Steel Fibers In Ultra-high Performance Concrete Research Articles

Effect of Steel Fibers on Tensile Properties of Ultra-High-Performance Concrete: A Review.

Ultra-high-performance concrete (UHPC) is an advanced cement-based material with excellent mechanical properties and durability. However, with the improvement of UHPC's compressive properties, its insufficient tensile properties have gradually attracted attention. This paper reviews the tensile properties of steel fibers in UHPC. The purpose is to summarize the existing research and to provide guidance for future research. The relevant papers were retrieved through three commonly used experimental methods for UHPC tensile properties (the direct tensile test, flexural test, and splitting test), and classified according to the content, length, type, and combination of the steel fibers. The results show that the direct tensile test can better reflect the true tensile strength of UHPC materials. The tensile properties of UHPC are not only related to the content, shape, length, and hybrids of the steel fibers, but also to the composition of the UHPC matrix, the orientation of the fibers, and the geometric dimensions of the specimen. The improvement of the tensile properties of the steel fiber combinations depends on the effectiveness of the synergy between the fibers. Additionally, digital image correlation (DIC) technology is mainly used for crack propagation in UHPC. The analysis of the post-crack phase of UHPC is facilitated. Theoretical models and empirical formulas for tensile properties can further deepen the understanding of UHPC tensile properties and provide suggestions for future research.

Recent development of innovative steel fibers for ultra-high-performance concrete (UHPC): A critical review

This paper presents a comprehensive review of recent advancements in innovative high-strength steel fibers for ultra-high-performance concrete (UHPC). Firstly, commercially available ordinary steel fibers for UHPC products are introduced, and the analysis of fiber aspect ratio on the mechanical properties of UHPC follows. Subsequently, the historical development of deformed steel fibers, including hooked, corrugated, twisted, half-hooked, and curvilinear steel fibers for UHPC, is reviewed, accompanied by a discussion of their impact on various properties of UHPC. In response to limitations associated with specific deformed steel fibers in UHPC due to stress concentration, recent development of surface-refined straight steel fibers is presented. The paper further highlights the development of surface-roughened steel fibers through physical and chemical methods aimed at increasing surface roughness and interfacial friction in UHPC, along with an explanation of the benefits of these surface-roughened steel fibers in enhancing both fiber-matrix interfacial bond and tensile performance. Additionally, the adoption of active and inert nanomaterial-based surface coatings on steel fibers for UHPC is explored to intensively reinforce the interfacial transition zone between the fiber and matrix. Hybrid surface refining methods and various chemical treatment techniques for modifying the surface properties of steel fibers in UHPC are reviewed and discussed. Finally, based on the available current database, optimal fiber types as reinforcements for UHPC, in terms of tensile and flexural performances, are suggested.

TDOF model for UHPC members under lateral low-velocity impact

The dynamic behaviors of ultra-high performance concrete (UHPC) members under lateral low-velocity impact loading have been intensively studied by experiment and numerical simulation, while the corresponding structural design approach is still limited and mainly concerned in this study. Firstly, through comprehensively considering the compression arch action, strain rate effect, and unloading stiffness, the dynamic resistance function of UHPC members, e.g., reinforced UHPC (R-UHPC) and UHPC-filled steel tube (UHPC-FST) beams/columns, are built and verified. Then, a two degrees of freedom (TDOF) model of the drop hammer test on UHPC members is improved by considering the P-δ effect and stress wave propagation effect. Furthermore, the impact force- and mid-span deflection-time histories of the existing five series of drop hammer tests on UHPC members are applied to verify the validity and applicability of the established TDOF model. The influence of considering the stress wave propagation effect, strain rate effect, and steel fibers in UHPC for predictions of the TDOF model are discussed by parametric analyses. Finally, based on the energy balance, the explicit formulae are proposed to predict the maximum and residual mid-span deflections of the flexural critical UHPC members under the lateral low-velocity impact, which can be helpful for the design of UHPC structures under lateral impact loadings.

