2001 Concrete Slab Track State of the Practice_D. Tayabji LIDO | Rail Transport

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  Railway track technology has evolved over a period of 150 years sincethe first railroad track on timber ties was introduced. For much of thisperiod, the conventional track system, commonly referred to as the bal-lasted track system, has consisted of certain components including rails,ties, ballast, and the subgrade (roadbed). Over the last 20 years, therehas been an increase in the use of concrete slab technology for transit,commuter, and high-speed train applications. Essentially, a slab trackconsists of a concrete slab placed on a subbase over a prepared sub-grade. The rails may be directly fastened to the concrete slab, or the railsmay be placed on concrete blocks or another slab system that is placedon (or embedded in) the underlying concrete slab. A version of the slabtrack, developed in the Netherlands, incorporates rails embedded in atrough in the slab and surrounded by elastomeric material. The slabtrack systems for passenger service applications incorporate severalrequirements to mitigate noise and vibration. The slab track system fortransit applications in the United States and for high-speed rail inEurope and Japan has performed well over the last 20 years. Also, thelimited application of the slab track system for mixed passenger service-freight operations has also exhibited good performance. Because of thecontinued increase in gross tonnage expected to be carried by railroadsand the expected growth in high-speed passenger rail corridors, with thesmaller deviation in the rail geometry allowed for high-speed rail, theneed for a stronger track structure is apparent. At-grade concrete slabtrack technology is expected to fill the need for stronger track in theUnited States. The state of the practice related to concrete slab tracktechnology is summarized. Railway track technology has evolved over a period of 150 yearssince the first railroad track on timber ties was introduced. For muchof this period, the conventional track system, commonly referred toas the ballasted track system, has consisted of certain componentsincluding rails, ties, ballast, and the subgrade (roadbed). Ties arepredominantly wood ties, but concrete ties are also widely used forboth transit and freight applications.Over the last 20 years, there has been an increase in the use of concrete slab technology for transit, commuter, and high-speed trainapplications. Essentially, a slab track consists of a concrete slabplaced on a subbase over prepared subgrade. The rails may be di-rectly fastened to the concrete slab or the rails may be placed on con-crete blocks or another slab system that is placed on (or embeddedin) the underlying concrete slab. A new version of the slab track,developed in the Netherlands, incorporates rails embedded in atrough in the slab and surrounded by elastomeric material. The slabtrack systems for passenger service applications incorporate severalrequirements to mitigate noise and vibration.The slab track system for transit applications in the United Statesand for high-speed rail in Europe and Japan has performed well overthe last 20 years. Also, the limited application of slab track formixed passenger service-freight operations has also exhibited goodperformance. Because of the continued increase in gross tonnageexpected to be carried by railroads and the expected growth in high-speed passenger rail corridors, with the smaller deviation in the railgeometry allowed for high-speed rail, the need for a stronger track structure is apparent ( 1 ). At-grade concrete slab track technology isexpected to fill the need for stronger track in the United States. Thispaper summarizes the state of the practice related to concrete slabtrack technology. HISTORICAL DEVELOPMENT OF SLAB TRACK TECHNOLOGIES Although slab track-type systems have been used in tunnels and onbridges for a much longer period, the application of slab track sys-tems for at-grade application has occurred only in the last 30 years.An excellent review of the at-grade concrete slab track system forrailroad and rail transit system as of 1980 is presented elsewhere ( 2 ).The use of slab tracks has further evolved over the last 20 years fortransit and high-speed applications, especially in Japan, the Nether-lands, Germany, and North America. However, only a limited appli-cation of slab tracks for dedicated freight service application hasbeen reported. Types of Slab Tracks The types of