• Journal of the Korea institute for structural maintenance and inspection
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• v.20 no.1
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• pp.1-8
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• 2016

The purpose of this study is to predict the cracking behavior and suggest the method of controlling the cracking in concrete box culvert for power transmission due to differential drying shrinkage. Drying shrinkage cracking is mainly influenced by the moisture diffusion coefficient that determines moisture diffusion rate inside concrete structures. In addition to the diffusion coefficient, surface coefficient of concrete surface and relative humidity of ambient air simultaneously affect the moisture evaporation from concrete inside to external air outside. Within the framework of drying shrinkage cracking mechanism, it is necessary to perform the numerical analysis, which involves these three influencing factors to predict and control the shrinkage cracking of concrete. In this study, moisture diffusion and stress analysis cor responding to drying shrinkage on concrete box culvert are performed with consideration of diffusion coefficient, surface coefficient, and relative humidity of ambient air. From the numerical results, it is found that cracking behavior due to differential drying shrinkage of box culvert shows the different feature according to three influencing factors and the methodology of controlling of drying shrinkage cracks can be suggested from this study.

• Journal of the Korea Concrete Institute
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• v.29 no.1
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• pp.11-21
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• 2017

The paper presents experimental results of self-healing effects of OPC-GGBFS paste with cracked and uncracked specimens. The self-healing of cracked specimens is processes of crack closing with re-hydration of unhydrated in crack surface. The OPC paste with GGBFS replacement ratios of 0%, 10%, 20% and 30% were prepared having a constant water-binder ratios of 0.5. The OPC-GGBFS paste specimens immersed in tap-water and sea-water. The temperature of tap and sea-water was $5^{\circ}C$, $15^{\circ}C$ and $25^{\circ}C$. The cracked specimens after deterioration were immersed for 60 days. The relationship between self-healing effect and age-effect was calculate based upon the experimental results. The self-healing effect was measured in ultrasonic pulse velocity (UPV) before and after loading. When the relative change rate of UPV is increases with the increase in GGBFS replacement ratios. Moreover, the self-healing effect is increased with the temperature of tap-water is increase. But the cracked specimens immersed in sea-water was unclear effects of different temperature. Furemore, most of the healing for OPC-GGBFS specimens immersed in tap-water and sea-water occurred during the first 30 days. The self-healing effect until 30 days is higher than that the age-effect. After 30 days, self-effect and age-effect was largely decreases. SEM/EDS analysis of crack on the surface of the specimens in tap-water were covered with aragonite, and sea-water were covered with brucite.

• 대한공업교육학회지
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• v.31 no.1
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• pp.200-210
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• 2006

This C-ring test, normally employed for evaluating susceptibility to stress-corrosion cracking, was determined to be a suitable small scale test to evaluate PWHT(Post-Weld Heat Treatment) cracking susceptibility. This test is possible to incorporate an actual weld, to introduce a notch into the coarse grained HAZ(Heat Affected Zone), to load the coarse grained HAZ any level of stress ad, most importantly, since the C-ring is an approximately constant strain type test, the stress decreases with time at temperature in a manner similar to that of an actual steel weldment. The procedure employed in making the C-ring was presented in the experimental procedure section, however, several points deserve further discussion. The walls of the weld groove are made along radial lines form the center of th var in order to obtain an HAZ which is oriented perpendicular to the walls of the machined C-ring. Therefore, the plane of maximum stress will be aligned through the HAZ and, therefore, crack propagation will not be forced to deviate form the plane of maximum stress in order to remain in the coarse grained HAZ as is the case with the Y groove test.

• Journal of Korea Foundry Society
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• v.41 no.5
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• pp.419-433
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• 2021

Al-Si-Cu alloys benefit from the addition of copper for better hardness and strength through precipitation hardening, which results in remarkably strong alloys. However, the addition of copper expands the solidification range of Al-Si-Cu alloys, and due to this, these alloys become more prone to hot tearing, which is one of the most common and serious fracture phenomena encountered during solidification. The conventional evaluation method of the hot tearing properties of an alloy is a relative and qualitative analysis approach that does not provide quantitative data about this phenomenon. In the present study, the mold itself part of a device developed in Instone et al. was partially modified to obtain more reliable quantitative data pertaining to the hot tearing properties of an Al-Si-Cu casting alloy. To assess the influence of Cu element, four levels of Cu contents were tested (0.5, 1.0, 3.0, and 5.0 wt.%) in the Al-Si-Cu system alloy and the hot tearing properties were evaluated in each case. As the Cu content was increased, the hot tearing strength decreased to 2.26, 1.53, 1.18, and 1.04 MPa, respectively. At the moment hot tearing occurred, the corresponding solid fraction and solidification rate decreased at the same temperature due to the increase in the solid-liquid coexistence range as the Cu content increased. The morphology of the fracture surfaces was changed from dendrites to dendrites covered with residual liquid, and CuAl₂ phases were observed in the vicinity of hot tearing.

• Journal of the Korean Geosynthetics Society
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• v.19 no.4
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• pp.41-50
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• 2020

Since cracks in concrete structures expedite corrosion of reinforced concrete over a long period of time, regular on-site inspections are essential to ensure structural usability and prevent degradation. Most of the safety inspections of facilities rely on visual inspection with naked eye, so cost and time consuming are severe, and the reliability of results differs depending on the inspector. In this study, a portable measuring device that can be used for safety diagnosis and maintenance was developed as a device that measures the width and length of concrete cracks through image analysis of cracks photographed with a camera. This device captures the cracks found within a close distance (3 m), and accurately calculates the unit pixel size by laser distance measurement, and automatically calculates the crack length and width with the image processing algorithm developed in this study. In measurement results using the crack image applied to the experiment, the measurement of the length of a 0.3 mm crack within a distance of 3 m was possible with a range of about 10% error. The crack width showed a tendency to be overestimated by detecting surrounding pixels due to vibration and blurring effect during the binarization process, but it could be effectively corrected by applying the crack width reduction function.

