• Magazine of the Korea Concrete Institute
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• v.10 no.3
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• pp.143-152
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• 1998

본 연구에서는 콘크리트 강도에 미치는 양생온도의 영향이 양생시점에 따라 어떻게 변하는지에 대한 실험과 기존의 모델식을 이용하여 분석을 수행하였으며, 양생시점의 영향을 고려한 수정된 등가재령식을 제시함으로써 새로운 강도예측식을 개발하기 위한 기초연구이다. 이를 위해 2종류의 물.시멘트비에 대하여 각각 11종류의 양생이력을 고려한 실험을 수행하였다. 실험변수로는 3종류의 양생온도 (5 $^{\circ}C$, 20 $^{\circ}C$, 4$0^{\circ}C$)와 4종류의 양생시점(0~1일. 1~2, 2~3, 6~7일)을 선정하였다. 또 기존의 Saul 및 Arrhenius 모델식을 이용하여 실험결과를 분석하여 양생시점의 영향을 도입한 각각 수정된 등가재령식을 제시하였다. 실험결과에서 초기재령에서 고온 양생한 경우에는 초기에는 높은 강도를 나타내지만 재령이 증가할수록 오히려 낮은 강도를 나타내었다. 또 초기에 저온으로 양생한 콘크리트는 그 반대의 경향을 보였다. 기존의 등가재령식에서는 같은 등가재령에서도 압축강도는 양생시점에 따라 달라짐을 알 수 있었다. 기존의 모델식은 특히 초기재령에서의 강도예측결과가 실험결과와 잘 맞지 않았으나 이 논문에서 제시된 수정된 등가재령식은 실험결과와 잘 일치하는 결과를 보여 주었다.

• Magazine of the Korea Concrete Institute
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• v.10 no.5
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• pp.89-100
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• 1998

본 연구는 양생온도이력이 콘크리트 강도에미치는 영향을 평가하기 위한 것으로 물 시멘트비가 60%, 45%, 26%인 3종류의 콘크리트에대하여 5$^{\circ}C$부터 5$0^{\circ}C$까지의 항온양생과 초기재령에 고온도이력을 변수로 한 변동온도양생을 실시한 공시체의 압축강도를 측정하였다. 또한 그 실험결과에 강도평가 방법의 하나인 Maturity 개념을 도입하여 강도평가에 미치는 재령, w/c, 온도이력 등에 대한 영향을 평가하였다. 항온양생 실험결과에 따르면 물시멘트비가 낮을수록 초기재령에서의 강도발현은 높게 나타나며 양생온도 5$0^{\circ}C$인 경우를 제외하고 재령7일까지의 강도발현은 양생온도가 높을수록 크게 나타나고 있다. 한편, 변동온도양생실험결과에 의하면 초기재령에서 고온양생한 콘크리트의 강도발현은 물시멘트비의 영향을 크게 받으며, 1주 이후의 양생온도가 강도발현에 미치는 영향은 1주까지의 고온도이력에 대한 영향에 비교해 2차적이다. 기존의 Maturity개념인 Saul-Bergstrom의 함수와 Ooi의 함수를 가지고 항온 및 변동온도 양생실험결과를 분석한 결과, 전체적으로 Saul-Bergstrom식에 의한 경우가 실험값과의 차이가 작게 나타났으나 두 식 모두 Maturity 가 큰 경우에는 계산에 의한 값이 실험에 의한 값보다 크게 나타나고 있어 장기 재령시 강도평가는 한계강도 개념을 고려한 새로운 Matruity함수를 제안할 필요가 있다.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.2
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• pp.99-107
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• 2014

In this study, the effects of curing procedures on the mechanical properties of recycled aggregate concrete (RAC) were investigated. The replacement ratios by recycled coarse aggregate were 0, 25, 50, 75 and 100% by mass of natural coarse aggregate. Steam curing was adopted to all recycled aggregate concrete mixtures. Compressive and split tensile strength, water porosity, chloride ions penetration resistance and drying shrinkage measurements were carried out to determine performance of the RACs. From the test results, it was found that the mechanical performance of RAC decreased as the recycled aggregate contents increased. Furthermore, steam curing reduced the compressive and split tensile strength, water porosity and total charge of RAC, especially at the early ages. However, at the later ages, the beneficial effect of steam curing was less prominent. This study clearly showed that initial steam curing could be one of practical methods to improve performance of RAC with higher replacement ratio of recycled aggregate.

