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Re: Fw: Vapor Barrier

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My own experience in NZ, Japan and here in Canada has been to put the barrier immediately below the slab but on top of sand consolidated by roller, jumping jack etc and/or water-deluging(?).  One opinion is that the barrier, being smooth, also allows the slab portions to shrink unrestrained.  Large slab areas (greater than 20'X20') were almost always cured by sprinklers, soaked gunny sacks, soaker hoses etc.  Proper reinforcing of corners for thick slabs reduces corner cracking such as in bridge decks.  Unreinforced slabs require adequate supervision.  

In all the cases I was ever involved with the engineer was adamant about proper curing.  The contractor is expected to include such cost in his price because it is "good" workmanship!  

I think that sand or free draining material is still needed as basecourse to mitigated capillary action and subsequent pressures.  It would seem from the thread that the site instruction will depend on the engineer, who will then have to ensure that his design assumptions are satisfied by diligent site inspection.

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: Thursday, October 15, 1998 9:13 PM
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 thee 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 too?
>> 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 difference.
>> >
>> >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 opinion)
>> >>
>> >> Thor A. Tandy   P.Eng.,  MCSCE
>> >> Victoria, BC, Canada
>> >> e-mail: <vicpeng(--nospam--at)>