FRP-reinforced Concrete Members Research Articles

FRP RC beams by collected test data: Comparison with design standard, parameter sensitivity, and reliability analyses

In the last few years, there has been a significant increase in the utilization of Fiber Reinforced Polymer (FRP) materials as reinforcing elements in concrete structures due to their excellent properties. Unlike traditional steel rebars, FRP rebars do not rust, which helps to prevent degradation and deterioration of the concrete structure over time. This growth has resulted in a rise in the application of design regulations for FRP-reinforced concrete (RC) members. There are currently no European standards that offer suggestions about FRP RC structures. This paper aims to assess the load- carrying capacity and deflection of FRP RC beams with large number of test data available against design standards. The results are compared with ACI 440.1 R-06 specifications and EC2 concepts available in fib Bulletin No. 40. It was found that both ACI and EC2 underestimate the shear flexural capacity. Both design codes presented 38% and 62% of the collected data that overestimated and underestimated the calculation of the deflection, respectively. A parameter influence analysis is performed considering the database collected, and a reliability analysis based on Annex D EN 1990 (2002) is conducted. The reliability analysis allowed suggestion new partial safety factors values of 1.45 and 1.65 for moment and shear capacities, respectively, which can be used by design engineering communities.

Shear and flexure of FRP-reinforced concrete beams and slabs – A review

The article reviews structural response of flexural members reinforced with fiber-reinforced polymer (FRP) as a sustainable alternative to conventional carbon steel. It overviews experimental studies and code-based models that characterize shear and flexural response of concrete beams and slabs reinforced with FRP bars. Discussion on lower resistance to fire being key limitation of FRP bars is also included. The most commonly used fibers to manufacture FRP bars include carbon, glass, basalt, aramid, or combinations of these materials. Consequently, each material plays its role in contributing to shear and flexure differently. Basalt FRP are characterized by higher tensile capacity and resistance to corrosion, carbon FRP bars can withstand exposure to harsh environments; glass FRP bars possess lower modulus of elasticity that may cause large deflection and wider cracks. FRP bars are generally preferred over carbon steel for use in concrete elements where exposure to chlorides or chemicals is expected, or places where non-magnetic properties are essential. Beams and slabs reinforced with FRP bars are likely to undergo larger deflections due to lower stiffness of FRP bars. The dominant failure mode of FRP-reinforced concrete flexural members is discussed in comparison to conventional steel-reinforced members. The article identifies increasing interest and research in using green concrete reinforced with FRP bars that helps to make construction serviceable and sustainable. However, premature de-bonding of bars remains a concern with more research needed to avoid deterioration of mechanical properties by underutilization of materials. Although concrete is practically fire-resistant, FRP is likely to undergo damage of resin due to the presence of combustible organic polymers. The bulk of research implies that the thermal instability of the resin in FRP reinforcing bars is not a disadvantage that outweighs the possible benefits of its usage in the construction industry.

Comparison of the Prediction of Effective Moment of Inertia of FRP Rebar-Reinforced Concrete by an Optimization Algorithm.

FRP (fiber-reinforced polymer)-reinforced concrete members have larger deflection than reinforced concrete members because of the low modulus of elasticity of the FRP bar. In this paper, we proposed a new effective moment of inertia equation to predict the deflection of FRP-reinforced concrete members based on the harmony search algorithm. The harmony search algorithm is used to optimize a function that minimizes the error between the deflection value of the experimental result and the deflection value expected from the specimen's specifications. In the experimental part, four GFRP (Glass Fiber-Reinforced Polymer)- and BFRP (Basalt Fiber-Reinforced Polymer)-reinforced concrete slab specimens were manufactured and tested. FRP-reinforced concrete slabs were reinforced with GFRP and BFRP rebars on spiral rib surfaces. The effects of the FRP reinforcement ratio and balanced reinforcement ratio (ρf/ρfb), the moment of inertia of the transformed cracked section and the gross moment of inertia (Icr/Ig), and the cracking moment and the maximum service load moment (Mcr/Ma) on the effective moment of inertia have been considered. The experimental results and predicted results of the flexural testing of concrete slabs reinforced with FRP rebars were compared, and the experimental results were in good agreement with the calculated values using the proposed effective moment of inertia equation.

