Strength characteristics of stone masonry

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 30, May 1997, pp Strength characteristics of stone masonry K. Venu Madhava Rao 1, B.V. Venkatarama Reddy 2 and K.S. Jagadish 3 (1) Research Scholar, (2) Senior Scientific Officer, (3) Professor Dept. of Civil Engineering, Indian Institute of Science, Bangalore , India SCIENTIFIC REPORTS A B S T R A C T This paper deals with an experimental investigation on the strength of stone and stone masonry. Granitoidgneiss is commonly used for masonry construction in India. The compressive strength of stone has been determined through 80 mm size cubes. It has been found that the compressive strength of granitoid-gneiss is greater when the load is parallel to the mineral bands. The compressive strength of stone masonry was studied through masonry prisms using 1:4 and 1:8 cement mortars. These tests have revealed that masonry strength is higher when the load applied is parallel to the mineral bands. The flexural bond strength of stone masonry walls was studied through full-scale tests. Flexural bond strength appears to play a major role in the failure of stone masonry walls. R É S U M É On présente une étude expérimentale de la résistance de la pierre et des maçonneries de pierre. En Inde, les pierres les plus souvent utilisées en maçonnerie sont des gneiss granitoïdes. La résistance à la compression de la pierre a été déterminée sur des cubes de 80 mm. Les résultats montrent que la résistance à la compression du gneiss granitoïde s augmente lorsque la charge est parallèle aux bandes minérales. La résistance à la compression de ces maçonneries a été étudiée à partir d éléments prismatiques utilisant des mortiers de ciment de 1:4 et de 1:8. Ces essais ont montré que la résistance de la maçonnerie est supérieure lorsque la charge est parallèle aux bandes minérales. Des essais complets ont été effectués sur la résistance de l adhérence en flexion des murs en maçonnerie de pierre. La résistance de l adhérence en flexion semble jouer un rôle important dans la détérioration des murs en maçonnerie de pierre. 1. INTRODUCTION Stone masonry construction is known since ancient times. Such construction can be seen all over the world. It has been used in the past for the construction of masonry walls, arches and domes. Stone slabs have also been used for flat roofs. Depending upon its local availability, stone is used even now in the construction of residential and public buildings. Normally, stone masonry walls are thick (greater than 350 mm), thus leading to higher cost and a reduction in available floor space. The stresses exerted on these thick stone walls are generally small. The use of thinner stone masonry walls required a clear understanding of the behaviour of such walls under various loading conditions. The information available on the strength characteristics of stone masonry is limited. This paper is an attempt to evaluate the strength of stone masonry in order to facilitate efficient construction with stone. Evaluating the strength of stone masonry has been undertaken through a series of test programmes. Firstly, the strength of stone was measured through compressive strength tests. Afterwards, the strength of stone masonry was assessed through prisms made using stone specimens and a bonding mortar. The bonding between the mortar and the stone was then checked through a flexural bond strength test. All these tests were carried out using fairly small specimens. An understanding of the strength characteristics of a stone masonry wall was then attempted using a compressive load test on a full-scale wall element. Granite is one of the most commonly used stones for building construction in India. Very often, the granite found in India is granitoid gneiss, which is granite in composition, but stratified in structure. This paper deals with the characteristics of masonry using granitoidgneiss stones. Granitoid-gneiss stones, locally available in Bangalore, have been used for the present study /97 RILEM 233

2 Materials and Structures/Matériaux et Constructions, Vol. 30, May COMPRESSIVE STRENGTH OF STONE AND STONE MASONRY 2.1 Compressive strength of stone cubes The compressive strength of the stone cubes was determined as per guidelines of the Indian standards code IS: [1]. Stone cubes of 80 mm in size were selected for compression testing. The cubes were made by manually chiselling from a larger, irregular piece of stone. They were capped with 1:1 cement mortar and cured. The specimens were then tested in a compression testing machine. Since stone is very brittle, the specimens were tested inside a metal enclosure to safeguard against explosive failure. It was, hence, not possible to record the first crack load. Granitoid-gneiss stones have visible mineral bands on their surface. Uniaxial compression tests were conducted on these stone cubes. In one set of tests, the loading was parallel to the mineral bands. In the other set, the loading was perpendicular to the mineral bands. Table 1 gives the results of the compressive strength test for loading parallel to and perpendicular to mineral bands. In all, 5 specimens were tested in each loading condition. The average compressive strength of the stone, when the load is parallel to and perpendicular to the bands, is MPa and 86.1 MPa, respectively. It can be observed that the strength of the granitoid-gneiss is greater in the case of loading parallel to mineral bands. This observation is in conformity with the results of Mohamedbhai et al. [2]. In both cases, the failure was by vertical splitting of the stone cube. There was no loss of bond between the specimen and the capping material. This seems to suggest that friction between the platen and the specimen must have played a negligible role at the time of failure and hence the vertical splitting of the cubes. Fig. 1 shows the typical failure patterns of the cube specimens. Normally, concrete cubes with strengths of the order of 20 MPa show the form of two intact pyramids with their apices within one another after failure. The stone with a much higher strength shows a columnar failure indicating that platen friction is not effective at such strengths. 