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unit of viscosity is the Pascal-second (Pa s). The conversion scribes an experiment in which Marshall specimens were
from centipoises to Pa s is made by multiplying by 0.001. compacted at temperatures ranging from 100°F to 350°F.
In some references cited in this report, the viscosity units Eleven sets of samples were compacted over the range at 25°F
used by the original author(s) were retained for clarity. increments. Results show that specimen densities were simi-
It is much less confusing to use a single unit of viscosity for lar for temperatures of 275°F and above, but decreased in a
the test measurement and the specification criteria. Since it is linear fashion for temperatures down to 150°F. Below 150°F,
more convenient to use Pa s and not have to worry about specimen density dropped significantly.
temperature-density corrections for asphalt binders, this unit Another reference dating to work in the mid 1950s alluded
will be used in this report for both measurements and crite- to specific ranges for Saybolt Furol viscosity as the basis for
ria. This requires the conversion of the kinematic viscosity to plant mixing temperature. In the description of constructing
absolute viscosity. If the density of an asphalt is assumed to the Michigan Test Road in 1954, Serafin et al. (4) state that
be 1.000 g/mm3, the equiviscous criteria convert to 0.17 ± 0.02 mixing temperatures were set so that the asphalts would have
Pa s and 0.28 ± 0.03 Pa s for mixing and compaction, respec- Saybolt Furol viscosities of 75 to 200 SSF. Six asphalts were
tively. Although most asphalts have densities between 0.93 used in the test road. At the midpoint of the viscosity range,
and 0.98 g/mm3 at temperature ranges used for mixing and mixing temperatures for the asphalts ranged from 290°F to
compaction, the small error due to the assumed density of about 312°F. Therefore, it is apparent that a recommended
1.000 g/mm3 is not considered to be significant. The Asphalt range for plant mixing viscosity was established prior to 1954.
Institute (AI) has used this practice in its Superpave mix de- According to Vyt Puzinauskas, former AI chief chemist, the
sign manual, SP-2. In 2009, AASHTO balloted a revision for viscosity range cited above was essentially based on the field
T 312 to include viscosity criteria for mixing and compaction experience of AI's field engineers.
as 0.17 ± 0.02 Pa s and 0.28 ± 0.03 Pa s. These studies may have provided the impetus for establish-
ing the equiviscous criterion for laboratory compaction.
The first edition of the AI's Mix Design Methods for Hot-Mix
Background on the Development of Mixing
Asphalt Paving (5) in 1956 does not include viscosity criteria
and Compaction Temperature Criteria
for mixing and compaction temperatures. In the 1960 paper
The origin of the equiviscous concept, which has been the titled "The Effect of Compaction Temperature on the Prop-
standard method for determining appropriate temperatures erties of Bituminous Concrete," (6) Kiefer states that "a mixing
for mixing and compacting asphalt mixtures for many decades, viscosity of about 100 seconds Saybolt Furol (SSF) was cho-
was a logical place to begin. However, there is scarce docu- sen in accordance with the recommendations of the Asphalt
mentation of the origin of this technique. It is clear that the Institute which advised a mixing viscosity of between 75 and
AI guided its development and evolution through several 150 sec Saybolt Furol." The reference cited in Kiefer's work
decades in the mid 1900s. was an informal paper by John W. Griffith, "The Effects of
The first mention of equiviscous temperatures found in the Viscosity of Asphalt Cement at the Temperature of Mixing
literature was a 1951 paper titled "Viscosity Effects in the on the Properties of Bituminous Materials," presented at the
Marshall Stability Test" (2) by Fink and Lettier of the Shell 38th annual meeting of the Highway Research Board in 1959.
Laboratories in Wood River, Illinois. They conducted re- Kiefer's work (6) with the Hveem mix design procedure also
search aimed at evaluating the influence of type and consis- concluded that compaction temperature also should be stan-
tency of asphalt binders on Marshall stability values. Experi- dardized. Samples were compacted in a Hveem kneading
mental factors included compaction temperature and the test compactor at temperatures of 150, 190, 230, 270, 310, and
temperature for measuring Marshall stability. A variety of 350°F. The range of temperatures was shown to affect sam-
types and grades of asphalt binders was incorporated in the ple density, air voids, Hveem stabilometer value, and cohesio-
mixes. They reported that compaction temperature had little meter value.
effect on the density of the specimens. However, stability val- The second edition of the AI's Manual Series No. 2 Mix De-
ues increased with increasing compaction temperature. Flow sign Methods for Asphalt Concrete (7) includes the equiviscous
values were influenced almost entirely by asphalt content, but criteria for mixing and compaction of laboratory samples. The
not affected by compaction temperature. They concluded that, publication states: "The temperature to which the asphalt
"neither stability nor flow was influenced by the consistency must be heated to produce 85 ± 10 seconds Saybolt Furol and
or source of the asphaltic binder if compaction temperature 140 ± 15 seconds Saybolt Furol shall be established as the mix-
is carried out at equiviscous temperatures above the soften- ing temperature and compaction temperature respectively."