FAI: Fast, accurate, and intelligent approach and prediction tool for flexural capacity of FRP-RC beams based on super-learner machine learning model

Fiber-reinforced polymer (FRP) composites have recently been considered in the field of structural engineering as one of the best alternatives to conventional steel reinforcement due to their high tensile strength, lightweight, cost-effectiveness, and superior corrosion resistance. However, the variation in FRP physical and mechanical characteristics among the different FRP types and manufacturers makes it difficult to predict the strength of FRP-reinforced concrete (RC) members. For that reason, an efficient prediction tool was developed for a fast, accurate, and intelligent (FAI) prediction of the flexural capacity of FRP-RC beams based on the result of an optimized super-learner machine learning (ML) model. A database of the experimental results on the flexural strength of FRP-RC beams was compiled and randomly split into 80% training and 20% test sets. Six factors were considered in the model; namely, width and effective depth of the beam, concrete compressive strength, FRP flexural reinforcement ratio, FRP modulus of elasticity, and FRP ultimate tensile strength. Grid search is combined with a 10-fold cross-validation to optimize the hyperparameters of the ML models. The prediction capability of the proposed super-learner ML model was benchmarked against boosting- and tree-based ML models, such as classification and regression trees, adaptive boosting, gradient boosted decision trees, and extreme gradient boosting. Moreover, a comparison with the existing code and guideline equations showed that the proposed super-learner ML model provided the most desirable prediction of the flexural capacity of FRP-RC beams.

Composite FRP reinforced concrete members with fiber reinforced polymer spirals

There is a limited design guidance for practitioners regarding the shear design equations of circular-concrete members with fiber reinforced polymer (FRP) reinforcements, especially the case of incorporating FRP bars and spirals. The objective of this study is to enhance the methods used for estimating the shear capacity of reinforced concrete (RC) beams with FRP bars and spirals. A database of 44 circular concrete beams reinforced with FRP bars tested under shear were compiled from different experimental studies. Two proposed models based on approaches developed for rectangular sections, to predict the shear capacity of beams without and with stirrups, were compared with the selected available shear design equations. The shear strength estimated using the proposed models are slightly more accurate. The comparison indicates that the shear capacity of FRP-reinforced concrete members with circular cross sections may be determined with the Vcfand Vsf approaches developed for rectangular sections provided that certain modifications are made to take into account the effective shear depth, equivalent breadth, the mechanical properties and geometry of GFRP spirals.

One-Way Shear Strength of FRP–Reinforced Concrete Members Without Stirrups: Design Provision Review

Abstract The American Concrete Institute recently updated their one-way shear provisions for steel–reinforced concrete (RC), and it is time to consider updating provisions for one-way shear in fibe...

Bond strength and bond stress-slip analysis of FRP bar to concrete incorporating environmental durability

Interfacial bond between fibre-reinforced polymer (FRP) composite bars and the surrounding concrete is an important factor in the composite performance of FRP-reinforced concrete beams under load. When exposed to harsh environmental conditions the interfacial bond has been shown in numerous small-scale experimental studies to significantly deteriorate. It is therefore essential that the magnitude of the deterioration is quantified in order for the long-term performance of FRP-reinforced concrete (FRP-RC) members to be determined. In this study, a database consisting of 544 observations of bond strength test results as well as a sub-set database of 315 observations of bond stress-slip results is compiled and statistically analysed in order to quantify reduction factors for both the bond strength and the bond stress-slip behaviour of FRP-RC members. The outcomes of this analysis are further applied to investigate the deterioration required to cause a change in failure mode from bar rupture to bar debonding in order for the deterioration of reinforcement to be considered in reinforcement detailing requirements.

Analytical method for derivation of stress block parameters for flexural design of FRP reinforced concrete members

This work derives stress block parameters for fibre-reinforced polymer (FRP) reinforced concrete (RC) members by applying the newly developed RC flexural theorems without doing structural tests. The equivalent rectangular stress block method is widely used for flexural design of RC members. Conventionally, stress block parameters are obtained from extensive flexural tests of RC members. Parameters of the stress block are closely related to material properties. Therefore, new stress block parameters are required when new materials, such as FRP, are used in construction. Structural testing is expensive, and test data is often insufficient for derivation of stress block parameters when a new material is applied in practice. The derived stress block parameters from this work and relevant RC flexural strength are compared with experimental results as well as those recommended by ACI 440.1R-15 and CSA-S806-12. More accurate results are obtained using the proposed parameters compared with those calculated from existing design codes. The method can also result in significant savings in FRP material due to more accurate calculation. The proposed approach facilitates more accurate yet convenient design for flexural strength of FRP-reinforced concrete members by engineers using the conventional stress block approach.

