Development Length with Dilled and Epoxyed Rebar

[AUCE98]

I am currently working on a project that is requiring an existing mat foundation to be extended to accommodate the new loading and revised column locations. The mat extension will be the same thickness as the existing foundation. The existing foundation has sufficient reinforcement for the modified stresses, and we can reinforce the extension as needed. At the concrete interface I am planning on drilling and epoxying rebar into the existing concrete and extending it into the new concrete and provide a class “B” lap length. This will allow for the transfer of the moment and shear between the two faces. The existing rebar is finished with a standard 180 degree hook. While the drilled and epoxyed rebar will be developed at the interface, my question is how deep should I extend the drilled rebar into the existing mat to ensure that it is developed with the existing rebar. Should I extend the doweled rebar equal to Concrete Cover + Ldh (hook development length) + embedment for epoxy anchoring, or is it sufficient to dowel it just the recommended embedment depth? Any thoughts would be appreciated.

 

[msquared48]

You need to dowell in sufficiently to be able to develop the required tension to develop the moment you need. Just make sure what situation you are dealing with, full or partial pullout cone, and adjust the forces you will attain accordingly.

Personally, I would not skimp on the dowel length, regardless of any complaints by the contractor. It's your a$$, not his.

Mike McCann
MMC Engineering

 

[DWHA]

How are you going to make sure that the new bars are located adjacent to the existing bars. You do not want the new bars to be centered in the spacing between the existing bars.

I would call you for the concrete to be removed at the ends of the slab. Then you can have the new bars tied to the exitsting bars and pour the two pieces together.

Might be easier/cheaper than drilling and epoxying a bunch of bars. Plus, you would get a much more controlled final product.

 

[asixth]

I guess that it would need to be drilled a minimum of (cover+0.5*development). I too would prefer a contact splice but think the contractor would not consider breaking away up to 4' of concrete to achieve that. And mat foundations are normally hard concrete.

 

[Ron]

Epoxied dowels do not usually fail with a conical failure interface like embedded rebar or expansion anchors. Their failure mode is usually direct shear at or near the interface of the epoxy-concrete bond. For this reason, your proximity to the developed rebar must be closer.....so locate the other rebar as accurately as you can.

 

[delagina]

I just use recommended depth per Manufacturers table to develop tensile strength of rebar/anchor.

It would also be a good idea to call the anchor company. I sometimes call Hilti.

 

[rowingengineer]

delagina,
he is trying to transfer moment as such he needs to get the forces from the doweled bar in to the existing reinforcing. just epoxying a bar to some nominal depth without considering the existing bar, will only provide a moment connection to the end of the dowel bar at attempts to take the moment.

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[DaveAtkins]

I would fully develop the hooked bar in the existing footing, AND develop the epoxied bar. So the embedment is the greater of ldh + cover OR the required embedment for the epoxied bar.

DaveAtkins

 

[rowingengineer]

"will only provide a moment connection to the end of the dowel bar at attempts to take the moment"

was meant to say:

"will only provide a moment connection to the end of the dowel bar as the concrete attempts to take the moment without reinforcing."


How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[hokie66]

My preference would be to design the extension to stand on its own, and only provide a nominal connection to the existing. Requiring so much drilling and glueing just seems illogical. I ask myself "how would this have been done prior to the development of epoxy?"