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Janowski A., Nagrodzka-Godycka K., Szulwic J., Ziolkowski P.: Remote sensing and photogrammetry
techniques in diagnostics of concrete structures. Computers and Concrete, pISSN 1598-8198,
eISSN 1598-818X, Vol.18, Iss. 3, 2016, pp. 405-420, DOI: 10.12989/cac.2016.18.3.405
Remote sensing and photogrammetry techniques in diagnostics
of concrete structures
Artur Janowski1,2a , Krystyna Nagrodzka-Godycka1, Jakub Szulwic1b, Patryk
Ziółkowski1c
1 Faculty of Civil and Environmental Engineering, Gdansk University of Technology, Gdansk, Poland
2 Faculty of Geodesy, Geospatial and Civil Engineering, University of Warmia and Mazury, Olsztyn,
(Received keep as blank , Revised keep as blank , Accepted keep as blank )
Abstract. Recently laser scanning technologies become widely used in many areas of the modern economy. In the
following paper authors show a potential spectrum of use Terrestrial Laser Scanning (TLS) in diagnostics of
reinforced concrete elements. Based on modes of failure analysis of reinforcement concrete beam authors describe
downsides and advantages of adaptation of terrestrial laser scanning to this purpose, moreover reveal under which
condition this technology might be used. Research studies were conducted by Faculty of Civil and Environmental
Engineering at Gdansk University of Technology. An experiment involved bending of reinforced concrete beam, the
process was registered by the terrestrial laser scanner. Reinforced concrete beam was deliberately overloaded and
eventually failed by shear. Whole failure process was tracing and recording by scanner Leica ScanStation C10 and
verified by synchronous photographic registration supported by digital photogrammetry methods. Obtained data
were post-processed in Leica Cyclone (dedicated software) and MeshLab (program on GPL license). The main goal
of this paper is to prove the effectiveness of TLS in diagnostics of reinforced concrete elements. Authors propose few
methods and procedures to virtually reconstruct failure process, measure geometry and assess a condition of structure.
Keywords: beam; cracks; deformation measurement; modes of failure; reinforced concrete; terrestrial laser
scanning
1. Introduction
A majority can say for sure that concrete is most common construction material in all sort of
widely understood engineering structures. Since the first concrete structure has been built,
properties of concrete materials and construction technology become remarkably diverse.
Simultaneously our knowledge about phenomena occurring in concrete has expanded. Despite
great efforts of many people, an inevitable fact is that concrete, as each material, decays by many
factors, such as rheology, high temperature or chemical environmental exposure. Deterioration of
concrete affects appearance and behaviour of the structure. Symptoms of poor concrete condition
might be visible as enormous deflections or cracking. Cracks may stimulate corrosion of rebars
and lead to loss of adhesion which consequence is a decline of load carrying capacity. In many
cases, preservation measures can augment structure lifetime cycle by many years. Proper repair
strategy should be based on accurate diagnostics and ongoing assessment of structural condition.
Appropriate decisions made by stakeholders, based on real information of concrete condition can
Corresponding author, Professor, E-mail: krystyna.nagrodzka-godycka@pg.gda.pl
a Ph.D., E-mail: artur.janowski@pg.gda.pl
b Ph.D., E-mail: jakub.szulwic@ pg.gda.pl
c M.Sc. Eng., E-mail: patryk.ziolkowski@pg.gda.pl
save money and reduce risk associated with an investment. Comprehensive analysis of concrete
elements is widespread issue that has been described in literature (Bencardino et al., 2014,
Godycki-Cwirko 1992, Kopanska et al. 2016, Nagrodzka-Godycka et al. 2014, Windisch 1998). In
literature dealing with teardown analysis of reinforced concrete elements, also reference to the
modern measurement methods, including methods of image analysis and laser scanning might be
found (Daliga et al. 2016, Dias-da-Costa et al. 2014, Diego et al. 2008, Gordon et al. 2007, Koken
et al. 2014, Kwak et al. 2013, Lichti et al. 2011, Olsen et al. 2010, Qi et al. 2014, Teza et al. 2009).
