Available via license: CC BY 4.0
Content may be subject to copyright.
Citation: Wang, C.; Han, Z.; Wang, Y.;
Wang, C.; Wang, J.; Chen, S.; Hu, S.
Fine Characterization Method of
Concrete Internal Cracks Based on
Borehole Optical Imaging. Appl. Sci.
2022,12, 9080. https://doi.org/
10.3390/app12189080
Academic Editor: Hwa Kian Chai
Received: 28 August 2022
Accepted: 6 September 2022
Published: 9 September 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
applied
sciences
Article
Fine Characterization Method of Concrete Internal Cracks
Based on Borehole Optical Imaging
Chao Wang 1,2 , Zengqiang Han 1,* , Yiteng Wang 1, Chuanying Wang 1, Jinchao Wang 1, Shuangyuan Chen 1,2
and Sheng Hu 1
1State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics,
Chinese Academy of Sciences, Wuhan 430071, China
2University of Chinese Academy of Sciences, Beijing 100049, China
*Correspondence: zqhan@whrsm.ac.cn
Abstract:
The internal cracks of concrete are very important in the safety evaluation of structures,
but there is a lack of fine characterization methods at present. Borehole cameras are a piece of in situ
borehole detection technology which can measure the structural elements of a borehole wall with
high precision. In this paper, borehole camera technology is used to measure the concrete cracks of
a tunnel floor, and the morphological characteristics (depth, width, and orientation) of the cracks
are analyzed. The results show that the average extension depth of the crack extending from the
orifice exceeds 1.195 m, and the width decreases with the increase in depth. The crack orientation
is basically stable, with the maximum deviation of 19
◦
at the orifice of different boreholes and 30
◦
at different depths of the same borehole. The crack inside the concrete (not extending to the orifice)
usually has a small extension depth and a relatively stable width, but the crack orientation changes
greatly. The coarse aggregate and concrete interface have different effects on the extension direction
of cracks. This paper also conducted a second measurement on two of the boreholes after an interval
of 15 days, and found the difference in crack development in the two measurements. The work of
this paper provides a new attempt for the detection and monitoring of concrete crack morphology.
Keywords: concrete cracks; crack morphological characteristics; crack detection; borehole camera
1. Introduction
Concrete is one of the most common building materials which is widely used in infras-
tructure construction such as roads, bridges, tunnels, dams, and housing construction [
1
–
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
–
7
]. Cracks in concrete structures will become channels for external aggres-
sive 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 sen-
sor technology, researchers have developed ground-penetrating radar [
11
–
13
], ultrasonic
technology [
14
–
16
], and a variety of pavement sensors to detect the morphological char-
acteristics 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 to
Appl. Sci. 2022,12, 9080. https://doi.org/10.3390/app12189080 https://www.mdpi.com/journal/applsci
Appl. Sci. 2022,12, 9080 2 of 15
couple 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, com-
mon detection methods include optical-fiber sensor detection technology, infrared-imaging
detection technology, etc. [
22
–
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
[26–28]
.
Thanks to the continuous breakthroughs of optical photography technology [
29
–
31
] and
digital-image-processing technology [
32
–
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
–
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
◦
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
–
44
]. In this study, digital borehole camera detection was carried out by using
core drilling which is necessary in the detection of concrete cracks [
45
]. The high-resolution
optical image of the borehole wall was obtained, and the depth, width, and orientation of
the crack in the image are quantitatively analyzed. This is a new attempt of crack detection,
which provides a new method for describing the morphological characteristics of internal
cracks in concrete, and the measurement results are satisfactory.
2. Crack Image Acquisition
2.1. Borehole Camera Technology
A borehole camera system is a kind of in situ optical measurement technology. This
technology records the structural information of the borehole wall by continuous photog-
raphy or video recording, and the measurement results are intuitive and low-cost. The
equipment used in this paper is the digital panoramic borehole camera system (DPBCS)
developed by the Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. The
system mainly includes a control box, optical probe, flexible cable, depth recorder, rigid
push rod (used in nonvertical downward boreholes), and power supply components. The
structural diagram of the DPBCS is shown in Figure 1.
The optical probe is the most critical component of the system, mainly including a
data-processing module, electronic compass, LED light source, glass window, camera, and
conical mirror, as shown in Figure 2a. The optical probe has three sizes, with diameters of
31 mm, 51 mm, and 73 mm, respectively, and corresponding lengths of 471 mm, 462 mm,
and 587 mm, respectively. The functions of the three types of probes are the same, and
the probe closest to the borehole diameter is usually selected for use. The use of conical
mirror technology is creative, so that the 360
◦
borehole wall reflects into a planar panoramic
image. The advantage of this technology is that the camera can capture a video image of
the 360
◦
borehole wall at the same time without rotating the probe. The LED light source
illuminates the borehole wall through the glass window, and the light is reflected into the
camera by the conical mirror. The conical mirror is located in the focal length range of
Appl. Sci. 2022,12, 9080 3 of 15
the fixed focus lens. The electronic compass is fixed coaxially with the camera, with an
accuracy of 0.1
◦
. Even if the probe shakes or rotates in the borehole, the electronic compass
can determine its orientation.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 16
Figure 1. Digital panoramic borehole camera system (1—Control box; 2—Optical probe; 3—Flexible
cable; 4—Depth encoder; 5—Rigid push rod; 6—Power supply).
