Water is present in the ground everywhere, in liquid form as ground water or attached to or between soil particles and in form of water vapor between the soil particles. While slabs on grade are unlikely to come in contact with liquid ground water, they definitely will come into contact with the water vapor that rises through the soil.
If a below slab vapor retarder is missing or defective, water vapor will be able to penetrate the bottom of the slab and accumulate in the slab if it cannot evaporate. Slabs on grade in contact with moist soil can also lead to capillary moisture transfer into the slab. Basement floor slabs may come into contact with liquid water if submerged in ground water.
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