Pre-tensioning and Post-tensioning both method are used under pre-stressing processwhich have few edge over the orthodox non-stressed structures like more span to depth ration, higher moment and shear capacity. Generally used PSC Girders , sleepers etc.

Pre-tensioning:

In this method the tendons(group of wires) are pulled between two end bulkhead prior to casting . This pre-stress is later transferred to concrete due to bonding between the concrete and tendons .

Post-tensioning:

In this process first the casting of structure is done and later the tensioning is done. Stressing in Post-Tensioning is done in two-stages in which the order of selection of tendon is done based on the drawing provided.

1st stage- is done after 7 days of casting.

2nd stage- after 28 days of casting.

This tendons are installed via duct pipe in the structure(PSC Girder) which after stressing process are sealed with a mix of cement+water+intraplaster (sika or civax-100). This process is known as Grouting.

Key point: Post-tensioning is preferred over Pre-tensioning method due to its ease and economic constraints.

select from Quora,origin links:https://www.quora.com/What-is-the-difference-between-pretension-and-post-tension-in-structural-members#MoreAnswers

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