57 CHAPTER – 3 MATERIAL PROPERTIES OF STRUCTURAL MASONRY 3.1 INTRODUCTION In engineered masonry, the compressive strength fpm and the modulus of elasticity Epm of the material are the two main components of the element. Compressive strength is important because it determines the bearing capacity of the element; the modulus of elasticity is important because it provides the deformation of the element under loading. The compressive strengths of masonry unit and mortars are two of the most tested properties for typical projects because, the specimens are relatively easy and inexpensive to prepare when compared with the testing for other properties. When structural masonry is subjected to vertical loading, the design parameters such as the stress-strain relationship and the elastic property are to be understood. In order to study the elastic properties of brick masonry in detail, mortar cubes and brick prisms were cast. In this research some preliminary investigations were determined for evaluating physical and mechanical characteristics of bricks, mortar and brick masonry. The parameters are, Brick strength, Mortar strength, Bond shear strength between brick - mortar and Masonry strength 3.2 FLY ASH Fly ash are the artificial pozzalona which is basically derived as the residue during combustion of pulverized coal used as fuel. During the combustion of coal, the products formed are classified into two categories viz. bottom ash and fly ash. The bottom ash is that part of the residue which is fused into particles. The fly ash is that part of the ash which is entrained in the combustion gas leaving the boiler. Most of this fly ash is collected in either mechanical collectors or electrostatic precipitators. In India, coal contains very high percentage of rock and soil and therefore ash contents are as high as 50%. Ash may be classified into two groups as class C and class F, based on the nature of their ash constituents. One is the bituminous ash (class F) and the other is the lignite ash (class C). The lignite ash (class C) in India is produced at Neyveli thermal power plant and most of the other power plants in India produce bituminous ashes (class F). Both class F and class C fly ash react to cement in similar ways and undergo a ―pozzolanic reaction‖ with the lime (calcium hydroxide) by the hydration (chemical reaction) of
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57
CHAPTER – 3
MATERIAL PROPERTIES OF STRUCTURAL MASONRY
3.1 INTRODUCTION
In engineered masonry, the compressive strength fpm and the modulus of elasticity Epm of the
material are the two main components of the element. Compressive strength is important because
it determines the bearing capacity of the element; the modulus of elasticity is important because
it provides the deformation of the element under loading. The compressive strengths of masonry
unit and mortars are two of the most tested properties for typical projects because, the specimens
are relatively easy and inexpensive to prepare when compared with the testing for other
properties. When structural masonry is subjected to vertical loading, the design parameters such
as the stress-strain relationship and the elastic property are to be understood. In order to study the
elastic properties of brick masonry in detail, mortar cubes and brick prisms were cast. In this
research some preliminary investigations were determined for evaluating physical and
mechanical characteristics of bricks, mortar and brick masonry. The parameters are,
Brick strength,
Mortar strength,
Bond shear strength between brick - mortar and
Masonry strength
3.2 FLY ASH
Fly ash are the artificial pozzalona which is basically derived as the residue during combustion
of pulverized coal used as fuel. During the combustion of coal, the products formed are
classified into two categories viz. bottom ash and fly ash. The bottom ash is that part of the
residue which is fused into particles. The fly ash is that part of the ash which is entrained in the
combustion gas leaving the boiler. Most of this fly ash is collected in either mechanical
collectors or electrostatic precipitators. In India, coal contains very high percentage of rock and
soil and therefore ash contents are as high as 50%. Ash may be classified into two groups as
class C and class F, based on the nature of their ash constituents. One is the bituminous ash
(class F) and the other is the lignite ash (class C). The lignite ash (class C) in India is produced at
Neyveli thermal power plant and most of the other power plants in India produce bituminous
ashes (class F). Both class F and class C fly ash react to cement in similar ways and undergo a
―pozzolanic reaction‖ with the lime (calcium hydroxide) by the hydration (chemical reaction) of
58
cement and water to form the calcium silicate hydrate which is the binder (ie) cement. In
addition, some class C fly ash may possess enough lime to be self- cementing, in addition to the
pozzolanic reaction with lime from cement hydration. In India, Tamilnadu has four major coal
based thermal power plants, they are Ennore thermal power station, Tuticorin thermal power
station, Mettur thermal power station and North Chennai thermal power station. The coal from
the mines Talcher and Ib Valley of MCL and Raniganj and Mugma of ECL is transported to the
load ports of Paradip (Orissa), Vizag (Andhra Pradesh) and Haldia (West Bengal) respectively
through rail. Thereafter, the coal is transported to the discharge ports of Ennore and Tuticorin by
ships. From Ennore port, the coal is transported again through rail to Ennore thermal power
station and Mettur thermal power station.
