Until relatively recently, concrete mix materials were selected and proportioned on the basis of certain general principles established through empirical tests, past experience with similar materials, and published tables. Increasingly, however, wc. need to produce high-strength concretes with what has been considered sub-optimal or even marginal quality materials. And, whereas in the past we could afford to achieve high strengths at age 28 or 56 days, there is much greater emphasis now on achieving strengths in excess of 8,000 psi in 24 hours or less.

In response to these new demands for high-strength concretes, the concrete industry is setting aside many of the traditional methods for selecting and proportioning materials. Now, mix materials are selected deliberately for the characterisitics of their microstructure (e.g., the mineralogy and particularly crystal structure of the aggregates), and are proportioned to optimize the microstructure of the concrete.

In this paper the author illustrates how the characteristics of a material's microstructure are used as tools in selecting and proportioning mix materials. The significance of the microstructure of the aggregate particles, mortar-aggregate interface, and mortar are discussed in detail.