• Korean Journal of Childcare and Education
• /
• v.13 no.6
• /
• pp.69-86
• /
• 2017

Objective: This study was conducted to investigate the effects of small-group arithmetic word problem activities with concrete materials on 5 year old children's mathematical ability and attitude. Methods: A total of 34 five-year-old children (control group 16 children, experimental group18 children) attending two kindergartens in P city participated in this study. Fifteen small-group arithmetic word problem activities with concrete materials were conducted in the classroom of the experimental group twice a week for eight weeks. Before and after the activities, all the participants individually took a basic arithmetic test, mathematical word problem solving test, and mathematical attitudes test. Results: First, we observed that the children in the experimental group achieved significantly higher scores on the mathematical ability tests, including the basic arithmetic test and mathematical word problems solving test when compared to the children in the control group. Second, we also found that children in the experimental group showed higher improvement in the mathematical attitudes test than their counterparts. Conclusion/Implications: The results of this study suggest that small-group arithmetic word problem activities with concrete materials are effective in improving children's mathematical ability and attitudes.

• KCI Concrete Journal
• /
• v.11 no.3
• /
• pp.127-140
• /
• 1999

In this paper, a microscopic analysis of prefinitelv strained cement paste specimen was carried out. The microscopic behavior of concrete under triaxial stress must be fully understood in order to explain the additional ductilitv that comes from lateral confinement and to get microstructural information in large deformed and large strained concrete. The so-called "tube-squash" test was applied to achieve enormously high shear and deviatoric strain of concrete under extremly high pressure without fracture. Then, microscopic analyses by focusing on hydration and microstructure of Prefinitely strained cement paste were carried out on cored-out deformed and virgin (undeformed) cement paste specimens : the first specimen being 40 days old, the second one being one year old. The microscopic analysis bv Field Emission Scanning Electronic Microscope (FESEM) was carried out for comparison between the specimens after 40 days and those arter one year. For one year old specimens, X-Ray Diffractometer (XRD) analysis, Energy Dispersive x-rav Spectrometer (EDS) analysis, and Differential Thermal Analysis/Thermo-Gravitv (DTA/TG) analysis were also carried out to study the hydration and the microstructures of prefinitely strained cement paste specimen by focusing on the methodologies of their microscopic analyses. analyses.

• Journal of the Earthquake Engineering Society of Korea
• /
• v.25 no.4
• /
• pp.161-168
• /
• 2021

Existing old reinforced concrete buildings could be vulnerable to earthquakes because they were constructed without satisfying seismic design and detail requirements. In current seismic design standards, the target collapse probability for a given Maximum Considered Earthquake (MCE) ground-shaking hazard is defined as 10% for ordinary buildings. This study aims to estimate the collapse probabilities of a three-story, old, reinforced concrete building designed by only considering gravity loads. Four different seismic design categories (SDC), A, B, C, and D, are considered. This study reveals that the RC building located in the SDC A region satisfies the target collapse probability. However, buildings located in SDC B, C, and D regions do not meet the target collapse probability. Since the degree of exceedance of the target probability increases with an increase in the SDC level, it is imminent to retrofit non-ductile RC buildings similar to the model building. It can be confirmed that repair and reinforcement of old reinforced concrete buildings are required.

• Proceedings of the Korean Institute of Building Construction Conference
• /
• 2022.11a
• /
• pp.197-198
• /
• 2022

Based on the carbonation depth measurement by the indicator for concrete collected from old structures and the quantitative analysis of Ca(OH)₂ and CO₂ in the carbonation section before and after the carbonation depth and in the non-carbonation section, the absorbable CO2 amount and carbonation degree measurement result is as follows 1) The carbonation depth of the 40-year-old reinforced concrete structure was measured to be about 22 mm. (basement interior wall, marble finish, strength 30MPa) 2) The amount of CO₂ absorbed by the concrete was about 4.3% of the sample weight, and the carbonation degree was estimated to be about 53%.

• Computers and Concrete
• /
• v.23 no.3
• /
• pp.161-170
• /
• 2019

This paper aims at presenting a numerical method for estimating the elastic modulus of recycled concrete with crushed aggregates. In the method, polygonal aggregates following a given sieve curve are generated, and placed into a square simulation element with the aid of the periodic boundary condition and the overlap criterion of two polygonal aggregates. The mesostructure of recycled concrete is reconstructed by embedding an old interfacial transition zone (ITZ) layer inside each recycled aggregate and by coating all the aggregates with a new ITZ layer. The square simulation element is discretized into a regular grid and a representative point is selected from each sub-element. The iterative method is combined with the fast Fourier transform to evaluate the elastic modulus of recycled concrete. After the validity of the numerical method is verified with experimental results, a sensitivity analysis is conducted to evaluate the effects of key factors on the elastic modulus of recycled concrete. Numerical results show that the elastic modulus of recycled concrete increases with the increase of the total aggregate content and the elastic moduli of old and new ITZ but decreases with increasing the replacement ratio of recycled aggregate and the thicknesses of old and new ITZ. It is also shown that, for a replacement ratio of recycled aggregate smaller than 0.3, the elastic modulus of recycled concrete is reduced by no more than 10%.

