Externally-bonded Fiber-reinforced Polymer Research Articles

Predicting the shear strength of rectangular RC beams strengthened with externally-bonded FRP composites using constrained monotonic neural networks

Fiber-reinforced polymer (FRP) composites bonded externally to reinforced concrete beams have shown promise for increasing shear load-carrying capacity. However, accurately predicting the shear strength contribution of FRP remains challenging due to the complexity of the shear failure mechanisms and interactions between the contributing components. Existing design provisions have limitations in modeling this behavior. Recently, machine learning has shown promise for addressing such complex problems but lacks transparency. Therefore, this study developed a data-driven Constrained Monotonic Neural Network (CMNN) approach implemented using Python libraries Keras and Tensorflow, which integrates domain knowledge to provide reliable FRP shear-strength predictions. This framework directly encodes expected monotonic input-output relationships through constraints, harnessing both predictive power and monotonicity guarantees. A comprehensive database of 273 beam tests was used to train and evaluate the model. Bayesian optimization through the Optuna framework and 4-fold cross-validation were used to tune the hyperparameters of the CMNN model. Extensive evaluation demonstrated the CMNN model's superior accuracy, yielding an R² value over 0.9, a CI above 0.93 and a low MAE under 16 kN on both training and unseen test data. The model also outperformed existing design code provisions, achieving significantly optimal performance metrics across various strengthening configurations. The analysis of the predictions revealed logical sensitivity trends aligned with the principles of shear mechanics. Such physics-informed machine learning presents a next-generation technique for performance-based structural design, contributing an effective and interpretable tool to advance knowledge on quantifying the shear capacity contribution of externally bonded FRP reinforcement in concrete beams.

Three-dimensional finite element modeling of debonding failure of skew FRP-bonded concrete joints

Despite the extensive investigations on the shear strengthening of reinforced concrete (RC) beams using externally bonded fiber-reinforced polymer (EB-FRP) composites, the critical problem of how the skew angle (angle between FRP and crack opening direction) influences the bond capacity of FRP has drawn little concern and remains unsolved. This paper presents the first three-dimensional (3D) finite element (FE) model for studying the failure of skew FRP-bonded concrete joints. The concrete was simulated using the concrete damaged plasticity model, and the FRP was modeled as an orthotropic material equipped with the LARC05 model as the damage criterion. The FRP-to-concrete interface was described using a linear elastic cohesive law. The 3D FE model, which provided a relatively large convergent mesh size of 2.5 mm, was calibrated and validated using the bond tests of aligned and skew FRPs. Both the tests and FE results revealed a notable decrease in the bond capacity of FRP at the skew angle of 45°. The parametric analysis demonstrated that the adverse skew effect was reduced when the FRP width increased or FRP thickness decreased, by delaying the onset of the stable peeling-off of concrete along the FRP fiber axis. Compared with FRP, steel plate suffered less from the skew effect, due to its isotropic nature. This study can provide valuable implications for more effective shear-strengthening methods of RC beams and facilitate novel insights for the development of more robust design models. The 3D FE framework can also serve as a versatile tool for analyzing the complicated failure modes of various FRP-strengthened concrete systems.

Nondestructive Testing (NDT) for Damage Detection in Concrete Elements with Externally Bonded Fiber-Reinforced Polymer

Fiber-reinforced polymer (FRP) composites offer a corrosion-resistant, lightweight, and durable alternative to traditional steel material in concrete structures. However, the lack of established inspection methods for assessing reinforced concrete elements with externally bonded FRP (EB-FRP) composites hinders industry-wide confidence in their adoption. This study addresses this gap by investigating non-destructive testing (NDT) techniques for detecting damage and defects in EB-FRP concrete elements. As such, this study first identified and categorized potential damage in EB-FRP concrete elements considering where and why they occur. The most promising NDT methods for detecting this damage were then analyzed. And lastly, experiments were carried out to assess the feasibility of the selected NDT methods for detecting these defects. The result of this study introduces infrared thermography (IR) as a proper method for identifying defects underneath the FRP system (wet lay-up). The IR was capable of highlighting defects as small as 625 mm2 (1 in.2) whether between layers (debonding) or between the substrate and FRP (delamination). It also indicates the inability of GPR to detect damage below the FRP laminates, while indicating the capability of PAU to detect concrete delamination and qualitatively identify bond damage in the FRP system. The outcome of this research can be used to provide guidance for choosing effective on-site NDT techniques, saving considerable time and cost for inspection. Importantly, this study also paves the way for further innovation in damage detection techniques addressing the current limitations.

