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38 A Manual for Design of Hot Mix Asphalt with Commentary
absorption. In HMA mix design it is often assumed that asphalt absorption will be one-half of
the water absorption. It is important to account for this absorption, since asphalt absorbed into
the permeable voids of the aggregate will not be available to fill the voids between the aggregate
particles. For this reason, the asphalt binder content for HMA mixtures made using aggregates
having high absorption values will tend to be significantly higher than those made using aggregates
with lower absorption values.
Aggregate Specification Properties
Angular and rough-textured aggregates are desirable within HMA to resist permanent
deformation and fatigue cracking. Very angular and rough-textured aggregates provide better
interlock between the aggregate particles which helps prevent plastic deformation (rutting) within
HMA layers. Angular and rough-textured aggregates also help improve the strength of HMA
mixtures, which can help prevent fatigue cracking. Angular aggregates with good surface texture
also improve the frictional properties of pavement layers, an important safety consideration in
the design of HMA for pavements.
The presence of flat or elongated particles within HMA is undesirable because these particles
tend to break down during production and construction. Aggregates that break during production
and construction will reduce the durability of the HMA layer, leading to raveling, pop-outs, and
potholes.
Another aggregate characteristic related to performance is cleanliness and the presence of
deleterious materials. Cleanliness is a term used to characterize the coatings on some aggregate
particles. These coatings are often very fine clay-like materials and can affect the adhesion between
the asphalt binder and aggregate particles leading to an increased potential for moisture damage.
Deleterious materials are particles in an aggregate stockpile that are weak, prone to freeze-thaw
damage or damage through repeated wetting and drying, or that otherwise can cause a pavement
to deteriorate. Some examples of deleterious materials are clay lumps, friable particles, shale, coal,
free mica, and vegetation. These types of materials are not as strong as mineral aggregates and break
down during the life of a pavement layer. When this happens, pop-outs and potholes can occur.
Aggregate toughness and abrasion resistance have also been shown to be related to pavement
performance. Aggregate particles that are tough and resistant to abrasion will not break down
during the construction process, which helps ensure that an HMA mix can be properly constructed,
placed, and compacted. Tough, abrasion-resistant aggregates also tend to produce a mix that is
resistant to pop-outs and raveling. Because aggregate pop-outs and broken aggregate particles
near the pavement surface make it easier for water to flow into a pavement, tough and abrasion-
resistant aggregates help improve the moisture resistance of HMA pavements. Aggregates with
poor abrasion resistance can also polish under the action of traffic. This can cause the pavement
surface to lose skid resistance, especially when wet.
Another aggregate property that is closely related to toughness and abrasion resistance is
durability and soundness. Freeze-thaw cycles and alternate periods of wetting and drying in a
pavement can weaken poor-quality aggregates, causing pop-outs and raveling. Aggregates that
possess good durability and soundness will resist the actions of wet-dry and freeze-thaw cycles
during the life of the pavement.
Superpave Consensus and Source Aggregate Properties
During the development of the Superpave mix design system for dense-graded HMA, aggregate
requirements were specified based on the experiences of a group of experts. Properties that
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Aggregates 39
were identified as important within HMA included the angularity of coarse and fine aggregates,
aggregate shape, cleanliness of the aggregates, toughness, soundness, and the proportion of dust
within the mixture. After some discussion, these experts reached an agreement, or consensus, that
four aggregate properties were most important to HMA performance and should be specified as part
of the Superpave system. A test method and specification limits were identified for each of these
consensus properties. The four Superpave consensus aggregate properties are coarse aggregate
angularity (CAA), fine aggregate angularity (FAA), clay content, and flat and elongated particles.
The expert panel identified several other aggregate properties as important to HMA pavement
performance, but could not reach agreement on the specification limits. These aggregate prop-
erties are toughness (Los Angeles Abrasion test), soundness (Sodium or Magnesium Sulfate
Soundness test), and deleterious materials. Test values for these properties vary significantly
across the United States and Canada, depending on the type of aggregates locally available.
The panel of experts therefore labeled these aggregate properties as "source aggregate properties"
and recommended that specification values for these properties be developed by individual
highway agencies.
The group of experts developed the aggregate requirements for HMA without the benefit of a
formalized research program. Since the early 1990s, when the group of experts met to develop
the aggregate requirements for the Superpave mix design system, a significant amount of work
has been conducted to evaluate various aggregate tests and their relationship with pavement
performance. This chapter provides aggregate requirements for the design of dense-graded HMA.
These requirements build on both the experiences of the group of experts and research that has
been conducted since the Superpave mix design system was developed. Because the Superpave
consensus aggregate properties are now firmly grounded in both experience and research, rather
than simply the consensus of an expert panel, the term "consensus properties" is no longer accurate.