concrete slab track systems that have been used areclassified into the following four major categories:1.Cast-in-place at-grade systems. These include jointed plain or reinforced concrete slabs or continuously reinforced concrete(CRC) slabs.2.Embedded tie systems. These include systems that use block ties or full ties with rubber boots embedded in concrete that is placedon a concrete base.3.Two-slab layer systems. These include a surface slab system(typically precast, but also cast in place) that is placed on a concretebase. The precast surface slab system is typically separated from theconcrete base with an asphalt sandwich layer or a cement-basedgrout material.4.Embedded rail systems (ERSs). These include rail embeddedin an elastomeric or cementitious material in a preformed trough ina concrete slab.The slab track systems need to incorporate adequate resiliencyand require a good-quality subbase and adequate provisions forsubsurface drainage and frost protection to mitigate excessive sub-grade settlement under train loading. The details of various types of  Concrete Slab Track State of the Practice Shiraz D. Tayabji and David Bilow S.D. Tayabji, Construction Technology Laboratories, Inc., 5565 Sterrett Place,Suite 312, Columbia, MD 21044. D. Bilow, Engineered Structures Group, PortlandCement Association, 5420 Old Orchard Road, Skokie, IL 60077. Transportation Research Record 1742  87 Paper No. 01-0240   slab tracks in use or tested experimentally in Japan, the Nether-lands, Germany, and North America are given in the following sec-tions. It should be noted that the most common use of slab track todate in the United States has been for passenger train operationsrequiring special track component features to minimize track main-tenance cost and maintain track geometry in areas where this wouldbe difficult. Slab Track Use in Japan The Japanese National Railways began production use of slab track more than 30 years ago. Slab track has been used both on theShinkansen Lines and narrow-gage lines for more than 2400 km,including tunnels and bridges ( 3 ). After experimentation, a slabtrack design, referred to as Slab Track Type A, was selected for rou-tine use for tunnels and viaducts. The slab tracks consist of precastconcrete slabs, 5 m long, and a cement asphalt mortar (CAM) layerbeneath the slab. On the roadbed concrete of a viaduct or in a tun-nel, lateral stopper concrete (400 mm in diameter and 200 mm high)is provided at intervals of 5 m. The track slabs are made of prefab-ricated reinforced concrete, prestressed concrete, or prestressed re-surfaced concrete. The track slab for the Shinkansen Lines is nom-inally 2340 mm wide, 4930 to 4950 mm long, and 160 to 200 mmthick. A one-track slab weighs approximately 5 tons. Direct-fixationfasteners are used to seat the rail on the slab. Recent modificationsto the slab track include use of a vibration-reducing grooved slabmat under the track slab.Although most of the slab track was initially used for tunnels andbridges, a slab track system, denoted Type RA, was tried on soil road-bed during the mid-1970s. However, no large-scale installations of this type of track were made because of concerns with excessive set-tlements. A current version of the slab track for at-grade application,referred to as the reinforced concrete roadbed system (RCRS), isshown in Figure 1 ( 4 ). This system has been undergoing experimen-tal testing and monitoring since the early 1990s and has been used onthe Hokuriku Shinkansen Line from Takasaki to Nagano, which 88 Paper No. 01-0240  Transportation Research Record 1742 opened for service in October 1997. The cost of the RCRS-type track was reported to be higher than that of ballasted track by 18 percent incuts and by 24 percent in fill sections. It was reported that because of low track maintenance, the extra costs would be recovered in about12 years of operation. It is also expected that the workforce requiredto maintain the slab track would be about 30 percent lower than thatrequired to maintain ballasted track.A frame-shaped slab track is being investigated to reduce initialconstruction costs.The slab tracks of the Shinkansen Lines carry 10 to 15 milliongross tons (MGT) per year (as of 1990). Although the overall per-formance of the slab track has been good, some problems have beennoted. These include damage to CAM layers, cracking in the slabtrack due to alkali-silica reaction, and warping of slab in tunnels.Overall, maintenance for slab track was found to be much less thanthat for ballasted track on