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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.
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TESTS AND EVALUATIONS OF IN-SERVICE ASPHALT TRACKBEDS
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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 5 to 8 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 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
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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 5 to 7 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
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INTRODUCTION
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.
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TYPICAL ASPHALT UNDERLAYMENT PRACTICES
The typical HMA layer is 12 ft (3.7 m) wide and is 5 to 6 in. (125 to 150 mm) thick (2). For
unusually poor roadbed support conditions and high impact areas, an 8 in. (200 mm) thickness is
used. Thickness of the overlying ballast normally ranges from 8 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 1 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 1 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 1 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 do 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.
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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
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.
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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 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
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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 2.
Peak pressures occur directly below the tie/rail interface.
Figure 3 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 2 to 4 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 6 psi (40 kPa) pressure while standing on a
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 4 is a fully loaded auto train. Note that the pressure at the top of the HMA is
increased by three orders of magnitude.
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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 highpressure 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
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 7 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
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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
Technology Center near Pueblo, Colorado. It was found that a track with track modulus of 2,000
lbs/in/in (13.8 MPa) or less (i.e., a conventional 18-in. (450-mm) granular layer over a soft
clayey subgrade) required frequent surfacing maintenance under 39-ton (36-metric ton) axle
loads. Therefore, various remedies aimed at limiting excessive subgrade deformation have been
tested and their effectiveness judged.
The soft subgrade test track was built in the 2.7-mile (4.3 km) HTL. The soft subgrade
was built by excavating into the native subgrade soil (silty sand). A 700-ft (213-m) long, 12-ft
(3.6-m) wide, and 5-ft (1.5-m) deep trench was then backfilled with Vicksburg (Buckshot) clay.
To prevent the loss of clay moisture over time, the sides and bottom of the clay subgrade are
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lined with a plastic membrane. Since its installation, this subgrade has remained at high moisture
content (approximately 33 percent).
To date, several methods to remedy soft subgrade deformation have been tested,
including an increased granular layer thickness, geocell reinforcement, and the application of
hot-mix asphalt (HMA) underlayments. Use of a 27-in. (685-mm) granular layer thickness 15in. ((375-mm) subballast) improved track performance, but did not prevent a rapid geometry
degradation following a heavy rainfall due to water building up under and in the thick and dense
subballast layer. Use of the granular layer with geocell (24-in. (600 mm) of total thickness)
improved track performance, with no surfacing maintenance needed in 200 MGT (180 MGt) of
traffic. The current HMA underlayment test started in the summer of 1999.
Design and Construction of HMA Underlayments at TTCI
In the summer of 1999, two HMA underlayments were placed as a course under the ballast but
above the soft subgrade. Each segment is about 350 ft (107 m) long. One segment has a 4-in.
(100-mm) HMA, and the other has a 8-in. (200-mm) HMA. Figure 8 illustrates the longitudinal
cross section of these two segments. For the entire test section, a 4-in. (100-mm) subballast layer
was used between the subgrade and the two HMA underlayments. At construction, the ballast
thickness above the HMA was 12 in. (300 mm) over the 4-in. (100-mm) HMA, but was 8-in.
(200-mm) over the 8-in. (200-mm) HMA. For both segments, the total granular/HMA thickness
was therefore 20 in. (500 mm).
The asphalt mix design was based on the guidelines recommended by the Asphalt
Institute (2). Table 1 gives the recommended and the actual compositions for the HMA mix,
which is essentially similar to a dense-graded, low voids base course for highway construction.
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The required HMA strength and the ability in reducing the access of water into the
subgrade were achieved by meeting the Marshall design criteria. Table 2 lists the recommended
design criteria as well as the actual test results for the mix composition as defined in Table 1.
During the construction, the HMA placement was done in one lift for the 4-in. (100-mm)
HMA, but in two lifts for the 8-in. (200-mm) HMA. To achieve the desired HMA density listed
in Table 2, a steel-wheeled, vibratory roller was used to compact the HMA layer while the mix
was still between 185 to 300ºF (85 to 150ºC). Following compaction, a nuclear density gage was
used to obtain the final in-situ HMA density results. In addition, a number of HMA core
samples were obtained for further laboratory testing.
Test Results of Track Performance at TTCI
This test is the first to apply HMA underlayment over a soft subgrade under 40 ton (36 metric
ton) axle loads. The use of HMA underlayment is intended to reduce traffic load induced
stresses to the subgrade and to provide a waterproof layer over the underlying soil. Since its
installation, the performance of this test track has been evaluated in terms of track geometry
degradation with traffic as well as the amounts of track modulus increase and subgrade stress
reduction compared to conventional granular layer construction.
Figure 9 gives the track modulus test results obtained at 92 MGT (83 MGt) and the
subgrade stress results under a static wheel load of 40,000 lbs (18 metric tons). As shown, the
average modulus values for the two HMA segments are 2,800 lb/in/in (20 MPa) and 3,300
lb/in/in (23 MPa) for the fully consolidated ballast (increased from 2600 and 2800 lb/in/in (18
and 19 MPa)), respectively, at 0 MGT (0 MGt). Obviously, the HMA underlayment application
significantly increased track modulus from the 18-in. (450-mm) granular track with an average
track modulus of 2000 lb/in/in (14 MPa). As a result, the measured subgrade stresses were lower
for the asphalt trackbeds than for the 18-in. (450-mm) granular track. Under 40 kip (18 metric
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ton) static wheel load, only 7 to 8 psi (50 to 55 kPa) of subgrade stress was generated under the
HMA underlayments, compared to 12 psi (83 kPa) under the 18-in. (450-mm) granular track
structure.
To show how stresses induced by wheel loads are reduced from the HMA to the
subgrade, Figure 10 shows the dynamic stress results under an actual train operation at 40 mph
(64 km/hr) measured on the 8-in. (200 mm) HMA surface as well as on the subgrade surface. As
illustrated, use of a 8-in. (200 mm) HMA underlayment reduced the subgrade stress by
approximately one half.
Figure 11 shows the results of average track settlement as a function of traffic for both
the segments. As illustrated, after the initial higher rate due to early ballast consolidation, the
settlements became gradual, characteristic of typical and normal track deformation. After almost
100 MGT (91 MGt), about 1.5 in. (37 mm) of total settlement was accumulated for the 4-in. (100
mm) HMA segment, while about 1.3 in. (33 mm) of total settlement was observed for the 8-in.
(200-mm) HMA segment. Nevertheless, the settlements (mainly due to ballast deformation)
have been uniform along and across the test track. No geometry maintenance has been required
to date.
Another benefit of using HMA underlayment beneath ballast is insulating the asphalt
layer from the air. This should keep the asphalt less susceptible (compared to highway
construction) to the oxidation and temperature effects, thus leading to longer asphalt life without
weathering and cracking. In Figure 12, temperature recordings were made over a span of about
one year for both the HMA underlayment and the air. As shown, HMA temperature experienced
much less variation than air temperature.
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FINDINGS AND CONCLUSIONS

