Fibre reinforced sands: Experiments and modelling

Sand reinforced with short polypropylene fibres has been tested in conventional triaxial compression and extension. The contribution of fibres to the strength was remarkable in compression while limited in extension confirming that it depends primarily on their orientation with respect to tensile strains. A modelling approach is proposed for coupling the effects of fibres with the stress–strain behaviour of unreinforced soil. It is necessary to define a fibre stiffness matrix and in this investigation it is assumed that fibres are working in their elastic domain. Any distribution of fibre orientations can be accounted for. The sand stiffness matrix is based on the Mohr Coulomb model although more elaborate models could be used. The constitutive model for the fibre-sand composite is calibrated against the results of drained triaxial compression and extension tests. Despite the simplistic nature of the Mohr-Coulomb assumptions for the sand matrix, simulations show how the influence of fibre orientation with respect to the strain conditions is captured and provide good agreement with experimental data.     

A new analytical solution to the mechanical behaviour of fully grouted rockbolts subjected to pull-out tests

A new analytical approach able to predict the mechanical behaviour of fully grouted rockbolts subjected to pull-out tests is proposed in this paper. Input parameters of such approach are: bolt radius, bolt’s Young modulus, displacement of the free end of the bolt and the constitutive law of the rockbolt–grout joint interface. The limited circumstances under which it is accurate to determine such constitutive law from pull-out tests are also presented. A solution for the load–displacement curve obtained during pull-out tests has been developed and is detailed in the case of a tri-linear bond-slip model. Comparison with experimental results obtained via in situ pull-out tests has led to the validation of this approach.     

Flowability of fibre-reinforced concrete and its effect on the mechanical properties of the material

Fibre distribution is an important factor, affecting both the fresh- and hardened-state properties of concrete. The paper describes a study in which the fibre distribution and orientation were investigated by means of a suitable technique, by using a translucent fluid model with a yield stress (Carbopol 940). The observation confirmed the ability of the developed method to provide data on the orientation and distribution of steel fibres within concrete. It was showed that orientation and distribution are dependent on the yield stress of the fluid material. Concrete specimens, containing 0.5% of 35 mm long hooked steel fibres, were also cast and tested under four-point loading, in order to study the correlation between rheology, fibre distribution and the mechanical properties of steel fiber-reinforced concrete. The results confirm that the rheology of concrete has an important influence on the orientation of the fibres. The flexural strength depends on the fibre distribution and orientation and is significantly improved when the fibres are oriented in the direction of the tensile stresses fresh (concrete with good workability). On the contrary, for concrete with poor workability, an inadequate orientation of fibres occurred, leading to a poor contribution of the fibres to the flexural behaviour of the tested specimens, despite the relatively higher compression strength of the tested concrete material.     

Effect of matrix strength on pullout behavior of steel fiber reinforced very-high strength concrete composites

This paper presents the results of single-fiber pullout tests for deformed and smooth steel fibers embedded in the newly developed very-high strength concrete (VHSC) matrixes. The pullout test program involved four types of steel fibers, eight compressive strengths of VHSC matrixes, and two normal concrete strengths. Test results have shown that pullout behavior of different steel fiber reinforced VHSC composites is influenced by the matrix strength and fiber end condition (smooth, flat end, or hooked). Results reveal that both maximum pull-out load and total pullout energy increases as matrix strength increases for all deformed fibers that did not rupture. The test results also indicated that the increase in total pullout energy is more significant than that in peak load.     

