Study on the Drying Shrinkage of Shotcrete Considering the Effects of Pore Structures and Aggregates Microcracks

The shrinkage of shotcrete under different drying directions were tested. The overall pore distribution and aggregates microcracks in shotcrete were tested using low field NMR. The pores, microcracks, hydration products and unhydrated cement particles in the interfacial transition zone (ITZ) of shotcrete were analyzed using image recognition technology based on BSE morphology test. The formation reasons of pores or microcracks in shotcrete were also analyzed. The results show that the drying shrinkage of shotcrete exhibits anisotropy under different drying directions. The drying shrinkage under the vertical spraying direction drying is about 2.16 times that along spraying direction. The spraying process will result in an increase in the average pore size, pore content and aggregates microcracks in shotcrete. There are pores and microcracks in the ITZ of shotcrete. The pore contents along the four directions of aggregate are significantly different. It is higher along the spraying direction and lower in the vertical spraying direction. There are microcracks in the coarse aggregate of shotcrete along the spraying direction which was directly contacted with the previous slurry layer. The pore and microcrack distribution characteristics in ITZ and aggregate are the two reasons for the anisotropy of drying shrinkage in shotcrete.