Interlaboratory Study for the Indirect Tensile at High Temperature Test and Ideal Rutting Test

About the report:

The Indirect Tensile at High Temperature (IDT-HT) test and Ideal Rutting (IR) test were recommended for screening rut-susceptible asphalt mixtures in the Balanced Mix Design (BMD) process, based on a research study by the Virginia Transportation Research Council (VTRC). The Virginia Department of Transportation (VDOT) is in the initial stages of implementing the IDT-HT test as part of the BMD initiative for dense-graded surface asphalt mixtures with unmodified asphalt binders. However, for full implementation, additional considerations such as fine-tuning the test procedure and determining precision estimates of the test method are necessary, and this study specifically addressed these aspects for the IDT-HT and IR tests. The work included three tasks: a sensitivity assessment, an interlaboratory study (ILS), and a proficiency study. All tasks were based on evaluation of specimens from four mixtures with varying rutting potentials.

The sensitivity assessment investigated factors such as specimen conditioning environment, loading rate, loading frame, and storage on test results. Findings revealed significant differences in specimen conditioning time between water bath and environmental chamber, and water bath-conditioned specimens consistently showing lower parameter/index values. Additionally, the repeatability characteristics of both tests were not significantly affected by conditioning environment, loading rate, loading frame, or storage. Moreover, loading rate, loading frame, and storage did not significantly affect IDT-HT and IR test results, especially in the context of single-operator precision estimates. Furthermore, preliminary performance criteria were established, requiring minimum strength and RT index values of 100 kPa and 62 for IDT-HT and IR tests, respectively, for specimens conditioned in a water bath at 54.4°C. Based on the outcome of this task, a Virginia Test Method was developed for the IDT-HT test.

The ILS was conducted with 10 and 9 laboratories for the IDT-HT and IR tests, respectively, employing compacted specimens conditioned in a water bath. Some laboratories were provided with additional specimens for testing after environmental chamber conditioning to evaluate further the effect of the conditioning environments on test results. The precision estimates and statements for both tests were developed. The results also confirmed the sensitivity assessment’s conclusion that different conditioning environments notably influence test results.

Proficiency testing was conducted for the IDT-HT test only, involving 31 VDOT and contractor laboratories in Virginia, and employed compacted specimens from a single mixture. The results revealed that 93.5% of the laboratories demonstrated satisfactory performance, indicating proficiency in the IDT-HT test.

The study recommends that VDOT should adopt (I) the developed Virginia Test Method for the IDT-HT test, (II) the developed precision estimates and statements for the IDT-HT test, and (III) the initial minimum strength criterion of 100 kPa for IDT-HT testing of dense-graded surface asphalt mixtures with unmodified asphalt binders in accordance with the developed Virginia Test Method.

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