Questions on Making a Concrete Slab

[JoeFahey]

Hey, I need to create a concrete slab to serve as the foundation for a small backyard metal shed. It will be about 7.5'x10' with a 4 thickness. I have developed some questions along the way:
1.) What should I put below the concrete slab? The installation guide recommends about 3-4 inches of gravel should be used.

2.) Should I put a vapor barrier like 6mil plastic above the gravel if it is used?

3.) For a slab this size (around 26 cubic feet), I apparently would need about 43 bags of the premix Quikrete sold at home depot. Is it as unreasonable as it seems to mix 43 bags of Quikrete in a wheelbarrow that can probably only hold a few at a time?

Any and all tips/recommendations would be appreciated.

Thanks!

 

[dank69]

Hey, I need to create a concrete slab to serve as the foundation for a small backyard metal shed. It will be about 7.5'x10' with a 4 thickness. I have developed some questions along the way:
1.) What should I put below the concrete slab? The installation guide recommends about 3-4 inches of gravel should be used.

2.) Should I put a vapor barrier like 6mil plastic above the gravel if it is used?

3.) For a slab this size (around 26 cubic feet), I apparently would need about 43 bags of the premix Quikrete sold at home depot. Is it as unreasonable as it seems to mix 43 bags of Quikrete in a wheelbarrow that can probably only hold a few at a time?

Any and all tips/recommendations would be appreciated.

Thanks!

1) Gravel just like it says.
2) No
3) How much is your time worth? For me, 43 bags seems unreasonable.

 

[nickbits]

I did 60 bags for my deck. It sucked. Look into getting it via cement truck.

 

[FoBoT]

some info
http://www.garagejournal.com/forum/showthread.php?t=40431

 

[herrjimbo]

why concrete? i'd just make a floor/base out of treated wood. tons easier and would serve the same purpose. plus anchoring the shed would be simple.

 

[T_Yamamoto]

We did like 3 bags to get out basketball net into the ground. Tough work.

 

[D1gger]

I would definitely strip all of the topsoil/sod down to very solid clay, then build it back up with 3/4" minus crushed aggregate depending on the depth of the topsoil and the desired final grade for the slab, this may be more than 3-4" of gravel. I definitely wouldn't go less that that 3-4 inches.

For a garden shed, I wouldn't worry about the vapour barrier.

You can order a ready mix truck for small quantities of concrete, and for 1 yd3 I would definitely do this.

We often order .6 or .8 m3 of 25 or 30MPa concrete and they charge by the m3 and for quantities this small will add an extra small load fee.

Typical cost for me: $160/m3, plus $70 small load fee.

Definitely better way to go than mixing 43 bags of quikrete. If you are set on doing it yourself, rent a portable cement mixer.


EDIT: Pro Tip -- if you want the slab to be extra strong and avoid cracks, add a layer of wire mesh prior to pouring.
http://www.youtube.com/watch?v=zW6uGjURuNo

 

[skyking]

Gravel depth depends on the current soil type and drainage. Where I live, for example, is so rocky that I need very little in the way of a gravel bed. Water does not pool unless it rains inches, and the pools are gone in a day.
If you go ahead with the wheelbarrow method, crew up. Get at least 4 people together to do it. The alternative is not going to be pretty.
Use a garden hoe and mix two bags at a time. A crew of 4 could mix that up in 20 minutes tops. a solo guy will have mud setting up before you can finish it.

 

[vi edit]

Quickcrete?

No.

Pour that sucker. Problem is that you likely won't get a truck out there for so little of a job. Gravel for the bed, plenty of rebar, and then finishing. Moisture barrier is usually an indoor thing. Guess it wouldn't hurt outdoors, but probably isn't entirely necessary. You could also consider doing a 2" layer of XPS foamboard between the gravel base and the slab. Insulate the slab a little more from frost and doubles as a psuedo-moisture barrier. Lots of reports that going that route results in less slab cracking.

 

[edro]

I just mixed 60 80lb bags for the post holes for the deck.
I bought a $120 cement mixer from Harbor Freight. It saved a ton of work... but it is still a huge labor intensive job.
Since you are pouring a single large slab, you should have it delivered by a truck and do a single pour.

I wouldn't want to keep dumping small batches together into a single large slab. It will take you at half a day to mix that many bags and it would all be curing at different rates.

Have it delivered. And since you have it delivered, make it thicker than 4" since you will probably be near the minimum quantity anyways.

 

[CPA]

Hey, I need to create a concrete slab to serve as the foundation for a small backyard metal shed. It will be about 7.5'x10' with a 4 thickness. I have developed some questions along the way:
1.) What should I put below the concrete slab? The installation guide recommends about 3-4 inches of gravel should be used.

2.) Should I put a vapor barrier like 6mil plastic above the gravel if it is used?

3.) For a slab this size (around 26 cubic feet), I apparently would need about 43 bags of the premix Quikrete sold at home depot. Is it as unreasonable as it seems to mix 43 bags of Quikrete in a wheelbarrow that can probably only hold a few at a time?

Any and all tips/recommendations would be appreciated.

Thanks!

1) Use a stamper to flatten the earth first, then add the gravel.
2) No
3) I've done it with about the same amount of concrete. Thing is you can't stop once you get going, no matter how tired you are AND you will be tired.

 

[vi edit]

I wouldn't want to keep dumping small batches together into a single large slab. It will take you at half a day to mix that many bags and it would all be curing at different rates. Have it delivered.

