https://doi.org/10.4334/JKCI.2026.38.1.003

박성환(Sung-Hwan Park) ; 김승환(Seung-Hwan Kim) ; 윤영묵(Young-Mook Yun)

The strut-and-tie model method is a simplified approach for designing disturbed stress regions (D-regions) in concrete structures. It represents complex stress states using struts, ties, and nodes to enable rational structural design. However, the model’s outcomes often depend on the designer’s experience and judgment, leading to variability in model configurations and design processes for independent D-regions. Defining load and boundary conditions for D-regions within complex or geometrically intricate structures remains a significant challenge. This study proposes a method for separating D-regions from a full concrete structure modeled using plane solid finite elements and for determining appropriate load and boundary conditions for the separated D-region’s finite element model. To support the proposed approach, an OpenGL-based pre- and post-processing program incorporating finite element analysis and graphical capabilities was developed. For validation, D-regions were separated from a beam test specimen, and the finite element analysis results of the separated regions were compared with those of the corresponding regions in the original, unseparated structure. The findings confirmed that the proposed method can effectively isolate D-regions and determine rational load and boundary conditions for their plane solid finite element models, demonstrating its practical applicability for strut-and-tie model design of non-independent D-regions.

https://doi.org/10.4334/JKCI.2026.38.1.013

김경민(Kyung-Min Kim) ; 박성우(Sung-Woo Park)

This study experimentally evaluated the flexural behavior of mortar specimens reinforced with grid-type carbon-fiber?reinforced polymer (CFRP), hereafter referred to as carbon grids. The experimental variables included the cross-sectional area and spacing of the longitudinal CFRP strands, as well as their resulting tensile capacities. The results showed that specimens with narrower CFRP strand spacing exhibited more uniformly distributed surface cracking and achieved the highest flexural strength, indicating that narrow CFRP strand spacing has a positive effect on flexural strength. However, their flexural efficiency coefficients were 0.86 and 0.60, and the post-peak absorbed energy accounted for the lowest proportion of total energy absorption, reflecting relatively brittle behavior. In contrast, specimens with larger CFRP strand cross-sectional areas exhibited widespread bond cracking. Their flexural efficiency coefficients were 0.41 and 0.28, indicating insufficient flexural strength development despite the highest tensile capacities of the longitudinal CFRP strands. Nevertheless, the post-peak absorbed energy accounted for 74 % and 177 % of the total absorbed energy, respectively, demonstrating relatively ductile behavior.

https://doi.org/10.4334/JKCI.2026.38.1.023

이준영(Jun-Young Lee) ; 홍승주(Seung-Ju Hong) ; 홍진영(Jinyoung Hong) ; 최하진(Hajin Choi)

In this work, a non-destructive method for evaluating the internal density of concrete using a non-contact ultrasonic transducer was proposed. In an experiment, ultrasonic signals were recorded while varying the angle of the transducer, from which the critical angle was calculated. The velocities of ultrasonic P-waves and S-waves were then calculated by applying Snell’s law. Based on these velocities, the internal density of concrete was estimated by using P-wave and S-wave velocity equations incorporating material coefficients and nonlinear regression analysis. The accuracy and reliability of the proposed method were verified through simulation and slab specimen experiments with varying material coefficients. Through this, the practicality of evaluating the internal density of concrete structures was evaluated.

https://doi.org/10.4334/JKCI.2026.38.1.031

조성원(Seong-Won Cho) ; 이문석(Moon-Seok Lee) ; 손동희(Dong-Hee Son) ; 배백일(Baek-Il Bae) ; 최창식(Chang-Sik Choi)

Flat-plate structures without beams are vulnerable to punching shear due to stress concentrations at slab?column connections. Current design codes assume a constant 45° crack angle and uniform concrete, which may not accurately capture the behavior of slabs with high-strength precast concrete (PC) near the column. This study evaluates punching shear strength by considering crack angle variation at the PC?cast-in-place (CIP) interface using a nonlinear finite element model validated against experimental data. Parametric analyses varied the compressive strength, PC head width, and flexural reinforcement ratio, revealing increased punching shear strength with all parameters. Observed crack angles ranged from 16° to 50°, deviating from the 45° assumption. Comparisons with ACI 318-19 and KDS 14 20 indicate that these codes may misestimate strength under composite slab behavior. The study consequently highlights the need to redefine the critical section to account for material discontinuity and PC head geometry, providing a basis for improved design of hybrid flat-plate systems.

