Influence of fibre type on flexural behaviour of self-compacting fibre reinforced cementitious composites

This paper investigates the flexural properties of self-compacting fibre reinforced cementitious composites that contain high fly ash volume. Seven types of fibres were compared at the same volume fraction and in similar matrices containing high-volume fly ash and having a high compressive strength of around 85 MPa at 28 days. Third-point bending test was conducted on beam specimens to obtain their load–deflection curves, and investigate their fracture behaviour, flexural strength, deflection and toughness. The results showed that using straight steel and micro-polyvinyl alcohol fibres produced composites demonstrating stable deflection-hardening with multiple-cracking phenomenon. This behaviour resulted in high flexural strength, along with large maximum deflection and toughness values, which are important for applications in cementitious composites. This study indicates that fibres with both sufficiently high aspect ratio and high tensile strength are necessary for achieving deflection-hardening in self-compacting cementitious composites with high-strength matrices containing high-volume fly ash.     

Modelling the influence of age of steel fibre reinforced self-compacting concrete on its compressive behaviour

Steel fibre reinforced self-compacting concrete (SFRSCC) can combine the benefits of self-consolidating concrete technology with those derived from adding steel fibres to quasi-brittle cement based materials. In a recent applied research project joining pre-casting industry, private and public research institutions, a method was developed to design cost-competitive SFRSCC of rheological and mechanical properties required for the prefabrication of SFRSCC façade panels. To assure safe demoulding process of the panels, the influence of the concrete age on the compression behaviour of the SFRSCC should be known. For this purpose, series of tests with specimens of 12 h to 28 days were tested in order to analyze the age influence on the compressive strength, strain at peak stress, Young’s modulus, and compressive volumetric fracture energy. The experimental program was divided in two groups of test series, one with SFRSCC of a volumetric fibre percentage of 0.38% and the other with 0.57%. To apply the obtained data in the design and numerical analysis framework, the influence of the age on these SFRSCC properties was modelled. This work describes the carried out experimental program, presents and analyzes the obtained results, and provides the derived analytical expressions.     

Time dependent behaviour of fibre pull-out in self-compacting concrete

In the present study, the effectiveness of fibres in transferring stress across cracks under a sustained load was assessed. Single fibre pull-out creep tests were performed, in which fibre slip was monitored as a function of the time. The influence of the fibre orientation angle (0, 30 and 60°), as well as pre-imposed fibre slip levels, s_pr, 0.3 and 0.5 mm on the creep response was investigated. Additionally, instantaneous fibre pull-out tests were carried out on undamaged-bond specimens in order to quantify the effects of the pull-out creep behaviour. The damage introduced by the pre-slip levels in the bond of the fibre/matrix interface influenced the long-term fibre pull-out behaviour and, consequently, accelerated the creep rate. However, the assembled pull-out creep behaviour did not differ considerably from the instantaneous pull-out behaviour for the adopted pre-imposed fibre slip levels.     

Mechanical behaviour of self-compacting concrete with hybrid fibre reinforcement

In this study, fibre-reinforced self-compacting concretes were developed for precast building components, incorporating either adherent metal fibres or polymeric synthetic slipping fibres or a combination of both. To achieve the warranted workability, compressive and splitting tensile strengths, compositions were determined by preliminary tests on self-compacting materials with various proportions of metal fibres. Bending tests in controlled deflection confirmed the positive contribution of fibres in the mechanical behaviour of self-compacting concrete. The comparison between vibrated and self-compacting concretes of similar mechanical characteristics indicated a possible better fibre-matrix bond in the case of self-compacting types. The results also showed that the properties of the hybrid fibre-reinforced self-compacting concrete could be inferred from the properties of the individual single-fibre reinforcements and their respective proportions through simple mix-rules.     

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.

---

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.

     

Assessment of fibre orientation in ultra high performance fibre reinforced concrete and its effect on flexural strength

Fibre distribution and orientation in a series of round panel specimens of ultra high performance fibre reinforced concrete (UHPFRC) was investigated using electrical resistivity measurements and confirmed by X-ray CT imaging. By pouring specimens in different ways, the orientation of steel fibres was influenced and the sensitivity of the electrical resistivity technique was investigated. The round panels were tested in flexure and the results are discussed in relation to the observed orientation of fibres in the panels. It was found that the fibres tended to align perpendicular to the direction of flow. As a result, panels poured from the centre were significantly stronger than panels poured by other methods because the alignment of fibres led to more fibres bridging the radial cracks formed during mechanical testing.     

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.

---

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.

     

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.     

Structural behaviour of basalt fibre reinforced glass concrete slabs

Small-scale slab tests at ambient and elevated temperatures, conducted on horizontally unrestrained simply supported slabs, are presented in this paper. The aim of this research is to investigate the structural behaviour of concrete produced from different percentages of glass sand (20, 40, and 60 % by weight) and reinforced with different volume fractions of basalt fibre (0, 0.1, 0.3, and 0.5 % by total mix volume), when subjected to large vertical displacement. The results were also compared against similar structural members with concrete that did not contain glass or fibres. The results showed that the fracture of the reinforcement was the mode of failure for all the slabs and the load carrying capacity was enhanced above the theoretical yield-line load. For the slabs tested at elevated temperatures, the enhancement due to membrane action was at least twice as high as that recorded in the ambient temperature tests. The slabs with higher glass sand and basalt fibre content also exhibited greater enhancement and failed at higher displacement. The results also showed that the enhancement in the concrete with glass aggregate and basalt fibre was greater than that in concrete that contained no glass or fibre by up to 26 and 31 % at ambient temperature and in fire respectively.     

