserva-tive approach is necessary.

The maximum chloride limits suggested in this report dif-fer from those suggested by ACI Committee 201 3.2 andthose contained in the ACI Building Code. When making acomparison between the figures, it should be noted that the

Category Chloride limit for new constructionAcid-soluble Water-soluble

Test method ASTM C 1152 ASTM C 1218 Soxhlet*

Prestressed concrete 0.08 0.06 0.06Reinforced concrete

in wet conditions 0.10 0.08 0.08

Reinforced concretein dry conditions 0.20 0.15 0.15

*The Soxhlet Method of Test is described in the appendix to this report.


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figures in this report refer to acid-soluble chlorides while theother documents make reference to water-soluble chlorides.As noted previously, Committee 222 has taken a more con-servative approach because of the serious consequences of cor-rosion, the conflicting data on corrosion threshold values,and the difficulty of defining the service environmentthroughout the life of a structure.

Various nonferrous metals and alloys will corrode in dampor wet concrete. Surface attack of aluminum occurs in the

presence of alkali-hydroxide solutions. Anodizing providesno protection.

Much more serious corrosion can occur if the concretecontains chloride ions, particularly if there is electrical (metal-to-metal) contact between the aluminum and steel reinforce-ment, because a galvanic cell is created. Serious cracking orsplitting of concrete over aluminum conduits has been re-

ported.3.28,3.29

Certain organic protective coatings have beenrecommended 3.30 when aluminum must be used and it is im-practicable to avoid contamination by chlorides. Other met-als such as zinc, nickel, and cadmium, which have beenevaluated for use as coatings for reinforcing steel, are dis-cussed elsewhere in this chapter. Additional information iscontained in Reference 3.31 .

Where concrete will be exposed to chloride, the concreteshould be made with the lowest water-cement ratio consistentwith achieving maximum consolidation and density. The ef-fects of water-cement ratio and degree of consolidation on therate of ingress of chloride ions are shown in Fig. 3.1 and 3.2.Concrete with a water-cement ratio of 0.40 was found to re-

sist penetration by deicing salts significantly better than con-cretes with water-cement ratios of 0.50 and 0.60. A lowwater-cement ratio is not, however, sufficient to insure lowpermeability. As shown in Fig. 3.2 , a concrete with a water-cement ratio of 0.32 but with poor consolidation is less resis-tant to chloride ion penetration than a concrete with a water-cement ratio of 0.60. The combined effect of water-cementratio and depth of cover is shown in Fig. 3.3 , which illus-trates the number of daily applications of salt before thechloride content reached the critical value (0.20 percentacid-soluble) at the various depths. Thus, 40 mm (1.5 in.) of 0.40 water-cement ratio concrete was sufficient to protectembedded reinforcing steel against corrosion for 800 appli-cations of salt. Equivalent protection was afforded by 70 mm(2.75 in.) of concrete with a water-cement ratio of 0.50 or 90mm (3.5 in.) of 0.60 water-cement ratio concrete. On the ba-sis of this work, ACI 201.2R recommends a minimum of 50mm (2 in.) cover for bridge decks if the water-cement ratiois 0.40 and 65 mm (2.5 in.) if the water-cement ratio is 0.45.Even greater cover, or the provision of additional corrosionprotection treatments, may be required in some environ-ments. These recommendations can also be applied to otherreinforced concrete components exposed to chloride ionsand intermittent wetting and drying.

Even where the recommended cover is specified, con-struction practices must insure that the specified cover isachieved. Conversely, placing tolerances, the method of construction, and the level of inspection should be consid-ered in determining the specified cover.

The role of cracks in the corrosion of reinforcing steel iscontroversial. Two schools of thought exist. 3.32,3.33 Oneviewpoint is that cracks reduce the service life of structuresby permitting more rapid penetration of carbonation and ameans of access of chloride ions, moisture, and oxygen to thereinforcing steel. The cracks thus accelerate the onset of thecorrosion processes and, at the same time, provide space forthe deposition of the corrosion products. The other viewpointis that while cracks may accelerate the onset of corrosion, such

Fig. 3.1Effect of water-cement ratio on salt penetration 3.5

Fig. 3.2Effect of inadequate consolidation on salt penetration 3.5

Fig. 3.3Effect of water-cement ratio and depth of cover

on relative time to corrosion 3.5
http://201-2r.pdf/http://201-2r.pdf/http://201-2r.pdf/http://201-2r.pdf/


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corrosion is localized. Since the chloride ions eventuallypenetrate even uncracked concrete and initiate more wide-spread corrosion of the steel, the result is that after a fewyears service there is little difference between the amount of corrosion in cracked and uncracked concrete.

