• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.649-652
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• 2008

In this paper, the applicability of hwangtoh, as an alternative of cement paste, is investigated for the solution of internal heat and shrinkage caused by the hydration of cement paste. Several small-sized specimens of hwangtoh and ordinary portland concrete(OPC) were compared as to compressive strength, heat of hydration, and shrinkage strain. Moreover, the applicability of mass structure was reviewed through the test of large-size specimens. The 28-day compressive strength of hwangtoh concrete(HBC), ranged 18 to 33 Mpa, can reach that of ordinary portland concrete. Not only the maximin internal temperature of HBC was read about 1/4 of OPC as it is cured, but also its drying shrinkage decreased as lower as 50% of OPC starting from 60 days. Therefore, hwangtoh binder is more favorable than cement one in the view of hydration heat and shrinkage under the construction of mass structures.

• Journal of the Korean Recycled Construction Resources Institute
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• v.9 no.3
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• pp.322-329
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• 2021

Nano-cementitious composites may have defects due to poor dispersion of nanomaterials and fabrication process. These defects can cause critical problems for nano-cementitious composites, but studies related to non-destructive analysis of defects sizes inside nano-cementitious composites are insufficient. This study aims to perform non-destructive analysis of nano-cementitious composites by utilizing ultrasonic and electrical resistance. Various sizes of defects were implemented inside the specimens and the specimens were subjected to ultrasonic non-destructive analysis and electrical resistance non-destructive analysis depending on the size of defects and curing days. As a result of the experiment, ultrasonic pulse velocity decreased by up to 11% as the defects size increased, and the electrical resistance increased by up to 14% depending on the defects size. For this reason, this study concluded that non-destructive analysis using ultrasonic and electrical resistance can predict defects inside nano-cementitious composites.

• Journal of the Korea Concrete Institute
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• v.15 no.1
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• pp.43-51
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• 2003

Recently, to consider financial and constructive aspect usage of Admixture such as Blast-Furnace Slag and Fly-Ash, are increased. Also the use of cold-weather-concrete is 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, so 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 frost damage for the cause of early age curing. The factors of this experience to give early frost damaged were Freezing temperature(-1, -10, $-15^{\circ}C$), Early curing age(0, 12, 24, 48hour), Freezing times(0, 12, 24, 48hour). According to this study, if early curing is carried out before haying frost damage, the strength of concrete used admixture, subjected to frost damage, is recovered. And that properties are considered, the effect of using admixture like blast-furnace-slag, is very high

• Tunnel and Underground Space
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• v.19 no.3
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• pp.181-189
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• 2009

This study is to evaluate an effect of supports with respect to these supports after comparing the characteristic of support between rock bolt of a widely used type and spiral bolt of a new type. For these purposes, we performed pull-out test in laboratory about rock and spiral bolts in the case of cement-mortar grout curing periods, 7 and 28 days, then calculated pull-out load, displacement, external pressure, inner pressure and shear stress using data obtained from the results of pull-out test, respectively. In relation between pull-out load and displacement, displacement of spiral bolt is larger than one of rock bolt. It is considered that mechanical property of rock bolt is due to larger than one of spiral bolt. In addition, displacement of supports shows nearly same or decreasing with curing periods. We found that because adhesive force between supports and cement-mortar grout is increasing with compressive strength of grout according to curing periods. The inner pressure of spiral bolt is represented larger than one of rock bolt at a step of same pull-out load. It is suggested that spiral bolt is more stable than rock bolt, maintaining stability of ground or rock mass, when supports are installed in a ground or rock mass under the same condition. Putting together with above results, we can consider that spiral bolt as a new support on an aspect of pull-out load and inner pressure is larger than rock bolt in a ground or rock mass under the same condition. Moreover, spiral bolt is more effective support than rock bolt, considering an economical and constructive aspects of supports, as well as ground or rock stability before or after installing supports.

• Journal of the Korean Geotechnical Society
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• v.31 no.3
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• pp.5-16
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• 2015

The characteristics of tensile strength of fiber-reinforced grouting (cement paste) injected into rocks or soils were studied. A tensile strength of such materials utilized in civil engineering has been commonly tested by an indirect splitting tensile test (Brazilian test). In this study, a direct tensile testing method was developed with built-in cylinder inside a cylindrical specimen with 15 cm in diameter and 30 cm in height. The testing specimen was prepared with 0%, 0.5%, or 1% (by weight) of a PVA or steel fiber reinforced mortar. A specimen with 5 cm in diameter and 10 cm in height was also prepared and tested for the splitting tensile test. Each specimen was air cured for 7 days or 28 days before testing. The tensile strength of built-in cylinder test showed 96%-290% higher than that of splitting tensile test. The 3D finite element analyses on these tensile tests showed that the tensile strength from built-in cylinder test had was 3 times higher than that of splitting tensile test. It is similar to experimental result. As an amount of fiber increased from 0% to 1%, its tensile strength increased by 119%-190% or 23%-131% for 7 days or 28 days-cured specimens, respectively. As a curing period increased from 7 days to 28 days, its strength decreased. Most specimens reinforced with PVA fiber showed tensile strength 14%-38% higher than that of steel fiber reinforced specimens.

