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BOND STRENGTH OF THE BRICK MASONRY

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									   International Journal of Civil Engineering and Technology (IJCIET),
                                                 OF CIVIL ENGINEERING (Print),
  INTERNATIONAL JOURNALJuly- December (2012), ©ISSN 0976 – 6308 AND
   ISSN 0976 – 6316(Online) Volume 3, Issue 2,                         IAEME
                              TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 3, Issue 2, July- December (2012), pp. 380-386
                                                                            IJCIET
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2012): 3.1861 (Calculated by GISI)                 © IAEME
www.jifactor.com




              BOND STRENGTH OF THE BRICK MASONRY

                     C.Freeda Christy1, R.Mercy Shanthi2 and D. Tensing3
                        School of Civil Engineering, Karunya University,
                            Coimbatore-641114, Tamil Nadu, India


   ABSTRACT

          This paper presents the experimental investigations of small burnt clay brick masonry
   samples. The shear bond strength has been obtained from 3 brick triplets. An experimental
   programme examining the bond strength of mortar-unit joints was therefore carried out, using
   mortars with and without pozzolans. It has been found that the strength and deformation
   characteristics of masonry constituents obtained from these tests are more representative of
   the actual composite behaviour of masonry. But, there is a need for energy efficient and
   environment friendly alternative materials for masonry. Fly ash blocks or bricks represent
   one such alternative for masonry applications. The deformation characteristics of individual
   brick and mortar have been determined and found to be different due to the composite action
   between the brick and mortar joint. The research quantified the differences in bond strengths
   achieved with various mortar/masonry unit combinations.

   Keywords: Bricks, bond, pozzalona, triplet

   INTRODUCTION

           Masonry is a material built with brick units and mortar. Behaviour of masonry greatly
   depends on the characteristics of masonry units, mortar and the bond between them. Bond
   strength is dependent on many interrelated factors that can directly affect bond development
   (e.g. unit surface absorption, pore structure, mortar composition, mortar water retentivity and
   curing conditions) or indirectly affect bond strength (e.g. unit surface texture and
   workmanship). It was also suggested that both mortar quality and surface absorption criteria
   of the masonry unit are the most significant parameters in developing good bond and bond
   strength, Goodwin and West (1982)1 and McGinley (1990)2. While the surface absorption
   characteristics define the rate and volume of water to move from the mortar to the unit and
   the quality of the mortar defines the amount of water available at the interface and the
   strength of hydration products deposited in the unit surface pores.



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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

BRICKS

        Brick is a ceramic material mainly used in construction industry and it is one of the
most important building materials. Its production process involves forming of clay into
rectangular blocks of standard size, followed by firing to temperature ranging from 900 -
1200°C. It is made of clay or shale and when given desired shape is dried and fired into a
durable ceramic product. Energy consumption and pollution are the two important
environmental and cost concerns related to the brick industry. Compressive strength of brick
is important as an indicator of masonry strength and as a result brick strength has become an
important requirement in brickwork design. Surface characteristics and suction are the most
important properties in determining bond capability. Other durability indices have also been
developed based on relationship of porosity and water absorption. (i) Major usage in the
world for construction is clay bricks; many researchers are presently looking for newer
options because they need low cost materials, which are also environmentally friendly. The
process of manufacturing clay bricks also requires high energy to burn due to the emission of
CO2 gas from this process. Fly ash, a waste industrial product is being used in cement and
building material industries. The effect of brick absorption property due to variable raw
materials used in its manufacturing was shown by Surej et al. (1998)3. The property of total
absorption capacity of the brick is also very important for the performance of the brick. A
high absorption results in vulnerability to volume changes that would result in cracking of the
bricks and structural damage in buildings. It also would lead to cracking in the event of
freezing and thawing of the water inside the pores. Too little absorption also not desired,
because rain water rather than getting partially absorbed by the brick would tend to run off
very quickly towards the joints and may find its way into the building as well as reduce the
durability of the mortar joints. The absorption is the amount of water which is taken up from
the mortar to fill pores in the clay brick. Water absorption tests were performed on fly ash
bricks and clay bricks as per IS 3495 (1992)4. Water absorption of bricks is usually measured
by 24 h cold immersion test allows water to be absorbed into pores, which are easily filled
under cold condition where all pores are filled up with water. The average comparison of
water absorption in the clay bricks and the fly ash bricks is shown in Fig.1.

