Rutting criteria for asphalt mixtures based on flow number analysis

The National Cooperative Highway Research Program (NCHRP) 9-19 panel recommended the Repeated Load Permanent Deformation (RLPD) test as a laboratory procedure that can be used for evaluating the resistance of Hot Mix Asphalt (HMA) mixtures to tertiary flow. Currently, there is no standard test protocol that addresses the required laboratory stress/temperature combination to be applied to the specimens. The test can take several hours until tertiary flow is reached, and in many cases, the sample may never fail. In addition, there are no permanent deformation criteria for asphalt mixtures that provide relationships between the tertiary flow (Flow Number) and rutting in the field.;A model capable of predicting and providing guidance on the Flow Number characteristics of a mix can be of great value. The model can be ideally used as a guide to determine the stress/temperature combination that will yield tertiary flow within a reasonable testing time.;In this research, an effort was undertaken to develop a Flow Number predictive model for asphalt mixtures. This model was based on a comprehensive database that contained HMA volumetric properties for 94 mixtures, as well as stress and temperature testing conditions. The comprehensive database included two major and significant data sources: the first one included tests conducted at Arizona State University; the second one included tests conducted by the Federal Highway Administration Mobile Asphalt Material Testing Laboratory (MAMTL). A total of 1801 Flow Number test results were included.;Various regression models were evaluated and a final model was recommended. This final model had fair statistical measures of accuracy, but it covered a wide range of mixtures, gradations, binder properties as well as laboratory applied stresses.;A major outcome of this research was the recommendation of a rutting design criterion that is a function of the Flow Number, traffic level and thickness of the asphalt mixture layer. A total of 1440 asphalt pavement sections were used in this analysis that included: 20 climatic locations, 3 traffic levels, 3 asphalt layer thicknesses, 4 aggregate gradations and 2 air void levels. For this rutting - Flow Number criterion, a power model form was recommended and had very good statistical measures of accuracy.