https://doi.org/10.4334/JKCI.2024.36.3.187

안성권(Sung Kwon Ahn) ; 방춘석(Chun Seok Bang) ; 강윤석(Yoon Seok Kang) ; 신윤지(Yun-ji Shin) ; 설유강(Yoo-Kang Seol)

The Earth’s temperature has risen more than 1 °C since the pre-industrial era, with an even more significant increase in urban areas during the summer due to the urban heat island effect, making air conditioning a common practice. However, there is a significant problem with the huge power consumption and subsequent greenhouse gas emissions associated with air conditioning. Therefore, to achieve carbon neutrality and mitigate the urban heat island effect, it is essential to use materials with good insulation properties to reduce energy consumption. This paper describes the process and results of experiments conducted to evaluate the feasibility of improving the thermal performance of concrete using moisture-absorbing nanomaterials. The thermal performance of nanoconcrete specimens with 0.08~0.24 % graphene oxide admixture was assessed using the hot disk transient plane source method. The thermal conductivity was calculated according to relevant international standards and verified using the thermal diffusivity equation. The thermal conductivity of nanoconcrete was found to be 1.5~1.9 W/m°C, while that of conventional concrete was 2.4~2.5 W/m°C, suggesting a 20~40 % reduction in thermal conductivity for nanoconcrete compared to conventional concrete. Additionally, differential scanning calorimetry was used to measure heat exchange during heating. While conventional concrete absorbed heat only up to about 75 °C, nanoconcrete absorbed heat up to approximately 100 °C, indicating a higher heat absorption capacity compared to conventional concrete. This work demonstrates that the thermal conductivity decreased, and the heat absorption increased with increasing graphene oxide content. The work also proposes further research, including numerical analysis using molecular dynamic simulations and experimental analysis using improved hydrophilic surface functional groups

https://doi.org/10.4334/JKCI.2024.36.3.197

유조형(Jo Hyeong Yoo) ; 권성준(Seung-Jun Kwon) ; 김형기(Hyeong-Ki Kim)

A series of studies was conducted to assess the carbonation-based service life of concrete mixtures produced in various regions of South Korea. In this article, as the first report on these research findings, the basic properties and carbonation rate values of representative concrete mixtures produced in different regions were investigated to confirm their distribution. Raw materials for producing concrete were selected for six representative regional groups across the country, and mix designs were carried out for various levels of compressive strength and slump. The results revealed that while practical concrete in South Korea varies in the mixture of mineral admixtures and aggregates depending on the region, it demonstrates more conservative resistance to carbonation than the values used in durability design, as long as the quality of the concrete is managed to achieve appropriate performance.

https://doi.org/10.4334/JKCI.2024.36.3.205

유조형(Jo Hyeong Yoo) ; 권성준(Seung-Jun Kwon) ; 김형기(Hyeong-Ki Kim)

A series of studies was conducted to assess the carbonation-based durability of concrete mixes produced in various regions of South Korea. This article presents the second part of the results from our study, in which the service life of concrete was evaluated based on the mix proportions and the measured carbonation rates. Utilizing three durability-based design codes?KDS 14 20 40, JASS 5 (2016), and fib MC2010?the durations required for carbonation to reach the target cover depths were calculated. A significant disparity was observed between the service lives calculated using the mix proportions per the standards and those determined based on the measured carbonation rates. The reliability and robustness of each method were assessed. In addition to determining the concrete mix proportion according to the minimum strength range for the exposure class, it was possible to establish a more effective mix proportion through accurate prediction of durability assessment.

https://doi.org/10.4334/JKCI.2024.36.3.217

최승하(Seung-Ha Choi) ; 오서은(Seo-Eun Oh) ; 정상엽(Sang-Yeop Chung)

This study investigated the use of cleaned cockle shells as fine aggregates in mortar specimens to evaluate their properties. The surfaces of cockle shells contain chloride and sulfate ions, which can affect the durability and hydration of concrete. The cleaning process involved the use of water, activated carbon, anion exchange resin, silver nitrate, and ultrasonic treatment, and mortar specimens containing the cleaned cockle shells were produced. The effectiveness of different cleaning materials was assessed using KS F 2713 standards and ion chromatography. The compressive strength of the cement mortar specimens incorporating cleaned cockle shells was measured. To analyze the impact of chloride and sulfate ions on the mechanical properties of the specimens, microstructural analyses, including scanning electron microscopy (SEM) and X-ray diffraction (XRD), were conducted. The obtained results confirmed that the removal of ions and organic matter from the cockle shell surface can affect both the microstructural features and the compressive strength of cement mortars.

https://doi.org/10.4334/JKCI.2024.36.3.225

김문길(Mun-Gil Kim) ; 심혜정(Hye-Jung Sim) ; 천성철(Sung-Chul Chun) ; 안영승(Yeong-Seung An)

