TYPES OF BRICKS
Bricks can be divided in two types
1.Traditional bricks
2. Modular bricks
The dimensions of traditional bricks vary from 21 to 25 cm. in length, 10
to 13 cm. in width and 7.5 cm. in height in different parts of the country. The
commonly adopted nominal size of a traditional brick is 23 X 11.4X 7.6 cm.
Masonry with modular bricks work out to be cheaper since there is saving
in the consumption of bricks, mortar and labour as compared with masonry with
traditional bricks.
Nominal thickness of wall:
The thickness of wall which is to be considered in evaluating the
quantity of brickwork in estimates is called nominal thickness of wall. In fact,
it is a fictitious dimension which represents neither the actual thickness of
wall nor the overall thickness of wall including plaster rendering etc. Thus
the nominal thickness of one brick wall using traditional brick of 23 cm. length
is taken as 23 cm. whereas its actual thickness varies from 21.8 cm to 22.4 cm.
Similarly, the nominal thickness of one brick wall using modular brick is taken
as 20 cm. whereas its actual thickness is of 19 cm.
GENERAL PRINCIPLES TO BE OBSERVED IN
BRICK MASONRY CONSTRUCTION
Brick masonry is classified as first class or second class according to
the type of bricks used and according to the method of laying. The strength of
masonry work, however depends very much upon the type of material used, nature
of workmanship and supervision. Bad workmanship assisted by lack of supervision
may lead to unsound construction inspite of the materials used being of the
best possible type.
1.The bricks used in a good work should be sound, hard and well burnt
with uniform size, shape and colour. They should have no cracks or flaws and a
fractured surface should be free from holes, grit or lumps of lime etc. The
surface of the bricks should be such that no mark should be made when scratched
by the finger nail. The bricks when struck together should produce metallic
ringing sound and they should not break when dropped from a height of 90 to 120
cm. on the ground. The bricks should not as a rule absorb water more than 20%
of their weight when dry, when immersed in water for twenty-four hours.
2. The bricks should be thoroughly soaked in clear water before use for
suitable period so that the water just penetrates the full depth of bricks.
This is not only helps in removing the dirt, dust and other soluble salts from
the bricks, but also reduces their tendency of suction of water from wet
mortar. In case, however, masonry is to be carried out in mud or fat lime
mortar the bricks should not be soaked in water before use.
3. The bricks should be laid on a full bed of mortar. They should be
slightly pressed into the bed mortar while laying so as to ensure proper
adhesion. All the courses should be laid truly horizontal and all the vertical
joints should be truly vertical.
4. All the joints should be properly flushed and filled with mortar so
that no cavity is left in between.
5. In case of walls two brick or more in thickness, the joints should be
grouted a every course in addition to bedding and flushing with mortar.
6. No brick bats should be used in the work except when it is absolutely
necessary for obtaining the specified bond.
7. Brick work is generally laid in English bond. In all cases, it should
be ensured that a proper bond is maintained throughout the work.
8. Unless brick on edge is specified, the bricks must be laid in their
proper beds with their frogs pointing upwards.
9. The buttress, counterforts etc. should be built up course by course,
maintaining proper bond with the main wall and should not be added or joggled
afterwards.
10. Plastering should be done after about 28 days of completion of brick
masonry. This permits adequate time for the shrinkage in masonry and concrete
to take place before plastering operation is carried out.
11. In works where plastering or pointing is not desired, the mortar
joints should be struck flush and finished at the time of laying.
12. All the finished masonry work should be kept wet for at least seven
days.
BONDS IN BRICK WORK
On account of their uniform size and shape, the bricks can be arranged in
a variety of patterns giving rise to different types of bonds. Bonding is
essential to eliminate continuous vertical joints both in the body as well as
in the face of wall there by imparting strength to masonry. A wall having
defective arrangement of bricks, reduces the strength and stability of the
structure. The different types of bonds commonly adopted are given below:
1.English Bond:
This bond consists of alternate course of headers and stretches. In this
arrangements, vertical joints in the header course come over each other and the
vertical joints in the stretcher course are also in the same line. For the
breaking of vertical joints in the successive course it is essential to place
queen closer, after the first header in each heading couse. The following
additional points should be noted in English bond:
1.A heading course should never start with a queen closer as I is liable
to get displaced I this position.
2. In the stretcher course, the stretchers should have a minimum lap of ¼
their longer over the headers.
3. In walls having their thickness equal to an odd number of half brick,
i.e. 1 ½ brick thick walls or 2 ½ brick thick walls and so on, the same course
will show stretchers on one face and headers on the other.
4. In thick walls the middle portion is entirely filled with header to
prevent the formation of vertical joints in the body of the wall.
