Glass Fiber Reinforced Polymer Bars Research Articles

Effect of confining stirrups and bar gap in improving bond behavior of glass fiber reinforced polymer (GFRP) bar lap splices in RC beams

Fiber-reinforced polymer (FRP) bars are becoming a popular replacement for steel rebars that are vulnerable to corrosion, especially under harsh corrosive environments. Despite several advantages for using FRP rebars, the bond between FRP bars and surrounding concrete has been a major concern that affects the behavior of FRP-reinforced structural members. The experimental program of this study comprises testing eight full-scale glass FRP (GFRP) reinforced beams under two-point loads. There were two control beams and six beams having rebar lap splices. The beams were reinforced using 2ϕ12 GFRP bars. The first control had continuous GFRP bars, and the 2nd control had GFRP lap splice conforming to the relevant ACI code. The lap splice was 65 times the bar diameter in six beams having lap splice. The contact lap and non-contact laps were used along with the variable spacing of shear stirrups in the lap zone (50, 100 mm, ∞, i.e., no shear stirrups). The lap splice, designed conforming to the ACI code, was found to be very conservative. The reduction in shear stirrups’ spacing in the lap zone and the gap between lapped bars was found to enhance the bond performance of GFRP bars by up to 31 %. The authors’ previous models for predicting development length and bond are found to show good predictions for the experimental results of the present study as well. Analytical models are developed for predicting with sufficient precision the peak load for GFRP reinforced beams with the two types of lap splices (non-contact and contact).

Experimental investigation of concentrically and eccentrically loaded circular hollow concrete columns reinforced with GFRP bars and spirals

Hollow concrete columns (HCCs) constitute a structural system with low self-weight, reduced material use, and high strength-to-weight ratio. Limited studies, however, have focused on investigating the concentrically loaded behavior of circular HCCs reinforced with glass fiber-reinforced polymer (GFRP) reinforcement. Moreover, there appears to be no research on the behavior of such columns under eccentric loading. This study investigated the performance of 10 large-scale GFRP-reinforced HCCs measuring 1,500 mm in height with outer/inner diameters of 305/113 mm. The test parameters were the effect of two longitudinal reinforcement ratios (2.5 % and 3.8 %) and five different levels of eccentricity (0 %, 8.2 %, 16.4 %, 32.8 %, and 65.6 % eccentricity) on the compressive and flexural behavior of GFRP-reinforced circular hollow columns. The test results show that the behavior of these columns was more significantly affected by the level of applied eccentricity than the longitudinal reinforcement ratio. The failure of specimens loaded under low eccentricity levels was compression controlled due to spalling of the concrete cover, followed by fracturing of the GFRP bars, attributed to most of the cross section being subjected to high compressive stresses. Conversely, initiation of failure for the specimens tested under high eccentricity levels was flexural tension followed by tensile cracking, which caused a secondary compression failure in the concrete. It should be noted that the contribution of the second-order effect on the total moment capacity was minor, approximately 9 %, compared to the primary bending moment induced by the initial eccentricity of 91 % on average. Lastly, the ductility of the hollow GFRP-reinforced concrete columns was higher than that of solid columns under eccentric loading, which significantly improved the energy absorption ability, leading to a decrease in the strength decay beyond the peak load.

Feasibility of using Static-Cast Concrete Transmission Poles fully reinforced with glass-fibre reinforced polymer bars and stirrups: A case study

The findings of cantilever beam bending tests conducted on a series of full-scale glass fibre-reinforced polymer (GFRP) and steel-reinforced concrete utility poles are presented. The poles were reinforced by (i) longitudinal steel bars and steel stirrups (ii) longitudinal GFRP bars and steel stirrups, and (iii) longitudinal GFRP bars and GFRP stirrups. Crack initiation and propagation as well as load carrying capacity and free end deflections of GFRP reinforced poles were obtained and compared with their counterpart steel reinforced poles. Based on the results all longitudinal GFRP reinforced poles have shown higher load-carrying capacity than that of steel reinforced poles, while due to the lower stiffness of GFRP bars in comparison to steel bars, at a certain load, higher free-end deflections were observed in GFRP reinforced poles. However, the permanent deformation of GFRP-reinforced poles was small. Furthermore, comparable results obtained for poles reinforced with GFRP stirrups than those of poles reinforced with steel stirrups, showed that GFRP stirrups could also be used safely instead of steel stirrups. However, due to the low elastic modulus, GFRP-reinforced poles likely fail to satisfy the deflection criterion specified in some standards. But, if adequate stiffness is provided for GFRP-reinforced concrete poles to satisfy the deflection criterion, due to the vulnerability of steel-reinforced utility poles when subjected to corrosive environments, using fully GFRP-reinforced concrete poles could be a feasible option.

