Durability study of concrete-covered basalt fiber-reinforced polymer (BFRP) bars in marine environment

Journal Pre-proofs Durability study of concrete-covered basalt fiber-reinforced polymer (BFRP) bars in marine environment Zhongyu Lu, Lizhu Su, Guijun Xian, Baihang Lu, Jianhe Xie PII: DOI: Reference:

S0263-8223(19)33031-4 https://doi.org/10.1016/j.compstruct.2019.111650 COST 111650

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

11 August 2019 16 October 2019 31 October 2019

Please cite this article as: Lu, Z., Su, L., Xian, G., Lu, B., Xie, J., Durability study of concrete-covered basalt fiberreinforced polymer (BFRP) bars in marine environment, Composite Structures (2019), doi: https://doi.org/10.1016/ j.compstruct.2019.111650

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Durability study of concrete-covered basalt fiber-reinforced polymer (BFRP) bars in marine environment Zhongyu Lu a, Lizhu Su a, Guijun Xian b,c,*, Baihang Lu b,c, Jianhe Xie a a

School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006,

Guangdong, China b

Key Lab of Structures Dynamic Behavior and Control (Harbin Institute of Technology), Ministry of Education,

Heilongjiang, Harbin 150090, China c

School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China

* Corresponding author. Postal address: 202 Haihe Road, Nangang District, Harbin 150090, China Tel: +86(451)8628-3120 Fax: +86(451)8628-3120 Email address: [email protected]

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Abstract: To understand the durability of basalt fiber-reinforced polymer (BFRP) bars for the reinforcement of concrete structures applied in marine environments, BFRP bars with 20-mm concrete covers were immersed in ocean water or simulated seawater at room temperature (~23 °C), and the evolution of the thermomechanical properties was studied. A set of uncovered BFRP bars immersed in ocean water and laboratory accelerated simulated seawater (23 °C, 40 °C, and 60 °C) was investigated for comparison. After one year, the mechanical properties of the uncovered BFRP immersed in 60 °C simulated seawater showed the most degradation, followed by the concrete-covered BFRP in the laboratory immersion. Concrete-covered BFRP showed more deterioration than uncovered BFRP in the ocean water. The results indicated that the alkalinity is the key factor causing the degradation of BFRP, especially for standing water in the laboratory environment. In addition, an alkali-aggregate reaction (AAR) of SiO2 in basalt fiber was found at the interface between the BFRP bars and concrete. The degradation of the resin matrix and sizing led to the direct exposure of basalt fiber to in alkaline solution, which established the foundation of the AAR.

Keywords: BFRP bars, Concrete covered; Seawater; Durability

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1. Introduction Reinforced concrete (RC) structures are currently the most widely used construction materials worldwide. However, the durability of RC structures in the ocean environment owing to the corrosion of steel rebars is a crucial issue [1, 2]. Many efforts to battle steel corrosion have been made, and the fiberreinforced polymer (FRP) bar is considered to be an ideal material to replace steel in the ocean environment [3]. Wei et al. [4] studied the durability of basalt fiber and glass fiber/epoxy resin composites in seawater. The results showed that the hydrolysis of fiber, a matrix, and their interface occurred in composites, and that the hydrolysis may break the molecular chain and change the performance of the material. We previously studied the durability of basalt fiber-reinforced polymer in a wet-dry cyclic condition in a chloride-containing environment. The results indicated that the degradation of the material was dominated by the damage to the interface between the basalt fiber and epoxy resin matrix. Generally, FRP composite used in civil engineering usually contains carbon FRP (CFRP), glass FRP (GFRP), aramid FRP (AFRP), and basalt FRP (BFRP). Among these FRPs, BFRP has gained increasing attention from both academia and industry as an emerging eco-friendly material [5-7]. Basalt fiber has high chemical stability and good mechanical properties [6, 8, 9], and is nontoxic and noncombustible [5]. Živković et al. [10] concluded that the impact behavior can be improved by using basalt fiber hybrid composites after salt-water conditioning. Based on these prominent merits, using BFRPs as structural materials is highly expected [7, 11]. Understanding the performance degradation of FRPs in ocean environments is necessary for their expanded use [7]. In fact, BFRP possess relatively strong resistance to seawater corrosion, moderate resistance to acid corrosion and severe degradation in an alkaline solution [12, 13]. However, FRPs are likely to be subjected to a strong alkaline solution with a pH value of approximately 12.5–13 owing to the concrete pore water [8, 14]. Many studies have been conducted on the deterioration of BFRP composites under the effects of alkaline solution [9, 15]. FRPs immersed in alkaline solution lead to the resin matrix hydrolysis of the ester linkages [16], fiber/resin interface debonding, and even porosity [17]. Beyond these degradations, the damages to basalt fiber in alkaline solution and seawater is confirmed. Guo et al. [9] studied the durability of CFRP, GFRP and BFRP. Specimens were immersed in 25 °C, 40 °C, and 60 °C simulated seawater sea-sand-concrete environments (pH was 12.7 and 13.4), and the results showed that the reaction of alkali ions with silicate 3

