Pullout Resistance Of Fibers Research Articles

Loading rate effects on multifiber pullout resistance of smooth steel fibers in ultra-high-performance concrete

This study investigated the multi-fiber pullout resistance of high-strength smooth steel fibers embedded in ultra-high-performance concrete (UHPC) at different loading rates. To discover the source of the fiber group effect on fiber pullout resistance, multi-fiber pullout tests were employed by changing the number of fibers, fiber-to-fiber distance, and difference in embedded lengths. The main source of the fiber group effect was found to be the reduced contact pressure per fiber, in the specimens containing more fibers, generated by matrix shrinkage: the fiber pullout stress at the static loading rate (0.0167 mm/s) decreased from 786.58 to 618.59 MPa (21.4 % reduction) as the number of fibers increased from 1 to 25. The equivalent bond strength of smooth steel fiber in UHPC generally increased from 4.5–5.2 to 5.9–7.1 MPa as the loading rate increased from 0.0167 to 1.67 mm/s. The favorable rate-sensitive pullout resistance of multi-fiber pullout specimens was notably affected by the number of fibers and fiber distance: the specimens containing nine fibers with a 2.7 mm fiber distance produced the highest dynamic increase factor (1.37) owing to enhanced interactions between interfacial cracks.

Optimization of the Shape of Hooked-End Steel Fiber Based on Pulling Out and Reinforcing Cementitious Composites.

Efficient steel fiber-reinforced cementitious composites (SFRCC) should improve not only the ultimate tensile strength but also the residual tensile strength (post-cracking tensile strength) of the SFRCC matrix. The degradation of the post-cracking tensile strength of SFRCC depends on the pullout properties of the steel fibers from the cementitious matrix. When the straight steel fiber was pulled out from the matrix, the pullout resistance was affected by the actions of bonding and friction. After debonding, the load was transferred only by friction, and the pullout resistance decreased rapidly, resulting in a weak reinforcing effect. In addition, changing the fiber shape can effectively slow down the decreasing rate of the pullout resistance of the steel fibers, thus improving their reinforcing effects. In this study, the shape of the steel fibers was optimized to slow down the decrease in the pullout resistance of the steel fibers from the cementitious matrix, thus achieving better tensile properties of SFRCC. First, a calculation model for the steel fiber pullout resistance was established. On this basis, a method to optimize the shape of the steel fibers was proposed. Finally, the pulling out behavior of steel fibers with different end hook shapes was tested, and the influence of the steel fiber shape on the decreasing rate of the residual pullout resistance was analyzed. The results showed that the optimized hooked-end steel fibers had better pullout resistance than ordinary hooked end steel fibers of the same diameter, tensile strength, and matrix.

Effects of double-arched geometry and tensile strength on the pullout resistance of fibers

This study evaluated the pullout resistance performance of a developed double-arched steel fiber. Pullout experiments were conducted to assess the resistance load, bond strength, and energy absorption capacity of the fiber according to the geometry and tensile strength of the fiber. Double-arched (DA) steel fibers demonstrated peak and residual resistance loads approximately 36–112% and 29–163% higher than hooked-end and single-arched steel fibers, respectively. The DA steel fiber also significantly increased the bond strength and energy absorption capacity by approximately 58–159% and 125–130%, respectively, compared to the hook-end and single-arched steel fibers. Moreover, the DA fiber exhibited a more significant increase rate in pullout resistance load, bond strength, and energy absorption capacity with increasing tensile strength. A prediction model was also developed to incorporate the influence of plastic bending and friction resistance on the pullout behavior of the DA fiber. The resistance loads and decreasing trend obtained from the prediction model exhibited a very close agreement with the experimental results.

Rate-sensitive pullout resistance of smooth-steel fibers embedded in ultra-high-performance concrete containing nanoparticles

Single-fiber pullout tests for smooth-steel fibers embedded in ultra-high-performance concrete (UHPC) containing nanoparticles (NPs) were conducted at various pullout speeds (0.0167–500 mm/s). Three types of NPs, namely, nano-CaCO3, nano-SiO2, and nano-CNT, were added to UHPC at dosages between 1 and 3 wt % of cement. The addition of NPs to UHPC notably increased the rate sensitivity of fiber pullout resistance. Among the NPs, the addition of nano-CaCO3 (3 wt %) produced the highest rate sensitivities for the dynamic increase factors (DIFs) of both τeq (1.95) and τpeak (2.06), whereas UHPC without NPs obtained the lowest rate sensitivity of τeq (DIF = 1.18), and UHPC containing 3 wt % nano-CNT had the lowest rate sensitivity of τpeak (DIF = 1.62). Nanoindentation, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests revealed that the higher amount of C–S–H at the fiber–matrix zone produced higher pullout resistance and rate sensitivity.

