Glass Fiber-reinforced Polymer Bars In Concrete Research Articles

Bond behaviour of staggered/non-staggered lap-spliced GFRP bars in concrete

This paper reports on the results of an experimental and analytical investigation into the bond behaviour of glass fibre-reinforced polymer (GFRP) splices in concrete. Eleven full-scale beam specimens in which GFRP bars were spliced within the constant-moment zone are examined. The study incorporates several key test variables, encompassing the ratio of bars lapped (33%, 50%, and 100%), surface characteristics (including sand-coated, ribbed, and wrapped bars), bar diameter (12 mm and 16 mm), and splice length (ranging from 30 to 40 times the bar diameter). The findings reveal a reduction in bond strength ranging from 12 to 24% for staggered bars when compared to their non-staggered counterparts. Furthermore, an increase in bar diameter is shown to decrease bond strength by 16%, whilst surface treatment also influences both bond strength and the pattern of cracking in the splice region. Increasing the splice length from 30 to 40 times the bar diameter enhances bar failure stress by 26%, but it also reduces the average bond strength by 9%. A new predictive equation for the design bond strength of fibre-reinforced polymer (FRP) bars is developed by utilizing a data base of 151 test results. The predicted bond strengths demonstrate greater consistency with experimental outcomes in comparison to previous models.

Effect of Bonding Area on Bond Stress Behavior of GFRP Bars in Concrete

The application of Glass Fiber Reinforced Polymer (GFRP) bars is suitable for concrete structures that are susceptible to corrosion, owing to their corrosion-resistant characteristics. Therefore, it is feasible to reduce the concrete cover on reinforced concrete beams by utilizing GFRP bars. However, this can reduce the bonding strength between GFRP bars and concrete. Therefore, this study aims to investigate the bonding behavior between GFRP bars and concrete as a preliminary test for structural applications. The bond stress behavior between GFRP bars and concrete was analyzed by 18 pull-out tests. The test specimens comprised GFRP bars with three different variations, namely GFRP bars with concrete cover (GFRP-C), GFRP bars without concrete cover (GFRP-E), and GFRP bars with a complete wrapping of GFRP sheet (GFRP-C-Sheet). The bond stress-slip curve, bond strength, and failure pattern were utilized to analyze the effect of each variation. The research results indicate that the bonding stress between GFRP bars and concrete was strongly influenced by the concrete cover, where the bonding strength decreased by 65%. Nevertheless, the utilization of a complete wrapping GFR) sheet resulted in a 26.4% increase in bonding stress. The present study has identified three distinct modes of failure, including pull-out (GFRP-C), concrete crushing (GFRP-E), and GFRP sheet debonding (GFRP-C-Sheet). Doi: 10.28991/CEJ-SP2023-09-010 Full Text: PDF

Durability of bond between carbon/glass hybrid fiber-reinforced polymer (HFRP) bar and concrete in water

Glass-fiber-reinforced polymer (GFRP) bars have been widely used as a reinforcement in concrete, and glass fibers are susceptible to reacting with alkali ions in concrete pores. Thus, carbon fibers are wrapped around GFRP bars to produce carbon/glass hybrid fiber-reinforced polymer (HFRP) bars so as to promote the durability of the internal GFRP bars in concrete. This work studies the durability of the interfacial bond between GFRP bars/HFRP bars and concrete using pullout tests. The specimens are tested after immersion in distilled water for 0, 150, 255, and 544 days. The failure of the GFRP bar–concrete composite is due to damage to the GFRP bar, while the HFRP bar–concrete composite fails because of a combination of the scratching of the HFRP bar and the failure of the concrete. Both HFRP and GFRP bars are scratched as the immersion period extends. Further, the strength of the bond between the HFRP bar and concrete is lower than that of the bond between the GFRP bar and concrete, while the bond stiffness of the HFRP bar–concrete composite is higher than that of the GFRP bar–concrete composite. The bond strength of the HFRP bar–concrete and GFRP bar–concrete composites increases in the early immersion periods but decreases after 544 days of immersion in water. The retention of the bond strength of the HFRP bar–concrete composite is superior to that of the GFRP bar–concrete composite. The carbon fiber coat is proven to improve the durability of the bond between HFRP bars and concrete in water. A formula for predicting the bond strength of HFRP bar–concrete and GFRP bar–concrete composites in water is also developed. Finally, the modeling results demonstrate that the bond strength retention of the HFRP bar–concrete and GFRP bar–concrete composite is 76% and 46% after 50 years of service time, respectively.

