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High Rapid RH readings??

Hi, I'm new to using rapid RH and concrete moisture testing and would appreciate any feedback on the following:
-New 2500sq.ft. slab 6" thick installed over vapor barrier over 12" gravel foundation.
-trowel finished with no other products added (no sealer or other)
-installed 3 RH probes at 40% depth after 1 month of pour. Sensors read 97% after 1 hour and 99% ever since.
-Its now been almost 3 months since the end of the water cure and still 99%RH from all 3 probes
-The RH in the gym was about 50% and is now 30% for about a month already. Temp. of gym has been around 70 F
-Is it normal to still have 99% while other test types have shown a drop?
-Are the probes malfunctioning and should I have waited longer prior to installing them?
-Should I go ahead and install another probe to be sure of the readings?


Dr. Demento?

All I do is Gym floors. So, I am very familiar with any issues you might have. I regularly see slabs that don't drop below 99% for many months.

1st - where are you and what is the exact finished floor to go on top? No offense but I don't like to help the competition.

1) What other test types have shown a drop? CaCl - worthless when it comes to gyms. Typically to get down to 92% rH I tell clients to wait 3 months for the first 4" and 1 month for each additional inch. And then depending on slab add another month or two.
2) Is the slab enclosed? Is there a roof on? I have had slabs tested at 1 month and had them be accurate over time. Did the slab / probes ever get re-weted? Sprinkler, rain, plumbing, etc... Any external moisture re-sets the 3m+1m/1" clock and also that clock runs slower on re-weting.
3) You can install another probe if it will make you feel better - they are so cheap in the grand scheme of things.


Yes, Dr. Demento. I used to listen to him all the time when I was a kid. Now he's doing gym floors.

Here is a thread that may interest you:

I would definitely install at least two more probes. If condensation may have formed on the existing probes at any time they may be problematic. Also that extremely humid and alkaline environment they have been sitting in is a difficult one. The accuracy of the sensors in that humidity range is subject to debate (again Admin, censor me as necessary Wink ). I think I would get agreement that nobody will bet their life on the exact RH when the probes get above 95%.

Read a little about hysteresis and put in two new probes and see what you have. Remember to clean the holes well with the brush, vacuum well, and let us know what you find.

JD Grafton
Concrete Answers for Flooring Problems

My floor is in montreal, Canada.

The floor will be a "floating" hardwood floor composed of a neoprene spacer, 2 plywoods, and hardwood to finish it off.

The other tests have been a nucleodensimeter done by my client.

Slab is brand new poured slab inside an old school, therefore enclosed and covered by a roof...no moisture or re-wetting either from other sources. Dehumidifiers and fans have been going for over 2 months now to keep the humidity down.

I think like cc solutions said, maybe the excessive moisture initially caused a malfunction of my probes. although it surprises me that all 3 would malfunction.

I will go ahead and install 1 more probe just to see the new reading. i will report back. Thanks for the help.

'Malfunction' is such a strong word... I'd prefer to say they have experienced high humidity and alkalinity and are now behind the curve of measuring. Did you get a chance to look at the other thread where we discus hysteresis?
JD Grafton
Concrete Answers for Flooring Problems

What is a nucleodensimeter?

You have to PM me your / project details. I suspect we know each other. Might your firm have a branch in Vermont or California?


(11-07-2011, 01:20 PM)eaadams Wrote:  What is a nucleodensimeter?

I've never seen one, but I am a young man yet. Tongue

Let's break it down, a densimeter is a device to measure density of objects, and a nucleo is a big bomb, so a nucleodensimeter would measure how dense a bomb is.

Or not. Huh

Not all words can be broken down to decipher their meaning, like military intelligence. That is a misnomer.

I suspect the tool ddemento is referring to measures density or porosity.

As long as we're discussing tools, and I have a propensity to wander off topic (Einstein did this commonly too... you know I am making that up.. unless it's true) did you know Jason cannot use a stud finder? They just point right back at him. Odd huh? Huh

Sorry, I'll now return you to your regularly scheduled post.
JD Grafton
Concrete Answers for Flooring Problems

A nuclear density gauge is a tool used in civil construction and the petroleum industry, as well as for mining and archaeology purposes. It consists of a radiation source that emits a directed beam of particles and a sensor that counts the received particles that are either reflected by the test material or pass through it. By calculating the percentage of particles that return to the sensor, the gauge can be calibrated to measure the density and inner structure of the test material.


I don't think it has anything to do with concrete moisture.

See? It measures density. Cool

The wetter the concrete is, the denser it will be.

But there is such a variation in concrete mixes, I can't imagine this form of testing is very useful for moisture content evaluation.
JD Grafton
Concrete Answers for Flooring Problems

(11-07-2011, 02:01 PM)CC Solutions Wrote:  See? It measures density. Cool

The wetter the concrete is, the denser it will be.

But there is such a variation in concrete mixes, I can't imagine this form of testing is very useful for moisture content evaluation.

Moisture content? I that like moisture content in hardwood?
Stephen Perrera dba
Top Floor Installation Co.

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