Tests and Evaluations of In-Service Asphalt Trackbeds

Rose, Li, & Walker TESTS AND EVALUATIONS OF IN-SERVICE ASPHALT TRACKBEDS by Jerry G Rose, Ph.D., PE Professor of Civil Engineering 161 OH Raymond Building University of Kentucky Lexington, Kentucky 40506-0281 USA 859 257-4278 859 257-4404 (Fax) jrose@engr.uky.edu Dingqing Li, Ph.D., PE Senior Engineer Transportation Technology Center, Inc PO Box 11130 Pueblo, Colorado 81001-4812 USA (719) 584-0740 (719) 584-0770 (Fax) dingqing_li@ttci.aar.com Lindsay A Walker, BSCE, EIT Graduate Research Assistant 161 OH Raymond Building University of Kentucky Lexington, Kentucky 40506-0281 USA 859 257-5927 859 257-4404 (Fax) lindsay_c83@yahoo.com To be presented at the American Railway Engineering and Maintenance-of-Way Association 2002 Annual Conference & Exposition, September 24, 2002, Washington, DC Rose, Li, & Walker TESTS AND EVALUATIONS OF IN-SERVICE ASPHALT TRACKBEDS Jerry G Rose, Dingqing Li, & Lindsay A Walker During the past twenty years the use of hot mix asphalt (HMA) as a sub-ballast layer within the track structure has steadily increased until it is becoming standard practice in many areas of the United States This asphalt-bound impermeable layer, typically to in (125 to 200 mm) thick, forms a superior “hardpan” to protect the underlying roadbed and to support the overlying ballast and track Long-term performance studies on numerous HMA installations attest to the improved attributes and economic benefits of the HMA layer, particularly on heavy tonnage lines traversing areas of marginal geotechnical engineering characteristics Previous investigations–involving core drilling, sampling, and characterization of trackbed materials–were conducted on twelve in-service HMA trackbeds These were widely scattered over six different states and averaged thirteen years of service The strength and bearing capacity values of the protected roadbed materials remain near optimum, thus assuring adequate support for the track The HMA layer–protected from temperature extremes, sunlight, and oxidation–maintains mechanical properties essentially unaffected after many years of exposure and loading The results of these investigations are summarized More recent studies involve instrumenting several HMA trackbeds with earth pressure cells and displacement transducers to measure trackbed pressures and deflections and to calculate track stiffness (modulus) These tests, conducted in the real time domain train operations, confirm the Rose, Li, & Walker positive attributes of the HMA layer Results are presented for several test installations on CSX Transportation heavy tonnage mainlines and for the Transportation Technology Center (Pueblo) low track modulus heavy tonnage test track For the 115 ton (105 metric ton) loaded hopper cars, track deflections are typically 0.25 in (6 mm) for wood tie track and 0.05 in (1.5 mm) for concrete tie track These equate to dynamic track modulus values of 3000 lb/in/in (20 MPa) and 7500 lb/in/in (50 MPa) respectively Vertical pressures at the ballast/HMA interface are a function of imposed loadings and range up to 17 psi (120 kPa) for 36 ton (33 metric ton) axle loads Pressures are further reduced to about to psi (35 to 50 kPa) under the HMA layer at the subgrade interface It is shown that the low trackbed stress level is due in part to the high sheer stress development in the ballast since the HMA layer provides optimum restraint and support for the ballast The low stress level assures a long fatigue life for the asphalt layer The results of these investigations and associated relationships which were developed are presented in detail The use of an asphalt layer within the track structure is appropriate for both the construction of new lines and the rehabilitation of existing lines The long-term economics are particularly beneficial for special trackworks and poor subgrade/drainage conditions in open track Key Words: railway, trackbed, asphalt, underlayment, pressure, deflection Rose, Li, & Walker INTRODUCTION1 During the past twenty years in the United States, the use of hot mix asphalt (HMA) as an underlayment (or sub-ballast) layer within the track structure has steadily grown It is becoming a standard procedure on heavy tonnage rail lines in certain areas of the U.S., particularly where prevailing subgrade materials and drainage conditions are not compatible with conventional open granular trackbed designs The HMA layer strengthens trackbed support and waterproofs the underlying roadbed It also provides a consistently high level of confinement for the overlying ballast and track These factors become increasingly more significant as axle loads and total tonnages increase on mainlines For example, the Association of American Railroad statistics (1) reveal that average freight car capacities have steadily increased over the years and presently average 92.7 tons (84 metric