Prestress is a technique to overcome the weakness of concrete during tension. In general, concrete undergoes tension on bottom flange and compression on top flange. In prestressing, the tendons are stressed along the axis and cement is charged; later, when the tendons are released the bottom is compressed and counter balance the compression at the top part of the beam.
The characteristic of prestressed concrete is that it allows preset engineering stresses to counteract stresses that occur during service loads. Prestressing eliminates a number of design limits that conventional concrete faces on load and span and it permits building of floors, bridges, roofs, and walls with longer unsupported spans. The global prestressed concrete market can be segmented based on prestressing types, force applied, application, and region.
In terms of prestressing type, the prestressed concrete market can be classified into external or internal; pre-tension or post-tension; linear or circular; full, limited, or partial; and uniaxial, biaxial, or multi-axial. When prestressing is achieved by tendons located outside the concrete, it is called external prestressing; when tendons are located inside the concrete, it is called internal prestressing. When tension is applied to the element before casting of the concrete, it is known as pre-tension prestressing; when tension is applied to the element after hardening, it is known as post-tension prestressing. Prestressing, when related to members that are flat or straight, is called linear prestressing, and when it is related to members that are curved, it is called circular prestressing. When the level of prestressing is such that there is no tensile stress in concrete under service load, it is termed as full prestressing.
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