Study On Initial Damage Detection And Evaluation Of Concrete Based On Nonlinear Acoustic Features

There are enormous applications of concrete in the infrastructure construction and its technique condition attracts much concerned as it becomes aging. Most of the structural concrete material in service is experiencing its initial and cumulative damage, it is very beneficial to identify and quantify its technical state. Ultrasonic scanning technique is able to sense the inner change of the materials, but the feasibility using traditional methods based on linear acoustic theory is limited for concrete material. The development of a quantitative nondestructive evaluation procedure to track and quantify damage in concrete, is still a challenging problem. Due to the complex structure and multiple constituents with different mechanical properties, concrete materials are inherently nonlinear. In general, when damaged, they become even more nonlinear. Thus it is very important and valuable on the theory and application to investigate the nonlinear acoustic features of the material so as to quantitatively evaluate the material in its service.This thesis is to develop a method for initial damage detection and quantitative evaluation of the concrete material based on its nonlinear acoustic features, including:Two methods of both a nonlinear harmonic generation and a nonlinear acoustic modulation using the relative nonlinear coefficient are presented, which respectively use the ratio change of the squared fundamental amplitude to the second harmonic amplitude, and also the ratio change of the sideband amplitude to the product of the two harmonic wave inputs as a measure to describe the inner structure change. Either method can be the verification for the other.The mechanism of the nonlinear acoustic modulation for cracking material is discussed and a modulation nonlinearity parameter method is presented with the assumption that the crack asperity behaviors as a spring model. When two waves at a high and a low frequency vary the contact within the interface, nonlinear harmonic generation and a nonlinear acoustic modulation occur. These features characterize the cracks due to the damage. It shows that the nonlinear features for the material with crack are similar to those in the classical nonlinear acoustic theory.A test configuration is presented for the nonlinear acoustic modulation experiment. This research uses the energy of the sideband (integration in the power spectrum) instead of the amplitudes of the modulation peaks as a quantitative measure of nonlinearity. The inherent high attenuation of cement-based materials is overcome with a procedure that uses the relative nonlinear modulation parameter and the sideband energy instead of measuring peak amplitudes. It is demonstrated that this sideband energy is reliable for concrete material with high attenuation. Detailed test setup is designed and some nonlinear signal process techniques are summarized.Experiments on the concrete specimens under the progressive damage due to the alkali-silica reaction are preformed. The presented methods, nonlinear acoustic modulation and harmonic generation, are very sensitive to the initiation of the microcrack, demonstrating a successful application of the two methods to quantitatively track the damage state throughout an entire life-cycle and to nondestructively identify the initiation time and the extent of microcracking in specimens for the first time. The methods can be used for characterizing the progressive microcracking and for quantitative evaluation of the concrete materials.Finally the relationship between the nonlinear coefficient and the damage variable is investigated. A methodology is reasonably presented to quantitatively evaluate the damage based on the nonlinear acoustic features. The feasibility using the elastic modulus to inversely deduce the strength of concrete is discussed, initially establishing a bridge between the degradation on the strength of concrete and its nonlinear coefficient.