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Long-Term Assessment of Asphalt Trackbed Component Materials’
Properties and Performance
by
Jerry G. Rose, PE Professor of Civil Engineering
161 OH Raymond Bldg University of Kentucky
Lexington, KY 40506-0281 859/257-4278, [email protected]
and
Henry M. Lees, Jr., PE
Sr. Engineer-Track & Structures BNSF Railway Company
920 SE Quincy Street Topeka, KS 66612-1116
785/435-6459, [email protected]
Submitted for Presentation at the 2008 AREMA Annual
Conference
Salt Lake City, September 2008 and Publication in the
Proceedings
Word Count: 5,794
June 1, 2008
mailto:[email protected]:[email protected]
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ABSTRACT
The uses of Hot Mix Asphalt as subballast layers within railroad
track structures for new
trackbed construction and trackbed maintenance applications have
grown steadily in the
United States during the past 25 years. The asphalt layer
(termed underlayment) is
used in lieu of an all-granular subballast layer. This paper
documents the results of a
characterization and evaluation study to ascertain the effects
of long-term exposure in
various trackbed environments on the material properties of the
trackbed materials –
asphalt and underlying (roadbed) subgrade. The primary purpose
of the testing program
was to determine if any weathering or physical/chemical
deterioration of the materials
were occurring that could adversely affect long-term performance
of the trackbeds. Six
asphalt trackbeds, ranging in age from 12 to 25 years; on heavy
traffic revenue lines in
three states were recently core drilled. Test data on the
trackbed materials were
compared to data obtained previously. The expected benefits and
trackbed life
projections are discussed relative to current basic design and
construction practices.
Keywords: hot-mix asphalt, railway trackbeds, trackbed
performance, subgrades,
subballast
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INTRODUCTION
From its beginnings in 1830, the railroads have been a primary
mode of freight transport
in this country. Its dominance is becoming significant in recent
years as train speeds,
gross ton-miles, and axle loads have increased. The latest
Association of American
Railroads statistics (1) indicate that in 2005 an all-time
record 1.7 trillion ton-miles of
freight was carried over the nation’s nearly 141,000-mile
(227,000 km) railroad network.
The average freight car weight has increased to 129 tons (117
metric tons) with most
new cars having gross weights of 143 tons (130 metric tons). The
importance of
developing and specifying premium track structures and
components to adequately
carry the increased tonnage is a current reality of the
industry. Failure of the track
structure and components results in difficulty maintaining track
geometric features
necessary for efficient and safe train operations. Maintenance
costs and track outages
increase due to frequent maintenance and renewal cycles.
The inability of the track structure to adequately carry the
imposed loadings can
be categorized into two primary failure types. The first one is
failure of the subgrade
when the pressure transmitted to the subgrade is higher than the
inherent hearing
capacity of the particular subgrade. The subgrade soil’s ability
to accommodate loading
pressures is a function of its shear strength, cohesion,
plasticity, density, and moisture
content. A well-compacted subgrade soil that is confined and
maintained reasonably dry
will normally perform adequately for an indefinite period of
time. A possible exception is
a highly compressible soil such as peat. Subgrade failures
adversely affect track
geometry and are normally difficult and expensive to
correct.
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The second type of trackbed failure occurs when one or more of
the trackbed
structural components fail to perform satisfactorily for a
reasonable period of time.
This is commonly manifested by the subballast, and particularly
the ballast, becoming
clogged (fouled) with excessive quantities of fine size
material. This lowers the shear
strength of the ballast and bearing capacity of the subballast.
Fouling is normally due to
degradation of the ballast, infiltration of subgrade soil
particles, extraneous droppings
from hopper cars, or an accumulation of wind-blown fine
particles. Track geometry is
adversely affected to varying degrees. It is difficult to
rectify track geometry in fouled
ballast with typical trackbed maintenance surfacing
equipment.
Periodic replacement of the track components (rails, ties,
fasteners, and special
trackworks) cannot be avoided (2). It is desirable to increase
the service life of the
components. The adequacy of the trackbed structural components
supporting the track
can have a significant effect on the life of the track
components by reducing impact
stresses and minimizing deflections of the track.
The solution for minimizing subgrade failures involves a
combination of reducing
the pressure on the top of the subgrade, improving drainage
(effectively improving the
properties of the subgrade), adding thickness to the trackbed
structural components, or
utilizing higher quality/load bearing trackbed components. The
solution for minimizing
structural component failure is designing and selecting
reasonable fasteners and
track components so that an optimum track structural support
stiffness will be achieved.
In order to design optimum track structural support stiffness,
it is necessary to
determine the applied pressures at different levels in the track
support structure and
select a combination of materials and thicknesses to withstand
the applied pressures.
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ASPHALT TRACKBEDS
The most common trackbed is composed of all-granular materials
consisting of layers of
ballast and subballast over a prepared subgrade, as noted in
Figure 1a. During the past
twenty-five years, the use of Hot Mix Asphalt as a subballast
layer within the track
structure has steadily increased until it is becoming standard
practice in many areas of
the United States. The asphalt-bound impermeable layer,
typically 5 to 6 in. (125 to 150
mm) thick, provides a “hardpan” to protect the underlying
roadbed and to support the
overlying ballast and track. Various tests and performance
evaluations have shown
numerous advantages over traditional all-granular (ballast)
trackbeds, particularly on
heavy tonnage lines traversing areas of marginal geotechnical
engineering
characteristics (3, 4, 5).
The most common asphalt trackbed, termed asphalt underlayment as
depicted in
Figure 1b, incorporates the layer of asphalt in lieu of the
granular subballast. Ballast is
used above the asphalt layer in a similar manner as conventional
all-granular trackbeds.
The ballast provides a protective cover for the asphalt by
blocking the sunlight,
protecting the surface from air and water, and maintaining a
relatively constant
temperature and environment. The ballast provides a means to
adjust the track
geometry, when necessary, with typical maintenance equipment and
procedures.
Recent studies involve instrumenting asphalt trackbeds with
earth pressure cells
and displacement transducers to measure pressure levels and
distributions within the
track structure and rail deflections under moving trains. These
tests, conducted in real
time domain train operations with 286,000 lb (130 metric ton)
cars, confirm the positive
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attributes of the asphalt layer (6, 7). Peak dynamic pressures
range from 13 to 17 psi
(90 to 120 kPa) on top of the asphalt layer. These are further
reduced to 7 to 8 psi (50
to 55 kPa) under the asphalt layer at the subgrade interface.
Dynamic track deflections
average 0.25 in. (6.4 mm) for wood tie track and 0.05 in. (1.3
mm) for concrete tie track.
These are considered optimum for quality trackbeds. Dynamic
track modulus values
consistently average 2,900 lb/in/in (20 MPa) for wood tie track
and 7,200 lb/in/in (50
MPa) for concrete tie track, also considered optimum stiffness
levels.
BASIC ASPHALT TRACKBED DESIGN AND CONSTRUCTION PRACTICES
The asphalt mix is similar to that used for highway
applications, but can be slightly
modified for optimum performance in the trackbed environment. It
is placed as a layer or
mat of specified thickness and the common term is “underlayment”
since the layer is
placed under the ballast and above the subgrade or old roadbed.
It basically serves as
a subballast. A lesser used technique, known as “full-depth or
overlayment” is
applicable for special situations and involves placing the track
directly on the asphalt
layer with no ballast between the ties or slab and the asphalt.
