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

 

[dfkrug]

......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......

Very well put, Tom.

Contractors pouring slabs have some motivations that differ from those of
the customer. Plastic sheet under the slab slows down the finishing process.
Many contractors want to put sand on top of the plastic. This serves the
same purpose: absorb water from the wet concrete so it can be finished
sooner. Good for the contractor, but bad for the customer.

Some other things I have learned from designing and pouring slabs:

> Use a large aggregate! At least 3/4". In our area, contractors want to
pour slabs with 3/8" pea gravel concrete and have it pumped. Less work,
but weaker concrete. Pumpers here charge more to pump 3/4", too.

> A stiff slump of 3" or even 2" makes a very strong
slab. Stiffer and it is very hard to move; wetter and it is weaker. I have poured with 1.5-2" aggregate at 0" slump and it is the best, but must be
tamped into place.

> Use at least 5-sacks Portland cement/cy. Don't try to save a few
cents by going 4-sack.

> Use rebar and not wire fabric. #3s at 18" OC is plenty, but #4s cost only
a little more and are usually straighter and easier to place. #3s often
arrive bent.

> Plan for shrinkage. Many times it makes more sense to design the slab
as a floating slab, rather than tied to walls, footers, etc. This can reduce
cracking as the slab shrinks.

> You do not need expansion joints if the slab is indoors. (No heating from
large outdoor temp swings.) If the contractor wants to cut or trowel a lot of joints to control cracks, it may be due to cheating on one of the above.

> And, as mentioned, keep it wet. This means for days, not hours. This is
an area where contractors do not have the customer in mind either. I have
seen forms for stem walls being removed the same day as the pour. Bad
for the ultimate strength of the wall. Concrete is specified by its 28-day
cured strength. If it dries out, it has stopped curing! Don't let your new
slab cook in the sun!

 

[EddieWalker]

You guys are making my point.

If you cover your garage floor with water, or even silicone a tube of some kind to your garage floor, will the water pass through the floor into the ground? Of course not. Then why does anybody think it will defy gravity and flow up through three to four inches of concrete?

Lets not compart basements to slabs. I have no experience or knowledge of basements, but can guess the reason a basement leaks is not that water is passing through solid concret, but the joints where the slab and walls meet. If it was poured all in one solid pour, floor and walls, than I doubt it would leak. But again, I don't know about these things.

Lets look at it another way. If after a heavy rain you notice water on your garage floor, why would you assume it traveld up through the floor? Why not through a leak in the wall, under your sills or even a leak in the ceiling?

The only time water travles up hill is when there is preasure on it to do so. I believe it's calledy hydraulic preasure. Swiming pools have this happen to them when a hole is dug and the pool is installed. The table water will fill this void in the ground if the table water is at that level. Table water doesn not rise above the grade of a flat slab above ground. If it did, the water would be forced out the sides of your foundation and you'd have a creek coming from your building. Not a puddle on the floor. Water is going to take the path of lease resistance, and passing through concrete is not that path!!!

Water does not pass from the ground into a building through concrete.

Now putting a vapor barrier down to hold water in the slab does make sense. If you loose more moisture out the bottom of your pour, than you will have an uneven cure of the concrete. I can see this happening on pads that drain really good. Gravel and sand would be perfect examples. Here we build on clay, which hold water very well. We hose the clay down if it's dry, but that's it. The reasoning is the same I believe. To stop the water in the mix from seeping into the ground. Not because some miniscule amount of moisture in teh ground will defy logic and travel against gravity and through a cured concrete floor.

Concrete takes months to cure fully. In a a day, it's almost there, but to fully cure takes a very, very long time. If you've ever put in a floating floor on a new pad, you know to tape a piece of plastic to the floor to check on moisture content. If the spot under the plastic is wet in any way after a day, than the concrete is still curing and you can't put down the floor yet. This moisture is not coming from the ground undernieth, but from teh concrete itself. When it's fully cured, the moisture will no longer be in the concrete. It will be dry all the way through and stay that way for ever.

Eddie

 

[gsganzer]

My brainium is starting the ache, trying to reason this one out.

I can see Eddie's point and I'm beginning to lean that way, but I've heard from an awful lot of different people over the years that the reason for the vapor barrier is to prevent water vapor from migrating up through the slab. We're talking about vapor, humidity, not actual running water. Maybe as the slab warms or cools, it draws the humidity through it. I don't know.

Listening to the reasoning about curing, makes more sense to me. We need to find one of those scientist type guru's on concrete knowledge. Or maybe I'll just stay at a Holiday Inn Express tonight and report back with the correct answer tomorrow.

