Second class bricks are supposed to have the same requirements as the first class ones except that
1. Small cracks and distortions are permitted.
2. A little higher water absorption of about 16–20% of its dry weight is allowed.
3. 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.
Third Class bricks : are underburnt. They are soft and light-coloured producing 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.
Fourth class bricks : are overburnt and badly distorted in shape and size and are brittle in nature.
Uses: The ballast of such bricks is used for foundation and floors in lime concrete and road metal.