Damage evolution and full-field 3D strain distribution in passively confined concrete

The damage evolution of concrete under multiple stress states is complicated because the triaxial stress on the heterogeneous concrete affects mesoscale interactions between aggregates and mortar, which further alters the internal cracking of concrete and its load bearing mechanism. Hence, probing the internal cracking evolution of concrete under a complex stress state is the key to better understanding the characteristics and failure mechanism of concrete material. This study conducted a series of X-ray computed tomography (X-CT) tests on fiber-reinforced polymer (FRP) confined concrete with varied in situ axial load levels. Different cross-sections of concrete specimens—circular, square with round corners, and square with sharp corners—were designed to achieve uniform or nonuniform passive confinement conditions. The obtained volumetric images from X-CT clearly exhibited the internal damage evolution of concrete, where crack growth, shear banding, and failure localization were displayed for the concrete under varying triaxial stress states. The development of damage to concrete due to cracking with increasing load was also quantified by image segmentation. In addition, the digital volumetric correlation technique (DVC) was employed to produce the volumetric displacement field and strain distribution in the confined concrete according to the image variation of natural speckles. A three-dimensional internal strain analysis revealed that strain localization was mainly caused by mesoscale concrete heterogeneity and nonuniform confinement. More importantly, this study provides the first database of full-field strain tensors for FRP-confined concrete under different axial load levels. These data are valuable for the development of concrete mechanics, failure criteria of materials, and computational modeling of concrete materials and their structures, especially for concrete under complex stress states.