- Email: [email protected]
Examination of the concrete from an old Portuguese dam: Texture and composition of alkali–silica gel
Materials Characterization 58 (2007) 1160 – 1170
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 b
a Departamento de Geologia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal 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 were examined by a Scanning Electron Microscope. A dense material with a smooth surface and drying shrinkage cracks or a spongy texture 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 white product in contact with an aggregate particle in a cone sampled from a pop-out was observed through use of the scanning electron microscope and it has characteristics similar to the gel present in the exudations and cavities. Reference is made to the potential alkali reactivity of the aggregate present in the concrete. The texture and composition of the products probably resulting from an alkali–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 which aims to evaluate the characteristics of granitic rocks as concrete aggregates. In the scope of the project, petrographic analyses of the rocks from the quarries are attempted to evaluate the potential alkali reactivity and old concrete structures are inspected in order to detect manifestations of deterioration due to alkali–aggregate reactions.
⁎ 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 early 1960s in an asymmetrical valley in Northern Portugal. Due to the width of the valley and the mechanical characteristics of the rock foundation, the dam is composed of two types of structures: an arch dam in the main valley, and two gravity dams. One of the gravity profiles was built in a secondary valley on the right bank, and the other closes the left bank of the main valley. There are also artificial concrete abutments linking each gravity section to the arch dam (Fig. 1). The arch dam is 94 m high, the gravity profiles 60 m high and the total crest length is 1970 m. There are three internal horizontal galleries across the arch dam and a drainage gallery along the arch and gravity structures, close to the foundation.
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1161
Fig. 1. (a) The Alto Rabagão dam. (b) Cross sections show internal galleries: the drainage galleries in the arch and gravity sections; three horizontal galleries in the arch dam.
The aggregate applied in the manufacture of the concrete was exploited in a quarry located close to the dam site. It is a medium to coarse two mica granite, showing a slight deformation. The texture and mineral composition of the rock were characterised by polarising microscopy in order to detect potential alkali reactive forms of silica. In a second phase, the dam was inspected in order to evaluate the preservation of the concrete. In places where signs of deterioration were found, samples were collected for characterisation in the laboratory. Samples of concrete were also extracted by diamond drilling. The petrographic examination was based mainly on the application of stereomicroscopy, the polarising
microscope and the scanning electron microscope, to evaluate the deterioration of the concrete and also to determine the morphology and composition of the products resultant from alkali–aggregate reactions. 2. Analytical methods The study of the Alto Rabagão dam took place between 2001 and 2004. Samples of the granite were collected from the quarry, which is still being worked, and thin sections were produced for the petrographic characterisation of the rock. The interior galleries of the three sections of the dam were inspected to detect signs of deterioration in the
1162
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
equipment, just before the examination. The examination took place in a scanning electron microscope (SEM) model JEOL JSM-6301F equipped with a NORANVOYAGER energy dispersive spectrometer (EDS) to obtain semi-quantitative standardless analyses. The accelerating voltage used was 15 kV with a working distance of 15 mm. In order to examine the concrete by petrographic methods, places were selected in the galleries for drilling cores, mainly on the sites where signs of deterioration were detected. Cores with 300 to 900 mm length and 90 mm diameter were extracted with a diamond drilling machine, Milwaukee model 4094-5, chilled by water circulation, with 230 W of power and a rotational
Fig. 2. Texture and composition of the gel present in a pop-out. Under SEM (a) it is amorphous and smooth with characteristic shrinkage cracks; (b) presents the EDS analysis.
structure, as they are ideal places for site investigation of the concrete. Samples of products from the pop-outs, exudations and efflorescence were collected and labelled. They were kept in plastic airtight containers in order to preserve them for examination and analysis. In the laboratory, the samples were taken out of the containers, glued with araldite to metallic cylinders 6 mm thick and sealed again in the airtight containers. Within a few days, the samples were sent to the scanning electron microscopy laboratory. They were submitted to vacuum and sputtered with gold in JEOL JFC 1100
Fig. 3. (a) Morphology and (b) composition of the calcium carbonate crystals being formed over the alkali–silica gel.
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
velocity of 450 cycles/min. According to the report [1], the pieces of concrete were labeled with a reference number and the sample orientations indicated with waterproof ink. To avoid desiccation, cling-film was wrapped around the samples and they were sealed in polythene bags. They were taken immediately to the laboratory. 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 araldite to a glass slide to produce thin sections. The samples were impregnated with resin by heating at T b 70 °C until dry, without the use of vacuum. The thin sections were produced totally by manual processes from the progressive grinding to the final polishing.
1163
Circular cavities identified as pop-outs were found along the gallery closer to the crest level of the arch dam, in the galleries of the artificial left abutment and also in the gravity section on the left bank. The most frequent are about 30 mm diameter and there are two instances with more than 70 mm diameter. In one of the cavities, a white product was found and sampled for examination under SEM and analysis by EDS. The product is composed mainly of silicon, calcium, sodium and potassium with some aluminium. It is amorphous and smooth in most of the sample area and it shows shrinkage cracks (Fig. 2). In some places, calcium carbonate crystals have formed over the gel (Fig. 3), probably resulting from carbonation.
