Evaluating Percent Within Limits (PWL) Specifications Volume 1: HMA Specifications (FHWA-OK 06(05)v1 2172)

Abstract

The Oklahoma Department of Transportation (ODOT) has let four paving jobs for construction where percent within limits (PWL) specifications were employed. The PWL specifications are intended to be used as part of the Quality Assessment program to determine the statistical probability of conformance to specified material properties and construction details, and to base the pay factor (PF) off the probability of conformance to the specifications. There is a need to evaluate the performance of the PWL specifications and assess the suitability of the PWL specifications for future jobs. Two hot mix asphalt (HMA) projects were let for construction using the proposed HMA PWL specifications. In addition to the normal quality control, acceptance and assurance sampling and testing, the ODOT Materials Division performed more extensive sampling and testing on randomly selected lots from each pavement which were designated "super lots." The data obtained were to analyzed 1) to determine whether the PWL specifications are working as intended, 2) to recommend changes to the PWL specification if necessary, including possible changes to the limits and ranges contained within the specifications, and 3) to recommend whether ODOT should continue to pursue PWL specifications as matter of policy in their overall QA/QC programs. The examination of the draft HMA PWL specification and analysis of the super lot data demonstrated that the specification concept is sound and will require very little additional work to develop fully. The F and t-test procedure for process verification is recommended for use.