Prestressed concrete, invented by Eugène Freyssinet in 1928, is a method for overcoming concrete's natural weakness in tension. It can be used to produce concrete panels, beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
Concrete has one major weakness - it cannot resist tension well, having perhaps a tenth of its compressive strength when it is pulled. So it is superb for arches, but not very good for beams. There is a solution to this problem - design the structure so that tension cannot occur in the concrete.
Pre-tensioned concrete is cast around already tensioned tendons. 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. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually 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, lintels, floor slabs, beams or foundation piles.
SiteWizard.co.uk