3. Results The minerals that constitute the granite from the quarry – quartz, microcline, orthoclase, plagioclase, muscovite, biotite, chlorite, sillimanite, apatite, sphene and zircon – are slightly oriented, especially the phyllosilicates and the acicular crystals of sillimanite included in the muscovite plates. Deformation is also revealed by the undulatory extinction of quartz and feldspar crystals. In some thin sections of the granite, the rock exhibits a cataclastic structure with microcrystalline quartz, subgrains and sutured boundaries, resulting from recrystallisation of the silica. This structure is not very common and it represents a low percentage of the total samples analysed. Nevertheless, it can be indicative of the potential alkali reactivity of the rock. Tests for the determination of alkali reactivity were also performed: the quick chemical method [2], the accelerated mortar-bar expansion test [3], and the accelerated mortar-bar test by autoclave [4]. The results of the tests led to the characterisation of the rock as innocuous or non-potentially reactive. The next phase consisted of the evaluation of the state of preservation of the Alto Rabagão dam, a structure where the granite was applied. During the site investigation of the galleries different signs of deterioration were registered in different places. It was verified that map cracking exists in a limited area of the wall of a gallery in the left abutment of the arch dam. The cracks are marked by a slight yellowish discoloration of the concrete. A core drilled from this wall showed that the cracks are very thin and are present only in the surface of the concrete, with less than 1 mm depth. The petrographic examination of the thin sections shows that there is no gel or micro-cracking related to this superficial cracking.
Fig. 4. (a) Texture and (b) semi-quantitative composition of the white product lining one of the cavities.
1164
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
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 cavity was produced to characterise the gel. Under the polarising microscope the gel is yellowish in plane polarised light, isotropic in crossed polarised light, and it shows shrinkage cracks which extend to the cement paste surrounding the cavity. White to yellowish compact products lining other pop-outs were studied and different textures and compositions were found, although the composition is similar to the former: – Massive gel with smooth, amorphous, cracked surfaces (Fig. 4a). The composition of the gel is homogeneous, with silicon, sodium and potassium.
– Gel-like material covering a semi-crystalline product. 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 the needle shaped and prismatic sodium-rich crystals (Fig. 5c) that are being formed from the gel. These crystals correspond to trona, resulting from the carbonation of the gel when exposed to the environment. – Rosette-like agglomerates of platy crystals (Fig. 6). These products are the only ones to show a crystalline form. They have a more complex composition than the former, with
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1165
the walls show signs of humidity. They form long needle-shaped filaments of white and translucent material. Under SEM/EDS this product shows clusters of globular and needle shaped sodium-rich crystals. The composition and texture of the crystals indicate they correspond to trona, as was confirmed by X-ray analysis. Exudations are not frequent. They were observed in the drainage gallery of the gravity dam built on the secondary valley. They are white, solid and hard, sometimes with a saccharoid character. In some places, besides the white product, there are translucent droplets viscous in appearance. These products, assumed to be alkali–silica gel, were analysed by SEM/EDS. They are composed mainly of silicon, potassium, sodium and some aluminium. In some samples calcium was also detected.
Fig. 6. (a) Rosette-like agglomerates of platy crystals; EDS analysis (b) indicates these are composed essentially of silicon and calcium.
silicon, calcium, potassium, aluminium, magnesium and sodium, Fig. 6b. The platy crystals are composed essentially of silicon and calcium. A concrete cone was detached from the ceiling of a gallery in the left artificial abutment. The top of this cone showed a translucent viscous product (Fig. 7). Part of this material, Fig. 8a, was prepared for analysis by SEM/EDS. It is composed of silicon, potassium and sodium, Fig. 8b, and exhibits a cavernous surface (Fig. 8a). Efflorescence exists on enlarged concrete joints in the artificial abutments galleries, in places where
Fig. 7. Alkali–silica gel occurring at the top of the concrete cone from a pop-out, under stereo microscope: (a) white to yellowish solid product (0.6×); (b) viscous translucent product (1.6×).
1166
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
varying especially in the concentration of calcium [8–13]. The products found in exudations and pop-outs of the Alto Rabagão dam, assumed to correspond to alkali–silica gel, are composed mainly of silicon (85.7 to 90.6%) with a low concentration of potassium (5.3 to 7.4%) and sodium (1.9 to 4.3%). The potassium content is always higher than sodium. In some samples calcium (0.2 to 4.7%) and aluminium (0.2 to 6.1%) were also found. The results of SEM/EDS analyses were recalculated on the basis of a fixed sum of the oxides SiO2–K2O– Na2O–CaO concentrations as 100%, excluding trace elements and water (Fig. 12). Most of the samples which
Fig. 8. (a) Cavernous gel occurring at the top of the concrete cone from a pop-out, as shown in Fig. 7. The EDS analysis is presented in (b).
