Experimental evaluation of the tensile bonding strength of the basalt fiber-reinforced polymer

Abstract

The bonding performance of basalt fiber-reinforced polymer and concrete substrate has a significant effect on the reliability of externally strengthened existing concrete structure, due to being the most vulnerable element to failure in this fiber-reinforced polymer–concrete strengthening system. Its failure can result in the failure of the whole structure. Although many previous researchers have been interested in the tensile bonding strength of carbon fiber-reinforced polymer and glass fiber-reinforced polymer–concrete interface, that of basalt fiber-reinforced polymer–concrete interface has been very limited. Thus, the objective of this study is to experimentally assess the tensile bonding strength of the basalt fiber-reinforced polymer–concrete interface. The effects of high temperature, freezing–thawing cycles, type of resin, and concrete crack widths on the tensile bonding strength are also investigated. The pull-off experiment is conducted according to ASTM D7522/D7522M-15. A total of 205 core specimens of 50 mm diameter and 10 mm depth were taken from 41 concrete beams. The experimental results illustrate that both freezing–thawing and high-temperature condition have a substantial effect on the bonding strength of the basalt fiber-reinforced polymer–concrete interface. Bonding strength was decreased within the range of about 9%–30% when the number of freezing–thawing cycles increases from 100 to 300; likewise, it was decreased up to 30% when the exposure temperature rises to 200°C. Also, the specimens which were repaired to close their cracks by epoxy resin had no significant effect on the bonding strength of basalt fiber-reinforced polymer–concrete interface, when the specimens had crack width of less than 1.5 mm.

Keywords

basalt fiber-reinforced polymer bonding strength crack width freezing–thawing high temperature

Introduction

Over the last two decades, fiber-reinforced polymer (FRP) materials have shown to be crucial and efficient, owing to their good mechanical properties, high resistance to corrosion, lightness, low thermal conductivity, excellent performance in fatigue loading, and easy installation procedures (Bakis et al., 2002; Sena-Cruz et al., 2012). As a result, FRP materials are commonly being used in the retrofitting and/or strengthening of deteriorating and deficient concrete structural elements (Bank, 2006; Fédération Internacionale Du Béton, 2001; National Research Council, 2013). This approach has been illustrated as an effective alternative solution to the traditional rehabilitation methods, which typically use steel to reinforced concrete (RC) structure. In particular, carbon fiber-reinforced polymer (CFRP) has been comprehensively applied in the retrofitting concrete structure, because it offers high tensile performance (Jia et al., 2014; Lee, 2015; Xie et al., 2018). In comparison with CFRP, basalt fiber-reinforced polymer (BFRP) has higher thermal resistance and cheaper cost (Hassan et al., 2016). Therefore, the use of BFRP to strengthen concrete structure has attracted increasing attention (Lu et al., 2017).

In strengthening with externally bonded FRP, the bonding of the FRP–concrete interface is not only the essential element in transferring stresses from the concrete structure to FRP but also vulnerable to debonding, which can cause failure of the whole structure. Environmental factors have been illustrated to have negative effects on the durability of concrete–FRP interface (Benzarti et al., 2011; Li et al., 2002). Environmental factors are temperature cycles, moisture, humidity, chemicals, and freeze–thaw cycles. As a result, numerous investigations of the mechanical properties of bonding between FRP and concrete interface have been studied. Fazli et al. (2018) studied the bond strength of the CFRP–concrete interface under full immersion in saltwater 3.5% at the temperature of 60°C and wet/dry cyclic exposure. The results indicated that 12 months of exposure in a marine environment had little effect on the bonding performance between CFRP and concrete. The results also confirmed the marginal increase of bonding strength values after exposure to 12 months of environmental conditions. Benzarti et al. (2011) studied the behavior of adhesive bond between CFRP and concrete under accelerated aging conditions, that is, at 40°C and 95% relative humidity. The results illustrated that the decrease of bonding strength of CFRP and carbon fiber sheet strengthened specimens prepared from non-carbonated concrete slab caused humid aging. In addition, the results showed decrease in mechanical strength during humid aging, and pronounced elasto-plastic behavior, because of the use of epoxy adhesive for the bonding.

