Investigation of aggregate properties influencing railroad ballast performance

Class I freight railroads nearly exclusively use premium, high-quality ballast aggregates such as granite, trap rock, and quartzite for their mainline ballast needs. These natural virgin aggregates are crushed stones with certain geologic origins, and their properties may vary significantly depending on quarry sources and production techniques. Findings are presented from a comprehensive laboratory study on ballast aggregates undertaken at the University of Illinois to investigate differences in engineering behavior of premium railroad ballast materials obtained from 13 quarries across the Union Pacific Railroad system. Laboratory research tasks consisted of gradation analysis; quantifications of shape, texture, and angularity properties with the University of Illinois aggregate image analyzer; shear strength evaluation by using large direct shear (shear box) tests; and ballast degradation and durability analyses with Los Angeles abrasion tests conducted at 400 and 1,000 turns and evaluations of subsequent ballast fouling. As the compacted ballast voids ratio increased, aggregate strength typically decreased because of lower solid densities obtained from shear box testing. Aggregate shape properties quantified through imaging-based shape indices were shown to be linked to the strength properties of different ballast density groups. One of the ballast aggregates with flat and elongated particles but very strong mineralogy had quite high strength properties despite the tendency for particles to break during shearing. Higher angularities that generally resulted in higher strength properties could also yield higher fouling and increased breakdown potential of the aggregates.