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Are capillary breaks really needed if there is a vapor barrier? 3

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joder

joder

Structural
Aug 5, 2003
22
I've been practicing for 26 years and never thought about this question recently posed to me by a contractor. Why do we need a "granular capillary break" if there is a vapor barrier? I can see the need if there is groundwater present; the capillary break will provide a conduit for ground water to drain to a lower level. Hydrostatic slabs are another subject altogether and beyond this discussion. I'm concerned about normal commercial and residential slabs where groundwater is not present.

I've been researching the subject and cannot find an answer to the question posed above. Most "authorities" indicate that a capillary break and a vapor barrier are required. The authorities also state that the capillary break won't stop vapor, only water. And we're all familiar with the controversy of whether the vapor barrier goes below or above the granular fill; it is well documented with authorities on both sides of that argument, so let’s not get into that question.

ACI 302.1R-04 Concrete Floor and Slab Construction, paragraphs 4.1.4, Base Material, recommends "a clean, fine-graded material with at least 10 percent to 30 percent of particles passing the No. 10 sieve ..." I copied that verbatim and have been using it in my General Notes for years. All this time I thought was that this was a spec for the "granular fill capillary barrier", but now I realize that it could not be with that many fines. I believe this is merely a good compactable base to pour a slab on. In fact, it is not a good capillary barrier at all.

In a post from “fattdad” on 7-6-06, he analyzed a “dense-graded aggregate (i.e., minus 3/4-in with about 5 percent passing the 200 sieve - D10 of about 0.15 mm) for the "capillary break" [typically used] beneath industrial floor slabs”. He concludes: “Looking at Terzaghi's book, the standard equation for capillary rise is Hc=c/(e*D10) using consistent units, where C is in the range of 0.1 to 0.5 cm. When I use this equation for the dense-graded aggregate, the calculated range for capillary rise is somewhere in the range of 23 to 115 cm (9 to 45 inches). As you can imagine this does not seem to offer a very effective "capillary break" when a typical layer thickness is 6 inches.
“
There is new ACI document ACI 302.2R-06 Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials. There is no mention of a capillary break in that document. They speak of the advantages of a "granular base" over the vapor barrier (7.2.1) acting as a blotter, but do not call it a capillary break. I don't believe it is a capillary break. I believe it is the same granular base described in ACI 302.1R above, which is more of a structural base than a capillary break.

So I ask, is there really a need for a capillary break? Is the term being misused? Is it really a compactable granular base used as a blotter for bleed water?

A capillary break is supposed to stop moisture from “wicking” up to the slab. We have begun specifying good quality vapor barriers, with very low perm ratings, taped joints, boots around pipe penetrations, etc. I wonder, with a good vapor barrier is there any need for a capillary break? Does it really matter if the soil under the vapor barrier is moist?
 
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For structures located in areas with seasonal water table far below the base slab, we usually only call for well graded gravel fill compacted to certain proctor. When vapor barrier is necessary, a few inches of fine sand is placed over the compact fill to avoid puncture. When ground water migration become an issue/concern, positive drainage system (coarse fill with/without sump pump, and/or linner) would be considered, or the slab would be designed accordingly (taking ground water effect into consideration). The above practices might not be the best, but leave no gray zone in between as you have experienced from trying to justify the necessity of a "capillary break" layer (personally I think the phrase is too vague and academic).
 
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VP and granular subbase "capillary barrier" are just two efforts to help minimize moisture penetration up through a slab - both are not fail-safe and so the two together just give you added protection.

kslee1000 - agree that in low water table areas you might not need the granular - however the granular also helps improve the subbase modulus and improve slab performance if you have wheel loads, rack leg loads, etc.

 
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Thanks for your replies.

I agree with the belt-and-suspenders approach. However, "a well graded compacted gravel fill" is going to be more of a wick than a capillary break (see fattdads's calcs and experiments showing that water climbed to the top of his 15" cylinder and his calcs showed it could go to 45"). So it's not helping keep water away, it's doing the opposite.

If its going to be a capillary break, it needs to be a poorly graded granular fill, like washed 3/4" stone. I'm not sure how well that would compact or improve subgrade modulus. Even a 6" layer of well compacted granular material may take the modulus from 200 to 225 and is generally not worth the cost.

