composition of alkali–silica reaction products at different locations within concrete structures

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Composition of alkali–silica reaction products at differentlocations within concrete structures

Isabel Fernandes⁎

Departamento e Centro de Geologia da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

A R T I C L E D A T A

⁎ Tel.: +351 220402456; fax: +351 220402490.E-mail address: [email protected].

1044-5803/$ – see front matter © 2009 Elsevidoi:10.1016/j.matchar.2009.01.011

A B S T R A C T

Article history:Received 6 October 2008Received in revised form5 January 2009Accepted 23 January 2009

Four concrete structures from Northern Portugal–three dams and one bridge–were studied.In the three dams concrete was produced with granitic aggregates. In the bridge, aggregatesfrom an aluvial deposit were used. Pebbles show a wide range of lithologies, thoughdominantly siliceous. Petrographic techniques were applied in the study of these structuresin which signs of deterioration were identified in order to confirm that the deterioration wascaused by alkali–silica reactions.Alkali–silica gel was found in all the structures under different forms. It was verified that thetexture and composition of the gel varies according to its location inside the concrete andalso when compared with extruded gel. This finding is of interest in regard to theassumption that the composition of the gel is related to its expansivity.From the analyses carried out it was verified that silicon is always the main component. Insome samples, calcium exists as the second most important component, but in otherscalcium is hardly detected. Alkalis show a low content in all the analyses. Sometimesaluminum is also detected.An attempt is made to correlate the composition of the gel to its location in concrete. Theresults are discussed in relation to data published by other authors.

© 2009 Elsevier Inc. All rights reserved.

Keywords:Alkali–silica reactionExudationSiliceous aggregatesGel composition

1. Introduction

Petrographic techniques used by geologists to characterizerocks are based on the determination of their mineralcomposition and texture, the latter being related to thegeological history of the rock. They comprise examination atdifferent levels of magnification and degrees of detail: thedescription of hand-specimens, the observation of thin orpolished sections and the application of complementaryanalytical techniques such as X-ray analysis, chemicalanalysis, Scanning Electron Microscope (SEM) studies andElectron Probe Microanalysis (EPMA). These methods havebeen applied since the first decades of the twentieth centuryon construction materials, and concrete in particular, for theevaluation of potential reactivity of aggregates and also forsystematic examination of concrete andmortars. Petrographic

er Inc. All rights reserved

techniques can provide useful information for the identifica-tion of concrete constituents, aswell as in the characterizationof products resulting from material deterioration.

It is known that alkali–silica reactions (ASRs) are delayedmechanisms that can occur 5, 10 or even 20 years afterconstruction. The present work is part of an investigativeprogramme developed in order to evaluate the potentialreactivity of some of the most common Portuguese rocks asaggregates for concrete and to characterize the products ofalkali–silica reaction. Four structures with ages between 15and 50 years, in which manifestations of ASR were observed,were studied using petrographic techniques. The examinationof the aggregates was carried out following RILEM AAR-1, 2003[1] and the Portuguese Especificação LNEC E415, 1993 [2]procedures. The main objective was to evaluate the potentialreactivity of granitic aggregates and then to compare the

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Fig. 1 – Photomicrographs of the granite used in concrete ofStructure 1 (NX): (a) fine tomedium two-mica granitewith smallcrystals of quartz due to deformation; (b)medium-to-coarse twomica granite with deformation of plagioclase crystals.

Fig. 2 – Photomicrographs of the granite applied as aggregatein Structure 2 (NX): (a) small crystals of quartz with suturedgrain boundaries; (b) deformation of quartz crystals withundulatory extinction and recrystallization.

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products resultant from the ASR with those that exist inconcrete manufactured with gravel as aggregate.

The granites applied as aggregates in the three dams comefromdifferent regions in thecountryandshowsomedifferencesin structure and geologic history. The fourth structure, a bridge,shows a wide variety of aggregate constituents, all siliceous,from an alluvial deposit, which has been intensely exploited forconcrete aggregates in the last decades. The last structure isdifferent from the others in regard to both in-service conditionsand aggregates composition.

