Microscopic investigations on the air-void characteristics of wet-mix shotcrete

This paper aims to perform microscopic investigations on the air-void characteristics of wet-mix shotcrete before and after the spraying process. Both air-void analysis (AVA) and image analysis (IA) were used to monitor the air-void characteristics of both fresh and hardened mixtures such as air content, spacing factor, and air-void specific surface area. To gain a comprehensive understanding on the air-void characteristics, a total of 28 mixtures were fabricated with different aggregate gradations, slumps, air entrainments, and the types and replacement levels of supplementary cementitious materials (i.e., fly ash, silica fume, metakaolin, and ground-granulated blast furnace slag (GGBFS)). The results have shown that: (1) both pumping and shooting significantly altered the air-void characteristics; (2) the pumping process increased the air content and air-void specific surface area while decreasing the spacing factor; and (3) a decrease in air content and an increase in air-void specific surface area were observed after shooting. In addition, the findings of this study revealed that the air-void characteristics of wet-mix shotcrete were more closely associated with the pumping and shooting operations rather than the mixture characteristics such as aggregate gradation, slump, air entrainment, and mineral admixtures addition.