An overview of progressive advancement in ultra-high performance concrete with steel fibers

A progressive advance in the construction sector is attained by employing ultra-high performance concrete (UHPC) technology. Rigorous efforts have been made in this research domain to have remarkable quality levels with 150 MPa or more strength and significant durability, which was impossible previously. Steel fiber incorporation in UHPC is vital in improving its mechanical characteristics. This review on the incorporation of steel fibers in UHPC evaluates, identifies, and synthesizes research outcomes for creating a summary of current evidence that can contribute to evidence-based practice. This study summarized a review of the literature on steel fibers’ effect on UHPC, intending to explore its essential aspects. The aim is to summarize the literature in this research domain and provide guidance for future research. Moreover, the basic requirements and materials, mixing and casting, mechanical properties, modern applications, advantages and disadvantages, and future perspectives associated with steel fibers reinforced UHPC in the construction sector are discussed. It is revealed from the conducted analysis that the most widely applied keyword is “steel fibers.” Due to the graphical illustration of the contributing studies, the current work may benefit academic scholars in sharing novel techniques and ideas and establishing collaborative efforts. Furthermore, the present work reveals that steel fibers have the potential to enhance the mechanical properties of UHPC; however, the large-scale production and applications of steel fiber-reinforced UHPC are controlled by parameters like fiber content and geometry.

Flexural behavior of FRP grid reinforced ultra-high-performance concrete composite plates with different types of fibers

Ultra-high performance concrete (UHPC), which has an excellent compressive strength and an extraordinary durability, has become increasingly popular in structural engineering. However, UHPC has an inferior tensile property to its compressive property, and thus appropriate reinforcing material is necessary in UHPC structures. To this end, fiber-reinforced polymer (FRP) grids have been proposed for UHPC plates as reinforcements (the composite plates are referred to as FRP grid-UHPC composite plates). As the steel fibers in UHPC would still suffer from corrosion problems in aggressive environments, non-metallic fibers can be in UHPC so that steel-free FRP grid-UHPC composite plates are developed. A total of 26 plates (600 mm in length × 120 mm in width × 40 mm in thickness) are prepared for three-point bending tests in this study. The effects of the fiber type, fiber content and the fiber length on the flexural behavior of FRP grid-UHPC composite plates are investigated. Additionally, an analysis into the microstructure based on scanning electron microscope (SEM) of the FRP-UHPC composite plates is conducted. Moreover, a simple evaluation between the costs and the flexural capacities of the FRP grid-UHPC composite plates is conducted. It is economical to adopt 12-mm-length polyethylene (PE) fibers with a 1 % volume fraction to manufacture composite plates. Finally, the ultimate flexural capacities of FRP grid-UHPC plates were predicted using the section analysis approach.

Use of engineered steel fibers as reinforcements in ultra-high-performance concrete considering corrosion effect

The feasibility of using steel fibers engineered through a hybrid treatment of ethylenediaminetetraacetic acid (EDTA)–electrolyte solution and nano-SiO2 coating as reinforcements for ultra-high-performance concrete (UHPC) was investigated by considering fiber corrosion effect. For this purpose, two different corrosion degrees of 2% and 5% were intentionally induced, and the interfacial bonding and tensile properties of UHPCs reinforced with plain and surface-reformed (or engineered) steel fibers were evaluated according to the degree of corrosion. The test results indicated that the EDTA–electrolyte solution treatment and nano-SiO2 coating are effective for improving the interfacial bonding of straight steel fibers in UHPC, leading to an 86% increase in the equivalent bond strength (24.7 MPa). The treatments also improved the tensile performance of ultra-high-performance fiber-reinforced concrete (UHPFRC), and more than twice higher energy absorption capacity could be achieved using the hybrid treatment method. In the case of plain steel fibers, the pullout resistance and tensile performance of UHPFRC both increased with fiber corrosion of up to 5%. However, UHPFRCs with surface-reformed steel fibers exhibited performance deterioration beyond a certain degree of corrosion. A longer immersion duration in the EDTA–electrolyte solution caused a higher susceptibility to steel fiber corrosion, regardless of the nano-SiO2 coating. Thus, fiber corrosion in UHPFRC must be prevented more strictly when surface-reformed steel fibers are used.

