12TH INTERNATIONAL BRICK/BLOCK. Masonry ON THE COMPRESSIVE STRENGTH OF STACKED DRY-STONE MASONRY

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1 12TH INTERNATIONAL BRICK/BLOCK Masonry c O N F E R E N C E Oli ON THE COMPRESSIVE STRENGTH OF STACKED DRY-STONE MASONRY D. V. Oliveira 1, P. B. Lourenço2, P. Roca 3 'Research Assistant, University of Minho, Guimarães, PORTUGAL (danvco@eng.uminho.pt) 'Assistant Professor, University of Minho, Guimarães, PORTUGAL (p.lourenco@eng.uminho.pt) ' Full Professor, Technical University of Catalonia, Barcelona, SPAIN (pedro.roca@upc.es) ABSTRACT The paper presents recent experimental results concerning dry stone masonry. Monotonic and cyc/ic uniaxial tests were performed on cylindrical stone specímens of two different sizes. The complete stress-strain diagram was obtained as wel/ as the cyc/ic behaviour. Results of monotonic tests on stacked dry-stone masonry are discussed and compared to the stone specímens in terms of Young's modulus and peak strength. Final/y, the main conc/usions concerning the results obtained are summarized. Key words: dry stone masonry, uniaxial compressive tests, cyc/ic behaviour. 2131

2 1. INTRODUCTION Even if today stone is an uncommon building material, stone masonry had been widely used along Human history and it is present in the most existing historical constructions. The preservation of this architectural heritage is a multidisciplinary field that involves naturally the structural and material characterization. In this domain, the mechanical characterization of dry stone masonry constitutes an important issue, fundamental for a suitable evaluation of the safety levei of the existing constructions. This material characterization has also a huge importance when accurate numerical models are intended to be used. The tests presented in this paper were recently carried out at the Technical University of Catalonia, Barcelona, Spain. Sandstone locally available, known as "Montjuic stone" which has been used for centuries in the construction of the monuments spread ali over Catalonia, was used for the tests. The stones were mechanically cut in with smooth faces and delivered into the laboratory in small prismatic pieces (200x200x100 mm J ). Macroscopically, the stone looks very homogeneous and has a very small grain size. Visually, it was impossible to define any kind of grain orientation or any other type of anisotropy. 2. TESTS ON CYLlNDRICAL SPEClMENS The complete load-displacement diagram for stones tested in compression has been obtained from decades (e.g. Wawersik (1968)) and provide valuable information about the stone's behaviour. During the initial development of the testing techniques, it became clear that, for some stones, it was impossible to obtain the full diagram without appropriate control techniques. Two types of diagrams in terms of the characteristics of the postpeak region were identified by Wawersik, see Figure 1. Class I diagrams are characterized by monotonical increase in strain, where fracture propagation is stable in the sense that work must be done on the specimen Figure 1. Two c/asses of stress-strain behaviour observed in uniaxial compression stone tests. Axial stress Axial strain 21 32

3 for each incrementai decrease in load-carrying ability (stable softening behaviour). In class 11 diagrams, fracture is unstable and the elastic strain energy absorbed in the material is sufficient to maintain fracture propagation until the specimen has lost ali strength (unstable softening behaviour). A key aspect in the uniaxial compressive testing of quasi-brittle materiais is the platen restrain effect. Different approaches have been used to obtain a uniaxial stress distribution on the specimens, namely steel brushes and teflon layers, e.g. van Mier (1984). Another possibility is to define a height/diameter ratio (h/d) of the specimen sufficiently high to reduce the platen restrain effect. In order to obtain a uniaxial behaviour at least at the center of the specimen, a ratio between two and three and a diameter preferably not less than 50 mm are recommended, see Fairhurst et 01. (1999). The diameter of the specimen should be at least 20 times the largest grain in the stone microstructure. Therefore, after grinding the top and bottom surfaces of the prismatic stones, cylindrical specimens of 0 50 x 120mm 3 were extracted by means of a drill, resulting in a h/ d ratio equal to 2.4. For this ratio, the confinement effect on the peak strength value was expected to be absent and no special provisions to eliminate this effect were taken. 50, no interposition material was inserted between the specimen and the machine platens. The stone specimens (55) were denoted by the stone number and by the specimen number. The reference represents thus the third specimen obtained from the prismatic stone n Q 2. To ensure correct sampling, the specimens were extracted randomly from different stone blocks. Ali the specimens were tested in a closed-ioop servo-controlled IN5TRON testing machine of 1000 kn load bearing capacity. Four stone specimens were tested under monotonic regime and six stone specimens under cyclic regime. At the beginning of the test, a small preload was applied, in force control, in order to adjust the upper platen to the top surface of the specimen. This platen had a hinge to prevent any unfavourable effect due to non-parallelism between the specimen faces. During the tests, the following control variables were used: - axial displacement control for small load values (5 Ilm/s); - force control during unloading (2kN/s); - circumferential displacement control in general (1.5 Ilm/s). The displacements were measured using a circumferential LVDT placed at the specimen mid-height and three axial LVDTs placed between the machine platens, see Figure 2. The applied load was measured by means of the machine load cell. The axial displacement of each tested specimen was defined by the average value obtained from the three LVDTs placed between the machine platens. Then, the axial strain and axial stress were calculated by dividing the change in average 2133

