Evaluation of Various Superpave Gyratory Compactors
At the conclusion of the Strategic Highway Research Program, the
FHWA Office of Technology applications developed a specification
for the pooled fund purchase of Superpave gyratory compactors
(SGC). The specification was largely based on the recommendations
of SHRP researchers (see "Development of the Superpave Gyratory Compactor")
as well as on early FHWA experience with prototype units. In
1994, FHWA evaluated the first articles delivered by two manufacturers
to meet this specification. These units were produced by Pine
Instrument Company and Troxler Electronic Laboratories, Inc.
The FHWA's first article evaluation consisted primarily of comparing
SGC test results among Pine, Troxler, and a modified Texas gyratory
unit. The modified Texas unit was the prototype device that was
used by SHRP researchers and was assumed to be the comparison
standard. This evaluation showed that the Pine and Troxler compactors
exhibited similar results as the modified Texas device. Overall,
FHWA judged the compactors to be in conformance with the specification
and thus, were suitable for the pooled fund purchase. Thirty
units each of the Pine and Troxler compactors were procured as
part of the first pooled fund purchase. In 1995, these units
were placed in state DOT laboratories, FHWA facilities, and one
in each of the five Superpave regional centers. Since then, a
considerable number of the compactors have been purchased by state
DOTs, industry, academia, and others.
Experience with the Pine and Troxler compactors has been good.
Both devices have been upgraded in numerous ways. The most significant
change to both compactors consisted of upgrade parts to allow
the compaction angle to be more closely held at the desired value,
1.25. A ruggedness experiment (see "Ruggedness Evaluation of AASHTO TP4 ")
was conducted on the Superpave gyratory compaction procedure (AASHTO
TP4). This experiment showed that the bulk specific gravity (Gmb)
of test specimens produced using the procedure was not unduly
sensitive to tolerable operator variation. The FHWA conducted
a study comparing the results of the two compactors using plant
produced mixes. This study showed that the Pine and Troxler compactors
were providing very similar test results. In general, the compactors
were subjected to considerable scrutiny by the asphalt materials
engineering community. The consensus appears to be overwhelming
in favor of their use as a mix design tool.
Because of this positive experience, it soon became clear that
the SGC would become the laboratory compactor of choice. Consequently,
since 1995 several additional manufacturers have developed SGCs
to meet the growing demand for such devices. While these units
generally meet the broad requirements of the original FHWA SGC
specification, their basic designs are somewhat different. Therefore,
potential users of the new SGCs must ask the question, "Will
I get the same test results from a new SGC as I would get from
either the Pine or Troxler?"
To ensure a systematic means for answering this question, the
FHWA developed a standard protocol for evaluation of SGCs. This
protocol was reviewed by the FHWA Superpave Mixtures Expert Task
Group (ETG) and changes were made based on this review. The protocol has now been standardized in AASHTO PP-35, Standard Practice for Evaluation of Superpave Gyratory Compactors. In recent months, the staff
of the South Central Superpave Center completed the evaluation
of three new Superpave Gyratory Compactors. This activity was
the initial use of a protocol for such evaluations that was developed
by the FHWA's Superpave Mixtures Expert Task Group. The manufacturers
of the new SGCs evaluated include:
|
Test Quip, Inc.
105 Old Highway 8, Suite 4 New Brighton, MN 55112 612-636-5510 fax 612-636-5448 testquip@mm.com |
Rainhart Company
P.O. Box 4533 Austin, TX 78765 512-452-8848 512-452-9883 fax bdurr@sig.net |
| Pine Instrument Company
101 Industrial Drive Grove City, PA 16127 412-458-6391 fax 412-458-4648 info@pineinst.com |
Troxler Electronic Laboratories, Inc.
P.O. Box 12057 Research Triangle Park, NC 27709 919-549-8661 919-549-0272 fax troxsale@troxlerlabs.com |
| Interlaken Technology Corporation
7600 Golden Triangle Drive Eden Prairie, MN 55344 612-942-7499 612-942-7599 fax sales@interlaken.com |
The compactor evaluation utilized a full-factorial design with
four asphalt mixtures and six specimens. A total of 48 specimens
per comparison (4 mixes 2 compactors 6 replicates) were produced.
Specimens were individually batched and stored in plastic bags
prior to use. As required by the protocol, the four mixes were
designed according to AASHTO PP28 and met all the Superpave requirements.
The mixes are:
Coarse mixes have gradations below the Superpave restricted zone
while the fine mix has a gradation above the restricted zone.
The design asphalt content was selected at four percent air voids
at the selected number of design gyrations using the existing
Pine compactor. The Pine compactor was employed for this purpose
because two of the mixes (Mixes 1 and 2) had previously been designed
using that device for roadway projects in Texas. Mixes 1 and
4 are composed of crushed limestone aggregate materials that serve
as standards at the South Central Superpave Center. Mixes 2 and
3 are composed of crushed limestone and sandstone aggregate materials
that were used on a Superpave project in Texas. The asphalt binder
met the requirements of PG 64-22.
As required by the protocol, a single operator was used throughout
the experiment for the compaction process. Working closely with
the compaction operator, a different operator was used for mixing.
A mixing and compaction cycle of 15 minutes was used. That is,
there was a constant 15-minute interval between mixing of specimens
and likewise, a constant interval of 15 minutes between compaction
of specimens. A short term aging period of four hours was used.
