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Slab on Slab Construction

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C Hietbrink

C Hietbrink

Structural
Joined
Aug 8, 2017
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13
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US
I need to bring the finished floor elevation of an existing elevated concrete slab up by anywhere from 2"-4" for new operating rooms within a hospital. The operating rooms are required to get a specialized epoxy flooring in which the substrate that it is installed over the existing elevated slab must meet several strength, flatness, and level requirements and can't have any cracks at all. The problem I am concerned about is that the new topper will span over an existing control joint between two separate building additions that are not tied together so I am worried about crack propagation through the topper due to differential movement of the two structures. To help mitigate this I was going to install a thin plate only anchored to one side to span the joint with two layers of poly over the entire floor to act as a slip sheet to minimize shrinkage cracks. Has anyone had any experience with a similar installation? If so what kind of concrete topper product would you recommend? The flooring contractor has stated that products like Ardex and other similar leveling compounds will not work. The topper must have a tensile strength greater than 219psi and a compressive strength greater than 4300psi as tested after installation. Any help or suggestions would be greatly appreciated.
 
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That's quite the predicament...

If that's the expansion joint between two structurally separate buildings, you don't want to be connecting them. The two structures move separately in the vertical and lateral planes. Depending on where you are geographically, this could be significant (high wind or seismic).

Does the layout of the new operating rooms require you to maintain a clean surface over that joint or is that the dividing line between two rooms? Typically expansion joints aren't located in the middle of a room (but I have seen it in some cases).

 
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The joint is located in the middle of the new operating room layout. At this point I doubt the client wants to go through a redesign of the layout. The plate that would span the joint would only be anchored on one side which would allow horizontal movement between the buildings. And the slip sheet would allow the topper to move independently of the existing slab again only in the horizontal direction. Unfortunately, I don't know if there is much I could do about differential vertical movement.
 
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I don't really like the slip sheet idea.

If the buildings move separately along the plane of the joint, where will the topping slab go? Will it even be able to slip based on the coefficient of friction? And if it could, which slab would it move with? And if all that is satisfied, won't the walls and other obstructions restrain it from slipping to begin with?

This doesn't even consider that with seismic or wind loads you really need that separation.

I don't think the slip sheet is viable.

For vertical movement, you probably have beams or a solid slab on both sides of the joint. Both probably have different stiffnesses and if there is something loading one side it will deflect, the other side won't. You'll form a crack at the joint. Maybe you can detail a way to prevent differential vertical movement in the beams/slabs but allow lateral movement in both planes? But that doesn't solve differential foundation settlement, which may not be a problem if you have similar buildings/foundations/age.



 
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You're right to be concerned. Unless you are considering connecting the structural frames of the two parts of the building together, I think you can expect cracking in your topping slab at the interface.

The plan that puts operating rooms over an expansion joint sounds like a set up for ongoing maintenance expense and disruption of the use of the facility.

I agree that a slip sheet is not viable. They are asking you to contradict physics.
 
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My thought was that I would have expansion joint material at all the interior walls to help prevent the walls from restraining the topper at all during any differential horizontal movement. There is a base plate in the middle of the operating room that secures the table to the existing slab structure so the topper would move with that portion of the building. The slip sheet would be two sheets of poly so the coefficient of friction would be between the two sheets of poly. I would have to look up the exact value. The slip sheet is also designed to help prevent shrinkage cracks when pouring the topper. One building is 1993 and the other was built in 2001 so I am hoping differential settlement will not be too much of an issue.
 
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How much movement is expected in this joint? If it's supposed to isolate the building systems, I would imagine that it's pretty big?
 
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I haven't had a chance to look at an approximate displacement yet, but the gap provided between structures is approximately 1". Thanks for everyone's input!
 
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You have been given an impossible assignment. No matter what you do, you will have a movement joint between the buildings, and that will inevitably transmit through your topping as a crack, but not necessarily directly above the joint. As structural engineers, we have to accept that we are not magicians.
 
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Been in the exact same situation. No solution.
 
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I believe you are correct hokie66! I have since discussed this with the client and architects to see what they want to do but I tend to agree that the likelihood of a crack forming is pretty much inevitable.
 
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Here's a thought - the doctor performing the operation could be on one side of the joint, and the table bolted to the other... there's a possibility there for differential movement between doc and patient. Not something I'd like, as the patient. Be sure to cover your bahooty liability-wise, if they do insist on a joint in the middle of the room.

Please remember: we're not all guys!
 
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The reason for a "no crack" policy in an operating is probably not just aesthetics (or anesthesia even) but somebody or some ruling organization's criteria for "no place where germs or fluids can get trapped" (to harm the next patient, raising the doctor's insurance bill and lawyers' fees). But a one-sided plate across the same joint will be worse, will be harder to clean of the dropped fluids now trapped under the moving plate.

Worse, no obvious solution allowing movement but no joint comes to mind.
 
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I'd put your lawyer hat on here. If this thing doesn't perform well, the owner may seek compensation remediation and downtime in an OR. And when an opposing council structural engineer sees that you've paved over a building expansion joint, it's going to be pay day. Either telegraph the movement joint through the new construction or justify truly stitching the joint up so that there's no more movement in the future.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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At first, I thought this was a slab on grade. With an elevated slab, I'd also worry about the consequences if the topping slab locked up the building separation joint. Maybe the architect can save the day and find a joint material that will allow movement and meet the hospital's infection control requirements.
 
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wannabeSE said:
Maybe the architect can save the day
Click to expand...
Laughed out loud at this one. The last time I mentioned something along these lines, i.e. if they could spec a joint treatment to accommodate some movement, they put "joint treatment, see structural" on the drawings the day before the drawing release date. I caught it and made them change it.
 
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Maybe get in touch with Sika or another similar company to see if they have developed engineered solutions like this that could work for your situation: Link
 
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Can you justify eliminating the expansion joint in the structure?

If not, can you install an expansion joint at each end of the operating room?
 
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After speaking with the owner and architects we will be shifting the expansion joints to fall under walls so that there will be no possibility of the expansion joint creating a crack that propagates through the epoxy floor. I just want to say thanks to all that replied.
 
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