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...     

CaCO_3晶须对钢-聚乙烯醇混杂纤维增强水泥基复合材料板弯曲性能的影响

将廉价的微米级CaCO_3晶须引入毫米级钢纤维与聚乙烯醇(PVA)纤维混杂纤维增强水泥基复合材料(HyFRCC),研究CaCO_3晶须对HyFRCC薄板力学性能、破坏形态和尺寸效应的影响,并使用SEM观察HyFRCC微观形貌。试验结果显示,引入晶须后,HyFRCC薄板呈现出良好的弯曲性能及比梁式试件更优良的假性应变硬化和多缝开裂特征,且可以更有效地减小尺寸效应对抗弯强度的影响。微观形貌观察证实掺加晶须后,混杂纤维体系可以在不同尺度上发挥多层次阻裂作用。研究认为,由廉价的CaCO_3晶须部分替代钢纤维和PVA纤维制备的HyFRCC呈现出对板式构件良好的适应性,实现了力学性能优化和经济性提高的双重目标。     

Influence of fiber content on the mechanical and dynamic mechanical properties of glass/ramie polymer composites

The combination of glass and ramie fibers with a polyester matrix can produce a hybrid material that is competitive to all glass composites (e.g. those used in the automobile industry). In this work, glass and ramie fibers cut to 45 mm in length were used to produce hybrid polymer composites by resin transfer molding (RTM), aiming to evaluate their physical, mechanical and dynamic mechanical properties as a function of the relative glass–ramie volume fractions and the overall fiber content (10, 21 and 31 vol.%). Higher fiber content and higher ramie fiber fraction in the hybrid composites yielded lower weight composites, but higher water absorption in the composite. The mechanical properties (impact and interlaminar shear strength) of the composites were improved by using higher fiber content, and the composite with 31 vol.% of reinforcement yielded the lowest value for the reinforcement effectiveness coefficient C, as expected. Although the mechanical properties were improved for higher fiber content, the glass transition temperature did not vary significantly. Additionally, as found by analyzing the adhesion factor A, improved adhesion tended to occur for the composites with lower fiber content (10%) and higher ramie fiber fraction (0:100) and the results for the adhesion factor A did not correspond to those found by the analysis of the tan delta peak height.     

Effect of accelerated freeze–thaw cycling on mechanical properties of hybrid PVA and PE fiber-reinforced strain-hardening cement-based composites (SHCCs)

In this study, the influence of rapid freezing and thawing actions on mechanical properties of hybrid fibers reinforced strain-hardening cement-based composites (SHCCs), which exhibit multiple cracking and strain-hardening behavior in direct tension, were investigated. Four SHCC mixtures with different water-to-binder (W/B) ratios and hybrid fiber combinations were assessed experimentally. The SHCC mixtures incorporating hybrid polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE) fibers at the 1.5% volume fraction were exposed to freezing and thawing according to ASTM C 666 (Procedure B). The freeze–thaw tests continued until the specimens achieved 300 freeze–thaw cycles. The results of these tests indicate that rapid freeze–thaw cycles in the laboratory have little effect on the compressive and tensile strength characteristics of the SHCC mixtures prepared in this study, whereas multiple cracking behavior and deformation capacity of SHCC specimens under direct tensile and flexural loadings indicate that freeze–thaw cycles have a negative effect on the these characteristics of the SHCC mixtures. A tendency toward reduced ductility is prominent for SHCC materials with higher W/B ratio and more hydrophilic PVA fiber.     

Effects of fiber–matrix interaction on multiple cracking performance of polymeric fiber reinforced cementitious composites

The effects of polymeric fiber addition on the multiple cracking performance of composites have been investigated. For this purpose, cement-based matrices incorporating fly ash and a latex emulsion have been designed. Prismatic samples have been prepared and subjected to four-point bending load. The load-midpoint deflection curves and crack patterns have been determined. Meanwhile, flexural strength and relative toughness values have been calculated. Finally, the number of visible cracks formed throughout the testing period has been analyzed.Test results showed that the toughening improvement mechanisms of PP and PVA fibers in a cement-based matrix are extremely different and matrix modifications significantly change the multiple cracking performance. The addition of a latex emulsion in a weak matrix decreased the multiple cracking tendency of PP fiber reinforced composites. However, the same modification attempt improved the multiple cracking capacity of weak matrix in case of PVA fiber reinforcement. The possible causes of this performance improvement have been discussed with the aid of microstructure investigations.     

