• Journal of the Korea Concrete Institute
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• v.23 no.5
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• pp.581-590
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• 2011

Since ultra-high performance cementitious composites (UHPCC) not only represents high early age shrinkage strain due to its low water-to-binder ratio (W/B) and high fineness admixture usage but also reduces the cross section of structure from the higher mechanical properties, it generally has more shrinkage cracks from the restraints of formwork and reinforcing bars. In this study, free and restrained shrinkage experiments were conducted to evaluate the suitability of incorporating both expansive admixture (EA) and shrinkage reducing agent (SRA). The test results indi-cated that approximately 40~44% of free shrinkage strain was decreased. Also, the results showed that 35% and 47% of residual tensile stresses were relieved by synergetic effect of SRA and EA, respectively. Residual tensile stresses from ringtest were relaxed by approximately 61% and 64% of elastic shrinkage stresses due to SRA and EA, respectively, because of the tensile creep effect. Therefore, the creep effect should be considered to precisely estimate the restrained shrinkage behavior of concrete structures. The degree of restraint of UHPCC was approximately in the range of 0.78~0.85. The addition of combined EA and SRA showed minute influence on the degree of restraint. However, the effect decreased when thicker concrete ring was used. Tensile creep strains were measured and compared to the predicted values from 4-parametric prediction model considering time dependent restrained forces.

• Journal of the Korea Concrete Institute
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• v.15 no.2
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• pp.273-279
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• 2003

The purpose of this study is to clarify the effect of the monomer ratio on properties of the polymer-modified mortars using methylmethacrylate-butyl acrylate(MMA/BA) latexes, and to obtain basic data necessary to develop appropriate latexes for cement modifiers. From the test results, we knew that the pore volume of polymer-modified mortars using MMA/BA latexes at bound MMA contents of 60 and 70 percent is 7.5∼75nm and the fine pore volume is increased with an increase in the polymer-cement ratio. The total pore volume of polymer-modified mortars using MMA/BA latexes is linearly reduced with an increase in the bound MMA content and increased in the polymer-cement ratio. In general, the superior compressive strength of polymer-modified mortars using MMA/BA latexes is obtained at a bound MMA content of 70 percent and a polymer-cement ratio of 15%. And, the water absorption and chloride ion penetration depth are greatly affected by the polymer-cement ratio rather than the bound MMA content. The important factors affecting the properties of polymer-modified mortars using MMA/BA latexes polymerized with various monomer ratios are the variations of the pore size distribution with changing bound MMA content and the polymer-cement ratio.

• Journal of the Korea institute for structural maintenance and inspection
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• v.12 no.6
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• pp.97-108
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• 2008

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement(OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical method, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash better results than the other methods of activation investigated in this study.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.4
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• pp.77-86
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• 1993

Recently it is reported in many countries that highway bridges are seriously damaged due to increasing volume of overloaded heavy vehicles. The safety of bridges are highly related to the design load level and the characteristics of extreme load effect induced by traffic loads during its lifetime. The maximum structural load effect during lifetime may be produced by simultaneous loading of trucks with moderate weights on a bridge rather than by single loading of extremely heavy trucks. In this study, a simulation technique to estimate extreme load effect due to traffic loadings has been developed, in which important characteristics of traffic loadings, such as heavy vehicle proportion, traffic mode, vehicle weights, headway distribution. daily traffic volume, etc., should be properly considered. In addition. sensitivity analysis on those factors have been performed.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2022.11a
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• pp.213-214
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• 2022

The purpose of this study is to compare and analyze the effect of the length and volume fraction of smooth steel fiber on the electrical conductivity and shielding effectiveness of cementitious composites. As the length and volume fraction of the fiber increase, the movement of electrons becomes active and the formation of a conductive path becomes advantageous, thereby increasing electrical conductivity. Accordingly, the electrical conductivity and the shielding effectiveness showed a very close relationship. Thereafter, it is judged that research is needed to increase the shielding effect.

• Journal of the Korea Concrete Institute
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• v.29 no.2
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• pp.169-177
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• 2017

Rather than causing damage from heat, blast, and radiation of a regular nuclear weapon, recently, it is predicted that North Korea has been inventing high altitude electromagnetic pulse (HEMP) missile in order to incapacitate electronic equipment. HEMP shielding facility is used for military purpose today. Despite the electromagnetic shielding effects from high quality compression plates, problems may include such as the possibility of electromagnetic influx resulting in the welding of the compression plates, and difficulties and high cost of construction. Therefore, in this study, a high electrical conducting material was added to the concrete experimental subject to ensure the shielding effect through electromagnetic waves to for the concrete structure, instead of building a shielding facility separately for the structure. Also, among the experimental subjects, 100 ${\mu}m$ of Iron-Aluminum alloy metal spraying coat was applied to two types with the highest shielding effect, and to two types with the lowest shielding effect. The result of the experiment indicates that experimental subjects added with a high electrical conductivity material did not meet the minimum shielding criteria of MIL-STD-118-125-1 standard, but all the experimental material applied to the metal spraying coating satisfied the minimum shielding criteria. In conclusion, it is considered that 100 µm of Iron-Aluminum alloy metal spraying coat contains high efficiency in the HEMP shielding.

