L IST OF S YMBOLS
Chapter 4 E XPERIMENTAL S TUDY ON URM W ALLS
4.2. T ESTS ON M ASONRY C ONSTITUENTS
The analytical and numerical evaluation of masonry walls and buildings are largely dependent on the various material parameters under the action of different loading conditions like tensile, compressive, shear, etc. Therefore, necessary tests were conducted to determine the various material properties of masonry, and the non-linear stress-strain curve was determined wherever necessary. At least seven sets of specimens were used to conduct each test so that the average values could be obtained with acceptable confidence.
4.2.1. Compressive Test on Bricks
Bricks are the major constituent materials of masonry walls and buildings. The solid fired clay bricks tested were the first-class bricks and had an average size of 220 mm × 110 mm
× 70 mm. The WA (Water Absorption) and IRA (Initial Rate of Absorption) was measured following the code IS 3495 (BIS, 1992) and ASTM C67-13 (ASTM, 2013b). The average water absorption capacity of the bricks was measured to be about 13%, and the average IRA was measured to be 2.35 kg/m2/min. The compressive strength was determined by placing the brick under direct uniaxial compressive loading (Figure 4.1) with a displacement rate of 0.16 mm/s which is as per the guidelines provided in ASTM C67-13 (ASTM, 2013b) and IS 3945 (BIS, 1992).
Figure 4.1. Compression test of bricks.
The frog of the brick was initially filled with mortar and the brick specimen was encased between soft plywood plates. The setup was then placed in the compressive testing machine between rigid steel plates with fog facing upwards. The average compressive strength achieved was about 21.20 MPa.
4.2.2. Compressive Test on Mortar
It is a common practice in India to use cement: sand mix mortar grade of ratio 1:6. As per ASTM C109/C109M-13 (ASTM. 2013b) and IS 2250 (BIS, 1995) the size of the mortar cube was fixed as 50 mm × 50 mm × 50 mm (Figure 4.2). The tests were carried out after 28 days of curing the mortar cubes. Before testing, the surfaces of the mortar cube and the bearing plates of the actuator were wiped clean to remove any unwanted loose particles.
The test was conducted with a slow loading rate of 0.11 mm/s under compression testing machine, and the average compressive strength of the mortar cube was found to be 6.56 MPa.
Figure 4.2. Compression test of cement mortar cubes.
4.2.3. Triplet Shear Test
Triplet shear test was conducted to determine the initial shear capacity of the mortar bond in zero compressive stress. The test was conducted as per the guidelines in BS-EN-1052 (2002). Before testing, the triplets constructed using 1:6 mortar grade were cured for 28 days. Two different sets of Triplets were constructed: one set was constructed in accordance with BS-EN-1052 (2002) represented as Triplet Shear Setup 1 (Figure 4.3a), and the other set constructed by shifting the central brick by about 10 mm represented as Triplet Shear Setup 2 (Figure 4.3b). The second set of arrangement was made because it requires much
less experimental setup, and therefore, it is easier to perform the test. The specimens were loaded up to failure under hydraulic pressure (Figure 4.3c).
(a) (b) (c)
Figure 4.3. Triplet Shear Test: (a) Setup 1, (b) Setup 2, and (c) Failure mechanism observed.
The initial shear strength (fvko) was calculated using the Eq. 4.1 as recommended in BS- EN-1052 (2002).
vko 2
p
f F
= A (4.1)
Where F is the peak shear strength obtained from the test, and Ap is the cross-sectional area of the specimen parallel to the bed joint. The first setup resulted in initial shear strength of 0.176 MPa, whereas the second setup resulted in a value of 0.164 MPa. The average value of the initial shear strength was considered as 0.17 MPa.
4.2.4. Tensile Bond Test
It has been observed from the past URM buildings that the failure occurs mainly at the joints between the mortar and the bricks. Hence, Khalaf (2005) suggested a procedure to determine the tensile bond strength between the mortar and the bricks (Figure 4.4). Khalaf assumed two types of stress distribution, one being linear and other being parabolic. For the present study, the linear stress distribution was considered since there was not much difference in the tensile stress using the two stress distributions. The flexural bond strength (ft) was determined by using the Eq. 4.2 recommended by Khalaf (2005).
