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.