Development of Two-Step Secant Method to Interpret the Flow Number Test Data of Asphalt Mixtures

The flow number test is widely utilized as a routine test for assessing the rutting resistance of asphalt mixtures. The objective of this study is to propose a novel method of determining the flow number of asphalt mixtures with better accuracy. The Dynamic Testing System (DTS) was employed to perform the flow number tests on four types of asphalt mixtures at multiple temperatures. With respect to the test data, the existing flow number calculation methods are firstly evaluated in terms of the accuracy of the determined flow number values. It is demonstrated that none of the existing methods are recommended to determine the flow number of asphalt mixtures. The two-step secant method is then developed based on the curve shape of the permanent strain versus load cycles graph. The characteristics of concavity and convexity shown in such a graph are utilized to determine the onset of the secondary stage and that of the tertiary stage. As the kernel of the two-step secant method, the secant method is employed twice to determine the load cycle at the minimum slope of secant line for selected permanent strain data. The two load cycles determined by the second step of the two-step secant method are demonstrated to be the onset of the secondary stage and tertiary stage. The termination criterion of flow number tests is then established based on the test data of four asphalt mixtures so that the two-step secant method can be applicable. In conclusion, the proposed two-step secant method is capable of determining the initiation points of the secondary stage and tertiary stage with better accuracy for different types of asphalt mixtures.