Rate-dependent tensile behavior of high performance fiber reinforced cementitious composites

High Performance Fiber Reinforced Cementitious Composites (HPFRCC) show strain hardening behavior accompanied with multiple micro-cracks under static tension. The high ductility and load carrying capacity resulting from their strain hardening behavior is expected to increase the resisting capacity of structures subjected to extreme loading situations, e.g., earthquake, impact or blast. However, the promise of HPFRCCs for dynamic loading applications stems from their observed good response under static loading. In fact, very little research has been conducted to investigate if their good static response translates into improved dynamic response and damage tolerance. This experimental study investigates the tensile behavior of HPFRCC using High strength steel fibers (High strength hooked fiber and twisted fiber) under various strain rates ranging from static to seismic rates. The test results indicate that the tensile behavior of HPFRCC using twisted fiber shows rate sensitivity while that using hooked fiber shows no rate sensitivity. The results also show that rate sensitivity in twisted fibers is dependent upon both fiber volume fraction and matrix strength, which influences the interface bond properties.     

Size and geometry dependent tensile behavior of ultra-high-performance fiber-reinforced concrete

This research investigated direct tensile stress versus strain response of ultra-high-performance fiber-reinforced concrete (UHPFRC) with various sizes and geometries. The UHPFRC in this research contained 1% macro twisted and 1% micro smooth steel fibers by volume. The effects of gauge length, section area, volume and thickness of the specimens on the measured tensile response of the UHPFRC were experimentally discovered. The different sizes and geometries of specimens did not generate significant influence on the post cracking strength of UHPFRC whereas they produced clear effects on the strain capacity, energy absorption capacity and multiple cracking behavior of UHPFRC. The strain capacity, energy absorption capacity and the number of multiple micro cracks within unit length obviously decreased as the gauge length, section area and volume of UHPFRC specimens increased. In contrast, as the thickness of the specimen increased, different tendency was observed.     

Tensile characteristics and fracture energy of fiber reinforced and non-reinforced ultra high performance concrete (UHPC)

In the framework of this study, various mixtures of fiber reinforced and non-reinforced ultra high performance concrete (UHPFRC and UHPC) were produced and tested with focus on the determination of the fracture energy and its comparison to standard mechanical material parameters. For some mixtures a compressive strength of more than 300 MPa was reached still retaining good fresh characteristics of the UHPC. These mixtures were examined for properties of fresh and hardened concrete, focusing on tensile strength properties and fracture energy. The fracture energy was determined to describe the work capacity, i.e. the potential energy intake until the failure of the material. Thereby, a significant increase of the work capacity could be achieved by the addition of steel fibers. Furthermore, the impact of a vacuum treatment of the freshly mixed concrete in regard to fresh and hardened concrete characteristics as well as the influence of aftertreatment (heat treatment and water storage) on compressive and tensile properties of the UHPC was investigated.     

Preparation of C200 green reactive powder concrete and its static–dynamic behaviors

In this paper, a new type of green reactive powder concrete (GRPC) with compressive strength of 200 MPa (C200 GRPC) is prepared by utilizing composite mineral admixtures, natural fine aggregates, short and fine steel fibers. The quasi-static mechanical properties (mechanical strength, fracture energy and fiber–matrix interfacial bonding strength) of GRPC specimens, cured in three different types of regimes (standard curing, steam curing and autoclave curing), are investigated. The experimental results show that the mechanical properties of the C200 GRPC made with the cementitious materials consisting of 40% of Portland cement, 25% of ultra fine slag, 25% of ultra fine fly ash and 10% of silica fume, 4% volume fraction of steel fiber are higher than the others. The corresponding compressive strength, flexural strength, fracture energy and fiber–matrix interfacial bonding strength are more than 200 MPa, 60 MPa, 30,000 J/m² and 14 MPa, respectively. The dynamic tensile behavior of the C200 GRPC is also investigated through the Split Hopkinson Pressure Bar (SHPB) according to the spalling phenomena. The dynamic testing results demonstrate that strain rate has an important effect on the dynamic tensile behavior of C200 GRPC. With an increase of strain rate, the peak stress rapidly increases in the dynamic tensile stress–time curves. The C200 GRPC exhibits an obvious strain rate stiffening effect in the case of high strain rate. Finally, the mechanism of excellent static and dynamic properties gains of C200 GRPC is also discussed.     

