• Steel and Composite Structures
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• 제18권2호
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• pp.375-394
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• 2015

This paper presents the results of four long-term experiments carried out to investigate the time-dependent behaviour of composite floor slabs with particular attention devoted to the development of non-uniform shrinkage through the slab thickness. This is produced by the presence of the steel deck which prevents moisture egress to occur from the underside of the slab. To observe the influence of different drying conditions on the development of shrinkage, the four 3.3 m long specimens consisted of two composite slabs cast on Stramit Condeck $HP^{(R)}$ steel deck and two reinforced concrete slabs, with the latter ones having both faces exposed for drying. During the long-term tests, the samples were maintained in a simply-supported configuration subjected to their own self-weight, creep and shrinkage for four months. Separate concrete samples were prepared and used to measure the development of shrinkage through the slab thickness over time for different drying conditions. A theoretical model was used to predict the time-dependent behaviour of the composite and reinforced concrete slabs. This approach was able to account for the occurrence of non-uniform shrinkage and comparisons between numerical results and experimental measurements showed good agreement. This work highlights the importance of considering the shrinkage gradient in predicting shrinkage deformations of composite slabs. Further comparisons with experimental results are required to properly validate the adequacy of the proposed approach for its use in routine design.

• Computers and Concrete
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• 제24권4호
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• pp.379-397
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• 2019

Steel-concrete composite beams are widely employed in constructions and their performance at the serviceability stage is of concern among practitioners and design regulations. In this context, an accurate evaluation of long-term deflections via various rheological concrete models is needed. In this work, the performance and predict capability of some concrete creep and shrinkage models ACI, CEB, B3, FIB and GL2000 are ascertained, and compared by using statistical bias indicators. Ten steel-concrete composite beams with existing experimental and numerical results are then modeled for this purpose. The proposed modeling technique uses the finite element method, where the concrete slab and steel beam are modeled with shell finite elements. Concrete is considered as an aging viscoelastic material and cracking is treated with the common smeared approach. The results show that when the experimental ultimate shrinkage strain is used for calibration, all studied rheological models predict nearly similar deflections, which agree with the experimental data. In contrast, significance differences are encountered for some models, when none calibration is made prior to. A value between twenty and thirty times the cracking strain is recommended for the ultimate tensile strain in the tension stiffening model. Also, increasing the relative humidity and decreasing the ambient temperature can lead to a substantial reduction of slab cracking for beams under negative flexure. Finally, there is not a unique rheological model that clearly excels in all scenarios.

• Structural Engineering and Mechanics
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• 제81권5호
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• pp.617-631
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• 2022

In this paper the viscoelastic responses of four experimental steel-concrete composite beams subjected to highly variable environmental conditions are investigated by means of a finite element (FE) model. Concrete specimens submitted to stepped stress changes are also evaluated to validate the current formulations. Here, two well-known approaches commonly used to solve the viscoelastic constitutive relationship for concrete are employed. The first approach directly solves the integral-type form of the constitutive equation at the macroscopic level, in which aging is included by updating material properties. The second approach is postulated from a rate-type law based on an age-independent Generalized Kelvin rheological model together with Solidification Theory, using a micromechanical based approach. Thus, conceptually both approaches include concrete hardening in two different manners. The aim of this work is to compare and analyze the numerical prediction in terms of long-term deflections of the studied specimens according to both approaches. To accomplish this goal, the performance of several well-known model codes for concrete creep and shrinkage such as ACI 209, CEB-MC90, CEB-MC99, B3, GL 2000 and FIB-2010 are evaluated by means of statistical bias indicators. It is shown that both approaches with minor differences acceptably match the long-term experimental deflection and are able to capture complex oscillatory responses due to variable temperature and relative humidity. Nevertheless, the use of an age-independent scheme as proposed by Solidification Theory may be computationally more advantageous.

