Development of a Tool for Determining Optimal Material Blends for Unpaved Road Wearing Courses

Experience over more than 100 years has shown that the appropriate choice of materials used on unpaved road wearing courses is critical for ensuring good and safe performance. Despite the availability of internationally developed performance-based specifications for unpaved road wearing course materials that will meet most requirements, many road agencies continue to use empirical specifications comprising grading envelopes and plasticity index ranges that do not provide an indication of expected performance. This complicates choosing the optimal blends of more than one material to obtain a better performing unpaved road wearing course, or determining the quantities of supplemental materials and the recycling depth when converting distressed, low-volume, paved roads to an engineered unpaved road standard. This paper introduces a performance-based approach to selecting unpaved road wearing course materials and a tool for optimizing blends of materials to meet specifications. The tool can be used in manual calculations, but rapid results can be obtained by using a web-based tool. The tool can be used for new unpaved road construction, regraveling existing unpaved roads, or for converting distressed paved roads to an engineered unpaved road standard. The tool is a companion to another tool that can be used for selecting chemical treatments for unpaved roads. It should be noted that the results from the tool are dependent on accurate input values for materials and road layer thickness, and consequently project investigations and materials testing are encouraged. Estimated values are likely to give misleading outputs.