• 한국분말재료학회지
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• 제30권6호
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• pp.463-469
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• 2023

Aluminum alloys, known for their high strength-to-weight ratios and impressive electrical and thermal conductivities, are extensively used in numerous engineering sectors, such as aerospace, automotive, and construction. Recently, significant efforts have been made to develop novel aluminum alloys specifically tailored for additive manufacturing. These new alloys aim to provide an optimal balance between mechanical properties and thermal/electrical conductivities. In this study, nine combinatorial samples with various alloy compositions were fabricated using direct energy deposition (DED) additive manufacturing by adjusting the feeding speeds of Al6061 alloy and Al-12Si alloy powders. The effects of the alloying elements on the microstructure, electrical conductivity, and hardness were investigated. Generally, as the Si and Cu contents decreased, electrical conductivity increased and hardness decreased, exhibiting trade-off characteristics. However, electrical conductivity and hardness showed an optimal combination when the Si content was adjusted to below 4.5 wt%, which can sufficiently suppress the grain boundary segregation of the α-Si precipitates, and the Cu content was controlled to induce the formation of Al₂Cu precipitates.

• 한국도로학회논문집
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• 제19권5호
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• pp.13-20
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• 2017

PURPOSES : This research was a laboratory study for evaluating the Reclaimed Asphalt Pavement (RAP) mixture added developed rejuvenator for warm mix recycling. Waste asphalt mixtures occupy about 18.2% of construction wastes in Korea. Moreover, most rejuvenators were imported from Europe or the U.S. Therefore, improving usage of RAP with a developed rejuvenator material provides environmental protection at a reduced cost. METHODS : The specimen used for this experiment was performed by only using RAP. A suitable rejuvenator for Target PG was then added. In addition, a conventional rejuvenator was selected to compare performance and specimens introduced with the same procedure as the developed rejuvenator was prepared. In order to evaluate rutting resistance and water susceptibility, we conducted a deformation strength test, a tensile strength ratio test, and a dynamic immersion test with the prepared mixtures. RESULTS :Laboratory test results indicated that both the developed additive and conventional additive improved performance of the recycled asphalt mixtures compared to mixtures without the rejuvenator. In addition, the deformation strength test and TSR test results satisfied standards for domestic recycling asphalt mixtures. The dynamic immersion test showed that the developed rejuvenator has superior scaling resistance than the conventional rejuvenator. CONCLUSIONS : In terms of rutting resistance and moisture susceptibility, the warm mix recycled asphalt mixtures with the developed rejuvenator appeared to effectively recovered performance.

• Geomechanics and Engineering
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• 제20권6호
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• pp.497-503
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• 2020

In the Kingdom of Saudi Arabia, the shallower depth of the earth's crust is composed of loose dune or beach sand with soluble salts. The expansive behavior of salt bearing soil, fluctuation of ground water table and extreme environmental conditions offer a variety of geotechnical problems affecting safety and serviceability of the infrastructure built on it. Despite spending money, time and other resources on repair and rehabilitation, no significant attention is paid to explore the root causes of excessive differential settlement and cracking to these facilities. The scientific solution required to ensure safety and serviceability of the constructed infrastructure is to improve the strength and durability properties of the supporting ground. In this study, shredded plastic is employed as a low cost and locally available additive to improve strength characteristics of the desert sand. The study shows a remarkable increase in the shear strength and normal settlement of the soil. A seven (07) degree increase in angle of internal friction is achieved by adding 0.4 percent of the shredded plastic additive. The effect of different proportions and sizes of the plastic strips is also investigated to obtain optimum values. Such a long-lived solution will seek to reduce maintenance and repair costs of the infrastructure facilities laid on problematic soil along with reduction of environmental pollutants.

