https://doi.org/10.4334/JKCI.2024.36.2.101

김창수(Chang-Su Kim) ; 노정휘(Jung-Hwi Noh) ; 김호영(Ho-Young Kim)

The use of precast concrete (PC) members is steadily increasing compared to cast-in-place (CIP) concrete, and the performance of joints is paramount when designing structures with PC elements. In the case of structures such as retaining walls, the use of PC segments is on the rise, and yet there remains a lack of suitable solutions for instances requiring simultaneous vertical and horizontal connections. In particular, the installation of PC members is necessary in tunnel projects, where vertical shafts must be quickly completed during the final stages of construction. Our work assessed the performance of vertical shaft connections through the utilization of the rotational connection method, which efficiently improves workability while maintaining connection performance. Various rotational connector details (such as bow-tie and loop types) were designed and presented, followed by a comparative review of the performance of the CIP and PC member connections. To evaluate the actual performance of these connection details, element-level loading experiments were conducted.

https://doi.org/10.4334/JKCI.2024.36.2.111

서동주(Dong-Ju Seo) ; 박종건(Jong-Gun Park) ; 권호찬(Ho-Chan Kwon) ; 송기창(Ki-Chang Song) ; 허광희(Gwang-Hee Heo)

Basalt fiber exhibits notably low bonding performance with cement-based composites, limiting its application as a construction material. We aimed to enhance the interfacial bond properties of basalt fibers in cement mortar by modifying their surface with nano-silica particles through chemical bonding of GPTMS (glycidoxypropyl trimethoxysilane) and TEOS (tetraethyl orthosilicate), a silane coupling agent. The composition and microstructure of the basalt fiber were characterized and analyzed using SEM, EDS, and FT-IR imaging techniques. The resulting nano-silica particles were completely covered and uniformly distributed on the surface of the basalt fibers, forming a thin layer. In addition, a single fiber pull-out test was performed to verify the effect of the new surface modification method on the interfacial bond strength of basalt fibers embedded in cement mortar. The experimental results showed that the interfacial bond strength of basalt fiber cement mortar (BFCM) modified with nano-silica was significantly enhanced compared to unmodified BFCM. Single fiber pull-out experiments and SEM analysis revealed that nano-silica particles on the surface of basalt fibers increased the interfacial bond strength between the fibers and the cement matrix. This enhancement facilitated the bridging effect of basalt fibers in the cement mortar, ultimately improving the interfacial bond strength of the cement mortar.

https://doi.org/10.4334/JKCI.2024.36.2.123

권유진(U-Jin Kwon) ; 백종하(Jong-Ha Paik) ; 백은림(Eun-Rim Baek) ; 이상호(Sang-Ho Lee)

The piloti building is a vertically irregular structure and is vulnerable to earthquake loads because the piloti story (weak story) lacks strength and stiffness compared to the upper floors. Moreover, if the centers of mass and stiffness are inconsistent due to the eccentric arrangement of cores in the piloti story, severe structural damage occurs due to stress concentration and torsional stress when an earthquake occurs. In particular, column of piloti story is subjected to experience axial force, shear force, bending moment, and torsional moment, and it is essential to understand the relationships between these loads, including the load-displacement behavior for each force. In this study, axial force, shear force, bending moment, and torsional moment were applied to the RC column of the piloti building with vertical and horizontal irregularities, and an experimental study was conducted to understand the structural performance. The experimental results showed that the maximum strength levels of the specimens with eccentricity distances of 0 mm and 150 mm were similar, but their ductilities were reduced. Furthermore, the specimen with a 300 mm eccentricity distance exhibited the maximum strength level at 50 % compared to the specimen without eccentricity. The specimen with axial load ratio of 0.24 showed about 10 % increase in maximum strength compared to the one with ratio of 0.12; however, after reaching the maximum strength, the load rapidly decreased, showing a brittle failure mode.

https://doi.org/10.4334/JKCI.2024.36.2.133

이승현(Seunghyun Lee) ; 이주원(Juwon Lee) ; 최하진(Hajin Choi) ; 강현구(Thomas H.-K. Kang)

