Experimental study on bond performance between carbon/glass-hybrid-fiber-reinforced polymer bars and concrete

Abstract

Coating GFRP bars with a layer of carbon fibers containing carbon/glass-hybrid-fiber-reinforced polymer (HFRP) bars has been proven to improved interlaminar shear durability. Thus, the bond performance of the HFRP bar-concrete interface depends on the behavior of the HFRP-reinforced concrete structure. In this context, the study investigates the effects of the compressive strength of concrete and the embedded length of HFRP bars on the bond behavior of the HFRP bar-concrete interface and compares the results with the data on the interfacial bond of GFRP bar to concrete. The results demonstrate that all the specimens fail at the FRP bar–concrete interface under the pullout. The bond strength and bond stiffness of the HFRP bar–concrete composites decrease with an increase in the embedded length of the HFRP bar from 4 to 10 times its diameter. Raising the compressive strength of the concrete enlarges the bond strength and bond stiffness of the HFRP bar–concrete composites. Furthermore, the outer layer of carbon-fiber on the GFRP bar decreases the impact of the embedded length of the HFRP bar on the bond strength and bond stiffness of the HFRP bar–concrete composites. Finally, the improved modified mBPE model can accurately predict the bond stress–slip relationship of GFRP/BFRP/HFRP bar–concrete composites.

Keywords

fiber-reinforced polymer bar-Concrete pullout test embedded length interfacial bond analytical modelling

Introduction

Steel-reinforced concrete is the most widely used in marine structures, and steel is susceptible to corroding by chloride ions in marine environments (Zhang et al., 2020, 2021; Dong et al., 2018). Glass-fiber-reinforced polymer (GFRP) bars have become an alternative to replace steel bars in concrete because of their high strength, light weight, low price, and excellent resistance to chloride ions (Benmokrane et al., 2020; Lu et al., 2021). The concrete pore solution is alkaline, and its pH ranges from 12.7 to 13.6 (Chen et al., 2007). Hydroxide ions etches glass fibers under alkaline conditions (Kamal and Boulfiza, 2011; Yu et al., 2021a), and GFRP bar deteriorates, followed by the weak bond of the GFRP bar to concrete (Alves et al., 2011; Micelli and Nanni, 2004). The application of a carbon fiber coating on the GFRP has been demonstrated to enhance the durability of the GFRP material (Pan and Yan, 2021a). HFRP bars exhibit superior resistance to alkaline solutions found in concrete compared to GFRP bars (Yu et al., 2021b; Pan and Yan, 2021b). While the carbon fiber volume fraction in HFRP is smaller than that in CFRP bars, HFRP remains cost-effective. This advantage positions HFRP with a promising outlook for applications in ocean engineering (Guo et al., 2022a, 2022b; Feng et al., 2022). The performance of FRP-reinforced concrete structure depends on the FRP-concrete interfacial bond (Wu et al., 2022; Altalmas et al., 2015). However, research on the HFRP bar-concrete bond behavior still very limited.

The bond behavior between FRP bar and concrete significantly influenced by the embedded length and diameter of GFRP bars and the compressive strength and cover thickness of concrete (Henin and Morcous, 2021; Nepomuceno et al., 2021; Zeng et al., 2022). The GFRP bar-concrete interface failed from concrete failure to the shear failure of the outer layer of GFRP bars with an increase in the concrete strength (Lee et al., 2008). It was reported that GFRP bar–concrete composites fail at the GFRP bars-concrete interface when the embedded length of the GFRP bars was five times their diameter. A combination of partial fiber failure and GFRP bar–concrete interface failure was found as the embedded length of the GFRP bars increased (Pecce et al., 2001). The strength of the interfacial bond drops, and the slip corresponding to the bond strength rises with the embedded length of the bars. Further, the compressive strength of concrete raises the interfacial bond strength (Achillides and Pilakoutas, 2004).

The CFRP (carbon-FRP) bar–concrete interface has a bond–slip relationship similar to the GFRP bar–concrete interface, but compared to the GFRP bar-concrete interface, the maximum stress of the CFRP bars to concrete is less (Baena et al., 2009). The maximum bond stress of CFRP bars to concrete is only 49% of that of GFRP bars to concrete (Nepomuceno et al., 2021). GFRP bars and concrete have a similar elastic modulus, and an increase of the concrete compressive strength enhances the initial bond stiffness between GFRP bar and concrete. Nevertheless, the interfacial bond stiffness was insignificantly influenced by the compressive strength of concrete since the modulus of elasticity of CFRP bars is larger than that of GFRP bars (Baena et al., 2009). HFRP bars consist of glass and carbon fibers, and the tensile modulus of elasticity of HFRP bars varies between the modulus of elasticity of GFRP bars and that of CFRP bars according to the rule of mixtures (You et al., 2007). The influence of a carbon-fiber coat on the bond performance of the HFRP bar-concrete composite is not evident.

The bond–slip relationship of FRP bars–concrete interface can be mathematically predicted (Gao et al., 2003). The typical bond stress–slip curves were shown in Table 1. The bond–slip relationship of the steel–concrete interface predicted by using the BPE model (Shen et al., 2016). The BPE model comprises an ascending curve, a plateau interval with bond strength, a linear descending branch, followed by a horizontal branch (Taha et al., 2020). However, the interfacial bond stress–slip for FRP bars in concrete does not have the plateau interval with bond strength. The double branch model, modified BPE (mBPE), is hence developed according to the BPE model and omits the second branch of the BPE model with constant bond strength (Solyom and Balázs, 2021; Lin and Zhang, 2014). The actual bond stress–slip relationship in the initial stages differs from the predicted curve fitted by the mBPE model; a power function may not sufficiently describe the initial stage of the curve of the mBPE model. It is reported that the accuracy of the ascending curve of the bond stress–slip relationship fitted by mBPE model is low (Liao et al., 2022; Wei et al., 2019). Gao Danying model and CMR model were developed to characterize the FRP bar–concrete bond behavior (Rolland et al., 2020; LUO et al., 2015). It was reported that Gao Danying model underestimated the initial stiffness of bond stress–slip curve, and the rising branch cannot be well consistent with the test results (Wenyuan et al., 2023). The CMR model has a higher fitting degree for the ascending phase of the bond slip constitutive, but it does not consider the descending phase, so it cannot effectively analyze the whole process of bond stress-slip behavior (Godat et al., 2021). The behavior of initial stages of the stress–slip curve depends on the chemical bonding of FRP bars–concrete composites, and the bond stress linearly increases with the slip. Once the slippage occurs, the bond stress nonlinearly enlarges with the slip to reach the maximum bond stress.