2. Flemish Bond:
In this arrangement of bonding brick work, each course consists of
alternate headers and stretchers. The alternate headers of each course are
centered over the stretchers in the course below. Every alternate course starts
with a header at the corner. For the breaking of vertical joints in the
successive courses, closers are in alternate courses next to the quoin header.
In walls having their thickness equal to odd number of half bricks, bats are
essentially used to achieve the bond.
Flemish bond is further divided into two different types.
a. Single Flemish bond
b. Double Flemish bond
Single Flemish bond:
This bond is a combination of English and Flemish bond. In this work the
facing of the wall consists of Flemish bond and the backing consists of English
bond in each course. This type of bonding cannot be adopted in walls less than
one and a half brick in thickness. This bond is adopted to present the
attractive appearance of Flemish bond with an effort to ensure full strength in
the brick work.
Double Flemish bond:
In this system of bonding brick work, each course presents the same
appearance both in the front and back elevations. Every course consists of
headers and stretchers laid alternatively. This type of bond is best suited
from considerations of economy and appearance. It enables the one brick wall to
have flush and uniform faces on both the sides. This type of bonding is
comparatively weaker than English bond. The alternative courses of the walls of
various thickness in Double Flemish bond.
COMPARATIVE MERITS AND DEMERITS OF
ENGLISH AND FLEMISH BOND:
· For walls thicker than 1 ½ brick,
English bond is stronger than Flemish bond.
· Flemish bond renders the appearance
of the face work more attractive and pleasing.
· Flemish bond is slightly economical
as a number of bats can be used. This renders the use of broken bricks
possible, but requires more mortar for additional joints.
· The adoption of Flemish bond requires
good workmanship and careful supervision.
OTHER TYPES OF BONDS
The bonds described below are not very common in use. Their use is
restricted to special types of construction.
Stretching bond:
In this arrangement of bonding, all the bricks are laid as stretchers.
The overlap, which is usually of half brick, is obtained by commencing each
alternate course with a half brick bat. Stretching bond is used for half brick
wall only. This bond is also termed as running bond and is commonly adopted in
the construction of half brick leaves of cavity walls, partition walls etc.
Heding bond:
In this type of bonding all the bricks are laid as headers on the faces.
The overlap, which is usually of half the width of the brick is obtained by
introducing a three quarter bat in each alternate course at quoins. This bond
permits better alignment and as such it is used for walls curved on plan. This
bond is chiefly used for footings in foundations for better transverse
distribution of load.
Facing bond:
This arrangement of bricks is adopted for thick walls, where the facing
and backing are desired to be constructed with bricks of different thickness.
This bond consists of heading and stretching courses so arranged that one
heading course comes after several stretching courses. Since the number of
joints in the backing and the facing differ greatly, the load distribution is
not uniform. This may sometimes lead to unequal settlement of the two thickness
of the wall.
Raking bond:
This is a bond in brick work in which the bonding bricks are laid at any
angle other than zero or ninety degrees. This arrangement helps to increase the
longitudinal stability of thick walls built in English bond. In this
arrangement of bonding, the space between the external stretchers of a wall is
filled with bricks inclined to the face of the wall. This bond is introduced at
certain intervals along the height of a wall.
Dutch bond:
This bond is a modification of the old English cross bond and consists of
alternate courses of headers and stretchers. In this arrangement of brick work,
each stretching course starts at the quoin with a three quarter bat and every
alternate stretching course has a header placed next to the three quarter brick
bat provided at the quoin.
Zig-Zag bond:
This is similar to herring bone bond with the only difference that in
this case the bricks are laid in a zig-zag fashion. This is commonly adopted in
brick paved flooring.
TEE-JUNCTION
A Tee junction is formed when two walls meet each other at right angles
forming the letter T in plan.
BONDS IN PIERS
Piers may be constructed in a variety of bonds. The selection of the type
of bond depends much upon shape of the pier as well as choice of the
individual. In common, brick piers are built either in English bond of in
Double Flemish bond.
SQUINT JUNCTION
A squint junction is formed when two walls meet each other at an angle
other than a right angle without forming a quoin. The junctions may be in
English or Flemish bond. Great difficulty is experienced in forming the
junction accurately and that is why they are rarely adopted.
RETAINING WALL
It may be defined as a wall built to resist the pressure of liquid, earth
filling, sand or other granular material filled behind it after it is built. It
is commonly required in the construction of hill roads, masonry dams, abutments
and wings walls of bridges and so on. Depending upon the site conditions, type
of material to be retained and the height of the wall to be constructed,
retaining wall may be built in dry stone masonry, stone masonry, brick masonry,
plain cement concrete and reinforced concrete.
