Effect of particle size distributions on the performance of unbound granular materials

Abstract

Particle size distribution (PSD) is a crucial factor influencing the mechanical behavior of unbound granular materials (UGMs). It also governs the drainage properties, as well as moisture and frost susceptibility, of UGMs. Therefore, optimizing the PSD is essential to achieve the desired properties for use in pavement structures. In this study, seven different PSDs of a crushed rock aggregate were evaluated for mechanical per-formance using repeated load triaxial (RLT) tests. Three of the PSDs were designed based on the widely used Fuller-Thompson equation, employing different shape factors. The other three were open-graded, drainable types intended for permeable pavements. The final PSD was generated using an optimization method originally developed for asphalt concrete mixtures. The RLT tests were conducted at the optimal moisture content and at 95% of the maximum dry density for each PSD, determined through the modified Proctor method. The analysis focused on the resilient modulus and resistance to permanent deformation for each PSD. The results demon-strated that the well-graded PSD with grading coefficient, n = 0.45 exhibited the highest resilient modulus and the greatest resistance to permanent deformation. The amount of fines influenced performance significantly: an excess of fines, as well as their removal to create drainable materials, negatively impacted mechanical behavior. Additionally, the maximum particle size also showed some effect. The optimization method applied in this study did not perform well, highlighting the need for further refinement of the approach for implementation in UGMs.