• Journal of the Korean Society of Safety
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• v.25 no.6
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• pp.128-136
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• 2010

In according with concrete structural design standard, it is common designing flexure reinforcement concrete to induce tension failure. So reinforcing ratio is limited to inducing tension failure. And maximum reinforcing ratio is regulated to protecting concrete compression strength caused by over reinforced building. Minimum reinforcing ratio is also limited in designing standard to protecting brittle failure as extremely using less reinforcing bar. But in minimum reinforcing ratio it is extremely conservative or it is sometimes impossible to induce stable tension-failure because they are depending on yield failure and experienced method and concrete designing standard strength. Therefore the purpose of the present paper is to evaluate the flexural behavior of minimum steel ratio of reinforced concrete of beams and to propose the guide-line of equation of minimum steel ratio by performing static flexural test of 16 beams according to size effect, number of steel, yielding stress of steel, and concrete compressive strength which are presumed effective variables. From experimental results, the equation of minimum steel ratio was newly proposed considered size effect.

• Steel and Composite Structures
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• v.34 no.3
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• pp.333-345
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• 2020

In the past, many efficient profiles have been developed for cold-formed steel (CFS) members by judicious intermediate stiffening of the cross-sections, and they have shown improved structural performance over conventional CFS sections. Most of this research work was based on numerical modelling, thus lacking any experimental evidence of the efficiency of these sections. To fulfill this requirement, experimental studies were conducted in this study, on efficient intermediately stiffened CFS sections in flexure, which will result in easy and simple fabrication. Two series of built-up sections, open sections (OS) and box sections (BS), were fabricated and tested under four-point loading with same cross-sectional area. Test strengths, modes of failure, deformed shapes, load vs. mid-span displacements and geometric imperfections were measured and reported. The design strengths were quantified using North American Standards and Indian Standards for cold-formed steel structures. This study confirmed that efficient profiling of CFS sections can improve both the strength and stiffness performance by up to 90%. Closed sections showed better strength performance whereas open sections showed better stiffness performance.

• Journal of Technologic Dentistry
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• v.31 no.2
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• pp.39-44
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• 2009

In the orthopedic field which firstly used zirconia as artificial joints, researchers had studied the reasons for collapsing zirconia used as restorative material by accumulated inner cracks in several years and they found out Low Temperature Degradation is one of the reasons. In the dentistry field, it has not been too long since they used zirconia as the cores of all-ceramic restoration; however, the study is needed as prophylactic measure against Low Temperature Degradation which can be caused by saliva wetting the mouth all the time and frictional forces such as bite pressure and masticatory pressure. Artificial aging by autoclaving is used because there are difficulties of testing in the patient's mouth. To study the changes in the material properties, the flexural strength of dental zirconia ceramic is measured before and after the test. The following are the result of the test. 1) The zirconia blocks in the autoclaves at $130^{\circ}C$ and $200^{\circ}C$ are phase-shifted tetragonal to monoclinic by Low Temperature Degradation. 2)The non-autoclaved specimens have the average fractural strength of 1346.4MPa, the specimens autoclaved at $130^{\circ}C$ have 1226.4Mpa and the specimens autoclaved at $200^{\circ}C$ have 1024.1MPa. The tests show that as the temperature increases, the flexural strength tend to decrease and the differences are noticeable(p<0.001). 3)Through the Duncan's post-hoc test, the differences in flexural strength of the 3 groups were listed in order of strength like normal temperature>at $130^{\circ}C$ autoclave low temperature degradation> at $200^{\circ}C$ autoclave low temperature degradation.

• The Journal of Korean Academy of Prosthodontics
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• v.42 no.1
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• pp.49-57
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• 2004

