• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2008년도 춘계 학술발표회 제20권1호
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• pp.29-32
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• 2008

리모델링은 친환경적 접근 방법으로써 현재 수요가 증가하고 있는 추세이다. 리모델링 사업시 구조 부재를 동반한다. 그러므로 리모델링시에 구조적인 안정성을 확보하기 위해서는 적절한 보강 방법이 요구된다. 이 연구의 목적은 전단벽에 개구부가 발생하였을 경우 Passive 보강 방식과 Active 보강 방식을 이용하였을 때의 거동 특성을 파악하는 것이다. Passive 보강 방식은 CFS나 강판 등을 부착하여 손상 부재를 보강하는 방식이고, Active 보강 방식은 프리스트레싱을 이용한 균열 제어를 통해 보강력을 도입하는 방식이다. 실험 결과 각 실험체들은 전단 파괴 형태를 보여주었고 두 가지의 다른 보강 방식은 강도나 연성에 있어서 다른 결과를 보여주었다.

• 한국지반공학회논문집
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• 제27권7호
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• pp.69-79
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• 2011

실내시험은 다양한 응력조건과 배수조건의 모사가 가능하므로 상용 유한요소 해석프로그램의 입력변수 산정에 필수적인 요소가 되나 시료의 운송, 시료추출, 트리밍, 함수비 손실, 응력이완으로 인한 교란과 채취가 관입시의 교란으로 양질의 시료를 얻는데 한계가 있다. 이러한 한계를 극복하기 위하여 교란에 따른 시료의 변화를 이해하고 이를 실내 시험 결과로부터 입력변수 산정에 적용할 수 있는 방법을 찾아내는 것이 필요하다. 따라서 본 연구에서는 정확한 입력정수의 산정이 필수적인 연약점성토의 교란에 대한 영향을 파악하고자 정규압밀 점성토의 교란에 따른 강도 및 강성의 변화를 삽축시험기를 이용한 응력경로시험을 통하여 파악하였다. 그 결과, 교란의 정도가 강도에 미치는 영향은 크지 않았으나, 전단강성계수와 커플링강성계수의 변형률에 따른 감쇠 경향에는 큰 영향을 미치는 것으로 나타났다.

• 전산구조공학
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• 제10권4호
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• pp.315-325
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• 1997

본 논문에서는 이산성 연속형 최적성규준방법을 이용하여 다지간 부분프리스트레스트 콘크리트보의 최적설계 알고리즘을 유도하였고, 최적설계프로그램을 개발하였다. 목적함수로서 건설 경비는 콘크리트 경비, 긴장재 경비, 철근 경비, 그리고 거푸집 경비를 포함하였으며 이를 최소화하였다. 설계제약조건으로는 시방서상의 최대처짐제약, 휨 및 전단강도제약, 연성제약 그리고 설계변수에 대한 상.하한계제약을 고려하였다. Kuhn-Tucker 필요조건을 이용하여 최적성규준을 설계변수의 항으로 명시적으로 유도하였으며, 이때 설계변수로는 보의 유효깊이, 긴장재의 편심거리 그리고 철근비를 취하였다. 긴장재의 형상은 포물선함수로 고려하였으며, 구조물 자중의 영향은 긴장력에 의한 이차효과와 마찬가지로 실제시스템의 평형방정식에서 고려하였다. 설계변수들의 개선을 위한 반복과정과 컴퓨터프로그램을 개발하였으며, 수치예를 들어 개발된 기법의 응용성 그리고 효율성을 보였다.

• The Journal of Advanced Prosthodontics
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• 제7권2호
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• pp.85-92
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• 2015

PURPOSE. This study aimed to investigate the efficacy of cleaning solutions on saliva-contaminated zirconia in comparison to air-abrasion in terms of resin bonding. MATERIALS AND METHODS. For saliva-contaminated air-abraded zirconia, seven cleaning methods)-no contamination (NC), water-spray rinsing (WS), additional air-abrasion (AA), and cleaning with four solutions (Ivoclean [IC]; 1.0 wt% sodium dodecyl sulfate [SDS], 1.0 wt% hydrogen peroxide [HP], and 1.0 wt% sodium hypochlorite [SHC])-were tested. The zirconia surfaces for each group were characterized using various analytical techniques. Three bonded resin (Panavia F 2.0) cylinders (bonding area: $4.5mm^2$) were made on one zirconia disk specimen using the Ultradent jig method [four disks (12 cylinders)/group; a total of 28 disks]. After 5,000 thermocycling, all specimens were subjected to a shear bond strength test with a crosshead speed of 1.0 mm/minute. The fractured surfaces were observed using an optical and scanning electron microscope (SEM). RESULTS. Contact angle measurements showed that groups NC, AA, IC, and SHC had hydrophilic surfaces. The X-ray photoelectron spectroscopy (XPS) analysis showed similar elemental distributions between group AA and groups IC and SHC. Groups IC and SHC showed statistically similar bond strengths to groups NC and AA (P>.05), but not groups SDS and HP (P<.05). For groups WS, SDS, and HP, blister-like bubble formations were observed on the surfaces under SEM. CONCLUSION. Within the limitations of this in vitro study, some of the cleaning solutions (IC or SHC) were effective in removing saliva contamination and enhancing the resin bond strength.

