If a house has concrete floor slabs, it is concrete that is cast on site, approximately 16-20 cm thick slab. The underside is then perfectly smooth (or almost, there may be imprints from the form on which the concrete was cast).
If it's lightweight concrete, it consists of pre-fabricated slabs, 25-60 cm wide, 17-25 cm thick. This can be seen on the underside by the beveled longitudinal joints between the slabs.
Additionally, there's lightweight clinker concrete (which is also a type of lightweight concrete in the sense that it weighs less per square meter than 'real' concrete), which also consists of pre-fabricated slabs, about 20 cm thick. This is also visible on the underside through beveled longitudinal joints between the slabs.
So take a look at the underside. You’ll probably find the answer there.
And indeed, it is true that (blue, blue-black) lightweight concrete can be radon hazardous. But it doesn't have to be. It depends on where the slabs were manufactured.
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Byggaren
