Fiber-reinforced Polymer Reinforcement Research Articles

Experimental investigation on shear behavior of FRP post-tensioned concrete beams without stirrups

To investigate the shear behavior of post-tensioned concrete beams with longitudinal fiber-reinforced polymer (FRP) reinforcements and without stirrups, seven large-scale beams, with a shear span-to-effective depth ratio of approximately 3.0, were tested. The test variables included the prestressing level, type of flexural reinforcement, and tendon profile. The shear crack width and slip were tracked using digital image correlation technique. It was found that the post-tensioned concrete beams failed in shear-compression or shear-tension, while the nonprestressed concrete beam failed in diagonal tension. The post-tensioned beams designed to have approximately the same longitudinal reinforcement stiffness, exhibited similar shear strength. Draping FRP tendons resulted in an increase of 8.8% in shear cracking strength, while it had slight effect on the maximum shear strength. In this study, it was demonstrated that arching action played a major role in the shear resistance of FRP post-tensioned beams, while the contribution of aggregate interlock was negligible.

Diagnosis of NSM FRP reinforcement in concrete by using mixed-effects models and EMI approaches

The use of fibre reinforced polymer (FRP) in civil construction applications with near-surface mounted (NSM) method has gained considerable popularity worldwide as suitable method for strengthening existing concrete structures. However, there is very little experience in the implementation of methods able to give a reliable prediction about the health of this type of structures even although sudden and brittle failure modes are likely to happen. Because of it, more contributions on this topic are really needed in order to prevent possible catastrophic failures. In this paper, a novel approach based on linear mixed effects models implemented on results from experimental tests performed on concrete beams strengthened with NSM FRP has been explored. In spite of their rapid growth in other areas, mixed effects models have barely been applied in structural problems. Impedance measurements captured from PZT sensors embedded and externally bonded to the beam are used for the quantitative and qualitative analysis. This research shows that the evolution of the progressive damage for this kind of repairing method can be effectively monitored using the proposed approach.

EFFECT OF ASPECT RATIO ON THE BEHAVIOR OF GFRP-RC CIRCULAR COLUMNS UNDER SIMULATED SEISMIC LOADING

The mechanical and physical properties of glass fiber-reinforced polymer (GFRP) reinforcement are different from steel, which requires independent code provisions for GFRP-reinforced concrete (RC) members. The currently available code provisions for GFRP-RC members still need more research evidence to be inclusive. For example, the available provisions for confinement reinforcement of FRP-RC columns do not consider the effects of column aspect ratio, which is not yet supported by any available research data. In this study, two full-scale spirally reinforced GFRP-RC circular columns were constructed and tested under concurrent seismic and axial loads. Both specimens had an aspect ratio (shear span-to-diameter ratio) of 7.0, while other two specimens with an aspect ratio of 5.0, from a previous stage of this study, were included for comparison purposes. For each aspect ratio, each specimen was loaded under one of two levels of axial load; 20 or 30% of the axial load capacity of the column section. All test specimens had a 35 MPa concrete compressive strength, 350-mm diameter, 85-mm spiral pitch and 1.2% longitudinal reinforcement ratio. The experimental results were analyzed in terms of hysteretic response, drift capacity and inelastic deformability hinge length. Based on the experimental results, it can be concluded that the aspect ratio affects the magnitude of secondary moments and inelastic deformability hinge length. In addition, the aspect ratio may affect drift capacity of GFRP-RC columns, depending on axial load level.

