Key words: Granite, anisotropy, physical properties, ultrasonics, deterioration, monument, Spain.

Transcription

1 205 PHYSICAL BEHAVIOUR ANO DEGRADATION TRENDS IN AN ANISOTROPIC GRANITE. PEREZ-ORTIZ, A, ORDAZ, J., ESBERT, R.M., ALONSO, F.J. and DfAZ-PACHE, F. Department of Geology, University of Oviedo (Spain). Key words: Granite, anisotropy, physical properties, ultrasonics, deterioration, monument, Spain. SUMMARY A texturally anisotropic granite -used in the building of Axeitos dolmen (NW of Spain)- has been studied in order to check the relationship among their intrinsic characteristics, physical properties and mechanical behaviour under certain weathering agents. It has been verified that the preferred orientation of biotites and cracks of this granite influences physical properties such as: velocity of wave propagation, mechanical strength, capillary suction and water vapour permeability. An accelerated ageing test -salt crystalization cycleshas been carried out in order to study the evolution of the induced decay. It shows that the anisotropic behaviour of the granite conditions its deterioration forms. 1.INTROOUCTION Granite is, among the building stones, a rock usually considered as homogeneous and relatively isotropic, especially in comparison with other types of rocks such as limestones and sandstones. The deterioration of granite is basically related to the selective behaviour of their mineral constituents (quartz, feldspars, micas) with respect to weathering agents (Casal et al., 1989; Delgado Rodrigues, 1991). However, the granite frequently shows some intrinsic factors, linked to petrogenetic processes that can mark directions of anisotropy (eg. preferred orientations of minerals or cracks). These textural anisotropies can notably contribute to the formation of certain forms of deterioration (e.g. scales or plaques) on the granitic stones im the monuments. Axeitos granite, used in the building of the dolmen of the same name located in the province of La Coruiia (Galice, NW of Spain) (Perez-Ortiz et al., 1994), has been selected in order to study the influence of the above mentioned characteristics on the degradation trends of granitic stones. 2. CHARACTERIZATION OF AXEITOS GRANITE. Axeitos granite shows a textural anisotropy caused by the combination of two concomitant features: the preferred orientation of discontinuities (fissures and cracks) and the alignment of the biotite grains (Esbert et al., 1994). Its mineralogical composition is: 43% quartz; 29% potasic feldspar-orthoclase and microcline-; 23% plagioclase (An 440 ); 3.8% biotite and 1,2% accesory minerals (apatite, zircon, muscovite, etc). The degre of weathering is relatively high. Feldspars and biotites are partially altered to sericite and clorite. Quartz grains are greatly affected by intragranular cracks. Transgranular and intergranular microfissures are also frequent (Fig. 1).

2 206 Some physical and mechanical properties of this granite are shown in Table I. Capillary suction, water vapour penneability or diffusivity, swelling and mechanical strength were measaured in the two anisotropy directions, z and Y; that is, perpendicular and parallel to the alignment of biotites, respectively. TABLE I Physical properties of Axeitos granite. Physical properties Average values 1 Density (rd) 2585 Kgm 3 1 0pen porositylno) 2.0 % 1 Coefficient of water saturation (Ws) 0.8 % 2 Coefficient of capillarity(c) Direction (Z) 2. 5x10-4 gcm 2. s 112 Direction (Y) 2.9x10-4 gcm 2.s Coefficient of water vapour Direction (Z) 0.25 gcm 2.24h permeability(kv) Direction (Y) 0.66 gcm 2.24h 4 Swelling (linear)(es) Direction (Z) 0.004± % Direction JYj 0.034±0.004 % 5 Compressive strength (sc) Direction (Z) 101±23 MPa Direction (Y) 107±7 MPa 5 Tensile strength (sr ) Direction (Z) 8.5±0.0 MPa Direction (Y) 9.0±0.5 MPa 1 RILEM 1980; 2 NORMAL, 1981 ; 3 NORMAL, 1985; 4 ISRM, 1979; 5 ISRM, ULTRASONIC BEHAVIOUR. Laboratory ultrasonic techniques have been applied in order to characterize Axeitos granite physically. Velocities of longitudinal wave propagation (Vp) have been measured in three perpendicular directions on two types of samples: prismatic blocks (30x20x1 Ocm) and cubic specimens (5x5x5cm) (Fig. 2). The equipment used was a New Sonicviewer, mod (Oyo Co.). The voltage used for the tests was 200 V; the adapted frequency was 54 KHz for the emitter and 40 Khz for the receiver.!tl"" --!! --y x x,. Receiver Receiver y Emitter z Direction Z Direction Y Fig. 2.- Scheme of the position of the transducers, for measuring Vp. in relation to the orientation of the specimens (Zand Y).

