Fatigue strength of steel fibre reinforced concrete in flexure

The paper presents a study on the fatigue strength of steel fibre reinforced concrete (SFRC). An experimental programme was conducted to obtain the fatigue-lives of SFRC at various stress levels and stress ratios. Sixty seven SFRC beam specimens of size 500×100×100 mm were tested under four-point flexural fatigue loading. Fifty four static flexural tests were also conducted to determine the static flexural strength of SFRC prior to fatigue testing. The specimens incorporated 1.5% volume fraction of corrugated steel fibres of size 0.6×2.0×30 mm. Concept of equivalent fatigue-life, reported for plain concrete in literature, is applied to SFRC to incorporate the effects of stress level S, stress ratio R and survival probability L_R into the fatigue equation. The results indicate that the statistical distribution of equivalent fatigue-life of SFRC is in agreement with the two-parameter Weibull distribution. The coefficients of the fatigue equation have been determined corresponding to different survival probabilities so as to predict the flexural fatigue strength of SFRC for the desired level of survival probability.     

Residual flexural behavior of fiber reinforced concrete after heating

In this study, the effects of fire on the flexural performance and residual strength of plain and fiber reinforced concrete are investigated. Three types of concrete are tested: plain, polypropylene (PFRC) and steel fiber reinforced concrete (SFRC). Prior to the flexural test, the specimens were exposed to fire for 15, 30, 45, and 60 min on a furnace. The burnt specimens were then tested under flexural load to measure their toughness and residual strength. Results indicate the reduction of flexural strength for both plain and FRC after being subjected to fire. For FRC, the effect of fire on the flexural response depends mainly on the fiber type and fire exposure duration. For PFRC, the flexural strength is found to drop significantly for every exposure duration, while toughness is found to increase at short exposure duration and then, drop quickly after long exposure duration due to the fiber evaporation effect. For SFRC, the flexural strength and toughness are found to drop gradually for every exposure duration due to the deterioration of cement paste and reduction in bond strength. SFRC exhibits a more consistent ability to maintain load carrying capacity after long exposure to fire than PFRC.     

Fatigue behaviour of steel fiber reinforced concrete

The results of an experimental investigation on the fatigue characteristics and residual strength of steel fiber reinforced concrete (SFRC) are reported. The testing program included flexural specimens as well as split-cylinders and cubes reinforced with two fiber types at a low volume content. One of the fibers was of the deformed slit-sheet type available at aspect ratios of 45 and 60. It is shown that SFRC has a better fatigue response than plain concrete and that the deformed slit-sheet fiber has an effect almost identical to hooked-end fiber of similar dimensions. There is no increase in residual strength measured by split-tension when specimens are subjected to fatigue stress above the endurance limit. Fatigue characteristics of SFRC from this testing program as well as previous works can be interpreted as a function of the fiber factor (i.e. a parameter accounting for volume fraction, aspect ratio and fiber type) to provide design charts. More experimental work is needed to provide an acceptable database for fatigue design of SFRC.     

Response of steel fiber reinforced high strength concrete beams: Experiments and code predictions

The shear-flexure response of steel fiber reinforced concrete (SFRC) beams was investigated.Thirty-six reinforced concrete beams with and without conventional shear reinforcement (stirrups) were tested under a four-point bending configuration to study the effectiveness of steel fibers on shear and flexural strengths, failure mechanisms, crack control, and ductility.The major factors considered were compressive strength (normal strength and high strength concrete up to 100 MPa), shear span-effective depth ratio (a/d = 1.5, 2.5, 3.5), and web reinforcement (none, stirrups and/or steel fibers).The response of RC beams was evaluated based on the results of crack patterns, load at first cracking, ultimate shear capacity, and failure modes.The experimental evidence showed that the addition of steel fibers improves the mechanical response, i.e., flexural and shear strengths and the ductility of the flexural members.Finally, the most recent code-based shear resistance predictions for SFRC beams were considered to discuss their reliability with respect to the experimental findings. The crack pattern predictions are also reviewed based on the major factors that affect the results.     

