Beyond the Build

Tips from the Field

Post-Tensioned Slabs On Grade

We have a project in the works that has potential for soil movement of subgrade because of wet and dry cycles. It may require a post-tensioned slab on grade. 

We will group the cables and space them throughout a slab, just as we would an elevated slab. After the concrete is poured, but before it permanently sets, the tendons are tightened to apply compression on the slab. This will then reduce the tension stresses due to soil movement of subgrade, thus minimizing cracking and increasing joint control.

The biggest potential disadvantage comes down the line, after a new tenant or building owner acquires the property. This could result in layout changes or core drilling the concrete for new or relocated plumbing, electrical, etc. It’s not fun to watch these cables get cut. In this case, an xray of the concrete to show cable locations is pricey, but becomes a must.

4 things to think about:

  1. Get the support steel and concrete accessories (chairs) for the post tensioned tendons just right.
  2. The slab edge pockets have to be grouted after the tendons are stressed to keep out the weather. It affects cable ends.
  3. Have your supplier provide two stressing tools just in case one breaks. It is well worth the cost to avoid shut down.
  4. Accurate as–built drawings indicating tendon final locations are a must.

*The image for this post comes from one of our jobs with a post-tensioned slab, but this one was elevated.

Merrill Stewart Jr.

Merrill Stewart is Founder and CEO of The Stewart/Perry Company, a commercial building contractor based in Birmingham.