Assessment of the fibre orientation factor in SFRC slabs

The design of steel fibre reinforced concrete (SFRC) structures is evolving towards a new approach that uses correction factors to consider differences between the small-scale characterisation specimens and the real-scale elements. Recently, the Model Code 2010 proposed an orientation factor (K) that accounts for the effects of the orientation in the structural response of elements. The present study focuses on the identification of this factor in SFRC slabs with different dimensions. For that, flexural tests on real-scale slabs were conducted and the fibre orientation was assessed with an inductive method. A finite element analysis showed the differences between the experimental curves and the prediction of the Model Code without considering K. Based on the results obtained, a range of values is proposed for K and validated. This study sheds light on possible modifications that this philosophy of design might require to better reproduce the behaviour of slabs.     

Tests of continuous concrete slabs reinforced with carbon fibre reinforced polymer bars

Although several research studies have been conducted on simply supported concrete elements reinforced with fibre reinforced polymer (FRP) bars, there is little reported work on the behaviour of continuous elements. This paper reports the testing of four continuously supported concrete slabs reinforced with carbon fibre reinforced polymer (CFRP) bars. Different arrangements of CFRP reinforcement at mid-span and over the middle support were considered. Two simply supported concrete slabs reinforced with under and over CFRP reinforcement and a continuous concrete slab reinforced with steel bars were also tested for comparison purposes. All continuous CFRP reinforced concrete slabs exhibited a combined shear–flexure failure mode. It was also shown that increasing the bottom mid-span CFRP reinforcement of continuous slabs is more effective than the top over middle support CFRP reinforcement in improving the load capacity and reducing mid-span deflections. The ACI 440.1R–06 formulas overestimated the experimental moment at failure but better predicted the load capacity of continuous CFRP reinforced concrete slabs tested. The ACI 440.1R–06, ISIS–M03–07 and CSA S806-06 design code equations reasonably predicted the deflections of the CFRP continuously supported slabs having under reinforcement at the bottom layer but underestimated deflections of continuous slabs with over-reinforcement at the bottom layer.     

Fibre orientation of novel dynamically sheet formed discontinuous natural fibre PLA composites

This paper describes work carried out to fabricate and to assess the fibre orientation in PLA reinforced by aligned discontinuous harakeke and hemp fibre mats produced using a dynamic sheet former (DSF). These mats were combined with PLA sheets to make composites with fibre contents of 5–40 wt% using a hot press. It was found that the fibre orientation factors (K_θ) for both harakeke and hemp fibre composites were higher than those values seen in the literature for composites prepared using injection moulding and hot pressed using randomly oriented fibre mats, but slightly lower than the highest values obtained with aligned fibre nonwoven preform composites utilising more processing stages. The highest K_θ values for harakeke and hemp fibres in this work were found to be 0.58 and 0.44 respectively. K_θ decreased, reflecting increased fibre misalignment as fibre content increased, believed to be due to fibre agglomeration and the higher pressure required during processing.     

An original approach for mechanical modelling of short-fibre reinforced composites with complex distributions of fibre orientation

In this paper, an original and effective model of behaviour for short-fibre reinforced composites is presented. In particular, complex fibre distributions of orientation can be dealt with in a very easy way, without orientation averaging or additional homogenisation steps. The matrix material has elastoplastic damage behaviour with non-isochoric plastic flow. Ductile damage can be fully anisotropic depending on the reinforcement characteristics. The model is validated for the case of a polypropylene reinforced with short flax fibres. In addition, simulations are performed to investigate the influence of key parameters like fibre length and interfacial shear strength, as well as the impact of progressive debonding at the fibre tips.     

Flexural stiffness modelling of RC slab strengthened by externally bonded FRP

This paper describes the behaviour of strengthened and unstrengthened reinforced concrete slabs, subjected to a single, specific, local, load at their centres. In the experimental stage, the influence of the reinforcement on the various slabs tested was analysed by studying their behaviour at failure and by making a bending stiffness analysis. In order to predict the mechanical behaviour of the slab, a model was developed to predict the value of the bending stiffness, the mid-span displacements and the strain on each material making up the slabs. The experimental results were compared to those of the model and there was good agreement.     

