Plate End Debonding Research Articles

Predicting FRP Plate End Debonding with a Neural Network Model Enhanced by Modified Sparrow Search Algorithm

Predicting FRP Plate End Debonding with a Neural Network Model Enhanced by Modified Sparrow Search Algorithm

Plate end debonding strength model for RC beams strengthened with FRP/SMA composites

Plate end debonding is a common failure mode for fiber reinforced polymer (FRP) strengthened beams. FRP sheets and shape memory alloy (SMA) composites can effectively slow down plate end debonding and improve beam ductility owing to the anchorage effect of SMA wires on the FRP ends. This work aims to comprehensively study the anchorage performance of FRP/SMA composite strengthened beams through both model analysis and experimental investigation. The plate end debonding strength models of existing FRP strengthened beams were initially summarized and analyzed, and an improved model was proposed to predict the debonding strength of FRP/SMA composite strengthened beams at the plate end. Moreover, the anchorage performance and flexural behavior of beams strengthened with carbon fiber reinforced polymer (CFRP) and SMA composites were studied through experiments. The failure mode of the FRP sheet strengthened beams was plate end debonding failure, while the plate end debonding failure of the CFRP/SMA composite strengthened beams occurred at the strengthening zone end, and the peeling failure was delayed owing to the anchorage effect of SMA wires on the ends of the strengthening CFRP sheets. The recovery effect of SMA wires could successfully introduce prestress into CFRP sheets. Moreover, the CFRP strain corresponding to plate end debonding increased because of the anchorage of SMA wires on the CFRP sheet in the strengthening zone. The strength and ductility of the CFRP/SMA composite strengthened beam significantly improved. The proposed model for predicting the plate end debonding strength of FRP/SMA composite strengthened beams was validated through experiments.

Machine learning-based model for recognizing the failure modes of FRP-strengthened RC beams in flexure

Plate end (PE) debonding and intermediate crack (IC) debonding are the main failure modes of fiber reinforced polymer (FRP) strengthened reinforced concrete (RC) beams in flexure. Different failure modes exhibit different failure characteristics. Therefore, accurately identifying the failure mode is of great significance in selecting methods to prevent the debonding of the strengthened beams. This study first established a primary indicator system through literature research. Then one hundred and eighty-eight FRP-strengthened RC beams containing PE and IC debonding were collected from the published literature of forty-eight researchers. After that, correlation analysis and grey correlation analysis were used to study the data of two failure modes. Finally, the indicator system for the prediction of failure modes was established. After the indicator system was established, six machine learning algorithms, including K-nearest neighbor algorithm (KNN), decision tree (DT), random forest (RF), back propagation neural network (BPNN), logistic regression (LR), and support vector machine (SVM), were used to build the prediction model of failure modes. The evaluation of the models shows that the coefficient of variation of the accuracy of the decision tree is merely 5.4%, which has the best robustness; the average accuracy of the random forest for the two failure modes reaches 93% and 98%, which has the highest precision.

Prediction of the Debonding Failure of Beams Strengthened with FRP through Machine Learning Models

Plate end (PE) debonding and intermediate crack (IC) debonding are the two main failure modes of beams strengthened with fiber-reinforced polymer (FRP) in flexure. Therefore, it is essential to clarify the force state of the structure when debonding occurs in strengthened beams. This paper collected 229 beams with debonding failure as the database, of which 128 were PE debonding and 101 were IC debonding. Correlation and grey correlation analysis were used to establish the indicator systems for predicting PE and IC debonding and to identify the critical indicators among them. Five machine learning models, linear regression, ridge regression, decision trees, random forests, and back propagation (BP) neural networks, were used to build the two debonding prediction models. Optimization of the best prediction among the five machine learning models took place using the Dung Beetle Optimizer (DBO) algorithm, which has competitive performance with state-of-the-art optimization approaches in terms of convergence rate, solution accuracy, and stability. Finally, the optimal prediction model was compared with the models suggested by codes, and it was found that the established model can well predict PE and IC debonding.

Finite element modeling of debonding failure in CFRP-strengthened steel beam using a ductile adhesive

Carbon fiber-reinforced polymer (CFRP) composites externally bonded to the tensile flange of the steel beam are an effective strengthening technique. Due to the apparent effect of the adhesive toughness on the debonding behavior between the CFRP and steel, this paper focuses on the CFRP-strengthened steel beam using a ductile adhesive and the adhesive selection in the strengthening work. Three-dimensional (3-D) finite element (FE) models were proposed to simulate the mechanical response of the CFRP-strengthened steel beam under a static four-point bending load. The failure of a ductile adhesive was modeled using the trapezoidal mixed-mode cohesive zone model (CZM) coded in the user subroutine (UMAT) of ABAQUS. The effectiveness of the trapezoidal CZM was validated by the experiment in the literature, and the debonding characteristics using different laminate thicknesses and lengths were clarified by the stress and damage analysis. In terms of plate-end debonding failure mode, the ductile adhesive is more appropriate for the CFRP-strengthened steel beam due to its reduced normal stress, increased mixed-mode fracture energy, and more uniform stress distribution.

