• Structural Monitoring and Maintenance
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• v.6 no.4
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• pp.291-315
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• 2019

Fatigue cracks are inevitable in circumstances in which the cyclic loading exists. Therefore, many of mechanical components are in a risk of being in exposure to fatigue cracks. On the other hand, renewing the facilities or infrastructures is not always possible. Therefore, retrofitting the structures by means of the available methods, such as crack arrest methods is logical and in some cases inevitable. In this regard, this paper considers three popular crack arrest methods (e.g., drilling stop-hole, steel welded patch, and carbon fiber reinforced (CFRP) patch), which have been compared by using extended finite element method (XFEM). In addition, effects in terms of the width and thickness of patches and the configuration of drilling stop holes have been evaluated. Test results indicated that among the considered methods, CFRP patches were the most effective means for arresting cracks. Besides, in the case of arresting by means of drilling stop holes, drilling two holes next to the crack-tip was more effective than blunting the crack-tip by drilling one hole. In other words, the results indicated that the use of symmetric welded metal patches could lead to a 21% increase in fatigue life, as compared to symmetric stop holes. Symmetric CFRP patches enhanced the fatigue life of cracked specimen up to 77%, as compared to drilling symmetric stop holes. In addition, in all cases, symmetric configurations were far better than asymmetric ones.

• Journal of the Architectural Institute of Korea Structure & Construction
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• v.35 no.11
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• pp.163-170
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• 2019

Most of the current crack investigation work consists of visual inspection using simple measuring equipment such as crack scale. These methods involve the subjection of the inspector, which may lead to differences in the inspection results prepared by the inspector, and may lead to a large number of measurement errors. So, this study proposes an image-based crack detection method to enhance objectivity and efficiency of concrete crack investigation. In this study, YOLOv2 was used to determine the presence of cracks in the image information to ensure the speed and accuracy of detection for real-time analysis. In addition, we extracted shapes of cracks and calculated quantitatively, such as width and length using various image processing techniques. The results of this study will be used as a basis for the development of image-based facility defect diagnosis automation system.

• International Journal of Highway Engineering
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• v.13 no.4
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• pp.51-59
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• 2011

Tensile stress occurs and random crack develops in concrete pavement slab when it contracts by variation of temperature and humidity. The tensile stress decreases and the random crack is minimized by sawcutting the slab and inducing the crack with regular spacing. The random crack, joint damage, decrease of load transfer efficiency are caused by too wide joint spacing while too narrow joint spacing leads to increase of construction cost and decrease of comfort. A mechanistic-empirical joint spacing design method for the concrete pavement was developed in this study. Structurally and environmentally weakest sections were found among the sections showing good performance, and design strengths were determined by finite element analysis on the sections. The joint width for which the load transfer efficiency is suddenly lowered was determined as allowable joint with referring to existing research results. The maximum joint spacing for which the maximum tensile stress calculated by the finite element analysis did not exceed the design strength were found. And the maximum joint width expected by the maximum joint spacing were compared to the allowable joint width. The new method developed in this study was applied to two zones of Hamyang-Woolsan Expressway being designed. The same joint spacing as a test section constructed by 8.0m of joint spacing wider than usual was calculated by the design method. Very low cracking measured at 6 years after opening of the test section verified the design method developed in this study.

• Journal of the Korea Concrete Institute
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• v.16 no.4 s.82
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• pp.511-520
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• 2004

The fracture energy evaluated from the previous experimental results can be simulated by using the modified singular fracture process zone (S-FPZ) model. The fracture model has two fracture properties of strain energy release rate for crack extension and crack close stress versus crack width relationship $f_{ccs}$ ( w ) for fracture process zone (FPZ) development. The $f_{ccs}$( w ) relationship is not sensitive to specimen geometry and crack velocity. The fracture energy rate in the FPZ increases linearly with crack extension until the FPZ is fully developed. The fracture criterion of the strain energy release rate depends on specimen geometry and crack velocity as a function of crack extension. The variation of strain energy release rate with crack extension can explain theoretically the micro-cracking, micro-crack localization and full development of the FPZ in concrete.

• Structural Engineering and Mechanics
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• v.69 no.4
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• pp.407-416
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• 2019

This paper develops a wireless sensor for online fatigue crack detection and failure warning based on crack-induced nonlinear ultrasonic modulation. The wireless sensor consists of packaged piezoelectric (PZT) module, an excitation/sensing module, a data acquisition/processing module, a wireless communication module, and a power supply module. The packaged PZT and the excitation/sensing module generate ultrasonic waves on a structure and capture the response. Based on nonlinear ultrasonic modulation created by a crack, the data acquisition/processing module periodically performs fatigue crack diagnosis and provides failure warning if a component failure is imminent. The outcomes are transmitted to a base through the wireless communication module where two-levels duty cycling media access control (MAC) is implemented. The uniqueness of the paper lies in that 1) the proposed wireless sensor is developed specifically for online fatigue crack detection and failure warning, 2) failure warning as well as crack diagnosis are provided based on crack-induced nonlinear ultrasonic modulation, 3) event-driven operation of the sensor, considering rare extreme events such as earthquakes, is made possible with a power minimization strategy, and 4) the applicability of the wireless sensor to steel welded members is examined through field and laboratory tests. A fatigue crack on a steel welded specimen was successfully detected when the overall width of the crack was around $30{\mu}m$, and a failure warnings were provided when about 97.6% of the remaining useful fatigue lives were reached. Four wireless sensors were deployed on Yeongjong Grand Bridge in Souht Korea. The wireless sensor consumed 282.95 J for 3 weeks, and the processed results on the sensor were transmitted up to 20 m with over 90% success rate.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.2
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• pp.173-182
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• 2007