Alkali–silica gel was observed in the four structures.Samples of exudations and of concrete were collected andthin-sections were produced. Thin-sections were observed bypolarizing microscope with the aim of characterizing theaggregates applied and detecting microscopic manifestationsof alkali–silica reactions. Inside concrete gel exists mostcommonly in cracks crossing aggregate particles and extend-ing through the cement paste, in voids and in the replacementof fine aggregate particles. To study the texture of the gel andobtain its qualitative composition, exudations and thin-

sections were analyzed in an SEM with Energy DispersiveSpectrometry (EDS). Owing to the very small dimensions of thesamples, other methods of chemical analysis could not beused. The texture varied from crystalline to amorphous gel inthe cracks inside the concrete. Also, gel in exudations showeddifferent features. Composition was found to be variableaccording to the location.

2. Materials and Methods

Concrete samples of 100 mm diameter and between 100 and600 mm length were obtained by coring with a diamonddrilling machine cooled by water circulation, with 230 W ofpower and a rotation velocity of 450 cycles/min.

The cores were labelled with a reference number and thesample's orientation indicated with waterproof ink. To avoiddesiccation, cling-film was wrapped around the samples andthey were sealed in polythene bags. They were takenimmediately to the laboratory.

The cores were first examined with the unaided eye in orderto detect possible manifestations of deterioration. They were

Fig. 3 – Photomicrographs of the granite present as aggregatein Structure 3 (NX): (a) crystals of quartz showing strongdeformation and preferential orientation; (b) deformation ofquartz and muscovite crystals, with orientation of flakyminerals and sub-grains of quartz.

Fig. 4 – Photomicrographs of the quartz particles present inthe concrete of Structure 4: (a) microcrystalline quartz in aquartzite particle (NX); (b) indented grains of quartz withopen boundaries and clay minerals in the interstices (N//).

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then cut lengthwise with a saw to analyze the fresh surfaces.Slices of the concrete were cut and glued to a glass slide toproduce 25×40 mm thin-sections. The samples were impreg-nated with resin by heating T<70 °C until dry; no vacuum wasapplied. The thin-sections were produced totally by manualprocesses from the progressive grinding to the final polishing.No fluorescent dye was used as the study was focused on theevaluation of aggregates and ASR gel composition.

The thin-sections were observed under a polarizing micro-scope inorder todetectpossible signsofASR.The siteswheregelwas detected were then marked with ink. Thin-sections weresubmitted to vacuum and coated with carbon for observationand analysis by SEM/EDS.

Samples of products from pop-outs and exudations werecollected and labelled. They were kept in plastic airtight contain-ers in order to preserve them for examination and analysis.

In the laboratory, the samples were taken out of thecontainers, glued to metallic cylinders 6 mm in diameter andsealed again in the airtight containers. Theywere submitted tovacuum and coated with gold just prior to examination.

The samples were examined in the SEM. The acceleratingvoltage used was 15 kV with a working distance of 15mm. Thecollection time for the microanalyses was 60 s with a deadtime of approximately 30%. The spot size was around 10 nm toproduce qualitative standardless analyses.

3. Results

3.1. Assessment of Aggregates

The observation of the thin-sections permitted the character-ization of aggregates as potentially alkali reactive withdifferent contents of reactive quartz:

- Structure 1–A fine to medium grained two mica granitewith signs of deformation and sutured grain contacts, anda medium to coarse two-mica granite, mainly muscovitic(Fig. 1), earlier described in [3];

- Structure 2–A medium to coarse two mica granite, show-ing deformation and small quartz crystals (Fig. 2), alreadydescribed in [4];


- Structure 3–A highly strained granite with large crystals offeldspar, preferential alignment ofminerals and sub-grainsof quartz (Fig. 3).

- Structure 4–This structure contains a wide variety ofaggregate constituents from an alluvial deposit. Thoseclassified as potentially reactive are quartzite with smallcrystals, sutured boundaries and sub-grains,microcrystal-line quartz, and quartz with permeable grain boundariesand micro-crystals of mica and clay minerals in theinterstices (Fig. 4).

