tensile bond strength for bonded structural topping that includes isolated repair areas

[ajk1]

What tensile bond strength should be specified for bonded structural topping where the topping contains rebar and is to act compositely with the base slab? The topping includes but is not limited to the top steel repair areas in the base slab which is a two-way flat slab.
CSA A23.1 specifies 0.9 MPa "minimum" for bonded toppings (see attached) but is vague about what this really applies to, leaving it up to the owner (i.e the structural engineer) to decide if more than the minimum should be specified.
0.9 MPa sounds like what might perhaps be appropriate where the topping is not structural, but seems low these days when 2.0 MPa can be achieved with a shotblast and good bonding agent. If the topping is bonded at 0.9 MPa, I wonder if that is good enough for the punching shear area around the supporting columns of a two-way flat slab. So my question is:


What tensile bond strength should be specified for bonded structural topping where the topping contains rebar and is to act compositely with the base slab?

 

[KootK]

This has been nagging at me something fierce. I think that I may have something. The clip below is from ACI 318-11, the horizontal shear provisions for composite members. Based on §17.5.3, you should be able to count on a horizontal shear capacity that would garner you and equivalent, factored, vertical shear capacity of;

0.75 x 0.8-ish x 12" x 80 lb/in^2 = 576 lb/ft per inch of waffle slab depth. That's not too shabby. For an 8" system, that's 4.6 k/ft or.

Interestingly, I think that this formulation suggests that you're only getting about 48 psi of factored direct shear capacity. I'm starting to feel that our problem here is not with the shear capacity of the topping interface but, rather, with our(my) formulation of the horizontal shear demand.

If one could make the argument that one way shear is all that matters here, then 4.6-ish k/ft is probably fine. It's only when you start trying to apply it to two way shear areas where things go to hell in a hand basket. This takes me back to the comment that I made near the top:

2) It's always bothered me that one way shear checks exist in two way slab design as it never governs and the lateral distribution of one way shear isn't uniform at all (yes, there would be some redistribution).

I wonder if the one way shear check on two way slabs isn't really intended to check horizontal shear like we're discussing here. And that would imply that you could spread your horizontal shear over the entire design panel.

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.

 

[Ingenuity]

ajk1,

The latest issue of ACI's CONCRETE INTERNATIONAL had an article on interfacial bond for repairs to ACI 562-16 code, that may be of interest to you.

Abstract:
This is another in a series of articles intended to explain the rationale behind some of the new content in the 2016 edition of the ACI 562 Code. This article discusses interface bond between existing concrete and repair materials. It also discusses the contribution of interface reinforcement, and the minimum QA/QC requirements that are included in the ACI 562-16 Code to improve the quality and service life of repairs.

Link

 

[ajk1]

For over 50 years I received CI, but I see that I am no longer receiving it...I guess I still have to formally renew my ACI membership to receive CI, even though I am a 50 year member and do not have to pay ACI membership fee. Thanks Ingenuity for this information..I will have to see if I can get that issue.