Concrete Fiber Reinforced Polymer Bars Research Articles

Development of an artificial neural network based-prediction model for bond strength of FRP bars in concrete

Fiber-reinforced polymer (FRP) bars have garnered increasing attention in recent years due to their superior corrosion resistance, offering a potential solution to the significant drawback of steel corrosion in concrete. For the widespread utilization of FRP bars in concrete structures, determining the bond strength between FRP bars and concrete is a crucial topic. This study seeks to develop a prediction model to estimate the bond strength of FRP bars in concrete, utilizing an extended dataset from 1010 pull-out tests. Initially, the study evaluates the applicability of several bond strength formulas from existing codes. Subsequently, two prediction models, namely a multivariate linear regression model and an artificial neural network (ANN) model, are introduced for estimating the bond strength of FRP bars in concrete. The results indicate that the correlation between the evaluation values of existing formulas and the experimental value is very low. This is because these formulas have not yet been updated to encompass the expanded usage scopes of FRP bars with various surface processing methods and types of concrete. While the multivariate linear regression model outperforms these formulas, its accuracy is still relatively low; in contrast, the ANN demonstrates superior performance, achieving an R^2 value for both the validation and test set of more than 0.92. The findings highlight that, when considering a broader range of applications, the ANN serves as a robust tool for accurately predicting the bond strength of FRP bars in concrete, in comparison to traditional formulas and linear regression models. This assessment approach provides engineers with a convenient, high-precision tool for designs utilizing various forms of FRP bars and diverse types of concrete in practical design scenarios

Bond performance of NSM FRP–concrete interfaces with epoxy under chloride‐salt conditioned environments

AbstractConcrete in coastal areas is susceptible to structural damage caused by corrosion and expansion of reinforcement. Near‐surface mounted (NSM) fiber‐reinforced polymer (FRP) bars have become effective for strengthening damaged reinforced concrete structures. The bonding performance of NSM FRP bars in concrete is a key factor limiting their application and promotion in civil engineering. In this study, the influences of the conditioned environment, such as the chloride salt concentration, immersion time, and bond length of the FRP bar, on the bonding performance were investigated experimentally. First, material properties in a conditioned environment were studied. Subsequently, the failure mode, bond stress–slip curve, and bond strength of the NSM FRP bar in concrete were investigated. Finally, the microstructure and chemical composition of the materials were revealed using scanning electron microscopy and energy‐dispersive spectroscopy (EDS) images of the materials under environmental conditions. The transfer mechanism of NSM FRP bars in concrete with epoxy was revealed. The results showed that the failure modes of the pullout specimens can be divided into epoxy splitting failure and basalt fiber‐reinforced polymer (BFRP) pulling failure. The chloride‐salt concentration was a critical factor affecting the bond properties, and the longer the bond length, the lower the bond strength. The microstructure clearly shows that the degradation of the bonding behavior at the interface of the NSM FRP bar in concrete in a conditioned environment is attributable primarily to the resin damage of the epoxy, resulting in pit corrosion. There was no significant damage to the fibers in the FRP bar, and the degradation was primarily due to resin matrix damage and interfacial debonding. The EDS results showed that the degradation of the epoxy resin and BFRP bars was caused by CO bond and SiO bond fractures, respectively.

Estimation of the Bond Strength of Fiber-Reinforced Polymer Bars in Concrete Using Artificial Intelligence Systems

