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

in the specimen C: F: FA

1 UCBP Clay brick 1: 0: 6 Unreinforced prism

2 UCBP10 Clay brick 1: 0.6: 5.4 Unreinforced prism

3 UCBP20 Clay brick 1: 1.2: 4.8 Unreinforced prism

4 RCBP Clay brick 1: 0: 6 Reinforced prism

5 RCBP10 Clay brick 1: 0.6: 5.4 Reinforced prism

6 RCBP20 Clay brick 1: 1.2: 4.8 Reinforced prism

7 UFBP Fly ash brick 1: 0: 6 Unreinforced prism

8 UFBP10 Fly ash brick 1: 0.6: 5.4 Unreinforced prism

9 UFBP20 Fly ash brick 1: 1.2: 4.8 Unreinforced prism

10 RFBP Fly ash brick 1: 0: 6 Reinforced prism

11 RFBP10 Fly ash brick 1: 0.6: 5.4 Reinforced prism

12 RFBP20 Fly ash brick 1: 1.2: 4.8 Reinforced prism

Stack bonded unreinforced clay brick prism (UCBP) and unreinforced fly ash brick prism

(UFBP) of size 230 x 110 x 420mm were prepared using clay brick and fly ash brick of size

230x110 x70mm in the ratio of 1:6 cement mortars with 0%, 10% and 20% replacement of fine

aggregate with fly ash (UCBP10, UCBP20, UFBP10 and UFB20P). The clay brick prism

(RCBP, RCBP10 and RCBP20) and fly ash brick prism (RFBP, RFBP10 and RFBP20) were

77

reinforced with hexagonal woven wire mesh at the alternate bed course and tested under

compression. Mortar joint thickness of 10 – 12mm was used for all the prism specimens. The

nature of the stresses developed in the masonry unit and the mortar when the brick masonry is

subjected to compression greatly depends upon its relative elastic modulus (E).

Fig 3.16 Stress distributions in the composite masonry

During compression of brick masonry prisms constructed with stiffer bricks, mortar of the bed

joint may have a tendency to expand laterally more than the bricks because of lesser stiffness of

mortar as stated by Hemant [2007]55

. However, the mortar is confined laterally at the brick

mortar interface by the bricks because of the bond between them; therefore, shear stresses at the

brick mortar interface result in an internal state of stress consisting of tri-axial compression in

bricks and bilateral tension coupled with axial compression in mortar as shown in Fig 3.16.

Under uni-axial compression, stack bonded brick masonry prism expands laterally in the plane

perpendicular to the direction of loading causing vertical splitting as shown in Fig. 3.17.

Fig 3.17 Clay brick prism (unreinforced) and fly ash brick prism (unreinforced and

reinforced)

x

tm

tb

y

z

y

y

y

x

z

y

z

x

z

x y

y x

z

78

The compressive strength of the brick masonry with clay brick prism and fly ash brick prism

having the ratio of 1:6 cement mortars with 0%, 10% and 20% replacement of fine aggregate

with fly ash are shown in Fig 3.18. It was found that, with partial replacement of fine aggregate

with the fly ash in 1:6 cement mortars resulted in increase in compressive strength of the brick

masonry.

Fig 3.18 Comparison of compressive strength of brick masonry

From Fig. 3.18, it was found that the clay brick masonry in 1:6 cement mortar with 20%

replacement of fine aggregate with fly ash resulted in higher load carrying capacity. However,

the fly ash brick masonry in 1:6 cement mortar with 10% replacement of fine aggregate with fly

ash resulted in higher load carrying capacity. From this, it was understood that fly ash content in

the mortar improves the interfacial zone microstructure as reported by Rafat [2003]120

and Chaid

et al [2004]17

, also the fly ash brick masonry has higher compressive strength than clay brick

masonry.

