Lecture

CHAPTER

1
Bricks

Bricks are one of the oldest and most popular building materials. The reasons for bricks being very popular
and widely used construction material are,

 Bricks are cheap

 Bricks are durable
 Bricks are easy to handle and work with

Bricks can be defined as,

“Bricks are blocks of tampered clay molded to suitable shapes and sizes while it is still in plastic
condition, dried in the sun and burnt, if desired so as to make them more strong, hard and durable.”

Bricks are normally rectangular in shape and size is set so as to make it easy for workers to handle it.
Bricks are usually available made up of three different materials,

 Burnt clay (Most common type in Pakistan)

 Mixture of sand and lime
 Portland cement concrete

The bricks made up of the last two types are usually called blocks and are available in sizes of
following proportions,

Length of brick = 2 × width of brick + thickness of mortar

Height of brick = width of brick

Commonly available size is,19 × 9 × 9 cm and 19 × 9 × 4 cm.

1
The bricks made up of clay usually have the size, 9" × 4½" × 3". Weight of such brick is around 3.0
kg. An indent called frog, 1–2 cm deep. The size of frog should be 10 × 4 × 1 cm. Thepurpose of
providing frog is to form a key for holding the mortar and therefore, the bricks arelaid with frogs on
top.

1 CLASSIFICATION OF BRICKS:

There are several categories to classify bricks. All those categories have been explained below.

1.1 ON FIELD PRACTICE:

Based upon the physical and mechanical properties the bricks are classified into four types such
as, first class, second class, third class and fourth class.

a. First Class Bricks:

 These are thoroughly burnt and are of deep red, cherry or copper colour.
 The surface should be smooth and rectangular, with parallel, sharp and straight edges and
square corners.
 These should be free from flaws, cracks and stones.
 These should have uniform texture.
 No impression should be left on the brick when a scratch is made by a finger nail.
 The fractured surface of the brick should not show lumps of lime.
 A metallic or ringing sound should come when two bricks are struck against each other.
 Water absorption should be 12–15% of its dry weight when immersed in cold water for
24 hours.
 The crushing strength of the brick should not be less than 10 N/mm2. This limit varies
with different Government organizations around the country.

Uses: First class bricks are recommended for pointing, exposed face work in masonry
structures, flooring and reinforced brick work.

b. Second Class Bricks:

These bricks are supposed to have the same requirements as the first class ones except that

 Small cracks and distortions are permitted.

 A little higher water absorption of about 16–20% of its dry weight is allowed.
 The crushing strength should not be less than 7.0 N/mm2.

Uses: Second class bricks are recommended for all important or unimportant hidden masonry
works and centering of reinforced brick and reinforced cement concrete (RCC) structures.

c. Third Class Bricks:

 These bricks are under burnt.

2
  They are soft and light-colored.
 They produce a dull sound when struck against each other.
 Water absorption is about 25 per cent of dry weight.

Uses: It is used for building temporary structures.

d. Fourth Class Bricks:

 These bricks are over burnt.

 Badly distorted in size and shape.
 Brittle in nature.

Uses: The ballast of such bricks is used for foundation and floors in lime concrete and road
metal.

1.2 ON STRENGTH:

On the basis of strength the bricks have been subdivided into the following categories (IS 1077).

1.3 ON THE BASIS OF USE:

On the basis of use the bricks have been classified into the following three types,

a. Common Bricks:

This is a general multi-purpose unit manufactured economically without special reference to

appearance. These may vary greatly in strength and durability and are used for filling,
backing and in walls where appearance is of no consequence.

b. Facing Bricks:

These are made primarily with a view to have good appearance, either of color or texture or
both. These are durable under severe exposure and are used in fronts of building walls for
which a pleasing appearance is desired.

3
 c. Engineering Bricks:

These are strong, impermeable, smooth, table molded, hard and conform to defined limits of
absorption and strength. These are used for all load bearing structures.

1.4 ON THE BASIS OF FINISH:

On the basis of finish the bricks have been classified into the following two types,

a. Sand Faced Bricks:

This typehas textured surface manufactured by sprinkling sand on the inner surfaces of
themould.

b. Rustic Bricks:

This type has mechanically textured finish, varying in pattern.

1.5 ON THE BASIS OF MANUFACTURE:

On the basis of manufacture the bricks have been classified into the following
two types,

a. Hand Made Bricks:

These bricks are hand molded.

b. Machine Made Bricks:

Depending upon mechanical arrangement, bricks are known as wire-

cut bricks—bricks cut from clay extruded in a column and cut off
into brick sizes by wires; pressedbricks— when bricks are
manufactured from stiff plastic or semi-dry clay and pressed into
molds; molded bricks—when bricks are molded by machines
imitating hand mixing.

