## 1 Introduction  
In the inspection of reinforced concrete, a combination of induction heating and IR thermography was first suggested by [PERSON] et al. in 1982 [1]. General applications of this inspection technique were summarized by [PERSON] et al. [2]. [PERSON] and [PERSON] focused on the detection of corrosion by applying induction heating [3]. [PERSON] et al. demonstrated that external optical stimulation of reinforcement in concrete enables detection of 10-cm structural elements in concrete at depths up to 7 cm [4]. Smaller thermal anomalies, such as 2 \(\times\) 2 cm, can be detected up to a depth of about 4 cm [5]. [PERSON] et al. described the use of microwave heating in the detection of rebars in concrete with a depth limit of about 2 cm [6]. This study covers the development of a thermal nondestructive testing (TNDT) technique for inspecting reinforced concrete supports of several types, such as overhead line poles, pipes, beams, pilings, etc., which are widely used in various industries, and in both the manufacturing and operation stages [7; 8; 9]. Historically, Russian railways represent one of the most important industrial areas in the country. It is worth mentioning that over 80% of cargo in Russia is being transported by means of railways (not counting the pipeline transport) [10]. Uncontrolled damage and full destruction of electrical supports may cause financial losses and even casualties, particularly in densely populated areas. Therefore, research intended to enhance the reliability of railway overhead lines is of a high priority in Russia, being supported by the corresponding federal directive documents. The mass electrification of Russian railways started in the 1980s. The lifetime of reinforced concrete poles is assumed to be about 50 years, hence, in the near future, efficient inspection techniques to evaluate the quality of supports will become increasingly important. Some poles may reveal an essential loss of their mechanical strength much earlier than within their normative lifetime. This may be conditioned by either poor workmanship or some ambient factors. Pole damage often starts in underground sections of supports, where corrosion appears because of varying soil moisture and high mechanical stresses. Corrosion damage is related to cracks that appear in concrete boundary layers adjacent to steel rebars because of enhanced pressure rendered by corrosion products. It is important to note that corrosion defects are located under the concrete surface layer and often hidden in the soil, thus are visually undetectable. Figure 1 shows a scheme of an electric railway pole and typical corrosion defects that appear at a pre-failure period of operation. The most dangerous is corrosion damage, which appears in underground pole sections (Figure 1b). A number of nondestructive testing (NDT) techniques have been applied to detect early corrosion in electric railway poles. They differ by sensitivity and test productivity but can hardly be applied to the evaluation of pole wear. The most usable ultrasonic inspection technique evaluates the strength of concrete by measuring ultrasound velocity in different directions or recording acoustic emission signals. Another ultrasonic test method involves the analysis of resonance features of the reinforcement. Both techniques are predominantly applicable for inspecting concrete surface layers and often require the use of special references. One more inspection technique involves electromagnetic (induction) heating of a pole to locate reinforcement in concrete, but this does not help with the evaluation of residual reinforcement thickness. Classical TNDT based on surface heating of test objects seems to be inappropriate in the inspection of  
Figure 1: (a) Electric railway pole scheme; (b,c) and examples of corrosion damage. concrete poles because of the considerable pole thickness and low thermal conductivity of concrete with regard to highly conductive steel.