SHEAR STRENGTH VARIATION DUE TO MORTAR STRENGTH VARIATION ... · SHEAR STRENGTH VARIATION DUE TO...

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15 th International Brick and Block Masonry Conference Florianópolis – Brazil 2012 SHEAR STRENGTH VARIATION DUE TO MORTAR STRENGTH VARIATION AND THE USE OF A TRIPLET SHEAR TEST SET-UP Vermeltfoort, A.T 1 1 PhD, Associated Professor, Eindhoven University of Technology, Department of the Built Environment, [email protected] ABSTRACT Together with a large masonry wall test program control tests to determine the mortar compressive and the masonry shear strength were conducted. It was shown that even for industrially made mortars the mortar compressive strength varied considerably. However, no clear relationship between mortar compressive and masonry shear strength could be established. Therefore a series of tests was performed to investigate the relationship between mortar compressive and masonry shear strength by varying the mortar properties under laboratory conditions. Mortar tests were performed according to EN 1052-11 and shear triplets were made using one type of soft mud bricks to be tested according to EN 1052-3. Some critical features of the used shear test set-up are discussed and suggestions for improvement are given. Results of the tests in which the mortar properties were deliberately varied are discussed. For comparison, the experiences with more than 400 masonry shear tests, performed earlier, are briefly summarized. Keywords: Triplet, shear test, mortar properties, laboratory made mortar, shear strength, fracture surface. INTRODUCTION In earlier studies mortar prisms and shear-triplets were made and tested together with the building of masonry walls for lintel-tests (Vermeltfoort and Martens, 2009). About four hundred mortar prisms were tested according to EN 1052-11 to validate the mechanical properties of the industrially made masonry mortar used, (Vermeltfoort 2010). A series of tests was performed in which the mortar properties were varied by adding sand to establish the strength variation due to a deliberate variation of mortar properties. In these earlier tests of Vermeltfoort (2010) shear strength increased and mortar compressive strength decreased when more sand was added. Experiments conducted earlier showed no clear relationship between mortar compressive and masonry shear strength. Therefore, the relationship between mortar compressive strength and shear strength was investigated again but with different conditions. A new series of tests was performed using mortars that were made in the laboratory by mixing sand and binder in a S/B ratio of 10 to 3 ratio. The binder was a mixture of cement and hydrated lime and their ratio (C/B) was varied. Table 1 shows the quantities used per batch. Table 1 Cement and lime quantities in kg used per batch of 13 kg. Mortar type M1 M2a M2b M3 M4 M5 Cement 0 1 1 1.5 2 3 Limes 3 2 2 1.5 1 0 Sand 10 10 10 10 10 10

Transcript of SHEAR STRENGTH VARIATION DUE TO MORTAR STRENGTH VARIATION ... · SHEAR STRENGTH VARIATION DUE TO...

Page 1: SHEAR STRENGTH VARIATION DUE TO MORTAR STRENGTH VARIATION ... · SHEAR STRENGTH VARIATION DUE TO MORTAR STRENGTH VARIATION AND THE USE OF A TRIPLET SHEAR TEST SET-UP Vermeltfoort,

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

SHEAR STRENGTH VARIATION DUE TO MORTAR STRENGTH VARIATION AND THE USE OF A TRIPLET SHEAR TEST SET-UP

Vermeltfoort, A.T1 1 PhD, Associated Professor, Eindhoven University of Technology, Department of the Built Environment,

[email protected]

ABSTRACT Together with a large masonry wall test program control tests to determine the mortar compressive and the masonry shear strength were conducted. It was shown that even for industrially made mortars the mortar compressive strength varied considerably. However, no clear relationship between mortar compressive and masonry shear strength could be established. Therefore a series of tests was performed to investigate the relationship between mortar compressive and masonry shear strength by varying the mortar properties under laboratory conditions. Mortar tests were performed according to EN 1052-11 and shear triplets were made using one type of soft mud bricks to be tested according to EN 1052-3. Some critical features of the used shear test set-up are discussed and suggestions for improvement are given. Results of the tests in which the mortar properties were deliberately varied are discussed. For comparison, the experiences with more than 400 masonry shear tests, performed earlier, are briefly summarized.

