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OCR for page 66
66 A Manual for Design of Hot Mix Asphalt with Commentary
Mixture Composition and Performance
The mixture design criteria for dense-graded and SMA mixtures presented in Chapters 8 and
10 place several requirements on mixture composition that are related to performance. These
requirements include
· Binder grade
· Aggregate angularity
· Nominal maximum aggregate size
· Mineral filler content
· Design air void content
· Design compaction level, and
· Design voids in the mineral aggregate (VMA)
These requirements are included in the design procedure to ensure that the resulting mixtures
will exhibit adequate performance when properly compacted in the field. It is important to
emphasize that field compaction is a critical factor affecting every aspect of pavement performance
and that the design procedures presented in this manual for dense-graded and SMA mixtures
assume that the HMA mixture will receive proper compaction during construction. Table 6-1
summarizes the effect of mixture composition on pavement performance.
In Table 6-1, an upward arrow indicates that a particular performance indicator improves
with an increase in the compositional factor. A downward arrow indicates that the performance
indicator deteriorates with an increase in the compositional factor. The relationship between
binder stiffness and fatigue resistance depends on the pavement structure; for thin pavement
structures, increasing binder stiffness will decrease fatigue resistance, while for thick pavement
Table 6-1. Effect of mixture composition on performance.
Typical Effects of Increasing Given Factor within Normal Specification Limits
While Other Factors Are Held Constant within Normal Specification Limits
Resistance to Resistance to Durability/
Rutting and Resistance to Low Resistance to Resistance to
Permanent Fatigue Temperature Moisture Penetration by
Component Factor Deformation Cracking Cracking Damage Water and Air
Increasing High
Temperature Binder Grade
Increasing Low
Asphalt Binder
Temperature Binder Grade
Increasing Intermediate
Temperature Binder
Stiffness
Increasing Aggregate
Angularity
Increasing Proportion of
Aggregates Flat and Elongated
Particles
Increasing Nominal
Maximum Aggregate Size
Increasing Mineral Filler
Content and/or
Dust/Binder Ratio
Increasing Clay Content
Increasing Design
Volumetric Compaction Level
Properties Increasing Design Air
Void Content
Increasing Design VMA
and/or Design Binder
Content
Increasing Field Air Void
Content
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Evaluating the Performance of Asphalt Concrete Mixtures 67
structures the reverse is true--that is why there are two arrows going in different direction for this
entry in Table 6-1. The relative importance of each of the factors is indicated by the number of
arrows shown in the table. The information presented in Table 6-1 and the more detailed discussion
on the relationships among binder properties, aggregate properties, mixture composition, and
pavement performance that is given later in this chapter are based on several sources, most
importantly NCHRP Report 539: Aggregate Properties and the Performance of Superpave-Designed
Hot Mix Asphalt and NCHRP Report 567: Volumetric Requirements for Superpave Mix Design.
Binder Characteristics and Performance
Performance Grading System
As discussed in more detail in Chapter 3, the Performance Grading system for asphalt binders,
AASHTO M 320, controls the properties of asphalt binders that are related to pavement per-
formance. This grading system includes requirements for the properties of the binder at high,
intermediate, and low pavement temperatures to address rutting, fatigue cracking, and low-
temperature cracking in pavements. Two conditioning procedures are used to simulate the effects
of binder aging during construction and the service life of the pavement.
In the Performance Grading system, asphalt binders are specified by two numbers, for example
PG 64-22. The first number, 64 in this example, is called the high temperature grade. It is the
pavement temperature in degrees Centigrade up to which the binder provides adequate stiffness
to resist excessive rutting for a properly designed HMA mixture exposed to a moderate volume of
fast-moving highway traffic. The second number, -22 in this example, is called the low temperature
grade. It is the pavement temperature in degrees Centigrade down to which the binder provides
adequate flexibility to resist low-temperature cracking. In the Performance Grading system,
the critical value of the performance-related property remains the same, but the temperature
where the binder provides the minimum or maximum property changes. A PG 70-22 must meet
the minimum high temperature properties at 70°C compared to 64°C for a PG 64-22. Similarly
a PG 64-28 must meet the low temperature properties at -28°C compared to -22°C for a PG 64-22.
Both of these binders provide a wider performance range than the PG 64-22. The performance-
related properties used in the Performance Grading system are summarized in Table 6-2. For
low temperature cracking, two criteria corresponding to Table 1 and Table 2 of AASHTO M 320
are given. Table 1 of AASHTO M 320 is based on low temperatures properties from the bending
beam rheometer, while Table 2 of AASHTO M 320 is based on the computed critical cracking
temperature of the binder. This computation uses data from both the bending beam rheometer
and the direct tension test. Most agencies use the Table 1 criteria. The test methods used in the
Performance Grading system were described in Chapter 3.
Rutting and Permanent Deformation
The high temperature grade of the asphalt binder is one of several important factors affecting
the rutting resistance of HMA. For a given pavement and HMA mixture, resistance to permanent
Table 6-2. Performance-related properties and criteria used in the
performance grading system, AASHTO M 320.
