Laboratory investigation of asphalt concrete dynamic modulus testing on the criteria of meeting linear viscoelastic requirements

In this study, asphalt concrete (AC) dynamic modulus testing is evaluated for meeting linear viscoelasticity (LVE) requirements. The evaluation is performed by performing stress sweep dynamic modulus testing at selected test frequencies. Three well-known linearity principles, namely superposition principle, proportionality principle and 5% stiffness reduction principle, are implemented to classify the AC response as a LVE. Analysis of the stress sweep dynamic modulus test results using these principles consistently showed that the stress and strain linearity limits for the studied mix are 155, 414, 506, 772, 470 and 462 kPa and 55, 69, 84, 99, 36 and 50 micro-strain each for loading frequency of 0.1, 0.5, 1, 5, 10 and 25 Hz, respectively. The current dynamic modulus testing standard, American Association of State Highway and Transportation Officials (AASHTO) TP-62, recommends maintaining test strains and stresses below 150 micro-strain and 700 kPa to make sure AC dynamic modulus testing is preformed within LVE range. However, these recommended limiting stress and strain values are considerably higher than the results found in this study. Moreover, the softening rate at which the dynamic modulus decreases is found to be approximately the same over the entire stress sweep range, which indicates the dependency of dynamic modulus on stress. This indicates that there is a significant chance of conducting the dynamic modulus test at stress and stain levels beyond the LVE region.