MECHANICAL PROPERTIES OF MASONRY SAMPLES FOR THEORETICAL MODELING

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1 MECHANICAL PROPERTIES OF MASONRY SAMPLES FOR THEORETICAL MODELING Sayari Arash PhD, Assistant Professor, Islamic Azad University, Sanandaj Branch, Iran, ABSTRACT Due to different geometries and material properties, masonry is considered as an anisotropic composite material. Mechanical properties of the masonry walls are very important parameters that affect the behaviour of masonry walls under loading. The mechanical properties of masonry are more complicated than mechanical properties of other construction materials. Elastic modulus (or Young s modulus) is one of the most important parameters in determination of the stiffness of structural elements prior to cracking and is calculated according to the linear part of stress-strain curves. In addition, in order to develop the theoretical modelling, mechanical properties including elastic modulus (Young s modulus) and compressive strength must be taken into account. In this research, different experiments are designed to measure the elastic modulus and compressive strength of masonry and mortar samples. The results are compared with the published results in this subject area. Keywords: Masonry, Mechanical properties, Elastic modulus, compressive strength INTRODUCTION Masonry is considered as an anisotropic composite material because of different geometries and material properties of masonry, including: shape of units (bricks/blocks), dimensions of units, perforations, slenderness ratio, strengths of materials, modulus of elasticity of materials, water absorption, etc. (Zilch et al., 21). The mechanical properties of masonry are more complicated than mechanical properties of other construction materials (Velazquez-Diams et al., 2 and 1998). Elastic modulus (or Young s modulus) is one of the most important parameters in determination of the stiffness of structural elements prior to cracking and is calculated according to the linear part of stressstrain curves. According to Wolde-Tinsae et al. (199), elastic modulus of masonry walls is not related to properties of brick units, mortar joints, grout or h/t ratio (of samples) individually; but is a function of all of mentioned parameters. Woulde-Tinsae concluded that it is the best option to calculate the value of elastic modulus according to the compressive strength of masonry samples, because compressive strength is also influenced by mentioned parameters. According to EC6, the compressive strength and elastic modulus of masonry samples should be determined from either results of or in the absence of tests in terms of equations 1 and 2 as below: = K.. (1)

2 Where: is characteristic compressive strength of masonry [N/mm²]; K: is a constant, which is a function of the type of masonry units and mortar is normalised average compressive strength of units [N/mm²]; is average compressive strength of mortar [N/mm²]; and : are constants, for general purpose mortar =.7 and =. and E=. (2) Where: E: is modulus of elasticity of masonry [N/mm²]; : is a constant equal to 1 according to UK National Annex to EC 6; is characteristic compressive strength of masonry [N/mm²]; For the linear and elastic behaviour of masonry walls, equivalent modulus of elasticity is a function of physical and mechanical characteristics of brick and mortar (Francis et al., 1971) given in equation : = ( + ) () =, = (4) Where is the elastic modulus of masonry; and are the thicknesses of mortar and brick, respectively; and are elastic modulus of mortar and brick respectively; and are the Poisson s ratio of mortar and brick, respectively According to Farshchi (28) for the lateral loading on masonry walls, elastic modulus can be calculated from equation 5. = (5) Where: : Elastic modulus of mortar = compressive strength of mortar; Elastic modulus of brick = 125 compressive strength of brick; : Elastic modulus of masonry sample; : Thickness of brick units; : Thickness of mortar joints; ICBO (1991) recommended the equation 6 for calculation of the elastic modulus of masonry walls. = 75 (6)