3.6.2 Elastic property of the brick masonry

Elastic properties of clay brick masonry and fly ash brick masonry for unreinforced (CBP,

CBP10 and CBP20) and reinforced (RCBP, RCBP10 and RCBP20) with wire mesh were

studied. Stress-strain characteristics of brick masonry were examined through prism test as per IS

1905 [1987]65

and ASTM C 67 [2009]6. The stress-strain curve of both unreinforced and

reinforced clay brick masonry with three types of mortar is shown in Fig.3.19. From the curve it

was derived that the compressive strength of reinforced clay brick masonry in 1:6 cement mortar

with 20% replacement of fine aggregate with fly ash exhibited higher compressive strength.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

UCBP RCBP UFBP RFBP

Co

mp

ress

ive

stre

ng

th,

f pm

(MP

a)

Unreinforced and reinforced clay brick and fly ash brick prism

Compressive strength of brick masonry, MPa

0%

10%

20%

79

Fig 3.19 Stress-strain curve of clay brick masonry

The stress-strain curve was found to be linear until 1/3rd

of the ultimate stress (fpm) after which

cracks began to form in the mortar introducing the non-linearity as shown in Fig 3.19. The stress-

strain curve of both unreinforced and reinforced fly ash brick masonry with three types of mortar

is shown in Fig.3.20. Secant modulus of elasticity at 60% of the ultimate strength of the

specimen is calculated from stress-strain curves.

Fig 3.20 Stress-strain curve of fly ash brick masonry

From the curve, it was found that the compressive strength of reinforced fly ash brick masonry in

1:6 cement mortar with 10% replacement of fine aggregate with fly ash exhibited higher

strength. Reda Taha and Shrive [2002]122

reported that the effect of fly ash on brick masonry is

attributed to its pozzolanic activity, by which the pozzolans chemically convert the weak CH

crystals to strong CSH fibrous gel. The pozzolanic activity depends mainly on the chemical

composition and the fineness of the pozzolans. The pozzolanic reaction of fly ash was reported to

have a significant effect on long-term strength development. The fly ash brick masonry prisms

0

0.5

1

1.5

2

2.5

3

0 0.005 0.01 0.015 0.02 0.025

Str

ess,

MP

a

Strain (mm /mmlength)

CLAY BRICK MASONRY - unreinforced and reinforced

CBP CBPR

CBP10 CBP10R

CBP20 CBP20R

0

0.5

1

1.5

2

2.5

3

3.5

0 0.005 0.01 0.015

Str

ess,

MP

a

Strain (mm / mm length)

FLY ASH BRICK MASONRY - unreinforced and

reinforced

FBP FBPR

FBP10 FBP10R

FBP20 FBP20R

80

were damaged with visible vertical cracks (macro cracking) along the entire surface as shown in

Fig 3.21. Lenczer [1972]81

and Mosalam [2009]106

reported that the mortar joints can develop

lateral tension while brick develops lateral compression in brick masonry. However, the stress-

strain curve of fly ash brick masonry was found to be non-linear.

Fig.3.21 Splitting of brick masonry

The compressive strength of the unreinforced clay brick prism varies in the range of 1.69 –

1.85MPa whereas the unreinforced fly ash brick prism varies from 2.4 – 2.68MPa. Mean

compressive strengths obtained on two types of bricks (fb) and three grades of mortar (fm) and

the ratio of brick compressive strength to mortar compressive strength obtained in this study are

detailed in Table 3.8.

Table 3.8 Property of the brick and the mortar in the brick masonry

Compressive strength, MPa Type of mortar (Cement: Fly ash : Fine aggregate)

1:00:06 1 : 0.6 :5.4 1 : 1.2 :4.8 1: 0.11: 6.66 1: 0.25 : 7.5

Mortar strength, MPa 5.47 8.5 12.7 4.67 4.33

Clay brick strength, MPa 3.29 3.29 3.29 3.29 3.29

Fly ash brick strength, MPa 7.18 7.18 7.18 7.18 7.18

Ratio of clay brick to mortar

strength 0.60 0.39 0.26 0.70 0.76

Ratio of fly ash brick to

mortar strength 1.31 0.84 0.57 1.54 1.66

Modulus of elasticity of clay

brick, MPa 200 200 200 200 200

Modulus of elasticity of fly

ash brick, MPa 420 420 420 420 420

Modulus of elasticity of

mortar, MPa 2330 1000 707 4982 8290

The implication of brick and mortar proportions on the structural behavior of masonry under

compressive stress were studied and reported in Table 3.8 and in Table 3.9. From the above

results, it was found that the elastic modulus of the cement mortar is greater than the modulus of

81

the brick. The elastic properties of the brick masonry in comparison with the reported data are

presented in Table 3.9.

