https://doi.org/10.4334/JKCI.2024.36.4.281

조창근(Chang-Geun Cho) ; 권승희(Seung-Hee Kwon) ; 김형기(Hyeong-Ki Kim)

A numerical study was conducted to enhance understanding of the flexural behavior of high-performance fiber-reinforced cement composites (HPFRCC). Based on experimental results, a stress-strain model considering strain softening in compression was adopted. Subsequently, a nonlinear behavior analysis of a flexural beam was performed using this model. The analysis encompassed various reinforcement ratios and strength change conditions. The study confirmed that when HPFRCC has a compressive strength of 120 MPa or more, its structural behavior was not significantly influenced by its compressive strength. Furthermore, it was observed that strain softening in the compression of HPFRCC only occurred under over-reinforced conditions. In practice, this strain softening contributes minimally to the flexural failure behavior of HPFRCC.

https://doi.org/10.4334/JKCI.2024.36.4.289

이연정(Yeon-Jeong Lee) ; 양근혁(Keun-Hyeok Yang) ; 이혜진(Hye-Jin Lee) ; 권희용(Hee-Yong Kwon)

This study examined the effectiveness of micro-steel fibers on the residual flexural strength of high-strength lightweight aggregate concrete (LWAC). The target compressive strength of the LWAC was designed to be 60 MPa. The volume fraction (V_f) of the micro-steel fibers varied up to 1.5 % in increments of 0.25 %. The residual flexural strength of beams was determined from the stress / crack mouth opening displacement (CMOD) curves measured in accordance with the RILEM TC 162-TDF guideline. Test results showed that the micro-steel fiber-reinforced LWAC beams exhibited a hardening response after the initiation of flexural cracking and ductile behavior after the peak stress, even at V_f of 0.25 %. As a result, all the reinforced beams displayed extensive CMOD propagation exceeding 5 mm. The residual flexural strength values of micro-steel fiber-reinforced LWAC could be expressed as a function of the flexural strength and fiber reinforcement index.

https://doi.org/10.4334/JKCI.2024.36.4.297

이재인(Jae-In Lee) ; 김채영(Chae-Young Kim) ; 윤주호(Joo-Ho Yoon) ; 이호준(Ho-Jun) ; 최세진(Lee)

Geopolymer composites obtained from industrial byproducts are sustainable alternatives to cement concrete for use in the construction industry. In this study, ground ferronickel slag (GFS) blended ground granulated blast furnace slag (GGBFS) based geopolymer composites are manufactured, and their heat of micro-hydration, compressive strength, split-tensile strength, ultrasonic pulse velocity, chloride-ion penetrability, dry shrinkage and microstructural anaylysis are investigated according to curing methods. As a result of the analysis, in the case of compressive strength at 7 days, when the curing conditions were the same, the sample containing GFS showed a compressive strength up to about 10.8 % higher than the sample containing only GGBFS. Additionally, for samples cured at high temperatures, chloride-ion penetration resistance was noticeably improved. Under the same curing conditions, the sample containing GFS showed up to approximately 34.7 % higher chloride-ion penetration resistance than the sample using only GGBFS.

https://doi.org/10.4334/JKCI.2024.36.4.309

심명화(Myunghwa Shim) ; 박장춘(Jangchun Park) ; 이민택(Mintaek Lee) ; 권이금(Iegeum Kwon)

Sulfur concrete has high strength properties due to its sulfur content, which is a byproduct of the crude oil refining process. In the Sudokwan landfill, bio-sulfur (SO3 93 %) is generated at a rate of about 20 tons a day as hydrogen sulfide from the landfill gas is oxidized by sulfur-oxidizing bacteria. This bio-sulfur, with a moisture content of about 50 %, was dehydrated using a filter press and subsequently dried in an oven. The water resistance of the bio-sulfur cement was ensured by mixing 10~15 % of dried bio-sulfur with rapid hardening cement. Calcium sulfate (CaSO4), formed from the sulfur in the bio-sulfur and the calcium in cement, effectively contributed to the compressive strength development. Bio-sulfur concrete has higher compressive strength, bond strength, and freeze?thaw resistance, as well as lower chloride penetration resistance and drying shrinkage, compared to conventional concrete. When the bio-sulfur rapid-set concrete (0.9 m3) was applied to a bridge (8.2 m2), it met the test standards for compressive strength, bond strength, chloride penetration resistance, and freeze?thaw resistance. After five months of testing, no cracks or breaks were observed. Similarly, when bio-sulfur rapid-set concrete (14.8 m3) was cast into a bridge for test construction, it satisfied the test standards for compressive strength, bond strength, chloride penetration resistance, and free-thaw resistance.

https://doi.org/10.4334/JKCI.2024.36.4.317

박재현(Jae-Hyeon Park) ; 임호(Ho Im) ; 백선우(Seon-Woo Baek) ; 김창수(Chang-Soo Kim)

As part of a research on the reuse of reinforced concrete (RC) members, a numerical study was conducted to evaluate the residual flexural strength of RC beams deteriorated by corrosion. To predict the residual flexural strength, a fiber analysis program was constructed based on strain compatibility using MATLAB. In the analysis, the reduction of effective rebar areas, degradation of rebar mechanical properties, degradation of bond strength between rebars and concrete, and degradation of concrete strength were considered. A total of 19 strength degradation models were generated through a combination of material models, and the predictions were compared with existing 85 test data for verification. The comparisons showed that the strength degradation models provided conservative estimates of the residual flexural strength. Furthermore, the accuracy of prediction was improved when the reduction of effective rebar areas, degradation of rebar mechanical properties, degradation of bond strength between rebars and concrete, and degradation of concrete strength were considered together.

