Constitutive behaviour of confined fibre reinforced concrete under axial compression

Steel fibre reinforced concrete is finding extensive use in field applications. The mechanism of delaying and arresting crack propagation by the fibres can be made use in passive confinement of concrete. Such concrete was termed as confined fiber reinforced concrete (CFRC). This paper presents an analytical model for predicting the stress–strain behaviour of CFRC based on the experimental results. A total of ninety prisms of size 150×150×300 mm were cast and tested under strain control rate of loading. The increase in strength and strain of CFRC were used in formulating the constitutive relation.     

Cyclic loading of laterally confined concrete columns

The behaviour under cyclic load of stub concrete columns confined laterally by a novel method of reinforcement is described. The columns were confined by an orthogonal system of steel bolts, and were loaded axially in compression in slow cycles. These were compared with similar columns loaded to failure monotonically in compression. It was found that the columns exhibited a large energy absorption and dissipation capacity. The concept of an envelope curve can be applied to the cyclic behaviour of the reinforced columns, both in terms of axial stress-strain behaviour and the development of confining pressures. The paper also deals with energy dissipation and stiffness degradation in the columns during cycling, together with the prediction of the residual strain.     

Shear strengthening of reinforced concrete beams using different configurations of externally bonded carbon fibre reinforced plates

This paper deals with research undertaken at Oxford Brookes University into shear strengthening of reinforced concrete beams using externally bonded carbon fibre reinforced plastic plates (CFRP). Thirty-eight reinforced concrete beams of 1.8 m length were constructed and tested and are described in this paper. The relative performance of a group of sixteen beams with the same steel reinforcement but with different amounts of shear strengthening is discussed. All the beams were designed to fail in shear using a spreadsheet program. The spreadsheets were designed to ensure that the beams' flexural capacity exceeded the shear capacity after strengthening. The variables were: main reinforcement ratio, spacing between links in the shear span and different configurations of CFRP plates on shear spans. The concrete had an average compressive strength of 61.76 N/mm². The majority of the beams tested showed a significant improvement in shear strength by the addition of CFRP plates, with increases of between 19–122% over the control beams.     

Impact of fibre orientation on tensile,bending and shear behaviors of a steel fibre reinforced concrete

Fibre orientation and density are known to have a significant influence on steel fibre reinforced concretes (SFRC) mechanical properties. In practice, parameters such as fresh state properties, restriction to concrete flowability and placing methods are likely to induce different fibre orientations in characterisation specimens and structural components. This difference in fibre orientation can impact the mechanical behavior of the structural component and therefore provide an unsafe design if not considered. This project consisted to produce a large SFRC slab, extract specimens with different fibre orientations, and submit specimens to tensile, bending and shear tests to evaluate the impact of fibre orientation and density on mechanical and post-peak strengths. Test results have shown that tensile and bending behaviors are mainly influenced by the fibre orientation, while the shear behavior is mainly impacted by fibre density. Test results were processed to allow comparison between tensile and bending tests. Linear correlations between tensile residual stresses and fibre orientation where found, linear or power type correlations according to bending residual stresses, as well as linear correlations between shear stresses and fibre density.     

Plastic shrinkage of steel fibre reinforced concrete

The paper presents the results of an experimental investigation on the plastic shrinkage of steel fibre reinforced concrete (SFRC). Tests were carried out on mixes with cement content ranging between 370 and 468 kg m^?3. Four types of steel fibres were used: melt extract, hooked, crimped and Harex. The volume of, fibres ranged between 0 and 4.47% of the mix volume. The results show that fibre reinforcement leads to significantly lower plastic shrinkage-reductions of up to 60% were observed. Crimped and Harex fibres were more effective in restraining plastic shrinkage compared with melt extract and hooked fibres, owing to their superior anchorage characteristics. A theoretical expression is also derived to predict the plastic shrinkage of SFRC, ε_fp, in terms of the shrinkage of concrete, ε_op. This is based on a schematic element of the composite comprising of an idealized, discrete fibre providing shrinkage restraint to a thick cylinder of matrix. The resulting expression is ε_fp=ε_op{1-0.306μv_f(l/d)[(s-d)/dl]}. The term μ represents the fibre-matrix interfacial property of shear resistance. The results indicate that μ increases with increasing v_fl/d ratio and that fibre with superior anchorage characteristics lead to higher values of μ.     

