Repair materials and techniques for concrete structures in nuclear power plants

Nuclear Engineering and Design 181 (1998) 71–89

Repair materials and techniques for concrete structures innuclear power plants

Paul D. Krauss a,*, Daniel J. Naus b

a Wiss, Janney, Elstner Associates, Inc., 330 Pfingsten Road, Northbrook, IL 60062-2095, USAb Martin Marietta Energy Systems Inc., Oak Ridge National Laboratory, Oak Ridge, TN 37831-8056, USA

Abstract

The paper summaries portions of work of the Structural Aging Program, sponsored by the Nuclear RegulatoryCommission (NRC). The paper addresses the assessment and repair of concrete structures in nuclear power plants.It presents the results of a survey of the the nuclear power plants in the United States to identify susceptible concretecomponents, rates of occurrence of deterioration, and to determine the durability of repairs. The paper describesdeterioration mechanisms and discusses their effect. Repair techniques are described. Evaluation techniques andnondestructive test techniques are also discussed. © 1998 Elsevier Science S.A. All rights reserved.

1. Introduction

The Structural Aging Program, sponsored bythe Nuclear Regulatory Commission (NRC), hasthe overall objective of developing an improvedbasis for evaluating nuclear power plant struc-tures as they age. The work conducted and man-aged by the Oak Ridge National Laboratoryincludes the development of a structural materialsproperty database, establishment of structuralcomponent assessment and repair methodologies,and the formulation of a methodology for contin-ued service determinations. Two reports havebeen recently issued concerning the evaluationand repair of concrete structures in nuclear powerplants: ‘Repair Materials and Techniques forConcrete Structures in Nuclear Power Plants’

(Krauss, 1994) and ‘Report on Aging of NuclearPower Plant Reinforced Concrete Structures’(Naus et al., 1996).

The report ‘Repair Materials and Techniquesfor Concrete Structures in Nuclear Power Plants,’addresses the assessment and repair of concretestructures in nuclear power plants. It presents theresults of a survey of the nuclear power plants inthe USA. The survey was performed to identifythe concrete components most susceptible todurability problems, to determine the rate of oc-currence of deterioration, and to determine thedurability of repairs. The report describes thevarious deterioration mechanisms and discussesthe effect of each deterioration mechanism. Em-phasis is placed on mechanisms that affect thelong-term durability of concrete. Repair tech-niques are described and practical experience isused to compare the various repair options as anaid in selection of the proper materials and tech-

* Corresponding author. Tel.: +1 847 2727400; fax: +1847 2915189.

0029-5493/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.

PII S0029-5493(97)00336-1

P.D. Krauss, D.J. Naus / Nuclear Engineering and Design 181 (1998) 71–8972

niques. Evaluation techniques and nondestructivetest techniques are discussed. This paper discussesportions of these two reports related to the repairof concrete structures. The two reports should bereviewed for more in-depth information.

Many of the nuclear structures in the USA areover 20 years of age. As these power plants con-tinue to age, there is an increased potential needfor sound, proven technical recommendations onthe repair of their reinforced and prestressed con-crete structures. Within a nuclear power plantthere are many different types of structures, eachof which may require repair procedures that areappropriate for its specific function.

2. Plant survey

A survey was distributed to solicit informationon the locations and types of concrete distresscommonly found in US nuclear power plants. Thesurvey included questions concerning the types ofrepairs performed and requested information orreports on the durability and service life of theserepairs. Twenty-nine plants returned surveys andthe surveys represent 41 reactor units. A widegeographical distribution of the United States wasrepresented by the completed surveys.

Concrete structure evaluations are usually lim-ited to an evaluation of prestressing systems ofthe post-tensioned concrete containments and ageneral visual survey of the exposed concrete sur-faces. Other than these limited inspections, regu-lar inspections of concrete structures are nottypically performed. All plants conducting inspec-tions rely mainly on visual inspection methodsalthough some periodically remove core samplesor perform chemical analyses on the concrete.Only a few plants reported making half-cell po-tential measurements or the use of other nonde-structive techniques such as pulse velocity, pulseecho, impact echo or acoustic emissiontechniques.

Twenty-six of the 29 plants reported that theyexperienced some type of damage or deteriorationto the concrete. Cracking was the most commontype of deterioration, with 25 plants reportingcracking situations. A summary of the reporteddeterioration is shown below.

Number ofDeterioration type %plants25Cracking 86

6519SpallingStaining 16 55

51Honeycombing 1510Reinforcing steel 34

corrosionEfflorescence 10 34

288PopoutsScaling 7 24

247Delaminations

Most of the honeycombing problems were dueto initial construction problems and repairs weremade. Many of the delaminations were located inconcrete domes. Other forms of deterioration re-ported by single plants were: damage due to icebuildup in the cooling tower fill, dusting, degrada-tion of the prestressing steel, deterioration ofgrout, and staining of concrete due to leakage oftendon grease.

The most common locations of deterioration inthe 19 PWR plants were in the containment domeand in the walls and slabs of auxiliary structures.The most common areas of damaged concrete forthe 10 BWR plants were in the slabs, walls, andequipment supports of the secondary containmentreactor buildings and auxiliary structures.

2.1. Plant repair procedures (brief re6iew)

Twenty-seven of the 29 plants reported thatthey have repaired damaged concrete at the plant.Fourteen plants reported using dry packing forcrack repairs or spall repairs. Other methods com-monly used to repair cracks included epoxy injec-tion (13 plants), grout injection (12 plants),flexible sealing (12 plants) and routing and sealing(8 plants). Proprietary epoxy materials were usedfor injection. Flexible sealing of cracks was per-formed with polysulfide, polyurethane, and otherunspecified materials. Plants also reported usingpolyurethanes, epoxies, and polymer-modifiedmortars for routing and sealing cracks. Drillingand plugging of cracks was reported by threeplants. Three plants reported stitching cracks, us-ing deformed reinforcing steel bars or steel plates.

P.D. Krauss, D.J. Naus / Nuclear Engineering and Design 181 (1998) 71–89 73

Spall or delamination repairs were most com-monly done by filling the cavity with normalconcrete, grout, or dry pack. Fourteen plantsreported using concrete replacement and 15 plantsreported using dry pack. One plant reported usingshotcrete, and none of the plants reported usingpreplaced aggregate techniques. The use of sealersor coatings was reported by only five plants. Thesealers or coatings used included silanes, epoxies,high molecular weight methacrylate, coal tar andunspecified materials. Four plants utilized ca-thodic protection for conventionally reinforcedstructures and two plants reported having ca-thodic protection on prestressed concrete mem-bers.

2.2. General comments from the sur6ey

Deterioration has generally been minor due tothe high initial quality of the original constructionand the relatively young age of most plants. Mostrepairs were associated with problems during ini-tial construction. Repairs are typically performedon an as-needed basis. Little information is avail-able on what was used for the repair, how therepair was installed or the durability of the re-pairs. Follow-up evaluations of these concreterepairs are not commonly performed.

