A Case Study for Developing Specification Limits for Hot-Mix Asphalt and the Impact on Pay Factors

Abstract

Specification limits utilized in percent within limits (PWL) specifications are developed by highway agencies, and used to determine the percent of hot-mix asphalt “Lot” within specified limits, and used later to make payments to contractors. Development of specification limits must be based on the typical variability of test results used in PWL specifications. Using overly wide or tight specification limits to evaluate the new pavement could cause risk for highway agencies or contractors, respectively. This study explains how to develop new specification limits and evaluate the current ones by using different data sets from projects conducted by the Oregon Department of Transportation (ODOT).

All the calculations and analysis performed in this study are based on data collected from 15 different projects and contractors. Typical standard deviations (within-process variability) and “target miss” variability (how the mean values of test results are variable around the target values) have been determined for aggregate gradation, asphalt content, and in-place density which are considered as pay-elements in ODOT standard specifications. New specification limits have been developed based on typical variability. Then current specification limits adopted by ODOT were evaluated and compared to the proposed new specification limits. Three projects have been utilized to investigate the impact of using the proposed new specification limits on final payment. Using the developed new limits will lead to a decrease in the final costs of the projects. Also, ODOT will see an improvement in contractors’ performance as these new limits will encourage the contractors to use high-quality materials with a low variability. It is recommended that DOTs consider regular evaluation of their specification limits by using new records and modify the specifications when needed.