Pre- and post-peak toughening behaviours of fibre-reinforced hot-mix asphalt mixtures

Recycled plastic fibre-reinforced hot-mix asphalt (HMA) mixtures have better fatigue resistance than plain HMA. The toughening effects of recycled plastic fibre-reinforced HMA were characterised using direct tensile loading tests. Adding a small quantity of recycled plastic fibres to HMA was found to significantly increase the mixture's fracture energy and toughness, which were calculated using the pre- and post-peak stages of tensile force–displacement curves. A theoretical model representing the pre-peak behaviour of fibre-reinforced HMA with direct tension-softening curves for various fibre contents is presented here. The enhanced toughness through post-peak analysis was also observed using toughness indices associated with fibre-bridging effect after the pre-peak composite stress. The pre-peak fracture energy model and post-peak toughness indices appeared to be governed by the direct tensile toughening of fibre-reinforced HMA's enhanced fibre-bridging effects. The pre-peak fracture energy model demonstrates the effect of fibre content on the strain energy density during the pull-out process within the pre-peak composite stress region. The maximum pre-peak fracture energy of a coarse-graded HMA mixed with recycled plastic fibres is achieved at a fibre content of 0.4% of the total weight of the HMA. The increases in the toughness indices within the post-peak composite stress region indicate that the fatigue resistance of fibre-reinforced HMA is at least 30% greater than that of control HMA.     

Fracture behaviour of plain and fiber-reinforced concrete with different water content under mixed mode loading

In the present paper, plain concrete and fiber-reinforced concrete are considered from the point of view of the mechanical characteristics, with particular emphasis on the fracture resistance, for different values of the water/cement ratio and different amount and type (metallic or polymeric) of reinforcing fibers. The main mechanical characteristics (such as compressive strength and tensile strength) of the examined materials have experimentally been determined, and several pre-cracked specimens have been tested under three-point bending up to the final failure in order to study the fracture behaviour by also evaluating the fracture energy. Furthermore, the crack paths for static tests under displacement control have been obtained, and the load–displacement deflection curves have been determined for different crack configurations. Assuming the fracture surface characterised by a fractal dimension, some quantitative evaluations of the fracture energy are carried out. Then, the fracture behaviour and the post-peak behaviour of plain and fiber-reinforced specimens are discussed, and the effects of reinforcing fibers are quantified. Some conclusions are finally drawn.     

Durability of textile reinforced concrete made with AR glass fibre: effect of the matrix composition

This paper presents the results of recent experimentation performed to study time-dependent changes in the mechanical performance of textile reinforced concrete (TRC) made with AR glass fibre and to specify the decisive mechanisms influencing the durability of this composite material. The effect of the matrix composition was investigated by varying hydration kinetics and alkalinity of the binder mix. At first, tensile tests on (accelerated) aged specimens made of TRC were performed. The results showed a pronounced decrease in the tensile strength and strain capacity for TRC whose matrix was most alkaline (Portland cement was used exclusively as binder in this composition). The performance of TRC made with modified, alkali reduced matrix composition was to a great extent unaffected by exposure to accelerated ageing. In order to investigate the mechanisms leading to such different behaviours, changes in the mechanical performance of the fibre–matrix bond were studied using double-sided pullout test specimens with under-critical fibre reinforcement after they had undergone accelerated ageing. Furthermore, the appearance of the microstructure in the interface between fibre and matrix was described by images obtained from SEM-investigations. Measured reductions in the toughness of the composite materials could be attributed mainly to the visual observed disadvantageous new formation of solid phases in the fibre–matrix interface, while the deterioration of the AR glass fibre seemed to play only a secondary role. It could be shown that the morphology of the formed solid hydration phases depends to a large extent on the matrix composition.     