Table 1.

The comparison of typical curves between the Gao Danying model, CMR model, BPE model and mBPE model.

It was reported that when the thickness of the concrete protective layer exceeds 5 times the diameter of the FRP reinforcement bar, the FRP bar may pull out from the concrete, resulting in failure. However, when the thickness of the concrete protective layer is less than 5 times the diameter of the FRP reinforcement bar, the concrete may be prone to splitting (Yan et al., 2016). In practical engineering, to prevent concrete splitting, it is common to ensure sufficient bond between the concrete and the FRP reinforcement by using a concrete protective layer thickness greater than 5 times the diameter of the FRP reinforcement bar. Thus, in this context, the current work examines the effects of the concrete compressive strength and the embedded length of HFRP bars on the HFRP bars–concrete bond, and compares the bond performance of the GFRP bar– and HFRP bar– concrete. Additionally, the effects of the outer layer of carbon–fiber on the bond properties between HFRP bar and concrete is shed light on. Finally, the mBPE model is enhanced and employed to predict the bond stress–slip relationship of the specimens with different FRP bar types, concrete compressive strengths, and embedded lengths of reinforcing bars under the pullout. By wrapping a thin layer of carbon fiber around the surface of GFRP, the issue of hydrogen ion corrosion of GFRP reinforcement within marine concrete can be addressed, thereby enhancing the bond durability performance at the GFRP reinforcement-concrete interface. This approach achieves a balance between cost and durability, which contributes to promoting the widespread application of GFRP reinforcement in marine structures.

Experimental program

Material properties

Figure 1(a) shows the images and schematic of the FRP bars, and the pultrusion process and composition of the FRP bars are detailed elsewhere (Pan and Yan, 2021a). Both GFRP and HFRP bars were fabricated at the Harbin FRP Institute of China, and the surface layer of the GFRP and HFRP bars was wrapped in spiral fibers. 8-mm-nominal-diameter of FRP bar was measured by a digital caliper. Figure 1(b) depicts a schematic of an FRP bar along with its dimensions. The axial tension test on the GFRP and HFRP bars was conducted according to standard GBT 30022–2013 (AQSIQ China’s General Administration of Quality Supervision, Inspection and Quarantine, 2013). Figure 1(c) depicts the preparation process of HFRP bars. Table 2 also shows the mechanical properties of the GFRP and HFPR bars. The GFRP radius of the HFRP core layer is about 3.3 mm, and the CFRP thickness of the cortex is about 0.7 mm. The detailed measurements and procedures referred to Ref. (Pan and Yan, 2021a).

Figure 1.

(a) The images and (b) schematic of the GFRP and HFRP bars (unit: mm), (c) formating of carbon fiber coat (Pan and Yan, 2021a).

Table 2.

The mechanical properties of the GFRP and HFPR bars.

Type of bars	Tensile strength (GPa)	Elastic modulus (GPa)
GFRP	1.13	37.3
HFRP	1.29	48.1

Two concrete grades with various compressive strengths, namely C30 and C40, were designed according to standard GB175-2007/XG3-2018 (SAMR State Administration for Market Regulation, 2018). Table 3 tabulates the mix proportions and compressive strength of the concrete cubes. A strength of 42.5 MPa of ordinary portland cement (OPC) was adopted. The fine aggregate was river sand with good gradation and a maximum particle diameter of 5 mm, and the coarse aggregate was granite gravel with a particle size of 5–20 mm. The concrete cubes with the dimensions 150 mm × 150 mm × 150 mm was tested by a Matest testing machine (Matest SpA, Italy) with a maximum load of 5000 kN according to Chinese Standard GB/T 50081 (MOHURD Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2019).

Table 3.

The mix proportions and compressive strength of the concrete specimens.

	Water (kg/m³)	Cement (kg/m³)	Sand (kg/m³)	Gravel (kg/m³)	Compressive strength (MPa)
C30	171.4	295.5	706.2	1246.9	29.1
C40	156.4	391.0	567.0	1325.6	38.3

Specimen preparation and testing procedure

Figure 2(a) illustrates a schematic of the bond specimen. The total length of the GFRP bar was 700 mm, and its free end was 20 mm. The loading end was also anchored by a seamless steel tube with a length of 140 mm. The plain concrete cylinder had an equal diameter and height of 200 mm. A preinstalled poly (vinyl chloride) pipe was employed to control the embedded length of the testing FRP bar, which was chosen to be 2, 4, 6, and 10 times its nominal diameter. The FRP bar–concrete composites were cured at room temperature in the Zhejiang Sci-Tech University (ZSTU) laboratory.

Figure 2.

The setup of the pullout test: (a) the specimen for testing the bond between FRP bar and the concrete; (b) the testing setup (unit: mm).

Figure 2(b) illustrates the pullout testing setup. A 100 kN hydraulic jack applied the load, and the applied load was recorded by using a pressure sensor. Linear variable differential transducer (LVDT) No. 2 measured the deformation of the top steel plate relative to the bond specimen. LVDT No. 3 recorded the slippage of the loaded end of the FRP bar in the concrete laminate, while LVDT No. 1 measured the FRP bar’s slippage in the concrete at the free end. The applied load and displacement were recorded by an acquisition instrument. The loading process is controlled by displacement to ensure that LVDT (LT20) collected at least once every 0.1 mm.

Table 4 lists the all FRP bar-concrete bond specimens. 36 specimens of FRP bar-concrete bond specimens were prepared. Each specimen of H_C40, H_C30, and G_C40 were divided into four groups for each type, with three specimens per group. The naming rules are: “H” and “G” represent the GFRP and HFRP bars respectively; “C30” and “C40” indicate concrete grades C30 and C40 respectively; “2X”, “4X”, “6X”, and “10X” denote the embedded length of the FRP bar equal to 2, 4, 6, and 10 times its nominal diameter respectively. For example, “H_C40_2X_1” represents an HFRP bar–C40 concrete specimen composed of an HFRP bar with an embedded length twice its nominal diameter.

Table 4.

The details and matrix of the specimen.

