Page 39 - MODELING OF ASPHALT CONCRETE

Modeling of Asphalt Binder Rheology and Its Application to Modified Binders 17

measure and to define a temperature susceptibility parameter. Again several types of
relations have been used: slope of log viscosity versus temperature (Traxler and Schweyer
1936), log viscosity versus reciprocal of temperature raised to a variable power (Traxler
and Schweyer 1936; Cornelissen and Waterman 1956), and log-log viscosity versus log
temperature (Fair and Volkmann 1943). The latter measure is probably the most widely
accepted and is referred to as viscosity-temperature susceptibility or VTS.
A third type of approach used a combination of consistency measures. The penetration
index (PI) for example combines penetration and softening point (Van der Poel 1954), and the
penetration viscosity number (PVN) combines penetration and viscosity (McLeod 1972).
The best indication of how well these different susceptibility measures represent the
rheological properties of asphalts is probably found in durability studies that used these
measures to define asphalt performance with age hardening. The conclusions about the
effect of aging on temperature susceptibility are controversial: Pfieffer (1950) and Blokker
and Hoorn (1959), using the penetration index, reported a decrease in temperature
susceptibility upon oxidative aging. Halstead and Zenewitz (1961) using plots of log-log
viscosity versus temperature, in the range of 60 to 95°F, indicated that some asphalts
show a decrease in temperature susceptibility while others show an increase upon aging.
McLeod (1972) used the penetration index (PI) and his penetration viscosity number
(PVN) and observed that after 9 years of service asphalts showed either no change or a
significant increase in temperature susceptibility. However, the PI did not show the same
change in temperature susceptibility. Puzinauskas (1979) used the VTS and concluded
that the majority of asphalts show an increase in temperature susceptibility. Anderson
and his coworkers (1983) used three different parameters (PI, PVN, and VTS) and
concluded that PI and VTS show a general increase in temperature susceptibility, while
PVN does not show significant changes with aging. The authors, experimenting on a
very large number of asphalts, indicated that these measures do not measure the same
property, and no simple explanation can justify the difference in the effect of oxidative
aging on the values of these parameters. Button and his coworkers (1983) confirmed
Anderson’s findings. The authors found that, using the PVN (estimated from pen77 and
vis140) as the temperature susceptibility measure, asphalts that were originally highly
temperature susceptible became more susceptible with oven aging, while those that
were less temperature susceptible became even less susceptible with aging.
Others have also used apparent viscosity as the measure of consistency. Moavenzadeh
and Stander (1967) measured apparent viscosity at a constant power input (1000 ergs) over
a range of temperatures between 10 and 160°C. The authors indicated that aged asphalts
are less temperature susceptible in the lower temperature range while more susceptible in
the higher temperature range. In other words, the temperature susceptibility shows
different changes at different ranges of temperature. Kandhal and his coworkers (Kandhal
et al. 1973; Kandhal and Wenger 1975) confirmed Moavenzadeh and Stander findings.
Here researchers measured apparent viscosity at 39, 77, 140, and 275°F and observed that
temperature susceptibility measured in terms of log-log viscosity versus log temperature,
decreased in the lower temperature range with age hardening while it increased in the
higher temperature range.
No doubt the above review is confusing. Two fundamental reasons can be given to
explain the problems with temperature susceptibility parameters:

1. The temperature susceptibility, as shown in linear viscoelastic characterization
studies, is both temperature range and time of loading dependent. The relations
are not linear nor can be approximated by a linear relation such that a few