Experimental study on seismic behaviours of hybrid FRP–steel-reinforced ECC–concrete composite columns

Owing to the high strength, noncorrosive property, and linear stress–strain relationship of fibre-reinforced polymers (FRPs), hybrid FRP–steel-reinforced concrete members exhibit higher strength, superior durability, and lesser residual deformation compared to steel-reinforced concrete members and higher ductility compared to FRP-reinforced concrete members. Hybrid-reinforced members have been widely used as flexural-dominated members rather than compression-dominated members owing to the poor compression performance of the FRP reinforcement. This paper proposes a novel approach for improving the seismic behaviour of such members through the use of high-ductility engineered cementitious composites (ECCs) in the plastic hinge zone. Numerous hybrid FRP–steel-reinforced ECC–concrete columns were tested under reversed cyclic loading. The effects of the matrix type, reinforcement type, axial force ratio, and reinforcement ratio on the seismic behaviours, including the failure mode, crack pattern, load-carrying capacity, residual deformation, ductility, and energy-dissipation capacity, of the columns were systematically investigated. The replacement of concrete with ECC in the plastic hinge zone efficiently eliminated the local buckling of the FRP bars and significantly improved the seismic performance of the column. Compared with the steel-reinforced ECC–concrete composite column, the hybrid-reinforced composite column exhibited significantly less residual deformation and higher post-yielding stiffness. With an increase in the axial force ratio, the ultimate strength increased, while the deformation capacity decreased.

Prediction of shear strength of FRP-reinforced concrete members using a rule-based method

Due to sudden and brittle shear failure of concrete members reinforced with fibre-reinforced polymer (FRP), shear design of these members is necessary. Various design equations have been developed to determine the shear strength with and without stirrup members. However, there is still no clear expression to predict the shear strength of FRP-reinforced concrete and the available design formulas have limited accuracy. Recently, soft computing methods such as artificial neural networks have been used for predicting the shear strength of FRP-reinforced concrete elements. However, these methods do not give enough insight into the generated models and are not as easy to use as the empirical formulas. In this study, new formulas based on M5′ and multivariate adaptive regression splines (MARS) model tree approaches for the prediction of shear strength are presented. In order to develop new models, a comprehensive database containing 176 and 112 test data for members with and without stirrups, respectively, is used. It is shown that the proposed models are compact, simple and physically sound. The most important parameters are specified based on sensitivity analysis, which is calculated using the MARS algorithm. Comparison between the developed and shear design formulas showed that the developed models are more accurate than existing equations.

Behaviour of Hybrid Steel and FRP-Reinforced Concrete-ECC Composite Columns under Reversed Cyclic Loading.

Fibre-reinforced polymer (FRP) is used widely in concrete structures owing to its noncorrosive, light-weight, nonmagnetic, and high tensile-strength properties. However, the FRP-reinforced concrete flexural member exhibits low ductility owing to the linear–elastic property of FRP reinforcement. Hybrid steel—FRP-reinforced concrete members exhibit good strength and ductility under flexure owing to the inelastic deformation of steel reinforcement. The existing investigations have focused on the mechanical behaviours of the hybrid steel—FRP-reinforced flexural members. Only few studies have been reported on the members under combined flexural and compression loads, such as columns, owing to the poor compressive behaviour of FRP bars. We herein propose a new type of hybrid steel—FRP-reinforced concrete—engineered cementitious composite (ECC) composite column with ECC applied to the plastic hinge region and tested it under reversed cyclic loading. The hybrid steel—FRP-reinforced concrete column was also tested for comparison. The influence of matrix type in the plastic hinge region on the failure mode, crack pattern, ultimate strength, ductility, and energy dissipation capacity, of the columns were evaluated systematically. We found that the substitution of concrete with ECC in the plastic hinge zone can prevent the local buckling of FRP bars efficiently, and subsequently improve the strength and ductility of the column substantially.

Predicting Shear Capacity of FRP-Reinforced Concrete Beams without Stirrups by Artificial Neural Networks, Gene Expression Programming, and Regression Analysis

The shear strength prediction of fiber-reinforced polymer- (FRP-) reinforced concrete beams is one of the most complicated issues in structural engineering applications. Developing accurate and reliable prediction models is necessary and cost saving. This paper proposes three new prediction models, utilizing artificial neural networks (ANNs) and gene expression programming (GEP), as a recently developed artificial intelligent techniques, and nonlinear regression analysis (NLR) as a conventional technique. For this purpose, a large database including 269 shear test results of FRP-reinforced concrete members was collected from the literature. The performance of the proposed models is compared with a large number of available codes and previously proposed equations. The comparative statistical analysis confirmed that the ANNs, GEP, and NLR models, in sequence, showed excellent performance, great efficiency, and high level of accuracy over all other existing models. The ANNs model, and to a lower level the GEP model, showed the superiority in accuracy and efficiency, while the NLR model showed that it is simple, rational, and yet accurate. Additionally, the parametric study indicated that the ANNs model defines accurately the interaction of all parameters on shear capacity prediction and have a great ability to predict the actual response of each parameter in spite of its complexity and fluctuation nature.