• Tunnel and Underground Space
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• v.32 no.6
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• pp.464-477
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• 2022

DFN (discrete fracture network) models that take account of spatial variability and correlation between rock fractures have been demanded for analysis of fractured rock mass behavior for wide areas with high reliability, such as that of underground nuclear waste repositories. In this regard, this report describes the spatial distribution characteristics of fracture networks, and the DFN modeling methodologies that aim to represent such characteristics. DFN modeling methods have been proposed to represent the spatial variability of rock fractures by defining fracture domains (Darcel et al., 2013) and the spatial correlation among fractures by genetic modeling techniques that imitate fracture growth processes (Davy et al., 2013, Libby et al., 2019, Lavoine et al., 2020).These methods, however, require further research for their application to field surveys and for modeling in-situ rock fracture networks.

• Journal of the Korean Geotechnical Society
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• v.24 no.9
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• pp.41-48
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• 2008

The aim of this study is to present a graphical method in order to evaluate stages in shrinkage cracking. Firstly, the distribution of crack openings is established by sorting the openings of individual cracks in the soil cracking system. Secondly, it is normalized in a range of 0 to 1 to obtain the normalized crack opening distribution. Thirdly, three S-shape curve models introduced by Brooks and Corey(1964), Fredlund and Xing(1994) and van Genuchten(1980) are chosen to fit the normalized crack opening distribution using a curve fitting method. The accuracy of fitting which is described through fitting parameters by the van Genuchten equation is much higher than that by the Brooks and Corey equation and slightly higher than that by the Fredlund and Xing equation; thus the van Genuchten model is used. Finally, the stages of shrinkage cracking are graphically evaluated by drawing three separate straight lines corresponding to three linear parts of the fitted normalized crack opening distribution. The proposed method is tested with different sample thicknesses. The measured data are fitted by the selected model with the fairly high regression coefficient and small root mean square error. The results show graphically that shrinkage cracking comprises three stages; namely, primary, secondary and residual stages. Subsequently, the ranges of evaluated crack opening for each of these stages are presented.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.4A
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• pp.329-335
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• 2009

The purpose of this study is to provide the efficient method and guideline of controlling the cracking in bridge deck concrete due to differential drying shrinkage. Drying shrinkage cracking is mainly influenced by the moisture diffusion coefficient that determines moisture diffusion rate inside concrete structures. In addition to the diffusion coefficient, surface coefficient of concrete surface and relative humidity of ambient air simultaneously affect the moisture evaporation from concrete inside to external air outside. Within the framework of cracking shrinkage cracking mechanism, it is necessary to conceive the numerical analysis, which involves these three influencing factors to predict and control the shrinkage cracking of concrete. In this study, moisture diffusion and stress analysis corresponding to drying shrinkage on bridge deck are performed with consideration of diffusion coefficient, surface coefficient, and relative humidity of ambient air. From the numerical results, it is found that cracking behavior due to differential drying shrinkage of bridge deck concrete shows different feature according to three influencing factors and the methodology of controlling of drying shrinkage cracks can be suggested from this study.

• Journal of Korean Tunnelling and Underground Space Association
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• v.25 no.6
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• pp.555-567
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• 2023

The cracks in the tunnel are currently determined through visual inspections conducted by inspectors based on images acquired using tunnel imaging acquisition systems. This labor-intensive approach, relying on inspectors, has inherent limitations as it is subject to their subjective judgments. Recently research efforts have actively explored the use of deep learning to automatically detect tunnel cracks. However, most studies utilize public datasets or lack sufficient objectivity in the analysis process, making it challenging to apply them effectively in practical operations. In this study, we selected test datasets consisting of images in the same format as those obtained from the actual inspection system to perform an objective evaluation of deep learning models. Additionally, we introduced ensemble techniques to complement the strengths and weaknesses of the deep learning models, thereby improving the accuracy of crack detection. As a result, we achieved high recall rates of 80%, 88%, and 89% for cracks with sizes of 0.2 mm, 0.3 mm, and 0.5 mm, respectively, in the test images. In addition, the crack detection result of deep learning included numerous cracks that the inspector could not find. if cracks are detected with sufficient accuracy in a more objective evaluation by selecting images from other tunnels that were not used in this study, it is judged that deep learning will be able to be introduced to facility safety inspection.

• Journal of the Korea Concrete Institute
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• v.18 no.3 s.93
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• pp.355-360
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• 2006

The Alkali-Silica Reaction(ASR) may cause a serious failure in the concrete pavements and structures. Several researches in some nations have conducted the continuous studies to prevent failure of the concrete structures by the ASR distress as well as the studies to manifest the mechanism. The researches on the ASR have not been performed affluently in Korea because the distress due to ASR has seldom been reported literarily. In this study, we tried to set up the systematic scheme practically for verifying the cause of distress due to ASR by using the visual inspections in field, the chemical method, petrographic analysis, and Electron Dispersive X-ray Spectrometer(EDX) method of Scanning Electron Microscopy(SEM) in laboratory. The chemical method, petrographic method using SEM, and X-ray method were used to verify the cause of pattern crack on the surface and internal crack in the plain concrete pavement. It can be concluded that the distress of a specific site in plain concrete pavement was mainly due to ASR. The chemical method, the petrographic method and EDX method using SEM may be the effective tools for verifying the cause of AAR distresses.