Prestressed Concrete - Basic Theory and Concept

What is Prestressing?

Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses that are arising from subsequent loads during its service period. This means that before the structure is subjected to any external loads, they will undergo some known stress.

The concept of prestressing existed before it has been applied in the concrete applications. The two such examples of methods are:

  • The force-fitting of the metal bands on the wooden barrels: Now the surrounding metal bands will induce initial hoop compression (C). The metal band is the element that promotes prestressing. The liquid that is filled inside the barrel will exert pressure which acts on the wall of the barrels. This is the tensional force (T). The force ' T ' will counteract the force ' C '.
Fig.1: Prestressing Action in Wooden Barrels

  • Pre-tensioning the spokes in a bicycle wheel: The spokes in the bicycle wheel are in tension. Because the spokes are tensioned for their arrangement. When the rider rides on the cycle, there is compression exerted on the spokes. As this compression is lesser than the tension on the spoke, easy resistance is carried out with spokes gaining a residual tension.

This principle of prestressing is applied in the prestressing of concrete. Hence the prestressed concrete can be defined as, concrete in which the effective internal stresses are induced ( usually by means of the tensioned steel) before the structure is loaded, in order to counteract the stresses resulting from the applied service loads.

Why Prestressing for concrete?

The tensile strength of concrete is only 8 to 14% of its compressive strength. So cracks are developed in the early stages of loading in the flexural members like beams and slabs. In order to prevent such cracks, compressive forces can be suitably applied in the longitudinal direction, either concentrically or eccentrically. This is called as Linear Prestressing.

This prestressing will enhance the bending, shear and torsional capacities of the flexural member. Other examples of prestressing are circular prestressing. Here the hoop tensile stresses are effectively counteracted by circular prestressing.