Linear and Non-Linear Guided Waves for Damage Detection in Reinforced Concrete Structures

Abstract

Reinforced concrete (RC) structures are a major part of our civil infrastructure and they provide valuable services in the sectors of residential, commercial, transport, defence, education and entertainment. RC structures can be subjected to debonding, corrosion and fire damage in unforeseen scenarios. The debonding and corrosion in RC reduce the bonding between steel rebar and concrete, which ultimately reduces the structural capacity of RC structure. Similarly, fire damage affects the structural and bond strength of RC structure. The fully established fire can cause temperature to rise above l,000°C, which can result in structural failure. Over the last decades, the fire damage has caused loss in structure, life and property. Structural health monitoring (SHM) of RC structures is a vital tool to detect damage in the early stages to avoid critical failure of such structures. Non-destructive testing using guided waves is an important technique for damage detection. Nonlinear features of guided waves are sensitive to material nonlinearity, cracks or debonding damage. The aim of this thesis is to utilize linear and nonlinear features of guided waves to detect debonding damage in rebar embedded in concrete. The work presented in this thesis has also investigated the effect of fire or heat damage on debonding between rebar and concrete in RC structures. The thesis comprises published and prepared journal articles under the scope of damage detection in RC structures using linear and nonlinear guided waves. Chapter 1 highlights the importance oflinear and nonlinear guided wave techniques to detect damage. The propagation of guided wave modes in cylindrical structures as re bar is also discussed.Nonlinear techniques, such as second harmonics and guided wave mixing, are introduced for damage detection, which has not been investigated for debonding damage in RC structures in the literature. Chapter 2 proposes to use the nonlinear feature of guided wave, second harmonic, to detect debonding damage in RC beam. Numerical and experimental investigations have shown the presence of second harmonics in debonded specimens, which are generated due to contact acoustic nonlinearity (CAN) between the rebar and concrete surfaces. Chapter 2 also presents study to locate debonding damage using nonlinear guided waves. In Chapter 3, an advanced nonlinear guided wave technique, guided wave mixing, is proposed to be used to detect debonding damage in ribbed reinforced concrete beams. Longitudinal mode pairs are mixed by satisfying synchronism conditions to generate sum combinational harmonics in longitudinal and torsional directions. Chapter 3 also proposes to use the phase reversal technique for magnifying the presence of sum combinational harmonics due to debonding damage. The sum combinational harmonics in the torsional direction are found to be more sensitive to damage than the longitudinal direction. The impact of heat damage on reinforced concrete beams is investigated in Chapter 4. Nonlinear guided wave techniques have shown that second harmonics are produced due to debonding between rebar and concrete as the CAN effect. The debonding is generated due to heat damage in RC beam from 100°C to 800°C. The absence of second harmonics in bare steel re bar heated from 100°C to 800°C shows that second harmonics in RC beam are sourced from debonding or contact effect as compared to material nonlinearity in steel rebar. It is found that nonlinear features are more sensitive to debonding type damage as compared to the linear features.