Analysis of the Relationship Between Air Voids Content, Hydraulic Conductivity, and Gas Permeability in Asphalt Mixtures

The air voids content in asphalt mixtures plays a critical role in influencing flexible pavements' mechanical and geometrical properties, which can lead to variations in hydraulic conductivity and gas permeability. This study presents an experimental investigation into the permeation characteristics of asphalt mixtures, with air voids content (Vm) varying from 1.2% to 14.1%. Two hot asphalt mixtures, designed for binder and wearing layers, were tested using over 100 specimens under different boundary conditions. Hydraulic conductivity (kw) was measured according to ASTM D 5084–00 under constant-head condi- tions. The results show that when V m is below 6%, kw ranges from 1 × 10 –10 to 1 × 10 –8 m/s, while for V m values above 4%, it ranges from 1 × 10 –7 to 1 × 10 –5 m/s. Specimens with V m between 4 and 6% exhibited random permeability to water, with the 5% air voids content emerging as a threshold that delineates low and high pressure sensitivity in the mixtures. Gas per- meability (kg) was evaluated following ASTM D 4525–90 at axial pressures from 0.1 bar to 0.5 bar, with kg values between 1.51 × 10 –23 m 2 and 3.44 × 10 –19 m 2 . Additionally, 15% of the specimens were impermeable to gas. Both hydraulic and gas permeabilities were positively correlated with air voids content, with a regression coefficient no lower than 0.76. Further investigation needs to describe water and gas permeation in the critical air voids range and interpret their role in moisture damage as blistering and water seeping.