Investigation of Five Synthetic Fibers as Potential Replacements of Steel Fibers in Ultrahigh-Performance Concrete

Investigation of Five Synthetic Fibers as Potential Replacements of Steel Fibers in Ultrahigh-Performance Concrete

Direct characterization of steel fibre corrosion in pre-cracked ultra-high performance concrete (UHPC) with different crack widths

The potentiodynamic polarization was used to directly characterize steel fibre corrosion, and the voltammetry was used as the complement, in pre-cracked ultra-high performance concrete (UHPC), during 36 wetting-drying cycles lasted for 180 days. Cracks of 5 different widths of 0 mm, 0.25 mm, 0.50 mm, 0.75 mm and 1.00 mm were pre-produced in specimens. Results showed that steel fibre on the surface of concrete easily corroded and the effect of chloride ion on steel fibre corrosion in UHPC was mainly limited to the vicinity of crack. In this experiment, after 30 wetting-drying cycles in NaCl solution, the corrosion degree of steel fibers in UHPC with crack widths of 0.50 mm and 0.75 mm was lower than that with other widths.

Determination of Axial Capacity for Enhanced Normal and Ultra - High Performance Concrete Columns

Abstract Concrete Technology has been developing for more than a century. One of the most exceptional achievements in concrete technology is the evolving of Ultra-High Performance Concrete (UHPC) which has been a research focus in a wide applications diversity. In this paper, an experimental work has been carried out for investigating the transverse and longitudinal reinforcements’ variation influence on the axial capacity of UHPC columns. Eight columns (five UHPC columns and three Normal Strength Concrete (NSC) columns) have been poured and tested under a concentric axial compression load till a failure is reached. Then, the results are reported herein. The experimental results show that UHPC columns failed in a controlled manner and no concrete chips or a concrete cover spalling are observed. Also, the longitudinal reinforcements have not buckled away beyond the peak load because of the presence of the reinforcing steel fibers in UHPC. Correspondingly, the steel ties spacing proportionally affects the load carrying capacity of columns as presented hereinafter.

Mechanical properties of steel-UHPC-steel slabs under concentrated loads considering composite action

Steel-concrete-steel (SCS) composite structure is widely used as the protective structure in the containment of nuclear power plants, offshore platforms, subsea tunnels, and so on. This study investigates the mechanical properties of SCS composite slabs with ultra-high performance concrete (UHPC) as the core material under concentrated loads. Five S-UHPC-S composite slabs were tested with different parameters of the spacing of headed studs and the thickness of the concrete core. Besides, one S-C-S composite slab with the normal concrete as the core material was tested for comparison. Two failure modes, i.e. punching shear failure and interfacial debonding failure, were defined considering different composite action between the concrete core and bottom steel plate. A punching shear cone was formed in punching shear failure, while interfacial debonding failure had an obvious interfacial slippage between the concrete core and bottom steel plate. The failure mechanism of these two failure modes was analyzed. Moreover, a design method to predict bearing capacities was developed, which could take the effect of the concrete core, steel fibers in UHPC, steel plates, and tie bars into consideration. In particular, a reduction coefficient was introduced to reflect the effect of composite action between the concrete core and bottom steel plate. The design method was suitable with six specimens in this study and other specimens in literature, which could provide theoretical guidance for the application of SCS composite structure in practical engineering.

