• Journal of the Korea Concrete Institute
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• v.23 no.1
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• pp.77-86
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• 2011

The concrete creep deformation of a hybrid composite section can cause additional deformation of the composite section and the stress relaxation of pre-compressive stress on the concrete section due to partial restraint of the deformation. In this study, the stress relaxation coefficient method (SRCM) is derived for simple analysis of complicate hybrid or composite sections for engineering purpose. Also, an equation of the stress relaxation coefficient (SRC) required for the SRCM is proposed. The SRCM is derived with the parameters of a creep coefficient, section and loading properties using the same method as the constant-creep step-by-step method (CC-SSM). The errors of the SRCM is improved by using the proposed SRC equation than the average SRC's which were estimated from the CC-SSM. The root mean square error (RMSE) of the SRCM with the proposed SRC equation for concrete with creep coefficient less than 3 was less than 1.2% to the creep deformation at the free condition and was 3.3% for the 99% reliability. The proposed SRC equation reflects the internal restraint of composite sections, and the effective modulus of elasticity computed with the proposed SRC can be used effectively to estimate the rigidity of a composite section in a numerical analysis which can be applied in analysis of the external restrain effect of boundary conditions.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.245-248
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• 2008

Concrete structures show time-dependent behavior due to creep and shrinkage of concrete and the uncertainties of creep and shrinkage are very huge. To reduce uncertainties of creep and shrinkage, it is substantially necessary to perform the long-term creep and shrinkage tests, but actual construction process doesn't allow it due to the limited time. Even though the tests are performed in laboratory, the values obtained from the tests could be different from the actual values in construction site because of the different environment between the laboratory and construction site and the model uncertainty itself. It is difficult to predict the long-term behaviors of concrete structures properly if the assumed creep coefficient obtained from Codes or the results of experiments is different from the real characteristics of concrete creep. In this study, for predicting the long-term behavior, the creep coefficients in reinforced concrete beams are estimated using creep sensitivity analysis from the measured deflections with time. And estimated creep coefficients using creep models of ACI Committee 209 and CEB-FIP MC90 are compared.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.10
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• pp.1229-1237
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• 2013

In this study, the theory of spherical indentation based on incremental plasticity is extended to an indentation method for evaluating creep properties. Through finite element analysis (FEA), the point where the elastic strain effect is negligible and the creep strain gradient constant is taken as the optimum point for obtaining the equivalent strain rate and stress. Based on FE results for spherical indentation with various values of creep exponent and creep coefficient, we derive by regression an equation to calculate creep properties using two normalized variables. Finally a program is generated to calculate creep exponent and creep coefficient. With this method, we obtain from the load-depth curve creep exponents with an average error of less than 1.5 % and creep coefficients with an average error of less than 1.0 %.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.1A
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• pp.45-54
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• 2006

To obtain realistic stress-strain relation in concrete, it is necessary to improve the constitutive model for creep and shrinkage of concrete. This study is made up with predicting model of creep using rheological approach and mathematical development which is solution for phenomenon of concrete creep. Long-term deformation components are combined based on traditional 4-parameters model. Creep deformation is obtained adequately using 4-parameters determined by considering aging effect and microprestress among gels. And coefficient of effective viscosity is able to represent both basic creep and total creep included drying creep. This study attempt to establish mathematical model considering effects of aging, hydration, and variations of pore humidity. It can predict both basic creep and total creep. Values of result between prediction and experiment have greater than correlation factor 99%. Additionally experimental results report bad consentaneity with highway design specification adopting FIB MC 90. Rather than those are similar to FIB MC 90 rev.99.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.293-296
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• 2008

Creep and shrinkage in concrete structures are very complex phenomena in which various uncertainties exist with regard to inherent material variations as well as modeling uncertainties. The creep and shrinkage models which are capable of predicting long-term structural response are specified in design codes such as ACI 209-92, CEB-FIP Model Code 90, etc. However, in the prediction formulas of creep and shrinkage effects of concrete, various kinds of parameters are involved to express the characteristics of concrete under consideration (i.e. the proportion of concrete, the shape of the structure, relative humidity, etc.). And the predicted values from each design code under same environment differ from each other. To predict the characteristics of concrete, the long-term experiments of creep and shrinkage is necessary but this is not suitable for a construction field. In this study, adjustment method of creep coefficient using sensitivity analysis is proposed to predict creep coefficient of concrete exactly and it is checked up on the validity of the predicting method by comparing to the assumed value and predicted one.