• 한국지반공학회지:지반
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• 제8권3호
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• pp.23-36
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• 1992

Flat DMT는 널리 사용되어지고 있는 현장시험기구로서 간단하고, 경제적이면서도 다양한 지반의 응력-변형-강도특성을 추정할 수 있으나, 이들 현장시험의 결과들은 지역적 특성에 의존하는 경향이 있기 때문에 기준값과 비교하여 많은 연구와 검증을 필요로 한다. 본 연구에서는 기존의 제안된 상관관계식의 우리나라 점토지반에 대한 적용성을 검토하기 위하여 FV 및 CPT의 현장시험을 실시하고, thin-walled tube sampler로 채취된 시료에 대한 비압밀비배수(UU) 실험. oedometer 실험 및 다른 기본적인 물성실험을 수행하여 DMT의 결과들 쏙 비교하였다. 세 장소의 점토지반에서 구한 DMT 결과들로부터 흙의 종류, 단위중량, 정지토 압계수, 과압밀비, 비배수 전단강도 및 변형계수를 추정하였으며 다른 시험결과들과 비교분석하여 우리나라 점토 지반에 대한 적용성을 검토하였다.

• 한국강구조학회 논문집
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• 제17권4호통권77호
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• pp.431-438
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• 2005

철골-콘크리트 합성 기둥(이하 SC 합성 기둥)은 철골 플랜지 사이를 연결재로 용접하고, 이런 플랜지 사이에 콘크리트를 채워 시공성과 경제성을 얻을 수 있는 새로운 합성 부재이다. 이 연구의 전단계로서 비조밀단면을 사용한 SC 합성 기둥의 축력 실험을 통해 구조 내력이 우수하다는 것을 입증하였으나, 기둥의 특성상 축력과 휨을 동시에 받고 있기 때문에 후속 실험 연구가 필요하게 되었다. 따라서 일정 축력을 받는 비조밀단면을 가진 SC 합성 기둥의 휨 성능을 강축 및 약축 방향으로 실험을 실시하여 각국 기준식과 비교하여 분석하였다. 실험 결과 모든 방향에 대해서 각국 기준식에서 제시하고 있는 축력-휨 모멘트 내력식을 만족하는 결과를 보이고 있었다. 특히 AISC-LRFD 기준식은 합성 기둥의 내력을 지나치게 안전측으로 고려하고 있다. 또한 추후 SC 합성 기둥의 축력-휨 모멘트 내력식은 EC4 기준식으로 산정하는 것이 철골과 콘크리트의 적합 조건을 고려하지 않은 일본 기준식과 약축 방향 내력을 산정하기 어려운 ACI 기준식보다는 바람직하다.

• 한국군사과학기술학회지
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• 제22권4호
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• pp.492-500
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• 2019

Equal channel angular pressing(ECAP) is a severe plastic deformation technique capable of introducing large shear strain in bulk metal materials. However, if an ECAPed material has an inhomogeneous microstructure and anisotropic mechanical properties, this material is difficult to apply as structural components subjected to multi-axial stress during use. In this study, extruded oxygen-free copper(OFC) rods with a large diameter of 42 mm are extruded through ECAP by route Bc up to 12 passes. The variations in the microstructure, hardness, tensile properties, and microstructural and mechanical homogeneity of the ECAPed samples are systematically analyzed. High-strength OFC rods with a homogeneous and equiaxed-ultrafine grain structure are obtained by the repeated application of ECAP up to 8 and 12 passes. ECAPed samples with 4 and 8 passes exhibit much smaller differences in terms of the average grain sizes on the cross-sectional area and the tensile strengths along the axial and circumferential directions, as compared to the samples with 1 and 2 passes. Therefore, it is considered that the OFC materials, which are fabricated via the ECAP process with pass numbers of a multiple of 4, are suitable to be applied as high-strength structural parts used under multi-axial stress conditions.

