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Epoxy dowel & shear friction

Epoxy dowel & shear friction

Epoxy dowel & shear friction

(OP)
If rebar is developed on either side of the shear plane, one side being with epoxy that develops the rebar with the epoxy but not long enough to develop the rebar if it was in bond with concrete only, should you be able to use shear friction by ACI or be forced to the concrete breakout checks in Appendix D?

RE: Epoxy dowel & shear friction

I always wonder how they get rebar to develop with a thin skin of adhesive in a much shorter distance than in the concrete. I don't buy it.

RE: Epoxy dowel & shear friction

Hokie66, out of curiosity, have the Australian codes imposed something similar to the notorious Appendix D that the haynewp mentioned? Just curious, that's all.

RE: Epoxy dowel & shear friction

No, not to my knowledge.

RE: Epoxy dowel & shear friction

Ah, thanks for the info. I was trying to get a feel for state of the industry, whether everyone's going down that road, or what the situation might be. That particular appendix is a lot to deal with but I'm willing to accept it if it really is an improvement, justified, or needs to be that complicated...that type of thing. When I hear that others are getting along just fine without it, well, it makes me sigh.

RE: Epoxy dowel & shear friction

(OP)
Requiring full concrete bond would eliminate Hilti and Simpson adhesives and all their testing. That's a large statement.

RE: Epoxy dowel & shear friction

hokie, I keep telling people this, but they always say "the manufacturer says..." The other side of the equation is how the force in the bar gets into the next length of reinforcement. Since these are presumably tension forces, they have to be put into other tension ties or compression struts, either of which would require full development length (or possibly an Ldh as with a headed anchorage.)

RE: Epoxy dowel & shear friction

I think that it points to a basic difference between a mechanical bond (straight embedment with air voids in the concrete) and a chemical bond (where there are no air voids). Same force in less embedment length. That would have to mean that the shear cone would be critical in a chemical bond, not the bond itself.

Just a thought...

Mike McCann
MMC Engineering

RE: Epoxy dowel & shear friction

But Mike, read the report of this investigation. The critical testing in haynewp's case is with the concrete restrained by a plate, therefore no cone is possible. And as the authors stated, there is much more room for error with chemically anchored bars than there is with those which are built in.

Haynewp, I don't eliminate chemical anchors, especially for shear. But I think their marketing has been based on half truths, and I don't mind making that "large statement".

RE: Epoxy dowel & shear friction

I agree with Hokie, I can't understand how this can be. sounds like black magic, use this stuff and all your problems disappear.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Epoxy dowel & shear friction

(OP)
I think they have to show test data based on ACI requirements in order to get ICC approval for their chemicals and what they are stating for lengths required to develop a bar. Without ICC approval they may not be legitimate for a certain mfr's product and maybe some are not being honest. It seems like they would say more length is required instead of less so they could sell more epoxy but I don't know.

But anyway, assuming the epoxy is correctly installed and inspected, and does actually develop the bar, it still seems to me like blowouts by Appendix D could occur for short embedment lengths. But then I can't find where there are any minimum edge distances to preclude blowouts under the shear friction section which doesn't seem right either even when you are using the longer concrete bond development lengths.

RE: Epoxy dowel & shear friction

Hokie:

It is nowhere near as apparent to me as it is to you. I will print out the paper though on Monday and read it in depth as I have time.

That being said though, this would be an excellent opportunity for you to write a paper and submit it to Ron's new site for further peer discussion. Just a thought.

Cheers. Have a good weekend.

Mike McCann
MMC Engineering

RE: Epoxy dowel & shear friction

I am confused. I thought appendix D is needed to calculate the capacity of reinforcing embedded in epoxy/adhesive. Do any reputable manufactures have other methods that ignore the concrete failure modes in Appendix D? If I am not mistaken ACI 355.4, Qualification of Post-Installed Adhesive Anchors in Concrete, is coordinated with Appendix D. Is there another ACI standard for adhesive embedded reinforcing? Is anyone disputing the capacity of epoxy embedded reinforcing when Appendix D is used?

For the original question concerning shear friction with epoxy embedded reinforcing, I assume the equation used is Vn = Avffyμ. Wouldn't one use Appendix D and the manufactures evaluation reports to determine the tension capacity for the Avffy used in the shear friction equation.

RE: Epoxy dowel & shear friction

There is a recent paper about the subject, proposing a design method for users of the ACI Code. I haven't accessed it or read it, but perhaps someone (TXStructural?) can give us a capsule summary.

http://www.concrete.org/PUBS/JOURNALS/AbstractDeta...

RE: Epoxy dowel & shear friction

(OP)
Prior to IBC 2012 (which I was not using for this project), adhesive anchors were excluded from Appendix D. Reinforcing used for embedment was required to be designed by "other parts of this Code" per ACI Appendix D, D2.2. By that abstract link above, epoxied reinforcing is still not part of Appendix D.

RE: Epoxy dowel & shear friction

Which to me means that there is no difference between reinforcement embedment requirements with or without epoxy...by your current Code.

RE: Epoxy dowel & shear friction

I have always looked at this like TXStructural. Yes, epoxy can develop a bar with a short embedment length into existing concrete, but the problem is the bars in the existing concrete are not developed.

DaveAtkins

RE: Epoxy dowel & shear friction

That the bars in the existing concrete are not developed is no mystery, and nothing new. If you need those bars, then you need to develop them such that force can transfer from the new bar to the existing bar. Typically that means much greater embedment depths for the new bar. Then you meet the depth to develop the new bar, and you have the development to get the existing bar as well. It's not the manufacturer's fault that many people overlook this.

RE: Epoxy dowel & shear friction

(OP)
I'm just trying to create a shear connection, not develop any existing bars.

RE: Epoxy dowel & shear friction

You cannot use shear friction unless the bar is fully developed on both sides of the shear plane, as you alluded to in your original post. The 318-11 11.6.8 language is "...shall be anchored to develop fy on both sides by embedment, hooks, or welding to special devices." The commentary says, in part, "Anchorage may be developed by bond...", which is slightly different from embedment. The mechanism required by 318-11 11.6.8 is tension in the reinforcement, without mention of direct shear transfer.

Appendix D offers a method to determine how well a post-installed anchor will resist forces. If the design considers all parts of the system and the load path is continuous and sufficient, the design would comply with the code.

RE: Epoxy dowel & shear friction

(OP)
I guess the question that has come up after my original question, since we have found out that expoxied rebar is not included in Appendix D, is this:

Are the short rebar embed lengths (as what is being provided by adhesive manufacturers) using epoxy to develop the bars accepted by Code through testing?

RE: Epoxy dowel & shear friction

The short answer is "no". The length of embedment that is required to bond bar to concrete is not the length required to comply with either Appendix D (which considers everything EXCEPT adhesive bond), or anchor the bar sufficiently to other reinforcement using development length. The method for determining adequate anchorage is Appendix D. If the adhesive (which is usually acrylic rather than epoxy) is tested to develop full bar yield strength along a length, AND the tables or software ALSO consider the Appendix D computations, then it should be good.

That said, short embedments using chemical anchors are MINIMUMS. There is no reason to adhere unreasonably to a minimum when life safety and structural integrity are on the line. You do not want to be excessive, but think the entire anchorage through carefully - it is the job of an engineer to use engineering judgement, not just get away with a minimum number published by a manufacturer. Look at supporting data and testing, understand the issues, and make an engineering decision. There is very little economy in minimal design, when a slightly deeper embedment of a limited number of bars eliminates all concern. We frequently spend lots of hours researching and discussing, when a more robust design solves the problem and costs less.

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