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CORROSION OF METALS 222R-1 ACI 222R-96 Corrosion of Metals in Concrete Reported by ACI Committee 222
  CORROSION OF METALS 222R-1 This committee report has been prepared to reflect 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. Keyword;  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 1—Introduction, p. 222R-1 1.1—Background1.2—Scope1.3—References Chapter 2—Mechanism of corrosion of steel inconcrete, p. 222R-3 2.1—Introduction2.2—Principles of corrosion2.3—Effects of the concrete environment on corrosion2.4—References Chapter 3—Protection against corrosion in newconstruction, p. 222R-11 3.1—Introduction3.2—Design and construction practices3.3—Methods of excluding external sources of chlorideion from concrete3.4—Methods of protecting reinforcing steel from chlo-ride ion   ACI 222R-96 Corrosion of Metals in Concrete Reported byACICommittee 222 ACI 222R-96 replaces ACI 222R-89 and became effective May 23, 1996.Copyright (c) 1997, American Concrete Institute. All rights reserved includingrights 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. Hope Chairman Keith A. Pashina Secretary John P. Broomfield 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 Members Stephen 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  222R-2ACI MANUAL OF CONCRETE PRACTICE 3.5—Corrosion control methods3.6—References Chapter 4—Procedures for identifying corrosionenvironments and active corrosion in concrete,p. 222R-21 4.1—Introduction4.2—Methods of evaluation4.3—References Chapter 5—Remedial measures, p. 222R-23 5.1—Introduction5.2—General5.3—Applicability5.4—The remedies and their limitations5.5—Summary5.6—References Chapter 6—References to documents of standard-producing organizations, p. 222R-28Appendix A—Standard Test Method for Water-Soluble Chloride Available for Corrosion ofEmbedded Steel in Mortar and Concrete Using theSoxhlet Extractor, p. 222R-28CHAPTER 1—INTRODUCTION1.1—Background  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 inconcrete 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  CORROSION OF METALS 222R-3 success in each of these areas though problems resulting fromcorrosion of embedded metals are far from eliminated. 1.2 — Scope  This report discusses the factors that cause and controlcorrosion 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 — References  1.1. Tremper, Bailey; Beaton, John L; and Stratfull, R. F. “Causes andRepair 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: Scientific Prin-ciples and Practical Applications , Edward Arnold, London, 1960, 303 pp.1.3. Biczók, Imre, Concrete Corrosion and Concrete Protection , 3rdEdition, Académiai Kiadó, Budapest, 1964. CHAPTER 2—MECHANISM OF CORROSION OFSTEEL IN CONCRETE2.1 — Introduction  This 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 concretevariables that influence corrosion, including concrete mixproportions, quality, and cover, and the effects of corrosioninhibiting admixtures and carbonation. 2.2—Principles of corrosion 2.2.1 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 metalsurface 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 isone 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  H 2 O + 1  /  4 O 2  + 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 the“polarizations” 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 diffusion through 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  222R-4ACI MANUAL OF CONCRETE PRACTICE 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.  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 isadded. 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.  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.  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. 2.2—Effect of concentration of sodium chlorideon corrosion rate 2.4 Fig. 2.1—Effect of water-cement ratio and thickness on diffusion of oxygen through mor-tar and concrete 2.2
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