## 1 Introduction  
Concrete cover has an important role in reinforced concrete (RC) structures. It protects reinforcement bars from the bad effects of weather, fire, and bad environmental conditions that cause the corrosion of the reinforcements and reduce the service life of the RC structures. Thus, it is an important durability indicator for RC structures, and an essential parameter to be considered for the structural health monitoring (SHM) of RC structures [1]. For this reason, the study on the concrete cover, as well as the development of different non-destructive evaluation (NDE) methods for detecting this parameter, have become crucial. The radar method is one of the NDE methods commonly used for the detection of the concrete cover [2, 3]. This electromagnetic method has also been used for determining the location of reinforcements and assessing moisture in RC structures [4, 5, 6, 7, 8]. However, this method is a contact method (contact between the wave transmitter and the detected element) while the infrared thermography method is a non-contact one with a simple and rapid implementation. The infrared thermography method is a temperature measurement technique based on heat transfer by thermal radiation. It is an NDE technique, that is based on measuring the temperature difference at the surface of materials or structures to detect the presence of problems, defects, or variations of thermal conduction properties beneath the surface [9]. The detection by infrared thermography is classified into two approaches: active thermography and passive thermography. The development of active infrared thermography methods has been accompanied by the development of interpretation methods and adopted stimulation techniques (heating source or excitation) [9; 10; 11; 12; 13; 14; 15; 16; 17]. Active infrared thermography methods have been developed with different types of excitation (halogen lamps, CO\({}_{2}\) lasers, microwaves, etc.) for various applications, such as spot weld inspection [18] and the detection of a metal element behind a wooden plate [19]. Other applications for defect detection include defects of roll-to-roll printed conductors [20], defects in reinforcement with a carbon fiber-reinforced polymer (CFRP) [21; 22; 23; 24], as well as defects in biomedical materials [25]. The latest development with a new microwave excitation system has enabled the detection of reinforcement in a reinforced concrete wall, which has highlighted the contributions of microwave excitation [26; 27; 28; 29; 30]. The microwave excitation is of great interest because of its volumetric heating, with a penetration depth of waves (which depends, in particular, on the dielectric constant of the detected material [31; 32; 33; 34]), allowing the visualization of steel bars in the RC structure during the heating phase. However, the detection of the concrete cover thickness (quantitative) by infrared thermography, especially in the heating phase, has not been accessed yet. The detailed analysis and discussions of the physical phenomena known as diffraction and interference affecting the thermograms during the detection of the steel bars by microwave thermography have given an essential key, leading to the resolution of this issue. As a result of the study, the effects of incident waves on the thermograms in both detection approaches (reflection and transmission) have also been clarified. On the thermograms in the reflection approach, concrete parts in the reflected areas are hotter than other areas, whereas the fractions in the areas of wave superposition (interference) are hotter than other areas in the transmission approach [35]. The present study shows an innovative methodology for determining the concrete cover thickness of one-layer reinforcements in a concrete wall with microwave thermography. In the methodology, the transmission approach (the infrared camera and the microwave antenna are situated on the same side) is used.