Use of HMA as an underlayment within the track structure is steadily growing in the U.S.,
particularly for heavy-haul applications.

Long-term performances of HMA trackbeds have been outstanding.

Peak dynamic pressures within the track structure occur directly under the tie/rail interface.

Peak dynamic pressures range from 13 to 17 psi (90 to 120 kPa) on top of the HMA mat
under 286,000 lb (130 metric ton) locomotives and heavily loaded cars—only two to three
times greater in magnitude than the pressure exerted by an average size person standing on
the HMA.

Peak dynamic vertical pressures under similar loading are further reduced to 5 to 7 psi (35 to
50 kPa) under the HMA layer at the subgrade interface.

Dynamic track deflections for HMA trackbeds under 286,000 lb (130 metric ton)
locomotives average 0.25 in. (6 mm) for wood tie track and 0.05 in. (1.5 mm) for concrete tie
track. These are considered optimum.

Dynamic track modulus (stiffness) values consistently average 2900 lb/in/in (20 MPa) for
wood tie track and 7200 lb/in/in (50 MPa) for concrete tie track. These are considered
optimum.

In-track test measurements on HMA underlayment trackbeds obtained for CSXT's heavyhaul revenue lines and at TTCI's heavy-haul research test facility are consistent with respect
to magnitude and time.

Moisture contents of old roadbeds/subballasts/subgrades under the HMA mat remain at or
near optimum after many years assuring optimum support for the HMA mat. This attests to
the waterproofing attributes provided by the HMA mat. An equally important attribute of the
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HMA mat is the confinement it provides for the ballast so that the ballast can develop
maximum shear strength and compactness.

The HMA mat in the insulated trackbed environment undergoes minimum variation in
temperature extremes throughout the year and it is not exposed to direct sunlight. The
modulus (stiffness) is
reasonably uniform throughout the year and weathering (oxidation) of the HMA is minimal
compared to highway applications.

The combination of low induced stress levels, minimal temperature changes and minimal
weathering in the trackbed environment assures a very long fatigue life for the HMA mat as
compared to
highway HMA applications.
ACKNOWLEDGMENTS
The research reported herein was supported financially by CSX Transportation, the Association
of American Railroads, the Federal Railroad Administration, and the Asphalt Institute. Technical
advice received from M.J. Hensley, retired Chief Engineer of the Asphalt Institute, greatly
benefited the research. Also, the efforts of three University of Kentucky Civil Engineering
graduate students–Bennett A. McElroy, Daniel M. Durrett, and William B. Long–during the
early phases of the project are gratefully acknowledged.
REFERENCES
1. Association of American Railroads (2001) Railroad Facts, 2001 Edition, 84 p.
2. Asphalt Institute (1998) HMA Trackbeds–Hot Mix Asphalt for Quality Railroad and
Transit Trackbeds, Informational Series 137, 10 p.
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3. Rose, J.G. & Hensley, M.J. (1991) Performance of Hot Mix Asphalt Railway Trackbeds.
Transportation Research Record 1300, Transportation Research Board, pp. 35-43.
4. Rose, J.G. (1998) Long-Term Performances, Tests and Evaluations of Asphalt
Trackbeds. Proceedings of the 1998 AREMA Technical Conference, Chicago,
September, 27 p.
5. 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.
7. 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, 4 p.
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LIST OF TABLES
TABLE 1. Composition of Dense-Graded HMA Mix
TABLE 2. Marshall Mix Design Criteria for HMA Underlayment
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TABLE 1. Composition of Dense-Graded HMA Mix
Sieve size
1.5 inch
¾ inch
3/8 inch
No. 4
No. 8
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
Actual
100
100
70 - 98
76
44 - 76
52
30 - 58
41
21 - 45
30
14 - 35
23
8 - 25
17
5 - 20
11
2-6
4.5
3.5 - 6.5
6.4
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TABLE 2. 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
1 - 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%**
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LIST OF FIGURES
Figure 1.Typical Cross-section
Figure 2. Pressure Cells
Figure 3. Empty Coal Train at Conway
Figure 4. Flat Wheel on a Loaded Auto Train at Conway
Figure 5. LVDT Configuration
Figure 6. Deflection under Loaded Coal Train
Figure 7. Conway Top of HMA Temperature vs. Time
Figure 8. Longitudinal Cross Section of HMA Test Track
Figure 9. 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
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Figure 1: Typical Cross-section
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Figure 2: Pressure Cells
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