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There is also a somewhat well founded suspi-cion that a wide range of highway pavements inthe midwestern United States are undergoingdeterioration at least partly induced by DEF;see for example Ref. 6.Pioneering laboratory studies on heat-induced DEF were carried out by Heinz andLudwig.7 Cases of DEF in non-steam curedpaste were studied by Odler and Gasser.Occurrences of DEF in steam cured con-cretes, especially in steam cured railwaysleepers, have been repeatedly reported. Inmany of these occurrences, there is strongevidence of simultaneous, or at least concurrentASR.9-2 Interestingly, in the first majorreported occurrence of railway sleeper DEFASR was not mentioned at all, but subsequentlyShayan and Quick14 showed conclusively thatASR was a major cause of the deterioration.Diamond and Ong have shown that hightemperature steam curing itself can induceimmediate ASR for some reactive aggregates,and that a further acceleration of ASR takesplace in steam cured mortar after cooling andexposure to moist conditions. DEF is facilitatedunder these conditions, and it was found that by6 months 50% more ettringite was present inthe mortars undergoing ASR than in com-panion mortars steam cured in an identicalfashion but lacking alkali reactive aggregate.The effectiveness of prior ASR-inducedcracking in stimulating the development of DEFhas been noted by the present writer in forensicinvestigations, and similar conclusions havebeen reported by Oberholster et ~1.~ and byShayan and Quick14 among others. Someevidence exists also that DEF may be facilitatedby the prior cracking induced by freezing andthawing.16In laboratory studies Fu et a1.7 re-affirmedthe effectiveness of prior cracking in promotingDEF distress, and suggested that nucleation ofdelayed ettringite takes place preferentially incrack tip zones. The significance of this conceptwill be discussed later.In addition to the promotion of DEF byASR-induced cracking, various workers haveraised the possibility of a chemical relationshipbetween ASR and DEF. For example, Brownand Bathe suggested that ettringite formationis retarded in the presence of pore solutionsthat have high concentrations of alkali hydrox-ide, such are produced with high alkali cements.Since ASR progressively reduces the alkali

hydroxide concentration, they postulated thatsuch reduction may trigger ettringite formation.In particular such an effect might occur atselected locations of more depressed alkalihydroxide concentrations, i.e. near alkali-react-ing aggregates.The relationship between chemical character-istics of the cement used and the developmentof DEF symptoms is of course not accidental.Specifically, many of the DEF occurrencesreported in the literature, and others investi-gated by the writer, have been associated withcements of high sulfate content (4-5% S03).Not uncommonly these cements have highclinker sulfate contents as well.In this connection a certain historical pers-pective is appropriate. In 1948, as a portion ofthe PCA long-time study of cement perform-ance, Lerch and Ford compiled analyses ofrepresentative American cements and of theclinkers from which they were produced.Clinker SOLi contents at that time were typicallyless than 0.5% and total cement sulfate contentsrarely exceeded 2%. In contrast, cements withtotal sulfate levels of 4-5%, incorporatingclinker sulfate levels of 3% or more are notunknown in current practice. The potential forDEF is thus much greater than it was in the eraof lower sulfate levels.

CHARACTERISTIC SYMPTOMS OF DEFDEF is clearly an expansive process, marked byenlargement of the affected concrete membersand by the development of gross cracking. Aswould be expected, the macroscopic crack pat-tern is controlled to a considerable extent bythe geometry of the restraint system and bysuperincumbent loads. Cracking in railroadsleepers tends to be primarily parallel to thelong axis, that is, parallel to the direction of theprestressing cables; it is normally most severe atthe ends of the sleepers, i.e. in regions whereprestress is not effective. DEF often inducesloss of prestress across the sleeper, and afterfailure of the steel-concrete bond and crackingin the vicinity of the steel, water may penetrateand corrosion of the steel often occurs.

Concretes undergoing DEF suffer a generalloss in dynamic elastic modulus, even if sampledin apparently sound areas between visiblecracks. Measured values may drop to as low as1.5 GPa, i.e. about one-third the normal mod-


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ulus of unaffected concrete. In extreme cases,the concrete becomes crumbly and soft, and it isevident that the effectiveness of the cementpaste binder has been destroyed.

MICROSTRUCTURAL FEATURESASSOCIATED WITH DEFThe microstructural features characteristicallyassociated with DEF have been described bymany investigators, including the present writerand his co11eagues.2 These are best illustratedin backscatter SEM, although optical micros-copy can also provide useful information. Itshould be stressed that in the writers experi-ence, identical microstructural features havebeen found in steam-curing induced DEF andin DEF where the concrete was not steamcured.Figure 1 shows a low magnification SEMdepicting the usual microscopic crack patterndeveloped in DEF. The concrete here wassteam cured. Features include (a) a rim orbond crack surrounding many sand grains (andcoarse aggregate pieces), forming an intrinsicpart of an interconnected network crack patternextending across the paste. The sizes ofuntracked blocks of paste between the crackmeshes vary; here they are roughly 300 pm.Figure 2 shows the corresponding crack pat-tern (at slightly higher magnification) of aconcrete undergoing DEF that had not beensteam cured. The characteristics of the responseare obviously identical in both cases.In the usual occurrences of DEF the rimcracks and much of the lengths of paste crack

Fig 1 Charactreristic crack pattern and other micro-structural features in a DEF-affected steam curedconcrete.

Fig 2 Charactreristic crack pattern and other micro-structural features in a DEF-affected non-steam curedconcrete.

are lined with ettringite oriented perpendicu-larly to the aggregate surface. Figure 3 shows atypical illustration, with the arrows pointing tothe ettringite-filled rim crack. The blockyappearance of the ettringite in this mode ofexamination does not necessarily imply an aber-rant crystal habit; examination at highmagnification, especially in secondary modeSEM indicates that the individual crystals aremostly high aspect ratio elongated needle orrods, somewhat fused together.The width of the rim crack in Fig. 3 is about20 ,um, which is fairly typical; such widthsgenerally range up to 40 pm in most DEFexamples examined by the writer. It should benoted that crack widths may vary from place toplace around the perimeter of individual aggre-gate grains. It is also not uncommon to see onlypartial rim cracks. Aggregate grains are often

Fig 3 Characteristic ettringite-filled rim crack around acoarse aggregate grain.


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found where the rim crack extends only aroundpart of the grain, the rest of the grain retainingclose paste-aggregate contact.The ettringite in the rim zone seen in Fig. 3does not appear to quite fill the space, but itshould be recalled that ettringite shrinks due toloss of water during the drying associated withsample preparation and with exposure to thehigh vacuum of the SEM. The blocky appear-ance of these deposits likely results from thatshrinkage.An EDS analysis was made at the spot in theettringite filled rim crack indicated, just abovethe center of the micrograph. The analysis indi-cates that these ettringite crystals, like mostsuch deposits, are almost stoichiometric ettrin-gite. The specific analysis here was 52.6% CaO,9.8% A1203 and 35.5% SO3 on a dry basis. Thedry-basis stoichiometric percentages are 49.6%CaO, 15.0% A1203 and 35.4% S03. Smallamounts of SiOZ and of K20 (about 0.2% each)were also detected.Figure 4 shows a similar ettringite deposit,this time filling a crack across the cement pasterather than a rim crack around an aggregateparticle. Note that other crack branches areempty. The dark gray aggregate particle in theupper left region shows a partial perimeter rimcrack filled with ettringite. Note that the righthand, generally vertical section of the perimetershows clear paste-aggregate contact, with thebond intact.

Many workers have observed ettringitedeposits in air voids, either as linings or some-times as apparently complete fillings. The upperright hand corner of Fig. 2 contains an example

