# Concrete shop floor thickness



## Pat of TN

Hey guys. I know this is probably a recurring question around the web, but I actually can't find a lot on HM.

I plan on making an extension to my current work... shed. It currently only has a wooden floor, with 2x4 joists on 24" centers and two layers of OSB sheet. Fine for my little 618 lathe, but I don't have the room nor faith in the floor to have any more.

I am looking at purchasing a mill no bigger than a PM-45/932-sized mill-drill, which apparently has a weight of 1100 pounds from the PM site, more likely a PM25 which is only 350 pounds.

I would also like to eventually have a larger lathe alongside the Atlas, something like a PM 10x30 or 11x27, which slightly exceeds 400 pounds.

The extension I would like to build will be 12 foot square, 12 x 12.

Pretty sure I need a concrete floor for machinery of that size along with the Atlas and other storage furniture and such. The question is thickness and specifics of the concrete.

Would a 4" thick slab work for this? I will not purchase larger machinery than mentioned, as for anything larger, I would need to build a better, standalone shop anyway.

Thanks fellers.


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## Andre

4" might be enough. My barn is has 4500 PSI (which is highly recommended) 6" slab where my 1800-2k# BP sits on. No cracks or problems in the year it's been sitting there. Keep in mind though, even with my 1/2HP mill (M head BP) I can feel the floor vibrate when taking a cut. And my dad says he can feel it 60 feet back in the barn. 

My advise, get a 6" slab. You won't regret it, but this might need a building code, as it could be interpreted as a foundation. But then again, my tree fort is illegal because it's higher than 32" off the ground. Go figure.


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## ronboley

A minimal slab is 3-1/2" of concrete over compacted native soil...2000 psi concrete is just about minimum and don't expect the slab to last forever...but better than 2x4's...

A good (proper?) concrete slab on grade consists of 4" thick, 2500 to 3000 psi concrete (a 6-sack mix will usually do it) reinforced with #4 rebar at 18" each way centered in the slab thickness over 4" to 5" of compacted base rock (not uniform size drain rock or pea gravel but "road base" that contains varying size of aggregates that can be compacted).  The ground below the base rock should be native or compacted soil that does not contain organic materials...don't put you slab on peat moss...

A "proper" slab like this should support just about anything a home shop on steroids would contain.  For really heavy stuff...10,000 lbs or so just go to 5" or 6" of concrete...or if you are in earthquake country and need to anchor heavy or tall equipment down to the slab go thicker....

The main concern is the soil the slab sits on...bad soil, bad slab...

Or so being a structural engineer for the last 30 years has taught me...

Good luck!


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## Grumpy Gator

Pat,                                                                                                                                If you know the footprint of your machines you can always dig a deeper spot in your 12' X 12' slab and put in more steel rebar  under where they will go.As long as you have a footer with rebar on all four sides and steel wire in between . You should be fine.                               Just go for higher mix like 3500 or 4000 . 2500 is standard for slabs.
*******************Just saying*********************Gator********************


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## Pat of TN

Hey guys, thanks for the help.

Andre, I see. Although the machines I'm looking at are a lot lighter than your B-Port, that's still surprising to me. I was hoping to get away with a 4" slab, but just off that, I'm leaning much more towards 6". Thanks buddy.

Ronboley, thanks as well. Structural engineer huh, well if that ain't perfect! Hah. A lot of good info in your post. Like I said, I'm looking at maybe 1500-2000 pounds total for all the major machinery, maximum.

The earth beneath where I plan on building this structure is solid. We have owned the land for 14 years, and the area has never been worked or dug. Before that and even up until about six years ago, that part of the land was basically woods, so it hasn't been worked or anything for quite some time.


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## JimDawson

My 3500 LB mill is sitting on a 3 1/2 inch floor.  I would like it 6 inch thick, but it is what it is.  I have not noticed any problems in that area, but running my 9500 LB, hard rubber tired, fork lift over the rest of the floor has taken it's toll.  The floor has a few cracks in it. When drilling in the floor, I have never run into any rebar, so I'm not sure that there is any in the floor.  The shop existed when I bought the place so I don't know how it was built.


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## 12bolts

grumpygator said:


> If you know the footprint of your machines you can always dig a deeper spot in your 12' X 12' slab and put in more steel rebar  under where they will go.As long as you have a footer with rebar on all four sides and steel wire in between.



I did exactly this for my slab. My shed has a dirt floor, and the couple of areas that I have already concreted for work areas and machinery I just dig a deeper foundation and put a bit more steel in for more substance. I also just leave some starter bars protruding from the edges of the slab to tie into the new slab as I pour it down the track.

