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。     

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.     

Influence of concrete matrix and type of fiber on the shear behavior of self-compacting fiber reinforced concrete beams

Steel fibers are known to improve shear behavior. The Design Codes (Eurocode 2 (EC2), Spanish EHE-08, Model Code 2010 and RILEM approach) have developed formulas to calculate the fiber contribution to shear, mainly focused on standard FRCs, i.e. medium strength concretes with a low content of normal strength steel fibers. However, in real applications other combinations are possible, such as high or medium strength concretes with high strength steel fibers of different lengths and geometry. An experimental program consisting of 12 self-compacting fiber reinforced concrete (SCFRC) I-type beams was carried out. All the beams had the same geometry and fiber content (50 kg/m³), and they were made with two different concrete compressive strength values and five different types of steel fibers and were tested for shear. The main conclusions reached were that the type of fiber substantially affects shear behavior, even when the Design Code formulas indicate similar contributions. The combination of high strength concrete matrixes with low strength fibers does not seem to be efficient. Also, the use of high residual flexural tensile strength values (e.g. f_R3 or f_R4) does not appear to be the most accurate reference value to calculate the beam shear strength in these cases. The present Design Codes consider standard FRCs, but their formulas should be revised for concretes with fibers of different strengths, slenderness and geometry, since these properties substantially affect shear behavior.     

Investigation of microstructure and flexural behavior of steel-fiber reinforced concrete

This paper presents microstructure and flexural behavior of steel-fiber reinforced concrete produced with different steel fibers volume fraction and aspect ratio. Prismatic concrete specimens of 100 × 100 × 350 mm were prepared with and without steel fiber. Two different steel fiber types (both is hooked-end) were used by ratio of 0% (control), 0.2, 0.4, 0.6 and 0.8% by volume. Specimens were de-molded after 24 h and cured in water until 7, 28, 56, 180 and 360 days. On the prisms, flexural strength has been defined for every age. The crack widths have also been measured after maximum bearing loads. Microstructure of SFRC was studied by scanning electron microscopy and optical microscopy for 180 aged specimens. The results showed that the polarized microcopy images may be used for observing the bond characteristic of SFRC as alternatively to SEM. A good bond was observed between steel fiber and concrete matrix interface zone by using polarizing microscopy, too. Flexural strength of SFRC increased with the concrete age and fiber volume fraction. Besides, the first crack development significantly decreased by increasing of fiber volume fraction in the all concrete ages.     

Flexural and impact response of woven glass fiber fabric/polypropylene composites

Impact tests with a falling dart and flexural measurements were carried out on polypropylene based laminates reinforced with glass fibers fabrics. Research has shown that the strong fiber/matrix interface obtained through the use of a compatibilizer increased the mechanical performance of such composite systems. The improved adhesion between fibers and matrix weakly affects the flexural modulus but strongly influences the ultimate properties of the investigated woven fabric composites. In fact, bending tests have shown a clear improvement in the flexural strength for the compatibilized systems, in particular when a high viscosity/high crystallinity polypropylene was used. On the contrary, the low velocity impact tests indicated an opposite dependence on the interface strength, and higher energy absorption in not compatibilized composites was detected. This result has been explained in terms of failure mechanisms at the fiber/matrix interface, which are able to dissipate large amounts of energy through friction phenomena. Pull-out of fibers from the polypropylene matrices have been evidenced by the morphological analysis of fracture surfaces after failure and takes place before the fibers breakage, as confirmed by the evaluation of the ductility index.     

多尺度纤维增强水泥基复合材料力学性能试验

基于水泥基材料多尺度的结构特征及破坏过程,设计了一种由钢纤维、聚乙烯醇（PVA）纤维以及碳酸钙晶须构成的多尺度纤维增强水泥基复合材料（MSFRCC）,研究了其抗压强度、抗弯强度、弯曲韧性、多缝开裂形态以及断裂过程等基本力学性能。结果表明：基体材料的强度和韧性均得到了显著提高;MSFRCC在弯曲荷载作用下表现出了硬化行为和多缝开裂模式。扫描电子显微镜和断裂试验结果证实了多尺度纤维在水泥基复合材料破坏过程中发挥了多尺度阻裂作用。研究认为：通过对纤维进行多尺度组合设计,可以显著改善水泥基复合材料的韧性,廉价的碳酸钙晶须可以适量取代钢纤维和PVA纤维。     

