Application of time–temperature superposition to develop master curves of cumulative plastic strain in repeated load permanent deformation tests

Abstract

The repeated load permanent deformation (RLPD) test is used to capture the permanent deformation characteristics of asphalt mixtures for prediction of pavement rutting. The full characterisation of permanent deformation behaviour as required by the National Cooperative Highway Research Program (NCHRP) Project 9-30A enhanced rutting model requires time-consuming RLPD tests at three temperatures – low, intermediate and high. This study investigates the application of time–temperature superposition (TTS) to reduce the material characterisation testing effort in the context of the NCHRP 9-30A models. Dynamic modulus and RLPD test data were collected for 27 mixtures, including mixtures from long-term pavement performance (LTPP) programme. The temperature shift functions obtained from dynamic modulus data were used to shift the associated RLPD test results at various temperatures to develop a master curve of cumulative permanent strain versus reduced number of loading cycles. The cumulative permanent strain curves at different temperatures collapsed fairly well into a single RLPD master curve for the evaluated mixtures, confirming the validity of TTS for the RLPD test. The TTS procedure was also applied individually to the test results at a single temperature to predict the permanent strains at the other temperatures. The results from the 27 mixtures suggest that the RLPD master curve obtained by testing only at the high temperature (LTPPBind high temperature at 50% reliability – 5°C) permits accurate and precise predictions of the permanent deformations at all other temperatures. RLPD testing at a single temperature has the real practical benefit of significantly streamlining the asphalt material characterisation testing programme.