( ) ( )
( ) ( )
2 2 2 2
2
0.5 0.5 0.75 1.25 0.5
0.33 1.5
b b bar bar v b b bar bar
t
mj b b bar
l l t t P l l t t W
f l w l t
− + + − +
= − (4.2)
Where, lb, wb, and lmj are the length and width of the brick specimen and length of the mortar joint, respectively; Pv is the failure load; W is the weight of the brick unit, and tbar represents the thickness of the bar. 1:6 mortar grade was used in the specimen, and the test was conducted after 28 days of curing. The average tensile bond stress was calculated to be 0.076 MPa.
(a) (b)
(c)
Figure 4.4. Tensile bond test: (a)Experimental setup of Z Specimen, (b) Free body diagram of Z-shaped specimen, and (c) Free body diagram of top brick displaying loads and stress distribution.
It is to be noted that during the tensile bond test, the material exhibited sudden brittle failure (Figure 4.5a), leaving no scope of capturing the post-peak softening behavior, which was required to define the tensile damage behavior of masonry material for the numerical analysis. Using the past results of Dhanasekar and Haider (2008) and Rai et al. (2014), a
bar
mortar brick
brick
supports F
W
W Ra
Rb F
1.5lb-tbar
lb lmj
lb/2 tbar
B A
tbar/2
W
Ra
F tbar
ffb
Ffb
A
lb/2=lmj
2/3lmj
H
lb – tbar/2 lb
tensile strain value of 0.0001 was therefore considered in the study corresponding to the peak tensile stress of 0.076 MPa. The post-peak behavior was assumed to be linearly falling till tensile stress of 0.015 MPa and corresponding strain value of 0.001, as shown in Figure 4.5b. The residual tensile stress of 0.015 MPa was further considered till strain value of 0.0015. These values in the tensile stress-strain curve were selected based on past literature to assist in smoother strength degradation required for convergence of numerical solution procedure during non-linear analyses.
(a) (b)
Figure 4.5. Tensile bond test: (a) tension bond failure of Z-shaped specimen (b) idealized tensile stress-strain curve.
4.2.5. Compressive Test on Masonry Prism
Compression tests of masonry prisms were carried out following ASTM C1314-12 specifications (ASTM, 2012) and IS 1905 ( BIS, 1987b). The prisms were constructed using first-class bricks and 1:6 cement: sand mortar grade. The specimens had an average height of about 425 mm, length of about 233mm, and thickness of about 115 mm. All the prisms were cured for minimum of 28 days before testing. The specimens were centrally placed between the loading plates of the compression testing machine (Figure 4.6a). A monotonic uniaxial compressive loading was applied to the specimen with a displacement rate of 0.16 mm/s. The average value of the compressive strength of the masonry prism (fm) was 3.40 MPa. The average modulus of elasticity was calculated using the equation suggested by Kaushik et al., (2007) and it comes out to be 1732 MPa. The average stress- strain curve obtained for the masonry prisms is shown in Figure 4.6b. Various mechanical properties obtained from the tests are summarized in Table 4.1.
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0 0.0005 0.001 0.0015 0.002
Axial Tensile Stress σ(MPa)
Axial Tensile Strain ε
(a) (b)
Figure 4.6. Masonry displaying: (a) compression test, and (a) average compressive stress-strain curve.
Table 4.1. Mechanical properties of masonry materials
Property Value Remarks
Compressive Strength of Brick Unit 21.2 MPa Experimentally obtained Compressive Strength of Mortar (1:6) 6.56 MPa Experimentally obtained Compressive Strength of Masonry Prism 3.40 MPa Experimentally obtained Tensile Bond Strength (Z test) 0.076 MPa Experimentally obtained Elastic Modulus of Masonry 1732 MPa Experimentally obtained Shear Strength (Triplet Shear Test) 0.17 MPa Experimentally obtained Specific Weight of Masonry 1870 kg/m3 BIS (1987a)
Elastic Modulus of Concrete 24768 MPa BIS (2000)
Density of Concrete 2400 kg/m3 BIS (1987b)