• Steel and Composite Structures
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• v.52 no.3
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• pp.357-376
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• 2024

In this paper, weakly bonded ultra-high-strength steel bars (UHSS) were used as longitudinal reinforcement in recycled aggregate concrete shear walls to achieve resilient performance. The study evaluated the repairability and hysteresis performance of shear walls before and after retrofitting. Quasi-static tests were performed on recycled aggregate concrete (RAC) and steel fiber reinforced recycled aggregate concrete (FRAC) shear walls to investigate the reparability of resilient shear walls when loaded to 1% drift ratio. Results showed that shear walls exhibited drift-hardening properties. The maximum residual drift ratio and residual crack width at 1% drift ratio were 0.107% and 0.01mm, respectively, which were within the repairable limits. Subsequently, shear walls were retrofitted with bonded X-shaped CFRP strips and steel plates wrapped at the bottom and retested. Except for a slight reduction in initial stiffness, earthquake-damaged resilient shear walls retrofitted with a composite method still had satisfactory hysteresis performance. A revised damage assessment index D, has been proposed to assess of damage degree. Moreover, finite-element analysis for the shear wall before and after retrofit retrofitting was established in OpenSees and verified with experimental results. The finite element results and test results were in good agreement. Finally, parametric analysis was performed.

• Journal of the Korean Institute of Landscape Architecture
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• v.40 no.3
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• pp.81-90
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• 2012

Improvements in the quality of life have resulted in a heightened awareness of safety and the environment. As a result, timber as an environmentally friendly material, is used for landscape facilities and a wide range of purposes. But there are a large number of defects since there are twists and cracks that can be found in wooden landscape facilities. This has led to the use of imported hardwood instead of the Western Hemlock which has been in widespread use. Hardwood is expensive. However, it is being used without any information or research on how much it reduces the actual defects. Construction contractors are in great need of information on the characteristics and defect rates of different types of timber. This study investigated and analyzed the cracks in four types of timber - namely the Western Hemlock, Burckella, Nyatoh and Malas - in order to provide basic information to construction contractor for them to be able to select and use the appropriate type of timber. The main results of this study are as follows. First, the Western Hemlock had 1.90 $cracks/m^2$, Malas had 0.83 $cracks/m^2$, Burckella had 0.14 $cracks/m^2$, and the Nyatoh had the least number of surface defects at 0.04 $cracks/m^2$. Second, while Malas has the highest degree of strength timber, Nyatoh had the smallest defect rate. This showed that having high timber strength does not necessarily mean it has less defects. Third, the Western Hemlock was the least expensive and Burckella was the most expensive. However, considering the cost of repairing defects, it would be economically advantageous to use Burckella and Nyatoh which have low defect rates. This study aimed to provide basic information to landscape construction contractors for them to be able to select and use the appropriate type of timber when constructing wooden outdoor rest furniture. The results are expected to contribute to quality enhancements and defect reduction in landscape facilities.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.2
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• pp.131-143
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• 2013

Vibration condition monitoring of low-speed rotational slewing bearings is essential ever since it became necessary for a proper maintenance schedule that replaces the slewing bearings installed in massive machinery in the steel industry, among other applications. So far, acoustic emission(AE) is still the primary technique used for dealing with low-speed bearing cases. Few studies employed vibration analysis because the signal generated as a result of the impact between the rolling element and the natural defect spots at low rotational speeds is generally weak and sometimes buried in noise and other interference frequencies. In order to increase the impact energy, some researchers generate artificial defects with a predetermined length, width, and depth of crack on the inner or outer race surfaces. Consequently, the fault frequency of a particular fault is easy to identify. This paper presents the applications of empirical mode decomposition(EMD) and ensemble empirical mode decomposition(EEMD) for measuring vibration signals slewing bearings running at a low rotational speed of 15 rpm. The natural vibration damage data used in this paper are obtained from a Korean industrial company. In this study, EEMD is used to support and clarify the results of the fast Fourier transform(FFT) in identifying bearing fault frequencies.

• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.3
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• pp.287-298
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• 2014

In order to develop SFRC TBM tunnel segment, evaluating the SFRC mixture was conducted through flexural tests of SFRC beams without ordinary steel reinforcement in this study. Considered variables were compressive strengths of SFRC, aspect and mix ratio of steel fibers and total 16 specimens were fabricated and tested until failure. The load-vertical displacement results demonstrates that the effect of aspect ratio is minor when compared to results form small beam test(Moon et al, 2013). A SFRC beam resists the vertical load until the width of crack reaches to 7 mm due to steel fibers across cracked surfaces. Moreover, it is found that flexural moment estimated by equation of TR No. 63(Concrete Society, 2011) is useful for prediction of nominal strength for SFRC structure. From the investigation of fiber distribution in cracked section, it is found that dispersion improved in actual size beam compared to in standard small beam for evaluation of flexural strength.

• Journal of the Korea Concrete Institute
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• v.19 no.6
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• pp.763-771
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• 2007

Over the last few decades, many researches have been conducted in order to find solution to the problem of corrosion in steel reinforced concrete. As a result, methods such as the use of stainless steel bars, epoxy coatings, and concrete additives, etc., have been tried. While effective in some situations, such remedies may still be unable to completely eliminate the problems of steel corrosion. Fiber reinforced polymer (FRP) elements are appealing as reinforcement due to some material properties such as high tensile strength, low density, and noncorrosive. However, due to the generally lower modulus of elasticity of FRP in comparison with the steel and the linear behavior of FRP, certain aspects of the structural behavior of RC members reinforced with FRP may be substantially different from similar elements reinforced with steel reinforcement. This paper presents the flexural behavior of one-way concrete slabs reinforced with FRP bars. They were simply supported and tested in the laboratory under static loading conditions to investigate their crack pattern and width, deflections, strains and mode of failure. The experimental results shows that behavior of the FRP reinforced slabs was bilinearly elastic until failure. Also, the results show that the FRP overreinforced concrete beams in this study can be safe for design in terms of deformability.

• Journal of the Korea Concrete Institute
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• v.26 no.2
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• pp.143-150
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• 2014

To avoid abrupt torsional failure due to concrete crushing before yielding of torsional reinforcement and control the diagonal crack width, design codes specify the limitations on the yield strength of torsional reinforcement of RC members. In 2012, Korean Concrete Institute design code increased the allowable maximum yield strength of torsional reinforcement from 400 MPa to 500 MPa based on the analytical and experimental research results. Although there are many studies regarding the shear behavior of RC members with high strength stirrups, limited studies of the RC members regarding the yield strength of torsional reinforcement are available. In this study, twelve RC beams having different yield strength of torsional reinforcement and compressive strength of concrete were tested. The experimental test results indicated that the torsional failure modes of RC beams were influenced by the yield strength of torsional reinforcement and the compressive strength of concrete. The test beams with normal strength torsional reinforcement showed torsional tension failure, while the test beams with high strength torsional reinforcement greater than 480 MPa showed torsional compression failure. Therefore, additional analytical and experimental works on the RC members subjected to torsion, especially the beams with high strength torsional reinforcement, are needed to find an allowable maximum yield strength of torsional reinforcement.

• Journal of the Korean Ceramic Society
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• v.43 no.2 s.285
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• pp.98-105
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• 2006

YBCO film was synthesized by using a new approach to the TFA-MOD method. In the fabrication process, $Y_2Ba_1Cu_1O_x\;and\;Ba_3Cu_5O_8$ powders were used as precursors (the so called '211 process'), instead of Y-, Ba-, and Cu-based acetates, and dissolved in trifluoroacetic acid followed by calcining and firing heat treatment. Consequently, we successfully fabricated YBCO film and evaluated the phase formation, texture evolution, and critical properties as a function of the calcining and firing temperature and humidity, in order to explore its possible application in coated conductor fabrication. The films were calcined at $430-460^{\circ}C$ and then fired at $750-800^{\circ}C\;in\;a\;0-20\%$ humidified $Ar-O_2$ atmosphere. We observed that $BaF_2$ phase was effectively reduced and that a sharp and strong biaxial texture formed under humidified atmosphere leading to increased critical properties. In addition, we found that the microstructure varied significantly with the firing temperature: the grain grew further, the film became denser, and the degree of texture and phase purity varied as the firing temperature increased. For the film fired at $775^{\circ}C$ after calcining at $460^{\circ}C$, the critical current was obtained to be 39 A/cm-width (corresponding critical current density is $2.0\;MA/cm^2$ which was probably attributed to such factors as the enhanced phase purity and out-of-plane texture, the moderate film density and grain size, and crack-free surface.

