Bond Behavior of Glass Fiber-Reinforced Polymer (GFRP) Bars Embedded in Concrete: A Review

Abstract

Glass Fiber-Reinforced Polymer (GFRP) bars are becoming increasingly common in structural engineering applications due to their superior material properties, mainly their resistance to corrosion due to their metallic nature in comparison to steel reinforcement and their improved durability in alkaline environments compared to CFRP and BFRP reinforcement. However, GFRP bars also suffer from a few limitations. One of the main issues that affects the performance of GFRP reinforcing bars is their bond with concrete, which may differ from the bond between traditional steel bars and concrete. However, despite the wide attention of researchers, there has not been a critical review of the recent research progress on bond behavior between GFRP bars and concrete. The objective of this paper is to provide an overview of the current state of research on bond in GFRP-reinforced concrete in an attempt to systematize the existing scientific knowledge. The study summarizes experimental investigations that directly measure bond strength and investigates the different factors that influence it. Additionally, an overview of the analytical and empirical models used to simulate bond behavior is then presented. The findings indicate the dependence of the bond on several factors that include bar diameter, bar surface, concrete strength, and embedment length. Additionally, it was concluded that both traditional and more recent bond models do not explicitly account for the effect of different factors, which highlights the need for improved bond models that do not require calibration with experimental tests.

Keywords: Glass Fiber-Reinforced Polymer (GFRP) Reinforcement; bond model; bond–slip; calibration; composites; experimental testing.

Figure 1

Different types of bar surfaces: (a) smooth bars; (b) sand-coated bars; (c) HW or braided bars; and (d) ribbed bars. Based on [24].

Figure 2

GFRP reinforcement cages for use in concrete corner joints.

Figure 3

(a) Pullout test; (b) beam end test; and (c) lap splice beam test.

Figure 4

Bond stress–slip relation for helically wrapped GFRP bars with varying diameters in unconfined concrete. Data from [37].

Figure 5

Bond stress–slip relation for 8 and 12 mm ribbed GFRP bars embedded in concrete of different strengths ($C1$ and $C2$ ($28$ and $48 \mathrm{M}\mathrm{P}\mathrm{a},$ respectively)). Data from [37].

Figure 6

Bond stress–slip relation for $8$ and $12 \mathrm{m}\mathrm{m}$ non-ribbed GFRP bars embedded in unconfined concrete of different strengths ($C1$ and $C2$ ($28$ and $48 \mathrm{M}\mathrm{P}\mathrm{a},$ respectively)). Data from [37].

Figure 7

Comparison between bond stress–slip relationships, sand-coated (HW, SC) and non-sand-coated helically wrapped (HW) $8$ and $12 \mathrm{m}\mathrm{m}$ GFRP bars embedded in concrete ($C1)$. Data from [37].

Figure 8

Comparison between bond stress–slip relationships for sand-coated helically wrapped $16$ and $20 \mathrm{m}\mathrm{m}$ GFRP bars embedded in concrete ($C1)$ with and without confining stirrups (Y represents confinement and N represents no confinement). Data from [37].

Figure 9

Comparison between bond stress–slip relationships for ribbed 16 and 20 mm GFRP bars embedded in concrete ($C1)$ with and without confining stirrups (Y represents confinement and N represents no confinement). Data from [37].

Figure 10

(a) Geometric properties of ribbed bars and (b) Comparison between bond stress–slip relationships for GFRP bars embedded in concrete with (WC) and without confining stirrups (NC). Data from [38].

Figure 11

Bond stress–slip relationships obtained for threaded (TB) and ribbed (RB) bars in unconfined concretes of different strengths. Note that the number in the notation represents the concrete strength in MPa. Data from [39].

Figure 12

Bond stress–slip relationships obtained for No. 4 GFRP bars (5 samples). Data from [40].

Figure 13

Bond stress–slip relationships obtained for GFRP bars of diameter $6.4$ and $9.5 \mathrm{m}\mathrm{m}$ ($G6$ and $G10,$ respectively). Data from [41].

Figure 14

Bond stress–slip relationships for round GFRP ($45Gr8D)$ and CFRP bars ($45Cr8D)$ embedded in $45 \mathrm{M}\mathrm{P}\mathrm{a}$ concrete. Data from [42].

Figure 15

Bond stress–slip relationships for $6$ and $8 \mathrm{m}\mathrm{m}$ GFRP bars (G6 and G8, respectively) embedded in $C4$ concrete (62 MPa) with varying concrete covers (10, 15, and 20 mm). Data from [43].

Figure 16

Load–slip relationships obtained from beam tests performed on GFRP and steel bars of different diameters. Data from [44].

Figure 17

Bond stress–slip relationships for $12$ and 18 mm GFRP bars with (a) shallow or (b) no ribs (S represents shallow rib). Note that the last number represents sample number. Data from [45].

Figure 18

Bond–slip relationships for $12 \mathrm{m}\mathrm{m}$ GFRP bars with different rib height (each color represents a different height as a percentage of the rebar diameter). Data from [46].

Figure 19

Effect of temperature on bond strength slip in CB and CPI GFRP bars. Data from [66].

Figure 20

Load–slip relationships obtained for the $8 \mathrm{m}\mathrm{m}$ GFRP bars at temperatures of $20$ and $80 °\mathrm{C}$. Data from [68].

Figure 21

Comparison between bond strength of smooth GFRP bars (notation A) with 0.5% steel fibers and $1\%$ steel fibers. Data from [113].

Figure 22

Schematic representation of BPE model. Data from [122].

Figure 23

Schematic representation of the modified BPE model. Data from [124].

Figure 24

Model proposed by [40].

Figure 25

Schematic representation of the secant modulus-based damage model proposed by [126].

Figure 26

Schematic representation of the exponential damage model proposed by [126].

Figure 27

Bond–slip–stress relationship for ribbed steel bars obtained from experimental results and from the proposed model by [128].

Figure 28

Bond–slip–stress relationships for $10 \mathrm{m}\mathrm{m}$ GFRP bars at different temperatures obtained from experimental results and from the proposed model by [128].

Figure 29

Bond–slip–stress relationships for GFRP bars of different diameters obtained from experimental results and from the proposed model by [128]. SC refers to sand-coated bars.

Figure 30

Calibrated modified BPE model and experimental results for bond–slip relationship for 16 mm GFRP bars of different rib depth. Data from [130].

Figure 31

Calibrated modified BPE model and experimental results for bond–slip relationship for 12 mm GFRP bars embedded in concretes of different strengths. Data from [130].

Figure 32

Calibration of mBPE and CMR for smooth GFRP bars. Data from [124].

Figure 33

Calibration of mBPE and CMR for ribbed GFRP bars. Data from [124].

Figure 34

Calibration of mBPE and CMR for twisted polyethylene GFRP bars. Data from [124].

Figure 35

Calibration of theoretical bond slip models (BPE, mBPE, and CMR) for ribbed bars. Data from [131].

Figure 36

Calibration of theoretical bond slip models (BPE, mBPE, and CMR) for helically wrapped sand-coated bars. Data from [131].

Figure 37

Comparison between BPE model and experimental results for rough $12 \mathrm{m}\mathrm{m}$ GFRP bars embedded in high (B5) and medium strength concrete (B1). Data from [123].

Figure 38

The effect of bar diameter on the bond slip of GFRP bars of different surfaces embedded in concrete of different strengths.

Figure 39

The effect of confinement on the bond slip of GFRP bars of different surfaces and diameters embedded in concrete of different strengths.