Damage detection of a reinforced concrete beam using the modal strain approach

Structural health monitoring using ambient vibration measurement has drawn significant attention owing to its ease of operation and lower cost compared with traditional measurement methods. Most techniques based on ambient vibration measurement employ measurement acceleration or velocity time-history data to compute dynamic characteristics such as fundamental periods, damping ratios, and vibration mode shapes, which are non-stationary and insensitive to small or local damage. In this study, we explored a scheme that adopts dynamic strain signals for the construction of modal strain patterns from a 5-m-long reinforced concrete beam to detect local damage in the early stages. Dynamic strain measurements were performed using piezoresistive strain gauges with a strain resolution of 0.02 & mu;e. Strain gauges were installed at the top and bottom steel reinforcements at different locations along the axis of the beam to capture the distribution of the strain. Furthermore, a finite element model of the beam was constructed for comparison with the experimental results. Signal processing of measurement data and frequency domain analysis in the numerical model was conducted in the frequency domain to obtain the modal strain pattern and neutral axis position, which indicates the level of damage. The location and the severity of the damage can be identified even at an early stage, in contrast to the conventional technique, which is based on a natural frequency that is insensitive to early damage levels. The results of the neutral axis position and the modal strain pattern between random excitation strain measurement and finite element analysis were in good agreement. Minor discrepancies between the experimental and numerical results could be due to the defects of the material properties in the numerical model and the imperfection of the geometry of the specimen.