Fiber-reinforced Polymer Contribution 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.

Shear strengthening of RC beams with NSM FRP. II: Assessment of strength models

The application of near-surface mounted (NSM) fiber-reinforced polymer (FRP) for shear strengthening of RC beams has attracted abundant research in the past few decades, and a number of strength models for predicting the contribution of NSM FRPs to the shear capacity of strengthened beam have been proposed. In the first of these two companion papers, a total of 12 strength models of NSM FRP shear strengthened RC beams collected from the existing literature have been comprehensively reviewed, while the present paper aims to provide an objective assessment of these models. To this end, the experimental results of 196 RC beams strengthened in shear with NSM FRP bars/strips are collected from the existing literature, as the database for the assessment. The comparisons between the experimental results and predictions by these strength models reveal that the existing strength models all fail to give accurate predictions. The reasons for the inaccurate predictions of these models are also analyzed.

Shear strengthening of RC beams with NSM FRP. I: Review of strength models

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) technique is known as a promising alternative to the externally bonded (EB) FRP technique for strengthening reinforced concrete (RC) members, on account of its many advantages such as high bond efficiency with concrete and good durability. Shear strengthening of RC beams by near-surface mounting the FRP bars/strips into the concrete cover on beam sides is one of the most prevalent applications of NSM FRP. Abundant experimental studies have been conducted to investigate the behaviour of NSM FRP shear strengthened beams, and many strength models for predicting the contribution of NSM FRPs to the shear capacity of the beam have been proposed. The present paper presents a comprehensive review of these strength models, in which all the models collected from the existing literature are first classified into three categories based on their used approach and then summarized and discussed. This paper not only aims to bring a deep understanding of the existing strength models for NSM FRP shear strengthened RC beams, but also provides a background and basis for the assessment of these models by using the newly-generated experimental database in the companion paper.

Shear Strength of Externally U-Bonded Carbon Fiber-Reinforced Polymer High-Strength Reinforced Concrete.

In this paper, we investigate the contribution of Fiber-Reinforced Polymer (FRP) to the load-carrying capacity of shear-strengthened Reinforced Concrete (RC) beams. Specifically, the investigation is focused on the FRP’s contribution in the presence and absence of shear stirrups. To this end, two sets of full-scale RC beam specimens were tested to failure in a simply supported setup. Set 1 consisted of specimens without shear stirrups whereas Set 2 included steel stirrups spaced at 170 mm. One and two layers of FRP discrete strips were bonded to the beams in a U-jacketing configuration. To investigate the contribution of FRP and its interaction with the stirrups, two different locations were considered when bonding the FRP strips: between the stirrups (referred to as Off-beams) and at the same level of the stirrups (referred to as On). Results of the experimental program showed that strengthening the beams with two layers of FRP does not necessarily translate to improved capacity. Furthermore, the location of FRP strips with respect to the location of shear stirrups has an influence on the beam’s overall behavior, especially its displacement ductility. This is an important parameter to consider to avoid premature failure of RC members. Test results were then used to assess the performance and accuracy of the predictions of ACI PRC-440.2-17 and fib-TG9.3. Both design codes were found to be conservative with an average prediction-to-test ratio of 0.7.

CFRP ropes for retrofitting heat-damaged concrete beams

The contribution of fiber reinforced polymer (CFRP) ropes for recovering mechanical performance of post-heated reinforced concrete beams is experimentally investigated. The ropes are implemented at variable profiles (parabolic, trapezoidal, and straight) within the side cover (NSM) or through the core of concrete beams (post-heated for 3 h to 400 and 500 °C). The mechanical response was evaluated under the effect of four-point loading with deflection and elongation measurements (in CFRP ropes) acquired versus loading up till failure. Meanwhile, load cracking formation and propagation until failure were observed and documented. Post-heating performance was also evaluated in terms of cracking severity and degradation in mechanical properties of concrete and reinforcing steel. The repaired beams recovered partially to fully their original load capacity while lost modest portion of their rotational ductility (at 17–37%). The best mechanical performance is attained when using the parabolic followed by the trapezoidal profiles. Higher damage extent had a more negative impact upon repair efficiency, evaluated by the different performance indicators of the study. Except that repaired with a straight profile, all beams showed sudden shear failure upon the rupture of the CFRP ropes near their anchorage ends. • It is advisable to strengthen concrete beams with SNSM CFRP ropes at variable profiles. • The best repair benefit was attained using a parabolic profile of SNSM CFRP ropes. • Anchorage of the CFRP ropes at their ends led to a significant improvement in repair efficiency. • Use of CFRP ropes at variable profiles altered the failure modes for the heat-damaged beams.