The differing viewpoints can be partly explained by thefact that the effect of cracks is a function of their origin,

width, intensity, and orientation. Where the crack is perpen-dicular to the reinforcement, the corroded length of inter-cepted bars is likely to be no more than three bardiameters. 3.33 Cracks that follow the line of a reinforcementbar (as might be the case with a plastic shrinkage crack, forexample) are much more damaging because the corrodedlength of bar is much greater and the resistance of the con-crete to spalling is reduced. Studies have shown that cracksless than about 0.3 mm (0.012 in.) wide have little influenceon the corrosion of reinforcing steel. 3.8 Other investigationshave shown that there is no relationship between crack widthand corrosion. 3.34-3.36 Furthermore, there is no direct rela-

tionship between surface crack width and the internal crack width. Consequently, it has been suggested that the controlof surface crack widths in design codes is not the most ratio-nal approach. 3.37

For the purposes of design, it is useful to differentiate be-tween controlled and uncontrolled cracks. Controlled cracksare those which can be reasonably predicted from a knowl-edge of section geometry and loading. For cracking perpen-dicular to the main reinforcement, the necessary conditionsfor crack control are that there be sufficient steel so that it re-mains in the elastic state under all loading conditions, andthat at the time of cracking, the steel is bonded (i.e., crackingmust occur after the concrete has set).

Examples of uncontrolled cracking are cracks resultingfrom plastic shrinkage, settlement, or an overload condition.Uncontrolled cracks are frequently wide and usually causeconcern, particularly if they are active. However, they can-not be dealt with by conventional design procedures, andmeasures have to be taken to avoid their occurrence or, if they are unavoidable, to induce them at places where theyare unimportant or can be conveniently dealt with, by sealingfor example.

3.3Methods of excluding external sources ofchloride ion from concrete

3.3.1 Waterproof membranes Waterproof membraneshave been used extensively to minimize the ingress of chlo-ride ions into concrete. Since external sources of chlorideions are waterborne, a barrier to water will also act as a bar-rier to any dissolved chloride ions.

The requirements for the ideal waterproofing system arestraightforward; 3.38 it should:

(a) Be easy to install,(b) Have good bond to the substrate,(c) Be compatible with all the components of the system

including the substrate, prime coat, adhesives, and overlay(where used),

(d) Maintain impermeability to chlorides and moisture un-der service conditions, especially temperature extremes,crack bridging, aging, and superimposed loads.

The number of types of products manufactured to satisfythese requirements makes generalization difficult. Any sys-tem of classification is arbitrary, though one of the most use-ful is the distinction between the preformed sheet systems

and the liquid-applied materials.3.38

The preformed sheetsare manufactured under factory conditions but are often dif-ficult to install, usually require adhesives, and are highly vul-nerable to the quality of the workmanship at criticallocations in the installation. Although it is more difficult tocontrol the quality of the liquid-applied systems, they areeasier to apply and tend to be less expensive.

Given the different types and quality of available water-proofing products, the differing degrees of workmanship,and the wide variety of applications, it is not surprising thatlaboratory 3.39-3.41 and field 3.14,3.42,3.43 evaluations of membrane performance have also been variable and some-times contradictory. Sheet systems generally perform better

than liquid-applied systems in laboratory screening tests be-cause quality of workmanship is not a factor. Although therehas been little uniformity in both methods of test or accep-tance criteria, permeability (usually determined by electricalresistance measurements) has generally been adopted as themost important criterion. However, some membranes do of-fer substantial resistance to chloride and moisture intrusion,even when pinholes or bubbles occur in the membrane. 3.41

Field performance has been found to depend not only on thetype of waterproofing material used, but also on the quality of workmanship, weather conditions at the time of installation, de-sign details, and the service environment. Experience hasranged from satisfactory 3.43 to failures that have resulted in themembrane having to be removed. 3.44,3.45

Blistering, which affects both preformed sheets and liquid-applied materials, is the single greatest problem encounteredin applying waterproofing membranes. 3.46 It is caused by theexpansion of entrapped gases, solvents, and moisture in theconcrete after application of the membrane. The frequencyof blisters occurring is controlled by the porosity and mois-ture content of the concrete 3.47 and the atmospheric conditions.Water or water vapor is not a necessary requirement for blis-ter formation, but is often a contributing factor. Blisters mayalso result from an increase in concrete temperature or a de-crease in atmospheric pressure during or shortly after appli-cation of membranes. The rapid expansion of vapors duringthe application of hot-applied products sometimes causespunctures (which are termed blowholes) in the membrane.

