This chapter delves into the bond characteristics of BFRP and GFRP bars in concrete, focusing on the results from a beam test study. It highlights the impact of mineral additives such as zeolite and metakaolin on the bond strength and slip behavior of these bars compared to traditional steel reinforcement. The study compares the bond behavior under various concrete conditions and stress levels, providing valuable insights into the potential of FRP bars as a sustainable and corrosion-resistant alternative in concrete structures. The research underscores the complex interplay between concrete properties, bar characteristics, and bond behavior, offering practical implications for engineers and researchers in the field of civil engineering and materials science.
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Abstract
This article presents a comparative analysis of the bond behavior of steel bars in concrete and bars made of basalt fiber-reinforced polymer (BFRP) and glass fiber-reinforced polymer (GFRP) in modified concrete. While steel bars have been the conventional choice for reinforcement in concrete structures, their bonding properties are well established. In contrast, FRP bars possess distinct mechanical and physical properties, which can lead to different bonding behavior in concrete. The study investigated the effects of concrete properties and bar characteristics on the bond behavior of GFRP and BFRP bars. Specifically, the study analyzed the relationships between bond stress-slip, modes and mechanisms of failure, and changes in bond strength of concrete with the addition of zeolite and metakaolin, with the presence of GFRP, BFRP, and steel bars. The findings of the study reveal that the adhesion of composite bars to modified concrete is enhanced to varying degrees. The bond stress of GFRP bars to concrete with metakaolin addition was found to be 50% higher than to normal concrete, while the bond stress to concrete with zeolite was similar. On the other hand, BFRP bars exhibited an increase in bond stress of 7% in the presence of concrete with metakaolin. Moreover, BFRP bars displayed a greater bond to steel reinforcement that underwent plasticization or rupture. The study also noted that the change in bond strength of GFRP and BFRP bars due to their linear deformability was gradual, characterized by a several times greater slip range compared to steel bars.
1 Introduction
The maintenance and repair of infrastructure remains one of the major challenges of civilization [1], with the global cost of infrastructure repair and maintenance estimated at over €100 billion [2]. Corrosion of the steel reinforcement of concrete structures as a result of carbonation of the concrete cover, the use of de-icing salts, as well as a combination of moisture, temperature and chlorides reducing the alkalinity of concrete are the most common causes of damage to concrete structures during their operation [3], while road surfaces, bridges, viaducts, tunnels, and underground garages are the main problems [4]. The cost of road and bridge corrosion is estimated at US$276 billion annually (approximately 3.1% of GDP) for US industry and government agencies [5].
To address this issue, fiber-reinforced polymers (FRP) have emerged as a promising alternative to traditional steel reinforcement [6]. Among the different types of FRP bars, basalt fiber reinforced polymer (BFRP) and glass fiber reinforced polymer (GFRP) bars are gaining increasing attention due to their high strength-to-weight ratio and excellent corrosion resistance. However, the bond behavior of FRP bars in concrete remains a subject of ongoing research.
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GFRP bars are the most commonly used FRP structural composites due to their cost effectiveness, with BFRP bars being a relatively new type of rebar that can provide an economical alternative to GFRP [7‐11]. BFRP bars composed of basalt fibers and epoxy matrix have been tested and demonstrated excellent resistance to environmental conditions. Test data for beams with BFRP bars confirmed their usefulness in this respect. It should be noted that both GFRP and BFRP bars exhibit different mechanical properties depending on their components, and the design of FRP-RC elements requires different considerations and measures than conventional reinforced concrete structures [3, 12‐15].
1.1 Zeolite and Metakaolin Properties
One of the key mechanical properties of rebar is its bond to concrete. The interaction between concrete and reinforcement is made possible by bond behavior—i.e. the ability to transfer forces between two building materials. On the one hand, the type of ribbing affects the improvement of the bond of the reinforcement, and on the other hand, the compressive and tensile strength of the concrete. Substitution of part of the cement with mineral additives in the form of zeolite and/or metakaolin increases the parameters of concrete and thus its bond behavior.
