Investigating meso-scale pore characteristics of unbound permeable aggregate base materials considering gradation effects

Unbound aggregate materials featuring large pores are increasingly used to construct permeable bases due to their desired drainage performance. However, the influence of particle packing structure for meso-scale three-dimentional pore characteristics of such materials remain unexplored. To address such deficiency, the original vibratory plate compaction tests were conducted on laboratory specimens of unbound permeable aggregate base (UPAB) materials with three different representative gradations (or particle packing structures). The compacted specimens were subjected to X-ray computed tomography (XCT) scanning for reconstructing their three-dimensional (3D) digital models and then extracting their internal pores. The parameters quantifying internal pore structure characteristics were computed accordingly including surface porosity, equivalent pore volume, and pore shape indices, while their three-dimentional morphology and distributions among different gradations were comparatively analyzed. The results are drawn. The internal surface porosity values of different specimens are generally symmetrically distributed along the height direction with those at the two ends being large and those in the middle being relatively small. The surface porosity values of skeleton dense specimens are at the lowest level and have the least distributed difference, but that of skeleton gap specimens is at the largest level and has the greater distributed difference along the height direction. As the particle packing structure of UPAB specimens gradually transitioned from floating dense type to skeleton dense type and further to skeleton gap type, the total number of pores inside the specimens greatly reduced. The number of isolated microv-and small-sized pores decreased, but the number of medium-and large-sized pores with better connectivity increased. The pores gradually evolved into irregular and elongated ones and transitioned from rounded and plump ones to those with broken boundaries. They have better connectivity and can provide channels for water and gas transport. Optimizing particle packing structure is beneficial for forming inter-connected internal pores and seepage channels, thus improving the hydromechanical behavior of UPAB materials. The findings could potentially provide theoretical basis and technical guidance for optimized design of UPAB materials. © 2024, Central South University Press. All rights reserved.