of a lining; here several bright crystals of CI-Iare present as well. Completely filled air voids,might presumably lose their effectiveness inpreventing freezing damage in subsequentfreezing and thawing exposure, but it is rarethat all of the air voids are filled.The existence of DEF-induced ettringite dep-osition in cracks and air voids is wellappreciated by those concerned with the sub-ject. What is not is the degree to whichettringite deposits within the space previouslyoccupied by the cement paste. Ettringitedeposits take various forms, but are commonly(but not always) morphologically detectable bythe shrinkage and cracking features discussedearlier.Figure 5 shows such a deposit, about 60 mmlong, in the lower left corner. Interestingly,there are no filled rim cracks around the aggre-gate particles in this concrete.Figure 6 shows a pair of smaller ettringitedeposits, circled, on opposite sides of the verybright sand grain.The deposits in Figs 5 and 6, while not spec-tacularly obvious, are reasonably noticeable to acareful observer. Ettringite may also bedeposited within the paste as more or less singlegrain inclusions of the order of 5 mm or so thatare not so easily detected. Figure 7 shows suchindividual single-grain ettringite inclusion inpaste. Each of the hand-circled areas has beenchecked by EDS and its identity confirmed. It isusual to find that EDS analyses for such indivi-dual grains are highly deficient in sulfate ascompared to normal ettringite composition, butthey show reasonable A1203 contents for ettrin-gite, typically 12-15%.

Fig. 4. Ettringite-filled paste crack and ettringite filledrim crack developed only part way around the peripheryof a sand grain. Fig. 5. Ettringite deposit within space formerly occupiedby cement paste.


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Fig. 6. Smaller but distinct ettringite deposits on oppo-site sides of a sand grain.

Ettringite (and monosulfate) are also thoughtto occur in minute crystals closely intermixedwith and occupying space within individualinner product or hydrated phenograin C-S-Hdeposits in ordinary cement pastes. Such closelyintermixed minute crystals can apparently befound using TEM but are not detectable at therelatively low magnifications possible in back-scatter SEM.

MECHANISMS OF DEF EXPANSION ANDCRACKINGGeneral considerationsIt is widely appreciated that exposure to of con-crete to temperatures of the order of 70C forseveral hours in steam curing destroys the nor-mal ettringite that has previously beendeposited, and that after cooling and for some

Fig. 7. Inconspicuous single ettringite crystals depositedin a nest within cement paste.

period of days or weeks thereafter no crystallinealuminosulfate hydrate component can bedetected. Subsequently ettringite begins toform, and progressive deposition of ettringite isin some fashion the basis for DEF-induced dis-tress. In non-steam cured concrete, because ofexcessive sulfate or slow availability of &A orsome kinetic factor, ettringite may continue toform long after the time that ettringite forma-tion is normally complete.

That the formation of large amounts ofettringite in cement pastes and concretes can beexpected to induce expansion (and cracking, ifunrestrained) has been known for many years.Successful and well-established commercialproducts such as shrinkage compensatedcements and expansive cements that derivetheir expansive properties from such massiveettringite formation are well known.The magnitude of these expansions can bevery great. Odler and GasseT measured linearpaste expansions of more than 175% in oneyear for never-heated pastes made of a suscep-tible clinker plus excessive gypsum addition(15%). For 10% gypsum the expansionobserved was more than 40%. For a normalgypsum content (5%) the expansion was 0.9%,about the same as the expansion induced bysteam curing and underwater storage in thestudies reported by Lewis et al. Odler andGasser monitored the ettringite content oftheir pastes, and unlike Lewis et al.* noted ageneral correlation between expansion and theamount of ettringite formed.Nevertheless, in the case of DEF associatedwith steam curing, the specific mechanism ormechanisms at work are, to say the least, highlycontroversial. If anything, intensive investiga-tions carried out over the last few years haveheightened, rather than ameliorated the con-troversy.On the one hand, highly respected investiga-tors such as Scrivener and Taylor,** Johansen eta1.23 and others have maintained that the basisfor the DEF effect in steam-cured concrete is auniform and homogeneous paste expansion tak-ing place sometime subsequent to the start ofpost steam curing exposure. According to thepicture proposed by Johansen et a1.23 his postu-lated uniform paste expansion results in rimcracks opening up around aggregates (which donot expand). The rim cracks are subsequentlyfilled in by recrystallized ettringite, but crystalli-zation pressure exerted by the ettringite is not


