Effective characterization of recycling agents for well-performing recycled asphalt pavements

Abstract: Use of Recycled Asphalt Pavements (RAP) has been on the rise in the United States, which is not only beneficial with respect to cost savings but also to the environment. In the US (as reported by National Asphalt Pavement Association), RAP usage during the 2021 construction season is estimated to have reduced the need for 4.7 million tons (26 million barrels) of asphalt binder and more than 89 million tons of aggregate with a total estimated value of more than $3.4 billion. Each 10 percent of RAP is estimated to save $3/ton and 5.9 percent GHG emissions. There has been lot of encouragement by industry and government for developing ways to construct pavements with high RAP and ultra-high RAP dosages. But to achieve this objective, recycling agents (RA) need to be added to the mix. Recycling agents help restore the maltene fractions, mitigate effects of the increase in asphaltenes in RAP and enable the longevity of the pavements. Given the rising demand and growth forecast of the Recycling Agents (RA) market, different types of RAs are being introduced, encompassing various sources and chemistries. Not all RAs are created equal and there need to be “blind” and effective approaches of evaluating and comparing these RAs, especially with respect to long-term aging performance. In order to help overcome this challenge, there is a need for a robust criterion encompassing appropriate methods and parameters to effectively evaluate the performance of RAs. In this study, comprehensive performance evaluation of six RAs including four commercial and two experimental RAs encompassing bio-based and non-bio-based sources was performed. A holistic approach was adopted, linking fundamental asphalt chemistry related aspects to physical, rheological and performance related parameters which help evaluate the effectiveness of a particular RA, with special focus on long-term aging performance. Experimental methods employed in this study consisted of chemical methods such as SAR-AD and Size Exclusion Chromatography (SEC); rheological characterization including advanced parameters from Dynamic Shear Rheometer (DSR) and Asphalt Binder Cracking Device (ABCD); and thermal characterization methods such as Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA). The approach included appropriate diagnosis of the aging trajectory of the RA material by itself as well as characterization of the RAP+RA blends at long term aging conditions. The research outcomes provide useful guidelines as well as essential do’s and don’ts that will help the asphalt industry in general and road agencies in particular, to adapt and navigate this ever-changing recycling agent material landscape with a futuristic and dynamic perspective.