Based on their physical and mechanical properties, clay bricks are classified into ____ classes.

Explanation:

Classification of bricks based on their physical and mechanical properties:

1. First class brick  2. Second class brick  3. Third class brick  4. Fourth class brick

1. First class brick: Following are properties of first class bricks:

• These are table moulded and burnt in kiln's.
• Smooth surface, rectangular in shape with parallel, sharp and straight edges with square corner. 
• Water absorption \(\ngtr \) 20% of its dry weight when immersed in cold water for 24 hours.
• Crushing strength \(\geq\) 10.5 N/mm²

• These bricks are recommended for pointing work, exposed face work in masonry structure, flooring and reinforced brick work.

2. Second class brick: Following are properties of Second class bricks:

• These are ground moulded and burnt in kiln.
• Small cracks and distortions are permitted.
• Water absorption = 22% of its dry weight in 24 hours immersion in cold water.
• Crushing strength \(\geq\) 7 N/mm²

• These bricks are recommended for hidden masonry works and centering of reinforced brick and reinforced cement concrete structure.

 3. Third class brick: Following are properties of Third class bricks:

• These are ground moulded and burnt in clamps.
• Soft and reddish yellow in colour.
• Water absorption = 25% of its dry weight in 24 hours immersion in cold water.

• These bricks are recommended for unimportant and temporary works where rainfall is not heavy.

4. Fourth class brick: Following are properties of Third class bricks:

• These are overburnt brick which are badly distorted.
• Used as ballast in foundation and floors in lime concrete.