Morphological characterization of railroad ballast degradation trends in the field and laboratory

Field and laboratory degradation trends of railroad ballast are investigated in this paper by using an aggregate image analysis approach. In addition to gradation changes, in-service ballast materials undergo degradation of particle shape properties (i.e., angularity, flatness and elongation, and surface texture), which can be quantified through imaging techniques. In the field, five ballast types were installed in eight steel boxes at a test section in the western heavy axle load revenue service test megasite near Ogallala, Nebraska. In-service performance evaluations consisted of sampling, sieving, and imaging-based shape property analyses of the field-collected ballast materials to monitor degradation twice a year. In the laboratory, following the Los Angeles (LA) abrasion test procedure, a sample of granite ballast was tested at increasing numbers of LA abrasion drum turns to assess changes in gradations and study degradation levels. All of the LA abrasion-tested granite samples and approximately 3,000 particles from the different sizes of the field-monitored ballast materials were collected and scanned with the Enhanced University of Illinois Aggregate Image Analyzer. Imaging-based particle shape indexes were determined at different degradation levels from both the LA abrasion and the field projects. The magnitudes and rates of measured degradation and breakage, as well as the captured changes in morphological properties of particles, were quite different between the LA abrasion test and the field degradation from repeated traffic loading. A comprehensive study with improved ballast sampling and testing techniques is recommended to further investigate mechanisms of ballast degradation.