Damage identification in FRP-retrofitted concrete structures using linear and nonlinear guided waves

Abstract

Structural health monitoring (SHM) involves the implementation of damage identification methods in engineering structures to ensure structural safety and integrity. The paramount importance of SHM has been recognised in the literature. Among different damage identification methods, guided wave approach has emerged as a revolutionary technique. Guided wave-based damage identification has been the subject of intensive research in the past two decades. Meanwhile, applications of fibre reinforced polymer (FRP) composites for strengthening and retrofitting concrete structures have been growing dramatically. FRP composites offer high specific stiffness and high specific strength, good resistance to corrosion and tailorable mechanical properties. On the other hand, there are grave concerns about longterm performance and durability of FRP applications in concrete structures. Therefore, reliable damage identification techniques need to be implemented to inspect and monitor FRPretrofitted concrete structures. This thesis aims to explore applications of Rayleigh wave for SHM in FRP-retrofitted concrete structures. A three-dimensional (3D) finite element (FE) model has been developed to simulate Rayleigh wave propagation and scattering. Numerical simulation results of Rayleigh wave propagation in the intact model (without debonding at FRP/concrete interface) are verified with analytical solutions. Propagation of Rayleigh wave in the FRP-retrofitted concrete structures and scattering of Rayleigh waves at debonding between FRP and concrete are validated with experimental measurements. Very good agreement is observed between the FE results and experimental measurements. The experimentally and analytically validated FE model is then used in numerical case studies to investigate the scattering characteristic. The scattering directivity pattern (SDP) of Rayleigh wave is studied for different debonding size to wavelength ratios and in both backward and forward scattering directions. The suitability of using bonded mass to simulate debonding in the FRP-retrofitted concrete structures is also investigated. Besides, a damage localisation method is introduced based on the time-of-flight (ToF) of the scattered Rayleigh wave. Numerical case studies, involving different locations and sizes of debonding, are presented to validate the proposed debonding localisation method. Nonlinear ultrasonics is a novel and attractive concept with the potential of baseline-free damage detection. In this thesis, nonlinear Rayleigh wave induced at debondings in FRPretrofitted concrete structures, is studied in detail. Numerical results of nonlinear Rayleigh wave are validated with experimental measurements. The study considers both second and third harmonics of Rayleigh wave. A very good agreement is observed between numerical and experimental results of nonlinear Rayleigh wave. Directivity patterns of second and third harmonics for different debonding size to the wavelength ratios, and in both backward and forward scattering directions, are presented. Moreover, a damage image reconstruction algorithm is developed based on the second harmonic of Rayleigh wave. This method provides a graphical representation for debonding detection and localisation in FRP-retrofitted concrete structures. Experimental case studies are used to demonstrate the performance of the proposed technique. It is shown that the proposed imaging method is capable of detecting the debonding in the FRP-retrofitted concrete structures. Overall, this PhD study proves that Rayleigh wave is a powerful and reliable means of damage detection and localisation in FRP-retrofitted concrete structures.