tons), double that of 1929 The 100-ton car (91 metric tons) having a gross weight of 263,000-lb (119 metric tons) has been standard for years, but is being rapidly replaced by the 286,000-lb (130 metric tons) car The 315,000-lb (143 metric ton) car is undergoing testing Furthermore, in 2000 the U.S railroads set volume records for ton-miles, tonnage, and intermodal traffic Revenue ton-miles increased 2.3% over the prior year to 1.47 trillion, a record high, while tonnage jumped to a record high of 1.74 billion Car loadings, which rose 2.5% in 2000, attained their highest level in three decades, using today’s higher capacity cars Also hauled were a record 9.2 million high priority, time sensitive intermodal trailers and containers Obviously today’s U.S heavy haul tonnage railroads require high performance track structures to minimize maintenance outages and enhance operating conditions Rose, Li, & Walker TYPICAL ASPHALT UNDERLAYMENT PRACTICES The typical HMA layer is 12 ft (3.7 m) wide and is to in (125 to 150 mm) thick (2) For unusually poor roadbed support conditions and high impact areas, an in (200 mm) thickness is used Thickness of the overlying ballast normally ranges from to 12 in (200 to 300 mm) HMA is used for new track construction and for rehabilitation/maintenance of existing lines It has a wide range of applications including open track, special trackwork (switches or turnouts, crossing diamonds, etc.) bridge approaches, tunnels and tunnel approaches, and highway/rail crossings Figure is a typical cross-sectional view The common HMA mixture specification is the prevailing dense-graded highway base mix in the area having a maximum aggregate size of to 1.5 in (25 to 37 mm) Normally the asphalt binder content is increased by 0.5% above that considered optimum for highway applications resulting in a low to medium modulus (plastic) mix having a design air voids of to 3% It is believed that this slight modification to the typical highway mix will impart the ideal properties to the track structure This mix is easier to densify to less than 5% in-place air voids assuring adequate strength and an impermeable mat Rutting of the plastic mix is not a concern in the trackbed since the pressures are applied through the ballast over a wide area Bleeding and flushing are also non-issues since the wheels not come in direct contact with the HMA layer and the temperature extremes are minimized in the insulated trackbed environment HMA TRACKBED STUDIES AT UNIVERSITY OF KENTUCKY Development of asphalt trackbed technology has been ongoing at the University of Kentucky since the early 1980s (3, 4, 5) Most of these endeavors have been supported by CSX Transportation and conducted on CSXT rail lines in the eastern portion of the United States Rose, Li, & Walker Additional studies have been supported by BNSF Railway in the midwest portion of the U.S These two railroads account for nearly 50 percent of the Class I railroad industry in the U.S Trackbed Materials Classifications Recent investigations–involving core drilling, sampling and characterization of trackbed materials–were conducted on twelve in-service HMA trackbeds on CSXT and BNSF revenue lines in six states (6) These HMA trackbeds, averaging 13 years of service, were providing essentially maintenance-free service and were selected to include varying geographical and geological conditions Of particular interest was determining the types, conditions, and moisture contents of the old roadbed/subgrade materials directly under the HMA mat The investigations involved a wide variety of substructures–from low-strength (high plasticity) clays to moisture-sensitive silts to higher quality granular materials The significant finding was that the in-situ moisture contents are very close to laboratory determined optimum values for maximum density of the respective materials The HMA mat does not appear to be performing as a membrane to collect and trap moisture, thus weakening support Actually, since the in-situ moisture contents are at or near optimum for maximum density, the strengths and load carrying capacities of the underlying materials remain uniformly high Furthermore, average moisture contents have remained essentially unchanged, at or near optimum, for the two projects from which previous data was available For design purposes, it is reasonable to base strength or bearing capacity values at optimum conditions (moisture content and density) for the material under the HMA mat Using strength or bearing capacity values Rose, Li, & Walker determined for the soaked condition, common for highway designs, is inappropriate for HMA trackbed designs The unsoaked, optimum moisture content condition is consistent with inservice trackbed conditions The HMA cores and extracted/recovered asphalt binders were extensively evaluated at the National Center for Asphalt Technology at Auburn University with