Energy absorption capacity of a sustainable Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) in quasi-static mode and under high velocity projectile impact

This paper investigates the energy absorption capacity of a sustainable Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) in quasi-static mode and under high velocity projectile impact. The design of the sustainable concrete mixtures aims on achieving a densely compacted cementitious matrix with a relatively low binder amount, employing the modified Andreasen & Andersen particle packing model. The experiments on UHPFRC are performed using a 4-point bending test and high velocity projectile impact tests. The obtained results show that although the utilization of hybrid steel fibre enhances the mechanical properties of the developed UHPFRC, the application of fibres with hooked ends is crucial in improving the energy absorption capacity of the sustainable UHPFRC in quasi-static mode. However, under high velocity projectile impact, the UHPFRC mixture with hybrid fibres shows a much better energy absorption capacity than the one with hooked steel fibres only, particularly in resisting the scabbing at the rear surface. The intrinsic mechanisms for the energy absorption capacity of the sustainable UHPFRC in quasi-static mode and under high velocity projectile impact are studied and analysed.     

热处理工艺对某低合金超高强度钢性能的影响

对某低合金超高强度钢采用四种不同温度淬火,而后进行低温回火,测定其静态力学性能,并采用动态压缩和强迫剪切方法对其动态压缩性能进行详细的研究,分析淬火温度对其性能的影响规律及机理。结果表明,在一定范围内淬火温度对此钢种的抗拉强度、屈服强度和硬度影响很小,而对其截面伸长率、断面收缩率、冲击功、动态压缩强度、应变率、最大剪切强度、产生绝热剪切带的时间和绝热剪切破坏程度影响很大。     

Mechanical, durability and microstructural characteristics of ultra-high-strength self-compacting concrete incorporating steel fibers

Few researches are carried out in the Gulf area to study the feasibility of producing UHSC using available local materials with the inclusion of steel fibers, and investigate its properties and durability. Local available materials and the inclusion of steel fibers with different volume fractions are investigated to produce UHSC. Different mechanical properties are evaluated (compressive strength and splitting tensile strength). Durability of the concrete in high sulfate and high temperature condition (i.e. resembling Gulf environment) is evaluated. Also, chloride permeability, bulk chloride diffusion and electrical resistivity are evaluated. Test results indicate that local material can produce UHS–FRC. The ductility of the concrete is greatly improved by the incorporation of steel fibers and increases as the fiber volume increases. Chloride permeability, bulk chloride diffusion and electrical resistivity are affected by the volume fraction of steel fibers. The inclusion of steel fibers did not have significant effect on the durability of the concrete in the sulfate environment. Microstructural investigations of UHS–FRC concrete were also performed. The microstructural investigations shed some light on the nature of interfacial bond of fibers and the cement paste and its effect on its mechanical and fracture properties.     

Partikel- und Faserverbundwerkstoffe für Flüssigkeitsfreie Lager

Composite materials including particles and fibers as oil-less bearing materials Oil-less bearing can be fabricated by polymers and metals including solid lubricants (up to 10 w %) like PTFE, MoS₂ or graphite. If it is necessary by application to incorporate a greater content of solid lubricants, the mechanical properties of the composite materials are too weak. The mechanical properties of bearing material increase, when high tensile strength fibers are incorporated. In the case of graphite fiberreinforced polymers wear rate as well as coefficient of friction decrease, while the ultimate flexural strength increase rapidly. In the case of metallic matrices containing graphite fibers or steel wires the ultimate strength increases (above all the combination white metal/steel wire) as well as the wear rate decreases.     

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.     

