Suggested Guidelines for Hot Mix Asphalt (HMA) Underlayment in Track by AREMA Committee l-sub Committee No. 2 - Ballast OVERVIEW June 11, 1998 The purpose of these Suggested Guidelines is to provide 1) site selection criteria, 2) HMA mixture design and production guidelines, 3) HMA and ballast dimensional requirements, and 4) construction procedures for placing Hot Mix Asphalt underlayment in the track substructure. These guidelines are provided to help the railroad engineer and / or designer determine if HMA can provide the desired benefit and perform as intended. The HMA underlayment is a mixture of paving grade asphalt cement and graded mineral aggregates, similar to that used for highway applications. HMA can be produced in a common hot mix asphalt mixing plant (either batch or drum type), hauled to the site in dump trucks, and spread mechanically to desired thickness and compacted while being maintained at an elevated temperature. As a railroad roadbed underlayment, HMA is placed as a course under the ballast but above the subgrade. The purpose of the HMA is to provide a support layer in the upper substructure, reduce pressures, and provide a waterproof layer over the underlaying soils. Another potential benefit of using HMA is a reduction in substructure thickness between the top of rail and the subgrade. Because HMA is typically stiffer than subballast, less HMA thickness is required to provide a given stress level on the subgrade. This may be particularly helpful where overhead clearance restrictions limit the track elevation.
HMA underlayment has been used for more stable track geometry in 1) the construction of new tracks, yards, and terminals, 2) total out-of-face rebuilding of existing roadbeds, and 3) rehabilitation of short sections in open track special track work sections, diamonds, turnouts, crossovers, and road crossings, bridge approaches, and tunnels. As normal with most common construction practice, HMA applications must have positive drainage so that surface water will drain freely from the HMA section and provide water elevation below the track sub structure. PLANNING This is a critical activity, particularly for any in-service track applications where a section of the track must be taken out of service and the track removed for a period of time. Adequate attention must be given to developing a means to economically transport and place the HMA. Access is a critical concern and varies in relative difficulty to topographic conditions and remoteness of the site. Availability of sufficient tracktime to remove and replace the track may limit the application to short sections of in-service track. follows: The critical steps in the planning process for an in-service track application are as Determine if conditions warrant a HMA underlayment 7 _. Determine the extent of the application, i.e., length of track to be removed estimate quantity of HMA and graded granular base required 2
3. Determine the optimum procedure for delivering the graded granular base and HMA to the track bed, i.e., dumped directly from highway dump trucks or hirailed and dumped, depending on access 4. Determine when HMA plant will be operating in order to schedule HMA work 5. 6. 7. Determine if a highway paving machine or a small track dozer will be used to spread and shape the granular stone base and HMA courses Determine the compaction procedure for the HMA layer, and what subgrade treatment, if any, is needed to provide a stable base on which to compact the HMA Develop the most efficient procedures to remove the existing track, excavate and dispose of the existing unsuitable roadbed material, prepare subbase with consideration for adequate drainage and place necessary stone base, place and compact HMA course, replace track, place ballast, clean ditches to assure positive drainage. These will largely depend on the extent of the applications, track time availability, equipment and labor availability, and access conditions. PRELIMINARY INVESTIGATION Use of HMA is most applicable where track instability may be attributed to weak deforming layers in the track sub structure. HMA has also been useful to provide a transition zone in areas of abrupt changes in track stiffness. (Bridge decks, crossings, diamonds, turnouts, and other special track works). Track instability should be investigated to determine if HMA could provide an appropriate benefit. For example, if track instability can be defined as a fouled ballast problem but a stable subgrade, a ballast renewal program may be required and HMA would not be necessary. A course of HMA could be effective in reducing the stresses passed onto a soft subgrade. Therefore, a site investigation should first determine if a deforming subgrade in the track structure is present and a major cause of roadbed instability. 3
In areas where water tables are high, pore water pressures could develop under the HMA course. Proper track maintenance practices should be performed to provide necessary track drainage to prevent the occurrence of high water pressures in the track substructure as detailed in AREMA chapter 1.2.4. SUBGRADE PREPARATION AND DRAINAGE CONSIDERATIONS Many applications of HMA as track underlayment will be in areas of soft subgrade support, a few guidelines for HMA use under these conditions are offered. Although HMA has been used successfully by placement directly on a fine-grained subgrade, consideration should be given to the subgrade to provide a well compacted, smooth, surface free of any depressions. A typical subgrade compaction goal for most projects should be at least 95% of modified proctor (as determined in the laboratory by ASTM D 1557T) and can be determined for compliance in the field by using nuclear density or other methods to assess the compaction level prior to HMA placement. Most road beds are many years old and have a layer of degraded ballast material which may perform as a subgrade / subballast. When soft or unsuitable (uncompactable) materials are encountered, undercutting and replacement with a compacted graded granular stone base would be desirable to assure a stable subgrade. This function would likely be best performed with crawler mounted excavation equipment to minimized any additional damage to the existing subgrade. The subgrade should present an equivalent minimum depth (combination of existing material plus any additional new base) of 4 inches, properly 4