2.2 Compressive strength of stone masonry The compressive strength of stone masonry was determined by testing stack-bonded prisms as per guidelines of the ASTM E447 [3]. IS 1905 [4] and the National Building Code [5] also specify similar guidelines regarding Table 1 Compressive strength of stone Cube size: 80 mm x 80 mm x 80 mm (approx.) SI Average compressive Standard Loading No. strength (MPa) deviation (MPa) 1 Parallel to mineral bands 2 Perpendicular to mineral bands Fig. 1 Typical failure pattern of stone cube under compression (loaded parallel to mineral bands). the prism tests on unreinforced masonry. A three-block high prism was prepared using stone cubes of 80 mm in size. The prisms were cast by laying stone cubes one on top of the other with a mortar bed between the masonry units. A mortar joint thickness of 10 mm was used throughout the experiment. The prisms were capped using a rich cement mortar with a proportion of 1:2 (cement : sand). The prisms were cured for 28 days by soaking in water and then tested in the wet condition. Generally, low-strength cement mortars such as 1:8 are often used for stone masonry construction in India. In the present investigation, two mortar combinations (1:4 and 1:8 cement mortars) have been used for the casting of prisms. The compressive strength of the mortar was determined by testing 70 mm cubes. Prisms were cast in two modes so that the load could be applied in directions both perpendicular to and parallel to the mineral bands of stone. In all, 6 prisms were cast using each combination. The test results of the stone masonry prisms are tabulated in Table 2. The table presents the details of loading pattern, mortar, masonry prism strength as well as masonry efficiency (η). It is clear from the table that masonry compressive strength increases with an increase in mortar compressive strength, irrespective of the orientation of the mineral bands. There is a 46% increase in the masonry compressive strength with a 3.3 fold increase in mortar strength for loading parallel to the bands. In the case of loading perpendicular to the bands, the increase is 27%. It Table 2 Strength of stone masonry prisms Prism size: 80 mm x 80 mm x 275 mm (approx.) (Values in parentheses indicates standard deviation) Average SI Mortar Mortar masonry Masonry Loading direction strength prism efficiency No. proportion (MPa) strength (η) (MPa) 1 Parallel to mineral bands 1: (6.8) Perpendicular to mineral bands 1: (8.5) Parallel to mineral bands 1: (2.6) Perpendicular to mineral bands 1: (2.9) η= Average masonry prism strength Cube strength 234

3 Venu Madhava Rao, Venkatarama Reddy, Jagadish Fig. 2 Typical failure pattern of stone masonry prism using 1:4 cement mortar under compression (loaded perpendicular to mineral bands). can also be observed that the strength of stone masonry is greater in the case of loading parallel to the bands rather than perpendicular to the bands, irrespective of the mortar. There is a marginal variation in masonry efficiency values for a given type of mortar. Masonry efficiency is comparatively high for 1:4 cement mortar, irrespective of the loading pattern/orientation of bands. The values for η range between and for both the mortars and the loading patterns considered. These values of masonry efficiency are similar to the values found in brick masonry [9]. However, the strength values are considerably higher than what could be expected from brick masonry in India. Stone masonry thus shows compressive strength values comparable to those of high-strength concrete. Its value in construction projects requiring high compressive strength should not be underestimated. Fig. 2 shows the typical failure patterns of stone masonry prisms. There is a vertical splitting of stone cubes as well as bond failure at the interface of stone and mortar. There is hardly any difference in load between the first crack and ultimate failure. Splitting of the stone and the bond failure were practically simultaneous for all the cases considered. It has been observed that only the central stone cube of the prism was getting split, and both joints at the middle of the prism were failing in bond for all cases except for the 1:4 cement mortar when the loading was parallel to the mineral bands. For 1:4 cement mortar with loading parallel to the bands, in addition to bond failure at both the joints, the top and central stone cubes were getting split, as shown in Fig FLEXURAL BOND STRENGTH OF STONE MASONRY Flexural bond strength is an important property that can influence the strength of walls in eccentric or lateral