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풍속과 일조량의 변화가 폴리카본산계 고성능감수제를 사용한 순환골재 콘크리트에 미치는 영향 Influence of Wind Speed and Sunlight on Recycled Aggregate Concrete Strength with Polycarboxylate Superplasticizer

https://doi.org/10.4334/JKCI.2019.31.6.525

양달훈(Dal-Hun Yang) ; 이태희(Tae-Hee Lee) ; 최승재(Seung-Jai Choi) ; 최지훈(Ji-Hun Choi) ; 김장호(Jang-Ho Jay Kim)

Recently, natural aggregate mining has been decreasing year by year, and shortages of aggregate required for construction are becoming increasingly serious. The government is trying to find alternatives to the use of aggregate and construction waste through the Law for Promoting Recycling of Construction Waste. Among provisions, recycling aggregate generated from waste concrete is considered an important alternative to solve aggregate shortage. There are various advantages to using recycled aggregate, but there is no standard for quality control and climate factors and recycled aggregate is not actively utilized. In this study, by improving the quality of recycled aggregate and setting the climate factors, we have developed recycled aggregate concrete that can be used in the actual field. For this purpose, concrete specimens were fabricated by applying a high performance water reducing agent to the recycled aggregate concrete, and the strength characteristics of the concrete were analyzed by setting various curing conditions such as wind speed and sunlight. As a result, it is considered that if the probabilistic performance based evaluation is carried out, the problems of recycled aggregate will be improved/predicted. Also, the workability and quality ensure that a large amount of construction waste can be recycled.

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조적요소의 사인장 응력-변형률 관계 평가 Evaluation of Stress-strain Relationship by Diagonal Tension of Masonry Assemblages

https://doi.org/10.4334/JKCI.2019.31.6.537

양근혁(Keun-Hyeok Yang) ; 이용제(Yongjei Lee) ; 황용하(Yong-Ha Hwang)

Diagonal shear stress-strain relationship at masonry assemblage was experimentally evaluated and generalized on the basis of test results. The main parameter investigated was the compressive strength of the masonry prism. The shear stress-strain curve of masonry assemblage was in proportion to the compressive strength of masonry element, as well as the shear modulus of elasticity, whereas the strain at the maximum strength of the masonry assemblage was in inverse proportion to the strength. Specimens showed diagonal tension failure or combined failure modes as expected for masonry assemblage with higher mortar strength than brick strength. The proposed model exhibited reasonable agreement with test results; as a result, it has a great potential for application as fundamental reference for nonlinear finite element analysis to assess structural performance of masonry walls.

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다중모델 평균화 기법을 활용한 콘크리트 시간변형 예측 Multi-model Averaging Method for Time-dependent Deformation in Concrete

https://doi.org/10.4334/JKCI.2019.31.6.545

차상률(Sang-Lyul Cha) ; 진승섭(Seung-Seop Jin)

Concrete creep and drying shrinkage are the most well-known time-dependent deformations of concrete. Since they are important factors for the design of concrete structures, their predictions should be estimated accurately during construction and service life. There are various prediction models, and these models are calibrated using short-term experiments (calibration period) to reflect uncertainties due to heterogeneity of concrete. Traditionally, a best model is selected to make a long-term prediction (extrapolation period). Although similar prediction performances are achieved over calibration period, predictions over the extrapolation are different depending on the selected model. To address this difficulty, a multi-model averaging method is proposed to improve the prediction accuracy. The proposed method is validated by comparing the predictive results with the experimental results. The results show that the proposed method provides consistent and reliable predictions over both calibration and extrapolation periods.

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프리스트레스트 콘크리트용 강연선을 위한 부식방지 공법의 보호성능 평가 Evaluation of Corrosion Prevention Systems of Strands for PSC Structures

https://doi.org/10.4334/JKCI.2019.31.6.557

윤인석(In-Seok Yoon) ; 강현구(Thomas H. K. Kang) ; 신형엽(Hyeong-Yeop Shin)

