纤维增强水泥薄板及其复合梁抗弯性能研究

实验研究了纤维对水泥基复合材料抗弯性能的影响.结果表明,聚乙烯醇(PVA)纤维增强挤压脱水成型板材在弯曲荷载作用下呈现多点开裂、应变硬化的特性,具有良好的延性,聚丙烯(PP)纤维增强挤压脱水成型板材呈应变软化的特性,木纤维增强挤压脱水成型板材则呈脆性破坏;与普通混凝土梁相比,冷浇和热浇纤维增强板-混凝土组合梁的抗弯强度...     

The spalling strength of ultra-fiber reinforced cement mortar

This is a research report about the effects of polypropylene fiber and wood fiber on mechanical properties of cement mortar. First, using advanced Hopkinson pressure bar (HPB) tests, it investigates the wave propagation in cement mortar comprised polypropylene fiber and wood fiber. Second, according to the experiment, the spallation position is recorded by high-speed camera. Thirdly, it analyzes the test data of ultra-fiber reinforced and common cement mortar by numerical method. Finally, it deduces the spalling strength of all kinds of cement mortar by integrating all experimental data above. The results indicate that, compared with the strength of common cement mortar, the dynamic spalling strength of ultra-fiber especially that of the polypropylene fiber reinforced cement mortar increases evidently. However, adding too much fibers will deteriorate the dynamic spalling strength of cement mortar specimen. So the results will provide a test basis for further optimizing performance of cement mortar.     

Aging effects on the structure and properties of recycled wastepaper fiber cement composites

Dry-processed wastepaper (magazine) fibers possess desirable mechanical characteristics as reinforcing fibers in cement-based matrices; up to 50% of virgin wood fibers have been replaced with wastepaper fibers in cement composites without any significant change in short-term performance characteristics. The large volumes of fibrous cement board used in building construction promise to provide wastepaper with a large-volume and high-value application. In this study, the effects of weathering on the performance of recycled wastepaper fiber-cement composites were investigated through accelerated aging tests simulating natural aging conditions. Microstructural studies were conducted in order to establish the mechanisms of aging in the composite material. These mechanisms provided the basis for the selection of certain refinements in the matrix composition, which were successfully evaluated for the control of weathering effects on the composite material structure and properties. The effects of aging and moisture on composites were best controlled by measures which reduced the calcium hydroxide content of hydration products and improved the watertightness and the structure of interface zones; these refinements were made using relatively high levels of cement replacement with silica fume or through full substitution of Portland cement with a special cement.     

Study of the degradation of non-conventional MgO-SiO2 cement reinforced with lignocellulosic fibers

This paper assesses the use of low alkaline composites based on magnesium oxide and silica (MgO-SiO₂) cement and reinforced with cellulose fibers for the production of thin elements to resist bending loads. The strategy adopted was to study the durability of lignocellulosic fibers in a lower pH environment than the ordinary Portland cement (OPC), by comparing the flexural performance of samples at 28 days and after 200 accelerated ageing cycles. Two types of vegetable fibers were used: eucalyptus and pine pulps. For both types of fibers, composites made out of MgO-SiO₂ cement after ageing treatment show a better mechanical performance than OPC samples (modulus of rupture of ∼10.5 and 9 MPa respectively). When used in MgO-SiO₂ cement matrices, eucalyptus fibers offer excellent specific energy (SE) values (∼5 kJ/m²) compared to OPC samples in which SE drastically decreases after ageing from 4.97 kJ/m² to 0.14 kJ/m². The preservation of the reinforcing capacity of the composite materials after ageing was also proved by SEM techniques. In the light of the results, the use of MgO-SiO₂ cements is an effective way to apply cellulosic fibers as reinforcement in fiber-cement products since no signs of degradation were found, even improving flexural properties over time.     

