Thermal damage detection in FRP-strengthened reinforced concrete structures using ultrasonic guided waves

This paper presents a study on the application of guided waves for detecting early-stage thermal damage in concrete structures strengthened with externally bonded fibre-reinforced polymer (FRP) composite plates. A theoretical model is firstly introduced, which includes both linear and nonlinear elastic coefficients in order to describe the material properties and behaviour of FRP plates. Thermal damage in turn is simulated by increasing the nonlinear elastic coefficients while a nonlinear parameter is proposed to quantify the extent of thermal damage. A user-defined subroutine is then presented for a three-dimensional (3D) finite element model that has the capability to simulate wave propagation behaviour across various frequencies. Experimental investigations are then reported on a carbon FRP (CFRP)-strengthened concrete beam, where thermal damage is induced by heating a specific area of the CFRP surface using a radiant heater. The sensitivity of the proposed nonlinear parameter for detecting various levels of the thermal damage is experimentally demonstrated. The study validates and demonstrates the reliability and sensitivity of the nonlinear guided wave method for early-stage detection of thermal damage in FRP-strengthened concrete structures.