Distress Mode Performance Related Criteria
Binder Property
Rutting G*/sin Minimum of 1.00 kPa for unaged binder at 10 rad/sec
Minimum of 2.20 kPa for RTFOT aged binder at 10 rad/sec
Low Temperature Creep stiffness, S Maximum of 300 kPa for PAV aged binder at 60 s.
Cracking, Table 1
m-value Minimum of 0.300 for PAV aged binder at 60 s
Low Temperature Critical cracking Equal to or lower than specified low temperature grade.
Cracking, Table 2 temperature
Fatigue Cracking G*sin Maximum of 5,000 kPa for PAV aged binder at 10 rad/sec
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68 A Manual for Design of Hot Mix Asphalt with Commentary
deformation increases as the high temperature performance grade increases. Additionally,
recent research has shown that for the same high temperature performance grade, the rutting
resistance of HMA made with polymer-modified binders is significantly improved over that for
neat (that is, undiluted or not mixed with other substances) asphalt binders.
Rutting in asphalt pavements increases with increasing traffic volume and decreasing traffic
speed. To counteract these effects, the high temperature binder grade must be increased or
"bumped" for pavements exposed to high traffic levels (trucks) and slow-moving traffic. Table 6-3
presents the recommended high temperature grade changes included in the mixture design
procedures presented in Chapters 8, 10, and 11.
When using grade bumping for different traffic levels, it is important to avoid binders that are
excessively stiff at the intermediate temperature for the binder when selected on the basis of
environmental conditions alone. As the high temperature performance grade increases, the
temperature where the binder is required to meet the maximum value of G*sin also increases.
This may result in the binder being too stiff for the intermediate temperature conditions under which
the pavement is expected to perform. For example, when bumping two grades from a PG 64-22 to
a PG 76-22, the temperature where the intermediate stiffness is normally tested increases 6°C,
from 25°C to 31°C. The appropriate intermediate temperature based on environmental con-
ditions is 25°C, not 31°C, and the binder should be expected to have a value of G*sin that
is less than or equal to 5,000 kPa at 25°C. The Performance Grading system does not ensure
that bumped binders will meet the intermediate temperature conditions required for the base
binder. Agencies usually place additional language in the HMA specification to require inter-
mediate temperature testing at the temperature obtained on the basis of environmental con-
ditions alone.
Fatigue Cracking
The intermediate temperature stiffness of the asphalt binder is one of several factors affecting
fatigue cracking in pavements. Top-down fatigue cracking was identified as an important form
of distress in thick asphalt pavements and overlays of portland cement concrete pavements in the
mid 1990s. Although this form of distress is not yet completely understood, it appears that binder
stiffness is at least a contributing factor. Surface courses made with binders that become excessively
stiff due to rapid age hardening are more susceptible to top-down cracking. The Performance
Grading system places a maximum limit on the stiffness of the binder after simulated long-term
aging. Although there is much debate over this requirement and its relationship to traditional
Table 6-3. Recommended high
temperature performance grade
changes to account for traffic
volume and speed.
Grade Adjustment for Average Vehicle
Speed in kph (mph):
Design Very Slow Slow Fast
Traffic < 25 25 to < 70 70
(MESALs) (< 15) (15 to < 45) ( 45)
< 0.3 --- --- ---
0.3 to < 3 12 6 ---
3 to < 10 18* 13 6
10 to < 30 22* 16* 10
30 --- 21* 15*
* Consider use of polymer-modified binder. If a polymer-
modified binder is used, high temperature grade may be
reduced one grade (6 °C) provided rut resistance is verified
using suitable performance testing.
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Evaluating the Performance of Asphalt Concrete Mixtures 69
bottom-up fatigue cracking, the requirement serves to limit age hardening of the binder and the
potential for top-down cracking.
Low-Temperature Cracking
The low-temperature cracking performance of asphalt pavements is almost completely con-
trolled by the environmental conditions and the low temperature properties of the asphalt
binder. Binder grade selection is, therefore, the most critical HMA design factor affecting the low
temperature performance of asphalt pavements. Since transverse thermal cracks cannot be repaired
and quickly reflect through future overlays, it is critical that binders be selected to have a high
reliability against thermal cracking. Reliability as applied to binder grade selection was discussed
in detail in Chapter 3.
Durability
Excessive age hardening of the binder during the service life of the pavement is a contributing
factor to several pavement distresses including raveling, top-down fatigue cracking, thermal
cracking, and moisture damage. The primary factors affecting age hardening are the environment,
the permeability of the HMA, and the characteristics of the binder. Age hardening is most severe
in high-temperature climates. Age hardening also occurs more rapidly in pavements that are
more permeable; therefore, it is critical to ensure that a high level of in-place density is achieved
to minimize the potential for interconnected air voids in the HMA. The Performance Grading
system includes tests on the binder after simulated long-term aging to control the age-hardening
characteristics of the binder.
