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Re: Capillary Breaks

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The granular material between the vapor barrier and the concrete is there to allow water to escape from the bottom of the slab for curing.  If the concrete is placed on the plastic sheeting the top will cure faster than the bottom causing uneven shrinkage and the tendency for curling up of the edges of the slab and resulting cracking at the surface.
 
Richard L. Hess
Hess Engineering Inc.
Los Alamitos, California
(562) 799-9787
RLHess(--nospam--at)HessEng.com
----- Original Message -----
Sent: Wednesday, February 09, 2005 11:53 AM
Subject: Re: Capillary Breaks

In a message dated 2/9/2005 2:45:58 PM Eastern Standard Time, James.Lutz(--nospam--at)earthtech.com writes:
I think maybe the "capillary break" above a vapor barrier is just to provide somewhere for bleed water to go so it doesn't accumulate on the bottom of your slab. If there were no vapor barrier, you would want a real "capillary break" to keep the soil from pulling water out of the mix.
That was kind of my assumption too - but then you get into the ACI 302/360 discussion about whether there should be a granular layer above the vapor retarder.
 
And a capillary break needs to be more or less the same size particle so that it is very free draining.  And that kind of material is not easy to compact well.  Whatever goes on top of the vapor retarder has to be compactible.  So to call for a "capillary break"  on top of the vapor retarder does not seem to make sense.
 
I am not sure who the guy that wrote the article is - he's a salesman, for all I know his qualifications are that he has a degree in automotive technology. 
 
Gail Kelley