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Prestressed concrete is concrete that has had internal stresses introduced to counteract, to the degree desired, the tensile stresses that will be imposed in service. The stress is usually imposed by tendons of individual hard-drawn wires, cables of hard-drawn wires, or bars of high strength alloy steel. Prestressing may be achieved either by pretensioning or by post-tensioning. To pretension concrete the steel is first tensioned in a frame or between anchorages external to the member. The concrete is then cast around it. After the concrete has developed sufficient strength the tension is slowly released from the frame or anchorage to transfer the stress to the concrete to which the tendons have by that time become bonded. The force is transmitted to the concrete over a certain distance from each end of a member known as the transfer length. Post-tensioned concrete is made by casting concrete that contains ducts through which tendons can be threaded. An alternative is to cast the concrete around tendons that are greased or encased in a plastic sleeve. When the concrete has sufficient strength the tendons are tensioned by means of portable jacks. The load is transmitted to the concrete through permanent anchorages embedded in the concrete at the ends of the tendons. Ducts are usually grouted later or filled with grease to protect the tendons against corrosion. In some applications the post-tensioning tendons are run alongside the concrete member. One advantage of post-tensioning is that it permits using tendons that are curved or draped. (This can be achieved in pretensioning but not so easily.) Post-tensioning can be done on the jobsite without any need of heavy temporary anchorages. Anchorages are needed for each tendon, however, which is a significant cost item.
