Bond-slip Law Research Articles

Influence of natural fibers characteristics on the interface mechanics with cement based matrices

Abstract The use of Natural Fibers in cementitious composites is an innovative technical solution but, they are characterized by a complex microstructure and significant heterogeneity, which influence their interaction with cementitious matrices, whose identification requires further advances in the current state of knowledge. The present study summarizes the results of a wide series of pull-out tests carried out on sisal, curaua and jute fibers. Then, the experimental results are employed in an inverse identification procedure aimed at unveiling the key features of the aforementioned bond-slip laws. Morphological, chemical, physical and mechanical characterization of the natural fibers were correlated with the resulting bond properties within the embedding matrix. The obtained results in terms of relevant parameters, such as bond strength and fracture energy (under pull-out stresses) of the fiber-matrix interface, pave the way for future studies intended for a better understanding of the structural response of Natural Fiber Reinforced Cementitious Composites.

Prediction of the Bond–Slip Law Between Externally Bonded Concrete Substrates and CFRP Plates Under Fatigue Loading

This study proposes a realistic model for the bond stress–slip relation between concrete and carbon fiber-reinforced polymer plates under fatigue loading. Double-lap shear experimental tests were implemented to explore the bond stress–slip behavior under fatigue loading. The bond stress–slip relation under fatigue loading was derived as a function of the loading/unloading linear slip, the stiffness of the unloading branch in slip and the number of fatigue cycles. In this model, the stiffness of the unloading branch in slip was derived from the simplest linear fitting of the experimental data based on the bond stress–slip relationship. Numerical analyses using the proposed model compared well overall to the experimental fatigue results.

An incremental inverse analysis procedure for identification of bond-slip laws in composites applied to textile reinforced concrete

Abstract Textile reinforced concrete (TRC) is a novel composite building material, its structural behavior is substantially influenced by the bond interface between the reinforcing textile fabrics and concrete. The bond interface can be characterized by a nonlinear bond-slip law. The pull-out test is a common experimental procedure for determination of the bond-slip law. In this paper, a general finite element procedure is proposed to calibrate the bond-slip law according to the results of pull-out tests. By adopting a generic multilinear bond-slip law and solving each piece of the law sequentially, the conventional curve fitting procedures employing optimization algorithms, which are computationally expensive and sometimes non-convergent, can be avoided. Pull-out tests of TRC specimens with varying anchorage lengths were carried out and the test results were used as the input data for the calibration procedure. It is found that the calibrated bond-slip law is independent of the specimen length. Using the calibrated bond-slip laws, the pull-out force vs. displacement curves are numerically reproduced. The numerical results agree well with the experimental data.

Textile reinforced concrete multiscale mechanical modelling: Application to TRC sandwich panels

The present study establishes a numerical strategy for describing the textile/concrete bond behaviour in textile-reinforced concrete (TRC) composites that separates the cohesive and coulomb friction contributions.The textile-concrete bond approach, validated on an existing pull out test in the literature, has been used to calibrate the textile-concrete bond slip law of an existing TRC tested in tension by an innovative inverse approach thanks to its pull-out mode of failure.The calibrated bond slip law has been used as an input parameter to produce an enhanced TRC multiscale numerical model that is based on the nonlinear behaviour of its constitutive components (concrete, textile, and textile-concrete bond slip law) and takes into account all the damage mechanisms of TRC, which are mainly characterized by matrix cracking and yarn pull-out. The model has been validated on the basis of the previous TRC experimental tensile test.Two approaches have been used to account for the mechanical behaviour of a textile-reinforced concrete sandwich panel under a four-point bending test: a macroscopic 3D finite element approach, which considers the composite TRC with its macro tri-linear stress – strain relation under tensile solicitation, and the proposed TRC multiscale 3D finite element approach which, involves definition of the textile-concrete interaction bond slip law. More accurate results have been achieved with the multiscale approach; furthermore, the experimental mode of failure of the sandwich panel has been captured.