• Computers and Concrete
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• 제27권2호
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• pp.99-109
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• 2021

Experimental and discrete element methods were used to investigate the effects of echelon non-persistent joint on the failure behaviour of joint's bridge area under uniaxial compressive test. Concrete samples with dimension of 150 mm×100 mm×50 mm were prepared. Uniaxial compressive strength and tensile strength of concrete were 14 MPa and 1MPa, respectivly. Within the specimen, three echelon non-persistent notches were provided. These joints were distributed on the three diagonal plane. the angle of diagonal plane related to horizontal axis were 15°, 30° and 45°. The angle of joints related to diagonal plane were 30°, 45°, 60°. Totally, 9 different configuration systems were prepared for non-persistent joint. In these configurations, the length of joints were taken as 2 cm. Similar to those for joints configuration systems in the experimental tests, 9 models with different echelon non-persistent joint were prepared in numerical model. The axial load was applied to the model by rate of 0.05 mm/min. the results show that the failure process was mostly governed by both of the non-persistent joint angle and diagonal plane angle. The compressive strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the joint angle. The strength of samples increase by increasing both of the joint angle and diagonal plane angle. The failure pattern and failure strength are similar in both methods i.e. the experimental testing and the numerical simulation methods.

• Computers and Concrete
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• 제29권1호
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• pp.31-45
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• 2022

Experimental and numerical simulation were used to investigate the effects of angle and number of T shape non-persistent crack on the shear behaviour of crack's bridge area under uniaxial compressive test. concrete samples with dimension of 150 mm×150 mm×40 mm were prepared. Within the specimen, T shape non-persistent notches were provided. 16 different configuration systems were prepared for T shape non-persistent crack based on two and three cracks. In these configurations, the length of cracks were taken as 4 cm and 2 cm based on the cracks configuration systems. The angle of larger crack related to horizontal axis was 0°, 30°, 60° and 90°. Similar to cracks configuration systems in the experimental tests, 28 models with different T shape non-persistent crack angle were prepared in numerical model. The length of cracks were taken as 4 cm and 2 cm based on the cracks configuration systems. The angle of larger crack related to horizontal axis was 0°, 15°, 30°, 45°, 60°, 75° and 90°. Tensile strength of concrete was 1 MPa. The axial load was applied to the model. Displacement loading rate was controlled to 0.005 mm/s. Results indicated that the failure process was significantly controled by the T shape non-persistent crack angle and crack number. The compressive strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. Furthermore, it was shown that the compressive behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the crack number and crack angle. The strength of samples decreased by increasing the crack number. In addition, the failure pattern and failure strength are similar in both methods i.e. the experimental testing and the numerical simulation methods (PFC2D).

• Computers and Concrete
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• 제29권 6호
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• pp.393-405
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• 2022

Lightweight concrete is a superior material due to its light weight and high strength. There however remain significant lacunae in engineering knowledge with regards to shear failure of lightweight fiber reinforced concrete beams. The main aim of the present study is to investigate the optimum usage of steel fibers in lightweight fiber reinforced concrete (LWFRC). Multi-scale finite element model calibrated with experimental results is developed to study the effect of steel fibers on the mechanical properties of LWFRC beams. To decrease the amount of steel fibers, it is preferred to reinforce only the middle section of the LWFRC beams, where the flexural stresses are higher. For numerical simulation, a multi-scale finite element model was developed. The cement matrix was modeled as homogeneous and uniform material and both steel fibers and lightweight coarse aggregates were randomly distributed within the matrix. Considering more realistic assumptions, the bonding between fibers and cement matrix was considered with the Cohesive Zone Model (CZM) and its parameters were determined using the model update method. Furthermore, conformity of Load-Crack Mouth Opening Displacement (CMOD) curves obtained from numerical modeling and experimental test results of notched beams under center-point loading tests were investigated. Validating the finite element model results with experimental tests, the effects of fibers' volume fraction, and the length of the reinforced middle section, on flexural and residual strengths of LWFRC, were studied. Results indicate that using steel fibers in a specified length of the concrete beam with high flexural stresses, and considerable savings can be achieved in using steel fibers. Reducing the length of the reinforced middle section from 50 to 30 cm in specimens containing 10 kg/m3 of steel fibers, resulting in a considerable decrease of the used steel fibers by four times, whereas only a 7% reduction in bearing capacity was observed. Therefore, determining an appropriate length of the reinforced middle section is an essential parameter in reducing fibers, usage leading to more affordable construction costs.