# Fine Characterization Method of Concrete Internal Cracks Based on Borehole Optical Imaging
## 5 Conclusions  
This paper introduces the borehole camera technology, which is an advanced technique for measuring in a borehole. It represents a new attempt to apply this technique to the measurement of the morphological characteristics of concrete cracks. The acquisition of a high-resolution borehole wall image provides a feasible scheme to accurately characterize the width, depth, orientation, and other valuable indicators of concrete internal cracks. Although this paper focused on the morphological characteristics of cracks in concrete, borehole camera technology can also be used to study the morphology and distribution characteristics of coarse aggregate. In addition, this technology can also be used for the fine measurement and characterization of internal structures or damages in other fields. For example, borehole cameras have been widely used for the measurement and statistical analysis of underground structures in geology and geotechnical engineering.  
In the application case of this paper, nine boreholes were measured and the crack morphology characteristics were quantitatively characterized. The measurement results show that the average depth of the crack extending from the orifice was 1.195 m, and the width decreased with depth. The orientation of the crack in different boreholes was basically the same at the orifice, with a maximum deviation of 19\({}^{\circ}\), and the orientation of the crack was basically stable from the orifice to the termination depth, with the deviation not exceeding 30\({}^{\circ}\). However, the crack orientation will change obviously after passing through the concrete interface. The average extension depth of cracks inside the concrete was 0.443 m, and the change in width was relatively small, but the orientation changed greatly. Coarse aggregate in concrete will force cracks to change orientation at the local position.  
Figure 11: Comparison and analysis of two measurement results.  
The induction effect of coarse aggregate characteristics (uniformity and morphological characteristics) on concrete orientation is a very meaningful topic. Obviously, borehole camera technology is an effective means for this research, and more attention will be paid to subsequent research.  
Borehole camera technology is also an effective method for the long-term monitoring of fracture development. Although limited by construction requirements in this paper, the long-term measurement of multiple boreholes was not carried out. However, the difference in crack propagation was still found in two measurements, which is very meaningful and helpful for the evaluation and maintenance of concrete buildings.  
Conceptualization, C.W. ([PERSON]); methodology, Z.H. and Y.W.; software, [PERSON] ([PERSON]); validation, Z.H. and Y.W.; formal analysis, [PERSON] ([PERSON]); investigation, [PERSON] ([PERSON]), S.C. and Z.H.; resources, Z.H. and J.W.; data curation, Y.W. and S.C.; writing--original draft preparation, [PERSON] ([PERSON]) and Y.W.; writing--review and editing, [PERSON] ([PERSON]), [PERSON] ([PERSON]) and Z.H.; visualization, J.W.; supervision, [PERSON] ([PERSON]); project administration, S.H.; funding acquisition, Z.H. and J.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Key Research and Development Program of Hubei Province, grant number 2021 BAA201, the Systematic Project of Guangxi Key Laboratory of Disaster Prevention and Engineering Safety, grant number 2020 ZDK015 and the National Natural Science Foundation of China, Grant numbers: 41731284, 41902294.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.