Assessing the Macro to Micro Properties of Recycled Ballast Mixtures by DEM Analyses for Enhanced Railroad Engineering

Despite the reduced size and angularity failing to meet the conservative ballast standards of some countries, the recycled ballast still retains significant mechanical characteristics for potential reuse. This study investigates the macro to microscopic geotechnical properties of fresh, recycled, and fresh-recycled (F-R) ballast mixtures to evaluate optimal mixtures for sustainable railroad construction. For this study, simulated recycled ballast was produced using Los Angeles abrasion tests conducted on fresh aggregates to smoothen their highly angular corners. Discrete element modeling (DEM) employing irregularly shaped particle clusters was then executed to mimic the altered angularity and the real particle shapes in these granular mixtures. A detailed testing program was conducted to evaluate the bulk density, angle of repose, compression, shearing, and microscopic internal structure of selected F-R mixtures. The results show that an increase in the recycling index (RI) introduced in this study represents a denser matrix having a smaller angle of repose and correspondingly diminished shear strength. However, although compromising internal friction to some extent, recycled grains can enhance the packing structure of the overall granular assembly by reducing the void space, eliminating stress concentrations, and causing a more uniform distribution of interparticle contacts and a diminished contact fabric anisotropy. The grain sizes of recycled content significantly impact these F-R ballast mixture properties, with smaller fractions of the recycled mixture leading to a greater angle of repose, higher peak stress ratio, and dilatancy. Given that the shear strength of a granular medium is governed mainly by the larger grain fraction, an optimal F-R mixture is proposed herein for strategic reuse in railways by replacing 30% of the small-sized aggregates with recycled particles, resulting in overall properties closely resembling those of freshly quarried ballast.