https://doi.org/10.4334/JKCI.2025.37.1.003

이연정(Yeon-Jeong Lee) ; 양근혁(Keun-Hyeok Yang)

This research examines the effect of micro-steel fibers on the carbonation resistance of lightweight aggregate concrete (LWAC) and the expected service life of RC walls exposed to carbonation-induced deterioration. The main test parameters include the design compressive strength of LWAC (21 MPa and 60 MPa) and the volumetric ratio of micro-steel fibers. Specimens were exposed to an accelerated carbonation environment in accordance with KS F 2584. From the carbonation depths measured at 28 and 84 days, the CO2 diffusion coefficients of each specimen were determined and empirically formulated as a function of compressive strength, unit weight of concrete, and fiber reinforcement index through regression analysis. The assessment of carbonation propagation in RC walls using the present model shows that the addition of micro-steel fibers at a volumetric ratio of 0.25 % extends the carbonation service life of the walls by approximately 1.6 times. Consequently, micro-steel fibers are promising for improving the carbonation durability of LWAC structural elements.

https://doi.org/10.4334/JKCI.2025.37.1.013

이정수(Jung-Soo Lee) ; 차수원(Soo-Won Cha) ; 장승엽(Seung-Yup Jang) ; 권승희(Seung-Hee Kwon)

In recent construction projects, frequent instances of slump loss in concrete after pumping have made it challenging to ensure workability during casting. Pumped concrete shows greater slump loss compared to concrete left in a static state for the same period. However, there is currently no quantitative method for predicting this loss. In this study, yield stress measurements before and after pumping, along with data on pumping distance, flow rate, and pumping pressure, were collected from 17 full-scale pumping tests performed in a previous experimental study. The correlations among these test results were analyzed, resulting in the proposal of a method to predict slump loss after pumping using the initial yield stress and pumping pressure. Additionally, a parametric study was conducted by varying the initial slump (150, 180, and 210 mm) and pumping conditions (horizontal and vertical pipelines). The initial slump values required to meet the target slump after pumping were determined based on pumping distance and height.

https://doi.org/10.4334/JKCI.2025.37.1.021

오창준(Chang-Jun Oh) ; 이정배(Jeong-Bae Lee)

As interest in energy conservation and carbon reduction continues to grow, the need to explore energy-saving methods in building structures has become increasingly important. Among the various types of energy used in construction, a significant amount can be saved through insulation, underscoring the need for effective strategies in this field. This study investigates the feasibility of producing insulation materials by incorporating lightweight aggregates into a geopolymer, which is made using fly ash and ground granulated blast-furnace slag (GGBS), both of which exhibit lower thermal conductivity than cement. The research examines the thermal conductivity, length change rate, bond strength, and physical properties of the insulation material based on the molar concentration of the alkaline activator, the amount of activator added, and the proportion of lightweight aggregates. The results indicate that geopolymer-based insulation material, when mixed with lightweight aggregates, meets the performance criteria set in the Korean Standard (KS F 4040). Furthermore, the material satisfies the required performance criteria for thermal conductivity, bond strength, and length change rate specified for three types of thermal insulation mortars. The study also confirms the material’s suitable fluidity and compressive strength, demonstrating its potential as an effective insulation material.

https://doi.org/10.4334/JKCI.2025.37.1.029

윤현도(Hyun-Do Yun) ; 박완신(Wan-Shin Park) ; 장영일(Young-Il Jang) ; 김선우(Sun-Woo Kim)

This work proposes variables for a regression equation related to the adiabatic temperature rise characteristics of concrete, based on experimental results involving the incorporation of ground granulated blast-furnace slag (GGBFS) or fly ash (FA). To achieve this, 21 concrete mixtures were designed, with key variables including the type of industrial by-product (GGBFS or FA), the cement replacement ratio, and the unit cement content. The cement used in this study was ordinary Portland cement, with Class F FA and three types of GGBFS. The experimental results showed that the 28-day compressive strength of concrete with GGBFS or FA was lower than that of conventional concrete. However, 91-day compressive strength tests confirmed a significant long-term strength development effect. When comparing the impact of different industrial by-products on adiabatic temperature rise characteristics, it was observed that both the final adiabatic temperature rise and the rate of temperature increase were lower in GGBFS-replaced concrete than in FA-replaced concrete. To quantitatively evaluate the rate of adiabatic temperature rise based on the replacement ratio of GGBFS or FA, the experimental results were analyzed through regression, following the equations presented in the standard specifications for mass concrete. The relevant variables were derived, and these variables exhibited a linear relationship with the replacement ratio of GGBFS or FA.

