Sulfate Attack on Concrete - Chapter 8 pot

The docu-ment clearly states that “low water cement ratio and high density concrete isimperative at all sulfate levels” and recommends the maximum w/cm, min-imum cement content, and ceme

8 Case histories

Since the introduction of relevant standards and codes in industrialized tries, occurrence of internal and external sulfate attack in properly designed,processed and executed concrete is rare When damage occurs, it is always theconsequence of incorrect construction that enables penetration into concrete

coun-of aqueous salt solutions needed to initiate and feed the attack Most coun-of thecodified recommendations are based on prescription of maximum values forwater–cement ratio, maximum levels of C3A in cement and, in some cases, ofminimum cement content and addition of supplementary materials such asselected pozzolanas or slags, or both

As the sulfate-generated distress is largely a function of concrete quality,the primary objective of the precautionary measures is to decrease theaccessibility of sulfate bearing solutions into concrete by decreasing its perme-ability A well-constructed, impermeable concrete structure will not sufferfrom sulfate attack regardless of the prevailing environmental conditionsand physico-chemical mechanisms (e.g potential for ettringite, thaumasite,gypsum, or efflorescence formation) According to Mehta and Monteiro (1993): The quality of concrete, specifically a low permeability, is the best pro-tection against sulfate attack Adequate concrete thickness, high cementcontent, low water/cement ratio and proper compaction and curing offresh concrete are among the important factors that contribute to lowpermeability In the event of cracking due to drying shrinkage, frostaction, corrosion of reinforcement, or other causes, additional safety can

be provided by the use of sulfate-resisting cements.1

In other words, properly designed and constructed concrete will be stableunder most aggressive conditions unless the concentration of sulfates in thesoil or the water in contact with the concrete is extreme Under such condi-tions additional measures have to be taken to prevent direct contact betweenthe concrete and the SO42+source

However, problems do occur, and sulfate attack may become a real issuewhen concrete is improperly proportioned, designed, cured and placed in

a hostile environment, or both (e.g Swenson 1968; Mehta 1992; DePuy1997; Figg 1999) The following case studies are examples that resulted frominadequate utilization of knowledge on concrete mixture design, concreteprocessing, and its inappropriate use in a potentially hostile environment

8.1 DETERIORATION OF RESIDENTIAL BUILDINGS IN

SOUTHERN CALIFORNIA

A well-publicized problem involving residential housing construction inSouthern California is an interesting case of external sulfate attack (e.g.

Reading 1982; Novak and Colville 1989; Rzonca et al 1990; Haynes and

O’Neill 1994; Travers 1997; Lichtman et al 1998; Haynes 2000) Numerouscourt cases were concluded or are still in progress The alleged violations ofbest concrete-making practices and codes seem to have lead to prematuredeterioration of relevant structures, including post-tensioned floor slabs,garage floors, footings, foundations, driveways, retaining walls, and streetcurbs The technical explanations of the observed damage, and even theanswers to the question whether there is any damage, differ from expert toexpert (e.g see presentations/discussions by Haynes, Diamond and Lee, andothers in references Marchand and Skalny (1999) and Haynes (2000)) Visible changes to concrete were observable often as early as 2–4 years aftercasting (see Figure 8.1) Structural and other problems unrelated to concretewere also encountered; these will not be highlighted in the following paragraphs

Figure 8.1 California residential house footing exposed to sulfate-containing ground

waters Note spalling and efflorescence (Photo: J Skalny)

It is known for many years, that wide areas of Southern California havesoils containing high levels of sulfates, often in form of gypsum (e.g Novakand Colville 1989; Rzonca et al 1990; Day 1995) Due to the geologicalhistory of Southern California – formerly sea beds with heavy salt deposits;earthquake zone – analyses of soil samples revealed variable sulfate concen-trations in a wide range from practically nil to well above 10,000 ppm Forthis reason, and probably others, the Cement Industry Technical Committee

of California issued in the 1970s a “Recommended Practice to MinimizeAttack on Concrete by Sulfate Soils and Waters” (CITC 1970) The docu-ment clearly states that “low water cement ratio and high density concrete isimperative at all sulfate levels” and recommends the maximum w/cm, min-imum cement content, and cement type to be used at various levels of sulfate

in ground water Generally, these recommendations are in line with mendations or requirements of ACI, Uniform Building Code, CaliforniaDepartment of Transportation, and other codes and standards

recom-The building boom of the 1980s and 1990s led to a situation in which therecommendations with respect to the type of cement were usually followed,but only the bear minimum cement content was used, and the requirementsfor the maximum allowed w/cm seems to have been often ignored Excessivew/cm can clearly result in higher concrete porosity and permeability than isappropriate for environment known to have high sulfate concentrations in soil

