Pre-stressed Concrete

The main objective of pre-stressed concrete is that to provide enough pre-compression (Compression that is generated by it’s own self weight, not due to any external/working load) in the concrete by tensioned steel reinforcements or cables / wires so that under working condition the concrete experiences no tensile stresses or low tensile strength so that no deformation / cracks is observed on the concrete surface.

Pre stressed Bridge Construction

Generally, in concrete members the top flange experiences compression & the bottom flange experiences tension, but in pre-stressed concrete the steel wire is subjected to tensile stress & then concrete is poured in the form work. After setting of concrete it is observed that due to the pre-stressing the bottom flange also experiences compression force, which counteracts with the top flange’s compression force. For this advantage the layout of the tendon (Steel wire / cables) is done by Structural Designer in such a way that the predetermined stresses can easily be dealt with the placement of the tendon.

Prestressed Beam

Individual steel wire / cable is stacked & twisted mechanically by machines to form a larger diameter steel wire. It has to be done mechanically as any loose steel wire can have a devastating effect on the structure. The amount of tensile stresses that it can withstand depends upon the external diameter of all the individual steel wire which as a single unit. Therefore, which the design limitation of prestress concrete is less comparatively to normal RCC. However high grade of concrete (greater than M50) shall be used during any pre-stressed concrete as if the concrete is weak then during or after tensioning the concrete might severely crack or deform.

Post tensioned Concrete work at project execution

There are two types of pre-stressing in a concrete member:

·         Pre-tensioned Concrete

·         Post-tensioned Concrete

Pre-tensioned Concrete-

In this process of casting the steel wire / cable is put under tensile force & locked before the concrete is poured in the formwork. Only after the final setting time of concrete the tendons are released, by which the prestress force of the steel is transferred to the concrete. In pre-tensioned concrete the bonding between the concrete & steel is much greater than post tensioned concrete as the steel is in direct contact with the concrete throughout.

Post-tensioned Concrete-

In this process, the steel wire is placed inside a conduit or tubes without any tensile force & concrete is poured in the formwork. After final setting of the concrete the steel wire /cable is stretched to give the required tensile forces. In post-tensioned concrete the bonding is not as good as pre tensioned concrete as the steel & concrete is separated by the conduits.

Some advantages of prestressed concrete:

•          Number of design limitations can be resolved in terms of span and loading which is not possible in case of normal RCC. Because of which longer unsupported span can be achieved, with high load bearing capacity.
•           Few columns are required for the structure as longer beam span can be obtained, this concept is used for auditoriums / roofs where there is a column restrictions and more open space is required & by normal RCC it is quite impossible. Bridges are also constructed by the use of pre stressed system.