3.2.1 XRD studies on fly ash
Chemical constituents of fly ash mainly depend on the chemical composition of the coal.
However, fly ashes that are produced from the same source have very similar chemical
composition and significantly different ash mineralogy depending on the coal combustion
technology used. The fly ash used for this study was collected from Mettur thermal power plant,
Tamil Nadu, India. XRD pattern obtained on fly ash material collected from Mettur thermal
power plant in India is shown in Fig. 3.1.
Fig 3.1 XRD pattern of fly ash
The XRD pattern (Fig 3.1) confirms the presence of Al2O3 and SiO2 as predominant materials in
the fly ash. Reda Taha and Shrive [2002]122
reported that the calcium bearing silica and silicate
minerals of ash occur either in crystalline or non-crystalline structures and are hydraulic in
nature; they easily reacts with water or hydrated lime and develop pozzolanic property. The
XRD detail for the fly ash used in the study is given in Table 3.1.
59
Table 3.1 – Comparison of XRD data obtained on fly ash material (collected from Mettur
thermal power plant, India) with standard JCPDS data
Standard XRD
data for
SiO2(JCPDS
No. 89-1668)
(2θ values)
Standard XRD
data for Al2O3
(JCPDS No. 88-
0107) (2θ values)
Standard XRD
data for CaO
(JCPDS No.
82-1690) (2θ
values)
Standard XRD
data for MgO
(JCPDS No. 89-
7746) (2θ values)
Powder XRD data
for fly ash material
(2θ values) I/Io
9.109 -- -- -- 8.8738 7
20.456 21.201 -- -- 20.9116 18
-- 21.315 -- -- 21.4366 3
26.773 -- -- -- 26.6957 100
27.365 -- -- -- 27.1553 5
27.756 -- -- -- 27.7921 8
-- 28.235 -- -- 28.0500 28
28.141 -- -- -- 28.6111 4
36.696 36.953 37.401 36.863 36.6172 6
39.517 40.023 -- -- 39.5304 5
45.756 45.988 -- -- 45.8548 7
-- 51.026 -- -- 50.8024 4
-- 57.139 -- -- 57.4901 12
-- 60.686 -- -- 60.0364 5
-- 67.910 67.467 -- 68.2064 4
-- 76.293 -- -- 76.8941 3
-- 81.136 -- -- 81.1921 5
-- -- 88.659 -- 90.8898 5
Katsioti et al [2009]75
studied the substitution of limestone filler with pozzolanic additives in
mortars and reported that the major portion of the fly ash material consists of SiO2 (48.09%),
reactive SiO2 (35%), Al2O3 (21.38%) and CaO (13.37%). Ozlem et al [2008]113
characterized
the fly ash material and studied its effect on the compressive properties of portland cement.
They studied the percentage of oxides present in five different fly ash materials and reported that
fly ash material consists of SiO2 (22- 57%), Al2O3 (5.9 – 23.2 %) and Fe2O3 (3.6 – 9.8 %). X-ray
diffraction study was carried out for the fly ash used in the study. The XRD data of the sample
was compared with the standard JCPDS for the XRD data of SiO2, Al2O3, CaO and MgO. The
major portion of 2θ values were matched with the JCPDS patterns for SiO2 (JCPDS card No. 89-
60
1668) and Al2O3 (JCPDS card No.88-0107). Few peaks of the samples matched with the JCPDS
pattern of CaO (JCPDS card No.82-1690) and MgO (JCPDS card No.89-7746). The comparison
data is indicated in Table 3.1. From the XRD measurements, it is concluded that the fly ash
material used in this study has the predominant oxides such as SiO2(S), Al2O3(A), MgO(M) and
CaO(C).
