examination of the concrete from an old portuguese dam: texture and composition of alkali–silica...

58 (2007) 1160–1170
Materials Characterization
Examination of the concrete from an old Portuguese dam:Texture and composition of alkali–silica gel

Isabel Fernandes a,⁎, Fernando Noronha a, Madalena Teles b

a Departamento de Geologia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugalb Departamento de Engenharia Civil, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

Received 16 December 2006; accepted 9 April 2007

Abstract

Exudations and pop-outs were identified in the interior galleries of a large dam built in the 1960s. The samples collected wereexamined by a Scanning Electron Microscope. A dense material with a smooth surface and drying shrinkage cracks or a spongytexture were observed in the samples. The semi-quantitative composition was obtained by energy dispersive spectrometry (EDS)and it was concluded that this material corresponds to alkali–silica gel, composed of SiO2–Na2O–K2O–CaO. A viscous whiteproduct in contact with an aggregate particle in a cone sampled from a pop-out was observed through use of the scanning electronmicroscope and it has characteristics similar to the gel present in the exudations and cavities. Reference is made to the potentialalkali reactivity of the aggregate present in the concrete. The texture and composition of the products probably resulting from analkali–silica reaction are presented, set out in ternary diagrams, and discussed.© 2007 Elsevier Inc. All rights reserved.

Keywords: Exudations; Pop-outs; Alkali–silica gel; Semi-quantitative composition

1. Introduction

The study presented is included in a project whichaims to evaluate the characteristics of granitic rocks asconcrete aggregates. In the scope of the project, petro-graphic analyses of the rocks from the quarries areattempted to evaluate the potential alkali reactivity andold concrete structures are inspected in order to detectmanifestations of deterioration due to alkali–aggregatereactions.

⁎ Corresponding author. Tel.: +351 220402456.E-mail addresses: [email protected] (I. Fernandes),

[email protected] (F. Noronha), [email protected] (M. Teles).

1044-5803/$ - see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2007.04.007

Alto Rabagão is a concrete dam built in the early1960s in an asymmetrical valley in Northern Portugal.Due to the width of the valley and the mechanical char-acteristics of the rock foundation, the dam is composed oftwo types of structures: an arch dam in the main valley,and two gravity dams. One of the gravity profiles wasbuilt in a secondary valley on the right bank, and theother closes the left bank of the main valley. There arealso artificial concrete abutments linking each gravitysection to the arch dam (Fig. 1).

The arch dam is 94 m high, the gravity profiles 60 mhigh and the total crest length is 1970 m. There are threeinternal horizontal galleries across the arch dam and adrainage gallery along the arch and gravity structures,close to the foundation.

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http://dx.doi.org/10.1016/j.matchar.2007.04.007

Fig. 1. (a) The Alto Rabagão dam. (b) Cross sections show internal galleries: the drainage galleries in the arch and gravity sections; three horizontalgalleries in the arch dam.

1161I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170

The aggregate applied in the manufacture of theconcrete was exploited in a quarry located close to thedam site. It is a medium to coarse two mica granite,showing a slight deformation. The texture and mineralcomposition of the rock were characterised by polaris-ing microscopy in order to detect potential alkali re-active forms of silica.

In a second phase, the dam was inspected in order toevaluate the preservation of the concrete. In placeswhere signs of deterioration were found, samples werecollected for characterisation in the laboratory. Samplesof concrete were also extracted by diamond drilling.

The petrographic examination was based mainlyon the application of stereomicroscopy, the polarising

microscope and the scanning electron microscope, toevaluate the deterioration of the concrete and also todetermine the morphology and composition of the pro-ducts resultant from alkali–aggregate reactions.

2. Analytical methods

The study of the Alto Rabagão dam took place be-tween 2001 and 2004. Samples of the granite werecollected from the quarry, which is still being worked,and thin sections were produced for the petrographiccharacterisation of the rock.

The interior galleries of the three sections of the damwere inspected to detect signs of deterioration in the

Fig. 2. Texture and composition of the gel present in a pop-out. UnderSEM (a) it is amorphous and smooth with characteristic shrinkagecracks; (b) presents the EDS analysis.

Fig. 3. (a) Morphology and (b) composition of the calcium carbonatecrystals being formed over the alkali–silica gel.

1162 I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170

structure, as they are ideal places for site investigation ofthe concrete.

Samples of products from the pop-outs, exudationsand efflorescence were collected and labelled. Theywere kept in plastic airtight containers in order to pre-serve them for examination and analysis.

