Experimental evaluation of damage in RC beams using natural frequency and mechanical behavior

To investigate the influence of deterioration on the load-carrying capacity and deformation performance of reinforced concrete (RC) members, four model beams were constructed, with three of them incorporating artificial damage such as bottom cover chipping and stirrup cutting to simulate typical degradation observed in aging bridge components. Static cyclic loading tests were conducted to examine the mechanical behavior of each beam under increasing levels of damage. Natural frequency was adopted as a quantitative indicator of structural soundness and measured at different stages using the non-destructive monitoring technique known as the impact vibration test (IVT). It was observed that higher-order vibration modes, especially the fourth mode, responded sensitively to reductions in bending rigidity and were less affected by boundary conditions. A numerical eigenvalue analysis model was developed to reproduce the variation in natural frequencies, incorporating localized stiffness reductions based on crack development in the mid-span region. The comparison between measured and simulated results validated the accuracy and applicability of the proposed method. Additionally, a health index based on high-order natural frequencies was introduced to quantify structural deterioration and support maintenance decision-making. Threshold values were discussed to distinguish between safe, repair-needed, and high-risk conditions. The findings confirm that natural frequency, particularly at higher modes, can serve as a reliable and practical tool for damage detection and structural health evaluation in RC beams. This approach provides a scientific basis for timely repair planning and contributes to the advancement of condition-based maintenance for deteriorated concrete infrastructure.