Bonding condition monitoring and evaluation of CFRP strengthened RC structures with piezoelectric sensors

Abstract

Carbon fiber-reinforced polymer (CFRP) is a composite material consisting of high-strength fibers and matrix. It has been commonly used to strengthen reinforced concrete (RC) structures. Improved structural performance is guaranteed only when perfect bonding between the CFRP and RC structure is secured. However, CFRP debonding can initiate and propagate in an invisible manner and lead to the brittle failure of the RC structure. Therefore it is of vital importance to monitor the CFRP bonding condition with reliable techniques. Current available global structural health monitoring (SHM) techniques based on the global-level structural vibration characteristics are usually insensitive to such local incipient damages. Therefore, local-level SHM techniques with high sensitivities are required. Among them, electromechanical impedance (EMI) and guided wave (GW) techniques based on the light-weighted, unobstructive piezoelectric wafer active sensor (PWAS) as both the actuator and sensor, have great potential. However, the studies on these techniques and their applications in the CFRP bonding condition monitoring are limited. This thesis is devoted to exploring the potential of the two techniques in this field. The first half of this thesis focuses on experimental and numerical studies on the EMI technique. In this technique, an electrically excited PWAS interacts with the structure and reflects the nearby structural mechanical impedance, which is influenced by the local structural stiffness, mass and damping. The EMI deviation from the baseline data obtained at the intact structural stage serves as the damage index. A preliminary experimental study is conducted in an RC beam. The performances of four commonly used PWAS configurations are comprehensively compared and evaluated for the first time, in which the surface bonded PWAS configuration presents the best performance among the single PWAS configurations. The EMI spectra of the PWASs on the surfaces of CFRP strips in four CFRP shear-strengthened RC beams are collected in the loading tests. The EMI is found more sensitive to the debonding initiation than the strain data collected close to the PWASs. The thresholds for the debonding initiation and complete debonding/final failure based on the EMI are summarized from both the experiment and the finite element analysis (FEA) results. Existing studies on the CFRP bonding condition monitoring using GW technique are mainly from the experimental and numerical aspects. The analytical GW fields under different bonding conditions are still unavailable. Therefore, the analytical formulae to compute the straight- and circular-crested wave fields in multi-layer structures are derived using the normal mode expansion (NME) method in the second half of the thesis. The participation factor of each wave mode and the tuning curves are obtained. The time domain responses are acquired through the inverse Fourier transform of the response spectrum, which is obtained by multiplying the tuning curves and the frequency spectrum of the excitation. With the derived formulae of the straight-crested waves, the wave fields in a free, bonded, and debonded CFRP plate are computed. The results match well with the numerical and experimental results. Both the wave amplitude and propagation time change with the change of the CFRP bonding condition and thus the degradation of the bonding condition can be detected through these changes. The computed circular-crested wave fields in the single- and multi-layer isotropic plates match well with the numerical and experimental results and demonstrate that the derived circular-crested wave computation formulae are capable of accounting for the amplitude induced by the geometry spreading. This PhD study evaluated different PWAS configuration comprehensively and brought the EMI technique in the CFRP bonding condition monitoring and evaluation to a direct application level. The theoretical formulae for computing the GW fields in the CFRP-strengthened RC structures were derived for the first time, with which the bonding condition degradation can be evaluated in both the baseline and baseline-free manner.