Materials and Structures

, Volume 41, Issue 10, pp 1697–1712

Influence of shear bond strength on compressive strength and stress–strain characteristics of masonry

Authors

    • Department of Civil EngineeringIndian Institute of Science
  • Ch. V. Uday Vyas
    • Department of Civil EngineeringIndian Institute of Science
Original Article

DOI: 10.1617/s11527-008-9358-x

Cite this article as:
Venkatarama Reddy, B.V. & Uday Vyas, C.V. Mater Struct (2008) 41: 1697. doi:10.1617/s11527-008-9358-x

Abstract

The paper is focused on shear bond strength–masonry compressive strength relationships and the influence of bond strength on stress–strain characteristics of masonry using soil–cement blocks and cement–lime mortar. Methods of enhancing shear bond strength of masonry couplets without altering the strength and modulus of masonry unit and the mortar are discussed in detail. Application of surface coatings and manipulation of surface texture of the masonry unit resulted in 3–4 times increase in shear bond strength. After adopting various bond enhancing techniques masonry prism strength and stress–strain relations were obtained for the three cases of masonry unit modulus to mortar modulus ratio of one, less than one and greater than one. Major conclusions of this extensive experimental study are: (1) when the masonry unit modulus is less than that of the mortar, masonry compressive strength increases as the bond strength increases and the relationship between masonry compressive strength and the bond strength is linear and (2) shear bond strength influences modulus of masonry depending upon relative stiffness of the masonry unit and mortar.

Keywords

Shear bond strengthMasonryCompressive strengthMasonry modulusStress–strain relation

1 Introduction

Masonry is a layered composite consisting of mortar and the masonry unit. Perfect bond between the masonry unit and the mortar is essential for the masonry to resist the stresses due to different types of loading conditions. For the masonry under compression the relative stiffness of the masonry unit and the mortar influence the nature of stresses developed in the masonry unit and the mortar. Elastic analysis proposed by Francis et al. [1] reveal the nature of stresses developed in the masonry unit and the mortar. Hilsdorf [2], Khoo and Hendry [3], Atkinson et al. [4] and McNary and Abrams [5] have proposed failure theories for masonry under compression. These theories are based on deformation of brick and mortar under multi-axial stress state and on the assumption that perfect bond exists between the brick and mortar till the ultimate failure of the masonry. In certain situations like very low brick–mortar bond strengths the masonry prism failure is accompanied by bond failure [6, 7].

Brick–mortar bond development is generally attributed to the mechanical inter-locking of cement hydration products into the surface pores of the bricks [811]. Masonry unit–mortar bond development is influenced by a large number of parameters, relating to characteristics of masonry unit and mortar, and bond morphology [11]. Surface characteristics of the masonry unit (surface texture, pore size, porosity, pore size distribution etc.) play a crucial role in the development of bond. Surface characteristics of the masonry unit do not have any bearing on the deformation characteristics (such as modulus, stress–strain relations etc.) of the masonry unit. Masonry unit–mortar bond strength can be altered or varied without altering the stiffness of the masonry unit and the mortar. It is worth examining the compressive strength of masonry when the masonry unit–mortar bond strength is varied over wide limits without altering the strength and deformation characteristics of the masonry unit and the mortar.

2 Earlier studies on masonry bond strength and scope of the study

A number of investigations can be found on the brick or block–mortar bond strength, addressing various aspects of masonry bond strength. But there are limited studies on bond strength and masonry compressive strength relationships. Sinha [12] obtained a relationship between the moisture content of the brick at the time of laying and tensile bond strength of masonry. This study showed that highest tensile bond strength is achieved when the bricks are saturated to about 80% at the time of construction, whereas use of dry and completely saturated bricks lead to poor bond strength.

Studies of Grandet et al. [8] throw light on the microstructure changes at the interface of cement paste and brick. They observed that pore size on the brick surface influences the bond development. Generally coarser pores give better bond strength and the bond development is due to mechanical interlocking of hydrated cement-products into the pores of the brick. Lawerence and Cao [9] examined the brick–mortar interface bond strength and attempted to understand the mechanism of bond development using burnt clay bricks with cement paste and cement–lime paste. They observed that the network of cement hydration products deposited on the brick surface and inside the brick pores helps in brick–mortar bond formation. They have concluded that the brick–mortar bond is essentially mechanical in nature since there is movement and penetration of hydration products into the pores of brick.

Groot [11] reports some of the earlier studies done on the influence of surface texture of bricks on bond strength [13, 14]. These studies show that rough surface texture gives better bond strength than the smooth surfaces. Ground surfaces of bricks can reduce the brick–mortar bond strength [15]. Studies of Saranagpani et al. [6] and Venkatarama Reddy et al. [16] show that increasing frog area on brick surface lead to improved bond strength.

Venu Madhava Rao et al. [17] have concluded that composite mortars like cement–soil and cement–lime mortars show better bond strength as compared to cement–sand mortars and masonry units with wider and deeper frogs give higher flexural bond strength. Walker’s studies [18] show that the block moisture content at the time of construction is the most important factor on resultant bond strength. Venkatarama Reddy and Ajay Gupta [19] have examined the tensile bond strength of soil–cement block masonry couplets using cement–soil mortars. They conclude that tensile bond strength is sensitive to initial moisture content of the block at the time of construction. Partially saturated blocks give higher tensile bond strength when compared to dry or saturated blocks.