loading. This section deals with the flexural bond Fig. 3 Typical failure pattern of stone masonry prism using 1:4 cement mortar under compression (loaded parallel to mineral bands). strength of the stone masonry prisms. The flexural bond strength was determined as per the guidelines of the ASTM C1072 with slight modifications in the bond wrench test [6]. Three-block, high stack bonded prisms were prepared using manually-dressed stones of size 305 x 125 x 100 mm with three different mortars. The mortar combinations tested were 1:4, 1:8 cement mortar and 1:1:6 soil-cement mortar. A constant joint thickness of 10 mm was used. The details of the strength of the mortar are reported in Table 3. The cube compressive strength of the mortar was determined at the time of testing for the flexural bond strength using 70 mm cubes. The prisms were cured for 28 days and tested for bond strength in a wet state. The results of the flexural bond strength are also reported in Table 3. Each result represents an average of 7 joints. The flexural bond strength of stone masonry varies from 0.12 MPa to 0.16 MPa. There is a 1/3 increase in flexural bond strength as the cement mortar is changed from 1:8 to 1:4. Combination mortar, such as 1:1:6 soil-cement mortar, gives better bond strength compared to a lean cement mortar of 1:8 proportion. There was a complete bond failure at the stone and mortar interface irrespective of the type of mortar considered. These flexural bond strength values may be compared with the corresponding values for brick masonry obtained by Venu Madhava Rao [6]. It was found that when the mortar is 1:4 cement mortar, the brick masonry has a flexural bond strength of MPa; with Table 3 Flexural bond strength of stone masonry Mortar proportions Mortar strength Flexural bond Coefficient of Cement : Soil : Sand (MPa) strength (MPa) variation 1 : 0 : % 1 : 0 : % 1 : 1 : % 235

4 Materials and Structures/Matériaux et Constructions, Vol. 30, May :1:6 soil-cement mortar, it has a bond strength MPa. These values indicate that stone masonry develops better flexural bond strength than that of brick masonry. 4. STRENGTH OF STONE MASONRY WALLS In general, where the strength of a wall is to be estimated, it is customary to relate the wall strength to the compressive strength of the masonry unit. The masonry unit s compressive strength is often represented by the strength of a small masonry prism. This approach of correlating unit strength seems to work well in brick and concrete block masonry. It has also been extended by Reddy [7] to soil-cement block masonry. It is now useful to examine this approach vis-à-vis to the strength of stone masonry walls. Strength tests were conducted on full-scale stone masonry walls in order to understand their behaviour under compressive loads. The details of the two wall specimens tested are given in Table 4. The walls were constructed using 1:8 cement mortar with an average joint thickness of 20 mm, and were then tested for compressive strength after curing for 35 days. The wall thickness given in Table 4 is an average value. It is very difficult to cut the stones to an exact thickness; hence, one face of the wall was built true to plumb, while small projections and depressions were present on the other face. Load was applied through a hydraulic jack and measured with a proving ring. Quarter-point loading was used to obtain a uniform load distribution. A reinforced block of 200 mm thickness was fixed on the top of the wall to distribute the load. The lateral deflection was measured using three dial gauges at mid-height of the wall. The properties of the stone used for the wall construction have already been discussed in the earlier sections. The mortar strength was measured by casting 70 mm size cubes, and its 7-day and 35-day strengths were 2.7 MPa and 2.9 MPa, respectively. 4.1 Test results and discussion Fig. 4 Possible modes of eccentricity and failure patterns. Results of the wall strength test are shown in Table 5. The compressive strength of the two walls turned out to be 0.71 and 0.98 MPa, respectively. Fig. 4 shows the two failure patterns of the stone masonry walls. Fig. 5 shows a wall after failure. One of the wall specimens failed purely in the cantilever mode. The other wall showed a failure initiation due to buckling as a pin-ended column. In both walls, the stones did not fail and remained intact even after the collapse. The low values of wall strengths and the failure modes observed may perhaps be attributed to the eccentricity caused on the wall while applying the load as well as to the non-uniformity of wall thickness. Results seem to suggest that the two walls failed essentially due to the bond failure of the stone mortar joint. Since the compressive stress at failure was very low, the stone blocks did not undergo any serious stressing. In other words, the compressive strength of the stone was largely unrelated to the failure stress of the wall. Table 4 Details of wall specimens Dimensions of wall (m) Mortar Slenderness No. of Bonding Height Width Thickness ratio (h/t) proportion specimens details (h) (w) (t) Cement:Sand : 8 1 stretcher bond : 8 1 stretcher bond Table 5 Compressive strength of stone masonry walls Mortar compressive strength: 2.80 MPa Specimen h/t Load Compressive No. ratio (kn) strength (MPa) Failure type Cantilever mode Failure initiation due to buckling as a pinned end column 236