Shear Strengthening of Reinforced Concrete T-Beams by Using Fiber-Reinforced Polymer Composites: A Data Analysis

Many studies have compiled experimental results of fiber-reinforced polymer (FRP) shear strengthened reinforced concrete (RC) beams into databases for the investigation of the effect of different parameters on the efficacy of FRP strengthening schemes. However, these studies are mainly comprised of the results of FRP shear strengthened rectangular RC beams. The RC T-beams are stronger in shear than the rectangular RC beams, represent the most realistic situation, and have more significance in the construction industry. However, no study comprising of a large number of results of FRP-strengthened RC T-beams has been found. Therefore, the present study represents the results of more than 250 tests performed on the shear deficient RC T-beams strengthened with FRP and also the essential material characteristics. It also compares the accuracy of seven widely used design guidelines for the prediction of shear contribution of FRP based on the experimental shear contribution of FRP. This study concludes that the efficacy of the FRP strengthening scheme varies with the adopted strengthening schemes, whereas the predictions of different design guidelines observed to be not promising. However, predictions by fib and TR55 guidelines have good correlation and standard deviation values, respectively, with the experimental results than the other guidelines.

A new formulation for prediction of the shear capacity of FRP in strengthened reinforced concrete beams

The use of fiber-reinforced polymer (FRP) to strength the concrete beams is an efficient method in retrofitting of preexisting structures. The application of FRP sheets makes to have higher shear strength, but the common equations in determining the shear strength are no longer effective. In this paper, a new formulation is presented to predict the shear contribution of FRP in strengthened reinforced concrete beams. The formula is produced using the multigene genetic programming (MGP) machine. For this purpose, a set of experimental data is collected from the literature. The shear capacity of FRP in reinforced concrete (RC) beams is considered as the output data, while other variables are considered as the input data. MGP is trained with the experimental data and a formula is produced. The results of the proposed formula are compared with the experimental data to show the ability of the proposed formula. Also, these results are compared with those obtained from the available formulas, approximation models and published researches. Results show that the proposed formula is able to predict the shear capacity of FRP in strengthened RC beams with a higher precision than the other evaluated methods such as CIDAR, Fib.TG9.3, ACI and CSA. The mean absolute percentage error for the MGP formula was reduced about 74% in comparison with the CIDAR equations. Also, the root-mean-squared-error of the MGP formula was decreased near 71% in comparison with the Fib.TG9.3 equations.

Models Reviewed for Predicting CFRP Shear Contribution of Strengthened Reinforced Concrete Box Beam

Eight various models for calculating the FRP (fiber-reinforced polymer) contribution to shear from different codes and studies were reviewed and discussed. From all the models, it is found that the effective strain is the most important parameter which lies in all equations except JSCE model. Besides, the six models from guidelines and two from latest literature with different parameters were discussed and applied for calculating shear contribution of 181 FRP shear strengthening specimens collected from existing literature. It reveals that all ratios of calculation related to experimental results showed distributions around the bisector in all equations of models. From the values of total members, the predictions of FRP shear contribution of the beam with a T-section or rectangular section using two models (fib Bulletin 14 and Sayed et al.) were very close to the experimental values and also stood safe sides. Furthermore, one control and four strengthened reinforced concrete (RC) box beam by carbon fiber-reinforced polymer (CFRP) were tested for shear strength. Compared with the experimental results, all theoretical values predicted by the reviewed models overestimated the FRP shear contribution of strengthened members, because the effective strain was overestimated. A coefficient of 1.31, therefore, was also proposed, which divided the formula of the effective strain in the fib Bulletin 14 model according to the experimental effective strain in fiber. The model with the modified effective strain presented a rational prediction in CFRP shear contribution of strengthened RC box beams.