shaped and compacted material as described in the section 2.11.2.5 in the AREMA Manual. To ensure positive drainage of the track substructure, the subgrade should be shaped and compacted with a slight crown, which would extend on through the HMA Course. The top surface of HMA will be above the surrounding grade to allow surface water to escape into the side ditches and be drained away from road bed so not to create future water / moisture damage. A well-prepared and compacted subgrade will provide a support platform for the HMA course plus allow for proper surface water drainage. And also will create a barrier, which will minimize the effect of pore water pressure under the HMA. HMA MIXTURE DESIGN AND PRODUCTION Hot Mix Asphalt designs are typically controlled by locally available materials and as specified by the local state Department of Transportation. The following information will provide some areas of consideration to determine the proper grade of HMA, modification to normal Hot Mix Asphalt mixes and requirements as necessary for railroad underlayment, types of HMA production plants, and quality control. Hot Mix Asphalt is typically graded in three basic products: Base Binder Surface (Topping) large course stone top size, for strength, variable % voids medium course stone top size, fair strength, variable % voids small top size stone, lower strength, most flexible, low % voids Note: Most railroad applications should conform to base mixes,
Tvpical Table of Composition Limits For Hot Mix Asnhalt Mix Designs Crushed Graded Aggregate Percent Passing by Weight Sieve Size Binder Surface I l/2 inch 314 inch l/2 inch 318 inch no. 4 no. 8 no. 16 no. 30 no. 50 no. 100 no. 200 100 70-98 100 44-76 57-85 30-58 37-68 21-45 25-52 14-35 15-38 8-25 9-27 5-20 5-20 3-10 3-10 2-6 2-6 100 80-100 55-80 35-60 20-45 lo-32 5-21 3-14 2-7 % Liquid Asphalt 3.5-6.5 4.0-7.0 4-8 These aggregate gradations are specified by the maximum nominal particle size and particle size distribution. Based on the chosen maximum nominal particle size the gradation should fall between the limits shown in the above ranges. This ensures there is not too much fine sand in relation to total sand and therefore the mix is not susceptible to compaction difficulties. Aggregate gradations may vary somewhat from region to region due to locally available materials, should be multi-faced crushed material. When 6
purchasing or specifying HMA, the engineer must identify and request normal materials and practices as required by the local state Department of Transportation, Bituminous asphalt cement is available in various grades based on local normal temperature ranges, i.e., PG 46-22 grade liquid asphalt is referenced to historic high and low temperatures Co locally. Best practice for railroad underlayment projects, especially for small low volume jobs, is to determine which grades are available from local producers and request the lower temperature grade for the area thus increasing the flexibility and lowering the percent of voids for the mix. Also, due to the lower pressures applied to the HMA in a railroad underlayment as opposed to a highway surface, it may be desirable to add another 0.5% liquid asphalt cement to the mix design to obtain greater flexibility and lower void percentage. This mixture should maintain adequate shear strength and reduce the access of water into a moisture-sensitive subgrade, assuming that other drainage guidelines are followed. Marshall mix design methods and tests are assumed as they are most common. Marshall* Mix Design Criteria For HMA Trackbed Property Value, Allowable RanPe, or Minimum Value Compaction (blows per face) 50 Stability (pounds) 1000 min. Flow (0.01 inches ) 12-24 Percent Air Voids (compacted HMA) 2-4% Voids Filled with Asphalt (VFA) 70-85% In-Place Density (% of solid) 94% min. 7
*Note: Comparable local mix design methods may be substituted. The HMA supplier / contractor should provide Hot Mix Asphalt properties ( e.g., asphalt content, % voids filled with asphalt, cement % voids in the mineral aggregate, aggregate gradation, and asphalt cement grade) to the engineer. These factors are required for the design phase of any HMA application. The HMA production facility, whether batch plant or continuous drum mix, should be capable of producing a mix meeting the specifications outlined herein and adequately sized to produce the anticipated volume of mix. Consideration should also be given to other projects and products the plant may have scheduled.for a given day as compared to plant capacity and raw material availability. The use of storage silos for surge control and multi-product production is preferred, but project planning and scheduling with the HMA producer better assures a consistent flow of material to the project. The temperature of the mix should be approximately 300-320 degrees F from the plant and covered en route the job site to minimize heat loss. Hot Mix Asphalt production facilities meeting the local state transportation department specifications are preferred. The pay item for furnishing the HMA mix normally is by the ton, measured by certified scales at the plant. The cost of obtaining the HMA will vary depending on: the cost of the aggregates and asphalt cement in the local area: the haul distance from the plant to the site: the size (tonnage) of the project, availability and cooperation of the local producer, and the ease of delivery access for the project. 8