Analytical and experimental investigations on the fracture behavior of hybrid fiber reinforced concrete

In the present study, Mode-I fracture tests of hybrid fiber reinforced concrete (HFRC) composite beams were conducted and the fracture properties and other post peak strength characteristics of the HFRC composites were evaluated and analyzed. The HFRC composite was produced using three types of fibers namely steel, Kevlar and polypropylene. A total of 27 HFRC composite beam specimens were cast and tested using the RILEM recommended three point bending test. The main variables were the fiber volume content and combinations of different fibers. The load versus crack mouth opening displacement (CMOD) curves of HFRC composite beams were obtained. Inverse analysis was carried out to determine the tensile strength and crack opening relationship. Analytical models based on comprehensive reinforcing index were developed for determining the influence of the fibers on fracture energy, flexural tensile strength, equivalent tensile strengths and residual tensile strengths of HFRC composites. Based on the experimental results and inverse analysis, a model for predicting the tensile softening diagram of HFRC composite mixes was also developed. The analytical models show conformity with the experimental results.     

Effects of coarser fine aggregate on tensile properties of ultra high performance concrete

This experimental research investigates the mechanical properties and shrinkage of ultra high performance concrete (UHPC) incorporating coarser fine aggregates with maximum particle size of 5 mm. To adequately design UHPC mixtures using various sizes of solid constituents, particle packing theory was adopted. UHPC mixtures containing either dolomite or basalt, and four fiber volume fractions up to two volume percent were investigated. Uniaxial tension test was performed to evaluate the first cracking tensile strength, ultimate tensile strength, tensile strain capacity and cracking pattern. The UHPC mixtures with dolomite and steel fibers with more than one volume percent achieved more than 150 MPa of compressive strength at the age of 56 days, and showed strain hardening behavior and limited decrease in tensile strength compared to typical UHPC without coarser fine aggregates. The experimental results highlight the potential of dolomite used as coarser fine aggregate in UHPC.     

不同纤维掺量下聚乙烯醇纤维增强工程水泥复合材料梁剪切韧性试验

为研究聚乙烯醇纤维增强工程水泥复合材料（PVA/ECC）无腹筋梁的剪切韧性,基于5组PVA/ECC梁受剪破坏试验结果,以剪切韧性指数和斜裂缝综合指数为指标,对不同纤维掺量下PVA/ECC梁的斜截面剪切韧性进行了研究与评价。结果表明:PVA纤维的掺入能改善梁的开裂性能,明显提高梁受荷全过程的变形能力及斜截面承载力,从而提高构件的剪切韧性;PVA纤维体积分数在0~2vol%范围内时,其值越大,加载过程中消耗的能量越多,斜截面抗剪承载力越高,破坏之前的总变形越大,梁的剪切韧性越好。      

混杂纤维增强超高性能混凝土弯曲韧性与评价方法

为了研究混掺纤维对超高性能混凝土(UHPC)的增韧效果, 通过161个三点弯曲梁的断裂试验, 测定了4种纤维和不同掺量下各UHPC试件的载荷-裂口张开位移(CMOD)曲线和载荷-挠度曲线。将素UHPC峰值载荷对应的CMOD视为混杂纤维增强UHPC的初裂CMOD值, 基于载荷-CMOD曲线提出了等效断裂韧度的韧性评价方法, 该方法具有明确的物理含义, 可用于分析混掺纤维品种和掺量对UHPC断裂韧性的影响规律。研究发现:在小变形(小于50倍素UHPC峰值载荷对应的CMOD值)时, UHPC韧性取决于钢纤维的掺率;粗合成纤维主要在中等变形和大变形阶段(大于50倍素UHPC峰值载荷对应的CMOD值)发挥其增韧效用。      

Modeling the effect of reinforcement discontinuity on the tensile strength of UD flax fiber composites

To exploit the potential of natural fibers as reinforcement of polymer matrix composites, aligned bast fiber composite materials are being produced and studied. Bast fiber reinforcement is discontinuous due to the limited length of natural fibers, which needs to be reflected in predictive models of mechanical properties of composites. The strength in tension in the fiber direction of an aligned flax fiber-reinforced composite is modeled assuming that a cluster of adjacent fiber discontinuities is the origin of fracture. A probabilistic model of tensile strength, developed for UD composites containing a microdefect, is applied. It follows from the theoretical analysis that the experimental tensile strength as a function the fiber volume fraction can be described with acceptable accuracy assuming the presence of a cluster of ca. 4 × 4 elementary fiber discontinuities.     