This technique is primarily
used in Europe and Japan (8, 9, 10).
The most common asphalt mix is produced as a hot mix asphalt,
thus the
acronym – HMA. Cold mix asphalt mixtures and in-place
stabilization of roadbeds with
liquid asphalts have been used sparingly. Normally the asphalt
mix is produced in a
local mixing plant, at a temperature around 275°F (135°C),
hauled to the site in dump
trucks, spread to the desired thickness, and compacted while
being maintained at an
elevated temperature.
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The asphalt underlayment system is equally applicable for heavy
tonnage freight
lines, high-speed passenger lines, commuter and transit lines,
freight and intermodal
yards, ballast loadout facilities, and practically all types of
special trackworks including
crossing diamonds, turnouts, tunnel floors, bridge approaches,
and highway crossings.
The majority of the asphalt trackbed applications are on
existing lines. The applications
number in the thousands and most have been used on in-service
lines in conjunction
with rehabilitation or renewal of special trackworks,
particularly when existing subgrade
support and drainage conditions are inferior. Current
installation practices, which require
removal of the track, are not applicable for long sections of
in-service lines since the
time required to remove and replace the track is not
commensurate with typical work
windows. Studies are underway to develop equipment to place
asphalt under a raised
track on in-service lines without removing the track.
New construction, particularly double-tracking and yard
installations, account for
the largest projects. At these selected locations, conventional
trackbed designs were
considered to be inadequate or uneconomical to provide the
required level of long-term
performance because of inherent poor qualities of the roadbed
support materials and
drainage conditions. The roadbed/subgrade is readily available
for regular highway
paving practices prior to track placement (11, 12).
Recommended asphalt mixture specifications and trackbed section
designs have
evolved over the years. Following is a summary of prevailing
practices. Detailed
information is available elsewhere (5, 13).
Normally a local dense-graded asphalt highway base mix is
specified, slightly
modified with an additional 0.5% asphalt (binder) cement
content. The ideal design air
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void content for the compacted asphalt layer is 2 to 3%. Typical
asphalt layer width is 12
ft (3.7 m) and thickness ranges from 5 to 6 in. (125 to 150 mm).
Ballast thickness above
the asphalt is from 8 to 12 in. (200 to 300 mm).
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
to purposefully improve
sub-surface drainage, or improve support with additional
granular material prior to
placing the asphalt, will depend on an analysis of the
conditions at the specific site.
ASPHALT TRACKBED MATERIALS TESTS AND EVAULATIONS
Eight asphalt trackbeds, located in five different states,
ranging from 12 to 29 years old
and having various asphalt thicknesses and trackbed support
materials, were selected
for materials characterization studies. Pertinent classification
and descriptive data for
the projects are presented in Table 1. Samples were obtained
during summer 2007.
Previous characterization studies, primarily conducted in 1998
(14, 15), were available
for selected projects and are included herein for comparison
purposes.
Samples normally were taken at three randomly selected locations
at each
project. Samples were removed from the field side crib area
(Figure 2). The following
sequence was followed at each location:
• Remove and sample ballast from crib area down to top of
asphalt layer
• Measure ballast thickness and observe condition
• Obtain 6 in. (150 mm) diameter core sample with core drill
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• Protect samples from core drilling water so as to not
contaminate the
underlying roadbed
• Measure asphalt core thickness, observe condition, and place
in sealed
plastic bag
• Auger out roadbed samples, note distance below asphalt,
separate if layered
conditions existed, place in sealed plastic bags
• Repeat drilling sequence, normally three cores were taken at
each location
• Fill core holes with cold mix patch and replace ballast
Geotechnical Tests and Evaluations
The following geotechnical laboratory tests and evaluations
using standard ASTM
procedures were conducted on the subgrade/roadbed samples:
• Moisture Content; in-situ condition – as sampled
• Grain Size Analysis; sieve and hydrometer
• Atterberg Limits; liquid limit, plastic limit, plasticity
index
• Soil Classification Determinations; unified system
• Standard Proctor Moisture-Density
• California Bearing Ratio; unsoaked and soaked
The samples were recorded by depth below the asphalt and placed
in separate
containers when differences in size, color, texture, or moisture
content were observed.
The sealed containers were transported to the geotechnical
laboratory at the Kentucky
Transportation Cabinet for subsequent tests.
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Table 2 contains the geotechnical evaluations for the
subgrade/roadbed
samples. Data from the 1998 sampling is included for comparison
with the recent 2007
data. Subgrade samples were obtained from four projects. The
subballast and subgrade
were sampled separately at the Hoover site. This was the only
project where granular
subballast was used below the asphalt. The Quinlan site had two
distinctly different
subgrades due to differing topography. Thus, six different
samples were analyzed for
the four projects.
The initial testing phase involved in-situ moisture content
tests, grain-size
analysis, and Atterberg limits tests followed by soil
classifications by the Unified
procedure. Based on the classifications, similar materials from
a site were combined to
accumulate samples of sufficient size for the subsequent
standard Proctor moisture-
density test to determine optimum moisture content for maximum
dry density and for the
California bearing ratio (CBR) test.
In-Situ Moisture Contents
There was significant interest in determining the existing
moisture contents of the
subgrade materials directly under the asphalt layer and
subsequently comparing these
with previous measurements with the optimum moisture contents
for the respective
materials. Every effort was made to remove core drilling water
to protect subgrade
samples. No significant water penetrated the soil (particularly
clay) subgrades. No
sample appeared to be overly wet or wet of optimum based on
initial observations.
In-situ moisture contents are provided in Table 2 for both the
1998 and 2007
sampling operations. The values varied relative to the type of
subgrade soil, but were
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very site specific comparable with values obtained during the
1998 sampling. These
data are shown in Figure 3. There was an average net decrease of
0.1% change in
moisture contents over the span of nine years.
Two of the projects had in-situ moisture tests taken during
similar coring
operations on several previous occasions, dating to the early
1980s. This data is
presented in Figure 4. The Oklahoma City trackbed has a highly
plastic clay under the
asphalt. The range in moisture values is minimal. The Conway
trackbed has the existing
old roadbed under the asphalt that is highly variable mixture of
large-size ballast, small-
size ballast, cinder, coal, soil, etc. The significance of the
data is that the average
moisture contents of the materials underlying the asphalt have
remained essentially
unchanged at each respective site over the years from the time
the asphalt was placed.
Previous concerns about pore water pressure, and its effects on
lowering subgrade soil
strengths, are not founded.
Unified Soil Classifications
The soil classifications, based on grain size analyses and
Atterberg limits tests, are
provided in Table 2. The test projects were selected to include
a wide variety of
subgrade materials, ranging from reasonably high plastic clays
to more silty/sandy
materials having little or no plasticity. As expected, little
difference in soil classifications
was noticed at individual sites for the samples taken in 1998
and 2007.
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Standard Proctor Moisture-Density
The standard Proctor moisture-density test was conducted to
determine the optimum
moisture content for achieving maximum density. The minus 0.50
in. (12.5 mm) size
material was removed. The optimum moisture content data is
included in Table 2.
Figure 5 shows the change in optimum moisture contents for the
six samples between
1998 and 2007 sampling. The changes were typically less than 1
percent, indicating
similar materials.
