International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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An algorithm for automatic analysis of deformation of reinforced concrete structures using photogrammetryBenning, Wilhelm, Geodetic Institute, University of Technology (RWTH) Aachen, Germany
Görtz, Stephan, Zerna, Köpper & Partner, Bochum, Germany
Lange, Johannes, Geodetic Institute, RWTH Aachen, Germany
Schwermann, Raimund, Geodetic Institute, RWTH Aachen, Germany
Chudoba, Rostislav, Chair of Structural Statics and Dynamics, RWTH Aachen, Germany
To understand the behaviour of cracks and its effect on the failure capacity, deformation tests are carried out on concrete structures. Using photogrammetry as measurement method and the evaluation software PHIDIAS[PHIDIAS was developed by the Geodetic Institute at the RWTH Aachen] is able to determine the deformations in a two dimensional way. Hence, the crack pattern will be extracted by the program RIZ[RIZ is used as a prototype for evaluation of cracks] using methods of intensification and tracing. The graphic representation and a finite element method, which is in development to improve the crack pattern in a recursive way, is realized by the program ORFEUS[ORFEUS is used by the Chairs of Statics and Concrete Structures] . Alternatively to the control of the program RIZ an expert system is scheduled.
To activate the full bearing capacity, a formation of cracks is inevitable at reinforced concrete structures. In view of the appearance, damages as a result of corrosion and also for the realisation of transmission of forces over already existing crack edges the crack width must be limited.Intensive research tries to generate suitable design and construction rules for development and progressive evolution of cracks. The research is part of a "Sonderforschungsbereich" - SFB 532- defined and encouraged by the DFG (German research association), for textile concrete at the RWTH Aachen. As a member the Geodetic Institute Aachen is developing methods by usingphotogrammetry. This is an easy manageable measurement method, to detect two-dimensional information about the crack pattern and about a single crack. In addition, the method allows statements of crack evolution and history.
Photogrammetry is a contactless measurement method. The main measurement instrument is a digital camera, to get the surveyed object with several measurement images from different directions (figure 1). The object is prepared on the surface with a grid of targets. In addition there exists calibration bars, which define the scale (figure 2).
The images are imported and analysed with the digital evaluation software PHIDIAS . By using redundant image information from several photo directions it is possible to calculate both the target coordinates with their standard deviation and the positions of the camera. As method of calculation the photogrammetric bundle adjustment with self calibration of the camera is used, whereby the optical path between object and photo cell is reconstructed mathematically. This method enables an accuracy of coordinates up to 3mm for a target point. That is important for the following calculation to detect very fine cracks, as they occur e.g. at the deformation of textile- reinforced concrete structures. Preparing the target points happens by a backing film. Its minimum distance amounts due to the fabrication 5m.
In the subject area "concrete structures" research on the basis of deformation tests for several phenomena at different concrete structures. The photogrammetry has accompanied several test series in the last years . All tests are progressively loaded, only interrupted for a short photogrammetric measurement. Different types of tests are presented in figure 3:
Tensile tests (1) are used on the one hand for basic tests of reinforcement and concrete. On the other hand they are well suitable for researching the photogrammetry and the automatic crack detection. The width of separation cracks can be easily estimated, because the test is only loaded by normal force.
Shear tests (2) are more complex. They are used by the institute for concrete structures Aachen(IMB) to research shear cracks. Reinforced and prestressed concrete beams as rectangle and I-beam section and smaller thin-walled textile-reinforced concrete I-beams are analysed by a photogrammetric measurement field at the area of shear cracks.
The following special tests can also be worked with the method: Plate tests (3), beams with openings (4), or tests with special moved areas to research the behaviour of the crack edges (5), show some more application of photogrammetry.
5 Analysis of deformation
The structures are loaded progressively and only interrupted for recording the photogrammetric images. The software PHIDIAS evaluates the images to get the three- dimensional coordinates of the measuring points and their standard deviation. This is the base for the following manipulation steps:
The program RIZ accesses the coordinates and calculates the difference of two loading steps to get the motion of every measuring point. The extension between neighbouring points is searched.
The gradient of the shifting vector from left to right in figure 4 indicates the deflection of the beam. Comparing the horizontal neighbouring vectors on a line, it shows that they don't arise continuously. Areas of discontinuity are caused by cracks. The crack pattern follows from the difference of two neighbouring vertical vectors (figure 5):
In figure 5 the structures of cracks may be recognised. An automatic, software-based analysis of cracks has to collect the structures to coherent cracks. In addition, it must regard that crack widths are very small, especially as they occur at textile-reinforced concrete structures. Or a big crack edge shift at shear cracks or crossing cracks at plate tests effects the detection.
6 Automatic crack extraction by the software RIZ
Figure 6 shows the overview about the automatic crack extraction. It will be described in detail in the following.
A routine for the calculation of crack width and shift is also available. Therefore the following method according to Görtz  is used (figure 9):
A cracked element, composed by four neighbouring measured points, is separated in two parts. The displacements of the measured points in both parts are summarized to a mean value The difference vector provides information about the relative displacement in the crack. The crack doesn't open orthogonal to the crack. For this reason the angle q differs to the real crack angle b, it is possible to convert the relative displacement by trigonometric functions into crack width and parallel crack shift.
7 Finite element simulation and recursion
At present the crack detection is expanded using a following finite element simulation by the program ORFEUS for the calculation of stresses in the cracked structure. Therefore a plate is generated sized as the measured field and the measured displacements are raised as external displacements. For this the areas, which are identified as crack locations, are approximated by crack elements, which can represent discontinuity in the displacement field.
The crack pattern analysis can be started recursively using the calculated tensions of the finite element simulation based on the predefinition of crack probabilities.
8 Result example
As a prototype for automatic crack detection the program RIZ was developed. It realizes the implementation of the presented methods. As an example a test of an pretensioned beam is shown, which is analysed in the web region for shear cracks by photogrammetry.
In figure 10 the crack pattern is shown at the ultimate limit state and the crack evolution of progressive load steps. The analysis automatically takes place. The formation of the crack pattern is clearly recognisable.