• Proceedings of the Korea Concrete Institute Conference
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• 2010.05a
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• pp.465-466
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• 2010

In this paper, an integrated system is proposed which can evaluate both the early-age properties and durability performance of concrete structures. This integrated system starts with a hydration model which considers both Portland cement hydration and chemical reactions of supplementary cementing materials (SCM). Based on the degree of hydration of cement and mineral admixtures, the amount of reaction products, the early age heat evolution, chemically bound water, porosity, the early age short-term mechanical behaviors, shrinkage and early-age creep are evaluated as a function of curing age and curing conditions. Furthermore, the durability aspect, such as carbonation of blended concrete and chloride attack, are evaluated considering both the material properties and surrounding environments. The prediction results are verified through experimental results.

• Journal of the Korean Home Economics Association
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• v.19 no.2
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• pp.1-9
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• 1981

The phenomena of memory have been shown to come out early in life and to develop drastically for early childhood. Therefore, the purpose of this study were examine the development of memory of Korean children and also to see the memory span of Korean children. the subjects in this study were 80 children (male and female each 40 children) from age 3 to age 6. These subjects were devide into two age groups (3~4 age, 5~6 age). the stimulus materials were the numbers and the words. The numbers in this study were chosen from the number problems of WISC. The 42 words which consisted of two phonemes were selected. The number of items was 3 to 9 on the first through the last trial. altogether 14 trials, 7 trials for numbers and 7 trials for words, were given to each subject. Free recall method was sued. The following conclusions could be made. 1. The memory span of Korean early childhood increased as the age of the children increased. 2. The primacy effect was appeared when the age increased. Therefore, rehearsal was appeared as the age of the Korean early childhood increased.

• Journal of Preventive Medicine and Public Health
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• v.45 no.4
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• pp.227-234
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• 2012

Objectives: The object of this study was to determine the relationship between body mass index (BMI) and early menarche in adolescent girls in Seoul. Methods: A retrospective study was conducted with 144 middle school students in Seoul who provided informed consent. We measured their body composition, and used the questionnaire survey method for data collection from November to December 2008. Past elemental body composition data were collected from elementary school health records of first year of middle school. Results: The early menarcheal group was taller and heavier than the late menarcheal group (p<0.05 from 8-12 years old). The body fat percentage (%), BMI were higher in the early menarcheal girls than the late-menarcheal girls (p<0.05, age at 13). In the result of multiple logistic regression, the BMI at the age of 8 and 9 was associated with early menarche after adjusting for birth weight, breast feeding and age at menarche of the mother (BMI at the age of 8: p for trend=0.01, BMI at the age of 9: p for trend=0.04). An increase in BMI from 7 to 8 year was associated with early menarche after adjusting for birth weight, breast feeding, age at menarche of the mother (p for trend=0.048). Conclusions: The BMI at the age of 8 and 9 was associated with the early menarche of girls and increase in BMI from 7 to 8 year was associated with the early menarche of girls. These results suggest that BMI and increase in BMI before menarche cause early menarche. Although this study does not represent all Korean adolescent girls, it is one of the few studies that have investigated the temporal relationship between BMI and early menarche.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.269-272
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• 2004

After concrete casts, temperature decent and shrinkage bring volume changes of concrete pavement. Microcracking and cracking in concrete pavement are caused by these volume changes. As a result, durability of concrete pavement is deteriorated. Recently, Very-Early Strength Latex Modified concrete(below:VESLMC) from the beginning of High-Way is used as urgent repair material for bridge deck. The advantage of VESLMC is that compressive and flexural strength at 3 hours age are 4.5MPa and 21MPa respectively. It allows the traffic to open in 3 hours. But, this material has the problem which is early-age shrinkage cracking caused by water self-dissipation with rapid hydration reaction and water evaporation with body dry. Unfortunately, until now, the research about early-age shrinkage of VESLMC leaves something to be desired. Therefore, the purpose of this study is to present the early-age shrinkage of VESLMC respect to latex contents and shrinkage ratio to maximum length change that can help field engineers' skill. Latex contents of 0, 5, 10, 15, $20\%$ in standard of same workability in VESLMC are selected by experimental variables. After initial set, shrinkage value was measured with 10mm LVDT for 3 days. The results of maximum shrinkage ratio were 0.019, 0.017, 0.023, $0.027\%$ respectively.

