Smooth Steel Fibers Research Articles

Electromechanical Response of High-Performance Fiber-Reinforced Cementitious Composites Containing Milled Glass Fibers under Tension.

The self-damage sensing capacity of high-performance fiber-reinforced cementitious composites (HPFRCCs) that blended long- (1 vol %) and medium-length (1 vol %) smooth steel fibers was considerably improved by adding milled glass fibers (MGFs) with a low electrical conductivity to a mortar matrix. The addition of MGFs (5 wt %) significantly increased the electrical resistivity of the mortar matrix from 45.9 to 110.3 kΩ·cm (140%) and consequently improved the self-damage sensing capacity (i.e., the reduction in the electrical resistivity during the tensile strain-hardening response) from 17.27 to 25.56 kΩ·cm (48%). Furthermore, the addition of MGFs improved the equivalent bond strength of the steel fibers on the basis of the higher pullout energy owing to the accumulated cementitious material particles attached to the surfaces of steel fibers.

Modelling UHPFRC tension behavior under high strain rates

The advantages of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) under static loads suggest it is a promissory material to withstand dynamic and especially extreme loads. However, the available results concerning dynamic behaviour of UHPFRC under high strain rates are still rather limited and there are some aspects that require further analysis and the development of numerical tools. A numerical model for UHPFRC is presented and applied to the simulation of high strain tension tests in this paper. The tension tests were performed in a Modified Hopkinson Bar with different strain rates and they include UHPFRC using different contents and orientations of smooth straight steel fibers. The numerical model is based on the modified mixture theory and takes into account the behaviour of the matrix and the fibers and the fiber/matrix sliding using a meso-mechanic pull-out model. The model was implemented in a non-linear dynamic finite element explicit code that constitutes a useful numerical tool for the design and analysis of structures made of this material. High strain rate tension tests were numerically simulated. The comparison of numerical and experimental results allows calibrating the material properties, validating the model and analysing the strain rate dependence of the composite and the contribution of each of the component materials and the pull-out mechanism. Tension dynamic amplification of UHPFRC is mainly due to the Ultra High Performance Concrete (UHPC) matrix and the pull-out mechanism. The dynamic amplification of the fibers pull-out mechanism is lower than that of the matrix and increases with fiber inclination.

Matrix-strength-dependent strain-rate sensitivity of strain-hardening fiber-reinforced cementitious composites under tensile impact

The effects of matrix strength on the rate-sensitive tensile responses of strain-hardening fiber-reinforced cementitious composites (SH-FRCCs) at high strain rates were investigated using an improved strain energy frame impact machine (I-SEFIM) overcoming the limitations of previous strain energy frame impact machine (SEFIM). The strengths of matrix were 56, 81, and 180MPa, while 1% long (30mm) hooked and 1% short (13mm) smooth steel fibers were blended in all matrices. Dynamic increase factors (DIFs) for the tensile parameters of the SH-FRCCs, as the strain rate increased, were clearly dependent on the matrix strength although they generally increased: a lower strength matrix produced higher DIFs for both the strain capacity and peak toughness, whereas a higher strength matrix generated higher DIFs for the post-cracking tensile strength. Besides, DIF equations were newly proposed for predicting the post-cracking strength.

Performance evaluation of corrugated steel fiber in cementitious matrix

Increasing the adhesion efficiency between the fiber and the concrete, in fiber reinforced concrete members, might be obtained by enhancing the properties of the concrete mixture using several parameters such as cement weight, water to cement ratio, grading curve of aggregates and admixtures, or by apply some changes on the fiber parameters such as the aspect ratio (l/d) or the outer roughness or the embedded length and the diameter value. Laboratory experiments lead to the fact that the use of smooth and straight steel fiber, in fiber reinforced concrete, produces weak bond strength between the fiber and the concrete. Therefore, to obtain better bond strength it is necessary to increase the total length of the fiber, which may cause unacceptable workability. Accordingly, it could be developed a new form of fiber has larger supporting region using the corrugated fiber shape. In this paper, the performance of this corrugated form will be monitored through new shear lag model to extract the traction separation relations between the corrugated fiber and the concrete matrix, as well as an experimental study using different values of embedded length of the fiber, in addition to several computer simulations which will be created using a finite element model and applying same dimensions and forces which are used in the laboratory experimental procedure. Through the results of nonlinear analysis of the computer simulations, the distribution of stresses in different directions for each of the concrete and the fiber can be shown, then the evolution of the bond strength between the fiber and the concrete can be monitored as an actual result of applying the corrugated shape.

