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Re: Vapor Barrier

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Jim Kestner wrote:
> I would like to get opinions on what I consider conflicting requirements
> regarding vapor barrier under slab-on-grade. ACI 302.1-R, 2.4.1 requires
> that if a vapor barrier be used that it be covered with 3" of sand
> before pouring the slab. This minimizes plastic shrinkage cracking and
> curling. It also controls bleed water coming to the surface. This is
> logical and well documented. However, this procedure is risky when
> pouring a slab not under cover. What are the chances of rain occuring
> after the sand is placed over the vapor barrier but before the slab is
> poured? Now, you may have water trapped which could be very expensive to
> deal with or if ignored, worse than having no vapor barrier at all.
> It seems like ACI requirements address the finishing and performance
> aspects of the slab, but have ignored the practical aspects of what the
> contactor must deal with. How do you other Engineers and Contractors
> deal with this?

What are the chances of it raining before placing the vapor barrier?? 
If the subgrade or sand barrier gets wet (saturated) prior to the pour,
delay the pour.

I personally like using the 3" barrier when possible; however, half the
contractors call my office and tell me I have an error on my plans.

Brian K. Smith, P.E.