Epoxy Anchors

[HardyParty]

I know this has been discussed fairly often in this forum, but I am looking for everyone 2 cents on epoxy anchors. I have a situation where the contractor has poured a footing wrong so I am trying to extend the footing back to contract dimensions. I am looking pour a new strip and to dowel and epoxy a small rebar (#3-#5) at 18" oc to tie the old concrete to the new. The rebar is essential to use the shear friction component of the concrete, so I have to fully develop the rebar on both sides.

In doing this, I have run in the "does appendix D apply"/"whats the bond strength of the epoxy" discussion. I am under the impression that I only have to look at developing the bar in pullout/bond. However, t looks to me as though Hilti (and others) do not provide load values for epoxied rebar in crackedconcrete (Hilti RE500-SD and similar). If I use the given bond strength, I get an embedment of ~15" or more. If I look at the epoxy anchors for uncracked concrete, load values are provided and it only takes ~6" to fully develop the rebar (Hilti RE500 and sim).

My conundrum here is, when can I use uncracked conrete? I typically always assume cracked..but why would they make the uncracked option if it were impossible. Could a footing ever be uncracked? It seems to me as anything over 12" is a long way to drill and epoxy (especially if the footing is only 24" wide).

I am planning on calling Hilti tomorrow, just wanted to see what others have to say. Thanks!

 

[JedClampett]

If your building code has implemented Appendix D, it applies. Almost all have. I wouldn't necessarily assume that a footing is cracked, but that doesn't get you off the hook to run the Appendix D capacities. It just changes the analysis.
You're going to need to figure out the tension in your reinforcing you need to get your shear friction to work. One you have that use Appendix D in conjunction with the Hilti ICBO report on the RE-500's.
Or, you can do what everyone else does and download Hilti's Profis software and run the numbers through that. In my opinion, if you're coming out with 15 inch embedment for #3 and #5 bars, you're making an error somewhere along the way.

 

[HardyParty]

I was under the impression that rebar is not included in the scope of Appendix D. If it were, then the rebar development lengths from ACI would be mute and most all would fail in some other failure mode. (Rebar at 4" apart would surely breakout). I believe the failure from rebar embedment is more of a shear failure between concrete and epoxy than a cone failure.

I did check to see what capacity I could get out of Hilti Profis. The kicker is that the rebar must develop fy. If I'm understanding correctly for a #5 that is (.31)(60)=18.6k. I cannot get that kind of force into a 24" wall footing.

 

[cliff234]

HardyParty, I was under the same impression (that development of reinforcing steel was not within the scope of Appendix D). I sure hope that ACI 318 does not "clarify" this. Take the case of headed anchors which are often used to alleviate congestion. If we had to do Appendix "D" calculations to compute the pullout capacity of closely spaced #11 bars with headed anchors, we would never get the numbers to work. Heck, for that matter ACI is also silent on computing the pullout capacity of closely spaced bars with standard hooks. We can compute the required embedment lengths for hooked bars to develop individual bars in tension, however the code does not explicitly require designers to compute the pullout capacity of groups of closely spaced hooked bars in tension.

 

[MrHershey]

Wouldn't just assume the concrete is cracked. Foundations are usually pretty hefty. Obviously have to check, but I wouldn't expect your typical spread footing on your typical job to crack on you.

 

[BONILL]

HardyParty,

Have you considered chipping out a portion of the footing (the amount required for developing your #3/#5 bar) and then just splicing the new rebar with the old using the new pour? I agree with you that it is more of a shear friction problem, and the problem with that is that you would have to develop the bar to its full capacity at the side of the existing footing (in case of using epoxy).

 

[sandman21]

Rebar used for anchorage does not follow within App. D when the bars comply with the deformation requirements of ASTM A615, ie. rebar used as anchor bolts. Post installing of rebar is covered under App. D. The likelihood that you can demonstrate that the section does not exceed fr for concrete for anything that sees main building loads is small. It's also not fy, its the capacity of the bar, typical A615 is ~10% over fy.

 

[HardyParty]

Thanks for all of your responses. I'm relatively new to the profession and still learning - all of theses responses help.

Sandman:

How would my typical rebar not fall within ASTM A615? On my general notes its states "All reinforcing shall be high strength deformed bars conforming to ASTM A615, Grade 60..." Whether it is post installed or cast in place, it is the same rebar.

Do you have a code reference stating that all post installed rebar falls under Appendix D?

Also, in ACI318-08 it states in 11.6.8 "Shear-friction reinforcement...shall be anchored to develop fy on both sides by embedment, hooks, or welding to special devices." I'm not sure where your reference to the capacity of the bar comes from.

BONILL:

Let's say I used a #3. A #3 class B splice in 3000psi concrete is 32.5". The existing footing is only 24" wide. There is not room to fully develop the bar (as it was originally only for t&s)

 

[BONILL]

HardyParty,

You are correct. I somehow missed the fact that you only had 24" to work with, and pictured a large isolated footing. It is important to note however that if you calculate your ldh (hooked) per ACI 12.5.2, that value would be reduced to about 17". Although this still wouldn't help you, you can use a percent of the capacity of the bar (As provided/ As required) x fy, in lieu of using full ldh, and just add more rebar to ccmpensate.

 

[TDIengineer]

Why not look at your max soil bearing pressure and determine the amount of load you need to transfer across your new cold joint. Don't attempt to use shear friction with epoxy. Use the allowable shear values based on the ICC report and check it based on demand.

 

[HardyParty]

TDIengineer:

That is a good idea. However, the ICC no longer gives shear (or tension) values for adhesives since the adoption of Appendix D. The only products that publish those kinds of values are for uncracked concrete, and have no ICC report to back them up. (ICC only supports adhesive tested to the new AC308).

What an awful runaround. I just don't quite understand it.

 

[sandman21]

There are two ways to use rebar to anchor, one is to directly weld the bar to a plate this is trandiotnal rebar, ASTM A706, the other is to use an "anchor" which complies with the requirements of ASTM A615 the threads on the "anchor" conform to the requirements of ASTM A615 making them "rebar" for App. D.

It's a round about way to getting back to ACI 318, the latest addition of ACi 318 now has most provision contained within it.

You are attempting to splice on a bar you should be under ACI 318-08 12.14.3.2 mechanical splice, or comply with App. D requirements for ductile failure.

I am looking at the ICC report for 500-SD and it gives values for cracked concrete both shear and tension along with any reduction factors you would need. Table 24 from ESR-2322 has kc.cr factor.