I like the key-in because I think the dowels could make the 3" concrete crack if there was any movement.
My concern would be that the ground under the new slab is compacted. Which it should be unless you disturbed it.

 

if it were me, i would bore "at least" 4 (2 per side) holes as deep as i reasonable could go. and rebar the whole thing. this way, the slab would be irrelevant .

 

I would dowel the two together without a doubt, and actually do in similar situations on a regular basis. There's no reason to believe the 12" thick slab is going to move if it's on hard gravel, so it should have no ill effect being connected to the thinner slab. The thinner slab AND the 12" thick pad will actually benefit from being tied together......

 

I wouldn't dowel in, there's too much risk of cracking the existing slab and it really won't help stabilize the new slab. Depending on what type of soul you have underneath the slab the idea that Fix had about adding to deep piers on each side wouldn't be a bad idea. Also as far as the rebar goes, the spacing looks good but I would probably put two layers instead of one or if I used one I would bring it up more instead of leaving it so near the bottom.

 

I think the concerns of pinning are over blown....though, 3" is kinda small.

If it was me....I'd drill 1/2 holes and use #3 rebar.

To be on the safe side I'd drill closer to the bottom....and then dig out the dirt under the edges and do what is called 'under pinning'. Basically, your putting concrete under the edge of your existing. This 'should' do two things. First, it supports the edge of your existing slab. Second, it gives your existing slab more meat below it to help prevent any blow out of the dowel.

But that is me.....What do I know.

 

we've done 2 retro but no doweling, no under-exist-floor digging , or full-size excavation, either,,, did you get this plan from the lift mfg'er ? HOWEVER, we did excavate 3'deep - all per lift mfg'er,,, both were installed in an existing commercial bldg w/5" exist conc floor

 

Thanks for the input guys. The slab info came from Mohawk Lifts, A7 model which is a two post, 7,000 lb max asymmetric lift, while the one I'm installing is a MaxJax 6,000 lb max symmetric lift.
Fix'n It, when you say bore 2 holes per side, do you mean down into the ground (like deck piers)?

 

Fix'n It, when you say bore 2 holes per side, do you mean down into the ground (like deck piers)?

yep . i think i would do 12"ers. i might even go to the point of having the anchor bolts welded to a rebar cage that goes down into these holes.

 

Regarding rebar placement, the cross section I posted previously is vague on how close the ends of rebar should get to the end of slab. Are there any general rules for this?

 

idk. but i would say 6" would be good.

 

Thanks!

 

2" clearance is the general rule, in all directions. Notice the note in the drawing calling for 2" minimum clearance at the bottom.......

 

Thanks guys. Pour on Monday!

 

All went well..

 

Nice work........... :thumbsup: