3" RC slab 1

[XR250]

I have a residential project where the existing front porch slab/paver system is to be overlaid with 3” concrete spanning 3’-4”. I have no way of verifying the existing slab capacity, however, the slab appears to be 3” thick with 1” of mortar and full-depth pavers. It vibrates more than expected when it is jumped on. I have run the numbers and it appears that a new 3” slab with #4 @ 18” O.C. meets the demand using 1” bottom cover. The ends of the new slab will be supported by the existing perimeter foundation. There is a roof over this area. Is there any practical issue with specifying a 3” slab? Seems thin. I will have the contractor provide a line of temporary shoring under the existing slab.

 

[werwiewas]

3" is the minimum thickness if there for non vehicle circulation.

 

[MIStructE_IRE]

Over here our building regulations require a minimum thickness of 150mm (6”) regardless of application. Do you guys not have anything like that?

From a pure engineering perspective I can’t see anything wrong really... If it works it works.. But its still a little too thin for me.

 

[r13]

It shall be ok as a topping slab. I would use low slump concrete, and provide protection throughout the curing period. Make sure the substrate is in moist state when pouring concrete over it.

 

[phamENG]

How detailed are you getting with specifying the concrete? If you keep 1" clear bottom cover, you'll have 1.5" clear top cover if they set the bars perfectly. Even with a roof, that's a little closer than I usually spec top cover for slabs - you're still outside. Anyway, as long as your aggregate size fits above and below comfortably so the "paste" sets up around the bars properly you're probably ok. Though with so little margin for error in placement, etc. I doubt the actual slab will perform exactly as you've analyzed it.

 

[XR250]

Though with so little margin for error in placement, etc. I doubt the actual slab will perform exactly as you've analyzed it.

If I spec the rebar 12" O.C., my demand is 1/3 of my capacity so I feel OK about that.
I guess i will spec a fine aggregate mix. Fortunately, the existing slab has a sag in it, so my actual depth will be more in the critical section.

I might check it with W2.0 WWF and see how it fares..

 

[Tomfh]

You should space the bars more closely than 12". That's four times the slab thickness. Our code has max spacing of 2x slab thickness. I'd space at 4"-6"

I'd also drill some L-bars into the other slab to pickup some composite action.

 

[phamENG]

Welded wire could be a good option. Look into the availability of deformed welded wire. I've only spec'd it a couple times, but didn't get push back. Those were commercial, though, so a residential contractor might baulk at that.

 

[Settingsun]

You're violating several 'rules' eg spacing and bar size relative to the effective depth but, on the other hand, you'll have two slabs where either suffices. You might also get some arching if the supports have lateral capacity.

RL718 ribbed mesh sounds ideal to me for design and construction. Not readily available in USA?

https://www.meshbar.com.au/reinforcing-mesh/

 

[EngineeringEric]

Welded wire mesh is they way to go for this size slab. it is very common in the USA everywhere i've done work. look at SDI (vulcraft for instance) and they will publish allowable loads for just the concrete deck for a 3" over 0.6C deck with welded wire fabric. again this is when they neglect the capacity of the deck... so very close to what you are saying. I would check it and see what they call for reinforcing at that span.

you may want AE concrete too?

 

[XR250]

Thanks for everyone's insight. Looks like 6x6W2.0 will work but not a lot of wiggle room. I'll check to see if 2.9 or 4.0 is available locally.

 

[Lomarandil]

I don't know if Tom and Steve are referring to other codes, but I would argue the ACI slab bar spacing requirements (2x thickness for two-way) don't apply because this is a SOG.

The other benefits of WWF aside for the thin slab here, I typically prefer to spec SOG bars at 12"+ spacing so they aren't stepped on.

----
just call me Lo.

 

[XR250]

but I would argue the ACI slab bar spacing requirements (2x thickness for two-way) don't apply because this is a SOG.

I am considering this a suspended slab as the slab below is suspended and my goal is to only use that as a form.

 

[Lomarandil]

Ah, missed that. My apologies.



----
just call me Lo.