Concrete joint with no surface prep

[canwesteng]

If the numbers work out for shear friction for a construction joint with concrete not intentionally roughened, is there any reason to roughen the joint? I called out the joint roughened to 1/4", but have roughly 60m2 of joint to hammer, so the contractor would prefer not to understandably. I could just make them roughen it, but the contract is T&M so the cost is born by the client, so to keep them happy I would like to allow this if it works.

 

[phamENG]

If it works, it works. Worst case is that if it does slip, another engineer will say "I would have just made them roughen it anyway to avoid this." How critical is the connection?

 

[canwesteng]

Well there is no redundancy if it slips, so critical in that case. The joint has a lot of bar crossing and dead load on it, so no uplift, so not critical in the sense that shear is close to enough to make it slip

 

[KootK]

60 m^2 is a lot. What kind of element are we talking about? Topping slab? Massive transfer slab pour joint? Retaining wall stem to footing?

Shear friction joints can have edge distance problems on the dowels which, of course, get worse when the shear resisting mechanism is primarily dowel action.

 

[hokie66]

Whether or not you roughen it, the joint should be clean. Like KootK, the type of elements to be joined and the reinforcement details would affect my decision. But then, he is a supporter of the shear friction concept, I am not.

 

[KootK]

Raft slab control joint!!

 

[canwesteng]

Wall on a footing, just very big. I would never check something like this through dowel action, only through shear friction. So it would be checked using the code value for friction for concrete cast against concrete not intentionally roughened.

 

[KootK]

I would never check something like this through dowel action, only through shear friction.

That right there is really the concern. As you migrate towards smoother surfaces, shear friction is dowel action.

Wall on a footing, just very big.

Cantilevered retaining wall?

Basement wall with lateral support top and bottom?

Is it accurate to assume that your concern is out of plane shear transfer?

Do you have reinforcing on the flexural compression side of the joint?

I'm currently in favor of the old school shear keys for most wall/footing joints. I've come to view that as the appropriate form of joint "roughening" in many situations.

 

[KootK]

But then, he is a supporter of the shear friction concept, I am not.

That remains an accurate statement but the needle on my personal "shear-frict-o-meter" has been moving back the other way over the last few years:

1) As I mentioned above, in some situations, I've come to view the right form of "joint roughening" to be the old school shear keys that utilize bearing.

2) I've come to have concern for dowel action edge distance problems in some situations.

3) I think that there are many, common situations where the real world state of joint stress sf a poor match for the testing setups that were done in support of shear friction (poor similitude).

4) I don't love the impression that the code shear friction provisions tend to give that shear friction is an un-nuanced, one size fits all solution to shear anywhere that one encounters it.

 

[canwesteng]

Close to a cantilever retaining wall, bar on both faces, I always aim to make shear friction bar symmetric. I've never like shear keys, since I've never figured out how they are detailed to take any flexure, even if the plain concrete works in shear.

 

[KootK]

I've never like shear keys, since I've never figured out how they are detailed to take any flexure, even if the plain concrete works in shear.

That is one of the situations where I always use shear keys nowadays. At reasonable proportions, I don't feel that any attention to flexure is required. Or shear. You're really using the keyway as the recipient of a diagonal concrete strut. Not that there aren't ways to check the keyway for shear and bending if you really felt compelled to do that.

...bar on both faces, I always aim to make shear friction bar symmetric.

I actually feel that compression face rebar worsens the shear friction situation. The presence of the compression steel tends to oppose the very clamping action that one hopes to rely upon for shear friction. And that further encourages dowel action.

Assuming that your footing has a meaningful toe to it, that should help to ameliorate any edge distance problems that might otherwise arise from dowel action.

 

[canwesteng]

Well, the bar can't break out in the footing, it would break out in the wall. I guess the compression steel would attract some of the compressive load that is providing friction, but I see it more as pre tensioning a bolt, at least where there is the standard 1/4" roughness - to make the joint slide, the bar needs to stretch 1/4" at the joint, which would remove all the preload.

 

[KootK]

...but I see it more as pre tensioning a bolt, at least where there is the standard 1/4" roughness - to make the joint slide, the bar needs to stretch 1/4" at the joint, which would remove all the preload.

I agree but, then, this thread exists precisely because you are proposing to omit the 1/4" roughness. My hierarchy of perceived joint performance would be this, ranked from best to worst:

1) Old fashioned shear key.

2) Properly roughened joint with no shear key.

3) Smooth joint with no shear key.

I like #1. I'll do #2 if pushed. It would take a pretty exceptional situation to have me giving serious consideration to #3 in a wall that wasn't built yet.

 

[KootK]

How thick is this wall? I don't feel that you'd need to roughen more than 1/3 of the wall width on the compression face. That would drop you from 60 m^2 to 20 m^2 and, at the least, make it clear that you're willing to try to be bold and helpful.

 

[hokie66]

KootK and I have argued the merits of the shear friction theory in the past. He has modified his opinions, but I take no credit for that.

The developers of the theory state that the capacity is part dowel action, part friction induced by clamping. They don't really say how much of each, and the test apparatus had no means of quantifying it. And they lump all the reinforcement together, no matter where it is placed. Like KootK, I think the compression face reinforcement cannot be counted on to provide a clamping force. We don't count that reinforcement in flexure, so why consider that it clamps?

For the base of a retaining wall, the shear can quite simply be turned into bearing by providing a 'key' the whole width of the wall and perhaps 3" deep. That's the way I do it. If I want to rely on dowel action, the dowels are big bars, placed central to the element.

 

[KootK]

Well, the bar can't break out in the footing

While it probably cannot break out in the footing, it can:

1) Shear the steel (unlikely).

2) Crush the concrete locally behind the dowels near the joint (more likely).

 

[KootK]

KootK and I have argued the merits of the shear friction theory in the past. He has modified his opinions, but I take no credit for that.

While that is gracious of you to say, know that you have, indeed, been a catalysis for my change of heart. Hearing a dissenting opinion from a respected mentor tends to make me more open to future sources of information that contradict my initial views. That is how this has unfolded for me.

 

[canwesteng]

Do you have any reference for the original development of the theory, research papers, or others?

I think if you exclude the reinforcement provided on the compression face, you should include the clamping force from the moment itself. In this case in particular, there is not going to be any real tension face either - since the retaining portion is small and there are large gravity loads above. Think unbalanced fill on a stem wall, only scale everything up.

 

[human909]

While that is gracious of you to say, know that you have, indeed, been a catalysis for my change of heart. Hearing a dissenting opinion from a respected mentor tends to make me more open to future sources of information that contradict my initial views. That is how this has unfolded for me.

One day I hope to be the recipient of such a grand form of flattery. (I say this out of respect for KootK and hokie66.)

I don't have much to say on this topic as I really don't know my concrete well but I'll try to listen and learn.