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Monitoring crack origin and evolution at concrete elements using photogrammetry
Abstract
Controlling and monitoring of crack origin and evolution of concrete elements is an important task in research of building materials. The efficiency of the reinforcement to the bearing capacity depends among other on the limited crack evolution. Regarding the research of crack evolution at the surface of the element, photogrammetry is approved, because it allows an expanded investigation of larger objects with high precision. With the presented experiments different structural elements of prestressed, reinforced and textile concrete are investigated. These are tension, shear beams and plate elements. For the photogrammetric measurements the surface is prepared by a grid of targets. Up to three digital cameras (Kodak DCS Pro 14n) are used as a multi exposure assembly or as a static one-camera-system. Cracks are causing local changes to the displacements between the targets. Repeating the measurement in time intervals the crack evolution can be observed. The digital images are evaluated by the photogrammetric software PHIDIAS; it yields a precision of the target coordinates up to 3µm. For analysing deformation, a software is developed to detect crack structures. Hereby the measurement data and the parameters of material are probabilistically regarded in the existence probability of a crack. The values are improved by intensification methods and then single cracks are automatically traced. Detail information like crack width and crack edge displacement are also calculated. In addition a Finite-Element-Module is developed, which simulates the test. Thus the results of photogrammetric measurements can be compared with the numeric tension calculation and iteratively improved. The described measurement and analysis system is developed at the Geodetic Institute of RWTH Aachen, University (Germany). The tests are realised at the Institute of Structural Concrete, the Institute of Building Material Research and Institute of Textile Technique, all RWTH Aachen. The work is part of the research activities referring to the "Sonderforschungsbereich 532 -Textilbeton" (Collaborative Research Centre financed by the DFG (German research association) for textile concrete) . KURZFASSUNG: Die Kontrolle und Beobachtung der Rissentstehung und Rissentwicklung von Betonbauteilen ist eine wichtige Aufgabe in der Bauforschung. Die Wirksamkeit der Bewehrung im Hinblick auf die Tragfähigkeit der Bauteile hängt unter anderem von der begrenzten Rissbildung ab. Bei der Untersuchung des Rissbildungsverhaltens an der Bauteiloberfläche hat sich die Photogrammetrie bewährt, da sie als einziges Verfahren die hochgenaue, flächenhafte Untersuchung eines größeren Objektbereiches ermöglicht. In den hier vorgestellten Versuchen wurden unterschiedliche Bauteile aus Spann-, Stahl-oder Textilbeton untersucht. Es handelt sich um Zug-oder Biegeträger sowie um Scheibenkörper. Für die photogrammetrischen Messungen wird die Oberfläche der Versuchskörper mit rasterförmig angeordneten Messmarken präpariert. Die Aufnahmen werden mit bis zu drei Digitalkameras (Kodak DCS Pro 14n) als Mehrbildverband oder als statisches Einkamerasystem erstellt. Risse bewirken lokale Änderungen in den Abständen zwischen den Messmarken. Durch die Wiederholung der Aufnahmen in zeitlichen Abständen kann auf diese Art die Rissbildung verfolgt werden. Die Aufnahmen werden photogrammetrisch mit der Software PHIDIAS ausgewertet, wobei die Koordinaten der Messmarken mit einer Genauigkeit von bis zu 3 µm ermittelt werden. Für die Deformationsanalyse wird eine eigene Software zur Ermittlung der Rissstrukturen entwickelt. Hierbei werden die Messdaten und die Parameter des Materials probabilistisch in der Existenzwahrscheinlichkeit eines Risses berücksichtigt. Die Messwerte werden mittels Verstärkungsmethoden verbessert, um danach einzelne Risse automatisch verfolgen zu können. Detailinformationen wie Rissbreiten und Rissuferverschiebungen werden zusätzlich in der Berechnung ermittelt. Außerdem wird an einem Finite-Elemente-Modul gearbeitet, das die Versuche parallel simuliert. Auf diese Weise kann das aus der photogrammetrischen Messung gewonnene Ergebnis mit den numerischen Spannungsberechnungen verglichen und iterativ verbessert werden. Die Entwicklung des hier beschriebenen Mess-und Analysesystems findet am (ITA), alle RWTH Aachen, durchgeführt. Die Arbeiten sind Bestandteil der Forschungsaktivitäten im Rahmen des Sonderforschungsbereiches 532 "Textilbeton".
... DIC methods are based on monocular or multiocular image sequences where features are tracked over time. Early contributions used artificial targets [4]. They compared lengths between adjacent targets to detect cracks between the time steps and also calculated crack widths and crack edge displacements using the method of [5] but ignored rotations between the time steps. ...
