Study: Vapor Barriers Under Concrete Slabs

lakeroadster · Mar 6, 2016

Excellent article with testing to back up various methods: Vapor Barriers Under Concrete Slabs

From the article:

The American Concrete Institute’s (ACI) Committee 302 “Guide for Concrete Floor and Slab Construction” suggests that a vapor barrier may not be necessary where no drainage or
soil problems exist and in arid regions where irrigation and heavy sprinkling are not done.

.... vapor barriers can affect the behavior of the concrete slab and significantly influence finishing time, cracking, and strength.

... ACI 302 recommends using a 3-inch-thick sand layer over the vapor barrier before concrete is placed.

Finishing Time

.. placing concrete directly on a vapor barrier increases finishing time and the possibility of surface defects. ACI 302 also says that a vapor barrier directly under a concrete slab aggravates plastic shrinkage cracking.

..There was extensive cracking in the slabs placed on polyethylene and little cracking in
the slabs placed over sand or cement-treated sand.

.. less water should be used in concrete that will be placed directly over a vapor barrier.

..Strength

Concrete placed over a sand bed was more than 30% stronger than concrete cast on the polyethylene.

kerr · Mar 6, 2016

Sand has always been the best thing under a slab if you ask me . The poly will turn to baby powder in a couple of years anyway , never understood its use . A tight finish is the best sealer on concrete .

Good read , Thanks .

Jess · Mar 6, 2016

Interesting assumptions but I would disagree with the suggestion that concrete placed over a sand bed was 30% stronger than concrete cast on polyethylene.
A good properly compacted base, sand or crush will support a slab that has reinforcement and compressive strength without failure, The poly won't be a factor unless one of these specifications is inadequate or not followed during placing.
In my climate, I would never pour a slab without a poly vapour barrier under and if it was in residential construction, it is a requirement. The concrete guys are always looking for shortcuts and that is where the issues largely come from.
Poly vapour barrier, under concrete is not subjected to UV light and will not deteriorate unless it is an inferior type, not suitable for the use. I have cut into slab on grade floors that were over 30yrs old, with foam over poly on a compacted base and it was like the day it was installed. Your experience may vary and my opinion is just that.

DougWil · Mar 6, 2016

The American Concrete Institute’s (ACI) Committee 302 “Guide for Concrete Floor and Slab Construction” suggests that a vapor barrier may not be necessary where no drainage or soil problems exist and in arid regions where irrigation and heavy sprinkling are not done.

^ That is probably the biggest problems with concrete slabs.
Improper drainage because of site leveling, instead of building on a mound where water drains away.
Water under the slab/footing from runoff, landscape watering causes all kinds of serious problems like non uniform settling, clay expansion, mold, etc...

lakeroadster · Mar 6, 2016

Jess said:
Interesting assumptions but I would disagree with the suggestion that concrete placed over a sand bed was 30% stronger than concrete cast on polyethylene.

Assumptions? They cut actual core samples and the 30% was based on actual testing.

That's pretty much the reason for this thread: Data based on actual testing.

Rookie2 · Mar 6, 2016

Published 1992

DougWil · Mar 6, 2016

lakeroadster said:
Assumptions? They cut actual core samples and the 30% was based on actual testing.

That's pretty much the reason for this thread: Data based on actual testing.

The slabs w/o the barrier used the available moisture to cure and increase strength for years after it was cast.

I have drilled holes in slabs that were quite soft years after being placed, yet closer the perimeter and available moisture were much harder.

The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Concrete continues to get stronger as it gets older. Most of the hydration and strength gain take place within the first month of concrete's life cycle, but hydration continues at a slower rate for many years.
http://www.cement.org/cement-concrete-basics/how-concrete-is-made

lakeroadster · Mar 6, 2016

Rookie2 said:
Published 1992

Perhaps the earliest known occurrence of cement was twelve million years ago

matt_i · Mar 6, 2016

Here is a .pdf discouraging the use of sand under slabs subject to heavy traffic.

http://www.concreteconstruction.net...se Sand Under Concrete Slabs_tcm45-343513.pdf.

