Prestress

Prestress is a technique in structural engineering where permanent compressive stresses are introduced into a material, typically concrete, to counteract tensile stresses from external loads. By keeping much of the section in compression, prestressed members exhibit higher strength, stiffness, and serviceability, with reduced cracking and deflection.

There are two main methods. In pre-tensioning, high-strength tendons are tensioned between fixed supports before pouring the concrete; once the concrete hardens and bonds to the tendons, the tendons are released, transferring the compressive force to the member. In post-tensioning, ducts or sleeves are cast into the concrete; after curing, tendons are threaded through the ducts, tensioned, and anchored at the ends, with grout often injected to protect and bond the system. Post-tensioning is common in cast-in-place elements, precast members, and long-span applications, while pre-tensioning is widely used in precast elements.

Materials used for tendons include high-strength steel strands or bars, modern non-metallic or FRP tendons, and protective grout. Anchors, ducts, and corrosion protection are essential components of the system. Design must account for losses due to creep, shrinkage, relaxation, and friction, as well as anchorage and transfer lengths.

Applications span bridges, floors, parking structures, and other elements requiring long spans, large load capacities, or reduced thickness. Prestressing enables slimmer sections, longer spans, and better crack control, contributing to durability and potential life-cycle savings, though it can involve higher initial costs and specialized construction practices.