Studies on the characterization of piassava fibers and their epoxy composites

This paper presents morphology, physical and strength properties of piassava fiber, a very rigid fiber having a potential to be used as composite reinforcement. Composites of continuous and aligned piassava fibers with and without alkali treatment dispersed in epoxy matrix were subjected to three point bend, tensile, and Izod impact tests. Composites with fibers above 20 vol.% showed an effective reinforcement behavior both in flexural and tensile tests, while the impact energy linearly increased for the amount of piassava fibers used in this study. Fractographic study revealed a relatively weaker fiber/matrix adhesion acting as preferential site for crack nucleation. Evidence was also found for crack arrest by the fiber above 20 vol.%. This, together with spiny surface protrusion in the piassava fibers, was found to be responsible for the reinforcement of the epoxy composites.     

Biocomposites from Musa textilis and polypropylene: Evaluation of flexural properties and impact strength

Abaca (Musa textilis)-reinforced polypropylene composites have been prepared and their flexural mechanical properties studied. Due to their characteristic properties, M. textilis has a great economic importance and its fibers are used for specialty papers. Due to its high price and despite possessing very distinctive mechanical properties, to date abaca fibers had not been tested in fiber-reinforced composites. Analysis of materials prepared showed that, in spite of reduced interface adhesion, flexural properties of the PP composites increased linearly with fiber content up to 50 wt.%. Addition of a maleated polypropylene coupling agent still enhanced the stress transfer from the matrix to the reinforcement fiber. As a result, composites with improved flexural properties were obtained. The mechanical properties of matrix and reinforcing fiber were evaluated and used for modelling both the flexural strength and modulus of its composites. In addition, the impact strength of materials was evaluated. Comparison with mechanical properties of composites reinforced with fiberglass points out the potentiality of abaca-reinforced polypropylene composites as suitable substitutes in applications with low impact strength demands.     

水泥基材料中倾斜端钩型钢纤维拔出力学性能计算模型

端钩型钢纤维是结构工程中应用最广泛的钢纤维品类之一,单根钢纤维拔出力学性能对于确定钢纤维混凝土的受拉本构及受拉韧性具有重要意义。为了得到能够有效预测倾斜端钩型钢纤维拔出荷载-端部位移曲线的理论模型,首先将倾斜端钩型钢纤维拔出过程分为完全黏结、脱黏和拔出滑移阶段三种受力状态,考虑不同拔出阶段及基体孔道损伤,建立了钢纤维黏结应力与纤维端部位移之间的关系,同时考虑钢纤维塑性变形、附加摩擦力及纤维拔出角度导致的基体剥落和挤压摩擦效应,建立了一种可以预测倾斜端钩型钢纤维拔出全过程的理论计算模型,在此基础上提出形式简单的简化模型,选取已有试验数据对提出的计算模型进行验证,结果表明:本文提出的两种模型均能够有效预测端钩型钢纤维拔出全过程,具有较高的计算精度且变异系数小,为进一步分析钢纤维对水泥基材料受拉性能的增强作用提供了理论依据。      

In-situ toughened CFRP composites by shear-calender orientation and fiber-bundle filtration of PA microparticles at prepreg interlayer

Epoxy matrix toughened by polyethersulfone (PES) and polyamide (PA) microparticles was designed and the in-situ interlaminar toughened carbon fiber/epoxy composites were fabricated. Synergistic toughening effect of PES and PA on epoxy matrix was achieved due to semi-IPN structure of PES toughened matrix and uniform dispersion of PA microparticles. Shear-calender orientation of PA microparticles was found during prepreg processing and the microparticles remained on the surface of prepreg due to fiber-bundle filtration. The in-situ formed toughening interlayer of PA microparticles and interfacial bonding between PA and epoxy matrix were detected, which resulted in enhanced fracture toughness, CAI, and transverse flexural strength of the composite based on the PES/PA synergistically toughened matrix. SEM images of fracture morphology of the composite showed evidence of enhanced plastic deformation created by PES and PA, and crack deflection and bridging by PA microparticles.     

Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites

Natural fiber reinforced polymer composites became more attractive due to their light weight, high specific strength, and environmental concern. However, some limitations such as low modulus, poor moisture resistance were reported. This study aimed to investigate the effect of glass fiber hybridization on the physical properties of sisal–polypropylene composites. Polypropylene grafted with maleic anhydride (PP-g-MA) was used as a compatibilizer to enhance the compatibility between the fibers and polypropylene. Incorporating glass fiber into the sisal–polypropylene composites enhanced tensile, flexural, and impact strength without having significant effect on tensile and flexural moduli. In addition, adding glass fiber improved thermal properties and water resistance of the composites.     

