influence of filler fractional voids on mastic and mixture performance

PROPOSED LIMITES ON FILLER RIGDEN VOIDS FOR WORKABILITY AND
RUTTING RESISTANCE
To incorporate the <strong>fractional</strong> <strong>voids</strong> into design <strong>of</strong> HMA, mastics were used as the bridge between
<strong>filler</strong> and mixture properties. Limits <strong>of</strong> mastic properties that result in acceptable mixture
performance are presented in this section. To relate the mastic limits to <strong>filler</strong> characteristics, the
regression equations relating Rigden Voids and mastic properties previously presented are used.
Thus, in design, one has two options:
• Conduct mastic testing to quantify the mastic property value and compare to
acceptable limit directly, or
• Perform binder and <strong>filler</strong> tests and use the regression equation to estimate mastic
property value and compare to acceptable limit.
In limit development analysis, it was necessary to sort limits by gradation. For the
purpose <strong>of</strong> this study, a coarse aggregate gradation is defined according to the Asphalt Institute’s
definition. Any aggregate gradation that crosses below 0.45 power maximum density line at or
before sieve #8 (2.36mm). The fine gradation is that which passes above the sieve #8.
Constructability
An extensive literature search was conducted to determine what limits can be used to define
satisfactory workability limits <strong>of</strong> mixture. No such limits for the number <strong>of</strong> gyrations to
92%Gmm could be found. The most comprehensive comparison between laboratory measured
and field measured compactability found was a study by Leiva and West (26). Leiva and West
evaluated the ability <strong>of</strong> multiple laboratory measured compactability indicators to predict actual
mixture compactability in the field. The authors did not recommend limits on number <strong>of</strong>
gyrations to reach 92%Gmm. They developed a multiple linear regression model to incorporate
the multiple factors found to affect mixture compactability.
Since no limits for number <strong>of</strong> gyrations to reach 92%Gmm exist in the literature, limits
are based on the results <strong>of</strong> mixture workability testing conducted within the scope <strong>of</strong> this study
by establishing the limit at the average number <strong>of</strong> gyrations to reach 92%Gmm measured for
coarse mixture plus one standard deviation measured for all coarse mixtures. Because all fine
mixtures exhibit low values <strong>of</strong> number <strong>of</strong> gyrations to reach 92%Gmm, the results for the fine
mixtures were excluded from the limits analysis. Using this strategy, the corresponding limit for
number <strong>of</strong> gyrations to reach 92%Gmm is 43 gyrations. Mixture with number <strong>of</strong> gyrations to
reach 92%Gmm less than 43 gyrations are deemed satisfactory. To increase the reliability <strong>of</strong> the
limit and account for variability in the results, the relative viscosity limit was developed based on
the corresponding relative viscosity minus one standard deviation, rounded to the nearest 0.5.
Using the trend line in Figure 2 for the coarse gradation minus one standard deviation, the
limiting relative viscosity corresponding to the proposed mixture limit is 5.0. A summary <strong>of</strong> the
proposed limits is provided in Table 6. Binder viscosity and Rigden <strong>voids</strong> can be measured and
the regression equation presented in Table 3 can be used to determine if the mastic meets the
proposed limit.
TABLE 6 Proposed Workability Limits
Parameter Value
Maximum N92 43
Maximum Relative 5.0
Viscosity
TRB 2012 Annual Meeting Paper revised from original submittal.
10