log(E *) = 3 + (Eq. 2-8)
Where
log(|E*|) = log of dynamic modulus,
6 = minimum modulus value,
fr = reduced frequency,
U. = span of modulus value,
P, y = shape parameters.
Note that 6 in this equation is not related to the phase angle it is just the notation
chosen by Pellinen and Witzcak (2002) for the minimum modulus value. The sigmoidal
function of the dynamic modulus master curve can be justified by physical observations
of the mixture behavior. The upper part of the function approaches asymptotically the
mixture's maximum stiffness, which depends on the binder stiffness at cold temperatures.
At high temperatures, the compressive loading causes aggregate interlock stiffness to be
an indicator of mixture stiffness. The sigmoidal function shown in Equation 2-8 captures
the physical behavior of asphalt mixtures observed in complex modulus testing
throughout the entire temperature range (Pellinen and Witzcak, 2002).
Sample Preparation
Currently, there is much discussion about the shape and size of specimen to be used
in complex modulus testing. In NCHRP Project 9-19, Witzcak and his colleagues
investigated the proper size and geometry of test specimens (Witzcak et al. 2000). Based
on numerous complex modulus test results, they recommended using 100-mm diameter
cored specimens from a 150-mm diameter gyratory compacted specimen, with a final
saw cut height of 150-mm. This recommendation came from a study (Chehab et al.,
2000) that considered the variation in air voids within specimens compacted using the
Superpave Gyratory Compactor (SGC). The studies showed that specimens compacted