The optical probe is the most critical component of the system, mainly including a
data-processing module, electronic compass, LED light source, glass window, camera,
and conical mirror, as shown in Figure 2a. The optical probe has three sizes, with diame-
ters of 31 mm, 51 mm, and 73 mm, respectively, and corresponding lengths of 471 mm,
462 mm, and 587 mm, respectively. The functions of the three types of probes are the same,
and the probe closest to the borehole diameter is usually selected for use. The use of con-
ical mirror technology is creative, so that the 360° borehole wall reflects into a planar pan-
oramic image. The advantage of this technology is that the camera can capture a video
image of the 360° borehole wall at the same time without rotating the probe. The LED
light source illuminates the borehole wall through the glass window, and the light is re-
flected into the camera by the conical mirror. The conical mirror is located in the focal
length range of the fixed focus lens. The electronic compass is fixed coaxially with the
camera, with an accuracy of 0.1°. Even if the probe shakes or rotates in the borehole, the
electronic compass can determine its orientation.
The depth encoder is fixed at the orifice to record the depth of the optical probe in
the borehole with an accuracy of 0.1 mm. The flexible cable descends the optical probe to
the bottom of the borehole at a constant speed through the depth encoder, and the lower-
ing speed shall not exceed 1.5 m/s. In addition, the flexible cable is also a channel for power
supply and data transmission between the control box and the optical probe. The control
box superimposes the video data, orientation data and depth data in real time to ensure
that the depth and orientation of each frame of the image are determined.
Figure 2a shows the measurement process. As the optical probe moves in the bore-
hole, the video image, orientation information, and depth information of the borehole wall
structure are collected and saved. Because the conical mirror technology reflects the pan-
oramic strip image, the image must be restored to a form convenient for viewing through
a certain algorithm. As shown in Figure 2b, the borehole wall image collected by the bore-
hole camera system is shown. The first picture is the plan expansion of the borehole wall
along the due north direction, and the other four are borehole histograms from different
perspectives. In the image, the aggregate and cracks on the borehole wall can be clearly
seen. The top of the image is the orientation data, and the left is the depth data; that is, the
coordinates of each pixel on the borehole wall are determined.
Figure 1.
Digital panoramic borehole camera system (1—Control box; 2—Optical probe; 3—Flexible
cable; 4—Depth encoder; 5—Rigid push rod; 6—Power supply).
Appl. Sci. 2022, 12, x FOR PEER REVIEW 4 of 16
(a)
(b)
Figure 2. Principle of borehole camera system. (a) Measurement process. (b) Borehole wall image.
2.2. Measurement Accuracy Analysis
The measurement accuracy of the concrete crack is also the resolution of the borehole
wall image. The hole wall image is a dot matrix composed of pixels, so it is necessary to
analyze the corresponding relationship between the image pixels and the real size of the
observed points. The axial resolution of the borehole wall image obtained by the digital
borehole camera system can reach 0.1 mm/pix. The circumferential resolution of the image
is related to the borehole diameter. The borehole wall images obtained from exploration
boreholes with different diameters correspond to different circumferential resolutions.
The maximum number of pixels in the circumferential direction of the borehole wall im-
age obtained by the digital borehole camera system is 2560, and the circumferential reso-
lution of the image in the conventional exploration boreholes with different diameters is
shown in Figure 3.
Figure 2. Principle of borehole camera system. (a) Measurement process. (b) Borehole wall image.
Appl. Sci. 2022,12, 9080 4 of 15
The depth encoder is fixed at the orifice to record the depth of the optical probe in the
borehole with an accuracy of 0.1 mm. The flexible cable descends the optical probe to the
bottom of the borehole at a constant speed through the depth encoder, and the lowering
speed shall not exceed 1.5 m/s. In addition, the flexible cable is also a channel for power
supply and data transmission between the control box and the optical probe. The control
box superimposes the video data, orientation data and depth data in real time to ensure
that the depth and orientation of each frame of the image are determined.
Figure 2a shows the measurement process. As the optical probe moves in the borehole,
the video image, orientation information, and depth information of the borehole wall
structure are collected and saved. Because the conical mirror technology reflects the
panoramic strip image, the image must be restored to a form convenient for viewing
through a certain algorithm. As shown in Figure 2b, the borehole wall image collected
by the borehole camera system is shown. The first picture is the plan expansion of the
borehole wall along the due north direction, and the other four are borehole histograms
from different perspectives. In the image, the aggregate and cracks on the borehole wall
can be clearly seen. The top of the image is the orientation data, and the left is the depth
data; that is, the coordinates of each pixel on the borehole wall are determined.
2.2. Measurement Accuracy Analysis
The measurement accuracy of the concrete crack is also the resolution of the borehole
wall image. The hole wall image is a dot matrix composed of pixels, so it is necessary to
analyze the corresponding relationship between the image pixels and the real size of the
observed points. The axial resolution of the borehole wall image obtained by the digital
borehole camera system can reach 0.1 mm/pix. The circumferential resolution of the image
is related to the borehole diameter. The borehole wall images obtained from exploration
boreholes with different diameters correspond to different circumferential resolutions. The
maximum number of pixels in the circumferential direction of the borehole wall image
obtained by the digital borehole camera system is 2560, and the circumferential resolution
of the image in the conventional exploration boreholes with different diameters is shown
in Figure 3.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 16
Figure 3. Relationship between borehole diameter and image accuracy.
It can be seen from Figure 3 that the circumferential resolution of the hole wall image
decreases with the increase in the hole diameter. The resolution is 0.037 mm/pix in 30 mm
diameter holes and 0.16 mm/pix in 130 mm diameter holes. The measurement accuracy is
fully sufficient in concrete crack detection.
