Numerical and Experimental Investigation of Production and Blending Mechanisms of Asphalt Mixtures with Reclaimed Asphalt Pavement
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This study investigated the production and blending mechanisms for asphalt mixtures that contain reclaimed asphalt pavement (RAP). The numerical and experimental methods used in this study can be used to assist asphalt plant design and operations to produce high quality RAP mixtures. Direct monitoring of the moisture and temperature of virgin aggregates and RAP materials during the production process is difficult. This study integrates models of heat transfer, moisture drying, binder transfer, and binder coating into a coupled computational fluid dynamics (CFD) and discrete element method (DEM) scheme to simulate the production process of RAP mixtures and to analyze quantitatively the critical parameters that could affect the drying and heating of virgin aggregate and RAP materials. The simulation results are then used to determine the optimal dry and wet mixing times that are needed to produce high quality RAP mixtures. Laboratory experiments were conducted to verify the numerical results. Poor mixing of RAP mixtures could produce inhomogeneous mixtures that may negatively affect their mechanical properties. The DEM is used to investigate the overall gradation segregation and the segregation between RAP and virgin aggregate particles during the dynamic mixing process. Gray-scale image analysis coupled with a finite element method is used to identify the correlation between the homogeneity of RAP mixtures and stiffness. The results show that a longer mixing time could cause gradation segregation, but could be beneficial to avoid RAP and virgin aggregate segregation. The results also show that the number of RAP elements within the middle area of the samples has a significant correlation with the stiffness of the RAP mixtures. This study also presents a laboratory mixing scheme that is designed to produce RAP mixtures that experience the different production stages, i.e., RAP binder transfer, mechanical blending, and diffusion. The results of such experiments conducted on mixtures containing 26 percent and 50 percent RAP show that the diffusion stage is the most dominant stage that affects the rheological and fracture properties of RAP mixtures. In addition, thorough mechanical blending contributes to improving the ductility of high percentage RAP mixtures.