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

Repeated impact loading caused progressive ballast degradation through settlement, stiffness reduction, particle breakage, and fouling under dry and water-saturated conditions. This study evaluated the deformation, stiffness, degradation, and fracture behavior of ballast modified with recycled plastic (RP) and crumb rubber (CR) under repeated impact loading. Drop-weight impact tests were conducted on ballast mixtures containing 5%, 10%, and 15% polymer by volume with 200 loading cycles. Settlement, stiffness, breakage ratio, breakage index, and fouling index were used to evaluate ballast performance. CR reduced settlement by 36.5% at 5% content under dry conditions and maintained stable performance under saturation, indicating strong damping and moisture-resistant behavior. RP increased stiffness by up to 32% at 15% content through improved stress redistribution and particle confinement. Both polymers reduced ballast degradation, with RP achieving approximately 60% reduction in particle breakage, while CR showed greater effectiveness in reducing fouling and suppressing crack propagation under wet conditions. Fracture observations indicated that RP improved structural confinement, whereas CR dissipated impact energy and reduced fracture development. Optimal performance occurred at 5% CR and 15% RP under dry conditions, and 15% CR under saturated conditions.