• Korean Journal of Social Welfare
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• v.42
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• pp.169-198
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• 2000

This paper attempts to analyze the early retirement in the OECD countries and discuss implication of that in the old age policy in Korea. The increase of the early retirement in the almost all OECD countries is a common fact. Especially the rate of early retirement rapidly increased in the 1980s, mostly reflecting the high rate of unemployment and states' policies to reduce it. However, it varies across countries: the unemployment compensation pathway in France, the mixture of social assistance and private insurance in England, VUT in Netherland, the privatization of the early retirement in the U. S., and partial retirement and labor market policy in Sweden. The early retirement in the advanced countries contributes to de-institutionalization and de-standardization in life course model. It resulted in the erosion of the ordinary conception that the retirement was the beginning of the old age. And the last phase of life course became blurred. With respect to the problem of the early retirement, there is a big difference between Korea and the OECD countries. Above all, the retirement age is 55 years in many companies and the public pension is not universalized in Korea. Accordingly the policy for income security of the old age in Korea should be connected with social security policy such as the gradual extension of the retirement age and the expansion of the public pension and labor market policy such as job training for the old age, transformation of the seniority wage system etc.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.809-814
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• 2002

Recently, the properties of early-age concrete are increasingly important because these properties directly influence the behavior of early-age concrete structures including stress and cracking behavior. Nevertheless, the studies on early-age concrete are limited to strength and temperature development. The purpose of present study is to propose a simple and rational method which can predict the stress and strain behavior of young age concrete. A series of test have been done to measure the temperature development, strains and stresses in concrete members. The concept of equivalent age was used to define the degree of hydration and this degree of hydration was used to calculate the strength and elastic modulus. The critical degree of hydration and thermal expansion coefficient were calculated using experimental data. It is seen that the critical degree of hydration range from 0.05 to 0.11 based on the measuring method. The thermal expansion coefficient was calculated based on the measured non-mechanical strain and it is found that the coefficient decreases slightly with the increase of age. The consideration of critical degree of hydration in calculating stresses gives more accurate results. The present study provides useful method and data in evaluating early-age behavior of concrete structure.

• International Journal of Concrete Structures and Materials
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• v.2 no.1
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• pp.63-68
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• 2008

High performance concrete (HPC) is widely used in civil engineering due to its high durability and low permeability etc. Compared with ordinary concrete, HPC may develop much higher AS (autogenous shrinkage) at early age due to the relative low water cement (w/c) ratio and adding of mineral admixtures, which is one of the main reasons for early age micro-cracking of HPC structures. This paper studies the early age property of HPC columns under similar construction-site surroundings by embedded strain transducers. Results show that for HPC structure, early-age autogenous shrinkage especially within the first day after concrete pouring is pretty large. AS within the first day are 60% larger than those for 14 days in this research for all specimens. Therefore it should be taken into account for structure durability. By comparison of PHPC (plain HPC column) and RHPC (reinforced HPC column) specimens, the effects of reinforced bars on AS and temperature distribution have been analyzed. Also the influence of w/c ratio on AS is demonstrated.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.05a
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• pp.189-194
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• 2002

Many researches in the past have shown that a majority initial cracking in concrete are caused during early age period. Therefore, the close examination of early age concrete behavior under various stress conditions is necessary to fully understand the cracking mechanism of concrete. In this study early age cement paste specimen is axially strained up to 20% of its original length by laterally reinforcing it. This type of test is called "Tube Squash Test" and has been previously used to apply up to 50% axial strain on concrete. Microscopic analyses (XRD, FESEM, EDS and DSE/TG) are performed on 20% axially strained early age cement paste specimen. The analysis results show that the microscopic structures and material characteristics of 20% axially strained cement paste remained same as the unstrained cement paste.

• Korean Journal of Materials Research
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• v.23 no.12
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• pp.687-694
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• 2013

This study was performed with an aim to improve the early-age strength of concrete containing fly ash, which is known to increase the long-age strength of concrete, reduce drying shrinkage, and enhance water tightness. The composition was partially substituted with calcium sulfoaluminate (CSA), from which ettringite is actively produced, in the early stages of hydration to verify its effect on improving the early-age strength and to determine the optimal mixing ratio. For this purpose, up to 30 % of the cement weight was substituted with fly ash, and the amount of CSA substitution was 8% of the fly ash weight. The mixtures were then fabricated into concrete specimens for compressive strength measurement and analysis of the correlation between the hydration products and the compressive strength.

• Proceedings of the KSR Conference
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• 2008.11b
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• pp.1065-1068
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• 2008

Concrete is still one most common construction materials even in railway structures. As structures become massive and mega-sized, the importance of early age concrete quality control becomes more significant. Among various factors, relative humidity and temperature are the primary factors governing the early age quality. Temperature raise due to cement hydration causes stress, which can develop to cracking with internal and/or external restraints. Exposure conditions including ambient temperature, humidity and wind also significantly affect the cracking behavior of early age concrete. Among many of studies on the early age concrete behavior, investigation on the variation of temperature and relative humidity internal of concrete is not common. That is in part because the difficulties in measuring the relative humidity and temperature inside the concrete. This study used a digital sensor with an appropriate logger to measure internal temperature and relative humidity. This direct measuring method is expected to provide more reliable and comprehensive data acquisition on the early age behavior of concrete.