Moisture is part of the original concrete mix. This moisture dissipates and evaporates through the surface as the slab cures. Moisture movement is an interactive process. As it evaporates from the surface, more internal moisture moves towards the surface. Under ideal conditions, the “rule-of-thumb” is to allow approximately 30 days of drying time for each inch of concrete thickness. The slab then reaches a moisture level at which installers can install flooring.
Unfortunately, the concrete drying process is seldom ideal. The conditions within and surrounding the slab are uncontrolled. Permanent doors and windows are not installed, and the HVAC is not in operation and the ambient air is saturated with moisture which slows down the drying process. The air can only hold so much moisture. If the air is saturated with moisture, it will not allow the con-crete to dissipate its moisture, which slows down the drying process. When the amount of slab sur-face moisture is lower than that of its ambient air, concrete can absorb moisture into the surface, also slowing the drying process considerably. Ground water sources, surface additives (sealers & curing compounds), temperature variations and poor drainage can all slow down the drying process. Fast-tracking a construction also does not allow the concrete to dry correctly. Now, we begin to see how flooring challenges are anything but ideal.
How can the construction industry help prevent future problems?
 When pouring new concrete, make sure drainage runs away from the slab and that the grade is adequate for local conditions.
 Placement of the vapor barrier under the slab to prevent moisture coming up through the ground.
 Use a concrete mixture with a lower water–to-cement ratio. Various admixtures may provide adequate workability while minimizing the initial moisture content.
 Allow the slab to dry naturally at service conditions. Protect it from the elements, and avoid maintenance or cleaning processes that will wet the slab.
 Test the slab’s moisture content reliably before installing the floor covering.

Various moisture tests are available, but not all satisfy ATSM’s flooring requirements.
Now considered the industry standard, Relative Humidity Test (ASTM F2170) measures the concrete’s internal relative humidity by installing a probe or sensor within the slab to a specified depth. For concrete slabs drying from top only (slab on ground with vapor retarder below, or slab on metal deck) the depth of the hole should be 40% of the thickness of the concrete slab (Example: 1.5 inches deep for a 4 inch thick concrete slab). A concrete slab drying from top and bottom (Example: elevated structural slab not in metal deck) he depth of the hole should be 20% of the thickness of the concrete slab (Example: 0.75 inches deep for a 4 inch thick concrete slab. For slabs on-grade and below-grade, include a test location within 3 feet of each exterior wall.
The Anhydrous Calcium Chloride Test (ASTM F1869), uses a measured amount of a calcium chloride sealed under a plastic dome to determine the moisture vapor emission rate (MVER) in the concrete slab. Weight gain is used to calculate the amount of moisture absorbed over a 60-72 hour period. Once frequently endorsed by manufacturers, ASTM recently disallowed calcium chloride testing of all lightweight aggregate concrete. The Calcium Chloride test only measures surface conditions and is greatly affected by the surrounding temperature and humidity condi-tions.
Moisture Meters using electromagnetic signal resistance (or impedance) to indicate slab mois-ture conditions. These meters do not read for depth, and are considered unreliable for overall con-crete assessments and generally not endorsed by the vinyl flooring industry.
A Mat Bond Test measures the reliability of an adhesive at the concrete’s current moisture con-ditions. A 3 foot by 3 foot piece of vinyl flooring is adhered to the concrete and seals around the edges. After 72 hours, the flooring is pulled up and checked for resistance. The mat bond test is not considered a reliable indicator of the moisture conditions within the slab.
The Polyethylene Sheet Test (ASTM D4263) produces a visual result. A 3 foot by 3 foot piece plastic sheet is taped to the concrete and left in place for a number of hours. If condensation is present under the plastic, it is considered a positive result that excess moisture is likely present. However, it is not known what the moisture levels are, and are they acceptable?

The best way to avoid issues is to accurately measure the concrete per the flooring manufacturer’s requirements before installing the flooring. This is to ensure that the concrete is within the re-quirements prior to installing the flooring. Once concrete is poured, installers need to await ser-vice conditions (or as close as they will get) before testing concrete moisture levels. Testing should be conducted at various locations throughout the slab, because levels may vary. With the RH Probe and the Calcium Chloride Test, the requirements are to run a minimum of three tests for first 1,000 square feet and one additional test for each 1,000 square feet. Normal placement is one test near the center with others being placed around the perimeter of the room.

Special Thanks to JJ Haines for the info!