Damage model for concrete reinforced with sliding metallic fibers

This contribution presents an effective and practical three dimensional (3D) numerical model to predict the behaviour of concrete matrix reinforced with sliding metallic fibers. Considering fiber-reinforced concrete (FRC) as two-phase composite, constitutive behaviour laws of plain concrete and sliding metallic fibers were described first and then they were combined according to anisotropic damage theory to predict the mechanical behaviour of FRC. The behaviour law used for the plain concrete is based on damage and plasticity theories able to manage localized crack opening in 3D. The constitutive law of the action of sliding metallic fibers in the matrix is based on the effective stress carried by the fibers. This effective stress depends on a damage parameter related to on one hand, on the content and mechanical properties of fibers and on the other hand, on the fiber–matrix bond which itself depends on the localized crack opening. The proposed model for FRC is easy to implement in most of the finite element codes based on displacement formulation; it uses only measurable parameters like Young’s modulus, tensile and compressive strengths, fracture energies and strains at peak stress in tension and compression. A comparison between the experimental data and model results has been also provided in this paper.     

Mechanical properties of steel fibre reinforced lightweight concrete with pumice stone or expanded clay aggregates

This paper presents basic information on the mechanical properties of steel fibre-reinforced light-weight concrete, manufactured using pumice stone or expanded clay aggregates. Results are presented for standard compressive tests and indirect tensile tests (splitting tests on cylinder specimens and flexure tests on prismatic beams using a three-point loading arrangement) under monotonically increasing or cyclically varying loads. The influence of steel fibres and aggregate types on modulus of elasticity, compressive and tensile strength and post-peak behaviour is evaluated. Test results show that compressive strength does not change for pumice stone aggregates, while an increase is observed for expanded clay; tensile strength and fracture toughness are significantly improved for both pumice stone and expanded clay. The results also show that with both expanded clay and pumice stone lightweight aggregates a suitable content of fibres allows one to obtain performances comparable with those expected from normal weight concrete, the important advantage of lower structural weight being maintained.     

Tensile and compressive damage coupling for fully-reversed bending fatigue of fibre-reinforced composites

ABSTRACT Due to their high specific stiffness and strength, fibre-reinforced composite materials are winning through in a wide range of applications in automotive, naval and aerospace industry. Their design for fatigue is a complicated problem and a large research effort is being spent on it today. However there is still a need for extensive experimental testing or large safety factors to be adopted, because numerical simulations of the fatigue damage behaviour of fibre-reinforced composites are often found to be unreliable. This is due to the limited applicability of the theoretical models developed so far, compared to the complex multi-axial fatigue loadings that composite components often have to sustain in in-service loading conditions.
In this paper a new phenomenological fatigue model is presented. It is basically a residual stiffness model, but through an appropriate choice of the stress measure, the residual strength and thus final failure can be predicted as well. Two coupled growth rate equations for tensile and compressive damage describe the damage growth under tension–compression loading conditions and provide a much more general approach than the use of the stress ratio R . The model has been applied to fully-reversed bending of plain woven glass/epoxy specimens. Stress redistributions and the three stages of stiffness degradation (sharp initial decline – gradual deterioration – final failure) could be simulated satisfactorily.     

Hierarchical composites reinforced with robust short sisal fibre preforms utilising bacterial cellulose as binder

A novel robust non-woven sisal fibre preform was manufactured using a papermaking process utilising nanosized bacterial cellulose (BC) as binder for the sisal fibres. It was found that BC provides significant mechanical strength to the sisal fibre preforms. This can be attributed to the high stiffness and strength of the BC network. Truly green non-woven fibre preform reinforced hierarchical composites were prepared by infusing the fibre preforms with acrylated epoxidised soybean oil (AESO) using vacuum assisted resin infusion, followed by thermal curing. Both the tensile and flexural properties of the hierarchical composites showed significant improvements over polyAESO and neat sisal fibre preform reinforced polyAESO. These results were corroborated by the thermo-mechanical behaviour of the (hierarchical) composites, which showed an increased storage modulus and enhanced fibre–matrix stress transfer. Micromechanical modelling was also performed on the (hierarchical) composites. By using BC as binder for short sisal fibres, added benefits such as the high Young’s modulus of BC, enhanced fibre–fibre and fibre–matrix stress transfer can be utilised in the resulting hierarchical composites.     

