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AGGREGATE PROPERTIES AND THE PERFORMANCE
OF SUPERPAVE-DESIGNED HOT MIX ASPHALT
SUMMARY Mineral aggregates make up between 80% and 90% of the total volume or 94% to
95% of the mass of hot mix asphalt (HMA). For this reason, it is important to maxi-
mize the quality of the mineral aggregates to ensure the proper performance of our
nation's roadways. The quality of mineral aggregates for road-paving materials has
generally been specified by the toughness, soundness (durability), cleanliness, particle
shape, angularity, surface texture, and absorption.
The Superpave® mix design method is a product of the Strategic Highway Research
Program (SHRP). Research to investigate aggregate's contribution to pavement perfor-
mance was intentionally excluded from the SHRP Asphalt Research Program. Instead,
the aggregate gradations and physical properties included in the Superpave mix design
method were developed through the use of a modified Delphi approach (1).* The Del-
phi process is designed to ascertain the consensus of a group of experts while avoiding
some of the negative aspects of group dynamics. The final results of the modified Del-
phi process used by SHRP included aggregate properties, test methods to determine
those properties, and specification criteria. Gradation limits were included as part of the
aggregate properties. The new gradation limits included definitions for nominal maxi-
mum and maximum aggregate size, control points for various nominal maximum aggre-
gate sizes, and the restricted zone. The remaining aggregate physical properties were
divided into two categories: consensus and source. The consensus properties--including
coarse aggregate angularity, flat and elongated particles, fine aggregate angularity, and
sand equivalent--were chosen to ensure the aggregate quality was sufficient to provide
satisfactory HMA performance for the design traffic level. Specification values were to
be uniform throughout the United States without regard for locally available materials.
The specification values for the source properties, including LA abrasion, sulfate sound-
ness, and deleterious materials, were to be set by the agency. This was done to allow for
variances in locally available materials. Finally, the modified Delphi process identified
volumetric properties of the resulting HMA mix including air voids, voids in mineral
aggregate (VMA), voids filled with asphalt, and dust-to-asphalt proportion. The aggre-
gate bulk specific gravity is required to calculate VMA, and the aggregate fines content
is required to calculate dust-to-asphalt proportion.
*See Summary References.
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Prior to the development of the Superpave mix design method, the aforementioned
aggregate properties had not been applied in concert. Some agencies found that the
Superpave aggregate specifications precluded the use of materials with long perfor-
mance histories, particularly with regard to gradation. Other agencies found the con-
sensus aggregate properties prevented the use of locally available materials. Others
questioned the precision of certain tests. Determination of aggregate bulk specific grav-
ity for calculation of VMA was a concern for quality control/quality assurance testing
during production. Since the conclusion of SHRP, these concerns have resulted in sev-
eral national research studies and in numerous smaller studies to better define the need
for various aggregate properties as well as the interaction among the properties. Eval-
uation of aggregate properties has also been included in some field and accelerated per-
formance studies.
Based upon performance histories of locally available materials or research projects
conducted to address concerns about the aggregate specifications included in AASHTO
M323, numerous agencies have modified their aggregate specifications for Superpave-
designed HMA. Often this experience is not shared with other agencies that are using
similar materials. A consensus among the states, based on their experience, may indi-
cate the need to alter aggregate test procedures or specifications on a national basis.
The objective of this study (NCHRP Project 9-35) was to review the technical liter-
ature and ongoing research to identify the consensus, source, or other aggregate prop-
erties that significantly impact HMA performance. The review also examined the effect
of production and crushing operations on aggregate properties. The review concen-
trated on the effect of aggregate properties on HMA designed using the Superpave
methodology, HMA construction, and HMA performance. New innovations in aggre-
gate testing were also examined, especially those related to aggregate shape, angular-
ity, and texture:
To accomplish the research objectives, five tasks were conducted:
· Task 1: State of the Practice,
· Task 2: Survey of Ongoing Research,
· Task 3: Survey of Agency Specifications,
· Task 4: Review of Performance Data from Field Test Sections and Full-Scale
Accelerated Testing, and
· Task 5: Final Report.
