Pretensionering

Pretensionering, or pre-tensioning, is a method of prestressing concrete where steel tendons are stretched before the concrete is cast. In this method, tendons are anchored to fixed ends on a pretensioning bed, and are tensioned by hydraulic jacks to the desired force. The tendons are held in that tension while the concrete is poured and gains strength around them. Once the concrete has achieved sufficient strength, the anchors are released or the tendons are cut, and the bond between steel and concrete transfers the prestressing force to the member, inducing compressive stresses that improve crack resistance and stiffness.

Pretensioning is primarily used in precast concrete elements, such as beams, slabs, and columns, where high early strength and dimensional precision are important. The process typically uses high-strength steel strands or bars and uses dedicated pretensioning beds, anchors, and jacks. Because the tendons lie within the hardened concrete, no post-installation ducts or grout is required, distinguishing pretensioning from post-tensioning.

Advantages include high early strength, good control of prestress, and efficient production for factory-made components. Limitations include restriction to precast fabrication, sensitivity to concrete quality and proper curing, and potential losses from elastic shortening and creep that must be accounted for in design. Compared with post-tensioning, pretensioning generally offers simpler fabrication but less flexibility for field adjustments after casting.