Garage concrete slab sweating

   / Garage concrete slab sweating #1  

flyer

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Apr 14, 2004
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Location
Ithaca, NY (upstate)
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1989 John Deere 855
I've got a problem that I am hoping to get some advice on. My garage's (detached 2-bay with workshop in the back) concrete slab is doing some serious sweating (i.e., condensation or what not), enough to create small puddles in certain places.

Can you recommend a course of action?

I was thinking that a concrete sealant might do the trick, but don't know any good brands.

Thanks in advance.
 
   / Garage concrete slab sweating #2  
If this is a brand new slab (<28 days typ.) that does not have a hard power trowelled finish, it could the free water remaining from the hydration (curing) process of the concrete working its way out through the open pores of the concrete. (not very common) occurs mostly when you spread a very wet concrete mix.

If this a slab that has been around for a while, is it possible that the slab is very close to ground water and the slab has very little or no underdrain capability? People do not realize that concrete in totally impervious to water, it can weep thru if it has not been sealed.

Just two quick thoughts, in all reality the could be many causes of this. Have you made any additional observations in which you elaborate on the detail? Do you in fact believe it is coming through the concrete or just collecting on the surface?

engineer
 
   / Garage concrete slab sweating #3  
I don't know that there is much you can do. /forums/images/graemlins/crazy.gif The cement slab is basically a giant heat sink, and is naturally going to be cooler than the surrounding air. If the air becomes really saturated, i.e. high humidity, then the cement is going to act just like your iced tea glass in the summer, and will get a ton of condensation. If the garage is fairly well sealed, you can consider running a de-humidifier, but I don't know of any (inexpensive) de-humidifiers that can handle a 500-600 sq ft room.

Sealing the cement might help to protect against any damage caused by the water, but I don't think it will stop it in any way.

Dave
 
   / Garage concrete slab sweating #4  
My shop is 50x64 and use a free standing dehumidifier. I keep it at about 60%. Works just fine and it only runs intermitently. Holds about gallon of water and I empty it once a day or two.
 
   / Garage concrete slab sweating #5  
</font><font color="blue" class="small">( My shop is 50x64 and use a free standing dehumidifier. I keep it at about 60%. Works just fine and it only runs intermitently. Holds about gallon of water and I empty it once a day or two. )</font>

Wow. I was about to ask if you are someplace really dry, but NE gets just as humid as IL. Just out of curiosity, have you ever placed a hygrometer at the other end of the shop, opposite of the de-humidifier? The reason I ask, is we have a free-standing one also in the house, and it pulls so much water that I had to run a drain line, other wise it kept shutting off after less than a day of operation.

Dave
 
   / Garage concrete slab sweating
  • Thread Starter
#6  
the slab is about 13 years old. the turf around the garage gets really wet and soggy after rainfall, so i imagine there's plenty of moisture below the slab. As far as whether or not the moisture is coming up through the concrete or not, i'm not sure. i do know, however, that the moisture is not coming from outside the garage (other than the air itself). it is either coming up through the concrete, or condensating from the surrounding air.

What I am hearing is there is nothing I can do if the moisture is condensation from the air, other than using a de-humidifier. In addition, it probably can't hurt to seal the concrete to protect it.

Can you guys suggest a good concrete sealer, or will any do?
 
   / Garage concrete slab sweating #7  
A simple, cheap way that might work if it is just condensation (sweating) is to put a fan in to keep the air moving across the floor. That will even out the temp between the air and the floor - some. Raising the concrete temp (heat at night) or lowering the daytime air temp (AC during day) will help in addition to the dehumidifier. Not always practical, but it will help.

If the ground around the slab is wet, then sloping it away and getting that ground drier would help lower the water table under the slab.

I don't think a concrete sealer will help, and may just make a mess if it is below-ground water coming up through the concrete. Just my feeling though.
 
   / Garage concrete slab sweating #8  
I had a drain line running across the floor at first until it dried up. It was a pain having the hose running to the floor drain. Seemed like it was always in the way. Now I just empty it once a day, on average probably 5 days out of 7.

The unit has a digital meter on it and I also have a cheap thermometer w/hygrometer in the shop. The unit is set at 55% and the hygrometer runs between 58-65%. I also have a fan running in on low all the time to keep the air moving.
 
   / Garage concrete slab sweating #9  
Dave; Could be temp differential causing the final problem, but my vote goes to moisture migration from under the slab. Do you know if a layer of visqueen was placed before the slab was poured? If so It would most likely just be from temp diff. If not it's probably migration aggravated by the temp/diff.

Also look at your grading outside, that's a pretty obvious fix. There are a number of good concrete sealers out there, although they're names escape me now. I'd use an epoxy based one if it was me.

Other than slowing down the curing process making a harder concrete slab, the visqueen also doubles as a moisture barrier under the slab.
 
   / Garage concrete slab sweating #10  
You asked for a recommendation for a concrete floor sealer. I decided not to use an epoxy based sealer. I have a Seal Krete product I am going to put down after construction is completed.

Seal Krete has a lot of products for sealing concrete and some advice too.

I'd recommend finding out where the water was comming from before you put down a sealer.
 
 
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