An analysis of aggregate crumb rubber mixture packing

Ballasted track has provided a cost-effective support for railway infrastructure. Modern trains generate high loads, increasing ballast degradation. The inclusion of crumb rubber has been proposed as a mitigation measure. Typically, the optimal crumb rubber content suggested in the literature is 10%, sometimes by weight and sometimes by volume. These represent significantly different material contents: 10% by weight is nearly double 10% by volume. This study investigates the packing of ¹/₃-scale ballast intermixed with two crumb rubber sizes at 10% by weight and by volume of aggregate particles. The analysis considers both a traditional two-phase framework, in which crumb rubber is considered part of the particulate skeleton, and a novel three-phase framework, where crumb rubber is considered as a separate phase from aggregate particles and voids. The two-phase framework showed reduced maximum and minimum void ratios when crumb rubber was mixed with scaled ballast. The three-phase framework, however, indicated increased intergranular void ratios, suggesting that crumb rubber disrupts aggregate packing or an effective crumb rubber content different from the target value. Observations from transparent box tests revealed segregation and stratification of rubber particles within the matrix. The results highlight the need for further study of mixing methods and segregation mechanisms.