Effects of filler treatments on the mechanical and morphological behavior of PP+wood flour and PP+sisal fiber

Composites were prepared from waste wood flour, sisal fiber and polypropylene (PP). The surface of the filler was modified to enhance the chemical affinity between hydrophilic cellulosic and hydrophobic polymer. The treatments studied were: a) The addition of a coupling agent; b) chemical treatment with NaOH; and c) the addition of functionalized polypropylene (Polybond 3150 and 3200). After treatment, mixtures of PP with 40% and 20% of the filler, wood flour and sisal fiber respectively, were extruded and injection molded. In the case of wood flour, a mixture of two particle sizes (mesh 20 and 40, 50% each) was used, and in the case of sisal, 10 mm long fibers were selected. Results showed that, regardless of the treatment to which the filler was previously submitted, Young’s modulus was always higher for mixtures prepared with wood flour, with values varying between 2839 and 3150 MPa. Whereas for mixtures with sisal fiber, the modulus’ values varied between 1704 and 2220 MPa. Values of breaking strength, elongation at break and impact strength for PP mixtures with treated sisal fiber were always higher than those for mixtures of PP with wood flour. Based on these results, we can conclude that sisal fiber is an organic filler which, due to its mechanical and morphological characteristics, has a reinforcing effect higher than that of wood flour. Received: 9 October 2000 / Reviewed and accepted: 10 October 2000     

Multiscale characterization of chemical–mechanical interactions between polymer fibers and cementitious matrix

Together with a series of mechanical tests, the interactions and potential bonding between polymeric fibers and cementitious materials were studied using scanning transmission X-ray microscopy (STXM) and microtomography (μCT). Experimental results showed that these techniques have great potential to characterize the polymer fiber-hydrated cement-paste matrix interface, as well as differentiating the chemistry of the two components of a bi-polymer (hybrid) fiber – the polypropylene core and the ethylene acrylic acid copolymer sheath. Similarly, chemical interactions between the hybrid fiber and the cement hydration products were observed, indicating the chemical bonding between the sheath and the hardened cement paste matrix. Microtomography allowed visualization of the performance of the samples, and the distribution and orientation of the two types of fiber in mortar. Beam flexure tests confirmed improved tensile strength of mixes containing hybrid fibers, and expansion bar tests showed similar reductions in expansion for the polypropylene and hybrid fiber mortar bars.     

The fluid transport properties of HCWA–DSF hybrid supplementary binder mortar

It has been demonstrated in several past studies that high calcium wood ash (HCWA) can be effectively used in combination with densified silica fume (DSF) as supplementary binder material to enhance the mechanical performance of concrete. The experimental investigation was conducted to study the effect of the inclusion of HCWA and DSF on the durability properties of high strength cement mortar produced. A total of twelve different mix designs of mortar were fabricated with the use of HCWA at various cement replacement levels of 0–20% in combination with 7.5% densified silica fume (DSF) and subjected to various durability tests. The durability assessments performed include tests on water absorption, air permeability, porosity and degree of carbonation. A significantly lower degree of water absorption, porosity and carbonation was observed for cement mortars with HCWA contents of 2–8% used in combination with 7.5% DSF by weight of binder as compared to an equivalent pure cement mortar.     

Supercritical carbonation treatment on extruded fibre–cement reinforced with vegetable fibres

The objective of this research was to evaluate the effects of supercritical carbonation treatment for 2 h on the main hydrated phases of the cement matrix (calcium hydroxide and calcium silicate hydrate) and durability of extruded fibre–cement reinforced with bleached eucalyptus pulp and residual sisal chopped fibres. The thermal analysis, bulk density, porosity, physical characteristics and mechanical performance were evaluated before and after 200 soaking and drying cycles for following the degradation of the material under accelerated ageing conditions. The higher carbonation rate during the early stage of curing period decreased the porosity by sealing the opened pores around vegetable fibres and, consequently, led to lower water absorption and higher bulk density in the composites. The average MOR-values showed a significant increase in the case of the supercritical carbonated extruded fibre–cement in the initial age and after accelerated ageing. Besides, after 200 soaking and drying ageing cycles, the average values of energy of fracture (γ_WoF) of the carbonated composites decrease only 28%, showing evidences of the preservation of microstructural stability and toughness of the fibre–cement composites after supercritical CO₂ treatment.     