Effect of temperature variation on the plate-end debonding of FRP-strengthened steel beams: Coupled mixed-mode cohesive zone modeling

Fiber-reinforced polymer (FRP) strengthened steel beams may experience significant temperature variation during their service life. Because of the different coefficients of thermal expansion (CTEs) of FRP and steel materials, thermal stresses can be generated by temperature variation at the FRP-to-steel interface and consequently influence the plate-end debonding mechanism. Therefore, an accurate prediction of the debonding failure of FRP-strengthened steel beams under combined mechanical and thermal loading is of great importance for the strengthening design. This paper proposes a closed-form analytical solution based on a coupled mixed-mode cohesive zone model (CZM) (i.e., with the consideration of Mode-I and Mode-II mixity), to analyze the effect of thermal stress on the debonding failure of FRP-strengthened steel beams. An excellent agreement has been achieved between the analytical solution and the finite element (FE) modeling in terms of interfacial full-range debonding behavior. Further parametric studies were conducted and indicated that the thermal stresses induced by elevated temperatures tend to reduce the plate-end debonding load and such effect becomes more significant when a thicker FRP plate is adopted.

Mitigation of Concrete Cover Separation in Concrete Beams Strengthened with Fiber-reinforced Polymer Composites

Strengthening of reinforced concrete (RC) beams using fiber reinforced polymers (FRP) is an effective method that is gaining more acceptance worldwide. However, failure of the strengthened beams at the plate-end region by concrete cover separation (CCS) compromises the effectiveness of such a strengthening system. Among available anchorage systems to remedy such problem, the use of FRP U-wraps at the end region of the externally bonded FRP has the potential to prevent CCS failure. Most studies in the literature suffer from poor design of test specimens and the use of random amounts of FRP anchorage. This resulted in nonconclusive findings and contradicting results on the real effectiveness of FRP U-wraps as an anchorage to prevent plate-end debonding failure. That in turn has been reflected in the lack of reliable design methods for the design of FRP U-wrap anchorage. In this paper, the effect of FRP U-wraps on the mitigation of CCS failure in FRP-strengthened RC beams was experimentally examined and quantified. The study included full-scale RC beams strengthened in flexure using carbon FRP (CFRP) sheets and anchored at the plate-end region using vertical CFRP U-wraps. The test variable was the amount of CFRP U-wrap that is required to achieve complete mitigation of CCS. The test results indicated the effectiveness of FRP U-wraps on mitigating CCS failure once the required area of CFRP U-wrap was placed. Relevant design models were presented and assessed against the experimental results of the current study. The comparisons indicated inaccurate predictions for the area of FRP U-wrap that is required to prevent CCS. This sheds the light on the need for developing a more reliable and accurate method for the design of an end anchorage to prevent CCS failure.

Effect of temperature variation on the plate-end debonding of FRP-strengthened beams: A theoretical study

Steel/concrete structures strengthened with externally bonded FRP plates may be subjected to significant temperature variations during their service time. Such temperature variation (i.e., thermal loading) may significantly influence the debonding mechanism in FRP-strengthened structures due to the thermal incompatibility between the FRP plate and the substrate as well as the temperature-induced bond degradation at the FRP-to-steel/concrete interface. However, limited information is available on the effect of temperature variation on the debonding failure in FRP-strengthened beams. This paper presents a new and closed-form solution to investigate the plate-end debonding failure of the FRP-strengthened beam subjected to combined thermal and mechanical (i.e., flexural) loading. A bilinear bond-slip model is used to describe the bond behavior of the FRP-to-substrate interface. The analytical solution is validated through comparisons with finite element analysis results regarding the distributions of the interfacial shear stresses, the interfacial slips and the axial stresses of the FRP plate. Given that a constant bond-slip relationship is adopted, it is observed that an increase in service temperature will lead to an increased interfacial slip at the plate end and consequently a reduced plate-end debonding load, and vice versa. Further parametric studies have indicated that the thermal loading effects become more significant when shorter and stiffer FRP plates are applied for strengthening.

Effect of Stirrups on Plate End Debonding in Reinforced Concrete Beams Strengthened with Fiber Reinforced Polymers.

