Performance-based mix design and low-temperature fracture behavior of cellulose nanocrystal (CNC)-modified HMA

Nanotechnology has emerged as a promising approach for enhancing asphalt pavement performance, and cellulose nanocrystal (CNC) offers a sustainable, bio-based alternative due to their high tensile strength, stiffness, and renewability. This study evaluates the effects of CNC modification on hot-mix asphalt (HMA) performance through an integrated Balanced Mix Design (BMD) and fracture mechanics framework, with particular emphasis on low-temperature cracking behavior. Asphalt binders were modified with CNC at 0.5%, 1.0%, and 1.5% by weight, and mixtures were tested for rutting resistance, intermediate-temperature cracking, moisture susceptibility, and low-temperature fracture behavior using Semi-Circular Bending (SCB) specimens under Mode I and Mode II loading at 0°C, –10°C, and –20°C. Results indicate that CNC significantly enhances low-temperature fracture properties, increasing fracture energy (G f ) and fracture toughness (K I , K II ). Mode I behavior showed consistent improvement across all CNC levels, reflecting enhanced crack initiation resistance and global fracture tolerance, whereas Mode II fracture exhibited a non-monotonic trend: G f increased up to 1% CNC and decreased at 1.5% likely associated with particle agglomeration, while K II continued to rise, indicating increased crack-tip resistance despite reduced post-peak ductility. Crack path analysis revealed that lower temperatures intensified brittleness and aggregate fracture, with Mode I cracks following nearly vertical paths and Mode II cracks showing angled, tortuous trajectories. Rutting and moisture performance were also positively influenced by CNC, with flow number peaking at 0.5% and flexibility index and tensile strength ratio maximized at 1%, highlighting the need to optimize CNC content for balanced performance. Statistical analysis confirmed significant effects of CNC on fracture, rutting, and moisture indicators, and minor temperature interactions in shear-dominated fracture. Overall, 1% CNC provides the optimal balance, enhancing low-temperature fracture resistance, intermediate-temperature cracking, rutting performance, and moisture durability. These findings establish a mechanistic understanding of CNC reinforcement at the nanoscale, demonstrating its potential to improve asphalt mixture fracture tolerance in cold climates while contributing to sustainable pavement design.