Repairs of parking garage floor and topping

[ajk1]

I am designing a repair for a 60 year old parking garage floor waffle slab. The floor has a 1" thick bonded concrete topping on it. The entire existing topping will be remove as part of the repair.

If the repair concrete is placed monolithically with the topping pour, the advantage is that there will be greater structural depth at the repair areas, but I am wondering if the shrinkage of the topping may create a horizontal force that could pull on the deeper concrete at the structural repair areas and cause debonding of those structurally critical areas. If the floor is 100 feet long, then the extremity of the topping will move about 0.2" based on a shrinkage strain of about 350 microstrain and assuming that the mid-length is at zero movement, so repairs near the extremities of the floor will have a significant pull on them. The topping will crack due to its mechanical anchorage by the deeper repair areas, but that is not so critical because it is going to all be covered by a waterproofing membrane and 3/4" thick mastic wearing surface.

Is it better to place the repair areas separately from the topping? In that case I would make the top of the repair areas at the same elevation as the surrounding topping.

 

[Ron]

Repairs require more meticulous attention, individually, than placing a topping. For this reason, I would repair first, then top.

 

[ajk1]

Hi Ron -- that is another reason for placing separately, and a good reason. Thanks.

 

[KootK]

Ron's logic makes sense to me. I wouldn't sweat the shrinkage aspect though. You're likely to have a high degree of restraint, and cracking, in the topping slab either way.

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.

 

[darthsoilsguy2]

similar to Ron's, i would think you would want the repair spots to be better detailed as far as surface prep and cleanliness, and perhaps patched with a more expensive small volume proprietary site batched mortar. It is hard to keep a large area surface prep at optimum conditions if you delay until the large slab pour. Lots of opportunity for dust and grime to accumulate back in the crevices. I would think you would prefer the structural depth to be better bonded to the existing slab than the topping slab. a 3rd possibility to consider is for the contractor to form the repair area and build a 1" table that the topping slab casts to later, it might reduce feathering issues when the repair is very shallow and undulates. Also you might want to have a pre-considered position on this approach since the Contractor may pitch the idea at a meeting.

 

[ajk1]

ok, thanks.

 

[hokie66]

Just curious, how do you intend to remove the existing bonded topping? If it is well bonded, it is just part of the slab, and won't just peel off.

 

[bridgebuster]

On NYSDOT bridge deck reconstruction projects we have two options for placement method.


A. Method 1 - Separate Placement. Place Class D slab reconstruction concrete and Class E
overlay concrete separately.

B. Method 2 - Integral Placement (Optional). When 100% of the top mat of bar reinforcement
is exposed or when all of the following conditions are satisfied, Class E overlay concrete and Class E
slab reconstruction concrete may be placed in a single lift.
1. The area of the exposed top mat of bar reinforcement is 5% or less of the placement area, per span.
2. No individual area of the exposed top mat of bar reinforcement exceeds 25 sf.
3. No dimension of any area of the exposed top mat of bar reinforcement exceeds 6 feet.

Class D has a slump of 2 1/2" to 3 1/2", with mostly 3/8" aggregate - 75 to 85%
Class E has a 3 to 4" slump with mostly 1/2" aggregate, 35 to 65%

 

[ajk1]

to Hokie66 - the existing topping is generally not well bonded and is coming off readily so far.

to bridgebuster - that is very useful information indeed, and something that I can use to back up my position that it should be separate pours. Thanks.

 

[ajk1]

to bridgebuster: when it is done as 2 separate pours, do they finish the repair area concrete flush with the top of the adjacent structural slab, and then place the topping on top of the repair area and on top of the adjacent structural slab?
Or do they do they finish the repair area to the same elevation as the top of the future topping, and then place the topping so that it fills in between the repair area concrete which is projecting above the top of the structural slab by the thickness of the topping?
Is there any specified wait period between the time of placing the repair areas and the placing of the topping, when they are placed separately?

 

[bridgebuster]

ajk1 The repair concrete is placed to the original top of slab, then the overlay is placed. The specs require a 3-day cure before placing the overlay.

In this type of project, we do the entire bridge area. The placement methods I cited are not for localized repairs.

 

[ajk1]

To bridgebuster - thanks again for the information. Very much appreciated.

 

[SkiisAndBikes]

I have completed several projects with similar parameters as yours. In all cases the waffle slab repair areas were poured separately from the topping. Generally we used hydrodemolition to remove the existing topping, which did result in a few additional areas of the waffle slab being blown out, however, the bonding surface for the new topping was, as a result of the hydro, very well prepared. A sandblasted or shotblasted surface would be good too.

 

[bridgebuster]

ajk1 - you're welcome