Response Variability of Diffusion Models Applied to Chloride Concentration Profiles from Concrete Structures

Chloride profiles can be used to assess the durability of concrete structures against corrosion in the marine environment. However, the models proposed to adjust these chloride profiles are diverse and have varied complexities. Therefore, the objective of this article was to compare the performance of five chloride penetration models using a database with different profiles found in the literature. For this, 63 chloride profiles were selected. The response of the models was evaluated through a strong statistical analysis to compare the accuracy of these equations. The results demonstrated that standardized models based on Fick’s second law should adjust only the data in the diffusion zone of the profiles by minimizing the sum of the squared residuals and applying a simple iteration, showing a coefficient of determination ($R^2$) above 0.90, mean absolute error (MAE) below 0.20, and root mean square error (RMSE) close to 0.20. However, traditional models were less effective to fit the entire profiles (convection and diffusion zones), especially when a triple iteration was applied. For this case, Holliday’s model showed better results ($R^2=0.84$, $\mathrm{MAE}=0.25$, and $\mathrm{RMSE}=0.30$). Therefore, a new analysis method is suggested in this article to better adjust the chloride profiles by the diffusion models.

The analysis of chloride profiles is an important tool to evaluate the service life of reinforced concrete structures. In this case, considering the reinforcement corrosion as the main degradation mechanism observed in these structures, the knowledge regarding the service life of field structures highlights the importance of research on this area, mainly if the structure is present in the marine environment, since the port infrastructure is vital to several international trade relations. Furthermore, artificial islands have been built to expand the marine infrastructure for tourists up to aeronautical purposes, thus increasing the number of structures exposed to this severe aggressive environment. Different models have been proposed to represent the chloride penetration in concrete structures, although many of them have difficulty representing the chloride penetration, adjusting only the diffusion zone by the solution of Fick’s second law of diffusion for a semi-infinite medium. The full representation of the chloride profiles, with clear physical variables, has been the subject of research in the last years. In this sense, a general model based on the modified Holliday equation was successfully applied to chloride profile modeling in different coastal structures around the world, attesting its worldwide applicability.