Fiber-reinforced Polymer Reinforcement Research Articles

Flexural behavior of concrete beams hybrid-reinforced with glass fiber-reinforced polymer, carbon fiber-reinforced polymer, and steel rebars

The utilization of fiber-reinforced polymer (FRP) reinforcements in structural design is increasing due to their non-corrosive nature. However, the anisotropic features and linear elastic behavior of FRP rebars have led researchers to explore the use of hybrid combinations of FRP and steel reinforcements. Current codes and guidelines predominantly focus on the design of glass FRP (GFRP) reinforced structural elements, leaving a gap in incorporating the hybrid use of FRP-steel combinations and different types of FRP materials, such as carbon FRP (CFRP). This study conducted experimental investigations on concrete beams reinforced with GFRP, CFRP, and hybrid (GFRP-steel and CFRP-steel in combination) rebars, comparing the results with theoretical models. Ten full-scale beams were tested under monotonic loading. Test results revealed that existing codes overestimate GFRP reinforced beam displacements while underestimating CFRP reinforced beam displacements. A reduction factor is proposed for the effective moment of inertia expression given by ACI 440.11-22 to predict the deflections of CFRP reinforced and hybrid reinforced beams. The experimental data for CFRP and hybrid reinforced concrete beams align well with the predictions calculated using the proposed equations.

Machine learning-based evaluation of punching shear resistance for steel/FRP-RC slabs

With the emergence of fiber-reinforced polymer (FRP) reinforcement as a substitute for conventional steel reinforcement, different design codes have been developed to account for the different mechanical properties of the FRP, specifically the elastic modulus. The ACI 440.11-22, CSA/S806-12, and JSCE-97 are well-known standards for FRP-reinforced concrete structures. In particular, these design standards show significant variations in estimating the punching shear resistance and accounting for the elastic modulus. This study provides a statistical and machine learning-based evaluation of the punching shear models in design standards. An Artificial neural network (ANN) framework is used to develop a generalized punching resistance model of flat slabs reinforced with steel and FRP bars utilizing a large experimental dataset with 539 tests. The study presents a parametric study to examine the effect of different factors on the punching shear strength. The parametric study provided a graphical presentation and comparison between the design models and the developed ANN model.

Structural Behaviour of FRP-Reinforced Tubular T-Joint Subjected to Combined In-Plane Bending and Axial Load

In this study, 90 finite-element models are used to explore the behaviour of fibre-reinforced polymer (FRP) reinforced joints under combined in-plane bending (IPB) and axial load (AX). The effects of joint geometry, FRP layer count, and AX levels of the chord or brace are considered. Three typical failure modes are observed: chord plastic failure, brace plastic failure, and brace buckling failure. Increasing the number of FRP layers can ensure that failure is chord-related failure in a ductility manner rather than the unexpectedly brace-related brittle failure. Depending on the stress distribution of fibres, FRP reinforcement can restrict the deformation of joints subjected to complex loading patterns. Moreover, added FRP layers efficiently reduce the effect of brace AX on the IPB resistance. Finally, a modified strength equation is established, including the influence of FRP reinforcement, chord AX, and brace AX.

Study on the Bonding Performance of BFRP Bars with Seawater Sand Concrete.

A total of 66 sets of pullout specimens were prepared to investigate the bonding properties of basalt fiber-reinforced polymer reinforcement (hereinafter referred to as BFRP) with seawater sand concrete (hereinafter referred to as SSC). The volume dosages of mono-doped glass fibers and mono-doped polypropylene fibers were 0.1%, 0.2%, and 0.3%; the total volume dosage was set to be constant at 0.3%; and the doping ratios of the hybrid fibers were 1:2, 1:1, and 2:1. The effect on the bonding performance of BFRP reinforcement with SSC was studied on the condition of the diameter D of the BFRP reinforcement being 12 mm; the bond length of SSC being 3D, 5D, and 7D; and the surface characteristics of the reinforcement being sandblasted and threaded. The research showed that due to internal cracks in the matrix, salt crystals in the pores, chloride salts with high brittleness and expansion, as well as sulfate corrosion products such as "Frederick salts" in SSC, the concrete became brittle, resulting in more brittle splitting failures during the pullout test. Doped fibers can increase the ductility effect of concrete, but the bonding effect between the threaded fiber reinforcement and the SSC was not as good as that of the sandblasting group. When the bond length was 5D, the bonding effect between the BFRP reinforcement and SSC was the best, and the bonding performance of the experimental group with doped fibers was better than that of the threaded group. Finally, by combining the ascending segment of the Malvar model with the descending segment of the improved BPE model, a constitutive relationship model suitable for the bond-slip curve between BFRP reinforcement and SSC was fitted, which laid a theoretical foundation for future research on SSC.

