• Journal of the Korea Concrete Institute
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• v.16 no.2 s.80
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• pp.205-212
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• 2004

Recently, as concerns over environmental issues are raised more and more trend to use recycled waste for producing construction materials is also raised. Especially, a case of waste resin is considerably destroying the environment due to disposal way that most waste resin produced is disposed of landfill. This study is performed by polymer concrete with recycled PET resin in terms of obtaining safely clean construction resources and protection of environment. High setting shrinkage and sensitivity to heat are main disadvantages of Polymer Concrete (PC) despites of a lot merits. The aim of this study is to investigate basic properties such as setting shrinkage, length change and sensitivity to heat about PET recycled polymer concrete. The other is to check the possibility of use of Montmorillonite as one of a lot of additive without special coupling agent. As results of experiments, various properties of polymer concrete with recycled PET resin are similar with conventional PC except that polymerization time is longer. Montmorillonite was efficiently used to reduce setting shrinkage, length change and coenicient of thermal expansion related to heat with enhanced strength

• Proceedings of the Korean Institute of Building Construction Conference
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• 2014.11a
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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.

• Polymer(Korea)
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• v.31 no.6
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• pp.491-496
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• 2007

In the polymeric dental restorative composites, the resin matrix mainly contains 70 wt% 2,2-bis[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane (Bis-GMA), as a base resin and 30 wt% triethylene glycol dimethacrylate (TEGDMA) as a diluent. Even though the viscosity of the resin matrix is rapidly decreased by adding TEGDMA, addition of TEGDMA to the Bis-GMA results in reduction in the mechanical properties and increase in the curing shrinkage of the dental composite. In order to fabricate dental composite exhibiting excellent properties by reducing TEGDMA content in the resin matrix, in this study, Bis-GMA derivatives, which do not contain hydroxyl groups, were used instead of Bis-GMA. The curing characteristics of Bis-GMA derivatives were similar with those of Bis-GMA, while the former exhibited lower viscosity and water absorption than the latter. Comparing the curing shrinkage of the dental composite containing Bis-GMA derivative with that prepared from Bis-GMA, the reduction in curing shrinkage was about 25%. Dental composites prepared from new resin matrices also exhibited low water uptake and better properties in mechanical strength.

• Journal of the Korea institute for structural maintenance and inspection
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• v.16 no.2
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• pp.95-103
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• 2012

In order to improve the dimensional stability and mechanical performance of cement-based composites, the effect of an expansive admixture based on calcium sulphoaluminate (CSA) on the shrinkage and mechanical properties of strain-hardening cement-based composite (SHCC), which exhibits multiple cracks and pseudo strain-hardening behavior in the direct tension, is investigated. Polyethylene fibers reinforced SHCC mixtures with three levels (30, 70, and 100MPa) of compressive strength were compared through free shrinkage, compressive strength, flexural strength, and direct tensile strength measurements. The SHCC mixtures were cast with and without replacing 10% of Portland cement content with CSA admixture. According to test results, CSA admixture is effective in reducing shrinkage of SHCC material. SHCC mixture with CSA admixture exhibited a little higher strength than companion mixture without CSA admixture.

• Proceedings of the Korean Fiber Society Conference
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• 2003.04a
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• pp.368-369
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• 2003

선진 복합재료의 매트릭스 수지로서 자장 많이 사용되고 있는 에폭시는 수지 및 경화제의 종류에 따라 여러 가지 물성을 나타낼 수 있다. 에폭시 수지는 기계적 물성 및 내화학성이 우수하고 경화시 수축변형이 적은 장점이 있으나 높은 가친밀도 때문에 순간적인 충격에 취약하다는 단점을 지니고 있다. 따라서 이러한 에폭시 수지의 취성 (brittleness)을 개선하기 위해 충격강도를 증가시킬 수 있는 강인화제를 혼합하여 경화시키는 방법이 사용되고 있다[1]. (중략)

• Composites Research
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• v.22 no.6
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• pp.45-51
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• 2009

The marginal integrity at the composite resin-tooth interface has been analyzed in real time through acoustic emission (AE) monitoring during the polymerization shrinkage of composite resin subjected to the light exposure. It was found that AE signals were generated by the polymerization shrinkage. Most AE hit events showed a blast type signal having the principal frequency band of 100-200kHz. Bad bonding states were indicated by many hit events in the initial curing period of 1 minute with high contraction rate. The quantity of hit events for the human molar dentin specimen was much less than that for the steel ring specimen but more than that for the PMMA ring specimen. The better the bonding state, the less the AE hit events. The AE characteristics were related with the tensile crack propagation occurring in the adhesive region between the composite resin and the ring substrate as well as the compressive behavior of the ring substrate, which could be used for a nondestructive characterization of the marginal disintegrative fracture of the dental restoration.

