https://doi.org/10.4334/JKCI.2024.36.6.541

정철우(Chul-Woo Chung) ; 최지웅(Ji-Ung Choi) ; 아비츄 미카엘 리에우(Gebremicael Liyew) ; 권성준(Seung-Jun Kwon) ; 장승엽(Seung Yup Jang) ; 김형기(Hyeong-Ki Kim)

A series of experimental studies was reassess to reconsider the maximum initial chloride contents specified in the relevant specifications. First, the current regulations regarding initial chloride content established in several countries were reviewed. next various properties, including mortar flow, porosity, strength, and chloride penetration resistance, were assessed for mortars with varying initial chloride content. Third, corrosion risks of rebars embedded in the mortar with various initial chloride contents were evaluated. The experimental results revealed that, in terms of mortar flow, setting time, porosity, compressive strength, and chloride penetration resistance, the initial chloride content had no adverse effects up to 0.4 wt.% of the binder. Furthermore, mortars with initial chloride contents of 0-0.4 wt.% of binder exhibited a low risk of rebar corrosion.

https://doi.org/10.4334/JKCI.2024.36.6.553

김형근(Hyung-Geun Kim) ; 양근혁(Keun-Hyeok Yang) ; 김용현(Yong-Hyun Kim) ; 김재포(Jae-Po Kim)

This study presents an original technique for in-situ cast concrete?filled precast hollow columns, incorporating supplementary ties that pass through the hollow core. As a fundamental experiment on the compressive behavior of hollow columns using the proposed technique, we evaluated the axial load?-strain relationship, the stress-strain behavior of confined concrete, the strength gain factor, and the compression ductility ratio. These results were compared with the corresponding responses compiled from previous extensive tests on conventional columns. The proposed hollow columns showed a strength gain factor and compression ductility ratio comparable to those of conventional columns with the same tie reinforcement index. The use of L-shaped steel and thin plates with burring holes, installed to form the hollow core, had an insignificant effect on the overall compressive performance of the columns and the stress?strain behavior of the confined concrete. Consequently, the proposed technique facilitates the seismic design of precast hollow columns conforming to the requirements of code provisions.

https://doi.org/10.4334/JKCI.2024.36.6.561

심재일(Jae-Il Sim) ; 문주현(Ju-Hyun Mun) ; 양근혁(Keun-Hyeok Yang) ; 이혜지(Hye-Ji Lee)

The objective of this study was to evaluate the flexural behavior of a composite wall comprising a cast-in-place (CIP) pile and a basement wall, connected using a socket-type shear connector (SSC). The main parameters investigated were the cross-sectional shape of the CIP pile, the type of tension reinforcement in the CIP pile, and the loading direction. A total of 12 composite basement walls were prepared and tested under monotonic 4-point loads. The test results showed that using SSC in the composite basement walls can achieve full composite action at the interface between the CIP pile and the basement wall. In addition, it was confirmed that a square cross-section with equivalent detailing exhibited flexural performance comparable to that of a circular cross-section. However, the AISC 360-16 standard, which is based on the concept of plastic stress distribution, was found to underestimate the maximum flexural moment of the composite basement walls using SSC. In particular, the installation of steel plates to replace H-shaped steel beams and deformed bars, which are commonly used in CIP piles, was effective in improving the ductility of the composite basement walls.

https://doi.org/10.4334/JKCI.2024.36.6.573

한상민(Sang-Min Han) ; 천성철(Sung-Chul Chun)

As the use of high-strength materials increases and the dimensions of strctural members decreases, shorter lap splice lenghts are required. To enhance the spliced strength of lapped hooked bars, transverse reinforcement should be placed across possible cracks along the splice length and details restraining prying action due to flexural deformation is required. This study suggested four details to enhance the splice strength of lapped hooked bars: extending a hook tail into the compression region, adding transverse confining reinforcement, including horizontal confining reinforcement, and arranging splices in a zigzag pattern. A total of 14 lap tests were conducted. The results indicated that all proposed details increased the deformation capacity, with transverse confining reinforcement proving to be the most effective in increasing splice strength. However, the increase of lap length is not effective. The average test-to-prediction ratios for all specimens exceeded 1.50 for three equations, except for the equation derived from the development length equation of hooked bars. This implies that the splice strength of lapped hooked bars is 1.5 times greater than the splice strength of lapped straight bars.

https://doi.org/10.4334/JKCI.2024.36.6.583

이주현(Joo-Hyun Lee) ; 이정배(Jeong-Bae Lee)