It should be noted that the use of mineral additives to concrete in the form of zeolite and metakaolin significantly reduces the energy consumption of RC structures. At the same time, it is a factor that significantly affects sustainable development due to the low energy-intensive mineral components that are a favorable alternative to some cement. The production of each ton of Portland cement requires the consumption of about 1.2 tons of limestone and 0.11 tons of standard coal. This results in the emission of about 0.85–0.92 tons of CO2 and a significant amount of NOx [16‐18]. A cement substitute in the form of zeolite or metakaolin without high-temperature calcination or sintering can reduce CO2 emissions by about 70% during production and use [19].
Zeolites are porous aluminosilicates containing large amounts of reactive SiO2 and Al2O3. Preliminary studies confirmed the beneficial effect of the modifier in the form of zeolite on the increase in compressive and flexural strength, but only with the share of this modifier below 15% of the cement mass. The addition of zeolite also increases the durability of conventional concrete not only by reducing the permeability of concrete, but above all by improving the resistance to the reaction of alkaline aggregate.
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Metakaolin, a highly reactive aluminosilicate material, has been found to exhibit high pozzolanic activity, making it a suitable substitute for cement in concrete, and an additional component for improving concrete's tightness [20]. The addition of metakaolin to concrete has been found to result in a 10% increase in compressive strength and a 50% increase in flexural tensile strength. In all cases, the use of metakaolin has shown a more favorable effect on tensile strength [21, 22]. The utilization of metakaolin as a partial replacement for cement in concrete is significant in the construction of sustainable development and environmental protection related to building structures.
1.2 Bond Behavior Research
Research on the bond behavior of GFRP bars to concrete has been conducted for several decades, with an emphasis on understanding the mechanisms governing the behavior of the FRP-concrete bond. Two commonly used experimental methods to test the bond of FRP bars in concrete are the pull out test and the beam test [23, 24].
In pull out tests, a single FRP bar is embedded in a cubic sample of concrete and a pull-out force is applied to the bar in a direction perpendicular to the concrete surface. The test provides information about the bond strength of the FRP bar to the concrete and the distribution of bonding stress along the bar length.
In order to evaluate the bonding behavior of fiber-reinforced polymer (FRP) bars in concrete, beam tests are commonly employed. In these tests, a concrete beam is reinforced with FRP bars and subjected to a bending load. The bonding behavior is then evaluated by measuring bond slip, which is the relative displacement between the FRP bar and the concrete under an applied load. Compared to other methods, such as pullout tests, beam tests provide a more realistic representation of the bonding behavior of FRP members in real structures because they take into account the effects of concrete compressive stresses and bar curvature [25, 26].
Studies have shown that the bond properties of FRP bars in concrete are affected by several factors, including concrete strength, surface roughness of FRP bars, surface treatment of FRP bars, and bar diameter [27, 28].
In general, the studies have shown that the bond strength of FRP bars in concrete is lower than that of steel bars, but the bonding slippage of FRP bars is much lower, indicating a more plastic bond behavior of FRP-concrete. In addition, studies have shown that the bonding properties of FRP bars in concrete can be improved by using surface treatments such as sandblasting or acid etching to increase the surface roughness of FRP bars and increase the bond strength [29, 30].
GFRP bar tests by Baena et al. [31] showed that the bond behavior of the bars does not depend on the strength of the concrete, but basically on the surface properties of the GFRP bars. The reason is the damage to the GFRP bars occurring mainly in the resin layer between the layers covered with sand concrete bars with a strength of about 50 MPa.
Achilledes and Pilakoutas [32] showed that the bond strength between GFRP bars and normal strength concrete mainly depends on the strength of the concrete when the strength is less than 15 MPa. However, in the case of concrete with a strength greater than 30 MPa, the destruction of adhesion occurs partly on the surface of the GFRP bars [33]. This was due to the control of the bond strength by the interlayer shear strength of the resin layer.