The morphology of the gel is identical to that found in some of the pop-outs and to that referred to by other workers [5–7], showing: – Smooth surface of amorphous alkali–silica gel (Fig. 9) with silicon, potassium, sodium and a low content of aluminium; – Gel-like material composed of silicon, calcium, aluminium and potassium (Fig. 10) covering semi-crystalline calcium carbonate; – Spongy, cavernous material (Fig. 11) composed of silicon, calcium, aluminium, potassium and sodium. A wide range of chemical composition of the gel is found in literature relating to alkali–silica reactions,
Fig. 9. Amorphous gel from an exudation showing (a) a smooth surface with shrinkage cracks; (b) presents the EDS analysis.
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1167
which showed macroscopically a white lining product, are partially or totally filled by ettringite as needleshaped crystals (Fig. 13a), observed also in small pieces of concrete cut from the cores. The composition of the ettringite was confirmed by SEM/EDS (Fig. 13b). In some spectra, in addition to the aluminium, calcium and sulphur, silicon and alkalis were also detected in the composition. No gel was identified under the microscope in the voids or cracks. 4. Discussion The rock from the quarry showed some characteristics that might indicate potential alkali reactivity. The
Fig. 10. (a) Gel-like material covering (b) partially crystalline calcium carbonate.
contain calcium also have some aluminium; these were not projected in these diagrams. The concrete cores extracted from different places in the galleries showed a compact and homogeneous concrete, with no cracks. The coarser particles of aggregate reach 200 mm in the larger dimension. Polished thin sections of the concrete were produced from the cores. The aggregate particles are angular to sub-rounded with low sphericity [14,15] due to the crushing of the granite to produce coarse and fine aggregates. There are rare thin micro-cracks at aggregate–cement interfaces. No reaction rims were detected in the aggregates or in cracks in the aggregates; the cement paste has no infilling material. Some of the air voids, especially those
Fig. 11. (a) Spongy, porous material from the exudations. Crystals are similar in composition (b) to the underlying spongy material.
1168
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
Fig. 12. Ternary diagrams of the composition of the gel identified in pop-outs and exudations.
deformation is revealed mainly in the quartz crystals and in the orientation of phyllosilicate minerals and sillimanite crystals. The quartz crystals show undulatory extinction, which is not recommended to quantify the degree of reactivity of the rock, and also limited portions of microcrystalline quartz [16–19]. The petrographic examination of the thin sections was the only method that suggested potential reactivity of the granite. All the reactivity laboratory tests performed characterised this rock as non-reactive. Granites are classified 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 and United Kingdom [20]. In Portugal the studies based on laboratory tests developed on some granites gave nonpotential reactivity [21]. Nevertheless, the petrographic examination of exudation products sampled from an old Portuguese dam built with granitic aggregates, classified as non-reactive, has shown the existence of superficial alkali–silica gel [22]. Therefore, this type of rock may have particular characteristics related to regional features that can make it potentially reactive. In the present case, evaluation of the performance of this granite as aggregate could be assessed because the rock was applied in the concrete of a large dam built four decades ago. According to references [1,23], the signs of deterioration detected on the walls of the interior galleries of the Alto Rabagão dam may correspond to the occurrence of alkali–silica reactions in the concrete. Although no gel or micro-cracking were identified in thin sections from the concrete cores extracted from the interior galleries of the structure, products composed of silicon and alkalis, and in some samples with calcium and aluminium, were identified in exudations and pop-
outs. The ternary phase diagrams show that silicon is predominant in all the products analysed and there are also low concentrations of potassium and sodium. The calcium content is low in all the analyses and it occurs mainly in samples which also contain aluminium. The relationship between the gel composition and expansibility has been discussed by many workers [10–13]. The results from the study of the Alto Rabagão dam lead to the conclusion that the gel lining the pop-outs must be expansive, though it has a low content of calcium; otherwise, the concrete cones would not detach and fall from the walls and ceilings of the galleries. The crystals composed of sodium present on the surface of the gel might result from the carbonation of the alkali–silica gel when exposed to the atmosphere. The existence of calcium-bearing or sodium-bearing crystals depends on the composition of the gel with which they are associated. 5. Conclusions The petrographic characterisation of the granite applied as concrete aggregate in the Alto Rabagão dam revealed signs of deformation in the rock, shown in particular by the presence of microcrystalline quartz. In the site investigation of the interior galleries of the dam different manifestations of alkali–silica reactions were detected. The use of SEM/EDS allowed us to assume that the products found in exudations and popouts correspond to alkali–silica gel with a variable morphology and composition. Silicon is the main component of the gel, with low contents of potassium and sodium. In a few samples calcium and aluminium were also detected, though the morphology of the gel is similar to that with SiO2–K2O–Na2O. The absence or very low content of calcium in the exudations is in
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1169
the Alto Rabagão dam, for the facilities during the fieldwork and also for permission to publish these results. The authors are grateful to reviewers for the constructive and interesting comments. References
Fig. 13. (a) Morphology and (b) composition of ettringite needleshaped crystals filling an air void. The needles show an erratic orientation and are detached from the void wall due to shrinkage.