• 한국지반환경공학회 논문집
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• 제16권10호
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• pp.15-22
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• 2015

본 연구에서는 앵커의 장기 거동특성과 인장력 손실을 평가하기 위하여 실제 시공된 인장형 앵커를 대상으로 장기 계측을 실시하고 이를 기존 예측결과와 비교 분석하였다. 영구앵커는 비탈면 안정 혹은 구조물의 부상방지 등을 목적으로 사용되는 보강재로써 가설 앵커와 달리 구조물의 공용기간 동안 기능을 유지해야 한다. 그러나 시간 경과에 따라 앵커의 릴렉세이션 및 크리프가 지속적으로 발생되기 때문에 안정적인 기능발휘를 위해서는 인장력 손실에 대한 관리가 필수적이다. 지금까지 인장력 손실은 탄성론을 이용한 프리스트레스 감소와 인장재 종류에 따른 릴렉세이션을 값을 이용하여 산정하여 왔으며 장기적인 계측결과를 이용한 검증은 제한적인 실정이다. 이에 본 연구에서는 실제 시공된 인장형 앵커를 대상으로 현장조건과 상세 지반조사를 실시하였으며 하중계, 경사계 및 지하수위계를 설치하여 최대 500일 이상의 장기 계측결과를 분석하였다. 또한 측정된 벽체의 변위 및 앵커의 인장력 손실을 기존 해석결과와 비교함으로써 앵커의 장기 거동특성을 평가하였다. 평가결과 대부분의 앵커력 손실은 90일 이내에 발생되며, 앵커력 손실은 예측된 값보다 작게 측정되는 것으로 나타났다.

• 비파괴검사학회지
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• 제28권2호
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• pp.177-183
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• 2008

고온-고압의 장시간 사용으로 나타나게 되는 재료열화현상을 이해하고 구조건전성 모니터링을 위한 기초연구로 사용하고자 대표적인 로터용 내열강인 2.25CrMo 강과 9CrMo 강 그리고 12CrW 강을 연구하였다. 재료열화를 모니터링하기 위해 2.25CrMo 강은 등온열화 시험을 9CrMo 강은 크리프-피로 시험을 그리고 12CrW 강은 크리프 시험을 각 단계별로 수행하였다. 이들 각 손상재에 대한 초음파전파특성, 비저항측정 그리고 보자력측정을 통하여 미세조직 의존성을 고찰하였고 이들 파라미터는 특정 미세조직의 변화에 상당히 민감하게 작용하였다.

• Computers and Concrete
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• 제19권5호
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• pp.579-588
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• 2017

Creep and shrinkage are the main types of volume change with time in concrete. These changes cause deflection, cracking and stresses that affect durability, serviceability, long-term reliability and structural integrity of civil engineering infrastructure. Although laboratory test may be undertaken to determine the deformation properties of concrete, these are time-consuming, often expensive and generally not a practical option. Therefore, relatively simple empirically design code models are relied to predict the creep strain. This paper reviews the accuracy of creep and shrinkage predictions of reinforced concrete (RC) shear walls structures strengthened with carbon fibre reinforced polymer (CFRP) plates, which is characterized by a widthwise varying fibre volume fraction. This review is yielded by three commonly used international "code type" models. The assessed are the: CEB-FIP MC 90 model, ACI 209 model and Bazant & Baweja (B3) model. The time-dependent behavior was investigated to analyze their seismic behavior. In the numerical formulation, the adherents and the adhesives are all modelled as shear wall elements, using the mixed finite element method. Several tests were used to demonstrate the accuracy and effectiveness of the proposed method. Numerical results from the present analysis are presented to illustrate the significance of the time-dependency of the lateral displacements and eigenfrequencies modes.

• 한국전기전자재료학회논문지
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• 제19권3호
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• pp.280-286
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• 2006

As a way to expand electric capacity in conductor with electric power demand, STACIR/AW (Super Thermal-resistant Aluminum-alloy Conductors Aluminum-clad Invar-Reinforced) conductor which has high electric current and heat resistance characteristics have been developed. STACIR/AW power line is mechanical composite wire composed of steel cores for dip control and aluminum conductors for sending electric current. Recently, to ensure stable operation and prediction of wire life span of STACIR/AW conductor, a heat property of STACIR/AW conductor have been investigated. In the present work, a change of essential property with long term-heat exposure of STACIR/AW conductor and its structure material, INVAR wire and Al conductor, have been investigated. INVAR/AW is approximately $3.2\;{\mu}m/m^{\circ}C$. thermal expansion coefficient of INVAR/AW wire increases with time of heat exposure. the thermal expansion coefficient of INVAR/AW is markedly influenced by heat and mechanical treatment. creep rate(0.242) of STACIR/AW $410\;mm^2$ conductor at room temperature is much higher than that(0.022) at $210\;^{\circ}C$ STACIR/AW $410\;mm^2$ conductor has minimum creep rate at operating temperature. To lower creep rate with increase temperature is more unique characteristics in STACIR/AW. It is expected that STACIR/AW turned its tension to INVAR/AW at the transition temperature. at room temperature, the tension apportionment of INVAR/AW in STACIR/AW is about $50\;\%$. but whole tension of STACIR/AW is placed on the INVAR/AW alone of core metal above transition temperature.

