Prestress Loss due to Friction in Post-Tensioned and Pretensioned Beams

Introduction:
Prestressing is a technique used to strengthen concrete structures by applying a pre-determined amount of compressive stress to counteract the anticipated tensile stresses. This technique involves the use of high-strength steel tendons that are tensioned before or after the concrete is cast. However, during the prestressing process, there can be losses in the initial prestress due to various factors, one of which is friction.

Friction in Prestressed Beams:
Friction occurs between the prestressing steel tendons and the surrounding concrete during the stressing operation. This frictional force can result in a loss of prestress due to the resistance encountered while applying or transferring the tension from the steel to the concrete. The amount of prestress loss due to friction depends on several factors, including the surface condition of the steel, the type of duct used to encase the tendons, the tendon profile, and the force applied during stressing.

Prestress Loss in Post-Tensioned Beams:
- Post-tensioned beams are those in which the tendons are tensioned after the concrete has hardened.
- In post-tensioned beams, prestress loss due to friction occurs primarily during the tensioning process, when the tendons are being pulled through the ducts.
- The frictional losses in post-tensioned beams are generally higher compared to pretensioned beams due to the additional frictional forces encountered while threading the tendons through the ducts.

Prestress Loss in Pretensioned Beams:
- Pretensioned beams are those in which the tendons are tensioned before the concrete is cast.
- In pretensioned beams, prestress loss due to friction occurs during both the tensioning process and the transfer of the prestress to the concrete.
- During tensioning, frictional losses occur as the tendons are being stretched and anchored.
- During transfer, frictional losses occur as the prestress is being transferred from the tendons to the concrete through bond and friction at the tendon-concrete interface.

Conclusion:
In summary, prestress loss due to friction occurs in both post-tensioned and pretensioned beams. However, the magnitude and distribution of these frictional losses may vary depending on the specific construction method and design details. It is important for engineers and designers to consider this frictional loss while determining the amount of prestress required and to account for it in the design calculations to ensure the desired level of prestress is achieved in the final structure.