1.6 ON THE BASIS OF BURNING:

On the basis of burning the bricks have been classified into the following three types,

a. Pale Bricks:

These are under burnt bricks obtained from outer portion of the kiln.

b. Body Bricks:

These are well burnt bricks occupying central portion of the kiln.

4
 c. Arch Bricks:

These are over burnt also known as clinker bricks obtained from inner portion of the kiln.

1.7 ON THE BASIS OF TYPES:

On the basis of types the bricks have been classified into the following four types,

a. Solid Bricks:

Small holes not exceeding 25 per cent of the volume of the brick are permitted; alternatively,
frogs not exceeding 20 per cent of the total volume are permitted.

b. Perforated Bricks:

Small holes may exceed 25 per cent of the total volume of the brick.

c. Hollow Bricks:

The total of holes, which need not be small, may exceed 25 per cent of the volume of the
brick.

d. Cellular Bricks:

Holes closed at one end exceed 20 per cent of the volume.

2 CHARACTERISTICS OF GOOD BRICKS:

The characteristics of a good brick are,

 Size and Shape: The bricks should have uniform size and plane, rectangular
surfaces with parallel sides and sharp straight edges.
 Color: The brick should have a uniform deep red or cherry color as indicative of
uniformity in chemical composition and thoroughness in the burning of the brick.
 Texture and Compactness: The surfaces should not be too smooth to cause slipping
of mortar. The brick should have precompact and uniform texture. A fractured
surface should not show fissures, holes grits or lumps of lime.
 Hardness and Soundness: The brick should be so hard that when scratched by a
finger nail no impression is made. When two bricks are struck together, a metallic
sound should be produced.
 Water Absorption: Water Absorption should not exceed 20 per cent of its dry
weight when kept immersed in water for 24 hours.
 Crushing Strength: Crushing strength should not be less than 10 N/mm2.
 Brick Earth: Brick earth should be free from stones, kankars, organic matter etc.

5
3 INGREDIENTS OF GOOD BRICK EARTH:

For the preparation of bricks, clay or other suitable earth is molded to the desired shape after
subjecting it to several processes. After drying, it should not shrink and no crack should
develop. Different ingredients present in the good quality brick earth have been shown in the
figure below with their percentages.

4 FUNCTIONS OF VARIOUS INGREDIENTS:

The functions of different substances present in the brick earth have been explained below,

4.1 SILICA:

It enables the brick to retain its shape and imparts durability, prevents shrinkage and
warping. Excess of silica makes the brick brittle and weak on burning.

4.2 ALUMINA:

Aluminaabsorbs water and renders the clay plastic. If alumina is present in excess of the
specified quantity, it produces cracks in brick on drying.

4.3 LIME:

Normally lime is added less than 10%. The benefits are,

 Reduces the shrinkage on drying.

 Causes silica in clay to melt on burning and thus helps to bind it.
 In carbonated form, lime lowers the fusion point.
 Excess of lime causes the brick to melt and the brick lose its shape.
 Red bricks are obtained on burning at considerably high temperature (more than
800°C) and buff-burning bricks are made by increasing the lime content.

6
 4.4 MANGNESIA:

Mangnesia is rarely exceeding 1 per cent, affects the color and makes the brick yellow,
in burning; it causes the clay to soften at slower rate than in most case is lime and
reduces warping.

4.5 IRON:

Iron oxide constituting less than 7 per cent of clay, imparts the following properties:

 Gives red colour on burning when excess of oxygen is available and dark brown or
even black colour when oxygen available is insufficient, however, excess of ferric
oxide makes the brick dark blue.
 Improves impermeability and durability.
 Tends to lower the fusion point of the clay, especially if present as ferrous oxide.
 Gives strength and hardness.

5 HARMFUL SUBSTANCES IN BRICK EARTH:

The harmful substances present in the brick earth have been explained below,

5.1 LIME:

When a desirable amount of lime is present in the clay, it results in good bricks, but if in
excess, it changes the colour of the brick from red to yellow. When lime is present in
lumps, it absorbs moisture, swells and causes disintegration of the bricks. Therefore,
lime should be present in finely divided state and lumps, if any, should be removed in
the beginning itself.