Each day 12 specimens were produced. An individual comparison
of two compactors was conducted over a period of 4 days.
Oven temperatures were verified prior to the experiment and daily
throughout the mixing and compaction process. A calibrated thermocouple
with digital readout was employed. The mixing temperature was
150 C. To reduce variability due to operator manipulation of
mixes, the compaction temperature was fixed at 135 C, which was
the same as the short term aging temperature.
Determination of bulk specific gravity (Gmb) was accomplished
according to AASHTO T166, Method A. Calibration of the balance
used for this activity was verified before and after the experiment.
Immediately, prior to the experiment, the candidate compactor
was thoroughly checked on-site by a manufacturer's representative
to ensure it was in proper working order. The operator was thoroughly
briefed in proper methods of verification of calibration. In
an effort to explain testing variation as much as possible, verification
of calibration items was accomplished before the experiment commenced
and before each day's compaction activities for both compactors.
Calibration items were: compaction angle, compaction pressure,
height measurement, and rotational speed.
The sequence of testing is not specified in the FHWA ETG protocol.
Thus, it was necessary to develop an experimental design that
would meet the ultimate goals of the protocol. A randomized test
sequence was used. This ensured that systematic, unnoticed errors
in the experiment would be randomly distributed among all the
data. Table 1 shows a typical randomized testing sequence.
| E - previously evaluated compactor, C - new candidate compactor | |||||||||||||
For each cell in Table 1, three response variables were collected
and analyzed:
Thus for any comparison of two compactors, 12 points of comparison
are possible (3 Gmb values 4 mixes 2 compactors).
The Gmb at Nmax was a measured value.
The other response variables were computed by the following equation:
Gmb at x gyrations = (hmax/hx)
Gmb at Nmax
where, hmax = specimen height at Nmax and
hx = specimen height at x gyrations.
Comparisons were conducted according to the FHWA ETG protocol.
For a given mix, the average and standard deviation in Gmb
for both compactors were computed. The protocol states that any
candidate compactor with an average Gmb more than 0.010
different than the evaluated SGC" will not be considered
"comparable." The FHWA ETG protocol also requires that
the variation in Gmb be analyzed, but the evaluation
is more qualitative. According to the protocol, the standard
deviations are observed and should be "comparable."
Table 2 shows typical test results in the form by which the comparison
is made for a given mix.
Tables 3 through 12 show the comparisons of the sample means and
standard deviations for all experiments conducted.
| Note: Averages based on absolute value of difference. | |||
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| Note: Averages based on absolute value of difference. | |||
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| Note: Averages based on absolute value of difference. | |||
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The comparison data in Tables 3, 5, 7, 9, and 12 indicate the new compactors
compare very favorably with the existing Pine unit. Out of 12
possible points of comparison the Interlaken compares 9 times
and the Test Quip and new Pine unit 11 times. The Rainhart compactor compares favorably 12 out of 12 times with the existing Pine unit. The new Troxler Compactor (model 4141) compares favorably 12 out of 12 times with the existing Troxler unit (model 4140).
The information in Tables 4, 6, 8, 10, and 12 indicate that the standard
deviations for a given mix are typically in the range from about
0.005 to 0.010, with only a few values outside this range.
As previously mentioned, the calibration of all of the compactors
was verified before and after each day's activities throughout
the experiment. This step showed that the devices remained within
tolerable limits as established in AASHTO TP4.
The SGC evaluation protocol developed by the FHWA Expert Task
Group is very workable and practical. Engineers and technicians
with a basic level of proficiency in SGC operation should be able
to conduct the experiment.
All of the candidate compactors compared favorably with an existing
SGC. Comparison of Gmb
values showed that they were largely within the 0.010 limit established
by the evaluation protocol. The variability in Gmb
for all candidate compactors was similar for existing units.
This experiment was highly controlled. Extreme care was taken
to ensure that variability in data was restricted, as much as
possible, to variability induced by the compactors themselves.
Users of these or any other laboratory compaction devices should
be aware that comparability of results is considerably influenced
by many other factors. Operator proficiency, oven size and quality,
compactor operating condition, adherence to standard test procedures,
and of course, material variability are among the many factors
that can influence the comparability of test results. As various
SGCs come into routine use, there will inevitably be systematic
differences in test results. While differences caused by
different compactors should always be investigated, users should
also consider that there are many other potential causes that
should be isolated.
Many agencies and organizations have expressed interest in an
"approved list" from which they can purchase new SGCs.
Discussions with FHWA and among personnel from the various Superpave
centers have indicated that no such list will be forthcoming.
It should be noted that approved lists of asphalt laboratory
equipment have generally not existed in the past, or if they have
existed, they were promulgated on a local level.
In fact, it is the intent of the evaluation protocol that an approved
list of SGCs not be necessary. Rather, prospective purchasers
of SGCs should require that a manufacturer show that their new
device has been rigorously evaluated according to the protocol
and shown to be comparable. Such data constitutes a base line
frame of reference beyond which other features may be used to
make a purchasing decision. These features include cost, availability,
operating features, field lab suitability, user friendliness,
data acquisition and management, etc. Should an agency or organization
prefer a particular model due to one of these or other factors,
they can develop a purchasing specification accordingly.