New Shop Concrete Thickness

Wenglish120 · Saturday at 7:19 PM

Just wondering if anyone has some recommendations on concrete thickness and PSI for a new shop. I'm looking to have a 30,000 pound horizontal mill brought in and my biggest nightmare is going too thin and the forklift popping the new slab. I'm leaning towards 6" 4000 PSI, and I wouldn't even question that with a normal 7,500-15,000 machine weight. Trying to avoid going to 8" if I can help it with the current price of concrete (almost $250 a yard where im at) Any help is appreciated. (I apologize if this was discussed in a previous thread, I couldn't find anything by doing a quick search)

Garwood · Saturday at 7:28 PM

What kind of mill? Horizontal is pretty vague. Have you had your soil tested? Why would you drive a versalift inside to set the mill instead of setting it on skates on an apron outside, or inside the doorway like is usually done when there are nice concrete floors?

Old broken factory floors- Who cares. Nice, shiny concrete and you normally use skates to set or remove a machine over 5ish tons.

Milland · Saturday at 7:29 PM

What's your budget? How much rebar?

My quick take is at least 8" for safety (it's rare that you don't get thin spots during soil prep), and two levels of rebar. If I could afford it, I'd go thicker still, at least where I plan on having heavy machines.

Wenglish120 · Saturday at 7:41 PM

Garwood said:
What kind of mill? Horizontal is pretty vague. Have you had your soil tested? Why would you drive a versalift inside to set the mill instead of setting it on skates on an apron outside, or inside the doorway like is usually done when there are nice concrete floors?

Old broken factory floors- Who cares. Nice, shiny concrete and you normally use skates to set or remove a machine over 5ish tons.

Looking at a used Mazak FH480. I know the standard approach is skating it in and jacking it up once in place to remove the skates. I'm not sure what approach the rigger would want to do, I'm just trying to protect myself from a worst case scenario. Mainly from needing to pick up the machine with the forklift at the final spot to remove the skates. I've seen that approach a bunch even in a nicer shop. I'm kinda limited on options as far as riggers as I'm in an area not really known for machining.

Milland-
As far as budget, I was quoted $32k for 8", I'm not opposed to going thicker, but I don't want to overbuild just for the sake of overbbuilding because I'm a small shop and every dollar counts.

Milland · Saturday at 8:45 PM

Wenglish120 said:
Milland-
As far as budget, I was quoted $32k for 8", I'm not opposed to going thicker, but I don't want to overbuild just for the sake of overbbuilding because I'm a small shop and every dollar counts.

Understood. What was the 6" quote? And how about a thicker pour where the machine will be, 6" elsewhere? I know it hurts up front, but chasing accuracy because of low foundation stiffness sucks too, and lasts a lot longer...

daringwilliams · Saturday at 10:06 PM

Grob just advised us to have 12" poured for a new machine install in April, horizontal 5 axis

Djstorm100 · Saturday at 10:38 PM

You are going about it the wrong way. The soil is the most important. Are you building a new slab in a existing building or is this a new building (slab and footing)?

How many feet does the machine have, it's a quick way how I go about designing pads for shops, then add a safety margin. Take the machine weight and divide it by how many feet it has. Then take that number and divide by the area of the feet to get your acting force on the concrete.
Thickness/ PSI only goes so far. How it reinfroces is the important way. I can build a 4" slab that is 3k psi that is wrong than 6" slab that's 3,500 psi.
Concrete is going to need reinforcement. At least 2 rebar mats (mat on the bottom and mat on the top. Rebar needs to be surrounded 3" all the way around. I would go up to #6 (3/4) 18" on center. On the corners of the mat's have L bars 3' x 3' long this reinforces the corners. Make *damn* sure you see teh guys out there with a viberator getting the air pocks out as they pour. Then cutting control joints at the right depth. Typically 1/4" thick and ever 12 to 15 feet.

Rebar is cheap. I do this for the living, machining is my secondary job.

I don't know where you live at but I would also put 6mil vapor barrier under the slab.

