Tying new concrete stem wall into existing foundation

[mgsb38]

I'm rebuilding my garage using the old 12' x 18' stem wall foundation, and I want to extend the length from 18' to 24'. Obviously I need to tie in the new footing / stem wall with the existing concrete. I thought I could anchor two or three pieces of 1/2" rebar horizontally into each side of the old concrete with epoxy and coat the face with concrete bonding adhesive before pouring the foundation extension, but I'm concerned that this won't be adequate. Most information I've found online says, "Talk to an engineer." Before I call the pros, I thought I'd check if anyone around here has seen or done this.

This will be a detached, single story garage to be used for storage and as a workshop (no vehicles inside).

I've attached views of the front and back of the foundation. The gray concrete is the existing foundation, and the red is what I want to add. I should mention that I didn't include the footing in the sketches, just the stem wall.

Thanks!

 

[Krodad]

You don't need to attach the spread footing to the old one at all. The stem wall can be attached with #5's at 16" O.C., epoxied at 12" depth per side. At least that's what they let me do here 10 years ago. Hilti HT150 I think was the epoxy I used.

 

[Krodad]

to be clearer...I know you won't be using epoxy on the wet set side. Sorry if that was confusing.

 

[mgsb38]

Hey Krodad, thanks for the posts. I understand what you mean about using epoxy on the existing foundation. That would be a neat trick to epoxy in wet concrete!

Just to clarify, you set 2' lengths of #5 rebar (1' into the old foundation with epoxy, 1' sticking out into the new concrete forms) every 16" OC vertically? How far below the top surface of the foundation did you start the first bar? My stem wall is 18" tall, so I don't have much room for 16" OC placement. I figured I should have at least 2 pieces of rebar connecting the stem walls.

 

[ConCretin]

If you've got a ftg under the stem wall, i'd run 3 #4 bars in the bottom that are drilled and epoxied into the existing ftg. I'd run two #4 bars in the top of the stem wall that are also drilled and epoxied into the existing walls. I'd skip the bonding agent.

 

[mgsb38]

The existing stem wall doesn't have much of a footing to speak of. Maybe 3" or 4" deep and 10" wide. My new footing will be to code at 6" deep and 12" wide.

LLWillysfan, are you talking about two bars in the stem wall parallel to each other? At what depth, and what spacing between? My stem wall is 6".

 

[Krodad]

Oh, I wrongly assumed this was a deeper frost footing and you had a lot more stem wall to attach. I'd go with Willy's reccomendation then...including the spread footing. If you were 4' below grade with the footer I would not worry about attaching, but in your cas I think I would go ahead and do that.

 

[mgsb38]

Yeah, I'm in Southern California, so footings have to only be a minimum of 6" deep.

The existing footing might even just be where the poured wall leaked out from the bottom of the form. It seems to have worked fine, though, since everything is level and the concrete still seems to be in good condition.

Assuming I don't have an existing footing to tie into, would two or three pieces of rebar connecting the old and new stem walls be enough? The new section of foundation is going to have horizontal rebar running in the footing and stem wall all the way around, with vertical rebar tying it all together every 2' OC. I was going to tie this to the rebar connecting the stem walls, too.

 

[Krodad]

Yes, tie the reinforcement together in the new pour...go ahead drill/epoxy the bar into the old stem, and have it extend into the new pour far enough to catch both a vertical and horizontal bar, and you'll have her locked in as good as can be done.

 

[mgsb38]

Got it. Thanks again for the advice!

 

[rancherbill]

The only thing I can add is that you have to do 100% compaction on the soil where you are pouring your new concrete.

You do not want any settling as it will pull apart with time.

 

[mgsb38]

Yeah, I was planning on compacting the bottom of the trench prior to pouring. The soil is pretty stable already, fortunately. Thanks for the tip.