Effect of interfacial thickness and stiffness on the stress distributions in fibre reinforced cementitious composites

The different microstructure of the fibre–cement interface might result in different failure mechanisms. It is expected that improvement of strength and toughness in fibre-reinforced cementitious composites will depend on their interfacial thickness and stiffness. A three-phase model, subject to a transversely uniform tensile stress, was utilized to investigate the effect of interfacial thickness and stiffness on the stress distributions near the fibre–cement interface and the corresponding failure mechanism. The results suggest that optimum interfacial microstructure of fibre-reinforced cementitious composites can be tailored to obtain a higher strength and toughness. Optimum interfacial thickness and stiffness was evaluated for various reinforcements, including steel, carbon, glass and polypropylene fibres. This revised version was published online in November 2006 with corrections to the Cover Date.     

Dynamic tensile test of fly ash concrete under alternating tensile–compressive loading

In order to obtain the post-peak stress–strain curve of plain concrete, the strain-controlled uniaxial tensile tests are carried out on cylindrical specimens. The tensile cyclic tests are performed between a lower compressive load and an upper load to the envelope curve. Test results have shown that residual strain accumulates slowly when the concrete is subjected to alternating tensile–compressive loading, during which the microcracks increase and aggregate. Meanwhile, the rates of stiffness degradation with cycles are similar in all cases. In the study, an analytical model of the post-peak cyclic behavior of concrete is proposed. The strain is decomposed into classical linear elastic strain and non-classical strain described in the Presach–Mayergoyz model, which describes the hysteresis behavior of concrete well from a microscopic point of view. A good agreement is obtained between the test data and calculated results.     

Evaluation of toughness of textile concrete

High Performance Fibre Reinforced Cementitious Composites (HPFRCC) are characterized by a stress–strain response in tension that exhibits strain-hardening behaviour accompanied by propagation of multiple cracks. This process is often referred to as pseudo-ductility due to multiple cracking with relatively large energy absorption capacity. The cracking characteristics are dependent on matrix strength, fibre/matrix bond, fibre volume fraction and the aspect ratio of the fibre used in the composite. The matrix cracking strength and interfacial bond vary with the degree of hydration of cement in the matrix, which is time and environment dependent. This study analyses the multiple cracking patterns formed in weathered Textile Concrete (TC) samples due to direct tensile testing, and links the cracking patterns to the tensile behaviour. The specimens used for the study were thin laminates which were produced by casting six layers of specially made polypropylene (PP) textile in fine-grained mortar. The samples were cured under controlled laboratory conditions for 28 days, and thereafter exposed to different weathering regimes for different periods. The weathered samples were tested in direct tension in a Universal Testing Machine (UTM) over a range of stresses. For all the samples tested, it was observed that the tensile behaviour was characterised by strain hardening and multiple cracking, which gave high tensile strains in excess of 20% at final failure. It was further found that the cracking patterns varied mainly with age, weathering history and stress levels. Other factors that contributed to the cracking characteristics were moisture state of the specimen and the fibre/matrix bonding strength. A strong bond and dense matrix resulted in wide crack spacings compared with samples with a weaker bond which developed closely spaced cracks. A general trend of increasing crack widths and crack spacings with ageing was observed which was accredited to increased hydration accompanied by an increase in fibre/matrix bond strength.     

Low-velocity impact response of fibre–metal laminates – Experimental and finite element analysis

In this contribution, the impact dynamic response and failure modes of fibre–metal laminated panels subjected to low velocity impact were investigated and presented. The fibre–metal laminate in this paper comprised of a layer of glass fibre-reinforced plastics sandwiched between two layers of aluminium alloy. Two different types of glass fibre-reinforced plastics were used for the fabrication: unidirectional and woven. A fairly extensive experimental investigation was conducted in conjunction with a detailed finite element analysis. The experiments were conducted using a standard drop-weight test machine and the finite element analysis was carried out using a commercially available finite element software. The results of maximum contact force, contact duration and corresponding failure modes are presented, compared and discussed in this technical paper.     