• Journal of the Korea Concrete Institute
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• v.14 no.1
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• pp.118-125
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• 2002

Hydration is the main reason for the growth of the material properties. An exact parameter to control the chemical and physical process is not the time, but the degree of hydration. Therefore, it is reasonable that development of all material properties and the formation of microstructure should be formulated in terms of degree of hydration. Mathematical formulation of degree of hydration is based on combination of reaction rate functions. The effect of moisture conditions as well as temperature on the rate of reaction is considered in the degree of hydration model. This effect is subdivided into two contributions: water shortage and water distribution. The former is associated with the effect of W/C ratio on the progress of hydration. The water needed for progress of hydration do not exist and there is not enough space for the reaction products to form. The tatter is associated with the effect of free capillary water distribution in the pore system. Physically absorption layer does not contribute to progress of hydration and only free water is available for further hydration. In this study, the effects of chemical composition of cement, W/C ratio, temperature, and moisture conditions on the degree of hydration are considered. Parameters that can be used to indicate or approximate the real degree of hydration are liberated heat of hydration, amount of chemically bound water, and chemical shrinkage, etc. Thus, the degree of heat liberation and adiabatic temperature rise could be determined by prediction of degree of hydration.

• Journal of the Korea institute for structural maintenance and inspection
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• v.15 no.6
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• pp.184-194
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• 2011

This study is to analyze the internal temperature and relative humidity of concrete at early age, as well as the mechanical behavior. Three different levels of cement unit content were cosidered as an experimental variable. In order to measure internal temperature and relative humidity immediately after concrete placement, this study developed a unique measuring device, which provided reliable results. Different cement content did not significantly affected the strengths including compressive, tensile and flexural strength and after 7 days of curing, strengths did not increase noticeably. Internal temperature reached the maximum about 11 hours later the placement and decreased after removal of forms. The internal temperature varied depending on the location and the exposure condition. In addition, the internal relative humidity was more affected by the exposure condition rather than the cement content.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.12
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• pp.7-13
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• 2018

The strength of concrete is affected significantly not only by the internal influence factors of cement, water, sand, aggregate, and admixture, but also by the external influence factors of concrete placement delay and curing temperature. The objective of this research was to predict the concrete strength considering both the internal and external influence factors when concrete is placed at the construction site. In this study, a concrete strength test was conducted on the 24 combinations of internal and external influence factors, and a neural network model was constructed using the test data. This neural network model can predict the concrete strength considering the external influence factors of the concrete placement delay and curing temperature when concrete is placed at the construction site. Contractors can use the concrete strength prediction neural network model to make concrete more robust to external influence factors during concrete placement at a construction site.

• 한국정보통신설비학회:학술대회논문집
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• 2007.08a
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• pp.315-319
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• 2007

지하에 설 치된 맨홀의 구조물이 노후되거나, 내부에 수 용된 시설물로 작업 공간이 협소하여 맨홀 구조물을 대개체 할 펼요가 있을 경우, 지금까지는 대부분 현장 콘크리트 타설에 의한 방법을 사용하였다. 이러한 방법은 거푸집 시공, 콘크리트 타설, 콘크리트 양생, 거푸집 철거 등의 공정이 소요되어 공사 기간이 장기화 되고, 도심지에 위치한 맨홀인 경우에는 차량과 보행자의 통행 불편을 초래하기도 하였다. 또한, 통상적으로 사용되고 있는 프리캐스트 맨홀에 비해 시공 품질이 낮아 향후 맨홀의 운용 및 유지관리에 많은 어려움을 겪는 원인이 되기도 하였다. 본 논문에서는 이러한 문제점들을 해결하기 위해, 폴리머 콘크리트를 이용하여 하부, 벽체, 상부로 구성된 다수의 세그먼트를 생산한 후, 현장에서 이들 세그먼트를 조립하여 맨홀을 설치하는 방법을 연구하였다. 가변형 거푸집을 이용한 세그먼트 제작과 조립식 블록에 의한 관구를 사용하여 현장 적용성을 높임은 물론, 프리캐스트로 생산되는 폴리머 콘크리트 맨홀 수준의 시공 품질을 확보할 수 있도록 하였다.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05b
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• pp.839-844
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• 1995

처분장 구조물의 주요방벽 재료중의 하나인 콘크리트에서의 핵종확산계수를 측정하기 위하여 내부확산법을 적용하였다. 핵종은 양이온과 음이온을 대표하는 Cs 과 I 이었으며, 고체 시료는 양생된 순수시멘트 또는 시멘트와 모레를 섞은 몰타르로 물과 시멘트의 비 (W/C) 를 변화시켰다. 용액은 표준 제조방법에 따라 제조된 합성지하수이었다. Curve fitting 법을 이용한 모델과 실험의 비교로부터 핵종의 확산계수를 계산하였다. 순수시멘트의 경우 Cs 의 겉보기 확산계수는 1.7 $\times$ $10^{-12}$~1.4 $\times$ $10^{-11}$ m$^2$/sec 의 범위로 W/C 가 증가할수록 컸으며 I 는 2.9 $\times$ $10^{-14}$ ~ 1.0 $\times$ $10^{-l3}$ $m^2$/sec의 범위로 또한 W/C 에 따라 증가하였다. 몰타르의 경우 W/C 가 클수록 Cs 의 확산계수는 증가하였으나 I 에 대해서는 큰 변화를 보여주지 않았다 본 실험에서 얻어진 핵종의 유효확산계수값이 $10^{-13}$ ~ $10^{-10}$ m$^2$/sec의 범위를 갖는 것으로 보아 공극확산이 핵종확산의 지배 메커니즘임이 예상되었다.