Studies of Venkatarama Reddy et al. [16] showed that the interfacial shear bond strength can be altered easily. They tried a number of artificial techniques such as surface coatings like epoxy resin and fresh cement slurry, rough textured block surface, introducing frogs, etc. to enhance the shear bond strength. Venu Madhava Rao et al. [20] have studied the effect of flexural bond strength on compressive strength of masonry. Their results indicate that the flexural bond strength and masonry compressive strength for a particular masonry unit has not varied with respect to strength of the mortar. Mortars with distinctly different compressive strength but having the same bond strength resulted in similar masonry compressive strength.

The study of Sarangapani et al. [6] was the first systematic effort to understand the influence of brick–mortar bond strength on masonry compressive strength. The study showed that increase in brick–mortar bond strength while keeping the mortar composition and strength constant leads to increased compressive strength for the masonry. A fourfold increase in bond strength resulted in doubling of masonry compressive strength and masonry compressive strength was more sensitive to brick–mortar bond strength than compressive strength of the mortar. In this study only the case of brick modulus lower than that of the mortar is considered. Venkatarama Reddy et al. [16] tried to correlate bond strength with the masonry compressive strength. In this case the block modulus and mortar modulus were in the same range. They concluded that there is only marginal variation in masonry compressive strength as bond strength is increased.

It is clear from the limited number of studies that there is some correlation between bond strength and masonry compressive strength. Hence, the present investigation is focused on examining the influence of bond strength on masonry compressive strength in greater detail. Thus the main objective of this study is to examine the masonry compressive strength while varying the shear bond strength between the block and mortar over wide limits for the cases of mortar modulus greater than block modulus and vice-versa. While keeping the block and mortar characteristics constant the shear bond strength was varied using bond enhancing techniques. The scope of the study involves exploring different methods of enhancing shear bond strength (without altering block and mortar characteristics), varying the ratio of block modulus to mortar modulus and then determining the masonry compressive strength.

3 Materials used in the investigation

Main objective of the present investigation is to understand the influence of brick–mortar bond strength on the compressive strength of masonry. This will necessitate: (a) varying the bond strength between masonry unit and the mortar without altering the characteristics of mortar and the masonry unit and (b) varying the masonry unit modulus to mortar modulus ratio. In order to achieve low values (≪1.0) of brick modulus to mortar modulus ratios, low strength bricks have to be used. Generally low strength burnt clay bricks have large coefficient of variation for any given mean strength [21, 22]. Hence in the present study soil–cement blocks were chosen. Use of soil–cement block for an exploratory study like this has the following advantages.
  1. 1.

    Strength and modulus of elasticity of the block can be easily varied by adjusting the cement content of the block during the manufacturing process.

     
  2. 2.

    Large deviations from the mean strength (especially for achieving low strength and low modulus can be avoided) by controlling the mix composition and density of the block during the manufacturing process.

     
  3. 3.

    Surface characteristics of the block (texture, porosity, pore size distribution, frog shape and size etc.) which can significantly influence the block–mortar bond development can be easily altered in soil–cement blocks.

     

3.1 Soil–cement blocks

Soil–cement blocks are solid blocks manufactured by compacting a soil–sand–cement mixture at optimum moisture using a machine. These blocks are used for load bearing masonry in India and elsewhere [18, 2330]. Studies of Venkatarama Reddy and Jagadish [31], Olivier and Mesbah Ali [32], Venkatarama Reddy and Peter walker [33] and Venkatarama Reddy et al. [34] give specifications and guidelines for soil composition and density for the manufacture of soil–cement blocks. These guidelines were followed while manufacturing the soil–cement blocks used in this study. Two types of soil–cement blocks with various types of surface finishes were prepared. Wet Compressive strength and water absorption of the blocks was determined using the procedure outlined in I.S. 3495 code [35]. The results of strength and water absorption characteristics of soil–cement blocks are given in Table 1. The results represent the mean of 6 specimens. Two types of soil–cement blocks designated as SCB1 and SCB2 contain 5% and 14% cement, respectively. Wet compressive strength of SCB1 and SCB2 blocks is 5.09 and 11.46 MPa, respectively. Water absorption is 11.74% and 9.10% for SCB1 and SCB2, respectively.
Table 1

Characteristics of soil–cement blocks

Sl. no.