                                                           Water absorption of bricks
                       Water absorption of bricks




                                                    15.0

                                                    10.0
                                 in %




                                                     5.0

                                                     0.0
                                                               Fly ash bricks     Clay bricks

       Fig.1 Comparison of water absorption in fly ash bricks and the clay bricks

The water absorption of both clay brick and the fly ash brick were within the limit of 20% of
its weight. The water absorption of the clay brick was observed as 13.7% higher than the fly
ash brick. From the results, it was understood that fly ash brick has moderate level of water
absorption behaviour and hence fly ash based construction may yield good structure
performance.


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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

INITIAL RATE OF ABSORPTION (IRA)

         The initial rate of absorption is of great importance for laying the bricks and bonding
with the mortar. Mariarosa Raimondo (2009)5 reported that a high IRA results in too quick
drying of the mortar and strung out for the bed joint and stiffens so rapidly that the bricks in
the next course cannot be properly bedded and thus weakens the mortar and reduces its
adherence to the brick. On the other hand, if the IRA is too low, the surface of the brick
adjacent to the mortar would absorb the excess water and the bricks tend to float on the
mortar bed, which makes it difficult to lay plumb walls at a reasonable rate and result in very
weak layer of the mortar that would not have penetrated enough into the surface crevices and
pores of the brick. In either case there will be poor bond. The bond between the brick and the
mortar is largely influenced by the capacity of the brick to absorb water and the ability of the
mortar to retain the water. This water is needed for the proper hydration of cement where the
mortar contacts the brick. The power of a brick to absorb water is measured by the initial rate
of absorption as per ASTM C 67 (2009)6. Masonry walls built using brick units with a low
initial rate of absorption (IRA) often have lower bond strength than walls built with moderate
IRA units because very little water is available to be absorbed into the unit during installation
into the wall. Therefore, high absorption brick should be wetted prior (3 hrs to 24 hrs) to
lying in order to reduce the absorption and allow the brick's surface to dry. Drysdale et al
(1992)7 observed that if IRA is less than 0.25g/cm2/min, which is a case for low absorption
bricks, then such bricks may tend to flow on mortar particularly if the bricks are damp. On
the other hand if IRA is more than 1.5g/cm2/min a poor brick mortar bond may result because
of rapid suction of water in mortar by bricks. The details of the initial rate of absorption
experiment are indicated in Fig.2.




                      Fig. 2 Test on brick for initial rate of absorption
The brick specimen is weighed as w1. Then the brick is placed into 1cm depth of water for 60
seconds. Finally, the brick is removed from water and weighed as w2. The initial rate of
absorption (IRA) or suction is the rate of absorption of water in the first minute after contact
of the bed surface with water. The IRA is calculated as,
    Initial Rate of Absorption (IRA) in (gram/cm2/ minute) = (w2 – w1) / contact area ---- Eq 1
Excessive water suction in the brick can lead to considerable reduction in brick masonry
strength, because bricks absorb excess amount of water from the mortar and thus interfere
with complete hydration of the cement. In this experiment initial rate of absorption obtained
for clay brick was 0.16g/cm2/min and for the fly ash brick was 0.63 g/cm2/min respectively.
From the results, it was understood that the bond between the clay brick and the cement
mortar is less when compared to the fly ash brick and the cement mortar.

MORTAR MIX

        Mortar is used as a means of sticking or bonding bricks together and to take up all
irregularities in the bricks. Although mortars form only a small proportion of a masonry wall
as a whole, its characteristics have a large influence on the quality of the brick masonry. The