https://doi.org/10.4334/JKCI.2026.38.1.041

이재기(Jae Gi Lee) ; 왕길환(Gil Hwan Wang) ; 전종수(Jong-Su Jeon)

This study investigates the dynamic response and damage state of reinforced concrete (RC) bridge columns with and without carbon fiber reinforced polymer (CFRP) jacket subjected to vehicle collision loads through numerical analysis. The numerical models of bridge columns were validated against experimental data available in literature, showing good agreement in impact force, lateral displacement, and energy dissipation. The validated model was then applied to full-scale bridge columns to simulate realistic column-truck collision scenarios. A parametric study examined ten variables that affect the post-impact damage of the columns. Among them, the column diameter, number of CFRP jacket layers, and fiber orientation were identified as key parameters. The extent of damage was evaluated in terms of rotation angle defined as the ratio of maximum lateral displacement to collision height. The results showed that CFRP jacket was especially effective for smaller-diameter columns. The findings demonstrate the potential of CFRP jacket to improve the collision performance of the RC bridge columns and provide valuable insights for future design and retrofit efforts.

https://doi.org/10.4334/JKCI.2026.38.1.051

송영민(Youngmin Song) ; 신지욱(Jiuk Shin)

Recent earthquakes have led to severe column damage in many existing reinforced concrete (RC) buildings with seismically deficient detailing. To mitigate such damage, a rapid seismic performance evaluation method is needed. However, conventional numerical simulations and experiment-based approaches are often limited due to their complexity. This study proposes a cGAN model in which an improved VAEGAN structure was implemented to rapidly predict lateral resisting capacities of RC rectangular section columns in the form of backbone curves. The effects of structural parameters on lateral resistance were considered, and eight input variables were selected, including material properties, geometric conditions, reinforcement details, and axial load. A total of 171 data samples generated from previous experimental studies were utilized to train and test the proposed model. The generated backbone curves from the proposed model have good correlation with the actual experimental data within slight variations (average error=8.15 %, MSE=0.13 %, R2=0.92) in the test dataset. The well-validated prediction model can serve as an efficient tool to rapidly estimate the lateral resisting capacities of RC columns and establish appropriate retrofit strategies using simplified structural information.

https://doi.org/10.4334/JKCI.2026.38.1.061

김현수(Hyeonsu Kim) ; 강준구(Jungu Kang) ; 정재현(Jaehyun Jeong) ; 한기표(Gi-Pyo Han) ; 송호민(Homin Song)

Bridge deck slabs are highly susceptible to rapid deterioration due to repeated traffic loading and environmental factors such as moisture and chloride penetration. However, early detection of internal damage is difficult through visual inspection alone, and reliance on a single nondestructive testing (NDT) method is insufficient for quantitative assessment. In this study, impact echo (IE), ground penetrating radar (GPR), and phased array ultrasonic testing (PAUT) were applied to deck slab specimens with simulated delamination and disintegration damage. For each NDT method, a machine learning?based deterioration evaluation model was developed. The IE data were converted into 2D spectrograms, while GPR and PAUT data were represented as B-scan images to construct 2D convolutional neural network (CNN) models based on ResNet-50. As a result, the classification accuracies of the IE, GPR, and PAUT models were 80.17 %, 75.32 %, and 80.80 %, respectively. This study identified the detectable damage types and limitations of each NDT method based on their wave propagation characteristics. The findings can serve as a foundation for developing quantitative condition assessment and multimodal fusion?based deterioration prediction models for bridge deck slabs.

https://doi.org/10.4334/JKCI.2026.38.1.071

우진석(Jin-Seok Woo) ; 정재용(Jae-Yong Jeong) ; 이진희(Jin-Hui Lee) ; 윤현도(Hyun-Do Yun) ; 박완신(Wan-Shin Park) ; 최원창(Won-Chang Choi)

