POST TENSION SLAB: It refers to the slab that is tensioned as soon as the slab is developed. Reinforcement is arranged to avoid the compression.
In Post tension slab, cables or steel tendons are utilized to substitute the reinforcement. Post-tensioning offers a solution to get rid of the natural weakness of concrete in tension as well as optimize its strength in compression.
In concrete structures, this is obtained by arranging high-tensile steel tendons/cables in the element prior to start the casting. If the concrete attains the required strength, the tendons are pulled with special hydraulic jacks and retained in tension with specially designed anchorages which are attached at each end of the tendon.
It creates compression at the edge of the structural member that increases the strength of the concrete for resisting tension stresses.
If tendons are correctly curved to a specific profile, they will exert, other than compression at the perimeter, a useful ascendant set of forces (load balancing forces) that will neutralize applied loads, alleviating the structure from a portion of gravity effects.
In this type of concrete slab, cables are affixed in place of reinforcement. In Steel reinforcement the gapping among bars is 4inch to 6inch while in Post tension slab the gapping is in excess of 2m.
Go through the following video tutorial, to get more details on post tension slab.