Determination of permeability evolution and preferential flow in the rock-concrete interfacial transition zone under the effect of seepage-dissolution

The rock-concrete interface under high hydrostatic pressure may lead to concrete leaching, which is crucial for the safety of concrete gravity dams. However, whether a seepage pathway forms in the interfacial transition zone (ITZ) and the mechanism of permeability evolution with pore structure was not fully understood. Therefore, this study aims to investigate the effects of concrete leaching on the pore structure and permeability of the rock-concrete ITZ. First, the seepage-dissolution experiment for rock-concrete specimens was accelerated by using ammonium chloride. The evolutions of permeability and leaching depth during this process were measured. Subsequently, the alteration of calcium compounds was determined through the dissolved amount of C-H (portlandite) and calcium silicate hydrate (C-S-H). The evolution of pore size distribution during the seepage-dissolution process was analyzed using NMR. Based on the analysis of these experimental results, we can conclude that: (1) The cement slurry dissolved most rapidly in the ITZ, forming a dominant flow pathway. (2) As the dissolution process progressed, the increase in capillary pores due to the continued dissolution of C-H and C-S-H enhanced the permeability in ITZ. (3) The pore size distribution was highly correlated with the evolution of permeability, suggesting that the flow in the ITZ was porous media flow rather than fracture flow. (4) The effect of hydrostatic pressure is not discussed in this study, but it is crucial for understanding the transition from porous media flow to fracture flow.