Moisture Damage
Some combinations of asphalt binder and aggregate exhibit greater potential for moisture
damage than others. For the same aggregate type, resistance to moisture damage improves
marginally with the use of a stiffer binder, particularly those modified with polymers.
Aggregate Characteristics and Performance
Excellent-performing pavements have been constructed using a wide variety of aggregate types.
Several characteristics of aggregates that are related to pavement performance are controlled in
the mixture design procedures presented in Chapters 8, 10, and 11.
Aggregate angularity and mineral filler content are important aggregate characteristics affecting
the rutting resistance of HMA. Resistance to rutting and permanent deformation improves with
increasing aggregate angularity and increasing mineral filler content--although excessive mineral
filler content will tend to produce a mixture that is very stiff and sticky and difficult to compact.
Rutting resistance also improves as the nominal maximum aggregate size (NMAS) of the HMA
increases because the design VMA decreases with increasing NMAS and the design VMA has a
major influence on the rutting resistance of HMA.
Aggregate characteristics are also important factors affecting the durability of HMA and its
resistance to moisture damage. Aggregates that are flat or elongated tend to break during
compaction, leaving uncoated surfaces, which decrease durability and increase the potential for
moisture damage. Clay particles disrupt the adhesion of the asphalt binder to the aggregates
making the HMA less durable and more susceptible to moisture damage. Durability improves
with decreasing NMAS because the design VMA increases with decreasing NMAS and, as discussed
in the next section, increasing the design VMA increases the effective binder content of the mixture,
which improves durability. Finally, increasing the mineral filler content of the HMA decreases
permeability for the same in-place air void content (again, understanding that there are practical
limitations to how much mineral filler can be used in HMA mixtures). Binder age hardening and
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70 A Manual for Design of Hot Mix Asphalt with Commentary
water infiltration are reduced in mixtures with lower permeability, leading to improved durability
and greater resistance to moisture damage.
Volumetric Properties and Performance
The volumetric properties of HMA have a major influence on the performance of HMA.
Volumetric properties affect an HMA mixture's resistance to rutting and fatigue cracking. They
also affect the durability of the mixture and its resistance to moisture damage.
Rutting and Permanent Deformation
Several volumetric factors affect the resistance of HMA to rutting and permanent deformation.
Although individually these factors are less important than high-temperature binder grade,
aggregate angularity, and mineral filler content, the volumetric factor effects are additive and,
if these act together in the same way, the results can be significant.
Rutting resistance tends to improve with decreasing design VMA and in-place air void content.
As discussed in Chapter 5, VMA is the volume of air and asphalt binder in the mixture. These are
the components of HMA that deform easily upon loading; therefore, rutting resistance improves
as VMA and in-place air void content decrease. Rutting resistance also improves with increasing
design compaction level. The resistance of the aggregate structure to deformation improves as
the number of gyrations used in the design increases. Finally, the rutting resistance improves
as the design air void content increases. At first, this effect might seem counter-intuitive, but by
increasing the design air void level while maintaining the in-place air void content constant, the
energy of compaction required to construct the pavement is increased significantly. Conversely,
decreasing design air void content under constant in-place air void content decreases the energy
required for field compaction. Even though decreasing VMA and increasing design air void content
will, in general, improve rut resistance, as discussed below, VMA values that are too low and
design air void values that are too high will often produce mixtures with poor durability. This
is why there are both minimum and maximum values for VMA and air void content. These
requirements are discussed in detail in Chapter 8 of this manual.
Fatigue Cracking
Several volumetric factors also affect the resistance of HMA to fatigue cracking. The most
important of these is the in-place air void content of the pavement. The fatigue life of typical HMA
pavements decreases with increasing in-place air void content. This occurs for several reasons.
Lower air void content will tend to produce a stronger pavement more resistant to cracking.
Lower air void content also will in general produce a pavement with lower permeability to both
air and water. This will reduce the amount of binder age hardening in the pavement and will
tend to minimize moisture damage, which can render the pavement weak and more prone to
fatigue damage.
The primary HMA mixture design factor affecting fatigue life is the effective volumetric binder
content of the mixture (VBE). For a given pavement, fatigue life increases with increasing VBE;
therefore, controlling VBE is an important consideration in mixture design. Since VBE is equal
to VMA minus the air void content, the mixture design procedures presented in Chapters 8 and 10
control VBE by controlling VMA and the design air void content of the mixture. As discussed
above, increasing VMA or decreasing air void content too much can significantly decrease rut
resistance; therefore, the requirements given in Chapters 8 and 10 provide both upper and lower
limits for VMA and design air void content. For dense-graded mixtures, the design procedure
in Chapter 8 provides the flexibility to increase the design VMA requirements up to 1.0% to
produce mixtures with improved fatigue resistance and durability. SMA mixtures, because they
have extremely high VMA, tend to produce mixtures with excellent fatigue resistance.