Fresh and hardened properties of steel fiber-reinforced grouts containing ground granulated blast-furnace slag

Fresh and hardened properties of steel fiber-reinforced grout (SFRG) containing ground granulated blast-furnace slag (GGBS) were investigated for applications to prestressed concrete substructures of offshore wind turbines. The addition of short smooth steel fibers (6mm in length, 0.2mm in diameter) generally decreased the flow, bleeding, and time of setting of SFRG, whereas the addition of GGBS for the replacement of 40% of the cement by weight ratio clearly increased its flow and the time of setting. The effect of adding GGBS on bleeding was different according to the type of cement. Despite these outcomes, it was observed that the addition of GGBS notably enhanced the chloride penetration resistance of SFRGs, while the addition of steel fibers significantly increased their flexural strength.

Monitoring of impact of hooked ends on mechanical behavior of steel fiber in concrete

The efficiency of hardened composite of Steel Fiber Reinforced Concrete (SFRC) is mainly related to the ability of its components to work together homogeneously. This homogeneously work of SFRC components might be obtained through sufficient bond between the fiber and the concrete matrix at its contact points on the interface surfaces. Usually, if a smooth and straight steel fiber is embedded in concrete matrix and subjected to tensile force, only weak bond may obtain at the interface between the fiber and the concrete. This weak bond decreases gradually parallel with increasing the value of the applied tensile force in the pull-out test, and the fiber can’t develop its yield strength, whereas debond length increases toward the depth of the concrete along the embedded length of the fiber until failure occurs in bond strength between the fiber and the concrete, then the fiber pulls out of the concrete through frictional sliding movement. The fracture mechanism of bond strength between the fiber and the concrete might be observed through pull-out tests. To enhance the bond strength performance of the fiber without change the concrete mix properties, it is necessary to find sophisticated form for the fiber such as end hooks. Monitoring of impact of hooked ends on mechanical behavior of steel fiber in concrete is observed during this research, where various pull-out experiments of single steel fiber in two forms (straight and hooked ends) are set using different values of embedded fiber length in concrete matrix. As well as computer simulations of single steel fiber with hooked ends embedded in concrete matrix are created using finite element model to monitor the development of stresses in different directions. Nonlinear results with contour maps and curves of different types of stresses are also obtained from the computer simulations, and numerical evaluation of the impact of enhancing the steel fiber shape has been done through this research.

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 shrinkage-reducing admixture on biaxial flexural behavior of ultra-high-performance fiber-reinforced concrete

This study aims to investigate the effect of shrinkage-reducing admixture (SRA) on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC). Three different SRA to cement weight ratios (0%, 1%, and 2%) were considered using the UHPFRC including 2% of smooth steel fibers by volume. The specimen without SRA exhibited the best performance in almost all aspects of the mechanical behaviors in compression, fiber pullout, and biaxial flexure including load carrying capacity, strain capacity, and energy absorption capacity (pullout energy and toughness). The mechanical performances deteriorated with the increase in the amount of SRA up to 2%. Finally, a suggestion was made to define the first cracking point of deflection-hardening UHPFRC under biaxial flexure stress.

High rate response of ultra-high-performance fiber-reinforced concretes under direct tension

The tensile response of ultra-high-performance fiber-reinforced concretes (UHPFRCs) at high strain rates (5–24s−1) was investigated. Three types of steel fibers, including twisted, long and short smooth steel fibers, were added by 1.5% volume content in an ultra high performance concrete (UHPC) with a compressive strength of 180MPa. Two different cross sections, 25×25 and 25×50mm2, of tensile specimens were used to investigate the effect of the cross section area on the measured tensile response of UHPFRCs. Although all the three fibers generated strain hardening behavior even at high strain rates, long smooth fibers produced the highest tensile resistance at high rates whereas twisted fiber did at static rate. The breakages of twisted fibers were observed from the specimens tested at high strain rates unlike smooth steel fibers. The tensile behavior of UHPFRCs at high strain rates was clearly influenced by the specimen size, especially in post-cracking strength.

