By Dr. Jerry Rose, P.E.
Because of unprecedented growth in traffic volumes, revenue ton miles and wheel loadings, the U.S. freight rail industry has had to find new technologies to provide stronger and longer lasting track and support structures.
To meet this challenge, the railroad companies and the asphalt paving industry have developed designs and applications for using hot mix asphalt (HMA) to replace a portion of the conventional granular material.
HMA is now being used in new mainline tracks, yards and terminals. It is also being used for trackbed maintenance and as a solution for instability problems in existing trackbeds. These trackbed solutions include rehabilitation of turnouts, railroad crossings, bridge approaches, hump tracks, tunnel floors and approaches, highway crossings, and loading facilities where conventional trackbed designs have not performed satisfactorily.
Two HMA designs are used to support the railtrack structure. The most common design is known as HMA underlayment, in which the HMA layer is placed directly on select subgrade or old roadbed. Then a layer of ballast is placed between the HMA layer and the railroad ties. This design changes little from normal trackbed design, since the HMA layer merely replaces the granular subballast layer.
The HMA layer is typically 5 to 8 inches (125 to 200mm) thick and 12 to 14 feet (3.7 to 4.3m) wide. It is similar to a paved lane of a highway, but it is contained within the track structure to form a structural hardpan layer between the ballast and the subgrade or existing roadbed.
HMA overlayment design involves placing the HMA layers in a similar manner, except no ballast is used between the HMA layer and the railroad ties. The ties are placed directly on the HMA surface. Cribbing aggregate is then placed between the ties and at the end of the ties to restrain track movement.
This design is used primarily in Europe as a version of slab track with two-block concrete ties. It is not as readily adapted to current U.S. railroad construction and maintenance practices as HMA underlayment design.
Benefits of HMA Underlayment
The benefits of an HMA underlayment, as determined by research tests and observations over the past 25 years include:
- A strengthened track support layer below the ballast to uniformly distribute reduced pressures to the roadbed or subgrade.
- A waterproofing layer and confinement to the underlaying roadbed to provide consistent load-carrying capability for track structures—even on roadbeds of marginal quality.
- An impermeable layer to divert water to side ditches, essentially eliminating roadbed moisture fluctuations, to consistently maintain adequate underlying support.
- A consistently high level of confinement for the ballast to provide high shear strength and uniform pressure distribution.
- A resilient layer between the ballast and roadbed to reduce the likelihood of subgrade pumping without substantially increasing track stiffness, and
- An all-weather, uniformly stable surface on which to place the ballast and track superstructure.
Heavy Freight and High Speed
Railways that most benefit from HMA underlayment are those having heavy freight traffic or high speed passenger traffic, where one or more of the following conditions exist or are anticipated:
- Difficulty in establishing and maintaining a sufficiently strong and stable roadbed (hardpan) to adequately support the ballast and track.
- Difficulty in establishing and maintaining proper surface drainage to convey surface water away from the track structure.
- Difficulty in preventing groundwater from weakening the track structure, and
- Abnormally high impact stresses at joints, bridge and tunnel approaches, other special track works, or open track where track stiffness varies abruptly.
- Areas where these conditions exist are likely to show rapid track contamination, excessive wear of track components and below standard track geometric parameters. Maintenance costs become too expensive to continue safe line speed operations so “slow orders” must be imposed, which reduce operating efficiency.
Recommended HMA specifications, trackbed section designs, and application practices have evolved over the years. Slight variations from the initial mix designs and construction techniques are typical and have not affected trackbed performance.
The HMA mix that has the best properties for the track structure is a low modulus (plastic) mix, having design air voids of 1 to 3 percent. The mix will easily compact to less than 5 percent air voids in place. A local dense-graded highway base mix with a minimum aggregate size of 1.0 to 1.5 inches (25 to 37mm) is typically specified.
Ideally, the asphalt binder content can be increased by about 0.5 percent above optimum for highway applications because rutting and bleeding are not concerns in the insulated trackbed environment.
Stiff but Resilient
Long-term monitoring and testing of in-service trackbeds indicate that this low voids, impermeable mix, undergoes minimal oxidation from the effects of air and water. The mix is also isolated from extreme temperature fluctuation within the insulated trackbed environment.
The mix provides a layer with reasonably consistent stiffness in hot weather but is slightly resilient in cold weather. Furthermore, the tendencies for the mix to rut and bleed in hot weather and crack in cold weather are significantly reduced, thus ensuring a long fatigue life for the mix.
Tests on subgrade/roadbed samples, obtained directly under the HMA layer, indicate that the in-situ moisture contents are very close to optimum values for maximum density of the materials. For structural design analyses, it is reasonable to base bearing capacity values at optimum conditions for the material under the HMA layer.
Width and Thickness
The predominate HMA layer width is 12 to 14 feet (3.7 to 4.3m) for open track, but the width is wider under special trackwork, such as turnouts, to provide support under the longer ties.
The thickness of the HMA layer varies depending on the quality of the roadbed (subgrade) support and traffic loadings. A 5- to 6-inch (125- to 150-mm) thick layer is normally used for average conditions. For unusually poor roadbed support conditions, and high impact areas, a minimum of 8 inches (200mm) is used. Ballast thickness normally ranges from 6 to 12 inches (150 to 300mm). A 6-inch (150mm) HMA thick layer that is 12 feet (3.6m) wide requires 0.42 tons per foot of track length (1.25 metric ton/track meter).
The HMA layer should extend several feet beyond the ends of special trackwork so that subsequent track surfacing operations and any impact from track stiffness changes will not infringe on the area.
The roadbed should be reasonably well-compacted, well-drained, and capable of accommodating the hauling and spreading equipment without excessive rutting or deformation. A slight crown or side slope is desirable. The need for sub-surface drainage or roadbed support improvements, prior to placing the HMA, depends on site specific conditions.
The structural design of railway trackbeds containing HMA underlayment can be performed using KENTRACK, a finite element computer program. The primary governing factor is limiting the vertical compressive stresses, or permanent deformation, on the subgrade.
For the HMA layer, the tensile strains at the bottom of the HMA layer control its service life. Damage analyses are conducted and used to predict the service life of the trackbed components for various combinations of traffic, tonnages, sub-grade support and component layer compositions and thicknesses.
The construction of new rail lines and extensions, classification yards and intermodal terminals are ideal conditions for HMA underlayment. These are normally large paving projects with exposed subgrades that are readily accessible to conventional asphalt paving equipment for placing the HMA layer prior to placing the ballast and track. Typical highway paving technology can be used.
New intermodal yards use HMA for all or portions of the new track construction. A particular advantage is the waterproofing characteristics of the HMA layer and the positive drainage system that can be used in the design of the loading area.
Maintenance and Rehabilitation
Maintenance and rehabilitation of existing rail lines, using current technology, requires that the track must be removed and the underlying material excavated to the desired grade. The existing roadbed material should be reasonably compact and capable of providing adequate support.
The HMA is hauled by dump trucks. For short track sections, it is common to dump the mix and spread it to the desired configuration. Paving machines can be used for longer sections where maneuverability permits. Conventional compaction equipment is used. Placing the ballast and tracks completes the process. It is not necessary for the HMA to cool prior to placing the ballast.
HMA underlayment provides excellent support for heavy freight rail traffic and high speed passenger traffic on subgrade that cannot sustain heavy weights and high speeds. HMA continues to be the working solution of choice to counter railtrack instability.
Jerry Rose is a professor in the Civil Engineering Department of the University of Kentucky. He specializes in transportation engineering with an emphasis on railway engineering.