Alternative interpretation of the adequate traffic levels of modified bitumens on Superpave®: a case study with crumb rubber and polyphosphoric acid (PPA)

This study proposes an alternative interpretation for the choice of modified bitumens on Superpave®, according to which heavier traffic levels described in the standardised bitumen classifications must comply with one additional requirement for the material to be graded as H, V or E. In order to show the application of this proposed interpretation, modified bitumens were subjected to MSCR tests at five temperatures (52°C, 58°C, 64°C, 70°C and 76°C) and four creep-recovery times (1/9, 2/9, 4/9 and 8/9 s). Crumb rubber and polyphosphoric acid (PPA) were used as modifiers to prepare three formulations: bitumen + PPA, bitumen + rubber and bitumen + rubber + PPA. The proposed criterion identified decreases in the traffic levels from E to V for the bitumen + rubber + PPA at 58°C and the bitumen + PPA at 64°C. This suggests that such bitumens may not be appropriate for heavier traffic levels, and a deeper investigation of their permanent deformation performance at the asphalt scale should be carried out.