Twisted Steel Fibers Research Articles

Size effect in ultra-high-performance concrete beams

In order to investigate the size effect on the flexural performance of ultra-high-performance concrete (UHPC), numbers of UHPC beams with three different sizes, including two with smooth steel fibers with aspect ratios of 65 and 100 and one with twisted steel fibers with an aspect ratio of 100, were fabricated and tested. The beams made of UHPC matrix without fiber were also considered for evaluating the effect of fibers on the size effect. In addition, image analyses at crack surfaces were performed to investigate the size effect on the fiber distribution characteristics and their implication on the size-dependent flexural performance. Test results indicated that the flexural performance of UHPC noticeably decreased with an increase in the specimen size, caused by a decrease in the number of fibers and poor fiber orientation. The use of longer steel fibers with a higher aspect ratio was effective in reducing the sensitivity to the size effect of UHPC beams compared with shorter steel fibers. Finally, by applying similar fiber distribution characteristics for all test specimens of different sizes, an insignificant size effect on the flexural strength was obtained for UHPC with 2% by volume of steel fibers.

Structural performance of ultra-high-performance concrete beams with different steel fibers

In this study, ten large ultra-high-performance concrete (UHPC) beams reinforced with steel rebars were fabricated and tested. The experimental parameters included reinforcement ratio and steel fiber type. Two different reinforcement ratios (ρ=0.94% and 1.50%) and steel fiber types (smooth and twisted steel fibers) were adopted. In addition, three different fiber lengths (Lf=13, 19.5, and 30mm) for the smooth steel fibers and one fiber length (Lf=30mm) for the twisted steel fiber were considered. For a control specimen, a UHPC matrix without fiber was also considered. Test results indicated that the addition of steel fibers significantly improved the load carrying capacity, post-cracking stiffness, and cracking response, but it decreased the ductility. Specifically, with the inclusion of 2% by volume of steel fibers, approximately 27–54% higher load carrying capacity and 13–73% lower ductility were obtained. In addition, an increase in the length of smooth steel fibers and the use of twisted steel fibers led to the improvements of post-peak response and ductility, whereas no noticeable difference in the load carrying capacity, post-cracking stiffness, and cracking response were obtained according to the fiber length and type. Sectional analysis incorporating the suggested material models was also performed based on AFGC/SETRA recommendations, and the ratios of flexural capacities obtained from experiments and numerical analyses ranged from 0.91 to 1.19.

Effect of fiber volume content on electromechanical behavior of strain-hardening steel-fiber-reinforced cementitious composites

This research investigated the effect of fiber volume content on the electromechanical behavior of strain-hardening steel-fiber-reinforced cementitious composites under direct tension. There is strong correlation between the change of electrical resistivity and the tensile response of strain-hardening steel-fiber-reinforced cementitious composites: the electrical resistivity of strain-hardening steel-fiber-reinforced cementitious composites clearly decreased during strain hardening as the tensile strain of them increased. The electrical conductivity, tensile resistance, and damage-sensing capacity of strain-hardening steel-fiber-reinforced cementitious composites were generally increased as the volume content of twisted steel fibers added in a mortar matrix increased from 0.0 to 2.0%. The strain-hardening steel-fiber-reinforced cementitious composites with fiber content more than 1% by volume produced high damage-sensing capacity with high nominal gauge factor: absolute value over 150. Besides, the addition of carbon black or ground granulated blast furnace slag in mortar matrices significantly reduced the electrical resistivity but slightly enhanced the damage-sensing capability of strain-hardening steel-fiber-reinforced cementitious composites.

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.

Flexural Behavior Characteristics of UHPC According to Twisted Steel Fiber

This study examines the effects of twisted steel fiber obtained by changing the shape of the fiber in order to improve the toughness of the conventional straight steel fiber. Three types of twisted fiber with different lengths (l) and diameters (d) are considered while maintaining the aspect ratio (l/d) to 100. Flexural tensile test is performed on the K-UHPC paste mixed with 1%, 1.5% and 2% of fiber relative to the volume of the matrix to analyze the flexural tensile performance, deflection capacity and energy absorption performance. The results show that the use of twisted fiber increases significantly the flexural toughness of K-UHPC compared to the use of the traditional straight fiber. Especially, the adoption of a quantity of twisted fiber reduced by 0.5% is seen to secure identical performances. The reduction of this quantity by 1% develops slightly lower flexural strength but provides remarkable deflection capacity. Accordingly, since the adoption of twisted fiber enables to reduce the mix ratio of fiber by 0.5 to 1% according to the intended purpose, such fiber is expected to allow further reduction of the fabrication cost of K-UHPC.

