Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete

In this paper single fiber pull-out performance of high strength steel fibers embedded in ultra-high performance concrete (UHPC) is investigated. The research emphasis is placed on the experimental performance at various pullout rates to better understand the dynamic tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC). Based on the knowledge that crack formation is strain rate sensitive, it is hypothesized that the formation of micro-splitting cracks and the damage of cement-based matrix in the fiber tunnel are mainly attributing to the rate sensitivity. Hereby, different pull-out mechanisms of straight and mechanically bonded fibers will be examined more closely. The experimental investigation considers four types of high strength steel fibers as follows: straight smooth brass-coated with a diameter of 0.2 mm and 0.38 mm, half end hooked with a diameter of 0.38 mm and twisted fibers with an equivalent diameter of 0.3 mm. Four different pull out loading rates were applied ranging from 0.025 mm/s to 25 mm/s. The loading rate effects on maximum fiber tensile stress, use of material, pullout energy, equivalent bond strength, and average bond strength are characterized and analyzed. The test results indicate that half-hooked fibers exhibit highest loading rate sensitivity of all fibers used in this research, which might be attributed to potential matrix split cracking. Furthermore, the effect of fiber embedment angles on the loading rate sensitivity of fiber pullout behavior is investigated. Three fiber embedment angles, 0°, 20°, and 45°, are considered. The results reveal that there is a correlation between fiber embedment angle and loading rate sensitivity of fiber pullout behavior.     

Multi-scale investigation of microstructure,fiber pullout behavior,and mechanical properties of ultra-high performance concrete with nano-CaCO3 particles

The mechanical properties of a fiber-reinforced concrete are closely related to the properties of the matrix, fiber, and fiber-matrix interface. The fiber-matrix bond property is mainly governed by the adhesion between the fiber and surrounding cement materials, as well as the strength of materials at the interfacial transition zone. In this paper, the effect of nano-CaCO₃ content, varying between 0 and 6.4%, by mass of cementitious materials, on microstructure development, fiber-matrix interfacial bond properties, and mechanical properties of ultra-high performance concrete (UHPC) reinforced with 2% steel fibers were investigated. The bond properties, including bond strength and pullout energy, were evaluated. Mercury intrusion porosimetry (MIP), backscattered electron microscopy (BSEM), optical microscopy, and micro-hardness testing were used to characterize the microstructure of matrix and/or interfacial transition zone (ITZ) around an embedded steel fiber. Test results indicated that the incorporation of 3.2% nano-CaCO₃ significantly improved the fiber-matrix bond properties and the flexural properties of UHPC. This was attributed to densification and strength enhancement of ITZ as observed from micro-structural analyses. Beyond the nano-CaCO₃ content of 3.2%, the fiber bond and mechanical properties of UHPC decreased due to increased porosity associated with agglomeration of the nano-CaCO₃.     

Simulation of single fiber pullout response with account of fiber morphology

A finite element model was developed at the single fiber length scale to predict the quasi-static pullout response of individual fibers from cementitious composites. The model accounts for energy dissipation through granular flow of the interfacial transition zone (ITZ) and matrix, plastic work in the fiber, and frictional dissipation at the fiber–ITZ interface. The considered fiber morphology was a triangular cross section that had been uniformly twisted along the fiber length. The model was calibrated to published experimental data for fiber pitches of 12.7 and 38.1 mm/revolution pulled from cement mortar with a 44-MPa unconfined compressive strength. The model was used to investigate slip-hardening behavior, tunneling of the cement mortar, in situ pullout behavior of helically twisted fibers at a crack plane, and provide an alternate explanation for the pullout response of twisted fibers from a 84-MPa unconfined compressive strength matrix containing silica fume. Calculations show that twisted fibers can provide up to 5 times the peak pullout force and 10 times the total work compared with straight fibers and infer work-hardening behavior during fiber pullout. The findings indicate that the tailoring of fiber morphology and control of constituent properties are important avenues for achieving significant improvements in the performance of fiber-reinforced cementitious composites.     

Crack/fiber interaction and crack growth resistance behavior in microfiber reinforced mortar specimens

Performance enhancement due to microfibers is well known. However, fracture processes that lead to strain hardening behavior in microfiber reinforced composites are not well understood. Crack growth resistance behavior of mortar reinforced with steel microfibers and polypropylene microfibers was investigated in-situ during load application. The polypropylene fibers were inter-ground in the cement mill to enhance the fiber/matrix interfacial frictional stress. A more homogeneous fiber distribution was observed in the inter-ground polypropylene composites compared to the steel microfiber reinforced composites. In steel microfiber reinforced composites the dominant toughening mechanisms were multiple microcracking and successive debonding along the fiber/matrix interface. Fiber pullout, the dominant mechanism in conventional macrofiber reinforced composites was rarely observed. In-situ observation of crack/fiber interaction in the inter-ground polymer fibers also revealed multiple microcracking along the length of the fibers followed by fiber pullout.     

钢纤维对掺花岗岩石粉UHPC的增强增韧:磷酸锌改性和纤维形状的影响及机理

采用自制的单根钢纤维拉拔试验装置等,通过拉拔试验和SEM-EDS等试验,开展钢纤维的磷酸锌(ZnPh)改性及其形状对在蒸压养护条件下的掺花岗岩石粉超高性能混凝土(UHPC)增强增韧影响机理的研究。所研究钢纤维形状包括:镀铜平直型S、镀铜单折线端钩型G1、镀铜双折线端钩型G2和镀铜波浪型L。研究表明,钢纤维的机械咬合力起主导作用,钢纤维平均粘结强度与拔出功大小顺序均为:G1G2LS。ZnPh改性后,钢纤维表面变粗糙,这增强了钢纤维与UHPC基体间的化学粘结力和静摩擦力,从而提高了钢纤维在UHPC中的平均粘结强度和拔出功。在UHPC韧性的提高方面,采用ZnPh改性,对S钢纤维最明显,而对异型钢纤维(G1、G2和L)则不明显。     

Pullout resistance of straight NiTi shape memory alloy fibers in cement mortar after cold drawing and heat treatment

In our study, we found cold drawing to be an effective method for enhancing the pullout resistance of NiTi shape memory alloy (SMA) fibers in concrete. The pullout resistance was observed to be dependent on the contact pressure and friction coefficient at the interface between the fibers and the mortar matrix. The drawing process increased the stiffness and yield stress of the fibers and consequently increased the contact pressure at the interface between the fibers and the mortar matrix. Moreover, heat treatment of the fibers after cold drawing was found to noticeably recover the fiber diameter, thereby significantly enhancing the pullout resistance. The enhancement of the interfacial bond strength by heat treatment verified the crack-closing capabilities of SMA-fiber-reinforced cement composites.     

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%.     

异型钢纤维拉拔界面模型

主要提出了一种新型的异型纤维拉拔界面模型,完成了整个拉拔过程中拉拔荷载与拉拔位移的对应计算。同传统的异型纤维拉拔计算模型相比,本模型不仅考虑到了拉拔过程中由于纤维的塑性变形而且考虑到了界面压力变化导致的界面摩擦变化所引起的荷载阻抗,此外,模型中还量化模拟了基体材料的剥落损伤行为及其对载荷释放的影响。最后从计算模拟结果与试验结果的对比来看,本模型具有较好的适应性。     

Fiber-end deformation effects in enlarged-end, fiber-reinforced composites

Composite materials reinforced by fibers with enlarged ends are known to have significantly better strength and toughness than those reinforced by flat-end fibers. The objective of this study is to develop an analytical model to determine the importance of deformation of the enlarged end on the reinforcement performance of ellipsoidal enlarged-end fibers. The resisting pullout load of the fiber is composed of a component due to interfacial bond at the fiber/matrix interface and a component due to mechanical anchorage at the embedded enlarged end of the fiber. The component due to mechanical anchorage at the enlarged end is due to both mechanical interlock and deformation of the enlarged end. In the past, little has been done to account for the deformation of the enlarged end. To account for this component of the mechanical anchorage resistance at the embedded enlarged end, a spring component is introduced to connect the embedded fiber with the enlarged ellipsoid. Analytical solutions were derived to predict the effects of the rigid enlarged end shape on the pullout load and stress distribution. These solutions were then compared to finite element solutions. It is shown that the shape of enlarged end has a significant influence on the stress distribution of the short fiber. Specially, the model demonstrates that the enlarged ends deform significantly for some shapes and are not effective for long fibers.     

