https://doi.org/10.4334/JKCI.2020.32.4.307

임채림(Chae-Rim Im) ; 양근혁(Keun-Hyeok Yang) ; 문주현(Ju-Hyun Mun)

The objective of this study is to estimate the flexural performance of lightweight aggregate concrete (LWAC) with minimum longitudinal reinforcement ratio () specified in KCI 2017 provision and propose a correction factor () for use in LWAC beams to retain a behavior equivalent to the flexural ductility of the companion normal-weight concrete (NWC) beams. The flexural ductility values of beams were assessed for four T-beam specimens and 2-dimensional non-linear analysis procedure. The displacement ductility ratio () of the LWAC T-beams tended to increase with the decrease in longitudinal reinforcement ratio () but decreased inversely when approaches . This trend was more notable for the beams with a lower concrete unit weight (). Thus, from the comprehensive parametric study conducted on the basis of the non-linear analysis, the value of could be proposed as a function of . This indicates that for LWAC beams needs to be increased with the decrease in with respect to an equivalent ductility of the companion NWC beams.

https://doi.org/10.4334/JKCI.2020.32.4.317

김기현(Ki-Hyun Kim) ; 윤석구(Seok-Goo Youn)

If voids occur because the inside of the sheath of a tensioned Prestressed Concrete structure is not filled with grout, water and chloride ions can penetrate, causing strand breakage and bridge collapse due to corrosion of the strands. The causes of voids in the sheath of PSC structures include material problems such as bleeding of grouting materials and hydrogen gas generation due to the use of metallic expanders, inadequate use of equipment, and insufficient installation of outlets due to insufficient skilled workers. In order to solve these problems, materials such as non-bleeding grout, non-hydrogen gas-expanded expansion material, and pre-mixed grout have been developed. However, in terms of grout equipment and grouting methods, and in consideration of constructability, this field is still insufficiently developed in Korea. In this study, an experiment was conducted to fill water and grout in a vertical tube to evaluate the field applicability of a vacuum grouting method in which grout was injected into a sheath tube using a vacuum grouting apparatus. Based on the experimental results, if the inside of the sheath pipe of the PSC structure is depressurized under vacuum pressure and grout is injected, it is considered that it is possible to fill the grout tightly in the sheath pipe by vacuum injection and the pumping injection using hydraulic pressure.

https://doi.org/10.4334/JKCI.2020.32.4.325

성민규(Min-Kyu Seong) ; 천성철(Sung-Chul Chun)

With high-strength reinforcing bars, more efficient construction is expected. However, high-strength materials have different mechanical properties from normal strength materials and experimental verifications of high-strength materials are required before the materials are applied to real structures. In this study, concentric compression tests were conducted for columns with high-strength concretes of 60, 80, and 90 MPa and spliced SD700 D22 reinforcing bars of 20, 25, and 30 lap lengths. Test results show that concrete compression failure occurred before yield stresses of bars developed, because as the strength of the reinforcing bars and concrete increased, the strain corresponding to the yield stress of the reinforcing bars increased but the ultimate strain of concrete did not increase. Comparing the test results of this study with previous studies shows that splice strength in compression increases in proportion to the concrete strength and splice length, but the strength converges to a certain value. When transverse reinforcing bars were placed within the lap splice length, the converged value increased by about 11.3 %. Upper limit strengths of 557 MPa and 620 MPa for unconfined and confined laps in compression, respectively, were proposed from this investigation. An equation to predict the splice strength of a compression lap splice is proposed by applying the upper limits to the equation of Chun et al.’s Model. In addition, design upper limit strengths of 462 and 514 MPa are suggested by incorporating a 5 % fractile coefficient.

https://doi.org/10.4334/JKCI.2020.32.4.333

조민수(Min-Su Jo) ; 김민준(Min-Jun Kim) ; 이진용(Chin-Yong Lee) ; 정찬유(Chan-Yu Jeong) ; 김길희(Kil-Hee Kim)

Fiber-reinforced cementitious matrix (FRCM) composites made of textile grids embedded in an inorganic matrix are advanced cement-based materials designed for repair and retrofitting of concrete structures. Composite materials are the result of combining basic materials into new material systems that have improved properties compared to the basic materials. In this respect, improving the performance of the components of the composit is important. In this study, basic research was carried out to derive the optimum mixture for improving the performance of FRCM inorganic matrix for repair and retrofitting of concrete structures. For the development of the inorganic matrix for the FRCM composite, four groups were sequentially determined and the basic properties were confirmed. From the experimental results, optimum results were obtained with 2.0 % GGBS, 1.0 % Silica fume, 0.2 % PVA fiber, and 0.2 % EVA Polymer. In terms of durability, the alkali resistance was reduced by about 70 % compared with that of conventional mortar, and the water absorption rates were almost identical.

https://doi.org/10.4334/JKCI.2020.32.4.341

성민규(Min-Kyu Seong) ; 천성철(Sung-Chul Chun)

The current design provision of development length of compression bars are based on very limited experimental results and cannot precisely predict compression anchorage strength. For hooked bars, the compression development length may be longer than the tension development length, although the compressive anchorage has more advantageous conditions than tensile anchorage of hooked bars has. As a result, an unnecessarily long development length is required and the column sectional dimension may be enlarged. In this study, equations to predict compression anchorage strength have been proposed based on test data on compression anchorage. In addition, design equations for compression development length of straight bars and hooked bars incorporating a 5 % fractile safety factor have been derived. The proposed equations have been compared with current design provisions and shows that required embedment lengths from the proposed equations are shorter than lengths calculated by current design provisions under same conditions.

https://doi.org/10.4334/JKCI.2020.32.4.351

김상희(Sang-Hee Kim) ; 안성룡(Seong-Ryong Ahn) ; 강현구(Thomas H.-K. Kang)

