Fineness Modulus ...

Okay, if we look at the def. on p. 184, we see the words `sum of the cumulative' percentage weight retained. What this means is that a bunch of sand with larger particles is gonna get more stuff hung up early, which gets counted again and again in the `summing of cumulative' process - thus higher FM means larger particles. Alternately, if the `handful' of sand is all really, really fine, and say it all pass through the sieves and gets hung on the one at the end (bottom), we will get a smaller cumulative amount, and thus smaller FM.

... that's why, then, Table 7.5 for example, demands less coarse agg in a mix with higher FM ... I think.