Our aim in designing an SMA’s aggregate structure has already been identified—a strong skeleton of coarse grains. Let us now consider what requirements an aggregate mix has to meet to create such a desirable skeleton. There is no room in it for too many fine or weak grains. The key solution for that question is gap gradation—that is, the right proportion of grains of defined sizes.
Let us start by examining a continuous gradation. If we want to design an aggregate mix with a maximum density (or otherwise, with a minimum void content), we should create it from an aggregate with a roughly equal share of grains from consecutive fractions. In other words, such a mix should contain a proportionally even quantity of all fractions. We would call this type of gradation a continuous gradation. The appearance of grains of different sizes makes closer packing in a volume unit possible. This also minimizes the volume of voids among grains. Asphalt concrete is an obvious example of a continuously graded mixture (Figure 2.5; solid line).
So what is a gap gradation? Gap gradation is a disruption in the occurrence of consecutive aggregate fractions in an aggregate blend; that disruption results from a lack or minimal amount of one or more aggregate fractions. Looking at Figure 2.4, we can see the formation process of a skeleton with coarse grains and some of the finest grains but without the sizes in between. Gap gradation means a lack or minimal share of specified fractions of intermediate aggregates. The role of the gap gradation is so essential that the lack of definite size grains must be evident. But which fraction or sizes of grains or fractions? Here we have a couple of definitions and methods.