3.3 BRICK UNIT
Building bricks are usually made with mixture of clay and sand, which are mixed and moulded
in various ways and are dried and burnt. Tutunlu Faith and Atalay Umit [2000]137
reported that
the clay for brick making must develop proper plasticity and be capable of drying rapidly
without excessive shrinkage, warping or cracking and of being burnt to desired texture and
strength. This process for making clay bricks, require heating of the bricks in kilns to more than
2000oF, which consumes much fossil fuel and generates air pollutants and carbon dioxides due to
the combustion of the fossil fuel; Fly ash is utilized to make bricks in one of several ways: (a) as
substitute for a portion of the cement and/or aggregates in making concrete bricks and blocks; (b)
as substitute for a portion of the clay used in making clay bricks. (c) as substitute for all the clay
used in making clay bricks, using the same process for making clay bricks which requires
burning fossil fuel to heat adobes in kilns at over 2000oF. This uses the same process and has the
same drawback of using 100% fly ash in making bricks; and (d) as the mixture of the fly ash
20% to 60%, lime, sand and gypsum in making pressed bricks and dried.
3.3.1 Clay bricks
Bricks are the standard units of traditional building construction. Bricks have been used since
ancient times for walls and columns of residential and non-residential buildings. Bricks are made
from soil and hence the property of bricks depends on the properties of soil. Raw materials
required for manufacturing of clay bricks are clay, silt and sand. As per IS 2117 [1991]66
alumina
(20 - 30%) to impart plasticity to the earth for mould; silica (50 – 60%) to prevent cracking,
shrinkage and warping during drying and burning; lime (small quantity) to prevent shrinkage of
raw bricks, iron oxide (5- 6%) to retain the red colour to bricks; and magnesia a small quantity to
impart yellow tint to bricks and to decrease shrinkages. The four distinct stages of manufacturing
the hand mould clay bricks are: (i) preparing the brick earth (ii) moulding clay in rectangular
blocks of uniform size (iii) drying in sun and air and (iv) burning them in brick kilns. Burning of
the brick during manufacture governs the quality and properties of brick and uses more fossil
fuels.
61
3.3.2 Fly ash bricks
Fly ash bricks manufacturing units can be set up near thermal power stations. Raw materials
required for manufacturing of fly ash bricks are fly ash, lime gypsum and sand (optional). The
general composition is: fly ash (50 -75%), lime (8 -20%), MgO content should be maximum of
5%, gypsum (2 - 5%) to accelerate hardening processes and acquiring early strength, sand (20 -
30 %) to enhance the gradation of the mix as per IS 13757 [1993]62
. In the presence of moisture,
fly ash reacts with lime at ordinary temperature and forms a compound possessing cementitious
properties. After reactions between lime and fly ash, calcium silicate hydrates are produced
which are responsible for the high strength of the compound. This process involves
homogeneous mixing of raw materials (generally fly ash, sand and lime), with chemical
accelerator like gypsum, then moulding of bricks and curing of the fly ash bricks. Bricks made
by mixing lime and fly ash are therefore chemically bonded bricks. These bricks are suitable for
use in masonry just like common burnt clay bricks. Generally, dry fly ash available from power
plants meets the properties specified in IS 3812 [1966]69
. After the processing, the bricks are
dried on applying required quantity of water on the bricks. After two days the dried bricks are
sold. Tayfun [2007]131
described that the manufacturing of clay brick requires kilns fired to high
temperatures that cause wastage of energy, air pollution and generate greenhouse gases that
contribute to global warming. It should be noted that the use of fly ash in the building material
also improves the properties of building material as indicated by Vyasa Rao and Raina [2005]140
.
Manufacturing each ton of fly ash bricks instead of clay bricks will reduce the emission of
carbon- di-oxide – the major greenhouse gas by 0.0434 ton as stated by Henry Liu [2007]57
.
3.3.3 Tests on brick
The clay bricks of size 230 x 110 x 70mm were procured from Sadivayal, near Coimbatore and
the fly ash bricks of size 230 x 110 x70mm were procured from Saravanampatti near Coimbatore
(Fig 3.2 and 3.3). These bricks were used in this study. The bricks were tested for their strength
and other properties and their results are discussed below:
62
Fig. 3.2 Clay bricks Fig. 3.3 Fly ash bricks
Conventional clay bricks and fly ash bricks were tested as per IS 3495[1976]68
and ASTM C 67
[2009]6 to obtain;
Mass of brick
Water absorption
Initial rate of absorption (IRA)
Compressive strength of the brick
Flexural strength of the brick
Elastic properties of the brick
3.3.3.1 Mass of brick
The tendency of an object to resist changes in its state of motion varies with the mass as it is
solely dependent upon the inertia of an object. The more inertia which an object has, the more
mass it has. More massive object in a structure has a greater inertia force on the structure when
acceleration is applied on the structure. The mass comparison of clay brick and fly ash brick is
shown in Fig. 3.4.