Flexural behaviour of small steel fibre reinforced concrete slabs

Influence of length and volumetric percentage of steel fibres on energy absorption of concrete slabs with various concrete strengths is investigated by testing 28 small steel fibre reinforced concrete (SFRC) slabs under flexure. Variables included; fibre length, volumetric percentage of fibres and concrete strength. Test results indicate that generally longer fibres and higher fibre content provide higher energy absorption. The results are compared with a theoretical prediction based on random distribution of fibres. The theoretical method resulted in higher energy absorption than that obtained in experiment. A design method according to allowable deflection is proposed for SFRC slabs within the range of fibre volumetric percentages used in the study. The method predicts resisting moment–deflection curve satisfactorily.     

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.     

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.     

Mixed mode fracture behaviour of steel fibre reinforced concrete

In this paper, results are reported for a series of discrete end hooked and straight fibre pullout tests subjected to mixed mode action with the results compared to that of discrete fibres pulled out in Mode I (tensile) and Mode II (shear) fracture. As has been previously observed from Modes I and II fracture tests, the snubbing effect dominates the behaviour of fibres at large fibre bending angles. At large fibre bending angles, considerable slip and crack separation occurred prior to the fibres being engaged in taking load and fibres that are inclined close to the cracked surface are ineffective in carrying load. The results of the test were compared with the fibre engagement and bond stress models in the Unified Variable Engagement Model (UVEM). A good correlation is observed for the UVEM model with the test data and provides further confirmation of the validity of the UVEM model to predict the mix mode fracture of steel fibre reinforced concrete.     

Predicting the flexural response of steel fibre reinforced concrete prisms using a sectional model

The material characterisation of steel fibre reinforced concrete (SFRC) continues to be an ongoing topic of debate in the scientific community. When designing a structural element made of SFRC, its defining characteristic is its post-cracking residual tensile strength. Theoretically, a uniaxial tension test is the ideal test in gathering these parameters; however these tests are expensive in time and testing. Consequently, much effort has been placed on inferring the post-cracking properties of SFRC from simpler tests, such as a notched prism in bending. In this paper, the sectional analysis procedure of Zhang and Stang (1998) is adapted with the inclusion of the variable engagement model to describe SFRC in tension. The model is shown to accurately capture the load–deformation characteristics of the tested specimens and allows for the explicit identification of the components resisting load.     

Long-term behaviour of plain and steel fibre reinforced concrete rings

The effects of long-term loading on plain (PLC) and steel fibre reinforced concrete (SFRC) rings under two-edge sustained load at five different levels were investigated. The results have led to the conclusion that a linear relationship between the initial and time-dependent deformations can be supposed. The intercept term of the regression straight line was found to be equal to the deformation caused by pure shrinkage. In spite of cracks appearing at higher loading levels the linearity was not distorted. The results indicate that an exponential function including one retardation time can be applied in practical computation of long-term deformations.

---

Resume On a étudie le comportement d’anneaux de béton non armé (PLC) et de béton renforcé de fibres d’acier (SFRC) chargés au bord dans deux directions différentes sous cinq charges différentes. Les résultats laissent supposer qu’il peut exister une relation entre les déformations initiales et celles qui se développent avec le temps. On a trouvé que l’intercept de la ligne droite de régression était égal à la déformation déterminée par le seul retrait. En dépit des fissures qui apparaissent à des niveaux de charges plus élevés, on n’a pas observé de distortion de la linéarité. Les résultats montrent qu’on peut appliquer une fonction exponentielle comprenant un temps de retard au calcul pratique des déformations à long terme.

     

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.     

Dynamic tensile behaviour of fibre reinforced concrete with spiral fibres

This paper performs drop-weight splitting tests to study the dynamic tensile properties of fibre reinforced concrete (FRC) materials with different steel fibres. A renovated splitting tensile testing method was developed to ensure a more qualified experimental process. The splitting tensile impact tests were conducted with an instrumented drop-weight impact system consisting of a hard steel drop weight, a fast-response load cell, a high-speed video camera and a high-frequency data acquisition system. The quasi-static compressive and splitting tests were also conducted to obtain the static properties of the FRC materials. The commonly used hooked-end steel fibre and a new spiral shaped steel fibre were tested in this study. The high-speed video camera was used to capture the detailed failure process, deformation and cracking process of the tested specimens. Average strain rates and the cracking extension displacement and velocity under impact loading were estimated by analysing the recorded high-speed images. The strains were also measured by the strain gages on the specimen surface. The dynamic stress–strain and stress–COD (cracking opening displacement) relations, the rate sensitivity of tensile strength and the corresponding energy absorption capacity of plain concrete and FRC with different fibres were obtained, compared and discussed. The advantage and effectiveness of the new spiral fibre in increasing the performance of FRC under dynamic tensile loading were examined. The results show that FRC with spiral fibres outperforms that with hooked-end fibres, and is a promising construction material in resisting dynamic loadings.