agreement with some of the literature about the composition of the gel in the surface of concrete. The absence of gel in the interior of the concrete, as shown by the examination of thin sections of concrete cores might indicate that alkali–silica reactions are superficial in this structure. The existence of pop-outs shows that the gel might be expansive thus causing the fall of the concrete cones. Acknowledgements We are particularly indebted to EDP — CPPE, Direcção de Produção Hidráulica for allowing access to
[1] British Cement Association. The diagnosis of alkali–silica reaction, Report of a working party. British Cement Association Publication; 1992. 45.042. [2] ASTM C 289–94. Standard test method for potential alkali — silica reactivity of aggregates (Chemical Method). Philadelphia: The American Society for Testing Materials; 2002. [3] ASTM, C 1260-94. Standard test method for potential alkali– aggregates (Mortar-Bar Method). Philadelphia: The American Society for Testing Materials; 2002. [4] Normalisation Française, P 18−590. Granulats — Stabilité dimensionelle en milieu alcalin. Essai accéléré sur mortier par autoclave. Paris: AFNOR; 1993. [5] Stievenard-Gireaud D. Etude de la réaction alcalis–silice dans des bétons. Laboratoire Central des Ponts et Chaussées, Rapport de recherche LPC, vol. 144. Paris: LCPC; 1987. [6] Shayan A, Quick GW. Microstructure and composition of AAR products in conventional standard and new accelerated testing. Proc 8th Int Conf on Alkali–Aggregate Reaction in Concrete, Kyoto, data; 1989. [7] Larive C. Les réactions de dégradations internes du béton — Où, quand, comment, pourquoi? Rapport des Laboratoire Central des Ponts et Chaussées, OA-6, Paris; 1990. [8] Knudsen T, Thaulow N. Quantitative microanalyses of alkali– silica gel in concrete. Cem Concr Res 1975;5:443–54. [9] Helmuth RA, Stark D. Alkali silica reactivity mechanisms. In: Skalny J, editor. Materials Science of Concrete, vol. III. Westerville, OH: American Ceramic Society; 1992. p. 131–208. [10] Thaulow N, Jakobsen UH, Clark B. Composition of alkali silica gel and ettringite in concrete railroad ties: SEM-EDX and X-ray diffraction analyses. Cem Concr Res 1996;26(2): 309–18. [11] Kawamura M, Arano N, Terashima T. Composition of ASR gels and expansion of mortars. In: Cohen M, Mindess S, Skalny J, editors. Materials Science of Concrete: Special Volume — The Sidney Diamond Symposium. Westerville, OH: American Ceramic Society; 1998. p. 261–76. [12] Thomas MDA. The role of calcium in alkali–silica reaction. In: Cohen M, Mindess S, Skalny J, editors. Materials Science of Concrete — The Sidney Diamond Symposium. Westerville, OH: American Ceramic Society; 1998. p. 325–37. [13] Diamond S. Chemistry and other characteristics of ASR gels. Proc 11th Int Conf on Alkali–Aggregate Reaction. Québec, June 11–16; 2000. [14] Krumbein WC, Sloss LL. Stratigraphy and sedimentation. 2nd ed. San Francisco: WH Freeman & Co; 1963. [15] Powers MC. A new roundness scale for sedimentary particles. J Sediment Petrol 1953;23(2):17–119. [16] Grattan-Bellew PE. Microcrystalline quartz, undulatory extinction and the alkali–silica reaction. Proc 9th Int Conf on Alkali– Aggregate Reaction in Concrete, London, July; 1992. [17] Wigum BJ, Hagelia P, Haugen M, Broekmans MATM. Alkali aggregate reactivity of Norwegian aggregates assessed by quantitative petrography. Proc 11th Int Conf on Alkali–Aggregate Reaction. Québec, June 11–16; 2000.
1170
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
[18] Broekmans MATM. The alkali–silica reaction: mineralogical and geochemical aspects of some Dutch concretes and Norwegian mylonites, Ph.D. Thesis, University of Utrecht, Utrecht, The Netherlands; 2002. [19] RILEM Recommended Test Method AAR-1. Detection of potential alkali–reactivity of aggregates — Petrographic method, TC 191-ARP: Alkali–reactivity and prevention — Assessment, specification and diagnosis of alkali-reactivity, prepared by I. Sims, P. Nixon. Mater Constr 2003;36:472–9. [20] RILEM. RILEM petrographic method, New edited draft. RILEM/TC-ARP/01/03a; 2001.
[21] Silva AS, Reis MOB. Avaliação da reactividade aos álcalis dos agregados para betão. Porto: Encontro Nacional Betão Estrutural; 2000. [22] Fernandes I, Noronha F, Teles M. Microscopic analysis of alkali– aggregate reaction products in a 50-year-old concrete. Mater Charact 2004;53(2-4):295–306. [23] ACI. State-of-the-art report on alkali–aggregate reactivity. Reported by ACI Committee, vol. 221. American Concrete Institute; 1998.
