Principle of Prestressing
This internal stress is induced into the member by either of the following prestressing methods:
Pre-Tensioning : Apply prestress to steel strands before casting concrete;
Post-Tensioning : Apply prestress to steel tendons after casting concrete
The function of prestressing is to place the concrete structure under compression in those regions where load causes tensile stress. Tension caused by the load will first have to cancel the compression induced by the prestressing before it can crack the concrete. Figure (a) shows a plainly reinforced concrete simple-span beam and fixed cantilever beam cracked under applied load. Figure (b) shows the same unloaded beams with prestressing forces applied by stressing high strength tendons. By placing the prestressing low in the simple-span beam and high in the cantilever beam, compression is induced in the tension zones; creating upward camber. Figure (c) shows the two prestressed beams after loads have been applied. The loads cause both the simple-span beam and cantilever beam to deflect down, creating tensile stresses in the bottom of the simple-span beam and top of the cantilever beam. The Bridge Designer balances the effects of load and prestressing in such a way that tension from the loading is compensated by compression induced by the prestressing. Tension is eliminated under the combination of the two and tension cracks are prevented. Also, construction materials (concrete and steel) are used more efficiently ; optimizing materials, construction effort and cost.
Prestressing can be applied to concrete members in two ways, by pretensioning or posttensioning. In pretensioned members the prestressing strands are tensioned against restraining bulkheads before the concrete is cast. After the concrete has been placed, allowed to harden and attain sufficient strength, the strands are released and their force is transferred to the concrete member. Prestressing by post-tensioning involves installing and stressing prestressing strand or bar tendons only after the concrete has been placed, hardened and attained a minimum compressive strength for that transfer.