https://doi.org/10.4334/JKCI.2024.36.5.399

김유진(Yu Jin Kim) ; 김태훈(Taehoon Kim) ; 김형기(Hyeong-Ki Kim)

A series of experimental evaluations was conducted to assess the extent of performance degradation in concrete resulting from various factors. These factors encompassed excessive use of mineral admixtures and air-entraining agents, an abundance of fines in the aggregate, exposure to low temperatures, and initial frost damage. The reduction in strength observed in both concrete and mortar, attributed to all factors except mineral admixtures, remained consistent regardless of the water?binder ratio. Concrete affected by most of these factors failed to reach the strength levels of the standard mix, even after 56 days of curing. Furthermore, the water absorption of hardened mortar was measured to elucidate its correlation with strength, indicating its potential as a quality estimation indicator. Additionally, concrete subjected to various factors demonstrated compromised rebar bond strength.

https://doi.org/10.4334/JKCI.2024.36.5.409

이재윤(Jae-yun Lee) ; 양근혁(Keun-hyeok Yang) ; 정기태(Ki-tae Jeong)

This study examined the compressive strength development and crack-healing performance of mortars containing hybrid inorganic and bacterial self-healing pellets. The pellets were sieved to achieve a particle size distribution between 600 μm and 1.7 mm. For the hybridization, the proportion of inorganic pellets ranged from 70 % to 90 %, while bacterial pellets constituted 30~10 %. The pellets were added as a 10 % volume replacement of fine aggregate in the mortar mixture. Test results revealed that the 28-day compressive strength of mortar specimens with the hybrid pellets increased by up to 1.23 times compared to plain mortar without pellets. Mortars with hybrid pellets exhibited efficient crack restoration, achieving self-healing ratios of 93~97 % for nominal crack widths of 0.3 mm. The hybridization of 80 % inorganic pellets and 20% bacterial pellets showed good promise concerning the 28-day compressive strength and self-healing of cracks.

https://doi.org/10.4334/JKCI.2024.36.5.417

심상락(Sang-Rak Sim) ; 고효진(Hyo-Jin Ko) ; 김용로(Yong-Ro Kim) ; 류동우(Dong-Woo Ryu)

This study evaluated the carbonation resistance of concrete with high-functional exterior wall paint. After applying the high-functional exterior paint, accelerated weathering test (KS M 6010) and accelerated carbonation test (KS F 2584) were conducted. As a result of the study, it was confirmed that the developed high-functional exterior wall paint significantly enhance the carbonation resistance when applied to concrete, thus improving the durability of concrete structure. Additionally, to examine the weathering resistance deterioration due to external factors (such as UV rays, moisture, and heat) when applied to actual buildings, the carbonation depth was measured after the accelerated weathering test. The results confirmed that even after 1,200 hours of harsh accelerated weathering conditions, the carbonation resistance did not significantly decrease.

https://doi.org/10.4334/JKCI.2024.36.5.427

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

In order to predict the fire resistance of reinforced concrete (RC) columns, a three-dimensional finite element analysis modeling technique was developed considering the axial load ratio, eccentricity, and end-rotation constraint. The fully coupled thermo-mechanical analysis method in Abaqus was used for the modeling, and the modeling was verified by comparing with ten test specimens subjected to concentric or eccentric load. Although there were some differences due to the relatively high axial load ratio, eccentric load effect, and end-constraint, the modeling gave conservative results in terms of temperature distribution, axial deformation, and fire resistance rating. These results indicate that the proposed modeling technique can be used in the fire resistance design of RC columns.

https://doi.org/10.4334/JKCI.2024.36.5.437

박연주(Yeon-ju Park) ; 이문석(Moon-seok Lee) ; 배백일(Beak-Il Bea) ; 최현기(Hyun-Ki Choi) ; 최창식(Chang-Sik Choi)

This study experimentally analyzed the impact of shear reinforcement ratio and positive/negative bending moments on the shear strength of precast (PC) and cast-in-place (CIP) concrete composite beams. The shear performance of composite beams with separate casting was evaluated using eight specimens with identical reinforcement layouts: four monolithically cast, and four separately cast. All specimens were designed to induce vertical shear failure by having high longitudinal reinforcement ratios and high horizontal shear strengths. The experiments were conducted using 4-point loading to assess the effect of the shear reinforcement ratio on the vertical shear strength of the PC and CIP concrete composite beams. The results showed that among the separately cast specimens, the specimen under negative moment with lower transverse reinforcement ratio had higher shear strength compared to the specimen under positive moment. The moment direction influenced the shear deformation and crack patterns. Additionally, the experimental results were compared with current design codes to evaluate shear performance.

