Abstract

In this paper, a fully non-contact second harmonic generation (SHG) technique using a pair of air-coupled ultrasonic transducers is developed and the feasibility of the technique is investigated through the evaluation of surface cracks of concrete beams subjected to the bending load. The reliability of developed technique is subsequently validated by comparing the non-contact nonlinear ultrasonic measurements with measurement results based on contact-type sensors, where the coefficient of variation of non-contact measurements is averagely about 46% of the contact measurements. The defined nonlinear parameter is found to have a monotonically increasing trend with the growth of concrete crack, and the nonlinear parameter corresponding to the largest crack increases about 7 times from its initial value corresponding to the sample in intact state. In contrast, the increase of linear parameter namely the time-of-flight of surface wave is only about 40%. The difference as high as one order of magnitude verifies the high sensitivity of developed air-coupled SHG technique. With consideration of the easily removable characteristic of air-coupled ultrasonic measurements, the developed SHG technique could be promising for the large scale quality control of concrete structures in engineering practice.

Highlights

  • The evaluation of health condition of aged concrete structures is of importance for the service security

  • Various nondestructive evaluation (NDE) techniques have been attempted to assess the damage of concrete structures and experimental techniques based on the ultrasonic wave propagation are the most commonly used in academia and industry [1,2,3,4,5]

  • The phase velocity of Rayleigh wave is considered as a typical parameter to characterize the structure defects and is used here for a comparison with the nonlinear parameter of second harmonic generation (SHG) measurements

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Summary

Introduction

The evaluation of health condition of aged concrete structures is of importance for the service security. Various nondestructive evaluation (NDE) techniques have been attempted to assess the damage of concrete structures and experimental techniques based on the ultrasonic wave propagation are the most commonly used in academia and industry [1,2,3,4,5]. There exist several types of nonlinear ultrasonic techniques according to the different ultrasonic phenomena. The first method is based on the investigation of higher order harmonic of primary signal, which could be excited due to the existence of structural damage. Nucera and Lanza di Scalea [10] used the second harmonic generation technique to monitor load levels in multi-wire steel strands in concrete.

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