The study presented in the paper, made by TLS and digital photogrammetry methods, focused on
the visual assessment of reinforced concrete beam, in contrary to studies using eg. ultrasound to
insight into internal concrete condition (Moradi-Marani et al. 2014, Rucka et al. 2013). The
authors decided to evaluate the mode of failure, through analysis of cracks on a surface of
reinforced concrete beams using TLS. Extended analysis and verification process of proposed
methods based on the use of TLS were carried out with synchronous photography supported by
photogrammetric solutions and computer vision. Previous research conducted by the authors
(Janowski et al. 2014.2, Nagrodzka-Godycka et al. 2014) indicate the importance of
photogrammetry and laser scanning data analysis (Janowski et al. 2013, Blaszczak-Bak et al.
2015) in an improvement of quality and accuracy improvement in reinforced concrete diagnostics.
The approach described in this paper can be used not only for analysis of reinforced concrete
beams but also for other structural elements as well, for study purposes test set-up was prepared.
Fig. 1 Test set-up, beam located under hydraulic jack and prepared for measurement.
2. Research object, environment.
Fig. 2 Static scheme and schematic longitudinal and a transverse cross-section of the reinforced concrete
beam.
The paper presents an analysis of data from the registered overloading process of reinforced
concrete beam (Fig. 1). Terrestrial laser scanner measurement was set up to gather as much
information as possible from registered failure process. The beam was designed to fail by shear. In
order to obtain an expected mode of the failure, the bending zone had strong longitudinal flexural
reinforcement while shear zone had insufficient transverse reinforcement (Fig. 3), this action
caused failure by shear. The beam was made of concrete C25/30 and reinforcing steel A-IIIN.
Schematic view of the beam is shown in Figure 2, test set-up in Figure 3.
Fig. 3 Test set-up.
Research have been carried out in the Regional Laboratory of Civil Engineering at Faculty of
Civil and Environmental Engineering, Concrete Structure Department, at Gdansk University of
Technology. Previously prepared concrete beam was installed in a special set-up by crane hoist.
The whole configuration simulated static scheme assumed by authors, whereby the position of the
beam was strictly set. To fulfil predefined condition, semi-circular steel supports and I-beam steel
traverse were used to imitate hinged support and two symmetrically point loads. Load from the
hydraulic stand was applied by I-beam steel traverse and two wooden pads in increments of 10kN
per stage until failure (Table 1). After each load increment, hydraulic stand was stopped and the
beam was scanned by the terrestrial laser scanner. Surface morphology and crack growth have
been controlled using Brinell Magnifier. Moreover, deflection control was provided by using dial
deflectometer. For further studies, two synchronous cameras were used in order to make digital
close range photogrammetry verification.
Table 1 The increase of load in the course of overloading (Legend: underline - failure load / italic – cracks
occurrence).
Stage 1 [kN]
Stage 2 [kN]
Stage 3 [kN]
Stage 4 [kN]
Stage 5 [kN]
Stage 6 [kN]
Stage 7 [kN]
Load
0.00
10.00
30.00
50.00
70.00
90.00
140.70
3. Data processing
The entire overloading process was captured by professional terrestrial laser scanner Leica
ScanStation C10. ScanStation C10 belong to TLS genus, which gives an ability to the independent
adjusting of horizontal and vertical resolution. This functionality allows to optimize scanning time
and can be an asset during measurement held by terrestrial as well as mobile scanning
(Burdziakowski et al. 2015, Nagrodzka-Godycka et al. 2014). To prevent an occurrence of crucial
linking and matching correlation errors, the position of scanner did not change during the whole
measurement process. Authors have made special preparations of lateral beam surface, engobed
and grillage has been made with regular 100 mm rectangular grid. In tangle points of the plotted
lines, circular tags with a diameter of 6 mm were deployed. Measurements were performed in two
resolutions: medium (default) and predefined high (distance: 100.00 m; horizontal: 0.05 m;
vertical: 0.01 m). Considering the close distance between object and scanner genuine resolution
was even higher (horizontal: 0.0025 m; vertical 0.0005 m). Due to selecting higher resolution
image detail has been improved in the post-processing stage. The medium resolution was
characterized by less image detail. High resolution is significant to capture cracks occurrence (Fig.
