Effect of Crumb Rubber Size on the Packing of 1/3 Scaled Ballast

Most of the world’s railways are on ballasted track—a versatile and cost-effective support solution that can be traced back to the nineteenth century. In the twenty-first century, heavier and faster trains (freight and passenger) create higher loads and maintenance requirements. Ballast degradation becomes an important issue and solutions to increase time intervals between maintenance interventions are necessary. Some proposals involve the incorporation of elastic elements such as crumb rubber mixtures in the ballast. The crumb rubber reduces dilatancy and particle breakage. However, there is a lack of consensus on some key parameters. For example, the optimum percentage of crumb rubber in ballast is often given as 10%, which results in beneficial changes in the stiffness and energy absorption properties of ballast. However, some studies refer to 10% by volume, while others refer to 10% by weight. This leads to very different outcomes, as 10% by weight is nearly double 10% by volume of the soil particles. The paper analyses the influence of incorporating crumb rubber with two different sizes of particle, at 10% by weight and 10% by volume of mineral particles, into 1/3 scaled ballast. The addition of crumb rubber densifies the natural volumetric packing of 1/3 scaled ballast. The maximum (e_max) and minimum (e_min) void ratios decreased for all situations tested. This is explained in part by voids filling. Under cyclic loading, crumb rubber segregation was observed.