PC wires are widely used in many industries. Prestressed concrete strand came into practice in the 1950s. However, the basic principles of prestressing are not only applicable to PC wire, but have also been used in many applications in ancient times. A good example is the iron hoop cylinder, which applies a certain amount of pressure to a loose cylinder wedge to form the cylinder and withstand sufficient lateral water pressure. This was the early principle of prestressing.
1. By the diameter: 4mm, 4.8mm, 5.0mm, 5.1mm, 6.0mm, 6.25mm, 7.0mm, 7.8mm, 8.0mm, 9.0mm, 9.5mm, 10.0mm, 11mm, 12mm, etc.
2. By surface coating: non-coated PC wire, epoxy coated PC wire and galvanized PC wire.
3. By strength level: medium strength PC wire 800~1200MPa; high strength PC wire 1470MPa, 1570MPa, 1670MPa, 1770MPa and 1860MPa, etc.
4. By treatment process: cold-drawn PC wire (no relaxation performance requirements) and low relaxation PC wire.
5. By the surface form: bright round PC wire, spiral ribbed or helix grooved PC wire (left or right rotation), crimped PC wire or indented PC wire (two-sided indentation, three-sided indentation, four-sided indentation), etc.
Cold-drawn prestressing is generally used for pipelines, especially those made with PCCP process. Low relaxation PC wire is widely used in railroad sleepers and prestressed concrete poles, a small amount is used in building components, and prefabricated beams are basically not used. Low relaxation galvanized PC wire is commonly used in cable stayed bridge, tie rod of steel pipe arch bridge and prestressing cable for large span building, galvanized steel wire without stabilization treatment is generally used in suspension bridge cable.
The advantages of prestressed concrete strand construction and the application of prestressing in concrete members are mainly to overcome the disadvantages of low tensile strength of concrete and to make full use of high strength steel. When the stress of the tensile steel reaches 250 MPa, the crack width reaches 0.2-0.3 m. At this time, the durability of the member is reduced and it is not suitable for high humidity or corrosive working environment. The role of prestressing strand is to make the member generate precompressive stress before stressing, so as to control the level of tensile stress in the member, or even make the member in a compressed state. In order to avoid early cracks in reinforced concrete members, high strength reinforcement and high strength concrete are fully utilized to improve their performance and durability.
The main results are as follows:
(1) Due to the effective use of high-strength PC wire and coagulants, prestressed concrete strands can be made into slender load-bearing structures with larger spans and less self-weight than ordinary reinforced concrete.
(2) Prestressed steel strand, is able to improve the serviceability of the concrete structure. It can effectively prevent the concrete from cracking. Or at least limit the width of cracks to a harmless level. Improving the durability of the structure.
(3) The deformation of the structure can be controlled to a small extent under load, even with partial prestressing.
(4) Cracks caused by overloading can be resealed after unloading, provided that the strain on the reinforcement is less than 0.01%.
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