• Journal of the Korea institute for structural maintenance and inspection
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• v.23 no.3
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• pp.43-50
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• 2019

Effects of tensile strength and aspect ratio of steel fiber on compressive and flexural behavior of steel fiber-reinforced concrete (SFRC) with high- and normal-strength were investigated. Also, this study explores compressive behavior of SFRC with different loading rate. For this purpose, four types of steel fiber were used for SFRC with specified compressive strength of 35 and 60 MPa, respectively. Cylindrical specimens with a diameter of 150 mm and height of 300 mm were made for compression test, and prismatic specimens with a $150{\times}150mm$ cross-section and 450 mm span length were made for flexural test. Test results from compression and flexural tests indicated that the toughness of concrete significant increased with steel fibers. Especially, using steel fiber with high tensile strength and aspect ratio can be lead to performance improvement of high-strength SFRC. In this study, equations are suggested to predict compressive toughness ratio of SFRC from flexural toughness ratio.

• The korean journal of orthodontics
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• v.28 no.5 s.70
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• pp.783-789
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• 1998

The purpose of this study was to evaluate the changes of shear bond strengths and failure patterns in orthodontic resin adhesives according to the water immersion time. Metal brackets were bonded to the specimens involving the premolars with chemical-cured($Concise^{\circledR}$) and light-cured($Transbond^{\circledR}$) adhesives. The shear bond strength was measured on universal testing machine and the failure patterns were assessed with the adhesive remnant index(ARI) after storage in distilled water at $37^{\circ}C$ for 1 day, 1 week and 1, 3, and 6 months, respectively. The results were as follows. 1. The shear bond strengths at the 6 month in both Concise and Transbond were significantly higher than those at the 1 day, 1 week and 1 month(p<0.05). There were positive correlations between shear bond strength and water immersion time in both Concise and Transbond(P<0.01). 2. There were no significant differences in shear bond strength between Concise and Transbond. 3. The brackets were failed primarily at the bracket base-adhesive interface and there was no significant difference in the incidence of ARI scores according to the water immersion time.

• 한국방재학회:학술대회논문집
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• 2011.02a
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• pp.83-83
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• 2011

도로의 동상방지층을 설치하기 위하여 동결관입깊이를 산정하며, 동결지수는 대기온도의 강도와 지속시간의 누가영향으로 표시된다. 대기온도는 일평균 대기온도를 사용하며 4개/일 평균(대기온도$_4$)을 사용하는 것이 일반적이지만, 현재 기상청에서 제공하고 있는 일평균 대기온도는 8개/일(대기온도$_8$)를 제공하고 있다. 이러한 일평균 대기온도의 산정하는 방법이 동결지수의 산정결과에 미치는 영향을 확인하기 위하여 한반도 남부지역(동결지수 $350^{\circ}C{\cdot}$일) 미만 6개지역을 저성토부, 절성경계부 및 절토부로 구분하여 15개소에 대하여 현장계측시스템을 구축하였다. 그리고 현장계측 시스템으로부터 얻어진 대기온도를 통하여 동결지수를 산정하여 비교하였다. 동결지수 비교결과 일반적으로 일평균 대기온도$_4$를 사용한 경우가 일평균 대기온도$_8$을 사용하는 경우보다 약 3% 전후 큰 것으로 나타나났으며, 6개 지역의 동결지수를 결정할 때 대기온도$_8$을 사용한 동결지수를 보정할 수 있는 보정식을 제안하였다.

• Journal of the Korean Geotechnical Society
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• v.20 no.2
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• pp.15-26
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• 2004

Based on an estimating method for post-cyclic strength and stiffness with cyclic triaxial tests proposed by one of the authors, cyclic Direct Simple Shear (DSS) tests were carried out to confirm whether the method can be adapted to DSS test on fine-grained soils: silty clay, plastic silt, and non-plastic silt. Results from cyclic and post-cyclic DSS tests were interpreted by a modified method as adopted for cyclic and post-cyclic triaxial tests. In particular, influence of plasticity index for fine-grained soils and initial static shear stress (ISSS) was emphasised. Findings obtained from the present study are: (i) liquefaction strength ratio of fine-grained soils decreases with decreasing plasticity index and increasing ISSS; (ii) plasticity index and ISSS did not markedly influence relation between equivalent cyclic stiffness and shear strain relations; (iii) the higher the plasticity index of fine-grained soils is, the less the strength ratio decreases with increment of a normalcies excess pore water pressure (NEPWP); (iv) stiffness ratio of plastic silt has large activity decrease rapidly with increasing excess pore water pressure; and (v) post-cyclic strength and stiffness results from DSS tests agree well with those predicted by the method modified from a procedure used for triaxial test results.

