https://doi.org/10.4334/JKCI.2026.38.5.583

윤태승(Taeseung Yoon) ; 우욱용(Ukyong Woo) ; 최하진(Hajin Choi)

The surface quality of concrete slabs, evaluated in terms of flatness and levelness, is a key factor in determining the usability and appearance of residential buildings. However, the ASTM E1155 standard requires a minimum measurement area of 30 m² and relies on linear measurements, limiting its applicability to typical apartment units in Korea. To address this limitation, this study proposes a new method for evaluating slab flatness and levelness using LiDAR-based point cloud data. The method involves extracting the region of interest, down-sampling at a 10 mm resolution, and applying cubic spline interpolation to create a uniform dataset. Flatness is defined as the standard deviation of the angles between mesh normal vectors and the ideal horizontal plane, while levelness is defined as the mean and standard deviation of slope angles derived from row- and column-wise regression. Simulation results confirmed that the proposed indices correlate strongly with those of ASTM E1155. Field application further showed that floating floor slabs achieved superior levelness compared to bare slabs, and inter-site comparisons enabled quantitative identification of construction quality differences. Therefore, this study confirms that the proposed method serves as a valid alternative for small-scale spaces where ASTM E1155 is inapplicable, and can be effectively applied for quality assessment and management at both the unit and construction-site levels.

https://doi.org/10.4334/JKCI.2026.38.5.593

한민규(Mingyu Han) ; 최원창(Wonchang Choi) ; 윤현도(Hyun-Do Yun) ; 서수연(Soo-Yeon Seo) ; 석승욱(Seungwook Seok)

This study summarizes representative direct tensile test results for biochar-based concrete, focusing on tensile softening behavior. Plain concrete exhibited an average tensile strength of 2.78 MPa and a fracture energy of 0.077 N/mm. In comparison, concrete with 3% biochar replacement displayed a tensile strength of 2.18 MPa and a fracture energy of 0.026 N/mm, while the 5% replacement resulted in a tensile strength of 2.36 MPa and a fracture energy of 0.044 N/mm. Although tensile strength partially recovered at the 5% replacement level, both biochar concretes exhibited a pronounced reduction in fracture energy, indicating significantly increased post-cracking brittleness. These results highlight that the structural applicability of biochar-based concrete cannot be reliably assessed based solely on strength, and that tensile softening and fracture energy must be explicitly considered in the design and evaluation of low-carbon concrete materials.

https://doi.org/10.4334/JKCI.2026.38.5.601

이대형(Dae-Hyoung Lee) ; 배수호(Su-Ho Bae)

This study proposes a practical equation for predicting the curvature ductility of doubly reinforced concrete flexural members. Nonlinear analysis of the moment–curvature relationship was performed using concrete compressive strength, steel yield strength, and tension/compression reinforcement ratios as the primary variables, and the influence of the ratio of tensile strain to yield strain on curvature ductility was systematically investigated. The results indicate that curvature ductility improves as the ratio of tensile strain to yield strain increases. Conversely, when equal amounts of reinforcement are added to both the tension and compression sides, the increased member stiffness raises the yield curvature, thereby reducing the curvature ductility. The proposed equation, which employs the ratio of tensile strain to yield strain as a single predictor variable, accurately estimates curvature ductility with a coefficient of determination (R²) of 0.9 or higher and a mean relative error within 2%. These results demonstrate that the proposed equation can be practically applied to the evaluation and design of ductile reinforced concrete flexural members within a performance-based design framework.

https://doi.org/10.4334/JKCI.2026.38.5.611

정희진(Hee-Jin Jeong) ; 이문석(Moon-Seok Lee) ; 김문희(Moon-hee Kim) ; 손동희(Dong-Hee Son) ; 최창식(Chang-Sik Choi) ; 배백일(Baek-Il Bae)

This study evaluated the seismic performance of precast concrete (PC) ductile beam-column connections incorporating steel fiber-reinforced concrete (SFRC). Three full-scale beam-column joint specimens were fabricated and tested: a cast-in-place RC specimen, a PC control specimen, and a 2 % SFRC specimen with 50 % reduced transverse reinforcement. The 2 % SFRC specimen demonstrated superior performance, exhibiting a higher peak load, greater initial stiffness, and improved displacement ductility ratios (2.92-3.34) relative to the control specimens. Steel fibers effectively suppressed joint cracking and concrete spalling throughout cyclic loading. All specimens satisfied the ACI 374.1 acceptance criteria, confirming that 2 % SFRC can replace conventional transverse reinforcement while maintaining seismic performance.