Fiber-reinforced polymer (FRP) bars have recently been introduced to the market as an alternative to steel for internal reinforcement for concrete construction exposed to situations that could cause corrosion. The bond behavior of FRP bars varies from that of steel bars, mostly due to variations in material properties and surface textures. Because of the unexpected nature of the crucial FRP–concrete interfacial (FCI) bond strength, the bond strength between FRP bars and concrete cannot be exactly determined. Numerous experimental investigations have been conducted with related empirical models established in an attempt to resolve this problem. These models were found to have a restricted capacity for generalization due to the small sample sizes of the experiments. Therefore, a more powerful numerical technique capable of processing large data sets with all possible parameters that may affect the relationship and considering the nonlinearity of data tendency is needed. In this study, the artificial neural networks technique and adaptive neuro-fuzzy inference system were utilized to predict the FRP–concrete bond behavior based on 238 data points collected from different studies in the literature. The performance of the ANN and ANFIS models in predicting the bonding strength was compared to other models published in the literature and codes. The results showed that the ANN and ANFIS models gave higher prediction performance than other models, with a slight advantage for the ANN model. For instance, the R-squared values of the proposed ANN and ANFIS were 0.94 and 0.92, respectively, for 20 data points that were not used to develop the ANN and ANFIS models. Based on the sensitivity analysis, the FRP diameter and compressive strength of concrete were found to be the most effective parameters on the bond strength in both the ANN and ANFIS models. In contrast, the bar position and surface texture had a lower importance index.

Bond performance of spliced GFRP bars in pre-damaged concrete beams retrofitted with CFRP and UHPC

Fiber-reinforced polymer (FRP) reinforced concrete structures are inevitably subjected to extreme loads. The retrofitting of damaged FRP-reinforced concrete structures will become an important issue, especially with the gradual application of FRP bars in concrete structures. For structures that can be retrofitted, the structural performance is significantly affected by the bond of FRP bars in pre-damaged concrete. For this reason, this paper investigated the effectiveness of retrofit methods of carbon FRP (CFRP) sheet and/or ultra-high performance concrete (UHPC) at restoring the bond strength of glass FRP (GFRP) bars in pre-damaged concrete, and twenty spliced GFRP-reinforced concrete beams were tested under four-point loading. The beams were pre-loaded until bond failure occurred, and then retrofitted and reloaded to assess the restoration of the bond strength. The effects of retrofit methods, spacing of confining stirrups, GFRP bar diameter, and pre-loading type were investigated. The bond strength of the 20 mm and 28 mm diameter GFRP bars was increased by 4~138% and 4~234% after retrofitting with CFRP sheet and/or UHPC, respectively. The modified flexural strength model and modified effective moment of inertia addressing the adverse effect of partial fracture of glass fibers and concrete cracks predicted well the load-carrying capacity and deflection of the retrofitted specimens showing flexural failure, respectively.

Surface-Dependent Interfacial Characteristics of Glass Fiber-Reinforced Polymer Bars in Concrete

Surface-Dependent Interfacial Characteristics of Glass Fiber-Reinforced Polymer Bars in Concrete

Bond Strength Prediction Model of Ribbed FRP Bars in Concrete

The bond behavior of Fiber-Reinforced Polymer (FRP) bars is a critical factor in the mechanical performance of FRP rebar reinforced concrete structures, but until now, there has been limited research on the effect of rib forming process and geometrical features of ribbed FRP bars on the bond behavior, and no relevant provisions are incorporated into the current main design codes. In the paper, the spirally-glued FRP bar was taken into consideration, and bond test data were collected to investigate the main test parameters on the bond failure modes and bond strength. Greater emphasis was placed on the effect of rib forming process and geometrical features of such ribbed FRP bars on bond behavior. Results shown the macroscopic failure mode was pullout failure. More specifically, the spirally-glued FRP bar mainly exhibited shearing off of FRP bar ribs, and bond strength increased by increasing concrete strength. Spirally-glued FRP bars shown higher bond strength by increasing concrete cover which contributes to better confinement to FRP bars. Bond strength of spirally-glued FRP bars could be improved by increasing relative rib height hrd and FRP bar rib width ratio Fr. Multivariate nonlinear regression was adopted to propose a suitable formula to estimate bond strength. The calculated results by the proposed equation were in good agreement with the test results with greater accuracy than the current design codes. This is because the effect of rib forming process and geometrical features of FRP ribs on bond strength is accurately accounted in the proposed equation.

