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What is Prestressed Concrete?
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What is Prestressed Concrete?

Prestressed Concrete is an architectural and structural material possessing great strength. The unique characteristics of prestressed concrete allow predetermined, engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads. This is accomplished by combining the the best properties of two quality materials: high strength concrete for compression and high tensile strength steel strands for tension.

Actually, prestressing is quite simple. High tensile strands are stretched between abutments at each end of long casting beds. Concrete is then poured into the forms encasing the strands. As the concrete sets, it bonds to the tensioned steel. When the concrete reaches a specific strength, the strands are released from the abutments. This compresses the concrete, arches the member, and creates a built in resistance to service loads.

Prestressed Concrete Beam
Prestressed or pretensioned before it leaves the plant, a slight arch or camber is noticeable. Energy is stored in the unit by the action of the highly tensioned steel which places a high compression in the lower portion of the member. An upward force is thereby created which in effect relieves the beam of having to carry its own weight.

The upward force along the length of the beam counteracts the service loads applied to the member.
Ordinary Concrete Beam
Even without a load, the ordinary concrete beam must carry its own considerable weight - this leaves only a portion of its strength available to resist added loads.
Under service loads, the bottom of the beam will develop hairline cracks.