Predicting the pullout response of inclined straight steel fibers

Fiber pullout tests have been used for decades to characterize and optimize bond strength on fiber reinforced concretes. However most of the investigations focus on the behavior of fibers aligned with load direction whose pullout mechanisms are not representative of the ones existing in real applications, where random orientation of fibers is likely to occur. In this paper a new predictive model for the pullout response of steel fibers embedded in cement matrices with any inclination respect to loading direction is provided. Comparisons with experimental results highlight the capacity of the model on describing appropriately the entire load–crack width behavior. The procedure differentiates itself from previous works by introducing clear and comprehensive concepts within a straightforward approach.     

Pullout tests of epoxy-coated reinforcement in concrete

The bond strength and slip of epoxy-coated reinforcing bars in concrete have been evaluated by carrying out single pullout and double pullout tests. In extended single pullout tests, slip measurements were made while tensile force was applied to reinforcing bars embedded in concrete. In double pullout tests, 20 cycles of load were applied at levels of steel stress between zero and 0·5 times characteristic steel strength. Strains were measured by electrical resistance strain gauges glued inside the bars. Both epoxy-coated and uncoated bars were used in the investigation, to obtain comparative results. The strain gradient along the bar was found to be less for the coated reinforcement. In general, the epoxy coating was found to increase slip in bond and thereby reduce the bond performance of coated bars.     

Bond behavior of GFRP and steel bars in ultra-high-performance fiber-reinforced concrete

The bond behavior of glass fiber-reinforced polymer (GFRP) and steel bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated according to embedment length and bar diameter. Post-peak bond stress-slip softening curve of the GFRP bars was obtained, and a wedging effect was quantitatively evaluated. Test results indicated that a normalized bond strength of 5 was applicable for steel bars embedded in UHPFRC, and the development lengths of normal- and high-strength steel bars were determined to be 2 and 2.5 times the bar diameter, respectively. The GFRP bars exhibited approximately 70% lower bond strength than the steel bars, and the bond stress additionally applied by the wedging effect increased almost linearly with respect to the slip. Based on dimensionless bond stress and slip parameters, an appropriate theoretical model for the bond stress and slip relationship of steel bars in UHPFRC was suggested, and it was verified through comparison with the test data.     

变形钢筋/超高性能混凝土搭接黏结性能

超高性能混凝土(UHPC)是一种高强度、高韧性和高耐久性的水泥基复合材料。为了研究钢筋/UHPC的搭接黏结性能,进行了21组考虑搭接长度、纤维掺量和配箍率影响的钢筋搭接对拉拔出试验,3组考虑锚固长度影响的钢筋直接拔出锚固试验;试验出现了劈裂拔出破坏和钢筋拉断破坏2种破坏模式;钢筋/UHPC平均黏结强度随钢筋埋置长度的增大而减小,随配箍率的增大而增大;钢纤维掺量的增大,有利于增大对UHPC的约束作用,增加配箍率和适当增大纤维掺量均能减小钢筋/UHPC的临界搭接长度;结合前人的试验结果,拟合得到平均锚固和搭接黏结强度计算公式及临界锚固和搭接长度计算公式,根据混凝土结构设计规范,建立了钢筋/UHPC锚固和搭接长度简化算法,计算结果较为准确。      

钢纤维类型对超高性能混凝土高温爆裂性能的影响

为了探寻可以有效改善超高性能混凝土（Ultra-high-performance concrete,UHPC）抗火性能的钢纤维类型,本文试验测定了不同类型钢纤维（3种普通钢纤维和2种来自于废旧轮胎的再生钢纤维）增韧UHPC及空白组混凝土的从常温至800℃高温爆裂行为和断裂能。结果显示,未掺入任何钢纤维的空白组UHPC试件全都发生了严重高温爆裂,钢纤维可以显著减轻其高温爆裂但却不能避免爆裂的发生,而掺入端钩型普通工业钢纤维（长度为35 mm,直径为0.55 mm）的UHPC呈现出最优的抗高温爆裂性能,其次是掺入未附着橡胶颗粒（RSF）的再生钢纤维（RSFR）增韧UHPC。可见,钢纤维自身性能特征显著影响了钢纤维增韧UHPC的高温爆裂,相同掺量情况下混凝土单位体积内分布密度较大的钢纤维或者分布密度较小但可以显著增加混凝土断裂韧性（断裂能）的钢纤维比较适合应用于具有较高抗火要求的UHPC结构中。     

硅烷偶联剂对玻璃织物/水泥复合材料界面行为的影响

本文通过织物抽拔试验和抽拔试验后残留在水泥基体中的纤维表面形貌扫描电镜照片分析,研究了在玻璃纤维织物表面涂覆硅烷偶联剂或涂蜡,对它增强的水泥基复合材料界面行为的影响.实验结果表明:在玻璃纤维织物表面涂覆硅烷偶联剂有利于它与水泥基体间的界面粘结,改善它们间的界面行为,在玻璃纤维织物表面涂蜡则不利于它与水泥基体间的界面粘结.     

