Page 70 - MODELING OF ASPHALT CONCRETE

48 Cha pte r T w o

FIGURE 2-18 Correlation between binder rutting parameter and the average mixture rutting
parameter. Slope measured in strain per stress (1/kPa).

∗
of the accumulated strain of the oxidized binder of the same grade. The G /sind of
the oxidized binder at the testing temperature is 15,900 Pa which is higher than the
∗
elastomeric binder that has a G /sind value of 13,000 Pa. This inversion of ranking is
very critical and can be well explained by the ability of the elastomeric binder to
recover under the testing conditions. The recovery, however, is not being captured
∗
by the G /sind due to the fact that the parameter cannot distinguish between total
energy dissipated and the energy dissipated in permanent flow. The results from the
creep and recovery test can also be explained by what is known about the molecular
nature for these materials.
To evaluate the effectiveness of using the creep and recovery binder test, 9 binders
of various grades were aged in the RTFO and tested at condition that matches the
temperatures and loading-time conditions at which the RSCH testing was conducted.
The rates of accumulation of permanent strain of the mixtures were plotted versus the
rate from the new binder test, as shown in Fig. 2-18.
The correlation between the mixture and binder properties has improved
significantly from 23 percent to approximately 68 percent. It appears that the new
approach based on the creep and recovery testing is very promising. The correlations
with the individual aggregate blends varied between higher than the average value for
the crushed limestone to very low correlations for the gravel aggregates. The poor
correlations for certain binders were expected since aggregates play a main role in the
rutting performance. An analytical approach was used to isolate the effect of binders
from the effect of aggregates based on modeling the relationship between mixture and
binder accumulation of permanent strain. This type of analysis enabled the separation
of the main effect of binders which showed correlations of the mixture and binder
rutting behavior in the range of 80 to 90 percent (Bahia et al. 2001).

Binder Fatigue Test
∗
As discussed earlier, it is found that the parameter G sind is not well related to the
accumulation of fatigue damage of mixtures as measured in a beam fatigue test, under
strain-controlled conditions. It is believed that the main reason is that the parameter
∗
G sind is measured in the linear viscoelastic range using small strains. There is a