In addition, it can be seen that elevated temperature presents a clear influence on the stress–strain curve of epoxy resin (Li et al., 2019). After exposure to temperature from 20°C to 50°C, the strength and stiffness of epoxy resin demonstrate further degeneration. Mechanical properties of the adhesive are reduced at high temperature, which is of relevant importance for the strengthened structure, typically in regard to the bond performance. In consideration of this, Triantafillou et al. (2001) recommended that the glass transition temperature of the adhesive used for bonding the concrete and FRP should equate 20°C in extra of the maximum ambient temperature at normal service conditions. Al-Salloum et al. (2011) were interested in studying the behavior of FRP-confined concrete exposure to high temperature. The results of the pull-off test conducted in this study showed that a significant degradation in the pull-off strength between the FRP and concrete substrate occurred at a temperature of 200°C. Also, the CFRP-overlaid specimens showed higher reduction in bond strength than the glass fiber-reinforced polymer (GFRP)-overlaid specimens. Leone et al. showed that, mostly, increasing the test temperature changed the type of failure mode (Leone et al., 2009). In addition, at the temperature similar to or higher than the glass transition temperature, the adhesion strength of the adhesive is smaller than that of the concrete, inducing bond failure at the FRP–adhesive interface.

Freeze–thaw cycling is one of the major concerns, as in cold regions, it is extremely threatening to the service life of concrete structure. When FRP composites are used to strengthen concrete structure, the adhesive, concrete substrate, and reinforcing material, as well as the interface of the FRP–concrete, are affected by the freeze–thaw environment. Temperature-induced stresses in the adhesive layers, due to differential contraction and thermal expansion of concrete and FRP, as well as the contraction and thermal expansion of water in the freeze–thaw environment, cause damage to the interface, and induce premature bond failure (Green et al., 2000). Davalos et al. (2005), Karbhari and Ghosh (2009), Silva and Biscaia (2008), Subramaniam et al. (2008), and Yun and Wu (2011) examined the durability of FRP–concrete interfaces in freeze–thaw environments. The results showed that with increasing freeze–thaw cycles, the bond capacity decreased remarkably. In contrast, Mukhopadhyaya et al. (1998) investigated the freeze–thaw response of GFRP–concrete interfaces using air-entrained concrete. They observed that after freeze–thaw exposure, the delamination force of the FRP–concrete interface was not decreased. Fava et al. (2007) also illustrated that the delamination force of the CFRP–concrete interface was not affected by freeze–thaw, but the bond–slip parameters (local bond shear strength, peak slip, and interfacial fracture energy) were reduced. Although previous researchers have offered valuable results in regard to freeze–thaw and high-temperature effects, the tensile bonding strength of the BFRP–concrete interface subjected to freezing–thawing, high temperature, and concrete crack repair conditions is much needed.

This article conducted a comprehensive investigation into the tensile bonding strength of the BFRP–concrete interface exposed to high temperature, freezing–thawing condition, type of resin, and concrete crack width. The pull-off experiment was conducted according to ASTM D7522/D7522M (2015). A total of 205 core specimens of 50 mm diameter and 10 mm depth were taken from 41 concrete beams.

Experimental methods

Preparation of test specimens

In the framework of an experiment, specific test specimens according to FRP RC beam were prepared. This test program constituted 41 concrete beam specimens strengthened using BFRP plates and three different types of resin, as shown in Tables 1 and 2. The concrete beams of dimension 100 mm (width) × 400 mm (length) × 100 mm (height) were cast with ready mix concrete (Figure 1). To determine the compressive strength of concrete beam, five cylinders of 200 mm height and 100 mm diameter were experimented with, using an MTS-290 testing machine. The mix proportion of concrete 24 MPa is of 1:2:4.45:5.29 for water, cement, sand, and gravel, respectively.

Table 1.

Mechanical properties of resin.

Properties	YD-128	KRF-120	APP-880
Equivalent weight (g/eq)	184∼190	172∼176	–
Density (g/mL)	1.17	1.1∼1.2	–
Mixing ratio(resin:hardener)	100:33	100:27	100:20
Viscosity (cps at 25°C)	11,500∼13,500	1100∼1300	300∼450
Glass transitiontemperature (°C)	78	70	67.08
Tensile strength (MPa)	79.9	70	75

Table 2.

Mechanical properties of concrete and BFRP.

Properties	Concrete	BFRP
Compressive strength (MPa)	24	–
Tensile strength (MPa)	–	780
Modulus of elasticity (MPa)	24,020	37,030
Shear modulus of elasticity (MPa)	–	2730
Poisson’s ratio	0.2	0.29

BFRP: basalt fiber-reinforced polymer.

