Degradation-Related Changes in Ballast Gradation and Aggregate Particle Morphology

Ballast degradation or fouling often associated with deteriorating railroad track performance refers to the condition of a new ballast layer changing its aggregate particle size and shape properties with usage. Fouling is mainly due to degradation or breakage of ballast aggregates under traffic loading, although other fine materials intruding can also contaminate a clean and uniformly graded ballast layer. Previous research efforts have focused on studying the degraded ballast behavior of clean ballast aggregates mixed with finer materials such as sand, mineral filler, subgrade clay, and coal dust. However, this approach cannot fully represent the cases where fouled ballast composition is made up of aggregate particle breakdown or intrusion of foreign materials. In the field, over two-thirds of the ballast fouling cases are due to ballast aggregate particle abrasion and breakage, which necessitates a better basic understanding of the field degradation trends and the related changes in ballast gradation and aggregate particle morphology. This paper describes an experimental approach to quantify ballast gradation and aggregate morphological properties at different stages of degradation studied through Los Angeles (LA) abrasion testing and aggregate image analysis technique in the laboratory. The results from this experimental study were intended to quantify changes in ballast gradation and aggregate morphological properties due to the LA abrasion-based degradation process and link different physical properties of the ballast material to the previously defined fouling index (FI) as a practical indicator of fouling conditions. A good correlation was found between percent changes in aggregate shape properties quantified by image analysis of ballast particles larger than 9.5mm (3/8in.) and the number of turns of the LA abrasion drum. Further, the study findings are also linked to the field degradation trends reported in the literature, which necessitates ballast gradation and aggregate shape property changes also be tracked in the field and better documented to the FI when studying fouled ballast behavior.