Comparative electromechanical damage-sensing behaviors of six strain-hardening steel fiber-reinforced cementitious composites under direct tension

This research investigates the electromechanical damage-sensing behavior of strain-hardening steel fiber-reinforced cement composites (SH-SFRCs) with six types of steel fibers (1.5% volume fraction content) within an identical mortar matrix (90MPa). The six types of steel fibers studied are long twisted (T30/0.3), long smooth (S30/0.3), long hooked (H30/0.375), medium twisted (T20/0.2), medium smooth (S19/0.2), and short smooth (S13/0.2) steel fibers. The damage-sensing behavior was evaluated by measuring the changes in the electrical resistance during direct tensile tests. The electrical resistivity of the SH-SFRCs clearly decreased as the tensile strain increased until the post-cracking point, owing to the generation of multiple micro-cracks during strain-hardening. All the SH-SFRCs investigated had nominal gauge factors ranging between 50 and 140; these values are much higher than the commercially conventional gauge factor, which involves metal and is around 2. Both T30/0.3 and T20/0.2 produced the highest gauge factor, i.e., the best damage-sensing capacity, whereas S13/0.2 produced the highest electrical conductivity.

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.

Size and geometry dependent tensile behavior of ultra-high-performance fiber-reinforced concrete

This research investigated direct tensile stress versus strain response of ultra-high-performance fiber-reinforced concrete (UHPFRC) with various sizes and geometries. The UHPFRC in this research contained 1% macro twisted and 1% micro smooth steel fibers by volume. The effects of gauge length, section area, volume and thickness of the specimens on the measured tensile response of the UHPFRC were experimentally discovered. The different sizes and geometries of specimens did not generate significant influence on the post cracking strength of UHPFRC whereas they produced clear effects on the strain capacity, energy absorption capacity and multiple cracking behavior of UHPFRC. The strain capacity, energy absorption capacity and the number of multiple micro cracks within unit length obviously decreased as the gauge length, section area and volume of UHPFRC specimens increased. In contrast, as the thickness of the specimen increased, different tendency was observed.

An experimental study on flexural strength enhancement of concrete by means of small steel fibers

Cost effective improvement of the mechanical performances of structural materials is an important goal in construction industry. To improve the flexural strength of plain concrete so as to reduce construction costs, the addition of fibers to the concrete mixture can be adopted. The addition of small steel fibers with different lengths and proportion have experimentally been analyzed in terms of concrete flexural strength enhancement. The main objectives of the present study are related to the evaluation of the influence of steel fibers design on the increase of concrete flexural characteristics and on the mode of failure. Two types of beams have been investigated. The force level, deflection and time to failure of beams have been measured. The shear crack, flexural crack and intermediate shear-flexural crack have been studied. The steel fiber content controlled crack morphology. Flexural strength and time to failure of fiber reinforce concrete could be further enhanced if, instead of smooth steel fibers, corrugated fibers were used.

Effect of Ultra-High-Performance Concrete on Pullout Behavior of High-Strength Brass-Coated Straight Steel Fibers

The objective of this research was to investigate the pullout behavior of straight high-strength steel fibers embedded in different ultra-high- performance concretes (UHPCs) with a compressive strength ranging from 190 to 240 MPa (28 to 35 ksi). Particular attention was placed on obtaining matrixes with high packing density to enhance the physicochemical bond with the embedded fiber. The parameters investigated included the use of different sand ratios, silica fume (SF) and glass powder with different mean particle sizes, different superplasticizers, and the addition of hydrophilic or hydrophobic nanosilica particles. Thus, by tailoring the matrix composition, significantly different bond stress versus slip-hardening behaviors were achieved. This is atypical for straight smooth steel fibers, which are normally characterized by a bond-slip softening behavior. Microscopical studies revealed that scratching and delaminating of the brass-coated fiber surface by fine sand and by abrading matrix particles is one reason for this phenomenon, and help explain the maximum equivalent bond strength observed of up to 20 MPa (2.9 ksi).

