## 1 Introduction  
In the past, the corrosion detection of steel bars in reinforced concrete structures used the chemical potential method as the measurement method, which could only measure the difference in the chemical potential of the deep corrosion of steel bars from the concrete surface, and it was necessary to destroy the drilled holes to the surface of the steel bars. Using the GPR electromagnetic wave reflection voltage method, the most important thing is that there is no need to damage the reinforced concrete structure. Secondly, GPR can increase the detection of a large area of reinforced concrete structures, and the corrosion status of each steel bar inside the structure can be measured. The traditional chemical potential method is used to predict the corrosion probability of steel bars, which is relatively inaccurate. This study uses the voltage reflected by the electromagnetic waves of ground penetrating radar to present the degree of corrosion of steel bars; this method can detect the corrosion state of individual steel bars, and there is no need to perform destructive opening detection on the concrete surface. Effective and comprehensive condition assessment can reduce the maintenance costs of concrete structures. The ground penetrating radar (GPR) can provide completely non-destructive results in real time and has been widely used in the condition assessment of reinforced concrete structures [1]. During the corrosion process, the GPR data were recorded. The results show that the relative amplitude between the reflection signals in both the parallel and the perpendicular polarization is decreasing as the corrosion degrades [2]. In Taiwan, the high humidity and corrosive factors caused by the extensive coastal environment or oceanic climate have caused serious corrosion to the structure of reinforced concrete bridges [3]. Most of the methods for detecting the degree of corrosion of the bars are based on corrosion potential or corrosion current [4]. However, these methods based on corrosion potential and corrosion current must establish electrochemical pathways by destructing the surface of the concrete components. Therefore, it is necessary to develop a detection technology that can improve detection efficiency and does not generate damage to concrete structures [5]. Corrosion is one of the complex mechanisms that most affects the resistance of reinforced concrete. A combined methodology using ground penetrating radar (GPR) and infrared inspection techniques for the detection and evaluation of corrosion is proposed [6]. This study proposes an improved repair and retrofit technique that includes the removal of rust from oxidized rebar parts by applying low viscosity epoxy resin to the slab base and allowing it to fully penetrate the concrete cracks and surface of the rebar's inside, thus producing a protective layer and repairing the bond [7]. The current research of ground penetrating radars detects the degree of corrosion of steel bars by using the methods of ground resistance and corrosion potential [8]. A technique for accelerating rebar corrosion using direct current with sodium chloride solution was used to induce corrosion into embedded rebars. A GPR was used to assess the corrosion of the rebar. The results show that corrosion of the rebar could be inspected [9]. Non-destructive testing, quasi-continuous results, and efficient data acquisition are the main advantages of mobile GPR systems. In this paper, the benefits and limitations of mobile GPR systems are discussed using examples from EMPA's recent work. The emphasis is on the investigation of pavement thicknesses and depths of rebar on concrete bridges [10]. This article proposes a deep learning-based autonomous concrete crack detection technique using hybrid images. The hybrid images combining vision and infrared thermography images are able to improve crack detectability while minimizing false alarms [11].