Keywords: Triplet, shear test, mortar properties, laboratory made mortar, shear strength, fracture surface. INTRODUCTION In earlier studies mortar prisms and shear-triplets were made and tested together with the building of masonry walls for lintel-tests (Vermeltfoort and Martens, 2009). About four hundred mortar prisms were tested according to EN 1052-11 to validate the mechanical properties of the industrially made masonry mortar used, (Vermeltfoort 2010). A series of tests was performed in which the mortar properties were varied by adding sand to establish the strength variation due to a deliberate variation of mortar properties. In these earlier tests of Vermeltfoort (2010) shear strength increased and mortar compressive strength decreased when more sand was added. Experiments conducted earlier showed no clear relationship between mortar compressive and masonry shear strength. Therefore, the relationship between mortar compressive strength and shear strength was investigated again but with different conditions. A new series of tests was performed using mortars that were made in the laboratory by mixing sand and binder in a S/B ratio of 10 to 3 ratio. The binder was a mixture of cement and hydrated lime and their ratio (C/B) was varied. Table 1 shows the quantities used per batch. Table 1 Cement and lime quantities in kg used per batch of 13 kg. Mortar type M1 M2a M2b M3 M4 M5 Cement 0 1 1 1.5 2 3 Limes 3 2 2 1.5 1 0 Sand 10 10 10 10 10 10

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THEORY Well designed masonry is loaded in compression perpendicular to the bed joints. Were possible, the direction of the bed joints is adapted to the line of thrust, e.g. in arches. However, it is not always possible or practical to build masonry with bed joints in the ideal direction. The load bearing capacity of masonry depends on the direction of the compressive stresses in relation to the direction of the bed joints. Using the components perpendicular to the bed joint (σ) and parallel to the bed joint (τ) of the applied compressive stress the failure envelope shown in Figure 1 is found. Four areas can be recognized: (A) tensile bond failure, (B) shear bond failure, of main interest in this study, (C) tensile brick strength failure and (D) compressive failure.

Figure 1 Masonry failure envelope, see e.g. Hendry (1990) or Drysdale et al. (1994).

Stress distribution and test set-up Different types of specimen have been used to establish the shear strength along mortar bed joints. Reviews have been published by Jukes and Riddington (1997), Abdou et al. (2006), Hendry (1990) and Drysdale (1994). The problem in this type of testing is to apply a reasonable uniform distribution of both shear and normal stress. Therefore, the shear force is applied as close as is possible to the joints, Edgell (2005) to minimize bending effects. A uniformly distributed shear stress load is almost impossible to apply. Therefore, shear test set-ups were developed by Van der Pluijm (1993), RILEM (1996) and Popal and Lissel (2010) and the idea behind all these set-ups was to apply the load (pre-stress and shear stress) as uniform as possible and in some cases to perform the tests with controlled deformation, like Van der Pluijm (1993) did. Figure 2 shows the expected stress distribution according to simulations of Popal and Lisel (2010).

Figure 2 Stress distribution in bed joint under shear and pre-compression (Popal & Lissel, 2010). However, some moment is introduced at the joint and stress concentrations occur close near the load introduction points. Van der Pluijm (1993) further postulates that introducing a pure shear stress distribution is nearly impossible. When applying a shear stress some variation in

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normal stress is introduced. Hofmann et. al. (1990), Jukes and Riddington (2000), and more recent, Popal and Lissel (2010) presented several test arrangements and showed, by means of FE calculations, that in all the simulated arrangements, both shear and normal peak stresses occur. Vermeltfoort (2010) discussed results of detailed measurements, which indicated that bending caused cracking at the bottom of the specimen. This process happened within one measuring cycle of one second, indicating that deformations are relative small as the loading speed of the vertical jack was 0.01 mm/second. After the first crack, the load resumed to increase as the jack moved further and then another peak level was reached. Cracking at the bottom, observed with laser speckle interferometry as discussed in Vermeltfoort (2010), indicates that even though the line of thrust is almost parallel to the brick mortar interface, some bending effects occur. Tests have shown that at zero pre-compression the variability in the results is too high for a standard test, Edgell (2005). Consequently the accepted procedure is to use more than one pre-compression and to plot the earlier mentioned Coulomb type relationship (area B in Figure 1) to establish initial shear strength (τini). MATERIALS AND SPECIMENS

Unit properties Walls and shear specimens were made with only one type of soft mud clay bricks, brand Rijswaard, with a sanded surface, Vermeltfoort (1997). The main brick properties were (in average): dimensions 206x96x50 mm3, free water absorption 15.5 mass%, dry density 1630 kg/m3, compressive strength 27 MPa and splitting strength ± 2MPa. Bricks were conditioned by submerging them in water for 200 seconds. This resulted in an absorption rate of 1.5 kg/m2 per minute.