Prediction of the Long-Term Tensile Strength of GFRP Bars in Concrete

The durability of two types of widely used glass fiber reinforced polymer (GFRP) bars, one without coating (G1) and one with slightly surface sand-coating (G2), were studied through accelerated aging. Concrete cylinders reinforced with GFRP bars were immersed in tap water in temperature-controlled tanks. The influence of different exposure temperatures, 20, 40, and 60 °C, and also different exposure times, 30, 60, 90, 120, and 180 days, on the degradation of the two types of GFRP bars was investigated. The tensile strengths of GFRP bars after different exposure times were evaluated with tensile tests, and the variation of the microstructure and elemental compositions of conditioned specimens was evaluated with scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS), respectively. The degradation rate of the tensile strength retentions of two types of GFRP bars decreased with an increase in the exposure time at all exposure temperatures. The tensile strength retentions of the GFRP bars were studied by three commonly used prediction models. Based on the degradation mechanism of fiber-matrix debonding, a new model was proposed. These four models were evaluated with the test results and a new model proposed was suggested as the best model to predict the residual tensile strength of the GFRP bars. The durability parameters of the GFRP bars were discussed. The tensile strength retention tended to converge to a constant value (52%) with the increase of exposure time, which contributes to the determination of the environmental reduction factor in relevant design guides, and the fiber-matrix debonding was found to be the main degradation mechanism due to the surrounding concrete environment. The sand-coating had some effect on the activation energy of the GFRP bars.

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.

BOND STRENGTH OF GFRP BARS IN FIBER CONCRETE IN COASTAL ENVIRONMENT CHARACTERISTIC OF THE MEKONG DELTA

Glass fiber-reinforced polymer (GFRP) is considered an alternative to steel reinforcement in concrete structures in highly corrosive environments. The bond characteristics of GFRP bars in concrete is an important parameter that determines the feasibility of using this material with non-corrosive concrete. Thus, the paper presents the experimental results of the bond strength of GFRP bars in fiber concrete in a coastal environment characteristic of the Mekong Delta in the south of Vietnam. Research results within 2 years showed that the bond strength of GFRP bars in fiber concrete increased over time and gave a maximum value corresponding to 5.0% silica fume content. In this study, the correlation between the bond strength of control specimens that were cured in the laboratory and specimens that were cured in actual conditions was established. In addition, the experimental results also showed that the impacts of salty water and alum (pH, salinity, temperature, wet-dry cycle) had almost no effect on the bond strength of GFRP bars in fiber concrete. These environmental influences need to be investigated further.

Influence of elevated temperatures on the bond behaviour of ribbed GFRP bars in concrete

FRP-reinforced concrete (RC) structures operating for several years in chemically aggressive environments have confirmed the good mechanical performance and durability of FRP reinforcement. However, the concerns and lack of knowledge about fire behaviour is still hampering its use in buildings; in fact, even moderately elevated temperatures are reported to cause premature loss of bond in overlapping rebars, potentially affecting the structural safety of load bearing members exposed to fire. This paper presents experimental and analytical investigations on the bond behaviour between glass fibre reinforced polymer (GFRP) rebars and concrete at moderately elevated temperatures. Steady-state tensile and pull-out tests were performed in two types of ribbed GFRP rebars, with different glass transition temperatures (Tg of 104 °C and 157 °C), from ambient temperature up to 300 °C. The results obtained were also compared with those of fibre wrapped-sand coated GFRP rebars (Tg of 98 °C) previously investigated by the authors. As expected, the tensile strength and elastic modulus of the ribbed GFRP rebars and especially the strength and stiffness of the GFRP-concrete interface were severely degraded with temperature. This study highlighted that the key parameters governing the bond to concrete at elevated temperatures are the surface finish and Tg of the rebars – at 100 °C, bond strength reductions between 20% and 34% were obtained for the ribbed rebars, while in the sand coated rebars such reduction had been 81%; most of the reduction of the GFRP-concrete interaction in the ribbed rebars occurred for temperatures above the rebars Tg, contrary to the sand coated rebars in which bond had been almost completely degraded below its Tg. Based on the test results, for each temperature, local bond stress vs. slip models were numerically calibrated for the specific ribbed GFRP rebars used in the experiments. These laws can be used in advanced numerical models to simulate the fire behaviour of GFRP-RC members.

