Jin-Keun Kim

An experimental study on thermal conductivity of concrete

Abstract Influencing factors on thermal conductivity of concrete are quantitatively investigated by QTM-D3—that is, a conductivity tester developed in Japan—and a prediction equation of thermal conductivity of concrete is suggested from the regression analysis of test results. To consider the interacted factors influencing thermal conductivity of concrete, mortar, and cement paste, seven testing variables such as age, water–cement (W/C) ratio, types of admixtures, aggregate volume fraction, fine aggregate faction, temperature, and humidity condition of specimen were adopted in this test. According to experimental results, aggregate volume fraction and moisture condition of specimen are revealed as mainly affecting factors on the conductivity of concrete. Meanwhile, the conductivities of mortar and cement paste are strongly affected by the W/C ratio and types of admixtures. However, age hardly changes the conductivity except for very early age. Finally, the conductivity of concrete is represented in terms of the aggregate volume fraction, fine aggregate fraction, W/C ratio, temperature, and humidity condition of specimen.

Effect of temperature and aging on the mechanical properties of concrete. Part I. Experimental results

Abstract This paper reports the results of curing temperature and aging on the strength and elastic modulus and the Part II paper suggests a prediction model based on these experimental results. Tests of 480 cylinders made of Types I, V, and V cement+fly ash concretes, cured in isothermal conditions of 10, 23, 35, and 50 °C and tested at the ages of 1, 3, 7, and 28 days are reported. According to the experimental results, concretes subjected to high temperatures at early ages attain higher early-age compressive and splitting tensile strengths but lower later-age compressive and splitting tensile strengths than concretes subjected to normal temperature. Even though the elastic modulus has the same tendency, the variation of elastic modulus with curing temperature is not so obvious as compressive strength. Based on the experimental result, the relationships among compressive strength, elastic modulus, and splitting tensile strength are analyzed, considering the effects of curing temperature, aging, and cement type.

Moisture diffusion of concrete considering self-desiccation at early ages

In concrete structures exposed to the ambient air at early ages, the moisture content in concrete decreases due to moisture diffusion. In addition, self-desiccation due to hydration of cement causes an additional decrease of moisture content in concrete at early ages, especially for high-strength concrete. In this study, the internal relative humidity in drying concrete specimens was measured at early ages. Furthermore, the variation of relative humidity due to self-desiccation in sealed specimen was measured. The moisture distribution in low-strength concrete with high water/cement ratio was mostly influenced by moisture diffusion due to drying rather than self-desiccation. In high-strength concrete with low water/cement ratio, however, self-desiccation had a considerable influence on moisture distribution. The results obtained from the moisture diffusion theory were in good agreement with experimental results.

Fabrication of LiMn2O4 thin films by sol–gel method for cathode materials of microbattery

Abstract LiMn2O4 thin films have received considerable attention as cathode materials for thin-film microbatteries. In this work, LiMn2O4 thin films are prepared by a sol–gel method using a spin coator. The precursor powder is investigated by TG–DTA and mass spectroscopy analysis in order to study the decomposition process prior to deposition. The coated films are dried at 310 to 360°C, and annealed at 700 to 800°C to obtain a spinel structure. Films annealed under appropriate conditions exhibit good crystallinity, smooth surface morphology, high capacity, and good rechargeability. This film is therefore suitable for use as a cathode for thin-film microbatteries.

Prediction of differential drying shrinkage in concrete

The non-uniform moisture distribution in concrete causes the differential drying shrinkage. From this type of differential drying shrinkage, tensile stress occurs on the exposed surface of concrete structures and may result in crack formation. This residual stress is significantly affected by the creep of concrete. In this study, for the purpose of predicting the differential drying shrinkage, the analysis method was suggested, in which the creep of concrete was also considered. In addition, the differential drying shrinkage strain was measured at various positions in concrete by using embedded strain gauges. The internal drying shrinkage strain differs significantly according to the depth from exposed surface. The validity of analysis method was verified by comparing test results with analytical results. Finally it was found that analytical results were in good agreement with test results.