This publication presents a digital image correlation (DIC) based technique applied to a shear test on a carbon-reinforced concrete member. DIC methods are based on image sequences where the first image is recorded under zero-load without deformations while further images are taken from deformed stages. The image processing starts with the computation of subpixel-precise displacement vector fields using photogrammetric image matching algorithms. In our method, instead of computing strains, another approach is used to quantify deformations and to detect cracks. The matching points are triangulated into a triangular mesh. Then, a deformation model is used that includes a split of the triangle into two parts. The relative translation between these parts is computed and is considered as deformation vector. The detection of cracks is performed by a thresholding applied to the norm of the deformation vectors. Furthermore, the deformation vector is decomposed into the components parallel and perpendicular to the crack. The decomposition requires the knowledge of the crack normal, for which a possible estimation scheme is proposed. The presented method is suitable for brittle material, and it was applied to a monocular image sequence of a shear test of a carbon reinforced concrete member. The test specimen was made of high-strength concrete and was reinforced with a multilayer carbon reinforcement. The selected shear test was intended to analyze the shear behavior of a multi-span system with a distributed load.KeywordsDeformation measurementCrack detectionImage Sequence AnalysisDigital Image Correlation (DIC)Shear testCarbon reinforced concrete (CRC)
... Many studies have made use of the two surveying technologies considered in this study in civil engineering applications such as monitoring the deformations of reinforced concrete beams, concrete brick walls, and reinforced concrete dams. Also, they were used for evaluating the shear transfer mechanism in reinforced concrete beams and evaluating corrosion in reinforced concrete structures [12,[21][22][23][24][25][26][27][28][29][30][31][32]. ...
Undoubtedly, the cracks in the reinforced concrete (RC) structures represent a great danger to those structures, and it was necessary to monitor these cracks with high accuracy. In this research, an experiment was done to investigate the use of modern surveying technologies (digital close-range photogrammetry and terrestrial laser scanning) to monitor such deformations. The experiment was done in the field on a real project under construction in a water treatment plant. In this plant, some cracks were found in an existing building and studied using the same proposed modern surveying technologies along with a traditional method (Digital Vernier). The results of the modern surveying methods are compared with those obtained by the traditional methods, proving that the proposed modern methods give accurate and reliable results. One of the most important advantages of these modern methods is that these methods do not require direct contact with the monitored structure. Moreover, these methods provide a three-dimensional model to show the exact details of the reinforced concrete structure. Ó 2023 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Alexandria University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).
... (Barazzetti and Scaioni, 2009) used a monocular system and an orientation frame to analyze cracks. (Benning et al., 2004, Görtz, 2004 observed artificial targets on concrete specimens with a trinocular camera system during load tests. They also computed crack widths in 4point elements (rectangles) without regarding rotations between the states. ...
As an extension to existing work on crack detection and subpixel accuracy crack width determination as a tool for civil engineering material testing, the paper shows an algorithmic approach to handle widening cracks with relative rotations between related crack borders. In the first time step under zero-load, a set of points to be tracked through consecutive frames of an image sequence is defined. Then, subpixel-precise displacement fields are computed for the image data of the following time steps using an 8-parameter least-squares matching approach. The points are triangulated into a mesh, and the changes of the inner geometry of the triangles are considered with a mathematical model assuming a split of each triangle. With this model, subpixel-precise deformation vectors are derived. Crack candidates are determined by a thresholding applied to the vectors’ lengths. After an estimation of the crack normal, a decomposition of the deformation vectors is applied, allowing to compute crack widths and shear movements. As a novel contribution to the technique, a model extension is proposed for the case of a relative rotation between the crack borders in order to reduce systematic errors. The model includes two separate rigid transformations for each crack side.
... For instance, digital photogrammetry was used to monitor cracks in concrete elements [21,22]. In the experiments, the surfaces of the concrete elements were covered by a grid of targets. ...
Determining the roughness of technical surfaces is an important task in many engineering disciplines. In civil engineering, for instance, the repair and reinforcement of building component parts (such as concrete structures) requires a certain surface roughness in order to ensure the bond between a coating material and base concrete. The sand patch method is so far the state-of-the-art for the roughness measurement of concrete structures. Although the method is easy to perform, it suffers from considerable drawbacks. Consequently, more sophisticated measurement systems are required. In a research project, we developed a novel camera‑based alternative, which comes with several advantages. The measurement system consists of a mechanical cross slide that guides an industrial camera over a surface to be measured. Images taken by the camera are used for 3D reconstruction. Finally, the reconstructed point clouds are used to estimate roughness. In this article, we present our measurement system (including the hardware and the self-developed software for 3D reconstruction). We further provide experiments to camera calibration and evaluation of our system on concrete specimens. The resulting roughness estimates for the concrete specimens show a strong linear correlation to reference values obtained by the sand patch method.
... An example of using this method for measuring the crack width of a concrete sample is provided by Benning et al. [30]. They targeted a 30×30 cm square concrete plate with 3481 (59×59) at a distance of 5 mm. ...
The traditional methods for calculating the width of the cracks in concrete structures are mainly based on the manual and non-systematic collection of information, and also depend on personal justifications and judgment. Due to the fact that these approaches are time-consuming and always there are some human errors inevitably, in recent years more attention is paid to the new methods for detection and monitoring of cracks. One of the most important new approaches is the application of image-based techniques. These schemes use field images and photos provided by the camera to determine specific parameters, such as damage occurrence, location, severity, length of cracks, width and depth of cracks. Moreover, tracking the crack propagation over time using a set of timed photos is among the design purposes of these methods. Image processing, and targeting are two common methods which have their own pros and cons. Results showed that the image processing approach detects some surface noises as cracks which is most challenging error in this method. On the other hand, targeting approach has shown weakness in determining the exact location of cracks. These limitations have pushed researchers to innovate more modern techniques such as Digital Image Correlation (DIC) and mathematical tools like Wavelet transform (WT) to eliminate these errors.