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deemed to either initiate or widen the rimcracks. Rather, the recrystallized ettringiteinfilling is deemed to be mechanically passive,and is considered to play no part in the expan-sion and cracking of the concrete. Similarly,deposition of ettringite in paste cracks and inair voids and other open spaces are consideredto be benign.These authors generally do not refer to thevarious cases of distress produced after variousdelays in non-steam cured concretes as illustra-tions of DEF. In the present writers experiencenon-steam concrete showing such distress candisplay symptoms and microstructures identicalto those developed in steam cured concretes,and are also examples of DEF.In contrast to the homogeneous paste expan-sion view, other investigators (including thepresent writer) contend that the expansion andcracking characteristic of DEF are the conse-quences of crystal pressure exerted by thegrowing ettringite crystals on their surround-ings. Proponents of this view consider thatettringite deposited in rims surrounding aggre-gate grains, and ettringite deposited in cracks ascontributing to the overall expansion, throughcrack development and propagation.A critique of the homogeneous pasteexpansion hypothesisThe mechanism of the postulated homogene-ous expansion following steam curing wasoriginally described in rather vague terms byJohansen et al.,23 who suggested that thedelayed ettringite formation may involve in situreactions between C3A and C4AF in homoge-neously distributed unhydrated, or notcompletely hydrated, cement particles and cal-cium aluminate hydrates with SO%- ions in thepore liquid and CSH.

More recently, Scrivener and Taylor22 indi-cated that the earliest ettringite found aftersteam curing and exposure to a wet environ-ment appeared to be microcrystalline ettringiteclosely intermixed with inner product CSH; onlylater were ettringite crystals found that weredistinguishable by SEM. The concept washinted at that the development of closely-inter-mixed ettringite might in some way beresponsible for the homogeneous expansionpostulated by various authors.This idea was further elucidated by Lewis et~1.~~They found that after heat treatment and

subsequent exposure, but prior to expansiontaking place, the C-S-H compositions analyzedby EDS suggested the presence of closely-mixedmicrocrystalline ettringite with the CSH; duringexpansion the compositions of the closely-mixedsulfoaluminate appeared to shift to lower sul-fate contents. At the end of the expansionprocess the analyses of the C-S-H gel indicatedthat the gel was intermixed only with mono-sulfate, although ettringite not closelyintermixed with C-S-H gel continued to exist.Subsequently Lewis and Scrivener24 suggestedthat the change-over from closely intermixedettringite to closely intermixed monosulfate lib-erated sulfate ions to the pore solution. Theyalso suggested that the time of this change-overwas important; the longer this process was post-poned during the underwater storage,apparently the greater the ultimate expansionmight be.The idea that ettringite closely intermixedwith the C-S-H is associated with homogene-ous expansion was further developed by Glasseret a1.25On the basis of investigations of thermo-dynamic stability at various temperatures in theCaO-A1203-CaS04-Na20-H20 system. Theysuggested the following sequence may occur insteam cured pastes: first, that pore solution sul-fate contents increase substantially at steamcuring temperatures (especially with high alkalicements), that the subsequent cooling leads tosupersaturation of the pore fluid with respect toettringite, and that this supersaturation resultsin massive ettringite precipitation. However,they viewed this ettringite precipitation asinnocuous, and argued that the expansion itselfderives from nucleation of ettringite within thepre-existing monosul ate crystals closely inter-mixed within inner product CSH.Unfortunately, the sequence of events postu-lated by Glasser et al. seems to be in directcontradiction to the direct pore solutionevidence of 0ng.26 Ong examined pore solu-tions expressed from steam cured cementpastes, and also determined the ettringite con-tents of those pastes by DSC at intervals.Determinations were made during the steamcuring cycle itself (to 95C), and subsequentlyduring room temperature exposure at 100%RH.In conflict with the predictions by Glasser eta1.,25 but in conformity with the experimentalresults of Brown and Bathe, no evidence ofmonosulfate was detected by DSC at any stage.


