Evaluation and Challenges of the Flow Number Simple Performance Test

The Simple Performance Test for asphalt mixtures has been under evaluation for the past five years under the NCHRP 9-19 project. The Flow Number test was one of the final three tests selected for further evaluation from the twenty-four test/material properties initially examined. This paper presents the results of the: selection, validation, simplification, and development efforts towards criteria for the Flow Number test. Over 1000 asphalt specimens including both DOT mixtures and mixtures from field validation studies were prepared and tested to enable the comparison of the materials Flow Number property against the actual field performance. The results showed that the Flow Number test has great potential for ranking the relative performance of asphalt mixtures, determining the performance history of asphalt mixtures (in terms of deformation resistance), determining the optimal binder content of asphalt mixtures, and for examining the effects of air voids on asphalt performance. Additionally, the study demonstrated that locally based criteria could readily be established to determine local performance criteria and history for asphalt mixtures. However, the study found that limitations still do exist, in determining the optimal air void content, as well as finding a common failure envelope or a method to correct to a common failure envelope. This limitation has currently made it difficult to establish global failure criteria for use with the Flow Number test.