VAA Virginia Asphalt Fall/Winter 2020

VAASPHALT.ORG 15 Stacey Diefenderfer, Senior Research Scientist, Virginia Transportation Research Council Ilker Boz and Jhony Habbouche, Research Scientists, Virginia Transportation Research Council ROUND ROBIN PHASE 1 Evaluating Indirect Tensile Test Data(IDT) for CT index As part of the Virginia Department of Transportation's efforts to implement Balanced Mix Design, the indirect tensile (IDT) cracking test at intermediate tem- perature (formerly called the IDEAL-CT test) is required to assess the cracking susceptibility of mixtures during mix design and production. As a relatively new test method, there is no precision statement available to date, leaving test operators unable to assess their test results' repeatability and reproducibility. In light of this issue, Virginia Transportation Research Council (VTRC) and Virginia Asphalt Association (VAA) began collaborating in 2019 to develop a round robin testing program for the IDT cracking test to generate data to create precision statements. Phase I of the program was initiated in early 2020, and two mix designs having two significantly different CTindex results were selected for use in the study. These designs were used to fabricate sets of test specimens shipped to participants in early May 2020. A total of 41 laboratories participated in the round robin. These labs consisted of VDOT, contractor, other DOT, and independent testing labs. Several labs received more than one set of test specimens per mix to perform testing usingmachines or load frames fromdifferent manufac- turers. 46 sets of test specimens for each mixture were distributed, and seven devices from different manufacturers were evaluated. Participants were asked to report detailed test results, including specimen measurements and bulk specific gravities, equipment manufacturer and model, loading mode (screw-drive or servo- hydraulic), and CTindex values. Perhaps most importantly, partici- pants were asked to submit the raw data files collected by their equipment in addition to reporting CTindex values for each specimen. These data files were used to perform quality checks on the data before further analysis. The quality checks revealed that only sixteen of the forty-six sets of test results fullymet the test requirements; an additional fourteen sets of data met all requirements except for the loading rate. Test Requirements ASTMD8225-19 Determination of Cracking Tolerance Index of Asphalt Mixture Using the Indirect Tensile Cracking Test at Interme- diate Temperature sets forth the requirements for conducting the IDT cracking test. In summary, the specification requires specimens to be 150mm ± 2mm diameter cylinders having a height of 62 ± 1mm. VDOT specifies specimen air void contents of 7% ± 0.5% and test temperature of 25ºC. Test specimens are placed in the test fixture, continues on page 16 △ FIGURE 1: Example of a compliant (a) load-displacement curve and (b) displacement-time curve.