Laboratory validation of a gradation design concept for sustainable applications of unbound granular materials in pavement construction

Unbound aggregates are becoming increasingly scarce and expensive due to the loss of rock quarries and gravel mines to other land uses; therefore, it is important to engineer and optimize aggregate gradations for the targeted end-performances. This paper is aimed at validating in the laboratory a newly introduced gradation design concept intended for controlling structural assembly strength (stability) and drainage characteristics (field drainability). Aggregate gradation optimizations were studied for two applications: (1) unbound permeable aggregate base (UPAB) and (2) mixing proportions of blending fine granite tailings (FGT), a typical crushed granite mining by-product that has long been considered “waste” materials, with coarse crushed granite aggregates (CCGA). To this goal, five representative gradations were first selected according to the current Minnesota DOT UPAB gradation band, and the effects of different UPAB gradation designs on the shear strength properties and particle breakage potential were investigated using a large-scale direct shear test device. In the second application, one of the common quarry byproduct wastes (i.e., FGT), were mixed with CCGA in varying percentages to explore their potential use for building pavement foundations. Both laboratory permeability and large-scale monotonic triaxial compression tests were performed to investigate the effects of blending proportions on the stress-strain behavior. Based on the test results, the optimum aggregate gradations recommended by the new gradation design concept provided enhanced stability without compromising drainability. The new gradation design concept, hence validated in this study with produced optimum gradations, is expected to achieve sustainable and beneficial unbound aggregate applications for cost-effective long-life pavements.