Concrete is a highly durable material widely used in the construction of structures. However, cracks can develop due to various internal and external factors, providing pathways for harmful agents to penetrate the concrete, thereby reducing its durability. This study aims to examine the self-healing performance of concrete containing natural pozzolan materials, where reaction products formed through contact with moisture act to autonomously heal cracks. To achieve this objective, the compressive strength, drying shrinkage, chloride ion penetration resistance, and absorption rate of the concrete incorporating natural pozzolan were evaluated at different curing ages. Additionally, artificial cracks were induced in the concrete and observed using a zoom optical microscope. The observations revealed the formation and growth of crystals within the cracks, confirming that when an appropriate amount of natural pozzolan material was incorporated, the cracks were effectively sealed.

https://doi.org/10.4334/JKCI.2024.36.6.591

최세진(Se-Jin Choi) ; 이재인(Jae-In Lee) ; 김채영(Chae-Young Kim) ; 윤주호(Joo-Ho Yoon) ; 김관호(Gwan-Ho Kim)

In this study, the mechanical, durability, and self-healing performance of mortars incorporating CMC, an cementitious materials-based capsule, were compared and analyzed. As a result of the analysis, the compressive strength at 56 days of age of the samples incorporating CMC was similar or relatively higher than that of the control sample, and the passing charge at 56 days of the B-FA sample was about 20.6 % lower than that of the control sample. In addition, the compressive strength recovery ratio of the samples incorporating CMC was superior to that of the control sample, and the compressive strength recovery ratio at 56 days of healing of the B-FF and FA-FS samples was the highest at about 126~127 %. In the case of healing ratio, the B-FF sample showed the highest healing ratio of about 81 %, which is about 38 % higher than the control sample.

https://doi.org/10.4334/JKCI.2024.36.6.603

진동식(Dong-Sik Jin) ; 곽상수(Sang-Soo Kwak) ; 홍용호(Yong-Ho Hong) ; 고봉진(Bong-Jin Ko)

For the development of a wet-process technology for the high-level reduction of radioactive concrete waste, characteristic evaluation experiments were conducted on coarse and fine aggregates generated through a dry process based on heat crushing, using a bench-scale coarse aggregate cleaning device and a fine aggregate cleaning device. In the case of the coarse aggregate cleaning device, it was confirmed that the dissolution rate of residual cement in the coarse aggregate exceeded 98% under optimal conditions. The residual cement content in the coarse aggregate increased slightly when the treatment amount in the reactor was increased and the agitation speed was decreased. For the fine aggregate cleaning device, the dissolution rate of residual cement in the fine aggregate also exceeded 98% under optimal conditions. The residual cement content in the fine aggregate increased slightly when the dissolution time was reduced and the treatment amount in the dissolution tank was increased. These experimental results will be considered when designing and manufacturing a commercial-scale wet-process device.

https://doi.org/10.4334/JKCI.2024.36.6.611

이상호(Sangho Lee) ; 이주형(Joo-Hyung Lee) ; 최지영(Ji-Young Choi) ; 곽임종(Imjong Kwahk) ; 조창빈(Changbin Joh) ; 조근희(Keunhee Cho)

Commercial ground penetrating radar (GPR) is widely used to obtain interior images of concrete structures. However, the preset design parameters of commercial GPR equipment make it difficult to optimize for specific situations or incorporate the latest technologies. Therefore, there is a need to develop radar equipment that can be customized for various purposes. This study conducted numerical simulations and experiments to develop a stepped frequency continuous wave (SFCW) radar for the nondestructive examination of concrete structures. First, numerical simulations of SFCW radar inspection of rebars within concrete were performed to investigate the effect of design parameters on the quality of interior images of structures. The main design parameters considered in the simulations included frequency range, antenna spacing, synthetic aperture distance, and beamwidth. B-scan data were obtained from the simulations, followed by image focusing on the B-scan data. From the focused images, the azimuth and range resolutions at the rebar positions were measured, and the influence of the main design parameters was analyzed. Based on these results, the SFCW radar system was constructed, and operational strategies were established. To validate the system, an internal inspection experiment on reinforced concrete specimens was conducted. The results demonstrated that the developed SFCW radar was effective in inspecting the interior of concrete structures.

https://doi.org/10.4334/JKCI.2024.36.6.625

김민선(Minsun Kim) ; 박예원(Yewon Park) ; 왕길환(Gil Hwan Wang) ; 전종수(Jong-Su Jeon)

