Cohesive fracture model for functionally graded fiber reinforced concrete

A simple, effective, and practical constitutive model for cohesive fracture of fiber reinforced concrete is proposed by differentiating the aggregate bridging zone and the fiber bridging zone. The aggregate bridging zone is related to the total fracture energy of plain concrete, while the fiber bridging zone is associated with the difference between the total fracture energy of fiber reinforced concrete and the total fracture energy of plain concrete. The cohesive fracture model is defined by experimental fracture parameters, which are obtained through three-point bending and split tensile tests. As expected, the model describes fracture behavior of plain concrete beams. In addition, it predicts the fracture behavior of either fiber reinforced concrete beams or a combination of plain and fiber reinforced concrete functionally layered in a single beam specimen. The validated model is also applied to investigate continuously, functionally graded fiber reinforced concrete composites.     

Orientation factor and number of fibers at failure plane in ring-type steel fiber reinforced concrete

Considering the probabilistic distributions of fibers in ring-type steel fiber reinforced concrete, the orientation factor and the number of ring-type steel fibers crossing the failure plane were theoretically derived as a function of fiber geometry, specimen dimensions, and fiber volume fraction. A total number of 24 specimens were tested incorporating different fiber types, specimen geometry, and fiber volume fractions of 0.2% and 0.4%: 5 beams and 5 panels containing straight steel fibers; and 6 beams and 8 panels containing ring-type steel fibers. Measurements were made to assess the number of fibers at fractured surfaces of steel fiber reinforced concrete. The developed theoretical expressions reasonably predicted the orientation factor and the number of ring-type steel fibers at failure plane: the average and the standard deviation for the ratios of the test to theory were 1.03 and 0.26, respectively. Theoretical investigations and comparisons were made for the values of orientation factor and the number of fibers at failure plane for straight steel fibers and ring-type steel fibers.     

玄武岩-粗聚丙烯纤维干硬性混凝土拉压性能试验研究

混杂纤维增强干硬性混凝土在国内外已有广泛的应用,纤维配比是影响其拉压性能的主要因素之一。为研究玄武岩纤维与粗聚丙烯纤维配比对干硬性混凝土拉压性能的影响,将玄武岩纤维与粗聚丙烯纤维单掺或按不同比例混合掺入干硬性混凝土中,开展不同养护龄期下纤维混凝土的抗压、劈裂抗拉试验,分析纤维混杂增强效应,并基于成熟度理论修正养护龄期,优化玄武岩-粗聚丙烯纤维干硬性混凝土的劈裂抗拉强度预测模型。结果表明:玄武岩纤维与粗聚丙烯纤维的掺入不仅提升了干硬性混凝土抗压、劈裂抗拉性能,而且纤维的桥接作用能明显改善混凝土的脆性破坏特征,其中玄武岩纤维与粗聚丙烯纤维混掺配比为1 ∶2(质量比)时最为明显,表现出了最优的纤维混杂正效应。根据等效龄期-抗压强度关系式计算得到的混凝土抗压强度与劈裂抗拉强度具有更好的幂函数关系,该模型便于计算及预测不同养护温度条件下玄武岩-粗聚丙烯纤维干硬性混凝土的拉压性能。     

玄武岩纤维钢筋混凝土梁弯曲性能试验研究

通过玄武岩纤维钢筋混凝土梁四点弯曲试验,分析其破坏形态和破坏机理.通过四点对称加载方式研究不同玄武岩纤维掺量下梁的承载力、挠度、韧性、混凝土应变、钢筋应变等变化规律.试验结果表明:玄武岩纤维掺入对钢筋混凝土梁的开裂荷载和极限荷载都有一定的提高,开裂荷载最大提高幅度为32％,极限荷载最大提高幅度为6.5％.与普通钢筋混凝土梁相比玄武岩纤维钢筋混凝土梁的挠度、韧性均有所提高.玄武岩纤维对梁的受压区混凝土具有阻止裂缝扩展的能力,当梁上部受压区混凝土被压碎时,混凝土碎块会在纤维的桥接作用下不剥落,梁仍保持较好的整体性.     

A theory for the flexural strength of steel fiber reinforced concrete

A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.     

