Optimizing the bio-based additive content considering bitumen microstructure and HMA mechanical properties

Lignin, a byproduct of paper production, shows potential for use in asphalt mixtures due to its stiffening effect on bitumen, though it may increase brittleness. This study investigated lignin (10 %, 15 %, 20 %) and cellulose nanofibers (CNF; 0.5 %, 0.75 %, 1.0 %) as bio-based additives, both individually and compositely. To evaluate the quality of additive mixing in the bitumen microstructure, the Energy Dispersive Spectroscopy mapping (EDS) method was used. The mechanical property evaluation of the asphalt mixture comprised indirect tensile strength testing (ITS) as per AASHTO T283, along with dynamic creep and beam bending tests. Dynamic creep was performed at 40°C, and beam bending was conducted at 20°C using the constant strain method. The results obtained from the EDS mapping analysis indicated the potential for better distribution of lignin compared to CNF in bitumen. For specimens containing only one of the additives, the maximum suitable amount of lignin was 15 %, and for CNF, it was 0.75 %. For composite specimens, the best microstructure was observed in the specimen containing 10 % lignin and 0.5 % CNF. In terms of durability criteria, the combination of 10 % lignin and 0.5 % CNF had the best performance. In creep resistance, the asphalt mixture containing lignin exhibited excellent performance, while CNF had no substantial impact on the results. Therefore, from the perspective of high-temperature behavior, using higher amounts of lignin is desirable. However, analyses conducted based on four different fatigue definitions showed that increasing lignin and CNF amounts continuously reduced and increased the fatigue life of the mixture, respectively. For mixtures modified with only one of these additives, the most suitable amount of lignin and CNF for fatigue resistance was 10 % and 1.0 % of bitumen weight, respectively. One of the interesting results was the behavior of the mixture containing the lignin and CNF composite. Irrespective of the fatigue life definition, the asphalt mixture containing 10 % lignin and 1.0 % CNF exhibited the highest fatigue resistance among all those studied. Overall, considering the combined results of tensile strength, durability, and high-temperature performance, the composite specimen LIG15-CNF0.75 can be identified as the most promising formulation, with LIG10-CNF0.75 also exhibiting strong performance, particularly in ITS and durability aspects.