Cast-in Place Bars versus Epoxied

[jike]

I have always wondered why there is such a large development length with rebar cast into fresh concrete versus a short one with rebar drilled and epoxied (Hilti and Powers) into existing concrete. After all, how can 1/16" of epoxy around the perimeter of the bar make such a huge difference in values without the surrounding concrete being affected?

I know the epoxy folks use safety factors on the order of 4 and the concrete folks use safety factors on the order of 1.7, but this would skew the differences the other way instead of making epoxy lengths so short.

Can someone help me see what I am missing here?

 

[nutte]

The rebar development lengths are based on mechanical anchorage (friction) between the bar deformations and the concrete. I presume the epoxy anchors develop chemical bond between the bar, epoxy, and concrete.

 

[csd72]

I believe it is to do with the crical concrete surface in shear.

Imagine that the critical failure surface is a cylinder around the bar.

For a cast in bar this cylinder is at the outside of the bars ribs. For an epoxied bar it is the outside surface of the drilled hole.

As the drilled hole is larger than the bar diameter, then the failure surface is larger and therefore it is stronger.

Another factor that comes in is that the epoxy engages the surrounding concrete along the whole surface, whereas the rebar ribs give more localised stresses.

 

[lkjh345]

We called Hilti one time about this very question, becasue it perplexed several of us in this office.

Their response was that the values listed in their catalog are based on pull tests of rebars epoxied into concrete not on mathimatical formulas.

It is also the reason that one of the footnotes in their HVA Capsules table reads 'HILTI DOES NOT RECOMMEND THE USE OF ALTERNATE EMBEDMENTS OTHER THAN THOSE TESTED AND LISTED ABOVE.' They do not have adequate mathimatical formulas to explain the behavior adequately, they just know it happens. There is no proper way to interpolate values between the embedment depths shown in their catalog.

 

[jike]

If I remember correctly Hilti uses 1/16" oversized holes and Powers uses 1/8" oversized holes. It seems logical that this slightly larger perimeter of a drilled hole versus the bar perimeter would not make much difference.

I can see where the localized stresses produced by the bar ribs versus the even stresses produced by the epoxy bonded to the concrete might make a big difference. However, the forces still have to eventually bleed into the concrete. And what about the difference in the factors of safety?

It still seems as if the criteria that ACI is using is quite a bit different than what the epoxy folks are using. Unfortunately, because of the propriatary nature of the epoxy products, we may never fully understand this without taking them at their word.

My question is not so much for the epoxy folks as it is for ACI. Are we way too conservative on our development lengths?

 

[Lion06]

My understanding is that the development length is based on a maximum achievable bond stress between the concrete and rebar. As NUTTE mentions, the epoxy gets a much higher bond stress (to both the concrete and rebar), hence the shorter length required.

 

[jike]

Another thought comes to mind......is there a better or more efficient rib design that could relieve localized stresses and therefore shorten our lap lengths? Is there any incentive for the industry or CRSI to study this? If the rib design is improved, they would sell less rebar.

 

[JLNJ]

The adhesive anchors are very sensitive to edge distances; therefore if your concrete is not thick (i.e. thin slab) then the bar isn't developed as quickly, or maybe at all. Just because one has the apparent embedment doesn't mean that the full bar strength can be developed into the concrete substrate.

 

[PackerFan]

The data that Hilti or Powers represents in their technical guide are based on rebar used as an "anchorage" in lieu of a splice. So these shorter embedments shouldn't be used if you need to transfer load from an existing rebar into a new rebar, these embedment's should only be used if you would like to simply develop the rebar tensile or yield strength like you would with a standard adhesive anchor. Also, be aware that these embedments are typically ultimate values instead of allowable values for rebar anchorage. If your desire is to lap the rebar into the existing slab and have load transfer from the new bar to the old bar in the concrete, the manufactures should recomend you to use the lap splice lengths from ACI, not the values from the technical guide. A description of this is on page 15 on this link from one manufacture.

http://www.us.hilti.com/data/techli....1 Anchor Principles & Design (70-88)r009.pdf

 

[yogia]

Interesting discussion ... I believe it has to do with the surface area of the cone formation which will have to fail, particulary in case of pull out for an anchor. And then, of course the properties of materials will come into play as well. Pull out tests will normally result in a conical chunk of concrete breaking away from the parent body.

Yogi Anand, D.Eng, P.E.
Energy Efficient Building Network LLC
ANAND Enterprises LLC

http://www.energyefficientbuild.com

 

[DaveAtkins]

PackerFan raises an excellent point--even if the new bar is developed into the existing concrete, the existing concrete will "pull off" of the existing rebars, if they are not developed per ACI.

DaveAtkins

 

[JedClampett]

Another important factor to consider is the minimum spacing and edge distances that epoxy doweling requires. Assuming Hilti HY-150, if you spaced #8's at six inches, they would violate the spacing requirement of 16 inches. Plus if you want to place them with two inches of cover, you're violating the edge distance criteria of 12 inches. You could reduce the allowables, but now you're back where you started.

 

[mitchelon]

What nutte said is correct. It has nothing to do with the surface area of the cone. The transfer of forces occurs by chemical adhesion between bar and concrete, frictional forces, and mechanical anchorage of the ribs. Pullout tests were the method of choice many years ago, but it is no longer used as the sole basis for determining the development length of reinforcing bars. But, with a FOS of 4, it should be OK. The bond mechanism is now well understood, but the equations and knowledge base on bond remains primarily empirical.