Shear behaviour of steel fibre reinforced self-consolidating concrete beams based on the modified compression field theory

A series of steel fibre reinforced self-consolidating concrete (SFRSCC) beams have been tested to investigate the influence of steel fibres and the combined effect of fibres and stirrups on the deflection and cracking, ultimate loads and failure pattern. The experiment indicates that the shear strength increases clearly with the increasing of fibre content. The combination of steel fibres and stirrups demonstrates a positive composite effect on the ultimate load, ductility and failure pattern of concrete beam. This study also examines the feasibility of applying the modified compression field theory (MCFT) for the suitable assessment of shear resistance in fibre and steel rebar reinforced self-consolidating concrete beams. For fibre reinforced concrete member, a theoretical method is proposed based on the MCFT. The proposed ultimate shear capacity model was verified by the comparison with different test results.     

A design model for fibre reinforced concrete beams pre-stressed with steel and FRP bars

This paper presents a design oriented model to determine the moment–curvature relationship of elements of rectangular cross section failing in bending, made by strain softening or strain hardening fibre reinforced concrete (FRC) and reinforced with perfectly bonded pre-stressed steel and fibre reinforced polymeric (FRP) bars. Since FRP bars are not affected by corrosion, they have the minimum FRC cover thickness that guaranty proper bond conditions, while steel bars are positioned with a thicker FRC cover to increase their protection against corrosion. Using the moment–curvature relationship predicted by the model in an algorithm based on the virtual work method, a numerical strategy is adopted to evaluate the load–deflection response of statically determinate beams. The predictive performance of the proposed formulation is assessed by simulating the response of available experimental results. By using this model, a parametric study is carried out in order to evaluate the influence of the main parameters that characterize the post cracking behaviour of FRC, and the pre-stress level applied to FRP and steel bars, on the moment–curvature and load–deflection responses of this type of structural elements. Finally the shear resistance of this structural system is predicted.     

Short and long-term cracking behaviour of GFRP reinforced concrete beams

A total of ten simply supported beams reinforced with different amounts of GFRP and steel bars were subjected to two consecutive test phases in order to evaluate their short and long-term cracking behaviour. The beams were initially tested up to service load and subjected to two additional load cycles. Subsequently, the specimens were subjected to two different levels of sustained load for 250 days. The effect of cyclic load during short-term tests resulted in an increase in crack width up to 25% more than the initial value. The sustained load led to an increase in crack width up to 2.9 times larger than that measured under the corresponding short-term load. A similar cracking behaviour was observed when reinforcing solutions with similar stiffness (GFRP or steel bars) were used.Existing models to estimate crack spacing and crack width for FRP and steel reinforced concrete elements, including ACI 440.1R-06, Eurocode 2 and Model Code 2010 are discussed and their performance is assessed against the experimental results. Model Code 2010 was found to yield more accurate predictions of the cracking behaviour of the test specimens under both short-term and long-term loading.     

CFRP-PCPs复合筋嵌入加固钢筋混凝土梁裂缝试验及计算方法研究

通过5根嵌入不同张拉控制应力的碳纤维增强塑料预应力混凝土棱柱体(CFRP-PCPs)复合筋加固钢筋混凝土梁受弯试验,对比分析试验梁的裂缝分布与发展,得到最大裂缝宽度与平均裂缝宽度在静力荷载作用下的变化特性。结果表明: 嵌入CFRP-PCPs复合筋能有效的减少被加固钢筋混凝土梁的裂缝宽度和高度。在试验基础上,根据国家现行混凝土规范,对平均裂缝间距和最大裂缝宽度计算公式进行参数修正,建立了CFRP-PCPs复合筋嵌入加固钢筋混凝土梁最大裂缝宽度计算公式,计算值与试验值吻合较好。     

Size effect in uncracked and pre-cracked reinforced concrete beams shear-strengthened with composite jackets