CONSENSUS AGGREGATE PROPERTIES
The Superpave method includes four consensus aggregate properties: coarse aggre-
gate angularity, flat and elongated particles, uncompacted voids in fine aggregate, and
sand equivalent. The consensus properties were to be uniformly adopted by all agen-
cies. Criteria for the consensus properties varied by traffic level and for properties
related to rutting depth in the pavement structure. The criteria for the consensus prop-
erties were to be applied to the blend of aggregates in the mixture.
Coarse Aggregate Angularity
Coarse aggregate angularity was identified by the Delphi panel as the second most
important parameter after gradation for the performance of HMA. The current test
method, ASTM D5821, is a subjective test that requires the technician to evaluate
whether the aggregate has fractured faces. The test method cannot distinguish between
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the angularity of aggregates with 100% two or more fractured faces (most quarried aggre-
gates). As such, NCHRP Project 4-19 (published as NCHRP Report 405: Aggregate Tests
Related to Asphalt Concrete Performance in Pavements [2]) recommended AASHTO
TP56, "Uncompacted Voids in Coarse Aggregate," as a replacement. AASHTO TP56
combines the effects of aggregate shape, angularity, and texture. To date, AASHTO
TP56 has not been adopted by state agencies. ASTM D5821, or a similar procedure, is
still used by 83% of the responding agencies. The criteria for coarse aggregate angular-
ity are based in part on work conducted by Cross and Brown (3) in the National Rutting
Study. Only 39% of the agencies that use ASTM D5821 specify the criteria outlined in
the Superpave method (AASHTO M323). Six states have lowered the fractured-face
requirements. This is most likely in recognition of the fact that it is nearly impossible
to achieve 100% particles with two or more crushed faces with crushed gravel sources.
Although there is extensive research that indicates improved rut resistance with increased
percentages of fractured faces, little work has been done to investigate the effect at high
levels of fractured faces (between 95% and 100%). Hand et al. (4) concluded that
coarse aggregate angularity did not have an effect on the rutting performance of the
WesTrack mixtures. Two coarse aggregate sources were used at WesTrack: the origi-
nal was crushed gravel having 98% one fractured face and 96% two fractured faces,
and the second was a crushed andesite with 100% one and two fractured faces.
The coarse aggregate angularity test appears to be useful for evaluating gravel sources
in terms of rutting potential. The current test method is highly subjective. Based on the
findings evaluated and current agency specifications, 95% two crushed faces would
appear to be a more reasonable target for high traffic pavements (greater than 30 million
equivalent single axle loads [ESALs]). Specifications in Arkansas, Louisiana, Missis-
sippi, and Utah currently support a small tolerance for the percent two fractured faces
for high traffic levels.
Flat and Elongated Particles
While the asphalt industry believes that excessive flat and elongated particles are
undesirable, perfectly cubical aggregates may also be undesirable. The Superpave
method specifies ASTM D4791, "Standard Test Method for Flat Particles, Elongated
Particles, or Flat and Elongated Particles," to evaluate aggregate shape with criteria for
a maximum percentage (10% by weight) exceeding the 51 ratio of maximum to min-
imum dimension.
A limited number of studies have been conducted to relate the percentage of flat and
elongated particles to performance since the implementation of the Superpave method.
None of the studies have addressed the relationship between flat and elongated particle
performance near the existing specification level of 10% particles exceeding the 51 ratio
of maximum to minimum particle dimension. The research conducted to date gener-
ally supports the following:
· Percentage of flat and elongated particles changes with handling of the stockpile
and mixing.
· Aggregate breakdown during compaction increases for higher percentages of flat
and elongated particles.
· VMA generally increases with increasing percent flat and elongated particles.
· There does not appear to be a relationship between the percentage of flat and elon-
gated particles exceeding the 31 ratio in the range of approximately 10% to 40%
and performance.
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· ASTM D4791 is a highly variable test procedure. Alternative methods of deter-
mining the percentage of flat and elongated particles should be developed. This
variability may mask relationships with performance.
ASTM D4791 or a similar procedure is specified by 86% of the responding agencies.
Sixty-three percent of the state agencies who use ASTM D4791 specify the same crite-
ria as AASHTO M323 (<10% particles exceeding the 51 ratio for traffic levels in
excess of 0.3 million ESALs). Seven states specify 31 criteria for Superpave criteria.