Fig. 3.4 Mass comparison of clay bricks and the fly ash bricks
0
0.5
1
1.5
2
2.5
3
3.5
4
1 2 3 4 5 6
Bri
ck m
ass
, K
g
Number of samples
Brick mass
Clay brick Fly ash brick
63
From the results, it is found that, generally fly ash bricks are 8.58% lighter than the clay bricks
used in this study. As the mass of the brick reduces, the loads on the structural elements also
reduce which may offer better strength to weight ratio. Due to this reason, the construction of
buildings using fly ash bricks can be done quickly and easily, in turn saves time and labour costs.
3.3.3.2 Water absorption test
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]68
. The specimens were immersed in water at room temperature
(22°C) for 24 h and the weight recorded as ws (saturated weight). All the specimens were dried
and the weight of dried specimens were recorded as wd (dry weight), where ws and wd are in kg.
The water absorption by the brick is calculated as,
Water absorption of brick (%) = [(ws-wd)/wd)] ×100 --------Eq. 3.1
The average comparison of water absorption in the clay bricks and the fly ash bricks is shown in
Fig. 3.5.
Fig.3.5 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
0.0
5.0
10.0
15.0
Fly ash bricks Clay bricks
Wate
r ab
sorp
tion
of
bri
cks
in %
Water absorption of bricks
64
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.
3.3.3.3 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]89
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 brick and 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]31
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 below.
Fig 3.6 Test on brick for initial rate of absorption
65
The brick specimen is weighed as w1. Then the brick is placed into 1cm depth of water for 60
seconds as shown in Fig 3.6. 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 3.2
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/cm
2/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.
3.3.3.4 Compressive strength of the bricks
Compressive strength tests were performed on clay brick and fly ash brick specimens for all four
different orientations such as frog upward, frog downward, brick placed on shorter edge and
brick placed on longer edge under constant loading rate and the results were compared in Fig 3.7.
The term frog represents the indentation on one bed of the brick. When the indentation is upward
it is called as frog upward and vice-versa.
Fig. 3.7 Compressive strength of bricks for different orientations
From the results, it was understood that the compressive strength of fly ash brick is at-least
46.8% better than that of the clay bricks available in Coimbatore. This is also of great
significance because the fly ash bricks can be used as the main load bearing elements that would
0
4
8
12
16
Frog upward Frog
downward
Brick placed
on shorter
Edge
Brick placed
on longer edgeCom
pre
ssiv
e st
ren
gth
in
MP
a
Orientation of brick
Compressive strength of bricks
Clay Brick Fly ash bricks
66
be able to carry several floors more than limits prescribed for the normal clay bricks based
construction.
Fig. 3.8 Compressive strength of clay bricks and the fly ash bricks with upward orientation
The average compressive strength of the fly ash bricks was 54.2% higher than the clay bricks as
shown in Fig. 3.8. One of the reasons for low strength of clay brick may be the presence of large
sized pores and high level of porosity as reported by Sarangapani [2002]125
.
3.3.3.5 Flexural strength of the brick
Compressive strength of masonry under uni-axial compression depends chiefly on the tensile
strength of the brick units. The flexural strength of the bricks was performed by single point tests
as per IS 3495 (Part – III – 1976)58
. The test specimen was placed centrally on self aligning
bearers with two steel rollers of 40mm diameters as reported by Dayaratnam [1987]25
. The
central point loading was applied as shown in Fig 3.9.
Fig. 3.9 Single point flexure test on brick
The rollers were mounted in such a manner that the load was applied axially and equally divided
between the two rollers. The load was applied at a uniform rate increasing continuously till the
specimen cracked and the maximum load applied to the specimen during the test was recorded
and the flexural strength was calculated as, F = 3PL/2BD2
0.0
2.0
4.0
6.0
8.0
10.0
1 2 3 4 5 6
Com
pre
ssiv
e st
ren
gth
, M
Pa
Number of samples
Compressive strength of brick
Clay brick Fly ash brick
67
where, F = Flexural strength of the brick in MPa
P = Load in Newtons
L = Span between the bearers in mm
B = Width of the brick in mm
D = Depth of the brick in mm
Fig.3.10 Flexural strength of clay brick and fly ash brick
The fly ash bricks had 56% higher flexural strength than the clay bricks on an average. The
tensile strength expressed in the form of the modulus of rupture value and is nearly 2.27 times
the value for normal clay bricks as shown in Fig 3.10. It is of considerable importance because, it
results in much less cracking in the fly ash bricks. Hence, the fly ash brick structure can
withstand flexure for higher load than the clay bricks.
