3.

BRICKS

Bricks are one of the most extensively used materials of construction. Their popularity as
building material lies in their strength, durability, insulating property against
heat/coldness and sound and their being handy to work with. They are also mostly
available at building centers at relatively cheap prices.

3.1 Raw materials

Bricks are manufactured from clays generally found in abundance in many parts of the
world. Clays are fine-grained soils that have resulted from the decay of rocks (they could
be residual clays formed from the decay of the underlying rocks or sedimentary if
removed from the parent rock, transported and deposited somewhere else by water or
wind).
They generally consist of the following chemical elements:

• Alumina (Al2O3)
• Silica (SiO2)
• Ferric Oxide (Fe2O3)
• Lime (CaO)
• Magnesia (MgO)
• Carbon Dioxide (CO2)
• Sulphur trioxide (SO3)
• Alkalies (K2O, Na2O)
• Water (H2O)

On account of the different phases they might have gone through and their history of
formation, clays are generally found mixed with other materials (impurities) that
influence their properties. A clay soil for brick making should be such that when prepared
with water, it can be moulded, dried and burnt without cracking or changing its shape or
warping. Such material should preferably have the following composition:

Clay----------------------------------------20-40%
Sand----------------------------------------30-50%
Others (lime, silt, loam etc.)------------20-35%

Each of the components and their constituents play different roles in the manufacture of
bricks and influence the characteristics of the final product.

3.1.1 Functions of the constituent materials

Each of the constituents of the raw material used for making bricks has a function that
can be summarized as follows:

a) Alumina: A fine-grained mineral that makes the major part of clay, becomes
plastic when mixed with water and is capable of being moulded to the desired

_______________________________________________________________________________ 10
Eyob Yilma, Constructiion Technology Department, NCTTE
 shape. On drying it loses its plasticity and becomes hard. This can be
accompanied with shrinkage that might result in warping and cracking depending
on the speed and magnitude of drying. When burnt, alumina becomes stronger
and harder as a result of the homogeneity created by fusion. Bricks of very high
alumina content are likely to be refractory.

b) Silica: A coarse-grained mineral, which can be present either inform of pure sand
or compound of silicate of alumina. It is useful in reducing shrinkage and warping
in burning. Its presence in bricks produces hardness and durability; however, a
large percentage of uncombined silica is undesirable because it leads to brittleness
of the product. Silica fuses only at very high temperature and hence increases the
refractoriness of low alumina clay and makes bricks resistant to heat. In firebricks
the silica content may rise to 98%.

c) Lime: When present in small quantities, lime acts as a flux and lower the fusion
point of silica. Lime in carbonate form breaks into CO2 and CaO at around 900oC.
It also acts as a binder to the clay and silica particles leading to greater strength.
Excess of lime may cause the bricks to melt and lose their shape.

d) Iron oxide: It lowers the fusion point of the clay and silica and hence, helps the
fusion of brick particles. It is this element that imparts the color of the clay and
the burnt product. Depending on the percentage of iron oxide present in the clay,
the color of the bricks may vary from light yellow to red. A high percentage may
make the bricks dark blue. If iron is present in the form of pyrites (sulphide of
iron), it can get oxidized, crystallized and split the bricks to pieces.

3.2 Manufacture of Brick

There are four basic stages in brick manufacture, though many of the operations are
interdependent. A particular brick will pass through these stages in away designed
specifically to suit the raw material used and the final product.

3.2.1 Clay preparation

After digging out (winning), the clay is prepared by crushing and/or grinding and mixing
until it is of a uniform consistency. Water may be added to increase plasticity (a process
known as ‘tempering’) and in some cases chemicals may be added for specific purposes,
for example, barium carbonate that reacts with soluble salt producing an insoluble
product, therefore reducing effloresence in the final product. At this stage sand and water
are added to produce the desired consistency for molding.

_______________________________________________________________________________ 11
Eyob Yilma, Constructiion Technology Department, NCTTE
3.2.2 Molding

The molding technique is designed to suit the moisture content of the clay.

The soft-mud process: This method utilizes a clay, normally from shallow surface
deposits, in a very soft condition, the moisture content being as high as 30%.

Consists of mechanically forcing wet, soft clay (moisture content 30%) into molds. The
molding machine forces the wet clay into several molds under pressure, cuts of excess
clay, and turns the molded bricks out onto a pallet or conveyor, to be carried away for
drying. The inside of the mold may be sprayed or dipped in water to prevent the clay
from sticking.

These bricks are called water-struck bricks. Sand-struck bricks generally have sharper,
cleaner edges than water struck bricks. The green bricks are very soft and must be
handled carefully prior to drying. A similar process produces hand-made bricks.

