Optimization of bituminous sub-ballast in railways infrastructures under sustainability criteria

Abstract

In recent years, the railway sector has focused on more sustainable constructive techniques and more durable solutions such as the inclusion of bituminous sub-ballast in order to reduce track degradation whilst improving its quality. The use of bituminous layer in railways tracks is regarded as an appropriate solution for improving the strength of the section. In comparison with traditional granular sub-ballast, these materials allow for both an increase in bearing capacity and greater protection of the substructure. However, the fact that these materials are manufactured at a temperature of 160°C means that their application can incur significant construction costs, along with a rise in pollution and energy consumption. In this regard, there is a need for the use of new techniques to produce asphalt, such as the reuse of waste materials or manufacturing with low-temperature technology, both of which allow for lower energy expenditure without changing its in-service mechanical performance, as well as reducing the general costs of this layer. In this context, this thesis therefore aims to, on the one hand, study the possibility of using sustainable bituminous sub-ballast manufactured from waste materials such as crumb rubber, which is recycled rubber produced from scrap tires and reclaimed asphalt pavement (RAP) from road pavement at the end of their service life. A further aim is to study the possibility of using WMA manufactured at lower temperatures, as bituminous sub-ballast. To this end, this thesis is focused on evaluating the mechanical behaviour of bituminous materials (under both routine and adverse temperatures) in comparison with that presented by conventional sub-ballast. In particular, performance is examined with respect to the main requirements that need to be met by these materials (resistance to plastic and punching deformations, bearing capacity, stress dissipation, cracking resistance, and waterproof properties) for their use in railway tracks. The results demonstrated that all the sustainable solutions studied, offer a suitable performance as subballast for railways tracks from a mechanical behaviour point of view. In addition, they can offer advantages such as reducing greenhouse emissions, fuel consumption and waste as in the case of HWMA 100% RAP. furthermore, the use of rubberized bituminous mixtures (DRA and WRA) make more durable sub-ballast layer, at the same time that allow for waste recovery. Finally, the use of WMA, despite its performances was slightly lower respect the others solutions studied, achieved acceptable results for its use as a subballast layer. Moreover, it is a solution that guarantees to reduce pollution and energy consumption thanks to its ability to reduce mixing and compaction temperatures.