Cyclic behavior of GFRP-reinforced concrete one-way slabs with synthetic fibers

This paper investigates the cyclic behavior of one-way glass fiber-reinforced polymer (GFRP)-reinforced concrete (GFRP-RC) slabs incorporating synthetic fibers. The study aims at improving the performance of GFRP-RC slabs both at serviceability (SLS) and ultimate limit states (ULS). A total of 8 slabs were tested under cyclic bending and the effect of fiber type (polypropylene (PP) and polyvinyl alcohol (PVA)), fiber dosage (0, 0.5%, and 1.0%), and the type of reinforcement (steel and GFRP) was investigated. The comparison and evaluation of the slabs’ behavior was made in terms of crack width and deflection performance, ductility, failure mode, and overall load and deflection capacity. The results showed that the effect of fibers was more pronounced in GFRP-RC slabs compared to steel-RC slabs, both at the SLS and ULS. This is due to their reduced neutral axis depth as a result of the lower elastic modulus GFRP bars. Moreover, the fibers were found to reduce crack widths and deflections and enhanced the energy absorption; however, they did not significantly enhance the ductility of the slabs. The addition of fibers also resulted in an elastic response with most of the total applied deflection recovered at the end of each cycle of loading. Compared with steel-RC members, the cost of repair of GFRP-RC members seems to be relatively lower because of the reduced permanent deflection of the slab.

Performance evaluation of normal- and high-strength concrete column specimens reinforced longitudinally with different ratios of GFRP bars

Glass fibre-reinforced polymer (GFRP) bars, as reinforcement in concrete structures, have several desirable properties over conventional steel bars, such as high tensile strength, low density and resistance to corrosion. However, there are still some concerns about using GFRP bars as longitudinal reinforcement in concrete columns primarily due to their low compressive strength and low modulus of elasticity. It is evident that the compressive behaviour of GFRP bars as longitudinal reinforcement in normal strength concrete (NSC) and high strength concrete (HSC) columns has not been extensively investigated to formulate adequate design guidelines. In this study, 12 reinforced concrete column specimens were tested under concentric compression loading. All the tested column specimens were 600 mm in height and 154 mm in diameter. The column specimens were divided into two groups as per the compressive strength of the concrete used in casting the column specimens, which were 45 MPa and 90 MPa. Each group consisted of six column specimens, one plain concrete column specimen and five column specimens reinforced transversely with 6 mm steel helices spaced at 40 mm pitch (transverse reinforcement ratio ρt = 2.2 %). Out of those five column specimens, one had no longitudinal reinforcement (ρ = 0), one was reinforced longitudinally with steel bars (ρ = 2.4) and the rest were reinforced longitudinally with GFRP bars (ρ = 2.7, 3.2, and 4.3 %). From testing results, it was observed that steel and GFRP bar reinforced NSC column specimens exhibited two peak axial loads, with a higher second peak axial load than the first peak axial load, whereas steel and GFRP bar reinforced HSC column specimens exhibited one peak axial load only. It was also observed that the contribution of the longitudinal steel bars was about two times the contribution of GFRP bars in both NSC and HSC column specimens. Finally, the ductility of GFRP bar reinforced NSC column specimens was greater than the steel bar reinforced NSC counterpart, whereas this trend was the opposite in the column specimens cast with HSC.

