• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.1
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• pp.67-72
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• 2022

Concrete prevents corrosion of reinforcing bars due to its strong alkalinity. However, in the sea, strong alkali components with a pH of 12 to 13 are eluted, which adversely affects the ecological environment and growth of marine organisms. In this study, the mechanical properties and durability of the low alkali mortar were evaluated for the development of a low alkali mortar for the 3D printed artificial reefs. As a result of evaluation of strength characteristics, the α-35 mixture, which were produced with fly ash, silica fume and α-hemihydrate gypsum, satisfied the strength requirement 27 MPa in terms of compressive strength. As a result of pH measurement, it was found that mixing with alpha-type hemihydrate gypsum resulted in minimizing pH due to the the formation of calcium sulfate instead of calcium hydroxide production. As a result of the chloride ion penetration resistance test, the α-35 mixture exhibited the best performance, 3844C. As a result of measuring the length change over time, the α-35 mixture showed the shrinkage 33.5% less compared to the Plain mix.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.32 no.5
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• pp.212-217
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• 2022

Large thermal stress due to the difference between silicon carbide and graphite's coefficients of thermal expansion could be formed during crystal growing process of silicon carbide (SiC) at high temperature. The large thermal stress could separate the SiC seed crystals from graphite components, which bring about the drop of the seed crystal during crystal growth. However, the bonding properties of SiC seed crystal module has hardly reported so far. In this study, SiC and graphite were bonded using 3 types of bonding agents and a three-point bending tests using a mixed-mode flexure test were conducted for the bonded samples to evaluate the bonding characteristics between SiC and graphite. Raman spectroscopy, X-ray Photoelectron Spectroscopy, and X-ray Computed Tomography were used to analyze the bonding characteristics and the microstructures of the SiC-graphite interfaces bonded with the bonding agents. As results, an excellent bonding agent was chosen to fabricate SiC seed crystal module with 50 mm in diameter. An SiC single crystal with 50 mm in diameter was successfully grown without falling out during top seeded solution growth of SiC at high temperature.

• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.4
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• pp.73-80
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• 2022

This study evaluated changes in fluidity and rheological properties over time for 3D printed composite materials, and evaluated compressive strength and splitting tensile strength properties for laminated and molded specimens. The composite material for 3D printing starts to change rapidly after 30 minutes of extrusion, and the viscosity of the material tends to be maintained up to 90 minutes, but it was confirmed that construction within 60 minutes after mixing is effective. The compressive strength of the laminated test specimen showed equivalent or better performance at all ages compared to the molded test specimen. In the stress-strain curve of the laminated specimen, the initial slope was similar to that of the molded specimen, but the descending slope was on average 1.9 times higher than that of the molded specimen, indicating relatively brittle behavior. The splitting tensile strength of the P-V laminated specimen was about 6% lower than that of the molded specimen. It is judged that this is because the interfacial adhesion force against the vertical load is affected by the pattern direction of the laminated test specimen.

• Journal of the Korean Wood Science and Technology
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• v.32 no.2
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• pp.90-95
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• 2004

Pitch pine (Pinus rigida) has been planted in Korean forests for several decades, primarily for erosion control and use as a fuel supply. To enhance its value, and especially potential use as lamina for structural glued laminated timber (glulam), log quality and lumber yield of pitch pine were evaluated in this study. Trees from pure pitch pine stands with an average diameter at breast height of 32 cm were felled and bucked into 3.6m long 15 cm minimum butt-end diameter logs. Over 80% of the logs were classified to No.2 or No.3 visual grade group. Upon sawing total lumber yield was 55.2%, 39.9% for structural glulam lamina, 7.2% for louver, and 8.1% for miscellaneous use. The final lumber yield for manufacturing structural glulam, after cross-cutting to eliminate knots and finger jointing, was only 15.3%. To enhance this manufacturing yield requires that the rate of knot-included lumber used as lamina be raised. However arrangement of the knot-included lamina, whose mechanical properties need to be accurately evaluated, must be optimized to minimize any reduction to the structural glulam strength. The log quality and lumber yield of pitch pine evaluated in this study are expected to facilitate proper planning for wood product manufacture in the Korean lumbering and glulam industrial field, which has not previously dealt with this species.

• Journal of the Korea Institute of Building Construction
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• v.22 no.6
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• pp.543-551
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• 2022

The mechanical performance and vibration damping ratio performance of a specimen according to the polymer mixing rate were evaluated for polymer modified mortar. As a polymer, Styrene Butadiene Rubber(SBR) liquid polymer with a solid content of about 49~51% was used, and the polymer content was increased by liquid 5%. The specimen was 40*40*160(mm), and after curing, compressive strength, flexural strength, and vibration damping ratio were measured using the ultrasonic pulse method. As a result, it was found that the compressive strength decreased as the polymer was mixed, but the flexural strength was increased. The vibration damping ratio increased by 11% at 5% polymer, 28% at 10% polymer, 33% at 15% polymer, and 72 at 20% polymer. I was found that the incorporation of the polymer was very effective to reduce the vibration of the mortar. In addition, through SEM and SEM-EDS analysis, it is determined that the cause of vibration reduction due to polymer mixing is that the polymer film formed in the transition zone of aggregate and internal voids buffered the vibration of the mortar inside. Taken together, in the scope of this study, the appropriate polymer mixing ratio for reducing the vibration of mortar is judged to be about 7.5%.