With reference to standards, the aggregates have increas-ing potential reactivity from Structure 1 to Structure 4. The lastis by far the most prone to reaction with alkalis.

3.2. Manifestations of Deterioration

The site investigation of the structures was performed inorder to identify the manifestations of deterioration, whichwere the following:

• In Structure 1, displacements were registered by monitoringequipment. There was a large number of small exudations,

Fig. 5 – Gel present in exudations of Structure 1 under SEM (CEM(b) EDS at Z1 in (a); (c) spongy gel; (d) EDS of Z3 in (c).

some spalling and a few pop-outs, all in the higher levelinterior gallery of the dam.

• In Structure 2, some pop-outs were observed and there wererare exudations in the galleries close to the abutments.

• Structure 3 showed displacements of the structural ele-ments of the crest and also cracking in the upstream faceand in the spillways. Spalling and pop-outs were evident inwide areas where humidity was visible. These featureswere evident in the higher-level gallery of the dam.Occasional exudations were detected.

• In Structure 4 there was some cracking as well as displace-ment of the structural elements.

3.3. Texture and Qualitative Composition of the Gel

The manifestations found during the site inspection suggestedthat ASR could be the cause of the deterioration. This was themain reason for the study of the concrete by petrographicmethods.

The observation of concrete thin-sections under polarizingmicroscope showedno gel in the thin-sections of Structure 1, gelin just one concrete core from Structure 2, in three cores of

UP). Samples are Au-coated. (a) Amorphous, massive gel;


Structure 3 and serious cracking with alkali–silica gel inStructure 4.

The locations where gel was identified in thin-sections weremarked with ink. Samples were prepared for observation andanalyses by SEM/EDS.

In the following paragraphs the results obtained by thequalitative analyses are presented.

3.3.1. Structure 1In the thin-sections of the concrete no gel was found.Exudations were characterized by SEM/EDS. They are com-posed of alkali–silica gel with various textures:

- Smooth surface of amorphous, massive, alkali–silica gel(Fig. 5a, b);

- Viscous gel covered by Na-rich crystals;- Spongy material (Fig. 5c, d).

The composition found is similar for all the samples:silicon dominates; there is some potassium and a littlesodium. No calcium was detected in the exudations.

Fig. 6 – Texture and composition of alkali–silica gel fromStructuregelwith smooth surface and shrinkage cracks froman exudation;(d) EDS of Z1 in (c).

3.3.2. Structure 2The exudations and the interior of a pop-out of Structure 2exhibit gel with variable textures, though similar to that foundin Structure 1:

- Smooth surface of amorphous alkali–silica gel, showingshrinkage cracks (Fig. 6a, b);

- Spongy material (Fig. 6c, d).

The composition was found to be similar to exudations ofStructure 1 for all the samples: silicon is predominant; there issome potassium and a little sodium. In the spongy materialaluminum was also found. No calcium was detected.

A thin-section produced across a pop-out cone showed,under polarizing microscope, the existence of a brownish toyellowish amalgam of indistinctive products with shrinkagecracks. These products were observed by SEM/EDS and it wasdeduced that they contain fragments of an aggregate particlemixed with alkali–silica gel (Fig. 7). The gel is composed ofsilicon with variable amounts of aluminum and minorquantities of potassium, sodium and calcium.

2 under SEM (CEMUP). Samples areAu-coated. (a) Amorphous(b) EDS at Z1 in (a); (c) spongy gel from the interior of a pop-out;

Fig. 7 – Photomicrographs of the thin-section crossing the cone of a pop-out in Structure 2. Thin-section C-coated. (a) Gelidentified by polarizing microscope, showing an amalgam of products (N//); (b) image of the thin-section under SEM (CEMUP);(c) EDS at Z2 in (b); (d) EDS at Z3 in (b).


