Fiber Reinforced Polymer Interface Research Articles

Analysis of the Impact of Sustained Load and Temperature on the Performance of the Electromechanical Impedance Technique through Multilevel Machine Learning and FBG Sensors.

The electro-mechanical impedance (EMI) technique has been applied successfully to detect minor damage in engineering structures including reinforced concrete (RC). However, in the presence of temperature variations, it can cause false alarms in structural health monitoring (SHM) applications. This paper has developed an innovative approach that integrates the EMI methodology with multilevel hierarchical machine learning techniques and the use of fiber Bragg grating (FBG) temperature and strain sensors to evaluate the mechanical performance of RC beams strengthened with near surface mounted (NSM)-fiber reinforced polymer (FRP) under sustained load and varied temperatures. This problem is a real challenge since the bond behavior at the concrete–FRP interface plays a key role in the performance of this type of structure, and additionally, its failure occurs in a brittle and sudden way. The method was validated in a specimen tested over a period of 1.5 years under different conditions of sustained load and temperature. The analysis of the experimental results in an especially complex problem with the proposed approach demonstrated its effectiveness as an SHM method in a combined EMI–FBG framework.

Size Effect on FRP External Reinforcement and Retrofit of Concrete Structures

The size effect on the strength of reinforced concrete (RC) beams flexurally strengthened by surface bonded sheets of fiber-reinforced polymer (FRP) is studied. As the failure is neither ductile nor brittle, but quasibrittle, occurring after stable growth of large cracks with large damage zones, the transitional size effect governed by the energetic size effect law (SEL) must be expected, which the present analysis confirms. Owing to scarcity of size effect experimental data, finite element (FE) analysis is used. A powerful microplane damage constitutive model M7 for concrete, coupled with the crack band model to suppress spurious mesh sensitivity, is calibrated by fitting the existing test data to FRP-strengthened RC specimens of diverse types, and consequently is trusted to simulate the size effect. The analysis captures the debonding near the concrete–FRP interface starting at midspan or at the end of the FRP sheet. It also captures the debonding between adjacent concrete shear cracks, and the delamination of concrete cover. The causes of these different delamination modes are explained. Simple beam-type formulas, in which the size characteristic is not the beam depth but the length of the shear span, are proposed and validated by FE results. The use of single-lap shear tests to determine the bond strength is critically discussed. Finally, a good fit of a few existing limited test data on the size effect of FRP-strengthened specimens is demonstrated.

Optimal design of damping layers in SMA/GFRP laminated hybrid composites

This work describes the optimization of the shape profiles for shape memory alloys (SMA) sheets in hybrid layered composite structures, i.e. slender beams or thinner plates, designed for the passive attenuation of flexural vibrations. The paper starts with the description of the material and architecture of the investigated hybrid layered composite. An analytical method, for evaluating the energy dissipation inside a vibrating cantilever beam is developed. The analytical solution is then followed by a shape profile optimization of the inserts, using a genetic algorithm to minimize the SMA material layer usage, while maintaining target level of structural damping. Delamination problem at SMA/glass fiber reinforced polymer interface is discussed. At the end, the proposed methodology has been applied to study the hybridization of a wind turbine layered structure blade with SMA material, in order to increase its passive damping.

Prediction Bond Strength between FRP and Concrete Interface by LEFM Method

To evaluate the interface strength of externally bonded fiber-reinforced polymer (FRP) composites to concrete structures, the method of the Linear Elastic Fracture Mechanic (LEFM) model is simply used. The parameters defining the material properties, describing bond action of the FRP-concrete interface is used , which was got recently by Obaidat from three-dimensional (3D) finite-element simulation results. Both the fracture energy and shear strength of the interface are determined by a function of concrete compressive strength and the adhesive shear stiffness, the maximum transferable load is predicted by LEFM interface bond-slip model. Comparison between the predicted and the experiment results shows good agreement and a certain degree of safe estimation.

