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Mixture Volumetric Composition 53
with decreasing apparent film thickness, as has been suggested by many engineers and observed
in various field studies. However, the relationship between rut resistance and film thickness is
not an inherent mechanism of asphalt films, but instead appears to be an indirect but useful
relationship. NCHRP Report 567 suggests that HMA mixtures with apparent film thickness
values greater than 9 to 10 microns can be prone to excessive rutting.
Although physically distinct films of asphalt binder cannot be separated from a compacted
specimen of asphalt concrete, such films do have physical meaning in loose uncompacted HMA.
Furthermore, these films serve an important purpose--they lubricate the aggregate particles and
allow the HMA to be placed and compacted properly. Apparent film thickness values that are
too low are indicative of mixtures that are difficult to place and compact, which in turn can cause
segregation, high in-place air void content, and a pavement that is permeable and prone to
raveling and surface cracking. Again, this relationship is not a direct one; the lack of durability
is not the result of low film thickness, but of segregation and/or poor compaction brought about
by the poor workability resulting from low film thickness in the loose hot mix. Unfortunately,
research linking apparent film thickness to the workability of HMA has not yet been performed.
Different ranges for minimum apparent film thickness have been suggested since this concept
was first proposed. It is suggested that mixtures with values below about 6 to 7 microns may be
difficult to place and compact properly.
The discussion above suggests that apparent film thickness can be a useful tool for designing
and analyzing asphalt concrete mixtures. Film thickness values in the range of 7 to 9 microns
appear to provide the best compromise between workability and rut resistance. Values below
6 microns or above 10 microns should be avoided. Although apparent film thickness is a poten-
tially useful concept, the relationships between apparent film thickness and performance are, at
best, indirect. Furthermore, equally good means of controlling mixture composition as it relates
to performance are available that do not involve the use of apparent film thickness. Controlling
VMA, design air void content, and aggregate fines is essentially equivalent to controlling film
thickness. Agencies that choose to specify film thickness for asphalt concrete mixtures should take
special care to ensure that there are no unintended conflicts with any simultaneous requirements
for VMA, design air void content, or aggregate gradation.
Mixture-Specific Gravity
Specific gravity has the same meaning when applied to asphalt concrete mixtures as it does
when applied to aggregates and other materials--the ratio of the density of a material to the
density of water at 25°C and at standard air pressure. Because the density of water under these
conditions is 1.000 gm/cm3, specific gravity is interchangeable with density in these circumstances.
However, specific gravity, since it is a ratio, is dimensionless. A mixture with a bulk specific gravity
of 1.352 has a bulk density of 1.352 g/cm3. Some agencies use density in units of kg/m3, which will
be 1,000 × the specific gravity. The specific gravity of the mixture in the previous example could
be given as 1.352 g/cm3 or 1,352 kg/m3. There is no link between specific gravity and performance.
However, measuring and calculating specific gravity and understanding how specific gravity is
used in volumetric analysis is critical to developing asphalt concrete mix designs and analyzing
paving mixtures.
Bulk Specific Gravity
The bulk specific gravity of a mixture refers to the specific gravity of a specimen of compacted
mixture, including the volume of air voids within the mixture. It is equivalent to the mass of a
given specimen in grams, divided by its total volume in cubic centimeters. The bulk specific gravity
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54 A Manual for Design of Hot Mix Asphalt with Commentary
Electronic balance
Wire hook
Mesh basket
Tub filled with distilled
water
Compacted specimen
Figure 5-7. Determining weight in water of compacted
specimen of HMA.
of an asphalt concrete mixture can be determined using either laboratory-compacted specimens
or cores or slabs cut from a pavement.
The standard procedure for determining the bulk specific gravity of compacted asphalt concrete
involves weighing the specimen in air and in water. Two slightly different laboratory techniques
are used, depending on the absorption of the specimen. For low absorption (less than 2.0%),
saturated surface-dry specimens are used (AASHTO T 166). For specimens having high absorption,
paraffin-coated specimens should be used in the specific gravity determination (AASHTO T 275).