GMDH-based prediction of shear strength of FRP-RC beams with and without stirrups

In the present study, group method of data handling networks (GMDH) are adopted and evaluated for shear strength prediction of both FRP-reinforced concrete members with and without stirrups. Input parameters considered for the GMDH are altogether 12 influential geometrical and mechanical parameters. Two available and very recently collected comprehensive datasets containing 112 and 175 data samples are used to develop new models for two cases with and without shear reinforcement, respectively. The proposed GMDH models are compared with several codes of practice. An artificial neural network (ANN) model and an ANFIS based model are also developed using the same databases to further assessment of GMDH. The accuracy of the developed models is evaluated by statistical error parameters. The results show that the GMDH outperforms other models and successfully can be used as a practical and effective tool for shear strength prediction of members without stirrups (R2=0.94) and with stirrups (R2=0.95). Furthermore, the relative importance and influence of input parameters in the prediction of shear capacity of reinforced concrete members are evaluated through parametric and sensitivity analyses.

Strength and Behavior of Circular FRP-Reinforced Concrete Sections without Web Reinforcement in Shear

AbstractConsiderable experimental research work has been conducted to quantify the shear strength Vcf of rectangular concrete members reinforced with fiber-reinforced polymer (FRP) bars. In contrast, no research seems to have investigated circular concrete members reinforced with FRP bars under shear loads. This paper reports on a study on the behavior and shear strength of full-scale circular concrete beams reinforced with longitudinal FRP bars. The beams, which measured 3,000 mm in length by 500 mm in diameter, were tested under four-point bending. The test parameters included the longitudinal reinforcement ratio and the modulus of elasticity of the reinforcing bars. Steel, glass-FRP, and carbon-FRP bars were used in the experimental program. The experimental results were compared with current codes and design guidelines as well as to recently developed shear design equations appearing in the literature. The comparison indicates that the shear capacity of FRP-reinforced concrete members with circular cr...

Shear Behavior of Circular Concrete Members Reinforced with GFRP Bars and Spirals at Shear Span-to-Depth Ratios between 1.5 and 3.0

In the last decade, the shear strength of concrete members with rectangular cross sections reinforced with fiber-reinforced polymers (FRPs) has received considerable attention. Yet no research seems to have investigated circular concrete members reinforced with FRP reinforcement under shear loads. This paper presents the results of an investigation of the shear strength and behavior of six circular concrete specimens reinforced with glass-FRP (GFRP) bars and spirals. The specimens, which measured 3,000 mm in length by 500 mm in diameter, were tested under four-point bending. The test parameters included the shear span-to-depth ratio (a/d) ranging from 1.5 to 3.0 and the GFRP spiral reinforcement ratio with different spiral spacings (100, 150, and 200 mm) and spiral diameters (13 and 15 mm). As designed, the specimens failed in shear due to GFRP spiral rupture or flexural-shear failure for the specimens with a/d>2.5 and strut crushing combined with spiral rupture for the specimens with a/d<2.5. The experimental results were compared to the current sectional models and the strut-and-tie model in codes and design guidelines as well as to the available analytical approach, which is based on the modified compression field theory. The comparison indicates that the shear capacity of FRP-reinforced concrete members with circular cross sections may be determined with the shear design provisions developed for rectangular sections within a variable degree of conservativeness.

Bond-dependent coefficient of glass- and carbon-FRP bars in normal- and high-strength concretes

The design of concrete members reinforced with fiber-reinforced-polymer (FRP) bars is typically governed by serviceability state rather than ultimate state. This necessitates verifying the crack width in FRP-reinforced concrete (FRP-RC) members at service load. Recent developments in the FRP industry led to the introduction of FRP bars with different surface configurations and mechanical properties, which are expected to affect their bond performance. In the absence of test data, the design codes and guides, however, recommend bond-dependent coefficient (kb) values considering the surface configurations. Thus, this study aims at investigating the kb values and verifying the dependency of the kb values on FRP bar type (glass [GFRP] and carbon [CFRP]), diameter, and concrete type, and strength limited to the first critical flexural-crack width. The investigation included 16 beams measuring 4250mm long×200mm wide×400mm deep. The beams were reinforced with sand-coated GFRP bars, helically-grooved GFRP bars, and sand-coated CFRP bars and were fabricated with normal- and high-strength concretes (NSC and HSC). The measured first critical-crack widths and measured strains were used to assess the current kb values recommended in FRP design codes and guides. The findings did not support using the same kb value for FRP bars of different types (carbon and glass) with similar surface configurations.