For this reason, the term "primary aggregate specification properties" is used herein to describe
these four critical characteristics. The term source aggregate properties is still appropriate for
the other aggregate tests, since specification values for these are still to be determined by individual
agencies.
Specification limits for the various primary aggregate specification properties are not uniform
for all HMA mixtures. Instead--as in the Superpave system--the specification requirements
for these test values are based on the expected amount of traffic over a 20-year pavement life, the
position of the layer being designed within the pavement structure, or both. Traffic is characterized
as equivalent single-axle loads (ESALs) and more stringent specification limits are provided for
pavements that will be subjected to higher traffic loads. Pavement layers that will encounter lower
traffic volumes or are within the lower portion of the pavement structure have less stringent
requirements.
The primary aggregate specification properties are designed to evaluate four critical charac-
teristics for aggregates used in HMA mixtures. These four characteristics are coarse aggregate
angularity, fine aggregate angularity, coarse aggregate particle shape, and cleanliness. Just as in
the Superpave system, requirements for these characteristics are intended to be enforced on the
aggregate blend and not on individual stockpiles. The following sections summarize the test
methods and specification limits for the four primary aggregate specification properties.
Coarse Aggregate Fractured Faces
Several research studies have shown that increasing the number of particles in an aggregate
blend that have been mechanically crushed increases resistance to permanent deformation.
The test recommended in this manual is the same as that used in the Superpave system--
a simple "crush count." The term coarse aggregate fractured faces (CAFF) is used instead of coarse
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40 A Manual for Design of Hot Mix Asphalt with Commentary
Table 4-6. Coarse aggregate fractured faces
requirements.
Percentage of Particles with at Least
One/Two Fractured Faces, for
Design ESALs (million)
Depth of Pavement LayerA, mm
0 to 100 Below 100
< 0.30 55 / --- --- / ---
0.3 to < 3 75 / --- 50 / ---
3 to < 10 85 / 80 60 / ---
10 to < 30 95 / 90 80 / 75
30 or more 98 / 98B 98/ 98B
ADepth of pavement layer is measured from pavement surface to top of
pavement layer within the pavement containing the given mixture.
BThe CAFF requirement for design traffic levels of 30 million ESALs or
more may be reduced to 95/95 if experience with local conditions and
materials indicate that this would provide HMA mixtures with adequate
rut resistance under very heavy traffic.
aggregate angularity because "fractured faces" is simpler and clearer, since it is the common term
used for this type of test. The procedure is described in ASTM D 5821, Standard Test Method
for Determining the Percentage of Fractured Particles in Coarse Aggregate. Aggregate particles
larger than 4.75 mm are visually examined to determine the percentage of particles that has at
least one fractured face, and the percentage that has at least two. A CAFF value of 76/53, for
example, means that 76% of the particles in a coarse aggregate have at least one fractured face,
and 53% have at least two fractured faces. Table 4-6 outlines the required minimum values for
CAFF as a function of traffic level and depth within the pavement structure. Note that the values
given in Table 4-6 are slightly different from the values currently specified within the Superpave
system; for the highest traffic level, the values for all mixtures are 98/98, whereas in the Superpave
system the required values for coarse aggregate angularity are 100/100. A footnote allows further
reduction of the CAFF requirement for this traffic level to 95/95 if local experience suggests that
the resulting HMA will have adequate rut resistance under very heavy traffic. These slightly lower
requirements for CAFF mean that high-quality crushed gravel can be used in HMA for high
traffic applications. In the Superpave system, because of the very high requirements for CAFF
at the highest traffic level, only crushed stone could be used for these applications. Experience over
the past 5 to 10 years suggests that high-quality crushed gravels will usually perform quite well
in properly designed HMA mixtures, even under extremely high traffic levels. Furthermore, the
mix design system described in this manual includes performance testing for HMA mixtures
designed for traffic levels of 10 million ESALs and greater. This performance testing provides
additional assurance that HMA mixtures will have adequate rut resistance. The slightly lower
values for CAFF recommended here should not be used unless performance testing is included
as part of the mix design process.