Specimens	Type of FRP bar	Grade of concrete	Embedded length
H_C30_2X_1∼3	HFRP	C30	2X
H_C30_4X_1∼3	HFRP	C30	4X
H_C30_6X_1∼3	HFRP	C30	6X
H_C30_10X_1∼3	HFRP	C30	10X
H_C40_2X_1∼3	HFRP	C40	2X
H_C40_4X_1∼3	HFRP	C40	4X
H_C40_6X_1∼3	HFRP	C40	6X
H_C40_10X_1∼3	HFRP	C40	10X
G_C40_2X_1∼3	GFRP	C40	2X
G_C40_4X_1∼3	GFRP	C40	4X
G_C40_6X_1∼3	GFRP	C40	6X
G_C40_10X_1∼3	GFRP	C40	10X

Experimental results

Failure mode

Figure 3 displays the interfacial bond failure. All specimens fail in the pullout mode. It is reported that the interfacial bond fails when the thickness of the concrete cover is more than 5 times the diameter of the FRP bar, and the thickness of the concrete cover in the present study is 25 times the diameter of the FRP bar. A thicker concrete cover confines the bar more, thereby resulting in a more dominant pullout failure (Yan et al., 2016).

Figure 3.

The failure mode of the FRP bar–concrete composites: (a) H_C40_2X, (b) H_C40_4X, (c) H_C40_6X, (d) H_C40_10X, (e) G_C40_2X, (f) G_C40_4X, (g) G_C40_6X, (h) G_C40_10X, (i) H_C30_2X, (j) H_C30_4X, (k) H_C30_6X, and (l) H_C30_10X.

Figure 3(a)–(d) illustrates the interfacial bond failure for HFRP bar pulled out from concrete. The failed concrete adheres to the carbon fiber and the matrix resin, and the failure of the fiber and resin is found on the concrete. It indicates that the HFRP bar–concrete interface failed by a coupled failure of scratches of the HFRP bar and concrete failure. The HFRP bar is scratched more severely with increasing its embedded length. Figure 3(d) and (l) shows the failure mode of specimens H_C40_10X and H_C30_10X. The higher the compressive strength of the concrete is, the more severely the surface of the HFRP bar is scratched. A higher concrete compressive strength causes more fibers to be scratched with concrete adhered to its surface. Figure 3(d) and (h) displays the failure mode of specimens H_C40_10X and G_C40_10X. Compared to HFRP bar, the GFRP bar has less elastic modulus, and is more susceptible to scratching. It indicates that the carbon-fiber coat results in primary failure of the concrete around the HFRP bar.

As shown in Figure 3(a), (e), and (i), the concrete failure of the specimens composed of an FRP bar with an embedded length twice its diameter is severer than that of the specimens made of an FRP bar with an embedded length 6–10 times its diameter, attributed to the concentration of stress on the short bond length. Comparing specimens H_C30_2X and H_C40_2X in Figure 3(a) and (i) reveals that the failure of H_C30_2X with concrete grade C30—decreased compressive strength—is severer than that of H_C40_2X.

Figure 4(a) illustrates the surface of the HFRP bar pulled out from C40 concrete scanned by using the SEM. The concrete scratches the matrix resin of the HFRP bar, and some failed concrete adheres to the surface of the HFRP bar.

Figure 4.

The SEM images of the surface of the HFRP bar of the (a) HFRP bar–C40 concrete and (b) HFRP bar–C30 concrete specimens after the pullout test.

Figure 4(b) displays the SEM image of the surface of the HFRP bar for the HFRP bar pulled out from C30 concrete. The observed phenomenon is similar to the HFRP bar–C40 concrete: the HFRP bar is scratched, and the failed concrete adheres to the HFRP bar surface. However, the surface of the HFRP bar pulled out from C30 concrete adheres to more failed concrete than the HFRP bar pulled out from C40 concrete, implying that the concrete fails more severely as its compressive strength declines.

Bond stress–slip relationship

The interfacial shear stress along the FRP varies from the loaded end to the free end, and it is difficult to obtain. The bond stress between FRP bar and concrete is averaged, and the equation is assumed to be expressed as follows (Zhou et al., 2022):

\bar{τ} = \frac{P}{π d l}

(1)

where P (kN) is the tensile load on the FRP bar and is recorded using the pressure sensor, d (mm) is the nominal diameter of the FRP bar, and l (mm) is the interfacial bond length of the FRP bar–concrete composites. Figure 5 plots the bond stress–slip curves of all specimens. The maximum bond stress is determined by equation (1). The bond stiffness (K) is defined as the secant stiffness, as expressed in (Li et al., 2019)

K = \frac{τ_{1 / 2}}{s_{1 / 2}}

(2)

where

τ_{1 / 2}

indicates half of the bond strength, and

s_{1 / 2}

is slip, and corresponds to

τ_{1 / 2}

Figure 5.

The bond stress–slip curves of the (a) HFRP bar–C40 concrete, (b) HFRP bar–C30 concrete, and (c) GFRP bar–C40 concrete composites.

The slip (s) is assumed to be expressed as follows:

s = s_{1} - s_{2} - s_{3}

(3)

where

s_{1}, s_{2}, s_{3}

is the slip recorded by LVDT NO.1, LVDT NO.2, LVDT NO.3. Table 5 lists the load capacity, bond strength, peak slip, and bond stiffness of the composites.

Table 5.

The results of the static pullout tests on the FRP bar–concrete composites.

Specimens	Capacity load/kN	Bond strength/MPa	Peak slip/mm	Bond stiffness/MPa/mm
H_C30_2X_1	2.56	6.36	1.13	10.42
H_C30_2X_2	3.39	8.42	2.88	9.01
H_C30_2X_3	2.35	5.85	4.21	7.79
H_C30_4X_1	6.48	8.06	1.74	8.92
H_C30_4X_2	6.10	7.59	4.29	8.11
H_C30_4X_3	6.88	8.56	4.93	3.50
H_C30_6X_1	9.88	8.19	3.61	3.24
H_C30_6X_2	10.26	8.51	3.93	4.46
H_C30_10X_1	14.32	7.13	5.40	1.80
H_C30_10X_2	14.97	7.45	4.99	2.16
H_C30_10X_3	16.07	7.99	4.01	/
H_C40_2X_1	4.06	10.10	3.01	9.24
H_C40_2X_2	5.52	13.72	2.02	11.38
H_C40_4X_1	10.77	13.40	3.54	8.78
H_C40_4X_2	10.79	13.42	5.18	8.77
H_C40_4X_3	9.20	11.44	5.72	9.91
H_C40_6X_2	12.22	10.14	4.72	6.87
H_C40_10X_1	22.37	11.13	5.39	2.77
H_C40_10X_2	19.06	9.48	6.36	2.90
H_C40_10X_3	25.98	12.93	4.80	5.53
G_C40_2X_1	4.42	11.01	2.22	22.26
G_C40_2X_2	4.34	10.81	3.46	23.12
G_C40_2X_3	4.12	10.25	1.94	27.24
G_C40_4X_1	14.77	18.38	5.37	22.53
G_C40_4X_2	13.72	17.07	5.72	19.17
G_C40_4X_3	14.91	18.55	2.99	23.10
G_C40_6X_1	20.79	17.24	6.33	3.97
G_C40_6X_2	17.20	14.26	6.11	7.84
G_C40_10X_1	28.22	14.04	8.95	2.27
G_C40_10X_2	28.36	14.15	7.44	3.18
G_C40_10X_3	31.47	15.73	10.50	2.05