Performance of RC beam-column joints with hybrid GFRP-steel reinforcement under cyclic and axial loads

Structural reinforced concrete (RC) elements incorporating glass fiber-reinforced polymer (GFRP) bars have been claimed to exhibit notable deformability under seismic loading. In this research, an attempt has been made to evaluate the influence of replacing steel bars with GFRP bars on the behaviour of RC beam-column joints. To this end, five ½ scale joints were fabricated and examined under cyclic and axial loading. The joints were the same in terms of specimens’ geometry, material properties and test setup while they differed in the number of beams’ steel bars replaced with GFRP bars. Otherwise noted, GFRP-to-steel bars ratio was changed by 0 (reference specimen), 25%, 50%, 75% and 100%. The crack propagation and force–displacement hysteresis curves are presented as the main test outputs. Moreover, seismic parameters namely displacement ductility, absorbed energy, stiffness and equivalent damping ratio are compared and discussed. Overall, the results demonstrated that incorporating GFRP bars by the ratio of less than 50% could enhance cyclic response of joints in terms of ductility, absorbed energy and equivalent damping ratio.

Torsional behavior of GFRP-reinforced concrete pontoon decks with and without an edge cutout

Using glass fiber-reinforced polymer (GFRP) bars in precast concrete pontoon decks in aggressive marine environments would eliminate corrosion problems encountered with steel reinforcement. Although wave action usually subjects pontoon decks to torsion, but little is known about the behavior of such structures. Moreover, nothing is known about the effects of cutouts on the torsional behavior of pontoon decks. This study experimentally investigated the torsional behavior of GFRP-reinforced concrete (RC) pontoon decks in terms of the effect of edge cutouts, reinforcement-bar distribution, and rotation direction. The results show that the bar configuration affected the failure behavior and torsional capacity of the GFRP-reinforced concrete decks under torsion. The decks with double-layer reinforcement exhibited slower and narrower cracking growth in the post-cracking stage than the decks with single-layer reinforcement. In addition, the edge cutouts reduced the cracking torque of the solid rectangular decks by an average 17%. The torsional behavior of the GFRP-reinforced planks can be accurately described by the ACI318-14 equation in the cracking stage, while the decks’ post-cracking torsional rigidity can be predicted accurately from the contribution of the GFRP bars.

Comparative study on the bond performance of near-surface mounted fiber-reinforced polymer bars

This paper presents the experimental and analytical results of a large study on the bond performance of near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars. The investigated parameters included the bar material (basalt-FRP (BFRP), glass-FRP (GFRP), carbon-FRP (CFRP), and stainless steel (SS) bars), the bar surface configuration (deformed and sand-coated bars), the filling adhesive (NSM-Gel, Sikadur-30, and Sika Grout-214), and the bonded length of the bar (6, 12, and 24 times its diameter). Sixty-six C-shape concrete specimens were tested under direct pullout loading configuration. The bond strength, the free-end and loaded-end slip, the strains in the NSM bar, and the modes of failure of the tested specimens are reported and discussed. The NSM-Gel adhesive outperformed other adhesives in developing the bond strength of the tested specimens regardless of their bar material. Both the deformed and sand-coated NSM-BFRP and GFRP bars showed almost similar bond strengths while the NSM-CFRP bars showed the highest strengths. The images obtained from the scanning electron microscope confirmed the obtained results in terms of the modes of failure and the bond failure mechanism. Analytically, the BPE model was calibrated using the experimental results to describe the bond stress-slip relationships of the FRP bars.

Fatigue behaviour of joint-free bridges with steel and GFRP-reinforced ECC link slabs

