Why moisture in concrete matters
Concrete keeps water long after it is hard enough to walk on, and slabs on grade can receive more from the ground. When a coating is applied, moisture vapor rising through the slab meets a barrier. Vapor pressure, dissolved alkalis and osmosis can then push the coating off, cause blistering, or prevent proper cure of moisture-sensitive products. Measuring moisture before coating, and comparing it with the data sheet limit, is a basic step in floor work.
How the calcium chloride test works
ASTM F1869 places a dish of anhydrous calcium chloride under a sealed plastic dome on the prepared slab. Over the test period, the salt absorbs the vapor emitted by the area under the dome. The weight gain, the exposed area and the time give the emission rate. The standard sets conditions for the test, including that the space be at service temperature and humidity for a period before and during the test, and the number of tests per floor area.
MVER and relative humidity
| Aspect | MVER (ASTM F1869) | In situ RH (ASTM F2170) |
|---|---|---|
| What it measures | Vapor emitted at the surface | Relative humidity inside the slab at a set depth |
| Depth represented | Upper portion of the slab | The depth defined by the standard for the slab type |
| Sensitivity to ambient conditions | High | Lower, once the probe has equilibrated |
| Result unit | Pounds per 1,000 ft² per 24 hours | Percent relative humidity |
Many manufacturers now refer to ASTM F2170, and some accept either method. The specification should state which test applies and which limit governs, taken from the coating data sheet. ASTM D4263 (plastic sheet) gives only a qualitative indication.
When the result is too high
- wait for further drying with the building conditioned, then retest;
- use a moisture mitigation primer or a system designed for high moisture, as the manufacturer allows;
- verify the presence and condition of a vapor retarder under slabs on grade;
- look for sources such as leaks, poor drainage or wet process areas, often signalled by efflorescence.
Moisture testing is part of the preparation plan for every industrial epoxy flooring project.