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These are rectangular artificial stones obtained by moulding clay in rectangular moulds, drying them and finally burning.
Composition of Brick Earth:
A good brick earth should contain silica (50-60 pc), Alumina (20-30 pc), lime (2-5 pc), iron oxide (5-6 pc) and magnesia (2-4 pc). Among the above, silica is the main constituent and imparts strength and durability. It also prevents cracking, shrinking or warping of bricks during drying. Alumina imparts plasticity to earth and imparts strength. Lime in small quantities is essential to smoothen the burning process. Lime prevents shrinkage of bricks during drying. Excess lime and lime in lumps form should be avoided since they will cause the bricks to melt during burning. Magnesia similar to lime, if in excess will cause unsoundness. While selecting the earth for brick manufacturing it should be ensured that presence of alkalis, small pebbles and organic matter which causes disintegration, be not present in it.
The selected brick earth is kneaded in pug mills duly adding water to it. Later it is molded into required size either by hand or by machine. The moulds are removed when the bricks have become sufficiently dry and taken to the final operation of burning. Burning is done either in clamps or in kilns. When bricks are required in large scale quickly, burning at kilns is adopted.
Qualities of a Good Building Brick
- It should have uniform size and shape. The sides should be true and square. The edges should be sharp and straight. It should be of standard size.
- It should be thoroughly burnt, but should not be over burnt.
- It should have uniform colour.
- It should have uniform texture.
- When two bricks are struck together, a metallic ringing sound should emanate.
- It should not break when dropped from a height of 120 cm.
- It should be hard, tough and durable. When scratched by finger nail no impression should appear on its surface.
- It should not have cracks, flaws or any loose fill material.
- When soaked in water for 24 hours, the percentage of water absorption by weight should not be more than 20 percent for first class bricks and 22 percent for second class bricks.
- It should have a minimum crushing strength of 35 kg/cm2. Basing on crushing strength the bricks are classified as
- Compressive strength not less than 70 kg/cm2 - Second class (Grade-B)
- Compressive strength not less than 105 kg/cm2 - First class (Grade-A)
- Compressive strength not less than 140 kg/cm2 - Grade-AA
Standard Size of Bricks
The standard size of bricks of modular size is 19 x 9 x 9 cm (length xwidth x height). With I cm thick mortar joint the nominal size of brick will become 20 x 10x 10 cm. On average a good brick weighs 2.5 to 3.5 kg.
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