DIY Home Improvement Forum banner

Question on pinning old & new slab

1 reading
24K views 16 replies 7 participants last post by  jomama45  
#1 ·
Newbie here from south eastern CT. My house was built in 1975 and building dept has nothing regarding the original permit, etc. on something that old. My question concerns the modification I'm doing to part of my garage (slab) floor to properly mount an auto lift I recently purchased. After drilling two test holes I determined that my slab was under min recommended thickness at only 3".

So I did some digging (figurative and literal) and am going with a floor mod recommended for a similar lift. 12" thick with rebar may sound like overkill but nobody wants to cheap out and have a car fall on them - right? The plan is posted below along with a photo of the excavation awaiting rebar.

So the thing I would like to get some 2nd opinions on is the pinning. It just doesn't seem like a very good idea to pin into a slab that is only 3" thick. Keying the thicker slab under the thinner one seems like the better way to go. Anyone have experience doing one or the other?
 

Attachments

#4 ·
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......
 
#5 ·
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.
 
#6 ·
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.
 
#8 ·
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)?