# Fine Characterization Method of Concrete Internal Cracks Based on Borehole Optical Imaging
## 1 Introduction  
Concrete is one of the most common building materials which is widely used in infrastructure construction such as roads, bridges, tunnels, dams, and housing construction [1, 2, 3, 4]. Concrete is a brittle material with low tensile strength. In the long-term operation process, cracks will inevitably occur due to uneven load, high-temperature thermal deformation, and erosion [5, 6, 7]. Cracks in concrete structures will become channels for external aggressive agents, thus accelerating the damage of buildings [8, 9]. When the depth and width of cracks exceed the critical value borne by the concrete structure, it will seriously affect the mechanical properties of the concrete structure [10], and may even lead to major safety accidents and unnecessary losses. The detection of concrete cracks is extremely important. The traditional way of visual and manual measurement can only be limited to the geometric shape measurement of concrete surface cracks, but cannot be applied to the description of the expansion shape of internal cracks. With the continuous breakthrough of computer technology and sensor technology, researchers have developed ground-penetrating radar [11, 12, 13], ultrasonic technology [14, 15, 16], and a variety of pavement sensors to detect the morphological characteristics of internal cracks in concrete. The ground-penetrating radar can only detect cracks with high accuracy within a limited depth range. When measuring deep cracks, the measurement accuracy will be seriously damaged, and the actual operation requires a high level of experience of inspectors [17, 18]. Ultrasonic testing technology uses couplants tocouple transducers to the surface of the concrete pavement, which leads to inconvenient operation in practice and requires longer testing time [19]. Due to the nonuniformity of concrete, this test method usually leads to high scattering and attenuation of transmitted pulses, and the final spatial resolution may be unsatisfactory [20; 21]. In addition, common detection methods include optical-fiber sensor detection technology, infrared-imaging detection technology, etc. [22; 23; 24; 25], but the complexity of field work and the inaccuracy and cost of test results make it difficult to meet the requirements of the fine description of concrete cracks. Digital image detection is an intuitive and economical crack characterization method [26; 27; 28]. Thanks to the continuous breakthroughs of optical photography technology [29; 30; 31] and digital-image-processing technology [32; 33; 34], the detection method based on digital image technology has attracted the attention of researchers. At present, the optical images of concrete cracks are mainly captured by handheld industrial cameras, vehicle-mounted cameras, and unmanned aerial vehicles. Obviously, these methods can only obtain the surface cracks of concrete buildings, so they can only characterize the geometric shape of concrete surface cracks. However, it is impossible to describe the hidden cracks and the extension and morphological characteristics of cracks in the concrete. In recent years, the research on image detection has focused on algorithm implementation to identify the width and orientation of cracks in the concrete surface image [35; 36; 37; 38]. However, there is a lack of research on the measurement and characterization of the internal crack morphology of concrete. Digital borehole camera technology [39; 40] is an in situ detection method that can go deep into the interior of the borehole and obtain 360\({}^{\circ}\) high-resolution optical images of the borehole wall. The equipment has excellent waterproof performance, so that it can work normally in a deep borehole with high-pressure water, but it needs to replace sewage with clean water [41]. This technology has already been successfully applied in the field of geotechnical engineering investigation, and has achieved very effective detection results [42; 43; 44].