DESIGN OF RETAINING WALL
The thrust from the backing which tends to overturn the wall or causes it
to slide, is the deciding factor in the selection of the section and type of
the wall. There are many conditions upon which thrust exerted by the backing
depends, such as cohesion of the soil, dryness of the backing material the
manner in which the material is filled against the wall and so on. There are
theories by the help of which the value of thrust under different conditions
can be worked out. Having known the thrust, the section of the wall is so
designed that the self-weight is sufficient to resist the tendency of the
thrust to slide the wall is such that the resultant force lies within the
middle third of the base. This condition is necessary to prevent the tendency of
thrust to overturn the wall and to ensure that there is no tension at the wall
base. It is equally essential to ascertain that the maximum stress at the toe
of the wall does not exceed the safe bearing capacity of the soil.
CONDITIONS OF STABILITY OF RETAINING
WALLS:
A satisfactory retaining wall must meet the following requirements for
ensuring its stability:
· The wall should be structurally
capable of resisting the pressure applied to it.
· The section of the wall should be so
proportioned that it will not overturn by the lateral pressure.
· The wall should be safe from
consideration of sliding, i.e. the wall should not be pushed out by the lateral
pressure.
· The weight of the wall together with
the force resulting from the earth pressure acting on it, should not stress its
foundation to a value greater than safe bearing capacity of the soil on which
it is founded.
· As far as possible, long masonry
retaining walls should be provided with expansion joints located at 6 to 8
metre apart.
STRENGTH OF BRICK MASONRY
The strength of bricks masonry depends mainly upon the strength of bricks
used in the masonry construction. The strength of bricks depends upon the
nature of soil used for brick making and the method adopted for moulding and
burning of bricks. Since the nature of soil varies from region, the average
strength of bricks varies from as low as 30 kg/cm^2 in Andhra Pradesh to as
high as 100 to 150 kg/cm^2 in U.P.
DEFECTS IN BRICK MASONRY
Brick work carried out by using good materials and by observing the
specified principles of construction should require no maintenance. Defects in
brick work may however, occur due to the following:
Sulphate attack:
Sulphate salts present in brick work react with alumina content of cement
and with hydraulic lime in the lime mortar causing considerable increase in the
volume of mortar. This in turn results in chipping and spalling of bricks and
formation of cracks in joints and rendering. This defect occurs in situations
where the brick work is exposed like boundary walls, parapets etc. or where it
is likely to be in contact with moisture like manholes, retaining walls etc.
this defect can be remedied by adopting suitable construction details and
choosing materials in such a way that entry of moisture into the body of brick
work can be checked.
Defect due to crystallization of
salts from brick:
This defect commonly occurs in masonry made out of bricks which contains
excessive soluble salts. When such bricks come in contact with water, the soluble
salts get dissolved and appear in the form of fine whitish crystals on the
surface of brick work. The phenomenon is also known as efflorescence. In this
defect the surface of brick work gets disfigured and presents an ugly look. Efflorescence
can be remedied by brushing and washing the effected surface repeatedly.
Defect due to corrosion of embedded
iron or steel:
Iron or steel embedded in brickwork gets corroded in the presence of dampness.
On corrosion the metal expands in volume and tends to crack the brick work. This
defect can be prevented by encasing the reinforcement or iron member in dense
cement mortar and providing a cover of 15 to 25 mm. around the embedded member.
Defect due to shrinking on drying:
Brick work normally get wells with the absorption of water and
subsequently shrinks when the water evaporates. In the process of shrinkage, it
develops cracks in the masonry joints. If the mortar used in the masonry is
lean, the cracks are distributed over large number of joints and in case of
rich mortar the cracks are few but wider. Such cracks do not affect the
structural strength of brick work and can be easily rectified. This defect can
be prevented by using good quality bricks and by protecting masonry from
moisture penetration.
REINFORCED BRICK WORK
Brick work strengthed by introduction of mild steel flats, hoop iron,
expanded mesh or bars is termed as reinforced brickwork. This type of masonry
is capable of resisting both compressive as well as tensile and shear stress. On
account of its ability to resist lateral forces, reinforced brick masonry is
extensively used in seismic areas.
As a matter of practice half brick walls are invariably reinforced by
providing 6 to 8 mm. dia bars or hoop iron at every third or fourth course.
Reinforced brick masonry is frequently adopted for the construction of
retaining walls especially in places where exposed brick work is necessary from
architectural considerations. It is seen that reinforced brick retaining wall up
to 4m. height works out to be cheaper as compared with R.C.C. retaining walls. Such
a wall is made by using special bricks in cement mortar 1:3 and reinforced with
vertical m.s. rounds placed near each face.
Construction of reinforced brick column is more or less similar to that of
reinforced brick wall except that the horizontal steel mesh is replaced by
stirrups.
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