Corrosion Monitoring of Reinforcing Bar in Concrete Under Different Corrosive Environments
Authors: Katayama Hideki , Yamada Maya , Doi Kotaro , Hoshi Yoshinao , Shitanda Isao , Itagaki Masayuki 
Published: February 2020

Reinforced concrete structures have been widely used for social infrastructures because of their high strength. However, it currently has been passed over 50 years since many infrastructures constructed, and the deterioration in durability is a problem. The corrosion of reinforcing bars due to neutralization of concrete and adhesion of airborne sea salt, and the concrete cracking by corrosion products is considered as the main cause of deterioration. Therefore, in order to properly maintain and manage the infrastructure structure, it is necessary to understand the corrosion deterioration behavior of the reinforcing bars in the concrete. In this study, wet and dry cyclic corrosion test was conducted using concentric ring type electrodes of carbon steel covered with mortar. The difference in corrosion behavior of reinforcing bars due to covering thickness and corrosion test conditions was investigated by continuously measuring the impedance between two electrodes. Furthermore, the diffusion behavior of water and chloride ions in mortar part was observed, and the correlation with corrosion behavior was investigated The concentric ring type electrodes of carbon steel were used as the monitoring electrodes, and they were covered with the different thickness of mortar (hereinafter, the mortar electrode). The composition of the mortar was coarse aggregate: cement: water = 3: 1: 0.6. 10 kg/m3 NaCl was added to the mortar to accelerate corrosion. The covering thickness was set to 3, 10 mm. The corrosion test was carried out by wet and dry cycle, and the electrochemical impedance at two frequencies of 10 mHz and 10 kHz was continuously measured during the corrosion test. A core having a height of 50 mm and a diameter of 40 mm was prepared with the same composition of mortar. The prepared cores were immersed in 0.1 mol/L NaCl solution for several hours. The permeation state of water and chloride ion was investigated by water leakage test agent and fluorescent X - ray analysis.

Changes in the corrosion rate of the mortar electrode prepared by underwater curing in the wet and dry cyclic corrosion process were continuously measured. Although there was some variation, corrosion rate decreased in the immersion process, regardless of the thickness of the mortar. The corrosion rate of the electrode with 3 mm mortar showed higher value than that with 10 mm mortar. The changes in corrosion rate and solution resistance in each cycle of corrosion test were examined. The solution resistance gradually decreases in the wet process, and the value showed a minimum in the drying process. This means that diffusion rate of the solution is slow in the mortar. Since the decrease in solution resistance corresponds to the increase in corrosion rate, it is considered that the increase in corrosion rate is due to wetting by the permeated solution. There was a large fluctuation in the electrode with 3 mm mortar, whereas the electrode with 10 mm mortar always showed a low solution resistance value regardless of process of the corrosion test. This difference in wetting environment can affect the change in corrosion rate and corrosion behavior.