Limitations of bitumen performance grading across temperature, frequency and shear rate history

Abstract: The grading system for bitumen is a standardised classification method used to evaluate and specify bitumen based on some measurable parameters. One such way of grading the bitumen is by the Superpave (Superior Performing Asphalt Pavements) Performance Grading (PG) system, in which the specification required to ensure resistance to rutting and fatigue is kept constant with the temperature varying. Parameters derived based on limiting the energy dissipation values in terms of the absolute shear modulus (|G^*|) and phase angle (\delta) are set to an upper bound to limit fatigue and a lower bound to limit rutting. These values are obtained by subjecting bitumen to an oscillatory shear loading at a constant amplitude and frequency. The bitumen is then graded based on the temperature range in which these parameters will not exceed the desirable values. Due to the variations in the temperature and loading rate sensitivity of bitumen, grading bitumen based on a rutting or fatigue parameter at a fixed temperature and frequency can lead to a biased ranking. Hence, it is necessary to grade bitumen based on its mechanical behaviour over a range of temperatures and frequencies. Beyond these factors, the bitumen is also expected to show varying responses when subjected to loading with different shear rate histories. These variations underscore the limitations of the PG system in fully capturing the unique characteristics of each bitumen across diverse conditions.
This study aims to address these limitations by examining three bitumen samples that share the same PG classification. They are evaluated across a range of PG temperatures, from 46 to 82 °C for high-temperature PG and from 4 to 40 °C for intermediate-temperature PG. The loading is applied through both frequency sweep and multi-wave experiments, with the multi-wave approach involving a simultaneous application of multiple sinusoidal waveforms with the same frequencies used in the frequency sweep experiment. This introduces a distinct shear rate history compared to that of frequency sweep testing, offering a comparative insight into the influence of shear rate history on the material response. Thus, the protocols are designed to assess temperature susceptibility, as well as the influence of frequency and shear rate history on the behavior of bitumen. Temperature susceptibility is evaluated by measuring PG parameters at a fixed frequency across temperatures. Frequency influence is assessed by comparing these parameters across different frequencies from frequency sweep or multi-wave tests at a given temperature. Finally, the influence of shear rate history is analysed by comparing PG parameters from frequency sweep and multi-wave tests at a constant frequency and temperature.
The results indicate significant differences in temperature susceptibility, frequency response and the effects of shear rate history across the bitumen samples despite their identical PG grading. These findings underscore the limitations of the PG grading system in adequately characterising the varied behaviors of bitumen. The study suggests the need for additional parameters or methods to capture the diverse responses of bitumen under varying field conditions.