Page 59 - MODELING OF ASPHALT CONCRETE

Modeling of Asphalt Binder Rheology and Its Application to Modified Binders 37

the different test systems and pavement distresses covered in the PG system. Workability
is measured at 135°C using a rotational viscometer; resistance to permanent deformation
and fatigue cracking is measured using a dynamic shear rheometer (DSR) at maximum
pavement temperature and intermediate pavement temperatures. For evaluating the
resistance to low-temperature cracking, the bending beam rheometer (BBR) and the
direct tension testing (DTT) device are used.
Numerous studies have been conducted to evaluate how different modifiers affect
the grades of conventional asphalts. Figure 2-10 depicts an example of effect of three
different types of crumb rubber based additives on performance measures. The effect is
calculated in terms of ratios of the value of the parameter after modification to the
∗
parameter before modification. It is apparent that for the rutting parameter (G /sind)
∗
the ratios are higher than one while for the fatigue parameter, the ratios of G sind is
much lower than one. S(60) ratios are also lower than one while the ratios of m(60) and
strain at failure are very close to the value of 1.0 (no change). The data in Fig. 2-10 show
that this type of modification has its main effects at high temperatures. This is expected
when the nature of the CRM modifier is considered. Crumb rubber acts mainly as a
flexible filler; at high temperatures it is stiffer than the asphalt and thus contributes
significantly to the increased moduli. With decreasing temperatures, the asphalt
becomes stiffer while the crumb rubber properties do not change significantly. At a
certain temperature, the asphalt may become stiffer than the crumb rubber and thus a
reduction in stiffness can be observed for the modified binder. Crumb rubber, at
moderate concentrations that are used in practice (10 to 20 percent), however, cannot
reduce stiffness by large margins because of its own relatively high stiffness at low
temperatures. It is, therefore, expected that the main effects of crumb rubber remain to
be seen at higher temperatures and to affect mainly the rutting parameter.
The use of the Superpave technology to evaluate modification is rather widespread
today and it became the standard for estimating the relative value of additives and to
justify some of the added initial cost to buy and use selected modifiers. Concerns,
however, were raised about using this technology without verification that it applies to
modified binders. The concerns were the results of a few field problems and because
the original SHRP did not include a variety of modified asphalts used in practice after
the program was completed. The following sections address these concerns (Bahia et al.
1998b; Bahia et al. 2001).

Relative change (CRM/neat) 10 RB1 RB2 RB3

8
6

4

2

0
G*/sind G*/sind S(60) m(60) F.strain

FIGURE 2-10 Typical effects of CRM-modifi er on the Superpave grading parameters.