Applications of numerical simulations for impact echo and ultrasonic testing on concrete structures

Non-destructive testing methods are effective tools to assess the condition of concrete structures without causing additional damage. As concrete is the most frequently used construction material in the world and all concrete structures are degrading over time, such testing methods are a crucial aspect of inspections of public infrastructure, which aim at ensuring public safety. Many non-destructive testing methods, such as impact echo and ultrasonic testing, utilize waves as the primary information carrier. These waves are reflected at all material interfaces, which poses some unique challenges for when the methods are applied to concrete with its heterogeneous mesostructure consisting of aggregates, cement matrix, and pores. A numerical simulation code based on the elastodynamic finite integration technique was implemented to investigate wave propagation for these two non-destructive testing methods. A heterogeneous concrete model was used to increase the realism of the simulations. This thesis investigates four different applications of the simulation code related to noise simulation during impact echo and ultrasonic testing.
During ultrasonic testing, strong noise amplitudes superpose with reflection signals originating from defects inside the inspected structure and might cause these defects to be overlooked during inspections. It is, therefore, necessary to investigate the capabilities of ultrasonic testing inspection systems. Typically, such investigations are performed by using probability of detection analyses. However, such analyses require data from defects, that can only be detected under certain boundary conditions. Under different circumstances these defects might get overlooked. Therefore, numerical simulations are used to emulate real-world inspection results and estimate the detectability of defects with varying sizes and depths. These results are then used to design a concrete specimen with artificial defects implemented inside it. Measurement results showed good qualitative agreement to the simulations. Approximately 68% of the defects implemented into the specimen could be detected, proving the high degree of realism in the simulations and enabling probability of detection analysis for ultrasonic testing data from a concrete structure.
The second application of numerical simulations in this thesis aims to investigate physical and numerical factors influencing the outcome of numerical impact echo simulations. The goal of impact echo is to measure the frequency of the zerogroup-velocity S1 Lamb wave mode. Numerical simulations of this non-destructive testing method are rarely performed. As the wavelengths during impact echo tests are of similar size as the thickness of a structure, many simulations approximate concrete as a homogeneous medium. However, it is known that pores and aggregates affect the simulation outcome. This study investigates the influence of material heterogeneity on wave speeds and the S1 Lamb wave frequency. Additionally, material attenuation and the frequency spectrum of the source function are investigated to increase the realism of simulations. Lastly, impact echo measurements are recreated numerically with 2D and 3D simulations. Simulation and measurement results are compared in time and frequency domain. It was found that 3D simulations show a high degree of realism, whereas 2D simulations only capture some of the features found in the measurements.

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