Most of the foundations we design are Post Tension. Post Tension Concrete Slabs or simply put, Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. Post-Tension slabs are a method of pre-stressing. Pre-stressed concrete or masonry has internal stresses (forces) induced into it during the construction phase for the purpose of counteracting the anticipated external loads that it will encounter during its lifecycle.
There are two methods of pre-stressing. One is called pre-tensioning. This method consists of stressing the reinforcing inside of large steel buttresses, and then casting the concrete around the reinforcing. This method can only be done at a pre-cast manufacturing facility and requires the completed pre-stressed concrete members to be trucked out to the job site and then assembled.
The other method mostly used for residential or smaller commercial jobs is called post-tensioning. Instead of stressing the reinforcing inside of large steel buttresses at a manufacturing plant, the reinforcing is simply installed on the job site after the contractor forms up the slabs or constructs the walls. The reinforcing steel is housed in a sheathing or duct that prevents the steel from bonding to the concrete so that it can be stressed after the concrete cures (hardens). Using the post tension method of pre-stressing enables a builder to get all the advantages of pre-stressed concrete or masonry while still enabling the freedom to construct the member (slab, wall, column, etc,) on the job site.