Impact Behavior of FRP-Strengthened RC Beams without Stirrups

AbstractThis study investigates the impact behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams and FRP contribution to shear strength. Thirteen RC beams (1,500×150×250 mm) were tested against static and impact loads. The beams were strengthened with FRP U-wraps and 45°-angle wraps to investigate the effectiveness of different wrapping schemes. The experimental results show that the debonding strain of FRP under impact loads is slightly smaller than that under static loads. The FRP contribution to shear strength is discussed and verified against a design code. In terms of using the same amount of FRP, 45°-angle FRP wraps provide better performance than the FRP U-wraps in terms of both the load capacity and displacement. Fully wrapping a RC beam with FRP is more efficient than wrapping distributed FRP strips. The impact behavior and failure modes of FRP strengthened RC beams are also discussed.

Evaluation of Ultimate Strength of Reinforced Concrete Beams Strengthened with FRP Sheets under Torsion

The ultimate torque of reinforced concrete (RC) members strengthened with fiber reinforced polymer (FRP) sheets does not only depend on the torque of RC members, but also on the FRP contribution to the torque. For structural design, predicting the accurate torsional capacity of the strengthened beams is considerably important. Three existing models for calculating the ultimate torsional moment of RC beams and two existing models for computing the FRP contribution to the ultimate torque are described and combined. Based on an experimental database collected from existing literature, six combinations were discussed and evaluated from the calculative values compared with the experimental results. The comparison shows that the combination of ACI 318 and fib Bulletin 14 models (Group 2), as well as Chinese and Ghobarah models (Group 6), can reasonably and accurately predict the ultimate torque of beams strengthened with FRP sheet. Furthermore, the ultimate torque of six boxsection beams strengthened with fully wrapping or U-wrap calculated by the Group 6 shows closely to the experimental results.

FRP-RC Beam in Shear: Mechanical Model and Assessment Procedure for Pseudo-Ductile Behavior

This work deals with the development of a mechanics-based shear model for reinforced concrete (RC) elements strengthened in shear with fiber-reinforced polymer (FRP) and a design/assessment procedure capable of predicting the failure sequence of resisting elements: the yielding of existing transverse steel ties and the debonding of FRP sheets/strips, while checking the corresponding compressive stress in concrete. The research aims at the definition of an accurate capacity equation, consistent with the requirement of the pseudo-ductile shear behavior of structural elements, that is, transverse steel ties yield before FRP debonding and concrete crushing. For the purpose of validating the proposed model, an extended parametric study and a comparison against experimental results have been conducted: it is proven that the common accepted rule of assuming the shear capacity of RC members strengthened in shear with FRP as the sum of the maximum contribution of both FRP and stirrups can lead to an unsafe overestimation of the shear capacity. This issue has been pointed out by some authors, when comparing experimental shear capacity values with the theoretical ones, but without giving a convincing explanation of that. In this sense, the proposed model represents also a valid instrument to better understand the mechanical behavior of FRP-RC beams in shear and to calculate their actual shear capacity.

An anticipated shear design method for reinforced concrete beams strengthened with anchoraged carbon fiber‐reinforced polymer by using neural network

SUMMARYUsing externally bonded carbon fiber‐reinforced polymer (FRP) for strengthening has been turned into a popular decision owing to its mechanical leads. Consequently, design guidelines and researchers have established several analytical equations to predict the contribution of FRP to ultimate shear capacity. The developed analytical equations projected the influence of FRP reinforcements within certain limits. However, not mentioned parameters such as the shear span‐to‐depth ratio and anchorage application influence the ultimate behavior of strengthened specimens. Accordingly, distant predictions between test results and code predictions are observed for the specimens in whom anchorage is applied. As an alternative method, artificial neural network (NN) can be used to predict the contribution of anchoraged carbon FRP to shear strength of deficient reinforced concrete beams. Accordingly, two NN models with back‐propagation are developed in this study. Unlike the existing design codes, the model considers the effect of anchorage and the shear span‐to‐depth ratio at the ultimate state. Artificial NN model is trained, validated and tested using the literature of 79 reinforced concrete beams. Then, NN results are compared with those ‘theoretical’ predictions calculated directly from International Federation for Structural Concrete, the American guideline (ACI 440.2R) and the Australian guideline. Within all theoretical predictions of design guidelines, fib14 provided the best predictions according to experimental results. Consequently, 25% of fib14 predictions are within ±10% of the experimental results, and also, 65% of the fib14 predictions are within ±25% of the measured values. Besides, executed comparisons indicated that the NN model is more exact than the guideline equations with respect to the experimental results and can be applied effectively within the range of parameters covered in this study. Copyright © 2014 John Wiley & Sons, Ltd.