Engineered Cementitious Composite (ECC) link slabs provide a feasible alternative to leaking expansion joints for improving long-term durability of slab-on-girder bridges. The use of Glass Fiber Reinforced Polymer (GFRP) rebars have become increasingly common for inducing superior chemical resistance in bridge decks in salt-prone locations. Existing literature lacks experimental research on effects of fatigue load on joint-free bridges with ECC link slabs reinforced with GFRP bars. This paper presents results of experimental testing of joint-free bridges with GFRP or steel-reinforced ECC link slabs subjected to static and fatigue (up to 1 million cycles at 4 Hz) loadings. Comparative performance between GFRP and steel-reinforced ECC link slab bridges is described based on load–deflection or moment-rotation behaviour, strain characteristics and crack development in pre-fatigue, fatigue and post-fatigue stages. After 1 million fatigue cycles, the GFRP-reinforced ECC link slab exhibited smaller crack widths and higher deformation capacity while its incorporation provided superior ductility and deformation capability of the full bridge. A finite element (FE) model was also developed using experimental results to simulate load–deflection behaviour of composite deck-steel girder joint-free bridge with ECC link slab subjected to fatigue loading. Parametric FE study showed that increasing mean stress level caused increased composite deck-steel girder deflection while link slab deflection was limited by bridge span deformation.

Near surface mounting technology with glass fibre reinforced polymer and sisal rope fibre for flexural strengthening of reinforced concrete beams

Using the near-surface mounted (NSM) technique, the purpose of this work is to investigate effectiveness of the flexural strengthening of the reinforced concrete (RC) beam. Extruded Glass Fibre Reinforced Polymer (GFRP) bars and sisal rope fibres are both being used as part of the research project in order to enhance the material. Following that, the purpose of this research is to evaluate the impact that embedment length has on the flexural behaviour of the beams. In order to accomplish this goal, the NSM reinforcements were either placed throughout the length of the beam or in the flexure zone (max bending moment area). In all, ten beams were put through the two-point load testing procedure. The results of the tests showed that beams strengthened with GFRP bars performed better than those that had been mended with sisal ropes. The GFRP strengthened beams showed increase of 28% in flexural capacity of the beam whereas the sisal rope strengthened beams showed 16% increase in the flexural capacity of the RC beam. In addition, beams that were retrofitted with NSM reinforcements over the whole length of the beam exhibited enhancement 8% in flexural capacities than beams that were just modified in the flexure zone (maximum bending moment region).

The mechanical properties of GFRP bars embedded in geopolymer concrete after high temperature exposure

Geopolymer concrete prepared with fly ash and slag has exhibited excellent thermostability and thermal insulation performance. Through combining geopolymer concrete with glass fiber reinforced polymer (GFRP) bars, the existing problems like high energy consumption, massive CO2 emission and steel bars corrosion in traditional reinforced concrete structures can be solved. To investigate the mechanical properties of GFRP bars embedded in geopolymer concrete after high temperature exposure, a test was conducted on 419 specimens in this study. Factors that influence the performance include exposure temperature, exposure time and type of concrete cover. The results demonstrated that the tensile properties of bare GFRP bars decreased significantly in the temperature range from 400 °C to 600 °C, and the ratio of tensile strength loss was as high as 50%–90%. The geopolymer concrete cover can effectively slow down and mitigate the degradation in tensile strength and elastic modulus of GFRP bars after high temperature exposure (400–600 °C), but this effect weakens following the rise in exposure temperature and time. In addition, the degradation mechanism of GFRP bars within geopolymer concrete was further analyzed by scanning electron microscopy (SEM) in the microscopic level. Finally, the contribution ratio of each influence factor to the residual tensile strength of GFRP bars after high temperature exposure was numerically illustrated using the statistical method of analysis of variance (ANOVA).

Long-term tensile performance of GFRP bars in loaded concrete and aggressive solutions

To better understand the long-term mechanical performance of glass fiber reinforced polymer (GFRP) bars under aggressive environments, tensile testing was conducted on GFRP bars after exposure to two environmental conditions: 1) immersed in alkaline and saline solutions both for 180 days; and 2) embedded in sustain-loaded concrete beams with or without NaCl solution wet-dry cycles for 366 days. Experimental results revealed that the failure modes and stress-strain curves of tested GFRP bars exhibited similar characteristics regardless of above various aggressive conditions and exposure periods. As to specimens immersed in solutions for 180 days, the degradation of tensile performance in saline solution was smaller than that in alkaline solution. Furthermore, no significant degradation of elastic modulus was observed in the experiment. For GFRP bars embedded in sustain-loaded concrete beams, the average retention of tensile strength was around 93% after one-year experience in concrete, which was nearly 5.0% lower than that of non-stressed GFRP bars in concrete. It is predicted from a comparative study that the degradation of GFRP bar placed in concrete (100% moist) for 7.5 years was equivalent to that exposed to alkaline solution for one year. Based on SEM observations, the degradation mechanism of tensile performance for GFRP bars was attributed to the reduction of bonding between fiber and matrix. Moreover, according to the Fib bulletin 40 method, the tensile strength retention of GFRP bar was predicted to be ranged from 62.8% to 75.1% after 100 years of service under various conditions.