https://doi.org/10.4334/JKCI.2025.37.1.037

민하은(Ha-Eun Min) ; 김희선(Hee Sun Kim)

Exposure to high temperature such as during a fire induces physical and chemical changes in concrete, leading to a significant reduction in its compressive strength. Experimental studies for the strength of heated concrete are not only time-consuming but also limited in experimental conditions to account for a wide ranges of parameters, including concrete mix ratios. Even though there have been attempts to predict the compressive strength of concrete exposed to high temperatures using machine learning (ML) models implementing ensemble algorithms, only a few studies have evaluated the prediction performance of different ensemble models. Hence, this study aimed to suggest the most suitable ensemble ML model for predicting the compressive strength of concrete having supplementary cementitious material (SCM) exposed to high temperatures by comparing five ensemble algorithms: gradient boosting regressor (GBR), extreme gradient boosting regressor (XGBR), categorical gradient boosting (CatBoost), random forest (RF), and extra trees (ET). The results revealed that the CatBoost algorithm had the highest predictive accuracy, as measured by the coefficient of determination (R2), root mean squared error (RMSE), and mean absolute error (MAE). A feature importance analysis was also performed to identify the most influential parameters, showing that the water-to-binder ratio and heating temperature were the most significant factors. Finally, the CatBoost and ET algorithms were chosen to predict the strength of heated concrete from new input data that had not been included in the training or testing process. Among them, CatBoost exhibited the better agreement with the experimental results.

https://doi.org/10.4334/JKCI.2025.37.1.049

이호진(Ho-Jjn Lee) ; 진수영(Su-Young Jin) ; 최가영(Ga-Young Choi) ; 윤영수(Young-Soo Yoon)

Enhancing the insulation performance of construction materials for improving building energy efficiency and addressing pollution caused by coal bottom ash (CBA), of which approximately 1.5 million tons is generated annually in Korea, making it a significant environmental issue. Although there are studies on using CBA to reduce the thermal conductivity of cement-based construction materials (CCMs) such as concrete and to promote sustainability in the construction industry, the resulting strength levels remain low. To address these challenges, this study experimentally evaluated the effects of replacing silica sand with CBA on the mechanical and thermal properties, as well as the microstructure, of ultra-high performance concrete (UHPC). The evaluation included measurements of density, compressive behavior, direct tensile behavior, and thermal properties, along with TG/DTG and MIP analyses. According to the experimental results, the use of CBA fine aggregate decreased the density, compressive strength, and direct tensile strength of UHPC. However, even when 100 % CBA fine aggregate was used, the resulting UHPC still exhibited a high compressive strength of 143?153.6 MPa. Additionally, the use of CBA significantly reduced the thermal conductivity of UHPC, and the 100 % CBA replacement variant showed a 49 % decrease in thermal conductivity compared to UHPC without CBA. Microstructural analysis further revealed that the substitution of silica sand with CBA increased the porosity of UHPC, but when the substitution rate exceeded 50 %, additional hydrates generated due to the pozzolanic reactivity of CBA contributed to a reduction in UHPC’s macro pores.

https://doi.org/10.4334/JKCI.2025.37.1.061

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

Based on surface chloride measurements under diverse exposure conditions, a chloride diffusion model was evaluated. Concrete specimens made from Portland and blended cement, with varying water-to-binder ratios, were exposed to the atmospheric, tidal, and submerged zones of Korea’s Western coast for a period of six years. Surface chloride content and its profiles were measured annually over this period. A model was developed to account for the time-dependent variations in surface chloride content and was incorporated into a numerical chloride diffusion model. A comparison between the numerical model results and the actual chloride content at various depths suggests that durability design, using a reasonably proposed constant surface chloride content, can serve as an effective alternative without requiring time-dependent surface chloride adjustments.