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.

Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing

Ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHP-FRC) were introduced in the mid 1990s. Special treatment, such as heat curing, pressure and/or extensive vibration, is often required in order to achieve compressive strengths in excess of 150 MPa (22 ksi). This study focuses on the development of UHP-FRCs without any special treatment and utilizing materials that are commercially available on the US market. Enhanced performance was accomplished by optimizing the packing density of the cementitious matrix, using very high strength steel fibers, tailoring the geometry of the fibers and optimizing the matrix-fiber interface properties. It is shown that addition of 1.5% deformed fibers by volume results in a direct tensile strength of 13 MPa, which is 60% higher than comparable UHP-FRC with smooth steel fibers, and a tensile strain at peak stress of 0.6%, which is about three times that for UHP-FRC with smooth fibers. Compressive strength up to 292 MPa (42 ksi), tensile strength up to 37 MPa (5.4 ksi) and strain at peak stress up to 1.1% were also attained 28 days after casting by using up to 8% volume fraction of high strength steel fibers and infiltrating them with the UHPC matrix.

Comparative flexural behavior of Hybrid Ultra High Performance Fiber Reinforced Concrete with different macro fibers

The flexural performance of four Hybrid (H-) Ultra High Performance Fiber Reinforced Concretes (UHPFRCs) with different macro fibers was investigated according to ASTM standards C1018-97 and C 1609/C 1609M-05. Four macro fibers were long smooth (LS-) steel fiber, two types of hooked (HA- and HB-) steel fibers, and twisted (T-) steel fibers while one type of micro fiber, short smooth (SS-) steel fiber, was blended. The enhancements in modulus of rupture, deflection capacity and energy absorption capacity were different according to the types of macro fiber as the amount of micro fiber blended increased. The order of flexural performance of H-UHPFRC according to the types of macro fiber was as follows: HB- > T- > LS- > HA-fiber. The influence of strain capacity in tension on the flexural performance of H-UHPFRC was also examined. The deflection capacity and the ratio between flexural strength and tensile strength were dependent upon the strain capacity of H-UHPFRC.

Strain-hardening UHP-FRC with low fiber contents

This research work focuses on the optimization of strength and ductility of ultra high performance fiber reinforced concretes (UHP-FRC) under direct tensile loading. An ultra high performance concrete (UHPC) with a compressive strength of 200 MPa (29 ksi) providing high bond strength between fiber and matrix was developed. In addition to the high strength smooth steel fibers, currently used for typical UHP-FRC, high strength deformed steel fibers were used in this study to enhance the mechanical bond and ductility. The study first shows that, with appropriate high strength steel fibers, a fiber volume fraction of 1% is sufficient to trigger strain hardening behavior accompanied by multiple cracking, a characteristic essential to achieve high ductility. By improving both the matrix and fiber parameters, an UHP-FRC with only 1.5% deformed steel fibers by volume resulted in an average tensile strength of 13 MPa (1.9 ksi) and a maximum post-cracking strain of 0.6%.

Effect of matrix strength on pullout behavior of steel fiber reinforced very-high strength concrete composites

This paper presents the results of single-fiber pullout tests for deformed and smooth steel fibers embedded in the newly developed very-high strength concrete (VHSC) matrixes. The pullout test program involved four types of steel fibers, eight compressive strengths of VHSC matrixes, and two normal concrete strengths. Test results have shown that pullout behavior of different steel fiber reinforced VHSC composites is influenced by the matrix strength and fiber end condition (smooth, flat end, or hooked). Results reveal that both maximum pull-out load and total pullout energy increases as matrix strength increases for all deformed fibers that did not rupture. The test results also indicated that the increase in total pullout energy is more significant than that in peak load.