4). The beam has been scanned along its entire length until it reaches failure by shear.
Fig. 4 View of reinforced concrete beam support in the Cyclone (stage 6, table 1). Measuring width of a
cracks.
Data collected during the measurement were post-processed in program Cyclone provided by
Leica Geosystems. Each sample has been placed in one unified coordinate system.
4. Scanning data analysis methods
Proper analysis of measurement data was carried out in two programs. The first program was
Cyclone which is dedicated software to operate with data from ScanStation C10 scanner. The
second program was MeshLab, an open source and extensible system to work with the point cloud.
4.1 Spheres Translation Method
Method is based on a conversion of physical virtual tags mapping to a virtual spherical mesh
object. In simpler terms method boils down to selecting characteristic points in point cloud model,
in the case of these studies, markers were presented in the form of discs with 6mm diameter
previously placed on the lateral surface of the beam (Fig. 5). After registration (scanning),
followed by mapping for subsequent stages of the beam load, points constituting markers should
be transformed into polygon meshes (constructed of primitives) approximating a sphere. Fixed
coordinate system (common to subsequent measurements) is essential. Next step is to transfer of
polygon objects (spheres) to subsequent measurements stage with an identical coordinate system.
However, before the spheres will be transferred to next model space (after load increment), it
should be borne in mind that in new model space characteristic tags should be prepared previously
in the same way as above. Consequently, as a result, differences in position of the spheres in the
same coordinate system should be seen. The difference in spheres coordinates for subsequent
stages should be identified as a translation spheres vector, which shows the change of marker
position in time. Classical methods of reinforced concrete elements diagnostics, such as dial
deflectometer measurement, allow obtaining only vertical displacement value. On the contrary to
traditional methods of measurement, spheres translation method allows to identify and measure
subtle changes in dimension and location of tags in every direction. One of the most important
advantages of this method is a complexity of obtained result. The method is illustrated in the
example of the beam geometry changes (Table 2).
Fig. 5 View at measurement station and lateral surface of beam with marked characteristic points - 12 points
were selected for analyses
Table 2 Comparison of deflection values, measured directly using dial deflectometer and based on spheres
position (Comparison of the results from model obtained by TLS and those by using measurements derived
from synchronous images - according to the method described in Chapter 5)
Measurement
epoch
Load
[kN]
Deflection [mm]
Deflection in
accordance with scan
model / photogrammet
ry model measurement
s [mm]
Deflection value
difference
[mm]
Scan view
2
30.00
1.81
1.00 / 1.25
+0.81 / +0.56
3
50.00
3.32
4.00 / 3.56
-0.68 / -0.24
5
90.00
5.89
6.00 / 5.92
-0.11 / -0.03
6
140.70
-
12.00 / 12.23
-
4.2 Colour Mapping Method
TLS data may be used to study deterioration of concrete surface. Points colour mapping allows
tracking cracks propagation in element over time. In the case of reinforced concrete beams, cracks
appear in middle part of the beam at failure due to bending, at failure due to shear, cracks occur in
support zone. Each point cloud has a certain range of intensity, resulting from maximum and
minimum intensity values of sample points. The maximum and minimum values are imposed on
marginal colour values of particularized range; this allocation is called a map of intensity.
Investigation of point cloud distribution without superimposed map of intensity is not sufficient to
monitor structure. During analysis, the authors determined the only faint outline of main failure
cracks, which could not constitute a ground for the precise study of this element (Table 3 – rows 1-
2). Point cloud with the superimposed map of intensity provides us much more accurate data.
