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Critical Grain Size of Fine Aggregates in the View of the Rheology of Mortar
Dongyeop Han,Jae Hong Kim,Jin Hyun Lee,Su-Tae Kang 한국콘크리트학회 2017 International Journal of Concrete Structures and M Vol.11 No.4
The aim of this research was to investigate the validity of the Krieger-Dougherty model as a quantitative model to predict the viscosity of mortar depending on various aggregate sizes. The Krieger-Dougherty model reportedly predicted the viscosity of a suspension, which includes cement-based materials. Concrete or mortar incorporates natural resources, such as sand and gravel, referred to as aggregates, which can make up as much as 80% of the mixture by volume. Cement paste is a suspending medium at fresh state and then becomes a binder to link the aggregate after its hydration. Both the viscosity of the suspending medium and the characteristics of the aggregates, therefore, control the viscosity of the cement-based materials. In this research, various sizes and gradations of fine aggregate samples were prepared. Workability and rheological properties were measured using fresh-state mortar samples and incorporating the various-sized fine aggregates. Yield stress and viscosity measurements were obtained by using a rheometer. Based on the packing density of each fine aggregate sample, the viscosity of the mortar was predicted with the Krieger-Dougherty model. In addition, further adjustments were made to determine the water absorption of fine aggregates and was transferred from successful experiment to simulation for more accurate prediction. It was also determined that both yield stress and viscosity increase when the fine aggregate mean size decreases throughout the mix. However, when the mean size of the fine aggregates is bigger than 0.7 mm, the yield stress is not affected by the size of the fine aggregate. Additionally, if aggregate grains get smaller up to 0.3 mm, their water absorption is critical to the rheological behavior.
Control of Viscosity of Cementitious Materials Using Waste Limestone Powder
Dongyeop Han,Jin Young Yoon,Jae Hong Kim 한국콘크리트학회 2019 International Journal of Concrete Structures and M Vol.13 No.4
Ultra high performance cementitious materials possess high solid volume fraction by a decreased water-to-binder ratio. The use of a high-range water reducing admixture allowed us to obtain the initial fluidity of ultra-high performance cementitious materials, but its poor workability and significant loss of fluidity remained. Incorporating a high volume of supplementary cementitious materials reportedly weakens workability because they reduce the plastic viscosity of a mix. Hence, in this research, waste limestone powder was tested to evaluate its effect on the rheological properties of cement paste. The results give new insight on controlling the fluidity and plastic viscosity when preparing the mix for ultra-high performance concrete.
Evaluation on Protection Performance and On-Site Applicability of Hybrid Fiber-Reinforced Concrete
Dongyeop Han,Yong-Jun Park,Min-Cheol Han,Seong-Tae Yi 한국콘크리트학회 2019 International Journal of Concrete Structures and M Vol.13 No.3
The aim of this research is to evaluate the protecting performance of hybrid fiber-reinforced concrete against lateral forces such as explosives or flying objects. The objective of this research is to better prepare this concrete for use in the ready-mix concrete industry. Even though the fiber addition gives it strong mechanical properties, it also decreases workability due to the inefficient dispersion of fibers. Therefore, it has been difficult to apply to the readymix concrete plant’s mixing, delivery and on-site placement. The authors have developed a combined steel and polyaramid fiber that gives the favorable protection needed against high-impact forces and provides a suitable workability for the ready-mix concrete system. To evaluate the required performance, the hybrid fiber-reinforced concrete specimen was subjected to high-speed projectiles and had its workability and performance tested on a construction site of an established ready-mix concrete company. The hybrid fiber-reinforced concrete successfully provided the protection capabilities sought for the finished product and provided a workable-ready mix applicability.
Influence of steel fiber content and aspect ratio on the strength of high strength concrete
Dongyeop Han 한양대학교 세라믹연구소 2015 Journal of Ceramic Processing Research Vol.16 No.S1
The aim of this research is evaluating the strength properties of high strength fiber-reinforced concrete depending on replacement ratio and aspect ratio. As a part of producing SUPER concrete which has high ductility with fiber-reinforcement, both high mechanical properties and improved ductility should be achieved. Hence, mainly mechanical properties of the concrete with fiber was studied and from the results of the experiment, it was not shown the significant influence on compressive strength with fiber replacement ratio and aspect ratio, while as the fiber replacement ratio and aspect ratio were increased, the tensile and flexural strengths were improved. Especially, for fiber aspect ratio of 100, when the fiber replacement ratio was 2%, the most desired ductility was obtained.
한민철 ( Han Min-cheol ),한동엽 ( Han Dongyeop ) 한국건축시공학회 2017 한국건축시공학회지 Vol.17 No.1
순환골재가 복토 및 성토용으로 활용되는 경우 순환골재표면의 미수화 시멘트로부터 발생하는 높은 pH의 침출수는 환경에 큰 영향을 주는 요소이다. 본 연구에서는 이러한 높은 pH의 침출수를 방지하기 위해 순환골재의 효율적인 알칼리 제거 방안에 대해 실험적으로 비교분석 하였다. 실험결과 순환골재의 높은 알칼리성을 감소시키기 위해서는 단순 야적 및 물을 이용한 씻기 방법보다는 비료를 사용한 처리방법이 가장 유리한 것으로 나타났다. 또한, 본 연구에서는 비료의 종류인 인산암모늄과 이인산암모늄을 활용하여 순환골재를 중화시켰고 그 결과 약 3주 만에 pH 8의 중성에 가까운 순환잔골재를 얻을 수 있었다. 본 연구를 통해 순환골재를 토공사용으로 사용할 때에 보다 환경에 안전한 지속가능한 재료로서 활용하는 데에 기여할 것으로 예상한다. The aim of the research is to suggest an economical and sustainable method of reducing pH of recycled aggregate as an embanking and a covering materials. Because of the unhydrated cement based materials on the surface of the recycled aggregate, it causes a severe problem on environment with leaked high alkali water from embankment and covering by using recycled aggregate. In this research, to reduce the pH of recycled aggregate, regarding the recycled fine aggregate, eight different methods were tested and analyzed with three different categorized: natural treatment, artificial treatment, and chemical treatment. From the results of experiment, it was considered that the most efficient method of reducing pH of recycled aggregate was the chemical treatment using acid such as CO<sub>2</sub> acceleration or monoammonium phosphate (MAP), and diammonium phosphate (DAP). Especially, using MAP and DAP, fertilizers, is the most efficient method of reducing pH with its time duration and performance.