When I first began in this industry I was taught that you didn't want to put anything over a floor until you tested to make sure that moisture wasn't going to come through and ruin your work. The test that I was taught to perform required taping down a piece of plastic for 24 hours and looking for condensation (ASTM D-4263.) If there was condensation we knew we had a problem, but the problem was that just because we didn't have condensation didn't mean that there wasn't a problem.
Eventually I was introduced to moisture meters (ASTM F-2659) and Calcium Chloride tests (ASTM F-1869). Moisture meters use conductivity to determine moisture content near the surface of the slab (not at depth) and can be affected by conductive metals and foreign objects in the slab. Calcium Chloride tests are widely used and provide a quantitative reading of the moisture vapor emissions, but only at the time of the test. They can be dramatically affected by ambient conditions (useless results without controlled air), and are inconvenient in occupied spaces. While both of these tests do provide a quantitative reading, they fail to test the moisture content at depth.
We care about the moisture content at depth because once a coating or floor covering with little to no permeability is placed on the slab, the moisture will begin to equalize (higher concentrations at depth will work their way to the surface.) As this happens, the moisture and alkalinity levels at the surface of the slab will increase, affecting the bond of the floor covering. This is not good and can lead to failure of the bond, osmotic blistering, mold, and large sums of money and down time to fix.
So how do we test what is down in the slab? At DMA Floors, we use In Situ probe testing (ASTM F-2170.) We drill holes in the slab to 40% depth (on grade and pan placed slabs), or 20% depth for suspended slabs (since they dry on two sides.) Then we place probes in the holes, allow them to acclimatize, and record their readings. Using those readings, we can determine the relative humidity of the slab and get a better idea of what will happen when the slab equalizes.
Calcium Chloride and Moisture meter testing are kind of like trying to calculate the volume of a body of water by simply measuring the surface area. In Situ probe testing is more like taking that surface area and the depth to calculate the proper volume. Calcium Chloride tests are often performed incorrectly as well (even by engineering firms), which can produce false test results.
If you have flooring options that are moisture vapor sensitive, it would be wise to budget for proper moisture testing and possibly mitigation. Though DMA Floors is certified by ICRI (international Concrete Repair Institute), we always recommend third party testing. Third party moisture testing provides you with an independent result from someone who has nothing to gain or lose from the results. It prevents a contractor from under-bidding a job and counting on a considerable change order to mitigate. Remember, the flooring contractor sells the solution to high moisture problems!