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74
CHAPTER 6
CONCLUSIONS AND RECOMMENDATIONS
The results of this review have emphasized the difficult 6.1.2 Fine and Elongated Particles
nature of conducting research to relate aggregate properties
and HMA performance. It is difficult to isolate the effects of The research revealed the following about F&E:
the aggregate properties from other interactions with grada-
tion and mixture volumetric properties. It appears as if the · Extreme levels (>10% 51 ratio) of F&E are most likely
shortcomings of a single property related to rutting resistance undesirable in HMA.
can be overcome by other supporting properties. · Increased levels of F&E increase aggregate breakdown
These interactions emphasize the need for laboratory per- during handling, mixing, and placement.
formance tests for HMA mixtures. If performance tests are · The current test for F&E, ASTM D4791, is extremely
adopted that have criteria in which agencies are confident, the variable (multilaboratory coefficient of variation of 35.3%
overall performance of the mixture could be assessed instead for the 31 ratio); however, precision improves as the
of relying solely on component screening tests--for exam- ratio of maximum-to-minimum dimension decreases.
ple, if the blend uncompacted voids in fine aggregate were · Seven states currently specify the 31 ratio; and one
43% for a given mixture to be placed on a high-volume road, province, the 41 ratio. A specification of a maximum
the rutting properties of this mixture could be tested (at the of 20% of particles exceeding the 31 ratio has been
contractor's expense) to show whether the mix should pro- adopted by five states.
· Research has been unable to establish that between 20%
vide acceptable performance.
and 40% F&E exceeding the 31 ratio is detrimental to
HMA performance. In fact, some level of F&E may be
6.1 CONSENSUS AGGREGATE PROPERTIES desirable to meet minimum VMA requirements.
· If ASTM D4791 continues to be used, specifications
The consensus aggregate properties have been adopted should be developed for the 21 or 31 ratio to improve
by the majority of the responding agencies. F&E, specified the precision of measurements. Up to 40% F&E exceed-
by 79% of the responding agencies, has the lowest level of ing the 31 ratio does not appear to be detrimental to
implementation. pavement performance. Therefore a specification level
of up to 40% particles exceeding the 31 ratio may be
appropriate.
6.1.1 Coarse Aggregate Angularity · Imaging methods have been developed to accurately
and precisely measure coarse aggregate shape, texture,
The research revealed the following about coarse aggre- and angularity and fine aggregate shape. The results with
gate angularity: these methods have not yet been correlated with the per-
formance of HMA.
· Increased coarse aggregate fractured faces provide · Research should be conducted to relate digital means for
increased rutting resistance. Increased particle index measuring aggregate shape, texture, and angularity to
value or uncompacted voids in coarse aggregate also pavement performance as replacements for both coarse
provide increased rutting resistance. The latter combine aggregate angularity and F&E tests.
the effect of shape, angularity, and texture.
· The current Superpave specification levels for coarse
aggregate angularity have been adopted by 39% of the 6.1.3 Fine Aggregate Angularity
agencies that specify ASTM D5821 or an equivalent.
Five states have more stringent criteria; and four states, The research revealed the following about FAA:
less stringent requirements.
· There is little research to support the need for two frac- · Currently, the Superpave mix design system does not
tured face counts in excess of 95%. address the shape, texture, and angularity of the material