Post-cracking behaviour of steel fibre reinforced concrete

Recently, RILEM TC 162-TDF has proposed equivalent,f _eq , and residual,f _R , flexural tensile strength parameters to characterize and simulate the post-cracking behaviour of steel fibre reinforced concrete (SFRC) structures. In the current work, more than two hundred flexural tests are carried out according to the RILEM TC 162-TDF recommendations and the corresponding values off _eq andf _R parameters are evaluated. In series of specimens reinforced with fibres of a distinct length/diameter ratio, similar values off _eq andf _R parameters were obtained in these series. Although a strong correlation betweenf _eq andf _R was determined, a larger scatter off _R values was observed thereby revealingf _eq to be more appropriate for design purposes. A numerical strategy involving a cross sectional layered model and an inverse analysis was developed to evaluate the post-cracking stress-strain and the stress-crack opening diagrams for the tested SFRC. This strategy was also used to determine a relation between the post-cracking strain, ɛ ^pcr , and the crack opening displacement,w, (ɛ ^pcr =w/L _p ) which is useful for evaluating the crack opening when numerical strategies based on a stress-strain approach are used. The obtainedL _p values range from half the specimen cross section height to half the distance between the tip of the notch and the top of the cross section.

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Résumé Récemment, pour caractériser et simuler le comportement post-fissuration en traction du béton renforcé des fibres d'acier, la Commission Technique 162-TDF de la RILEM a proposé des paramètres désignés par résistance équivalente, f_eq, et résistance résiduelle, f_R, à la contrainte en flexion. Dans le travail présent, des valeurs de ces paramètres sont obtenues sur plus de deux cents essais de flexion effectués en accord avec les recommandations du TC 162-TDF de la RILEM. Des valeurs semblables de f_eq et f_R ont été obtenues dans des séries d'éprouvettes renforcées avec des fibres d'un rapport longueur/diamètre distinct. Bien qu'une forte corrélation entre f_eq et f_R ait été déterminée, une plus grande dispersion de valeurs du f_R a été observée, en démontrant que f_eq est plus approprié pour les buts du projet. Pour évaluer les diagrammes contrainte-déformation et contrainte-ouverture après fissuration, une stratégie numérique a été développée, en utilisant un modèle de section et en effectuant une analyse inverse. Cette stratégie a aussi été utilisée pour déterminer une relation entre la contrainte après fissuration, ɛ ^pcr , et l'ouverture de fissure, w, (ɛ ^pcr =w/L _p ) utile pour évaluer l'ouverture de la fissure quand les stratégies numériques sont basées sur une approche contrainte-déformation. Les valeurs de L_p obtenues ont varié entre la demi-hauteur de la section de l'éprouvette et la demi-distance entre l'extrémité de l'entaille et le sommet de la section.

     

Splitting tensile test for fibre reinforced concrete

The splitting tensile test is a much used method to determine the tensile strength of concrete. The conventional test procedure is known to have a number of limitations related to size effect and boundary conditions. Furthermore, it has been reported to be impossible to determine the tensile strength of Fibre Reinforced Concrete (FRC) using the standard splitting tensile test method. The objective of this paper is to present a methodology to obtain a close estimate of the true tensile strength of FRC from an adjusted tensile splitting test procedure. Splitting tests were performed on cylindrical specimens of four FRC mixes. The transversal deformation perpendicular to the load direction was recorded during the tests. The experimental load-deformation curves thus obtained have two peaks, an initial one as a result of the tensile stresses at the centre of the specimen and a second peak due to secondary cracking outside the loading axis. The tensile strength can be calculated from the first peak which represents the elastic limit state for the material. The method is validated through numerical simulation of the splitting tests using a cohesive crack approach. It is concluded that it is possible to obtain a close estimate of the true tensile strength of FRC using the procedure developed in the paper.     

