What is concrete?

Concrete is one of the most useful building materials ever developed by man. It is strong, long-lasting, fairly cheap to use, and easy to handle. It is not harmed by fire, water, weather, or heavy pressures.

Huge dams, bridges, and skyscrapers, as well as highways, homes, and airport runways, are built of concrete.

Concrete is made from Portland cement, water, sand, gravel, or crushed stone. The materials are measured and mixed together to make concrete. After mixing, the concrete can be given any shape that is wanted by placing it in molds that are called forms.

The mixing turns the water and cement into a paste that coats the pieces of sand and gravel. When this paste hardens, it holds the pieces together in a solid, rock-like mass. Keeping the concrete moist after putting it in the forms makes it even harder. Because of a chemical reaction between the cement and the water, the concrete keeps getting harder as it ages.

Concrete is treated in various ways to make it suitable for special purposes. For example, when concrete is used in long, slender parts, it may snap or be pulled apart. To make concrete structures hold up under forces that would bend them or pull them apart, steel rods or mesh can be set in the concrete. This is called reinforced concrete. Concrete can also be strengthened by casting (pouring) it around high-tension steel wires. When these wires are tightened_-before the concrete hardens-the concrete is placed in a squeeze that makes it stronger. Such concrete is called prestressed concrete.

A kind of concrete is now made that contains billions of tiny air bubbles in each cubic centimeter. It is called air-entrained concrete.

Roads built of this concrete are not harmed by freezing and thawing.

So you can see how much can be done with concrete to make it serve special needs in construction.