Shear-Friction and Waterproofing

[daiv]

I have a question on whether the shear-friction section of ACI is applicable when a layer of liquid waterproofing membrane is applied between two concrete placements?
ACI commentary states that when using coefficient of friction µ=0.6, the shear force is resisted “primarily due to dowel action” and the value is confirmed with tests.
From the commentary I assume that some of the shear is resisted by friction of unroughened concrete against the second pour. (in my case it's the joint between footing and stem of the 20ft tall cantilever retaining wall)
I would appreciate any thoughts

thanks
David

 

[KootK]

I see your logic and mostly agree with it. My concern with relying on dowel action is that it then depends very much on where within the concrete section the dowels are placed (spalling behind the dowels). In your connection, I'm guessing that your dowels will be dirt side and conditions will be unfavourable. As such, I'd be hesitant to use this strategy.

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.

 

[daiv]

I agree that location is important. I would not use the heel side reinforcing since it's in max. tension. the toe side is not working in tension and can be used for shear-friction resistance. also the toe side rebar is favorable and spalling behind bars should not be expected.
I am questioning the applicability of ACI guidance for shear -friction with applied waterproofing. It becomes a standard detail for waterproofing consultants and I'm not sure most structural engineers pay enough attention to it.

 

[KootK]

I would not use the heel side reinforcing since it's in max. tension. the toe side is not working in tension and can be used for shear-friction resistance.

For the heel side bars, I don't believe that the bar being utilized for other purposes is the primary concern. Rather, it is the fact that you're only likely to have 50 mm of cover and, when the dowels push against that thin layer of concrete, there is a high propensity for it to spall off.

the toe side is not working in tension and can be used for shear-friction resistance.

Now that I think of it, this may not work either. I can't remember the exact clause but I'm pretty sure that ACI has a provision that prevents bars in compression from being used in shear friction. It makes sense as a compressed bar is actually resisting the clamping force that you'd want in the joint. Of course, if you're going with dowel action instead of true friction, perhaps this is not a concern. The "dowel action" statement in ACI is one of my least favorite statement in that entire document. It's an entirely different mechanism from standard shear friction and, really, little guidance is given for its application.

It becomes a standard detail for waterproofing consultants and I'm not sure most structural engineers pay enough attention to it.

I don't doubt it. Would it be possible to waterproof over top of a shear key? That would fix things up in my opinion.

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.

 

[daiv]

theoretically it could work, however in practice, the shear key was filled with liquid membrane material (in some areas up to 1/2" layer) creating an unacceptable condition, so I ordered to remove it

 

[hokie66]

Now you may have created another unacceptable condition...leakage. But I have never seen "liquid membrane material" used in the way described. Waterstops are the way it is normally done. You didn't mention a shear key before.

KootK's "least favourite" statement about dowel action is my "most favourite", because I am not a shear friction advocate, but then he knows that.

 

[KootK]

In my opinion, ACI lacks a solid method for true dowel shear transfer. I ran into such a method in a European precast manual a while back but it would take some digging to find my way back to it. Given how much energy I seem to expend on shear friction related discussions, I'm quite disappointed in myself for not bookmarking it somehow.

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.