Evaluation of shear behavior of short-span reinforced concrete deep beams strengthened with fiber reinforced polymer strips

The application of externally bonded fiber-reinforced polymers (EB-FRP) is known to improve the shear behavior of reinforced concrete beams. Nevertheless, the studies available in the literature are not sufficient to explain the shear behavior of the beams that have low shear span to effective depth ratios (av/d) and are strengthened with FRP and the interaction between the horizontal steel shear reinforcement and FRP strips. To fill these gaps in the literature, an experimental study involving twenty-two tests of deep beams with a low av/d and with different amounts of vertical and horizontal steel shear reinforcement was carried out. Both unstrengthened and strengthened with fully wrapped FRP strips beams were tested where the number of FRP layers, the spacing between them and the type of FRP material were varied. The experimental results showed that the FRP strips improved the shear and ductility capacities of beams with low av/d. In addition, as the vertical shear steel reinforcement ratio in the beams strengthened with FRP strips increased, the percentage increases in the shear and deflection capacities decreased. Also, as the horizontal shear steel reinforcement ratio in the strengthened beams increased, the percentage increase in the shear capacity increased. Finally, a new equation was proposed to estimate the FRP contribution to the shear capacity. To prove the reliability of the proposed equation, a comparative study was conducted using twenty existing equations.

Numerical Study on the Influence of Bubble Defection on the Bond Strength of Externally Bonded Fiber-Reinforced Polymer-to-Concrete Joints

The influences of bubble defects on the bond strengths of externally bonded fiber-reinforced polymer (EB-FRP) concrete joints were investigated using numerical method in this research. Studies of the influences of increasing bubble defect areas on interfacial bonding performance were first conducted. It was observed that the influence of a bubble deficiency located in the middle of the bond region is not significant when the size of the bubble area is less than 2.5% of the effective bond area. Then, further investigation was conducted on the influences of the distances between the bubble and the loaded end, as well as the distance between the bubble and the side of the FRP, the ratio of the bubble’s width to the width of the FRP, the number of bubbles with a fixed total area and the distribution of multiple bubbles. It was found that the distances of the bubble to the loaded end and different numbers of bubbles distributed within a fixed area do not significantly affect the bond strength. When the bubble is positioned closer to the side of the FRP, a decrease in load capacity tends to occur earlier, and the amount of decrease also slightly increases. The bond capacity can be influenced by the shapes of the bubbles, and as the bubble width in the lateral direction increases while keeping the bubble areas constant, the load capacity decreases.

Shear strengthening of a concrete trough bridge using embedded through‐section (ETS) FRP bars

AbstractAs worldwide infrastructure is ageing, significant efforts have been paid to the development of strengthening techniques that will restore or increase the initial capacity of existing structures. Considering that it is expected that shear failure happens in a less ductile mode than that observed under bending actions, special attention has been devoted to shear strengthening methods. These methods included externally bonded fiber reinforced polymers (EB‐FRP), near‐surface mounted (NSM‐FRP) method, and the embedded trough‐section (ETS) technique, among others. In this paper, ETS method is used for the strengthening in shear of a reinforced concrete railway bridge located in Finland. The ETS method consists in the embedment of FRP or steel bars through predrilled holes into the concrete core. The bars are bonded to the concrete using adhesives. The paper includes a brief review of recent advances in the use of ETS and comparison with other available techniques, the description of the case study, instrumentation of the bars using fiber optic sensors (FOS) for strain monitoring, the procedure used for the installation of the bars in the field, and a preliminary analysis of the data collected.

