Pre-stressing concrete where high-strength steel tendons (cables or bars) are tensioned after the concrete has been poured and has achieved sufficient strength.
This process introduces an internal compressive force into the concrete element, which actively counteracts the tensile forces that will be created by the structure’s own weight (dead load) and traffic/use (live load).
The Process of Pre-Stressing:
The procedure occurs in three main stages, differentiating it from pre-tensioning:
- – Casting with Ducts: Flexible plastic or steel ducts (sheathing) are accurately positioned and secured within the formwork before the concrete is poured. These ducts define the path of the future tendons and are often curved or “draped” to follow the line of tension created by the load. The concrete is then poured and allowed to harden.
- – Tensioning: Once the concrete has reached the required strength (typically 3 to 7 days after pouring), high-strength steel tendons (strands or bars) are threaded through the ducts. Specialized hydraulic jacks are then used to pull or stress the tendons from the ends of the concrete member.
- – Anchoring and Grouting:
- – The stressed tendons are permanently secured in their extended (tensioned) state using anchorage devices (like wedges and bearing plates) embedded at the ends of the concrete.
- – The hydraulic jacks are removed, and the large compressive force is transferred to the concrete through these anchorages.
- – For bonded post-tensioning (most common in bridges), a cementitious grout is then injected into the duct to fill the void, bond the tendon to the concrete, and protect the steel from corrosion.