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The erosion of chloride ions is a significant factor that causes the fracture of the passive film of steel bars in reinforced concrete. To investigate the time-dependent growth and fracture mechanism of the passive film on steel bars under the influence of chloride ions, open circuit potential method and Mott-Schottky measurements were employed to analyze its evolving electronic characteristics. Additionally, X-ray Photoelectron Spectroscopy(XPS) analysis was employed to characterize the progressive changes in physical and chemical properties of the passive film at the microscopic level. Then, a temporal growth and fracture model for the passive film under the influence of chloride ions erosion was developed. The research indicates that the growth and fracture process of passive films follows a distinct temporal sequence, specifically divided into formation, stabilization, binding, and fracture stages. The progression of structural variation in the passive film is determined by the concentration of chloride ions in the solution. When the concentration of chloride ions is less than the threshold value, the passive film undergoes limited bonding with chloride atoms. However, once the concentration surpasses the threshold, chloride ions progressively react with the outer layer of the passive film, initiating a dissolution and fracture process. This work provides new insights into the temporal evolution of chloride-induced depassivation of steel bars and establishes a time-based framework for understanding the breakdown mechanism.

Crack location and type identification of reinforced concrete columns in service is of great interest to researchers, since it influences the decision-making of maintenance and retrofit. Due to the existing of load and corrosion, the complex stress state leads to complicated crack development which has been a challenge for researchers to determine the crack type. Acoustic emission (AE) method gives a promising solution to non-destructively locate and identify the crack position and type. In this work, corroded reinforced concrete columns were subjected to compression test with different eccentric degree. AE signals during the loading process were recorded. The traditional RA-AF method and a newly proposed Gaussian mixture model (GMM) method were applied to process the data for crack type identification. It was found that due to the large data set, RA-AF cannot identify the crack type in between the shear crack and the tensile crack types. The probability density distribution of GMM result gives an attractive offer to determine the crack type. A simple 4-step method based on GMM plot is proposed to identify the crack type which can calculate the ratio of shear crack and tensile crack in collected signals.

A long-term elevated temperature environment is typically found in the metallurgy industry, which has adverse effects on the properties and microstructure of concrete structures. A few studies investigated the effect of short-time high temperature on the properties and microstructure of concrete, but the effect of long-term elevated temperature is seldom analysed. In this study, the deterioration mechanism of concrete under long-term elevated temperature was investigated by determining the thermal stresses on basis of finite element model (FEM) and analyzing the changes in its chemical composition and microstructure through X-ray diffraction (XRD), pore structure analysis (PSA) and scanning electron microscope (SEM). The results indicated that the maximum temperature stress of surface concrete could exceed 4 MPa, which was close to the ultimate tensile strength of normal concrete. The results also showed that calcium hydroxide (CH) could decompose at less than 200 ℃ because of the crystallization of tiny CH crystals, doping with other ions, extended heating time, and xonotlite (α-C2SH), which was formed on the internal surface of the concrete and decomposed on its external surface. Owing to these reasons, cracks and holes were generated, resulting in severe deterioration of the concrete structure.