Refined modeling of the interfacial behavior between FRP bars and concrete under different loading rates

Fiber-reinforced polymer (FRP) bar can be served as a promising alternative to steel rebars due to its lightweight, high corrosion resistance and wider working temperatures, etc., especially for the structure sensitive to weight or in aggressive environments. In order to lay the foundation for a more scientific design of structures reinforced with FRP bars, the bond behavior of FRP bars to surrounding concrete subjected to pull-out loadings with different loading rates was investigated with a refined numerical model in the present study. In the model, the surface features of FRP bars and the heterogeneity of concrete were explicitly reflected. The interaction between the two materials was modeled with a surface-to-surface contact strategy. The effectiveness of the model was verified by the good agreement between the simulation results and the available experimental observations. With the help of the numerical model, taking basalt FRP (BFRP) bars as an instance, the cracking process and failure patterns, bond-slip curves, bond strength and peak slip at the FRP bar to the concrete interface were simulated and analyzed. The influences of fiber types and surface characteristics of FRP bars were discussed. Based on the simulation results, an empirical formula was given to predict the dynamic bond-slip relationships of FRP bars in concrete. It was indicated that the failure patterns of the FRP bar to concrete interface under various strain rates do not show much difference within the scope of the present study. The elastic modulus of FRP bars mainly affects the bond stiffness and peak slip instead of bond strength. The density of FRP bars has a slight influence on the bond strength and peak slip while ignorable on bond stiffness. The variation in rib height of FRP bars changes the bond strength and the descending part of bond-slip curves, showing no regularity on bond stiffness and peak slip. The influence of rib spacing on bond performance is irregular. As the strain rate increases, the bond strength of FRP bars to concrete is increased continuously while the peak slip is reduced firstly and then increased. The empirical formula in the form of two power functions, considering the strain rate effect of bond strength and peak slip, can reasonably reproduce the dynamic bond-slip relationships between FRP bars and concrete to a certain degree.

Effect of surface condition on the bond of Basalt Fiber-Reinforced Polymer bars in concrete

Basalt fiber reinforced polymer (BFRP) bars are used in reinforced concrete structures as an alternative to conventional mild steel bars due to their excellent strength and durability properties. The bond of BFRP bars with concrete is a critical design criterion for both flexural strength and crack control. In this paper, the effect of two surface conditions (primary sand coating and secondary sand coating) on the bond of BFRP bars is evaluated experimentally in three investigations: first, twelve pull-out specimens (six specimens for each surface condition) are tested to determine the bond strength; second, eleven beams (three beams with primary sand coated BFRP bars, three beams with secondary sand coated BFRP bars, three beams with bundled BFRP bars, and two beams with mild steel bars) are tested to determine the bond-dependent coefficient (kb) of different surface conditions compared to that of deformed steel bars; third, eleven beams are tested in flexure to determine the ultimate flexural strength and failure modes of beams with different surface conditions and reinforcement ratios. It was observed that the surface condition of BFRP bars has significant effect on bond strength, kb, and flexural strength. Test results are compared to those predicted using ACI 440-15 and ISIS 2007, which found to be conservative.

The tensile performance of FRP bars embedded in concrete under elevated temperatures

In this research, the mechanical properties of glass and carbon fiber reinforced polymer (FRP) bars with epoxy resin matrices embedded in concrete were investigated under an extensive range of elevated temperatures (i.e., 25–800 °C). Embedded FRP bars with various bar diameters were studied in order to determine bar diameter influence on the results. In addition, analysis of variance (ANOVA) was performed on the experimental results to investigate the contribution of exposure temperature and bar diameter to the tensile behavior of embedded in concrete FRP bars at elevated temperatures. The results show that the tensile strength of embedded FRP bars generally decreases with increasing temperature; however, the rate of decrease of the tensile strength varies within different temperature ranges due to different failure modes and states of the FRP material. The influence of the bar diameter was not significant in the tensile behavior of embedded FRP bars at various elevated temperatures. However, the influence of the bar diameter increased within the temperature range of 150–500 °C compared to the temperatures lower than 150 °C and above 500 °C. The results also show that the concrete cover prevented direct heat and oxygen from reaching the bars; as a result, embedded FRP exhibited improved tensile performance at elevated temperatures compared to bare bars that were directly exposed to heat. Two Bayesian regression models were developed for predicting the tensile performance of embedded GFRP and CFRP bars at elevated temperatures. The models have been shown to be in good agreement with the experimental results.

