• Proceedings of the Korea Concrete Institute Conference
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• 2000.10b
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• pp.1189-1194
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• 2000

The purpose of this investigation is to establish which consequence is happen about initial curing concrete's compressive strength of 7days, 28days which changes W/C ratio by the change of vibrating speed and vibrated time. An experimental parameter is fixed 4 degrees of W/C ratio(45%, 50%, 55%, 60%), 3 degrees of vibrated time(3hr, 6hr, 12hr) and different vibrating speed(0.25kine, 0.5kine, 1kine). As the result, compressive strength of 7days was increased when vibrating speed and vibrated time is 1kine, 12h and also compressive strength of 28days was increased when vibrating speed and vibrated time is 0.25kine and 3hr.

• Journal of the Korea Concrete Institute
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• v.17 no.4 s.88
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• pp.551-558
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• 2005

The setting and hardening of concrete is accompanied with nonlinear temperature distribution caused by development of hydration heat of cement. Especially at early ages, this nonlinear distribution has a large influence on the crack evolution. As a result, in order to predict the exact temperature history in concrete structures it is required to examine thermal properties of concrete. In this study, the convection heat transfer coefficient which presents thermal transfer between surface of concrete and air, was experimentally investigated with variables such as velocity of wind, curing condition and ambient temperature. At initial stage, the convection heat transfer coefficient is overestimated by the evaporation quantity. So it is essential to modify the thermal equilibrium considered with the boiling effect. From experimental results, the convection heat transfer coefficient was calculated using equations of thermal equilibrium. Finally, the prediction model for equivalent convection heat transfer coefficient including effects of velocity of wind, curing condition, ambient temperature and boiling effects was theoretically proposed. The convection heat transfer coefficient in the proposed model increases with velocity of wind, and its dependance on wind velocity is varied with curing condition. This tendency is due to a combined heat transfer system of conduction through form and convection to air. From comparison with experimental results, the convection heat transfer coefficient by this model was well agreed with those by experimental results.

• Journal of the Korea institute for structural maintenance and inspection
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• v.7 no.1
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• pp.139-145
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• 2003

Ternary blended concrete, which contains both fly ash and granulated blast furnace slag, has an initial cost effective and is environment friendly. Furthermore, it has a lot of technical advantages such as the improvement of long term compressive strength, high workability, and the reduction of hydration heat. However, as the use and study on the performance of ternary blended concrete is limited, it is worthwhile studying the actual performance of this technology. This study examined the durability performance of ternary blended concrete, compared to binary blended concrete and ordinary portland concrete. It led to the conclusion that ternary blended concrete is very suitable for submerged members under marine environment. However, it should be noticed that ternary blended concrete becomes weak on carbonation, when it is situated on combined deterioration environment of carbonation and chloride. Therefore, the curing duration of ternary blended concrete should be prolonged in order to enhance the resistance of carbonation.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2013.05a
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• pp.334-336
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• 2013

In this study, the temperature history was evaluated for the improved bubble sheets combining hot wires and PE films, which were developed under the extreme environmental condition of -10℃ and applied on the top surface of slab to prevent initial damage by freezing. Results can be summarized as follows. If improved bubble sheets combining hot wires with different capacity on double and quadruple bubble sheets are used, the temperature history for all materials decreased to 2~3℃ below zero but all test materials except Type 1 secured the accumulative temperature of 45° D·D at 7 days of material age, required for the prevention of initial freezing damage. This indicates the bubble sheets can prevent the initial damage by freezing.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2016.05a
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• pp.11-12
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• 2016

The contemporary buildings are becoming super-taller gradually as the industry evolves. Accordingly, winter concrete also became an important element for the year-round construction since shortening of a construction duration became important. Accordingly, this research team once developed a double bubble sheet as concrete cover curing compound during winter. But since there is no proper countermeasure for the exposed reinforcement bars, it is worried that the concrete can be damaged by exposed frozen reinforcement bars at a low temperature during the initial period. Therefore, in this study, it is intended to review the temperature history of the reinforcement bars depending on changes of the covering methods of the exposed reinforcement bars by using bubble sheets.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2020.06a
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• pp.73-74
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• 2020

In this study, the feasibility of the durometer into super retarding concrete was studied by comparing the penetration resistance with the hardness of each durometer using the penetration resistance and the improved Durometer and Durometer A-Type according to the ultra-delay mixture rate. The test results showed that initial setting time by improved Durometer and Durometer A-Type were fixed at 25, 50 HD, respectively, and the 35, 80 HD showed at final setting time. It was also found that the use of the durometer can be available to measure the setting time of the concrete.

