What is Post Tensioning?
Essentially, Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. Post-Tensioning is a method of Pre-Stressing. Pre-Stressed concrete or masonry is made up of internal stresses (forces) induced into it during the construction phase with the purpose of counteracting the anticipated external loads that it will encounter during its lifecycle.
There are two methods of Pre-Stressing
Pre-Tensioning & Post-Tensioning
Post-Tensioned reinforcement consist of very high strength steel strands. Typically, strands are used in horizontal applications like foundations, slabs, beams, and bridges; and bars are used in vertical applications like walls and columns. A typical steel strand used for Post-Tensioning has a tensile strength of 1861 N/mm2. In comparison, a typical non-prestressed piece of reinforcing (rebar) has a tensile strength of 413 N/mm2. Strands commonly have a diameter of 12.7mmØ or 15.2mmØ and are stressed to a force of 228 N/mm2 using a hydraulic jack.
The Pre-Stressing steel is housed in sheathing or duct to allow it move as the tensioning force is applied after the concrete cures. The steel stretches as it is tensioned, and it is locked into place using an anchoring component that forms a mechanical connection and keeps the force in the strand for the life of the structure. After the tendons have been stressed and engineering approval given, they are cut off and permanently sealed. A free flowing cementitious grout is then pumped into the ducts to lock the cables in position permanently. This is classified as bonded Post-Tensioning.