Ultimate behavior of axially loaded RC wall-like columns confined with GFRP

The assessment of the effectiveness of the fiber reinforced polymer (FRP) confinement on rectangular reinforced concrete (RC) columns with high aspect ratio (wall-like) still represents an unresolved issue. The present paper aims at providing more experimental evidence about the behavior of such members confined with both uni-directional and quadri-directional glass FRP laminates. Particular attention is devoted to issues related to the premature failure of confining fibers experimentally observed in wall-like columns. Test results on nine axially loaded columns are herein presented; emphasis is also given to the analysis of FRP strain profiles along the sides of the cross-section. The analysis of test results highlights that glass FRP (GFRP) confinement could determine significant strength and ductility increases; the discussion of failure modes points out that the failure of GFRP confined wall-like columns is controlled by the shape of the cross-section and occurs at transverse strains in the jacket much lower than those ultimate of the fibers. Theoretical–experimental comparisons are performed using some available models for strength prediction of such members.     

Behaviour of ±45° glass/epoxy filament wound composite tubes under quasi-static equal biaxial tension–compression loading: experimental results

The primary aim of this paper is to present results describing in detail the behaviour of ±45° E-glass/MY750 (GRP) tubes, of various wall thicknesses, subjected to equal biaxial tension–compression loading, generated under combined internal pressure and axial compression. The role played by the non-linear lamina shear has also been assessed by comparing various shear stress–strain curves for embedded laminae (extracted from tests on ±45° tubes subjected to circumferential: axial stress ratios SR=1:0, 1:−1 and 2.3:−1) with that of an ‘isolated’ lamina (measured from torsion of 90° tubes). Extracted shear failure strains, for embedded laminae, were more than four fold larger than those measured at ultimate failure for an ‘isolated’ lamina. Soft characteristics were observed in the embedded lamina and these were believed to be due to interaction between early matrix damage initiation (and propagation) and shear. Factors affecting the behaviour of the tubes, such as bulging, scissoring, thermal stresses and stress variation through the thickness are discussed.     

Assessing the effect of fibre extraction processes on the strength of flax fibre reinforcement

A number of factors impede the direct translation of fibre properties from plant crop species to natural fibre composites. Commercially available fibre extraction processes introduce defects and degrade the mechanical properties of fibres. This study reports on a novel image based approach for investigating the effect of fibre extraction processes on flax fibre bundle strength. X-ray micro Computed Tomography (μCT) was coupled with uniaxial tensile testing to measure the in-situ fibre bundle cross-section area and tensile strength in flax plant stems. The mean tensile strength result was 50% higher than that of the fibres extracted through the standard commercial process. To minimize fibre damage during fibre extraction, a pre-treatment was proposed via saturating flax plant stems in 35% aqueous ammonia solution. By environmental scanning electron microscopy (ESEM), it was evident that ammonia treatment significantly reduced the extent of damage in flax fibre knots and the optimum treatment parameter was identified.     

Guidelines for flexural resistance of FRP reinforced concrete slabs and beams in fire

Several building codes are currently available for the design of concrete structures reinforced with fiber-reinforced polymer (FRP) bars. Nevertheless, there is little information provided about structural behavior in case of fire and no reliable design methods are available for FRP reinforced concrete (RC) members in fire. The goal of this paper is to provide guidelines for the calculation of the resistant bending moment of FRP-RC members exposed to fire in compliance with the provisions of Eurocodes, based on studies recently carried out by the authors. The paper provides a conceptual approach to fire safety checks for bending moment resistance of FRP-RC members. With reference to thermo-mechanical analysis, a simplified design method (for both thermal and mechanical analyses) for sagging bending moment resistance of FRP-RC slabs in fire situations is finally suggested.     

Mechanical performance of innovative GFRP-bamboo-wood sandwich beams: Experimental and modelling investigation

Innovative GFRP-bamboo-wood sandwich beams were developed and investigated experimentally and by modeling. The effects of the thickness of the GFRP and bamboo layers on the overall structural performance in bending were clarified. It was shown that an increase of thickness of the bamboo and GFRP layers could significantly increase the flexural stiffness and ultimate load of the sandwich beams. ANSYS was used to parametrically analyze the material efficiency and to obtain optimal solutions for the thickness of the GFRP, wood, and bamboo layers. The total depth of 60 mm and the thickness of 6 mm for bamboo and of 4.5 mm for GFRP presented the best material efficiency in terms of stiffness enhancement. A simplified model based on Timoshenko beam theory was proposed to predict the load-deflection behavior of the sandwich beams, where the section transformation method was used to calculate the stress distribution along the depth of the sandwich beams. The calculated results showed good correlation with the experimental and numerical results. Design optimization in terms of self-weight and cost of the proposed sandwich beam was conducted using MATLAB and ANSYS, and the optimized thicknes was obtained with minimized self-weight, cost, and acceptable mechanical performance.     

