• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.2
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• pp.100-107
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• 2018

The material and geometrical nonlinear finite elment analysis of UHPFRC 50M composite box girder was carried out. Constitute law in tension and compressive region of UHPFRC and HPC were modeled based on specimen test. The accuracy of nonlinear FEM analysis was verified by the experimental result of UHPFRC 50M composite girder. The UHPFRC 50M segmental composite box girder which has 1.5% steel fiber of volume fraction, 135MPa compressive strength and 18MPa tensile strength was tested. The post-tensioned UHPFRC composite girder consisted of three segment UHPFRC U-girder and High Strength Concrete reinforced slab. The parts of UHPFRC girder were modeled by 8nodes hexahedron elements and reinforcement bars and tendons were built by 2nodes linear elements by Midas FEA software. The constitutive laws of concrete materials were selected Multi-linear model both of tension and compression function under total strain crack model, which was included in classifying of smeared crack model. The nonlinearity of reinforcement elements and tendon was simulated by Von Mises criteria. The nonlinear static analysis was applied by incremental-iteration method with convergence criteria of Newton-Raphson. The validation of numerical analysis was verified by comparison with experimental result and numerical analysis result of load-deflection response, neutral axis coordinate change, and cracking pattern of girder. The load-deflection response was fitted very well with comparison to the experimental result. The finite element analysis is seen to satisfactorily predict flexural behavioral responses of post-tensioned, reinforced UHPFRC composite box girder.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.2
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• pp.138-145
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• 2018

This paper studies the bending behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams focused on the effect of variation in major material design parameters such as tensile strength, elastic modulus of UHPFRC, and rebar ratio. Analytical results show that the variation in the range of ${\pm}20%$ in the tensile strength of UHPFRC causes the significant difference in ${\pm}8{\sim}9%$ of bending strength compared to the reference condition. The variation of elastic modulus in UHPFRC rarely causes the effect on the bending strength of the UHPFRC section, whereas causes the difference in the slopes of moment-curvature curves, indicating different bending stiffness of UHPFRC sections. For the rebar with yield strength of 400MPa, the bending strength of SC120f is increased by 30, 67, and 99% when the rebar ratio is 1.0, 1.5, and 20%, respectively, compared to the rebar ratio of 0.5%. Therefore, it is observed that the variation of rebar ratio significantly affects the difference in bending strength of UHPFRC beams. However, as the compressive strength of UHPFRC becomes greater, the effect of rebar ratio on the increase of bending strength is decreased.

• Journal of the Korean Recycled Construction Resources Institute
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• v.7 no.4
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• pp.95-103
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• 2012

Precast concrete method is known to have advantages of minimizing works in the construction, controlling concrete quality easily and saving construction period due to only fabrication work in the construction field, but it needs to apply steam curing to accelerate early concrete strength. In the meanwhile, the oil cost for steam curing has been continuously increased because of political instability in the middle East and international economic shaky. Thus, this study addresses the development of precast/ prestressed concrete which has over 14MPa at 1 day age and specified concrete strength of 40MPa at low temperature, not applying steam curing. Tests were carried out in terms of material characteristics in fresh concrete and compressive strength using 3 types of cement such as Type I, Type III and rapid hardening compound cement. As results of tests, it is found that cements for rapid hardening had disadvantages with respect to slump, slump loss, and air content, but showed higher compressive strength than specified one, especially the highest value when using rapid hardening compound.

• Journal of the Korean Recycled Construction Resources Institute
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• v.2 no.3
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• pp.173-179
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• 2014

This is an experimental study for recycling coal ash left over from coal use as a potential fine aggregate in concrete. Coal ash is generally divided into either fly ash or bottom ash. Fly ash has been utilized as a substitution material for cement in concrete mixes. On the other hand, bottom ash has the problem of low recycling rates, and thus it has been primarily reclaimed. This study partially substituted fine concrete aggregates with bottom ash to increase its application rate and therefore its recycling rate; its suitability for this purpose was confirmed. The concrete's workability dropped noticeably with increasing bottom ash content when a fixed water-cement ratio of concrete mix was used. Thus, concrete mixes with higher ratio levels are required. To address this problem, concrete was mixed using a polycarboxylate high-range water reducing agent. The fluidity and air entrainment immediately after mixing the concrete and 1 h after mixing were measured, thereby replicating the time concrete is placed in the field when produced either in a ready-mixed concrete or in a batch plant. As a result of this research, the workability and air entrainment were maintained 1 h after mixing for a concrete mixture with approximately 30% of its fine concrete aggregates substituted with the bottom ash. A slight drop in compression strength was seen; however, this confirmed that potential of using bottom ash as a fine aggregate in concrete.

• Journal of the Korean Recycled Construction Resources Institute
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• v.2 no.3
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• pp.255-264
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• 2014

This study analyzes the cause and the repairing method of water leak by parts of basement parking lot which is recorded to have a high defect frequency in apartment buildings. It has been assessed that the cause of water leakage on the first floor upper substrate is due to such factors as landscaping and weights. During construction or through other cases, it has been determined that cracks were produced in the apartment structure because the structure was weak and exposed to the effects of the substrate movement. The base floor and underground external walls are areas that are exposed to water pressure (uplife pressure), thus in normal cases the rear surface repair of the structure using sythetic rubberized polymer gel should be considered as an effective method. However, in cases where application of waterproofing layer is required in the structure due to high water pressure, using asystolic cement milk grout to form the waterproofing layer and applying water-swelling acrylic material into the cracked areas is considered to be highly effective.

