Experimental Pull-out Tests Research Articles

Meso-scale Study of Bond Behavior between Ribbed Steel Rebar and Fiber-reinforced Cementitious Composite Considering Rebar Rib Geometry

This study aims to develop a meso-scale finite element approach to investigate the pull-out behavior of ribbed steel rebar and fiber-reinforced cement composites. We considered separately the three phases of steel fibers, cement composite matrix, and rebar within the meso-scale finite element model. A random distribution of fibers was generated in the matrix based on the geometric characteristics of the fibers and their volume fraction. By employing the interfacial transition zone (ITZ) model, the interaction between rebar and fibers with cement composite was simulated. Experimental pull-out tests were conducted to determine the parameters of the model. A meso-scale finite element model was validated and the effect of rebar diameter and fiber volume fraction on the bond behavior of rebar with fiber-reinforced cement composites was examined. In accordance with the experimental results, the bond-slip curve obtained with the meso-scale finite element model can be divided into five areas: non-slip, slight slip, splitting, decreasing, and residual. Additionally, it can be observed that fiber-free cement composites fail by splitting, whereas specimens containing fibers fail by sliding. This numerical model is therefore able to predict and estimate the influence of a variety of parameters on the pull-out response between fiber-reinforced cement composite and ribbed bar and failure mechanisms without the need for expensive and time-consuming testing.

Numerical evaluation of dimension effects of cold-drawn shape memory alloy crimped fiber on pullout resistance

A finite-element (FE) pullout model was developed and validated by a previous experimental pullout test of cold-drawn shape memory alloy (SMA) crimped fiber. Then, how the crimped fiber dimensions, such as the diameter, wavelength, embedded wave, and wave depth, influence its pullout behavior is investigated. The FE results indicate that the fiber diameter must be large enough to produce a good flexural rigidity. The recommended diameter is about 1.0 mm. However, it should note that the pullout resistance is not improved much for thicker fiber. The short wavelength is more effective than the long one because of the higher maximum pullout stress and the more peak points. The more embedded waves produce the higher pullout resistance; however, the increasing pullout resistance is slowed down when the maximum pullout stress exceeds the yield strength. A 5-embedded wave is proposed to produce a reasonable pullout resistance whereas satisfying the economically used material. Finally, deep-crimped fiber generates a higher pullout resistance than shallow-crimped fiber. Based on the effect of the dimensions on the pullout behavior, an optimum dimensions concept is proposed for crimped fiber.

A finite element model for investigating the influence of keel design and position on unicompartmental knee replacement cementless tibial component fixation

ObjectivesThe cementless Oxford Unicompartmental Knee Replacement (OUKR) tibial component relies on an interference fit to achieve initial fixation. The behaviour at the implant-bone interface is not fully understood and hence modelling of implants using Finite Element (FE) software is challenging. With a goal of exploring alternative implant designs with lower fracture risk and adequate fixation, this study aims to investigate whether optimisation of FE model parameters could accurately reproduce experimental results of a pull-out test which assesses fixation. Materials and methodsFinite element models of implants with three methods of fixation (standard keel, small keel, and peg) in a bone analogue foam block were created, in which implants were modelled using an analytical rigid definition and the foam block was modelled as a homogenous linear isotropic material. The total interference and elastic slip were varied in these models and optimised by comparing simulated and experimental results of pull-out tests for two (standard and peg) implant geometries. Then the optimised interference and elastic slip were validated by comparing simulated and experimental data of a third (small keel) implant geometry. ResultsThe optimisation of parameters established an interference of 0.16 mm and an elastic slip of 0.20 mm as most suitable for modelling the experimental force–displacement plots during pull-out. This combination of parameters accurately reproduced the experimental results of the small keel geometry. The maximum pull-out forces from the FE models were consistent with experimental data for each implant design. ConclusionsThis study shows that experimental pull-out tests can be accurately modelled using adjusted interference values and non-linear friction and outlines a method for determining these parameters. This study demonstrates that complex problems in modelling implant behaviour can be addressed with relatively simple models. This can potentially lead to the development of implants with reduced risk of failure.

A novel method for assigning bone material properties to a comprehensive patient-specific pelvic finite element model using biplanar multi-energy radiographs

The increasing prevalence of adult spinal deformity requires long spino-pelvic instrumentation, but pelvic fixation faces challenges due to distal forces and reduced bone quality. Bi-planar multi-energy X-rays (BMEX) were used to develop a patient-specific finite element model (FEM) for evaluating pelvic fixation. Calibration involved 10 patients, and an 81-year-old female test case was used for FEM customization and pullout simulation validation. Calibration yielded a root mean square error of 74.7 mg/cm3 for HU. The simulation accurately replicated the experimental pullout test with a force of 565 N, highlighting the method’s potential for optimizing biomechanical performance for pelvic fixation.

