Abstract
The effect of tertiary flow length on asphalt concrete (AC) flow number (FN) has been studied in this paper. The standard FN test (AASHTO T378) designated by American Association of State Highway and Transportation Officials (AASHTO) is currently among the widely adopted test for assessing the rutting performance of AC mixtures. The standard adopts the Fracken model (FM) for fitting the permanent deformation curve (PDC), prior to FN estimation. The FN was observed to vary as tertiary flow progresses. The FM, along with other PDC models (MFM-1 and MFM-2) obtained by modifying the FM, was utilized to analyze and minimized this variation. Variation of FN resulted because of mathematization of the PDC data. Instead of representing the actual initiating point of permanent shear deformation of the strain data, estimated values of FN were observed to indicate the inflection points of the fitted parent curve. As per standard FN range suggested by AASHTO T378, the observed variation in FN leads to the situation, where a single asphalt mix specimen can be regarded as appropriate for two different traffic levels, depending on the tertiary flow point at which the test is terminated. Supplementary steps in the FN estimation methods were proposed for refinement of FN values. For the refinement and further standardization of FN value and FN test respectively, FN:T (flow number to test duration ratio) has been recognized as the key and most simple solution. Two potential options for employing FN:T in the estimation of FN have also been highlighted. Several important correlations have been reported herein as well. Comparatively, MFM-1 was found to be more robust in FN:T curve utilization than MFM-2 and FM.
•Effect of tertiary flow length on asphalt concrete flow number has been studied.•Francken model (FM) estimated FN values were observed to vary as tertiary flow progresses.•Supplementary steps in the FN estimation methods were proposed for refinement of FN values.•Flow number to test duration ratio has been identified as the key FN test termination criteria.