Experimental Investigation on the Cyclic In-Plane Behavior of GFRP-Reinforced Concrete Shear Walls

The current study presents an experimental investigation performed on slender reinforced concrete shear walls, representing a common lateral-load resisting system of mid-rise buildings. The walls were reinforced with steel and glass fiber-reinforced polymer (GFRP) bars and tested up to failure under reversed quasi-static cyclic loading to investigate the capability of GFRP bars in reinforcing RC shear walls under seismic loads. Moreover, the effect of the GFRP reinforcement ratio on the structural response, deformation performance, and failure mode resulting in RC walls, compared with its behavior when reinforced with steel bars, is also investigated. Six full-scale shear walls with an aspect ratio of 3.25 were constructed. The reference wall was entirely reinforced with steel bars. Two specimens were reinforced by hybrid scheme of GFRP–steel bars. The remaining three shear walls were entirely reinforced with GFRP bars. The overall performance of each wall was characterized by investigating the lateral load capacity, hysteretic response, cracks propagation, ductility, and the behavior of energy dissipation. The experimental results showed that GFRP-reinforced concrete walls had an elastic behavior characterized by a stable hysteretic response with recoverable deformation of more than 80% of the ultimate load. However, sudden and brittle failure was attained for the wall with a high GFRP reinforcement ratio. GFRP decreases the displacement ductility of the shear walls by an average of 32.9%, depending on the reinforcement ratio, compared to that reinforced by steel bars. Moreover, lower energy dissipation through inelastic deformation was obtained for the walls reinforced with GFRP bars. Nonetheless, when GFRP bars are combined with steel bars, acceptable levels of dissipated energy are attained compared to the steel-reinforced wall.

Bond behavior of helically wound FRP bars with different surface characteristics in fiber-reinforced concrete

Bond behavior of fiber reinforced polymer (FRP) bar in concrete is critical for structural performances. This study investigates the effects of bar type, steel fiber content and surface characteristics on the bond behavior of FRP bars. Pull-out tests of basalt and glass FRP bars with different rib heights (smooth, shallow and deep) were conducted in concretes with various steel fiber contents (0%, 1.0% and 1.5%). Experimental results indicate that bond strength of FRP bars is significantly affected by the height of rib. The bond strength can be significantly increased from less than 2.5 MPa to 30 MPa by increasing rib height from 0 to 0.76 mm. The inclusion of 1.0% steel fibers in volume can prevent the splitting failure. A novel bar geometry-based model for calculating the bond strength of helically wound FRP bars is proposed by combining the experimental data in this study and existing data in literatures.

Seismic behavior and restoring force model of GFRP-RC beam-column interior joints

In this paper, the beams-column interior joint in the concrete frame structure was taken as the main research object. First, four joints reinforced with GFRP (glass fiber reinforced plastic) bars were used to analyze the influences of reinforcement ratios under reversal cyclic loading. Then, the failure mechanism of the joints with GFRP bars was analyzed based on the damage of concrete and the bars. Finally, the corresponding restoring force model was proposed to show the bearing capacity and deformation characteristics of joints. The results show that the seismic behaviors of the GFRP-RC (glass FRP reinforced concrete) joints are good due to the high drift ratio, which can reach to about 6%. The energy dissipation of the joints mainly comes from the compression damage of the concrete. Compared with steel reinforced concrete, the high ultimate drift rate increases the range of concrete involved in energy consumption, which makes up for low energy consumption of GFRP bars. The stiffness degradation of the skeleton curve is related with the damage of concrete and the surface damage of the reinforcement. Besides, the damage of the bars was concentrated on the surface of the rib marks. The four-stage hysteretic model proposed in this paper can better reflect the seismic behaviors of joints. This study aims to provide research basis for the researches of GFRP-RC structures from the component level to the structural level.

