• Proceedings of the Korea Concrete Institute Conference
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• 2005.05b
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• pp.29-32
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• 2005

In this study, the experiment was carried out to investigate and analyze the strenth properties and flowability of ultra-high strength concrete accroding to types of mineral admixtures. The main experimental variables were water/binder ratio 25.0, 27.5 and 30.0$\%$, water content 155, 160, 165, and 170kg/$m^{3}$ and mineral admixtures such as fly ash, silica fume and meta kaolin. According to the test results, the principle conclusions are summarized as follows. 1) In case of using admixtures, superplasticizer amount need more than plain concrete. 2) According to kinds of admixtures, the viscosity of concrete show much difference. 3) The compressive strength of concrete that use admixtures becomes low in early-age strength, but appeared by higher than plain concrete in long-term strength. 4) Meta kaolin is excellent in side but has viscosity enlargement efficiency a little. But, problem estimates that is not to make design strength 600 and 700kgf/$cm^{2}$ if use mixing condition with water-binder ratio properly.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2020.06a
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• pp.25-26
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• 2020

In this study, by applying the concrete compressive strength estimation system Concrete IoT Management System (hereinafter referred to as CIMS) to the concrete slab concrete in the domestic field, the purpose of this study is to confirm the practical use of CIMS and to verify the accuracy of estimating the initial strength of concrete. As a result, it shows a high correlation when the compressive strength and CIMS estimated strength of the specimen for structural management are converted and compared with the integrated temperature. However, in order to determine a more accurate experimental constant, it is necessary to consider the results up to 28 days.

• Journal of the Korea institute for structural maintenance and inspection
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• v.26 no.6
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• pp.112-119
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• 2022

Recently, as carbon neutrality has been important factor in the construction industry, many studies have been conducted on the high-volume fly ash concrete. High volume fly ash concrete(HVFC) is usually made by replacing more than 50% of cement with fly ash. However, HVFC has a disadvantage of low compressive strength in early age. To overcome this shortcoming of HVFC, improve this, interest in techonolgy using nanomaterials is increasing. Nano silica is expected to improve the early age strength of HVFC as a pozzolanic material. This study investigated the effect of nano silica on the early hydration reaction and microstructure of HVFC. The early hydration reaction of HFVC was analyzed through setting time, isothermal calorimeter, compressive strength and thermal weight analysis. In addition, the microstructure of HVFC was measured by mercury intrusion porosimetry. From the test results, it was confirmed that nano silica increased the early age strength and improve the microstructure of HVFC.

• Journal of the Korean Society for Nondestructive Testing
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• v.22 no.3
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• pp.292-303
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• 2002

This paper presents experimental results for early-age properties of concrete such as the setting time and strength, evaluated via the ultrasonic pulse velocity (UPV). Developing and using an automatically-recording monitoring system, the UPV's of mortar and concrete with various water to binder ratios (W/B) were measured during the first 24 hours. In addition, probe penetration and compression tests were conducted to measure the setting time and compressive strength, respectively. It was observed that the UPV's of mortar with high W/B remained constant during the first 6.5 hours and then abruptly began to increase at constant rates. On the other hand, the UPV of mortar with low W/B increased relatively slowly and gradually due to the setting retardation caused by the use of high range water reducing agent (HRWR). It was found that setting of concrete occurs when the UPV reaches a certain value. Moreover, it was concluded that the estimation formulas should incorporate the effects of W/B to more accurately estimate the early-age strength of concrete from the UPV.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.05a
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• pp.1023-1030
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• 2002

From the early 1980s, the New Austrian Tunnelling Method (NATM) has been developed as a one of the standard tunneling method in Korea. Approximately 10 years ago, wet-mix shotcrete with sodium silicate accelerator (waterglass) was introduced and widely used to tunnel lining and underground support. However, this accelerator had some disadvantages due to the decrease of long-term strength compared to plain concrete (without accelerator) and low quality of the hardened shotcrete. In order to compensate for these disadvantages, recently developed alkali-free accelerator has been successfully demonstrated in numerous projects and applications as a new material to make tunnels more durable and safer. An experimental investigation was carried out in order to verify the strength behavior of wet-mix Steel Fiber Reinforced Shotcrete (SFRS) with alkali-free accelerator. Compressive strength, flexural strength and equivalent flexural strength were measured by testing specimens extracted from the shotcrete panels. From the results, wet-mix SFRS with alkali-free accelerator exhibited excellent strength improvement compared to the conventional shotcrete accelerator.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2013.05a
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• pp.56-58
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• 2013

In order to develop concrete generating compressive strength of 15MPa~30MPa aging for 6~12 hours in the room temperature curing, Hardening accelerator containing Ca²⁺ mixed with rapid hardening portland cement containing C₃S in quantity. The result was that the more addictive contents of Hardening accelerator is, the more greatly early compressive strength was improved. That s because the composition of Ca(OH)₂ was mass-produced at early-ages.