Ductility and Performance Assessment of Glass FiberReinforced Polymer-Reinforced Concrete Deep Beams Incorporating Cementitious Composites Reinforced with Basalt Fiber Pellets

Ductility and Performance Assessment of Glass FiberReinforced Polymer-Reinforced Concrete Deep Beams Incorporating Cementitious Composites Reinforced with Basalt Fiber Pellets

Seismic performance of fibre-reinforced polymer and steel double-reinforced bridge piers

The re-centring ability and durability of important bridges have been the main objectives in recent provisions of seismic design codes. To achieve better recoverability and durability for piers, this article presents a double-reinforced pier configuration, in which the outer layer is reinforced with fibre-reinforced polymer (FRP) rebar and the inner layer is reinforced with steel rebar. An equal flexural capacity design method was used to determine the double-reinforced configurations. The effects of the potentially influential design parameters on the seismic response of piers were studied and compared to those of conventional RC piers. The results show that the proposed design method can satisfy the ultimate capacity and durability demands. The double-reinforced piers had a better performance than conventional piers in terms of the ductility, post-yield stiffness and residual deformation. The maximum deformation and energy dissipation capacity of the double-reinforced piers were comparable to those of the conventional RC piers. The results also demonstrated that the proportion of steel reinforcement had a clear effect on the performance of double-reinforced piers. However, FRP reinforcement with a high modulus of elasticity should be avoided due to the adverse effect on ductility.

Residual bond strengths of epoxy and cement-bonded CFRP reinforcements to concrete interfaces after elevated temperature exposure

This paper presents the results from a comparative study on the residual bond performance of epoxy and cement-bonded carbon fiber-reinforced polymer (CFRP) reinforcement to concrete interfaces that experienced different levels of elevated temperature exposure. In total twelve groups of concrete beams strengthened with CFRP sheet and grids of the same tensile stiffness, through epoxy and cement-based adhesive bonding, respectively, were carefully prepared and exposed to different test temperature levels from 20 °C to 600 °C. Both polymer cement mortar (PCM) and ductile engineered cementitious composite (ECC) were adopted as the cement binder. The strengthened beams were tested under three-point monotonic bending after different levels of elevated temperature exposure. It was found that with increasing the exposure temperature, the failure modes of the strengthened beams shifted from debonding in a thin layer of concrete substrate to debonding near the fiber-adhesive interface and tearing off of CFRP grids from the ECC matrix. The peak loads of concrete beams strengthened with cement-bonded CFRP grids are at least 50% higher than their counterparts strengthened with epoxy-bonded CFRP sheets given the same temperature exposure.

Flexural behaviour of ECC and ECC–concrete composite beams reinforced with hybrid FRP and steel bars

Owing to the good ductility of steel and high strength and excellent corrosion resistance of fibre-reinforced polymer (FRP), concrete beams reinforced with hybrid steel and FRP bars exhibit better ductility than FRP-reinforced concrete beams as well as higher load-carrying capacities and better corrosion resistance than steel-reinforced concrete beams. However, the inherent brittleness of concrete in tension results in steel corrosion because of wide cracks and accelerated fracture of FRP reinforcement because of crack-induced stress concentration. This study investigated the effects of ultra-high ductile engineered cementitious composites (ECCs) on the flexural behaviour of hybrid steel and FRP-reinforced beams. Six hybrid-reinforced beams with various reinforcement ratios, matrix types and ECC pouring positions were tested in four-point bending. The flexural behaviours of the beams in terms of failure modes, crack patterns and developments, load versus deformation relationships and ductility are discussed herein in detail. We observed that substituting ECC with concrete results in a higher load-carrying capacity and better ductility of the hybrid reinforced beams owing to the excellent characteristics of ECC materials. When a layer of ECC is poured in the tension zone, the average crack width and crack spacing along the beam decrease; therefore, the longitudinal reinforcements can be adequately protected.

Numerical and experimental study of externally reinforced RC slabs using FRPs subjected to close-in blast loads.