Die Faser- und Fadenverstärkung von plastischen und spröden Matrixmaterialien

Fibre- and Filament-Reinforcement of Plastic and Brittle Matrix Materials. The tensile strength of a fibre-reinforced material depends on the stress-strain behaviour of the two components and on the volume percentage, the orientation and the geometry of the fibres. For production and application of the composite material at elevated temperatures the chemical reactions between fibre and matrix are of critical importance. It may be necessary to incorporate an intermediate layer in order to inhibit an excessively strong reaction or to increase the bond strength. Feasible fibre-reinforced composites with metallic matrices are discussed. Nonmetallic inorganic materials are rather difficult to reinforce because of their brittle fracture properties. Development trends in the field of fibre-reinforced materials are outlined.     

Stress transfer in the fibre fragmentation test: Part III Effects of interface debonding and matrix yielding

The micromechanics of stress transfer is presented for the fibre fragmentation test of microcomposites containing debonded fibre–matrix interface and yielded matrix at the interface region. Results from the parametric study are discussed for carbon fibre composites containing epoxy and polyetheretherketone (PEEK) matrices, representing respectively typical brittle debonding and matrix yielding behaviour at the interface region. The stress transfer phenomena are characterized for the two interface failure processes. The sequence of interface failure and fibre fracture as a function of applied stress are also identified. Maximum debonded and yielded interface lengths are obtained above which a fibre will fracture into smaller lengths. There are also threshold fibre fragment lengths above which fibre will fracture without interface debonding or matrix yielding. The applied stresses for these conditions are governed by three strength properties of the composite constituents, namely interface shear bond strength, matrix shear yield strength and fibre tensile strength for given elastic constants of the fibre and matrix, and the geometric factors of the microcomposite. The ineffective length, a measure of the efficiency of stress transfer across the fibre–matrix interface, is shown to strongly depend on the extent to which these failure mechanisms take place at the interface region. This revised version was published online in November 2006 with corrections to the Cover Date.     

Single-fibre Broutman test: fibre–matrix interface transverse debonding

The single-fibre Broutman test was used to study the fibre–matrix interface debonding behaviour when subjected to a transverse tensile stress. During testing, damage was detected using both visual observation under polarized light and acoustic emission (AE) monitoring. Separation of failure mechanisms, based on AE events, was performed using time domain parameters (amplitude and event width) and fast Fourier transform (FFT) frequency spectra of the AE waveforms. The latter can be considered as a fingerprint allowing to discriminate fibre failure, matrix cracking, fibre–matrix interface debonding, friction and ‘parasite noise’. Stresses in the specimens were evaluated using a two-dimensional finite element model (FEM) and monochromatic photoelasticity was used to verify the simulated stress distribution.Two failure mechanisms appeared to be in competition in the Broutman test: fibre failure under compressive stresses and fibre–matrix interface debonding under transverse tensile stresses. For systems in which the interfacial adhesion is not so ‘good’, like glass fibre–polyester systems for instance, fibre–matrix debonding was observed, and the progression of the debonding front with the interfacial transverse stress was recorded. Thermal stresses are also discussed, and a FEM simulation shows that they encourage fibre failure under compressive stresses.     

Age effect on the characteristics of fibre/cement interfacial properties

The experimental findings of an investigation on the age development of interfacial properties of polyethylene fibers in a cementitious matrix are reported. It was found that the interfacial bond strength matures much faster, in less than 7 days, in comparison with bulk property development, which typically takes 14–28 days. A parallel ESEM study of the microstructure development in the interfacial transition zone supported the fact that the rapid early age saturation of the interfacial adhesive bond strength is associated with the growth of a CH rim around the fibre periphery. The formation of the CH layer appears to be completed much earlier than the hydration process of the bulk material. The slip-dependent friction after initial debonding was found to develop with age up to 28 days, gradually converting from a slip-weakening to a slip-hardening behaviour. These findings should be useful in interpreting early age fibre-reinforced cementitious composite properties. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical characterization of rubberized concrete using an image-processing/XFEM coupled procedure

