Pre-stressed concrete elements are finished parts, in which tensioned wires have been inserted. The stressing of the wires compresses the concrete. The tensioned bead wires allow compensation for subsequent loads, effects of temperature or traction forces, which may arise.
Pre-stressed concrete elements can be manufactured using a variety of tensioning techniques.
In pre-tensioning technology the steel bead wires are tensioned and anchored on a production bed. A mould, which gives the finished part its prospective shape, is located on the tensioning bed. The concrete is introduced into the mould with the tensioned wires by using either slipformers or extruders. After curing, the wires are slackened. They transfer their pre-tension to the prefabricated unit by the bond with the concrete and thus compress the concrete. They thus remain stretched and tensioned.
In post-tensioning the pre-stressing steel is tensioned after the concrete has cured. For this purpose tubes are inserted into the loosely reinforced structural component. After the concrete has cured, steel stranded wires are fed though these tubes as tendons and then tensioned and anchored against the structural component. Afterwards cement mortar can be injected into the tubes to produce a bond with the structural component. This technology is used principally in bridge construction.