Q: This may be fundamental, but why is it possible for a 1:2 1/2:4 concrete mix, volume batched, to be stronger than a 1:2:4 mix?
The proportion of cement in the 1:2:4 mix is after all greater.
The answer to this one rests in the fact that the strength of concrete depends on its density; thus a mix containing more sand may be stronger because it is denser.
For example a coarse aggregate graded from 3/4 to 3/16 inch has about 40 percent voids. A 1:2:4 mix is designed on the assumption that if 50 percent sand is included it will, as a sand-cement mortar, both fill these voids and provide a coating for all the larger aggregate particles where they touch and there are no voids between them. The sand used for the mix may however have been damp when delivered; its volume could then be anything between 20 and 40 percent greater than sand that is thoroughly dry or completely saturated with water. During mixing the sand will become saturated and its volume is therefore reduced by say, some 25 percent. It can thus be seen that, if a damp sand is used in the proportion of say 1 cubic foot to 2 cubic feet of coarse aggregate, the actual volume of sand included is only three-fourths of a cubic foot. In other words the sand content is 25 percent less than was assumed, that is 37 1/2 percent and not 50 percent of the volume of the coarse aggregate. The result is that the voids in the coarse aggregate are insufficiently filled and the concrete is less dense. Conversely increasing the proportion of damp sand to 2 1/2 parts means that there will actually be 2 parts of sand to 4 parts of coarse aggregate, which gives the desired 50 percent figure and a denser, and consequently stronger, concrete.