Questions and Answers About Slab-on-Ground Foundations

Frequently Asked Questions About Slab-on-Ground Foundations

What are post-tensioned slab-on-ground foundations? What are conventionally reinforced slab-on-ground foundations? What are the significant differences between a conventionally reinforced and post-tensioned slab-on-ground foundations? Should it make any difference to me what kind of slab-on-ground foundation a house has? How do slab-on-ground foundations work? What are active and dormant areas of a slab-on-ground foundation?

What are post-tensioned slab-on-ground foundations?

A post-tensioned slab-on-ground foundation is simply a ground-supported concrete slab foundation that is reinforced with flexible cables that are tensioned after the concrete hardens. When the cables are tensioned, the concrete is placed in compression. The cables are not normally tensioned until at least 7-days (and sometimes not until 30-days) after concrete placement. Since the cables cannot provide any crack control until after the cables are tensioned, every post-tensioned slab experiences anywhere from 7 to 30-says during which there is no crack control at all. This allows for the development of large, visible curing cracks called restraint-to-shrinkage (RTS) cracks.

In spite of this, post-tensioning is generally considered to be a superior method of bending crack control as compared to conventional reinforcing.

(return to top)

What are conventionally reinforced slab-on-ground foundations?

A conventionally reinforced slab-on-ground foundation is simply a ground-supported concrete slab foundation that is reinforced with what are called deformed steel bars. Unlike post-tensioning cables in a post-tensioned slabs, the concrete bonds to the conventional reinforcement as the concrete cures. The conventional reinforcement provides crack control almost immediately.

(return to top)

What are the significant differences between a conventionally reinforced and post-tensioned slab-on-ground foundations?

The most important differences between post-tensioned slabs-on-ground and conventionally reinforced slabs are listed below:

	Post-tensioned slabs-on-ground are usually thinner than conventionally reinforced slabs-on-ground. This fact alone makes the post-tensioned slab potentially more flexible than a conventionally reinforced slab. There are other aspects, however, that, in practice, mitigate this difference.
	Post-tensioned slabs are less likely to develop cracks due to bending. Since the concrete in a post-tensioned slab is placed in compression, cracks due to bending are less likely than in conventionally reinforced slabs.
	Post-tensioned slabs are more likely to develop cracks due to restraint-to-shrinkage. Since the cables cannot be tensioned for at least 7-days after the concrete is placed, visible RTS cracks are almost inevitable in a post-tensioned slab. These cracks will tighten when the cables are tensioned. However, certain conditions, including debris lodging in the crack, may prevent an RTS crack from fully closing. These cracks are not significant structurally to the performance of the foundation whether they close or remain open.
	Post-tensioned slabs-on-ground can be less expensive. If several foundations are to be constructed using the same design, a post-tension design will almost always be less costly to the builder. This situation common with large builders; constructing post-tensioned slabs can bring significant cost savings for such builders.

(return to top)

Should it make any difference to me what kind of slab-on-ground foundation a house has?

The reality is that the large majority of house foundations constructed in the Greater Houston Area in the last 25-years have been post-tensioned foundations. In addition if you are looking for a house within a specific price range in a particular subdivision or area of town, every house that fits your criteria may be either post-tensioned or conventionally reinforced. From a practical perspective, what is important is how well or how poorly a specific foundation has performed in the past rather than how it is reinforced.

One of the authors has grew up in a house founded on a conventionally reinforced foundation and has lived in a house founded on a post-tensioned foundation for the almost 30-years. Neither house has ever shown any significant distress due to foundation movement with the exception of the house the author grew up in and that house displayed problems only after it was rented and the occupants did not water the soil adjacent to the foundation during dry periods.

The bottom line is that, in our opinion, the question of whether a resale house has a conventionally reinforced foundation or a post-tensioned foundation should be a non-issue.

(return to top)

How do slab-on-ground foundations work?

The structural function of a slab-on-ground foundation is to act as a buffer that mitigates the differential distortions between the supporting soil and house supported on the foundation. Slab-on-ground foundations do this by resisting the moisture-induced distortion of the supporting soil and by spanning over moisture-induced distortions in the supporting soil.

The foundation is intended to do this while maintaining the surface levelness within permissible levelness tolerances. The levelness tolerances are dependent on the as-constructed levelness of the foundation surface and construction of the house. The intent of the design protocol is for the foundation surface to distort within a range that:

	Does not result in significant structural damage to the house frame.
	Does not distort the frame structure so that numerous doors and windows cannot function.

It is extremely important to understand that slab-on-ground foundations are not designed to eliminate the possibility of cosmetic damage or minor door problems.

(return to top)

What are active and dormant areas of slab-on-grade foundations?

When a slab-on-ground foundation is placed in the ground, the presence of the slab inhibits the ground under the foundation from wetting up or drying out due to wet and dry weather respectively. This is most pronounced in the central area of the foundation. Since this area is protected from seasonal weather related changes in the soil moisture, the ground supporting the central area of the foundation does not shrink and swell very much compared to the area of the foundation near the perimeter or edge of the foundation. Most of the soil shrinkage and swelling occurs in the area of the foundation within around 8 to 10-feet of the foundation edge. This area near the edge of the foundation is called the active area since this where most of the soil distortion due seasonal weather changes occurs. The following sketch shows the relative locations of the active and dormant areas of a rectangular slab-on-ground foundation.

(return to top)

© 2004 R. Michael Gray, P.E. and Matthew T. Gray. This material may be reprinted for personal and educational non-commercial use only. This material is based on generally accepted engineering principles and practices; it is for general information only. The information contained herein should not be used without first securing competent professional advice with respect to its suitability for a general or specific application. Anyone using this information assumes all liability for such use.