Prestressed concrete is basically concrete in which internal stress of a suitable magnitude and distribution are deliberately introduced so that the stresses resulting from external loads are counter acted in a desired degree For example let's try this prestressing wires are placed eccentrically in a simple beam produce an axial compression and hogging moment . Under service loads, the same beam will develop sagging moment making it possible that entire beam will be in compression when service loads are imposed on beam

So this will make that there will be no cracking, as reinforced concrete cracks in tension

Methods:

PRE TENSIONING :Tension is done prior to the casting of concrete ex: Railway sleepers, Electric poles.

POST TENSIONING : Tensioning after casting of concrete ex: For longer span girders.