how much sand/dry cement for shed base?

• posted on May 21, 2012, 10:45 am
hi, i have prepared my shed base in my garden and now i am about to lay a sand/dry cement (researching on this site i have read people using a 10 parts sand to 1 part dry cement mix).
i would like to know the following if anyone can help me please:
what type of sand/cement should i use?
how much do i need for a hole which is 10 ft by 10 ft and approx 4.5inches deep?
do i need to protect the walls of the hole with wood? (i havent and it seems fine but im curious all the same=)).
chris
--
braanbatondi

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• posted on May 22, 2012, 4:53 pm
On Mon, 21 May 2012 10:45:31 +0000, braanbatondi

The type appropriate to pouring slabs. You don't use play sand or beach sand. Surely your concrete supplier will be able to deal with this for you.
I own a concrete mixer, which is handy when doing posts and small projects (not just for mixing it, but also to keep it from setting while you're digging the next post hole, etc), but the amount of concrete you need, you may as well drive off and purchase a load of cement in one of those towable mixers. Let a pro deal with making the mixture.
Cement is easy - if you're buying it as a separate component rather than as a pre-mix concrete, you'd be getting "portland cement". Most likely "Type II" (which in this case relates to reactivity with sulfur in the soil).

You mean, this is the size of the _slab_ you're making. Do the math:
convert your inch reference to feet: 4.5/12 = 0.375
10x10x0.375 = 37.5 cubic feet of fill
A 60# sack of concrete mix is 0.45 cubic feet of cured concrete, and an 80# yeilds 0.60 cubic feet. So, assuming absolutely no losses to sloppiness and prep, you'd need either (84) 60# or (63) 80# bags of concrete (you should budget 5% more than that though - it'd suck to run a bit short in the middle of a slab pour). Without the 5% padding, that's a smidge over 5,000 lbs of bagged concrete mix - keep that in mind while you're thinking about how many loads you'll need to do in your SUV, pickup, or compact car, as well as how many bags you'll need to lift onto the cart at the homecentre, then load into the vehicle, and unload and mix at home, then dump into the slab space.
Obviously, larger aggregate (rock) has more voids in it than sand, but basically, you're looking at a (cubic) yard and a half of concrete mix. Aggregate lends strength to the concrete, and fines (sand) allow the surface to be trowelled smooth.
For a garden shed, why are you making the floor so thick? It's too small to park a car in. You could shave 30% off your materials by going 3.5" thick. Use reinforcing mesh (kind of like laying a wire fence down on the ground).
Are you setting up for a drain (say, trench out and away and set up a circular drain with cover in the middle of the floor, with the slab gently tapering down towards the drain, or even just a trough drain (like you might see in front of some garages), and are you setting up to have a raised perimeter (where you might frame some 2x6' so that the pour around the perimeter is higher than the floor), which would keep your sill above grade. Going to set anchors around the perimeter to affix the sills for the walls (use pressure treated lumber for that)?

Framing the slab helps to keep it well formed - generally more of an aesthetic concern (say on sidewalks), but slag around the perimeter of a foundation is a bother when you're doing rennovation work later (such as trying to drop a conduit for power straight down the side of a structure instead of several inches out because the slag is in your way).

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• posted on May 22, 2012, 8:59 pm
On Tue, 22 May 2012 09:53:32 -0700, Sean Straw