Tighiouart et al. after performing adhesion tests of FRP bars, they found that the bond strength between the GFRP bars and the surrounding concrete does not increase with increasing concrete strength [34].
Research results by Dai et al. suggest that an increase in temperature and humidity can lead to a decrease in bond strength, while aging can have a significant effect on the bonding behavior of FRP bars in concrete. The study also showed that the type of FRP used and the type of resin used to bond the FRP-to-the concrete can also affect the bonding behavior of the FRP rods in the concrete [35].
Research results by Wang et al. and Xiong et al. [36, 37] showed that BFRP and GFRP bars have excellent bonding properties in concrete, and their bond strength is comparable or even higher than that of steel bars. The bond strength of FRP bars in concrete is influenced by several factors, including the type of FRP used, the type of resin used to bond the FRP to the concrete, the concrete mix and the surface roughness of the FRP bars.
These findings suggest that FRP bars could potentially be a suitable alternative to steel bars as a reinforcement material in concrete structures. However, it should be noted that specific results may vary depending on the type of FRP used, concrete mix and testing conditions.
2 Research Program
2.1 Purpose and Course of the Research
In the present investigation, the effect of additives in the form of zeolite and metakaolin to concrete on the bond stress and slip behavior of BFRP and GFRP bars was compared and, for comparison, steel bars. The use of steel bars as reinforcement in concrete structures is a well-established practice, and their bonding properties are well documented. However, due to the distinct mechanical and physical properties of FRP bars, their bonding behavior in concrete may differ from that of steel bars. The results of this study will provide an understanding of the distinctive bonding properties of FRP bars to concrete, and compare them to the conventional bonding of steel bars to concrete. Such information will be of great value to engineers and researchers in comprehending the potential advantages and constraints of using FRP as reinforcement in concrete structures.
Three types of concrete were subjected to bond strength tests utilizing BFRP and GFRP composite reinforcement, as well as steel reinforcement, which served as the reference reinforcement in the beam test. The research was conducted to assess the feasibility of utilizing unconventional modifiers, in the form of mineral additives, to increase the adhesion of composite reinforcement made of FRP bars in selected concrete elements used in building infrastructure facilities. The optimal concrete mix was developed by selecting the ingredients to increase adhesion to FRP bars. Two modified concrete mixes were chosen, with 10% addition of zeolite (Z) and 10% addition of metakaolin (K), respectively, in relation to the ordinary concrete mix (C).
2.2 Strength Characteristics of Concrete and Reinforcement
The concrete mixes used in the beam test were composed of CEM I 42.5R cement provided by Lafarge. The density of the concrete was 2270 kg/m3, and the water-to-cement (w/c) ratio was 0.45. The consistency of the concrete was of class S3, as specified by [38]. The detailed compositions of the concrete mixes, including the proportions of cement, water, aggregates, and mineral additives, are presented in Table 1.
Table 1.
Summary of concrete mix compositions
Mix proportion
C (ordinary)
Z (zeolit)
K (metakaolin)
Cement
360
324
324
Water
162
162
162
Gravel 2/8
574
574
574
Gravel 2/16
631
631
631
Sand 0/2
708
708
708
Zeolit
–
36
–
Metakaolin
–
–
36
In order to determine the strength parameters of the concrete, 150 mm cubic samples and 150 × 300 mm cylindrical samples were prepared. Three types of strength tests were performed on these samples, which enabled the determination of the compressive and splitting tensile strength, as well as the modulus of elasticity of the concrete. The test results are presented in Table 2.
Table 2.
