Abstract Summary
Concrete is one of the most widely used construction materials in the world. As it is brittle and has a random, heterogeneous, multi-phase structure, it is very susceptible to cracking. Therefore, the detection of cracks at the earliest possible stage is of great importance. In recent years, various non-destructive testing (NDT) and structural health monitoring (SHM) techniques have been investigated to improve safety and control the current state of structures made of concrete. In this study, the emphasis is on micro-crack monitoring in concrete beams. The experimental analysis was carried out on concrete elements with dimensions of 4 cm × 4 cm × 16 cm. The beams were subjected to three-point bending in a testing machine under monotonic quasi-static loading. During the tests, the fracture process was characterized by ultrasonic waves. The monitoring was conducted using a set of PZT transducers. One of them was an actuator while the others acted as sensors. The excitation was a wave packet composed of a 5-cycle sine wave modulated by the Hann window. The procedure assumed excitation and registration of wave signals at selected time intervals during the whole process of mechanical degradation of concrete specimens in laboratory conditions. The recorded signals were further processed by coda wave interferometry (CWI). The technique allowed the detection of cracks using the decorrelation between ultrasonic wave signals collected at different stages of degradation. Different values of excitation frequencies in the range from 100 kHz to 500 kHz were used to investigate the influence of frequency selection on the effectiveness of the damage indication based on the decorrelation of the coda. The CWI monitoring was integrated with the optical tracking of the fracture using the digital image correlation (DIC) technique. The front side of the beam has been covered with the speckle pattern and the photographs of the samples were taken at selected time intervals during mechanical degradation. Then, local strain changes were calculated enabling the observation of growing cracks. The integrated use of CWI and DIC methods allowed effective fracture monitoring and early-stage damage detection. The combination of these two methods appeared to be successful in the detection of micro-cracks and upcoming macro-type damage. The results obtained from the experiment were intended to highlight the effect of applied frequencies on the coda wave interferometry.
