Relation between fibre distribution and post-cracking behaviour in steel fibre reinforced self-compacting concrete panels

In this research, the influence of the fibre distribution and orientation on the post-cracking behaviour of steel fibre reinforced self-compacting concrete (SFRSCC) panels was studied. To perform this evaluation, SFRSCC panels were cast from their centre point. For each SFRSCC panel, cylindrical specimens were extracted and notched either parallel or perpendicular to the concrete flow direction, in order to evaluate the influence of fibre dispersion and orientation on the tensile performance. The post-cracking behaviour was assessed by both splitting tensile tests and uniaxial tensile tests. To assess the fibre density and orientation through the panels, an image analysis technique was employed across cut planes on each tested specimen. It is found that the splitting tensile test overestimates the post-cracking parameters. Specimens with notched plane parallel to the concrete flow direction show considerable higher post-cracking strength than specimens with notched plane perpendicular to the flow direction.     

An integrated approach for modelling the tensile behaviour of steel fibre reinforced self-compacting concrete

The present work resumes the experimental and numerical research carried out for the development of a numerical tool able of simulating the tensile behaviour of steel fibre reinforced self-compacting concrete (SFRSCC). SFRSCC is assumed as a two phase material, where the nonlinear material behaviour of SCC matrix is modelled by a 3D smeared crack model, and steel fibres are assumed as embedded short cables distributed within the SCC matrix according to a Monte Carlo method. The internal forces in the steel fibres are obtained from the stress–slip laws derived from the executed fibre pullout tests. The performance of this numerical strategy was appraised by simulating the tensile tests carried out. The numerical simulations showed a good agreement with the experimental results.     

Prediction of the plastic viscosity of self-compacting steel fibre reinforced concrete

Micromechanical constitutive models are used to predict the plastic viscosity of self-compacting steel fibre reinforced concrete (SCFRC) from the measured plastic viscosity of the paste. The concrete is regarded as a two-phase composite in which the solid phase is suspended in a viscous liquid phase. The liquid matrix phase consists of cement, water and any viscosity modifying agent (VMA) to which the solids (fine and coarse aggregates and fibres) are added in succession. The predictions are shown to correlate very well with available experimental data. Comments are made on the practical usefulness of the predicted plastic viscosity in simulating the flow of SCFRC.     

Shape and orientation effects on volume determination by microscopy of lath-shaped fibres modeled by trivariate log-normal size distributions

Fibre size distributions have been described by bivariate log-normal distributions, neglecting fibre height. However, fibre height in general is different from fibre width for lath-shaped particles, as seen in the microscope. This has implications for volume determination. The trivariate log-normal distribution, which is completely specified by nine parameters, is used to derive volume estimators and to determine their properties by numerical simulation. The addition of individual fibre volumes determined as length times width squared, overestimates volume, unless all fibres have a square cross-section, even if there is no preferred orientation upon collection on a substrate, and there is no correlation between width and height. A volume estimator, which circumvents this problem is suggested, but the correlation must be known a priori. Volume determination by sizing fibres in two dimensions can give biased results. Joint determination of length, width, and height thus should be performed. Because individual fibre volumes have a large variance, the precision of total volume estimation can be only improved by sizing sufficiently many fibres.     

Interfacial adhesion in short-fibre composites

For the evaluation of the interfacial adhesion in short-fibre composites, a simple method based on fibre pull-out length distribution was proposed. The experiments with three composites of isotactic polypropylene with 32 wt.-% of short-glass fibres confirm the potentials of the method for a sensitive determination of differences in the interfacial adhesion. The effects of temperature, crystallinity and fibre orientation angle on the interfacial adhesion were investigated.     

自密实橡胶混凝土性能研究综述

自密实橡胶混凝土是将废弃轮胎加工成橡胶颗粒并掺加到自密实混凝土中制成的新型建筑材料,相比于常规混凝土,其在硬化后具有脆性低、变形能力强、节能环保的显著优点.为了全面了解自密实橡胶混凝土的最新研究现状,从自密实性能(流动性、粘聚性、间隙通过性、抗离析性)、力学性能(抗压强度、劈裂抗拉强度、弹性模量、抗折强度、抗冲击性能、...     