Figure 5 shows the interfacial bond stress–slip response between FRP bar and concrete: all curves have similar shapes. The bond behavior comprises chemical bonding, mechanical interaction, and frictional forces (Wei et al., 2019). The bond stress linearly increases to approximately half of the bond strength (Li et al., 2019). At this stage, the slip is primarily caused by the tensile deformation of the FRP bar, and the chemical bonding of the FRP bar-concrete interface controlled the bond (Borosnyói, 2015). The chemical bonding fails once the slippage of the FRP bar in the concrete occurs. Then, a softening slope appears, and the bond stress nonlinearly increases with an increase in the slip until the bond stress reaches the bond strength. Friction forces and mechanical interlock primarily control the bond of the FRP bar-concrete interface. The cracks in the concrete around the FRP bar propagate with the applied load at this stage (Won et al., 2008). After exceeding the maximum bond strength, the bond stress fast plummets with further slip, attributed to the shear failure of the interfacial bond. The frictional forces primarily control this stage.

Factors affecting bond strength and bond stiffness

Embedded length of fiber-reinforced polymer bar

Figure 6(a) shows the bond strength varies with the length of the FRP bar in concrete. The bond strength of the composites made of an FRP bar with an embedded length twice its diameter is less than that of the specimens composed of an FRP bar with an embedded length 4, 6, and 10 times its diameter, attributed to the concentration of stress on the bond between the FRP bar and concrete, resulting in the severe failure of the concrete around the FRP bar. In the case of the specimens composed of an FRP bar with an embedded length 4, 6, and 10 times its diameter, the bond strength of both GFRP bar–concrete and HFRP bar–concrete composites decrease with the embedded length of the FRP. Raising the embedded length of the FRP bar from twice its diameter to 4, 6, and 10 times its diameter increases the bond strength of the GFRP bar to C40 concrete by 68.4%, 47.3%, and 36.9% respectively, enhances the bond strength of the HFRP bar to C40 concrete composite by 7.06%, 1.49%, and 0.16% respectively, and enlarges the bond strength of the HFRP bar to C30 concrete by 17.3%, 21.4%, and 9.3% respectively, which can be explained as follows.

Figure 6.

The effect of the embedded length of the FRP bar on the (a) bond strength and (b) bond stiffness of the FRP bar–concrete composites.

On the one hand, the shear stress decreases from the loaded end to the free end. On the other hand, the applied load reduces the diameter of the FRP bar due to the effect of Poisson’s ratio (Baena et al., 2009). The effect of Poisson’s ratio improves with an increase in the embedded length of the FRP bar, reducing the frictional forces and mechanical interaction.

Figure 6(b) shows that the bond stiffness of the FRP bar–concrete composite declines with an increase in the embedded length of the FRP bar. Increasing length of the FRP bar in concrete from twice its diameter to 10 times its diameter reduces the bond stiffness of the GFRP bar to C40 concrete, HFRP bar to C40 concrete, and HFRP bar to C30 concrete by 10.33%, 40.93%, and 21.81% respectively. A short length of the FRP bar in concrete can mobilize the maximum bond stress along the entire (or most of) the length, and a small slip can easily reach a relatively significant average bond stress. Thus, the bond stress–slip curve shows a greater slope. Otherwise, for a longer embedded length of the FRP bar (e.g., 6 and 10 times its diameter), the load on the bond is borne by a partial bond length, and the bond strength of the remaining length should be activated through relative sliding. Hence, a smaller slope is observed in the early stages of the bond stress–slip curve. The longer the embedded length of the FRP bar is, the smaller the ratio of the load-bearing length to the embedded length of the FRP becomes. This phenomenon can be called “lag of load transfer” (Liao et al., 2022).

Compressive strength of concrete

Figure 7(a) shows comparison of the bond strength of HFRP bar in C40 concrete and C30 concrete. The bond strength increased with an increase of concrete compressive strength at a similar length of FRP bar in concrete. Further, the effect of the concrete compressive strength on the bond strength of the FRP bar-concrete composites lessens with enlarging the length of the FRP bar in concrete. Compared to the HFRP bars in C30 concrete, an FRP bar with an embedded length 2, 4, 6, and 10 times its diameter increases the bond strength of the HFRP bar–C40 concrete by 73.1%, 58.0%, 21.4%, and 37.4% respectively.

Figure 7.

The effect of the compressive strength of the concrete on the (a) bond stress and (b) bond stiffness of the FRP bar–concrete composites.

Figure 8 depicts the effect of the compressive strength of the concrete on the bond strength of the FRP bar–concrete composites. The strength of the bond mainly depends on the interfacial failure of the FRP bar reinforcement in concrete. According to Lee et al. (2008), the FRP bar is confined by the concrete. A higher concrete compressive strength provides more significant hoop confinement. The interfacial bond of the FRP bar to C40 concrete primarily fails at the fiber–matrix interface, while the bond between the FRP bar and C30 concrete mainly fails at the concrete–matrix interface, as shown in Figure 8. Thus, the strong loop confinement by C40 concrete shifts the failure mode from the concrete–matrix interface to the fiber–matrix interface.

Figure 8.

The failure of the interfacial bond.

Figure 7(b) delineates the impact of the concrete compressive strength on the bond stiffness of the composite specimens. Compared to the HFRP bar reinforcement in C30 concrete, the HFRP bar–C40 concrete composites has a greater bond stiffness at the same length of FRP bar in concrete. Compared to HFRP bar–C30 concrete composites, the HFRP bar–C40 concrete composites composed of an FRP bar with an embedded length 2, 4, 6, and 10 times its diameter increases the bond stiffness by 13.6%, 33.9%, 78.4%, and 113.1% respectively. The higher the compressive strength of the concrete is, the denser the concrete structure becomes. A denser structure of the concrete also causes the loop deformation of the specimens made of C40 concrete to be more efficient, and such an effect improves when the length of the FRP bar in concrete rises.

