After considering the much appreciated advice given here, I've now decided to bite the bullet and pour a concrete base for my shed.
AIUI C20 is a suitable mix. If I use all-in ballast, would I need to mix 1:5 or 1:6?
As far as quantities go, it's about 0.75m3. So, if using say 1:5, I'd need:
50 x 25Kg all-in ballast 10 x 25kg bags of cementDoes this sound about right?
1:6 IIRC with all in one ballast:
If you say 1m^3 is 1800kg of ballast, and 320kg of dust, then 0.75 should be 54 x 25kg bags of ballast, and about 8 x 25kg of cement.
Check that 2 big bags of ballast isn't cheaper, though.
The densities are different and the usual ratios are by volume, not weight.
NT
See if you can find a contact that will get you the tail end of a mixer load. You've got a reasonable amount of concrete to mix there, even if you use a mixer and some friends. OK I've done bigger volumes on a manual basis but then I was young and foolish. It cost me =A390 for 1 m^3 the other day with it being shot straight through the hedge onto the site so I had no barrowing, no storage, no beer to give friends, no backache, etc, etc and it was all done within an hour.
Beware of course that you may only get half and hours warning. Rob
No, C20 is 1:2:4, or 1:4 with all-in ballast. 1:6 will only give you a C7-ish mix, too weak for slabs and bases.
For 0.75 m^3 of C20 the calculator at
Not sure I follow that...
My understanding was that 1:2:4 was 1 part cement, 2 parts fine aggregate, and 4 parts coarse aggregate.
So if you are using ballast (which is the coarse and fine aggregates mixed in about the right proportion), then that worked out at 1 part cement to six ballast?
So 54 bags of ballast at 1350kg, and 8 bags of cement at 200kg - sounds like the same ballpark?
(we agree on the quantities of beer, friends and mixers though)
Sounds more like the M25 than a shed base.
Yes.
No. When you mix 2 vols of sharp sand and 4 vols of gravel the result is (not much more than) 4 vols of ballast. The sand fills the gaps between the stones and doesn't bulk out the overall volume very much. Therefore when using ready mixed (all-in) ballast the mix is 1 of cement to 4 of ballast.
By the same argument your 1:6 mix is equivalent to 1:3:6 ('foundation mix').
Fairy nuff...
Ah yup, just spotted the error in my sums - I did the sums based on weights required to make a cube of concrete rather than volume, which in theory should have arrived at the right quantities - however at the last step calculated the cement on the 1:6 ratio rather than just multiplying
320 by 0.75.The message from Andy Wade contains these words:
My ancient (1965) Collins Architects and Builders diary has it that
1:2:4 is the same as 1:5 ballast. It has 1:2:3 as equivalent to 1:4 1/4.FWIW I have just had delivered 2 tonnes loose of Ballast and 12 bags of cement as the first stage of reworking my 1 in 4 alternative exit. I had intended to use 1:6 as I was under the impression that that was what McCormack recommended.
In the past I have used 1:2:5 for floors but am fairly sure that was a bit wasteful of cement.
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