Strength properties of concrete types
C (ordinary)
Z (zeolit)
K (metakaolin)
average
SD
COV
average
SD
COV
average
SD
COV
MPa
MPa
%
MPa
MPa
%
MPa
MPa
%
fc
46,36
1,01
2,19
48,37
0,98
2,02
49,55
0,93
1,87
fct
2,70
0,35
12,84
3,22
0,33
10,39
2,96
0,43
14,68
Ec
30017
340
1,13
31958
1138
3,56
36614
1339
3,66
Notefc—cube compressive strength, fct—splitting tensile strength, EL—modulus of elasticity, SD—standard deviation, COV—coefficient of variation
Table 3 presents the mechanical properties of GFRP, BFRP, and steel bars used in the beam tests. The properties include the tensile strength, yield strength (for steel bars only), and modulus of elasticity. The standard deviation and coefficient of variation are also provided to indicate the variability in the test results.
Table 3.
Mechanical properties of bar types.
GFRP
Steel
BFRP
average
SD
COV
average
SD
COV
average
SD
COV
MPa
MPa
%
MPa
Mpa
%
MPa
MPa
%
fy
–
–
–
519,7
3,50
0,67
–
–
–
ft
1033,3
30,49
2,95
615,8
3,64
0,59
1024,7
69,65
6,80
EL
41892
3410
8,14
227293
25783
11,32
41470
3570
8,62
Notefy yield strength, ft—tensile strength, EL—modulus of elasticity, SD—standard deviation, COV—coefficient of variation
2.3 Test Procedure
The bond test, utilizing the beam test, was conducted following the guidelines of the PN-EN 10080 standard [39]. The beam test methodology is used to determine the bonding characteristics of reinforcing bars to concrete. The test configuration consists of two concrete beams, each with dimensions of 80 mm × 160 mm × 375 mm, linked in the tension zone by the tested bar and in the compression zone by a steel joint, which is in the form of a cylinder with a diameter of 30 mm (as illustrated in Fig. 1).
Fig. 1.
Set-up for the test: 1—tested bar, 2—steel joint Ø30 mm, 3—PVC pipe, 4—slip measurement sensor
×
The bond between the reinforcing bar and the concrete was evaluated at a location in the middle of the 10d beams, where d represents the diameter of the bar. The remainder of the bar was enclosed in PVC pipes and had no adhesion with the concrete of the beam. The bond test was conducted under load using two concentrated forces, with the displacement of the tested bars at the ends of the beams being recorded during the test, as illustrated in Fig. 1. The test beam, supported by two rotating roller bearings, was loaded with two equal forces applied symmetrically about the center of the span. The concrete age of the tested beam was required to be within the range of 21 to 35 days. The load was applied incrementally corresponding to the stress σ in the bar, starting from 0 MPa and increasing in successive increments of 80 MPa and up to 240 MPa. For each increment, the total force applied to the set of beams was recorded.
where: An is the nominal cross-sectional area of the bar, σ is the tensile stress in the test bar, z is the distance from the center of the hinge to the center of the test bar, and a is the shear distance.
The test shall continue until complete loss of adhesion in both beams. The adhesion stresses τb for a given value of the slip value for the force Fa are:
$$ \tau_{b} = \frac{\sigma }{40} $$
(2)
where: σ is the stress in the tested bar.
The bond stress must be computed for four measured slip values: τ0.1—bond stress at 0.01 mm slip, τ0.1—bond stress at 0.1 mm slip, τ1—bond stress at 1 mm slip, and τmax—bond stress at maximum force.
A total of 27 sets of beams were prepared to measure the bond behavior of GFRP and BFRP bars with a nominal diameter of 12 mm, and for comparison, steel bars with a nominal diameter of 12 mm. The dimensions and reinforcement of all the beams in the test sets were identical. Particular attention was paid to the proper placement of the tested rods and the appropriate length of the rod ends to enable the installation of IL065 laser gauges for slip measurement. After casting the beam sets, the formwork elements were left in place to avoid stresses in the bars that could arise during the transfer of the element to the test stand.
