Burnt-clay bricks have good resistance to moisture, insects and erosion and create a good room environment. They are medium in cost and have medium-to-high compressive strength.
Bricks can be made with sophisticated factory methods, simple labour-intensive methods or a range of mechanized technologies in between. The labour-intensive production methods are most suitable for rural areas where the demand for bricks is limited. The bricks produced by hand will have relatively lower quality, especially compressive strength, and will tend to have irregular dimensions. However, they are economical and require little capital investment or transportation cost. Bricks made in this manner have been used in buildings which have lasted for centuries. Their longevity has depended on the quality of the ingredients, the skill of the artisans and the climate in which they were used.
Brick Making
Four main ingredients are required for brick making: suitable clay and sand, water, fuel and manpower. The clay must be easily available, be plastic when mixed with small amounts of water, develop strength upon drying and develop hard and durable use-strength when burned.
Suitable soils contain 25 to 50% clay and silt and 50 to 75% coarser material as determined by the simple sedimentation test. The soil must be well graded. Another test consists of rolling out by hand on a flat surface a long cylinder with a 10mm diameter from moistened soil and then picking it up by one end and letting it hang unsupported. A soil is adequate for brick making if the piece of cylinder that breaks off is between 50 and 150mm long. In the bar shrinkage test, using a mould 300mm long and 5Omm wide and deep, a suitable soil should show no cracking or only a little on the surface and shrink less than 7%, i.e., less than 20mm.
The clay is obtained by chipping it out of a clay bank and when necessary, mixing it with sand to a mixture that will not crack during drying. Water is gradually added to make the clay plastic.
In making bricks, the mould must be cleaned periodically with water. Before each brick is formed, the mould is sprinkled with sand. A lump or clot of clay just slightly larger than required for a brick is rolled into a wedge shape and then in sand before it is thrown, point down, into the mould. Thrown correctly, the mould will be completely filled and the excess clay is then shaved off the top with a bowcutter. The sand in the mould and on the clot helps release the newly formed brick.
The bricks should be left to dry for about three days in the place where they were made. They will then be strong enough to be stacked, as shown in Figure 3.17, for at least one week of further drying. Clay tends to become lighter in colour when dry and, when sufficiently dried, the brick, upon being broken in half, will show no color differential throughout the section area. During drying the bricks should be protected from rain.