For many people the surface appearance of an SMA course gives the impression of being excessively water permeable. Indeed, SMA differs from AC due to a substantially deeper macrotexture. SMA courses with an increased content of voids may occasionally be seen in practice, most frequently when the mixture was improperly compacted on a work site (such cases were elaborated on in Chapter 11). However, there is more to permeability than work site error.
Permeability is related to the content of air voids and the size, distribution, and existence of interconnections between internal pores. The permeability of a mixture depends on the maximum aggregate size, the thickness of the course, and the level of compaction (WsDOT, 2005). Research has shown that the larger the maximum aggregate size in a mixture, the higher its permeability (Cooley et al., 2002), which means an increase in maximum aggregate size results in the growth of the pore sizes and an increased probability of their connection (Cooley and Brown, 2003). The influence of VMA is also seen; permeability decreases as VMA increases for constant air voids (Brown et al., 2004).
To reduce the permeability of courses, one should appropriately match the mixture gradation to the course thickness so that the ratio of thickness to the maximum aggregate size in a mixture is not less than 3.0 (a ratio closer to 4.0 is preferred for SMA with a strong gap gradation). This should make the compaction of the course easier, reduce the content of voids, and limit connections among pores (Cooley et al., 2002). The following list summarizes the important points regarding permeability:
• The larger the maximum aggregate size in a mixture, the higher its permeability.
• The higher the content of air voids in a course, the higher its permeability; however, mixtures with the same content of voids can have different levels of permeability.
• The gradation of the SMA mixture (more or less gap graded) influences the permeability.
• The thicker the course, the lower its permeability.
