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Pre-tensioning and Post-tensioning in Prestressed Concrete

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Pre-tensioning

In pre-tensioning, the prestressing reinforcement (strands or bars) are tensioned before the casting of concrete. Once the concrete gains sufficient strength, the prestressing force is transferred from the tendons to the concrete.

Stages of Pre-tensioning:
  1. Tendon Placement & Tensioning
    • High-strength steel strands are stretched between two end abutments (bulkheads) within a prestressing bed.
  2. Concrete Casting
    • Concrete is cast around the tensioned tendons and left to cure.
  3. Transfer of Prestress
    • After the concrete attains the required strength, the tendons are cut loose from the abutments.
    • The prestress is transferred to the concrete due to the bond between the tendons and the hardened concrete.
  4. Effects of Prestressing
    • The concrete undergoes elastic shortening due to the prestressing force.
    • If tendons are placed eccentrically, the member develops a camber (upward deflection).

Post-tensioning

In post-tensioning, concrete is cast first, and after it gains the required strength, the prestressing steel are tensioned.

Stages of Post-tensioning:
  1. Duct Placement
    • Before casting, ducts for tendons are positioned along with conventional reinforcement.
  2. Concrete Casting & Curing
    • Concrete is poured and allowed to gain adequate strength before tensioning begins.
  3. Tendon Placement & Tensioning
    • Steel tendons are inserted into the ducts and then tensioned, with the reaction force acting against the hardened concrete.
  4. Bonded vs. Unbonded Post-tensioning
    • Bonded Post-tensioning (common): The ducts are later filled with grout to establish a bond between the tendons and concrete.
    • Unbonded Post-tensioning: The tendons remain unbonded, relying only on end anchorages to maintain tension.






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