And since you have it delivered, make it thicker than 4" since you will probably be near the minimum quantity anyways.

:thumbsup:

 

[CPA]

Quickcrete?

No.

Pour that sucker. Problem is that you likely won't get a truck out there for so little of a job. Gravel for the bed, plenty of rebar, and then finishing. Moisture barrier is usually an indoor thing. Guess it wouldn't hurt outdoors, but probably isn't entirely necessary. You could also consider doing a 2" layer of XPS foamboard between the gravel base and the slab. Insulate the slab a little more from frost and doubles as a psuedo-moisture barrier. Lots of reports that going that route results in less slab cracking.

I have on a foundation for my outdoor kitchen. Cost me about $200.

 

[BoomerD]

For a yard of concrete, (your specs are 25 cu/ft) rather than mix sacks, I'd find a place that offers "you-haul" concrete...IF you have a truck that can haul the trailer...and IF you can get the wheelbarrow to the work place...and IF you can handle a wheelbarrow full of mud.
It would be best if there are at least two of you doing this...and 3 or 4 would be better, but it's doable by yourself if you have to...

 

[vi edit]

Lots of little things you can do to burn up extra concrete. Form up some sidewalks, extend a walkout patio, make some slabs for rainwater barrels, ect. Or you talk to a neighbor and see if they need any work done. Figure about $100 a yard, give or take $10 depending on location.

 

[skyking]

Around here I can rent a 1 yard mixer trailer. Gotta have at least a 3/4 ton truck per rental agreement. Cost about 125 per yard last time I tried it. had a Honda on it that mixed the mud the whole time, and dumped it out.

 

[BoomerD]

Lots of little things you can do to burn up extra concrete. Form up some sidewalks, extend a walkout patio, make some slabs for rainwater barrels, ect. Or you talk to a neighbor and see if they need any work done. Figure about $100 a yard, give or take $10 depending on location.

Plus, it doesn't take much "imperfection" in forms or ground leveling to eat up a bunch of concrete. Not to mention that the "u-haul" concrete companies don't always give you a full load.

When we had the pad for our garden shed poured, the guy who did the work set up the forms at exactly 8' x 10'...and one yard left us short...I had to go buy another 1/4 yard to finish the job...and had a bit left over.

 

[rudder]

Since you are pouring a single large slab, you should have it delivered by a truck and do a single pour.


Have it delivered. And since you have it delivered, make it thicker than 4" since you will probably be near the minimum quantity anyways.

Most of those cement trucks have a minimum fee... probably like $1000. OP could always look for houses under construction and ask if they have a little left over after a job... but of course hit or miss there.

 

[BoomerD]

Most of those cement trucks have a minimum fee... probably like $1000. OP could always look for houses under construction and ask if they have a little left over after a job... but of course hit or miss there.

Nah, but there is usually a small charge for "short loads."

I had my driveway widened 3 years ago...took about 2 yards of concrete. IIRC, there was a $50 "short load" charge for less than 4 yards of concrete.

 

[highland145]

A little help.

What's the gravel for? Drainage? Why if the shed is over the slab keeping the rain off?

Would he use a barrier if he poured on the ground to keep moisture from wicking up?


Have a 12'X20' old garage, brick perimeter, that I tamped/put down plastic and had 4" poured. It's fine/dry for the last 10+ years.

 

[OutHouse]

OP could always look for houses under construction and ask if they have a little left over after a job... but of course hit or miss there.

taht is exactly what i did for my side walk in in my back yard. i had it all framed up and ready to go. neighbor up the street was getting a new driveway and i talked to the foreman and he charged me 200 bucks for the extra cement and his crew poured, leveled and put the seams in it. hell of a deal.

sidewalk is 4' wide, 5"thick and about 30' long.

 

[vi edit]

What's the gravel for? Drainage? Why if the shed is over the slab keeping the rain off?

Ground run off? A roof over the top won't stop water from seeping in around the edges or from natural ground water leaking up. You just want room for that water to go so it's not getting trapped under there and either erroding out your base or getting stuck during the winter and having frost heave your slab up.

 

[BoomerD]

I'd also recommend adding fiber to the mix. It works very well at holding the slab together.
Concrete cracks...it's the "nature of the beast," but fiber can prevent a small crack from getting out of control.

 

[IndyColtsFan]

I had an 8 X 8 X 6" (IIRC; might have been 4") with rebar poured for around $700. I could've gotten it cheaper (the cheapest guy was going to charge around $500 IIRC) but this was through my patio contractor and due to coordination, I decided to pay the extra money. I'd hire it done unless money is really tight.

 

[iroast]

Hey, I need to create a concrete slab to serve as the foundation for a small backyard metal shed. It will be about 7.5'x10' with a 4 thickness. I have developed some questions along the way:
1.) What should I put below the concrete slab? The installation guide recommends about 3-4 inches of gravel should be used.

2.) Should I put a vapor barrier like 6mil plastic above the gravel if it is used?

3.) For a slab this size (around 26 cubic feet), I apparently would need about 43 bags of the premix Quikrete sold at home depot. Is it as unreasonable as it seems to mix 43 bags of Quikrete in a wheelbarrow that can probably only hold a few at a time?

Any and all tips/recommendations would be appreciated.

Thanks!

Before you go digging, check with the town/city to see if there are any pipes in the way.