Elevated Slab Cracking

[jplay2519]

I have a storage facility that is being constructed. Floor plan consists of 3 floors of storage (slab on grade, elevated level, elevated level, roof). The elevated levels are supported by 2" composite steel deck with 2" concrete with 6x6 W1.4xW1.4 1" below slab. the slab is cracking like crazy, every 12" or 36" parallel with the span. The deck spans 10ft continuous spans and bear on cold formed metal walls. I'm not sure why its cracking so much but with the reinforcing in and we are 10 days old it seems like something with the mix maybe. The contractor said they have seen this but not as severe when finishing with large riding trowels and stay on just a bit too long. Any ideas?

 

[hokie66]

They are drying shrinkage restraint cracks. You only have 2" of concrete over each rib, with minimal reinforcement. To be expected, but perhaps not this severe.

A lot of things could have contributed, with the concrete mix itself probably a minor factor. Weather, finishing technique, lack of curing, etc. But you have narrow, evenly distributed cracking rather than large, widely spaced cracks, and that is the good news.

 

[dik]

Minimal topping thickness... what was the slump, strength, etc. Where is the WWM located? in flat sheets? Do you require any fire resistance rating? Storage facility... how are you moving stuff on it?

What was the weather like and was it exposed to sunlight and/or wind? What curing?

Dik

 

[jplay2519]

4" slump, 4000 psi specified strength, around 3000 psi around the time cracks started appearing, WWM is 1" below top of slab in flat sheets. We used a UL assembly to get fire rating (may have been 2 1/4" instead of 2"). It's people bringing things in and storing it there so walking, rolling, dolly etc. It was rainy during the 1st week but sunny and windy directly after. Not sure how they cured it. Is there anything to do to repair these at this point or just leave it be? Trying to grout the cracks or something seems unreasonable. Maybe sealing it somehow, but that won't do anything for the cracks.

 

[dik]

So under ACI, that could be 5" slump... For concrete topping, I'd usually go for a 3" max, and let the contractor use SuperP. If not cured, with wind and sunshine, after a week, you could get excessive drying since the thinness is there. No easy fix... an epoxy finish to 'fill in the cracks' may be possible, else, live with the cracks. I'd not normally use that kind of floor construction for storage areas.

dik

 

[bones206]

My first thought was excessive restraint against shrinkage/drying due to those channels fastened to the slab.

 

[bhiggins]

The area of wire reinforcement seems way too low. I would use 3" as average slab thickness: As(req)=3"x12"x0.0018=0.065 in^2/ft

Wire reinforcement also tends to be stepped on and mangled before concrete placement so it is often at the very bottom of the slab unless extreme care is taken.

 

[epoxybot]

Maybe it is the photos but some of those cracks look to be quite large (wide). It suggests an excessive amount of water or they also may have started too soon with the finishing. I'd be curious to know if, when an ice pick/awl/kinife is scraped inside a crack, if the concrete feels weaker than one might like. The real question is how this concrete will hold up in the main traffic areas. If the concrete is soft under the steel trowel finish, then the finish will be short lived and dusting will be the issue. As Dik states it is not an easy issue to address. The steel trowel finish makes the application of "penetrating sealers" intended for surface strengthening via consolidation unlikely of any satisfactory outcome.

What is the condition of the slab on grade? If the concrete of the slab on grade is of the same specification & finish, yet doesn't display the same defects; you could do a 60 day Elcometer Pull Off Test comparison. If there is a marked difference, floor to floor (lower on the elevated decks) there may be a need to discuss some form of compensation to the owner for a long-term durability issue. These only makes sense to be addressed at a later date, as the finish in the traffic areas diminishes. Of course there will be some finger pointing. Only you know the extent of what the contractor's attitude was, when directed to a defect and replied "That's what always happens". Sometimes, it is the truth because the specs are lean (Undernourished) & sometimes it is just bad work.

 

[rapt]

As bhiggins suggested, the reinforcement supplied is below half of what would be required for unrestrained slabs. Add some restraint (from the decking itself and the walls) plus a little excess concrete depth and it is far too low.

As I understand it, your WWF is very low ductility, so when a crack has occurred the amount of reinforcement is so low that the tension strain the WWF has to resist is too large and the steel fractures, giving very bad cracking!

Has anyone checked flexural strength, punching shear, deflection or vibration for this slab?

 

[Wallache]

That size of WWF is spec'ed by the deck manufactures for composite deck. I wouldn't call it under-reinforced since thousands of floors have been constructed that way with out the cracking described.

 

[hokie66]

Wallache,
If you depend on deck manufacturers to specify crack prevention reinforcement, that is what you get. I think your statement about "thousands of floors" is misleading. The cracking is inevitable in the scenario described by the OP.

 

[Celt83]

If it is a true composite the deck, the deck itself serves as a portion if not all of the minimum reinforcement for positive bending. SDI notes a minimum of 0.00075 times the area of concrete above the deck for temperature and shrinkage, however the commentary does note this won't prevent cracking but instead help to control it.

Negative bending regions on the other hand should follow standard ACI rules.

If these are single span deck conditions that 10ft span would be pushing it, was it unshored construction?

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[rapt]

Celt83,

This cracking appears to be transverse to the spanning direction, over the top of the deck ribs. Depending on the deck profile, the decking will probably provide no minimum reinforcement in that direction!

.00075 is a joke, especially if the steel is of very low ductility.

And 2" thick slab over the decking ribs!

Hope everyone's insurance is paid up!

 

[bhiggins]

Does the crack spacing more or less match the deck rib width? This appears to be cracking parallel to the deck span because the deck would provide restraint for cracks perpendicular to the span.

 

[hokie66]

In addition to the cracks parallel to the deck ribs, there appears to be some cracking in the other direction, and some crazing. This would indicate that the slab was cast in low humidity and perhaps windy conditions.

Placement, finishing, curing, all contributors. And with effectively no crack control reinforcement, this slab is a classic of everything not to do.

 

[jplay2519]

I'm going to tell them to increase the reinforcement to W6x6 4.0x4.0 to get 0.0018 of the full depth. They need to do something about the finishing and curing too. They were rushing to get it finished so they could put anchors in. I've had issues with this PM the whole time with his pushing timelines and trying to keep things lower priced. He needs to pull back and listen now instead of trying to do "what they do on their other projects".

 

[hokie66]

Good luck with that.

 

[dik]

I second that... I'd probably look at your mix design and curing for a jumping off place.

Dik

 

[haynewp]

The lower ratio on metal deck currently comes from SDI C1.0, but the lower ratios go back a long time before this document. The document “Designing with Steel Form Deck” has the below comment. I assume they have a similar stance for composite deck.

“Another influence on designing the reinforcing is that the deck does provide some unknown amount of shrinkage reinforcement. Many successful slabs have been con-structed with less than the ACI recommended amount of temperature steel.”