Slab over slab

[joncroke]

Hello,

I have a 4" concrete slab in my shop in excellent condition. Every summer there is a tremendous amount of pooling water condensation that results in a mess and hazard. It is also causing the epoxy paint coating to come off. There was no vapor or insulating barrier installed when it was poured.

I intend to solve the problem by covering the slab with an approved vapor sheet, and then an approved insulating board (blue DOW Styro or similar) and then pouring another layer of concrete over this.

My question: what should be the minimumm thickness for the pour over this existing slab?? (Shop only has automobiles as the heaviest equipment.) Is 2" OK?? I was thinking of the fiber reinforced material to avoid metal screen for this top layer.

Any opinions/ advice greatly appreciated!

Thanks
Jon

 

[Ron]

Jon
Considering you wish to put a compressible material between two hard materials, you will need to increase your topping thickness. Two inches is not sufficient to withstand vehicle loading for your conditions. Why do you need the insulation in there? You indicated summer was the time you have the most problem.

If you leave out the insulation and put a vapor barrier on the existing floor, you can then place a two-inch topping. Fiber enhancement is OK, but don't forget that proper joint spacing will be critical to minimizing "random" cracking.

 

[joncroke]

Ron,

Thanks for your prompt reply. The reason Im so bent on getting that insulation in there is that I am convinced that the pooling water (condensation) in summer is caused by the cool earth maintaining the slab at such a low temp during the summer months. With a layer of insulation, I can more easily keep the slab at 'room' temperature which should be alot warmer during summer weather.

But I see your advice is that I cannot go so thin (2&quot because of that insulation..... so much for my 'economizing' approach... so I need another 4".. ouch! But thanks for the info... this is what I need to know!

 

[boo1]

Jon,
Is it condensation or wicking of ground moisture though the slab? Are the problems concentrated near the perimeter of the slab, external water may be entering beneath the slab? Condensation only occurs when the temperature drops below the dewpoint.

I think the real problem is the lack of a vapor retarder (I assume u allowed the mix to cure before u painted). If a vapor retarder or vapor barrier was installed (low- or high-density polyethylene 6 mil), then no problem. You can also help the situation by improving drainage of rainwater.

I dont like the idea of placing the Dow board (compessable) under the slab. Most location the ground tempertures are a desireable heat sink (cool in summer, warm in winter). Consider using 6-mil poly with a 2" thick 3000 psi fiber reinforced mix. Two inch concrete should have control joints 6' apart or in the original locations .

 

[joncroke]

Hi Boo1,

I guess the plot thickens! My shop floor is approx 1500 sq' and about half is covered with epoxy paint and the other half is original uncoated concrete finish. I used epoxy coating on the half where I tinker with autos/planes and spillage of oil/grease is common -- cleanup is always a snap. The pooling of water almost completely covers the painted surface in summer on high humidity days - slippery and as soon as you mop up -- returns by next day. There seems to be no corelation with rain/ground moisture. The entire unpainted area of slab is PERFECTLY DRY! Pretty amazing to see this in person. I guess it is dry because it soaks up the water?? There is truly no change in appearance of this unpainted half of shop floor while the painted area begins the water pooling.

This was supposed to be my dream shop but I feel I really blew it when I have half of the area a mess and slippery during the summer months when you want to use it!
A friend suggested I close the shop door, install an air conditioner/dehumidifier of proper size ($$) and use during the summer months!

Thanks for your analysis and additional suggestions in this matter -- I'll consider anything!

Jon

 

[boo1]

Your comment "in summer on high humidity days" elliminates condensation source. The water is wicking up through the slab.

Have you tried a masonry waterproofer/sealer?

 

[Ron]

Jon,
Before you spend any money on anything, take 2 or 3 pieces of plastic about 1 foot square and tape them down onto the floor. Do this in both the coated and uncoated areas. If you have condensation under the plastic, moisture is likely coming from the concrete. If you have condensation over the plastic, then it's from the air.

Find out the humidity in the area 1 foot above the floor as compared to 5 or 6 feet off floor.

Ron

 

[jheidt2543]

It is unlikely that moisture is coming up through the slab and through the epoxy coating to rest on top of the painted surface. The moisture under the epoxy paint would make the epoxy paint peel. Use the "plastic patch test" suggested by Ron to verify this (the patches should stay on the floor for 24 hours). My guess is that it is condensation on the painted surface. The uncoated concrete is pourous enough to "sponge" up the moisture.

My suggestion is to use a dehumidifier to reduce the moisture in the air and a fan to mix the air in the room.

Let us know how the "plastic patch test" works out.

 

[joncroke]

Thanks for everyone's help.
I fear that there may not be too many warm, humid days left up here in Wisconsin to get the condensation again, but will be ready to try the test as soon as possible.

Interesting, I researched dehumidifiers and related their water removal performance to power consumed. If you compare this ratio to that of an air conditioner (most have water removal specs also) there is little difference!! THIs is great news in that for the SAME electric cost I can use an airconditioner instead of a dehumidifier -- and cool the place for 'free'. What do you think the wife's first words will be next summer (no airconditioning in the home) when she sees that the SHOP is now airconditioned?? !!!

Jon

 

[jheidt2543]

If your choice is between air conditioning the shop and your wife, I think you are in for a cool reception in more ways than one. As a fellow "cheese head", your on thin ice!

By the way, the plastic patch test will tell if moisture is migrating up through the slab regardless of the relative humidy of the air. Just tape it down and check to see if there is moisture trapped between the plastic and the concrete after 24 hours.