Effects of eccentric loading on performance of concrete columns reinforced with glass fiber-reinforced polymer bars.

Glass fiber-reinforced polymer (GFRP) reinforcements are superior to traditional steel bars in concrete structures, particularly in vertical elements like columns, and offer significant advantages over conventional steel bars when subjected to axial and eccentric loadings. However, there is limited experimental and numerical research on the behavior of GFRP-reinforced concrete (RC) columns under eccentric loading having different spacing of stirrups. In this study, six specimens were cast under three different values of eccentricities (25mm, 50mm, and 75mm) with two groups of stirrups spacing (50mm and 100mm). The experimental results showed that by increasing the eccentricity value, there was a reduction in the load-carrying capacityof the specimens. The finite element ABAQUS software was used forthe numerical investigation of this study.The results from the finite element analysis (FEA) were close to the experimental results and within the acceptable range. The maximum difference between theexperimental and FEA resultswas 3.61% for the axial load and 12.06% for the deformation.

Experimental and statistical investigations of the material properties of FRP reinforcement in compression

Internationally leading standards currently do not permit to consider the contribution of reinforcement made of fibre reinforced polymers (FRP) to a concrete member’s compressive load-bearing capacity due to a lack of reliable knowledge regarding FRP’s material properties in compression. Thus, there is a very high demand for carefully specifying an optimised test setup for FRP bar reinforcement that accounts for both theoretical considerations and established standards from other industry branches. This paper presents an extensive literature review on mechanical approaches to describe FRP material properties in compression, existing test setups from various industry branches as well as experimental studies that have been conducted in order to research the compressive material properties of FRP reinforcement. Based on experimental results from literature a database was compiled. Following a reasonable choice of a test setup based on the literature study, an experimental campaign was setup, in order to investigate the influence of different fibre materials (carbon, glass, basalt), polymer matrices and bar diameters on the compressive material properties of FRP reinforcement bars. In addition to the experimental determination of the material properties, the joint evaluation of the total sample size of n = 95 own experimental test results and the results from the database generated from literature allows for statistical investigations. Thus, the determination of material scatter and the estimation of the distribution type by means of a statistical analysis applying the Kolmogorov-Smirnov test provides the basis for further research towards the reliability of concrete structures reinforced with FRP bars in compression.

Study on damage characteristics and application of gypsum pillar strengthened with FRP

The instability of mine pillars has posed a serious threat to the safety of mines in China. In order to reduce the risk of pillar instability, this study used fiber-reinforced polymer (FRP) to reinforce gypsum specimens and conduct mechanical loading experiments. The study investigates the reinforcement effect of gypsum specimens with different height-diameter ratios and different FRP wrapping thicknesses, and analyzed the uneven distribution of constraint force along the height direction of FRP-reinforced gypsum specimens. An optimized wrapping method was proposed, and a stress–strain constitutive model for gypsum materials under optimized wrapping was established. By utilizing a statistical method based on strain energy density for heterogeneous rock damage identification, a simulation study was conducted on FRP-reinforced gypsum-like specimens. A comparative analysis was carried out with experimental results to verify the feasibility of the constitutive model. The research findings indicate that the use of FRP reinforcement significantly improves the uniaxial compressive strength and strain values of gypsum-like specimens. Moreover, the smaller the aspect ratio, the more pronounced the effect of FRP reinforcement on gypsum-like specimens. The stress–strain curve of FRP-reinforced gypsum-like specimens shows a distinct combination of curves and straight lines, which can be divided into two distinct stages based on different elastic moduli. The peak value of acoustic emission energy of FRP-reinforced gypsum specimens appears at the junction of the first and second stages of the stress–strain curve, and then the acoustic emission energy curve becomes relatively flat, with an increase in peak energy before failure. The constraint force acting on FRP-reinforced gypsum specimens exhibits an obvious uneven distribution along the height direction, based on which an optimized reinforcement scheme is proposed. Based on the established constitutive model, numerical simulation was conducted using real data from a specific mine. The research findings demonstrate that when the optimized reinforcement method is employed, the load-bearing capacity of the mine pillars was significantly enhanced. The maximum displacement was reduced by 33%, the maximum vertical stress was reduced by 3.2%, and the volume of the plastic zone was reduced by 39.87%.

Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) panel reinforced with FRP bar/grid: Development and flexural performance

In this study, Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) panels with Fiber-Reinforced Polymer (FRP) reinforcement (i.e., FRP bars and girds) were proposed for the construction of sustainable marine structures. Based on four-pointed bending tests, the mechanical performance of FRP-reinforced UHS-ECC [with 2% polyethylene (PE) fibers] and FRP-reinforced ultra-high-strength concrete (UHSC, without fibers) panels were investigated and compared to understand the composite action between UHS-ECC and FRP. Compared with the FRP-reinforced UHSC panel, the FRP-reinforced UHS-ECC panel showed significantly higher ultimate load (139-173% higher), stiffness, and deformation capacity. It was also found that the use of seawater as the raw material had almost no effect on the mechanical performance of FRP-reinforced UHS-ECC (UHSC) panels. An analytical investigation was conducted to predict the load capacities of the tested panels, and the prediction error was acceptable. For FRP-reinforced UHS-ECC (UHSC) panels, it was revealed that using UHS-ECC to replace UHSC improved the stress transfer and deformation compatibility with FRP because the multiple cracking behavior of UHS-ECC lowered the crack-induced shear stress concentration along the FRP reinforcement. The developed FRP-reinforced UHS-ECC system showed great potential in the construction of durable and sustainable marine infrastructure.

The first attempt of utilizing mineral-impregnated carbon fibre reinforcements as load sensors for structural health monitoring

A novel reinforcement of mineral-impregnated carbon fiber (MCF) composite has been introduced to replace traditional steel or fiber-reinforced polymer (FRP) reinforcements, primarily attributed to its high-temperature endurance, cost-effectiveness, and sustainable utilization. Previous studies have investigated the advantages of MCF in terms of its processing and exceptional mechanical performance over a wide temperature range, however, superior inherent electrical conductivity and related piezoresistivity of carbon fibers (CFs) embedded within the matrix have not yet been explored systematically. In this study, we explore the utilization of MCF as highly sensitive stress sensors for structural health monitoring (SHM), including the electrical resistance, piezoresistivity, and underlying mechanisms of MCF composites under cyclic flexural stresses. Additionally, the effect of two different electrode configurations is examined. The findings reveal that the MCF exhibits a significant stress-sensing capacity under cyclic flexural loading within an elastic regime. Specifically, MCF with outer electrodes shows superior electrical conductivity, albeit with less significant fractional changes of resistance (FCR) and stress-sensing sensitivity. Conversely, MCF with inner electrodes exhibits the opposite trend. These outcomes provide valuable insights into the intrinsic self-sensing capability of MCF and propel its potential applications as a multifunctional composite, particularly in fields such as 3D-printed smart sensors and SHM for concrete infrastructures.

Applicability of available NDT methods for damage detection in concrete elements reinforced or strengthened with FRP

The application of Fiber Reinforced Polymer (FRP) materials in concrete structures has been rising due to their several advantages, including lightweight, high tensile strength, ease of installation, and corrosion resistance. They have been mostly implemented for strengthening and repairing existing structures in the form of an externally bonded system, i.e., sheet, jacket, near surface mounted. Furthermore, they have been recently utilized as internal reinforcement of concrete elements in the form of strands, bars, tendons, etc. Although higher durability and performance are associated with the FRP material in some aspects compared to steel, concerns remain regarding damages and defects in this material, many of which are related to their unique features. Importantly, debonding of FRP materials from a concrete surface or within a concrete element has always been an issue resulting in the premature failure of the structure. To this end, concrete elements strengthened or reinforced with FRP materials has to be inspected periodically to detect potential issues and hence prevent any premature failures. This study first determines all possible or potential damages and anomalies attributed to FRP reinforced/strengthened concrete (FRP-RSC) elements. It then investigates Non-Destructive Testing (NDT) methods that can be applicable to the inspection of FRP-RSC elements from a literature survey of past studies, applications, and research projects. Furthermore, this study evaluates the ability of two of the most commonly used NDT methods, Ground Penetrating Radar (GPR) and Phased Array Ultrasonic (PAU), in detecting FRP bars/strands embedded in concrete elements. GPR and PAU tests were performed on two slab specimens reinforced with GFRP (Glass-FRP) bars, the most commonly used FRP bar, with variations in their depth, size and configuration, and a slab specimen with different types of available FRP reinforcements. The results of this study propose the most applicable methods for detecting FRP and their damage/defects in FRP-RSC elements. This study further investigates the feasibility of two new methods for improving the detectability of embedded FRP bars. By providing the inspection community with more clarity in the application of NDT to FRP, this study offers means for verifying the performance and, therefore, help the proliferation of FRP materials in concrete structures.

