Guide on the selection of appropriate laboratory stress levels for the flow number test

This study examines the use of the triaxial shear strength parameters (c and /) in providing guidelines for selecting the appropriate laboratory stress level for the flow number test of asphalt mixtures. The approach is based on the concept that the flow number (tertiary flow) is inversely related to the applied stress-to-strength ratio of the material. The correlation of the flow number with the stress-to-strength ratio can provide a good estimate of the stress level that will yield tertiary flow within a reasonable testing time. It was also realized that the triaxial shear strength test is not routinely conducted by Department of Transportation agencies and testing laboratories. Therefore, a database of triaxial shear strength test data was used to develop predictive models of the shear strength parameters for a particular asphalt mixture. The models' predictor variables were based on the volumetric properties of 46 different asphalt mixtures and a total of 276 test results. Regression models to estimate the c and friction (φ) parameters had good statistical measures of model accuracy. The models were used to develop guidelines for laboratory stressto-strength ratios to achieve tertiary flow within a reasonable testing period.