Classification of Bricks
1. On the basis of Field Practice:
- Clay bricks are classified as first-class, second class, third class and fourth class based on their physical and mechanical properties.
First Class Bricks
- These bricks are table moulded and burnt in kilns.
- These are thoroughly burnt and are of deep red, cherry or copper colour.
- The surface should be smooth and rectangular; parallel, sharp and straight edges and square corner.
- It should be free from flaws, cracks, and stones.
- It should have a uniform texture.
- No impression should be left on the brick when a scratch is made by a fingernail.
- Fractured surface of the brick should not show lumps of lime.
- Metallic ringing sound should come when two bricks are struck against each other.
- On allowing immersed in cold water for 24 hours, water absorption should not be more than 20% its dry weight.
- Compressive strength >10.5 N/mm2.
Uses: First class bricks are recommended for pointing exposed face work in masonry structure flooring and reinforced brickwork.
Second Class Bricks
- These bricks have the same requirements as first-class brick except for the ones listed below.
- These bricks are ground moulded and they are burnt in kilns.
- Small cracks and distortions are permitted.
- Water absorption 22% of its dry weight is allowed.
- Compressive strength is 7.0 N/mm2 or greater.
Uses: Second class bricks are recommended for all important or unimportant hidden masonry Works and centring of reinforced brick and reinforced cement concrete (RCC) structures.
Third Class Bricks
- These bricks are ground moulded and they are burnt in clamps.
- These bricks are Soft and Reddish yellow coloured.
- Produce a dull sound when struck with each other.
- Water absorption is about 25% of dry weight is allowed.
- These have the rough surface with irregular and distorted edges.
Uses: For unimportant and temporary structures and at places where rainfall is not heavy.
Fourth Class Bricks
- These bricks are over burnt and badly distorted in shape and size and are brittle in nature.
Uses: Ballast of such brick is used for foundation and floors in lime concrete and road metal because burnt bricks have a compact structure and found to be stronger than even first class bricks.
2. On the basis of Strength
- Bureau of Indian Standards (BIS) has classified the bricks on the basis of compressive strength and is as given in Table below.
| Class Designation | Average compressive strength not less than (N/mm2) |
|---|---|
| 35 | 35.0 |
| 30 | 30.0 |
| 25 | 25.0 |
| 20 | 20.0 |
| 17.5 | 17.5 |
| 15 | 15.0 |
| 12.5 | 12.5 |
| 10 | 10.0 |
| 7.5 | 7.5 |
| 5 | 5.0 |
| 3.5 | 3.5 |
- As per IS-1077-1976, common building brick shall have a minimum strength of 35 kgf/cm2.
3. On the Basis of IS Classification
- As per IS: 3102-1965 Bricks are classified according to various physical properties as shown below.
| Class Designation | Compressive strength(N/mm2) |
Water absorption after 24 hrs immersion in cold water (%) max. |
Efflorescence | Shape and texture(N/mm2) |
|---|---|---|---|---|
| H I | 44 | 5 | NIL | Smooth rectangular faces with sharp corners shall emit a clear ringing sound when struck |
| H II | 44 | 5 | NIL | Slight distortion and slightly rounded edges permissible provided no difficulty will arise on this account in laying of uniform courses. |
| F I | 17.5 | 12 | Very Slight | Smooth rectangular faces with sharp corners shall emita clear ringing sound when struck |
| F II | 17.5 | 12 | Slight | Slight distortion and slightly rounded edges permissible provided no difficulty will arise on this account in laying of uniform courses. |
| M I | 7.0 | 20 | Very Slight | Smooth rectangular faces with sharp corners shall emit a clear ringing sound when struck |
| M II | 7.0 | 20 | Slight | Slight distortion and slightly rounded edges permissible provided no difficulty will arise on this account in laying of uniform courses. The ringing sound may be clear or dull |
| L I | 3.5 | 25 | Very Slight | Smooth rectangular faces with sharp corners, the ringing sound may be clear of dull. |
| L II | 3.5 | 25 | Slight | Slight distortion and slightly rounded edges permissible provided no difficulty will arise on this account in laying of uniform courses. The ringing sound may be clear or dull. |
Note: The burnt clay bricks having compressive strength more than 40.0 N/mm2 are known as heavy duty bricks and are used for heavy duty structures such as bridges foundations for industrial buildings, multistory buildings, etc The water absorption of these bricks is limited to 5 per cent.