BFRP筋钢纤维高强混凝土梁受弯承载力试验与理论

通过11根玄武岩纤维增强聚合物复合材料（BFRP）筋钢纤维高强混凝土梁的受弯性能试验,研究了钢纤维混凝土层厚度、钢纤维体积分数和BFRP筋配筋率对BFRP筋钢纤维高强混凝土梁受弯破坏形态及其承载力的影响。结果表明,BFRP筋钢纤维高强混凝土梁的破坏模式可分为受压破坏、受拉破坏和平衡破坏3种;钢纤维混凝土层厚度和钢纤维体积分数的变化对于BFRP筋钢纤维高强混凝土梁受弯承载力具有一定程度的影响,当BFRP筋配筋率为0.77%时,掺加体积分数为1.0%钢纤维的梁受弯承载力较无钢纤维梁提高了22.7%,在受拉区0.57倍截面高度内掺加1.0vol%钢纤维的梁受弯承载力达到全截面钢纤维混凝土梁受弯承载力的86.7%;增大BFRP筋配筋量可显著提高BFRP筋钢纤维高强混凝土梁的受弯承载力,BFRP筋配筋率为1.65%的试验梁受弯承载力较配筋率为0.56%的试验梁提高了39.4%。针对不同的破坏模式,提出了BFRP筋钢纤维高强混凝土梁受弯承载力和平衡配筋率的计算方法,并结合安全配筋率的概念对试验梁的破坏模式进行了预测,试验结果与分析结果吻合良好。     

Preliminary study on bullet resistance of double-layer concrete panel made of rubberized and steel fiber reinforced concrete

In this study, the impact resistance of double-layer concrete panels made of rubberized and steel fiber reinforced concrete subjected to direct fire weapon (11 mm or 0.45 magnum bullet size) is investigated. Concrete panels with dimensions of 400 × 400 × 50 mm are subjected to impact forces from 11 mm-diameter bullets at a distance of 10 m. Three types of concrete panels are tested: single-layer steel fiber reinforced concrete (SFRC), single-layer crumb rubber concrete (CRC), and double-layer CRC/SFRC. For a double-layer CRC/SFRC, the CRC layer of 12.5 mm is added to the front surface by partially replacing part of the SFRC panel. The CRC layer is expected to act as a cushion layer to absorb impact energy from the bullet and to reduce damage to the concrete panel.     

An experimental study on the tensile strength of steel fiber reinforced concrete

This paper deals with steel fiber reinforced concrete mechanical static behaviour and with its classification with respect to fibers content and mix-design variations. A number of experimental tests were conducted to investigate uniaxial compressive strength and tensile strength. Different mixtures were prepared varying both mix-design and fiber length. Fibers content in volume was of 1% and 2%. Mechanical characterization was performed by means of uniaxial compression tests with the aim of deriving the ultimate compressive strength of fiber concrete. Four-point bending tests on notched specimens were carried out to derive the first crack strength and the ductility indexes. The tensile strength of steel fiber reinforced concrete (SFRC) was obtained both from an experimental procedure and by using an analytical modelling. The experimental tests showed the different behaviour of SFRC with respect of the different fiber content and length. Based on the experimental results, an analytical model, reported in literature and used for the theoretical determination of direct tensile strength, was applied with the aim of making a comparison with experimental results. The comparison showed good overall agreement.     

高强型钢超高性能混凝土梁受弯性能试验研究及有限元分析

为研究高强型钢超高性能混凝土梁的受弯性能,以配钢率、型钢位置和钢纤维体积分数为变化参数,设计了6个试件,并对其进行了静力加载试验,获得了试件的破坏形态和荷载-跨中挠度曲线,分析了试件的承载能力和变形能力,以及型钢、纵向钢筋和超高性能混凝土的应变变化规律。基于试验研究,建立了高强型钢超高性能混凝土梁受弯性能的有限元分析模型,计算结果与试验结果吻合较好,进而进行了参数分析。结果表明：所有试件均发生的是适筋破坏,纵向受拉钢筋和型钢下翼缘率先屈服,然后受压区超高性能混凝土被压碎;在试件的破坏阶段,所承担的荷载会依次经历陡降、波动、缓慢上升和缓慢下降四个阶段;试件的变形能力系数超过5,呈现出较强的变形能力;试件开裂前,超高性能混凝土的应变符合平截面假定,但开裂后,只有受压区和受拉区在中和轴附近的一小部分超高性能混凝土应变呈线性分布;配钢率和型钢强度增大,试件的承载能力和变形能力均提高;超高性能混凝土抗压强度增大及型钢从截面居中位置下移,试件的承载能力提高,但变形能力下降;钢纤维体积分数增加,试件的抗裂能力和变形能力均提高,但承载能力变化不显著。     

