How do I patch a hole in the slab under a kitchen island?
My answer from this question will work for your situation
How should I finish around this hole in my slab?

Starting at the lowest level:


vapor barrier (6 mil polyethelene plastic)
if you have cold winter, you can optioally add an insulation layer: 1" or more of XPS insulation
Something to cover the plastic (or insulation).  Pea gravel is nice for removability.  Concrete will prevent anything from burrowing (in or out)