Dispersed and Co-Continuous Morphologies of Epoxy Asphalt Bond Coats and Their Effects on Mechanical Performance

The co-continuous microstructure represents an ideal configuration for polymer-modified asphalts. Consequently, determining the optimum polymer content hinges on establishing this critical network between polymer and bitumen. In this study, epoxy asphalt bond coats (EABCs) exhibiting three distinct morphologies (epoxy-dispersed, co-continuous, and bitumen-dispersed) were prepared. Phase structure evolution and the final cured morphology were analyzed using a laser scanning confocal microscope (LSCM). Rotational viscosity–time characteristics, tensile properties, single-lap shear strength, and pull-off adhesion strength were characterized using various techniques. Results indicated that the viscosity of EABCs at the late stage of the curing reaction increased with increasing epoxy resin (ER) concentration, whereas the time required for EABCs to reach a viscosity of 5 Pa⸱s decreased. LSCM analysis revealed that EABCs exhibited three distinct morphologies dependent on ER concentration: (1) a bitumen-continuous morphology with dispersed epoxy domains (41–42 vol.% ER) formed via a nucleation and growth mechanism; (2) a co-continuous structure (43–45 vol.% ER); and (3) an epoxy-continuous structure with dispersed bitumen domains (46 vol.% ER). Furthermore, the EABC with 42 vol.% exhibited a transitional morphology between bitumen-continuous and co-continuous structures. A significant improvement in mechanical properties occurred during the transition from the bitumen-continuous (41 vol.% ER) to the co-continuous morphology (43 vol.% ER): tensile strength, elongation at break, and toughness increased by 524%, 1298%, and 2732%, respectively. Simultaneously, pull-off adhesion strength and single-lap shear strength rose by 61% and 99%, respectively. In contrast, mechanical properties increased only gradually during the co-continuous phase and the subsequent transition to an epoxy-continuous morphology (45–46 vol.% ER). Considering cost, rotational viscosity–time dependence, and mechanical performance, an ER concentration of 43 vol.% (within the co-continuous region) is optimal for EABC production.