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Questioning the results of RH probes
10-13-2011, 01:06 PM,
#1
Questioning the results of RH probes
I have come across this situation on many if not all of my long term moisture testing jobs and it is starting to get to me. I find that when taking initial readings (after the 72 hours) my results are typically high (90s or at least high 80’s). When I return to the site for regular readings and plot all of the data I find that the test locations in general will gradually come down in RH until we stop testing or until they reach the desired levels and they are covered. Occasionally we get some that do not come down at all but the gradual lowering of the RH over time is more common.

While this sounds good and appears to be doing what the concrete does over time under the right conditions, the part that scares me about the validity of the RH testing is that this will be typical on my jobs where the slab was poured 8 months ago just the same as slabs that were poured 20 years ago.

Yes I know that the slab poured 20 years ago may have been covered immediately back then and never dried out, had some recent problems where the concrete was wet or some other strange occurrence to make the concrete have a higher RH again but it is a regular occurrence that they start off with high RH values and slowly lower over weeks or months of testing.

The main point is that I have not seen a slab that is initially at a reasonable RH and has already stabilized. It always seems to be starting it’s drying out process the moment I start to monitor it.

Any thoughts?
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10-13-2011, 02:17 PM,
#2
RE: Questioning the results of RH probes
Brian, need a lot more info.

Where are you? Slabs change drastically across geographic areas as different designs become popular for different regions. And just because they are an Architect or Engineer doesn't mean they know what they are doing.

What sort of slabs? Medical? They do it one way. Tilt up / Warehouse? They do it another way. Residential? All bests are off, run for the hills.

When are you starting your testing? You say it starts to dry out when you start to monitor. Well that is quite vague.

Are they poured indoors or with a roof on. People don't realize that that 8 month old slab usually only gets enclosed sometimes weeks before you show up.

A slab will never completely dry out. That is a misnomer. Concrete needs a certain amount of moisture in it to 'be'. They will eventually reach an equilibrium with the other moisture they are exposed to.

Also a 20 year old slab doesn't necessarily mean it is a good slab. Does it have a vapor membrane?
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10-13-2011, 02:44 PM,
#3
RE: Questioning the results of RH probes
Thanks for the reply and I will try to answer all the questions you have.

My jobs are primarily in the mid-Atlantic region. The majority of the buildings are office buildings with elevated slabs (the basements are usually not floored so no need for testing). This includes rehabbing buildings (the 20 year old part) where they are being gutted on the inside and re-designed for offices or residential.

When I mentioned “It always seems to be starting it’s drying out process the moment I start to monitor it.” What I mean is when I look at the data, the RH is high (90’s and occasionnally high 80’s) right when I first start to monitor and it gradually lowers, regardless of how long ago the slab was poured or how long it has been in controlled climate conditions.

For instance, I installed several probes at a job and watched as the RH values slowly lowered over time. We were then asked to monitor another area in the building that was in a controlled environment just as long as our current test area. (now I know everyone is saying that it is a different area with different circumstances and I know and understand that but that is not what I am getting at) The second set of probes again started off with high RH values and slowly went down. Don’t you think that at some point I would end up with a slab that had already dried out somewhat and had RH levels that weren’t in the 90’s or high 80’s prior to my starting to monitor it?

Thanks again for your comments
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10-13-2011, 02:51 PM,
#4
RE: Questioning the results of RH probes
(10-13-2011, 02:44 PM)brian Wrote:  Don’t you think that at some point I would end up with a slab that had already dried out somewhat and had RH levels that weren’t in the 90’s or high 80’s prior to my starting to monitor it?

No I don't. It really takes a lot for a slab to read down in the 80-75 area.

However, I think a lot of flooring has requirements at 80-75 which is artificially low. But they have to assume worst case scenario. There are floors now that can go down @ 92% rH.

I'm sure others can chime in if they have ever tested a slab at below 80%
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10-13-2011, 03:09 PM,
#5
RE: Questioning the results of RH probes
Hi Brian, 'Others' here, glad to have you...

I have tested many slabs that were less than 75%, in fact just recently I was handed a job for mitigation and when I conducted my testing I found the slab was in the upper 60% range. Believe me, I wanted it to be higher and the owner was ready to pay me to mitigate this large facility, but the numbers were fine.

I know exactly what you are thinking and that's why you need to hear from those who do read low numbers, and I read them often.

One question that would explain your situation quite readily: You don't wet drill do you?
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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