Asphalt with different aging degrees requires different rejuvenation methods. However, current applications of hot in-place recycling (HIR) for Reclaimed Asphalt Pavement (RAP) do not consider the differences in the aging degree of asphalt binder across different layers of RAP. Additionally, there is limited understanding of the changes in asphalt binder and aggregate properties during the HIR process. Changes in the properties of RAP materials can lead to inaccuracies in the mix design, potentially causing suboptimal performance. This study compares the performance of asphalt binders at different depths within RAP and clarifies the effects of the grinding and heating processes during HIR on both asphalt and aggregate properties. The aging gradient of RAP asphalt was assessed using macroscopic performance tests (bending beam rheometer (BBR), dynamic shear rheometer (DSR)) and microscopic techniques (scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR)). The effects of heating and milling on RAP materials were evaluated using conventional performance tests (DSR, BBR) and sieving analyses. The results show that the RAP asphalt exhibits an aging gradient under natural conditions, with the upper surface asphalt aging more than the lower layers. The heating process during HIR accelerates the secondary aging of RAP asphalt, reducing penetration by 25.3%, increasing the softening point by 7.4%, and decreasing ductility by 36.4%. The milling process causes gradation failure of RAP, with a damage rate of 14.4% of the coarse aggregates. Therefore, it is recommended that when using HIR for maintenance of severely aged pavements, the upper layer of the RAP should be separately milled and treated. The HIR mix design should consider the impact of heating and milling on RAP materials.

To address the insufficient interlayer bonding performance and high preparation costs associated with conventional prime coat materials for semi-rigid base asphalt pavement, in this study, we utilized waste bio-oil to enhance asphalt–aggregate separation in recycled asphalt pavement (RAP) and collected the filtrate after separation. The RAP–bio-oil filtrate acts as a diluent, which is reintroduced into the asphalt to form a novel prime coat material: RAP–bio-oil filtrate cut-back asphalt (RFCA). The optimal proportions of each component in RFCA were determined through penetration tests, interlayer shear tests, and interlayer tensile tests. Subsequently, interlayer shear strength tests were conducted to evaluate the bond strength of RFCA compared to traditional prime coat materials, such as PC-2 emulsified asphalt (PC-2 EA) and kerosene cutback asphalt (KCA). Additionally, interlayer shear fatigue tests were performed to assess the durability of the bond provided by RFCA between the surface and base layers. The experimental results indicate that the optimal formulation for RFCA comprises 60% asphalt by mass of the RAP–bio-oil filtrate and 10% penetrant agent by mass of the RAP–bio-oil filtrate. Under conditions of 15 °C, 40 °C, and 15 °C water immersion, the interlayer shear strength of RFCA demonstrates an enhancement compared to PC-2 EA and KCA. Compared to PC-2 EA, the interlayer shear fatigue life of RFCA improved by 39.7% at a stress ratio of 0.2, 40.8% at 0.3, 51.2% at 0.4, and 40.3% at 0.5. The application of RFCA as a prime coat material significantly enhances interlayer durability. Based on the cost analysis, the application cost per square meter for RFCA is lower than that of PC-2A. Consequently, RFCA presents a viable option as a green, renewable, low-cost, and high-performance prime coat material for application on semi-rigid base asphalt pavement.

In practical applications, semi-flexible pavement materials need to achieve a balance between mechanical properties, high-temperature stability, cracking resistance, and water stability. However, these performance requirements are strongly influenced by the void content of the matrix asphalt mixture, which determines the grouting space and the composite structure of semi-flexible pavement materials. In this study, matrix asphalt mixtures with three different void contents (20.6%, 24.9%, and 29.2%) were prepared. The effects of the matrix void content on the perfusion characteristics and overall performance of semi-flexible pavement materials were systematically evaluated through perfusion tests, Marshall stability and splitting tests, three-point bending tests, rutting tests, and freeze–thaw splitting tests. Subsequently, the impact of the void ratio on each parameter was analyzed. The findings indicate that, within the tested void content range (20.6%–29.2%), increasing the void ratio of the matrix asphalt mixture continuously improved the mechanical properties of the semi-flexible pavement material: the Marshall stability was enhanced by a factor of up to 4.8, and the splitting strength by up to 1.4 times. Results from the trabecular three-point bending test reveal a more pronounced effect of void fraction on the crack resistance of semi-flexible pavement materials with rising temperatures. At 15 °C, lower void fractions lead to superior crack resistance. Conversely, when the void fraction reaches 24.9% at −10 °C, semi-flexible pavement materials exhibit enhanced crack resistance. Rut dynamic stability and freeze–thaw splitting tests demonstrate that higher void ratios in the matrix correlate with superior resistance to high-temperature deformation and water stability. When the void ratio falls below 24.9%, increasing it significantly enhances resistance to high-temperature deformation, with an improvement range of up to 63.6%. Therefore, 24.9% is identified as the recommended balanced air-void content of the matrix asphalt mixture, at which the semi-flexible pavement material achieves the most balanced comprehensive performance in terms of mechanical properties, high-temperature deformation resistance, water stability, and cracking resistance.