How much sand is in 1 cubic yard of concrete using 6 portlands per cubic yd?

Answer 1

Where I am, concrete would be made in the proportions of 1 of cement, 2 of sand, and 4 of gravel. River gravel, by definition is stronger material than the fines and sand, for it has better withstood the travel down the river. Thus river gravel will always make a stronger concrete that material quarried from a hillside, even if nominally the same rock type.

The cement should cover every material surface inside the concrete. That is why too much sand (with its proportionately higher surface to volume ratio) will weaken the concrete.

The setting reaction is a complex set of calcium carbonate, sulphate, and silicate reactions which take considerable time to form the crystals which give the strength to the concrete. And, of course, any water surplus to the crystal formation will leave (minute) voids that will give a weaker concrete.

So for concrete constructions in which strength is vital (bridges, slabs) a long drying period is preferred, and the concrete should be kept moist for as long as possible.