Evaluation of area- and volume-based gradations of sand-crushed stone mixture by 2D images

Ballast fouling is a challenging issue in ballasted railway tracks due to frequent maintenance works. At present, sieve analysis is conducted to obtain the gradation curve of fresh and fouled ballasts at laboratories. The investigation time is quite long since it requires a sampling. The information obtained from sieve analysis are not sufficient to propose appropriate maintenance plans since it does not produce shape characteristics of the particles properly. In this research, area- and volume-based gradation curves of fresh and fouled ballasts, simulated by laboratory-scale particles, were studied using an image analysis method. A simple method was proposed to obtain the volume-based gradation curve by 2D images. This method can be practised as an in-situ test method since it needs only a camera and a computer. The results suggest that the grain size should be defined appropriately according to the shape assigned for the particles. It was also found that area-based gradation curve produced by an ellipse shape for the crushed stone particles match sieve analysis closer than circle and rectangle shapes. The results also indicate that area-based gradation match sieve analysis closer than number- and volume-based gradations for the crushed stone particles. It was also found that the volume-based gradation curves evaluated by parallel and diagonal passing overestimate and underestimate respectively its grain size compared to sieve analysis. The gradation curve evaluated by an equivalent passing with a grain size correction factor match the sieve analysis for crushed stone, sand and sand-crushed stone samples. The image analysis results also indicate that the crushed stone particles are more angular than sand particles, which is important in numerical analysis to simulate fresh and fouled ballasts.