Development length for drilled and grouted reinforcement

[Enginerdad]

I frequently have to detail drilled and grouted reinforcement to be installed into existing concrete structures. Standard Gr. 60 rebar and common non-shrink grout, non-proprietary. It's always been my understanding that the required tension development length for this application would be the same as for cast-in-place rebar, but one of my reviewers questioned it and now I'm having trouble finding a reference to back up that assumption. I work in bridges so AASHTO LRFD is my primary code, but I know they reference the ACI 318 standards when it comes to post-installed anchors. Does anybody know where I can find some guidance on the proper way to calculate the development length?

 

[phamENG]

Are these bars in tension?

For adhesives/epoxy anchorage products, the bond between the concrete and the adhesive is tested and reasonably well understood - so development length is the greater of the embedment required by the manufacturer and the lap length spec'd by ACI/AASHTO.

For standard non shrink grout I'd say you're not so much looking at development length as shear friction across an unreinforced cold joint. You may be able to develop the bar in the grout, but then you have a failure plane between the grout and the concrete.

I wouldn't rely on drilled and grouted bars as you describe for tension - only dowel action.

 

[Enginerdad]

phamENG

My application is removing the tops of existing abutments and constructing new bridge seats on them for superstructure replacement. The connection is primarily shear friction between the old abutment and new bridge seat for lateral forces. But in order for shear friction to work the reinforcement has to be fully developed on both sides of the joint, so tension development still matters. I think I understand what you're saying; there are two conditions to check for tension. The development of the rebar in the grout, but also the bond between grout and concrete, which would be its own shear friction check. Thanks, that's a helpful direction to go.

 

[SJBombero]

It's a fairly common detail. The failure between the grout and concrete is covered by the manufacturers data sheet that claims the bond will be at least as strong as the shear strength of the concrete.

 

[phamENG]

No problem. I imagine some non-shrink grouts that are, in fact, expansive, could work as the expansion would provide a normal force to produce the friction.

If you're doing it as a generic detail, you may want to specify that the grout to concrete interface be as strong as monolithic concrete as SJBombero mentioned.

 

[bones206]

I think grout manufacturers should provide test reports for code acceptance similar to adhesive anchor manufacturers if they want their products to be used in an anchorage application. Without an ESR report or similar I would be reluctant to assume anything. The characteristic bond strength at the grout/concrete interface is pretty critical and should have some kind of reliable basis for design.

This article cites some studies for grout/concrete bond strength, but I haven't vetted them myself:

https://www.structuremag.org/?p=18593

 

[Enginerdad]

All great suggestions, thanks everybody!

 

[EZBuilding]

Enginerdad, any objection to utilizing an epoxy product?

 

[Enginerdad]

any objection to utilizing an epoxy product?

None at all, but unfortunately it's not my call. Working for a DOT they have standard specs on what materials to use. This DOT maintains a list of preapproved non-shrink grouts that the Contractor can choose from, but I can't jump to epoxy because it's not a standard item and I have no reason to say that a regular grout wouldn't work for this application. It's literally one of the most common applications of drilled and grouted dowels in the industry.

 

[gte447f]

In 20 years I don't think I've ever done this or seen this (i.e., rebar drilled and grouted with non-shrink grout). All I've ever known has been drill and "epoxy" with proprietary adhesives. I am curious to learn from others in this thread about the possibility of drilling and grouting.

 

[Tomfh]

How do you grout the holes and install the reinforcement?

 

[KootK]

1) "Cored and grouted" is probably more accurate terminology.

2) I've done some of this and a lot of something similar in precast where you grout bars into pre-installed corrugated sleeves. You can find some research papers in this space such as this.

I like this better from the perspective of bond stress. Told that way, and for general applications, you've got the following to worry about primarily:

a) Bond stress between the bar and the grout a the circumference of the bar.

b) Bond stress between the grout and the concrete substrate at the circumference of the cored hole which, obviously, is an improvement over regular development.

c) General, appendix D style anchorage. Technically this is a non-issue for shear friction although, in many applications, I question the validity of that assumption.

d) Sometimes -- but rarely -- splitting.

3) I've seen manufacturers like Five Star recommend that grouting be used only for static or lightly dynamic loading. It's particularly applicable for high heat applications where most epoxies will perform poorly. For your situation, I also would prefer epoxy where it one of the options available to you.

4) Williams provides a bit of design guidance for their grouted in anchors here: Link

5) One of the issues of concern is just where the dowels wind up within area of the cored hole.

 

[BridgeSmith]

This DOT maintains a list of preapproved non-shrink grouts that the Contractor can choose from, but I can't jump to epoxy because it's not a standard item

Interesting. At my DOT, we've eliminated grout as an option for anchoring rebar dowels and threaded anchor rods. We spec epoxy adhesive anchorage systems, although we don't use them for tension applications, due to typically poor installation practices resulting in pullout failures.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enginerdad]

So if you can't use grout and can't use epoxy for tension, what do you use to bond rebar into existing concrete?

 

[KootK]

We spec epoxy adhesive anchorage systems, although we don't use them for tension applications, due to typically poor installation practices resulting in pullout failures.

Under ACI, this really does come back to OP's previous comments. If you're not using dowels in tension then, presumably, you're using them in shear. In ACI, there is no true "dowel" methodology so shear transfer is shear friction. But, then, shear friction also implies using the dowels in tension -- for fy no less.

I know that other parts of the world have true "dowel" design methods. Does AASHTO?

 

[Enginerdad]

No, AASHTO takes their post-installed section directly from ACI

 

[bugbus]

Couldn't you treat this the same way as a dowel in a road pavement? I.e. one end effectively debonded (therefore acting as a 'true dowel', as KootK described it)?

 

[KootK]

Couldn't you treat this the same way as a dowel in a road pavement? I.e. one end effectively debonded (therefore acting as a 'true dowel', as KootK described it)?

You certainly could but, in north America, we don't actually have codified ways of doing this that I know of. Do you guys have something in AU for dowel action? I recall there being some Eurocode stuff in the world of precast that deals with this but, off hand, I don't recall where to find it. Our wood codes have dowel stuff and I've seen folks attempt to Jerry-rig that for use with concrete dowels. It's a bit of a stretch in my opinion however.

 

[BridgeSmith]

So if you can't use grout and can't use epoxy for tension, what do you use to bond rebar into existing concrete?

We don't dowel rebar into existing concrete for tension application. We chip the concrete back and provide a lap with the existing reinforcing, either a standard lap length or sometimes just the few inches needed for a mechanical splice.

It may not be feasible for some situations, but we've found a way to do it so far on our bridge rehabs.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

In ACI, there is no true "dowel" methodology so shear transfer is shear friction.

It's been a while since I did a doweled shear joint, but I believe we limit the shear capacity of the joint to the shear capacity of the doweled steel.

We've done a few seismic applications where we provided a bond breaker sheet, in order to limit the shear capacity of a shear key to the capacity of the reinforcing, so that the failure force would be more predictable.

Rod Smith, P.E., The artist formerly known as HotRod10