Digital Concrete Physics – Microstructural Characterization of Concrete under Confining Pressure: Insights from X-ray Computed Tomography and Microscopy

The investigation of microstructural characteristics in concrete constitutes a fundamental basis for advancing its performance in civil engineering construction. Existing expertise in digital rock physics (DRP), developed for natural rock materials, is transferred and adapted for concrete. DRP utilizes non-destructive X-ray computed tomography (XRCT) to examine the internal microstructure of concrete, allowing for the visualization of features such as phase distributions, pore space, and microcracks. In this study, high-resolution digital concrete twins are created to capture and quantify internal microstructural changes induced by external mechanical loading. To overcome limitations in phase and microstructure identification caused by the restricted resolution of XRCT, these digital investigations are complemented by detailed microstructural analyses using standard polarization microscopy and scanning electron microscopy (SEM). The results show that externally applied stresses significantly influence the microstructural response of concrete and thus affect the accuracy of physical measurements conducted under high-pressure conditions.

XRCT datasets with varying spatial resolutions were acquired under in-situ confining pressures ranging from 0.1 MPa to 46 MPa. CT images of concrete in unloaded and mechanically loaded states were subsequently analyzed and compared to identify stress-induced microstructural changes, with particular emphasis on the segmentation workflow. Here, particular focus is on large and small concrete aggregates, grain/phase boundaries within the aggregates, (micro-)porosity, and especially the interfacial transition zone (ITZ), which represents a major source of uncertainty in phase assignment during segmentation.

Image quality was first assessed by identifying artifacts and evaluating grayscale histograms. Subsequently, global thresholding was applied for phase assignment and initial segmentation, which was iteratively refined using complementary microscopic analyses of thin sections, including SEM, as reference data. The resulting segmentation of the concrete subvolume (600x600x769) distinguishes large and small aggregates (<80 % quartz, ca. 20 % phyllosilicates), pore space, phyllosilicate-composed matrix, silica-composed matrix, and inclusions (mainly rutile, zircon, apatite, iron oxides). Small changes can be seen in the distribution of the individual phases at the different pressures. With increasing pressure, the porosity decreases, and partially areas with characteristic phase arrangements arise along the large aggregates, potentially indicating the influence of the ITZ.

However, the quantitative determination of the interfacial transition zone remains challenging using XRCT data, and microcracks are likewise difficult to reliably resolve and segment. Therefore, the high-resolution microstructural investigations are also required to adequately capture these features. Overall, the study highlights the necessity of detailed microstructural characterization for the reliable interpretation of XRCT data and the assessment of stress-induced changes in concrete.