Q.: We repeatedly hear that the strength of concrete depends almost entirely on the water-cement ratio. Yet I cannot understand why two mixes, one containing four bags of portland cement per cubic yard and the other containing six bags per cubic yard, could have the same strength. The second has 50 percent more cement than the first. Isn't the second mix really stronger at the same water-cement ratio?
A.: The two mixes will be very close to the same strength. Suppose that you make two mixes of cement and water only. One has four bags of cement and the other six. Both contain, let us say, half as much water as cement by weight. When they have set and cured they will both have the same strength, though one will occupy a larger volume than the other.
Now mix two more batches exactly like the first ones. Into each one add enough saturated surface-dry aggregate to make a total volume of one cubic yard. Obviously one mix will take a little more aggregate than the other. Now when they have cured the weakest link in both masses will be the cement-water portion: the cement paste. The cement paste will determine the strength. But we've noted before that both pastes have the same strength; consequently both concretes will have the same strength.
There are, of course, some slight differences in strength of the two mixes, because there is less aggregate surface to be bound in one mix than the other, but the difference is minor.
There is a qualification to the above discussion: The explanation does not apply to concretes in which the aggregate is weaker than the cement paste. This is likely to be true, for example, of concretes made with perlite, vermiculite or polystyrene aggregate.