• 열처리공학회지
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• 제24권4호
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• pp.193-198
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• 2011

Effect of CaO addition on age hardening response has been studied by using optical microscopy, scanning electron microscopy and differential thermal analysis in AZ91 and CaO-containing ECO-AZ91 alloys. After solution treatment, the ${\beta}$($Mg_{17}Al_{12}$) phase formed during solidification mostly disappeared in the microstructure in the AZ91 alloy, whereas numerous ${\beta}$ precipitates containing Ca were still observed in the ECO-AZ91 alloy due to its enhanced thermal stability. The ECO-AZ91 alloy showed the delayed peak aging time and higher peak hardness compared with those of the AZ91 alloy. The activation energies for ${\beta}$ precipitation calculated by means of Kissinger method increased from 71.4 to 85.6 kJ/mole by the addition of CaO, which implies that CaO plays a role in reducing ${\beta}$ precipitation rate in the AZ91 alloy.

• 한국주조공학회지
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• 제19권5호
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• pp.433-439
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• 1999

Rapidly solidified Mg-Al-Si base alloys containing Ca were obtained by melt spinning. The melt-spun ribbons were aged isochronally or isothermally to investigate age hardening phenomena and microstructural change according to the alloy composition. Age hardening occurred after aging at $200^{\circ}C$ for 1h mainly due to the precipitation of $Al_2Ca$ and $Mg_2Ca$, which have coherent interfaces with the matrix. With the increase of Ca content, the hardness values of the alloy ribbons were increased. Among the alloys, Mg-10Al-2 Si-3Ca alloy showed a good thermal stability at elevated temperature.

• 한국전산구조공학회논문집
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• 제24권4호
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• pp.391-398
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• 2011

철근콘크리트 구조물은 타설 후 시간이 경과함에 따라 물리적인 요인과 화학적인 요인으로 인하여 열화가 진행된다. 열화된 구조물의 효율적인 구조해석을 수행하기 위하여 구조물의 거동과 밀접한 관련이 있는 중요열화변수를 정의하는 것은 필요하다. 본 연구에서는 경년열화된 철근콘크리트 전단벽의 중요변수를 분석하기 위하여 일계이차모멘트법을 이용하여 민감도해석을 수행하였다. 콘크리트의 경화현상을 고려하지 않을 경우 구조물의 열화성능이 과소평가될 수 있으므로 콘크리트의 경화에 따른 해석변수들의 민감도를 분석하였다. 열화된 전단벽에서 변수의 중요도는 토네이도 다이어그램으로 나타내었다.

• Architectural research
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• 제14권4호
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• pp.153-161
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• 2012

This paper presents an integrated procedure to predict the temperature and moisture distributions in hardening concrete considering the effects of temperature and aging. The degree of hydration is employed as a fundamental parameter to evaluate hydro-thermal-mechanical properties of hardening concrete. The temperature history and temperature distribution in hardening concrete is evaluated by combining cement hydration model with three-dimensional finite element thermal analysis. On the other hand, the influences of both self-desiccation and moisture diffusion on variation of relative humidity are considered. The self-desiccation is evaluated by using a semi-empirical expression with desorption isotherm and degree of hydration. The moisture diffusivity is expressed as a function of degree of hydration and current relative humidity. The proposed procedure is verified with experimental results and can be used to evaluate the early-age crack of hardening concrete.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2014년도 추계 학술논문 발표대회
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• pp.135-136
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• 2014

In this study, the experimental study on the early age compressive strength of cement mortar using anti freezer and hardening accelerator at low temperature was conducted. For this study, all of materials for experiment were kept in a low temperature for 24 hours before mortar mixing. After mortar curing at low temperature, compressive strength was measured at the early ages. Furthermore, properties of hardened cement material was analysed using TG-DTA and MIP.

• 한국기계가공학회지
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• 제12권2호
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• pp.49-54
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• 2013

Recently welding of aluminum material is actively carried out to make lightweight in the fields of LNG vessels, aircraft, chemical plants, etc. To obtain high strength, hardness and elongation, elements such as manganese, zinc, silicon, etc should be added in aluminum alloy, which has been improved on the mechanical properties like precipitation hardening, age hardening, loosening, corrosion resistance acid resistance. Ar gas is used as a shielding gas of MIG welding for aluminum, also $N_2$, $O_2$, $CO_2$, $H_2$ etc can be added depending on the composition of the alloy. In this study, Ar + $O_2$, Ar, and He were used for welding, hardness, penetration status and changes in composition of penetrated parts were compared and analyzed. This made it possible to know the status and changes of the process in the penetrated parts depending on used gas throughout this study.

