Enhancement of Contact Acoustic Nonlinearity Effect in a Concrete Beam using Ambient Vibrations

Contact acoustic nonlinearity (CAN) is generated when oscillating crack faces open and close while a wave passes through it. However, reliably assessing the nonlinear effect due to micro-scale defects is challenging, especially in concrete structures, due to their large size, high attenuation and low signal-to-noise ratio. However, concrete facilities vibrate due to ambient excitations such as vehicle movement, wind, and water flow. These ambient vibrations can be utilised in amplifying CAN. For example, a vehicle can be moved at a particular velocity over a bridge to amplify a particular natural mode of vibration. This paper illustrates a method of enhancing contact acoustic nonlinearity with the help of ambient vibrations of the structure. A finite element (FE) model of a concrete beam with a thin crack is developed. The base of the beam was oscillated at 100 Hz. Simultaneously, a 200 kHz ultrasonic excitation was applied on the beam to monitor its propagation through the crack. The closing and opening of the crack generate the nonlinear behavior of the ultrasonic pulse. A considerable increment of nonlinearity was observed demonstrating the efficacy of the proposed method. The time windows for the nonlinear zone have been identified. A laboratory experiment has been performed to demonstrate the proposed method in reinforced concrete beams. This investigation demonstrates that CAN can be utilised in monitoring concrete structures when ambient vibrations are taken into account.