The paper deals with the size effect on shear behaviour of reinforced concrete beams strengthened with fiber reinforced polymer jackets. Continuous U-jackets were made of glass or carbon fiber fabrics and epoxy composite materials. Twelve uncracked or pre-cracked strengthened reinforced concrete beams and six beams without strengthening, all of them in 6 different sizes, were tested. The results indicate that fabric-epoxy continuous U-jackets have reduced the brittleness of the shear failure of beams, tensile strains in stirrups, and, in a significant way, also the width of shear cracks at the failure state. Although similar strengthening was used for both, uncracked and pre-cracked beams, activation of jackets significantly differed. While jacket strains and their strengthening effectiveness were affected by the sizes of uncracked, retrofitted beams, they remained almost constant in pre-cracked, repaired beams of varying sizes. In contrast to repaired beams, stirrups in retrofitted beams did not yield at failure. Degree of strengthening, defined as the ratio of strengthened-to-unstrengthened beam shear capacities, was studied. It was found out that consideration of the degree of strengthening would provide relations reflecting real behaviour of reinforced concrete beams strengthened with fiber reinforced polymer U-jackets or U-jacketed strips.     

Prediction of intermediate crack debonding failure in FRP-strengthened reinforced concrete beams

The present paper addresses with intermediate crack (IC) debonding failure modes in FRP-strengthened reinforced concrete beams; a non-linear local deformation model, derived from a cracking analysis based on slip and bond stress, is adopted to predict the stresses and strains distribution at failure. Local bond-slip laws at the longitudinal steel-to-concrete and FRP-to-concrete interfaces, as well as the tension stiffening effect of the reinforcement (steel and FRP) to the concrete, are considered. Model predictions are compared to experimental results available in the literature together with predictions of other models. Reasonable agreement with experimentally measured IC debonding loads and FRP strains is observed for all examined strengthened beams. Results of a parametric analysis, varying geometrical and mechanical parameters involved in the physical problem are also presented and discussed.     

Shear design of reinforced concrete beams, slabs and walls

The paper presents the background to the shear design provisions for reinforced concrete beams and slabs used in the Australian practice. Correlation of design equations with experimental results are given. The design provisions are illustrated by examples. The importance of shear strength in the design of structural walls is discussed. A new expression to calculate the shear strength of walls is presented.     

Behaviour of high volume fibre cement mortar in flexure

Tests are reported on the behaviour of high volume percentage steel fibre mortar specimens subjected to flexure. Flexural and cyclic load tests were conducted; in addition, comparison tests were made on conventional fibre mortar and ferrocement specimens. Both strength and deflection characteristics were studied. The results of the investigation indicate that with 8% high volume, steel fibre mortar specimens possess a flexural strength of about 40 MPa.     

Development of early age shrinkage stresses in reinforced concrete bridge decks

This paper describes the instrumentation and data analysis of a reinforced concrete bridge deck constructed on 3-span continuous steel girders in Evansville, West Virginia. An instrumentation system consisting of 232 sensors is developed and implemented specifically to measure strains and temperature in concrete deck, strains in longitudinal and transverse rebars, the overall contraction and expansion of concrete deck, and crack openings. Data from all sensors are automatically collected every 30 minutes starting at the time of placing the concrete deck. Measured strain and temperature time-histories were used to calculate the stresses, which were processed to attenuate the thermal effects due to daily temperature changes and isolate the drying shrinkage component. The results indicated that most of concrete shrinkage occurs during the first three days. Under the constraining effects from stay-in-place forms and reinforcement, early age shrinkage leads to elevated longitudinal stress, which is the main factor responsible for crack initiation.     

Mechanical properties of date palm fibres and concrete reinforced with date palm fibres in hot-dry climate

The study examines four types of date palm surface fibres and determines their mechanical and physical properties. In addition, the properties of date palm fibre-reinforced concrete, such as strength, continuity index, toughness and microstructure, are given as a function of curing in water and in a hot-dry climate. The volume fraction and the length of fibres reinforcement were 2–3% and 15–60 mm respectively. Increasing the length and percentage of fibre-reinforcement in both water and hot dry curing, was found to improve the post-crack flexural strength and the toughness coefficients, but decreased the first crack and compressive strengths. In hot-dry climate a decrease of first crack strength with ageing was observed for each concrete type. Water curing decreased the global degree of the voids and cracks with time for each concrete type, but increased it in hot-dry climate.     

Microstructure,flexural properties and durability of coir fibre reinforced concrete beams externally strengthened with flax FRP composites

This study investigated the flexural behaviour of plain concrete (PC) and coir fibre reinforced concrete (CFRC) beams externally strengthened by flax fabric reinforced epoxy polymer (FFRP) composites. PC and CFRC beams without and with FFRP (i.e. 2, 4 and 6 layers) reinforcement were tested under three- and four-point bending. The microstructures of coir fibre, coir/cement matrix, flax/epoxy matrix, and FFRP/concrete interfaces were analysed using scanning electronic microscope (SEM). Test results indicated that the peak load, flexural strength, deflection and fracture energy of both PC and CFRC specimens enhanced proportional to an increase of FFRP layers. Coir further increased load, strength and energy of the specimens remarkably. It was also found that the thickness and coir influenced the failure modes while the test method influenced the load and energy of the specimens remarkably. SEM studies showed effective bond at coir/cement, flax/epoxy and FFRP/concrete interfaces. Therefore, it concluded that natural FFRP composites can be used to repair or retrofit existing concrete structures.     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

Automatic image analysis and morphology of fibre reinforced concrete

Automatic image analysis is an efficient tool to quantify the morphology of materials. Moreover, it can aid to understand their mechanical behaviour. Several applications of automatic methods are presented to investigate concrete reinforced by ribbon shaped amorphous cast iron fibres. Introducing ribbons into the plain matrix entrapped air voids. This affected the workability and, later on, the compressive strength of the fibre reinforced concrete (FRC). Both were improved by additions of superplasticizer in order to keep the water to cement ratio constant. The influence of the superplasticizer and fibre contents on the compactness of the FRC was characterized by the dimensional and the spatial distributions of the air voids. The orientations of fibres and microcracks were quantified by Fourier image transforms. Due to the casting procedure of the FRC, the fibres were located in “horizontal layers”, perpendicular to the casting axis. Whatever the direction of compression with respect to the layers of fibres, the microcrack network was getting more and more oriented in the direction of compression as stresses increased. The analysis of fibre and microcrack orientations suggests that, under uniaxial compression, the inelastic strain domain should be characterized by an anisotropic biaxial damage model, whose principal axes are the orthogonal and parallel directions to the layers of fibres.     

A unified approach for the static and dynamic analyses of intermediate debonding in FRP-strengthened reinforced concrete beams

A spectral method is proposed for solving static and dynamic problems in FRP-strengthened reinforced concrete beams in a unified way. In order to appropriately simulate the debonding failure a mechanical model considering nonlinear stress–strain relationships for concrete and steel is used. The FRP-to-concrete interface is modelled using a realistic bilinear local bond-slip law. Numerical results with the proposed model for the interfacial shear stress distribution and the load–displacement response are derived for beams statically tested. Using the same spectral model the influence of interfacial delaminations on the dynamic characteristics of the structures is studied. The feasibility of the proposed method for performing dynamic analyses for high frequency excitations in a very simple and non-expensive way makes this study very useful in non-destructive testing analyses as a tool for diagnoses and detection of debonding in its initial stage by monitoring the change in dynamic characteristics.     

Finite element modelling of multiple cohesive discrete crack propagation in reinforced concrete beams

This paper presents a finite element (FE) model for fully automatic simulation of multiple discrete crack propagation in reinforced concrete (RC) beams. The discrete cracks are modelled based on the cohesive/fictitious crack concept using nonlinear interface elements with a bilinear tensile softening constitutive law. The model comprises an energy-based crack propagation criterion, a simple remeshing procedure to accommodate crack propagations, two state variable mapping methods to transfer structural responses from one FE mesh to another, and a local arc-length algorithm to solve system equations characterised by material softening. The bond-slip behaviour between reinforcing bars and surrounding concrete is modelled by a tension-softening element. An example RC beam with well-documented test data is simulated. The model is found capable of automatically modelling multiple crack propagation. The predicted cracking process and distributed crack pattern are in close agreement with experimental observations. The load-deflection relations are accurately predicted up to a point when compressive cracking becomes dominant. The effects of bond-slip modelling and the efficiency and effectiveness of the numerical algorithms, together with the limitations of the current model, are also discussed.     

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.     

