In post-tensioned work the concrete is cast in a way that allows the concrete to harden before the tendons are stressed. This can be done by using tendons that have been coated with grease and encased in plastic sheathing during manufacture. These covered tendons are cast in the concrete. Another method is to cast ducts in the concrete through which tendons are later threaded. When the concrete is strong enough the tendons are stressed and anchored. There are other specialized, less widely used methods of post-tensioning, such as winding tendons around the exterior of a tank, stressing the tendons, and then protecting them with a layer of plaster or shotcrete. Post-tensioning tendons can also be mounted on the outside of beams or other members.
Post-tensioning may conveniently be divided into categories, depending on whether the stressing tendon is single-strand, multistrand, bar, or wire. Of all these, single-strand, multistrand and deformed bar systems are the most widely used in the United States. Post-tensioning construction is classified as bonded or unbonded. In bonded post-tensioned construction, the tendon ducts are filled with a mortar grout after stressing. In unbonded construction, the tendons are greased and wrapped with paper or covered with plastic.
Lightweight hydraulic jacks designed for the purpose are used to apply tension to the steel. Such jacks or stressing rams are designed to fit over the tendon and bear against the anchor plate. The magnitude of the prestress force is monitored both by watching the load on a hydraulic gage and by measuring the amount of elongation of the tendon. Force is applied to the jack by a hydraulic pump.