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. In the mid-1990s, an IDT protocol was developed for evaluating tensile strength and creep properties of asphalt concrete mixtures, as specified by the American Association of State Highway Transportation Officials (AASHTO) in AASHTO T322. However, with the increased use of finer aggregate gradations and polymer modified asphalt binders in asphalt concrete mixtures, the validity of IDT strength results can be questioned in instances where significant crushing occurs under the narrow loading heads. Therefore, a new specimen configuration is proposed for indirect tension testing of asphalt concrete. In place of the standard loading heads, the specimen was 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 three asphalt concrete mixtures in two flattened configurations. This integrated modeling and testing study showed that when using fine aggregate gradations and compliant asphalt binders, crushing is significantly reduced while maintaining tensile stresses near the center of the specimen. Furthermore, creep compliances were evaluated using the flattened IDT and compared with those obtained following AASHTO T322. Some variation was observed between the creep properties evaluated from the different geometries, particularly for higher compliance values. As a preliminary assessment, the flattened IDT seems to be a suitable geometry for the evaluation of indirect tensile strength of asphalt concrete. Further testing and analysis should be performed on the flattened IDT arrangement for evaluation of the creep compliance. This study provides an initial step towards a possible revision of the current AASHTO standard for IDT testing of asphalt concrete mixtures.