https://doi.org/10.4334/JKCI.2025.37.1.073

김시현(Si-Hyun Kim) ; 김훈민(Hoon-Min Kim) ; 최경규(Kyoung-Kyu Choi)

This paper presents the results of punching shear tests conducted on three GFRP-reinforced concrete slab?column connections, with the reinforcement ratio of the slabs and column sizes as variables. The experimental results confirmed that as the GFRP reinforcement ratio increased, the punching shear strength of the specimens also increased. In contrast, as the column size and critical section length decreased, both deflection and punching shear strength decreased. Additionally, crack patterns and strains were analyzed based on the results observed in the experiments. The existing design equations for punching shear strength in ACI.440.11-22, JSCE, and CSA underestimated the punching shear strength, indicating that improvements are needed to more accurately account for variations in punching shear strength according to the GFRP reinforcement ratio and column size of the slabs.

https://doi.org/10.4334/JKCI.2025.37.1.083

민근형(Geunhyeong Min) ; 김기환(Kihwan Kim) ; 정유석(Yoseok Jeong) ; 김우석(WooSeok Kim)

Deteriorated concrete bridges undergo periodic repairs such as section repairs. However, cracks frequently occur after these repairs. This study evaluated the performance of repair materials based on the inclusion of coarse aggregate and the maximum aggregate size, as well as to assess the effects of crack reduction during each stage of section repair. Breaking and water jetting were applied to assess the effect of different deteriorated area removal techniques. In addition, the saturated surface dry (SSD) condition of the substrate was compared between the applied and unapplied methods, and the effect of coarse aggregate was checked by incorporating aggregates of 3, 10, and 13 mm sizes. The results showed that the inclusion of coarse aggregate in repair materials helps suppress drying shrinkage, thereby leading to fewer cracks. Field experiments confirmed that, during section repair, the combination of using a water jet for deteriorated part removal, applying the SSD condition to the parent concrete, and using polymer cement concrete with a maximum aggregate size of 10 mm as the repair material resulted in the fewest cracks.

https://doi.org/10.4334/JKCI.2025.37.1.093

김민숙(Min Sook Kim) ; 이영학(Young Hak Lee)

This study investigates the flexural performance of Partially Encased Composite (PEC) beams integrated with replaceable steel joints, specifically designed to facilitate efficient installation, disassembly, and reinstallation. Five specimens were tested, comprising one monolithic beam without joints and four beams equipped with steel joints that varied in bolt type and slit configurations. Flexural tests revealed that beams with steel joints achieved approximately 49?63 % of the maximum load capacity of the monolithic beam. Among these, specimens incorporating high-strength bolts and slit-applied joints demonstrated superior maximum load capacities compared to those utilizing conventional threaded bolts or lacking slit configurations. Furthermore, the energy dissipation capacity of the jointed specimens reached approximately 55 % of that observed in the monolithic beam. These findings underscore the potential of the proposed joint system for practical applications, balancing structural performance with reusability and ease of assembly.

https://doi.org/10.4334/JKCI.2025.37.1.101

문도영(Do Young Moon) ; 이재훈(Jae-Hoon Lee)

The effect of crack defects formed during the manufacturing process of domestically produced glass fiber reinforced polymer (GFRP) rebars on moisture absorption was investigated through experimental analysis. Two types of GFRP rebars, made of from epoxy and vinyl-ester, were studied. Before the experiment, the width and length of crack defects in the cross-section were assessed. The experiment was performed in accordance with ASTM D 570, and both 24-hour moisture absorption and saturated moisture absorption were analyzed. It was confirmed that the larger the crack defect, the greater the 24-hour and saturated moisture absorption. However, the increase in moisture absorption due to crack defects was confirmed to occur much more significantly in vinyl-ester rebar compared to epoxy rebars. Therefore, the effective management of crack defects during manufacturing is especially important for GFRP rebars made of vinyl-ester.