• Journal of the Korea Institute of Building Construction
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• v.16 no.4
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• pp.305-311
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• 2016

This study examined the long-term inelastic characteristics, including unrestrained shrinkage and creep, of low-heat cement concrete under different ambient curing temperatures. To achieve the designed compressive strength of 42MPa, water-to-binder ratios were selected to be 27.5, 30, and 32.5% for curing temperatures of 5, 20, and $40^{\circ}C$, respectively. Test results showed that the shrinkage strains of concrete mixtures tended to decrease with the decrease in curing temperature because of the delayed evaporation of internal capillary and gel waters. Meanwhile, creep strains were higher in concrete specimens under lower curing temperature due to the occurrence of the transition temperature creep. The design models of KCI provision gave better accuracy in comparison with test results than those of ACI 209, although a correction factor for low-heat cement needs to be established in the KCI provision.

• Journal of the Korea Institute of Building Construction
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• v.16 no.4
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• pp.297-303
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• 2016

Shrinkage Reducing Agent(SRA) was developed in order to control drying shrinkage cracks in concrete, and the use of SRA is increasing since it can control drying shrinkage cracks and improve the quality of concrete structures. Although there are many types of prediction equations of drying shrinkage strain, there is no prediction method which can consider the effect of SRA up to the present. Therefore, it is impossible to predict the tensile stress generated by drying shrinkage of SRA concrete, and to investigate the quantitative serviceability limit state of SRA concrete. In this study, the drying shrinkage of SRA concrete was investigated by experiment and analysis in order to suggest the predictability of drying shrinkage of SRA concrete. As a result, AIJ model, ACI model, GL2000 model showed there was a correlation between the predicted values and the experimental values within the error range of ${\pm}10%$. However, CEB-FIP model and B3 model underestimated the experimental values.

• Geophysics and Geophysical Exploration
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• v.13 no.4
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• pp.388-396
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• 2010

As a part of basic studies on monitoring and saturation estimation of carbon dioxide ($CO_2$) storage using resistivity survey, laboratory experiment has been conducted to measure the change of the electrical resistivity through repeated experiments of supercritical $CO_2$ and brine water injection into homogeneous and heterogeneous sandstones. The $CO_2$ saturation is estimated by using resistivity index based on the resistivity measurements. The experimental results of two types of sandstones show that the effect of pore structure in the rock and the effect of contained clay minerals in the rock can be affected to calculate the $CO_2$ saturation. The result can be useful to evaluate the $CO_2$ saturation based on resistivity survey at the site where $CO_2 sequestrates.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.984-990
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• 2017

Mesh screen modeling and liquid propellant discharge simulation of surface tension tank were performed using commercial CFD software Flow-3d. $350{\times}2600$, $400{\times}3000$ and $510{\times}3600$ DTW mesh screen were modeled using macroscopic porous media model. Porosity, capillary pressure, and drag coefficient were assigned for each mesh screen model, and bubble point simulations were performed. The mesh screen model was validated with the experimental data. Based on the screen modeling, liquid propellant discharge simulation from PMD tank was performed. NTO was assigned as the liquid propellant, and void was set to flow into the tank inlet to achieve an initial volume flow rate of liquid propellant in $3{\times}10^{-3}g$ acceleration condition. The intial flow pressure drop through the mesh screen was approximately 270 Pa, and the pressure drop increased with time. Liquid propellant discharge was sustained until the flow pressure drop reached approximately 630 Pa, which was near the estimated bubble point value of the screen model.

• Polymer(Korea)
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• v.33 no.6
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• pp.575-580
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• 2009

Carbon fabric-reinforced silicon carbide composites (C/SiC) have attracted a considerable attention for high temperature structural application because of their outstanding oxidation resistance property and thermal shock resistance. In this study, we reported on the preparation of C/SiC composites by the polymer impregnation and pyrolysis (PIP) method. For this, polycarbosilane solution was impregnated into the carbon fabric and then cured by electron beam irradiation under argon atmosphere. Afterwards, the cured composite was pyrolyzed at $1300^{\circ}C$ for 1 h under argon atmosphere to produce the C/SiC composite. The porosity and density of the C/SiC composite were 13.5% and $2.44\;g/cm^3$, respectively, when the impregnation of the carbon fabric with the 30 wt% polycarbosilane solution conducted four times. In addition, in the isothermal experiment at $1500\;^{\circ}C$ in air for 5 h, the 95.9 wt% of the C/SiC composite was remained, indicating that the prepared C/SiC composite has a outstanding oxidation resistance.

