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Prestressed concrete, invented by Frenchman Eugène Freyssinet in 1928, is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams or slabs with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete member would otherwise experience due to a bending load. Prestressing can be accomplished in two ways:
- Pre-tensioned concrete is cast around an already tensioned steel tendon. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned elements may be balcony elements, floor slabs, beams or foundation piles.
- Post-tensioned concrete is cast around a duct. The duct may be curved, to follow the area where otherwise tension would occur in the concrete element. A set of tendons is fished through the duct and the concrete is placed. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. Anchors which are locked onto the tendon and which also react against the ends of the beam maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendon from corrosion. Because the concrete member itself provides the reaction for tensioning the tendon, post-tensioned members can be built on-site.
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