Carbon-fiber coat

Figure 9(a) depicts the effect of a carbon-fiber coat on the bond strength of the FRP bar to concrete. For the specimens made of an FRP bar with an embedded length twice its diameter, the interfacial failure of the FRP bar reinforcement in concrete is mainly dependent of the concrete around the FRP bars. Compared to specimens of the length of FRP bars in concrete ranged from 4 to 10 times its diameter, the bond strength of the specimens with an embedded length twice its diameter shows less difference for the GFRP bar and HFRP bar reinforcement in C40 concrete.

Figure 9.

The effect of the carbon-fiber coat on the (a) bond strength and (b) bond stiffness of the FRP bar–concrete composites.

Figure 9(a) shows that the effects of the FRP type on the bond strength between FRP bars and C40 concrete. In the case of FRP bar to C40 concrete, the bond strength of the specimens reinforced with HFRP bar is 29.2%, 35.6%, and 23.6% of that of the specimens reinforced with GFRP bar at an length of FRP bar in concrete 4, 6, and 10 times its diameter respectively.

The elastic modulus of concrete ranges from 25.4 to 38.5 GPa (Sun and Fanourakis, 2022), while that of GFRP bars and the carbon-fiber coat is 37.3 and 145.0 GPa respectively. The modulus of elasticity of GFRP bars is similar to that of concrete, so the compatibility between the deformation of GFRP bars and concrete is better than that of CFRP bars and concrete. Thus, the specimens reinforced with GFRP bars shows larger bond strength than the specimens reinforced with HFRP bar.

Furthermore, the bond strength of the specimens reinforced by GFRP bar decreases more severely than that of the specimens reinforced with HFRP bar as the embedded length of the FRP bar rises. Raising the length of the FRP bar in concrete from 4 to 10 times its diameter reduces the bond strength by 18.7% and 6.1% for of the GFRP bar- and HFRP bar-C40 concrete, respectively. The effect of Poisson’s ratio improves with an increase in the embedded length of the FRP bar, reducing the friction between the GFRP bar and the concrete. The outer layer of carbon-fiber resists the transversal deformation under the applied load, reducing the friction force of interface bond of the specimens reinforced with HFRP bar less than that of specimens reinforced with GFRP bar. Thus, compared to the bond interface of GFRP bar in concrete, the HFRP bar-concrete interface has a stabler bond strength.

Figure 9(b) depicts the impact of the outer layer of carbon-fiber on the GFRP bars on the bond stiffness of the FRP bar reinforcement in concrete. Raising the embedded length of the FRP bar from 2 to 10 times its diameter reduces the bond stiffness of the GFRP bar- and HFRP bar-C40 concrete by 89.67% and 59.07% respectively. For an FRP bar with an embedded length 10 times its diameter, the bond stiffness of the specimens reinforced with HFRP bar is 40.76% larger than that of specimens reinforced with GFRP bar. The slip is primarily due to the elongation of the FRP bars at this stage. Raising the embedded length of the GFRP bars enlarges the load on them, causing them to be significantly elongated under sustained loading. Nonetheless, the outer layer of carbon-fiber has larger stiffness to prevent the core layer, i.e., GFRP, from elongating, and the specimens reinforced with HFRP bar shows a larger bond stiffness.

Constitutive models of bond stress–slip relationship

The mBPE model is usually used to predict the bond stress–slip relationship of the FRP bar reinforcement in concrete (Cosenza, 1996) and consists of an ascending and a softening branch for the pre- and post-peak bond behavior respectively. It can be expressed by:

{\begin{cases} τ / τ_{\max} = {(s / s_{\max})}^{α} (0 \leq s \leq s_{\max}) \\ τ / τ_{\max} = 1 - p (s / s_{\max} - 1) (s_{\max} \leq s \leq s_{r}) \end{cases}

(4)

where (

τ_{\max}, s_{\max}

) and (

τ_{r}, s_{r}

) are the point of the bond strength and the residual point, respectively.

α

and

p

are determined by the tested results, and

α

must be smaller than 1.0 to be physically meaningful (Baena et al., 2009).

As discussed above, the curve of the bond stress to slip in the initial stage is linear. The slip enlarges by the elongation of the FRP bar. Once the slippages between FRP bar and concrete occur, the bond stress of FRP bar-concrete interface nonlinearly increases until the bond strength reaches. It is reported that the mBPE model does not agree well with the experimental data before the peak point. It is attributed to that a power-law function was adopted to characterize the micro slip and slip segment before the peak point (Liao et al., 2022; Wei et al., 2019). In the actual service environment, FRP reinforced concrete structures are in the elastic state, and the bond between FRP reinforcement and concrete at the interface is also in the elastic state. The descending portion has not been taken into consideration, and is of little significance in practical engineering research. At the same time, the descending branch of the existing mBPE model has been well satisfied to the actual demand (Solyom and Balázs, 2021). Thus, an improved mBPE (imBPE) model is proposed. The curve of the ascending branch of the mBPE model is decomposed into a linear segment and a segmented model of an offset curve segment so as to improve the prediction of the bond stress–slip curve before the peak. The equation is expressed as follows:

{\begin{cases} τ = Ks (0 \leq s \leq s_{τ_{1 / 2}}) \\ τ / τ_{\max} = {(s / s_{\max})}^{α_{m}} + m (s_{τ_{1 / 2}} \leq s \leq s_{\max}) \\ τ / τ_{\max} = 1 - p (s / s_{\max} ‐ 1) (s_{\max} \leq s \leq s_{r}) \end{cases}

(5)

where (

τ_{1 / 2}, s_{1 / 2}

), (

τ_{\max}, s_{\max}

) and (

τ_{r}, s_{r}

) are the peak point of the first line, peak point of second curve and the residual point of the third line, respectively.

K, α_{m}

, and

m

are the curve-fitting parameters.

Figure 10 plots equations (4) and (5).

Figure 10.

The typical curve of the imBPE model.

Table 6 compares our data on the specimens under the pullout with the related literature (Ma et al., 2019; Rolland et al., 2018; Godat et al., 2022; Xie et al., 2022). Equations (4) and (5) fit all the results, and Table 6 lists the curve-fitting parameters. Figure 11 compares the tested results with the predictions of the mBPE model. Figure 11(c) and 11(d) shows that the results predicted by the mBPE model agree well with the data on the CFRP bar–concrete composite estimated by the mBPE model. The coefficient of determination (R²) of the results predicted by the mBPE model is higher than 0.95, as presented in Table 6. The slip in the initial stages primarily results from the deformation of the CFRP bar. Thus, chemical bonding fails with a tiny slip of the CFRP bar due to its large elastic modulus, and the initial line is short. The power function of the mBPE model can accurately estimate the initial curve.

Table 6.