in basalt fibers leads to an intense degradation of BFRP. Wang et al. [15] studied the long-term performance of BFRP and GFRP bars in seawater and sea-sand-concrete environments (at room temperature, 40 °C, 48 °C, and 55 °C). The immersion solution also considered pH value as an index for simulating high-performance seawater and sea-sand-concrete pore solution (pH was set at 12.7 and 13.4). The tensile modulus of the FRP bars did not change significantly, and the reaction between Cl- and Fe2+ (contains in basalt fiber) accelerated the degradation of BFRP. Based on an extensive literature investigation, experiments were carried out to reveal the alkaline resistance of BFRPs. Most of the research studies immersed the specimens in a constant-pH solution [6-9, 13-15, 17]. However, chloride ions and sulfate contained in seawater can affect the alkalinity of concrete by physical and chemical processes, leading to a change in the pH value of the concrete [18]. It was proven that seawater slightly accelerates the early strength of concrete [19]. The dissolution of compounds of cement is accelerated because chloride and sulfate ions presenting in seawater and sea sand concrete, particularly tricalcium silicate in water, facilitates a more rapid hydration of concrete [20]. During cement hydration, the release of alkaline ions into pore water leads to a high pH of concrete pore water [21]. With regard to sulfate, the mode of attack is crystallization. Potassium and magnesium sulphates (K2SO4 and MgSO4) presenting in seawater can initially react with calcium hydroxide Ca(OH)2, which is present in the set cement formed by the hydration of dicalcium silicate (C2S) and tricalcium silicate (C3S). The reaction produces insoluble magnesium hydroxide (Mg(OH)2), which forces the reaction to form gypsum, and the value of the pH decreases [22]. Thus, the degradation of BFRP bars in actual concrete is different from that in a simulated concretepore alkaline solution. The pH inside the concrete changes, but the pH value is constant in the laboratorysimulated alkaline solution of concrete. Experiments conducted in a real ocean environment are more valuable for validating the data obtained in the laboratory. The work presented in this study aims to investigate the performance degradation of concrete-covered BFRP bars exposed in laboratoryaccelerated simulation seawater and ocean water (full immersion zone). This experiment is expected to reveal the long-term performance and degradation mechanisms of BFRP. 2. Experimental 2.1. Raw materials

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The BFRP bars used in this study were provided by TuoXin Aerospace Basalt Industrial Co., Ltd (Chengdu City, China). The resin matrix was an epoxy based on bisphenol-A (DGEBA, similar to EPON 828) and an anhydride hardener. The nominal diameter of the BFRP bar was 8 mm, and the fiber volume fraction (Vf) is approximately 67%. The constituents of the basalt fiber provided by the supplier are listed in Table 1. The tensile strength, tensile modulus, and elongation at the break of the BFRP were 1121 MPa, 50.2 GPa, and 2.2%, respectively. Table 1 Components of basalt fibers. Component Proportion (%)

