Risks with Multiple Pay Factor Acceptance Plans

It is rare that a state highway agency (SHA) uses only a single quality characteristic to determine the pay factor for a lot. This paper deals with two specific types of risk associated with the use of multiple quality characteristics: remove and replace provisions and the effect on pay factor determination if the quality characteristics are correlated. First, computer simulation studies showed that typical acceptance plan provisions that call for lot removal and replacement at a given quality level, such as a percentage of material within specification limits of 60 or 50, can place much greater risk on the contractor when they apply to multiple characteristics. An approach to correct this problem was developed. Next, a total of 1,742 sets of asphalt content (AC), air voids (AV), and voids in the mineral aggregates (VMA) test results were analyzed for correlations. VMA values were positively correlated with both AV and AC values. Additional simulation studies showed that while this correlation does not affect the long-term average pay factor for a lot, it increases the variability in the individual lot composite pay factors. The positive correlation also increases the weighting of AV and AC in a composite pay equation. The use of multiple quality characteristics for determining the pay factor for a lot introduces sources of risk that are not present when only one characteristic is used. Any SHA that uses multiple pay factors should consider performing the types of remove and replace and correlation analyses presented in this paper.