# Application of Thermal Image Data to Detect Rebar Corrosion in Concrete Structures
## 1 Introduction  
In general, it would be improved the structural performance of the concrete, when rebars in the reinforced concrete are uncorroded and intact [1; 2; 3; 4; 5]. This is because concrete has strong alkaline properties, which indicate a pH of 12.5 [2; 3; 4; 5]. Thus, rebars would be protected from corrosion by the passive film that is formed around them. However, if concrete cracks occur in the surface of the concrete, chlorine ions, moisture, and oxygen could infiltrate the concrete and distribute inside, eventually reaching the rebars within the concrete [5; 6; 7; 8; 9]. In such cases, the passive film that protects the rebars would be destroyed and the rebars start to corrode. When this happens, corrosion products are chemically created during the rebar corrosion process. Because the volume of the corrosion products can expand several times faster than the volume of the rebars, tensile stress acts on the concrete cover, and the structure's durability is significantly reduced by cracks, delamination, and failures in the concrete as well as the reductions in the rebar's cross-sectional area [3; 4; 6; 10; 11; 12]. Therefore, there is a necessity to develop methods for quantitatively measuring the corrosion of rebars that occurs in reinforced concrete structures as well as methods for accurately evaluating the extent to which the strength of the corroded reinforced structure is reduced.  
Rebar corrosion monitoring techniques have been used in studies involving embedded sensors that can detect changes in the corrosion currents and the polarization resistance of rebars in large structures. The data from these sensors are used to predict rebar corrosion and evaluate the integrity of the structures. Studies are being continuously conducted to improve the reliability of monitoring techniques and create efficient maintenance systems [3; 7; 12; 13].  
In addition, several studies have been conducted to develop techniques for measuring the rebar corrosion within concrete [14; 15; 16; 17]. These techniques include spontaneous potential, polarization resistance, electrical resistance, ac impedance, electrochemical noise, dynamic polarization, and dynamic potential difference bipolar polarization methods. The results that have been obtained from these studies have identified the electrochemical mechanism of rebar corrosion, established theories related to various types of rebar corrosion, and measured the speed of rebar corrosion.  
Defect detection methods that employ infrared cameras are useful non-destructive testing methods to detect defects in concrete structures [18; 19; 20; 21; 22; 23; 24]. These methods are non-contact, stable, prompt, remote detection, and continuous testing methods [25; 26; 27]. However, the measurement results from these methods would vary depending upon several factors that affect thermography. Because of these reasons, they have not been able to provide sufficient reliability. To obtain clear measurement results during infrared thermography, it has been found that efforts must be made to minimize the target location's temperature and humidity to reduce the impact on the measurement results as well as the target structure's surface temperature.  
The ultimate goal of this study is to develop a technique that uses infrared cameras to quantitatively measure rebar corrosion rates. Rather than a qualitative examination that determines whether or not rebar corrosion is occurring, the aim is to construct a system that provides accurate and efficient information on rebar corrosion in reinforced concrete structures.