Cellulosic fibers from rice straw and bamboo used as reinforcement of cement-based composites for remarkably improving mechanical properties

The cement-based composites reinforced with cellulosic fibers isolated from rice straw were fabricated by a slurry vacuum de-watering technique. The physical structures and mechanical properties of the composites with fiber contents ranging from 2% to 16% by weight (wt.%) were investigated. Moreover, the composites reinforced with bamboo cellulosic fibers and the control cement paste, sample without cellulosic fibers, were also fabricated as reference materials. As a result, the cement-based composites reinforced by cellulosic fibers showed a remarkable improvement in the mechanical properties. The measurements of the flexural strength and the fracture toughness of the optimal sample were found to be increased by 24.3% and 45 times, respectively. The bulk density of the composites was decreased by 12.4–37.3% as a result of the introduction of cellulosic fibers. Additionally, the field emission scanning electron microscope (FSEM) observations and energy dispersive spectroscopy (EDS) analyses revealed that the hydration products of Portland cement migrated to the fiber lumens, resulting in mineralizing the cellulosic fibers and decreasing the fracture toughness of the composites.     

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.     

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。      

含粗骨料超高性能混凝土力学性能研究及机理分析

为探索含粗骨料超高性能混凝土的各项力学性能,研究了粗骨料体积掺量(0kg/m~3、280kg/m~3、400kg/m~3、480kg/m~3、560kg/m~3)、纤维掺量(2%、2.5%)以及纤维形态(平直型、端钩型)对超高性能混凝土抗压强度、弹性模量以及四点弯曲强度的影响,并引入纤维取向系数和纤维有效长度,探索粗骨料掺量对弯曲强度影响的微观机理。结果表明,粗骨料体积掺量对含粗骨料超高性能混凝土抗压强度的影响不大(0.4%~4.5%);对弹性模量的提高效果显著,最高可提高7.8%;对抗弯强度具有不利影响,并且随着粗骨料掺量增大,纤维取向系数下降,纤维有效长度减小,负面影响扩大。当粗骨料体积掺量为560kg/m~3时,弯曲强度下降了21.2%。增加纤维掺量或者掺入端钩型纤维可提高弯曲强度,掺入端钩型钢纤维可显著增大纤维有效长度,从而大幅度提高弯曲强度。     

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical C_f/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.     

Static bending and impact behaviour of areca fibers composites

Natural fibers are considered to have potential use as reinforcing agents in polymer composite materials because of their principle benefits: good strength and stiffness, low cost, and be an environmental friendly, degradable, and renewable material. A study has been carried out to evaluate physical, flexural and impact properties of composite made by areca fibers with randomly distributed fibers. The extracted areca fibers from the areca husk were alkali treated with potassium hydroxide to get better interfacial bonding between fiber and matrix. Then composite laminates were fabricated by using urea formaldehyde, melamine urea formaldehyde and epoxy resins by means of compression molding technique with varying process parameters, such as fiber condition (untreated and alkali treated), and fiber loading percentages (50% and 60% by weight). The developed areca fiber-reinforced composites were then characterized by physical, bending and impact test. The results show that flexural and impact strength increases with increase in the fiber loading percentage. Compared to untreated fiber, significant change in flexural and impact strength has been observed for treated areca fiber reinforcement.     

Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding

Sisal fibers were subjected to various chemical and physical modifications such as mercerization, heating at 100 °C, permanganate treatment, benzoylation and silanization to improve the interfacial bonding with matrix. Composites were prepared by these fibers as reinforcement, using resin transfer molding (RTM). The mechanical properties such as tensile, flexural and impact strength were examined. Mercerized fiber-reinforced composites showed 36% of increase in tensile strength and 53% in Young’s modulus while the permanganate treated fiber-reinforced composites performed 25% increase in flexural strength. However, in the case of impact strength, the treatment has been found to cause a reduction. The water absorption study of these composites at different temperature revealed that it is less for the treated fiber-reinforced composites at all temperatures compared to the untreated one. SEM studies have been used to complement the results emanated from the evaluation of mechanical properties.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.