Evaluating the Effects of Nanomaterials on Thermal Cracking of HMA in the Presence of Moisture with Different Degrees of Acidity

Several factors, including constituents, pavement structure geometry, and environmental conditions, influence the low-temperature performance of hot-mix asphalt (HMA). Moisture is a critical factor related to environmental conditions. It directly weakens the mastic phase of the HMA or leads to loss of aggregate-bitumen adhesion, thereby diminishing the integrity of the HMA structure. The moisture characteristics are caused by run-off changes in the presence of surface contaminations; one of the most significant changes is the run-off acidity or basicity (pH) variation. This study evaluated the effect of acidic (pH=5 and pH=6), basic (pH=8 and pH=9), and neutral moisture conditions (pH=7) on bitumen and HMA performance against thermal cracking. Given the unique properties of multiwalled carbon nanotubes (MWCNTs) and nanographene oxide (NGO), including hydrophobicity, high tensile strength, high modulus of elasticity, and effective outcomes in water treatment industries, these two materials were used as bitumen additives at 0.3% and 0.6 wt.% to enhance the bitumen and HMA performance against thermal cracking under different moisture conditions. The bending beam rheometer (BBR) and the semicircular bending (SCB) tests were conducted on bitumen and HMA samples under different moisture conditions. The BBR test results showed that changing the pH of water leads to a 43% rise in creep stiffness and a 24% decline in m-value, on average. The SCB test results revealed that acidic, basic, and neutral moisture conditions decreased the fracture toughness (FT) and fracture energy (FE) of HMA samples and diminished their low-temperature performance. The acidic and basic moisture conditions had a greater impact on decreasing the FE and FT of HMA, respectively. Moreover, the HMA samples containing limestone performed better under different moisture conditions against thermal cracking, compared with the specimens that contained granite. The incorporation of MWCNTs and NGO and increasing their weight percentage improved the strength of HMA against thermal cracking under different moisture conditions; NGO at 0.6% by weight of bitumen had the best performance in enhancing the FE and FT of the HMA. In addition, MWCNTs and NGO increased the m-value and creep stiffness of the bitumen. However, these nanomaterials are extremely effective in the interaction between bitumen-aggregate and have improved the performance of the HMA against thermal cracking despite the increase in bitumen creep stiffness. According to the statistical analysis results, changing the water pH significantly affected the creep stiffness and m-value of the bitumen. The application of acidic, basic, and neutral moisture conditions significantly reduced the FE and FT of HMA. However, the use of NGO at 0.6% by weight of bitumen significantly elevated the FE and FT of HMA.