Corrosion of prestressing tendons is closely related to the safety and durability of prestressed concrete. Because PSC strands under large tension stress are vulnerable to corrosion, choice of corrosion prevention systems is quite important. The current study investigated electrochemical estimation and metal loss reduction rate so that the most effective system to prevent steel corrosion can be decided. The types of test specimens are as follows: (a) bare strand, (b) standard cementitious grouting, (c) grease, (d) double protection of grease and grouting, (e) double protection of HDPE sheath after coating with grease, and (f) triple protection of HDPE sheath after double protection of standard cementitious grouting and coating with grease. Acceleration experiments were divided into two types: those with only chloride solution, and those with chloride solution with dissolved oxygen. As for the experimental results, the HDPE sheath had an excellent anti-corrosion performance compared to other methods. However, tendon protected by grouting showed a decreasing corrosion current density with elapsed time, while tendon protected by grease showed a rising trend. In particular, the grease showed poor corrosion protective performance under chloride solution and dissolved oxygen. This was in part due to physical damage of the grease coating by oxygen bubbles. Finally, corrosion was initiated at corrosion current density of 0.15 μA/cm2 under chloride solution, and at 0.17 μA/cm2 under chloride solution and dissolved oxygen.

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프리캐스트 콘크리트 모멘트골조의 지진하중계수 평가 Evaluation of Effects of Earthquake Load for Precast Concrete Moment Frames

https://doi.org/10.4334/JKCI.2019.31.6.567

이정윤(Jung-Yoon Lee) ; 신동익(Dong-Ik Shin)

Although use of precast concrete structures is increasing, current design codes do not provide a clear seismic design method for precast concrete frames. The ACI 318-14 design code and the ASCE 7-10 standard specify seismic coefficients (response modification factor, system overstrength factor, and displacement amplification factor) for precast concrete structures. However, the KCI-17 design code does not provide seismic coefficients and connection details for precast concrete structures. In this study, effects of earthquake for precast concrete frames were evaluated by comparing the shear strength of connections simulated by FEM analysis and the probable shear strength of connections. Analytical results indicated that considering the effects of earthquake was suitable for the beams of precast concrete intermediate moment frames, while taking 1.5 times was suitable for columns. In case of precast concrete special moment frame, it was suitable to double the earthquake effects.

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고강도 대구경 철근을 사용한 철근콘크리트 부재의 전단마찰거동 Shear Friction Behavior of RC Members with High Strength and Large Diameter Steel Bars

https://doi.org/10.4334/JKCI.2019.31.6.577

이재훈(Jae-Hoon Lee) ; 김호영(Ho-Young Kim) ; 이병수(Byung-Soo Lee)

In order to use large diameter and high strength steel bars as shear friction reinforcement, direct shear tests were performed on specimens with reinforcements of 43.0 mm diameter and specified yield strength 550 MPa (610~698 MPa). According to the test results, friction coefficient of high strength reinforcement is 17.5 percent less than that of normal strength re-bars. If the areas of reinforcement are equal, diameter of reinforcement does not affect shear friction strength. Current design equation of shear friction strength could not ensure a constant safety margin for varying design conditions. Therefore, to maintain the safety margin, design equation should be modified by considering the cohesion component of concrete and decreasing the reinforcement component.

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강재 U형 단면과 채움 콘크리트로 형성된 압축요소를 갖는 부분 PC 합성바닥구조의 시공단계 성능 평가 Performance Evaluation of Truss Encased Slab with Compression Element Composed of U-shaped Steel and Infilled Concrete in Construction Stage

https://doi.org/10.4334/JKCI.2019.31.6.589

김문길(Mun-Gil Kim) ; 천성철(Sung-Chul Chun) ; 김영호(Young Ho Kim)

A design procedure for truss encased slab was developed that doesn’t require slab formwork and shoring system, can be applied to parking lots. Truss encased slab is composed of half PC, tension rebar, truss rebar, U-shaped channels and filling concrete. In construction state, the half PC and tension rebars resist the tensile force of the lower part, and the U-shaped channels and filling concrete resist the compression force of the upper part. Among the flexural, shear and truss buckling strengths, the smallest strength is determined as the design strength against the construction load. Flexural tests were conducted to evaluate the performance of the truss encased slab in the construction state. The variables include the widths of the U-shaped channel and the half PC, and the strength of the half PC concrete. Test results show that truss buckling occurs after flexural cracks in all specimens. All specimens had strengths higher than expected truss buckling strength. The average [Experimental Strength]/[Expected Truss Buckling Strength] ratio is 1.25 and the coefficient of variation is 10.1. The [actual cracking strength]/[expected cracking strength] ratios is 0.79, showing that the first crack occurred earlier than expected. However, for all specimens, no cracks occurred in the construction load. The average of the [actual cracking strength]/[construction load] ratio is 3.12, meaning that it had strength 3 times the design load. Test results show that developed truss encased slab secures proper structural behavior against construction load.

