Complex Modulus as a Design Parameter
The 2002 AASHTO Guide for the Design of Pavement Structures recommends the
complex modulus as a design input parameter for the mechanistic-empirical design
procedure (NCHRP 1-37A 2002 Design Guide Draft, 2002). Level 1 Analysis requires
actual dynamic modulus test data to develop master curves and shift factor based on
Equations (2-7) and (2-8). This testing is performed on replicate samples at five
temperatures and four rates of loading per temperature. Binder testing must be performed
at this level to shift the data into smooth master curves. Level 2 Analysis constructs a
master curve using actual asphalt binder test data based on the relationship between
binder viscosity and temperature. Level 3 Analysis requires no laboratory test data.
Instead, the Witczak modulus equation (NCHRP 1-37A 2002 Design Guide Draft,
2002) is used with typical temperature-viscosity relationships established for all binder
grades.
Witczak Predictive Modulus Equation
The complex modulus test is relatively difficult and expensive to perform.
Therefore, numerous attempts have been made to develop regression equations to
calculate the dynamic modulus from conventional volumetric mixture properties. For
example, a predictive regression equation is proposed as a part of the 2002 Design Guide
(Witczak et al., 2002) to calculate the dynamic modulus, E*|, based on the volumetric
properties of any given mixture. The predictive equation developed by Witczak et al.
(2002) is one of the most comprehensive mixture dynamic modulus models available
today (Equation 2-11).