Concrete slab

[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.