Calcium Chloride Test

Understanding the Calcium Chloride Test

The calcium chloride test, standardized under ASTM F1869, is one of the most widely used methods for checking moisture vapor emission from a concrete slab before a coating goes down. A sealed dish of anhydrous calcium chloride is placed on the slab under a plastic dome for 60 to 72 hours. Since calcium chloride absorbs moisture from the air, weighing the dish before and after shows how much vapor moved through the surface during that window.

Reading the Results

Results are reported as pounds of moisture vapor emitted per 1,000 square feet over 24 hours, often written as MVER. Many coating manufacturers set an upper limit, commonly in the 3 to 5 pound range, above which a standard coating system is not recommended without a moisture mitigating primer. The specific threshold always depends on what the coating manufacturer allows for that product.

Limitations Compared to Other Moisture Tests

The calcium chloride test only measures moisture at the surface of the slab and can be affected by ambient humidity and temperature during the test window, which is why many contractors now pair it with, or replace it with, an in-situ relative humidity test using probes drilled into the slab. RH testing is considered more consistent, but calcium chloride testing remains common because it is inexpensive and does not require specialized probes.

When to Run This Test

Testing should happen after the slab has had time to acclimate to the job site’s ambient conditions, and manufacturers typically require multiple test locations per square footage of floor. Running this test before ordering material, rather than after prep is already finished, gives contractors time to plan for a moisture mitigating system if the results come back high.