Its often best practice, or a remedial requirement, to lay a membrane (aka a moisture barrier) over the site concrete.
Are you referring to an oversite concrete cover, then a ventilated subfloor space/crawl space, and then suspended floor joists.
If the answer to above is yes: then no, you dont have to lay a membrane, and then pour the oversite concrete over the membrane.
Its often best practice, or a remedial requirement, to lay a membrane (aka a moisture barrier) over the site concrete.
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