A concrete floor or polyethylene sheet can serve an air barrier. The floor air barrier must be sealed to the air barrier in the below-grade walls. This can be done by caulking the joint between the floor and basement walls.
Dampproofing and air barrier protection against radon leakage into conditioned spaces is usually provided by a sealed polyethylene barrier under the basement slab. This arrangement also constitutes a vapour barrier.
Source : Canada Mortgage and Housing Corporation (CMHC)