... Other techniques base on the analysis of image sequences that allow the comparison of a deformed state to an undeformed state. Benning et al. (2004) used a trinocular camera system to record image sequences during load tests with concrete specimens. First, displacements of a grid of artificial targets on the surface of the specimens were determined. ...
This publication presents a procedure for the determination of all three components of crack opening vectors from stereoscopic image sequences of a specimen under load in civil engineering material testing. The method is based on analyzing stereoscopic image sequences of a concrete specimen with a surface texture, which is suitable for applying image matching techniques. Spatio-temporal correspondences are established by applying sub-pixel accuracy area based image matching techniques to a grid of surface points. Data acquisition starts at zero load. The load is stepwise or continuously increased during the experiment. The surface points are matched between the stereo images and tracked through each camera image sequence. As an intermediate result, we obtain a set of 3D object surface points for each epoch by spatial intersection. These 3D object points are triangulated into a mesh. Then, the mesh triangles are tested for deformations by transforming the triangles into 2D space and computing the norm of the 2D relative translation vector. Connected components of deformed triangles are determined and crack normals are computed. In the next step, the 3D relative translation vector can be derived for each deformed triangle. Defining local crack opening coordinate systems for the deformed triangles, the three components of the crack opening vectors can be computed. The method has been tested and validated in practical experiments. The technique is capable of quantitatively analyzing cracks with a width of less than one pixel in image space.
... During the last few years, several contributions about photogrammetry in material testing were published. Early photogrammetric applications in the field of deformation measurement dealt with artificial targets that were tracked in an image sequence of monocular or multi-ocular camera systems (Maas 1998;Fraser and Riedel 2000;Whiteman et al. 2002;Benning et al. 2004;Hampel and Maas 2003; Barazzetti and Scaioni 2010). For example, Barazzetti and Scaioni (2009) presented a 2D image-based method for crack analysis using a digital camera, an orientation frame, and a pair of signalized supports. ...
An image sequence analysis procedure is developed to quantitatively analyze complex multiple crack patterns in tension tests of fiber-reinforced composite specimens. Planar textured surfaces of such specimens can be observed with a monocular image sequence using a camera of suitable spatial and temporal resolution. Due to the narrow crack paths, a dense high-precision displacement vector field is computed applying least-squares image matching techniques. Some uniformly distributed matching points are triangulated into a mesh. To measure deformations, principal strains and crack widths are computed for each face. Stretched triangles presumably containing one or multiple cracks are subdivided into three new triangles in order to densify the mesh in critical regions. The subdivision is repeated for some iterations. The crack width computation of the triangles requires at least three vertices and its displacements. Due to the dense displacement vector field, there are more points available. In this paper, an algorithm for the crack width computation in a least-squares fit is presented.
... Advances have been made to produce crack detection from image sequences. Benning et al. [3] first proposed to compute deformation of material under stress using photogrammetry. Since then, a number of researchers employed digital image correlation (DIC) to detect microscale cracks in 2D displacement fields [6] [13] [20] [22]. ...
We present a novel geometry-based approach for the detection of small-scale cracks in a temporal series of 3D-reconstructions of concrete objects such as pillars and beams of bridges and other infrastructure. The detection algorithm relies on a geometry-derived coloration of the 3D surfaces for computing the optical flow between time steps. Our filtering technique identifies cracks based on motion discontinuities in the local crack neighborhood. This approach avoids using the material color which is likely to change over time due to weathering and other environmental influences. In addition, we detect and exclude regions with significant local changes in geometry over time e.g. due to vegetation. We verified our method with reconstructions of a horizontal concrete beam under increasing vertical load at the center. For this case, where the main crack direction is known and a precise registration of the beam geometries over time exists, this approach produces accurate crack detection regardless of substantial color variations and is also able to mask out regions with simulated growth of vegetation over time.
... Abdel-Qader et al. [3] used images to identify cracks and the state of conservation of pavements in bridges, and proposed algorithms for crack patterns in pavements of bridges. Benning et al. [8] used photogrammetry to measure reinforced concrete deformations and monitor cracks. ...
Image processing methods are increasingly used in civil engineering, especially in the maintenance of concrete structures. Current digital cameras and post-processing methods allow verifying qualitatively the state of conservation of wide areas of concrete in dams and bridges. When dealing with building refurbishments and rehabilitation, it is important to verify that existing structural elements fit the requirements of the standards; in the case of structures formed by traditional RC joists, cracking of the bottom-face provides information about the serviceability of these elements. This research proposed and put in practice through experimental tests an image post-processing method for quantification of cracking (five specimens were used and calibrated). Based on a sequence of shots and through a complex step-by-step post-processing, cracks were identified and measured to calibrate this method for real purposes. The method quantifies the crack opening width and spacing by analyzing the bottom-face of the joists through the shots. Measured values of crack spacing are very similar to those predicted by the standards, while the values of crack opening width differ more from theoretical ones due to the scattering of results. However, the proposed method has been proved as suitable and useful for fast inspections of RC elements under bending.
