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Hairpin clear cover and vapor barrier

Hairpin clear cover and vapor barrier

Hairpin clear cover and vapor barrier

thread167-250813: Clear Cover to Flexural Reinforcement in Structural Slab on Grade

In a prior thread, it was stated that "For all those reading this, ACI and Bruce Suprenant (Mr. Concrete) agree that 1 1/2-inches of cover on the bottom of a slab "exposed to earth" is the ABSOLUTE minimum WITH A VAPOR BARRIER! 2-inches is recommended without. There is no specific criterion for cover on rebar in an enclosed garage, but ACI suggests 2-inches for "exposure to de-icing salts". ACI qualifies "exposed to weather" in the commentary as "direct exposure to moisture changes and not just temperature changes...unless subject to alternate wetting and drying, including that due to condensation conditions or direct leakage from exposed top surfaces, run off, or similar effects."

Is there any documentation to support this? I have a small PEMB building with a 4" slab on grade and WWF/Hairpin reinforcing. This slab was placed directly on a vapor barrier. The slab is poured and the building is up. The city has now gone back and looked at it, and they want 3" of concrete cover at the hairpins.

I am looking for documentation that this is not required in a situation like this. Does anyone know of any ACI or other documentation to support the statement above?


RE: Hairpin clear cover and vapor barrier

No ACI documentation, however have a read of PCA IS001.08t. Also Canadian Code: CSA A23.3-04 "concrete cast against and permanently exposed to earth" requires 75mm (2"), BUT "slabs, walls, joists, shells and folded plates" require 40mm (1-1/2"). The distinction is the exposure to moisture...

I think you're okay with a vapour barrier, as you aren't technically cast directly against the earth.

This is supported directly by NZS 3101:Part 1:2006, the latests New Zealand concrete code. Cl Casting Against Ground "When concrete is cast on or against ground and compacted in accordance with NZS 3109, the minimum cover for a surface in contact with the ground shall be 75mm,or 50mm if using a damp-proof membrane between the ground and the concrete to be cast."

This is discussed in the commentary to NZS 3101 "The increase in cover requirements relates to the casting of items directly agaist the ground i.e. not agaist forwork constructed to NZS 3109. Where blinding concrete or sand blinding treatment of a base course has been used to produce a surface similar in tolerance to formwork to NZS 3109, then the cover requirements may be determined by direct reference to Tables 3.1 and Table 3.6 or Table 3.7."

Effectively they are arguing that if you protect from moisture and have a good, level surface, there is no reason for the increased cover.

Good Discussion here (which actually disagrees!): http://www.eng-tips.com/viewthread.cfm?qid=28866

RE: Hairpin clear cover and vapor barrier

Oh, and to be perfectly clear: I wouldn't make a habit of this.... In fact where I have placed hairpin bars into slabs, I've either increased them to 5" throughtout, or required 6" depressions around the columns and hairpins. The extra inch is for the fact that it is very hard to profile the compacted fill, and I don't trust any contractor to provide the extra inch.

In NZ it is very common to have 4" slabs with inclusions, and there's been no rash of failed slabs.

RE: Hairpin clear cover and vapor barrier

Refer to ACI 301 (Specifications), Section 3, which has a table of mandatory minimum cover values. Note that these are not clear cover minimums, but nominal values, subject to tolerances according to ACI 117.

If the slab is exposed to deicing salts, you should expect problems as the salts penetrate cracks in such a thin slab. If the exposure is sufficient, the barrier below the slab may act to retain moisture and salts, increasing corrosion potential. However, since this is a slab on ground, the structural ramifications would seem to be nil.

It is typically acceptable to use a traffic membrane to supplement thin cover on exposed surfaces of traffic-bearing slabs.

RE: Hairpin clear cover and vapor barrier

Thanks for both of your help.

I am looking for some sort of documentation that a 20 mil vapor barrier (placed directly under the slab) is adequate to consider the slab as "Concrete not exposed to weather or in contact with ground".

RE: Hairpin clear cover and vapor barrier

Well Hawkaz, that's what the New Zealand code states... But whether you can sell that you an Americal plan checker/authority having jurisdiction, I don't have the foggiest.

RE: Hairpin clear cover and vapor barrier

I would think that using a vapor barrier would by definition mean that the concrete is not in contact with the ground.
The ground is the ground, not a vapor barrier.

RE: Hairpin clear cover and vapor barrier

ACI 318-11, in the commentary R7.7, explains that providing a barrier to prevent exposure to weather (including wetting and drying) is acceptable to allow reduction of the minimum cover.

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