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Specifications of Prestressing Steel in Prestressed Concrete

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Specifications of Prestressing Steel in Prestressed Concrete

For 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:

Hard-drawn steel wires which are indented or crimped are preferred for pretensioned members as their bond characteristics are superior to the plain wires. Strands normally comprise two, three or seven wires of 2 to 5 mm size. The helical form of twisted wires in the strand improves the bond strength. Two and three-ply strands are made up of 2 mm and 3 mm diameter wires while a seven-ply strand is made by twisting 2 to 5 mm wires. High-tensile steel bars commonly used in prestressing are manufactured in nominal sizes of 10, 12, 16, 20, 22, 25, 28 and 32 mm diameter. The ultimate tensile strength of bars does not vary appreciably with the diameter. This is because the high strength of the bars is due to alloying rather than due to cold-working as in the case of wires. The minimum characteristic tensile strength of high-tensile strength bars as per code is 980 N/mm2. Their proof stress should not be less than 80% of the minimum specified tensile strength. Elongation at rupture should be 10% for the specified gauge length. It is specified that all prestressing steel should be free from splits, harmful stretches, surface flaws, rough, jagged and imperfect edges and other defects likely to impair its use in prestressed concrete. Slight rust on the surface of reinforcement may be permitted provided there is no visible surface pitting. If any coupling is made or any other similar fixture is used in conjunction with wires or bars, it should have an ultimate strength not less than the individual strength of the wires or bars which are being joined. It is specified in the code that the modulus of elasticity of steel tendons should be determined by tests on samples of steel to be actually used in construction. Otherwise the value may be obtained from the manufacturer of the steel. If it also is not possible, then the following values may be used.
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
Strands 195
The ultimate tensile strength of a plain hard-drawn steel wire varies with its diameter. The tensile strength decreases with increase in the diameter of the wires. Tensile strengths and elongation characteristics of cold-drawn stress relieved wires as per IS: 1785 (Part1)-1983 is as given in the following table:
Nominal Diameter (mm) Minimum tensile strength (N/mm2) Elongation (%)
2.5 2010 2.5
3.0 1865 2.5
4.0 1715 3.0
5.0 1570 4.0
6.0 1470 4.0
7.0 1375 4.0

Permissible Stresses in Steel

As 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 Reinforcement

In 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:
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