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 b
a Departamento de Geologia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal 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 were examined by a Scanning Electron Microscope. A dense material with a smooth surface and drying shrinkage cracks or a spongy texture 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 white product in contact with an aggregate particle in a cone sampled from a pop-out was observed through use of the scanning electron microscope and it has characteristics similar to the gel present in the exudations and cavities. Reference is made to the potential alkali reactivity of the aggregate present in the concrete. The texture and composition of the products probably resulting from an alkali–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 which aims to evaluate the characteristics of granitic rocks as concrete aggregates. In the scope of the project, petrographic analyses of the rocks from the quarries are attempted to evaluate the potential alkali reactivity and old concrete structures are inspected in order to detect manifestations of deterioration due to alkali–aggregate reactions.
⁎ 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 early 1960s in an asymmetrical valley in Northern Portugal. Due to the width of the valley and the mechanical characteristics of the rock foundation, the dam is composed of two types of structures: an arch dam in the main valley, and two gravity dams. One of the gravity profiles was built in a secondary valley on the right bank, and the other closes the left bank of the main valley. There are also artificial concrete abutments linking each gravity section to the arch dam (Fig. 1). The arch dam is 94 m high, the gravity profiles 60 m high and the total crest length is 1970 m. There are three internal horizontal galleries across the arch dam and a drainage gallery along the arch and gravity structures, close to the foundation.
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1161
Fig. 1. (a) The Alto Rabagão dam. (b) Cross sections show internal galleries: the drainage galleries in the arch and gravity sections; three horizontal galleries in the arch dam.
The aggregate applied in the manufacture of the concrete was exploited in a quarry located close to the dam site. It is a medium to coarse two mica granite, showing a slight deformation. The texture and mineral composition of the rock were characterised by polarising microscopy in order to detect potential alkali reactive forms of silica. In a second phase, the dam was inspected in order to evaluate the preservation of the concrete. In places where signs of deterioration were found, samples were collected for characterisation in the laboratory. Samples of concrete were also extracted by diamond drilling. The petrographic examination was based mainly on the application of stereomicroscopy, the polarising
microscope and the scanning electron microscope, to evaluate the deterioration of the concrete and also to determine the morphology and composition of the products resultant from alkali–aggregate reactions. 2. Analytical methods The study of the Alto Rabagão dam took place between 2001 and 2004. Samples of the granite were collected from the quarry, which is still being worked, and thin sections were produced for the petrographic characterisation of the rock. The interior galleries of the three sections of the dam were inspected to detect signs of deterioration in the
1162
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
equipment, just before the examination. The examination took place in a scanning electron microscope (SEM) model JEOL JSM-6301F equipped with a NORANVOYAGER energy dispersive spectrometer (EDS) to obtain semi-quantitative standardless analyses. The accelerating voltage used was 15 kV with a working distance of 15 mm. In order to examine the concrete by petrographic methods, places were selected in the galleries for drilling cores, mainly on the sites where signs of deterioration were detected. Cores with 300 to 900 mm length and 90 mm diameter were extracted with a diamond drilling machine, Milwaukee model 4094-5, chilled by water circulation, with 230 W of power and a rotational
Fig. 2. Texture and composition of the gel present in a pop-out. Under SEM (a) it is amorphous and smooth with characteristic shrinkage cracks; (b) presents the EDS analysis.
structure, as they are ideal places for site investigation of the concrete. Samples of products from the pop-outs, exudations and efflorescence were collected and labelled. They were kept in plastic airtight containers in order to preserve them for examination and analysis. In the laboratory, the samples were taken out of the containers, glued with araldite to metallic cylinders 6 mm thick and sealed again in the airtight containers. Within a few days, the samples were sent to the scanning electron microscopy laboratory. They were submitted to vacuum and sputtered with gold in JEOL JFC 1100
Fig. 3. (a) Morphology and (b) composition of the calcium carbonate crystals being formed over the alkali–silica gel.
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
velocity of 450 cycles/min. According to the report [1], the pieces of concrete were labeled with a reference number and the sample orientations indicated with waterproof ink. To avoid desiccation, cling-film was wrapped around the samples and they were sealed in polythene bags. They were taken immediately to the laboratory. 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 araldite to a glass slide to produce thin sections. The samples were impregnated with resin by heating at T b 70 °C until dry, without the use of vacuum. The thin sections were produced totally by manual processes from the progressive grinding to the final polishing.
1163
Circular cavities identified as pop-outs were found along the gallery closer to the crest level of the arch dam, in the galleries of the artificial left abutment and also in the gravity section on the left bank. The most frequent are about 30 mm diameter and there are two instances with more than 70 mm diameter. In one of the cavities, a white product was found and sampled for examination under SEM and analysis by EDS. The product is composed mainly of silicon, calcium, sodium and potassium with some aluminium. It is amorphous and smooth in most of the sample area and it shows shrinkage cracks (Fig. 2). In some places, calcium carbonate crystals have formed over the gel (Fig. 3), probably resulting from carbonation.