HMA PLACEMENT The HMA layer should be placed on a new subgrade or existing roadbed that is compacted and well-drained as here to fore described, and capable of accommodating the hauling and spreading equipment without excessive rutting or deformation. HMA paving work is typically weather sensitive and is recommended to be performed in temperatures above 45 degrees F and in dry conditions for best results. These requirements are no more stringent than those typically imposed for conventional graded granular roadbeds. A 95% compaction level is desirable..it is sometimes necessary to place an initial course of graded granular material to serve as a working platform on which to compact the required thickness of HMA. A slight crown or side slope is desirable. The need to purposefully improve surface or sub-surface drainage will depend on an analysis of the situation at the specific site. HMA layer thickness should be no less than a minimum of 4 inches or as specified by project design engineer. The HMA course width should extend at least to the bottom of the ballast shoulder. This normally requires a 11 to 12 foot wide course on single track installations. An additional foot width or more beyond the nominal 11 or 12 feet may be desired on a curved track to accommodate alignment changes in curves and spirals, under turnouts, crossovers, or other special track work to provide support under the longer ties. When using HMA in relatively short sections with high impact loading, such as under special track work, the HMA course should extend beyond the ends of the special 9
track work to provide a transition zone so that subsequent track surfacing operations do not infringe on the special track work area. It also helps ensure that the vehicle dynamics are sufficiently damped out when wheel loads reach the end of the HMA section. This transition length will vary as related to vehicle speed and characteristics but as a general guideline 25 to 100 feet from the location of high impact load should be considered. The following table provides the quantities of HMA required for various compacted course thickness. It is assumed that the layer is 12 feet wide and is compacted to density of 140 lb/cu.ft. For estimating purposes, it is common to use 0.50 ton per track foot. Quantities of Asphalt Underlayment Layer Thickness Tons of Mix (US Tons) 4 ins mm Per Track Foot Per Square Yard 4 10 0.28 0.21 5 125 0.35 0.26 6 150 0.42 0.32 7 170 0.49 0.37 8 200 0.56 0.42 Construction of new lines, extensions, classification and intermodal yards provide opportunities for HMA since the exposed subgrade is available for placing the HMA 10
underlayment with conventional highway paving and compaction equipment prior to placing the ballast and track. For existing lines, the track must first be removed and the underlying material excavated and properly prepared to the desired subgrade. The depth of the excavation will vary, depending on the replacement thickness of the HMA and ballast layers, and the desired elevation, top of rail. The HMA is hauled by dump trucks from the mix plant. For small projects where track time availability is limited, it is desirable to have all the HMA on the site prior to the anticipated placement time. The placement of the HMA requires a minimum period of time and should be continuous. Delays are normally experienced when trucks have to make additional rounds to the plant. At sites accessible to rubber- tired trucks, the mix can be dumped directly into a standard asphalt paver or back dumped on grade and spread with a small dozer blade, loader bucket, excavator bucket. A standard asphalt paver should be used for longer sections of track, if access will permit. For short sections and special track work, it is generally more economical and expeditious to backdump on grade and spread with on-site equipment.. At sites only accessible by rail, it may be necessary to transload the HMA from the highway trucks to a hi-rail dump truck, which is backed into the site. The mix can be dumped into a loader bucket and spread or dumped on grade and spread with a blade or bucket. Procedures for transporting, spreading and compacting the mix should minimize temperature loss. A major consideration is being able to obtain sufficient track time to properly install the HMA layer in addition to normal track and ballast installation, 11
The HMA layer is normally placed in 4 inch compacted lifts, although lifts of up to 6 inches can be adequately compacted. Compaction is best achieved with a standard roller, preferably a steel- wheeled, vibratory type, while the mix is between 250 to 300 degrees F. Other means of obtaining compaction may be applied if a roller is not available, such as a hand tamper or repeated passes with a crawler mounted machine as long as surface drainage is maintained and rutting does not develop on the surface. A well-compacted layer with minimum air voids (less than 5%) is desirable, but is difficult to achieve if the mix chills excessively before compaction, or if the compactive effort is insufficient, or if the underlying support is weak. A compaction level of 94% or higher of the theoretical maximum density determined in accordance with ASTM designation D2041 is desired. Ideally, the top surface of the HMA should be slightly crowned or sloped to one side to facilitate positive surface drainage of water from within the ballast to the track side. This may be accomplished by adjusting the screed on the paver or by tilting the blade of the dozer. If during placement of HMA a cold joint becomes necessary, a typical HMA break should be layered over a 3-4 linear foot area. After the track substructure is restored, the track superstructure is rebuilt or dragged back on the Hot Mix Asphalt Course using rubber-tired equipment. Cranes can be used to lift short panels and special trackwork. Snaking techniques are applicable for longer sections of track. Adequate space must be available to facilitate removal and replacement of the track and provide access for the HMA paving operations. 12
The ballast is distributed with conventional unloading and spreading equipment. The track is raised to provide the specified ballast depth below the ties. The ballast should fill the crib areas between the ties and provide shoulder to restrain lateral movement of the track and drainage for the track structure.