Numerical Analysis of an Innovative Double-Strap Joint for the Splicing of Near-Surface Mounted Fiber-Reinforced Polymer Bars for Reinforced Concrete Beam Strengthening

The issue of the cut-off splicing of an additional fiber-reinforced polymer (FRP) bar in the near-surface mounted (NSM) technique for reinforced concrete (RC) beam strengthening exposed dominantly to bending is insufficiently investigated. A possible solution of this issue is a new proposed technique: a double-strap joint. It implies the widening of the groove at the cut-off location and the symmetrical installing of additional supplements of FRP reinforcement. In this research, beam strength has been determined for the following cases: additional NSM FRP reinforcement without a cut-off, with a cut-off, and without overlapping, and with different lengths of splice overlapping. A nonlinear analysis based on the finite element method (FEM) has been applied. The length of the cut-off splice of the additional FRP reinforcement with glass fibers (GFRP) was 20Ø, 40Ø, and 60Ø. The validation of the numerical model and a comparison of the results were conducted by using the authors’ experiments. It has been shown that, in the case of a cut-off of NSM GFRP bars, a significant loss in strengthening efficiency occurs, and that, with an increase in the overlapping length, this loss decreases. An overlapping length of 60Ø provides full strengthening. An efficiency assessment was carried out via the use of a parametric study, varying the FRP bar material type and its diameter for a constant splicing length.

Analysis of the transfer, flexural bond and anchorage lengths of pretensioned FRP reinforcement based on Eurocode

Current design codes in North America have guidelines for assessing the anchorage zone of pretensioned fiber reinforced polymer (FRP) reinforcement. However, there is no comparison of the Eurocode approach with experimental data or suggestions for its application for prestressed FRP reinforcement. Therefore, the main objective of this article is to provide a comparison of available data in the literature on the transfer, flexural bond, and anchorage length of different types of FRP reinforcement with Eurocode and to provide insight on the adaptation of this code. The database of more than 300 and 100 specimens with the results of the transfer and flexural bond lengths is used, respectively. This database is used to derive and propose the parameters that describe the type and surface of pretensioned FRP reinforcement based on the Eurocode approach. Furthermore, the influence of shear reinforcement and the type of pretensioned reinforcement release (gradual and sudden) is taken into account. The Eurocode approach with the coefficients proposed for different FRP reinforcements on average gave the best prediction of the experimental transfer, flexural bond, and anchorage length results compared to the North American design codes for FRP reinforcement and the Eurocode for steel strands.

Effect of temperature on epoxy resin deterioration for fibre-reinforced polymer reinforcement in simulated concrete pore solutions

Fibre-reinforced polymer (FRP) tendons have excellent mechanical properties and corrosion resistance and are widely used in concrete structures; however, corrosion deterioration of FRP tendons is a serious problem in a highly alkaline concrete environment owing to the hygrothermal ageing effect of the resin. Accordingly, this study investigated the macro and micro properties evolution of epoxy resin in a simulated concrete pore solution at different temperatures. The results showed that the resin properties deteriorated significantly under long-term hygrothermal immersion. However, short-term high-temperature soaking caused some of the resin to undergo post-curing, improving its properties.

