Effect of steel fiber on the mechanical properties of oil palm shell lightweight concrete

This paper reports the results of a study conducted to investigate the effect of low volume content of steel fiber on the slump, density, compressive strength under different curing conditions, splitting tensile strength, flexural strength and modulus of elasticity of a grade 35 oil palm shell (OPS) lightweight concrete mixture. The results indicate that an increase in steel fiber decreased the workability and increased the density. All the mechanical properties except the modulus of elasticity (E) improved significantly. The 28 day compressive strength of steel fiber OPS lightweight concrete in continuously moist curing was in the range of 41–45 MPa. The splitting tensile/compressive and the flexural/compressive strength ratio for plain OPS concrete are comparable with artificial lightweight aggregate. The (E) value measured in this study was about 15.5 GPa on average for all mixes, which is higher than previous studies and is in the range of normal weight concrete. Steel fiber can be used as an alternative material to reduce the sensitivity of OPS concrete in poor curing environments.     

Material and structural performance evaluation of recycled PET fiber reinforced concrete

Most PET bottles used as beverage containers become waste after their usage, causing environmental problems. To address this issue, a method to recycle wasted PET bottles is presented, in which short fibers made from recycled PET are used within structural concrete. To verify the performance capacity of recycled PET fiber reinforced concrete, it was compared with that of polypropylene (PP) fiber reinforced concrete for fiber volume fractions of 0.5%, 0.75%, and 1.0%. Appropriate tests were performed to measure material properties such as compressive strength, elastic modulus, and restrained drying shrinkage strain. Flexural tests were performed to measure the strength and ductility capacities of reinforced concrete (RC) members cast with recycled PET fiber reinforced concrete. The results show that compressive strength and elastic modulus both decreased as fiber volume fraction increased. Cracking due to drying shrinkage was delayed in the PET fiber reinforced concrete specimens, compared to such cracking in non-reinforced specimens without fiber reinforcement (NF), which indicates crack controlling and bridging characteristics of the recycled PET fibers. Regarding structural member performance, ultimate strength and relative ductility of PET fiber reinforced RC beams are significantly larger than those of companion specimens without fiber reinforcement.     

Comparative impact behavior of four long carbon fiber reinforced concretes

The addition of long carbon fibers (fibers more than 10 mm in length) to traditional reinforced concrete is proposed as a method to improve the impact spalling resistance of concrete. A series of experimental tests were conducted to compare the impact resistance of plain concrete (PC), steel reinforced concrete, and four different types of long carbon fiber reinforced concrete (LCFRC) panels. The plain and conventional steel reinforced concrete panels served as control specimens. Of the four types of long carbon fibers tested in this study, the first fiber type consisted of an epoxy-impregnated, bidirectional weave (Type A), while the remaining types consisted of fiber tow with three different variations of a polypropylene support system (Type B). To determine the properties and performance of the LCFRC, experimental testing included a drop weight impact test of the panels as well as a standard ASTM test method for flexural performance of fiber-reinforced concrete. The results from each test in terms of impact energy, time histories of impact load and deflection, strain energy, failure crack pattern, and flexural properties were then compared to one another. This comparison indicated that adding long carbon fibers to concrete increases the post-cracking behavior of the concrete and decreases fragmentation during an impact test. Of the four fibers tested, Fiber Type B3 exhibited the highest performance, absorbing more energy during impact. This result is most likely related to the unique shape of this type of fiber in comparison to the others, which allowed more extensive wetting of the fiber with cement paste and thus improved bond to the cementitious matrix.     

Effects of steel fibers and mineral filler on the water-tightness of concrete pipes

Water-tightness of concrete and reinforced concrete pipes used to convey sewage flow of any kind is extremely important from the aspects of (1) groundwater contamination and (2) durability of the pipes. In this study, water-tightness tests were applied on plain concrete, reinforced concrete, and steel fiber concrete pipes of 500 mm diameter. Standard seepage tests were applied on many pipes of various materials and combinations with the objective of determining the pipes with the best water-tightness. Tests on plain concrete pipes whose concrete included finely ground limestone passing the no. 100 sieve (d < 0.15 mm) at an amount of 7% by dry weight of the total aggregates revealed that the water-tightness of former was 57% better than that of pipes manufactured without the filler. The water-tightness values measured on steel fiber concrete pipes with a fiber dosage of 25 kg/m³ turned out to be 47% and 15% better than those of plain concrete pipes and reinforced concrete pipes, respectively. Those findings tangibly reveal that the addition of steel fibers and mineral filler in concrete pipes improve their seepage property.     

