Nonlinear ultrasonics-based technique for monitoring damage progression in reinforced concrete structures

In reinforced concrete (RC), material nonlinearity is evident even in its undamaged state due to the inherent microstructure. In the present work, damage progression in RC structure at different levels of damage is investigated using linear and nonlinear ultrasonic techniques. The primary focus of this study is to monitor the structure from its initiation stage(s) of damage to advanced stages. Ultrasonic velocity tomography is first implemented to identify the weaker regions and map any damage occurring at various levels of loading. Two critical regions are identified from ultrasonic tomography and further damage characterization is carried out using various ultrasonic techniques to quantitatively assess the progression of damage in these two regions. The linear ultrasonic techniques such as time-of-flight (TOF) and attenuation, and the nonlinear ultrasonic techniques such as sub- and super- harmonic, energy distribution, etc. are employed to detect the damage progression. It is found that the changes in linear parameters due to damage progression in RC structure are often insignificant and inconsistent. However, some of the nonlinear ultrasonics-based techniques are found to be very efficient to monitor the damage progression. A relatively new and promising nonlinear ultrasonic technique, namely the sideband peak count-index (or SPC-I) provides a very clear and consistent indication of damage at the early stage. The present study shows that during the initial stages of damage, SPC-I based nonlinear technique performs significantly better (at both regions as identified through ultrasonic tomography) than other linear and nonlinear techniques, whereas at higher damage stage the superiority of this nonlinear ultrasonic technique slowly diminishes. The present study also shows that out of all nonlinear ultrasonics-based techniques considered here, SPC-I technique provides the highest sensitivity to the damage progression and can be effectively used as a very robust nonlinear ultrasonic tool for identifying the onset and progression of damage in RC structures.