As discussed earlier, depending on the concrete quality and environmentalconditions, the complex sulfate attack mechanisms may lead to variouschemical and physical changes in concrete Chemical changes may include:

1 removal of Ca2+ from some of the hydration products (e.g decomposition

of calcium hydroxide and C-S-H, or both);

2 unusual changes in pore solution composition;

3 formation of hydrated silica (silica gel);

4 decomposition of still unhydrated clinker minerals;

5 dissolution of previously formed hydration products;

6 formation of ettringite (in excess of that formed from original sulfate inthe cement), gypsum, and thaumasite;

7 formation of magnesium-containing compounds such as magnesiumhydroxide (brucite) and magnesium silicate hydrate;

8 repeated recrystallization of sodium sulfate unhydrate (thenardite) to/from sodium sulfate decahydrate (mirabilite); and

9 penetration into concrete of ionic species and subsequent formation andcrystallization of salts such as NaCl, K2SO4, MgSO4, etc

The observable physical changes are the consequence of the above chemicalchanges and may include:

1 complete restructuring of the pore structure and solid microstructure;

2 increased porosity and permeability;

178 Case histories

3 volumetric expansion and the associated microcracking;

4 formation of complete or partial circumferential rims or gaps (pasteexpansion cracks) around the aggregate particles;

5 surface spalling, delamination, exfoliation;

6 paste softening, decreased hardness;

7 deposition of salts on surfaces and exfoliation cracks;

8 loss of strength; and

9 decreased modulus of elasticity

By themselves, neither of the above chemical, physical, and tural changes are necessarily an adequate sign of sulfate attack However, incombination, there can be little doubt It should be noted that initially, due

microstruc-to pore filling by the reaction products, the reactions of sulfate attack might lead

to decreased porosity and even increased compressive strength (e.g Jambor1998) However, as the chemical and microstructural changes proceed, the trendreverses and the concrete gradually loses its required engineering properties The following information is available from Southern California regardingthe relevant conditions and observed phenomena (e.g Haynes and O’Neill1994; Day 1995; Deposition Transcripts 1996–2000; Lichtman et al 1998;

Diamond and Lee 1999; Brown and Badger 2000; Brown and Doerr 2000;Diamond 2000):

• Large amounts of concrete were designed and placed using w/cm as high

as 0.65 (in apparent violation of applicable codes and recommendations);occasionally, concretes with w/c of 0.7 or higher were identified;

• Typical cement content used was about 250–320 kg/m3 (400–500 lb percubic yard) In some instances, the cement content was as low as 220 kg/m3(350 lb per cubic yard) Mostly ASTM Type V, in some instances Type IIcements with pulverized fly ash were used;

• Compressive strength required at twenty-eight days, depending on theapplication, was about 13–20 MPa (c 2,000 to c 3,000 psi);

• Sulfate concentration in ground water is variable, often even within thesame construction locality; typically between 150 and 10,000 or more ppm;

• Presence in ground water of Mg2+, Na+, K+, Cl−, and HCO−3and otherions, in addition to SO2−4 ;

• Depth of ground water variable from locality to locality, from surface to several meters below the surface;

near-• Typical (summer) ambient temperature: 10–20°C at night, 25–35 °C, ormore at direct sun exposure, during daytime;

• Humidity variable from very low at daytime to above dew point at night;

• Visually observable damage includes efflorescence, delamination ofmortar, exposed aggregate, spalling, and limited cracking;

