• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.1
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• pp.1-6
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• 2021

Concrete structures gradually age due to deterioration of materials or excess loads and environmental factors, and their performance decreases, affecting the usability and safety of structures. Although external tension construction methods are widely used among the reinforcement methods of old bridges, it is insufficient to identify the effects and effects of reinforcement depending on the level of aging. Therefore, in this study, a four-point loading experiment was conducted on the subject with the non-reinforced and external tensioning method to confirm the reinforcement effect of the external tensioning method, assuming the aging of the structure as a reduction in the compressive strength and tensile reinforcement of concrete, to analyze the behavior of the reinforcement and confirm the reinforcement effect. As a result of the experiment, it was difficult to identify the amount of reinforcement in the extreme condition due to early elimination of the anchorage. Therefore, compliance with the regulations on anchor bolts is required when applying the external tension reinforcement method. Crack load and yield load increased depending on whether external tension was reinforced, but before the crack, the stiffness before and after reinforcement was similar, making it difficult to confirm the reinforcement effect.

• Journal of the Korean Geosynthetics Society
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• v.20 no.1
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• pp.9-19
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• 2021

This paper deals with the reinforcement performance of acrylate for reinforcing the settled railway roadbed. Concrete tracks have the advantage of reducing track maintenance costs and high resistance to track destruction. However, roadbed settlement is occurring in some construction sections, and the safety of railways is a serious concern because of difficulties in maintenance. Currently, maintenance through the track restoration method is being carried out in Korea as a way of roadbed settlement in concrete tracks, but continuous re-settlement can occur because the roadbed itself cannot be reinforced, and there are very few cases of reinforcement of railway roadbeds and field application. So the development of reinforcement materials and construction methods to reinforce railway roadbeds is required. Therefore, in this paper, acrylate was selected as reinforcement material for railway roadbed, and the reinforcement performance of acrylate was analyzed through experiment. As a result, it was analyzed that the acrylate can penetrate into a permeability coefficient of 1×10^-4 cm/sec, and secure uniaxial compression strength of 0.5 MPa/30min or more and stiffness of 80 MPa or more.

• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.6
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• pp.68-75
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• 2021

In this study, the anchoring performance of 57mm headed bars anchored at the external beam-column joint under cyclic loading was evaluated. A total of 6 external beam-column joint test specimens were planned, and anchorage performance was evaluated by setting concrete compressive strength, side covering thickness, lateral reinforcement ratio, and fracture type as major experimental variables. As result of cyclic loading test, it was found that the factors that had the greatest influence on the anchoring capacity of the large-diameter headed bar anchored at the joint were the side cover thickness and the transverse reinforcing bar. It was confirmed that the 57mm large-diameter headed bar anchored at the external beam-column joint showed sufficient anchoring capacity even under cyclic loading.

• Journal of the Korean Recycled Construction Resources Institute
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• v.9 no.2
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• pp.216-222
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• 2021

This is an experiment to complement new ways of using concentrated water discharged from the seawater desalination plant. In this study, metakaolin, which has excellent chloride ion immobilization effect, was used as the main binder, and 10% and 20% of calcium oxide were substituted with the activator. In addition, tap-water(TW) and reverse osmosis brine water(RW) were used as mixed water. As a result of the experiment, the mixture using RW showed higher compressive strength than TW. It also showed low water absorption and high density. In the mixture using RW as mixed water, a hydration reaction substance called Friedel's salt could be observed. Considering the corrosion problem of steel, RW is considered to be applicable to products such as non-reinforced concrete, brick, and curb stone. Through this study, it is thought that it is meaningful to propose a new application method other than the ocean release of RW.

• Journal of the Korean Recycled Construction Resources Institute
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• v.10 no.4
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• pp.489-497
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• 2022

In this study, the fluidity, mechanical properties and microstructure of cement pastes with carbon nanotube (CNT) were experimentally investigated. The 6 types of cement paste mixes with different PCE:CNT and w/b had been manufactured, and several tests including flow, compressive strength, absorption and water porosity were performed on cement pastes with or without CNT.Additionally, microstructural observations such as x-ray diffraction (XRD) and scanning electron microscopy (SEM) were carried out to examine hydrates formed in cement paste with CNT. As a result, it was found that the performance of cement pastes with CNT was better compared to that of control cement paste (OPC) due to both of hydration acceleration effect and filling effect. Furthermore, the SEM images clearly showed that CNT can bridge cracks formed in cement matrix. Conclusively, it is believed that the CNT, if mixed appropriately, could be an option as nono-materials to improve performance of concrete structures.

• Journal of the Korean Recycled Construction Resources Institute
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• v.11 no.1
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• pp.55-61
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• 2023

This study aims to develop CLSM fill material for emergency restoration using landfill coal ash. As a result of examining physical properties such as particle size distribution and fines content of landfill coal ash, bottom ash, fly ash, and general soil were mixed, and SP was found to have a density of 2.03 and a residual particle pass rate of 7.8 %. CLSM materials that secure fluidity in unit quantities without using chemical admixtures such as glidants and water reducing agents have a high risk of material separation due to bleeding. As a result of this experiment, it was found that the bleeding ratio did not satisfy the standard in the case of the specimen with a large amount of fly ash and a lot of addition of mixing water. As a result of the compressive strength test, the strength development of 0.5 MPa or more for 4 hours was found to be satisfactory for the specimens using hemihydrate gypsum with a unit binder amount of 200 or more, and the remaining gypsum showed poor strength development. Although it is judged that landfill coal ash can be used as a CLSM material, it is necessary to identify and apply the physical and chemical characteristics of coal ash buried in the ash treatment plant of each power generation company.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.18 no.5
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• pp.211-216
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• 2008