3.3.3.6 Elastic property of brick
Stress-strain curves are an extremely important graphical measure of a material‘s mechanical
properties. Stress-strain curve of clay brick and the fly ash brick shows a non linear behaviour as
shown in Fig.3.11. The failure compressive strain (mm / mm length) developed is in the range of
0.014 to 0.022 for clay brick and 0.022 to 0.027 for fly ash brick.
0
0.5
1
1.5
2
2.5
3
3.5
Clay brick Fly ash brick
Fle
xura
l st
rength
of
the
bri
ck i
n
MP
a
Flexural strength of the brick
68
Fig.3.11 Stress-strain curve of brick
The initial tangent modulus and the tangent modulus at 60% of ultimate stress are presented in
Table3.2 and the values are compared with values obtained by Mathana [2002]90
and
Sarangapani [2002]125
. Evaluating the ratio of lateral strain to the axial strain, the average value
of poisson‘s ratio was determined.
Table 3.2 Elastic properties of bricks
S.
No
Type of brick
Initial
tangent
modulus
(MPa)
Tangent
modulus at
60% of ultimate
stress (MPa)
Poisson’s
ratio
Ultimate
stress
(MPa)
Peak
strain
mm/ mm
length
1 Clay brick 108 200 0.11 3.29 0.023
2 Fly ash brick 266 420 0.15 7.18 0.029
3 Table moulded brick,
Mathana [2002]90 408 408 0.16 2.3 0.0074
4 Table moulded brick,
Sarangapani [2002]125 500 467 0.05 - 0.0081
The Tangent modulus at 60% of ultimate stress of clay brick is about 200MPa and the peak
strain is 0.023mm per mm length with ultimate stress of 3.29MPa. Whereas, the tangent modulus
at 60% of ultimate stress of fly ash brick is about 420MPa and peak strain obtained is 0.029mm
per mm length for the ultimate stress of 7.18MPa. The fly ash bricks have a pleasing colour like
cement, uniform in shape and smooth in finish, also they require no plastering for building work.
High compressive strength eliminates breakages/wastages during transport and handling. The
cracking of plaster is reduced due to lower thickness of joints and plaster and basic material of
the bricks, which is more compatible with cement mortar. Due to its comparable density, the
0.0
2.0
4.0
6.0
8.0
0.000 0.010 0.020 0.030 0.040
Str
ess,
MP
a
Strain (mm/mm length)
Stress - strain curve of brick
Clay brick
Fly ash brick
69
bricks do not cause any extra load for the design of structures and provides better resistance for
earthquake loads due to panel action with high strength bricks. The properties obtained for fly
ash bricks and clay bricks used in this study are discussed in the Table 3.3.
Table 3.3 Properties of fly ash bricks and clay bricks
S.No Properties Fly ash bricks Clay bricks
1 Basic Raw Material pozzolana – fly ash clay
2 Fuel Not required Required
3 Size and Quality Uniform(Factory made) Uneven(mould made)
4 Number of Joints in construction Less (uniform size) More (uneven size)
5 Mortar requirement Less More
6 Plastering Less More
7 Direct Gypsum Plaster Possible Not viable
8 Compressive strength MPa 7.18 MPa 3.29 MPa
9 Flexural strength, MPa 2.91 1.28
10 Standards IS: 13757 [1993]62
IS:3495(Pt.I) [1992]68
11 Water absorption (%) 11.0 12.8
12 Initial rate of absorption (IRA)
(g/cm2/min)
0.63 0.16
13 Poisson‘s ratio 0.15 0.11
14 Green material rating [Paresh,
2010]117 A Scale B Scale
15 Mercury from environment [Henry
Liu, 2007]57 Adsorbs Emits
16
Thermal conductivity, Michele
[2004]94
and Gangadhara Rao
[1998]46
0.9 – 1.05 W/m2 0C 1.25 – 1.35 W/m
2 0C
3.4 MORTAR
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 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]39
. The use
of fly ash is accepted in recent years primarily due to saving cement, consuming industrial waste
and making durable materials, especially due to the improvement in the quality stabilization of
70
fly ash, as stated by Li Yijin [2007]82
. Fly ash is another type of pozzolanic material widely
being used as a cement/fine aggregate replacement as reported by Rajamane [2007]121
. Many
researchers, viz. Rafat [2003]120
and Chaid et al [2004]17
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]74
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. Hydration of tri-calcium-aluminate in the ash provides
one of the primary cementitious products in many ashes. The rapid rate at which the hydration of
the tri-calcium-aluminate results in the rapid set of these materials and is the reason for the delay
in lower strengths of the stabilized material, as reported by Dattatreya et al [2002]23
. Use of the
waste material like fly ash as partial replacement with cement and fine aggregate as 0%, 10%,
20%, 25% and 30% was investigated to obtain the substitutes for the cement/ fine aggregate in
the mortar. The ordinary portland cement with fine aggregate of zone II and the fly ash obtained
from the Mettur thermal power plant were used for this study. The basic properties of mortar like
compressive strength, modulus of elasticity and poisson's ratio were determined. Mortar with
different proportions of ingredients as four set of mixture proportions were prepared as given in
Table 3.4.