Resilience of Medium-to-High-Rise Ductile Coupled Shear Walls Located in Canadian Seismic Zones and Strengthened with Externally Bonded Fiber-Reinforced Polymer Composite: Nonlinear Time History Assessment

Coupled shear walls (CSWs) are structural elements used in reinforced concrete (RC) buildings to provide lateral stability and resistance against seismic and wind forces. When subjected to high levels of seismic loading, CSWs exhibit nonlinear deformation through cracking and crushing in concrete and yielding in reinforcements, thereby dissipating a significant amount of energy, leading to their permanent deformation. Externally bonded fiber-reinforced polymer (EB-FRP) sheets have proven to be effective in strengthening RC structures against various loading and environmental conditions. In addition, their high strength-to-weight ratio makes them an attractive solution as they can be easily applied without significantly increasing the structure’s weight. This study investigates the effectiveness of using EB-FRP sheets to reduce residual displacement in CSWs during severe earthquake loadings. Two series of 15-story and 20-story CSWs in Western and Eastern Canadian seismic zones, which serve as representative models for medium- and high-rise structures, were evaluated through nonlinear time history analysis. The numerical simulation of all CSWs and strengthened elements was carried out using the RUAUMOKO 2D software. The findings of this study provided evidence of the effectiveness of EB-FRP sheets in reducing residual deformation in CSWs. Additionally, significant reductions in the rotation of the coupling beams (CBs) and the inter-story drift ratio were observed. The results also revealed that bonding vertical FRP sheets to boundary elements and confining enhancement by wrapping CBs and wall piers is a very effective configuration in mitigating residual deformations.

Bending performance of RC slabs strengthened by CFRP sheets using sprayed UHTCC as bonding layer: Experimental study and theoretical analysis

Debonding failure is one of the main drawbacks of reinforced concrete (RC) structures strengthened with externally bonded (EB) fiber reinforced polymer (FRP) systems. Due to the huge application potential of sprayed ultra-high toughness cementitious composite (UHTCC) in strengthening and retrofit, this study, with the aim of delaying or preventing intermediate crack-induced debonding of EB FRP sheets, experimentally and analytically investigated the flexural behavior of RC slabs strengthened by EB CFRP sheets with sprayed UHTCC as bonding layers, which has been rarely reported in previous research works. For this purpose, five RC slabs with low strength concrete and low reinforcement ratio, including one control specimen and four strengthened specimens repaired by EB CFRP sheets with sprayed UHTCC as bonding layers, were manufactured and tested under four-point bending load. Variable parameters in the four strengthened specimens included the thickness (10 mm, 20 mm, and 30 mm) and length (600 mm and 900 mm) of the sprayed UHTCC bonding layer. The experimental results indicated that the specimens strengthened by EB FRP sheets with sprayed UHTCC as bonding layers experienced higher load-bearing capacity (24.43 %–120.92 %) and toughness (43.15 %–166.14 %), compared to the control specimen. The thickness of the UHTCC bonding layer had a significant effect on the structural performance. By increasing the thickness from 10 mm to 20 mm and from 20 mm to 30 mm, the ultimate load was enhanced by 32.69 % and 33.81 %, while the ultimate deflection was decreased by 14.99 % and 41 %, respectively. The test findings also showed that sprayed UHTCC had a strong cracking-control ability and that no debonding failure occurred between the UHTCC layer and the CFRP sheet, while interface slip was observed between the concrete substrate and the UHTCC layer. The sectional analysis method was then used to propose an analytical model for analyzing the composite action between the concrete substrate and the strengthening material, as well as a model for predicting the load-bearing capacity of strengthened specimens. Comparing the analytical results to the experimental ones revealed that the proposed models can properly predict the flexural behavior of RC slabs strengthened by EB CFRP sheets with sprayed UHTCC as bonding layers under four-point bending load.

Fiber Reinforced Polymer Debonding Failure Identification Using Smart Materials in Strengthened T-Shaped Reinforced Concrete Beams.

