THE METHOD OF ANALYSIS OF DAMAGE REINFORCED CONCRETE BEAMS USING TERRESTRIAL LASER SCANNING
Krystyna Nagrodzka-Godycka Jakub Szulwic
Patryk Ziółkowski
Faculty of Civil and Environmental Engineering, Gdansk University of Technology, Poland
ABSTRACT
The authors present an analysis of the possibility to assess deformations and mode of failure of R-C beams using terrestrial laser scanning. As part of experiments carried out at the Regional Laboratory of Construction (at Gdansk University of Technology), reinforced concrete beams were subjected to destruction by bending and by shear. The process of press impact on the reinforced concrete beam was recorded using terrestrial laser scanner. Development of scanning effects was performed using Leica Cyclone software and MeshLab. In order to verify results, the independent recording of beam deflection with use of extensometer and the recording with non-meter synchronous digital cameras were carried out, photos from these cameras were subjected to photogrammetric processing. However, the principal subject of this paper is to show the usefulness of laser scanning for the analysis of reinforced concrete beams damages and authors of this paper focused on this subject.
In the description of the experiment there are six stages of measurement mentioned for the reinforced concrete beam: from the zero condition (no force), through intermediate conditions up to the stage of destruction. Scanning was performed from one stable measurement station to avoid in processing the effect of correlation errors (matching and linking) of ScanWorld.
Analysis of the materials from laser scanning allowed to assess the geometry of element subjected to destruction with an accuracy not worse than 1 mm. For this purpose, the method of spheres translation was developed and in its characteristic groups of points from scanning the spheres were described. Spheres centers grew to be geometrical element to be assessed on scans in subsequent measurement epochs. Additionally, through the use of automated analysis of intensity map of laser light reflection, it was indicated the possibility of rapid identification of the beam, in which there may occur microcracks and changes in the structure. The usefulness of the evaluation mechanism can be appreciated in the mass studies or inspective examinations of constructional components.
As a result of the authors' research and in the course of measurements analysis it was possible to establish the concise procedure to specify the mode of the reinforced concrete destruction and it was defined the minimum size of cracks that may be recorded in the cloud of points from laser scanning.
Keywords: laser scanning, deformation measurement, reinforced concrete, beam, cracks, mode of failure
INTRODUCTION
Cracks in reinforce concrete structures, particularly with excessive width, significantly determine the durability of reinforced concrete elements, because they can accelerate the corrosion of reinforcing steel. Moreover, cracks may lead to loss of adhesion of the concrete reinforcing bars which results in the decrease of the load carrying capacity or in extreme cases may even result in damage. Among others, in case of shear, the damage of beams can be rapid. Problem of concrete elements analysis (especially beams) is common and we find numerous references in literature [1, 2, 3]. Within the frames of experimental tests, the experimental station for the analysis of bent reinforced concrete beams was prepared. In the experiment, steel and concrete strains were examined along with widths of cracks and deflections. The method presented in the paper is not only dedicated to the analysis of reinforced concrete beams.
DESCRIPTION OF THE EXPERIMENT
In two experiments, three beams with varying degrees of reinforcement were prepared in such a way as to force the occurrence of predetermined mode of failure mechanism of destruction different for each beam. It was assumed that the two beams will be destroyed by exhausting the bending resistance, in turn for the concrete and steel. This is achieved by the use of alternating reinforcement in the bend zone. The third beam should be destroyed because of the exhaustion of shear capacity. For the preparation of beams, C 25/30 concrete and A-III N steel were used. Within the frames of the experiment and analyses presented in this paper we focused on the analysis of beam No.
3. Recording of changes was carried out using the terrestrial laser scanner (TLS).
Scanning technology finds its documented usage in the monitoring and analysis of the structure [4, 5, 6, 7]. Due to the combination of laser scanning method and digital photography supported by photogrammetric analyses, the beam was additionally prepared. The side surface of the beam has been bleached and symmetrical rectangular grid with a width of 10 cm has been applied on this surface. In points of vertical and horizontal lines intersections, metal markers with a diameter of 6 mm were applied. The beam was placed in a hydraulic press in such a way as the points of application and loadings and support points correspond to the assumed static scheme. Appearance of the experimental station is shown in Fig. 1.
