Modern road construction constantly faces the challenge of balancing long-term durability with environmental sustainability. Traditional asphalt binders are non-renewable petroleum resources that degrade over time through oxidative aging, traffic-induced rutting, and fatigue cracking. To address these issues, researchers are increasingly exploring circular economy strategies that valorize agricultural and industrial waste streams into useful engineering materials. By transforming organic by-products into carbon-rich biochar, pavement engineering can benefit from sustainable modifiers that enhance mechanical behavior while mitigating environmental impacts.

European hazelnut shells represent an abundant agricultural residue that can be converted into high-value biochar via slow pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More. When produced at optimized temperatures, this biochar retains significant antioxidant properties and oxygen-containing surface functional groups. Evaluating how different particle sizes and modifier concentrations influence asphalt binder rheology provides vital data for scaling up green infrastructure solutions. Smaller particle fractions allow for superior physical dispersion and intimate contact within the bitumen matrix, optimizing the overall structural reinforcement of the road material.

Operational testing shows that introducing biochar increases rotational viscosity and requires slightly higher mixing and compaction temperatures, depending on the modifier concentration and particle dimensions. Despite the higher flow resistance, all modified binders exhibit excellent storage stability, remaining homogeneous without significant separation over extended periods at elevated temperatures. This stability ensures that the biochar remains uniformly dispersed throughout the binder matrix during storage and handling at industrial scales.

At high service temperatures, biochar significantly improves rutting resistance by increasing the rutting parameter and reducing non-recoverable creep compliance under repeated loading cycles. Smaller particle sizes yield superior high-temperature performance compared to larger fractions, effectively restricting permanent deformation under heavy traffic loads. Conversely, low-temperature testing demonstrates that while biochar slightly increases creep stiffness, it preserves stress relaxation capabilities and satisfies all standard specification limits required to prevent thermal cracking.

Master curve analysis and rheological aging evaluations reveal that biochar-modified binders exhibit lower susceptibility to long-term aging-induced stiffening. The inherent antioxidant compounds and phenolic content present in the hazelnut shell biochar help inhibit oxidative hardening during both short-term and long-term aging simulations. Furthermore, fatigue evaluations under cyclic loading conditions show that the optimized biochar formulation sustains higher accumulated dissipated energy and withstands more loading cycles before significant structural degradation occurs. Ultimately, utilizing 5.0 weight percent of fine hazelnut shell biochar offers a balanced, high-performing, and sustainable alternative to conventional asphalt modifiers.

Source: Martínez-Toledo, C., Valdés-Vidal, G., Calabi-Floody, A., Leiva-Villacorta, F., Concha, J. L., & Norambuena-Contreras, J. (2026). Balanced asphalt binder performance with hazelnut shell biochar. Cleaner Materials, 100442.