Mortar constituents In this project, the mortar was made in the laboratory using sand, cement and lime. Sand of Dutch origin was used. The main component of this type of sand is Quarts (SiO2), with a volumetric mass of 2.65 gr/cm3. Sand grading is shown in Table 1. So called “Mekal luchtkalk” was used (Ca(OH)2. This type of lime contains 70% active calcium hydrate and the fineness is characterized by the fact that 99% or more passes a sieve with openings of 0.2 mm. The lime was produced by Carmeuse Natural Chemicals. ENC I 42.5 N cement was used. This type of Portland cement was produced by ENCI BV, a firm part of the Heidelberg Cement Group. Table 2. Sand grading Sieve 0.125 0.25 0.5 1 2 4 8 16 Proportion % m/m passing 0.20 5.20 45 86 96 100 100 100

Preparation of mortar specimens Mortar was prepared in batches of 13 kg with a target value for the mortar flow of 170 mm. Per mortar type, three 40x40x160 mm3 prisms were made according to EN 1015-11 using a

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steel mould. Then the filled mould was covered with polyethylene sheets for at least one day after which the prisms were taken out of the mould and stored at 100% RH and approximately 20°C. Seven days after making, the prisms were moved to a climate chamber at 60% RH and 20°C where they were stored until testing. The mortar prisms were tested in flexure and subsequently the two rest pieces were tested in compression via a steel platen of 40 x 40 mm2, according to EN 1052-11. The specimen’s age at testing varied between 56 days and 70 days.

Preparation of shear specimens Shear specimens were made in the same way as larger wall-type specimens. Shear specimens were built of three clay brick units with two (bed) joints. Loose material (sand) was wiped off with a broom. The specimens were constructed according to EN-NEN 1052-3. After building, the specimens were stored under more than 90% RV. This condition was obtained by covering the specimens with damp clothes and wrapping them in airtight polyethylene sheets. After a few hours, condensation was observed inside the cover indicating that the RV inside was above 90% as required. Seven days after building the shear specimens were unwrapped and subsequently stored in a climate room under 60% RH and 21 °C. MORTAR TEST RESULTS The results of tests on mortar samples are given in Table 3 and the Modulus of rupture (frup) was plotted versus C/B ratio in Figure 3. Compressive strength (f’d) was plotted versus C/B ratio in Figure 4. In Figure 4 also the results from Cizer et al. (2008) and Hoath et al. (1970) are indicated. Table 3 Mortar properties Mortar type

proportion of binder Sand Cement Lime frup StDev # f'd StDev #

kg kg kg MPa MPa MPa MPa M1 10 0 3 0.42 0.10 3 0.75 0.03 6 M2a 10 1 2 0.87 0.20 6 2.84 0.79 12 M2b 10 1 2 0.48 0.08 3 2.33 0.13 6 M3 10 1.5 1.5 2.19 0.20 3 8.96 0.22 6 M4 10 2 1 3.35 0.23 3 10.59 1.06 6 M5 10 3 0 4.03 0.39 3 14.38 1.11 6 *) batches of 13 kg per mortar type were prepared.

Studies on mortar recipes and consequences on compressive strength Hoath et al. (1970) and Cizer et al. (2008) studied the properties of blended lime-cement mortars for conservation purposes. In the mortars used, the sand-binder ratio was equal and the cement-lime ratio in the binder varied. Both flexural and compressive strength were established for mortars with varying amounts of lime. From the graphs in Hoath et al. (1970) and in Cizer et al. (2008) values for compressive strength after 28 days were derived and plotted versus C/B ratio in Figures 3 and 4.

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Figure 3 Modulus of rupture versus cement / binder (C/B) ratio.