• Journal of the Korean GEO-environmental Society
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• v.23 no.12
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• pp.5-15
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• 2022

Conventionally, because evaluation methods of the bearing capacity for double steel pipe-concrete composite pile design have not been established, the conventional vertical bearing capacity equations for steel hollow pile are used. However, there are severe differences between the predictions from these equations, and the most conservative one among vertical bearing capacity predictions are conventionally adopted as a design value. Consequently, the current prediction method for vertical bearing capacity of composite pile prediction composite pile causes design reliability and economical feasibility to be low. This paper investigated mechanical behaviors of a new composite pile, with a cross-section composed of double steel pipes filled with concrete (DSCT), vertical bearing capacities were analyzed for several DSCT pile conditions. Axisymmetric finite element models for DSCT pile and surrounding ground were created and they were used to analyze effects on behaviors of DSCT pile pile by embedding depth, stiffness of plugging material at pile tip, height of plugging material at pile tip, and rockbed material. Additionally, results from conventional design prediction equations for vertical bearing capacity at steel hollow pile tip were compared with that from numerical results, and the use of the conventional equations for steel hollow pile was examined to apply to that for DSCT pile.

• Journal of Science Education
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• v.46 no.1
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• pp.1-16
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• 2022

The purpose of this study is to explore learning progressions for global warming at middle school level. For this purpose, we conducted a construct modeling approach that specifies constructs, item designs, outcome spaces, and measurement model steps from April to October, 2021. In order to develop student assessment items, we analyzed the 2015 revised curriculum and textbooks of middle school and categorized a concept hierarchy for each construct to create a construct map. The assessment items were developed into multiple-choice, short answer, and essay questions according to the selected constructs to strengthen the linkage between the constructs and the items. Based on the three-step grading criteria for each item, an online assessment of 21 minor items developed for middle school students show that many students met 'high' level, but none met 'low' level. In this manner, the initial set lower anchor was reset to level 0, the original set upper anchor was lowered from level 4 to level 3, and the hypothetical learning progression for global warming was presented in the following order: phenomenal, conceptual, and mechanical understandings. The results of the research have raised implications for reorganizing the next science curriculum and improving the assessment system.

• Journal of Christian Education in Korea
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• v.70
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• pp.141-173
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• 2022

The purpose of this study is to illuminate depersonalization in the flow of technological revolution and to present a Christian SARAMDAUM education that aims for a new human image. It represents the Christian SARAMDAUM education that adapts to, mediates, and offers alternatives to the technological and human evolutionary flow of the machine age. The purpose of education for this purpose is to aim for 'Homo Creators', creative human beings presented as a new human image in the age of technological revolution. The educational goal is to nurture creative human beings through creative interpretation, creative integration between disciplines, and personal dialogue in the post-mechanical/ post-conventional paradigm. The content of the education is a conversation with the SARAMDAUM that consiliences the characteristics of post-machine and post-convention. The educational method utilizes Edu-Tech and AIED(Artificial Intelligence in Education) to realize systemic thinking and SARAMDAUM dialogue of technology. In addition, the composition of teachers and learners, educational environment and educational evaluation is presented. The significance of this study is that from the point of view of Christian education, the identity of human beings in the era of the technological revolution has been identified, and research on the creative image of the human being is newly attempted, and the direction of Christian SARAMDAUM education aimed at this is presented. This can be said to be a Christian education that emphasizes the essential characteristics of human beings while accommodating the era of technological revolution.

• Clean Technology
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• v.16 no.3
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• pp.172-181
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• 2010

Hydrofluoroethers (HFEs) with fluoride molecules in their structure which are evaluated as the third generation replacement alternatives to chlorofluorocarbons (CFCs) are known to be excellent for removal of nanoparticles and fluoride-type soils due to their low surface tension and high wetting index. In addition, HFEs have good physical properties with no flash point and excellent drying characteristics. But, HFEs also have shortcomings in that they are not effective for removal of organic soils due to their poor solubility in soil. In this study, $C_5$ HFE-based cleaning agents were formulated through addition of solvents such as isopropyl alcohol (IPA), ethyleneglycol monoether (EG), propyleneglycol monoethylether (PM) to HFE-7100 [$CF_3CF_2CF_2CF_2OCH_3$] or HFE-mec-f [$CF_3CHF=CF_2OCH_2CF_3$] with its maximum amount, respectively, in order to have no flash point for the safety in the working environment. These solvents are known to be excellent for dissolving organics in soil. Their physical properties and cleaning abilities for fluxes, water-insoluble cutting oils, and fluoride-type oils were evaluated and compared with those of other cleaning agents with single components. The experimental results show that the HFE-based formulated cleaning agents have various good physical properties which are almost similar to those of a single type of HFE cleaner. They show excellent cleaning ability for fluxes, water-insoluble cutting oils, and fluoride-type oils. These results indicate that the HFE-based formulated cleaning agents can be applicable to various industrial cleaning fields because of their good physical properties and cleaning abilities for various soils.

• Journal of Korean Tunnelling and Underground Space Association
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• v.25 no.2
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• pp.141-155
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• 2023

Monitoring technology based on electrical resistivity is widely used for non-destructive data collection and health analysis of underground structures and tunnels. Vulnerable sections such as fault zone generates many problems during construction of the tunnel. These problems cause displacement and stress changes of the ground. Therefore, it is necessary to predict the state of the fault zone section to ensure the mechanical stability of the underground structure. Monitoring the size of joints and the porosity of the fillings is essential for rocks. Previous studies have not considered the variety of fillings in rock joints. In this study, electrical resistivity tests were conducted according to the particle mixing state of the sandy fillings. When the size of fillings is decreased at the constant porosity, the electrical resistivity tends to increase. The results of this study are expected to be useful as basic electrical resistivity data for predicting the ground conditions and evaluation of the ground behavior that is containing sandy fillings in the rock joint for tunnels.