Influence of seawater and sea sand on early-age performance and cracking sensitivity of concrete

Due to resource constraints, there has been a growing demand for alternative sources of water and sand in construction engineering. Recent advancements in technology, such as the use of fiber-reinforced polymers (FRP) reinforcement or impressed current cathodic protection-structural strengthening (ICCP-SS), have enabled the utilization of seawater and sea sand, garnering significant research attention. The study investigated various aspects of four types of early-age concrete based on the temperature stress testing machine (TSTM) method, including fresh properties, basic mechanical properties, hydration temperature history, restraint stress, autogenous shrinkage, creep, and cracking sensitivity. The results showed that seawater significantly increased the early tensile strength of concrete, while sea sand had the opposite effect. Compared with ordinary concrete, seawater and sea sand increased the early autogenous shrinkage of concrete, and the impact of seawater was more significant. For the early creep of concrete, seawater and sea sand reduced the maximum compressive creep of concrete, and the effect of sea sand was more remarkable. Seawater increased the tensile creep of concrete, while sea sand decreased the tensile creep of concrete. The comprehensive evaluation index indicates that seawater and sea sand have no significant effect on concrete's cracking sensitivity. This research may serve as a valuable reference for the use of seawater and sea sand in concrete.

Development of multi-fiber approach finite element model for reinforced concrete beam using frp reinforcements under pure torsion

A finite element model using a multi-fiber approach is proposed in this paper for the analysis of reinforced concrete (RC) members using fiber-reinforced polymer (FRP) reinforcements, with a specific focus on the effect of pure torsion. The proposed model is formulated using a displacement-based approach and a kinematic assumption involving a two-node Timoshenko beam. The compatibility and equilibrium between concrete and FRP materials in the membrane elements are formulated based on a discretization of the cross-section into several areas following its stress state and the principle of the Modified Compression Field Theory. The nonlinear responses of RC elements with FRP bars can be predicted using an appropriate constitutive material law with internal equilibrium of transverse reinforcement and concrete. The pure torsional response is implemented using an enhanced formulation of concrete's tensile behavior, which is based on experimental tests on torsion and the characteristics of FRP materials. The good agreement between the numerical results and experimental data confirms the validity of the proposed model

Torsional behaviour of RC beams strengthened by NSM GFRP bars

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement is an innovative, serviceable, and beneficial method for strengthening reinforced concrete (RC) structures. Previous studies have proven that the epoxy-resin bonded NSM FRP method effectively achieved shear and flexural strengthening. However, only a few researches have employed this technique for improving torsion but, no research has been conducted on the use of NSM with GFRP bars and epoxy adhesives. This study investigates the behavior of RC beams subjected to combined torsion when they have been enhanced with NSM GFRP U-shaped and closed stirrups with different overlap sites. Four RC beams with dimensions of (150×250×2000) mm (width X depth X length) were considered in the experimental tests. One RC beam is not strengthened and used as a control beam (CB), while three beams are enhanced with NSM GFRP bars in different shapes and overlap configurations. The findings of the tests in terms of twist angle_applied torque curves, the torque of initial cracking, the ultimate torsional carrying capacity, and the ultimate angle of twist were all recorded and evaluated. It has also been shown that a considerable improvement in the performance of RC beams in twisting can be achieved by using NSM GFRP stirrups. From the different NSM strengthening configurations used in the tests, results have demonstrated that beams strengthened by closed GFRP stirrups at the opposite overlap with 100 cm spacing outperformed other strengthened beams of about 181.5% compared to the CB. In contrast, the beam, strengthened by U-shaped GFRP stirrups with spacing at 200 cm, had the lowest torsional strength of about 48.1% relative to the CB.

Surface crack growth in metallic pipes reinforced with Fibre-Reinforced Polymers subjected to cyclic loads: An analytical approach

This paper introduces a novel analytical approach aimed at predicting the growth of surface cracks in metallic pipes reinforced with Fibre-Reinforced Polymers (FRPs) subjected to cyclic bending and/or tension loads. The primary objective of this study is to develop a comprehensive analytical model that accounts for multiple factors influencing crack growth, namely stress reduction, crack-bridging effect, stiffness degradation, and fatigue damage of the FRP-to-metal interface simultaneously. By considering these simultaneous effects, our proposed approach enables accurate evaluations of the stress intensity factors (SIFs) at both the surface point and the deepest point of a surface crack. To facilitate practical implementation, we have developed an in-house program that automates crack growth rate and residual fatigue life predictions. The proposed analytical method has been validated through a series of comparisons with experimental data and finite element results, demonstrating its accuracy in estimating fatigue lives. The key novelties of this research lie in the holistic consideration of multiple dominating and influencing factors, the achievement of precise SIF evaluations, and the development of an automated prediction tool for practical applications. Overall, our findings confirm the suitability of the proposed analytical approach for predicting crack growth and provide valuable insights for guiding the design of FRP reinforcement in surface-cracked metallic pipes. This work contributes to advancing the understanding of crack growth behaviour in FRP-reinforced metallic pipes and opens new possibilities for the safe and efficient design of such structures.