SiO2 48.4

Al2O3 14.7

CaO 7.7

MgO 4.7

K2O 1.6

Na2O 3.0

FexOy 15.3

TiO2 3.8

B2O3 0.8

2.2. Immersion media The specimens were immersed in ocean water (Wendeng City, Shandong Province, China) or laboratory-accelerated simulated seawater. The average seawater temperature of the ocean water was 12.5 °C with a high of about 24 °C in August and a low of 0.9 °C in February. The pH value ranged from 7.5 to 8.5. The components of the seawater are listed in Table 2. The covered and uncovered BFRP bars were completely immersed in the seawater even during the ebb-tide period. The immersion conditions are shown in Fig. 1. The laboratory-accelerated simulated seawater environments were set at room temperature (~23 °C), 40 °C, and 60 °C. The uncovered BFRP bars were immersed in the acceleration environments, and the concrete-covered specimens were immersed at room temperature, as shown in Fig. 2. The simulated seawater was prepared according to ASTM D1141-13 [23], and the details are listed in Table 3. Table 2 Components of ocean water. Element Content (g/L)

Cl 17.3

SO4 2.72

F 0.03

Na 10.2

Mg 1.92

Al 8 × 10-7

K 0.31

Ca 0.47

Fe 9.8 × 10-8

Cu 4.8 × 10-7

Fig. 1. Location of specimens in ocean water and laboratory: (a) overview map; (b) details. 5

Fig. 2. Immersion environment for specimens. Table 3 Chemical composition of simulated seawater [23]. Compound H2O NaCl Na2SO4 MgCl2 CaCl2 SrCl2 KCl NaHCO3 KBr H3BO3 NaF

Concentration (g/L) 1000 24.53 4.09 5.20 1.16 0.025 0.695 0.201 0.10 0.027 0.003

2.3. Mechanical tests The tensile properties of the BFRP were tested according to ASTM D 3171-15 [24]. An electronic universal testing machine (WDW-100D model, Hengyi Company, Shanghai, China) equipped with a 100-kN load cell was used for the tensile test. The anchorage length of the specimen was set at 140 mm, and the gauge length was 320 mm. A tension rate of 5 mm/min was used. An extensometer at a gauge length of 50 mm was employed to measure the tensile strain. The flexure behavior of the BFRP bars was tested based on ASTM D 4476-14 [25]. The load was controlled by an electronic universal testing machine (identical to that used in the tensile test), and the details are shown in Fig. 3. The specimens were extracted with a designer knife, and the length of the specimen was set at 80 mm. The span was 16 times the thickness of the specimen. The specimen is shown in Fig. 4. The flexure test was measured at a crosshead speed of 3 mm/min under displacement control.

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Fig. 3. Flexure test setup: (a) sketch map; (b) physical map.

Fig. 4. BFRP specimens for flexure test. The flexure strength was calculated by S=

𝑃𝐿𝐶 4𝐼

(1)

where S is the flexure strength (MPa), P is the ultimate load (N), L is the clear span (mm), C is the distance from the center of the section to the outer edge (mm), and I is the moment of inertia (mm4). The flexure modulus was calculated by 𝑃𝐿3

𝐸𝑏 = 48𝐼𝑌

(2)

where Eb is the flexure modulus (GPa), P is the selected load (kN), L is the clear span (mm), Y is the maximum deformation at the selected load (mm), and I is the moment of inertia (mm4). A transverse shear test was performed in accordance with ASTM D7617 [26]. The length of the specimen was set at 300 mm, and the test was performed at a crosshead speed of 1 mm/min under displacement control. The details are shown in Fig. 5.

