• Journal of the Architectural Institute of Korea Structure & Construction
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• v.33 no.12
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• pp.63-70
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• 2017

When industrial by-products like slag and fly ash are using in concrete with cement, it improves strength and durability against external deterioration factors by densifying the structure through potential hydraulic and pozzolanic reaction. But it has been pointed out that high dependence on the quality variation and the curing condition using a admixure material for concrete. In this study, the characteristics of internal micropore structure according to curing condition were analyzed for pastes and mortar specimens under using blast furnace slag powder. As a result, the variation of compressive strength and the internal microstructure were observed according to curing conditions by binder type. Particularly, using blast furnace slag powder, decrease in compressive strength were clearly observed in indoor and carbonation curing compared with water curing. The pore structure analysis also clearly observed the decrease of the gel pore existing in the CSH hydrate layer and the increase of the capillary pore in indoor and carbonation curing compared with water curing condition.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.3D
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• pp.325-330
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• 2008

This study carried out the repeated freezing-thawing test in order to understand the long-term degradation mechanism of GFRP dowel bars. The mechanical property measured by shear test. In addition, analyzes repeated freezing-thawing degradation mechanism of GFRP dowel bars by observe the microstructure through Scanning Electron Microscope (SEM) and Gas Physisorption techniques. As the result of test, it was found that the mechanical property didn't decreased as the exposure time to water and repeated freezing-thawing environment. It shows clearly observed microstructure investigations.

• Computers and Concrete
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• v.34 no.1
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• pp.123-135
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• 2024

Strain-hardening cement-based composites (SHCC) under cracked condition exhibits remarkable capillary absorption due to water ingress from multiple cracks. Surface treatment using water repellent agents is an effective way for improving water resistance of SHCC, but the water resistance may remarkably decrease when cracks penetrate impregnation depth. Another way is to add water repellent agents directly into the mixture, offering SHCC integral water repellency even if cracks form later. However, although integral water repellent treatment has been proved feasible by previous studies, there is still lack of simulation work on the treated SHCC for evaluating its durability. This study presents a simulation method for capillary absorption of cracked SHCC with integral treatment based on a multi-scale approach proposed in the authors' previous work. The approach deals with water flows in bulk matrix and multiple cracks using two individual transport equations, respectively, whereas water absorbed from a crack to its adjacent matrix is treated as the mass exchange of the two equations. In this study, the approach is enhanced for the treated SHCC by integrating the influencing of water repellency into the two transport equations as well as the mass exchange term. Using the enhanced approach, capillary absorption of water repellent SHCC under cracked condition is simulated, showing much more reduced water ingress than the untreated concrete, which is consistent with total absorption data from previous tests. This approach is also capable of simulating water spatial distribution with time in treated SHCC reasonably.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.5
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• pp.21-29
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• 2017

Cold joint caused by construction delay is vulnerable to shear stress and it allows more rapid chloride penetration and diffusion. In the paper, investigation of chloride diffusion coefficient is performed for 1-year cured concrete considering compressive and tensile loading level and cold joint. The results are compared with the previous results in 91-day cured concrete. In the 1-year cured concrete without loading, 10.7% and 10.5% of diffusion reduction are evaluated for those in 91-day cured concrete, respectively. The reduction ratios are almost similar however the result in cold joint concrete shows much higher values. The results in 1-year cured concrete under 30% and 60% of compressive loading show reduction of chloride diffusion by 10.9% and 5.8% compared with 91-day cured results, which is caused by steady hydration of cement particles, so called, time effect. In the case of tensile loading, the differences in results are not significant regardless of time effect and cold joint since micro cracks which is weak point of concrete is much dominant despite of long term curing.

• Journal of the Korea Concrete Institute
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• v.16 no.6 s.84
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• pp.741-750
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• 2004

Air voids in hardened concrete have an important influence on concrete durability such as freeze-thaw resistance, surface scaling resistance, and water permeability, and they have been characterized by spacing factor Linear traverse and point count methods in ASTM standard have been used in estimating an air void system in hardened concrete. However, these methods require lots of time and efforts, further they are not repeatable. Image analysis method could be utilized In estimating an air void systems in hardened concrete with a developments of microscope, digital camera and computer program. The purpose of this study was to develope image analysis method and provide a guideline by comparing the results from ASTM method and image analysis method. The concerns were at air void content and diameter distribution, air voids system as well as spacing factors. The experimental variables included air content by air entrained agent (0, 0.01, $0.03\%$) and depth of specimen (top, middle, bottom). The result showed that it was possible to calculate spacing factor using image analysis technique, as well as air content, air diameter distribution, and air structure. This study also contributed in developing an reasonable and repeatable image analysis method.

