Cementing out a machine shed

Thinking of putting a concrete floor in a 50 x 80 machine shed. Going 5" thick, 1/2" rebar on 3' centers, 3000# mix.

What about stands under the rebar? Is that a good thing to do? Also, what is a fair price for labor? There will be some leveling to do.

What else do I need to consider? Kinda feel like a bare foot pilgrim when it comes to concrete. Thanks for any advice!!

Hello, If you are not already pretty good with concrete hire a pro company and see if you can help labor. You really do not want a sea of bad concrete. Bet you cant guess how I know. Dave

The bars going in at 3' centers will do as much good as no bars. The more the better, 12" minimum. 6" minimum on the concrete. Do it right and hire a professional. You get what you pay for taking shortcuts.

We always used wire mesh and only rebar in critical areas depending on the base. One thing I would use 4000 mix with absolutely no fillers of any kind, and be sure to seal it. The 4000 over the 3000 should be only a few bucks a yard more. Do it once and do it right.

You want #5 bars one foot on center with expansion joint in middle.Wire mesh on top of rebar mat. tie the 5 bars to one inch chairs ,plastic underneath.
Big job I would charge u bout 11,300 for the floor finished and cut!

Good luck

I put my rebar on 2 ft both ways. Took a post hole digger and dug me 30 holes about 3 ft deep. Poured them level with the grade below the floor. Then poured 5,000 Lb concrete 4 inches thick. All this was done in about 2 feet of fill dirt packed with a truck load of rock. This was done in 1990. I have not seen a crack of any kind yet.

1) Yes the stands are a big help in keeping the rebar up in the concrete where it needs to be.

2) 1/2 rebar on 3 foot centers is too far apart. I usually use mesh/old cattle panels/wire corn cribs. Anything as long as it is not rusty. If rebar alone then 1 foot on center in the door ways and edges. Then maybe 18 inches out in the lower stress areas.

3) Going 5 inches is the minimum. If you have heavy equipment then go 6-8 inches. Also dig footers at all door ways. I usually go at least three feet. It will hold up under loads. You will want to put loads of grain/hay/stuff in. The edges will break off is you don't put a footer at the doors.

4) Go with a #4000 lbs mix plus fiber. It will only add about $8-10 per yard and it will be much stronger. Plus the fiber will help it not crack. I just poured a cattle shed this last week. The concrete was right at a hundred dollars a yard delivered. That was #4000 mix with fiber.

5) As for cost to lay the concrete. Around me the standard rate on flat work is about double the concrete cost. Walls and basements are 2.5 times the concrete cost.

The most common thing done wrong in pouring concrete is its poured too wet,all concrete finishers want it sloppy so its easy to put down.
Concrete should never be poured at more then a 5"
slump(4" is better) not a 'finishers 5" which will be more like a 7".To get a professional job hire a concrete technician to take samples for slump and a few test cylinders even if you throw the test cylinders away it'll keep the concrete supplier honest about what they're sending out to you.Be sure to let them know it'll be tested.Also you might want to use superplastizer that will increase slump but not cut the strength of the concrete keeps the finisher happy.I worked in the concrete business for 25 years and its proabably the most misunderstood common product around and one where the most shennagians goes on.

The advise from a forum you would get could vary depending on soil type from different parts of the country. It would be better to go to your nearest city and get specifications from their building permit department. As far as the grade of cement the #3000 cement is rated for 3000 lbs per sq. in. The #4000 cement is rated for 4000 lbs per sq. in. The chairs under the rebar are a good idea however good concrete people can manually pull the rebar up into the cement as they go. At 50'x 80' unless you use a pumping truck or a cement truck has access to both sides the cement truck will have to drive in on the rebar to make the pour. If you set up chairs in advance they will get crushed into the ground. Where I live in Texas the soil is sand. I poured the floor in my shop 3" thick in the center with remesh and 6" thich around the edges. I regularly drive vehicles in on the floor as well as a fork lift and there is no cracks. I know it wouldn't have worked in Dallas where the soil in gumbo.

As someone else said, your soil may determine your pour. Similar to painting over loose paint, if you don"t match the reinforcing/thickness/etc to the soil conditions, you may have problems.
After that, I usually will take my concrete sub"s recommendations and balance them with my own experience/education. (But I have also had the same sub now for about 10 years, so we know each other well). They have a very good feel for the site conditions as well as what supplier they will be using, and that sometimes will change how we place reinforcement.
I doubt you will ever get a no-crack guarantee, and if you do, be wary. Shrinkage cracks will occur, but structural ones shouldn"t.
The common guarantee in concrete being "it will get hard, it will crack, and no one is going to steal it."
Good luck

Tony

what you think about adding fiberglass to it?

There it is Gordy. I assume you are planning to pour this with some neighborhood help. If that is so the chairs are nice but pricy for what they do. I will agree with most in that 24" center, 5/8 bar, and 4000#. Set up your bar as you pour out. RELIEF CUTS. There are two types of concrete, Cracked and Gonna crack. Cuts at 15' to 20' will control the expansion do to thermals there in Iowa. Moisture is your friend, keep it moist or wet for 3-5 days in the shadded shed and there will be alot less curling and cracking. The chemical cure of concrete needs hydration.