the Shinkansen Lines, ranging from about18 to about 33 percent of that for ballasted track. Slab Track Use in the Netherlands Several innovative slab track-type systems have been developed inthe Netherlands. These include the ERS, the Edilon Block Track,and the Deck Track. Embedded Rail System  The ERS, in use in the Netherlands since the 1970s, involves theprovision of continuous support for the rail by means of a compoundconsisting of Corkelast (a cork-polyurethane mixture) ( 5 ). The sys-tem shown in Figure 2 has been used on a limited basis on bridgesand level crossings.Recently, a 3-km length of the ERS concrete slab track on gradewas built in the south of the Netherlands. The structure consists of a CRC slab resting on a cement-stabilized base, which was placedover a sand subbase. An advantage of this system is that the finaltrack geometry is not influenced by the geometry of the supporting FIGURE 1Japanese-type RCRS slab track on grade (dimensions are given in millimeters; CA  cement asphalt; RC  reinforced concrete) (  4 ).  slab. The use of the ERS for HSL-Zuid, the high-speed line fromAmsterdam to the Belgian border, is now being considered. Edilon Block Track  The Edilon block track is mainly used for bridges and tunnels ( 6  ).It is a “top-down” construction system. Under this system, the firststep is to place the rails and blocks in position. The blocks are thencast in the concrete slab with Corkelast (to provide the necessaryelastic support). This is similar to other systems in which rubber-booted tie blocks are cast into the concrete (e.g., the Stedef system,the Sonneville low-vibration track system, and the Swiss Walo sys-tem). The Edilon system has been used for more than 100 km onrailways and light rail transit systems in the Netherlands and formore than 100 km of the metro system in Madrid, Spain. Deck Track  The deck track is a recent innovation developed for use with embed-ded rail ( 7  ). A 200-m test section of the track was constructed dur-ing the spring of 1999 near Rotterdam, the Netherlands, and wasopened to traffic in July 1999. Many heavy freight trains use thetrack every day. Although it is too early to judge the performance of the track, the constructibility of the track has been demonstrated.Construction cost data are not available. Slab Track Use in Germany  Slab track use has been undergoing development in Germany formany years. Approximately 330 km of slab track has been con- Tayabji and Bilow Paper No. 01-0240  89 structed throughout the German Railway (DB) network. In 1996,DB began operating in Karlsruhe a test track consisting of sevennew types of ballastless track. One of the best-known designs is theRheda system ( 8  ). In the Rheda system, the concrete ties are castinto a continuously resurfaced concrete slab. The Rheda system,developed in the 1970s, is shown in Figure 3. During construction,track consisting of rail, ties, and fastenings is assembled on the baseslab. After laying and lining of the slab panels, concrete is placedinto the cribs and spaces below the ties. It is required that the slabtrack be constructed over load-bearing, frost-protected subgrade andthat the groundwater be greater than 1.5 m below the slab. The jus-tifiable cost factor for the slab track in Germany is considered to beless than 1.4 of that for ballasted track. Slab Track Use in North America Although slab track has been in limited use for many years, use of slab track in North America has been steadily increasing over the last10 years. The predominant use of slab tracks has been in transit sys-tems. Early use on transit systems was for tunnel and bridge sectionswith systems customized to reduce noise and vibration. Some of thistrackage included the floating slab design, which was used forthe Washington, D.C., Metro subway system. The embedded track system, which consisted of dual tie blocks set in rubber boots andwhich used microcellular pads locked in with a second pour of con-crete, has been used for transit systems in San Diego, California;Atlanta, Georgia; Toronto, Ontario, Canada; San Francisco, Califor-nia; St. Louis, Missouri; Portland, Oregon; Philadelphia, Pennsylva-nia; Baltimore, Maryland; and Dallas, Texas. The rubber boots andmicrocellular pads provide vibration isolation and electrical isolation.A discussion of selected at-grade slab track projects follows. FIGURE 2Embedded rail slab track system (  6  ). (Corkelast is a trademark.) (b) FIGURE 3Rheda slab track system: ( a  ) cross section, (  b ) asconstructed (UIC  International Union of Railways) (  8  ).  