• Food Science and Preservation
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• v.8 no.1
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• pp.47-53
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• 2001

In order to investigate the effect of two type of ceramic film(15% SiO$_2$ incorporated LDPE, 10% zeolite incorporated LDPE), perforated film and LDPE film on freshness of shiitake mushroom, weight loss, surface color and marketability of mushroom were measured during MA storage at 5 $\^{C}$ and 20$\^{C}$. The effect of packaging films on weight loss and marketability index of shiitake mushroom at 20$\^{C}$ was negligible due to its rapid physiological changes. However, the effect of packaging films on surface color of shiitake mushroom was significant at 20$\^{C}$ storage, but not at 5$\^{C}$ storage. During MA storage of shiitake mushroon, discoloration were reduced effectively by ceramic films and weight loss were by LDPE film, but they had little difference between packaging 51ms. However, there were significant differences among the packaging films in marketability of mushroom induced by water vapor inside package and accelerate its spoilage and browning. Ceramic film(15% SiO$_2$ incorporated LDPE) showed best result to maintain marketability of shiitake mushroom and zeolite incorporated film, perforated film, LDPE film in order.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 1999.05a
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• pp.254-260
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• 1999

Meteor Burst Communication can provide effective and economical means of communication where long distance VHF NLOS data transmission is needed; often ranges more than 1000 km. It has been, however, so far considered unsuitable for short distance application because of phenomenal decrease in burst durations, which leads to decreation of total duty rate of the system. This paper extensively analyzes characteristics of shot distance MB(Meteor Burst) path and shows the low duty rate may be improved by increasing burst rate through adapting antenna beam width to cover entire hot-spot region in the space and, by compensating effective burst length through cutting down man-made noises introduced by antenna. Based on the analysis, we are developed a small-opening-cavity antenna, especially designed for short distance MB path. In operation, the antenna is to be buried under ground surface so as to improve directivity and reduces noise introduction. The antenna exhibits power gain of 3 dB with 90 degree beam width and thus enables to illuminate entire hot-spot regions with the elevation angle of 8-90 degree which is the case of transmission less than 100 km. Directivity horizontal to earth surface is suppressed to minimum which enables to cut man-made noises from near-by sources down to more than 3 dB from the level reported with conventional 4 element Yagi. A series of experiments performed on 100 km MB paths have conformed that, with the antenna installed at receiving site, the burst rate and duration time have been noticed to increase by 10 and 20 percent respectively from the values obtained by conventional 4-element Yagi antenna under same testing condition.

• Journal of the Korea institute for structural maintenance and inspection
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• v.23 no.4
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• pp.23-30
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• 2019

Fiber-Reinforced Cement Composite (FRCC) is known to be effective in mechanical effects such as cracking width control as well as steel anticorrosive effect. In this study, we examined about steel anticorrosive effect by using metal fibers including zinc fibers by accelerated corrosion test. Moreover, it was examined for salt penetration, sacrificial anode effect and formation of electric circuit that was significant effect on the steel anticorrosive effect. As a result, Steel anticorrosive effect was confirmed with mixed metal fibers on FRCC. Especially, In the case of zinc fibers with a high sacrificial anode effect, it was confirmed that the suppression of penetration and corrosion resistance were improved.

• Korean Journal of Soil Science and Fertilizer
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• v.16 no.3
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• pp.223-227
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• 1983

This study was conducted to investigate the effect of supplement of Zeolite mineral whose C.E.C. is high, on the nutrients content in the compost and changes of compost status during its stacking period in 1982. The results obtained were summarized as follows : 1. By the supplement of Zeolite, the decomposing temperature in the compost was tended to be continued higher rather than that of nonsupplemented plot. 2. The contents of $NH_4-N$, $P_2O_5$, and $SiO_2$ in the compost were conspicuously increased by the supplement of Zeolite, poultry manure and urea etc, during the stacking period. It was considered that the supplement of Zeolite was much effective on the prevention of the waste of nutrients in the compost because the nutrients content was shown much high along with the progress of decomposition. 3. It was accepted to be the effect of Zeolite on shortening the period of compost decomposition and the prevention of the waste of nutrients content because the C/N ratio of the compost plots to supplement with the mixture of poultry manure, urea and Zeolite were 21.7 - 25.0 at 30 days after stacking but its of non-Zeolite treated plots were 21.3 - 24.8 at 60 days after stacking. 4. The C/N ratio in the compost and decrease rate of compost volume at 90 days after stacking was found to be significantly negative correlation.