Properties and other details

Type of block

Block designation

SCB1

SCB2

1

Block size (mm)

255 × 122 × 80

255 × 122 × 80

2

Cement content (by weight)

5%

14%

3

Wet compressive strength (MPa)

5.09 (0.66)

11.46 (0.72)

4

Water absorption (%)

11.74 (0.44)

9.10 (0.86)

5

Initial tangent modulus (MPa)

6650

14500

6

Strain at peak stress

0.00164

0.00143

7

Poisson’s ratio (at 25% peak stress)

0.13

0.18

Values in parenthesis indicate standard deviation

The stress–strain characteristics of soil–cement blocks were obtained by testing the blocks in a displacement controlled universal test rig. The experimental set-up for the measurement of lateral and longitudinal strains is shown in Fig. 1. Prior to test, the specimens were soaked in water for 48 h. Stress–strain relationships for the two types of soil–cement blocks are shown in Fig. 2. Compressive stress versus lateral strain relationships are also shown in this figure. The stress–strain relationship for SCB1 block is curvilinear showing a more softening behaviour with hardly any linear portion. The stress–strain curve remains almost flat and parallel to strain axis for the strain values between 0.0008 and 0.0025. The stress–strain relationship for SCB2 block is linear up to 4 MPa stress (∼60% of peak stress), and then it becomes non-linear with a well defined drooping portion after the peak. The modulus and strain at peak stress values for the blocks are given in Table 1. The strain at peak stress is 0.0016 and 0.0014 for SCB1 and SCB2 blocks, respectively. Both the types of blocks show nearly similar values of strain at peak stress, but their modulus values are distinctly different. Initial tangent modulus values for SCB1 and SCB2 blocks are 6,650 and 14,500 MPa, respectively. Modulus of SCB2 block is more than double of that of SCB1 block. Poisson’s ratio at 25% of peak stress is 0.13 and 0.18 for SCB1 and SCB2 blocks, respectively.
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Fig. 1

Experimental set-up for stress–strain measurements of the soil–cement block

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Fig. 2

Stress–strain relationships for soil–cement blocks

3.2 Mortars

Ordinary Portland cement conforming to I.S. 8112 [36], commercial grade hydrated lime and natural river sand were used for the preparation of mortars. Cement–lime mortars of two different proportions were used in the investigations. Table 2 gives details of mortar proportions, mortar designation, flow value, and water/cement ratio (by weight). Thus we have two types of mortars: viz. CLM1 and CLM2. Both strength and stress–strain characteristics were obtained by keeping the mortar flow constant at 100%, thus fixing the w/c ratio as given in Table 2. Compressive strength of mortar was obtained by testing 100 mm size cube specimens. Mortar cubes were prepared as per the guidelines given in I.S. 2250 [37]. The cubes after 28 days curing were tested in a compression testing machine in saturated condition. The compressive strength is 3.42 and 9.40 MPa for CLM1 and CLM2 mortars, respectively.
Table 2

Characteristics of mortars

Sl. no.

Properties and other details

Type of mortar

Mortar designation

CLM1

CLM2

1

Proportion (cement:lime:sand) (by volume)

1:1:6

1:0.5:4

2

Flow

100%

100%

3

Water–cement ratio

1.88

1.17

4

Cube compressive strength (MPa)

3.42

9.40

5

Initial tangent modulus (MPa)

6450

11600

6

Strain at peak stress

0.0020

0.0027

7

Poisson’s ratio (at 25% peak stress)

0.16

0.18

Stress–strain relationships for the mortars were obtained by testing mortar cylinder of size 150 mm diameter and 305 mm height. After curing for 28 days the cylinders were soaked in water for a period of 48 h prior to testing. Cylinders were tested in a displacement controlled universal test rig. The longitudinal strains and lateral strains were recorded using extensometers attached externally as shown in Fig. 3. Eight specimens were tested for each mortar proportion and the mean values are reported. The stress–strain characteristics for mortars are given in Table 2. Stress–strain curves for the two mortars are shown in Fig. 4. This figure also shows the plot of lateral strain variation with the compressive stress. The initial tangent modulus is 6,450 and 11,600 MPa for CLM1 and CLM2 mortars, respectively. Poisson’s ratio at 25% of peak stress is 0.16 and 0.18 for CLM1 and CLM2 mortars, respectively.
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Fig. 3

Experimental set-up showing extensometers for stress–strain measurements of mortar

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Fig. 4

Stress–strain relationship for cement–lime mortars

4 Enhancing block–mortar bond strength

Shear bond strength of masonry couplets has to be altered/varied without altering the block as well as mortar characteristics. Different artificial methods and techniques were employed to improve the interfacial shear bond strength of the couplet specimens. Details of the techniques/methods adopted are as follows.

4.1 Altering the surface texture of the masonry unit

Earlier studies indicate that bond development between masonry unit and the mortar is purely mechanical in nature and is attributed to the inter-locking of hydration products of fresh mortar into the masonry unit pores. Hence, attempts were made to alter the surface texture of the masonry unit. It is easy to alter the surface texture during the manufacturing of soil–cement blocks. The procedure used for obtaining rough textured surface and introducing frogs on the block surface is outlined below.

Two major steps followed in the soil–cement block production process are: (a) filling the metal mould with the requisite quantity of soil–cement mixture (at optimum moisture content) and (b) compacting into a dense block through a piston movement. Top and bottom surfaces of the block (during the compaction process) are in contact with the lower face of the lid and the top of the bottom plate, respectively. The following types of blocks can be obtained during the block compaction process.
  1. 1.

    When the lid and the bottom plate surfaces are plain, the soil–cement block will have plain surfaces at the top and bottom.

     
  2. 2.