Predicting tensile properties of strain-hardening concretes containing hybrid fibers from single fiber pullout resistance

Tensile properties of strain-hardening fiber-reinforced concrete are the key engineering parameters in determining bending resistance of the material. In this paper, an analytical model to predict tensile properties of ultra-high-performance fiber-reinforced concrete (UHPFRC), a type of strain-hardening fiber-reinforced concretes, was performed based on single fiber pullout test. The studied UHPFRCs contained hybrid fiber system, including macro steel fiber combined with micro steel fiber. Three types of macro steel fibers were used, including long smooth fiber (LS), hooked A fiber (HA), and hooked B fiber (HB); they had different lengths and geometries but same volume content (1.0 %). The only short smooth fiber (SS), one type of micro steel fiber,was employed with various volume content (0.5 %, 1.0 %, 1.5 %). The experimental data from the fiber pullout tests in the available references were used to predict the first crack/post crack strength and cracking parameters of UHPFRCs with hybrid fibers. The predictive equations for strengths and crack resistance of UHPFRC containing hybrid fibers were proposed with modified coefficients.

Liquid crystal display glass powder as a filler for enhancing steel fiber pullout resistance in ultra-high-performance concrete

This study investigated the feasibility of using waste liquid crystal display (LCD) glass powder as a partial replacement for the silica flour that is typically used as a filler in ultra-high-performance concrete (UHPC). In this study, 50% of the silica flour was replaced with LCD glass powder, and its effects on the interfacial bond properties of two steel fiber shapes—circular and triangular—were evaluated. The test results indicated that the LCD glass powder effectively enhanced the bond between the steel fibers and the UHPC, which created a denser interfacial zone; this could be attributed to the filling effects of the LCD glass powder and secondary hydrates. The use of the LCD glass powder increased the pullout energy by 15–40% and the average bond strength by 16%, along with formation of additional scratches on the fiber surfaces. For the same matrix, as compared to the circular steel fibers, the triangular steel fibers yielded superior pullout resistance owing to their higher intrinsic fiber efficiency ratio. Thus, the highest average bond strength and pullout energy of 11.2 MPa and 744 mJ could be obtained from the UHPC mixture containing the LCD glass powder and triangular steel fibers.

A Study of Waterproof Reinforcement Layers for the Post-Cracking Behavior of Fiber Reinforced Concrete

In this study, in order to protect the concrete layer from slab sinking cracks in the factory floor layer, four types of reinforcing materials with a notable waterproof performance are fixed at the bottom. Furthermore, short fibers mixed with concrete in order to evaluate the load transfer mechanism and residual stress retention ability are used. The change in flexural strength due to the waterproof reinforcement varied from about 10 to 48% depending on the type of reinforcing material, and the flexural strength of the specimen reinforced with Typar and Preprufe was demonstrated to be the best. Additionally, the increase in flexural strength due to the combination of the SF20 + Typar and MF2.8 + Preprufe specimens was remarkable. After the concrete matrix cracking, the toughness resulting from the fiber pull-out resistance and the increase in the reaction force of the waterproof reinforcement showed a marked improvement in all the test specimens. The test specimen reinforced with Typar demonstrated the best crack resistance regardless of the fiber type. The crack transfer mechanism in the concrete floor can be summarized in that the fiber pull-out resistance and the reaction force of the waterproof reinforcement immediately after cracking causes a reduction of the crack length (l) from the rapid load transfer, and as a result, the fiber bridging zone (lf) is widely protected. Therefore, it is determined that the residual stress rises, maintains, and slows, as the resistance of the fiber bridging in the cracked section and the effect of the waterproofing reinforcement layer is combined.

