How Prestressed Concrete Works
There are three major implementations of prestressed concrete:
Pre-Tensioned Concrete: In this form, concrete is cast around steel bars or cables under tension. The concrete naturally bonds to these "tendons" while it cures. Compression by static friction transfers the tension to the concrete once it is released. Subsequently, any tension on the concrete transfers readily to the tendons. Pre-tensioned concrete elements are common in beams, lintels, and floor slabs.
- Bonded Post-Tensioned Concrete: In this form, compression is applied in situ during curing. A duct of aluminum, plastic, or steel is used in casting and follows the area where tension would occur in the concrete. Tendons are pushed through the duct, then tensioned via hydraulic jack after hardening. Once tendons' stretching meets design specifications, they are wedged in place and the duct is grouted.
- Unbonded Post-Tensioned Concrete: Here, the individual tendons retain freedom of movement relative to the concrete. Tendons are prepared with a coating of lithium-based grease, then given a plastic-based "shell" formed through extrusion. The steel cables are tensioned against anchors placed in the slab's perimeter. This design provides the ability to de-stress the embedded tendons prior to repair.