are capillary breaks really needed if there is a vapor barri

huangyhg

are capillary breaks really needed if there is a vapor barri
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.
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).
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.

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.

joder:
i think you mis-read my response.
jae: agreed.
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.
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.

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.

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.
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.

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?