... Wenyu et al [5] presented a method which uses shape based image processing algorithm which uses average smoothing filtering, thresholding and morphological opening for only detecting and classifying defects in a structure. A more robust picture was painted by the use of visual inspection system (VIS) described by Quingyoung et al [6].It focused on contrast enhancement combined with the segmentation by thresholding algorithm. ...
... Photogrammetry has demonstrated its potential for monitoring structural tests in the past (Cooper and Robson, 1990;Maas, 1998;Benning et al., 2004). In particular, it allows instantaneous measurement of a very high number of monitoring points with a single arrangement of image sensors. ...
Photogrammetry has been used in the past to monitor the laboratory testing of civil engineering structures using multiple image-based sensors. This has been successful, but detecting vibrations during dynamic structural tests has proved more challenging because they usually depend on high-speed cameras which often results in lower image resolutions and reduced accuracy. To overcome this limitation, a novel approach has been devised to take measurements from blurred images in long-exposure photographs. The motion of the structure is captured in individual motion-blurred images without dependence on imaging speed. A bespoke algorithm then determines each measurement point's motion. Using photogrammetric techniques, a model structure's motion with respect to different excitation frequencies is captured and its vibration envelope recreated in 3D. The approach is tested and used to identify changes in the model's vibration response.
... The system consists of three digital cameras which map the deformations in adjustable intervals of 5 to 30 s. The precision of the measurement data amounts to about 3 to 5 μm [11]. ...
... Photogrammetric targets were observed with a trinocular camera system over 2 h. Hampel and Maas (2003) and Benning et al. (2004) presented multi-ocular photogrammetry systems for civil engineering material testing. The test bodies were prepared with a grid of targets that were tracked in an image sequence using image correlation techniques. ...
The paper shows advanced spatial, temporal and spatio-temporal filtering techniques which may be used to reduce noise effects in photogrammetric image sequence analysis tasks and tools. As a practical example, the techniques are validated in a photogrammetric spatio-temporal crack detection and analysis tool applied in load tests in civil engineering material testing. The load test technique is based on monocular image sequences of a test object under varying load conditions. The first image of a sequence is defined as a reference image under zero load, wherein interest points are determined and connected in a triangular irregular network structure. For each epoch, these triangles are compared to the reference image triangles to search for deformations. The result of the feature point tracking and triangle comparison process is a spatio-temporally resolved strain value field, wherein cracks can be detected, located and measured via local discrepancies. The strains can be visualized as a color-coded map. In order to improve the measuring system and to reduce noise, the strain values of each triangle must be treated in a filtering process. The paper shows the results of various filter techniques in the spatial and in the temporal domain as well as spatio-temporal filtering techniques applied to these data. The best results were obtained by a bilateral filter in the spatial domain and by a spatio-temporal EOF (empirical orthogonal function) filtering technique. © 2015 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS).
... In this situation, new measurements techniques are providing a significant breakthrough to obtain better interpretation of the experimental evidence [19]. In particular, digital measurement from high-resolution photography (photogrammetry) constitutes a consistent tool to obtain accurate measurements of strains and crack widths at high frequencies (even higher than 1 Hz). ...
Shear in concrete members without transverse reinforcement can be carried by various potential shear-transfer actions, whose activation depends much on the actual cracking pattern and kinematics at failure. Failures can occur in a progressive manner (at the end of a stable propagation of a critical shear crack) or in a sudden manner (by an unstable progression or development of a new crack). In addition, the development and shape of the failure crack may also be very different from case to case. These differences influence which shear-transfer actions may be governing for a given member and loading situation. Despite the large number of specimens tested in shear, almost no information on the actual crack development during the process of failure is yet available. This paper presents the results of an experimental programme consisting of thirteen beams. The tests were designed to investigate different structural systems and loading conditions commonly found in practice (cantilevers with concentrated and distributed loading, single span beams with distributed loading and continuous beams). The cracking patterns and their associated kinematics were tracked in detail by using photogrammetric techniques at high frequencies during testing and particularly during the process of failure, providing data on the actual crack development leading to shear failure. The observations show that very different cracking patterns may be found and that they might be also developed in different manners. The results are interpreted with reference to the measured crack kinematics and related to the various potential shear-transfer actions, with the aim of providing a useful material towards the development of rational approaches for shear design.
... DIC has been used previously to analyze bridges and concrete test objects. Several researchers have conducted tests on laboratory scale concrete and steel beams to study cracking behavior [6][7][8][9][10][11][12] and displacement [13][14][15][16][17][18] during bending tests. Most researchers located cracks using the displacement or strain fields. ...