5 Venu Madhava Rao, Venkatarama Reddy, Jagadish 4.2 Analysis of the failure The failure pattern of the walls made it clear that the development of flexural stresses due either to eccentricity or to the process of buckling must have caused the failure. It is now possible to determine a plausible eccentricity measure assuming that the flexural bond strength of the masonry is known. It is possible to conceive of two types of eccentric loading: (a) The axis of load is inclined to the vertical axis of the wall the maximum flexural stress occurs at the bottom of the wall. (b) The axis of load is parallel to the vertical axis of the wall, but there is a constant eccentricity e. (A) Inclined loading Let the inclination of the loading with the vertical axis be α and the load be P. If the flexural bond strength is f and the wall area is A, then: f = 6Pl tan α bd P 2 A Substituting the values P = kn, A = m 2, b = m, d = 0.16 m and l = 2.85 m, we obtain α = The exceedingly small value of α suggests that this could be a possible failure mode. The failure of the first wall indeed initiated at a joint very close to the base of the wall. (B) Vertical but eccentric loading Let the eccentricity be equal to e. The extreme fibre tension due to eccentric loading can be calculated as: Pe 6 P = f 2 bd A assuming that f equals the flexural bond strength (0.12 MPa) and substituting the other values for wall specimen 2, P = kn, A = m 2, b = 0.90 m, d = 0.16 m and e = m. This shows that an eccentricity of 23 mm is adequate to cause failure due to flexural bond in a wall of 160-mm thickness. The difficulties in applying the load truly at the centre and the undulating surfaces of the wall resulting in a shift of the centroid explain the occurrence of eccentricity. The average wall strength is 0.9 MPa. This is very low compared to the prism strength values. In the case of stone masonry prism, the ultimate failure was due to the vertical splitting (material failure) of stone in combination with shear bond failure. However, when the stone masonry failed, the failure was entirely due to loss of bond between the stone and mortar. The stone block did not show any cracking. It is therefore clear that prism strengths and the strength measurement of stone are not very useful in assessing the strength of stone masonry walls. In the case of brick masonry, the wall strength could be predicted using prism strength values with a correction factor accounting for eccentricity and the slenderness ratio. This is mainly due to the fact that both the brick wall and the brick prism ultimately fail due to material failure. Brick masonry walls fail in the buckling mode without causing material failure only when slenderness ratios are larger than 30 and 35 [8]. In the present case of stone walls, a slenderness ratio of the order of 20 is incapable of causing material failure. Further studies need to be carried out to confirm the possible use of flexural bond strength in estimating or predicting stone masonry wall strength. 5. CONCLUSIONS The following conclusions emerge from the investigations on the strength of stone and stone masonry. (i) The strength of granitoid-gneiss stone is relatively higher when loaded parallel to the mineral bands. The average compressive strength of the stone when the load is parallel to and perpendicular to the bands is MPa and 86.1 MPa, respectively. (ii) The strength of masonry increases with an increase in mortar strength. Stone masonry strength is again higher when loaded parallel to the mineral bands. The masonry efficiency varies between 0.32 and (iii) The flexural bond strength of stone masonry increases with an increase in the strength of cement mortars. Combination mortars, such as 1:1:6 soilcement mortar, provide better bond strength compared to lean cement mortars such as 1:8. (iv) The average strength of a stone masonry wall is 0.9 MPa. When the slenderness ratio is 18, failure appears to be initiated due to the loss of flexural bond strength under eccentric loading. REFERENCES [1] IS:1121, Indian Standard Code of Practice for Method of Test for Determination of Strength Properties of Natural Building Stones: Part 1: Compressive Strength (Bureau of Indian Standards, 1976). [2] Mohamedbhai, G.T.G., Chan Chin Yuk, C.W. and Baguant, B.K., Use of stone masonry in housing construction in Rodrigues, Proceedings of 3rd International Seminar on Structural Masonry for Developing Countries (Mauritius, 1990) [3] ASTM E447, Standard Test for Compressive Strength of Masonry Prisms (American Standards for Testing and Materials, 1984). [4] IS:1905, Indian Standard Code of Practice for Structural Use of Unreinforced Masonry (Bureau of Indian Standards, 1987). [5] National Building Code of India (Bureau of Indian Standards, 1970). [6] Venu Madhava Rao, K., Some studies on flexural and compressive strength of masonry, M. Sc. (Engg.) Thesis, Dept. of Civil Engineering, Indian Institute of Science, Bangalore, India, [7] Venkatarama Reddy, B.V., Studies on static soil compaction and compacted soil-cement blocks for walls, Ph. D. Thesis, Dept. of Civil Engineering, Indian Institute of Science, Bangalore, India, [8] Hendry, A.W., Structural Masonry (MacMillan Education Ltd., London, 1990). [9] Shrinivas Rao, S., Studies on soil-cement blocks and block masonry, M. Sc. (Engg.) Thesis, Dept. of Civil Engineering, Indian Institute of Science, Bangalore, India,