• Journal of the Korea Concrete Institute
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• v.26 no.3
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• pp.315-321
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• 2014

Concrete as a construction material is widely used in nuclear vessel and plant for excellent radiation shielding. However the isolation characteristics in concrete may affect adversely in the case of fire and melt-down in nuclear vessel since temperature cooling down is very difficult from outside. This study is for development of high thermal conductive concrete, and its mechanical and thermal properties are evaluated. Magnetite aggregates with volume ratio of 42.3% (maximum) and steel powder of 1.5% are replaced with normal aggregates and thermal properties are evaluated. Thermal conductivity little increases by 30% addition of magnetite but rapidly increases afterwards. Finally thermal conductivity is magnified to 2.5 times in the case of 42.3% addition of magnetite. Steel powder has a positive effect on high thermal conduction to 106~113%. Several models for thermal conduction like ACI, DEMM, and MEM are compared with test results and they are verified to reasonably predict the thermal conductivity with increasing addition of magnetite aggregates and steel powder.

• Journal of the Korea Concrete Institute
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• v.19 no.2
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• pp.179-188
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• 2007

Recently, the concern on the receipt of poor ready-mixed concrete in the construction field and the durability of concrete has been increased. Based on the such background, a large number of measuring methods of water content for fresh concrete have been developed and enforced in a developed country. In this study, to investigate practicality for quality control of ready-mixed concrete among various water content measurement techniques, microwave range method, air meter method and capacitance measurement method as measuring methods of water content were selected. Then, it was evaluated estimating performance of water content according to the change of binder types, fine aggregate types, absorption ratio, water content and water-binder ratio in series I and II. Also, it was examined influence on error occurrence of water content according to change of properties of used materials in series III. Finally, based on this study, it was proposed fundamental data to utilize measurement technique of water content to quality control of ready-mixed concrete in construction field.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.6
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• pp.617-625
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• 2009

This study is devoted to the problems of thermal and autogenous expansion stresses in order to avoid cracking using chemically prestressing method. The chemical prestress can be induced by autogenous expansion characteristics of MgO concrete made in specific burning temperature. The volume change induced cracking has great influence on the long-term durability and serviceability. To evaluate risk of cracking, the computer programs for analysis of thermal and autogenous expansion stresses were developed. In these 3-D finite element procedures, long-term autogenous expansive deformation is modeled and its resultant stress is calculated and then verified by comparison with manual calculation results. In this study, the stress development is related to thermal and autogenous expansive deformation. Using the developed program, residual stresses of MgO concrete were compared and analysed in the example From the numerical results it is found that long-term, and temperature dependent expansive concrete with light-burnt MgO is most effective in controlling the risk of cracking of mass concrete because it has high temperature for long period. The developed analysis program can be efficiently utilized as a useful tool to evaluate the thermal and autogenous expansion stresses in mass concrete structures with lightly burnt MgO.

• Journal of the Korea Concrete Institute
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• v.20 no.2
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• pp.185-191
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• 2008

To fundamentally solve the problem of deterioration of concrete structures, it has been researched that the high durable concrete structure reinforced with the FRP rebar can be one of major solution to the newly-developed concrete structure. FRP rebar has lots of advantages such as non-corrosive, high performance and light weight against the conventional steel rebar. Among these kinds of FRP rebars, GFRP rebar has usually been considered as the best reinforcement because of its economic point of view. Even though the material capacity of the GFRP rebar was already investigated, there are some problems such as low modulus of elastic that will be cause for degrade of the serviceability of flexural concrete member reinforced with the GFRP rebar. Thus, the deflection characteristics of the GFRP rebar reinforced concrete structure should be considered then investigated. In this study, ACI 440 guideline (2003), ISIS Canada Design Manual (2001) and Toutanji et al. (2000) was considered for predicting the moment of inertia of the concrete beam reinforced with the GFRP rebar. And it was also evaluated that load-deflection relationship had a good accordance with the test and analysis result. In the result of this study, it could be estimated that the load-deflection relationship using the suggested equation of moment of inertia in this study indicated better accordance with the test result than that of the others until failure.

• Journal of the Korean Recycled Construction Resources Institute
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• v.7 no.2
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• pp.109-115
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• 2019

In case of the cracks in concrete, the penetration of deterioration ions such as chloride ions in to cracks is accelerated. According to the penetration of chloride ions, structural and durability problems to RC(Reinforced Concrete) structures are caused. In this study, the accelerated chloride diffusion coefficient which is in steady state is evaluated for 2 year aged normal and high strength FA(Fly Ash) concrete, after a range of crack depths are induced up to 1.0 mm in 56 aged day. Considering crack effect by linear regression analysis, high strength concrete has slightly less increasing ratio of diffusion coefficient by crack than normal strength concrete, and diffusion coefficient increases non-linearly as crack width is increased. Also, In two types of concrete, crack effect decrease as the curing period increase. In the case of quantifying crack and curing effect by using exponential function form, the coefficients of determination are higher than those of linear regression analysis. Under steady state, it is thought that there is not a high correlation between the crack effect and the curing effect, and considering the two independent effects, it is believed that reasonable prediction equation for diffusion of concrete with crack can be proposed.