Plate end (PE) debonding is one of the critical debonding failure modes that may occur in reinforced concrete (RC) beams strengthened with externally bonded fiber reinforced polymers (FRPs). This study investigated the effect of internal steel stirrups on the PE debonding failure load of FRP-strengthened RC beams. The dimensions of the beams were 3400 × 400 × 200 mm. The beams were strengthened with carbon FRP (CFRP) sheets bonded to the soffit of the beams. The beams were divided into two series based on the distance of the cutoff point of the CFRP sheets from the nearest support. This distance was 50 mm or 300 mm, and the amount of steel stirrups was varied in terms of varying the stirrup diameter and spacing. The beams were simply supported and tested under four-point bending. The test results indicate that the effect of stirrups on the load carrying capacity of the beams was more pronounced for the beams with CFRP sheets extended close to the supports. It was also indicated that beams with larger amounts of stirrups failed in PE debonding by concrete cover separation while beams with lower amounts of stirrups failed in PE by either PE interfacial debonding or critical diagonal crack-induced debonding. The beams were analyzed using several analytical models from design guidelines and the literature. The result of analysis indicates that most of the available models failed to reflect the effect of stirrups in predicting PE debonding failure load of the beams. On the other hand, the models of El-Sayed et al. and Teng and Yao were able to capture such an effect with the best predictions provided by El-Sayed et al. model.

Cracking behavior of sea sand RC beam bonded externally with CFRP plate

CFRP is an alternative technique for cracking control of high-chloride reinforced concrete (RC) beams. This research, therefore, investigates the strength performance and failure mode and cracking behaviour of RC beams incorporated with sea sand bonded externally with the carbon fibre reinforced polymer (CFRP) plate. Sea sand is used as a 100% replacement of fine aggregate. Three batches of RC beams were carried out in this research, including the control beam (no sea sand neither CFRP), RC beam with normal sand bonded with CFRP plate, and RC beam with sea sand and bonded with CFRP. A four-point bending test was performed under static loading for the specimens. Finite element simulation was modelled for further comparison. The experimental findings showed that the flexural capacity of the sea sand RC beam bonded externally with CFRP plate is 5.50% greater than the flexural strength of the beam without CFRP (control beam). Besides, results demonstrated that RC beams bonded externally with CFRP were failed by plate end debonding (PED) while the control RC beam without bonding was failed at the mid-span by concrete crushing. However, the bonded RC beams were stiffer, which could lead to lower crack spacing. Finite element simulation showed very acceptable results compared to the experimental results.

EMPIRICAL SHEAR BASED MODEL FOR PREDICTING PLATE END DEBONDING IN FRP STRENGTHENED RC BEAMS

This paper presents the development of a simplified model for predicting plate end (PE) debonding capacity of reinforced concrete (RC) beams flexurally strengthened using fiber reinforced polymers (FRP). The proposed model is based on the concrete shear strength of the beams considering main parameters known to affect the opening of the shear cracks and consequently affect PE debonding. The model considers also the effect of the location of the cut-off point of FRP plate along the span of the beam. The proposed model was verified against experimental database of 128 FRP-strengthened beams collected from previous studies that failed in PE debonding. In addition, the predictions of the proposed model were also compared with those of the existing PE debonding models. The predictions of the model were found to be comparable to the best predictions provided by the existing models, yet the proposed model is simpler. Furthermore, the proposed model was combined with the ACI 440 IC debonding equation to provide a procedure for predicting the governing debonding failure mode in FRP strengthened RC beams. The procedure was validated against 238 beam tests available in the literature, and shown to be a reliable approach.

Experimental Investigation of Fatigue Debonding Growth in FRP-Concrete Interface.

An externally bonded fiber reinforced polymer (FRP) plate (or sheet) is now widely used in strengthening bending members due to its outstanding properties, such as a high strength to weight ratio, easy operating, corrosion and fatigue resistance. However, the concrete member strengthened by this technology may have a problem with the adhesion between FRP and concrete. This kind of debonding failure can be broadly classified into two modes: (a) plate end debonding at or near the plate end, and (b) intermediate crack-induced debonding (intermediate crack-induced (IC) debonding) near the loading point. The IC debonding, unlike the plate end debonding, still needs a large amount of investigation work, especially for the interface under fatigue load. In this paper, ten single shear pull-out tests were carried out under a static or fatigue load. Different load ranges and load levels were considered, and the debonding growth process was carefully recorded. The experimental results indicate that the load range is one of the main parameters, which determines the debonding growth rate. Moreover, the load level can also play an important role when loaded with the same load range. Finally, a new prediction model of the fatigue debonding growth rate was proposed, and has an excellent agreement with the experimental results.