Fibre orientation distribution in turbulent fibre suspensions flowing through an axisymmetric contraction

The Reynolds averaged Navier–Stokes equation was solved numerically with the Reynolds stress model to get the mean fluid velocity and the turbulent kinetic energy in turbulent fibre suspensions flowing through an axisymmetric contraction. The fluctuating fluid velocity was represented as a Fourier series with random coefficients. Then the slender‐body theory was used to predict the fibre orientation distribution and orientation tensor. Some numerical results are compared with the experimental ones in the turbulent fibre suspensions flowing through a contraction with a rectangular cross‐section. The results show that the fibres with high aspect ratio tend to align its principal axis with the flow direction much easier. High contraction ratio makes the fibre alignment with the flow direction much easier. The contraction ratio has a strong effect on the fibre orientation distribution. Only a small part of the fibre is aligned with the flow direction in the inlet region, while most fibres are aligned with the flow direction when they approach to exit. The fibres are aligned with the flow direction rapidly in the inlet region, after that the fibre orientations change little in the most of the downstream region. The fibres with high aspect ratio are aligned with the flow direction faster when they enter the contraction. The randomising effect of the turbulence becomes significant in the downstream region because of the high turbulent intensity.     

Crystallographic texturing in single poly(p-phenylene benzobisoxazole) fibres investigated using synchrotron radiation

A micro-focus synchrotron beam has been used to investigate crystallographic texturing in poly(p-phenylene benzobisoxazole) fibres. By generating diffraction patterns across single fibres, it is possible to produce profiles showing scattering intensity as a function of beam position across the fibre. A straightforward model has been developed to allow the degree of texturing to be quantified for direct comparison between fibre types. The experimental results are found to fit a radial fibrillar-texturing model, which incorporates a distribution in radial orientation about the fibre axis. Previous studies reporting the a-axis of the PBO unit cell to be aligned radially within fibrils about the fibre axis are found to be correct.The degree of radial fibrillar texturing is in the same fibre order as tensile modulus and crystalline domain orientation for PBO fibres with different processing histories. It is proposed that the degree of radial fibrillar texturing is therefore related to fibre homogeneity. An extrapolation of tensile modulus to that of a perfectly homogeneous fibre results in a value in good agreement with the PBO crystal modulus. This further supports the proposal that the degree of radial fibrillar texturing is related to fibre homogeneity.     

Orientation effects during the flow of short-fibre reinforced thermoplastics

A study has been carried out to investigate molecular orientation in injection moulded bars of short glass fibre reinforced polypropylene and polyethylene. For the range of fibre concentrations encountered commercially, the fibres do not appear to have any direct effect on the matrix orientation. As the fibre concentration increases, however, the matrix orientation becomes dominated by the orientation of the fibres. These effects are interpreted in terms of current ideas of the rheology of polymer melts during injection moulding and the crystallization of polymers at fibre surfaces.     

Direct meso‐scale simulations of fibres in turbulent liquid flow

A procedure for direct, meso‐scale simulations of flexible fibres immersed in liquid flow is introduced. The fibres are composed of chains of spherical particles connected through ball joints with the bending stiffness of the joints as a variable. The motion of the fibres and the liquid is two‐way coupled with full resolution of the solid–liquid interface. First the simulation procedure is validated by means of an analytical solution for sphere doublets in zero‐Reynolds simple shear flow. Subsequently we use the numerical method to study inertial flows with fibres, more specifically the interaction of a fibre with isotropic turbulence.     

The thermal conductivity of fibre-reinforced concrete

The effect of copper and steel fibre inclusions on the thermal conductivity of mortar and concrete is investigated. The experimental technique is based on the conventional steady-state method using desiccated specimens. The results indicate that copper fibres significantly increase thermal conductivity while steel fibres have a lesser effect. Vibration of the fresh concrete, during specimen manufacture, produces some fibre alignment. As a result, it is found that theoretical methods based on the assumption of a random fibre distribution under-estimate the experimental values.     

Morphology and Structure of Constituent Fibres in Thermally Bonded Nonwovens

Summary: A new process has been developed to produce three‐dimensional nonwovens directly from staple fibres. In order to establish suitable windows of the process parameters to achieve high‐quality nonwoven products, the effects of thermal bonding temperature, dwell time and mould material on the morphology and structure of the fibre have been investigated using PP/PET bi‐component fibres. It was evident from both scanning electron microscope images and Raman spectra that thermal‐induced shrinkage of the PP sheath fibre occurred in the thermal bonding process, leading to deformation and cracking of the PP sheath and exposure of the PET core. X‐ray diffraction results revealed crystal imperfection and/or less ordered polymer chains, more γ‐form and thermal contraction of the crystal lattice for the PP sheath fibre, while birefringence measurements indicated that both the birefringence and the orientation factor for the PP fibre decreased after the thermal bonding process. The degrees of the thermal‐induced shrinkage increased, and the crystallinity, birefringence and orientation factor of the PP sheath fibre decreased with increasing thermal bonding temperature, dwell time and thermal conductivity of the mould material. All these can be attributed to the different levels of modification of chemical composition caused by thermal oxidative degradation and thermal‐induced relaxation of the orientation during the thermal bonding process.

Changes of morphology and crystalline features of PP/PET fibre after thermal bonding process.     