BOB-OO · Saturday at 10:44 PM

8" wont meet the certified foundation plans for that beast. I just ripped up 8" to replace it w 12" compacted base & 24" of 4000psi w #5 rebar on 12" centers for a machine only 12k# heavier. The pad is also isolated from the rest of the floor with a rubber type material. I hate vibrations and its way cheaper then what's sitting on it.

If possible layout the pad for the new machine and pour that deep 12"-18". Isolate it from the rest, pour your floor. Then Also make a friend at Mazak- they'll sell you the manuals, ask for the foundation plans for that model, they'll give($) them to you.

We use 8" min for anything we pour- forklifts, versalifts, pallets of steel, anchoring racks, edm machines, grinders -it all needs a solid foundation.

Enjoy your build!

sfriedberg · Saturday at 11:40 PM

Both of these points have already been made, but it's worth repeating. 1) Get the foundation specs from the machine tool maker. It may require a special foundation slab. 2) Once you get past a 6" good quality concrete slab, the subgrade condition makes a bigger difference in load bearing capacity than additional inches of concrete in the slab.
If memory serves, the WWII field construction manuals didn't call for much over 6" even for tank parks, with multiple 100T concentrated static and dynamic loads. But they were real clear on what had to go under the concrete. And when I was stationed in Sheridan Kaserne, Augsburg, in the 80's, they happily replaced the streets with cobblestone (real cobblestone laid by real German street masons) for the US 5th Armor tanks. All the load bearing capacity was provided by the subgrade; the hard stones (basically free to move up and down) just kept the tracks from tearing anything up.

Milland · Sunday at 12:58 AM

Djstorm100 said:
Rebar is cheap. I do this for the living, machining is my secondary job.

I don't know where you live at but I would also put 6mil vapor barrier under the slab.

DJ, I've heard of additives that feature a controlled expansion of the concrete which offsets drying shrinkage, the upshot being not needing to cut control joints. Do you have an opinion on those, is there a downside?

Djstorm100 · Sunday at 1:37 AM

I would stay away from it, it's something that has to be measured. Add to much, takes forever for curing to take place enough to walk on it or set machines. When a pad is pour keep it wet as possible for the first 2-3 days, this is a one thing I see not done often. When concrete cures it makes heat. The layer at the bottom cures before the top. By wetting you are getting the concrete to cure more evenly.

Concrete is going to crack and cracks because of many variables. Control joints midigate it. Our shop is 80 x 120. I've got 1 crack maybe 2 feet long.

DDoug · Sunday at 7:29 AM

Yup, I've talked with Mazak several times when I was to layout foundations for new machines my customer purchased.
Not a problem, and have them send you the CAD file of the machine as well, so you can locate all the anchors ahead of time, as well as the utility stub outs.
Crane coverage in your building, I show the actual hook travels
as a dotted grey line, and place the machine so it can service both the table, and anything else maint, deems needing crane service.

gbent · Sunday at 8:24 AM

Concrete is a rug. A hard surfaced rug, but a rug never the less. It will telegraph the subgrade conditions to the surface. The thickness is to give the concrete the strength to spread out the surface loads to an sufficiently large subgrade area, and to bridge small discontinuities in the subgrade. The strength of the concrete will directly affect the longevity of the surface. I would want 4500 psi minimum, 5000 even better. That way when you drop a heavy part it doesn't spall out a large area.

If you look at the component costs of your building and then factor in the cost of upgrading, you will see the floor is one area you can't afford to skimp. If the building is to small, its easy to add on. If you need more insulation, its easy to add, same with electrical. Skimp on the floor, you just as well move to a new building.

But concrete is a local thing, the same with subgrade. Talk to experienced contractors who have a history in the area. Aggregate makes the concrete. Some places have good aggregate close by, others have lousy natural aggregate and have to truck in good aggregate to make concrete that they know has to pass sampling inspection. Depending on the size of your shop, hiring an experienced local civil engineering firm to check the soils at your location and design an appropriate pad could be money well spent.

