https://doi.org/10.4334/JKCI.2026.38.4.425

이태민(Taemin Lee) ; 김영근(Young-Gun Kim) ; 최하진(Hajin Choi)

This study proposes an algorithm for estimating rebar cover depths using ground-penetrating radar (GPR) for use during quality inspections of pretensioned centrifugal-type prestressed concrete (PC) poles. The accuracy and applicability of the proposed method were experimentally verified using actual PC pole specimens. Conventional destructive testing methods face limitations in terms of representativeness and structural damage, making it difficult to perform a quantitative quality assessment across the entire pole length. They are also known for low efficiency. In this study, a signal-processing technique combined with the synthetic aperture focusing technique (SAFT) was used to reconstruct signals reflected from internal rebars, after which the signals were analyzed to determine the cover depth. Two different types of PC poles were tested, and a total of nine repeated measurements per specimen confirmed high accuracy within an error range of 0.27 mm to 2.4 mm. In addition, the method demonstrated its potential to evaluate structural characteristics such as variations in rebar spacing distances and cover depths. This study presents a non-destructive quality evaluation approach capable of offsetting the limitations of KS F 4304-based destructive testing and provides a quantitative basis for objective cover depth assessments using GPR.

https://doi.org/10.4334/JKCI.2026.38.4.433

김우석(Wooseok Kim) ; 신경준(Kyung-Joon Shin)

The rebound hammer test is the most widely used non-destructive testing method in Korea for estimating the compressive strength of concrete. This method has the advantages of simplicity and rapid testing but is influenced by various factors such as the test procedure, environmental conditions, and structural deterioration. In aged structures, the effects of carbonation are particularly significant. While, the ‘age coefficient’ is applied to compensate for these effects; however, this coefficient is based on guidelines established in Japan in 1956 and has been criticized by many researchers as being unreasonable. This study aims to conduct an in-depth review of the current application of the rebound hammer method and related domestic regulations. To this end, the influence of carbonation on the rebound hardness is analyzed, and the approaches adopted by international standards and guidelines from Japan, the United States, Europe, China, and the ISO are compared to propose improvements for the Korean standards.

https://doi.org/10.4334/JKCI.2026.38.4.443

박요셉(Yo-Seb Park) ; 엄태성(Tae-Sung Eom)

The current design code (KDS 14 20 20) provides simplified models for the effective stiffness of reinforced concrete columns; however, these models often result in overly conservative estimates when applied to the moment-magnification method. In contrast, ACI 318-25 promotes a more rational approach to column design by adopting an alternative effective stiffness model. This study investigates the suitability of various effective stiffness models for columns specified in current design codes, in terms of strength design and structural analysis. The theoretical foundations and characteristics of these effective stiffness models were compared, and their accuracy in capturing the increases in column deformations and moments due to second-order effects was assessed. The alternative models provided reasonable predictions of column deformations and moments under serviceability conditions, but significantly underestimated them near the ultimate limit state, specifically, in the high-load region approaching the PM interaction diagram. Nevertheless, they produced accurate estimates as long as the second-order moment increase remained below 40 % of the first-order moment. Based on these findings, recommendations for column effective stiffness are proposed.

https://doi.org/10.4334/JKCI.2026.38.4.455

김민희(Min-Hee Kim) ; 김철구(Chul-Goo Kim)

The behavior of reinforced concrete structures strongly depends on the bond performance between reinforcing bars and concrete. Low compressive strength can lead to brittle failure due to deterioration of bond performance. In this study, 20 pullout tests were conducted to quantitatively evaluate the bond performance of low-strength concrete, with concrete compressive strength (C10, C14, C17, C21, and C26), rebar diameter (D10 and D13), and bond length(10db and 15db) as test variables. The results showed that both the average bond stress at the first free-end slip and the maximum average bond stress generally increased with increasing compressive strength. In addition, higher compressive strength and longer bond length tended to result in bond failure after yielding of the rebar. However, at concrete compressive strength levels in the range of 10~14 MPa, sufficient bond performance to allow the rebar to develop its tensile capacity could not be achieved. A comparison with an empirical bond strength equation, which was derived from pullout test data, revealed that the equation tends to overestimate bond strength in the low-strength concrete range, and that a nonlinear consideration of bond length is required.