Bond performance of NSM FRP bars in concrete with an innovative additional ribs anchorage system: An experimental study

Abstract Bond failure due to a lack of anchorage capacity is often observed when using FRP reinforcement as a strengthening method in RC structures, thus restricting the efficiency in upgrading their structural performance. To address this issue, our research group has recently developed a simple and innovative additional ribs anchorage system for FRP bars, which has been verified to have the high efficiency of improving the anchorage performance of FRP bars reinforced in concrete. As a promising strengthening method in RC structures, the near-surface mounted (NSM) fiber-reinforced polymer (FRP) technique has become popular. However, bond failure also exists in the NSM FRP strengthening system. It is also important to initiate a study on the bond performance of NSM FRP bars which are mounted in concrete using the proposed additional ribs. In this study, a total of 36 direct pull-out specimens and 12 beam pull-out specimens were employed. In particular, the influences of additional ribs on the bond behavior of FRP bars mounted in concrete in terms of the failure modes, local bond stress distribution, and load-slip curves were discussed. Several important parameters expected to affect the bond performance were investigated, namely, the type and the bonded length of the FRP bars, groove size and section configuration, and adhesive species. The test results and corresponding analysis in this paper demonstrated the feasibility and significant efficiency of using the proposed additional ribs anchorage system to enhance the bond performance of NSM FRP bars in concrete.

Experimental study on the enhancement of additional ribs to the bond performance of FRP bars in concrete

The deficient bond performance of Fiber-reinforced polymer (FRP) bars has impeded their application in concrete structures. To enhance this weak characteristic of FRP bars, the additional ribs as a new anchorage system applied on the bars was proposed in this paper. Its effect on the bond performance of the FRP bars was experimentally investigated through 36 pull-out tests with a centric FRP bar placement in concrete blocks. After the tests, the failure mode and bonding mechanism were discussed. The effects of the types of FRP bar, the anchorage length and the number of additional ribs on the bond properties of bars in concrete were also evaluated. Based on the results, it can be concluded that the additional ribs with radial force contributed greatly to the anchoring force. The utilization of additional ribs also changed the bonding force transferring mechanism between the FRP bars and the concrete. The end bearing that arose from the additional ribs reduced the occurrence of concrete splitting, and thus, the tensile strength of the FRP bars can be fully utilized. The test results also indicated that the additional ribs significantly improved the anchorage performance of the FRP bars in concrete. This improvement would continue with the increase in the number of additional ribs.

Improved Bond Equations for Fiber-Reinforced Polymer Bars in Concrete.

This paper explores a set of new equations to predict the bond strength between fiber reinforced polymer (FRP) rebar and concrete. The proposed equations are based on a comprehensive statistical analysis and existing experimental results in the literature. Namely, the most effective parameters on bond behavior of FRP concrete were first identified by applying a factorial analysis on a part of the available database. Then the database that contains 250 pullout tests were divided into four groups based on the concrete compressive strength and the rebar surface. Afterward, nonlinear regression analysis was performed for each study group in order to determine the bond equations. The results show that the proposed equations can predict bond strengths more accurately compared to the other previously reported models.

Beam-Testing Method for Assessment of Bond Performance of FRP Bars in Concrete under Tension–Compression Reversed Cyclic Loading

AbstractThis paper presents a novel beam-testing method to assess the bond performance of fiber-reinforced polymer (FRP) bars in reinforced-concrete resisting systems subjected to tension–compression reversed cyclic loading. Special attention was paid to ensuring the stability of the proposed test setup, limiting all unwanted forces generated in the system and allowing for the application of tension and compression forces to the FRP bar without premature bar buckling. Preliminary test results are presented specifically for the cyclic bond stress-slip relationship under different loading conditions—tension–tension and tension–compression cyclic loading—in addition to the monotonic-tension loading to assess the suitability of the novel testing method.