In this study, the STA/LTA algorithm with a specific time interval applied randomly for determining signal arrival time in concrete was developed and validated. The performance evaluation included comparing deviation values of the AIC (akaike information criterion), Hinkley Criterion, and Amplitude Threshold-picker algorithms, as well as the STA/LTA algorithm with a specific time interval applied randomly. This comparison relied on ultrasonic signals measured through both direct and indirect methods in models of non-reinforced and reinforced concrete. The signals were measured in simulations and experiments using the Comsol Multiphysics software along with actual UPV equipment. Following this, a comparison of the results between non-reinforced and reinforced concrete was conducted to assess the impact of material heterogeneity on the signals. Additionally, a comparison between the direct and indirect methods was performed to evaluate the effect of the indirect method on ultrasonic signals. As a result, the STA/LTA algorithm with a specific time interval applied randomly setting exhibited superior performance compared to other algorithms. It had lower bias values, especially in simulations as opposed to experiments, in non-reinforced concrete compared to reinforced concrete, and with the direct method rather than the indirect method.

https://doi.org/10.4334/JKCI.2024.36.2.145

문정호(Jeong-Ho Moon) ; 오영훈(Young-Hun Oh) ; 이강철(Gang-Chul Lee) ; 이창현(Chang-Hyun Lee) ; 박현석(Hyun-Suk Park)

This work involves an experimental study on WBS-II. WBS-II, aimed at shortening construction periods and achieving cost-efficiency in PC structures, comprises a dry beam-column connection with a wide beam. The main variables examined in the experimental study included the shape of the panel zone, the type and distribution method of girder reinforcement, and the amount of tie reinforcement. Nonlinear analysis was also performed alongside experiments on five specimens categorized into Types A, B, and C. Type A represents a fully dry connection, while Types B and C are partial dry connections, with upper and lower story columns dry-connected, except for the space reserved for column penetration of the bottom reinforcements of the partial dry type girder. Types B and C had variations in the type of girder bottom rebar, with additional consideration given to the amount of panel zone ties as an experimental variable. The findings revealed that the WBS-II frame system showed performance that satisfies the requirements for intermediate moment resisting frames in seismic design criteria. It was observed that adopting a partial dry connection, where the bottom reinforcements of the girder penetrate the column, is preferable to achieve unity between columns and beams. In addition, it was concluded that it is better to use lower-strength rebars than high yield-strength rebars to prevent excessive adhesion stress in the panel zone.

https://doi.org/10.4334/JKCI.2024.36.2.157

이유상(Yousang Lee) ; 박홍근(Hong-Gun Park)

A quantitative assessment of air flow rate is required for infrastructure where its airtight performance is crucial. In this study, a test was conducted to assess the air leakage rate of cracked reinforced concrete elements. A hollow beam specimen was designed to withstand pressures up to 200 kPa, employing a relatively simple set-up for airtight pressure test. While the bottom flange was subjected to tension cracking under transverse loading, air pressure was applied into the air chamber of the beam, and the air leakage rate was measured as the magnitude of loading was increased. The test variables included plate thickness and reinforcement ratio. The test results showed that the leakage rate was proportional to the pressure difference and the cube of the average crack width, and was inversely proportional to plate thickness and reinforcement ratio.

https://doi.org/10.4334/JKCI.2024.36.2.165

이대형(Dae-Hyoung Lee) ; 배수호(Su-Ho Bae)

This study proposes a simplified ductility formula for predicting the ductile behavior of reinforced concrete flexural members. The formula takes into account concrete strength, rebar yield strength, and the quantity of reinforcement. In reinforced concrete structures, ensuring ductile failure at the ultimate state is a crucial design concept. To achieve this, revised design standards specify minimum allowable tensile strain and tension-controlled strain limits based on rebar yield strength. Consequently, a formula for calculating curvature ductility is presented, using the ratio of tensile strain at compressive concrete failure to yield strain as a variable. In this context, the influence of concrete strength and yield strength of reinforcement on curvature ductility was investigated. It was found that the difference in curvature ductility based on concrete strength was insignificant when considering tensile strain of rebar as a reference, and with higher yield strength of reinforcement, the curvature ductility decreased. The proposed formula aligns with previous research results and provides a more convenient means to predict curvature ductility.