Numerische und experimentelle Untersuchungen zur zerstörungsfreien Charakterisierung von Hochleistungsbetonen mittels linearer und nichtlinearer Ultraschallwellen

Increasing damages on building structures in recent times, as well as the deteriorating state of civil infrastructure resulting from the growing number of severe natural catastrophes and higher traffic load, are causing immense costs for refurbishment and maintenance work in the construction sector worldwide. To ensure the durability and to prevent serious damage events, the development of modern high-performance materials and health monitoring of buildings is becoming increasingly important. Non-destructive testing using ultrasound-based methods can make a valuable contribution on this. In addition to the conventional and proven linear ultrasound methods, such as the impact-echo method and the pulse-velocity measurement, especially the so-called non-linear ultrasound methods offer the potential for reliable material characterization and damage assessment of construction materials. In this case, nonlinear ultrasound methods are related to the nonlinear elastic parameters of a material and are characterized by a high sensitivity to early material degradation in the form of micro-damage, which is usually hidden from the conventional linear ultrasound methods.
For the practical application of novel ultrasound methods in the construction sector, it is first necessary to understand the relevant relationships and acoustic effects of non-linear wave propagation in construction materials. Numerical simulation methods, which can be used in the case of complex material properties and 3-D problems, are suitable for this purpose. Therefore, in the numerical part of this thesis, based on the classical Chebyshev-pseudospectral-collocation method, an efficient numerical method of high accuracy is developed, which allows to implement arbitrary nonlinear material laws. Subsequently the nonlinear wave propagation in concrete under the consideration of acoustic attenuation and damage is treated numerically. In the subsequent experimental part of this work the linear and nonlinear ultrasound techniques are tested for characterization and damage assessment in unreinforced and steel-reinforced high-performance concrete. Their applicability for the ultrasonic determination of the linear
and nonlinear properties of high-performance concrete is shown by means of examples. The sensitivity of nonlinear ultrasound technology for the detection of mechanically induced damage in the form of microcrack formation and growth is examined and demonstrated.