Bond behavior of helically wrapped sand coated deformed Glass Fiber-Reinforced Polymer (GFRP) bars in concrete

Using Fiber-Reinforced Polymer (FRP) bars in reinforcing concrete structures requires understanding the bond behavior between the two materials to ensure adequate force transfer and crack control. Currently, there are no standards for characterizing the surface condition of FRP bars, which results in significant variability on the bond behavior of FRP bars. Thus, this paper presents the experimental investigation conducted to evaluate the bond behavior of helically wrapped sand coated deformed glass fiber-reinforced polymer (GFRP) reinforcing bars. The bond-slip relationship of 96 pullout test specimens were analyzed to determine the effects of bar size and production variability. Test results demonstrated that force transfer between GFRP bars and concrete is achieved by chemical adhesion, mechanical interlock (bearing), and friction. All tested bar sizes demonstrated the same bond-slip relationship; however, small bars developed higher bond strength than large bars. Also, test results indicated that production variability could result in significant differences in the bond behavior of GFRP bars in concrete. Test data were used to calibrate and compare the existing bond-slip models of FPR bars and a new model for the bond-slip relationship of the helically wrapped sand coated deformed GFPR bars was proposed.

Using genetic programming to model the bond strength of GFRP bars in concrete under the effects of design guidelines

Purpose This paper aims to use a derivative of genetic programming to predict the bond strength of glass fiber-reinforced polymer (GFRP) bars in concrete under the effects of design guidelines. In developing bond strength prediction models, this paper prioritized simplicity and meaningfulness over extreme accuracy. Design/methodology/approach Assessing the bond strength of GFRP bars in concrete is a critical issue in designing and building reinforced concrete structures. Findings Ultimately, the equation of a linear form of a particular design guideline was suggested as the optimal prediction model. Improvements to the current design guidelines suggested by this model include setting a 1.31 magnification and considering the effects of the three significant parameters of bar diameter (db), minimum cover-to-bar diameter (C/db) and development length to bar diameter (l/db) under an acceptable root mean square error accuracy of around 2 MPa. Furthermore, the model suggests that the original influence parameter of concrete compressive strength (fc) may be removed from bond strength calculations. Originality/value The model suggests that the original influence parameter of concrete compressive strength (fc) may be removed from bond strength calculations.

Effect of temperature on the bond behaviour of GFRP bars in concrete

Glass Fibre Reinforced Polymer (GFRP) bars have been employed as internal reinforcement for concrete members when corrosion of the commonly used steel bars is expected to be an issue. While a good bond is anticipated between GFRP bars and concrete at ambient temperature, the bond performance at high temperature is expected to be reduced due to the physical and mechanical changes that the matrix undergoes at temperatures approaching the glass transition temperature (Tg). Up to date this phenomenon has only been marginally investigated and most of the available bond tests are performed at ambient temperatures after cooling of the heated specimens.This paper presents the results of an experimental investigation on the bond behaviour of GFRP bars in concrete and exposed to temperature levels ranging from ~20 °C to 300 °C. The test specimens, consisting of an indented GFRP bar embedded in a cylindrical concrete block, were heated in an electric furnace. The pull-out tests were carried out within the furnace only after the temperature level, measured with thermocouples at the interface of GFRP and concrete, stabilized to the desired value.The paper discusses the effect of temperature on bond behaviour in terms of bond strength, bond stress-slip relationships and failure modes. A contactless technique measuring the free-end slip during pull-out tests at high temperatures was developed and its effectiveness demonstrated. Finally, the experimental results were used to calibrate the parameters of the two most widely used analytical models: mBPE and CMR.

Establishment and experimental validation of an updated predictive equation for the development and lap-spliced length of GFRP bars in concrete

Glass fibre-reinforced polymer (GFRP) reinforcement due to its physical and mechanical behavior has a completely different bond behavior compared to the steel bars. In this paper, an equation is proposed to predict the splice length in GFRP-reinforced concrete beams. First, equations for the local bond strength and displacement modulus of GFRP bars are obtained by using the eccentric and concentric pull-out test results given in the literature. Then, an equation is derived for the bond strength of spliced GFRP bars in beams. In the derivation of this equation, the non-uniform distribution of the bond stress along the splice length and the effect of elastic modulus of GFRP bars are taken into account. Compared to other available equations and design guidelines, the proposed equation for bond strength calculation shows good agreement with the experimental results. Using the proposed equation for bond strength, an equation is also proposed for the splice length of GFRP bars.