Quantitative characterization of accelerated aging in cement composites using flexural inverse analysis

A constitutive model consisting of a tri-linear tensile stress-strain with residual strength was applied in characterization and prediction of long term flexural behavior of several cement-based composite materials. Flexural test results were back-calculated to obtain material parameters and establish their relationship with aging. The material behavior is described by tensile stress-strain parameters consisting of elastic modulus, first cracking strain, post cracking stiffness, ultimate strain, and a residual strength parameter. The relationships between the material parameters and age were established by studying the time dependent flexural performance of various composites with glass and natural fibers as reported by Litherland et al. (1981), Marikunte et al. (1997), Bartos et al. (1996), and natural fibers reported by Toledo-Filho et al. (2000). An analytical model for prediction of rate and extent of damage as a function of time and temperature is proposed for degradation of flexural behavior of strain softening and hardening fiber reinforced concrete subjected to aging. This model is applicable to long-term durability of different classes of materials subject to accelerated aging under different environmental conditions.     

Bending behavior of mortar reinforced with steel meshes and polymeric fibers

The main purpose of this research was to study the effects of combining reinforcing steel meshes with discontinuous fibers as reinforcement in thin walled Portland cement based mortar beams. The term ‘thin’ implies thicknesses of less than about 25 mm. The underlying idea behind this combination is to satisfy the ultimate strength limit state through the steel mesh reinforcement (main reinforcement) and to control cracking under service loads through fiber reinforcement (secondary reinforcement).

An extensive experimental program with bending tests was undertaken. Specimens were 127 × 457 × 12.7 mm. The following variables were investigated: (a) the reference mesh size — 25.4 × 25.4 mm and 50.8 × 50.8 mm; (b) the transverse wire spacing — 25.4 mm, 50.8 mm, and no transverse wires; (c) the type of fibers — polyvinylalcohol (PVA) and polypropylene (PP); and (d) the fiber volume fraction — 1 and 2% for PVA fibers, and 0.5 and 1% for PP fibers.

Some of the main conclusions are: (a) for the same fiber volume fraction, the use of PVA fibers led to a better overall performance than that of PP fibers; (b) an increase in cracking moment and a decrease in crack spacing was observed when 1% PVA, 2% PVA, and 1% PP fibers were used; (c) when 0.5% PP fiber was used, no noticeable change in behavior was observed in comparison to specimens without fibers; and (d) for 1% PVA fibers the transverse wire spacing had little effect on the crack spacing and for 2% PVA fibers, the transverse wire had no influence.     

Freeze-thaw influence on the flexural properties of ductile fiber-reinforced cementitious composites (DFRCCs) for durable infrastructures

Ductile fiber-reinforced cementitious composites (DFRCCs) are innovative cementitious materials characterized by multiple cracking and pseudo strain-hardening behavior under static flexure. This paper investigates the effects of freeze-thaw cycles, and water-to-binder ratio (W/B) as well as reinforcing fiber combination on flexural properties and cracking procedure of DFRCC prismatic specimens. The DFRCC materials used in the present study are reinforced with hybrid polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE) at the 1.5% volume fraction. These DFRCC materials were tested for modulus of rupture (MOR), relative dynamic modulus of elasticity, and change in mass. The test results for freezing and thawing actions within 300 cycles indicate that freeze-thaw cycles have little effect on the MOR of the DFRCC materials, whereas freeze-thaw cycles have a negative effect on multiple cracking behavior and deformation capacity of DFRCC prismatic specimens under flexural loadings. The results of durability tests show that the DFRCC specimens remain durable after 300 cycles of freezing and thawing actions.     

Micromechanisms of damage in unidirectional fiber reinforced composites: 3D computational analysis

Numerical micromechanical investigations of the mechanical behavior and damage evolution of glass fiber reinforced composites are presented. A program code for the automatic generation of 3D micromechanical unit cell models of composites with damageable elements is developed, and used in the numerical experiments. The effect of the statistical variability of fiber strengths, viscosity of the polymer matrix as well as the interaction between the damage processes in matrix, fibers and interface are investigated numerically. It is demonstrated that fibers with constant strength ensure higher strength of a composite at the pre-critical load, while the fibers with randomly distributed strengths lead to the higher strength of the composite at post-critical loads. In the case of randomly distributed fiber strengths, the damage growth in fibers seems to be almost independent from the crack length in matrix, while the influence of matrix cracks on the beginning of fiber cracking is clearly seen for the case of the constant fiber strength. Competition between the matrix cracking and interface debonding was observed in the simulations: in the areas with intensive interface cracking, both fiber fracture and the matrix cracking are delayed. Reversely, in the area, where a long matrix crack is formed, the fiber cracking does not lead to the interface damage.     

