Cement Slab Thinckness and Rebar

[VocaTexas]

I'm finally getting around to finishing my shop. It started out as a 24x34 carport slab. I added 6 feet to the 24 foot side to have room for toolboxes and work tables. Since then I've bought a fairly extensive set of machine tools, so my original slab was too small. Due to space limitations, I decided the best route would be to mirror this slab with another of the same size and have one side set up for my machine tools and the other for welding/mechanical jobs with a dividing wall down the middle.

My question is this: I have a lathe and a milling machine that both weigh around 14,000 lbs. apiece. Would a 4 1/2 inch slab with 3/8 rebar on one foot centers be enough to support these machines, or should I go with a 5 1/2 inch slab with 1/2 inch rebar on one foot centers?

 

[ItsNemo]

A 2 inch slab without rebar could hold it up probably without problem, concrete has huge compressive strength (think, 3000psi concrete means 3000 pounds in one square inch). Given it's a static compressive load, the 4.5" slab with 3/8" rebar is plenty.

 

[ConCretin]

Without doing the engineering, I'd probably go with something closer to a 6" slab. I'd also make sure the base under the slab is up to the task. ItsNemo is correct about the compressive strength of concrete being adequate, two 7 ton pieces of equipment will introduce additional loads. The cost of a couple inches of concrete will be minimal.

Rebar won't make any difference to the load bearing i.e. crack resistance of the slab but it will help hold everything together of you happen to get a crack. I'd probably do #4's at 18" centers because it's easier to support than #3's

 

[ItsNemo]

Without doing the engineering, I'd probably go with something closer to a 6" slab. I'd also make sure the base under the slab is up to the task. ItsNemo is correct about the compressive strength of concrete being adequate, two 7 ton pieces of equipment will introduce additional loads. The cost of a couple inches of concrete will be minimal.

Rebar won't make any difference to the load bearing i.e. crack resistance of the slab but it will help hold everything together of you happen to get a crack. I'd probably do #4's at 18" centers because it's easier to support than #3's

Yeah I should have probably clarified, but it's not the concrete thickness that's really going to make the big difference, but the sub base below it...a well compacted solid base is going to keep things more true than some thicker concrete will.

 

[NUTTSGT]

I'd go for the 5 1/2" thick slab over the 4 1/2" thick. Looking at a 6'x34' pour, it's only going to take like a half yard more. Probably money well spent if you have the truck coming.

 

[JoeMcGov]

Each 1" of slab thickness appears to be worth about $1,000.

So.....if it was my slab I would do 5" thick. With 1/2" (aka: #4 bars) rebar at 18" centers, each way.

But by far the most paramount aspect, as mentioned above, is the subgrade. And having a properly compacted & prepared subgrade. Everything below must hold up everything above.

 

[PNWguy]

I'm curious about a 14,000# mill and lathe.

What did you get?

 

[strutaeng]

I would advice to select a spot and do a "pad," say 8" on top of your standard slab. You can also pour your pad first. Basically, a thickend slab.

By the way, it is "concrete," not "cement."

If you anticipate having heavy loading elsewhere, then just make the entire floor thicker. You really need a geotech. report to size slab...

 

[GMCGarage]

I'm finally getting around to finishing my shop. It started out as a 24x34 carport slab. I added 6 feet to the 24 foot side to have room for toolboxes and work tables. Since then I've bought a fairly extensive set of machine tools, so my original slab was too small. Due to space limitations, I decided the best route would be to mirror this slab with another of the same size and have one side set up for my machine tools and the other for welding/mechanical jobs with a dividing wall down the middle.

My question is this: I have a lathe and a milling machine that both weigh around 14,000 lbs. apiece. Would a 4 1/2 inch slab with 3/8 rebar on one foot centers be enough to support these machines, or should I go with a 5 1/2 inch slab with 1/2 inch rebar on one foot centers?

I would say the biggest stress the floor will see is when you move those items into the shop.

Proper sub base prep will ensure the slab it up to the task. Closer spacing with smaller bars is better than largers spacing and bigger bars. If you truly want crack control, use a wire mesh up to the proper size, and at the top face.

#4@18 will just keep the cracked concrete together, and hopefully not create trip hazzards.

 

[skulldrinker]

All you need to remember is 4" is for backyard patios. 6" is for garages. The more rebar the better. 5 bag/3500# is fine but hey what the hell go with 6 bag/4000# for your shop.