Prediction of compressive strength of fly ash concrete by new apparent activation energy function

Abstract A new prediction model using apparent activation energy is proposed to estimate the variation of compressive strength of fly ash concrete with aging. After analyzing the experimental result with the model, fly ash replacement content and water–binder ratio influence on apparent activation energy was investigated. According to the analysis, the model provides a good estimation of compressive strength development of fly ash concrete with aging. As the fly ash replacement content increases, limiting relative compressive strength and initial apparent activation energy increase. Concrete with water–binder ratio smaller than 0.40 gives nearly constant limiting relative compressive strength and initial apparent activation energy when analyzed with various water–binder ratios. However, concrete with water–binder ratio larger than 0.40 increases limiting relative compressive strength and initial apparent activation energy.

The behavior of reinforced concrete columns subjected to axial force and biaxial bending

Abstract When stress is beyond elastic limit or cracking occurs in a reinforced concrete member subjected to axial force and biaxial bending, curvature about each principal axis of gross section may be influenced by axial force and bending moments about both major and minor principal axes. It is mainly due to the translation and rotation of principal axes of the cross section after cracking. In this study, a numerical method for predicting the behavior of reinforced concrete columns subjected to axial force and biaxial bending is proposed considering curvature localization. To verify the proposed numerical method, a series of tests was also carried out for 16 tied reinforced concrete columns with 100×100 mm square and 200×100 mm rectangular sections under various loading conditions. The boundary conditions at both ends of the column were hinged and eccentricities (40 mm) were equal and of the same directions. The angles between the direction of eccentricity and the major principal axis of gross section were 0°, 30°, 45° for the square section and 0°, 30°, 45°, 60°, 90° for the rectangular section. A comparison between the numerical predictions and the test results shows good agreements in ultimate loads, axial force-lateral deflection relations, and lateral deflection trajectories. It is also found, in this limited investigation, that ACIs moment magnifier method is conservative in both uniaxial and biaxial eccentric loading conditions.

SIZE EFFECT ON COMPRESSIVE STRENGTH OF PLAIN AND SPIRALLY REINFORCED CONCRETE CYLINDERS

Many experimental and theoretical investigations have been carried out to examine the reduction phenomenon of compressive strength of cylindrical concrete specimens with size, but up until now, an adequate analysis technique has not been developed. In this paper, the fracture mechanics type size effect on the compressive strength of cylindrical concrete specimens was studied, with the diameter, the height/diameter ratio, and the volumetric spiral ratio of the cylinder considered as the main parameters. For this purpose, theoretical and statistical analyses were conducted. First, a size effect equation was proposed to predict the compressive strength of cylindrical concrete specimens with various diameters and height/diameter ratios. Second, the model equation derived from the plain concrete was extended for predicting the compressive strength of spirally reinforced concrete cylinders. The proposed equation showed good agreement with the existing test results for concrete cylinders with and without spiral reinforcement.

Effect of temperature and aging on the mechanical properties of concrete: Part II. Prediction model

In Part I, empirical relationships between compressive strength and splitting tensile strength or elastic modulus with temperature and aging were proposed. This paper investigates new prediction models estimating splitting tensile strength and elastic modulus without knowing compressive strength. The prediction model is suggested on the basis of the equation that was suggested to predict compressive strength. The mechanical properties calculated by the model are compared with empirical results presented in Part I. To evaluate in-place applicability of the model, the empirical data on strength and elastic modulus of concrete cured at variable temperature are compared with the values estimated using the prediction model. The prediction model properly estimates the strength and elastic modulus of Types I and V cement concretes cured at constant and variable temperature conditions.

Experimental study of the fatigue behavior of high strength concrete

In this study, cylindrical concrete specimens with various strength levels were tested to investigate the fatigue behavior of concrete in compression. Selected test variables were compressive strength with 4 levels (26 Mpa, 52 Mpa, 84 Mpa, 103 Mpa) and maximum stress with 4 levels (75%, 80%, 85%, 95%), 160 specimens (φ 100 × 200 mm) were cast for the test. The reference for the fatigue strength was the ultimate static strength acquired just before the fatigue testing. The moisture contents of specimens were preserved during the fatigue testing. In fatigue tests, the first cycle of loading was loaded at standard rate, and the other cycles were loaded in the frequency of 1 Hz. Test results show that the fatigue life decreased with increasing the concrete strength, and a model for Smax-Nf relationship considering the effect of the concrete strength was proposed. While fatigue strain of high strength concrete was smaller than that of low strength concrete, the rate of fatigue strain increment of high strength concrete was greater than that of low strength concrete.