A two-phased modelling approach for corrosion-induced concrete cracking and bond deterioration in reinforced concrete

Bond deterioration is one of the major consequences of reinforcement corrosion in reinforced concrete (RC) structures. In this paper, a two-phased numerical modelling approach is presented that aims to determine the constitutive behaviour (bond–slip) at the reinforcement-concrete interface at a certain corrosion level. The novelty of the approach is that it consists of a crack model and a bond model, and that the flow of corrosion products into the pores and corrosion-induced cracks as well as the effect of the bonded length and concrete cover are taken into account in the 2D crack model. The resulting expansion of corrosion products from the crack model is used as input in the 3D bond model. The combination of both models leads to a procedure that balances computational time and modelling detail. The model is validated on a substantial amount of experimental pull-out test results. A good agreement is obtained between the experimental data and the models for different corrosion levels in terms of crack width, crack pattern, corrosion-induced bond loss, and failure mode.

Bond assessment between rebars embedded into a parent material using a single-function bond-slip model

The pull-out test is usually used to evaluate the local bond behaviour between a rebar such as a ribbed steel rebar or a Fiber Reinforced Polymer (FRP) and a parent material, like concrete or timber. Although some codes recommend using a piecewise bond-slip model for ribbed or round steel rebars embedded in concrete, there is no globally accepted bond-slip model for FRP composites. Consequently, this piecewise bond-slip model is usually adapted for FRP composites embedded in a parent material (substrate). The present work therefore aims to propose a novel single-function bond-slip model that may be calibrated and used to model the pull-out test and the detachment process of an embedded rebar from a parent material. The model requires the calibration of three parameters that may vary according to the type of rebar and its parent material. Since the governing equation of the detachment between the rebar and the parent material during a pull-out test has no known analytical solution, the Finite Difference Method (FDM) was applied to find the interfacial slips throughout the embedded length. The results were compared and validated with the experimental pull-out tests of ribbed steel rebars embedded into a concrete block as well as with the numerical results obtained from the use of the Finite Element Method (FEM). To cover a broader range of materials and show the versatility of the novel bond-slip model, the results were also compared with other experimental data available in the literature. The results showed a very high precision of the new bond-slip model when applied to several pull-out tests especially when ribbed steel rebars-to-concrete and FRP-to-concrete interfaces or FRP-to-timber interfaces were used.

Temperature effects on the behaviour of liquid-laminated embedded glass connections

Embedded load-bearing laminated glass connections have gained popularity in recent years due to their mechanical performance and aesthetic appeal. However, there is a paucity of data on their structural behaviour across a range of temperatures that may arise in building applications and there is also no simplified mechanics-based model for predicting their load–displacement response. This study addresses these gaps directly through experimental pull-out tests on steel inserts encapsulated in resin-laminated glass performed at various temperatures. The experimental results confirm that the response of the resin interlayer is time / temperature-dependent which therefore significantly affects the connection behaviour. In particular, both the stiffness and strength of the connection decrease with increasing temperature. Similarly, temperature also governs the failure mechanism of the connection. Specifically, temperatures at or below ambient indoor temperature (-10 °C and + 22 ± 2 °C) result in glass fracture whereas at + 50 °C the connection fails due to insert delamination. The numerical (FE) simulations of these tests show that a complex stress/strain state is set up in the vicinity of the embedded insert which correlates well with the experimentally observed failure mechanisms at different working temperatures. Finally, the insights gained along with the data generated from the experimental and numerical work were used to develop a simple analytical tool that predicts the pull-out load–displacement response of the embedded connection at different temperatures and load durations.

Performance assessment of bamboo bond strength in cement–fly ash mortar

The novelty of this study is the combined experimental and statistical approach, which is showcased to predict and optimise bamboo–mortar bond strength, incorporating the effects of various material factors. The design of experiments method was used to quantify the impact of multiple factors affecting the bond strength. Firstly, different experimental pull-out tests were conducted in the laboratory to determine the effect of various factors (water/cement ratio, curing duration, cement/sand ratio, fly ash content and bamboo treatment) on bond strength development. Three failure patterns were observed – bamboo failure, bond failure and mortar failure. Secondly, using a two-level full factorial design method, different statistical models were formulated. These models were developed based on the surface treatment of the bamboo (model M1 (untreated) and model M2 (treated and untreated)). The effects of single factors and their interactions on bond strength development were quantified. The developed models were validated for the prediction of bond strength based on different factors. The developed statistical models were found to predict and simulate the experimental behaviour of bond strength with sufficient accuracy.

