Application of vibrothermography to visualise hidden cracking process in concrete at the component scale

A thorough understanding of the cracking process in concrete structures, especially the initiation and expansion processes in the hidden stage, plays a decisive role in the maturity and completion of fracture mechanics theory. However, there exist few simple methods to detect the multi-crack development under the condition of unknown initiation area at the component scale and associated behaviours of the cracks during crack development. In this study, ultrasonic thermal excitation to visualise the multi-point hidden cracking process in concrete components was studied and hidden cracking detecting experiments were conducted. Multi-point initiation and expansion within the range of the components were achieved by destroying a suitably reinforced concrete beam using four-point bending tests with step-by-step cyclic loading. The cracking processes were monitored in real time via the auxiliary use of strain gauge groups and digital image correlation (DIC). The loading process was controlled based on the strain rules, and typical cracking stages were observed. The ultrasonic thermal excitation visualisation effect at typical cracking stages was studied via local monitoring using DIC. The results show that the ultrasonic thermal excitation visualisation method and the corresponding device proposed herein can be used to monitor hidden cracking processes in concrete at the component scale, while multi-point initiation and expansion of 0.01-mm-level crack groups can be detected in a simple and efficient manner.