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Re: gravel under slab on grade

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Keep the vapor barrier, too.  Without it, any non-porous floor covering
will become the new barrier for water vapor moving upward (into the dryer
air of the interior above).  These floor coverings will eventually fail
to adhere.  When they do, a very wet concrete will be seen due to the
accumulation of vapor.

Russ Nester
On Thu, 09 Oct 1997 10:05:16 -0500 dennismc <dennismc(--nospam--at)>
>Keep the gravel.
>The gravel serves the purpose of allowing ground water to flow
>horizontally rather that swell up the soil (not so likely with sand).
>The vapor barrier keeps the moisture out and the sand allows for a
>cushion from expansive materials.
>If he bid the job with the, have him put it in, sand or no sand base.
>If you want to take it out, have the a soils engineer make that
>decision, (the contractor should pay for this if no soils engineer on
>the job initially).  If it is warranted by the soils engineer to take 
>out, ask the contractor how much he is saving by eliminating it, then
>tell him you want that much credit back for the owner.
>Jeff Smith wrote:
>> I have a situation where I have specified a 5" reinforced 
>> garage slab over 2 inches clean sand, 6 mil vapor barrier and 4"
>> crushed
>> aggregate. As it turns out the existing subgrade is sand, probably
>> dune
>> sand from the area that was brought in as fill 100 years ago. The 
>> is
>> fairly clean, very deep and appears cohesionless. The contractor 
>> to
>> pour the slab without the gravel base. He wants to moisture 
>> the
>> existing sand and compact it with a light roller, then place the 
>> directly over the sand and place the 2" sand cushion over the V.B.
>> He believes the gravel is not necessary due to the existing sand
>> subgrade.
>> We all know he simply wants to save money, but is the gravel 
>> justified. Any thoughts on this?
>> Regards,
>> Jeff Smith. S.E.
>> phone: (415) 543-8651
>> fax: (415) 543-8679
>> email: smthengr(--nospam--at)
>> Smith Engineering
>> 27 South Park
>> San Francisco, CA 94107