Concrete slab on metal deck cracking. Any insight?
Concrete slab on metal deck cracking. Any insight?
(OP)
We have a client who's elevated slab on metal deck and bar joists is cracking. Thankfully it was not our design. It appears to be shrinkage cracks but I cannot find much information on control joint locations for elevated slabs. Any help or direction to authoritative source would be appreciated. Ken






RE: Concrete slab on metal deck cracking. Any insight?
If they're not very large and are over beams and girders at column lines, then they're not unusual. Assuming that it was screeded level, the slab's thinnest there. We tried everything at my old job and we could never get rid of these. They only caused major problems when we used sheet vinyl flooring. Even tiny cracks cause little poofed up areas that we called ant trails in that kind of flooring.
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
Written by J. Thomas Ryan, PE.
Could not find the direct link. I have pdf on disk, be glad to email it to you, or post it somewhere.
Here is some info to back up you assessment.
http://www.aci-int.net/FAQ/afmviewfaq.asp?faqid=65
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Bottom line, all slabs crack. The cracks in elevated slabs on steel deck usually do not compromise structural integrity.
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
I agree that in commercial buildings the crack can be filled and be covered with finished floor material although if it a hard material, a working crack sometimes can mirror thru.
It is a trade off between adding more steel or accepting cracks. Get the Owner's direction on what he wants.
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
The 3rd Floor was occupied and had ant trails (little ridges in the sheet vinyl) above beams and girders at every column line.
An upper floor was shell space, so had practically no load above its self weight, and it had cracks in the same locations, mostly small, but some pretty good size. We had seen these before, so provided #3@12 top, IIRC. That did a heck of a lot of good, LOL.
I figured these were from negative moment, but one of the older guys pointed out that there was no negative moment in the slab from the slab weight because it cured in that position--duh.
He said that the cracks simply form there because that's where the slab is thinnest and the studs provide stress risers, almost like the perforated edge of a paper designed to be torn out. There's also lots of restraint against shrinkage and the slab was probably 150' long. I had to think about it a while, but I'm sure that's right.
Unless you have a shored system or some serious superimposed loads, I have serious doubts if your cracks are from negative moment. Not sure that it matters now--they're there.
That being typed, I did cause negative moment cracks in a lab specimen once. It was a 3-span footbridge. When I put 20 psf on two adjacent spans, it cracked over an interior support. I think it did not crack earlier from shrinkage because it was so stiff that it wasn't much thinner over the supports and because it had no studs. It also wasn't as restrained against shrinking as a real slab would be.
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
Thank you very much for the backup.
I think you'd think I went to a good school.
RE: Concrete slab on metal deck cracking. Any insight?
I would just caution that composite steel girder-beam construction, posted above, produces different issues than composite steel girder-bar joist construction.
RE: Concrete slab on metal deck cracking. Any insight?
If the joist deflections are due primarily to concrete weight, then these deflections would also occur before the concrete cures, so no negative moment. If there's a significant superimposed load AFTER the concrete cures (if this is the case, then how can he see the cracks?), then that could cause negative moment cracks.
But then again, this is also the case for a composite slab on beams. I think these cracks occur with or without any load being applied. Also, like I typed earlier, reinforcement won't prevent these cracks.
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
Just from looking at the huge number of cracked slabs and the absence of problems, it seems that they probably don't do much.
RE: Concrete slab on metal deck cracking. Any insight?
Lin, Yiching, J.Y. Yen, and Chen-Fun Chen "Composite Beams Subjected to Static and Fatigue Loads," ASCE Journal of Structural Engineering, June 1997.
These guys discussed the subject a little, but it's not very helpful. There are lots of refs in those papers if anybody cares enough to go digging.
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
The next time I talk to Dr. Easterling at VT, I'll ask him too. He's a composite slab guru.
As for betting a license on it, I don't think one has much of a choice other than to not assume composite action at those locations. That one paper even said that rebar performed WORSE than WWR, so there's no help there!
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
The post above regarding the lab test on the 3 span deck tested the deck capacity of a narrow walkway is quite different than a longer spanning support system. Just something to ponder.
RE: Concrete slab on metal deck cracking. Any insight?
Why would there be a time lag? There is no creep component of deflection in steel at atmospheric temperatures.
RE: Concrete slab on metal deck cracking. Any insight?
Boiler8019 - I don't think there'd be a time lag with steel. The deflection would be instantaneous wouldn't it?
RE: Concrete slab on metal deck cracking. Any insight?
One idea is that the concrete at the top shrinks faster than the concrete on the deck. This would cause curvature and perhaps negative moments over supports.
I'd have to think about the moment diagram with the top of the continuous beam trying to shrink more than the bottom.
The concrete is so weak during those early times that it might crack due to this.
