Fiber Reinforced Polymer Rebars Research Articles

Explainable Boosting Machine Learning for Predicting Bond Strength of FRP Rebars in Ultra High-Performance Concrete

Aiming at evaluating the bond strength of fiber-reinforced polymer (FRP) rebars in ultra-high-performance concrete (UHPC), boosting machine learning (ML) models have been developed using datasets collected from previous experiments. The considered variables in this study are rebar type and diameter, elastic modulus and tensile strength of rebars, concrete compressive strength and cover, embedment length, and test method. The dataset contains two test methods: pullout tests and beam tests. Four types of rebar, including carbon fiber-reinforced polymer (CFRP), glass fiber-reinforced polymer (GFRP), basalt, and steel rebars, were considered. The boosting ML models applied in this study include AdaBoost, CatBoost, Gradient Boosting, XGBoost, and Hist Gradient Boosting. After hyperparameter tuning, these models demonstrated significant improvements in predictive accuracy, with XGBoost achieving the highest R2 score of 0.95 and the lowest Root Mean Square Error (RMSE) of 2.21. Shapley values analysis revealed that tensile strength, elastic modulus, and embedment length are the most critical factors influencing bond strength. The findings offer valuable insights for applying ML models in predicting bond strength in FRP-reinforced UHPC, providing a practical tool for structural engineering.

Bond-slip behavior of lapped sand-coated deformed GFRP rebars in UHPC under double-row splice test

Novel Ultra-High-Performance-Concrete (UHPC) structures reinforced with Fiber-Reinforced Polymer (FRP) rebars are promising candidates for applications in important infrastructures under exposed environments where normal concrete and steel rebars may falter. This paper aims to assess the bond-slip behavior between lapped sand-coated deformed Glass FRP (GFRP) rebars and UHPC using double-row splice tests, with parameters including bar diameter, splice length and lap clearance. Failure modes including the pullout of GFRP rebars and the splitting of UHPC were identified. For cases of pullout failure, the average bond strengths in samples with splice lengths of 5db were reduced by 17.6–22.1 % compared to those of 2.5db. Increasing the lap clearance from 0 to 1db and 2db led to 11.1 % and 30.2 % increases in average bond strengths. Furthermore, average bond-slip models for lapped sand-coated deformed GFRP rebars in UHPC were developed. The predicted curves matched the experimental ones, showing errors within 20 % for both average bond stresses and slips. When the cover is not less than 2db, the splice length is recommended to be at least 15db for sand-coated deformed GFRP rebars with diameters of 10 mm–16 mm in UHPC, approximately 1.25 times the corresponding development length proposed by the existing research.

Durability of partially cured GFRP reinforcing bars in alkaline environments with or without sustained tensile load

Corrosion of steel reinforcing bars (rebars) in reinforced concrete structures can be addressed by using corrosion-resistant materials like Fiber-Reinforced Polymers (FRP). However, in France, FRP rebars are primarily restricted to temporary applications due to lack of comprehensive durability data. While numerous studies have explored FRP durability in alkaline environments, understanding the combined effect of alkali ions and sustained loading remains limited. Furthermore, the impact of incomplete matrix curing on the long-term performance of FRP rebars is limited.Accordingly, this paper investigates the durability of Glass FRP (GFRP) rebars exposed to alkali environments at varying temperatures. A batch of partially cured rebars was selected for this study, with part of them undergoing post-curing to investigate the effect of curing state on the ageing behaviour. Additionally, some partially cured specimens were subjected to combined sustained tensile loading at 20 % and 40 % of ultimate strength. Mechanical, physico-chemical and microstructural characterizations were conducted after various exposure durations. Results revealed that incomplete curing minimally impacted long-term tensile properties but significantly affected the interlaminar shear strength of aged rebars. Moreover, applying a combined load had little influence on residual properties at 20°C but led to rapid reduction in GFRP residual tensile strength at higher ageing temperatures (40 and 60°C).

Shear performance and strength of reinforced concrete non-prismatic beams with basalt fiber reinforced polymer rebars

Tapered beams, which are horizontal structural elements, possess varying moments of inertia across their length. Among these, the pier cap mode is widely employed in engineering applications. This research explores the influence of incorporating basalt fiber reinforced polymer (BFRP) bars on the shear strength of eight concrete haunched beams. While two of these beams were straight beams, the others have different inclination angles. The study aimed to examine two main factors: the angles of tapered part in the compression chord and the effect of BFRP stirrups. Results from the experiment revealed a direct correlation between increasing tapered angles and the enhancement of shear capacity in haunched beams. Consequently, a comprehensive analysis was conducted using three different codes, leading to the formulation of proposed equations derived from semi-empirical methods. The analysis demonstrated a high level of accuracy in predicting shear strength with a satisfactory safety margin.

