Concrete taking too long to dry

[oldiemotors]

I am about to apply Rust Bullet on my new (several months old) concrete slab. I used a diamabrush with water and followed with power washing. After one week of running the AC to help dry out the building/slab I started a calcium chloride moisture test. After a three day test interval I pulled the pellets and weighed them.

The results showed an emission rate of 10.1 lbs. Rust Bullet says the concrete should have a 3lbs or less emission rate before applying so I waited a week and started another test. The second test show almost an identical rate at 10.3lbs.

During these tests I had the AC running at 78 degrees and my humidity measurement was constant at about 50% (that's low for south Texas). I also did a low-budget test of taping a sheet of plastic to the floor and when I removed the sheet there was no dark area under the sheet.

So my question is, can these calcium chloride test be wrong? It has been several weeks since I pressure washed and have maintained a 78 degree 50% humidity in the building. It has to be dry by now...right? It has been raining during the last 3-day test, but I don't think this is affecting the test results because the building is closed and insulated and the humidity is remaining constant.

 

[BlackTalon]

There is no hard and fast rule as to how long the concrete will take to dry sufficiently. Running A/C, etc. may dry out right at the surface, but moisture will still be a little deeper in the concrete, which could lead to coating failure. I am guessing you did not test prior to washing the slab, so it is hard to tell if the slab is wetter now then it was before the wash, or if it is steadily drying.

I am not familiar with the calcium chloride tablets. There at RH probes available for commercial use that give goods indications of the actual moisture level in slabs. They require drilling holes though so the probes can be dropped down into the concrete cross-section.

 

[Radix2]

What is below the slab ? vapor barrier? Foam? Sand bed ?

maybe there is a good sized reservoir that needs to be worked through ?

 

[oldiemotors]

What is below the slab ? vapor barrier? Foam? Sand bed ?

maybe there is a good sized reservoir that needs to be worked through ?

They laid a plastic sheet on top of sand so I would like to believe I have a decent moisture barrier under the concrete.

Radix2, I didn't test before pressure washing so I don't know how moist it was before pressure washing.

 

[Radix2]

They laid a plastic sheet on top of sand so I would like to believe I have a decent moisture barrier under the concrete.

Radix2, I didn't test before pressure washing so I don't know how moist it was before pressure washing.

yeah too bad about not having a before reading. I wouldn't think that you would saturate the slab really well just from a few hours of grinding anyway.

Maybe the coatings guys will chime in with some more insight into calcium testing...

 

[Evilunclegrimace]

How about running a dehumidifier in the shop for a few days? See if it helps pull any moisture from the slab.

 

[oldiemotors]

I was hoping that my AC would be enough of a dehumidifier, but is only getting the humidity down to 50%.

If I did the math right, for my 1500 sq/ft with a calcium chloride test emissions reading of 10, that is 1.85 gallons of moisture/water coming from the slab every 24 hours. Sounds like a lot for two weeks. I was expecting to see a significant drop in the emissions reading for my second test. Am surprised to see almost the exact same reading.

I see a lot of folks testing by just using the plastic sheet taped down. If I went by that test I would be good to go. But the calcium chloride test is saying I am 3x too wet to seal the floor!

 

[pstnbly]

The chloride is absorbing moisture from the atmosphere.

 

[Shea]

The chloride is absorbing moisture from the atmosphere.

No, the kit is sealed from the atmosphere if the OP is performing the test correctly.

I believe someone touched on the issue earlier. Since you have a vapor barrier under the slab, the only direction for moisture to escape is up. By grinding wet and using a pressure washer, you most likely have a small reservoir (relatively speaking here) of moisture that is going to take some time to work its way out of the concrete.

You may be better off just running a fan across the surface and leave the AC off. Warm air wants to rise and when it does the moisture will come with it. Keeping it cool is most likely slowing down the drying out process.

 

[theoldwizard1]

First, the curing process removes all water that was in the mix (assuming a proper mixture). But a full cure is 28 days. And concrete does not "dry". The water combines with the chemical in the cement, so the water is technically absorbed.

I believe someone touched on the issue earlier. Since you have a vapor barrier under the slab, the only direction for moisture to escape is up. By grinding wet and using a pressure washer, you most likely have a small reservoir (relatively speaking here) of moisture that is going to take some time to work its way out of the concrete.

You may be better off just running a fan across the surface and leave the AC off. Warm air wants to rise and when it does the moisture will come with it. Keeping it cool is most likely slowing down the drying out process.

This gets my vote !

 

[oldiemotors]

The test kit had a dome that seals to the floor. Was definitely a good seal as now i am having to really work to get that seal goo off my pristine surface.

Thanks for the comments/replies everyone. I think i favor the idea that the water from my pressure washing has been trapped and is slowly working its way out. I still struggle with why taping down a sheet of plastic does not show any moisture.

Might also have to look into a dehumidifier and turning off the AC. Been raining for a week now so that isn't helping.

 

[BlackTalon]

FWIW, the 'window pane test' hasn't been considered a reliable test in the commercial floor coating/ covering industry for several years now...

 

[oldiemotors]

FWIW, the 'window pane test' hasn't been considered a reliable test in the commercial floor coating/ covering industry for several years now...

I was not aware of that. Thanks.

 

[Armorpoxy]

We have seen false positives where the plastic dome wasn't affixed properly to the floor and moisture migrated in. Perhaps redo the test, they are very inexpensive.

 

[Radix2]

I disagree about turning the ac off unless you have low humidity outside. At 78 degrees with the ac on, I don't think it is slowing you down. Adding a fan is an excellent idea if you have not done so, it will increase the surface evaporation many fold. If your ac has a dehumidify mode, use it with an external fan.

Good luck!

 

[oldiemotors]

We have seen false positives where the plastic dome wasn't affixed properly to the floor and moisture migrated in. Perhaps redo the test, they are very inexpensive.

I started a 3rd test and this time I made sure it is sealed to the surface. In addition to the dome's sealer I used some AC foil tape around the dome's perimeter.

I do have my 42" fan running constantly along with my AC. It is the only way I can keep the humidity down, short of buying a dehumidifier.

 

[FullRaceMerc]

What type of A/C unit? How does condensation leave the A/C?

 

[oldiemotors]

What type of A/C unit? How does condensation leave the A/C?

It is a 4 ton R22 split system. The compressor sits outside and I suspended the air handler and condensor from the ceiling. The condensor coils have a 3/4" pvc pipe exiting at the bottom that I connected to a vent pipe. From there the condensate goes into my septic system.

Here is a photo, the AC in the top right.