Post-Tension Concrete Slab Foundations

We want the home buyers and homeowners of Plano to understand some of the construction aspects of their homes. We have been very critical of most home builders in Texas because they build residential concrete foundations that we consider minimally functional. Most of these foundations use steel rebar to reinforce the concrete slab with the dual goals of providing support for the home and resisting soil movement. In our opinion, home builders use too little concrete (and perhaps too little steel rebar) too build a concrete slab that will resist the natural forces of soil movement. When builders use less concrete they increase their profits.

More recently and unfortunately a different method of building residential foundations has become popular with home builders. This is the post-tension slab and in our opinion it is less suitable and less than minimally functional for home buyers and homeowners in Plano. We are not the only ones who feel this way. Structural engineers will tell you similar things. Below is a professional opinion from a structural engineer.

Oh, by the way, if you buy a new home in Plano and the foundation fails within the first 10 years you cannot sue the builder. You have to use mandatory arbitration. Remember that sales contract you signed? Well, the builder selects the arbitration company – which is a for-profit company that is cozy with businesses like home builders. And the historical probability of you winning in mandatory arbitration ? 10% or less.

Thanks to the home builders in Plano and throughout Texas, we have a large and busy foundation repair industry. Read on, brave home buyer.

In addition to slabs with steel rebar reinforcement, post-tensioned slabs are intended by design to be minimally functional.

As a separate issue, the post-tensioning technique has an inherent weakness. Post-tensioning is intended to eliminate the use of steel rebars by achieving a minimum of 50 psi compression of the concrete at the middle of the slab. Theoretically, this compression is accomplished by “squeezing” the slab together from the edges toward the middle. The “squeezing” force is applied by the post-tension cables at their anchored ends on the edges of the slab.

As long as the slab is free to slide along the surface of the supporting soil while the “squeezing” occurs, the theory is achieved in practice. This sliding requires that the slab have a smooth flat bottom. However, in the Greater Plano area, post-tensioned residential slab foundations do NOT have a smooth flat bottom; the grade beams – perimeter and interior – extend down into the soil below the bottom of the slab that is continuous across the tops of the grade beams. These downward extensions act as concrete cleats that are built in trenches dug into the soil.

Thus, when the post-tension cable tension is applied to the edges of the slab, the inward side of each grade beam that runs across each post-tension cable is pushed against the side of the trench in which it was cast. This acts as a very effective brake to prevent each such grade beam and the monolithic slab on top from “squeezing” toward the middle of the slab. As a result, the middle of the slab is not compressed and is certainly not compressed to the required 50 psi minimum. This leaves the middle of a post-tensioned slab as an unreinforced (plain) concrete slab, just like an improperly built wire mesh or steel bar reinforced slab with the steel lying on top of the plastic moisture barrier and not embedded in the thickness of the slab. Such a plain concrete slab only provides a hard surface, but has no significant strength to resist bending – either up or down – at the edges.

If the weight of the house causes one or both opposite edges of such a slab to bend down, due to the weight of brick or stone veneer exterior walls, or a brick fireplace hearth and chimney, or the “edge half” of a gable roof, or some combination of any or all of these, the plain concrete slab cracks across the middle between the downward bending edges.

These comments were written by C. Randall Riddell, structural engineer. We hope you found them enlightening.

 

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