Concrete floor for workshop questions

[ejb]

OK guys (and gals if there are any out there), I have a question that is only semi-tractor related, but can't think of a better place on the web to ask it....

I have a 24 X 30 shop, with a dirt floor. I want to put a cement floor in it before winter comes, and I am going to do most of it myself to save money, but don't want to make any mistakes, so here goes the questions:

Any recommendations on how thick the slab should be? The building is wooden, on a concrete knee walls, and is only heated occaisonally in the winter. I have a heavy tractor (a JD 5410 and various other very heavy attachments) and a oversized pickup truck I bring in sometimes and I don't want the floor to crack on me. My dad suggested 3-4 inches, I am not sure if that is going to be strong enough to resist cracking.

Besides digging down the required dept (3-6 inches) and laying in some wire mesh, anything else I need to do ahead of time?

I have 6-8 rolls of wire fencing that the previous owner left here, think I could use this as the wire meshing? I can't see why not...

Does the wire go at the bottom of the slab or do I support it enough so that it ends up in the middle?

Lastly: I am considering digging a pit in the middle of the floor and building forms so that I can stand upright when I work under my vehicles...pros/cons on this? what would be the right dimensions for the pit?

Any other suggestions would be appreciated!

Thanks all.

 

[MChalkley]

I'm no expert, but will repeat some advice I was recently given. Around here, fiber-reinforced concrete is not much more expensive than plain concrete and supposedly eliminates the need for wire reinforcing and resists cracking considerably more, even at thinner thicknesses. Might be worth checking into locally to see what the consensus of opinion is.

Mark

 

[W5FL]

ejb,

What is the frost line in your area? Normally you would dig down the outer wall and beams about every 8 to 10 feet and below frost line. The beams are normally about 12 inches wide and 18 inches deep (depends on the frost line where you live) and reinforced with 4 5/8 in rebars tied with tie wire to hold them in place and a minimum of 1 inch inside the concrete. The reinforcing for the actual slab is not nearly as important as people think. I would use 3/8 rebar on 18 inch centers but 24 inch is also used, just not as strong. You can buy concrete "chairs" that are about 2 1/2 inches high to hold the rebar off the bottom. Depending on soil conditions, several inches of cushion sand under the slab lets it float instead of crack. Usually 2 to 3 inches of sand is adequate. The thickness of 3 1/2 inches is commonly used because you can frame it with a 2x4. I frame everything with 2x12 on the outside wall and by the time you dig out the beams and leave the dirt in then fill with 4 inches of sand, the forms will hold 4 to 6 inches of concrete. That is a strong slab.

Pits look better than they work. Think of 20 gallons of gasoline in a pit and it gets set on fire. Takes a good fire extinguisher to put it out. Then there is the problem of draining the thing. You have to have a slope of about 1 in 12 for the drain pipe and you have got to come out somewhere that is lower than the shop. I use 2 good jacks and stands instead. /w3tcompact/icons/smile.gif

You can use reinforcing wire (not fence wire) instead of the 3/8 rebar, but the savings are minimal. The rebar should wind up about 1 1/2 inches from the TOP of the slab. Concrete is very good in compression, but very poor in tension. The ONLY purpose of the rebar is to provide a tension carrying member in the concrete.

In actuality, the weight of the John Deere tractor and implements is almost neglible in the compression load the slab will carry. Slabs normally crack because they are build over expansive soil or don't have adequate beams running through the concrete, or they are built on fill and not piered to solid stable soil.

 

[Mike H]

Mesh definitely does NOT go on the bottom. It's there to take the tension load that will crack concrete (which is great in compression and lousy in tension). Generally will go 1/3 of the way up although I'm sure lots of folks will put it in the middle.

For garage and pole barn slabs, I think most references will say 4" (which means 3.5" since you'll form it w/ 2x4's) and that mesh isn't req'd. If it was me, I'd form it w/ 2x6's and mesh it. This is how I did a 24'x36' slab in front of my barn a couple of years ago. I figured for the extra few hundred $ it cost, I didn't want to crack it in a couple of years driving loaded horse trailers, trucks, tractors (now!), etc. Then again, this is a driveway that gets loaded w/ something several times a day.

There are lots of different concrete mixes. Talk to your supplier and make sure you get something appropriate. I think you'll want the fiberglass in it and may want a 6 bag vs. 5 bag (they may quote it as compression strength). Tell them what your application is - if they're at all customer service oriented, they'll help.

I'd also shell out $5 for some general concrete reference book (or hit the library) at Lowes, Home Depot, etc. - if nothing else you'll learn some of the language. (A "come-along" meant something very different in the concrete world). I spent $100-$150 on several tools as well (a decent magnesium float w/ 12' pole wasn't cheap). You'll need helpers - it's not a one man job. But it's doable and pretty fun.

Mistakes are rather permanent so I commend your approach - ask lot's of questions and be prepared to digest a few conflicting opinions. And remember, all concrete cracks, it's just a matter of when and where. You minimize it w/ steel and control it w/ joints but it will crack - it's a 'brittle' material.

 

[richh]

Did this 2 yrs. ago. Everything posted looks good, just dff. ways. My 2 cents, insulat the floor if your going to heat it. slab will act as a heat sink and hold some of your heat, or better yet put the heat in the floor, this is as good as it gets, a warm floor just think how the bones will feel!!!/w3tcompact/icons/smile.gif

 

[W5FL]

Mike,

My barn floor is 50 x 90 feet and does not have a single crack in it after 6 years. It is built on 4 1/2 foot fill on one side and the floor is smooth enough to hold an old fashioned barn dance on. The stem wall is 12 inches and the slab is 4 inches (not 3 1/2 inches) thick. The column mounting piers are 2 ft x 2 ft x 4 ft deep. The barn is 14 feet high and rated for 90 mph winds.

My dad could not get a concrete truck to his place due to low gross weight bridges. He poured the floor as flatwork using only concrete mesh on chairs and made 6 separate pours separated with wood stringers mixing all the concrete in a mixer and finishing it all with only a simple hand float. Not quite as elegant, but inexpensive and works just as well. /w3tcompact/icons/smile.gif

 

[Bird]

I've had a bit of experience along these lines and would strongly recommend you follow Wen's advice. He described the proper method quite well.

Bird

 

[JimBinMI]

Mike H,

The way I heard it, "All concrete in Michigan cracks"!
Never knew about the other states!

JimBinMI

We boys and our toys!

 

[Mike H]

I definitely believe you. Read your post after I sent a few words. Exellent comments on the importance of the base. My comment on "everything cracks" had to do with a general fact of life in frost zones! In my area footings need to be 48" deep and very few pole barns are built with footings anything like this. So, they're designed to float. I will say that in doing things like this (flatwork), I certainly develop and appreciation for the pros who can make the surface texture any way they want!

 

[Mike H]

Nah, the "cracks" I'm talking about are the grooves, or expansion joints, or those new-fangled plastic joint thingies that I tried and hated. I suspect the further North you go the more of these we need, but if you do them right then the concrete isn't cracking however it feels like.

You ever try to watch any of the concrete work on I-94 near us (over about the past 10 years!). Now that's engineered a little differently! You know, our gas tax money's paying for all that concrete work.