Long Island Railroad Projects  The most significant use of slab track on grade in the United Stateshas been on the Long Island Railroad (LIRR), as discussed next. Massapequa Station Project The Massapequa Station Project,constructed just east of the Massapequa Station, consists of 2.06 kmof two parallel, CRC slab tracks on an embankment section with con-tinuously welded rail and direct-fixation fasteners. The project wasconstructed during the late 1970s and opened for traffic in Decem-ber 1980 ( 9 ). Details of the slab track sections are shown in Figure 4.Each year the slab track carries 12 MGT, made up of commuterpassenger trains and short freight trains.The slab track design incorporated the following: ã Concrete slab (CRC):–Thickness, 300 mm,–Slab width, 3.2 m,–Longitudinal reinforcement, 0.9 percent (two layers), and–Placement, with side forms; ã Subbase, 150-mm-thick and 4.4-m-wide hot-mix asphalt; ã Fastening system:–Spacing, 762 mm,–Clips, Pandrol “e” clips, and–Base plate, 178 × 381 × 38 mm with a neoprene pad betweentwo steel plates; ã Rail, 119RE; and ã Expansion joints at slab ends (at structures).After 20 years of revenue service, the slab track is performing verywell. The concrete slab exhibits no distress and the transverse cracksin the CRC slab are very tight, with crack spacing ranging from 0.8to 2 m. Only minor problems with the direct-fixation fasteners havebeen reported after 20 years of service.  West Side Storage Yard The West Side Storage Yard, located just west of Amtrak’s Penn Station in New York City, was con-structed during 1984 and consists of 27 concrete slab tracks, whichvary in length from 0.24 to 0.37 km, to hold commuter trains ( 10 ).The total length of the slab track is about 8.2 km. The slab track designused here was similar to that used at the Massapequa Station site. Theslab is continuously reinforced, 3.2 m wide, and 330 to 343 mm thick.The slab was placed over a 100-mm-thick asphalt base, which was 90 Paper No. 01-0240  Transportation Research Record 1742 placed over 863 mm of granular subbase. The reinforcement wasplaced in two layers, with the top layer epoxy coated. The concretestrength was specified to be 27.6 MPa, with a slump of 63 to 75 mm.After 16 years of service, the slab track is performing well and hasexhibited only fastener-related problems. Richmond Hill Yard The Richmond Hill Yard, used as a mainte-nance facility, contains two sets of slab tracks. One set of tracks,constructed during 1992–1993, is used for engine and car repair.Another set of track (a single track) was constructed during 1997and is used as a wash facility, with trains operating at about 8 km/h.This track includes a 12-degree curve. The unreinforced slabs at thewash track are very thick, up to 660 mm, and have a joint spacing of 6.1 m. Both sets of tracks use 119RE rail. Fastener problems devel-oped along the wash track as a result of the severe curve, and inter-mediate fasteners were installed to provide an effective fastenerspacing of 381 mm. Fastener problems have also occurred on theolder repair slab tracks. Other than the fastener problems, both setsof slab tracks are performing well, with no distresses exhibited bythe concrete slab or the substructure. Canadian Pacific Rail Slab Track Sections  Canadian Pacific (CP) Rail constructed two test sections near RogersPass in British Columbia during 1984 and 1986. The first section, atAlbert Canyon, was constructed as a test section, and the secondsection is a 15.8-km single-track section in McDonald Tunnel.  Albert Canyon Test Section The Albert Canyon Test Sectionwas a 0.28-km slab track section built during late 1984 by using thepatented PACT-TRACK system developed by British Rail andMcGregor Paving Limited in the United Kingdom ( 11 ). The track test section was built to investigate the use of the PACT-TRACK forthe 15.8-km McGregor Tunnel and simulated the tunnel track con-ditions. A thick concrete slab was first placed on a prepared sub-grade to simulate the tunnel’s floor. The slip-formed continuouslyreinforced surface slab was then placed.The PACT-TRACK surface slab is 229 mm thick and 2.43 mwide. Concrete is placed with a customized slip-form pavingmachine, which rides on two rails (which are later used for the track)and which feeds the concrete into the front of the paving machinewith a conveyer system. After the concrete has cured, the continu- FIGURE 4LIRR slab track (1 in.  25.4 mm; 1 ft  305 mm) ( 10  ).
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