Post-cracking behaviour of steel and macro-synthetic fibre-reinforced concretes

The present paper describes the results of an experimental investigation on the performances of concrete specimens reinforced with either steel or macro-synthetic fibres under three-point bending. Steel fibres are often used to improve the flexural toughness of concrete and are used in various structural applications while synthetic fibres are more often used to reduce crack opening due to shrinkage. Macro-synthetic fibres have been proposed more recently with the aim of creating an alternative to steel fibres in structural applications but their use is still limited.In the tests performed, specimens cast with the same concrete mix, but containing different dosages of either steel or macro-synthetic fibres, were used and compared. In general steel fibres were more efficient in increasing the toughness of concrete than macro-synthetic fibres, even though their results were significantly more scattered.Test results were used to calculate the parameters of stress-crack opening relations via inverse analysis using a cracked hinge model. This numerical model provided results, in terms of force-Crack Mouth Opening Displacement (CMOD) curves, in very good agreement with the experimental data.     

非均质软土地基上吸力式沉箱抗拔承载力数值分析

针对非均质软土地基上吸力式沉箱的抗拔极限承载力,运用大型有限元分析软件ABAQUS进行了三维非线性数值分析。通过一定数量的数值计算与分析,较系统地考察了沉箱承受拉拔荷载过程中主动侧土体与结构接触状态对沉箱极限承载力的影响。由此表明:软土地基的不排水抗剪强度、吸力式沉箱结构的长径比、荷载作用点位置等因素对于主动侧土体与结构的接触状态具有一定影响。通过与有关模型试验结果及极限分析上限解的对比分析,在一定程度上验证了有限元计算模型与数值分析结果的合理性。     

Influence of yield stress and compressive strength on direct shear behaviour of steel fibre-reinforced concrete

This study aims in examining the influence of the paste yield stress and compressive strength on the behaviour of fibre-reinforced concrete (FRC) versus direct shear. The parameters studied are the steel fibre contents, the aspect ratio of fibres and the concrete strength. Prismatic specimens of dimensions 10 × 10 × 35 cm made of concrete of various yield stress reinforced with steel fibres hooked at the ends with three fibre volume fractions (i.e. 0%, 0.5% and 1%) and two aspects ratio (65 and 80) were tested to direct shear. Three types of concretes with various compressive strength and yield stress were tested, an ordinary concrete (OC), a self-compacting concrete (SCC) and a high strength concrete (HSC). The concrete strengths investigated include 30 MPa for OC, 60 MPa for SCC and 80 MPa for HSC.The results show that the shear strength and ductility are affected and have been improved very significantly by the fibre contents, fibre aspect ratio and concrete strength. As the compressive strength and the volume fraction of fibres increase, the shear strength increases. However, yield stress of concrete has an important influence on the orientation and distribution of the fibres in the matrix. The ductility was much higher for ordinary and self-compacting concretes (concrete with good workability). The ductility in direct shear depends on the fibre orientation and is significantly improved when the fibres are perpendicular to the shear plane. On the contrary, for concrete with poor workability, an inadequate distribution and orientation of fibres occurred, leading to a weak contribution of the fibres to the direct shear behaviour.     

Numerical simulation of the nonlinear bending response of fibre-reinforced cementitious matrix beams and comparison with experimental results

The aim of this work is the efficient numerical simulation of the bending behaviour of fibre reinforced cementitious matrix (FRCM) beams. For the production of the FRCM specimens, a high-strength cementitious matrix with hooked-end steel fibres is used. Two types of FRCM beams are considered. The first type has no conventional steel reinforcement, while the second one includes longitudinal and transverse reinforcement steel bars. For comparison reasons, conventional reinforced concrete (RC) beams are also studied. The beams are tested under static and cyclic loading. The response of the tested beams is simulated by means of effective two-dimensional finite element models, in which the contribution of the FRCM is taken into account by means of two different layers of finite elements. The first one represents the cementitious matrix while the second one accounts for the contribution of the steel fibres in a homogenized manner. The presented models are able to follow the nonlinearities that appear in the corresponding physical models. The validity of the proposed methodology is established by comparing the numerical results with the corresponding experimental results.     