Properly applied map of intensity scheme was a significant contribution to extract from TLS data
full information of cracks propagation. Imposed intensity map might by modifying, mapping for
various point clouds are selected severally by the software. Colour mapping intensity ranges, the
so-called “schemes”, can be modified due to particular needs. Change of colour scope in intensity
map significantly improve a visibility of cracks propagation (Table 3 – rows 3-6, Table 4).
Table 3 Analysis of point cloud distribution with and without superimposed maps of intensity units (Leica
Cyclone view).
Scheme
MODE OF FAILURE ANALYSIS
View of the scan
No scheme
No scheme
with marked crack
Multi-Hue/Rainbow
Multi-Hue/Rainbow with
marked cracks
Topo 3
Topo 3
with marked cracks
Table 4 The progress of the beam overloading process (MeshLab view).
Load stage No.
BEAM (SCHEME: MESHLAB RGB)
View of the scan
1
2
3
4
5
6
In order to identify most suitable scheme for research purposes, the analysis was conducted.
Compiled data come from the survey of the beam, which reached failure by shear. Information
about the geometry of the beam was exported from Leica Cyclone .PTX format and imported into
software MeshLab for further analysis. Below authors summarized a couple of schemes that were
selected as the best-matching (Table 5).
Table 5 Comparative analysis of experimental results with the results obtained from TLS scans
No.
BEAM (SCHEME: SAWTOOTH GRAY 8)
Load [kN]
Cracks in real
element
Cracks in virtual
model
View of the scan
1
0.00
-
-
2
10.00
-
-
3
30.00
-
-
4
50.00
A
-
5
70.00
B
-
6
90.00
B
B
7
140.70
C
C
*A: Flexural (bending) cracks; B: Shear cracks; C: Failure mode onset.
5. Close range photogrammetry and image analysis of geometry as a tool for verifying
experiment with TLS.
5.1 Brief introduction
To evaluate results of measurements obtained from TLS authors used the proven method of
digital photogrammetry. Photogrammetry involves using the stereo pair of synchronous images.
Synchronous images are snapshots taken at one time by two cameras placed in various positions
represented by projections centre of the camera lens
1
O
and
2
O
. Further images were taken
with continuous loading. On the cameras imaging planes location of the actual point on the object
(P) in left (
1
p
) and right (
2
p
) image coordinates shall be identified and fixed by camera
calibration matrix due to meet the requirements of a stereoscopic model. Obtained representation
of points (
1
p
,
2
p
) with camera projections centres (
1
O
,
2
O
) shall be combined in order to
generate
1
V
and
2
V
vectors. The offset of a centre of the right camera
2
O
in relation to the
centre of left the camera
1
O
is T vector (Fig. 6).
The combination of these three vectors is positioned on one plane surface. Realization of 3D
model reproduction from stereoscopic images by using computer vision is based on a fundamental
assumption that two vectors
1
V
and
2
V
are co-planar. Co-planarity of normalized vectors of
points
1
p
and
2
p
is essential using stereoscopy and photogrammetry (Hartley 1997, Paszotta et
al. 2010, Kohut et al. 2012, Hejmanowska et al. 2015).
Fig. 6 Photogrammetry registering with use of synchronous digital cameras - scheme.
5.2 Mathematical apparatus – description
After the images acquisition process, point position measurement in pixel coordinates system
of each image (
pix
X
,
pix
Y
) can be performed. Later in the mathematical description, general
formulas for the cameras will be indicated, the formulas should be applied separately for the
camera 1 and camera 2. In theoretical coordinate system, it can be assumed that, spatial position of
the point P (
T
X
,
T
Y
,
T
Z
) is known in the camera coordinate system whose centre is located in the
middle of camera projection, Z axis runs perpendicular to the plane of the images and is aimed
from the centre of the plane projection in the direction of the image surface. X and Y axes coincide
with the directions of pixel columns and rows, which forms together with the Z axis clockwise
scheme (Fig. 7).
Y
X
Z
PT(Xt,Yt,Zt)
OPC(Xc,Yc,Zc)
IMAGE
Fig. 7 Theoretical scheme of point P registration on the plane of the image.
