Elevated Exterior Concrete Deck

[XR250]

Working on an elevated exterior concrete deck for a restaurant. It will be open below. The deck spans approximately 6 ft. between supports and is 20' x 25' in rectangular dimension.
The framing is I-beams. The slab is proposed to be 6" total thickness and tapering to 4 1/2" at the internal floor drain. I am considering using 18ga inverted composite deck, G90 (unshored) and adding sufficient flexural reinforcement to carry the concrete if the deck rusts out. Also, adding a significant amount of temp. and shrinkage reinf. to minimize cracking. A collegue of mine suggested using shored 22ga. 0.6C G90 conform and constructing it as only a reinforced slab. His contention is there will be less cracking potential due to the smaller rib height. Any thoughts? Also, the client does not want to add any topping,tile or other substrates to the slab. I was thinking about spec'ing a hardnener/densifier in lieu of any other topping. He is aware that the deck may no last as long it would otherwise. Thanks for your help in advance. FWIW, I have witnessed many exterior suspended slabs in our area that are 60 years old and show no signs of issues.

 

[RHTPE]

If you are designing the slab reinforcing as if the metal deck did not exist, why incur the cost of composite metal deck?


Ralph
Structures Consulting
Northeast USA

 

[JAE]

Also - why not use a traffic bearing waterproof membrane on top? (look up Neogard - Pedagard online)



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[XR250]

@RHTPE - Was considering the composite deck just as belts and suspenders. Also, would not have to shore it.

@JAE - I will pass this along to the client - Thanks


FWIW, I called W.R. Meadows and they recommended a penetrating sealer applied annually instead of the densifier. I like the idea of the Neogard better - hopefully the client will as well.
My concrete consultant also recommended using only 3000 psi concrete with a mid-range water reducer and 6" max. slump to further help prevent cracking.

 

[BUGGAR]

And don't forget a good long, wet, flood cure immediately after pouring. And no steel trowelling if you have air entrainment.

 

[KootK]

I would expect a deeper slab to produce wider cracks if the deep and shallow options were designed to the same deflection limits (identical curvature). That's rarely likely to be the case however.

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.

 

[XR250]

@BUGGAR:

I was def. going to specify wet curing for 7 days. How do you finish the concrete w/o a trowel - broom?

 

[Canuck67]

I would also consider 'vented' composite metal deck, especially if you are considering applying a waterproofing membrane. This will allow some relief for the water that will eventually find it's way past the concrete and sit trapped on the metal deck. Venting will allow some form of relief for trapped moisture and lengthen the life span of the metal deck.

 

[XR250]

@Canuck67 - never heard of that. Thanks for the info.

 

[Canuck67]

XR250,

See attached excerpt from CANAM catalogue. You definitely don't want moisture trapped in the system. Galvanizing will also help.

 

[XR250]

Thanks. Was going to spec G90 regardless.

 

[BS2]

I too am currently working on an elevated exterior concrete deck. The deck is much larger however at approximately 50'x80'. My plan is to use 2" non-composite deck (galvanized) with 5.5" total slab thickness. The concrete will be reinforced to handle the 100 psf live load. As JAE mentioned, we are planning to use a Peda-Gard membrane to waterproof the system. The deck will also be sloped at approximately 1/4" per foot to one side to shed water.