Digital concrete physics - High-resolution X-ray computed tomography (XRCT) and microstructural investigations of concrete under confining pressures

Understanding the microstructural details of concrete is crucial for improving their material properties in structural applications within civil engineering. Digital Rock Physics (DRP) refers to a modern technique that enhances the understanding of the physical behavior of rocks or respectively concrete through microscale imaging of their internal structures. Based on the non-destructive method of high-resolution X-ray computed tomography (XRCT), which is still widely underestimated, it is possible to obtain information e.g., on phase distributions, volume characteristics like pore spaces and furthermore microstructures such as microcracks can be visualized. Here we used the XRCT to investigate the influence of external mechanical loading on concrete. XRCT images with different resolutions were performed under confining pressures of 0.1 MPa to 46 MPa. The generated and analyzed CT images of unloaded and loaded (i.e., due to external stress) concrete are compared with respect to any potential microstructural changes. We specifically examined the segmentation process and its impact on the determined effective material properties. Despite the many possibilities enabled by XRCT technology, there are still challenges in identifying microstructures or phases correctly, due to its relatively low resolution, which also complicates the assignment of their physical properties based on numerical simulations. For the interpretation of the concrete’s CT images, additional methods are needed. This applies in particular to grain and phase boundaries of individual aggregates, transition zones or very fine-pored phases. For this reason, the high-resolution image-based data from XRCT is combined with standard polarization and scanning electron microscopy (SEM). Together with these additional fundamental laboratory techniques, it is possible to receive more detailed information of the structure, detect internal changes at all scales and to get an optimal spatially segmented image of the concrete samples. It is possible to determine realistic synthetic scenarios for different loading situations, which enables the application of advanced numerical approaches. This study demonstrates the importance of understanding the internal microstructure of concrete-based structures to analyze the XRCT images and to identify the effects of external stresses in concrete and thus the factors that influence the accuracy of physical measurements at elevated pressure conditions.