PRESTRESSED CONCRETE- MERITS AND DEMERITS
Prestressed concrete has the following merits:
- Since the technique of prestressing eliminates cracking of concrete under all stage of loading, the entire section of the structures takes part in resisting the external load. In contrast to this, in the reinforced concrete, only portion of the concrete above neutral axis is effective.
- Since concrete does not crack, the possibility of steel to rust and concrete to deteriorate is minimized.
- Absence of cracks results in higher capacity of the structure to bear reversal of stresses, impact, vibration and shock.
- In prestressed concrete beams, dead loads are practically neutralized. The reactions required are therefore much smaller than required in reinforced concrete. The reduced dead load weight of the structure results in saving in the cost of foundations. The neutralization of dead load is of importance in large bridges.
- The use of curved tendons and the pre-compression of concrete helps to resist shear.
- The quantity of steel required for prestressing about 1/3 of that required for reinforced concrete, though the steel for the former should have high tensile strength.
- In prestressed concrete, precast blocks and elements can be assumed and used as one unit. This saves in the cost of shuttering and centering for large structures.
- With the advent of prestressed concrete, it has been possible now to construct large size liquid retaining structures not economical to build otherwise. Such structures have low cost and are preferably safe against cracking and consequent leakage.
- Prestressed concrete can be used with advantage in all those structures where tension develops, such as tie and suspender of a bow string girder, railway sleepers, electric poles, upstream face of gravity dam etc.
- Prestressed concrete beams have usually low deflection.
Prestressed concrete construction has following demerits:
- It requires high quality dense concrete of high strength. Perfect quality concrete in production, placement and compaction is required.
- It requires high tensile steel, which is 2.5 to 3.5 times costlier than mild steel.
- It requires complicated tensioning equipment and anchoring devices, which are usually covered under patented rights.
- Construction requires perfect supervision at all stages of construction.