Prestressed concrete is a form of concrete used in construction that while under construction is substantially "prestressed" ...

Design of Prestressed Girder for Bridge

Prestressed concrete is a form of concrete used in construction that while under construction is substantially "prestressed" (compressed) in the areas that will be subjected to tensile forces while in service to strengthen it against these forces.
This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete volume and is done to improve the performance of the concrete in service.Tendons may consist of single wires, multi-wire strands or threaded bars that are most commonly made from high-tensile steels, carbon fiber or aramid fiber.

The essence of prestressed concrete is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces and of ductile high-strength steel when subject to tension forces. 

This can result in improved structural capacity and/or serviceability compared with conventionally reinforced concrete in many situations. In a prestressed concrete member, the internal stresses are introduced in a planned manner so that the stresses resulting from the superimposed loads are counteracted to the desired degree.

Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow for longer spans, reduced structural thicknesses, and material savings compared with simple reinforced concrete. Typical applications include high-rise buildings, residential slabs, foundation systems, bridge and dam structures, silos and tanks, industrial pavements and nuclear containment structures.

First used in the late-nineteenth century, prestressed concrete has developed beyond pre-tensioning to include post-tensioning, which occurs after the concrete is cast. Tensioning systems may be classed as either monostrand, where each tendon's strand or wire is stressed individually, or multi-strand, where all strands or wires in a tendon are stressed simultaneously. Tendons may be located either within the concrete volume (internal prestressing) or wholly outside of it (external prestressing). While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons.

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