Wafflemat is a concrete foundation forming system used on expansive soil and on other types of soil where greater slab rigidity is required. Wafflemat creates a pattern of deep voids on the underside of concrete foundation slabs. The voids are formed with Waffleboxes that are made with polypropylene co-polymer plastic–the same tough material used in automotive parts such as bumpers and battery boxes. Concrete is poured over the Waffleboxes to form the slab, and to form a series of narrow beams in two directions between the Waffleboxes. The beams have center-to-center dimensions that do not exceed 4’-2”, and that support the foundation directly on the soil. They create a very rigid foundation slab.
It’s important to observe that a 4” thick concrete slab with closely spaced beams, as used in the Wafflemat foundation, is much more rigid than a 4” thick concrete slab with widely spaced beams, as is commonly used in Texas.
Note: Foundations using the Wafflemat system must be designed by a licensed Civil or Structural Engineer experienced in the design of concrete foundations on expansive soil, using soil test data provided by a licensed Geotechnical Engineer.
How Does Wafflemat Work?
In a Wafflemat foundation, voids are created under the slab immediately after the concrete is poured. There is no waiting time, such as with carton forms that require the paper in the forms to disintegrate, in order to create voids. With Wafflemat, expansive soil can start expanding into the voids immediately after the concrete sets. By absorbing the expanding soil volume, the Wafflemat foundation minimizes the uplift effects of expanding soil on the foundation and house structure.
The following drawings show how Wafflemat works:
The above drawing shows how the Wafflemat foundation looks when it is constructed. Wafflemat foundations are usually built on a flat graded building site. The native soil is not disturbed. Only the top layer of vegetative material has to be removed, just as in any foundation. No removal of the expansive soil on the site and its replacement with a non-expansive soil cap is usually required with Wafflemat.
The above drawing demonstrates the simple principle of Wafflemat. When soil under a Wafflemat foundation expands, it moves into the voids created by the Waffleboxes. The width of the beams in contact with the soil is minimized in the foundation design, so that the force of expanding soil on those beams is also minimized. Yet, the beams are designed to be wide enough so that the load of the foundation on the soil does not exceed the allowable soil bearing pressure.