Surface modification of steel fibers using chemical solutions and their pullout behaviors from ultra-high-performance concrete

To improve the pullout resistance of conventional straight steel fiber from ultra-high-performance concrete (UHPC), its surface was modified using various chemical solutions. Calcium carbonate (CaCO3) particles were precipitated on the surfaces of the steel fiber in an aqueous solution. The morphology and particle size of CaCO3 were controlled by ethylenediaminetetraacetic acid (EDTA) and investigated via scanning electron microscopy; further, its chemical composition was verified through energy-dispersive X-ray spectroscopy. The test results indicate that longer duration of immersion in a solution of CaCl2 and Na2CO3 yield larger CaCO3 particles on the surface owing to Ostwald ripening. The addition of EDTA changes the morphology of the CaCO3 particle and decreases its grain size from 16.2 to 3.1 μm. In order to investigate the effects of the precipitated CaCO3 particles on the fiber pullout resistance, single fiber pullout test was carried out; the modified steel fibers were embedded in UHPC matrix. The CaCO3 particles precipitated on the fiber surface improved the pullout resistance of the steel fiber from the UHPC. The smaller CaCO3 particles were more effective for enhancing the pullout resistance. The average bond stress and pullout work were increased by up to 15% and 37%, respectively, owing to the precipitated CaCO3 particles. The EDTA solution could peel off the surface of steel fiber regularly and render it rough; thus, it greatly increased the pullout resistance (more than 80%) than the plain fiber.

Effect of calcium sulfoaluminate-based expansive agent on rate dependent pullout behavior of straight steel fiber embedded in UHPC

To use a calcium sulfoaluminate (CSA) expansive agent (EA), beneficial for volume stability of concrete, its effect on the pullout behavior of straight steel fiber in ultra-high-performance concrete (UHPC) needs to be analyzed under various loads, from static to impact (0.018 to 1244 mm/s). Adding CSA EA enhanced the static pullout resistance of only inclined fibers, whereas the average bond strengths and pullout energies of both the aligned and inclined fibers improved by it under impact loading. Thus, adding the CSA EA is more effective in enhancing the dynamic pullout resistance. The fiber pullout resistance improved at a higher loading rate, and 45°-inclination of the fibers improved the dynamic pullout resistance. A higher rate sensitivity on the bond strength was observed with the CSA EA addition and fiber's inclination, and the pullout energy was more dependent on loading rate than the average bond strength as fiber rupture was prevented.

Steel Fibre Performance in Concrete with Expansive Additives under Restrained Hardening Conditions

Steel moulds in the form of a rigid cubical shell were developed in order to investigate single steel fibre pull-out resistance in concrete with expansive additive under restrained hardening conditions. The cubical shell (100 x 100 x 100 mm) with wall thickness of 5 mm was designed with two openings – a small 4 mm hole for fibre embedment in concrete and a larger opening for filling the concrete. Standard beam (100 x 100 x 400 mm) and cube (150 x 150 x 150 mm) samples were also manufactured and hardened under and without restrained conditions, where the restraint was realised by rigid standard steel moulds. All the restrained conditions realized by either the developed cubical steel shell (for single fibre pull-out) or existing beam and cube moulds simulate internal (from steel fibres in concrete) and external (from friction against sub-base) restraints that hinder expansion of the concrete due to the use of special expansive additives in a flooring slab structure installed on ground. Samples with a single hooked-end steel fibre (50 mm long and 0.75 mm in diameter), with and without expansive additive were manufactured and tested in the developed mould geometry. The results show that restrained expansion in concrete with expansive additives positively affects concrete compressive strength, single fibre pull-out and flexural behaviour. Concrete compressive strength increases by 7.5 %, single fibre delamination resistance increases by 24 %, the peak pull-out load by 10.8 % while the flexural strength increases by 3.1 %.

An integrated approach for predicting the shear capacity of fibre reinforced concrete beams

This paper describes the development of an integrated design approach for determining shear capacity of flexurally reinforced steel fibre reinforced concrete members. The approach considers fibre distribution profile, fibre pull-out resistance and the modified compression field theory integrated using a comprehensive strategy. To assess the performance of the developed model, a database consisting of 122 steel fibre reinforced and prestressed concrete beams failing in shear was assembled from available literature. The model predictions were shown to correlate well with the test data. The performance of the analytical model was also compared to predictions attained by the two approaches recommended by the fib Model Code 2010, one based on an empirical equation and the other on the modified compression field theory approach. The predictive performance of the proposed approach was also assessed by using the Demerit Points Classification (DPC), being the prediction as better as lower is the total penalty points provided by the classification. The model developed in this paper demonstrated a superior performance to those of the Model Code, with a higher predictive performance in terms of safety and reliability.