Enhancing the tensile performance of ultra-high-performance concrete through novel curvilinear steel fibers

This study developed a novel curvilinear steel fiber to improve the tensile behavior of ultra-high-performance concrete (UHPC) and to mitigate the stress concentration observed in conventional deformed steel fibers. To achieve this, four curvilinear steel fibers of different curvatures (0.02–0.10mm range) and a commercial smooth, straight steel fiber were employed. The average and equivalent bond strengths could be improved using the curvilinear steel fibers than the straight fiber from the UHPC matrix. The tensile performance of UHPC was improved by using the curvilinear fibers of curvature (κ) up to 0.04mm. The tensile strength and energy absorption capacity were significantly increased up to 52% and 174%, respectively, by replacing the straight steel fiber to the moderately curved steel fibers with κ=0.04mm, which is the optimum fiber type. The highly curved steel fibers, i.e., with curvatures beyond 0.04mm, slightly deteriorated the tensile performance because of the poorer fiber dispersibility and excessive matrix damage. A comparison of the pullout and tensile parameters showed that the equivalent bond strength of inclined steel fibers in UHPC is the most appropriate indicator for predicting the tensile performance.

Spacing and bundling effects on rate-dependent pullout behavior of various steel fibers embedded in ultra-high-performance concrete

This study examines the effects of fiber geometry, spacing, and loading rate on the pullout resistance of steel fibers in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, four different fiber spacings, and three different loading rates ranging from 0.018 to 740 mm/s were considered. Test results indicated that the single straight fiber in UHPC was most rate sensitive for pullout resistance, followed by the single twisted and then hooked fibers. The bond strengths and pullout energy of specimens with multiple straight fibers were improved by increasing the loading rate but were not affected by fiber spacing. Closer fiber spacing had a detrimental effect on the dynamic pullout resistance of multiple hooked steel fibers in UHPC, while no enhancement of average bond strength of multiple twisted fibers was observed as fiber spacing and loading rate varied. The average bond strengths of single and bundled hooked and twisted steel fibers in UHPC were clearly improved by increasing the loading rate. Bundling of fibers enhanced the impact pullout resistance of all the steel fibers in UHPC. The highest dynamic increase factors for the bundled straight, hooked, and twisted fibers were approximately 3.78, 1.57, and 1.41, respectively, at the impact loads.

Effects of rust layer and corrosion degree on the pullout behavior of steel fibers from ultra-high-performance concrete

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.

Dynamic Pullout Behavior of Multiple Steel Fibers in UHPC: Effects of Fiber Geometry, Inclination Angle, and Loading Rate.

This study examined the influences of fiber geometry, inclination angle, and loading rate on the pullout behavior of multiple steel fibers in ultra-high-performance concrete (UHPC). For this, two different steel fiber types, i.e., straight (S-) and hooked (H-), four different inclination angles (0°–60°), and four different loading rates (0.018 mm/s to 1200 mm/s) were considered. Test results indicated that the pullout performance of S-fibers in UHPC was improved by increasing the loading rate. The highest maximum pullout load of the S-fiber was obtained at the inclination angle of 30° or 45°. The maximum pullout loads of H-fibers also increased with increases in the loading rate, while their slip capacities rather decreased. No specific inclination angle was identified in the case of H-fibers that caused the highest maximum pullout load. The H-fibers yielded higher average bond strengths than S-fibers, but similar or even smaller pullout energies under the impact loads. The aligned S-fiber in UHPC was most sensitive to the loading rate compared to the inclined S-fiber and aligned H-fiber. The rate sensitivity became moderate with the fiber inclination angle. Consequently, the aligned S-fiber was recommended to achieve the best energy absorption capacity and interfacial bond strength at various impact loads.

Effect of steel fibers with galvanized coatings on corrosion of steel bars embedded in UHPC