3. Results The minerals that constitute the granite from the quarry – quartz, microcline, orthoclase, plagioclase, muscovite, biotite, chlorite, sillimanite, apatite, sphene and zircon – are slightly oriented, especially the phyllosilicates and the acicular crystals of sillimanite included in the muscovite plates. Deformation is also revealed by the undulatory extinction of quartz and feldspar crystals. In some thin sections of the granite, the rock exhibits a cataclastic structure with microcrystalline quartz, subgrains and sutured boundaries, resulting from recrystallisation of the silica. This structure is not very common and it represents a low percentage of the total samples analysed. Nevertheless, it can be indicative of the potential alkali reactivity of the rock. Tests for the determination of alkali reactivity were also performed: the quick chemical method [2], the accelerated mortar-bar expansion test [3], and the accelerated mortar-bar test by autoclave [4]. The results of the tests led to the characterisation of the rock as innocuous or non-potentially reactive. The next phase consisted of the evaluation of the state of preservation of the Alto Rabagão dam, a structure where the granite was applied. During the site investigation of the galleries different signs of deterioration were registered in different places. It was verified that map cracking exists in a limited area of the wall of a gallery in the left abutment of the arch dam. The cracks are marked by a slight yellowish discoloration of the concrete. A core drilled from this wall showed that the cracks are very thin and are present only in the surface of the concrete, with less than 1 mm depth. The petrographic examination of the thin sections shows that there is no gel or micro-cracking related to this superficial cracking.
Fig. 4. (a) Texture and (b) semi-quantitative composition of the white product lining one of the cavities.
1164
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
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 cavity was produced to characterise the gel. Under the polarising microscope the gel is yellowish in plane polarised light, isotropic in crossed polarised light, and it shows shrinkage cracks which extend to the cement paste surrounding the cavity. White to yellowish compact products lining other pop-outs were studied and different textures and compositions were found, although the composition is similar to the former: – Massive gel with smooth, amorphous, cracked surfaces (Fig. 4a). The composition of the gel is homogeneous, with silicon, sodium and potassium.
– Gel-like material covering a semi-crystalline product. 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 the needle shaped and prismatic sodium-rich crystals (Fig. 5c) that are being formed from the gel. These crystals correspond to trona, resulting from the carbonation of the gel when exposed to the environment. – Rosette-like agglomerates of platy crystals (Fig. 6). These products are the only ones to show a crystalline form. They have a more complex composition than the former, with
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1165
the walls show signs of humidity. They form long needle-shaped filaments of white and translucent material. Under SEM/EDS this product shows clusters of globular and needle shaped sodium-rich crystals. The composition and texture of the crystals indicate they correspond to trona, as was confirmed by X-ray analysis. Exudations are not frequent. They were observed in the drainage gallery of the gravity dam built on the secondary valley. They are white, solid and hard, sometimes with a saccharoid character. In some places, besides the white product, there are translucent droplets viscous in appearance. These products, assumed to be alkali–silica gel, were analysed by SEM/EDS. They are composed mainly of silicon, potassium, sodium and some aluminium. In some samples calcium was also detected.
Fig. 6. (a) Rosette-like agglomerates of platy crystals; EDS analysis (b) indicates these are composed essentially of silicon and calcium.
silicon, calcium, potassium, aluminium, magnesium and sodium, Fig. 6b. The platy crystals are composed essentially of silicon and calcium. A concrete cone was detached from the ceiling of a gallery in the left artificial abutment. The top of this cone showed a translucent viscous product (Fig. 7). Part of this material, Fig. 8a, was prepared for analysis by SEM/EDS. It is composed of silicon, potassium and sodium, Fig. 8b, and exhibits a cavernous surface (Fig. 8a). Efflorescence exists on enlarged concrete joints in the artificial abutments galleries, in places where
Fig. 7. Alkali–silica gel occurring at the top of the concrete cone from a pop-out, under stereo microscope: (a) white to yellowish solid product (0.6×); (b) viscous translucent product (1.6×).
1166
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
varying especially in the concentration of calcium [8–13]. The products found in exudations and pop-outs of the Alto Rabagão dam, assumed to correspond to alkali–silica gel, are composed mainly of silicon (85.7 to 90.6%) with a low concentration of potassium (5.3 to 7.4%) and sodium (1.9 to 4.3%). The potassium content is always higher than sodium. In some samples calcium (0.2 to 4.7%) and aluminium (0.2 to 6.1%) were also found. The results of SEM/EDS analyses were recalculated on the basis of a fixed sum of the oxides SiO2–K2O– Na2O–CaO concentrations as 100%, excluding trace elements and water (Fig. 12). Most of the samples which
Fig. 8. (a) Cavernous gel occurring at the top of the concrete cone from a pop-out, as shown in Fig. 7. The EDS analysis is presented in (b).