Figure 6 is a graphical comparison of the measured in-situ
moisture contents and
the Proctor optimum moisture values. The linearity of the
relationship is shown in Figure
7. Note that the R2 value is in excess of 0.9 indicating very
good correlation. The in-situ
moisture contents were very close to optimum values. These
findings indicate that the
subgrade materials under the asphalt layer can be considered,
for design purposes, to
have a prevailing moisture contents very near optimum for
maximum compactability and
strength.
In addition, strength or bearing capacity values used in design
calculations
should be reflective of optimum moisture content values. It is
common practice, when
designing conventional all-granular trackbeds, to assume the
subgrade is in a soaked
condition, which for most soils is a weaker condition than when
the soil is at optimum
moisture.
California Bearing Ratio
California Bearing Ratio (CBR) specimens were prepared at
moisture contents
determined from previous Proctor tests to be optimum for maximum
density. Specimens
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were tested immediately in the unsoaked condition. Companion
specimens were
soaked in water for 96 hours prior to testing. Tests were
conducted at 0.1 in. (2.5 mm)
penetration.
The CBR data is presented in Table 2. The values were typical
for the types of
materials tested. For example, the highest CBR value was in the
50 range, which was a
select river gravel used as a subballast (locally known as
“Tex-Flex” base), for the
Hoover project. A select crushed stone product is considered to
have a CBR value of
100. The other subgrade materials have CBR values significantly
lower, as expected,
even for the unsoaked condition.
A comparison of unsoaked and soaked CBR test values is presented
graphically
in Figure 8. CBR values were significantly lower for the soaked
samples, particularly
those containing clay size material, which had values in the low
single digits. Test
results for the 1998 and 2007 sampling were reasonably close
considering that
materials sufficient for only one unsoaked and one soaked
specimen per site were
available for tests. Likely the 1998 and 2007 test comparisons
would have been less
variable had additional tests been conducted to obtain averages
based on several
replicable tests.
As noted previously, the in-situ moisture contents for
individual samples were
very close to the those determined from the Proctor test to be
near optimum. This
relationship is shown graphically in Figure 7. Since the
unsoaked CBR values are
derived from tests on samples at optimum moisture contents, and
the test results from
samples under asphalt trackbeds were determined to be at or very
near optimum
moisture contents, it is obvious that the unsoaked CBR bearing
capacity values are
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appropriate to use for structural design calculations. The
soaked (lower) CBR values
result in a conservative overdesign. The preceding statements
are not necessarily
applicable to the open all-granular trackbeds, which are prone
to variable moisture
contents depending on the amount of rainfall and surface
drainage conditions, and
corresponding variations in support strength. The
subgrade/roadbed materials
underlying the asphalt layers were at moisture contents near
optimum, and based on
long-term monitoring at two sites, maintain optimum moisture
conditions for indefinite
periods.
Asphalt Mixture and Core Tests and Analysis
The following laboratory tests were conducted on the asphalt
mixtures and cores at the
National Center for Asphalt Technology (NCAT) at Auburn
University:
• Density and Voids Analysis
• Asphalt (binder) Content
• Extracted Aggregate Gradation
• Resilient Modulus @ 5°C (41°F) and 25°C (77°F) @ 1 loading
cycle per
second
• Dynamic Modulus @ 5°C (41°F) and 25°C (77°F) @ 1 hertz load
frequency
• Recovered Asphalt Binder Properties
Penetration @ 25°C (77°F)
Absolute Viscosity @ 60°C (140°F)
Kinematic Viscosity @ 135°C (275°F)
Dynamic Shear Rheometer @ 25°C (77°F)
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Figure 9 depicts typical asphalt cores as obtained from the
trackbeds. Table 3
contains results for the Mix Extraction Tests and Core Analysis.
Table 4 contains test
results on the Recovered Asphalt Binders. The most recent test
results are listed in the
far right columns. This represents 2007 data for six of the
projects. The significance of
the prior tests is so that the changes in the properties and
weathering characteristics of
the asphalt layers can be evaluated over a period of time.
Mix Extraction Tests and Core Analysis
The extraction test results (Table 3) are indicative of
dense-graded base mixes with 1.0
in. (25 mm) maximum size aggregate and about 6 percent passing
the No. 200 sieve.
These are basically in conformance with guidelines previously
described (5, 15).
Asphalt binder contents vary somewhat, ranging from 4.5 to 7.0
percent. No particular
changes are evident in aggregate gradations or asphalt binder
contents over the period
of years.
Tests on the asphalt cores included density and voids analyses
and dynamic and
resilient modulus tests. The air voids were typically higher
than desirable for five of the
sites ranging from 5 to 9 percent. The air voids were
purposefully maintained at 2 to 3
percent range at three of the sites. This range is considered to
be optimum to resist
premature oxidation of the binder. Average air voids for each
site were less than the 8%
maximum normally believed to represent the upper limit to
provide an impermeable
layer.
The industry standard dynamic and resilient modulus tests were
used to measure
the modulus of elasticity of the asphalt cores. In both tests,
repeated loads were applied
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to a cylindrical specimen and the displacements were measured.
The values, reported
in Table 3, were measured under uniaxial compression loading for
the dynamic modulus
and under indirect tensile loading for the resilient modulus.
Tests were conducted at two
standard temperatures which represent the nominal lowest, 5°C
(41°F) and highest,
25°C (77°F), temperature asphalt experiences in the insulated
trackbed environment.
Recent tests were limited to resilient modulus since it is now
considered as more
representative of the actual stiffness of the asphalt core.
Values were typically several orders of magnitude higher at the
lower
temperature, which is normal for a viscoelastic, thermoplastic
material – and is
characteristic of the asphalt binder in the mix. At lower
temperatures, the asphalt
becomes stiffer, as reflected in higher modulus (or stiffness)
values. At higher
temperatures, the asphalt becomes less stiff. Obviously, for
asphalt highway
environments, where the asphalt is exposed to greater
temperature extremes, the
stiffness differences from winter to summer are significantly
greater than those existing
in the insulated trackbed environment.
Figure 10 is a plot of Resilient Modulus versus Age of the
asphalt mixes. The
circled symbols represent data for cores (obtained from the
trackbed in 1998) that cured
the final nine years in the laboratory environment. They are
plotted directly above the
railroad cured data for similar ages. Note that the modulus
values for the cores cured
the last nine years in the laboratory were higher than the cores
in the railroad
environment.
The measured modulus values are reasonably consistent for the
various sites.
There is no particular trend or changes in modulus as a function
of time. The mixes vary
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in asphalt contents, densities, aggregate gradations, and binder
properties from site-to-
site, which can be expected to produce variations in modulus
values. However, these
variations are minimal. The significant factor is that the
values are reasonably typical for
new, unweathered mixes not exemplifying fatigue and cracking –
thus low values, or
exemplifying hardening/weathering of the binder – thus high
values. The values are
basically intermediate in magnitude, even after many years of
loading and weathering in
the trackbed. The asphalt appears to be undergoing little, if
any, weathering or
deterioration in the trackbed environment.
Recovered Asphalt Binder Tests
Test results for Penetration, Absolute and Kinematic
Viscosities, and Dynamic Shear
Rheometer on the recovered asphalt binders are contained in
Table 4. Plots of
Penetration and Absolute Viscosity versus Age of the Asphalt
Underlayments are
contained in Figures 11a and 11b. The data points circled at the
ends of the trend lines
represent the 2007 values. The preceding data points are nine
years prior, or 1998
values.