3. Mitigating ageing of concrete containmentbuildings

In-service inspection techniques are availablethat can indicate the occurrence and extent ofage- or environment-stressor related deterioration.Periodic application of these techniques can mon-itor the progress of deterioration. Results ob-tained from these programs can be used todevelop and implement remedial action prior tothe structure achieving an unacceptable level ofperformance. Depending on the degree of deterio-ration and the residual strength of the structure,the function of remedial measures may be struc-tural, protective, cosmetic, or any combination ofthese three requirements. Basic components of aremedial measures program include diagnosis(damage evaluation), prognosis (can repair be

made and is it economical), scheduling (priorityassignments), method selection (depends on na-ture of distress, adaptability of proposed method,environment, and costs), preparation (function ofextent and type of distress), and application(Waddell, 1980). The basic steps of a typicalrepair strategy include the following (Interna-tional Union of Testing and Research Laborato-ries for Materials and Structures, 1994):� Initial assessment of the condition of the

structure� Decision on strategy to be based on:

determination of the cause of the distressthe environmentconsequences of the damagestructural aspectstimingeconomyremaining period of uselocal availability of materials andexperience for repair under consideration

� Detailed design plus choice of materials� Execution including quality control� Assessment of completed work� Definition of maintenance requirements3.1. Initial repair considerations

The deterioration of concrete structures is par-ticularly accelerated when heat and moisture arereadily available. This has been repeatedly ob-served with most mechanisms, including corro-sion, alkali-silica reactivity, and sulfate attack.While nuclear plant structures contain many in-door structures and members that are protectedfrom rain water, these same structures and mem-bers are generally thick and in many cases utilizesteel liner plates that prevent drying from oneface. As a result, many members maintain highwater content and relative humidities for years,even decades. These high internal humidities cansupply the necessary moisture to sustain numer-ous deterioration mechanisms. Another potentialaccelerating factor is the high concrete tempera-tures which, when combined with high internalhumidities, further accelerate deterioration.Therefore, condition surveys should certainly fo-cus extra attention on large members exposed toelevated temperatures.


While this paper provides general guidelines onrepair and durability, such a document cannotreplace a skilled engineer or materials technologistwith experience in the detection and repair ofdefects in concrete structures

The first step in any repair activity is a thor-ough assessment of the damaged structure orcomponent including evaluation of the (1) causeof deterioration, (2) extent of deterioration, and(3) effect of deterioration on the functional andperformance requirements. Basic elements of theassessment include (1) preplanning and accumula-tion of background data (e.g. age, previous condi-tion surveys, design documents, as-built drawings,materials data sheets, etc.), (2) visual examination,(3) in situ and laboratory testing, and (4) evalua-tion of collated survey data and determination ofcause(s) of deterioration. The extent of the dam-age must be identified, and the condition of theportions of the structure to remain in service mustbe determined. The extent of deterioration is oftenunderestimated. Therefore, estimates should beconservative; shoring and additional support forload-carrying members should be carefully con-sidered prior to demolition or removal of deterio-rated concrete. Based on careful examination andanalysis, the repair methodology is selected. Thismust be done on an individual case basis andrequires careful judgement by an experiencedengineer.

From this information a remedial measuresstrategy is developed as based on the consequenceof damage (e.g. affect of degradation on struc-tural safety), the time requirements for implemen-tation (e.g. immediate or future safety concern),economic aspects (e.g. partial or complete repair),and the residual service life requirements (e.g.desired residual service life will influence actiontaken (Price et al., 1993)). Basic remedial mea-sures options include (1) no active intervention;(2) carry out repairs to restore deteriorated ordamaged parts of structure to a satisfactory con-dition; (3) if safety margins are presently accept-able, take action to prevent deterioration fromgetting worse; and (4) demolish and rebuild all orpart of structure. Quite often options (2) and (3)are considered jointly.

When selecting a repair material, a specific onemay not be entirely suitable for all the uses rec-ommended by the manufacturer. Dozens of repairformulations have been encountered that weretotally unsuitable for the recommended repair, fora variety of reasons. Also, a material should notbe specified unless its ingredients are known. Sev-eral fast-setting materials currently on the marketare unstable under moist conditions. This is be-cause they depend on the setting of plaster fortheir early strength and on Portland cement forlater strength development. Plaster consists of cal-cium sulfate, which can result in severe internalsulfate attack later on if sufficient moisture isavailable. This causes expansion, cracking andloss of strength. The patches usually grow untilthey either fall out of the excavation or crack thesurrounding concrete.

The most appropriate materials for patchingare those that are closest in composition to thematerial to be patched. Usually, this means Port-land cement concrete for large patches or Port-land cement mortar for small ones. By patchingwith a cementitious material, the final thermaland structural properties of the repair will besimilar to the base concrete. Many proprietaryformulations are based on Portland cement, andmany of these include a latex emulsion. Theselatex emulsions may be acceptable and offer someadvantages if the proper formulation is used.Non-Portland cement binders have also been usedsuccessfully. One example is magnesium phos-phate cement, which shrinks much less than Port-land cement during hardening. This is a decidedadvantage in patching materials. However, aswith all materials, magnesium phosphate concretealso has limitations.

Repair materials should have low shrinkage.Other properties, such as thermal coefficient ofexpansion, strength, modulus of elasticity, andcreep should be close to those of the substrateconcrete. The thermal coefficient of expansion is ameasure of the expansion and contraction of amaterial subjected to temperature changes. Themodulus of elasticity is a measure of the stiffnessof a material. A low modulus material will deflectand deform more under load than a high modulusmaterial. Creep is a measure of long-term defor-


mation under substantial load. Generally, a repairmaterial with a slightly lower thermal coefficient,lower modulus of elasticity and higher creep, isoptimum so that cracking stresses are minimized.

Commonly, repairs are performed with highcement content, high strength mortars. This re-sults in a repair material with a high thermalcoefficient, high shrinkage, high modulus and lowcreep. The advantages of higher strength con-cretes are that they are generally less permeableassuming they do not crack. A lower cementcontent material may be more compatible anddurable if the lower strength can be tolerated.High strength is typically not as important asgood bond and no cracking.

Polymers used as binders in patching mortarscan be effective when used in small quantities.However, when used to fill large cavities, thedifferences in thermal coefficients between theseresin-based materials and the substrate concretebecome important. Even if mixed with largeamounts of aggregate, most polymer concretesexpand and contract several times the amountexperienced by Portland cement concrete. In largequantities, this can cause enough stress at theinterface between patch and concrete to causebond failure. Polymers should be considered foroverlays and coatings, especially in environmentsaggressive to Portland cement materials.

Special attention should be given to repairs thatare exposed to aggressive fluids. The chemicalcomposition of the fluids should be known andthe repair materials must be compatible. Acidsand sulfates will attack and degrade Portlandcement materials but not most polymers. Strongsolvents may attack or soften some polymer mate-rials. Dilute acids can sometimes be more aggres-sive than concentrated acids to Portland cementconcrete. Hard water that contains large amountsof calcium salts are less corrosive than soft water.Ultra-pure water can aggressively leach Portlandcement concrete. Low concentration solutions ofsodium chloride (1–3%) may be more corrosive toexposed steel than high concentration solutions(10–20%).

The location and environment of repairs mustalso be considered. Limited access may restrictwhat type of equipment can be used for installa-

tion. Steel congestion may determine the consis-tency of the repair material required. Goodventilation is required when using most polymermaterials. Some resins may become brittle afterlong-term exposure to elevated temperatures whileother resins may flow. Elevated temperatures alsoincrease the solvent attack on polymer resins.

The design of the repair and the material selec-tion can be difficult, but problems that result inloss of durability are often due to poor installa-tion. Proper surface preparation, batching, mix-ing, placement and curing are all important forlong-term durability. Specifying materials thathave previous success within the plant is goodpractice since workers are familiar with the mate-rial and problems during installation are lesslikely. Review of manufacturer’s data, contactwith others who have used the materials, anddiscussions with consultants specializing in repairare all valuable in establishing the anticipateddurability of the repair. Within reason, the bestrepair technique and material should be selectedregardless of cost, and conservative estimates ofperformance should always be made.

If the durability of the repair is of major impor-tance, or the volume of the repair is large, testingand trial installations of the repair material arerecommended. A material that performed well inthe past may not perform well now due tochanges in formulation or raw material supplies.Material testing to ensure that materials meet theproject and manufacturer’s specifications, and ac-celerated testing under the exposure conditionsexpected in-service are good practices. Tests forcementitious materials might include set time atexpected temperature extremes, flow, density,strength gain, bond, freezing and thawing resis-tance, resistance to aggressive media, and abra-sion resistance. Additional tests for polymermaterials might include viscosity, specific gravity,solids content, flash point, vapor pressure, bondto wet and dry concrete, and properties at ele-vated temperatures.

