Characterization of Reclaimed Asphalt Pavement Material Properties for Hot In-Place Recycling.

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.