Epoxy (or adhesive) rebar anchor - depth of embedment

[Dennis59]

I am curious as to what folks in this forum are specifying in regard to this issue. In my own specifications, I require a depth of embedment such that the bar develops 125% of yield strength (ala ACI 318-05, 12.14.3.2). In doing this, I am basically saying that I want the same kind of reliability/capacity from an adhesive anchor that I would expect from a mechanical anchor.
The problem (as you might imagine) is getting submittals that clearly show what kind of embedment depth is needed for this strength. I usually receive a manufacturer's catalog sheet which shows a table of test-derived ultimate pull-out loads in xxxx psi concrete, at such and such embedment depths. Then I usually do some interpolating of my own, and tell the Contractor what embedment to use for each bar size.
Sometimes the manufacturer's embedment/capacity tables make this very difficult - there is not enough information, or the test values are all over the map and interpolation seems questionable.
I would appreciate any feedback you care to offer on this approach, or perhaps you would just like to reply with the method you personally use.
Thank you for any thoughts you care to offer.

 

[Lion06]

What kind of adhesive anchors do you see? Hilti and Powers both have tables showing the embedment required to develop Fy and Fu for each bar size.

I don't let the contractor tell me the embed to use. I spec it on the drawings.

 

[sdz]

I would specify the type of adhesive (e.g. "EAA - Epoxy Adhesive Anchor, Hilti HIT RE-500 or approved alternative") and embedment depth required for the grade of concrete. Some adhesives are better than others, e.g. Hilti HIT RE-500 is an epoxy that is suitable for diamond cored holes and for wet holes. Some adhesives are not suitable for those conditions. You can't always be sure what site conditions will be.

The contractor will probably try to use the cheapest adhesive with the least embedment.

 

[asixth]

I have always used tables that are provided by the manufacturer of the epoxy (Hilti or Ramset). These tests are performed in laboratory conditions and are assuming that the epoxy is installed using best installation techniques. This may not always be the case in the field. I had a hilti load test performed on some hold-down anchors today which returned a pull-out strength of 41kN (9.2kips) while the tables published say that the pull-out capacity for this design is 50kN (11.2kips).

I am satisfied that bar development is achieved as long as the minimum embedment set out in the product literature is achieved.

 

[jike]

Reject any submittal that does not clearly provide the info that you are looking for.

I limit the approved epoxy to only a couple that I am familiar with. I also specify the embedment depth on the design drawings based upon the approved epoxies.

 

[Dennis59]

I thank you kindly for your responses thus far.
I am particularly interested to know your opinion about my use of the "125% of Fy" idea, and the concept of trying to achieve a kind of equivalence with ACI 318-05, 12.14.3.2. No one has commented on that yet. Some of you have said that you specify an embedment depth on your drawings, based on some good manufacturers - do you specify the depth required for Fy, Fu, or something in between?
As far as your other comments, yes, I also list good well-known manufacturers in my specs, but I don't always get what I ask for. With public work, I sometimes need to evaluate an anchor from an off-brand manufacturer. The "depth to achieve 125% of Fy" provision in my specs gives me a basis for acceptance or rejection of some off-brand anchor. Several times I have told a contractor, "you know, if you were to use the more expensive epoxy from company X, you wouldn't have to drill such deep holes as you will with the cheaper epoxy you just submitted from company Y". Sometimes the contractor changes to a stronger, better epoxy because of that. Other times, they don't seem to care and they just go with the cheaper epoxy, even though the holes end up significantly deeper. I'm interested to know your experience, if you care to share it.

Thank you again.

 

[IJR]

Dennis59

To my limited knowledge and with a quick look at manufacturers embeddment data, I notice that their embeddment depths are well below 12diameters and their capacities are well below yield strengths. Failures often occur as cone failures and steel is usually understressed.
This is perhaps why their safety factors are large.

Next, those manufactures also have to supply accessories including nozzles and brushes and carefully design and standardize them for efficiency when used on again standard embeddments and hole sizes.

I wouldnt specify 125%Fy for an adhesive anchor for the following reasons, unless it is a very special application:

The accompanying embeddment(can be calculated theoretically because manufacturers publish bond strengths too) will be too large for practical use, drilling through concrete will be tough, and application of the adhesive through that depth will lead to poor quality, making your safety valve of 125%fy obsolete within minutes.

When loads are that high to demand 125%fy, I would vote for cast in anchors with/without additional reinforcement. When practical and reasonable load, I would choose adhesive or mechanical anchors.

respects
ijr