2.3. Measuring Project
The measurement project is a highway tunnel with open excavation and concealed
burial construction. After backfilling with soil and stone, a long and deep crack appeared
on the bottom plate along the axis of the tunnel, as shown in Figure 4. In order to quanti-
tatively evaluate the cracks, several drilling and coring operations were carried out at the
crack location. However, the drilled concrete core cannot accurately describe the charac-
teristics of cracks. Because some concrete cores are broken and damaged, it is impossible
to determine whether the damage is caused by drilling factors or crack expansion. More
importantly, it is difficult to determine the width of cracks and the extension direction
inside the concrete, and it is impossible to evaluate the development of cracks with time
through the concrete core.
Figure 4. Concrete crack and concrete cores.
Figure 3. Relationship between borehole diameter and image accuracy.
It can be seen from Figure 3that the circumferential resolution of the hole wall image
decreases with the increase in the hole diameter. The resolution is 0.037 mm/pix in 30 mm
Appl. Sci. 2022,12, 9080 5 of 15
diameter holes and 0.16 mm/pix in 130 mm diameter holes. The measurement accuracy is
fully sufficient in concrete crack detection.
2.3. Measuring Project
The measurement project is a highway tunnel with open excavation and concealed
burial construction. After backfilling with soil and stone, a long and deep crack appeared on
the bottom plate along the axis of the tunnel, as shown in Figure 4. In order to quantitatively
evaluate the cracks, several drilling and coring operations were carried out at the crack
location. However, the drilled concrete core cannot accurately describe the characteristics of
cracks. Because some concrete cores are broken and damaged, it is impossible to determine
whether the damage is caused by drilling factors or crack expansion. More importantly, it is
difficult to determine the width of cracks and the extension direction inside the concrete, and
it is impossible to evaluate the development of cracks with time through the concrete core.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 16
Figure 3. Relationship between borehole diameter and image accuracy.
It can be seen from Figure 3 that the circumferential resolution of the hole wall image
decreases with the increase in the hole diameter. The resolution is 0.037 mm/pix in 30 mm
diameter holes and 0.16 mm/pix in 130 mm diameter holes. The measurement accuracy is
fully sufficient in concrete crack detection.
2.3. Measuring Project
The measurement project is a highway tunnel with open excavation and concealed
burial construction. After backfilling with soil and stone, a long and deep crack appeared
on the bottom plate along the axis of the tunnel, as shown in Figure 4. In order to quanti-
tatively evaluate the cracks, several drilling and coring operations were carried out at the
crack location. However, the drilled concrete core cannot accurately describe the charac-
teristics of cracks. Because some concrete cores are broken and damaged, it is impossible
to determine whether the damage is caused by drilling factors or crack expansion. More
importantly, it is difficult to determine the width of cracks and the extension direction
inside the concrete, and it is impossible to evaluate the development of cracks with time
through the concrete core.
Figure 4. Concrete crack and concrete cores.
Figure 4. Concrete crack and concrete cores.
Therefore, in order to quantitatively characterize the morphological characteristics of
internal cracks in concrete, the cracks on the borehole wall were quantitatively measured
using borehole camera technology in the borehole after coring. Due to the large number of
on-site boreholes, we selected nine representative boreholes for measurement. The borehole
number and measurement depth are shown in Table 1. The diameter of all boreholes was
57 mm, and the diameter of the optical probe used in this paper was 51 mm. Figure 5shows
the measurement site and measurement equipment.
Table 1. Borehole number and measurement depth.
Borehole zk15 + 025 zk14 + 896 zk14 + 931 zk15 + 018 zk14 + 994 zk14 + 819 zk14 + 858 zk14 + 870 zk14 + 903.5
Depth/m 1.02 1.70 1.06 1.85 1.80 1.78 1.84 1.50 1.69
Appl. Sci. 2022,12, 9080 6 of 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 16
Therefore, in order to quantitatively characterize the morphological characteristics of
internal cracks in concrete, the cracks on the borehole wall were quantitatively measured
using borehole camera technology in the borehole after coring. Due to the large number
of on-site boreholes, we selected nine representative boreholes for measurement. The
borehole number and measurement depth are shown in Table 1. The diameter of all bore-
holes was 57 mm, and the diameter of the optical probe used in this paper was 51 mm.
Figure 5 shows the measurement site and measurement equipment.
Table 1. Borehole number and measurement depth.
Borehole
zk15 + 025
zk14 + 896
zk14 + 931
zk15 + 018
zk14 + 994
zk14 + 819
zk14 + 858
zk14 + 870
zk14 + 903.5
Depth/m
1.02
1.70
1.06
1.85
1.80
1.78
1.84
1.50
1.69
Figure 5. Measurement site and measurement equipment.
After measurement, high-resolution borehole wall images of nine measurement
holes were obtained, as shown in Figure 6. In the borehole wall image, structural infor-
mation, such as aggregate, damage, and cracks, can be clearly observed. For the conven-
ience of viewing, the shape of the crack is depicted with red lines in Figure 6. It should be
reminded that the wide black vertical defects in the figure are not cracks, but borehole
wall defects caused by the overlapping of adjacent boreholes. There are two types of
cracks in the nine boreholes, the cracks that expand from the orifice (zk15 + 025, zk14 +
896, zk15 + 018, zk14 + 994, zk14 + 858, and zk14 + 870) and the cracks inside the concrete
(not extending to the orifice; zk15 + 025, zk14 + 931, zk14 + 819, and zk14 + 903.5).
Figure 5. Measurement site and measurement equipment.
