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Re: Slab on grade

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Slabs-on-grade (particularly, residential) are usually reinforced for shrinkage and do not generally fall under ACI318. 
The pertinent ACI360R and ACI302.1R do not reference any cover below the rebars in the slab, but specifically require rebars to be placed 2" below slab surface or within upper 1/2 of thickness, whichever is closer to the surface.  
According to ACI 360R, SOG reinforcement should be spaced not more than at 3 times the slab thickness to be effective (welded wire fabric should not be spaced more than 14 inches). 
This means that in your 4" slab, rebars at 12" spacing do not necessarily have to be #4, they can be #3 placed about 1.25" below TOC. 
It should be noted that ACI302.1R discusses the vapor barrier issue that may completely eliminate any relevancy of the 3" cover. 
V. Steve Gordin, SE
Irvine CA  
----- Original Message -----
Sent: Friday, November 07, 2008 10:29
Subject: Slab on grade

For our typical 4? slab on grade (non-structural slab) we call out #4 at 24?o.c. each way.


We just got by a building official plan review for not meeting the 3? min. clearance for concrete cast next to earth.  For a non-structural slab I have never worried about the bottom clearance, maybe I should be?


Any thoughts?  Is there a exception for non-structural concrete not needing the 3? clearance?