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1 S U M M A R Y Mix properties that deviate appreciably from design during the production and con- struction of asphalt mixtures often lead to premature pavement distress or even failure. The objective of this project was to quantify sources and causes of variability in the mea- surements of volumetric and mechanical properties of dense-graded asphalt mixtures for three types of specimens that may be encountered during design, production, and con- struction. In addition, the effects of variation among specimen types on pavement perfor- mance prediction were evaluated. This was accomplished by evaluating common volumetric and mechanical properties of the three specimen types (design, production, and construc- tion) from a nationwide compilation of 11 mixtures from various states throughout the United States. Variations in key production process factorsâspecifically the return of bag- house fines, delay in specimen fabrication, aggregate absorption, aggregate hardness, and stockpile moisture contentâwere evaluated in this study. For each mixture, the following volumetric and mechanical properties were evaluated for the three specimen types: ⢠Volumetric properties: air voids, voids in the mineral aggregate, voids filled with asphalt, aggregate bulk specific gravity, mixture maximum specific gravity, asphalt binder content, and gradation. ⢠Mechanical properties: loaded-wheel test (LWT) rut depth, axial dynamic modulus, and indirect tensile test (IDT) dynamic modulus. Based on the experimental, statistical, and analytical analyses conducted in this study, the following conclusions may be drawn: ⢠The effects of the process-based factors (i.e., return of baghouse fines, delay in specimen fab- rication, aggregate absorption, aggregate hardness, and stockpile moisture content) on the volumetric and mechanical properties were not as pronounced as originally hypothesized. Results of a contractor survey showed that contractors are actively making adjustments based on their experience with the processes in their region. ⢠With respect to the effects of process-based factors on mechanical properties, it was con- cluded that these factors did not have a significant effect on the differences of mechanical properties among the three specimen types. The lack of the observed effects of process- based factors may result from the variations in the mechanical properties being strongly controlled by compaction effort. Many of the individual mixture comparisons showed that plant-mixed, field-compacted (PF) specimens were significantly softer than laboratory- mixed, laboratory-compacted (LL) and plant-mixed, laboratory-compacted (PL) speci- mens, even though the air voids were the same for both sets of specimens. This finding was Comparing the Volumetric and Mechanical Properties of Laboratory and Field Specimens of Asphalt Concrete
2attributed to differences in compaction effort and confinement conditions between the two compaction processes (laboratory and field). ⢠Slight differences in gradation, while within state tolerances, may lead to significant differences in important volumetric properties (e.g., air voids and voids filled with asphalt). ⢠Tolerance recommendations were developed based on the average difference among spec- imen comparisons for the 11 national mixtures evaluated. Based on these findings, speci- fying agencies should evaluate and adjust their current tolerance values. These tolerance values encompass mixtures from around the country. Therefore, more regional values may be appropriate. ⢠Conversion factors among the three specimen types were developed for the loaded-wheel test (LWT). Conversion factors can be used to assess whether or not an as-built mixture will be expected to meet performance indicators developed with the laboratory design. The conversion factors (see Table S-1) indicate that laboratory-compacted specimens typically resulted in 33% less rut depth than field-compacted specimens. Therefore, if the LWT rut depth of a PF specimen is required to be 6 mm at 20,000 passes, the laboratory- compacted mixture should have a rut depth of 4.5 mm at 20,000 passes. This relationship will be important as agencies transition toward performance-based specifications. ⢠The modulus determined from IDT was generally 80% of the modulus determined from axial testing. The difference between axial and IDT moduli determined at high tempera- ture was much more variable, which was likely due to the increased influence of the load- ing mode at high temperature; some mixtures exhibited both higher and lower values of modulus when comparing IDT dynamic modulus with axial dynamic modulus. Comparison Conversion Factor Design (LL) / Producon (PL) 1.0 Design (LL) / Construcon (PF) 0.75 Producon (PL) / Construcon (PF) 0.75 Table S-1. Conversion factors.
