Moisture or hydrostatic pressure, is a real problem if you want to coat a grade concrete floor or apply tile, etc. that will be bound with glue/cement.
From the Quikrete website:
http://www.quikretecoatings.com/quik...ges-workshops/
Test for moisture:
The presence of excess moisture in concrete can inhibit a coating’s ability to penetrate the surface. Do not apply this product to concrete areas with moisture coming from within the surface. You can test for moisture by taping a 2' x 2' piece of plastic wrap to the concrete using duct tape on all edges. Allow the plastic to sit for 24 hours. If water droplets appear on the underside of the plastic, or if the concrete is damp, the surface contains excess moisture and should not be painted. Moisture problems must be corrected before coating.
I've been there before with a commercial concrete floor that was below sea level just off the beach. The hydrostatic pressure lifted/popped all the tile off the floor within 6 months of the install. The two genius engineers who inspected it and offered recommendations to "successfully" cover the floor were all proven wrong when the floor failed again within 1 year. I'm sure this was an extreme example being a few feet below sea level and very close to the beach.
So, yes, test for moisture before coating any unfinished floor with a nonporous finish.