Granular flexible recycling of semi-rigid base asphalt pavements using foamed and emulsified asphalt

The rehabilitation of semi-rigid base asphalt pavements generates both reclaimed asphalt pavement (RAP) from the surface layer and reclaimed inorganic-binder-stabilized material from the base layer. This study investigated a granular flexible recycling approach using foamed asphalt and emulsified asphalt for two practical scenarios: recycling of surface-layer RAP and composite recycling of surface RAP with reclaimed semi-rigid base material. Binder preparation, gradation reconstruction, optimum water content, recycling-binder content and cement-assisted mixture design were followed by scanning electron microscopy (SEM), unconfined compressive strength, compressive resilient modulus, indirect tensile strength (ITS), moisture resistance and dynamic modulus/master-curve evaluation. Type-I 70# asphalt showed the best foaming performance at 160 °C and 3.0% foaming water, with an expansion ratio of 17.8 and a half-life of 14 s. The selected emulsified asphalt was prepared using 95 °C water, 150 °C asphalt, 1.5 min emulsification, 1.5% Type-A emulsifier, 62% asphalt content, 2% stabilizer and pH 2. The optimum foamed asphalt content was 3.0% for both reclaimed-material systems, whereas the optimum emulsified asphalt contents were 4.5% and 4.0% for the surface-layer and surface-base systems, respectively. The optimized mixtures achieved dry ITS values of 0.57–0.63 MPa and wet/dry strength ratios of 90.6–94.2%, exceeding the JTG/T 5521–2019 base-layer design thresholds of 0.50 MPa and 80% for heavy-and-above traffic. At 20 °C and 10 Hz, the EACR systems exhibited higher dynamic modulus, whereas the FACR systems exhibited higher compressive and dry tensile strength under the tested curing condition. Because Type-I 70# asphalt was used for FACR and Type-I 90# asphalt was used for EACR, these differences are interpreted as system-level differences rather than effects attributable solely to recycling-binder form. The results support laboratory-scale applicability of the optimized mixtures as recycled flexible base or lower structural layers; low-temperature cracking and rutting resistance require further verification.