The use of woven Fiber Reinforced Polymer (FRP) as strengthening material on full-scale Reinforced Concrete (RC) slabs was studied in this paper. Carbon (CFRP) and E-glass (GFRP) fibers were tested for comparison. The specimens of the trials were subjected to blast loads at scaled distances of 0.83 m/kg1/3, 0.42 m/kg1/3 and 0.21 m/kg1/3, varying both charge and distance to the slab. Furthermore, Finite Element (FE) numerical models have been developed to study, in-depth, the response of the structural elements. A non-reinforced slab facing the highest scaled distance has been used as calibration test. It was monitored with pressure gauges and accelerometers, obtaining errors of around 2% and 5%, respectively, compared with the models’ predictions. Seven other trials were carried out and used as validation tests. The explosive charge model was developed using the *LOAD_BLAST_ENHANCED (LBE) tool available in the LS-DYNA software. Two concrete material models have been tested, CSCM and RHT, achieving better results with the first one. To model the FRP reinforcement, *MAT_058 (LAMINATED_COMPOSITE_FABRIC) was used. Applying CFRP resulted in a generally reduced damage area on both surfaces especially at intermediate scaled distances. Concerning the specimens reinforced with GFRP, further investigation needs to be conducted as the field test outcomes were not entirely conclusive due to the bonding of the fiber on concrete. The experimental results were closely reproduced with the developed numerical models, with errors in terms or surface damage below 15%.

Development of a Robot-Based Multi-Directional Dynamic Fiber Winding Process for Additive Manufacturing Using Shotcrete 3D Printing

The research described in this paper is dedicated to the use of continuous fibers as reinforcement for additive manufacturing, particularly using Shotcrete. Composites and in particular fiber reinforced polymers (FRP) are increasingly present in concrete reinforcement. Their corrosion resistance, high tensile strength, low weight, and high flexibility offer an interesting alternative to conventional steel reinforcement, especially with respect to their use in Concrete 3D Printing. This paper presents an initial development of a dynamic robot-based manufacturing process for FRP concrete reinforcement as an innovative way to increase shape freedom and efficiency in concrete construction. The focus here is on prefabricated fiber reinforcement, which is concreted in a subsequent additive process to produce load-bearing components. After the presentation of the fabrication concept for the integration of FRP reinforcement and the state of the art, a requirements analysis regarding the mechanical bonding behavior in concrete is carried out. This is followed by a description of the development of a dynamic fiber winding process and its integration into an automated production system for individualized fiber reinforcement. Next, initial tests for the automated application of concrete by means of Shotcrete 3D Printing are carried out. In addition, an outlook describes further technical development steps and provides an outline of advanced manufacturing concepts for additive concrete manufacturing with integrated fiber reinforcement.

The impact of percentage content of basalt fiber-reinforced polymer and steel reinforcement on the strength indicators of experimental beams with hybrid reinforcement

The use of composite reinforcement in the structures subject to bending is limited due to their excessive deformation. One way to overcome this shortcoming is to use hybrid reinforcement where steel and composite reinforcement are used in combination. The effectiveness of this approach has been proven by previous studies. There are no studies on the establishment of the effective ratio between steel and composite elements. This paper presents the results of tests of prototype beams with hybrid reinforcement made of basalt fiber-reinforced polymer (BFRP) and steel. A total of 12 series of samples of beams, three beams in each series, were studied. The percentage of both types of reinforcement was variable; it varied from a ratio of 100% / 0% to 0% / 100% in increments of ~ 20% (the numerator of the fraction corresponds to the percentage of steel reinforcement, and the denominator – to the percentage of BFRP reinforcement). According to the results of tests of beams by static loading, it was found that the use of hybrid reinforcement allowed to increase the bearing capacity of the samples by 33% – 74%, depending on the percentage of reinforcement in the cross section, compared with the beams of the control series. Based on the nature of breaks and load-bearing capacity, the most optimal ratio of steel and BFRP reinforcement was 60% – 40%. The optimal percentage of cross section reinforcement is 1.07%.

Numerical simulation of application effect of FRP on torsional-flexural behavior of RC columns

Regarding to significant effects of torsional loads as well as axial and flexural loads on columns under earthquake loads, strengthening and confinement of the concrete columns with Fiber Reinforced Polymer (FRP) layers is accounted as an appropriate technique to enhance the torsional strength of the column. In the present paper, the numerical simulation of torsional performance of the columns, flexural and axial loads are simultaneously considered. The experimental results from the authors on columns with and without FRP reinforcement with different configurations are used to validate the numerical results. Finite element method and software ABAQUS are applied for simulation. The behavior of concrete is simulated using damaged plasticity model and the behavior of the steel is defined with coordination of yield and ultimate points using bilinear model. Whereas the cyclic loads are applied to the column, the kinematic hardening is used and also the effects of large deformations on the behavior are considered. Moreover, the nonlinear static analysis is applied and the displacement based loads are applied to the models in accordance with experimental loads. The numerical and experimental results are compared and shows less than 5% error.

