Page 40 - MODELING OF ASPHALT CONCRETE

18 Cha pte r T w o

measurements, even if done properly, can be used to represent them. Therefore,
any two measures should consider both time of loading and temperature range. In
addition oxidative hardening may change the shape of the relation differently at
different times of loading. This observation dictates then that unless the parameters
of susceptibility are referring to the same narrow temperature range and times of
loading they will naturally give different indications of the effect of aging. Except
for the VTS, none of these requirements are satisfied in the used parameters.
2. The penetration, softening point, and apparent viscosity have been shown in the
previous section to involve serious problems. Using them for temperature
susceptibility measurements may only lead to more difficulties. The simple reasoning
is that as the temperature decreases, or as the asphalt is aged, the time required to
reach the specified shear rate will be different which means we are using measures
at the same temperature but two different times of loading are being used.

In summary, it is rather difficult to consider the temperature susceptibility parameters
discussed here as reliable measures to characterize asphalts or to even measure the effect
of oxidative hardening.

Shear Susceptibility Parameters
Two types of parameters have been used to represent the shear susceptibility of asphalt
cements: The degree of complex flow “C” and the shear index.

Degree of Complex Flow Traxler and coworkers (1944) were the first to introduce the use
of this parameter for paving asphalts. They indicated that measurements on different
asphalts show that the slope of log shear stress versus log strain rate may be considered
constant and, therefore, concluded that asphalt rheological properties may be
approximated by a complex flow equation that is usually used for power low fluids:

M = T (2-1)
S C
where M = constant
T = shear stress
S = shear rate
C = degree of complex flow
For C = 1, the asphalt is a Newtonian fluid and M is the steady-state coefficient of
viscosity. Therefore, C was considered a good measure of non-Newtonian behavior. A
large number of researchers accepted the approximation and the change in the C value
was used as an indicator of the asphalt rheology and an indicator of effect of oxidative
aging on the properties of asphalts (Gallaway 1957; Moavenzadeh and Stander 1967;
Jimenez and Gallaway 1961).
The degree of complex flow C, in fact, looks very attractive for the study of asphalt
rheology. However, the measure also has its own problems that have been shown in
several studies:
1. It assumes the relation between shear stress and shear rate to be linear when
C = 1. This is true only over a small range of stress or strain rate. At very low
shear rates or at very small stress levels almost all asphalts will show Newtonian
behavior (C = 1.0). This behavior, depending on the type of asphalt, will start