Corrosion of reinforcing steel in concrete structures is an expensive corrosion problem. Rebar in concrete structures exposed to seawater is susceptible to corrosion, which can influence the life span of concrete structures in marine environments. Typically, reinforcing steel in concrete would remain indefinitely under usual atmospheric exposure as long as the concrete cover protects corrosive factors such as moisture, oxygen, and chloride from the embedded reinforcement. However, concrete structures exposed to seawater deteriorate over time due to the large amount of chloride ions in seawater. Salt damage is a critical cause for concrete bridges and harbor structures [1]. Recently, the performance of reinforced concrete structures has worsened due to severe environmental pollution and poor construction [2]. The maintenance costs for industrial infrastructure continues to increase year over year [3]. Rebar corrosion of concrete is a major parameter related to the durability of reinforced concrete structures. Generally, this type of corrosion is difficult to visualize in its early stages because the corrosion of the rebar initiates inside the concrete and propagates stealthily [4]. To establish the most economical repair methods, the corrosion of the inner reinforcement must be accurately and quantitatively detected at an early stage. Significant efforts have been devoted to corrosion measurement of reinforced concrete in an attempt to develop a reliable technology, but no satisfactory methodology has been reported to date [5]. Corrosion of reinforced concrete structures is a sensitive phenomenon that depends on many factors including the concrete cover and grade, temperature, humidity, carbonation, and chloride contamination. This multiplicity of factors significantly influences the data obtained based on either in-situ observations or experimental investigations [6]. Comparing the several hundred studies regarding the corrosion of reinforced concrete, corrosion data is often obtained from a variety of experimental setups and parameters. Almost all of the reported data were obtained from laboratory studies, whereas only several studies have reported long-term field tests in-situ. This large difference in the literature is related to the different experimental procedures, measurement methods, and variety of parameters that affect the corrosion of steel in concrete. Stefanoni et al. concluded that the main parameter controlling the corrosion rate of steel in concrete is exposure conditions, and an inverse relation between concrete resistivity and corrosion rate provided an empirical correlation depending on the degree of concrete pore saturation [7]. Corrosion measurement techniques using electrochemical methods have attracted attention as accurate and fast non-destructive technologies.

However, many difficulties including the interpretation of results, environmental effects, and problems with measurement methods for the use of appropriate electrochemical methods on reinforced concrete structure persist.

To date, two major methods for accurately measuring the corrosion of reinforced concrete have been developed: measuring from the outside of the concrete structure or using embedded sensors [8].

Corrosion sensors that feature half-cell potential mapping which measures corrosion potential of rebar at the surface of the concrete are widely used. These sensors are useful for rapid and facile surveying and evaluation of rebar corrosion with simple measurement equipment in the field. For embedded sensors, measurements of current density, corrosion rate, concrete resistivity, and chloride content can be achieved [9].

Herein, electrochemical measurements of embedded corrosion-monitoring sensors, including corrosion potential, corrosion current density, concrete resistivity, and potentiodynamic polarization tests were performed to investigate the corrosion behavior of reinforced concrete over time both qualitatively and quantitatively.

Figure 3 shows the half-cell potential measurements between the embedded Pt/Ti reference electrode probe and Ag/AgCl reference electrode. These results showed potential variations of the Pt/Ti reference electrode in the cylindrical mortar specimen exposed to three corrosive conditions, i.e. tap water, 3% saltwater, and 15% saltwater with a silver/silver chloride reference electrode. The half-cell potential measurements were performed over 10 days. Variations of the potential of the embedded Pt/Ti reference electrode ranged from -100 to -130 mV under all environments. Thus, this setup was suitable for use as a reference electrode since the deviation of the half-cell potential was small. Thus, it was consider being usable as a reference electrode embedded in mortar specimen.

Figure 3: 
Potential measurements of the Pt/Ti reference electrode in the cylindrical mortar specimen exposed to various corrosion conditions

Figure 4 shows a comparison of the obtained corrosion current densities under four test conditions. The corrosion current densities measured using the sensors imbedded in the mortar specimens exposed to atmospheric and tap water conditions were negligible at approximately 4.2 × 10^-4 mA/cm² over the course of the measurement. However, the current densities of the mortar specimens exposed to 3% and 15% saltwater conditions were significantly higher due to the absorbed salinity from the saltwater. The current density under the 15% saltwater conditions (the most saline environment) was significantly increased to 1.1 × 10^-1 mA/cm² after 90 days of exposure.

Figure 4: 
Current density measurements of the sensor in the cylindrical mortar specimen exposed to various corrosion conditions

The magnitudes of the current densities under the atmospheric and tap water conditions were the smallest, followed by the 3% saltwater condition, and the 15% saltwater condition was the largest, similar to previously reports [13]. The sensor with a Pt/Ti reference electrode provided good indication of corrosion status when applied to the reinforced concrete structure via current density results.

The concrete resistivity measurement was performed simultaneously with the corrosion rate test. Figure 5 shows the concrete resistivity of the sensor embedded in the mortar specimens under the four test conditions. Under atmospheric conditions, the concrete resistivity was increased to approximately 1.3 kohm·cm over time due to the hydration reaction of the cement. Under the other conditions, i.e. tap water, 3% saltwater, and 15% saltwater conditions, the concrete resistivities were noticeably lower because of the conductive environment. In particular, the 15% saltwater condition exhibited the lowest resistivity of all conditions due to the diffusion rate of chloride ions induced by the large concentration difference.

Figure 5: 
Concrete resistivity results of the sensor embedded in the cylindrical mortar specimens exposed to various corrosion conditions

Figure 6 shows the potentiodynamic polarization tests results. Under atmospheric and tap water conditions, positive corrosion potentials and reduced current densities were observed in the polarization diagram. In addition, the corrosion current density was lower than those of the saltwater conditions, and the pitting potential where the current density significantly increases at a noble potential was not observed. Thus, it was assumed that a dense passive layer on the steel surface of the sensor was formed due to the high alkalinity of the cement mortar and absence of chloride ions.

Figure 6: 
Potentiodynamic polarization results of the sensor embedded in the cylindrical mortar specimens exposed to various corrosion conditions

The sensors under the 3% and 15% saltwater conditions exhibited corrosion behavior with higher current densities approximately 104 times higher than that observed under atmospheric conditions. The corrosion current density of the sensor exposed to 15% saltwater conditions was enormously high. It was assumed that the chloride ions in saltwater penetrated into the specimen, breaking the passive film on the metal surface of the sensor.

From the electrochemical behavior results, the corrosion-monitoring sensors embedded in the cement mortar specimen could effectively monitor the corrosion behavior of reinforced concrete over time both qualitatively and quantitatively.

From the experimental results regarding the electrochemical behavior of the corrosion-monitoring sensors, the corrosion-monitoring sensors embedded in the cement mortar specimens effectively monitored the corrosion behavior of the reinforced concrete over time both qualitatively and quantitatively. The major conclusions of this study are as follows.

• (1) The corrosion potential of the Pt/Ti reference electrode embedded in the mortar specimen was measured using a silver/silver chloride reference electrode over 10 days. The corrosion potential ranged from -100 to -130 mV in all environments tested.
• (2) The corrosion rate as measured by the linear polarization resistance test increased in order of environmental severity with increasing chloride content in the mortar specimen.
• (3) Concrete resistivity decreased with increasing diffusion rate of chloride ions induced by concentration differences.