https://doi.org/10.4334/JKCI.2026.38.4.463

정승태(Seung-Tae Jeong) ; 이기훈(Gi-Hun Lee) ; 양인환(In-Hwan Yang)

In this paper, the thermal characteristics of concrete pebbles containing graphite were quantitatively evaluated to determine the feasibility of using them as thermal energy storage media in a high-temperature sensible heat storage system. Thermal charging and discharging tests were conducted to simulate the operating conditions of a thermal energy storage (TES) system. The mass flow rate of the heat transfer fluid and the rate of temperature increase were considered as test variables, and their influence on heat transfer and the thermal storage performance of the concrete pebbles was examined. As the mass flow rate of the heat transfer fluid increased, heat moved more rapidly through the storage bed, reducing the temperature difference between the sections and improving the temperature uniformity. A higher flow rate also intensified heat exchange between the heat transfer fluid and the concrete pebbles, thereby shortening charging and discharging times. These results show that both short-cycle and long-cycle operation of thermal charging and discharging can be achieved by adjusting the rates of increases and decreases in the temperature of the supplied heat transfer fluid in the storage system.

https://doi.org/10.4334/JKCI.2026.38.4.473

남진원(Jin-Won Nam) ; 이현우(Hyeon-Woo Lee) ; 박우현(Woo-Hyun Park) ; 권성준(Seung-Jun Kwon)

Cement mortar is a construction material with pores those are formed through hydration reactions and have a considerable effect on the durability performance of the material. In this study, changes in temperatures on cement mortar surfaces under cooling or heating conditions were obtained through averaged IRTI (infrared thermal image) over the surface area, and compared with durability performance outcomes. The porosity, permeability coefficient, and carbonation rate were derived from cement mortar samples with w/c ratios in the range of 0.4~0.6 and their relationships with the average IRTI rate over test duration were obtained. To take into account the characteristics of the entire cement mortar sample in each case, gradients of the changing temperatures were derived and compared with the durability test results. Durability performance depending on the porosity showed close relationships with surface temperature changes, and the proposed technique demonstrates good applicability to durability evaluation as a non-destructive technique.

https://doi.org/10.4334/JKCI.2026.38.4.481

오채연(Che-Youn Oho) ; 이문석(Moon-Seok Lee) ; 손동희(Dong-Hee Son) ; 최창식(Chang-Sik Choi) ; 배백일(Baek-Il Bae)

For accurate predictions of the triaxial failure of steel-fiber-reinforced concrete (SFRC), this study proposes an improved Lubliner-Ottosen strength criterion (LOSC) that accounts for the strength improvement and bridging effects of steel fibers. The Ottosen parameter x, y, and correction factor ? were derived through triaxial stress tests, enabling the proposed model to accurately capture the failure surface expansion on both the meridian and deviatoric planes. This refined framework demonstrates high correlations with experimental data, providing a robust solution for the precise nonlinear analyses of SFRC structures.

https://doi.org/10.4334/JKCI.2026.38.4.491

양근혁(Keun-Hyeok Yang)

This study examines the crushing strength of aggregates as an additional governing parameter, alongside the compressive strength and unit weight of concrete, and proposes a new rational model for predicting the modulus of elasticity of concrete. A comprehensive database of 11,946 experimental results was compiled from the literature and systematically analyzed. The results demonstrate that the aggregate crushing strength is an independent and significant parameter affecting the modulus of elasticity, with higher elastic moduli obtained for concretes incorporating aggregates with greater crushing strength levels at the same unit weight and compressive strength. Comparisons with existing design equations and empirical models show that the proposed model achieves higher accuracy and lower dispersion over a broad range of concrete types and strength classes.

https://doi.org/10.4334/JKCI.2026.38.4.499

남우석(Woo Suk Nam) ; 임홍재(Hong Jae Yim)

