Cold Mix Asphalt for sustainable bituminous sub-ballast for railway: mechanical, vibratory and environmental assessment

Abstract

This study focuses on developing cleaner and functional bituminous sub-ballast for sustainable railway infrastructure using rubberized Cold Mix Asphalt (CMA) technologies. The proposed approach integrates high contents of Reclaimed Asphalt Pavement (RAP), and rubber from end-of-life tires, aiming to enhance vibration damping capacity while maintaining high load-bearing capacity and sub-layer protection. The research identifies optimal design parameters to improve stiffness, stress dissipation, vibration damping, and sustainability. A total of 17 sustainable mixtures were analyzed, assessing the influence of cement content, mineral skeleton, rubber content and size, and emulsion type and content, in comparison to two reference materials: granular sub-ballast from natural aggregates, as the most common solution and Hot Mix Asphalt (HMA) as conventional bituminous sub-ballast. A comprehensive multivariable analysis was conducted through mechanical characterization, vibrational assessment, and Life Cycle Assessment (LCA) techniques, allowing for an integrated evaluation of structural, functional, environmental, and economic performance. The results demonstrated that all sustainable mixtures significantly outperform conventional granular sub-ballast in mechanical performance, with improvements of up to 500 % in stiffness and 59 % in stress dissipation. Rubberized mixtures achieved damping improvements of up to 195 %, while the best-performing CMA alternatives reduced Global Warming Potential (GWP) by approximately 20–40 % compared to HMA. Cement content, optimized mineral skeleton, and rubber content emerged as the most influential variables. The findings highlight the need for balanced designs integrating moderate cement content (2–3 %) and optimized mineral skeleton to improve the mechanical performance while incorporating high rubber dosages (around 3.0 %) to achieve multifunctional performance. CMA demonstrates the potential to serve as a competitive alternative for railway infrastructure compared to granular sub-ballast, offering higher structural and functional properties, aligning with goals of durability, vibration mitigation, and sustainability while advancing circular economy practices.