Damage identification in reinforced concrete beams using damage index derived from stiffness reduction

Reinforced concrete structures are widely used in civil construction and need regular monitoring during their comparatively long service period. Timely detection of damages is one of the most important strategies for successful health monitoring. Experimental modal testing and subsequent analysis is a popular vibration-based technique to determine structural damage in a non-destructive manner. In the present investigation, experimental modal testing was carried out over six reinforced concrete beams to understand the correlation between the changes in dynamic responses and structural damages at the onset of concrete cracking, as well as during the yielding of reinforcing bars and finally, at the ultimate load. Damages were inflicted on the reinforced concrete beams quasi-statically using a universal testing machine and modal testing was conducted in between each loading and unloading cycle. The results obtained were used to update a representative finite element model of the tested beams at each stage of damage to assess the equivalent stiffness loss. A damage index based on the percentage variation of the stiffness properties of the beam along the length has been used. Investigations were conducted on six reinforced concrete beams, three having a length of 1.8 m and the remaining three slightly longer- 3.3 m, and the damage indices thus identified were found to be effective in locating and quantifying both damaged and impending damage regions in terms of loss of stiffness.