Gel was also found as a rim around a quartz particle (Fig. 8).No important cracking in the surrounding cement paste wasobserved in connection with this particle. The EDS analysisidentified the presence of silicon, calcium, aluminum, potas-sium and sodium inside the particle (Z4), indicating that it isbeing affected by the concrete fluids. The surrounding gel hasa similar composition but with a variation in the relativecontents of the elements, in particular in the amount ofcalcium, which is considerably higher than that found in theproduct inside the quartz particle. This gel shows shrinkagecracks and contains a small amount of aluminum (Z3).

3.3.3. Structure 3Only one of the exudations sampled in this structure wasidentified as alkali–silica gel. This gel shows a smoothamorphous surface with some cracks (Fig. 9). The compositionis similar to that of the exudations in Structures 1 and 2. Nocalcium was detected.

In this structure, six concrete cores were drilled. Gel wasfound in three of the cores. One was collected close to the

location where the exudation was sampled and the otherswere related to the spalling and wide pop-outs. It was verifiedthat gel replaces fine aggregate particles and is alwaysassociated with needle shaped ettringite. Though the rest ofthe concrete cores show some ettringite, this is much moreabundant in the cores containing alkali–silica gel. In placeswhere alkali–silica gel was identified, the ettringite forms rimsat the interface with the cement paste. The colorless-to-brownish gel detected in the thin-sections has a compositionin which the calcium content is greater than that of potassiumand sodium (Fig. 10). There is also some aluminum.

Gel was also observed partially filling cracks in theconcrete. This gel is crystalline and exhibits a brownish colorwhen inside the aggregate particles, where it is moreabundant. The cracks start inside the quartz particles, inplaces where there are sub-grains, and then grow through thecement paste, sometimes crossing fine aggregate particles.SEM/EDS analyses showed that besides silicon, calcium,potassium and sodium, there are also traces of aluminumand magnesium (Fig. 11).

Fig. 8 – Photomicrograph of the gel present in a thin-section of the concrete of Structure 2. (a) Gel under polarizing microscope(N//); (b) image of the thin-section under SEM (CEMUP); (c) EDS at Z3 in (b), obtained at the exterior rim; (d) EDS at Z4 in (b),obtained in the interior of the reacted particle.


3.3.4. Structure 4In Structure 4, gel is much more abundant than in the firstthree structures, as expected, according to the characteristicsof the aggregates. Some of the quartz particles are reacted andpartly or wholly dissolved and replaced by gel, others showdarkened open interstices between grains. There are alsosome quartz particles with a dark border in contact with thecement paste. Cracks exist in different places: they start in theaggregate particles and extend into the cement paste, wherethey form a network of fine cracks. The origin of these cracksis, in most of the cases, related to microcrystalline quartz butthere are also cracks associated with large crystals of quartzshowing undulatory extinction and indented boundaries.There is also partial dissolution of coarse aggregate particles,in particular those composed of quartz grains with openinterstices filled by small mica crystals and clay minerals.

Gel was identified in different places, showing a brownishcoloration under parallel polarizers:

- Filling voids;- Partially filling the cracks;

- As rims around aggregate particles;- Replacing fine aggregate particles.

SEM/EDS analyses were carried out in order to assess thecomposition of the alkali–silica gel. The gel is sometimes veryporous, which is reflected in the content of carbon in some ofthe analyses. This is due to the glue used in the thin-sectionpreparation which is filling the voids. It was verified thatbesides silicon, calcium is always present in the compositionof gel as well as potassium and sodium. The content of theelements varies according to the location of gel (Fig. 12).Calcium exists at a low concentration in cracks inside thequartz particles (Z2) and increases when cracks extend to thecement paste (Z4), in this case containing also small amountsof aluminum and magnesium. The later composition issimilar to that found in rims around aggregate particles, inthe interface with cement paste.