3 207 Measurements were canied out at different moisture contents. The humidity of the blocks extracted from the quarry corresponds to its natural water content, whereas the cubic specimens were tested under dry and water saturated conditions. Table II shows the results of the ultrasonic tests. The results obtained verify the anisotropic character of the Axeitos granite. Most of the properties (especially capillary suction and water vapour permeability or diffusivity), show higher values in the Y direction than in the Z one. Vp also shows higher values in the Y direction. The higher values correspond to the main direction of microfissuring and biotite alignment. Water content also influences Vp, showing the water saturated specimens higher values than the dried ones. TABLE II. Average values of Vp for the Axeitos granite Samples Moisture content \&(ms) z x y Blocks (30x20x10 cm l Quarry Humidi!Y. 2200± ± Specimens Dry 2250± (5x5x5 cm) Water Satured 3990± ARTIFICIAL WEATHERING: CORRELATIONS WITH THE INTRINSIC CHARACTERISTICS Procedure. An accelerated ageing test -salt crystallization cycles- was canied out in laboratory on specimens of Axeitos granite, in order to study the evolution of the induced decay and the deterioration forms generated throughout the test. Two series of specimens, cubic (5cm side) and prismatic (5x15x25cm), were tested according to the mentioned anisotropy directions. An aqueous solution of decahydrated sodium sulphate (Na 2S04.10H20) at 14% was used. The absorption was by capillary suction, and the procedure followed was the one proposed by RILEM (1980). The number of cycles was 50. The evaluation of damages was made taking into account the observation of superficial modifications of the specimens, as well as weight variations and values of ultrasonic waves velocities after every five cycles Results. The damages induced by the salt crystallization test changes according to the shape, size and orientation of the specimens. a) Cubic specimens. The specimens Z-oriented in the sense of the rise of the saline solution, the degradation is mainly due to fissuring. Fissures also accumulate in the central part of the specimens, coinciding with the limit of the dry section of the specimen and the one impregnated with the saline solution. In the 50th cycle the specimens eventually split. In the specimens Y-oriented in the sense of the rise of the saline solution tend to deteriorate by granular disaggregation. The first minerals to disaggregate are the biotites, followed by plagioclases and K-feldspars; the last ones in detaching are the quartz grains. The final result is an arenization that is preferably located in a band, of one centimeter wide, in the central part of the specimens. In general, the losses of material are greater in Z direction than in the Y one, due to the detachment of the material involved in the processes of fissuration and final breaking. Vp decreases along the cycles with respect to the initial ones (non-tested specimens) (Fig. 3). This decrease is very apparent from the 35th cycle, and it is greater in the specimens Z-oriented than in the Y ones. b) Prismatic specimens. There was no significant damage in Y orientation, only slight pitting. In the Z orientation, detachment of grains (mainly of the assemblage plagioclase-k feldspar) were observed from the 12th cycle, as well as a progressive increase of fissuring, until scaling and traces of plaques took place. A staining by lixiviation of iron from biotites was also evident after the test. The loss of material is greater in the prisms Z-oriented than in the Y ones. Ultrasonic waves along the cycles show a similar behaviour in both directions Y and Z, with no appreciable decrease (Fig. 5).

