# Fine Characterization Method of Concrete Internal Cracks Based on Borehole Optical Imaging
## 3 Morphological Characteristics Analysis of Cracks  
### Analysis of Width and Depth  
Depth and width are important parameters to describe the morphological characteristics of cracks, and are also the main parameters for the safety evaluation of concrete buildings. In this paper, the crack depth and width in nine boreholes were measured and statistically analyzed, as shown in Figure 7.  
(1) In borehole zk15 + 025, there were two discontinuous cracks; the first crack started from the orifice and extended to 0.28 m. The second crack started from 0.42 m and extended to 0.97 m. The crack width of the first crack was relatively large, and the extension of the two cracks was relatively small. There was no obvious characteristic of the change in crack width. (2) In zk14 + 896, the crack started from the orifice and extended to 0.81 m. The crack width reached 7.37 mm at the orifice, and the crack width gradually decreased with the increase in depth. (3) In borehole zk14 + 931, the crack started from 0.07 m and extended to 0.33 m. The crack seemed to form a closed oval, which may have been caused by the intersection of adjacent boreholes. The crack extension depth was small, and the crack  
Figure 7: Statistical analysis of crack depth and width. (**a**) Boreholes zk15 + 025, zk14 + 896, and zk14 + 931. (**b**) Boreholes zk15 + 018, zk14 + 994, and zk14 + 819. (**c**) Boreholes zk14 + 858, zk14 + 870, and zk14 + 903.5.  
width increased with the depth. (4) In borehole zk15 + 018, the crack started from the orifice and extended to 1.78 m. The crack width was large at the orifice, reaching 8.54 mm, and then rapidly decreased. The fluctuation of the crack width at local positions was obvious, but decreased with the increase in depth in macro. (5) In borehole zk14 + 994, due to the influence of the intersection of adjacent boreholes, the morphology of cracks at the orifice was not observed. In the borehole wall image, the crack started from 0.23 m and extended to 1.65 m. The crack width was relatively small at the initial position, and also showed a law of decreasing with the increase in depth. (6) In borehole zk14 + 819, there was no obvious crack before 1 m depth. In the borehole wall image, the crack started from 1.09 m and extended to the concrete interface (1.64 m). The crack extension depth was not large, and the change law of crack width was not obvious. (7) In borehole zk14 + 858, the crack started from the orifice, penetrated the concrete interface, and extended to 1.38 m. The crack was wide at the orifice, reaching 11.42 mm, and then rapidly decreased. The crack width also decreased with the increase in depth. (8) In borehole zk14 + 870, the crack started from the orifice, penetrated the concrete interface, and extended to 1.36 m. The crack was also relatively large at the orifice, reaching 9.01 mm. The crack width fluctuated greatly at local positions, but it still showed a trend of decreasing with the increase in depth. (9) In borehole zk14 + 903.5, the upper half of the borehole wall was complete, and the crack started from 0.96 m and extended to 1.36 m. The crack width was basically stable and changed little. Except for borehole zk15 + 025, the extension depth of cracks extending from the orifice was relatively large. There were five boreholes with a crack depth of more than 1 m and three boreholes with a crack depth of more than 1.5 m, accounting for 55.6% and 33.3%, respectively. The extension depth of cracks inside the concrete was relatively small. The width of the crack at the orifice was large, but it often decreased rapidly after a short distance from the orifice. The larger crack width at the orifice may have been caused by the falling block caused by the interference of external factors such as vehicles and pedestrians. The cracks extending from the orifice showed a trend where the width gradually decreased with the increase in depth. However, the fluctuation of the width of cracks inside the concrete was relatively small (except for borehole zk14 + 931), which was not significantly affected by the depth.