Cracks in Slab over Conform Deck

[Engineer CFL]

I was called out to inspect a interior mezzanine in a retail stock room where there were cracks in the concrete deck. The owner wants to know if the cracks can be cosmetically repaired or the slab needs replaced. Upon looking at it, there were mapped pattern cracks all over. A few cracks had the reinforcement exposed and were missing the concrete along the cracks. A few other areas the reinforcement was visible at the top of the slab - likely a QC issue.

The spans are fairly short, varying 2' to 2'-6" between the bar joists below. The deck is 9/16 24ga conform and the net slab thickness is 1.5" with 6X6 W1.4XW1.4.

Fortunately, I had access the the original plans to confirm this along with field measurements.

Per the SDI Engineering manual, the concrete slab is the structural component. The manual makes this clear even though in the field there is bond between he deck and the slab. However, at 1.5" thick and poor QC I can understand why it cracked with bottom restraint at the deck and dynamic live loading from rolling carts, etc. It looks like it has been this way a while. Under ideal conditions the slab should have the capacity to support the 125PSF LL and the DL. Additionally, the form deck also has the capacity to support the DL & LL loads with L/240 (<1/8") defection with a SF of approximately 1.4 in the 2 or 3 span condition.

My engineer brain says that a structural system is damaged and needs to be replaced. I understand that all cracks aren't necessarily bad but the its the combination of poor qc, missing concrete, exposed reinforcement that makes me deem it unacceptable. I don't think grinding out WWM is going to be an option.

However, I wonder that since there is no "mathematical" threat of collapse, replacing the entire slab might be overkill for this application.

Any perspectives on how you would handle this are appreciated.

I am sort of stuck in this place of trying to communicate this to the client without a)creating a panic but also b)informing that a structural system has been damaged and should be repaired/replaced.

 

[hokie66]

That type system is prone to cracking as described, especially if exposed to dynamic wheel loading. And then, the concrete was probably not the best. A cheap and dirty system used in the wrong application. That being said, serviceability is now the issue, and something needs to be done. My first thought is some type of flexible overlay, perhaps one of the trafficable membranes. Suggest you talk to Sika or the like.

 

[bones206]

Maybe you could look into some type of repair overlay material. Something that has good durability as a wearing surface but flexible enough to bridge cracks.

 

[jhnblgr]

Is it possible they used a ride on power trowel to finish the concrete at the time of construction and it caused cracks in the concrete due to the loading exceeding capacity at the time of finishing? The joist deflection caused the cracks and the concrete had already set but the trowel worked the cream into the cracks hiding them. The concrete in this case is structural and thus the metal decking is the only thing holding the floor up now. Also could be the stone used was too large for that depth. Recently saw a slab with cracks following the reinforcement everywhere perfectly and I told the person the cover over the bar caused the issue all because the design engineer had no idea about concrete design and maximum aggregate size or minimum cover for construct ability.

 

[hokie66]

Actually, I would consider the form deck as the structural element now, with probably some composite action due to friction between the deck and concrete. The OP said the deck has enough capacity on its own, and if it is in a protected environment, that should not change. Possibly not enough for purists, but not a life safety issue. A trafficable membrane could be the solution.