Cheers Phil


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## Andre

Pat of TN said:


> Hey guys, thanks for the help.
> 
> Andre, I see. Although the machines I'm looking at are a lot lighter than your B-Port, that's still surprising to me. I was hoping to get away with a 4" slab, but just off that, I'm leaning much more towards 6". Thanks buddy.



NP, the barn is built to store a backhoe, a JD410. Weights around 8 tons, and it sits on a concrete section 12" thick. No cracks and it's been there 15 years. Then off the 12" section we branched out on either side and reduced the thickness to 6" to conserve concrete. And the milling machine sits on one of the wings. 

I've been told (I wasn't born when the barn was built) that we threw in bed frames and all kinds of scrap metal we could find to help strengthen the rebar mesh. It's working good so far.


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## dave2176

My pickup weighs 4,400 pounds.  I bet the front wheels take about 70% of that so about 1,500 pounds per tire.  Not seeing any issues on the 4" slab I poured 20+ years ago. I wouldn't think twice about loading it with several more 1,000+ pound machines or even a full size Bridgeport thrown in.

Dave


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## ody

I spent an obscene amount of time figuring out the best thickness for the concrete of my slab.  In the end the most consistent bit of advice I was seeing was to make sure the ground you are pouring on is solid (all top soil removed) and has a god few inches of hardcore well packed down with a whacker plate.  As with most things in life, preparation is key.  I went overboard and poured 9inches with A142 reinforcing mesh on well packed (I spent hours with a whacker plate) 3inches of hardcore/MOT .. but I don't ever plan on moving so want it to last.


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## chuckorlando

When I was doing mud work some years back we parked many cars on 4in parking lots. Hell multi level parking garages were no more then 8in. Lotta steel in that though. At work the slabs 4in and in my shop it's 4in both hold full size machines


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## John Hasler

Whatever you do don't let the woodchucks get under your slab.  Just sayin'.


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## GA Gyro

Concrete is a wonderful floor product, especially for a shop!

When weight holding capacity of concrete is important, a few things should be taken into account:
*First, and this is important:  The soil under the concrete needs to be hard!  The concrete is a great surface, however if the dirt below it is soft, the concrete will crack and possible break.  Better if the dirt has not been disturbed, and even better if uneven sized rock (road base rock) are added and compacted.  Google road bed construction... 
*Concrete can be produced to different strengths, and the thickness determines weight capability.  Contact a ready-mix concrete company for information on various 'mixes' and strengths.
*Generally, a concrete slab is poured with a 'beam' around the edge, sometimes a beam through the middle for a load bearing wall or a thicker place under a specific load (machine tool) can be added.
* A layer of plastic sheeting is usually placed under the concrete on the soil, to keep moisture from seeping up through the slab. 
*The re-enforcing wire (steel) in the concrete serves to keep it from cracking... a google search will explain proper concrete slab design and steel placement.
*When the slab is actually 'poured' the water content of the concrete is very important!  Too dry, and it is difficult to work... too wet and the strength goes down quickly.  Something similar to waffle dough is about right... I would suggest getting a concrete crew to do the slab, it requires hard work and quickly... as the concrete must be placed and finished in a given time. 

The last time I built a garage shop, I designed the slab to be roughly the thickness of a coke can (about 5.5"), used 6x6x6 mesh, and did the beam with 1/2" rebar.  The slab was quite strong, it held the intended loads with no cracking issues. 

Hope this helps.

GA


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## coolidge

IMO its not the thickness of the concrete so much as the base the concrete sits on. Example my brother had three CNC machines weighing 8000-11000 pounds in a light industrial park, the concrete was only about 4 inches thick, the machines sat there for years and there was never an issue. A 1,000 pound lathe and mill is a nothing burger really. I have 20 feet of concrete in front of my 3 car garage, new construction house 2 years ago. The concrete is nothing special grade. I have 2 full size 5,000+ pound trucks parked on it and no cracks or issues, the concrete has a base of sand and rock under it. I have a 4,000 pound tractor parked inside the garage, again no issues and its base is also sand over native soil.

I'm going to have a shop built on my property in the next couple of years. I'm likely to have the base prepared like the prepared the base for my asphalt drive, 3/4" crushed gravel over 3" crushed rock rolled then maybe some sand. I have clay soil, its like concrete dry but wet its mushy mashed potatoes. Key is keeping water away from the structure so I'll put in French drains and gutters so the shop space stays dry as a bone.

If you live in the north where the soil freezes hard as a rock in winter and frost zones are deeper consult an expert, the ground for example in Upstate NY where I used to live can heave up and down several inches during freezes and thaws. 

Just my 2 cents worth.


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## ken4570

I have a 4"  pour, ground is stable and rebar in the pour.
Lathe is 13lbs, Mill 1100, no problems. in 8 years. Not even a hairline crack.
I think there is plastic underneath as temps. can go to zero.