    Welding a protruded mild steel piece on the top of bottom plate and lower face of the lid gives a soil–cement block with top and bottom surfaces having frogs.

     
  3. 3.

    Rough surface texture for the top and bottom surfaces of the soil–cement block can be obtained by introducing a thin layer (6 mm) of gravel–cement mixture at the top and bottom surfaces as shown in Fig. 5.

     
Figure 6 shows the three types of block surfaces used in this study. Centre line average (CLA) index was obtained to quantify the surface roughness of the blocks. CLA index for the plain block surface and the rough textured surface was measured using Profilometer technique. CLA index is a universally recognized parameter for quantification of surface roughness. CLA index represents the arithmetic mean of the absolute departure of the roughness profile from the mean line. Surface profiles of both plain and rough textured surfaces of the blocks are shown in Figs. 79. The CLA index values for plain surfaces are 20.09 and 12.29 μm for SCB1 and SCB2 blocks, respectively. Whereas the CLA index of the rough textured surface is 29.5 μm.
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Fig. 5

Introducing rough textured gravel-cement mixture on soil–cement block surfaces

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Fig. 6

Different types of block surfaces (L to R: plain surface, rough textured surface, surface with a frog)

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Fig. 7

Surface profile of plain surface of SCB1 soil–cement block

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Fig. 8

Surface profile of plain surface of SCB2 soil–cement block

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Fig. 9

Surface profile of rough textured soil–cement blocks (SCB1 and SCB2)

4.2 Application of bond enhancing coatings on the block surface

Studies of Venumadhava Rao et al. [17, 20], Sarangapani et al. [6] and Venkatarama Reddy et al. [16] have shown that shear bond strength can be enhanced by applying a coat of fresh cement slurry or epoxy resin on the brick or block surface during construction of couplets or prisms. Cement slurry coating as well as epoxy resin coating was adopted in this investigation. During the construction of couplet specimen a coating of fresh cement slurry is applied on the block surface, which is going to be in contact with the fresh mortar. Cement slurry was prepared by mixing one part of water with one part of ordinary Portland cement (by weight) in fresh condition while constructing the specimens. In the case of epoxy coating a fresh coat of epoxy resin is applied using a brush while constructing the specimens. The coating is applied on the block surface which will be in contact with the fresh mortar surface. Thus five different types of bond enhancing techniques were used for each block–mortar combination. Details of type of bond enhancing technique and its designation are given in Table 3.
Table 3

Details of bond enhancing techniques

Type of bond enhancing method

Designation

1. Plain soil–cement block surface (Fig. 6)

Type A

2. Rough textured block surface (Fig. 6)

Type B

3. One frog of 80 × 50 mm2 in the block surface (Fig. 6)

Type C

4. Fresh cement slurry coating on the plain block surfaces while casting the couplet specimen

Type D

5. Epoxy coating on the plain block surfaces while casting the couplet specimen

Type E

5 Experimental programme and testing procedure

Two types of mortars (CLM1 and CLM2) and two types of soil–cement blocks (SCB1 and SCB2) were used in these investigations. Block–mortar combinations of SCB1–CLM1, SCB1–CLM2 and SCB2–CLM1 were attempted with all the five bond enhancing techniques. Tests were performed to obtain: (a) shear bond strength of block–mortar interface, (b) compressive strength of masonry and (c) stress–strain characteristics of masonry. Details of test methods and procedures adopted to evaluate shear bond strength, masonry compressive strength and stress–strain relations of masonry are discussed in the following sections.

5.1 Shear bond strength

Shear bond strength of masonry joints was measured using masonry couplets. Fig. 10 shows the details of soil–cement block couplet used in the study. Block couplets with one mortar joint sandwiched between the two blocks were cast. Moisture content of the blocks was kept at 75% of saturation value while constructing the couplet specimens. Mortar flow was maintained constant at 100% during construction of the couplet specimens. A total of 90 couplets were made with various bond enhancing techniques and different types of block–mortar combinations consisting of six couplets in each category. After 28 days curing under wet burlap the couplets were tested for shear bond strength in saturated state by soaking them in water for 48 h prior to the test. Figure 10 shows the test set-up for shear bond strength and details of the couplet. This set-up is similar to that employed to test shear strength of soil using direct shear box apparatus with modifications to accommodate a masonry couplet. The masonry couplet mounted in the set-up does not come in contact with the metal mould as shown in the Fig. 10. The two blocks of the couplet are gripped by a series of bolts on either faces of the block as shown in the figure. The horizontal force applied through the loading arm is transferred to the couplet through these bolts attached to the mould and are very close to the block–mortar interface. Thus the set-up facilitates to transfer the applied shear force to the block–mortar interface without causing any significant bending moment on the joint between the two blocks. If the shear bond strength of the block–mortar interface is larger than the shear strength of the block or the mortar, the set-up allows for shearing failure of either the block or mortar depending upon their relative strength.
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Fig. 10

Experimental set up for determination of shear bond strength and couplet details

Some shear bond tests were performed on the masonry couplets using the above mentioned test set-up with pre-compression values of 0.10, 0.25 and 0.50 MPa. A Mohr–Coulomb relationship was established to predict the shear bond strength at zero pre-compression for one case. It was found that the shear bond strength obtained through experiments was 0.22 MPa as against the predicted value of 0.21 MPa from the Mohr–Coulomb relationship. This substantiates the fact that there is hardly any bending stress acting on the block–mortar interface under shear in the test set-up.