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Furthermore, the pore solution results did notappear to be as Glasser et aL2 had predicted. Itwas clear that the pore solution sulfate concen-trations did not increase substantially at steamcuring temperatures. For a normal alkalicement, the maximum sulfate concentrationthat was reached during the steam curing cyclewas only about 0.2N, and this was reachedduring the early ramping up of the heatingcycle, before the destruction of the early ettrin-gite. The pore solution sulfate content thenactually dropped slightly (to 0.18N) as heatingprogressed. When the maximum temperaturehad been reached and all early ettringite hadbeen destroyed, the sulfate concentration in thepore solution had actually dropped to about0.13N. This concentration was maintainedthrough the cooling part of the steam curingcycle and did not change appreciably for manyweeks of room temperature exposure at 100%RH.Parallel experiments were also carried out ona high alkali cement paste. In conformity withthe predictions of Glasser et a1.25 and in linewith normal expectations, the sulfate concentra-tions were indeed significantly higher at allstages. However, again, no increase in sulfateconcentration was noted as the temperaturereached the hot part of the steam curing cycleconcomitant with the destruction of the ettrin-gite.Nor was there any evidence of massive pre-cipitation of ettringite from solution on coolingfor either paste. No ettringite was detected forat least 28 days in either case, and the develop-ment of delayed ettringite was just that -delayed. It occurred only slowly and progres-sively over months of exposure. Solution sulfateconcentrations were stable for periods ofmonths after cooling, while the slow precipita-tion of ettringite (and the consequentexpansion) were occurring.These results will be submitted for publica-tion elsewhere.An idea frequently cited in support of thehomogeneous expansion following steam curinghypothesis is the observation reported byJohansen et a1.23 that for an experimental mor-tar the width of the gap around aggregategrains is proportional to the size of the grain. Itappears to the writer that this is a considerableoversimplification. In the affected concretes hehas examined, most rim cracks are between 10and 30 mm across, and within this limited range

do not appear to vary appreciably with the sizeof the aggregate.Further to the point, the postulated homo-geneous expansion of the paste appears to bequite incompatible with three obvious featuresof the microstructure of DEF-affected con-cretes. First, as pointed out specifically for Fig.3, and visible also in Figs 1 and 2, for manyaggregate grains the rim crack does not go allthe way around the aggregate, but only part-wayaround. Can it be argued that paste expandedhomogeneously on one side of the aggregateparticle but not on the other?Secondly, as is visible in most fields examinedby the present writer, some aggregate particlesin a given field of view have rim cracks; otherssimply do not have rim cracks at all. Can this beconsistent with the effects of homogeneouspaste expansion?The third point is most important. Figure 8 isreproduced from Johansen et a1.,27 and is usedby these authors to illustrate the basic conceptsof the postulated effect of the homogeneousexpansion, i.e. that peripheral gaps around theparticles are created when the matrix is expand-ing relative to the particles.The present writer invites comparison of thisfigure with the actual crack pattern of Figs 1and 2 of the present paper, or indeed with thecrack pattern exhibited in the optical micro-graph of DEF-affected concrete shown as Fig. 2of the same Johansen et al. paper. In suchmicrographs it is seen that the rim cracks are infact portions of a continuous crack networkrunning across the paste, as well as aroundsome aggregate grains, par t l y around others, andentirely missing from the perimeter of stillothers. In contrast, the pattern of crackingobserved seems readily compatible with the idea

Fig 8 Schematic illustration of concept of effect ofhomogeneous paste expansion, after Johansen et a1 27


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of inhomogeneous, locally varying expansionassociated with ettringite deposition in weakareas or in pre-existing cracks - i.e. the crystalpressure hypothesis.The ettringite crystal pressure hypothesisSimply stated, this hypothesis maintains thatexpansion and cracking found in DEF is theobvious and necessary result of growth of mac-roscopic ettringite crystals in restricted spaces,the growing crystals exerting expansive stressesagainst the restraint offered by the pre-existingmicrostructural elements. Under this hypothesismost of the ettringite crystals depicted in themicrographs shown in this paper would beexpected to participate in this process; the onlyobvious exception would be ettringite depositedas incomplete linings in partly filled air voids orother open spaces.That expansive pressures of enormous magni-tude can be developed by crystal growth isproven every day with the highly successful useof so called silent explosives. These materials,produced as specialty products by variouscement companies, depend on crystal pressureexerted by CaO hydrating to Ca(OH),. In prac-tice holes are drilled within hard rock bouldersor other masses that need to be broken up.Products containing free lime and water areused to fill the holes, and the holes are fittedwith strong plugs. The silent explosive thenproduces expansive stresses by crystal pressuresufficient to break up the rock mass, withoutthe necessity of employing dynamite or otherexplosives.Proponents of the homogeneous expansionhypothesis have tended to reject crystal pressurepartly on the grounds of thermodynamic analy-sis. This usually takes the form of thewell-known thermodynamic argument that thepressure that can be developed is limited by thedegree of supersaturation with respect to thegrowing crystals, and it is maintained that theneeded degree of supersaturation cannot beproduced by the ettringite formed under theconditions existing in DEF.The supersaturation argument has beenaddressed by several workers. Taylor28 cal-culated that for a reasonable degree ofsupersaturation (2.4), and assuming isotropicbehavior, the pressure that can be exerted isonly 3 MPa. However, results of others, notablyPing and Beaudoin29Y30 and, very recently, Deng