3 Where: : Elastic modulus of masonry; : Compressive strength of masonry walls; According to the above discussion, different researchers suggested different amount for mechanical properties of the masonry samples. In this research, different experiments are designed to measure the elastic modulus of masonry and mortar samples. In addition, according to direct relation between compressive strength and the elastic modulus, compressive strength of these samples is also measured. TEST SET UP FOR MEASUREMENTS OF ELASTIC MODULUS AND COMRESSIVE STRENGTH Three different categories of samples were constructed for measurement of the elastic modulus and compressive strength as below (Photo 1): ۰ Masonry cubes of 215mm 215mm 215mm dimensions. ۰ Mortar cubes of 1mm 1mm 1mm dimensions. ۰ Mortar cylinders with diameter equal to 15mm and height equal to mm. Masonry cubes Mortar cubes Mortar cylinders Photo 1: Samples used for experimental study According to UK National Annex (NA to BS EN :25) three types of mortar M2, M4 and M6 were used for production of the masonry and mortar samples (Table 1). The cement type was Portland cement and the type of lime was Hydraulic lime. Table 1. Different types of mortars (Table 2 of UK National Annex to EC6) a) MASONRY CUBES The frog type of London brick (Hanson, 21) and Ibstock brick were used for construction of the masonry cubes. Nine masonry cubes were constructed in the material laboratory with three different types of mortars (M2, M4 and M6) and two different types of bricks (Table 2). The thickness of the mortar layers between bricks rows was nominal 1mm.

4 To check possible variation of compressive strength, the masonry samples constructed from London bricks are tested at two different ages (4 days and 4 months). For Ibstock it was decided to use 4 months period of curing. No. Of Sample Table 2. Detail of the masonry samples Brick type Mortar Ratio (Sand; Cement; Lime) Mortar type according to EU6 1 London Brick 6,1,1 M4 4 2 London Brick 6,1,1 M4 4 London Brick 6,1,1 M4 4 4 Ibstock 8,1, M Ibstock 4,1, M Ibstock 6,1,1 M London Brick 8,1, M London Brick 4,1, M London Brick 6,1,1 M4 115 Curing period (Day) All samples were checked to have a horizontal surface using a bubble level. After the construction of each sample, it was left for more than 28 days to allow for curing and to achieve its maximum design capacity. Two different tests were performed for each masonry sample: direct measurement of elastic modulus and measurement of uni-directional compressive strength. In order to directly measure the elastic modulus of masonry samples, three pairs of bases for DEMEC gauges with 15 mm distance were attached to each sample using a special glue (Photo 2). Before attaching the bases to the samples, exact position of bases on the samples were indicated and using a steel brush loose and uneven areas were removed from the location of the DEMEC bases, and all dust was removed using a vacuum cleaner. 15 mm 5 mm 54 mm Photo 2. Attaching DEMEC bases to a sample After curing of the glue joining DEMEC bases to the sample, axial compressive load with the rate of 1kN/sec was applied to each sample. The load was applied in the steps of 1 kn, distributed on the surface of the sample. After each step of loading, the displacements were measured using a DEMEC gauge.

5 For all cubic masonry samples in Table 2, compressive strength was measured using a compression-testing machine. The load was applied at the same rate as before to the horizontal surfaces of the samples. b) MORTAR CYLINDERS Due to the required information for calculation of the elastic modulus of masonry according to equation and equation 5 it has been decided to conduct additional testing of cylinder mortar samples. Three mortar cylinders were constructed with type M4 of mortar (sand 6; lime 1; cement 1). All samples were checked to have a horizontal surface using a bubble level. After construction of each sample, it was left for more than 28 days for curing and developing its design capacity. Two different tests were performed on each sample: direct measurement of elastic modulus and measurement of uni-directional compressive strength. In order to directly measure the elastic modulus of mortar samples, axial compressive load with the rate of 1kN/sec was applied to each sample. The load was applied in the steps of 1 kn, distributed on the surface of the sample. After each step of loading, the deformation was measured using a mechanical strain gauge attached to the sample. For all samples, the compressive strength was measured using a compression-testing machine. The load was applied at the same rate as before to the horizontal surface of the samples. c) MORTAR CUBES Twelve mortar cubes (1mm 1mm 1mm) were constructed with three different types of mortars. Six samples were of type M4, three of M6 and three of M2 (Table ). To check possible variation of compressive strength, the cubic samples constructed from type M4 of mortar are tested at two different ages (4 days and 4 months). For types M2 and M6 of mortar, it was decided to use 4 months period of curing. Table. Detail of the mortar cubes Mortar Rate (Sand, Mortar type Cement, Lime) according to EU6 1 6,1,1 M ,1,1 M4 4 6,1,1 M ,1,1 M ,1,1 M ,1,1 M ,1, M ,1, M ,1, M ,1, M ,1, M ,1, M2 115 No. of Samples Curing period (Day)