Fundamental Investigations of Bond Behaviour of High-Strength Micro Steel Fibres in Ultra-High Performance Concrete under Cyclic Tensile Loading.

The objective of the contribution is to understand the fatigue bond behaviour of brass-coated high-strength micro steel fibres embedded in ultra-high performance concrete (UHPC). The study contains experimental pullout tests with variating parameters like load amplitude, fibre orientation, and fibre-embedded length. The test results show that fibres are generally pulled out of the concrete under monotonic loading and rupture partly under cyclic tensile loading. The maximum tensile stress per fibre is approximately 1176 N/mm2, which is approximately one third of the fibre tensile strength (3576 N/mm2). The load-displacement curves under monotonic loading were transformed into a bond stress-slip relationship, which includes the effect of fibre orientation. The highest bond stress occurs for an orientation of 30° by approximately 10 N/mm2. Under cyclic loading, no rupture occurs for fibres with an orientation of 90° within 100,000 load changes. Established S/N-curves of 30°- and 45°-inclined fibres do not show fatigue resistance of more than 1,000,000 load cycles for each tested load amplitude. For the simulation of fibre pullout tests with three-dimensional FEM, a model was developed that describes the local debonding between micro steel fibre and the UHPC-matrix and captures the elastic and inelastic stress-deformation behaviour of the interface using plasticity theory and a damage formulation. The model for the bond zone includes transverse pressure-independent composite mechanisms, such as adhesion and micro-interlocking and transverse pressure-induced static and sliding friction. This allows one to represent the interaction of the coupled structures with the bond zone. The progressive cracking in the contact zone and associated effects on the fibre load-bearing capacity are the decisive factors concerning the failure of the bond zone. With the developed model, it is possible to make detailed statements regarding the stress-deformation state along the fibre length. The fatigue process of the fibre-matrix bond with respect to cyclic loading is presented and analysed in the paper.

Experimental and numerical studies on progressive debonding of grouted rock bolts

Understanding the mechanism of progressive debonding of bolts is of great significance for underground safety. In this paper, both laboratory experiment and numerical simulation of the pull-out tests were performed. The experimental pull-out test specimens were prepared using cement mortar material, and a relationship between the pull-out strength of the bolt and the uniaxial compressive strength (UCS) of cement mortar material specimen was established. The locations of crack developed in the pull-out process were identified using the acoustic emission (AE) technique. The pull-out test was reproduced using 2D Particle Flow Code (PFC2D) with calibrated parameters. The experimental results show that the axial displacement of the cement mortar material at the peak load during the test was approximately 5 mm for cement-based grout of all strength. In contrast, the peak load of the bolt increased with the UCS of the confining medium. Under peak load, cracks propagated to less than one half of the anchorage length, indicating a lag between crack propagation and axial bolt load transmission. The simulation results show that the dilatation between the bolt and the rock induced cracks and extended the force field along the anchorage direction; and, it was identified as the major contributing factor for the pull-out failure of rock bolt.

A Finite Element Investigation into the Cohesive Properties of Glass-Fiber-Reinforced Polymers with Nanostructured Interphases.

Glass-fiber-reinforced polymer (GFRP) composites represent one of the most exploited composites due to their outstanding mechanical properties, light weight and ease of manufacture. However, one of the main limitations of GFRP composites is their weak inter-laminar properties. This leads to resin delamination and loss of mechanical properties. Here, a model based on finite element analysis (FEA) is introduced to predict the collective advantage that a GF surface modification has on the inter-laminar properties in GFRP composites. The developed model is validated with experimental pull-out tests performed on different samples. As such, modifications were introduced using different surface coatings. Interfacial shear stress (IFSS) for each sample as a function of the GF to polymer interphase was evaluated. Adhesion energy was found by assimilating the collected data into the model. The FE model reported here is a time-efficient and low-cost tool for the precise design of novel filler interphases in GFRP composites. This enables the further development of novel composites addressing delamination issues and the extension of their use in novel applications.

Textile-to-mortar bond behavior: An analytical study

Reliable design and application of textile-reinforced mortar (TRM) composites for the repair of existing masonry and concrete structures requires a fundamental understanding of the textile-to-mortar bond behavior as one of the main mechanisms controlling their nonlinear response and cracking behavior. Suitable test setups and analytical models are needed to extract the bond-slip laws from the experimental pull-out tests. This paper proposes a new bond-slip law and analytical model, which predicts the bond behavior of lime and cement-based TRM composites considering the slip hardening and softening effects observed in experimental tests. For this purpose, the pull-out response of experimental specimens with different fiber types (steel and glass), bond lengths, and mortar age are analyzed, and their bond slip-laws are extracted. The accuracy of the developed model is shown by comparing analytical and experimental results.