• 전자공학회논문지A
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• 제28A권12호
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• pp.31-38
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• 1991

The current-voltage characteristics at annealed Be-Cu alloy(1.8-2 wt% Be, 0.2 wt% Co+Ni) interfaces have been studied by means of the cyclic voltammetric method. The specimens were annealed in nitrogen gas($N_{2}$) furnace at 36$0^{\circ}C$ for 1.5 hours. After annealing, the vickers hardness(HV) was increased from 210 to 385. The used solutions were distilled water(H$_{2}$O), 10$^{-3}M\;CsNO_{2},10^{-2}M\;KCl,10^{-2}M\;KOH,10^{-4}M\;H_{2}SO_{4}$ aqueous electrolytes, and ethylalcohol ($C_{2}H_{5}OH$), etc. The cyclic voltammograms showed significant current-voltage characteristics between the annealed and unannealed specimens at the same conditions. The age hardening and the related surface potential barrer and dissolution effects have been observed during the whole experimental process. The dissolution process of annealed Be-Cu alloys was effectively retarded by the age hardening phenomenon. The age hardening effect also raised the surface potential barrier of Be-Cu alloys. The interfacial phenomena of Be-Cu alloys seem to be one of good models for understanding the activation process.

• 콘크리트학회논문집
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• 제28권3호
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• pp.309-316
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• 2016

본 연구에서는 21~27 MPa 수준의 콘크리트를 대상으로 하여, 양생온도 및 단위결합재량에 따른 초기강도 발현 특성을 검토한 후, 초기강도를 개선하기 위한 방안으로 시멘트의 응결 및 경화를 촉진할 수 있는 경화촉진제의 종류에 따른 성능을 검토하였다. 검토 결과 본 연구의 범위에서는 수용성 무기염이 유기염에 비해 상대적으로 경화촉진 효과가 우수하였으며, 사용량이 낮은 범위에서는 수용성 무기염에 의한 $C_3S$의 수화 반응촉진효과가 초기강도 발현에 효과적인 것으로 나타났다. 또한, 경화촉진제 중에서는 Pt(Potassium thiocyanate)의 촉진효과가 가장 우수하였으며, 양생온도 $15^{\circ}C$ 조건에서도 약 25시간에 초기동해 제어 강도 확보가 가능하여 공기단축 효과를 기대할 수 있고, 양생온도 $5^{\circ}C$에서도 양생온도 $10^{\circ}C$ 조건의 Plain과 유사한 시간에 5MPa 확보가 가능하여, 동일한 공기를 고려할 경우 연료비용 및 $CO_2$ 가스 발생 저감 등의 효과를 기대할 수 있을 것으로 판단된다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2005년도 봄학술 발표회 논문집(II)
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• pp.413-416
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• 2005

The super early hardening cement is widely used for reducing construction period. But there are some problems with handling the cement because the loss of workability is so big to control. In this study, the fluidity properties of super early hardening cement paste was evaluated at early age of hydration by using gel-time determination method. 4 types of set controlling agent were selected and combinations of them were used for gel-time test. As a result, the gel-time of super early hardening cement paste was extended up to 20 minutes by using the combinations of several types set controlling agent.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2015년도 춘계 학술논문 발표대회
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• pp.50-51
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• 2015

In marine and coastal environments, penetration of chloride ions is one of the main mechanisms causing concrete reinforcement corrosion. Currently, most of experimental investigations about submerged penetration of chloride ions are started after the four weeks standard curing of concrete. The further hydration of cement and reduction of chloride diffusivity during submerged penetration period are ignored. To overcome this weak point, this paper presents a numerical procedure to analyze simultaneously cement hydration reaction and chloride ion penetration process. First, using a cement hydration model, degree of hydration and phase volume fractions of hardening concrete are determined. Second, the dependences of chloride diffusivity and chloride binding capacity on age of concrete are clarified. Third, chloride profiles in hardening concrete are calculated. The proposed numerical procedure is verified by using chloride penetration test results of concrete with different mixing proportions.