Stages of Post Tensioning

In post-tensioning systems, the ducts for the tendons (or strands) are placed along with the reinforcement before the casting of concrete.The tendons are placed in the ducts after the casting of concrete.The duct prevents contact between concrete and the tendons during the tensioning operation.

Unlike pre-tensioning, the tendons are pulled with the reaction acting against the hardened concrete.If the ducts are filled with grout, then it is known as bonded post-tensioning.The grout is a neat cement paste or a sand-cement mortar containing suitable admixture.The grouting operation is discussed later in the section.

In unbonded, as the name suggests, the ducts are never grouted and the tendon is held in tension solely by the end anchorages.The following sketch shows a schematic representation of a grouted post-tensioned member.The profile of the duct depends on the support conditions. For a simply supported member, the duct has a sagging profile between the ends. For a continuous member, the duct sags in the span and hogs over the support.

Post-tensioning

Among the following figures, the first photograph shows the placement of ducts in a box girder of a simply supported bridge.The second photograph shows the end of the box girder after the post-tensioning of some tendons.

Post-tensioning of a box girder

The various stages of the operation are summarised as follows.

1) Casting of concrete.

2) Placement of the tendons.

3) Placement of the anchorage block and jack.

4) Applying tension to the tendons.

5) Seating of the wedges.

6) Cutting of the tendons.

The stages are shown schematically in the following figures.After anchoring a tendon at one end, the tension is applied at the other end by a jack.The tensioning of tendons and pre-compression of concrete occur simultaneously.A system of self-equilibrating forces develops after the stretching of the tendons.

Stages of post-tensioning