Prestressed Concrete (PSC)
Since the strength of concrete in tension is very low, it is ignored in R.C.C. designs. Concrete in the tension zone only acts as cover to reinforcements and helps in keeping the steel at desired distance. In R.C.C. construction therefore, concrete is not fully utilized. Prestressing the concrete, is one of the ways to make sure that concrete used in construction is fully utilized. The priniciple behind the prestressing of concrete entails the introduction of calculated compressive stresses into the structural member. These stresses are introduced into the zones where tensile stresses are expected in the member. Hence, in such members, when loading is applied, the stresses developed by such loads, will have to nullify or counteract the compressive stresses before their tensile effects come into play.
This ensures that the entire concrete is utilized or used to resist the applied load in prestressed concrete. Similarly, hair cracks which reduce durability of structural members is avoided in prestressed concrete. Another importance of prestressed concrete is that deflections are low in this type of construction. This makes it suitable for longer span constructions than reinforced concrete.
Prestressed concrete is commonly used in the construction of bridges, free slabs, large columns and roofs. PSC sleepers and electric piles are also common structural members made from this method.
High tensile steel and high strength steel are materials used in prestressed concrete. Tensioning of wires may be by pre-tensioning or post-tensioning. Pretensioning involves stretching the wires before concreting, thereafter, releasing them. In posttensioning, ducts are made in concrete elements through which prestressing wires are passed after the hardening of concrete. The wires are then stretched and anchored to the structural members by special anchors.