5.2 PEBBLES AND GRAVELS:

Pebbles and Gravels do not allow the clay to be mixed thoroughly and spoil the
appearance of the brick. Bricks with pebbles and gravels may crack while working.

5.3 IRON PYRITES:

This tends to oxidize and decompose the brick during burning. The brick may split into
pieces. Pyrites discolourise the bricks.

5.4 ALKALIS:

These form less than 10 per cent of the raw clay, are of great value as fluxes, especially
when combined with silicates of alumina. These are mainly in the form of soda or
potash. However, when present in excess, alkali makes the clay unsuitable for bricks.
They melt the clay on burning and make the bricks unsymmetrical. When bricks come in
contact with moisture, water is absorbed and the alkalis crystallize. On drying, the
7
 moisture evaporates, leaving behind grey or white powder deposits on the brick which
spoil the appearance.

5.5 ORGANIC MATTER:

On burning green bricks, the organic matter gets charred and leave pores making the
bricks porous; the water absorption is increased and the strength is reduced.

5.6 CARBONACEOUS MATERIALS:

Present in the form of bituminous matter or carbon greatly affects the color of raw clay.
Unless proper precaution is taken to effect complete removal of such matter by
oxidation, the brick is likely to have a black core.

5.7 SULPHUR:

Sulphur is usually found in clay as the sulphate of calcium, magnesium, sodium,

potassium or iron, or as iron sulphide. Generally, the proportion is small. If, however,
there is carbon in the clay and insufficient time is given during burning for proper
oxidation of carbon and sulphur, the latter will cause the formation of a spongy, swollen
structure in the brick and the brick will be discolored by white blotches.

5.8 WATER:

A large proportion of free water generally causes clay to shrink considerably during
drying, whereas combined water causes shrinkage during burning. The use of water
containing small quantities of magnesium or calcium carbonates, together with a
sulphurous fuel often causes similar effects as those by sulphur.

6 MANUFACTURING OF BRICKS:

6.1 PREPARATION OF BRICK-EARTH

Following steps are involved in preparing Brick-earth.

a. Digging

If the area from where soil is to be taken is grassy or has other vegetation then the top layer
(about 20 cm deep) is excavated and thrown away as it contains roots of vegetation and
other organic matter. The excavated lumps of soil are broken. It is ensured that the soil is
free from gravel, coarse sand, lime and kankar particles, vegetable matter etc.

b. Weathering

Excavated soil, after the lumps have been broken, is mixed with a little water and is left in
heaps to weather for a period varying from a few weeks to as long as it can be left. This

8
 improves its plasticity and strength. To keep the soil wet water may be sprayed on the heap
from time to time and the heap turned over.

c. Blending

The earth is then thoroughly broken and mixed with sandy soil if needed. The whole mass is
thoroughly mixed up and reasonable amount of water is added if needed.

d. Tempering

Blended soil is kneaded under the feet of men or

cattle after desired quantities of water have been
added to it. The whole mass becomes homogeneous
and plastic. It is then left covered with mats and
allowed to dry gradually in layers about 30 cm
thick for not less than 36 hours, till it is just soft
enough for moulding, pug mill is used for
tempering earth needed for the manufacture of
bricks either on large scale or for use on superior
works like arches etc.

Pug Mill

e. Moulds

Moulds are rectangular boxes of wood or steel without top bottom and the longer sides
projecting a few centimeters to act as handles. The
edges of the wooden moulds should be protected
with strips of brass or steel screwed on them.

Steel moulds keep their shapes and last longer than

the wooden moulds and are used for heavy works.

Inside dimensions of the moulds are kept a little larger than the size of burnt brick (generally
about 1/10th of the size of brick each way). It is done to allow for the shrinkage of the
moulded brick on drying and burning. The exact allowance to be made for shrinkage can be
ascertained by field tests.

6.2 MOULDING QF BRICKS

Giving the required shape to the prepared brick earth is known as moulding of btieks. There
are two different ways of moulding.

a) Hand moulding

b) Machine moulding.

a. Head moulding:

9
Hand moulding of bricks is extensively used and could be done either on ground or on table.

b. Ground moulding:

This method is adopted when a large and level area of land is available for the purpose. The
area of land on which moulding is to be done is levelled, plastered smooth and sprinkled
over with sand.

i. To prevent the moulded bricks from sticking to the moulds either sand is sprinkled on the
inner sides of the mould or the mould is dipped in water each time before moulding is done.
When sand is used to prevent the sticking of earth to moulds the moulded bricks are known
as sand moulded and if the mould is dipped in water each time before moulding a brick then
the bricks are known as slop moulded bricks. Sand moulded bricks have better finish and
sharper edges.

