RC balcony repair (cracks)

[n3jc]

Hello,

Im wondering what is the best way to repair a balcony - RC slab that is cracked at the top surface above a support (wall bellow).

I was thinking about adding another layer of concrete above existing slab (lets say 50 mm thick) - but first properly clean and roughen the existing surface and adding some agent for better connection between both layers of concrete. Or maybe it would be better to partly remove existing slab - lets say that slab is 140 mm thick so Id remove 40 mm of this and then add a new layer of concrete on top? I was also thinking about adding some vertical rebars (for horiz. shear between an existing and new layer of concrete).

I would design new sufficient reinforcement rebars bellow the top surface of new concrete layer.
The lenght 'X' in attached image is a lenght that is sufficient for top rebars to be properly anchored (anchored lenght of top rebars).

Mind you I have never do or seen something like this.

What do you think? Is this OK? Better options?

Thanks for help.

 

[hokie66]

Is there a reason to believe that the flexural cracking has reduced the strength of the cantilever? Do you know the existing reinforcement, size, spacing, and effective depth? Is there any spalling indicating corrosion of the bars? The reason for all these questions is that flexural cracking does not always indicate a severe problem.

Photographs of the slab showing the cracks would be helpful.

 

[n3jc]

Thanks for reply. Someone was asking me to come and give my opinion about this issue so I have almost no data, but I want to prepare myself. The only thing I know about it is that slab is cracked at the top. As far as I know I wont be able to know what kind of rebars are put inside of a slab.

What can be other reasons for cracking apart from insufficient top reinforcement? Too much/not enough concrete cover? Insufficient concrete mix?

 

[hokie66]

Flexural cracking, as I said, is not necessarily indicative of a serious issue.

In addition to the amount of reinforcement, placement within the slab depth is very important. So if the top bars are too low, or as you say too much cover, that would decrease the slab capacity and increase cracking. Concrete strength would be a less likely factor.

But there is no substitute for experience in judging these things, and there is no substitute for knowing the amount and location of reinforcement in assessing strength.

 

[n3jc]

I understand and agree with what you are saying.
Since we dont have all necessary data / experiences to judge this we want to be on a safe side.
What is your opinion on my solution in the 1st post?

 

[hokie66]

No encouragement from me as to your proposed solutions.

 

[Ron]

Agree with hokie66. Just because you see cracks doesn't mean you have an issue. The corrective action might simply be sealing the cracks and providing a traffic coating on the balcony to protect the rebar, but first you have to evaluate the structural conditions.

The rebar pattern, size and depth must be determined. You need to know the strength of the concrete, but that is, as hokie66 noted, less of an issue overall...but you do need to know what it is.

You mentioned you have never seen nor done this before. Balconies are not structural items to practice on. They carry significant life safety issues and should be evaluated by an experienced structural engineer.

 

[mats12]

Let's say that we find out that existing reinforcement (size, spacing)is not sufficient or that rebars are placed too low.

What options / solutions are there?

 

[JAE]

I had a similar concrete balcony issue some time ago - once we determined that a lot of the rebar had some significant corrosion and section loss (chipped away concrete in various locations to check) we determined that the whole balcony needed to be rebuilt.

This involved complete removal of the concrete, salvaging the existing rebar coming out of the back-span supporting floor inside the building, adding new rebar, reforming and placing a new concrete balcony.

An expensive solution but so much of the rebar was corroded (beyond the support mostly) and the concrete itself was in poor shape (for shear failure concerns)

But if the existing rebar has not lost significant area, if the concrete material is still in good condition, and if you can successfully seal the cracks from future moisture infiltration you most likely don't need to go to the extent I did on my project.

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[BridgeSmith]

If it's determined that the existing slab capacity is inadequate, the solution you propose I think needs anchorage of the new reinforcement into the existing slab. I would not rely on friction or bond to provide composite action.

If it needs more capacity and raising the slab isn't an option (or maybe even if it is), consider removing the top half of the existing slab beyond the support and splicing new reinforcing onto the existing (mechanical splices are a good option).

Anyway, the width of the cracks can give you a good indication of what's going on. A few hairline cracks are probably no big deal. Several 1/8" wide cracks? I wouldn't stand on it. That usually means yielded reinforcement.