Penetration values will tend to decrease and viscosity values
will tend to increase
with time due to expected oxidizing and hardening of the asphalt
binders. There is
indication of this phenomenon when comparing the 1998 and 2007
test values.
However, the Abson method (ASTM D1856) was used for the 1998 and
prior asphalt
recoveries; whereas, the Rotary Evaporator method (ASTM D5404)
was used for the
2007 recoveries. The Rotovapor method is considered more
effective at removing the
solvent. Therefore, the 2007 penetration values would be
expected to be lower and the
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2007 absolute viscosity values would be expected to be higher
than their respective
1998 values. These trends are evident from Figures 12a and 12b
respectively.
It is likely that the original asphalt binders were PAC 60-70
penetration or AC-20
viscosity graded. The effects of short-term aging (elevated
temperatures) during the
pavement construction process and long-term aging for several
years will reduce the
binder penetration to the 25 to 40 range and the absolute
viscosity at 60°C (140°F) will
be maintained to less than 15,000 poises (17). These samples
meet these criteria,
indicating minimal oxidation and weathering.
The Dynamic Shear Rheometer (DSR) procedure for evaluating
asphalt binders
was developed in the mid-1990s. Fortunately this test was
conducted in 1998 on
samples from 5 of the 6 sites and this data is compared to the
2007 data in Figure 13.
The standard for performance grade asphalt binders, after short-
and long-term aging, is
that the DSR at 25°C (77°F) should be less than 5,000 kPa. Note
in Figure 13 that all of
the samples are well below 5,000 kPa, another indication that
the asphalt binders in the
trackbed cores are not oxidizing and hardening excessively
(17).
Discussion
It is not surprising that the asphalt binder in the trackbed
cores are not oxidizing and
hardening to the extent normally observed for asphalt highway
pavements. This is
largely due to two factors. The surface of the asphalt is
typically submerged 20 in. (500
mm) from the surface (atmosphere) by the ballast/tie cribs and
the depth of ballast
below the ties. The lack of sunlight and reduced oxygen largely
negates normal
weathering which occurs in highway pavements exposed to
sunlight.
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Secondly, the range in temperature extremes which the HMA mat
undergoes
from summer to winter is significantly less in the insulated
trackbed environment than
for exposed highway pavements. This information was developed
initially during 1982
and 1995 tests in Kentucky from buried thermistors, and reported
previously (14) and
reproduced in Table 5. Additional tests during 2000 at the AAR
Pueblo test site
confirmed the previous tests (6).
SUMMARY AND CONCLUSIONS
The primary purpose of this investigation was to determine,
based on test results,
current materials properties of the asphalt and underlying
materials in order to assess if
any weathering or deterioration of the materials was occurring
in the trackbed
environment which could adversely affect long-term
performance.
Material characterization evaluations were conducted on asphalt
cores and
subgrade/roadbed samples from eight asphalt trackbeds. The
trackbeds were from 12
to 29 years old when tested and were distributed over five
states. The inherent
conditions varied significantly from site-to-site. These
included asphalt thickness and
composition, ballast thickness, trackbed support, and traffic.
Previous characterization
evaluations were available for the projects and the results were
included for
comparisons with recent evaluations.
The significant finding relative to the materials (old
roadbed/subgrade) directly
under the asphalt layer, is that the in-situ moisture contents
are very close to laboratory
determined optimum values for maximum density of the respective
materials. The
asphalt layer is not performing as a membrane to collect and
trap moisture, thus
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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 are also at or near optimum. Furthermore, average
moisture contents remain
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 asphalt layer. Using strength or bearing capacity
values determined for the
soaked condition, common for highway designs, is inappropriate
for asphalt trackbed
designs. The unsoaked, optimum moisture content condition is
consistent with in-
service trackbed conditions.
An equally significant finding, relative to the asphalt cores
characterizations, is
that the asphalt binders and asphalt mixes do not exhibit any
indication of excessive
hardening (brittleness), weathering, or deterioration even after
many years in the
trackbed environment. This is considered to be primarily due to
the insulative effects of
the overlying ballast which protects the asphalt from excessive
temperature extremes
and oxidation and hardening of the asphalt binder. These factors
will contribute to a
long fatigue life for the asphalt layer. There is no indication
that the asphalt layers are
experiencing any loss of fatigue life based on resilient modulus
test on the extracted
cores.
The typical failure modes experienced by asphalt highway
pavements are 1)
rutting at high temperatures, 2) cracking and fatigue at low
temperatures, 3)
stripping/raveling under the suction of high tire pressures on
wet pavements, and 4)
progressive fatigue cracking due to inadequate subgrade support,
generally augmented
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by high moisture and improper drainage. These conditions do not
exist in asphalt
railroad trackbeds. For example, the temperatures are not
sufficiently high to promote
rutting. Conversely, the temperatures are not sufficiently low
to promote low
temperature cracking and decreased fatigue life, nor does the
asphalt binder weather or
harden excessively in the insulated trackbed environment which
would have further
negative influence on cracking and fatigue life. Obviously the
tendency to strip/ravel is
essentially eliminated in the trackbed environment since there
is no rubber suction
action. Also, the moisture contents of the underlying
subgrade/roadbed support
materials are maintained at or near optimum for maximum density
and support strength.
In addition, peak dynamic vertical pressures on top of the
asphalt layer are
typically less the 20 psi (138 kPa) under 286,000 lb (130 metric
ton) locomotives and
heavily loaded cars. (16) This is only two to three times larger
than the pressure exerted
by an average-size person standing on an asphalt pavement, and
much less than
pressures exerted by heavily loaded highway tracks, which can be
in excess of 100 psi
(690 kPa). These peak dynamic pressures are further reduced to
less than 10 psi (69
kPa) under the asphalt layer at the subgrade interface (6).
Based on the findings and analyses of the research reported
herein, asphalt
underlayments installed in conformance with the basic design and
construction
practices also reported herein, should have an extremely long
service life as a premium
subballast to properly support railroad tracks. There is no
indication of any deterioration
or cracks of the asphalt after many years of heavy traffic under
widely varying
conditions.
-
Ancillary benefits of a long-lasting premium subballast support
material for
railroad tracks include the following: increased strength,
decreased abrasion, and
increased life of the ballast; decreased wear and improved
fatigue life of the ties, rail,
and premium-cost track components such as special trackworks; a
consistent level of
track stiffness (modulus) designed for optimum levels; reduced
maintenance activities
and associated track closures; and improved adherence to track
geometric parameters.
All of these benefits impact favorably on achieving efficient
operation of the rail
transportation system.
ACKNOWLEDGEMENTS
The research was primarily supported by CSX Transportation and
the BNSF Railway
Company. The geotechnical laboratory testing was performed by
the Geotechnical
Branch of the Kentucky Department of Transportation. The asphalt
laboratory testing
was performed by the National Center for Asphalt Technology at
Auburn University.
William (Zack) Dombrow, BNSF Summer Intern from the University
of Illinois, assisted
with the sample collections and tests.
REFERENCES
1. Association of American Railroads (2006) Railroad Facts, 2006
Edition, 84 pages.
