Post-tensioned is a type of Pre-stressed concrete. Another type is pre-tensioned. 

I am assuming you know what pre-stressing is. In Post-tensioned structure, the tensioning of reinforcement is done after the concrete has been placed. So, you have ducts running through the concrete, in which the reinforcement steel is passed through. The steel is anchored on one end and then tension is applied from the other end with the help of a jack. Since the duct is slightly larger in diameter than the steel, the duct can also be grouted, which is called bonded post-tensioning. 

Purpose of a post-tensioned structure: 

• The structure can take higher loads for same section without pre-stressing.

• Section size can be reduced.

• Shear capacity is increased.

• Suitable for dynamic loads (Bridges).

• Useful for precasting of sections, leading to faster constructions. (Eg. Delhi Metro girders)

In comparison to Pre-tensioned structure; 

• Post-tensioning is better suited for large/heavy members.

• Post-tensioning is suitable for longer spans, as the transfer of loads takes place through the end anchorages.

from quora

original links https://www.quora.com/What-is-the-purpose-of-a-post-tensioned-structure

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