3.2. Repair document

The American Concrete Institute (ACI) Com-mittee Reports 201.2 (ACI, 1968) and 546 (ACI,


1988) discuss the repair of concrete and areexcellent references. The ACI has also pro-duced a number of documents used in semi-nar series. Two documents of particularinterest are entitled ‘Troubleshooting ConcreteProblems—And How to Prevent Them in theFuture’ (ACI, 1987a) and ‘Concrete RepairBasics’ (ACI, 1987b). Persons involved in re-pair of concrete structures should review thesedocuments to become familiar with typical re-pair procedures and techniques.

Both the US Corps of Engineers and theUS Bureau of Reclamation have recently pro-duced concrete repair manuals and specifica-tions (US Army Corps of Engineers, 1986;US Department of the Interior, 1990). Themanual by the Corps of Engineers entitled‘Evaluation and Repair of Concrete Structures’(US Army Corps of Engineers, 1986) providesa standard format for repair techniques. Thedocument includes chapters on evaluation ofconcrete, causes of distress and deteriorationof concrete, selection of materials and meth-ods for repair and rehabilitation, concrete re-moval and preparation for repair, materialsand methods for repair and rehabilitation,maintenance of concrete, specialized repairsand case histories. The document does not,however, provide extensive information as tothe types of materials available for repairs.The discussion on material applications andlimitations tends to be brief. No informationis provided on the service life of a repair,nor does it provide a ranking system for thedurability of repairs of different types.

The US Corps of Engineers REMR Note-book (US Army Corps of Engineers, 1992)includes material data sheets on specific prod-ucts and test data. The manufacturer’s dataand test results by the Corps of Engineers areincluded. A computer database of the Note-book has been established and is accessible bycomputer modem. The database has over 1700products listed by application or type. Specificinstruction on the REMR database can be re-ceived from the US Army Engineer Water-ways Experiment Station, Vicksburg, MS.

4. Typical remedial measures techniques forreinforced concrete containment structures

Application of the basic remedial measuresstrategy includes the repair of damaged concreteand mitigation of the cause of deterioration. Dete-rioration of reinforced concrete structures gener-ally will result in cracking, spelling, ordelamination of the cover concrete. Of these, sur-veys of general civil engineering construction (Mc-Donald and Cambell, 1985) and nuclear powerplant (NPP) structures (Krauss, 1994; Gregor andHookham, 1993; International Atomic EnergyAgency, 1995) indicated that cracking was by farthe most frequently occurring problem. Seepageof water through construction joints or crackswas also reported in the NPP surveys as well asthe presence of honeycombs and voids.

4.1. Cracks

Cracking in concrete structures can be expectedto occur for a number of reasons, including plas-tic and drying shrinkage, thermal effects, fatigue,reactive aggregates, and excessive loads. Concretestructures have cracks that can be classified intotwo categories: microcracks and macrocracks. Mi-crocracks form within the cement paste adjacentto the aggregate particles and are discontinuous,very narrow, and require no repair action. Themicrocracks are important from the standpointthat under increased loadings they become widerand propagate, and can eventually reach a size(i.e. macrocracks) sufficient to deteriorate the con-crete, accelerate corrosion of embedded steel, orproduce leakage. Macrocracks may be importantto service life.

The loss of mixing water from newly cast concreteduring exposure to air at less than 100 percentrelative humidity (RH) causes drying shrinkage.Drying shrinkage is the most common cause ofcracking. The majority of the drying shrinkage willoccur within the first several years after construc-tion, except for massive concrete members and thosemembers partially covered with steel liner plateswhich may take decades. The rate of water loss fromconcrete depends on the evaporation rate and thesurface-to-volume ratio of the concrete members.


Cracks reduce the service life of reinforced con-crete structures as they permit access of chlorideions, moisture, and oxygen to the steel, accelerat-ing the onset of corrosion. Factors that mayinfluence corrosion occurrence include crack ar-rangement, width, depth, shape, orientation withrespect to steel, intensity, origin; type of structure;and service environment (Campbell-Allen andLau, 1978).

Researchers have provided a number of relation-ships between maximum permissible crack widthand exposure condition, but many are conflicting.Larger crack widths typically increase the proba-bility of corrosion: however, crack widths are notalways reliable indicators of corrosion and thedeterioration to be expected. In general, the follow-ing guidance is representative: for severe exposureto deicing chemicals or for water tightness, widths50.1 mm; for normal exterior exposures or inte-rior exposures subjected to high humidities, widths50.2 mm; for internal protected structures, widths50.3 mm; and for structures containing chemicalsor fluids that must remain leak tight, widths 50.05mm. However, experience shows that in environ-ments of high chloride or other aggressive agents,any visible crack reduces durability and repairshould be considered.

4.1.1. Selection of crack repair techniqueAfter identifying that a crack is of sufficient size

to require repair, it is important to determine ifthe crack is active or dormant. Active cracks arethose for which the mechanism causing crackingis still at work, whereas dormant cracks are thosecaused by a condition that is not expected torecur. Having established the cause of cracking,several questions should be addressed:� Is repair necessary? Repair of cracking caused

by expansion products of internal chemical re-actions may not be practical.

� Should repair be treated as spelling rather thancracking? If the damage is such that loss ofconcrete mass is probable, treatment of thecracks may not be adequate. For example,cracking due to corrosion of embedded metalor freezing and thawing would be better treatedby removal and replacement of concrete thanby one of the crack repair methods.

� Is it necessary that the condition causing thecrack be corrected? Is doing so economicallyfeasible?

� What will be the future movement of thecrack?

� Is strengthening across the crack required?� What is the moisture environment of the crack?

With these questions answered, a repair tech-nique can be selected. Repair methodologies differfor dormant and active cracks. Detailed descrip-tions of techniques available for repair of dor-mant or active cracks in reinforced concrete areavailable elsewhere (Krauss, 1985; Naus, 1986;Krauss, 1994; US Army Corps of Engineers,1986; US Department of the Interior, 1990).

Dormant cracks can be resin injected usingepoxy or high molecular weight methacrylate(HMWM). HMWM resins were developed fortopical treatment of bridge decks that containnumerous fine cracks (Krauss, 1985). Due to thelow viscosity of the HMWM resin (8–20 cps) itreadily flows into very fine cracks, even less than0.02 mm (0.001 in.) wide, by gravity, so pressureinjection is not needed. Epoxy resins are alsoformulated to repair cracks by gravity feed.

Active cracks must be treated as if they werecontrol joints. It must be determined if it is neces-sary to restore the tensile or flexural strengthacross the crack. If the strength must be restored,it is best to install an expansion joint nearby priorto bonding the crack. If the crack is simplybonded, a new crack will occur adjacent to theold. Active cracks that are leaking and must bebonded present the most difficult problem. Firstthe leakage must be stopped and the crack dried.If the crack can not be dried prior to injection,other methods of strengthening should be consid-ered, such as stitching or external stressing.

Active cracks that must be made watertightcannot be easily repaired, because they change inwidth in response to changes in temperature orhumidity. The required seal must have the properelongation and shape factor. ACI 504 (ACI,1990a) contains an excellent discussion of crackrepair and shape factors. To provide a watertightseal in a crack, the crack must be routed out to awidth of at least 10 mm (3/4 in.) to provide somestretching length. In addition, bond must be pre-


vented to the bottom of the routed area to achievean unbonded length.

Elastomer sealants include urethanes, poly-sulfides, acrylics, silicones and epoxy. The selec-tion of the sealant is based on the exposureconditions and the amount of movement of thecrack or joint. Sealants such as asphalt mastics,general purpose latex sealants, and butyl-basedsealants can be used to repair cracks or joints thatare not moving and not continuously wet.