FRP筋异强混凝土叠浇梁挠度与延性研究

为提高纤维增强聚合物(FRP)筋异强混凝土叠浇梁的抗弯性能与延性,研究了钢纤维掺量、钢纤维混凝土叠浇层厚度对FRP筋异强混凝土叠浇梁的影响。以钢纤维体积掺量(0%、0.5%、1.0%、1.5%)与钢纤维混凝土叠浇层厚度(0 mm、180 mm、210 mm、300 mm)为变量,对6根FRP筋异强混凝土叠浇梁进行三分点弯曲试验,并对试验梁的破坏过程、破坏形态、裂缝宽度以及跨中挠度进行分析。研究结果表明：钢纤维的掺入改善了FRP筋异强混凝土叠浇梁的受力性能,使其由脆性破坏向延性破坏发展;随着钢纤维掺量、钢纤维混凝土叠浇层厚度的增加,FRP筋异强混凝土叠浇梁的极限承载力提高了9%～33%,抗弯性能提升了4%～21%,延性提升了22%～89%。基于试验与理论分析,建立了钢纤维作用下的FRP筋异强混凝土叠浇梁挠度计算公式与延性评价方法。     

Estimation of the volume resistivity of electrically conductive composites

The modeling of the electrical conductivity of polymer composites reinforced with conductive fibers is investigated. Existing models generally can be divided into percolation theories and non-percolation theories. The basis of the percolation theory is the fact that the conductivity of the composite increases dramatically at a certain fiber concentration called the percolation threshold. This theory can be used to model the behavior of the composite or to predict the percolation threshold itself. Non-percolation theories include terms, which account for microstructural data such as fiber orientation, length, and packing arrangement. A comparison of experimental data with predictions from the various models reveals that only the percolation theory is able to accurately model the conductive behavior of an actual composite. Two alternative new models, which predict the volume resistivity of a composite using microstructural data, are evaluated. The first model relates resistivity to the concentration and orientation of the fibers, while assuming perfect fiber-fiber contact. The relationship between resistivity and fiber concentration predicted by the model is in qualitative agreement with actual data, and predictions of the anisotropy in volume resistivity compare well with experimental results. The second model accounts for the effect of fiber-fiber contact and fiber length on composite resistivity. Predictions are in excellent agreement with experimental data for polypropylene composites reinforced with nickel-coated graphite fibers.     

纤维聚合物筋混凝土梁正截面承载力的计算方法

以3根粘砂碳纤维聚合物筋混凝土梁和2根粘砂玻璃纤维聚合物筋的试验为依据,分析了纤维聚合物筋混凝土梁的挠度变形和破坏形式,以及界限配筋率和正截面承载力;并结合我国混凝土结构设计规范对梁的正截面承载力计算公式进行了推导。试验结果表明:该公式可以用来对纤维聚合物筋混凝土梁的正截面承载力进行预测。     

芳纶纤维加固钢筋混凝土梁抗弯疲劳性能试验研究

本文采用芳纶纤维加固钢筋混凝土梁进行室内疲劳试验,分析了混凝土和钢筋的应变滞回变化规律及加固构件刚度随循环次数的衰减变化规律.试验表明采用芳纶纤维进行加固后,梁的疲劳抗裂性能得到极大改善,有效地延长了损伤混凝土结构的使用寿命,验证了芳纶纤维用于加固承受疲劳荷载结构的可靠性.     

Properties of wet-mixed fiber reinforced shotcrete and fiber reinforced concrete with similar composition

Fiber reinforced shotcrete (FRS) is commonly used in slope protection, tunnel linings as well as structural repair and rehabilitation. For the design of shotcrete mixes, it is of interest to see if data on fiber reinforced concrete (FRC) can be employed as an initial guideline. In this study, various properties of FRS, including its compressive strength, flexural behavior, permeability and shrinkage behavior, are compared with FRC of similar composition. The results, based on five different mixes, indicate that the fabrication process (i.e., shotcreting vs. casting) can significantly affect compressive strength and permeability, but has relatively little effect on shrinkage behavior. The flexural strength of FRS is slightly higher than that for FRC in most cases, but the residual load carrying capacity in the postcracking regime can be significantly lower. Based on the differences in the properties of FRC and shotcrete, implications to material design are discussed.     

