Concrete Mix

[Mr-H]

I want to put in a solid concrete shed base, measuring 8x6ft, by approx 4-5 inches deep. This seems to suggest approx 3/4 cubic yard of concrete, but I can only buy 1 tonne bags (I don't want to have 3/4 ton delivered as loose gravel, and to use to 23kg bags would work out expensive I believe. My question(s) revolve around the concrete mix, which I am told should be 3 or 4:1. Would I order what my local guy calls 'ballast'? Does it come 'ready to use' for my purpose, just adding cement and water? If this is the case, how many bags of cement should I order please? I would happily make the base big enough to suit all of the concrete these loads would make, but would I make it larger in 2 dimension (length and width) or should I go down deeper? By making the base larger, I would have a concrete surround; this would be ok since my garden, in that area, is Cotswold gravel and could be scraped to cover the concrete if necessary.

[Svejk]

I make that .43 of a cubic metre.

[Clanad]

Could I make a suggestion? You're only talking about 12 to 14 cubic feet which is roughly half a cubic yard. Here in the U.S., the ready mix concrete companies (you know the ones... the large trucks with a rotating cylinder on the back that deliver... well... ready mixed concrete) are always looking to get rid of what's left over from an order. If they don't sell it to someone they have to dispose of it otherwise and that involves cost and waste. On several occasions I've called some of ours and asked them to call me when they want to deliver half a yard or so. They've willing done this and the cost has been significantly below their usual charges.

As I see it, you're going to need, in addition to the materials, a cement mixer, unless you intend to mix it by hand or rent a mixer. Problem is it's difficult, even for an experienced cement mixer to get the mix consistent throughout the run of concrete... some will be more watery than others and some will have more cement in the mix than others. This is a recipe for disaster or at least unsightly when finished.

It may be more cost effective and efficient to have the area excavated and framed up, with reinforcing wire ready to be poured and wait for the one you've made arrangements with to call.

This all presupposes access to your area by the truck...

Best of luck!

[The Builder]

For all practical purposes, Mr H, I would call that 0.5 cubic metres. Thickness of 100mm, plus an allowance for extra needed where the hardcore base is not even. (0.43 cu.metre plus allowance.)

If you can get a 1 tonne bag, then go for it. Should be about right. Check this out. Normally, "Dumpy bags" are around 850kg ........ 0.85 cu.m.

Terminology varies around the country. I guess by "ballast", you mean what we refer to as "all-in mix". 20mm chippings and sand mixed together in one bag, in a ratio of 2:1 ....... chippings:sand.

That's the stuff. Just add cement and water. A ratio of 6:1 would be plenty strong enough. (Mix:cement).

The critical thing with shed bases isn't the mix etc, but the "substrate". In simple terms............. what's it sitting on?

Clanad's idea with surplus ready-mix is a really good one. For some reason, American companies tend to be rather more helpful than ours. Over here, some guy in an office may not feel inclined to help out. Often, if someone orders a 5 cu.m. load from a 6 cu.m. truck, they make him pay for the missing bit anyway.

Another way is to hire a mixer truck that actually carries dry mix and cement (separate) on a truck with a mixer on board. They can batch as much as you like, with no surplus to throw away.

[The Builder]

^^^^ nonsense alert ^^^

///If you can get a 1 tonne bag, then go for it. Should be about right. Check this out. Normally, "Dumpy bags" are around 850kg ........ 0.85 cu.m. /////

Should read ............

If you can get a 1 tonne bag, then go for it. Should be about right. Check this out. Normally, "Dumpy bags" are around 850kg ...... 0.42 cu. m.

[bookbinder]

Do you really have to lay a concrete base for your shed? Try laying concrete slabs instead. Much quicker, much easier, and you can calculate the number of slabs exactly.

[-Talbot-]

4-5 inches deep....for a shed base?