Application of numerical procedure for thermal diagnostics of the delamination of strengthening material at concrete construction Wojciech Piotr Adamczyk Institute of Thermal Technology, Silesian University of Technology, Gliwice, Poland Marcin Gorski Faculty of Civil Engineering, Department of Structural Engineering, Silesian University of Technology, Gliwice, Poland Ziemowit Ostrowski and Ryszard Bialecki Institute of Thermal Technology, Silesian University of Technology, Gliwice, Poland Grzegorz Kruczek and Grzegorz Przybyla Department of Energy and Environmental Engineering, Politechnika Slaska, Gliwice, Poland, and Rafal Krzywon and Rafal Bialozor Silesian University of Technology, Gliwice, Poland Abstract Purpose – Large structural objects, primarily concrete bridges, can be reinforced by gluing to their stretched surface tapes of fiber-reinforced polymer (FRP). The condition for this technology to work requires the quality of the bonding of FRP and the concrete to be perfect. Possible defects may arise in the phase of construction but also as a result of long-term fatigue loads. These defects having different forms of voids and discontinuities in the bonding layer are difficult to detect by optical inspection. This paper aims to describe the development of a rapid and nondestructive method for quantitative assessment of the debonding between materials. Design/methodology/approach – The applied technique belongs to the wide class of active infrared (IR) thermography, the principle of which is to heat (or cool) the investigated object, and determine the properties of interest from the recorded, by an IR camera, temperature field. The methodology implemented in this work is to uniformly heat for a few seconds, using a set of halogen lamps, the FRP surface attached to the concrete. The parameter of interest is the thermal resistance of the layer separating the polymer tape and the concrete. The presence of voids and debonding will result in large values of this resistance. Its value is retrieved by solving an inverse transient heat conduction problem. This is accomplished by minimizing, in the sense of least squares, the difference between the recorded and simulated temperatures. The latter is defined as a solution of a 1D transient heat conduction problem with the already mentioned thermal resistance treated as the only decision variable. Findings – A general method has been developed, which detects debonding of the FRP tapes from the concrete. The method is rapid and nondestructive. Owing to a special selection of the compared dimensionless The delamination of strengthening material 2655 Received 2 April 2019 Revised 25 June 2019 Accepted 22 July 2019 International Journal of Numerical Methods for Heat & Fluid Flow Vol. 30 No. 5, 2020 pp. 2655-2668 © Emerald Publishing Limited 0961-5539 DOI 10.1108/HFF-04-2019-0278 The current issue and full text archive of this journal is available on Emerald Insight at: https://www.emerald.com/insight/0961-5539.htm