• 한국도로학회논문집
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• 제19권4호
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• pp.19-25
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• 2017

PURPOSES : In this paper, the flow of construction material was simulated using computational fluid dynamics in a 2D axisymmetric condition to evaluate the effect of initial or varying material properties on the final shape of a specimen. METHODS : The CFD model was verified by using a well-known analytical solution for a given test condition followed by performing a sensitivity analysis to evaluate the effect of material properties on the final shape of material. Varying dynamic viscosity and yield stress were also considered. RESULTS : The CFD model in a 2D axisymmetric condition agreed with the analytical solution for most yield stress conditions. Minor disagreements observed at high yield stress conditions indicate improper application of the pure shear assumption for the given material behavior. It was also observed that the variation of yield stress and dynamic viscosity during curing had a meaningful effect on the final shape of the specimen. CONCLUSIONS : It is concluded that CFD modeling in a 2D axisymmetric condition is good enough to evaluate fluidal characteristics of material. The model is able to consider varying yield stress and viscosity during curing. The 3D CFD-DEM coupled model may be required to consider the interaction of aggregates in fluid.

• 한국재료학회지
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• 제23권10호
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• pp.599-605
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• 2013

CSA, a cement mineral compound that is mainly composed of $3CaO{\cdot}3Al_2O_3{\cdot}CaSO_4$, generates ettringite as a hydration product after a reaction with glass (lime), gypsum and water to speed up the hardening process and enhance the strength and degree of expansion. When used as a cement admixture, there is increased production of ettringite, which can improve the initial strength in the first three days and ameliorate the reduction in the initial strength caused by the use of fly ash in particular. In this study, a hydrate analysis was performed using XRD and SEM after substitution with fly ash (30%) and CSA (8%) with the goal of observing the effect of CSA on the initial strength of a cement mixture containing fly ash. The results of the analysis showed that an addition of CSA promoted the production of ettringite and improved the initial strength, resulting in the generation of hydrates, which can effectively enhance the long-term strength of these materials.

• Geomechanics and Engineering
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• 제17권5호
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• pp.439-444
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• 2019

It is generally known that high strength soil is indicative of well-graded particle size distribution. However, there are some special cases of firm ground despite poor grade distribution, especially a specific gap-graded soil. Based on these discoveries, this study investigated the development of an additive of gap-graded soils designed to increase soil strength. This theoretical concept was used to calculate the mixed ratio required for optimal soil strength of the ground sample. The gap-graded aggregate was added according to Plato's polyhedral theory and subsequently calculated ratio and soil strength characteristics were then compared to characteristics of the original soil sample through various test results. In addition, the underground stress transfer rate was measured according to the test conditions. The test results showed that the ground settlement and stress limit thickness were reduced with the incorporation of gap-graded soil. Further field tests would confirm the reproducibility and reliability of the technology by using gap-graded soil to reinforce soft ground of a new construction site. Gap-graded soil has the potential to reduce the construction cost and time of construction compared to other reinforcing methods.

• 콘크리트학회논문집
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• 제18권2호
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• pp.205-211
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• 2006

본 연구는 팽창재와 수축저감제를 조합 사용하여 수축변형률이 저감된 저수축 고성능 콘크리트에 대하여 내구성을 평가하였다. 그리고 저수축 고성능 콘크리트와 비교를 위해 동일한 물-결합재비에서 팽창재와 수축저감제를 사용하지 않은 고성능 콘크리트와 일반 콘크리트에 대해서도 시험을 실시하였다. 그 결과, 저수축 고성능 콘크리트는 동일한 물-결합재비를 가진 고성능 콘크리트에 비해 압축강도와 인장강도가 다소 크게 나타났다. 염소이온 침투 저항성과 탄산화에 대한 저항성은 고성능 콘크리트와 일반 콘크리트에 비해 우수하며, 동결융해 저항성은 동결융해시험 600사이클에서도 내구성 지수가 거의 100으로 나타났다. 또한 수밀성과 공극분포를 검토한 결과, 저수축 고성능 콘크리트는 고성능 콘크리트 또는 일반 콘크리트에 비해 시멘트 경화체 조직이 치밀해져 내구성이 향상되는 것으로 분석되었다. 따라서 팽창재와 수축저감제를 조합사용한 저수축 고성능 콘크리트를 구조물에 적용한 경우, 콘크리트의 수축과 균열을 저감시킬 수 있을 뿐만 아니라 내구성능이 향상되는 결과를 얻을 수 있을 것으로 분석되었다.