Sensitivity of lateral impact response of RC columns reinforced with GFRP bars and stirrups to concrete strength and reinforcement ratio

This is the first study in the literature which experimentally and numerically investigates the impact behavior of concrete columns reinforced with fiber-reinforced polymer (FRP) reinforcements. The effect of concrete strength (50 MPa vs 100 MPa) and longitudinal FRP ratio on the lateral impact response of the columns was investigated. The experimental results showed that the longitudinal reinforcement ratio strongly affected the failure modes and impact-resistant capacity while the use of high strength concrete (HSC) did not effectively improve the performance of the columns, which on the contrary, might have caused spalling failure due to its brittleness. The peak impact force and displacement of the columns increased linearly with the impact velocity up to their maximum capacity. The longitudinal reinforcement ratio slightly affected the peak impact forces while the concrete strength showed marginal variation. The use of HSC did not effectively reduce the maximum displacement of the columns. The energy absorption of the columns and the impact velocity exhibited an approximately linear relationship regardless of the reinforcement ratio and concrete strength. Different from the static case, the numerical results show three critical sections, i.e. at the impact location and column ends, which need to be carefully designed.

Numerical simulation of the influence of bond strength degradation on the behavior of reinforced concrete beam-column joints externally strengthened with FRP sheets

Limited information is known about the effects of bond strength degradation during installation on the bond's quality and performance between fiber-reinforced polymer (FRP) reinforcement and substrate material. This research study's primary focus is to investigate the efficiency of the external FRP composites in rescuing the structural performance and controlling the mode of failure of the reinforced concrete (RC) beam-column joint with different bond strength degradation percentages nonlinear finite element analysis (NLFEA). Firstly, the RC beam-column model was validated against the published experimental results and then was expanded to consider the effect of the degradation percentages in bond strength between concrete and FRP composite (0 % (Fully bond), 10 %, 20 %, 30 %, 40 %, 50 %, 60 %, 70 %, 80 %, 90 %, and 100 % (control or un-strengthened joint). The structural performance was evaluated in terms of failure mode, stress distribution, pulling and pushing ultimate load capacity and corresponding displacement, horizontal load-displacement hysteretic loops, horizontal load-displacement envelopes, displacement ductility, energy dissipation, stiffness degradation, and equivalent hysteretic damping factor. The NLFEA results showed that the FRP strengthening technique with bond strength degradation percentages less than 30 % enhanced the cyclic performance (higher load capacity, larger horizontal displacement, higher displacement ductility, higher energy dissipation, and slower secant stiffness degradation). Also, the utilized FRP method with bond strength degradation percentage less than 30 % performed well in eliminating any surface debonding or buckling in the FRP composite because of the proper lateral support provided for the strengthening sheets. Finally, the bond strength degradation percentage less than 30 % could significantly enhance the deficient joints' seismic performance under strong beam-weak column conditions by changing its behavior to a more ductile one, including the beam flexural hinging. Moreover, the relocation of a plastic hinge in the beam provided more lateral strength for the joint specimens.

Shear Behavior of Post-Tensioned Concrete Beams with Draped FRP Tendons and without Transverse Reinforcement

To investigate the shear behavior of post-tensioned concrete beams with fiber-reinforced polymer (FRP) reinforcements, six large-scale post-tensioned beams without transverse reinforcement and with a shear span-to-effective depth ratio of approximately 3.0 were tested to failure. All beams were longitudinally reinforced with draped prestressed carbon FRP tendons and non-prestressed glass FRP bars. The test variables included the amount of flexural reinforcement and the prestressing level. With the aid of full-field measurement on the beam surface using digital image correlation, the kinematics of the critical shear crack of each beam were tracked. Two shear failure modes, including shear compression and shear tension, were observed in the tested beams. Generally, when the amount of flexural reinforcement increased, there was a corresponding increase in the maximum shear force. When the total prestressing force was increased from 360 to 440 kN, the shear cracking strength and the maximum shear strength increased by 6.9% and 10.0%, respectively. It was demonstrated that an arch mechanism formed in the tested FRP post-tensioned beams, although the contribution of aggregate interlock to the shear capacity was negligible. The predictions of the shear capacity calculated from various shear design models showed that the American and Japanese recommendations were highly conservative, whereas the Canadian recommendations were more consistent with the experimental results.