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 the loading frequency on the compressive fatigue behavior of plain and fiber reinforced concrete

This paper presents the recent experimental results aimed at disclosing the loading frequency effect on the fatigue behavior of a plain concrete and two types of fiber-reinforced concrete, using polypropylene and steel fibers. Compressive fatigue tests were conducted on 123 cubic specimens (100 mm in edge length). Four different loading frequencies, 4 Hz, 1 Hz, 1/4 Hz and 1/16 Hz, were employed. The maximum stress applied on the specimen was 85% of its compressive strength and the stress ratio was kept constant as 0.3. The results show that the loading frequency effect on the fatigue behavior of the plain concrete is pronounced. The fatigue life (the number of cycles to failure) at lower frequencies is less than that at higher frequencies. However, the fibers do improve the fatigue behavior significantly under low loading frequencies. Such trend can be attributed to the effectiveness of the fibers in bridging cracks, and thus inhibiting the crack extension under cyclic loads.     

高应变率下多尺寸聚丙烯纤维混凝土动态压缩力学性能研究

采用Φ74 mm的分离式霍普金森压杆(Split Hopkinson pressure bar,SHPB)试验装置,对两种尺寸聚丙烯细纤维和一种尺寸聚丙烯粗纤维单掺及混掺的混凝土试件进行冲击压缩试验,对比分析粗、细纤维及不同纤维掺量比的多尺寸纤维混凝土试件在五种不同应变率下的动态压缩强度、动态压缩变形、动态压缩韧性和破坏特征,研究聚丙烯纤维混凝土的动态压缩力学性能.结果表明:随应变率的增加,素混凝土及纤维混凝土的动态压缩强度、动态压缩变形和动态压缩韧性表现出显著的应变率效应;在试验应变率范围内,粗聚丙烯纤维混凝土的动态抗压强度最高,相对素混凝土增幅为132.36％～213.85％;多尺寸聚丙烯纤维混凝土的动态强度增长因子与素混凝土基本一致;掺入多尺寸聚丙烯纤维可有效增大混凝土在不同应变率下的动态峰值应变和动态极限应变;多尺寸聚丙烯纤维混凝土的动态极限韧性较高,其中细聚丙烯纤维含量为1.2 kg/m3时混凝土动态极限韧性最高,增幅为121.11％.     

Strain rate dependent properties of ultra high performance fiber reinforced concrete (UHP-FRC) under tension

The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.0001 1/s to 0.1 1/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.     

Interlaminar shear strength of SiC matrix composites reinforced by continuous fibers at 900 °C in air

To reveal the shear properties of SiC matrix composites, interlaminar shear strength (ILSS) of three kinds of silicon carbide matrix composites was investigated by compression of the double notched shear specimen (DNS) at 900 °C in air. The investigated composites included a woven plain carbon fiber reinforced silicon carbide composite (2D-C/SiC), a two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composite (2.5D-C/SiC) and a woven plain silicon carbon fiber reinforced silicon carbide composite (2D-SiC/SiC). A scanning electron microscope was employed to observe the microstructure and fracture morphologies. It can be found that the fiber type and reinforcement architecture have significant impacts on the ILSS of the SiC matrix composites. Great anisotropy of ILSS can be found for 2.5D-C/SiC because of the different fracture resistance of the warp fibers. Larger ILSS can be obtained when the specimens was loaded along the weft direction. In addition, the SiC fibers could enhance the ILSS, compared with carbon fibers. The improvement is attributed to the higher oxidation resistance of SiC fibers and the similar thermal expansion coefficients between the matrix and the fibers.     

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.     

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。      

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.     

