Minimum cover to bottom rebar in concrete cast against void box

[livewire9]

ACI 318 requires concrete cover to be 3" for concrete cast against and permanently exposed to earth OR 2" for concrete exposed to earth or weather (not cast against). I believe it could be argued that a void box (used to provide separation between bottom of concrete and expansive clays) acts similar to earth. It can wick moisture. Therefore 3" cover would be prudent. Is this too conservative?

 

[JAE]

The 3" clearance takes into account the fact that soil surfaces will be highly irregular when concrete is cast against them, thus causing areas where cover is less than 3".

For cardboard void forms you will not have that degree of irregularity - thus the 2" might be a more appropriate choice.

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