Statement of problem : The increased awareness of esthetics in dentistry has brought the esthetic consideration in prosthetic restorations . Dental ceramics offer better esthetics for use of prosthetic restorations. Unfortunately, dental ceramic materials are not always the most suitable candidate materials since their inherently brittle nature. In recent years, basic research in ceramic science has led to the recognition or several approaches to strengthen and to enhance esthetics of ceramics. Several all ceramic systems use ceramic core and porcelain build up structures . Ceramic cores influence to strength of all ceramic crowns . So the strength of ceramic cores is important to all ceramic crowns. Purpose : The purpose of this study is to estimate the flexural strength of ceramic cores in some all ceramic systems. Material and method : A biaxial flexure test was conducted on three groups(Cergo, Empress 2, In-Ceram). Each group consisted of 30 discs of nearly identical dimension with a 0.5mm, 1.0mm, 1.5mm thickness and 12mm in diameter. The fracture load was recorded by Instron. Analysis of valiance(ANOVA) and Tukey's tests were performed using SAS statistical software. Results : 1.5mm thickness of specimens were significantly stronger than 0.5mm and 1.0mm thickness of specimens in Cergo and In-Ceram. But each sepecimen group of Empress 2 was no significantly strength by thickness. In order of In-Ceram, Empress 2 and Cergo has significantly stronger strength in the same thickness. Conclusion : In-Ceram is the strongest ceramic material in 3 materials. All the materials can be used according to the required characters.

• Journal of the Korea Concrete Institute
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• v.19 no.1
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• pp.103-111
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• 2007

Experimental study was carried out to investigate the structural performance of composite beams with steel fiber concrete and angle. For this purpose, seven specimens composed of two RC beams with or without steel fiber and five composite beams with steel fiber and angle were constructed and tested. All specimens had no web shear reinforcement. Main variables for the specimens were tensile reinforcement ratio and fiber volume fraction. Based on the test results, structural performance such as strength, stiffness, ductility and energy dissipation capacity was evaluated and compared with the predicted strength. The prediction of flexure and shear strength gives a good relationship with the observed strength. The strength, ductility and energy dissipation capacity are increased, as the fiber volume fraction is increased. Meanwhile, high tensile reinforcement ratio resulted in the reduction of ductility and energy dissipation capacity for the composite beams.

• The Journal of Korean Academy of Prosthodontics
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• v.34 no.4
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• pp.665-674
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• 1996

This study was designed to evaluate the effect of processing method and surface design on the transverse strength of repaired denture base resin. Three heat-cured denture base resins(Vertex, Lucitone, Lang), one cold-cured resin(Lang), and one light-cured resin(Dentacolor gingiva material) were used for repair purpose. The specimens for 3-point flexure test were fabricated by five processing methods such as self-curing, pressure pot, boiling water, processing, and light curing. Finally to evaluate the effect of surface designs for repaired resin, three surface designs(butt, bevel, inverse bevel) were tested. Within the limit of this study, following conclusions were drawn. 1. Lucitone denture base material showed highest flexural strength of $131.37{\pm}2.15MPa$, and there were significant differences in stength between Lucitone and other resins. 2. Between two different self curing methods, self curing repair resin, Lang, cured by pressure pot method showed highest flexural strength, $58.49{\pm}4.89MPa$. 3. Among the heat cured repair resins, maximum transverse strength value of $88.69{\pm}16.60MPa$ was recorded in Lucitone group cured by processing method. 4. Inverse bevel joint design showed significantly higher bond strength than butt joint group, Maximum bond strength was $59.36{\pm}1.33MPa$ in inverse bevel joint design group.

• Korean Journal of Dental Materials
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• v.43 no.1
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• pp.43-50
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• 2016

The aim of this study was to investigate the effect of multiwall carbon nanotube functionalized with carboxyl group (MWCNT-COOH) on the mechanical properties of dental glassionomer cement (GIC). MWCNT-COOH was prepared by the acid oxidative method. The MWCNT-COOH was incorporated into a commercial GIC powder or liquid at 0.5 wt% or 1.0 wt%. The net setting time of the cements was measured in accordance with ISO 9917 (Dental water-based cement). Specimens for compressive strength ($4mm{\varphi}{\times}6mm$), diametral tensile strength ($6mm{\varphi}{\times}4mm$) and flexure strength with modulus ($2mm{\times}2mm{\times}25mm$) were prepared by mixing with the cement liquid and kept in water bath of $(37{\pm}1)^{\circ}C$. Mechanical tests were conducted in 1 d, 7 d, and 14 days at a cross-head speed of 1 mm/min. Compressive strength of GIC mixed with 0.5 wt% MWCNT-COOH increased significantly at 7 d. However, overall mechanical properties of GIC modified with MWCNT were not significantly increased with a delayed setting time, in comparison with control cement. Overall results indicated that the MWCNT/GIC composite cements showed a limited strengthening effect for dental glassionomer cement.