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## tweinke

Use good common sense, get as much info as you can, remember spend the money once because cheap is not always best. Oh and remember there only seems to be two kinds of concrete, the kind that is cracked, and the kind that will crack


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## brav65

Pat,

A lot of great advice listed above, her are my 2 cents.  I am a 3rd generation home builder, and have built over a 1000 homes in my career, and had only one slab with problems due to incorrect slump on a pour.  The slab ultimately came up to design after 6 months.  Pad prep is the single most important item when it comes to the performance of a slab.  You can do an excellent job with rebar, pour the perfect mix at the perfect slump at the perfect temp, and have the slab fail if the substrate is not prepared correctly.    You could do  a 12" mat pour with rebar 12" on center over any substrate, but it is easier to prep the earth right.

These are the steps that I would follow:


Scrape off top soil
Place 4" to 6" of ABC (used in road beds, compacts to 95% compaction)
Water the area down and use a plate compactor to compact area
Place plastic down on area, lap seams by 24"
Place #4 rebar 24" on center tie off every other intersection and lift rebar off plastic  1 1/2"
Place 4" of a 5 or 6 bag mix (approx. 3,000#) make sure you do not add too much water as it weakens the concrete.
Do not allow concrete to set up while pouring, as a cold joint will form and be guaranteed to crack

Some other things to consider are a turndown (thickened area) around the perimeter of the slab.  You can also thicken the area where the machines will sit, but I do not think that is necessary.  You should consider ordering your concrete rather than mixing your own.  The time/labor savings and better quality product justify the cost increase.  Good luck!


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## silence dogood

One more thing that you need to take into account is curing.  This slab will not only be supporting some heavy machinery but also will be subject to vibration.  A driveway needs 5 to 7 days to cure before one can drive a vehicle on it.  Many sealers (which is a good idea, keep the dust down for one) call for the curing time of at least 28 days before application.  
Mark


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## brav65

Excellent addition, concrete will continue to harden it's entire life.  To achieve full cure means that the concrete has achieves it's designed strength.  In fact on higher psi concrete in very large pours it can be many months before concrete reaches designed strength.  

 I built a group of homes on expansive soils which required a 12" mat pour with #5 repar 18" on center, a 12" turn down at all penetrations, and around the perimeter of the slab.  The engineer required a hammer test to confirm design strength prior to loading the roof and crush tests on cylinders pulled during the pour at 6 and 12 months.  We also had to have pad certs for compaction on each lot prior to the pour.  A real PIA, but no cracked slabs!



silence dogood said:


> One more thing that you need to take into account is curing.  This slab will not only be supporting some heavy machinery but also will be subject to vibration.  A driveway needs 5 to 7 days to cure before one can drive a vehicle on it.  Many sealers (which is a good idea, keep the dust down for one) call for the curing time of at least 28 days before application.
> Mark


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## RWL

I have had a Bridgeport, Clausing 12x36" lathe, etc.  sitting on a standard thickness (probably 3.5-4" thick) cellar floor for 28 years and the floor hasn't cracked near the machines.  It was laid on native excavated ground and there was only a minimum of crushed stone beneath the concrete.  I don't believe they put reinforcing wire or rebar in the floor.  I couldn't argue with some of the suggestions above, particularly adding reinforcing wire, and putting a couple inches of stone beneath the concrete, but unless you have heavier machinery or your soil isn't very firm, I don't think you need to do much more than what I have.


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## Cami

If an installation is probably OK and a little downtime down the road won't kill you, why not gamble?  If things go bad you could yard it out of the hole  and put in a new thicker section.  We're not talking about hydro turbines or power hammers, right?

On wood floors:  I recall a coworker talking about a machine shop he worked in years ago (New Westminster/Coquitlam I think) that was on the second floor of a timber building.  The engineers signed off on it...


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## Falcon67

I live where we have some expansive type soil but no frost line.  I used 12" x 12" footer with depth 4" minimum.  3 beams across 12x12 and one longways down the middle. 2 ea 1\2 rebar in the footer, 3\8 bar 18" oc.   24x40, think we dropped 16 yards 4000 psi min.  Second truck was late.  The slab has the usual shrinkage cracks for this area.  No changes in any of the few visible cracks in 3 years.  Slab is good for a two post lift, 4 post lift or any machine I could possibly afford.  If I bought a 10 ton punch press then I'd just cut a hole and pour a separate pad for that lol.


