Steel and concrete are the basic materials of construction of prestressed concrete.
Ordinary mild steel and deformed bars used in R.C.C. are not used in PSC (Prestressed concrete) because their yield strength is not very high. In the PSC, loss of prestress (about 20%) occurs due to many factors. If mild steel or HYSD bars are used then very little prestress will be left after the losses and will be of no use for us. Therefore, high tensile strength steel is used for prestressing. In addition to the high strength, the steel used in prestresing must have a higher ultimate elongation. Various forms of steel used for prestressing are as follows:
Tendons are high strength tensile wires available in various diameter from 1.5mm to 8mm. Table gives the ultimate tensile strength of steel wires used for prestressing.
|Diameter of wire (mm)
|Ultimate tensile strength (N/mm2)
(b) Wire Strands or Cables
A strand or cable is made of a bundle of wires spun together. The overall diameter of a cable or stand is from 7 to 17mm. They are used for post-tensioning systems.
High tensile steel bars of diameter 10mm or more are also used in prestressed concrete.
Since high tensile steel is used in PSC, the concrete used should also be of good quality and high strength. Therefore, IS code recommends a minimum mix of M40 for pretensioned system and M30 for post tensioned system. These mixes have high strength and a high value of modulus of elasticity of concrete which results in less deflection.
The concrete used in PSC should be well compacted. High strength concrete is used in PSC for following reasons:
- Use of high strength concrete results in smaller sections.
- High strength concrete offers high resistance in tension, shear, bond and bearing.
- Less loss of prestress occurs with high strength concrete.