Time-dependent deterioration of shotcrete under accelerated calcium leaching at varying flow velocities

Abstract

Understanding the deterioration of shotcrete exposed to flowing water is essential for improving the durability of tunnel support structures. This work investigated the effects of alkali-free accelerator dosage, flow velocity, and curing duration on shotcrete deterioration using accelerated leaching tests in 6 mol/L ammonium chloride solution. Accelerator dosages of 0–8%, flow velocities of 0–2 m/s, and curing durations of 7–28 days were examined. Compressive strength, mass loss, and apparent leaching depth were measured, while low-field nuclear magnetic resonance, X-ray diffraction, and backscattered electron imaging with energy-dispersive X-ray spectroscopy characterized changes in pore structure, phase composition, and calcium distribution. Exposure increased porosity, particularly within the 0.01–0.1 μm pore-size range, and depleted calcium-bearing phases in the affected regions. Flow velocity induced the largest deterioration changes within the investigated ranges. For specimens containing 8% accelerator and cured for 28 days, increasing flow velocity from 0 to 2 m/s increased compressive strength loss from 69.3% to 91.7%, mass loss from 1.5% to 8.2%, and apparent leaching depth from 6.8 mm to at least 15 mm after 28 days of exposure. Higher accelerator dosages and shorter curing durations also intensified deterioration, consistent with their greater initial porosity and less compact microstructures. The findings support a mechanism in which flowing solution promotes calcium removal, while dissolution of calcium-bearing phases increases porosity and weakens the cementitious skeleton. Under the accelerated conditions investigated, controlling accelerator dosage, reducing flow velocity, and extending curing before exposure mitigated deterioration.