###### Abstract  
Concrete structures are considered as durable construction material, but corrosion of the embedded steel reinforcement occurs under chloride exposure as concrete has porous properties. Herein, a cyclic drying-wetting test was performed for two years using saltwater to accelerate steel corrosion in a reinforced concrete (RC) member. The open-circuit potential (OCP) was measured using a newly developed and replaceable agar sensor. The corrosion potential was measured considering the chloride concentration, water-to-cement (\(w/c\)) ratio, and cover depth at three levels. Furthermore, its relationships with influential parameters were evaluated using averaged OCP results. The measured OCP showed a linear relationship with the cover depth, and this tendency was more distinct with increasing retention period and higher chloride concentration. For the highest \(w/c\) ratio (0.6), values below \(-100\) mV were monitored after only six months regardless of the cover depth, and values below the critical potential level (\(-450\) mV) were evaluated at lower cover depths (30 and 45 mm). The results of regression analysis considering the exposure environment showed a clear relationship in the case of high chloride concentration (7.0%). A linear relationship between cover depth and OCP was derived with a reasonable determination coefficient ranging from 0.614 to 0.771.  
choride ingress; corrosion monitoring; OCP; cover depth; RC member +
Footnote †: journal: Applied sciences  
studies on corrosion detection for RC structures have many limitations because of the difference in corrosion behavior in a laboratory experiment and that of an actual structure, the non-homogeneity of the concrete material, and the influence of local conditions. The nondestructive technique (NDT) has been used in many fields, as it evaluates the performance of a structure by collecting the representative index without destroying it. It has also been applied for evaluating the corrosion feasibility of steel reinforcement. As reported in [11], the results from electrochemical sensors were summarized as OCP (open-circuit potential), surface potential, concrete resistivity, noise analysis, and galvanic current. For OCP measurement, not only conventional reference electrodes such as SCE and CSE but also new material such as activated titanium and MnO\({}_{2}\) were attempted for measurement [12; 13; 14]. Concrete resistivity is a special characteristic of the material that can permit the amount of carrying current. It shows qualitive results of corrosion feasibility but is very useful for field assessment [15; 16]. Polarization resistance and EIS (Electrochemical Impedance Spectroscopy) measurement can provide more accurate and quantitative results such as corrosion current [17]. For galvanic current measurement, many studies have been performed considering environmental conditions and residual chloride contents [18; 19]. Measurements of influencing parameters on corrosion have been carried out such as pH [20], oxygen [12], and chloride content [21; 22]. In the previous research [11], the sensor mechanism and classification of the measurement method were well explained. Monitoring sensors that are conventionally installed on embedded steel reinforcement during the construction stage can evaluate the relatively quantitative corrosion behavior (potential and corrosion density) through measuring the potential difference among the reference electrode (RE), working electrode (WE), and counter electrode (CE) connected to the embedded steel reinforcement. Although the embedded steel reinforcement and the related measuring electrodes are safe, significant misleading errors from actual corrosion potentials may occur when the electrolyte life of the embedded sensors expires or the internal medium is supersaturated by high-concentration ion penetration. In this study, a sensor that can be replaced and calibrated outside was fabricated using agar as an ion transfer material.