Micromechanics of crack bridging in fibre-reinforced concrete

The stress-crack width relationship has been determined experimentally for concretes reinforced with two types of fibres, steel and polypropylene, of various fibre volume fractions. A micromechanics-based theoretical model is proposed which captures the essential features of the stress-crack width relationships at small crack widths (less than 0.3 mm). Micromechanisms accounted for include the bridging actions due to aggregates and fibres, Cook-Gordon interface debonding and fibre pre-stress. The fibre bridging action involves interface slip-dependent friction as well as snubbing friction for fibres bridging at inclined angles. Theoretical predictions based on independent parametric inputs compare favourably with experimental measurements of the stress-crack width relationship. Findings in this research provide confidence in the use of the proposed model for materials engineering targeted at prescribed structural performance.     

Wedge splitting test for a steel-concrete interface

This paper presents a test method designated for the determination of the stress-crack opening relationship of a steel-concrete interface. The method is based on the well known wedge splitting test (WST), and it is illustrated how to obtain the stress-crack opening relationship through an inverse analysis. This inversion method utilizes the cracked hinge model, modified such that it describes the problem at hand. In this paper, pure concrete and steel-concrete composite specimens are tested and compared. It turns out that interfacial cracking of a bimaterial specimen usually behaves as one of the parent materials, in this case concrete. The stress-crack opening relationship of both the concrete and bimaterial specimens are obtained through the proposed inverse analysis. The results show, that interfacial cracking is dominated by the so-called wall-effect and its behavior can be described as quasi-brittle. However, due to the wall-effect, interfacial cracking is more brittle than for the pure concrete.     

A review on durability properties of strain hardening fibre reinforced cementitious composites (SHFRCC)

This paper reviews and presents various durability properties of strain hardening fibre reinforced cementitious composites (SHFRCC). Published research results show that, due to its tight crack width properties compared to ordinary concrete and ordinary fibre reinforced concrete, SHFRCC significantly resists the migration of aggressive substances in to the concrete and improves the durability of reinforced concrete (RC). It is also reported that, due to the strain hardening and multiple cracking behaviours, SHFRCC meets the tight crack width limits for durability of RC structures proposed by different design codes. Based on the reviewed durability properties it is argued that SHFRCC materials can be used in selected locations of RC structural members to improve their overall durability performances.     

Shear capacity of steel and polymer fibre reinforced concrete beams

This paper deals with the application of a plasticity model for shear strength estimation of fibre reinforced concrete beams without stirrups. When using plastic theory to shear problems in structural concrete, the so-called effective strengths are introduced, usually determined by calibrating the plastic solutions with tests. This approach is, however, problematic when dealing with fibre reinforced concrete (FRC), as the effective strengths depend also on the type and the amount of fibres. In this paper, it is suggested that the effective tensile strength of FRC can be determined on the basis of the tensile stress-crack opening relationship found from wedge splitting tests. To determine the effective compressive strength of FRC, it is proposed to adopt the formula used for conventional concrete and modify it by introducing a fibre enhancement factor to describe the effect of fibres on the compressive softening behaviour of FRC. The enhancement factor is determined as the ratio of the areas below the stress–strain curves for FRC and for conventional concrete. The outlined approach has been verified by shear testing of beams containing no fibres, 0.5% steel fibre volume and 0.5% polymer fibre volume. The tests results are compared with estimations and show satisfactory agreements, indicating that the proposed approach can be used.     

超高性能混凝土单轴拉压本构关系研究

超高性能混凝土（UHPC）是一种具有超高强度、超高耐久性和超高韧性的新型水泥基复合材料,因其优越的性能而受到国内外土木工程领域的广泛关注,具有广阔的工程应用前景。单轴受拉和受压应力-应变本构模型是进行UHPC构件力学性能分析的前提和基础。为了进一步深入研究UHPC力学性能,本文归纳总结了国内外不同学者提出的UHPC本构模型,包括单轴受压应力-应变关系和单轴受拉应力-应变、应力-裂缝宽度关系,对其进行了分类和比较,发现了现有模型的共同点和存在的差异,并对这种差异进行了原因分析,最后将部分本构关系应用于ABAQUS有限元分析软件,对高强钢筋UHPC梁进行数值模拟,并与试验结果对比,验证了部分本构模型的合理性,最后提出了一些研究结论。本文研究成果将为UHPC的结构性能研究进行结构分析和结构设计提供依据。      

Optimizing the performance characteristics of beams strengthened with bonded CFRP laminates

