Punching test for mechanical characterization of asphalt railway sub-ballast

The use of asphalt mixtures for sub-ballast layers in railway infrastructure, which is becoming a preferred design solution in in the high-speed and high-capacity lines in some European countries and in the United States, offers several structural, functional and economic benefits. The assessment of physical-mechanical characteristics of these mixtures still occurs with methods that are well-established in the road paving industry, though these methods are not consistently capable of highlighting the peculiarities of the railway operations, such as the ballast/sub-ballast interaction and the granular behavior of the overlying unbound layer. Specifically, the interface between the crushed stone elements of the ballast bed and the underlying asphalt layer deserves specific attention. Thus, this study presents a new conceived experimental method for the mechanical characterization of mixes intended for asphalt sub-ballast, based on punching test. The test employs an adaptive indentation plate (AIP), which was designed to replicate the interaction between ballast particles and the sub-ballast. Cylindrical asphalt specimens (phi 150 mm) were subjected to vertical point loads using the AIP, considering different temperatures (5, 20, 35 degrees C) and deformation rates (5.08, 25.4, 50.8 mm/min). The validation of the experimental procedure involved the analysis of two asphalt mixes of different stiffness, which conformed to the Italian standard for asphalt sub-ballast. The results showed that the punching test offers an effective evaluation of resistance to plastic deformation and additional information on the indentation phenomenon, which is not currently considered in existing specifications. This approach could be used alongside standard tests to better evaluate the performance of different materials in railway sub-ballast applications.