An extended high order cohesive interface approach to the debonding analysis of FRP strengthened beams

Abstract The paper studies the debonding mechanisms in concrete beams strengthened with externally bonded fiber reinforced polymer (FRP) composite strips. The paper develops a nonlinear analytical model that combines an extended high order multi-layer consideration of the strengthened beam with a cohesive interface modeling of its physical interfaces. The handling of the adhesive layer uses 2D elasticity and considers the layer as an isotropic medium with shear, vertical-normal, and longitudinal-normal stiffness. This allows for capturing the variation of all stress components through the thickness of the adhesive layer and for implementing two distinct cohesive interfaces, one at the adhesive–concrete interface and one at the adhesive–FRP interface. By means of this combination, the model responds automatically with the nucleation and evolution of debonding mechanisms in each physical interface. The study looks into the debonding mechanisms triggered by the localized effects near the edge of the bonded layer and near a flexural crack in the concrete beam. The numerical results, the quantitative comparison with reference results, and the qualitative comparison with experimental observations reported in the literature gain insight into the nucleation and evolution of the failure mechanism. This effort takes another step towards the understanding of the debonding phenomenon and towards the development of a high-resolution analytical platform for its handling.

Influence of Fiber-Reinforced Polymers on Pile–Soil Interface Strength in Clays

Abstract A series of direct shear tests were carried out in order to characterize the pile–soil interface strength for various pile materials including steel, concrete, and grout and to investigate the influence of fiber-reinforced polymer (FRP) materials on the pile–soil interface strength in soft clay. The study investigated both pile–soil interface friction and interface adhesion by simulating drained and undrained conditions. The results among the traditional pile materials indicated the superior performance of grout and concrete relative to steel. FRP interfaces were shown to perform at a level the same as or higher than that of traditional steel piling under both drained and undrained conditions in clays. The FRP–clay interface friction angles were 5 % to 19 % greater than those in traditional steel–clay interfaces and 12 % to 23 % smaller than that of concrete. In addition, FRP interface adhesion was observed at between 86 % and 135 % of the interface adhesion of steel and between 65 % and 75 % of the interface adhesion of concrete.

Three-dimensional contact-damage coupled modelling of FRP reinforcements – simulation of delamination and long-term processes

In recent years FRP (Fiber Reinforced Polymer) technology has been developed to repair damaged concrete structures. This work proposes to investigate the complex mechanism of stress–strain evolution at the FRP interface until complete debonding as well as when long-term loads are accounted for. The study has been performed by means of a fully three-dimensional approach within the context of damage mechanics, to appropriately catch transverse effects and normal stresses, developing a realistic and comprehensive study of the delamination process. The adhesion properties have been reconstructed through a contact model incorporating an elastic-damaged constitutive law, relating inter-laminar stresses acting in the sliding direction. A F.E. research code (FRPCON3D) has been developed, including a numerical procedure accounting for Mazars’ damage law inside the contact algorithm. The code is also able to describe the delamination process when considering the effects from different concrete surface preparations. Additionally, it has been upgraded to describe the long-term behaviour of composite structures by means of two visco-elastic formulations: (i) Bažant’s B3 law has been considered for the concrete component, where creep effect is composed by three different terms, i.e. the elastic part, basic creep and drying creep; (ii) for FRP’s fibres and matrix a micromechanical approach has been implemented. The numerical results have been calibrated and validated against available experimental evidences.

Analytical identification of bond–slip relationship of EB-FRP joints

Bond–slip relationship of externally-bonded (EB) fiber reinforced polymer (FRP) joints can be determined by either directly or indirectly from a pull-off test. The indirect analytical method is gaining popularity and has been recommended in recent literature because of the more consistent and accurate results it yields. To date, loaded end slip vs applied load curve is used for indirect identification of the bond–slip relationship, ignoring the free end slip. This work shows that ignoring free end slip may result in significant errors in measuring the bond–slip relationship. The analytical method is used in this work to derive closed form solutions for FRP strain, interface slips, and bond strength of EB-FRP joints. The analytical solutions are validated by experimental results. Using the analytical solution, it is clearly shown in the paper that free end slip affects results and should be considered in deriving bond–slip relationship of EB-FRP joints, especially when bond length is short.