• Journal of the Korean Recycled Construction Resources Institute
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• v.11 no.1
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• pp.62-69
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• 2023

For the nuclear power concrete plant structures in the UAE, it is necessary to consider the deterioration from high sulfate ions in the atmosphere and high chloride ions from the coast. In this study, two strength grade concrete mixture (40 MPa and 27 MPa) and two curing/diffusion temperatures (20 ℃ and 50 ℃) were considered for evaluating the temperature effects on diffusion and strength due to high average temperature above 38 ℃ a year in UAE. When the initial curing temperature was high, the compressive strength increased in high-temperature curing to 7 days, but the strength slightly increased in the 20 ℃ curing condition at 28 days. Regarding diffusion test, unlike the compressive test results, reduced chloride diffusion coefficients were evaluated both in 40 MPa and 27 MPa grade at 28 days. In the case of 91 days of curing, an increase in diffusivity due to high temperature and a decrease in diffusivity due to age effect occur simultaneously. Compared to the results of the curing and diffusion tests at 20 ℃ and 28 days, when the curing and diffusion tests were conducted at 50 ℃ in 91 days, the diffusion coefficients decreased to 76.2 % in 40 MPa grade and 85.4 % in 37 MPa grade with increasing curing period, respectively.

• Journal of the Korea Institute of Building Construction
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• v.17 no.2
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• pp.175-181
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• 2017

The purpose of this study is evaluating the characteristics of tensile strength of UHPC and examining tensile performance of notched specimens by direct tensile test. For test variables, 120, 150, and 180MPa of target design standard strength were aimed at. With general water curing and $90^{\circ}C$ high temperature steam as curing conditions, the properties were reviewed. Overall, it was represented that the specimens of notch-type direct tensile strength concrete was effective in inducing central cracks compared with existing direct tension specimens. Through this, it was judged that data construction with high reliability was possible. Above all, in a graph of direct tensile strength and strain, in the case of steam curing at high temperature, there was great difference of initial tensile strength compared with water curing. As passing of ages, an aspect that the difference gradually decreased was shown. Maximum tensile strength was found to increase steadily with increasing age for all target design strengths in water curing, in the case of steam curing, the tendency to increase significantly due to the initial strength development effect at 7 days of age. The initial crack strength increases with age in case of underwater curing, in the case of steam curing, it was higher than that of water curing in 7 days, while the strength of 28 days was lowered. In this part, it is considered necessary to examine the arrangement condition of the steel fiber.

• Journal of the Korea Concrete Institute
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• v.25 no.1
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• pp.73-80
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• 2013

Polymer cement slurry (PCS) made from organic polymer dispersion and cement has good adhesion to steel, waterproofness and acid resistance due to being of polymer films formed in cement slurry. The purpose of this study is to evaluate the mix design conditions at early curing age of PCS-coating material effected on improvement in bond strength of coated rebar. The test pieces are prepared with two types of polymer dispersions such as St/BA and EVA, four polymer-cement ratios, two types of cement, four coating thicknesses and three curing ages, and tested for the bond strength test. From the test results, in general, bond strength of PCS-coated rebar is better than that of uncoated rebar and epoxy-coated rebar. It is also high bond strength at curing ages of 7-day or less, and coating thicknesses of $75{\mu}m$ and $100{\mu}m$. The maximum bond strength of PCS-coated rebar at curing age of 3-hour is almost same as that of curing age of 1-day and 7-day. The maximum bond strength of PCS-coated rebar with ultra high-early strength cement and St/BA at polymer-cement ratio of 80%, and coating thickness of $100{\mu}m$ is about 1.52 and 1.58 times respectively, the strength of uncoated rebar and epoxy-coated rebar. It is apparent that the curing age, coating thickness, type of polymer and cement are very important factors to improve the bond strength of PCS-coated rebar to cement concrete. We can have basic information that PCS-coated rebar with curing age at 3-hour and coating thickness of $100{\mu}m$ can replace epoxy-coated rebar.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.5A
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• pp.737-746
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• 2008

