Center for Transportation Research and Education

About the research

Granular-surfaced roads and shoulders frequently experience extensive surface damage caused by heavy agricultural traffic loads, freeze-thaw cycles, and wet-dry cycles, which increases maintenance requirements and reduces safety. The goal of this study was to cost-effectively recycle existing degraded granular surface materials while improving them to the optimum gradations and plasticity that will provide the best performance and durability. In this study, a series of laboratory tests was first conducted to quantify the influence of gradation and index properties on the mechanical performance of commonly used materials for granular-surfaced roads and shoulders. Using the optimum gradations and plasticity index ranges as design targets, field demonstration test sections were constructed and tested over the 2016-2017 seasonal freeze-thaw period to assess the performance of several selected materials and construction methods.

Results of this study clearly demonstrate how index properties of granular materials, such as the maximum aggregate size, gradation, plasticity, and aggregate quality, significantly influence mechanical characteristics and field performance. Based on the laboratory and field evaluations, a performance-based free design method for determining the gradation and plasticity of granular surface materials was developed. To help secondary roads agencies implement the proposed method, a Microsoft Excel-based program was also developed to optimize the mixing ratios of two or three available quarry materials with a chosen thickness of existing surface material to reach the optimum gradation in terms of strength. To help secondary roads agencies more easily and accurately determine the plasticity of the existing granular surface materials to be recycled, various laboratory testing methods were statistically evaluated, and recommendations are provided. In addition to the optimized gradation and plasticity design, a new laboratory testing method was developed in this study to evaluate the quality, morphology changes, and compaction characteristics of granular materials under simulated compaction and traffic loading conditions.