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。     

Prediction of first matrix cracking in micro/nanohybrid brittle matrix composites

The classical Aveston–Cooper–Kelly shear-lag model for predicting the first matrix cracking strength in a brittle matrix composite is extended to the case of a hybrid brittle matrix composite containing both micro-scale and nano-scale fibers. First, closed-form solutions for the stresses in the two types of fibers and the matrix are derived. These are then used along with an energy analysis to predict the matrix cracking stress as a function of relevant material parameters. The analysis is applied to a typical Nicalon-SiC/CVI-SiC ceramic matrix composite containing additional nanofibers, for a wide range of nanofiber properties. A few volume percent of small diameter, moderate-stiffness nanofibers is predicted to provide significant strengthening and reduced crack opening while maintaining acceptable post-cracking fiber stresses. Various issues in the design of such micro/nanohybrid composites are then discussed.     

胶层厚度对碳纤维/双马来酰亚胺树脂复合材料平-折-平混合连接接头力学性能的影响

以碳纤维/双马来酰亚胺(BMI)树脂复合材料平-折-平(FJF)连接接头为对象,通过试验对比分析了特定胶层厚度下碳纤维/BMI树脂复合材料FJF连接接头的静强度和疲劳性能,并探究了胶层厚度对碳纤维/BMI树脂复合材料FJF混合接头力学性能的影响。利用背面应变技术对碳纤维/BMI树脂复合材料FJF混合接头搭接区端部胶层开裂进行监测。利用有限元软件ABAQUS对不同胶层厚度下碳纤维/BMI树脂复合材料FJF混合接头搭接区胶层应力分布进行了分析。结果表明,碳纤维/BMI树脂复合材料FJF混合接头的平均拉伸极限载荷、搭接区端部胶层开裂平均循环次数和平均疲劳寿命均随着胶层厚度在0.1~0.3 mm范围内增加而增大。不同胶层厚度的碳纤维/BMI树脂复合材料FJF混合接头均经历相同的失效阶段,即搭接区胶层端部开裂,胶层沿搭接区断裂扩展,最终靠近加载端孔边拉伸断裂,呈±45°断口。随着胶层厚度在0.1~0.3 mm范围的增加,搭接区端部胶层剥离应力、剪切应力及孔边胶层压缩应力均减小。在胶层厚度为0.1~0.3 mm范围内,剪应力是胶层破坏的控制因素。      

CVD SiC fiber-reinforced barium aluminosilicate glass—ceramic matrix composites

Unidirectional CVD SiC (SCS-6) monofilament reinforced BaOAl₂O₃2SiO₂(BAS) glass—ceramic matrix composites have been fabricated by a tape lay-up method followed by hot pressing. The glass matrix flows around fibers during hot pressing resulting in nearly fully dense (95–98%) composites. Strong and tough composites having first matrix cracking stress of 250–300 MPa and ultimate flexural strength as high as 900 MPa have been obtained. Composite fracture surfaces showed fiber pullout with no chemical reaction at the fiber/matrix interface. From fiber push out, the fiber/matrix interfacial debond strength and the sliding frictional stress were determined to be 5.9 ± 1.2 MPa and 4.8 ± 0.9 MPa, respectively. The fracture surface of an uncoated SiC (SCS-0)/BAS composite also showed fiber/matrix debonding, fiber pullout, and crack deflection around the fibers implying that the SiC fibers may need no surface coating for reinforcement of the BAS glass-ceramic. Applicability of micromechanical models in predicting the first matrix cracking stress and the ultimate strength of these composites has also been examined.     

Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence

Engineered cementitious composites (ECC) is a class of ultra ductile fiber reinforced cementitious composites, characterized by high ductility and tight crack width control. The polyvinyl alcohol (PVA) fiber with a diameter of 39 μm and a length of 6-12 mm is often used. Unlike plain concrete and normal fiber reinforced concrete, ECC shows a strain-hardening behavior under tensile load. Apart from the mix design, the fiber distribution is another crucial factor for the mechanical properties of ECC, especially the ductility. In order to obtain a good fiber distribution, the plastic viscosity of the ECC mortar before adding fibers needs to be controlled, for example, by adjusting water-to-powder ratio or chemical admixtures. However, such adjustments have some limitations and may result in poor mechanical properties of ECC. This research explores an innovative approach to improve the fiber distribution by adjusting the mixing sequence. With the standard mixing sequence, fibers are added after all solid and liquid materials are mixed. The undesirable plastic viscosity before the fiber addition may cause poor fiber distribution and results in poor hardened properties. With the adjusted mixing sequence, the mix of solid materials with the liquid material is divided into two steps and the addition of fibers is between the two steps. In this paper, the influence of different water mixing sequences is investigated by comparing the experimental results of the uniaxial tensile test and the fiber distribution analysis. Compared with the standard mixing sequence, the adjusted mixing sequence increases the tensile strain capacity and ultimate tensile strength of ECC and improves the fiber distribution. This concept is further applied in the development of ECC with high volume of sand.     

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.