Post-tensioning is the process by which tendons (either steel strands or high tensile steel rods) are placed through preformed holes on the concrete and tensioned, once the concrete has gained sufficient strength. Construction requires the installation of some form void former, at the required location, rigidly fixed so that it does not move during the concreting process. Generally this is a permanent liner to the void (either steel or polyethylene) though various forms of removable void former are available.
Once the concrete has been cast and gained sufficient strength, the tendon is passed though the void, anchorages installed at both ends and the tendon tensioned. Generally the void between the tendon and the concrete is subsequently filled with a cement-based grout to ensure composite action and to improve durability. This approach is known as the use of ‘bonded tendons’. In some applications, such as some multi-storey buildings, the tendons are supplied in grease-filled sheaths and are no grouting is used. This approach is the use of ‘unbonded tendons’.
See also related category ´Prestressing´ and associated sub-categories.