Post-tensioning and its phases
The ducts for the tendons (or strands) are arranged together with the reinforcement prior to the casting of concrete in post-tensioning systems.
The tendons are arranged in the ducts as soon as the casting of concrete is completed. The objective of duct is to avoid connection among concrete and the tendons throughout the tensioning operation.
Contrary to pre-tensioning, the tendons are pulled with the reaction that operates against the hardened concrete. It is defined as bonded post-tensioning when the ducts are filled with grout. The grout belongs to a neat cement paste or a sand-cement mortar that comprises of proper admixture.
In unbonded post-tensioning, the ducts are not at all grouted and the tendon is set in tension individually with the end anchorages. In the sketch given below, there is a schematic illustration of a grouted post-tensioned member. The profile of the duct is based on the support conditions. For a simply supported member, the duct contains a sagging profile among the ends. For a continuous member, the duct bends in the space and hogs over the support.
The following figures demonstrate the assessment of ducts in a box girder of a simply supported bridge. The second image demonstrates the end of the box girder as soon as the post-tensioning of some tendons is completed.
Given below, the different phases of the post-tensioning operation :-
1) Casting of concrete.
2) Arrangement of the tendons.
3) Arrangement of the anchorage block and jack.
4) Use tension to the tendons.
5) Seating of the wedges.
6) Cutting of the tendons.
The stages are demonstrates schematically in the following figures. Once a tendon is anchored at one end, the tension is employed at the other end with a jack.
The tensioning of tendons and pre-compression of concrete happen concurrently. A system of self-equilibrating forces forms once the tendons are expanded.