4 Figure 2. Experimental test se-up for the stone specimens. measured axial length by the initial axial length and the load by the initial crosssectional area, respectively, see e.g. ASTM (1999), Fairhurst et 01. (1999). Table 1 exhibits the Young's modulus and the peak strength values obtained for the ten specimens. The Young's modulus was computed in the (30%-60%) stress interval by linear regre5sion. It can be observed that some 5catter is pre5ented in the results due to the fact that samples were extracted from distinct stone pri5ms. This fact can be explained by differences exhibited by the stone nature. Even if for stones delivered in a single batch, it was not possible to ensure that ali stone5 were submitted to the same conditions or even to ensure that they were extracted from the same location. Table 7. Young's modulus and compressive strength of the stone specimens (h = 720mm). Specimen E '~60 [Gpa) (J.... [Mpa) Specimen E"... [GPa) (J... [Mpa) The stres5-strain diagram5 for four of the stone specimens te5ted are shown in Figure 3. Ali the curves exhibit the common initial adjustment between the specimen and the machine platens. In ali the tests, the prepeak behaviour wa5 very easy to follow. The specimens showed a reasonable linear behaviour almost until the peak load. But just after the peak, a very pronounced fragile behaviour was exhibited by ali the specimens. The applied load decreased in a very non-smooth way. 2134

5 Figure 3. Stress-strain diagrams obtained for the stone specimens (h = 120 mm) l ~60 i(l ~ - 40.~ <{ SS5.1 O'-"''--'---~~-~~-~~-----' o AxiaJ Strain [nun/m: 20 o o Axial Strrunlmm/m l SS ~ 60 ~ ~ 40 >< -( o o Axial Srrain (0101/01, 20 Ax ial Strain[mmJm SS7.1 The initial macroscopical cracks were visible only for a load very close to the peak load. Macroscopic crack initiation took place at the extremities, progressing through the entire specimen. Some of the cracks had a sudden formation and in some specimens were accompanied with a clear sound. In the cyclic tests, the specimens had to be unloaded from defined locations and then reloaded in arder to generate "new" stress-strain curves. The local Young's modulus (positive slope of the ascending branches) of these new curves was calculated by linear regression. Its evolution can be considered as a measure of the damage in the material. Table 2 shows the Young's modulus computed for ali the reloading branches (rb) of the specimens tested cyclically. The values computed in the postpeak portion are typed in italic. Table 2. Local Young 's modulus obtained by linear regression of the reloading branches. Spedmen E ['!'al rbl rb2 rb3 rb4 rb5 lii

6 In the prepeak region, a slight increase of the Young's modulus can be observed (e.g. specimens and 5S4.2). This result is in agreement with results obtained from tests using strain gauges, Oliveira (2000), and results presented by other authors, e.g. Rocha (1981). On the other hand, a continuous decrease of the Young's modulus in the postpeak region can be observed (see specimens and 557.1). This decrease is related to the progressive damage growth exhibited by the specimens. In order to check the reliability of the use of LVDTs on the evaluation of the Young's modulus, two stone specimens were tested using three double electric resistance strain gauges rosettes. The axial deformations were measured by the strain gauges and LVDTs. It has showed that the elastic modulus ca lculated using strain gauges was always larger than the value obtained using the data from the LVDTs, but the differences were relatively small (Iess than 12%), see Oliveira (2000). Therefore, it can be accepted that the information obtained by means of LVDTs may be used to evaluate the Young's modulus in the specimens tested without strain gauges. Typical failure modes of the stone specimens tested are shown in Figure. 4. Due to the confinement effect caused by the machine platens, the top and bottom surfaces did not present any visible crack. For a load dose to the peak value, the first macrocracks became visible. The observation of the collapsed specimen suggested that colapse may be attributed to failure along a shear band, formed by the coalescence of the major cracks. In ali specimens tested, the formation of shear bands took place when postpeak region was reached. Its development seems to be the cause of specimen failure. This described behaviour is well known in rock mechanics, e.g. Rocha (1981) and Li et 01. (1998). 2.1 Influence of the slenderness ratio of the specimens In order to check the influence of the slenderness ratio on the peak load and on the postpeak behaviour, it was decided to perform some tests on specimens Figure 4. Typical observed failure modes. 2136