The favorable contribution of externally bonded fiber-reinforced polymer (EB-FRP) sheets to the shear strengthening of reinforced concrete (RC) beams is widely acknowledged. Nonetheless, the premature debonding of EB-FRP materials remains a limitation for widespread on-site application. Once debonding appears, it is highly likely that brittle failure will occur in the strengthened RC structural member; therefore, it is essential to be alerted of the debonding incident immediately and to intervene. This may not be always possible, particularly if the EB-FRP strengthened RC member is located in an inaccessible area for fast inspection, such as bridge piers. The ability to identify debonding immediately via remote control would contribute to the safer application of the technique by eliminating the negative outcomes of debonding. The current investigation involves the detection of EB-FRP sheet debonding using a remotely controlled electromechanical admittance (EMA)-based structural health monitoring (SHM) system that utilizes piezoelectric lead zirconate titanate (PZT) sensors. An experimental investigation on RC T-beams strengthened for shear with EB-FRP sheets has been performed. The PZT sensors are installed at various locations on the surface of the EB-FRP sheets to evaluate the SHM system's ability to detect debonding. Additionally, strain gauges were attached on the surface of the EB-FRP sheets near the PZT sensors to monitor the deformation of the FRP and draw useful conclusions through comparison of the results to the wave-based data provided by the PZT sensors. The experimental results indicate that although EB-FRP sheets increase the shear resistance of the RC T-beams, premature failure occurs due to sheet debonding. The applied SHM system can sufficiently identify the debonding in real-time and appears to be feasible for on-site applications.

Using end buckles to improve debonding resistance in FRP-bonded precracked RC beams

The strengthening effect of reinforced concrete (RC) members externally bonded (EB) with fiber reinforced polymer (FRP) laminates is significantly affected by interfacial debonding failure. To improve the connection between FRP sheets and concrete, a novel type of anchor device for FRP sheets called buckle was proposed in this study. This device can effectively lock the end of the sheet using a patented method of wrapping. In addition to conventional external bonding, bolts were installed on the buckles to form hybrid anchored (HA) FRP sheet. To verify the effectiveness of this new method, a precracked RC beam was prepared in comparison with two undamaged beams strengthened with EB or HA FRP sheets. As observed, end buckles can limit the development of cracks regardless of initial cracking. Despite the occurrence of intermediate debonding due to initial cracks, both the ultimate capacity and failure ductility were significantly improved. Owing to the end buckles, the debonding resistance was excellent, and the initial cracking of the beam hardly affected the strengthening effect. In addition, existing formulas were used to calculate the lower limit of the ultimate load of the specimen strengthened with HA FRP, which was still higher than that of the specimen strengthened by EB FRP. Therefore, the results confirmed that the debonding resistance was greatly enhanced due to the locking of the FRP sheet via end buckles, and the proposed method is useful to guarantee the strengthening effect.

Long-term performance of 15-year-old full-scale RC beams strengthened with EB FRP composites

Despite the well-known advantages of fiber-reinforced polymer (FRP) composites, concerns of the long-term performance of reinforced concrete (RC) structures strengthened with FRP composites have been an obstacle to the growing applications of FRP composites due to the lack of knowledge and scarce test data about long-term performances. Thus, the long-term performance of three 15-year-old full-scale (6.8 m long) RC beams was investigated in this study. A non-strengthened beam was served as a control specimen and two were strengthened with externally bonded (EB) glass FRP (GFRP) laminates. The three beams that had been left outside for exposure to harsh environmental conditions for over 15 years were tested under the typical three-point bending condition. It was found that the EB GFRP-strengthened RC beams were superior to the non-strengthened RC one from the standpoint of load-carrying capacity, initial stiffness, ductility, and toughness (energy absorption capacity), which indicates that the EB GFRP strengthening was still effective even over 15 years. Furthermore, the long-term effective tensile strength reduction of the GFRP composites due to the over 15-year exposure to the harsh environment was calculated to be 26.5 % in comparison with the design tensile strength, which is crucial information for the EB FRP strengthening technique.

Structural Performance of EB-FRP-Strengthened RC T-Beams Subjected to Combined Torsion and Shear Using ANN.