ii. After either sprinkling sand on the inside of the mould or dipping the mould in water take
a lump of well prepared earth, the volume of which is a little more than that of the brick.
This lump is shaped in hands to the size and shape of the brick.

iii. Now it is rolled in sand and with a jerk the lump is dashed into the mould in such a
manner that the mould is completely filled with earth. The moulder then gives blows with
his fists and press in the corners and edges with the
thumbs.

iv. The surplus soil is then scrapped off and the top
surface is levelled. A metal plate with a sharp edge,
known as strike is used for removing the surplus soil.
Generally a thin wire stretched on a frame is used for this
purpose.

v. After the brick has been moulded the mould is given a gen stroke with something hard
and then mould is lifted leaving the brick dry on the
ground. The mould is placed nearby to mould another
brick and the process is repeated.

Bricks moulded directly on the ground have their lower

face rough and can have no frog.

To avoid it bricks are moulded on a block of wood

known as the moulding block, having a projection 0.5 cm
thick. A moulder can mould between 500 to 1000 bricks per day.

c. Table Moulding:

In it the moulder carries out the moulding of brick on a table. He does so while standing by
the side of the table. He moulds bricks on boards known as stock boards. Stock boards are of
the same size as the moulds and have a projection for the frog. Sand is sprinkled inside the
mould and on the stock board. The mould is placed to fit the stock board and then filled with
10
 earth. Sufficient quantity of earth is dashed into the mould
pressed hard and the surplus earth is removed with a strike or a
thin wire. A Pallet is then placed on the mould. The mould
containing the brick is then smartly lifted off the stock board
and inverted so that the whole rests on the pallet. The mould is
then given a gentle blow and lifted leaving the brick on the Stock Board

pallet. One more pallet is placed on the brick and it is carried to

drying site between the two pallets. It is allowed to dry on side.

d. Machine moulding:

There are a variety of moulding machines and these machines are capable of manufacturing
large number of bricks quickly. The bricks moulded in machine have better / sharp edges /
smooth surface, stronger than hand moulded bricks etc.

6.3 DRYING OF BRICKS

Before burning it is essential that the bricks have dried and have
become sufficiently hard to be handled and stacked in kilns
without getting damaged.

Also if the bricks have not completely dried then they are likely
to get cracked and distorted when burnt in the kiln.

The following points are kept in view to ensure successful

completion of drying operation.

i. As soon as the moulded bricks become dry enough so that they

do not get damaged on handling they should be turned on edge
and left for a day or two more t o further harden. In the initial
stage of drying, bricks should be protected from sever sun and
winds as otherwise rapid drying of bricks might result in their
developing cracks.

ii. They should then be stacked in such a way that each brick gets full circulation of air all

11
 around it. Best form of stack is of a breadth equal to two bricks laid longitudinally with
interval between them. The alternate layers being along and across the stack and all placed
on edges.

iii. The drying area should be higher than the surroundings so that it does not get flooded due
to any occasional rain. It should have a gentle slope to facilitate drainage of rain water.

iv. A layer of sand should be spread at the drying area so as to keep it dry in wet weather.

v. To protect the drying, bricks from damage caused by occasional rains temporary bamboo
frames and sirki should be provided. The sirkis should be weighed down with heavy planks
to stop them from being blown away by winds. The height of stack may be of eight to ten
layers of bricks.

A gap of about 1 m should be left between adjacent stacks so as to facilitate free movement
of workers.

Length and height of all the stacks should be kept the same. Every stack should contain
bricks in multiples of a thousand. This shall make it easy to count the number of bricks.
Depending upon weather it takes three to eight days for bricks thoroughly dry.

6.4 BURNING OF BRICKS

Burning of bricks is done in a clamp or kiln. A clamp is a temporary structure whereas kiln
is a permanent one.
Burning in Clamp or Pazawah:
A typical clamp is shown in Fig. The bricks
and fuel are placed in alternate layers. The
amount of fuel is reduced successively in the
top layers. Each brick tier consists of 4–5
layers of bricks. Some space is left between
bricks for free circulation of hot gasses. After
30 per cent loading of the clamp, the fuel in
the lowest layer is fired and the remaining
loading of bricks and fuel is carried out
hurriedly. The top and sides of the clamp are
plastered with mud. Then a coat of cow dung
is given, which prevents the escape of heat.
The production of bricks is 2–3 lacs and the
process is completed in six months. This
process yields about 60 per cent first class Clamp or Pazawah
bricks.