Table 3.4 Mortar compositions
Mortar mix
(Cement : Sand) Mix ingredients
1:3 Partial replacement of cement with fly ash as 0%, 10%, 20%, 25% &30%
1:4.5 Partial replacement of cement with fly ash as 0%, 10%, 20%, 25% &30%
1:6 Partial replacement of cement with fly ash as 0%, 10%, 20%, 25% &30%
1:6 Partial replacement of fine aggregate with fly ash as 0%, 10% &20%
The first mix (control mix) was prepared without the addition of fly ash and the other mixes were
prepared with the addition of class F fly ash obtained from Mettur thermal power plant, India. In
the first three sets, the mortars were prepared with the partial replacement of cement with fly ash
in 1:3, 1:4.5 and 1:6 cement mortar ratios. The fly ash was blended with the mixed cement at
replacement ratios of 100:0, 90:10, 80:20, 75:25 and 70:30. In the fourth set, the partial
71
replacement of fine aggregate with fly ash in the ratio of 1:6 cement mortars was prepared with
100:0, 90:10 and 80:20 ratios and the results were compared.
3.4.1 Compressive strength of the mortar
The strength of the brick masonry is much depended on the quality and strength of the brick and
the mortar used in the construction of the walls. Thus, there is an optimum relationship between
the strength of the masonry brick unit and the strength of the mortar. For maximum strength and
for the whole mass of the wall to act together, the bricks should be bonded together properly.
The joints should not be very thick. If the bricks are stronger than the mortar, then mortar
determines the strength of the brick masonry. Compressive strength tests on cement mortar were
performed in a compression testing machine with cube sized samples.
3.4.1.1 Fly ash as a substitute for cement in the mortar
The comparative studies were performed for different cement mortar ratio as 1:3, 1:4.5 and 1:6
with partial replacement of cement with fly ash as 0%, 10%, 20% and 30% is shown in Fig.3.12.
Fig. 3.12 Mortar strength with partial replacement of fly ash with cement
Specimens with cement mortar ratios of 1:3, 1:4.5 and 1:6 with 10% replacement of cement with
fly ash produced a compressive strength slightly higher than the control mix at 28 days curing
period. Compressive strength of the cement mortar specimens with 20% and 30% replacement of
cement with fly ash exhibited lower values than the control mix. Pitre [1985]119 reported that the
mortar specimens with replacement of cement with fly ash may gain strength after long days of
curing.
3.4.4.2 Fly ash as a substitute for fine aggregate in the mortar
The comparative studies were made on their characteristics for cement mortar ratio of 1:6 with
partial replacement of fine aggregate with fly ash as 0%, 10% and 20% at 3, 7, 28, 56 and 90
0
5
10
15
20
25
30
0%FA 10%FA 20%FA 30%FA
Mo
rta
r st
ren
gth
, M
Pa
partial replacement of cement with fly ash
Compressive strength of mortar with partial
replacement of cement with fly ash
cm 1:3
cm 1:4.5
cm 1:6
72
days. Specimens with cement mortar with the ratio of 1:6, the mortar strength increases with the
increase in fly ash content and the results are depicted in Fig 3.13.
