A normal slab is basically a RCC (Reinforce Cement Concrete) slab whereas a post-tensioned slab is the PSC (Prestressed Concrete) slab.

PSC is applied with compressive stress on the cross-section in advance before the application of loading on the member so that the stresses after loading are fully or partially balanced by the pre-stressed(stresses developed due to the tensioning of steel wires, cables, bars, strands etc.).

1- A dead load of PSC member is lesser than RCC member and hence PSC beam or PSC slab transfers less load on columns.

2- The shear resistance of PSC member is more and the full area of concrete is utilized.

3- The most important is that the deflection and cracks in PSC members are much lesser compared to a RCC member. (As higher the horizontal compressive force acting on the member lesser will be the deflection).

Types of prestressing

1- Pre-tensioning: Steel tendon is tensioned before the construction of member and then the concrete member is cast over it. The tendon is cut off after getting the required strength of concrete (28 days) to transfer the prestress on the concrete member.

1- The minimum grade required for pre-tensioning is M-40.

2- The bond b/w concrete and steel is fully effective in pre-tensioned member and hence loss in prestressing due to friction is not present.

 

2- Post-tensioning: The concrete member is the first cast by leaving a hollow duct along the centroidal axis. Steel tendon is tensioned after getting the full strength of concrete (28 days) in the hollow duct provided and anchored at the ends of the member.

1- The space remains in the duct is filled with mortar by grouting.

2- The minimum grade required for post-tensioning is M-30.

3- The bond b/w concrete and steel is not effective in post-tension member and hence loss in prestressing due to friction is present.

4- The stressing force is transferred from end bearing in post-tension member and the zone where the variation of stress is present is called end block zone.

1- The grade of steel needed in PSC is of a higher grade than used in RCC.

2- PSC is based on the process of stress balancing which is shown in the diagram below-

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