Due to the aging global civil infrastructure (e.g. bridges), there is a critical need for monitoring and assessing structural integrity of large scale structures. According to the ASCE, in 2008, the average bridge in the U.S.A. was 43 years old and 161,892 bridges were structurally deficient or obsolete. Currently, bridge health is assessed primarily using qualitative visual inspection, which is not always reliable because some damage is difficult to detect, quantify visually, or is subject to human interpretation. Traditional sensors such as strain gages, and displacement sensors, have been recently used to monitor bridges. These sensors only measure at discrete points or along a line, making it difficult to detect damage that is not in the immediate vicinity of the sensor or is difficult to interpret. To address these issues, this paper investigates the use of three-dimensional (3D) digital image correlation (DIC) as a sensing approach for improved bridge structural health monitoring. 3D DIC is a non-contact, full field, optical measuring technique that uses digital cameras to measure surface geometry, displacement, and strain. It is proposed that DIC can be used for monitoring by imaging a bridge periodically and computing strain and displacement from images recorded at different dates or operating conditions. In this paper, DIC is shown to locate non-visible cracks in concrete, quantify spalling, and measure bridge deformation. These techniques are first demonstrated in the laboratory. Field measurements are also made on three full-scale bridges. This paper discusses challenges and solutions to implementing DIC on large structures in the field. The results reveal that DIC is an effective approach to monitor the integrity of large scale civil infrastructure.
... This showed that the values of crack dimensions where approximately plus or minus 62 in 2 (0.04 m 2 ) for this study. Benning et al. (2004) presented work where the crack monitoring on a structure was accomplished using photogrammetry. Using the photogrammetric software, PHIDIAS, cracks were monitored by measuring the displacement of targets placed on the structure. ...
The bridge inspection industry has yet to utilize a rapidly growing technology that shows promise to help improve the inspection process. This thesis investigates the abilities that 3D photogrammetry is capable of providing to the bridge inspector for a number of deterioration mechanisms. The technology can provide information about the surface condition of some bridge components, primarily focusing on the surface defects of a concrete bridge which include cracking, spalling and scaling. Testing was completed using a Canon EOS 7D camera which then processed photos using AgiSoft PhotoScan to align the photos and develop models. Further processing of the models was done using ArcMap in the ArcGIS 10 program to view the digital elevation models of the concrete surface.
Several experiments were completed to determine the ability of the technique for the detection of the different defects. The cracks that were able to be resolved in this study were a 1/8 inch crack at a distance of two feet above the surface. 3D photogrammetry was able to be detect a depression of 1 inch wide with 3/16 inch depth which would be sufficient to measure any scaling or spalling that would be required be the inspector. The percentage scaled or spalled was also able to be calculated from the digital elevation models in ArcMap. Different camera factors including the distance from the defects, number of photos and angle, were also investigated to see how each factor affected the capabilities. 3D photogrammetry showed great promise in the detection of scaling or spalling of the concrete bridge surface.
... Then, cross-curvature evaluation followed by linear filtering is performed in order to differentiate cracks from analogous background patterns. For higher accuracy measurements of crack changes in [5] digital photogrammetry measures local changes to the displacements between targets around the cracks with a precision down to the range of 3µm. In [6] some related aspects of the orientation analysis in images are studied. ...
One of the most important monitoring tasks of tunnel inspection is the observation of cracks. This paper describes an approach for crack following using mid-resolution (2-5mm per pixel) images of the tunnel surface. A mosaic on the basis of the tunnel design surface is built from images taken with a mobile platform. On this image representing the unwrapped tunnel surface texture the starting points of each crack are found semiautomatically using a modified Hough transform. Crack following takes place on the basis of local line fitting and exhaustive search in both directions of the crack, taking into account several restrictions, rules and optimization criteria to find the correct crack trajectory. A practical implementation polygonizes the extracted cracks and feeds them into a tunnel inspection data base. The method is applicable to various types of background texture as expected in the tunnel environment.
... The tracking and comparison of homologous points in temporally successive images enable the measurement and analysis of material deformations. The application of two or more cameras allows the three dimensional evaluation of an observed scene and was described in [3], [18] and [24]. The profit of information by using a second or third camera is associated with the increasing costs of the equipment and the time of analysis. ...
This paper focuses on space- and time-resolved crack detection in concrete structures by combining photogrammetric techniques with acoustic emission analysis. For the photogrammetric measurements, the surface of reinforced concrete members is textured with a random pattern. A consumer-grade digital camera is used to observe the region of interest during the loading tests. In a sequence of images, cracks are visualized by detecting discrepancies in local displacement vector fields, which are obtained from matching algorithms applied to consecutive images. Critical areas of concrete samples are additionally equipped with several acoustic emission sensors to monitor the crack formation and propagation by acoustic emission analysis. The parameter-based method is used to record specific parameters in real time and enables the distinction between bending or shear cracks depending on the signal energy and duration. During the loading tests, information about the crack prolongation is used to derive the time of transition from stable to unstable phase. The acoustically detected shear cracks are monitored in the images to track growth and to stop the experiment before a critical bearing status is reached. Thus, shear cracks can be localized temporally and locally on the surface and inside the structure. In a next step, the detected cracks have to be checked for certain properties providing information about the condition of the structure. Indicators with high level significance referring to structures with no or low advance notice of failure constitute a focal point of further research.