After measurement, high-resolution borehole wall images of nine measurement holes
were obtained, as shown in Figure 6. In the borehole wall image, structural information,
such as aggregate, damage, and cracks, can be clearly observed. For the convenience of
viewing, the shape of the crack is depicted with red lines in Figure 6. It should be reminded
that the wide black vertical defects in the figure are not cracks, but borehole wall defects
caused by the overlapping of adjacent boreholes. There are two types of cracks in the
nine boreholes, the cracks that expand from the orifice (zk15 + 025, zk14 + 896, zk15 + 018,
zk14 + 994
, zk14 + 858, and zk14 + 870) and the cracks inside the concrete (not extending to
the orifice; zk15 + 025, zk14 + 931, zk14 + 819, and zk14 + 903.5).
Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 16
Figure 6. Borehole wall images of 9 measuring boreholes.
3. Morphological Characteristics Analysis of Cracks
3.1. Analysis of Width and Depth
Depth and width are important parameters to describe the morphological character-
istics 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.
Figure 6. Borehole wall images of 9 measuring boreholes.
Appl. Sci. 2022,12, 9080 7 of 15
3. Morphological Characteristics Analysis of Cracks
3.1. Analysis of Width and Depth
Depth and width are important parameters to describe the morphological charac-
teristics 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.
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.
(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
Appl. Sci. 2022,12, 9080 8 of 15
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.
3.2. Analysis of Crack Orientation
The extension orientation of the crack is very important for describing the morpholog-
ical characteristics. An accurate understanding of the crack orientation is very important
for analyzing the cause of crack initiation and subsequent repair. When the crack intersects
with the borehole, two cracks will appear on the expanded image of the borehole wall,
as shown in Figure 8. The crack may intersect with the borehole at any angle, and when
the crack does not pass through the borehole center, the orientation of the crack cannot be
directly seen on the borehole wall image. Figure 8shows the calculation principle of the
crack orientation at any position on the borehole wall. Suppose that at a certain depth in
the borehole wall image, the two points on the crack are
A1
and
A2
, respectively, then the
orientation of the crack at this depth can be expressed as Equation (1):
γ=1
2L(x1+x2)×360◦±90◦(1)
where
γ
is the orientation of the crack.
x1
and
x2
are the distances from
A1
and
A2
to the
leftmost side (0
◦
) of the borehole wall image, respectively.
L
is the perimeter of the borehole.
Appl. Sci. 2022,12, 9080 9 of 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 10 of 16
Figure 8. Calculation principle of crack orientation in borehole wall image. 1 and 2 are two
points on the crack at the same depth in the borehole wall image, respectively. 1 and 2 are the
distances from 1 and 2 to the leftmost side (0 °) of the borehole wall image, respectively. is
the perimeter of the borehole.
In this paper, the crack orientation at different depths in nine boreholes was calcu-
lated, and the statistical results are shown in Figure 9. It should be pointed out that the
orientation can be calculated only when two cracks appear in the borehole wall image. (1)
In borehole zk15 + 025, although the upper and lower cracks were not continuous, the
orientation of the two cracks was very close, both distributed in the range of [142°,158°],
and the crack orientation at the orifice was 151°. (2) In borehole zk14 + 896, the change in
crack orientation was relatively gentle, distributed in the range of [114°,143°], and the frac-
ture trend at the orifice was 143°. (3) In borehole zk14 + 931, the crack orientation was
distributed in the range of [133°,144°]. (4) In borehole zk15 + 018, affected by the intersec-
tion of adjacent boreholes, only one crack appeared in the borehole wall image of the first
0.5 m, so the orientation of the crack cannot be calculated before 0.5 m. After 0.5 m, the
crack orientation continued to change, gradually deflecting from the initial position of
145° to the due east direction until 80°. (5) In borehole zk14 + 994, there was only one crack
in the borehole wall image of 0.5-0.8 m due to the intersection of adjacent boreholes, so
the crack orientation could not be calculated in this section. The crack orientation of this
borehole was distributed in the range of [94°,129°], and the crack orientation at the initial
position was 112°. Before 0.9 m, the change in crack orientation was very small, and the
crack orientation suddenly changed significantly within the range of 0.9–1 m, increasing
from 94° to 129°. (6) In borehole zk14 + 819, the crack orientation continued to decrease
from the initial position of 184° to 125°. (7) In borehole zk14 + 858, the orientation of the
crack changed little before 0.9 m, and the orientation of the crack at the orifice was 132°.
After passing through the concrete interface, the crack orientation changed significantly,
continuously decreasing from 114° to 19°. (8) In borehole zk14 + 870, the crack orientation
changed little, distributed in the range of [103°,132°], and the crack orientation at the ori-
fice was 132°. (9) In borehole zk14 + 903.5, the crack orientation was distributed within the
range of [122°,165°], and the crack orientation at the initial position was 165°.
Figure 8.
Calculation principle of crack orientation in borehole wall image.
A1
and
A2
are two points
on the crack at the same depth in the borehole wall image, respectively.
x1
and
x2
are the distances
from
A1
and
A2
to the leftmost side (0
◦
) of the borehole wall image, respectively.
L
is the perimeter
of the borehole.