Five states allow 20% maximum, one state allows 10% maximum, and a final state spec-
ifies flat or elongated particles. It should again be noted that research studies have been
unsuccessful in relating percentages of flat and elongated particles at the 31 ratio to per-
formance. However, testing flat and elongated particles at the 31 or even 21 ratio prob-
ably provides a better indication of the shape of the source and would more readily
identify changes in shape resulting from the crushing process.
The current test method for flat and elongated particles, ASTM D4791, is highly vari-
able, particularly at low percentages of flat and elongated particles (typically found at
the 51 ratio). The percentage of flat and elongated particles has been related to volu-
metric properties; therefore, changes in percentages of flat and elongated particles may
be related to changes in volumetric properties. However, flat and elongated aggregate
tests performed at the 51 ratio are generally insensitive to production changes in aggre-
gate shape. Based on research conducted on aggregate having 10% to 40% flat and
elongated particles exceeding the 31 ratio, there is no evidence that these levels are
detrimental to performance. However, monitoring the percentage of flat and elongated
particles that exceed the 31 or 21 ratio would provide more information about pro-
duction changes in aggregate shape and the changes' influence on volumetric proper-
ties. The 31 criteria adopted by six states (five states 20% maximum and one state 10%
maximum) are probably more restrictive than necessary from a performance standpoint.
Fine Aggregate Angularity
To measure the angularity of fine aggregate, the Superpave method specifies AASHTO
T304, "Uncompacted Void Content in Fine Aggregate, Method A." The test is included
to ensure that there is sufficient internal friction--resulting from particle shape, angu-
larity, and texture--to provide rut-resistance in the HMA. The uncompacted voids test
is an indirect measure of aggregate shape, angularity, and texture and works under the
assumption that particles that are more flat and elongated, are more angular, have more
texture, or are a combination thereof will not pack as tightly and therefore will have a
higher uncompacted void content.
The uncompacted voids test may be the most controversial of the consensus proper-
ties, and several concerns have been expressed regarding its use. The primary concern
is that some fine aggregates, which are 100% crushed, do not meet the minimum require-
ments (>45%) for mixes used in the upper 100 mm of the pavement structure for traffic
levels greater than 3 million ESALs. Typically, these are extremely cubical limestone
sources. A second concern is that particles that pass the 4.75-mm sieve but are retained
on the 2.36-mm sieve are not evaluated for angularity or shape under the current Super-
pave aggregate properties (5). A third concern is related to the variability of the test pro-
cedure and its dependence on the fine aggregate dry bulk specific gravity (5). Finally,
there is concern that the fine aggregate angularity test may not be related to the rutting
propensity of the HMA mixture.
Twelve studies were discussed in detail. The following is a summary of the key find-
ings of these studies. Numerous test procedures are available to assess fine aggregate
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angularity and texture. Several of the imaging techniques and the compacted aggregate
resistance (CAR) test appear to be promising. Two studies found fair relationships
between the direct shear test (ASTM D3080) and laboratory measures of rutting resis-
tance. However, researchers using the direct shear test have indicated that it is difficult
to obtain consistent results. In addition to the new methods examined, to date, the
majority of the work to correlate fine aggregate shape and texture to performance has
been completed using AASHTO T304 Method A.
The results of studies relating the uncompacted voids content from AASHTO T304
Method A to performance are mixed. Generally, studies indicated a trend between
uncompacted voids content and improved rutting performance, but in some cases the
trend was weak. Subtle differences in uncompacted voids content can be overwhelmed
by the effect of the coarse aggregate or other HMA mixture properties. Several studies
supported the 45% uncompacted voids criteria for high traffic, but several also indi-
cated that performance was unclear between 43% and 45% (or higher) uncompacted
voids. There is clear evidence that good performing mixes can be designed with uncom-
pacted voids contents between 43% and 45%, but evaluation of these mixes using a rut-
ting performance test is recommended. Although the results for the uncompacted voids
tests were mixed, an alternative test was not clearly identified as being related to per-
formance. Also, higher uncompacted void contents generally resulted in higher VMA
and lower densities at Ninitial.