Moreover, two clamps made of 15 mm thick steel sheet were fabricated to transfer force from the steel pin to the concrete, connecting two beams with the tested rod. The load was transferred symmetrically through the traverse onto the beam set at two points 150 mm apart, as shown in Fig. 2.
Fig. 2.
A example of the beam test—the bond testing of BFRP bar
×
For slip measurement, a CMOS laser sensor IL065 with a measuring range of 50 mm and a measuring accuracy of 2 μm was installed at both ends of the tested bars. To immobilize the tested bars against the PVC sheaths and enable their axial displacement, a special assembly foam was used at the ends of the beams.
To measure the slip, a CMOS multi-function analog laser sensor, specifically the Keyence IL065, was installed at both ends of the tested bars. The sensor had a measuring range of 50 mm and a high measuring accuracy of 2 μm. Each of the tested rods was immobilized at the ends of the beams against the PVC sheaths using special assembly foam, which enabled their axial displacement for accurate measurements.
For the beam tests, three types of concrete were used with BFRP and GFRP composite reinforcement, as well as steel reinforcement which served as the reference reinforcement in the beam test. The load was transmitted through an actuator with a load range of 200 kN, mounted on an articulated joint in the ZD20 testing device. Throughout the test, the slippage of the tension bar at both ends of the beam set was continuously recorded. The results were recorded every second throughout the study period, as the load increased monotonically until maximum slip was reached.
3 Results and Discussion
Table 4 presents the bond stress for successive slip values: 0.01 mm, 0.1 mm, 1 mm and the maximum τmax and average τm bond stress.
Table 4.
Bond stresses of bars to concrete for successive slip values in MPa.
Type
Side
τ0,01
τ0,1
τ1,0
τmax
τm
BG-1
Left
7,27
9,15
10,57
13,28
8,99
Right
5,32
7,63
9,16
7,37
BG-2
Left
6,66
8,05
9,69
10,52
8,13
Right
5,77
6,49
9,08
7,11
BG-3
Left
5,73
8,30
11,60
11,77
8,54
Right
7,73
8,95
10,71
9,13
ZG-1
Left
7,22
9,45
-
11,76
–
Right
3,21
8,47
9,76
7,15
ZG-2
Left
7,16
9,96
10,84
12,70
9,32
Right
7,29
10,49
7,68
8,49
ZG-3
Left
5,72
7,62
9,98
10,87
7,77
Right
1,54
7,7
10,02
6,43
MG-1
Left
13,24
13,74
–
14,31
–
Right
10,26
12,20
12,20
11,55
MG-2
Left
15,15
15,74
20,13
21,93
17,01
Right
14,07
14,70
17,37
15,38
MG-3
Left
12,50
13,46
16,73
16,79
14,23
Right
12,93
13,82
14,96
13,90
BS-1
Left
10,85
13,23
–
16,14
–
Right
11,08
13,38
–
–
BS-2
Left
0,14
11,88
–
15,18
–
Right
6,31
10,33
15,10
10,58
BS-3
Left
8,75
11,93
11,93
14,99
10,87
Right
6,11
9,90
–
–
ZS-1
Left
15,35
-
–
16,03
–
Right
12,45
15,33
–
–
ZS-2
Left
10,34
14,60
–
16,00
–
Right
10,31
14,18
–
–
ZS-3
Left
12,02
13,81
–
16,08
–
Right
9,45
15,14
–
–
MS-1
Left
10,97
14,58
–
16,26
–
Right
9,77
12,68
16,16
12,87
MS-2
Left
9,79
11,02
–
16,86
–
Right
8,80
12,13
–
–
MS-3
Left
7,79
10,28
16,68
16,73
11,58
Right
7,90
11,73
–
–
BB-1
Left
13,30
14,77
–
22,12
–
Right
15,66
19,25
21,54
18,82
BB-2
Left
15,57
16,48
20,92
21,28
17,66
Right
12,82
14,50
–
–
BB-3
Left
13,93
17,00
20,29
20,42
17,07
Right
12,73
17,54
–
–
ZB-1
Left
18,79
18,80
20,87
21,80
19,49
Right
16,86
19,79
–
–
ZB-2
Left
21,49
22,75
–
23,67
–
Right
11,55
18,35
23,59
17,83
ZB-3
Left
20,21
21,18
–
22,73
–
Right
15,36
18,65
22,47
18,83
MB-1
Left
15,84
18,97
22,17
22,91
18,99
Right
14,52
15,88
19,20
16,54
MB-2
Left
14,67
17,72
–
19,51
–
Right
13,17
14,29