Fig. 1. View on the laboratory station – beam located under the hydraulic press and prepared for recording.
Before concreting, the series of bonded wire strain gauges was attached to rebars of the beam. Strain gauges were connected with wires through the electronic system with computer having a software dedicated to the measurement of steel deformations. The beam was placed in a hydraulic press. For the reflection of the hinged support, the semi- circular steel supports were used. Whereas the force coming from a hydraulic press was transferred through the steel beam of a I-bar section onto metal supports, and from them to the beam. Such a system was designed to reflect the beam load with two symmetrical point loads. Force was applied gradually in steps of 20 kN until the destruction (ca. 140 kN). After each applied load, the measurement was carried out using geodetic terrestrial laser scanner from Leica Geosystems - C10 model.
Zero scan and scan after the first applied load was performed at medium scanning resolution. Subsequent scans were performed in the increased resolution. Increasing the resolution in subsequent measurements significantly improved reception of the model in the subsequent treatment at the expense of longer scan time. After each applied stress, the data on stress in the reinforcement were recorded and the deflection was determined and the presence of cracks was checked. Throughout the entire course of the study, the position and orientation of the scanner has not been changed or tampered. Additional photographic documentation was made using two synchronous cameras. This documentation was used to verify the measurement with the methods of close -range photogrammetry, also using algorithms taken from the computer vision and spatial analysis of distributed points [ 8, 9, 10]. Finally, load caused shear failure, which is in accordance with the established mode of failure.
RESULTS PREPARATION
Measurement data from laser scanning were processed in Leica Cyclone v. 7.3.3.
ScanWorlds from individual measurement epochs were combined into a single object (in a common coordinate system) where it was possible to perform geometric analyses.
In order to assess the size of beam deflection, the two ways of analyses were assumed.
The first method was aimed to identify cracks in the beam (the evaluation of scratches), which has been subjected to load impact). In this approach, an analysis of the intensity of the laser light reflection was applied. The second method was aimed to measure the beam geometry. Here, it was required to use the method of translation of spheres generated on the basis of points indicated or identified on the beam.
Data were exported from Leica Cyclone in .PTX format, and then imported into the MeshLab software. As a result of the experiments, the rule of Red-White-Blue Scale from the Color Mapping tool of MeshLab software was selected and this turned out to be better than the rules of MeshLab RBG, Sawtooth Gray 8 - also possible to be used in the analyses of concrete structures scans. This rule allows you to optimally expose the cracks, so it is possible to determine mode of failure of R-C beam. By visual inspection of the model in the program, we are able to conclude from the cracks arrangement what type of destruction occurred. In the Fig. 2. 2 there are presented beam conditions in selected (1, 4, 5 and 6) observation epochs.
However, to determine whether the scratch occurred may be obtained automatically when the program is able to analyze modifications in color histograms. When preparing
properly crafted model, we are able to (in MeshLab software [Program options: Render / Show Quality Histogram]), obtain colour histograms.
1
4
5
6
Fig. 2. View on the laboratory station – beam located under the hydraulic press and prepared for recording.
In the version presented in these study, due to the possibility of visual assessment, the histogram of beam destruction level is indicated (post-critical condition). However, computer analysis of the histogram allows you to note the occurrence of changes in the histogram already at the third (ambiguous indication) and fourth (unambiguous indication) epoch of observation. Authors experiences allowed to refer to the previous works and to choose the computer histogram analysis methodology (Fig. 3., Fig. 4.).
Geometry measurement was performed using the author's method of spheres translation.