2. Lopresti, J., Davis, D., and Kalay, S. (2002) Strengthening
the Track Structure for Heavy Axle Loads, Railway Track &
Structures, September, pp. 21-26.
3. Rose, J. and Anderson, J. (2006) Long-Term Performance of
Asphalt
Underlayment Trackbeds for Special Trackbed Applications,
American Railway Engineering and Maintenance-of-Way Assoc. 2006
Annual Conference Proceedings, Louisville, KY, September, 27
pages.
-
4. Rose, J., Li, D., and Walker, L. (2002) Tests and Evaluations
of In-Service Asphalt Trackbeds, Proceedings of the American
Railway Engineering and Maintenance-of-Way Association, 2002 Annual
Conference & Exposition, September, 30 pages.
5. Rose, J. (2006) Hot-Mix Asphalt in Railway Trackbeds,
ASPHALT, Asphalt Institute
Magazine, Vol. 21, No. 3, pp. 22-25.
6. Li, D., Rose, J., and LoPresti, J. (2001) Test of Hot-Mix
Asphalt Trackbed over Soft Subgrade Under Heavy Axle Loads,
Technology Digest-01-009, Assoc. of American Railroads, April, 4
pages.
7. Rose, J., Su, B., and Twehues, F. (2004) Comparisons of
Railroad Track and
Substructure Computer Model Predictive Stress Values and In-Situ
Stress Measurements, American Railway Engineering and
Maintenance-of-Way Assoc. 2004 Annual Conference Proceedings,
Nashville, TN, September, 17 pages.
8. European Asphalt Pavement Association (2003) Asphalt in
Railway Tracks,
www.eapa.org, October, 11 pages.
9. Teixeira, P., Pita, A., Ubalde, L. and Gallego, I. (2005) New
Possibilities to Reduce Track Maintenance Costs on High-Speed Lines
by Using a Bituminous Sub-ballast Layer, Proceedings of Railway
Engineering 2005, London, June, 11 pages.
10. Momoya, Y., Horiike, T., and Ando, K. (2002) Development of
Solid Bed Track on
Asphalt Pavement, Quarterly Report, Railway Technical Research
Institute, Vol. 43, No. 3, September, pp. 113-118,
11. Frailey, F. (2004) BNSF Reborn, TRAINS, Vol. 64, No. 10,
October, pp. 34-49.
12. Lustig, D. (2007) Paving a Way for a Railroad Line, TRAINS,
Vol. 67, No. 3, March,
pp. 26-27,
13. Rose, J. and Hensley, J. (2000) Design, Construction, and
Maintenance Practices for Asphalt Trackbeds, Proceedings,
Transportation Systems 2000 Workshop, San Antonio, February, pp.
275-281.
14. Rose, J., Brown, E., and Osborne, M. (2000) Asphalt Trackbed
Technology
Development; The First 20 Years. Transportation Research Record
1713, Transportation Research Board, pp. 1-9.
15. Rose, J. (1998) Long-Term Performances, Tests, and
Evaluations of Asphalt
Trackbeds. Proceedings of the American Railway Engineering and
Maintenance-of-Way Association 1998 Conference, September, 27
pages.
http://www.eapa.org/
-
16. Rose, J. (2008) Test Measurements and Performance
Evaluations of In-Service Railway Asphalt Trackbeds, Proceedings of
the Transportation Systems 2008 Workshop, Phoenix, April, 24
pages.
17. American Society for Testing and Materials (2007) Standard
Specification for
Performance-Graded Asphalt Binder, ASTM D6373, Book of Standards
Volume: 0403, 5 pages.
-
LIST OF TABLES
Table 1. Asphalt Test Trackbeds
Table 2. Subgrade/Roadbed Geotechnical Evaluations
Table 3. Mix Extraction Tests and Core Analyses from Asphalt
Trackbeds
Table 4. Tests on Recovered Asphalt from Asphalt Trackbeds
Table 5. Temperature Range from Winter to Summer in Trackbed
Environment
-
LIST OF FIGURES
Figure 1. Cross-Sectional Views of Typical All-Granular and Hot
Mix Asphalt Trackbeds.
Figure 2. Core Drilling Operation to Obtain Asphalt Cores and
Underlying
Roadbed/Subgrade Samples. Figure 3. Changes in In-Situ Subgrade
Moisture Contents Between 1998 and 2007. Figure 4. Subgrade/Roadbed
In-Situ Moisture Tests After Coring. Figure 5. Changes in Optimum
Subgrade Moisture Contents Between 1998 and 2007. Figure 6.
Comparison of 1998 and 2007 Measured In-Situ Moisture Contents
and
Optimum Moisture Contents for the Roadbed/Subgrade Samples.
Figure 7. Relationships for Roadbed/Subgrade In-Situ and Optimum
Moisture
Contents. Figure 8. Comparison of 1998 and 2007 Unsoaked and
Soaked CBR Test Values for
the Roadbed/Subgrade Samples. Figure 9. Typical Asphalt Cores of
Various Compositions and Thicknesses. Figure 10. Resilient Modulus
versus Age of Asphalt. Figure 11. Penetration and Absolute
Viscosity versus Age of Asphalt. Figure 12. Penetration and
Absolute Viscosity Values for Railroad and Laboratory Cured Asphalt
Cores. Figure 13. Dynamic Shear Rheometer Values for 1998 and 2007
Tests.
-
Table 1. Asphalt Test Trackbeds Location
(Railroad) Conway, KY
(CSXT) Deepwater, WV
(CSXT) Cynthiana, KY
(CSXT) Guthrie, OK
(BNSF) Oklahoma City,
OK (BNSF) Quinlan, OK
(BNSF) Hoover, TX
(BNSF) Raton, NM
(BNSF)
Type of Facility
High Speed Mainline
Open Track
High Speed Mainline Bridge
Approaches
High Speed Mainline Open Track/ Road Crossings
High Speed Mainline Bridge
Approaches
Slow Speed Yard Lead
High Speed Mainline
Open Track
High Speed Mainline
Open Track
Slow Speed Branch Line Open Track
Traffic Type (million gross tons per year)
Unit Coal Intermodal Mixed
freight (40+)
Unit Coal Intermodal Mixed
Freight (40+)
Unit Coal Intermodal Mixed
freight (40+)
Unit Coal Intermodal Mixed
freight (40+)
Mixed Freight (10)
Unit Coal Intermodal Mixed
freight (40+)
Unit Coal Intermodal Mixed
freight (40+)
Unit Coal (3)
Year Roadbed Constructed
≈1900 (original)
≈1900 (original)
≈1900 (original)
1989 (new alignment)
1980 (new yard)
1995 (new double track)
1994 (new double
track)
1969 (new coal spur)
Year HMA Placed (Age of HMA)
1983 (24 years)
1984 (23 years)
1984 (23 years)
1989 (18 years)
1982 (25 years)
1995 (12 years)
1994 (13 years)
1969 (38 years)
HMA Section Length and Thickness
1000 ft, 8 in. (305 m, 200 mm)
1000 ft, 5 in.
(305 m, 125 mm)
200 ft, 8 in. (61 m, 200 mm)
280 ft, 4 in.
(85m, 100 mm)
1300 ft, 6 in. (396 m, 150 mm)
3100 ft, 4 in. (945 m, 100 mm)
532 ft, 8 in. (162 m, 200 mm)
7.9 miles, 6 in. (12.7 km, 150 mm)
4.4 miles, 4 in. (7.1 km, 100 mm)
700 ft, 2 ½ in. (213 m, 65 mm)
700 ft, 5 in.
