Bonding a 3" topping slab to an existing waffle slab

[kngpenn]

I have a project which involves a hospital designed in 1978. The client wants to add another floor above a section of the hospital which was never designed for future expansion. The roof is waffle slab construction design for a 55 psf live load. We need to get 105 psf live load for the new addition and analysis has indicated the waffle slab does not have sufficient moment capacity to accomodate this additional loading. A suggestion would be to roughen the surface of the existing waffle slab, pour 3" of reinforced topping slab with hopes this system would bond as one and the additional reinforcement and concrete would give the system more moment capacity to handle the additional loading.

Any thoughts on this idea?

 

[ishvaaag]

If I had to go for this solution I quite likely would add some kind of bonding agent and some embedded studs maybe at corners of waffle modules to entirely prevent defoliation at limit load.

Maybe I would try to prove test that the approach is effective to limit design (factored) loads. However, if on-site, the test quite likely would be restricted to the less stringent service level loading, then extrapolation would be used to guess the behaviour at limit loads.

Also, the effects of the modified structure on what existent would be examined through models as complete as to warrant the significant aspects are under control.

There is literature on concrete overlays for roads that talk about diverse issues on overlays that are also useful for your case. Particularly the rate of evaporation must be kept low whilst the concrete is curing, and the overlay must not be placed when the daily variation of temperature is expected to be bigger than 15ºC (max temp to min temp in 24 hr).

 

[dhengr]

Kngpenn:

You might want to add FRP (Fiber Reinforced Plastics) materials and methods to your options for strengthening that roof system. They can do some amazing things to increase the strength of that waffle slab system, at a much smaller DL, all else being workable.

 

[MiketheEngineer]

What about the existing beams and columns?? Can they take the added loads. If so - you could simply layout new beams and pour a new floor.

Bonding (successfully) to existing roof seems rather iffy to me..

 

[kngpenn]

This is a waffle slab with no beams, just dropped panels at the columns.

 

[hokie66]

What is the condition of the top of the slab? If it is contaminated by roofing materials, I would agree that it is problematic. But if not, it will just need to be thoroughly cleaned by shotblasting. This, along with placing the new concrete on a moistened surface, is really all that is required for bonding. Done all the time with bonded toppings on hollow core slabs. Be careful with useing "bonding agents". If the timing is not exactly right, they can serve as debonding agents.

 

[ron9876]

Check ACI 318 for composite design. They have criteria for this type of condition.

Not a believer in bonding agents myself. If not properly installed they can become bond breakers. I like the old surface to be roughened enough to expose clean concrete for the new concrete to bond to.

 

[asixth]

Can be done. I would use embedded studs to achieve composite action and bonded frp to the underside to increase flexural capacity. No suggestions for how to increase shear capacity.

 

[SkiisAndBikes]

I have sucessfully used both shotblasting and hydrodemolition on waffle slabs to prepare surfaces to accept a topping. In all our cases we specified a bonding agent but had to insist on full time inspection to ensure the bonding agents were properly installed. We stayed away from epoxy type bonding agents due to their short working time. There are other bonding agents with much longer working times.

Of note, sometimes the hydrodemolition blew panels out between the ribs, generally if there were preexisting cracks or other defects. Definitely need to have the underside hoarded to prevent anyone from entering the lower area when the hydro equipment was working.

On each project we started with test patches to confirm the materials and procedures worked, plus we adopted a system for bond testing with a Locktest machine to confirm results as the project progressed.