For me its pretty simple. Compact washed limestone and its very hard, as measured with your heel as an indenter as you walk around. Sand on the other hand, can barely get traction.

Leaflessshadetree · Mar 6, 2016

lakeroadster said:
Perhaps the earliest known occurrence of cement was twelve million years ago

It doesn't mention if the 12 million year old concrete had a vapor barrier or sand base. :lol_hitti

Tejay · Mar 6, 2016

The vapour barrier is a building code requirement for moisture migration and radon gas preventative.

ms fowler · Mar 7, 2016

matt_i said:
Here is a .pdf discouraging the use of sand under slabs subject to heavy traffic.

http://www.concreteconstruction.net...se Sand Under Concrete Slabs_tcm45-343513.pdf.

For me its pretty simple. Compact washed limestone and its very hard, as measured with your heel as an indenter as you walk around. Sand on the other hand, can barely get traction.

I think you are confusing bearing with shear strength. It is true that sand has no shear strength, but it also has the ability to transfer the load directly to whatever lies beneath.

The sand layer above the vapor barrier will also decrease the tendency of concrete to curl near the construction joints. Measuring FF and FL is now a (seldom enforced) requirement in the newer ACI specs that are typically cited in most commercial building specs.

buening · Mar 7, 2016

There are industry standards on the minimum thickness for a vapro barrier, at least in commercial construction. The thin sheeting will puncture and deteriorate over time, whereas the stuff actually made to be a vapor barrier will last and not get punctured as easily.

ACI has since updated their recommendations on vapor barriers and the sand blotting atop the vapor barrier. See the following links for more info:

http://www.cement.org/for-concrete-...rete-construction/vapor-retarders-in-concrete

http://www.concreteconstruction.net...n-new-aci-3021-concrete-floors-guide_o_2.aspx

From the 2nd link: Basically, if the intended use of the building above the slab involves anything that could be sensitive to moisture (flooring, adhesives, underlayments, coatings, goods stored on the slab), or if the building will have a climate-controlled environment, a vapor barrier is required. And in almost all cases the slab should be placed directly in contact with the vapor barrier. The old way of placing the vapor barrier and covering it with a blotter layer of granular material, that inevitably ends up trapping water next to the slab, has largely been abandoned by Committee 302.

SH7mi · Mar 7, 2016

I built our house in SE Pa. In 1996. Idk if the vapor barrier was code at the time but I had the concrete company omit it in basement and garage due to surface cracks and weakening the barrier causes. As for the 'radon issue'... That's a topic for a different thread.

Jazz1 · Mar 7, 2016

I use the Stegowrap vapour barrier on floor of portable garage to keep moisture out. That stuff does not puncture easy at all. I think its 8mm

lakeroadster · Mar 7, 2016

buening said:
ACI has since updated their recommendations on vapor barriers and the sand blotting atop the vapor barrier. See the following links for more info:

http://www.cement.org/for-concrete-...rete-construction/vapor-retarders-in-concrete

http://www.concreteconstruction.net...n-new-aci-3021-concrete-floors-guide_o_2.aspx

Thanks for those links. Here's the flow chart, for folks that don't like to read links. Make sure to read the notes at the bottom of the chart...

I'm a lefty, in this instance

cbracer · Mar 8, 2016

Wow, tell someone no vapor barrier and they sometimes just won't believe you. What was originally posted is true. Vapor barriers are not required everywhere, and should be avoided whenever possible. But hey if you want cracks then use one.

joes169 · Mar 8, 2016

buening said:
There are industry standards on the minimum thickness for a vapro barrier, at least in commercial construction. The thin sheeting will puncture and deteriorate over time, whereas the stuff actually made to be a vapor barrier will last and not get punctured as easily.

