Detection of reinforcement bars in concrete slabs by infrared thermography and microwaves excitation by F. Brachelet*, S. Keo*, D. Defer* and F. Breaban* * Laboratoire de Génie Civil et Géo-Environnement, Université Lille Nord de France Université d’Artois, Technoparc Futura, 62400 Béthune, France. franck.brachelet@univ-artois.fr Abstract This paper presents a NDT method by infrared thermography with a microwave excitation system applied to the detection of metallic parts: a metal ring behind a wooden plate of 1cm thick, a reinforcement bar in front of a concrete wall and the detection of reinforcement bars equally spaced in a concrete slab of 6.5 cm thick. The excitation device is based on a magnetron associated with a horn pyramidal antenna. A contrast algorithm applied to the sequences of thermograms highlights metal ring or bars location. 1. Introduction Many methods of structural inspection are based on the propagation of electromagnetic waves and are able to provide qualitative and quantitative information on the internal structure of the inspected elements [1]. Among the existing techniques such as radiography, gammagraphy or radar applied to civil engineering, active infrared thermography often requires relatively long test duration because of high time constant of thermal diffusion within the inspected elements [2]. Moreover, thermal stimulations applied from the external surface limit the auscultation depth or impose long-term tests. This paper aims to present the development of a new active infrared thermography method based on a microwave excitation system [3]. Compared to other sources traditionally used in active infrared thermography, the microwaves can propagate into the volume of a concrete element. The waves undergo the absorption, the reflection and the diffraction effects while they impact any metal parts [4,5]. This new method has been applied to defects detection in the case of a metal element placed behind a wooden plate of 1cm thick, and more particularly to vertical reinforcement bars in a concrete wall where radiothermal interference phenomena can be observed. 2. Excitation and detection device A microwave source was set up based on a commercial magnetron of 800 W at a 2.45 GHz frequency associated to a pyramidal horn antenna with aperture dimensions of A = 59 cm and B = 56 cm. As shown in figure 1 a., the generator was adapted to a horn antenna which serves to guide the microwave beam onto the tested samples. This system is placed on a tripod to ease the antenna orientation. Figure 1.b shows the radiation pattern for the horn antenna providing an aperture around 40° in the sagittal and azimuthal plans. Considering all the safety, a protective enclosure was built in order to protect operators from the radiation. Thermograms are recorded with a CEDIP Silver 420 cooled camera equipped with a 320x240 pixels InSb detector. The camera placed at a distance of 1.5m far from the system, provides a field of view of the heated area. The thermograms are recorded at a frame rate of 1 Hz during the tests. Fig. 1 a&b. Microwaves horn antenna and the resulting radiation pattern diagram (principal lobes)