VTRC UPDATE On the other hand, the O-J mix test results statistically differed from most of the variation mixes, primarily due to variations in asphalt content. In some combinations, the differences even exceeded the d2s limits of the multi-laboratory precision estimate, a considerably wider boundary than the d2s single operator boundary. The decrease and increase in asphalt content with no change in gradation resulted in statistically different outcomes for 78.6% and 64.3% of the L-J and H-J variation mixes, respectively. Although the interaction mixes had statistically different results, this was also mainly driven by the asphalt content changes. Generally speaking, the results indicate that the variations in CT index values caused by asphalt content variation within the production tolerance limits are significantly wider than the test variability, increasing the risk of failing the IDT-CT test during production, even when asphalt content varies within acceptable tolerances. Industry Implications What does this mean for the industry? Overall, the variability for each material being used in a mix needs to be understood, monitored, and accounted for during design and production to minimize the risk of a BMD failure. While the study recommendations address the IDT-CT test, they demonstrate the importance of good design practices and quality control to ensure passing results for all BMD tests. Designing to account for material and test variations, along with improved aggregate stockpile management, RAP management, and use of consistent asphalt sources will be keys for success in producing consistent, acceptable BMD mixes. Study Conclusions: • From the perspective of the IDT-CT test, current material tolerance limits on aggregates are also appropriate for the BMD concept. The risk of IDT-CT failure is minimal when mix gradations remain within the tolerances for acceptance. However, sudden deviations or continuing drift from target values may result in test failure. • From the perspective of the IDT-CT test, asphalt content during production is the single most influential factor influencing potential failures. Designing a mix with both suitable asphalt content for performance and consistent control of the asphalt content during production is extremely important to meet IDT-CT requirements. Also, a consistent asphalt source will minimize the likelihood of changes in the asphalt binder performance that may adversely impact test results. • Asphalt mixes designed with an average performance result within the single-operator precision estimates of performance thresholds may risk failures during production due to inherent material and test variability. This means that, especially for mixes with CT index values within a single operator tolerance of the CT index limit, additional quality control efforts may be necessary to ensure the mixes remain balanced during production. The single operator variability limits for the design CT index is 70 to 88 CT units. If mixes are designed within this range, the risk for failures due to material and test variability increases significantly. These risks can be mitigated by designing for a CT index value of at least one d2s above the minimum criteria (a value of 88) or by improving quality control of materials and production. △ continued from page 11 12 FALL/WINTER 2023
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