掺PVA纤维的抗裂改性水泥的性能与应用研究

抗裂性差是水泥基材料存在的主要问题之一,严重影响水泥基材料物理力学性能和耐久性。本文对掺PVA纤维的抗裂改性水泥的性能与应用进行了研究。结果表明,与普通水泥砂浆相比,掺PVA纤维的抗裂砂浆的强度、变形性能、抗裂性和耐久性均具有明显改善。PVA纤维增强抗裂砂浆技术在工程中得到了实际的应用。     

Investigation into engineering properties and strength mechanism of grouted macadam composite materials

To further understand engineering properties of grouted macadam composite materials (GMCM) used as a surfacing layer in pavement, the mechanical properties and durability characteristics of GMCM were evaluated, and the relevant strength mechanisms were investigated at the micro level. Results indicate that GMCM has better high-temperature stability, fatigue performance and moisture stability than that of conventional asphalt mix, while it shows an acceptable decrease in low-temperature crack resistance due to the relative brittleness of hardened cement paste. The hardened cement paste also generates a spatial network crystalline lattice in asphalt mix skeleton to form a three-dimensional integral coagulation-crystalloid structure. This facilitates the asphalt mix skeleton and hardened cement paste to bear loads in unison and increase durability of the GMCM. Further, the fibre-like hydrated products of fresh cement slurry on the bitumen film surface increase the interfacial strength between bitumen and hardened cement paste due to toughening and bridging effects, which plays an important role to enhance mechanical properties and durability of GMCM. Finally, GMCM strength is from the internal friction of asphalt mix skeleton, the network structure of hardened cement paste and the adhesion between porous asphalt mix and hardened cement paste. It is concluded that GMCM can better meet the requirements of mechanical properties and durability characteristics than the conventional asphalt mix.     

Application of high performance polypropylene fibers in concrete lining of water tunnels

In this study, the application of high performance polypropylene fibers (HPP fibers) in concrete lining of water tunnels, was investigated experimentally. A comparison between the behavior of steel fiber reinforced concrete and HPP fiber reinforced concrete with ordinary concrete is drawn. Advantages and shortcomings of HPP fibers used for concrete lining of water tunnels are also presented.The obtained results showed that the HPP fibers were not effective in compressive strength when compared to steel fibers, but the effects of HPP fibers on tensile strength, flexural strength, toughness and energy absorption of concrete were significant. Based on the results, the effects of HPP fibers on concrete characteristics such as the flexural toughness, concrete permeability and resistance to chloride penetration were higher than those of steel fibers. The results also showed that with application of HPP fibers, durability and serviceability of the concrete linings can be improved.     

Impact of Cement Hydration on Durability of Cellulosic Fiber‐Reinforced Cementitious Composites in the Presence of Metakaolin

Degradation of cellulosic fiber in the alkaline environment of concrete generated in the process of cement hydration is the primary reason for the low durability of such composites. However, the impact of cement hydration on cellulosic fiber's degradation in cementitious systems has not been thoroughly understood. This paper presents the dependence of deterioration behavior of cellulosic fiber‐cement systems on cement hydration in the presence of metakaolin. Experimental investigations, such as isothermal calorimetry, thermogravimetric analysis and energy dispersive X‐ray spectroscopy, and thermodynamic simulations are carried out to investigate cement hydration kinetics and hydration products. Durability of cellulosic fiber‐reinforced cement composite is assessed based on the degradation in flexural properties. The results indicate that, in the presence of metakaolin, the hydration of cement is enhanced accompanied by consumption of calcium hydroxide, low release of hydration heat, decreased Ca/Al and Ca/Si ratios of C–S–H phase, and reduced OH^‐ and Ca²⁺ amounts in pore solution. A cement substitution by 30 wt% metakaolin results in an improvement of flexural toughness and durability of cellulosic fiber‐reinforced cement composites by 42 and 269%, respectively. The correlations between composite durability and hydration of Portland cement are established.

     

Effect of a novel coupling agent,alkyl ketene dimer,on the mechanical properties of wood–plastic composites

The objective of this study was to investigate the incorporation of poplar wood fibers both with and without a novel coupling agent, alkyl ketene dimer (AKD), on the mechanical properties of wood fiber/polypropylene (PP) composites. The resulting properties were compared to those obtained with the most commonly used coupling agent, maleic anhydride grafted PP (MAPP). Tensile and impact strengths of the composites decreased with increasing poplar wood fibers content. Tensile modulus of the composites increased by the incorporation of the wood fibers content up to 70 wt% but further increment in the wood fibers decreased the tensile modulus. At the constant content of poplar wood fibers (70 wt%), the tensile strength determined for the coupled composites with 5% AKD increased by 41% in comparison with the non-coupled composites while the tensile modulus increased by 45%, the impact strength of the coupled composites increased by 38%. The performance of 5% AKD on the mechanical properties of the composites is a little better than 3% MAPP. The good performance of 5% AKD is attributed to the enhanced compatibility between the poplar wood fibers and the polymer matrix. The increase in mechanical properties of the composites demonstrated that AKD is an effective coupling agent for wood fiber/PP composites.     