• Tunnel and Underground Space
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• v.22 no.5
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• pp.330-338
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• 2012

Though the uniaxial compressive strength (UCS) of rock material is one of the key properties for rockmass characterization purposes, core samples for the test cannot always be obtained from the field. Indirect tests such as the point load test (PLT) can be a useful alternative. In this study, correlation between the UCS and the point load strength index of limestone of Imgye area was analyzed, and the linear regression equation obtained from regression analysis of two variables was suggested. The results of this study were also compared with previous studies on limestone. It was suggested that conversion factors for the same rock type from diverse areas in Korea may have different values, and more data should be obtained to increase the accuracy of regression analysis.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.989-992
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• 2008

In this paper, we are presenting a case that integrates ultra high strength concrete(150MPa) with surface-exposed concrete. Ahead of the field application, we carried out laboratory experiment and B/P Test for a basic property of concrete(slump flow, air content, 50cm flow time, elapse time change and compression strength) and productivity. The next, we conducted Mock-up Test using simulation specimen to evaluate infilling, surface-finishing and hydration heat of concrete. We had satisfactory results for a basic property and hydration heat of concrete. However at the time of field application, it was occurred rupture of formwork because of high lateral pressure of concrete, and then formwork was reinforced and case-in-place time was adjusted. And regardless of low and high frequency vibration, it occurred to surface-pockmark. In case that applies ultra high strength concrete to surface-exposed concrete, we estimate that it is important of systematic management and improvement of construction.

• Journal of the Korean Wood Science and Technology
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• v.30 no.2
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• pp.134-143
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• 2002

According to the result of the experiment, except that Oebal Hanji making, which does not consider the fiber orientation, has a smaller difference between its strength of width and length direction than of Ssangbal Hanii making, there is not a significant difference between Oebal Hanji making and Ssangbal Hanji making. In addition, even though Oebal Hanji making produced indigenous technique is generally felt sturdy because its absolute strength is enhanced by applying fine materials and overlaying several sheets of paper, its relative strength, which is expressed in an index, is not far beyond that of Ssangbal Hanji making Study on the effect of Dochim revealed that Dochim improves remarkably folding endurance and enhances breaking length and the burst strength. Especially, Dochim increase tensile strength. It was also revealed that, indigenous Oebal Hanji making, the higher the number of sheets of paper overlaid is, the higher the tear strength is the increase relatively higher than those of other types of strength. Consequently, Dochim treatment greatly reinforces the imperfect strength of indigenous Hanji. We consider that if we apply Dochim treatment to Ssangbal Hanji making it may bring about abnormal imbalance of strength. We can benefit from the characteristic strength of Oebal Hanji making only when performing paper making and Dochim considering fiber orientation.

• The Korean Journal of Financial Management
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• v.16 no.2
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• pp.337-364
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• 1999

