In 1928 AD, Eugene Freyssinet introduced a revolutionary material, the prestressed concrete. In prestressed we overcome the limitation of concrete that it has a low tensile strength.
In prestressed concrete, concrete is under compressive stress before it is loaded. This could be done by tensioning steel cables before loading by means of either a pre-tensioned or post-tensioned system. Both these systems are explained below.
In this type of system, first, the mould in which member is to be cast is prepared and then tendons are placed and then tensioned then after the concrete is placed in it.
Pre-tensioning is being broadly used for the mass production of railway sleepers.
In this system ducts for tendons are provided before placing concrete. After that concrete is placed in the mould. After concrete gains strength, tendons are passed through the duct and by means of anchorage, it is tensioned.
Post tensioning system could be divided into two types
- Bonded members
- Unbonded members
Post tensioned prestressed concrete is very popular in prestressed bridges.
Advantages of prestressed concrete
- As in prestressed concrete, high strength materials are used, it leads to saving in materials and hence also cost.
- Slender sections can be designed.
- The entire cross section area of the member is utilised to overcome bending moment as compared with RCC in which only half the area was utilized.
- High strength of materials leads to achieve higher durability.
- Prestress concrete members are less prone to cracks as the entire section is under compression. (no tension in the cross section). This crack free behavior makes it suitable to make structures for liquid storage with prestressed concrete.
- Prestress concrete members could effectively carry impact load.
- The design life of prestressed is longer.
Disadvantages of Prestressed concrete
- The technology of prestressed concrete is costlier. The cost of cables and anchorage systems is very high at present.
- Skilled labors are needed for the construction.
- Prestressed concrete is brittle as compared with RCC.
- The initial cost is also higher than RCC.
- Prestress losses are also undesirable up to some extent.
- Indian standard code (IS 1343-2012 pdf) for Prestressed concrete
- Losses of prestress in prestressed concrete.