Investigation on viscoelastic behavior of modified asphalt binder with a composition of waste edible oil and biochar: A laboratory study

While waste edible oil (WEO) shows promise as an asphalt rejuvenator, its practical application is hindered by poor moisture resistance and thermal instability. Prior studies have mainly targeted acid value reduction, neglecting high-temperature performance and moisture degradation. This study proposes a synergistic solution, combining transesterified WEO methyl ester (WEOME) with treated biochar to address these fundamental limitations. Initially, a dual-stage catalytic modification process involving acid-catalyzed esterification followed by base-catalyzed transesterification was employed to reduce waste edible oil's acid value from 2.06 to 0.16 mg KOH/g. Then, olive kernel-derived biochar was produced via slow pyrolysis at 450 °C. Three binder formulations were systematically evaluated: virgin binder (VB), 6% WEOME-modified binder (K2B), and 6% WEOME + 3% biochar composite (K3B). In order to investigate the chemical formation, viscoelastic behavior, and quantify moisture susceptibility of this dual-modifier system, a comprehensive laboratory evaluation was performed including Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), binder fast characterization (BTSV), master curves, multiple stress creep and recovery (MSCR), and Glover-Rowe (G-R) parameters. Also, to evaluate mixture durability, the indirect tensile strength (ITS) test was conducted on Marshall specimens in dry condition and after one and three freeze-thaw (F-T) cycles. Chemical and molecular analysis showed that K3B exhibited superior oxidative resistance with lower carbonyl and sulfoxide indices, effectively retained larger molecular fractions than VB and K2B, and showed improved phase stability with a modified glass transition temperature. The rheological performance of K3B showed superiority, with the highest softening temperature and lowest phase angle, confirming the higher complex modulus master curves, indicating a 50% strain reduction and improved recovery compared to VB and K2B. The analysis also highlighted a 10% reduction in shear thinning of K3B compared to 23% for K2B and 28% for VB, while the parameters placed K3B below the fatigue cracking threshold, in contrast to the higher values for K2B and VB. The ITS results showed that K2B reduced tensile strength and increased susceptibility to F-T damage, whereas the K3B composite significantly improved mixture durability. The composite K3B formulation, combining WEOME with olive kernel-derived biochar, effectively integrates the rejuvenating effect of WEOME with the stabilizing role of biochar. This complementary action improves high-temperature stability, elastic recovery, oxidative resistance, and moisture susceptibility compared to the VB and the K2B, providing a more balanced viscoelastic performance suitable for sustainable pavements.