Prestressed concrete is basically a block of concrete in which internal stress of suitable magnitude and distribution are introduced so that the stresses resulting from external load are counteracted to a desired degree.
Prestressed concrete is different from a conventional RCC structure due to the application of an initial load on the structure prior to its use.
For Concrete internal stress are induced (usually by mean of tensioned steel) due to the following reasons
Tensile strength of concrete is less and hence cracks may develop at the early stages of loading in flexural members such as beams and slabs. Hence if the members are compressed prior to application of flexural load, the cracking will not occur.
Prestressing enhances the bending, shear, and torsional capabilities of the flexural members.
In pipes and liquid storage tanks subjected to internal fluid pressure, the hoop tensile stresses can be effectively counteracted by circular prestressing.
How do we apply to prestress by means of tensioned wire?
There are two ways (1) Pretensioning (2) Post-tensioning
(1)Pre-tensioning
In pre-tensioning system, the high-strength steel tendons are pulled between two end abutments (also called bulkheads) prior to the casting of concrete.
The abutments are fixed at the ends of a prestressing bed.
Once the concrete attains the desired strength for prestressing, the tendons are cut loose from the abutments.
The prestress is transferred to the concrete from the tendons, due to the bond between them.
The stages of pre-tensioning are shown schematically in the following figures.
(2)Post-tensioning
In post-tensioning systems, the ducts for the tendons (or strands) are placed along with the reinforcement before the casting of concrete.
The tendons are placed in the ducts after the casting of concrete.
The duct prevents contact between concrete and the tendons during the tensioning operation.
After anchoring a tendon at one end, the tension is applied at the other end by a jack. The tensioning of tendons and pre-compression of concrete occur simultaneously. Finally, the jacking end is anchored.
