Minimum thickness for concrete floor slab?

[SecondMonkey]

I'm moving into a new shop, which is a nice concrete building, but out back is a big shed that's basically like an old barn. It smells moist and musty with a hint of urine and has a gravel floor.

I'd like to cover the gravel with concrete, just to provide a nicer work surface, and hopefully seal out some of the moisture and smell. I don't want to spend a lot of money on the concrete because I lease the building and it's kind of hard to bring the concrete with me when I move.

So, what would the minimum thickness of concrete be that I can lay in there? It doesn't need to be that sturdy, I won't be parking on it, it's just a floor covering. I'll put a big sheet of plastic between the concrete and the gravel.

 

[Gary Allan]

I think 3.5-4" would be more than enough. You can probably get by with less, but I think it will just mean how soon it cracks. I can't find the tables, but I thought my neighbor (who was a concrete person) said that 3.5" provides a 3500lb^2' capacity. My memory could surely be flawed, but you have to figure with your single foot, many of us are putting something like 1200lbs^2' on the floor.

 

[Audi Junkie]

Depends on the size of the slab. Bigger needs more thickness to remain stable. That's in an ideal engineered situation. Considering your situation and the fact the gravel has been there long enough to settle and compact, maybe 2" with control joints every 8 feet might work. Asphalt/fibre joints are best, but you can use some treated plywood ripped into strips.

Now bear in mind, nothing is worse than a bad cement job, a LOT of work to tear it out if it crumbles. As a trick to harden the mix further, cure it with a sheet of plastic on it to retard drying.

What is the overall size anyway?

 

[hate2work]

Don't put plastic over the concrete, don't put plastic under it.

You want the concrete to mold and adhere to the gravel, can't do that through plastic.

Remove all vegetation.

If you want to slow the drying process, spread some hay on it and sprinkle it with water.

Don't use less than a 5 sack mix for best results.

Use chicken wire or fencing as reinforcement to help prevent cracking.

One thing I'd consider is using treated T&G plywood instead. At least you could take that with you when you leave, and would be easier to put down than concrete.

How many square feet is the building?

 

[dwendt44]

+1 on the 5 bag mix.
I would think 4" is the minimum you'd want.
If you are not going to drive a car/truck on it that is.

 

[SecondMonkey]

The space is roughly 24'x24'. I don't think cracks are a major issue, even a totally spiderwebed floor would be way better than the dirty gravel that's there now. I would probably get my own mixer and section it off into about 9 different pours. The floor on the other side of the wall is 12" thick so I'll do all my heavy stuff in there.

Still thinking about other options too...maybe just prefab cement board? Concrete would be the best at sealing out moisture and smells though...(and that body I've been meaning to hide....)

 

[robshelton]

I'm a long way from you, but around here, a 4" slab is standard. And I agree with the above in that I'd use floor reinforcing wire and retard the hardening with hay. I would not put plastic under the slab - it wouldn't serve any purpose but would provide a place for moisture to collect and freeze, then lift the slab.

Why don't you ask the owner if he would knock off some rent in exchange for your floor? Might be a win/win situation.

 

[SecondMonkey]

Isn't plastic normal under concrete for indoor areas? That's what I found in my last shop when I dug a hole for a sump pump. I guess maybe if I just seal the concrete it would acomplish the same thing? I'm also thinking if 4" is "normal", then I could probably get away with 2 or 3 inch. I've seen my fair share of 2x4s (3.5") as sidewalk forms. I don't think this area will have any issues with freezing water, we have pretty mild winters and the hope is to keep it as dry as possible.

I'm going to talk to the owner about sharing the cost, but, it seems unlikely. She's pretty cheap and totally oblivious to the idea that she could charge more rent if she fixed the place up some more....

 

[Amkeer]

You should place plastic under the slab before the pour. Also you are dealing with a fairly large area so you will probably need a concrete truck to deliver. A 4 inch thickness would be the minimum. You can use wire mesh or fibermesh.

 

[eljefino]

+1 on fiberglass impregnated concrete, it doesn't cost much more than regular and is much stronger.

4" in the middle, good idea to dig a trench around the outside edge though so it can go to 12".

At this scale, DIY with quikrete will kill you. Get a truck.

Concrete doesn't seal against moisture, but does hold cold so condensation will be an issue.

 

[dwendt44]

Some of the odor is in the wood, not just the dirt/gravel.
Sealing the walls with a cheap barn paint should help that.
Use white to brighten the interior.

 

[SecondMonkey]

Originally Posted By: dwendt44
Some of the odor is in the wood, not just the dirt/gravel.
Sealing the walls with a cheap barn paint should help that.
Use white to brighten the interior.

Yeah, I'm going to go in there with a pump up sprayer full of bleach and just soak the place. Right now the "windows" are open to the atmosphere, they just have bars on them, I'm going to put some plexyglass over that.

As for the concrete job, I already want to buy a big mixer for other jobs. I was thinking I'd have somebody deliver a pallet of dry mix and only do small sections at a time - like maybe 6x6. There's lot of unemployed people who will probably be glad to help me out for minimum wage too.

There's also a local place that rents out gas powered trailer mounted mixers, which they prefill with wet concrete for you...that might be an option.

Or I could just leave it stinky and get my own cow. I've always wanted a cow.

 

[LS2JSTS]

Be careful buying a cow....They have a lot of personality and grow on you and your family.

I thought I was buying meat and now I have another family pet!

 

[tom slick]

I say 4" minimum with 5-6 sack concrete, no plastic, use wire mesh at a minimum but #3 rebar would be preferred.

If it's in the open you can "tail gate" it off the truck, otherwise get a pump. A 24' x 24 ' x 4" slab is just over 7 yards of concrete, a full mixer truck is 9 yards, so you have almost a truck load. With a portable mixer your first mixed batch is going to start setting up before your last batch is mixed.

Bagged concrete sucks for jobs over 1 yard, DAMHIK.

 

[SecondMonkey]

Last night I decided to rinse the gravel off as best I could...the sludge that came out of it was disgusting. More surprising was the number of little plastic bags with, umm, "residue" inside that floated up to the top. Previous occupant is currently in jail on drug charges...

I definately want to get this gravel sealed off!

 

[SecondMonkey]

What do you guys think about this idea instead of concrete:

I could buy pre cast 12x12 and I THINK 2.5" thick blocks meant for stepping stones and such. Space them 1/2" apart, level with a blob of mortar in the center, and then fill the cracks with mortar?

This would be a lot easier to do one peice at a time as I get the time...

 

[Kruse]

About a month ago, I poured a 40' X 50' slab for a new building. I had it poured at 5 1/2" thick.
Of course, this slab will have to hold up some pretty heavy loads like an occasional overloaded semi truck a couple times a year.

 

[Amkeer]

Sounds like you do not intend on doing the job properly. It is better to do nothing.

You should have plastic under it if you do concrete and it is an interior space.

 

[Audi Junkie]

What about plastic and more gravel? You could always go over it later.

 

[eljefino]

Originally Posted By: SecondMonkey
What do you guys think about this idea instead of concrete:

I could buy pre cast 12x12 and I THINK 2.5" thick blocks meant for stepping stones and such. Space them 1/2" apart, level with a blob of mortar in the center, and then fill the cracks with mortar?

This would be a lot easier to do one peice at a time as I get the time...


That's a great idea. Another thing you can do is put some pressure treated 2x8s down (look at the dump wood pile etc) then the pavers on top of those, it will keep heaving down. I would just put sand between the pavers and let some flexibility happen.

FWIW, Lowes or HD had these pavers on clearance last winter, down from a couple bucks each to 25 cents. I'd keep an eye peeled.