I don't see any way that it can be due to deflection of slabs, joists, beam, whatever, though.
RE: Concrete slab on metal deck cracking. Any insight?
I'm interested about hearing the final outcome on this.
RE: Concrete slab on metal deck cracking. Any insight?
From a quick site visit and conversations with the contractor here is what I understand the system to be:
-3" normal wt concrete slab, total thickness.
-WWF 6x6-W2.9xW2.9
-3/4"+- corrugated metal deck, non-composite
-Steel joists at 2' o.c. spanning between beams
-Steel beams spanning between columns, non-composite and simple spans.
As noted in my earlier post, the contractor made a poor choice of construction joint locations. He located them parrallel with the beams at roughly the quarter points of the joist span with the 1/4 placed first with adjacent bays. Then when he placed the remaining 3/4 of the slab, the dead load created negative moment in the hardened slab over the beams. Loading the beams also created negative moment over the columns and caused cracking in the orthogonal direction. Throw in a few hairline shrinkage cracks and you can see why the owner questioned the product.
It seems that placing the construction joints on the column lines parrallel to the joists would have eliminated most of the cracks, especially the larger ones.
Most concrete in the area contains fly ash. All else equal, and it never is, how does fly ash effect the shrinkage as compared to a straight portland mix?
Ken
RE: Concrete slab on metal deck cracking. Any insight?
Properly designed fly ash mix should not shrink more than all portland cement mix. If anything, a bit less. More likely culprit is inadequate (or zero) curing.
I did a lot of floors like this one years ago and never had problems with this type cracking. Wonder if the floor is as stiff as it should be. What does the engineer who designed it say?
RE: Concrete slab on metal deck cracking. Any insight?
Casting the slab in strips parallel to the joists would eliminate negative moment cracking over the beams since the concrete would not have set and over the columns there would be a nice straight joint.
RE: Concrete slab on metal deck cracking. Any insight?
Maybe you didn't read all the posts, especially those by 271828. He and I, and perhaps others, believe that your cracks are probably not due to negative moment but rather to drying shrinkage which occurs above the beam because that is where the slab is thinnest. If we are correct and you pour strips parallel to the joists, the cracking over the beam will still occur. Thus my suggestion to pour the strips parallel to the beam.
RE: Concrete slab on metal deck cracking. Any insight?
But I wouldn't worry about the cracks in suspended slabs. They aren't all that critical. Control joints aren't, and in my opinion shouldn't, be cut into concrete on metal deck slabs.
RE: Concrete slab on metal deck cracking. Any insight?
I believe I had read the posts carefully and agree that the slab is thinnest and would be most likely crack over the supports both parallel to the joists and perpendicular to the joists assuming that the beams deflect too. However one of the cracks over the beam was within about 8 feet of a construction joint and it was one of the larger cracks. If it was only shrinkage induced, I would not expect it to be that close to the end and that large. As BEFORT and the references stated "typically no control joints in slab on deck" since the deck acts as resistance to shrinkage. With this in mind, I would not have expected the large crack.
If the joists deflect enough to make the slab significantly thinner then they deflect enough to cause negative moment in the hardened concrete over their end. If a lightly reinforced 3" concrete slab was cantilevered 8 feet, I would expect it to crack. If there was a little bonding of the concrete to the deck then the concrete was trying to stop the end rotation of the joists to boot.
While I agree that thinner slabs over beams are typically the cause of the cracks, in this case I stand by my assumption of negative moment being the major cause.
Thanks for the references and discussion. If I had more time I would measure all the crack widths and document their location along with the construction joint locations, joist sizes, beam sizes, spans etc and we could look at it analytically, but that doesn't pay the bills.
Ken
RE: Concrete slab on metal deck cracking. Any insight?
A very interesting post. I have seen a great number of cracked slabs with composite steel deck slabs. Some were office floors and some 125 psf mezzanines.
In two separate cases, one office and one storage mezzanine slab cracked a couple of years after the initial pour with a very load noise that alarmed the occupants and called for our investigation. It appeared that the cracks were negative moment stresses that belatedly let go.
Recently a viewed a large mezzanine slab framed with open web joists about 3 ft oc. The slab crack at almost every joist location, most were hair line, but some started to unravel under pallet jack wheel traffic.
I think the best solution to eliminate or at least keep the cracks tight is to limit beam deflections to L/480 to L/600 if you can.
An tell your Clients that this is the beauty of concrete "it cracks where it wants to!"
Any other thoughts on limiting deflection?
RE: Concrete slab on metal deck cracking. Any insight?
RE: Concrete slab on metal deck cracking. Any insight?
Good point. Keep the reinforcing in the top of the slab to limit cracking. I am not sure you can drape it in both directions with the steel joists at 2' o.c. but just keeping it in the top should help reduce crack width.