Compression creep behaviour of steel fibre reinforced cement composites

The paper presents the results of an experimental investigation to determine the influence of steel fibre reinforcement on the creep of cement matrices under compression. Creep tests were carried out at a number of applied stress-strength ratios ranging between 0.3 to 0.9. Melt extract and hooked steel fibres were used at volume fractions ranging between 0 and 3% by volume of a mix. Three types of cement matrices were used namely cement paste, mortar and two mix proportions of concrete. The results indicate that steel fibres restrain the creep of cement matrices at all stress-strength ratios, the restraint being greater at lower stresses and at higher fibre contents. Steel fibres are effective in restraining only the flow component of creep of cement matrices, the delayed elastic component being unaffected. The reduction in creep of cement pastes, due to fibre reinforcement, is much greater than that for mortar or concrete matrices. Mathematical expressions are given for the creep of steel fibre reinforced cement matrices.

---

Résumé Cet article présente les résultats d’une étude expérimentale en vue de déterminer l’influence du renforcement de fibres d’acier sur le fluage de matrices de ciment en compression. On a exécuté les essais de fluage à un certain nombre d’intervalles correspondant à des rapports contrainte/déformation allant de 0,3 à 0,9. On a utilisé des fibres d’acier dans des proportions se situant entre 0 et 3% par volume de mélange, et on s’est servi de trois types de matrices de ciment: pate de ciment, mortier et béton selon deux dosages. Les résultats montrent que les fibres d’acier empêchent le fluage des matrices de ciment dans tous les rapports de contrainte/déformation, l’effet étant plus important pour les contraintes faibles et les teneurs en fibres plus élevées. Les fibres d’acier n’agissent qu’en s’opposant à la déformation plastique des matrices de ciment sans que l’élasticité différée en soit affectée. La diminution du fluage des pates de ciment due au renforcement des fibres est beaucoup plus importante que pour le mortier ou les matrices de béton. On donne des formules mathématiques pour le fluage des matrices de ciment renforcé de fibres d’acier.

     

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.

---

     

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.     

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.     

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.     

Investigation of mixed-mode fracture of aligned steel fibre reinforced cementitious composites

International Journal of Fracture - Mixed-mode fracture experiments were conducted on aligned steel fibre reinforced cementitious composite (ASFRC) and ordinary steel fibre reinforced cementitious...     

A theory for the free shrinkage of steel fibre reinforced cement matrices

The paper presents a theoretical model to predict the free shrinkage of cement matrices reinforced with randomly oriented discrete steel fibres. The model is based on the consideration that the equivalent aligned length of a random fibre is responsible for restraining the shrinkage of a thick matrix cylinder of diameter equal to the fibre spacing, through the fibre-matrix interfacial bond strength. The validity of the model is established by means of extensive experimental data for different types of steel fibres in cement, mortar or concrete matrices. The theoretical model is also used to determine the values of coefficient of friction,, and the average bond strength,, of the fibre-matrix interface. It is shown that is a basic property of the matrix and fibre interface, which is affected by the surface roughness and mechanical deformation of the fibres., however, is greatly influenced by the shrinkage of the matrix and volume fraction of fibres. Finally, an empirical expression is derived to determine the shrinkage of steel fibre reinforced cement matrices based on the shrinkage of unreinforced matrices and fibre properties.     

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.     

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.     

Processing stainless steel fibre reinforced NiAl matrix composites by reactive hot pressing

Stainless steel fibres (SSF) were first wound and then plated with electrical nickel film. Alternative layers of these properly spaced Ni-coated stainless steel fibres (SSF–Ni) and aluminum foils were then stacked and diffusion bonded at 500 °C, 100 MPa in vacuum for 10 min to produce an SSF–Ni–Al precursor. Lower-temperature reactive hot pressing (RHP) at 700 °C, a process employed in this work, allowed the nickel and aluminum to react to form an NiAl matrix at a temperature approximately 500 °C lower than that accomplished by hot pressing, hot extrusion or hot isostatic pressing (HIP). RHP at 650 °C was also undertaken in order to investigate the mechanism leading to the formation of a nickel aluminide matrix composite. A dense specimen with complete NiAl reaction was obtained by hot pressing at 900 °C. The variance in microstructure that resulted from the different conditions of heat treatment and hot pressing was analysed by optical microscopy, scanning electron microscopy and X-ray diffraction (XRD). This revised version was published online in November 2006 with corrections to the Cover Date.     

Fretting of glass fibre reinforced composites

Favourable specific mechanical properties of polymer matrix composites make them an attractive material for application in many engineering structures for which they offer substantial improvements over metals. The paper deals with fretting behaviour of unidirectional glass epoxy composites/metal contacts. Fretting is a plague for many industries: failures, loss of matter, loss of function can be induced by fretting. It occurs in all quasi-static contacts and appears as a complex wear phenomenon where a lot of parameters have been studied. From the interface tribology concept, the velocity accommodation mechanisms are discussed for different fibre orientations versus the contact surface of the glass fibre reinforced epoxy material. Results were analysed in two steps. From friction logs, Running Conditions Fretting Maps (RCFM) were first plotted in order to give an analysis of contact conditions and determine the associated material responses. The tribological degradations were then analysed. Differences between the different fibre orientations are mainly discussed on the basis of the stiffness of the anisotropic material and the velocity accommodation in the contact.     

Growth kinetics of interface intermetallic compounds in stainless steel fibre reinforced aluminium matrix composites

Growth kinetics of binary intermetallic compounds in the fibre/matrix interface has been studied in stainless steel fibre reinforced aluminium matrix composites fabricated by the P/M hot pressing, squeeze casting, and infiltration techniques. As expected in most binary diffusion couples, more than one intermetallic compound of the type Fe_xAl_y forms at the interface. However, not all the iron-aluminide intermetallic compounds possible as dictated by the binary phase diagram are present. This is primarily the result of the non-equilibrium conditions at the interphase boundaries as the activation-controlled and diffusion-controlled interfacial reactions progress between the fibre and the matrix. Two equations have been established for the growth kinetics of the interface; one relates to hot pressing, the other to squeeze casting and infiltration. Parabolic rate constants have been determined. A rate constant of about 0.7 × 10^–16 m² sec^–1 for hot-pressed composites produces an optimum thickness of the interface of about 3 m and results in the maximum strength of the composites. In addition to the FeAl and Fe₂Al₅ that form at the interface, the presence of NiAl₃ intermetallic compound is also predicted. Further investigation is suggested for the determination of the rate constants in squeeze-cast and infiltrated composites.