In this paper, the crack orientation at different depths in nine boreholes was calculated,
and the statistical results are shown in Figure 9. It should be pointed out that the orientation
can be calculated only when two cracks appear in the borehole wall image. (1) In borehole
zk15 + 025, although the upper and lower cracks were not continuous, the orientation of
the two cracks was very close, both distributed in the range of [142
◦
,158
◦
], and the crack
orientation at the orifice was 151
◦
. (2) In borehole zk14 + 896, the change in crack orientation
was relatively gentle, distributed in the range of [114
◦
,143
◦
], and the fracture trend at the
orifice was 143
◦
. (3) In borehole zk14 + 931, the crack orientation was distributed in the
range of [133
◦
,144
◦
]. (4) In borehole zk15 + 018, affected by the intersection of adjacent
boreholes, only one crack appeared in the borehole wall image of the first 0.5 m, so the
orientation of the crack cannot be calculated before 0.5 m. After 0.5 m, the crack orientation
continued to change, gradually deflecting from the initial position of 145
◦
to the due east
direction until 80
◦
. (5) In borehole zk14 + 994, there was only one crack in the borehole wall
image of 0.5–0.8 m due to the intersection of adjacent boreholes, so the crack orientation
could not be calculated in this section. The crack orientation of this borehole was distributed
in the range of [94
◦
,129
◦
], and the crack orientation at the initial position was 112
◦
. Before
0.9 m, the change in crack orientation was very small, and the crack orientation suddenly
changed significantly within the range of 0.9–1 m, increasing from 94
◦
to 129
◦
. (6) In
borehole zk14 + 819, the crack orientation continued to decrease from the initial position of
184
◦
to 125
◦
. (7) In borehole zk14 + 858, the orientation of the crack changed little before
0.9 m, and the orientation of the crack at the orifice was 132
◦
. After passing through the
concrete interface, the crack orientation changed significantly, continuously decreasing
from 114
◦
to 19
◦
. (8) In borehole zk14 + 870, the crack orientation changed little, distributed
in the range of [103
◦
,132
◦
], and the crack orientation at the orifice was 132
◦
. (9) In borehole
zk14 + 903.5, the crack orientation was distributed within the range of [122
◦
,165
◦
], and the
crack orientation at the initial position was 165◦.
Appl. Sci. 2022,12, 9080 10 of 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 11 of 16
(a)
(b)
(c)
Figure 9. Statistical analysis of crack orientation. (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.
There was little difference in the orientation of cracks at the orifice. The orientation
of cracks at the orifice was measured in boreholes zk15 + 025, zk14 + 896, zk14 + 858, and
z14 + 903.5, which were 151°, 143°, 132°, and 132°, respectively, and the maximum differ-
ence was only 19°. The orientation change of cracks at different depths in the same bore-
hole was relatively small, and, basically, the deviation did not exceed 30°. However, after
passing through the concrete interface and entering another different type of concrete, the
orientation of cracks changed significantly. The change in crack orientation inside the con-
crete was relatively large. For example, in boreholes zk14 + 819 and zk14 + 903.5, the
change in crack direction reached 59 ° and 43 °, respectively, in a short extension depth.
The crack extension in the concrete is affected by the coarse aggregate structure. The crack
extends along the interface between coarse aggregate and mortar, and rarely directly pen-
etrates the coarse aggregate, which affects the orientation of the crack to a certain extent,
as shown in Figure 10. However, in this paper, the influence of coarse aggregate on the
Figure 9.
Statistical analysis of crack orientation. (
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.
There was little difference in the orientation of cracks at the orifice. The orientation
of cracks at the orifice was measured in boreholes zk15 + 025, zk14 + 896, zk14 + 858,
and z14 + 903.5, which were 151
◦
, 143
◦
, 132
◦
, and 132
◦
, respectively, and the maximum
difference was only 19
◦
. The orientation change of cracks at different depths in the same
borehole was relatively small, and, basically, the deviation did not exceed 30
◦
. However,
after passing through the concrete interface and entering another different type of concrete,
the orientation of cracks changed significantly. The change in crack orientation inside the
concrete was relatively large. For example, in boreholes zk14 + 819 and zk14 + 903.5, the
change in crack direction reached 59
◦
and 43
◦
, respectively, in a short extension depth.
The crack extension in the concrete is affected by the coarse aggregate structure. The
crack extends along the interface between coarse aggregate and mortar, and rarely directly
penetrates the coarse aggregate, which affects the orientation of the crack to a certain extent,
as shown in Figure 10. However, in this paper, the influence of coarse aggregate on the
orientation of cracks that extended from the orifice was limited. After bypassing the coarse
aggregate, the crack soon continues to extend along the previous orientation.
Appl. Sci. 2022,12, 9080 11 of 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 12 of 16
orientation of cracks that extended from the orifice was limited. After bypassing the coarse
aggregate, the crack soon continues to extend along the previous orientation.
Although the electronic compass inside the optical probe has a measurement error of
0.1 °, the error can be ignored relative to the influence of coarse aggregate on the orienta-
tion. In addition, the orientation of the borehole also causes measurement errors, so the
verticality of the borehole should be guaranteed as much as possible. If the borehole is not
vertical, the orientation of the borehole axis and the included angle with the horizontal
direction are necessary to correct the error.
Figure 10. Crack extension characteristics at the edge of coarse aggregate.
4. Discussions
The development of a crack is a dynamic process, and the change in its morphological
characteristics with time is an important factor to be considered. The stable or slow devel-
opment of crack morphology is the desired result of engineers. If the cracks continue to
extend and widen over time, this will bring great difficulties to the evaluation and repair
of concrete structures. Therefore, it is necessary to regularly observe the crack morphol-
ogy. Due to the construction requirements, most of the boreholes were quickly blocked
after the first measurement. Only boreholes zk15 + 018 and zk14 + 994 were allowed to
remain as observation boreholes. Fifteen days after the first measurement, boreholes zk15
+ 018 and zk14 + 994 were measured again with the same borehole camera equipment.