The following formula is used for calculating bulk specific gravity of a saturated surface-dry
specimen:
A
Gmb = (5-1)
B-C
where
Gmb = bulk specific gravity of compacted specimen
A = mass of the dry specimen in air, g
B = mass of the saturated surface-dry specimen in air, g, and
C = mass of the specimen in water, g
As a general rule, if the water absorption of a compacted HMA mixture is above 2.0%, the bulk
specific gravity should be determined using paraffin-coated specimens. The calculation of specific
gravity in this case is more complicated because the mass and volume of the paraffin film must
be accounted for; details can be found in AASHTO T 275. Figure 5-7 is a sketch of a typical
weight-in-water determination. Some agencies determine mixture bulk specific gravity using a
pycnometer method, which involves calibrating a container and then determining the weight of
the container with the compacted specimen and filled with water. Although theoretically this
method will provide results equivalent to the weight-in-water method, no AASHTO standard
exists for this procedure. Furthermore, use of multiple procedures for determining specific gravity
of aggregates and HMA mixtures should be discouraged, since this can only increase the variability
in the test results and in the subsequent volumetric analyses.
Theoretical Maximum Specific Gravity
The theoretical maximum specific gravity of an asphalt concrete mixture is the specific
gravity of the mixture at zero air void content. It is one of the most difficult tests performed in
paving materials laboratories and also one of the most important. Like bulk specific gravity,
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Mixture Volumetric Composition 55
theoretical maximum specific gravity in and of itself does not affect the performance of a paving
mixture. However, it is essential in determining volumetric factors that are good indicators of
performance, such as air void content and VMA.
Maximum specific gravity is determined by measuring the specific gravity of the loose paving
mixture, after removing all of the air entrapped in the mixture by subjecting the mixture to
a partial vacuum (vacuum saturation). The loose mix is prepared by gently heating the sample
in an oven until it can be easily broken apart. The mixture is then removed from the oven and
occasionally stirred while cooling, to make sure that it remains broken up as much as possi-
ble into separate particles of asphalt-coated aggregate. After determining the weight in air of
the sample, it is placed in a tared, calibrated vacuum container. The container is then con-
nected to a vacuum pump, and the pressure in the container gradually reduced to 30 mm Hg
or less--about 4% of normal atmospheric pressure. This partial vacuum is maintained for
5 to 15 minutes, and the container is occasionally tapped or rolled to help release entrapped air
from the loose mixture. The vacuum is then carefully released, the container topped off with
water to the calibration mark, and the weight of the container, specimen, and water determined.
The theoretical maximum specific gravity of the specimen is calculated using the following
formula:
A
Gmm = (5-2)
A+ D- E
where
Gmm = theoretical maximum specific gravity of loose mixture
A = mass of oven-dry specimen in air, g
D = mass of container filled with water at 25°C to calibration mark, g
E = mass of container with specimen filled with water at 25°C to calibration mark, g
Because of the importance of theoretical maximum specific gravity determinations, and because
the measurements are difficult to perform with great precision, some additional comments
concerning this procedure are warranted. In a well-run laboratory, every effort should be made to
perform this procedure as much as possible in the same way every time it is run. Many laboratories
use small sieve shakers to agitate the specimen while the vacuum is applied, because of the
variability in hand rolling and tapping. Although the procedure allows for a time range of
5 to 15 minutes for vacuum saturation, a much narrower time range should be used. Initial
tests with typical local materials and with the specific vacuum saturation equipment to
be used in running the procedure will help determine the most effective time for applying
the vacuum. Because this test involves pulling a vacuum on a container filled with water, care
should be made to ensure that the pump is suitable for this use. Many vacuum pumps are
quickly damaged by water vapor when water condenses within the interior of the pump,
mixing with the vacuum oil and ruining its effectiveness. If such a pump is not available, a
series of traps should be installed between the specimen and the pump to prevent water vapor
from entering the pump. Because water will boil at 30 mm Hg at a temperature of 29°C (84°F),
the area in which this test is performed and the water used in the procedure should be kept
cool. It will be impossible to reach a vacuum of 30 mm Hg if the temperature of the water
within the container is 84°F or higher. Laboratory personnel should also make certain that the
pump used can quickly reach a partial vacuum of at least 30 mm Hg. Good-quality, accurately
calibrated gages should be used to monitor the vacuum during the procedure. The complete
procedure for performing the theoretical maximum specific gravity test can be found in
AASHTO T 209. Figure 5-8 is a diagram of a sample of loose mix being vacuum saturated as
part of this procedure.