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## 110octane

You have received some excellent advise in these posts.  I cannot find fault with any of the suggestions (I, like ronboley, am a structural engineer with a little over 40 years experience) but I can comment on some of the details that may help with economy.  Your soil sounds good and firm and I would not anticipate that you would have a bearing problems with it.  If you need to level the base of the slab and want a little "bridging" for slab, you can use a little stone under the slab (No. 57 is what most contractors prefer because it is easy to rake level, I prefer a more dense graded stone, but it costs more and is more difficult to place level).  You can use "Visqueen" or construction fabric over the stone if you feel you want to keep moisture down, sealing the surface is arguably just as good.  (The construction fabric can add strength to the slab by spreading the load over the stone, it's not especially costly.)  A four inch thick slab should prove adequate.  I have used No. 3 rebar on 16" centers with No. 4 bar on 8" centers for the last foot around the perimeter.  One time we did this for heavy tracked machines with no cracking.  In that case with did use 4 X 3 A36 steel angle with studs at the edges where the machines entered/exited.  These same angles served as the top of the forms which saved some time.  You should no need anything like this, but if you ever encounter that situation.  The slab was abused beyond imagination but it never cracked.

I personally do not prefer the wire "mesh".  In a 4" slab this stuff is hard to keep level and will float up when you try to finish the surface.  The rebar is comparable in price (might be less!) and will stay put during pouring and finishing.  Just lap any splices about 20 bar diameters.  That's not but about 8" for No. 3 and 10" for No. 4.  If you want to assure that you don't get any little surface "crazing" cracks you can have the concrete supplier add fiberglass shards to the mix.  This costs ('round here) about $4 per cubic yard. The "glass" makes the slab more difficult to finish and there will be little "fuzzies" on the finished surface.  This does not last long in travel areas and the slab is as tight as a drum.  You can treat the cured slab with everything from acrylic form treatment to fiberglass and epoxy coatings.  That's the Cadillac end of things.  (Note:  Thompson's Water Seal is an acrylic material that can be duplicated and exceeded in quality for a lot less cost by purchasing the heavy form sealer from any Construction Supply Company-they know this and will sell you what you need.)

Where frost heaving and highly plastic soils are present (or slimy alkaline soils that won't drain), or earthquakes, there are big problems to overcome; but they can be overcome, it just costs more.  If anyone has a particular problem with these conditions, just send me a PM and will send you back my highly biased opinion.  There is no need to go into that amount of detail here, unless someone wants to.


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## Pat of TN

My goodness, fellas. Thanks for all the help.

I think I'm going to go with a four or six inch slab, with some crushed aggregate and such beneath and rebar reinforcement. A slightly larger addition may be in the cards, thus opening up space for slightly larger machinery, though not by much. Not talking a one-ton Bligeport or some such.

Fortunately I have got my father on board with this as well. Despite his old age, he's got the know-how when it comes to structural integrity. So between him and this board, I believe I'm pretty well set.

HM rocks!


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## Leggman

I have poured two slabs, one for my shop (24' x 36') and a 24' x 24' for my garage. I made them both 3-1/2 inches thick on compacted sand/gravel/soil which is what I have here in central Kentucky. In both cases I used 4000 pound concrete with fiber and NO filler ash.
All concrete will crack so I put saw cuts every eight feet about 1 inch deep and that way the concrete will crack in the saw cuts so no unsightly crack anywhere.
Its been ten years since I poured the shop slab and it has held up just fine.
I did make the perimieter of the slab a little deeper to lock it in.
I think a four inch slab will be fine for you if the base under the concrete is good and you don't go cheap on the mix. 
I did put two rows of 1/2" rebar around the perimeter in the deepened part but no mesh. You really dont need mesh if you use the fiber in the mix


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## oldhank60

I'd go no less than 6' thick and add rebar or find some heavy wire fence to put in the mix while pouring .  just make sure its not rusty, most of my garage / shop floor is 8" but there is one area than is only 6" thick.  I added rebar panels and wire fencing , after more than 20 years I have only 1 crack, that happened about 4 months after the pour .  I did cut relief joints every 12 foot  (2)  the worst part is troweling a smooth finish with out power paddle.   better to have it to thick than to thin.    I knew I would have lathe and mill before I built building. also install what looked like a pi symbol  in 1 corner to chain items down that could be wheeled off.   but I had stuff in shop before I had doors. recommend steel frame and steal doors with heavy locks.   Way back then we didn't even lock house when we went to town. now we lock doors even when we are home.  times are changing every where.


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## Scruffy

I know this is a old post. My shop floor is 4 in with fiber on a well compacted base. I did this 10 yrs ago and holding up well. 16 by 53 American pacemaker  6600 lbs, 9 j gorton mill. 4200 lb and 16 in cinn shaper haven't had any problems.
  If I was doing again ,I would put chain pots or eye bolts in the floor priemter every 8 ft or so to help in moving machines.
Thanks ron


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