Evaluation of the Direct Tensile Behavioral Characteristics of UHPC Using Twisted Steel Fibers

This paper investigates the improvement of the toughness of UHPC through the use of conventional straight steel fibers and twisted fibers. The conventional straight steel fiber has length of 13 mm and diameter of 0.2 mm. Besides, twisted fibers with diameters of 0.2 mm, 0.22 mm and 0.3 mm, respective lengths of 20 mm, 22 mm and 30 mm and made of high elastic steel fibers with aspect ratio (l/d) of 100 are used at volume fractions of 1%, 1.5% and 2%. The results of the examination of the direct tensile behavior reveal the occurrence of strain hardening after early cracking. K-UHPC using twisted fiber exhibits significantly improved mechanical characteristics compared to UHPC using conventional straight fiber. Equivalent or superior performance is developed when small proportion of about 0.5% of steel fiber is adopted and 90% of the performance is developed when small proportion of approximately 1.0% of fiber is admixed. Consequently, the use of twisted fiber in K-UHPC opens the possibility to achieve economically efficient fabrication by enabling significant reduction of the amount of fiber.

Influence of sand to coarse aggregate ratio on the interfacial bond strength of steel fibers in concrete for nuclear power plant

The interfacial bond strength of three high strength steel fibers (smooth, hooked, and twisted fiber) in concrete of nuclear power plants was investigated to develop fiber reinforced concrete for containment building. Sand to aggregate ratio (S/a) was adjusted to compensate reduction in the workability due to adding fibers; the influence of S/a ratio on the interfacial bond strength was investigated. As the S/a ratio increased from 0.444 to 0.615, the bond strength of twisted steel fiber was significantly improved while smooth and hooked steel fiber showed no clear difference. The different sensitivity according to the S/a ratio results from the different pullout mechanism: twisted steel fiber generates more mechanical interaction during fiber pullout at the interface between fiber and matrix than smooth and hooked fibers. The microscopic observation by scanning electron microscope back-scattered electrons images discovered lower porosity at the interfacial transition zone between fiber and concrete with higher S/a ratio.

비틀림 강섬유의 비틀림 횟수가 고성능 섬유보강 시멘트 복합재료의 인장거동에 미치는 영향

Dept. of Civil and Environmental Engineering, Sejong University, Seoul 143-747, KoreaABSTRACT This research investigated the influence of the number of twist on single fiber pullout behavior of Twisted steel(T-) fiber and tensile behavior of high performance cementitious composites reinforced with the (T-) fibers (HPFRCC). Micro-mechanical pullout model for T- fibers has been applied to analytically investigate the influence of various fiber parameters includ-ing the number of twist on single fiber pullout behavior; and, to optimize the number of twist to generate larger pullout energyduring fiber pullout without fiber breakage. In addition, an experimental program including single fiber pullout and tensile testshas been performed to investigate the influence of twist ratio experimentally. Two types of T- fiber with different twisted ratios,T(L)- fiber (6ribs/30 mm) and T(H)- fiber (18ribs/30 mm), were tested. T(L)- fiber produced higher equivalent bond strength (larg erpullout energy) although T(H)- fiber produced higher pullout stress during pullout since T(H)- fiber showed fiber breakage duringpullout. Tensile test results confirmed that T(L)- fiber in high strength mortar generates better tensile performance of HPFRCC,e.g., load carrying capacity, strain capacity and multiple micro-cracking behavior.Keywords : twisted fiber, high performance fiber reinforced cementitious composites, pullout energy, number of twist

High Performance Fiber Reinforced Cement Composites with Innovative Slip Hardending Twisted Steel Fibers

This paper provides a brief summary of the performance of an innovative slip hardening twisted steel fiber in comparison with other fibers including straight steel smooth fiber, high strength steel hooked fiber, SPECTRA (high molecular weight polyethylene) fiber and PVA fiber. First the pull-out of a single fiber is compared under static loading conditions, and slip ratesensitivity is evaluated. The unique large slip capacity of T-fiber during pullout is based on its untwisting fiber pullout mechanism, which leads to high equivalent bond strength and composites with high ductility. Due to this large slip capacity a smaller amount of T-fibers is needed to obtain strain hardening tensile behavior of fiber reinforced cementitious composites. Second, the performance of different composites using T-fibers and other fibers subjected to tensile and flexural loadings is described and compared. Third, strain rate effect on the behavior of composites reinforced with different types and amounts of fibers is presented to clarify the potential application of HPFRCC for seismic, impact and blast loadings.

Tensile Response and Fracture Process of High-Performance Hybrid Fiber-Reinforced Cement Composites

Five twisted steel fibers (steel cord, SC) are used to improve the post-cracking behavior of cement composites. In addition to these steel cord macrofibers, microfibers are used to enhance the pre- and post-cracking strength and behavior. The three specific hybrid fiber blends pursued in this investigation are SC and polyethylene (PE), SC and polypropylene (PP) as well as SC and polyvinyl alcohol (PVA) fibers. The research results show that SC macrofibers with high ultimate strain, strength and bond stress can bridge the macrocracks in the matrix. Therefore, cement-based composites reinforced with SC fibers exhibit a pseudo-strain hardening in tension as a result of the formation of multiple cracks. The microfibers improved pre-peak mechanical performance, strength, and fracture process by delaying the formation of a macrocrack. The tensile strength, energy consumption capacity and ductility of SC macrofibers and high elastic modulus microfiber PE reinforced cement-based composites are tremendously improved compared to SC macrofibers with lower elastic modulus microfibers PVA and PP.