Shear design of reinforced concrete beams with FRP longitudinal and transverse reinforcement

The shear resisting mechanisms of reinforced concrete (RC) beams with longitudinal and transverse FRP reinforcement can be affected by the mechanical properties of the FRP rebars. This paper presents a mechanical model for the prediction of the shear strength of FRP RC beams that takes into account its particularities. The model assumes that the shear force is taken by the un-cracked concrete chord, by the residual tensile stresses along the crack length and by the FRP stirrups. Failure is considered to occur when the principal tensile stress at the concrete chord reaches the concrete tensile strength, assuming that the contribution of the FRP stirrups is limited by a possible brittle failure in the bent zone. The accuracy of the proposed method has been verified by comparing the model predictions with the results of 112 tests. The application of the model provides better statistical results (mean value V_test/V_pred equal to 1.08 and COV of 19.5%) than those obtained using the design equations of other current models or guidelines. Due to the simplicity, accuracy and mechanical derivation of the model it results suitable for design and verification in engineering practice.     

Structural performance of ballastless track slabs reinforced with BFRP and SFCB

Because of the inductive impedance caused by steel meshes in traditional reinforced ballastless track slabs, the electrical properties, primarily the rail resistance and inductance, of jointless track circuits are affected by electromagnetic induction between the slabs and the electric current in the rail. This problem results in poor transmission performance throughout the track circuit. Insulating sleeves or cards between the steel meshes have been used to improve the insulation capability of steel meshes in slabs; however, they reduce the bonding performance between the steel bars and concrete. Because of the good insulation properties of fiber-reinforced polymer composite bars (FRPs) and steel-fiber reinforced polymer composite bars (SFCBs), these composite materials have shown potential to overcome this insulation problem. However, the structural performance of the ballastless track slabs reinforced by basalt fiber reinforced polymer composite bars (BFRPs) and SFCBs, which play a key role in the structure and transportation safety, needs to be investigated. In this paper, six ballastless track slabs reinforced with BFRPs, SFCBs, and steel bars were constructed and tested. The following results were obtained. (1) Shear failures were observed for all slabs, both the BFRP and SFCB slabs meet the load level requirements, and SFCBs reinforcements have higher strength utilization compared with BFRPs reinforcements. (2) The bond-quality of SFCBs and BFRPs reinforcements proved slightly poorer than that of the steel bars. Because of the good corrosion resistance of the FRP, the maximum crack width limits can be slightly larger than that of the RC slabs. (3) Bischoff’s equation was initially used to calculate the deflection of partially prestressed concrete slabs under service loads. The results demonstrated a good agreement between the theoretical and experimental analysis. (4) Considering the tensile stiffness, the modified ACI equation was used to calculate the slabs’ crack width and the theoretical and experimental results showed a good agreement.     

Theoretical prediction and experimental verification of pulling carbon nanotubes from carbon fiber prepared by chemical grafting method

This article reports on experiments for measuring pulling forces and displacements of a carbon nanotube (CNT) grafted on carbon fibers (CFs) and modeling for predicting the pulling force–displacement curves. In the experiments, the pulling force and displacement for different grafting configurations are measured. In the present analytical model, a power function relationship between a curvature radius and critical failure angle of the CNT was firstly established on the basis of the experimental data, and then the critical failure angle, maximum pulling forces and displacements were determined by using a free length, initial grafting length, initial grafting angle, the Hamaker constant and friction coefficient between the CNT and CF. By using the present model, pulling mechanical behaviors of CNTs with different grafting configurations are investigated and verified with results obtained from images taken in the experiment.     

A continuum damage model for glass/epoxy laminates in tension

The present work is concerned with the study of the damage behaviour of a composite material based on glass fibre reinforced polymer (GFRP). The main goal is to predict the rupture force using model equations that combine enough mathematical simplicity to allow their usage in engineering problems with the capability of describing a complex nonlinear mechanical behaviour. A model for tensile developed within the framework of Continuum Damage Mechanics that accounts for the effect of the load rate and temperature of the system is proposed and analyzed. The predicted values of tensile stress for different values of the load rate and temperature are compared with experimental data, showing a good agreement.     