Effect of BFRP ties on the axial performance of RC columns reinforced with BFRP and GFRP rebars

The adoption of fiber-reinforced polymer (FRP) bars in lieu of traditional steel reinforcement for concrete structures has gained prominence. Nevertheless, prevailing international standards have not incorporated provisions to assess the significant contribution of FRP materials to the axial capacity and deformability of compression members. This study investigates the impact of Basalt (BFRP) ties on the axial performance of full-scale concrete columns reinforced with BFRP and Glass (GFRP) rebars, alongside traditional steel longitudinal rebars. Ten columns were subjected to concentric loading, with parameters including the type of longitudinal rebars (BFRP, GFRP, and steel), spacing of BFRP ties (180 mm, 120 mm, and 60 mm), and diameter of steel rebars (16 mm and 20 mm). Varied steel rebar diameters aimed to discern the influence of axial stiffness on column behavior. Results indicate an 18 % to 26 % higher axial load capacity in steel-reinforced concrete (RC) columns compared to FRP-RC columns. Both types of FRP rebars contributed approximately 12 % to the axial capacity. Reducing BFRP tie spacing from 180 mm to 60 mm significantly increased deformability by up to 250 % in FRP-RC columns and ductility by up to 14 % in steel-RC columns. The study highlights conservative predictions in existing design equations (ACI 440.11–22 and CSA S806-12) for the axial load capacity of BFRP and GFRP RC columns, underestimating by 14 % to 20 %, as they neglect the contribution of FRP rebars. Utilizing a concrete crushing strain of 3,000 µε and 3,500 µε as the ultimate FRP compressive strain produced predictions closest to the experimental axial load capacity.

Feasibility Study of using Glass Fiber Reinforced Polymer rebars as Reinforcement in Concrete

Feasibility Study of using Glass Fiber Reinforced Polymer rebars as Reinforcement in Concrete

Numerical assessment of using FRP rebars as an alternative passive reinforcement in RC beams

AbstractRecent sustainable building strategies make the right decisions to be environmentally friendly and reduce carbon emissions. In some reinforced concrete (RC) elements, fiber-reinforced polymer (FRP) bars have been proposed as an alternative to conventional steel bars. The demand for using noncorrosive and/or nonmetallic reinforcing bars in various reinforced concrete projects has increased. Although these concrete elements are lightweight, have a long lifespan, and need little maintenance, their non-ductile nature and bond with the surrounding concrete create significant challenges. Several experimental investigations have been conducted to evaluate the behavior of RC elements, even with their high cost. This study aims to assess numerically the viability of using FRP bars instead of traditional steel ones in simply supported reinforced concrete beams (RCBs) as longitudinal reinforcement (passive reinforcement). Utilizing the three-dimensional (3D) nonlinear finite element analysis (FEA) software (ABAQUS), a total of eighteen models were carried out to validate the results available in reference case studies with FRP bars. The verification of the numerical results has been verified by comparing them with the reference experimental data. Next, in order to assess the rigidity of such RCBs with FRP bars, parametric research was carried out. The numerical results proved that RCBs with FRP bars have a positive impact on enhancing load-carrying capacities. But on the other hand, the strain energy of such RCBs with CFRP bars is reduced to about 75% of the strain energy of RCBs with steel bars, which leads to low beam ductility.

Comparison between microstructural analysis of GFRP and CFRP rebars using micro computed tomography (μCT), scanning electron microscope (SEM), and acoustic emission (AE) techniques

The internal micro-structure of glass and carbon fibre-reinforced polymer (FRP) rebars subjected to tensile loading was investigated using micro computed tomography (μCT) and scanning electron microscopy (SEM) images. Three dimensional (3D) and two dimensional (2D) reconstructed μCT images were used to study and quantify the void volume and its distribution along with FRP rebar samples after being subjected to tensile loads. The void volume was observed to increase in all samples as the tensile load on the samples was increased. Acoustic emission (AE) monitoring during the tensile loading was also employed to establish a correlation between the AE parameters and damage evolution in the FRP samples. Cumulative energy of AE signal in frequency domain was used to monitor the progression of the internal damage in the FRP reinforcing rebars. A high correlation was observed between the void volume measured by μCT and SEM and the cumulative energy of the AE signal.