• Petrographic observations (light optical and SEM microscopy; dispersive spectrometry):2 formation of ettringite “nests” in the paste,microcracking of the cement paste, expansion of the paste (formation of

energy-circumferential gaps around aggregate particles), formation of gypsumveins, removal of calcium hydroxide from the paste, decomposition ofC-S-H, increased and irregular porosity or both, severe carbonation ofexternal and of some buried concrete surfaces, decalcification of the stillunhydrated calcium silicates, deposition of reaction products in poresformed as a result of hydration or decalcification of the clinker minerals,formation of Mg-rich layers in Headley grains, formation of brucite andmagnesium silicates, presence of Friedel’s salt (calcium chloroaluminatehydrate, a chloride analog of calcium monosulfate 12-hydrate), surfaceefflorescence (predomiantly sodium sulfate; occasionally also sodiumchloride, magnesium sulfate, other salts); and some corrosion of rein-forcement;

• X-ray diffraction data: presence in the efflorescing material of thenardite

or mirabelite; occasionally other salts, including Friedel’s salt, NaCl, andMgSO4

• Physical testing data: decreased hardness of concrete with depth, pressive strengths variable (higher, lower or as designed for twenty-eightdays, depending on local conditions and age of concrete exposure tothe environment), decreased tensile strength, very high permeability(ASSHTO T 277-831 rapid chloride permeability test, water vapor andwater permeability), decreased modulus of elasticity

com-In the following, we will present microscopic evidence that has been used

in interpretation of some of the observed external sulfate attack phenomena.The set of micrographs in Figure 8.2 is typical of ettringite forms found inconcrete exposed to external sulfate attack As has been discussed earlier,the observed ettringite morphologies found in internal and external sulfateattack situations are similar This is not surprising considering that themechanisms are based on the same chemical principles As emphasized onprevious pages, the presence of ettringite per se is not a sign of sulfate attack.Ettringite is found usually in the form of:

1 “nests” located throughout the paste in the C-S-H mass, dominating thelocal morphology and often being accompanied by microcracking anddevelopment of a network of microcracks (micrographs a and b);

2 deposits located in gaps around the aggregate particles and in cracks(micrographs c and d);

3 deposits in air voids, usually filling the void only partially (micrograph e);and

4 microcrystalline ettringite, not detectable by microscopic techniques;this form of ettringite is most probably responsible for the paste expan-sion evidenced by the formation of the gaps around aggregate, and thesubsequent deterioration of physical properties

Under specific environmental conditions such as lower ambient temperatureand presence of carboxyl ions, thaumasite may form in addition to ettringite.

It is believed by some that damage caused by thaumasite may be even moresevere that that caused by ettringite An example of thaumasite crystallitesfound in concrete exposed to sulfate-containing ground water is given in

dc

e

Figure 8.2 (a,b) Formation of ettringite “nests” and cracking of cement paste; (c,d)

ettringite in paste and gaps at the paste-aggregate interface; (e) ettringitepartially filling an air void SEM, backscattered mode (Photos courtesy of

RJ Lee Group)

Figure 8.3 Note that this concrete was produced and located in an arid zonewhere low temperatures, assumed by some to be needed for thaumasiteformation, are uncommon

One of the common observations in sulfate attack is change in pasteporosity Depending on the age of concrete, severity of the sulfate attackand, possibly other variables, the porosity at the time of observation may beunchanged (usually slightly-damaged concrete) or changed dramatically(highly-damaged concrete) Micrographs of Figure 8.4 show typical examples

of high (a) and inhomogeneously distributed (b) porosity The given examplesrepresent concrete made with initial (mix) w/cm of about 0.65

One of the characteristic features of severe external sulfate attack is tion of gypsum It is usually found in the form of layered deposits parallel tothe surface that is in contact with the sulfate-bearing ground water or soil.Examples of gypsum deposits found in Southern California concrete areshown in Figure 8.5

forma-Thaumasite

Gypsum

Thaumasite

Figure 8.3 Simultaneous formation of thaumasite (square) and gypsum (triangle) in

concrete exposed to external sulfate EDAX pattern: thaumasite SEM,backstattered mode (Photo courtesy of RJ Lee Group)

In permeable concrete, especially in situations where a part of the aboveground concrete is exposed to repeated temperature and humidity fluctua-tions, the sulfate-bearing solutions may penetrate to the exposed concretesurface where they crystallize According to ACI Guide to Durable Con-crete (ACI 1992), under such conditions concrete may be exposed to severe

a

b

Figure 8.4 Extremely high (a) and inhomogeneous distribution (b) of porosity SEM,

backscattered mode (Photo courtesy of RJ Lee Group)

a b

Figure 8.5 Formation of gypsum veins parallel with the horizontal concrete surface in

contact with sulfate-containing ground water Note well developed gypsumcrystals shown on left (Photo courtesy of RJ Lee Group)