Evaluation of debonding failure of reinforced concrete girders strengthened in flexure with FRP laminates using finite element modeling

The use of Fiber-Reinforced Polymer (FRP) materials dates back to the early 1940s when they were used in aerospace and naval applications. During the 1970s and early 1980s, FRP started being used in civil engineering applications for new construction, but more importantly for repair and strengthening of existing structures. However, experimental research showed that the typical failure mode of reinforced concrete (RC) structures strengthened with FRP composite materials is due to the debonding that occurs at the interface between concrete and FRP. The bond between FRP and concrete is therefore the key factor controlling the behavior of these structures since it limits the full use of the FRP strength. The paper evaluates the effect of the debonding failure on the response of FRP-strengthened RC beams. A nonlinear RC beam element with bond-slip between the concrete and the FRP laminates is developed and used to analyze several test specimens and to investigate their corresponding failure mode. The model was also used to study the reduction factor of FRP tensile strength of simply supported strengthened RC girders due to debonding failure. This reduction factor proved to be affected by several parameters: (a) the bond strength between FRP and concrete interface; (b) the concrete strength; (c) the thickness of FRP; (d) the modulus of FRP; (e) the width of FRP laminate; and (f) the development length of the FRP sheet. A large number of beam specimens were analyzed in order to conduct a thorough evaluation of debonding failure of RC beams strengthened with FRP laminates. Based on these studies, new equations that account for the aforementioned parameters were proposed to address the reduction in FRP strength due to debonding failure.

Numerical Investigation of Creep Effects on FRP-Strengthened RC Beams

Numerical analysis using a finite-element model was performed to simulate and investigate the long-term behavior of two RC beams with similar steel reinforcement, cast from the same batch of concrete. One beam was a plain RC beam and the other beam was strengthened using carbon fiber-reinforced polymer (FRP) strips. The deflections of both beams have been monitored for 5 years after loading. The finite-element model included both creep of concrete and viscoelasticity of the epoxy adhesive at the concrete-carbon FRP (CFRP) interface. The results of the finite-element analysis are compared to experimental observations of the two beams. The finite-element analysis was found to be able to simulate the long-term behavior of the CFRP-strengthened beam and help us understand the complex changes in the stress state that occur over time.

Experimental and Computational Modeling of Low Cycle Fatigue Damage of CFRP Strengthened Reinforced Concrete Beams

Improvement in the short-term behavior of deficient reinforced concrete beams is very well known through extensive testing of reinforced concrete beams strengthened by externally bonded carbon fiber reinforced polymer strips. However, long-term performance of such beams needs to be assessed before the method can gain full acceptance. Prominent modes of failure in strengthened reinforced concrete beams reported in literature are concrete crushing in compression, carbon fiber reinforced polymer plate rupturing in tension and peeling of the carbon fiber reinforced polymer plate due to high interfacial shear and peeling stresses at the plate cut-off point. Modeling of the overall system of strengthened reinforced concrete beams requires modeling of the individual components that ultimately leads to failure i.e., concrete, carbon fiber reinforced polymer and concrete - carbon fiber reinforced polymer. Component elasto-damage constitutive models are developed for concrete crushing, carbon fiber reinforced polymer rupture and concrete-carbon fiber reinforced polymer interface failure, calibrated by data from both suitably designed experiments and from experiments reported in literature, and are utilized to investigate the low cycle fatigue behavior of reinforced concrete beams strengthened by carbon fiber reinforced polymer strips. The predictive ability of the model is tested by comparison to data from two different beam strengthening schemes i.e., with end anchorage and without end anchorage.

Experimental Investigation and Fracture Analysis of Debonding between Concrete and FRP Sheets

The last few years have witnessed a wide use of externally bonded fiber reinforced polymer (FRP) sheets for strengthening existing reinforced and prestressed concrete structures. The success of this strengthening method relies on the effectiveness of the load-transfer between the concrete and the FRP. Understanding the stress transfer and the failure of the concrete–FRP interface is essential for assessing the structural performance of strengthened beams and for evaluating the strength gain. This paper describes an experimental investigation of the interfacial bond behavior between concrete and FRP. The strain distributions in concrete and FRP are determined using an optical technique known as digital image correlation. The results confirm that the debonding process can be described in terms of crack propagation through the interface between concrete and FRP. The data obtained from the analysis of digital images was used to determine the interfacial material behavior for the concrete–FRP interface (stress...