RH Numbers Rising - Printable Version
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RH Numbers Rising - COD - 04-05-2011 09:40 AM
I recently entered the 21st Century and started using the RH test in lieu of the Calcium Chloride tests. I have a new addition project where the slab is about 3 months old. The HVAC is not on yet, there is temporary heat in the building and the temperature is in the mid to low 60s. There is a moisture barrier under the slab.
I installed 4 probes at various locations in the slab, waited about 90 minutes and checked the results. The readings were between 91% and 94%. When I went back a week later all the readings had risen to 99%. Is this unusual?
RE: RH Numbers Rising - eaadams - 04-05-2011 10:49 AM
It makes sense for the readings to go up a week later. The drill creates heat which provides an artificially low initial reading.
A slab on grade with moisture membrane in direct contact has a rough guestimate of 1 month per inch once the building is closed in @ operational levels and assuming the slab has not been rewetted.
Mid to low 60's for a temp isn't drying out that slab. reference Concrete Construction's field test update #3 where the slab has sat for years and never dried out. http://www.concreteconstruction.net/curling-and-shrinkage/discovering-the-unexpected.aspx
RE: RH Numbers Rising - Ernesto - 04-05-2011 11:49 AM
I'd crank up the heat and rum some fans. Still I think your months away from being dry enough to be safe. Unless you put on some crazy over priced moisture mitigation system.
RE: RH Numbers Rising - eaadams - 04-05-2011 12:19 PM
I had a GC tell me the other day that he once was working on a prison hospital that was to receive vinyl flooring. He said what they did (and apparently worked) was super heat with heaters all night, come in in the morning, vent the room out with fans, close it up, super heat all day, vent at end of day, and repeat. Claims they dropped their moisture enough to get floor covering in a few weeks.
Sounds a lot like this Munters / Best Buy story ... I'd love to know how much such a system / process costs: http://www.munters.us/upload/Case%20studies/Best%20Buy%20Case%20Study.pdf
RE: RH Numbers Rising - Ernesto - 04-05-2011 12:27 PM
Neato eaadams. Out here is the southwest we can do the same thing in most of the seasons by simply keeping the doors and windows open.
RE: RH Numbers Rising - COD - 04-05-2011 01:16 PM
I was on the site again today to check the readings for a third time and they are exactly the same as they were the 2nd time.
RE: RH Numbers Rising - CC Solutions - 04-05-2011 05:26 PM
The slab is probably quite wet! It has only had 3 months to dry, conditions are not ideal, and I'd bet a nickel that the surface has either been sealed or is burned black. If that is the case, even a Munters unit will not dry it out.
Speaking of dehumidifiers (Munters), there is no guarantee they will do anything. Especially if the concrete isn't open and porous. If the slab is open, they can dry it out faster than the 1" per month rule we use.
As for what one GC did on one job, please take those stories with a grain of salt. The whole reason I got into this line of work was because of a job my company did where we dried the slab out (remember this was 17 years ago) and had the CaCl down to 2.5lbs. Definitely good for vinyl flooring right? Wrong. Complete failure within 3 years. The slab was 12" thick lightweight concrete poured in a pan and it was at 98% RH. When we opened the floor there was standing water under the vinyl and we were on third floor!!
(04-05-2011 11:49 AM)Ernesto Wrote: I'd crank up the heat and rum some fans. Still I think your months away from being dry enough to be safe. Unless you put on some crazy over priced moisture mitigation system.
Oh... This hurt...
Which reminds me.... even at 99% RH I can have that floor ready for ANY flooring in one day, guaranteed.
RE: RH Numbers Rising - Ernesto - 04-05-2011 08:42 PM
Well I got another annoying pertinent question. Whats a healthy rh percentage for a concrete slab? See where I am going? No, ok how about, where does all the mver's possibly h20 go when your slab is 99% and is only sealed around the perimeter of the walls. Does it go up into the walls?
Might not be a big deal in commercial apps but in a home, could lead to SHS. Can concrete stay 99% and keep it's strength and not deteriorate
RE: RH Numbers Rising - CC Solutions - 04-06-2011 06:04 AM
Concrete can stay wet forever and it just gets harder and harder as it continues to hydrate. Bridge pilings are poured and flooded with water all the time. They live underwater for hundreds of years.
As far as water going up the walls, well it was going to anyway. A sealed slab will not 'breathe' up the walls meaning moisture in the center will not wick to the edges. So whatever area the walls cover will breathe the same whether the slab is sealed or not. It is far more important to have a good vapor retarder under the slab in the first place. The small amount of moisture under the wall footprint won't harm anything (unless there is no vapor retarder and the moisture is unlimited!) .
RE: RH Numbers Rising - Ernesto - 04-06-2011 07:40 AM
Hmmm, I'm going to have to do some resaerch on that. However I do believe the admixtures are quite different when placing pilings that will be underwater verses a residential slab. Apples and oranges