GENERAL DISCUSSION ON BRICK AND BRICK TESTING

1. GENERAL DISCUSSION ON BRICK AND BRICK TESTING

1. Introduction:

1.1 Brick is an artificial kind of stone made from clay. Here clay is the type of soil whose chief

Characteristics are plasticity when wet and stone-like hard as heated to high temperature. Burnt clay

bricks are one of the main construction materials in Bangladesh. Broken bricks or brick aggregates

are also used in concrete or in road layers.

1.2 Since a number of factors affect the strength/durability of bricks and brick aggregates, testing of

bricks is very important before use. As an example, the compressive strength of burnt clay brick can vary some hundred psi to 5000 psi or even higher strength. Therefore, the strength and durability

of a brick wall depend very much on the quality of the bricks. The strength of concrete (using brick

aggregates) may also vary widely due to the quality of brick aggregates. Testing of bricks is

therefore important to maintain quality. Testing of bricks and other related items has been

discussed in this chapter.

2. Factors Affecting the Quality of Bricks:

The following are the factors on which the quality of bricks depends:

i. Chemical properties of the clay used

ii. Preparation of the clay

iii. Process of drying

iv. Different degrees of burning

3. Constituents of Brick-Clay and their Functions:

3.1 A good brick clay should be such a mixture of pure clay and sand that when prepared with

water, it can easily be moulded and dried without cracking or warping. It should also contain

Alumina ( Aluminium Oxide), Lime, Iron Oxide and Magnesia (Magnesium Oxide). Chemical

analysis of a good brick clay should give the following chemical composition:

Silica 55%

Alumina 30%

Iron oxide 8%

Magnesia 5%

Lime 1%

Organic matters 1%

100%

BRICKS TESTING, Field Test of Bricks, Characteristics of Bricks, ingredient component of Brick, properties of bricks, Tests for Bricks

3.2 Functions of the Constituents:

i. Silica: Silica exists in all clays in a state of chemical combination and sometimes in a free state

when it is called flint or sand. The presence of sand prevents cracking, shrinking, and warping.

The higher the proportion of sand, the more shapely and uniform in texture will the brick be. Too

much sand however makes the brick brittle and weak.

ii. Alumina (Aluminium Oxide): It imparts plasticity to clay which is very essential for the purpose of moulding. But too much alumina may cause crack and wrap during drying Brick and brick becomes very hard under the influence of heat.

iii. Iron Oxide: The presence of Iron Oxide in clay enhances the impermeable and durable qualities.

iv. Magnesia (Magnesium Oxide): The presence of Magnesia in a small quantity decreases shrinkage.

v. Lime: Lime reduces shrinkage of bricks during drying and enables the silica to melt in burning and thus binds the particles of brick together. In excess, however, it will cause the brick to fuse too readily and the shape will be lost. Lime should be present in a very fine state. If present in the form of lumps, it is very injurious to bricks, since on burning it becomes quick-lime and absorbs moisture hence causing disintegration.

vi: Organic Matter: A small quantity of organic matter will assist burning bricks. Excess is bad because if it is not completely burnt, the bricks will be porous.

4. Harmful Constituents of Brick-Clay:

i. Iron Pyrites: The presence of Pyrites cause crystallisation and disintegration of bricks on burning.

ii. Alkalis: These are mainly the Chlorides and Sulphates of Calcium, Magnesium, Sodium and Potassium. They produce a dark greenish hue on the surface of bricks on drying. They cause the bricks to fuse, twist and wrap. Alkalis in bricks absorb moisture from the air and drying will cause efflorescence.

ii. Stone Particles: Small particles of stones do not allow the clay to be mixed thoroughly and uniformly. These make bricks porous, weak and non-uniform.

5. Characteristics of Good Bricks:

The following are the characteristics of good bricks:

i. Bricks should be uniform in colour, size and shape. The standard dimension is 240x115x 70mm.

ii. They should be sound and compact.

ii. They should be free from cracks and other flaws such as air bubbles, stone nodules, etc.

iv. They should not absorb more than about 1/5 of their own weight of water when immersed

in water for 24 hours (15 to 20 % of dry wt.).

v. The average compressive strength of bricks should be in the range of 2500 psi.

vi. The percentage of soluble salts (Sulphates of Calcium, Magnesium, Sodium and Potassium)

should not exceed 2. 5% in burnt bricks. Because the presence of excess soluble salts causes

efflorescence.

vii. They should be neither over-burnt nor under-burnt.

viii. The weight should be generally 2.7Kg (6Ib.) per brick and the unit weight should be

generally 2000 Kg/m' (125Ibs./cft).

ix. They should have low thermal conductivity as it is desirable that the buildings made of them

should be cool in summer and warm in winter.

x. They should be non-inflammable and incombustible.

xi. Bricks should not change in volume when wet.

xi. They should emit a metallic sound when struck by another brick.

6. Field Testing of Bricks:

The following tests may be performed for quick and rough determination of the quality of brick:

i. Take a brick and try to make a mark on its surface with a nail. If you can do this, it is not a

good brick. If not, it is sufficiently hard and compact.

ii. Take a brick and strike it with a hammer or another brick. If it gives a clear ringing or metallic

sound, it is a good brick.

iii. Take two bricks and form a 'T as shown in Figure 2.1 and drop from a height of 1.2m to 1.5 m on a more or less solid surface. If they break, they are not good bricks. If they remain unbroken, they are good bricks.

iv. Surfaces of good bricks should be smooth have square edges, and be free from cracks and voids.

v. Colour, shape, size and structure of bricks should be uniform.

NOTE: It should always be remembered that Field Test is not the alternative to 'Laboratory Test'. But it gives a preliminary idea of the quality of Brick before going through detailed laboratory testing.