Seung-Jun Kwon

Effect of W/C Ratio on Durability and Porosity in Cement Mortar with Constant Cement Amount

Water is often added to concrete placing for easy workability and finishability in construction site. The additional mixing water can help easy mixing and workability but causes increased porosity, which yields degradation of durability and structural performances. In this paper, cement mortar samples with 0.45 of W/C (water to cement) ratio are prepared for control case and durability performances are evaluated with additional water from 0.45 to 0.60 of W/C. Several durability tests including strength, chloride diffusion, air permeability, saturation, and moisture diffusion are performed, and they are analyzed with changed porosity. The changing ratios and patterns of durability performance are evaluated considering pore size distribution, total porosity, and additional water content.

Prediction of Durability for RC Columns with Crack and Joint under Carbonation Based on Probabilistic Approach

Carbonation in RC (reinforced concrete) structure is considered as one of the most critical deteriorations in urban cities. Although RC column has one mix condition, carbonation depth is measured spatially differently due to its various environmental and internal conditions such as sound, cracked, and joint concrete. In this paper, field investigation was performed for 27 RC columns subjected to carbonation for eighteen years. Through this investigation, carbonation distribution in sound, cracked, and joint concrete were derived with crack mappings. Considering each related area and calculated PDF (probability of durability failure) of sound, cracked, and joint concrete through Monte Carlo Simulation (MCS), repairing timings for RC columns are derived based on several IPDF (intended probability of durability failure) of 1, 3, and 5%. The technique of equivalent probability including carbonation behaviors which are obtained from different conditions can provide the reasonable repairing strategy and the priority order for repairing in a given traffic service area.

Strength and Durability Performance of Alkali-Activated Rice Husk Ash Geopolymer Mortar

This paper describes the experimental investigation carried out to develop the geopolymer concrete based on alkali-activated rice husk ash (RHA) by sodium hydroxide with sodium silicate. Effect on method of curing and concentration of NaOH on compressive strength as well as the optimum mix proportion of geopolymer mortar was investigated. It is possible to achieve compressive strengths of 31 N/mm2 and 45 N/mm2, respectively for the 10 M alkali-activated geopolymer mortar after 7 and 28 days of casting when cured for 24 hours at 60°C. Results indicated that the increase in curing period and concentration of alkali activator increased the compressive strength. Durability studies were carried out in acid and sulfate media such as H2SO4, HCl, Na2SO4, and MgSO4 environments and found that geopolymer concrete showed very less weight loss when compared to steam-cured mortar specimens. In addition, fluorescent optical microscopy and X-ray diffraction (XRD) studies have shown the formation of new peaks and enhanced the polymerization reaction which is responsible for strength development and hence RHA has great potential as a substitute for ordinary Portland cement concrete.

Engineering properties of cement mortar with pond ash in South Korea as construction materials: from waste to concrete

Among the wastes from coal combustion product, only fly ash is widely used for mineral mixture in concrete for its various advantages. However the other wastes including bottom ash, so called PA (pond ash) are limitedly reused for reclamation. In this paper, the engineering properties of domestic pond ash which has been used for reclamation are experimentally studied. For this, two reclamation sites (DH and TA) in South Korea are selected, and two domestic PAs are obtained. Cement mortar with two different w/c (water to cement) ratios and 3 different replacement ratios (0%, 30%, and 60%) of sand are prepared for the tests. For workability and physical properties of PA cement mortar, several tests like flow, setting time, and compressive strength are evaluated. Several durability tests including porosity measuring, freezing and thawing, chloride migration, and accelerated carbonation are also performed. Through the tests, PA (especially from DH area) in surface saturated condition is evaluated to have internal curing action which leads to reasonable strength development and durability performances. The results show a potential applicability of PA to concrete aggregate, which can reduce consuming natural resources and lead to active reutilization of coal product waste.

Experimental Study on the Relationship between Time-Dependent Chloride Diffusion Coefficient and Compressive Strength

Dept. of Civil Engineering, Hannam University, Daejeon 306-791, KoreaABSTRACT Since strength and diffusion coefficient of concrete, representative concrete properties that in change with age, thetime effect must be considered in the analysis of chloride penetration. In this study, an evaluation of correlation between accelerateddiffusion coefficient, apparent diffusion coefficient, and compressive strength in high performance concrete with various mineraladmixtures such as ground granulated blast furnace slag, fly ash, and silica fume was performed. For this work, thirty mix prop ortionswere prepared. Accelerated diffusion coefficients at the age of 28, 91, 180, and 270 days were evaluated. For apparent diffusion coef-ficient, submerging test for 6 months was performed. For evaluation of compressive strength with ages, the compressive strengthtest was carried out at an age of 7, 28, 91, 180 days. The results of accelerated diffusion coefficient, apparent diffusion coefficient,and strength were compared, and the correlation was analyzed considering time dependency. From this study, linear relationshipbetween accelerated diffusion and apparent diffusion coefficient were obtained regardless of concrete age. The linear relations werealso observed in strength-accelerated diffusion coefficient and strength-apparent diffusion coefficient regardless of concrete age.Keywords : high performance concrete, time-dependency, accelerated diffusion coefficient, strength, apparent diffusion coefficient

