In pre-tensioning the prestressing tendons are tensioned and anchored between fixed supports before the concrete is cast. The concrete is either cast in moulds or formed by an extrusion or slip-form process to form the required cross-section. When the concrete has gained sufficient strength, the tendons are released from the supports. The prestressing force is transferred from the tendons to the concrete by the bond between the tendon and concrete. The transfer occurs over a short transmission length at each end of the concrete member. Elastic shortening of the member will lead to some loss of the prestressing force.
Pre-tensioning is usually carried out in a factory. Large single units (such as bridge beams) can be cast in steel moulds against which the tendons are stressed and anchored. Alternatively, a number of units can be fabricated at the same time using long-line prestressing beds.