7.16 : プレストレストコンクリート

Under a load, a beam experiences compression on one side and tension on the other. Steel reinforcement resists the tension forces in a reinforced concrete beam.

The concept of prestressed concrete is applying an initial high tension to the high-strength steel strands used as reinforcements within a concrete beam or slab and then releasing.

These high-strength steel strands, specifically engineered for prestressing, consist of cold-drawn steel wires assembled into slender cables.

The technique of prestressing is carried out either by pretensioning or posttensioning.

Pretensioning starts by stretching the steel strands across the casting area. Then, concrete is poured around these strands and allowed to cure, and lastly, the strands are cut to put the concrete under compression, causing the beam to arch upwards slightly.

In posttensioning, the strands are sheathed in casing to avoid bonding with the concrete.

These strands are tensioned only after the concrete gains sufficient strength, using a hydraulic jack at one end and anchoring at the opposite end to compress the concrete, inducing a camber in the beam.