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RE: Fw: Vapor Barrier

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When I built my house 5 years ago, I put a 4" slab with 6x6 1.4 WWF 
directly on 6 mil Poly.  The contractor put a curing compound on it but 
over the next 2 weeks I wet cured it with a hose as much as I could, which 
was a fair amount.  I saw no noticeable curl or cracks due to curl up to 
the time I put on the floor covering and I still haven't seen any curl at 
the door openings.  I live in a VERY dry climate and this was done in July. 
 I've always been a believer in a vapor barrier, and it seems that good 
curing has got to be easier than messing with sand on plastic. Do they 
REALLY get it compacted?

-----Original Message-----
From:	raranous(--nospam--at) [SMTP:raranous(--nospam--at)]
Sent:	Thursday, October 15, 1998 10:03 PM
To:	seaint(--nospam--at)
Subject:	Re: Fw: Vapor Barrier

Now I think we are getting into the realm of theory vs. practicality.  It 
has been my experience that a slab poured in contact with the barrier is 
subject to curl because of the differential migration of the the water.  If 
a contractor were to cure a slab

like the specifications called--keep it moist at all times, there should 
not be any curl and there would be no need to use sand or gravel. 
 Unfortunately, to cure a slab like the specifications call is expensive. 
 Contractors want to pour the slab and get

on with the rest of construction.  This is one reason why they want to 
sawcut joints instead of using zip strips or similar products.

This is a very interesting concept, Thor.  I would pose the general 
question--would a slab perform better if it were cured properly and used a 
barrier directly under the slab?  Secondly, in the overall scheme of 
things, would this system be more
economical than using sand or gravel?

Any takers on this one?

T wrote:

> Wouldn't the barrier in direct contact with the slab be even better at 
enhancing curing?
> I saw on another posting that this would encourage slab curl due to 
differential curing?  That might encourage keeping the upper surface moist 
> Thor A. Tandy   P.Eng.,  MCSCE
> Victoria, BC, Canada
> e-mail: <vicpeng(--nospam--at)>
> -----Original Message-----
> From: raranous(--nospam--at) <raranous(--nospam--at)>
> To: seaint(--nospam--at) <seaint(--nospam--at)>
> Date: Wednesday, October 14, 1998 7:49 PM
> Subject: Re: Vapor Barrier
> >>From what I understand, the initial moisture migration during curing is 
both up and down through the slab.  The sand allows that initial moisture 
to be pulled away from the slab.  However, a properly cure slab is kept 
moist.  Having water trapped in
> >the sand below the slab, allows that moisture to keep the bottom of the 
slab moist during the whole curing process.  The same process can occur 
with gravel.  However, contractors have told me that it is more economical 
(cheaper) to use sand.  My
> >experience with both sand and gravel has shown no significant 
> >
> >T wrote:
> >
> >> I probably didn't understood your explanation, however, although I 
have seen both ways done without obvious detriment to the slab, wouldn't it 
be desirable to have the water contained for the concrete to cure rather 
than the sand taking it away?
> >> (this is meant as a point of discussion and not necessarily my 
> >>
> >> Thor A. Tandy   P.Eng.,  MCSCE
> >> Victoria, BC, Canada
> >> e-mail: <vicpeng(--nospam--at)>
> >>
> >> -----Original Message-----
> >> From: raranous(--nospam--at) <raranous(--nospam--at)>
> >> To: seaint(--nospam--at) <seaint(--nospam--at)>
> >> Date: Tuesday, October 13, 1998 9:00 PM
> >> Subject: Re: Vapor Barrier
> >>
> >> >For what it's worth!  I have always ...  typical response from me: 
> >> >I would explain that the sand absorbed the water migrating from the 
concrete so
> >> >it cured from top and bottom; and 2) now you can ...