Assessment of plate-end debonding design provisions for RC beams strengthened with FRP

Despite the enhancement in the flexural capacity of reinforced concrete (RC) members externally strengthened with fiber reinforced polymer (FRP) composites, debonding of the FRP from the concrete substrate at its ends is a major concern. This premature failure mostly occurs at a low fraction of the FRP rupture strain, thus compromises the effectiveness of FRP-strengthening. This paper presents a review on plate-end (PE) debonding as a critical failure mode in FRP-strengthened beams. The available models for predicting this mode of failure are also presented and assessed against experimental database established from previous studies. The precautions recommended by the relevant design codes to prevent PE debonding are also discussed and assessed. Based on the assessment, several conclusions are emphasized, including the inaccuracy in predictions of PE debonding given by most available models when compared with the experimental database and the inconsistency among these models. The assessment also revealed that the precautions recommended by most of the design codes are not adequate to prevent PE debonding.

Debonding analysis of curved RC beams externally bonded with FRP plates using CZM

External bonding of fiber-reinforced polymer (FRP) plates has been widely used to strengthen reinforced concrete (RC) structures. The effectiveness and efficiency of this technique largely depend on the bonding behavior between the strengthening FRP plate and the RC substrate. This work initially extends the state space method combined with its numerically stable solution to present an elastic analysis of the interfacial shear stress in curved RC beams externally strengthened by FRP plates. The processes of the plate end debonding and intermediate crack-induced (IC) debonding in the FRP-strengthened curved beam is analyzed for the first time by introducing the cohesive zone model (CZM) for simulation of the adhesive layers. Compared with other analytical methods, the present method is advantageous in its application to much more complex situations, such as the general bond-slip laws, arbitrary boundary conditions and load cases, and variable cross-sectional properties of the curved beams and FRP plates. Finally, some numerical examples are given, and the results demonstrate that the tapering at the ends of FRP plates can significantly reduce the interfacial shear stress, and the debonding area will extend to the whole interface immediately once the load reaches the peak value for the plate end debonding.

Improved uncoupled closed-form solution for adhesive stresses in plated beams based on Timoshenko beam theory

Premature failure due to plate-end debonding or plate-end separation can take place in reinforced concrete (RC) beams with a bonded plate at soffit, which results from concentration of adhesive stresses at the plate ends. Many theoretical studies based on the Euler-Bernoulli beam theory for simply supported beams bonded with plates and subjected to a partial load have been conducted to investigate the adhesive stresses. However, the results of these studies lack accuracy especially for depth beams because shear deformation in the plate and the beam is neglected. Only a few studies have considered the effect of shear deformation of both the plate and the beam, but the general solutions and problem solving procedures are extremely complicated owing to the fact that the two governing equations for the normal and shear adhesive stresses are coupled. In this paper, a new theoretical method is proposed to derive two uncoupled governing equations, and the exact general solutions are obtained. The proposed model is validated through comparisons of the adhesive stresses evaluated with another two existing models and measured from test. The effects of elastic modulus of the adhesive and plate, thickness of the adhesive and plate, and length of the bonded plate on adhesive stresses are investigated with the proposed and validated model, from which useful conclusions are provided for potential applications.

Flexural strengthening of RC beams with CFRP grid-reinforced ECC matrix

This paper investigates the flexural performance of a series of RC beams externally bonded with carbon fiber-reinforced polymer (CFRP) grid-reinforced engineered cementitious composite (ECC) matrix. A total of 15 RC beams, including three control and twelve strengthened, were prepared and tested. The test variables included the longitudinal reinforcement ratio, the strengthening configurations that consisted of different cementitious matrices (ECC versus epoxy mortar), different installation methods (prefabricated versus cast-in-place), and different stiffness of CFRP grids. The test results showed that ECC is an ideal cementitious matrix for the strengthening applications where FRP grids are used as the external reinforcement. The flexural strengthening configuration using the epoxy adhesive to bond prefabricated CFRP grid-reinforced ECC plate proved to be the most efficient solution. For such configuration, the plate-end debonding can be avoided and the mid-span debonding can be almost suppressed. Flexural capacity analysis was conducted and demonstrated that the plane section assumption is valid and the full strength composite action can be nearly achieved for the strengthening system. The average ratio of the predicted peak loads to the experimental ones of the strengthened RC beams was 1.05.