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Fig. 5. Transverse shear test device. The transverse shear strength was calculated by 𝑃

(3)

τ = 2𝐴

where τ is the transverse shear strength (MPa), P is the ultimate load (N), and A is the cross-sectional area of the specimen (mm2). 2.4. Preparation of concrete-covered BFRP bars A PVC tube was employed as a mold for the concrete cover. The external diameter of the PVC tube was 50 mm, and the thickness was about 1.5 mm. Two caps at each end with a hole in the core were used to keep the specimen in the middle of the mold. The mixture proportion by weight of the constituents was determined as cement:water:fine aggregate = 405:200:590. The specimens were sealed with a plastic film to maintain a wet condition in the laboratory for 24 h. After demolding, they were cured twice a day with tap water for a total of 28 days at indoor temperature. The specimens were removed after immersion for 15 days, 1 month, 2 months, 6 months, and 1 year (ocean water only 6 months and 1 year), and the BFRP bars were extracted after crushing the concrete cover. A summary of all specimens used in this study is listed in Table 4. Table 4 Summary of all specimens used in this study.

Specimen

Uncovered/Covered

Immersion condition

Lab.-C-RT Lab.-U-RT Lab.-U-40 °C Lab.-U-60 °C Ocean-C Ocean-U

Covered Uncovered Uncovered Uncovered Covered Uncovered

Laboratory-Room temperature Laboratory-Room temperature Laboratory-40 °C Laboratory-60 °C Ocean water Ocean water

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Tensil e 5 5 5 5 5 5

Number of specimens Flexure Transverse shear 5 5 5 5 5 5

5 5 5 5 5 5

2.5. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) The morphologies of the contact interface of the concrete and BFRP bar-coated platinum were observed with a focused ion beam field-emission scanning electron microscope (LYRA 3 XMU, Czech). Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the chemical elements of the concrete and BFRP surfaces on the BFRP-concrete interface with Oxford Instruments’ Aztec software. 2.6. Alkaline activity test of basalt fiber The basalt fiber was first placed in a chamber at 450 °C for 3 h to remove the sizing covered on the fiber, and ground to a powder after it was cooled to room temperature. The alkaline activity test was in accordance with ASTM C1260 [27]. The mixture proportions by the weight of the constituents were as follows: cement (1.0):water (0.47):basalt fiber (0.045). The length of the prism was set at 285 mm, and the immersion media was 80 °C NaOH solution with a concentration of 1 mol/L. The specimens were immersed in the solution after demolding. Fig. 6 shows the specimens in the immersion media. The length of the specimens was recorded with a vernier caliper. In addition, a set of control specimens without basalt fibers was tested for comparison. Three specimens were tested, and the averages were reported.

Fig. 6. Photograph of specimens immersed in NaOH solution. 3. Results and Discussion 3.1. Variation of mechanical properties The evolution of the tensile property as a function of the immersion time of the BFRP bar in ocean water or laboratory-accelerated simulated seawater is shown in Fig. 7. As shown in Fig. 7a, the tensile strength decreased with the immersion time for all immersion media, the biggest drop was found for the uncovered BFRP bar immersed in 60 °C solution, and the lowest drop was for the uncovered specimens immersed in ocean water. An interesting phenomenon can be found from the tensile strength: the degradation of the uncovered BFRP bar immersed in ocean water was consistent with the specimens 9

immersed in 23 °C simulated seawater. This indicates that the degradation rates of the BFRP bars in ocean water and in simulated seawater are similar. The deterioration mechanism including degradation of the resin [6, 16] and interfacial debonding between basalt fiber and resin matrix [11, 17, 28]. Another reason for the degradation is that ferric iron reacts with chloride ions leading to basalt fiber damage [9, 11, 15]. This may be insignificant because the content of ferric iron in basalt fiber is low (see Table 1), and the sizing protects the basalt fibers to a certain extent [9, 10, 12, 29]. With regard to the concrete-covered specimens, the degradation of the specimens immersed in simulated seawater was consistent with the uncovered BFRP bar immersed in 60 °C simulated seawater. The degradation of the specimens immersed in 23 °C simulated seawater was more serious than that of the specimens immersed in ocean water. The tensile strength decreased from 1091.7 MPa to 446.3 MPa and 702.4 MPa after 360 days’ immersion in simulated seawater and ocean water, respectively. The alkaline-solution density change is an important factor in BFRP degradation [the damage mechanism is shown in Eq. (1)] [9, 11, 13]. Seawater is moving in ocean water, but the laboratorysimulated seawater is static. Thus, the specimens were immersed in a higher-pH-value solution than that of the ocean water [21]. The destructive effects of alkalinity on basalt fiber can be indicated by the decomposition of a Si-O-Si bone, as shown in Eq. (4).  Si  O  Si  OH    Si  OH   Si  O 

