Maximum Moisture Vapor Transmission Rate for Applying Sealers

By Chris Sullivan – ChemSystems, Inc.

Question:

What is the standard allowable moisture vapor transmission (MVT) rate when applying sealers?

Answer:

ASTM states that a moisture vapor transmission rate of 4 pounds per 1,000 square feet over 24 hours is the maximum allowable for concrete to be covered with a coating, sealer, or adhesive. You can determine the MVT rate by performing a calcium chloride test (ASTM F 1869). This simple test involves placing a preweighed tablet of unhydrated calcium chloride on the concrete surface and then covering it with a sealed, domed plastic lid. After 72 hours, the tablet is weighed again to determine how much moisture left the concrete and was absorbed by the calcium chloride. A simple math calculation provides an extrapolation of the MVT rate in a 1,000 square foot area over 24 hours.

A new trend in moisture testing, which has been used in Europe for years, is in-situ relative humidity testing (ASTM F 2420). This test can be completed in minutes and provides a more realistic moisture reading from within the concrete. The downside: It requires drilling a 1/2-inch-diameter hole into the concrete for insertion of the probe that takes the humidity reading. The maximum relative humidity reading allowable by ASTM is 75%.

What happens if you apply a sealer, coating, or adhesive to a slab with an MVT rate exceeding 4 pounds per 1,000 square feet or 75% relative humidity? Chances are good that the application will fail, turn white, or peel. You can find more information on moisture testing and moisture vapor transmission on the Internet. Simply conduct a search using the terms "concrete moisture vapor transmission" or "concrete moisture testing."

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