CONCEPT OF PRE-STRESSED CONCRETE
As mentioned before, prestressing is the application of an initial load on the concrete structure, so that the structure is able to counteract or withstand the stresses coming due to the service loads. The concept can be clearly understood by the example of a barrel. A barrel used in olden periods to transports liquids and grains are tightly bound by metal bands as shown in figure-1. These metal bands are fitted so tightly that it creates a hoop compression around the barrel. When this barrel is filled with liquid, it exerts hoop tension. The hoop compression that is created by the metal bands helps to counteract the hoop tension created by the fluid within. This is a prestressing system.
Need for Prestressing Concrete
The need for prestressing in concrete can be justified by the following issue:
- Concrete is weak in tension and strong in compression. This is a weak point of concrete that results in early flexural cracks mainly in flexural members like beams and slabs. To prevent this, the concrete is induced with compressive stress deliberately (prestressing) and this stress counteracts with the tensile stress the structure is subjected to during service condition. Hence the chances of flexural cracks are reduced.
- The pre-compression that is induced as a part of prestressing helps to enhance the bending capacity, the shear capacity and the torsional capacity of the flexural members.
- A compressive prestressing force can be applied concentrically or eccentrically in the longitudinal direction of the member. This prevents cracks at critical midspan and supports at service load.
- A prestressed concrete section behaves elastically.
- The full capacity of the concrete in compression can be used over entire depth under full loading in the case of prestressed concrete.
Advantages of Prestressed Concrete
The major advantages of Prestressed Concrete are:
- The prestressing of concrete by using high tensile steel improve the efficiency of the materials
- The prestressing system works for a span greater than 35m.
- Prestressing enhance shear strength and fatigue resistance of concrete
- Dense concrete is provided by prestressing systems thus improving the durability
- Best choice for the construction of sleek and slender structures.
- Prestressing helps to reduce the dead load of the concrete structure
- Prestressed concrete remains uncracked even at service load conditions which proves the structural efficiency
- Composite construction by using the prestressed concrete unit and cast-in-unit derives the economical structure
Disadvantages of Prestressed Concrete
- Higher material costs
- Prestressing is an added cost
- Formwork is more complex than for RC (flanged sections, thin webs) – thus, precast not as ductile as RC