I had to look all over to find answers to my question on how to make an approx. C20 concrete mix with the above mixer, and do some basic calculations. I thought I would put the information I collected here to save other people the trouble. NB the 'recipe' I give depends upon how wet the sand is, so use this information at your own risk. C20 is a general purpose concrete grade of 20 N/mm^2 or 20 MPa. strength. To make it you mix by volume 1 part cement, 2 parts sharp sand to 4 parts of aggregate, say 20 mm gravel for coarse concrete, or 10mm gravel for fine concrete. First confusion comes when all-in ballast is substituted for the separate sand/aggregate ingredients. Here you will find people recommending anything from a cement :ballast ratio of 1:4 to 1:6, for a C20 mix. The reason why it is not 1:6 by volume is that the finer sand can sit in the some of the holes between the larger gravel/aggregate particles, so the mixture is denser than its constituents. However having looked into the quoted densities for ballast mixtures, it does not appear to get that dense on mixing so that a cement:ballast rato of 1:4 would probably not be correct. That would assume all the sand can be accomodated within the pores between the gravel chunks. Based on the densities quoted, a ratio somewhere between 1:5 - 1:5.5 would seem about right. In any case the required all-in ballast for a single load should weigh about (59 + 112 kg, see below) 171 kg, and in theory this should have a volume of about 96 litres, plus or minus ??? The most confusing bit is that the change in density is much larger when going from the dry mix to the wet mixture, but anyway here are the figures for a single load in a Belle 150 minimix. Cement 25 kg ~ 18 litres Water 12.5 kg ~ 12.5 litres (use less if sand and ballast is very wet) Sharp sand 59.5 kg ~ 35 litres Gravel 112 kg ~ 70 litres Total 209 kg ~ 135.5 litres However when mixed together the volume drops down to the 85-90 litres mark, but obviously the weight stays the same, so you end up with concrete of density around 2.4 tonnes per cubic metre. 35 litres is reckoned to be 6 No. 2 shovels, but the cautious newcomer is probably best using a bucket, a typical size is 14 litres, but if you make a mark at 12 litres and fill to this mark, then it’s 3 sand plus 6 of aggregate. Use a watering can to ‘calibrate’ the bucket. (works out about 7.5 -8 buckets [12 litres] of all-in ballast to a 25 kg bag of cement) VERY IMPORTANT To start the mix put about 7 litres of water into the mixer, then add the 3 buckets of gravel/aggregate, now you can either (1) add all the cement or (2) add some of the cement, then some of the sand, alternating until it is all (cement and sand) added, however after that stage you add the rest of the gravel/aggregate. NB keep your cement away from anything wet before it enters the mixer, i.e. separate bucket etc.. You can not dry mix like you do when doing it by hand on a mixing board, you need the drop in volume for it to fit, that’s the key! Adding the rest of the water is the tricky bit, as you don’t really know how wet everything was before you started, so you might not need all the remaining 5.5 litres or so of water. Gradually add some water and give it chance to mix (but not talking 10 minutes here!). Pointers, if the mix goes over the top it is too dry, ideally the paddles should lift it to about ¾ around the drum, if it just runs through the paddles you have added too much water*. Once all the water has been added, 2-4 minutes further mixing is all that is required, do not over mix, it can cause segregation of the aggregate and reduce the strength of the concrete. Main observation here is that at the magic ratio the mix suddenly goes from dry to just right, so you have to be cautious near to this ‘tipping’ point, as you can easily go too wet, but the concrete should be smoothish and flowing when at the correct cement/water ratio. *Only thing you can try to rectify this is adding more cement and ballast in the correct 1:5 ratio (the excess will probably slop out of the mixer, just take more care on the next mix). The readymix technicians perform a standard slump test to check this. Basically they fill a metal cone with the mixed concrete, upturn it on a flat surface and see how much the top drops down. Should be about 50mm. Slump test The freshly mixed concrete is packed into a 300 mm (12 in.) high cone, 200 mm (8 in.) wide at the bottom and 100 mm (4 in.) wide at the top, which is open. The concrete is smoothed off level with the top rim of the cone, and the cone is then carefully lifted, so that the concrete is left unsupported. The slump is the distance that the centre of the cone top settles. In a so-called ‘true’ slump test the base of the concrete does not spread excessively. If the concrete collapses or shears to one side the test results will be unreliable. Although the slump test does not directly measure the work needed to compact the concrete, it gives a reasonable indication of the how easily a mix can be placed and is simple to perform. The test is only suitable for reasonably workable, cohesive mixes. Very stiff mixes do not settle enough for useful measurements to be made and uncohesive mixes tend to shear or collapse. A slump of less than 25 mm (1 in.) indicates a stiff concrete and a slump of more than 125 mm (5 in.) indicates a very runny concrete. For the normal range the slump is 30-60 mm, best you can do is just get used to the correct degree of self support normal concrete exhibits when you make deep valleys in it using a shovel and a spot board. Hope this is helpful to someone.