Can concrete reinforcing cover be reduced to 3/4" if a roofing membrane is present? 6

[Geoffre14]

In a concrete roof slab, can concrete reinforcing cover be reduced from 1-1/2" to 3/4" if a roofing membrane is present? ACI vaguely alludes to this but I haven't found a straight answer anywhere.

 

[JAE]

I always have reduced it to 3/4”

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[KootK]

Agreed. Under the membrane, I'd consider the roof slab to be as "indoors" as anything else. I supposed designers are at liberty to provide more cover if they feel that the risk of roof leakage getting to the concrete warrants it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MrHershey]

Will give it a third yes. ACI distinction here is 'exposed to weather or in contact with ground'. If there's waterproofing (I assume properly designed) between slab and the exterior, then it's not exposed to weather.

 

[JAE]

Sorry...corrected to 3/4”

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[IDS]

In my opinion, given:

1) The probability of the waterproofing failing somewhere over the life of the structure (high)
2) The potential consequences of corrosion (very high)
3) The cost of providing the additional cover (very low)

it's best to provide the cover specified for external exposure, regardless of waterproofing.

Doug Jenkins
Interactive Design Services

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[dik]

I don't use 3/4" cover for parkades... but have used it often without adverse effect... and there is a marginal saving, but, a saving none the less... Have to check FRR and also there is required maintenance to ensure no water penetration...

Dik

 

[XR250]

Seems like spec'ing 3/4" cover would get you somewhere between 1/4" and 1 1/2" of actual cover depending on the QC. Does not give the contractor much wiggle room.
Isn't concrete spalling and the subsequent reduced bite an issue?

 

[dik]

With concrete cover and concrete strengths and slumps... I don't spec a value, except that I spec a minimum and/or maximum value if required. For example cover would be 3/4" min and 1" max or slump would be 4" max...

Dik

 

[dcarr82775]

I tend to go with IDS and use 1.5". There is a bit in the ACI 318 commentary that allows you to go less with approval of building official, and some locations such as South Florida specifically allow it on balconies provided some other measures are taken.

 

[dik]

Why would the building official have anything to do with this engineering decision?

Dik

 

[kipfoot]

I'd go with 3/4" cover. As I understand, this is for bottom reinforcing in a roof slab that has a membrane over the top of it. I would classify that as 'not exposed to weather.'

 

[KootK]

It's hard to argue IDS's logic from a technical perspective. Solid. It would be a perception/competitiveness thing for me in many applications. Take a mid to high-rise concrete condo:

- The architect and contractor usually expect that the roof slab will be the same thickness as the floor slabs. It's not their wheelhouse but they're always happy to chime in.

- I find that that I usually need a roof slab to be 1" thicker than the floor slabs owing to no columns above and somewhat different loading.

- Bump cover up to 2" and now the structurally useless weight of that probably means that I need yet another inch of "d".

So, all tolled, I've now got a 11-12" roof slab when I've got 8" floor slabs. That'll raise some eyebrows. And, as evidenced by the concensus here, you know that the guy down the road is going to have 9". This kind of thing probably shouldn't dictate smart engineering decisions but, obviously, does. As with many things, an early conversation with the owner about risk probably makes sense.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Building officials have a legitimate interest in preventing spalling. Concrete cancer, as it is called in coastal areas, presents a big headache not only for property owners, but for the authorities as well. So I don't object when a building official wants to have an input into the quality specification.

Specifying cover it a tricky thing. What you get in a slab depends on the size of chairs used, so the method of support needs to be reviewed during reinforcement inspection. If you just specify a minimum cover, you can get anything more than that, which you don't want as too little effective depth affects strength.

 

[cliff234]

XR250 - You noted that specifying 3/4" cover will get you an actual cover that might vary from 1/4" to 1 1/2" cover. ACI 318 and ACI 117 provide allowable tolerances (on slab depth and bar placement), and the range you noted is outside what's permitted. I've also wrestled with this over the years, and I've concluded that if your bars are placed within the permitted range of tolerances, you do not have to concern yourself with having too little cover or a "d" dimension slightly smaller than what you used in design. The code and the phi factors take the tolerances in to account. If the contractor can't build the structure in accordance with the ACI tolerances, then that's not my problem. (I have enough things to worry about.) It's up to the owner and the construction manager to hire competent contractors - an the competent contractor is not always the low bidder.

I also use 3/4" cover to top steel in slabs with roofing membranes. It is up to the owner to maintain the building (including the roofing system).

 

[dcarr82775]

Dik, ask ACI. I didn't write 318 so I can't speak to their reasons.

I will admit I didn't read things close enough in that it is clearly limited to roof membrane and roof membranes alone. In that case I would go with 3/4".

Out of curiosity, what if it is a traffic coating instead of a roofing membrane? Do folks consider a traffic coating protection from weather?

 

[dik]

hokie... so does the engineer, who in my estimation, is a lot more knowledgeable than the AhJ.

Dik

 

[hokie66]

dik,
Would that it were so all the time, but in some cases, it is not. I think many of the comments on this site confirm my opinion.

 

[dik]

Hokie... your comments are correct, as are mine.

Dik

 

[cliff234]

dcarr82775 - When design parking decks with traffic bearing membranes, I assume the membrane is not there when determining the concrete cover. Traffic bearing membranes do deteriorate (wheel loads and snow plow blades) and must be replaced every so often.