(213 m, 125 mm)
700 ft, 7 ½ in. (213 m, 190 mm)
Ballast Thickness
5 – 7 in. (125 – 175 mm)
8 – 12 in. (200 – 300 mm)
10 in. (250 mm)
10 in. (250 mm)
8 in. (200 mm)
12 in. (300 mm)
12 in. (300 mm)
10 in. (250 mm)
Type of Roadbed existing mixture existing mixture
existing mixture
select subgrade
clay soil subgrade
clay & silt soil subgrades
select subballast soil mixture subgrade
Select subgrade
-
Table 2. Subgrade/Roadbed Geotechnical Evaluations* Test Project
Location
Grain Size Analysis Atterberg Limits Unified Soil Classification
Proctor California Bearing Ratio Values
In-situ % Moisture
Content % Retained No. 4 Sieve
% No. 4 to No. 200
Size Material
% Passing No. 200 Sieve
LL PL PI
Group Symb
ol Group Name
Optimum %
Moisture Content
Unsoaked CBR, %
2.5 mm (0.1 in.)
Soaked CBR, %
2.5 mm (0.1 in.)
Guthrie, OK Select Subgrade
10.8 - 16.0 (13.2)** 1 67 32 Non Plastic SM
Silty Sand 11.5 16.0 6.0
Select Subgrade
10.1 - 13.5 (11.8) 0 67 33 Non Plastic SM
Silty Sand 12.5 12.1 3.9
Oklahoma City, OK Clay Subgrade
16.7 - 20.4 (18.1) 1 6 93 38 20 18 CL
Lean Clay 17.6 8.5 3.2
Clay Subgrade
15.1 – 22.4 (17.6) 0 4 96 34 18 17 CL
Lean Clay 18.0 8.2 2.8
Quinlan, OK Clay Subgrade
15.9 – 20.5 (18.0) 4 11 85 37 19 18 CL
Lean Clay 17.0 10.0 3.8
Clay Subgrade
15.4 – 19.8 (17.6) 0 12 88 30 17 13 CL
Lean Clay 17.0 8.8 4.2
Silt Subgrade
8.4 – 15.2 (11.6) 12 38 50 Non Plastic ML
Sandy Silt 13.0 23.1 22.7
Silt Subgrade
10.6 – 13.2 (11.9) 0 50 50 20 18 2 ML
Sandy Silt 13.2 42.1 30.0
Hoover, TX
Subballast (river gravel)
5.1 – 11.4 (6.9) 51 43 6 Non Plastic
GP-GM
Poorly graded
gravel w/ silt and sand
9.2 59.1 54.1
Subballast (river gravel)
6.2 – 9.6 (7.6) 39 50 11 Non Plastic SM
Silty sand 9.1 51.7 38.6
Subgrade 7.6 - 14.2 (10.7) 39 36 25 27 18 9 GC-GM
Silty clayey
gravel w/ sand
11.4 4.8 2.8
Subgrade 9.1 – 13.6 (11.0) 36 34 30 21 14 7 SC Clayey sand 10.0
8.6 4.7
* 1998 data in normal print, 2007 data in bold print ** Test
data in parenthesis represents averages
-
Table 3. Mix Extraction Tests and Core Analyses from Asphalt
Trackbeds Project Location (Date Constructed) Conway, KY (1983)
Hoover, TX (1994)
Age After 1 Day After 2 Years After 11 Years After 15 Years*
After 24 Years** After 4 Years* After 13 Years**
Exposure RR RR RR RR RR RR RR Lab (9 Years)
Extraction Results
Maximum Aggregate Size, mm (in.)
25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1)
Percent Passing No. 200 Sieve
3.5 – 5.3 4.6 – 5.9 5.8 – 6.9 4.5 – 11.7 Avg. 6.3 -- 4.8 – 5.0
Avg. 4.9 -- --
Asphalt Binder % by Weight of Total Mix
4.8 – 4.9 4.5 – 4.8 5.1 – 5.3 5.0 – 5.4 Avg. 5.3 4.4 – 4.7 Avg.
4.6
5.7 – 6.4 Avg. 6.2
6.6 – 6.8 Avg. 6.7 6.8
Core Analysis
Thickness, mm (in.)
108 – 213 (4 1/4 – 8 3/8)
121 – 216 (4 ¾ - 8 ½)
114 –210 (4 ½ - 8 ¼)
102 – 210 (4 – 8 ¼)
114 – 216 (4 ½ - 8 ½)
64 – 102 (2 ½ - 4)
51 - 102 (2 – 4)
64 – 102 (2 ½ - 4)
Density, kg/m3 (lb/ft3)
2260 – 2340 (141 – 146)
2225 – 2340 (139 – 146)
2305 – 2420 (144 – 151)
2327 – 2427 (145 – 151)
2242 – 2391 (140 – 149)
2171 – 2286 (136 – 143)
2213 – 2325 (138 – 145)
2267 – 2286 (141 – 143)
Gmb bulk 2.390 2.242 – 2.391 Avg. 2.316 2.237 2.213 – 2.325
Avg. 2.270 2.267 – 2.286
Avg. 2.277
Gmm max 2.492 2.511 – 2.534 Avg. 2.522 2.372 2.350 – 2.405
Avg. 2.383 2.383
Air Voids, % 7.0 – 10.1 6.9 – 13.2 3.5 – 10.9 2.4 – 6.4 Avg. 4.4
4.8 – 11.5 Avg. 8.2
3.3 – 8.5 Avg. 5.8
2.5 – 7.9 Avg. 5.1
4.1 – 5.1 Avg. 4.5
Dynamic Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
517 – 914
84 - 171
--
--
921
230
225 – 400 Avg. 308
101 – 186 Avg. 143
--
--
--
127 – 174 Avg. 154
--
--
--
--
Resilient Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
--
--
--
--
--
--
655 – 976 Avg. 796
267 – 508 Avg. 363
--
206 – 508 Avg. 387
328 – 728 Avg. 580
140 – 232 Avg. 185
--
228 – 384 Avg. 309
--
386 – 465 Avg. 423
*1998 data **2007 data
-
Table 3 (Cont.). Mix Extraction Tests and Core Analyses from
Asphalt Trackbeds Project Location (Date Constructed) Cynthiana, KY
(1984) Deepwater, WV (1984) Raton, NM (1969)
Age After 1 Year After 10 Years After 14 Years* After 23 Years**
After 1 Year After 14 Years* After 14 Years After 29 Years*
Exposure RR RR RR RR Lab (9 Years) RR RR RR RR
Extraction Results
Maximum Aggregate Size, mm (in.)
25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25
(1)
Percent Passing No. 200 Sieve
6.1- 8.6 8.1 - 8.4 5.1 - 9.3 Avg. 7.0 -- -- 1.8 – 2.0 1.5 – 1.9
Avg. 1.7 9.3 – 10.1
8.8 – 10.4 Avg. 9.5
Asphalt Binder % by Weight of Total Mix
4.7 – 5.0 4.9 – 5.3 4.5 – 5.2 Avg. 5.0 5.1 4.9 – 5.3 Avg. 5.1
4.0 – 4.4
4.7 – 5.1 Avg. 4.9 6.9 – 7.3
6.6 – 7.4 Avg. 7.1
Core Analysis
Thickness, mm (in.)