In the laboratory, the samples were taken out ofthe containers, glued with araldite to metallic cylinders6 mm thick and sealed again in the airtight containers.Within a few days, the samples were sent to the scanningelectron microscopy laboratory. They were submittedto vacuum and sputtered with gold in JEOL JFC 1100

equipment, just before the examination. The examinationtook place in a scanning electron microscope (SEM)model JEOL JSM-6301F equipped with a NORAN-VOYAGER energy dispersive spectrometer (EDS) toobtain semi-quantitative standardless analyses. The ac-celerating voltage used was 15 kV with a working dis-tance of 15 mm.

In order to examine the concrete by petrographicmethods, places were selected in the galleries for drillingcores, mainly on the sites where signs of deteriorationwere detected. Cores with 300 to 900 mm length and90 mm diameter were extracted with a diamond drillingmachine, Milwaukee model 4094-5, chilled by watercirculation, with 230 W of power and a rotational

Fig. 4. (a) Texture and (b) semi-quantitative composition of the whiteproduct lining one of the cavities.


velocity of 450 cycles/min. According to the report [1],the pieces of concrete were labeled with a referencenumber and the sample orientations indicated withwaterproof ink. To avoid desiccation, cling-film waswrapped around the samples and they were sealed inpolythene bags. They were taken immediately to thelaboratory.

Places for thin sections were selected along the cores,especially where infilled air voids were detected.

Slices of the concrete were cut and glued with aralditeto a glass slide to produce thin sections. The sampleswere impregnated with resin by heating at Tb70 °C untildry, without the use of vacuum. The thin sections wereproduced totally by manual processes from the progres-sive grinding to the final polishing.

3. Results

The minerals that constitute the granite from thequarry – quartz, microcline, orthoclase, plagioclase, mus-covite, biotite, chlorite, sillimanite, apatite, sphene andzircon – are slightly oriented, especially the phyllosili-cates and the acicular crystals of sillimanite included inthe muscovite plates. Deformation is also revealed by theundulatory extinction of quartz and feldspar crystals. Insome thin sections of the granite, the rock exhibits acataclastic structure with microcrystalline quartz, sub-grains and sutured boundaries, resulting from recrystalli-sation of the silica. This structure is not very common andit represents a low percentage of the total samples an-alysed. Nevertheless, it can be indicative of the potentialalkali reactivity of the rock.

Tests for the determination of alkali reactivity werealso performed: the quick chemical method [2], the ac-celerated mortar-bar expansion test [3], and the accel-erated mortar-bar test by autoclave [4]. The results ofthe tests led to the characterisation of the rock as innoc-uous or non-potentially reactive.

The next phase consisted of the evaluation of thestate of preservation of the Alto Rabagão dam, a struc-ture where the granite was applied.

During the site investigation of the galleries differentsigns of deterioration were registered in different places.

It was verified that map cracking exists in a limitedarea of the wall of a gallery in the left abutment of thearch dam. The cracks are marked by a slight yellowishdiscoloration of the concrete. A core drilled from thiswall showed that the cracks are very thin and are presentonly in the surface of the concrete, with less than 1 mmdepth. The petrographic examination of the thin sectionsshows that there is no gel or micro-cracking related tothis superficial cracking.

Circular cavities identified as pop-outs were foundalong the gallery closer to the crest level of the arch dam,in the galleries of the artificial left abutment and also inthe gravity section on the left bank. The most frequentare about 30 mm diameter and there are two instanceswith more than 70 mm diameter.

In one of the cavities, a white product was found andsampled for examination under SEM and analysis byEDS. The product is composed mainly of silicon, cal-cium, sodium and potassium with some aluminium. It isamorphous and smooth in most of the sample area and itshows shrinkage cracks (Fig. 2). In some places,calcium carbonate crystals have formed over the gel(Fig. 3), probably resulting from carbonation.

Fig. 5. (a) Porous surface of fine nodules of gel from a pop-out. Over the gel (b), needle-shaped prismatic sodium-rich crystals (c) are forming.


A thin section of the concrete close to the cavitywas produced to characterise the gel. Under the polar-ising microscope the gel is yellowish in plane polarisedlight, isotropic in crossed polarised light, and it showsshrinkage cracks which extend to the cement pastesurrounding the cavity.

White to yellowish compact products lining otherpop-outs were studied and different textures and com-positions were found, although the composition is sim-ilar to the former:

– Massive gel with smooth, amorphous, cracked sur-faces (Fig. 4a). The composition of the gel ishomogeneous, with silicon, sodium and potassium.