Method development to evaluate steel fiber performance in concrete with expansive additives under restrained hardening conditions: preliminary results

Novel steel molds in form of a rigid cubical shell were developed in order to investigate single steel fiber pull-out resistance in concrete with expansive additive under restrained hardening conditions. The developed steel molds simulate internal (from steel fibers) and external (from friction against sub-base) restraints that hinder expansion of the concrete in a flooring slab structure installed on ground. Samples with a single hooked-end steel fiber, with and without expansive additive were manufactured and tested in the developed mold geometry. The results show that restrained concrete expansion positively affects single fiber pull-out behavior, i.e. fiber delamination resistance is increased by 29.7 % and pull-out peak load by 10.8 %.

Twin-twist effect of fibers on the pullout resistance in cementitious materials

As a part of ongoing research to develop a new type of a twin-twisted steel fiber and characterize its tensile performance in cementitious materials, an initial experimental study was performed to evaluate the pullout behavior of the fiber. Comparative analyses were carried out with straight and hooked-end fibers to examine the twin-twist effect on the pullout resistance of fibers. The maximum and residual pullout capacity, bond strength, and energy absorption capacity were evaluated. The twin-twisted fiber showed higher maximum and residual pullout load compared to the straight and hooked-end fibers. In particular, the new type of the twin-twisted fiber retained the residual pullout resistance until the final failure of the fiber, which could significantly improve the toughness and resistance of cement-based materials.

Fracture behaviour of steel fibre-reinforced concrete at a wide range of loading rates

Three-point bending tests on notched beams of steel fibre-reinforced concrete (SFRC) have been conducted using both a servo-hydraulic machine and a drop-weight impact device. The shape and geometry of the beams followed the RILEM recommendation, i.e., 150 mm × 150 mm in cross section, 700 mm in length, notch-depth ratio was around 1/6 and the span of the tests was kept constant 500 mm. The peak load and the fracture energy were measured over a wide range of loading rates (loading point displacement rates), spanning six orders of magnitude. Under low loading rates, from 10−3 mm/s to 100 mm/s, the tests were performed with the servo-hydraulic machine; from 102 mm/s to 103 mm/s, the drop-weight impact machine was used instead. The results show that the fracture energy and the peak load increase as the loading rate increases. Furthermore, such a trend is relatively mild under low rates. The gain of the fracture energy and peak load is around 10% compared with its quasi-static values. It could be attributed to viscous effects mainly originated by the presence of free water in voids and porous structures in the matrix, and the weak fibre pullout resistance. However, under high rates the increases in the fracture energy and the peak load are pronounced due to the inertia effect and the greater fibre pullout energy. The dynamic increase factors of the peak load and the fracture energy for the fastest loading rate are approximately 3.5 and 2.5, respectively.

Effect of discrete fibre reinforcement on soil tensile strength

The tensile behaviour of soil plays a significantly important role in various engineering applications. Compacted soils used in geotechnical constructions such as dams and clayey liners in waste containment facilities can suffer from cracking due to tensile failure. In order to increase soil tensile strength, discrete fibre reinforcement technique was proposed. An innovative tensile apparatus was developed to determine the tensile strength characteristics of fibre reinforced soil. The effects of fibre content, dry density and water content on the tensile strength were studied. The results indicate that the developed test apparatus was applicable in determining tensile strength of soils. Fibre inclusion can significantly increase soil tensile strength and soil tensile failure ductility. The tensile strength basically increases with increasing fibre content. As the fibre content increases from 0% to 0.2%, the tensile strength increases by 65.7%. The tensile strength of fibre reinforced soil increases with increasing dry density and decreases with decreasing water content. For instance, the tensile strength at a dry density of 1.7 Mg/m3 is 2.8 times higher than that at 1.4 Mg/m3. It decreases by 30% as the water content increases from 14.5% to 20.5%. Furthermore, it is observed that the tensile strength of fibre reinforced soil is dominated by fibre pull-out resistance, depending on the interfacial mechanical interaction between fibre surface and soil matrix.

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.