4.2. Water seepage repair

Water seepage through construction joints andcracks may result in leaching of the concrete,entry of aggressive environments into the concretematrix, or unacceptable flow of fluids either intoor out of a facility. Additionally, long-term reac-tions that require the presence of moisture, suchas efflorescence, sulfate attack, or alkali–aggre-gate reactions, also may initiate as seepage intothe concrete occurs. Implementation of a repairactivity, therefore, can prevent possible future de-terioration and the unacceptable migration offluids either into or out of the facility.

A properly implemented repair procedure firstwill identify the source and then repair the path.Chemical grouting using silicate, acrylamide,lignin, or resin (i.e. epoxy, polyester, and ure-thane) systems is the most effective repair tech-nique when there is moisture present. Thechemical grouts consist of solutions of two ormore chemicals that react to form a gel or solidprecipitate, as opposed to cement or clay groutsthat consist of suspensions of solid particles in afluid. The reaction of the chemical grout, whichmay be purely chemical or physicochemical, pro-duces a decrease in fluidity and a tendency tosolidify and form occlusions in channels or fillvoids in the material (US Department of theInterior, 1990). Reaction of the chemical groutcan form either soft flexible, semirigid, or rigidgels. When seepage is intermittent and the paththrough the concrete periodically dries, it can beinjected with epoxy resins, or water can be incor-porated into a urethane injection system to pro-mote expansion and curing to form a flexiblefoam material.

4.3. Spalls

Spalls can occur due to impact, corrosion ofembedded metals, erosion, or problems such asalkali–aggregate reactions, freeze–thaw, and fireexposure. Surface preparation is critical to a suc-cessful spell repair. The concrete substrate mustbe sound, and the exposed surface dry and free ofgrease, oil, and loose particles. Suitable techniquesfor surface preparation include use of small chip-ping hammers (followed by abrasive blasting, andremoval of dust and chips by compressed air) andhigh-pressure water blasting. If steel reinforce-ment is exposed during the removal of degradedmaterial, the excavation should be extended sothat the steel will be enclosed in the patch mate-rial. If the steel reinforcement is corroded, thecorrosion products must be removed and the steelcoated with a barrier material such as epoxy resin,or a high electrically-resistant patch materialutilized.

Shallow spalls (520 mm) are generally re-paired using Portland cement-based mortar mate-rials. Polymer concretes containing epoxies ormethyl methacrylates also have been successfullyutilized. Deep spalls are treated in a similar man-ner to shallow spalls except coarse aggregate isadded to the repair mortar To ensure gooddurability of the repair it is important that therepair material have mechanical and physicalproperties similar to the inplace concrete and thatit is properly consolidated and cured. For massconcrete requiring extensive replacement of mate-rial, the repair patch may be built up in two ormore layers to prevent excessive heat build-up dueto cement hydration. Fly ash can be used as apartial replacement for the cement to reduce themaximum temperature build-up. For vertical oroverhead surface repairs, special precautions arerequired because of the increased difficulty inapplication. Generally, dry packing is used inwhich a very harsh mix (i.e., dry) is applied andcompacted using a blunt instrument (ACI, 1988).Typically properties of materials commonly usedfor spall repairs and additional information onspall repair techniques is available elsewhere(Naus, 1986; ACI, 1988; US Army Corps of Engi-neers, 1986; US Department of the Interior, 1990;Krauss, 1994).


4.3.1. PlacementPrior to patching, unsound concrete can be

identified by sounding with a hammer or dragchain. The edge of the spall should be sawcut orchipped to a near vertical edge, several inchesoutside of the distressed area. Feather edges onspall repairs do not work since they tend to ravel.The surface should be carefully roughened toremove the contaminants and low strength con-crete. It is recommended that chipping hammersno larger than 30 lbs be used.

When batching mortar, proper handling, accu-rate proportioning and thorough mixing are re-quired. The exposed steel should usually beprimed, and some of the mortar should bescrubbed into the surface prior to placing thebalance of the repair material. Excess moisture onthe base concrete should be avoided prior topatching. Good consolidation of the repair mate-rials is essential to ensure good bond strengths,intimate contact between the patch and the sub-strate, good consolidation and long-termdurability.

The best adhesion for cementitious repairs isachieved on a saturated surface dry (SSD) surfacethat is continuing to dry. The slightly dry concretesubstrate pulls minor amounts of moisture fromthe repair material and provides the best bond.The substrate for polymer grouts must be kept asdry as possible prior to topping.

Overlays can provide a high strength surfaceand add protection from aggressive environmentsand to the embedded steel from corrosive environ-ments. Overlays are often designed to be an inte-gral part of the load-carrying capacity of thestructure and can be used to improve drainageand grades. For thicknesses between 6 mm (3/4in.) to 25 mm (1 in.), polymer concretes or groutsare typically used. For overlays between 25 mm (1in.) to 40 mm (1.5 in.) thick, latex-modified con-crete (LMC) is typically used. ACI Committee548 recently published a document entitled Stan-dard Specification for Latex-modified Concrete(LMC) Overlays (ACI, 1992). This document in-cludes recommended specifications for LMC over-lays. Overlays thicker than 40 mm (1.5 in.) arecommonly done with normal Portland cementconcrete.

4.3.2. Repair materialsInorganic spall repair materials include Port-

land cement concrete, latex-modified concrete(LMC), modified high alumina cement concrete,and magnesium phosphate concretes. Organicspall repair materials include epoxy, acrylic,polyester, and urethanes. If the repair must berapid setting, magnesium phosphate, modifiedhigh alumina, and most polymer concretes cansupport loads within 2–3 h. Normal Portlandcement concrete and LMC generally require 1–3days of wet curing. Rapid-setting magnesiumphosphate concretes and polymer concretesshould be placed on a dry substrate and shouldnot be wet cured.

Portland cement-based materials have severaladvantages for repairs. They are relatively inex-pensive, easy to use, and readily available. Theyhave been used extensively in the field and havefamiliar handling characteristics. The main disad-vantages may be occasional poor bonding charac-teristics, slow strength gain, high modulus ofelasticity, high abrasion loss, poor chemical resis-tance, and the potential for excessive shrinkageand cracking.

To withstand the rigors of freezing and thawing(particularly in the presence of deicers), the use ofan air-entraining admixture is mandatory. It iscommonly specified in new concrete but is oftenforgotten in concrete used for repairs. Basically, itshould be used in all concrete to be exposed tofreezing weather, either during construction or inrepair of completed structures.4.3.3. Preplaced aggregate repairs

Preplaced aggregate repairs have the advantageof minimizing the paste volume and reducing theshrinkage of the repair. Preplaced aggregate con-crete is a system in which coarse aggregate isplaced in the form, and a special cement–sandgrout is then pumped into the form, starting atthe bottom, to fill the voids between the aggregateand create a solid concrete unit. The point-to-point contact of the preplaced aggregate particlesreduces shrinkage to a negligible amount. Dryingshrinkage, following proper curing, is normally inthe range of 200 to 400 millionths as compared toordinary concrete, which ranges between 500 and800 millionths.


4.3.4. ShotcreteShotcrete is defined in ACI 116R (ACI, 1990b)

as ‘mortar or concrete pneumatically projected athigh velocity onto a surface; also known as air-blown mortar, pneumatically applied mortar orconcrete, sprayed mortar, and gunned concrete’.Shotcrete can be used for large volume repairsand for general applications over large surfaceswhen increased cover is needed. In a repair pro-ject where thin repair sections less than l50 mm (6in.) depth and large surface areas with irregularcontours are involved, shotcrete is generally moreeconomical than conventional concrete because ofthe saving in forming costs. Due to their impor-tance, nozzlepersons should be evaluated prior toserving in that capacity on projects. This evalua-tion should include a workmanshipdemonstration.

