Concrete shop floor thickness

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

  1. Scrape off top soil
  2. Place 4" to 6" of ABC (used in road beds, compacts to 95% compaction)
  3. Water the area down and use a plate compactor to compact area
  4. Place plastic down on area, lap seams by 24"
  5. Place #4 rebar 24" on center tie off every other intersection and lift rebar off plastic 1 1/2"
  6. 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.
  7. 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!
 
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
 
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
Click to expand...
 
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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