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Concrete slab on metal deck cracking. Any insight?
7

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?

2
Where are the cracks and how big are they?

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?

(OP)
I have not seen the slab yet.  Just doing a little back ground before I meet with the client at the site.  Ken

RE: Concrete slab on metal deck cracking. Any insight?

If the cracks are over a structural member then they may be a result of lack of reinforcement for negative moment.

RE: Concrete slab on metal deck cracking. Any insight?

Normally you don't use control joints in elevated slabs.

RE: Concrete slab on metal deck cracking. Any insight?

Need to know more.  Thickness of slab, reinforcing in slab, type of metal deck, spacing of joists, whether cracks are in any sort of pattern or random, width of cracks.

RE: Concrete slab on metal deck cracking. Any insight?

2
There was an article written titled "Let it Crack"
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

http://www.carrollconcrete.com/Images/Others/Images/cpp23.pdf

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?

(OP)
Thanks ironmon.  The links were spot on.  We are ACI members, I should have searched their site.  Ken

RE: Concrete slab on metal deck cracking. Any insight?

I would add one caveat: Get the Owner's agreement and buy in before you just "let it crack". It has been my experience that Owners most often expect to have an "uncracked" slab in new buildings.

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?

(OP)
Visited the site today.  Most cracks appear to be due to negative moment over the beams where the joists butt.  A few other random (shrinkage) cracks.  Contractor did a poor job locating his construction joints and selecting the direction of placement.  I think he is ready for some help on the next one.  Thanks for the help.  Ken

RE: Concrete slab on metal deck cracking. Any insight?

In years past I have discussed this very issue with the metal deck manfgs. I was told it was not necessary to provide control joints. Think about it, theres a solid layer of metal reinfg below the concrete slab (the deck) and typically the mesh will run thru the joinst anyway.

RE: Concrete slab on metal deck cracking. Any insight?

Don't count on them being from negative moment, not that it probably matters much.  We had one slab that developed these cracks and the owner had us look at it.  

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?

Don't know if there are studs on these beams, but the rest of 271828's reasoning is spot on.  Deflection of joist under wet concrete, but floor finished level, therefore floor thinnest at support.  So they are shrinkage cracks of no structural import.  I suppose construction joints over beams would at least tend to make the cracks straight, but have never seen it done.  

RE: Concrete slab on metal deck cracking. Any insight?

You may want to review the AISC design guide for steel parking garages. They address the cracking on slabs with metal decks with some details.

RE: Concrete slab on metal deck cracking. Any insight?

hokie66 typed: "Don't know if there are studs on these beams, but the rest of 271828's reasoning is spot on."

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?

Dont have all the necessary info here to make a judgement, but there will be tension (negative moment) induced into the slab at the girders from the deflection of the bar joists.  It's not negative moment caused by the deck span.  If there's no bars or properly placed WWF in the slab then there's the possibility of pattern cracking....fibermesh doesnt count.  

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?

Boiler8019, I don't see how the joist floor is any different from a slab on beams for the current discussion.

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?

Ken, since someone has flagged this thread as useful for posterity, it would be helpful if you would give the slab thickness, type of deck, and joist spacing.  I took it as a thin slab on corrugated deck spanning about 2 feet, but others have thought it may be something different.

RE: Concrete slab on metal deck cracking. Any insight?

More importantly to me would be if these are composite beams located under all these cracked locations, what do you think this is doing to your composite action?

RE: Concrete slab on metal deck cracking. Any insight?

haynwp, that is an excellent question, one that I've heard before.  I did a quick literature search and didn't find any papers on the subject, so I doubt that anybody has ever done any tests using cracked slabs.  Pushout tests are always with pristine little specimens.

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 "Tracing Initiation and Propagation of Cracks in Composite Slabs," ASCE Journal of Structural Engineering, July 1996.

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?

I had {heard} from a good source that some testing that was done did not show much of a difference, but I would not bet my license on it. I need to call that guy out in the Midwest that does testing for Vulcraft, Lloyd I think? And see if he knows anything. I have come across a couple of composite cracked slabs over girders myself though not of my design. Or I may decide to research it out on my own.

RE: Concrete slab on metal deck cracking. Any insight?

haynewp, I'd be interested to see what he says.

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?

Curious to know why you think the beams supporting the joists may be composite.  What would be the detail for the depth of the joist seat?  We didn't do composite beams with bar joists when I worked in the US, and in Australia, we don't have bar joists at all, so I may be in the dark.  If the beam is composite, there must be a web of concrete over the flange and surrounding the joist seats, in which case I would not expect the cracks to align with the studs.

RE: Concrete slab on metal deck cracking. Any insight?

The beam almost certainly isn't composite.  If the floor is screeded level and the joists deflected, then I'm assuming that the slab is thicker away from column lines and thinner at them.  The principle would be similar--that cracks would probably form at the thinnest locations.

RE: Concrete slab on metal deck cracking. Any insight?

Joist construction doesnt typically space supporting members as much as beam-girder construction.  Therefore pattern cracking can be attributed to the joist span more likely than deck span.  It's obvious that the joists will deflect due to the weight of the wet concrete, though, I'd recommend reading into how quickly the deflection occurs.  I'll have to dig and find it, but I have read that there is time lag.  If the joists were somewhat underdesigned then he'd may want to research that avenue.

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?

Boiler,
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?

yes...I wondered that too.
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?

I don't see how there could be a time lag either, but perhaps Boiler will dig up a ref.

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?

My bad guys, now I remember my reference.  It came from a 30yr+ senior engineer I work with.  Though, not everything he says makes sense and I think he's making a bogus claim on this one.  Boiler Materials Class 101 says steel has minimal creep...shouldve stuck to the basics.

I'm interested about hearing the final outcome on this.    

RE: Concrete slab on metal deck cracking. Any insight?

(OP)
Wow, this has sparked a lot of replies.

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?

The location of the construction joints doesn't prove whether the cracks formed due to bending or shrinkage.  The section over the beam is still the thinnest, so where the shrinkage cracks would occur.  I suggest directly over the beam is the best location for construction joints in this system.  Makes a nice straight crack.

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?

(OP)
I sent the designer a courtesy email and did not get a reply.
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?

MWPC,
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?

This discussion is interesting because it stimulates the mind in figuring out why the slab may have cracked.  

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?

(OP)
hokie66 and 271828,

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?

MWPC

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?

I have always had fairly good luck achieving uncracked slabs by using draped WWF in commercial construction where I require continuous bolsters over all joists and beams. I have had similar good luck achieving uncracked slabs with draped WWF over composite beams plus #4 rebar over the composite girders. Make sure the As of the draped WWF can resist the calculated negative moment. I calculate the required negative moment over the composite girders using Appendix A, Design of Two Way Slabs, Method 2 from ACI 318-63. The effective slab thickness over the composite girder is just the slab above the deck which does not give much to work with.



RE: Concrete slab on metal deck cracking. Any insight?

(OP)
jike,

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.

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