Figure 1.

Dimensions of tested specimens.

The accurate preparation of the concrete substrate to reach appropriate bonding at the BFRP–concrete interface is of high significance. Initially, a grinding machine was used to remove any loose particles and uneven surfaces of the concrete beam, as shown in Figure 2(a). After grinding, the concrete surface was smoothed using sandpaper, after which resins were applied. These resins were applied to the concrete surface using a small paint brush (Figure 2(b)). A hand roller was used in order to carefully squeeze out any excess air voids between the BFRP plates and concrete surfaces. To ensure optimal performance, the minimum thickness of BFRP plate of 1 mm was used, according to ASTM D7522/D7522M (2015) recommendation. The concrete beam specimens were bonded to the BFRP plates on the top surface, as shown in Figure 2(c). After the BFRP plate was completely attached to the concrete beam, the specimens were cured for 24 h in the laboratory at ambient temperature, before the accelerated aging process, as explained in the next section.

Figure 2.

Preparation of specimens: (a) grinding surface of concrete, (b) paint brush, and (c) attaching BFRP plate.

Configuration of the accelerated aging process condition

To accelerate the deterioration of the specimens subjected to environmental exposure conditions, an accelerated aging procedure was applied in this study. Environmental exposure conditions of freezing–thawing, temperature, and crack width were applied to evaluate the behavior of the bonding strength of the BFRP–concrete interface. Figure 3 shows the test program of the experiment. Each exposure condition was presented as follows:

Exposure to freezing–thawing: the tensile pull-off test of the BFRP–concrete interface was divided into two conditions: case (1) bonded BFRP to concrete substrate before exposure to freezing–thawing, while case (2) bonded BFRP to concrete substrate after exposure to freezing–thawing. In this case, YD-128 epoxy resin was used to bond the concrete and BFRP. The freezing–thawing was conducted using the freezing and thawing apparatus chamber model CKF-3000, as shown in Figure 4(a). To obtain 100, 200, and 300 freezing–thawing cycles from +4°C to −18°C, the specimens were kept in the chamber for 17, 34, and 50 days, respectively.

Exposure to high temperature: to perform the tensile pull-off test in this condition, the BFRP bonded to concrete used three types of resin: YD-128 epoxy resin, KRF-128 epoxy resin, and APP-880 phenol resin. The specimens were exposed to temperature of 50°C, 100°C, 150°C, and 200°C for 1 h with the MG Indus system, as indicated in Figure 4(b). In this case, the BFRP was attached to the concrete, before placement in the temperature room.

Exposure to crack width: the specimens were divided into two cases: the crack on specimens with, and without, reinforced epoxy resin. The crack widths of specimens were artificially created by inserting a thin plate with the width of 0.3, 0.6, 1.2, 4.0, and 10 mm while casting concrete. Then, YD-128 epoxy resin was used to reinforce the crack on the specimens with injector, as shown in Figure 4(c).

Figure 3.

Tensile pull-off test program.

Figure 4.

Specimens exposure to: (a) freezing–thawing, (b) temperature, and (c) reinforcing crack with epoxy.

Pull-off bond test

The pull-off test is one of the most frequently used tests to measure the bonding strength of BFRP–concrete substrate. This method has been carried out as a standard BS 1881 part 207 (1992). To progress the pull-off bond test until failure, the test requires a loading fixture (aluminum dolly disk, Figure 5(a)) to be attached to the surface of the specimens. Based on ASTM D7522/D7522M (2015), the partial core surrounding the test zone is suggested to be between 6 and 12 mm. A core drilling machine was used to create the circular core of the same diameter as the dolly disk, in order to facilitate the distinguishability of the individual test.

Figure 5.

(a) Aluminum dolly disks and (b) Proceq DY-216 pull-off tester.

In this study, a drill press was used to core through the strengthened surface of the concrete beam with a diameter of 50 mm and 10 mm depth (as illustrated in Figure 1), as recommended by ASTM D7522/D7522M (2015). To avoid cross-influence between the cores, each core was drilled separately with 50 mm (Karbhari and Ghosh, 2009), as shown in Figure 5(a). The pull-off test was performed using a Proceq DY-216 pull-off tester (Figure 5(b)) accordingly to ASTM D7522/D7522M (2015), so that an aluminum dolly was bonded to the surface of the specimen. Finally, a tensile loading was applied to the aluminum dolly disk by means of a dynamometer device, until debonding occurred. These tests were performed on a testing machine with 0.01 mm s⁻¹ displacement rate. According to ASTM D4541-09 (2009) requirements, at least three specimens are needed to describe the test results. Thus, in the study, five rounds of pull-off testing for each surface of the specimen were performed.

