• Proceedings of the Korea Concrete Institute Conference
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• 1999.10a
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• pp.285-288
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• 1999

Creep is a major parameter to represent long-term behavior of concrete structures concerning serviceability and durability. The effect of creep is recently taking account into crack resistance analysis of early-age concrete concerning durability evaluation. Since existing creep prediction models were proposed to predict creep for hardened concrete, most of them cannot consider effectively the information on microstructure formation and hydration developed in the early-age concrete. In this study, creep tests for early-age concrete made of the type I cement and the type V cement are carried out respectively and creep prediction models are evaluated for the prediction of creep behavior in early-age concrete. A creep prediction model is modified for the prediction of creep in early-age concrete and also verified by comparing prediction results with results of creep tests on early-age concrete.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.215-220
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• 2001

The objective of this study is to examine the fracture characteristics of concrete at early ages such as critical stress intensity factor, critical crack-tip opening displacement, fracture energy based on the concepts of the effective-elastic crack model and the cohesive crack model. A wedge splitting test for Mode I was performed on cubic wedge specimens with a notch at the edge. By varying strength and age, load-crack mouth opening displacement curves were obtained and the results were analyzed by linear elastic fracture mechanics. The results from the test and analysis showed that critical stress intensity factor and fracture energy increased, and critical crack-tip opening displacement decreased with concrete age from 1 day to 28 days. The obtained fracture parameters at early ages may be used as a fracture criterion and an input data for finite element analysis of concrete at early ages.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.623-628
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• 2001

The objective of this study is to find out the effect of Horizontal vibration on the compressive strength for 7 days and 28 days cured concrete specimens according to the kind of fine aggregates and the variation of vibration velocities, and times began to vibrate. Three kinds of fine aggregate(river sand, sea sand, crush sand), three types of vibration velocities(0.25, 0.5 0.1kine(cm/sec)), and four steps of times(0, 3, 6, 9 hours after concrete casting) were chosen as the experimental parameters in this study, the vibrations are applied for 30 minutes in each case. From this study, the variation with type of fine aggregate doesn't show tendency and the compressive strength decreases with increasing vibration velocity. When the vibration force time is more than 6 hour, It shows that compressive strength decreases under the condition of vibration velocity which the value is 0.5 kine and 1 kine, respectively.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.635-640
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• 2001

In this study, a mathematical model is established for prediction of chloride penetration in unsaturated cracked early-age concrete. The model is combined with models for thermo-hygro dynamic coupling of cement hydration, moisture transport and micro-structure development. Chloride permeability and water permeability at cracked early-age concrete specimens are evaluated using a rapid chloride permeability test and a low-pressure water permeability test, respectively. Then, a homogenization technique is introduced into the model to determine equivalent diffusion coefficient and equivalent Permeation coefficient. Increased chloride transport due to cracks at the specimen could be predicted fairly well by characterizing the cracks using proposed model. Proposed model is verified by comparing diffusion analysis results with test results.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.641-646
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• 2001

In early-age concrete, volumetric deformations due to thermal expansion and moisture transfer are restrained by various boundary conditions, and then restraint stresses occur in proportion to developed stiffness. With increase of the age, these stresses are gradually relieved by significant relaxation behavior of early-age concrete. Therefore, it is necessary to consider the stress relaxation in order to analyze the behavior of early-age concrete more accurately. In this paper, we propose a unified algorithm which combines a relaxation model with hydration model, heat conduction model, micropore structure formation model, moisture diffusion model and mechanical properties development model and develop a finite element program based on the algorithm. The program is applied to evaluate stress development if a temperature-stress test machine (TSTM) specimen and a massive concrete structure, and then validity of the program is discussed and evaluated.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.05a
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• pp.668-671
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• 2004

Recently, the concern for mechanical properties at early age concrete are increasing because of the importance of the thermal stress and the determination of removal time of form work and prestressing work. In this study, an estimation for the development of compressive strength and elastic modulus with age in concretes isothermally cured $(10^{\circ}C,\;20^{\circ}C)$ and having W/C ratio of 30, 40, and $50\%$ were investigated. According to experiment results, the development of compressive strength and elastic modulus shows higher values at early ages as the W/C ratio decreases and curing temperature increases. When the maturity concept, for estimation of the strength, is adopted, a modification for W/C ratio is required at early ages.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.04a
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• pp.123-128
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• 1999

Recently, as a performance based design concept is introduced, assurance of expected performances on serviceability and safety in the whole span of life is exactly requested. So, quantitative assessments about durability related properties of concrete in early-age long term are come to necessary, Especially in early age, deterioration which affects long-term durability performance can be occurred by hydration heat and shrinkage, so development of reasonable hydration heat model which can simulate early age behavior is necessary. The micor-pore structure formation property also affects shrinkage behavior in early age and carbonations and chloride ion penetration characteristic in long term, So, for the quantitative assessment on durability performance of concrete, modelings of early age concrete based on hydration process and micor-pore structure formation characteristics are important. In this paper, a micromechanics based hydration heat evolution model is adopted and a quantitative model which can simulate micro-pore structure development is also verified with experimental results. The models can be used effectively to simulate the early-age behavior of concrete composed of different mix proportions.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.04a
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• pp.361-366
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• 1999

The purpose of this study is to evaluate the structural performance of Reinforced steel fiber concrete structures using early age concrete. Reinforced concrete structures using early age concrete are result in the degradation of structural performance due to crack, overload, unexpected vibration and impact load. Specimens, designed by the over 0.75% of steel fiber incorporated, were showed the ductile behavior and failed slowly with flexure and flexure-shear. Increasing the percent of steel fiber incorporated(0.25~2.0%), the ultimate shear stress of each specimen were increased 12~40% than that of specimen SSS.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.11a
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• pp.309-312
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• 2003

Quasi-instanteneously performed creep loading is probably one way to seperate the viscous and the plastic creep from the elastic deformation. For mature concrete no differences seem to exist depending on whether the loading is rapidly applied or not. At early age the plastic deformations probably dominate the deformation. A series of test have been done to measure the quasi-instanteneous deformation in concrete specimens according to loading durations and concrete ages. The present study indicates that even very short-term loading contains creep deformation. As concrete is younger and load duration is longer, the proportion of creep deformation in quasi-instanteneous deformation is increased.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.04a
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• pp.303-308
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• 1998

In the concrete structures exposed to environmental conditions at early ages, water movement occurs by moisture diffusion in the concrete, and self-desiccation of concrete is also occurred. Thus the internal relative humidity is changed from moisture diffusion and self-desiccation. Thus the internal relative humidity at each location in concrete includes the decrease by self-desiccation. Especially, for high-strength concrete the much unit cement content is used, so that the non-uniform relative humidity distribution is affected form self-desiccation at early ages. In this study, the internal relative humidity in concrete was measured at early ages, and the moisture diffusion component and self-desiccation component of total relative humidity were discussed.