Study on the mesoscopic failure and fractal characteristics of concrete with holes and cracks

This study investigates the influence of concrete's mesoscopic heterogeneity and internal defects on its mesoscopic mechanical behavior and macroscopic nonlinear response. Using defective concrete as a model system, the research systematically examines the impact of mesoscopic heterogeneity on fracture processes and damage evolution. This is accomplished through an integrated approach combining micro-CT scanning, digital image processing, physical experiments, and numerical simulations. Fractal dimension analysis is then employed to provide an in-depth characterization of mesoscopic damage evolution. It is important to note that the acoustic emission (AE) results discussed are derived from numerical simulations, and the results demonstrate that: (1) The shape, size, and spatial distribution of internal constituents contribute to mesoscopic heterogeneity, which can be effectively quantified using digital image processing techniques. The resulting mesoscopic structural model, when implemented within the RFPA2D (Rock Failure Process Analysis System), provides reliable numerical simulation results. (2) The reduced mechanical strength of the interfacial transition zone between aggregate and mortar matrix severs as a preferential location for crack initiation. Crack initiation and subsequent propagation are preferentially observed in regions of high stress concentration, typically induced by pre-existing cracks and pores. Aggregate particles impede crack propagation, causing deviations in the crack path. The inherent mesoscopic heterogeneity of the material is a fundamental driver of this irregular crack propagation. (3) Analysis of the fractal dimension of the damage zone within the concrete reveals that the fractal dimension increases with applied stress up to the point of peak stress. The fractal dimension reaches its maximum value at the peak stress. The fractal dimension of the undamaged specimen is 1.342, representing the maximum observed value, whereas the specimen containing holes and cracks exhibits a minimum fractal dimension of 1.211. This indicates that the presence of defects such as holes and cracks leads to stress concentration, resulting in a more localized accumulation of failure elements and, consequently, a lower fractal dimension value. (4) The fractal dimension of the damage zone provides a valuable metric for characterizing the internal damage development within concrete. This approach offers a novel perspective for quantitatively investigating the mesoscopic damage mechanism in concrete and establishing a direct link between internal structural modifications and the macroscopic mechanical behavior of the material.