Membranes can be placed without blisters if the atmo-spheric conditions are suitable during the curing period.Once cured, the adhesion of the membrane to the concrete isusually sufficient to resist blister formation. To insure goodadhesion, the concrete surface must be carefully preparedand be dry and free from curing membranes, laitance, andcontaminants such as oil drippings. Sealing the concrete pri-or to applying the membrane is possible, but rarely practi-cal.3.48 Where the membrane is to be covered (for example,with insulation or a protective layer), the risk of blister for-


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mation can be reduced by minimizing the delay betweenplacement of the membrane and the overlay.

Venting layers have been used in Europe, but rarely inNorth America, to prevent blister formation by allowing thevapor pressures to disperse beneath the membrane. The dis-advantages of using venting layers are that they require con-trolled debonding of the membrane, leakage through the

membrane is not confined to the immediate area of the punc-ture, and they increase cost.3.3.2 Polymer impregnation Polymer impregnation con-

sists of filling some of the voids in hardened concrete with amonomer and polymerizing in situ. Laboratory studies havedemonstrated that polymer impregnated concrete (PIC) isstrong, durable, and almost impermeable. The properties of PIC are largely determined by the polymer loading in theconcrete. Maximum polymer loadings are achieved by dry-ing the concrete to remove nearly all the evaporable water,removing air by vacuum techniques, saturation with a monomerunder pressure, and polymerizing the monomer in the voidsof the concrete while simultaneously preventing evaporationof the monomer.

In the initial laboratory studies on PIC, polymerizationwas accomplished by gamma radiation. 3.49 However, this isnot practical for use in the field. Consequently, chemical ini-tiators, which decompose under the action of heat or a chem-ical promoter, have been used exclusively in fieldapplications. Multifunctional monomers are often used to in-crease the rate of polymerization.

The physical properties of PIC are determined by the ex-tent to which the ideal processing conditions are compro-mised. Since prolonged heating and vacuum saturation aredifficult to achieve, and increase processing costs substan-

tially, most field applications have been aimed toward pro-ducing only a surface polymer impregnation, usually to adepth of about 25 mm (1 in.). Such partially impregnatedslabs have been found to have good resistance to chloridepenetration in laboratory studies, but field applications havenot always been satisfactory. 3.5,3.50

There have been a few full-scale applications of PIC toprotect reinforcing steel against corrosion and it must still beconsidered largely an experimental process. Some of the dis-advantages of PIC are that the monomers are expensive andthat the processing is lengthy and costly. The principal defi-ciency identified to date has been the tendency of the con-crete to crack during heat treatment.

3.3.3 Polymer concrete overlays Polymer concrete over-lays consisting of aggregate in a polymer binder have beenplaced experimentally.

Most monomers have a low tolerance to moisture and lowtemperatures; hence the substrate must be dry and in excessof 4 C (40 F). Improper mixing of the two (or more) compo-nents of the polymer has been a common source of problemsin the field. Aggregates must be dry so as not to inhibit thepolymerization reaction.

Workers should wear protective clothing when workingwith epoxies and some other polymers because of the potentialfor skin sensitization and dermatitis. 3.51 Manufacturers rec-

ommendations for safe storage and handling of the chemi-cals must be followed.

A bond coat of neat polymer is usually applied ahead of the polymer concrete. Blistering, which is a common phe-nomenon in membranes, has also caused problems in the ap-plication of polymer concrete overlays. A number of applications were reported in the 1960s. 3.52,3.53 Many lasted

only a few years. More recently, experimental polymer over-lays based on a polyester-styrene monomer have beenplaced, using heavy-duty finishing equipment to compactand finish the concrete. 3.54

3.3.4 Portland cement concrete overlays Portland ce-ment concrete overlays for new reinforced concrete are ap-plied as part of a two-stage construction process. The overlaymay be placed before the first-stage concrete has set or sev-eral days later, in which case a bonding layer is used betweenthe two lifts of concrete. The advantage of the first alterna-tive is that the overall time of construction is shortened andcosts minimized. In the second alternative, cover to thereinforcing steel can be assured, and small construction tol-

erances achieved because dead load deflections from theoverlay are very small. No matter which sequence of con-struction is employed, materials can be incorporated in theoverlay to provide superior properties, such as resistance tosalt penetration and wear and skid resistance, than possible us-ing single stage construction.

Where the second stage concre te is placed after the firststage has hardened, sand or water blasting is required toremove laitance and to produce a clean, sound surface.Resin curing compounds should not be used on the first-stage construction because they are difficult to remove.Etching with acid was once a common means of surfacepreparation. 3.55,3.56 but is now rarely used because of thepossibility of contaminating the concrete with chloridesand the difficulty of disposing of the runoff.

