In recent years, we have learned how to produce a remarkable range of concrete products: ultra-high strength concrete, self-compacting concretes, corrosion inhibiting concretes, “tough” concretes (with the addition of ﬁbres), and now even sustainable concretes. In fact, we can now largely “tailor-make” concretes for virtually any project. However, in our approach to mixture proportioning, we still rely largely on prescriptive speciﬁcations, such as those described in Chapter 14. That is, speciﬁcations that generally include requirements such as maximum w/b ratios, minimum cement contents, cement types, limitations on the types and/or amounts of both chemical and mineral admixtures, on the amount of ﬁller material in the cement, and so on. These types of speciﬁcations have served us reasonably well in the past, when the cement and concrete industries as a whole were much less sophisticated than they are now. However, such speciﬁcations also tend to inhibit the most efﬁcient use of the materials now available to make up a concrete mixture.