This study quantitatively analyzed the effects of various climatic factors on the chloride diffusion coefficient of marine concrete bridges by integrating 141 chloride profile datasets from 19 coastal bridges along the west and south coasts of Korea with bias-corrected ERA5 reanalysis data. A statistical analysis confirmed that the geographic location is a more dominant variable than the exposure zone (ANOVA, p = 0.0007). Seven climate?ocean variables were selected; six were calibrated from ERA5 using variance scaling, linear scaling, and quantile mapping, while salinity was obtained from observational data. From these, 21 static climate indicators (mean, extreme, and cumulative) were derived. A correlation analysis revealed that wave variables were most strongly associated with cumulative indicators, temperature?humidity with extreme indicators, and wind speed?salinity with mean indicators. After multicollinearity screening, five variables were retained for multiple regression, improving R2 by 6.1 percentage points and reducing the RMSE and MAE by 19.4 % and 21.2 %, respectively. Extreme seawater temperatures (p = 0.027) and mean salinity levels (p = 0.035) were identified as significant governing variables. The south coast exhibited diffusion coefficients approximately 14 % higher than those of the west coast due to the combined effects of higher temperature, salinity, wind speed, and wave energy levels, highlighting the need for regional differentiation in the current Korean durability design standards.

https://doi.org/10.4334/JKCI.2026.38.4.515

김유진(Yu Jin Kim) ; 김진호(Jinho Kim) ; 권성준(Seung-Jun Kwon) ; 장승엽(Seung Yup Jang) ; 김형기(Hyeong-Ki Kim)

This paper presents a comprehensive review of concrete durability design codes and guidelines currently applied worldwide, with a focus on the structural characteristics and practical issues of existing durability design frameworks. Particular attention is given to the Korean durability design code, KDS 14 20 40, which adopts a hybrid system allowing both prescriptive design and performance-based designs. The fundamental concepts of these two approaches and the characteristics of their resulting design outcomes are comparatively examined. The review indicates that, even under identical exposure conditions and target service lives, the concrete cover depths determined by different design approaches can differ significantly. In addition, key durability indicators, such as the chloride diffusion coefficient and carbonation rate coefficient, exhibit considerable variability depending on the testing methods and conditions. Field measurement data obtained from long-term in-service concrete structures further suggest that laboratory-based durability indicators may not fully represent the long-term behavior of actual structures. Furthermore, considering recent changes in design environments, including external exposure conditions, deicing salt exposure, and the increasing use of alternative binders, the applicability of conventional experience-based durability design approaches is shown to be limited in certain cases.

https://doi.org/10.4334/JKCI.2026.38.4.531

이하은(Ha-eun Lee) ; 우한수(Han-soo Woo) ; 허무원(Moo-won Heo)

With the recent increase in the frequency of earthquakes, securing the seismic performance of existing low-rise reinforced concrete (RC) structures has emerged as a critical issue. In particular, vertical extension remodeling entails an increase in the building’s mass and an elevation of its center of gravity, which can degrade the seismic performance of the existing structure, thereby necessitating a rational retrofitting strategy. This study investigates the effects of applying a seismic isolation system on the seismic performance of a low-rise RC structure built in the 1990s undergoing vertical extension. The seismic isolation device was designed using a combination of Lead Rubber Bearings (LRB) and Rubber Bearings (RB), with the target isolation period set to 2.0 seconds based on the design displacement. Subsequently, boundary nonlinear dynamic analyses were performed using seven input ground motions selected through a site response analysis, and the response characteristics of isolated and non-isolated structures were compared and analyzed. The analytical results indicate that while the overall displacement increased when the seismic isolation system was applied, the deformation was concentrated in the isolation layer, leading to effective control of the deformation of the superstructure. Furthermore, the maximum reponse acceleration was reduced by more than 70 %, the story shear force decreased by approximately 55 % compared to that of the non-isolated structrue, demonstrating that the seismic energy was effectively absorbed and dissipated within the isolation layer. In conclusion, the seismic isolation system was evaluated and found to be highly effective in reducing seismic responses and improving the seismic performance of the structure, even under the structurally disadvantageous conditions caused by vertical extension. Therefore, vertical extension remodeling utilizing a seismic isolation system is considered a practical alternative that can enhance the retrofitting efficiency while ensuring the structural safety of existing RC structures.