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Figure 4 Compressive strength versus cement / binder ratio. Comparison of mortar M1-M5 results with results from Hoath et al. (1970) and Popal and Lissel (2010). A linear descending trend for mortars with more than approximately 25% cement (C/B > 0.25). As expected, a decreasing trend was found with a decreasing cement content both for bending and compressive strength. A linear relationship between mortar compressive strength and cement-binder ratio may be assumed when the cement-binder ratio is above approximately 25%. The relationship for poor cement mortars is different. Also the failure behaviour in compression of mortars with low cement ratio is different from that of cement rich mortars. SHEAR TEST SET-UP AND COURSE OF TESTS

Performing the test Shear strength was established according to NEN EN 1052-3 using the set up shown in Figure 6. The brick in the middle is sheared. Therefore, the two outer bricks are supported at the bottom while the one in the middle is vertically loaded on top, Figure 5a. A varying pre-compression load is applied perpendicular to the shear surface, i.e. horizontally. Three pre-compression levels, 0.2 MPa, 0.6 MPa and 1.0 MPa, were applied. Soft board sheets, 10 mm in thickness, were used to distribute the load. The load introduction and support elements had possibilities to rotate in several directions and allowed for some adjustment for warp and dimensional variation in the specimens. Figure 5 shows some details of the set-up. The pre-compression load was applied via a 10 mm thick soft board layer which allowed for some (vertical) movement of the specimen under shear loading due to closing of unintended openings between the contact shear loading supports. First pre-compression was applied with a hydraulic jack and a manually operated pump to the required level. Then the shear load was applied using a separate hydraulic system, with two other jacks. The jack used to apply the shear load was connected with a similar second jack. This second jack was placed in a loading machine. This allowed the use of the equipment of the loading machine to control the increase of the shear load. The vertical jack that produced the shear loading was moved with a speed of 3mm/min. Each test took about 300 seconds in total. After shear occurred, the test was continued for approximately 100 seconds to obtain friction values.

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Figure 5 Parts of shear equipment in detail. The pre-compression loads fluctuate most when the specimen cracks and shearing starts. Therefore, while testing, both vertical shear load and horizontal pre-compression were measured every second and plotted versus time. Figure 5b shows an example. Maximum shear load, friction load and corresponding pre-compression loads were derived from these graphs as described in detail in Vermeltfoort (2010). These four forces per test were used for further analyses. In some of the tests described in this paper, first the load dropped and instantly resumed until the second drop occurred, usually at a higher load level than the first drop. After the second drop the shear-load remained more or less constant. The first and second dropping of the load indicates fracture of the joints. However, cracks were hardly visible, certainly not when the first crack occurred. The two-peak-phenomenon occurred more often; usually the first peak was lower than the second one.

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Figure 6 a) Shear test set up b) Pre-compression and shear loads versus time. SHEAR TEST RESULTS For each series, shear-pre-stress relationships were established, both for the initial shear strength and for the friction shear values, using the four forces mentioned earlier. Stress-calculations were based on a loaded gross area with dimensions of the bricks used, i.e. in average 96 x 206 mm2. The net area is usually smaller and varies in shape and dimensions, Figure 8. However, the ratio between pre-stress and shear stress remains the same when either the gross area or the net area is used in the stress calculation.

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The shear and pre-stress loads were plotted versus time and ultimate shear and friction loads established. When for each group of specimens the results per test are plotted in a graph, the two shear-strength functions can be characterized with three key-values: a) the value for the initial shear strength, b) the slope in the ‘initial’ relationship and c) a friction coefficient, i.e. the slope in the friction relationship. The shear test results are given in Table 4. The shear load versus time graphs for two mortar types are shown in Figure 7.

Figure 7 Load deformation diagrams for a) mortar M1 (100% lime) and b) mortar M5 (100% cement). Nine tests per mortar type.