Shrinkage of fibre and reinforced fibre concrete beams in hot-dry climate

The influence of steel fibre inclusion on the shrinkage of 16 full-size plain and reinforced concrete beams was assessed. Shrinkage measurements, at three levels over the depth of the beams, were carried out for 200 days. Half of the beams were cured in a controlled laboratory environment and the other half cured under hot, dry and windy climatic conditions.

Test results show that under laboratory curing conditions adding 1% by volume of steel fibres reduced the ultimate shrinkage at the top, mid-height, and bottom of the plain concrete by 16, 23, and 28%, respectively. However, in the reinforced concrete beam the presence of longitudinal reinforcement rendered it less significant.

Under the uncontrolled severe curing environment, the addition of 1% by volume of fibres produced a reduction of 30% in shrinkage at the bottom level of both the plain and the reinforced concrete beams. At the top level, however, the geometry constraints and the compaction techniques influenced the fibre contribution to shrinkage.     

Seismic behaviour of steel fibre reinforced concrete piles

This paper provides test data on seismic behaviour of SFRC piles. Four large-scale specimens are subjected to a combination of constant axial load and alternating cyclic flexure. A similar number of specimens are subjected to alternating cyclic flexure without axial load. Twin-cone fibres and new twin-crimped fibres in weight fraction between 40 kg/m³ and 50 kg/m³ are used.     

Long-term properties of steel fibre reinforced marine concrete

The paper presents some results from a continuing research programme which aims to develop steel fibre reinforced concrete (sfrc) for marine applications and to investigate its durability. A mix of proportions by weight of 1:1.5:0.86 was adopted which was reinforced with three types of steel fibres, namely, low carbon steel, corrosion resistant and melt extract. Prism specimens were cured under marine exposure, both in the laboratory and at Aberdeen beach, for up to 2000 wet-dry cycles (1250 days) and were tested at regular intervals of age. The paper presents results on long-term compressive strength, flexural strength and ductility as measured from flexural load-deflection curves. The state of fibres, with respect to corrosion, is also discussed. The results indicate that melt extract fibres, although least effective from the mechanical strength point of view, are the most suitable for marine applications.     

Shear behaviour of steel fibre reinforced concrete beams

The possibility of substituting traditional transverse reinforcement (stirrups) for steel fibres in precast elements can significantly reduce production costs. In the present paper, the shear behaviour of prestressed elements has been investigated by means of experimental tests on full scale beams. Tests concern beams with conventional as well as steel fibre reinforcement. Experimental results show that the shear behaviour of fibre reinforced concrete beams without conventional reinforcement is similar to, or even better than that of beams with stirrups. When used in beams with stirrups, steel fibres significantly improve their shear strength. A discussion on the contribution of steel fibres on the shear strength is also presented, with reference to the latest RILEM provisions.

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Résumé La possibilité de remplacer des armatures transversales traditionnelles par des fibres d'acier dans des éléments pré-tendus peut apporter des améliorations considérables sur les performances structurales. Dans ce rapport, le comportement en cisaillement des éléments précontraints a été étudié à l'aide de tests expérimentaux sur des poutres à échelle grandeur nature. Les essais portent aussi bien sur des poutres avec armatures traditionnelles que sur des poutres renforcées à l'aide de fibres. Les résultats expérimentaux montrent que les performances des poutres en béton de fibres sans armatures traditionnelles sont similaires, sinon meilleures, à celles des poutres avec armature ordinaire de cisaillement. Lorsqu'elles sont utilisées dans des poutres avec armatures traditionnelles les fibres d'acier améliorent considérablement leur résistance en cisaillement.

     

Investigation of structural behaviours of laterally restrained GFRP reinforced concrete slabs

The corrosion of reinforcement in concrete bridge deck has been the cause of major deterioration and of high costs in repair and maintenance. Glass fibre reinforced polymer (GFRP) reinforcement is a more durable alternative to steel reinforcement and has higher strength to weight ratio. Due to the low value of elasticity and brittle behaviour of GFRP, the service behaviour of GFRP reinforced concrete structure is critical. However, laterally restrained slabs, such as those in bridge deck slabs, exhibit arching action or compressive membrane action (CMA) which has a beneficial influence on the service behaviour such as the deflection. This paper presents the results of experimental tests and numerical analysis of laterally restrained GFRP reinforced concrete slabs with varying some structural variables. The analysis results are discussed and conclusions on the compressive membrane action in GFRP reinforced concrete slabs are presented.     