                                              382
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

primary mechanism for the development of bond strength is cement hydration (the chemical
action initiated when water is added to cement). High cement-content mortars are thus
beneficial, as discussed by Samia Guirguis (2003)8 in which a considerably higher bond
strength was achieved using a 1:1/4:3 (cement:lime:sand) mortar than when using a 1:1:6
mortar. Also discussed that the movement of fine particles (cementitious components and
fine sand) towards the brick/mortar interface while the mortar is still fluid is extremely
important in developing bond between the mortar and the masonry units. The utilization of
fly ash as cement replacement material in mortar or as additive in cement introduces many
benefits from economical, technical and environmental points of view as per Erdog Du
(1998)9. The use of fly ash is accepted in recent years primarily due to saving of cement,
consuming industrial waste and making durable materials, especially due to the improvement
in the quality stabilization of fly ash, as stated by Li Yijin (2007)10. Fly ash is another type of
pozzolanic material widely being used as a cement/fine aggregate replacement as reported by
Rajamane (2007)11. Many researchers, viz. Rafat (2003)12 and Chaid et al (2004)13 indicated
that low-calcium fly ash (class F) improves the interfacial zone microstructures. Portland
cement hydrates to produce calcium hydroxide as much as 20% to 25% by weight. Joshi and
Lohitia (1997)14 reported that, when the pozzolanic materials in the form of fly ash are added
to the cements, the C-H of hydrated cement is consumed by the reactive SiO2 portion of these
pozzolanas. This pozzolanic reaction improves the microstructure of cement composites as
additional C-S-H gel is formed and also the pore size refinement of the hydrated cement
occurs.

BOND STRENGTH

        The function of mortar in masonry wall is to bind the individual bricks or blocks
together to form a single element to resist the movement and stress; and (in external walls)
provide as a weather proof barrier. The bond between the mortar and the masonry units is one
of the most important properties of masonry construction, particularly when it is load bearing
such as in low-rise buildings. The bond between the mortar and the masonry units is one of
the most important properties of masonry construction. Poor bond and low bond strength is a
major weakness of brickwork. This bond is affected by many interrelated factors associated
with both masonry units and mortar. The mortar with partial replacement of fine aggregate
with fly ash is varied (0%, 10% and 20%) and studied for the bond strength. The intent was to
enhance the bond strength of the masonry by altering the microstructure of the mortar-unit
interface. The shear characteristics of the brick masonry and the interfacial interaction
parameters of brick/mortar joint were determined on masonry prism; by triplet prism test as
reported by Sarangapani (2002)15. There are two types of bonds between the mortar and the
brick units: chemical and friction. Tensile strength at the interface is primarily due to the
chemical bond. Hence, the chemical bond depends upon the absorption rate of the brick units
as reported by Reda Taha and Shrive (2002)16. Therefore, high absorption rate of the brick
units decreases the strength of the bond. Thus, brick units are usually wetted with water
before they are laid. The shear strength at the interface between the surface of mortar layer
and the surface of the brick unit is by the friction and the chemical bond between the mortar
and the brick units. The purpose for testing an assemblage of triplet brick prism is to
determine the maximum bond-shear strength retained by the joint between the mortar and the
brick. The bond shear strength is determined by testing a triplet specimen such that only shear
stresses develop in between the mortar and the masonry unit contact planes as shown in Fig.
3.


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International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
                                             July
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME




                         Fig. 3 Triplet bond test on brick masonry
Bond strength of brick masonry triplet prisms was investigated in this test. The unreinforced
triplet brick prisms of size 230mm x 220mm x 110mm were used in this study. The
reinforced brick prisms were made with woven mesh at the bed joints of the brick masonry.
The mortar used for the construction has the ratio of 1: 6 cement mortar mix with 0%, 10%
and 20% replacement of fine aggregate with fly ash. The clay brick masonry (CBM) and the
                                                               of
fly ash brick masonry (FBM) with partial replacement of fine aggregate with fly ash
(CBM10, CBM20, FBM10 and FBM20) were tested for both unreinforced and reinforced
with woven wire mesh (CBMR, CBM10R, CBM20R, FBM, FBM10R and FBM20R). The
                                                                            re
shear strength was obtained from the triplet test as shown in Fig.3, where the brick in the
middle is sheared and the upper and lower bricks are supported. The vertical shear load (P  (Pv)
was applied at the uniform rate with a hydraulic jack until shear failure occurred. The
                                                     subjected
masonry specimen is considered as a short beam subjected to an average bond stress and
evaluated as;
                         Bond stress, τb = Pv / 2A --------------- Eq 2
Where,
Pv = Vertical compressive load in N;
                                                 mm
A = Cross sectional area of the triplet prism in mm2;
                                                                             )
The triplet shear prism detail with the breaking load and the bond stress (τb) for various brick
prisms are reported in Table 1.