Predicting punching shear in RC interior flat slabs with steel and FRP reinforcements using Box-Cox and Yeo-Johnson transformations

This study presents a new approach for predicting the punching shear strength of reinforced concrete flat plates reinforced with steel and fiber-reinforced polymer (FRP) bars, using four machine learning regression models. A dataset of 505 interior flat plates from previous literature was used to develop the models. Input variables were transformed using Box-Cox and Yeo-Johnson transformations to improve predictive power by reducing white noise. The SHapley Additive exPlanations (SHAP) framework was utilized as the Explainable Artificial Intelligence (XAI) method to decipher model execution and feature the main info factors for anticipating the punching shear capacity. Additionally, design codes and equations from the literature are compared to the efficiency and accuracy of the power transformation featured XGBoost model. The findings demonstrate that the suggested prediction model performed very well and is appropriate for estimating the punching shear capacity of slab-column connections reinforced with steel and FRP bars. Moreover, the power transferred XGBoost model performed better than other numerical equations because it had the highest mean R2 (0.93) and lowest MAPE (0.20) for testing, which suggests that machine learning models may be able to offer an elective strategy to currently used mechanics-based models for configuration practice.

Quantifying the Residual Stiffness of Concrete Beams with Polymeric Reinforcement under Repeated Loads.

Current technology development ensures a variety of advanced materials and options for reinforcing concrete structures. However, the absence of a uniform testing methodology complicates the quantification and comparative analysis of the mechanical performance of the composite systems. The repeated mechanical loads further complicate the issue. This research extends the recently developed residual stiffness assessment concept to the repeated loading case. It provides an engineer with a simplified testing layout and analytical model to quantify the residual flexural stiffness of standardized laboratory specimens subjected to repeated cycling loads. This model explicitly relates the particular moment and curvature values, requiring neither iterative calculations nor the load history. Thus, this feature allows residual stiffness quantification under repeated loading conditions, including complete reloading of the beam samples imitating the structural strengthening procedure; the proposed technique is equally efficient in quantifying the residual stiffness of the beam samples with any combinations of fiber-reinforced polymer (FRP) reinforcements, i.e., embedded bars, near-surface-mounted strips, and externally bonded sheets. This study employs 12 flexural elements with various reinforcement and loading layouts to illustrate the proposed methodology's efficiency in quantifying the residual strength of the tension concrete, which estimates the efficiency of the reinforcement system. The explicit quantifying of the residual resistance of the FRP reinforcement systems under repeated load cycles describes the essential novelty of this work.

Behavior of high-strength concrete (HSC) box girders reinforced with GFRP bars, ties, and spirals under torsion

To date, the torsional behavior of high-strength concrete (HSC) box girders reinforced with glass fiber-reinforced polymer (GFRP) reinforcement has not been investigated. This study reports on the experimental results of HSC box girders reinforced with GFRP bars, ties, and spirals under pure torsion. Seven full-scale reinforced concrete (RC) box girders with a total length of 4,000 mm, width of 380 mm, height of 380 mm, and wall thickness of 100 mm were examined over a clear span of 2,000 mm. The test parameters included the type, configuration, and amount of the web reinforcement and the concrete type. All specimens have the same longitudinal reinforcement ratio. The test specimens comprised three box girders reinforced with GFRP bars and individual ties, three specimens reinforced with GFRP bars and continuous spirals, and one box girder entirely reinforced with steel reinforcement as a reference specimen. Two girders were fabricated with normal-strength concrete (NSC)—one reinforced transversally with GFRP spirals; the other reinforced with ties—to investigate the influence of concrete type on torsional behavior. The test findings indicate that, at the same web reinforcement ratio and reinforcement configuration, the HSC specimens achieved higher pre- and post-cracking torsional strength and stiffness than their counterpart NSC specimens. The continuous spiral configuration instead of individual ties improved the overall torsional performance of the tested box girders. Moreover, the stirrup's capacity strongly influenced the torsional strength of the GFRP- and steel-reinforced concrete box girders, but the stirrup stiffness did not affect the ultimate torque.