Prestressed concrete is widely utilized for the construction of various structures such as bridges, buildings, and nuclear facilities. However, studies of impact resistance on prestressed concrete members are still lacking, and there is no applicable design equation in particular. In this study, impact test data of the prestressed concrete panel tests performed by other researchers were analyzed to assess the effect of prestressing force in a concrete panel, and nonlinear finite element analyses were performed for parametric study. Based on this, a new empirical equation was proposed in order to predict the ballistic limits of bonded prestressed concrete panel. The proposed equation was verified with the use of test data and analytical studies. As a result, it is expected to be effectively used for the design of prestressed concrete structures.

https://doi.org/10.4334/JKCI.2020.32.4.359

윤영묵(Young Mook Yun)

For an appropriate strut-and-tie model design of structural concrete, the effective strengths of nodal zones must be determined with sufficient accuracy. Many values and equations for effective strengths have been suggested. However, since the recommended strength values have been determined based on experimental works with specific geometry and loading conditions, it is inappropriate to use them in the strut-and-tie model design of general structural concretes. In this study, a new numerical method, that evaluates the effective strengths of two-dimensional nodal zones accurately and consistently by reflecting the state of stresses in the nodal zones, the effects of stresses and details of reinforcing bars placed inside and around nodal zones, and the compressive strength of concrete, is proposed. The ultimate strengths of 434 reinforced concrete deep beams tested to failure were estimated to verify the validity of the proposed method using the ACI 445 strut-and-tie model method with the existing and proposed effective strengths of nodal zones.

https://doi.org/10.4334/JKCI.2020.32.4.371

서지석(Ji-Seok Seo) ; 이봉춘(Bong-Chun Lee) ; 김윤용(Yun-Yong Kim)

This study presents a method for evaluating the output quality of 3D printing mortar through layer thickness, and analyzed the effect of mortar flowability and void-ratio on uniformity and accuracy of layer thickness. As a result of observing the appearance of the 3D printer output specimen, the difference in the width of the upper layer and the lower layer of the specimen increased as the flowability and void ratio increased. It was also observed that the surface of the specimens became rough as the flowability and void ratio decreased. Using image analysis, the layer thickness was measured in units of pixels; the layer quality properties were analyzed using the proposed equation. The X-axis and Z-axis uniformity values of the layer thickness were in ranges of 0.79 to 0.98 and 0.75 to 0.83, respectively. The uniformity increased as the flowability and void ratio increased, while uniformity decreased with increase of flowability and void ratio after reaching maximum uniformity. The flowability and void ratio of the mortar showing maximum uniformity are 1.89 and 0.70, respectively. The accuracy of the layer thickness was in a range of 0.86 to 0.94, and showed the same tendency as the uniformity with respect to flowability and void ratio. The mortar showing the maximum uniformity exhibited a maximum accuracy of 0.94.

https://doi.org/10.4334/JKCI.2020.32.4.379

김형국(Hyeong-Gook Kim) ; 이용준(Yong-Jun Lee) ; 김민준(Min-Jun Kim) ; 안정희(Jung-Hee An) ; 김길희(Kil-Hee Kim)

Recently, studies on the development and application of new construction methods for seismic retrofitting of existing school buildings built in the 1980s have been actively conducted. When applying extruded concrete panels to a reinforced concrete frame, as in the case of constructing a masonry-infilled wall, meticulous structural examinations are needed since the bending and shear performances and the hysteretic behavior of the reinforced concrete (RC) frame may have changed. In this study, in-plane static-cyclic loading tests were performed on non-seismic detailed RC frames infilled with existing concrete blocks and newly developed lightweight concrete blocks to evaluate the effect of the type of infilled concrete blocks on the seismic performance of RC frames, with comparisons of load-displacement relationship, displacement ductility factor, energy dissipation and strain distribution of specimens. The test results showed that the maximum strength and the ductility of the specimens increased regardless of the type of infilled concrete blocks and that an increase in unit volume of the masonry object improves the lateral load resistance of RC frames by reducing the number and width of cracks, even if the compressive strength of concrete is small. Besides this, the results of the comparative study showed that the specimen infilled with the newly developed concrete block has a seismic performance equal to or higher than that of the specimen infilled with an existing concrete block.

https://doi.org/10.4334/JKCI.2020.32.4.387

오근영(Keunyeong Oh) ; 이강민(Kangmin Lee) ; 이경구(Kyungkoo Lee)

The purpose of this study was to evaluate the seismic performance of existing RC frames retrofitted with steel chevron bracing and infilled frame by conducting an experiment study. Although the steel chevron bracing and infilled frame require high constructability, these approaches known to be more effective than other seismic retrofitting methods to provide lateral strength and stiffness. However, specific design procedures and methods for retrofitting steel chevron bracing and infilled frames are not included in the current guidelines. In this study, an experimental study was conducted to produce fundamental data for developing seismic retrofitting design methods for existing RC frames with steel chevron bracing and infilled frame. In order to evaluate the seismic performance of the existing RC frames retrofitted with steel members, a total of four specimens were manufactured, and the flexural stiffness of all specimens was calculated using the direct stiffness method. Two of the specimens were reinforced with a steel chevron bracing system, and designed to be two to three times the flexural stiffness of the existing RC frames, respectively. In addition, one specimen was retrofitted with an steel infilled frame and was designed to be four times the flexural stiffness of the existing RC frame, and one specimen was not reinforced. Test results revealed that the specimens retrofitted with the steel member almost similar target flexural stiffness to that calculated by the direct stiffness method. It was confirmed that retrofitted specimens showed a notable increase in strength and stiffness compared to the existing RC frame.