Evaluation of the Effect of Dust and Soot on Runoff Acidity and Moisture Sensitivity of Asphalt Mixtures Using Thermodynamic and Mechanical Methods

AbstractThe loss of strength and durability because of the presence of moisture leads to moisture damage in asphalt mixtures. Differences in the drinking water characteristics, used in moisture sensitivity tests, and runoff water, which causes moisture damage in the asphalt pavements, motivated the present study on the effect of air pollutants, dust, and soot on pavement surface (as the surface contamination) on the acidity (pH) of rainfall-runoff. In addition, the effect of a change in the pH of runoff was investigated on the moisture damage of HMA samples. In this study, the modified Lottman method was employed as the common test of moisture damage in HMA and concepts of the thermodynamic theory were used to investigate the mechanism of changes in the process of stripping. Based on the results, the pH of the rainfall-runoff was acidic in areas with heavy traffic in which the accumulation of soot was dominant, and basic in areas with light traffic in which the accumulation of dust particles was more probable. The results of mechanical tests revealed that pH fluctuations reduced the asphalt mixture’s strength in wet conditions and increased the potential for moisture sensitivity. Also, more differences could be observed in the performance of asphalt mixtures at neutral pH and higher and lower values of pH during higher freeze–thaw cycles, which can be very important in areas with more rainfall cycles. The surface free energy (SFE) method results showed that the acidic and basic runoff increased the energy released at the asphalt–aggregate interface and the tendency of both acidic and basic aggregates to moisture, which led to decreasing the moisture strength of asphalt mixtures. In addition, the nonpolar properties of surface free energy of runoff were increased in both acidic and basic environments. Variation in water properties, in addition to the change in the runoff acidity and weakening of the aggregate-bitumen adhesion, also caused a weakening in the bitumen film and increased the cohesion failure. Also, statistical analysis results showed that in addition to the type of contamination, the type of aggregate affects the characteristics of runoff and consequently the performance of an asphalt mixture against moisture damage in different chemical conditions.