Prestressed Concrete

What It Means

Prestressed concrete is a structural concrete technology in which high-strength steel tendons (strands or bars) are stressed before the concrete carries external load, placing the concrete section in a state of pre-compression. Because concrete is strong in compression but weak in tension, pre-compressing the concrete counteracts the tensile stresses that live loads would otherwise create, allowing for longer spans, thinner members, less reinforcement, and reduced dead load compared to conventional reinforced concrete. Two prestressing methods are used: pretensioning (tendons are tensioned between abutments, concrete is cast around them, and the tension is released after curing to transfer force to the concrete, typical for plant-cast girders) and post-tensioning (concrete is cast with ducts, tendons are threaded through after curing, then stressed against the hardened concrete, typical for cast-in-place segmental bridges). Prestressed concrete came into widespread U.S. bridge use in the 1950s and now accounts for roughly 40% of the NBI inventory, particularly for standard AASHTO I-girders, bulb-tees, box beams, and adjacent box beams on short-to-medium spans (50-180 feet). Modern prestressed concrete can span up to 300+ feet with post-tensioned segmental construction. The technology has excellent durability when tendons remain protected, but strand corrosion can be catastrophic: if chlorides penetrate to the prestressing steel, localized corrosion can cause strand fracture with disproportionate capacity loss because there is no yielding reserve. The Lake View Drive Bridge collapse in Pennsylvania in 2005 was traced to prestressed concrete beam failure from chloride-induced strand corrosion, leading to enhanced inspection protocols for adjacent box beam bridges nationwide.

Frequently Asked Questions