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OCR for page 23
Effect of Vacuum Duration on Gmm FINDINGS AND CONCLUSIONS
Measurement
This report presents the results of research to
For the purpose of improving accuracy and preci- evaluate the effects of key equipment and method-
sion of Gmm measurements, the effect of vacuum du- ological variables on the measurement of the theo-
ration on Gmm and its variability was investigated. The retical maximum specific gravity (Gmm) of asphalt
three SMA field mixtures were used for this evalua- mixtures for possible refinement of the AASHTO T
tion. Two replicates of each mixture were tested at 209 test method. The variables examined include
Setting 6 of the Gilson device for vacuum/agitation agitation and device type, vibration intensity of me-
durations of 5, 10, 15, 20, and 25 minutes. It was chanical shaking tables, order of placing water and
found that Gmm increased with the increase in the mixture in vacuum container, and duration of the
vacuum/agitation time until a maximum was reached vacuum/agitation process. This section summarizes
at or about 20 minutes. Increasing the vacuum/ the findings and conclusions of the research.
agitation time to 25 minutes resulted in a decrease The Gmm measurements at various settings of the
in Gmm. Visual observation indicated that the water devices evaluated in the research indicated that for
was slightly cloudy after 20 minutes of vacuum/ each vibratory device, Gmm of the mixture increases
agitation and became substantially cloudy after with the increase in intensity of vibration until the
25 minutes. Analysis of the variability of the Gmm mea- highest Gmm of the mixture is reached. From that
surements for the five agitation durations indicates point on, a further increase in vibration intensity re-
that higher variability is usually observed at higher sulted in a decrease in Gmm. This phenomenon may
vacuum/agitation durations, but that there was no be related to stripping of the asphalt.
specific trend of increase or decrease in variability Gmm values from manual agitation were always
with increasing time. smaller than the highest Gmm values from mechani-
The significance of the difference between the cal agitation devices. In most cases, manual agita-
Gmm obtained at various durations of vacuum/agitation tion produced Gmm values that were equivalent to
should indicate if a higher duration is necessary to pro- Gmm produced by the mid-range intensity settings of
duce a more accurate measurement of Gmm. From a the mechanical devices. The results of the statistical
practical point of view, the significance of the differ- analysis indicated that for four out of nine mixtures
ence between the Gmm is derived from an evaluation of tested in the research, measurements from manual
the difference in air voids. Air voids were calculated agitation were significantly different from those of
using assumed Gmb values of 2.532, 2.357, and 2.339 at least one mechanical device. In addition, the dif-
for the 9.5-mm, 12.5-mm, and 19.0-mm SMA mix- ference between air voids from manual agitation and
tures, respectively. The difference between the air from mechanical devices ranged from 0.2% to 0.4%,
voids from 15 minutes of agitation, which is specified
which could be practically significant. Therefore,
in AASHTO T 209, and the air voids from 20 minutes
use of manual agitation for the measurement of Gmm
of agitation, which produced the highest Gmm, was less
is not suggested.
than 0.1%. This difference is not considered practi-
cally significant. Investigation of the change in Gmm resulting from
The statistical comparison of Gmm values for change in the device type indicated that, statistically,
various vacuum/agitation durations was conducted the differences between the Gmm of the nine mixtures
using a Scheffé test. F values were computed for measured using various devices were not significant.
comparisons of Gmm of all combinations of vacuum/ Therefore, based on statistical results, it could be
agitation durations. Of the computed F values, none concluded that if vibrating devices are operated at
was greater than the critical F value of 5.192 (for a their optimum settings, they should produce Gmm val-
5% level of significance). Therefore, the vacuum/ ues that are statistically the same. Even devices with
agitation duration does not significantly affect Gmm. a single, constant setting (HMA Vibrating Table, Ag-
Based on the above findings, a vacuum/agitation gregate Drum Washer, and Corelok) would produce
period of 15 minutes appears appropriate for Gmm Gmm values that are statistically the same as the Gmm
measurement. Although a higher Gmm value was mea- values produced by the optimum settings of vibrat-
sured at 20 minutes of agitation, there was no prac- ing devices with variable settings.
