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Classification Of Bricks
On 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
1. These are thoroughly burnt and are of deep red, cherry or copper colour.
2. The surface should be smooth and rectangular, with parallel, sharp and straight edges and square corners.
3. These should be free from flaws, cracks and stones.
4. These should have uniform texture.
5. No impression should be left on the brick when a scratch is made by a finger nail.
6. The fractured surface of the brick should not show lumps of lime.
7. A metallic or ringing sound should come when two bricks are struck against each other.
9. Water absorption should be 12-15% of its dry weight when immersed in cold water for 24 hours. The crushing strength of the brick should not be less than 10 N/mm2. This limit varies with different Government organizations around the country.
Uses: First class bricks are recommended for pointing, exposed face work in masonry structures, flooring and reinforced brick work.
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.
The Bureau of Indian Standards (BIS) has classified the bricks on the basis of compressive strength and is as given in Table 2
Table 2 Classification of Bricks based on Compressive Strength (IS: 1077)
Class : Average compressive strength not less than (N/mm2)
Notes: 1. The burnt clay bricks having compressive strength more than 40.0 N/mm 2 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.
2. Each class of bricks as specified above is further divided into subclasses A and B based on tolerances and shape. Subclass-A bricks should have smooth rectangular faces with sharp corners and uniform colour. Subclass-B bricks may have slightly distorted and round edges.
Dimension(cm) Tolerance(mm) Dimension(cm) Tolerance(mm)
Length 380 ± 12 380 ± 30
Width 180 ± 6 180 ± 15
(i) 9 cm 180 ± 6 180 ± 15
(ii) 4 cm 80 ± 3 80 ± 6
On the Basis of Use
Common Brick is a general multi-purpose unit manufactured economically without special reference to appearance. These may vary greatly in strength and durability and are used for filling, backing and in walls where appearance is of no consequence.
Facing Bricks are made primarily with a view to have good appearance, either of colour or texture or both. These are durable under severe exposure and are used in fronts of building walls for which a pleasing appearance is desired.
Engineering Bricks are strong, impermeable, smooth, table moulded, hard and conform to defined limits of absorption and strength. These are used for all load bearing structures.
On the Basis of Finish
Sand-faced Brick has textured surface manufactured by sprinkling sand on the inner surfaces of the mould.
Rustic Brick has mechanically textured finish, varying in pattern.
On the Basis of Manufacture
Hand-made: These bricks are hand moulded.
Machine-made: Depending upon mechanical arrangement, bricks are known as wire-cut bricks-bricks cut from clay extruded in a column and cut off into brick sizes by wires; pressed-bricks-when bricks are manufactured from stiff plastic or semi-dry clay and pressed into moulds; moulded bricks-when bricks are moulded by machines imitating hand mixing.
On the Basis of Burning
Pale Bricks are underburnt bricks obtained from outer portion of the kiln. Body Bricks are well burnt bricks occupying central portion of the kiln.
Arch Bricks are overburnt also known as clinker bricks obtained from inner portion of the kiln.
On the Basis of Types
Solid: Small holes not exceeding 25 per cent of the volume of the brick are permitted; alternatively, frogs not exceeding 20 per cent of the total volume are permitted.
Perforated: Small holes may exceed 25 per cent of the total volume of the brick.
Hollow: The total of holes, which need not be small, may exceed 25 per cent of the volume of the brick. Cellular: Holes closed at one end exceed 20 per cent of the volume.
Note: Small holes are less than 20 mm or less than 500 mm2 in cross section.
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