The variability of AASHTO T304 Method A appears to be larger than reported in
the test method. Much of this variability appears to be related to variability in the fine
aggregate specific gravity measurements used to calculate the uncompacted voids. On-
going research to improve fine aggregate specific gravity measurements may also ben-
efit AASHTO T304.
Based on the research evaluated, the test for uncompacted voids in fine aggregate
appears to be a reasonable screening tool for fine aggregates with respect to their rutting
potential. The research supports the fact that some crushed fine aggregates with uncom-
pacted voids contents between 43% and 45% can be used to produce rut-resistant mix-
tures. Currently, four agencies' specifications allow either uncompacted void contents
of 43% or 44% up to 10 million ESALs, one agency allows 44% for all traffic levels,
and one agency allows 43% for all traffic levels if the volumetric properties are met.
Further research is recommended to evaluate alternatives to the uncompacted voids
test, such as the CAR test. This study should include an examination of the interactions
that allow rut-resistant mixes to be produced with uncompacted voids contents between
43% and 45%.
Several new tests are being developed to directly measure aggregate size, shape, angu-
larity, and texture (NCHRP Project 4-30). Measurements are made from digital images
or laser scans. These techniques should eliminate subjectivity and improve testing preci-
sion. Once the measurement techniques are perfected, additional research will be required
to relate these parameters to the performance of HMA and, thereby, to develop criteria.
These methods have the potential to replace coarse aggregate angularity, flat and elon-
gated particles, and fine aggregate angularity. However, because of the cost and com-
plexity of the new methods, the existing consensus properties may have applicability
in field labs for some time to come.
Sand Equivalent
Many factors influence moisture damage. HMA characteristics (aggregate, asphalt
binder, and type of mixture), weather during construction, environmental effects after
construction, and pavement surface drainage properties all affect pavement moisture
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damage. Aggregate tests related to moisture damage generally fall into two categories:
tests to identify clay-like fines and tests that evaluate the surface properties of the aggre-
gate related to the adhesion of the binder to the aggregate.
Clay-like fines may coat the fine aggregate and prevent the asphalt from adhering to
the aggregate surface. In the presence of water, such coatings may lead to moisture
damage. The Superpave method currently specifies the sand equivalent test (AASHTO
T176) to identify clay-like fines in fine aggregate. Controversial results and findings
exist for the sand equivalent test. In some cases, the sand equivalent test identifies
crusher fines as harmful clay-like particles. The sand equivalent test is specified by 92%
of the responding agencies. The majority of the responding agencies have adopted the
same criteria specified in AASHTO M323, although several states have more restric-
tive criteria for low traffic levels.
It appears that the methylene blue test may be the best method to quantify the amount
of harmful clays in fine aggregate. However, there is concern that the methylene blue
test is too variable for routine specification work and is better suited to research and
forensic investigations.
The net adsorption test was developed during SHRP to evaluate the interaction
between the asphalt binder and aggregate in the presence of water. However, valida-
tion work conducted as part of SHRP indicated a poor predictive ability for the test, and
it has not been widely used since. At present, the surface energy techniques appear to
be promising. The procedures are relatively new. Results and efforts from NCHRP Proj-
ect 9-37, "Using Surface Energy Measurements to Select Materials for Asphalt Pave-
ments," can be used to apply the energy surface theory in the future.
SOURCE PROPERTIES
Source properties address two categories: aggregate durability and deleterious mate-
rials. Deleterious materials are organics or other unsuitable materials such as coal and
lignite. Criteria for source properties were to be set by the specifying agency to allow
for regional differences in geology. Aggregate durability generally encompasses two
categories of tests: tests that measure aggregate abrasion resistance and breakdown dur-
ing handling, mixing, laydown, and under traffic and tests that address aggregate weath-
ering when exposed to freezing and thawing or wetting and drying. These tests are
employed in concert to ascertain that the aggregate used in the production of HMA will
be durable. Specifically, tests related to durability are selected to address the following:
· Aggregate breakdown during handling, mixing, and placement. This breakdown
can generally be accounted for in the design process.
