Guidance for Doweling new concrete to existing concrete section

[CWEngineer]

I am looking for some guidance on determining the bar size, bar spacing, hole diameter and depth for doweling a new concrete section onto an existing concrete section. Basically, I am planning to increase the height of a 3.5 ft retaining by 1 ft, so will be providing dowels on top of the wall stem. I have ran into several details for doweling but cannot seem to find guidance/calculations on how these are obtain.

Appreciate your help.

 

[racookpe1978]

How thick is the existing retaining wall?
How is it constructed (blocks and mortar? Filled blocks with concrete poured in the hollows? As a single cast concrete wall with ??? internal rebars spaced ???? apart and tied together ?? by ?? at ??? intervals?)

 

[CWEngineer]

The stem of the wall is 10 inches thick, it is cast-in place concrete. #4 vertical bars on the stem are placed with 2 in. clear cover on one face and 2.5 clear cover on the other face, they are spaced at 12 inches on center. The horizontal bars in the stem are also #4 bars and they are also spaced 12 inches on center.

 

[Once20036]

I think that the dowels and additional height at the top of the wall is the easiest part and there's more to the question than just the dowels at the top.
Increasing the dirt/height will:
- Increase the sliding forces.
- Increase the overturning forces.
- Increase the shear at the base of the wall.
- Increase the forces in the existing vertical bars at the base of the wall
Can the existing wall safely resist all these additional forces?

To answer your original question, I would epoxy the new dowels into the top of the existing wall with a Hilti epoxy.
Bar size and spacing can vary and are up to the designer to determine. #4@12 seems like a reasonable place to start.
The bar spacing will determine the tension in the bars, which would be the design load to use with the Hilti product.
Hole diameter is based on bar size and Hilti recommendations.

If you want to go through the calculations by hand, use ACI Appendix D.

 

[CWEngineer]

Yes, I was planning to analyze the existing wall for stability and strength due to the 1 ft height increase. Thanks, I will contact the epoxy manufacture for recommendations and look at ACI 318.

Was also kind of looking for a reference or guidance that provide an example or went through the design process.

Thanks

 

[TXStructural]

CRSI's Design Handbook provides wall design calculations.
It sounds like the wall, at 3.5' tall and 10" thick, has minimum reinforcement (about .22 sq in/ft is required.)
I would make the 1 ft extension along the lines of the following:
- roughen and expose aggregate along the top of the existing wall with a bush hammer or by cutting a shallow keyway
- drill, clean holes, and use adhesive for #4 rebar dowels either a) embedding the dowels 14 inches into the old concrete (28 db minimum, uncomplicated development length; this seems excessive, but without further assessment, you cannot develop old bars with the new ones without this), or b) compute the required anchor embedment using 318 Appendix D;
- dowels should be located about 3" or 1/3rd of thickness from the soil side of the wall to clear the existing reinforcement and make drilling easier. Forces on the new part of the wall will be low, but the crack along the new cold joint probably needs to remain as tight as possible - more reinforcement across the joint will keep the crack tighter.
- Because you will only have about 10 inches of embedment into the new concrete, these bars need to be hooked since the design relies on shear friction along the old-new joint. Again, probably excessive, but meets the minimum code requirements. I would probably order these bars with 180 hooks to make construction easier. It gives a 4" out-to-out dimension which will fit well into the wall.

From a field perspective, there is no big savings in using 8" deep holes instead of 14" deep holes on small jobs. Every inch costs pennies, and on reasonably small jobs, I sometimes just make the adhesive anchor embedment the same as minimum development length. It sometimes seems that it costs more to do the App D calcs than buy a few inches of bar and adhesive. It is always conservative but seldom terribly excessive.

 

[CWEngineer]

Thanks for your guidannce, it was very helpful!

 

[CWEngineer]

TXStructural,

In your recommendation you suggested to use 28db. For a #6 bar this equates to 21 inch embedment depth. If I use ESR-2508 (Table 5B - Set-XP Epoxy Adhesive Anchor Rebar Bond Strenght Design Information), for a #6 bar, the maximum embedment depth is 15 inches. So I am planning to use 15 inches for the embeddment depth of the #6 bar instead of 21 inches, since I will not get any benefits by going deeper if I use the SET-XP Epoxy. Does this make sense? Appreciate your feedback.

 

[dcarr82775]

I believe the max embedment depth refers to the maximum tested. You can certainly go beyond that.