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73
ever, when aggregate type and gradation were held con- Draft AASHTO format procedures for the SSF method and
stant, the binder ID and compaction temperature signifi- the Phase Angle method in are included in Appendix C. Mix-
cantly affect the mix density at 25 gyrations. Maximum ing and compaction temperatures determined by these meth-
shear ratio was not a useful indicator of compactability. ods are only applicable to the laboratory setting for mix de-
8. Compaction temperature has a significant effect on low sign work, quality assurance testing of HMA, and fabricating
temperature properties of mixtures: lower compaction tem- HMA samples for laboratory performance tests. The mixing
peratures resulted in slightly higher creep compliance val- and compaction temperatures determined by this method
ues. Conversely, aging of the binders due to high mixing should not be used to control plant production or pavement
temperatures stiffened mixtures (lower creep compliance), construction temperatures. Greater latitude in mixing tem-
which reduces the ability of the pavement to dissipate peratures is necessary in the field to allow for different ambi-
thermal stresses. The effect of aging was more evident for ent conditions, haul distances, and other mix characteristics
binders with lower PG grades. In other words, a PGXX-34 that affect coating and compactability. Although excessive
is affected more by increases in mixing and compaction temperatures may cause emission problems for some binders,
temperatures than a PGXX-16. no evidence of this was found in the laboratory work nor did
9. Correlations of the mixing and compaction temperatures tests demonstrate degradation. At this time, the best guidance
determined from the candidate methods with laboratory for plant mixing temperatures is the EC 101 guidance (38).
tests on mixtures for coating and workability were poor.
The weak correlations between the binder tests and the
Recommendations for Further Work
mix tests are partially attributable to the poor precision of
the coating and workability tests. On the other hand, com- There are several additional key steps that should be con-
paction temperatures from both of the candidate methods sidered in order to validate, refine, and eventually implement
correlated well with the results of the laboratory com- the use of the SSF and/or Phase Angle methods. Four addi-
paction tests. Results of the candidate methods also agree tional steps are outlined as follows:
reasonably well with the binder suppliers' recommended
mixing temperatures. 1. Independent validation of the methods by asphalt suppliers;
2. Refinement of the SSF and Phase Angle methods;
3. Interlaboratory studies to determine precision informa-
Recommendation of a New Method
tion for the new procedures; and
for Determining Mixing and
4. Training on the new method(s) for full implementation
Compaction Temperatures
by the asphalt paving industry.
The objective of this research was to identify or develop a
simple, reliable, and accurate procedure for determining mix-
Independent Validation
ing and compaction temperatures that is applicable to modified
and unmodified binders in HMA. Two candidate procedures, The first major step is to validate the SSF and Phase Angle
the SSF method and the Phase Angle method, were thoroughly methods with many more asphalt binders in other laboratories.
evaluated. This would logically begin with asphalt suppliers conducting
both methods in parallel with grading tests on their current
· An advantage of both candidate methods is that they can be slate of paving grade asphalt products. Simple DSR control and
set up and performed using existing standard DSR equip- analysis programs to run the tests and analyze results need to be
ment used in most asphalt binder labs in the United States. developed and distributed for the variety of DSR's being used in
Limitations of both methods include the restrictions nor- the asphalt industry. To begin this effort, preliminary training
mally applied to parallel plate DSR testing, such as the binder on the methods would be necessary. A single organization
test sample must be homogenous and free of particulate should be identified to collect the results of this broader field
matter (e.g., ground rubber particles) that may interfere with validation effort in an organized fashion. Ideally, an online data-
or distort the rheological response of the instrument. base would be established to facilitate entry of results from
· Both methods also appear to provide reasonable tempera- users anywhere in the world. The following list shows basic in-
tures for mixing and compaction temperatures for a variety formation that would be useful to gather in such a database:
of modified and unmodified asphalt binders being used
across the United States. · Binder supplier.
· Correlations of the mixing and compaction temperatures · Binder PG grade.
with laboratory coating, workability, and compactability · Modification type(s).
were similar for both methods. · Crude source.