The artificial lightweight aggregate (ALA) was manufactured in a rotary kiln at $1125^{\circ}C$ using green body formed by pelletizing the batch powder composing of coal bottom ash (CBA) produced from power plant, clay and dredged soil (DS). The TCLP (Toxicity characteristic leaching procedure) results showed that the dissolution concentration of heavy metal ions of ALA fabricated in this study was below the limitation defined by the enforcement regulations of wastes management law in Korea. The ALA containing 60$\sim$70 wt% CBA had a bulk density of 1.45$\sim$1.49 and a water absorption of 17.2$\sim$18.5 %. The impact values for oven-dry state and saturated-surface dry state of ALA were 27.4$\pm$1.3 and 23.4$\pm$2.6 % respectively. The 28-days compressive strength of concrete made with various ALA was $22.7\sim27.8 N/mm^2$. The slump of concrete with ALA containing CBA 60 and 70 wt% were 7.9 and 14.3 cm respectively. The unit weight of concrete made with any ALA fabricated in this study was satisfied with the standard specifications of lightweight concrete for the civil engineering and construction presented by Korea as below $1.84 ton/m^3$.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.5
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• pp.251-258
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• 2018

In construction site, conventional methods such as jackhammer or explosive methods(dynamite) have been often used for the demolition of structures. Use of those methods are more carefully treated in environmentally and historically sensitive area. For those reasons, use of Soundless Chemical Demolition Agent(SCDA) is getting the spotlight. The SCDA is a powder which has expansive strength when it is mixed with water. In these Characteristics, SCDA can destroy the concrete or rock as it is poured into boreholes of the concrete or rock structures. However, there is no industrial standard for the use of SCDA effectively yet. In this study, experimental study to measure the expansive pressure was conducted depending on various boundary conditions such as waterproof, length of the steel pipe, submerged of steel pipe. Furthermore, computational analysis using damage plasticity model to predict the minimum required pressure of the SCDA for the concrete demolition depending on spacing between holes(k-factor) and compressive strength of the concrete was conducted. Obtained results indicates that water heat dissipation with submerged steel pipe shows the stable pressure for measuring the SCDA and hole distance(k-factor) is the most important factor for crack initiation of concrete.

• Journal of the Korea Institute of Building Construction
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• v.15 no.3
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• pp.265-271
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• 2015

This study is intended to produce a PC (Precast Concrete) member without a steam curing process in developing the high early strength concrete satisfying the condition of 10MPa in compressive strength at the age of 6 hours, and is intended to ensure economic feasibility by increasing the turnover rate of concrete form. Hence, high early strength cement with high $C_3S$ content and the hardening accelerator of powder type accelerating the hydration of $C_3S$ was used. And the properties of concrete were evaluated according to the hardening accelerator mixing ratio (0, 1.2, 1.6, 2.0). No big difference was found from the tests of both slump and air content. When 1.6 % or higher amounts of the hardening accelerator were mixed, the compressive strength of 10MPa was achieved at the age of 6 hours. From the test results of autogenous (drying) shrinkage and plastic shrinkage, it can be seen that there was a difference according to hydration reaction rate due to the addition of the hardening accelerator. However, it was shown that no problem arose with crack and durability. And it was shown that resistance to freezing-thawing, carbonation, and penetration were excellent.

• Journal of the Korea Institute of Building Construction
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• v.9 no.4
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• pp.55-61
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• 2009

Recently, the number of high-rise buildings being built in Korea by major construction companies for residential and commercial use has been increasing. When constructing a high-rise building, it is necessary to apply massive amounts of concrete to form a mat foundation that can withstand the huge load of the upper structure. However, it is of increasing concern that due to limitations in terms of the amount of placing equipment, available job-sites and systems for mass concrete placement in the construction field, it is not always possible to place a great quantity of concrete simultaneously in a large-scale mat foundation, and for this reason consistency between placement lift cannot be secured. In addition, a mat foundation Is likely to crack due to the stress caused by differences inhydration heat generation time. To derive a solution for these problems, this study provides test results of a hydration heat crack reduction method by applying placement lift change and setting time control with a super retarding agent for mass concrete in a large-scale mat foundation. Mock-up specimens with different mixtures and placement liftswere prepared at the job-site of a newly-constructed high-rise building. The test results show that slump flow of concrete before and after adding the super retarding agent somewhat Increases as the target retarding time gets longer, while the air content shows no great difference. The setting time was observed to be retarded as the target retarding time gets longer. As the target retarding time gets longer, compressive strength appears to be decreased at an early stage, but as time goes by, compressive strength gets higher, and the compressive strength at 28 days becomes equal or higher to that of plain concrete without a super retarding agent. For the effect of placement lift change and super retarding agent on the reduction of hydration heat, the application of 2 and 4 placement lifts and a super retarding agent makes it possible to secure consistency and reduce temperature difference between placement lifts, while also extending the time to reach peak temperature. This implies that the possibility of thermal crack induced by hydration heat is reduced. The best results are shown in the case of applying 4 placement lifts.