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RISS 인기검색어
- 검색키워드
- 저자 : Nemkumar Banthia (검색결과 13 건)
- 무료
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Numerical simulation on structural behavior of UHPFRC beams with steel and GFRP bars
류두열,Nemkumar Banthia 사단법인 한국계산역학회 2015 Computers and Concrete, An International Journal Vol.16 No.5
This study simulates the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams reinforced with steel and glass fiber-reinforced polymer (GFRP) rebars. For this, micromechanics-based modeling was first carried out on the basis of single fiber pullout models considering inclination angle. Two different tension-softening curves (TSCs) with the assumptions of 2-dimensional (2-D) and 3-dimensional (3-D) random fiber orientations were obtained from the micromechanics-based modeling, and linear elastic compressive and tensile models before the occurrence of cracks were obtained from the mechanical tests and rule of mixture. Finite element analysis incorporating smeared crack model was used due to the multiple cracking behaviors of structural UHPFRC beams, and the characteristic length of two times the element width (or two times the average crack spacing at the peak load) was suggested as a result of parametric study. Analytical results showed that the assumption of 2-D random fiber orientation is appropriate to a non-reinforced UHPFRC beam, whereas the assumption of 3-D random fiber orientation is suitable for UHPFRC beams reinforced with steel and GFRP rebars due to disorder of fiber alignment from the internal reinforcements. The micromechanics-based finite element analysis also well predicted the serviceability deflections of UHPFRC beams with GFRP rebars and hybrid reinforcements.
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Geometrical and Boundary Condition Effects on Restrained Shrinkage Behavior of UHPFRC Slabs
Doo-Yeol Yoo,Nemkumar Banthia,Young-Soo Yoon 대한토목학회 2018 KSCE JOURNAL OF CIVIL ENGINEERING Vol.22 No.1
Six large Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) slabs were fabricated and tested to investigate the restrained shrinkage and cracking behaviors. The use of expanded polystyrene and Teflon sheets with two different slab thicknesses was considered to improve the shrinkage crack resistance. Free shrinkage was simultaneously measured to evaluate the degree of restraint according to the above test parameters. The test results showed that free shrinkage strains of -689 με to -723 με were obtained after 9 days, and prismatic specimens with a higher exposed surface area-to-volume ratio (S/V) had slightly higher free shrinkage strains than those with a lower S/V. Increasing the concrete slab thickness and using expanded polystyrene and Teflon sheets were effective at reducing the degree of restraint and improving the shrinkage crack resistance of the UHPFRC slabs. Among the various specimens, the slabs with the expanded polystyrene exhibited the lowest degree of restraint by 0.45 after 9 days.
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Yoo, Doo-Yeol,Banthia, Nemkumar Elsevier 2017 Construction & building materials Vol.149 No.-
<P><B>Abstract</B></P> <P>This study comprehensively investigates impact and blast resistances of ultra-high-performance fiber-reinforced concrete (UHPFRC) by considering various influential factors. At a material level, rate-dependent fiber pullout behavior, dynamic compressive behavior, and impact tensile and flexural behaviors were examined in detail, and the benefits of using UHPFRC to improve the impact resistance of ordinary concrete were discussed. It was obvious that (1) UHPFRC is able to dissipate much higher energy by impact than ordinary concrete with and without fibers, (2) the use of long straight steel fiber is effective in improving the impact resistance of UHPFRC compared to that of deformed steel fibers at high volume fractions, (3) fiber orientation significantly influences the impact resistance of UHPFRC: when more fibers are aligned in the tensile load direction, better impact resistance is achieved, and (4) size effect on the dynamic increase factor versus strain-rate relationship is insignificant. Impact and blast resistances of UHPFRC beams, slabs, columns, and composite structures were also examined at structural level, and several useful conclusions were drawn. (1) UHPFRC is favored for impact- or blast-resistant structures as compared with ordinary concrete due to its much better impact and blast resistance at identical dimensions, reinforcement configuration, and load magnitude, (2) the use of high-strength steel rebar provides the better blast resistance of UHPFRC beams or slabs as compared with that of normal-strength steel rebar, and (3) seismic detailing applied in UHPFRC columns leads to better blast resistance than is seen for columns without seismic detailing. Further research is suggested to address the remaining complicated problems or conflicts and to inspire proper design of structural UHPFRC members in an attempt to increase the use of UHPFRC.</P> <P><B>Highlights</B></P> <P> <UL> <LI> UHPFRC dissipates much higher energy by impact than ordinary FRC. </LI> <LI> Fiber orientation significantly influences the impact resistance of UHPFRC. </LI> <LI> Long straight steel fiber is effective in improving the impact resistance of UHPFRC. </LI> <LI> Using high-strength steel in UHPFRC is efficient in enhancing the blast resistance. </LI> <LI> Adopting seismic detailing in UHPFRC columns improves the blast resistance. </LI> </UL> </P>
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Impact Resistance of Reinforced Ultra-High-Performance Concrete Beams with Different Steel Fibers
Yoo, Doo-Yeol,Banthia, Nemkumar,Yoon, Young-Soo American Concrete Institute 2017 ACI structural journal Vol.114 No.1
