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.     

A simple material model for unidirectionally aligned fibre reinforced composites

A material model for the nonlinear behaviour of composite materials is proposed. The model combines the linear elastic fibre properties with the nonlinear behaviour of the matrix in order to obtain the average properties of the composite. The interaction between the fibre and the matrix is considered using stress partitioning factors, which are based on the experimental behaviour of the material.     

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.

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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.

     

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...     

The theoretical prediction of the cracking stress of glass fibre reinforced inorganic cement

Glass fibre-reinforced inorganic cement is increasingly being seen as a promising new building material. For any structural use, its cracking stress will be significant because of its resemblance to the yield point of mild steel. In this paper, the use of fracture mechanics to formulate a prediction of the cracking stress is proposed. Experimental evidence is also presented in support of this theory.     

Crystallization of carbon fibre reinforced polypropylene

Polypropylene (PP) was compounded with carbon fibre of various contents (0, 5, 10, 15, 20 vol%) using a single and a twin screw extruder. The influence of both the carbon fibre content and the compounding method on the thermal behaviour and characteristics of crystallization was studied using differential scanning calorimetry (DSC), polarizing optical microscopy and scanning electron microscopy (SEM). Melting and crystallization temperatures increased with the amount of carbon fibre. Isothermal crystallization was observed using DSC and it was found that crystallization was accelerated by the presence of carbon fibres. Using polarizing optical microscopy, it was found that the nucleation of polypropylene started at the crossing point of two or more fibres.     

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.     

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.

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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.

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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.     

The effects of age up to 18 years under various exposure conditions on the tensile properties of a polypropylene fibre reinforced cement composites

An 18 year test programme has been completed on the tensile stress-strain performance of cement composites reinforced with two types of polypropylene fibrillated networks. Three storage conditions have been used —natural weathering storage inside in laboratory air, and under water storage. Some composites have been precracked before exposure and others left uncracked before testing. The parameters measured included the elastic modulus and cracking stress of the matrix, complete tensile stress-strain curves and crack distribution in the composite, and the effects of the different weathering conditions on the tensile strength, strain to failure and bond strength of the polypropylene reinforcement. In general terms, it was shown that where the fibre volume remained above the critical fibre volume, the strength, ductility and toughness of the composite was maintained regardless of exposure conditions for very long periods of time. In particular, the material has remained ductile, with a failure strain in excess of 5% after 18 years under water which is unusual for fibre reinforced cement composites.

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Résumé Un programme d’étude d’une durée de 18 ans vient de s’achever sur le fonctionnement en tension des composites ciment renforcés avec deux types de réseaux de polypropylène fibrillé. Trois conditions de conservation ont été utilisées—vieillissement naturel en plein air, conservation ambiante dans le laboratoire, et conservation sous l’eau. Quelques-uns des composites ont été pré-fissurés avant d’être exposés et d’autres sont restés non fissurés jusqu’à l’essai. Les paramètres messurés ont compris le module élastique et la résistance en traction de la matrice, des courbes de contrainte-déformation et la distribution des fissures dans le composite, et les effets de diverses conditions de vieillissement sur la résistance en traction, la déformation à la rupture et la force d’adhérence de l’amature en polypropylène. Il a pu être démontré que lorsque le volume de fibres dépasse le volume critique, la résistance, la ductilité et la dureté du composite ont été maintenues quelles que soient les conditions de l’exposition pendant de très longues périodes. En particulier, le matériau est resté ductile, avec une déformation à la rupture augmentée de 5% après 18 ans sous l’eau, ce qui est rare pour des composites ciment renforcés de fibres.

     

Engineering and evaluation of hemp fibre reinforced polypropylene composites: Micro-mechanics and strength prediction modelling

The strength of a composite consisting of 40 wt% NaOH/Na₂SO₃ treated hemp fibre, polypropylene and 4 wt% MAPP was evaluated by means of mathematical modelling and mechanical testing. Interfacial shear strength, single fibre tensile strength and fibre length distribution within the composite were obtained, and theoretical composite strengths were determined by means of the Modified Rule of Mixtures and Bowyer–Bader models. The experimentally obtained composite tensile strength of 50.5 MPa was found to be one-third of the theoretical strength determined by means of the Bowyer–Bader model, and this difference was thought to be mainly due to the non-axial planar-random orientation of the fibres within the composite.     

Multiple cracking response of plasma treated polyethylene fiber reinforced cementitious composites under flexural loading

The effects of low frequency cold plasma treatments on the microstructure and chemistry of Polyethylene (PE) have been investigated. PE plates and fibers were exposed to plasmas of argon and oxygen gases. The surface wettabilities of plasma-treated plates were monitored. Possible physical changes on fiber surfaces were observed by a scanning electron microscope (SEM) at micrometer scale and by an atomic force microscope (AFM) at nanometer scale after this process. The effects of plasma treatment on surface chemistry of PE fibers have been analyzed by using an X-ray photoemission spectroscope (XPS). The fibers modified by plasma treatments were used in prismatic cementitious composites. The flexural performance of samples were characterized at two different ages (28 days and 8 months). Results showed that plasma treatment caused significant modifications on fibers’ surface structure and composites’ performance. Proper plasma treatment conditions significantly leads to improvement of multiple cracking behavior of fiber reinforced composites.     

Chemical modification of flax reinforced polypropylene composites

This paper presents an experimental study on the static and dynamic mechanical properties of nonwoven based flax fibre reinforced polypropylene composites. The effect of zein modification on flax fibres is also reported. Flax nonwovens were treated with zein coupling agent, which is a protein extracted from corn. Composites were prepared using nonwovens treated with zein solution. The tensile, flexural and impact properties of these composites were analysed and the reinforcing properties of the chemically treated composites were compared with that of untreated composites. Composites containing chemically modified flax fibres were found to possess improved mechanical properties. The viscoelastic properties of composites at different frequencies were investigated. The storage modulus of composites was found to increase with fibre content while damping properties registered a decrease. Zein coating was found to increase the storage modulus due to enhanced interfacial adhesion. The fracture mechanism of treated and untreated flax reinforced polypropylene composites was also investigated from scanning electron microscopic studies.