Comparing our data on the specimens under the pullout with the related literature.

Specimen	Type of bars	Bar diameter (mm)	Elastic modulus (GPa)	Embedded length (d)	Concrete compressive strength (MPa)	BPE		mBPE
Specimen	Type of bars	Bar diameter (mm)	Elastic modulus (GPa)	Embedded length (d)	Concrete compressive strength (MPa)	$α$	R²	$K$	$α_{m}$	$m$	R²
G_C40_2X	GFRP	8	37.3	2	38.3	0.29	0.994	25.50	0.43	2.94	0.989
G_C40_4X	GFRP	8	37.3	4	38.3	0.22	0.994	28.48	0.53	8.77	0.998
G_C40_6X	GFRP	8	37.3	6	38.3	0.68	0.978	4.10	0.66	−0.85	0.996
G_C40_10X	GFRP	8	37.3	10	38.3	0.64	0.952	2.35	0.53	−3.50	0.993
GuoweiMa et al. Helically wrapped GFRP specimen 2	GFRP	10	48.07	5	35	0.48	0.876	20.00	0.47	1.03	0.969
Arnaud Rolland et al. GLASS-S-9.5-2	GFRP	12	53±2	6	28.5	0.39	0.850	54.96	0.43	0.92	0.955
Ahmed Godat et al. B12-NC30-L5	BFRP	12	48	5	34	0.24	0.816	125.56	0.23	1.76	0.958
Ahmed Godat et al. B12-NC30-L10	BFRP	12	48	10	34	0.33	0.943	67.38	0.28	0.41	0.984
Ahmed Godat et al. B12-NC30-L15	BFRP	12	48	15	34	0.31	0.870	23.49	0.21	−0.03	0.975
Ahmed Godat et al. B12-NC45-L5	BFRP	12	48	5	47	0.36	0.947	14.38	0.29	0.65	0.993
Ahmed Godat et al. B12-NC45-L5	BFRP	12	48	5	63	0.22	0.748	273.90	0.19	3.37	0.945
Jian Xie et al. C10-5	CFRP	12	48	2.5	24	0.81	0.990	1.85	0.79	0.02	0.992
Jian Xie et al.C10-1	CFRP	10	153	5	24	0.35	0.895	8.99	0.32	0.34	0.981
Jian Xie et al.C10-7	CFRP	10	153	10	24	0.98	0.976	1.57	0.94	−0.09	0.976
Jian Xie et al.C10-11	CFRP	10	153	5	47.3	0.43	0.979	4.81	0.37	−0.15	0.955
Jian Xie et al.C10-12	CFRP	10	153	5	46.5	0.39	0.993	8.32	0.43	−0.13	0.940
H_C30_2X	HFRP	8	48.1	2	29.1	0.40	0.979	14.33	0.30	0.09	0.988
H_C30_4X	HFRP	8	48.1	4	29.1	0.51	0.928	11.56	0.41	0.89	0.993
H_C30_6X	HFRP	8	48.1	6	29.1	0.64	0.927	3.37	0.51	−2.47	0.987
H_C30_10X	HFRP	8	48.1	10	29.1	0.61	0.934	1.97	0.41	−4.11	0.988
H_C40_2X	HFRP	8	48.1	2	38.3	0.51	0.983	8.48	0.50	0.20	0.993
H_C40_4X	HFRP	8	48.1	4	38.3	0.46	0.913	9.89	0.54	2.41	0.985
H_C40_6X	HFRP	8	48.1	6	38.3	0.39	0.953	7.97	0.33	0.31	0.984
H_C40_10X	HFRP	8	48.1	10	38.3	0.76	0.914	2.78	0.55	−3.14	1.990

Figure 11.

Comparing the predictions of the mBPE model with the tested results: (a) and (b) BFRP bar–concrete composites; (c) and (d) CFRP bar–concrete composites; (e) and (f) GFRP bar–concrete composites; (g) and (h) HFRP bar–concrete composites.

Figure 11(a), (e), (g), (b), (f), and (h) compares the tested results with the data on basalt-fiber-reinforced polymer (BFRP) bar–, HFRP bar–, and GFRP bar–concrete composites predicted by the mBPE model. Figure 11(a), (e), and (g) depicts the effect of the embedded length of the FRP bar. The deviation of the predictions of the mBPE model from the experimental data decreases with increasing the embedded length of the FRP bar. The R² of the specimens reinforced with HFRP bar composed of an FRP bar with an embedded length twice its diameter is 0.983, while that of the specimens reinforced with HFRP bar made of an FRP bar with an embedded length 10 times its diameter is 0.914, as tabulated in Table 6. Chemical bonding primarily contributes to the deviation of the initial section of the bond stress–slip curve. The tensile deformation of the FRP bar before the slippage of the FRP bar in concrete primarily causes the slippage at this stage. The longer the embedded length of the FRP bar is, the more significant the load-bearing capacity of the FRP bar–concrete composite becomes. Thus, a long length of the FRP bar in concrete causes a large slip at initial stage I (i.e., 0A in Figure 10).

Figure 11(b), (f), and (h) depicts the effect of the compressive strength of the concrete on the comparisons of the predictions of the mBPE model with the tested results. The deviation of the predictions of the mBPE model from the tested results declines with reducing the compressive strength of the concrete. The R² of the HFRP bar to C30 and C40 concrete composed of an FRP bar with an embedded length 10 times its diameter is 0.934 and 0.914, respectively, as presented in Table 6. The low compressive strength of concrete gives rise to the premature failure of the bond between the FRP bar and the concrete because of the propagation of cracks in the concrete, and the mechanical interaction controls the concrete failure. Thus, the compressive strength of concrete mainly influences the softening slope at initial stage II (AB in Figure 10). In the case of the BFRP bar–, GFRP bar–, and HFRP bar–concrete composites with a longer embedded length of the FRP (higher than four times its diameter) and concrete strength more than 34 MPa, R² ranges from 0.75 to 0.95 in Table 6. The error of the mBPE model results in the ascending segment at initial stages I and II, and the mBPE model cannot accurately predict the bond stress–slip relationship.

Figure 12 compares predictions of the imBPE model with the tested results. Figure 12(a), (e), and (g) illustrates the effect of the embedded length of the FRP bar on the comparisons between the tested results and the predictions of the imBPE model. The present study and the literature results include the BFRP bar–, HFRP bar–, and GFRP bar–concrete composites with a length of the FRP bar in concrete ranging from 2 to 15 its diameter. The coefficient of determination higher than 0.958 (R² > 0.958) in Table 6 confirms that the proposed imBPE model can accurately predict the bond stress–slip curve of BFRP bar–, HFRP bar–, and GFRP bar–concrete composites with different embedded lengths of the FRP bar.

