Concrete Through Slab Repairs

[CuriousEng13]

I have recently been questioned as to our through slab repair details by a building department. They are simply going off what they have seen from other consultants details.

a) Our typical concrete through slab repair shows straight (or perpendicular to the surface) removal/repair lines.

b) Other consultants have detailed this as a wedged repair surface.

I have tried to rationalize why this approach has been taken by other but have yet come up with a sound explanation.
Personally, I see the wedged repair surface as potentially creating a failure plane. The only thing I can imagine is trying to create a "shear key" of sorts.
I also prefer the straight surface for stress block bearing.

Ultimately if we am repairing in a high shear zone we typically do one of the following:

1) remove concrete as required to have monolithic in this area
2) provide a design to make the shear work
3) dowels to create interface shear friction that works.

I have attached a simple sketch for reference.

I am interested to how others here have approached.

 

[BridgeSmith]

This is similar to our full-depth bridge deck repairs. We don't specify the wedge-shaped repair, and our details show vertical sides of the 'typical' repair. We do require an epoxy resin bonding compound on the faces of the existing concrete if we specify standard concrete for the repairs, but not when using silica fume modified or latex modified concrete. They use chipping hammers for removal, so the sides are not even close to being smooth; specifying a wedge shape is considered unnecessary. We also have alot of reinforcing (#5 bars at 6" transverse top and bottom, and longitudinal bars at 8" to 10" spacing), but they carry 16 kip wheel loads all day long.

 

[Enable]

Looks like you're from Ontario, which is where I practice; I can definitely say that it's commonly done both ways in our market. However, in the last few years the most common approach I've seen is a partial slope / wedge at the top and then a straight cut from midway down. This helps with facilitating shear dowel installation (flatish surface to drill against) and also reduces the possibility of microfractured concrete at the toe (i.e. the end of the wedge will be in very precarious shape after demo if it's tapered as shown). The partial sloped / partial straight also gives people the warm and fuzzies about having the new sit on the old, which ironically should often be reversed depending on where your repair line is.

Anyways, either approach is fine for our market and my suggestion would be to put on paper what you need to satisfy the building department. During construction tell the contractor what you want, and they'll get it done. Don't worry about inspection flagging anything since they are unlikely to even come to site. For structural restoration work all they need is a sign-off from the EOR, which I know because I've done 100s of parking garages all over Ontario and have never called for an inspection. Actually, there was one garage I had to but that was in Ottawa and we were a Toronto contractor. So we got the short end of the stick (even then it was only at the start and at the end - no repair inspections).

 

[CuriousEng13]

BridgeSmith
- This is pretty similar to our approach and i agree.

Enable,

- Ontario, yes I am. I agree it is common but in discussions with contractors and engineers, I am just curious if they can explain why? The constant push back for the wedge is surprising to me.
- You mention the irony of the repair line, I think it to be plain wrong to blindly apply this detail because of this.
- At the end of the day, it doesn't matter what others do. We instruct the contractors as we see fit, and provide technical justification to building officials and all is well.

 

[TLHS]

It seems like you'd want to be careful with the wedge. You've got some potentially weird stuff happening with your load transfer when loads work out so that the wedges are in the non-helpful alignment in comparison to your shear.

The fact that your bars aren't perpendicular to your failure plane definitely seems to have an effect, but it's too late on a friday for me to decide exactly what it does when combined with the fact that your shear force is perpendicular to your bar but not the failure surface.

 

[lexpatrie]

I really don't like the idea of a deceptive detail "for permit" and a verbal change in the field later on. There's got to be some research that can be provided as reference for how you want to do it via the references in ACI 318. Or it's in the concrete rehabilitation guide (352?). The bit about the toe getting more heavily damaged and tapering the cut makes physical sense to me. Ultimately you are the EOR and it's on you to do it appropriately. I'm not convinced the shear friction provisions require a flat cut or an angle, I think both can be done. It sounds like a designer preference item.

 

[Enable]

I suppose what I was trying most to get across is that it does not matter. No detail your furnish can be constructed as shown since you’re at the mercy of the existing concrete and how it responds to the chipping hammer. You can try to obtain a particular profile all you want but when you’re at the repair extent, you can’t just chase for days, and you’re stuck with what’s there for the most part. Even if you restrict final chipping to 15lbs hammers it’s impossible to get it to a particular shape. What is most important is to avoid undercuts, which is something you can control(ish).

So one can spend time contemplating what’s a better detail or the ethics of not giving a flying monkey about what’s on the drawing. But to me this is a complete and utter waste of time.

 

[BridgeSmith]

I suppose what I was trying most to get across is that it does not matter. No detail your furnish can be constructed as shown since you’re at the mercy of the existing concrete and how it responds to the chipping hammer.

Bingo!

 

[bridgebuster]

I remember many years ago an older engineer told me the wedge was there to prevent the repair from falling through. Hit the submit botton by mistake.

Use vertical edges; it's not easy to do a wedge and it avoids a feathered edge. I've been doing bridge rehab work since the late 70's and back then everyone had their own concrete repair details. Now, at least on NYSDOT work the repair details are the same.