Effects of short-term aging of base and nano-modified bitumens on surface free energy and their relationship with moisture sensitivity in warm-mix asphalt

Abstract

The warm-mix asphalt (WMA) has become increasingly popular because it reduces the possibility of aging during construction and paving. However, its susceptibility to moisture-related distresses remains a concern, as such damages lead to additional pavement distresses, and the aging that still occurs can further intensify these effects. To this end, this research examined the effect of two types of nano materials, Nano Hydrated Lime and Nano Calcium Carbonate on the moisture sensitivity of base and aged WMA and explored the effects of these additives on thermodynamic parameters. To evaluate these effects, twenty different asphalt mixtures were evaluated using the modified Lottman test under multiple freeze-thaw cycles, along with complementary surface free energy measurements. The results indicate that the additives increased the debonding energy at both base and aged states, indicating a considerable improvement in moisture sensitivity of WMA, which aligns with the indirect tensile strength results. Moreover, bitumen modification with these additives remarkably increased the cohesion free energy by 63% and 53% for PG 64-14 and 58-22 bitumens, respectively. Regarding bitumen-aggregate adhesion, bitumen modification increased the adhesion free energy by 12% on average, indicating the additives' proper performance for low-adhesion WMA. Finally, the correlation coefficient between the mechanical index of moisture susceptibility and the bitumen-aggregate energy released during the stripping process was 89% and 85% in the fifth freeze-thaw cycle for the base and aged states, respectively. This strong correlation between the results demonstrates the need to use thermodynamic concepts in estimating the effect of bitumen aging on moisture susceptibility of WMA.