Rheological and Morphological Characterization of Asphalt Cement Hybrid-Modified with Waste-Derived Cellulose Fibers and Crumb Rubber

Conventional bitumens may not adequately withstand increasing traffic loads and environmental stresses, necessitating the development of more durable and sustainable modified binder systems. This study investigated the rheological, workability, and morphological characteristics of a 50/70 penetration-grade asphalt binder hybrid-modified with waste-derived cellulose fibers and crumb rubber. Penetration, softening point, Brookfield viscosity, Dynamic Shear Rheometer (DSR), Scanning Electron Microscopy (SEM), and Fourier Transform Infrared Spectroscopy (FTIR) analyses were performed. The hybrid-modified binder exhibited a penetration value of 55 ± 2 (0.1 mm) and a softening point of 53 ± 1 °C. The G*/sinδ criterion was satisfied at 66 ± 1.5 °C, while the corresponding critical temperature reached 71.2 ± 1.2 °C after ageing, indicating enhanced high-temperature deformation resistance. However, the viscosity reached 2367 ± 45 mPa·s at 150 °C, indicating a trade-off between high-temperature performance and workability. SEM analysis revealed that modification transformed the relatively smooth and homogeneous surface of the base bitumen into a rougher, more heterogeneous, and structurally complex morphology containing partially embedded modifier-rich domains. Although localized agglomeration indicated that completely uniform dispersion was not achieved, the observed morphological changes supported the development of a physically reinforced binder structure. FTIR analysis showed that the principal chemical framework of the base bitumen was preserved following modification. Variations in the fingerprint region indicated contributions from the cellulose and crumb rubber phases, whereas the absence of distinct new absorption bands suggested that the modification mechanism was predominantly governed by physical blending, crumb-rubber swelling, mechanical interlocking, and weak secondary intermolecular interactions rather than the formation of new covalent bonds. Overall, the combined rheological, SEM, and FTIR findings demonstrate that waste-derived cellulose fibers and crumb rubber can improve asphalt binder performance while promoting waste valorization and supporting circular economy principles.