Experimental investigations on the influence of size on the performance of RC T-beams retrofitted in shear with CFRP fabrics

More than 85% of the available test data, related to the shear strengthening of reinforced concrete (RC) with externally bonded fiber-reinforced polymer (FRP) composites, have been obtained on beams with effective depths less than 450mm. Whether or not the beam size influences the shear resistance of RC members retrofitted in shear with FRP, in particular the resistance contributed by the FRP, is a timely question of great interest. This is because the models forming the basis of current guidelines for calculating the contribution of FRP to shear resistance are based on experimental test data. To investigate this question, beams of two different sizes were tested. In total, 12 tests on RC T-beams strengthened in shear with carbon fiber-reinforced polymer (CFRP) composites were performed. The beams were geometrically similar, and the size range was 1:2. The test results clearly showed that the beam size influences the shear resistance contributed by both concrete and the CFRP, but not the shear resistance contributed by the transverse steel reinforcement. Therefore, the shear resistance of the beams with no transverse steel, retrofitted in shear with FRP, is influenced by the beam size. In the beams with a relatively high transverse steel ratio, the shear capacity is strongly dominated by the transverse steel contribution. Hence, the beams with transverse steel appear to be less influenced by the beam size. However, further research is needed to explain fully, and ultimately provide a reliable model for the beam size effect on the contribution of FRP to shear resistance.

Repair of shear-deficient and sulfate-damaged reinforced concrete beams using FRP composites

Reinforced concrete structures often demonstrate nonstructural cracking caused by the action of sulfates from external sources. These cracks are signs of concrete damage that reduces the flexural and shear capacity of reinforced concrete structural elements and require therefore immediate attention. In this paper, the potential of using advanced composite materials in repairing shear deficient reinforced concrete prototype beams that are damaged by a sulfate cyclic treatment to a predetermined level is investigated. Prototype sulfate-damaged beams were repaired using varying configurations of (carbon and glass) fiber reinforced polymer (FRP) sheets or strips. Experimental data on shear strength capacity, stiffness, deflection, FRP strain, crack opening and mode of failure of the repaired beams were obtained, and compared with those of control, and sulfate-damaged ones. The results confirmed that FRP composites applied to shear spans not only enhanced the shear capacity but also improved deflection characteristics, stiffness, toughness, profitability index, crack resistance, performance factor, and FRP composites strain. However, the effectiveness of the present FRP repair schemes in terms of load capacity was negatively affected by the level of sulfate damage. Finally, a suitable analytical model was developed and examined for the prediction of FRP contribution to shear capacity of sulfate-damaged beams.

Strength Reduction Factor for Flexural RC Members Strengthened with Near-Surface-Mounted Bars

AbstractCurrent American Concrete Institute (ACI) guidelines for the design of flexural RC members strengthened with externally bonded fiber-reinforced polymer (FRP) systems assign an additional partial strength reduction factor to the contribution of FRP, marking a deviation from ACI’s approach in building codes. This conservative method finds its rationale in the novelty and the higher variability of FRP, because of its nature as a material (compared to steel) and the conditions of its installation (externally bonded). Using the case of near-surface-mounted (NSM) FRP bars, this paper demonstrates that a single strength reduction factor can be formulated, while maintaining the same reliability and safety required in conventional RC members. Using a comprehensive test matrix of flexural members processed with a computerized Monte Carlo simulation technique, the probabilistic implications of strengthening RC beams and slabs with NSM FRP bars are investigated. The generated statistical data are employed to ...

Revisiting the shear design equations for concrete beams reinforced with FRP rebar and stirrup