Neural Networks—Deflection Prediction of Continuous Beams with GFRP Reinforcement

Deflections on continuous beams with glass fiber-reinforced polymer (GFRP) reinforcement are calculated in accordance with the appropriate standards (ACI 440.1R-15, CSA S806-12). However, experimental research provides results which differ from the values calculated pursuant to the standards, particularly when it comes to continuous beams. Machine learning methods can be applied for predicting a deflection level on continuous beams with GFRP (glass fiber-reinforced polymer) reinforcement and loaded with a concentrated load. This paper presents research on using artificial neural networks for deflection estimation and an optimal prediction model choice. It was necessary to first develop a database, in order to train the neural network. The database was formed based on the results of the experimental research on continuous beams with GFRP reinforcement. Using the best trained neural network model, high accuracy was obtained in estimating deflection, expressed over the mean absolute percentage error, 9.0%. This result indicates a high level of reliability in the prediction of deflection with the help of artificial neural networks.

Investigation of ANN architecture for predicting shear strength of fiber reinforcement bars concrete beams.

In this paper, an extensive simulation program is conducted to find out the optimal ANN model to predict the shear strength of fiber-reinforced polymer (FRP) concrete beams containing both flexural and shear reinforcements. For acquiring this purpose, an experimental database containing 125 samples is collected from the literature and used to find the best architecture of ANN. In this database, the input variables consist of 9 inputs, such as the ratio of the beam width, the effective depth, the shear span to the effective depth, the compressive strength of concrete, the longitudinal FRP reinforcement ratio, the modulus of elasticity of longitudinal FRP reinforcement, the FRP shear reinforcement ratio, the tensile strength of FRP shear reinforcement, the modulus of elasticity of FRP shear reinforcement. Thereafter, the selection of the appropriate architecture of ANN model is performed and evaluated by common statistical measurements. The results show that the optimal ANN model is a highly efficient predictor of the shear strength of FRP concrete beams with a maximum R2 value of 0.9634 on the training part and an R2 of 0.9577 on the testing part, using the best architecture. In addition, a sensitivity analysis using the optimal ANN model over 500 Monte Carlo simulations is performed to interpret the influence of reinforcement type on the stability and accuracy of ANN model in predicting shear strength. The results of this investigation could facilitate and enhance the use of ANN model in different real-world problems in the field of civil engineering.

To the design of the deflections of flexural concrete members reinforced with fiber-reinforced polymer bars