This research study applied Artificial Neural Networks (ANNs) to predict and evaluate the structural responses of externally bonded FRP (EB-FRP)-strengthened RC T-beams under combined torsion and shear. Previous studies proved that, compared to reinforced concrete (RC) rectangular beams, RC T-beams performance in shear is significantly higher in structural analysis and design. The structural response of RC beams experiences a critical change while torsion moments are applied in load conditions. Fiber Reinforced Polymer (FRP) is used to retrofit the structural elements due to changing structural design codes and loadings, especially in earthquake-prone countries. We applied Finite Element Method (FEM) software, ABAQUS, to provide a precise numerical database of a set of experimentally tested FRP-retrofitted RC T-beams in previous research works. ANN predicted structural analysis results and Mean Square Error (MSE) and Multiple Determination Coefficients proved the accuracy of this study. The MSE values that were less than 0.0009 and values greater than 0.9960 showed that the ANN precisely fits the data. The consistency between analyzed experimental and numerical results demonstrated the accurate implication of ANN, MSE, and in predicting the structural responses of EB-FRP- strengthened RC T-beams.

Shear Strengthening of RC Beams with FRP Composites: Database of FE Simulations and Analysis of Studied Parameters

The use of externally bonded fiber-reinforced polymer (EB-FRP) composites for shear strengthening of reinforced concrete (RC) beams presents many challenges given the complex phenomena that come into play. Premature bond failure, the behavior of the interface layer between FRP composites and the concrete substrate, the complex and brittle nature of shear cracks, and the adverse interaction between internal steel stirrups and EB-FRP are some of these phenomena. Compared to experimental investigations, the finite element (FE) technique provides an accurate, cost-effective, and less time-consuming tool, enabling practicing engineers to perform efficient, accurate nonlinear and dynamic analysis as well as parametric studies on RC beams strengthened with EB-FRP. Since 1996, many numerical studies have been carried out on the response of RC beams strengthened using FRP. However, only a few have been related to RC beams strengthened in shear using EB-FRP composites. In addition, the analytical models that have been reported so far have failed to address and encompass all the factors affecting the contribution of EB-FRP to shear resistance because they have mostly been based on experimental studies with limited scopes. The aim of this paper is to build an extensive database of all the studies using finite element analysis (FEA) carried out on RC beams strengthened in shear with EB-FRP composites and to evaluate their strengths and weaknesses through various studied parameters.

Evaluation on the effect of anchor embedment depth on the flexural capacity of concrete prisms

Using fiber-reinforced polymer (FRP) composites to strengthen and retrofit existing reinforced concrete (RC) structures has gained wide acceptance owing to the superior properties of FRPs such as high strength-to-weight ratios and corrosion resistance. Externally bonded FRP (EB-FRP) laminates can be applied to enhance the flexural and shear capacities of RC beams and improve the ductility of RC columns. However, the premature debonding of FRP laminates from the concrete substrate is a major concern and usually results in a brittle member failure. To tackle this issue, many studies proved that adequate anchorage systems could improve the FRP-to-concrete bond, and therefore results in delaying the FRP debonding and further developing the tensile strength of the FRP laminates. One of the most commonly used anchorage systems is FRP spike anchors. Compared to other anchorage systems (for e.g., metallic anchors), FRP anchors have high compatibility with the FRP laminates. In addition, FRP spike anchors are noncorrosive and have high tensile strengths. Although several studies addressed FRP spike anchors in flexural and shear strengthening of RC beams and slabs, the literature still lacks comprehensive information on the effect of all anchor parameters on the performance of the strengthening system. As a result, the aim of this study is to investigate the effect of anchor embedment depth on the behavior, strength, and effectiveness on the FRP spike anchorage system. Six concrete prisms were cast and strengthened with carbon FRP (CFRP) laminates and spike anchors then tested in flexure under four-point bending tests. Test results showed that installing anchored CFRP laminates increased the capacity of the plain concrete prism by 80-120%. Anchoring the laminates at different embedment depths of 50mm, 75mm, 100mm, and 125mm enhanced the capacity of the unanchored prisms by 11%, 13%, 15%, and 33% respectively. Thus, it was concluded that increasing the anchor embedment depths increased the ultimate load carrying capacity of the prisms.

