One of the most common foundation systems used in Texas is the Ribbed Slab, as detailed in the Post-Tensioning Institute’s “Design of Post-Tensioned Slabs-on-Ground” manual.
Depending on the expansion potential of the soil, this type of foundation is either placed on imported non-expansive fill or directly on the existing expansive soil. In highly expansive soil, carton forms are sometimes used in Ribbed Slab foundation designs to create void space under the slab, in order to isolate the slab element from expansive soil.
When Wafflemat is used in the Ribbed Slab design, as a substitute for carton forms under the slab, the foundation immediately has the ability to absorb expanding soil, including non-expansive soil caps pushed upward by remaining underlying expansive soil. Note that the use of non-expansive fill is usually specified by Geotechnical Engineers to reduce the amount of potential vertical rise of underlying expansive soil.
When Wafflemat is not used in the Ribbed Slab design, but carton forms are used, carton forms must first disintegrate before void space is created under the slab. The amount of time for this to happen can vary greatly. Carton forms do not immediately create void space. Carton forms must also be protected from getting wet until the concrete poured over them has set. With Wafflemat, void space under the slab is created immediately; and Wafflemat is not affected by damp ground or rain.
Note that non-expansive fill does not eliminate the effect of any remaining underlying expansive soil on foundations. Non-expansive fill transmits the force of underlying expansive soil to foundations. Expansion potential of expansive soil is usually only completely eliminated if all the underlying expansive soil is removed and replaced. This can be a very expensive option.
Ribbed Slab Foundation with Carton Form Void Space:
Ribbed Slab Foundation with Wafflemat Substitute for Carton Form Void Space:
What happens when you use Wafflemat as Substitute for Carton Forms in Ribbed Slab Foundations?
- Void space is created under slab immediately after concrete is set.
- Not affected by water.
- Wafflebox plastic forms heavy and permanent moisture barrier under slab, in addition to the plastic sheeting.
And without Wafflemat?
- Carton forms must first disintegrate before void space is created.
- Carton forms can be damaged by water before concrete pour, thereby requiring costly replacement.
- Cartons forms use only thin plastic sheeting to form moisture barrier.