A post tensioning “tendon” is defined as a complete assembly consisting of the anchorages, the prestressing strand or bar, the sheathing or duct and any grout or corrosion-inhibiting coating (grease) surrounding the prestressing steel. There are two main types of posttensioning: unbonded and bonded (grouted). An unbonded tendon is one in which the prestressing steel is not actually bonded to the concrete that surrounds it except at the anchorages. The most common unbonded systems are monostrand (single strand) tendons, which are used in slabs and beams for buildings, parking structures and slabs-on-ground.

A monostrand tendon consists of a seven-wire strand that is coated with a corrosion-inhibiting grease and encased in an extruded plastic protectivesheathing. The anchorage consists of an iron casting and a conical, two-piece wedge which grips the strand. In bonded systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete. T e strands are stressed with a large, multi-strand jack and anchored in a common anchorage device. The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct. Bonded systems are more commonly used in bridges, both in the superstructure (the roadway) and in cable-stayed bridges, the cable-stays. In buildings, they are typically only used in heavily loaded beams such as transfer girders and landscaped plaza decks where the large number of strands required makes them more economical.

Rock and soil anchors are also bonded systems but the construction sequence is somewhat different. Typically, a cased hole is drilled into the side of the excavation, the hillside or the tunnel wall. A tendon is inserted into the casing and then the casing is grouted. Once the grout has reached sufficient strength, the tendon is stressed. In slope and tunnel wall stabilization, the anchors hold loose soil and rock together; inexcavations they hold the wood lagging and steel piles in place.

There are several critical elements in a post tensioning system. In unbonded construction, the plastic sheathing acts as a bond breaker between the concrete and the prestressing strands. It also provides protection against damage by mechanical handling and serves as a barrier that prevents moisture and chemicals from reaching the strand. The strand coating material reduces friction between the strand and the sheathing and provides additional corrosion protection.

Anchorages are another critical element, particularly in unbonded systems. After the concrete has cured and obtained the necessary strength, the wedges are inserted inside the anchor casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments, the anchorages and exposed strand tails are usually covered with a housing and cap for added protection.

In building and slab-on-ground construction, unbonded tendons are typically prefabricated at a plant and delivered to the construction site, ready to install. The tendons are laid out in the forms in accordance with installation drawings that indicate how they are to be spaced, what their profile (height above the form) should be, and where they are to be stressed. After the concrete is placed and has reached its required strength, usually between 3000 and 3500 psi (“pounds per square inch”), the tendons are stressed and anchored. The tendons, like rubber bands, want to return to their original length but are prevented from doing so by the anchorages. The fact the tendons are kept in a permanently stressed (elongated) state causes a compressive force to act on the concrete. The compression that results from the posttensioning counteracts the tensile forces created by subsequent applied loading (cars, people, the weight of the beam itself when the shoring is removed). This significantly increases the load-carrying capacity of the concrete. Since post-tensioned concrete is cast in place at the job site, there is almost no limit to the shapes that can be formed. Curved facades, arches and complicated slab edge layouts are often a trademark of post-tensioned concrete structures. Post-tensioning has been used to advantage in a number of very aesthetically designed bridges.