## 1 Introduction  
Reinforced concrete has been a core material in structural engineering for over a century, combining the compressive strength of concrete with the tensile strength of embedded steel reinforcement. Despite its robustness, concrete is susceptible to time-dependent degradation processes, such as cracking, delamination, corrosion of rebars, and moisture ingress. These issues, often developing below the surface, may remain invisible until structural integrity is compromised. Therefore, accurate and regular non-destructive inspection of reinforced concrete is essential for the safe operation and maintenance of infrastructure systems [1, 2, 3, 4]. Detecting and characterizing embedded steel reinforcement in concrete remains a critical aspect of structural diagnostics, particularly in aging or deteriorating infrastructure. Accurate localization of rebars is vital not only for assessing current structural conditions, but also for guiding repair, retrofitting, or demolition work. Inaccurate detection can result in ineffective interventions, increased maintenance costs, or even unintended damage to the structure during mechanical work. Furthermore, detailed knowledge of the reinforcement layout is often essential in safety assessments and finite element modeling of structural behavior. Traditional non-destructive testing (NDT) methods--including ultrasonic pulse velocity, ground-penetrating radar (GPR), and electromagnetic sensing--offer important insights into the internal condition of concrete elements [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. However, these methods present several limitations. GPR performance, for instance, is highly sensitive to the electrical properties of concrete, which vary with moisture content, material heterogeneity, and aggregate composition. Electromagnetic methods may struggle in reinforced or saturated structures, especially when rebars are closely spaced or located deep beneath the surface. Moreover, most traditional NDT techniques require physical contact or close-range scanning (in the case of ultrasonic testing), are time-consuming over large areas, and often demand expert interpretation of ambiguous signals. Active infrared thermography (IRT) has emerged as a promising alternative that enables visualization of near-surface subsurface features based on thermal responses to controlled heating. Conventional heating sources, such as flash lamps or hot air systems, are typically limited to shallow inspections due to their surface-constrained heat delivery [15; 16; 17; 18; 19]. To overcome this limitation, recent studies have proposed using microwave excitation as a volumetric heat source for active thermography. Microwave energy interacts with the material, allowing localized heating of internal volumes, particularly in heterogeneous systems like concrete [20; 21; 22; 23]. However, in the case of metallic elements such as steel rebars, direct microwave penetration is minimal due to the very low skin depth of conductive materials at GHz-range frequencies. As such, rebars do not heat directly under microwave exposure. Instead, they behave as thermal and electromagnetic discontinuities that strongly influence the surrounding material response. Specifically, the cement matrix immediately above the reinforcement bars heats more intensely than the adjacent areas. This occurs because steel rebars, having high thermal conductivity, act as barriers to heat diffusion, causing heat accumulation in the thin volume of concrete cover directly above them. In addition, the presence of metal inclusions introduces interference and diffraction phenomena, altering the electromagnetic field distribution in complex ways. These effects manifest as distinctive surface temperature patterns that are observable through infrared imaging, which can indicate the presence and location of embedded steel elements [24; 25]. Previous research has explored these mechanisms in controlled settings, analyzing microwave-concrete-metal interactions through both simulations and experiments [26; 27].