In order to evaluate the pull-off strength, the failure mode displayed in each test was closely examined and recorded. Different failure modes were observed at the bond surface after testing. The failure characteristics were classified into seven different types, in accordance with ASTM D7522/D7522M (2015). Table 3 classifies the observed failure modes in this study, while Figure 6 shows a schematic of the test (the interface between the adhesive, the epoxy resin, the FRP layers, and the concrete is extended for limpidity). Failure took place at the feeblest plane within the system, which could have been the bond interface, the concrete substrate, the epoxy used to bond FRP to dolly, or a combination of these failure modes (Benzarti et al., 2011; Fazli et al., 2018; Green et al., 2000; Leone et al., 2009; Mikami et al., 2015). When a failure occurs at the adhesive material, it is considered to be the true bond strength. Therefore, in order to express that there is a reduction of bond strength between the FRP and concrete substrate owing to the environmental conditions, failure at the adhesive was expected (Mikami et al., 2015).

Table 3.

Pull-off test failure mode classification.

Failure mode	Failure type	Cause of failure
G	Concrete substrate failure	Desirable failure mode
F	Combination of Modes G and E failure	Inconsistent FRP–concrete adhesive. Failure is partially on substrate and partially adhesive
E	FRP–concrete interface failure	Incomplete epoxy resin cure, contamination of adhesive during application
A	Bonding adhesive failure atloading fixture	Improper adhesive bonding of dolly. This mode is not acceptable as failure mode

Figure 6.

Failure modes of the pull-off test for the BFRP–concrete interface of the specimen.

Results and discussion

Failure modes

General observation on the effects of freezing–thawing cyclic exposure on failure modes

In the case of BFRP bonded to concrete substrate before exposure to freezing–thawing, visual observation of the control specimen test results showed that the dominant failure mode was Mode G, as shown in Figure 7(a). This failure mode indicated acceptable and complete bond between the BFRP and concrete. It also displayed that the bond strength of BFRP–concrete was greater than the tensile cracking strength of concrete. The 100 and 200 cycles conditioned specimens showed various failure modes: Mode G (three tests) and Mode E (two tests), as shown in Figure 7(b). The failure of Mode E exhibited deterioration of the bond-line as a result of exposure. It revealed the degradation of the resin/adhesive layer at the BFRP–concrete interface. The 300 cycles conditioned specimens demonstrated that the dominant failure at the concrete substrate was primarily indicative of Mode G (three tests) and Mode D (two tests). This failure of Mode D can be attributed to the increasing brittleness of the adhesives and resins as a consequence of continued exposure to subzero environments, and the increased propensity for matrix cracking (Dutta, 1988; Lord and Dutta, 1988).

Figure 7.

Pull-off test failure: (a) Mode G, (b) Mode E, and (c) Mode F.

In the case of BFRP bonded to concrete substrate after exposure to freezing–thawing, after 100 cycles of exposure, the dominant failure of partial failure in the top layer of concrete at the FRP–concrete interface and partial failure at the concrete substrate was primarily indicative of Mode F, as presented in Figure 7(c). This failure was due to inconsistent FRP–concrete adhesive. The 200 and 300 cycles conditioned specimens demonstrated that the dominant failure at the BFRP–concrete interface was Mode E. This was because of the increasing brittleness of the concrete substrate surface, and the increased propensity for concrete cracking.

General observation of the effects of temperature exposure on failure modes

The result of the 50°C conditioned specimens indicated that the dominant failure mode was Mode G. For the 100°C conditioned specimens, the dominant failure was Mode E. This was due to the loss of adhesive joint performance (Stratford et al., 2009). Moreover, this failure occurred because the temperature applied to the specimens exceeded the critical temperature (glass transition temperature) of the epoxy, which induced change of the polymer matrix from a rigid glassy material to a soft or rubbery material (Bank, 1993; Blontrock et al., 1999). Many commonly used thermoset matrices revealed the glass transition temperature range of 65°C–82°C (ACI 440.2R-02, 2002). The 150°C and 200°C conditioned specimens exhibited various failure modes: Mode E (three specimens) and Mode A (two specimens), as shown in Figure 8. The failure of Mode A was owing to the debonding of the aluminum dolly disk from the composite. This can be attributed to improper adhesive bonding of the dolly. Upon finalization of all the pull-off testing, the various failure modes of G, E, and A were observed, which is as reported by Karbhari and Ghosh (2009).