Projected ray which runs through the point P and centre of coordination system intersects
the plane of the image in point PC. Assuming that PT and PC points are in homogeneous
coordination system and the distance between plane of image and projection centre is f (focal
length of camera) the following equation is true:
1
0100
000
000
T
T
T
C
C
C
Z
Y
X
f
f
Z
Y
X
(1)
Due to the occurrence of registered images influence of radial and tangential distortions and
eccentric position of projection centres (in relation to the image) in the process of obtaining vector
components
C
X
,
C
Y
should be corrected by using previously determined camera calibration matrix
K (Duane 1971, Fryer et al. 1986).
100
00
0
y
x
Ky
x
(2)
0
x
,
0
y
- coordinates of origin point in pixels,
- represents the skew coefficient between the x and y-axis, and is often 0,
xx mf
,
yy mf
- represent focal length in terms of pixels,
x
m
,
y
m
- scale factors relating pixels to distance.
Then corrected coefficients
C
X
,
C
Y
,
C
Z
will be expressed by the formula:
1
0|
T
T
T
C
C
C
Z
X
X
IK
Z
Y
X
(3)
Expression (3) is performed separately for the registrations of point P by camera 1 and by camera
2 using two proper matrices K (respectively for camera 1 and camera 2). Moving on to flat
homogeneous, normalized (taking into account notation from Fig. 6) coordinates, it can be
assumed that nonlinear vectors V1, V2, T are co-planar which is expressed by the equation:
0
21 VTV
(4)
whereas:
3
2
1
2
2
21
1
1;
1
;
1T
T
T
Ty
x
Vy
x
V
(5)
1
V
- normalized vector of point p1 (projected point P on image1) described in camera 1
coordination system.
2
V
- normalized vector of point p2 (projected point P on image2) described in camera 2
coordination system.
T - translation vector of centre of projection of camera 2 in relation to the position of the centre of
projection of camera 1.
Implementation of the condition (4) requires presentation of V2 in camera 1 coordination
system. If both the rotation and translation T will be presented as transformations matrices in
homogeneous coordinate system, then (4) can be described:
0
21 EVVT
(6)
where:
333231
232221
131211
EEE
EEE
EEE
RTE x
(7)
moreover:
333231
232221
131211
RRR
RRR
RRR
R
(8)
is unknown in terms of the value of rotation matrix. E matrix is called an essential matrix and
illustrates the interrelation of homologous points in the shared 3D space, expressed in the camera1
system (using pinhole cameras). When the matrix E is formed as multiplication of translation and
rotation matrix includes two equal non-zero singular values and one singular value equal to zero.
Constraint (6) can be rewritten as
0eaT
(9)
where
T
EEEEEEEEEe 333231232221131211
(10)
and
T
yxyyyxyxyxxxa 1
221212112121
(11)
When you have given a set of n corresponding image points you have matrix A
T
n
aaaA ...
21
(12)
This gives finally linear equation
0Ae
(13)
and the possibility to obtain the vector e and matrix E (7).
We make the additional constraints: ||e|| = 1,
1
33 E
and solving (13) as a linear least squares
minimization problem – it needs at least 8 point matches on two images.
Errors of measurement and point identification result that this assumption is not entirely
numerically correct. By adopting a substitute, for the matrix E, matrix E':
T
UDVE'
(14)
such that:
)0,1,1(diagD
(15)
and assuming matrix
100
001
010
W
it is possible to separate matrices
x
T
and R from E’.
