Placement of slab vapor barrier

[Iamsam]

I have read several articles about the placement of vapor barriers below a concrete slab. There is some controversy about whether or not concrete should be placed directly on the vapor barrier or should a layer of stone be placed on the vapor barrier first, then the concrete. The slabs we are preparing to place are 4" thick, 4000 PSI concrete, no entrained air, .44 W/C, and approx 20% of the portland type II cement is replaced with ground blast furnace slag cement, #4 rebar each way at 16" on centers. The slabs are approx. 60 feet X 150 feet w/shrinkage joints.

Does anybody know of any serious problems that may be encountered from placement of the concrete directly on the plastic vapor barrier, like maybe slab curling? At this time we are planning on placing the concrete directly on the plastic which is placed over 4" of 57 stone, which is sitting on compacted 2A modified structural fill. At the time the 4" of stone is placed it may or may not be in a covered building.

 

[concretedoc]

Will the slab receive a moisture-sensitive floor covering? If so, ACI 302.1R-96 recommends placing the concrete directly on the vapor retarder. An experienced floor designer recommends keeping the #4 bars 1-in. below the top surface. You may still get some curling, but it's easier (and usually cheaper) to grind curled joints than it is to try to fix a floor moisture problem. For more information on the vapor retarder location question (with pros and ocns for the granular blotter layer) go to

http://www.worldofconcrete.com,

Concrete Construction magazine, article archive search. Use "vapor retarder" for the subject and "Suprenant" for the author. For the curling problem enter "curling" and "Holland" for the subject and author, respectively.

 

[haynewp]

IAMSSAM, there is a flow chart addendum to ACI 302 where in some cases it is better to put the slab on the vapor barrier and some cases it is better not to.