huangyhg

concrete cover w/ vapor barrier
can the required 3" of concrete cover be reduced for a slab that is cast against the ground if a vapor barrier is used?
i'm not sure - it makes sense that its not technically "exposed to earth or weather" but in most slab applications i have the reinforcing in the upper portion of the slab so i've never really worried about it.
vb's are not always placed perfectly so i would probably still require the 3".
i would opt on the side of error...vapor barriers are easily omitted. keep the 3"...low cost, but could protect a large error
i would think that it is the variation in the soil profile that forces the need for a cover of 3" and not the exposure to the soil. observe the bottom of a footing or a slab when it is removed.
from as3600 (australian) for concrete in contact with the ground;
-add 10mm cover if cast onto a vapour barrier (compared to cast against formwork).
-add 20mm cover if cast directly onto the ground.
the 3" cover requirement does not apply to slabs cast on ground, unless it is transmitting loads to the ground. aci 360, not 318, would govern. if it is a structural slab, then yes, i think you could reduce it with a vapor barrier, but i agree that you take a chance hoping that the vb remains intact.
this will be a structural slab that is cast against the ground. this is a beachfront home so i am accounting for scour. all things considered, i think my best option will be to increase my slab debth in order to get the 3" cover. thanks for all the input.
the 3" cover requirement is meant to account for three things:
1. the potential exposure to moisture from the soil.
2. the reduction in cover due to variable subgrade elevation.
3. the mixing of soil and concrete at the interface.
placing the slab on a good quality vapor barrier such as 15 mil polyolefin over a well-compacted cushion layer will help address some of these concerns. a reduction in cover is reasonable, but is not directly addressed in the aci 318 code. i guess that is where "engineering judgment" comes in.
i like this 3" rule. our australian code as3600 typically prescribes far less than this. i feel our code does not adequately account for these items taro mentioned.
stick to 3", vb will just add protection for your slab againts moisture and undesirable chemical contamination of your subgrade.

i do not recall coming across this specifically during a plan review but i would have to say, reading the code literally, you can consider the vapor retarder as preventing the concrete from being in direct contact with the earth.
don phillips

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