Elevated Floor Slabs 2

[Zulak]

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.

 

[LPPE]

I've heard for a 5" slab with 2" metal deck, you need a control joint roughly every 1000 sf. Just a thought.

 

[Taro]

Control joints will not do much good in a composite concrete on metal deck slab. The restraint of the steel deck will not allow the joints to be effective. In fact, cutting control joints every 30 feet or so (to meet your 1000 SF requirement) will do more harm than good in terms of reducing composite beam strength and diaphragm capacity. You are much better off eliminating the control joints and providing well distributed slab reinforcing to minimize crack widths. If you're using wire mesh reinforcing, specify that it be chaired up on rebar to keep it near the top of the slab where it will be effective, instead of laying flat on the metal deck.

 

[JAE]

Right, we don't use control joints in elevated slabs on metal decking either. In most cases, the elevated slab is not exposed (cracks don't matter that much) and the shrinkage is not resisted by the elevated structure to the extent that a slab on grade is.

 

[LPPE]

Good point. I guess depending on desired use, control joints would be optional. If you have a warehouse, then who cares about cracks.

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.

 

[Taro]

Let me reiterate. Control joints in composite slabs are fairly useless. The idea behind control joints is to let the slab shrink unrestrained between the joints so that all of the movement is concentrated at the joint and cracks do not develop between the joints. This is impossible with composite slabs because the concrete is bonded to the steel deck. The deck will not allow the concrete to shrink unrestrained. You will be doing more harm than good by cutting the slab.

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.

 

[dik]

I've sawcut 1" deep parallel to the centreline of beams with good effect and have used the beams for composite action at the same time with transverse top reinforcing holding the concrete 'together' so composite action can occur.

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.

 

[LPPE]

Dik - good idea on the rebar on top of the studs. And, yes, almost every concrete contractor wants to lift up the mesh as they pour. That would seem OK, but then the concrete guys walk all over and through the fresh concrete, therefore sinking the mesh back to the bottom. Then again, it all depends on who the contractor is. Some guys do exactly what you want with no problems, while other guys do whatever they want.

 

[JAE]

pylko -
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.

 

[Taro]

JAE wrote:
"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.

 

[JAE]

Taro
Yes....non-composite form deck. When we use composite decking, the WWF would only serve as temp-shrinkage reinforcing...or we use fiber reinforcing.