Table 1 Triplet shear prism detail
Specimen Hexagonal wire Specimen dimension                Breaking load     Bond    strength,
                                                                                    stre
           mesh                (mm)                       (N)               (τb) MPa
                                                                             τ
CBM        Unreinforced        250x220x110                3234              0.064
CBM10      Unreinforced        250x220x110                3498              0.069
CBM20      Unreinforced        250x220x110                4752              0.094
CBMR       Reinforced          250x220x110                5742              0.113
CBM10R Reinforced              250x220x110                5544              0.110
CBM20R Reinforced              250x220x110                7260              0.143
FBM        Unreinforced        250x220x110                11946             0.236
FBM10      Unreinforced        250x220x110                24948             0.493
FBM20      Unreinforced        250x220x110                20988             0.415
FBMR       Reinforced          250x220x110                44946             0.888
FBM10R Reinforced              250x220x110                72996             1.443
FBM20R Reinforced              250x220x110                62964             1.244


                                              384
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

The shearing load at failure is recorded as the maximum capacity of shear force retained by
brick – mortar bond. Lourenco (2004)17 reported that the shear strength of brick masonry
along the bed joint is the function of the bond strength between the mortar and the brick units
under zero compressive load. The comparison of bond stress of the brick masonry is depicted
in Fig.4.
                                                Bond strength on brick masonry
                  Bond strength, MPa   1.6       0% FA   10%FA    20% FA
                                       1.4
                                       1.2
                                       1.0
                                       0.8
                                       0.6
                                       0.4
                                       0.2
                                       0.0
                                                CBM          CBMR           FBM           FBMR
                                             Unreinforced and reinforced clay brick and fly ash brick
                                                                    masonry
                                             Fig. 4 Bond strength of brick masonry
From the results, it was found that the bond strength of reinforced clay brick masonry in the
ratio of 1:6 cement mortar was increased to 43.68% than unreinforced clay brick masonry.
Also, the bond strength of reinforced fly ash brick masonry in 1:6 cement mortar was
increased to 73.42% than the unreinforced fly ash brick masonry. The clay brick masonry
with 20% replacement of fine aggregate with fly ash in 1:6 cement mortar gave the higher
bond strength. Further, the fly ash brick masonry with 10% replacement of fine aggregate
with fly ash in the ratio of 1:6 cement mortar gave the higher bond strength as it react with
the pozzalona to produce strong calcium silicate hydrates. Reda Taha and Shrive (2002)16
reported that the reactivity of the pozzolanas affects the interface bond development as the
high reactive pozzolanas allow for early formation of the CSH gel and these strong hydrates
will provide the mechanical interlock between the unit and the mortar enhancing the bond
strength.
CONCLUSIONS
   •   Based on the triplet shear test, the presence of fly ash had a strong influence on the
       brick-mortar joint. The bond strength of unreinforced clay brick masonry in the ratio
       of 1:6 cement mortar with 20% replacement of fine aggregate with fly ash was 1.45
       times more than the unreinforced clay brick masonry in the ratio of 1:6 cement
       mortar.
   •   The bond strength of reinforced clay brick masonry in the ratio of 1:6 cement mortar
       with 20% replacement of fine aggregate with fly ash was 1.5 times more than the
       unreinforced clay brick masonry.
   •   The bond strength of reinforced fly ash brick masonry in the ratio of 1:6 cement
       mortar with 10% replacement of fine aggregate with fly ash was twice than the
       unreinforced fly ash brick masonry.
   •   Incorporation of fly ash in the brick masonry resulted in the reaction of pozzolanas
       with the calcium hydrate which produced strong calcium silicate hydrates, thus
       enhancing the bond strength of the brick masonry with the modification of the
       microstructure of the mortar-brick unit interface.

                                                                 385
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME

REFERENCES

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   3. Surej Rk, Fazio P, Feldman D (1998). Development of new Durability Index For Clay
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       Construction and Building Materials, Vol 18, 2004, p 125




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