纳米SiO2对钢纤维/混凝土高温后力学性能及微观结构的影响

研究了掺纳米SiO₂的钢纤维混凝土(NSFC)、 钢纤维混凝土(SFRC)和普通混凝土(NC)三种材料在不同加热温度后的抗压、 劈裂和抗折强度等力学性能, 对不同温度热处理后的微观结构进行了SEM分析, 对钢纤维与过渡区界面的相结构进行了XRD分析。结果表明: 在测试温度范围内, NSFC的抗压、 劈裂和抗折强度均高于SFRC和NC的强度, 且在400 ℃时达到最大值。在常温下, NSFC的抗压、 劈裂和抗折强度较NC分别提高27.01%、 63.28%和54.12%, 400 ℃高温热处理后比NC分别高35.09%、 84.62%和87.23%; SEM分析表明, 在钢纤维与过渡区的界面处, 致密度提高, 显微硬度提高。由于固相反应, 使界面区结构发生变化, 在钢纤维表层形成扩散渗透层(白亮层), 即化合物层, 呈锯齿状, XRD分析证明, 白亮层主要由FeSi₂和复杂的水化硅酸钙组成, 从而增强了钢纤维与基体的粘结力, 提高了混凝土的高温力学性能。     

钢-聚丙烯纤维增强人造花岗岩复合材料的制备与性能

为深入研究钢-聚丙烯纤维增强人造花岗岩复合材料（钢-聚丙烯纤维/人造花岗岩）抗压、抗弯强度的影响因素,通过排水法实验研究了骨料堆积的空隙率,确定了骨料级配和实验指数q并对大量试件进行了抗压、抗弯强度测试,分析了钢-聚丙烯纤维/人造花岗岩复合材料各组分质量分数、骨料堆积空隙率等因素对钢-聚丙烯纤维/人造花岗岩复合材料抗压、抗弯强度的影响。实验结果表明:钢纤维与聚丙烯纤维能够明显增大钢-聚丙烯纤维/人造花岗岩复合材料的抗弯强度,随着钢-聚丙烯纤维质量分数的增加,钢-聚丙烯纤维/人造花岗岩复合材料试件的抗压和抗弯强度都逐渐增大;当钢纤维与聚丙烯纤维质量比为30∶1、钢-聚丙烯纤维质量分数为1.7wt%时,钢-聚丙烯纤维/人造花岗岩复合材料试件的抗压强度达到最大,当钢-聚丙烯纤维质量分数为1.9wt%时,钢-聚丙烯纤维/人造花岗岩试件的抗弯强度达到最大;黏结剂质量分数越接近骨料堆积空隙率,钢-聚丙烯纤维/人造花岗岩复合材料试件的抗压和抗弯强度越大,当骨料质量分数为80wt%、黏结剂质量分数为11wt%时,钢-聚丙烯纤维/人造花岗岩复合材料试件的抗压、抗弯强度同时达到最大。      

Effects of polypropylene twisted bundle fibers on the mechanical properties of high-strength oil palm shell lightweight concrete

In this study, the effects of a new type of non-metallic fiber (polypropylene twisted bundle (PPTB)) on the slump and mechanical properties of oil palm shell (OPS) concrete have been investigated. The results showed that increasing the volume fraction of PPTB fibers, it slightly decreases the workability and density of the concrete. It has found that the compressive strength of OPS concrete increases with increasing PPTB fiber volume fraction. The results revealed that the reinforcement of OPS concrete with steel and PPTB fibers reduces the strength loss of OPS concrete in poor curing environments. In addition, the fiber with low volume fraction (up to 0.25 %) is more efficient in improving the flexural strength of OPS concrete compared to its splitting tensile strength. The average modulus of elasticity (E value) is obtained to be 17.4 GPa for all mixes, which is higher than the values reported in previous studies and is within the range for normal weight concrete. The performance of the PPTB fibers is comparable to that for steel fibers at a volume fraction (V_f) of 0.5 %, which provides less dead load for lightweight concrete. The findings of this study showed that the PPTB fibers can be used as an alternative material to enhance the properties of OPS concrete. Hence, PPTB fibers are a promising alternative for lightweight concrete applications.     

高延性混凝土与带肋钢筋黏结性能试验研究

为研究高延性混凝土(HDC)与带肋钢筋的黏结性能,设计制作了20组试件。通过中心拔出试验,研究单调和重复荷载作用下试件的破坏形态及黏结滑移破坏机理,分析了HDC的抗压强度、纤维掺量、纤维种类及保护层厚度对带肋钢筋与HDC黏结性能的影响。结果表明：单调与重复荷载作用下,普通混凝土试件发生了脆性劈裂破坏,HDC试件发生劈裂和拔出破坏;试验结果表明,带肋钢筋与HDC的黏结强度随HDC抗压强度增加而提高;相同HDC抗压强度时,纤维掺量的增加可改善带肋钢筋与HDC的黏结性能;相比PP纤维,相同体积掺量的PE纤维与PVA纤维可有效限制混凝土内部径向裂缝的开展,提高带肋钢筋与HDC的黏结强度;依据破坏形态和黏结强度,得出临界相对保护层厚度为2.0;单调与重复荷载作用下,试件黏结强度比由纤维种类和相对保护层厚度主导,残余黏结强度比由抗压强度和纤维掺量主导;根据试验结果,建立了带肋钢筋与HDC的黏结强度计算公式和黏结-滑移本构模型。     