Mechanical behaviour and fibre dispersion of hybrid steel fibre reinforced self-compacting concrete

In this study, mixture design, workability, fibre dispersion/orientation, mechanical properties and fracture behaviour of hybrid steel fibre reinforced self-compacting concretes (HSFRSCCs) were investigated. Three different types of steel fibres with and/or without hooked-ends were added to the mixtures in two different volume fractions (0.75 and 1.5% of the total volume of concrete). The results of slump flow, U-box, V-funnel and J-ring tests have shown that increasing the fibre content of the concretes slightly reduced the workability of HSFRSCC, and the main influencing factor on flowability is the geometry of fibres. The addition of fibres, although did not change the final flowability, decreased the rate of flowability. The results from the experimental tests showed that the flexural strengths increased slightly with increasing strength of long fibres, whereas the splitting tensile strength remained unchanged. The concretes with high strength, long steel fibres show behaviour of enhanced toughness and ductility compared to that with normal strength steel fibres. The orientation and distribution of fibres in concrete have been investigated by image analysis and it was observed that fibres dispersed homogeneously in all concrete series without any clumping. With increasing the amount of fibres, the fibres were more vertically orientated relative to the bending loading direction, resulting in enhancement in the mechanical properties of concrete.     

界面特性及填料粒径影响格栅加筋性能的离散元研究

为研究筋土界面细观结构演化及填料粒径对加筋效果的影响,采用“clump”方法开发了可模拟砂土性状的椭球形颗粒,建立三维离散元模型并结合室内试验结果验证了模型的正确性,系统分析了拉拔阻力、格栅应变、局部孔隙率等力学响应并揭示了其发展规律。拉拔试验结果显示大粒径填料置换后表观黏聚力提高显著而摩擦角变化不大,进行宏细观分析后发现,置换体系颗粒发生了更大程度的位置重排,拉拔力增量主要来源于摩擦阻力。研究成果可为从细观角度探究筋土界面机理提供新的认识。     

Rissbildung von biegebeanspruchten Bauteilen aus Ultrahochfestem Faserbeton

Das Konstruieren und Bemessen von Bauwerken aus faserbewehrtem UHPC erfordert sehr detailliertes Wissen über das Zugtragverhalten von Faserbeton. Es ist beeinflusst von der Fasergeometrie, der Fasermenge, der Schubverbundfestigkeit zwischen Faser und Matrix, der Matrixfestigkeit, dem Schwinden und der Faserorientierung. Leutbecher beschreibt ein Modell für reine Zugbeanspruchungen, indem all diese Parameter Eingang finden. Im hier vorliegenden Artikel wird dieses Modell für Biegebeanspruchungen erweitert und anhand von Biegeversuchen an Platten aus UHPC verifiziert. Dabei werden sowohl Faserbewehrung als auch konventionelle Stabbewehrung berücksichtigt. Crack Formation of Flexural Members made of UHPFRC For the design and planning of buildings made of fibre reinforced ultra high performance concrete one needs very detailed knowhow about the tensile carrying behaviour of fibrated concrete. It is affected by various parameters such as the fibre geometry, the content of fibres, the bond strength between fibre and binder matrix, the strength of the matrix, the shrinkage of the concrete and the orientation of the fibres. All these parameters are considered in a theoretical model for UHPFRC under tension (Leutbecher). In the paper presented here the range of validity of this model is extended to bending stress. Experimental investigations on UHPC‐plates under bending complete and verify the theoretical results. The tests cover fibre reinforcement as well as conventional passive reinforcement.     

国家体育场桁架柱柱脚锚固性能试验研究

国家体育场钢柱脚锚固于钢筋混凝土承台中,其受力复杂,以往国内外类似的设计及试验均很少。本文共进行了4个钢柱脚-钢筋混凝土承台试件锚固性能的试验研究,其中2个为混凝土承台中配置抗拔钢筋的试件,2个为混凝土承台中未配置抗拔钢筋的试件。通过单向重复加载下的钢柱脚锚固性能试验,对试件的锚固承载力、延性、合理配筋型式等进行了研究。试验表明,混凝土承台中配置抗拔钢筋的试件与未配置抗拔钢筋的试件相比其锚固承载力和延性显著提高。在试验基础上,建立了柱脚锚固承载力计算的桁架模型,计算结果与实测结果符合较好。     

