Investigation of the Modified Ballast Breakage Index for a Laboratory Test Series using the Proctor Compactor Machine

Abstract

The rock physics properties of the crushed railway ballast include resistance against breakage and wear. The qualification of such materials and their compliance with requirements is mainly considered by two standard tests: The Los Angeles abrasion and Micro-Deval wear resistance tests. These tests are indispensable for the legally mandatory qualification of aggregates, but several measurement methods have been developed that better simulate operating conditions and provide an even more accurate classification of these materials. A Proctor compactor machine was applied to induce a top impact load more similar to the operating conditions of ballast. Pre-screened, sorted, washed, and dried samples of andesite aggregate from four quarries with different rock physics characteristics were used to conduct the test series. The impact load was applied with increasing numbers of blows, i.e., 64, 128, 256 and 1024. Several parameters and indexes were calculated to reveal different relationships, which had to be modified in some cases to obtain estimates as close as possible to those obtained in individual tests. The original Ballast Breakage Index seemed to be an appropriate measure of the change in ballast material quality during deterioration for over a decade. The original calculation method was not consistent with the newly introduced test method. Keeping the principle, however, a modified BBI index calculation method has been developed that fundamentally simplifies the calculation of ballast fragmentation. The whole series of measurements aimed to provide, by material and number of impacts, a series of fragmentation and degradation curves for each of the three repeated measurements, which would give the degradation of samples with different rock physics and grain sizes.