Bond strength of glass fiber-reinforced polymer bars in concrete after exposure to elevated temperatures

This paper presents the test results of an experimental study to investigate the effect of elevated temperatures on the bond properties of glass fiber-reinforced polymer bars in concrete. For this purpose, a total of 39 pullout specimens with glass fiber-reinforced polymer bars were cast for bond strength tests. In addition to the laboratory temperature, specimens were exposed to heating regimes of 100, 200, 300 and 350℃ for a period of 1, 2 or 3 h. The test results are presented in terms of bond strength, bond–slip relationship, and mode of failure. All specimens failed by shearing of the concrete corbels surrounding the bars and almost no damage was seen in the glass fiber-reinforced polymer bars. The results showed that the bond strength decreased as the temperature or exposure period increased. Reductions of about 20% of the original bond strength were recorded after exposure to 100 and 200℃ for 3 h. Significant reductions of about 50% in the bond strength were recorded after exposure to 350℃ for periods of 2 and 3 h. A modification to the ACI (American Concrete Institute) and CEB-FIP (Comité Euro-International du Béton-Fédération Internationale de la Précontrainte) equations was developed to consider the effect of elevated temperatures and it showed good agreement with test results.

An experimental study of different factors affecting the bond of NSM FRP bars in concrete

Near-surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement has proven to be an efficient technique for strengthening concrete members. The bond behavior of the NSM system depends on the bond in the two existing interfaces, while the response in terms of load–slip curves and load capacity may be strongly affected by many other parameters. As a consequence of the variety of factors affecting the bond, different studies into the behavior of NSM FRP reinforcement have been carried out in recent years. In this paper, an experimental study is performed in which the effect of adhesive properties, bar type, bar size, FRP properties, groove geometry and the use of mechanical interlocking on the capacity and bond behavior of a NSM joint is investigated. The failure load, average bond stress, loaded end slip, free end slip, transverse strains (on adhesive and concrete), and the mode of failure for the tested joints are reported and discussed. The results show that the adhesive properties, FRP bar size and bar surface treatment were critical factors in the capacity and mode of failure of the specimens tested.

Effect of Construction Details on the Bond Performance of NSM FRP Bars in Concrete

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement has proven effective for strengthening reinforced concrete structures and is gaining increasing attention. The NSM reinforcement is installed by grooving the surface of the member to be strengthened and embedding FRP bars or strips in the grooves with an appropriate binder. This paper reports the results of an investigation designed to evaluate the influence of some construction parameters on the bond performance of NSM FRP round bars. These parameters include the groove depth and width-to-depth ratio, the groove distance from the edge of the member, the mechanical properties of the groove-filling epoxy, and the use of transverse FRP sheets for confinement of the joint. During the tests, conducted with a direct shear setup, the loaded-end and free-end slips and the strain distributions along the reinforcement and in the transverse plane were monitored. Tests were conducted on both short and long NSM bar anchorages. Results are here reported and analyzed to determine the effect of the test variables on local bond-slip behavior and development capacity. The local bond-slip curves obtained from the tests with short bond length are modelled analytically and used to predict the capacity of the specimens having long bond length and the same other test variables.

Development Length of Glass Fiber-Reinforced Polymer Bars in Concrete

Glass fiber-reinforced polymer (GFRP) reinforcing bars have become a viable option for reinforcement in concrete when corrosion is a concern. The objective of this research was to investigate the bond performance of GFRP reinforcing bars in concrete, to evaluate the existing ACI Committee 440 recommendations for development length of FRP bars in concrete, and to develop new design recommendations if needed. An equation for the development length of GFRP reinforcing bars in concrete was formulated by applying a methodology similar to one used to create the development length equation for steel reinforcing bars. Data collected from beam-based bond tests in the literature were used to construct two equations for development length: one based on a splitting mode of failure and the other on a pullout mode of failure. The effects of bar diameter, bar tensile strength, concrete compressive strength, cover, and the presence of transverse reinforcement were investigated. The proposed equations appear to be a conservative yet reasonable means to calculate the development length of GFRP reinforcing bars in concrete given the available test data. The proposed equations are compared to those found in ACI 440.1R-03 and the Japanese Design Guidelines.