Page 58 - MODELING OF ASPHALT CONCRETE

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material. Another hypothesis is that the dispersed polymer particulates may serve as
reinforcements, arresting microcrack propagation and increasing toughness of binders.
The typical trend that can be observed from review of polymer modification works,
however, is that not many polymers used currently improve low-temperature failure
properties. This may be attributed to the fact that until recently there has been no simple
technique to measure the brittle failure of asphalt, and also to the fact that none of the
used binder specifications address the brittleness of asphalt in a rational and fundamental
form. These issues did not encourage many polymer modifier producers to concentrate
on designing a modifier to mainly enhance low-temperature failure properties.
Figure 2-8(b) depicts failure plots for an asphalt before and after modification with
crumb rubber at 10 percent (CRM1) and 20 percent (CRM2) concentration by weight of
total binder. The effect of the CRM is similar to the polymer modification with respect to
the strain at failure values; higher strains are observed at low temperatures but similar
strains are observed as the flow region is reached by the binders. The effect also represents
a shift of the failure curve along the temperature scale toward lower temperatures. The
shift is larger for the higher CRM content. The stresses at failure, however, show a trend
different from the polymer modification. The CRM results in stress values that are
significantly higher than the unmodified asphalt at all temperatures. This behavior can
be attributed to the reinforcing effect of the rubber particles. The crumb rubber particles
do not dissolve in asphalt; the particles maintain their integrity and tend to swell in
asphalts resulting in effective volumes that are larger than their initial volume (Bahia
and Davies 1994; Chehoveits 1982). It is speculated that the swelling results in selective
absorption and/or adsorption of certain components of the asphalt. Such interactions
are expected to reinforce the matrix of the binder and result in higher strength, as
observed in the figure. The increase in strain and stress at failure is favorable for paving
grade asphalts, particularly when it is not accompanied by an increase in stiffness.

Effects of Modification on the Superpave Grading Properties
The Superpave performance grading (PG) system was developed to evaluate binder
properties at specific temperatures with specific testing systems (Anderson and
Kennedy 1993; Anderson et al. 1994; McGennis 1995). The AASHTO MP1 procedure
includes details of testing and grading of binders. Figure 2-9 is a schematic describing

FIGURE 2-9 The Superpave grading system.