Concrete Slab

[FMC1959]

Looking at buying a piece of land, 2 acres, that has a 30 x 40 foot barn which needs some work....first and foremost, a concrete floor. Currently a mix of dirt and gravel, which hasn't been touched in years.

Many questions for any concrete experts.

1 - Being undisturbed soil/ gravel, would 2 inches of gravel be sufficient bed for a poured concrete floor?
2 - I live the land of cold, freezing and long winters; what thickness would the concrete need to be, to be used as a garage for cars and trucks....4" or maybe 6"....more?
3 - Would you pour as one 30 x 40 floor, or separate into smaller sections with expansion joints? If so, how many sections or what is the largest you would go before adding some sort of expansion joint?
4 - I would like a nice solid floor that I can rebuild the walls to be set on the floor, without suffering cracks in the cement, AND spend the least amount possible; any suggestions to add that I have not asked?

 

[Red 17]

AFAIK: You need to excavate to a certain depth, and use vapor/moisture barriers under the concrete. Compacted gravel with clear, full drainage to start, sand, barrier, sand, rebar, then pour.

If you pour over what you have, you run a very high risk of problems that will be expensive to fix and aggravating to live with.

https://www.wagnermeters.com/concrete-moisture-test/concrete-info/getting-the-vapors/

https://www.constructionprotips.com/jobsite-tips/prep-crack-free-concrete/

No matter how careful you are and what steps you take, you should expect cracks. Not big ones, but some. One guy told me "..if you don't want it to crack, leave it in the truck..."

4" should get you by, 6" if you are running heavy stuff.

30x40 will be multiple trucks and you can have them put the joints in, or the slab will make them for you. (cracks)

If you are going to set the walls on the slab, I would, instead, pour the slab and have your rebar hanging out on the edges. You will probably have to do a section at a time for setting the walls, and you should do those as footings tied into the slab. This gets the building up away from the ground and ground water and allows you to tie it down and avoid uneven mating, gaps, skunk passageways, drafts, etc.

OK I am a rank amature and have watched more concrete work than most and pour just a little. Wait and see on more answers, always a good bet. Good luck with it.

 

[Lynden]

A good source for information is ConcreteNetwork.com.

https://www.concretenetwork.com/concrete/slabs/

https://www.concretenetwork.com/concrete-subgrades-subbases/

Click on and read all of the information under Concrete Slabs Overview on the left side of the page.

 

[Kaizen]

Yea disturbed soil is what you will find after removing a few feet of stone, animal bedding, horse **** that has accumulated since that was built if it’s old. If not still need to dig to make sure it’s not fill. Base and compaction are 90 percent of the game.

 

[ConCretin]

I'll take a shot at your questions;

1 - Being undisturbed soil/ gravel, would 2 inches of gravel be sufficient bed for a poured concrete floor?

I would dig a couple test holes to verify what's there. If you have undisturbed granular base material, you can place right over it. A thin layer of compacted crushed gravel would make it easier to level up.

2 - I live the land of cold, freezing and long winters; what thickness would the concrete need to be, to be used as a garage for cars and trucks....4" or maybe 6"....more?

A 4" slab is adequate for normal vehicle loading even in cold climates.

3 - Would you pour as one 30 x 40 floor, or separate into smaller sections with expansion joints? If so, how many sections or what is the largest you would go before adding some sort of expansion joint?

There is no need for expansion joints. You will however want to control cracking from shrinkage. Saw cut or grooved control joints will give the slab a place to crack where you won't see them.

4 - I would like a nice solid floor that I can rebuild the walls to be set on the floor, without suffering cracks in the cement, AND spend the least amount possible; any suggestions to add that I have not asked?

A normal wood framed wall won't impart enough load to cause problems. You could thicken the slab under the walls if you anticipate higher loads.

You can give my Guide to Floor Slabs a read for an overview of the whole process.

 

[matt_i]

Saw cut into 10-12' squares the next morning after the pour, do not delay!

Personally I think the stone bed should be equal thickness to the concrete, and I'm a fan of washed, crushed limestone 3/4" nominal, whatever that's called in your area. Plate compact.

A rotary laser level makes DIY leveling of stone and dirt much easier.

I recommend a 10 mil vapor barrier.

You might be lucky enough to pour during the winter inside the enclosure if you dont wait until its 20 below zero.

I also believe in wet curing or sealer-curing to promote higher strength in the concrete.

 

[FMC1959]

Thanks for the advice as well as links, lots of good info.

 

[Kevin54]

Depending on what was actually in the barn in each area, to be safe, I would dig down at least 6" or more. Add crushed finer limestone with a lot of stone dust to the level of the bottom of where you want your concrete. Wet the gravel down and run a plate compactor over everything....over and over. Wait a day, dampen down and repeat. Do this for a week. Lay poly down, then concrete on top. 2" is what you put down to lay paver blocks. Concrete needs a good base.

 

[earthmover1980]

To FMC 1959,
I am in the Excavating Business in SW Michigan.
What the standard is here is pour 4" concrete on fill sand with rolled wire, or just fiberglass mesh . I don't like this, it is the cheap shortcut way out. If you want it right try this. Here is my way: scrape out enuff hay, leaves, and sod to allow for 5" of concrete, and 6" of aggregate (compacted). What kind of aggregate you ask. Well use a quality product that is available to you. Limestone is the highest quality here, but spendy. Native, or recycled aggregate is fine. They all serve the same purpose. Use no less than 4" and ideally 6"+. Lay it in, smooth it out, wet it slightly, and compact it tight. Take some grade shots with laser, or pull strings if you don't have an instrument. Make sure it is level, and or
Make provisions for a floor drain(s) if applicable. We use 6"-8" of crushed concrete, and pack it in with a vibratory roller compactor. Maintain a perfect consistant grade, with no holes, or high spots to allow for a 5" thick slab. Now, obtain #4(1/2") or bigger rebar and put it up on cement bricks so it is in the middle of the slab. Put the rebar on 24" square centers.
Tie with wire, or weld if you are super fussy. Be sure to drill horizontal holes in any adjacent slabs that will be butting up to the new slab. Then put rebar in the holes, and epoxy them in if you are fussy. This attaches new slab to old ones, and prevents toe stubbers. We never pour on plastic, however we do put high density insulation around the inside perimeter of the barn between poles 4' deep to prevent frost from getting under the floor. Use a high quality redi mix concrete. Higher the psi the stronger. #7000 would be great. Finish it smooth, and burnish with power trowel. We used this exact method on a large workshop addition for our maintenance shop, and have no cracks (zip strips on 10'sq). We run heavy equipment , and gravel trucks in there daily. No problems ever. My 2 cents
Earthmover1980

 

[nelstomlinson]

If you plan to heat the structure, ever, put 2 inches of foam down between your dirt and the concrete. Put 1/2 inch heat tube in the floor, one linear foot per square foot. Make all your loops the same length, none over 300 feet long.

The suggestion above to dig a couple of test holes is a great one.

Use #4 rebar two feet on center in both directions, and ask for fiber in your concrete.