Bonded Fiber-reinforced Polymer Reinforcement 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.

Intermediate Crack Debonding of Externally Bonded FRP Reinforcement-Comparison of Methods.

Many researchers around the world have made extensive efforts to study the phenomenon of fiber-reinforced polymer (FRP) debonding. Based on these efforts, code provisions and various models have been proposed for predicting intermediate crack (IC) debonding failure. The paper presents a comparison of seven selected models: fib bulletin 14 approach, Teng et al. model, Lu model, Seracino et al. model, Said and Wu model, Elsanadedy et al. model and ACI 440. The accuracy of each model was evaluated based on the test results of 58 flexural specimens with IC debonding failures of externally bonded (EB), carbon FRP plates or sheets found in the existing literature. The experimental database was prepared to include a wide range of parameters affecting the issue under consideration. A comparison of the measured and predicted load capacity values was made to evaluate the prediction accuracy of the considered models. The analysis included the limitation of the load capacity estimated based on IC debonding models as well as concrete crushing and FRP rupture types of failure. The results indicate that the latest models proposed for direct implementation in design guidelines—the Said and Wu model and the Elsanadedy et al. model—offer the best accuracy in predicting the load capacity. In contrast, the fib bulletin 14 approach shows a wide dispersion of predictions and a large proportion of highly overestimated results.

Generalized Evaluation of Bond Behavior of the Externally Bonded FRP Reinforcement to Concrete

This paper presents a generalized evaluation method for determining the bond slip relationship of an external reinforcement bonded to concrete based on the experimental global force versus the displacement curve. The proposed method also provides several advantages, including predicting the global force versus displacement curve with any bond slip relationship, deriving the bond strength model, predicting the strain distribution along the fiber reinforced polymer (FRP)–concrete interface, and evaluating the effective bond length. An introductory example shows detailed implementation of the proposed method on an FRP–concrete interface reported in the literature. Since no prior assumption is made on the detailed configuration of the bond slip relationship using the proposed method, the obtained bond slip relationship can be seen more generalized and representative. The obtained bond slip relationship differs from the available bond slip models and leads to predictions that best approximate the experimental results. In addition, the reliability of the proposed method in determining the effective bond length is also verified against the experimental and analytical results.

End cover separation in RC beams strengthened in flexure with bonded FRP reinforcement: simplified finite element approach

The flexural performance of RC beams can be improved using externally bonded (EB) or near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. One of the commonly observed failure modes of such FRP-strengthened RC beams is the end cover separation failure mode which involves the detachment of the bonded FRP reinforcement together with the cover concrete along the level of the longitudinal steel reinforcement. This paper presents a simplified 2-D (plane stress) nonlinear finite element (FE) approach for end cover separation failure in RC beams strengthened in flexure with either EB or NSM FRP reinforcement. In this simplified FE approach, only the part of the RC beam between the two cracks near the critical end of the FRP reinforcement is considered. This approach leads to the determination of the FRP debonding strain at the cracked section in the strengthened region adjacent to the crack at the end of the FRP reinforcement, which can then be used to determine the moment acting on this cracked section at debonding failure through conventional section analysis. The validity of the proposed simplified FE approach is demonstrated through comparisons with results from the full 2-D (plane stress) nonlinear FE approach and laboratory tests.

Long-term behavior of wide shallow RC beams strengthened with externally bonded CFRP plates

The use of fiber reinforced polymer (FRP) for flexural strengthening by adhesively bonding to the tension soffit of reinforced concrete (RC) beams has become a popular retrofit technique. Extensive research work is available in the literature on the short term behavior of RC beams strengthened with externally bonded FRP reinforcement. On the other hand, the long term behavior of FRP-strengthened beams has not yet been fully explored. This paper investigates the long term deflection and cracking characteristics of wide shallow RC beams strengthened with carbon FRP (CFRP) plates. Five full scale wide shallow beams were constructed: two beams were tested under static loading and three beams were tested under sustained load for a period of 600days. Three of the beams were strengthened with CFRP plates bonded to the soffit of the beams and two beams were unstrengthened serving as control. The long term beam deflection was compared with the predictions of the effective modulus approach considering two different concrete creep models. Beams strengthened with CFRP plates showed significant improvement in the short term deflection and crack width compared to the unstrengthened beam. The strengthened beams, however, did not show that much improvement in the long term behavior. The effective modulus approach was found to reasonably predict the additional long term deflection of the beams depending on the incorporated concrete creep model. Further, an analytical procedure for predicting the long term crack width of the beams was presented.

Preparation and Characterization of Steel Surfaces for Adhesive Bonding

AbstractIn fiber-reinforced polymer (FRP) strengthened steel structures, debonding of the bonded FRP reinforcement from the steel substrate may result from adhesion failure at the steel/adhesive interface or the FRP/adhesive interface, cohesion failure in the adhesive, or a combination of these two modes. Of these failure modes, cohesion failure in the adhesive is the preferred mode of failure as it facilitates the development of a design theory based on the adhesive properties; the other two failure modes should be avoided if at all possible. This paper presents a systematic experimental study to identify a surface-adhesive combination that will avoid adhesion failure at the steel/adhesive interface. Different steel surface preparation methods, including solvent cleaning, hand grinding, and grit blasting, and different commonly used adhesives were examined in the study. Surface characterization using three key parameters (namely surface energy, surface chemical composition, and surface roughness and topo...

