The Experts below are selected from a list of 2232 Experts worldwide ranked by ideXlab platform
Doo-yeol Yoo - One of the best experts on this subject based on the ideXlab platform.
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benefits of curvilinear straight steel fibers on the rate dependent Pullout Resistance of ultra high performance concrete
Cement & Concrete Composites, 2021Co-Authors: Jaejin Kim, Doo-yeol Yoo, Nemkumar BanthiaAbstract:Abstract This study examines the effectiveness of using curvilinear steel fibers to enhance the Pullout Resistance of conventional straight steel (S-) fiber for an ultra-high-performance concrete (UHPC) matrix. Because structures composed of UHPC are subjected to various loading conditions and have random fiber orientations, three different loading rates ranging from static (0.018 mm/s) to impact (up to 906.2 mm/s) and two inclination angles of 0° and 45° were adopted. To fabricate the novel curvilinear steel fibers, five different curvatures were also utilized. Test results indicated that the curvilinear fibers provided a clearly higher Pullout Resistance than that of the conventional S-fiber, and the effectiveness increased with the curvature, regardless of the inclination angle and loading rate. Higher bond strengths were obtained for the S- and curvilinear fibers when they were inclined at 45° rather than aligned. In general, the bond strengths and Pullout energies of the S- and curvilinear fibers in UHPC were improved by increasing the loading rate, and a higher loading rate sensitivity on the bond strengths was observed in aligned fibers than in inclined fibers. The matrix spalling phenomenon was only observed in an inclined state and became more significant with an increasing curvature and loading rate. The developed curvilinear fibers provided noticeably smaller matrix spalling areas than the conventional deformed steel fibers under both static and impact conditions, owing to the mitigated stress concentration. Thus, these may be worthy of attention as a novel reinforcing fiber type for UHPC composites.
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Liquid crystal display glass powder as a filler for enhancing steel fiber Pullout Resistance in ultra-high-performance concrete
Journal of building engineering, 2020Co-Authors: Doo-yeol Yoo, Ilhwan YouAbstract:ABSTRACT This study investigated the feasibility of using waste liquid crystal display (LCD) glass powder as a partial replacement for the silica flour that is typically used as a filler in ultra-high-performance concrete (UHPC). In this study, 50% of the silica flour was replaced with LCD glass powder, and its effects on the interfacial bond properties of two steel fiber shapes—circular and triangular—were evaluated. The test results indicated that the LCD glass powder effectively enhanced the bond between the steel fibers and the UHPC, which created a denser interfacial zone; this could be attributed to the filling effects of the LCD glass powder and secondary hydrates. The use of the LCD glass powder increased the Pullout energy by 15–40% and the average bond strength by 16%, along with formation of additional scratches on the fiber surfaces. For the same matrix, as compared to the circular steel fibers, the triangular steel fibers yielded superior Pullout Resistance owing to their higher intrinsic fiber efficiency ratio. Thus, the highest average bond strength and Pullout energy of 11.2 MPa and 744 mJ could be obtained from the UHPC mixture containing the LCD glass powder and triangular steel fibers.
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analysis on enhanced Pullout Resistance of steel fibers in ultra high performance concrete under cryogenic condition
Construction and Building Materials, 2020Co-Authors: Min Jae Kim, Doo-yeol YooAbstract:Abstract This study aims to investigate the implication of cryogenic condition on the Pullout properties of straight and deformed steel fibers embedded in ultra-high-performance concrete (UHPC). For this, two environmental conditions, i.e., ambient and cryogenic, approximately −170 °C, were considered along with three steel fiber types, i.e., straight, half-hooked, and twisted, and two inclination angles of 0° and 45°. In order to rationally explain the test results obtained, optical micrograph and scanning electron microscope images were captured and analyzed. The test results indicated that the bond strength of all steel fibers generally increased at cryogenic temperatures, and the effectiveness was higher when they were aligned than inclined. The straight fiber led to a much higher enhancement of the Pullout Resistance in terms of strength and energy when compared to the hooked and twisted fibers. The matrix damage became more severe as the fiber was geometrically deformed, inclined, and tested under cryogenic conditions. The fiber deformation improved the initial Pullout properties but deteriorated the surrounding matrix and later Pullout properties due to the fiber rupture or severe matrix spalling. Consequently, a lower efficiency of the deformed fibers as compared to that of the straight fiber was obtained under inclined or cryogenic conditions.