Bond mechanism and bond strength of GFRP bars to concrete: A review

Abstract Glass fiber-reinforced polymer (GFRP) reinforcements are taken as an alternative solution for the deterioration of civil infrastructures. GFRP bars have received increasing attention due to low cost compared to carbon fiber-reinforced polymer (CFRP) bars. Bond characteristic of GFRP bars in concrete is the most critical parameter for implementation of the material to the corrosion-free concrete structures. Unlike steel reinforcement, GFRP materials behave anisotropic, non-homogeneous and linear elastic properties, which may result in different force transfer mechanism between reinforcement and concrete. With the purpose of covering the most valuable contributions regarding bond mechanism in the past work, a comprehensive review focusing on the failure mode and bond strength is carried out in this paper. A database consisted of 682 pullout-test specimens was created to observe the factors affecting bond behavior. Basic relationship between bond strength/slip and factors was analyzed accordingly. In addition, the development of bond degradation under environmental conditions, such as freezing-thawing cycling, wet-dry cycling, alkaline solutions and high temperature was presented thereafter. These environmental influences need to be further investigated.

Fatigue Behavior of GFRP Reinforcing Bars in Air and in Concrete

Although the use of glass fiber-reinforced polymer (GFRP) reinforcing bars is gaining acceptance as an alternative to conventional steel reinforcement in concrete structures, little empirical data exist on the long-term fatigue performance of GFRP bars in concrete. Abrasion at the bar-concrete interface is known to have an adverse effect on bonds and on the overall member response under cyclic loading, although this effect was not adequately quantified until now. This paper presents the results of an experimental study comparing the fatigue performance of GFRP bars in air with the behavior of similar bars embedded in concrete. The experimental findings showed that the GFRP bars tested in air survived fatigue lives that were approximately a full order of magnitude longer than similar bars in concrete beams. The results were used to calibrate a statistical fatigue life model and a residual stiffness model for GFRP bars in reinforced concrete flexural members.

Effect of different environments on bond strength of glass fiber-reinforced polymer and steel reinforcing bars

The long-term durability of Glass Fiber-Reinforced Polymer (GFRP) bars in concrete need to be investigated and analyzed. This paper presents an experimental investigation into comparison of the durability of the bond between GFRP and steel reinforcing bars in concrete under different environment. For all bars, five environmental conditioning media were used. The bond strength after exposure was considered as a measure of durability of the bond between the bars and concrete. Ninety pullout specimens were used to study the effect of different environment on bond strength of GFRP and steel bars to concrete. Experimental results showed no indication of any bond degradation due to the continuous immersion in tap water, alkaline and salt solutions for ninety days. To the contrary the bond strength values of the conditioned specimens were comparable or slightly higher than the bond of the controlled specimens. The acid+chloride solutions have more detrimental effect on bond strength of GFRP bars in concrete. GFRP and steel bars have similar bond strength retention under alkaline environment. And the rate of bond degradation is very slow due to concrete reinforcement.

Long-term durability prediction models for GFRP bars in concrete environment

This article presents a durability study of glass fiber reinforced polymer (GFRP) bars within concrete environment and provides recommendations for effective accelerated ageing methods, data reduction and validation, and formulation of master curves for long-term predictions. Concrete beams reinforced with GFRP bars were immersed in tap water inside temperature-controlled tanks. Sustained load was applied to beam samples, and elevated temperatures were used to accelerate the ageing of GFRP bars. After exposure, GFRP bars were tested for residual tensile strength, which was considered as a measure of the durability performance of the specimens. Scanning electron microscopy (SEM) images were used to evaluate the microstructural changes of conditioned specimens. Short-term (up to 270 days) results showed that compared to alkalinity and low sustained load, moisture and elevated temperature played much more important roles in accelerating environmental attack to GFRP bars. Based on the short-term test results, the dominant degradation mechanism for GFRP bars in concrete was found as the deterioration of fiber/matrix interface. A degradation model based on Arrhenius concept and prediction procedures are given, and correlations with other studies and field data are provided. The tensile strength retentions for GFRP bars in saturated concrete subjected to sustained load were predicted. It was found that degradation rates decreased rapidly, but tensile strength retentions at different exposure temperatures converge to a constant value with the increase of exposure time. Beyond fiber-matrix debonding, there is a possibility that other degradation mechanisms such as fiber corrosion may start to dominate the deterioration process of GFRP bars with time.