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