• Advances in concrete construction
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• v.11 no.4
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• pp.335-345
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• 2021

It is known from the literature that there are relatively few studies on the engineering properties of ultra-high performance concrete (UHPC) in early age. In fact, in order to ensure the safety of UHPC during construction and sufficient durability and long-term performance, it is necessary to explore the early behavior of UHPC. The test parameters (test control factors) investigated included the percentage of cement replaced by silica fume (SF), the percentage of cement replaced by ultra-fine silica powder (SFP), the amount of steel fiber (volume percent), and the amount of polypropylene fiber (volume percentage). The engineering properties of UHPC in the fresh mixing stage and at the age of 7 days were investigated. These properties include freshly mixed properties (slump, slump flow, and unit weight) and hardened mechanical properties (compressive strength, elastic modulus, flexural strength, and splitting tensile strength). Moreover, the effects of the experimental factors on the performance of the tested UHPC were evaluated by range analysis and variance analysis. The experiment results showed that the compressive strength of the C8 mix at the age of 7 days was highest of 111.5 MPa, and the compressive strength of the C1 mix at the age of 28 days was the highest of 128.1 MPa. In addition, the 28-day compressive strength in each experimental group increased by 13%-34% compared to the 7-day compressive strength. In terms of hardened mechanical properties, the performance of each experimental group was superior to that of the control group (without fiber and without additional binder materials), with considerable improvement, and the experimental group did not produce explosive or brittle damage after the test. Further, the flexural test process found that all test specimens exhibited deflection-hardening behavior, resulting in continued to increase carrying capacity after the first crack.

• Journal of the Korea Concrete Institute
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• v.22 no.2
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• pp.237-245
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• 2010

Recently, deflection of the slab during construction periods becoming one of the important issues because of increasing the large-span structures. Early removing the form and support of the slab to achieve the rapid construction cause falling-off in quality of the structures. To reduce these deterioration and make rapid construction, construction of strength and stiffness gain model is needed by the research about the early-age concrete properties. Previous research results indicated that concrete model in existing design codes could not provide the mechanical properties of early age concrete. This paper carried out the concrete compressive strength tests on the curing age at early age stage. Evaluation of the accuracy of compressive strength and modulus of elasticity gain formula in existing various design codes was performed based on this test results, and new design model was proposed. This new model will be useful to develop the new rapid construction methods or prevent the deterioration of the deflection at construction periods. Material tests were performed at 1, 3, 7, 14, 28 curing days, total 159 cylinder style specimens were tested. Based on analyzing the test results, the relationship between compressive strength and modulus of elasticity at early age was proposed.

• Journal of the Korean Society of Child Welfare
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• no.54
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• pp.79-111
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• 2016

The purpose of this study was to examine the structural models in which early cumulative risk factors affect children's language(indicated by expressive vocabularies) and social development(indicated by peer competence) at age 3 thorough their effects on the home learning environment. To examine the hypothesized models, the data of 1,725 families from the second and the fourth waves of the Panel Study of Korean Children was used. Correlation analysis and structural equation modeling were conducted to test the models. First, the cumulative risk factors at age 1 and 3 were highly correlated, implying the stability of the risk factors over time. The more cumulative risk factors at age 1 predicted the lower level of the home learning environment at age 3, which, in turn, was significantly related to both language and social development at age 3. However, the early cumulative risk factors did not directly influence later developmental outcomes. Moreover, the cumulative risk factors at age 3 were directly related to the child's language development, but neither social development northe home learning environment. In addition, the mediational role of the home learning environment (i.e., cumulative risk factors at age 1${\rightarrow}$home learning environment${\rightarrow}$language and social development) was statistically supported. In conclusion, the early cumulative risk factors in infancy indirectly predicted children's development at age 3 through the home learning environment. The practical implications for the early intervention and support for the families with infants who are experiencing multiple risk factors were discussed.

• Korean Journal of Comparative Education
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• v.26 no.4
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• pp.153-172
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• 2016

