Refinement of Flow Number as Determined by Asphalt Mixture Performance Tester

Abstract

Flow number determined with the repeated-load test is used as a criterion to characterize the rut resistance of hot-mix asphalt. The repeated-load test is performed with the asphalt mixture performance tester (AMPT). The algorithm currently used in the AMPT to determine flow number was found to be extremely sensitive to noise in the data and identifies erroneous flow number results, especially for modified mixes. A new algorithm that uses the Francken model to fit the flow number data is considered for implementation. The robustness of the Francken model is verified in this study by fitting flow number data obtained from field projects. The flow number test is time-consuming, especially for high-stiffness binders and, in many cases, can take as long as 6 h. In this study, other parameters are examined to determine whether the test can be terminated early without loss of information on rut resistance. Steady-state slope and slope at 2% strain are found to correlate well with flow number; the correlation indicates that they may be robust indicators of rut resistance. The flow number test can be terminated once steady-state slope or slope at 2% strain is reached; this substantially reduces test time. This paper discusses the findings from analysis of flow number data from the Francken model and the development of a refined, more expedient method for determining flow number.