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Effects of rust layer and corrosion degree on the Pullout behavior of steel fibers from ultra-high-performance concrete
Journal of Materials Research and Technology, 2020Co-Authors: Doo-yeol Yoo, Jae Young Gim, Booki ChunAbstract:Abstract This study investigates the effect of surface corrosion on the Pullout behavior of straight steel fibers embedded in ultra-high-performance concrete (UHPC). To this aim, straight steel fibers, either with or without surface corrosion, were utilized, and various corrosion degrees from 2% to 15% by weight were considered. To evaluate the implication of rust layer on the Pullout behavior of corroded steel fibers from UHPC, both washed and unwashed conditions were considered. The surface roughness of plain and corroded steel fibers was analyzed by means of scanning electron microscope and atomic force microscope (AFM) images. The test results indicated that surface corrosion is effective in enhancing the Pullout Resistance of straight steel fibers in UHPC when the fibers are completely pulled out from the matrix without breakage. The maximum average bond strength and Pullout energy of moderately corroded fibers in UHPC were found to be 18.5 MPa and 715.7 N·mm, approximately 2.7 and 1.8 times higher than those of plain fibers in the same matrix at the aligned condition. The benefits of moderate surface corrosion on improving the Pullout Resistance were mitigated by inclining the fibers. A higher corrosion degree led to a better Pullout Resistance up to a certain value (2 or 5%); however, beyond such value, the Resistance decreased significantly due to the rupture of fibers. A threshold value of 2% for the corrosion degree was thus suggested to achieve an excellent fiber bridging capability. The washed corroded fibers exhibited higher bond strength and Pullout energy than the unwashed ones with the same degree of corrosion at aligned condition; however, the benefits of washing vanished when the fibers were inclined and ruptured prematurely. An obvious correlation between the bond strength and the surface roughness was observed from the AFM images.
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effects of fiber shape and distance on the Pullout behavior of steel fibers embedded in ultra high performance concrete
Cement & Concrete Composites, 2019Co-Authors: Jaejin Kim, Doo-yeol YooAbstract:Abstract This study examines the implications of the fiber type and the distance between fibers on the Pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and four different distances between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7% and a fiber bundle, were considered. To evaluate the effect of the distance between fibers, four individual fibers were included in a single dog-bone specimen, and a single fiber specimen was also fabricated and tested as a control specimen. Test results indicate that the twisted steel fiber exhibited the greatest Pullout Resistance, followed by the hooked and straight steel fibers. Approximately 30% lower bond strengths were obtained for the specimens with multiple fibers as compared to those with a single fiber, regardless of the fiber type and distance between fibers. The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the distance between fibers up to 1 mm, corresponding to a volume fraction of 7%, while the bundled fiber specimens provided the poorest Pullout Resistance in terms of bond strength and energy absorption capacity for all types of fibers. The reduction rate of Pullout Resistance was the most significant for the straight fiber, relative to the hooked and twisted fibers. Minor matrix damage was obtained for the straight fiber specimen, and its Pullout performance was not influenced by the surrounding fibers. In contrast, severe matrix damage was observed for the hooked and twisted fibers, and they were overlapped, causing a larger spalling area with a closer fiber distance.
Eunsoo Choi - One of the best experts on this subject based on the ideXlab platform.
