Round Robin Testing Program for the Indirect Tensile Cracking Test at Intermediate Temperature: Phase I

About the report:

The indirect tensile cracking test (referred to herein as “the IDT-CT”) was recommended for use in balanced mix design (BMD)specifications to address cracking susceptibility of dense-graded surface mixtures with A and D designations in Virginia. The test method for the IDT-CT, ASTM D8225-19, does not currently contain a precision statement for the test. This creates potential issues if different test results are measured by individual laboratories conducting testing on the same asphalt mixture.

The purpose of this study was to determine and develop precision estimates and statements for the cracking tolerance index (CT index) of asphalt mixtures determined by performing the IDT-CT at intermediate temperature in accordance with ASTM D8225-19.  In addition, precision estimates and statements were developed for the fracture strain tolerance index (FST index), strength (St), and cracking resistance index (CRI)from the same test data.  The effects of device and loading rate on the selected IDT-CT indices were also investigated.  Moreover, a preliminary assessment of the impact of the shelf life of compacted specimens on the selected IDT-CT indices was conducted. These objectives were achieved by conducting a two-stage round robin study.  Stage I focused on non-VDOT(Virginia Department of Transportation) laboratories, and Stage II, conducted 1year later, focused on VDOT laboratories.

In Stage I, only 14 of 41 participating laboratories submitted results (16 datasets) for both mixtures in full accordance with ASTM D8225-19.  The initial data quality resulted in performing the analysis on two groups of data to calculate precision estimates.  The precision estimates for the CT index, FST index, St, and CRI were calculated, and the corresponding statements were developed.  A significant drop in the precision parameter (i.e., coefficient of variation or standard deviation) of the IDT-CT indices for both single-operator and multi-laboratory conditions were observed when data trimming was performed. 

The relatively higher variability observed for data collected in Stage II when compared to the variability for data collected in Stage I could be attributable to a relative lack of operator experience; the need for training; and potential changes in material properties during the storing, handling, shipping, or testing process.  However, the analyses and comparisons of data collected in Stage I and Stage II indicated that there was no significant impact of 1 year of climate-controlled storage of compacted specimens on the calculated IDT-CT index.

The study recommends that a second phase of the round robin for the IDT-CT be conducted to assess the impact of variability induced because of specimen preparation to better reproduce the actual state of the practice during design and production.  Further, the study recommends that a more comprehensive effort to assess the impact of loading rate on the IDT-CT results be initiated. Further, the study recommends that the impact of testing IDT-CT specimens under saturated surface dry conditions as compared with dry conditions be assessed.  Finally, hands-on training and demonstration of the laboratory tests (e.g., the IDT-CT)being considered by VDOT as part of the BMD initiative are recommended.   

Authors