Effect of size factor and tapered angles on the shear behavior and strength of haunched beams reinforced with basalt fiber reinforced polymer rebars: Experimental and analytical study

Haunched beams are beams that exhibit varying effective depth over their span. This study includes an experimental and analytical analysis aimed at investigating the influence of size factor on the shear behavior of tapered beams reinforced with basalt fiber reinforced polymer BFRP bars. The main focus of this study revolves around investigating the effects of both inclination angle and size factor as key variables. In this study, a total of eight simply supported beams were tested. The beams were longitudinally reinforced with a 1% ratio of BFRP reinforcement, and separated into two equal groups. Each group consisted of one straight beam and three haunched beams. The haunched beams had varied tapering angles, namely 4⁰, 7.96⁰, and 11.86⁰. The first group is designated for the allocation of large beams, while the second group is designated for the allocation of small beams, which have a cross-section that is one-fourth the size of the large beams. The test results indicate that an increase in tapered angles leads to an increase in shear strength. Additionally, it was observed that smaller beams exhibited a greater normalized shear strength compared to larger beams with a significant rate as compared with straight beams. Subsequently, two equations were developed using the tested data, whereby one of them represents an optimal equation including a logical safety margin, while the other equation is characterized by a conservative equation, providing a substantial safety margin for predictions.

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.

Shear Strength Evaluation of Concrete Beams with FRP Transverse Rebar

Rebar corrosion in traditional reinforced concrete (RC) components may lead to a decrease in service life and carrying capacity. This condition is one of the reasons of the growing popularity of Fiber Reinforced Polymer (FRP) rebars as a corrosion-resistant alternative, particularly in RC infrastructure projects. Because the material properties and behavior of FRP rebar are very different from conventional steel rebar, the calculations used for reinforced concrete with conventional steel reinforcement should be updated for this material. The aim of this study is to propose a new shear strength prediction model for RC beams with transverse steel rebar in order to calculate the shear strength of RC beams with FRP transverse rebar according to TS-500, which is the Turkish Building Code. To achieve this goal, Finite Element Method (FEM) models were created for 27 RC beams with FRP transverse rebars and 9 RC beams without transverse rebars. Furthermore, for RC beams with FRP transverse rebars, a prediction model has been developed. Additionally, 13 prediction models obtained from regulations or scientific studies were compared to the proposed prediction model using a database of 105 tests obtained from previous experimental studies. It was observed that the proposed prediction model provides more consistent results with the test database from the literature compared to the models suggested by other regulations or studies. Therefore, by modifying the shear strength relations recommended in TS-500 for RC beams with transverse steel rebar, they can also be applied to RC beams with transverse FRP rebars.

GFRP reinforced concrete curb: design, economic and regulatory considerations

This paper discusses the use of GFRP reinforcement application for concrete curb design and bridge barrier replacement projects. Unlike conventional steel bars, Glass Fiber Reinforced Polymer rebars have the considerable advantage of not being susceptible to corrosion, preserving the durability of the structures and reducing the maintenance costs during the service life of the bridge. An additional advantage is the low specific weight of the GFRP, which makes it possible to speed up the installation of the reinforcements on site, ensuring greater safety for operators. The paper compares, in terms of structural performance and overall costs, two curbs of the same geometry, one reinforced with classic steel bars, the other with GFRP bars, providing evidence that current code provisions can be practically implemented in typical design situations.

Experimental investigation of concrete frames reinforced with GFRP bars under reversed cyclic loading

Fiber reinforced polymers (FRP) have been widely investigated and successfully used as internal reinforcement in new reinforced concrete (RC) members and structures such as slabs, beams, bridges, multi-storey car parks and others as means to overcome the corrosion problem of steel. The linear-elastic behaviour, brittle failure and low modulus of elasticity are some of the significant mechanical characteristics of FRP rebars which raise concerns on their capability to allow structural members to dissipate energy in seismic zones and thus limit their use in earthquake resistant RC structures (such as moment resisting frames). The behaviour of concrete frames reinforced with FRP bars is still in its early stages of research and development with only a few researchers exploring such structures under in-place cyclic loading (seismic loading). As a result, design codes do not provide guidance for RC moment frames internally reinforced with FRP. This study attempts to partially fill the gap by investigating the behaviour of glass fiber reinforced polymers (GFRP) RC frames under in-plane loading. Six 1/3 scaled RC frames were constructed and tested under reversed cyclic loading to simulate seismic loading conditions. Three were reinforced with GFRP bent bars and three (their counterparts) with conventional steel. The testing was conducted in displacement-controlled mode with the loading history according to ACI 374.1–05. Test parameters included longitudinal reinforcement ratio and arrangement in the columns and presence of links in the joints. The experimental results are presented as hysteretic curves, lateral load / drift graphs, stiffness degradation and cumulative energy dissipated with the overall indication of the feasibility of GFRP RC frames in seismic regions. All GFRP specimens successfully reached 2.75% loading drifts. The energy dissipation of GFRP specimens was lower at initial drifts compared to the steel reinforced samples, but it become greater at final drift. GFRP frames had approximately the same load bearing capacity as their counterparts and even greater in one case. Ultimate loads for G frames were reached at higher level of displacement than for S frames.

