Classification of clay bricks: Sun-dried bricks & Burnt bricks and their uses

Classification of clay bricks

Bricks are broadly classified into two broad categories as follows: 

1. Sun-dried bricks and 

2. Burnt bricks 

1. Sun-dried bricks -

Sun-dried bricks also called un-burnt or katcha bricks, and these are dried directly under the sun after the process of moulding. These bricks are of inferior quality and are used for the construction of temporary and cheap structures. Such bricks should not be used in areas exposed to heavy rains.

2. Burnt bricks -

Burnt bricks are a type of clay brick that are made by molding, drying, stacking, and then burning to harden. Burnt bricks are more durable than sun-dried bricks. Burnt bricks are of superior quality, which are generally used for civil engineering constructions. 

These are classified into four categories: 

1. First-class Bricks

2. Second-class Bricks

3. Third-class Bricks

4. Fourth-class Bricks

1. First-class Bricks -

• First-class bricks are table-moulded bricks and are burnt in kilns.

• 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 fingernail.

• 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 not be more than 10 –15% of its dry weight when immersed in water for 24 hours.

• The crushing strength of the brick should not be less than 10 N/mm2.

Uses: 

These bricks are used for superior quality works and works of permanent nature. First class bricks are recommended for pointing, exposed face work in masonry structures, flooring and reinforced brick work.

2. Second-class Bricks -

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

• Second-class bricks are ground moulded and are burnt in kilns.

• They may have a slightly irregular shape, rough, uneven surfaces or may have slight crack.

• These bricks are used where brickwork is to be provided with coat of plaster.

• 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.

3. Third-class Bricks - 

Third-class bricks are ground moulded and burnt in clamps. These bricks have irregular edges with less sharpness, uneven surfaces and are not hard enough. They give a dull sound when struck against each other. Water absorption is about 25 per cent of dry weight. 

They are used for unimportant and temporary constructions.

4. Fourth-class Bricks - 

Fourth-class bricks are either over burnt or under burnt with irregular shapes, edges and surfaces and are brittle in nature. 

These are used as aggregates for concrete in road, floor and foundation construction.