In thin-section locations where a crack is present bridgingtwo coarse aggregate quartzitic particles, extending into thecement paste and surrounding a fine particle of aggregate,analyses were performed along the crack in two places: inside

Fig. 9 – Exudation from Structure 3. Sample Au-coated. (a) Gel with smooth surface (CEMUP); (b) EDS from area Z1.


the aggregate particle and at the interface between cementpaste and fine aggregate particle (Fig. 13). The analyses showthat calcium is very abundant in the latter location and has alow content inside the particle. This variation is accompaniedby a decrease in potassium and sodium contents from theaggregate particle to the crack in the cement paste. Gel in theinterface cement paste-aggregate also contains aluminum.

4. Discussion

Identification of the mineral composition of a rock is notsufficient to identify the aggregate as potentially reactive. Theclassification is conditioned by local and regional factorswhich have, in the geological history of the rock, modulatedthe structure and strained the minerals.

Although granites are classified in many countries as notpotentially reactive, there are indications of reactivity in someregions and countries [1].

Granitic rocks have been reported to have caused alkali-aggregate reactions both in concrete structures and inlaboratory experiments [5–11].

The granites described in an earlier section (see “Assess-ment of Aggregates”), exhibit distinct degrees of strain. Thegranite from Structure 3 shows stronger deformation thanthose in Structures 1 and 2. The quartz is present as elongated,orientated crystals, sub-grains, and sometimes as microcrys-talline quartz. Shayan [6] noted that themore highly deformedthe granite is, themore prone it is to AAR in comparison to theless deformed rock. This trend was confirmed in the presentwork. Manifestations of alkali–silica reaction are more fre-quent and serious for Structure 3. Gel is found replacing fineaggregate particles and filling cracks both inside aggregateparticles and across the cement paste.

Structure 4 is the most recent of the four structures but it isthe one inwhich aggregates show features of stronger potentialalkali reactivity. The structure also shows major effects of ASR.Gel is ubiquitous, although showingheterogeneous distribution

Fig. 10 – Photomicrograph of the alkali–silica gel replacing two particles of fine aggregate in Structure 3. Thin-section C-coated.(a) and (b) Gel surrounded by ettringite as a rim between the gel and the cement paste. Images obtained by polarizingmicroscope (N//); (c) image of the gel and ettringite at the bottom of image (a) obtained by SEM (CEMUP); (d) EDS at Z5 in (c).


Fig. 11 – Photomicrograph of a thin-section of the concrete in Structure 3. Thin-section C-coated. (a) Gel partially filling cracks inthe aggregate particle and extending to the cement paste, observed by polarizing microscope (N//). Cracks are in some placesparallel to each other and to the exposed surface of the concrete; (b) image of the same cracks by SEM (CEMUP); (c) detail of thewidest crack in (b); (d) EDS at Z1 in (c).


Fig. 12 –Gel in Structure 4. Thin-section C-coated. (a) Gel filling a crack crossing a quartz particle by SEM (CEMUP); (b) EDS fromZ2 in (a); (c) crack expanding to the cement paste bySEM; (d) EDS from Z4 in (c).

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Fig. 13 – Photomicrograph of a thin-section in Structure 4. Thin-section C-coated. (a) Gel in cracks bridging two coarse aggregate particles and extending to the cement paste,observed by polarizing microscope (N//); (b) image of the same place by SEM (CEMUP); (c) EDS from Z1 in (b), inside the aggregate particle; (d) EDS from Z2 in (b), at the interfacebetween cement paste and an aggregate particle.

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amongst the cores and within each core. Alkali–silica reactionsare associated not only with microcrystalline quartz, but alsowith quartz grains with moderate to strong undulatory extinc-tion [12]. Undulatory extinction isnot recommended toquantifythe degree of reactivity of a rock, but it can be indicative of thepresence of reactive forms of silica such as microcrystallinequartz [13,14].

Reaction also occurs in association with impure quartziticparticles exhibiting open interstices filled with small micacrystals and clay minerals. The mica and clay may haveenhanced the reaction [15] and consequently the dissolutionof particles in which they are present. Dissolution may havebeen aided by some of the alkalis and aluminum found in thecomposition of the particles.