assistance from the Asphalt Institute Selected samples were forwarded to the Western Research Institute for indepth tests and evaluations The primary purpose was to determine if any significant weathering or deterioration of the HMA (insulated from sunlight and temperature extremes) was occurring in the trackbed environment, which could adversely affect long-term performance A variety of HMA mixture compositions and mat thicknesses were evaluated It was concluded that the various asphalt binders and HMA mixes did not exhibit any indication of excessive hardening (brittleness), weathering, or deterioration even after many years in the trackbed environment This is primarily due to the insulative effects of the overlying ballast This protects the HMA from sunlight and excessive temperature extremes, which significantly reduces oxidation and hardening of the asphalt binder The mat remains slightly flexible, which contributes to a long fatigue life for the HMA layer There is no indication that the HMA mats are experiencing any loss of fatigue life These findings were further confirmed by extensive chemical analyses of the recovered asphalt binders, which were conducted at the Western Research Institute It has been observed that mixes specifically designed to be more viscous (plastic) are conducive to the angular ballast particles slightly penetrating or imbedding into the top surface of the Rose, Li, & Walker asphalt mat This increases the interfacial shear strength and improves overall structural value of the track structure Furthermore, the uniformly high level of support provided by the HMA mat maintains a high degree of ballast compaction which results in increased modulus, reduced wear, and increased life of the ballast This is a primary contributor to the extended excellent track geometry indicators provided by the HMA mat and confined ballast layer The combined supports provided by the HMA mat and the confined ballast layer are believed to be primary contributors to the excellent track geometry indicators routinely measured over long periods of time Trackbed Pressure/Stress Measurements Trackbed pressure (stress) measurements have been obtained at prevailing speeds under heavy tonnage railroad loadings Pressure measurements were recorded using hydraulic type (Geokon model 3500-2) earth pressure cells (Figure 2) These are imbedded in the track structure above and below the HMA mat The location of one of these on the mat can also be seen in Figure Peak pressures occur directly below the tie/rail interface Figure is a typical plot of the pressures exerted on top of the HMA mat for an empty coal train Vertical pressures imposed by typical 286,000 lb (130 metric ton) locomotives range from 13 to 17 psi (90 to 120 kPa) on top of the HMA mat The average locomotive wheel load is 35,000 lb (16 metric tons) Pressures are reduced to to psi (15 to 30 kPa) under the 62,000 lb (28 metric ton) empty cars which have an average wheel load of 8000 lb (3.5 metric tons) The beam action of the track, which distributes the concentrated wheel loadings over several ties and the confined, high modulus ballast layer, serve to effectively reduce the heavy wheel loadings By comparison, a 180 lb (82 kg) person will exert about psi (40 kPa) pressure while standing on a Rose, Li, & Walker level surface Furthermore, typical tire pressures imposed on highway asphalt surfaces under loaded trucks range from 100 psi (700 kPa) to over 200 psi (1400 kPa) depending on the magnitude of loading and tire configurations The effect of flat wheels on pressures exerted within the track structure has also been evaluated Figure is a fully loaded auto train Note that the pressure at the top of the HMA is increased by three orders of magnitude It can be concluded that trackbed vertical stress levels on top of the HMA mat under heavy tonnage railroad loadings are very low and only a fraction of those imposed by high-pressure truck tires on highway pavements The HMA mat should have an extremely long fatigue life at the load-induced pressure levels existing in the trackbed environment Trackbed Deflection Measurements Dynamic track deflections have been recorded in conjunction with the pressure measurements using linear variable displacement transducers referenced to a fixed datum (Figure 5) Rail deflections under the 286,000 lb (130 metric ton) locomotives and loaded cars average 0.25 in (6 mm) for wood tie track and around 0.05 in (1.5 mm) for concrete tie track (Figure 6) These are considered optimum for both track types Calculated dynamic track modulus (stiffness) values are in the 2500 lb/in/in (17 MPa) range for wood tie track and around 7500 lb/in/in (52 MPa) for concrete tie track These are also considered optimum The concrete tie track deflects much less than the wood tie track and is thus much stiffer This increases pressure values within the ballast The ballast