• Journal of Dental Rehabilitation and Applied Science
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• v.18 no.3
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• pp.205-215
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• 2002

This study investigated the compressive fracture strength of Targis ceromer crown by the difference of occlusal thickness on a maxillary first premolar. Control group was a castable IPS-Empress all-ceramic crown with occlusal thickness of 1.5 mm constructed by layered technique. Experimental groups were Targis crowns having different occlusal thicknesses of 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, respectively. The classification of Targis group is T10, T15, T20, T25 and T15N (for no-thermocycling and occlusal thickness of 1.5mm). Ten samples were tested per each group. Except occlusal thickness, all dimension of metal die is same with axial inclination of $10^{\circ}$and marginal width 0.8mm chamfer. All crowns were cemented with Panavia F and thermocycled 1,000 times between $5^{\circ}$ and $55^{\circ}$ water bath with 10 sec dwelling time and 10 sec resting time. The compressive fracture strength was measured by universal testing machine. The results were as follows : 1. Fracture strength was increased as the occlusal thickness increased : compressive fracture strength of Group T10, T15, T20, T25 was $66.65{\pm}4.88kgf$, $75.04{\pm}3.01kgf$, $87.07{\pm}7.06kgf$ and $105.03{\pm}10.56kgf$, respectively. 2. When comparing material, Targis crown had higher fracture strength than IPS-Empress crown : the mean compressive strength of group T15 was $75.04{\pm}3.01kgf$ and the value of group Control was $37.66{\pm}4.28kgf$. 3. Fracture strength was decreased by thermocycling : the compressive fracture strength of T15 was $75.04{\pm}3.01kgf$, which is lower than $90.69{\pm}6.88kgf$ of group T15N. 4. The fracture line of crowns began at the loading point and extended along long axis of tooth. IPS-Empress showed adhesive failure pattern whereas Targis had adhesive and cohesive failure. In the SEM view, stress was distributed radially from loading point and the crack line was more prominent on Targis crown.

• Tunnel and Underground Space
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• v.28 no.2
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• pp.170-185
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• 2018

The effect of fault on the stability of the closely-spaced twin tunnels located in fault zones was investigated by numerical analyses and scaled model tests on condition of varying widths, inclinations and material properties of fault. When obtaining the strength/stress ratios of pillar between twin tunnels, three different stresses were used which were measured at the middle point of pillar, calculated to whole average along the pillar section and measured at the left/right edges of pillar. Among them, the method by use of the left/right edges turned out to be the most conservative stability estimation regardless of the presence of fault and reflected the excavating procedures of tunnel in real time. It was also found that the strength/stress ratios of pillar were decreased as the widths and inclinations of fault were increased and as the material properties of fault were decreased on condition using the stresses measured at the left/right edges of pillar. As a result of scaled model tests, it was found that the model with fault showed less crack initiating pressure than the model without fault. As the width of fault was larger, tunnel stability was decreased. The fault had also a great influence on the failure behavior of tunnels, such as the model without fault showed failure cracks generated horizontally at the left/right edges of pillar and at the sidewalls of twin tunnels, whereas the model with fault showed failure cracks directionally generated at the center of pillar located in the fault zone.

• Journal of Korean Tunnelling and Underground Space Association
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• v.20 no.2
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• pp.235-253
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• 2018

Groundwater flow induced by cracks in a buried pipe causes ground loss in the vicinity of it which can lead to underground cavities and sinkhole problems. In this study, the ground collapse mechanism and the failure mode based on an aperture in the pipe located in cohesionless ground were investigated through a series of physical model studies. As the influence parameters, size of the crack, flow velocity in the pipe, groundwater level, ground cover depth and ground composition were adopted in order to examine how each of the parameters affected the behavior of the ground collapse. Influence of every experimental condition was evaluated by the final shape of ground failure (failure mode) and the amount of ground loss. According to the results, the failure mode appeared to be a 'Y' shape which featured a discontinuous change of the angle of erosion when a groundwater level was equal to the height of the ground depth. While in the case of a water table getting higher than the level of ground cover depth, the shape of the failure mode turned to be a 'V' shape that had a constant erosion angle. As the height of the ground depth increased, it was revealed that a mechanism where a vertically collapsed area which consisted of a width proportional to the ground height and a constant length occurred was repeated.