Figure 8.

Pull-off test failure, Mode A.

General observation of the effects of crack width exposure on failure modes

In the case of crack width without epoxy resin reinforcement, the 0.3 and 0.6 mm crack width conditioned specimens showed the dominant failure mode of Mode G. This failure mode depicted adequate and complete bond between the BFRP and concrete. It also revealed that the BFRP–concrete bond strength was greater than the tensile cracking strength of concrete. The results of the 1.2 and 4.0 mm crack width conditioned specimens showed the dominant failure mode of Mode G, with one Mode F. This failure mode was partially in the top layer of concrete at the BFRP–concrete interface, with the remaining failure in the concrete through the aggregates (ASTM D7522/D7522M, 2015). The 10 mm crack width conditioned specimens presented failures mode of Mode G (two tests), Mode F (one test), and Mode C (two tests). The failure of Mode C was adhesive failure, which occurred at the BFRP–adhesive interface. This failure can be attributed to inadequate bonding of the BFRP to concrete that caused the existence of void at the BFRP–concrete interface.

In the case of crack width with epoxy resin reinforcement, the results of 0.3, 0.6, 1.2, and 4.0 mm crack width specimens displayed the failure mode of Mode G that was a desirable failure mode. For the 10 mm crack width conditioned specimens, the bonded surface demonstrated failures of Mode A (dominant), Mode G (one test), and Mode F (one test). In this case, the results for Mode A were neglected in calculating the average pull-off strength of the specimens.

Effect of freezing–thawing

Figure 9 shows the results of global strength in the form of the average bonding strength under freezing–thawing for the five multi-material specimens. The average bonding strength values were obtained from the average of the five pull-off test results. Table 4 shows the standard deviation of the pull-off strength and the coefficient of variation (CV), in which CV = 100 × (standard deviation/average). Figure 9 and Table 4 demonstrate that the CV analysis for all of the specimens varies in the range 4.07%–25.45%. However, it is evident that large scatter in the pull-off bond test results is typical behavior. In addition, the maximum difference in the results recorded using the same machine is 27.8%, according to ASTM D4541-09 (2009). Therefore, note that these results were acceptable, as they appear to be crucial in regard to the CV and visual inspection from experimental test.

Figure 9.

Average pull-off strength versus number of cycles.

Table 4.

Average pull-off test, standard deviation (SD), and coefficient of variation (CV) under the effects of freezing and thawing.

Number of cycles	Case 1 (24 MPa)				Case 1 (30 MPa)				Case 2 (24 MPa)				Case 2 (30 MPa)
Number of cycles	0	100	200	300	0	100	200	300	0	100	200	300	0	100	200	300
Average	2.27	2.31	2.07	2.05	2.50	2.61	2.19	2.12	2.27	1.70	1.59	1.58	2.50	2.05	1.88	1.71
SD	0.09	0.11	0.37	0.10	0.18	0.27	0.56	0.14	0.09	0.23	0.08	0.27	0.18	0.40	0.46	0.15
CV	4.08	4.91	17.90	5.04	7.40	10.23	25.46	6.38	4.08	13.45	5.36	17.29	7.40	19.32	24.25	8.50

In the case of BFRP bonded to concrete substrate before exposure to freezing–thawing (case 1), using the control specimens (0 cycle) as the reference, the average pull-off strength for 100 cycles conditioned specimens was observed to exhibit a minuscule increase of 1.94% and 4.38% for concrete strength of 24 and 30 MPa, respectively, which is in agreement with Baumert et al. (1996), Green et al. (1997), and Kaiser (1989). They confirmed that after 100 cycles freezing–thawing from +25°C to −25°C, there was no detrimental influence on the overall structural performance of the beams. It can be observed that increasing the exposure to 200 cycles led to decrease in the average strength of 8.78% and 12.47% for 24 and 30 MPa, when the dominant failure mode was Mode G and Mode E, respectively. On increasing the exposure to 300 cycles, the average bonding strength decreased by 9.38% and 15.31% for 24 and 30 MPa, respectively, when the failure mode was Mode G (three tests), with Mode D (two tests). This decrease in bonding strength is probably due to the increasing brittleness of the adhesive/resin and the increased propensity for matrix cracking (Dutta, 1988; Lord and Dutta, 1988), which led to change in the pull-off strength of the adhesive as mechanical properties.

