Concrete slab

[CurlyDave]

My experiance has been that its the moisture coming up through the ground not the humidity in the air.

You are absolutely right.

Even if concrfete is waterproof, it does not stop all water vapor from migrating thorugh it.

If upper surface of the concrete is cooler than the earth underneath it, water vapor will condense on the upper surface under many conditions.

In a garage it is just a nuisance.

If you put a non-permeable floor over the concrete, such as linoleum, vinyl, etc., you will soon have a nice crop of mold under there.

 

[EddieWalker]

If upper surface of the concrete is cooler than the earth underneath it, water vapor will condense on the upper surface under many conditions.

Dave, forgive me for being so dense here, but doesn't your statement prove my point?

When air temps come into contact with a surface cool enough to lower the tempature to 100 percent humidity, you have condensation. It doesn't matter if the concrete is 20 feet thick and is also one hundred percent waterproof. You will have condensation on the surface of the concrete.

People use the term or expresion that the concrete is sweating when in fact it is just collecting condensation. I've seen concrete on lakes in all sorts of situations and I've never seen water penitate it to the extent that you have standing water on top of it. This only happens when you have condensation or you have massive cracks or holes in it.

How much moisture is there in the ground under a home slab that it's coming through 4 inches of concrete???? I don't get it.

The same thing is true for a glass of water. When it gets moisture on the outside of the glass, it's condensation. The glass isn't allowing the water to pass through it. The same is true for a concret slab. In fact, how many water tanks and ponds are made from concrete?? They don't leak standing water out of them. Concrete is a wonderful material for making septic tanks and it's used for pipes. I realize that there are different grades of concrete, but even the 80 pound sacks from the store will make a waterproof pond.

Since concrete is waterproof, what does a vapor barrier accomplish?

Eddie

 

[CTyler]

Concrete is waterproof?

I can tell you that if you do not put down plastic, waterproof the walls, block or poured concrete, and have drain tile you will have a damp basement. Unfortunately I have had personal experiance on this as well.

 

[gsganzer]

Eddie, I see your line of reasoning.
It's been explained to me that standard concrete isn't truly waterproof. Sort of like a terracotta pot, but not quite so purous. I think the concrete does leak water "vapor". Kind of like Goretex I think. Water doesn't run through it, but the "sweat vapor" will. The only time you actually see it as condensate is when there isn't enough air movement to keep it evaporating from the immediate surface, allowing it to condense and form droplets on the surface.

I think most of us are missing the point that there has to be water present under the slab for there to be a problem in the first place. here in north Texas with my black clay soil, I guaranty that water migrates under my slab. I can water 10 feet away and the water will wick in all directions. In sandy soils, the water will permeate the sand and travel downward. In clay soils, it tends to travel furthur horizontally. It has to do with the surface tension of water, the pore spaces in the soil and the capillary actions of some soils.

If you're sure you have properly drained soils, adequate slope from the slab etc, etc, then no vapor barrier is probably alright. If you can't be sure of all of those things, then a vapor barrier is very cheap insurance.

 

[mopacman]

In kansas City in the spring when the ground thaws out is when the concrete will swet. The roads will do this to, It will make them slick.

 

[Tom_Veatch]

There is a statement in this thread to the effect that the dryer the mix the stronger the concrete. That is true only to a point. Obviously, the driest mix possible is no water at all which, just as obviously, has no strength at all.

Concrete should be mixed with no more water than necessary for the chemical bonds to form with the cement and aggregate during the cure. But, that is a very stiff mix which is difficult to work. Extra water is usually added to improve the workability of the mix but that does reduce the ultimate strength of the cured concrete.

What you do not want is for water near the boundaries, i.e. top, bottom, or sides, to be leached out as the concrete sets. That adversely affects the chemistry of the cure and the resulting strength of the cured concrete.

Concrete poured on a dry, absorbent surface will lose a significant portion of its moisture to that surface. It will also lose moisture to a dry atmosphere. The concrete "drys" instead of curing. You need to keep the water in the mix until the concrete cures. Believe it or not, all other things being equal, the strongest concrete is that cured underwater.

That's why you need to keep a pour's exposed surface moist - resorting to a fine, misting water spray, if necessary. It's also why you should place a vapor barrier under the concrete to restrict moisture loss to the soil. That does cause a poured slab to take longer before it's ready to finish, which is why, as one person noted, many finishers don't like vapor barriers - they don't get to go home as quickly. But it does result in a better, stronger slab.

Whether you mix the concrete yourself or buy it pre-mixed and delivered to the site, you are paying for a certain strength. In my opinion, it's pointless to effectively throw away much of what you've already paid for by allowing the concrete to dry out before it cures.

Or, to put it another way - a vapor barrier won't hurt, and it can help.

 

[Egon]

http://http://www.cement.org/tech/cct_floors_moisture.asp

Check out " The Portland Cement Association " Could be interesting and give some proven advice. The site has proper information backed by emperical testing.

 

[BB_TX]

The slab in my metal building (30'x40') has plastic under it and it "sweats" in cool humid weather. Actually condensation forms on it.

 

[Farwell]

Eddie,
I live in Mid Michigan and in the early Spring when the ground is thawing out my attached garage actually has puddles of standing water on it. The moisture is coming up through the concrete or out of the air. There is one other possibility, all of the years that previous owners parked their salt covered vehicles on the slab leaving behind 60 years worth of salt deposit could have something to do with the moisture problem. The slab has been washed many times but that does not seem to make any difference.
None of my other slabs have a moisture problem with or without a moisture barrier.
Farwell

 

[RobJ]

After a few months, the ground under a slab is so dry that when you go to break out any part of a slab, you normally need a jackhammer just to dig in it. There is none, to very, very little moisture in the soil under a slab.

I would think the moisture under a slab remains fairly constant. Otherwise the ground would shrink causing slab problems. A case and point is this past weekend my son and I repaired the slab inside our garage. Previous owners had patched but it had failed and looked bad. I dug it out to the dirt(black plastic was there). Found a couple roots. There was a half inch gap between the concrete and the dirt as far as I could stick my hand it. I presume it's from the tree removing the moisture and the ground shrinking. My house was build in 75. The break happened in an area where a cars drives over it AND where they decided to stop putting in some 6x6 wire. Yep of the 3' diameter hole I dug out, only about a foot had any wire. 5 80# bags to fill the hole. In a couple places the frigging concrete was less than 3".

Rob