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effect of crimped sma fiber geometry on recovery stress and Pullout Resistance
Composite Structures, 2020Co-Authors: Eunsoo Choi, Hee Sun Kim, Taehyun NamAbstract:Abstract This study investigates the recovery stress and Pullout Resistance of crimped fiber made by cold drawn shape memory alloy (SMA) wires considering the geometry of the crimped fiber, namely, wave height/depth. Moreover, it assesses how the shape memory effect influences the Pullout Resistance of the crimped fiber. For this purpose, four types of crimped fibers with different wave heights as well as the as-received fiber without any crimping are prepared with six specimens, and half of them are heated to induce the shape memory effect. The recovery stress is measured with several heating temperatures ranging from 100℃ to 300℃, and the residual stress at room temperature is also measured. When the crimped fibers are heated to induce phase transformation, the ratio of wave height to wire diameter is reduced from 33% to 48%. In this study, two types of crimped fibers with relatively small wave height do not show yielding, while the other two fibers with relatively large wave height show yielding of the fiber. Thus, this study suggests a critical wave depth for yield of the fiber. If the difference between wave height and thickness, namely wave depth, exceeds 0.1 mm, the crimped fibers experiences yield.
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active reinforcing fiber of cementitious materials using crimped niti sma fiber for crack bridging and Pullout Resistance
Materials, 2020Co-Authors: Eunsoo Choi, Jongsu JeonAbstract:This study investigated the recovery stress and bond Resistance of cold drawn crimped SMA fiber using two different initial diameters of 1.0 and 0.7 mm. These characteristics are important to the active prestressing effect and crack-closing of the fiber. NiTi SMA fiber was used for the cold drawing, and then crimped shapes were manufactured with various wave heights. After that, tensile, recovery, and Pullout tests were conducted. The cold drawn crimped fiber showed softening tensile behavior more clearly than the cold drawn straight fiber when not subjected to heating, whereas they had the same tensile behavior under heating. The recovery stress and the residual stress of the crimped fibers were less than those of the straight fiber with the same diameter. Moreover, crimped fibers with a large diameter and higher wave height would induce more recovery stress and residual stress. The maximum Pullout Resistance of the crimped fiber was a function of the wave depth, embedded length, yield strength, and flexural rigidity of the fiber.
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Pullout Resistance of deformed shape memory alloy fibers embedded in cement mortar
Journal of Intelligent Material Systems and Structures, 2015Co-Authors: Dong-joo Kim, Youngsoo Chung, Hee Ae Kim, Eunsoo ChoiAbstract:In this study, the Pullout Resistance of deformed shape memory alloy fibers embedded in a mortar matrix is investigated to develop self crack-closing capacity. Three types of deformed shape memory alloy fibers (dog bone–shaped, end-deformed, and crimped) and one type of smooth shape memory alloy fiber, fabricated from two different alloys, NiTi and NiTiNb, were embedded in a mortar matrix with a compressive strength of 55 MPa. The Pullout Resistance differed considerably depending on the geometry of the fiber and composition of the alloy. The Pullout Resistance was generally higher for deformed shape memory alloy fibers than for the smooth shape memory alloy fiber. Among the deformed shape memory alloy fibers, dog bone–shaped fibers showed the highest enhancement in bond strength after heat treatment. The Pullout Resistance was higher for the NiTiNb alloy than the NiTi alloy when the shape memory alloy fiber was deformed, whereas the relationship was reversed when the shape memory alloy fiber was smooth.
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Pullout Resistance of straight niti shape memory alloy fibers in cement mortar after cold drawing and heat treatment
Composites Part B-engineering, 2014Co-Authors: Youngsoo Chung, Eunsoo ChoiAbstract:Abstract In our study, we found cold drawing to be an effective method for enhancing the Pullout Resistance of NiTi shape memory alloy (SMA) fibers in concrete. The Pullout Resistance was observed to be dependent on the contact pressure and friction coefficient at the interface between the fibers and the mortar matrix. The drawing process increased the stiffness and yield stress of the fibers and consequently increased the contact pressure at the interface between the fibers and the mortar matrix. Moreover, heat treatment of the fibers after cold drawing was found to noticeably recover the fiber diameter, thereby significantly enhancing the Pullout Resistance. The enhancement of the interfacial bond strength by heat treatment verified the crack-closing capabilities of SMA-fiber-reinforced cement composites.
Jinchun Chai - One of the best experts on this subject based on the ideXlab platform.
