Deteriorated concrete ledge

[Robbiee]

Hi all,
Need your input please. The attached is a concrete beam with a ledge supporting adjacent floor. The rebars drawn in the beam are what I expect to be, but I don't have that info, because we don't have existing drawings. The ledge has concrete spalled from the soffit and signs of corroded rebars. I am thinking of cutting and replacing the concrete (of course after shoring), and for the corroded rebars provide epoxy anchored new bars with a hook at the end to look like J placed around the horizontal bar.
Should I worried about the existing rebars in the beam if they are not properly details? The concern I have is because when you analyze the ledge with Strut and Tie model, you end up needing to know what is in the beam to verify the model.
Or do you make the assumption that this ledge is a small cantilever and if tension bars are OK and the interface shear is OK then you're OK?

 

[JAE]

Do you know why the ledge failed in the first place? Was it mostly (entirely) due to rebar corrosion?

Seems like I'd want to ensure that the "new" condition met current standards with respect to either ACI corbel design (318 chapter 11) or PCI spandrel ledge requirements.

The beam stirrups do participate as a hanging element so you'd have to know what you have there. ACI's corbel design doesn't directly deal with this as I recall.



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[Robbiee]

Thanks JAE. The deterioration is mainly due to failed expansion joint and the infiltration of salty water. This is a parking floor.
I am in Ontario, so we use the CSA A23.3. Now, if you don't have as-built drawings, how would you know what rebars are in the beam at the ledge? would you remove concrete to left of the ledge to confirm?

 

[Teguci]

I'd be seriously tempted to cut the existing ledge clean off and provide a steel bracket that anchors into the top third of the L beam and wraps under the stem. The problem with replacing the concrete will be the shear friction (especially with epoxy coating) at the cold joint. I also suspect that the slip pads (2 korolath plastic shims?) aren't performing well enough to keep the tension load off of the top of the ledge and helped cause the initial cracking.

Is this for each double tee stem or is this for an IT beam (uh-oh)?

 

[KootK]

It's tempting to trust that the hanger bars were designed correctly but I'd be looking to verify that somehow. You can probably have the beam scanned to get an estimate of the rebar size and spacing. Some other possibilities for ledge reinforcement:

1) The ledge flexural bars might full hoops.

2) There might be a diagonal hanger in there similar to retaining wall T-joint reinforcing.


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.

 

[Robbiee]

Teguci,
I think I misled you with the sketch. The inclined line is not a crack, but the line of cutting the old concrete and replacing it with new. A long section of the ledge needs to be replaced. I am told by the restoration firm I am helping, that this is a typical repair that they do all the time. I wonder if there is a "restoration structural engineer" here to comment.
Would you be satisfied if the removing 2" to the left of the ledge from the beam showed the vertical legs of the stirrups. This way you have new ledge and the interface shear is much greater?

 

[Teguci]

this is a typical repair

I've never seen a precast(?) L-beam need this kind of repair. If the chlorides are high in the soffit then I'd think the top of the ledge is also contaminated, and I'd expect the top rebar to have expanded as well.

Would you be satisfied if the removing 2" to the left of the ledge

No, for a concrete repair option, I prefer the inclined cutline as you've shown. If the numbers work then go for it. The advantages for concrete replacement are numerous (fire, corrosion, similar construction, etc...). However, lock down why this ledge failed in the first place. During the years I designed parking structures (and some resto), I did not encounter the bottom surfaces of precast beams spalling (carbonation?).

 

[JAE]

Teguci - that sounds like a good idea (using a steel bracket) in terms of the confidence I'd have with it.
However, I'd also worry about future on-going salt-water exposure on the steel.

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[hokie66]

This type of ledge is susceptible to cracking due to failure of the bearings to work as intended. Then when cracks form, water gets in, and corrosion results. I wouldn't look at this as a concrete repair, but rather as providing new support as Teguci suggested.

 

[Archie264]

...so, as a rough analogy:

With functioning friction pads = pinned connection?

Without functioning friction pads = mimics a fixed connection at the ledge, for which it was not designed?

Teguci, Hokie66, is that what you gentlemen are saying?

 

[Robbiee]

Teguci, Sorry, I should have noted that this is a cast-in-place concrete beam not precast.
I'd be more comfortable cutting along line 2 than 1, because we can confirm the presence of the stirrups, and we have longer surface to transfer shear. One concern with this is the development length of the horizontal tension bar beyond the cut line.

 

[hokie66]

Archie,
With a properly working slide bearing, the support should act as a roller, neither fixed or pinned.

 

[Robbiee]

Archie264, I think what Teguci and Hokie66 are saying is that when these pads, that have high compressive strength but small shear strength, fail to function, the friction between the two causes tension. All ledges and corbels, however, need to be design for a minimum tension unless this tension is somehow relieved (sliding bearing maybe?).

 

[Teguci]

@ Archie264:
Still missing some detail on this.

If this is a ledge supporting double tee stems with an expansion joint between the double-tees and L beam, then you need to allow for rotation of the double tees and allow for sliding at the joint. Ideally, there would be a neoprene pad adhered to a steel/stainless steel plate over a Teflon surface. In the real world, that's not gonna happen for this lightly loaded support. Typically, 2 plastic shims are stuck in there to allow for some sliding and a very little bit of rotation. It is usually good enough for this redundant and lightly loaded connection. In this case, we have specific evidence of a failure that may be due to the usual not being good enough. Further investigation should be pursued or provide a repair that adds rotational flexibility and better sliding.

Of course, I don't know without additional detail. If its just spalling, then replacing the concrete makes the sense.

Update: - OK this is CIP concrete. I'm sure looking at the big picture, this might make more sense. But, I'm having a tough time figuring out why a CIP deck is detailed this way. Oh well, no matter. As noted above, the bearing may be the reason for the failure and should still be looked at or provide a detail that relieves the needed motion.

 

[Archie264]

>>>With a properly working slide bearing, the support should act as a roller, neither fixed or pinned.<<<

Yeah, I guess I should have written "roller", though it can't be roller on both ends. (Yeah, I know it can in the real world but I'm just referring to idealized conditions.)

I think I've got it. I guess I was just surprised that a seized up roller would make that big of a difference since a ledge is designed for top tension due to negative moment. I guess it's a matter of magnitude and, not having run any numbers, it surprised me. But clearly the bearing pads are there for a reason.

 

[hokie66]

Robbiee,

The constructability of your "Line 2" is problematic, due to working in such close quarters. I think demolition and recasting of the entire beam would be simpler. That way, you can do it correctly.

 

[Robbiee]

hokie66, do you agree that cutting at line 2 is preferred over line 1 if constructability is not problematic?

 

[KootK]

I see no advantage to cutting at line two other than the visual confirmation of the stirrups. And you should be able to accomplish that by way of scanning / non-destructive testing. The limiting shear friction condition will be at line one regardless of whether or not you take your demolition to line two.

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.

 

[Archie264]

It occurred to me that if CRSI research has demonstrated the need for diagonal bars at the interior joint of retaining walls then perhaps it would also be appropriate for ledges such as this. Just a thought.

 

[hokie66]

Robbiee,
I wouldn't answer that question because I think the premise is false. Constructability is obviously problematic.