The morphology of the gel is identical to that found in some of the pop-outs and to that referred to by other workers [5–7], showing: – Smooth surface of amorphous alkali–silica gel (Fig. 9) with silicon, potassium, sodium and a low content of aluminium; – Gel-like material composed of silicon, calcium, aluminium and potassium (Fig. 10) covering semi-crystalline calcium carbonate; – Spongy, cavernous material (Fig. 11) composed of silicon, calcium, aluminium, potassium and sodium. A wide range of chemical composition of the gel is found in literature relating to alkali–silica reactions,
Fig. 9. Amorphous gel from an exudation showing (a) a smooth surface with shrinkage cracks; (b) presents the EDS analysis.
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1167
which showed macroscopically a white lining product, are partially or totally filled by ettringite as needleshaped crystals (Fig. 13a), observed also in small pieces of concrete cut from the cores. The composition of the ettringite was confirmed by SEM/EDS (Fig. 13b). In some spectra, in addition to the aluminium, calcium and sulphur, silicon and alkalis were also detected in the composition. No gel was identified under the microscope in the voids or cracks. 4. Discussion The rock from the quarry showed some characteristics that might indicate potential alkali reactivity. The
Fig. 10. (a) Gel-like material covering (b) partially crystalline calcium carbonate.
contain calcium also have some aluminium; these were not projected in these diagrams. The concrete cores extracted from different places in the galleries showed a compact and homogeneous concrete, with no cracks. The coarser particles of aggregate reach 200 mm in the larger dimension. Polished thin sections of the concrete were produced from the cores. The aggregate particles are angular to sub-rounded with low sphericity [14,15] due to the crushing of the granite to produce coarse and fine aggregates. There are rare thin micro-cracks at aggregate–cement interfaces. No reaction rims were detected in the aggregates or in cracks in the aggregates; the cement paste has no infilling material. Some of the air voids, especially those
Fig. 11. (a) Spongy, porous material from the exudations. Crystals are similar in composition (b) to the underlying spongy material.
1168
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
Fig. 12. Ternary diagrams of the composition of the gel identified in pop-outs and exudations.
deformation is revealed mainly in the quartz crystals and in the orientation of phyllosilicate minerals and sillimanite crystals. The quartz crystals show undulatory extinction, which is not recommended to quantify the degree of reactivity of the rock, and also limited portions of microcrystalline quartz [16–19]. The petrographic examination of the thin sections was the only method that suggested potential reactivity of the granite. All the reactivity laboratory tests performed characterised this rock as non-reactive. Granites are classified 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 and United Kingdom [20]. In Portugal the studies based on laboratory tests developed on some granites gave nonpotential reactivity [21]. Nevertheless, the petrographic examination of exudation products sampled from an old Portuguese dam built with granitic aggregates, classified as non-reactive, has shown the existence of superficial alkali–silica gel [22]. Therefore, this type of rock may have particular characteristics related to regional features that can make it potentially reactive. In the present case, evaluation of the performance of this granite as aggregate could be assessed because the rock was applied in the concrete of a large dam built four decades ago. According to references [1,23], the signs of deterioration detected on the walls of the interior galleries of the Alto Rabagão dam may correspond to the occurrence of alkali–silica reactions in the concrete. Although no gel or micro-cracking were identified in thin sections from the concrete cores extracted from the interior galleries of the structure, products composed of silicon and alkalis, and in some samples with calcium and aluminium, were identified in exudations and pop-
outs. The ternary phase diagrams show that silicon is predominant in all the products analysed and there are also low concentrations of potassium and sodium. The calcium content is low in all the analyses and it occurs mainly in samples which also contain aluminium. The relationship between the gel composition and expansibility has been discussed by many workers [10–13]. The results from the study of the Alto Rabagão dam lead to the conclusion that the gel lining the pop-outs must be expansive, though it has a low content of calcium; otherwise, the concrete cones would not detach and fall from the walls and ceilings of the galleries. The crystals composed of sodium present on the surface of the gel might result from the carbonation of the alkali–silica gel when exposed to the atmosphere. The existence of calcium-bearing or sodium-bearing crystals depends on the composition of the gel with which they are associated. 5. Conclusions The petrographic characterisation of the granite applied as concrete aggregate in the Alto Rabagão dam revealed signs of deformation in the rock, shown in particular by the presence of microcrystalline quartz. In the site investigation of the interior galleries of the dam different manifestations of alkali–silica reactions were detected. The use of SEM/EDS allowed us to assume that the products found in exudations and popouts correspond to alkali–silica gel with a variable morphology and composition. Silicon is the main component of the gel, with low contents of potassium and sodium. In a few samples calcium and aluminium were also detected, though the morphology of the gel is similar to that with SiO2–K2O–Na2O. The absence or very low content of calcium in the exudations is in
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
1169
the Alto Rabagão dam, for the facilities during the fieldwork and also for permission to publish these results. The authors are grateful to reviewers for the constructive and interesting comments. References
Fig. 13. (a) Morphology and (b) composition of ettringite needleshaped crystals filling an air void. The needles show an erratic orientation and are detached from the void wall due to shrinkage.
