Comparative study on the moisture transfer properties of shotcrete and casting concrete

A comparative study was conducted on moisture transfer of shotcrete and casting concrete under different drying directions. The differences in pore distribution, cement stone pore structure and interface transition zone (ITZ) characteristics were analyzed by surface pore structure test, mercury intrusion pore test (MIP), microhardness test and scanning electron microscopy test (SEM). A theoretical model for two-direction moisture transfer in shotcrete was established. The two-direction water diffusion coefficients were also calculated. The moisture transfer of shotcrete has significant anisotropic characteristics under the drying conditions of along or vertical spraying direction. The moisture transfer speed along the spraying direction is slow, while the transfer speed in the vertical spraying direction is fast. However, the two-direction moisture transfer speeds of casting concrete are almost the same. The pores in shotcrete are mainly distributed in the transition zone between aggregate and matrix. The width of ITZ for shotcrete is about twice of concrete. The hardness of ITZ is significantly lower than that of casting concrete. And the hardness of ITZ along the spraying direction is lower than that in the vertical direction. There are obvious gaps between aggregate and matrix in shotcrete by the SEM tests. The two-direction moisture diffusion coefficients of shotcrete were obtained by inverse calculation. The diffusion coefficient along spraying direction was small, and vertical spraying direction was large. The diffusion coefficient of casting concrete was between the above two. The two-direction moisture transfer theoretical model established can effectively describe the differences in moisture transfer in different directions for shotcrete.