ACI has since updated their recommendations on vapor barriers and the sand blotting atop the vapor barrier. See the following links for more info:

http://www.cement.org/for-concrete-...rete-construction/vapor-retarders-in-concrete

http://www.concreteconstruction.net...n-new-aci-3021-concrete-floors-guide_o_2.aspx

From the 2nd link: Basically, if the intended use of the building above the slab involves anything that could be sensitive to moisture (flooring, adhesives, underlayments, coatings, goods stored on the slab), or if the building will have a climate-controlled environment, a vapor barrier is required. And in almost all cases the slab should be placed directly in contact with the vapor barrier. The old way of placing the vapor barrier and covering it with a blotter layer of granular material, that inevitably ends up trapping water next to the slab, has largely been abandoned by Committee 302.

Excellent post, a lot of things have changed in the last 24 years. As a concrete contractor, it actually hurt to read the info in the original post, as it goes agianst everythign we now practice. Concrete certainly does not cure and behave the way it did 24 years ago, since the EPA stuck it's nose into the cement producer's business, it's an entirely different animal. THe last ~9-10 years have seen some huge changes in cementicious material characteristics.......

BlackTalon · Mar 8, 2016

x2. Lots of misinformation in this thread.

wssix99 · Mar 9, 2016

^ The flow chart, above is what I used to plan my garage slab design a while back when I was calculating the reinforcing and having the "where to put the vapor barrier" debate with my wife. (My wife is a GC.)

The chart and a detailed discussion of this issue, floor curling, etc. is also in ACI 360 Design of Slabs-On-Ground. There is a ton of information there - so much so, it's probably great bed time reading material.

While we are "geeking-out" on concrete cracking, here's some more goodies. A Plastic Cracking Risk Calculator Web App: https://plasticcracks.siue.edu/

View media item 58802
^ You can figure out which days, based on ambient conditions, (temp, humidity, wind, etc.) that you need to work harder to keep the slab moist, etc. during curing.

kerr · Mar 9, 2016

Im no builder but have done some concrete work . I poured my shop pad in 1996 . 3500 fibermesh .10"deep and wide footings .I have zero cracking anywhere . Its a 27x40 with NO expansion cuts . The only seam is where I added a 20x16 addition .

Where a pad can be built with drainage I see no need whatsoever for poly . I am sure it has its place and also that all poly is not alike .

The best way to have a dry slab is to build your pad up say 2 feet high . Buy the time you grade and add gravel for a driveway it wont look high at all and will be dry .

YMMV

ms fowler · Mar 9, 2016

The number one rule in concrete work is, "Concrete will crack"--sometimes, by use of sofcut we can almost influence where it will crack.

oilslick · Mar 9, 2016

So does taped 2" foam form vapor barrier? I am talking the good extruded stuff, not expanded.

matt_i · Mar 9, 2016

My thoughts are that the EPS (blue, pink) foam is going to be as good as a vapor barrier when it comes to keeping moisture in the concrete. Water beads on the surface of this material and does not saturate it. Its probably very close to a sheet of plastic depending on how well the seams are taped. However its my belief that the natural voids in the compacted washed stone fill below is an equal component in breaking the surface tension with the ground. Not sure how you filled below the foam.

slowzuki · Mar 10, 2016

Such nonsense in this thread. The vast majority of areas require a vapour barrier if you are going to occupy the space above. Poly does not degrade when covered by your slab, at least not fast enough to worry about.

Yes if you pour with too much water in your mix, poly is a problem and the surface will have too much water when its worked. The solution is don't let the monkey on the truck or the placers add water!

It simply comes down to this, if you live in an area where even part of the year water comes out of the soil via evaporation, you need vapour barrier or your junk is going to get wet where it covers the slab and prevents airflow from carrying away the moisture.

There are a few very arid areas in North America that are dry enough for this. There are a few shoulder areas where ventilation and dehumidification can keep an open shop floor dry and pull moisture sideways from below cabinets etc.

