The Investigation Of Ionic Transport In Cement-Based Materials By Mono-profile Regression Method And X-CT

The durability of reinforced concrete structures is very important because they are used in various environment for decades and even hundreds years. The corrosion of reinforcing steel is an important durability concern, which is mainly controlled by the chloride ingress in concretes. However, the chloride ions would be fixed to the cement matrix physically or chemically as bound chloride during the diffusive ingress and form a chloride ions concentration profile in matrix eventually. So accurately and conveniently obtaining the chloride ions concentration profile and the diffusion and binding properties of chloride in concretes have important theoretical significance and application value for understanding the chloride transport and service life prediction.Firstly, we proposed a mono-profile regression method to obtain the diffusion and binding parameters of chloride ions in cement-based materials. The method is based on the causal relationship between total chloride ion concentration distribution and diffusion and binding capacity which controls the ingress of the chloride ion. The proposed method only needs a total chloride ion concentration profile to simultaneously obtain both the effective diffusion coefficient and binding isotherm parameters by regression calculation. In addition, this method can obtain the confidence intervals of various parameters, the curve of apparent diffusion coefficient varies with time and space, and the profiles of total, bound and free chloride ions vary with time and space.Secondly, we performed numerical experiments and applied the method to literature data. The results of numerical experiments present a good convergence of the proposed method. The results of the calculation of the 9 total chloride ion concentration profiles from the literatures show following findings. The obtained chloride ions Freundlich isotherms are close to linear, which is mainly due to the influence of hydroxyl ions concentration gradient. In addition, the obtained free chloride ion concentrations are larger than the literature values.Finally, based on the similarity between iodide ion and chloride ion diffusion in cement-based material, we applied iodide ion to substitute chloride ions in this thesis. We further quantified the iodide ion concentration profiles of cement pastes using the CT grayscale values of X-ray tomography (X-CT) images. The diffusion coefficient of iodide ion were obtained by three different methods. Using the CT grayscale profiles, the mono-profile regression method was used to calculate the iodide ion effective diffusion coefficients and binding isotherms. Using the CT grayscale profiles and the iodide ion concentration profiles respectively, the traditional error function(ERFC) method was applied to calculate the apparent diffusion coefficients. The diffusion coefficients obtained from the three methods are comparable.