Evaluation of Percent-within-limits Construction Specification Parameters

Various state agencies, including the Florida Department of Transportation are rapidly moving towards a more practical percent-within-limit (PWL) specification. The current specification is based on unknown standard deviation because the range of variability is high and difficult to estimate. The parameters of PWL specifications, the acceptable quality level (AQL) and the rejectable quality level (RQL), which are selected based on engineering judgment, could lead to buyer and seller risk level greater than the values recommended by American Association of State Highway Transportation Officials (AASHTO). The purpose of this paper is to provide guidelines to determine AQL, RQL and appropriate sampling frequency using operation characteristic (OC) curves based on known standard deviation to achieve the recommended risk levels. This was demonstrated by selecting two parameters (a) asphalt content and (b) the percent passing 2.36 mm sieve, of ten Superpave mixtures. The study showed that an excessive sampling frequency of ten per lot was required to achieve the AASHTO recommended buyer's risk of 5%. In addition, a framework was developed to evaluate the sampling plan and its associated the risk level based on the construction and material variability observed in the field.