Wenglish120 · Sunday at 5:19 PM

My bad guys, I left out the detail on the machine pad. I do plan to have a 18" 4500 psi isolated pad with 3 layers of rebar. That part I'm sure of. It's the rest of the shop that has me concerned as to just getting the machine across the floor. I obviously want to do this right and not have any regrets. My knowledge of structural loads of concrete slabs is limited. I know the isolated pad specs because I was at a shop that had a similar machine of similar weight installed and I'm basically copying their formula for that.
My concrete guy suggested 3" of gravel under the the slab with a layer of plastic (don't know the thickness, will have to ask in the morning). Is this substantial or should ask for thicker gravel layers or different substrate material?
This is a personal home shop that will have my business ran out of it until its too small and I need to expand (hopefully) at which point it will be turned into a car garage/general shop/ storage building with a 4 post lift. This is why im trying to avoid over building and trying to go with 6"because it will most likely be unnecessary for the long term life of the building.

Rob F. · Sunday at 5:40 PM

As others have said before it is more important to have good solid subgrade than this or that concrete. If you put that 3" of gravel on top of soft dirt nothing will work. It is critical to have a solid subgrade before doing anything. You may be removing 10 feet or more of poor soil, or you may scrape off less than a foot of topsoil and hit solid ground. It ALL depends on what is in (or under) your yard.
Imagine that you put 3" of gravel on top of a farmers plowed field, would that be any good? Even 12" gravel would not help...

Wenglish120 · Sunday at 5:49 PM

Rob F. said:
As others have said before it is more important to have good solid subgrade than this or that concrete. If you put that 3" of gravel on top of soft dirt nothing will work. It is critical to have a solid subgrade before doing anything. You may be removing 10 feet or more of poor soil, or you may scrape off less than a foot of topsoil and hit solid ground. It ALL depends on what is in (or under) your yard.
Imagine that you put 3" of gravel on top of a farmers plowed field, would that be any good? Even 12" gravel would not help...

We are cutting into a hillside(6-9 feet of height) on some family land thats known to have random patches of sandstone. Hoping this will help support the slab. I'm gonna do my best to find a local civil engineer. That's easier said than done for the remote area I live in lol

Djstorm100 · Sunday at 6:02 PM

Wenglish120 said:
My bad guys, I left out the detail on the machine pad. I do plan to have a 18" 4500 psi isolated pad with 3 layers of rebar. That part I'm sure of. It's the rest of the shop that has me concerned as to just getting the machine across the floor. I obviously want to do this right and not have any regrets. My knowledge of structural loads of concrete slabs is limited. I know the isolated pad specs because I was at a shop that had a similar machine of similar weight installed and I'm basically copying their formula for that.
My concrete guy suggested 3" of gravel under the the slab with a layer of plastic (don't know the thickness, will have to ask in the morning). Is this substantial or should ask for thicker gravel layers or different substrate material?
This is a personal home shop that will have my business ran out of it until its too small and I need to expand (hopefully) at which point it will be turned into a car garage/general shop/ storage building with a 4 post lift. This is why im trying to avoid over building and trying to go with 6"because it will most likely be unnecessary for the long term life of the building.

Get soil test
Compact the absolute hell out of it
Put 3" gravel
6mill plastic at least, the plastic is a vapor barrier, to keep moisture in the soil from going up in the the concrete (concrete is porous)
The machine will be on stakes. The load will be spread out, Have the riggers put metal plates down, the load will be even more spread out then.
Your pad configuration for the machine is over kill but will work well.

gbent · Sunday at 6:35 PM

The center mat of rebar in your pad does nothing for you but waste your money. If you want more steel, put bigger bar or more bars in the top and bottom.

Djstorm100 · Sunday at 6:44 PM

To big of bar, space to close together can be a bad thing too

john.k · Sunday at 7:03 PM

You can slide ,machines on two layers of greased plates ,this works on a asphalt floor ,and wont overload your concrete.....Its easier than using skates ,provided you have the space to put down the plates.

New Shop Concrete Thickness

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