Billy Nagle, age 12, of Nokomis, Fla., for his question:

WHAT IS PRESTRESSED CONCRETE?

To make prestressed concrete, wet concrete is put into a casting around steel cables which have been 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 made stronger when it is stretched or when it is in tension. In this way, builders combine the strongest qualities of both 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 downward pressure of the weight of the bridge.

Prestressed concrete is used for beams, roofs, floors, and bridges.