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Specifications of Prestressing Steel in Prestressed ConcreteFor prestressed concrete members, the high-tensile steel, used generally, consists of wires, bars or strands. The high tensile strength of steel is generally achieved by marginally increasing the carbon content in steel in comparison to mild steel. High-tensile steel usually contains 0.6 to 0.85% carbon, 0.7 to 1 % manganese, 0.05% of sulphur and phosphorus. The high carbon steel ingots are hot rolled into rods and cold drawn through a process of dies to reduce the diameter and increase the tensile strength. The durability of wires gets improved due to the cold-drawing operation. The cold-drawn wires are then tempered to improve their properties. Tempering or ageing or stress relieving by heat treatment of wires at 150-420°C improves the tensile strength. These cold-drawn wires are generally available in nominal sizes of 2.5, 3, 4, 5, 7 and 8 mm diameter.
The prestressing steel, as per the code, should be any one of the following types:
- Plain hard-drawn steel wire conforming to IS: 785(Part 1)-1966 and IS: 1785(Part 2)-1967,
- Cold-drawn indented wire,
- High tensile steel bar conforming to IS: 2090- 1962, and
- Uncoated stress relieved strand conforming to IS: 6006-1970.
|Type of steel||Young’s Modulus of Elasticity (E) in kN/mm2|
|Plain cold-drawn wires||210|
|High tensile steel bars rolled or heat treated||200|
|Nominal Diameter (mm)||Minimum tensile strength (N/mm2)||Elongation (%)|
Permissible Stresses in SteelAs per the Indian code, the permissible stresses in the tendons at the time of initial prestressing should not exceed 80% of the characteristic tensile strength of the tendons. The final prestresses after allowing for all losses of prestress should not be less than 45% of the characteristic tensile strength of tendons.
Untensioned ReinforcementIn prestressed concrete construction, use of untensioned reinforcement also has to be made along with tendons. This is necessary as tensile stresses produced due to prestress act in lateral directions. These stresses are taken up by untensioned reinforcement which is provided in addition to tendons in lateral as well as longitudinal directions. The untensioned reinforcement is also needed to take on stresses produced during the transport of the prestressed concrete members. Reinforcement used as untensioned steel should be one of the following types:
- Mild steel and medium tensile steel bars,
- Hot-rolled deformed bars,
- Cold-twisted bars, and
- Hard-drawn steel wire fabric