Effects of prestress parameters of fiber reinforced polymer rebar on the stiffness and crack resistance of bending concrete elements. Experimental studies

Introduction. In contrast to steel reinforcement, high tensile strength and relatively low elasticity modulus of fiber reinforced polymer rebar (FRP) determine the feasibility of its prestressing. However, the issues of its prestressing technology and the design of structures with its application are insufficiently studied. The current regulatory documentation contain no exhaustive scope of provisions and requirements in relation to the conditions and parameters of FRP prestressing and their account in the design and calculation of structures. In this regard, conducting a set of clarifying studies on this topic appears to be relevant and represents a considerable practical interest.Aim. To assess the effect of various FRP prestressing parameters on the stiffness and crack resistance of bending concrete elements.Materials and methods. As a part of the study, an experimental research program was developed and implemented, including the manufacture and bending testing of six concrete sample series with various FRP prestressing parameters.Results. The destructive loads were determined; patterns of deformation, cracking and destruction of bending concrete elements for various FRP prestressing parameters were established. The prerequisites for improving the system of urban planning activities in terms of clarifying and supplementing the existing regulatory documents for the design of FRP-reinforced concrete structures were established.Conclusion. The technology of FRP mechanical tensioning was laboratory successfully tested. Nevertheless, an adaptation of tensioning devices and grippers is required for its implementation at contemporary production enterprises. FRP prestressing was concluded to be an effective way of increasing the crack resistance and stiffness of bending concrete elements. Within the framework of the study, increase in the time of crack formation (relative to structures without prestressing) and a decrease in deflections in the middle of the span of up to 2.25 and 0.5 times, respectively, was recorded.

Analysis of the bond behavior of a GFRP rebar in concrete by in-situ 3D imaging test

Computed tomography combined with mechanical tests offers completely new insight into the behavior of concrete samples under stress. Particularly the development of new fiber reinforcement materials for concrete elements requires appropriate material models and thus for investigating the interior of the concrete structure. In 3D image data obtained by computed tomography, local structural changes within the sample due to mechanical loading can be observed without further altering the sample. We applied this state-of-the-art approach to a concrete core with an embedded glass fiber reinforced polymer rebar under increasing forces applied to pull out the rebar. In this paper, authors describe a novel in-situ setup for non-destructively 3D imaging during the pull-out test. Conducting the pull-out test leads to the formation of local pore volume changes along the rebar. These pore volume changes are not only visualized but quantified analytically based on the images. Interpreting these volume changes, we derive a novel method for calculating strain and normal stresses in the rebar. Our new method captures the detailed distribution of the bond stresses between rebar and concrete and consequently describes the bond behavior more accurate. It turns out that the observed bond behavior cannot be explained completely by commonly assumed material laws. This emphasizes the need for further, more extensive research combining 3D imaging, mechanical testing, and quantitative image analysis.

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.

Experimental Investigation on Post-Fire Mechanical Properties of Glass Fiber-Reinforced Polymer Rebars

Glass fiber-reinforced polymer (GFRP) rebars are commonly used as an alternative to conventional steel reinforcement in a variety of structural applications due to their superior low cost, strength-to-weight ratio, and durability. However, their mechanical properties after exposure to elevated temperatures, particularly in fire-prone environments, remain a significant concern. To address this concern, the present study focuses on investigating the residual tensile behavior, specifically the tensile strength and elastic modulus, of GFRP rebars exposed to high temperatures that are realistically encountered during fire incidents. The temperature range considered in this analysis spans from 100 °C to 400 °C, with a heating rate of 20 °C/min. The fire duration of 1 h is used. This comprehensive analysis is essential for enhancing our understanding of the performance and applicability of GFRP rebars in fire-prone environments. Based on their actual application in the construction industry, five specimens of three different rebar sizes (16, 20, and 25 mm) were examined for the effect of rebar size on tensile behavior after fire exposure. In addition, the effects were investigated of air- and water-cooling methods on residual tensile behavior. The nominal tensile strength, elastic modulus, and ultimate strain of GFRP rebars at ambient temperature are 930 MPa, 50.2 GPa and 1.85%, respectively. The test results indicated that as the temperature increased to 400 °C, the ultimate tensile strength of the GFRP bars decreased by up to 55%, while the ultimate strain increased by up to 44%, regardless of the cooling method. In addition, when rebars of sizes 16–25 mm were subjected to a 400 °C fire treatment, the smaller the rebar, the greater the percentage of ultimate tensile and strain reduction. These findings hold great significance for the utilization of GFRP bars within the construction industry. This study offers valuable insights into the design of fire-resilient structures, emphasizing the importance of considering rebar size and cooling methods due to their impact on the post-fire tensile strength and strain of GFRP rebars.