Each of them adopts one of two values thus giving the four possible solutions of matrices
combination
x
T
and R:
33
32
31
1U
U
U
Tx
or
33
32
31
2U
U
U
Tx
(16)
T
UWVR
1
or
TTVUWR
2
. (17)
All pairs of values (
x
T
, R) should be examined and the right one shall be chosen. It may be
1
p
and
2
p
imaging was generated as a result of projections determined by M1 and M2 matrices
(size of 3x4) of P on the imaging planes of camera 1 and camera 2 (Fig. 6) assumed that
PMp 11
or
PMp 22
(18)
0100
0010
0001
1
M
(19)
whereas:
1
2
HM
(20)
moreover:
10 xj
iTR
H
for i=1..2, j=1..2 (21)
then the following system of equations is true:
0
0
0
0
22
11
22
11 PMp
PMp
PMp
PMp
x
x
(22)
which can be symbolically expressed as:
0BP
(23)
P vector may be calculated by subjecting B matrix to SVD decomposition:
T
USVB
(24)
P vector will be equal to the last column of the V matrix. After its normalization, it is necessary to
calculate the position of P point with respect to the camera 2, i.e. P’:
PMP 2
'
. (25)
If the components of the vectors P and P' are respectively upstream camera1 and camera 2, this
pair of [T]x and R (the only one among the four) is correct and they are used in formulas (18) to
calculate the coordinates in the model system of all points measured in the image. The model
system is a system in which there is an isometry of points position in relation to their equivalents
from the real world but the scale is not determined. The scaling effect may be achieved by
pointing, at least two points from the point cloud (distance between this two points has to be
known variable). The model might be also levelled, by using, at least three non-collinear points
from the point cloud. Nowadays on the market there is a wide range of solutions based on
photogrammetry and Computer Vision - eg. 3D Scanner ARAMIS, adaptations of Microsoft
Kinect or Microsoft Kinect 2.0 and even cameras in smartphones, which allows to obtain point
cloud model (Goszczynska B et al. 2015, Nagrodzka-Godycka et al. 2015, Qi et al. 2014).
Photogrammetric measurement solution presented in this paper has been implemented and coded
as a standalone desktop application (Janowski et al. 2014.1). This standalone application uses
synchronous photographs as an input. In presented solution, the algorithm applied to the
application has been improved in the term of accuracy and effectiveness. Embedded solution is
consistent with the general theory described in the literature (Hartley 1997 W. Wang et al. 2000).
Mentioned method has been used as a verification of the measurement results obtained from TLS.
Scaling was performed using data from TLS and actual measurements. The accuracy obtained
using digital photogrammetry methods are much higher than measurements using point clouds
(Janowski et al. 2005, Janowski et al. 2014.2).
6. Summary and conclusions
The leading goal of this particular study was to determine whether and how TLS might be used
in diagnostics of reinforced concrete elements. Authors show that this technology can be used to
register a deformation process, examine and measure geometry of reinforced concrete (RC) beams.
However, all of these advantages have certain restrictions. TLS boundary conditions and
limitations shall designate a level of its potential applications in diagnostic of RC element.
Through fuse of photogrammetry and photogrammetrically prepared synchronous photos, it has
been established that TLS might be used for the failure analysis of reinforced concrete beams and
other concrete elements. Prerequisites described in section 3 designate usability areas of TLS,
authors conclude that links usage for signal tags locating is incorrect because it leads to dilution of
global coordination system. Signal tags lead themselves to micro-shifts which in macro scope
geodesy measurements are meaningless, but they are crucial in deformation measurement.
Exclusion of inaccuracies associated with scans bonding and adverse albedo (fraction of reflected
shortwave radiation) of concrete surface requires that previously scanned the object and the
scanner had to be in the same local coordination system. The position of the scanner during
measurement process cannot be changed in relation to the scanned element. At the stage of
element preparation, it is recommended to engobe scanned surface in order to provide better
contrast between the surface of concrete and expanding cracks, white colour reflects light much
more intense than different colours. Resolution of measurement have an impact on the density of
point cloud, scan resolution should be predefined before measurement and maintained at fairly
high level. Data from TLS about cracks propagation enable to infer which mode of failure
occurred, but only in the pre-critical stage. This technology allows creating full three-dimensional,
specific, virtual model of the structure with full operability. An important advantage of laser
scanning is a speed of measurements, the standard scan may be formed even in 3 minutes.
Furthermore, objects might be investigated without any physical contact and laser scanning can
penetrate inaccessible areas. The most important advantage of TLS over the classical methods of
deformation measurement is complexity and immersion of obtained results.
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