Most people know the difference between a concrete slab and raised-floor foundation. (It’s common for garages to be on slabs with a couple steps up to the raised-floor portion of the house, while some buildings are built completely on concrete slabs.) One foundation type that is not so well known is the post-tension (PT) wafflemat slab. Having designed hundreds of thousands of square feet of PT wafflemat slab over the past several years, I have come to appreciate its relative benefits over other foundation types.


Most concrete slabs have mild steel reinforcement bar (“rebar”) placed at the center of their cross sections. This rebar helps to prevent the concrete from cracking due to temperature-induced expansion and contraction. It also holds the slab together in the event of differential settlement caused by underlying soils that expand and contract due to changes in moisture content. Soils that are high in clay mineral content such as kaolinite and montmorillonite are highly expansive and require additional measures to prevent the foundation from cracking.

Post tensioning employs the use of steel tendons that are pulled into tension after the concrete is poured and partially cured, causing the concrete to go into compression and thereby giving it additional stiffness. In the case of both post-tension and mild-steel reinforced slabs, it is common practice for the underlying soil to be prepared by means of excavating an upper layer and replacing it with a combination of engineered back fill, gravel, sand and/or vapor retarder. Even after this costly process, lower levels of clay can expand, causing the slab to be pushed upwards.

When material savings in concrete is desired, a ribbed slab can be used that exploits the strength of deep concrete beams only where they are needed. Not only does this solution not solve the heaving issue (the soil is still pushing up on the complete floor area of the slab), this approach also requires skilled excavation and formwork that is often cost prohibitive.

So, what if a foundation could have the high-performance characteristics of a PT slab, use less concrete by employing deep beam-like ribs, not be susceptible to heaving, but not require the expensive earthwork?

The Wafflemat

The PT wafflemat slab foundation allows builders to easily place bottomless polyethylene box forms directly on grade, then pour concrete between them, forming a waffle-like slab. The bottomless boxes yield air voids into which expanding soil can “bulge” when the soil is wet. This reduces movement of the slab even when underlain by severely expansive soils.

While the idea of using on-grade forms is not a new one (the practice of using solid polystyrene forms in other parts of the world is not uncommon), the added benefit of the wafflemat’s air void, ease of transportation (they nest inside of each other) and relatively recent emergence as an accepted building technology cannot be understated. One of the reasons wafflemats will be utilized more and more in the future is that many of the “ideal” building locations with more favorable soil have already been developed, leaving the remaining sites with poor quality soils. Additionally, labor costs are not getting any cheaper. Developers are now more than ever seeking quick, easy and cost-effective solutions for their projects.

Even when a PT wafflemat is not warranted for structural reasons, there are some not-so-obvious benefits to their use having to do with a topic I discuss a lot with people – drainage. Despite the focus that engineers give to the natural forces of gravity, wind and earthquakes, water causes far more damage to buildings than anything else. The use of a wafflemat form on grade automatically raises the finished floor elevation and allows for easy grading away from the building.

Contact Us

If you have a building project in an area that is prone to expansive soils or simply want a high-performance solution without a lot of hassle and haven’t been educated about the PT wafflemat slab by your contractor or architect, please feel free to contact Simmons Engineering to further discuss this effective option.

3 Thoughts on “A New Take on an Old Idea: Post-Tension Wafflemat Slab Foundations”

  • Clay, How would this work for a driveway? Would it work on our Mont. Village clay soil? Would it be cost prohibitive? If the answers are yes, and no, respectively, do you know anyone locally who incorporates this process in their exterior concrete work?
    Could it be used with the “stamping” effect? Thanks, Carol

    • Carol, thanks for your comment. I know the soil you speak of . . . Were Montgomery Village (a 1950s subdivision in Santa Rosa, CA) to be built today, it would be a very good candidate for this system throughout. To answer your question, stamping can certainly be used with PT slabs. However, I wouldn’t recommend this system for a driveway or any kind of architectural flat work unless, perhaps, it was to be poured together with the rest of the house’s foundation. It is cost prohibitive for such a small job. Also, your driveway needs to be slightly lower than and sloping away from your garage entrance for drainage. With the 12″ +- overall depth of a PT wafflemat, you’d have to excavate down quite a bit, taking away a lot of the advantage of this system. I would recommend something like a typical 4″ slab with #3 bar at 18″ O.C. (or #4 at 24). Prepare the subgrade with some engineered backfill and use 4″ of free-draining crushed rock. If the city hassles you, let me know and I’ll sketch something up for you quickly. Thanks again for your comment. -Clay

  • I had no idea that post tension helped to compress concrete more and make it more stiff. It’s amazing that a method like this has been developed to improve the strength of concrete. This is especially important because of how much we use concrete in construction. It helps to have ways to better enforce that material.

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