• Journal of the Korea Concrete Institute
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• v.23 no.3
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• pp.295-301
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• 2011

Recently, the use of UHPC made of superplasticizers, silica fume, and steel fibers has been increasing worldwide. Although UHPC has a very high strength as well as an excellent durability performance due to its dense microstructures, earlyage cracks may occur due to the high heat of hydration and autogenous shrinkage caused by low W/B and high unit cement content. The early-age shrinkage cracking of UHPC can be controlled by using the shrinkage reducers and expansive admixtures having autogenous shrinkage compensation effect. In this paper, ultrasonic pulse velocity of UHPC containing shrinkage reducers and expansive agents was measured to predict its stiffness change. Also, the effect of shrinkage reducers and expansive agents on the autogenous shinkage of UHPC was investigated through the shrinkage test of UHPC specimens. Furthermore, the material coefficients of autogenous shrinkage prediction model were determined using the autogenous shrinkage values of UHPC with age. Consequently, the test results showed that, by adding shrinkage reducers and expansive agents, the stiffness of UHPC was rapidly developed at early-ages and the autogenous shrinkage was considerably reduced.

• Journal of the Korean Recycled Construction Resources Institute
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• v.3 no.4
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• pp.307-312
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• 2015

This paper addresses mechanical properties and length change performance of the recycled aggregate concretes(RAC) in which natural coarse was replaced by recycled coarse aggregate(RCA) by compressive strength levels(20, 35, 50 MPa). A total of 9 RAC were produced and classified into three series, each of which included three mixes designed with three compressive strength levels of 20 MPa, 35 MPa and 50 MPa and three RCA replacement ratios of 0, 50 and 100%. Physical/Mechanical properties of RAC were tested for slump test, compressive strength, and length change. The test results indicated that the workability of RC could be improved or same by RCA replacement ratios, when compared with that containing no RCA. This is probably because of the RCA shape improving the workability of RAC. Also, the test results showed that the compressive strength was decreased by 9~10% as the RCA replacement ratios increase. However, the length change ratio by the RCA replacement ratios increased regardless of compressive strength levels. At 20 MPa level, the length change ratio was 8~40% which was much higher than that of 4~17% at both 35 and 50 MPa levels. Therefore, it was considered that such admixture addition preventing dry shrinkage is required in order to improve the properties of the RAC at 20 MPa level.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.1
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• pp.128-135
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• 2011

In this study, in order to develop ultra rapid hardening mortar(URHM) for tunnel repairs using bottom ash of low recycle ratio and Admixture as Eco concept, fundamental properties of URHM on temperature condition of construction field were performed. Test result, URHM of three types for fluidity and setting time were as in the following : B > C > A. Those for low temperatures were later than the standard condition. Compressive, bending and bond strength were similar with three types as follow. In compressive strength, initial strength of the low were smaller than the standard but the low in the long-term were similar with the standard. On the contrary to this, bending strength were similar in initial strength but the low in the long-term were smaller than the standard. The low in bond strength was average 35% less than the standard. Length changes was as in the following : A > C > B. the low is two times much as the standard but the case using blast furnace slag particles noticeably reduced length changes. Water absorption coefficient and water vapor resistance were as in the following : C > A > B. In case of URHM added bottom ash, water absorption coefficient and water vapor resistance were increased because bottom ash is porous material.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.853-856
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• 2008

Lightweight foamed concretes are mainly used in apartment building construction for building room floor insulation, sound proof and height difference adjustment, etc. However, existing lightweight foamed concretes have problems like volume reduction by foam removal and excessive crack occurrence, etc, and for compensation, they developed improved concrete binders for lightweight foamed concrete with special characteristics by adding admixture materials used in concrete manufacturing. Therefore, this study reviewed the possibility of its practical use by analyzing all the engineering characteristics after producing imitation member proposed as actual binders and piling lightweight foamed concrete as improved lightweight foamed concrete binder through prior study, the results are as follows. Plain in which various pulverulent materials are mixed showed about 230mm of flow value, satisfying the target flow value, and at 100mm member, about 4mm of settlement occurred, showing a settlement depth reduction effect double the OPC. On strength, OPC showed highest value, but the three levels all showed strengths above the specified value of KS standard 0.5 grade. From the analysis of drying shrinkage member crack, plain, about 0.1mm, was shown very excellent against drying shrinkage crack.

• Maxillofacial Plastic and Reconstructive Surgery
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• v.22 no.1
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• pp.70-75
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• 2000

The cemento-ossifying fibroma is a well-demarcated and occasionally encapsulated neoplasm composed of fibrous tissue that contains varying amounts of calcified tissue resembling bone, cementum, or both. The greatest number of cases are encountered during the third and fourth decades of life and there is a definite female predilection, with female-to-male ratios as high as 5 : 1 being reported. The mandible is involved far more than the maxilla, and 90 percent of all cases are located in the mandible. The mandibular premolar-molar area is the most common site. Radiographically, the lesion is most often well defined and unilocular. Depending on the amount of calcified produced in the tumor, it may appear as completely radiolucent ; more often it shows varying degrees of radiopacity. Some lesions may be largely radiopaque with a radiolucent halo. On microscopic study, the tumor is composed of fibrous tissue of varying degrees of cellularity containing calcified material. This may be in the form of trabeculae of osteoid and bone or basophilic ovoid calcifications that resemble cementum-like material. Admixture of the two types of calcifications are commonly seen. On this case, large cemento-ossifying fibroma has grown in the mandible and this lesion was treated by complete surgical excision and posterior iliac bone graft. A review of literature and a report of a case of cemento-ossifying fibroma are presented.

