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From my ACI Seminar for "Slabs on Grade," I believe the instructors would state that you are both wrong. Albeit you would be closer to the correct answer, they stated that the problem with pouring directly on the vapor barrier or sand is that it creates a high zone of moisture in the bottom of the slab, which will cure at a much slower rate than on the top surface. This can create the greater chance of slab problems such as curling. These set of instructors recommended a graded aggregate base and then the vapor barrier. This assists in creating a zone on the bottom for the concrete to cure. The problem they state with sand is that there is very little air space for moisture to occupy.

As for your clients concern, it sounds like they have been placing a lot of water on the sand previously. Certainly it is important for the base to have some moisture and not be "thirsty," but just as important for it not to be drenched.

Finally, I would state that it has been a challenge in my career to help architects, contractors, and clients appreciate proper slab on grade placement.  

Richard Lindenberg
Davis, Bowen & Friedel, Inc.

On Tue, 25 Apr 2000 12:35:32 EDT, eaint(--nospam--at) wrote:
>Just hoping I can get a basic question clarified. I  have a client  I'm doing  
>a series of homes for, and he wants me to specify that the slab on grade be  
>placed directly over the vapor barrier, (no sand between) He claims that when  
>sand over the vapor barrier is moistened prior to pouring concrete, sand will  
> never dry out,  the moisture from the sand will travel through the slab for  
>a very long time causing bubbling in the vinyl flooring, prevents  wood  
>flooring from sticking to concrete, etc. I've always believed that the sand  
>aids the curing of the slab.  The caveat is that vapor barrier is installed  
>properly and not punctured during placement of concrete. Any help is  
>Andy Vidikan
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-- Richard Lindenberg, rel(--nospam--at) on 04/25/2000 at 1:24:47 PM