Performance of asphalt mixtures modified with bio-based hydrogenated styrene poly(β-farnesene) copolymer through morphology, mixture-scale evaluation, and mechanistic pavement simulation

This study evaluates asphalt mixtures modified with hydrogenated styrene poly(β-farnesene) copolymer (HSF), an 80% bio-based thermoplastic elastomer derived from sugarcane fermentation. HSF-modified binders contained 12.0 wt% HSF in a 50/70 binder and 10.5 wt% HSF in a 30/45 binder. These performance-selected laboratory dosages had previously provided rheological responses close to those of a commercial SBS-modified binder (60/85-E). Morphology was assessed by atomic force and fluorescence microscopy, and chemical characterization by FTIR spectroscopy. Mixtures were evaluated for fatigue resistance, permanent deformation, and moisture susceptibility, while mechanistic simulations employed the Huet-Sayegh model and ViscoRoute 3.1. HSF modification increased the dynamic modulus at 20 °C and 10 Hz by 42.4% and 34.5% for the 50/70 and 30/45 mixtures, respectively. Rut depth after 30,000 cycles decreased from 10.8% to 4.6% and from 7.8% to 4.4%, while the tensile strength ratio increased from 0.73 to 1.01 and from 0.81 to 1.01, respectively. The HSF-modified mixtures exhibited a stiffer and more strain-sensitive response under strain-controlled fatigue loading, reducing the admissible strain at one million cycles by 7.9% and 5.9% for the 50/70 and 30/45 mixtures, respectively. Mechanistic simulations indicated that under lower-strain structural conditions, the HSF-modified mixtures presented higher model-predicted fatigue lives than their corresponding unmodified mixtures, with fatigue-life ratios reaching up to 4.5. Overall, HSF should not be considered a direct replacement for SBS, but may be regarded as a candidate modifier for lower-strain structural conditions in which rutting and moisture-damage resistance are prioritized. These findings also highlight the need to improve polymer-binder compatibility.