Jerry G. Rose

Asphalt trackbed technology development: the first 20 years

The evolution of hot-mix asphalt (HMA) trackbed technology is documented as presently practiced in the United States. Criteria used in selecting sites for and the attendant benefits of HMA trackbeds, based on long-term performance evaluations, are discussed. Prevailing practices for selecting ideal HMA mix parameters, trackbed section designs, and application procedures are described in detail. Primary attention is directed at the “underlayment” procedure in which the HMA serves as a premium subballast layer within the track structure to enhance the support, waterproofing, and confinement properties of the subballast. The roadbed or subgrade materials underlying the HMA mats maintain near-optimum moisture content. The HMA appears to undergo little if any weathering or deterioration in the trackbed environment. The resultant benefits are decreased maintenance costs, fewer slow orders, fewer operational interferences, and improved operational efficiency of the rail network overall.

UTILIZATION OF ASPHALT/BITUMINOUS LAYERS AND COATINGS IN RAILWAY TRACKBEDS - A COMPENDIUM OF INTERNATIONAL APPLICATIONS

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.

In-Situ Test Measurement Techniques Within Railway Track Structures

Recent changes in national transportation needs have placed increased burden on railroad infrastructure. To meet the increased demand for efficient freight transport, the railroad industry has increased traffic volume and maximized axle loadings. Increased axle loads have forced railroads to reevaluate existing infrastructure to ensure their ability to accommodate the additional traffic loads. It is imperative to design and maintain tracks such that they can withstand high volume and increasing axle loads over an extended service life, considering the track structure is the most significant capital expense for railroad companies. It has been desirable for years to develop non-intrusive procedures to directly measure pressures and stresses at various levels and interfaces in the railroad track structure in order to optimize track designs and improve subsequent track performance. Methods for measuring both pressures and deflections have been presented in recent research focusing on assessing the performance of trackbeds with increased track modulus, primarily through the addition of asphalt underlayment. These studies involve instrumenting HMA trackbeds with earth pressure cells and displacement transducers to measure pressure levels and distributions within the track structure and rail deflections under moving trains. Additional test methodologies have been developed to include pressure readings at interfaces like the rail/tieplate interface and the tieplate/tie interface using very thin pressure sensitive Tekscan sensors. The Tekscan Measurement System uses a piezoelectric film sensor composed of a matrix-based array of force sensitive cells, similar to mini strain gauges, to obtain accurate pressure distributions between two surfaces in the track. The procedure appears applicable for a wide variety of specific track related measurements to include: 1) analyzing pressure distribution patterns at the rail base/tie plate/tie interfaces to minimize wear and eliminate pressure points, 2) validating and optimizing horizontal curve geometric design criteria relative to superelevation, 3) assessing crossing diamond, other special trackwork, and bridge approach impact pressures, and 4) evaluating the advantages/disadvantages of various types of tie plates, fastenings, and tie compositions with the objective of equalizing pressure distributions over the interface areas. Results of testing are presented in detail for test installations on CSX Transportation heavy tonnage mainlines and at the Transportation Technology Center (Pueblo) low track modulus heavy tonnage test track.Copyright

Selected In-Track Applications and Performances of Hot-Mix Asphalt Trackbeds

The use of a layer of asphalt within railway trackbeds has steadily grown since the early 1980s in the United States. Its primary use has been for maintenance and rehabilitation applications in existing tracks, particularly at special trackworks, to improve trackbed performance, and to a lesser extent for new trackbed applications where the projected longterm performance of the asphalt trackbed is anticipated to be economically justified. Normally the asphalt layer is 6 in. (150 mm) thick, placed on a prepared subgrade or granular subballast, and is subsequently topped with a layer of typical ballast. Accepted highway/railway construction practices are adhered to, including adequate preparation and compaction of the support layers. In addition, surface and sub-surface drainage aspects are evaluated on a site-specific basis and improvements are specified based on accepted engineering practices. This application does not deviate significantly from conventional all-granular trackbed designs, except the asphalt layer is substituted for a portion of the thickness of the granular subballast and ballast support materials.

Numerical Investigation of Vibration Reduction of Ballast Track with Asphalt Trackbed over Soft Subgrade

A methodology of analyzing track vibration by Fourier transform technique is presented. It offers a means of establishing the four layer continuous elastic beam model for vibration analysis of ballast track with asphalt trackbed. The random irregularity of the vertical railway surface is considered and the asphalt trackbed is simulated as Euler beam with resistant bending modulus. First, the vibration equations of the ballast track are transformed by Fourier transform. The vibration displacements in Fourier transform domain are solved with this transformed equation. Then, the vibration responses of the track are obtained by performing inverse Fast Fourier Transform. As an application example, analysis of vibration reduction of ballast track with asphalt trackbed over soft subgrade is presented. Influences of different asphalt thicknesses and different rubber contents as well as different soft subgrade moduli on track vibration are investigated. The studies show that the use of asphalt trackbeds as a substitute for sub-ballast in conventional ballast track with soft subgrade can significantly reduce the deflections and accelerations of the rail, tie, ballast and asphalt and the pressures on the subgrade. For deflections and accelerations of the rail, tie, ballast and asphalt the maximum reduction can be up to 14.1% and 18.6% respectively compared with the conventional ballast track with soft subgrade. The maximum reductions of pressures on the subgrade can be up to 14%.