高延性混凝土加固RC梁抗剪性能试验研究

剪跨比对FRP抗剪加固梁的裂缝开展和破坏模式有重要影响,但对FRP加固梁抗剪强度及尺寸效应的影响研究较少。采用三维细观数值模拟方法,考虑混凝土细观组成的非均质性及碳纤维布(CFRP)与混凝土之间的相互作用,建立了CFRP加固无腹筋钢筋混凝土梁剪切破坏力学分析模型。在验证细观模拟方法合理性的基础上,拓展模拟与分析了剪跨比对CFRP加固钢筋混凝土梁剪切破坏及尺寸效应的影响机制与规律。研究结果表明：剪跨比对CFRP抗剪加固梁剪切破坏模式影响较大,剪跨比越大,加固梁愈趋近于延性较好的斜拉破坏;剪跨比对CFRP加固梁抗剪承载力有较大影响,对抗剪强度尺寸效应影响较小;剪跨比对加固梁中的CFRP剪切贡献影响较大,剪跨比越大,CFRP对加固梁的抗剪效果越好,其中对中型剪跨比(λ=2.5)的梁加固效果最有效。     

Textile-reinforced mortar (TRM) versus fiber-reinforced polymers (FRP) in shear strengthening of concrete beams

This paper presents an experimental study on shear strengthening of rectangular reinforced concrete (RC) beams with advanced composite materials. Key parameters of this study include: (a) the strengthening system, namely textile-reinforced mortar (TRM) jacketing and fiber-reinforced polymer (FRP) jacketing, (b) the strengthening configuration, namely side-bonding, U-wrapping and full-wrapping, and (c) the number of the strengthening layers. In total, 14 RC beams were constructed and tested under bending loading. One of the beams did not receive any strengthening and served as control beam, eight received TRM jacketing, whereas the rest five received FRP jacketing. It is concluded that the TRM is generally less effective than FRP in increasing the shear capacity of concrete, however the effectiveness depends on both the strengthening configuration and the number of layers. U-wrapping strengthening configuration is much more effective than side-bonding in case of TRM jackets and the effectiveness of TRM jackets increases considerably with increasing the number of layers.     

Minimum reinforcement in concrete structures and material/structural instability

The problem of the assessment of minimum reinforcement in concrete members has been examined both theoretically and experimentally by the bridged crack model. The model has been demonstrated to be an efficient numerical tool for investigating the behavior of structural elements in bending, and allowed to show the minimum reinforcement percentage depends on the structural element size, and decreases with increasing beam depths. In the model, Linear Elastic Fracture Mechanics concepts are used to determine the equilibrium and the compatibility equations of a beam segment subjected to bending in presence of a mode I crack. Recently, the model has been extended to include the presence of closing stresses as a function of the crack opening in addition to steel reinforcement closing traction. This allows to characterize the mechanical behavior of fiber reinforced structural elements. A criterion for accounting for crushing in compression has been introduced as well, to bound from below (minimum reinforcement) and from above (maximum reinforcement) a region of stable and ductile mechanical behavior as a function of the mechanical properties as well as of the size of the structural element. Some experimental results are commented under this light.     

Chloride ingress into marine exposed concrete: A comparison of empirical- and physically- based models

In establishing the reliability of performance-related design methods for concrete – which are relevant for resistance against chloride-induced corrosion - long-term experience of local materials and practices and detailed knowledge of the ambient and local micro-climate are critical. Furthermore, in the development of analytical models for performance-based design, calibration against test data representative of actual conditions in practice is required. To this end, the current study presents results from full-scale, concrete pier-stems under long-term exposure to a marine environment with work focussing on XS2 (below mid-tide level) in which the concrete is regarded as fully saturated and XS3 (tidal, splash and spray) in which the concrete is in an unsaturated condition. These exposures represent zones where concrete structures are most susceptible to ionic ingress and deterioration. Chloride profiles and chloride transport behaviour are studied using both an empirical model (erfc function) and a physical model (ClinConc). The time dependency of surface chloride concentration (C_s) and apparent diffusivity (D_a) were established for the empirical model whereas, in the ClinConc model (originally based on saturated concrete), two new environmental factors were introduced for the XS3 environmental exposure zone. Although the XS3 is considered as one environmental exposure zone according to BS EN 206-1:2013, the work has highlighted that even within this zone, significant changes in chloride ingress are evident. This study aims to update the parameters of both models for predicting the long term transport behaviour of concrete subjected to environmental exposure classes XS2 and XS3.