Abstract:Cracks are the main damages of concrete structures. Since cracks may occur in areas that are difficult to reach, non-contact measurement technology is required to accurately measure the width of cracks. This study presents an innovative computer vision system combining a camera and laser rangefinder to measure crack width from any angle and at a long distance. To solve the problem of pixel distortion caused by non-vertical photographing, geometric transformation formulas that can calculate the unit pixel length of the image captured at any angle are proposed. The complexity of crack edge calculation and the imbalance of data in the image are other problems that affect measurement accuracy, and a combination of the improved U-net convolutional networks algorithm and Canny edge detection method is adopted to accurately extract the cracks. The measurement results on the different concrete wall indicate that the proposed system can measure the crack in a non-vertical position, and the proposed algorithm can extract the crack from different background images. Although the proposed system cannot achieve fully automated measurement, the results also confirm the ability to obtain the crack width accurately and conveniently.Keywords: computer vision; crack measurement; U-net; convolutional neural network; structural health monitoring; artificial intelligence
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Culvert is a common reinforced concrete structure in highway engineering, which is often used for drainage or as a passage for people and cars. As the number and weight of vehicles in highways increase, the safety of culvert operations is threatened. Therefore, it is necessary to regularly test the mechanical properties of culverts in highways [1]. Reinforced concrete box culverts (BCs) are an important part of highway industrial construction, which has the characteristics of factory prefabrication and on-site assembly construction. However, any inadequacy in the design or construction of prefabricated BCs can cause problems such as misalignment and deep cracks in the structure [2]. Therefore, an accurate analysis of the mechanical properties of reinforced concrete BCs should be established.
BC structure experienced three stages of elasticity, cracking, and failure with the change of loading. The cracking load was used to distinguish the elastic stage from the cracking stage, while the ultimate load was used to define the failure load of the structure. The load results were shown in Table 3. In order to study the cracking of the top slab concrete, the side surface of the top slab of the BC was taken as the recording surface, and the intersection of the internal surface of the top slab and the external surface of the side wall were taken as the origin of the coordinate axis to record the initiation and development of the crack. Figure 4 shows the distribution of cracks.
It can be seen from Figure 5 that the deflection curves of the midspans of various types of BCs exhibit a nonlinear growth trend with the increase of the load, and four curves can be divided into elastic working state and plasticity working state. In the elastic working stage, there is no crack on the culvert, and the midspan deflection of the top slab in the culvert is linear with the load. Except for the initial stiffness of the round hinged type BC, the initial stiffness of the other three types of BCs is maintained at a high level. As the load continues to increase, BC enters the second working state, which is the plasticity working state. Cracks begin to appear in various types of BCs. Moreover, the number of cracks begins to increase and the stiffness of BC begins to decay. BC loses its working ability when the deflection of the midspan of the top slab of BC increases sharply. It can be seen from Figure 5 that, under the same reinforcement ratio and concrete strength, the structural performance of the integral type BC is the best, followed by the flat seam type BC and the mortise type BC, and the worst is the round hinged type BC. Therefore, it is recommended to use integral type structure in the design of the BC structure to avoid the round hinged type structure. Moreover, it is necessary to focus on observing the deflection curve of the top slab in the health detection process of the BC structure. And the working state of BC can be measured to determine the safety of BC by plotting the long-term midspan of top slab deflection curve.
A positive strain indicates that the concrete is being pulled, and a negative value indicates that the concrete is under pressure. Analysis of Figure 7 shows that, from loading to cracking load, the bending strain variation law of top slab across midspan section of all types of BC structure meets the flat cross-section hypothesis. After the flat seam BC structure cracking, the strain distribution of the top slab across the midspan section shows the nonlinear distribution law, and the position of the neutral axis increases in the vertical direction with the increase of the load, indicating that the bending and tensile action of the concrete under the section is more obvious. The top slab of the round hinged BC structure has a small degree of strain change in the elastic working state. There is no obvious change in the neutral axis position, and the bending strain of the upper and lower sections is contrapuntal, indicating that the force of the structure in this working state is more uniform. Before loading to structure cracking, the neutral axis of the midspan section of the integral BC is in the middle of the section. And as the load increases, the neutral axis of the midspan section is raised up in the direction of the wall height. After the mortise BC structure cracking, the strain distribution law of the top slab across the midspan section is linear distribution. The absolute value of the upper and lower bending strains of the section is quite different, and the neutral axis has a large degree of upward shift.
As can be seen from Figure 11, the experimental value of the top slab of round hinged BC and integral BC is basically the same as the simulation value; with the increase of load, the experimental value of the top slab deflection is bigger than the simulation value. In the elastic working stage of culverts, the relative errors between the experimental value and the simulated value in round hinged BC and integral BC are 12.5% and 17.4, respectively. However, the relative error between two culverts reached 30.2% and 29.1% in the plasticity work phase. Considering that this situation is related to the structural stress redistribution of culverts after the cracking load, the influence of cracking on the bearing capacity of the culvert is not considered in the finite element, so the relative error in the plasticity work phase is large. In summary, the finite element model is consistent with the experimental conclusions of the scale model test by comparing the displacement data of the round hinged BC structure and the integral BC structure, which indicates that the finite element model is effective.
The mechanical properties of reinforcement in the round hinged BC and the integral BC are compared and analyzed with the cracking load of the round BC as the loading value. The axial force diagram of reinforcement in the two directions of BC is extracted by taking the direction of top slab span as X-axis and the direction of side wall height as Y-axis, and the results are shown in Figures 12 and 13.
The first principal stress cloud diagram of the concrete in the calculation results of the round hinged BC and the integral BC is extracted with the round hinged BC cracking load as the loading value. The result is shown in Figure 14.
As a new type of structural form, prefabricated reinforced concrete BCs need to be further researched on its mechanical properties, such as the influence of vehicle live load on BCs and the influence of reinforcement ratio on the cracks on the internal surface of top slab. Furthermore, the mechanical properties of the BC in the engineering are also tested.
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