Effect of pozzolans on the performance of fiber-reinforced mortars

Randomly oriented short fibers have been shown to increase tensile strength and retard crack propagation of cement based materials such as fiber-reinforced mortars for diverse applications, especially in aggressive environments. In the case of reinforced concrete, it is very important to produce a “high quality” cover in order to prevent corrosion of the rebars. In order to obtain a high performance material the use of a pozzolan is advisable because low permeability is achieved. The objective of this research was to determine the effect of pozzolans such as silica fume (SF), fly ash (FA), and metakaolin (MK) on the properties of fiber-reinforced mortars. Different types of natural and synthetic fibers were used. A superplasticizer was used to keep the same workability as that of the control mortar. Results of the mechanical and durability properties of the fiber-reinforced mortars are reported. The results show that a loss of resistance due to embedding fibers in mortar is compensated for by the increase in strength caused by silica fume or metakaolin additions to the mortar. The addition of 15% of SF or MK produces an improvement of up to 20% and 68%, respectively, when compared with those mortars without addition. There is a significant decrease in the coefficient of capillary absorption and chloride penetration when a highly pozzolanic material is incorporated into the matrix. In general, these materials, especially SF and MK, improve the mechanical performance and the durability of fiber-reinforced materials, especially those reinforced with steel, glass or sisal fibers. The fly ash addition had a different performance, which could be attributed to its low degree of pozzolanicity.     

再生钢纤维增韧超高性能混凝土的力学性能

采用来自于废旧轮胎的两种再生钢纤维制备含粗骨料的超高性能混凝土,并测定其抗压强度、劈裂抗拉强度、断裂能和静弹性模量等力学性能,空白组及普通钢纤维增韧超高性能混凝土作对比性能试验。结果显示,未附着橡胶颗粒的再生钢纤维使超高性能混凝土的抗压强度略微下降,降低幅度为3.91%,其余各类型钢纤维均有利于提高超高性能混凝土的力学性能;而附着橡胶颗粒的再生钢纤维显著提高了超高性能混凝土的断裂能,约为普通钢纤维增韧超高性能混凝土的4倍。此外,再生钢纤维对超高性能混凝土的劈裂抗拉强度和静弹性模量的提高效果均优于普通钢纤维。再生钢纤维,尤其是附着橡胶颗粒的再生钢纤维,可以作为一种增韧材料替代普通钢纤维应用到超高性能混凝土工程结构中。      

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.     

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.     

钢纤维超高强混凝土动态力学性能

采用分离式霍普金森压杆装置对不同纤维体积率的钢纤维超高强混凝土进行不同应变率的冲击压缩试验,结果表明钢纤维超高强混凝土是应变率敏感材料,并测出其应变率敏感阀值,当应变率超过阀值后,钢纤维超高强混凝土的强度、韧度与弹性模量都随纤维体积率的增加而显著提高,在高应变率下,超高强混凝土基体成粉碎性破坏,而钢纤维超高强混凝土呈现出“裂而不散”的破坏形态。     

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.     

Effect of shrinkage reducing admixture on flexural behaviors of fiber reinforced cementitious composites

The use of shrinkage reducing admixture (SRA) at various concentrations was investigated in fiber reinforced cementitious composites. Both mortar and high strength concrete (HSC) matrices were tested. Two types of fibers—steel and polypropylene—were assessed. The effect of SRA was measured on the fundamental properties such as surface tension of the bulk fluids and the contact angle developed between the fibers and the bulk fluids, on the fresh properties such as the air content and the density, and finally on the hardened mechanical properties, specially the flexural behaviors. It was noted that SRA enhances the wettability of fibers and reduces the air content of fiber reinforced cement mortars, while critical SRA concentrations are existing. SRA with critical concentration can significantly improve the flexural toughness and residual strength of steel fiber reinforced cement mortar. In the case of polypropylene fiber, SRA is not as effective in enhancing the flexural behaviors as it is in the case of steel fiber. SRA is generally ineffective in reducing the air content of HSC and the properties of steel fiber reinforced HSC with SRA are inferior to those without SRA.     