5.2 Compressive strength of masonry and stress–strain relationships

Compressive strength of soil–cement block masonry was determined by testing the stack bonded masonry prisms. Five blocks height masonry prisms were cast using the appropriate block–mortar combination. The size of the prisms used was 255 × 122 × 440 mm. The mortar joint thickness of 10 mm was maintained for all the prisms. The prism was capped with rich cement mortar at both the ends for facilitating application of uniform load during testing. The blocks were soaked in water for a definite period of time, such that at the time of constructing the prism, the water content of the block is maintained at 75% of saturation value. Mortar flow was kept constant at 100% while constructing the specimens. In each block–mortar combination a total of 25 prisms were prepared consisting of five prisms for a particular bond enhancing method. The prisms were cured for a period of 28 days under moist burlap. Prior to testing, the prisms were soaked in water for a period of 48 h. The stress–strain curves were generated by testing the saturated specimens in a displacement controlled universal testing machine. The strains were measured using an extensometer.

6 Results and discussions

6.1 Shear bond strength of masonry couplets

Table 4 gives details of shear bond strength for different block–mortar combinations. Details of bond enhancing technique, maximum–minimum and mean values of shear bond strength and type of couplet failure are given in the table. The following observations can be made from the results given in Table 4.
  1. 1.

    Shear bond strength can be enhanced by using bond enhancing techniques such as altering the surface texture of the blocks and surface coatings like cement slurry coating and epoxy resin coating.

     
  2. 2.

    Shear bond strength of couplets ranges between 0.12 and 0.83 MPa for various block–mortar combinations with and without bond enhancing techniques. The highest bond strength values were obtained when fresh cement slurry coat was applied to the block surface. The lowest bond strength values are noticed for plain surface blocks without the use of any bond enhancing techniques.

     
  3. 3.

    Changing the bed face of the block from plain to rough texture (i.e. CLA index 30 μm) lead to considerable increase in shear bond strength. There is 2–2.75 times increase in shear bond strength between plain block surface and rough textured surface for various block–mortar combinations attempted.

     
  4. 4.

    Introducing one frog on each face of the block is also quite effective in increasing the shear bond strength. Bond strength with frog and rough textured surface are comparable for certain block–mortar combinations.

     
  5. 5.

    Large increase in shear bond strength is noticed due to the application of a coat of fresh cement slurry on the block face for all the block–mortar combinations attempted. There is a fourfold increase in shear bond strength when compared to plain block surface for SCB1–CLM1 and SCB2–CLM1 combinations and nearly three fold increase for SCB1–CLM2 combination.

     
  6. 6.

    Use of epoxy coating has lead to increase of 3–3.75 times for SCB1–CLM1 and SCB2–CLM1 combinations and about two times for SCB1–CLM2 combination.

     
  7. 7.

    SCB2–CLM1 combination exhibits higher bond strength (65–100%) when compared to SCB1–CLM1 combination.

     
Table 4

Shear bond strength of soil–cement block masonry couplets

Type of bond enhancing technique

Shear bond strength (MPa)

SCB1 block

SCB2 block

CLM1 mortar

CLM2 mortar

CLM1 mortar

Mean

Type of failure

Mean

Type of failure

Mean

Type of failure

A

0.12 (0.08–0.18)

a

0.15 (0.10–0.22)

a

0.22 (0.10–0.34)

a

B

0.27 (0.18–0.36)

a

0.32 (0.26–0.39)

a

0.51 (0.41–0.62)

a, d

C

0.24 (0.14–0.34)

a, d

0.25 (0.20–0.31)

a

0.49 (0.36–0.77)

a, d

D

0.51 (0.47–0.59)

b

0.41 (0.26–0.57)

b

0.83 (0.45–1.17)

c

E

0.45 (0.36–0.59)

b

0.28 (0.25–0.32)

b

0.73 (0.63–0.86)

c

Number of specimens tested in each category: 6, range of values in parenthesis

a, Interface bond failure; b, Block failure; c, Mortar failure; d, Partial block or mortar failure

These results clearly indicate that rough textured block surface, introducing frogs and surface coatings lead to considerable increase in bond strength when compared to plain block surface. There is a drastic increase (3–4 fold) in shear bond strength when bond-enhancing techniques such as cement slurry and epoxy coatings were used. Fresh cement slurry coating on the block face is very effective in increasing the shear bond strength and it is practically feasible to use cement slurry coating. Use of epoxy coatings may not have practical significance as such coatings will be expensive and cumbersome.

6.2 Failure patterns of shear bond couplets

The failure patterns of shear bond test couplets can be classified into four types:
  1. Type a:

    Interface failure, with clear separation of the bond between the block surface and the mortar at the interface (Fig. 11a).

     
  2. Type b:

    Block failure, with the shearing of the block surface in the horizontal plane. In this type of failure the bond at the block–mortar interface is intact and no failure of the mortar is observed (Fig. 11b).

