Risk and expected pay factor analysis for assessing gap and dense-graded Superpave mixture specifications

Abstract

With the implementation of the Superpave mix design method, some state highway agencies have experienced significant problems in durability of hot mix asphalt (HMA) mixtures due to lower binder content. With the adoption of revised HMA specifications for the state of Maryland, it was desired to evaluate the potential risks to both agency and contractors. This was achieved by calculating the α and β risks through the construction of operating characteristic curves. Furthermore, simulation analyses were conducted for assessing the impact of the current HMA production quality on the specification tolerances percent within specification limits and mixture pay factor. The findings of this study are based on a large set of quality control and quality acceptance data since the analyses were based on 7 consecutive years of production and paving of Superpave mixtures in the state. The risk analysis indicated that the agency bears a significant risk on accepting low-quality material, and thus changes in the current State Highway Administration specification are needed and discussed in this paper. The simulation analysis illustrated that the correlation among mixture parameters had minimal impact on pay factors. Furthermore, it was concluded that the asphalt content has a more pronounced impact on the pay factor than any other mixture property. Overall, the analysis indicated that significant changes in the acceptance specifications are needed in order to reduce the risks of accepting lower quality, or rejecting good quality, HMA mixtures. Furthermore, changes of the order of 10%, 20% even 50% in specification tolerances represent feasible and currently achievable levels of production quality by the paving industry, and thus no increase in the production cost of HMA is expected. Such change in the specification tolerances is expected to promote an overall improvement in quality of Superpave mixtures in the state and the region. The approach used in this study can be used by highway agencies and the pavement industry to evaluate risks related to the acceptance of hot asphalt mixtures. The pay factor analysis can aid in adjusting pay schedules and specification tolerances for accepting higher quality materials.