Capturing the Durability of High Recycled Binder Ratio Asphalt Mixtures (2024)
Chapter: Summary
SUMMARY
Capturing the Durability of High Recycled Binder Ratio Asphalt Mixtures
Significant economic and environmental benefits provide motivation to use more recycled asphalt materials (RAM) in asphalt mixtures. The National Asphalt Pavement Association estimates that 98.1 million tons of reclaimed asphalt pavement (RAP) and 0.7 million tons of recycled asphalt shingles (RAS) were reused in approximately 442 million tons of asphalt mixtures produced in 2022. Use of these RAM preserved virgin asphalt binder and aggregates and saved approximately $4.7 billion in material costs and 68 million cubic yards of landfill space. However, these sustainability benefits can be fully realized only if durability can be maintained by engineering each unique combination of virgin materials, RAM, and any additives. To control durability, state highway agencies (SHAs) currently limit RAP and RAS contents or recycled binder ratio (RBR).
Currently, AASHTO has no uniform standard practice for assessing the component materials, their proportions, or their combination in binder blends or mixtures with high RBRs between 0.3 and 0.5. The current mix design specifications focus on the quantity of binder in the mixture primarily through volumetric requirements. However, as RAM content (and the need for additives) increases, the quality of the effective blended binder and its interaction with any additives must also be considered. Balanced mix design (BMD) approaches consider both the quantity and the quality of binder by requiring adequate performance through practical tests of mixtures to assess their resistance to the most prevalent distresses. Mixture characterization captures the effects of partial RAM binder availability (RBA), incomplete blending of virgin and recycled binders and additives, and anti-stripping agents (ASAs) for moisture mitigation, as well as cracking mitigation strategies that help offset the effects of stiff, brittle, aged RAM. These high RAM cracking mitigation strategies include using a substitute virgin binder that includes polymer-modified asphalt (PMA), incorporating a recycling agent (RA) or warm-mix asphalt additive, reducing the RBR or RAM content, and increasing effective binder content (Pbe) by gradation adjustment or alternate mix design requirements like decreased RBA.
BMD approaches are gaining significant interest, and their development and implementation are progressing rapidly. According to the National Center for Asphalt Technology, as of 2024, 35 SHAs have a draft, provisional, or standard BMD specification. Per AASHTO PP 105, four BMD approaches have been adopted, with varying levels of volumetric requirements expected to decrease as implementation progresses. When using RAM in a BMD approach, guidelines and evaluation tools are needed to design a materials combination (the type and properties of selected component materials and their proportions) with balanced rutting and cracking performance. NCHRP Project 09-58, “The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios,” met this need by drafting a Standard Practice for high 0.3–0.5 RBR mixtures with
recycling agents. The draft was revised on the basis of NCHRP Project 20-44(24), “Pilot Test of Proposed Standard Practice for Use of Recycling Agents in Asphalt Mixtures with Recycled Asphalt Materials.”
NCHRP Project 09-65, “Capturing Durability of High Recycled Binder Ratio Asphalt Mixtures,” has broadened the scope by drafting a comprehensive Standard Practice for the design and evaluation of durable asphalt mixtures regardless of RAM content or the presence of additives. The following tools were expanded and revised to produce durable mixtures resistant to traffic and environmental conditions like aging and moisture:
- Component material selection guidelines for virgin and RAM binders and aggregates and their proportions, including an RA dose selection method and corrected optimum asphalt content considering decreased RBA
- Binder and blend rheological evaluation tools, including blend preparation procedures, aging protocols, and thresholds or criteria for adequate performance
- Mixture evaluation tools, including specimen fabrication procedures, aging protocols, and thresholds or criteria for adequate performance
Four laboratory experiments were conducted to evaluate several materials combinations according to 10 field mix designs with eight virgin binders, four RAP sources, two RAS sources, four aggregate sources, and three additives, namely, an RA, a liquid anti-stripping (LAS) agent, and lime. A stepwise approach was used first to design materials combinations with adequate cracking resistance, followed by a check of balanced rutting performance, adjustments if needed to ensure sufficient moisture resistance, and, finally, evaluation of raveling resistance and low-temperature cracking resistance in cold climates. Two performance modeling approaches were also used to assess the robust, durable mixtures designed by the stepwise approach with respect to their resistance to repeated traffic loading after aging and moisture conditioning.
The comprehensive laboratory testing results facilitated engineering of robust, durable mixtures and highlighted that the best high RAM and moisture strategies depend on the materials combination. The use of the high RAM strategies explored in this project to enhance cracking resistance with longer-term aging did not result in rutting, moisture susceptibility, or raveling issues. Virgin and RAM binder quantity and quality were both important in influencing mixture moisture resistance and degree of activation (DoA) of the RAM binder RBA, respectively. Aggregate type also influenced mixture moisture resistance in some instances, and specific aggregate and binder properties and proportions were shown to be tied to improved durability. RAM with lower stiffness and greater flexibility provided a higher DoA of the RAM binder and facilitated higher RAM contents and ASA effectiveness.
Analyses across all mixtures allowed for the development of the evaluation tools and component materials selection guidelines contained in the revised Standard Practice draft. These tools and guidelines provide a framework with a selection of candidate tests to assess resistance to common distresses.
Additional research is proposed on the following topics:
- Application of combined aging and moisture conditioning to a wide range of materials combinations
- Characterization of PMA and mixtures to capture enhanced performance documented in the field and shown in pavement performance modeling
- Rheological evaluation of binders at intermediate temperatures to tie to corresponding mixture cracking resistance
The following implementation activities are suggested for applying these tools:
- Construction of field pilot projects to demonstrate the stepwise evaluation approach to producing robust, durable mixtures with high RBR
- Continued performance monitoring of existing field projects constructed in conjunction with NCHRP Project 09-58 and NCHRP Project 20-44(24) to provide additional data for adjusting binder blend and mixture performance thresholds
- Review of existing performance thresholds in SHA specifications based on benchmarking, existing field projects, and new field pilot projects