· Abrasion or weathering of the aggregates in the pavement structure. Gross aggre-
gate wear or weathering can occur in the form of raveling, popouts, or potholes.
· Freeze-thaw durability, although this is generally less of a concern with HMA as
compared with aggregate base or Portland cement concrete because the aggregate
particles in HMA should be coated with asphalt.
The Superpave method includes two methods to measure aggregate durability: the Los
Angeles (LA) abrasion test and sulfate soundness.
LA Abrasion Test
The LA abrasion test subjects the aggregate sample to impact and crushing. It has
been correlated with other impact tests such as the Aggregate Impact Value and Aggre-
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gate Crushing value, both of which are British standards. The LA abrasion test is prob-
ably most related to the expected breakdown during handling, mixing, and placement.
Recent studies have only indicated a fair correlation with in-place performance
although early developmental studies indicated better correlations. The LA abrasion
test is used by 96% of the responding agencies. Agency specification values range from
less than 30% to less than 55% loss, with 40% loss being the most frequently cited spec-
ification. Some research has been conducted to evaluate the micro-deval test as an alter-
native to LA abrasion. However, the two tests measure different deterioration methods.
There is no evidence that the LA abrasion test should be replaced for assessing break-
down during construction. Individual agencies may wish to examine their criteria based
on other agencies' experience.
Sulfate Soundness
Aggregates can deteriorate from wetting and drying or freezing and thawing cycles.
The sulfate soundness test simulates the effects of the expansion of water in the aggre-
gate pores during freezing. Two sulfates can be used in the sulfate soundness test
(AASHTO T104): magnesium or sodium. Seventy-five percent of the responding agen-
cies specify sulfate soundness. Sodium sulfate soundness is specified by 64% of these
agencies, and magnesium sulfate soundness by 30% of these agencies. Two agencies
(6%) allow either magnesium or sulfate soundness. More than 50% of the agencies spec-
ifying sodium sulfate soundness specify a maximum loss of 12%. There is no consen-
sus for magnesium sulfate soundness; however, NCHRP Project 4-19 recommended the
use of magnesium sulfate soundness with a maximum loss of 18%. Other recent research
regarding the sulfate soundness test has been conducted in conjunction with evalua-
tions of the micro-deval test, discussed below.
Micro-Deval Test
The Superpave method did not specify a test method to evaluate the abrasion of
aggregates under traffic, although the sulfate soundness test evaluates disintegration of
aggregates caused by environmental exposure. Several studies have evaluated the
micro-deval test for inclusion as a durability test for aggregates. In the micro-deval test,
the aggregate is loaded in a jar with water and a charge of steel shot and then rotated at
100 RPM for 2 hours. This is not an impact test; however, as the aggregate breaks
down, abrasive slurry is created in addition to the steel shot.
Although originally intended to assess degradation from freezing and thawing, sul-
fate soundness tests (AASHTO T104) have been widely used to assess aggregates' resis-
tance to weathering. Several studies indicated good correlations between magnesium
sulfate soundness loss and micro-deval abrasion loss (AASHTO TP58). Several studies
have also indicated that the strength of some aggregates is significantly lower when wet.
The micro-deval test offers improved precision over sulfate soundness. The micro-deval
test also indicates abrasion resistance. This suggests that the micro-deval test may be
more suitable to predicting aggregates' performance in relation to weathering and abra-
sion than is the sulfate soundness test. NCHRP Project 4-19 recommended a criterion
of a maximum of 18% loss may separate good- and poor-performing aggregates. How-
ever, data suggests specifications for micro-deval loss may have to be based on aggre-
gate type. In regions where freeze-thaw is a concern, equipment now exists to perform
actual freeze-thaw tests on aggregate, such as the equipment used in AASHTO T103.
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AGGREGATE GRADING
The aggregate grading has significant effect on the constructability and performance
of HMA mixtures. Numerous research studies have indicated that the restricted zone,
included when the Superpave method was originally developed during SHRP, was
redundant when considering the other Superpave properties, particularly uncompacted
voids in fine aggregate and Ninitial. This was evidenced by the good historical perfor-
mance of many agency-used mixes that passed through the restricted zone by specifica-
tion prior to the adoption of the Superpave method. Based on these studies, the restricted
zone was removed from the 2004 AASHTO Superpave specification (AASHTO M323).