– Gel-like material covering a semi-crystallineproduct. It is composed of silicon, potassium,sodium and aluminium, Fig. 4b.

– Skeletal, porous material, in very fine nodules(Fig. 5). The gel (Fig. 5b) is a precursor of theneedle shaped and prismatic sodium-rich crys-tals (Fig. 5c) that are being formed from the gel.These crystals correspond to trona, resulting fromthe carbonation of the gel when exposed to theenvironment.

– Rosette-like agglomerates of platy crystals(Fig. 6). These products are the only ones toshow a crystalline form. They have a morecomplex composition than the former, with

Fig. 6. (a) Rosette-like agglomerates of platy crystals; EDSanalysis (b) indicates these are composed essentially of siliconand calcium.

Fig. 7. Alkali–silica gel occurring at the top of the concrete cone froma pop-out, under stereo microscope: (a) white to yellowish solidproduct (0.6×); (b) viscous translucent product (1.6×).


silicon, calcium, potassium, aluminium, magne-sium and sodium, Fig. 6b. The platy crystals arecomposed essentially of silicon and calcium.

A concrete cone was detached from the ceiling of agallery in the left artificial abutment. The top of thiscone showed a translucent viscous product (Fig. 7). Partof this material, Fig. 8a, was prepared for analysisby SEM/EDS. It is composed of silicon, potassiumand sodium, Fig. 8b, and exhibits a cavernous surface(Fig. 8a).

Efflorescence exists on enlarged concrete jointsin the artificial abutments galleries, in places where

the walls show signs of humidity. They form longneedle-shaped filaments of white and translucentmaterial. Under SEM/EDS this product shows clustersof globular and needle shaped sodium-rich crystals.The composition and texture of the crystals indicatethey correspond to trona, as was confirmed by X-rayanalysis.

Exudations are not frequent. They were observed inthe drainage gallery of the gravity dam built on thesecondary valley. They are white, solid and hard, some-times with a saccharoid character. In some places,besides the white product, there are translucent dropletsviscous in appearance. These products, assumed to bealkali–silica gel, were analysed by SEM/EDS. They arecomposed mainly of silicon, potassium, sodium andsome aluminium. In some samples calcium was alsodetected.

Fig. 8. (a) Cavernous gel occurring at the top of the concrete cone froma pop-out, as shown in Fig. 7. The EDS analysis is presented in (b).

Fig. 9. Amorphous gel from an exudation showing (a) a smoothsurface with shrinkage cracks; (b) presents the EDS analysis.


The morphology of the gel is identical to that foundin some of the pop-outs and to that referred to by otherworkers [5–7], showing:

– Smooth surface of amorphous alkali–silica gel(Fig. 9) with silicon, potassium, sodium and a lowcontent of aluminium;

– Gel-like material composed of silicon, calcium,aluminium and potassium (Fig. 10) coveringsemi-crystalline calcium carbonate;

– Spongy, cavernous material (Fig. 11) composed ofsilicon, calcium, aluminium, potassiumand sodium.

A wide range of chemical composition of the gel isfound in literature relating to alkali–silica reactions,

varying especially in the concentration of calcium[8–13]. The products found in exudations and pop-outsof the Alto Rabagão dam, assumed to correspond toalkali–silica gel, are composed mainly of silicon (85.7 to90.6%) with a low concentration of potassium (5.3 to7.4%) and sodium (1.9 to 4.3%). The potassium contentis always higher than sodium. In some samples calcium(0.2 to 4.7%) and aluminium (0.2 to 6.1%) were alsofound.

The results of SEM/EDS analyses were recalculatedon the basis of a fixed sum of the oxides SiO2–K2O–Na2O–CaO concentrations as 100%, excluding traceelements and water (Fig. 12). Most of the samples which

Fig. 10. (a) Gel-like material covering (b) partially crystalline calciumcarbonate.

Fig. 11. (a) Spongy, porous material from the exudations. Crystals aresimilar in composition (b) to the underlying spongy material.


contain calcium also have some aluminium; these werenot projected in these diagrams.

The concrete cores extracted from different places inthe galleries showed a compact and homogeneous con-crete, with no cracks. The coarser particles of aggregatereach 200 mm in the larger dimension. Polished thinsections of the concrete were produced from the cores.The aggregate particles are angular to sub-rounded withlow sphericity [14,15] due to the crushing of the graniteto produce coarse and fine aggregates. There are rarethin micro-cracks at aggregate–cement interfaces. Noreaction rims were detected in the aggregates or incracks in the aggregates; the cement paste has no in-filling material. Some of the air voids, especially those

which showed macroscopically a white lining product,are partially or totally filled by ettringite as needle-shaped crystals (Fig. 13a), observed also in small piecesof concrete cut from the cores. The composition of theettringite was confirmed by SEM/EDS (Fig. 13b). Insome spectra, in addition to the aluminium, calcium andsulphur, silicon and alkalis were also detected in thecomposition. No gel was identified under the micro-scope in the voids or cracks.