FRP-Strengthening of Curved Masonry Structures: Local Bond Behavior and Global Response

The aim of the paper is to propose and assess the reliability of a modeling strategy which combines the homogenization of the masonry material and the use of zero-thickness interface elements. This strategy is specifically proposed for numerically investigating the structural response of FRP-reinforced curved masonry structures. Indeed, in order to consider the influence of the geometry curvature of the masonry substrate on the local bond behavior of the FRP-strengthening system, bond-slip laws which specifically account for the geometric curvature of the substrate are introduced at the FRP/substrate interface layer. Numerical analyses concerning masonry arches selected from the current literature are presented in the paper in order to assess the reliability of the proposed modelling approach.

Tension stiffening approach for interface characterization in recycled aggregate concrete

This study proposes a comprehensive analysis on the structural performance of reinforced Recycled Aggregate Concrete members. Particularly, it summarizes the results of an experimental investigation aimed at analyzing the tension stiffening behavior of normal and high strength class concretes produced with Recycled Concrete Aggregates (RCAs). The mixtures were proportioned in order to achieve 25 and 65 MPa of compressive strength and, moreover, several recycled-to-natural coarse aggregates replacement ratios were considered: 0%, 25% and 50%. The results derived from this type of test furnish a comprehensive analysis on both the steel-to-matrix interaction and the crack formation and propagation on concrete elements as well as distributed cracking mechanisms. Using a finite difference numerical model, the experimental results are used to back-calculate and identify the steel-to-concrete bond slip law. Also, it is an alternative mean of developing the stress-crack-width law for concrete in tension. The results showed that the use of recycled concrete aggregate does not affect the resulting concrete performance and, therefore, the RCAs can be successfully employed, up to the levels analyzed herein, for the production of structural elements made with normal and high strength class concrete mix.

Development of Reinforcing Bars in SRCC Matrix: Modeling and Interpretation

Experiments on deformed reinforcing bar anchorages developed in strain-resilient cementitious composites (SRCCs) with high tensile deformation capacity illustrate that the bar-matrix assembly may respond in a ductile manner marked by pullout failure with no cover splitting—even in the absence of external confinement. This ductile bond-slip response is owing to the extremely high tensile fracture energy of the matrix, which is attributed to the reinforcing action of the dispersed microfibers in the cementitious matrix. This enhances the associated bond-slip law with higher strength and a slowly descending branch, very similar in form to the response curve of confined anchorages that demonstrate ductile, resilient response. To understand and model the structural response of an elastic bar anchorage in a SRCC matrix, the analytical solution of the field equations that govern the bond problem are established with reference to the entire range of the bond-slip law up to large levels of slip, including the postpeak descending branch, which quantifies the fracture energy of the matrix. The accuracy of the mathematical solution is verified through correlation with laboratory evidence, benchmark finite-element analysis examples, and numerical solutions of the discretized problem. The mathematical solution is used in order to conduct a parametric investigation that accounts both for the bond toughness and the anchorage geometry, to illustrate their significance as prerequisite for development of high-strength reinforcement.

Effect of Cyclic Loadings on the Shear Strength and Reinforcement Slip of RC Beams

Numerous studies of the response of reinforced concrete members under cyclic loadings, many of which have been summarized and have indicated that, in general, the flexural strength of under-reinforced beams remains unimpaired under cyclic loadings consisting of a reasonable number of cycles. However, there is a body of evidence indicating that their shear strength may suffer under such loadings. The first objective of the current study is to construct an accurate 2D shell finite element model of reinforced concrete beams under cyclic loadings. The second objective is carrying out a parametric study on reinforced concrete beams, using the suggested 2D shell model. The objective of this study was to observe the effect of the stirrup spacing, steel-to-concrete bond properties on the performance of reinforced concrete beams under cyclic loadings. For this purpose, an efficient and accurate finite element model was established taking into account the compression and tensile softening introducing damage in the concrete material, the Baushinger effect using nonlinear isotropic/kinematic hardening in the steel and an adequate bond-slip law for the concrete–steel interface. The simulated results of numerical models were verified by experimental results available in literature in order to validate the proposed model, including hysteretic curves, failure modes, crack pattern and debonding failure mode. The model provided a strong tool for investigating the performances of reinforced concrete beam. The results showed that: Cyclic loadings may change the failure mode of the beam to bond failure even though it has sufficient bond length to resist static loadings. So that under cyclic loadings additional anchorage length must be taken, cyclic loadings also influence the ductility and peak load for beams fail in shear. All these topics are of the utmost importance to RC behaviour to be considered by construction codes.