Consideration of creep coefficient of RC beam reinforced by GFRP rebar with minimum 900 MPa guaranteed tensile strength

Since FRP rebars developed up to present are produced without specific guidelines on their content ratio, fiber type, and surface shape, the performance capacity of FRP reinforced concrete (RC) members cannot be fully guaranteed even though the concrete members were designed according to the existing design guidelines. Moreover, since investigations of FRP RC members have focused mainly on their short-term behavior, knowledge of their long-term behavior (i.e., creep and shrinkage effects) of FRP RC members is greatly needed for a proper design of the members. In order to verify material and structural creep behavior of a deformed glass FRP (GFRP) rebar with a guaranteed tensile strength of minimum 900 MPa manufactured from modified braidtrusion process, long-term material and structural creep tests of the GFRP rebar and GFRP RC beam, respectively, were conducted. The material creep test result showed that a service life of 100 years can be guaranteed when the sustained load magnitude is less than 50 % of the maximum tensile load capacity of the rebar. The structural creep test results showed that current design guidelines either overestimate or underestimate the long-term deflection of GFRP RC members. Therefore, a coefficient accounting for the time-dependent characteristics must be properly selected.

Torsional Moment Capacity and Failure Mode Mechanisms of Concrete Beams Reinforced with Carbon FRP Bars and Stirrups

AbstractFiber-reinforced-polymer (FRP) bars and stirrups have emerged as internal flexural and shear reinforcement for reinforced-concrete (RC) members in different applications. Nonetheless, the torsional behavior of FRP RC members has not yet been defined. This paper presents the results of an investigation of the torsional strength and behavior of full-scale concrete beams reinforced with carbon-FRP (CFRP) bars and stirrups. The beams measured 4,000 mm long, 250 mm wide, and 600 mm deep and were tested under pure torsion loading. The test specimens included four beams reinforced with CFRP bars and stirrups and one control beam reinforced with conventional steel reinforcement. The test variables were the type of reinforcement and CFRP stirrup spacing. The test results indicated that the CFRP RC beam exhibited similar strength, cracking behavior, and post-peak torsional stiffness compared with the counterpart steel RC beam. The hollow-tube, space-truss analogy with the 45° inclination of diagonal compres...

Prediction of shear strength of FRP-reinforced concrete flexural members without stirrups using artificial neural networks

A theoretical model based on an artificial neural network (ANN) was presented for predicting shear strength of slender fiber reinforced polymer (FRP) reinforced concrete flexural members without stirrups. The model takes into account the effects of the effective depth, shear span-to-depth ratio, modulus of elasticity and ratio of the FRP flexural reinforcement and compressive concrete strength on shear strength. Comparisons between the predicted values and 106 test data showed that the developed ANN model resulted in improved statistical parameters with better accuracy than other existing equations. From the 2k experiment, the influence of parameters was identified in the order of effective depth, axial rigidity of FRP flexural reinforcement, shear span-to-depth ratio and compressive concrete strength. Using the ANN model and based on the results of the 2k experiment, predictive formulas for shear strength of slender FRP-reinforced concrete beam without stirrups were developed for practical applications. These formulas were able to predict the shear strength better than other existing equations.

Estimation of the crack width and deformation of FRP-reinforced concrete flexural members with and without transverse shear reinforcement

As the general acceptance of FRP reinforcing bars in the concrete construction industry continues to rise, it is increasingly imperative that the structural behavior of FRP-reinforced concrete members be predicted with good accuracy. The current crack width equation used in North American design codes for FRP-reinforced concrete members assumes a constant value of the bond coefficient which results in inconsistent accuracy of crack width predictions depending on the stress level in the reinforcement relative to the stress at which the bond coefficient was calibrated. Meanwhile, effective moment of inertia equations which have been shown to be reasonably accurate for FRP-reinforced concrete members with transverse shear reinforcement do not account for the additional deformation which may result from the vertical displacement of inclined cracks in members without shear reinforcement. Simple modifications are proposed to the current deflection and crack width equations to improve their accuracy at all reinforcement stress levels within the service range and are compared to the experimental results of four full-scale FRP-reinforced concrete slabs tested in flexure.