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정진훈 한국도로학회 2005 한국도로학회논문집 Vol.7 No.3
Predicting distribution and variation of humidity inside concrete is essential to improve curing quality of concrete at field. The concrete humidity is predicted by numerical analysis using surface humidity as boundary condition. However, ambient humidity has been used instead of the surface humidity because the surface humidity could not be accurately measured. Because it is hard to accurately measure the surface humidity, however, the ambient humidity has been used instead of the surface humidity in the numerical analysis. In this paper, a methodology to accurately measure the surface humidity is suggested, and the ambient humidity and the humidity at the surface and inside the concrete measured by a series of laboratory tests are presented. The cause of low concrete humidity immediately after placement was investigated by a separately performed test. A surface humidity prediction model was developed using the measured humidity, and consequently validated through an additional test. 콘크리트 내부의 습도분포와 변화를 예측하는 것은 현장에서 콘크리트의 양생품질을 향상시키기 위하여 필수적이다. 콘크리트의 습도는 주로 표면습도를 경계조건을 사용하는 수치해석을 통하여 예측된다. 하지만, 표면습도를 정확히 측정하기 어려우므로 거의 모든 수치해석에 표면습도 대신 대기습도를 사용하여 왔다. 본 논문에서는 표면습도를 정확하게 측정할 수 있는 방법을 제시하고 일련의 실내실험을 통하여 측정된 대기습도와 콘크리트의 내부 및 표면습도를 보여준다. 이와는 별개의 실험을 통하여 타설 직후의 콘크리트가 낮은 습도를 나타내는 원인을 조사하였다. 측정된 습도를 이용하여 콘크리트의 표면습도를 예측할 수 있는 모형을 개발하였으며 추가적인 실험을 통하여 모형의 유효성을 검증하였다.
Aggregate Gradation Effects on Cracking-Related Displacements in Concrete Pavement
정진훈,김낙석 한국콘크리트학회 2005 콘크리트학회논문집 Vol.17 No.4
Aggregate gradation effects on cracking-related displacements of concrete are investigated in the laboratory using the German cracking frame. Concrete workability was assessed by use of the slump and drop tests for two different concrete mixtures consisting of gap-graded and dense-graded aggregates. Shrinkage strain, cracking frame strain, and concrete strain were measured and used to compare to strength gain and creep development. The measured and calculated strains of the different aggregate gradations were compared each other. Gradation effects on strength and stress development relative to tensile cracking at saw-cut tip were also investigated. Test results revealed that the gap-graded concrete has indicated larger shrinkage and creep strains than dense-grade concrete perhaps because of its higher volume concrete of cement mortars in the mixture.
A Thermal Conductivity Model for Hydrating Concrete Pavements
정진훈,김낙석 한국콘크리트학회 2004 콘크리트학회논문집 Vol.16 No.1
Hydrating concrete pavement is typically subjected to temperature-induced stresses that drive cracking mechanisms at early concrete ages. Undesired cracking plays a key role in the long-term performance of concrete pavement systems. The loss of support beneath the concrete pavement due to curling caused by temperature changes in the pavement may induce several significant distresses such as punch out, pumping, and erosion. The effect of temperature on these distress mechanisms is both significant and intricate. Because thermal conductivity dominates temperature flow in hydrating concrete over time, this material property is back-calculated by transforming governing equation of heat transfer and test data measured in laboratory. Theoretically, the back-calculated thermal conductivity simulates the heat movements in concrete very accurately. Therefore, the back-calculated thermal conductivity can be used to calibrate concrete temperature predicted by models.
정진훈 한국도로학회 2005 한국도로학회논문집 Vol.7 No.1
Concrete has higher vapor pressure than its surrounding ambient air immediately after placement. The moisture at the concrete surface evaporates to the ambient air to adjust the equilibrium of vapor pressure between them. The moisture inside the concrete moves to the surface because the evaporation at the surface causes the gradient of vapor pressure inside the concrete. Plastic cracking, degree of hydration, strength development, and others caused by the velocity of the moisture movement significantly influences the quality of concrete. In this paper, the moisture diffusivity of early-age concrete was back-calculated using the governing equation of the moisture diffusion, and temperature and relative humidity of concrete measured in a laboratory. The moisture diffusivity of concrete was modeled using the back-calculated moisture diffusivity. The relative humidity of concrete calculated by finite element method (FEM) using the modeled moisture diffusivity as input data coincided with the measured relative humidity well.