Parallel cracking of a suspended composite slab

[KeyEng]

The contractor has made me aware of cracking to a recently poured suspended composite slab. The slab is 5.5" total thickness over 3" composite deck, supported on load bearing walls at 10'-0" o.c.. Deck sheets span, minimum 2 spans. The hairline cracks are 2'-0" o.c. approximately, parallel to the deck flutes, perpendicular to the load bearing walls and parallel in nature. The subcontractor was contractually forbidden from using ride-on power floats to finish the slab, but did use such equipment. Has anyone encountered such cracking, was it caused by the ride-on power floating equipment and is the structural integrity of the suspended composite slab compromised.

 

[jayrod12]

What is in the slab for crack control reinforcing?

 

[KeyEng]

The slab is ultimately designed to support a 125 psf live load. The slab is reinforced with the deck manufacturer's recommended 6"x6" W1.4xW1.4 WWF. The power float was 1052#'s + operator.

 

[jayrod12]

Even spaced cracking like that sure feels like shrinkage cracking. Are you sure the mesh was installed? and chaired up off the deck?I bet those cracks line up directly over the high flutes where you've only got 2" of concrete. and if the reinforcing wasn't chaired, then it's not doing you any damn good for crack control.

 

[KeyEng]

I don't disagree, but 2'-0" o.c. seems excessively close, especially when poured in near perfect fall temperatures. I've been involved in dozens of these type pours in apparently harsher conditions and have not seen this to date. Ultimately, I concerned about the structural integrity of the suspended composite slab.

 

[jayrod12]

If they are over the high flutes, then I wouldn't necessarily worry about the composite action of the slab and deck, as the composite nubs are only in the low flutes.

Others here may have more insight.

 

[hokie66]

Restraint by the bearing walls, too thin concrete over the ribs, inadequate shrinkage reinforcement, maybe some contributions from the concrete mix and placement. All in all, a cheap and nasty way to build a slab. You get what you pay for. But then, the cracks would not affect the ultimate capacity of the composite deck.

 

[JAE]

But then, the cracks would not affect the ultimate capacity of the composite deck.

I also agree with this statement.

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[Tomfh]

I've seen parallel shrinkage cracks like that on topping slabs. I don't believe shrinkage reinforcement can save you in these instances. I agree it's not generally a structural problem - it just upsets people.

 

[jayrod12]

I've never experienced that level of cracking from shrinkage, however I tend to allow no less than 3" of concrete above the high flutes and reinforce it with 10m (#3) @ 12" o/c perpendicular to the flutes.