Railway ballast aggregate characterization through a new strength index: Preliminary suitability evaluation of an industrial by-product for a railway application

Ballast is a layer of the railway infrastructure that ensures the correct geometric position of track equipment, distributing vertical loads on the underlying layers. Ballast is composed by aggregate grains of adequate grading, containing a consistent number of voids and a medium–low fine content. The aggregates of ballast must be tested in the laboratory to ascertain the suitability for their use under construction or maintenance. In this work, the physical–mechanical characteristics of different types of aggregate (natural and “artificial”) were evaluated to verify their acceptability as ballast under the Italian specifications. All the aggregates came from Italian production plants. One “artificial” aggregate (steel slag) was used to assess if it can be used instead of the natural aggregate on a railway line in Italy. A mixture of natural and artificial aggregates was also tested for comparison purposes. An index from Point Load Test was also assessed to predict the main physical–mechanical characteristics provided by the laboratory tests. The artificial aggregate shows excellent physical–mechanical properties, compared to the natural one, indicating that the tested by-product can replace the natural aggregate. The mixture of natural and artificial aggregates can guarantee the preservation of natural resources and solve supply difficulties of by-products, maintaining very good physical–mechanical characteristics. In general, the Los Angeles index (both before and after the freeze-thawing cycles) shows a good correlation with the Point Load Strength Index for the tested ballast aggregates. Starting from this result, the Point Load Test could be used to characterize ballast aggregates. A statistical analysis was also performed on the other physical–mechanical parameters: a good correlation was found in some cases, indicating that the Point Load Strength Index can provide a first indication of the other physical–mechanical characteristics of ballast aggregates.