Assessing low shear viscosity as the new bitumen Softening Point test

A large amount of research has been published over the last decade in response to the inadequacy of the traditional Softening Point test and the more recent SHRP G∗/sin δ criteria to accurately characterise the rutting behaviour of bitumens for use in road construction. This is particularly evident when considering polymer modified bitumens with a high proportion of delayed elasticity. In this paper, the concept of Zero Shear Viscosity (ZSV) is explored and a newly proposed test variant on ZSV referred to as the Low Shear Viscosity (LSV) test has been examined in some detail. The LSV proposed test protocol, which essentially consists of measuring complex viscosity using dynamic oscillatory loading at a pre-determined equi-viscous temperature, was found to be suitable for characterising the creep performance of two penetration grade bitumens, whereas for an SBS modified bitumen used in this investigation, the protocol was shown to be entirely inadequate. An alternative method of data interpretation is proposed in this paper based on measurement of the phase angle values at low shear rates. The phase angle, or damping factor, has been shown to be a better candidate for characterising the three bitumens assessed in this investigation and is worthy of further investigation.