Figure 11 shows the comparison of borehole wall images obtained from the two measure-
ments. There was only a slight difference between the two measurement results. In bore-
hole zk15 + 018, the second measurement found that the crack continued to extend 6 mm
downward, while in borehole zk14 + 994, no significant crack extension was found. The
crack width of the two boreholes at the orifice basically increased by 2 mm. This may be
due to the vibration caused by vehicles and pedestrians on the tunnel surface, resulting in
the falling off of mortar and aggregate. However, below the orifice, the crack width had
no obvious expansion trend.
The development of cracks can be monitored by using the borehole camera technol-
ogy to conduct periodic measurements in the same borehole. In this paper, subtle differ-
ences in the development of cracks were found in the two measurements, and the cracks
observed in the two boreholes were basically in a stable state. However, since the number
of boreholes measured in the second time was too small, it is arbitrary to give the devel-
opment law of cracks in a large area. If the construction conditions permit, the number of
observation boreholes and the measurement period shall be increased in similar monitor-
ing schemes.
The borehole camera technology can accurately measure the morphological charac-
teristics of the cracks on the borehole surface, but it cannot measure the cracks in the
Figure 10. Crack extension characteristics at the edge of coarse aggregate.
Although the electronic compass inside the optical probe has a measurement error of
0.1
◦
, the error can be ignored relative to the influence of coarse aggregate on the orientation.
In addition, the orientation of the borehole also causes measurement errors, so the verticality
of the borehole should be guaranteed as much as possible. If the borehole is not vertical,
the orientation of the borehole axis and the included angle with the horizontal direction are
necessary to correct the error.
4. Discussion
The development of a crack is a dynamic process, and the change in its morphological
characteristics with time is an important factor to be considered. The stable or slow
development of crack morphology is the desired result of engineers. If the cracks continue
to extend and widen over time, this will bring great difficulties to the evaluation and repair
of concrete structures. Therefore, it is necessary to regularly observe the crack morphology.
Due to the construction requirements, most of the boreholes were quickly blocked after
the first measurement. Only boreholes zk15 + 018 and zk14 + 994 were allowed to remain
as observation boreholes. Fifteen days after the first measurement, boreholes zk15 + 018
and zk14 + 994 were measured again with the same borehole camera equipment. Figure 11
shows the comparison of borehole wall images obtained from the two measurements. There
was only a slight difference between the two measurement results. In borehole
zk15 + 018
,
the second measurement found that the crack continued to extend 6 mm downward, while
in borehole zk14 + 994, no significant crack extension was found. The crack width of
the two boreholes at the orifice basically increased by 2 mm. This may be due to the
vibration caused by vehicles and pedestrians on the tunnel surface, resulting in the falling
off of mortar and aggregate. However, below the orifice, the crack width had no obvious
expansion trend.
The development of cracks can be monitored by using the borehole camera technology
to conduct periodic measurements in the same borehole. In this paper, subtle differences in
the development of cracks were found in the two measurements, and the cracks observed in
the two boreholes were basically in a stable state. However, since the number of boreholes
measured in the second time was too small, it is arbitrary to give the development law of
cracks in a large area. If the construction conditions permit, the number of observation
boreholes and the measurement period shall be increased in similar monitoring schemes.
Appl. Sci. 2022,12, 9080 12 of 15
Appl. Sci. 2022, 12, x FOR PEER REVIEW 13 of 16
concrete around the borehole. By increasing the borehole density, a comprehensive anal-
ysis of multi-borehole data can help reduce the impact of this limitation. In addition, it is
difficult for a single detection technology to fully meet the requirements, and the com-
bined use of multiple measurement methods can make the results more detailed and com-
prehensive.
Figure 11. Comparison and analysis of two measurement results.
5. Conclusions
This paper introduces the borehole camera technology, which is an advanced tech-
nique 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 charac-
terize the width, depth, orientation, and other valuable indicators of concrete internal
cracks. Although this paper focused on the morphological characteristics of cracks in con-
crete, borehole camera technology can also be used to study the morphology and distri-
bution 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 basi-
cally the same at the orifice, with a maximum deviation of 19 °, and the orientation of the
crack was basically stable from the orifice to the termination depth, with the deviation not
exceeding 30°. 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. The induction effect of coarse aggregate characteristics (uniformity and morpho-
logical characteristics) on concrete orientation is a very meaningful topic. Obviously,
Figure 11. Comparison and analysis of two measurement results.
The borehole camera technology can accurately measure the morphological character-
istics of the cracks on the borehole surface, but it cannot measure the cracks in the concrete
around the borehole. By increasing the borehole density, a comprehensive analysis of
multi-borehole data can help reduce the impact of this limitation. In addition, it is difficult
for a single detection technology to fully meet the requirements, and the combined use of
multiple measurement methods can make the results more detailed and comprehensive.
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
◦
, and the orientation of the
crack was basically stable from the orifice to the termination depth, with the deviation not
exceeding 30
◦
. 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.
Appl. Sci. 2022,12, 9080 13 of 15
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.
Author Contributions:
Conceptualization, C.W. (Chao Wang); methodology, Z.H. and Y.W.; software,
C.W. (Chao Wang); validation, Z.H. and Y.W.; formal analysis, C.W. (Chuanying Wang); investi-
gation, C.W. (Chao Wang), S.C. and Z.H.; resources, Z.H. and J.W.; data curation, Y.W. and S.C.;
writing—original
draft preparation, C.W. (Chao Wang) and Y.W.; writing—review and editing, C.W.
(Chao Wang), C.W. (Chuanying Wang) and Z.H.; visualization, J.W.; supervision, C.W. (Chuanying
Wang); project administration, S.H.; funding acquisition, Z.H. and J.W. All authors have read and
agreed to the published version of the manuscript.