Thermo-mechanical properties of fiber reinforced raw perlite concrete

The effect of hooked steel, wavy steel and polypropylene fibers on the thermo-mechanical properties of raw perlite aggregate concrete was investigated. In order to determine the effect of fiber ratio on the thermo-mechanical properties of 100% raw perlite concrete, 0.25%, 0.75%, 1.25%, and 1.75% fiber ratios were used by volume of the sample and also, 350 kg/m³ cement dosage and 3 ± 1 cm slump were used. When compared to the control sample that contains no fiber, (1) with the increase of steel fiber ratio in the mixtures thermal conductivity (TC), unit weight, splitting-tensile strength, and flexural strength of concretes increased, (2) with the increase of steel fiber ratio in the mixtures compressive strength of concretes decreased, and (3) with the increase of polypropylene fiber ratio in the mixtures TC, unit weight, compressive strength, splitting-tensile strength, and flexural strength of concretes decreased.     

钢纤维混凝土多轴损伤比强度准则EI北大核心CSCD

以损伤比强度理论为基础,建立了钢纤维混凝土真三轴损伤比强度准则,并根据钢纤维混凝土试验资料,推荐了钢纤维混凝土损伤比变量表达式中的6个经验参数。利用钢纤维混凝土在单轴、双轴和三轴受力状态下的应力-应变曲线试验结果验证了损伤比取值合理性,对比了单轴受拉、单轴受压和双轴等压等典型受力状态下钢纤维混凝土和普通混凝土损伤比变量取值的差异。通过与国内外共104组钢纤维体积率为0.5%~2.5%的钢纤维混凝土三轴强度试验资料的比较,表明六经验参数钢纤维混凝土损伤比强度准则的三维破坏包络面接近已有认识;通过与国内外强度准则比较,表明损伤比强度准则与钢纤维混凝土三轴试验数据有较高的吻合度。对于围压三轴受力状态,提出简化的钢纤维混凝土常规三轴强度准则,并与已有常规三轴强度准则进行比较分析。此外,对于材料处于二轴受力,推荐了简化的损伤比二轴强度准则中的经验参数取值。     

Flexural behavior of geopolymer composites reinforced with steel and polypropylene macro fibers

Like ordinary Portland cement concrete, the matrix brittleness in geopolymer composites can be reduced by introducing appropriate fiber reinforcement. Several studies on fiber reinforced geopolymer composites are available, however there is still a gap to understand and optimize their performance. This paper presents the flexural behavior of fly ash-based geopolymer composites reinforced with different types of macro steel and polypropylene fibers with higher aspect ratio. Three types (length-deformed, end-deformed and straight) of steel fibers and another type of length-deformed polypropylene fiber with optimum fiber volume fraction of 0.5% are studied. The effects of different geometries of the fibers, curing regimes (ambient cured and heat cured at 60 °C for 24 h) and concentration of NaOH activator (10 M and 12 M) on the first peak strength, modulus of rupture and toughness of the geopolymer composites are investigated. The quantitative effect of fiber geometry on geopolymer composite performance was also analyzed through a fiber deformation ratio. The compressive strength, splitting tensile strength and flexural toughness are significantly improved with macro fibers reinforcement and heat curing. The results also show that heat curing increases the first peak load of all fiber-reinforced geopolymers composites. End-deformed steel fibers exhibit the most ductile flexural response compared to other steel fibers in both heat and ambient-cured fiber reinforced geopolymer composites.     