https://doi.org/10.4334/JKCI.2024.36.5.445

박완신(Wan-Shin Park) ; 장영일(Young-Il Jang) ; 김선우(Sun-Woo Kim) ; 윤현도(Hyun-Do Yun)

In the last several decades, hybrid coupled shear walls with link beams have become increasingly recognized as efficient lateral load-resisting systems for mid- to high-rise buildings. The purpose of this study was to investigate the feasibility of alternative detailing for steel link beam-to-reinforced concrete (RC) wall connections, to simplify the complex reinforcement details outlined in the ACI CODE-318-19(22) design standards and to reduce the risk of brittle fractures in the connection region. The test variables of this study included the presence or absence of fiber, the embedment length of the steel coupling beam, and the inclusion of stud bolts and horizontal ties in the connection region. The results show that incorporating PVC fibers into the connections significantly enhances both the shear capacity and damage tolerance of the steel coupling beam-to-wall connection.

https://doi.org/10.4334/JKCI.2024.36.5.455

남궁경욱(Kyeong-Wook Namgung) ; 김창혁(Changhyuk Kim)

In this study, the flexural strengthening effect of reinforced concrete (RC) beams using two layers of prestressed carbon fiber reinforced polymer (CFRP) sheets was considered. A commercial finite element analysis software, ATENA 3D, was utilized to construct the CFRP-strengthened RC beams. The main variables in the analysis included the prestressing level, combinations of prestressing, and the timing of reinforcement application. The ultimate load, ductility, material deformation, and crack patterns of each specimen were compared. From the analysis, the maximum strength of the specimens was similar, regardless of the prestress level applied to the carbon fiber. Although higher prestressing levels enhanced the reinforcement effect, the specimens exhibited brittle behavior. In contrast, specimens reinforced with both prestressed and non-prestressed CFRP sheets, demonstrated a reinforcement effect similar to that of specimens with two layers of prestressed CFRP sheets, but with ductile behavior. After the application of the initial load, specimens reinforced with a combination of prestressed CFRP sheets showed an increase in maximum strength compared to the control specimen, along with reduced material deformation, deflection, number of cracks, and average crack width. Additionally, the camber effect that occurred immediately after reinforcement contributed to crack reduction, thereby verifying the superior crack control of the reinforced specimens.

https://doi.org/10.4334/JKCI.2024.36.5.465

최재우(Jae-Woo Choi) ; 경제환(Jae-Hwan Kyung) ; 김문길(Moon-Gil Kim) ; 전상현(Sang-Hyeon Jeon) ; 천성철(Sung-Chul Chun)

Joist floors with deep deck plates do not need concrete formwork nor shoring support or minimum supports, which can reduce the construction period as well as improve construction safety. Typical 4-point loading flexural tests were conducted on composite joist floors with CAP II and CAP III to evaluate the flexural behavior of composite joist floors under positive moment. A total of eight specimens were tested: four composite joist floors and four reinforced concrete (RC) joist floors. The ratios of the measured maximum loads of the composite joist floors to the calculated loads, ignoring the effects of the steel deck plate, ranged from 1.60 to 3.34 indicating superior flexural strengths of the composite joist floors. Especially, the specimens with topping concrete of 110 mm and 120 mm depth had high maximum loads of 3.34 and 3.13 times calculated loads, respectively. The deflections at service loads of the composite joist floors were reduced due to the composite action of the steel deck plate and concrete. The deflections of the composite joist floors were only 0.48~0.75 times those of the RC joist floors with the same configuration. The average ratio of measured to calculated deflections at service loads, including the deep deck plate and reinforcing bar, was 1.04 implying that deflections at service loads can be properly estimated.

https://doi.org/10.4334/JKCI.2024.36.5.475

이정배(Jeong-Bae Lee) ; 최현덕(Hyun-Deuk Choi)

The durability and structural stability of concrete structures are crucial in the construction industry. However, performance degradation due to environmental factors such as sulfate attack can pose significant challenges. Sulfate attack is divided into chemical corrosion, which causes expansion and cracking due to the formation of ettringite and gypsum, and physical corrosion, which involves the growth of salt crystals. These processes shorten the lifespan of concrete and threaten structural safety. We investigated concrete’s resistance to sulfate attack by adjusting the water?binder ratio and the proportion of mineral admixtures. Experiments confirmed that a lower water?binder ratio increases resistance to sulfate attack. These results provide a basis for developing new concrete mix designs to enhance durability. Additionally, using appropriate mineral admixtures and additional protective measures such as surface coatings can further protect concrete from sulfate attack. This research offers important findings that can contribute to extending the lifespan and enhancing the safety of concrete structures. It highlights the importance of exploring new approaches to mitigate sulfate attack and improve concrete durability through innovative mix designs and protective strategies.