tical or statistical difference in Gmm between 15- and Evaluation of the air voids from various devices
20-minute durations of vacuum/agitation. indicated that for three out of nine mixtures, the
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differences between the air voids from at least two vacuum/agitation duration resulted in a decrease of
devices were greater than 0.2%, which could be prac- Gmm. Although the highest Gmm was obtained after the
tically significant. Differences between the air voids 20-minute vacuum/agitation period, statistical analy-
measured by a state DOT and by a contractor using a sis of Gmm values and evaluation of the air voids indi-
different vibrating device could impact the accep- cated that the difference in Gmm and air voids between
tance of a project. Therefore, it is proposed that the the 15-minute and 20-minute agitation periods was
same method and apparatus used for measuring Gmm not significant. In addition, the variability of mea-
for a mix design be used for quality assurance testing surements was slightly greater after 20 minutes than
of that mix during production. after 15 minutes of vacuum/agitation. Therefore, it is
The relationship between the energy of vibration suggested that the 15-minute vacuum/agitation time
and the highest Gmm produced by a device indicated specified in T 209 be maintained.
that, although the highest Gmm values from various Based on the data gathered in the study, several
vibrating devices were very similar, the vibration proposals are made related to the optimum settings of
properties of the mechanical devices were very dif- four vibration devices. Table 5 provides the proposed
ferent. For example, the Gmm values measured using settings and their corresponding vibration properties.
Syntron and Humboldt devices are comparable, but For the Humboldt device, the highest Gmm of the
the kinetic energy of the Syntron table is two orders nine mixtures were produced over the range of Set-
of magnitude greater than that of the Humboldt device. tings 7 to 10. Based on the concern with water clar-
It is speculated that the amount of energy produced ity at Settings 8 through 10, however, and given the
by a device is not necessarily the same as the amount lack of significant difference between the Gmm from
of energy transferred to the mixture. those settings and Setting 7, Setting 7 was selected
In selecting the optimum setting for each device, as the optimum setting of Humboldt device.
it was found that the variability of Gmm was not a For the Gilson device, the majority of the highest
defining factor as there was no correlation between Gmm measurements occurred at Settings 6 and 7;
measured Gmm and the vibration settings. For all Gmm however, the occurrence of substantial cloudiness at
measurements, the difference between replicate Settings 6 and above resulted in selecting Setting 5
measurements at any setting was smaller than 0.007, as the optimum setting.
which is much less than the acceptable difference For the Syntron device, a few optimum readings
between two replicate measurements as specified in occurred at Setting 8; however, based on issues with
AASHTO T 209. water cloudiness at higher settings and given a lack
The change in Gmm values arising from changing of significant differences in Gmm between Setting 7
the order of placement of water and mixture in the vac- and Setting 8, Setting 7 was selected as optimum.
uum container indicated that adding the mixture to For the Orbital device, the highest Gmm were ob-
water produced higher Gmm values. Statistical analysis tained at vibration levels in the range from 210 rpm
of Gmm values and evaluation of the computed air to 300 rpm. Based on increasing water cloudiness at
voids confirmed the significance of the increase in Gmm higher settings, however, a vibration level of 240 rpm
as a result of placing the water first. It is speculated that was selected as optimum for the Orbital device.
the release of air is facilitated by adding the mixture Laboratories are advised to adjust their vibration
to water as opposed to adding water to the mixture. devices to the settings recommended in Table 5 to en-
Therefore, it is proposed that AASHTO T 209 be re- sure accurate measurement of the Gmm of their asphalt
vised to specify placing the water in the vacuum con- mixtures. Finally, agencies should note that any pro-
tainer prior to adding the mixture. posed changes to the current test procedures resulting
The effect on Gmm of duration of vacuum/agitation from this research may result in increased Gmm values
of the three SMA mixtures indicated that Gmm in- that can or will affect air voids of laboratory- and field-
creased with increasing the vacuum/agitation time compacted specimens. Agencies should consider the
until the highest Gmm was achieved, after a 20-minute effect of such changes on acceptance criteria and pay
vacuum/agitation period. Further increase of the factors in their specifications.
24
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