Chloride ion transport in unsaturated ultra-high performance concrete cracks under high hydraulic pressures

[Display omitted] •The transport of Cl is hindered by the gas bubbles in the cracks.•Capillary force within the cracks is not the primary factor affecting Cl transport.•Hydraulic pressure compresses the volume of air and increases the depth of Cl transport.•The moisture distribution in the cracks is successfully visualized by X-CT. The high strength, durability, and impermeability of ultra-high performance concrete (UHPC) make it a vital material for the construction of deep underground facilities, such as tunnels, nuclear waste repositories, and research laboratories. However, the presence of cracks and high hydraulic pressure in deep underground greatly accelerates the transport of chloride ions, which can shorten the service life of UHPC. Unfortunately, the underlying mechanism for this is still unclear. Here, we investigated the influence of hydraulic pressure and capillary force on chloride ion transport in cracked UHPC using a self-designed testing module. It was found that the presence of trapped gas creates a gas–liquid two-phase state within the cracks. Theoretical models were established using the Laplace equation, which effectively describe the hindrance of gas within cracks on the transport of chloride ions. Contrary to current understanding, the primary driving force for chloride ion transport within the crack is the capillary pores along the crack walls, rather than the capillary forces associated with the crack itself. In addition, the unhydrated regions within the UHPC interior expedite the adsorption of water molecules compared to ordinary concrete. Hydraulic pressure significantly accelerates the depth of chloride ion penetration. Subsequently, the visualization of the solution within cracks was carried out through the application of X-ray computed tomography (X-CT) and capillary tests, thereby providing a means to validate the accuracy of the model. This study contributes to the advancement of our knowledge regarding the influence of capillary force and high hydraulic pressure on the transport of chloride ions. The findings have important implications for the analysis of the durability of UHPC structures situated in deep underground environments.