A coupled (two-step) numerical procedure to characterize the mechanical behaviour of Rubberized Concrete (RuC) is proposed and validated in this paper. In particular, the splitting tensile strength test is described in detail. In the first step, MATLAB Image Processing is used to obtain the model geometry and the RuC heterogeneous configuration (distribution of rubber particles within the concrete matrix). In the second step, the Extended Finite Element Method (XFEM) included in ABAQUS software is used to simulate the inelastic behaviour of the concrete matrix and allow the nucleation and development of cracks, as well as the damage evolution and ultimate strength of the RuC specimen cross-section. Additionally, a set of experimental results on mechanical behaviour of RuC is presented. This shows that RuC has both lower strength and stiffness but higher ductility (less brittle behaviour) than normal concrete (NC). Finally, a good agreement between the two-step procedure results and the experimental results (in terms of indirect tensile strength, stiffness and failure mode) is observed.     

Die Faser- und Fadenverstärkung von plastischen und spröden Matrixmaterialien

Fibre- and Filament-Reinforcement of Plastic and Brittle Matrix Materials The tensile strength of a fibre-reinforced material depends on the stress-strain behaviour of the two components and on the volume percentage, the orientation and the geometry of the fibres. For production and application of the composite material at elevated temperatures the chemical reactions between fibre and matrix are of critical importance. It may be necessary to incorporate an intermediate layer in order to inhibit an excessively strong reaction or to increase the bond strength. Feasible fibre-reinforced composites with metallic matrices are discussed. Nonmetallic inorganic materials are rather difficult to reinforce because of their brittle fracture properties. Development trends in the field of fibrereinforced materials are outlined.     

纤维增强聚合物基复合材料的疲劳损伤模型

从微观离散分子力学出发,考虑力学化学的交互作用和材料微观组织的影响,建立了纤维增强聚合物基复合材料的力学化学分子链疲劳损伤模型在模型中引入表示基体树脂和界面分子链断裂数占材料分子链总数的比例Am和Al来描述基体断裂主导和界面断裂主导的损伤,给出剩余强度与疲劳过程中微观断裂机理、结构参数、物理化学参数和力学性能变化之间的关系与短玻璃纤维增强树脂基复合材料(SMC)的恒载荷疲劳实验结果比较,本模型预测的疲劳剩余强度与实验值吻合得比较好,     

Evaluation of fracture parameters of concrete from bending test using inverse analysis approach

In this study, an inverse analysis approach is developed to obtain the fracture parameters of concrete, including stress–crack opening relationship, cracking and tensile strength as well as fracture energy, from the results of a three-point bending test. Using this approach, the effects of coarse aggregate size (5–10, 10–16, 16–20 and 20–25 mm) and matrix strength (compressive strength of 40 and 80 MPa, respectively) on the fracture parameters are evaluated. For normal strength concrete, coarse aggregate size and cement matrix strength significantly influence the shape of σ–w curve. For a given total aggregate content, small aggregate size leads to a high tensile strength and a sharp post-peak stress drop. The smaller the coarse aggregate, the steeper is the post-peak σ–w curve. By contrast, in high strength concrete, a similar σ–w relationship is obtained for various aggregate sizes. The post-peak stress drop for high strength concrete is more abrupt than that for normal strength concrete. Also, the smaller the coarse aggregate size, the higher is the flexural strength. For both normal and high strength concrete, fracture energy and characteristic length are found to increase with increase of coarse aggregate size.     

Structural behaviour of composite sandwich panels with plain and fibre-reinforced foamed concrete cores and corrugated steel faces

This paper studies the four-point bending response and failure mechanisms of sandwich panels with corrugated steel faces and either plain or fibre-reinforced foamed concrete core. Mechanical properties of both plain and polyvinyl alcohol fibre-reinforced foamed concrete were obtained, which are needed for the design of sandwich panel and numerical modelling. It is found that the fibre-reinforcement largely enhances the mechanical behaviour of foamed concrete and composite sandwich panels. Finite element code Abaqus/Standard was employed to investigate the influence of face/core bonding and fastening on the four-point bending response of the sandwich panels. It was found that face/core bonding plays a crucial role in the structural performance while the influence of fastening is negligible.