Fig. 3.13 Compressive strength of 1:6 cement mortar with partial replacement of
fine aggregate with fly ash
With the increase in days of curing, the compressive strength of the mortar also increased. At 28
days with 10% and 20% fly ash for fine aggregate replacement in mortar resulted in 35.6% and
56.9% higher than the control mortar. Cement normally gains its maximum strength within 28
days. During that period, lime produced from cement hydration remains within the hydration
product. Generally, this lime reacts with fly ash and imparts more strength as reported by Yilmaz
[2010]143
. Cement mortar ratio of 1:6 with partial replacement of fine aggregate with fly ash as
0%, 10% and 20% are designated as F1, F2 and F3. Cement mortar ratio of 1:6 with partial
replacement of cement with fly ash as 10% and 20% is designated as F4 and F5. Table 3.5 gives
the recommended mortar mixes and their strengths similar to the mortar designated as per
IS1905 [1987]65
.
Table 3.5 Mix proportions recommended for construction
Cement mortar
Mortar
desig-
nation
Cement
(C)
Pozza-
lona
(F)
Fine
aggregate,
(FA)
Compressive
strength,
MPa
Mortar
designation
as per IS
1905 [1987]65
1:6 cement mortar F1 1 0 6 5.47 M2
10% FA replaced
with fly ash F2 1 0.60 5.40 8.50 near to H1
20% FA replaced
with fly ash F3 1 1.20 4.80
12.70 near to H1
10% cement
replaced with fly ash F4 1 0.11 6.66
4.67 near M1
20% cement
replaced with fly ash F5 1 0.25 7.50 4.33 near to M1
0.0
5.0
10.0
15.0
20.0
25.0
30.0
3 days 7 days 28 days 56 days 90 days
Mo
rta
r st
ren
gth
, M
Pa
Curing days
Strength of 1:6 cement mortar with partial
replacement of fine aggregate with fly ash
0% 10% 20%
73
The compressive strength in these mixes is attributed to both the continued hydration of portland
cement and the pozzolanic reactions between the fly ash and the calcium hydroxide compound of
portland cement gains more strength in 28 days and were compared in Fig 3.14. It can be seen
that there is increase in strength with the partial replacement of fine aggregate with fly ash in the
cement mortar. Yucel [2006]144
also reported that replacement of the cement by fly ash decreases
the compressive strength of the concrete mortar. However, maximum strength occurred with
20% replacement of fine aggregate with fly ash in the 1:6 cement mortar. This increase in
strength due to the replacement of fine aggregate with fly ash may be attributed to the pozzolanic
action of fly ash as reported by Joshi and Lohitia [1997]74
.
Fig. 3.14 Comparison between the substitution of fly ash with cement / fine aggregate in 1:6
cement mortar
Cement mortar of ratio 1:6 with 20% replacement of fine aggregate with fly ash showed higher
strength because of the inclusion of fly ash as the partial replacement of fine aggregate and
pozzolanic action starts to densify the matrix and due to this the strength of the fly ash mortar is
higher than the strength of control mix (1:6). Reda Taha and Shrive [2002]122
showed that a
strong CSH fibrous network can significantly enhance the masonry bond when fly ash is
incorporated in the mortar mix. Replacement of cement by fly ash results in lower compressive
strength, since fly ash exhibits very little cementing effects and acts as fine aggregate as reported
by Rajamane [2007]121
. Mortar with 20% replacement of cement with fly ash in cement mortar
1:6 is suggested for the brick masonry having the brick strength of 2 – 5 MPa and 10%
replacement of fine aggregate with fly ash in the ratio of 1:6 cement mortar is suggested for brick
masonry having the brick strength of 5- 10 MPa.
0
3
6
9
12
15
0% 10% 20%
Mort
ar
stre
ngth
, M
Pa
Partial replacement of cement / fine aggregate with fly
ash
Mortar strength with fly ash
Fine aggregate replaced with fly ash
Cement replaced with fly ash
74
3.4.4.3 Improving earthquake resistance behaviour of masonry buildings
Miha Timozevic [2009]96
has reported that seismic forces may cause sliding of a part of the wall
along one of the bed-joints, if the vertical compressive stresses in the wall are low and the quality
of mortar is poor. Sliding shear failure of unreinforced walls usually takes place in the upper part
of the masonry buildings below rigid roof structures, where the compressive stresses are low and
the response accelerations are high. For the purpose of specifying the earthquake resisting
features, the buildings have been categorized in five categories as A to E with respect to the
seismic coefficient and the recommended mortar mixes for different categories of masonry
buildings as per IS 13828[1993]63
based on the value of seismic coefficient is reported in
Table3.6:
Table 3.6 Recommended mortar mix in seismic zones
Building
Category
Seismic coefficient,
Ah
Mortar mixes [IS
13828]63
Recommended mortar
designations
A 0.04 to 0.05 M2 or M3 F1
B 0.05 to 0.06 M2 F2
C 0.06 to 0.08 M2 F2
D 0.08 to 0.12 H2 or M1 F3, F4
E More than 0.12 H2 or M1 F3, F5
The earthquake response of a masonry wall depends on the relative strengths of the bricks and
the mortar. Fly ash mortar can provide satisfactory or higher strength as compared with the plain
cement mortar as suggested by Rafat [2003]120
and Chaid et al [2004]17
.