본 연구에서는 국내 주가지수선물시장과 현물시상간의 일중 가격 및 가격변동성의 선-후행관계를 실증적으로 분석함으로써 양 시장간의 동적관련성을 살펴보고자 하였다. 먼저, 상관관계분석의 결과는 KOSPI 200 주가지수선물수익률과 현물수익률, 그리고 주가지수선물수익률자승과 현물수익률 자승간에 유의한 교차상관관계가 존재하는 것으로 나타났다. 수익률의 선-후행관계를 살펴보기 위한 주가지수선물수익률의 시차변수들과 현물수익률간의 다중회귀분석의 결과는 주가지수선물수익률이 현물수익률을 약 15분 정도 선행하는 것으로 나타났으며, 이러한 현상은 현물수익률에 존재할 수 있는 비동시적 거래의 영향을 통제한 경우에도 비록 그 강도가 약하기는 하지만 여전하였다. 다음으로, 수익률 변동성의 선-후행관계를 살펴보기 위해 Grammatikos-Saunders (1986)가 제시한 무조건부 변동성의 추정치인 로그수익률자승을 사용하여 분석한 결과 주가지수선물수익률의 변동성이 현물수익률의 변동성을 약 10분 정도 선행하는 것으로 나타났으며, 이러한 결과는 비동시적 거래의 영향을 통제한 경우에도 동일하였다. 또한, Nelson(1991)의 EGARCH모형을 사용하여 수익률의 변동성을 추정한 후 이를 갖고 분석한 결과, 특히 비동시적 거래의 영향을 통제한 경우에는 주가지수선물시장과 현물시장의 수익률 변동성간에 선-후행관계가 존재한다는 것을 부정할 수 없었다.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.196-196
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• 2023

가뭄은 발생 시점과 종점을 정확히 파악하기 어려우며, 피해 면적이 광범위하기 때문에 수자원시스템 전반을 비롯한 사회, 경제적 측면에서 심각한 영향을 줄 수 있다. 우리나라의 가뭄 발생경향은 2000년 이후로 급증하고 있으며, 2022년 전라남도 지역의 경우, 평년 대비 강수량이 60%에 그쳐 50년 관측 사상에서 가장 낮은 수준으로 나타나면서 극심한 가뭄이 발생하여 현재까지도 지속되고 있다. 미래에도 기후변화로 인한 가뭄의 강도와 빈도가 증가될 것으로 예측됨에 따라 가뭄을 예방하기 위한 미래 가뭄 상황의 예측에 대한 필요성이 대두되고 있다. 따라서 다양한 기후모델 및 미래 기후변화 시나리오를 활용해 미래 가뭄에 대한 전망을 분석하고 적응 전략을 수립해야 한다. 본 연구에서는 CMIP6 (Coupled Model Intercomparison Project 6)에서 제공하는 18개의 전 지구적 기후모델별로 산출한 SSP (Shared Socioeconomic Pathways) 시나리오를 기반으로 기상학적 가뭄지수인 표준강수지수 (Standardized Precipitation Index, SPI), 유효가뭄지수(Effectvie Drought Index, EDI)와 강수량 및 기온의 변화에 따른 증발산량을 고려하여 가뭄을 판단하는 표준강수증발지수 (Standardized Precipitation Evaportranspiration Index, SPEI), 증발수요 가뭄지수 (Evaporative Demand Drought Index, EDDI)를 적용하여 미래 가뭄지수별 가뭄 예측 및 변동성을 분석하였다.

• Journal of Korean Society of Forest Science
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• v.107 no.2
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• pp.193-204
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• 2018

This study was carried out to select the optimal method of competition index computation according to the competitor selection methods and distant-dependent competition index models, and to analyze the characteristics of competition indices in terms of thinning intensity and tree density targeting Pinus densiflora, Pinus koraiensis, and Larix kaempferi, which are the major coniferous species in Korea. Data was the re-investigated tree information from 240 permanent plots of 80 sites in the stands of P. densiflora, P. koraiensis, and L. kaempferi, which were located in the national forest of Gangwon and North Gyeongsang provinces. The number of subject trees with competition index calculated were 1126 trees for P. densiflora, 4093 trees for P. koraiensis, and 3399 trees for L. kaempferi. For the best competition index computation method, three kinds of competitor selection methods were considered: basal area factor, angle of height, angle of height to crown base. Also, six kinds of competition index models were compared: Lorimer, Martin-EK, Braathe, Heygi, Daniels, and Modified Daniels, which was developed in this study. Correlation coefficient was the best when the competitor selection method of basal area factor $4m^2/ha$ and the competition index model of Modified Daniels were used, and thus, it was selected as the best method for computing competition index. According to the best method by stand characteristics, competition index decreased in all species as thinning intensity was high and tree density was low.