Selecting the binder content in a design SMA mixture is relatively easy. With a correctly designed aggregate mix, it is enough to remember an appropriate content of voids in compacted samples. A thorough understanding of that subject will surely be made much easier by reading Chapter 7, including the description of both the U. S. and Dutch methods, and the section in Chapter 8 on preparing samples.
The majority of SMA guidelines have stipulated minimum contents of binder for a specific SMA mixture, and a limitation on the maximum quantity of binder has occasionally appeared. In each case, it should be kept in mind that these limits have been introduced in relation to the expected density of an aggregate mix (see Section 6.3.1.4).
Designing the binder content in SMA is the next stage of work after fixing the composition of an aggregate mix (using any method). Normally, the aim is to determine the content of the binder, that enables achieving the expected level of voids in compacted mix samples.
The method of compaction (Marshall versus gyratory) influences the final optimum binder content, therefore it is very important to use equivalent compactive efforts. For example, the number of rotations should equal 2 x 50 blows in Marshall or, alternatively, the number of gyratory revolutions should be standardized and used consistently. Improper parameters of gyratory compaction lead to misleading results of optimum binder content. The description of this topic is in Chapter 8.
Using analytical formulae that enable the determination of the optimum quantity of binder in a mixture is increasingly rare. These equations were invented based on the conversion of the specific surface area of an aggregate mix, and the determination of the film thickness needed to coat the aggregate. Nevertheless, it is necessary to say clearly that the probability of finding the optimum quantity of binder is not high because the most frequently used conversion factors were adopted for AC but not for SMA. Naturally, they do not take into account the specificity of forming voids among particles of a skeleton as we saw in Part I.
The a priori assumption of a specific content of binder in SMA is another very interesting aspect of selecting an optimum quantity of binder. Given an optimum binder content, an adequate aggregate mix is selected to allow the required amount of binder, making use of rules already known by the reader. The first of these relations is between the content of voids and gradation of the coarse aggregate fraction. This approach is used in the Dutch method (see Chapter 7).