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Pullout Resistance of bearing reinforcement embedded in coarse grained soils
Geotextiles and Geomembranes, 2013Co-Authors: Cherdsak Suksiripattanapong, Suksun Horpibulsuk, Avirut Chinkulkijniwat, Jinchun ChaiAbstract:Abstract The bearing reinforcement was developed as a cost-effective earth reinforcement. It is composed of a longitudinal member and transverse members. The longitudinal member is made of a steel deformed bar and the transverse members are a set of equal angles. The present article studies the influence of soil properties (friction angle, grain size and gradation) and dimension and spacing of the transverse members on the Pullout mechanism of the bearing reinforcement. The total Pullout Resistance is the sum of the Pullout friction and the Pullout bearing Resistance. The tan δ /tan ϕ ratio, where δ is the friction angle between soils and the longitudinal member and ϕ is the internal friction angle of soil, is greater than unity because of the roughness and rigidity of the steel deformed bar. The bearing failure mechanism of a single transverse member is dependent upon the B / D 50 value, where B is the leg length of the transverse member and D 50 is the average grain size of the soil. The transverse member interference is dependent upon the ratio of spacing between transverse members and the leg length of transverse members, S / B . Based on a critical analysis of the test results, the Pullout Resistance equations of the bearing reinforcement with different dimensions and spacing between transverse members embedded in different coarse-grained soils are introduced and verified. These equations were developed based on a limit equilibrium analysis, which is a simple rational method for analyzing the internal stability of bearing reinforcement earth walls.
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prediction of Pullout Resistance and Pullout force displacement relationship for inextensible grid reinforcements
Soils and Foundations, 1996Co-Authors: Dennis T. Bergado, Jinchun Chai, Norihiko MiuraAbstract:A new analytical method is proposed for determining the inextensible grid reinforcement Pullout Resistance and Pullout force/Pullout displacement curve by using basic backfill soil and grid reinforcement properties. The Pullout skin friction Resistance/Pullout displacement relationship is simulated by linear elastic-perfectly plastic model. A hyperbolic model has been proposed to represent the Pullout bearing Resistance/Pullout displacement relationship in which the maximum bearing Resistance of a single bearing member is determined using a new bearing capacity equation proposed in this paper. The influences of the grid bearing member spacing ratio, S/D, the bearing member deflection rigidity, and the Pullout softening behavior on the mobilization of Pullout bearing Resistance are explicitly included in the proposed model. Good agreement has been obtained between calculated values and laboratory test results.
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Pullout force displacement relationship of extensible grid reinforcements
Geotextiles and Geomembranes, 1994Co-Authors: Dennis T. Bergado, Jinchun ChaiAbstract:Abstract A model for predicting the Pullout Resistance of polymer-grid reinforcement has been proposed. The influence of bearing member rigidity and spacing ratio (S/D) are explicitly expressed in the hyperbolic model. A new bearing capacity equation is incorporated for calculating the maximum Pullout force. The displacement along the reinforcement is calculated by using the proposed Pullout bearing Resistance model together with the elongation of the grid longitudinal member. The validity of the method is confirmed by good agreement between calculated values and actual test data. The analytically determined effective reinforcement embedment lengths (i.e. the length of the reinforcement in tension) and Pullout displacement to mobilize the desired Pullout Resistance of polymeric grids under different backfill conditions and under different applied normal pressures, provide useful information for the design of reinforced earth structures against Pullout failure.