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The difference in the failure load per group of three tests, depending on the pre-load and mortar type is clear. Also significant is the difference in failure speed. For mortar type 1 (little cement, small strength) the shape of the curve is more rounded than the shape of e.g. mortar type 5 (much cement, higher strength). Some graphs have two peaks; this may be caused by different strength of the two joints of the specimens as discussed earlier, indicating a strength difference in both joints of one specimen. The saw-tooth-shape of the curves for the higher pre-loaded specimens of mortar type 1 is caused by the fact that while cracking the pre-stress decreased. However, it was tried to keep this load as constant as possible using a manual pump. Each time a drop in the pre-load was observed the load was increased. The bond of this mortar type was relatively poor. Under maximum shear load and pre-stress the mortar crumbled and the section decreased in size. This required constant pumping of the pre-stress jack to keep the pre-load on the required constant level. However, as the loaded section decreased in size the actual pre-stress increased for these relatively soft, lime-rich, mortars. The richer cement mortars itself were stronger, their bond with brick however was the weakest link. Their friction surface remained more or less the same.

Failure patterns Three shear failure modes are recognized in triplet specimens, Edgell (2005) and EN-NEN 1052-3. Shear failure may occur in a) the unit/mortar bond area, either on one or divided between two unit faces, b) only in the mortar, or c) in the unit. The shear failure mode for the brick-mortar interface of cement rich mortars is of type a), characterized by shear slip along the bed joints as a result of the sanded brick surface, while the mortar fails for the lime rich mortars, i.e. failure mode b).

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It is noted that when shear failure occurred in the mortar-brick bond area, this was either on one or divided between two unit faces, like in Edgell (2005) on page 117. Shear failure in the mortar only occurred in the triplets made with the weakest mortar. Shear failure in the unit only (type c mentioned above) was not observed. Figure 8 shows typical fracture surfaces. As mentioned earlier, the lime rich mortars crumble while the brick-mortar interface of the cement rich mortars failed.

Figure 8 Failure mode b) for specimens of mortar type M1, a lime rich mortar.

Shear strength versus pre-compression For each type of mortar the shear strength was plotted versus the applied pre-stress and key values established using linear best fit relationships. The key values per mortar type, like initial shear strength, coefficient of friction and R2 values are given in Table 3.

Table 4 Results shear tests, τini, α and R2. S:C:L τini α R2 friction coef. M1 0 0.08 0.46 0.97 0.48 M2a 10:2:1 0.09 0.66 0.88 0.75 M2b 10:2:1 0.22 0.61 0.90 0.77 M3 10:1.5:1.5 0.33 0.71 0.88 0.82 M4 10:1:2 0.25 0.68 0.97 0.84 M5 10:0:3 0.21 0.69 0.90 0.80

α = coefficient in τ = τini + α∗σ The initial shear strength is plotted versus cement / binder ratio in Figure 9a and versus mortar compressive strength in Figure 9b. In both cases, the graphs show a maximum value for the 50/50 cement-lime mortar. The mean value for the friction coefficient was 0.80 when the M1 result is omitted. The M1 friction value was considerably smaller than those of the other mortars with more cement; similar to the α-value in Table 3. The friction values are more or less the same for mortars with more than a minimum amount of cement, i.e. C/B > 0.25. Probably this is due to the two different types of failure discussed earlier. In Figure 8b also the data from Vermeltfoort (2010) are plotted. These data show a different trend than the mortars in this paper. A larger amount of sand added to the pre-fab factory mortar i.e. less binder, showed an increase in initial shear strength with smaller compressive strength with smaller compressive strength. The compressive strength of the mortar of Vermeltfoort (2010) was relatively small compared to the laboratory made blended mortars in this project. Adding sand to a pre-fabricated factory made mortar not only reduces the C/B ratio but also the proportion of additives in the mixture. This may have changed other

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properties as well. Two types of mortar (factory made mortars with sand added Vermeltfoort (2010) versus laboratory made mortars) clearly show different behavior as shown in Figure 9.

