Developing Specification Limits for Hot Mix Asphalt Properties and Impact on Pay Factors

Abstract

Specification limits are developed and employed by the departments of transportation to determine the percent of Lot quantity within specified limits, and then used later to make payment to contractors. Developing these limits must be based on the typical variability of data used in percent within limit (PWL) specification. Using overly wide or tight limits could create risk for highway agencies or contractors. This paper explains how to develop new specification limits for hot mix asphalt pay-elements. Data sets used in this paper are representing most of the contractors who are working with the Oregon Department of Transportation (ODOT). The data sets were collected from 15 projects performed in the state. Two types of variabilities have been considered in the development process; within-process variability and target-miss variability [how the mean value of test results is variable around the target value (TV)]. One project was selected as a case study to present the impact of the new proposed specification limits on the payment. The new proposed specification limits contained with this paper will balance the risk between ODOT and its contractors. With the new specification limits, it is likely less bonus will be paid; only awarding high performing contractors with a maximum bonus of 5%. Over time, this has the potential to motivate others to construct pavements closer to specification TV and with a lower variability.