Observations on the electrical resistivity of steel fibre reinforced concrete

Steel fibre reinforced concrete (SFRC) is in many ways a well-known construction material, and its use has gradually increased over the last decades. The mechanical properties of SFRC are well described based on the theories of fracture mechanics. However, knowledge on other material properties, including the electrical resistivity, is sparse. Among others, the electrical resistivity of concrete has an effect on the corrosion process of possible embedded bar reinforcement and transfer of stray current. The present paper provides experimental results concerning the influence of the fibre volume fraction and the moisture content of the SFRC on its electrical resistivity. The electrical resistivity was measured by alternating current (AC) at 126 Hz. Moreover, an analytical model for the prediction of the electrical resistivity of SFRC is presented. The analytical model is capable of predicting the observed correlation between the fibre volume fraction and the electrical resistivity of the composite (the SFRC) for conductive fibres and moisture saturated concrete. This indicates that the steel fibres were conducting when measuring the electrical resistivity by AC at 126 Hz. For partly saturated concrete the model underestimated the influence of the addition of fibres. The results indicate that the addition of steel fibres reduce the electrical resistivity of concrete if the fibres are conductive. This represents a hypothetical case where all fibres are depassivated (corroding) which was created to obtain a conservative estimate on the influence of fibres on the electrical resistivity of concrete. It was observed that within typical ranges of variation the influence of the moisture content on the electrical resistivity was larger than the effect of addition of conductive steel fibres, but also that the relative impact on the electrical resistivity due to conductive steel fibres increased when the moisture content of the concrete was reduced.     

Material and bond properties of ultra high performance fiber reinforced concrete with micro steel fibers

For investigating the effect of fiber content on the material and interfacial bond properties of ultra high performance fiber reinforced concrete (UHPFRC), four different volume ratios of micro steel fibers (V_f = 1%, 2%, 3%, and 4%) were used within an identical mortar matrix. Test results showed that 3% steel fiber by volume yielded the best performance in terms of compressive strength, elastic modulus, shrinkage behavior, and interfacial bond strength. These parameters improved as the fiber content was increased up to 3 vol.%. Flexural behaviors such as flexural strength, deflection, and crack mouth opening displacement at peak load had pseudo-linear relationships with the fiber content. Through inverse analysis, it was shown that fracture parameters including cohesive stress and fracture energy are significantly influenced by the fiber content: higher cohesive stress and fracture energy were achieved with higher fiber content. The analytical models for the ascending branch of bond stress-slip response suggested in the literature were considered for UHPFRC, and appropriate parameters were derived from the present test data.     

Long term properties under marine exposure of steel fibre reinforced concrete containing pfa

The paper presents results on the long-term mechanical properties and durability under marine exposure of a steel fibre reinforced concrete (SFRC) mix containing pulverized fuel ash (PFA) which was developed for marine applications. The mix was of proportions by weight of PFA:OPC:fine aggregate:coarse aggregate of 0.26:0.74:1.51:0.84 with a water/(OPC+PFA) ratio of 0.4. The resulting cement content of the mix was 435 kg m⁻³. Theconcrete was reinforced with low-carbon steel, corrosion-resistant (galvanized) or melt-extract (stainless) steel fibres. Prism specimens were cured in the tidal zone at Aberdeen beach, under wet-dry cycles of sea-water spray in the laboratory, in a water-tank in the laboratory and in the laboratory air. The specimens were cured for up to 1200 marine cycles of exposure (640 days) and were tested at regular intervals of age. The paper presents results on long-term compressive strength, flexural strength and energy absorption capacity as measured from the load-deflection curves. The state of corrosion of fibres is also described. The results indicate that fibres embedded within concrete remain free from corrosion under marine exposure. In the case of fibres exposed at the concrete surface during casting, extensive corrosion occurs in low-carbon steel fibres, isolated rust spots appear in corrosion-resistant fibres and no corrosion is evident in melt-extract fibres. This corrosion, however, remains a surface phenomenon and does not penetrate the concrete. The long-term mechanical properties indicate no deterioration due to possible corrosion. In general the compressive strength of concrete increases significantly with fibre reinforcement.Increases in flexural strength and post-cracking ductility due to fibre reinforcement are of the order normally expected of SFRC.     

局部高密度钢纤维混凝土弯曲疲劳损伤演变规律

在混凝土弯曲构件底部用高密度钢纤维局部增强称为局部高密度钢纤维混凝土(PHPFRC)与同样纤维掺量的传统钢纤维混凝土(SFRC)比较它可以用相近的价格获得高得多的刚度﹑承载力和抗疲劳断裂性能为了能够预测PHPFRC在循环荷载作用下的疲劳寿命需要确定其疲劳损伤演变规律本文通过试验发现局部高密度钢纤维混凝土的弯曲疲劳损伤表现出韧性材料所具有的性质基本上接近韧性损伤这与素混凝土及传统钢纤维混凝土有本质的不同根据其疲劳特性的实验结果探讨了PHPFRC弯曲疲劳损伤阈值结合损伤理论建立了适合于纤维体积掺量为1.2%的局部高密度钢纤维混凝土试件的弯曲疲劳损伤演变方程     

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress–strain response of high strength concrete and the nonlinear stress–strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model, tension stiffening and bond slip between concrete and reinforcement (fibers, prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However, beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams.