Pullout Behavior Of Steel Fibers Research Articles

Interfacial bond behavior of steel fibers embedded in manufactured sand-based ultra-high performance concrete

The interfacial bond region between matrix and fibers is a non-negligible weak link that can significantly affect the mechanical properties of manufactured sand-based ultra-high-performance concrete (UHPMC). This study investigates the interfacial bond characteristics between steel fiber-UHPMC matrix using single-fiber pullout tests. Four variable parameters were considered: manufactured sand (MS) replacement ratio, stone powder content, fiber embedment length, and steel fiber geometry. Additionally, the microstructure of the steel fiber-UHPMC matrix interface was evaluated using backscattered electron microscopy (BSEM) and scanning electron microscopy (SEM). The results indicate that two typical failure modes including fiber pullout slip failure (in straight and hooked-end fibers) and fiber rupture failure (in corrugated fibers) were observed. The analysis of interfacial evaluation indicators shows that increasing the MS replacement ratio from 25 % to 100 % results in a decreasing trend in peak load, peak tensile stress, and energy consumption. Peak bond strength decreases with increasing embedment length. With the increase in stone powder content, the peak load, peak bond stress, and energy consumption increase initially and then decrease. Additionally, the interfacial failure mechanisms between straight/hooked-end steel fibers and the UHPMC matrix were further elucidated through bond-slip behavior characterization and microstructure analysis. Finally, a design-oriented bond-slip model was developed to predict the interfacial pull-out behavior of steel fiber -UHPMC matrix, showing favorable agreement between predicted and test results. These findings contribute to understanding the interfacial pullout behavior of steel fibers-UHPMC matrix, providing data reference for the performance optimization of UHPMC.

Pull-out behaviour of steel fibres embedded in ultra-high-performance concrete after exposure to high temperatures

The paper examined the bond-slip behaviour of steel fibres embedded in ultra-high-performance concrete (UHPC) after exposure to elevated temperatures. Different types of steel fibres were cast into the halved UHPC dog-bone-shaped samples, including straight fibres, three types of hooked-end fibres and corrugated fibres. The bond response was investigated through single fibre pull-out tests on the samples unheated and heated to 200, 400 and 600 °C. In addition, the effects of polypropylene (PP) fibre addition and heating rate on steel fibre’s bond behaviour were also investigated. The results revealed an increase in bond strength when the straight and hooked-end fibre samples were heated to 200 °C, while the bond strength of the corrugated fibre decreased at the same temperature. Bond strength reductions were observed at higher temperatures. In addition, the straight fibres led to the lowest pull-out resistance as both the hooked-end and the corrugated fibres significantly enhanced the bond strength. With the hooked-end or corrugated fibres, the failure mode gradually shifted from pull-out failure to fibre rupture. Besides, adding PP fibres increased the bond strength of straight fibres at 400 °C but reduced the bond strengths of hooked-end fibres at ambient and high temperatures. High heating rate also led to a greater bond strength for the straight fibres at 200 °C and 400 °C.

Pullout behavior of steel fiber embedded into cement matrix under low humidity curing

An experimental was presented in this article to investigate the impact of the low relative humidity (low RH) on the pullout properties of the straight and hooked-end fibers embedded into cement matrices. It was shown that decrease in curing RH results in a reduction in the interfacial carrying capacity and the pullout energy absorption ability, owing to the microstructure of the fiber-matrix ITZ deteriorating as the humidity decreased for all types of fibers. Besides, a novel model was developed to simulate the bond-slip behaviors of the steel fiber-matrix interface under low RHs, and a theoretical method was carried out to estimate the parameters in the model. Furthermore, various analytical models were proposed based on the bond-slip model to simulate the pullout behaviors of the steel fibers embedded in matrix under low RHs, and the validation results revealed that the suggested model is consistent with the experimental data.

Influence of fiber orientation on the microstructures of interfacial transition zones and pull-out behavior of steel fiber in cementitious composites

The reinforcement of steel fibers on cementitious composites is mainly influenced by the bonding between the fiber and matrix. The steel fibers and matrix bonding depend on the interfacial transition zone (ITZ). A major cause of ITZ formation is microbleeding, i.e., the flow of free water around steel fibers at the early stage of bleeding may cause the redistribution of water and cement particles around the fibers, leading to the formation of ITZs. The fiber orientation in the matrix affects the flow of water and further influences the microstructure of the ITZ around the steel fiber and the bonding properties. This study proposes a theoretical model based on fluid mechanics to investigate the influence of fiber orientation on cement-water suspension migration around steel fibers. The microstructure of the ITZ around the steel fiber is then estimated based on the Powers model according to the water-cement ratio distribution. Finally, the interrelationships among the fiber orientation, ITZ porosity and bond strength are established by curve fitting. The results indicate that since free water with small particles flows past the steel fiber by flowing past circular (or elliptical) cylinders, a difference in the ITZ microstructures at the upper and lower surfaces of the steel fiber arises. As the inclined angle of the fiber increases, the upper surface porosity and the average porosity of the ITZ around the steel fiber decrease, leading to an increase in the fiber-matrix bond strength.

Pullout Behavior of Steel Fiber in Ultra-high-performance Concrete Subjected to Freeze-thaw

Pullout Behavior of Steel Fiber in Ultra-high-performance Concrete Subjected to Freeze-thaw

Benefits of chemically treated steel fibers on enhancing the interfacial bond strength from ultra-high-performance concrete

This study investigated the effects of various chemical treatments on the pullout behavior of steel fibers in ultra-high-performance concrete (UHPC). Various methods used in previous studies were adopted and reproduced using macro straight steel fibers. The methods included acetone and acid washing, zinc phosphating, nanosilica coating, and ethylenediaminetetraacetic acid (EDTA) electrolyte solution treatment. The surface conditions and roughness of each treated fiber were analyzed using scanning electron microscopy and atomic force microscopy. The fiber pullout test results indicated that the surface roughness increased between 1.5 and 24.9 times, and that a roughened fiber surface leads to an improvement in the average bond strength. Acetone washing was the most effective in improving the pullout resistance of the fiber due to its cleansing effect. Acid washing resulted in a slip-hardening behavior, which improved the pullout energy significantly. The zinc phosphating treatment exhibited the lowest average bond strength increase rate. Nanosilica coating was found to considerably improve adhesion; however, it caused fractures as inclined. EDTA electrolyte solution treatment significantly improved the average bond strength in proportion to the surface roughness. By considering the energy absorption capacity, the optimum treatment time of EDTA electrolyte solution was proposed to be less than 6 h.

Pull-out behaviour of steel fibres in cracked concrete under wet–dry cycles – deterioration phenomena

This paper presents an experimental investigation into the mechanisms altering the pull-out behaviour of partially pulled hooked-end steel fibres inside an artificial crack exposed to wet–dry cycles of chlorides and carbon dioxide for 6 months. Mechanical and corrosion damage at the surface of the steel fibre was investigated using three-dimensional optical interferometric profiling, and petrographic analyses were used to describe damage and healing processes at the matrix surrounding the fibre. Mechanical damage observed in the cementitious matrix and at the fibre's surface confirmed that the pull-out process was governed by yielding of the fibre, fracture of the adjacent matrix, and friction between the hook and the matrix. Increases in the pull-out force of partially pulled-out fibres after exposure were connected to accumulation of corrosion products from the steel fibre and to alteration of the cement matrix surrounding the fibre. This study concluded that autogenous healing and carbonation of the damaged cement matrix around the fibre are the main mechanisms responsible for the increase in the fibre–matrix bond strength of hooked-end steel fibres bridging cracks in concrete exposed to wet–dry cycles.

Pull-out and bond-slip performance of steel fibers with various ends shapes embedded in polymer-modified concrete

The bond-slip behavior between matrix and fibers highly depends on the geometric and mechanical characteristics of fibers. Thus, by considering various geometric of fiber end shapes, the pull-out behavior of steel fibers from polymer-modified concrete is studied in the present study. For that, a three-dimensional non-linear finite element method is employed for simulations. For considering more real guess, interaction among matrix and fibers is simulated via Interfacial Transition Zone (ITZ), and its factors are attained from experimental results of the fiber pull-out test. After validating the results, the effect of geometric parameters of the fibers end shape on the bond-slip properties with polymer composite is investigated. The results from the load-slip curves indicate that there are different mechanisms in the behavior of the fibers depending on the diameter and angle of the fibers. For fibers with 2 mm diameter, increasing the angle of the hook from 0 to 90 degrees increases the pull-out force of hook and half-hook fibers by about 35% and 15%, respectively. Whereas a further increase in hook angle reduces pull-out parameters of the hook-shaped fibers.

Influence of embedment length on the pullout behavior of steel fibers from ultra-high-performance concrete

Single fiber pullout behavior has been investigated extensively at various embedment lengths, and to generalize the test results of the existing studies, the implication of embedment length on the pullout behavior needs to be analyzed. To this end, experiments were conducted with two types of steel fibers in ultra-high-performance concrete (UHPC) at three embedment lengths and two inclination angles. The effect of embedment length on the pullout behavior was more significant in the half-hooked fiber than the straight one. Its pullout performance was also better than that of the straight fiber up to an embedment length of 10 mm; however, the effectiveness decreased with an increase in the embedment length. At the longest embedment length of 15 mm, the pullout energy and equivalent bond strength obtained for the half-hooked fiber samples were poorer than those obtained for the straight fibers. Therefore, its maximum length is suggested to be below 30 mm at a diameter of 0.375 mm to prevent premature rupture in UHPC.

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.

Cryogenic pullout behavior of steel fibers from ultra-high-performance concrete under impact loading

This study investigates the effect of impact loading condition on the pullout property of steel fibers from ultra-high-performance concrete (UHPC) under various temperatures. For this, static and impact loads, ambient and cryogenic (−170 °C) temperatures, three steel fiber types, i.e., straight, half-hooked, and twisted, and two inclination angles of 0° and 45° were considered. From the test results and analysis, it was found that the steel fibers mostly had positive values of dynamic increasing factor (DIF) for the average bond strength at ambient temperature, but these were significantly reduced when the fibers were geometrically deformed, inclined, or tested under cryogenic temperature. Slip capacities generally decreased under the influence of the impact loading condition, and this effect was more severe at cryogenic temperature. There was no obvious impact loading rate effect on the probabilities of fiber and UHPC matrix damages at both ambient and cryogenic temperatures. The DIF values for pullout energies normally decreased or even became negative when the fibers were inclined, geometrically deformed, or tested at cryogenic temperature.

Elasto‐plastic phase‐field model for pullout tests of steel fiber embedded in high‐performance concrete: numerical calibration and experimental validation

AbstractIn this paper, the pullout behavior of steel fiber embedded in high‐performance concrete (HPC) is investigated. First, a constitutive framework of elasto‐plastic phase‐field model of fracture is constructed and applied to simulation of pullout tests for different embedded lengths of steel fibers. Thereby, the description of the mechanical behavior the Drucker‐Prager plasticity model is used. The numerical calibration of the above mentioned model is performed by simulating pullout tests and validated by data obtained from experiments. Therefore, the mechanical behavior of high‐performance concrete is studied in experiments.

Effects of superabsorbent polymer on interfacial transition zone and mechanical properties of ultra-high performance concrete

In this study, two particle sizes of superabsorbent polymer (SAP) with different contents (0–0.6%) were incorporated in the UHPC samples cured under two regimes: water and steam curing. The interfacial transition zone (ITZ) associated properties, namely compressive strength, flexural strength, and pullout behavior of steel fiber were evaluated. Micro-hardness method and backscattered electron (BSE) imaging examinations were also applied to investigate the underlying mechanisms of SAP performance in UHPC. The results show that the use of SAP densified the ITZ and reduced micro-cracks around the fibers. However, the pores left from SAP impaired the bonding strength and ITZ of UHPC, when high contents and large size of SAP particles were incorporated. Steam curing, though, partly compensated the bond strength loss of fibers caused by the SAP.

Effects of fiber shape and distance on the pullout behavior of steel fibers embedded in ultra-high-performance concrete

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.

Pullout behavior of steel fibers with different shapes from ultra-high performance concrete (UHPC) prepared with granite powder under different curing conditions

Abstract Granite powder (GP) was used to completely replace quartz powder (QP) as the fine filler to prepare ultra-high performance concrete (UHPC). Pullout behavior of steel fibers from the target UHPC, i.e., UGP, was investigated by a single-fiber pullout test and was compared to the control concrete. The influence of the fiber shape and the curing conditions with different thermal treatments on the pullout resistance from UGP was discussed. It is found that the replacement of QP by GP can improve the pore structure and the density of the cement matrix. As a result, the compressive and flexural strengths of UHPC are enhanced; meanwhile, the pullout resistance with the straight & smooth (S) fiber is also enhanced. Compared to standard curing (SC), warm water curing (WWC) and autoclaved curing (AC) can improve the peak pullout load, the average bond strength, the pullout energy and the fiber utilization rate of the S fiber and most deformed fibers, but the brittleness of failure by fiber fracture also increases. Deformed fibers demonstrate superior pullout resistance compared to the S fiber, however, the excessively deformed fibers (e.g, the more corrugated C2 fiber with dense waves) tend to fracture when the cement matrix cured under WWC and AC presents ultra-high strength. It is suggested that it should be feasible to prepare UHPC by replacing QP with GP, but the effects of curing and the fiber shape on the pullout resistance from UGP should be comprehensively considered. The appropriate fibers employed by UGP in practical use should be determined according to the curing to be provided.

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.