Three-dimensional finite element modelling of the fire behaviour of insulated RC beams strengthened with EBR and NSM CFRP strips

This paper presents numerical simulations of the fire behaviour of reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) strips installed according to the externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques. Three dimensional (3D) finite element (FE) models of the beams were developed, in which the temperature-dependent thermophysical and mechanical properties of the constituent materials was considered. The CFRP-concrete interaction was modelled by means of bi-linear bond-slip laws previously calibrated by the authors for different temperatures. Comparisons between numerical and previous experimental results confirmed the accuracy of the models in predicting the thermo-mechanical response of insulated CFRP-strengthened RC beams exposed to fire. The results obtained highlight the possibility of exploring the CFRP mechanical contribution in fire through a “cable” behaviour by applying a thicker insulation in the CFRP anchorage zones. Moreover, the numerical results confirmed the better fire performance of the NSM strengthening technique when compared to EBR: with the latter technique CFRP debonding occurred after considerably shorter periods of fire exposure and for much lower average temperatures in the adhesive in the anchorage zones, from 1.2×Tg to 1.4×Tg, with such range being 2.4×Tg to 4.2×Tg for the NSM system.

Single-lap shear bond tests on Steel Reinforced Geopolymeric Matrix-concrete joints

Abstract Nowadays Fiber Reinforced Polymers (FRPs) represent a well-established technique for rehabilitation of Reinforced Concrete (RC) and masonry structures. However, the severe degradation of mechanical properties of FRP under high temperature and fire as well as poor sustainability represents major weak points of organic-based systems. The use of eco-friendly inorganic geopolymeric matrices, alternative to the polymeric resins, would be highly desirable to overcome these issues. The present work aims to investigate the bond characteristic of a novel Steel Reinforced Geopolymeric Matrix (SRGM) strengthening system externally bonded to a concrete substrate having low mechanical properties. SRGM composite material consists of stainless steel cords embedded into a fireproof geopolymeric matrix. Single-lap shear tests by varying the bonded length were carried out. The main failure mode observed of SRGM-concrete joints was debonding at the fiber-matrix interface. Test results also suggest the effective bond length. On the basis of the experimental results, a cohesive bond-slip law was proposed.

General cohesive zone model for prediction of interfacial stresses induced by intermediate flexural crack of FRP-plated RC beams

In this paper, a general model is developed to predict the distribution of interfacial shear and normal stresses induced by intermediate flexural crack of FRP-plated reinforced concrete beam. For that, a new theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth. The stress deformation relationship is generally referred to as bond-slip law since the deformation of the interface is mainly the relative displacement (slip) between the FRP plate and the reinforced concrete beam (RC beam). Adherent shear deformations have been included by assuming a parabolic shear stress through the thickness of the adherents, verifying the cubic variation of the longitudinal displacement function, whereas all existing solutions neglect this effect. To obtain interfacial normal stress, the adhesive layer is assumed in linearly elastic stage in normal direction. This assumption is adopted in this study, because the FRP plate and RC beam are in contact during the whole debonding process. A complementary experimental study is made in order to show the debonding process according to mechanical and geometrical characteristics of adhesive and FRP plate. Analytical results obtained through the closed-form of interface stresses are in good agreement with those given by numerical and experimental models. Finally, parametric studies are carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding.