 

[HomeBrew2]

... defy gravity ...

No "capillary action/motion" in Texas It truely is a unique state

 

[RobJ]

Eddie, The moisture can come up through small cracks in the concrete over time. Here is some Google stuff...

Concrete is naturally porous (12 to 18% air). When concrete cures, surplus water creates a network of interconnected capillaries. These pores then let in liquid water, water vapor, and radon gas. Like a sponge, concrete even draws in water by wicking action from several feet away. And as it ages the pores get bigger and bigger.

Efflorescence - that white powder stuff that appears on concrete..
Efflorescence -- White Fluffy Crystalline Powdery Deposit on Concrete

Concrete Technology | Efflorescence | Portland Cement Association (PCA) Note paragraph 3 and 4.

Vapor barrier - Wikipedia, the free encyclopedia

Building Science Corporation - Technical Resources - Foundations - Sand Layers Should Not Be Placed Between Polyethylene Vapor Barriers and Concrete Floor Slabs

Just a few...

 

[tamu94]

I am having a 30'X50' building built in Lampasas right now. Slab will be poured on Monday. It DOES have a vapor barrier and all three contracters who submitted bids said they always install one.

BTW, concrete is porous and water will leach through it like a sponge. It wont pour out but significant amounts of water vapor will come through (unless its against standing water in which case you cab sometime see water drip through). That is why Thompsons and others make concrete sealers for basement applications. Check out those old concrete cisterns you see around the hill country. You can see the water stain.

My dad made the mistake of finishing a basement without sealing the walls and floor. During the spring rains enough water came through to cause mildew damage.

 

[HomeBrew2]

A fun experiment, I am told, to demonstrate the ability of concrete to store water in it's pores is to just lay a lit cutting torch down on a slab for a few minutes ... please have medical personel handy or pray for good luck

 

[dfkrug]

A fun experiment, I am told, to demonstrate the ability of concrete to store water in it's pores is to just lay a lit cutting torch down on a slab for a few minutes ...

Fully cured Portland cement-based concrete can indeed absorb liquid
water. Concrete also contains substantial water that is chemically
bonded to the Portland cement in what is called a hydrate. Heating
concrete can cause the evaportation of liquid water in the pores, but
it can also break the hydration bonds which give the Portland the
ability to bind the aggregates together.

Fully cured concrete is slightly alkaline, which serves to protect steel
rebar from rusting due to the moisture that inevitably gets absorbed.
This convenient property is overwhelmed when the moisture absorbed
is an electrolyte like salt water in marine environments.

 

[bugstruck]

Good thread...Visible moisture on the surface of a portland based concrete slab that is above grade is often a condensation event, not usually moisture permeating up through the slab. You can determine condensate moisture as typically not visible but visible during periods of temperature and humidity swings as others noted. My garage slab does that in my current residence with no vapor barrier and my rental property that has a 6 mil barrier does the same exact thing. That said, moisture will surely permeate a slab with no moisture barrier. If I set something that traps moisture on the slab in my current residence (no vapor barrier) it will form a whiteish colored mold or mildew growth (not efflorescence as apparant by the odor and ability to wipe clean easily) beneath the object, if not moved over time. The slab with the vapor barrier never does that, no matter how long an object sits there. That growth is fed from moisture that is permeating up through the slab, not atmospheric moisture. May have already been mentioned... but many paints or topical finishes for concrete advise to tape a piece of cellophane to the slab for a relatively brief period of time and confirm whether moisture is emanating up through the slab prior to application. Many topical finishes can't be applied without the vapor barrier installed. Without the vapor barrier some amount of moisture will permeate through to the surface unless you are in a very,very arid and well drained locale> IMO.

Let's see if I can get Eddie to change his mind on this permeation thing? He's got another Contractor on the mix now.... I bet he doesn't get that much permeation in the locales he's lived (California and Texas) so it's hard to imagine what you've never seen or experienced. If he were from the east coast he could actually see it occur here. I didn't used to think it possible either.

EDIT Forgot to mention, the garage slab with the moisture barrier is in a basement back entry (below grade) and the my current residence garage is fully above grade and very well drained. So the one below grade is drier. My above grade slab won't pass the cellophane test and it's been there 34 years.

 

[Egon]

Talked to my son this morning and asked him about the membrane barrier below a concrete slab. [ He is qualified to answere this question.]

He state's that it is to keep moisture from moving upwards through the slab. He gave me an example of a commercial building where they had to install some pumps after the fact to keep this from happening.

Also, rolled plastic is not the best membrane for this type of work.

Sorry Eddie.