Bond Behavior Of Fiber-reinforced Polymer Bars Research Articles

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.

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.

A Review on the Physical Parameters Affecting the Bond Behavior of FRP Bars Embedded in Concrete

The present study is a detailed literal survey on the bond behavior of FRP (Fiber Reinforced Polymer) reinforcing bars embedded in concrete. There is an urgent need for the accurate assessment of the parameters affecting the FRP–concrete bond and quantification of these effects. A significant majority of the previous studies could not derive precise and comprehensive conclusions on the effects of each of these parameters. The present study aimed at listing all of the physical parameters affecting the concrete-FRP bond, presenting the effects of each of these parameters based on the common opinions of the previous researchers and giving reasonable justifications on these effects. The studies on each of the parameters are presented in detailed tables. Among all listed parameters, the surface texture was established to have the most pronounced effect on the FRP–concrete bond strength. The bond strength values of the bars with coarse sand-coating exceeded the respective values of the fine sand-coated ones. However, increasing the concrete strength was found to result in a greater improvement in bond behavior of fine sand-coated bars due to the penetration of concrete particles into the fine sand-coating layer. The effects of fiber type, bar diameter and concrete compressive strength on the bar bond strength was shown to primarily originate from the relative slip of fibers inside the resin of the bar, also known as the shear lag effect.

Bond behavior of FRP bars in CR concrete

This paper evaluated the bond behavior between recycled crumb rubber (CR) concrete and fiber reinforced polymer (FRP) bar. An experimental study consisting of fifty-seven pull-out specimens with fine recycled crumb rubber aggregate content of 0%, 5%,10% and 15% was described. The results showed that recycled crumb rubber concrete pull-out specimens exhibited similar bond stress-slip curves, which had four stages: the micro-slip, concrete internal cracking, pullout and residual stages. The use of crumb rubber in concrete reduced the bond strength. The bond strength decreased with the increase of the compressive strength of recycled crumb rubber concrete. The type and content of recycled crumb rubber also affected the bond strength. The glass fiber reinforced polymer (GFRP) bars tended to have stronger bonds than that of basalt fiber reinforced polymer (BFRP) bars.

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.

Review on the bond behavior and durability of FRP bars to concrete

In the past two decades, the use of Fiber Reinforced Polymer (FRP) bars in reinforced concrete structures has been increasing, replacing conventional steel reinforcement, especially in areas with adverse environmental conditions. This paper presents a literature review on the bond behavior of FRP bars to concrete, which includes studies on their (i) instantaneous and (ii) long-term (durability) properties. This work also included a database created from the collection of experimental data in the existing literature, with results from a total of 1002 pullout tests, intended to clarify the most relevant parameters that influence the bond of FRP bar to concrete. The analysis conducted on the database yielded the conclusion that FRP bar exhibits a good performance as reinforcement in concrete structures. Moreover, the collected data allowed the development of FRP bar to concrete bond (durability) long-term predictions considering the type of exposure environment.

Feasibility of using geopolymers to investigate the bond behavior of FRP bars in seawater sea-sand concrete

To fully exploit the application of marine resources in island engineering construction and to improve the durability of fiber-reinforced polymer (FRP) reinforced concrete structures, this paper explores the feasibility of using geopolymers as alternatives to ordinary Portland cement (OPC) to investigate the bond performance of FRP bars in seawater sea-sand concrete (SWSSC). Two types of FRP bars (i.e., glass-FRP and basalt-FRP bars) and three grades of concrete (C40, C50, and C60) were also considered. The experimental results indicated that the cracking or rupture of the FRP bars dominated the failure modes of the pull-out specimens when the compressive strength (fcu) of seawater sea-sand geopolymer concrete (SWSSGC) exceeded 50 MPa. The utilization of the geopolymer binders enhanced the splitting tensile strength (ft) of the concrete owing to the improvement of interfacial microstructures between the binders and the aggregates, which delayed the propagation of internal cracks in concrete, thereby effectually enhancing the bond behavior of FRP bars, especially its bond rigidity that defined by the slop of bond stress-curves at ascending stage. Compared with the cement-based SWSSC, there was approximately three times improvement in the initial bond stiffness for SWSSGC under the same concrete strength. Additionally, as the fcu of SWSSGC increased, the ft/fcu ratio was gradually declined, but the initial bond rigidity and the bond strength were improved. These results manifested that the combination of the FRP bars and SWSSGC is feasible, which can provide a promising approach for the application of geopolymers in SWSSC structures.

A study of the bond behavior of FRP bars in MPC seawater concrete

Using magnesium potassium phosphate cement (MPC) and fiber-reinforced polymer (FRP) bar to produce reinforced concrete can overcome the durability problems facing conventional steel reinforced PC concrete. In addition, FRP bar reinforced MPC concrete can also mitigate the CO2 emission issues caused by Portland cement (PC) production and the shortage of natural resources such as virgin aggregates and freshwater. This paper, therefore, is aimed at investigating the bond behavior of the FRP bars in MPC seawater concrete. The direct pullout tests were conducted with a steel bar, BFRP bar, and GFRP bar embedded into different concretes. The effects of reinforcing bars, type of concrete and mixing water on the bond behavior of FRP and steel bars were investigated and discussed. The results showed that the MPC concrete increases the bond strength of BFRP and GFRP bars by 51.06% and 24.42%, respectively, compared with that in PC concrete. Using seawater in MPC concrete can enhance the bond strength of GFRP bar by 13.75%. The damage interface of the FRP bar -MPC is more severe than that of PC with a complete rupture of the FRP ribs and peeling-off of the resin compared to that in steel reinforced MPC specimens. Moreover, the bond stress-slip models were developed to describe the bond behavior of MPC-FRP specimen, and the analytical results match well with the experimental data. In conclusion, the FRP bars showed better bond behavior in the MPC seawater concrete than that in the PC counterparts.

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.

Post-tension near-surface mounted strengthening system for reinforced concrete beams with changes in concrete condition

The concrete capacity of bridges gradually decreases due to the external environment and aging of the structure. Recently, near-surface mounted (NSM) systems have been studied to reinforce concrete bridges. The NSM system can reinforce existing concrete structures with less labor and without changing its dimensions. The post-tension NSM system was applied to improve the flexural capacity of concrete structures that are damaged due to aging or by the external environment. In this study, 6.4-m reinforced concrete (RC) beam strengthened with a post-tension NSM system was fabricated, and a four-point loading experiment was performed. The strengthening effect of the RC beam was investigated experimentally with respect to the initial concrete strength, bonded and unbonded fiber reinforced polymer (FRP) bar, and RC beam damage conditions. This study proposed a finite element (FE) analysis model by simulating the bond behavior of FRP bars and the filler. Parametric study was performed on the bond length of the FRP bar, and pre-stressing was performed using the proposed FE model.

Discussion of “Bond Behavior of Fiber Reinforced Polymer Bars under Direct Pullout Conditions” by Zenon Achillides and Kypros Pilakoutas

This article comments on earlier research on bond behavior of fiber reinforced polymer (FRP) bars (Achillides and Pilakoutas, 2004). The author seeks to clarify the bond behavior of FRP bars with concrete, specifically the discussion of the shear lag effect. Shear lag has a noticeable effect on the tensile strength of FRP rebars. Larger diameter rebars have a more significant shear lag effect and consequently a lower tensile strength. Wang points out that the diameter effect also exists in steel rebars and that normal stress distribution on rebar cross sections does not have a direct relationship to bond strength, refuting Achillides and Pilakoutas' contention that higher stresses caused by shear lag lead to lower than normal bond strength.