• Journal of the Korea Institute of Building Construction
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• v.10 no.4
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• pp.113-122
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• 2010

In this study, 150MPa ultra-high-strength concrete was manufactured, and its performance was reviewed. As technically meaningful autogenous shrinkage reportedly occurs at a W/B ratio of 40% or less, although it occurs in all concrete regardless of the W/B ratio, the effects of the use of expansive admixture and shrinkage reducer, or of the friction and restraint of forms that may result in the effective reduction of autogenous shrinkage, were reviewed. As a result, considering the flow and strength characteristics, it was found that the slump flow time was shorter with expansive admixture, and shortest with shrinkage reducer. All specimens with $30kg/m^3$ expansive admixture showed high strength at early material age. Their strength decreased due to the expansion cracks when there was excessive use of expansive admixture, and the use of shrinkage reducer did not influence the change in the strength according to the material age. The expansive admixture had a shrinkage reduction effect of 80%, while the shrinkage reducer had a shrinkage reduction effect of 30%, indicating that the expansive admixture had a stronger effect. It seems that mixing the two will have a synergistic effect. The shrinkage reduction rate was highest when the W/B ratio was 20%. The form suppressed the expansion and shrinkage at the early period, and the demolding time did not significantly influence the shrinkage. The results of the study showed that the excessive addition of expansive admixture leads to expansion cracks, and the expansive admixture and shrinkage reducer have the highest shrinkage reduction effect when they are mixed.

• International Journal of Highway Engineering
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• v.11 no.2
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• pp.185-194
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• 2009

The autogenous shrinkage of high-performance concrete, including very-early strength latex-modified concrete(VES-LMC), is generally bigger than that of normal strength concrete because of the low water/cement ratio, high binder contents, and usage of superplasticizer. Mix. proportion of VES-LMC has low water/cement ratio(0.38), high cement content(390kg/m$^3$), and aid of latex(15% of cement weight). Thus, these factors of VES-LMC, rapid water self-dissipation and evaporation within 3 hours of concrete placement would increase the autogenous shrinkage. The purpose of this study was to evaluate the early-age shrinkage, thermal deformation and autogenous shrinkages of VES-LMC with retarder contents(retarder solids-cement ratio, by weight) using to secure working time in field. The experimental results showed that retarder contents do not affect of the maximum hydration temperature. Early-age expansion of VES-LMC was mostly caused by thermal expansion and partly by autogenous expansion. The autogenous shrinkage is decreased by increasing the retarder contents within this study. On the other hand, the usage of retarder should be decided carefully considering the field conditions because an excessive usage of retarder can cause handful early-age expansion.

• Journal of the Korea Concrete Institute
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• v.15 no.1
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• pp.43-51
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• 2003

Recently, to consider financial and constructive aspect usage of Admixture such as Blast-Furnace Slag and Fly-Ash, are increased. Also the use of cold-weather-concrete is increased. Blast-Furnace Slag, a by-product of steel industry, have many advantage to reduce the heat of hydration, increase in ultimate strength and etc. But it also reduces early-age strength, so it is prevented from using of Blast-Furnace Slag at cold-weather-concrete. In this study, for the purpose of increasing usage of Blast-Furnace Slag at cold-weather-concrete, it is investigated the strength properties of concrete subjected to frost damage for the cause of early age curing. The factors of this experience to give early frost damaged were Freezing temperature(-1, -10, $-15^{\circ}C$), Early curing age(0, 12, 24, 48hour), Freezing times(0, 12, 24, 48hour). According to this study, if early curing is carried out before haying frost damage, the strength of concrete used admixture, subjected to frost damage, is recovered. And that properties are considered, the effect of using admixture like blast-furnace-slag, is very high

• Advances in concrete construction
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• v.5 no.4
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• pp.345-358
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• 2017

This paper describes the experimental investigation carried out to develop geopolymer concrete using rice husk ash (RHA) along with alccofine. The study reports the fresh and hardened properties of the geopolymer concrete (GPC) activated using alkaline solution. GPC were prepared using different RHA content (350, 375 and $400kg/m^3$), the molarity of the NaOH (8, 12 and 16M). The specimens were cured at $27^{\circ}C$ and $90^{\circ}C$. GPC was activated using NaOH, $Na_2SiO_3$, and alccofine. Prepared GPC samples were tested for compressive and splitting tensile strengths after 3, 7 and 28 days. RHA was suitable to produce geopolymer concrete. Results indicate that behavior of GPC prepared with RHA is similar to fly ash based GPC. Workability and strength can be improved by incorporating the alccofine. Further, alccofine and heat curing improve the early age properties of the GPC. Heat curing is responsible for the initial polymerization of GPC which leads to high workability and improved mechanical properties of the GPC. High strength can be achieved by using the high concentration alkaline solution in terms of molarity and at elevated heat curing. Further, RHA based geopolymer concrete has tremendous potential as a substitute for ordinary concrete.