... Moreover, the technique will deliver only an accumulated crack width when multiple cracks are present between two targets, and cracks passing through targets may cause indistinctness. An example of a study on crack width measurement via discrete targets affixed to a concrete probe is given by Benning et al. (2004). They marked a reinforced concrete plate of 30 × 30 cm 2 with 3481 (59 × 59) targets at a target spacing of 5 mm. ...
Concrete probes in civil engineering material testing often show fissures or hairline-cracks. These cracks develop dynamically. Starting at a width of a few microns, they usually cannot be detected visually or in an image of a camera imaging the whole probe. Conventional image analysis techniques will detect fissures only if they show a width in the order of one pixel. To be able to detect and measure fissures with a width of a fraction of a pixel at an early stage of their development, a cascaded image analysis approach has been developed, implemented and tested. The basic idea of the approach is to detect discontinuities in dense surface deformation vector fields. These deformation vector fields between consecutive stereo image pairs, which are generated by cross correlation or least squares matching, show a precision in the order of 1/50 pixel. Hairline-cracks can be detected and measured by applying edge detection techniques such as a Sobel operator to the results of the image matching process. Cracks will show up as linear discontinuities in the deformation vector field and can be vectorized by edge chaining. In practical tests of the method, cracks with a width of 1/20 pixel could be detected, and their width could be determined at a precision of 1/50 pixel.
... In laboratories, loading tests are commonly performed on individual structures, such as concrete beams (Parkash et al., 2006). Artificial stressing is typically used for validating the elastic properties of a concrete beam: in Mattock et al. (1971) a comparative study of pre-stressed concrete beams, with and without bond, is reported; Tan and Ng (1997) describe the effects of deviators and tendon configuration on behaviour of externally pre-stressed beams; several authors deal with detecting cracks during loading tests (Benning et al., 2004;Hegger et al., 2004;Chen et al., 2006). ...
In this paper, terrestrial laser scanning (TLS), photogrammetric and total station measurements were compared with dial gauge observations in two different loading experiments on a reinforced concrete beam. In the first test, the T-beam was stressed in several loading phases resulting in deformations of up to 13·63 mm. All measuring methods were able to detect deformations with an accuracy of better than 0·38 mm. The theoretical calculations of deflections based on a form of Euler-Bernoulli beam equation, however, underestimated the maximum bending of 4·08 mm. A similar loading experiment was applied on a rectangular concrete beam. In this case, the maximum deformation of the beam was 14·94 mm and measuring accuracies of all methods were better than 0·44 mm. The accuracy of theoretical calculations was better than 2·07 mm. The results indicate that laser scanning could be used as an alternative or complementary method to photogrammetric and total station measurements for detecting structural deformations in buildings. © 2009 The Authors. Journal Compilation © 2009 The Remote Sensing and Photogrammetry Society and Blackwell Publishing Ltd.
... Die digitale Photogrammetrie hat sich im Verlauf der letzten Jahre für eine Vielzahl von Messaufgaben als ein effizientes Messverfahren bestätigt (HAMPEL, MAAS [1], LUHMANN ET AL. [5], BENNING ET AL. [6]). Für definierte Anwendungen werden Systeme angeboten, die Bilddatensätze vollautomatisch auswerten und Resultate in Echtzeit bzw. ...
Die digitale Nahbereichsphotogrammetrie ermöglicht eine effiziente Erfassung dreidimensionaler Objektoberflächen bei experimentellen Untersuchungen. Besonders für die flächenhafte Erfassung von Verformungen und die Rissdetektion sind photogrammetrische Verfahren – unter Beachtung entsprechender Randbedingungen – prinzipiell geeignet. Der Beitrag geht unter Einbeziehung aktueller Untersuchungen an textilbewehrten Betonproben auf die Problematik der Rissdetektion ein und gibt einen Überblick über den Entwicklungsstand und das erreichbare Genauigkeitspotential. In Bezug auf die praktische Anwendung der vorgestellten Verfahren wird abschließend auf verschiedene Möglichkeiten der Optimierung eingegangen.
p>The paper summarizes the investigations on shear resistance of I-girders longitudinally stiffened by closed trapezoidal stiffeners, performed in the frame of a European RFCS project (RFS-CR-03018) designated as “ComBri” (Competitive Steel and Composite Bridges by improved Steel Plated Structures), with configurations typical of what may be expected in the design of modern steel bridges. It first reports on four full-scale tests performed at RWTH Aachen, on stiffened panels with strong closed stiffeners where the deformations have been measured with photogrammetric devices. As expected, failure in the subpanels has been governing. A devoted FEA shell model is then described and validated towards these results and other experimental sources. The numerical simulations demonstrate a very good agreement with the test-results. Parametric studies performed with a sophisticated numerical tool help pointing out several aspects where the recommendations of EN 1993-1-5 [1] on stiffened panels in bending and shear may be improved.</p
Concrete sleepers, installed on majority of railroad track in this nation can, if not maintained properly, threaten the safety of running trains. In this paper, an algorithm for automatically identifying cracks in a sleeper image, taken by high-resolution camera, is developed based on Adaboost, known as the strongest adaptive algorithm and most actively utilized algorithm of current days. The developed algorithm is trained using crack characteristics drawn from the analysis results of crack and non-crack images of field-installed sleepers. The applicability of the developed algorithm is verified using 48 images utilized in the training process and 11 images not used in the process. The verification results show that cracks in all the sleeper images can be successfully identified with an identification rate greater than 90%, and that the developed automatic crack identification algorithm therefore has sufficient applicability.