7 with slightly different height. Thus, two cylindrical specimens of 0 50 x 100 mm 3 (h/d= 2.0) were tested under monotonic compression. Table 3 shows the results, in terms of Young's modulus and ultimate strength. It is noted that these results do not have statistical meaning due to the reduced number of specimens tested. Table 3. Young's modulus and compressive strength of the stone specimens (h = 700mm). Speclmen BO 60 [GPa] speak [MPa] With such decrease in the specimen's height (about 17 %) no remarkable differences were found when comparing the peak strength. With respect to the elastic modulus, it turns out to be very difficult to extract any conclusion because this property seems to show a wide scatter. However, if a comparison between the postpeak branches is made, it can be observed that to the smaller height is associated a better stability until complete loss of strength capacity, see Figure 5. Figure 5. Stress-strain diagrams obtained for the stone specimens (h = 700 mm)..j Axial Sl.rain lmm/m l Axial St rain jmm/ml It is known that under axial compression, due to localization of the deformations after peak load, the complete stress-strain diagram becomes dependent of the specimen size. This phenomenon is characteristic of quasi-brittle materiais, like stones and concrete, see Labuz et 01. (1991) and van Mier (1984). The results presented seemed to show that a smaller slenderness factor (from 2.4 to 2) provides a more stable behaviour to the specimen in postpeak loading. 2737

8 3. TESTS ON STACKED DRY-STONE MASONRY In order to evaluate the superposition effect of the stone blocks, uniaxial compressive tests on stacked dry-stone masonry prisms were performed. The stone blocks utilized were from the same batch used to make the cylindrical specimens. Two prisms made of three 20xl Oxl O cm 3 pieces and two other prisms made of four 20x20xl0 cm 3 pieces were tested. The slenderness ratios obtained (h/d) were three and two, respectively, where h is the height and d is the lowest base dimension of the prism. These ratios allow a uniaxial compressive behaviour at the center of the prisms. Figure 6. Adopted prisms of stacked dry-stone masonry. l I 10 L 40 l~ J j X10-! [em] x20 ---J r 30 L The prisms were built just by superposition of the stone pieces and no additional surface treatment was done. The stone pieces that presented some kind of visible defect were removed apart. The prisms were denoted by the letters SP (stone prism) and by an order number. SPl and SP2 designated the prisms made of three pieces and the prisms made of four pieces were denoted by SP3 and SP4. Ali the prisms were tested with their natural water content. A SUZPECAR testing machine controlled by a MTS 458 system (5 MN load bearing capacity) was used for these tests. Three vertical LVDTs, equally spaced, were used to measure the displacements between the machine platens. The applied load was measured by means of the machine load cell. The axial displacement of each prism was defined by the average value obtained from the three LVDTs and the axial strain was calculated by dividing the average axial displacement by the initial axial length of the prism. Likewise, the axial compressive stress was computed as the axial load divided by the initial cross-sectional area. Initially, the prism was put into the lower platen and a small preload was applied, under force control, to adjust the upper platen to the top surface of the specimen. This platen has a hinge in order to avoid any unfavourable effect due to the non-parallelism between the prism faces. The four tests were performed under ax- 2138

