An apparatus for forming a concrete slab to be supported on a surface of load bearing members, and a method of use of the apparatus, are provided. The apparatus may includes a support element having a layer of active material bonded on each side by a backing sheet. The active material is chosen to slowly compress over time when placed under a load of the concrete when poured onto the apparatus. The rate of compression is chosen such that the layer of active material continues to support the slab until such time as the slab becomes self-supporting (i.e., the concrete has cured/set) on the load bearing members. The active material is chosen such that the load supported by the active material reduces gradually from an original value of 100% when the concrete is poured to less than 40% of its original value after 10 days as the concrete cures. Another feature is the inclusion of a surface of the backing sheet with a relatively low coefficient of friction. When two support elements are laid one on top of the other with the surfaces in contact, the upper support element may slide over the lower support element, for example during a seismic event.