• 한국산학기술학회논문지
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• 제22권6호
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• pp.19-26
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• 2021

격자 구조체는 강도 및 강성, 초경량 및 에너지 흡수 능력 등의 우수성을 가지고 있어서 전방위 산업에서 주목을 받고 있으나, 다양한 장점에도 불구하고 복잡한 형상에 따른 제조 공정의 어려움으로, 현재까지 광범위한 상용화 및 사용은 제한되고 있다. 한편 적층 제조는 전통적인 제조 방법으로는 불가능한 복잡한 기하학적 형상 제조가 가능한 기술로써, 격자 구조체 제조를 위한 최적 기술로 주목을 받고 있다. 본 연구에서는 3차원 좌표 방법으로 단위 셀을 형성하고, 단위셀의 바운더리 박스 크기 및 스트럿 반경에 따른 상대 밀도의 관계식을 도출하였다. 본 연구에서는 Simple Cubic(SC), Body-Centered Cubic(BCC) 및 Face-Centered Cubic(FCC)을 모델링 소프트웨어를 사용하여 상대 밀도 맞춤형 구조체를 설계하였다. 본 연구에서 제안한 상대 밀도 산출식 정확도는 SC, BCC 및 FCC에서 98.3 %, 98.6 % 및 96.2%의 신뢰성을 확보하였다. 격자 구조체를 대상으로 시뮬레이션 수행 결과, 동일한 셀 배열 조건에서는 상대 밀도가 커짐에 따라 항복 하중이 커지고, 동일 배열 조건에서는 SC, BCC, FCC 순으로 압축 항복 하중이 작아지는 결과가 나타났다. 최종적으로 20 mm × 20 mm × 20 mm 크기의 구조체는 SC 단위 셀을 3×3×3 배열로 구성하는 것이 압축 하중에 대한 구조적 최적화가 가능한 것으로 나타났다.

• 한국건축시공학회지
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• 제22권6호
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• pp.597-606
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• 2022

본 논문은 발수제 혼입에 따른 시멘트 모르타르 표면의 얼음 제거성능에 다양한 요인 중 발수제 첨가 및 종류에 대한 영향을 검토하였다. 그 결과 발수제 첨가 시멘트 모르타르의 압축강도는 무첨가 시멘트 모르타르와 비교하여 감소하였고, 올리고머계 발수제 첨가 모르타르의 압축강도는 모노머계 발수제와 비교하여 높게 나타났다. 발수제 첨가 모르타르의 접촉각은 무첨가 모르타르와 비교하여 증가하였고 모노머계 발수제보다는 올리고머 발수제가 상대적으로 증가하였다. 발수제 혼입에 따른 시멘트 모르타르의 얼음형성시간 측정 결과 발수제를 첨가하였을 때 25분까지 지연시키는 것으로 나타났다. 얼음 제거하중 측정결과 발수제를 첨가하지 않은 모르타르는 342N을 발현하였고 발수제를 첨가한 모르타르는 31N을 발현하여 발수제를 모르타르에 첨가하는 경우 얼음제거성능이 감소하였다.

• 한국농공학회논문집
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• 제49권6호
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• pp.93-101
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• 2007

In general, geotechnical properties have many uncertain aspects, thus probabilistic analysis have been used to consider these aspects. It is, however, quite difficult to select an appropriate target probability for a certain structure or construction process. In this study, minimum expected cost design method based on probabilistic analysis is suggested for design of vertical drains generally used to accelerate consolidation in soft clayey soils. A sensitivity analysis is performed to select the most important uncertain parameters for the design of vertical drains. Monte Carlo simulation is used in sensitivity analysis and probabilistic analysis. Total expected cost, defined as the sum of initial cost and expected additive cost, varies widely with variation of input parameters used in design of vertical drain systems. And probability of failure to get the minimum total expected cost varies under the different design conditions. A minimum value of total expected cost is suggested as a design value in this study. The proposed design concept is applicable to unit construction process because this approach is to consider the uncertainties using probabilistic analysis and uncertainties of geotechnical properties.