Bricks and blocks

Burnt clay bricks have been India's main walling unit for centuries. Clay is moulded, dried and fired in a kiln, which fuses the particles into a hard unit. Good bricks are uniform in colour and size, give a ringing sound when struck together, and do not show white salt deposits (efflorescence) after wetting.

Brick quality is checked by compressive strength, water absorption, efflorescence and dimensional tolerance. Absorption matters because a very thirsty brick sucks water from the mortar before it can set; that is why bricks are soaked before laying.

For common burnt clay bricks, the Indian specification caps 24-hour cold-water absorption at 20 percent by mass for the lower strength classes (up to class 12.5) and at 15 percent for the higher classes. The test dries the brick to constant mass, soaks it for a day and weighs it again.

Alternatives are now common. Fly ash bricks use a by-product of power stations. Autoclaved aerated concrete (AAC) blocks are light and insulating, reducing dead load on frames. Solid and hollow concrete blocks speed up walling. Compressed stabilised earth blocks need no firing at all.

In a framed building most walls are infill and carry only their own weight, but they still interact with the frame in an earthquake. In load-bearing masonry, the walls carry floors and roofs and need bands at lintel and roof level in seismic zones, as described in IS 4326.