| Summary: | There has long been an understanding that differences exist between the asphalt mixture designed in the laboratory and the asphalt mixture produced in the asphalt plant. Additionally, variations between the mixtures compacted in the laboratory and mixtures extracted from the roadway have been observed. There is concern that asphalt mixtures produced through the plant and further compacted on the roadway are not representative of asphalt mixtures designed in the laboratory. This is especially a concern when field specimens (i.e. roadway cores) are used to evaluate the quality of the asphalt mixture. For the first time, a national effort has been made to quantify the magnitude and cause of the differences of commonly measured parameters of asphalt mixtures among specimen types. This was accomplished by evaluating the volumetric and mechanistic properties of three specimen types (design (LL), production (PL), and construction (PF)) from 11 mixtures from various states throughout the country. Variations in the production process were identified and varied throughout the mixtures. Specifically, the return of baghouse fines, time delay in specimen fabrication, aggregate absorption, aggregate hardness, and stockpile moisture content were the processes evaluated. The results of the research provided typical magnitudes of differences of many volumetric and mechanistic properties. These differences were summarized into recommended single operator tolerance values and conversion factors among the three specimen types. The effects of process based factors were not as pronounced as originally hypothesized. The results of a contractor survey showed that the contractors are making adjustments based on their experience with the processes in their region. For example, a contractor who uses soft aggregate accounts for the aggregate breakdown during design. Also, contractors who typically return baghouse tend to use baghouse in their mixture design specimens. Furthermore, the process-based factors evaluated did not have a significant effect on comparisons between production and construction. However, significant impact on mechanistic properties were noted for specimens compacted in the laboratory when compared to specimens compacted in the field. The effects of these mechanistic differences on predictive pavement performance were also evaluated.
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