A variety of inventions have attempted to solve either the problem of slab movement due to soil expansion or the problem of heat loss through a slab, but none has provided a single solution to both of these problems.

Daniel P. Gallagher Jr. of Niwot, Colo., has now patented a lightweight rigid slab assembly that can be used over expansive soils without movement or damage to itself or to interior walls resting on the slab. It also provides thermal insulation to substantially reduce heat loss through the slab.

Construction of Gallagher's cast-in-place invention results from concrete placement into thermally insulating preshaped forms made of a material such as foamed polymer. In one type of assembly, the support members are made of a degradable material, which disintegrates eventually leaving the slab unsupported by the soil. In another style, the forms use resilient rails or rails with degradable pads resting on the soil as support members. The rails permit the soil to expand without applying damaging up-lift pressure to the slab.

In both cases, after casting, the forms remain in place and function as thermal insulation for reducing heat loss through the slab and into the underlying soil. During casting, concrete flows into the separation between the panels and forms the reinforcing ribs. Under extreme conditions where the soil expands sufficiently, the triangular edge of the rib can be designed to split or collapse the rail before forces become sufficient to damage the slab.

This second embodiment is lower in cost than the first embodiment because most of the material in the insulating forms is in the inexpensive rectangular panels. Only the insulating rails have a complex shape.