In some countries, SMA mixes have been applied in lower layers of the pavement structure. They are usually coarse-graded SMA mixes from 0/16 to 0/22 mm. Such solutions have been tested in the United States and recently in Germany, where they are called Splittmastixbinder (SMB) (Gartner et al., 2009; Schunemann, 2006). Because of the high binder contents, commonly a polymer modified binder with a stabilizer as well, the fatigue properties of the pavement are definitely better than those of conventional asphalt concrete. The application of a strong aggregate skeleton increases the resistance to rutting.
In many cases, SMB 0/16 with a hard modified binder may be better than a conventional asphalt concrete layer. Experimental roadway sections in Bavaria (Germany) on the highway A73 are good examples of such an application (Gartner et al., 2009). The SMB mix has been designed with target air voids in Marshall specimens between 3.5 and 4.0% (v/v) and a minimum binder content of 5.0%. A hard modified binder (pen@25°C = 10/40, SP > 65°C) has been used. Figure 13.4
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FIGURE 13.4 Grading curves (limits) of the SMB mix used on Highway A73 in Germany. (Data from Gartner, K., Graf, K., and Schunemann M., Asphaltbinderschichten nach den Splittmastixprinzip. Strasse und Autobahn, July 2009 With permission.)
shows the grading curves of the designed SMB mix. The thickness of the compacted layer has been defined as 7.5 cm, the required compaction factor greater than or equal to 98%, and the content of air voids in the finished course from 2.0 to 6.0% (v/v).
The test results on SMB mixes with hard grade polymer modified binder have confirmed that an intermediate course of this type can be a better solution than the conventional asphalt concrete; with a similar resistance to rutting, the fatigue life of SMB is remarkably better (Schunemann M., 2006).