Figure 12.

Comparing the predictions of the imBPE model with the tested results: (a) and (b) BFRP bar–concrete composites; (c) and (d) CFRP bar–concrete composites; (e) and (f) GFRP bar–concrete composites; (g) and (h) HFRP bar–concrete composites.

Figure 12(b), (f), and (h) depicts the effect of the compressive strength of the concrete on the comparisons between the tested results and predictions of the imBPE model. The present study and literature data include the specimens reinforced with BFRP bar, HFRP bar, and GFRP bar with the concrete compressive strength ranging from 28.5 to 63.0 MPa, respectively. The coefficient of determination higher than 0.958 (R² > 0.958) in Table 6 demonstrates that the proposed imBPE model can accurately predict the bond stress–slip relationship of specimens reinforced with BFRP bar, HFRP bar, and GFRP bar with different compressive strengths of concrete, respectively. In conclusion, the proposed imBPE model can predict the bond stress–slip relationship of BFRP bar–, HFRP bar–, and GFRP bar–concrete composites, while the mBPE model is suitable for describing only the bond stress–slip relationship of specimens reinforced with CFRP bar.

Conclusions

This work analyzed the impacts of the length of the FRP bar in concrete and the concrete compressive strength on the bond behavior of specimens reinforced with HFRP bar and compared it with the bond performance of GFRP bar–concrete interface. Based on the previous results and discussions, several conclusions can be drawn:

• All FRP bar–concrete specimens failed by a coupled failure of the scratches of FRP bars and damage concrete. Compared to the HFRP bar pulled off from concrete, the appearance of GFRP bar was scratched more severely, and shows more crucially as its embedded length and the concrete compressive strength increased.

• Compared to the bond interface of HFRP bar to concrete, the GFRP bar–concrete interface has a higher bond strength at a similar embedded length of the FRP bar. A less differences in the interfacial bond strength. An increasing length of the FRP bar in concrete ranged from 4 to 10 times its diameter reduced the interfacial bond strength for the specimens reinforced with GFRP bar and HFRP bar. Moreover, the bond strength of the specimens reinforced with HFRP bar–decreased with an increase in the concrete compressive strength.

• Raising the length of the FRP bar in concrete reduced the bond stiffness of the specimens reinforced with GFRP bar and HFRP bars, and raising the compressive strength of the concrete also enlarged the bond stiffness of the HFRP bar–concrete composites.

• The outer layer of the carbon-fiber around the GFRP bars reduced the effect of the embedded length of the FRP bar on the bond strength and bond stiffness of the composites and stabilized the bond strength and bond stiffness of the HFRP bar–concrete interface.

• The proposed imBPE model could accurately predict the bond stress–slip relationship of the specimens reinforced with GFRP bar, BFRP bar, and HFRP bar. In contrast, the mBPE model was suitable for estimating the bond stress–slip relationship of CFRP bar–concrete composites.

Footnotes

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China [grant numbers 51908507, 52278280], the Zhejiang Postdoctoral Foundation (CN) [grant number ZJ2022150], and the Zhejiang Provincial Natural Science Foundation (CN) [project number LY19E080029].

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China [grant numbers 51908507, 52278280], the Zhejiang Postdoctoral Foundation (CN) [grant number ZJ2022150], and the Zhejiang Provincial Natural Science Foundation (CN) [project number LY19E080029].

ORCID iD

Yunfeng Pan

Data Availability Statement

Some or all the data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.

References

Achillides

Pilakoutas

(2004) Bond behavior of fiber reinforced polymer bars under direct pullout conditions. Journal of Composites for Construction 8: 173–181.

Altalmas

El Refai

Abed

(2015) Bond degradation of basalt fiber-reinforced polymer (BFRP) bars exposed to accelerated aging conditions. Construction and Building Materials 81: 162–171.

Alves

El-Ragaby

El-Salakawy

(2011) Durability of GFRP bars' bond to concrete under different loading and environmental conditions. Journal of Composites for Construction 15: 249–262.

AQSIQ (China’s General Administration of Quality Supervision, Inspection and Quarantine) (2013) Test method for basic mechanical properties of fiber reinforced polymer bar. GB/T 30022. Beijing, China: China Planning Press.

Baena

Torres

Turon

, et al. (2009) Experimental study of bond behaviour between concrete and FRP bars using a pull-out test. Composites Part B: Engineering 40: 784–797.

Benmokrane

Hassan

Robert

, et al. (2020) Effect of different constituent fiber, resin, and sizing combinations on alkaline resistance of basalt, carbon, and glass FRP bars. Journal of Composites for Construction 24: 04020010.

Borosnyói

(2015) Influence of service temperature and strain rate on the bond performance of CFRP reinforcement in concrete. Composite Structures 127: 18–27.

Chen

Davalos

Ray

, et al. (2007) Accelerated aging tests for evaluations of durability performance of FRP reinforcing bars for concrete structures. Composite Structures 78: 101–111.

Cosenza

(1996) Bond characteristics and anchorage length of FRP rebars. In: Proceedings of the 2nd international conference on advanced composite materials in bridges and structures, ACMBS-II, Montreal 1996: National Academics.

10.

Dong

Z-Q

Zhao

X-L

, et al. (2018) Long-term bond durability of fiber-reinforced polymer bars embedded in seawater sea-sand concrete under ocean environments. Journal of Composites for Construction 22: 04018042.

11.

Feng

Zhu

Guo

, et al. (2022) A review on mechanical properties and deterioration mechanisms of FRP bars under severe environmental and loading conditions. Cement and Concrete Composites 134: 104758

12.

Gao

Zhu

Xie

(2003) The constitutive models for bond slip relation between FRP rebars and concrete. Industrial Construction 33: 41–43.

13.

Godat

Aldaweela

Aljaberi

, et al. (2021) Bond strength of FRP bars in recycled-aggregate concrete. Construction and Building Materials 267: 120919.

14.

Godat

Alghafri

Al Tamimi

, et al. (2022) Bond behavior of basalt fiber reinforced polymer bars in recycled coarse aggregate concrete. Sustainability 14: 1374.

15.

Guo

Jin

Xiong

, et al. (2022a) Deterioration of mechanical properties of basalt/carbon hybrid FRP bars in SWSC under seawater corrosive environment. Construction and Building Materials 317: 125979.

16.

Guo

Xiong

Jin

, et al. (2022b) Deterioration of basalt/carbon-based hybrid-FRP bars in a simulated pore solution of seawater sea-sand concrete. Journal of Building Engineering 54: 104634.