Funding:
This research was funded by the Key Research and Development Program of Hubei
Province, grant number 2021BAA201, the Systematic Project of Guangxi Key Laboratory of Disaster
Prevention and Engineering Safety, grant number 2020ZDK015 and the National Natural Science
Foundation of China, Grant numbers: 41731284, 41902294.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Damasceno, I.I.R.; Ferreira, M.D.P.; De Oliveira, D.R.C. RC beams with steel fibers under impact loads. Acta Sci. Technol.
2013
,
36, 23. [CrossRef]
2.
Tarefder, R.A.; Ahmad, M. Evaluating the Relationship between Permeability and Moisture Damage of Asphalt Concrete
Pavements. J. Mater. Civ. Eng. 2015,27, 04014172. [CrossRef]
3.
Soumya; Pandey, A.D.; Das, R.; Mahesh, M.J.; Anvesh, S.; Saini, P. Structural analysis of a historical dam. Procedia Eng.
2016
,144,
140–147. [CrossRef]
4.
Cui, X.; Wang, Q.; Dai, J.; Zhang, R.; Li, S. Intelligent recognition of erosion damage to concrete based on improved YOLO-v3.
Mater. Lett. 2021,302, 130363. [CrossRef]
5.
Galouei, M.; Fakhimi, A. Size effect, material ductility and shape of fracture process zone in quasi-brittle materials. Comput. Geotech.
2015,65, 126–135. [CrossRef]
6.
Wang, H.L.; Dai, J.G.; Sun, X.Y.; Xiao, Y.; Zhang, X.L. Characteristics of concrete cracks and their influence on chloride penetration.
Constr. Build. Mater. 2016,107, 216–225. [CrossRef]
7.
Jiang, W.; Zhou, G.; Wang, C.; Xue, Y.; Niu, C. Synthesis and self-healing properties of composite microcapsule based on sodium
alginate/melamine-phenol–formaldehyde resin. Constr. Build. Mater. 2020,271, 121541. [CrossRef]
8.
Zhou, C.; Li, K.; Pang, X. Geometry of crack network and its impact on transport properties of concrete. Cem. Concr. Res.
2012
,42,
1261–1272. [CrossRef]
9.
Li, M.; Chen, H.; Liu, L.; Lin, J.; Ullah, K. Permeability of concrete considering the synergetic effect of crack’s shape- and
size-polydispersities on the percolation. Constr. Build. Mater. 2021,315, 125684. [CrossRef]
10.
Li, K.; Li, L. Crack-altered durability properties and performance of structural concretes. Cem. Concr. Res.
2019
,124, 105811.
[CrossRef]
11.
Fernandes, F.M.; Pais, J.C. Laboratory observation of cracks in road pavements with GPR. Constr. Build. Mater.
2017
,154,
1130–1138. [CrossRef]
12.
Tong, Z.; Yuan, D.; Gao, J.; Wei, Y.; Dou, H. Pavement-distress detection using ground-penetrating radar and network in networks.
Constr. Build. Mater. 2019,233, 117352. [CrossRef]
13.
Hong, S.; Chen, D.; Dong, B. Numerical simulation and mechanism analysis of GPR-based reinforcement corrosion detection.
Constr. Build. Mater. 2021,317, 125913. [CrossRef]
Appl. Sci. 2022,12, 9080 14 of 15
14.
Choi, P.; Kim, D.-H.; Lee, B.-H.; Won, M.C. Application of ultrasonic shear-wave tomography to identify horizontal crack or
delamination in concrete pavement and bridge. Constr. Build. Mater. 2016,121, 81–91. [CrossRef]
15.
Cook, K.; Garg, N.; Singh, A.; Flynn, M. Detection of Delamination in the HMA Layer of Runway Pavement Structure Using
Asphalt Strain Gauges. J. Transp. Eng. 2016,142, 04016047. [CrossRef]
16.
Grabke, S.; Clauß, F.; Bletzinger, K.-U.; Ahrens, M.A.; Mark, P.; Wüchner, R. Damage Detection at a Reinforced Concrete Specimen
with Coda Wave Interferometry. Materials 2021,14, 5013. [CrossRef]
17.
Xu, X.; Zeng, Q.; Li, D.; Wu, J.; Wu, X.; Shen, J. GPR detection of several common subsurface voids inside dikes and dams.
Eng. Geol. 2010,111, 31–42. [CrossRef]
18.
Song, X.; Xiang, D.; Zhou, K.; Su, Y. Fast Prescreening for GPR Antipersonnel Mine Detection via Go Decomposition. IEEE Geosci.
Remote Sens. Lett. 2018,16, 15–19. [CrossRef]
19.
Tsai, Y.-C.; Kaul, V.; Mersereau, R.M. Critical Assessment of Pavement Distress Segmentation Methods. J. Transp. Eng.
2010
,136,
11–19. [CrossRef]
20.
Asadollahi, A.; Khazanovich, L. Numerical investigation of the effect of heterogeneity on the attenuation of shear waves in
concrete. Ultrasonics 2018,91, 34–44. [CrossRef]
21.
Chekroun, M.; Le Marrec, L.; Abraham, O.; Durand, O.; Villain, G. Analysis of coherent surface wave dispersion and attenu-ation
for non-destructive testing of concrete. Ultrasonics 2009,49, 743–751. [CrossRef] [PubMed]
22.
Chapeleau, X.; Blanc, J.; Hornych, P.; Gautier, J.L.; Carroget, J. Assessment of cracks detection in pavement by a distributed fber
optic sensing technology. J. Civil. Struct. Health Monit. 2017,7, 459–470. [CrossRef]
23.