聚丙烯纤维增强混凝土研究进展

聚丙烯纤维具有耐化学腐蚀、加工性好、质轻、蠕变收缩小、价格低廉和在低掺量时对混凝土的塑性干缩开裂改善效果显著、抗渗性好、防止火灾中混凝土爆裂等优良的技术经济性能。但同时也存在一些缺点,如随着纤维掺量的增加,混凝土工作性降低、弹性模量低、增韧效果差以及低掺量下对混凝土的强度(抗拉、抗压、抗折)无显著影响等等。综合论述了改善聚丙烯纤维增强混凝土目前的研究进展,展望了其发展趋势。     

Crack growth resistance in fibre reinforced alkali-activated fly ash concrete exposed to extreme temperatures

This paper examines the crack growth resistance of alkali-activated fly ash concrete under extreme temperatures. Plain and hybrid fibre reinforced alkali activated concrete prepared with fly ash were subjected to a range of temperatures from ? 30 to 300 °C, sustained for 2 h. The alkali activation was effected with a blend of sodium hydroxide and sodium silicate. A fibre blend of steel to polypropylene in the volume ratio of 4:1 and a total as high as 1% by volume fraction reinforced the mixtures. The resulting systems were examined for residual strength under compression and splitting tension. Further, notched prisms were loaded under 4-point flexure to evaluate the residual fracture toughness. Based on the results, four different stages for fracture behaviour were identified with superior fibre efficiency seen at sub-zero temperatures. Across the breadth of temperatures examined, adding fibres led to higher residual fracture toughness for those systems that displayed a narrow range of thermal conductivity.     

Uni-axial tension test for steel fibre reinforced concrete

RILEM Technical CommitteesRILEM TC 162-TDF: Test and Design Methods for Steel Fibre Reinforced Concrete

Uni-axial tension test for steel fibre reinforced concrete Recommendations     

Tests of continuous concrete slabs reinforced with carbon fibre reinforced polymer bars

Although several research studies have been conducted on simply supported concrete elements reinforced with fibre reinforced polymer (FRP) bars, there is little reported work on the behaviour of continuous elements. This paper reports the testing of four continuously supported concrete slabs reinforced with carbon fibre reinforced polymer (CFRP) bars. Different arrangements of CFRP reinforcement at mid-span and over the middle support were considered. Two simply supported concrete slabs reinforced with under and over CFRP reinforcement and a continuous concrete slab reinforced with steel bars were also tested for comparison purposes. All continuous CFRP reinforced concrete slabs exhibited a combined shear–flexure failure mode. It was also shown that increasing the bottom mid-span CFRP reinforcement of continuous slabs is more effective than the top over middle support CFRP reinforcement in improving the load capacity and reducing mid-span deflections. The ACI 440.1R–06 formulas overestimated the experimental moment at failure but better predicted the load capacity of continuous CFRP reinforced concrete slabs tested. The ACI 440.1R–06, ISIS–M03–07 and CSA S806-06 design code equations reasonably predicted the deflections of the CFRP continuously supported slabs having under reinforcement at the bottom layer but underestimated deflections of continuous slabs with over-reinforcement at the bottom layer.     

Flexural behaviour of RC beams in fibre reinforced concrete

Even though a number of research studies have demonstrated the effectiveness of Fibre Reinforced Concrete (FRC) in improving the structural response of RC members under different loading conditions, some concerns recently arose on the sectional ductility under flexure which can be reduced under specific conditions. In fact, fibres do not significantly increase the ultimate moment of RC members and, with rather tough FRC and low strain-hardening ratio of the longitudinal rebars, the rotation capacity can substantially decrease owing to a cracking localization at ultimate limit state.This paper focuses on this topic with a number of experimental results on full-scale FRC beams tested under flexure.Experimental results evidence that fibres, when provided in sufficient amount, are able to move the beam failure from concrete crushing to steel rupture. Under certain circumstances, the overall ductility, measured in terms of displacements, may decrease.On the other hand, in all cases the addition of fibres determines a stiffer and in general enhanced post-cracking behaviour in service conditions.