Investigating the effect of aging on the fatigue cracking mechanisms of modified asphalt concretes based on surface free energy theory and mechanical tests

Fatigue cracking is a major distress in asphalt concrete, with aging identified as a key factor. This study investigated the effects of aging on bitumen–aggregate combinations by evaluating surface free energy (SFE) components of bitumen and aggregates, and calculating adhesion and cohesion energies. Also, the direct Pull-Off tensile test (in cohesive and adhesive failure modes) and indirect tensile fatigue (ITF) test were performed to assess the influence of aging and additives. To mitigate aging effects, ceramic fibers (CF) and granular hydroxy functionalized ultra-high-molecular-weight Polyethylene (FUHMWPE) were incorporated at 1.5% and 3% dosages. Results from SFE analysis were compared with Pull-Off and ITF outcomes to establish a clear relationship between adhesion/cohesion properties and fatigue life of mixtures. The SFE results indicated that aged bitumens, due to their lower non-polar component, form weaker non-polar (covalent) bonds with aggregates, which reduces the bitumen–aggregate adhesion free energy. Consistent with these findings, the pull-off test also showed that the mechanical adhesive strength of aged bitumens with aggregates is lower. The weakened bitumen–aggregate adhesion under aging conditions allowed micro-cracks caused by fatigue in the asphalt concretes to propagate along the bitumen–aggregate interface, thereby reducing the mixture's fatigue life. Furthermore, a decrease in ambient temperature within the intermediate service temperature range of pavements, combined with aging conditions, exacerbates the loss of mixture resistance against fatigue cracks occurring at the bitumen–aggregate interface. In contrast, CF significantly increased the cohesive and adhesive strength of the bitumen–aggregate system under aging conditions, improving the fatigue life of the asphalt concretes by an average of 22%. Similarly, the significant effect of FUHMWPE on enhancing bitumen–aggregate adhesion and cohesion led to an average 27% improvement in fatigue life. This indicates that FUHMWPE has a greater effect than CF on improving the intermediate-temperature properties of the asphalt concrete, owing to the polar functional groups in its polymer structure, possesses higher SFE. This promotes stronger polar interactions with aggregates, resulting in increased adhesive interfacial energy at the bitumen–aggregate boundary and, consequently, greater resistance to fatigue cracks. Statistical analysis results also confirm that both additives at a dosage of 1.5% significantly improve adhesion/cohesion-related properties and fatigue performance. • Aging reduced adhesion energy, increasing fatigue cracks at the bitumen–aggregate. • CF and FUHMWPE improved SFE and reduced cohesion failure in aged asphalt mixtures. • Modified binders showed enhanced interface bonding, improving fatigue resistance. • Pull-Off and SFE results showed strong agreement in assessing cracking mechanisms. • Limestone showed better adhesion than granite, reducing fatigue cracking potential.