     
  3. Type c:

    Mortar failure, with the shearing of the mortar surface in the horizontal plane. In this type of failure the bond at the block–mortar interface is intact and no failure of the block is observed (Fig. 11c).

     
  4. Type d:

    Partial block/mortar failure, with partial failure of both the block and the mortar (Fig. 11d).

     
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Fig. 11

Failure pattern of shear bond test couplets

Failure patterns of couplets given in Table 4 indicate that, interface failure (Type a) is dominant when the shear bond strength is about 0.25 MPa. For higher bond strengths the failure is either in the block or in the mortar (Type b and c) depending upon the relative strength of the mortar and block.

6.3 Influence of shear bond strength on compressive strength of masonry

Compressive strength of soil–cement block masonry was determined by testing the stack-bonded prisms. Prisms with SCB1–CLM1, SCB2–CLM1 and SCB1–CLM2 block–mortar combinations were prepared and in each category the shear bond strength of masonry was varied by adopting the bond enhancing techniques (Type A–E) as explained in previous sections. Thus five different shear bond strength values were used for each block–mortar combination. The block–mortar combinations chosen represent Eblock to Emortar ratios of 0.57, 1.03 and 2.25 for SCB1–CLM2, SCB1–CLM1 and SCB2–CLM1 combinations, respectively. Here Eblock and Emortar represent the initial tangent modulus of the soil–cement block and the mortar, respectively. Thus we have Eblock to Emortar ratio equal to one, less than one and greater than one. The results of masonry compressive strength using different bond enhancing methods for both the mortars are given in Table 5. Details of bond enhancing parameter, shear bond strength values, mean values of prism compressive strengths along with maximum and minimum values for the two types of mortars are given in the table. Figure 12 shows a plot of bond strength versus compressive strength of masonry for Eblock to Emortar ratio of 0.57, 1.03 and 2.25. Shear bond strength–compressive strength relationships obtained by Sarangapani et al. [6] (Ebrick/Emortar = 0.09) are added for comparison. The following points are clear from the results of Table 5 and Fig. 12.
  1. 1.

    Shear bond strength of couplets varies between 0.12 and 0.51 MPa for CLM1–SCB1 combination. For the same block–mortar combination the prism compressive strength varies in a very narrow range between 2.30 and 2.65 MPa. These results indicate that even though there is fourfold increase in bond strength the compressive strength does not vary much. It is to be noted here that Eblock/Emortar = 1.03. For the case of CLM1–SCB2 combination, where Eblock/Emortar = 2.25, again fourfold increase in shear bond strength (0.22–0.83 MPa) does not cause any significant variation in masonry compressive strength (5.41–6.16 MPa).

     
  2. 2.
    Masonry compressive strength increases as the shear bond strength increases and the relationship is linear (Fig. 12) for the case of Eblock to Emortar ratio of 0.57 (SCB1–CLM2 combination). For a change in shear bond strength from 0.15 to 0.41 MPa, the masonry compressive strength increased by about 50%. Sarangapani et al. [6] studied the influence of bond strength on masonry compressive strength for the case of Ebrick to Emortar ratio = 0.09. They noticed doubling of masonry compressive strength for fourfold increase in bond strength. The experimental results discussed here clearly indicate that masonry compressive strength is sensitive to bond strength only when Eblock to Emortar ratio is less than one. Comparing the results of Sarangapani et al. [6] and the present investigation, the following points emerge:
    1. (a)
      Bond strength between masonry unit and the mortar has significant influence on masonry compressive strength only when mortar is stiffer than the brick or block (Emasonry unit to Emortar ratio less than one). For very low Emasonry unit to Emortar ratios, there will be considerable increase in masonry compressive strength as the bond strength is increased. The correlation between compressive strength and bond strength is as follows. Masonry compressive strength = σ in MPa, Shear bond strength = τ in MPa.
      $$ \sigma = 1.457 + 5.01\tau \;\;({\text{coefficient of correlation}} = 0.89)$$
      present study
      $$ \sigma = 1.796 + 9.02\tau \;\;({\text{coefficient of correlation coefficient}} = 0.90)$$
      Sranagpani et al. [6]
       
    2. (b)

      Masonry compressive strength is not sensitive to bond strength variation when the masonry unit is stiffer than that of mortar.

       
     
Table 5

Compressive strength of soil–cement block masonry prisms

Bond enhancing technique

SCB1 block

SCB2 block

CLM1 mortar, X = 1.03

CLM2 mortar, X = 0.57

CLM1 mortar, X = 2.25

Shear bond strength (MPa)

Compressive strength (MPa)

Shear bond strength (MPa)

Compressive strength (MPa)

Shear bond strength (MPa)

Compressive strength (MPa)

A

0.12

2.65 (2.40–2.97)

0.15

2.39 (2.16–2.79)

0.22

6.16 (5.53–6.87)

B

0.27

2.40 (2.05–2.76)

0.32

3.12 (2.84–3.42)

0.51

5.41 (5.10–5.72)

C

0.24

2.30 (2.01–2.45)

0.25

2.50 (2.37–2.61)

