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CHAPTER 2

Modeling of Asphalt Binder

Rheology and Its Application

to Modified Binders

Hussain U. Bahia

Introduction
Rheology is the science of flow and deformation. It is a science for the study of materials
whose deformation characteristics vary not only with load but also with time rate of load
application. Asphalt binders are rheological materials; that is, their behavior depends on
temperature and rate (or time) of loading. At any combination of time and temperature,
the time-dependent (i.e., viscoelastic) behavior of asphalt binders, within the linear
range, is best characterized by two properties: the total resistance to deformation under
load and the relative distribution of that deformation between elastic and viscous parts
(Bahia and Anderson 1995). Although there are many methods of characterizing
viscoelastic properties, cyclic (oscillatory) testing and creep testing are two of the best
techniques to represent the uniqueness of the behavior of this class of materials.
What is unique about asphalt binders is their high sensitivity to temperatures within
the range of applications. The stiffness of asphalt can vary by as much as eight orders of
magnitude, and their phase angle (relative distribution of response between elastic and
viscous) by as much as 85°, between peak summer and peak winter conditions. It can also
vary by similar amounts in response to standing traffic and high-speed traffic (Anderson
et al. 1994).
Similar to viscoelastic properties, failure properties and damage resistance properties of
asphalt binders are also very sensitive to temperature and loading rate. Stress and strain at
failure can change by an order of magnitude by a change of only 10°C (Dongre et al. 1995).
Fatigue life of binders can change by orders of magnitude in response to a change of a few
degrees of temperature or by changing loading frequency within the application range (fast
versus slow traffic) (Pell and Cooper 1975; Bahia et al. 1999; Bonnetti et al. 2002).
Conventional methods of asphalt refining have their limitations which prompted
the introduction of modified binders. In 2005 it is estimated that 20 to 25 percent of the

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