Min and Tang Mingshu31 are not in accord withthese calculations. Deng and Tang calculatedthat crystallization pressures of ettringite inportland cement paste at 1 day could reachlevels in excess of 55MPa, certainly far morethan is needed to generate the expansion andcracking observed in DEF.However, the writer believes that thermo-dynamic calculations based on overall averagepressures miss the point. Concrete is a quasi-brittle material. As such, the concepts offracture mechanics apply. In much simplifiedform, fracture mechanics suggests that any ten-sile stress exerted near the tip of a crack ismagnified by a stress intensity factor thatdepends on the length of the crack and by thegeometry of the crack tip. A common formula-tion, found in most current undergraduateengineering materials texts, is given as eqn (1):

whereKt = s /s = 2(~/r,)~ (1)

Kt is the stress concentration or stress ampli-fication factor at the tip of the crackS, is the maximum stress, that is the stressexerted at the tip of the cracks, is the applied tensile stressa is half the length of the crack, andr, is the radius of curvature of the crack tip.In the analysis, the stress may result fromexternal applied stresses or from pressuresgenerated by crystals growing within the crack.Cracks in concrete are normally very longwith respect to the radius of curvature of thecrack tip, which is typically very sharp; accord-ingly the stress concentration factor is extremely

large. Thus, only modest thermodynamicallycalculated overall pressures developed by crystalgrowth within cracks are needed to translate toextremely large stresses at the crack tips. Pro-gressive extension and widening of the cracks isthus a predictable consequence, even undermodest degrees of supersaturation.Johansen et aZ.27 have raised an interesting _point in opposition to the crystal pressure hypo-thesis. They called attention to the fact thatcracks are not typically seen radiating out fromair voids filled with ettringite, and noted that ifcrystal pressure were at play, such filled voidswould behave like expanding particles; thus,cracks radiating from into the surrounding pasteshould be observed. In point of fact, the geo-


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metry of spherical voids is such the stressintensity factor developed for cracks does notoperate. Accordingly, stresses generated againstthe surrounding paste are limited to thermo-dynamically calculated pressure, which dependson the supersaturation attained.A much more sophisticated analysis incor-porating both fracture mechanics andthermodynamic considerations was recently pro-vided by Fu et a1.7 Based on free energyconsiderations, it was shown that ettringitenucleation will take place preferentially incracks, a conclusion well supported by experi-ence. More to the point, according to thisanalysis, nuclei should first form near the tips ofthe cracks, where the lowest free energy condi-tions exist. After nucleation the driving force ofany continued supersaturation then results inthe growth of these ettringite crystals and subse-quent propagation and opening of the crack.The local expansive stresses at the crack tipclearly reflect a very large stress intensity factor.

One final graphic argument might beadduced from the writers experience. He offersFig. 9 as evidence that ettringite precipitationexerts expansive force and results in expansion.The micrograph was taken from a steam curedconcrete in which mica constitutes a minorcomponent of the aggregate. Layers of ettrin-gite are seen to have been deposited betweenthe lamellae of the mica, apparently forcingthem apart and inducing obvious expansion inthe direction normal to the layers. Examplessimilar to Fig. 9 have been seen in both steamcured and not steam cured concretes undergo-ing DEF.

Fig 9 Illustration of ettringite deposits seemingly pro-ducing separation of mica lamellae by crystal pressure in aDEF-affected steam cured concrete.