4.3.5. Polymer-modified Portland cement-basedmaterials

The addition of a latex admixture to normalPortland cement concrete enhances its bondstrength, tensile strength, improves its resistanceto chemicals and reduces its permeability. Thelatex emulsions include styrene butadiene,polyvinyl acetate (PVA), acrylics and epoxy emul-sions. Re-emulsifiable PVA bond coats can beplaced long before the application of the concretesince the fresh concrete will soften and bond tothe PVA. However, they should not be used inareas that are subjected to future moisture as thePVA will re-emulsify and lose its bond strength.Non-emulsifiable PVA bonding agents are avail-able, but the patch must be applied over the wetbonding agent.

4.3.6. Organic resin concretesOrganic polymer concretes (polyester-styrene,

epoxy, methacrylate, and urethane) can be eitherformulated on the job site or obtained commer-cially. Polymer patching materials have severaladvantages over inorganic materials. They aregenerally rapid-setting and quickly develop highcompressive, flexural, tensile and bond strengths.The rapid-hardening properties of polymer con-cretes reduces the time required for installationand cutting. They have good adhesion to most

surfaces, are impervious to water and deicingsalts, are not subject to freezing and thawingdamage, and are highly abrasion resistant.

The thermal coefficient of expansion of polymerconcretes can be over 3 times as much as conven-tional Portland cement concrete. The inclusion oflarge well-graded aggregates will reduce the resincontent, initial exothermic temperature gain, andthe thermal coefficient of expansion of the patch,thereby improving the thermal compatibility ofthe patching material to the substrate.

Shrinkage strains vary for each polymer. Linearshrinkage of poorly formulated polyester con-cretes can be as high as 30000 microstrain (3percent) and for properly formulated epoxies itcan be negligible. Therefore, shrinkage of somepolymer concretes can be significant and must beconsidered.

4.4. Delaminations

Delaminations are horizontal voids in concretedomes or slabs that commonly occur due to cor-rosion of steel reinforcement or separation ofconcrete layers that do not develop adequatebond. Spalls will usually occur if the delamina-tions are not repaired. Delaminations can be re-paired by removal and replacement of thedelaminated concrete using procedures similar tothose for repair of spalls. In areas where removalof concrete is not required, the delaminated areacan be repaired by injection of epoxy or HMWMacrylic resin. Several holes are made into thedelamination using a drill with a vacuum attach-ment. If water is used, the concrete should bepermitted to dry prior to resin injection. Dowel-pins can be used to enhance shear transfer. Addi-tional information on delamination repairtechniques is available elsewhere (Naus, 1986; USArmy Corps of Engineers, 1986; US Departmentof the Interior, 1990; Krauss, 1994).

4.5. Honeycomb and 6oids

Nonvisible voids such as rock pockets, honey-comb, or excessive porosity can be repaired bydrilling small diameter holes to intercept thevoids, determining the extent and configuration of


the void system by injection of compressed air orwater into the void system, or by visual inspectionusing a borescope and, depending on the magni-tude of the delamination, injecting either epoxyresin, expansive cement grout or mortar, orepoxy-ceramic foam. Proper injection of the ce-ment grouts requires prewetting of the substratumwith excess water removed prior to injection.

4.6. Thermal shock and fire

The ability of concrete structures to performexceptionally well both during and after exposureto elevated temperatures has been well docu-mented. Principally due to its high capacity forheat absorption and relatively low thermal con-ductivity, concrete can sustain its strength andintegrity during fire events, as well as other hightemperature excursions.

Evaluation of a fire-damaged structure involvesa systematic process similar to that employedwhen investigating more ‘conventional deteriora-tion’. A key difference is the need to gain anaccurate estimate of the maximum temperaturethat various parts of the affected structure weresubjected to, so that engineering judgements canbe made to determine overall residual strength.This information is often obtained by insightfulobservations from the fire damaged area, perfor-mance of in-place nondestructive testing of sus-pect concrete and steel, and use of detailedpetrographic examinations, supplemented by con-trolled heating tests of undamaged concrete. Anumber of nondestructive tests (NDT) can beperformed on the concrete to determine its resid-ual engineering properties, including reboundhammer, ultrasonic pulse velocity, and impact-echo.

4.7. Freezing and thawing

Freeze–thaw damage to concrete is caused bythe pressures resulting from the increase in vol-ume during the phase change from water to icewithin the concrete. It can result in severe deterio-ration of non-air-entrained or marginally air-en-trained concrete. Certain types of aggregates arealso susceptible to damage from freezing and

thawing. Air entrainment of concrete reduces oreliminates such damage by providing tiny pres-sure-relief sites, distributed uniformly throughoutthe concrete.

Visual examination of concrete damaged byfreezing and thawing may reveal surface scalingand delaminations parallel to the freezing surface.The deterioration can range from light scaling,exposing only the surface sand grains, to heavyscaling where the coarse aggregate is exposed.

The service life of non-air-entrained or moder-ately air-entrained concrete can be extended bydrying the concrete and preventing additionalmoisture ingress. This can be done by applyingsealers, membranes or coatings, so long as theconcrete is not in contact with the ground orother sources of moisture. Any method to extendthe life of non-air-entrained concrete that iswetted in a freezing environment should be con-sidered beneficial but not curative.

4.8. Erosion

ACI 116R (ACI, 1990b) defines erosion as a‘progressive disintegration of a solid by the abra-sion or cavitation action of gases, fluids, or solidsin motion.’ There are two primary causes of con-crete erosion: abrasion and cavitation. Abrasionerosion is caused by repeated rubbing and grind-ing of particles on the concrete surface. Cavitationerosion is due to repeated impact forces caused bythe collapse of vapor bubbles in rapidly flowingwater or other fluids.

A number of methods are available for prevent-ing or at least reducing abrasion–erosion damage.Unless the adverse conditions that cause abrasionerosion damage are minimized or eliminated, itwill be extremely difficult for any economicalmaterial currently available to prevent continuedabrasion loss. To minimize damage, the concreteshould contain the maximum amount of the hard-est available coarse aggregate and have the lowestpractical water–cementitious materials ratio. Itmay be beneficial to use silica fume as a mineraladmixture, and a high range water reducer as achemical admixture, in order to increase thestrength of the concrete. The abrasion erosionresistance of vacuum-treated, polymer, polymer-


impregnated, and Polymer-Portland cement con-cretes is significantly superior to that of compara-ble conventional concrete, but at an increased costassociated with materials, production, andplacing.

Cavitation erosion damage cannot be preventedas easily as abrasion erosion damage. The highwater velocity and surface irregularities create avacuum on the concrete surface, resulting in ten-sile fracture. Consequently, prevention must beprimarily concerned with reducing the water ve-locity, increasing the smoothness and streamliningof the surface, and maintaining the front edge ofjoints approximately 1.5 mm (1/16 in.) above thedownstream edge.

4.9. Irradiation

Irradiation of concrete by fast or thermal neu-trons emitted by the reactor core or by gammarays produced by the capture of neutrons byadjacent members can cause deterioration. Fastneutrons cause atomic displacements in certaintypes of aggregates such as flint. Gamma raysproduce radiolysis of water within the concrete,which can affect the concrete’s creep and shrink-age properties. Damage to concrete by prolongedexposure to radiation typically occurs as cracksand spalls on the surface. However, the shieldingproperties of the concrete are not significantlyaffected by such exposure.

Steel can be degraded by neutron irradiationcaused by the displacement of atoms within thesteel. NUMARC (Nuclear Management and Re-serves Council Inc., 1991) concluded that damageto the embedded reinforcing steel due to radiationis not expected since the radiation fluence and fluxlevels anticipated during normal operations is 1014

neutrons cm−2, well below the threshold fordegradation of 1019 neutrons cm−2.

A threshold level for neutron fluence damage toconcrete of 1×1019 neutrons cm−2 and 1010 rads,for gamma radiation, has been reported(Hookham, 1991). NUMARC (Nuclear Manage-ment and Reserves Council Inc., 1991) reportedthat the levels of neutron fluence in a PWR at thecontainment wall would be less than thisthreshold and no larger than 1017 neutrons cm−2.

