prestress

Prestressing is the practice of deliberately introducing internal stress into a structural member, usually tension, before it bears its service load. The goal is straightforward: create an internal force that opposes and reduces the stresses that will occur once the structure is in use. In concrete work, this is the dominant application. A prestressed concrete beam may be designed to experience significant bending stress under its own weight and imposed loads; prestressing introduces a compressive force that partially or wholly neutralizes that bending stress, allowing thinner or longer spans than conventional reinforced concrete permits.

Two main methods exist. In pretensioning, steel tendons or cables are tensioned against external anchorages before concrete is cast around them. When the concrete hardens and bonds to the steel, the concrete is released from the external tension, pulling inward on the tendons. The tendons pull back, placing the concrete in compression. This method is most common in precast plants where controlled conditions and repetitive production justify the fixed infrastructure. Post-tensioning applies tension after the concrete has cured. Ducts are cast into the concrete at predetermined locations; after strength develops, high-strength steel cables are threaded through the ducts, tensioned with hydraulic jacks anchored to the concrete, and then grouted in place. This method suits on-site casting and complex geometry because the tensioning apparatus can be positioned and operated at the job.

The materials are critical. Prestressing steel, whether in wire, strand, or bar form, must have far higher tensile strength than ordinary reinforcement, typically 1,500 to 2,100 megapascals depending on the grade. Ordinary reinforcing steel would creep or yield unacceptably under the sustained high tension required. The steel is also highly susceptible to stress corrosion cracking if exposed to moisture and certain contaminants, which is why grouting in post-tensioned ducts, or protective coatings on pretensioned members, is essential.

Losses and the practical calculation

Initial tension is always higher than final tension because prestress is lost over time through several mechanisms. Elastic shortening of the concrete immediately upon tensioning reduces tension in the steel. Creep and shrinkage of the concrete continue the loss over weeks and months. Relaxation of the steel itself, particularly at high stress levels, causes a gradual drop in tension. Depending on conditions, total losses can range from 10 to 25 percent of the initial tension. The designer must apply these loss factors to ensure adequate final prestress remains after all effects have acted.

Prestressing enables longer spans, shallower beams, reduced cracking, and sometimes elimination of internal reinforcement in certain members. It shifts the economy of concrete construction when large clear spans or heavy sustained loads demand it. Bridges, parking structures, long-span floor systems, and precast concrete products from utility poles to railway sleepers all routinely use prestressing. However, the process demands precision: errors in tendon placement, inadequate grouting, under-tensioning, or over-tensioning can compromise the structure. The high forces involved also create significant safety hazards during tensioning operations.