Aging road facilities experience a reduction in their service life. Without appropriate maintenance, the performance of aging structures can deteriorate, diminishing their utility. Predicting the safety performance of these structures with time characteristics is an effective strategy for maintenance planning and can appropriately pre-determine the duration and cost for their safety performance. This research predicts the safety grade of domestic road facilities including bridges, tunnels, and retaining walls. A database for each facility type was constructed to perform the safety performance evaluations, detailed safety inspections, and precision safety diagnostics. Ordinary bridges, NATM tunnels, and reinforced concrete retaining walls were selected since they are the most common structural types in the database. The safety grade prediction model was developed based on machine learning techniques considering the service life of the structures. Three regression-based machine learning algorithms were used to develop an optimal prediction model: decision tree, random forest, and extreme gradient boosting (XGBoost). The accuracy and confusion matrix for the three machine learning models were compared. In the comparative results, the XGBoost model resulted in the highest accuracy of 100 % for the road bridges, road tunnels, and retaining walls, effectively reflecting the time characteristics.

https://doi.org/10.4334/JKCI.2024.36.6.637

윤인석(In-Seok Yoon)

Chloride ion adsorption experiments were performed according to the chemical development characteristics of cement pore water at each time step. The adsorption amount of the major hydrates for chloride ion adsorption decreased as the pH of the pore water increased. The stability of Friedel’s salt, formed by the reaction of the AFm phase with chloride ions, decreased as the pH increased. Specifically, the decrease in the adsorption capacity of the C-S-H phase with increasing pH was more pronounced as the Ca/Si ratio increased. Based on the experimental results, a correlation between the chloride adsorption density of cement hydrates, the pH of the pore water, and the amount of chloride ions infiltrated was proposed using a multiple regression analysis equation. The experimental results were combined with a model according to the chemical development of cement pore water to estimate the chloride ion adsorption density of the major cement hydrates at each time step.

https://doi.org/10.4334/JKCI.2024.36.6.647

김예은(Ye-Eun Kim) ; 김수빈(Subin Kim) ; 이기학(Kihak Lee) ; 신지욱(Jiuk Shin)

This study developed a rapid blast damage assessment engine based on displacement-based blast performance evaluation methods. The engine uses a Multi-Step Learner model combining two machine learning models to enhance the accuracy of blast damage prediction according to failure types. The input variables for the model include column details, blast loading scenarios, and failure types derived through machine learning models. The output variable is the blast damage grade, based on displacement ductility. To develop the model, a training and validation dataset was generated using finite element analysis models from previous studies. Seven classification learners were trained, and the best-fit model was selected based on its superior prediction performance. The ensemble method showed outstanding performance, with F-score and AUC values 41.05 % and 14.65 % higher, respectively, compared to other classifiers on the test dataset.

https://doi.org/10.4334/JKCI.2024.36.6.657

이재현(Jae-Hyun Lee) ; 김우혁(Woo-Hyuk Kim) ; 민근형(Geun-Hyeong Min) ; 김우석(WooSeok Kim)

This work aims to develop predictive models for the efficient maintenance of bridge decks using various machine learning algorithms, including random forest, XGBoost, k-NN, SVM, neural networks, LSTM, and GRU. The models predict the deck defect index and condition rating, with LSTM, random forest, and XGBoost showing superior performance in defect index prediction, and XGBoost, GRU, and LSTM excelling in condition rating prediction. The models were evaluated using MSE, RMSE, MAE, and other metrics, and cross-validation was conducted to assess overfitting risks. The results demonstrate that these models can contribute to more accurate and efficient bridge deck maintenance, enabling proactive interventions and optimizing maintenance costs. Future research will focus on incorporating additional factors, such as environmental impacts, to further enhance the predictive accuracy and practicality of the models.

https://doi.org/10.4334/JKCI.2024.36.6.665

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 CFRP strands, constituting the carbon grids manufactured using different methods. The results showed that specimens with larger cross-sectional areas of longitudinal CFRP strands exhibited relatively poorer bond properties between the carbon grid and the mortar. All specimens reached flexural failure due to mortar crushing at the compressive edge, and it was found that the higher the tensile force of the longitudinal CFRP strand, the higher the flexural strength. The coefficient of efficiency for flexural strength of the specimens ranged from 0.24 to 0.35, indicating similar tensile performance across the samples. The behavior after reaching flexural strength varied depending on the extent of damage to the mortar and longitudinal CFRP strands. Specifically, the carbon-grid?reinforced mortar specimens with a cross-laminated structure failed in a brittle manner, while those with a biaxial warp knitting structure failed in a more ductile manner. It was also considered to be advantageous for flexural behavior to arrange smaller cross-sectional areas of CFRP strands at narrower spacings in the case of similar tensile forces of the longitudinal CFRP strands.