Types of Concrete

There are special ways of strengthening concrete or of making concrete building materials. These include (1) reinforced concrete, (2) prestressed concrete, (3) precast concrete, and (4) concrete masonry. Engineers have also developed special kinds of concrete for certain uses. These include (1) air-entrained concrete, (2) high-early-strength concrete, and (3) lightweight concrete.
Reinforced concrete is made by casting concrete around steel rods or bars. The steel strengthens the concrete. Almost all large structures, including skyscrapers and bridges, require this extra-strong type of concrete.
Prestressed concrete usually is made by casting concrete around steel cables stretched by hydraulic jacks. After the concrete hardens, the jacks are released and the cables compress the concrete. Concrete is strongest when it is compressed. Steel is strong when it is stretched, or in tension. In this way, builders combine the two strongest qualities of the two materials. The steel cables can also be bent into an arc, so that they exert a force in any desired direction, such as upward in a bridge. This force helps counteract the weight of the bridge. Prestressed concrete beams, roofs, floors, and bridges are often cheaper for some uses than those made of reinforced concrete.
Precast concrete is cast and hardened before being used for construction. Precasting firms make concrete sewer pipes, floor and roof units, wall panels, beams, and girders, and ship them to the building site. Sometimes builders make such pieces at the building site and hoist them into place after they harden. Precasting makes possible the mass production of concrete building materials. Nearly all prestressed concrete is precast.
Concrete masonry includes many shapes and sizes of precast block. It is used to make about two-thirds of all the masonry walls built each year in the United States. Some concrete masonry is decorative or resembles brick.
Air-entrained concrete contains tiny air bubbles. These bubbles are formed by adding soaplike resinous or fatty materials to the cement, or to the concrete when it is mixed (see RESN). The bubbles give the water in concrete enough room to expand as it freezes. The bubbles also protect the surface of the concrete from chemicals used to melt ice. Such qualities make air-entrained concrete a good material for roads and airport runways.
High-early-strength concrete is chiefly used in cold weather. This concrete is made with high-early-strength portland cement, and hardens much more quickly than ordinary concrete. It costs more than ordinary concrete. But it is often cheaper to use, because it cuts the amount of time the concrete must be protected in cold weather.
Lightweight concrete weighs less than other kinds of concrete. Builders make it in two ways. They may use lightweight shales, clays, pumice, or other materials instead of sand, gravel, and crushed rock. Or they may add chemicals that foam and produce air spaces in the concrete as it hardens. These air spaces are much larger than the air spaces in air-entrained concrete.

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