Measuring and testing activities have been performed from the beginning of the twentieth century within engineering. Early activities in structural health monitoring were damage identification in aerospace and mechanical engineering. Aircrafts and military vehicles needed monitoring, and a lot of sensors were developed for these purposes. It was also common to perform damage identification with rotating machines, which was called condition monitoring. Organized structural health monitoring (SHM) activities became more acknowledged in the last decades and emerged various engineering disciplines. Today, also many civil engineering structures are monitored continuously and provide true real‐time information of these structures.
Technical development of sensory technology has been rapid and is still ongoing. Several new techniques are available on the market and provide higher performance and innovative, more reliable measurements.
Fiber‐optic sensors, advanced NDT methods such as acoustic emission, radar‐based techniques, in addition to micro electromechanical systems (MEMS) technology together with advanced data acquisition techniques and database automation supply for beneficial SHM. This paper presents new and emerging technologies and new areas of usage; mostly for the civil engineering structures. It highlights their advantages and also brings up with challenges. Some real applications are presented in order to give a true picture about SHM with new and emerging technologies and complexity of the subject.
Der Beitrag stellt die Grundprinzipien der Nahbereichsphotogrammetrie und einige Anwendungsbeispiele im bautechnischen Versuchswesen vor. Das Ziel der Photogrammetrie ist die messtechnische Auswertung von Bilddaten mit hoher Genauigkeit und Zuverlässigkeit. Ein entscheidender Vorteil des Einsatzes photogrammetrischer Verfahren liegt in der Möglichkeit simultaner Messungen an einer großen Anzahl von Messpunkten. Durch stereoskopische Aufnahme können aus mehreren Aufnahmen eines Objekts dreidimensionale Informationen gewonnen werden, und durch die Anwendung von Bildverarbeitungsverfahren lassen sich Messabläufe weitestgehend automatisieren. Dank Subpixeloperatoren der Bildverarbeitung in Kombination mit einer strengen geometrischen Modellierung können auch beim Einsatz preiswerter Kameras hohe Genauigkeiten in vollautomatischen Messsystemen erzielt werden.
On the potential of photogrammetric techniques in civil engineering material testing
Photogrammetry is a technique to derive metric information from images with high precision and reliability. The paper presents the fundamentals and some application examples of close range photogrammetry in civil engineering material testing. A clear advantage of photogrammetric techniques is in the fact that it allows for simultaneous measurements at a large number of positions in an image. Processing stereoscopic images, 3D object information can be obtained. Subpixel accuracy image processing operators together with strict geometric sensor modeling allows for obtaining high accuracies in fully automatic measurement systems based on off-the-shelf camera components.
Since the time Brown introduced the concept of self-calibration, it was known that there was no impediment in using consumer grade
devices for metric purposes. Today, dSLR cameras are knowingly the standard photogrammetric tool in applications when time is not
an issue, thus images can be taken sequentially. Nonetheless, albeit available with standard video signal, there has been little interest
in applying them to observe dynamic scenes. In this paper we present a methodology to use dSLR cameras for shape and motion
reconstruction at frequency of 30Hz. Particular focus is put on calibration and orientation issues, in static and dynamic cases i.e.
cameras also undergoing a change in position during the measurement. Performance of the system was validated with results obtained
by a system of superior quality.
Visual inspection of structures is a highly qualitative method in which inspectors visually assess a structure’s condition. If a region is inaccessible, binoculars must be used to detect and characterize defects. Although several Non-Destructive Testing methods have been proposed for inspection purposes, they are nonadaptive and cannot quantify crack thickness reliably. In this paper, a contact-less remote-sensing crack detection and quantification methodology based on 3D scene reconstruction (computer vision), image processing, and pattern recognition concepts is introduced. The proposed approach utilizes depth perception to detect cracks and quantify their thickness, thereby giving a robotic inspection system the ability to analyze images captured from any distance and using any focal length or resolution. This unique adaptive feature is especially useful for incorporating mobile systems, such as unmanned aerial vehicles, into structural inspection methods since it would allow inaccessible regions to be properly inspected for cracks. Guidelines are presented for optimizing the acquisition and processing of images, thereby enhancing the quality and reliability of the damage detection approach and allowing the capture of even the slightest cracks (e.g., detection of 0.1 mm cracks from a distance of 20 m), which are routinely encountered in realistic field applications where the camera-object distance and image contrast are not controllable.
