Development of a Flow Number Predictive Model

Abstract

The NCHRP 9-19 panel recommended the repeated load permanent deformation test as a laboratory procedure that could be used to evaluate the resistance of a hot-mix asphalt (HMA) to tertiary flow. No standard test protocol addresses the required laboratory stress to be applied. The test can take several hours until tertiary flow is reached and in many cases the sample may never fail. A model capable of predicting or providing general guidance on the flow number characteristics of a mix can be of great value. The model can be ideally used as a guideline to determine the stress–temperature combination that will yield tertiary flow within a reasonable testing time. In this study, an effort was undertaken to develop a flow number predictive model. The model uses HMA mixture volumetric properties and stress–temperature testing conditions as predictor variables. The laboratory test data used are a combination of two valuable databases. The first one included tests conducted at Arizona State University; the second one included tests conducted by the FHWA Mobile Asphalt Material Testing Laboratory. Ninety-four mixtures were evaluated, and 1,759 flow number test results were available. Various regression models were evaluated by combining several independent variables. The final model selected had fair statistical measures of accuracy, and it covered a wide range of mixtures, gradations, and binder properties, as well as laboratory-applied stress. As more testing data become available, the model could be refined and recalibrated for better accuracy.