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interaction between cohesive frictional soil and various grid reinforcements
Geotextiles and Geomembranes, 1993Co-Authors: Dennis T. Bergado, Jinchun Chai, Marolo C. Alfaro, H O Abiera, A. S. BalasubramaniamAbstract:Abstract A total of 52 large-scale laboratory Pullout and 24 large-scale direct-shear tests were conducted to investigate the interaction behavior between the different reinforcements and cohesive-frictional soil. The reinforcements used were steel grids, bamboo grids, and polymer geogrids. The backfill material used was locally available weathered Bangkok clay. The test results show that the inextensible reinforcements, such as steel grids, move approximately as a rigid body during the Pullout test, and the maximum Pullout Resistance was reached within a relatively small Pullout displacement. For extensible reinforcements, such as Tensar geogrids, the degree of Resistance mobilization along the reinforcement varies, and the Pullout-Resistance achieved in the tests was controlled by the stiffness of the reinforcement. For steel grids, the friction Resistance from the longitudinal member contributed only to about 10% of the total Pullout Resistance of the grids. The Pullout of the bamboo and Tensar geogrids without transverse members yields 80–90% of the Pullout Resistance of the corresponding grids with transverse members, attributed to the nodes or ribs on longitudinal members. The bond coefficient as calculated for steel and bamboo grids demonstrated that the steel grids yielded a higher bond coefficient than that of the bamboo grids with the same grid size. However, for a polymer geogrid, the bond coefficient cannot be calculated from a Pullout test because of the complicated Pullout-Resistance-mobilization mechanism along the reinforcement. The large-scale direct-shear-test results showed that, for the soil/grid-reinforcement interfaces, shear Resistance can exceed the direct-shear Resistance of the soil itself owing to the influence of the apertures on the grids. Finally, for compacted weathered clay, the strength parameters obtained from large-scale direct-shear tests were found to be substantially smaller than the results of triaxial UU tests. This may be because the failure plane in the large-scale direct-shear test was formed progressively, and the peak soil strength along the predetermined shear plane may not have been mobilized simultaneously.
Dennis T. Bergado - One of the best experts on this subject based on the ideXlab platform.
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Analytical model of interaction between hexagonal wire mesh and silty sand backfill
Canadian Geotechnical Journal, 2001Co-Authors: Dennis T. Bergado, P Voottipruex, A SrikongsriAbstract:The interaction behavior between hexagonal wire mesh and silty sand backfill can be evaluated from Pullout tests. The Pullout Resistance of the hexagonal wire mesh reinforcement consists of two components, namely friction Resistance and passive bearing Resistance. The friction Resistance relative displacement relationship of a hexagonal wire mesh can be simulated by a linear elastic perfectly plastic model. The passive bearing Resistance of an individual bearing member can be modelled by a hyperbolic function. The friction Resistances for galvanized and PVC-coated hexagonal wire mesh were 25 and 21%, respectively, of the total Pullout Resistance. A new analytical model for predicting the Pullout Resistance of hexagonal wire mesh reinforcement has been proposed. The proposed solution can estimate the maximum Pullout force at different reinforcement levels from observed horizontal movement of a hexagonal wire mesh reinforcement.Key words: hexagonal wire mesh, necking phenomena, bearing Resistance, analy...
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prediction of Pullout Resistance and Pullout force displacement relationship for inextensible grid reinforcements
Soils and Foundations, 1996Co-Authors: Dennis T. Bergado, Jinchun Chai, Norihiko MiuraAbstract:A new analytical method is proposed for determining the inextensible grid reinforcement Pullout Resistance and Pullout force/Pullout displacement curve by using basic backfill soil and grid reinforcement properties. The Pullout skin friction Resistance/Pullout displacement relationship is simulated by linear elastic-perfectly plastic model. A hyperbolic model has been proposed to represent the Pullout bearing Resistance/Pullout displacement relationship in which the maximum bearing Resistance of a single bearing member is determined using a new bearing capacity equation proposed in this paper. The influences of the grid bearing member spacing ratio, S/D, the bearing member deflection rigidity, and the Pullout softening behavior on the mobilization of Pullout bearing Resistance are explicitly included in the proposed model. Good agreement has been obtained between calculated values and laboratory test results.
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Pullout force displacement relationship of extensible grid reinforcements
Geotextiles and Geomembranes, 1994Co-Authors: Dennis T. Bergado, Jinchun ChaiAbstract:Abstract A model for predicting the Pullout Resistance of polymer-grid reinforcement has been proposed. The influence of bearing member rigidity and spacing ratio (S/D) are explicitly expressed in the hyperbolic model. A new bearing capacity equation is incorporated for calculating the maximum Pullout force. The displacement along the reinforcement is calculated by using the proposed Pullout bearing Resistance model together with the elongation of the grid longitudinal member. The validity of the method is confirmed by good agreement between calculated values and actual test data. The analytically determined effective reinforcement embedment lengths (i.e. the length of the reinforcement in tension) and Pullout displacement to mobilize the desired Pullout Resistance of polymeric grids under different backfill conditions and under different applied normal pressures, provide useful information for the design of reinforced earth structures against Pullout failure.