Ductility is of fundamental importance in the design of concrete structures. With structures using conventional materials such as concrete and steel, ductility of a member as a whole can be satisfactorily defined in terms of deflection, curvature or energy absorption capacity as examplified by the area under the load-deflection curve. However, when structural members strengthened with externally bonded fibre reinforced polymer (FRP) laminates are considered, conventional definitions of ductility become less precise because of the brittle behaviour of FRP materials, and their resultant effect on the performance of the strengthened beam. Furthermore, such beams when stregthened without the provision of external anchorages will fail very suddenly, with abrupt debonding of the laminate and substantial loss of load capacity. This paper intends to propose a new criterion to evaluate the structural performance of such strengthened composite beams, and the efficiency of the external anchorage system. The design criterion termed, Peformance Factor, incorporates both the deformability and strength of composite beams. Unlike the concept of toughness as applied to materials, the Performance Factor incorporates the effect of numerous parameters which influence structural design. To examine the reliability of this parameter a series of eleven reinforced concrete beams were tested to evaluate the structural performance of beams strengthened with and without externally bonded carbon fibre reinforced polymer (CFRP) laminates, and with different types of internal reinforcement and external anchorage systems. The structural behaviour of these beams was then evaluated using the Performance Factor.     

Prediction of steel fibre reinforced concrete under flexure from an inferred fibre pull-out response

Steel fibre-reinforced concrete (SFRC) is being used in a variety of structural applications, yet there is still considerable debate how to express and evaluate flexural toughness for design purposes. This is holding back the material's development as a permanent structural material. Existing beam and slab test methods have problems with variability or their application in structural design. Furthermore, existing models of SFRC flexural behaviour do not fully capture what happens at the cracked section in terms of the fibre-matrix interactions. Typical of these approaches is the modelling of the tension zone from single fibre pull-out tests, which is problematic in measurement of the load-displacement relationship, the interaction of groups of fibres and the extensive testing required to cover all permutations of fibre geometry. An alternative approach is proposed where the average pull-out response of the fibres bridging the cracked zone is inferred from flexural beam tests. The characteristic load versus crack-mouth opening displacement behaviour for a particular fibre concrete then forms part of the stress and strain/displacement profile in a flexural analysis to predict moment capacity in a design calculation. The model is explained together with its validation by comparing the predicted load-displacement response for a range of fibre volumes in sprayed and cast SFRC. It is concluded that the analysis of beam load/deflection curves to infer the fibre pull-out response is a viable approach. It offers a promising solution to the need for a flexural design model combined with a practical method of characterizing the tensile contribution of steel fibres.     

反分析法确定钢纤维水泥砂浆拉应力与裂缝张开位移关系

在数值模拟钢纤维混凝土结构或构件和用基于断裂力学理论的设计方法设计钢纤维混凝土结构或构件时,钢纤维混凝土材料σ-w关系是一个重要的材料参数。该文根据三点弯曲缺口梁的荷载与裂缝张开位移(CMOD)曲线用反分析法确定了钢纤维水泥砂浆的σ-w关系。试验制作了五种不同体积含量的钢纤维水泥砂浆单轴拉伸试件、圆柱体压缩试件和梁试件,钢纤维体积含量分别为0.5%、1.0%、1.5%、2.0%和2.5%。试验发现,当钢纤维体积含量较小时,三点弯曲缺口梁的荷载与裂缝张开位移曲线呈现CMOD软化特性;而当钢纤维体积含量较大时,三点弯曲缺口梁的荷载与裂缝张开位移曲线呈现CMOD强化特性。对有CMOD软化特性的钢纤维水泥砂浆,可用三折线软化模型来模拟钢纤维水泥砂浆的σ-w关系;对有CMOD强化特性的钢纤维水泥砂浆,可用应力跌落-常残余强度模型来拟合。     

Evaluation of ductility requirements in current design guidelines for FRP strengthening

Advanced composites are widely used for the strengthening of existing concrete structures. Current design guidelines give basic requirements on how to model the enhancement of structural performance of concrete members using surface bonded FRP (fibre reinforced polymer) reinforcement. With respect to this, it is of interest to evaluate the ductility requirements which are explicitly or implicitly imposed by design guides. Based on an evaluation of four major design guidelines in Europe, Japan and North-America, and a small parametric study, the ductility aspect of the design of FRP strengthened concrete members is verified. It appears that the ductility of flexural members strengthened with FRP should be considered with care, as reduced deformability is obtained at ultimate, though generally a minimum deformability is implicitly obtained in a proper design. At the other hand, ductility enhancement by means of FRP confinement is explicitly considered in the design guidelines.     