Analytical evaluation on uniaxial tensile deformation behavior of fiber metal laminate based on SRPP and its experimental confirmation

Uniaxial tensile tests have been carried out to accurately evaluate the in-plain mechanical properties of fiber metal laminates (FMLs). The FMLs in this paper comprised of a layer of self-reinforced polypropylene (SRPP) sandwiched between two layers of aluminum alloy 5052-H34. In this study, nonlinear tensile and fracture behavior of FMLs under in-plane loading conditions has been investigated with numerical simulations and theoretical analysis. The numerical simulation based on finite element modeling using the ABAQUS/Explicit and the theoretical constitutive model based on the volume fraction approach using the rule of mixture and the modified classical lamination theory, which incorporates the elastic–plastic behavior of the aluminum alloy and SRPP, are used to predict the in-plain mechanical properties such as stress–strain response and deformation behavior of the FMLs. In addition, the pre-stretching process is used to reduce the thermal residual stresses before the uniaxial tensile tests of the FMLs. Through comparing the numerical simulations and the theoretical analysis with the experimental results, it is concluded that the adopted numerical simulation model and the theoretical approach can describe with sufficient accuracy of the actual tensile stress–strain curve.     

Mechanical behaviors of carbon fiber composite sandwich columns with three dimensional honeycomb cores under in-plane compression

Mechanical properties and failure modes of carbon fiber composite egg and pyramidal honeycombs cores under in plane compression were studied in the present paper. An interlocking method was developed for both kinds of three-dimensional honeycombs. Euler or core shear macro-buckling, face wrinkling, face inter-cell buckling, core member crushing and face sheet crushing were considered and theoretical relationships for predicting the failure load associated with each mode were presented. Failure mechanism maps were constructed to predict the failure of these composite sandwich panels subjected to in-plane compression. The response of the sandwich panels under axial compression was measured up to failure. The measured peak loads obtained in the experiments showed a good agreement with the analytical predictions. The finite element method was used to investigate the Euler buckling of sandwich beams made with two different honeycomb cores and the comparisons between two kinds of honeycomb cores were conducted.     

Tensile behaviour of mortar-based composites for externally bonded reinforcement systems

Textile reinforcements applied with inorganic matrices are currently receiving great attention for strengthening reinforced concrete and masonry structures, especially when preservation criteria need to be fulfilled for safeguarding cultural heritage. As the development of mortar-based reinforcements is still at an early stage, their mechanical properties need to be investigated and standardized testing methodologies have to be defined. The paper presents an experimental study on the tensile behaviour of strengthening systems comprising two different textiles and five mortar matrices. Various clamping methods and testing setups have been experimented and their effect on the results is discussed. Monotonic and cyclic tests have been carried out to derive strength and stiffness, crack pattern, failure mode, and response stages under tension, which have been related to the mechanical properties and the layout of the matrix and the textile.     

Basalt FRP rods for reinforcement and repair of timber

Limited research has been undertaken into the use of basalt fibre reinforced polymer (FRP) materials for the strengthening and repair of structural timber elements. This paper describes an experimental test programme in which the flexural performance of low-grade glued laminated timber was reinforced using bonded-in basalt FRP rods. Tension test results show that basalt FRP rods compare extremely well to the mechanical characteristics of glass FRP rods. Strengthened and repaired beams exhibited considerable ductility in contrast to brittle tension behaviour of the unreinforced sections. With the use of a modest reinforcement percentage of 1.4% strategically located in circular routed out grooves at the soffit of the beam, mean stiffness enhancements of 8.4% and 10.3% for the global and local measurements were achieved respectively and a mean improvement in the ultimate moment capacity of 23% was achieved in comparison to the unreinforced glulam beams. The distance of the reinforcement to the neutral axis was shown to be highly influential on the mechanical enhancements. The use of basalt FRP rods is seen to be highly effective as a repair technology for damaged timber elements. Strain profile readings from the beams which included the reinforcement demonstrated improved utilisation of the compression characteristics of the timber. In all testing, a good quality bond is reported between the basalt FRP and wood. There exists significant potential for the development of environmentally friendly engineered structural elements by combining timber based products with other natural materials such as basalt fibre reinforced polymers.     

Computational fatigue life prediction of continuously fibre reinforced multiaxial composites

The potential of a fatigue-life prediction method for continuously fibre reinforced carbon/epoxy laminates has been investigated. Stress analysis conducted with a finite element solver in combination with the experimentally measured anisotropic S–N curves was used as input parameters. Subsequently, lifetime of a unidirectional and a multidirectional composite was calculated for a cyclic tension–tension load case and validated with experimental fatigue tests. The predicted lifetime of the unidirectional laminate correlated well to the experimental results. For the fatigue-life calculation of multidirectional composites, the software underestimated the experimental data. Results and possible improvements based on the presented calculations are discussed in detail.