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SHEAR CHARACTERISTIC OF INTERLOCKING MORTARLESS BLOCK MASONRY
JOINTS
Ahmed H. Alwathaf1, Waleed A. Thanoon2, Mohd Saleh Jaafar2, Jamaloddin Noorzaei2 and
Mohd Razali Abdul Kadir2
1Ph.D. Student, Faculty of Engineering, (UPM), 2Associate Professor, Faculty of Engineering, Universiti
Putra Malaysia (UPM)
E-mail: walid@eng.upm.edu.my
Abstract
Recently, interlocking mortarless masonry system has been developed as an alternative to the
conventional masonry system for wall construction. Shear characteristics in an interlocking
mortarless block masonry system are yet to be investigated. As a matter of fact, there is no code
provision or design guides available for the design of interlocking and mortarless wall system.
This paper presents laboratory experiments on shear characteristics of the interlocking joints in
mortarless masonry. Modified triplet tests on mortarless interlocking panels with different levels
of axial compression were carried out to ascertain the deformation and shear strength
characteristics of the system at the joint interface. Failure criterion for the interlocked bed joints
under combined normal-shear load is proposed.
Notation
The following symbols are used in this paper:
σn Normal stress from pre-compression load.
τu Shear strength of masonry joint.
τo Shear strength at zero pre-compression stress (σn = 0).
μ Friction coefficient in joint interface.
2
1. Introduction
Interlocking masonry blocks are different from those of conventional mortared
masonry in that the mortar layers are eliminated and instead the block units are
interconnected through interlocking keys (protrusions and grooves). Interlocking
between the blocks provides self-alignment; increase speed of wall construction and
improve wall stability during erection. The main characteristic in the interlocking system
is to assure efficient construction where the wall could be well-aligned even without
skilled masons.
There have been several attempts to develop interlocking mortarless hollow blocks
in different parts of the world in the recent past. These blocks however, widely varied in
dimensions, shapes and their interlocking mechanisms [1, 2, 3, 4, 5]. Most of developed
interlocking systems are used mainly for block alignment and to temporarily keep the
block in position before filling the voids with concrete and reinforcing steel [1, 2, 3]. The
usage of interlocking blocks as a load bearing wall is uncommon and requires further
experimental studies [4]. The mechanical characteristic of load bearing interlocking
block masonry together with mortarless interlocked joints is still unexplored because
their basic behaviour and complete response to loads up to failure is not well
understood.
Characteristic values of the shear strength for structures with mortar bedding
masonry are regulated by design codes. For interlocking mortarless masonry a design
code is not available and the shearing behaviour of interlocking mortarless masonry
structures needs to be investigated. Valuable parameters such as shear stiffness
characteristics can be obtained by establishing the shear load deformation relationships
3
through laboratory experiments. The tests could also contribute towards a better
understanding of the load carrying behaviour of the system.
Comprehensive tests had been carried out to study shear characteristics and
behaviour of conventional mortared masonry systems and dry stacked masonry by
using different test methods [6, 7, 8, 9, 10, 11, 12]. The shear tests were carried out
under different pre-compression forces to investigate the shear behaviour of masonry
bed joints for small block units without interlocking projections. These test methods to
determine the strength parameters for conventional/dry stacked masonry need to be
modified for the interlocking mortarless block system. The main objective of this
investigation is to ascertain the shear characteristics of the mortarless interlocking joints
in mortarless load bearing masonry using the modified triplet test set up.
2. Failure Criterion
The failure of mortared masonry joint under shear with moderate pre-compression
levels can be represented by the Coulomb friction law, which establishes a linear
relationship between the shear strength τu and the normal stress σn on the bed joint
area, being given by
τu= τo +μ σn (1)
where
σn is normal stress from pre-compression load
τo is the shear strength at zero pre-compression stress or cohesion strength
(σn = 0)
μ is the actual friction coefficient in joint interface.
4
The linear Coulomb type is valid for normal stresses σn less than 2 MPa [13, 14, 15].
In the case of higher axial stresses, masonry walls may fail although the friction
resistance in the joints is not activated to the full capacity. The failure is due to the
principal tensile stress reaching the diagonal tensile strength of the units, thus inducing
the units to crack. For mortarless interlocking masonry, the failure criteria for joint under
combined normal-shear envelop need to be developed in order to understand the
behaviour of the interlocking system. Furthermore, the failure criteria are essential for
the prediction of joint failures and analysis of the interlocking wall using finite element
method.
