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Relative Humidity Probe Depth
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03-24-2011, 07:00 AM #11
CC Solutions Concrete Moisture Evangelist *******
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(03-23-2011, 05:12 PM)mwoody Wrote:  We utimately used the old rule of thumb the CC mentioned for the probe depth, but does that meet the intent of the ASTM and Wagner for acceptabitily?

I found this in the do's and don't thread:

"When measuring concrete poured in a fluted pan, take the thickness of the concrete above the flutes (i.e. 4") and add 1/2 of the depth of the flutes (2" fluted pan means you would add 1" to the 4" for a total of 5") and drill your hole 40% of that measurement. I have not seen official documentation on this issue released yet, so please check beforehand. I believe the new ASTM will address this."

Yes, that was my quote and it may have been from before the official release of Wagner's specification.

I suggest we follow Jason's specification to find the thickest point of the concrete and install probes there.

Now for the arguable points: (because it's me!! Big Grin )

We always must install 3 probes for the first thousand square feet and 1 per thousand after that. We always take the highest reading in consideration. We always investigate why our readings are what they are.

That said, when you see concrete poured in a fluted pan, most of the time the center of a bay will be thicker than the column line beams. Many slabs will be 2" thicker in the center. So when you are told you have a 5" slab, look at some cores a plumber or electrician drilled through the slab and measure them.

Part of an installer's job is to recognize these variances and spread probes out to read these areas. You should always take readings within 3' of an outside wall, the center, each different pour, and anywhere the concrete changes depth.

We don't always know for sure what the concrete thickness is. Even a slab on grade may vary by several inches! The path the cement truck drove over while delivering concrete will be a deep trench, irregular grading and utility trenches will cause variations too. But by sprinkling probes through a job site we will get a solid reading overall, and we can go back and revisit a high reading to find out if something is wrong there.

My next point is, and don't expect any flooring manufacturers to admit this, the 75% RH standard is really low. There's some buffer room built in there. There has to be. I have worked on a lot of projects where we pushed the limit, the RH envelope if you will, and the testing gets really precise because we really need to be sure our numbers are spot on. If a manufacturer agrees to allow 80% or 85%, they want to be sure the slab isn't actually 88% and the readings were off a bit. This is how I first came to depend on Wagner Rapid RH, when we started working on projects where we needed precision RH readings because we were working on multi-million dollar floors where a point or two in RH could mean hundred$ of thousand$ in change orders. (Keep in mind I do not do flooring, this isn't my money we're talking about Tongue )

So I want to leave you with this: Floors aren't perfect, spreading RH probes around minimizes compounding errors, and look into discrepancies for the reason.

JD Grafton
Concrete Answers for Flooring Problems

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