Comparative experimental study of mechanical properties of concrete prepared by different fibres

This paper reports a comparative experimental study on the mechanical properties of concrete containing different types of fibres: polyvinyl alcohol (PVA), polypropylene (PP) and steel fibres. Compression and three-point bending tests are performed on both plain concrete and each type of fibre-reinforced concrete (FRC). The experimental results show that the presence of fibres has less of an effect on the FRC's compressive strength. The tensile strength is commonly increased by the addition of fibres, but an appropriate fibre content of PVA or PP fibres should be selected. PVA and PP fibres decrease the concrete's elastic modulus, but steel fibres increase the modulus due to the steel's higher Young's modulus. The FRC containing PVA shows brittle characteristics, but the FRC containing steel or PP fibres have load-deflection curves with flattened descending paths. Flexural behaviour of the concrete is improved by addition of steel or PP fibres, but not by PVA fibres, and the concrete's fracture toughness is increased by the addition of steel fibres.     

Numerical simulation on seismic retrofitting performance of reinforced concrete columns strengthened with fibre reinforced polymer sheets

This paper evaluates the seismic performance of reinforced concrete columns retrofitted with fibre reinforced polymer (FRP) sheets through numerical simulations of the load–deformation response using two-dimensional finite element analysis (2D-FEA). The relatively rational mesh configuration is verified through comparison of analysis results obtained from the different mesh configurations. The seismic performance of three reinforced concrete (RC) columns strengthened with FRP sheets is assessed through a series of parametric studies, and the applicability of existing crack models and constitutive relationships on crack discontinuity and concrete compressive behaviour are validated. Comparisons of analysis results with tests shows that an equivalent uniaxial strain model and a failure criterion can be used to accurately simulate nonlinear behaviour and the failure of concrete under a biaxial stress state, respectively. Moreover, it is shown that a modified confinement model can be simply adopted to evaluate confined effects from hoop steel and FRP on concrete, which generally operate in three-dimensional confinement. Lastly, the seismic retrofitting performance of RC columns wrapped with FRP sheets is verified by analysing load–deformation responses and the progression of stress–strain at inflection points and bottoms of the columns.     

Influence of the strip–backfill interaction model in the analysis of the behavior of a mechanically stabilized earth wall

To improve the modeling of friction between the ground and reinforcing inclusions, such as steel strips in reinforced earth walls, a specific type of finite element was introduced in the CESAR-LCPC finite element code. Simulations were carried out, in which these elements were used in combination with a linear elastic-perfectly plastic model to describe the interaction between the ground and the inclusions. With this simple interaction model, however, it was not possible to reproduce the observed behavior of a full-scale, experimental wall or the results of pull-out tests carried out on the same wall under the same set of parameters. Therefore, two more complex models were introduced for the soil–inclusion interaction, which led to a much better agreement between the numerical results and the experimental observations.     

Modelling of I-shaped beam-to-tubular column connection subjected to post-fire conditions

This paper presents numerical results of structural post- fire bahaviour of I-shaped beam-to-chord joints in offshore platforms topside. Considering the high risk of fire events in offshore oil/gas platforms, this study focuses on the structural behaviour of these connections at post- fire condition. A highly detailed three-dimensional (3-D) finite element (FE) model of this connection has been created using the ABAQUS software. Steel members and connection components are considered to behave nonlinearly. The results of finite element and experimental tests conducted on I shape beam-to-tubular column connections in furnace fire conditions are compared, and the obtained failure modes and moment-rotation-temperature characteristics are in good agreement with those associated with experimental tests. The validated model has been used to conduct numerical parametric studies to generate theoretical data to help develop detailed understanding of the joint behaviour in post-fire condition.     