3. Interlocking block properties
The interlocking blocks used in this investigation are load bearing mortarless blocks
named as Putra Block, which had been developed at Universiti Putra Malaysia [4]. The
block units used in the test specimens have been produced especially for the current
study using a semi automatic block making machine in the university campus.
The block unit dimensions are 300 mm (length), 150 mm (width), and 200 mm
(height) as shown in Figure 1. The top protrusions are an extension for the webs which
have a width of 66 mm and a height of 20 mm. Side protrusions and grooves at the
ends of face-shells are also provided as shown in Fig. 1. The interlocking keys and
face-shell of the block have a small tapered shape of 2 mm to allow for ease of casting,
in addition the top keys have 2 mm tolerance for ease assembly.
Uniaxial compression and splitting tests were carried out to find the compressive and
splitting tensile strength for the individual block units according to ASTM C140-99b and
ASTM C 1006-84 [16,17] respectively. The average of compressive and tensile strength
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for the block units were 23.6 N/mm2 and 2.09 N/mm2 respectively. The compressive
strength of the block was obtained based on the face-shells bedded area.
4. Test Set-Up
The development of an ideal shear test method that reflects the absolute shear
strength value is still a challenge to masonry researchers. Unavoidable non-uniformity in
shear stress could occur in different test setup used for shear test due to flexural action
[7, 8, 9]. Hence the test setup used must reflect an accurate prediction of shear
strength.
In the absence of an established test method for shear in the interlocking block
system, triplet test set up [6, 7, 8] used for masonry joints had been modified to simulate
the actual interlocking features of the block. Different test set up used for the masonry
are shown in Figure 2(a)-2(d). The modified shear test set up for the interlocking
mortarless blocks is shown in Figure 3. This test set up is capable to include both face-
shell bed joints and the interlocking protrusions in resisting applied shear loads. The
main differences between the modified shear tests set up the existing triplet test set up
are the specimen size and loading arrangement. The shear test set up for the
conventional masonry had to be modified to allow for correct representation of the
masonry wall assembly in the interlocking block system. The actual test up carried out
for the interlocking masonry system is shown in Fig. 4. This modified test method can
also be used for different design of masonry system to predict their strength
characteristic under combined Shear-Normal load.
The specimen panels consist of 3-masonry courses constructing by stacking the
blocks on top of each other (see Fig.5) without using any mortar layers. The top and
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side protrusions and grooves of the blocks provide the necessary alignment of the
panel.
The top and bottom masonry courses are restrained to move horizontally whereas
the middle course is allowed to move under applied horizontal load. When the applied
shear load is greater than the shear resistance of the joints, the middle course will slide
providing the value of the shear strength of the two joints simultaneously. However,
since horizontal applied load and restraining loads are applied over the height of the
blocks; unavoidable minor flexural effect will be induced. This is a common problem in
most shear test-setup [7, 8, 9].
During the construction of test panels, cement mortar capping was used at the top
and bottom of the panel to ensure the evenness of the end surfaces. Cement mortar
capping was also used on the blocks that were in contact with side plates.
The test frame was fabricated and prepared using main steel frame and a strong
floor system. To restrain the horizontal movement of the top and bottom block courses,
two end plates were connected to vertical UB 250x100mm sections as shown in Fig. 4.
The stability of the lateral supports and the test frame were assured and confirmed by
monitoring their movements during the tests. Vertical and horizontal loads were applied
using two hydraulic jacks with maximum loading capacity of 500 kN each. Thick steel
plate of total thickness equal to 100 mm was fixed at the top of test specimen. A
spreader beam of 300 mm height was used to distribute the jack load as shown in Fig.4.
Two dial gauges were kept on both sides of the jack to detect any uneven movements.
5. Test Procedure and Measurements
7
In order to define the coefficient of friction of the interlocking mortarless bed joints,
four different pre-compression stress levels were adopted namely 0.5, 1.0, 1.5 and 2
N/mm2. Two panels for each pre-compression stress level (series I & II) were tested
under increasing shear load, giving a total of eight panel specimens tested in this study.