0.49

5.75 (5.04–6.29)

D

0.51

2.62 (1.76–3.12)

0.41

3.62 (3.11–4.13)

0.83

6.05 (3.56–7.77)

E

0.45

2.46 (2.13–2.71)

0.28

2.73 (2.42–2.93)

0.73

5.88 (4.60–6.29)

No. of specimens tested for shear bond strength: 6; for masonry compressive strength: 5

Range of values in parenthesis, Eblock/Emortar = X

https://static-content.springer.com/image/art%3A10.1617%2Fs11527-008-9358-x/MediaObjects/11527_2008_9358_Fig12_HTML.gif
Fig. 12

Bond strength versus compressive strength for various block–mortar combinations (figures in the graph refer to Ebrick/Emortar ratios)

6.4 Nature of stresses developed in the mortar and the masonry unit for masonry under compression

Figure 13 shows a masonry prism under compression and the nature of stresses developed in the block as well as mortar for the cases of Emasonry unit to Emortar ratio less than one and greater than one. Earlier investigations on masonry failure theories [15] are for the cases of Emasonry unit to Emortar ratio greater than one, where mortar is under triaxial compression and masonry unit under biaxial tension. For the case of Emasonry unit to Emortar ratio less than one, the mortar will be under biaxial-tension and compression, and the masonry unit is under triaxial compression. The biaxial horizontal compression in the masonry unit is due to the stiffer mortar pulling it inwards for strain compatibility. The horizontal compression developed in the masonry unit is due to horizontal shear stress at the block–mortar interface. Suppose if bond failure takes place at the interface, the horizontal compression induced by shear stresses will also vanish and the masonry unit will fail by lateral tension. Thus one of the failure mechanisms of soft masonry unit–stiff mortar is dependent on the shear bond strength at the interface. Higher bond strength means that the masonry unit will develop a large horizontal compression as long as high shear bond stress in the masonry unit–mortar interface is sustained. This could probably explain the reason for increase in bond strength leading to increased masonry compressive strength when Emasonry unit to Emortar ratio is less than one.
https://static-content.springer.com/image/art%3A10.1617%2Fs11527-008-9358-x/MediaObjects/11527_2008_9358_Fig13_HTML.gif
Fig. 13

Nature of stresses developed in the masonry unit and mortar

6.5 Failure pattern of masonry prisms

Generally the masonry prisms under uniform compression fail by the development of vertical splitting cracks. For the case of Eblock to Emortar ratio less than one the mortar will be under biaxial tension–compression, therefore the first vertical splitting crack appears in the mortar joint. As the compressive load on the prism increases these vertical splitting cracks propagate and extend into the block. Ultimately large numbers of vertical splitting cracks appear before the prism collapses. In case where Eblock to Emortar ratio is greater than one, the vertical splitting crack appear first in the brick and extend over the prism height. Ultimately prism fails by developing large number of vertical splitting cracks. A typical failure pattern of the prism is shown in Fig. 14.
https://static-content.springer.com/image/art%3A10.1617%2Fs11527-008-9358-x/MediaObjects/11527_2008_9358_Fig14_HTML.jpg
Fig. 14

Typical crack pattern of the masonry prism

6.6 Stress–strain characteristics of masonry

Figure 15 shows stress–strain relationships for the soil–cement block masonry for the case of SCB1–CLM2 combination as the bond strength is varied over wide limits. Similar relationships were obtained for SCB1–CLM1 and SCB2–CLM1 combinations. Table 6 gives the stress–strain characteristics for the soil–cement block masonry. Details of shear bond strength, initial tangent modulus and strain at peak stress for SCB1–CLM1, SCB1–CLM2 and SCB2–CLM1 masonry are given in the table. A plot of shear bond strength with initial tangent modulus is shown in Fig. 16 for Eblock to Emortar ratio ranging from 0.57 to 2.25. The following points emerge from the results of these figures and Table 6.
  1. 1.

    For SCB1–CLM2 combination, (Eblock to Emortar ratio = 0.57) modulus of masonry is lower than that of the block as well as mortar. In this case strain at peak stress for the masonry is 0.0068 which is 3–4 times more than that of the mortar and the block. Thus soft block and stiff mortar combination leads to a more ductile masonry than that for the block and mortar separately. In case of SCB1–CLM1 combination (Eblock to Emortar ratio = 1.03) the modulus of masonry is lower than that of block and mortar, whereas strain at peak stress is more than that for the block and the mortar. As the Eblock to Emortar ratio increases to 2.25 (SCB2–CLM1 combination) the modulus of masonry lies in between that of block and the mortar. In this case strain at peak stress for masonry is 0.0025 which is more than that of the block and the mortar separately. Strain at peak stress for masonry is always more than that of the block and the mortar irrespective of Eblock to Emortar ratio greater than one or less than one.

     
  2. 2.

