• 한국건축시공학회지
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• 제15권3호
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• pp.265-271
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• 2015

본 연구는 재령 6시간에 압축강도 10MPa 이상을 만족하는 조강형 콘크리트를 개발함에 스팀양생 공정을 생략한 PC(Precast Concrete)부재를 생산하고자 하였으며, 거푸집의 회전율을 높임에 경제성을 확보하는데 목적이 있다. 이에, $C_3S$의 함유량이 높은 조강시멘트와 $C_3S$의 수화작용을 촉진시키는 분말형 경화촉진제를 사용하였으며, 경화촉진제 혼입율 0%, 1.2%, 1.6%, 2.0%에 따른 콘크리트의 특성을 평가하였다. 경화촉진제 혼입율에 따라 슬럼프 및 공기량 시험에는 큰 차이를 나타내지 않았으며, 1.6%이상을 혼입하였을 때, 재령 6시간에 10MPa를 만족하였다. 자기(건조)수축 길이변화 시험 및 소성수축 시험결과, 경화촉진제의 혼입으로 수화반응 속도에 따른 차이는 있지만, 균열 및 내구성에 대한 문제가 발생되지 않을 것으로 나타났으며, 동결융해 및 탄산화 침투에 대한 저항성도 우수한 것으로 나타났다.

• 한국건축시공학회지
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• 제14권2호
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• pp.156-162
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• 2014

본 연구는 PC(Precast Concrete)부재 생산 시 스팀양생 공정을 생략하여 경제적 및 환경적 문제를 해결하고자 상온양생으로 탈형강도(10MPa이상)를 만족시킬 수 있는 조강형 콘크리트를 개발하는데 목적이 있다. 따라서, 조기강도 발현에 반응성이 높은 $C_3S$를 많이 함유하고 있는 조강 시멘트와 $C_3S$의 수화반응을 높이는 경화촉진제를 사용한 조강콘크리트를 평가하였으며, 경화촉진제를 사용한 콘크리트 실험결과는 다음과 같다. 유동성 확인을 위한 슬럼프 플로 시험과 공기량은 목표값을 만족하였다. 경화촉진제를 혼입함에 있어 압축강도는 12시간까지 급격한 발현성상을 나타내었으며, 6~9시간 만에 목표를 만족하였다. 건조수축 및 자기수축의 최대 수축량은 ($-800{\times}10^{-6}$)이하의 값을 나타내어 양호한 것으로 나타났다. 또한, 간이단열온도상승 시험으로 24시간 이내에 Peak 온도를 나타내고 감소하는 것을 확인하였다.

• 환경위생공학
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• 제18권4호
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• pp.36-44
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• 2003

The cement-based system among S/S(Solidification/Stabilization) is widely used to treat hazardous wastes. In this study, tannery sludge was solidified to evaluate the stabilization effects of using admixtures in the cement-based S/S. Fly ash as substitute also used to increase the strength of the S/S of hazardous waste. The compressive strength measurement and leaching experiment of chromium metal of solidified mortar were carried out to compare and evaluate the physical and chemical characteristics of solidified hazardous waste sludge. From the result of this study, there was increased of compressive strength by using AEW-3(early-hardening AE water reducing agent), and leaching concentration of chromium became low enough to satisfy the regulatory criteria. The successful solidification for the organic contaminant and heavy metal in hazardous waste should enable to treat by cement-based system using early-hardening AE water reducing admixture and fly ash as substituted cement.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2001년도 가을 학술발표회 논문집
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• pp.245-250
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• 2001

Ultrasonic pulse velocity(UPV) is a useful tool for examining the property of early-age mortar or concrete. Thus, UPV has been used for a long time to characterize setting and hardening of cementitious materials. In this study, in order to investigate the characteristics of setting for mortar, UPV was measured using automatic monitoring system up to 3 days after casting. Test results show that UPV of high water to binder ratio(w/b) mortar remained constant at the beginning of hydration and then abruptly began to increase. However, UPV of low w/b mortar gradually increase due to setting retard caused by use of superplasticizer. Furthermore, the development of UPV for mortar with fly ash is slower than that of mortar without fly ash. It was concluded that the property change of mortar or concrete, such as setting and hardening can be assessed by monitoring of UPV.

• 한국건축시공학회지
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• 제14권3호
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• pp.273-284
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• 2014

Ultra-high-performance concrete (UHPC) consists of cement, silica fume (SF), sand, fibers, water and superplasticizer. Typical water/binder ratios are 0.15 to 0.20 with 20 to 30% silica fume. In the production of ultra-high performance concrete, a significant temperature rise at an early age can be observed because of the higher cement content per unit mass of concrete. In this paper, by considering the production of calcium hydroxide in cement hydration and its consumption in the pozzolanic reaction, a numerical model is proposed to simulate the hydration of ultra-high performance concrete. The heat evolution rate of UHPC is determined from the contributions of cement hydration and the pozzolanic reaction. Furthermore, by combining a blended-cement hydration model with the finite-element method, the temperature history in the hardening of UHPC is evaluated using the degree of hydration of the cement and the silica fume. The predicted temperature-history curves were compared with experimental data, and a good correlation was found.