Nominal Flexural Strength of High Strength Fiber Reinforced Concrete Beams

This paper presents the development of simple semi-empirical formulae for the analysis of nominal flexural strength of high strength steel fiber reinforced concrete (HSFRC) beams. Such developed formulae were based on strain compatibility and equilibrium conditions for fully and partially HSFRC sections in joint with suitable idealized compression and tension stress blocks. The stress blocks were given by suitable empirical functions for the compressive and post-cracking strengths of HSFRC. The enhancement in compressive strength due to fibers inclusion is proposed as a function of concrete matrix strength and fiber reinforcing index. To account for the pullout resistance of fibers in tension, the post-cracking strength was evaluated as a function of fiber reinforcing index and bond strength. The fiber reinforcing index was considered as a function of volume content, aspect ratio, orientation and length efficiency factors of the fibers. In view of the degenerative nature of the pullout resistance of the fibers with the increase of crack width, a limit was placed on the useful tensile strain extent, depending on fiber length and crack spacing. It was found that there is a good agreement between the flexural strengths for HSFRC beams predicted by the proposed formulae and the experimental results reported in the literature, while the predicted flexural strengths as computed by ACI Committee 544.4R (ACI Struct J 85:563–580, 1988) and ACI Committee 544.1R (ACI Struct J 94:1–66, 1997) were very conservative. The parametric studies indicate that the nominal moment section capacity increases with the increase of fiber content and fiber aspect ratio.

Mechanical behaviour of strain hardening cement-based composites under impact loading

This paper describes the material behaviour of a strain hardening cement-based composite (SHCC) at high strain rates. The results of highly dynamic spall experiments using a Hopkinson bar at strain rates 140–180 s −1 are arrayed against the results of quasi-static uniaxial tensile tests at strain rates of 0.001 s −1. This comparison is based on the values of tensile strength, Young’s modulus, and fracture energy of the specimens. In addition, the experimental results of SHCC are related to the characteristic values of other concrete types. Differences in material behaviour are explained by the phenomena of crack formation and fibre pullout resistance.

ＰＥＴ繊維・鋼繊維補強モルタルの一軸引張特性

Concrete or mortar is one of the fragile materials that possess low tensile strength and relatively high compressive strength. The addition of fibers randomly distributed in the matrix, substantially improves their mechanical performance such as post-cracking strength in compression, strength/toughness (energy required up to failure) in bending, impact strength, fatigue strength and also wear resistance. Fiber reinforced concrete/mortar (FRC/FRM) had been gaining various application fields; tunnel support, dam site, river facilities, road pavement and floor in building.In this study, uniaxial tension tests of steel/PET fiber reinforced mortar (SFRM/PET-FRM) were conducted by servo-controlled testing machine to obtain stress-displacement curves. The experimental results can be summarized;1) After mortar was cracked, stress induced by pullout resistance of fibers was observed.2) The displacement of PET-FRM at which stress took the post-cracking peak value (post-cracking strength) was considerably larger than that of SFRM.3) The stress/force-displacement curves of SFRM/PET-FRM can be estimated by the results of a single steel/PET fiber pullout tests.4) As far as SFRM is concerned, the number of fibers observed on the fractured surface is smaller than that on mechanically sectioned surface. In case of PET-FRM, no difference is found in the number of fibers between the fractured and the sectioned surface.

コンクリート補強用ＰＥＴ繊維とポリプロピレン繊維の引抜特性

Fiber reinforced concrete (FRC) or mortar (FRM) is one of the future promising composite materials and suits, for example, today's rapid excavation by a tunnel boring machine, a boom header or a continuous miner. It is well known that FRC or FRM is improved in toughness and crack resistance mainly by the pullout resistance of fibers. Though some pullout tests have been published, fundamental data concerning pullout resistance are still insufficient and pullout process of fibers is not clear.In this study, pullout test was conducted pulling a single PET or polypropylene fiber out of the mortar sample at a constant displacement rate by a servo-controlled testing machine. The maximum pullout resistance of PET / polypropylene fiber was considerably lower than that of steel fiber. The displacement at the peak value was larger than that of steel fiber. The coefficient of variation of the maximum pullout resistance was smaller than that of steel fiber. After the peak pullout resistance, pullout resistance-displacement curve was decreasing linearly with displacement though undulations synchronized with the indent interval were observed.