Figure 9 Initial shear strength a) versus cement proportion and b) versus mortar compressive strength DISCUSSION AND CONCLUSION Modulus of rupture and mortar compressive strength decrease with reduction of the amount of cement in the mortar. The failure mechanism for mortars with little cement is different from the mechanism for mortars with much cement. The found relationship between mortar compressive strength and cement amount is confirmed by results from other research, like Hoath et al. (1970) and Cizer et al. (2008). The mean initial shear strength according to EN 1052-3:2001 varied with cement-lime ratio. More cement results in larger shear (and compression) strength. The mean slope in the Mohr-Coulomb relationships was 0.66. The friction coefficient (after failure) was 0.80. Comparable values were found by Vermeltfoort (2010). Peculiar is that below a cement/binder ratio of 25 to 30% the relation between mortar compressive strength and cement binder ratio is less steep compared to the relation of mortars with more cement. It seems that there is a critical minimum amount of cement in the mortar required to be fully effective. Below that value, the skeleton, formed by sand particles dominates mortar behaviour. Shear failure of the mortar occurs in lime rich mortar specimens contrary to brick-mortar interface failure in specimens with a higher cement proportion. A test set-up was built, according to EN1052-3, to allow for a quick and simple performance of the test. Soft board was used to introduce the pre-compression load. A movable vertical shear jack and adjustable supports, that also could rotate, allowed for vertical alignment after pre-compression was applied. Dimensional variation of a few mm’s was allowed while the position of the supports was adjustable by using shims. To reduce the amount of work, capping of the contact surfaces could be considered. The load-platens for the pre-compression load were mounted to the frame and it was assumed that the soft board would allow for some vertical movement. The pre-compression load was applied first and then the shear load was increased under constant pre-compression. Simultaneously increasing shear load and pre-compression load may give different results.

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REFERENCES Abdou L., Ami Saada, R., Meftah, F., and Mebarki, A., 2006, “Experimental investigations of the joint-mortar behaviour”, Mechanics research communications 33, pp. 370-384. Cizer O., Van Balen, K. and Van Gemert, D., 2008, “Blended lime-cement mortars for conservation purposes. Microstructure and strength development”, Struct. Anal. of Historic Construction, Taylor & Francis group, London, ISBN 978-0-415-46872-5, pp 965-972. Drysdale, R., Hamid, A.A. and Baker, L.R., 1994, “Masonry Structures, behaviour and design”, Chapter 5, Prentice Hall. Inc. ISBN 0-13-562026-0. Edgell, G., 2005, “Testing of Ceramics in construction”, Whittles Publishing Ltd., Scotland, ISBN 1-870325-43-5. Hendry, A.W., 1990, “Structural Masonry”, chapter 4, Macmillan Education LTD, London ISBN 0-333-49748-1 Hoath SB, Lee HN and Renton KH, “The effect of mortars on the strength of brickwork cubes”, Proceedings of 2nd IBMaC, 1970 pp 113 116. Hoffmann, P., Stockl, S. and Mainz, J., 1990, “A comparative finite element evaluation of mortar joint shear tests”, Masonry International, vol. 3(3), pp. 101-104. Jukes, P. and Riddington, J.R., 1997, “A review of masonry joint shear strength test methods”, Masonry International, 11, (2), pp. 37-43. Jukes and Riddington, “Finite element prediction of block triplet shear strength”, Proc. 12th IBMaC, Madrid, pp 1523..1531. Popal R. and Lissel, S., 2010, “Numerical evaluation of existing mortar joint shear tests and a new test method”, 8th Int. Mas. Conf. Dresden, pp. 1-8. Rilem, 1996, “Determination of shear strength index for masonry unit/mortar junction”, Materials and structures, Vol 29 Oct. 1996 pp 459 475. Van der Pluijm, R., 1993, “Shear behaviour of bed joints” Proc. 6th North American Masonry conference, pp 125 136. Vermeltfoort A.T. (1997) “Properties of some clay bricks under varying loading conditions” Masonry International, vol.10, no.3, pp.85-91. Vermeltfoort, A.T., 2010, “Variation in Shear properties of masonry”, 8th Int. Masonry Conference, Dresden. Vermeltfoort, A.T., Martens, D.R.W (2009), ”Variation in mechanical properties of mortar and masonry”, in Proc. 11th Canadian Masonry Symposium, W.W. El-dakhakhni, R.G. Drysdale (Eds.), McMaster University, (pp. 1-10). EN 1015-11 (1999) “Methods of test for mortar for masonry - part 11: Determination of flexural and compressive strength of hardened mortar” European committee for standardization, Brussels. EN 1052-3 (2001) “Methods of test for masonry – Part 3: Determination of initial shear strength” European committee for standardization, Brussels.