Risk analysis of ODOT's HMA percent within limits (PWL) specification.

Final report.PDFTech ReportFHWA/07(03)Item 2182Paving materialsHot mix paving mixturesPavingRoad constructionRisk analysisSpecificationsSimulationMonte Carlo methodProbabilityStatistical analysisOklahomaOklahoma. Dept of TransportationMcTernan, William F.Cross, Stephen A.Oklahoma State UniversityOklahoma. Dept. of Transportation. Planning & Research DivisionUnited States. Federal Highway AdministrationNTL-HIGHWAY/ROAD TRANSPORTATION-MaterialsNTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and PerformanceNTL-HIGHWAY/ROAD TRANSPORTATION-Construction and MaintenanceNTL-REFERENCES AND DIRECTORIES-StatisticsUS Transportation CollectionThe Oklahoma Department of Transportation (ODOT) is considering switching its method of paymentfor hot mix asphalt (HMA) construction from their current procedure to a Percent Within Limits (PWL)specification. Quality characteristics are percent asphalt content (%AC), percent laboratory compacted airvoids (%AV) and percent roadway density (%RD). As the PWL methodology is new to both ODOT andits contractors, there is concern that differences between the methodologies could result in significantdifferences in the payments awarded for contracted HMA work.A risk based simulation approach, using Monte Carlo simulation, was structured to evaluate theprobabilities that PWL\u2019s generated randomly from a representative data base structured from ODOTpaving projects would be within \u201cacceptable\u201d limits. In this way ODOT would be able to assess theexpected performance and pay structure resulting from adoption of the PWL methodology against a set ofpreviously collected data.The data set provided by ODOT showed that contractors are, on average, hitting the target value forasphalt content but running 0.37 and 0.9 percent below the target value for %AV and %RD, respectively.Comparisons of the ODOT generated data to data sets found in the literature indicate that the ODOT dataset contains a high measure of control, while the simulation efforts showed that average performance forboth %AC and %RD would earn bonuses but less than full pay for %AV, and less than full pay for thelot.The draft PWL specification as currently written appears to not fairly reward a contractor for a highlevel of quality control. Recommendations for adjustments in the target limits and specification limitswere made. It is recommended that ODOT proceed slowly with the implementation of PWLspecifications until a sufficiently large data base can be obtained and the analysis repeated to verify anychanges to specification and target limits