• The Journal of Korean Academy of Prosthodontics
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• v.32 no.3
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• pp.411-430
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• 1994

The denture may be fractured accidentally by an impact while outside the mouth, or may be cracked or broken while in service in the mouth. The latter is generally a fatigue failure caused by repeated flexure over a period of time. This investigation compared the flexural fatigue resistance, the impact force and the transverse strength of two denture base materials with and without the grid strengthener, the T300, the T800 and the Kevlar fiber to evaluate the fracture resistance. The distribution and behavior of fibers across fracture lines were examined by Hi-Scope Compact Microvision System. Through analyses of the data from this study, the following conclusions were obtained. 1. The flexural fatigue resistance, impact strength and transverse strength of high impact strength resin were higher than those of conventional heat polymerizing resin, but statistically there was no significant difference(p>0.05). 2. All specimens with and without the grid strengthener did not show significant differences in the flexural fatigue, the impact and the transverse strength test(p>0.05). 3. All specimens reinforced with the T300, the T800 and the Kevlar fiber showed significant increase of the fatigue resistance and the impact force(p<0.05). 4. All specimens reinforced with the T800 and the Kevlar fiber showed significant increase of the transverse strength(p<0.05). 5. All specimens reinforced with the T300, the T800 and the Kevlar fiber exhibited greenstick fractures. The fibers tended to remain enveloped in the resin, resisting pull-out.

• Structural Engineering and Mechanics
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• v.82 no.3
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• pp.295-312
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• 2022

This study investigates the effects of nano silica (NS) and micro steel fiber on the properties of ultra-high-performance concrete (UHPC). The experimental consists of three groups, each one with five percentages of NS content (0%, 2%, 4%, 6% and 8%) in addition to the 20% silica fume and 20% quartz powder proportioned according to the weight of cement added to the mixtures. In addition, three percentages of micro steel fibers (0%, 1% and 2%) were considered. Different mixtures with varying percentages of NS and micro steel fibers were prepared to set the water-to-binder ratio, such as 0.16% and 1.8% superplasticizer proportioned according the weight of the binder materials. The fresh properties, mechanical properties and elevated temperatures of the mixtures were calculated. Then, the results from the microstructure analyses were compared with that of the reference mixtureand it was found that 6% replacement of cement with NS was optimum replacement level. When the NS content was increased from 0% to 6%, the air content and permeability of the mixture decreased by 35% and 39%, the compressive and tensile strength improved by 21% and 18% and the flexural strength and modulus of elasticity increased by 20% and 11.5%, respectively. However, the effect of micro steel fibres on the compressive strength was inconclusive. The overall results indicate that micro steel fibres have the potential to improve the tensile strength, flexure strength and modulus of elasticity of the UHPC. The use of 6% NS together with 1% micro-steel fiber increased the concrete strength and reduce the cost of concrete mix.

• Structural Engineering and Mechanics
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• v.89 no.6
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• pp.579-588
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• 2024

The present paper investigates the curvature ductility of confined reinforced concrete (RC) beams with normal (NSC) and high strength concrete (HSC). For the purpose of predicting the curvature ductility factor, an analytical model was developed based on the equilibrium of internal forces of confined concrete and reinforcement. In this context, the curvatures were calculated at first yielding of tension reinforcement and at ultimate when the confined concrete strain reaches the ultimate value. To best simulate the situation of confined RC beams in flexure, a modified version of an ancient confined concrete model was adopted for this study. In order to show the accuracy of the proposed model, an experimental database was collected from the literature. The statistical comparison between experimental and predicted results showed that the proposed model has a good performance. Then, the data generated from the validated theoretical model were used to train the artificial neural network (ANN) prediction model. The R² values for theoretical and experimental results are equal to 0.98 and 0.95, respectively which proves the high performance of the ANN model. Finally, a parametric study was implemented to analyze the effect of different parameters on the curvature ductility factor using theoretical and ANN models. The results are similar to those extracted from experiments, where the concrete strength, the compression reinforcement ratio, the yield strength, and the volumetric ratio of transverse reinforcement have a positive effect. In contrast, the ratio and the yield strength of tension reinforcement have a negative effect.