Initially, the vertical normal compressive load was applied incrementally by means of
the vertical hydraulic jack until the desired pre-compressive load was attained. Upon
reaching the desired compressive stress level, the vertical load was maintained and
horizontal (shear load) was applied incrementally. The vertical load was continuously
monitored to ensure the pre-compressive stress was kept at the desired level for the
whole duration of the shear load application.
Demec gauges were aligned in several locations to trace the deformation of the
joints under the vertical and shear loads (see Fig.6). The horizontal shear movement
(shear slip) was measured using 100mm demec gauge points that were installed
diagonally across the top and bottom bed joints of the middle course at specified
locations. This would allow for the shear deformation measurements in the sliding
stage. 50mm demec points were also installed vertically at the respective locations to
measure the vertical displacement of the joints during pre-compression stage. The
horizontal displacements of the joints were obtained from the changes in the length of
the diagonal lines that were given by the 100mm demec gauge points as shown in
Fig.7. In the slip calculation, the change in the angles in the inclined demes will be very
small and thus ignored.
6. Experimental Test Results
8
6.1. Deformation and Strength Characteristics
The behaviour of the test specimens can be studied from the recorded deformation
and their load carrying capacity. Fig.8 (a, b) shows typical curves of the shear slip
versus shear load at different points on the bed joints of the middle course.
Because the demec points were installed on different locations on the joint, they
appear to display different behaviour. Generally, two different patterns of shear slip
were observed. In the first pattern, joint movement was detected at low applied shear
forces and continued to increase until failure. This behaviour was observed mainly at
local points of DP1 and DP4 as shown in Fig.8 (a). In the second pattern of behaviour,
negligible movement (almost zero) was observed until shear load reach about 80% of
the failure load. Once the slip took place at this load level, the shear slips (given by local
points of DP3 and DP6) increased in a similar way to the first pattern as shown in Fig.8
(b). This is because of part of the applied horizontal load will be resisted by friction
between the top and bottom beam and specimen. However, these differences are
minor and not significant. Hence, the average readings of all the six locations are then
considered in detailed discussion on the shear load-slip behaviour.
Fig. 9 shows the relation between the shear load versus the shear slip of the bed
joints for series I and II specimens. Here, the shear slip represents an average of the
horizontal movement for the top and bottom joints of the middle course (DP1-DP6). In
general, similar to the mortared masonry system, increasing the pre-compressive stress
develops higher shear strength. Identical results were obtained from series I and II
except for the panel specimen under pre-compressive load of 142 kN in series II
(normal stress is 1.97 N/mm2). This is due to the development of two cracks in the
specimen which will be highlighted in section 6.2.
9
A major part of shear strength for this system is from the friction forces, however, the
interlocking between block plays its role after the peak strength is reached when the
interlocking parts came into contact. From Fig. 9, this happens after about 1mm slip.
The slip continues to increase in the post-peak range with no significant increase in the
shear load indicating elasto-plastic behaviour, as shown in Fig.9. This observation is
one of the main differences between the interlocking mortarless and conventional
mortared joints. In the conventional joint, the shear stress is relatively decreasing in the
post-peak range as shown in Fig.10 [7, 9]. However, in pre-peak loads, the shear
deformation characteristics for both masonry systems are quite similar.
6.2. Mode of Failure
The failure mode was generally that of sliding along the bed joints of the middle
course as shown in Fig. 11 (a-d), where the slip is distinct between the movable and
restrained courses. However, the sliding failure of panel SH4 series II is displayed by
the cracking of the blocks at the top and middle course as shown in Fig.12 (a) and (b).
Generally, the uneven beds from block to block at the joints occurred during casting
process of the block affects the shear strength. The friction resistance of the bed joint
area was not fully utilized, which in turn causes stress concentration at localized areas.
This observation is seen as in Fig. 13. Therefore, the unevenness of bed joint
significantly decreases the shear strength of the system. For higher pre-compressive
stress and higher shear stress, the principal tensile stress may reach the tensile
strength and inducing cracks in the units as well as slipping in the joints. The
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interlocking keys between the block courses failed partially after shear slip of about 2
mm as shown in Figs.14 and 15.
6.3. Normal-Shear Stresses Failure Envelop
According to the Coulomb failure criterion, the shear strength increases as the axial
compression increases and the respective curves of combined shear-compression
stresses are ascending proportionally.
