What is Transition Temperature for Bitumen and How to Measure It?

In a typical year and for any typical location in India, the lowest temperature of the pavement can go up to 0 °C or below and the highest temperature can reach up to 70 °C or above. During such temperature changes, the binder exhibits behaviour ranging from a glassy material (at lower temperature) to a viscoelastic/non-Newtonian fluid (at higher temperature). The range of temperature change which is of interest to a pavement engineer is the transition between viscoelastic solid to viscoelastic fluid. Two measurements are normally used for quantifying the transition temperature and they are the G′–G″ cross over point and the point when tan δ is independent of frequency (Winter-Chambon criterion). These measures are applicable when the material has a single relaxation time (typically a single constituent material). Since bitumen is a mixture of several complex hydrocarbons each with different relaxation time, it is natural that such measures are also frequency dependent. This investigation will focus attention on determining the frequency dependence of G′–G″ cross over point as well as the relationship of tan δ with frequency. An unmodified binder of VG30 grade as per IS73-2013 and a crumb rubber modified binder of meeting CRMB 60 grade as per IS:15462-2004 were used in this investigation. To estimate the viscoelastic solid–fluid transition temperature, frequency domain tests were performed in the temperature range of 25–75 °C for frequencies in the range of 50–1 Hz. All the tests discussed in this study were performed at two aging conditions, unaged and short-term aged. Due to the frequency dependent nature of bitumen, the transition temperature is not a sharp point as in the case of polymers but occurred gradually over a temperature range. Contrary to what one expected, modification process did not drastically change the transition temperature and one could see a change of only 3 °C for CRMB 60 when compared to VG30. Aging of the modified bitumen proportionally shifted the transition temperature in a similar manner to that of the unmodified bitumen. It was also clearly seen that the Winter-Chambon criterion was not met for all the binders investigated in this study.