6 Comressive strength(n/mm²) Elastic modulus (N/mm²) 15 th International Brick and Block For all samples in Table, the compressive strength was measured using a computerised compressive testing machine. The load was applied at the rate of 1 kn/sec on the horizontal surface of the samples. ANALYSIS OF THE RESULTS OF MASONRY SAMPLES For the samples that were constructed from London bricks and mortar type M4, the average elastic modulus is 75 N/mm² (Figure 1). In addition, the average value for compressive strength for these samples is.7 N/mm² (Figure 2). Masonry sample with 4 months curing had 6% higher strength in comparison to the average strength of the samples with 4 days curing. 4 London Brick, Mortar M Average=751. N/mm² 1 Sample 1 Sample 2 Sample Sample 4 Figure 1. Elastic modulus of masonry samples (London Brick, Mortar M4) 4.7 London Brick, Mortar M Average=.7 N/mm² Sample 1 Sample 2 Sample Sample 4 Figure 2. Compressive strength of masonry samples (London Brick, Mortar M4) Average value of elastic modulus and compressive strength of different masonry samples obtained from the experimental study are presented in Table 4. The results show that elastic modulus and compressive strength of masonry samples constructed with London bricks and mortar type M6, are larger than those of other masonry

7 Compressive strength (N/mm²) Elastic modulus (N/mm²) 15 th International Brick and Block samples. In addition, the masonry samples constructed with London bricks and mortar type M2, showed the lowest elastic modulus and compressive strength compared to the other masonry samples. Table 4. Elastic modulus and compressive strength for masonry samples (Test) Bricks Mortar Elastic modulus (N/mm²) Compressive strength (N/mm²) London brick M Ibstock M Ibstock M Ibstock M London brick M2 12. London brick M ANALYSIS OF THE RESULTS OF CYLINDER MORTAR SAMPLES The average value for elastic modulus and compressive strength of cylinder mortar samples (M4) are 287 N/mm² and.6 N/mm², respectively (Figures, 4). 4 Cylinder, Mortar M Average=287.8 N/mm² 1 Sample 1 Sample 2 Sample Figure. Elastic modulus of mortar cylinders (Mortar M4) 4 Cylinder, Mortar M Average=.6 N/mm² 1 Sample 1 Sample 2 Sample Figure 4. Compressive strength of mortar cylinders (Mortar M4) ANALYSIS OF THE RESULTS FOR CUBIC MORTAR SAMPLES For three different types of mortars M2, M4 and M6; the average uni-directional compressive strengths for cubic samples are presented in Figures 5 to 7 and Table 5.

8 Compressive strength (N/mm²) Compressive strength (N/mm²) Compressive strength (N/mm²) 15 th International Brick and Block 4 Cubic samples, Mortar M Average=.65 N/mm² 1 Sample 1 Sample 2 Sample Sample 4 Sample 5 Sample 6 Figure 5. Compressive strength of cubic mortar samples (Mortar M4) 5 Cubic samples, Mortar M Average=5. N/mm² 1 Sample 1 Sample 2 Sample Figure 6. Compressive strength of cubic mortar samples (Mortar M6) 2.6 Cubic samples, Mortar M Average=2.6 N/mm² Sample 1 Sample 2 Sample Figure 7. Compressive strength of cubic mortar samples (Mortar M2) Table 5. Compressive strength of cubic mortar samples (Test) Mortar Compressive strength, N/mm² M6 5. M4.65 M2 2.6