The influence of freeze–thaw cycles on the unconfined compressive strength of fiber-reinforced clay

Freeze–thaw cycling is a weathering process that frequently occurs in cold climates. In the freeze state, thermodynamic conditions at temperatures just below 0 °C result in the translocation of water and ice. Consequently, the engineering properties of soils such as permeability, water content, stress–strain behavior, failure strength, elastic modulus, cohesion, and friction angle may be changed. Former studies have been focused on changes in physical and mechanical properties of soil due to freeze–thaw cycles. In this paper, the effect of freeze–thaw cycles on the compressive strength of fiber-reinforced clay is investigated. For this purpose, kaolinite clay reinforced by steel and polypropylene fibers is compacted in a laboratory and exposed to a maximum of 10 closed-system freezing and thawing cycles. The unconfined compressive strength of reinforced and unreinforced specimens is then determined. The results of the study show that for the soil investigated, the increase in the number of freeze–thaw cycles results in the decrease of unconfined compressive strength of clay samples by 20–25%. Moreover, inclusion of fiber in clay samples increases the unconfined compressive strength of soil and decreases the frost heave. Furthermore, the results of the study indicate that fiber addition does not decrease the soil strength against freeze–thaw cycles. Moreover, the study shows that the addition of 3% polypropylene fibers results in the increase of unconfined compressive strength of the soil before and after applying freeze–thaw cycles by 60% to 160% and decrease of frost heave by 70%.     

聚丙烯-钢纤维增强高强混凝土高温性能

通过对聚丙烯-钢纤维增强高强混凝土(混杂纤维/高强混凝土)试块的高温试验, 研究不同目标温度后混凝土表观特征、高温爆裂、质量损失及力学性能。结果表明: 高强混凝土在600 ℃时发生爆裂, 混杂纤维/高强混凝土直至800 ℃未出现爆裂, 混杂纤维有效抑制了高强混凝土的高温爆裂。混杂纤维/高强混凝土的质量损失随所受温度的升高而增大, 其抗压强度、抗折强度随温度的升高而降低, 并且400 ℃以后显著降低。相同温度下, 混杂纤维的加入提高了高强混凝土高温后强度。通过对试验结果的统计分析, 分别建立了混杂纤维混凝土质量损失、抗压强度和抗折强度随温度变化的关系式。     

Assessment of factors influencing mechanical properties of steel fiber reinforced self-compacting concrete

In the last decade the steel fiber reinforced self-compacting concrete (SFRSCC) has been used in several partially and fully structural applications. This study investigates how the inclusion of steel fibers affects the properties of SFRSCC. For this purpose, an extensive experimental program including different cement contents of 400, 450 and 500 kg/m³, two maximum aggregate sizes of 10 and 20 mm along with steel fiber volume fractions of 0%, 0.38%, 0.64% and 1% was conducted. The water/cement ratio was kept constant at 0.45 for all the mixes studied. Mechanical properties were tested for compressive, splitting tensile and flexural strengths and modulus of elasticity. The results showed that mixture characteristics and volume fraction of steel fibers can significantly affect these major properties. Furthermore, this study represents extensive comparisons using database that have been gathered from a wide variety of international sources reported by many researchers and data obtained experimentally, which came up with about some discrepancies in the results.     

Flexural toughness characteristics of steel–polypropylene hybrid fibre-reinforced oil palm shell concrete

Hybridization of steel–polypropylene leads to improvements of both the mechanical and ductility characteristics of concrete. In this investigation, the effect of steel, polypropylene (PP) and steel-PP hybrid fibres on the compressive strength, tensile strength, flexural toughness and ductility of oil palm shell fibre reinforced concrete (OPSFRC) was studied. The comparison on the above said properties between the specimens prepared with crushed and uncrushed oil palm shell (OPS) as lightweight coarse aggregate was also carried out. The experimental results showed that the highest compressive strength of about 50 MPa was produced by the mix with 0.9% steel and 0.1% PP hybrid fibres. The highest increments in the splitting tensile and the flexural strengths of the OPSFRC were found up to 83% and 34%, respectively. However, the mixes with 1% PP fibres produced negative effects on both the compressive and tensile strengths. The results on the toughness indices showed that the OPSC possess no post-cracking flexural toughness. Though, the flexural deflection and toughness of the OPSC was significantly enhanced by the addition of fibres; the dominance of the steel fibre on the first crack flexural deflection and toughness of OPSFRC was evident. The mixes with 0.9% steel and 0.1% PP hybrid fibres reported the highest improvement in toughness index and residual strength factor.     

