Experimental investigation of damage evolution in concrete beams under cyclic loading: an integrated approach using DIC and modal identification

The reliable detection and monitoring of damage in reinforced and prestressed concrete structures subjected to cyclic loading remains a major challenge in structural engineering. Early-stage damage is often localized, while global stiffness degradation changes only slightly, making it difficult to capture under limited instrumentation and operational conditions. Accurately assessing structural integrity therefore requires combining local measurements of crack development with global dynamic behavior. This work presents an experimental integrated approach for investigating damage evolution in concrete beams, combining vibration-based structural monitoring and local Digital Image Correlation (DIC), and including static four-point bending tests as well as dynamic tests with impulsive excitation. Natural frequencies, damping ratios, and mode shapes were identified, enabling characterization of global stiffness evolution. Vibration-based damage indicators derived from modal shapes and, particularly, from modal curvature were analyzed alongside global modal parameters. Local DIC measurements, focused on crack-prone regions, allowed extraction of Crack Opening Displacement (COD), monitoring of crack initiation and growth, and derivation of load–opening (P-COD) curves and local stiffness metrics. Integration of DIC data with global modal indicators enabled direct correlation between local stiffness loss and changes in dynamic behavior, supporting modal and finite element model updating. This integrated approach highlights the potential of combining local DIC measurements with vibration-based indicators for reliable structural health monitoring, even when instrumentation is limited. By linking local crack evolution to global dynamic response, it enables early damage detection and improved structural assessment under realistic operational conditions.