My local gravel pit sells a 60/40 mix for mixing concrete. The problem is, I don't know how much portland cement to mix in. All I remember is years ago, my dad would do a shovel of this and couple of shovels of that and a few more of the other thing I have a regular trailer type electric motor driven mixer I bought that I will use to do the mixing, I just need the ratio. The cement will be used for a sidewalk. Thanks for any input.