Assessment of Low-Temperature Fracture Behavior in Bituminous Mixtures Incorporating Sustainable Waste Additives: SCB Testing and Finite Element Analysis

In this study, the combined effect of waste lubricating oil (WLO) and waste pine branch ash (WPBA) was investigated to improve the low-temperature fracture behavior of asphalt mixtures. To investigate the low-temperature fracture behavior of the mixtures containing 2%–6% WLO and 3%–12% WPBA, the semi-circular bending (SCB) test was conducted at −15 °C, and the experimental results were analyzed using a statistical approach. Fracture toughness (K(IC)), total fracture energy (GF) and its pre-peak and post-peak components, crack resistance index (CRI), flexibility index (FI), and toughness index (TI) were determined. The findings indicate that the optimal combination depends on the fracture parameter under consideration. The best fracture toughness results, K(IC) = 24.96 MPa·mm0.5, indicating increased resistance to crack initiation, was obtained at 4% WLO and 12% WPBA content. On the other hand, the best fracture energy results, GF = 1.202 N/mm, reflecting an increase in energy dissipation during crack propagation, was obtained at 4% WLO and 9% WPBA content. The highest FI, CRI, and TI values were obtained with 6% WLO and 9% WPBA, corresponding to increases of approximately 66%, 69%, and 16%, respectively, compared with the reference mixture. This resulted in greater crack-initiation resistance, improved post-peak deformation capacity, and higher energy-dissipation capacity, although the greater WLO content led to a decrease in structural integrity. When the results are evaluated overall, it is understood that the use of WLO in the mixtures increases deformation capacity, while the use of WPBA controls crack propagation through its hardening and filling effects. In mixtures containing 4% WLO and 9%–12% WPBA, a balanced behavior was achieved in terms of crack initiation, crack propagation, and stiffness under low-temperature conditions. © 2026 by the authors.