• Applied Microscopy
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• 제45권2호
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• pp.37-43
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• 2015

In present work, work-hardening behavior of TiCu-based bulk metallic glass composite with B2 particles has been studied by systemic structural and mechanical investigations. After yield, pronounced work-hardening of the alloy was clearly exhibited, which was mainly related to the martensitic transformation as well as the deformation twinning in B2 particles during deformation. At the early plastic deformation stage (work-hardening stage), the stress-induced martensitic transformation from B2 phase to B19' phase and deformation-induced twinning of B19' phase was preferentially occurred in the around interface areas between B2 phase and amorphous matrix by stress concentration. The higher hardness value was observed in vicinity of interface within the B2 particles which are probably connected with martensitic transformation and deformation twinning. This reveals that the work-hardening phenomenon of this bulk metallic glass composite is a result of the hardening of B2 particles embedded in amorphous matrix.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2014년도 추계 학술논문 발표대회
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• pp.44-45
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• 2014

On this study, initial crack index was evaluated by performing FEM analysis to crack propagation from hydration heat for development of precast concrete. On the result, as increased the usage of hardening accelerator, initial compressive strength were improved and setting time also was shortened. Additionally, central temperature of concrete was increased, the reaching time for the highest temperature could be shortened. By the result to assess crack index, there was no problem about crack despite of growth of initial high hydration heating. This result guessed because of small size element when analyzed trough FEM, realization for mass concrete's crack index should be performed.

• Journal of Welding and Joining
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• 제22권4호
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• pp.35-42
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• 2004

Contact tips are required to have a higher resistance to wear and thus to have an extended life time under the advanced GMAW welding process. Several requirements have been specified and employed by domestic industries for selecting their tips for such a purpose. However no attempt has been made to justify their requirements based on the experimental data of wear resistance or life time of contact tips. In this study, five different contact tips with three different compositions were employed for actual GMA welding up to 4 hours and were evaluated their wear resistance by measuring in every one hour the area of enlarged hole at the exit side. Experimental results clearly showed that the Cr-containing tips strengthened by precipitation hardening have much better resistance to wear than those made by work hardening. It was further noticed that Cr is an excellent alloying element for improving the wear resistance of contact tips only when it is in an properly aged condition. Initial hardness may play some role in the early stage of wear but not in the later stage of welding because the microstructure of tip changes significantly by the prolonged exposure to welding arc heat. Based on these results, critical review has been made on the current requirements employed by domestic industries. Of importance is that a new guideline has been confirmed to be more reasonable.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2022년도 가을 학술논문 발표대회
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• pp.127-128
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• 2022

In order to realize carbon neutrality in the international society, research on supplementary cementitious materials(SCMs) has been actively conducted as a way to reduce carbon dioxide emissions in the cement industry. However, the use of SCMs causes problems of initial hydration delay and strength reduction due to the reduction of tricalcium silicate(C3S) in the cement clinker. Therefore, in this study, the initial hydration and basic characteristics of cement mortar were confirmed by adding a C-S-H based hardening acceleration to blended cement mixed with Portland cement, blast furnace slag, fly ash, and limestone power. As a result of the heat of hydration and compressive strength test, it was confirmed that when hardening acceleration was added, the initial reactivity was high, so the heat of hydration was promoted, and the initial strength was increased. It is considered to be due to C-S-H seeding effect. Therefore, it is judged that the use of C-S-H based hardening acceleration can supplement the problem of initial hydration delay of blended cement in Korea.

• 대한치과기공학회지
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• 제34권4호
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• pp.345-352
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• 2012

Purpose: The age-hardening mechanism of an Au-Ag-Cu-Pt-Zn alloy for crown and bridge fabrication was investigated by means of hardness test, X-ray diffraction study and field emission scanning electron microscopic observation. Methods: Before hardness testing, the specimens were solution treated and then were rapidly quenched into ice brine, and were subsequently aged isothermally at $400-450^{\circ}C$ for various periods of time in a molten salt bath and then quenched into ice brain. Hardness measurements were made using a Vickers microhardness tester. The specimens were examined at 15 kV using a field emission scanning electron microscope. Results: By the isothermal aging of the solution-treated specimen at $450^{\circ}C$, the hardness increased rapidly in the early stage of aging process and reached a maximum hardness value. After that, the hardness decreased slowly with prolonged aging. However, the relatively high hardness value was obtained even with 20,000 min aging. By aging the solution-treated specimen, the f.c.c. Au-Ag-rich ${\alpha}_0$ phase was transformed into the Au-Ag-rich ${\alpha}_1$ phase and the AuCu I ordered phase. Conclusion: The hardness increase in the early stage of aging process was attributed to the formation of lattice strains by the precipitation of the Cu-rich phase and then subsequent ordering into the AuCu I-type phase. The decrease in hardness in the later stage of aging process was due to the release of coherency strains by the coarsening of tweed structure in the grain interior and by the growth and coarsening of the lamellar structure in the grain boundary. The increase of inter-lamellar space contributed slightly to the softening compared to the growth of lamellar structure toward the grain interior.