4. Discussion

The rock from the quarry showed some character-istics that might indicate potential alkali reactivity. The

Fig. 12. Ternary diagrams of the composition of the gel identified in pop-outs and exudations.


deformation is revealed mainly in the quartz crystals andin the orientation of phyllosilicate minerals andsillimanite crystals. The quartz crystals show undulatoryextinction, which is not recommended to quantify thedegree of reactivity of the rock, and also limited portionsof microcrystalline quartz [16–19].

The petrographic examination of the thin sections wasthe only method that suggested potential reactivity of thegranite. All the reactivity laboratory tests performedcharacterised this rock as non-reactive. Granites areclassified in many countries as not potentially reactive,but there are indications of reactivity in some regions/countries like Argentina, Australia, Belgium, Canada,Hong Kong, India, Italy, Romania, South Africa andUnited Kingdom [20]. In Portugal the studies based onlaboratory tests developed on some granites gave non-potential reactivity [21]. Nevertheless, the petrographicexamination of exudation products sampled from an oldPortuguese dam built with granitic aggregates, classifiedas non-reactive, has shown the existence of superficialalkali–silica gel [22]. Therefore, this type of rock mayhave particular characteristics related to regional featuresthat can make it potentially reactive.

In the present case, evaluation of the performanceof this granite as aggregate could be assessed becausethe rock was applied in the concrete of a large dam builtfour decades ago.

According to references [1,23], the signs of deteri-oration detected on the walls of the interior galleries ofthe Alto Rabagão dam may correspond to the occur-rence of alkali–silica reactions in the concrete.

Although no gel or micro-cracking were identified inthin sections from the concrete cores extracted from theinterior galleries of the structure, products composed ofsilicon and alkalis, and in some samples with calciumand aluminium, were identified in exudations and pop-

outs. The ternary phase diagrams show that silicon ispredominant in all the products analysed and there arealso low concentrations of potassium and sodium. Thecalcium content is low in all the analyses and it occursmainly in samples which also contain aluminium.

The relationship between the gel composition andexpansibility has been discussed by many workers[10–13]. The results from the study of the Alto Rabagãodam lead to the conclusion that the gel lining the pop-outsmust be expansive, though it has a low content of cal-cium; otherwise, the concrete cones would not detach andfall from the walls and ceilings of the galleries.

The crystals composed of sodium present on thesurface of the gel might result from the carbonation ofthe alkali–silica gel when exposed to the atmosphere.The existence of calcium-bearing or sodium-bearingcrystals depends on the composition of the gel withwhich they are associated.

5. Conclusions

The petrographic characterisation of the graniteapplied as concrete aggregate in the Alto Rabagão damrevealed signs of deformation in the rock, shown inparticular by the presence of microcrystalline quartz.

In the site investigation of the interior galleries of thedam different manifestations of alkali–silica reactionswere detected. The use of SEM/EDS allowed us toassume that the products found in exudations and pop-outs correspond to alkali–silica gel with a variablemorphology and composition. Silicon is the main com-ponent of the gel, with low contents of potassium andsodium. In a few samples calcium and aluminium werealso detected, though the morphology of the gel issimilar to that with SiO2–K2O–Na2O. The absence orvery low content of calcium in the exudations is in

Fig. 13. (a) Morphology and (b) composition of ettringite needle-shaped crystals filling an air void. The needles show an erraticorientation and are detached from the void wall due to shrinkage.


agreement with some of the literature about the com-position of the gel in the surface of concrete. The ab-sence of gel in the interior of the concrete, as shown bythe examination of thin sections of concrete cores mightindicate that alkali–silica reactions are superficial in thisstructure. The existence of pop-outs shows that the gelmight be expansive thus causing the fall of the concretecones.

Acknowledgements

We are particularly indebted to EDP — CPPE,Direcção de Produção Hidráulica for allowing access to

the Alto Rabagão dam, for the facilities during thefieldwork and also for permission to publish these results.The authors are grateful to reviewers for the constructiveand interesting comments.

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examination of the concrete from an old portuguese dam: texture and composition of alkali–silica...

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