What is the Concept of Prestressed Concrete ?

In case of big concrete construction projects like bridges, steel reinforcement bars alone won’t be sufficient to provide the necessary tensile strength. Pre-stressed concrete is used in all kinds of structures from bridges to buildings, silos and tanks. It’s a great way to minimize cracking and take fuller advantage of the incredible strength of reinforced concrete.

In this case pre-stressed concrete is being used in pre-stressing the rods or tendons are stretched along the axis and cement is poured later.when the tendons are released the compression is generated at the bottom which tries to counterbalance the compression due to loading at the top part of the beam.

The basic principle behind pre stressed concrete is that compressive stresses induced by the high strength steel tendons in a concrete member before loads are applied.

This will balance the stresses imposed in the member during the service pre stressed concrete can be used for the construction of roofs, floors, walls and bridges with longer unsupported spans. Pre-stressing is generally done by two methods Pre-tensioned concrete and Post-tensioned concrete.

Methods of Prestressed Concrete.

1. Pre-tensioned concrete :

Pre-tensioned concrete is those in which the tendons are tensioned before pouring of concrete. The tendons are initially tensioned and anchored at both end and then concrete is poured over it and cured. When the concrete attains desired strength the tendons are released and projections are cut out. The Pre-stress imparts into beam through bond action.

Also Read : Different Types of Grades of Concrete

2. Post-Tensioned Concrete :

In Post-tensioned concrete the tendons are tensioned after the concrete is poured. Initially the concrete is cast around the sheathing. The ducts are formed in concrete in which tendons are placed after the concrete is hardened. The tendons are tensioned by hydraulic jacks and anchored, In case of bonded type tendons the duct is grouted with sufficient slurry. In the unbounded type of tendons grouting is not necessary.

Advantages of Prestressed Concrete :

• Prestressed concrete members have higher resistance capacity than RCC against impact, shock and stresses. Moreover they are free from cracks.
• Prestressed members are light in weight. They can be easily transported from one place to another.
• Shear resistance of the Prestressed concrete members can be increased by using curved tendons at the time of casting of concrete.
• Diagonal tension in concrete is also reduced by using Prestressed concrete.

Disadvantages of Prestressed concrete :

• In manufacturing of Prestressed concrete member different items or equipments like anchor, jacks etc are required, Which increases additional material cost.
• High tensile steel is required in Prestressed concrete, Which is quite difficult to obtain.
• Much more costlier than RCC Structures.
• Skilled workers and expert supervision is required.

Also Read : Cement Concrete Specification for Design Mix 1:2:4