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interaction between cohesive frictional soil and various grid reinforcements
Geotextiles and Geomembranes, 1993Co-Authors: Dennis T. Bergado, Jinchun Chai, Marolo C. Alfaro, H O Abiera, A. S. BalasubramaniamAbstract:Abstract A total of 52 large-scale laboratory Pullout and 24 large-scale direct-shear tests were conducted to investigate the interaction behavior between the different reinforcements and cohesive-frictional soil. The reinforcements used were steel grids, bamboo grids, and polymer geogrids. The backfill material used was locally available weathered Bangkok clay. The test results show that the inextensible reinforcements, such as steel grids, move approximately as a rigid body during the Pullout test, and the maximum Pullout Resistance was reached within a relatively small Pullout displacement. For extensible reinforcements, such as Tensar geogrids, the degree of Resistance mobilization along the reinforcement varies, and the Pullout-Resistance achieved in the tests was controlled by the stiffness of the reinforcement. For steel grids, the friction Resistance from the longitudinal member contributed only to about 10% of the total Pullout Resistance of the grids. The Pullout of the bamboo and Tensar geogrids without transverse members yields 80–90% of the Pullout Resistance of the corresponding grids with transverse members, attributed to the nodes or ribs on longitudinal members. The bond coefficient as calculated for steel and bamboo grids demonstrated that the steel grids yielded a higher bond coefficient than that of the bamboo grids with the same grid size. However, for a polymer geogrid, the bond coefficient cannot be calculated from a Pullout test because of the complicated Pullout-Resistance-mobilization mechanism along the reinforcement. The large-scale direct-shear-test results showed that, for the soil/grid-reinforcement interfaces, shear Resistance can exceed the direct-shear Resistance of the soil itself owing to the influence of the apertures on the grids. Finally, for compacted weathered clay, the strength parameters obtained from large-scale direct-shear tests were found to be substantially smaller than the results of triaxial UU tests. This may be because the failure plane in the large-scale direct-shear test was formed progressively, and the peak soil strength along the predetermined shear plane may not have been mobilized simultaneously.
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Pullout Resistance of Steel Geogrids with Weathered Clay as Backfill Material
Geotechnical Testing Journal, 1992Co-Authors: Dennis T. Bergado, H. C. Hardiyatimo, C. B. Cisneros, Chai Jin Chun, A. S. Balasubramaniam, Marolo C. Alfaro, Loren R. AndersonAbstract:Laboratory Pullout tests were conducted on various reinforcement sizes, mesh geometry, and compaction conditions of the backfill material. Field Pullout tests were also conducted to investigate the Pullout Resistance of reinforcements embedded at representative overburden, field moisture, and density conditions. The soil-reinforcement interaction indicated the dominant role of passive or bearing Resistance contributed by the transferse members to the total Pullout Resistance. The frictional Resistance of the longitudinal members was found to contribute only about 5 to 15 percent of the total Pullout Resistance. It was observed that the reinforcement moved nearly as a rigid body and that the Pullout Resistance along the reinforcement is uniformly mobilized. Comparison of the predicted Pullout bearing Resistance with the observed data indicated that the prediction based on the bearing failure model formed the upper boundary while the prediction associated with the punching failure model provided the lower boundary. An empirical equation was proposed to predict the bearing Resistance of the transverse members with reasonable accuracy.
Jaejin Kim - One of the best experts on this subject based on the ideXlab platform.
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benefits of curvilinear straight steel fibers on the rate dependent Pullout Resistance of ultra high performance concrete
Cement & Concrete Composites, 2021Co-Authors: Jaejin Kim, Doo-yeol Yoo, Nemkumar BanthiaAbstract:Abstract This study examines the effectiveness of using curvilinear steel fibers to enhance the Pullout Resistance of conventional straight steel (S-) fiber for an ultra-high-performance concrete (UHPC) matrix. Because structures composed of UHPC are subjected to various loading conditions and have random fiber orientations, three different loading rates ranging from static (0.018 mm/s) to impact (up to 906.2 mm/s) and two inclination angles of 0° and 45° were adopted. To fabricate the novel curvilinear steel fibers, five different curvatures were also utilized. Test results indicated that the curvilinear fibers provided a clearly higher Pullout Resistance than that of the conventional S-fiber, and the effectiveness increased with the curvature, regardless of the inclination angle and loading rate. Higher bond strengths were obtained for the S- and curvilinear fibers when they were inclined at 45° rather than aligned. In general, the bond strengths and Pullout energies of the S- and curvilinear fibers in UHPC were improved by increasing the loading rate, and a higher loading rate sensitivity on the bond strengths was observed in aligned fibers than in inclined fibers. The matrix spalling phenomenon was only observed in an inclined state and became more significant with an increasing curvature and loading rate. The developed curvilinear fibers provided noticeably smaller matrix spalling areas than the conventional deformed steel fibers under both static and impact conditions, owing to the mitigated stress concentration. Thus, these may be worthy of attention as a novel reinforcing fiber type for UHPC composites.
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effects of fiber shape and distance on the Pullout behavior of steel fibers embedded in ultra high performance concrete
Cement & Concrete Composites, 2019Co-Authors: Jaejin Kim, Doo-yeol YooAbstract:Abstract This study examines the implications of the fiber type and the distance between fibers on the Pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, i.e., straight, hooked, and twisted, and four different distances between fibers, corresponding to fiber volume fractions of 1%, 2%, and 7% and a fiber bundle, were considered. To evaluate the effect of the distance between fibers, four individual fibers were included in a single dog-bone specimen, and a single fiber specimen was also fabricated and tested as a control specimen. Test results indicate that the twisted steel fiber exhibited the greatest Pullout Resistance, followed by the hooked and straight steel fibers. Approximately 30% lower bond strengths were obtained for the specimens with multiple fibers as compared to those with a single fiber, regardless of the fiber type and distance between fibers. The average bond strengths of the hooked and twisted steel fibers were improved by decreasing the distance between fibers up to 1 mm, corresponding to a volume fraction of 7%, while the bundled fiber specimens provided the poorest Pullout Resistance in terms of bond strength and energy absorption capacity for all types of fibers. The reduction rate of Pullout Resistance was the most significant for the straight fiber, relative to the hooked and twisted fibers. Minor matrix damage was obtained for the straight fiber specimen, and its Pullout performance was not influenced by the surrounding fibers. In contrast, severe matrix damage was observed for the hooked and twisted fibers, and they were overlapped, causing a larger spalling area with a closer fiber distance.
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effect of calcium sulfoaluminate based expansive agent on rate dependent Pullout behavior of straight steel fiber embedded in uhpc
Cement and Concrete Research, 2019Co-Authors: Doo-yeol Yoo, Booki Chun, Jaejin KimAbstract:Abstract To use a calcium sulfoaluminate (CSA) expansive agent (EA), beneficial for volume stability of concrete, its effect on the Pullout behavior of straight steel fiber in ultra-high-performance concrete (UHPC) needs to be analyzed under various loads, from static to impact (0.018 to 1244 mm/s). Adding CSA EA enhanced the static Pullout Resistance of only inclined fibers, whereas the average bond strengths and Pullout energies of both the aligned and inclined fibers improved by it under impact loading. Thus, adding the CSA EA is more effective in enhancing the dynamic Pullout Resistance. The fiber Pullout Resistance improved at a higher loading rate, and 45°-inclination of the fibers improved the dynamic Pullout Resistance. A higher rate sensitivity on the bond strength was observed with the CSA EA addition and fiber's inclination, and the Pullout energy was more dependent on loading rate than the average bond strength as fiber rupture was prevented.