Effect of different alkali salt additions on concrete durability property

Abstract Friedel’s salt (FS) is a versatile material and has several advantages in constructional and metallurgical sectors. In this paper, FS is formed in situ in concrete. The addition of Al2O3 (5% by weight of cement) was maintained constant and the formation of FS was proceed through the various ranges of NaCl, KCl and CaCl2. Among them, CaCl2 addition was found effective in the large formation of FS in concrete. The influence of FS on the mechanical, permeability and corrosion resistance properties of concrete was studied by electrochemical techniques. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron micrographs were used to characterize the formation of FS in concrete.

Standardization, Calibration, and Evaluation of Tantalum-Nano rGO-SnO2 Composite as a Possible Candidate Material in Humidity Sensors

The present study focuses the development and the evaluation of humidity sensors based on reduced graphene oxide—tin oxide (rGO-SnO2) nanocomposites, synthesized by a simple redox reaction between GO and SnCl2. The physico-chemical characteristics of the nanocomposites were analyzed by XRD, TEM, FTIR, and Raman spectroscopy. The formation of SnO2 crystal phase was observed through XRD. The SnO2 crystal phase anchoring to the graphene sheet was confirmed through TEM images. For the preparation of the sensors, tantalum substrates were coated with the sensing material. The sensitivity of the fabricated sensor was studied by varying the relative humidity (RH) from 11% to 95% over a period of 30 days. The dependence of the impedance and of the capacitance with RH of the sensor was measured with varying frequency ranging from 1 kHz to 100 Hz. The long-term stability of the sensor was measured at 95% RH over a period of 30 days. The results proved that rGO-SnO2 nanocomposites are an ideal conducting material for humidity sensors due to their high sensitivity, rapid response and recovery times, as well as their good long-term stability.

Durability Evaluation of Inorganic-Impregnated Concrete Exposed to Long-Term Chloride Exposure Test

표면 침투제를 이용한 보수기법은 열화된 콘크리트구조물에 대해서 효과적이므로, 최근들어 많은 연구가 수행되고 있다. 특히 무기계 표면 보호재를 제조할 경우나 현장에서 적용할 경우, 공기오염이 없으므로 환경친화적이며, 침투층과 구콘크리트간의 물리적 성능 차이에 의한 박리가 발생하지 않는다. 침투된 콘크리트의...

Analysis Technique for Chloride Behavior Using Apparent Diffusion Coefficient of Chloride Ion from Neural Network Algorithm

Dept. of Civil Engineering, Hannam University, Daejeon 306-791, KoreaABSTRACT Evaluation of chloride penetration is very important, because induced chloride ion causes corrosion in embeddedsteel. Diffusion coefficient obtained from rapid chloride penetration test is currently used, however this method cannot provide a cor-rect prediction of chloride content since it shows only ion migration velocity in electrical field. Apparent diffusion coeffici ent of chlo-ride ion based on simple Ficks Law can provide a total chloride penetration magnitude to engineers. This study proposes an analysistechnique to predict chloride penetration using apparent diffusion coefficient of chloride ion from neural network (NN) algorithmand time-dependent diffusion phenomena. For this work, thirty mix proportions with the related diffusion coefficients are studied.The components of mix proportions such as w/b ratio, unit content of cement, slag, fly ash, silica fume, and fine/coarse aggregateare selected as neurons, then learning for apparent diffusion coefficient is trained. Considering time-dependent diffusion coefficientbased on Ficks Law, the technique for chloride penetration analysis is proposed. The applicability of the technique is verifie d throughtest results from short, long term submerged test, and field investigations. The proposed technique can be improved through NNlearning-training based on the acquisition of various mix proportions and the related diffusion coefficients of chloride ion.Keywords : chloride attack, neural network algorithm, learning, apparent diffusion coefficient of chloride ion, concrete

An Experimental Study on Evaluation of Compressive Strength in Cement Mortar Using Averaged Electromagnetic Properties

A non-destructive testing (NDT) method for evaluating physical properties of concrete including the compressive strength is highly desirable. This paper presents such an NDT method based on measurement of electromagnetic (EM) properties of the material. Experiments are carried out on cement mortar with different water/cement (W/C) ratios. Their EM properties including the conductivity and the dielectric constant are measured at different exposure conditions and curing periods over a wide frequency range of the EM wave. The compressive strength of these specimens is also tested. It is found that both the conductivity and the dielectric constant increase as the W/C ratio decreases and the curing period increases, which lead strength development in the specimens. A linear correlation is observed between the averaged EM properties over the 5 to 20 GHz frequency range and the measured compressive strength, demonstrating the effectiveness of the EM property-based NDT method in evaluating strength of OPC mortar.