Shrinkage of steel fibre reinforced cement composites

The paper presents results of an experimental investigation on the influence of steel fibres on the free shrinkage of cement-based matrices. Shrinkage tests were carried out on cement paste, mortar and two types of concrete mixes for a period of up to 520 days. Melt extract, crimped and hooked steel fibres were used for reinforcement at volume fractions ranging between 1 and 3%. The results indicate that fibres restrain the shrinkage of the various cement matrices to a significant extent, resulting in reductions of up to 40%. Crimped fibres are the most efficient in providing shrinkage restraint. The paper also presents a theoretical expression and an empirical expression which can be used to predict shrinkage strains of steel fibre reinforced cement matrices. The analysis requires a knowledge of the values of coefficient of friction, μ, at the fibre-matrix interface, which are also derived in this paper. The μ values for steel fibres in normal concrete, mortar and cement paste range between 0.07 and 0.12.     

Impression creep behaviour of TiB2 particles reinforced steel matrix composites

Impression creep behaviour of the powder metallurgy processed steel matrix composites was investigated under constant stress at different temperatures in the range of 873–973?K. By using the power-law relationship, the estimated activation energy for unreinforced steel was found to be 149?kJ?mol^?1 and steel reinforced with 2 and 4?vol.-% TiB₂ was found to be 298 and 338?kJ?mol^?1, respectively indicating better creep resistance of the reinforced steel matrix composites. Dislocation creep is the dominant creep mechanism based on the calculated values of stress exponent and activation energy. Hence, this method can be used to assess the potential of steel matrix composites for use as structural materials for high-temperature application.     

A theory for the creep of steel fibre reinforced cement matrices under compression

The paper presents a theoretical model to predict the creep of cement matrices reinforced with randomly oriented discrete steel fibres. The theory considers the composite to be represented by an aligned steel fibre which is surrounded by a thick cylinder of the cement matrix. The fibre provides restraint to the flow component of creep of the matrix through the fibre-matrix interfacial bond strength. The delayed elastic strain component of creep is unaffected by the fibre. The fibre-matrix interfacial bond strength,, is shown to be primarily a function of the shrinkage of the cement matrix and the radial deformation caused by the sustained axial stress. In addition, the state of stress in the matrix at the interface is suggested to influence greatly the bond strength,. The validity of the theory is established by means of experimental data on concrete and mortar matrices reinforced with melt extract and hooked steel fibres, at sustained stress-strength ratios of 0.3 and 0.55. Finally an empirical expression is derived to determine the creep of steel fibre reinforced concrete, based on a knowledge of the creep in unreinforced matrices and fibre size and volume fraction.     

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.

     

Fibre spacing in steel fibre reinforced composites

A method is presented which makes it possible to evaluate the fibre content or to estimate the distribution of fibres in steel fibre reinforced composite materials from the analysis of X-ray pictures. The basic notion in this method is an apparent fibre spacing defined as the average spacing between the intersections of individual fibre projections upon a certain plane and an arbitrary base line drawn on that plane. Such apparent spacing may be estimated analytically, it may also be measured directly on the radiogram. The comparison of analytical and experimental data shows satisfactory agreement.

---

Résumé On présente une méthode qui, par l'analyse des diagrammes de rayons-X, permet d'évaluer la teneur en fibre ou d'estimer leur distribution dans des composites renforcés de fibres d'acier. On suppose que la distribution des fibres est homogène et isotropique, toutes les fibres ayant même longueur et même diamètre. En tant que notion de base de cette méthode, on définit un espacement de fibres apparent comme l'espacement moyen entre d'une part les intersections des projections des fibres individuelles, et d'autre part une ligne de base arbitraire tracée sur le même plan. Un tel espacement apparent (s _{app. anl.} ) peut être évalué analytiquement comme une fonction de la longueur de la fibre (l), du diamètre (d) de la teneur en volume (β) et de l'épaisseur de l'échantillon (w)-équation (7). On peut aussi le mesurer directement sur le radiogramme commes _{app. exp.} [voirfig. 2, équation 12)]. La comparaison des résultats analytiques et expérimentaux (tableau I) montre une bonne concordance. On peut aussi se servir de la méthode pour évaluer l'épaisseur d'échantillons soumis aux rayons-X, ce qui prouverait une bonne lisibilité du radiogramme.

---

     

Fatigue behaviour of continuous glass fibre reinforced composites

A study has been undertaken of fatigue in glass fibre reinforced composites. Two matrix resins were tested: a standard polyester and a polyurethane-vinyl-ester, which was designed to have a higher toughness. Three different types of glass fibre fabrics were used for reinforcement: a conventional woven roving and two stitch-bonded cloths. The glass cloths were combined into various lay-ups, in order to consider the effects of matrix, cloth and lay-up on the fatigue strength. Additionally, a study was undertaken to evaluate the micromechanisms that occurred during fatigue and how damage accumulated throughout the sample lifetime. This involved measuring stiffness changes during fatigue cycling, followed by microscopic study of the samples. It was found that similar damage micromechanisms occurred in each lay-up regardless of resin and cloth type, and these included matrix cracking, delamination and fibre breakage. However, differences were observed in the extent, location and rate of damage, and these were consistent with the variations seen in the fatigue strengths.     