Sodium sulfate

Figure 8.6 Micrograph of Na2SO4 efflorescing material and corresponding EDAX

patterns SEM, secondary electron mode (Photo courtesy of RJ Lee Group)

chemical sulfate attack Examples of efflorescing material formed under

such conditions are given in Figure 8.6

Deposition of various salts may occur not only at the concrete surfacebut also in the interior of a concrete structure exposed to ionic solution.Such depositions of NaCl, Na2SO4, and Friedel’s salt are presented in Fig-ure 8.7

Under conditions where both chloride and sulfate ions are a part of theaggressive solution, one can identify microstructures in which the reactionproducts of the reinforcement corrosion penetrate, and possibly replace, thelocalities formerly occupied by hydration products See Figure 8.8, micro-graphs a, b Occasionally, one may encounter evidence of both sulfate attackand reinforcement corrosion (micrograph c)

b

c a

Figure 8.7 (a) Deposition of sodium chloride in the paste; (b) Friedel’s salt within the

paste; (c) deposit of sodium sulfate on a partially decalcified calcium ate particle SEM, backscattered mode (Photos (a) and (c) courtesy of RJLee Group; photo (b): J Skalny)

silic-Of the “fingerprints” characterizing external sulfate attack, the most vincing in the discussed cases are the presence of gypsum and, if magnesiumsulfate is present, of magnesium-containing reaction products, and deteriora-tion of some, though not all, physical properties Formation of brucite andhydrated magnesium silicates, taking into account the presence of both SO2−and Mg2+ ions in the ground water, is the most damaging Their formation isclosely associated with decrease in calcium hydroxide concentration anddecalcification of the C-S-H Presence of gypsum in the concrete cannot beexplained by any other damage mechanism It should be noted that of theobserved reaction products only brucite and gypsum are stable phases; theother phases present, such as ettringite and monosulfate, are metastable andtheir presence depends on the local micro-conditions Gypsum, ettringiteand monosulfate can co-exist in cement paste only due to the paste matrixheterogeneity

c

Figure 8.8 Products of reinforcement corrosion (lightest color) (a,b) Note intermixed

paste hydration and corrosion products; (c) penetration of corrosionproducts into paste and gypsum crystallites SEM, backscattered mode(Photo courtesy of S Badger)

Although volumetric changes at macro-scale do not seem to predominate,observation within a few years after casting of micro-scale cracking caused

by ettringite formation in the paste is an indication of progressing sulfateattack As explained above, the lesser importance of the usual ettringiteform of sulfate attack is believed to be preconditioned by the environmentalconditions and ionic composition of the ground water

The adequate compressive strength of many of the tested concrete coreshas been taken by some as an indication of limited or no damage However,

as is now accepted by most experts (Mehta 1997; Neville 1998; Jambor1998), strength, especially compressive strength, is an inappropriate measure

of durability Most of the tested concrete had allegedly inadequate tensileand flexural properties, diminished hardness, and lowered modulus of elasti-city The finding that the usual ratio of compressive and tensile properties ofthe concrete, believed to be c 10:1, increased well above the expected due tomicrocracks formation may be by itself a sign of internal damage (Ju et al.1999) Such finding was reported earlier (e.g Harboe 1982) It remains awell-established fact that durable concrete also exhibits adequate mechanicalstrength, but the reverse may not be the case

Another reported observation is decomposition of the still unhydratedclinker calcium silicates and their transformation into hydrated (?) magnesiumsilicate or silica gel The fact that the original shape of the clinker minerals ismaintained indicates that the decalcification happened before these mineralshad a chance to hydrate (see micrographs of Figure 8.9) In other words, theaggressive sulfates must have had access to C3S or C2S particles in very earlystages of hydration, possibly within hours after concrete was placed in thehigh-sulfate environment This supports the experimentally obtained datarevealing high porosity and permeability

