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1 More than 90% of highways and roads in the United States are built using hot-mix asphalt (HMA) or warm-mix asphalt (WMA) mixtures, and these mixtures now recycle more than 99% of some 76.2 million tons of reclaimed asphalt pavement (RAP) and about 1 million tons of recycled asphalt shingles (RAS). According to the National Asphalt Pavement Asso- ciation (NAPA), cost savings in 2017 totaled approximately $2.2 billion with these recycled materials replacing virgin materials. The use of RAP in HMA dates back to the early 1900s, with renewed focus on research and implementation in the 1970s and 1980s and again in 2008 with significant increases in the cost of petroleum products including asphalt binders. Thus, highway agencies and the paving industry have developed a renewed interest in using larger quantities of recycled materials (RAP and RAS) to maximize economic and environmental benefits that include conservation of natural resources (aggregate, binder, fuel, etc.), reduction in energy con- sumption, and reduction in emissions (including greenhouse gases). In spite of these sym- biotic benefits, state departments of transportation (DOTs) limit the use of RAP and RAS in asphalt mixtures for reasons that include variability of the recycled materials and concerns about long-term mixture performance. In addition, mix design is more complicated and more time consuming, particularly with large quantities of recycled materials identified by high recycled binder ratios (RBRs) between 0.3 and 0.5. The potential for compactabil- ity issues during construction and decreased mixture cracking resistance is also increased as RBRs increase and corresponding mixtures become stiffer and more brittle. Thus, the environmental and economic benefits must be compared to the potential increased risks associated with construction and performance to ensure engineering benefits can also be realized. Mitigation of these construction and performance issues can be addressed through mix design with the use of higher binder contents, material selection with the use of softer binders that may be polymer modified, or additives such as recycling agents, as long as there are not compatibility concerns and mixture resistance to rutting and moisture damage is maintained. National Cooperative Highway Research Program (NCHRP) Project 9â58, âThe Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios,â focused on using recycling agents to facilitate increasing RBR and met the following study objectives: â¢ Assess the effectiveness of recycling agents at a selected dose to partially restore binder blend rheology and improve mixture cracking performance without adversely affecting rutting resistance, â¢ Evaluate the evolution of recycling agent effectiveness with aging, and â¢ Recommend evaluation tools for assessing the effectiveness of recycling agents initially and with aging for mixtures with high RBRs in specific climatic regions. S U M M A R Y Evaluating the Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios
2 Evaluating the Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios Many materials combinations were used; these included 10 base binders, 6 RAP sources, 5 RAS sources, 5 aggregate types, and 10 recycling agents. Laboratory testing results and field performance at five field projects in Texas, Indiana, Nevada, Wisconsin, and Dela- ware facilitated engineering rejuvenated binder blends and corresponding mixtures and the development of evaluation tools for assessing the effectiveness of recycling agents initially and with aging for binders and mixtures with high RBRs. The comprehensive results generated and documented in this report and in multiple pub- lications and presentations indicated that the following factors are not distinct but instead contribute concurrently to determine the performance of mixtures with high RBRs and recycling agents initially and with aging: â¢ Base binder performance grade (PG) and quality (ÎTc); â¢ Binder modification by polymers or WMA or other additives; â¢ Proportions of recycled materials (RAP binder ratio [RAPBR] and RAS binder ratio [RASBR]); â¢ Recycling agent type; â¢ Recycling agent dose; and â¢ Recycled binder availability, which is a function of its aging state and production temperature. These overlapping factors and their interplay necessitated the development of the fol- lowing tools that are included in the draft American Association of State Highway and Transportation Officials (AASHTO) standard practice as an appendix to facilitate the evalu- ation of the effectiveness of recycling agents in high RBR binder blends and corresponding mixtures initially and with aging: â¢ Component materials selection guidelines, â¢ Recycling agent dose selection method and materials proportioning, â¢ Binder blend rheological evaluation tools, â¢ Mixture performance evaluation tools, and â¢ Recycled binder availability factor. The evaluation tools for binder blends and mixtures include aging protocols, recycling- agent blending methods, and laboratory tests and corresponding thresholds for adequate performance. Additional investigations of chemical compatibility of recycling agents with base and recycled binders and representative binder blending were also completed, and laboratory aging and climate effects were explored for both binder blends and mixtures. The majority of the field projects in this study utilized significantly lower recycling agent doses than those determined by the method developed and included in the draft AASHTO standard practice. Thus a field demonstration project with a higher recycling agent dose selected by the proposed method is needed for validation of the evaluation tools also devel- oped in this study. Additional implementation activities to apply these tools include the following: â¢ A review of state specifications for limiting RAP and RAS and proposed revisions as needed and â¢ Continued performance monitoring of the field projects established in this study to pro- vide additional data to adjust binder blend and mixture performance thresholds. Additional research is also proposed on the following topics: â¢ Evaluation of climate effects to refine mixture performance thresholds based on climate, â¢ Exploration of the effects of long-term aging on mixtures with high RBRs and recycling agents and how to capture these effects in the laboratory,
Summary 3 â¢ Assessment of the effects of moisture susceptibility for mixtures with high RBRs and recycling agents and possible addition of another performance threshold, â¢ Rheological evaluation of modified binders to determine how to capture their benefits when used in mixtures with high RBRs and recycling agents, and â¢ Chemical assessment of recycling agents to revise specifications for these component materials.