https://doi.org/10.4334/JKCI.2026.38.5.621

박산하(San-Ha Park) ; 권승희(Seung-Hee Kwon) ; 이정수(Jung-Soo Lee) ; 김형기(Hyeong-Ki Kim)

This study re-examines the suitability of the time-dependent strength development model specified in the Korean Design Standard (KDS 14 20 01) and proposes rational improvements reflecting the characteristics of modern concrete mixtures. An analysis of a large-scale compressive strength dataset collected from domestic and international literature revealed that the strength development of ordinary mixtures, utilizing ordinary Portland cement and low volumes of supplementary cementitious materials (SCMs), exhibited trends similar to the current standard. In contrast, mixtures with high replacement ratios of slag and fly ash showed delayed early-strength development compared to the predictions of the current model. These findings suggest that the current standard risks overestimating the early-age strength of such mixtures. Consequently, this study suggests parameter adjustments for high-volume SCM mixtures to ensure safety-side design and proposes a continuous prediction model incorporating SCM replacement ratios as a variable.

https://doi.org/10.4334/JKCI.2026.38.5.631

김태경(Tae-Gyeong Kim) ; 양근혁(Keun-Hyeok Yang)

This study investigated the practical applicability of low-cement concrete for precast members by replacing Portland cement with high volumes (60% and 70%) of PC1200, a ground granulated blast-furnace slag (GGBS) -based cementitious material. A conventional precast mix containing 25% GGBS was prepared as a reference at the same target compressive strength. All specimens were steam-cured under typical precast-plant conditions (4 h pre-curing followed by curing at 60°C for 6 h). The results indicated that the low-cement mixes achieved 1- and 28-day compressive strengths and mechanical properties (stress?strain behavior, elastic modulus, splitting tensile strength, and shear resistance) comparable to those of the reference concrete. In addition, the low-cement mixes exhibited approximately 17-33% lower drying shrinkage strain than the conventional mix. Overall, the proposed low-cement concrete is a promising option for precast applications, meeting practical performance requirements while enabling reduced carbon emissions associated with concrete production.

https://doi.org/10.4334/JKCI.2026.38.5.639

오영훈(Young-Hun Oh) ; 김준삼(Jun-Sam Kim) ; 김정욱(Jeong-Wook Kim) ; 전병갑(Byong-Kap Jeon) ; 최재관(Jae-Kwan Choi) ; 문정호(Jeong-Ho Moon)

The seismic performance of beam–column connections using a dry jointing method was evaluated. To achieve dry connections, a construction method was developed employing two types of connectors–B-brackets and S-brackets–such that construction loads are supported by the brackets, while service and factored loads are transferred by enabling the brackets to function as mechanical splices for reinforcing bars. Analytical investigations were conducted for each bracket, and the splice performance of the two bracket types, acting as mechanical splices, was evaluated in accordance with KCI-ST103. For the experimental study, structural designs were carried out for precast concrete (PC) apartment buildings and general PC buildings, and based on these designs, three full-scale specimens were prepared. The objective of the experiments was to assess whether the beam?column connections using dry joints exhibit seismic performance corresponding to that of an intermediate moment-resisting frame. In addition, nonlinear analysis was performed to provide a comparative evaluation of the experimental results. The findings of this study indicate that the proposed PC structural system with dry joints satisfies the requirements of KDS 41 20 10 in terms of seismic performance.

https://doi.org/10.4334/JKCI.2026.38.5.647

이현호(Hyun-Ho Lee) ; 이소민(So-Min Lee) ; 허무원(Moo-Won Heo)

This study investigates the statistical characteristics of compressive strength based on field quality control data under varying seasonal conditions, using 632 sets of field quality control data. The relationship between nominal strength and measured compressive strength exhibited a strong linear correlation, with a coefficient of determination (R²) of 0.757. Analysis of strength ratios revealed that the average strength ratio exceeded 1.0 across all seasonal conditions, indicating that the measured compressive strength generally surpassed the nominal strength. A two-way analysis of variance (ANOVA) was performed to evaluate the effects of seasonal conditions and strength grade, and a statistically significant interaction between the two factors was identified. Based on these findings, a season-adjusted regression model was proposed, yielding an improved coefficient of determination (R²) of 0.828. The results emphasize the importance of adopting an approach that explicitly considers seasonal conditions in the evaluation and quality control of concrete compressive strength.

