concrete question for a lift

[rfuerst911sc]

I have a scissor lift that I want to recess into the concrete floor of my garage. I have the basic outline on the floor where the lift will go. I will rent a concrete saw from one of the local box stores and cut it out. Once I get rid of the cut out concrete I'll dig down aprox. a foot into that red Georgia clay. Now the questions.

1.I assume I should put down plastic in the freshly dug soil for a vapor barrier ?
2.Should I put any gravel on/under the plastic?
3.How much rebar should I use for an area aprox. 4 foot by 7 foot ?
4.Should high strength concrete be used or regular ?

I will be putting a perimeter " rim " so I can cover the lift when not in use with 3/4 plywood. Did I forget anything ?

[URY914]

1. yes
2. not needed
3. I would make a mat of #3 or #4 bars 8" a part but you need to drill holes into the edge of the existing slab to tie the new walls to the slab
4. Sure you might as well, but not really needed.


Some will now reply different from me.....

[VINMAN]

I'm planning on doing the same thing, but im also going to put a 3/8 or 1/2 inch steel plate on the bottom of the pit to sit the lift on. and use a 1/4 inch plate to cover the hole when its not being used.

[TimT]

Are doing the job yourself?

You could go for high early strength concrete, or high strength but its not needed a regular #3000 psi mix is fine..

You could place rebars as mentioned, or you could just use some wire mesh..

Vapor barrier=Yes

Oops on edits I see you are DIY'ing this..

[TimT]

3. I would make a mat of #3 or #4 bars 8" a part but you need to drill holes into the edge of the existing slab to tie the new walls to the slab

Thing is most residential garage slabs I have seen are 4" ... so nothing for him to drill into.

When we did Bills new shop the slab was 6" so I recommended that the contractor undermine the slab a bit as a "key"

[javadog]

Originally Posted by rfuerst911sc

1.I assume I should put down plastic in the freshly dug soil for a vapor barrier ?
2.Should I put any gravel on/under the plastic?
3.How much rebar should I use for an area aprox. 4 foot by 7 foot ?
4.Should high strength concrete be used or regular ?

1. Wouldn't hurt. Make sure the soil is well compacted and level.
2. Wouldn't hurt, if you put it underneath the plastic. Of course, you'll want to go deeper then, which means you'll have to deal with some other issues, so maybe it's not worth it.
3. I'd use #4, doweled into the adjoining slab, on 12" centers each way. You'll likely have two layers, the bottom one needs to be 3'' off of the bottom of the excavation. The dowels will be above this, as your existing slab is not going to be 12'' thick. The dowels don't have to extend all the way across but I would so so, as it takes very little time and costs almost nothing. Rebar is so cheap, it's nearly free. Of course, I buy mine from a steel supplier, not Home Depot.
4. I'd use at least 4000psi. Around here, I can get that for $10/yard more than 3,000psi. Given your small quantity, that's almost free. 4000 high/early is even better and about as cheap. Use that and you get to play with your lift sooner. Most concrete suppliers have a mix they use for highway road patches that need to cure quickly. It's usually pretty good. Feel free to put a curing agent on top of the finished slab.

Make sure the bottom of the hole is well compacted and the moisture content of the soil is not too high or low. Concrete likes to sit on something stable.

JR

[URY914]

See, I told you....

[vash]

what about drainage? that hole will fill with water..or trash.

[TimT]

See, I told you....

And who replied with anything but agreement with your post?

More accurate advice could be given when he has the hole cut in the slab.. and sees what he has to work with

"field measurements"

If you excavate 1 foot from top of existing slab, to bottom of excavation, allowing for the lift, for a proposed 4 in slab... you get to dowel your rebar into dirt...

Unless the OP has a to code industrial garage I dont think he will have the option to drill and grout,

what about drainage? that hole will fill with water..or trash.

Empty joint compound bucket in the center

[URY914]

Drainage? It's inside his garage.

[TimT]

Drainage?

I know counter-intuitive... but these are Porsches, they like to mark territory by leaking fluids

anyway...

I found a pic of our alignment system.. it sits flush with the floor...drive the car on, perform alignment back the car off.

When we had this equipment installed, I watched the contractor saw cut, remove concrete slab, remove fill yada , prepare the sub-grade yada yada... place the concrete.. yada yda yada...

The center line elevations of each slab prohibited doweling into the existing slab, which is the situation I think the OP will find himself in




I have some work in progress photos of the pit but cant remember where they are

[widebody911]

I can't really help you, since I did mine as part of the foundation, but here's my pix anyway. The ABS pipe is for the lift controls.

[vash]

every hole seems to get filled with some liquid sooner or later. coolant, water..whatever. being indoors, they really seem to evaporate slowly.

[vash]

thinking about this..the idea is really cool. sawcut oversized, so you can form up the sides. i would pour a 4 inch slab with standard, home depot rebar. drill and dowel into the sides of the existing slab. bend it down to match the new slab. i like gravel beds below concrete.

then have a box cover fabricated so you can keep it covered when not in use. it would be cool.

[rfuerst911sc]

Thanks for all the replies. Now I just need some time off from work to get this done.

[azasadny]

Great job, Thom!

[rfuerst911sc]

I thought I would give an update. I marked the outline on the floor for the cutout and the first two pics are the cuts. I decided to do some crosscuts to make it easier to bust up. Then rented an electric jackhammer and busted it up. That was a real pain in the butt as the existing floor is 6+ inches thick and may be high strength concrete. The third pic is some of the busted up concrete. Finally this morning I dug out the dirt. The new pad will be 8 inches thick minimum and will be 5 1/2 inches below the existing surface. I will also pour a ledge around the perimeter, this ledge will protrude about 2 inches and will be 1 1/2 inches below the existing floor. I plan on taking 3/4 plywood and doubling it up for a cover when the lift is not in use. The plywood will sit on the ledge. The last pic is todays dig. Next will be the rebar.

[A930Rocket]

WOW! Somebody went overboard on the concrete slab at 6", although it's nice to have I guess.

Keep the updates coming.

[Racerbvd]

Originally Posted by vash

every hole seems to get filled with some liquid sooner or later. coolant, water..whatever. being indoors, they really seem to evaporate slowly.

He is in Central FL, it won't take long to evaporate, and if there is a big puddle, a big Wet/Dry vac will suck it up....

[Paul K]

Basically, you'll need one of these;

Built an inspection pit...

Except shallower!