The chemical composition of the alkali–silica gel has beenstudied by a number of researchers and it is recognised that itvaries widely. The results found in the literature are basedmainly in qualitative analyses carried out by SEM/EDS [16–24].As stated by Thaulow et al. [20], gel has high and varyingcontents of silica, lower and varying contents of calcium andlow and relatively constant contents of alkalis. The resultsobtained in this study agree with those of other authors: silicais dominant and calcium shows a wide range of variation.

Gel was found as exudations and inside the concrete and inparticular replacing aggregate particles and in cracks. Asstated in the literature [17,18,24], the composition of the gelvaries with its texture and the location in concrete structure.

Gel forming exudations is composed of silicon and alkalis,but calcium is not present, which was also found by Knudsenand Thaulow [16] and referred in [25]. The samples were in aviscous state when collected, which might indicate that theywere recent and not yet carbonated. The absence of calciumcan be attributed to a short transport within the concrete andonly brief contact with cement paste before extrusion. Asobserved in the spectra from SEM/EDS, potassium is moreabundant than sodium, usually by more than a factor of two.

The gel observed in thin-sections has a different composi-tion, showing a variable content of calcium and, sometimes,small amounts of aluminum, in agreementwith otherworkers[10,26,27]. Calcium content is more prevalent in cracks foundin the cement paste [17] than inside aggregates, developing areverse trend to that of silicon. In the cracks crossing thecement paste aluminum is also found; this element is notpresent inside the aggregate.

A difference in gel compositionwas to be expected betweenStructure 3 and Structure 4, which are of a very different age[17], different type of structure with different in-serviceconditions and have aggregates of different composition.However, the compositions found are similar, though indifferent locations inside the concrete. The similarity incomposition is probably due to the fact that granitic rocksdevelop delayed reactions (Structure 3) when compared toaggregates composed of strongly strained and microcrystal-line quartz (Structure 4).

Concerning the possible relationship between the exis-tence of calcium in the gel and its role in ASR expansion[17,21,22], it was verified that displacements in Structures 1and 3 were recorded with monitoring equipment and thatStructure 2 showed pop-outs; Structure 3 also showed cracksrepresentative of the expansion due to ASR. These features

were noted to occur in the three concrete dams even with lowcontents of calcium found in the gel's composition. It may beconcluded that the role of calcium in the expansivity of the gelis not yet totally understood.

The existence of ettringite, also referred in [17,18], in cracksand surrounding the gel that replaces aggregate particles inStructure 3, can be explained by the potential for crystalgrowth in open spaces developed from expansion due toalkali–silica reactions [28].

5. Conclusions

The results obtained in this study are in some aspects inagreement with existing data in the literature but in otheraspects they lead to different conclusions than those acceptedby most researchers. The petrographic examination of con-crete samples from four structures permitted the character-ization of aggregates as potentially reactive to alkalis. In threeof the structures granitic aggregates were used, previouslycharacterised as innocuous by laboratory tests [29].

All the structures show manifestations of deteriorationprobably resulting from alkali–silica reactions, with strongerintensity according to an increasing content of reactive silica.The alkali–silica gel formed was observed by light opticalmicroscope and characterized by SEM/EDS, to establishmorphology and qualitative composition.

In the samples studied, it was confirmed that thecomposition of gel varies according to its location in concrete.In the gel sampled from exudations at concrete surfaces nocalciumwas identified. Inside the concrete, calcium is presentin a wide range of (low level) concentrations.

Gel present in cracks shows usually a higher content ofcalcium when crossing the cement paste than inside anaggregate particle. It was verified that expansion occurs in allthe structures, even when a low or zero amount of calcium ispresent.

The composition of the gel does not vary directly withageing in the structures. The composition and potentialreactivity of the aggregates used in the concrete productionshould also be considered.

Acknowledgments

The present work was developed in the scope of the researchproject GRANMAT-POCTI/CTA/45936/2002 and the pluri-annual project GEOREMAT, both financed by Fundação para aCiência e a Tecnologia.

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composition of alkali–silica reaction products at different locations within concrete structures

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