• Geomechanics and Engineering
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• v.20 no.6
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• pp.517-525
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• 2020

The long-term stability of rock engineering is significantly affected by the time-dependent deformation behavior of rock, which is an important mechanical property of rock for engineering design. Although the hard rocks show small creep deformation, it cannot be ignored under high-stress condition during deep excavation. The inner mechanism of creep is complicated, therefore, it is necessary to investigate the relationship between microscopic creep mechanism and the macro creep behavior of rock. Microscopic numerical modeling of sandstone creep was performed in the investigation. A numerical sandstone sample was generated and Parallel Bond contact and Burger's contact model were assigned to the contacts between particles in DEM simulation. Sensitivity analysis of the microscopic creep parameters was conducted to explore how microscopic parameters affect the macroscopic creep deformation. The results show that the microscopic creep parameters have linear correlations with the corresponding macroscopic creep parameters, whereas the friction coefficient shows power function with peak strength and Young's modulus, respectively. Moreover, the microscopic parameters were calibrated. The creep modeling curve is in good agreement with the verification test result. Finally, the creep curves under one-step loading and multi-step loading were compared. This investigation can act as a helpful reference for modeling rock creep behavior from a microscopic mechanism perspective.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.10a
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• pp.579-582
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• 1999

This paper represents the results of experiments designed to investigate the time-dependent response of concrete and steel tube in circular concrete-filled steel tubes, as are deployed extensively in high-rise building construction. The experiments were performed for creep of concrete and CFT column specimens with three loading cases. The creep coefficient and specific creep(unit creep) obtained from the test results were used for estimating and comparing the time-dependent response of each case. From these analyses, it is show that CFT-column has many merits for long-term behavior.

• Proceedings of the KSME Conference
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• 2000.04a
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• pp.31-36
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• 2000

High temperature and pressure materials in power plant are degraded by creep damage, if they are exposed to constant loads for long times, which occurs in the load bearing structures of pressurized components operating at elevated temperatures. Many conventional measurement techniques such as replica method, electric resistance method, and hardness test method for measuring creep damage have been used. So far, the replica method is mainly used for the Inspection of High temperature and pressure components. This technique is, however, restricted to applications at the surface of the testpieces and cannot be used to material inside. In this paper, ultrasonic evaluation for the detection of creep damage in the form of cavaties on grain boundaries or integranular microcracks are carried out. And the absolute measuring method of quantitative ultrasonic velocity technique for Cr-Mo material degradation is analyzed. As a result of ultrasonic tests for crept specimens, we find that the sound velocity is decreased as the increase of creep life fraction$({\Phi}_c)$ and also, confirmed that hardness is decreased as the increase of creep life fraction$({\Phi}_c)$ but the coefficient of ultrasonic attenuation is increased as the increase of creep life fraction$({\Phi}_c)$. Finally based on the result in this paper, it can be recognized that the ultrasonic techniques using velocities and attenuation coefficient factor are very useful non-destructive methods to evaluate the degree of material degradation in fossile power plants.

• Journal of Ocean Engineering and Technology
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• v.11 no.2
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• pp.39-47
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• 1997

Creep rate the elevated temperature is known to be controller by the softening process of microstructure especially in the solid solution alloys such as 125 Cr rotor steel. The change of structure is a decreasing process of the free energy of the state including stress, diffusivity of the material, and tmeperature. This study shows that diffusion coefficient, D of 12% Cr rotor steel at 953K with 74.8 MPa is 1.084~3.140*$10^{15}mm^2sec^1$ compared to $1.658*10^{24}mm^2sec^1$at 963K without stress. During creep, the growth of martensite laths accelerates the diffusion coefficient under stress due to incoherency of interface between carbides and matrix.

• Journal of Ocean Engineering and Technology
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• v.13 no.3 s.33
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• pp.100-107
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• 1999

Due to the high temperature and pressure, the materials of pipeline in power plant are degraded by creep damage. So far, many conventional measurement techniques such as replica method, electric resistance method, adn hardness test method for creep damage have been used. Among them, the replica method has mainly been used for the inspection of components. But this technique is restricted to the applications at the surface of the objects and cannot be used to material inside. In this paper, the measuring methods of evaluation by using ultrasonic attenuation and electric resistance for the creep detection of creep damage in the form of cavities on grain boundaries or intergranular microcracks were carried out. Absolute measuring method of quantitave ultrasonic attenuation technique for 1Cr0.5Mo material degradation was analyzed for determining the creep degradation steps using life prediction formula. As a result of measurement for creep specimens, we founded that the coefficient of utrasonic attenuation was increased as the increase of creep life fracton(${phi}c$) and the decreasing rate of wlwctric resistance was also increased.