Modeling the Shear Strength of FRP-Strengthened Rc Beams Using Artificial Neural Networks

Externally strengthening reinforced concrete (RC) structures has been a desired practice in both research community and the industry over the last few decades. This application entails bonding composites to the surfaces of RC members to upgrade their strength, stiffness, and ductility. This study will attempt to use Machine Learning (ML) techniques to study and predict the shear strength of RC beams strengthened in shear with externally-bonded fiber-reinforced polymer (EB-FRP) laminates. An extensive database consisting of 511 tested specimens and 17 test parameters were collected. An appropriate artificial neural network (ANN)-based model was used to predict the shear capacities (Vu) of the FRP-strengthened beams, including the specific contributions of the EB-FRP (Vf) to these shear capacities. The obtained results indicate that the ANN-based model provided reasonable predictions for both and Vu and Vf.

Performance of externally bonded fiber-reinforced polymer retrofits in the 2018 Cook Inlet Earthquake in Anchorage, Alaska

As part of the effort to improve the seismic performance of buildings in Alaska (AK), many of the deficient structures in Anchorage, AK, were retrofitted—some with externally bonded fiber-reinforced polymer (EBFRP) composite systems. The 2018 magnitude 7.1 Cook Inlet earthquake that impacted the same region offered an opportunity to evaluate the performance of EBFRP retrofits in a relatively high-intensity earthquake. This study summarizes the following findings of this field investigation: (1) the performance of EBFRP-retrofitted structures in the Cook Inlet earthquake and (2) the observations concerning the condition of FRP retrofits from over a decade of exposure in a subarctic environment. A deployment team from the National Institute of Standards and Technology (NIST) in collaboration with the University of Delaware (UD) Center for Composite Materials conducted post-earthquake inspections of EBFRP retrofits in multiple buildings to assess their performance during the earthquake and condition with respect to weathering. EBFRP debonding was documented with infrared thermography and acoustic sounding and the bond quality between EBFRP and concrete was assessed using pull-off tests. Visual inspections showed no major signs of earthquake damage in the EBFRP-retrofitted components. However, evaluation of debonding and pull-off test results suggested that outdoor conditions may have led to bond deterioration between EBFRP and concrete from installation defects that grew over time, freeze–thaw expansion from moisture present at the FRP/concrete interface, differences in thermal expansion of the materials, or a combination thereof. The carbon fiber–reinforced polymer (CFRP) bond to concrete was found to be more vulnerable to outdoor exposure than the glass fiber–reinforced polymer (GFRP) bond. Earthquake effects on FRP/concrete bond could not be assessed due to the lack of baseline data.

Strengths of RC beams with a fibre-reinforced polymer (FRP)-strengthened web opening

For the passage of utility ducts and/or pipes, openings often need to be created in the web of a reinforced concrete (RC) beam. Such a web opening can lead to a significant decrease in the ultimate load (i.e., strength) of the beam due to the reduced cross-sectional area and/or the severing of some of the existing steel reinforcement (particularly stirrups). In such cases, an externally-bonded fibre-reinforced polymer (FRP) strengthening system may be installed around the web opening to ensure the safety of the weakened beam. While existing experimental and numerical studies have provided useful information on the structural behaviour of RC beams with an FRP-strengthened web opening, this paper presents a theoretical study on the strength of such beams. First, a relatively simple, iterative numerical procedure (referred to as the iterative method) based on the static method of plastic limit analysis for predicting the plastic limit load (i.e., strength) of RC beams with an FRP-strengthened web opening is proposed, and then a closed-form, algebraic strength equation (referred to as the strength model) for such beams is established as a simplified approach of the iterative method. The accuracy of the proposed iterative method and strength model is verified with test results.

