Evaluating ECHO2 Biochar as Sustainable Bitumen Binder Modifier in Road Pavements: High-Temperature Performance Characterisation

The incorporation of bio-derived modifiers in bitumen binders presents a practical pathway toward sustainable, carbon-sequestering road pavement infrastructure. This study evaluates commercially produced ECHO2 softwood biochar as a modifier for Australian viscosity-graded C170 bitumen, combining microstructural, thermal, physical, and rheological characterization to assess its suitability as a high-temperature reinforcing modifier. In this study, biochar was incorporated at 3%, 6%, 9%, and 12% by weight, utilizing particles smaller than 75 µm to maximize interfacial interaction. Characterization via SEM-EDS, XRD, and TGA revealed a highly stable, carbon-rich, amorphous material with a rough, porous morphology, favourable for physical interlocking with the bitumen matrix. Physical and rheological investigations demonstrated that ECHO2 biochar measurably enhances binder stiffness and high-temperature deformation resistance. Compared with the control, 12% biochar modification reduced penetration by approximately 27% and increased the softening point by approximately 10%, indicating a reduction in temperature susceptibility. Dynamic shear rheometer (DSR) temperature sweeps highlighted substantial increases in the complex shear modulus (G*) and rutting factor (G*/sinδ) without altering the phase angle (δ), confirming the modifier acts as a rigid, particulate reinforcing agent rather than an elastomer. Multiple stress creep recovery (MSCR) testing supported these findings; non-recoverable creep compliance (Jnr) decreased progressively. Critically, under the AASHTO M 332 specification, while the neat bitumen binder barely met the standard traffic (S) criteria, the progressive reduction in Jnr (particularly at 12%) delivered a substantially higher factor of safety against rutting within the standard traffic designation. Finally, ECHO2 biochar demonstrates strong potential as a sustainable modifier that restricts viscous flow through particulate stiffening, enhancing high-temperature rutting resistance at elevated temperatures.