Data from the 2000 National Center for Asphalt Technology (NCAT) Test Track indi-
cates that fine and coarse gradations can be equally rut resistant.
EFFECTS OF FINES AND FILLERS
It is widely believed that depending on the particle size, fines can act as a filler or an
extender of asphalt cement binder. Some fines have a considerable effect on the asphalt
cement, making it act as a much stiffer grade when compared with the neat asphalt
cement. Early work indicated that both the size of the filler and the asphalt binder com-
position had an impact on the stiffening effect. As much as a 1,000-fold increase in vis-
cosity of the neat asphalt cement was measured when certain fillers were added to asphalt
cement. Some fines may also make HMA mixtures more susceptible to moisture-induced
damage.
Numerous studies have evaluated the effects of fines, filler, and mortar on HMA per-
formance in the laboratory and in the field. Efforts to characterize fillers have gener-
ally followed three paths: characterization of particle size or packing, binder tests per-
formed on a mortar, or modeling of the overall interaction between the filler and binder.
Several research studies have been conducted to develop suitable test parameters
related to particle size or packing to evaluate the fines and fillers. D60 (the particle size
of P200 at 60% passing) and methylene blue values were found to be related to rutting,
and D10 and methylene blue values to stripping. The modified Rigden voids test has
been used to characterize the stiffening potential of baghouse fines. Superpave binder
tests including the dynamic shear rheometer, bending beam rheometer, and direct ten-
sion test have been used to characterize the fine mortar or voidless mastics properties.
Recommended criteria have been developed for fillers for stone matrix asphalt mixtures.
EFFECT OF CRUSHING OPERATIONS ON AGGREGATE PROPERTIES
The implementation of the Superpave method impacted the aggregate industry.
Some of the areas that highlighted the importance of crushing operations include the
following:
· Requirements for coarse aggregate angularity for high-traffic pavements;
· Increased emphasis on particle shape and specifications for flat and elongated par-
ticles using the 31 ratio;
· Uniform utilization of product sizes, particularly with the preference for coarse-
graded Superpave mixes early in the implementation process; and
· The divergent requirements for aggregate properties for different end users (HMA,
hydraulic cement concrete, and base).
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For gravel sources, it can be impossible to meet high levels (100% two or more frac-
tured faces) of coarse aggregate angularity for high traffic levels. Fractured faces are
produced through the size reduction of the aggregate. If the feed aggregate is broken in
half, one fractured face is produced on each of the resulting particles. If the particle size
of the gravel feed stock is not large enough to allow significant particle size reduction
in the production of the final product size (say, one-third or one-quarter of the feed
size), then it is impossible to consistently produce very high levels of fractured faces.
Mississippi DOT's specifications account for this reality. Mississippi requires higher
percentages of fractured faces for smaller nominal maximum aggregate sizes. These
smaller nominal maximum aggregate sizes are typically used in surface mixes where
stresses are higher.
Particle shape is affected by the geology of the aggregate as well as the crushing
process. Impact type crushers tend to produce the best particle shape. Horizontal shaft
impact crushers are only suitable for low abrasion feed; their operation would be cost
prohibitive for hard aggregates such as granite. Vertical shaft impact crushers, partic-
ularly autogenous ones, can be used on harder aggregates, but they produce a relatively
small size reduction. In an autogenous crusher, the rotor propels aggregate particles
against a cascade of aggregate particles reducing wear on the crusher liner.
Compression crushers are more commonly used on harder aggregates. A number of
factors can improve the shape of aggregate crushed in compression crushers:
· The crusher should be run with a full or choked feed cavity to promote inter-particle
crushing.
· Crushers should be operated in closed circuits where a recirculating feed can be
used to fill the crusher cavity.
· The reduction ratio should be reduced, which can be accomplished by reducing the
feed size or increasing the circulating load.
· The close-side setting should be approximately equal to the desired product size.
Unfortunately, most of the techniques that improve particle shape also generate more
fines. Most quarries already produce more fines than they can sell. The majority of the
techniques used to improve particle shape also increase production cost. Finally, HMA
is just one use for aggregate. In some cases, there are product requirements that are not
consistent for HMA, hydraulic cement concrete, and aggregate base. While it would be
possible to produce specialty products for each application, this may be cost prohibitive.
