Post Installed Rebar - Anchor vs Development Length

[RFreund]

Let's say for example you have a retaining wall repair. The top of the wall is going to be saw cut, removed and replaced. The wall is only 6" thick (it's short) and you will drill and epoxy bars into the existing wall. Would you find the development length required and call it good or are you checking appendix D (aka embedded anchor)? Say you have a very shallow required development length (lap length) of less than 12". Is there reason to check Appendix D?

Relevant:

http://www.structuremag.org/?p=8651

EIT

http://www.HowToEngineer.com

 

[dsweet1984]

If I'm understanding this correctly, you will need to calculate the pullout strength of the adhesive anchor in the concrete. See attached. Then you will have to base your tension force in the steel on this strength when you are calculating your available moment. This may or may not yield the rebar, so you may have a compression controlled flexural member.

Dustin

 

[KootK]

Link.

The most common method is to effectively non-contact lap the new bars with the old. That may require more, or less, than Ld depending on the situation. If you do it this way, there's no need for appD.

If you don't lap the bars, you can use appD as an alternate.

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.

 

[MacGruber22]

KootK is correct - it is only one or the other, not both. As embedment depth increases, appendix D becomes inefficient at predicting a practical lower bound to the rebar capacity.

Say you have a very shallow required development length (lap length) of less than 12".

12" is the minimum development length required for deformed bar unless something has changed.

As dsweet mentioned, you can check the fully-developed bar capacity with the available bond strength, but I do not believe you have to worry about that for ICC-approved concrete adhesives. I am fairly certain, that the cracked adhesive bond strength at the minimum development length for any given bar size will be greater or equal in capacity to phi*As*fy.

"It is imperative Cunth doesn't get his hands on those codes."

 

[MacGruber22]

eh..substitute "lap" for "development" in my response.

"It is imperative Cunth doesn't get his hands on those codes."

 

[DaveAtkins]

If you are not truly lapping the bars, make sure you check the development of the new bars being embedded, AND the development of the existing bars in the wall.

DaveAtkins

 

[RFreund]

So if you are lap splicing you can use lap lengths and not worry about appendix D, what about minimum edge distances? This to me, is what seems to muddy the waters. If you are lapping tension reinforcement that has 3/4" cover then appendix D is not going to work. I suppose I'm having a hard time transitioning between the two. Maybe that is because I'm not fully understanding the behavior.

If you are developing a bar into a mass on concrete but not necessarily lap splicing, you still have the option to use either?

Side question:
ACI says that 12" is the minimum length for Ld. However Ld can be reduced by Asrd/Asprovided. Can this be reduced below 12"?

EIT

http://www.HowToEngineer.com

 

[TehMightyEngineer]

That's because appendix D was written with the intent of allowing engineer to dowel bars into blocks of concrete without transferring the load directly into another bar (via a non-contact lap splice).

Thus, when you lap splice the bar with low cover the stress in the bar isn't going to need to transfer to the free edge (3/4" cover side); it's going to transfer through the bond and deformations of the bar into the concrete and then back into the adjacent bar.

If you are developing the bar into a block of concrete with a nearby edge then I would consider using Appendix D but you are right that this situation does muddy the waters a lot. I'd carefully read the code in this situation to decipher what is legally required but also to try find the intent of ACI 318 (good luck).

Maine Professional and Structural Engineer.

 

[wannabeSE]

RFreund,
Hilti HIT-RE 500 SD is the only adhesive that I know that has an ESR report that includes provisions for reinforcement connections (rather than anchor provisions only). The ESR includes cover requirements and allows deep embedment needed to develop reinforcing. For more information and requirements see

http://www.icc-es.org/Reports/pdf_files/load_file.cfm?file_type=pdf&file_name=ESR-2322.pdf

 

[KootK]

Given that the wall is only 6" thick, I would dowel into the centre of it to avoid spalling. That means that you'll be non-contact in both directions, including the bad one (transverse). It shouldn't be a problem for such a small scale application however.

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.

 

[TehMightyEngineer]

Side question:
ACI says that 12" is the minimum length for Ld. However Ld can be reduced by Asrd/Asprovided. Can this be reduced below 12"?

