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

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

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

(OP)
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.

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

I always have reduced it to 3/4”

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

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.

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

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.

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

Sorry...corrected to 3/4”

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

4
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
http://newtonexcelbach.wordpress.com/

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

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

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

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?

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

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

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

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.

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

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

Dik

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

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.'

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

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.

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


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.

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

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).

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

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?

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

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

Dik

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

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.

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

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

Dik

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

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.

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

I'd say that, per cliff234, with a high-traffic area with a bonded membrane, then a 3/4" cover might not be appropriate.

However, for a concrete slab on a building, with a roofing system,
I'd go with the 3/4" as it is not "exposed to earth or weather" and,
while roofs do leak, it is not always excessive in frequency or area.

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

I'd never use 3/4" with parkades... more like 1-1/2" absolute minimum...

Dik

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

>>>When design parking decks with traffic bearing membranes, I assume the membrane is not there when determining the concrete cover.<<<

Cliff234, could you expound on that a bit? I've never really understood the point of a membrane on parking garages yet I see that so many of them have it. What is it's function? Is it to protect leaking at the joints? If so I suppose it's easier to provide a membrane over the whole deck instead of just at the joints. Is that the rational? Otherwise it would seem to me that protection of reinforcing steel would be most cost -effectively provided by adequate cover and high enough f'c, same as any other concrete. There must be a reason it's done in the majority of the cases I've seen. Thanks.

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

Whatever decision you make, be sure that the plumbing details are copacetic. Years ago I walked a high rise roof that had a 2 inch architectural topping slab. Some how the drains for the roof were set to the topping slab elevation with no accommodation for the roof slab to drain. When the waterproof membrane on the roof slab started to fail, trying to find the actual location of the leak was impossible. When it rained the leaks went on for days afterwards.

If you go with a 3/4" cover, what kind of surface prep do you specify if a cold applied membrane waterproof/roofing membrane is going to be used? I have no faith in contractors doing a proper job of acid etching. Relying on waterproofing warranties is just building misery, legal & consulting fees into the structure.

Archie264 - As we all know concrete cracks. Often initially only the top floor of a parking structure receives a traffic membrane. Owners of parking structures don't like having to pay for paint work on luxury autos, so the parking bays on flat deck parking structures eventually get a membrane, especially in snow country. Also, flat decks are never flat. Open sided, flat deck parking structures have bird baths. The coarse broom finish on parking garage ramps doesn't last forever and if the garage is prone to cars standing backed up, while entering or existing, they are depositing fluids in the drive path. It is sometimes better to get out in front of the wear & tear problem than have concrete so contaminated that the friction coarse membranes won't stay down.

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

Archie264 - For cast-in-place concrete parking structures, we always specify traffic-bearing membranes on slabs that are above non-parking spaces below (i.e., slabs that are over retail, offices, etc.). The purpose is to provide a greater level of protection against leaks into those spaces. We don't coat all of the slabs. Only the slabs over non-parking areas. Our precast garages rarely have occupied spaces (i.e., non-parking) below the double tees - so I've never seen the membrane coating used in precast garages.

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

Epoxybot and Cliff234, thank you very much.

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

Unprotected concrete roofs leak, and leakage also occurs through floors at cracks if there is water present. And water is inevitably present in parking garages, due to cleaning if nothing else. Don't assume that cars below can tolerate water leaking from above...so I agree with epoxybot.

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

Archie264: I'm not sure where you practice, but Google "Elliot Lake mall collapse" if you want to know why there should be a membrane on a parking garage slab.

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