How does pretensioning differ from posttensioning in prestressed concrete?

In prestressed concrete, both pretensioning and posttensioning are techniques used to place concrete into compression, which helps counteract tensile bending stresses and allows for lighter, shallower, or longer-spanning structures. The primary difference lies in the timing of the steel reinforcement tensioning relative to the concrete curing process.12

Pretensioning

In this method, steel wires or cables are placed in a mold and stretched to a desired tension before the concrete is poured. Once the concrete has set and gained sufficient strength, the anchors holding the steel are released. As the steel attempts to shrink back to its original length, it transfers a compressive force to the concrete. This process is commonly used for precast concrete members fabricated in a factory setting.123

Posttensioning

In this process, the concrete is poured with ducts or tubes already embedded within it. The steel reinforcement is placed through these ducts, and the concrete is allowed to harden. Only after the concrete has cured are the steel wires stretched using hydraulic jacks and anchored to the exterior of the member. This allows for precise regulation of the compressive force applied to the concrete.321