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Preferred Method for Testing RH?
12-17-2009, 10:40 AM
Post: #11
RE: Preferred Method for Testing RH?
If I accept your statements as correct without verifying, I believe there are flaws in your examples as they relate to concrete moisture testing.

Yes absorptive materials will retain water. This is why additional water is needed in the box with cotton in it. Once the water saturates the cotton to a point above 50% - 70% any excess water will become suspended in the atmosphere. Over time, the air and the cotton will equilibrate, as do the absorptive materials in concrete.

While you may get your box of water down to 20% Rh or less, you will not get concrete that low. Very old and very dry concrete will measure 50% - 75% RH. The absorptive materials in concrete will retain water and absorb moisture from the air if they fall below this level. The concrete will seek equilibrium with the environment.

What we are testing for is free water, and that will show up as RH levels above 80% or so. If we see a reading of 100%, there very well may be condensation as in your kitchen example (I really enjoy your ability to simplify your examples, it makes things quite easy to understand) but we are not concerned how much water is in a liquid state as this must all evaporate and the RH must come down to 75% - 80% before we know the concrete has stopped hydrating and is ready for flooring.

Concrete that is ready for flooring will have many many gallons of water in it, but this water alone will not create the vapor drive which results in flooring failure. Concrete can live quite happily at 75% relative humidity and be considered 'dry'.

JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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Messages In This Thread
Preferred Method for Testing RH? - george - 08-12-2009, 09:24 AM
RE: Preferred Method for Testing RH? - jf222 - 08-13-2009, 11:30 AM
RE: Preferred Method for Testing RH? - jf222 - 08-13-2009, 03:15 PM
RE: Preferred Method for Testing RH? - CC Solutions - 12-17-2009 10:40 AM