• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1992년도 가을 학술발표회 논문집
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• pp.67-72
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• 1992

The heat of hydration of cement causes the internal temperature rise at early age, particulary in massive concrete structures such as a footing of nuclear reactor building or a dam. As the result of the temperature rise and restraint of foundation, the thermal stress may induce cracks in concrete. Therefore, the prediction of the thermal stress is very important in the design and construction stages in order to control the cracks developed in massive concrete structures. And, in case of young concrete, creep effect by the temperature load is larger than That of old concrete. Thus the effect of creep must be considered for checking the cracks, serviceability, durability and leakage. This study is composed of two items. The first, it is to develop a finite element program which is capable of simulating the temperature history in mass concrete. The second, when the thermal stress of mass concrete structures considering creep is calculated by using the modified elastic modulus due to the inner temperature change. It is shown that the analytical results of this study is in comparably good agreement with JCI's analytical results.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2008년도 춘계 학술발표회 제20권1호
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• pp.857-860
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• 2008

본 연구는 잠실제2롯데월드에 적용된 50, 60, 70MPa 콘크리트 강도에 대한 크리프 및 건조수축 특성 파악을 위한 재료연구의 일환으로 진행된 콘크리트 테스트 프로그램의 준비 단계에 대한 일련의 체계화된 과정을 나타내고 있다. 콘크리트 테스트 프로그램의 준비단계는 국내와 국외에서 진행되었다. S사 실험실에서 진행된 국내 진행일정은 총 5일간 진행되었으며 그 진행과정은 공시체 제작, 몰드탈형, 공시체 마킹, 수중양생, 포장, 및 운송으로 이루어져 있다. 국외에서는 PCA(Portland Cement Association)산하 CTL사에서 진행되고 있으며 테스트 항목은 탄성계수 측정테스트(정탄성 & 동탄성), 압축강도 테스트, 크리프 건조수축 테스트, 및 할렬인장강도 테스트가 진행되고 있다. 크리프 및 건조수축 실험의 경우에는 하중 재령일에 따라 총 18개월간 테스트가 진행될 예정이다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 춘계학술대회 논문집
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• pp.273-276
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• 2008

Thermoplastic resin takes place stress relaxation and creep according to temperature and time. In room temperature, time dependent deformation (TDD) of polymer was carried out at previous study. In this study, it evaluates time dependent deformation to relate temperature. Nanoscale indents can be used as cells for molecular electronics and drug delivery, slots for integration into nanodevices, and defects for tailoring the structure and properties. Therefore, it is important to control pattern depth for change of indent depth by creep when using Nanoindenter. For evaluating TDD at high temperature, it is occurred thermal-nanoindentation test by changing hold time at maximum load. Temperature is putted at $90^{\circ}C$, hold time at maximum loads are putted at 1, 10, 50, 100, 200, 300 and 500s.

• 한국도로학회논문집
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• 제17권1호
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• pp.25-33
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• 2015

PURPOSES : This paper, presents the results of a laboratory study aimed to verify the suitability of a particular type of Electric Arc Furnace (EAF) steel slag to be recycled in the lithic skeleton of both dense graded and porous asphalt mixtures for flexible pavements. METHODS : Cyclic creep and stiffness modulus tests were performed to evaluate the mechanical performance of three different asphalt mixtures (dense graded, porous asphalt, and stone mastic) prepared with two types of EAF steel slag. For comparison purposes, the same three mixtures were also designed with conventional aggregates (basalt and limestone). RESULTS : All the asphalt mixtures prepared with EAF steel slag satisfied the current requirements of the European standards, which support EAF steel slag as a suitable material for flexible pavement construction. CONCLUSIONS : Based on the experimental work, the use of waste material obtained from steel production (e.g. EAF steel slag) as an alternative in the lithic skeleton of asphalt mixtures can be a satisfactory and reasonable choice that fulfills the "Zero Waste" objective that many iron and steel industries have pursued in the past decades.

• 한국식품과학회지
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• 제24권2호
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• pp.165-170
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• 1992

호화된 쌀 전분 젤의 creep 거동을 농도($30{\sim}45%$)와 온도($20{\sim}85^{\circ}C$)를 달리하여 측정하였으며 전분 젤의 점탄성에 미치는 온도와 농도의 영향은 shift factor($a_{T},\;a_{C}$)를 이용하여 분석하였다. Compression creep test의 결과 쌀 전분 젤은 순간변형 부분, 지연탄성 부분 및 유동부분으로 구성된 6-element의 역학적 모델로 해석할 수 있었으며 점탄성 거동은 순간변형 부분이 주체를 이루었다. 전분 농도가 $30{\sim}45%$로 증가함에 따라 creep compliance는 감소하였고 농도 증가는 주로 순간변형 부분에 영향을 미쳤다. 각 농도에서의 total compliance는 shift factor($a_{C}$)를 이용하여 일정한 기준농도(38%)로 변환시켜 농도불변 곡선으로 정확히 나타낼 수 있었다. 온도를 $20{\sim}85^{\circ}C$로 조절하여 얻은 creep compliance는 온도의 증가에 따라 증가하여 젤의 조직이 점점 연화되었다. $20^{\circ}C$를 기준온도로 shift factor($a_{T}$)에 의해 각 온토에서의 단편적 total compliance 곡선을 log(t)에 대해 수평이동시키면 전반적으로 부드럽게 연결되는 master curve를 얻을 수 있었으므로 시간-온도 환산법칙이 잘 성립되었다. 수정된 WLF식을 이용하여 $a_{T}$의 온도 의존성을 분석한 결과 전분 젤의 활성화에너지는 일정하지 않고 온도의 증가에 따라 감소하였으며, 공유결합에 의해 망상구조를 이루는 다른 젤들에 비해 그 값이 상대적으로 작았다.

• Computers and Concrete
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• 제32권4호
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• pp.383-392
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• 2023

The mechanical properties of Recycled Aggregate Concrete (RAC) with 100 percent Recycled Coarse Aggregate (RCA) under loading rates were investigated in depth. The theoretical model was validated utilizing the RAC elastic modulus obtained from cylindrical specimens subjected to various strain rates. Viscoelastic theories have traditionally been used to describe creep and relaxation of viscoelastic materials at low strain rates. In this study, viscoelastic theories were extended to the time domain of high strain rates. The theory proposed was known as reversed viscoelastic theory. Normalized Dirichlet-Prony theory was used as an illustration, and its parameters were determined. Comparing the predicted results to the experimental data revealed a high level of concordance. This methodology demonstrated its ability to characterize the strain rate effect for viscoelastic materials, as well as its applicability for determining not only the elastic modulus for viscoelastic materials, but also their shear and bulk moduli.

• 콘크리트학회논문집
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• 제18권1호
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• pp.109-116
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• 2006

크리프와 건조수축에 의한 비역학적 변형이 구조물의 내적 혹은 외적 경계조건에 의하여 억제됨으로써 발생하는 역학적 변형률은 내적 응력상태의 변화를 초래한다. 이러한 내적응력 상태의 변화는 크리프 변형의 초기조건을 변화시킴으로써 비역학적 변형에 영향을 미치게 되며 또 다시 내적응력 상태의 변화를 초래하는 순환루프를 형성하게 된다. 콘크리트의 재령종속적 재료특성을 고려한 장기거동해석에서 요구되는 구성관계는 이러한 응력상태의 지속적 변화로 초래되는 크리프 변형 미케니즘의 변화를 고려할 수 있어야한다. 비역학적 변형의 영향을 받는 합성형 거더의 거동 예측을 위하여 본 연구에서는 콘크리트의 재령종속적 특성을 고려한 총 응력-변형률 관계를 Taylor 급수확장을 이용하여 기준시간에 관해 선형으로 확장하는 과정을 통해 응력-변형률의 점증관계를 정의하는 증분유효접선계수를 재령종속적 관계로 유도하였다. 콘크리트의 건조 수축과 크리프 및 탄성계수발현으로 야기되는 합성형 거더의 재령종속적 거동을 분석하기 위해 제안된 증분유효접선계수에 근거하여 유한요소 정식화를 수행하였다. 개발된 해석 알고리즘을 이용하여 부모멘트 구간 하단부에 콘크리트를 타설한 이중합성 거더에 대하여 최근 수행된 재령종속적 거동 실험을 예측하였고, 두 결과를 비교하여 매우 근접한 예측 결과를 얻을 수 있었으며, 합성형 거더에서 발생하는 지점부 부반력을 비교과정을 통해 분석하였다.

• Structural Engineering and Mechanics
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• 제22권2호
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• pp.197-222
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• 2006

In this paper, the non-linear time-dependent closed-form, discrete and combined solutions for the post-elastic response of a geometrically and physically non-linear suspended cable to a uniformly distributed load considering the creep effects, are presented. The time-dependent closed-form method for the particularly straightforward determination of a vertical uniformly distributed load applied over the entire span of a cable and the accompanying deflection at time t corresponding to the elastic limit and/or to the elastic region, post-elastic and failure range of a suspended cable is described. The actual stress-strain properties of steel cables as well as creep of cables and their rheological characteristics are considered. In this solution, applying the Irvine's theory, the direct use of experimental data, such as the actual stress-strain and strain-time properties of high-strength steel cables, is implemented. The results obtained by the closed-form solution, i.e., a load corresponding to the elastic limit, post-elastic and failure range at time t, enable the direct use in the discrete non-linear time-dependent post-elastic analysis of a suspended cable. This initial value of load is necessary for the non-linear time-dependent elastic and post-elastic discrete analysis, concerning incremental and iterative solution strategies with tangent modulus concept. At each time step, the suspended cable is analyzed under the applied load and imposed deformations originated due to creep. This combined time-dependent approach, based on the closed-form solution and on the FEM, allows a prediction of the required load that occurs in the post-elastic region. The application of the described methods and derived equations is illustrated by numerical examples.

• International Journal of Aeronautical and Space Sciences
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• 제13권4호
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• pp.458-467
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• 2012

In this study, the linear viscoelastic response of a rectangular laminated plate is investigated. The viscoelastic properties, expressed by two basic spring-dashpot models, that is Kelvin and Maxwell models, is assumed in the range to investigate the influence of viscoelastic coefficients to mechanical behavior. In the present study, viscoelastic responses are performed for two popular equivalent single-layered theories, such as the first-order shear deformation theory (FSDT) and third-order shear deformation theory (TSDT). Compliance and relaxation modulus of time-dependent viscoelastic behavior are approximately determined by Prony series. The constitutive equation for linear viscoelastic material as the Boltzmann superposition integral equation is simplified by the convolution theorem of Laplace transformation to avoid direct time integration as well as to improve both accuracy and computational efficiency. The viscoelastic responses of composite laminates in the real time domain are obtained by applying the inverse Laplace transformation. The numerical results of viscoelastic phenomena such as creep, cyclic creep and recovery creep are presented.

• Structural Engineering and Mechanics
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• 제62권6호
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• pp.725-737
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• 2017

An analytical method considering axial equilibrium is proposed for the short- and long-term analyses of shear lag effect in reinforced concrete (RC) box girders. The axial equilibrium of box girders is taken into account by using an additional generalized displacement, referred to as the longitudinal displacement of the web. Three independent shear lag functions are introduced to describe different shear lag intensities of the top, bottom, and cantilever plates. The time-dependent material properties of the concrete are simulated by the age-adjusted effective modulus method (AEMM), while the reinforcement is assumed to behave in a linear-elastic fashion. The differential equations are derived based on the longitudinal displacement of the web, the vertical displacement of the cross section, and the shear lag functions of the flanges. The time-dependent expressions of the generalized displacements are then deduced for box girders subjected to uniformly distributed loads. The accuracy of the proposed method is validated against the finite element results regarding the short- and long-term responses of a simply-supported RC box girder. Furthermore, creep analyses considering and neglecting shrinkage are performed to quantify the time effects on the long-term behavior of a continuous RC box girder. The results show that the proposed method can well evaluate both the short- and long-term behavior of box girders, and that concrete shrinkage has a considerable impact on the concrete stresses and internal forces, while concrete creep can remarkably affect the long-term deflections.