Investigation into CFRP plate end anchorage utilising uni-directional fabric wrap

Carbon fibre reinforced polymers (CFRP) has become a very popular method of improving the capacity of structural elements. Failure of CFRP strengthening systems when applied to concrete structures is usually typified by de-lamination of the CFRP from the concrete substrate. Research has shown that anchoring the ends of the CFRP plates or sheets can result in a significantly higher load/stress being reached before de-bonding occurs and that when sufficiently anchored, the CFRP material strain at failure can approach its ultimate strain at rupture. The following is continuation of experimental studies into CFRP anchorage systems used to retrofit concrete structures. It follows from previous investigations conducted by the authors into the bond behaviour of a new anchorage system utilising a mechanically strengthened substrate. The paper presents two alternative anchorages; both utilising uni-directional fabric wrap oriented horizontally across and parallel to the direction of the laminate. Both forms of anchorage are found to be effective in increasing the CFRP to concrete bond strength by distributing contact stresses over a greater area of concrete.     

Fire behaviour of FRP-strengthened reinforced concrete structural elements: A state-of-the-art review

The last two decades have seen increasing applications of fibre reinforced polymer (FRP) materials in civil engineering structures due to their many advantages over traditional strengthening and reinforcing materials. Among the most common applications is bonding or wrapping FRP products (strips or sheets) to the exterior of reinforced concrete (RC) members to increase their strength or deformability. However, widespread application of FRP strengthening systems in buildings, where structural fire ratings are required, is hindered due to unknowns surrounding the reduction in their mechanical and bond properties at elevated temperatures. This paper presents a state-of-the-art review on the fire performance of FRP-strengthened RC structural elements. The review addresses first the mechanical behaviour at high temperature of the constituent materials of FRPs and how their bond to concrete is affected when heated. The paper then discusses available experimental and numerical studies on the fire behaviour of FRP-strengthened RC beams, slabs, and columns. Available design guidance is also discussed. Finally, recommendations for future research are given.     

Development of kenaf fibre reinforced polymer laminate for shear strengthening of reinforced concrete beam

Fabrication of green fibre composite laminate for strengthening of reinforced concrete structure is one of the current interests in the field of construction industry. The aim of this research was to develop kenaf fibre reinforced polymer (KFRP) laminate for shear strengthening of reinforced concrete beam. Comprehensive design and theoretical models were also proposed for KFRP laminate shear strengthened beam. In the experimental programme, KFRP laminate had been fabricated with various fibre content to obtain optimal mix ratio. Physical and mechanical properties of KFRP laminates were experimentally investigated. Three reinforced concrete beam specimens were prepared for structural investigations. Results showed that KFRP laminate with maximum fibre content had the highest tensile strength and the laminate was found to be elastic isotropic in nature. The KFRP laminate strengthened beam had 100 % higher shear crack load and 33 % ultimate failure load as compared to un-strengthened control beam. It reduced the numbers and width of cracks and had shown strain compatibility behavior with shear reinforcement. The failure load, ductility, crack patterns and strain characteristics of KFRP laminate strengthened beam were found to be closely comparable with CFRP laminate strengthened beam. The experimental results satisfactorily verified the proposed design and theoretical models.     

Mechanical performance of composite-strengthened concrete structures

The interest of using fibre reinforced plastic (FRP) materials in rehabilitating damaged concrete structures respectively has been increased rapidly in recent years. In this paper, the structural behaviours of the glass–fibre composite strengthened concrete structures subjected to uni-axial compression and three point bending tests are discussed through experimental studies. Two types of concrete structure are used in present study, they are concrete cylinder and rectangular concrete beam. Discussion on the environmental effects of composite strengthened reinforced concrete (RC) structures is also addressed. Experimental results show that the use of glass–fibre composite wrap can increase the load carrying capacity of the plain concrete cylinders with and without notch formation. The flexural load capacity of the concrete beam increases to more than 50% by bonding 3 layers of glass–fibre composite laminate on the beam tension surface. Direct hand lay up method gives better strengthening characteristic in term of the ultimate flexural load compared with pre-cured plate bonding technique. The flexural strengths of composite strengthened RC beams submerged into different chemicals solution for six months are increased compared with the RC beams without strengthening. The strength of the concrete structure is seriously attacked by strong acids.     

Fire‐damaged R/C Members Repair With High‐Performance Fibre‐Reinforced Jacket

Abstract: A new repair technique, developed for fire‐damaged R/C structures and based on the application of jackets made of high‐performance fibre‐reinforced concrete, is presented and discussed. The proposed technique may be in principle applied to both beams and columns, as shown in this article. At first, the residual bearing capacity of the reference members is determined for different values of the fire duration. At this aim, an analytical model based on the direct integration of the hot or residual mechanical properties of the three materials (concrete, steel and fibre‐reinforced concrete), throughout the member cross section, is here adopted. Then, after discussing the pros and cons of the application of concrete jacketing to the structural cases in question, the performance of the composite structures is analytically investigated, to make comparisons with the performance of the original undamaged structures, as well as with that of damaged structures. Finally, the fire resistance of the repaired members is evaluated, to show the effectiveness of the proposed technique.     