The idea of the method is to indicate on the beam the signalled points and to put there spheres, automatically generated in Leica Cyclone. Finally, the assessment of the geometry comes down to measuring points displacement vectors. As part of the verification of the method, it was checked the correctness of generating spheres, taking
into account the manual determination of geometrical points, recorded in scans. The assessment was performed for points, presented in the Fig. 5. Each characteristic element, captured by the scanner is reflected by the points in the model.
Fig. 3. Reflection intensity maps histogram which indicates the scratches
Fig. 4. Histogram of the reflection intensity maps (combination of two epochs of observation), under conditions of beam overload, with a marked characteristic instep of the indication that scratches are present
The aim of the analysis is to determine the coordinates of all compound points describing the characteristic point in the points cloud model. Then, using the arithmetic mean of coordinates of all compound points, the very center of each characteristic point was determined. The assumption of the method is such that the obtained center should be center of the sphere with a diameter of characteristic point.
Tab. 1 presents selected points and the analysis of accuracy of the sphere center identification. In total, in the full analysis, the coordinates of sphere center and coordinates of very center of the point cloud part corresponding to signalled tag on the beam gave discrepancies not larger than 1 mm. A separate issue not raised in this article, and to which authors are also devoted to is the method of spheres approximation
for areas of the non-uniform coverage by scanning points. However, in case of the described experiment, the covering of signalled tags on the beam was recognized as uniformly dense.
Fig. 5. Surface of the beam with marked characteristic points 12 points were selected for analyses
Table 1. Fragment of the statement from the comparison of sphere center coordinates and the center of points cloud recorded on the scan at the location of the signalled point on the beam
Item Number of points
Points coordinates taken from the scan
Difference between sphere center and average coordinates of points from
scan
Points view on the scan
1 7
(0.169 -2.808 -0.842) (0.169 -2.808 -0.843) (0.169 -2.806 -0.843) (0.168 -2.808 -0.844) (0.168 -2.807 -0.843) (0.168 -2.805 -0.841) (0.168 -2.809 -0.842)
Cloud gravity point:
(0.168 -2.807 -0.843)
Sphere position:
[0.169 -2.806 -0.842]
Cloud gravity point:
[0.168 -2.807 -0.843]
Difference:
[0.001 0.001 0.001]
6 7
(-0.330 -2.740 -0.847) (-0.327 -2.740 -0.849) (-0.330 -2.740 -0.850) (-0.330 -2.739 -0.848) (-0.329 -2.739 -0.848) (-0.330 -2.739 -0.848) (-0.329 -2.739 -0.847)
Cloud gravity point:
(-0.329 -2.739 -0.848)
Sphere position:
[-0.329 -2.738 -0.848]
Cloud gravity point:
[-0.329 -2.739 -0.848]
Difference:
[0.000 0.001 0.000]
Using the method of spheres translation, the measurement of beam geometry was performed (Tab. 2.). Validation of the obtained results was performed by referring to the deflections, recorded on the dial gauge located under the beam. In the sixth observation
epoch, when load caused shear failure of the beam, observation was possible only with the use of data from scanning, due to the early removal of the dial gauge endangered by the damage by pressure of broken beam
Tab. 2. Comparison of beam deflection value, measured directly on the laboratory station and on the basis of spheres centers, determined on scans (for the chosen observation epochs)
Measurement epoch
Force load [kN]
Deflection [mm]
Deflection in accordance
with measurements
on the model [mm]
Differences of deflection value [mm]
View on the scan
2 30 1.81 1.00 +0.81
3 50 3.32 4.00 -0.68
5 90 5.89 6.00 -0.11
6 140.7 12
SUMMARY
In the authors’ opinion, such method of beams cracks examination, based on the intensity of laser light reflection and spheres translation method may be the effective tool in the evaluation of reinforced concrete objects geometry. Essentially, it is
necessary to use high-speed terrestrial scanners and to strengthen the method by using the scans filtration [11] and developing methods of analyses outside the lab, including mobile experiments supported by additional surveying [12]. These studies are a further stage of works, realized within the fames of authors cooperation with centers in Gdansk and Olsztyn.
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