Elevated Floor Slabs
Elevated Floor Slabs
(OP)
Is there any published criteria for control joints on elevated concrete floor slabs on steel floor deck? If control joints are not necessary, how large of a floor area can be poured at one time, without worrying about overall shrinkage? The floor in question is 100' x 150', and is a 4" total slab depth with 1-1/2" composite floor deck.






RE: Elevated Floor Slabs
RE: Elevated Floor Slabs
RE: Elevated Floor Slabs
RE: Elevated Floor Slabs
In my experience, you can spec putting the mesh on chairs, but when the mesh goes down and the workers are walking on top constantly, the chairs invariably fall into the flutes and the mesh winds up sitting on the deck. Then they put hte chairs back, and then the concrete guys trample them down. Its lose-lose. And I've seen two layers of mesh (top and bottom) and the slab still cracks.
Back to control joints - If you dont put in control joints, and your pouring a 15000 sf floor, there's gotta be shrinkage cracks somewhere. Usually they appear near the girder lines. Anyway, to the layman, these cracks may be a bit unnerving.
RE: Elevated Floor Slabs
I agree with your point about the difficulties of using wire mesh reinforcing. In the past, I have used rebar instead (spaced far enough apart so the workers can step between the bars) or fiber reinforcing in combination with rebar over the beams. The reality is there WILL be cracking in the slab, even with control joints. The best you can do is provide reinforcing to limit the crack widths. If that's an aesthetic problem, lay down some carpet.
RE: Elevated Floor Slabs
I've also poured topping slabs in panels to 'break up' large pours. The biggest problem seems to be preventing random cracking when brittle tile floor finishes are used.
Regarding support of top/temp reinforcing, I've tack welded rebar to studs to act as support bars for using WWF/rebar in topping slabs to help ensure it stays at the top. Often have contractors want to 'lift' the mesh as they place the concrete instead of using proper accessories.
RE: Elevated Floor Slabs
RE: Elevated Floor Slabs
Exactly...this is the reason we DO NOT specify welded wire fabric for slabs-on-grade anymore. We have gone to either fiber reinforcing or chaired rebar. Contractors don't seem to understand that WWF needs chairing just like the bars.
For elevated slabs, we do use WWF on top of the metal decking as bottom reinforcing. Here, we don't mind the WWF being in direct contact with the deck and on the bottom as this generally is where we like it. In some instances, it still should be chaired. And usually, these chaired situations call for a heavier wire as well.
RE: Elevated Floor Slabs
"For elevated slabs, we do use WWF on top of the metal decking as bottom reinforcing. Here, we don't mind the WWF being in direct contact with the deck and on the bottom as this generally is where we like it."
Are you talking about noncomposite form deck? If you are using composite deck, the deck itself will function as the bottom reinforcing.
RE: Elevated Floor Slabs
Yes....non-composite form deck. When we use composite decking, the WWF would only serve as temp-shrinkage reinforcing...or we use fiber reinforcing.