Short Answer:
In a post-tensioned member, the prestressing force is applied after the concrete has hardened. Ducts are placed inside the formwork before pouring concrete. Once the concrete gains strength, high-strength steel tendons are inserted into the ducts, stretched using hydraulic jacks, and anchored at both ends to apply the prestressing force.
This process compresses the concrete, allowing it to resist tensile stresses during service. The ducts are then filled with grout to bond the tendons and protect them from corrosion, ensuring the structure remains strong and durable over time.
Detailed Explanation:
Application of prestressing force in a post-tensioned member
Post-tensioning is a popular method of prestressing concrete used in civil engineering structures such as bridges, slabs, and beams. The goal is to introduce compressive stress into the concrete to counteract tensile stresses caused by external loads. In a post-tensioned system, this compressive force is applied after the concrete has hardened, which is different from pre-tensioning where it is applied before concreting.
The prestressing process in post-tensioned members involves several carefully planned steps:
Step-by-step application of prestressing force
- Duct placement:
Before casting the concrete, ducts or sheaths (usually made of plastic or steel) are placed in the formwork where the tendons will pass. These ducts define the path and profile of the tendons. - Concrete casting:
Concrete is then poured and allowed to cure around the ducts. It must reach a sufficient compressive strength, usually around 75% of the design strength, before post-tensioning can begin. - Insertion of tendons:
Once the concrete is hardened, high-tensile steel tendons or strands are inserted into the ducts. These tendons are specially made to withstand high stress without permanent deformation. - Stressing of tendons:
Hydraulic jacks are used to pull or stretch the tendons from one or both ends. This creates tensile stress in the steel and simultaneously develops compressive stress in the concrete as the tendons try to contract but are held in place. - Anchorage:
After reaching the desired tension, the ends of the tendons are anchored using steel grips or anchor plates. These hold the tendons tightly, maintaining the prestress in the concrete. - Grouting (for bonded post-tensioning):
To protect the steel from corrosion and to bond it to the concrete, the ducts are filled with cement grout under pressure. This completes the post-tensioning process.
Purpose and Benefits
- Improves crack resistance by keeping concrete in compression.
- Allows longer spans with thinner structural elements.
- Reduces deflection and material usage.
- Provides flexibility for in-situ applications and complex geometries.
Post-tensioning is widely used in multi-span bridges, parking garages, buildings with large column-free spaces, and elevated slabs due to its efficiency and strength benefits.
Conclusion:
In post-tensioned members, the prestressing force is applied after the concrete sets. It involves inserting tendons into preformed ducts, tensioning them with jacks, anchoring, and finally grouting. This method enhances the load-carrying capacity, reduces cracks, and provides durable and cost-effective structural solutions for modern construction.