• 터널과지하공간
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• 제14권1호
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• pp.69-77
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• 2004

암석의 시간 의존 변형거동은 암석의 가장 근본적인 역학적 성질 중의 하나이며, 크립거동의 특성과 메커니즘에 대한 연구는 지하 암반 구조물의 장기적 안정성을 평가하는데 필수적이라 할 수 있다. 이에 본 연구에서는 국내 화강암을 대상으로 단축압축하중 하에서 크립시험을 실시하였고, 수분과 응력 수준을 달리하여 각각의 조건에 따른 크립 특성을 비교, 분석하였다. 크립시험 결과 화강암은 1차, 2차 및 3차로 확연히 구분된 크립거동을 보였다. 1차 크립거동은 대체로 24시간 내에 종결되었으며, 2차 크립 변형률은 단축압축강도에 대한 재하응력의 비율이 증가함에 따라 증가하였다. 일정 응력하에서 화강암의 최대강도는 시간에 따라 감소하는 경향을 보였다. 완전 포화된 시료는 자연 건조된 시료보다 훨씬 큰 크립 변형량 및 변형률을 보였으며, 포화시료의 파괴강도 및 파괴시간은 급격히 감소하였다. 본 연구의 크립시험 결과를 버거모델(Burger's model) 및 이전 연구자들에 의해 제안된 두 개의 경험적 모델에 적용하여 크립의 유동상수를 구하였다. 세 개의 모델 모두 화강암의 크립거동을 잘 나타내었으나, 그 중 버거모델이 실험결과와 가장 잘 일치하였다. 각각의 모델에서 결정된 유동상수들은 함수량과 응력 수준에 영향을 받았다. 또한 본 실험결과에 기초하여 자연건조 및 완전 포화된 화강암 시료에 대하여 응력 수준과 시간의존변형의 경험적 관계식을 유도하였다.

• 콘크리트학회논문집
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• 제15권2호
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• pp.280-287
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• 2003

산업폐기물 중 하나인 굴패각(Oyster Shell, OS)에 대한 건설재료로써의 활용성을 검토하기 위해 굴패각을 혼합한 콘크리트의 장기역학적 특성과 내구성을 실험적으로 평가하였다. 실험결과에 따르면 굴패각을 10% 대체한 콘크리트의 장기강도는 굴패각을 대체하지 않은 콘크리트와 유사한 결과를 보였으나, 굴패각을 20% 대체한 콘크리트의 장기강도는 감소하는 것을 보였다. 즉, 일정 한도 이상의 굴패각 대체는 콘크리트의 장기재령 강도에 열화요인으로 작용할 수 있는 것으로 나타났고, 굴패각의 대체에 따른 탄성계수 저하는 대체율에 거의 비례적으로 저하하였으며 굴패각의 대체율 20%에서 약 10∼15% 저하하였다. 굴패각의 대체율이 증가할수록 건조수축 발생량은 증가하였으며 기존의 건조수축과 크리프 모델식은 굴패각의 증가에 따른 영향을 잘 반영하지 못하였고 이에 대한 예측식의 보정이 필요하다. 잔골재의 일부를 굴패각으로 대체하여도 콘크리트의 동결융해 저항성, 탄산화 저항성, 및 화학침식 저항성에는 나쁜 영향을 미치지 않으며, 투수 저항성은 오히려 크게 개선되었다.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 추계 학술발표회
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• pp.1358-1365
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• 2010

A case study deals with Squeezing behavior under tunneling. Squeezing stands for large time-dependent convergence during tunnel excavation. Squeezing can occur in both rock and soil as long as the particular combination of induced stresses and material properties pushes some zone around the tunnel beyond the limiting shear stress at which creep starts. Under squeezing rock conditions, If the support installation is delayed the rock mass moves into the tunnel and a stress redistribution takes place around it. On the contrary, if deformation is restrained, squeezing will lead to long-term load build-up of rock support. This paper shows analysis case mutually with monitoring and numerical analysis result of squeezing behavior of Pinglin tunnel in Taiwan.