智能混凝土与结构

智能混凝土是通过将极少量具有某种特殊功能的材料复合于传统的混凝土材料中形成的具有多功能特性或某一种特殊功能特性的新型土木工程结构材料。与其他智能材料相比,智能混凝土是多功能本征性智能材料,可从本质上提高工程结构的性能。自感知混凝土(纳米混凝土、碳纤维混凝土)、自阻尼混凝土和自愈合混凝土等是智能混凝土研究的热点。总结了近年来作者在智能混凝土的制备、性能、机理,以及智能混凝土结构等方面的研究成果,并指出需进一步研究的问题。     

A multiplanar model for the pore radius distribution in isotropic near-planar stochastic fibre networks

A model is presented for the pore radius distribution in isotropic near-planar stochastic fibre networks. At a given areal density, the mean pore radius of two-dimensional random networks is shown to decrease with increasing fibre width and to increase with increasing fibre linear density.For structures with a structural component in the third dimension the standard deviation of pore radii is shown to be proportional to the mean for changes in areal density and porosity in agreement with data reported in the literature. At a given porosity, near-planar networks exhibit an increase in mean pore radius with increasing fibre width and linear density.     

Influence of casting condition on the anisotropy of the fracture properties of Steel Fibre Reinforced Self-Compacting Concrete (SFRSCC)

Identification of the tensile constitutive behaviour of Fibre Reinforced Concrete (FRC) represents an important aspect of the design of structural elements using this material. Although an important step has been made with the introduction of guidance for the design with regular FRC in the recently published fib Model Code 2010, a better understanding of the behaviour of this material is still necessary, mainly for that with self-compacting properties. This work presents an experimental investigation employing Steel Fibre Reinforced Self-Compacting Concrete (SFRSCC) to cast thin structural elements. A new test method is proposed for assessing the post-cracking behaviour and the results obtained with the proposed test method are compared with the ones resulted from the standard three-point bending tests (3PBTs). Specimens extracted from a sandwich panel consisting of SFRSCC layers are also tested. The mechanical properties of SFRSCC are correlated to the fibre distribution by analysing the results obtained with the different tests. Finally, the stress-crack width constitutive law proposed by the fib Model Code 2010 is analysed in light of the experimental results.     

Non-linear analysis of RC beams using a SIFCON matrix

The results of a non-linear analysis model of reinforced concrete beams using a high-performance fibre-reinforced cement (FRC) based matrix are described and compared with experimental observations. The constitutive relationship of the FRC materials used in both compression and tension and the logical flow chart for the non-linear analysis model are described. Analytical predictions of moment versus curvature and moment versus deflection curves are compared with experimentally observed results of reinforced concrete beams using a SIFCON (slurry infiltrated fibre concrete) matrix. The use of a fibre-reinforced matrix in the compression zone of a reinforced concrete beam allows for other over-reinforced sections to achieve significant increases in structureal ductility while resisting loads close to their ultimate load. Ductility and energy ratios, normalized by the control beam without fibres, ranged from 2.47 to 3.60 and 3.28 to 5.69, respectively; that is several hundred per cent in improvement. The analytical model is shown to predict, with reasonably good agreement, experimental observations of moment-curvature and moment-deflection curves. Thus it can be used to evaluate the effects of various parameters in order to optimize structural performance.     

Holistic design of RC beams and slabs strengthened with externally bonded FRP laminates

Structural strengthening with externally bonded reinforcement is now recognized as a cost-effective, structurally sound and practically efficient method for rehabilitating deteriorated and damaged reinforced concrete structures. Although a variety of worldwide on-site applications using composite materials have been realized for the rehabilitation and reinforcement of structural elements, the technology is now at a stage where its future development and competitiveness with conventional methods will depend on the definition of valid design guidelines based on sound engineering principles rather than on the availability of new materials or production processes.The main objective of this paper is to present a general design philosophy for externally plated reinforced concrete beams and slabs, based on a holistic approach, in which appropriate strategies for achieving durable and safe strengthened structures are described.Essential to the design for safety, durability and ductility is the availability of structural models which are: (i) based on sound engineering principles; (ii) capable of reflecting the physical behaviour of strengthened members; (iii) of general applicability, irrespective of the type of external reinforcement material (steel or fiber-reinforced polymer), and the reinforcement configuration (web or tension plate); (iv) capable of describing all possible failure modes, in order to predict the weakest link chain of resistance of a structural member.It will be shown, with a series of numerical/experimental comparisons, that such requirements can be conveniently obtained with a unified approach in which materials and structures, calculation and experimental verification, modelling and analysis are integrated.