Recycled Asphalt Materials: Binder Availability and Its Impact on Mix Performance (2026)
Chapter: 1 Background
CHAPTER 1 BACKGROUND
1.1 Problem Statement
Using RAM in asphalt mixtures provides economic, environmental, and engineering benefits (Stroup-Gardiner, 2016; Epps Martin et al., 2024; Williams and Willis, 2025). However, asphalt mixtures with high RAM contents—corresponding to recycled binder ratios (RBR) between 0.3 and 0.5 as defined by AASHTO PP 127—may experience cracking and durability issues due to excessive stiffness and poor relaxation properties (McDaniel and Anderson, 2001; West et al., 2013; Epps Martin et al., 2019). From a binder quality perspective, the asphalt binder in RAM is significantly aged compared to virgin binder, making it more susceptible to fatigue and thermal cracking. From a binder quantity perspective, the asphalt binder in RAM may not be fully activated and mobilized to provide aggregate coating and mixture flexibility. Nevertheless, many DOTs currently assume 100% binder activation when designing asphalt mixtures with RAM. This can result in mixtures, especially those with high RAM contents or heavily aged RAM, with poor workability and cracking resistance.
Common mix design approaches to improve the cracking resistance and durability of high-RAM asphalt mixtures include using a softer binder or an RA, focusing on improving the overall binder quality in the mixture (Hajj et al., 2012; Sias et al, 2022; Epps Martin et al., 2024). RBA offers an alternative approach through increasing the binder quantity in the mixture. In this project, RBA is defined as “the amount of recycled binder from RAP or RAS that activates and contributes to the total effective binder content in an asphalt mixture (Epps Martin et al., 2021),” expressed as a percentage ranging from 0% to 100%. It is an intrinsic property of the mixture that can be affected by the type and quality of the RAM and other mix design and production variables. Considering RBA requires incorporating additional virgin binder into the mixture to compensate for the inactive RAM binder during mix design and production, with the quantity contingent upon the determined RBA value. Utilizing excessively high RBA values may result in RAM asphalt mixtures with inadequate cracking resistance, whereas unreasonably low RBA values could produce mixtures with poor rutting resistance. Therefore, an appropriate RBA value is crucial to achieving balanced performance in high-RAM asphalt mixtures.
As of January 2020, nine state DOTs had adopted the RBA approach, including two for both RAP and RAS, two for RAP only, and five for RAS only (Epps Martin et al., 2021). The RBA values used by these agencies varied from 60% to 100% for RAP and 60% to 85% for RAS. The determination of RBA has been extensively studied worldwide among the asphalt research community. Notable efforts in this field include the Corrected Optimum Asphalt Content (COAC) study by the Georgia Department of Transportation (DOT) (Horan, 2020), the binder availability experiment described in NCHRP research report 927 (Epps Martin et al., 2019), and an interlaboratory study on the degree of binder activity by the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (RILEM) (Menegusso Pires et al., 2021), among others. Despite these initiatives, systematic guidance on incorporating the RBA approach into mix design and production for high-RAM asphalt mixtures remains limited. This project aimed to address several key questions, including:
- How can RBA be quantitatively determined?
- What is the sensitivity of RBA to different mix design and production variables?
- What is the impact of RBA on the performance of asphalt mixtures containing RAM?
- How can RBA be incorporated into the existing volumetric mix design and quality assurance practices?
1.2 Research Objectives
The objective of this research was to identify or develop methods to (a) quantify binder availability from recycled asphalt materials and (b) based on availability, measure the impact of recycled asphalt materials incorporated in asphalt mixtures prepared with a range of virgin, rubberized, and modified asphalts on mixture performance. These methods shall be used to improve mix design procedures and specifications by accounting for binder availability and its impact on performance.
1.3 Organization of the Report
This report is organized into four chapters. Chapter 1 defines the problem statement and research objectives. Chapter 2 describes the research approach followed to address the problem. Chapter 3 presents detailed documentation of the research results, findings, and applications. Finally, Chapter 4 presents a summary of the project, including key findings and conclusions, and recommendations for suggested research. The report also includes the following appendices that can be found on the NCHRP Project 09-68 page at https://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=5141.
- Appendix A. Literature Review
- Appendix B. State DOT Survey and Responses
- Appendix C. Draft Method of Test for Determining the Recycled Binder Availability for Asphalt Mixtures Containing Recycled Asphalt Materials
- Appendix D. Draft Method of Test for Determining the Effects of Recycled Binder Availability on Performance Properties of Asphalt Mixtures Containing Recycled Asphalt Materials
- Appendix E. Proposed Revisions to AASHTO PP 127, Provisional Standard Practice for Development of Balanced and Durable Asphalt Mixtures with High Recycled Asphalt Materials Contents
- Appendix F. Proposed Revisions to AASHTO M 323, Standard Specification for Superpave Volumetric Mix Design
- Appendix G. Proposed Revisions to AASHTO R 35, Standard Practice for Superpave Volumetric Design for Asphalt Mixtures
- Appendix H. Recommended Process for Incorporating RBA in the Design and Production of Asphalt Mixtures Containing RAM
