What is a Post-Tension Slab?
The prefix post, which means “after”, indicates that the foundation is tensioned after the concrete has hardened. What this means is that the cables or tendons are stressed after the concrete has been placed in the foundation. The post-tensioning industry is a spin off of the pre-stress industry which has been around since the early 1900’s. The pre-stress industry is used for bridge beam type construction like most elevated bridge structures like the Atchafalaya and Lake Ponchetrain long-span bridges. These long-span concrete pre-stressed concrete beams are used for the loads and spanning ability. This type of construction or pre-tensioning was converted to “after” construction via a plastic coating or encapsulating applied to the tendons to prevent bonding of the concrete. This coating allowed the cables to be stressed after and thus the post-tension industry was born back in the lat 1960’s. This concept of “after “construction tensioning was widely accepted in the construction indust
Even though concrete is a very rigid material, it has a natural weakness when it comes to tension. It is limited with respect to the length of a beam, floor, or bridge that can be made out of it. One way to be able to build these structures with longer spans than would be possible with ordinary concrete is through a technique called pre-stressing. A post-tension slab is a slab of concrete that has been pre-stressed using a specific method to increase the strength of the concrete. Several methods exist for pre-stressing concrete, with post-tensioning being a very common one. Before a post-tension slab is poured, high-strength steel strands or cables, called tendons, are laid in a tight grid. These help support and give strength to the slab once it has cured. The tendons are sheathed in plastic so that they do not directly touch the concrete. After the grid is made, the concrete is poured, with extra care taken to make sure that the tendons remain at the correct depth. The concrete is al
