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Preferred Method for Testing RH?

#11
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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#12
Thanks everyone for your thoughts on this forum. i agree with cc solutions. I am much more confident with my RH readings than i ever was with CaCl.
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#13
I guess the point of sorptive materials was lost somewhere in the translation...sorptive materials will "steal" humidity from the air..but through thermal changes, this moisture can condense and or evaporate under changing conditions.

As far as vapor drive..there is very insignificant vapor drive, usually far less than 1.0 p.s.i. Consider it takes at least 2 p.s.i. to inflate a childs latex balloon...I think there is agreement that "drive" is of no consequence..the facts remain:

Unless the amount of space is known...the water content is unknowable

Humidity changes approximately 5% for each temperature change of ONLY 2 degrees F

If I am incorrect in these two significant points, then a LOT of textbooks need to be rewritten.
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#14
so,
is the discussion on the physical characteristics of concrete and hygroscopic nature, in relation to it environment?

We can drive out moisture to affect CC readings,
and Rh/t can also affect the probes Rh measurements,

As a flooring person, what am I most concerned? The moisture or the pH?
if the pH, what will bring it out? Rh in the concrete or the vapor emission?

Are both tests Qualitative or are they both, Quantitative? or, is one is or one is not?

Using meters, what should be done to the surface of the concrete first? I witnessed a so called tech person from XXXX, checking on some XXXX modular tiles, put his meter directly on the pressure release adhesive then declare in a authoritative voice the adhesive was re-emulsifying when there was no empirical physical evidence of moisture condensing under the vinyl backed tiles,

I use both methods to give my clients a total picture, and, I use the meter to find "hot spots" that I map out, to show differences from one area to another, and, I always use the GE system, if the holes are drilled correctly and the area around it treated correctly, then, others who have also tested the same slab using the Wanger found similar readings, no two meters, even of the same brand, will read the same, in fact, I have found, using the same Wanger probe at different times will give different readings, as will the GE,,,,
meters, if you know the huge range of differential between their readings and oven dry test methods for wood, would understand they are not accurate but should only be used as indicators and to show, if using the same meter, differences between locations at a one time reading,,, using Wanger with different machines seems to me, a sure fired method to be wrong.

Pin wood meters, thermal hydrometers and concrete probes all work on similar designs and technology, so, when using meters, what is our differential range and how will we calculate? Does the density and hardness affect any of the readings?

Dew point? temp of the surface vs interior vs the air vs our understanding we are not concrete engineers, or at least, I am not, who here is? Who here has worked for concrete correction companies?

maybe they can help.
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#15
(12-23-2009, 11:54 AM)RCConcrete Consulting Wrote:  I guess the point of sorptive materials was lost somewhere in the translation...sorptive materials will "steal" humidity from the air..but through thermal changes, this moisture can condense and or evaporate under changing conditions.

Absorbent materials will absorb moisture until they are in equilibrium with their environment. If you drop a cotton ball in a jar of water, the same amount of water is in the jar, but some is absorbed by the cotton ball. And heating the air in the jar will allow the air to become more absorbent and it will pull moisture from the cotton ball and equilibrate once again.

Where concrete is concerned, we look for 75% - 80% RH. At this point concrete will not cause a problem with moisture sensitive flooring.

(12-23-2009, 11:54 AM)RCConcrete Consulting Wrote:  Unless the amount of space is known...the water content is unknowable
Yes this is true, but does not concern us. We want to know the RH of the slab. The amount of moisture will only give us an indication as to how much water needs to come out of the slab.

This can and has been tested. We can take two identical slabs, and one slab can be weighed and then baked dry to determine the amount of moisture in the concrete. Then knowing how much water is in the slab, and measuring the moisture lost each day, we can estimate how long the slab will take to dry to equilibrium with the environment. After years of testing and research, we can apply this knowledge to a stable water content that doesn't cause problems with flooring. This is called the RH test. Smile

(12-23-2009, 11:54 AM)RCConcrete Consulting Wrote:  Humidity changes approximately 5% for each temperature change of ONLY 2 degrees F

If I am incorrect in these two significant points, then a LOT of textbooks need to be rewritten.

The textbooks don't show this to be true. I don't know how to post pictures here yet, but if you Google 'Psychrometric Chart' you will see the correlation between moisture and relative humidity. It is not linear as you suggest. And luckily indoor concrete slabs are relatively stable in temperature, and we are to test at service temperature.
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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#16
Quote:The textbooks don't show this to be true. I don't know how to post pictures here yet, but if you Google 'Psychrometric Chart' you will see the correlation between moisture and relative humidity. It is not linear as you suggest. And luckily indoor concrete slabs are relatively stable in temperature, and we are to test at service temperature.

But people do go away from home and sections of commercial buildings HVAC get shut down, even overnight when these buildings are inhabited. So tetsting at service temp is simply a snapshot in time and does not mean that will be the same forever.

I have to say if I am going to test ASTM-2170 method I believe the RapidRh is the best because your testing only down at the bottom of the hole as instructed,. The other probes that hang in the holes even though the air pocket is equilibriated must have a diffency factor because the top part of the concrete is dryer so not a true reading at 40%. Yes, I have watched the infomercial. Big Grin
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