A newly developed stainless steel post-installed expansion anchor, to enhance durability and seismic performance, was validated according to evaluation guidelines through two service-condition tests: a test at minimum edge distance and a simulated seismic-tension test. Tests at minimum edge distance were conducted for M12 and M16, while seismic-tension tests were performed for M12, M16, and M20. The tests at minimum edge distance indicated that both M12 and M16 exhibited strengths statistically equal to the reference tests, thus equivalent to the current anchor design standards regarding edge distance effects. Unlike the reference tests where concrete breakout failure occurred, the tests at minimum edge distance resulted in splitting failures of concrete blocks. This discrepancy can be attributed to the excessive conservatism of the current guidelines regarding the thickness of concrete blocks compared to actual conditions. Amendments are required in the tests at minimum edge distance to better reflect actual conditions. The simulated seismic-tension tests conducted for M12, M16, and M20 exhibited residual tensile capacity after cyclic loading, exceeding the current evaluation guidelines by more than 10 %, indicating excellent seismic resistance performance.

https://doi.org/10.4334/JKCI.2024.36.3.235

권준석(Jun-Seok Kwon) ; 허무원(Moo-Won Hur) ; 이현호(Hyun-Ho Lee) ; 류정식(Jeong-Sik Ryu)

In this paper, a seismic reinforcement method using a double frame (double frame method, DFM) is proposed. The performance test involves a material test of a post-installed anchor that directly connects the existing frame to the DFM, and a shear performance test of the vertical connector. In addition, in order to verify the improvement of seismic performance, a two-story, one-span reinforced concrete existing frame test specimen and a double-frame reinforced frame test specimen were manufactured to examine the effectiveness through repeated loading experiments. As a result of the anchor’s pull-out test, the average tensile strength was 27.9 kN, exceeding the average design strength of 21.7 kN as suggested by the manufacturer. The shear strength test results of the anchor showed an average of 28.7 kN. From the vertical connector test, the internal force per anchor was 46.83 kN, which is approximately 163 % greater than the single anchor design shear strength of 28.7 kN. Therefore, it is considered appropriate to reflect the single anchor test results in a design. As a result of the two-story frame experiment, the double-frame reinforced specimen showed a maximum strength of about 4.0~4.5 times that of the non-seismic specimen. Furthermore, the ratio of energy dissipation of the non-seismic specimen to the energy dissipation of the double-frame reinforced specimen is about 4.00~4.58, showing that energy dissipation by double-frame reinforcement is significantly larger.

https://doi.org/10.4334/JKCI.2024.36.3.243

이명훈(Myung-Hun Lee) ; 우욱용(Ukyong Woo) ; 이주원(Juwon Lee) ; 최하진(Hajin Choi) ; 김종찬(Jong-Chan Kim)

The risk of complex disasters and accidents is increasing due to the rapid increase in infrastructure and aging structures worldwide. Accordingly, as a non-destructive inspection technique to inspect internal cracks and damage in the finishing materials of old buildings, a portable thermal imaging camera (FLIR One Pro) consisting of two camera modules (RGB and thermal imaging sensor) was used to measure the shape and width of cracks on the concrete surface inside the finishing materials. The crack shape and features were extracted through a 2D convolution operation between the measured thermal image and the filter, and then the crack width within the thermal image was derived using pruned exact linear time (PELT), a change point detection technique. A correlation was derived between the crack width measured with the thermal imaging camera and the actual crack widths (0.5 mm, 0.7 mm, 1.0 mm) by counting the number of pixels in the extracted crack area. It was confirmed that the number of pixels in the crack section increased as the crack width increased. As a result, the applicability of thermal imaging inspection techniques to detect internal cracks in finishing materials was verified, and it was demonstrated that quantitative evaluation was possible by deriving correlations based on crack size.

https://doi.org/10.4334/JKCI.2024.36.3.255

전동호(Dongho Jeon) ; 오홍섭(Hong Seob Oh) ; 권석민(Sukmin Kwon) ; 김동주(Dong Joo Kim) ; 표석훈(Sukhoon Pyo) ; 윤세윤(Seyoon Yoon)

Cracking is one of the visually noticeable signs of deterioration when concrete structures face durability problems. Various crack mechanisms exist in concrete, and cracks occur due to the combined actions of these mechanisms. However, the first step to understanding concrete cracks is to understand intrinsic cracks. Therefore, in the present study, cracks that are not generated by external forces were classified as intrinsic cracks, and a review study was conducted on intrinsic cracks due to drying and plastic shrinkage, plastic settlement, alkali-silica reaction, thermal contraction, and reinforcement corrosion. Furthermore, in this work, various methods to control the aforementioned intrinsic cracks were also reviewed, and if properly applied in practice, it will be possible to reduce the damage to concrete structures caused by intrinsic cracks.