CONCLUSIONSThe arguments elucidated have ranged over avariety of topics and approaches, indicating thecomplexity of the DEF problem. They may besummarized as follows:Concretes undergoing DEF following steamcuring, and concretes undergoing DEF associ-ated with excess sulfate or other factors thatwere not steam cured seem to show similarcrack patterns and microstructural features,despite the difference in history. The fact thatin steam curing much or all of the early ettrin-gite is destroyed and must be constituted anewdoes not appear to change the pattern of crack-ing and ettringite deposition that ensues.Concepts that maintain that DEF followingsteam curing reflects a special homogeneousexpansion in the paste by virtue of which non-shrinking aggregates develop empty rim cracksproportional to their size, only later to be filledin by ettringite deposits, seem not to bereflected by close examination of real concretes.Rather than developing a system of isolatedempty rim cracks, it appears that the rim cracksdo not go all the way around some aggregateparticles, and are entirely absent from others inthe same field. Furthermore the writers experi-ence suggests that the claimed relationship ofcrack width with aggregate size may not holdgenerally.The observed crack pattern is that of a net-work with component crack segments runningpartly along aggregate peripheries (rim cracks),but generally connecting through segments run-ning through the cement paste (paste cracks).The pattern appears to reflect local and inho-mogeneous crack propagation rather thangeneral and homogeneous paste expansion.The association of DEF with prior crackinginduced by ASR, by freezing, or even by shrink-age is not accidental but may be fundamental tothe DEF processes. While the overall stressesthat can be thermodynamically calculated toresult from ettringite precipitation are limitedby the limited degree of supersaturation sus-tainable in the pore solution, free energyconsiderations appear to preferentially promoteettringite precipitation near crack tips. In anyevent stress intensity factors associated withfracture mechanics considerations shouldgrossly magnify the mechanical effects of ettrin-gite crystallizing in cracks.The common association between ASR and


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DEF may reflect chemical factors in addition tothe crack-inducing effect of ASR. The reductionin alkali hydroxide concentration attendant onASR may also serve to promote ettringite pre-cipitation.The practical consequences of DEF are morevarious and more complex that simple expan-sion. Foss of prestress in prestressed railwaysleepers followed by debonding and subsequentcorrosion are important factors in the DEFeffects on sleepers. In any affected concrete thecrack network induces a severe loss in dynamicelastic modulus, which has important structuralimplications. Filling up of fine air voids byettringite may interfere with frost resistance.In the last analysis, the spate of modern DEFproblems stems from high contents of sulfate inmany modern cements. These may involve highlevels of intergound calcium sulfate, but in thewriters experience they may also reflect high(and often variable) clinker sulfate contents. Insuch cases, the need to add sufficient inter-ground sulfate to assure normal settingbehavior and strength gain thus can result inhigh total sulfate contents.

ACKNOWLEDGEMENTSThe writer is pleased to acknowledge the con-tributions of several former associates atPurdue University, including Shaode Ong, Yut-ing Wang, and David Bonen. The work wassupported by the National Science FoundationCenter for Advanced Cement Based Materials.REFERENCES

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Pettifer, K. & Nixon, P. J., Alkali metal sulfate - afactor common to both alkali aggregate reaction andsulfate attack on concrete. Cem. Concl: Res., 10(1980) 173-181.Jones, T. N. & Poole, A. B., Alkali silica reactions inseveral UK concretes: the effect of temperature andhumidity on expansion and the significance of ettrin-gite development. In Proc. 7th Int l . Con on Al kaliAggregat e Reacti ons, O t t aw a, 1986, edited by P. Grat-tan Bellew. Noyes Publications, Park Ridge, NJ, pp.446-450.Larive, C. & Louarn, N., Diagnosis of alkali aggregatereaction and sulphate reaction in French structures.Proc. 9th Znt l. Conf on Alk ali Aggregate Reactions,London, 1992, Concrete Society, pp. 587-598.Saloman, N., Claude, J. & Hasni, L., Diagnosis ofconcrete structures affected by alkali aggregate reac-tion. Proc. 9th Znt l. Co@ on Alk ali Aggregatefixzixz~~, London, 1992, Concrete Society, pp.Hime, W. F., personal communication, October 1995.

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