Assessment of shift factor for predicting asphalt performance on Western Australian local roads
Flexible granular pavements with thin asphalt surfacings are widely used around Perth in Western Australia. The pavement thickness design is commonly undertaken using linear elastic theory and asphalt fatigue life is initially assessed using the Austroads model. This originally had used a “shift factor” to go from laboratory testing to field performance, and previous researches indicated that shift factor can range between 5 and 20. It now uses reliability factor, which incorporates both shift factor and desired project reliability, and this can range from 0.67 to 2.5. This paper presents a case study on performance of a local road pavement comprising granular crushed limestone basecourse with thin asphalt surfacing near Perth. Field and laboratory tests were undertaken to characterise the pavement materials, and modulus values of the pavement layers were back-calculated. Based on the analysis presented in this paper, the lower bound of shift factor values was between 5 and 15. At this level though, the road has not failed and shows no fatigue distress at all, which suggests that its structural capacity is much greater than these shift factors imply. A shift factor of at least 5 can be suggested for design of medium trafficked pavements with a thin asphalt surfacing say up to 75 mm in thickness. The broader question is raised for light to medium trafficked pavements with thin asphalt layers as to whether the structural capacity of thin asphalt layer should be considered in their structural analysis. At the least, it might be that the Austroads approach not to consider fatigue cracking for lightly trafficked roads could be extended to include the lower end of medium trafficked roads, say up to 3 x 106 ESA.