Originally Posted by JoeD
I don't understand the comment about zero PSI on the concrete surface.
Uncured concrete cannot support weight since it is a semi-liquid so
-unless concrete expands while curing (and it very well may)-
the surface of the concrete will just barely be in contact with the undersurface of the materials above it, so no pressure will be on the cured concrete surface.
If you drive a wedge between the cured concrete and the material it is supporting, that wedge may be seeing hundreds of pounds of compressive force, depending on how thick it is and how "elastic" the structure of the building is.
At least that is how I imagine it.
Strain gages can confirm or deny this idea and that's a bit beyond DIY stuff.