Permanent deformation prediction of asphalt concrete mixtures - A synthesis to explore a rational approach

Exploring a reliable laboratory test parameter to predict the rutting (permanent deformation) susceptibility of asphalt concrete (AC) mixtures remains a challenge to asphalt industry. Flow Number (FN) test as part of the HMA mix- and structural- design processes is used to characterize the AC mixtures rutting resistance and optimize field performance. Cognizant of the noise-characteristic of FN test data and need to apply post-processing techniques, this study explores a new parameter - FN Index as a ratio of traditional FN (cycles) and corresponding accumulated permanent strains. The FN and dynamic modulus (DM) tests are conducted on the Superpave gyratory compacted specimens. Twelve (six wearing course and six base course) mixtures were selected including Superpave, Asphalt Institute, British Standard dense bituminous macadam, and Pakistan's National Highway Authority gradations. Under the hypothesis of axial compression applied on the cylindrical specimen, first-order multiple linear regression models are developed for FN and FN Index as a function of the binder, volumetric parameters, gradation, and DM to estimate permanent deformation. The results indicate that FN models are strongly correlated with DM values at 38 °C and 54.4 °C temperature, gradation, volumetric, and Superpave rutting parameter. The predictive capability of developed models is validated using data of 16 different AC mixtures acquired from published literature. The findings of this study provide guidance to asphalt mix designers for providing cost-effective designs within specified levels of rutting resistance.