Influence of Confinement Pressure and Rest Periods on Measurement of Dynamic Modulus of Bituminous Mixtures

In the AASHTO T-378 protocol for computing dynamic modulus and phase angle, the bituminous mixture sample is subjected to a continuous loading from high frequency to low frequency. The postprocessing method involves fitting a regression equation to the stress and the strain data. The influence of confinement pressure and rest period between the frequencies on the overall mechanical response of the material within the context of the current test method needs detailed investigation, and this paper discusses some of the interesting aspects. Two dense graded bituminous mixtures (unmodified and polymer-modified) are subjected to repeated haversine compression loading at six different temperatures (5°C–55°C) and ten test frequencies ($25–0.01\mathrm{Hz}$). The tests are conducted with and without confinement pressure and rest period. The analysis of the data revealed that confinement pressure strongly influenced the dynamic modulus and phase angle in the temperature range of 35°C–55°C and frequencies in the range of $0.01–2\mathrm{Hz}$. The influence of the rest period on dynamic modulus and phase angle is observed to be negligible. For all the loading conditions, the dynamic modulus master curve is constructed using the sigmoidal model, and the phase angle master curve is constructed following a free shifting procedure. The results show that for the dynamic modulus master curve, the root mean square error is considerable for the tests conducted with confinement conditions. Finally, the collected data that fulfilled the Kramers–Kronig relationship is used for generating a master curve for dynamic modulus and phase angle in the temperature range of 5°C–35°C.