Cracks in concrete structures should be measured to periodically assess potential problems in durability and serviceability. Conventional crack measurement systems depend on visual inspections and manual measurements of the crack features such as width, length, and direction using microscope and crack gage. However, conventional methods take long time as well as manpower, and lack quantitative objectivity resulted by inspectors. In this study, an evaluation technique for concrete surface cracks is developed using image processing and artificial neural network. Developed technique consists of three major parts: (1) crack detection (2) crack analysis and (3) pattern recognition. To examine validity of the technique developed in this study, crack analyzing tests were performed on the images obtained from various types of concrete surface cracks. The test results revealed that the system is highly effective in automatically analyzing concrete surface cracks in terms of features and patterns of cracks.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.04a
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• pp.481-486
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• 1999

According to results of this research Fly-ash Cement permanent-form production was found to be possible by fly-ash mortal. The compress strength 350kg/$\textrm{cm}^2$, banding strength 120kg/$\textrm{cm}^2$ were possible material separting and bleeding by excessive W/C rate was decreased permanent-form made by polymer solved high price of polymer by fly-ash. Model material was made by result of first research. There were no minute-crack on beam form and out surface of form was very smooth, So filling degree seemed desirable length of form after steaming curing was maintained as expected. with these results production of form seemed possible. In the banding load test, fly-ash showed increase of maxim load 12% than RC. in the case of minute-crack, comparing with RC, fly-ash showed no crack at connect. at the first stage under continuing loading size of crack increased. These phenomena seemed to be based on contribution of stress of inner bars in permanent-form. in the test of defection, fly-ash shower about 10% beam load increase than RC. in the case of beam defection, RC showed sudden decrease of tolerance at maxim load and total breaking, but permanent-form showed breaking of bending maintaining defection with contribution of steel stress ($\Phi$6 wire-mash). There phenomenic seemed to be attributed to increase of surface and steel tolerance of form. According to construction explacemaion, it was guessed that each panel was constructed by conner-steels in form edge. so cohesiveness was small. on these bases. keeping width of horizontal band 30cm, form-panel of 20mm width was found to be of use. Permanent-form was found to be efficient in compressibleness, defection, safety and use of Fly-ash mortal.

• Journal of the Korea Concrete Institute
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• v.26 no.3
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• pp.299-306
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• 2014

Coating is one of the methods used to solve the problem of corrosion of reinforcement in concrete structures. There are few research reported in the literature regarding the effect of zinc-coating on flexural behavior compared to epoxy coating. The objective of this study was to determine whether zinc-coated rebar adversely affects flexural behavior. Concrete beams reinforced with black or zinc-coated steel were tested in flexure. The test variables included the presence of rebar surface coating with zinc, steel ratio used and cover depth. The study concentrated on comparing crack pattern, crack width, deflection and strain. The ultimate flexural capacity of beams reinforced with zinc-coated bars was not different from that of black steel reinforced beams. The results from deflection and crack width measurements were indicative of no significant variation for the different rebar surface conditions. In addition, it was found that load-strain curve of beam reinforced with zinc-coated steel was similar to that of beam reinforced with zinc-coated steel. Therefore, the test results indicated that the use of zinc-coated rebar had no adverse effect on flexural behavior compared to the use of black rebar.

• Journal of the Korean Wood Science and Technology
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• v.37 no.4
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• pp.293-299
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• 2009

In manufacturing the crack-free carbonized boards using fiberboards, shrinking ratio, weight loss and density variation of carbonized boards at each carbonization temperature were investigated. Fiberboards with thickness of 3, 4.5, 6, and 18 mm were carbonized while pressed with pressure plates at different temperature from $400^{\circ}C$ to $1,000^{\circ}C$ using a ordinary laboratory furnace. Either of crack or twist was not observed in fiberboards by adapting the pressing carbonization method. The ratios of shrinkage of length, width, and thickness were 10~25%, 12~25%, and 28~48%, respectively, and shrinkage ratio of thickness was higher than those of length and width with increasing the carbonization temperature. Weight loss tended to increase with increasing the carbonization temperature, but low correlation between weight loss in thickness of fiberboards and carbonization temperature was observed. Density of 3 mm carbonized hardboard had the highest value and it tended to increase with increasing the carbonization temperature.

• Computers and Concrete
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• v.13 no.6
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• pp.709-737
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• 2014

Fibre-reinforced self-compacting concrete (FRSCC) is a high-performance building material that combines positive aspects of fresh properties of self-compacting concrete (SCC) with improved characteristics of hardened concrete as a result of fibre addition. To produce SCC, either the constituent materials or the corresponding mix proportions may notably differ from the conventional concrete (CC). These modifications besides enhance the concrete fresh properties affect the hardened properties of the concrete. Therefore, it is vital to investigate whether all the assumed hypotheses about CC are also valid for SCC structures. In the present paper, the experimental results of short-term flexural load tests on eight reinforced SCC and FRSCC specimens slabs are presented. For this purpose, four SCC mixes - two plain SCC, two steel, two polypropylene, and two hybrid FRSCC slab specimens - are considered in the test program. The tests are conducted to study the development of SCC and FRSCC flexural cracking under increasing short-term loads from first cracking through to flexural failure. The achieved experimental results give the SCC and FRSCC slabs bond shear stresses for short-term crack width calculation. Therefore, the adopted bond shear stress for each mix slab is presented in this study. Crack width, crack patterns, deflections at mid-span, steel strains and concrete surface strains at the steel levels were recorded at each load increment in the post-cracking range.