Know More About Cement And Concrete

Cement is one of the most important raw materials used in construction work because it is the essential ingredient in concrete. Millions upon millions of tones of concrete are used every year in building roads, bridges, dams, and so on. It is cheap, easy to make, and is waterproof and fireproof. It can also be used under water. When concrete is first mixed, it forms a pasty mass that can be cast into any shape. But it quickly sets rock-hard.

Cement Manufacturing Plant

How concrete is made

Concrete is made by adding water to a mixture of cement, sand, and gravel or stone. The sand is called fine aggregate and the stones are called coarse aggregate. The sand fills in the gaps between the larger stones so that there will no air trapped between them. The presence of air would weaken the concrete. The most common type of cement is Portland cement, which is a fine, grey powder made by roasting a crushed mixture of limestone, clay, and other materials. Large lumps of chalk or limestone rock from a quarry are broken down into smaller lumps by powerful mechanical crushers. Then they are mixed with the other materials and water to form slurry. The slurry enters a ball mill where tumbling steel balls grind it finely. From there it passes into a long, cylindrical kiln that slopes slightly downwards and rotates so that the materials gradually move along it. In this kiln the materials are first dried and then heated while-hot to form a hard, coak-like mass, called clinker. After cooling, this clinker is finely crushed into the familiar fine powdery form of cement. A certain amount of gypsum is added to control the rate at which the cement will eventually set.

Concrete Mixer

Concrete is usually mixed on the site where it is to be used in machines called concrete mixers. They have a rotating drum which mixes the ingredients thoroughly together. The wet concrete may be taken to where it is needed in huge buckets, or skips carried by cranes, or it may be pumped into place through a pipeline. Sometimes concrete is supplied ready-mixed in huge mixer trucks, which pour it directly into place.

Concrete may be cast in its final position in moulds made of wood. Or it may be precast into shape and then taken to the site to be assembled.

Reinforced concrete

Ordinary concrete is strong when it is being squeezed, or under compression. But it is weak when it is being stretched, or under tension. If, say a concrete beam is supported only at its ends, this weakness may cause it to crack and collapse. To prevent this weakness, steel wires or rods are cast into concrete. It is then known as reinforced concrete. The strength can be increased further by stretching the wires or rods either before or after they are forced in the concrete.

In pretensioning, the concrete is cast around rods that are being stretched. When the concrete sets, the rods are released and compress the concrete, making it stronger. In post-tensioning, the concrete beam is cast with holes, pulled out, and then anchored so that the beam is under compression. This again strengthens the beam.