The rest of us get to have mould growing under everything if their cheap **** contractor heard you could omit vapour barrier and make a better floor.

SH7mi · Mar 10, 2016

slowzuki said:
Such nonsense in this thread. The vast majority of areas require a vapour barrier if you are going to occupy the space above. Poly does not degrade when covered by your slab, at least not fast enough to worry about.

Yes if you pour with too much water in your mix, poly is a problem and the surface will have too much water when its worked. The solution is don't let the monkey on the truck or the placers add water!

It simply comes down to this, if you live in an area where even part of the year water comes out of the soil via evaporation, you need vapour barrier or your junk is going to get wet where it covers the slab and prevents airflow from carrying away the moisture.

There are a few very arid areas in North America that are dry enough for this. There are a few shoulder areas where ventilation and dehumidification can keep an open shop floor dry and pull moisture sideways from below cabinets etc.

The rest of us get to have mould growing under everything if their cheap **** contractor heard you could omit vapour barrier and make a better floor.

Thank you Mr Wizard. You apparently know more than others doing this for 30 plus years.

lakeroadster · Mar 10, 2016

The funny thing is... I started this thread because the article had data, data backed up by actual testing. I thought others would find it informative too.

Instead, it just kicked up another "vapor barrier" **** storm :wtf:

kerr · Mar 10, 2016

LOL . And a good read it is , thanks . Sorry if I added to it .

ssdave · Mar 10, 2016

Thanks, John, for trying to educate on this. A lot of engineers and architects really need to understand this stuff. With the push to insulate floors, the foam sheets act like a vapor barrier, and are causing many problems that could be eliminated by burying the foam under an inch of sand. The push to use poly to stop radon infiltration and to cure wet floors has also caused a lot of concrete slab problems to develop. I've seen a lot of plastic shrinkage cracking and slab curling/tipping that was caused by water entrapment in placed slabs, as well as, or exacerbated by improper surface curing techniques.

The one thing that is left out of a lot of the anecdotal experience things that come up with concrete is the fact that 95% of the concrete placed by non-specialty contractors is not placed in anything resembling good practice by ACI standards. I am an engineer with some considerable qualifications in concrete construction, and I cringe at what I see as ordinary practice.

Some of the things that most concrete contractors do that deviate from good practice:

1) Segregating materials by poor handling or by deliberate technique.
2) Improper use of moisture barriers.
3) Excessive water.
4) Improper wait times to finish, or improper timing of finishing steps.
5) Excessive finishing or emphasis on too smooth of a final surface or too much emphasis on a pretty looking surface.
6) Finishing techniques that lead to delamination.
7) Improper reinforcement placement and location control in slabs.
8) Poor choice of concrete mixture for the application.
9) Improper curing. This is the most significant of all the problems, most concrete placed has 30 to 70% of it's possible strength and durability because of this.
10) Incorrect jointing, edging, separation.
11) Poor reinforcement and joint detailing.

kerr · Mar 10, 2016

I dont understand the radon issue . If its in the earth , wont it be in the gravel in the concrete thats on top of the poly ? Honest question .

trinaussie · Mar 10, 2016

Some very interesting discussion points. I am going to add my 2 cents.
Concrete strength is dependent on water cement ratio. Good mix designs have the correct amount of water added for the required strength, actually a little more to ensure that all of the cement reacts with the water. A common reason for low strength concrete is too much water added at the jobsite. In commercial construction I have sent concrete back to the batchers because it sat too long in traffic and the driver added more water to try and cover it up. It is surprising how many concrete truck drivers (and concrete customers) have no idea that concrete curing is a chemical reaction (hydration) and not a drying out. If you do not have a vapour barrier under your slab, water from the wet mix is drawn (sponge action) into the underlying granular altering the W/C ratio. Maybe you get lucky and enough water is left that you actually increase the strength because you have lowered the W/C ratio, but usually you do not leave enough water to meet you design strength. I would always insist on the barrier for this reason in addition to its obvious purpose to prevent wicking up of moisture into the building once cured. Incidentally as one poster already pointed out poly does not deteriorate under the slab because it is not exposed to UV.

rburke65 · Mar 10, 2016

When I had my shop built in '12 I had to bust out a section of concrete floor in the nearby pole barn to feed power and air to the shop. The pole bard was built in '82 ......33 years ago...and the plastic vapor barrier was still there....still doing its job.