(4)

Fig. 7b shows the variation of the tensile modulus of the specimens immersed in various media as a function of the immersion time. As expected, the degradation trend of the tensile modulus is similar to that of the tensile strength, but the tensile modulus is less influenced by the immersion. This is because the tensile modulus mainly depends on the basalt fibers and is much less sensitive to the degradation of the resin and fiber-matrix interface [14, 17, 30]. The greatest degradation of the tensile modulus occurred in uncovered BFRP bars immersed in 60 °C simulated seawater (a decrease from the original value of 50.2 to 36.6 GPa), followed by the concrete-covered BFRP bars immersed in 23 °C simulated seawater (a decrease from the original value of 50.2 to 39.6 GPa). As the distribution of basalt fibers is uneven in BFRP bars [31, 32], the main framework of Si-O-Si bonds in basalt fiber is called a short-distance order, and no periodicity occurs over longer distances [5, 6, 33]. The strain obtained during the tensile test was an average over a range of lengths. Therefore, the tensile modulus is close after 360 days’ immersion.

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Scanning electron microscopy (SEM) photographs of the tensile fracture surface of the BFRP bars in various immersion media after 180 days are shown in Fig. 8. Fig. 8a shows the tensile fracture surface of the uncovered BFRP bars immersed in 23 °C simulated seawater. Many resin matrices are attached to the basalt fiber surfaces, which indicates a good bonding of the fiber to the resin matrix [34]. Figs. 8b and 8c show the tensile fracture surface of the uncovered BFRP bars immersed in 40 °C and 60 °C simulated seawater, respectively. As can be seen, the resin attached to the basalt fibers decreased as the immersion temperature increased, which indicates the degradation of the resin matrix [11, 13, 14, 16]. With regard to the specimens of BFRP bars covered by concrete, the results for the specimens immersed in simulated seawater and ocean water are shown in Figs. 8d and 8e, respectively. A similar phenomenon can be found for the uncovered BFRP bars immersed in 40 °C and 60 °C simulated seawater (Figs. 8b and 8c), and much less resin appears on the fiber surfaces. These phenomena can be used to reveal the cause of the tensile strength degradation of the concrete-covered BFRP bars immersed in simulated seawater and ocean water.

Fig. 7. Tensile properties of BFRP specimens: (a) tensile strength; (b) tensile modulus.

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Fig. 8. SEM micrographs of tensile fracture surfaces of BFRP bars after 180 days’ immersion: (a) uncovered bars in 23 °C simulated seawater; (b) uncovered bars in 40 °C simulated seawater; (c) uncovered bars in 60 °C simulated seawater; (d) concrete-covered bars in 23 °C simulated seawater; (e) concrete-covered bars in ocean water. Fig. 9 shows the flexural behavior of the specimens immersed in various immersion media as a function of the immersion time. The flexure strength results of the BFRP bar are shown in Fig. 9a. A similar degradation trend to that of the tensile strength can be found. The flexure strength of BFRP bars after 360 days’ immersion in various environments followed a trend: uncovered BFRP bars immersed in 60 °C simulated seawater (a decrease from 1587.9 MPa to 292.6 MPa, or 81.6%) > concrete-covered BFRP bars immersed in 23 °C simulated seawater > uncovered BFRP bars immersed in 40 °C simulated seawater > concrete-covered BFRP bars immersed in ocean water > uncovered BFRP bars immersed in ocean water > uncovered BFRP bars immersed in 23 °C simulated seawater. The test results also indicated the degradation of the resin, resin-basalt fiber interface, or basalt fiber damage. The higher the immersion temperature the more degradation can be found [6, 8, 13]. Different from the tensile modulus, which is less affected by defects. The flexure modulus results of the BFRP bar exhibited more serious degradation, as shown in Fig. 9b. After 360 days of immersion, the flexure modulus decreased from its original value of 55.5 GPa to 30.6 GPa, 26.2 GPa and 5.1 GPa for 12