"Road Mesh" will be by far easier to drop in and step-down, than Re-Bar and trying to pull it up...
Besides, I think it works better on a thin pour, like a floor..
If you don't want it to crack, go on and put enough concrete in for a Double Matt of Road-Mesh..
There, the Chairs would be handy to maintain the top-to-bottom spacing..
If you are planning to keep Heavy equipment on the floor, expect it to crack..May just want to grind lines across the floor and hope it cracks there..otherwise, better go with 12" of Concrete (or more)..Then, 1/2" to 5/8" Re-Bar would hardly be necessary..the Road-Mesh + 3/8" or 1/2" would be good and cheaper..

Ron..

1/2 inch rebar on 1 foot square, 6 inches concrete. Hire professionals to do the work. You only have one kick at the can when doing something like that. Oh yeah, 30 mpa concrete

You can have thinner slab if you spend lots of money on good compacted base. The concrete is just a cap on top of the prep work and can't make up for bad prep.

POUR the mud DRY. I saw some PRO's pour the bay floor on our four bay fire station. It looked so dry I did not think they would ever get it spread. They also used fiber in the mix. NO Rebar used. I think it was a 4 inch pour but it may have been 6. We park trucks with between 1000 and 2000 gallons of water on the truck plus the weight of the truck. No cracks yet. the Pro's saw joint before we ever park a truck on the floor.
Have pros do the job. This will cost more but you will have a better floor that you will likely be happier with. Also when my nephew built his house He had a Pro do the foundation. It was some of the hardest concrete I ever had to drill to mount things like the breaker box. This was two years ago. No cracks in the foundation yet.

Kent

That is a huge floor to screw up. You will be money ahead over the long run to hire it out. Make sure you put some drains in before you pour.

5 inch is better, you are the one paying for it tho.

never heard of 3000 for such, 4000 or 5000 is what's used.

Never heard of 3 foot spacing; 1 foot is great, 2 foot is common for cost. 3 foot - why bother. Rebar is best in a floor, no more than 2 feet. Chairs are good, but then you can't wheelbarrow over the area, how you gonna pour? Without experienced folk, you gonna remember to pull the bar up as you go without chairs, or let it lay at the bottom....

Concrete cracks, cut it on 12 foot sections or so to control where the cracks happen.

Mesh likes to rust out where cracks happen, it likes to sink to the bottom of the pour and do nothing, it is kinda weak to hold a floor crack together.

Fiber is good to control surface spawling/ minor surface cracks, but won't hold a floor together. As well, some folks have issues with the fibers sticking out & scratching you as you kneel/ lay on the floor.

Solid tamped soild underlayer is most important. Do that right, period or the floor won't hold up no matter what you do.

Those here saying they have no cracks did the bottom surface right an well, and will see cracks some day - all concrete does. A shed my dad floored in the 1970's only 12x18, solid floor, last year asll of a sudden big crack through it, and offset 1/2 inch drop. Dad overbuilt stuff, but he didn't believe in wasting money on rebar. If he'd a done the rebar, the crack would hasppen, but the rebar would hold the floor from settling.

They all crack sometime, the rebar & cuts help you control that. Rebar needs to be close enough to bridge the problems that show up over the years.

--->Paul

Thank you guys for the suggestions. I now have the info needed to do the floor right.

I am a civil engineer and design slabs, retaining walls, and pier type foundations everyday.

For the average farm tractor weight, a 6" slab will be more than sufficient. You could get away with using 4x4 wire mesh, but I would use #4 rebar on 12" centers to create a grid pattern. The bars should be 2" below the surface. Rebar gives the concrete strength in the upper portion of the slab. Any lower than 1/2 way and it is not helping you with the tension the slab sees. Concrete works great in compression, not tension. Control joints should be cut at no more than 15' spacings. Remember, it is a given all concrete will crack somewhere. You just want to control where. Compacted soil is a must to keep it from settling. If you are filling the interior with subgrade, compact the stone or soil in 6" lifts. Rent a compactor. Driving a tractor over it is too risky and allows uncompacted voids. Use a standard 4000 psi concrete mix with 5%-7% air entrainment. Also use a vapor barrier under the concrete. Concrete should not be placed on dirt. It needs a minimum of 4" of crushed stone. I suggest to thicken the edges to go below frost line depths. This will help in eliminating heaving during freezing and thawing times if water runoff is a problem. If water isn't an issue, the slab will weigh enough that any water that freezes under it will not be able to push the slab upward.

Soil conditions, unless mush really won't play a large part with your type of slab. Thickness of concrete has very little to do with soil conditions. It is mostly the square footage (footprint) that matters. The thickness is determined by what the punch through loading is. 6" of concrete will hold a lot of weight if the footprint is large enough.

If you are pouring it in separate sections on different days, put a 1/2" foam strip expansion board between the slabs. Think of the earth's plates during an earthquake.

Check your local and state building codes. They have them for a reason.

Keep heavy equipment off the floor for at least 7 days. 4000 PSI concrete will cure over 28 days, but after 7 days it has about 75% of its strength.