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.     

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.     

Practical prediction model for shrinkage of steel fibre reinforced concrete

A practical prediction model is proposed for predicting the shrinkage of steel fibre reinforced concrete (SFRC) from the composition of concrete mix, strength, age when drying begins, conditions of environment, size and shape of structures, fibre volume, aspect ratio of fibre, etc. The formulas were developed according to an extension of the well-recognized BP prediction model for the shrinkage of plain concrete (OPC). All important features of the BP prediction model, such as the diffusion-type size dependence of humidity effects and the square-root hyperbolic law for shrinkage, are automatically adopted for SFRC. In order to supplement the insufficient test data collected from the literature, a recently finished experimental series was used to expand the databank and hence the authors have a better command of the prediction formulas. Results of shrinkage from the literature and from the test series are reported, together with comparisons with predictions using the proposed model.     

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.     

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.

     

Restrained shrinkage cracking in steel fibre reinforced and rubberised cement-based mortars

The paper focuses on the combined effect of Steel Fibre Reinforcement and of Rubber aggregates on the resistance to restrained shrinkage cracking of cement-based mortars. The kinetics of restrained shrinkage cracking of a control mortar is compared both to the one incorporating a single fibre content as reinforcement and to the above fibre reinforcement combined to rubber aggregates substitution. Two rates of substitution were considered in the case of Steel Fibre Reinforced and Rubberized Mortar (SFRRM). The used rubber aggregates are obtained by grinding used tyres, a way that may address the demand for the conservation of a clean environment by recycling an industrial by-product. Fibre-reinforced mortar was based on the control mortar and one metal fibre content was studied: 40 kg/m³ (0.5% by volume). The used fibres have a high bond with the cementitious matrix. SFRRM composites were cast using two contents of rubber aggregates: 20 and 30% by volume replacing mineral aggregates and the one fibre content reported here above (40 kg/m³). Tests were conducted using ring type specimens to simulate restrained shrinkage cracking according to ASTM C 1581-04 standard. Additionally, after the cracking occurred, the development of the crack widths was measured by video-microscope. Ring tests demonstrated that the SFRRM exhibit high strain capacity prior to macro-cracking localization and the effectiveness of rubber aggregates along with their positive synergistic effect when combined with fibre reinforcement to improve resistance to shrinkage cracking. It is a promising solution to improve the durability of large surface area such as pavements and thin bonded cement-based overlays, whose durability is often limited by shrinkage cracking.     

A static fatigue model for the durability of glass fibre reinforced cement

This paper presents a model for predicting service lives for glass-fibre reinforced cement (grc) components using hot-water accelerated ageing. It improves on previous models, being derived from consideration of a specific proposed micro-mechanical strength loss mechanism based on static fatigue principles and can be applied from time = 0. The model fitted well to all available strength vs. time data pertaining to various grc formulations. The activation energies thus derived for the strength loss process (80–90 kJ mol^–1) were consistent with those derived previously and those proposed for general glass dissolution mechanisms. Updated acceleration factors for predicting service lives for grc are advanced. The model was also applied to grc made with modified cement matrices. For metakaolin modified matrices, the activation energy appeared similar to that for OPC-grc, thus the use of similar acceleration factors appears justified. There is some evidence that calcium sulphoaluminate modified grc degrades according to a different activation energy. More data are required for modified matrix grcs if the model is to be applied thereto with confidence.     

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.     

A high performance fibre reinforced cement based plaster for retrofitting RC members

A high performance fibre-reinforced cementitious composite (HPFRCC) material is developed to be used for retrofitting reinforced concrete members. It can be applied to the face of a concrete member to the desired thickness as a wet mix or as an adhesively-bonded prefabricated slab or strip. The material is compatible with concrete and possesses favourable strength and ductility properties, desirable for seismic retrofit. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and fibre-reinforced polymer (FRP) laminates caused mainly by the mismatch of their tensile strength and stiffness with that of the concrete member being retrofitted. An extensive rheological analysis is undertaken to develop the appropriate mixes using different types and mix proportions of constituent materials including; fine steel fibres, fine quartz sand, silica fume, cement and superplasticizer. Much reduced amounts of steel fibres are used compared to the previous studies so that ordinary mixing procedures could be applied and a more cost-effective retrofitting material could be developed. Samples made of the optimum mixes are shown to posses very high compressive and tensile strengths and sufficient ductility for the composite plaster to be used externally for strengthening and seismic retrofitting of concrete members.     

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     

Fatigue endurance of aged glass fibre reinforced cement

The resistance to flexural fatigue of glass fibre reinforced cement (GRC) stored in water for six years, has been studied. Peak stresses of between 6.0 and 18.2 MN m^–2 were used. At stresses of 10.0 and 18.2 MN m^–2 the median times before failure were 1.95×10⁵ and 2.0×10³ cycles, respectively. At a stress of 8.1 MN m^–2, six out of sixteen samples tested survived 4.65×10⁶ cycles. At a stress of 6 MN m^–2, all of the samples survived 1.75×10⁶ cycles. An unreinforced mortar specimen was also studied and its fatigue endurance showed greater scatter than the GRC samples.     

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.

     

A quantitative study on the plastic shrinkage cracking in high strength hybrid fibre reinforced concrete

High strength concrete using silica fume is prone to plastic shrinkage cracking in dry and windy conditions. Addition of fibres is known to restrict the growth of shrinkage cracks. The present study was aimed at controlling plastic shrinkage cracks in high strength silica fume concrete by means of adding fibre reinforcement up to 0.5% by volume of concrete. Individual steel fibres as well as hybrid combinations of steel and non-metallic (polyester, polypropylene and glass) fibres were evaluated for their influence on plastic shrinkage cracking. Results showed that hybrid fibres were most effective in reducing shrinkage cracks. Among the hybrid fibre combinations, the steel and polyester combination was found to reduce plastic shrinkage cracks by more than 99% compared to the plain concrete. Increased fibre availability and low stress levels at early ages were the main factors contributing to the good performance of hybrid fibre mixtures.     

The work of fracture of random fibre reinforced cement

From considerations of the pull-out behaviour of individual inclined wires from model composites, the work of fracture and post-cracking stress of random fibre reinforced composites have been estimated. Results are presented for cement composites reinforced with short, high-strength steel wires. It was found that when the fibres were short and/or the interfacial shear stress low, a substantial part of the composite work of fracture was due to plastic deformation of those fibres which were not perpendicular to the matrix crack face. Under such conditions random fibre composites showed superior properties than aligned composites of the same volume fraction of reinforcement.

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Résumé On a évalué le travail de rupture et la contrainte après fissuration de matériaux renforcés de fibres à répartition aléatoire à partir de l'étude de modèles de composites dans lesquels on a considéré le comportement à l'arrachement de filaments individuels inclinés. Les résultats donnés ici se rapportent à des composites à base de ciment renforcé avec de courts filaments d'acier de haute résistance. On constate que lorsque les fibres sont courtes et/ou la contrainte de cisaillement à l'interface faible, une part substantielle du travail de rupture est due à la déformation plastique des fibres qui ne sont pas perpendiculaires à la surface fissurée. Dans de telles conditions, les composites à fibres réparties au hasard montrent des proprietés meilleures que les composites à fibres alignées pour le même rapport de volume de renforcement.