102 – 254 (4 –10)
127 – 229 (5 – 9)
127 – 279 (5 – 11)
102 – 203 (4 – 8)
127 – 279 (5 – 11)
102 – 178 (4 – 7)
76 – 178 (3 – 7)
67 – 194 (2 5/8 – 7 5/8)
127 – 190 (5 – 7 ½)
Density, kg/m3 (lb/ft3)
2194 – 2339 (137 – 146)
2179 –2355 (136 – 147)
2196 – 2375 (137 – 148)
2217 – 2343 (138 – 146)
2236 (139)
2115 – 2243 (132 – 140)
2132 – 2317 (133 – 145)
2180 – 2225 (136 – 139)
2232 – 2278 (139 – 142)
Gmb bulk -- -- 2.302 2.217 – 2.343 Avg. 2.272 2.236 -- -- --
--
Gmm max -- -- 2.456 2.438 – 2.506 Avg. 2.483 2.482 -- -- --
--
Air Voids, % 6.2 – 12.6 6.9 – 8.1 4.0 – 11.2 6.2 4.9 – 11.2 Avg.
8.5 9.9 9.4 –14.1
6.7 – 13.0 Avg. 8.8 3.1 – 4.7
0.9 – 4.2 Avg. 2.2
Dynamic Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
--
--
678
261
--
90 – 108 Avg. 97
--
--
--
--
197 – 340
52 - 60
--
46 – 66 Avg. 56
895 – 1461
95 - 127
--
51 – 90 Avg. 64
Resilient Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
--
--
--
--
500 – 600 Avg. 555
185 – 236 Avg. 206
--
199 – 391 Avg. 270
--
417 – 511 Avg. 464
--
--
428 – 804 Avg. 693
168 – 347 Avg. 287
--
--
479 – 748 Avg. 609
172 – 576 Avg. 401
*1998 data **2007 data
-
Table 3 (Cont.). Mix Extraction Tests and Core Analyses from
Asphalt Trackbeds Project Location (Date Constructed) Guthrie, OK
(1989) Quinlan, OK (1995)
Age After 9 Years* After 18 Years** After 3 Years* After 12
Years**
Exposure RR RR RR RR Lab (9 Years)
Extraction Results
Maximum Aggregate Size, mm (in.)
19 (3/4) 25 (1) 25 (1) 25 (1) 25 (1)
Percent Passing No. 200 Sieve
7.3 -- 4.0 – 5.8 Avg. 4.9 -- --
Asphalt Binder % by Weight of Total Mix
5.7 5.3 – 5.6 Avg. 5.5 4.5 – 4.5 Avg. 4.5
3.8 –3.9 Avg. 3.9 4.1
Core Analysis
Thickness, mm (in.)
102 – 140 (4 – 5 ½)
102 – 114 (4 – 4 ½)
152 – 171 (6 – 6 ¾)
127 – 190 (5 – 7 ½)
152 – 171 (6 – 6 ¾)
Density, kg/m3 (lb/ft3)
2458 – 2463 (153 – 154)
2399 – 2423 (150 – 151)
2292 – 2389 (143 – 149)
2293 – 2341 (143 – 146)
2293 – 2341 (143 – 146)
Gmb bulk -- 2.399 – 2.423 Avg. 2.410 2.355 2.293 – 2.341
Avg. 2.322 2.293 – 2.341
Avg. 2.312
Gmm max -- 2.445 – 2.478 (2.465) 2.539 2.510 – 2.525
Avg. 2.516 2.513 – 2.520
Avg. 2.516
Air Voids, % 1.7 – 4.3 Avg. 2.8
1.7 – 3.0 Avg. 2.2
6.0 – 9.4 Avg. 8.3
5.2 – 7.5 Avg. 6.7
7.1 – 8.8 Avg. 8.1
Dynamic Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
--
20 – 25 Avg. 21
--
--
68 – 177 Avg. 132
46 – 68 Avg. 54
--
--
--
--
Resilient Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
1431 – 2130 Avg. 1680
351 – 415 Avg. 383
--
232 – 315 Avg. 276
1142 – 1441 Avg. 1262
419 – 571 Avg. 502
--
366 –572 Avg. 450
--
589 – 644 Avg. 628
*1998 data **2007 data
-
Table 3 (Cont.). Mix Extraction Tests and Core Analyses from
Asphalt Trackbeds Project Location (Date Constructed) Oklahoma City
Yard (1982)
Age After 2 Months After 3 Years After 7 Years After 16 Years*
After 25 Years**
Exposure RR RR RR RR RR Lab (9 Years)
Extraction Results
Maximum Aggregate Size, mm (in.)
25 (1) 25 (1) 25 (1) 25 (1) 25 (1) 25 (1)
Percent Passing No. 200 Sieve
7.0 6.0 – 6.5 -- 5.2 – 6.4 Avg. 5.9 -- --
Asphalt Binder % by Weight of Total Mix
5.7 5.5 – 5.6 -- 5.3 – 5.7 Avg. 5.5 5.7 5.9 – 6.2 Avg. 6.1
Core Analysis
Thickness, mm (in.)