• Journal of the Korea Institute of Building Construction
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• v.2 no.4
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• pp.177-182
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• 2002

The purpose of this study is to develope the strength prediction model by Maturity Method. A maturity function is a mathematical expression to account for the combined effects of time and temperature on the strength development of a cementious mixture. The method of equivalent ages is to use Arrhenius equation which indicates the influence of curing temperature on the initial hydration ratio of cement. For the experimental factors of this study, we selected the concrete mixing of W/C ratio 45, 50, 55 and 60% and curing temperature 5, 10, 20 and $30^{\circ}C$. And we compare and evaluate with logistic model that is existing strength prediction model, because we have to verify adaption possibility of new strength prediction model which is proposed by maturity method. As the results, it is found that investigation of the activation energy that are used to calculate equivalent age is necessary, and new strength prediction model was proved to be more accurate in the strength prediction than logistic model in the early age. Moreover, the use of new model was more reasonable because it has low SSE and high decisive factor.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.6
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• pp.52-58
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• 2017

Concrete made with ground granulated blast-furnace slag(GGBS) has many advantage, including improved durability, workability and economic benefits. GGBS concrete is that its strength development is considerably slower under standard $20^{\circ}C$ curing conditions than that of portland cement concrete, although the ultimate strength is higher for same water-binder ratio. GGBS is not therefore used in application where high early age strength is required. In this study, to overcome the limitation of the initial strength decrease due to the use of slag, the slag substitution rate was changed to 30% under the low temperature curing temperature condition and the slag used concrete composition with the same or higher strength performance as OPC(Ordinary Portland Cement).

• Computers and Concrete
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• v.31 no.6
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• pp.501-511
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• 2023

The strengthening efficiency of biopolymer treated soil depends on biopolymer type, concentration ratio, soil type, initial water content, curing time and mixing method. In this study, the physical and mechanical properties of xanthan gum (XG) treated kaolinite were investigated through compaction test, Atterberg limit test, triaxial test and unconfined compression test. The results indicated that the optimum water content (OWC) increased from 30.3% of untreated clay to 33.5% of 5% XG treated clay, while the maximum dry density has a slight increase from 13.96 kg/m³ to 14 kg/m³ of 0.2% XG treated clay and decrease to 2.7 kg/m³ of 5% XG treated clay. Meanwhile, the plastic limit of XG treated clay increased with the increase of XG concentration, while 0.5% XG treated clay can be observed the maximum liquid limit with 79.5%. Moreover, there are the ideal water content about 1.3-1.5 times of the optimum water content achieving the maximum dry density and curing time to obtain the maximum compressive strength for different XG contents, which the UCS is 1.52 and 2.07 times of the maximum UCS of untreated soil for 0.5% and 1% XG treated clay, respectively. In addition, hot-dry mixing can achieve highest UCS than other mixing methods (e.g., dry mixing, wet mixing and hot-wet mixing).

• Structural Engineering and Mechanics
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• v.77 no.1
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• pp.1-17
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• 2021

The influence of prestress force on the fundamental frequency and static deflection shape of uncracked Prestressed Concrete (PC) beams with a parabolic bonded tendon was examined in this paper. Due to the conflicts among existing theories, the analytical solutions for properly considering the dynamic and static behavior of these members is not straightforward. A series of experiments were conducted for a total period of approximately 2.5 months on a PC beam made with high strength concrete, subsequently and closely to the 28 days of age of concrete. Specifically, the simply supported PC member was short term subjected to free transverse vibration and three-point bending tests during its early-age. Subsequently, the experimental data were compared with a model that describes the dynamic behavior of PC girders as a combination of two substructures interconnected, i.e., a compressed Euler-Bernoulli beam and a tensioned parabolic cable. It was established that the fundamental frequency of uncracked PC beams with a parabolic bonded tendon is sensitive to the variation of the initial elastic modulus of concrete in the early-age curing. Furthermore, the small variation in experimental frequency with time makes doubtful its use in inverse problem identifications. Conversely, the relationship between prestress force and static deflection shape is well described by the magnification factor formula of the "compression-softening" theory by assuming the variation of the chord elastic modulus of concrete with time.