RES2016-04: Evaluating Recycling Efficiency of Asphalt Plants

This study aims to determine recycling efficiency of mixtures containing recycled asphalt pavement (RAP) and recycled asphalt shingle (RAS) from different asphalt plants and to investigate the influence of mixture design and production conditions on recycling efficiency. A new procedure was developed by using fluorescence microscopy to differentiate between RAP/RAS and virgin binders as well as their blends, in which a new parameter, mean grey value (MGV) based on fluorescence image, was utilized to quantitatively determine the mobilization rates of aged asphalt binder in RAP/RAS. By generating the “Blending Chart” to show the change of MGV with RAP/RAS binder content, the mobilization rate of aged binder in RAP/RAS can be directly measured. A total of 14 plant-produced asphalt mixtures were tested for their RAP/RAS binder mobilization rate using fluorescence microscopy and GPC. Linear regression analyses were conducted to evaluate the influence of mixture design and production condition parameters on the mobilization rate. In addition, a preliminary study was performed to investigate the influence of different rejuvenator incorporation methods on the rheological and aging properties of asphalt binders containing RAP/RAS. The major conclusions from the study are summarized as follows: (1) The mobilization rates were different for large and small aggregates because of different RAP/RAS contents. The overall mobilization rate of a mixture could be determined by considering the surface area of large and small aggregates. (2) The overall mobilization rates of 14 plant mixtures were in the range of 0.4 to 1.0 for the fluorescence microscopy method and 0.37 to 0.83 for the GPC method. The mobilization rates from fluorescence microscopy and GPC method were different but showed similar trend among 14 plant mixtures. The R2 of the linear regression between GPC and fluorescence results is 0.557. (3) Among the production condition parameters of asphalt mixture plants, the mixing temperature and the percentage of RAP/RAS showed significant influences on the mobilization rate. In addition, the aging level of the RAP/RAS binder also had a significant influence on the mobilization rate. (4) The sensitivity of MGV to the aged binder content was affected by the test conditions. An index, Differentiation Factor, was used to measure the sensitivity of MGV to aged binder content. Results indicate that the MGV measured at the wavelength of 450-490 nm and the exposure time of 0.5-1.5 s was most sensitive to the aged binder content. For the construction of a blending chart, it was found that there existed a linear relationship between the MGV of a binder blend and its RAP binder content, and a quadratic relationship between the MGV of a binder blend and its RAS binder content.