Some characteristics of high strength fiber reinforced lightweight aggregate concrete

The effect of polypropylene and steel fibers on high strength lightweight aggregate concrete is investigated. Sintered fly ash aggregates were used in the lightweight concrete; the fines were partially replaced by fly ash. The effects on compressive strength, indirect tensile strength, modulus of rupture, modulus of elasticity, stress–strain relationship and compression toughness are reported. Compared to plain sintered fly ash lightweight aggregate concrete, polypropylene fiber addition at 0.56% by volume of the concrete, caused a 90% increase in the indirect tensile strength and a 20% increase in the modulus of rupture. Polypropylene fiber addition did not significantly affect the other mechanical properties that were investigated. Steel fibers at 1.7% by volume of the concrete caused an increase in the indirect tensile strength by about 118% and an increase in the modulus of rupture by about 80%. Steel fiber reinforcement also caused a small decrease in the modulus of elasticity and changed the shape of the stress–strain relationship to become more curvilinear. A large increase in the compression toughness was recorded. This indicated a significant gain in ductility when steel fiber reinforcement is used.     

Permeability of ultra high performance fiber reinforced concretes (UHPFRC) under high stresses

Ultra High Performance Fiber Reinforced Concretes (UHPFRC) present outstanding mechanical properties and a very low permeability which make them very attractive for the rehabilitation of existing structures and for new conceptions. UHPFRC are characterized by a significant tensile strain hardening (multiple cracking stage) that can be used to optimize the mechanical performance of composite structural elements. In order to validate this assumption, permeability tests were carried out on UHPFRC specimens previously submitted to various levels of tensile deformation with progressive damage. Based on permeability results, it was possible to define maximal tensile deformations whereby the water permeability of a specific UHPFRC remains low for various exposure conditions.     

新拌超高性能纤维增强混凝土流动性能对其抗压强度的影响

钢纤维掺入能提高超高性能纤维增强混凝土(Ultra-high performance fiber reinforced concrete,UHPFRC)的抗压强度,但削弱新拌浆体的流动性能,降低了对抗压强度的增强效果,且影响UHPFRC的工作性能。为研究这种不利影响,以钢纤维体积分数和长径比为变量,进行了A、B两组试验。A组固定水胶比为0.18,不控制流动性能,主要研究钢纤维对流动性能和抗压强度的影响。试验结果表明,新拌UHPFRC流动性能随钢纤维的体积分数、长径比增加而下降;当钢纤维体积率超过一定值(2.00vol%)时,流动性能明显下降,抗压强度增强效果也相应下降。通过X-ray CT扫描发现钢纤维掺入减弱浆体的自密实能力,导致硬化后的基体内部孔隙尺寸增大和孔隙率增加,进而削弱抗压强度。综合考虑钢纤维掺入对抗压强度的正、负效应,提出了抗压强度的半经验预测公式。B组改变水胶比,控制扩展度为240 mm,对比A组研究流动性能控制后,钢纤维体积分数和长径比对抗压强度的影响规律。结果表明,钢纤维体积分数较大时,增大水胶比,保持一定流动性能,能有效提高纤维的增强效果;钢纤维体积分数较小时,在满足流动性能要求的前提下,减小水胶比,可以进一步提高抗压强度。在UHPFRC配合比设计时,应考虑钢纤维对流动性能的不利影响,以提高纤维的增强效应并保证其良好的工作性能。      

低成本船用EH36高强钢板的研制及其性能

不添加价格昂贵的铌、钒元素,仅在C-Mn钢成分体系上添加钛元素细化晶粒,采用热机械控制工艺研制出低成本的船用EH36高强钢板,并对其力学性能、显微组织等进行了分析.结果表明:该船用EH36高强钢板成本较原来低,其性能满足标准要求,且强度和韧性均有较大的富余量;经人工应变时效后,其低温冲击韧性仍较好;经两种热输入焊接后,...