• Journal of the Korea Concrete Institute
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• v.23 no.5
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• pp.581-590
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• 2011

Since ultra-high performance cementitious composites (UHPCC) not only represents high early age shrinkage strain due to its low water-to-binder ratio (W/B) and high fineness admixture usage but also reduces the cross section of structure from the higher mechanical properties, it generally has more shrinkage cracks from the restraints of formwork and reinforcing bars. In this study, free and restrained shrinkage experiments were conducted to evaluate the suitability of incorporating both expansive admixture (EA) and shrinkage reducing agent (SRA). The test results indi-cated that approximately 40~44% of free shrinkage strain was decreased. Also, the results showed that 35% and 47% of residual tensile stresses were relieved by synergetic effect of SRA and EA, respectively. Residual tensile stresses from ringtest were relaxed by approximately 61% and 64% of elastic shrinkage stresses due to SRA and EA, respectively, because of the tensile creep effect. Therefore, the creep effect should be considered to precisely estimate the restrained shrinkage behavior of concrete structures. The degree of restraint of UHPCC was approximately in the range of 0.78~0.85. The addition of combined EA and SRA showed minute influence on the degree of restraint. However, the effect decreased when thicker concrete ring was used. Tensile creep strains were measured and compared to the predicted values from 4-parametric prediction model considering time dependent restrained forces.

• Journal of the Korean Geotechnical Society
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• v.24 no.5
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• pp.107-115
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• 2008

This paper is performed to examine the effect of creep during the primary consolidation and the applicability of the Yin's EVP (Elasto-Visco-Plastic) model. In ordinary consolidation theories using the elastic model, the primary consolidation process can be expressed but the secondary consolidation process cannot. It is due to the viscosity, which can express the secondary consolidation, and is sometimes related to the scale effect (difference of the thickness of clay layer between laboratory sample and field condition) such as hypotheses Type A and Type B shown by Ladd et al. (1977). Usually, the existence of the creep during the primary consolidation has been conformed and the Type B is well acceped. On the other hand, from the large-scaled consolidation tests the intermediate characteristic between Type A and Type B was proposed as Type C by Aboshi (1973). In this study, to clarify the effect of creep on the settlement-time relation during the primary consolidation in detail, Type B consolidation tests were performed using the separate-type consolidation test apparatus for a peat and clay. Then the test results were analyzed by using Yin's EVP Model (Yin and Graham, 1994). In conclusion, followings were obtained. At the end of primary consolidation, the compression for the subspecimens should not be the same because of the difference of the excess pore water pressure dissipation rate. And the average settlement measured by the separate-type consolidometer coincides with the analyzed one using the Yin's EVP model. As for the dissipation of the excess pore water pressure, however, the measured excess pore water pressure dissipates faster compared with the Yin's model.

• Journal of Korean Association for Spatial Structures
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• v.10 no.1
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• pp.75-84
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• 2010

According to the available study and experimental data about the long term behavior of CFT(Concrete Filled Tube) columns, the creep and of concrete in CFT columns are smaller than those of RC columns because of the confinement effect of outer steel columns. In this study, the uncertainties associated with assumed values for concrete properties such as strength, creep coefficients, and service load have been considered and analyzed for the prediction of time-dependent column shortening of CFT column. The CFT column shortening analysis using Monte Carlo method is proposed and an of a 37 story tall building with CFT columns is studied for illustration. According to the results obtained by the probability analysis with multi parameters, the effect of variation coefficient for 3 parameters is investigated considering confidence interval.

• Progress in Superconductivity and Cryogenics
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• v.25 no.3
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• pp.28-33
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• 2023

Magnetic relaxation properties of pure MgB₂ powder samples and diluted water-treated MgB₂ powder samples were investigated. The magnetic field H-dependence, m(H), and the time t-dependence, m(t), of the magnetic moment m were measured and analyzed using the PPMS-VSM magnetometer equipment, respectively. The m(t) reduction rates of pure MgB₂ powder samples and diluted water-treated MgB₂ powder samples decreased to about 0.7 ~ 1.8% and 0.6 ~ 1.0% for about 7200 s, respectively, at temperature T = 15 K. The magnetic relaxation properties of the two types of MgB₂ powders were analyzed by calculating the magnetic relaxation rate S = -dln(M_irr)/dln(t) values according to Anderson-Kim theory. The magnetic relaxation ratio S values of the two types of MgB₂ powder samples were almost similar. As a result of the quantum creep effect, the constant magnetic relaxation rate S characteristic was confirmed at a temperature range of T = 10 K or less.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.10a
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• pp.263-263
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• 1998

• Proceedings of the Korean Geotechical Society Conference
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• 2000.11a
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• pp.327-334
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• 2000

응력-변형률 관계의 모델링에 있어서 creep, stress relaxation, strain rate effect 등의 묘사는 중요한 지반거동중의 하나인 시간 의존적 거동의 simulation은 있어서 대단히 중요한 요소라 할 수 있다. 특히 지반은 변형률 속도에 대하여 때로는 매우 다른 거동 특성을 보이기 때문에 지반의 모델링에 있어서 변형율 속도를 고려한 구성방정식의 제시는 큰 비중을 차지한다 하겠다. 본 연구에서는 변형율에 따라 변화하는 지반의 거동특성을 보다 현실에 가깝게 묘사하기 위한 구성모델을 제시하였다. 이를 위하여 Bounding Surface Model의 점탄소성 부분을 Perzyna(1966)와 Adachi and Oka(1982)의 구성방정식 이론을 이용하여 발전시켰다. 제안된 구성모델은 기존의 모델에 비하여 다양한 변형율 속도에 적용할 수 있는 모델 정수를 비교적 간단히 결정할 수 있다는 장점이 있으며, 변형율 속도의 영향뿐 아니라 creep, stress relaxation등의 현상도 잘 simulation 할 수 있다. 본 모델은 후에 엄격히 실시되는 실내시험을 통하여 검증될 예정이다.

• Journal of the Korea Concrete Institute
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• v.12 no.4
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• pp.59-67
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• 2000

With increased height of structure, the effect of column shortening need special consideration in the design and construction of high-rise buildings. The shortening of each column affects nonstructural members such as partitions, cladding, and M/E systems, which are not designed to carry gravity forces. The slabs and beams will tilt due to the cumulative differential shortening of adeacent vertical members. The main purpose of estimating the total shortening of vertical structural member is to compensate the differential shortening between adeacent members. This paper presents effect of parameters for phenomenon of column shortening in vertical members. The paper presents effect of parameters for phenomenon of column shortening in vertical members. The conclusions obtained from this study are follow as ; Strength of concrete and steel ratio effected on column shortening caused by elastic and inelastic shortening. Also, it is known that Ultimate-shrinkage-Value, Specific-Creep-Value, and volume to surface ratio effected on inelastic shortening only. Particularly, Ultimate-Shrinkage-Value and Specific-Creep-Value effected considerable on the amount of total column shortening.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.11 s.254
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• pp.1494-1501
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• 2006

Mechanical and physical properties of a copper alloy for a liquid rocket engine(LRE) combustion chamber liner application were tested at various temperatures. All test specimens were heat treated with the condition they might experience during actual fabrication process of the LRE combustion chamber. Physical properties measured include thermal conductivity, specific heat and thermal expansion data. Uniaxial tension tests were preformed to get mechanical properties at several temperatures ranging from room temperature to 600$^{\circ}C$. The result demonstrated that yield stress and ultimate tensile stress of the copper alloy decreases considerably and strain hardening increases as the result of the heat treatment. Since the LRE combustion chamber operates at higher temperature over 400$^{\circ}C$, the copper alloy can exhibit time-dependent behavior. Strain rate, creep and stress relaxation tests were performed to check the time-dependent behavior of the copper alloy. Strain rate tests revealed that strain rate effect is negligible up to 400$^{\circ}C$ while stress-strain curve is changed at 500$^{\circ}C$ as the strain rate is changed. Creep tests were conducted at 250$^{\circ}C$ and 500$^{\circ}C$ and the secondary creep rate was found to be very small at both temperatures implying that creep effect is negligible for the combustion chamber liner because its operating time is quite short.

• Tunnel and Underground Space
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• v.14 no.1
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• pp.69-77
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• 2004

Investigation of the time-dependent behavior of rock and the associated mechanisms are of key interest in long-term stability analysis of many engineering applications. In this study, creep tests were performed on Daejeon granite samples of 25.4mm diameter under uniaxial compression at varying stress levels. The effect of moisture was investigated by testing both air-dried and fully water-saturated samples. The creep behavior of Daejeon granite exhibited three distinctive stages of primary, secondary and tertiary creep. The ultimate strength of granite under a constant stress decreased considerably with time. Saturation and immersion of the test specimen in water markedly increased the total creep strain as well as the secondary creep rate. The experimental creep curves are fitted to Burger's model as well as two other empirical models suggested by previous researchers. A number of the parameters determined for each model are dependent on stress and influenced by the presence of water. Based on the experimental results, an empirical relation between the applied stress and the time-dependent strain is established separately for each air-dried and fully water-saturated Daejeon granite.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.1 no.1
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• pp.53-68
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• 1981

Most clays under sustained load exhibit time-dependent deformation because of creep movement of soil particles and many investigators have attempted to relate their findings to the creep behavior of natural ground and to the long-term stability of slopes. Since the creep behavior of clays may assume a variety of forms depending on such factors as soil plasticity, activity and water content, it is difficult and complicated to analyse the creep behavior of clays. Rheological models composed of linear springs in combination with linear or nonlinear dashpots and sliders, are generally used for the mathematical description of the time-dependent behavior of soils. Most rheological models, however, have been proposed to simulate the behavior of secondary compression for saturated clays and few definitive data exist that can evaluate the behavior of non-saturated clays under the action of sustained stress. The clays change gradually from a solid state through plastic state to a liquid state with increasing water content, therefore, the rheological models also change. On the other hand, creep is time-dependent, and also the effect of thixotropy is time-function. Consequently, there may be certain correlations between creep behavior and the effects of thixotropy in compacted clays. In addition, the states of clay depend on water content and hence the height of the specimen under drained conditions. Futhermore, based on present and past studies, because immediate elastic deformation occurs instantly after the pressure increment without time-delayed behavior, the factor representing immediate elastic deformations in the rheological model is necessary. The investigation described in this paper, based on rheological model, is designed to identify the immediate elastic deformations and the effects of thixotropy and height of clay specimens with varing water content and stress level on creep deformations. For these purposes, the uniaxial drain-type creep tests were performed. Test results and data for three compacted clays have shown that a linear top spring is needed to account for immediate elastic deformations in the rheological model, and at lower water content below the visco-plastic limit, the effects of thixotropy and height of clay specimens can be represented by the proposed rheological model not considering the effects. Therefore, the rheological model does not necessitate the other factors representing these effects. On the other hand, at water content higher than the visco-plastic limit, although the state behavior of clays is visco-plastic or viscous flow at the beginning of the test, the state behavior, in the case of the lower height sample, does not represent the same behavior during the process of the test, because of rapid drainage. In these cases, the rheological model does not coincide with the model in the case of the higher specimens.