Compressive stresses are introduced in prestressed concrete either by pre-tensioning or post-tensioning the steel reinforcement.
Concrete, although strong in compression, is weak in tension. For this reason it needs help in resisting the tensile stresses caused by bending forces from applied loads which can result in cracking and ultimately failure. In service bending stresses (tensile) can therefore be counteracted by prestressing.
In pre-tensioning the reinforcement, in the form of tendons or cables, is stretched (put into tension) across the concrete formwork before the concrete is placed. After the concrete has hardened and a suitable strength developed, the tendons are released. The tendons attempt to shorten and transfer the energy stored to the concrete through the bond between the concrete and the reinforcement. A compressive force is therefore induced into the concrete.