agreement with some of the literature about the composition of the gel in the surface of concrete. The absence of gel in the interior of the concrete, as shown by the examination of thin sections of concrete cores might indicate that alkali–silica reactions are superficial in this structure. The existence of pop-outs shows that the gel might be expansive thus causing the fall of the concrete cones. Acknowledgements We are particularly indebted to EDP — CPPE, Direcção de Produção Hidráulica for allowing access to
[1] British Cement Association. The diagnosis of alkali–silica reaction, Report of a working party. British Cement Association Publication; 1992. 45.042. [2] ASTM C 289–94. Standard test method for potential alkali — silica reactivity of aggregates (Chemical Method). Philadelphia: The American Society for Testing Materials; 2002. [3] ASTM, C 1260-94. Standard test method for potential alkali– aggregates (Mortar-Bar Method). Philadelphia: The American Society for Testing Materials; 2002. [4] Normalisation Française, P 18−590. Granulats — Stabilité dimensionelle en milieu alcalin. Essai accéléré sur mortier par autoclave. Paris: AFNOR; 1993. [5] Stievenard-Gireaud D. Etude de la réaction alcalis–silice dans des bétons. Laboratoire Central des Ponts et Chaussées, Rapport de recherche LPC, vol. 144. Paris: LCPC; 1987. [6] Shayan A, Quick GW. Microstructure and composition of AAR products in conventional standard and new accelerated testing. Proc 8th Int Conf on Alkali–Aggregate Reaction in Concrete, Kyoto, data; 1989. [7] Larive C. Les réactions de dégradations internes du béton — Où, quand, comment, pourquoi? Rapport des Laboratoire Central des Ponts et Chaussées, OA-6, Paris; 1990. [8] Knudsen T, Thaulow N. Quantitative microanalyses of alkali– silica gel in concrete. Cem Concr Res 1975;5:443–54. [9] Helmuth RA, Stark D. Alkali silica reactivity mechanisms. In: Skalny J, editor. Materials Science of Concrete, vol. III. Westerville, OH: American Ceramic Society; 1992. p. 131–208. [10] Thaulow N, Jakobsen UH, Clark B. Composition of alkali silica gel and ettringite in concrete railroad ties: SEM-EDX and X-ray diffraction analyses. Cem Concr Res 1996;26(2): 309–18. [11] Kawamura M, Arano N, Terashima T. Composition of ASR gels and expansion of mortars. In: Cohen M, Mindess S, Skalny J, editors. Materials Science of Concrete: Special Volume — The Sidney Diamond Symposium. Westerville, OH: American Ceramic Society; 1998. p. 261–76. [12] Thomas MDA. The role of calcium in alkali–silica reaction. In: Cohen M, Mindess S, Skalny J, editors. Materials Science of Concrete — The Sidney Diamond Symposium. Westerville, OH: American Ceramic Society; 1998. p. 325–37. [13] Diamond S. Chemistry and other characteristics of ASR gels. Proc 11th Int Conf on Alkali–Aggregate Reaction. Québec, June 11–16; 2000. [14] Krumbein WC, Sloss LL. Stratigraphy and sedimentation. 2nd ed. San Francisco: WH Freeman & Co; 1963. [15] Powers MC. A new roundness scale for sedimentary particles. J Sediment Petrol 1953;23(2):17–119. [16] Grattan-Bellew PE. Microcrystalline quartz, undulatory extinction and the alkali–silica reaction. Proc 9th Int Conf on Alkali– Aggregate Reaction in Concrete, London, July; 1992. [17] Wigum BJ, Hagelia P, Haugen M, Broekmans MATM. Alkali aggregate reactivity of Norwegian aggregates assessed by quantitative petrography. Proc 11th Int Conf on Alkali–Aggregate Reaction. Québec, June 11–16; 2000.
1170
I. Fernandes et al. / Materials Characterization 58 (2007) 1160–1170
[18] Broekmans MATM. The alkali–silica reaction: mineralogical and geochemical aspects of some Dutch concretes and Norwegian mylonites, Ph.D. Thesis, University of Utrecht, Utrecht, The Netherlands; 2002. [19] RILEM Recommended Test Method AAR-1. Detection of potential alkali–reactivity of aggregates — Petrographic method, TC 191-ARP: Alkali–reactivity and prevention — Assessment, specification and diagnosis of alkali-reactivity, prepared by I. Sims, P. Nixon. Mater Constr 2003;36:472–9. [20] RILEM. RILEM petrographic method, New edited draft. RILEM/TC-ARP/01/03a; 2001.
[21] Silva AS, Reis MOB. Avaliação da reactividade aos álcalis dos agregados para betão. Porto: Encontro Nacional Betão Estrutural; 2000. [22] Fernandes I, Noronha F, Teles M. Microscopic analysis of alkali– aggregate reaction products in a 50-year-old concrete. Mater Charact 2004;53(2-4):295–306. [23] ACI. State-of-the-art report on alkali–aggregate reactivity. Reported by ACI Committee, vol. 221. American Concrete Institute; 1998.