9 Elastic modulus (N/mm²) 15 th International Brick and Block Comparision of the above results show that mortar cubes M6 have higher value of compressive strength than other cubic samples. APPLIED ELASTIC MODULUS For calculation of the elastic modulus of masonry according to Francis (1971) in the equation and Farshchi (28) in the equation 5, the value of elastic modulus for mortar is assumed to be N/mm² which is the estimated value from our experiments. In addition, value of elastic modulus for the brick according to Farshchi is assumed to be 125 times of the compressive strength of the brick. As the compressive strength of London brick is 25 N/mm² (Hanson, 21), thus elastic modulus of this type of brick is equal to 125 N/mm². The elastic modulus for masonry walls (constructed from London bricks and mortar type M4) achieved from different sources is presented in Table 6. Table 2. Elastic modulus of masonry from different sources Source Elastic Modulus of masonry, E (N/mm²) Equation Direct measurement 75 - (Figure 1) EC6 E= 1. = 1. 7= 7 2 ICBO (1991) = = Francis (1971) = + +2 ( + ) E= 696 Farshchi (28) = E= 65 5 The elastic modulus measured in this research is closest to the one calculated by equation suggested by EC6 (Figure 8) Direct measurment EC 6 ICBO (1991) Francies Source (1971) Farshchi (28) Figure 8. Comparison of the elastic modulus from different sources for masonry samples

10 CONCLUSION The main results of this study are summarised as follows: 1. Estimation of elastic modulus for masonry samples and mortar is confirmed to be close to the suggested values from EC6. 2. The results for ultimate compressive strength of the mortar M4 via cylinders and via cubes are almost identical which is different from testing of corresponding concrete samples. REFERENCE ۰ EC6, EN :25 Eurocode 6, (25). Design of masonry structures-part 1-1: General rules for reinforced and unreinforced masonry structures. CEN. ۰ Farshchi, D.M., Motavali, M., and Marefat, M.R., (28). A theoretical investigation on the seismic retrofitting of historical masonry buildings using FRP post-tensioned systems. PhD thesis, Tehran university. ۰ Francis, A.J., Horman, C.B., and Jerrems, L.E., (1971). The effect of joint thickness and other factors on the compressive strength of brickwork, proceeding of the 2ed international brick masonry conference, H.W.H. west, ed, British Ceramic Association, Stoke on Trent, 1-7, UK. ۰ Hanson, (21). Guide to London Brick., Available at < [Accessed on 1, September 21], Hanson building products. ۰ ICBO Evaluation Services, Inc., (1997). Acceptance criteria for concrete and reinforced and unreinforced masonry strengthening using fibre reinforced composite system. ACI 25-R2-497 (BCG/BNH), International conference of building officials, Whittier, California. ۰ UK National Annex to Eurocode 6, (25). Design of masonry structures Part 1-1: General rules for reinforced and unreinforced masonry structures. (NA to BS EN :25). ۰ Velazquez-Dimas, J.I., Ehsani, M. R., and Fellow, (2). Modelling out-of-plane behaviour of URM walls retrofitted with fibre composites. Journal of composites for construction, November 17. ۰ Velazquez-Dimas, J.I., (1998). Out-of-plane behaviour of URM walls retrofitted with fibre composites. PhD Thesis, Faculty of civil engineering and engineering mechanics, The University of Arizona, USA. ۰ Wolde-Tinsae, A.M.R., Atkinson H., and Hamid A. A., (199). State-of-the-Art Modulus of Elasticity of Masonry. In Proceedings of Sixth North American Masonry Conference. The Masonry Society. Boulder, Colorado. USA. ۰ Zilch, K., Schatz, M., (21). Masonry construction manual. Published by institute fur international architektur documentation GmbH, Munich, pages