Impacts of carbon nanotubes and nano-graphene oxide on fatigue cracking in HMA exposed to runoff with varying acidity.

Hot-mix asphalt (HMA) fatigue cracking is governed by external factors, including temperature, precipitation, and traffic loading, alongside internal factors, such as bitumen type, bitumen percent, air void, and the aggregate's mineralogy and physical properties. As a critical external factor, moisture, particularly runoff with varying acidity, exacerbates fatigue cracking through physical and chemical interactions. The use of advanced additives such as nanomaterials in HMAs enhances their performance against various types of distress. Carbon nanotubes (CNTs) and nano-graphene oxide (NGO) are nanomaterials that exhibit exceptionally high elastic resistance and desirable hydrophobic properties. Their application has shown promising results in the contaminated water treatment industry. Given that the providing solutions to mitigate the negative effects of acidic and alkaline waters on the intermediate-temperature performance of HMAs has not been widely discussed, this study employs CNTs and NGO to mitigate the damaging effects of acidic and alkaline runoff from surface pollutants on the fatigue performance of bitumen and HMAs. This study employed dynamic shear rheometer (DSR) and semi-circular bending (SCB) tests under dry, neutral wet, acidic wet (pH = 5 and 6), and alkaline wet (pH = 8 and 9) conditions to evaluate the fatigue cracking potential in different bitumens and asphalt mixtures. Limestone and siliceous aggregates, differing in moisture susceptibility, were combined with PG 58-22 bitumen to fabricate HMA. Carbon nanotubes (CNTs) and nano-graphene oxide (NGO) were also incorporated at 0.3% and 0.6% by the bitumen's weight. Test results on the bitumen and HMAs under different moisture conditions show that altering the pH of pure water significantly degrades the bitumen's rheological properties and the mixture's mechanical fracture resistance. Conversely, CNTs and NGO substantially reduced G*Sinδ while boosting fracture energy and toughness. In other words, these two nanomaterials reduced the fatigue parameter of the bitumen under dry, acidic, and alkaline moisture conditions, demonstrating their outstanding performance in enhancing the fatigue resistance of the modified bitumen against intermediate temperature cracking under various moisture exposures. Furthermore, the mixtures modified with CNTs and NGO withstood higher energy for cracking at intermediate temperatures when exposed to acidic and alkaline moisture conditions, while also increasing the critical stress threshold required for final failure. This indicates that CNTs and NGO, even when the detrimental effects of acidic and alkaline runoff dominate their properties, improve the performance of both the bitumen and the mixture against intermediate-temperature cracking. The specimens with 0.6% NGO exhibited exceptional fatigue resistance across all conditions. These findings highlight NGO at 0.6% concentration as a superior enhancer of asphalt resilience against runoff-induced fatigue deterioration.