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

Concrete features significant microstructural heterogeneity which affects its mechanical behavior. Strain localization in the matrix phase of concrete has received significant attention due to its relation to microcracking and our ability to quantify it with X-ray computed tomography (XRCT). In contrast, stresses in sand and aggregates remain largely unmeasured but remain critical for micromechanics-based theories of failure. Here, we use a combination of in-situ XRCT, 3D X-ray diffraction (3DXRD), and scanning 3DXRD to directly measure strain and stress within sand grains in two samples of mortar containing different sand volume fractions. Our results reveal that, in contrast to inclusion theories from continuum micromechanics, aggregates 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 improvements in micromechanics models for concrete which can capture stress heterogeneity.