Bond behavior and ultimate capacity of notched concrete beams with externally-bonded FRP and PBO-FRCM systems under different environmental conditions

The influence of environmental conditions on the bond behavior and ultimate capacity of strengthening composite systems applied to concrete substrates is investigated. Two different systems are analyzed, namely fiber-reinforced polymer (FRP) system with carbon sheet and epoxy resin (CFRP), and fiber-reinforced cementitious matrix (FRCM) system with polybenzoxole (PBO) grid and cement-based mortar. The experimental campaign comprises three-point bending tests on 24 notched beams with the strengthening system being externally bonded onto the bottom face of the beam in order to selectively study the bond behavior with no tensile contribution of concrete. In addition to classical curing process in air at environmental conditions, two wet curing procedures are studied, namely partial immersion in water at 30 °C and 50 °C and 100% relative humidity for a period of 31 days. Results in terms of load-displacement curves, failure modes and ultimate capacity are critically analyzed so as to highlight the influence of the conditioning factors (humidity and temperature) on the bond behavior at the concrete-composite interface of the two strengthening systems comparatively. For the relatively short duration of the partial immersion process considered in this experimental campaign, the FRCM system is not sensitive to environmental conditioning, whereas the FRP system is only sensitive to curing temperatures close to the glass transition temperature of the resin/primer. For the PBO-FRCM system, an indirect method to evaluate the stress-global slip curve is elaborated in this study through an analytical procedure based on kinematics considerations, which allows a comparison between results from the proposed notched beam test setup and results from alternative single-lap or double-lap shear tests that are more widely used in the literature.

Structural Materials: Identification of the Constitutive Models and Assessment of the Material Response in Structural Elements Strengthened with Externally-Bonded Composite Material.

This article investigates the material behavior within multiple-component systems. Specifically, a structural concrete element strengthened to flexure with externally-bonded fiber-reinforced polymer (FRP) material is considered. Enhancements of mechanical performances of the composite structural element resulting from synergies in the framework of the multiple-component system are studied. The research work comprises the determination of the constitutive relations for the materials considered separately as well as the investigation of materials’ response within a complex system such as the composite structural element. The definition of the material models involves a calibration of the model constants based on characterization tests. The constitutive relations are integrated into the finite element model to study the material behavior within the multiple-component system. Results obtained by finite element analysis are compared with experimental results from the literature. The finite element analysis provides valuable information about the evolution of some internal variables, such as mechanical damage accumulation. The material synergies find expression in the load-carrying capacity enhancement and the delay in the damage accumulation in concrete.

Experimental study of the steel/CFRP interaction in shear-strengthened RC beams incorporating macro-synthetic fibers

This paper presents an investigation of the performance of RC beams incorporating macro-synthetic fibers and rehabilitated using Externally Bonded Fiber Reinforced Polymer (EB-FRP) strips. A series of tests were conducted to investigate the EB-FRP contribution to the shear strength recovery of pre-damaged shear-deficient beams. The investigation included studying the effect of macro-synthetic fiber reinforcement on the contribution of EB-FRP to the shear strength of retrofitted beams, as well as the interaction between EB-FRP and steel stirrups in the presence of macro-synthetic fiber reinforcement in the concrete. The key parameters of the investigation included: (a) beam condition (damaged beams and virgin beams), (b) beam shear reinforcement (stirrups and no stirrups), and (c) macro-synthetic fiber dosage (0.5%, 0.75%, and 1.0%). The test results showed that the retrofitting of severely shear-damaged RC beams using EB-FRP significantly enhanced the original shear capacity of the beams with strength gains varying between 50% and 111% depending on the amount of macro-synthetic fibers and/or conventional shear reinforcement present in the specimens. The test results have also provided evidence for the existence of interaction between the steel stirrups, the macro-synthetic fibers and the EB-FRP strips in carrying part of the applied shear force in the retrofitted beams. Such interaction is such that with use of EB-FRP, the conventional shear reinforcement and/or macro-synthetic fibers have lesser contribution to the total shear force in the retrofitted beams. However, the presence of such additional shear reinforcement leads to enhanced crack control thereby delaying the premature debonding of EB-FRP strips and increasing the deflection capacity of the retrofitted beams.