Fig. 16 shows the relation between the normal stress and the shear strength for all
test specimens, as well as a linear regression line obtained to fit all the test results. The
average shear stresses are based on the two face-shell bedded areas in the shear
planes at the top and bottom joints and the normal stresses applied were based on the
normal area of bedded face-shell.
The slope of the correlation yields the coefficient of friction, μ, whereas the
interception gives the cohesion parts (τo). For interlocking mortarless masonry, the
predicted cohesion value, τo, is zero which is confirmed in the present tests as shown in
Fig. 16. The regression indicates a friction coefficient value equal to 0.603. For different
mortared hollow block masonry, the value of friction coefficient highly varies from 0.21
to 1.54 [9]. In the BS standard the friction coefficient is taken as 0.60 for the concrete
and masonry faces [14]. Although the shear resistance is not fully utilized due to
unevenness of bed joints, the interlocking masonry system provides sufficient shear
resistance as required by BS standard.
7. Conclusion
11
Modified triplet test presented in the study for small panel was used successfully to
assess the shear behaviour of mortarless interlocking masonry.
The experimental tests show that the observed friction behaviour for the interlocking
mortarless joints under different axial compression follows the linear Coulomb type
failure criterion and having coefficient of friction of 0.603 with zero cohesion
contribution.
The unevenness of the bed joints significantly affects the shear strength of the
system because the area for friction resistance is not fully utilized. The tolerances
provided in the interlocking keys for the ease of block assembly affects the transfer of
shear stress between adjacent block units.
In the pre peak range of loads, the average shear-slip response of the interlocking
masonry joint is characterized by insignificant slip at shear load lower than 80% of the
peak value. While in the post-peak range the joint revealed a relatively constant shear
load which is the major difference between this system and the conventional masonry
system.
8. References
1. THALLON, R. Dry-Stack Block. Fine Homebuilding Magazine, August, pp. 50-57,
1983.
2. HAENER, .Stacking Mortarless Block System. Engineering Design Manual by
lkinson Engineering, Inc. Hamilton, Ontario, 1984.
3. CETHOLIC, Mortarless Masonry – The Mecano System. Housing Science, 12, (2),
pp.145-157, 1988.
4. THANOON, W. A., JAAFAR M. S., ABDUL Kadir, M. R., Ali, A. A., TRIKHA, D.N.
and NAJM, A. M. Development of an Innovative Interlocking Load Bearing Hollow
Block System in Malaysia. Construction and Building Materials, 18, pp 445-454,
2004.
12
5. OH, K., HARRY, H.G. and HAMID, A.A. Development of New Interlocking and
Mortarless Block Masonry Units for Efficient Building System. 6th Canadian Masonry
Symposium, Saskatoon, Canada, pp.723-734, 1992.
6. JOHNSON, F.B. and THOMPSON, J.N. Development of Diametral testing
Procedures to Provide a Measure of Strength Characteristics of Masonry
Assemblages, Designing Engg. And Const. With Masonry Products, Johnson, F.b.,
Gulf Publishing Co., Houston, USA, pp.51-57, May 1969.
7. HAMED, A. A., DRYSDALE, R. G., and HIDEBRECHT, A. C. Shear Strength of
Concrete Masonry Joint. Journal of structural division Proc. of ASCE, 105, pp 106-
113, 1979.
8. MARZAHN, G. The Shear strength of Dry-Stacked Masonry Walls, Leipzig Annual
Civil Engineering Report, 3, pp. 247-261 ,1998.
9. GUO, P. Investigation and Modelling of the Mechanical Properties of Masonry. Ph.
D. Thesis, McMaster University, 1991.
10. Yokel, F.Y. and Fattal, S.G. Failure Hypothesis for Masonry Shear Wall, Jnl. Of
Structural Division, ASCE, 102, No.ST3, pp. 515-532, 1976.
11. PAGE, A. W. A Finite Element Model for Masonry, Jnl. Of Structural Division,
ASCE,104, (8), pp. 1267-1285 ,1978
12. EL-SAKHAWY, N. R. , ABDEL RAOF, H. and GOUHA,R A. Shearing Behaviour of
Joints in Load-Bearing Masonry Wall, J. Mat. in Civ. Engrg., ASCE,14, (2), pp. 145-
150 , 2002.