https://doi.org/10.4334/JKCI.2026.38.5.655

민병호(Byoung-Ho Min) ; 허무원(Moo-Won Heo) ; 이현호(Hyun-Ho Lee)

This research investigates the dynamic response characteristics of a large-span stadium by proposing a structural system in which seismic isolation devices are installed at the roof-substructure interface, followed by non-linear dynamic boundary analyses under various seismic loadings. Based on these analyses, the acceleration and displacement responses of the fixed-base and isolated structures are comparatively evaluated to examine the applicability and structural efficiency of roof isolation systems for large-span spatial structures. The results demonstrate that the maximum horizontal response displacement of the isolation layer does not exceed the design allowable limit (400 mm) under any input ground motion or direction. Furthermore, the displacement distribution remains relatively uniform, without localized concentration at specific supports. The evaluation of peak response accelerations indicates a substantial reduction in the acceleration transmitted to the superstructure due to the isolation devices. Specifically, the roof-level peak acceleration is reduced by approximately 65% in the X-direction and 55% in the Y-direction compared to the input peak ground acceleration (PGA), clearly demonstrating the effectiveness of the seismic isolation system. Consequently, this study verifies that implementing a roof isolation system in large-span spatial structures?which are typically characterized by uneven support spacing and substantial superstructure mass?is a highly efficient structural design approach. It significantly mitigates overall seismic responses while simultaneously ensuring both structural safety and serviceability under extreme seismic events.

https://doi.org/10.4334/JKCI.2026.38.5.665

서수연(Soo-Yeon Seo) ; 문정호(Jeong-Ho Moon) ; 오영훈(Yeong-Hun Oh) ; 윤현도(Hyun-Do Yun)

In this study, in order to investigate the hysteretic behavior of precast concrete (PC) intermediate structural walls according to the shear reinforcement details of the vertical joint, repeated lateral force tests were conducted. on PC walls with three details of horizontal reinforcement within the vertical joint: conventional loop reinforcement, loop reinforcement rotated 90°, and head reinforcement confined by spiral reinforcement. The results showed that, regardless of the detail of the vertical connection, all specimens exhibited similar failure patterns and hysteretic responses, with displacement ratios at maximum strength of approximately 1 %. The vertical connection with head reinforcement confined by spiral reinforcement showed the highest strength and reinforcement efficiency. All three details were found to satisfy the performance requirements of PC intermediate structural walls specified by current Korean standards.

https://doi.org/10.4334/JKCI.2026.38.5.675

박준호(Junho Park) ; 김지수(Ji-Su Kim)

This work presents a CFD-based numerical simulation model incorporating time-dependent material property evolution for evaluating the buildability of 3D concrete printing (3DCP). Conventional numerical models assuming constant material properties exhibit limitations in capturing the progressive hardening behavior of cementitious materials during the printing and deposition processes. To overcome this limitation, a thermal-based internal state parameter was introduced to represent the time-dependent evolution of yield stress, and the deposition behavior was simulated using a FLOW-3D?based Elastic?Visco?Plastic (EVP) model. The simulation results demonstrated that the proposed model effectively reduced the lateral deformation of lower layers and improved height retention during three-layer deposition. In addition, the stability of the deposited geometry increased with increasing degree of material property evolution. Furthermore, an extended model incorporating a phase-change concept was investigated to evaluate the applicability of simulating post-setting solidification behavior. The proposed approach is expected to provide a useful numerical framework for evaluating early-age buildability and optimizing process parameters in 3DCP applications.

https://doi.org/10.4334/JKCI.2026.38.5.685

박성완(Sung-Wan Park) ; 황찬울(Chan-Wool Hwang) ; 김형기(Hyeong-Ki Kim) ; 권성준(Seung-Jun Kwon)

The corrosion characteristics and durability performance of carbon-eating concrete (CEC), cured under high CO₂ concentration and temperature, were evaluated. An accelerated corrosion test using the impressed current method (ICM) was performed on concrete specimens with different carbonation depths, and several durability parameters–including corrosion current, cumulative charge, and split tensile strength–were investigated. The test results showed that CEC exhibited lower corrosion current and cumulative charge compared to normal concrete in the initial corrosion stage, indicating improved corrosion resistance. The effect of durability improvement through pore densification was also evaluated. However, as the accelerated corrosion period increased, the corrosion rate also increased, and cracking and rebar-fracture occurred in some samples due to manufacturing defects. The split tensile test results showed a reasonable trend consistent with the corrosion resistance behavior. CEC maintained higher tensile strength than normal concrete, although the split tensile strength significantly decreased after 7 days of the accelerated corrosion period.

