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RISS 인기검색어
- 검색키워드
- 저자 : Dietmar Stephan (검색결과 5 건)
- 무료
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- 유료
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정상엽,Mohamed Abd Elrahman,Dietmar Stephan 한국콘크리트학회 2017 International Journal of Concrete Structures and M Vol.11 No.4
Insulating concrete is a material designed to reduce heat conduction with pores/insulations, and these pores strongly affect the material characteristics. In general, the insulation effect is directly proportion to the pore volume, while the material strength decreases as the porosity increases. To overcome this contrary, anisotropic insulations with different spatial distributions are proposed and investigated in this study. A set of mortar specimens with different arrangements of coin-shaped insulations are produced to examine the anisotropic insulation effect on the material characteristics. In addition, different types of insulation materials and their effect on the materials are also investigated here. X-ray computed tomography images and probabilistic description methods are used to confirm the arrangement of the insulations. The thermal and mechanical responses for different directions are investigated using both experimental and numerical methods. From the results, it is demonstrated that the use of anisotropic insulations for a specific direction can enhance the insulation efficiently as well as minimizing the loss of compressive strength.
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Kim, Ji-Su,Chung, Sang-Yeop,Stephan, Dietmar,Han, Tong-Seok Elsevier 2019 Construction and Building Materials Vol.202 No.-
<P><B>Abstract</B></P> <P>There are issues on the microstructure characterization of cement paste obtained from μ -CT due to resolution limits, and evaluation of properties through virtual experiments. A phase separation procedure between the solid and pore phases, which can be used for pure cement paste microstructures, is proposed. The problems of underestimation of microstructural characteristics such as porosity in virtual specimens from μ -CT, as compared to real specimens, are addressed. Reflecting such underestimation, the process of input modeling parameter determination for virtual experiments on mechanical property evaluation using the phase field fracture model is elaborated. Through virtual tests, the effects of domain size and mesh resolution on the evaluated properties are investigated, and the correlation between the microstructural characterization parameters and mechanical properties is reconfirmed. It is shown that the virtual experiment framework proposed in this study can be used as a loading tool to supplement time and effort consuming real experiments for evaluating the mechanical properties of cement paste at the micro-scale.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Issues on domain size and resolution of μ-CT images of cement paste are identified. </LI> <LI> Phase separation procedure for μ-CT images of cement paste is extended. </LI> <LI> Phase field fracture model performs adequately as a virtual loading tool. </LI> <LI> Virtual experiment shows a potential to supplement real experiment of cement paste. </LI> </UL> </P>
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Chung, Sang-Yeop,Kim, Ji-Su,Stephan, Dietmar,Han, Tong-Seok Elsevier BV 2019 Construction and Building Materials Vol.229 No.-
<P><B>Abstract</B></P> <P>Cement-based materials are generally highly heterogeneous, and their material properties are strongly affected by the characteristics and spatial distributions of each constituent. However, it is difficult to characterize the compounds inside these materials without destroying them. Micro-computed tomography (micro-CT) is an approach that enables the investigation of the inner structure of a material without damaging the specimen. From micro-CT, a series of cross-sectional images of a material with a pixel size of a few micrometers can be achieved, which can be used to evaluate the characteristics of the material. In this study, the use of micro-CT for various cement-based materials, e.g., lightweight concrete and high-performance concrete, is summarized. The characteristics of pores and solid phases, such as the volume fraction and spatial distribution, are also analyzed, and their relations with the physical properties of cement-based materials are reviewed and presented. It is shown that micro-CT has a great potential not only for investigating the performance of existing materials but also for developing new cement-based materials.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Micro-CT has been used to investigate various cement-based materials. </LI> <LI> Threshold determination from micro-CT images is crucial for phase identification. </LI> <LI> Microstructure and property relations are effectively correlated through micro-CT. </LI> </UL> </P>
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Pawel Sikora,Levent Afsar,Sundar Rathnarajan,Morteza Nikravan,정상엽,Dietmar Stephan,Mohamed Abd Elrahman 한국콘크리트학회 2023 International Journal of Concrete Structures and M Vol.17 No.6
The use of alternative and locally available materials is encouraged in the construction industry to improve its sustainability. Desert regions with shortages in freshwater and river sand as fine aggregates in concrete have to search for alternative materials such as seawater, dune sand, and waste glass powder to produce lightweight concretes. The potential negative effects of adding these alternative materials can be reduced by adding nanosilica to the cementitious system at very low quantities. This study evaluates the feasibility of using these alternative materials and nanosilica (NS) in producing lightweight aggregate concretes (LWACs). A systematic study was carried out to understand the synergistic effect of nanosilica and seawater in improving the hydration characteristics of the developed cementitious systems. Also, the effect of these alternative materials on the fresh properties of the cementitious system was assessed by slump flow tests. The evolution of compressive strength at early ages was investigated after 2, 7, and 28 days of moist curing and an improvement in the strength development in concretes with seawater was observed. Furthermore, the integrity of the developed LWACs was analyzed using oven-dry density, thermal conductivity, water porosity and shrinkage measurements. Moreover, the capillary porosity and sorptivity measurements revealed the denser microstructure in the nano-modified seawater lightweight concretes. In the end, the life-cycle assessment study calculated the benefit of alternative materials in terms of carbon footprint and water consumption. As an outcome, a sustainable solution for producing LWACs containing seawater, dune sand or glass powder was proposed.
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