17,85
15,10
MB-3
Left
14,34
16,53
–
18,43
–
Right
12,50
14,73
–
–
Note The initiation of slip of the bars relative to the surrounding concrete was observed first on one side of the sample beam system due to the slight variation in the concrete mix, and it is equally important to consider the side of the beam system where slippage occurred later. This is crucial because it affects the course of bar slip after exceeding the slip for maximum bond stress
The interaction between concrete and reinforcement is facilitated by bond—the ability to transfer forces between two building materials. The formation of cracks in the concrete is necessary to activate the bond effect and make the reinforced concrete (e.g., FRP bars, steel bars) useful. Bond analysis was conducted for GFRP, steel, and BFRP bars in normal concrete, concrete with the addition of zeolite, and concrete with the addition of metakaolin. GFRP and BFRP bars with braid ribbing were compared to ribbed steel bars with similar equivalent diameter. As the composite bars differed in equivalent diameter due to technological conditions, it was decided to compare the bond behavior using the ratio of equivalent to nominal diameter. Since the bars had similar diameters, the bond stress could be accurately assessed. According to EN 1992–1 [40], sufficient bond stress are ensured if the average bond stress τm ≥ 6.42 MPa and the maximum bond stress τmax ≥ 10.51 MPa for a bar diameter of 12 mm. All tested bars met the requirements of the above standard. The tests were conducted under standard conditions for both average and maximum bond stress.
For ordinary concrete, the average slip at maximum bond stress for GFRP bars was 2.20 mm, which was higher than that for steel (0.35 mm) and BFRP (0.61 mm). In concrete with the addition of zeolite, the average slip for GFRP bars at maximum bond stress was 0.82 mm, which was higher than the average slip for steel (0.11 mm) and BFRP (0.37 mm).
In concrete with the addition of metakaolin, the average slip for GFRP bars at maximum bond stress was 0.83 mm, which was higher than the slip for steel (0.35 mm) and greater than that for BFRP (0.67 mm).
Figure 3 presents the maximum bond stress for GFRP, steel, and BFRP bars in ordinary concrete, concrete with zeolite addition, and concrete with metakaolin addition.
Fig. 3.
Relationship of bond stress of GFRP, steel and BFRP bars to the type of concrete (a) ordinary concrete, (b) concrete with zeolite, (c) concrete with metakaolin
×
The results showed that for ordinary concrete, the maximum bond stress τmax for GFRP bars was 11.86 MPa, which was lower than the bond strength of other types of bars. Specifically, for steel bars, τmax was 15.44 MPa, which was 30.2% larger than GFRP bars. On the other hand, the maximum bond stress was observed for BFRP bars, with a value of 21.27 MPa, which was higher than GFRP bars by 79.3% and steel bars by 37.8%. It is noteworthy that the steel bars were plasticized, which resulted in the inhibition of slip due to plastic deformation of the middle section of the bar (Fig. 3a).
In the case of concrete with the addition of zeolite, the maximum bond stress τmax for GFRP bars was 11.78 MPa, which was lower than other types of bars. For steel bars, τmax was 16.04 MPa, which was 36.2% larger than GFRP bars. The highest bond strength was again observed for BFRP bars, with a value of τmax = 20.64 MPa, which was higher than GFRP bars by 75.2% and steel bars by 28.7% (Fig. 3b).