https://doi.org/10.4334/JKCI.2026.38.4.541

모바헤디 나스타란(Nastaran Movahedi) ; 전시우(Siwoo Jeon) ; 주민관(Minkwan Ju) ; 한동석(Tong-Seok Han) ; 박경수(Kyoungsoo Park)

This study presents a coupled experimental?numerical approach to identify the crack tip position on a bilinear softening model in concrete. Three-point bending tests of concrete specimens were conducted, and load?crack mouth opening displacement (CMOD) responses were measured at different stages of crack propagation (e.g., CMOD = 0.06, 0.12, and 0.24 mm). Crack paths were identified using a threshold-based image analysis, and crack lengths were measured for each stage. A two-dimensional finite element model with a predefined crack path and bilinear softening model was employed to simulate the concrete fracture process. Traction profiles along the crack path were evaluated and correlated with experimentally measured crack lengths. Based on this comparison, the position of the crack tip was identified as the kink point in the bilinear softening model, associated with the ratio of the initial and total fracture energies.

https://doi.org/10.4334/JKCI.2026.38.4.549

김민성(Min-Seong Kim) ; 전나경(Na-Kyung Jeon) ; 김성현(Sung-Hyun Kim) ; 강수민(Su-Min Kang)

Conventional cyclic loading protocols (e.g., ACI 374.1 and FEMA 461) were developed based on earthquake records from high-seismic regions and therefore impose excessive cumulative damage demands when applied to structures in moderate-seismic regions such as Korea. In this study, 25 input ground motions were selected for Sc and Sd site conditions in each case while considering the shear wave velocity, magnitude, source-to-site distance, significant duration, and dominant frequency to reflect the short-period characteristics of Korean seismic environments. A total of 200 nonlinear time-history analyses (=25×2 site conditions×2 structural models×2 directions) were conducted for 3- and 5-story reinforced concrete moment frames using the selected ground motions. Repeated deformation cycles were extracted from the interstory drift responses of the critical floor through rainflow cycle counting. The cumulative damage characteristics of actual seismic responses were quantitatively evaluated using the Cumulative Distribution Function (CDF) and Cumulative Damage Effect (CDE), and loading protocols were developed based on these characteristics. Compared to the ACI 374.1 standard protocol, the proposed protocols required approximately 33?41 % (median), 53?67 % (84th percentile), and 91?116 % (98th percentile) of the cumulative damage effect. The 98th percentile protocols showed a cumulative damage demand comparable to that of ACI 374.1. The proposed protocol can provide an appropriate seismic performance evaluation method for reinforced concrete moment-resisting frames by reflecting the cumulative damage characteristics of actual seismic responses in Korea.

https://doi.org/10.4334/JKCI.2026.38.4.561

박주현(Ju-Hyun Park) ; 조중훈(Jung-Hun Cho) ; 이은솔(Eun-Sol Lee) ; 이종한(Jong-Han Lee)

This study proposed a diamond-shaped strength control model to determine whether a CSG dam concrete mixture satisfies both the compressive strength and the vebe test criteria at the mix-design stage. To overcome the limitation of conventional quality management relying on 28-day strength test results, this study constructed and integrated an XGBoost-based compressive strength prediction model and a vebe test decision model. The compressive strength prediction model achieved an accuracy rate of 93.0 % within ±10 %, while the vebe decision model achieved accuracy of 98.1 %. The integrated diamond-shaped strength control algorithm achieved accuracy of 90.6 % in a measurement-based validation assessment. Applying the integrated algorithm to each aggregate type and varying the W/C ratio, this study predicted and visualized the feasible mix region satisfying both the target strength and the workability criteria as a diamond-shaped region, providing a basis for proactive quality management at the mix-design stage.