VTRC UPDATE HOW PRODUCTION AND VARIABILITY IMPACT BALANCED MIX DESIGN Stacey Diefenderfer, Ph.D., PE, Associate Principal Research Scientist, Virginia Transportation Research Council Ilker Boz, Ph.D., Research Scientist, Virginia Transportation Research Council The Virginia Transportation Research Council (VTRC) recently completed the second phase of a project investigating how materials variability impacts balanced mix design (BMD) test results during production. While the first phase of the work was performed at the National Center for Asphalt Technology (NCAT) and used lab-produced mixes to evaluate the impact of acceptable volumetric variations on BMD test results, this second phase overlaid the information with actual production data. The results of this effort demonstrate how variability in mix properties during production can impact BMD test results. Only Cantabro mass loss and Indirect Tensile Cracking Tolerance (IDT-CT) test results are addressed; and only one sample in the study failed the asphalt pavement analyzer (APA) rut depth requirement, which supports the anecdotal concerns that mixes are being under-asphalted. Thirteen production mixes, of which ten were BMD mixes and three were non-BMD mixes, are discussed herein. Naming conventions were used for the mixes that consist of a letter indicating a single plant (A-F), reclaimed asphalt pavement or RAP content xR (where x indicates RAP content by weight), asphalt binder type PG yy-yy (where yy-yy indicates performance grade), and, if present, an additive (recycling agent, RA, or For charts, black stars indicate the design JMF values and black plus signs indicate the O-J lab-produced values. Solid circles and squares indicate individual sample results, while hollow blue circles and red squares show the mix average. Green and orange symbols indicate the lab-produced mix variations. Mass Loss Figure 1 shows the lab-produced mix test results for mass loss overlaid with the production and reheat sample values. From design and production, one JMF and seven reheat samples failed the mass loss requirement of 7.5%. The failing JMF (B 30R PG 64S-22) was a non-BMD mix. No production samples failed the mass loss requirement. The average production mass loss values were always less than average reheat mass loss values, likely due to the additional aging introduced during the reheating process. Average production values were also less than JMF mass loss values, except for three mixes: C 35R PG 58-28, E 35R PG 58-28 RA, and E 35R PG 58-28 Softening Oil + Fiber. In seven of the thirteen cases, the reheat sample mass loss values bracket the JMF values. Only two mixes (E 35R PG 58-28 Softening Oil + Fiber and F 40R PG 58-28) are shown to have all production samples, reheat samples, and lab-produced variations pass the mass loss requirement. The lab-produced mixes show the influence of materials variability on the mass loss results. Only seven of the thirteen O-J mixes passed the mass loss criterion. All lab- produced mixes were made from aggregate and RAP sampled at production, except for a few instances when resampling was required. The variations in stockpile gradation and RAP likely resulted in the O-J mass losses being different from those submitted with the JMF. Considering the controlled variations of the lab-produced mixes, the L-J and O-C variations were most likely to fail mass loss, together comprising 53% of the failures and followed by the O-J and O-F variations at 19% other additive). Production samples were not reheated before specimen fabrication; reheat samples cooled after sampling and were reheated for specimen fabrication. For the lab-produced mixes, O indicates optimum asphalt content; L and H signify higher and lower asphalt content, respectively; C and F designate coarser and finer gradation, respectively; and J denotes the job-mix formula (JMF) gradation. Figure 1. Mass Loss. Dashed line indicates a maximum acceptable mass loss. R = RAP content; PG = performance grade; RA = recycling agent; JMF = job mix formula; O = optimum asphalt content; L = low asphalt content; H = high asphalt content; C = coarse gradation; F = fine gradation J = JMF gradation. 10 FALL/WINTER 2023
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