https://doi.org/10.4334/JKCI.2026.38.5.693

양지수(Ji-Su Yang) ; 김우혁(Woo-Hyeok Kim) ; 민근형(Geun-Hyeong Min) ; 김우석(Woo Seok Kim)

Bridge substructures have received limited attention in deterioration prediction research, with existing models facing two key limitations: non-physical deterioration patterns over long-term extrapolation intervals, and an inability to reflect structure-specific initial conditions. We developed an integrated short- and long-term deterioration prediction framework for bridge substructures that incorporates environmental and regional characteristics. Eight machine learning and deep learning algorithms were applied for short-term prediction (1-5 years), while four survival analysis models were employed for long-term prediction (beyond 5 years). The key contribution lies in linking short-term prediction results to the survival analysis models as bridge-specific initial conditions, enabling structure-level deterioration trajectory estimation. Light Gradient Boosting Machine (mean absolute error: 0.0276; mean absolute percentage error: 11.09 %) and the Weibull model (concordance index: 0.9916; integrated Brier score: 0.0037) were identified as optimal models, with generalization performance validated using independent field inspection data.

https://doi.org/10.4334/JKCI.2026.38.5.701

이빛나(Binna Lee) ; 이유진(Yu Jin Lee) ; 이종석(Jong-Suk Lee) ; 김유진(Yu Jin Kim) ; 김형기(Hyeongki Kim)

In this study, the basic properties of mortars incorporating two types of boron carbide (BC1 and BC2) and two types of boron nitride (BN1 and BN2) were evaluated. Each boron compound was mixed at 5 wt% of cement, and flow and setting time tests were performed. The mortar test results showed that the boron carbide mortars exhibited flow comparable to or higher than that of the reference, whereas the boron nitride mortars exhibited a flow reduction of about 30 mm due to their finer particle size. The initial setting time of the boron-containing mixtures was delayed compared with that of the reference mixture, whereas the final setting time showed no marked difference. Based on the mortar test results and material cost, BC2 and BN1 were selected and applied to concrete. The BC2 concrete showed a compressive strength similar to that of the reference concrete, while the BN1 concrete exhibited an approximately 13% reduction in compressive strength. The neutron shielding efficiencies of the boron carbide and boron nitride concretes were approximately 33% and 31%, respectively?about twice that of the reference concrete (15%). These results can serve as fundamental data for selecting boron-based shielding materials under space-limited conditions such as those in small modular reactors.

https://doi.org/10.4334/JKCI.2026.38.5.709

이진희(Jin-Hui Lee) ; 우진석(Jin-Seok Woo) ; 김선우(Sun-Woo Kim) ; 박완신(Wan-Shin Park) ; 서수연(Soo-Yeon Seo) ; 윤현도(Hyun-Do Yun)

Current design codes for the shear strength of precast concrete (PC) wall wet vertical joints are largely based on shear friction theory, which assumes shear transfer across a single interface. However, these joints transfer shear through a grout region and a series of shear keys, making that assumption insufficient to represent the actual shear-transfer mechanism. This study therefore evaluates the concrete/shear-key and horizontal reinforcement contributions separately, and proposes a shear strength prediction equation derived from 402 test specimens. The normalized shear strength increased with the shear-key width-to-height ratio and decreased with the bar diameter-to-projected-length ratio, but showed no clear relation to the number of shear keys. At peak load, the horizontal bars reached only 52% of the yield stress, contradicting the yielding assumption of shear friction theory. Confined specimens developed an average of 1.63 times the strength of identical unconfined specimens, giving a correction factor of 0.6 for unconfined joints. The current design codes either overestimated (KDS 41, MEAN=1.42) or underestimated (fib MC 2010, MEAN=0.59) the shear strength. Compared with ACI 318 (RMSE=156.57, MAE=125.64), the most accurate code, the proposed equation (RMSE=93.95, MAE=71.21) showed about 40% lower error, and despite a slight overestimation (MEAN=1.26) it showed low dispersion (IQR=0.56) and the highest R2 (0.67) among the codes (R2=0.50?0.61). This performance is attributed to the separate evaluation of the two contributions and the confinement correction.