Recently, NDTs (Non-Destructive Techniques) using electromagnetic(EM) properties are applied to the performance evaluation for RC (Reinforced Concrete) structures. Since nonmetallic materials which are cement-based system have their unique dielectric constant and conductivity, they can be characterized and changed with different mixture conditions like W/C (water to cement) ratios and unit cement weight. In a room condition, cement mortar is generally dry so that porosity plays a major role in EM properties, which is determined at early-aged stage and also be affected by curing condition. In this paper, EM properties (dielectric constant and conductivity) in cement mortar specimens with 4 different W/C ratios are measured in the wide region of 0.2 GHz~20 GHz. Each specimen has different submerged curing period from 0 to 28 days and then EM measurement is performed after 4 weeks. Furthermore, porosity at the age of 28 days is measured through MIP (Mercury Intrusion Porosimeter) and saturation is also measured through amount of water loss in room condition. In order to evaluate the porosity from the initial curing stage, numerical analysis based on the modeling for the behavior in early-aged concrete is performed and the calculated results of porosity and measured EM properties are analyzed. For the convenient comparison with influencing parameters like W/C ratios and curing period, EM properties from 5 GHz to 15 GHz are averaged as one value. For 4 weeks, the averaged dielectric constant and conductivity in cement mortar are linearly decrease with higher W/C ratios and they increase in proportion to the square root of curing period regardless of W/C ratios.

• Journal of the Korea Institute of Building Construction
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• v.17 no.4
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• pp.331-339
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• 2017

The purpose of this study is to evaluate the effect of curing age and thickness of coating material on the bond strength of polymer cement slurry(PCS)-coated rebar that can replace epoxy-coated rebar. The test specimens were prepared with two types of cement, two types of polymer dispersion as St/BA and EVA, two polymer-cement ratios, two coating thicknesses and three curing ages, and tested for bond strength test to cement concrete. The flexural behavior of RC beam that is made by optimum conditions such as polymer-cement ratio of 80%, coating thickness of $100{\mu}m$ and curing age of 7 days of PCS recommended from the bond strength test is also conducted. From the test results, The maximum bond strength of PCS-coated rebar at curing age of 7-day and coating thickness of $100{\mu}m$ was about 1.52 and 1.58 times respectively, the strength of plain and epoxy-coated rebar. The ultimate loads of RC beam using PCS-coated rebar were range of 81.1% to 102.3% of that of plain rebar, and 98.4% to 124.1% of that of epoxy-coated rebar. It is apparent that PCS-coated rebar with EVA, curing age at 7-day and $100{\mu}m$ can replace epoxy-coated rebar in construction field.

• Journal of the Korea Concrete Institute
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• v.14 no.2
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• pp.137-147
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• 2002

Maturity models involving curing temperature and curing ages have been widely used to predict concrete strength, which can accurately estimate concrete strength. However, they may not consider physical quantities such as the characteristics of hydrates and the capillary porosity of microstructures associated with strength development. In order to find out the effects of both factors on a strength increment, the hydration model and the estimation method of the amount of capillary porosity were established, and the compressive strength test of concrete nth various water/cement ratios was carried out considering two test parameters, curing temperature and curing age. In this study, by analyzing the experimental results, a strength estimation model for early-age concrete that can consider the microstructural characteristics such as hydrates and capillary porosity was proposed. Measured compressive strengths were compared with estimated strengths and good agreements were obtained. Consequently, the proposed strength model can estimate compressive strength of concrete with curing age and curing temperature within an acceptable error.

• Journal of the Korea Institute of Building Construction
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• v.20 no.5
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• pp.409-416
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• 2020

In this paper, strength correction factors of the concretes incorporating ordinary Portland cement(OPC), fly ash(FA) and blast furnace slag(BS) with 50% of water to binder ratio due to temperature drop for standard room temperature(20±3℃) are provided. For this, strength development was done based on equivalent age method. For calculating the equivalent age, apparent activation energy was obtained with 24.69 kJ/mol in OPC, 46.59 kJ/mol in FA, 54.59 kJ/ol in BS systems. According to the estimation of strength development of the concretes, the use of FA and BS resulted in larger strength drop than that of OPC under low temperature compared to standard room temperature. Hence, strength correction factors(Tn) for OPC, FA and BS are suggested within 4~17℃ with every 3MPa levels.