Size of concrete pad for G4003g lathe?

[dsymes]

I took delivery of a G4003g last week and have realized that my existing concrete floor is not stable enough at the place I want to put the lathe. It was poured about 20 years ago as a series of slabs separated by 2x4 pressure treated lumber. Also, the floor slopes about 1" in 6 ft. Unfortunately, the place I want to put the lathe is at the joint between two floor sections, and when I step on floor on either side of where the stands will go, the floor moves about .004” or so (see photos). Since this is the best place to put the lathe in a crowded home shop, the obvious solution is to cut out a section of the existing slabs and to pour a 24sq ft x 4” or 5” pad, reinforced with rebar.



My question is how large a pad will be adequate. My thought is to make the pad about 3.5 ft x 7.5 ft, or a little more than a foot larger than the footprint of the lathe in all directions. It would be nice to do the whole floor, but at this point, that isn’t practical.

The soil around here is clay, but I would not be surprised if I find the builder used sand to level this ground under the slab – I just don’t know what I’ll find. I plan to dig down a bit, add a layer of crushed rock, and tamp it down before pouring.

So, my question at this point is whether or not this slab is big enough not to move when I step on it while operating the lathe? Or should I pour a smaller pad, say 4” larger than the footprint?

Thanks in advance for all suggestions.
Douglas

 

[Al 1]

Douglas, I would build my pad so it extends at least 3' in front of your machine area. That will give you room to walk on a level surface.
cut out old concrete and Dig down about 8 to 10", Fill with stable fill 4 to 5" and tamp well ( Clay is not stable. ). Try to have your finish concrete even as possible with your walking area. Install your re-bar and wire. make sure your wire and rebar are somewhat centered in depth of concrete.
If this concrete does move it will not flex so your machine will not flex. So you should be OK. Al.

 

[Trip]

Dig a post hole at each end if the pad a couple of feet deep 8" to 12" diameter . The post holes will act like pilings it wont move

 

[westerner]

I think I would remove sections, rather than cut them. Very, Very messy either way. Looks like 4 sections would give you a plenty big enough pad to hold the lathe.

 

[dsymes]

Al 1 - I don't plan to make the pad level - it will have the same slope as the rest of the floor, so I don't have to consider having a level area to stand on, just whether or not stepping on the edge of the pad will tend to make it move.

Trip - I like it. I think it would work with a slab that was 2.5 ft x 6.5 ft (6" larger than footprint in all directions).

Westerner - your suggestion would make a better floor, but would involve way more concrete than I'm considering.

 

[Al 1]

Douglas, Yes Trips idea is good. Al.

 

[Radials]

Not exactly along the same lines as your question but if I was in the same position I might consider making a leveling platform out of steel like the one shown with more points of contact than just the corners. This would allow you to straddle that crack and compensate for the slope in the floor as well.

Nick

 

[dsymes]

Not exactly along the same lines as your question but if I was in the same position I might consider making a leveling platform out of steel like the one shown with more points of contact than just the corners. This would allow you to straddle that crack and compensate for the slope in the floor as well.

Nick

The problem isn't the crack, it is that the edges of each slab flex up and down when I put my weight on them. I'm resigned to going through the mess of cutting an inch or two down into the existing concrete with a skill saw at the edges of the new pad and then jack-hammering it out. Then pouring a new slab with rebar and probably some "pilings" as Trip suggested.

Thanks for the photo - I'm planning on welding some 1/2" x 2" steel onto the bottom of the stands and having it stick out 2" on each side so my leveling bolts will be outside the footprint of the stands. Haven't decided whether to have them run front to back like you have or side to side (to have less to trip on). In any case, I will anchor the threaded rod into the concrete with epoxy.

 

[Eddyde]

I'm resigned to going through the mess of cutting an inch or two down into the existing concrete with a skill saw at the edges of the new pad and then jack-hammering it out. Then pouring a new slab with rebar and probably some "pilings" as Trip suggested.

Hi, Douglas,

I am a contractor and have a lot of experience with concrete remediation.

If your are pouring a new slab, I would skip the pilings. Unless you put them down 4' or so they probably won't add much support and are unnecessary. As suggested above, dig the soil out about 6" below the bottom of the slab, tamp it well, place a bed of gravel, tamp well and pour 4" of 5000# concrete mix. 4" or 6" wire mesh embedded about an inch from the bottom is all you really need but a couple of pieces of rebar wouldn't hurt.

Or, Instead of cutting out the old slab you could pour a new slab on top. Either just doing the lathe footprint, raising it up 3.5" (assuming a 2x4 form) or doing a larger area, to include adequate space to stand on. I personally find most lathes kind of low (and I'm only average height) so I would probably want to raise it a bit anyway. That avoids the hellacious job of cutting and jackhammering concrete indoors.

Eddy

 

[Logan 400]

All good advice above. If it is movement between slabs that is the main concern use rebar or at least 1/2 slick rod and horizontally pin the new slab to the existing slabs about every 16" or so.