Posted by 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. Prestressed concrete has its inherent advantages and disadvantages.
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.
Pre-Tensioning System
In this type of system, first, the mold in which the member is to be cast is prepared and then tendons are placed and tensioned, after that the concrete is placed in it.
Pre-tensioning is being broadly used for the mass production of railway sleepers.
Post-Tensioned System
In this system ducts for tendons are provided before placing concrete. After that concrete is placed in the mold. After concrete gains strength, tendons are passed through the duct, and using anchorage, it is tensioned.
Post-Tensioning system could be divided into the following two types.
- Bonded Members
In Bonded Members, after the strands or tendons are tensioned, the duct is filled with cementitious grout. - Unbonded Members
In unbonded Post Tensioned Prestressed concrete, the strands are coated with grease and allowed to move freely relative to surrounding.
Post-Tensioned prestressed concrete is very popular in prestressed bridges.
Advantages of Prestressed Concrete
- As in prestressed concrete, high strength materials are used, which leads to savings in materials and hence also cost.
- Due to higher bending strength slender sections can be designed and this will give an aesthetic look.
- The entire cross section area of the member is utilized to overcome bending moment as compared with RCC in which only half the area was utilized.
- High strength of materials leads to achieving higher durability.
- Prestressed 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.
- Prestressed Concrete members could effectively carry impact load.
- The design life of the prestressed concrete structures 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 laborers 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.
Read Also-
- Indian standard code (IS 1343-2012 pdf) for Prestressed Concrete
- Losses of Prestress in Prestressed Concrete
- Grades of Concrete and Mix Ratio Explained in Detail
- Lightweight Concrete – Making And Applications
- Different Types of Admixtures Used in Concrete (Concrete Technology)
External Sources-
NPTEL Prestressed concrete courseware