Permeability of Intact Asphaltic Mixtures

The permeability of asphaltic mixtures is controlled by the size and interconnec­tion of the void space. To illustrate this, some recent data is given in Fig. 5.1. This presents results for various hot-mix asphaltic specimens as described in Table 5.1. Figure 5.1 shows that permeability values of intact asphaltic materials are typically in the 0 to 40 x 10-6 m/s range. It is apparent that permeability is insignificant at less than approximately 7% air voids but can then rapidly increase. Probably this is be­cause interconnection of voids becomes possible at these high air void ratios and be­cause mixtures that exhibit such air void proportions may be inadequately prepared leaving permeable fissures in the material’s structure. Other authors (Zube, 1962; Brown et al., 1989 and Santucci et al., 1985) conclude that a limit of 8% air voids should be adopted to avoid rapid oxidation and subsequent cracking and/or ravelling and to keep permeability low.

Furthermore, higher permeability values are associated with asphaltic mixtures having larger voids. Larger voids are found both in fine-grained mixtures having high in-situ air void contents or in coarser grained mixtures at lower void contents. For example, Table 5.2 summarises the air void content at which a threshold be­tween essentially non-permeable and permeable behaviour was observed in-situ,

together with the permeability coefficient at that point (Cooley et al., 2001). The laboratory derived data of Fig. 5.1 tells a similar story although with a threshold void content of 8-9% for a permeability of 10 x 10-6 m/s. Both data sets reveal that the coarse, low-fines mixtures are least well-performing. In Fig. 5.1, Mixture 3, the poor performance is seen in the rapid increase in post-threshold permeability, while in Table 5.2 the threshold air-void content at which permeability increases is much lower.