While the guys were unloading supplies I looked at the recommended mix ratios on a sack of mastercrete, abd was surprised to see recommended a mix of 1 cement to just 1.5 sand! Plus 2.5 coarse aggregate. They're taking the michael surely.
NT
That's 1 cement to 5 ballast - it's what I remember from my concrete mixing days - over 30 years ago.
Now my maths isn't brilliant but 1.5 + 2.5 = 4
so wouldn't that be 1 cement to 4 ballast?
That seems too strong for normal use
as coarse aggregate is specified I would be looking at about 1 cement to 5.5 ballast (2 sand 3.5 aggregate)
Regards
yes - too late at night - I woke at 4 this morning and couldn't get back to sleep.
What is normal?
C20
If you were using brick sand, the people who are selling cement would be about right, on the generous side. And I'd stick with it if the gravel was river gravel not broken in a quarry's mill.
You can make concrete a lot weaker. It all depends on what you are using it for. And how much trowelling up you are going to give it.
The more you mess with the concrete the more likely the fat is to come out of the mix. Tamping it and leaving it will be OK with a weaker mix but if you are going to tamp it then float it...
But I think I'd go for a coarse sand, replacing half a shovel of that with gravel: 2 cement 3 coarse sand 8 mixed size aggregate.
Can we drop that ino the wiki somewhere?
NT
There is nothing wrong with good quality gravel and it would be really stupid for some-one in the South-East to go chasing after crushed granite for concrete. Flint pebbles are hard as, err, nails, well harder than nails then. And, with a crushed aggregate (rather than a naturally rounded gravel) you need to up the cement ratio to get the same strength of concrete.
That's C35 - good enough for a road.
good enough for a nuclear bunker probably.
A quick google gave some examples of purposes for which dredged gravel was OK - Sizewell B and the Dartford bridge being the two most memorable ones.
I think it'll be ok for the slab I am pouring today in my garden. I am not sure whether it is marine or pit sourced, but it would be ridiculous to care.
I find it difficult to believe that rounded gravel can interlock they way sharp edged stuff can.
You need to add more water to crushed, angular, aggregate to get a workable mix. More water in the mix =3D weaker concrete, so you need to add more cement to compensate.
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