W_A_Watson_II · Mar 10, 2016

I've always wondered about the use of foam insulation directly under the slab causing the same issues.

ssdave · Mar 10, 2016

W_A_Watson_II said:
I've always wondered about the use of foam insulation directly under the slab causing the same issues.

It does!

So, you have to manage slab curling and plastic shrinkage cracking if you use under slab foam directly under the concrete. ACI has good recommendations on how to handle this.

Dagny · Mar 10, 2016

I am going to confess many years ago I did flat work and we use to poke as many holes in it as we could so we would not have to stay up all night finishing it.

matt_i · Mar 10, 2016

trinaussie said:
Some very interesting discussion points. I am going to add my 2 cents.
Concrete strength is dependent on water cement ratio. Good mix designs have the correct amount of water added for the required strength, actually a little more to ensure that all of the cement reacts with the water. A common reason for low strength concrete is too much water added at the jobsite. In commercial construction I have sent concrete back to the batchers because it sat too long in traffic and the driver added more water to try and cover it up. It is surprising how many concrete truck drivers (and concrete customers) have no idea that concrete curing is a chemical reaction (hydration) and not a drying out. If you do not have a vapour barrier under your slab, water from the wet mix is drawn (sponge action) into the underlying granular altering the W/C ratio. Maybe you get lucky and enough water is left that you actually increase the strength because you have lowered the W/C ratio, but usually you do not leave enough water to meet you design strength. I would always insist on the barrier for this reason in addition to its obvious purpose to prevent wicking up of moisture into the building once cured. Incidentally as one poster already pointed out poly does not deteriorate under the slab because it is not exposed to UV.

Excellent points! Just to add, typically one is advised to wet-cure the surface of the concrete (see also: curing sealer) for several days to prevent water from diffusing/evaporating away when its needed to complete the reactions. That's on the top, underneath, the same thing happens but its a little harder to spray the undersurface of your concrete

The barrier keeps it from wicking/draining to the dry stone.

Evilunclegrimace · Mar 10, 2016

lakeroadster said:
Assumptions? They cut actual core samples and the 30% was based on actual testing.

That's pretty much the reason for this thread: Data based on actual testing.

And they stated that this test was for high water content cement and that lower water content cement would not have the same results.

Evilunclegrimace · Mar 10, 2016

buening said:
There are industry standards on the minimum thickness for a vapro barrier, at least in commercial construction. The thin sheeting will puncture and deteriorate over time, whereas the stuff actually made to be a vapor barrier will last and not get punctured as easily.

ACI has since updated their recommendations on vapor barriers and the sand blotting atop the vapor barrier. See the following links for more info:

http://www.cement.org/for-concrete-...rete-construction/vapor-retarders-in-concrete

http://www.concreteconstruction.net...n-new-aci-3021-concrete-floors-guide_o_2.aspx

From the 2nd link: Basically, if the intended use of the building above the slab involves anything that could be sensitive to moisture (flooring, adhesives, underlayments, coatings, goods stored on the slab), or if the building will have a climate-controlled environment, a vapor barrier is required. And in almost all cases the slab should be placed directly in contact with the vapor barrier. The old way of placing the vapor barrier and covering it with a blotter layer of granular material, that inevitably ends up trapping water next to the slab, has largely been abandoned by Committee 302.

This makes the most sense and is how I have done cement work with zero problems.

Study: Vapor Barriers Under Concrete Slabs

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