This study aims at comparing and analyzing early childhood education systems in both South and North Koreas and suggesting educational alternatives for integrating in various aspects based on it, in order to build directions of early childhood education system in the preparation of Unification. A comparative study on the early childhood education system in South and North Korea was done in terms of early childhood education, public education of early childhood. As a result, there were differences rather than similarities in many areas due to the differences of political ideology between the South and the North. First, early childhood is not compulsory, but becomes public in South, while it is partially compulsory and has an overall public system in North in terms of its compulsory education and Pulic education. Second, as a result of comparing operating systems depending on early childhood education institutions in the South and North, there were differences in various areas such as subjects of foundation, classification of age, operating types, etc. Third, looking at its administrative support system, two Koreas has a similarity of a overall dual system, but showed a specific difference. Fourth, its public support had a similarity that early childhood education is done free of charge in two Koreas. In light of the results of comparing the early childhood education system in South and North Korea, this study suggests the following implications. First, as part of an effort to restore similarities of the early childhood education system between two Koreas, a challenge that must be addressed with priority is to integrate early childhood education-nursery in the early childhood education for age 3-5. Second, we'll take an effort to include the early childhood education for age 3-5 in the basic disciplines with 'Making early childhood education public' being currently focused. In addition, we should an attitude to recover a similarity through mutual exchange and to have advantages of the early childhood education in two Koreas as a challenge of taking an effort to integrate the early childhood education in the South and the North various studies and debates, discussions will be made for the meaning of the early childhood education and the integration.

• PNF and Movement
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• v.16 no.2
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• pp.239-248
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• 2018

Purpose: This study aimed to examine the elderly physical fitness test for elderly people living in urban areas and to evaluate the physical fitness level according to gender and age groups. Methods: The subjects were 180 elderly people aged 65 and over living in urban areas. The elderly people were classified into early old age, middle old age, and advanced old age groups. Five items of the physical fitness test for the elderly (arm curl, chair stand, 2 min step test, back scratch, and chair sit and reach). Results: In analyzing the average of each item by gender and age, a significant difference was found in the results of each item as age increased in all sports, but no difference was observed according to gender except dumbbell lifting. Conclusion: Although the physical fitness level of all ages decreased from early age to late age, the difference in physical fitness according to gender was not significant except the arm curl test. The results of this study can be used as basic data for a new "age-specific exercise program" for the elderly.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.391-396
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• 2002

The so called setting is defined as the onset of rigidity in fresh concrete. In the analysis of the early-age concrete behavior, we consider fresh concrete as a structural element immediately after mixing. But for the activation of real structural behavior of fresh concrete, it takes some time after the beginning of hydration reaction. So, the very early age deformations due to hydration heat and shrinkage which occur before the setting do not produce restraint stresses. In this paper, we propose a setting characteristic model based on the so called percolation theory. From the analysis using the model, the influence of curing temperature is investigated and analytical results are compared with experimental results. From the comparison, the validity of proposed model is verified. This model is also applied to evaluate stress development in a temperature-stress test machine (TSTM) specimen and then the effect of setting time on the stress development is discussed.

• International Journal of Concrete Structures and Materials
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• v.18 no.2E
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• pp.73-77
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• 2006

This paper shows a study of the efficiency of expansive additive and shrinkage reducing admixture in controlling restrained shrinkage cracking of high performance concrete at early age. Free autogenous shrinkage test of $100{\times}100{\times}400mm$ concrete specimens and simulated completely-restrained test with VRTM(variable restraint testing machine) were performed. Creep and autogenous shrinkage of high-performance concrete with and without expansive additive and shrinkage reducing admixture were investigated by experiments that provided data on free autogenous shrinkage and restrained shrinkage. The results showed that the addition of expansive additive and shrinkage reducing admixture effectively reduced autogenous shrinkage and tensile stress in the restrained conditions. Also, it was found that the shrinkage stress was relaxed by 90% in high-performance concrete with and without expansive additive and shrinkage reducing admixtures at early age.

• 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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• 2002.05a
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• pp.143-148
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• 2002

Recently, to consider economical and constructive aspect, Usage of Admixture, like Blast-Furnace Slag and Fly-Ash, are increased. Blast-Furnace Slag, a by-product of steel industry, have many advantage, to reduce the heat of hydration, increase in ultimate strength and etc. But it also reduces early-age strength, it is prevented from using of Blast-Furnace Slag at cold-weather-concrete. In this study, for the purpose of increasing usage of Blast-Furnace Slag at cold-weather-concrete, it is investigated the strength properties of concrete subjected to time and period of frost damage for early age curing. According to this study, if early age curing is carried out before having frost damage, the strength of concrete, subjected to frost damage, is recovered. And that properties is not connected with the frost cause.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.05a
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• pp.806-809
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• 2004

The early-age cracks have bad effects on the diffusion movement of chloride ions and oxygen. In this study, a corrosion analysis algorithm for cracked concrete is proposed to examine the influence of early-age cracks on corrosion of RC structures. For different environmental exposure conditions of RC structures, a corrosion model is combined with models for activation polarization and concentration polarization. From the finite element corrosion analysis using the proposed algorithm and the models, the effects of early-age cracks to the corrosion is simulated.