Compressive behaviour of unidirectional flax fibre reinforced composites

The compressive strength of unidirectional flax fibre epoxy composites was studied. The compressive strength is influenced negatively by the presence of kink bands in the flax fibres. Improvement of the adhesion between the fibres and the epoxy resin can be achieved easily by removing the thin wax layer which covers the surface of the flax fibres. However, improving the adhesion between fibres and matrix only improves the compressive strength to a very limited extent. Stabilisation of the kink bands present in the fibres and improvement of the compressive properties of the fibres can be achieved by impregnating the fibres with melamine formaldehyde (MF) resin. This results in a large increase in the compressive strength of the resulting composite. The increase in compressive strength is proportional to the amount of MF resin present in the composite. However, the presence of the resin in the fibres lowers their tensile strength, and subsequently the tensile strength of the resulting composite.     

The stress-strain behaviour of glass-fibre reinforced cement composites

Equations describing the stress-strain behaviour of continuous, aligned, brittle-fibre-brittlematrix composites have been modified to take account of the construction of practical choppedstrand glass-fibre reinforced cement composites. The effects on the composite strain to failure of a number of factors have been considered and some detailed comparisons made between theoretical predictions and experimental results.     

Multiple cracking in aligned polypropylene fibre reinforced cement composites

Multiple cracking has been shown to occur in hardened cement paste reinforced with aligned polypropylene fibres of elastic modulus considerably lower than that of the cement paste. The effect of fibre volume fraction on the distribution of matrix cracks has been studied and good agreement found with existing theory. Factors which enable fibre/matrix contact to be maintained during the multiple cracking process, despite the unfavourable Poisson's ratio contraction of polypropylene have been discussed. These include the lateral displacement of one surface of a crack relative to the other and also the lateral displacement of matrix material next to the crack surface relative to the fibre array. This latter mechanism is shown to apply to other aligned composites and calculations based on a simple theory predict that multiple cracking should occur in all composites provided that a critical size is exceeded.     

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.

     

Plant-based natural fibre reinforced cement composites: A review

The quest for sustainability in construction material usage has made the use of more renewable resources in the construction industry a necessity. Plant-based natural fibres are low cost renewable materials which can be found in abundant supply in many countries. This paper presents a summary of research progress on plant-based natural fibre reinforced cement-based composites. Fibre types, fibre characteristics and their effects on the properties of cement-based materials are reviewed. Factors affecting the fresh and hardened properties of cement-based composites reinforced with plant-based natural fibre are discussed. Measures to enhance the durability properties of cement-based composites containing plant-based natural fibres are appraised. Significant part of the paper is then focused on future trends such as the use of plant-based natural fibres as internal curing agents and durability enhancement materials in cement-based composites. Finally, applications and recommendations for future work are presented.     

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.     

Carbon fibre-cement adhesion in carbon fibre reinforced cement composites

The addition of small amounts of short carbon fibres to cement causes a great increase in the composite material toughness and tensile, flexural, and impact strength. In order to understand how cement properties are improved by carbon fibres and to understand the level of adhesion and interfacial failure mode which are necessary to obtain optimum carbon fibre reinforced cement (CFRC) properties, various admixtures were included in cement and CFRC. Their effects on the carbon fibre-cement adhesion and the composite material properties were determined using fibre pull-out and composite material flexural tests. The addition of latex to CFRC, and hot water curing of CFRC dramatically increase fibre-matrix adhesion. Both latex (with an anti-foam agent) and hot water curing increase flexural strength by 40% over adhesion changes the failure mode from fibre pull-out to fibre rupture. Optimum strength and toughness of CFRC result from an intermediate level of fibre-matrix adhesion.     

Charpy impact behaviour of Sigma fibre reinforced titanium composites

Abstract

Charpy impact behaviour of SiC fibre reinforced titanium composites (TMCs)has been examined at a range of temperatures on specimens with fibres orientated longitudinally [0]₈ and transversely [90]₈ to the impact direction. Corresponding monolithic alloys have also been studied. Although monolithic Ti–6Al–4V has the superior ‘bend’strength at room temperature, Timetal 834 is superior at the operating temperatures of gas-turbine components. The TMCs follow the same trends as the monolithic alloys, but their impact strengths are inferior to the monolithic alloys by an order of magnitude. This is attributed to the low impact strength of the SiC fibres, which dominates in [0]₈ specimens, and the low strength of the fibre/matrix interface, which dominates in [90]₈ specimens. For [0]₈ specimens, Ti-6-4 reinforced with carbon coated SiC fibres (Sigma SM1140+) has consistently superior impact strength compared with the other three TMCs. For [90]₈ specimens, Timetal 834 reinforced with TiB_x/carbon coated SiC fibres (SM1240) exhibits the best impact strength at intermediate temperatures.     

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.