152 – 241 (6 – 9 ½)
241 – 267 (9 ½ - 10 ½)
203 – 229 (8 – 9)
178 – 229 (7 – 9)
190 – 229 (7 ½ - 9)
178 – 229 (7 – 9)
Density, kg/m3 (lb/ft3)
2385 – 2420 (149 – 151)
2385 – 2405 (149 – 150) --
2352 – 2414 (147 –151)
2381 – 2406 (149 – 150)
2368 – 2382 (148 – 149)
Gmb bulk -- -- -- 2.388 2.381 – 2.406 Avg. 2.393 2.368 –
2.382
Avg. 2.374
Gmm max -- -- -- 2.445 Avg. 2.422 Avg. 2.415
Air Voids, % 0.9 – 2.7 0.9 – 2.3 -- 0.9 – 4.0 Avg. 2.3 0.7 – 1.7
Avg. 1.2
1.4 – 1.8 Avg. 1.7
Dynamic Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
1090 – 1110
145 - 163
--
151
--
125 - 175
--
69 – 244 Avg. 137
--
--
--
--
Resilient Modulus psi x 103 @ 1 Hz 5ΕC (41ΕF) 25ΕC (77ΕF)
--
--
--
--
--
--
703 – 940 (827)
199 – 341 Avg. 262
--
335 – 474 Avg. 377
--
424 – 966 Avg. 695
*1998 data **2007 data
-
Table 4. Tests on Recovered Asphalt from Asphalt Trackbeds
Project Location (Date Constructed) Conway, KY (1983) Hoover, TX
(1994)
Age After 1 Day After 2 Years After 11 Years After 15 Years*
After 24 Years** After 4 Years* After 13 Years**
Exposure RR RR RR RR RR RR RR Lab (9 Years)
Recovered Asphalt
Penetration, dcm 25ΕC (77ΕF) 50 35 41 - 44
39 – 44 Avg. 42 25
50 – 54 Avg. 52 31 34
Viscosity, P 60ΕC (140ΕF) 4400 - 4410 6250 - 14060 9780 -
12034
6334 – 9378 Avg. 7983 13214
3020 – 3358 Avg. 3210 6022 5107
Viscosity, cSt 135ΕC (275ΕF) 530 - 540 610 - 840 750 - 760
650 – 932 Avg. 731 1752
596 – 627 Avg. 612 778 722
Dynamic Shear Rheometer, G*/sin 1.00 kPa, ΕC
72.6 68.6 66.6
Dynamic Shear Rheometer, kPa 25ºC (77ºF)
1213 1819 – 2249 Avg. 2043 1080 2084 1790
*1998 data **2007 data
-
Table 4 (Cont.). Tests on Recovered Asphalt from Asphalt
Trackbeds Project Location (Date Constructed) Cynthiana, KY (1984)
Deepwater, WV (1984) Raton, NM (1969)
Age After 1 Year After 10 Years After 14 Years* After 23 Years**
After 1 Year After 14 Years* After 14 Years After 29 Years*
Exposure RR RR RR RR Lab (9 Years) RR RR RR RR
Recovered
Asphalt
Penetration, dcm 25ΕC (77ΕF) 43 - 51 41
30 – 51 Avg. 40 42 21 53 -60
25 – 35 Avg. 29 62 - 82
61 – 77 Avg. 68
Viscosity, P 60ΕC (140ΕF) 6440 - 9177 13880 - 14480
8440 – 15405 Avg. 11855 13290 4193 - 5699
19201 – 33891 Avg. 25129 1060 - 1610
1314 – 1477 Avg. 1361
Viscosity, cSt 135ΕC (275ΕF) 631 - 688 886 - 894
760 – 1159 Avg. 936 763 1347 496 - 543
1003 – 1104 Avg. 1050 270 - 310
290 – 318 Avg. 301
Dynamic Shear Rheometer, G*/sin 1.00 kPa, ΕC
77.3 78.3
Dynamic Shear Rheometer, Kpa 25ºC (77ºF)
1188 1111 3706
*1998 data **2007 data
-
Table 4 (Cont.). Tests on Recovered Asphalt from Asphalt
Trackbeds Project Location (Date Constructed) Guthrie, OK (1989)
Quinlan, OK (1995)
Age After 9 Years* After 18 Years** After 3 Years* After 12
Years**
Exposure RR RR RR RR Lab (9 Years)
Recovered
Asphalt
Penetration, dcm 25ΕC (77ΕF) 42 28
28 – 34 Avg. 31 22 21
Viscosity, P 60ΕC (140ΕF)
5922 – 6136 Avg. 6029 8276
7827 – 10751 Avg. 8927 11745 12949
Viscosity, cSt 135ΕC (275ΕF)
678 – 782 Avg. 730 826
856 – 1085 Avg. 941 1127 968
Dynamic Shear Rheometer, G*/sin 1.00 kPa, ΕC
70.0 75.9 73.5
Dynamic Shear Rheometer, kPa 25ºC (77ºF)
1387 – 2378 Avg. 1883 2197 3308 3842
*1998 data **2007 data
-
Table 4 (Cont.). Tests on Recovered Asphalt from Asphalt
Trackbeds Project Location (Date Constructed) Oklahoma City Yard
(1982)
Age After 2 Months After 3 Years
After 7 Years
After 16 Years* After 25 Years**
Exposure RR RR RR RR RR Lab (9 Years)
Recovered
Asphalt
Penetration, dcm 25ΕC (77ΕF) 58 57 59
45 – 67 Avg. 54 28 24
Viscosity, P 60ΕC (140ΕF) 3870 3490 2495
2197 – 4482 Avg. 3368 8678 12735
Viscosity, cSt 135ΕC (275ΕF) 580 700 - 730 471
482 – 834 Avg. 620 1105 869
Dynamic Shear Rheometer, G*/sin 1.00 kPa, ΕC
71.6 75.3
Dynamic Shear Rheometer, kPa 25ºC (77ºF)
1172 2821 2478
*1998 data **2007 data
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Table 5. Temperature Range from Winter to Summer in Trackbed
Environment* Year Tested 1982 1982 1995
Location and System
Average Within HMA Layer
4 in. (100 mm) Below HMA
4 in. (100 mm) Below HMA
Ravenna, Ky Overlayment
35ºF - 80ºF (2ºC - 27ºC)
37ºF - 77ºF (3ºC - 25ºC) —
Cynthiana, KY Underlayment — —
39ºF – 67ºF (4ºC - 19ºC)
Conway, KY Underlayment
41ºF - 74ºF (5ºC - 23ºC)
44ºF - 70ºF (7ºC - 21ºC)
39ºF - 65ºF (4ºC - 18ºC)
*Typical range in Kentucky from winter to summer for highway
pavement expected to be from 14ºF to 122ºF (-10ºC to 50ºC).
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Figure 1. Cross-Sectional Views of Typical All-Granular and Hot
Mix Asphalt
Trackbeds.
-
Figure 2. Core Drilling Operation to Obtain Asphalt Cores and
Underlying
Roadbed/Subgrade Samples.
-
Figure 3. Changes in In-Situ Subgrade Moisture Contents Between
1998 and 2007.
-
Figure 4. Subgrade/Roadbed In-Situ Moisture Tests After
Coring.
-
Figure 5. Changes in Optimum Subgrade Moisture Contents Between
1998 and 2007.
-
Figure 6. Comparison of 1998 and 2007 Measured In-Situ Moisture
Contents and Optimum Moisture Contents for the Roadbed/Subgrade
Samples.
-
Figure 7. Relationships for Roadbed/Subgrade In-Situ and Optimum
Moisture Contents.
-
Figure 8. Comparison of 1998 and 2007 Unsoaked and Soaked CBR
Test Values for the Roadbed/Subgrade Samples.
-
Figure 9. Typical Asphalt Cores of Various Compositions and
Thicknesses.
-
Figure 10. Resilient Modulus versus Age of Asphalt. Figure 10.
Resilient Modulus versus Age of Asphalt.
-
Figure 11. Penetration and Absolute Viscosity versus Age of
Asphalt.
-
Figure 12. Penetration and Absolute Viscosity Values for
Railroad and Laboratory-Cured Asphalt Cores.
-
Figure 13. Dynamic Shear Rheometer Values for 1998 and 2007
Tests.
Rose, LeesbyAREMA Annual ConferenceASPHALT TRACKBEDSBASIC
ASPHALT TRACKBED DESIGN AND CONSTRUCTION PRACTICESASPHALT TRACKBED
MATERIALS TESTS AND EVAULATIONS
Geotechnical Tests and EvaluationsIn-Situ Moisture
ContentsUnified Soil ClassificationsStandard Proctor
Moisture-DensityCalifornia Bearing Ratio
Asphalt Mixture and Core Tests and AnalysisMix Extraction Tests
and Core AnalysisRecovered Asphalt Binder TestsDiscussionSUMMARY
AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES
Rose, Lees Tables & FiguresTable 3. Mix Extraction Tests and
Core Analyses from Asphalt TrackbedsTable 3 (Cont.). Mix Extraction
Tests and Core Analyses from Asphalt TrackbedsTable 3 (Cont.). Mix
Extraction Tests and Core Analyses from Asphalt Trackbeds *1998
dataTable 3 (Cont.). Mix Extraction Tests and Core Analyses from
Asphalt Trackbeds *1998 dataRecovered AsphaltRecovered
AsphaltRecovered AsphaltRecovered Asphalt