I've seen it all drove mixer for over 20 years.

 

[like2wheel]

All you need to remember is 4" is for backyard patios. 6" is for garages

Is this true?
Seems like overkill for most garages, but I'm no expert.

 

[GMCGarage]

Is this true?
Seems like overkill for most garages, but I'm no expert.

Might be a good rule of thumb, but even a 6" slab is useless on crappy sub base.

I would rather have a properly prepared sub base, and 4" than the above.

 

[VocaTexas]

I'd go for the 5 1/2" thick slab over the 4 1/2" thick. Looking at a 6'x34' pour, it's only going to take like a half yard more. Probably money well spent if you have the truck coming.

With my rambling above I wasn't clear; it's actually a 30x34 slab I'm pouring. I added six feet to one side of the original slab three years ago and am basically mirroring that slab to double my square footage. I want my machine tools separated from dirt and grit from welding and grinding by a wall.

It seems most are of the opinion to go with the thicker slab, and I probably will. I just wanted to make sure it wasn't overkill. Concrete here has gone up tremendously the last three years. I'm looking for ways to keep this project on budget and not spend money needlessly.

 

[VocaTexas]

I'm curious about a 14,000# mill and lathe.

What did you get?

I bought a 36x72 Lodge and Shipley last fall. It was built in 1917 but has hardly seen any use. From what I can tell it basically needs taken apart and cleaned really well before I start using it. The lathe weighs 14,000 lbs.

I recently bought a Kearney and Trecker TF-17 horizontal mill with a vertical head. That machine weighs 13,500 lbs.

 

[Jimbo*]

Are you going to add a bridge crane or at least a jib crane to be able to utilize your new turning capacity?

 

[matt_i]

How are you rigging this? Pipe & rollers, machine skates, forklift/riggers?

The forklift is where you have to be worried. Think about a very simple example. I have 15,000 lbs supported on the forks. I need an equivalent 15,000 lbs object placed behind it (in the forklift's structure) to equalize the see-saw [ignoring moment balance here for simplicity].

Now the front tires have a total of 30,000 lbs bearing down on the slab in a couple of tightly spaced pads. If you have a 4" slab and you have voids in the subgrade I think you would find them.

I poured a 6" slab, 4ksi, 6" of crushed washed limestone, vapor barrier, and rebar on 16" centers on 2" chairs. For machines up to around 10,000 lbs.

 

[NUTTSGT]

With my rambling above I wasn't clear; it's actually a 30x34 slab I'm pouring. I added six feet to one side of the original slab three years ago and am basically mirroring that slab to double my square footage. I want my machine tools separated from dirt and grit from welding and grinding by a wall.

It seems most are of the opinion to go with the thicker slab, and I probably will. I just wanted to make sure it wasn't overkill. Concrete here has gone up tremendously the last three years. I'm looking for ways to keep this project on budget and not spend money needlessly.

Ah,, I misunderstood, I thought you were adding six feet to an existing slab

 

[VocaTexas]

How are you rigging this? Pipe & rollers, machine skates, forklift/riggers?

The forklift is where you have to be worried. Think about a very simple example. I have 15,000 lbs supported on the forks. I need an equivalent 15,000 lbs object placed behind it (in the forklift's structure) to equalize the see-saw [ignoring moment balance here for simplicity].

Now the front tires have a total of 30,000 lbs bearing down on the slab in a couple of tightly spaced pads. If you have a 4" slab and you have voids in the subgrade I think you would find them.

I poured a 6" slab, 4ksi, 6" of crushed washed limestone, vapor barrier, and rebar on 16" centers on 2" chairs. For machines up to around 10,000 lbs.

Nobody around here has a forklift that will pick up these machines. I'll be moving them on skates. I do have a skid steer that I can do a bit of pushing with, but lifting either machine is out of the question. I hauled them in here with a tilt bed trailer.

 

[matt_i]

If it were me I'd go 6" and call it a good day. The skates should distribute the load reasonably well.

Rebar helps keep it all together and cracks tight so the saw cut joints can do their jobs. I also recommend a plan to keep the slab flooded for a month or apply curing sealer as soon as you can walk on it without damage (couple hours after the last finisher touches it). Saw cut the next morning after the pour.

 

[rayra]

proper compaction / subgrade at least as important as the thickness of the reinforced slab.

4-1/2" ought to be plenty for those sort of no-impact machines.