13. HENDRY, A. W. Structural Brick Work, Macmillan, London, 1990.
14. British Standard Institution Code of Practice for Use of Masonry: Part1: Structural
Use of Un-reinforced Masonry, BS 5628: Part 1, BSI, London, 1992.
15. RIDDINGTON, J. R. and JUKES, P. A. Masonry Joint Shear Strength Test Method,
Proc. Instn Civ. Engrs Structs & Bldgs, 104 ,(8), pp 267-274. , 1994.
16. American society for testing and materials .Sampling and Testing Concrete Masonry
Units and Related Units. ASTM C140-99b, Philadelphia, PA, 1999.
17. American society for testing and materials. Splitting Tensile Strength of Masonry
Units, ASTM C 1006-84, Philadelphia, PA, 1996.
13
List of Captions of Figures
Figure 1: Interlocking block unit dimensions
Figure 2: Different types of shear tests set-up
Figure 3: Test set-up
Figure 4: A specimen under test
Figure 5: Blocks arrangement in a specimen
Figure 6: Dimensions of the panel specimen and location of displacement measuring
points, DPs.
Figure 7: Diagonal demec point's displacement (d) and horizontal slip(s)
Figure 8 (a): Typical shear slip at points DP1 and DP4 on the bed joints.
Figure 8 (b): Typical shear slip at points DP3 and DP6 on the bed joints.
Figure 9: Relation between the shear loads versus the shear slip of the bed joints for all
panels
Figure 10: Typical shear test results of conventional mortared joint
Figure 11 (a-d): Typical slip failure at the bed joints
Figure 12: Slipping and cracking of panel SH4 (II)
Figure 13: Failure surface of a face-shell bed joint
Figure 14: Cross section view interlocking system after failure
Figure 15: Interlocking projections failure of bottom course
Figure 16: Relation between normal stress and shear strength
14
List of Figures
Fig. 1. Interlocking block unit dimensions
15
Fig.2: Different types of shear tests set-up
Fig.3. Test set-up
End Plates
Supports
Vertical load
Shear load
a) Triplet test (set-up I) [6]
b) Triplet test (set-up II) [7, 8]
c) Couplet test [9]
d) Off-Axis compression test [10, 11]
16
Fig.4. A specimen under test
Fig.5. Blocks arrangement in a specimen
17
Fig.6. Dimensions of the panel specimen and location of displacement measuring
points, DPs.
Fig.7. Diagonal demec point's displacement (d) and horizontal slip(s)
Ψ=45
Li
L
s
d
Demec Point, DPs.
All dimensions in mm.
18
0
20
40
60
80
100
120
140
160
180
200
-1 0 1 2 3 4 5 6
Shear slip mm
Shear Load kN
DP1
DP4
4
1
Norma load
Shear
load
Fig.8 (a). Typical shear slip at points DP1 and DP4 on the bed joints.
0
20
40
60
80
100
120
140
160
180
200
-1 0 1 2 3 4 5 6
Shear slip mm
Shear Load kN
DP3
DP6
3
6
Normal load
Shear
load
Fig.8 (b). Typical shear slip at points DP3 and DP6 on the bed joints.
19
0
20
40
60
80
100
120
140
160
180
200
0 1 2 3 4 5 6 7
Average Shear slip mm
Shear Load kN
P=34 kN (I)
P=34 kN (II)
P=69 kN (I)
P=69 kN (II)
P=108 kN (I)
P=108 kN (II)
P=142 kN (I)
P=142 kN (II)
Fig.9. Relation between the shear loads versus the shear slip of the bed joints for all
panels
Fig.10. Typical shear test results of conventional mortared joint [9]
20
Fig.11 (a, b). Typical slip failure at the bed joints
Fig.11. (c, d).Typical slip failure at the bed joints
Fig.12. Slipping and cracking of panel SH4 (II)
(c)
(d)
(a)
(b)
(a)
(b)
21
Fig.13. Failure surface of a face-shell bed joint
Fig.14. Cross section view of interlocking system after failure
High friction
stress
22
Fig.15. Interlocking projections failure of bottom course
y = 0.603x
R2 = 0.98
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0.00 0.50 1.00 1.50 2.00 2.50
Normal stress N/mm2
Max shear stress N/mm
2
Present test results
Linear (Present test
results)
Fig.16. Relation between normal stress and shear strength