Current design guidelines and codes use modified shear equations for calculating the shear strength contribution of fiber reinforced polymer (FRP) transverse reinforcement (stirrup) in concrete beams reinforced with longitudinal steel or FRP rebar. These equations, originally developed for steel as longitudinal and transverse reinforcement, are semi-empirical in nature and were developed with a core analytical equation where the coefficients were determined from regression analysis. Here, a comparative study among various code equations was conducted to predict the shear strength of FRP reinforced concrete beams. To facilitate this comparison a database was established from the published literature, which was composed of slender concrete beams (shear span to depth ratio, a/d >2.5) with FRP longitudinal and transverse reinforcement. The database contained 114 beams without transverse reinforcement and 46 beams with transverse reinforcement. The guidelines, codes and models that were implemented and compared in this study consisted of ACI 440.1R-06, JSCE 1997, CNR-DT 203, CSA S806-02, CSA S6-06, unpublished CSA S6-06 Addendum, ISIS-M03-01, simplified Modified Compression Field Theory, cracked section analysis model and modified Zsutty equations. It was observed from the statistical analysis that the CSA S806-02 produced greater coefficients of variation than the CSA S6-09. The ACI 440.1R-06 and JSCE 1997 produced more conservative results in calculating the transverse shear strength. The CSA S6-09 Addendum exhibited the best all-around performance in predicting the shear contribution of FRP reinforced beams compared to that of other design codes and guidelines.

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1) the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2) the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3) the effect of FRP strip location with respect to internal transverse-steel location; (4) the effect of FRP strip width; and (5) the effect of internal transverse-steel reinforcement on the CFRP shear contribution.

Predicting the Capacity of RC Beams Strengthened in Shear with Side-Bonded FRP Reinforcements Using Artificial Neural Networks

The application of artificial neural network (ANN) to predict the shear capacity of reinforced concrete (RC) beams retrofitted in shear by means of side-bonded fiber-reinforced polymer (FRP) is investigated in this paper. An extensive literature review has been carried out. In addition, ten shear deficient RC beams with different carbon fiber-reinforced polymer (CFRP) configurations were tested and added as data to the collected data. It was aimed to build an efficient and practical ANN model with parameters which can easily be obtained without any calculation and/or experimental investigation. The results are compared with the design guideline equations that emerge as predictions of the FRP contribution using the trained neural networks: these are in good agreement with the experimental results and better than those calculated from the theoretical guideline equations. Based on ANN results, a parametric study has been carried out to study the importance of different influencing parameters on the FRP contribution. Thereafter, a new simple expression is proposed for determining the contribution of externally bonded side-bonded FRP. Accordingly, the suggested design formula is capable of predicting the experimental FRP satisfactorily so that it can be admitted as an alternative to the existing guideline equations within the range of parameters covered in the study.

Behavior of RC Beams Shear Strengthened with Bonded or Unbonded FRP Wraps

Reinforced concrete (RC) beams shear-strengthened with fiber-reinforced polymer (FRP) fully wrapped around the member usually fail due to rupture of FRP, commonly preceded by gradual debonding of the FRP from the beam sides. To gain a better understanding of the shear resistance mechanism of such beams, particularly the interaction between the FRP, concrete, and internal steel stirrups, nine beams were tested in the present study: three as control specimens, three with bonded FRP full wraps, and three with FRP full wraps left unbonded to the beam sides. The use of unbonded wraps was aimed at a reliable estimation of the FRP contribution to shear resistance of the beam and how bonding affects this contribution. The test results show that the unbonded FRP wraps have a slightly higher shear strength contribution than the bonded FRP wraps, and that for both types of FRP wraps, the strain distributions along the critical shear crack are close to parabolic at the ultimate state. FRP rupture of the strengthened ...

Novel Technique for Inhibiting Buckling of Thin-Walled Steel Structures Using Pultruded Glass FRP Sections

The use of composite materials for strengthening the ailing infrastructure has been steadily gaining acceptance and market share. It can even be stated that this strengthening technique has become main stream in some applications such as strengthening concrete structures. The same cannot be said about steel structures; for which research on composite material strengthening is relatively new. Several challenges face strengthening steel structures using composite materials such as the need for high-modulus composites to improve the effectiveness of the strengthening system. This paper explores a new approach for strengthening steel structures by introducing additional stiffness to buckling-prone regions. The proposed technique relies on improving the out-of-plane stiffness of buckling-prone members by bonding pultruded fiber-reinforced polymer (FRP) sections as opposed to the commonly used approach that relies on in-plane FRP contribution. The paper presents results from an experimental investigation where shear-controlled beam specimens were tested to explore the feasibility of the proposed technique. Bar specimens were also tested in tension to compare between in-plane and out-of-plane contributions of FRP to the behavior and strength of thin steel plates. Based on the results, it can be concluded that this strengthening technique has great potential for altering failure modes by delaying the initiation of undesirable local buckling of thin steel plates. Recommendations for future research efforts are made to expand the knowledge base about this unexplored strengthening technique.