De Maeijer, P.K.; Luyckx, G.; Vuye, C.; Voet, E.; Bergh, W.V.D.; Vanlanduit, S.; Braspenninckx, J.; Stevens, N.; De Wolf, J. Fiber
Optics Sensors in Asphalt Pavement: State-of-the-Art Review. Infrastructures 2019,4, 36. [CrossRef]
24.
Zheng, D.; Tan, S.; Li, X.; Cai, H. Research on the Infrared Thermographic Detection of Concrete under Solar Heating. Adv. Civ.
Eng. 2021,2021, 6692729. [CrossRef]
25.
Sirca, G.F.; Adeli, H. Infrared Thermography for Detecting Defects in Concrete Structures. J. Civ. Eng. Manag.
2018
,24, 508–515.
[CrossRef]
26.
Payab, M.; Abbasina, R.; Khanzadi, M. A Brief Review and a New Graph-Based Image Analysis for Concrete Crack Quanti-fication.
Arch. Comput. Methods Eng. 2019,26, 347–365. [CrossRef]
27.
Shan, B.; Zheng, S.; Ou, J. A stereovision-based crack width detection approach for concrete surface assessment. KSCE J. Civ. Eng.
2015,20, 803–812. [CrossRef]
28.
Zhao, P.; Zsaki, A.M.; Nokken, M.R. Using digital image correlation to evaluate plastic shrinkage cracking in cement-based
materials. Constr. Build. Mater. 2018,182, 108–117. [CrossRef]
29.
Ahmed, M.; Haas, C.T.; Haas, R. Toward low-cost 3D automatic pavement distress surveying: The close range photogrammetry
approach. Can. J. Civ. Eng. 2011,38, 1301–1313.
30.
Ouyang, W.; Xu, B. Pavement cracking measurements using 3D laser-scan images. Meas. Sci. Technol.
2013
,24, 105204. [CrossRef]
31.
Zhang, C.; Elaksher, A. An Unmanned Aerial Vehicle-Based Imaging System for 3D Measurement of Unpaved Road Surface
Distresses1.Comput. Civ. Infrastruct. Eng. 2011,27, 118–129. [CrossRef]
32.
Fu, R.; Xu, H.; Wang, Z.; Shen, L.; Cao, M.; Liu, T.; Novák, D. Enhanced Intelligent Identification of Concrete Cracks Using
Multi-Layered Image Preprocessing-Aided Convolutional Neural Networks. Sensors 2020,20, 2021. [CrossRef] [PubMed]
33.
Zakeri, H.; Nejad, F.M.; Fahimifar, A. Image Based Techniques for Crack Detection, Classification and Quantification in Asphalt
Pavement: A Review. Arch. Comput. Methods Eng. 2016,24, 935–977. [CrossRef]
34.
Kim, H.; Ahn, E.; Shin, M.; Sim, S.-H. Crack and Noncrack Classification from Concrete Surface Images Using Machine Learning.
Struct. Health Monit. 2018,18, 725–738. [CrossRef]
35.
Calderón, L.S.; Bairán, J. Semi-automatic detection and measurement of cracks in concrete elements in digital photos using image
processing. Hormig. Acero. 2020,71, 21–27.
36.
Li, S.; Zhao, X. Automatic crack detection and measurement of concrete structure using convolutional encoder-decoder network.
IEEE Access 2020,8, 134602–134618. [CrossRef]
37.
Bayar, G.; Bilir, T. A novel study for the estimation of crack propagation in concrete using machine learning algorithms.
Constr. Build. Mater. 2019,215, 670–685. [CrossRef]
38.
Zhang, Y.; Wang, Z. Concrete Surface Crack Recognition Based on Coordinate Attention Neural Networks. Comput. Intell.
Neurosci. 2022,2022, 7454746. [CrossRef]
39.
Li, S.J.; Feng, X.-T.; Wang, C.Y.; Hudson, J.A. ISRM Suggested Method for Rock Fractures Observations Using a Borehole Digital
Optical Televiewer. Rock Mech. Rock Eng. 2012,46, 635–644. [CrossRef]
40.
Zou, X.; Song, H.; Wang, C. A High-Precision Digital Panoramic Borehole Camera System for the Precise Analysis of In Situ Rock
Structures. Rock Mech. Rock Eng. 2021,54, 5945–5952. [CrossRef]
41.
Han, Z.; Wang, C.; Liu, S.; Zhu, H. Research on Connectivity of Deep Ore-Lodes of Borehole based on Digital Borehole Camera.
Disaster Adv. 2013,6, 41–46.
42.
Han, Z.; Wang, C.; Hu, S.; Wang, Y. Application of Borehole Camera Technology in Fractured Rock Mass Investigation of a
Submarine Tunnel. J. Coast. Res. 2018,83, 609–614. [CrossRef]
43.
Zou, X.; Wang, C.; Wang, Y.; Song, H. Morphological Feature Description Method of Structural Surface in Borehole Image during
In-Situ Instrumentation. Rock Mech. Rock Eng. 2020,53, 2947–2956. [CrossRef]
Appl. Sci. 2022,12, 9080 15 of 15
44.
Wang, J.-C.; Wang, C.-Y. Analysis and Evaluation of Coral Reef Integrity Based on Borehole Camera Technology. Mar. Georesources
Geotechnol. 2015,35, 26–33. [CrossRef]
45.
GB/T 50344-2019; Technical Standard for Inspection of Building Structure. National Standard of the People’s Republic of China:
Taiwan, China, 2019. Available online: https://www.chinesestandard.net/PDF/English.aspx/GBT50344-2019 (accessed on
27 August 2022).
