Axial stress measurement of concrete with nonlinear ultrasonic harmonic method

The working stress is an important indicator of the reliability of concrete structure. Nondestructive testing of the concrete working stress can directly evaluate the stress state of concrete structure, which plays an important role in the health diagnosis of concrete structure. When ultrasonic wave propagates in concrete, it is affected by the cracks and the harmonic component is generated. The concrete working stress has a direct influence on the number and development trend of the cracks, which provides a potential way to evaluate the working stress in the concrete by analysis the harmonic component. In this paper, a nondestructive testing method of concrete working stress based on nonlinear ultrasonic harmonic method is proposed. Specifically, the theoretical expression of the nonlinear coefficient of ultrasonic wave propagation in concrete is derived, and the relationship between the nonlinear coefficient and the concrete working stress is established. The sensitivity of the nonlinear coefficients is studied when the ultrasonic wave propagates perpendicular to the concrete stress direction. The relationship between the concrete working stress and the ultrasonic nonlinear coefficient is compared using four different strength grades of concrete. It is found that the nonlinear coefficient of ultrasonic wave increases with the concrete working stress, and the fitting relationship between the concrete working stress and the nonlinear coefficient is consistent with the theoretical formula. This study provides a possible way for the nondestructive detection of the concrete working stress.