The stiff-mud process- here enough water is used to form the clay into a cohesive mass,
which is then forced or extruded in a column through dies in a brick-making machine.
The column of clay is forced into a wire-cutting table, where it is cut into appropriate
length by taut wires. This produces a wire-cut face. Brick may be end cut or side cut,
depending on the size and shape of the die.

Dry-pressed brick is manufactured of relatively dry or non-plastic clays. The material is

fed into the machine by hoppers, where it is compressed into molds under high pressure.
Dry pressed bricks are compact, strong, and well formed.

3.2.3 Drying

When the bricks come to the brick-making machine, they contain from 7 to 30%
moisture, depending on the process used. They may be stacked in open sheds for periods
of 7 days to 6 weeks for final drying. Most brick is now dried in chambers under
controlled conditions of heat, moisture and air velocity for 2 to 4 days. Drying enables
the bricks to be stacked higher in the kiln without lower bricks becoming distorted by the
weight of bricks above them. Drying is also enables the firing temperature to be increased
more rapidly without problems such as bloating, which may result when gases or vapor
are trapped within the brick.

3.2.4 Firing

The object of firing is to cause localized melting (sintering) of the clay, which increases
strength and decrease the soluble salt content without loss of shape of the clay unit. The
main constituents of the clay-silica and alumina-do not melt, since their melting point are
very high; they are merely fused together by the lower melting point minerals such as
metallic oxides and lime. The main stages of firing are:

_______________________________________________________________________________ 12
Eyob Yilma, Constructiion Technology Department, NCTTE
 100oC Evaporation of free water
400oC Burning of carbonaceous matter
700oC Dehydration
900oC Oxidation
900o – 1000oC Sintering of clay

Fuels may assist the latter stages of firing whether present naturally in the clay or those
added during processing.

The control of the rate of increase of temperature and the maximum temperature is most
important in order to produce bricks having satisfactory strength and quality; in
particular, too-rapid firing will cause bloating and over burning of external layers, while
too low a temperature seriously impairs strength and durability. Stronger bricks, such as
engineering bricks, are normally fired at higher temperature.

Firing of ordinary-quality bricks or common bricks is at 900oC and for that of

engineering brick is greater than 1000oC.

3.3 Brick kilns

Kiln may be either intermittent or continuous. In the intermittent kiln, the bricks must be
fired, the fires extinguished, the bricks allowed to cool, the kiln dismantled, and the
bricks removed before a new pile of green bricks is piled to be fired. The development of
the continuous kiln greatly speeded up the process. The continuous kiln may consist of
several compartments fired by a single oven. The heat is regulated in each section so that
while the remaining water is being removed from the brick in one compartment, bricks
are being fired in a second compartment and cooled in a third. The continuous kiln is now
widely used. The kiln consists of either a straight or a curved tunnel, with several zones
in which heat is carefully controlled. Bricks are loaded on to special cars and pulled
through the preheating, firing, and cooling zones at a constant rate of speed. The tunnel
kiln is very efficient and produces a more uniform product.

3.4 Types of Bricks

a) Common Bricks: -these are ordinary bricks that are not designed to provide good
finish appearance or high strength. They are, therefore, in general, the cheapest
bricks available.

b) Facing Bricks: - these are designed to give attractive appearance; hence they are
free from imperfections such as cracks. Facing bricks may be derived from
common bricks to which a sand facing and/or pigment has been applied prior to
firing.

c) Engineering Bricks: - these are designed primarily for strength and durability.
They are usually of high density and well fired.

_______________________________________________________________________________ 13
Eyob Yilma, Constructiion Technology Department, NCTTE
 Types of Bricks (According to ESC)

Two types of clay bricks are manufactured in Ethiopia at present. These are the

- Solid clay bricks

- Hollow clay bricks and beam tiles

Solid Clay bricks

Ethiopian standard ESC. Dr. 026.1973 (1) specifies requirements for burnt solid clay
bricks manufactured from suitable clay material and used in the building industry.
According to the standard, the solid bricks are of the following three types.

a) Brick without holes or depression (Type TS)

b) Brick with holes up to 20mm in diameter each and having a total cross-sectional
area not exceeding 25 percent of the base area of the brick (Type TH)

c) Brick with depression not exceeding 25 percent of the base area and having a
maximum depth of depression not more than 10mm (Type TD).

The nominal dimensions of solid bricks are 60 mm x 120 mm x 250 mm or 55 mm x 120

mm x 245 mm with dimensional tolerances of 2.5 mm, + 5 mm and +8.10 mm for the
height, the width, and the length respectively.

Hollow Clay bricks and beam tiles

According to ESC. D4.026.1973 (2) burnt hollow clay bricks and beam tiles
manufactured from suitable clay material and used in the building industry are of the
following three types:

a) With two faces keyed (combed or scared) for plastering or rendering (Type KK)

b) With two faces smooth and suitable for used without plastering or rendering on
either side (Type SS)

c) With one face smooth and another face keyed for plastering (Type SK).