Parameters related to fiber length and processing in cementitious composites

Effects of fiber length on the tensile and flexural performance of cast and extruded PVA fiber reinforced cement composites were investigated. Microstructural characterization, image analysis, and statistical tools were used to study the influence of processing and fiber length on fiber-matrix bond, fiber dispersion and fiber orientation in the composites. In the extruded composites, shorter fibers improved the performance. In the cast composites, longer fibers gave the best performance. This contradictory trend was found to be a result of differences in fiber failure mechanism, fiber distribution and fiber orientation. Microstructural observations indicated a strong matrix-fiber bond for the extruded composites. Statistical quantification of image analysis indicated a better distribution and alignment of shorter fibers in extruded composites.

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Résumé Les effets de la longueur des fibres sur les performances de tension et de flexion des composites moulés et des composites extrudés, à base de ciment renforcé et de fibres d’acétate de polyvinyle, ont été étudiés. La caractérisation microstructurale, l’analyse d’images et des outils statistiques ont été utilisés pour étudier l’influence du traitement et de la longueur des fibres sur le lien fibres-matrices, la dispersion et l’orientation des fibres dans les composites. Dans les composites extrudés, des fibres plus courtes ont amélioré les performances. Dans les composites moulés, des fibres plus longues ont engendré de meilleures performances. Cette tendance contradictoire s’est avérée être le résultat de différences dans le mécanisme de rupture des fibres, la distribution des fibres et l’orientation des fibres. Les observations microstructurales ont indiqué un lien matrice-fibres important pour les composites expulsés. La quantification statistique de l’analyse d’images a montré une meilleure répartition et un meilleur alignement des fibres plus courtes dans les composites extrudés.

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Editorial Note Prof. Dr. Surendra P. Shah is RILEM Senior Member. He participates in RILEM TCs 162-TDF (Test and design methods for steel fibre reinfored concrete) and EAS (Early age shrinkage induced stresses and cracking in cementitious systems). Prof. Shah is a member of the Management Advisory Committee of RILEM. He is also Editor in chief of Concrete Science and Engineering, a scientific journal published quarterly by RILEM Publications S.A.R.L.     

Fatigue-induced in-situ strength deterioration of micro-polyvinyl alcohol (PVA) fiber in cement matrix

For soft fiber and brittle matrix system such as polymeric fiber-reinforced cementitious composites, the fiber strength deterioration dominates the performance of composites subject to fatigue loading. The fatigue-induced in-situ fiber strength deterioration in brittle matrix, however, has rarely been studied. In this paper, fatigue-induced in-situ strength deterioration of micro-polyvinyl alcohol (PVA) fiber in cement matrix was experimentally investigated. The effects of fiber embedment, fiber inclination, and fiber surface treatment on the in-situ strength of micro-PVA fibers are reported. The results show that fiber embedment into cement matrix not only reduces the in-situ strength of fiber but also changes the fatigue stress-cycle (S-N) curve and failure mode of fiber. Fiber inclination further decreases the in-situ strength of embedded fiber due to local stress concentration of bent fibers. Oil-treatment on fiber surface can effectively delay fatigue-induced in-situ strength deterioration of micro-PVA fiber.     

Crack growth resistance of hybrid fiber reinforced cement composites

Crack propagation in cement-based matrices carrying hybrid fiber reinforcement was studied using contoured double cantilever beam (CDCB) specimens. Influence of fiber type and combination was quantified using crack growth resistance curves. It was demonstrated that a hybrid combination of steel and polypropylene fibers enhances the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing high performance hybrid fiber composites. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper emphasizes the desired durability characteristics of these composites and discusses their current and future applications.     