Corrosion effects on pullout behavior of hooked steel fibers in self-compacting concrete

Fiber reinforced concrete structures are subjected to chloride and carbonation penetration that could initiate corrosion of steel fibers, with eventual pernicious consequences in terms of structural and durability performance. Cracks in concrete are known to hasten initiation of steel corrosion in reinforced concrete structures. The investigation of the impact of cracks on the corrosion initiation and the associated interfacial damage between concrete and steel fibers is important for understanding the mechanical behavior of steel fiber reinforced concrete.In the present work, with the aim of studying the corrosion action on the mechanical behavior of cracked Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC), an experimental program was performed to characterize the corrosion of hooked-end steel fibers and to assess the fiber pullout behavior in cracked concrete, previously subject to the action of corrosion by exposure to aggressive chloride environment.     

Fracture behavior and analysis of fiber reinforced concrete beams

Fiber reinforced concrete beams with varying notch depths and different volume fractions of steel fibers were tested. The results were analyzed to examine the applicability of various fracture mechanics approaches including: critical stress intensity factor, J-integral, critical crack opening displacement, compliance technique for determining the slow crack growth and R-curve analysis. Attempts were made to identify a fracture parameter which is independent of test-specimen geometry and which can correctly predict the effects of fiber addition. R-curve method appears to be promising.     

内埋碳纤维砂浆调节混凝土梁承载能力的试验研究

基于碳纤维水泥基材料的电热效应,通过内埋碳纤维砂浆块对普通混凝土梁进行变形调节,获得了相应的温度-变形规律.在此基础上实施了对混凝土梁预加反向变形,使混凝土梁内部产生预应力,从而提高了混凝土梁达到相同挠度时的承载能力.     

结合均匀化理论对钢纤维混凝土梁的疲劳性能分析

钢纤维混凝土(SFRC)在工程中应用日益广泛,为了探究其疲劳破坏现象内在机理,本文结合Mori-Tanaka均匀化理论预测了不同体积掺量下钢纤维混凝土的弹性模量,并以此为基础建立钢纤维混凝土梁四点弯曲有限元模型,采用Miner疲劳损伤准则,分别进行了静力抗弯试验和疲劳试验的数值模拟。模拟结果与相关试验结果拟合较好,验证了模型的可靠性。利用疲劳分析软件,预测了钢纤维混凝土梁的疲劳寿命和疲劳强度,分析了纤维掺量、尺寸效应和纤维长度对疲劳寿命的影响。结果表明:钢纤维可以大幅提高混凝土梁的疲劳寿命,应力水平越低、纤维掺量越大,增幅越大;尺寸效应对疲劳强度和疲劳寿命有一定影响,但随着构件尺寸增加对疲劳性能影响减小;纤维长度越长,梁的抗疲劳性能越好。     

纤维混凝土梁研究综述

对纤维增强混凝土梁的国内外研究进展进行了归纳总结,从试验分析方面阐述了纤维对梁力学性能的影响;在理论研究方面对纤维混凝土梁的抗弯和抗剪理论及抗剪承载力计算公式进行了总结并对公式的实用性做了进一步的阐述;在数值模拟方面介绍了纤维混凝土梁数值计算使用的的软件及成果.最后,基于现有的研究成果,提出了目前存在的不足,探讨了未来纤维混凝土梁的发展方向.     

Flexural properties after exposure to elevated temperatures of a ground granulated blast furnace slag concrete incorporating steel fibers and polypropylene fibers

Experiments were carried out to investigate the flexural properties of fiber‐reinforced ground granulated blast furnace slag (GGBFS) concrete after exposure to high temperatures. On the basis of experimental observation, the effect of GGBFS content, the steel fiber dosage, the polypropylene (PP) fiber dosage, and the strength grade on the residual strength of concrete after exposure to elevated temperatures were systematically examined. Test data indicate that exposure to high temperatures causes deterioration in the flexural strength of concrete; inclusion of GGBFS, PP fibers, and steel fibers, all effectively improve the residual flexural strength of concrete after fire. The optimum amounts of GGBFS, PP fibers, and steel fibers are identified respectively for better fire resistance of concrete. The strength losses of concretes characterized by different strength grades are very close to one another. Equations are proposed to predict the residual flexural strength of concrete incorporating GGBFS, PP fibers, and steel fibers after being heated to temperatures up to 800°C. Copyright © 2013 John Wiley & Sons, Ltd.     