• Journal of the Korean Recycled Construction Resources Institute
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• v.8 no.2
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• pp.204-211
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• 2020

Poly-Lactic Acid(PLA) fiber is an eco-friendly material and is biodegradable, so it can be utilized for manufacturing porous construction materia ls with interna l pore connection. In this study, domestic PLA fiber products(0.5mm india meter, 1.0mm in length, 10mm in length) were tested for melting at high temperatures and high alkality, and they were incorporated with FA-based geopolymer. Compressive strength was obtained through high temperature curing and alkali activator, however the complete melting of the PLA fiber was not ensured. The previous study handling PLA fiber with 0.003mm in diameter was completely dissolved, but 0.5mm and 1.0mm in diameter showed 42.5% and 33.3% of dissolution ratio, respectively. In addition, the increasing fiber volume led floating fibers during curing, which had a negative effect on its workability and solubility. Although the properties of PLA fiber may vary depending on the raw materials and production conditions, PLA fiber with 0.1mm or less diameter is recommended for porous construction material.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.6
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• pp.4192-4200
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• 2015

In this paper, dispersibility of steel fiber is improved mixing with form for material development of protection and blast resistant structure sprayed concrete. And it is developed a high toughness cellular sprayed concrete material using steel fiber. Oversupply form for dispersibility improvement of steel fiber is mostly fade away through sprayed, finally it is satisfied with the proper mixing ratio under 3 % ~ 6 %. This is considered for compressive strength and flexural toughness. Test results of compressive strength showed superior strength capability in 28, 56 days, also flexural strength and flexural toughness is great. Then oversupply form is enhanced for dispersibility of steel fiber and I think that it did not cause decreasing of strength. But analysis results of pore structure through image analysis failed for a great spacing factor and specific surface area. This is largely measured in spacing factor because air content have a grate evaporation effect for sprayed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1529-1534
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• 2013

A voice coil actuator with a rotating moving magnet has been developed for a miniaturized mobile robot. The actuator has simple structure comprising a magnet, a coil, and a yoke. Actuator performance is predicted using a linearized theoretical model, and dynamic performance based on the air-gap between the magnet and the coil is predicted using motor constant and restoring constant obtained through finite element simulations. The theoretical model was verified using a prototype with 60 Hz resonance and 80 Hz bandwidth. We found that an input of 1.5 V can make the actuator rotate by $20^{\circ}$ statically. The driving configuration of the proposed actuator can be simplified because of its implementation of open-loop control.

• The Journal of Engineering Geology
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• v.32 no.3
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• pp.339-350
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• 2022

There are a variety of types in tafoni formed in Miocene tuff from Golgulsa, Gyeongju. Tuff bearing tafoni was quite weathered, composed of quartz, feldspars, micas, vermiculite, chlorite, smectite, and analcite. In the early stage of the tafoni development, tafoni preferentially formed from cavities where volcanic breccias were removed or from microcavities where microcrystals were chemically altered. Small tafoni grew into large one by merging each other. The orientation of tafoni is inversely arranged to slopes, with slight inclination toward the inner cavity. Height, width, and depth of tafoni are closely interrelated: the correlation coefficients are 0.839 (width-height), 0.900 (width-depth), and 0.856 (height-depth), respectively. Removal of walls between tafoni resulted in lenticular or crescent forms, and small tafoni laterally combined to large tafoni. Large tafoni is weak because of high porosity and low strength compared to normal slope. Therefore, systematic monitoring for slope strength, pore proportion and volume, and growth of cavity needs to secure the slope stability where tafoni in Golgulsa is widespread.

• Journal of the Korean Recycled Construction Resources Institute
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• v.10 no.3
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• pp.269-275
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• 2022

In this study, the setting time and compressive strength of cement paste composites applied with nano-micro pozzolanic binders were experimental analyzed. The pozzolanic binder was reduced initial and final setting time and the compressive strength was increased. Micro silica was effective in decrease the initial setting and final setting time and impressing the compressive strength. When two or more cement binders were used, the using of silica fume and a small amount of nano silica at reduced the setting time to 62-64 % to OPC cement and the compressive strength was increased to 117 %. A small amount of mixing the nano silica was effect to pore filling and pozzolanic activation. However, the addition of a chemical admixture should be considered when mixing table design because pozzolanic binders high specific surface area causes a decrease in cement composites flow.