https://doi.org/10.4334/JKCI.2024.36.4.329

임형진(Hyung Jin Lim)

Due to recent structural safety issues stemming from catastrophic collapses of concrete structures in South Korea, the interest in the safety and quality control of aging concrete bridges is increasing. Accordingly, Ground Penetrating Radar (GPR), a non-destructive testing technology, is emerging as a promising tool for diagnosing concrete structures. In this work, the research status of laboratory and field application of GPR technology for concrete bridge diagnosis was systematically investigated and analyzed. In addition, literature on the use of GPR for non-destructive estimation of concrete compressive strength and recent advancements in GPR signal processing techniques was reviewed. It was concluded that GPR is an effective tool for the non-destructive and accurate diagnosis of large concrete bridge structures. In addition, the possibility of developing a technique for non-destructively estimating the compressive strength of concrete was confirmed by utilizing the correlation between the electromagnetic wave characteristics of GPR and the water content during the hydration (strength development) process of concrete. Based on the results of this study, it is expected that GPR will be used to evaluate and improve the integrity of concrete bridges in the future.

https://doi.org/10.4334/JKCI.2024.36.4.337

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

This study evaluates the blast resistance performance of reinforced concrete columns by applying two assessment methods: ductility-based evaluation and residual strength-based evaluation. The primary design variables selected for analysis include the longitudinal reinforcement ratio, transverse reinforcement ratio, and axial load ratio, examining their impact on the columns’ blast resistance. Additionally, by comparing the outcomes of both evaluation methods, a significant inconsistency was observed, indicating the limitations of relying solely on a single assessment method for blast resistance evaluation. This inconsistency suggests the need for further research to develop more reliable solutions for assessing blast resistance.

https://doi.org/10.4334/JKCI.2024.36.4.347

정유석(Yoseok Jeong) ; 이일근(Ilkeun Lee) ; 민근형(Geunhyeong Min) ; 김우석(WooSeok Kim)

It is imperative to propose effective maintenance strategies for deteriorated bridge concrete deck, given the increasing cases of maintenance failures. This study aims to elucidate the deterioration mechanism and patterns of bridge concrete deck degradation. Additionally, it presents various repair methods and flowcharts for determining the most suitable repair method for each type of deterioration. The deterioration mechanisms include initial defects, cracks, heavy vehicle loads, and the infiltration of water and deicing chemicals through cracks. These factors lead to the expansion and deepening of deterioration, exacerbated by freeze-thaw cycles and heavy vehicles. Repair methods are categorized into concrete overlay repair (Type A), partial-depth deck repair (Type B), and full-depth deck repair (Type C). The study also proposes flowcharts for determining the appropriate repair method based on results obtained from visual and in-depth inspections of deteriorated concrete decks. By following this structured approach, the maintenance and rehabilitation of bridge decks can be conducted more efficiently, reducing long-term costs and enhancing the resilience of bridge infrastructure.

https://doi.org/10.4334/JKCI.2024.36.4.357

윤세윤(Seyoon Yoon)

In this study, the influence of contact impedance distribution between electrodes and a specimen on the image reconstruction of Electrical Resistance Tomography (ERT) in cement-based materials was analyzed, and an improved image reconstruction technique considering this effect was proposed. For this purpose, cement mortar specimens were fabricated, and ERT measurements were performed. Based on the measured data and finite element analysis, the contact impedance of each electrode was estimated using a nonlinear optimization technique, and its distribution characteristics were statistically analyzed. The results showed that the contact impedance values of individual electrodes, determined through the nonlinear optimization technique, exhibited deviations within 10% of the mean value. When these derived contact impedances were applied to the ERT image reconstruction, the difference between the maximum and minimum values of the reconstructed image decreased compared to the case where a single value of contact impedance was used. This means that the measurement sensitivity of the electrodes is improved by individually considering the non-uniform contact conditions between the electrodes and the specimen. These findings are expected to contribute to improving the accuracy and reliability of ERT image analysis of cement-based materials.

https://doi.org/10.4334/JKCI.2024.36.4.367

오경수(Gyeong-Su Oh) ; 손동희(Dong-Hee Son) ; 최현기(Hyun-Ki Choi) ; 배백일(Baek-Il Bae) ; 최창식(Chang-Sik Choi)

This study experimentally compares the shear strength and size effect of plain concrete (PC) beams and steel fiber-reinforced concrete (SFRC) beams without shear reinforcement. Beam specimens with heights of 350, 650, and 950 mm were fabricated, and steel fiber volume ratios were set at 0 % and 1 %. The experimental results indicated that SFRC beams exhibited higher shear strength compared to PC beams, and the reduction in shear strength due to size increase was relatively lower. This was attributed to the bridging effect of steel fibers across crack faces, effectively controlling crack propagation. However, current design standards do not adequately reflect the enhanced shear strength and reduced size effect provided by steel fibers. Therefore, there is a need for a new shear strength formulation that properly accounts for the size effect in SFRC beams.

https://doi.org/10.4334/JKCI.2024.36.4.375

이정윤(Jung-Yoon Lee)

Structural design codes specify the strength limits for RC deep beams. To reasonably limit the maximum strength, the factors affecting strength must first be identified. The maximum strength limits in the KDS 140 20 00 and ACI 318-19 codes proportional to the square root of the compressive strength of concrete, but they do not account for the effects of shear span-to-depth ratio and size effect. Therefore, for deep beams with short spans or using high-strength concrete, the strength is limited by the maximum strength, which may result in uneconomical design. In this study, the test results of 705 RC deep beams with various variables were analyzed to evaluate the factors affecting the maximum strength limit. The analysis shows that the maximum strength limit of deep beams should be proportional to the compressive strength of concrete and should reflect the influence of shear span-to-depth ratio.