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'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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[dik]

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

Dik

 

[Archie264]

>>>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.

 

[epoxybot]

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.

 

[cliff234]

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.

 

[Archie264]

Epoxybot and Cliff234, thank you very much.

 

[hokie66]

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.

 

[OldBldgGuy]

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.