Experimental investigation and numerical study on chloride diffusion coefficient of long-term fly ash concrete

Owing to the specific hydration characteristics of fly ash, reasonable replacement of fly ash in concrete can optimize the microstructure of concrete and significantly enhance its resistance to chloride ion penetration. In this paper, both experimental investigations and numerical simulations are performed to study the variation regulation of the chloride diffusion coefficient in fly ash concrete. In the experiments, the influence of the water to binder ratio, aggregate volume fraction, fly ash replacement rate, and curing age on the chloride diffusion coefficient is studied. The experimental results show that, as the water to binder ratio increases or aggregate volume fraction decreases, the chloride diffusion coefficient of concrete increases. When the curing age is 60, 240, and 720 d at a fly ash replacement rate of 30%, the chloride diffusion coefficients are 30.2%, 33.5%, and 51.1% lower than that at a fly ash replacement rate of 10%, respectively. For fly ash replacement rates of 10%, 20%, and 30%, the average decreasing rates of the chloride diffusion coefficient at different aggregate volume fractions and water to binder ratios account for 93.8%, 93.5%, and 93.6% of the total decreasing rates, for a curing-age range from 28 to 540 d. In the numerical simulations, the microstructure of blended system is constructed according to the hydration kinetics model. The reliability of the hydration model is verified by the experimental results. The mesostructure of three-phase concrete is constructed by increasing the scale. A continuous-based random walk method is proposed and applied to the blended bulk paste and concrete scale by scale to obtain the corresponding chloride diffusion coefficients. The reliability of the numerical simulation method is verified by 18 groups of experimental results. The simulation results indicate that the average reduction in the chloride diffusion coefficients when the curing age increases from 720 to 1800 d contributes merely 1.9%, 2.0%, and 2.2% of the total reduction observed over an extended period from 28 to 1800 days, for fly ash replacement rates of 10%, 20%, and 30%, respectively.