Numerical simulation of bond-slip interface and tension stiffening in GFRP RC tensile elements

Bond between reinforcement and concrete highly affects the structural behaviour of reinforced concrete (RC). Introduction of bond-slip models into numerical simulation allows taking into account the bond interaction and analyse its effect on local and global behaviour. Unlike conventional steel reinforcement, no standard bond-slip law exists for FRP reinforcement, as bond is the result of a combination of parameters such as reinforcing material and bar surface configuration, among others. Therefore, there is a need in developing a methodology to easily implement bond-slip response from experimental bond tests.In this work, a methodology to implement bond-slip behaviour between concrete and reinforcement into a FEM model is presented. The inverse analysis is used to characterise the bond mechanisms active in a pull-out test. The obtained constitutive behaviours are thereafter implemented into a FE program by using connector elements and surface-to-surface contacts, and GFRP RC tensile elements are modelled. Numerical results are compared to experimental ones available in the literature, showing good accuracy in terms of load-deformation, crack width and crack spacing, as well as strains and bond stress and slip distributions along the reinforcing bar.

Creep response of intermediate flexural cracking behavior of reinforced concrete beam strengthened with an externally bonded FRP plate

In this study, a creep response model for the long-term behavior of interfacial shear stresses induced by intermediate flexural crack of FRP-plated RC beams is developed. A theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth with time increments. The creep behavior of the RC beam, adhesive layer and FRP plate have been included by considering the time-dependent mechanical properties. The time-dependent stress-deformation relationship caused by creep is referred to as bond-slip law. Consequently, a new interfacial law which combines time-dependent mechanical properties of all retrofitted beam constituents is proposed by considering constant the total energy during the creep response with intermediate crack. Obtained results are in good agreement with those given in literature. A parametric study is carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding. Indeed, the creep behavior, activates the debonding process in long term. Size of the softening zone is reached quickly and the bearing capacity of the retrofitted beam will be relatively affected by the time increase.

3D FE Analysis of RC Beams Externally Strengthened with SRG/SRP Systems

The purpose of this study is to evaluate, through a nonlinear Finite Element (FE) analysis, the structural behavior of Reinforced Concrete (RC) beams externally strengthened by using Steel Reinforced Grout (SRG) and Steel Reinforced Polymer (SRP) systems. The parameters taken into account were the external strengthening configuration, with or without U-wrap end anchorages, as well as the strengthening materials. The numerical simulations were carried out by using a three-dimensional (3D) FE model. The linear and nonlinear behavior of all materials was modeled by appropriate constitutive laws and the connection between concrete substrate and external reinforcing layer was simulated by means of cohesive surfaces with appropriate bond-slip laws. In order to overcome convergence difficulties, to simulate the quasi-static response of the strengthened RC beams, a dynamic approach was adopted. The numerical results in terms of load-displacement curves, failure modes, and load and strain values at critical stages were validated against some experimental data. As a result, the proposed 3D FE model can be used to predict the structural behavior up to ultimate stage of similar strengthened beams without carrying out experimental tests.

Inverse identification of the bond behavior for jute fibers in cementitious matrix

Abstract This paper analyzes the bond behavior of natural fibers in cement-based matrix. First, it presents the results of pull-out tests carried out on jute fibers embedded in a cementitious mortar. Then, the experimental results are employed in identifying the bond-slip relationship which describes the interaction between fiber and matrix. A theoretical model, capable of simulating the various stages of a pull-out test, is formulated and applied for an inverse identification of the bond-slip law. Finally, the obtained results demonstrate the soundness of the proposed theoretical interpretation for the bond behavior of jute fibers embedded in a cementitious matrix.