Moreover, for concrete with the addition of metakaolin, the maximum bond stress for GFRP bars was 17.67 MPa, which was lower than other types of bars. For steel bars, τmax was 16.61 MPa, which was smaller than GFRP bars by 6%. Once again, the highest bond strength was noted for BFRP bars, with a value of τmax = 21.21 MPa, which was higher than GFRP bars by 20.0% and steel bars by 27.7% (Fig. 3c).
The changes in the average bond stress τm, depending on the influence of zeolite and metakaolin additions to the concrete of GFRP, steel, and BFRP bars, are presented in Fig. 4.
Fig. 4.
Comparison of the average bond stress of bars for the samples with ordinary concrete, with the addition of zeolite and metakaolin: (a) GFRP bars, (b) Steel bars, (c) BFRP bars
×
The results of the study revealed that the average bond stress for GFRP bars decreased by 4% in concrete with the addition of zeolite. In contrast, in the presence of metakaolin, a significant increase in the average bond stress of 77.4% was observed compared to ordinary concrete (Fig. 4a).
For steel bars, an increase in the average bond stress by 19% was observed for concrete with the addition of zeolite. However, for concrete with the addition of metakaolin, a modest increase of 14% was noted compared to normal concrete (Fig. 4b).
In the case of BFRP bars, the average bond stress increased by 5% in the presence of zeolite addition to concrete. However, in the case of metakaolin addition, a decrease of 11% in the average bond stress was observed compared to normal concrete (Fig. 4c).
4 Conclusions
This research study was designed to investigate the bond behavior of BFRP and GFRP bars in modified concrete using beam tests. The effects of concrete properties and bar characteristics on the bond behavior of FRP bars were thoroughly analyzed and compared. The findings of this study provide important information for the design and implementation of FRP reinforced concrete structures and contribute to the development of improved models of FRP bar bond in concrete.
1.
It was ensured that standard conditions of average and maximum bond stress were met in all tests.
2.
For the tested types of bars, the maximum bond stress (τmax) and the average bond stress (τm) determined for the slip of 0.01mm, 0.1mm, and 1mm were sufficiently high and met or exceeded those required by EN 1992–1-1.
3.
The results showed that the highest maximum bond stress (τmax = 22.73 MPa) was recorded for BFRP bars in concrete with zeolite addition, which were 75.2% and 28.7% higher than GFRP and steel bars, respectively. However, in the presence of metakaolin addition in the concrete, the maximum bond stress of BFRP bars was τmax = 21.21 MPa, which was higher by 27.7% and 20.0% compared to steel and GFRP bars, respectively.
4.
In terms of average bond stress, the highest values were recorded for BFRP bars in concrete with zeolite addition (τm = 18.72 MPa), which were 47% and 154% higher than steel and GFRP bars, respectively. However, in the presence of metakaolin in the concrete, the average bond stress of BFRP bars was τm = 15.82 MPa, which was higher by 24.2% and 16.2% compared to steel and GFRP bars, respectively.
5.
The addition of zeolite and metakaolin to concrete reduced the slippage at maximum bond stress in the case of GFRP bars by more than two and a half times. For BFRP and steel bars, reduced slip was observed in the presence of concrete with the addition of zeolite.
6.
A diametrically different behavior in the case of GFRP and BFRP bars was observed after reaching the stress peak. Bond behavior on the “retarded slip” side decreased gradually and remained at the level of over 80% with the slip several times higher than the slip at maximum bond stress.
In conclusion, the bonding behavior of FRP bars in concrete is a complex and ongoing area of research, and further research is necessary to fully comprehend and optimize the bonding of FRP bars in concrete structures.
Acknowledgements
The authors gratefully acknowledge Astra company for generously providing GFRP and BFRP bars, as well as the concrete additives, zeolite and metakaolin, for the purpose of this study. This research was conducted under the Grant 35.2022 RND from Warsaw University of Technology, for which the authors express their deep appreciation.
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