FEM Simulations of Bond Behaviour between Concrete and Seven-Wire Prestressing Strand

The article deals with the subject related to numerical tests of the concrete bond to seven-wire strands. Nonlinear contact concrete-steel calculations with using Finite Element Method (FEM) is applicable for concrete bond to reinforcing steel analyses. At the moment, the analyzes are the subject of independent research projects. The research using several scientific and commercial computing systems allows us to look deeper into the sample and obtain information about cracks development, stresses, and internal deformations. Such analyzes are impossible when tests only experimentally. The concrete-steel behaviour of materials can be modeled with a bond-slip mechanism where the relative slip of reinforcement and concrete is phenomenologically described. The behaviour of the slip zone is then mapped with a zero thickness interface (contact) element. The constitutive laws of bond-slip adhesion are based mainly on the theory of total deformation, which expresses the pulling force as a function of total relative displacement. The correct construction of the bond model requires consideration of physical phenomena occurring not only at the interface of the materials but also inside within each of them. When designing numerical research, it is necessary to keep in mind the limits in computing memory of computers. Performing static calculations for complex numerical models containing a number of geometric, material and contact nonlinearities may prove to be too time-consuming on a PC. For this reason, it is now reasonable to construct approximate, simplified models. FEM models with a strand modeled as a solid bar (with a sidewall accurate to the real one) made it possible to observe and research, destructive mechanisms identified in experimental pull-out tests.

Experimental pull-out tests and design indications for strength anchors installed in masonry walls

This study deals with the identification of the mechanical behavior of chemical anchors embedded in masonry walls. 108 pull-out tests are carried out in five types of masonry walls built with clay brick or vertically perforated units with cement mortar. Different parameters are taken into account: embedment depths, masonry type, anchor position (injection either in brick units or in mortar joints). The axial load capacity and the failure mode are observed for each test. The results are examined by means of elastic and plastic models assessing the efficiency of anchors installed in headers, stretchers or mortar joints. The anchors injected in mortar joints are shown to have much greater pull-out capacity than that found for anchors in bricks. Passing from 90 to 160 mm of embedment depth, a minimum increase by 40% of pull-out strength is observed. The most common failure modes are the sliding failure, which occurs for short anchors or weak masonry, and mixed sliding/cone failure, for long anchors or strong masonry. An analytical model is proposed to design anchors in order to avoid or at least to limit brittle masonry failures and to identify the field of application of uniform stress models.

GFRP as an alternative for steel nails, development of a model for prediction of creep displacements using experimental tests

During recent decades, fiber-reinforced polymers have been widely used as an alternative to steel in different engineering fields. The purpose of the current paper is to simulate nail-grout interface in soil-nail systems and to evaluate glass-fiber-reinforced polymers nail as an alternative for conventional steel nails. Although numerous studies have been performed on pullout behavior and creep tests of fiber-reinforced polymer and steel rebar in concrete, the behaviors of glass-fiber-reinforced polymer nails are not well investigated yet. A model to predict the maximum creep displacements of glass-fiber-reinforced polymer nails was developed and calibrated based on experimental creep tests. As such, beginning with presenting the results of several experimental pullout tests with different embedded lengths, to reach the length in which bond stress increment will be negligible, bond stress at nail-grout interface was evaluated with steel and glass-fiber-reinforced polymer nails. Then, time-dependent tests with both glass-fiber-reinforced polymer and steel nails have been carried out under sustained loads which were fractions of ultimate pullout loads. It was clearly noticed that creep displacements are lower with glass-fiber-reinforced polymer nail than steel.

Bond-slip behavior between ultra-high-performance concrete and carbon fiber reinforced polymer bars using a pull-out test and numerical modelling

In the present study, the impact of carbon nanotube (CNT) on the bonding behavior of carbon fiber reinforced polymer (CFRP) and steel rebars in carbon nanotube reinforced ultra-high-performance concrete was studied numerically and experimentally. The finite element (FE) analyses, was used to numerical modelling of the pull-out tests. Mechanical properties of the samples, were extracted from the experimental tests performed on standard samples with different quantities of CNT. In order to consider more realistic assumptions, the interaction and bond between concrete and rebar was simulated using cohesive zone modelling method, whose parameters were obtained by model-updating between the FE models and the experimental pull-out test results. The three-dimensional FE model was validated with the experimental pull-out tests. Then, the effect of the diameter and material of rebar as well as the contents of CNT on the pull-out behavior of bar and UHPC were considered. The results show that for 0.02 wt% CNT the maximum pull-out stress of CFRP rebar with a diameter of 12 mm and 16 mm increased by 176% and 407% compared to the control sample (UHPC without CNT), respectively; also, this increase for steel rebars was 5.7 times and 1.7 times, respectively.

