This report clarifies the Washington State Department of Transportation (WSDOT) hot-mix asphalt (HMA) statistical acceptance specification’s statistical basis and how its key components influence overall specification performance and contractual pay.
The WSDOT specification uses a variable sampling plan to measure in-place density, asphalt content, and aggregate gradation. Specification bands on these three quality characteristics are reasonable and consistent with typical material, sampling, testing, and construction variability. Pay is determined by calculating a percentage within limits (PWL), then applying a series of parabolic pay equations, depending on sample size.
The methods used by WSDOT balance risk between the contractor and WSDOT well but result in two issues that differ from similar specifications: (1) Expected pay for material produced at acceptable quality level (AQL) is greater than 1.0, and (2) WSDOT’s AQL is 95 PWL, but contractors seem to consistently produce material near 90 PWL The issues are not critical, and therefore, the specification should not be changed. However, if the third and final report in this series, which studies quality characteristics to be measured for Superpave design mixes, results in major recommended changes, then we recommend reviewing the AQL and pay factors to bring the specification into line with actual practice.