Establishing Strategies to Improve Cracking Resistance of High Recycled Binder Ratio (RBR) Asphalt Mixtures

Abstract

With the increasing emphasis on sustainability within the asphalt industry, the adoption of Recycled Asphalt Materials (RAM) in asphalt mixtures is on the rise. Incorporating RAM into the mixtures offers several advantages, including savings in raw materials, cost reduction, decreased energy consumption, and reduced greenhouse gas emissions. Even a mere one percent increase in Reclaimed Asphalt Pavement (RAP) in new asphalt mixtures nationwide could lead to a reduction of 0.14 million metric tons (MMT) of CO2e emissions annually, equivalent to the emissions of 30,000 passenger vehicles (Shacat et al., 2022). Particularly, the National Asphalt Pavement Association (NAPA) has announced an initiative to achieve net zero carbon emissions by 2050, thereby paving the way for increase RAM utilization in asphalt mixtures. However, RAP, being reclaimed from pavements that have undergone extensive aging and loading during their design life, often renders the binder too stiff for reuse without mitigation strategies. The primary challenge associated with incorporating high stiff binder in mixtures lies in the loss of flexibility, making the mixture brittle and prone to initial cracking issues. To address these challenges, various mitigation strategies are available in the market, which not only tackle cracking problems but also enhance the overall lifecycle performance and durability of mixtures. Nonetheless, asphalt contractors encounter a major hurdle due to the absence of comprehensive guidelines or standards governing the use of these strategies. Limited studies have been conducted to evaluate different strategies for improving performance on the same mixture, thereby assessing the effectiveness of these strategies based on various factors such as RAM content, quality and quantity of RAM binder, properties of virgin aggregates and virgin binder. This study aims to investigate various mitigation strategies for enhancing the cracking performance of asphalt mixtures with high recycled binder ratios (RBR). Different aggregates and binders were sourced from two climatic zones, namely south and north. The south-moisture resistant (SR) mixtures were designed with typical low 0.16 and high 0.29 RBRs with RAP, while the north having two mixture types i.e., north-moisture resistant (NR) mixtures included 0.21 and 0.37 RBRs with RAP and 0.44 RBR with RAP/RAS and north-susceptible (NS) mixtures with typical 0.20 and high 0.29 RBR mixes. The mitigation strategies evaluated included the use of softer binder, different binder sources with higher ΔTc, recycling agent (RA), reduced recycled binder availability (RBA), polymer-modified asphalt (PMA), and hybrid approaches including softer binder + RBA and PMA+RBA. The Indirect Tensile Asphalt Cracking Test (IDEAL-CT) and Disc-shaped Compact Tension (DCT) test were conducted to evaluate intermediate-temperature and low-temperature cracking performance, respectively. In addition to the cracking tolerance index (CTIndex), an IDEAL-CT interaction diagram analysis was incorporated to further know the effect of test result variables on cracking performance. Utilizing the results from cracking tests, mixtures meeting defined thresholds were selected for further analysis through pavement performance modeling. FlexPAVETM was employed for this modeling, with inputs derived from Dynamic Modulus (E*) and Cyclic Fatigue (CF) tests. The findings indicate that while no single strategy proves universally effective for all RBR mixtures tested, RA, RBA, softer binder, or their combinations demonstrate satisfactory cracking performance depending upon factors such as RAP binder stiffness and quantity, RA dosage, climatic zone, and RBA.