Adjusting top of deck elevations on a pre-topped twin tee

[MJC6125]

I have a project where a client wants to adjust the grading/slopes inside one area of a precast parking garage to meet ADA accessibility slope limits. To achieve this they are proposing raising the deck surface by anywhere from 0" - 4". I still need to check if this extra weight of concrete on top of the twin tees will work or if the structure will need to be reinforced. Assuming it did work, I'm wondering how this could actually be achieved because I'm dealing with pre-topped twin tees in this case. See image below for an example elevation view of how the top of deck surface needs to be adjusted (idealizing the twin tee surface as level). Where the extra topping thickness tapers down to 0", I'm assuming they need to demo out at least 2" of existing twin tee deck to be able to pour back a minimum 2" thick new concrete layer for this new surface.

Has anyone ever done this before? This case is for an area that is approximately 6,000 sf (approximately 14 twin tees with the majority of their span needing elevation adjustments). I know I can run some checks for the extra weight on the structure. However, the things I'm worried about are:

1. Can some thickness of a pre-topped twin tee deck be demoed away temporarily (2" max out of a 4 3/4" thick flange)? I'm guessing the contractor may need to shore the tees in this case because the tee will at least temporarily be weaker.

2. The extra demoing away is necessary that way the new topping layer is at least 2" thick, is that correct? Does anyone know of a concrete/grout that can instead taper down to 0" and will be durable enough for vehicle traffic?

3. If they roughen the surface of the deck that will receive the new topping layer, including the exposed surface after the demoing away, is it safe to assume that this new layer of concrete will act compositely with the precast twin tee, so it will be at least as strong in bending as it originally was (assuming matching concrete strengths)? Not sure how weak that shear flow surface is compared to the original monolithic flange.

 

[BridgeSmith]

The minimum thickness of the overlay depends on the material used. We use a 1" minimum thickness for our silica fume modified and latex modified concrete bridge deck overlays. You could probably go less in a parking garage, where the deck isn't being pounded by 70,000 lb trucks doing 80 mph, like our bridges are.

You'd have to check the design capacity of the twin tees with a reduced top flange to see what the limits of live load would be during construction, but I doubt you'll find it to be terribly restricting.

 

[MJC6125]

Good to know, thank you for the response. Do you have any thoughts on my question #3? Is it safe to assume the new topping can re-bond with the original concrete deck and act compositely for the final compression flange loading condition? Not sure if you guys have to do assumptions like that in the bridge world often? I guess that is how precast double tees with topping are usually designed, so if it works for that condition, I'm leaning towards it working for this re-pour condition as well.

 

[BridgeSmith]

We deal with overlays on bridges all the time, and the question of whether a concrete overlay can be considered structural or not is a matter of much debate. We have opted to take the conservative assumption that if the new concrete does not extend below the top layer of reinforcing, we don't consider it part of the composite section. However, several state DOTs do consider thin bonded overlays to be part of the structurally composite deck.

 

[HTURKAK]

The following excerpt is from PCI Parking Structures: Recommended Practice for Design and Construction.

The double tee surface receiving composite topping should be roughened . The use of light mesh reinf. at topping should be considered.

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[JohnRwals]

Generally, 2% double tee slope is adopted due to floor drainage.
On top of 2% drain slope, there should be 2"~3" camber due to pretensioning.
So, it is hardly possible to satisfy 2% ADA floor slope requirement with pre-topped double tees.
Should these double tees be field-topped, so topping concrete should be adjusted according to ADA requirements?
Repair should not be easy to do.
_JRW

 

[MJC6125]

Unfortunately, this is an existing pre-topped double tee building that does not meet 2% ADA requirements, and they are now trying to go back and get it to meet the limits. So we are looking at repair options at this stage. No chance to install them as field topped anymore.

Thanks for that reference HTURKAK. It seems like there's some merit to making the topping at least 2" thick to ensure any reinforcing that needs to be placed in it has proper clear cover.