Pretensioned

Pretensioned describes a method of prestressing concrete in which reinforcing tendons are tensioned before the concrete is cast or gains sufficient strength, and the resulting prestress is transferred to the concrete when the tendons are anchored and released. In typical pretensioning, high-strength steel strands or bars are stretched between fixed end blocks, with the concrete poured around them. After the concrete hardens, the tendons are released, producing a compressive force within the member that improves its serviceability and strength.

Pretensioned components are usually precast in controlled plant environments, enabling precise fabrication and rapid production. The induced compressive stress counteracts service loads, reduces crack widths, enhances stiffness, and can allow longer spans or thinner sections. Common applications include prestressed concrete beams, slabs, hollow-core planks, and bridge girders. The practice relies on durable bond between the tendons and concrete, along with properly designed end anchorages to transfer forces.

Materials typically involve high-strength steel strands or wires, sometimes complemented by reinforcement to handle shear and other demands. The technique is contrasted with post-tensioning, where tendons are placed in ducts within hardened concrete and tensioned after curing, allowing greater flexibility for field adjustments.

History and context: pretensioning emerged in the early 20th century and was advanced by pioneers such as Eugène Freyssinet as a foundational method of prestressed concrete. Today, pretensioned elements remain a major part of precast construction, though post-tensioning is often preferred for on-site or more complex geometries.