KENTRACK 4.0: A RAILWAY TRACKBED STRUCTURAL DESIGN PROGRAM

The KENTRACK program is a finite element based railway trackbed structural design program that can be utilized to analyze trackbeds having various combinations of all-granular and asphalt-bound layered support. It is applicable for calculating compressive stresses at the top of subgrade, indicative of potential long-term trackbed settlement failure. Furthermore, for trackbeds containing an asphalt layer, it is applicable for calculating tensile strains at the bottom of the asphalt layer, indicative of potential fatigue cracking. The program was recently expanded to include both English and international units. A procedure has been incorporated to provide a path to save results in a text formation in post-Windows XP operating systems. More importantly, properties of performance graded (PG) asphalt binders and the Witczak E* predictive model have been incorporated in the 4.0 Version of the program. Component layers of typical trackbed support systems are analyzed while evaluating the significance of layer thicknesses and material properties on design and predicted performance. The effect of various material parameters and loading magnitudes on trackbed design and evaluation, as determined and predicted by the computer program, are presented. Variances in subgrade modulus and axle loads and the incorporation of a layer of asphalt within the track structure have significant effects on subgrade vertical compressive stresses and predicted trackbed service lives. The parameter assessments are presented and evaluated using sensitivity analysis.Copyright

Track vibration analysis for railways with mixed passenger and freight traffic

Abstract By means of establishing the continuous elastic three layer beam model for track structures with random irregularity of the track vertical profile, a methodology of analysing track vibration by Fourier transform technique is presented. First, the vibration equations of the track structure are transformed by Fourier transform. The vibration displacements in Fourier transform domain are solved with this transformed equation. Then, the vibration responses of the track structure are obtained by performing inverse Fast Fourier Transform. As an application example, vibration analysis of the track for railways with mixed passenger and freight traffic is carried out. Influences of different speeds of passenger and freight trains and different line grades of the track irregularity on track vibration are investigated. The studies show that the worst situation for railways with mixed passenger and freight traffic is from freight trains. The track displacement and the track acceleration as well as the track dynamic forces on the ground resulting from freight traffic are larger by 54—57, 39—47, and 52—56 per cent, respectively than those from the passenger trains when both passenger and freight trains are running at the same speed. Also, the irregularity of the track vertical profile has great impact on track vibration. With the development of track irregularity, dynamic responses induced by freight trains increase more rapidly than those induced by passenger trains.

Comparative analysis on dynamic behavior of two HMA railway substructures

A numerical analysis using a finite element program was performed on three structures: hot mix asphalt (HMA) reinforced trackbed (RACS-1), HMA directly supported trackbed (RACS-2), and traditional Portland Cement Concrete (PCC) slab track (SlabTrack). Although the comprehensive dynamic responses of RACS-1 were similar with SlabTrack, HMA layer can positively affect the stress distributions. In particular, the horizontal stresses indicate that the resilience of RACS-1 was improved relative to SlabTrack. In addition, HMA reinforced substructure has the capacity to recover the residual vertical deformation. The effective depth for weakening dynamic loadings is mainly from 0 to 2 m, this being especially true at 0.5 m. The results from the analysis show that HMA is a suitable material for the railway substructure to enhance resilient performance, improve the stress distribution, weaken dynamic loading, and lower the vibration, especially at the effective depth of 2 m. The HMA constructed at the top of the stone subbase layer allows the vertical modulus a smooth transition. In terms of the comprehensive dynamic behaviors, RACS-1 is better than SlabTrack, while the results for RACS-2 are inconclusive and require further research.

KENTRACK, A PERFORMANCE-BASED LAYERED ELASTIC RAILWAY TRACKBED STRUCTURAL DESIGN AND ANALYSIS PROCEDURE - A TUTORIAL

KENTRACK is a layer elastic finite element based computer program that can be utilized for a performance-based structural design and analysis of railway trackbeds. Kentrack was initially developed to analyze traditional all-granular layered trackbeds and asphalt layered trackbeds. The versatility was recently expanded to analyze trackbeds containing a combination of granular and asphalt layers. The principle factor in the analysis is to limit vertical compressive stresses on the subgrade. In addition, it is possible to consider the fatigue lives of the various layers relative to the effects of wheel loads, tonnages, environmental conditions and other factors. The service lives of the individual components of the trackbed are predicted by damage analysis for various combinations of traffic, tonnages, subgrade support, component layer properties and thicknesses. The latest version, KENTRACK 3.0, is coded in C#.NET, a popular computer language for achieving accuracy and efficiency. The graphical user interface in the KENTRACK 3.0 provides a technique to analyze trackbeds as structures. It is possible with KENTRACK 3.0 to select trackbed layers and associated thicknesses to satisfy roadbed and trackbed performance requirements. In addition, it is possible to performance-rank different track sectional designs based on the relative importance of the particular track section and track type. The types of roadbed and trackbed configurations are selected to meet each of the various performance ranks. The various steps involved in the calculations are highlighted during the tutorial phase of a sample design calculations and analysis.Copyright

Evaluating tie support at railway bridge transitions

This paper compares the behavior of three different railway bridge transition zones to illustrate how poor tie support affects track performance. The three bridge transitions consist of a high-speed passenger line, a freight line, and a spur track. All bridge transitions were instrumented with accelerometers that allow tie support and track performance to be non-invasively evaluated by analyzing the measured acceleration magnitudes and vibration frequencies in the frequency domain. The results show tracks with good tie support display tie accelerations below 5 g and small vertical displacements during train loading whereas approaches with poor tie support display accelerations generally greater than 5 g. These results are used to evaluate other transition zones and identify problematic track locations that require repair procedures to retain acceptable track geometry.