Spacing between existing and new reinforcement

[sb79]

I am working replacing an old silo is replaced with a higher one (68' old and 100' new) but same diameter. At door the old silo had #4 dowels @ 3" o/c.

My question is if the old bar are cut (at top of foundation) and new dowel bars are to be placed (w/ Epoxy) in the foundation, the spacing between the old bar and the new bar will be 1.5" on o/c. Is this acceptable?
Or is there any other way?

Thank you

 

[CELinOttawa]

If you want any real help with a problem like this, please provide a sketch.

 

[sb79]

Here is the sketch for reference

 

[TDIengineer]

If the existing rebar is cut and left in place, then the remaining piece of bar is a non-issue. Think of the existing rebar that is left in place as a long piece of steel sggregate within your concrete medium. Just be sure your new dowels still figure for 3" o.c. for the epoxy dowel.. That seems to be very tight spacing for epoxy dowels. I am assuming you have such tight spacing because this is for a vessel containing some liquid.

 

[Ron]

It depends on the nominal size aggregate you are using in the concrete mix. If you have 2 to 3 times the nominal aggregate size in spacing, should be ok. (for 1-1/2" spacing, course aggregate nominal size should be 1/2" to 3/4")

 

[a2mfk]

what about keeping the old bars and adding to them as needed?

 

[CANPRO]

Do the new bars have to be centered? You could stagger them a bit in the wall to get a larger clear spacing. Does the taller silo have a thicker wall? The location of your new dowels may also be dictated by design assumptions and whether you want to keep the inner/outer/average diameter constant in the new silo.

 

[CELinOttawa]

The problem you are going to encounter is that the existing rebar and your new rebar will be creating a plane of weakness in your existing concrete.

I would not recommend this approach, and always work to retain original bars where possible. Better would be to create an interface zone where you chip to expose existing bars and lap these with new, casting both into a joint zone. This, obviously, comes at a cost... But I don't think anyone can tell you that what you propose is okay based on what you've presented here.

Contact Hilti, Ramset, Powers, etc, and get their input. I think you'll find that with the right epoxy this may work, but this is beyond the standard "look it up for yourself" solution.

 

[TLHS]

Why can't you reusue the existing? Are the old bars too small?

Even if you need to cut flush to the foundation you can still chip back a little bit and do a mechanical coupling to the existing bars.

 

[TXStructural]

You're planning to reuse a foundation from a 68' silo to support a 100' silo? Interesting. That will more than double the wind load forces at the foundation, and will mean a roughly 50% increase in gravity load to the foundation, based on height and what appears to be a similar footprint.
As to use of reinforcement and spacing, what grade are the old bars? What is the purpose of the bars in question (shear friction development, tension, direct shear, or other?

In the scenario described, I would recommend using the old bars, leaving them sticking up into the new silo wall, and lapping them with new bars that extend upward. This will save substantial amounts of money. If the bars are already cut or the demolition requires it (do not let the contractor tell you it is the best or only way to go), then drilling between the old bars and using an adhesive (very few of which are actually epoxy chemistry) will work just fine. The drilled-in dowels need to extend a distance comparable to the development length of the bar in order to form a lap with the adjacent, old bars. It is not a function of the adhesive, but to assure that once the forces enter the concrete, they can fully develop into the existing reinforcement, as a (non-contact) lap splice. DO NOT rely on short adhesive embedments required by the manufacturer to fully develop the bars. These values only determine how much adhesive is required to get development, but not how much embedment is required to allow the concrete and surrounding bars to accept the load, particularly when you really do not know what is in the old foundation.