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slab on grade bottom bar location
how much concrete cover is generally used for bottom slab bars in slab on grade?
aci generally requires 3" for concrete cast against soil, but how about in the case when the slab is cast on a significant vapor barrier?
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whats the depth of grade, and whats the slab being used for?
usually we use 50mm cover but with a high strength concrete.

if the vapor barrier is thought to stand well for the environmental attack for the life of the building, a cover as interior placement or as provided by the code for such case can be used; this is the case for our cte (código técnico de la edificación).
if you are casting onto a vapor barrier, it is not on the ground; however, due to breaches in the barrier it might be "exposed to earth", so 1-1/2" is acceptable (assuming you are using no. 5 or smaller bar or wire).
if the steel is for temperature, not bending, put it in the middle of the slab.
if you have 2 layers of reinforcing steel with a pevb on your soil beneath the slab 1-1/2" cover is adequate with sufficient cover on the top layer to permit sawcutting the slab without cutting the bars. for example, with a 6" slab, i would use 1-1/2" sawcuts with 1-1/2" or 2" clear cover to the top layer. for a 6" slab, i would only use a single layer of rebar. is there a reason for two layers? is this flexural reinforcing or just ts. if temperature steel, then you might reconsider using 2 layers.
dik
there are 2 layers of bars in an interior 8" thick garage floor for tanker trucks.
the detail appeared odd with the bottom bars up 3".
the crsi manual was showing 2" cover top and bottom, but that generated the question on the aci requirement.
aci specifically excludes itself for the design of sogs under the scope section. depending on the building use, the mid-depth of the slab is usually a good place for the reinforcement.
for anyone interested, the military generally has pretty good references:
teguchi...
i tend to place the reinforcing nearest the top as noted. visible cracks form on the top surface and become less noticeable with the rebar closest to the visible surface.
dik

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