Introduction. Fiber-reinforced polymers (FRP) reinforcement has a relatively low modulus of elasticity compared to steel. In this connection, the bent members with such reinforcement have a higher deformability. The stress-strain diagram for FRP under short-term loading is almost straight and does not have a yield line, as in steel reinforcement. At the same time, modern methods of calculating structures with for FRP reinforcement are based on existing approaches for reinforced concrete structures. In this regard, the current direction for study is to assess the reliability and improve the existing methods for calculating members with FRP reinforcement for the serviceability limit states.
 Materials and methods. Experimental studies were carried with regard for and in compliance with the provisions of National State Standard 8829-94. Tested samples represented concrete beams that were 1,810 mm long and had a cross section of 120 × 220 mm. Their tensile side was reinforced with two bars. Steel, glass fiber-reinforced polymer (GFRP) and basalt fiber-reinforced polymer (BFRP) bars were used to reinforce the beams. The value of the reinforcement ratio varied. Deflections calculation methods, applied according to Construction rules and regulations 295.1325800.2017 (Russia) and ACI 440.1R-06 (USA) were analyzed.
 Results. The results of the theoretical and experimental studies of the deformability of flexural members having FRP reinforcement are obtained. The inaccurate determination of the cracking moment Mcrc has a negative impact on the results of the deflection calculation. The deviation of the theoretical values of the cracking moment according to the method of Construction rules and regulations 295.1325800.2017 from the experimental ones is 26.2–59.1 %, in the method of ACI 440.1 R-06 — 20.1–57.1 %. For the ACI 440.1 R-06 method, the calculation is more accurate, in which the short-term deflection was multiplied by the factor λ = 0,6ξ = 0,6 · 2 = 1,2, for the Construction rules and regulations 295.1325800.2017 method — when using ψf = 1 – 0,8Mcrc / M.
 Conclusions. The results obtained showed the need to improve the considered calculation methods. To increase the accuracy of the calculation of deflections according to the method of SP 295.1325800.2017, at the first stage, it is possible to approximate the theoretical values of the cracking moment to the experimental data by introducing correction factor in the equation according to the calculation of Mcrc: for beams with a GFRP high-bond bars — 0.7, for BFRP with a sanded surface — 0.5.

Size Effect on Strength of Reinforced Concrete Cylinders Confined by Carbon Fiber-Reinforced Polymer and Transverse Stirrup Reinforcement

Size Effect on Strength of Reinforced Concrete Cylinders Confined by Carbon Fiber-Reinforced Polymer and Transverse Stirrup Reinforcement

Bond durability of epoxy and cement- bonded CFRP reinforcement to concrete interfaces subject to water immersion

Durability is a critical concern for concrete structures externally bonded with fiber-reinforced polymer reinforcement. This paper presents the results from an experimental study which compared the bond durability of epoxy-bonded CFRP sheets and cement-bonded CFRP grids to concrete interfaces subjected to water immersion. Totally twelve groups of CFRP sheet or grid-strengthened concrete beams with a notch at their midspan were carefully fabricated through epoxy- and cement-based bonding systems and tested under monotonic loading before and after water immersion exposure, respectively. The cement binder included polymer cement mortar (PCM) and ductile engineered cementitious composite (ECC). The CFRP grids and sheets were designed with the similar tensile stiffness. The duration of water immersion ranged from 0 to 6 months. The test results indicated that debonding in concrete adjacent to the concrete-adhesive interface in accompany with a diagonal concrete crack near the notch was the main failure mode of the beams strengthened with epoxy-bonded CFRP sheet and ECC-bonded CFRP grid, while the tearing off of CFRP grids from the PCM matrix was the main failure mode of the beams strengthened with PCM-bonded CFRP grids. After water immersion exposure, the ultimate loads of the beams with cement-based CFRP grids were at least 50% higher than their epoxy-bonded counterparts, confirming the superior moisture durability of cement based bond to epoxy-bonded ones. The interfacial fracture energy was found to exhibit an exponential decline trend with water immersion duration.

Finite element analysis of rectangular RC beams strengthened with FRP laminates under pure torsion

AbstractComposite materials have attracted wide attention in strengthening of structural members. In this study, the behavior of rectangular reinforced concrete (RC) strengthened with fiber reinforced polymer (FRP) laminates under pure torsion was investigated numerically and validated by existing data from literature. Various practical strengthening configurations were considered and the efficiency of them was illustrated. Concrete damage plasticity constitutive model was used and required parameters was drawn. Good agreement in terms of torque–twist behaviors of RC beams strengthened with FRP laminates before and after cracking was found. Reasonable steel and FRP reinforcement responses as well as crack patterns were achieved and the failure modes of all the specimens were modeled correctly. A parametric study was carried out and parameter such as number of FRP plies, concrete compressive strength, and FRP strip orientations were considered. The results showed that the torsional capacity could improves by increasing the number of FRP plies and concrete compressive strength. Moreover, no significant change in the torsional capacity of RC beams when it was strengthened with a 45° or with a 90° FRP laminate was observed.