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During the past thirty years the use of a layer(s) of hot-mix asphalt pavement within railway track structures has steadily increased until it is becoming a common consideration or practice for specific conditions and areas in several countries throughout the world. This practice augments, and for certain designs replace, the traditional granular support materials. It is considered to be a premium trackbed design. The primary documented benefits are to provide additional support to improve load distributing capabilities of the trackbed components, decrease load-induced subgrade pressures, improve and control drainage, insure maintenance of specified track geometric properties for heavy tonnage freight lines and high-speed passenger lines, and decrease subsequent expenditures for trackbed maintenance and component replacement costs. The asphalt layer is normally used in combination with traditional granular layers to achieve various configurations. This paper presents a compendium of International Asphalt Trackbed Applications. The various factors are discussed that are considered in the design phases and subsequent performance-based tests and analyses. Illustrations include typical sectional views of the trackbed/roadbed components and thicknesses and photographs of construction and finished views for various asphalt trackbed applications in several countries. Following are brief accounts for selected significant international activities emphasizing high-speed and intercity passenger rail line applications. In the United States the use of asphalt trackbeds has steadily grown since the early 1980’s. It is primarily used for maintenance (cure-all) applications in existing tracks to improve trackbed performance and for new trackbed construction where the projected superior performance of asphalt trackbeds can be justified economically. Typically the asphalt layer is 15 cm thick and is topped with conventional ballast. This application does not deviate significantly from typical designs, except the asphalt is substituted for a portion of the granular support materials. Several other countries are actively involved with the construction of new segments or complete rail lines using asphalt (frequently termed – bituminous) trackbeds. For instance, Japan has used asphalt trackbeds on certain test sections for their high-speed rail lines since the 1960’s, but since the 1970’s asphalt trackbeds with ballast cover is a standard on newly constructed rail lines. The 5-cm thickness of asphalt primarily serves as a waterproofing layer and facilitates drainage. The Japanese believe that this will assist in reducing subsequent maintenance costs associated with ballast fouling from subgrade pumping. The Japanese have recently instigated a performance-rank design system. Asphalt trackbed designs are either required or are an option for the two premium trackbed performance ranks. Italy represents another country heavily involved with incorporating asphalt trackbeds in their rail lines. In the late 1970’s Italy placed test sections of both asphalt and concrete on their original Rome to Florence high-speed line. From the Italian perspective the asphalt out-performed the other test sections, leading to standards requiring the use of asphalt trackbeds on all newly constructed high-speed passenger rail lines. The typical asphalt layer thickness is 12 cm. Germany has focused on using asphalt for ballastless trackbed designs. The main asphalt track in use in Germany consists of concrete ties or slab track placed on a 26 to 30-cm thick layer of asphalt. Various designs are incorporated into the system. Recently France installed a 3-km test section of asphalt on their Paris to Strasbourg Eastbound High-Speed Line. The French are currently observing the effects of high-speed trains traversing various test sections to determine how beneficial the use of asphalt trackbeds will be for future high-speed passenger lines. The sections are heavily instrumented for analyzing numerous trackbed induced effects on ride quality and other aspects. Other countries, a recent addition includes Spain, are involved to varying degrees with the development of asphalt trackbed technology, particularly for high-speed and intercity passenger rail lines. Pertinent information and documentation of recent findings and results are included in the paper.

This paper presents the first phase laboratory test results of a research project at the University of Kentucky A number of resonant column tests were conducted to measure the stiffness and damping ratio of rubber-modified asphalt (RMA) samples with different rubber contents and mixed by different methods. The measurements were conducted under a wide range of confining pressures and shear strains to simulate the stress conditions that would be expected in a typical railway foundation. Data from the tests show that RMA has stiffness much higher than compacted soils while providing significantly higher damping ratio than most soils under the same strain level. Therefore, the results from this study indicate that RMA has great potential as a foundation material for high-speed railway trackbeds.

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The pressure distribution at the ballast–tie interface of conventional railroad track plays a key role in overall track support. Loads exceeding the strength of the ballast or tie can contribute to degradation of track quality. In this study, matrix-based tactile surface sensors (MBTSS) were used to study the load distribution at the ballast–tie interface. MBTSS allows for fine-scale pressure distributions to be measured unobtrusively and in a dynamic load environment. In this application, the loads imparted by individual ballast particles can be measured. Laboratory ballast box testing and in-track testing were conducted at the Transportation Technology Center. Ballast gradation at the interface was varied for both laboratory and in-track testing. Laboratory results indicated that under nominal heavy axle loads, average peak ballast–tie pressures ranged from 284 psi (1,960 kPa) on sand to 1,450 psi (10,000 kPa) on new ballast. In-track testing found that six of the 10 ties tested showed higher pressures adjacent to the rail and not directly underneath it. In both cases, the contact area was shown to increase under an increasing applied load, in part because of additional ballast particles being engaged as the tie deflects. The high peak pressures observed in the laboratory and the variability of pressure distribution along the tie observed in-track significantly varied from the ballast–tie pressure distribution recommended by the American Railway Engineering and Maintenance-of-Way Association's Manual for Railway Engineering. Ballast–tie interface characterization has implications for tie structural design, ballast degradation, and under-tie pad design.