Under compression, concrete is considered a high strength material but, it possesses an inherent weakness when under tension. Prestressed concrete has stress applied to it during manufacture so as to improve the tensile strength of the finished concrete.
Traditional Reinforced Concrete
This is simply placing steel reinforcement materials in place and pouring the concrete over them. They do provide additional strength but no balancing forces have been applied.
Prestressed Tensioners
A better way can be to insert stretched metal rods or wires into freshly poured concrete before it has set. In this way, prestressed metal shares its stress with the setting concrete.
Unbonded Post-Tensioned Tendons.
The (reinforcement) is still metal – usually in cable form and this is currently considered the best for concrete slabs used as the base, floor, flat roof or even walls of buildings. As the name implies, this method uses unstressed metal tendons put in place before the concrete is poured. The same principle but with the tendons bonded to the setting concrete was once popular but it has gone into decline following serious problems developing when used in bridge construction.
Post tensioning is only applied to the metal cable after it has been encased in (but not bonded to) the concrete for several days to allow the concrete to set. Furthermore, the tendons are greased and then encased in a plastic sheath. Because of this, the tendon can move (as it is stretched) relative to the set concrete.
After the concrete has set and the tendons have been tensioned by use of hydraulic jacks, the loads on the tendons are maintained by connecting them to steel anchors embedded in the outer edges of the concrete slab. This ensures that the set tension will be permanently applied to the concrete. It is worth noting that this method allows for subsequent adjustments to be made.
Suncoast Post-Tension is the leading supplier of post tensioned cables for Houston, TX. They can also supply all the necessary additional components needed for a successful installation. Follow them on Facebook
