Interlaboratory Study for the Indirect Tensile Cracking Test at Intermediate Temperature: Phase II

About the report:

The indirect tensile cracking test (IDT-CT) is performed in accordance with ASTM D8225-19.  This test method does not currently contain precision estimates and associated statements. This creates potential issues when test results are different among individual laboratories conducting testing on the same asphalt mixture.  In2020, Phase I of an IDT-CT interlaboratory study was conducted by researchers at the Virginia Transportation Research Council to establish precision estimates and statements for several indices associated with the IDT-CT (i.e., the cracking tolerance index, fracture strain tolerance index, strength [S_t],and cracking resistance index) through the evaluation of two asphalt mixtures.  Phase I involved the evaluation of specimens fabricated and compacted by a third party laboratory and sent to participant laboratories for testing only.

The purpose of the current study (i.e., Phase II) was to build on the efforts undertaken as part of the Phase I study. The major objective of both phases was to determine acceptable variability and establish precision estimates and statements for IDT-CT results.  Phase II also evaluated the impact of additional critical factors and their interactions on the IDT-CT results, such as specimen fabrication and preparation and equipment type (PhaseII.1); specimen conditioning method (Phase II.2); and loading rate and data collection frequency (Phase II.3).

In Phase II.1, 24 of 50 participating laboratories submitted results (29 of 55 data sets, or about 53% of the submitted data) for both mixtures that were in full accordance with the requirements of ASTM D8225-19.  As compared to Phase I.1, this was a significant decrease in the percentage of participants that submitted non-compliant data and a significant increase in the percentage of participants that submitted data that fully conformed with the requirements of ASTM D8225-19.  Phase II.1 involved the evaluation of test results obtained from specimens that were fabricated by the participant laboratories from loose mixture that was produced and distributed by a third party laboratory along with detailed instructions for specimen fabrication and testing.  The precision estimates for the IDT-CT indices were determined. The precision estimates for single-operator conditions were similar whether or not specimens of a given mixture were fabricated by the same laboratory.  Specimen preparation introduced additional variability in the precision estimates for multi-laboratory conditions.

Phase II.2 showed that the indices tested were not dependent on the type of conditioning method used (i.e., dry vs. wet). Moreover, Phase II.3 showed that the indices were not dependent on the loading rate applied within a range of 50 ± 3 mm/min or on the frequency of data collection.

The study recommends that the Virginia Department of Transportation (VDOT) (1) include the developed precision estimates and statements in their balanced mix design specifications and adopt them for acceptance; (2) allow use of the wet conditioning method with the IDT-CT; (3) extend the allowable loading rate tolerance for the IDT-CT from50 ± 2 mm/min to 50 ± 3 mm/min; (4) establish an annual proficiency testing program for the IDT-CT; and (5) routinely offer hands-on training and demonstrations of the laboratory tests being considered by VDOT as part of the balanced mix design initiative.  

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