Page 187 - MODELING OF ASPHALT CONCRETE

VEPCD Modeling of Asphalt Concr ete with Gr owing Damage 165

been proven valid for asphalt concrete by Chehab et al. (2002) for tension and by others
in compression (Zhao 2002; Gibson et al. 2003; Kim et al. 2005).
The underlying principles of the VEPCD model have been characterized and
verified in the last 15 years through a series of research projects, and readers are referred
to the reports (Kim and Lee 1997; Kim et al. 2002; Kim and Chehab 2004; Kim et al. 2005)
and dissertations (Chehab 2002; Daniel 2001; Lee 1996) for theoretical details of these
principles. In the following sections, these principles are briefly described.

TTS with Growing Damage
It is well known that the behavior of asphalt concrete depends on time and temperature
and that, when in its linear viscoelastic range, asphalt concrete is thermorheologically
simple (TRS); that is, the effects of time or frequency and temperature can be expressed
through one joint parameter. The viscoelastic material property as a function of time (or
frequency), such as the relaxation modulus (or dynamic modulus) at various temperatures
can be shifted along the horizontal log time (or log frequency) axis to form a single
characteristic mastercurve. If this principle can be extended to outside of the LVE range,
its impact is significant in terms of testing requirements and efficiency in modeling.
The TTS with growing damage can be verified using a simple technique shown in
Fig. 7-1. In short, stress and time are determined at a strain level from the constant
crosshead rate monotonic tests at different rates and temperatures. The corresponding

FIGURE 7-1 Schematic representation for a single strain level of the technique used to verify
time-temperature superposition with growing damage. (Underwood et al. 2006b, with
permission from Association of Asphalt Paving Technologists.)