Table 3.9 Elastic properties of brick masonry

S.

No

Type of brick

masonry Type of

brick

Type of

mortar

Initial

tangent

modulus

(MPa)

Secant

modulus

at 60% of

ultimate

stress

(MPa)

Ultimate

stress

(MPa)

Peak

strain

(mm/mm

length)

1

Unreinforced clay

brick prism

(UCBP)

Clay

brick 1:6

cement

mortar

(control

mix)

292 300 1.69 0.0059

Reinforced clay

brick prism (RCBP) 190 388 2.19 0.0125

Unreinforced Fly

ash brick prism

(UFBP)

Fly ash

brick

420 400 2.4 0.009

Reinforced clay

brick prism (RFBP) 639 739 2.8 0.0095

2

Unreinforced clay

brick prism

(UCBP10)

Clay

brick

1:6

cement

mortar

with

10%

partial

replace

ment of

fine

aggregat

e with

fly ash

300 167 1.73 0.010

Reinforced clay

brick prism

(RCBP10)

185 222 2.35 0.020

Unreinforced Fly

ash brick prism

(UFBP10)

Fly ash

brick

500 500 2.95 0.007

Reinforced clay

brick prism

(RFBP10)

667 500 3.22 0.0075

3

Unreinforced clay

brick prism

(UCBP20)

Clay

brick

1:6

cement

mortar

with

20%

partial

replace

ment of

fine

aggregat

e with

fly ash

325 300 1.85 0.0148

Reinforced clay

brick prism

(RCBP20)

190 225 2.47 0.0175

Unreinforced Fly

ash brick prism

(UFBP20)

Fly ash

brick

487 338 2.68 0.0095

Reinforced clay

brick prism

(RFBP20)

664 550 2.85 0.010

4

Clay brick Prism,

Sarangapani

[2002]125

Table

moulded

clay

brick

1:6

cement

mortar

166.6 170 2.2 0.0110

82

With partial replacement of fine aggregate in the mortar with the fly ash, the load carrying

capacity has been increased and the strain yielded much more indicating ductility in the mortar.

The average strength value of the mortar was much higher when compared to the prism masonry

specimens, but near to the average strength of the bricks. Based on experimental observations,

modulus of elasticity of masonry is derived by the least best fit of regression analysis as,

Elastic modulus of brick prism, Epm = 250 fpm ------------- Eq 3.3

At the macroscopic scale, the assumption is that the heterogeneous masonry material can be

represented as a homogeneous material. For the masonry under compression, the nature of the

stresses developed in the brick unit and the mortar depend upon the relative modulus of the brick

and the mortar. Thus, the elastic modulus of the bricks (Eb) and the elastic modulus of the mortar

(Em) can be determined by the standard tests. While the masonry prism (Epm) can be calculated

using the former moduli and considering that the total vertical displacement of the prism ( prism)

with mesh is the sum of the displacements of the joints ( mortar) and of the bricks ( brick).

Considering the same compressive stress in all the components of the brick masonry, the elastic

modulus of the brick masonry was derived. Hence,

Elastic modulus of brick masonry, b

bm

t

tpm EpE

1

1

---------- Eq 3.4

b

mt

t

t

; b

m

bmE

E

; mesh

b

E

Ep

t – Thickness ratio between the mortar and the brick

mb – Elastic modulus ratio between the mortar and the brick

p – Reinforcement constant as elastic modulus ratio between the brick and the mesh

For, Unreinforced clay brick masonry, CBP, p = 1.0

Reinforced clay brick masonry, CBPR, p = 0.65

Unreinforced fly ash brick masonry, FBP, p = 1.0

Reinforced fly ash brick masonry, FBPR, p = 1.35

tm – Thickness of the mortar in mm

tb – Thickness of the brick in mm

Em – Elastic modulus of the mortar in MPa

Eb – Elastic modulus of the brick in MPa

Emesh – Elastic modulus of the woven wire mesh in MPa

The comparison of the equivalent homogenized elastic modulus of brick masonry with the

observed experimental elastic modulus of the brick masonry is shown in Fig. 3.22.