Damage mechanisms other than external sulfate attack, such as ASR, acidrain, etc., were also considered, but experimental evidence does not supportthese options The issue in Southern California seems to be inadequate con-crete quality in an environment rich in sulfates and chlorides Whereas mostparties agree that the sulfate levels in California are high and the used w/cmwere excessive, the interpretations of the observed damage to concrete vary(e.g Haynes 2000; Deposition Testimonies 1996–2000) Among others theclaim is made that only very few, if any, cases of sulfate attack were docu-mented; those admitted are categorized by some as “physical” salt attack Incontrast, other experts are of the opinion that both chemical sulfate attackand repeated mirabilite–thenardite recrystallization are responsible for thedamage, and argue that visual observation is an inadequate technique toassess the sulfate damage to concrete (Diamond and Lee 1999; DepositionTestimonies 1996–2000)

The observed damage to the concrete at macro- and microstructural levelsseems to be the result of complex sulfate attack mechanisms involving bothchemical and physical processes enabled by high concrete porosity and per-meability Repeated cycles of wetting–drying and of low–high temperatures

enabled physical mechanisms to supplement the well-known chemical cesses of destruction: formation of internal cracking due to excessive ettringite,decalcification of the paste and C-S-H, and formation of magnesium-bearingcompounds

pro-In addition to the above sulfate-triggered changes, the high porosity of theconcrete resulted in excessive carbonation and chlorination Carbonation isknown to affect the stability of ettringite; in carbonated parts of the examinedconcrete no ettringite was present, and an ettringite layer (or front) ahead ofthe carbonated zone was observed The synergistic effect of sulfates andchlorides is not entirely clear, but there are reports on decreased effective-ness of ASTM Type II and Type V cements to sulfates in the presence ofchlorides

Figure 8.9 Decalcified pseudomorphs of unhydrated clinker calcium silicates: (a)

partial decalcification (Photo courtesy of B Erlin); (b) complete cification to hydrated (?) silica; (c) partial tranformation to magnesiumsilicate hydrate; (d) complete transformation to magnesium silicatehydrate SEM, backscattered mode (Photos courtesy of RJ Lee Group)

decal-In conclusion, we would like to share our opinion regarding the observeddamage to concrete in residential houses of Southern California The issue isnot which of the experts are right or wrong What is unfortunate, however, isthat the knowledge on sulfate attack, generated since the 1900s by some ofthe best “concrete” minds in North America, including California, was some-how forgotten or ignored All described problems could have been avoided ifthe most basic principles of concrete making were remembered and adopted.Codes, standards and concrete-making guidelines are clear: sulfate attackcan be prevented by production of low-permeability, properly proportioned,and adequately cured concrete It is the non-compliance with these standardsand best practices of concrete making that apparently lead to the encounteredproblems and subsequent need for expensive rehabilitation

8.2 SULFATE ATTACK DAMAGE BROUGHT ABOUT BY

HEAT TREATMENT (DEF)

In the period between 1980 and 1984, an extended occurrence of damage toprefabricated, pre-stressed steel-reinforced concrete railway ties and someother concrete products was observed in what was then West Germany(Association of German Cement Manufacturers 1984) Several millions ofties were affected

The damage became apparent several years after the products have beenmanufactured and in use It was characterized by development of cracks thatstarted at the corners and edges of the concrete element and graduallyspread into deeper regions as the time progressed Also observed were gapsbetween aggregate particles and the cement paste, associated with a loss ofbond between the two The cracks and especially the gaps tended to bepartially or completely filled with crystals of thaumasite alone or a combination

of thaumasite and ettringite There were indications that the cracks and gapswere not created by the formation of thaumasite or ettringite or both, andthat these phases only precipitated in already preexisting empty spaces.Remarkably, any damage was observed only in ties that were steam-cured inthe course of production and were exposed to rain No damage was seen inties located in tunnels or under bridges

Tests performed on the used cements and aggregates proved that allmaterials complied with the existing specifications None of the aggregateswas alkali sensitive and alkali–silicate reaction mechanism could be ruledout as the cause of the problem The strength of the produced concreteexceeded significantly the required value

In laboratory experiments, triggered by the existing situation in the field, itwas found that concrete mixes steam cured at 80°C exhibited consistently adistinct expansion and even cracking regardless on whether they were storedsubsequently at 5°C or 40 °C, partially immersed in water The extent ofdamage was somewhat enhanced in mixes made from cements with elevated