Question for the concrete guys

[ptgb]

I want to inset a motorcycle table lift into my garage floor. Will end up being a 9'x4' hole cut into the 4" floor, then dug down to about 15". Heavy clay soil.

What I was planning on doing is using 2x12 treated lumber around the perimeter of the hole, as forms. Pour a 4" slab on top of about 3" of gravel. So basically have a completely separate slab for the lift to rest on. I will then leave the boards in the hole and paint them. There will be no attachment between the floor and the new poured slab.

Having no concrete experience whatsoever, is this a viable way to do this? Anything I should keep in mind if it's ok to do it like this?

Thanks

 

[flat tire]

if it were me I would use a hanging form
make the side of you hole 2" bigger than needed. attach 2x4s to the existing floor extending to the original opening minus 1/2" plywood that will be fastened to the ends of the 2x4s. extend the plywood down in the hole 4" from the dirt. do this all the way around the hole
use some 2x4s for bracing inside the box at the bottom of your plywood.
hope this makes sence
you will then have no wood and all concrete

 

[matt_i]

I don't like the idea of leaving wood "in the ground" even if its under a roof, etc. Moisture will attack it eventually.

I would cut the slab about 4" oversized from each wall (8" total across), drill the edges for #4 rebar. Excavate, bend and tie a "cage" of rebar, ending with the tails going into the edges of the cut slab. Then pour the bottom of the lower slab. Then form the plywood box suggested above, and fill the rest in. around the center. Use Mazola to coat the form wood. Strip the forms at the end, and enjoy.

I don't think the gravel is necessary, just don't over-dig and scrape the clay with something like a hoe, down to the required dimension. So you are pouring over more or less undisturbed soil. A vapor barrier is a good idea as it will also help keep the concrete hydrated on the back surface as it cures....you can wet down the top/working surfaces as it cures.

 

[homebuilt burner]

These guys are right ^^^ Also, make sure whatever you build you can disassembled from the top only. To be able to remove after the concrete is poured.

 

[nfk]

I also would agree with making it all concrete and not leaving any lumber in the hole. You will be much happier that way.

 

[Dan in Pasadena]

I agree with everyone about not leaving wood in the hole. Especially when making a hanging form as described above is so easy. Also, do NOT forget to spray the for work with a good release agent!