17.

Henin

Morcous

(2021) Bond behavior of helically wrapped sand coated deformed Glass Fiber-Reinforced Polymer (GFRP) bars in concrete. Construction and Building Materials 288: 123120.

18.

Kamal

Boulfiza

(2011) Durability of GFRP rebars in simulated concrete solutions under accelerated aging conditions. Journal of Composites for Construction 15: 473–481.

19.

Lee

J-Y

Kim

T-Y

Kim

T-J

, et al. (2008) Interfacial bond strength of glass fiber reinforced polymer bars in high-strength concrete. Composites Part B: Engineering 39: 258–270.

20.

Chen

Ouyang

, et al. (2019) Synergistic effects of steel fibres and expansive agent on steel bar-concrete bond. Cement and Concrete Composites 104: 103380.

21.

Liao

Zeng

J-J

Bai

Y-L

, et al. (2022) Bond strength of GFRP bars to high strength and ultra-high strength fiber reinforced seawater sea-sand concrete (SSC). Composite Structures 281: 115013.

22.

Lin

Zhang

(2014) Evaluation of bond stress-slip models for FRP reinforcing bars in concrete. Composite Structures 107: 131–141.

23.

Yang

(2021) Mechanical and durability properties of GFRP bars exposed to aggressive solution environments. Science And Engineering Of Composite Materials 28: 11–23.

24.

Luo

Tang

Kuang

, et al. (2015) Test research on anchorage adhesive property of GFRP anchor bolt. Journal of Railway Science and Engineering 12: 521–527.

25.

Huang

Aslani

, et al. (2019) Tensile and bonding behaviours of hybridized BFRP–steel bars as concrete reinforcement. Construction and Building Materials 201: 62–71.

26.

Micelli

Nanni

(2004) Durability of FRP rods for concrete structures. Construction and Building Materials 18: 491–503.

27.

MOHURD (Ministry of Housing and Urban-Rural Development of the People’s Republic of China) (2019) Standard for test methods of concrete physical and mechanical properties. GB/T 50081. Beijing, China: China Architecture & Building Press.

28.

Nepomuceno

Sena-Cruz

Correia

, et al. (2021) Review on the bond behavior and durability of FRP bars to concrete. Construction and Building Materials 287: 123042.

29.

Pan

Yan

(2021a) Study on the durability of GFRP bars and carbon/glass hybrid fiber reinforced polymer (HFRP) bars aged in alkaline solution. Composite Structures 261: 113285.

30.

Pan

Yan

(2021b) Study on the durability of GFRP bars and carbon/glass hybrid fiber reinforced polymer (HFRP) bars aged in alkaline solution. Composite Structures 261: 113285.

31.

Pecce

Manfredi

Realfonzo

, et al. (2001) Experimental and analytical evaluation of bond properties of GFRP bars. Journal of Materials in Civil Engineering 13: 282–290.

32.

Rolland

Quiertant

Khadour

, et al. (2018) Experimental investigations on the bond behavior between concrete and FRP reinforcing bars. Construction and Building Materials 173: 136–148.

33.

Rolland

Argoul

Benzarti

, et al. (2020) Analytical and numerical modeling of the bond behavior between FRP reinforcing bars and concrete. Construction and Building Materials 231: 117160.

34.

SAMR (State Administration for Market Regulation) (2018) Portland cement for general use. GB175-2007/XG3. Beijing, China: China Standards Press.

35.

Shen

Shi

Zhang

, et al. (2016) Experimental study of early-age bond behavior between high strength concrete and steel bars using a pull-out test. Construction and Building Materials 113: 653–663.

36.

Solyom

Balázs

(2021) Analytical and statistical study of the bond of FRP bars with different surface characteristics. Composite Structures 270: 113953.

37.

Sun

Fanourakis

(2022) An assessment of factors affecting the elastic modulus of concrete. Structural Concrete 23: 593–603.

38.

Taha

Alnahhal

Alnuaimi

(2020) Bond durability of basalt FRP bars to fiber reinforced concrete in a saline environment. Composite Structures 243: 112277.

39.

Wei

Liu

Xiong

, et al. (2019) Bond performance between fibre-reinforced polymer bars and concrete under pull-out tests. Construction and Building Materials 227: 116803.

40.

Wenyuan

Xiang

Weichen

(2023) Bond-slip constitutive model for glass fiber-reinforcedpolymer rebars in ultrahigh-performance concrete. Harbin Gongcheng Daxue Xuebao/Journal of Harbin Engineering University 44: 5.

41.

Won

J-P

Park

C-G

Kim

H-H

, et al. (2008) Effect of fibers on the bonds between FRP reinforcing bars and high-strength concrete. Composites Part B: Engineering 39: 747–755.

42.

Wang

, et al. (2022) Experimental study on bond properties between GFRP bars and self-compacting concrete. Construction and Building Materials 320: 126186.

43.

Xie

Liu

Qiao

, et al. (2022) Bond behaviors of ribbed CFRP bars in concrete exposed to low temperatures. Construction and Building Materials 341: 127910.

44.

Yan

Lin

Yang

(2016) Bond mechanism and bond strength of GFRP bars to concrete: a review. Composites Part B: Engineering 98: 56–69.

45.

You

Y-J

Park

Y-H

Kim

H-Y

, et al. (2007) Hybrid effect on tensile properties of FRP rods with various material compositions. Composite Structures 80: 117–122.

46.

Liu

Pan

, et al. (2021a) Durability of glass fiber-reinforced polymer bars in water and simulated concrete pore solution. Construction and Building Materials 299: 123995.

47.

Pan

Zhou

, et al. (2021b) Effects of water and alkaline solution on durability of carbon-glass hybrid fiber reinforced polymer bars. Polymers 13: 3844.

48.

Zeng

J-J

Liao

Zhuge

, et al. (2022) Bond behavior between GFRP bars and seawater sea-sand fiber-reinforced ultra-high strength concrete. Engineering Structures 254: 113787.

49.

Zhang

Wei

Bai

, et al. (2020) A novel seawater and sea sand concrete filled FRP-carbon steel composite tube column: concept and behaviour. Composite Structures 246: 112421.

50.

Zhang

W-P

Chen

J-P

Q-Q

, et al. (2021) Corrosion evolution of steel bars in RC structures based on Markov chain modeling. Structural Safety 88: 102037.

51.

Zhou

, et al. (2022) Experimental investigations on bond behavior between FRP bars and advanced sustainable concrete. Polymers 14: 1132.