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OCR for page 191
Design of Gap-Graded HMA Mixtures 191
Tests are performed on specimens cored and trimmed from large gyratory specimens to final
nominal dimensions of 100 mm diameter by 150 mm high. In the flow number test, a 600 kPa
load is applied to the specimen every second, until the flow point is reached, representing failure of
the specimen as seen in an increasing rate of total permanent strain during the test. Flow number
tests are run at the average, 7-day maximum pavement temperature 20 mm below the surface, at
50% reliability as determined using LTPPBind version 3.1. Specimens should be prepared at the
expected average air void content at the time of construction, typically 7.0±0.5%. Because GGHMA
mixtures are high-performance materials, usually intended for very demanding applications,
it is likely that the required test values used for these mixes will be those for the highest design
traffic levels. For example, most GGHMA mixtures should probably meet or exceed a flow number
of 580--representing the minimum flow number for design traffic levels of 30 million ESALs and
higher--when tested using the flow number test as described in Chapter 8.
As noted in Chapter 8, the minimum flow number values given in Table 10-13 are for fast
traffic; it is suggested that, to account for the greater damage associated with slow traffic, the test
temperature be increased for slower traffic speeds. The specifics of such adjustments are given in
Chapter 8. Although the suggested required test values given in Table 10-13 and in the related
tables presented in Chapter 8 are based either on a careful analysis of laboratory and field data,
or on existing standards, it is quite possible that they will need to be adjusted by the specifying
agency for optimum results for GGHMA mixtures in its region. Factors that need to be considered
when making such adjustments are climate, the types and grades of binders commonly used in
a given locale, aggregates with unusual properties, and typical traffic mixes and traffic levels. For
various reasons, some agencies might wish to alter the conditions a test is run under, which will
significantly alter the resulting test values and the appropriate specification values. For details on
the proper procedures for performing each test, engineers and technicians should refer to the
appropriate standard test method.
Trouble Shooting GGHMA Mix Designs
If the designer cannot produce a mixture that meets all requirements, remedial action will be
necessary. Some suggestions to improve mixture properties are provided below.
Air Voids
The amount of air voids in the mixture can be controlled by the asphalt binder content.
However, a problem occurs when low voids exist at the minimum asphalt binder content.
Lowering the asphalt binder content below the minimum to achieve the proper amount of air
voids violates the required minimum asphalt binder content (Table 10-11). Instead, the aggregate
gradation should be modified to increase the VMA so that additional asphalt binder can be added
without decreasing the voids below an acceptable level.
Voids in the Mineral Aggregate
The VMA may be raised by decreasing the percentage of aggregate passing the breakpoint sieve
or by decreasing the percentage passing the 0.075-mm sieve. Changing aggregate sources or
stockpiles may also be required to solve the problem.
Voids in the Coarse Aggregate
If the VCAMIX is higher than that in the dry-rodded condition (VCADRC) then the mixture
gradation must be modified. This is typically accomplished by decreasing the percent passing the
breakpoint sieve.
OCR for page 192
192 A Manual for Design of Hot Mix Asphalt with Commentary
Moisture Susceptibility
If the mixture fails to meet the moisture susceptibility requirements, lime or liquid anti-strip
additives can be used. If these measures prove ineffective, the aggregate source, asphalt binder
source, or both can be changed to obtain better aggregate and asphalt binder compatibility.
Draindown Sensitivity
Problems with draindown sensitivity can be remedied by increasing the amount of stabilizing
additive or by selecting a different stabilizing additive. Fibers have been shown to be very effec-
tive in reducing draindown.
Rut Resistance
As mentioned earlier in this section, the rut resistance tests and recommended minimum
or maximum values for test results have been selected so that most GGHMA designs developed
following the procedures given in this manual will meet the requirements, and no additional
laboratory work will be needed. However, some mix designs may fail to meet requirements for
rut resistance. In such cases, the test results should first be checked to make sure there were no
errors in either the procedures used or in the calculation of the test results. If no errors are found,
and the test results are close to meeting the requirements, the test can be repeated. In this case,
the results of both tests should be averaged and compared to the test criteria. If the mix still fails
to meet the requirements for rut resistance testing, the mix design will have to be modified. The
rut resistance of a GGHMA mix design can be improved in the following ways:
· Increase the binder high-temperature grade.
· If the binder is not modified, consider using a polymer-modified binder of the same grade or
one high-temperature grade lower.
· If the binder is polymer modified, try a different type of modified binder.
· Increase the amount of mineral filler in the mix, adjusting the aggregate gradation if necessary
to maintain adequate VMA.
· Decrease the design VMA value, if possible, by adjusting the aggregate gradation.
· Replace part or all of the aggregate (fine or coarse or both) with a material or materials having
improved angularity.
If a different asphalt binder is used in the mix, the volumetric composition should not change.
However, if other aspects of the mix design are changed, the volumetric composition might change
significantly, which will require further refinement of the mix prior to further rut resistance testing.
Bibliography
AASHTO Standards
M 17, Mineral Filler for Bituminous Paving Mixtures
M 320, Performance-Graded Asphalt Binder
M 325, Standard Specification for Designing Stone Matrix Asphalt (SMA)
R 46, Standard Practice for Designing Stone Matrix Asphalt (SMA)
R 30, Mixture Conditioning of Hot-Mix Asphalt
T 19, Bulk Density ("Unit Weight") and Voids in Aggregate
T 96, Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles
Machine.
T 104, Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate
T 166, Bulk Specific Gravity of Compacted Asphalt Mixtures Using Saturated Surface-Dry Specimens
T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalency Test.
OCR for page 193
Design of Gap-Graded HMA Mixtures 193
T 209, Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures.
T 245, Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus.
T 283, Resistance of Compacted Asphalt Mixtures to Moisture-Induced Damage.
T 304, Uncompacted Void Content of Fine Aggregate
T 305, Determination of Draindown Characteristics in Uncompacted Asphalt Mixtures.
T 326, Uncompacted Void Content of Coarse Aggregate (As Influenced by Particle Shape, Surface Texture and
Grading).
TP 79-09, Determining the Dynamic Modulus and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt
Mixture Performance Tester (AMPT)
ASTM Standards
C 612, Mineral Fiber Block and Board Insulation
D 4791, Flat Particles, Elongated Particles, or Flat and Elongated Particles in Coarse Aggregate
Other Publications
Bonaquist, R. F. (2008) NCHRP Report 629: Ruggedness Testing of the Dynamic Modulus and Flow Number Tests
with the Simple Performance Tester, Transportation Research Board, National Research Council, Washington, DC,
130 pp.
Brown, E. R., and L. A. Cooley, Jr. (1999) NCHRP Report 425: Designing Stone Matrix Asphalt Mixtures for Rut
Resistant Pavements. Transportation Research Board, National Research Council, Washington, DC.
NAPA (1999) Designing and Constructing SMA Mixtures--State of the Practice (QIP-122), Lanham, MD, 43 pp.
NAPA (1999) Evaluation of Baghouse Fines for Hot Mix Asphalt (IS-127), Lanham, MD, 36 pp.