Characterization of the adhesion of single-walled carbon nanotubes in poly(p-phenylene terephthalamide) composite fibres

Poly(p-phenylene terephthalamide)/single-walled carbon (PPTA/SWNT) composite fibres with different draw ratios have been spun using a dry-jet wet spinning process and their structure and deformation behaviour analysed using Raman spectroscopy. The dispersion of nanotube has been examined by Raman scattering intensity mapping along the fibre. The nanotubes improved the polymer orientation in composite fibre with a draw ratio of 2 but degraded the orientation at higher draw ratios. The mechanical reinforcing effect by nanotubes is related to the change of polymer orientation, suggesting a dominant role of polymer orientation in mechanical performance of the composite fibre. High efficiency of stress transfer within the strain range of 0-0.35% and breakdown of the interface at higher strains has been found in the composite fibres through an in situ Raman spectroscopic study during fibre deformation. Cyclic loading applied on the fibre has indicated reversible deformation behaviour at low strain and gradual damage of the interface at high strains.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

Tensile deformation of poly(p-phenylene terephthalamide) fibres,an experimental and theoretical analysis

The tensile deformation of poly(p-phenylene terephthalamide) fibres has been investigated. Functional relationships observed between stress, crystallite orientation distribution, dynamic modulus and strain are derived from an analysis of the deformational behaviour of a series model consisting of a linear arrangement of crystallites. It is shown that the deformation of these fibres is largely brought about by the elastic strain and irreversible rotation of the crystallites. A formula is derived for the stress—strain relationship of a crystalline polymeric fibre with a narrow crystallite orientation distribution.     

全统计法测量复合材料中短纤维的长度及其分布

提出了一种测量短纤维橡胶复合材料中短纤维长度及其分布的方法———全统计法。先用溶剂将未硫化复合材料中的短纤维分离出来 ,并使其均匀分布于有多个长条形区域的表面皿上 ,然后对部分长条形区域中的纤维进行全统计测量。该测试方法简便、数据准确     

Determination of the Orientation of Crystallites in PET Fibres

Abstract

A method for the determination of the average angle of orientation of crystallites φ from the azimuthal record of the meridional reflection of the 1 05 diatropic plane for PET fibres is widely used. The 1 05 reflection is the closest to the meridian and the normal to this plane making an angle of approximately 10[ddot] with the c axis. The intensity distribution measured by this method reflects the distribution of the 1 05 plane normals and therefore it comprises: the distribution of the 1 05 plane normals relative to the c crystallographic axis, the distribution of the crystallites relative to the reference fibre axis and the tilted orientation; that is the molecular chain axis inclined by some degrees with respect to the fibre axis. By the studying of the diffraction intensity distribution from the 1 05 diatropic plane this particularly should be taken into account. The sub-meridional (1 05) reflection was scanned in two different geometries: omega and chi scan.

Supercritical CO₂ fluid as an alternative dyeing medium changes the fibre structure to a certain extent in dependence on the treatment temperature and pressure used. Therefore the changes of crystalline orientation in poly(etbylene terephthalate) (PET) fibres as brought about under the influence of supercritical CO₂ fluid were investigated.     

Adiabatic Extension and Strength of Fibres

A method is proposed for the simultaneous determination of the stress wave propagation rate in high-modulus fibres, the density, and the strength of the fibres. A setup for investigating the dynamic deformation of fibre materials is described.     

The effect of measuring procedure on the apparent rheological properties of self-compacting concrete

Torque versus time during testing of the rheological properties of fresh concrete has been investigated. The testing was performed in a BML viscometer and on a self-compacting concrete (w/c=0.45, 70% rapid hardening Portland cement, 3% silica fume, 27% fly ash, third generation superplasticizer). The relaxation period needed to obtain steady-state flow may affect the rheological properties estimated and should be taken into account in the selection of measuring procedures. Nonsteady state is likely to cause an overestimation of the plastic viscosity and an underestimation of the yield value. Furthermore, lack of steady state may explain the apparent shear-thickening behaviour of self-compacting concrete reported elsewhere.     

A new model of turbulent fibre suspension and its application in the pipe flow

A new model of turbulent fibre suspension in pipe flow is developed by deriving the equations of Reynolds averaged Navier‐Stokes, turbulence kinetic energy and turbulence dissipation rate with the additional term of the fibres, and the equation of probability distribution function for mean fibre orientation. The equations are solved numerically. The numerical mean velocity is in agreement with the experimental data. The effects of Reynolds number, fibre concentration, and fibre aspect‐ratio on the mean velocity, turbulent kinetic energy and turbulent dissipation rate are analysed. The results show that the effect of Reynolds number on the flow behaviour is insignificant. The turbulent kinetic energy and turbulent dissipation rate increase with an increasing fibre concentration and fibre aspect‐ratio. © 2012 Canadian Society for Chemical Engineering