The construction industry faces serious environmental challenges, including excessive CO2 emissions and the depletion of natural aggregates, with approximately 39 % of global carbon emissions attributed to construction activities. In this context, biochar?a carbon-rich byproduct with potential for carbon sequestration and structural reinforcement?has emerged as a promising alternative construction material. This study presents the development of a biochar-based lightweight aggregate (BA) utilizing silica fume (SF) as a binding agent. The BA was manufactured through drying and grinding processes in accordance with the KS F 2527 standard. Its physical properties and structural suitability were evaluated, and the mechanical performance of mortar incorporating BA was assessed under 14-day and 28-day curing conditions. The results demonstrated that BA exhibited significantly improved physical strength and a 70~80 % reduction in water absorption compared to uncoated biochar (BC). Notably, the SF30 aggregate achieved a crushing strength of 2.8 MPa, representing around 55 % improvement over commercial lightweight aggregate (LWA), which had a strength of 1.8 MPa. Mortar tests further confirmed the superior performance of BA: mortar incorporating SF30 achieved a compressive strength of 46.2 MPa after 28 days of curing, marking a 204 % increase compared to BC mortar (15.2 MPa) and a 63 % increase compared to LWA mortar (28.2 MPa). Additionally, the specimen recorded a high compressive energy of 205.3 J, indicating excellent energy dissipation capacity, ductile fracture behavior, and favorable microstructural characteristics. Analysis of stress?strain curves revealed that BA mortar exhibited a more stable and gradual failure pattern, suggesting advantages in structural durability and resilience. Overall, this study experimentally demonstrates that biochar can serve not merely as an additive but as a structurally viable lightweight aggregate, highlighting its potential as a high-value, eco-friendly construction material.

https://doi.org/10.4334/JKCI.2026.38.1.081

김원우(Won-woo Kim) ; 유승한(Seung-han You) ; 문재흠(Jae-heum Moon) ; 신현섭(Hyun-seop Shin) ; 김성욱(Sung-wook Kim)

Under extreme loading conditions, such as explosions and high-velocity impacts, the mechanical behavior of concrete differs significantly from that observed under quasi-static loading. This study investigates the impact resistance of a high-performance fiber-reinforced cementitious composite (HPFRCC) subjected to high strain-rate loading through Split Hopkinson Pressure Bar (SHPB) tests and high-velocity gas-gun experiments. The SHPB tests were conducted to characterize the dynamic material properties of HPFRCC under high strain rates, while the gas-gun experiments were performed to evaluate its resistance under impact conditions. In addition, experimentally obtained results were compared with predictions from existing empirical models proposed in previous studies to assess their applicability to ultra-high performance concrete. The results demonstrated that HPFRCC exhibited significantly enhanced impact resistance compared to conventional concrete; however, a substantial discrepancy was observed between the experimental results and the predictions obtained from existing models. These findings highlight the need for developing improved constitutive models and impact resistance prediction methods that explicitly account for the unique material behavior of HPFRCC under high strain-rate loading.

https://doi.org/10.4334/JKCI.2026.38.1.091

조상환(Sanghwan Cho) ; 김민욱(Min Ook Kim)

In this study, tree-based machine learning models were developed to quantitatively predict the compressive strength development of CO2-cured cementitious composites and to identify the key influencing factors based on experimental data collected from the literature. A dataset comprising 333 experimental results was compiled from 23 published studies, incorporating variables related to binder composition, mixture proportions, CO2 curing conditions, environmental parameters, and curing age. The predictive performance of Random Forest and Gradient Boosting algorithms was evaluated, and the results showed that CatBoost and XGBoost achieved high prediction accuracy and stable generalization performance on the test dataset. SHAP-based sensitivity analysis revealed that CO2 curing duration, coarse aggregate-to-binder ratio, and CO2 concentration were the dominant variables governing compressive strength development, exhibiting non-monotonic and nonlinear influence characteristics. This study provides an interpretable, data-driven framework for understanding compressive strength development under CO2 curing and offers a foundation for future multi-objective optimization studies that integrate carbon uptake efficiency and durability performance.

https://doi.org/10.4334/JKCI.2026.38.1.103

문도영(Do Young Moon) ; 이강진(Kang Jin Lee)

This study investigates the rheological behavior of steel fiber reinforced mortar (SFRM) under electromagnetic fields (EMF) and evaluates the magnetic force required for fiber alignment. Rheological tests were conducted on normal mortar and SFRM with and without EMF application. Normal mortar exhibited linear Bingham behavior regardless of EMF, whereas SFRM showed pronounced non-linear stick?flow behavior under EMF. This behavior is attributed to magnetic-field-induced fiber orientation, which obstructs flow and increases viscous resistance. An effective viscosity was defined based on the median value of the viscosity distribution under EMF, and the required magnetic flux density for fiber alignment was evaluated accordingly. The results indicate that the magnetic force required for fiber alignment increases by approximately 1.6?1.9 times compared to the reference state without a magnetic field. These findings indicate that changes in rheological behavior should be carefully considered in the design of steel fiber alignment control techniques using magnetic fields.