The paper presents an Image-based Method for Crack Analysis (IMCA) which is capable of processing a sequence of digital imagery to perform a twofold task: (i) the extraction of crack borders and the evaluation of its width across the longitudinal profile; (ii) the measurement of crack deformations (width, sliding and rotation). Here both problems are solved in 2-D, but an extension to 3-D is also addressed. The equipment needed to apply the method is made up of a digital camera (or a still video-camera in case a high frequency in data acquisition is necessary), an orientation frame which establishes the object reference system, a pair of signalized supports to be placed in a permanent way on both sides of the crack to compute deformations; however, permanent targets are mandatory only for case (ii). The measurement process is carried out in a fully automatic way, a fact also that makes this technique highly operational for unskilled people in engineering surveying or photogrammetry. The accuracy of the proposed method, evaluated in experimental tests adopting different consumer digital cameras, is about ± 5–20 μm, like the accuracy of most deformometers, but with the advantage of automation and of augmented achievable information; moreover, the image sequence can be archived and off-line measurements could be performed at any time.
The localisation and calibration of damage in a structure are often difficult, time consuming, subjective and error prone. The importance of a simple, fast and relatively inexpensive non-destructive technique (NDT) with reliable measurements is thus greatly felt. The usefulness and the efficiency of any such technique are often affected by environmental conditions. The definition of damage and the subsequent interpretation of the possible consequences due to the damage introduce subjectivity into an NDT technique and affect its performance. It is of great importance in terms of practical application to find out the efficiency of an NDT technique in a probabilistic way for various damage definitions and environmental conditions through the use of receiver operating characteristic (ROC) curves. Such variations of performance of an NDT tool can be predicted through simulation processes, and the test conditions conducive to good detections can be isolated and ranked according to their relative efficiency. This paper considers a camera based image analysis technique to identify, quantify and classify damage in structures at various levels of scale. The general method has been applied to identify the affected areas on aluminium due to pitting corrosion. The method depends on the optical contrast of the corroded region with respect to its surroundings, performs intelligent edge detection through image processing techniques and computes each affected and closed region to predict the total area of the affected part, together with its spatial distribution on a two-dimensional plane. The effects of various environmental factors on the quality of such images are simulated from an original photograph. The objectivity and the amount of available information, quantification and localisation and the extent of pitting corrosion are observed, together with the various constructed ROC curves. The method provides the engineer, the owner of the structure and the end-user of the NDT technique with a tool to assess the performance of the structure in an as-built condition and decide on the appropriateness of a certain NDT, under a given environmental condition and a certain definition of damage. Moreover, it allows the findings of the NDT results to be introduced in the decision chain and risk analysis.
This paper presents an enriched finite element representation of crack bridges suitable for simulating the complex damage
process of textile reinforced concrete. The heterogeneity of both the matrix and the reinforcement occurs at similar length
scales of the material structure and demands for an improved accuracy to capture the relevant damage mechanisms. For this
purpose, the eXtended Finite Element Method (X-FEM) is utilized by augmenting the finite elements with special functions reflecting
the discontinuities both in the matrix displacement and in the slip between the matrix and the reinforcement. The approach
is presented for the 1D example of a tension bar with two interacting cracks. A second example demonstrates the modeling concept
on the 2D analysis of shear zones with a detailed approximation of the local deformation fields in the crack bridge.
The new composite material Textile Reinforced Concrete (TRC) is a promising development which may open up entirely new fields for the application of the construction material
concrete. The possible more filigree structures with high quality surfaces make TRC an attractive choice for the architect
and give the engineer more freedom in design. However, the use of TRC requires design rules which are currently being developed
at RWTH Aachen University, Germany. In this article, recent experimental results as well as modeling techniques are described.
At present there is a rising interest of architects and engineers in the application of Textile Reinforced Concrete (TRC) as a construction material. Filigree, self-supporting and ventilated façade systems are state-of-the-art in the application of TRC. In current investigations potentials for light-weight structural members are developed. The required models for a secure design of structural members are deduced within the framework of the research activities in the collaborative research centre 532 at RWTH Aachen University [Collaborative Research Centre 532: Textile Reinforced Concrete–Development of a new technology. RWTH Aachen University, Germany. http://sfb532.rwth-aachen.de]. The article outlines fundamental research results as well as their realisation in first applications.
Measurement of the extent of damage in a real structure is extremely important in terms of any maintenance strategy. However, this measurement often turns out to be difficult, time consuming and error – prone. The necessity of a simple, fast and relatively inexpensive damage monitoring system with reliable measurements is growing for quite sometime. The paper proposes a camera based image analysis technique to quantify and classify damage in structures at various levels of scale. The general method has been applied to corroded plate specimens in the laboratory with the aim to identify the affected areas on a steel pile due to pitting corrosion. The method depends on the contrast of the corroded region with respect to its surroundings, performs intelligent edge detection through image processing techniques and computes each affected and closed region to predict the total area of the affected part along with its spatial distribution on a two dimensional plane. Moreover the performance of the camera allows defining a detection threshold and the so-called probability of detection (PoD) and probability of false alarms (PFA). PoD are suggested as functions of the area of the pitting for the construction of Receiver-Operating-Characteristic (ROC) curves. The methodology can be used as a tool for the owners/managers of the structure for objectively quantifying and localising the extent of pitting corrosion, rather than providing information through a subjective visual assessment. Moreover, it allows introducing the probability of detection and probability of false alarms in the decision chain and in risk analysis. The method is shown to be robust, reliable, simple and inexpensive. Accepted Version Peer reviewed
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