9 ial displacement control at a constant rate of 3 Ilm/s. Ali prisms failed just after the peak load, exhibiting a very pronounced fragile behaviour. Therefore, postpeak could not be obtained. The Young's modulus of the four prisms was evaluated in the (30%-60%) stress interval by linear regression. Table 4 summarizes the Young's modulus and the compressive strength values of the four stone prisms. Again, it can be observed a wide scatter present in the results. The average value of the Young's modulus measured for the stone specimens (050x120mm l ) and prisms presented close values (difference of 5%). This result could be probably expected because the stone blocks used had the surfaces polished. 50, it seems that the evaluation of the Young's modulus of stacked drystone masonry can be obtained from tests on simple stone specimens. On the other hand, the peak strength value of the prisms showed an important decrease with respect to the specimens (about 31 % in terms of average value). The scatter presented in the results of the specimen tests may have led to a decrease on the strength of the stone pieces associated together. However, an interpretation purely based on statistics, Mosteller (1973), cannot explain completely such decrease in strength. Being the compression failure controlled by mode I behaviour, the discontinuity between the stone pieces (horizontal joints) led to stress concentrations in a few contact points that originated failure for a load lower than the values achieved with the stone specimens. Table 4. Initial Yaung's madulus and peak strength af the stane prisms. Prism BO 60 [GPal speak[mpal SP SP SP SP Due to the high load values and the brittleness of the stones, careful safety practices were adopted. Therefore, the four tests were performed by placing a cylindrical metallic box around the machine platens, so, it was not possible to accompany their macroscopic behaviour. The peak load and consequent failure were preceded by crack formation detected only by the clear sounds produced. From the failure modes observed, collapse may be attributed to relative displacements along shear bands, for both prism types. 4. CONClUSIONS Tests on stone specimens and stacked dry-stone masonry were carried out by the authors. Ali the tested specimens showed pronounced fragile behaviour after 21 39

10 peak load. The cyclic tests allowed observing that stiffness degradation occurred specially during postpeak domain. The different postpeak behaviour observed for the two different cylindrical stone specimens as wel l as the fai lure modes obtained suggest that localization occurs for stones specimens loaded in uniaxial compression. This means that postpeak branch is not independent on specimen size. Then, the descending part of the stress-strain diagram should not be identify as a material property. The Young's modulus computed for the cylindrical specimens and for the prisms presented very similar average va lues. Therefore, it seems possible to evaluate this elastic property of dry-stone masonry based on the results obtained from stone specimens. The scatter concerning the mechanical properties constitutes an important issue in the sense that a significant decrease of resistance took place when shifting from stone specimens to masonry (in terms of average values). Thus, this subject should be further studied since existing design codes, EC6 (1995), do not take stacked dry-stone masonry into account. The results described along this paper show that when dealing w ith historical structures, the intrinsic variability of the mechanical properties of stone is an important issue that should be kept in mind. 5. ACKNOWLEDGEMENTS The experimental work presented in this paper was carried out at the Structural Technology Laboratory, Technical University of Cata lonia, Barcelona, Spain. The work was partially supported by the projects "Mechanical characterization of traditional or historical factories of bricks and stone masonry -DGES PB " and "Computational strategies for historical structures PRAXIS/C/ECM/13247/1998". The author is grateful to the Portuguese Science and Technology Foundation for making his stay at the Technical University of Catalonia possible through the PRAXIS XXI BD/16168/98 grant. 6. REFERENCES ASTM (1999), "Standard Test Method for Elastic Moduli of Intact Rock Core Specimens in Uniaxial Compression", Annua l Book of ASTM Standards, Section 4, Volume 04.08, D CEN - "Eurocode 6: Oesign of masonry structures" ENV :1995, CEN, Brussels, Belgium. Fairhu rst, C. E.; Hudson, ]. A. (1999), "Oraft ISRM suggested method for the complete stress-strain curve for intact rock in uniaxial compression", International ]ournal of Rock Mechanics and Mining Sciences, 36, pp Labuz, ]. F.; Biolzi, L. (1991), "Class I vs class /I stability: A demonstration of size effect", International ]ournal of Rock Mechanics and Mining Sciences, 28, pp Li, Chun lin; Prikryl, Richard; Nordlund, Erling (1998), "The stress-strain behaviour of rock material related to fracture under compression", Engineering Geology, 49, pp

11 Mosteller, F.; Rourke, R.E.K. (1973), "Sturdy statistics (nanparametrics and arder statistics)", Addison-Wesley, Reading, Massachusetts, USA (Appendix A). Oliveira, D. (2000), "Experimental characterizatian af stane and brick masanry", Report OO-DEC/ E- 4, University of Minho, Portugal. Rocha, Manuel (1981), "Rack Mechanics", Laboratório Nacional de Engenharia Civil, Lisboa, (in portuguese). Van Mier, I.G.M. (1984), "Strain-saftening af concrete under multiaxiallaading conditians", Dissertation, Eindhoven University of Technology, Eindhoven, The Netherlands. Wawersik, W. R. (1968), "Detailed analysis af rock failure in labaratary compressian tests ", Ph. D. thesis, University of Minnesota. 2141

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