In the case of BFRP bonded to concrete substrate after exposure to freezing–thawing (case 2), in comparison with control specimens, the average bonding strength decreased 24.76% and 18.12% for 24 and 30 MPa, where the dominant failure mode was Mode F for exposure to 100 cycles. In this exposure, it can be observed that the average bonding strength of case 2 decreased, while that of case 1 increased. This was because the bonded surface of concrete substrate in case 2 was directly exposed to the freezing–thawing that induced deterioration by freezing of the pore water inside the concrete. If the pores are tiny, the expansion caused by freezing can utilize stresses on the concrete that crack the concrete, and deteriorate its strength (Neville, 1995). For exposure to 200 and 300 cycles of freezing–thawing, the average bonding strength decreased as consistently as that of case 1.

When considering the pull-off strength value for both cases 1 and 2, it can be observed that the pull-off strength of case 1 was about 19%–27% greater than that of case 2 for both concrete strengths of 24 and 30 MPa. This was because in case 2, the bonded surface concrete substrate was exposed directly to the freezing–thawing condition that increasing the brittleness of the concrete. As a result, the concrete became the most vulnerable material among others, which resulted in the dominant failure mode being induced by the concrete.

Effect of temperature

To highlight the effects of temperature on the pull-off bonding strength of the BFRP–concrete interface, tests of three different types of resin—YD-128 epoxy resin, KRF-120 epoxy resin, and APP-phenol resin—were performed. Figure 10 and Table 5 show the average bonding strength of the BFRP and concrete interface, the standard deviation, and the CV. Using the control specimens as the reference (under 20°C), for YD-128 and APP-880, it can be seen that increasing the exposure temperature to 50°C led to the increment of the average pull-off strength of 3.47% and 1.02%, respectively. This may be due to the accelerated curing process of resin and concrete (Cao and Detwiler, 1995). It also displayed that increasing the exposure temperature to 100°C reduced the average pull-off strength by 0.87% and 6.82% for YD-128 and APP-880, respectively. This was due to the reduction of the epoxy-matrix strength, while with direct exposure to the elevated temperature (Al-Salloum et al., 2011). With increasing temperature exposure to 150°C and 200°C, the average pull-off strength decreased 6.81% and 28.04% for YD-128 and 19.90% and 21.6% for APP-880, respectively. For KRF-120, the average pull-off strength decreased to 8.45%, 19.51%, 34.01%, and 35.13% when the temperature increased to 50°C, 100°C, 150°C, and 200°C, respectively. These results were also in agreement with Al-Salloum et al. (2011).

Figure 10.

Average pull-off strength versus temperature.

Table 5.

Average pull-off test, standard deviation (SD), and coefficient of variation (CV) under the effect of temperature.

Temperature (°C)	YD-128 epoxy resin					KRF-120 epoxy resin					APP-880 phenol resin
Temperature (°C)	20	50	100	150	200	20	50	100	150	200	20	50	100	150	200
Average	2.30	2.38	2.28	2.14	1.66	2.15	1.97	1.73	1.42	1.40	2.13	2.13	1.98	1.70	1.67
SD	0.14	0.10	0.17	0.21	0.21	0.22	0.09	0.42	0.28	0.15	0.47	0.24	0.28	0.28	0.15
CV	6.31	4.30	7.45	9.71	12.39	10.25	4.79	24.31	19.78	10.92	22.20	11.48	14.35	16.41	9.18

From the above results and the discussion of the pull-off test implement, it can be observed that the adhesion strength using YD-128 epoxy resin provided the highest resistance, while KRF-120 phenol resin provided the lowest bonding resistance. Considering material properties, this was because YD-128 epoxy resin has greatest tensile strength and glass transition temperature if compare to KRF-120 epoxy resin and APP-880 phenol resin. For phenolic resin, although the impregnated material itself has excellent resistance to temperature, its bonding strength test in our research is lower than that of epoxy resin.