Several different types of concrete have been used as con-crete overlays including conventional concrete, 3.57 concretecontaining steel fibers, 3.57 and internally sealed con-crete.3.57,3.60 However, two types of concrete, low-slumpand latex-modified concrete, each designed to offer maxi-mum resistance to penetration by chloride ions, have beenused most frequently.

3.3.5 Low-slump concrete overlay sThe performance of low-slump concrete is dependent solely on the use of con-ventional materials and good quality workmanship. Thewater-cement ratio is reduced to the minimum practical(usually about 0.32) through the use of a high cement content(over 470 kg/m 3 or 800 lb/yd 3) and a water content sufficientto produce a slump less than 25 mm (1 in.). The concrete isair-entrained and a water-reducing admixture or mild retard-er is normally used. The use of such a high cement factor andlow workability mixture dictates the method of mixing, plac-ing, and curing the concrete.

Following preparation of the first-stage concrete, a bond-ing agent of either mortar or cement paste is brushed into thebase concrete just before the application of the overlay. Thebase concrete is not normally prewetted. The overlay con-crete is mixed on site, using either a stationary paddle mixer


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or a mobile continuous mixer, because truck mixers are notsuited to producing either the quantity or consistency of con-crete required. The concrete must be compacted and screed-ed to the required surface profile using equipment speciallydesigned to handle stiff mixtures. Such machines are muchheavier and less flexible than conventional finishing ma-chines and have considerable vibratory capacity. The perme-

ability of the concrete to chloride ions is controlled by itsdegree of consolidation, which is often checked with a nucle-ar density meter as concrete placement proceeds. Wet burlapis placed on the concrete as soon as practicable without dam-aging the overlay (usually within 20 min of placing), and thewet curing is continued for at least 72 hr. Curing compoundsare not used, since not only is externally available water re-quired for more complete hydration of the cement, but thethin overlay is susceptible to shrinkage cracking and the wetburlap provides a cooling effect by evaporation of the water.

Low-slump concrete was originally proposed as a repairmaterial for concrete pavements 3.55 and was developed forpatches and overlays on bridges 3.61-3.63 in the early 1960s.More recently, concrete overlays have been used as a pro-tection against reinforcing steel corrosion in new bridges. Ingeneral, the performance of low-slump overlays has beengood.3.64-3.66 Local bond failures have been reported, butthese have been ascribed to inadequate surface preparation 3.64

or premature drying of the grout. 3.66 The overlays are suscep-tible to cracking, especially on continuous structures, thoughthis is a characteristic of all rigid overlays.

3.3.6 Latex-modified concrete overlays Latex-modifiedconcrete is conventional portland cement concrete with theaddition of a polymeric latex emulsion. The water of sus-pension in the emulsion hydrates the cement and the poly-

mer provides supplementary binding properties to produce aconcrete having a low water-cement ratio, good durability,good bonding characteristics, and a high degree of resistanceto penetration by chloride ions, all of which are desirableproperties in a concrete overlay.

The latex is a colloidal dispersion of synthetic rubber par-ticles in water. The particles are stabilized to prevent coagu-lation, and antifoaming agents are added to preventexcessive air entrapment during mixing. Styrene-butadienelatexes have been used most widely. The rate of addition of the latex is approximately 15 percent latex solids by weightof the cement.

The construction procedures for latex-modified concreteare similar to those for low-slump concrete with minor mod-ifications. The principal differences are:

1. The base concrete must be prewetted for at least 1 hr pri-or to placing the overlay, because the water aids penetrationof the base and delays film formation of the latex.

2. A separate bonding agent is not always used, becausesometimes a portion of the concrete itself is brushed over thesurface of the base.

3. The mixing equipment must have a means of storingand dispensing the latex.

4. The latex-modified concrete has a high slump so thatconventional finishing equipment can be used.

5. Air entrainment of the concrete is believed not requiredfor resistance to freezing and thawing.

6. A combination of moist curing to hydrate the port-land cement and air drying to develop the film formingqualities of the latex are required. Typical curing timesare 24 hr wet curing, followed by 72 hr of dry curing. Thefilm-formation property of the latex is temperature sensi-

tive and film strengths develop slowly at temperatures be-low 13 C (55 F). Curing periods at lower temperaturesmay need to be extended and application at temperaturesless than 7 C (45 F) is not recommended.

Hot weather causes rapid drying of the latex-modified con-crete, which makes finishing difficult and promotes shrinkagecracking. Some contractors have placed overlays at night toavoid these problems. The entrapment of excessive amountsof air during mixing has also been a problem in the field. Mostspecifications limit the total air content to 6.5 percent. Higherair contents reduce the flexural, compressive, and bondstrengths of the overlay. Furthermor