two way slab composite topping 1

[structSU10]

Looking at treating a new concrete topping slab as composite with an existing two way slab as a part of a rehabilitation effort. The issue I get is development of the shear reinforcement across the interface - the code requires this to be developed but I don't have enough depth to do that in the existing slab. My only thought was a through bolt and plate so it is anchored but it makes me wonder what I do to develop it into the topping.

Is anyone aware of a system I can count on to meet the code requirement for development?

 

[KootK]

1) One way or another, this is almost always done without explicit mechanical connection between the slab and the topping. Just roughening and bond with pull-off tests and that kind of jazz.

2) In my mind, the provisions for horizontal shear friction stuff for this is mostly intended for "flangey" stuff where you're aggregating shear load into one location.

3) I've dabbled in precast some and, there, have found some information leading me to believe that, even for flangey stuff, straight bond without dowels probably gets the job done.

 

[KootK]

Check this thread out for some related information: Link.

 

[le99]

I think you may need to explore other methods for the retrofit, such as external posttensioning.

 

[ALK2415]

Once we did this old sandwich technique [Not cost effective But works for important structure]
Recently they use similar technique 'double skin composite structures'

 

[MotorCity]

With the sandwich plate idea, I would expect to see diminishing returns. You are adding 40 psf to a structure that you are trying to reinforce.

 

[ALK2415]

@ MotorCity
you could use same concept but with CFRP Laminates (Bonding has durability issues !!!)
CFRP laminates will reduce the added weight ...
but still an effective tool to use

 

[hokie66]

Just as with any bonded topping, the existing surface just needs to be clean. Shot blasting does the job. And to add capacity, the topping for a continuous slab has to be reinforced.

 

[OldDawgNewTricks]

Looking at treating a new concrete topping slab as composite with an existing two way slab as a part of a rehabilitation effort. The issue I get is development of the shear reinforcement across the interface - the code requires this to be developed but I don't have enough depth to do that in the existing slab. My only thought was a through bolt and plate so it is anchored but it makes me wonder what I do to develop it into the topping.

Is anyone aware of a system I can count on to meet the code requirement for development?

You didn't say which code requirement you are referring to. In the U.S., shear reinforcement is not required across the interface. This is done all the time for composite topping slabs over precast members. You just need to provide a clean, intentionally roughened (1/4" amplitude) surface. The relevant ACI 318 code provisions are 8.5.3.2, 16.4.1.3, and 16.4.4.2.

 

[structSU10]

I am at around 70 psi bond stress required and felt some shear reinforcement would be a good idea - it is an old slab so perhaps a bond test will help verify we can get what we need.

If I don't meet the intent of the code for developed bars am I just asking for something totally useless to make myself feel better? It just feels like some nominal dowels would be a good idea for this situation.

 

[le99]

Two potential problems here 1) inadequate development length renders the dowel less useless, and the result can't be evaluated, or predicted. 2) drilling holes on the existing concrete with closer spacing increases the chance to damage the concrete, unless under tight control.