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Light Weight Concrete on Pan Deck

What recommendations do you have to bring down the relative humidity of a light weight concrete pan deck that was installed over a year ago but is still reading 95% RH with a Wagner probe. The slab is 2-1/2" over a 9/16" steel deck for a total slab thickness of 3".


Lightweight can be a real bear to get dried out, but the process is the same as for standard weight concrete.

Be sure the surface is open and porous. If it is sealed with chemicals or burned black by a steel trowel you should grind or blast the surface open.

Warm air holds more moisture than cold air. Warming the air and the slab will promote moisture transfer out of the concrete.

Moving the air and bringing in drier air also promotes moisture movement out of the slab. Dehumidifiers can help a lot.

If all else fails or you don't have the time you can always use a mitigation system.
JD Grafton
Concrete Answers for Flooring Problems

Thanks for the feedback. We are using a Wagner model with separate probes. Given that the slab is 3". Are we correct in that the depth of the hole for the probe should be appx 1-1/6" deep? We were reading and came across a post that said the hole is to be 40% the slab depth. The holes that were used for the previous test are over 2" deep.

The depth of the hole will be 40% of the slab thickness when the slab is able to dry in one direction only.

On a 3" slab, 40% is 1.2 inches or about 1 3/16" deep.

The slab will be wetter at the bottom than at the top because it dries from the top. So drilling deeper ( as you said 2" was drilled) should result in higher than expected readings.

Remember to drill the hole dry, keep your bit straight so it doesn't wobble and elongate the hole, clean the hole out well and then re-clean to be sure using a vacuum and the brush, then insert the probe fully into your 1 3/16" deep hole. A little bit of the probe tube will be sticking out of the slab and that's okay.

Wait a couple of hours for a good reading.

Let me know if you need more help, I can be reached at my email addy almost always. And let us know what your new readings are!
JD Grafton
Concrete Answers for Flooring Problems

We installed another probe being extra careful to keep the proper 40% depth but it made no difference regarding the RH. We read 100% RH after 1 hour. We added several commerical fans through out the floors which help dropped the RH by appx 5-7% within 1 week. In addition, after some research we found that although the Fasol Tile by Toli requires 75% RH for installation, the adhesive recommended by Toli for installation is CBC 5100 acrylic adhesive which allows for a 90% RH limit. Therefore, we were able to proceed with flooring installation upon reaching 90% RH. Hopes this helps everyone.


Cute so if an attorney were to read this they could say the material is good to X rH but the adhesive is good to Y rH. ... X% < Y% .. So, if you have a failure, the X product has an out and the Y product is covered.

Or, at least, that is how I read the Warranty.


I hope the installation works well for you! (now stop reading arlington Tongue )

For others reading this, I have come across adhesives that are supposed to work to 90% and when they fail the manufacturer points to alkalinity, slab cleanliness or a myriad of other 'issues'.

From the manufacturer's data sheet for CBC 5100:
Concrete substrates must comply with limitations of moisture and alkalinity, with in-situ Relative Humidity per ASTM F2170 not to exceed 90%, and substrate readings between 7.0 and 11.0 pH. The concrete slab must be clean, dry smooth and flat.

From the above disclaimer I can see if the surface reads more than 11pH they can deny warranty, or if the slab is dirty, dusty, etc. they can deny warranty, and the word that really bugs me is 'dry'. What do they consider 'dry'? Every time a floor fails and we open it up there seems to be a good chance there will be high alkalinity and the adhesive will be destroyed and 'wet'.

Adhesive manufacturers will also commonly tie their moisture requirements to the flooring limits. Meaning you can have an adhesive good to 90% but if the floor is only good to 75% then that's all they will warrant to....

Speaking of warranty, there is none listed on the CBC website, in fact, they don't list the CBC 5100 adhesive either. They list no adhesive warranties at all. I wondered if the warranty is a replacement bucket of adhesive, which is pretty common for most manufacturers.

Okay I just wrote to CBC to get the warranty using the product with another manufacturer's product, such as a VCT or whatever. I asked just exactly what is your warranty? Is it a new bucket of glue or will you replace any defective areas or what? They replied:

Hi JD it s incorporated in our flooring products warranties. We do not offer a stand alone adhesive warranty.

I'm kind of a pessimist because I see these things play out every day. People WANT to do the right thing and a good job, then something happens and they get the fine print thrown back at them..... Sad
JD Grafton
Concrete Answers for Flooring Problems

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