Types of prestressing
Pre-tensioning: Pretensioned: in this type, the cables tensioned before the casting of concrete then load transferred after casting of concrete
Stages of Pre-tensioning
In the pre-tensioning system, the high-strength steel tendons are pulled between two end abutments (also called bulkheads) prior to the casting of concrete. The abutments are fixed at the ends of a prestressing bed.
Once the concrete attains the desired strength for prestressing, the tendons are cut loose from the abutments
The prestress is transferred to the concrete from the tendons, due to the bond between them. During the transfer of prestressing force, the member undergoes elastic shortening. If the tendons are located eccentrically, the member is likely to bend and deflect (camber).
Post-tensioning concrete: in this type concrete casted first and after it gains the required strength cables tensioned.
Stages of Post-tensioning
In post-tensioning systems, the ducts for the tendons 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 grouting operation is discussed later
In unbonded post-tensioning, the ducts are never grouted and the tendon is held in tension solely by the end anchorages. The following sketch shows a representation of a grouted post-tensioned member.