Concrete Moiture Test Video
Time: 07:09
Bob explains the proper function and use of the moisture test, calcium chloride test, and pH test when pouring a concrete overlay or epoxy.

There are a couple of simple tests that tell us a few things about how much vapor is coming out of our floor and a more difficult test that tells us more. The old standby is simply to duct tape an 18-inch square of clear plastic on the surface of the slab (ASTM D 4263). Come back in 16 hours and if there is moisture under the plastic, it's too wet for an overlay or a sealer or anything else that could delaminate. But temperature and dew point can affect this test and a dry sheet test isn't necessarily a sure sign.

A common test that has been used for many years is the calcium chloride test (ASTM F 1869). This test is done with kits that are available from a variety of sources (such as Vaprecision, or Engius). This test indicates how much moisture is coming out of the slab. However, there is no reliable way to calibrate the test and it only indicates what's happening at the surface and at that moment-ambient conditions can change the results and it only tells us the moisture in the top ½ inch of the slab. If an impermeable coating is applied, everything can change. But if you have no choice, you might have to rely on this test.

Calcium chloride test kits measure MVER. Engius in Stillwater, OK
RH probes measure the relative humitiy in the slab. Wagner Electronics in Rogue River, OR
Relative humidity in a slab decreases over time. Engius in Stillwater, OK.

The best way to test for slab moisture is with relative humidity testing (ASTM F2170). RH probes are embedded into the slab or inserted through small drilled holes. This method indicates the moisture condition throughout the slab. The ideal is to have a slab with an average relative humidity below 75%, although floors as high as 90% are acceptable for some floor coatings. For details on this, talk with the folks at Wagner Electronics or at Engius. Another great source of information is Howard Kanare's book, Concrete Floors and Moisture, available from the Portland Cement Association.

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