https://doi.org/10.4334/JKCI.2024.36.5.483

서수연(Soo-Yeon Seo) ; 하니프 엠 우스만(M. Usman Hanif) ; 트란 하이 반(Hai Van Tran) ; 윤현도(Hyun-Do Yun)

The purpose of this study is to identify cracks and bond damage in fiber reinforced polymer (FRP) sheets applied to reinforced concrete (RC) beams, using the acoustic emission (AE) technique. First, in order to investigate the reinforcing effect of FRP sheets, RC beams with and without FRP strengthening were made. Additionally, a beam was strengthened with an FRP sheet that included partially debonded areas on the bottom surface. AE sensors were attached to the lower surface of all specimens to obtain relevant information. The AE responses were calibrated using the pencil-lead break test, and the responses were monitored during the loading test. The results showed that the strength of the beam increased due to the FRP sheet, although somewhat brittle fracture behavior was observed. The AE technique successfully detected the initiation and propagation of concrete cracks, as well as the location of FRP sheet debonding.

https://doi.org/10.4334/JKCI.2024.36.5.493

김윤정(Yun-Jeong Kim) ; 문주현(Ju-Hyun Mun)

This study examined the ductility performance of precast lightweight aggregate concrete (PLC) shear walls and proposed a calibration factor (ξ) relating to equivalent concepts. The main parameters investigated were the methods of wall-to-base connection and the yield strength (fy) of longitudinal reinforcing bars in boundary elements. All PLC shear wall specimens were tested under constant axial load and cyclic lateral loads. The test results showed that PLC shear walls designed with a lower yield strength and connected using bolts, rather than the conventional spliced sleeve method, exhibited higher displacement ductility ratios (μΔ) and cumulative work damage indicators. To achieve ductility equivalent to that of precast concrete (PC) shear walls made of normal-weight concrete, a simplified model was proposed. For PCL shear walls designed with a yield strength of 600 MPa, it was recommended that the transverse reinforcing bar ratio (Ash) in boundary elements be 2.31 times greater than the minimum amount specified in KDS 14 20 00.

https://doi.org/10.4334/JKCI.2024.36.5.505

이상훈(Sang-Hoon Lee) ; 김재민(Jae-Min Kim) ; 김재현(Jae Hyun Kim) ; 김강수(Kang Su Kim)

Deriving the response of structures subjected to blast loads is essential for protecting human lives and ensuring structural safety. Structural safety can be assessed through blast-resistant analysis, and the behavior of blast-resistant structures can be derived using a single-degree-of-freedom numerical analysis method based on reasonable assumptions. Generally, blast loads are treated as uniformly distributed in such analyses; however, in the case of close-in explosions, where the blast source is near the structure, the blast pressure does not act uniformly on the structure. In this study, a single-degree-of-freedom numerical analysis response database considering the effects of close-in explosions was established, and a close-in explosion response correction artificial neural network (ANN) model was developed and validated to adjust the responses derived from the single-degree-of- freedom analysis method, assuming uniformly distributed loads, for the specific effects of close-in explosions.

https://doi.org/10.4334/JKCI.2024.36.5.515

오영훈(Young-Hun Oh) ; 김준삼(Jun-Sam Kim) ; 김정욱(Jeong-Wook Kim) ; 전병갑(Byong-Kap Jeon) ; 이강철(Gang-Chul Lee) ; 문정호(Jeong-Ho Moon)

This study focuses on the seismic performance of high-rise apartment building structures composed of dry-jointed precast (PC) moment-resisting frames. The objective is to ensure that a dry-jointed PC frame meets the performance level of a PC intermediate-moment frame. To this end, a steel bracket-type connection device was developed to connect the bottom rebars of the PC beam to the column, and both experimental and analytical studies were conducted to evaluate its performance. The experimental study was conducted in two stages. In the first stage, the performance of the connecting device was evaluated according to the KS D 0249 standards for mechanical joints of reinforcing bars. In the second stage, three full-scale test specimens of the PC structure were manufactured and tested to determine whether they met the performance conditions outlined in the draft KDS earthquake-resistant design code. In parallel, nonlinear analysis and performance evaluation using ASCE/SEI-41 were conducted to further assess the experimental results. The results demonstrated that the beam-column dry joint of the PC apartment building frame, which is the subject of this study, can achieve earthquake-resistant performance equivalent to that of a PC intermediate moment frame