Pre-Tensioning Method
- Pre tensioning is one method of applying prestress.
- Tendons either pass through a single mould or a line of moulds for multiple members arranged end to end and can be attached at one end to fixed anchorage.
- The tendons are then tensioned from the fixed anchorage between an external independent anchorage to give the required tensile force in the tendon.
- The tendons are then held in place while the concrete is poured.
- When the concrete has hardened sufficiently the ends of the tendons are slowly released from the anchorages. The tendons are restrained from gaining their original length by the development of bond stresses between the concrete and the tendon, and it is these bond stresses that transfer the compressive stress to the concrete.
- The tendons are then finally trimmed off.
Post-Tensioning Method
- Post-Tensioning is another method of applying prestress to a concrete member.
- The tendon is placed in the correct position in the formwork with the dead-end anchorage and live end anchorage, through which the tendon passes.
- The concrete is then poured and left to harden.
- When the concrete has gained sufficient strength a jack is attached to the live end anchorage and the tendon stressed to the required force. The operation is to be carefully done as any error could impair the structural integrity of the member.
- The tension force in the tendon is transferred to the concrete as a compressive force by the reactions at the anchorages. The jack is then removed.
Precautions to avoid aircracks in slab Concreting.