• Computers and Concrete
• /
• v.21 no.1
• /
• pp.1-9
• /
• 2018

Electrical resistivity is a property associated with both the physical and chemical characteristics of concrete. It allows the evaluation of the greater or lesser difficulty with which aggressive substances penetrate the concrete's core before the dissolution of the passive film process and the consequent reinforcement's corrosion begin. This work addresses the steel fiber addition to concrete with two types and various contents from 0% to 1.3%, correlating it with its electrical resistivity. To that effect, 9 different mixes of steel fiber reinforced concrete (SFRC) were produced. The electrical resistivity was evaluated on the on six years aged SFRC by direct measurement at different frequency from 0.1 kHz to 100 kHz. The results indicate that steel fiber content is strongly conditioned by the type and quantity of the additions used. It was also found that long type of fibers has more effect on decreasing the electrical resistivity of concrete than short fibers. Therefore, they increase the corrosion risk of concrete depending on fiber volume fraction and moisture percentage.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2003.05a
• /
• pp.53-58
• /
• 2003

Many researchers have rigorously studied the nonlinear behavior of stress-strain relationship of concrete using mathematical curves. Most of model equations for stress-strain relationship, however, have been focused on old age concrete, and were not able to adequately represent the behavior of concrete at an early age. A wide understanding on the behavior of concrete from early age to old age is very important in evaluating the durability and service life of concrete structures. In previous study by authors of this paper, a stress-strain model equation for low- and medium-strength concretes was suggested. In this paper, to extend the application region of compressive stress-strain curve to high-strength concrete, an analytical research was performed. An analytical expression of stress-strain curve with strength and age was developed using regression analyses on the experimental results. For the verification of the proposed model equation, it was compared to the experimental data. The result showed that the proposed model equation was not only compatible with the experimental data quite satisfactorily but also describing well the effect of strength and age on stress-strain curve.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2002.05a
• /
• pp.587-592
• /
• 2002

Fiber reinforced concrete has been used for tunnel lining and rehabilitation of old structures. Recently, structural synthetic fiber was developed to overcome the corrosive properties of steel fibers. Fibers play a role to increase the tensile and cracking resistance of concrete structures. The Post cracking behavior must be clarified to predict cracking resistance of fiber reinforced concrete. The purpose of the present study is to develop a realistic analysis method for post cracking behavior of synthetic fiber reinforced concrete members.

• Proceedings of the Korea Concrete Institute Conference
• /
• 1995.10a
• /
• pp.164-168
• /
• 1995

As nation's infrastructure is getting old, nondestructive evaluation of existing structures and construction quality control are getting important. In this paper non-destructive evaluations of concrete members using impact echo and SASW methods are introduced. Both techniques are based on the stress wave propagations. Experimental tests were performed using beam type concrete member where voids and cracks are included. Within reasonable accuracy, void locations were detected using impact echo method and the dynamic modulus of concrete were measured using SASW method. Both NDT methods showed a feasibility for the implementation into quality evaluaton of concrete members in practice

• Land and Housing Review
• /
• v.11 no.3
• /
• pp.83-92
• /
• 2020

In South Korea, problems caused by material deterioration of time-worn concrete structures have been increased recently. Because severe material deterioration could damage the structure's safety, it's important to evaluate the old structure's condition and structural capacity regularly to keep its proper performance. The safety evaluation of concrete structures has been initiated and performed periodically since 1995 according to a guideline in accordance with a law in Korea. The guideline prescribes the evaluation types, intervals and methods of the target structure. A lot of cost and labor have been invested every year to carry out the regular safety evaluation. However, it's not clear that the current manual could inspect the old structure's condition and assess the structural capacity precisely. Thus, the verification study initiated to figure out the Korean safety evaluation manual's practicalness. First, the Korean manual was analyzed and then compared with that of other countries for concrete bridges which are representative concrete structure. After that, the previously written evaluation reports were collected and analyzed to find out how the safety evaluation has been carried out. Based on the study results, the parts requiring verification of the manual were drawn. A research program is in progress in order to verify the parts by performing tests with actual structural members from decommissioned concrete bridges.