12
Kiln Burning

Intermittent Kiln:
The example of this type of an over ground, rectangular
kiln is shown in Fig. After loading the kiln, it is fired,
cooled and unloaded and then the next loading is done.
Since the walls and sides get cooled during reloading and
are to be heated again during next firing, there is wastage
of fuel. Intermittent Kiln

Continuous Kiln:
The examples of continuous kiln are Hoffman’s kiln and
Bull’s trench kiln .In a continuous kiln, bricks are
stacked in various chambers wherein the
bricks undergo different treatments at the same time.
When the bricks in one of the chambers is fired, the bricks in the next set of chambers are
dried and preheated while bricks in the other set of chambers are loaded and in the last are
cooled.

13
7 TESTING OF BRICKS:

Will be covered in lab portion

Hoffmann`s Kiln
It is a continuous kiln and has a permanent roof as a result of which it can function even during
rains. The first Hoffmann kilns were in the form of a great circular ring chamber, with massive
walls and a large chimney at the centre, to which underground radial flues converged from the
inside walls of each of the twelve chambers. The chambers were barrel arched (like a railway
tunnel), and in the roof arches there were several small feed holes through which fine coal could
be fed into spaces made among the bricks to be fired. Around the outer wall or the kiln were the
twelve openings for loading and unloading the individual firing chambers. These chambers were
separated from each other by very large metal dampers that could be raised and lowered as the
fire moved around the kiln from chamber to chamber.
With the kiln is in full operation two chambers will be open, and the other ten sealed up at the
door and by the interconnecting steel dampers. If the chambers are numbered from 1 to 12, then
bricks are being unloaded from 2 and loaded in 1. The damper is closed between 1 and 12, but
opens between all other chambers. Air is drawn through the open door of 2 and through the
bricks in 3, 4, 5, and 6, cooling them down and at the same time being heated itself.

Chamber 7 is being fired, with fuel being fed at intervals through the roof, which is immediately
ignited by the hot air from 6. The products of combustion pass on to 8, 9, 10, and 11 drying and
pre-heating the bricks in these chambers The chambers being loaded and unloaded move forward
in sequence, and this way heat is extracted from the cooling bricks and also from the hot
combustion gases.

The Bull's trench kiln

The kiln can be made circular or elliptical in shape. It is constructed on dry land, by digging a
trench, 6 - 9 m wide, 2 - 2.5 m deep, and 100 - 150 m long. An alternative method is to build up
the sides of the kiln with bricks, especially where drainage is a problem. Gaps are left in the
outer wall for easy access to the trench during setting and drawing of bricks.The green bricks to
14
be fired are set in rows, two to three bricks wide, with holes in between that allow feeding of
coal and a sufficient flow of air through the setting (Figure 2). On top of the bricks, two layers of
bricks, covered with ash or brick dust, seal the setting. A large piece of canvas, paper or metal
sheet is placed vertically across the brick setting to block air from entering from the wrong side
of the chimneys (Figure 3). The trench contains 200 - 300,000 bricks at a time.

Chimneys, 6 - 10 m high, made of sheet metal, are placed on top of the brick setting. They are
moved around as the firing progresses and they have to be light, so that the firing crew can carry
them. Wires attached to the top of the steel chimneys support them. The need for lightness and
the cost of replacing the chimneys often have the effect that the height of the chimneys becomes
too low. That means the exhaust temperature has to be higher in order to maintain sufficient
draught and the chimneys are placed closer to the firing zone. Thereby, less heat of the exhaust
gases can be reutilized. Small circular Bull's trench kilns use only one chimney, whereas the
larger elliptical kilns need two chimneys.

The firing in a Bull's trench kiln is continuous, day and night. Green bricks are loaded and
finished bricks are drawn all the time. The fuel saving is achieved by reusing part of the energy
that is otherwise lost in intermittent kilns. The air for combustion is drawn through the already
fired but still hot bricks. The cooling bricks transfer their heat to the combustion air, pre-heating
it before it enters the firing zone. After combustion, the hot exhaust gases pass through the yet
unfired bricks on their way to the chimneys. This pre-heats the bricks, so less fuel is needed to
bring the bricks to the maximum temperature. Once every 24 hours the chimneys are moved
forward 5 to 7 m. Daily output is 15 - 25,000 bricks.

15