Effects of environmental aging on the mechanical properties of bamboo–glass fiber reinforced polymer matrix hybrid composites

Short bamboo fiber reinforced polypropylene composites (BFRP) and short bamboo–glass fiber reinforced polypropylene hybrid composites (BGRP) were fabricated using a compression molding method. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to improve the adhesion between the reinforcements and the matrix material. By incorporating up to 20% (by mass) glass fiber, the tensile and flexural modulus of BGRP were increased by 12.5 and 10%, respectively; and the tensile and flexural strength were increased by 7 and 25%, respectively, compared to those of BFRP. Sorption behavior and effects of environmental aging on tensile properties of both BFRP and BGRP systems were studied by immersing samples in water for up to 1200 h at 25°C. Compared to BFRP, a 4% drop in saturated moisture level is seen in BGRP. After aging in water for 1200 h, reduction in tensile strength and modulus for BGRP is nearly two times less than that of BFRP. Use of MAPP as coupling agent in the polypropylene matrix results in decreased saturated moisture absorption level and enhanced mechanical properties for both BFRP and BGRP systems. Thus it is shown that the durability of bamboo fiber reinforced polypropylene can be enhanced by hybridization with small amount of glass fibers.     

Reinforcement effects of polyvinyl alcohol and polypropylene fibers on flexural behaviors of sulfoaluminate cement matrices

The fracture behavior of unoiled/uncoated polyvinyl alcohol (PVA) fiber reinforced sulphoaluminate cement (SAC) matrices was experimentally investigated and compared with those of polypropylene (PP) fiber reinforced SAC and PVA fiber reinforced Portland cement (PC) matrices in this study. In the experimental investigation, three-point bending tests were carried out for notched fiber reinforced cement beams. Special attentions were paid on their deflection-hardening and multiple crack patterns. The different flexural behaviors between the plain SAC and PC matrices were evaluated using the double-K fracture model. The results indicate that the PVA fiber reinforced SAC matrices exhibited better flexural behaviors when compared with the PVA fiber reinforced PC matrix having comparable matrix strength. The bond strength between SAC matrix and PVA fiber are relatively better than that between the counterpart PC matrix and PVA fiber, while the bond strength between SAC matrix and PVA fiber is obviously stronger than that between the SAC and PP fibers.     

Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike

Damage is inflicted in a series of carbon fiber/epoxy composite specimens using a simulated lightning strike generator in the effort to understand the fundamental damage response of this material form. The strikes up to 50,000 A and 28,000 V are inflicted on both pristine specimens and specimens containing a Hilok stainless steel fastener. Damage area is evaluated via ultrasonic scanning, and advanced optical microscopy is used to gain further understanding in the morphology of damage. Subsequent mechanical testing to assess the residual tensile and compressive strength and modulus of the material is performed according to ASTM standards. Results show that residual tension strength counter intuitively increases after the infliction of damage, while residual compressive strength is much more dramatically and negatively affected. Furthermore, the presence of the fastener influences dramatically both the state of damage in the specimen and its residual strength by spreading throughout the thickness rather than limiting it to the specimen surface.     

Physical and mechanical characterization of Fiber-Reinforced Aerated Concrete (FRAC)

Fiber-Reinforced Aerated Concrete (FRAC) is a novel lightweight aerated concrete that includes internal reinforcement with short polymeric fibers. The autoclaving process is eliminated from the production of FRAC and curing is performed at room temperature. Several instrumented experiments were performed to characterize FRAC blocks for their physical and mechanical properties. This work includes the study of pore-structure at micro-scale and macro-scale; the variations of density and compressive strength within a block; compressive, flexural and tensile properties; impact resistance; and thermal conductivity. Furthermore, the effect of fiber content on the mechanical characteristics of FRAC was studied at three volume fractions and compared to plain Autoclaved Aerated Concrete (AAC). The instrumented experimental results for the highest fiber content FRAC indicated compressive strength of approximately 3 MPa, flexural strength of 0.56 MPa, flexural toughness of more than 25 N m, and thermal conductivity of 0.15 W/K m.