<P>Ten large reinforced ultra-high-performance concrete (UHPC) beams were fabricated and tested under drop-weight impacts. The test parameters included the potential energy, fiber volume content, and steel fiber type and length. The important parameters obtained from an experimental program were summarized to provide a fundamental data set belonging to a research area that is limited within the literature. The test results showed that the addition of 2% (by volume) steel fibers was effective in decreasing the maximum and residual deflections by impact, improving residual capacities after impact damage, redistributing the tensile stress associated with microcracking, and preventing local failure at the contact surface. The use of long smooth steel fibers also resulted in the improvement of both the impact and residual capacities-that is, a decrease in the maximum and residual deflections by impact and an increase in the residual moment capacity and deflection capacity at the ultimate state. In contrast, the fiber content and type had negligible influences on the ratios of the moment capacities under impact and quasi-static loadings. Finally, a step-by-step procedure to assess the residual capacities after impact damage was proposed based on the quasi-static flexural response and the maximum deflection by impact.</P>
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Impact resistance of fiber-reinforced concrete – A review
Yoo, Doo-Yeol,Banthia, Nemkumar Elsevier 2019 CEMENT AND CONCRETE COMPOSITES Vol.104 No.-
<P><B>Abstract</B></P> <P>This paper reviews the state of the art of the impact resistance of ordinary fiber-reinforced concretes (FRCs) containing various fibers. First, various types of impact test methods that are current available are addressed as well as some concerns about them based on extensive literature reviews and our perspective. Then, common properties of FRCs under impact loading regardless of fiber type, such as the reasons for their enhanced strength under impact, the effect of size on impact resistance, and several factors (i.e., matrix strength, loading conditions, and fiber existence) that influence strain-rate sensitivity, are discussed. Furthermore, the comprehensive impact resistances of FRCs with various fibers (i.e., steel, polymeric, carbon, basalt, natural, and hybrid fibers) are investigated under different loading conditions. After summarizing the impact properties of FRCs with various fibers, the comparative impact resistance of FRCs according to the fiber type is evaluated to determine which type gives the best improvement of impact resistance. Lastly, the effect of supplementary cementitious materials (SCMs), i.e., fly ash, silica fume, and slag, on the impact resistance of FRCs is examined, and some combinations of SCM and fiber types that lead to enhanced impact resistance are suggested.</P>
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Size-dependent impact resistance of ultra-high-performance fiber-reinforced concrete beams
Yoo, Doo-Yeol,Banthia, Nemkumar Elsevier 2017 Construction and Building Materials Vol.142 No.-
<P><B>Abstract</B></P> <P>This study examines the rate dependent flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams with three different sizes. Two different loading rates (static and impact), fiber aspect ratios (l<SUB>f</SUB>/d<SUB>f</SUB> of 65 and 100), and fiber types (straight and twisted) were considered. Test results indicated that the static flexural performance, including the flexural strength and toughness, were improved by increasing the fiber aspect ratio or through the use of twisted steel fibers. The static flexural strength clearly decreased with an increase in specimen size due to a decrease in the number of fibers at the crack surface. The use of straight steel fibers with a higher aspect ratio of 100 provided the best impact resistance in terms of the highest post-cracking flexural strengths and the largest normalized energy dissipation rates, compared to those of twisted steel fibers and straight steel fibers with a reduced aspect ratio of 65. Thus, the use the straight steel fibers with high aspect ratios was recommended to improve the impact resistance of UHPFRC. Dynamic increase factor (DIF) on the flexural strength of UHPFRC beams was properly investigated with strain-rate, regardless of specimen size. In addition, there were no effects with regard to the fiber aspect ratio and type on the relationship between the DIF of the first-cracking flexural strength and the stress- (or strain-) rate.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Flexural strength of UHPFRC decreases with increasing specimen size. </LI> <LI> Using twisted or long straight fibers improves static flexural performance of UHPFRC with short straight fibers. </LI> <LI> Deflection-hardening response of UHPFRC under impact is successfully captured. </LI> <LI> Long straight steel fiber is most effective in improving impact resistance of UHPFRC. </LI> <LI> Strain-rate is proper to analyze DIF on <I>f</I> <SUB>LOP</SUB> of UHPFRC excluding size effect. </LI> </UL> </P>
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Yoo, Doo-Yeol,Banthia, Nemkumar,Yoon, Young-Soo National Research Council of Canada 2017 Canadian journal of civil engineering Vol. No.
<P> Flexural behaviors of reinforced ultra-high-performance fiber-reinforced concrete (UHPFRC) beams were experimentally and numerically investigated in terms of reinforcement ratio. To do this, four UHPFRC beams with different reinforcement ratios (0%-1.71%) were fabricated and tested. Since we focused on the placement technique of the steel reinforcing bars, only a small number of reinforced UHPFRC beams were deliberately considered. Test results indicated that with an increase in the reinforcement ratio, post-cracking stiffness and load carrying capacity were increased, whereas first cracking load was decreased. The cracking behavior was characterized by numerous vertical micro-cracks up to near the peak, followed by crack localization with a gradual decrease in load carrying capacity. The number of cracks and average crack spacing were marginally influenced by the reinforcement ratio. Sectional analysis incorporating a linear compressive model and tension-softening curves obtained from inverse analyses and direct tensile test were performed and verified through comparison with the experimental moment-curvature responses. </P>
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