Quasi‑static Cyclic In‑plane Testing of Slender GFRP-Reinforced Concrete Shear Walls

Using Glass fiber-reinforced polymer (GFRP) bars as a replacement for conventional steel bars is one of the most potential solutions to steel-corrosion-related problems in concrete. Their durability and high strength-to-weight ratio make them a cost-effective and applicable alternative to conventional steel bars. This study investigates the characteristic behavior of concrete shear walls reinforced with steel, GFRP, and a hybrid scheme of steel and GFRP bars under seismic loading. Six full-scale RC shear walls with an aspect ratio of 3.25 were tested under pseudo-static reversed-cyclic lateral load to investigate the potential of a hybrid reinforcement scheme of steel-GFRP to improve the seismic behavior of slender RC shear walls. The overall performance of each tested wall was characterized by investigating the hysteretic response, crack propagation, lateral load capacity, and energy dissipation behavior.

A case study on the feasibility of using static-cast fibre-reinforced concrete electric poles fully reinforced with glass fibre reinforced polymer bars and stirrups

This study experimentally investigates the structural performance of full-scale reinforced concrete utility poles fully reinforced with glass fibre reinforced polymer (GFRP) bars. The weak bond strength between GFRP bars and concrete in full-scale structures has been addressed by using an innovative anchorage system. Furthermore, fibre-reinforced concrete (FRC) was successfully used to enhance the crack propagations of GFRP-reinforced concrete poles. Three poles were constructed from FRC, while one was constructed with normal concrete (NC) as a benchmark. Polypropylene (PP) and crimped polyolefin-based macro-synthetic (CPO) fibres were used. An innovative anchorage system was used to increase the bond strength between longitudinal bars and the concrete. Crack initiation and pattern, load-carrying capacity and vertical deflections of GFRP reinforced FRC poles were obtained and compared with their normal concrete counterparts. According to the results, the PP-FRC pole showed better crack propagation performance, while the CPO-FRC showed higher stiffness than the normal concrete poles. In addition, regardless of the concrete type, the permanent deformation of all GFRP reinforced concrete poles after different stages of loading were significantly smaller than those of typical steel reinforced concrete poles.

Thermal Performance of Concrete Slabs Reinforced with Glass Fiber-Reinforced Polymer Bars

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Effect of Compressive Glass Fiber-Reinforced Polymer Bars on Flexural Performance of Reinforced Concrete Beams

Effect of Compressive Glass Fiber-Reinforced Polymer Bars on Flexural Performance of Reinforced Concrete Beams

Investigation on mechanical properties of lightweight sandwich exterior wall with thermal blocking shear connector

In order to solve the problems of obvious thermal bridge effect for metal shear connectors and the lower shear bearing capacity for non-metal shear connectors, two new types of thermal blocking shear connectors (TBSC) were investigated. One is made by bond stress between glass fiber reinforced polymer (GFRP) sleeve and reinforcement, and the other is made by mechanical connection between the steel sleeve and GFRP bar. The goal is to study the mechanical properties of the lightweight sandwich exterior wall with TBSC. There are 18 specimens were conducted based on the mechanical properties, and the parameters of different types, different embedded depths and different bending angles were considered. The pull-out and direct shear tests were carried out, and each parameter's influence on the result was compared and analyzed. The test results show that with the increase of bending angle, the bearing capacity of GFRP sleeve TBSC increases. When the embedded depth of the TBSC increases from 25 mm to 40 mm, the pull-out test and direct shear test values increase by 95 % and 59 %, respectively. The main failure mode of specimens is the concrete anchorage failure. According to the phenomenon of concrete anchorage failure, the calculation formulas of flexural and direct shear capacity were proposed. Moreover, the finite element simulation is used to extend and validate, providing a theoretical reference for designing and applying a lightweight sandwich external wall with TBSC.