Detection of flaw in FRP-concrete composite system using nonlinear ultrasonic wave propagation characteristics

Abstract

Retrofitting of existing structures is one of the most environment friendly, sustainable, and effective ways to improve a structure’s energy performance. High strength Fiber Reinforced Polymer (FRP) offers colossal potential for lightweight and economic retrofitting of concrete infrastructure through external bonding of concrete components and enhance their load carrying capacity (both flexural and shear). In FRP strengthened structures, defects can initiate during or after the retrofit process, and several factors can attribute to those defects, such as improper installation, lack of adhesion due to environment conditions, concrete cover separation, intermediate shear or flexure crack induced interfacial debonding, etc. The common and major source of defect in FRP strengthened concrete beam takes place between the composite and concrete. Hence, the integrity and durability of FRP strengthened concrete structure remains as foremost concern in ensuring the safety and performance of the FRP retrofitted structures. In the present study, ultrasonic wave propagation technique is used for nondestructive evaluation (NDE) of FRP retrofitted beam. During specimen preparation, CFRP laminate is pasted on the concrete beam (150 mm × 200 mm × 1300 mm) using epoxy as the adhesive and the artefacts are used to simulate the flaw in the epoxy layer. An attempt has been made in this research to detect the flaw in CFRP laminate strengthened concrete beam by employing the linear and nonlinear ultrasonic methods: Time of Flight (TOF), attenuation, wavelet energy, dynamic time warping, hilbert phase, side-band energy and Sideband Peak Count Index (SPC-I). The results show that, the methods that are failing to show good trend in lower frequency excitation, may exhibit extraordinarily good trend with higher frequency. The study infers that, the waves change their travel path due to de-bonding and propagate rapidly until it reaches the bonding phase and hence results in reduction in TOF. The sensitivity analysis is also performed to figure out the most efficient ultrasonic methods for identifying the flaw in the epoxy layer. SPC-I has given a clear and steady trend of increment in lower as well as higher frequency transmission. The proposed non-invasive NDE using the nonlinear ultrasonic (NLU) technique is found to be very promising for integrity assessment on the underlying flaws in the FRP-concrete composite system.