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Evaluating Specification Limits

The results of a study to evaluate the existing South Carolina Department of Transportation (SCDOT) hot-mix asphalt quality assurance specification are presented. When the existing specification was developed, assumed values were used for the standard deviations needed to establish the specification limits. Acceptance test results from 39 different projects were analyzed to determine standard deviation values that were being obtained for asphalt content, air voids, voids in mineral aggregate, and density. For each project, standard deviation values were calculated for each lot, and these were then pooled to obtain a typical value for the project. The target miss variability, that is, the ability of contractors to center their processes on the target, was also determined for each acceptance characteristic. The project and target miss values were then used to establish a typical process standard deviation value for each of the four acceptance characteristics. All these standard deviation values proved to be smaller than those initially assumed and resulted in the SCDOT narrowing the specification limits for each of the acceptance characteristics. The project standard deviation values justified that these changes were made to reflect the actual variability that was present and not simply to tighten the specifications.