REVIEW OF PERFORMANCE DATA FROM FIELD TEST SECTIONS
AND FULL-SCALE ACCELERATED TESTING
The earliest Superpave projects were constructed 1992. A number of experimental
field sections have been built and documented by agencies. Unfortunately, the consen-
sus and source aggregate properties are generally not documented in these reports.
There are a number of accelerated loading facilities in the United States; however,
aggregate properties have not been experimental factors in the majority of testing com-
pleted to date. One exception is the Indiana DOTPurdue University Accelerated Pave-
ment Testing Facility in West Lafayette, Indiana, where a large number of aggregate
studies have been completed. In addition, there are three test tracks that have been
active since the completion of the Superpave mix design system: Minnesota Road
Research Project (MnRoad), WesTrack, and the National Center for Asphalt Technol-
ogy (NCAT) Test Track.
The Long-Term Pavement Performance (LTPP) Program contains data for more than
773 SPS-1, -5 and -9 HMA sections. The majority of the SPS-9 sections were designed
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using the Superpave method. Unfortunately, of the consensus aggregate properties,
only the uncompacted void content is currently available in the database (DataPave 3.0)
and then only for a limited number of sections. Traffic data is also missing in many
cases. A weak relationship (R 2 = 0.46) was found between the uncompacted voids con-
tent and the measured rut depth divided by the square root of ESALs.
Aggregate properties were not an experimental variable at MnRoad or WesTrack.
There were over eight combinations of aggregate types, three nominal maximum aggre-
gate sizes, and a wide range of gradations placed at the 2000 NCAT Test Track. All of
the sections performed well. Strong correlations were not evident between aggregate
properties and rutting, VMA, construction density, or densification under traffic. Com-
parisons could be made between the performance of fine and coarse graded mixes using
an unmodified binder for three aggregate types. Statistical analysis indicated that gra-
dation (coarse or fine) did not have a significant effect on the rut depth of the pavement
sections.
One overall conclusion from the review of in-service and accelerated loading sites
was that aggregate property data was not as readily available as expected. Additional
efforts need to be made to collect and make readily available aggregate property data
from in-service pavements.
FUTURE RESEARCH NEEDS
The results of this review have emphasized the difficult nature of conducting research
to relate aggregate properties and HMA performance. It is difficult to isolate the effects
of the aggregate properties from other interactions with gradation and mixture volu-
metric properties. It appears as if the shortcomings of a single property related to rut-
ting resistance can be overcome by other supporting properties.
These interactions emphasize the need for laboratory performance tests for HMA
mixtures. If performance tests are adopted that have criteria in which agencies are
confident, the overall performance of the mixture could be assessed instead of relying
solely on component screening tests. For example, if the blend uncompacted voids in
fine aggregate were 43% for a given mixture to be placed on a high-volume road, the
rutting properties of this mixture could be tested (at the contractor's expense) to show
whether the mix should provide acceptable performance.
There is also a need to emphasize the collection and reporting of aggregate property
data for both in-service pavements and accelerated loading facilities. More effort needs
to be placed on capturing aggregate property data in national studies related to HMA
performance.
Two areas selected for immediate research are an investigation of alternatives to, and
specification limits for, the uncompacted voids in fine aggregate tests and the develop-
ment of performance relationships and criteria for the new imaging methods to mea-
sure aggregate shape, angularity, and texture. However, even where the test method
may not be in question, there is still room for additional research to refine specification
limits for such tests as coarse aggregate angularity, flat and elongated particles (both of
which could one day be replaced by imaging), LA abrasion, and micro-deval. Addi-
tional refinement of the methylene blue test could make it a likely and more discrimi-
nating candidate to replace the sand equivalent test.
SUMMARY REFERENCES
1. Cominsky, R., R.B. Leahy, and E.T. Harrigan. Level One Mix Design: Materials Selection, Com-
paction, and Conditioning, SHRP A-408. Strategic Highway Research Program, National Research
Council, Washington, DC, 1994.
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