How can I model floor thermal mass in BEopt with a slab floor

I am trying to model floor thermal mass in BEopt. This works fine for a crawlspace or pier and beam foundation but is not available for a slab on ground construction. This appears to be a BEopt design decision.

Most of the floor will be polished concrete and a proportion of the thickness will act as thermal mass. Any ideas of a workaround or insights as to why this cant be modeled.

Please read the description of the Floor Mass category -- it only defines the top layer of non-slab, conditioned floor surfaces (e.g., the floor between the first and second story, the floor above a crawlspace, etc.). Slab-on-grade surfaces are already assumed to be concrete and their properties (thickness, density, etc.) can be changed in the Slab category.