 
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joder:

I think you mis-read my response.

JAE: Agreed.
 
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joder,

Thanks for your post and the reference to the earlier one started by our friend fattdad.

I tend to think that the concept of a capillary break is overrated, and that the vapor barrier is the important component. But then, as Ron pointed out in the earlier post, workmanship in installation of the plastic sheet and maintaining it through concrete placement is problematic.
 
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I believe that kslee1000 is agreeing with me that this granular layer should not be called a capillary break. My point is that it is OFTEN mislabelled as one. Compacted well graded granular material does not provide a capillary break. Only a poorly graded material will provide a capillary break.

I agree with JAE saying that the capillary barrier is one of just two efforts to help minimize moisture penetration up through a slab - both are not fail-safe and so the two together just give you added protection. But I am starting to believe that effort is in vain unless you get real specific about the grading of the granular fill.

I believe JAE is thinking, as I have been for many years, that the compacted granular fill that we have been specifying is a capillary break. I believe we've been wrong about that all along. Can someone set me straight on this? Maybe the answer is that it is a BETTER capillary break than a clayey subbase and that is some help. Where I practice, there are only sands however, and the granular base is not any improvement from a capillary action standpoint.

There are times when a capillary break is needed, but those are rare and should not be confused with the standard compacted well graded fill that makes a good subbase for slabs on grade. I'm thinking that the only times a true capillary break is needed is if you need a drainage layer to move water out from under a slab, or if you are in moist clayey soil.
 
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When I specify a capillary layer - I specify a uniformly graded stone or gravel...not a well graded crushed stone. If there are fines then the capillary break doesn't happen - you just have a nice granular sponge which isn't what you want.

While well graded crushed stone does a better job of compacting and improving the subgrade modulus, the uniformly graded material does compact and does help too.

 
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The performance of slabs on ground is one of the least understood areas of structures. We specify them based largely on previous experience, and in the case of capillary barriers, on the basis of speculation.
 
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I was about to hit Submit and accidentally hit some key that made my post disappear. I had typed several paragraphs. Maybe it got submitted and thus I'll be repeating myself. It went something like this...

JAE, we're are in total agreement that to be a capillary break it needs to be uniformly graded and not have any fines. And they can be compacted fairly well.

To ensure everyone understands the terminology, "uniformly graded" means that all the particles are the same size. That's the opposite of "well graded".

I discussed this with a geotechnical engineer buddy and he said that to be a capillary break, not only does it need to be uniformly graded, but the pore size needs to be relatively large, like #57 (3/4") stone. He said that clays wick moisture several feet above the water table, sand several inches, and gravels not at all.

Here's the rub:

ACI 302.1R-04 Concrete Floor and Slab Construction, paragraphs 4.1.4, Base Material, recommends "a clean, fine-graded material with at least 10 percent to 30 percent of particles passing the No. 10 sieve ..." That is not a capillary break at all.

ACI 302.2R-06 Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials. There is no mention of a capillary break or anything like it in that document.

So if you're specifying a slab on grade that has a base that is in accordance with ACI 302, remove the word capillary break from your notes and details.

I have learned to correct my terminology and only call it a capillary break when I mean it, which in my neck of the woods is almost never.
 
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For interior slab on grade, can anyone enlighten me when, why and how a "capillary break" becomes important factor on top of such typical concerns over strength of sub-base, and wet floor? We specify compact fill for strength concern, vapor barrier to prevent wet surface, drainage layer to divert seepage/uplift, what does "capillary break" serve?
 
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it minimizes wicking of moisture up to the underside of the slab. even when there's a "vapor barrier", moisture can still transmit through to the slab. vapor barrier is a relative term unless you're really using something like a 30+ mil vapor barrier. the heavier the vapor barrier, the less moisture will transmit from one side to the other. the typical vapor barrier is very thin, easily torn and can transmit a fair amount of moisture. in other words, if moisture is present on one side of the "vapor barrier", some amount of that will likely be present on the slab side as well.
 