• Korean Journal of Environment and Ecology
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• v.26 no.4
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• pp.475-483
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• 2012

Tribolodon hakonensis(Cypriniformes; Leuciscinae) is anadromous; they are born in freshwater, migrate back to the ocean, then return to their home stream for spawning from mid-March to early-June. Here, five microsatellites were used to assess the level of gene flow among T. hakonensis populations from the Samcheok-Oship Stream, South Korea. The frequencies of dominant alleles across several loci differed between down-and upstream populations divided by several weirs, and pairwise multilocus $F_{ST}$ estimate was significantly high(0.083). However, there were no signs of any loss of genetic variation in the upstream population. Assignment tests of individuals in admixture model(K=2) to a set of baseline samples showed fairly correct assignment to each cluster; all of upstream individuals sere included in the first cluster, while the majority of downstream individuals(65%) comprise the second cluster. These results indicate reduced gene flow between up- and downstream populations but allowing passive downstream drift. It is likely that man-made structures might at least partially be a factor for creating and consolidating the current distribution patterns of genetic variation among T. hakonensis populations in the Samcheok-Oship Stream. This information will assist governing agencies in making informed decisions regarding conservation of anadromous fishes in Korean drainage systems.

• Journal of the Korea Concrete Institute
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• v.17 no.4 s.88
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• pp.543-550
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• 2005

In the present work, modulus of rupture (MOR), flexural toughness properties $(I_{30}\;and\;W_{2.0})$ and workability (slump) of high performance hybrid fiber reinforced concrete (HPHFRC) mixed with micro-fiber (carbon fiber) and macro-fiber (steel fiber), and replaced with a fine mineral admixture such as silica fume (SF) are characterized through the analysis of variance (ANOVA). Data of MOR, $I_{30}(or W_{2.0})$ and slump are used as the characteristic values to estimate flexural performance and workable property of HPHFRC. Specially, an experimental design was Planned according to the fractional orthogoanl nay method to reduce experimental number of times. The experimental results show that steel fiber is a considerable significant factor in MOR and I30 $(W_{2.0})$. Based on the significance of experimental factors about each characteristic factors, the following evaluation can be used: Experiment factors which reduce slump most remarkably are carbon fiber, steel fiber, silica fume order.; Those that improve MOR most significantly are silica fume $({\fallingdotseq}\;carbon\;fiber)$, steel fiber order; Those that increase flexural toughness most distinctly are silica fume, carbon fiber, steel fiber order. It is obtained that the combination of steel fiber $1.0\%$, carbon fiber $0.25\%$ and silica fume $5.0\%$ is the experimental condition that improve MOR and flexural toughness excellently with workability ensured within the experiment.

• Journal of the Korean Recycled Construction Resources Institute
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• v.4 no.2
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• pp.97-106
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• 2009

self-loading, various problems related to construction can be solved as well as the save of construction cost. Thus, this study has an aim of applying foam concrete to structural purpose by adding bottom ash as a reinforcing material like fine aggregate, in contrast to conventional non-structural usage such as soundproofing or insulating materials. In addition, it was evaluated in terms of unit volume weight, flow value, air void, water absorption and dosage of foam agent wether replacement of cement by granulated blast furnace slag or fly-ash has an effect on the material characteristics of foam concrete. As results of experiments, it can be found that the increase of fine aggregate ratio, that is to say, the increase of bottom ash results in the increase of unit volume weight, while decreasing air void and flow value. But, appropriate addition of bottom ash to foam concrete makes it easy to control a homogeneous and uniform quality in foam concrete due to less sensitive to bubbles. As the replacement ratio of mineral admixtures such as granulated blast furnace slag and fly-ash increases, as unit volume weight tends to decrease. In the meanwhile, serious effects were shown on fluidity of foam concrete when more than limit of replacement ratio was applied.

• Journal of the Korea Institute of Building Construction
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• v.17 no.3
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• pp.279-285
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• 2017

In this study, the development of concrete preventing initial frost damage and durability about that were evaluated by using anti-aging agent and admixture(reduced slag). As a result of experiment, initial hydration heat was increased by $C_{12}A_7$ of reduced slag components but it was not effective to development of strength. Also fluidity decreased with increasing replacement of reduced slag. This suggested that fluidity was low by rapid setting due to absent of gypsum in reduced slag components. In case of CR2G specimen that added 4% gypsum, the flow ability was higher than plain. It is considered that concrete developed using reduced slag should use $SO_3$. Result of durability experiments, the durability decreased with increasing replacement amount of reduced slag.

• Journal of the Korea Institute of Building Construction
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• v.17 no.3
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• pp.245-252
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• 2017

Recently, structures such as large retailers, outlets and warehouses have been increasing in accordance with changes in consumption patterns. Since these structures include ultra-flat slab members, they are thoroughly managed to control slab cracking by the plastic and drying shrinkage. In order to control the cracking of the slab member, a chemical crack reduction method is used. In particular, the use of the shrinkage reducing agent has been examined. However, domestic research results are limited. In this study, the shrinkage properties of concrete using shrinkage reducing agent and the drying shrinkage properties according to the mixing factors were investigated. The performance of domestic shrinkage reducing agent was appeared similar to that of overseas high-grade shrinkage reducing agent. As the shrinkage reducing agent usage increased, the drying shrinkage reduction effect increased. At the age of 100 days, the dry shrinkage rate of specimen with the shrinkage reducing agent of 1.5%was shown about half that of the specimen without the shrinkage reducing agent. The shrinkage reducing agent was gound to have no specific performance change for the use of the admixture.