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 tensile stresses caused by bending forces from applied loads which can result in cracking and ultimately failure. In service bending stresses (tensile) can be counteracted by prestressing.
For post-tensioning the reinforcement is not normally in the form of a metal bar and a tendon or cable is normally used. In post-tensioning the concrete is cast around tubes or ducts which will contain the reinforcing tendon. Once a suitable concrete strength has developed, the tendonsare threaded through the ducts and stretched (put into tension).
This is achieved by anchoring the tendons against one end of the concrete element and using hydraulic jacks to pull the tendon from the other end. The second end is then anchored and the jacks released. The tendons attempt to shorten transferring the stored energy to the concrete as a compressive force through the end anchors thus putting the concrete in compression.
The ducts are normally then filled with a cement grout to protect the steel from corrosion and to aid the transfer of the prestressing forces into the concrete. Post-tensioning can also be applied by using external bars suitably connected to the concrete section to allow the transfer of compression force. This system of ´external post-tensioning´ allows access to the reinforcing steel for inspection and monitoring.