Garage concrete slab sweating

flyer · Aug 27, 2004

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.

engineer · Aug 27, 2004

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

DaveNay · Aug 27, 2004

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

CTyler · Aug 27, 2004

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.

DaveNay · Aug 27, 2004

( 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. )

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

flyer · Aug 27, 2004

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?

beenthere · Aug 27, 2004

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.

CTyler · Aug 27, 2004

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.

johnday · Aug 28, 2004

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.

v8dave · Aug 30, 2004

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.

jfh230 · Aug 30, 2004

It sounds like a vapor barrier problem. Typically when pouring a concrete slab you put down 4" stone over the compacted dirt, then a layer of poly vapor barrier and then pour the concrete. The stone base helps with draining away any underground moisture and the vapor barrier prevents the moisture from reaching the slab. If you don't use at least 6 mil poly then the poly will get destroyed with holes when pouring the slab. Holes in the poly allow the moisture thru.

A quick check is seal down a two foot section of the slab with plastic being careful to fully tape all edges securly. Come back in 24 hours. Moisture on the top of the plastic is condensation, moisture under the plastic is vapor barrier problems.

If you have a vapor barrier problem most sealers will not help and will make any future attempts at fixing more difficult because you will have to remove the sealer (sandblasting or shotblasting). The only product I have had any type of success with in fixing vapor barrier problems is hydraloc. Its not so much a sealer as a penetrant which soaks into the concrete and fills up all the little voids. If I remember it will go down a couple of inches into the concrete. As it cures it turns solid and becomes a permanent part of the concrete. I've had some sucess with this in older basement slabs that I know don't have a vapor barrier.

The company is enduroseal www.concretesealer.com

I think it was about $150/5 gal pail. You buy it direct from manufacturer. Keep in mind that when dealing with a vapor barrier problem the only 100% solution is to rip up the slab and install it again. Nothing you put on top is going to work 100%.

Good luck
Jack

DaveNay · Aug 30, 2004

( A quick check is seal down a two foot section of the slab with plastic being careful to fully tape all edges securly. Come back in 24 hours. Moisture on the top of the plastic is condensation, moisture under the plastic is vapor barrier problems.)

Hey, that is an excellent method! I never thought of that....nice and simple! /forums/images/graemlins/wink.gif

Dave

flyer · Aug 30, 2004

Thanks Jack. Great idea on determining the problem! I will try that over the next day or so. I don't know how the slab was originally constructed, and of course I would like to do anything short of tearing it up and re-doing.

Junkman · Aug 30, 2004

another quick check is to lift a box or can that has been sitting on the floor for a few days, or months in my case. If it is wet underneath, then it is coming from underneath. If dry, then from above...... If you have a neat garage, then this probably won't help..... /forums/images/graemlins/grin.gif

DaveNay · Aug 30, 2004

( another quick check is to lift a box or can that has been sitting on the floor for a few days, or months in my case. If it is wet underneath, then it is coming from underneath. If dry, then from above...... If you have a neat garage, then this probably won't help..... /forums/images/graemlins/grin.gif )

....or coming from something inside the box that is now leaking. /forums/images/graemlins/tongue.gif

Dave

Egon · Aug 30, 2004

After finding out where the water is comming from you may be able to install a little sump that gets pumped out.

Think I'd stay away from a sealer if the water is comong from below.

Drainage trench ending in sump outside may also be a solution. Probably work the best .

Then again maybe you are working the slab too hard and its just natural sweat!!!

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

Egon

johnday · Aug 30, 2004

jfh20; Right you are, up here we try to use clean sand in place of gravel, using that minimizes even more the chance of not puncturing the visqueen. BTW, the trick of laying a piece of plastic on the floor also works for basement walls. I remembered that all by myself once you jogged my mamory. /forums/images/graemlins/cool.gif

engineer · Aug 31, 2004

A product that I have used to seal manholes and pump stations both old and new is Aquron 2000, see the following address: http://www.aquron.com/products/aqu2000.htm

Have had quitre good success with the product. It will soak in to the concrete similar to a wood preservative on pine, this is where the similarity ends. The Aquron 2000 will bond the free water remaining in the concrete (even after 14yrs.) essentially creating a pseudo-crystline gel thus sealing the concrete. The surface of the slab will remain unchanged.

If moisture truely is seeping through the concrete an epoxy type surface finish could cause the moisture to build up in the concrete and potentially cause the epoxy finish to spawl.

engineer