For a simply supported beam portion below neutral axis is in tension and above NA is in compression, if we apply a compressive stress at tension zone before loading, when loading the tension made will be neutralized by earlier applied compression. This is the principle of prestressed beam.

The maximum concrete strength is 40 N/mm2 in compression, concrete will not take tensile stress, its tensile strength is 2.5 N/mm2 only. If we need high strength, we may give heavy sections. For large span we could design prestressed concrete beams. By tendons with high strength steel wires apply tensile stress. Earlier it is applied with MS wire and it failed.

The defects of reinforced concrete member, is overcome by prestressing. In order to get the maximum advantages of prestressed concrete member it is necessary to give high strength concrete. And high strength steel wires.

It requires high strength concrete because

  • Since large prestressing forces are applied to the member by tendons, high bearing stresses are developed at the ends.
  • This high strength will not take by low strength concrete.
  • High bond strength also needed
  • Shrinkage cracks is also little to high strength concrete
  • Due to high modulus of elasticity the elastic and creep strains are small
  • Loss of prestress is smaller
  • Light section can be used

High tensile steel wires are used and it is called tendons. It should be used because if mild steel is used loss of prestress is the result due to creep and shrinkage.

RCC and Prestressed Concrete Beam Comparison

In prestressed beam tension is taken by steel and compression is taken by concrete. In RCC beam two opposite forces form a couple and the moment of the couple is its moment of resistance. Lever arm of the couple is constant but tension and compression depends on external loading. In prestressed beam , lever arm changes with external loading.

In RCC beam concrete in the compression zone only is used, concrete in the tension side is ineffective, but in prestressed beam whole section is effective.

RCC beams are heavy where as prestressed beams are lighter.

For prestressed beam auxiliary units are necessary so it is economical when repeated use of these units

In the case of heavy load and large spans it is economical

Water cement ratio for PCC is 0.4, using admixtures it can be lowered high strength concrete shall be made.

Assumptions in the Design of PCC

A transverse plain section before bending will be plain after bending also

Hooks law is applicable

The stress in the rebars does not change with length of rebars

Stress changes take place for the concrete component only

Variation of stress in the rebars with external load is ignorable

Applying high tensile strength in tendons and concreting is done and releasing the force concrete will be compressed, when loading the tensile stress is neutralized with compressive stress already in the concrete