Modeling the effect of fly ash on alkali-silica reaction in concrete considering the reduction of alkali concentration in pore water

Alkali-silica reactions (ASRs) in concrete have severe deteriorative effects on concrete structures. In cold regions, ASRs can combine with other deteriorative actions (i.e. frost damage and chloride-induced corrosion) of deicing salt and cause severe damages to concrete bridges. The addition of fly ash in concrete is a measure for mitigating ASRs. Previous studies proposed various mechanisms of ASR suppression using fly ash based on chemical reactions; however, an integrated model for evaluating the extent of ASR reduction by fly ash has not yet been developed. Hence, it is difficult to determine the precise amounts of fly ash needed to suppress ASR using various types of fly ashes under different environmental conditions. This study develops a model to investigate the effect of fly ash on ASRs in concrete based on the chemical reactions and hydrate properties of concrete, particularly focusing on the alkali concentration of pore water. First, a model of alkali adsorption in an aggregate was built to accurately evaluate the alkali concentration. Second, models for considering the adsorption of alkali ions by C-S-H were developed for cases in which the Ca/Si ratio was low owing to fly ash addition. The quantitative modeling of these phenomena can estimate the concentration of sodium and potassium ions in the pore water in cement-based materials, which can lead to the performance evaluation of fly ash on ASR expansion considering the fly ash characteristics and environmental conditions. By applying the model to a multiscale chemo-hygral analytical scheme developed by the authors, the structural performance can be assessed, and the system can be used to develop the optimal mix proportion of concrete containing fly ash against the risks of ASRs.