Grading highly modified binders by multiple stress creep recovery

The multiple stress creep recovery (MSCR) test was developed in the USA to evaluate the high-temperature permanent deformation resistance of bitumens. It is credited as representing world best practice for high-temperature shear response of binders for asphalt production. However, some researchers have questioned the suitability of the current test protocol for characterisation of highly modified binders. Five highly polymer-modified elastomeric and plastomeric Australian grades of binder and one conventional binder were tested by MSCR at various temperatures (64–76°C) and the results compared with laboratory asphalt mixture properties indicative of asphalt field performance. The results indicated the current MSCR protocols are not suited to highly modified binders and that an increased level of applied stress and longer recovery periods between stress cycles should be considered for improved characterisation. Furthermore, the MSCR creep compliance, intended to be indicative of asphalt permanent deformation, was found to be poorly correlated with asphalt mixture wheel track rutting and repeated shear flow test results. This lack of agreement between bitumen and asphalt mixture test results may reflect the limitations of the current MSCR protocol for highly modified binders. However, further investigation is required to understand the impact of base bitumens imported into Australia and other contributing factors.