In post-tensioning, the prestressing tendons, which may be located inside or outside the concrete, are tensioned after the concrete has hardened. Internal tendons are contained within ducts or sheaths cast into the concrete and can be arranged to follow the most appropriate profile to suit the applied external loading. External tendons are arranged in a series of straight lines with changes of direction at specific locations. In all cases the tendons are tensioned using jacks that bear against the exposed face of cast-in anchorages at the ends of the tendon. When the required force has been attained, the tendons are secured at the anchorages, using a system of wedges. The elastic shortening of the concrete that occurs during the tensioning process has no effect on the force in the tendons, but some loss may occur due to draw-in of the anchoring wedges.
Internal tendons may be bonded to the concrete or may remain unbonded, depending on the design. In either case, durability of the tendons is essential. For bonded tendons, protection is achieved by injecting cement grout into the duct after the tendons have been tensioned and anchored. Unbonded tendons are supplied encased in plastic tubing filled with grease.