A description of Post Tensioning

Post-tensioned concrete, a form of pre-stressed concrete, is the descriptive term for a method of inducing pre-compression into a concrete section after the concrete member has been cast and before the concrete member is loaded with dead and live loads.

High tensile strands are reamed into hollow ducts which are profiled to reflect the bending moment of the concrete member. The concrete is then cast around the ducts and once the concrete achieves sufficient compressive strength the strands are tensioned using a hydraulic jack. The strands are then locked off at the jack and the load from the jack is then transferred into the concrete member, creating a level of pre-compression in the concrete member.

Once the stressing operation is completed for all strands the ducts are then grout filled under pressure to provide a load path for the strand and to provide corrosion protection to all the strands.

Materials used in post-tensioning

Post-tensioning strands are made from 7 wire high strength low relaxation super strand. Where ordinary deformed bars have a tensile strength of 460N/mm2, high tensile strands used for pre-stressing have a tensile strength of 1840N/mm2.

The strands are housed inside either flat galvanised metal ducts, or circular galvanised metal ducts. For special applications such as bridges the ducts are made from HDP(high density polyethylene).

The stressing anchors which provide the mechanism to transfer the load from the stressing jack into the concrete member are made from cast iron to strict design specifications.