PRESTRESSED CONCRETE

PRESTRESSED CONCRETE

                       In this case of ordinary reinforced concrete which consists of concrete which consists of concrete and mild steel, compressive stresses are borne by concrete while tensile stresses are borne by steel. The concrete surrounding steel reinforcement does not take tension as concrete is weak in tension. Thus only that portion of concrete, which lies above the neutral axis is considered to be useful in resisting compressive stress. This results in heavy section.

                       Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the pre-compressive stress in concrete will balance the tensile stress produced in concrete (surrounded steel) due to an external force. due to this, the whole of the concrete can participate in consisting of the external forces. In reinforced concrete members, the prestress is commonly introduced by tensioning the steel reinforcement.

 Prestressed Material

    Principal Materials used in PSC

•  High Tensile Steel
•  High Strength Concrete

                         Ordinary mild steel wires are not useful because no prestress will be left in wires after losses. the initial tensile stain in steel is reduced after tensioning by as much as 15 to 20% due to various losses.

                    concrete used in PSC of good quality and high strength. A well compacted dense concrete has a less elastic strain and has less shrinkage plastic flow thus reducing the loss of prestressing considerably.

 Classification of prestressed concrete 

1.   External and Internal prestressing system
2.  Linear or Circular prestressing 
3.  Predestining and Post Tensioning System
4.  Full Prestressing or Partial prestressing

 External and Internal Prestressing system

  External prestressing is achieved by external reaction introduced through different support conditions.

Prestensioning and post-tensioning system

In Prestensioning system, the tendons are pre-stressed before the concrete is placed. In the post-tensioning system, the member is concreted and a duct is formed in member either with a tube or with a metal sheet. When the concrete is sufficiently hard, the tendons are tensioned and anchored to the concrete member of its each.

Linear or Circular Prestressing

 Linear prestressing is applied in straight members such as beams, slabs, piles etc. Circular prestressing is applied to the circular structure such as tanks, pipes, silos etc. where prestressing tendons are wound around the circumference.

Full Prestressing or Partials Prestressing 

If there are no tensile stresses in concrete under working load, concrete is said to be prestressed. If some tensile stresses are produced I concrete under working load conditions. It is said to be partially prestressed.