We will quantify the microscale stress and strain fields generated during the macroscopic compression of concrete. Currently, the macroscopic modulus and strength of concrete are predicted accurately by “mean-field” micromechanics theories and numerical models based on the postulate that each sand particle (aggregate) in the microstructure experiences the same stress state during macroscale loading and is perfectly bonded to the surrounding cement-paste matrix. Recent work by the proposers demonstrates that the aggregates instead experience significant stress variability. Our proposed experiments will exploit combined 3DXRD, scanning 3DXRD and x-ray computed tomography, with digital volume correlation, to quantify stress variability and aggregate-matrix debonding during elastic and inelastic stages of compression in-situ. Results will improve micromechanics theories and provide first-of-its-kind information on aggregate-matrix interface mechanisms previously inaccessible in-situ.