The nominal dimensions of the hollow clay bricks and hollow clay beam tiles are given
in Table 1.

_______________________________________________________________________________ 14
Eyob Yilma, Constructiion Technology Department, NCTTE
 Table 1: Nominal dimensions of hollow clay bricks and hollow clay beam tiles

Nominal Dimensions in millimeter (mm)

Height (h) Width (w) Length (l)
Hollow Clay Bricks
100 200 300
100 150 350
100 250 300
120 250 300
150 200 300
Hollow Clay Beam Tiles
140 250 250
140 400 250
160 250 250
160 400 250

Indentations & perforation in bricks

- They assist in forming strong bond between the brick & the remainder of the
structure.
- They reduce effective thickness of the brick and hence reduce firing time.
- They reduce the material and hence the overall cost of the brick with out
serious in – situ strength loss.

3.5 Tests and classification of Bricks

Two classes of Test are used to determine the quality of building bricks: field tests and
laboratory test.

a) Field Tests

Field tests such as appearance, hammer test and hardness test can easily be made at the
construction site.

Appearance tests such as shape, planeness using vernier caliper, color, checks and blister
form valuable indications of quality.

When struck with a hammer, a properly burnt dry brick free from cracks emits a highly
metallic ring.

The hardness of a brick sample can be checked by scratching its surface or broken section
with a nail. A well-burnt brick will be scratched with difficulty.

_______________________________________________________________________________ 15
Eyob Yilma, Constructiion Technology Department, NCTTE
b) Laboratory Tests.

The Ethiopian Standard specifies a number of tests including visual inspection, checking
of dimensions and planeness, compressive strength, water absorption, saturation
coefficient and efflorescence tests on solid clay bricks. Infact, according to the standard,
solid clay bricks are classified according to the numerical values of their compressive
strength, water absorption, saturation coefficient and efflorescence as follows:

Table 2. Minimum Compressive Strength

Minimum Compressive Strength

Class Average of 5 Bricks N/mm2 Individual Brick N/mm2
A 20 17.5
B 15 12.5
C 10 7.5
D 7.5 5.0

Table 3 Maximum Water Absorption

After 24 hrs. Immersion After 5 hrs. Boiling

Class Average of Individual Average of 5 Individual
5 bricks % brick % bricks % brick %
A 21 23 22 24
B 22 24 23 24

C, D No limit No limit No limit No limit

In the case of hollow clay bricks and beam tiles the minimum compressive strength and
maximum allowable value for water absorption are given in Tables 4 and 5.

Table 4. Minimum Compressive Strength

Minimum Compressive Strength

Individual
Type Average of 5 Bricks N/mm2
Brick N/mm2
KK 7 5.5

SS 7 5.5

SK 7 5.5

_______________________________________________________________________________ 16
Eyob Yilma, Constructiion Technology Department, NCTTE
 Table 5. Maximum Water Absorption

After 1 hr. Submersion in Boiling Water

Type Average of 5 Bricks % Individual Brick %

KK 21.5 23

SS 21.5 23

SK 21.5 23

Efflorescence

This is the name given to the build-up of white surface deposits on drying out. It results
from dissolved salts in the brick and quite commonly spoils the appearance of new brick
Work, especially of exposed to weather, as in parapet, or to the prevailing wind. The
effect is most noticeable after periods of wet weather.

In the test for efflorescence, bricks are saturated with distilled water in order to dissolve
any salts present and allowed to dry such that salts are carried to one exposed face. The
apparatus is shown in the figure below.

_______________________________________________________________________________ 17
Eyob Yilma, Constructiion Technology Department, NCTTE
 The efflorescence is assessed as follows. (BS)

No perceptible deposit of efflorescence………………….. Nil

No more than 10% of area covered with

thin deposit of salts but unaccompanied
by powering or flaking of the surface……………………. Slight

A heavier deposit than slight covering

up to 50% of the area of the face but
unaccompanied by powdering or flaking
of the face ………………………………………………. Moderate

A heavy deposit of salts covering more

than 50% of the area of the face and/or
powdering or flaking of the surface………………………. Heavy

Expansion on wetting

Fired clay products of many types undergo a progressive irreversible expansion as

moisture penetrates pores and is absorbed on to internal surfaces. Over a period of years,
this expansion may amount to over 1000x10-6, especially in more porous bricks, though
the movement of the brickwork is normally only about 60% of this unless, for some
reason, the mortar is expansive. Movement joints should be provided at least every 12m
for clay brickwork and more frequently where openings, such as window, might act as
crack initiators.

Thermal Expansion

The coefficient of thermal expansion of clay brickwork is approximately 7x10-6 per 0C,
considerably less than that of most other building materials, so that thermal movement is
not normally a problem.

_______________________________________________________________________________ 18
Eyob Yilma, Constructiion Technology Department, NCTTE