What is Post-Tensioning

Post tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.

Post-tensioning is a form of prestressing. Prestressing simply means that the steel is stressed (pulled or tensioned) before the concrete has to support the service loads. Most precast, prestressed concrete is actually pre-tensioned-the steel is pulled before the concrete is poured. Post-tensioned concrete means that the concrete is poured and then the tension is applied-but it is still stressed before the loads are applied so it is still prestressed.

Example:

I was on summer training after pre final year on NH-1 with Unitech and there was a bridge was under construction on river Markanda in Shahabad town. I remember correctly they used heavy machine ( motor type ) to stretched outward the tendons after  concreting and used wedge shaped anchors to freeze it. It was new technique in year 1999 -2000 in India. The span of bridge is more than 50 mtr. may be arround 54 mtr. and of box type. Tendons were run through ducts exist in the sides of box.

( Photo's are taken from google search image)

There is one method which is to stretch the tendons out ward( apply tension) and anchored then pour concrete, after concreting release the anchor and concrete is stressed. In this method cracking at ends or place of anchor usually occurred. This post tensioning method is improvement over it.  There was a screw nut method may be its name was on LEE Method for prestressing but may be not that popular.

A very high level bridge and of good span like 50 mtr., prestressing  is good and proper method. I think this would help you to understand the topic.

from Quora