Full-scale field experimental investigation on the interfacial shear capacity of continuous slab track structure

This paper presents the experimental observations and results of six full-scale field ballastless track structure specimens, and tested under longitudinal and transverse shear load. The tests aimed to examine the interfacial shear capacity of the continuous slab track structure and investigate the interfacial bond–slip behaviour. The results show that bond strength of the two interfaces which were on the top and bottom of mortar layer, respectively, have a large difference. Until the top interface of the mortar layer fractured, no slip displacement was observed in the bottom interface. In addition to the experimental study, a finite element model using nonlinear interface elements was employed to simulate the tests. The numerical calculated capacity agreed well with the experimental results, showing that the proposed bond–slip law is reliable. Finally, the track slab's evenness with the bond–slip effect under the dynamic load was studied.

Experimental Study on Bond Behavior in Fiber-Reinforced Concrete with Low Content of Recycled Steel Fiber

The use of recycled steel fibers from waste tires as reinforcement in concrete matrix appears to be a promising solution, thanks to the performance of the material in terms of toughness and postcracking behavior. The main objective of this paper is to analyze the bond behavior of recycled steel fiber–reinforced concrete (RSFRC) and steel bars, and compare the results with those of industrial steel fiber–reinforced concrete (ISFRC). The paper focuses on the characterization of the mechanical properties of concrete reinforced with short steel fibers from waste tires and on the results of pull-out tests executed both on RSFRC and ISFRC. The experimental results, in terms of failure mode, maximum bond stress, and bond stress versus slip behavior are analyzed and discussed. Finally, a theoretical analysis of the bond-slip behavior was performed. The experimental results show that most of the known performance of the ISFRC can be extended to RSFRC. Referring to the bond performance, an improved behavior of RSFRC with respect to ISFRC in terms of bond-slip law was observed.

A cohesive interface crack model for the matrix–textile debonding in FRCM composites

Existing masonry structures often need to be strengthened or repaired. Among non-traditional retrofitting techniques, Fibre Reinforced Cementitious Matrix (FRCM) materials are becoming more and more interesting, especially for the reinforcement of historical buildings. The bond strength of the FRCM reinforcement is one of the key factors affecting the behaviour of the reinforced member and failure mode mostly involves the failure of the textile/mortar interface. Based on experimental results on single lap pull–push shear tests on FRCM materials bonded to masonry blocks, a tri-linear bond–displacement behaviour at the textile–mortar interface is proposed. The last phase of the bond–slip law is characterized by a constant shear stress in order to model the friction phenomena between the matrix and the textile observed in the experimental tests. Analytical expressions for the load–displacement response and interfacial shear stress distribution at different loading stages are derived taking into account the effects of the friction phenomena. The model is discussed in detail and the analytical results are compared to experimental outcomes. Finally, a parametric analysis is performed to highlight the effect of bond length, cohesive bond–slip law parameters and friction on the load–displacement response.

Generalization of the interface law for different FRP processing techniques in FRP-to-concrete bonded interfaces

Abstract Recent trends in the application of advanced processing techniques, such as vacuum assisted resin infusion (VARI) or heated vacuum bag only (HVBO), compel generalization of the bond–slip laws for prediction of the interface behaviour in adhesively bonded joints. In this research, a new analytical approach is presented to determine the interface behaviour. Since the model is developed solely based on boundary conditions, it can be applied to any types of the FRP processing technique. With this method, the interface characteristics can be obtained from the applied load at each stage and the properties of the materials. Comparisons between experimental and analytical results of strain, shear stress and slip profiles indicate that the proposed analytical model is capable of predicting the interfacial behaviour between FRP-to-concrete bonded joints manufactured with different the FRP processing techniques (FPTs).

Full Debonding Process of Adhesively Bonded Composite and Metallic Pipe Joints under Torsion

Bonded joints are very common in many pipeline systems, in which the bond behavior of the joint interface is of crucial importance. This paper presents two analytical solutions for the debonding process of a pipe joint under torsion, assuming that the bond interface follows either an exponential softening bond-slip law or a simplified bilinear model. The solutions are general, applicable to composites and metallic and indeed other pipes. Based on the analytical solutions, the influences of the bond length and stiffness on the torque-displacement curve and ultimate load are investigated. The solutions can be used to explain the stress transfer mechanism, the interface crack propagation, and the ductility of the joint.