The basic and universal rules of stone matrix asphalt (SMA) design were described in the previous chapter. Chapter 7 provides an overview of SMA design methods developed in various countries. Undoubtedly, there are many of them, so their description could be the subject of a separate book. We will focus here on the most distinctive or the most interesting ones available in the technical literature.
The literature about SMA design methods can be both instructive and creative. You may judge for yourself which method most closely fits your needs or seems to have the most merit.
7.1.1 Description of the Method
The German method is based on long-standing experience in the application of repeatable materials and mixes. Such an approach not only makes analyzing cases of successful and unsuccessful SMA much easier but also drawing conclusions and ultimately proposing changes to technical specifications.
It was discussed earlier that the recommended ratios of SMA ingredients (see Table 2.1), combined with precisely determined gradations of each aggregate fraction supplied by quarries, enable SMA design in principle almost without the use of boundary gradation curves. Obviously, such gradation curves are being published— the first one for SMA was ZTVbit-StB 84—and then widely applied in practice. The new ZTV Asphalt-StB 07 and TL Asphalt-StB 07 standards have been in use since 1st January 2009.
The following stages may be identified in the German method:
• Design composition of an aggregate mix according to gradation limits
• Determination of a series of binder contents in the mixture
• Preparation of Marshall samples (2 x 50 blows) for each variant of SMA mixture
• Determination of the volumetric parameters of the SMA specimens
• Selection of an optimum variant of the mixture meeting requirements
• Air voids in compacted asphalt samples at 2.5-3.0% (v/v)[27]
• Voids filled with binder
• Draindown testing with Schellenberg’s method
• Wheel-tracking (rutting) test (for selected types of SMA)
The content of the coarse aggregate fraction specified in ZTV Asphalt-StB 07 amounts to 70-80% (m/m) for SMA 8S and SMA 11S but only 60-70% (m/m) for SMA 5N.*