• Journal of the Korean Recycled Construction Resources Institute
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• v.1 no.2
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• pp.114-119
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• 2013

The present study tested 5 concrete mixes to develop reliable mixing proportions for the sustainable alkali-activated(AA) foamed concrete as a thermal insulation material for the floor heating system of buildings. The AA binder used was composed of 73.5% ground granulated blast-furnace slag, 15% fly ash, 5% calcium hydroxide, and 6.5% sodium silicate. As a main variable, the unit binder content varied from $325kg/m^3$ to $425kg/m^3$ at a space of $25kg/m^3$. The test results revealed that AA foamed concrete has considerable potential for practical applications when the unit binder content is close to $375kg/m^3$, which achieves the minimum quality requirements specified in KS F 4039 and ensures economic efficiency. In addition, lifecycle assessment demonstrated the reduction in the environmental impact profiles of all specimens relative to typical ordinary portland cement foamed concrete as follows: 99% for photochemical oxidation potential, 87~89% for global warming potential, 78~82% for abiotic depletion, and 70~75% for both acidification potential and human toxicity.

• Journal of the Korean Recycled Construction Resources Institute
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• v.1 no.2
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• pp.136-143
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• 2013

In the sewage structures and wastewater facilities, concrete is exposed to hydrogen sulfide ($H_2S$) which acts as an acid material in a solution, and a strongly acidic sulfate ion ($SO{_4}^{-2}$) is generated by a sulfuric bacteria. Hence, a degradation of concrete with biochemical corrosion would be accelerated. Finally, durability of concrete and concrete structures may be greatly reduced. In this study, in order to remove the hydrogen sulfide which is used by the sulfuric bacteria organic-biologically, the antibiotic metal and metallic salt powders were mixed to concrete, and a suppressing performance of the sulfate ion was assessed. For the sulfuric acid bacteria, a comparative evaluation of antimicrobial performance on neutralized concrete specimens were carried out, also by a rapid chloride penetration test, chloride penetration depths and diffusion coefficients were measured for antibiotic concrete in accordance with the amount of metal and metallic salt-based antibacterial agents. Eventually, by an observation of the biochemical state of the surface of concrete specimens exposed outdoors, the performance and applicability of antibiotic concrete were confirmed.

• Journal of the Korean Recycled Construction Resources Institute
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• v.5 no.1
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• pp.53-58
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• 2017

FRP Hybrid Bar, composed of an embedded steel and the coated composites with epoxy and glass fiber, is an effective construction material with tension-hardening performance and lightweight. The epoxy exposed to UV(Ultra Violet Rays) and FT(Freezing and Thawing) action easily shows a surface deterioration, which can cause degradation of bonding strength between inside-steel and outside-concrete. In the present work, surface inspection for 3 different samples of normal steel, FRP Hybrid Bar before UV, and FRP Hybrid Bar after UV test was performed, then concrete samples with 3 reinforcement types were prepared for accelerated FT test. Through visual inspection on 3 typed reinforcement, no significant deterioration like chalking was evaluated. The results from FT test to 120 and 180 cycles showed FRP Hybrid Bar exposed to UV test has higher bonding strength than normal steel by 106.3% due to enlarged bond area by silica coating. The 3 cases showed a similar bond strength tendency with increasing FT cycles, however a relatively big deviations of bond strength were evaluated in FRP Hybrid Bar after UV test due to loss of silica coating.

• Journal of the Korean Recycled Construction Resources Institute
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• v.1 no.1
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• pp.81-88
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• 2005

The fly ash has been widely used in the latest to complement the performance and economical efficiency of the concrete which uses only a normal portland cement, the pulp ash gained through the incineration of paper sludge is possible to be used as the material of concrete because it contains the properties similar to the previous fly ash in ingredients and physical characteristics. Therefore, this research has tested physical characteristics by replacing 20% of fly ash used with the paper ash to solve the problem which lowers the early strength caused when the fly ash was used. As a result, it showed that the fluidity becomes lower and the compressive strength becomes increased by using paper ash. In addition, after mixing the paper ash with the fly ash, it showed that time and heating amount of the 2nd peak of the minor heat of hydration affecting the revelation of strength was equivalent to the combination for normal portland cement, and also indicated that the compressive strength for 3 days is superior to the combination of the fly ash. Therefore, if the paper ash having a regular fineness is used, it was effective in improving the early strength of concrete used the fly ash.

• Journal of the Korean Recycled Construction Resources Institute
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• v.3 no.1
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• pp.50-57
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• 2015

Portland cement production is under critical review due to high amount of $CO_2$ gas released to the atmosphere. Attempts to increase the utilization of a by-products such as fly ash and ground granulated blast-furnace slag to partially replace the cement in concrete are gathering momentum. Many researchs on alkali-activated concrete that does not need the presence of cement as a binder have been carried out recently. Instead, the sources of material such as fly ash, that are rich in Silicon(Si) and Aluminium(Al), are activated by alkaline liquids to produce the binder. Hence concrete with no cement is effect reduction of $CO_2$ gas. In this study, we investigated the influence of the fluidity, air content and compressive strength of mortar on alkaline activator in order to develop cementless fly ash and ground granulated blast-furnace slag based alkali-activated mortar with superplasticizer. In view of the results, we found out that Pn of fluidity and compressive strength is the best in four type of superplasticizer, and PNS of powder type of fluidity is better than that of liquid type in the case of AA.