    Modulus of masonry increases as the shear bond strength increases when Eblock to Emortar ratio is less than or equal to one (Fig. 16). There is about 160% increase in modulus as the bond strength increased by 160% for SCB1–CLM2 combination where Eblock to Emortar ratio is 0.57. For Eblock to Emortar ratio of 1.03 (SCB1–CLM1 combination) the increase in modulus is only about 20% as the bond strength increases by 400% from 0.12 MPa. As the Eblock to Emortar ratio is increased further to 2.25 (SCB2–CLM1 combination) the modulus decreases with increase in bond strength (Fig. 16). There is a 50% decrease in modulus for 400% increase in bond strength.

     
https://static-content.springer.com/image/art%3A10.1617%2Fs11527-008-9358-x/MediaObjects/11527_2008_9358_Fig15_HTML.gif
Fig. 15

Stress–strain curves for SCB1–CLM2 masonry with various bond enhancing techniques

Table 6

Stress–strain characteristics of masonry

Type of bond enhancing technique

SCB1 block ITM: 6651 MPa, ∈0: 0.00164

SCB2 block ITM: 14532 MPa, ∈0: 0.00143

CLM1 ITM: 6450 MPa, ∈0: 0.002

CLM2 ITM: 11600 MPa, ∈0: 0.0027

CLM1 ITM: 6450 MPa, ∈0: 0.002

Shear bond strength (MPa)

ITM of masonry (MPa)

0

Shear bond strength (MPa)

ITM of masonry (MPa)

0

Shear bond strength (MPa)

ITM of masonry (MPa)

0

A

0.12

5300

0.0028

0.15

1670

0.0068

0.22

13100

0.0025

B

0.27

7200

0.0012

0.32

3800

0.0025

0.51

9915

0.0035

C

0.24

6100

0.0015

0.25

1730

0.0050

0.49

7938

0.0042

D

0.51

5413

0.0019

0.41

3620

0.0033

0.83

9900

0.0030

E

0.45

8200

0.0012

0.28

2606

0.0078

0.73

8500

0.0023

ITM; Initial tangent modulus, ∈0; Strain at peak stress

https://static-content.springer.com/image/art%3A10.1617%2Fs11527-008-9358-x/MediaObjects/11527_2008_9358_Fig16_HTML.gif
Fig. 16

Shear bond strength versus initial tangent modulus for masonry

It is clear from the above discussion that the modular ratio (Eblock/Emortar) of materials and shear bond strength of the masonry influence the stress–strain characteristics of masonry. As the bond strength increases the modulus of masonry increases when Eblock to Emortar ratio is less than one and modulus decreases with increase in bond strength when masonry unit is stiffer than that of the mortar. Soft masonry unit–stiff mortar combination leads to more ductile masonry than that of its constituent materials.

7 Conclusions

Shear bond strength of masonry couplets, methods of enhancing shear bond strength, influence of shear bond strength on masonry compressive strength for the cases of stiff block–soft mortar and soft block–stiff mortar combinations, and stress–strain characteristics of masonry for a range of bond strengths were explored. The following conclusions emerge from these exploratory studies.
  1. 1.

    Shear bond strength can be varied without varying the characteristics of the masonry unit and the mortar, through the manipulation of surface texture of blocks, introduction of frogs on the block surface and by using surface coatings such as fresh cement slurry coating and epoxy resin coating. Shear bond strength varied between 0.12 and 0.83 MPa for various block–mortar combinations. Fresh cement slurry coating is very effective in increasing the shear bond strength. Shear bond strength increased by 3–4 times when plain block surface was coated with fresh cement slurry during specimen construction for 1:1:6 cement–lime mortar. In case of 1:0.5:4 cement–lime mortar the shear bond strength increases by 2.25 times when cement slurry coat is applied. Use of rough textured surfaces, introducing frogs on block surfaces and application of an epoxy resin coating were also effective in increasing the shear bond strength significantly.

     
  2. 2.

    Masonry compressive strength increases as the shear bond strength increases only for the case of soft block–stiff mortar (Eblock to Emortar ratio is less than one) combination. The compressive strength increase due to increase in bond strength is significant for very small values of Eblock to Emortar ratio. Sarangapani et al. [6] noticed doubling of compressive strength when bond strength is increased by four times where Ebrick to Emortar ratio = 0.09, whereas in this investigation compressive strength increased by 50% for 270% increase in bond strength where Eblock to Emortar ratio = 0.57. Masonry compressive strength is not sensitive to bond strength variation when the masonry unit is stiffer than that of the mortar (i.e. Eblock to Emortar ratio greater than one).

     
  3. 3.

    Modulus of masonry is dependent on the relative stiffness of the masonry unit and the mortar (Eblock to Emortar ratio). Modulus of masonry is less than that of the block and the mortar when Eblock to Emortar ratio is less than one. For Eblock to Emortar ratio greater than one the modulus of masonry lies in between the block and mortar modulus. When Eblock to Emortar ratio is ∼1, the modulus of masonry is marginally less than that of the block and the mortar separately. The strain at peak stress is always more than that of the masonry unit and the mortar irrespective of Eblock to Emortar ratio.

     
  4. 4.

    Shear bond strength of the masonry has an influence on stress–strain characteristics of masonry. As the bond strength increases the modulus of masonry increases when Eblock to Emortar ratio is less than one and the modulus decreases with increase in bond strength when Eblock to Emortar ratio is greater than one.

     

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