3.5 REINFORCEMENT
Durgesh Rai [2005]35
reported that the use of the reinforcement in masonry improves the load
carrying capacity and most importantly its flexure and shear behaviour under earthquake loads.
Horizontal reinforcement should be provided in walls to strengthen them against horizontal in-
plane loading. This also helps to tie together the perpendicular walls. Bed joint reinforcements
can be easily placed in the horizontal mortar layers without any significant modification to the
construction scheme. The presence of even slight horizontal reinforcement is very effective in
controlling crack, strength, displacement capacity and energy absorption as reported by Maria
Rosa [2005]88
. Masonry piers with horizontal reinforcement significantly enhance the seismic
response in particular; damage reduction and enhances the in-plane and out-of-plane lateral load
carrying capacity. Horizontal bed joint reinforcement in alternate mortar bed joints was carried
75
out using hexagonal woven wire mesh made of galvanized iron drawn wire mesh (fabric). This
may improve the structural performance of masonry walls. Woven wire mesh placed along the
bed joint in alternate course of the brick masonry is shown in Fig 3.15. Hexagonal wire mesh
fabric formed by twisting wires with a series of hexagonal openings and the length depends on
the purchaser and the manufacturer. Hexagonal woven wire mesh is the least expensive and had
the higher tensile strength among the meshes.
Fig 3.15 Woven wire mesh placed along the bed joint in alternate course
The thickness of the woven wire mesh strand used was 0.67mm and the opening was found to be
19mm. Since the wire mesh (reinforcement) is much stronger in tension compared to the matrix
(mortar), the role of the matrix is to properly hold the mesh in place and to give a proper
protection and to transfer stresses by means of adequate bond. Compressive strength of this
composite is generally a function of the compressive strength of the matrix (mortar), while the
tensile strength is the function of the mesh content with the elastic property as 310 MPa. The
main requirement of the composite is that, it is easy to handle and flexible enough to be bent
around sharp corners. The main function of the wire mesh is to act as a lath providing the form
and to support the mortar in its green state. In the hardened state, its function is to absorb the
tensile stresses on the structure, which the mortar on its own would not be able to withstand.
3.6 MASONRY ASSEMBLAGES
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. Analysis and design
of buildings with masonry require material properties of masonry; modulus of elasticity of
masonry is required in the case of linear static analysis. In general, masonry walls are primarily
subjected to vertical gravity force and lateral in-plane shear forces during an earthquake. Direct
compression and direct shear tests were carried out to obtain mechanical properties of masonry
with various combinations of brick and the mortar. The strength and the elastic modulus of brick
masonry under compression have been evaluated. Two types of bricks viz. clay brick and fly ash
brick and the cement mortar with partial replacement of fine aggregate with fly ash were used in
76
this study. The properties of different bricks and mortars adopted for casting the masonry
specimens were also studied. In particular, modulus of elasticity is a mechanical property
influenced by different factors, such as compressive strength of unit, shape of unit, compressive
strength of mortar and state of stress developed during loading.
3.6.1 Compressive strength of the brick masonry
Masonry is commonly used for the construction of foundations and superstructure throughout the
world. Variety of masonry units (stones, burnt clay bricks, concrete blocks etc) and mortars are
used for masonry construction. Codes of practice on masonry design give the guidelines to assess
the compressive strength of the brick masonry by considering compressive strength of the
masonry unit, height of the masonry unit and the type of the mortar (cement (C): fly ash(F): fine
aggregate(FA)). The compression testing was performed according to Indian masonry code IS:
1905[1987]65
. Five brick stack bonded masonry prism tests were performed under axial
compression tests to obtain the basic compressive strength of the brick masonry. The tests were
conducted with suitable prism assemblages with different combinations of masonry units and
mortars as given in Table 3.7.
Table 3.7 Specimen details for compressive strength of the brick masonry
S.No Designation of
the prism
Types of brick Mortar Details of the reinforcement