In which method is prestressing steel placed under tension after the concrete member has been erected?

The method where prestressing steel is placed under tension after the concrete member has been erected is known as posttensioning. In this process, ducts are typically embedded within the concrete before it sets, and once the concrete has achieved sufficient strength, the prestressing steel cables or strands are threaded through these ducts. They are then tensioned using hydraulic jacks and anchored at the ends to create compressive forces in the concrete.

This technique is particularly advantageous because it allows for more flexibility in the design and can lead to longer spans and reduced structural depth. It also helps to mitigate the effects of tensile stresses that may develop in the concrete, thereby enhancing durability and performance.

In contrast, pretensioning involves tensioning the steel before the concrete is poured, which is quite different from posttensioning. Vacuum dewatering and consolidation refer to alternative processes related to concrete handling and finishing, but do not involve prestressing techniques.