Experimental investigation of a ballastless asphalt track mockup under vertical loads

Abstract

This study presents experimental results from a laboratory investigation into the mechanical behaviour of a ballastless asphalt track under vertical loads and isothermal conditions. A full-scale test section was constructed inside a steel box, consisting of three wide-base sleepers resting on an asphalt layer that was underlain by an unbound granular layer (UGL) supported on a rubber mat (representing subbase and subgrade). Sensors were installed to measure diverse responses, consisting of vertical stresses at the bottom of the UGL, horizontal strains at the bottom of the asphalt layer, relative vertical displacements between various track components, and vertical surface accelerations. Sleepers were loaded directly on top of the rail pads by using servo-hydraulic actuators. Cyclic loads were applied to investigate the effects of different excitation amplitudes and frequencies. It was found that all measured responses displayed a strong frequency dependence. Vertical stresses below the UGL varied linearly with the load amplitude, while other responses showed a non-linear relationship. Train passages with a maximum speed of 200 km/h and axle loads up to 200 kN were simulated by sequentially loading the three sleepers. From this load type, it was found that ballastless asphalt track exhibited time-dependent behaviour such as delayed recovery of strains in-between axle passes. Furthermore, the majority of the vertical actuator displacement was absorbed by rail pad compression. Lastly, measured stresses and strains were of very low magnitudes, suggesting marginal long term mechanical damage under service loads for such a ballastless asphalt track structure.