Analytical study on RC beams strengthened for flexure with externally bonded FRP reinforcement

This paper presents analytical study on reinforced concrete (RC) beams strengthened for flexure with externally bonded fiber reinforced polymer (FRP) reinforcement. A simple yet rational numerical model is developed and proposed for this purpose. The model is based on cross-sectional analysis satisfying strain compatibility and equilibrium conditions. The moment–curvature relationship can be generated for an RC beam section using an incremental strain technique. The model can also generate the load–deflection relationship of the beam with respect to its configuration, loading system, and preloading conditions. The model can predict the flexural capacity of FRP-strengthened section based on full composite action and IC debonding failure modes. This also allows for designing the FRP strengthening area according to the desired failure mode. Various IC debonding criteria were adopted in the model and compared with test results from the literature. The result of comparison indicated that the accuracy of the model is dependent on the adopted IC debonding criterion. Furthermore, the model was verified against test data related to full composite action failure mode and good agreement was found.

FRP-to-concrete interface debonding: Experimental calibration of a capacity model

Externally Bonded FRP Reinforcement (EBR) has become a popular technique for Reinforced Concrete (RC) structures strengthening. Several theoretical models, based on the fracture mechanics, have been developed by researchers in the last years to estimate the debonding load of Fiber Reinforced Polymer (FRP) reinforcement applied on concrete surface. Theoretical fracture energy relationships generally include parameters calibrated with experimental data obtained by FRP-concrete bond test. The main available experimental FRP-concrete bond tests show that the most used test procedure is the Single Shear Test (SST) and the largest part of tests involved FRP sheets rather than plates. The results of 34 SST tests carried out by authors on both CFRP sheets (18) and plates (16) are reported and discussed in the present paper; a comparison between plates and sheets reinforcements in terms of debonding load is also given. Based on an enlarged database of FRP-concrete bond tests, a refined calibration of the experimental coefficient to be used in the design equations provided by CNR-DT200/2004 to compute fracture energy for EBR FRP strengthening has been performed; the calibration has been separately performed for sheets and plates in order to specifically take into account the influence of the type of reinforcement on fracture energy. Finally a comparison between experimental results and theoretical predictions provided by the re-calibrated CNR-DT200/2004 equation, fib bulletin 14 and ACI 440.2R-08 is reported.

Constitutive Model for Time-Dependent Behavior of FRP–Concrete Interface

A fundamental understanding of fiber-reinforced polymer (FRP) laminate bonding behavior, including bond strength and effective bonding length, is of primary importance for the development of design guidelines and codes for concrete structures strengthened with externally bonded FRP reinforcement as a bond-critical application. However, the long-term serviceability of such FRP-strengthened structures is still a concern due to a lack of both long-term performance data and a suitable model to represent these performances. This study aims at presenting a viscoelastic model describing the time-dependent behavior of the FRP–concrete interface. The proposed model has been calibrated using strain measurements of the designed specimen for the experimental investigation of the time-dependent behavior of the FRP–concrete interface, including the development of the effective bonding length. Afterward, the proposed model satisfactorily predicts the time-dependent bonding length of the FRP sheet in comparison with the ...

Fatigue Behavior of Carbon Fiber Reinforced Polymer-Strengthened Reinforced Concrete Bridge Girders

This study examines the effects of one-dimensional fiber-reinforced polymer (FRP) composite rehabilitation systems on the flexural fatigue performance of reinforced concrete bridge girders. Eight 508 mm deep and 5.6 m long reinforced concrete T-beams, with and without bonded FRP reinforcement on their tensile surfaces, were tested with a concentrated load at midspan under constant amplitude cyclic loading. The objective of this investigation is to establish the effect that these repair systems have on the fatigue behavior and remaining life of the girders. Results indicate that the fatigue behavior of such retrofit beams is controlled by the fatigue behavior of the reinforcing steel. The fatigue life of a reinforced concrete beam can be increased by the application of an FRP retrofit, which relieves some of the stress carried by the steel. The observed increase in fatigue life, however, is limited by the quality of the bond between the carbon FRP and concrete substrate. Debonding, initiating at midspan and progressing to a support, is common and is driven partially by the crack distribution and shear deformations of the beam.

Near-Surface-Mounted Fiber-Reinforced Polymer Reinforcements for Flexural Strengthening of Concrete Structures

Use of fiber-reinforced polymer (FRP) materials to strengthen bridges has been adopted extensively in the last decade. FRP has been used in different configurations and techniques to use the material effectively and to ensure long service life of the selected system. One of these innovative strengthening techniques is near-surface mounting (NSM) that consists of placing FRP reinforcing bars or strips into grooves precut into the concrete cover in the tension region of the strengthened concrete member. This method is relatively simple and considerably enhances the bond of the mounted FRP reinforcements, thereby using the material more effectively. This paper presents test results of reinforced concrete (RC) T-beams strengthened in flexure with different strengthening systems using FRP reinforcing bars and strips as NSM reinforcement and externally bonded FRP strips. The FRP reinforcements used in this investigation include carbon FRP (CFRP) reinforcing bars and strips and glass FRP (GFRP) thermoplastic strips. Behavior and effectiveness of the materials used for the various strengthening systems are compared. The structural performance and modes of failure of the tested beams are presented and discussed. Test results indicated that using NSM FRP reinforcing bars and strips is practical, significantly improves the stiffness, and increases the flexural capacity of RC beams. The limitations of using NSM FRP reinforcing bars and strips are controlled by serviceability requirements in terms of overall deflections and crack widths rather than delamination, observed by many researchers, of externally bonded FRP reinforcement. Strengthening of RC beams using NSM FRP strips provided higher strength capacity than externally bonded FRP strips using the same material with the same axial stiffness.