Percent within Limits for Hot-Mix Asphalt on Low-Volume Agricultural Roads in Egypt

Abstract

Replacement of traditional, method-based specifications, which use average deviations to determine payment for materials, with percent-within-limits (PWL) specifications is essential for highway agencies. The PWL approach recognizes that construction material properties follow a normal distribution. When the appropriate standard deviations for each material property are known, then upper and lower test limits can be set around the target value of the material property. The contractor's payment is then based on the percentage of the test results that fall within these upper and lower limits. The main goal of this research study was to investigate the potential of developing a PWL specification system for low-volume agricultural roads in Egypt that included the selection of the material properties used for payment, the frequency of testing for these properties, development of the standard deviations and specification limits, and development of a payment system. The proposed system is capable of providing guidelines for determining acceptable quality level, rejectable quality level, and appropriate sampling frequency with operating characteristic curves based on known standard deviation to achieve the recommended risk levels. Field data were used to evaluate the implications of implementing PWL and the risk between PWL and the current specifications.