shear bond strength on compressive strength

1. ORIGINAL ARTICLE
Influence of shear bond strength on compressive strength
and stress–strain characteristics of masonry
B. V. Venkatarama Reddy Æ Ch. V. Uday Vyas
Received: 24 April 2007 / Accepted: 14 January 2008 / Published online: 29 January 2008
Ó RILEM 2008
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 stiff-
ness of the masonry unit and mortar.
Keywords Shear bond strength
Masonry Compressive strength
Masonry modulus Stress–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 compres-
sion 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
B. V. Venkatarama Reddy () Ch. V. Uday Vyas
Department of Civil Engineering, Indian Institute of
Science, Bangalore 560012, India
e-mail: venkat@civil.iisc.ernet.in
Ch. V. Uday Vyas
e-mail: vyas_y2k@yahoo.co.in
Materials and Structures (2008) 41:1697–1712
DOI 10.1617/s11527-008-9358-x

2. [8–11]. 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 char-
acteristics 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 com-
pressive 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. Venkat-
arama 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
1698 Materials and Structures (2008) 41:1697–1712

3. masonry compressive strength and masonry com-
pressive 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. Venkat-
arama 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. 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. Large deviations from the mean strength (espe-
cially 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. 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, 23–
30]. 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 charac-
teristics 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.
The stress–strain characteristics of soil–cement
blocks were obtained by testing the blocks in a
displacement controlled universal test rig. The exper-
imental 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
Materials and Structures (2008) 41:1697–1712 1699

4. 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 differ-
ent. Initial tangent modulus values for SCB1 and
SCB2 blocks are 6,650 and 14,500 MPa, respec-
tively. 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.
Table 1 Characteristics of
soil–cement blocks
Values in parenthesis
indicate standard deviation
Sl. no. Properties and other details Type of block
Block designation SCB1 SCB2
1 Block size (mm) 255 9 122 9 80 255 9 122 9 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
Fig. 1 Experimental set-up for stress–strain measurements of
the soil–cement block
Fig. 2 Stress–strain
relationships for soil–
cement blocks
1700 Materials and Structures (2008) 41:1697–1712

5. 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 charac-
teristics 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.
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.
4 Enhancing block–mortar bond strength
Shear bond strength of masonry couplets has to be
altered/varied without altering the block as well as
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
Fig. 3 Experimental set-up showing extensometers for stress–
strain measurements of mortar
Materials and Structures (2008) 41:1697–1712 1701

6. 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. When the lid and the bottom plate surfaces are
plain, the soil–cement block will have plain
surfaces at the top and bottom.
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. 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 Profilom-
eter 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. 7–9. The CLA index values for plain surfaces
are 20.09 and 12.29 lm for SCB1 and SCB2 blocks,
Fig. 4 Stress–strain relationship for cement–lime mortars
Fig. 5 Introducing rough textured gravel-cement mixture on
soil–cement block surfaces
Fig. 6 Different types of block surfaces (L to R: plain surface,
rough textured surface, surface with a frog)
1702 Materials and Structures (2008) 41:1697–1712

7. respectively. Whereas the CLA index of the rough
textured surface is 29.5 lm.
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.
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 combina-
tions 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.
Fig. 7 Surface profile of plain surface of SCB1 soil–cement
block
Fig. 8 Surface profile of plain surface of SCB2 soil–cement
block
Fig. 9 Surface profile of rough textured soil–cement blocks
(SCB1 and SCB2)
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 9 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
Materials and Structures (2008) 41:1697–1712 1703

8. 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 con-
sisting 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.
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 estab-
lished 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 substan-
tiates 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 9 122 9 440 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
Fig. 10 Experimental set
up for determination of
shear bond strength and
couplet details
1704 Materials and Structures (2008) 41:1697–1712

9. 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 observa-
tions can be made from the results given in Table 4.
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. 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. Changing the bed face of the block from plain to
rough texture (i.e. CLA index 30 lm) 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 com-
binations attempted.
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. 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. 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. SCB2–CLM1 combination exhibits higher bond
strength (65–100%) when compared to SCB1–
CLM1 combination.
These results clearly indicate that rough textured
block surface, introducing frogs and surface coatings
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
Materials and Structures (2008) 41:1697–1712 1705

10. 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:
Type a: Interface failure, with clear separation of
the bond between the block surface and the
mortar at the interface (Fig. 11a).
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).
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).
Type d: Partial block/mortar failure, with partial
failure of both the block and the mortar
(Fig. 11d).
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 tech-
niques (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
Fig. 11 Failure pattern of
shear bond test couplets
1706 Materials and Structures (2008) 41:1697–1712

11. 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 compres-
sive 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 Sa-
rangapani 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. Shear bond strength of couplets varies between
0.12 and 0.51 MPa for CLM1–SCB1 combina-
tion. 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. Masonry compressive strength increases as the
shear bond strength increases and the relation-
ship 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 experi-
mental 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 Saranga-
pani et al. [6] and the present investigation, the
following points emerge:
(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
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
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
0
Shear bond strength (MPa)
)
a
P
M
(
h
t
g
n
e
r
t
s
e
v
i
s
s
e
r
p
m
o
c
m
s
i
r
P
Sarangapani et al [6]
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Fig. 12 Bond strength versus compressive strength for various
block–mortar combinations (figures in the graph refer to Ebrick/
Emortar ratios)
Materials and Structures (2008) 41:1697–1712 1707

12. considerable increase in masonry compres-
sive strength as the bond strength is
increased. The correlation between com-
pressive strength and bond strength is as
follows. Masonry compressive strength = r
in MPa, Shear bond strength = s in MPa.
r ¼ 1:457 þ 5:01s
ðcoefficient of correlation ¼ 0:89Þ
present study
r ¼1:796 þ 9:02s
ðcoefficient of correlation
coefficient ¼ 0:90Þ
Sranagpani et al. [6]
(b) Masonry compressive strength is not sensi-
tive to bond strength variation when the
masonry unit is stiffer than that of mortar.
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 investiga-
tions on masonry failure theories [1–5] 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 hori-
zontal compression in the masonry unit is due to the
stiffer mortar pulling it inwards for strain compatibil-
ity. 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 strengthmeans 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.
6.5 Failure pattern of masonry prisms
Generally the masonry prisms under uniform com-
pression 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. Ulti-
mately 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
Fig. 13 Nature of stresses
developed in the masonry
unit and mortar
1708 Materials and Structures (2008) 41:1697–1712

13. 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.
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. 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
Fig. 14 Typical crack pattern of the masonry prism
Fig. 15 Stress–strain
curves for SCB1–CLM2
masonry with various bond
enhancing techniques
Materials and Structures (2008) 41:1697–1712 1709

14. 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. 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 combina-
tion 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.
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 charac-
teristics of masonry for a range of bond strengths
were explored. The following conclusions emerge
from these exploratory studies.
1. Shear bond strength can be varied without
varying the characteristics of the masonry unit
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
Fig. 16 Shear bond strength versus initial tangent modulus for
masonry
1710 Materials and Structures (2008) 41:1697–1712

15. 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 coat-
ing 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 applica-
tion of an epoxy resin coating were also effective
in increasing the shear bond strength
significantly.
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 dou-
bling of compressive strength when bond
strength is increased by four times where Ebrick
to Emortar ratio = 0.09, whereas in this investi-
gation 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 varia-
tion when the masonry unit is stiffer than that of
the mortar (i.e. Eblock to Emortar ratio greater than
one).
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. 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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