No, I'm almost positive that 12" is the hard minimum for that. Also, note that the Required/Provided ratio reduction only applies to flexural rebar.

Maine Professional and Structural Engineer.

 

[RFreund]

Spalling seems like a concern during the actual installation, but once the epoxy and bar are in place is this really still a concern?

That is what I had thought too, but I can't seem to find anything definitive, seems like you could make a case either way. I'll stick with 12" but I'd like to see it be made clear.

This is opening the door to the shear friction debate....
Did we ever decide if it was reasonable to reduce the development lengths for shear friction?

My example is simple but but I'm just using it to try and understand the underlying fundamentals of when and how to use what, appropriately.

Thanks again for the replies!

EIT

http://www.HowToEngineer.com

 

[KootK]

Spalling seems like a concern during the actual installation, but once the epoxy and bar are in place is this really still a concern?

Only an issue during install.

That is what I had thought too, but I can't seem to find anything definitive, seems like you could make a case either way. I'll stick with 12" but I'd like to see it be made clear.

12" minimum if you're using reinforced concrete principles and need to lap/develop. This won't apply if you're using anchorage principles (appD).

Did we ever decide if it was reasonable to reduce the development lengths for shear friction?

We decided that ACI clearly expects full development for fy. We're still a little fuzzy on why.



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.

 

[TehMightyEngineer]

Only an issue during install.

To expand on this; it's almost the exact same issue as driving a screw in a wood member near to an edge where splitting can occur; once the screws in there the risk for splitting goes down a lot.

The impact hammer drills commonly used to drill out a hole for anchors have a pretty heavy impact combined with removing material from the concrete. Until the hole is filled with epoxy and a dowel this is an area of reduced section and may not take to the impact of the drill.

Maine Professional and Structural Engineer.

 

[MacGruber22]

The impact hammer drills commonly used

Specify that diamond bit coring has to be used?

Also, what about routed channel slots on the tension face? (if there is access). Similar to stitching new negative bar across concrete joists in a parking garage.

"It is imperative Cunth doesn't get his hands on those codes."

 

[wannabeSE]

The problem with cored holes is that the design values are typically based on testing with hammer drilled holes. I suggest contacting the adhesive manufacture anytime the product is being a used differently than the published documentation.

 

[MacGruber22]

Yes, agreed, wannabe. But, if you use Hilti Profis software (for example), you can choose which method of installation and it applies a reduction (if required). You might be able to extract the reduction from there, as long as you don't exceed the allowable core diameter for the given bar size.

"It is imperative Cunth doesn't get his hands on those codes."

 

[TehMightyEngineer]

What they said, coring helps with the install problems but then hurts the anchor capacity. I've always wondered if coring could be employed and then use a impact hammer drill to "roughen" the interior surface of the cored hole allowing the benefits of a cored hole with no reduction.

Maine Professional and Structural Engineer.

 

[Ingenuity]

TME,

Your solution of coring then percussion re-drill would work. Doubles the work, but reaming the hole with a percussion bit works.

Back about 15 years ago we did a repair project where the general contractor installed 160 each x 1-1/4" dia anchor bolts x 6 feet long into a large octagonal-shaped concrete pedestal for a 150' high steel chimney stack for a power plant. Unfortunately they PAINTED the embedded anchor bolt length (straight shaft). It was in a typhoon region so wind loads were significant.

We load tested the installed bolts and they slipped out at about 10% of required capacity.

Long story, short. We removed the AB's (that was relatively easy) then core drilled each hole to slightly over-size the resulting hole, then ACID ETCHED the hole, neutralized the acid, checked the pH to make sure it was alkaline, then installed the sand-blasted AB's, then epoxy injected the annular space, then load tested after cure.

We also did some trials on non-etched holes (just core drilled) and they also tested to the same magnitude as those that were etched. BUT - the holes were cleaned repeatedly to remove the coring slurry. The slurry, if not removed immediately, causes significant loss of bond.

 

[MacGruber22]

Using RE-500, table 35 shows little difference between #4 bars and no difference in #3 bars

http://www.icc-es.org/Reports/pdf_files/ESR-2322.pdf

I don't think it is worth doubling the work for such a relatively small wall.




"It is imperative Cunth doesn't get his hands on those codes."