must be properly supported from below so it can develop high shear strength to reduce the higher than normal Rose, Li, & Walker imposed loading pressures The high modulus HMA mat provides increased support and confinement for the ballast in concrete tie track Temperatures at the ballast/HMA layer have been periodically monitored using thermisters which are an integral part of the pressure cells Figure shows the relationship between temperature and time during the year measurements were taken Since the HMA is insulated from the atmosphere by the overlying ballast and track, the temperature extremes in summer and winter are minimized The maximum temperature recorded in the summer was 75ºF (24ºC) and the minimum in the winter was 36ºF (2ºC) Pavements exposed to the atmosphere and direct sunlight will typically experience temperature extremes of 120ºF (50ºC) to 0ºF (17ºC) in the Kentucky climate HMA TRACKBED STUDIES AT TRANSPORTATION TECHNOLOGY CENTER The Association of American Railroads subsidiary, Transportation Technology Center, Inc (TTCI), has been involved with additional measurements and evaluations of HMA underlayment trackbeds (7) Explanations of these recent research efforts are detailed in the following sections Introduction and Background One of the main causes for track geometry deterioration is the deterioration of soft subgrade support Without remedy, a subgrade of fine-grained soils will develop excessive deformation under heavy axle loads, which in turn will lead to excessive track maintenance costs Geometry deterioration due to soft subgrade support will worsen with an increase in train axle loads or operating speeds In recent years, the effects of heavy axle loads upon track substructure performance have been studied at the High Tonnage Loop (HTL) at the Transportation Rose, Li, & Walker 14 Rose, J.G (2000) Asphalt Trackbeds: Selection, Design, Installation Practices, LongTerm Performances & Material Properties Proceedings of Railway Engineering-2000 3rd International Conference and Exhibition, London, July, 12 p 6 Rose, J.G., Brown, E.R & Osborne, M.L (2000) Asphalt Trackbed Technology Development: The First 20 Years Transportation Research Record 1713, Transportation Research Board, pp 1-9 Li, D., Rose, J & LoPresti, J (2001) Test of Hot-Mix Asphalt Trackbed Over Soft Subgrade Under Heavy Axle Loads Technology Digest 01-009, Transportation Technology Center, Inc., April, p Rose, Li, & Walker LIST OF TABLES TABLE Composition of Dense-Graded HMA Mix TABLE Marshall Mix Design Criteria for HMA Underlayment 15 Rose, Li, & Walker 16 TABLE Composition of Dense-Graded HMA Mix Sieve size 1.5 inch ¾ inch 3/8 inch No No No 16 No 30 No 50 No 200 37.5 mm 19 mm 9.5 mm 4.75 mm 2.36 mm 1.18 mm 0.60 mm 300 µm 75 µm Asphalt Amount finer, weight % Recommended 100 70 - 98 44 - 76 30 - 58 21 - 45 14 - 35 - 25 - 20 2-6 3.5 - 6.5 Actual 100 76 52 41 30 23 17 11 4.5 6.4 Rose, Li, & Walker 17 TABLE Marshall Mix Design Criteria for HMA Underlayment Property Required Range Compaction 50 blows Stability lbs (N) Minimum 750 (3300) Flow inch (mm) 0.15 – 0.25 (3.8 - 6.4) Percent air voids - 3% Voids filled w/asphalt 80 - 90% In-place density* 92 - 98% *Maximum density = 151 ptc (2424 kg/m³) ** Average nuclear density test results Actual Test Results 50 blows 1730 (7700) 0.24 (6.1) 2% 86% 94%** Rose, Li, & Walker 18 LIST OF FIGURES Figure 1.Typical Cross-section Figure Pressure Cells Figure Empty Coal Train at Conway Figure Flat Wheel on a Loaded Auto Train at Conway Figure LVDT Configuration Figure Deflection under Loaded Coal Train Figure Conway Top of HMA Temperature vs Time Figure Longitudinal Cross Section of HMA Test Track Figure Test Results in Track Modulus (consolidated ballast) and Subgrade Stress (under 40 kip (18 metric ton) static wheel load) Figure 10 Reduction of Dynamic Stresses from 8-in (200 mm) HMA to Subgrade under 39 ton (35 metric ton) Axle Cars Figure 11 Track Settlement as a Function of Traffic Figure 12 HMA Temperature vs Air Temperature Rose, Li, & Walker 19 Figure 1: Typical Cross-section Rose, Li, & Walker 20 Figure 2: Pressure Cells Rose, Li, & Walker 21 Rose, Li, & Walker 22 Rose, Li, & Walker 23 Rose, Li, & Walker 24 Rose, Li, & Walker 25 Rose, Li, & Walker 26 Rose, Li, & Walker 27 Rose, Li, & Walker 28 ...Rose, Li, & Walker TESTS AND EVALUATIONS OF IN-SERVICE ASPHALT TRACKBEDS Jerry G Rose, Dingqing Li, & Lindsay A Walker During the past twenty years the use of hot mix asphalt (HMA) as a sub-ballast... Performance of Hot Mix Asphalt Railway Trackbeds Transportation Research Record 1300, Transportation Research Board, pp 35-43 Rose, J.G (1998) Long-Term Performances, Tests and Evaluations of Asphalt Trackbeds. .. sampling and characterization of trackbed materials–were conducted on twelve in-service HMA trackbeds on CSXT and BNSF revenue lines in six states (6) These HMA trackbeds, averaging 13 years of service,