Effect of operating parameters and chemical treatment on the tribological performance of natural fiber composites: A review

The demand for high-performance engineering products made from natural resources is increasing because of the low-cost, low-density, biodegradability, renewable nature and lighter than synthetic fibers. With these characteristics, the tribological performance of natural fiber composite has become an important element to be considered in most industrial and manufacturing functions. This paper presents an overview of the factors that influence the tribological performance of natural fiber composites, which include applied load, sliding distance, sliding velocity and fiber orientation. Influences of chemical treatment is also reviewed and illustrated through scanning electron microscope (SEM) observations. This review will focus on kenaf fibers (KFs) and oil palm fibers (OPFs) which have been widely exploited over the past few years among the available natural resources. The results show that the operating parameter, fiber orientation and chemical treatment has significant effects on the tribological performance of natural composite. A clear understanding of the factors that affect the tribological performance is very essential in performance improvement on natural fibers reinforced polymer composite for potential applications.     

Mix design of concrete for prestressed concrete sleepers using blast furnace slag and steel fibers

The application of ground granulated blast furnace slag (GGBFS) and steel fibers in prestressed concrete railway sleepers was investigated in this study. The use of GGBFS was considered as an eco-friendly material aimed at reducing CO₂ emissions and energy consumption as well as to enhance the durability performance of railway sleepers. Steel fibers improves the durability and structural performance in terms of crack control and reduction of spalling and can replace shear reinforcement. The mix proportions of the concrete incorporating GGBFS (56% GGBFS) and GGBFS with steel fibers (56% GGBFS and 0.75% steel fibers) were determined through a series laboratory tests and a life cycle assessment. These mixes satisfied the requirements of the Korean Railway Standard and resulted in improved flexural capacity as well as less CO₂ emissions compared with current railway sleepers. Using these mixes, a total of ninety prestressed concrete sleepers were produced in a factory under the same manufacturing process as current railway sleepers, and their mechanical properties as well as durability performance were evaluated. The mix with partial replacement of Type III Portland cement by GGBFS showed an improved resistance to chloride ion penetration and freeze-thaw cycles compared with the concrete used for current railway sleepers. However, these mixes were more vulnerable to carbonation. The mix with GGBFS and steel fibers (mix BSF) showed a slightly better durability performance than the mix with GGBFS only (mix BS), including better carbonation and freeze-thaw resistances. The mix BSF showed decreased chloride ion penetration depth than mix BS but showed a slightly higher chloride ion diffusion coefficient.     

A methodology for quantifying the impact of casting procedure on anisotropy in fiber-reinforced concrete using X-ray CT

Fiber-reinforced concretes (FRCs) offer significant improvements in tensile strength and durability compared to most other concrete mixes. However, for safe and efficient use of FRC in large structures, anisotropy of fiber orientation needs to be understood and properly controlled. In this project, both cored samples extracted from a FRC slab and FRC samples cast individually in molds were assessed using X-ray computed tomography (CT) and measurements of fiber orientation were extracted from the resulting CT images. These results showed that fibers within the slab were highly anisotropic in orientation while fibers in individually cast samples showed a much more heterogeneous distribution of orientations. This indicates that fiber orientation is highly dependent on the casting process and suggests that FRC can only be safely and efficiently utilized if anisotropic fiber orientation is properly accounted for during design and optimized casting methods are used during construction.     

喷射FRP加固震损钢筋混凝土柱抗震性能试验

通过12组72件喷射纤维/树脂复合材料(FRP)试样的拉伸强度试验,研究了纤维种类、树脂基体材料、纤维体积分数、纤维混杂比及纤维长度等因素对喷射FRP拉伸强度、弹性模量和断裂伸长率等性能的影响。通过8根钢筋混凝土(RC)柱试件的拟静力试验,研究了喷射玄武岩纤维/树脂复合材料(BFRP)和混杂玄武岩-碳纤维/树脂复合材料(BF-CFRP)加固震损RC柱的抗震性能,分析了喷射FRP层厚度、纤维混杂比、柱预损程度和柱轴压比等对加固试件的极限承载力、抗侧变形能力、刚度退化特征和滞回特性的影响。结果表明:玻璃纤维与乙烯基酯树脂基体的协同工作性能最优,而玄武岩纤维具有耐久性高、延性好、与乙烯基酯树脂基体协同工作性能好等优良性能,可以作为玻璃纤维的良好替代品;玄武岩纤维混杂少量比例的碳纤维作为树脂基体增强材料,可以有效提高喷射FRP的拉伸强度和变形性能;震损RC柱经喷射FRP加固后,可以基本恢复其震损前设计极限承载力,并有效提高其延性和耗能能力。该加固方法可以对地震区已震损RC柱进行快速加固,有效防止整体结构在余震中发生倒塌等严重破坏。