Prevention of premature failures of plate bonded flexurally strengthened RC slab using end anchor and connector

Flexural strengthening of reinforced concrete (RC) slab using externally bonded plate is found to be the most common and popular in retrofitting of deficient structure. However, premature failures due to plate end debonding of steel plate and shear could be the major drawbacks of the system to obtain the highest flexural capacities. The aim of this research was to propose a comprehensive strengthening technique to eliminate premature failures of flexurally strengthened RC slab. In the experimental programme, five full scale RC slabs including un-strengthened control slab and flexurally strengthened slabs using steel plates were fabricated and tested. The strengthened slabs were further strengthened with end anchor and embedded steel bar connector to eliminate premature end peeling and shear failure respectively. Theoretical and numerical models were also provided to design and to analyse the structural behaviour of steel plate strengthened slabs. Results showed that end anchor and embedded connectors prevented premature end peeling and shear failure of flexurally strengthened RC slabs completely and allowed the slab to fail by flexure with the maximum strength and ductility, whereas, their corresponding strengthened slabs without end anchor and embedded connector failed with premature plate end debonding and shear respectively. Both premature failures were found to be brittle in nature. The proposed theoretical model predicted the capacities of strengthened slabs satisfactorily. The results based on the numerical analysis were found to be comparable with the experimental findings.

Finite element modelling of debonding failures in steel beams flexurally strengthened with CFRP laminates

A steel beam may be strengthened in flexure by bonding a carbon fibre-reinforced polymer (CFRP) plate to the tension face. Such a beam may fail by debonding of the CFRP plate that initiates at one of the plate ends (i.e. plate end debonding) or by debonding that initiates at a local damage (e.g. a crack or concentrated yielding) away from the plate ends (intermediate debonding). This paper presents the first finite element (FE) approach that is capable of accurate predictions of such debonding failures, with particular attention to plate-end debonding. In the proposed FE approach, a mixed-mode cohesive law is employed to depict interfacial behaviour under a combination of normal stresses (i.e. mode-I loading) and shear stresses (i.e. mode-II loading); the interfacial behaviour under pure mode-I loading or pure mode-II loading is represented by bi-linear traction–separation models. Damage initiation is defined using a quadratic strength criterion, and damage evolution is defined using a linear fracture energy-based criterion. Detailed FE models of steel beams tested by previous researchers are presented, and their predictions are shown to be in close agreement with the test results. Using the proposed FE approach, the behaviour of CFRP-strengthened steel beams is examined, indicating that: (1) if the failure is governed by plate end debonding, the use of a CFRP plate with a higher elastic modulus and/or a larger thickness may lead to a lower ultimate load because plate end debonding may then occur earlier; (2) plate end debonding is more likely to occur when a short CFRP plate is used, as is commonly expected; and (3) the failure mode may change to intermediate debonding or other failure modes such as compression flange buckling if a longer plate is used.

An analytical cohesive crack modeling approach to the edge debonding failure of FRP-plated beams

This paper focuses on the prediction of edge debonding for a beam retrofitted with a Fiber-Reinforced Polymer plate. This failure mechanism, also known as plate-end debonding, stems from the concentration of interfacial stresses arising at the termination of the strengthening plate. Early models of edge debonding adopted failure criteria based on interfacial stresses. However, due to the typically catastrophic nature of this failure mechanism, approaches based on Linear Elastic Fracture Mechanics (LEFM) became increasingly established. In this paper, the problem is addressed by means of the Cohesive Crack Model (CCM) and of the Finite Fracture Mechanics (FFM). These models are able to bridge the gap between the stress- and the energy-based approaches and nevertheless have been used in a limited number of analytical studies to date. Based on a cohesive interface law with linear softening, closed-form solutions are derived for the interfacial stresses and the load–displacement curves, as well as for the ultimate load. It is found that LEFM usually overestimates the debonding load, thus justifying the need for the proposed approach; on the other hand, debonding load estimates based on FFM are in excellent agreement with the CCM predictions. Finally, a parametric analysis highlights the effect of the geometry/material properties on the structural response, as well as the ductile-to-brittle transition and the possible occurrence of structural instabilities depending on the test controlling parameters.

Comparison of Moment-Curvature Models for Fiber- Reinforced Polymer Plate-End Debonding Studies Using Global Energy Balance Approach

This paper compares a number of different moment-curvature models for cracked concrete sections that contain both steel and external fiber-reinforced polymer (FRP) reinforcement. The question of whether to use a whole-section analysis or one that considers the FRP separately is discussed. Five existing and three new models are compared with test data for moment-curvature or load deflection behavior, and five models are compared with test results for plate-end debonding using a global energy balance approach (GEBA). A proposal is made for the use of one of the simplified models. The availability of a simplified model opens the way to the production of design aids so that the GEBA can be made available to practicing engineers through design guides and parametric studies.