Prestressed Concrete

1. What is prestressed concrete

Simply put, it is concrete formed under stress. Reinforcement bars are placed in a form and stressed by the stretching of the bars at each end, inducing tension in the bar. Concrete is poured into the form and all around the bars while they are still being stretched. When they are released, the steel tries to resume its original, shorter, length, and adds a compressive force to the concrete laterally, giving it the strength to span longer distances than normal reinforced concrete.

2. The uses of prestressed concrete

Prestressing is used to make composite beams and piers in large-scale construction such as highway overpasses and commercial buildings. It enables a concrete beam to support weight between piers on either side; without such reinforcing, concrete’s lack of tensile strength would cause it to collapse without support in the middle.

Here are three major implementations of prestressed concrete:

• Pre-Tensioned Concrete: 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 floor slabs, beams, and lintels.
• Bonded Post-Tensioned Concrete: 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 the 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 an 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.