Complex Modulus Testing of Asphalt Concrete Under Very Small Strain Amplitudes

The complex modulus norm |E*| of asphalt concrete (AC) materials is commonly measured under strain amplitudes of the order of 50 microstrains (με). Despite the small strain regime, |E*| values are known to exhibit strain-level sensitivity; within allowable protocol limits, they decrease as strain amplitudes increase and increase as strain amplitudes decrease. This nonlinear effect is usually substantial, rendering all reported |E*| values apparent |E*| values. The stated objective of this work was to measure |E*| under very small strain amplitudes of the order of a few microstrains and evaluate whether the behavior of |E*| becomes linear or whether nonlinearity persists. A targeted experimental campaign was carried out over an asphalt concrete specimen made of a standard mix. The testing included two temperature levels and a combination of four frequencies under several very small strain amplitudes. Ultimately, |E*| results were obtained for strain amplitudes smaller than 10 με with some below 5 με. It was found that the nonlinear behavior of |E*| persists, leading to the conclusion that the steady-state response of asphalt concrete to harmonic excitation is not truly sinusoidal.