Punching test for mechanical characterization of asphalt railway sub-ballast
The adoption of asphalt sub-ballast layers in high-speed and high-capacity railway lines has gained traction across Europe and the United States, driven by structural, functional, and economic advantages. However, current mechanical characterization methods, inherited from road paving practice, fail to capture railway-specific interactions, particularly the ballast/sub-ballast interface and the granular behavior of the overlying unbound layer. This study introduces a novel punching test employing an adaptive indentation plate (AIP) to replicate the contact between ballast particles and the sub-ballast. Cylindrical asphalt specimens (150 mm diameter) were subjected to vertical point loads via the AIP at temperatures of 5, 20, and 35 °C and deformation rates of 5.08, 25.4, and 50.8 mm/min. Two asphalt mixes compliant with Italian sub-ballast standards—one with conventional B50/70 binder (SSB) and one with polymer-modified PmB 45/80-65 (HSB)—were tested to validate the procedure. Force–displacement curves revealed three interaction phases, and key parameters such as peak force (Fmax), displacement at peak force (uy,*), force at 1.5 mm deflection (Fu,1.5), and secant slope (m) between 1.25 and 1.75 mm were extracted. Results demonstrate that the punching test effectively evaluates resistance to plastic deformation and indentation behavior, offering insights beyond existing specifications. The lowest deformation rate (5.08 mm/min) best captures viscoelastic properties. This methodology provides a valuable tool for assessing bituminous sub-ballast performance under simulated railway interface conditions.