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Ambient humidity vs. RH

#2
G'Day Mate!!!

Always nice to meet a new fellow from the land of roos. Glad you posted, please pull up a chair, grab a cup of coffee and enjoy! Smile

Concrete and moisture. Let's step back and take a look at the big picture before we get into details. It's really not as mysterious as some folks think it is, you just have to look at what is happening.

When concrete is poured in place, it has a lot of extra water in the mix that helps the workers put it in place and finish it. Even at a low water / cement ratio of .45, the concrete has twice as much water as it will ever need to fully hydrate. So what we have with all concrete slabs is a tendency for moisture in the concrete to want to come out of the concrete allowing the slab to equilibrate with its environment.

Now here's where the environment plays a role. A slab that is poured under water (bridge columns for instance) will not tend to emit water vapor (technically they could due to the heat of the cement reaction, but let's keep this simple for now). The underwater concrete piers are in equilibrium with their underwater environment when they are extremely wet.

A slab poured on a raised deck in the sahara desert where the average humidity is between 3% and 30% will emit water vapor until it is in equilibrium with the environment there.

Concrete poured in more hospitable living climates will tend to equilibrate somewhere around 50% -75% relative humidity.

The amount of moisture in a slab and its predisposition to absorb or emit moisture to equilibrate with the environment varies based on just a few factors. The RH of the slab and the RH of the environment. And of course these two factors change constantly, not just seasonally, but daily, even hourly. Now we can factor in moisture from the ground that slowly moves into the slab, slab porosity, floor covering porosity, temperature variations, I think the list of influences is quite long, but I don't need to worry about all that. Let the scientists work on the technicalities, let us keep our eye on the big picture.

So to answer your question, I would tell you that a slab poured in a drier climate will obviously dry faster than one poured in a damp location, but the slab in the dry climate is doing the same thing the damp climate slab is doing, they are both trying to equilibrate with their environment.
While the damp climate slab may be at equilibrium at 75% RH or 80% RH, the dry climate slab may need to be at a much lower RH before it reduces its push to rid itself of excess water.

Note: I almost said 'before the vapor loss is low enough to install a moisture sensitive floor' but I don't want to muddy that creek.... Here in Beaver Dam, we understand that even with a low vapor emission if we have a high RH pushing moisture, we have a high potential for failure.

And one more point if I may, when we say the temperature has little effect on the RH testing, we need to qualify that statement... put a little asterisk next to it...

RH, relative humidity, is a function of the amount of water in a sample of air and the temperature of the air. Obviously temperature will affect an RH reading. But the ambient temperature swings a room experiences will have little effect on the slab RH reading. What we have found in our area is that slabs poured on grade in a commercial facility generally stay at 67*F to 69*F. If I take an RH reading and my slab is at that temperature, I have a trusted reading, even if the room I'm in is being blasted by a heater temporarily and the air temp is 85.

On the other hand I have refused to allow installation on slabs that read 75% RH but they are very warm, say 85*F. This can happen in multi story structures where heat is being forced directly under the slab being tested. In those cases as the slab cools to the expected operating temperature of 72*F the RH reading will increase.

Sorry I'm so long winded... Tongue
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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Messages In This Thread
Ambient humidity vs. RH - Rubensgt40 - 10-04-2011, 09:29 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-06-2011, 06:41 AM
RE: Ambient humidity vs. RH - Ernesto - 10-06-2011, 03:43 PM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-06-2011, 05:16 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-06-2011, 06:43 PM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-06-2011, 09:20 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-07-2011, 06:07 AM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-08-2011, 12:24 AM
RE: Ambient humidity vs. RH - Ernesto - 10-08-2011, 01:42 PM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-09-2011, 12:58 AM
RE: Ambient humidity vs. RH - eblakewi - 10-10-2011, 11:21 AM
RE: Ambient humidity vs. RH - CC Solutions - 10-10-2011, 11:37 AM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-10-2011, 02:53 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-10-2011, 03:47 PM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-11-2011, 04:32 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-12-2011, 06:15 AM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-13-2011, 09:12 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-14-2011, 06:12 AM
RE: Ambient humidity vs. RH - Rubensgt40 - 10-16-2011, 02:42 PM
RE: Ambient humidity vs. RH - CC Solutions - 10-16-2011, 06:09 PM

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