I notice that the quantities/ratios are mass rather than volume.
This is of course fine if it involves lobbing 25kg bags of cement whole or half into a mixer. And SBR and water can be assumed to be 1kg/l
But with stuff like sand and aggregate, when I'm going to be shovelling them out of a 1 ton crane bag, how do you normally gauge the mass?
Would you just assume a standard density and go by the bucket full, given density may vary according to dampness of materials (mine have been well rained upon and are thus damp) - or would you mark up a bucket line according to "10kg" for both, say, using scales to set that up. This seems a bit pedantic, so I wondered how much it actually mattered...
If you look at Cormack's site they do it by volume which can make it simpler:
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This is of course fine if it involves lobbing 25kg bags of cement whole or
Most builders etc will simply do it approximately by volume. X shovels of sand to 1 of cement. This works well enough if you don't need exact control over mix or colour. Just try to keep the shovels consistent - especially when doing the cement since it can be harder to get a nice rounded shovel full of cement out of a small bag.
The only time I would make an attempt at converting to mass is for working out approx ordering quantities. If you need a very precise mix then use a receptacle to gauge the mix - say a bucket or similar.
One area to watch is when converting between concrete mixes that are specified in gravel, sand, and cement, to use "all in one ballast" and cement. the volume translations are not always obvious - depending on what "all in one ballast" is actually like from your local suppliers.
A unit of gravel + a unit of sand is equal in volume to something less than 2 units, since the sand will occupy spaces not available to the gravel alone. So, if your ballast is a mix of gravel and sand - you may need more cement than first thought to allow for the rise in density of the ballast. If however you ballast is similar to what we get round here, a mixture of sharp sand and rounded pebble type stones, you can do a rough translation of say 4:2:1 -> 6:1 (although its probably closer to 5.5:1)
Final comments would be - if doing any quantity at a time (say more than a cube or so), its probably cheaper and easier to get it mixed for you (either delivered mixed or mixed on site). If you go for a mix on site, try and pick a supplier that has a decent accuracy mixer and not one with a big rotating drum on the back!
John Rumm coughed up some electrons that declared:
I'm reckoning on using a medium C20 (1:2:4) concrete for everything, unless the BCO calls for something better. He's coming on Wednesday anyway.
OK - I will probably do it by the bucket which is fairly practical - don't trust my shovel-fulls(!)
Indeed.
I've run up a spreadsheet and it seems the biggest single session lay will be about 0.3 m3 - I have a high volume 2nd hand builder's mixer - should manage with that.
Tell me more about high accurracy mixers. My mate used Mixamate once for a driveway slab and they did indeed have a drum mixer.
BTW - as two sets of floor screed call for an SBR screed (25mm screed) and SBR is 40 quid/5l in the builder's yards, I since discovered:
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is a far more sensible price (I need about 25l including the cement/SBR slurry primer layer).
Unless you are doing big volumes fast, get aggregate in, but buy cement as needed. Nothing goes nasty faster than a bag of opened cement. Buying one at a time to put in the car boot is less expensive than throwing away half a dozen spoilt half bags..
The Natural Philosopher coughed up some electrons that declared:
True - that's how I've been buying plaster.
I cheated though recently and hired a skilled labourer for a few weeks to help me through the heavy work (wise move in retrospect, should have done that a month ago - the man is gifted at serious hard graft, reasonably priced and I still control everything).
So I reckon we'll deal with all the concreting and screeding work in one hit.
Just to pick up on John R's comment about concrete quantities larger than a cube, I was in the situation of requiring near enough exactly a cube for a shed floor..
I went to my next door neighbour who is in the drainage and landscaping business and asked if he had contacts for getting the tail end of a concrete lorry load.. " no bother,just tell me when".
I told him "when" a week later, and got about 40 minutes notice of the delivery. This was just enough time to phone a friend for assistance and get the dpm down. The lorry parked on the curb and shot the concrete down its chute through the garden hedge. So a cube of concrete arrived all at once at one end of the site, with a certain amount not making over the shuttering and having to be rescued. Then having got it into place and started to level it we found that it was about 0.1 cube short; fortunately I had cement and ballast at hand to make that up, but that wasn't really part of the equation.
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