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As to kslee1000's question about why and how a "capillary break" becomes an important factor on top of such typical concerns over strength of sub-base, and wet floor?. I don't think it is AS important as strength of sub-base, but it is related to wet floor. Here's a link to an article that explains it well.
It says, "If recommended in the geotechnical evaluation of the jobsite, install a 6 to 8 inch layer of coarse gravel or crushed stone as a capillary break. Note that the coarse stone capillary break will not reduce moisture vapor transmissions from the subgrade."

It also describes the subgrade compaction VERBATIM out of ACI 302.1R, "a clean fine-graded, preferably crushed, material with about 10 to 30 percent passing the No. 100 seive..."

So they distinguish between two separate layers: a capillary break and a compacted subgrade. They recommend the capillary layer IF geotechnical conditions require it. Both layers are granular. The article has a graphic detail that shows a capillary break that is hatched like gravel and a compacted subgrade that is hatched like earth. It is not clear that there are two separate granular layers unless you read the text.

I believe that the capillary break is only needed when you are building over clay that wicks moisture up.

I believe the best answer to the question I first posed, "Can a high quality vapor barrier eliminate the need for a capillary break"?, is: No, you need a capillary break if it is recommended by the geotechnical conditions, for the reasons given by msucog and JAE, because vapor barriers aren't perfect and it helps to have two means of keeping the slab dry.

Here is a link to some good references on the subject, many written by Bruce Suprenant, former ACI president.

Continuing to beat this dead horse, I gleaned this from the Stego website (keeping in mind that they are selling a product):

Though it is true that many people in the construction industry will use these terms [vapor barrier and vapor retarder] synonymously, there is a very big difference between the two. A vapor barrier needs to have a low enough permeance to mimic the permeance of a floor covering. Otherwise the potential exists for the buildup of relative humidity/dew point within the slab/under the floor covering.


What is the difference between a vapor barrier and a vapor retarder?
To be considered a vapor barrier, testing results from ASTM F 1249 or ASTM E 96 must state a water vapor transmission rate (WVTR) of less than 0.01 perms (grains/[hour * ft2 * in. Hg])
A vapor retarder is classified by ASTM E 1745 as a plastic material having a permeance of less than 0.3 perms as tested by ASTM F 1249 or ASTM E 96.

Over and out!
 
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Joder & JAE:

Thank you both. I think we all can agree on that a coarser granular fill should be placed over a wet/moist clay layer for both site improvement (a strength concern), and capillary/moisture control (a functional concern) purposes.
As it serves dual purposes, namewise, I stay with my old practice - just call it the "coarse gravel fill" rather than given a function specific name. For material specifications, I will seek advice from the geotechnics, or from a more sophisticate sources such as DOT, not the ACI.
Thanks again for the worthy discussion and view points.
 
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kslee1000,

That sounds like a good practical way to go. Specify a coarse granular fill that will serve both functions; a capillary break and a subbase.

The well graded ACI subbase could be underneath that IF needed for high performance slabs.

I've got some General Notes to edit now!
 
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As others have pointed out, there is often a need for both. When the water table has the potential to rise to within a few feet of the the slab bottom, and the subgrade soils are clayey or silty, then a capillary break is certainly indicated. A capillary break should be, as JAE noted, gap graded to keep out most of the fine material, and should have a fineness modulus of at least 3.0. The vapor barrier serves a similar purpose, but generally is discontinous and breached during the construction process, so using both is truly a better way to go if your goal is to mitigate the upward migration of moisture.

 
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I thought there was some argument that granular fill over the vapor barrier reduced slab curling. Moisture is trapped by the vapor barrier on the bottom of the slab which made it cure much slower than the surface hence curling.

Personally I never thought the argument held water. I’ve always felt vapor barriers or retarders are useless since they are always ripped and punctured. The only structural benefit is that they keep the welded wire out of subgrade.
 
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The vapor retarder, when coupled with a pipe below the retarder and vented to the outside, is intended to create a path for the radon gas to be passively vented and reduce the amount of radon getting into a basement.

The next step, if the radon level is in excess of 4 picocuries per liter, is to install an exhaust fan on this pipe to create a low pressure area around the pipe below the retarder so that the radon migrates towards the pipe.

Some municipalities adopt Appendix J of the 2003/2006 IRC, when the map indicates it has a potential for radon.


Don Phillips
 
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