NUMARC also reported that gamma radiationon containment concrete is mitigated due to dis-tance and shielding, making its effect insignificant.Hookham (1991) reports that radiation levels ap-proaching these thresholds may occur in the Cate-gory I primary shield wall after 40 years or moreof operation. Heat caused by radiation effectsmay also cause reduction in the mechanical prop-erties of concrete, loss of moisture and volumechanges. Research showed that the tensilestrength may be decreased more significantly thanthe compressive strength when subjected to neu-tron radiation. The neutron radiation resistancedepends on the mix proportions, type of cement,and type of aggregate in the concrete althoughspecific recommendations could not be made. Theradiation dose decreases rapidly with increasingdistance from the exposed surface and normallyhas limited effect at distances more than 0.5 m (20in.) from the exposed face. Change in concreteresistance to irradiation due to exposure magni-tude and duration may be of importance forbiological shield and prestressed concrete reactorvessel applications.

4.10. Fatigue and 6ibration

Fatigue and vibration may rapidly deteriorateconcrete. Cyclic or fatigue loads can occur atlocations such as nuclear steam supply system(NSSS) equipment supports, pump and turbinesupports, and local areas in the containment suchas liner anchors. These fatigue loads may causeconcrete to fail at a substantially lower load thanthat required to cause damage during a singleload application. For example, the load requiredto cause a failure of concrete at 10000000 cycles isapproximately 55% of that required for failure ina single loading. As the load range increases, thenumber of cycles required for failure decreases.This loss of strength under repeated loading issensitive to many influences including, but notlimited to: load range, load rate, material proper-ties, and environmental conditions.

Fatigue problems typically become very obvi-ous before any type of failure occurs, with largedeflections and very wide cracks becoming notice-able. Also, fatigue problems are generally slow in


developing, allowing the damage to be detectedbefore problems can occur.

4.11. Alkali–aggregate reactions

Three basic requirements exist for the occur-rence of swelling and cracking of concrete due toalkali-aggregate reactions: (1) reactive silica orsiliceous components in the aggregate, (2) suffi-ciently high hydroxyl ion concentration in theconcrete pore solution, and (3) sufficient moistureavailability. Mitigation procedures generally in-volve elimination of one (primarily elimination orminimization of exposure to moisture) or all ofthese requirements. In new construction, controlof alkali–aggregate reactions is done by eliminat-ing deleteriously reactive aggregate materials fromconsideration through petrographic examinations,laboratory evaluations, and use of materials withproven service histories. Additional mitigationprocedures for new construction include the useof pozzolans, restricting the cement alkali con-tents to less than 0.6% Na2O equivalent, andapplication of barriers to restrict or eliminatereactions. Organic (e.g. amines, alkyl-alkoxy-silanes, and cryptands) and inorganic (e.g. phos-phates, lithium compounds, and sodiumsilicofluoride) chemical agents have been tried inthe laboratory to reduce or alter the course ofalkali–aggregate reactivity in concrete. Lithiumhydroxide, lithium carbonate, and lithium nitritehave been found to be somewhat effective.

Alkali–silica reactivity (ASR) is more commonthan alkali–carbonate reactivity. The reactiveforms of silica in aggregates are opal; chalcedony;micro-crystalline quartz; crystalline tridymite; andrhyolitic, dacitic, latitic, or other glass-like aggre-gates. These aggregates are highly siliceous, con-taining large amounts of SiO2. Some of the slowerreacting aggregates include granite gneisses, meta-morphosed sub graywackes, and fractured andstrained quartz. These reactive siliceous aggre-gates combine with alkalies and calcium hydrox-ide from the cement to form solid alkali–silica orcalcium–alkali–silica gel complexes. The cal-cium–alkali–silica gel may be non-expansive, butthe alkali–silica complex shrinks and swells withchanges in moisture. Which of these compounds

is formed depends upon the relative concentra-tions and surface areas of the reactiveconstituents.

Map cracking, popouts and spalls are typicalsigns of ASR. Such indications should be takenseriously. Concrete susceptible to ASR should beidentified early so that mitigation techniques canbe applied as soon as possible. Repairs may besubstantial if deterioration occurs and is allowedto continue.

The repair of ASR damaged structures is notstraightforward. Most repair methods must beconsidered as attempts to extend the service life ofthe structure. Practicality may dictate that theonly solution is to remove and replace the struc-ture. The current expansion rate can be tested bymonitoring the concrete using strain gage pointsor by testing core samples in accelerated labora-tory tests. Under some circumstances, the reactivecomponent of the aggregate is depleted beforedestruction occurs. Before replacing moderatelydeteriorated ASR-affected structures, the concreteshould be petrographically evaluated and labora-tory tested to determine if the reactivity hasslowed or ceased. The development of an effectiverepair strategy is dependent upon determining ifthe expansion and cracking will continue and ifmoisture can be reduced.

Mitigating alkali–aggregate reactions in exist-ing concrete structures can be done by interferingwith the reaction mechanisms (i.e. treatment withlithium salts, drying, sealants) or treatment ofsymptoms (i.e. restraint and crack filling). Appli-cation of surface film coatings (e.g. acrylic/polyvinylacetate) to prevent moisture penetrationhas been found to be ineffective over the longterm. However, using sealers that allow goodvapor transmission, non-film formers such assilane, may be effective in slowing the distress ofstructures in low humidity environments by allow-ing the concrete to dry. If the alkali–aggregatereactivity has stopped, the only repair generallyrequired is to fill the surface cracks with cementgrout or resin. Other methods that have beenutilized to not mitigate but to counteract theeffects of alkali–aggregate reactions includestrengthening by addition of external steel rein-forcement, and cutting joints into the structure toaccommodate expansion.


4.12. Delayed ettringite formation (DEF)

The phenomenon of delayed ettringite forma-tion (DEF) is very recent, or only recently recog-nized, and our studies have shown that there areseveral causes other than the usually attributed‘excessive steam curing’. Problems that were firstreported in the early 1980s in Europe were associ-ated with distressed precast concrete. The theorypostulated was that the distress was due to ‘exces-sive’ heat curing that prevents the formation, orcauses decomposition, of ettringite(3CaO·Al203 ·CaSO4 ·32H2O) that is formed dur-ing the early hydration of the Portland cement.Ettringite is a product of the reaction of sulfateions, calcium aluminates and water. If suchsteam-cured concrete members are later exposedto water, ettringite reforms as a massive develop-ment of needle-like crystals, causing expansiveforces that result in the cracking of the particularprecast concrete members. The phenomenon hasbeen termed ‘delayed ettringite formation’ (DEF)by most investigators.

Historically, clinker sulfates levels have beenless that 1%. In the last decade or so, changes inclinkering processes in some cement plants haveled to clinker sulfate contents up to 5%. As anincrease in the SO3 level in the clinker may lead toan increase in the SO3 level in the Portland ce-ment because tests for optimum sulfate may not‘recognize’ this clinker sulfate. Further, even if thesulfates are relatively soluble, high ‘available’ sul-fate amounts may not react completely before theconcrete gets hard.

Based on our knowledge today, DEF may beminimized or prevented by lowering the curingtemperature, limiting clinker sulfate levels, avoid-ing excessive curing if there is a certain ratiobetween sulfate and aluminates, preventing expo-sure to substantial amounts of water in service,and using proper air entrainment. More researchis needed to establish the influence of these andother factors on the development of DEF.

WJE/EHA studies, as performed during the lastseveral years, suggest several other causes of DEFdevelopment. Distress in concrete is not caused bythose needle-like ettringite crystals that areformed in fresh concrete during early hydration,

or by the fine ettringite crystals formed in airvoids and open cracks in old concrete, but iscaused by formation of ettringite in the paste ofhardened concrete when the concrete is exposedto wetting and drying for months for severalyears. If such development is caused by exposureof the concrete to sulfate solutions, it is called‘sulfate attack’.

