Impact-Induced Degradation and Fracture Behavior of Polymer-Modified Ballast under Dry and Saturated Conditions

Railway ballast degradation under repetitive impact loading remains a critical issue affecting track stability, particularly under varying moisture conditions. While crumb rubber (CR) has been widely studied as a damping additive, the role of recycled plastic (RP) in mitigating ballast degradation under impact loading remains poorly understood. This study investigates the deformation, stiffness, degradation, and fracture behavior of ballast modified with RP and CR under impact loading, both dry and water-saturated conditions. A series of drop-weight impact tests was conducted on ballast mixtures containing 5%, 10%, and 15% of CR and RP by volume of ballast aggregate. Four performance indicators were evaluated to propose a suitable mix, including settlement, stiffness, breakage ratio, breakage index, and fouling index. The results demonstrate that polymer modification significantly enhances ballast performance. CR reduced settlement by up to 36.5% at 5% content under dry conditions and maintained strong performance under saturation, indicating excellent damping and moisture resilience. RP improved stiffness by up to 32% at 15% content through enhanced load distribution and structural confinement. Both polymers effectively reduced degradation, with RP achieving approximately 60% reduction in breakage, while CR showed superior performance in reducing fouling and maintaining stability under wet conditions. Fracture analysis revealed distinct mechanisms: RP enhances inter-particle confinement, whereas CR dissipates impact energy and suppresses crack propagation. Optimal performance was achieved at 5% CR and 15% RP under dry conditions, and 15% CR under saturated conditions, supporting the development of sustainable, performance-based railway materials aligned with circular economy principles.