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 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.
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.
I agree with post that indicate the 3" clearance is for concrete cast on soil, due to the irregularties of grading the surface. Five inch minimum concrete slabs on grade reinforced with #4 rebar @12" each way are are often used, specifying 1 1/2" clearance from top of slab. Therefore, the clearance would idealy be only 2 1/2" to the vapor barrier (now the preferred termonology is vapor retarder). The Vapor Retarder is placed on sand or directly on 6-8" stone capillary water barrier, depending on whose recommendations you follow. Regardless the top surface should be graded to a more pecise grade than the soil. I review commercial buildings at numerours loacations throughout the United States, and designers in arid regions seem ignore the need for a vapor retarder under interior building slabs, stating the floors are not moisture sensative as outlined in ACI. I think PTI and NRMCA are correct in recommending a Vapor Retarder for all on grade building slabs, as the use may change and most VCT, Carpet, etc. are considered moisture sensative flooring. While this is my opinion I would be interested in others comments on this subject.
I was involved in a project south of Fresno, CA in 1991 with a slab on grade first floor, very sandy, arid soil, and the contractor wanted to delete the vapor retarder so we executed a deduct change order. In hindsight, this required an adjudication since it is a code requirement. If I knew then....
The floor finish was a vinyl backed carpet tile. About 9 months later, a portion of the vacant floor was being built out and when the tile was lifted, mold was growing under the vinyl. We normally did a full spread glue but on this project, we again executed a deduct change order to use the grid method of glue, as recommended by Milliken.
We did a mositure test of the salb, inconclusive; monitored temperature and humidity for a couple of weeks, inconclusive; reviewed the concrete test reports to ensure the maximum slump was not exceeded; inconclusive.
We cleaned up the mold, replaced the carpet, and laid it down. We found the main lobby also had a lot of moldy areas as well. They were cleaned up and the carpet replaced.
The only thing we could think of - since the floor one slab was placed well after floor two - since it had an underfloor cellular system like that used on floor 2, hydration may not have been complete and the concrete was moist enough foster mold growth under the vinyl.
The next project in Portland, OR in 1992, we did not delete the vapor retarder, we had the concrete tested to ensure it was "dry" enough (by that time, glue was becoming water based and sensitive to moisute in the concrete) and we went back to the full spread glue method. We monitored it and did not find a recurrence.
In my opinion the vapor barrier will prevent soil mixing with concrete so I reduce the cover to less than 3". However, I allow around 1/2"-1" for the irregularity of the soil surface. If you have a large structural ground floor slab (have a large footprint condo under construction that has a slab to resist rising water pressure) using the full 3" cover will be overly costly.