Development of DEF can usually be observedby the unaided eye or by the microscope, either asa gel-like material or as needle-like crystals. Thegel-like material can be misinterpreted as alkali–silica gel, and elemental analysis (e.g. the X-rayemission spectra obtained when using a scanningelectron microscope) is usually necessary forproper DEF identification.

Structures undergoing DEF usually exhibit ex-pansion and cracking. The extent of developmentof DEF in concrete and of the subsequent con-crete damage (from cracking) is dependent on theamount of sulfate available for late ettringite de-velopments in the particular concrete, and on thepresence of water during the concrete service life.Elevated temperatures, especially a early ages,also increase the potential for damage due toDEF.

Sources of high temperature can be:� ‘overheating’ during steam curing� development of high temperature in mass

concrete� exposure conditions (hot weather, fire damage)

Source of excessive amounts of sulfate mayinclude:� slowly soluble sulfate originating in the clinker

phase of the Portland cement� high amounts in the cement� mineral admixtures� aggregates

4.13. External sulfate attack

Certain materials combine with cement to formrelatively insoluble compounds of high volumethat disrupt the concrete. Salts such as sodiumsulfate may cause surface disintegration by exert-ing crystallization pressure in the pores of theconcrete. In highly impervious concretes thesetypes of attack would be largely superficial. On


the other hand, porous or highly permeableconcrete may be affected throughout the mass.

Sulfate levels of 1500 ppm or higher areconsidered aggressive to concrete. The rateand degree of attack by sulfate-containing wa-ter or soil is dependent on the concentrationof sulfate ions, the cation present (sodium,potassium, ammonium, or magnesium), theC3A content of the cement, and the perme-ability and quality of the concrete.

Concrete can be manufactured to be resis-tant to sulfate attack by using an adequatecement content, a low C3A cement, a mineraladmixture such as fly ash, low water-to-ce-ment ratio, good consolidation, and propercuring. Any action to reduce the permeabilityof the concrete will increase the sulfate resis-tance.

Repair of concrete deteriorated by sulfateattack requires removing all of the soft anddeteriorated concrete by mechanical means orhigh pressure water jetting. The spalled areascan be built up with dry pack, patching, oroverlay materials. The entire exposed areashould be repaired and treated with a qualitysealer or coating to prevent further exposureto the high sulfate waters or soils.

4.14. Leaching and efflorescence

ACI 116 (ACI, 1990b) defines efflorescenceas ‘a deposit of salts, usually white, formedon a surface, the substance having emerged insolution from within either concrete or ma-sonry and subsequently been precipitated byevaporation’. It is unsightly but harmless un-less it accumulates within the pores of theconcrete. Such accumulation can disrupt thesurface. The calcium hydroxide in the con-crete, masonry or mortar can leach to thesurface. Then, when exposed to the carbondioxide in the atmosphere it becomes calciumcarbonate, which appears as a whitish deposit.This is water-insoluble and must be removedwith acid or by abrasion. Hydrochloric acidshould be avoided on reinforced concretesince it may lead to future corrosion of theembedded steel.

4.15. Acid and chemical attack

Acidic deterioration of concrete is a commonproblem, although it is generally recommendedthat Portland cement concrete not be exposed toacids. However, nuclear plants and spent fuelprocessing plants, must utilize acidic solutions.The containment of these solutions is critical.Acids rapidly react with the calcium hydroxidesand carbonates at the surface of concrete, causingsurface deterioration.

Concrete is very vulnerable to strong acids suchas sulfuric, nitric, sulfurous, hydrochloric and hy-drofluoric. Concrete is attacked, to a lesser de-gree, by weaker acids such as acetic, carbonic, andtannic, and many organic oils. Also, some sub-stances that contain or produce acids will attackconcrete. Organic acids are often produced duringthe microbial fermentation of organics in water.Acids such as sulfuric and carbonic acid can bepresent in groundwater. The reaction betweenmost acids and concrete results in soluble saltsthat can be leached from the concrete. This resultsin increased permeability, which intensifies theattack of acids or other aggressive solutions.

Acid cannot penetrate far into concrete beforebeing neutralized by its high pH. Therefore, theinterior of concrete cannot deteriorate without theexternal surface of the concrete being severelydamaged or destroyed. The rate of deteriorationis, in a large part, a function of the type andconcentration of the acid and the length of expo-sure. The rate of penetration is also affected bythe permeability and the constituents of the con-crete, and the quantity of neutralizing materials inthe concrete such as calcium hydroxide and lime-stone aggregate. Sulfuric acid converts the cal-cium hydroxide to calcium sulfate, causing furtherdetrimental expansion due to the formation ofettringite. Other acids may not result in expansionand swelling, but may slowly dissolve the cementpaste.

Acid resistance of Portland cement concrete canbe increased by reducing the permeability of theconcrete and by selecting acid-resistant or sacrifi-cial aggregates. Concrete having the lowest watercontent and best compaction possible is best inresisting any type of chemical attack. If strong


acid resistance is desired, Portland cement con-crete should not be used, or if it is, it should becoated with a barrier material. The ACI 515.1R‘A Guide to the Use of Waterproofing, Damp-proofing, Protective, and Decorative Barrier Sys-tems for Concrete’ (ACI, 1990c) contains anextensive list of materials and their effect onconcrete. The document also provides excellentadvice on surface preparation, application proce-dures, and barrier system selection where suchbarriers are required.

4.16. Carbonation

Carbonation occurs when CO2 present in theatmosphere reacts with moisture and hydrationproducts. The main reaction is with Ca(OH2)forming calcium carbonate. Carbonation of Port-land cement paste has two distinct effects, onechemical and one physical. The chemical effect isto drop the pH of the pore liquid from approxi-mately 13 to somewhat less than 9. The physicaleffects are irreversible shrinkage, which can beequal to that due to drying, and a moderateincrease in strength.

The carbonation process is slow because of therelatively low levels of CO2 in the air (in non-ur-ban areas, about 0.03%) and the normally lowpermeability of concrete. The chemical effect ofconcrete carbonation is important in that the dropof pH to less than 9 changes any embedded steelsystem from an anodically controlled, passivecondition, to one in which the corrosion canbecome active. This is not the rapid, chloride-in-stigated electrochemical type of corrosion, butgiven sufficient time, the same sort of damage canbe done by corrosion of the steel in carbonatedconcrete.

4.17. Corrosion of embedded steel

One of the major mechanisms of deteriorationin concrete structures is corrosion of embeddedreinforcing steel. Structures in contact with salt orsea water or chloride-containing ground or cool-ing tower water are most susceptible to corrosion.Structures subjected to airborne saltwater sprayare also susceptible to corrosion. Cracks in struc-

tures will greatly increase the potential forcorrosion.

Corrosion of steel reinforcement is by far thegreatest threat to the durability of reinforced con-crete structures. Fortunately, incidences of corro-sion at nuclear power plants concrete structureshave been limited (Krauss, 1994) probably due tomore detailed considerations associated with ma-terial selection and construction workmanship,and effective use of quality assurance/quality con-trol procedures. However, the history of thesestructures is somewhat limited and, as they age,incidences of corrosion can be expected toincrease.

Basically three processes are necessary for cor-rosion occurrence: anodic, cathodic, and elec-trolytic. Repair activities are directed at haltingone or more of these processes. Basic principlesfor halting the anodic and electrolytic processesand information on each of these principles, in-cluding proper application, effectiveness, advan-tages, and disadvantages, and any limitations isprovided elsewhere (Pullar-Strecker, 1987; ACI,1989; International Union of Testing and Re-search Laboratories for Materials and Structures,1994).

Corrosion of steel reinforcement can be ex-pected if the concrete is sufficiently moist andeither carbonation or sufficient chlorides havereached the surface of the steel. The most costeffective approach to treating corrosion of rein-forced concrete structures obviously is to providepreventative protection or, if necessary, interven-tion early in the process. The application of barri-ers in the form of sealers, coatings, or membranesto exposed surfaces provides one commonly usedmeasure of intervention.

