Environmentally/mechanically caused changes in microstructure and their effect on coda signals

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Coda Wave Interferometry (CWI) is particularly suitable for the long-term assessment of reinforced concrete (RC) structures.The objective of this project is to develop and understand the Coda technique as a tool for assessing the structural integrity and durability of RC structures. Using well-controllable one-dimensional experiments, this subproject aims to determine the effect of mechanical load and environmental impacts on the Coda signal. On the one hand, calibration curves for the perturbation of the Coda because of direct impacts will be established. On the other hand, typical concrete phenomena, e.g. the long-term processes carbonation or frost attack, will be characterized in the time domain. At the end of the project, an entire spectrum of calibration curves and concrete-specific features will be provided to enable calibration and signal compensation at real engineering structures.In order to validate the origin of signal perturbations, changes in concrete microstructure are characterized using conventional and innovative techniques. The relationships between impacts and microstructural changes, together with the corresponding analysis and interpretation of the Coda signals, provide important input parameters for modelling wave propagation, material properties and structural changes as they are developed within the scope of other individual projects within this research unit.During the second phase of this project, relationships from one-dimensional tests from the first phase are extended systematically by the effect of specimen geometry, surface condition, and reinforcement, all related to the corresponding changes in the Coda. In addition, practically relevant moisture and temperature gradients will be investigated following the characterization of homogeneous states. Special focus will be put into the separation between perturbations from reversible compared to irreversible changes on the micro structural level by means of the validating methods computed tomography, acoustic emission, and thin section microscopy.Furthermore, the research unit intends to conduct benchmark tests for CWI monitoring at two engineering structures, the Gänstorbrücke, which connects Ulm and Neu-Ulm, and the underground station Scheidplatz in Munich. TUM1 will take responsibility for the coordination of the instrumentation of the Scheidplatz as well as the preparation and maintenance regarding data acquisition to a common database.With this project, the basis for many applications of the CWI method in modern construction technology and our built environment will be provided.