Interfacial changes of optical fibers in the cementitious environment

Embedded optical fiber sensors have recently been employed for strain and crack monitoring in concrete structures. The performance of the sensor is strongly affected by the fiber/matrix interface. For strain monitoring, effective stress transfer between fiber and matrix is required. A high interfacial bond is therefore desirable. On the other hand, crack sensing may rely on fiber debonding and bending, which is only possible with a weak interfacial bond. In the cementitious environment, the interfacial properties are known to vary with time, and this may affect the long-term performance of embedded optical sensors. The objective of the present investigation is to study the interfacial changes when specimens containing embedded optical fibers (with different coatings) are subject to different environmental conditions including wet curing, wetting/drying and freezing/thawing. Fibers removed from the matrix are examined under the SEM. Also, fiber pull-out specimens are prepared and tested. The results show that the fiber pull-out test can reveal significant changes in interfacial behaviour that cannot be detected from SEM examination. The pull-out test is therefore demonstrated to be a useful technique for the characterization of time dependent interfacial behavior for embedded optical fiber sensors.     

Effects of poly(vinyl alcohol) on fiber cement interfaces. Part I: Bond stress-slip response

This is the first part of a two-part article describing the effects of adding poly(vinyl alcohol) (PVA) to a cement based matrix to improve the bond at the fiber-matrix interface. Two types of fibers were used, steel and brass fibers (simulating brass-coated steel fibers) in a series of pull-out tests where the load versus global slip up to complete pull-out was recorded. The measured slips was that at the section where the fiber penetrates the matrix. The first article describes the mechanical effects of the addition of PVA, while the second article presents the microscopic observations. Correlation between the two studies is pointed out in the second part and conclusions are drawn. In particular, it is observed that the addition of PVA in the amount of 1.4% by weight of cement matrix leads to a significant improvement in the bond strength as well as in the frictional resistance, thus pull-out work, after the peak load.     

非直纤维的细观动态拔出模型分析

在Chanvillard关于非直纤维静态拔出模型基础上,建立非直纤维的细观动态模型。此模型通过纤维与基体接触界面剪应力的传递考虑纤维的动态拔出过程,并考虑应变率效应。同时,适当选取阈值,建立基于细观损伤演化的纤维脱粘和拔出动态模型。此模型可以很好地模拟Chanvillard关于非直纤维的静态实验结果,并能反映复合材料的应变率影响。      

Rebar pullout testing under dynamic Hopkinson bar induced impulsive loading

The dynamic behavior of the bond slip between a deformed reinforcing bar and plain concrete has been experimentally investigated by employing Hopkinson bar techniques. Pullout tests with various specimen types (unconfined, confined, cast-in-place, post-installed etc.) have been performed. Pullout of the steel rebar and splitting of the concrete cylinder have been the failure modes induced. Test results comprise peak pullout forces and complete bond stress–slip diagrams. They clearly show that the dynamic pullout forces and curves are well above the static ones, and that the pullout work of bond failure is considerably greater for the dynamic impact loading. Confinement, provided by a steel tube, leads to improved bonding; peak loads increase up to 2.5 times. The effects of bond length and concrete strength have also been put into evidence. Finally it has been verified that post-installed rebars, depending upon the particular adhesive employed, can achieve the same bond resistance as the cast-in-place ones.     

Pull-out work of steel fibers from cementitious composites: Analytical investigation

The main objective of this study is to provide a parametric evaluation of the pull-out work of smooth steel fibers embedded in cementitious matrices. The various parameters controlling the behavior of the bond stress versus slip relationship are analyzed; their effects on the entire pull-out load versus slip response and the corresponding pull-out energy up to full debonding and/or up to total pull-out are investigated. Also discussed are the effects of the fiber geometric parameters such as fiber diameter, length, and aspect ratio. Finally, a brief section addresses the relation between pull-out work and the critical strain energy release rate G_c, a fracture mechanics variable, which can be calculated if the P-Δ curve is known.