Small specimen geometries have been gaining attention in recent years to enable the characterization of as-built pavement layers using the Asphalt Mixture Performance Tester (AMPT). Small specimens also offer a significant opportunity to improve the efficiency of laboratory-fabricated specimen AMPT testing. The goal of this project was to rigorously evaluate small specimen geometries for AMPT dynamic modulus and cyclic fatigue AMPT testing to enable the development of standardized procedures. Two small specimen geometries were evaluated: 38-mm diameter cylindrical specimens and 25-mm thick by 50-mm wide prismatic specimens. An assessment of specimen geometry effects was first conducted using five mixtures values to identify the material types and testing conditions where the small specimen geometries yield representative results. The results demonstrated that the dynamic modulus test results acquired from the standard and small specimen geometries are statistically equivalent at the temperatures outlined in AASHTO PP 61. The cyclic fatigue test results of the small and standard specimens were very similar for the mixtures evaluated. The specimen-to-specimen variability of the standard and small specimen dynamic modulus and cyclic fatigue test results are comparable. While the small prismatic specimens yield equivalent results to the small cylindrical specimens,
A comparison between the AMPT dynamic modulus and cyclic fatigue test results of small specimens that were cored horizontally and vertically from gyratory-compacted samples was conducted to optimize the laboratory fabrication of the 38-mm diameter specimens. The comparison of dynamic modulus and fatigue test results of small specimens that were horizontally- and vertically-extracted from gyratory-compacted samples indicates that the effects of anisotropy are minimal. However, all of the horizontally-extracted small specimens exhibited fatigue failure at the specimen ends, outside the range of the gauges; the failure was likely due to the peripheral air void gradients in the gyratory-compacted samples. Therefore, small specimens should be vertically-cored from gyratory-compacted samples for the laboratory fabrication of small specimens. The optimal laboratory small specimen fabrication procedure yields four small specimens from a single gyratory-compacted sample.
To facilitate technology transfer, the research team collaborated with Instrotek, Inc. and IPC Global to develop gluing jig adapters and end platens to facilitate small specimen testing in the AMPT that are now commercially available. Proposed AASHTO standards for 38-mm diameter small specimen fabrication, AMPT dynamic modulus, and AMPT cyclic fatigue testing were also developed.