Prediction of failure and fracture mechanisms of polymeric composites using finite element analysis. Part 2: Fiber reinforced composites

A robust finite element scheme for the micro‐mechanical modeling of the behavior of fiber reinforced polymeric composites under external loads is developed. The developed model is used to simulate stress distribution throughout the composite domain and to identify the locations where maximum stress concentrations occur. This information is used as a guide to predict dominant failure and crack growth mechanisms in fiber reinforced composites. The differences between continuous fibers, which are susceptible to unidirectional transverse fracture, and short fibers have been demonstrated. To assess the validity and range of applicability of the developed scheme, numerical results obtained by the model are compared with the available experimental data and also with the values found using other methods reported in the literature. These comparisons show that the present finite element scheme can generate meaningful results in the analysis of fiber reinforced composites.     

Comparative behavior under compression of concrete columns repaired by fiber reinforced polymer (FRP) jacketing and ultra high-performance fiber reinforced concrete (UHPFRC)

This paper summarizes the experimental results from a comprehensive research program to study the fundamental stress–strain behavior of damaged concrete repaired by two techniques: increased concrete section and bonding fiber reinforced polymer (FRP). In this work, two types of FRP composite jackets were used, carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer, and two types of concretes were used to repair the damaged concrete by increased concrete section: ordinary concrete and ultra high-performance fiber reinforced concrete (UHPFRC). Fifteen circular columns of concrete (110 × 220) cm³ were initially pre-damaged up to intense cracking, repaired by increased concrete section and by bonding FRP, and tested under uni-axial compression by loading up the damage. The impact of different design parameters, including plain concrete strength, types of composites, and type of concrete used for increasing section, was considered in this study. The strength enhancement and ductility improvement of specimens are discussed. A simple model is presented to predict the compressive strength of repaired damaged concrete columns. A significant strength and an increase in ductility were achieved, particularly when the columns were repaired by increasing section with UHPFRC and by bonding CFRP. These preliminary tests indicate that the use of UHPFRC is an effective technique for rehabilitating damaged concrete columns, highly competitive with the repaired concrete by wrapping specimens with FRP composite jackets.     

Introducing two simple models for predicting fiber‐reinforced asphalt concrete behavior during longitudinal loads

Scientists and engineers are constantly trying to improve the performance of asphalt pavements. Modification of the asphalt binder is one approach taken to improve pavement performance. The idea of using fibers to improve the behavior of materials is an old suggestion, so different researchers reported the results of adding a large variety of fibers to asphalt concrete (AC) as fiber‐reinforced asphalt concrete (FRAC). However, there are few comments about the mechanism of reinforcement and fiber performance in the inner structure of AC and/or exposing some models to predict fiber recital as a modifier in FRAC. So this article is going to introduce two simple models for predicting FRAC behavior during longitudinal loads. The former is called “Slippage Theory” and the latter is “Equal Cross‐Section.” Finally, four types of fibers (glass, nylon 6.6, polypropylene, and polyester) were used in AC to evaluate the two theories. “Marshall Test,” as stability and flow outcomes, and “Specific Gravity” were carried out on specimens in the next stages followed by an artificial neural network (ANN), which was developed in the system to recognize important fiber parameters effective in the FRAC specifications. In the end, the two theories predicted each fiber performance in FRAC as well as ANN. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

碳/玻混杂纤维的混杂效应及其受力性能研究

碳纤维与玻璃纤维进行层间混杂后用来进行混凝土结构的加固,可以产生较好的正向混杂效应．就混杂纤维的混杂方式．混杂效应和受力性能进行了研究,结果表明：较之单一纤维,混杂纤维复合材料表现出了明显的混杂效应,还可以降低加固成本,综台效益较好。并提出了混杂纤维复合材料在当前工程应用中存在的问题。