4 208 i ; looo?-?!~ ~Jl z CAPlLLARY t t tt sucnon 40 ~o Fig. 3. Variation of Vp along the salt crystallization cycles, in cubic specimens y i ;.. looo 10 z y )iz I - l J. J. t CAPII..LAR y I I SUCTION t 20 lo 40 ~ 3000 z 30 "6 Fig. 4. Variation of Vp along the salt crystallization cycles, m prismatic specimens. The Vp variations are closely related to the loss of material, the development of the induced fissuration by the action of salts, as well as the crystallization of the salts in the preexisting or new formed voids. When the formed voids have a size that can not be filled by the crystallizing salts, the Vp decreases in an abrupt or slowly way. In the first case, the loss of material by granular disaggregation is generalized; and in the second case, fissuring is the main factor responsible for the physical deterioration. 5. CONCLUSIONS Intrinsic characteristics of the granitic rocks clear1y influence some of its physical properties at intact rock scale, as well as its weatherability and degradation as a building stone. Axeitos granite shows an anisotropic behaviour that can be closely related to the preferred orientation of discontinuities at different scales and with the alignment of the biotites. This behaviour is evidenced through the variation of some physical properties, mainly the velocity of sonic waves, capillary suction and water vapour permeability. It has been experimentally proved, by means of an accelerated ageing test (salt crystallization cycles), that the anisotropic character of the rock material conditions or favours the appearance of some typical deterioration mechanisms usually shown by the granitic stones: sand disaggregation, scaling and plaque shedding. The simulation, in laboratory, of these deterioration forms depends largely on the shape, size and orientation of the specimens with respect to the weathering agent. Vp measurements have been especially useful to evaluate indirectly the induced damages. It has also been observed that the textural anisotropy of the granite condition the preferred growth of cracks while progressing its physical deterioration, giving rise to planes of mechanical weakness. This would explain the tendency of some granitic stones in monuments to develop plaques over other deterioration forms. Thus,

5 209 the anisotropic character of the granite will be kept in mind in order to understand its degradation trends as a building stone. The different levels and features of decay shown by exposed ashlars in monuments can be explained in many cases by the influence of some particular intrinsic factor, such as the preferred orientation of cracks. ACKNOWLEDGEMENTS. We are grateful to the European Economic Community for its financial support of the research project: "Conservation of Granitic Rocks. Application to Megalithic Monuments" (Contract No. STEP-CT ). Also to the "Comisi6n lnterministerial de Ciencia y Tecnologia" (CICYT, Spain) Project: PAT C REFERENCES. Casal, M., B. Silva, J. Delgado (1989). Agents and forms of weathering in granitic rocks used in monuments. Science and Techn. in European Cultural Heritage, Commision of the European Communities, Bologna, pp Delgado Rodrigues, J. (1991). lnc6gnitas e problemas relativos a conservayao de rochas graniticas ea sua abordagem nos projectos STEP. Alteraci6n de Granitos y Rocas Afines (M.A. Vicente, E. Molina and V. Rives, Eds.), CSIC, Madrid, pp Esbert, R.M., A. Perez-Ortiz, J. Ordaz, F.J. Alonso (1994). Intrinsic factors influencing the decay of the granite as a building stone. Proceed. Seventh Int. Cong. of the Int. Assoc. of Eng. Geol. (R. Oliveira, L.F. Rodrigues, A.G. Coelho and A.P. Cunha, Eds.}, Vol. 5, A.A. Balkema, Rotterdam, pp Perez-Ortiz, A., J. Ordaz, R.M. Esbert, F.J. Alonso (1994). Microfissuring evolution of the granite from Axeitos dolmen along the salt crystallization test. Ill Int. Symp. on the Conservation of Monuments in the Mediterranean Basin (V. Fassina, H. Ott and F. Zezza, Eds.}, Soprintendenza ai Beni Artistici e Storici di Venezia, pp