Effect of concrete crack

Figure 11 shows the influence of crack width on the variation of the pull-off strength. In this study, two types of specimens with and without reinforced epoxy resin were considered. The dashed line and solid line indicate the fitting curve from the experiment of specimens with and without reinforcement, respectively. It can be seen that the maximum value of bonding strength was obtained for the specimens without crack, while the minimum was achieved from specimens with 10 mm crack width. This was due to the reduction of effective bonding area that caused the bonding stiffness to also decrease. The results also indicate that when the crack width of the specimens was increased from 0.0 to 0.6 mm (from without crack to with crack), the average bonding strength decreased rapidly. Whereas, for crack width 0.6–10 mm, the bonding strength decreased gradually. As a result, it can be concluded that the crack width had a significant effect on the bonding strength of the FRP–concrete interface when the specimen had a crack with width of less than 0.6 mm, for both specimens with and without resin reinforcement. The effect of crack width on the bonding strength can be well captured by the following exponential function

σ_{w} = 0.85 \times e^{- 2.45 x} + 1.32

(1)

σ_{wt} = 0.94 \times e^{- 0.899 x} + 1.01

(2)

where $σ_{w}$ and $σ_{wt}$ are the bonding strength of specimens with and without reinforced epoxy resin, respectively; and x is the crack width.

Figure 11.

Average pull-off strength with regard to crack width.

According to curve fitting in Figure 11, it can be noted that the reinforcement of specimens with epoxy resin had no significant influence on the bonding strength of the FRP–concrete interface for the specimens with crack width smaller than 1.5 mm. This was due to the high viscosity of resin that can hardly penetrate the small crack of specimens. In comparison for the crack width greater than 1.5 mm, the bonding strength of specimens reinforced with epoxy resin provided higher performance than that of specimens without the reinforcement. Thus, it can be concluded that when the crack width was greater than 1.5 mm, the epoxy resin reinforcement had a substantial role in improving the bonding strength of the FRP–concrete interface.

Conclusion

This article conducted the experiment of bonding between BFRP and concrete at their interface to investigate the performance of the pull-off strength of the BFRP–concrete interface. The effects of freezing–thawing, high temperature, type of resin, and concrete crack width on the tensile bonding strength were also investigated. Based on the present experimental study, the following conclusions can be drawn:

From the tests, six different failure modes were identified at the interface between BFRP and concrete: (G) concrete substrate failure, (F) partial concrete substrate failure and partial adhesive failure, (E) FRP–concrete interface failure, (D) cohesive failure at adhesive, (C) adhesive failure at the FRP–adhesive interface, and (A) bonding adhesive failure at loading fixture.

It can be seen that the average bonding strength of the concrete specimen bonded to BFRP after exposure to freezing–thawing provided 19%–27% smaller strength than that of the concrete specimen bonded to BFRP before exposure. In the case of BFRP bonded to concrete substrate before exposure to freezing–thawing, when increasing exposure to 300 cycles of freezing–thawing, the failure mode changes from failure at the concrete substrate to cohesive failure in the adhesive. This is due to increasing brittleness of the resin/adhesive, and propensity for matrix cracking.

Strengthening of the concrete structure using FRP as externally bonded reinforcement was sensitive to the influence temperature variations. To avoid performance reduction of bonding strength during repairing work, the specimens should only be exposed to the temperature that is less than the glass transition temperature of the polymer matrix.

In strengthening concrete structure using FRP, it was suggested to use YD-128 epoxy resin in bonding FRP to concrete since it had the highest bonding strength. However, for concrete structure where extreme temperature (over 150°C) was expected, APP-880 phenol resin should be used instead because it showed the lowest strength variation between the low and high temperature.

Note that when the crack width was less than 0.6 mm, the crack had a strong influence on the reduction of the bonding strength of the BFRP–concrete interface for both with and without epoxy resin reinforcement. Since the reinforcement had no significance effect on the bonding strength of specimens with crack less than 1.5 mm, the reinforcement with epoxy resin should apply to structure component which had cracks at least 1.5 mm.

Footnotes

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: This work was supported by the National Research Foundation (NRF) grant, funded by the Korea government (MEST) (no. 2017R1A2B3008623).

ORCID iD

WooYoung Jung

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Experimental evaluation of the tensile bonding strength of the basalt fiber-reinforced polymer–concrete interface

Abstract

Keywords

Introduction

Experimental methods

Preparation of test specimens

Configuration of the accelerated aging process condition

Pull-off bond test

Results and discussion

Failure modes

General observation on the effects of freezing–thawing cyclic exposure on failure modes

General observation of the effects of temperature exposure on failure modes

General observation of the effects of crack width exposure on failure modes

Effect of freezing–thawing

Effect of temperature

Effect of concrete crack

Conclusion

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References