Bond-slip behaviour of steel fibres in concrete after exposure to elevated temperatures

The bond-slip mechanisms, associated with the pull-out behaviour of steel fibres embedded in concrete after exposure to elevated temperatures, are experimentally investigated. A series of pull-out tests have been performed on straight and hooked-end steel fibres embedded in four different types of concrete, namely, normal strength concrete (NSC), medium strength concrete (MSC), high strength concrete (HSC) and ultra-high performance mortar (UHPM). Ninety days after casting, the specimens were heated to a target temperature of either 100, 200, 300, 400, 500, 600, 700 or 800°C. The effect of temperature on the mechanical and thermal properties of the steel fibres and concrete was also studied. The results showed that the bond behaviour of straight fibres is significantly influenced by heating. The influence of elevated temperatures on the bond characteristic of hooked-end fibre was twofold: the bond strength does not vary significantly for all matrixes in 20–400°C, while the bond dramatically degraded in 400–800°C, especially at temperatures greater than 600°C. The reduction in bond strength at elevated temperatures is found to be strongly related to the degradation in properties of the constituent materials, i.e. the fibres and concrete.

Pull-out behaviour of spiral-shaped steel fibres from normal-strength concrete matrix

Reinforcing concrete with steel fibres has been proven being able to improve the properties of concrete such as strain capacity, impact resistance, energy absorption and tensile strength because the discrete steel fibres act as dispersed reinforcement in mitigating crack propagation. A number of laboratory tests in recent years have indicated the superiority of spiral geometry of steel fibres as compared to other fibre types in terms of ductility, crack controllability, dynamic strengths and energy absorption. Understanding fibres’ pull-out behaviour is essential to evaluate the properties of steel fibre reinforced concrete (SFRC). The pull-out behaviour of straight, hooked-end and twisted steel fibres have been investigated by many researchers. However, there has been no study that focuses on pull-out behaviour of spiral fibres in the literature yet. The pull-out behaviour of steel fibre with spiral geometry from concrete matrix is experimentally investigated in the present study. In addition, steel fibres with hooked ends were also prepared and tested for comparison. The spiral fibres with various geometric characteristics such as pitch length, embedment length and coil diameter were pulled out from concrete matrix at different rates. Test results showed several peak loads before reaching a final maximum pull-out force. Based on the experimental results, the influences of embedment length, pull-out rate, coil diameter and pitch length were investigated. Through analysing the testing results, combining hooked-end fibres and spiral fibres was proposed as a hybrid steel fibre reinforcement technique in order to further improve the performance of SFRC materials and structures. Laboratory tests in this study proved the effectiveness of this technique in enhancing the capacity of the reinforced concrete structure.

Effect of elevated temperature on pull-out behaviour of 4DH/5DH hooked end steel fibres

This paper presents the effect of elevated temperature on the bond mechanisms associated with the pull-out behaviour of steel fibres. A series of pull-out tests have been performed on 4D and 5D hooked end steel fibres embedded in four different types of concrete, namely, normal strength concrete (NSC), medium strength concrete (MSC), high strength concrete (HSC) and ultra-high performance mortar (UHPM). At the age of 90days, the specimens were heated to target temperatures of 100, 200, 300, 400, 500, 600, 700 and 800°C respectively. The influence of elevated temperature on the mechanical and thermal properties of concrete was investigated. The results showed that the pull-out response of both fibres does not vary significantly throughout 20–400°C temperature range, but within the temperature range of 600–800°C, the pull-out strength decreases significantly for all concretes. The comparisons between the two fibre types show that the mechanical anchorage contribution provided by the 5DH fibre is significantly higher than that of the 4DH fibre, especially for higher strength concretes. The reduction in bond strength of both fibres after elevated temperature exposure is found to correlate closely with the degradation in compressive strength of the concretes.

Fiber-Matrix Bond Characteristics of Alkali-Activated Slag Cement–Based Composites

AbstractFiber-matrix bond properties of alkali-activated slag cements (AASC)–based composites have not been studied comprehensively in literature. Within the scope of this study, the effects of AASC matrix phase composition and the silicate modulus of activator solution on the pullout behavior of steel fiber were investigated under water- and steam-curing conditions. Test results showed that AASC have a superior bond strength compared to portland cement (PC). Bond strength of AASC mortars with a compressive strength greater than 18 MPa were found higher than that of PC mortar with a compressive strength of 55 MPa. When the compressive strength of AASCs is higher than 40 MPa, the bond strength of AASC exceeds the bond strength of PC mortars that have compressive strength of 120 MPa. Microstructural analysis revealed that the map cracking in the fiber-matrix transition zone, which was induced by high drying shrinkage of AASC, can be eliminated by incorporating pozzolans. Consequently, it is possible to prod...