Sealers are liquids applied to the surface ofhardened concrete to either prevent or decreasethe penetration of liquid or gaseous media (e.g.water, carbon dioxide, or aggressive chemicals). Anumber of materials have been applied to con-crete (e.g. boiled linseed oil, sodium or potassiumsilicates, stearates, silicones, asphaltic emulsion,and cementitious formulations). Five categories ofsealers have been found to be effective in bridgedeck applications: polyurethanes, methylmethacrylates, certain epoxy formulations, rela-


tively low molecular weight siloxane oligomers,and silanes (Pfeifer and Scali, 1981). Of these, thesilanes and oligomers presently are most com-monly used. Formulations of these materials pen-etrate the concrete surface to some degree, butstill permit the transmission of air or water vapor.This can be beneficial for drying but coatings alsohave to be applied to protect against carbonation.

Silanes are commonly used for protectingbridge structures from deicers. New higher solidsformulations are available that improve perfor-mance. Water-based silanes are also available thatprovide a safer and non-polluting alternative tothe alcohol solvent carrier typically used. Theiradvantages include ease of application, excellentchloride screening properties, good vapor trans-mission, good penetration, and they do notchange the appearance of the concrete.

Coatings and membranes differ from sealers inthat they are applied in some thickness, generallymeasured in hundredths of a millimeter, and gen-erally do not penetrate the concrete. Coatingtypes include epoxy resins, polyester resins,acrylics, vinyls, polyurethanes, and cementitiousmaterials. Membrane types include liquid appliedacrylics, urethanes, neoprenes, vinyls, rubberizedasphalts, silicones, and preformed membranessuch as rubberized asphalts, neoprenes, and butylrubbers, hypalons, vinyls, and ethylene propylenediene. Properties of the various resins used forsealers and coatings are discussed in Krauss(1994).

Selection of sealer, membrane, or coating mate-rials involves a number of factors (e.g. compatibil-ity with new or old concrete, compatibility withjoint sealant materials, crackbridging ability, ef-fective service life, weatherability, etc.). Surfacepreparation is also extremely important in the useof any sealer or coating material (i.e. cleanlinessand moisture condition).

4.18. Corrosion of prestressing tendons

Corrosion of prestressing tendons and pre-stressing losses have been determined to be apotentially significant degradation mechanism byNUMARC (Nuclear Management and ReservesCouncil Inc., 1991). The corrosion of prestressing

can be managed by programs of visual inspectionof tendon anchor heads as well as periodic exami-nation of the corrosion protection medium. Lossof prestressing can be detected by inspection andload monitoring programs.

Naus (1986) reports that the performance ofprestressing tendons has been quite good; how-ever, stress corrosion cracking has been seen inseveral plants. Most ungrouted tendons have per-formed well, with minimal corrosion or wire fail-ures. Generally, the grease and corrosioninhibitors within the ducts are working well.

Corrosion of prestressed steel may be aggra-vated by hydrogen embrittlement, fatigue crack-ing, moisture access to bundled tendons, andstress corrosion. Elevated temperatures on pre-stressing steel have a far greater consequence thanon mild steel because the highly stressed steel mayundergo stress relief (relaxation and creep). Thiseffectively reduces the compressive forces that aretransmitted to the concrete, which may lead tostructural problems.

A number of studies on corrosion of a 7-wireprestressing strand have concluded that the time-to-corrosion and the chloride ion corrosionthreshold for strand are both substantially greaterthan for conventional reinforcing bar (Pfeifer etal., 1987; Clear, 1991). The 1987 FHWA report,‘Protective Systems for New Prestressed and Sub-structure Concrete’ (Pfeifer et al., 1987) discussedthese observations. A conclusion from this studywas as follows:

‘‘Corrosion activity of unstressed grey prestress-ing strands with 1 in. cover in moist-curedconcrete began when the chloride content at thestrand level reached an average of almost 1.2percent by weight of Portland cement (acid-sol-uble). This unusually high corrosion thresholdlevel is 6 times greater than that determined forgrey bars. In addition, the average time-to-cor-rosion for these unstressed prestressing strandswas 3 times as long as that measured for greybars.’’

These data suggest that the unstressed 7-wirestrand was able to tolerate approximately 6 timesthe reported reinforcing bar chloride ion corro-


sion threshold which was about 0.20% (acid-solu-ble) chloride ion by weight of cement.

This unusual corrosion resistance may be be-cause the prestressing strand possess a naturalcorrosion protection after the manufacturing pro-cess. During the wire drawing processes, the wiresare coated with zinc phosphate and calciumstearate materials which impart an unintentionalbut effective corrosion protection system directlyonto the wire. These materials are also verydifficult to remove. Therefore, prestressing strandcan possess a significant corrosion protection sys-tem from lubricants used to draw and manufac-ture strand. Similar lubricants are not used tomanufacture reinforcing bars.

4.19. Remedial measures for liners and coatings

Reinforced and prestressed concrete contain-ments are typically lined with metallic or non-metallic materials to provide a leak-tightboundary and to aid cleanup operations shoulddecontamination be required. Protective coatings(e.g. inorganic zinc and polyamide epoxy) gener-ally are applied to exposed interior surfaces of thecontainment metallic liners.

Several incidences of corrosion of metallic linershave been reported. The primary location wherecorrosion has been observed is along the circum-ference of the liner adjacent to the upper portionof the concrete basement. The cause was at-tributed to a breakdown of the waterstop thatpermitted fluids to accumulate in this region. Re-medial measures generally involved inspecting theregion, cleaning, recoating, and reapplication ofthe waterstop. The most extensive repair of thistype was undertaken in France where corrosionhad progressed to the state that 1 cm diameterholes had penetrated the liner. Although corro-sion had completely penetrated the liner in thisregion, the leak-tight integrity of the containmentwas not compromised as the containment passedthe leak-rate test that was conducted prior to therepair (Naus et al., 1996). The repair procedureinvolved removing some of the concrete slab adja-cent to this region, sandblasting the liner, inspect-ing, welding plates over areas where holes werepresent, and painting. In addition, corrosion in-

hibitor was injected into the space between theliner and floor slab, and a redesigned (i.e. moredurable) waterstop installed.

The primary form of nonmetallic liner degrada-tion has been occurrence of cracks due to local-ized effects (e.g. stress concentrations) or physicalor chemical changes of the concrete. Leak tight-ness is generally re-established through surfacepreparation and application of an additional coat-ing such as polyurethane. As coatings are sub-jected to a number of potential deteriorationfactors (e.g. temperature, abrasion, and high hu-midity), the coatings are inspected and areas ofdeterioration are repaired by cleaning and recoat-ing with a compatible material.

5. Summary

The relatively excellent performance of concretestructures in nuclear power plants reflect the ini-tial quality control, the young age, and the gener-ally benign environment within a plant. Mostdistress has been construction related or associ-ated with exterior structures such as seawaterintakes or discharges and cooling towers. As theplants age, it becomes more important to monitorthe aging of the concrete structures to identifypotentially deleterious reactions that may ad-versely effect the continued serviceability of theplant.

References

ACI, 1986. Guide for Making a Condition Survey of ConcreteIn-Service. Proc. Am. Concr. Inst. 11 (65) 905–918.

ACI, 1987a. Troubleshooting Concrete Problems And How toPrevent Them in the Future, SCM–17. American ConcreteInstitute, Detroit, MI.

ACI, 1987b. Concrete Repair Basics, SCM 24-91, CT92.American Concrete Institute, Detroit, MI.

ACI, 1988. Guide for repair of concrete bridge superstruc-tures. Im: Manual of Concrete Practice, Part 2. ACICommittee 546, American Concrete Institute, Detroit, MI.

ACI, 1989. Corrosion of Metals in Concrete. ACI Committee222, American Concrete Institute, Detroit, MI.

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Repair materials and techniques for concrete structures in nuclear power plants