Development of a flattened indirect tension test for asphalt concrete

The Indirect Tension Test (IDT) is frequently used in civil engineering because of its benefits over direct tension testing. During the Strategic Highway Research Program (SHRP), in the mid-1990's, an IDT protocol was developed for evaluating tensile strength of Hot Mix Asphalt (HMA) mixtures. However, with the increased use of finer aggregate gradations and polymer modified asphalt binders in HMA mixtures, the IDT results can be misleading because of crushing failure under the narrow loading heads. For such mixtures the 150-mm diameter, 50-mm thick, cylindrical specimens tends to fail in crushing beneath the loading heads versus the desired indirect tension at the center of the specimen. Therefore, a new specimen configuration is proposed for strength testing of HMA. In place of the loading heads at the top and bottom, the specimen is trimmed to produce flat planes with parallel faces, creating a "flattened-IDT." A viscoelastic finite element analysis of the flattened configuration was performed to evaluate the optimal trimming width. In addition, the numerically determined geometry was verified by means of laboratory testing of 3 different HMA mixtures. This integrated modeling and testing study shows that for the HMA mixtures with fine aggregate gradations and compliant asphalt binders used in this study, the flattened IDT eliminates the severe crushing observed in the regular IDT. It is recommended that further testing and analysis be performed on the flattened IDT arrangement, leading to a revision of the current AASHTO standard for IDT testing as asphalt mixtures.