Bond performance of carbon fiber reinforced polymer rebars in ultra-high-performance concrete

This paper investigated the bond performance between carbon fiber reinforced polymer (CFRP) bars and ultra-high-performance concrete (UHPC) by a hinged beam test investigation. A total of 11 sets of beam tests were carried out to investigate the effect of different parameters on the bond performance, including steel fiber volume fraction, bonded length, reinforcement diameter, and concrete cover. The tests showed that the bond damage modes between CFRP bars and UHPC were dominated by pull-out damage, with the surface of the bonded section of CFRP bars being peeled off from the internal core and no damage observed in the UHPCs. An increase in the cover and steel fiber volume fraction improves the bonding performance, while an increase in diameter reduces the bond performance. Based on this study, the formulae for calculating the ultimate bond strength and development length that integrated the effects of CFRP bar diameter, bonded length, and cover thickness were proposed. The bond-slip curve shows three stages, i.e., ascending section, descending section, and a residual section showing periodic fluctuations. The fluctuation period showed a certain degree of linear correlation with the rib spacing of CFRP bars. Based on the characteristics of tested curves, a bond-slip constitutive model was proposed to simulate the bond-slip behavior of CFRP bars in UHPCs with good agreement to test results.

RC beams strengthened in shear with FRP-Reinforced UHPC overlay: An experimental and numerical study

In this study, a hybrid strengthening technique consisting of ultra-high performance concrete (UHPC) overlay reinforced by fiber reinforced polymer (FRP) bars is proposed to strengthen reinforced concrete (RC) beams in shear. Eleven beams, five of which are physical samples tested under static load, and the other six are virtual ones derived from a robust finite element (FE) analysis, are used to assess the effectiveness of this retrofitting technique. Several parameters are examined, namely overlay thickness, overlay configuration (continuous vs. discontinuous), FRP reinforcement ratio in the overlay, and type of FRP rebar (Glass-FRP vs. Carbon-FRP). Results showed that the system was very effective in increasing the beam capacity (Pu) and altering the failure mode from shear to flexure. Increasing the overlay thickness (toverlay) from 0 to 30 mm for plain and FRP-reinforced overlays results in a significant increase in Pu of 76–117%, compared to the control beam. Using discontinuous UHPC strips, although does not alter the shear failure, results in 89% increase in Pu. Reinforcing the overlay with GFRP and CFRP bars results in reducing the overlay thickness needed to shift the failure mode from shear to flexure. The energy absorption index, which is a measure of ductility, of the strengthened specimens was 4.0 to 6.9 times that of the control beam. FRP bars resulted in 11–25% increase in energy absorption compared to plain overlays, with GFRP showing more ductility than CFRP. An analytical procedure is also included and used to predict Pu from modifying existing code equations to include the contribution of UHPC overlay to shear.

Mechanical behavior and its variability analysis of fiber reinforced polymer rebars after high temperatures

This study investigated the influence of high temperatures on the mechanical properties of glass and basalt fiber reinforced polymer (GFRP and BFRP) rebars. These rebars with a wide range of diameter from 10 to 25 mm were heated up to 400 ℃ and tested under shear and compression. Based on the data from the extensive experiments, the variability of shear and compressive strength was assessed. Results indicate that the reduction of both shear and compressive strength with high temperatures did not show notable dependence on the rebar diameter and type. High temperatures, especially 350 and 400 ℃, would induce more reduction of compressive strength than that of shear strength. Using the combined probability test and the Bayesian information criterion (BIC) test, this study proposed an approach to establishing appropriate probabilistic strength models for FPR rebars after high temperatures. The Gamma distribution outstood all chosen models for capturing the variability of residual shear strength of FRP rebars, while both Gamma and Weibull distributions were needed for compressive strength. Results herein can facilitate a better understanding of high-temperature induced change in properties of GFRP and BFRP rebars, and provide useful probabilistic strength models for probability-based evaluation of FRP rebars reinforced concrete structures after a fire.