Grain-scale stress heterogeneity in concrete from in-situ X-ray measurements

Concrete features significant microstructural heterogeneity which affects its response to mechanical loading. Strain localization in the matrix phase of concrete has received significant attention due to its relation to microcracking and failure and because of our ability to quantify it with a combination of X-ray computed tomography (XRCT) and digital image correlation tools. In contrast, strains and stresses in sand and aggregates embedded in the matrix phase remain largely unmeasured but remain critical for micromechanics-based theories of failure and mesoscale modeling. In this communication, we discuss direct strain and stress measurements made within sand inclusions in two samples of mortar containing different sand volume fractions. We used a combination of in-situ XRCT, 3D X-ray diffraction (3DXRD), and scanning 3DXRD to image the microstructure and quantify stresses and stress fields within sand inclusions in each sample. Our results reveal that, in contrast to mean-field theories and inclusion theories from continuum micromechanics, aggregates across the sample feature a broad distribution of average stresses and significant gradients in their internal stress fields. Our work furnishes the first known dataset with these quantitative stress measurements and motivates an improvement in continuum micromechanics models for concrete which can capture stress heterogeneity.