Slab Cover requirement for different conditions 1

[WayneRuin10]

Hello All,

I have a concrete slab where the top surface is exposed to weather, however the bottom surface is not exposed to weather. Keeping that in mind, does the ACI allow me to have a smaller cover at the bottom? Since bottom surface is not exposed to weather?

Thank you!

 

[dik]

With many codes, there are requirements for concrete cover depending on the use. Beams, columns and slabs have different cover requirements to ensure sufficient bond to the concrete and reinforcing. In addition there are other requirements for concrete cover related to corrosion issues, fire resistance issues, etc. You may require 1-1/2" conc cover on the top for corrosion issues and 1" on the bottom for fire resistance issues. It depends on the end use and the requirements. Different codes may stipulate different values.

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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

Keeping that in mind, does the ACI allow me to have a smaller cover at the bottom? Since bottom surface is not exposed to weather?

Yes, I believe that's correct per code. It is a weird situation though so I can definitely see why you're asking.

I also struggle to imagine how one would detail the envelope in this situation such that the top of the slab was exposed but, at the same time, moist air would not make it's way around the slab edges and to the slab soffit. I'm happy to leave the details of that to you and your architect however.

To the extent that additional cover provides additional rebar passivation, I suppose that one could also argue that more cover ought to be provided at the slab soffit for this reason in case water migrates from the top of the slab to the bottom.

It's a risky proposition to leave a slab over conditioned space exposed to the elements. You're just asking for moisture to make is way down through the slab thickness and into that conditioned space.

 

[JoshPlumSE]

I also struggle to imagine how one would detail the envelope in this situation such that the top of the slab was exposed but, at the same time, moist air would not make it's way around the slab edges and to the slab soffit. I'm happy to leave the details of that to you and your architect however.

I wonder if this might be related to freeze / thaw cycles and such. Where a small crack could get moisture in it and freeze to expand that crack. That being said, I might just use the larger cover for both. I've always thought that water coming up from the ground could have more salt and corrosive contaminants than regular water or such.

 

[OldDawgNewTricks]

Here's what ACI 318-19 commentary R20.5.1.1 has to say on the subject:

 

[hokie66]

A concrete slab should in most situations not be considered as a membrane. If it is left unprotected, moisture gets through to the bottom. When concrete spalling occurs, which surface? The bottom, usually.

 

[WayneRuin10]

Thank you all for your insight and inputs!

 

[KootK]

Here's what ACI 318-19 commentary R20.5.1.1 has to say on the subject

So how do you interpret that commentary for the condition at hand? I read it as implying that a slab with an exposed topside pretty much always has an exposed condition soffit. In which case the statement "slab soffits are not usually considered directly exposed" doesn't make much sense to me.

 

[OldDawgNewTricks]

So how do you interpret that commentary for the condition at hand? I read it as implying that a slab with an exposed topside pretty much always has an exposed condition soffit. In which case the statement "slab soffits are not usually considered directly exposed" doesn't make much sense to me.

For most common cases, if the top of the slab is exposed, I would design the soffit for exposed conditions also. But there could be exceptions to this if it can be shown that significant moisture will not migrate through the slab. Concrete with crystalline admixture might be one example. Another example that comes to mind is a structure that I am currently designing. It is an open structure that is mostly protected by an upper roof, but the lower slab could occasionally get some wind-driven rain. The lower slab is fairly heavily reinforced so any cracks that might form are expected to be small. The lower is slab is also sloped so even if a bit of water gets in, standing water cannot puddle on it. I might feel comfortable considering the top of the lower slab as exposed but the soffit as not exposed.

 

[BridgeSmith]

AASHTO spec specifies 3" cover to main reinforcement where the concrete is cast against earth, 2" for concrete faces exposed to weather. That said, at the DOT where I work, our standard details for approach slabs call out 1 1/2" cover for the exposed top, and the bottom that's cast against granular backfill (compacted crushed base).

 

[KootK]

@OldDawgNewTricks:

Thanks for the clarification. Out of curiosity, how would you apply this to a typical, balcony slab on a residential condo tower? Drip reglet and all that jazz...

 

[OldDawgNewTricks]

KootK, for a typical condo balcony, I would consider the soffit exposed because cracks at the top surface are likely to allow water to migrate through the slab. I suppose you could provide some type of durable membrane that could bridge the cracks and prevent water getting through but it's probably not worth the trouble considering that bottom reinforcing is not usually critical in cantilever balconies. I would just provide the extra cover and be done with it.

 

[hokie66]

If it is a simple cantilever, just don't reinforce the bottom. But most condos have some degree of support at the outer edge of balconies, or else double cantilever action, which can produce stress at the bottom.