Fine Aggregate Angularity
The angularity of the fine aggregate fraction is as important as the angularity of the coarse
aggregate fraction to the performance of dense-graded HMA. In combination, the coarse and
fine aggregates provide strength to HMA, which helps minimize the potential for permanent
deformation. AASHTO T 304, Method A, Uncompacted Void Content of Fine Aggregate, is used
to measure fine aggregate angularity. A graded sample of fine aggregate (passing the 2.36-mm sieve)
is placed within a specially made funnel which allows the aggregate particles to freely drop into
a cylinder of known volume (Figure 4-8). Using the combined bulk specific gravity of the fine
aggregate blend, the percent voids between the aggregate particles is determined. Results from
the fine aggregate angularity test represent this percent of uncompacted voids in the fine aggregate;
higher values of uncompacted voids indicate greater angularity of the fine aggregate. Requirements
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Aggregates 41
Fine aggregate
Funnel sample
Cylinder of
known volume
Figure 4-8. Fine aggregate angularity test.
for fine aggregate angularity are given in Table 4-7. The requirements in this table are nearly
identical to those given in the Superpave system. The only exception is that in Table 4-7 where
the minimum FAA value is 45, it can be lowered to 43 if experience with local conditions and
materials suggests that this will produce mixtures with adequate rut resistance.
Flat and Elongated Particles
The percentage of flat and elongated particles in a coarse aggregate is determined using
procedures described in ASTM D 4791, Flat Particles, Elongated Particles, or Flat and Elongated
Particles. As in the Superpave system, this manual recommends a maximum value of 10% for flat
and elongated particles exceeding a 5:1 ratio. To conduct this test, aggregate particles are measured
with a proportional caliper (Figure 4-9) using a specified ratio of 5:1. The larger caliper opening
is set to the particle length; if the width of the particle can fit within the smaller opening, it is
considered flat and elongated. Coarse aggregates that fail this requirement are rare. Some state
agencies use slightly different versions of this test--using different limits, or specifying a maximum
value for flat or elongated particles for coarse aggregates. Technicians should check the applicable
specifications to make sure they are using the proper test and limits when evaluating aggregates
for an HMA mix design.
Table 4-7. Fine aggregate angularity requirements.
Depth of Pavement Layer from
Design ESALs (million) SurfaceA, mm
0 to 100 Below 100
< 0.30 ---B ---
0.3 to < 3 40 ---
3 to < 10 45C 40
10 to < 30 45C 40
30 or more 45C 45C
Criteria are presented as percent air voids in loosely compacted fine aggregate.
ADepth of pavement layer is measured from pavement surface to top of
pavement layer within the pavement containing the given mixture.
BAlthough there is no FAA requirement for design traffic levels below 0.30
million ESALS, consideration should be given to requiring a minimum
uncompacted void content of 40 % for 4.75-mm nominal maximum aggregate
size mixes.
CThe FAA requirement of 45 may be reduced to 43 if experience with local
conditions and materials indicate that this would produce HMA mixtures with
adequate rut resistance under the given design traffic level.
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42 A Manual for Design of Hot Mix Asphalt with Commentary
5:1 pivot point fixed post (B)
fixed post (A) swinging arm
Figure 4-9. Flat or elongated test.
Requirements for flat and elongated particles are not based on the traffic level or the antic-
ipated depth within the pavement structure. Flat and elongated particles are considered to
be detrimental within an HMA mixture during production and construction, irrespective of
traffic loadings or depth within the pavement; therefore, a single maximum percentage of flat
and elongated particles is required. Table 4-8 presents the requirements for flat and elongated
particles.
Clay Content
The presence of dust or clay coatings on aggregates can prevent the asphalt binder from prop-
erly coating the aggregates within an HMA. This can lead to water penetrating the asphalt binder
film and, therefore, stripping of the asphalt binder from the aggregate. The Sand Equivalent test
(AASHTO T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test)
is used to evaluate the cleanliness of aggregates to identify when harmful clay-sized particles exist
in an aggregate blend. The procedure is conducted on the aggregate fraction of the blend that
passes the 4.75-mm sieve. If hydrated lime is used in the mixture, it should not be included in
the fine aggregate used during the sand equivalent test. The aggregate sample is placed within
a graduated, transparent cylinder that is filled with a mixture of water and flocculating agent.
The combination of aggregate, water, and flocculating agent is then agitated for 45±5 seconds.
After agitation, the combination is allowed to sit at room temperature for 20 minutes. After
the 20 minutes, the heights of the sand particles and the sand plus clay particles are measured
(Figure 4-10). The sand equivalent value is then calculated as the ratio of the height of the sand
to the height of sand plus clay, expressed as a percentage.
High sand equivalent values are desirable, since this indicates that the aggregate is relatively
free of dust and clay particles. Therefore, minimum values for sand equivalency are specified.
These minimum values do not change with depth within the pavement, but do vary somewhat
with design traffic level. Table 4-9 summarizes the requirements for the sand equivalent test.
Table 4-8. Criteria for flat and elongated particles.
Maximum Percentage of Flat and
Design ESALs (million)
Elongated Particles at 5:1
< 0.30 ---
0.3 to < 3 10
3 to < 10 10
10 to < 30 10
30 or more 10
Criteria are presented as percent flat and elongated particles by mass.
