Reinforcing Existing Concrete Beam with... More Concrete?

[metalchair]

Good Afternoon All,

I have a concrete beam that is failing in bending and was wondering if it was possible to reinforce it by adding an extra layer of concrete and tension reinforcement to the already existing beam. Additionally, I planned to put concrete up the two sides and (1) bar of top reinforcement to allow for new shear steel; essentially creating a shell around the existing rectangular beam. I am unsure if this is an actual practical solution to this problem, as I do not have much experience working with concrete.

I am aware I will have to dowel the two parts (shell and existing) together in order to resist transverse shear. But beyond that I am not aware of what other things I must do (besides adequately designing the new final beam). This solution feels too simple and I fear I am missing something.

Any help appreciated.

 

[driftLimiter]

I suggest you run through the numbers here a bit. Since we don't have a sketch or any dimensions to go off. I am skeptical that you will be able to add enough concrete to engage reinforcement for the transverse shear whilst not adding too much concrete.

What kind of shear dowels would you aim to use? You need pretty substantial embedment to develop a rebar with epoxy. Then you need to provide sufficient development on the new side of the joint as well.

Its kind of a losing game adding concrete because your adding significantly more load with it as well you may end up kind of chasing your own tail. Even with a #4 bar you would need about 6" of concrete min to anchor the bar with a std. hook.

 

[Tomfh]

It can be made to work but there's a fair bit to assess if you want composite action.

Is it failing on paper, or literally failing?

 

[bugbus]

Carbon fibre strips essentially do what you're describing without the additional weight.

 

[metalchair]

The beam is only failing on paper as of now, as the structure has not been brought up to the full loading it is expected to see during it's design life.

Oddly enough the beam is in negative flexure, so additional self-weight may actually help in counteracting the governing moment. Additionally, space is not really an issue, as this beam in question is below grade.

Tomfh, assuming no space constraints, what would one need to do to achieve composite action between the two?

 

[Lomarandil]

The other consideration -- how much load are you able to take off the existing beam (or shore out) while building the repair?

It's a lot like a composite beam -- sequencing is important, because the stresses won't go elsewhere unless you make them.

 

[KootK]

This has certainly been done. Just how to do it is tricky. I'd start by envisioning the finished product using the conventional truss model, acknowledging the cold joint. Then imagine how to get all of the internal forces properly across the cold joint boundaries to where they need to go. Some obnoxious things to consider:

1) As nifty as shear friction is, by the book you have to develop the dowels on both sides of the joint. That isn't always easy or practical.

2) Ideally, you stirrups would connect your tensile rebar to your concrete compression block. This, also, is not always easy or practical given the likely presence of a slab.

One of the nice features of post-tensioned reinforcement, particularly if you can anchor it to/over the beam supports, is that you get get some additional shear resistance via the balancing loads without all of the rigmarole of trying to create new, ideal stirrups. It's also pretty spiffy for deflection control.

 

[metalchair]

I would like to thank everyone who took the time with explanations and suggestions. Definitely very helpful and very much appreciated.