• Journal of the Korea Concrete Institute
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• v.17 no.4 s.88
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• pp.581-586
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• 2005

High-performance concrete (HPC), which is particularly sensitive to self-desiccation, is required to be durable even in severe environmental conditions, i.e. costal area, cold district, etc. However, in recent years, some attention was particularly given to cracking sensitivity of high performance concrete at early age. It has been argued and demonstrated experimentally that such concrete undergoes autogenous shrinkage due to self-desiccation at early age under restrained condition, nd, as a result, internal tensile stress may develop, leading to micro cracking and macro cracking. This shrinkage-introduced crack produces a major serviceability problem for concrete structures. One possible method to reduce cracking due to autogenous shrinkage is the addition of expansive additive. Tests conducted by many researches have shown the beneficial effects of addition of expansive additive for reducing the risk of autogenous shrinkage-introduced cracking. However, the research on hydration model of expansion additive has been hardly researched up to now. This paper presents a study of the hydration model of Ettringite-Gypsum type expansive additive. As a result of comparing forecast values with experiment value, proposed model is shown to expressible of hydration of expansive additive.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1993.10a
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• pp.27-33
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• 1993

천마 연소관 및 Bracket는 고강도 특수강의 일종인 Maraging steel C-250 Grade로서 유동성형공정 (flow forming process: F/F) 및 용접공정을 거쳐 시효경화후의 경도(HRC 48-52)가 높아 절삭가공 및 Tapping에 어려움이 있다고 판단되어 브라켓 밀링가공(Tapping포함)을 시효경화공정 이전에 완성하는 것으로 공정 FLOW를 설정하였으나, 시효경화 시 유동성형공정 잔류응력 및 재질특성에 의한 수축, 변형 등으로 도면상 요구된 품질(형상 및 위치공차) 만족이 미흡하였을 뿐만 아니라 오히려 전체 공정 수만 증가하였음. 따라서 연소관 및 Bracket 완성가공을 시효경화 후에 실시하는 것으로 공정 개선 하고자 시험 작업한 결과, 선삭, 밀링작업등 다른 기계가공 공정의 문제점은 대부분 해결할 수 있었으나, Tapping공정만은 해결할 수 없어서 진정한 공정개선을 기할 수 없었음. 그러나, 제품의 품질 및 생산성을 고려 시효경화 후 Tapping 공정실시 필요성이 강력히 대두되어 Maraging steel 재질특성에 적합한 공구 및 작업조건을 검토, 설정 시험작업 함으로써 공정개선을 이룰 수 있었음.

• Journal of the Korea Institute of Building Construction
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• v.14 no.1
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• pp.110-117
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• 2014

The objective of this study is to investigate an effect of curing temperature on autogenous shrinkage of high strength cement mortar with 0.15 of W/B incorporating fly ash and silica fume in terms of equivalent age. The contents of fly ash and silica fume are varied from 10% to 30%. Non linear regression model applying equivalent age was used to estimate the autogenous shrinkage evolution. To obtain apparent activation energy($E_a$), setting time method by Pinto and existing method were calculated and compared respectively. Test results showed that use of silica fume increased autogenous shrinkage while use of fly ash decreased it. It was also found that poor agreements were obtained when $E_a$ by setting time was applied. But, application of existing $E_a$ resulted in a good agreement between calculated autogenous shrinkage and measured one.

• Journal of the Korean Recycled Construction Resources Institute
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• v.7 no.4
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• pp.367-374
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• 2019

Drying shrinkage in the hardened cement is known to change in volume by decreasing the moisture content in the hardened body, and it is known that the higher the W / C and the higher the content of the paste, the larger the drying shrinkage. In the case of ECC, more drying shrinkage occurs compared to concrete, since it does not contain coarse aggregate. Since ECC is an important material for tensile performance, the effect of restrained tensile stress on mechanical tensile behavior should be considered. The purpose of this study is to analyze the effect of stress caused by restraint on the tensile behavior of ECC. The mechanical properties of the specimens were tested by uniaxial tension tests with different restraints. As a result, the difference of tensile behavior according to restraint stress was observed and the cause was analyzed.