83

Fig. 3.22 Equivalent homogenized elastic modulus of brick masonry

From the Fig. 3.22, it was understood that the existence of horizontal mesh reinforcement

distributed the strain at the region of reinforced clay brick masonry which resulted in reduction

of the elastic modulus of the clay brick masonry. Further, the mesh reinforcement effectively

influenced the distribution of the total strain through the clay brick masonry. It was noted that the

effect of mesh reinforcement on the strain distribution of the fly ash brick masonry was found to

be less. After observing the failure of the prism in the case of clay brick masonry with mesh

reinforcement, the composite action was found to be less effective whereas in the case of fly ash

brick masonry with mesh reinforcement, the composite action was very effective. The observed

experimental elastic modulus of the brick masonry was found to have the average variation

percentage of 5.9% with the theoretical elastic modulus of the brick masonry.

3.6.3 Bond strength of the brick masonry

Masonry walls are intended to resist shear force due to in-plane lateral load in addition to the

effect of compressive load and bending. 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]125

. 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]122

. 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

0

100

200

300

400

500

600

700

800

Ela

stic

mo

du

lus,

Ep

m(M

Pa

)

Unreinforced and reinforced clay brick and fly ash brick masonry

Homogenized elastic modulus, Epm

Epm(theo)

Epm(exp)

84

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 3.23.

Fig 3.23 Triplet bond test on brick masonry

Shear strength of brick masonry triplet prisms were investigated in this test. The unreinforced

and reinforced 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 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 shear strength was obtained

from the triplet test as shown in Fig.3.23, where the brick in the middle is sheared and the upper

and lower bricks are supported. The vertical shear load (Pv) was applied at the uniform rate with

a hydraulic jack until shear failure occurred. The masonry specimen is considered as a short

beam subjected to an average bond stress and evaluated as;

Bond stress, τb = Pv / 2A --------------- Eq 3.5

Where,

Pv = Vertical compressive load in N;

A = Cross sectional area of the triplet prism in mm2;

The triplet shear prism detail with the breaking load and the bond stress ( b) for various brick

prisms are reported in Table 3.10.

85

Table 3.10 Triplet shear prism detail

Specimen Hexagonal wire mesh Specimen

dimension (mm)

Breaking load

(N)

Bond stress,

( b) MPa

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

The shearing load at failure is recorded as the maximum capacity of shear force retained by brick

– mortar bond. Lourenco [2004]83

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.3.24.

Fig. 3.24 Bond stress 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%

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

CBM CBMR FBM FBMR

Bo

nd

str

ess

, M

Pa

Unreinforced and reinforced clay brick and fly ash brick

masonry

Bond stress on brick masonry

0% FA 10%FA 20% FA

86

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. Reda Taha and Shrive [2002]122

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.

3.7 CONCLUSIONS

i. The manufacture of fly ash bricks may save more energy as the manufacture of clay brick

needs coal for burning the bricks in order to attain good strength. Also, the natural

resource such as fertile soil is not utilized for making fly ash bricks.

ii. The fly ash bricks have good mechanical properties than the clay bricks. The compressive

strength of the fly ash bricks was 54.2% more than the clay bricks and the flexural

strength of the fly ash bricks was 56% more than the clay bricks. The fly ash bricks are

8.58% less in weight than the clay bricks.

iii. The cost of the fly ash brick is 40% less than the clay brick.

iv. The fly ash bricks are cost effective, energy-efficient and environment friendly and they

are suggested for the construction purposes.

v. The mortar with the ratio of 1:6 cement mortar with 20% replacement of fine aggregate

with fly ash exhibited a higher compressive strength than the control mix after 28 days of

curing.

vi. The compressive strength of unreinforced fly ash brick masonry was 34% more than the

unreinforced clay brick masonry. The reinforced fly ash brick masonry was 20.7% more

than the reinforced clay brick masonry.

vii. The introduction of wire mesh in the clay brick masonry resulted in an increase of load

carrying capacity by 25%, while the introduction of mesh in fly ash brick masonry

resulted in an increase of load carrying capacity by 10% as the strength of the fly ash

brick contributed more in the brick masonry strength.

viii. 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.

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ix. 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.

x. 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.

xi. 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.

xii. The elastic modulus of the brick masonry (Epm) was determined with the prism strength

(fpm) as 250 fpm.

xiii. The equivalent homogenized elastic property of the masonry was derived with the elastic

properties of brick, mortar and the reinforcement.