Is there an error on the mortars and concrete page

[ringi]

A C20 mix is given as 4 x Gravel plus + 2 x Grit Sand plus + 1 x Cement OR 6 x 'ballast' + 1 x Cement.

However when you mix grit sand and gravel the combined volume is a less then the separate volumes as the sand fills the space between the gravel.

[Brucieboy]

Ringi - technically, you're correct. A 4:2:1 by volume mix will equate to somewhere in the region of 5.25 - 5.5 "all-in" by volume ballast because, as you say, some of the sand will fill the voids in the gravel. Assuming a typical concrete density of 2350 kg/m3 at S2 (70mm) slump, a 4:2:1 by volume has a cement content of 275 kg/m3 compared to about 240 kg/m3 for a 6:1 all-in by volume.

The reason a C20 is shown on the website as 4:2:1 or 6:1 is that both have sufficient proportions to achieve the required strength (assuming you don't go silly with the water and make the concrete too sloppy).

[lutonlagerlout]

our volumetric company bring 20mm gravel,grit sand and cement all separate and knock it up there and then
its a very good mix
with all in ballast one of the biggest problems is when it gets wet,the water sinks to the bottom and the last few gauges end up sloppy
LLL

[Tony McC]

We don't tend to use 'all-in-ballast' up here in t'north because we have plenty of proper aggregates, and that allows us to ensures the proportions are more-or-less right.

All-in-Ballast is very much a case of "you get what you're given". I've seen bags in a local DIY shed that are at least 90% sand or grit with almost no 'gravel' or stone >6mm.

And consequently, because A-i-B is so bloody hit-and-miss, there's no point in being overly accurate regarding the ratios. 6:1 is close enough!

[lutonlagerlout]

I have actually dug ballast out of the ground near st albans
thought it was a trick hitting ballast 6 inches below ground,but out it came all in ballast
lot of sand and gravel pits around that area
LLL