Practical methodology of determining the in situ dynamic (complex) moduli for engineering analysis

The viscoelastic behavior of asphalt mixtures can be conveniently presented in the form of a stiffness master curve in various engineering applications. However, the methods to obtain the in-service mixture stiffness are limited to the falling weight deflectometer (FWD) testing and back-calculation procedure because the pavement layer thicknesses in many cases are too thin to allow laboratory testing. The test protocol to measure the axial dynamic modulus requires cylindrical specimens with a height-to-diameter ratio higher than 2, while the cores need to be at least 38 mm thick for measuring shear stiffness. This technical note describes and compares the methods for constructing asphalt mixture master curves utilizing stiffness predictive equations for asphalt binders and mixtures, in addition to using laboratory measurements, as an alternative to FWD backcalculation analysis. This technical note also recommends constraining the low and high mixture stiffness values measured from simple shear testing to obtain stiffness comparable to the uniaxial dynamic modulus testing.