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국내 기존 철근콘크리트 건물의 내진성능평가법 및 지진위험도 추정 Seismic Capacity Evaluation and Risk Estimation of Existing Low-rise Reinforced Concrete Buildings in Korea

https://doi.org/10.4334/JKCI.2019.31.6.599

정원희(Won-Hee Jung) ; 정주성(Ju-Seong Jung) ; 이강석(Kang Seok Lee)

The main purpose of this study is to propose a method for evaluating the seismic capacity of existing low-rise reinforced concrete buildings in Korea by reviewing the applicability of the Japanese Standard. Based on the seismic capacity of Korean buildings evaluated by this standard, we modified the basic structural index, which is calculated in terms of the ultimate horizontal strength and ductility. We compared our experiment results to the values calculated using the strength equations for columns and walls from the Japanese Standard. The flexural strength equations could be applied without modification, while the shear strength equations required modification. We evaluated these modified strength equations and proposed unit average shear stresses of columns and walls based on statistical data relating to the structural characteristics of Korean buildings. The values of the ductility indices, which are primarily calculated based on the shear-to-flexural capacity ratio, of Korean buildings with hoop spacing wider than 300 mm, as computed by the proposed method, were more reasonable than those calculated by the Japanese Standard. We used a probabilistic approach to estimate the structural damage ratios of Korean buildings due to 0.1 g, 0.15 g and 0.2 g earthquakes as 7 %, 27 % and 55 % respectively. We believe that the proposed method provides a useful strategy for identifying Korean buildings that are vulnerable to seismic activity, and recommend urgent earthquake preparedness measures.

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하이브리드 FRP 시트로 보강된 RC 기둥의 휨 성능평가 Evaluation of Flexural Performance of RC Columns Reinforced with Hybrid FRP Sheets

https://doi.org/10.4334/JKCI.2019.31.6.611

김민준(Min-Jun Kim) ; 김형국(Hyeong-Gook Kim) ; 이진용(Chin-Yong Lee) ; 조민수(Min-Su Jo) ; 김길희(Kil-Hee Kim)

In this study, experiments were conducted to evaluate the flexural performance of the RC column reinforced with hybrid FRP sheet. The experiments were carried out under reversed cyclic antisymmetric moment, such as seismic loads. The variables of the specimens is reinforcement with hybrid FRP sheet which was made up of aramid fiber and glass fiber. Experimental results showed that the specimen reinforced with hybrid FRP sheet exhibited a flexural performance equal to or higher than that of the specimen without hybrid FRP sheet. Moreover, ductility capacity increased by 1.9 times and energy dissipation capacity increased by 1.75 times, respectively, compared with the non-reinforced specimen. It was confirmed that the specimen reinforced with hybrid FRP sheet have sufficient structural performances which exceed requirements recommended by the design codes; KCI 2017 and ACI 318-14.

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콘크리트 크리프 및 건조수축 실험변수 결정 방법에 관한 연구 A Study on a Method to Determine Experimental Variables for Creep and Shrinkage of Concrete

https://doi.org/10.4334/JKCI.2019.31.6.619

조선두(Seon Doo Jo) ; 송은석(En Seok Song) ; 김진근(Jin Keun Kim) ; 권승희(Seung Hee Kwon)

Concrete long-term behavior prediction models have many errors. In order to increase the accuracy of the long-term behavior prediction of structures long-term behavior experiments are performed at the laboratory level to correct the prediction model. However, long-term behavior experiments are time-consuming and costly, and there are many constraints on the experimental conditions, so it is difficult to test all of the various experimental variables that affect long-term behavior. Therefore, this study proposed an optimum determination method of creep and shrinkage experimental variables to construct a concrete long-term behavior prediction model with minimum experimentation. The general properties of concrete long-term behavior with specified compressive strength of 40 MPa were analyzed using the existing long-term behavior prediction model. From the properties of the concrete long-term behavior, optimum experimental variables of creep and shrinkage were proposed. It was found that it is reasonable to construct a long-term behavior prediction model by experimenting with three variables for age of concrete at loading (1~14 days, 14~84 days, 84~365 days), two variables for relative humidity (40~60 %, 60~80 %), and three variables for volume-surface ratio (37.5 mm, 75~200 mm, over 200 mm) for creep experimental variables; two variables for age of concrete when drying starts (1~14 days, 14~365 days), two variables for relative humidity (40~60 %, 60~80 %), and two variables for volume-surface ratio (same size as creep specimens) for shrinkage experimental variables.

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