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 of steel cables before loading by means of either pre-tensioned or post-tensioned system. Both these systems are explained below.
In this type of system, first mould in which member is to be casted 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 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 bridge.
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 is utilised to overcome bending moment as compared with RCC in which only half area was utilised
- High strength material leads to achieve higher durability
- Prestress concrete members are less prone to cracks as the entire section is under compression. (no tension)
- Prestress concrete members could effectively carry impact load.
- Design life of prestressed is longer.
Disadvantages of Prestressed concrete
- Technology of prestressed concrete is costlier.
- Skilled labours are needed for the construction.
- Prestressed concrete is brittle as compared with RCC.
- Initial cost is also higher than RCC.
- Prestress losses are also undesirable up to some extent.