Ultra-high-performance concrete (UHPC) is widely accepted for its super-high durability. However, the corrosion performance of steel reinforcing bars embedded in UHPC and the improved protection of UHPC cover to steel bars are seldom studied. Furthermore, whether using steel fibers in UHPC will accelerate or slow down the corrosion of steel bars is of special interest. In this study, the effects of two types of steel fibers with different galvanized coatings (brass and zinc coated fibers) on the electrical resistance of UHPC and corrosion resistance of embedded steel bars were investigated. Electrochemical tests were conducted for 1 year in 3.5 wt % NaCl solution. The tests included open circuit potential (OCP), linear polarization resistance (LPR), Tafel polarization, and electrochemical impedance spectroscopy (EIS). Test results showed that ASTM C876 was not applicable to UHPC to determine corrosion state of embedded steel bars based on the OCP values. This is because the dense microstructure of UHPC can limit the penetration of oxygen and significantly lower OCP values. The corrosion rates in terms of corrosion current densities of steel bars from the LPR and Tafel polarization tests showed a decreasing trend with time, hence indicating that the steel bars were in passive state. Furthermore, chloride was not detected in the UHPC matrix after the electrochemical tests. This proved that corrosion did not occur on the steel bar surface in the testing period. The electrochemical resistance of UHPC matrix, the resistance of passive film, and the charge transfer resistance of the steel bars from the EIS test displayed a long-term increasing trend. However, when the conductive steel fibers increased from 0 to 2%, by volume, the UHPC matrix resistance decreased by more than 40%. It was also found that the fiber type and fiber content did not affect the resistance of passive film and charge transfer resistance of embedded steel bars. Galvanic corrosion did not happen due to uniform distribution of the fibers along the cast specimens. When proper casting method was employed, both brass and zinc-coated steel fibers can be safely used in UHPC for fiber volume up to 2%.

Chloride-induced corrosion of steel fiber near the surface of ultra-high performance concrete and its effect on flexural behavior with various thickness

The results of experimental investigation of ultra-high performance concrete (UHPC) by evaluating the flexural response are presented in order to qualitatively evaluate the effects of chloride-induced corrosion. The third-point bending test is carried out using the hydraulic servo-controlled testing machine. The experimental variables are thickness of specimen (10, 25 and 50 mm), immersion duration, and mixture design of UHPC. The specimens are immersed in 10 wt% NaCl solution up to a year to induce corrosion. The effect of chloride-induced corrosion of steel fiber of UHPC is evaluated in terms of compressive strength, flexural strength and flexural toughness. The experimental results indicate that there is no significant loss in flexural strength and toughness for the specimens which were thicker than 25 mm over a period up to 365 days of immersion in chloride solution; whereas, the specimen with thickness of 10 mm showed about 10% decrease in maximum stress and corresponding toughness after 180 days of the immersion. Furthermore, it is revealed by comparing the compressive strength with and without fibers that the presence of steel fibers in UHPC under corrosive environments do not lead to substantial strength decrease.

Effect of fiber spacing on dynamic pullout behavior of multiple straight steel fibers in ultra-high-performance concrete

Abstract In this study, the effects of fiber spacing and loading rate on the pullout behaviors of single and multiple straight steel fibers embedded in ultra-high-performance concrete (UHPC) were investigated. Hence, three different fiber spacings, corresponding to the fiber volume fractions of 1, 2, and 7%, and loading rates, i.e., quasi-static and impacts, were considered. Bundled fiber specimens were also fabricated and tested to examine the fiber bundling effect at both the static and impact loading conditions. The test results indicated that the pullout resistance of straight steel fibers in UHPC improved by increasing the loading rate, regardless of their number, spacing, and bundles. Approximately 33–68% greater bond strengths were obtained in the single fiber specimen compared to the multiple fiber specimens at both the static and impact loads. The fiber bundles deteriorated the static pullout resistance, whereas both positive and negative bundling effects were found on the interfacial bond strength and pullout energy, respectively, under the impact loads. The strongest rate sensitivity on the dynamic increase factors of the pullout parameters, such as bond strength, maximum fiber tensile stress, and pullout energy, was found as the fibers were bundled, followed by the single and multiple fiber specimens. For the case of multiple fiber specimens, the loading rate sensitivity increased by decreasing the fiber spacing in general, such that the multiple fibers with closer spacings, corresponding to 2 and 7% by volume, were more sensitive to the rate of loading than their counterparts with greater fiber spacings.