Development and application of an interface constitutive model for fully grouted rock-bolts and cable-bolts

This paper proposes a new interface constitutive model for fully grouted rock-bolts and cable-bolts based on pull-out test results.A database was created combining published experimental data with in-house tests.By means of a comprehensive framework,a Coulomb-type failure criterion accounting for friction mobilization was defined.During the elastic phase,in which the interface joint is not yet created,the proposed model provides zero radial displacement,and once the interface joint is created,interface dilatancy is modeled using a non-associated plastic potential inspired from the behavior of rock joints.The results predicted by the proposed model are in good agreement with experimental results.The model has been implemented in a finite element method(FEM) code and numerical simulations have been performed at the elementary and the structural scales.The results obtained provide confidence in the ability of the new model to assist in the design and optimization of bolting patterns.     

Analytical prediction of transfer length in prestressed self-consolidating concrete girders using pull-out test results

While self-consolidating concrete (SCC) is comparable to conventional concrete (CC) in terms of strength, the comparability of SCC’s bond to steel is less well-defined. A keen understanding of SCC’s bond strength is essential to advance SCC within the prestressed concrete industry. This study presents an analytical method for predicting the transfer length of steel strands in prestressed girders using pull-out test results. The experimental data from a series of 56 pull-out tests is utilized to derive bond stress–slip relationships for 12.7 mm steel strands embedded in SCC and CC. Modification factors are used to correlate pullout bond stresses to transfer bond stresses in prestressed members, and the modified relationships are integrated in three-dimensional finite element models to predict transfer lengths in prestressed SCC girders. The analytical predictions correlate well with experimental results and transfer length requirements of current US design codes.     

High Strength Polypropylene Fibre Reinforcement Concrete at High Temperature

Concrete is an inherently brittle material with a relatively low tensile strength compared to compressive strength. Reinforcement with randomly distributed short fibres presents an effective approach to the stabilization of the crack and improving the ductility and tensile strength of concrete. A variety of fibre types, including steel, synthetics, and natural fibres, have been applied to concrete. Polypropylene (PP) fibre reinforcement is considered to be an effective method for improving the shrinkage cracking characteristics, toughness, and impact resistance of concrete materials. Also, the use of PP fibre has been recommended by all of the researchers to reduce and eliminate the risk of the explosive spalling in high strength concrete at elevated temperatures. In this study, constitutive relationships are developed for normal and high-strength PP fibre reinforcement concrete (PPFRC) subjected to high temperatures to provide efficient modelling and specify the fire-performance criteria for concrete structures. They are developed for unconfined PPFRC specimens that include compressive and tensile strengths, elastic modulus, modulus of rupture, strain at peak stress as well as compressive stress–strain relationships at elevated temperatures. The proposed relationships at elevated temperature are compared with experimental results. These results are used to establish more accurate and general compressive stress–strain relationships prediction. Further experimental results for tension and the other main parameters at elevated temperature are needed in order to establish well-founded models and to improve the proposed constitutive relationships, which are general, rational, and fit well with the experimental results.     

The composite effect of steel fibres and stirrups on the shear behaviour of beams using self-consolidating concrete

Based on the investigation of the influence of steel fibre on the workability of fresh self-consolidating concrete (SCC), this paper presents the experimental results carried out on a series of simply supported SCC rectangular beams, using steel fiber reinforcement with and without stirrups, and subjected to four-point symmetrically placed vertical loads. The major test variables are the steel fibre content and stirrup ratios. The current study on the shear strength of conventional reinforced concrete (RC) beams verifies the shear strength of SCC beams with steel fibres. The investigation indicates that the shear strength significantly increases by increasing the fibre content; the addition of steel fibres in an adequate percentage can change the failure mode from a brittle shear collapse into a ductile flexural mechanism. The stirrups can be partially replaced by steel fibres. The combination of steel fibres and stirrups demonstrates a positive composite effect on the mechanical behaviour. The shear strength recorded experimentally is compared with the value obtained from the proposed formula, and the correlation is satisfactory.