ing point of the binder." In 1965, Bahri and Rader reported on experiments with the
However, different results were obtained by Parker. In his recommended mixing and compaction viscosities for the
1950 paper titled "Use of Steel-Tired Rollers," (3) Parker de- Marshall method (8). The recommended ranges in ASTM D
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1559 were the same as the MS-2 criteria. Effects of tempera- pactor (SGC) over a wide range of compaction temperatures
ture and mineral filler content were studied. The results and found that except for strain-controlled fatigue testing
showed that variations in mixing and compaction viscosities using a Superpave shear tester, the shear properties of the
produced significant changes in Marshall stability, flow, spe- mixture improved with increasing compaction temperature.
cific gravity, and voids. They concluded that the mixing and The strain-controlled fatigue results were not significantly
compaction viscosities recommended by ASTM were satis- affected by mixing temperature.
factory for the mixture with 8.6% mineral filler (bitumen A number of studies have shown that laboratory com-
filler ratio 1.81). For the mixture with 11.3% mineral filler paction temperature has little to no influence on the volu-
(bitumen filler ratio 2.38), an optimum mixing viscosity of metric properties of samples compacted in a SGC. Findings
54 SSF and an optimum compaction viscosity of 250 SSF were of NCHRP Project 9-10 (14) indicated very minor changes in
recommended. density of SGC samples when compaction temperature var-
In 1984, Kennedy et al. (9) conducted a study to analyze the ied from 80° to 155°C (176° to 311°F). Over this same tem-
effect of lower compaction temperatures on the engineering perature range, the binder viscosity increased by three orders
properties of HMA. The study was prompted by an investiga- of magnitude. It is interesting to note, however, that when the
tion of premature rutting of a recycled asphalt concrete over- mixtures were compacted with a U.S. Army Corps of Engi-
lay that had met in-place density specifications even though neers gyratory testing machine and with a Marshall hammer,
unusually low compaction temperatures had been used. Field the air voids increased by 56% and 44%, respectively, over the
records showed that the average delivery temperature to the same temperature range. Huner and Brown (15) investigated
roadway was 93°C (200°F). Laboratory experiments involved the effect of reheating HMA and the effect of varying com-
compacting samples over the range of temperatures during paction temperature over a range of 28°C (50°F) on volumet-
construction and determining the tensile strengths of the sam- ric properties for mixtures compacted in the SGC. Their work
ples. They concluded that the low compaction temperature included eight mixtures using two aggregate types, two grada-
had an adverse effect on the properties of the HMA and thus tion types, and two binders. None of the mixtures were affected
contributed to the early pavement failure. by changing the compaction temperature. In a ruggedness
In 1985, Crawley (10) conducted a field evaluation of lower study of the SGC compaction procedure, McGennis et al. (16)
compaction temperatures and reached a different conclusion. also found that compaction temperature was not a significant
This research evaluated engineering properties of HMA placed factor on the compacted density of mixtures for unmodified
on a field project in which about half the asphalt base and asphalts, but it was a significant factor for mixtures containing
binder courses were produced at the normal temperature of modified binders.
149°C (300°F) and half were produced at 107°C (225°F). After De Sombre et al. (17) conducted research to determine the
3.5 years, cores were taken from the project, and it was found range of temperatures over which the compactive effort of
that there was no significant difference in mixture properties HMA is maximized. In this study, an Intensive Compaction
or performance, although mixtures were placed at the differ- Tester (ICT) gyratory compactor produced in Finland was
ent temperatures. The study further quantified a reduction of used to compact samples at different temperatures. Density
20.6% in energy consumption by using the lower production data were recorded at several points during compaction. The
mixing temperatures. density data, the change in height of the sample during com-
Other studies have demonstrated the effects of laboratory paction, the size of the sample, and the pressure used to com-
compaction temperature on the results of mechanical tests. pact the sample were used to calculate the total work energy
Newcomb et al. (11) conducted a lab and field study to eval- during compaction. By plotting the shear stress and power
uate asphalt mixtures with plastic and latex modifiers. Part of used during compaction against the number of gyrations for
their work examined the effect of compaction temperature each temperature, the compactability of the mixtures was
on air voids and resilient modulus. Compaction was accom- evaluated. By testing samples at several temperatures, it was
plished with a California kneading compactor at four tempera- possible to determine a desirable range of compaction tem-
tures: 79, 116, 154, and 191°C (175, 240, 310, and 375°F). perature for a given mixture. Six laboratory mixes using three
The lowest air voids were achieved when compaction tempera- binder grades and two aggregate gradations, dense-graded
ture was 154°C (310°F). Resilient moduli increased linearly and Stone Matrix Asphalt (SMA), as well as five field mixes
with increasing temperature. Aschenbrener and Far (12) eval- were used in the study. For both the laboratory and field
uated the influence of compaction temperature on results of mixes, there was an attempt to establish a relationship be-
the Hamburg Wheel Tracking Device. They reported that a tween temperature and shear stress during compaction at dif-
higher compaction temperature improved the resistance to ferent temperatures for each mix. A comparison of shear
deformation in the Hamburg test. Azari et al. (13) compacted stress versus temperature showed that there was very little
samples of an HMA mixture in a Superpave Gyratory Com- change in shear stress although the temperature changed