Calibration of bar-Concrete Bond Stress Relationships for Bond Stress Prediction of GFRP Soil Nails Using Experimental Pullout Tests

Even though steel bar is a conventional reinforcement in soil stabilization systems, the problem of corrosion of steel may lead to vast damages especially in aggressive environments. In the past decades, Fiber Reinforced Polymer (FRP) materials have offered an effective solution to overcome the corrosion problem. Despite numerous bond stress-displacement models for reinforcements in concrete, there is a lack of models for FRP nails in grout. In this paper, the usability of four bond stress models (Malvar, EPB, CMR and Soroushian) of reinforcements in concrete was evaluated to predict the bond stress of FRP nails in grout. For this purpose, the results of several experimental pullout tests were used to calibrate the reinforcement-concrete bond stress models and the constant parameters were obtained. To evaluate the accuracy of the calibrated models, four statistical criteria of R2, SSE, RMSE and MAPE have been also determined for each model. Results showed that Malvar model with R2 of 0.94 and MAPE of %21 was deemed suitable for the prediction of bond stress of GFRP nails while CMR model is not recommended.

Effectiveness of design formulations for injected anchors in masonry elements

The paper is aimed to carry out a critical analysis of the available experimental results of pull-out tests related to several types of injected anchors in masonry elements. The database is also comprehensive of the results of some experimental pull-out tests directly carried out by the Authors. Both data on traditional steel bars and on anchors made by innovative materials embedded in several types of masonry made of natural stones and clay bricks are included in the database. Firstly, the main parameters influencing the experimental pull-out forces in the collected database were analyzed. Successively, the experimental values of the pull-out forces were compared with theoretical predictions provided by existing formulations and their reliability was discussed. Finally, regression analyses were carried out on the available database in order to assess new design formulations taking into account the main parameters influencing the performance of injected anchors.

An Equivalent Method for Bar Slip Simulation in Reinforced Concrete Frames

An extreme loading on a reinforced concrete frame leads to severe increase in cracks in concrete as well as large strains in reinforcing bars. This, in turn, results in reduction of steel–concrete bond as well as an increase in the subsidiary effects of slip. In the current study, to include the slip effects in the numerical analyses, an indirect approach based on the results of pull-out test modeling is proposed. The proposed approach suggests a steel strength-reduction factor of 0.6 so as to apply to the bars prone to slip. This value is obtained based on the results of micro-modeling of available experimental pull-out tests using ABAQUS. To account for the bond-slip effect between concrete and rebar, adjacent nodes of concrete and rebar were connected using nonlinear springs. To estimate the length of slip prone zone, a sensitivity analysis on the collapse behavior of two reinforced concrete sub-frames from a 10-story building was conducted. The experimental data for the sub-frames were available. Corroborated with the experimental data revealed that if the length of the slip prone zone is considered to be twice the depth of the beam, the presented indirect approach will be capable of predicting the load–deformation response with a good accuracy. The obtained results showed the significance of considering slip effect in the analyses of reinforced concrete frames as well.

Effect of tensile loading rate on interfacial properties of SMA/polymer composites

In the present paper, the effect of loading rate on the interfacial properties of shape memory alloy (SMA) reinforced polymer composites is investigated using experiments and theoretical methods. First, experimental pull-out tests were performed at three different loading rates with the crosshead speeds of 0.3, 25 and 400 mm/min. The effect of loading rate on the tensile properties of SMA wires was also examined using tensile tests. For each loading rate, three pre-strain levels (0, 2 and 4%) were considered. A previously developed theoretical model was modified to determine the effect of loading rate on the interfacial behavior between SMA wires and polymer. According to results obtained, increasing the loading rate led to a reduction in the tensile strength and Young's modulus of SMA wires. On the other hand, interfacial shear strength between SMA and polymer was improved in higher loading rates. Enhancement of 70% was obtained for the interfacial shear strength at 400 mm/min in comparison with 0.3 mm/min crosshead speed. By comparing the results of the theoretical model and experiments good agreement was obtained. Moreover, the optical microscopy was utilized to investigate the effect of the generated recovery force in SMAs, on the compressive zone in SMA/polymer specimens.