Numerical simulation of calcium leaching and damage in concrete pipes under flowing water

To accurately evaluate the evolution of calcium leaching in underground concrete pipes for water and their service degradation mechanisms under flowing water conditions, this study established a variable-coefficient transport model for the calcium leaching, based on the calcium leaching mechanism under flowing water, the law of mass conservation, and Fick’s second law, and the solid-liquid equilibrium relationship of calcium in concrete. The model considered the effects of porosity and pore tortuosity changes caused by leaching on the diffusion coefficient of calcium ions in concrete, as well as factors such as environmental temperature. In addition, the degradation of wall thickness under the combined action of calcium leaching and flowing water scouring was investigated. Based on experimental data and literature, the relationship between flow velocity and critical solid-phase calcium content was fitted, and a damage and spalling criterion was proposed. A predictive model for damage and spalling that reflects boundary movement in concrete pipes for water was constructed. The finite difference method was used to numerically solve the variable-coefficient calcium leaching and damage–spalling models. The results showed that the predictions of the variable-coefficient transport model were consistent with both experimental and literature results, with errors controlled within 20 %. Analysis of influencing factors showed that increasing the water-to-cement ratio, porosity, and leaching environment temperature led to a decrease in calcium ion concentration in the concrete pore solution, while increasing the geometric tortuosity of pores led to a rise in calcium ion concentration. The study of water flow velocity in concrete pipes for water indicated that the higher the velocity, the earlier the initial spalling time and the greater the spalling depth. For example, when the velocity increased from 0.5 m/s to 2.5 m/s, the initial spalling time was advanced by 113.4 years, and the spalling thickness increased by 8.5 mm.