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. 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.