Dynamic pullout behavior of half-hooked and twisted steel fibers in ultra-high-performance concrete containing expansive agents

This study investigates the effects of matrix shrinkage, fiber geometry, and loading rate on the pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For varying the matrix shrinkage, different quantities of calcium sulfoaluminate-based (CSA) expansive agent (EA) were adopted, ranging from 0% to 8%. In addition, moderately and highly deformed, i.e., half-hooked (HH) and twisted (T), steel fibers were used along with three different loading rates ranging from 0.018 (static) to 793 mm/s (impact). The test results indicated that the addition of the CSA EA is effective in reducing the shrinkage strains of UHPC only when its quantity is greater than 6%. The addition of the CSA EA improved the static average bond strengths of all the HH- and T-fibers; however, its effectiveness on the pullout energy was only valid when these fibers were aligned. The addition of the CSA EA enhanced the dynamic bond strengths of HH- and T-fibers, and the improvement was more obvious in the T-fiber specimens than in the HH-fiber specimens and in the aligned fibers than in the inclined ones. However, its implication on the dynamic pullout energies of the HH- and T-fibers was ambiguous. Both the addition of the CSA EA and a faster loading rate increased the probability of rupture failure of the deformed steel fibers in UHPC. The loading rate sensitivity was the highest in the straight steel fiber in UHPC, followed by the HH- and T-fibers, respectively. Consequently, straight or moderately deformed steel fibers are recommended for use in reinforcing UHPC under extreme loads, such as impact and blast.

Comparative pullout behavior of half-hooked and commercial steel fibers embedded in UHPC under static and impact loads

This study investigates the pullout behaviors of various steel fibers embedded in ultra-high-performance concrete (UHPC) under static and impact loading conditions. For this purpose, four types of steel fibers, i.e., straight, hooked, twisted, and half-hooked, and three different loading rates applied by static and impact pullout test machines were adopted. To examine the effect of inclination angle on the pullout behavior, four different inclination angles of 0°, 30°, 45°, and 60° were considered. Test results indicate that the highest average bond strengths were found for the hooked and twisted fibers for static and impact loads, respectively, whereas the straight fibers exhibited the lowest bond strength at all inclination angles. The effectiveness of using half-hooked fibers increased when they were inclined, and maximized at an inclination angle of 45° compared to straight and highly deformed fibers. The use of the twisted and half-hooked fibers was also more effective in static pullout energies than the hooked and straight fibers. The pullout resistance of all the steel fibers in UHPC was improved under impact loading conditions, and the order of the loading rate effectiveness regarding both the average bond strength and pullout energy was as follows: straight fibers > half-hooked fibers > twisted fibers > hooked fibers. The change in the failure mode from pullout to rupture, due to the increase in the loading rate, adversely affected the rate sensitivity of the bond strength and energy absorption capacity. Since the deformed steel fibers were easily ruptured under impact loads, their rupture failure needs to be prevented to achieve an excellent bond strength and energy absorption capacity.

Numerical simulation and visualization of motion and orientation of steel fibers in UHPC under controlling flow condition

To improve fiber orientation in ultra-high performance concrete (UHPC), one L-shape device was developed to control the flow of fresh mixture. A smoothed particle hydrodynamics (SPH) method was used to analyze and simulate the motion of steel fibers in UHPC with flow through this L-shape apparatus. SPH simulation indicated that steel fibers tended to rotate to be parallel with local streamlines and most fibers were aligned in the horizontal channel of this L-shape device, inducing an improvement of fiber orientation in hardened specimens. This modification effect of fiber orientation can be a little strengthened with the decreasing height of the horizontal channel. On the other hand, a colorless, transparent and viscous concentrated suspension was obtained by adding super absorbent polymer (SAP) into water for representing fresh UHPC matrix and the visualization of fiber motion was achieved. This visualization approach, coupled with image analysis of steel fibers in hardened UHPC specimens, well verified the SPH numerical simulation results. Therefore, this type of L-shape apparatus provides a potential method to manually improve the steel fiber orientation in UHPC for better mechanical properties.