the uncovered BFRP bar specimens immersed in 23 °C, 40 °C, and 60 °C simulated seawater, respectively. With regard to the concrete-covered BFRP bar specimen, the specimen immersed in ocean water showed a higher flexure modulus, and the value decreased to 28.6 GPa after 360 days’ immersion. The specimen immersed in simulated seawater decreased to 16.5 GPa. As the degradation of the resin, the interoperability decreased, and basalt fibers were broken during the flexure test [6, 9, 12]. Thus, the flexure modulus showed more serious degradation than the flexure strength.

Fig. 9. Flexure properties degradation of BFRP specimens: (a) flexural strength; (b) flexural modulus. Fig. 10 shows the transverse shear-strength degradation of the specimens as a function of the immersion time. As shown, the transverse shear strength decreased as the immersion temperatures increased after 180 days and 360 days of immersion, while more severe degradation was found for the uncovered BFRP bars immersed in 60 °C simulated seawater. After 360 days of immersion in 23 °C, 40 °C and 60 °C simulated seawater, the degradation trend of the transverse shear strength of the uncovered BFRP bars was similar: a reduction to 172.0, 148.3, and 107.5 MPa from the original value of 195.0 MPa, and a reduction to 130.0 and 134.8 MPa for the concrete-covered specimens immersed in simulated seawater and ocean water, respectively. The minimum degradation was found in the uncovered BFRP bars immersed in 23 °C simulated seawater (172.0 MPa), followed by the uncovered BFRP bars immersed in ocean water (165.5 MPa). Water tended to easily penetrate the BFRP bars because of the increased void content and possible formation of cracks by the resin degradation process [5, 11, 13] or basalt fiber damage [6, 9]. This led to a quick degradation of the transverse shear strength.

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Fig. 10. Transverse shear strength as a function of immersion time. 3.2. Microstructural and chemical element analysis An interesting phenomenon was observed with the BFRP bars extracted from the covered concrete. White gel was found at the interface of the BFRP bars and concrete, as shown in Fig. 11. Thus, efforts were made to study the composition and formation.

Fig. 11. White gel at interface of BFRP bars and concrete. SEM photographs are shown in Fig. 12. Figs. 12a, 12b, and 12c correspond to spectrogram 1, spectrogram 2, and spectrogram 3. The content of each element is given in Fig. 13 and is summarized in Table 5. A large amount of C, O, Si, and Ca were observed in all three areas because 3CaO·SiO2, 2CaO·SiO2, and CaCO3 are the main contents of cement and sand. In addition, small amounts of Mg, Al, and Fe are involved in the composition of concrete. Specially, Na and K can be found in the areas of the contact interface of concrete and BFRP bars (Figs. 13a and 13b and Table 5). Compared with Table 5, an abundant amount of Na was found in the bulge (rib of BFRP bars) of the BFRP bar-concrete interface. The position is shown in Fig. 12a. This occurred because fewer resin matrices at the ribs led to weaker protection of the basalt fibers [35]. SEM photographs are shown in Fig. 14, fewer resin matrices can be found at the basalt fiber-rib interface and under the ribs (Figs. 14a and 14b), and more resin matrices can be found at the bulge of BFRP bars (Fig. 14c). Thus, few elements of Na and K could be found at the BFRP bar-concrete interface, as shown in Fig. 12b. 14

Based on a comparison of Fig. 13c and Table 5 (concrete substrate shown in Fig. 12c), it can be concluded that an alkaline aggregate reaction (AAR) occurred at the interface between the concrete and the BFRP. The alkali-silica reaction, one of the causes of the degradation of BFRP bars, occurs when the amorphous or poorly crystallized silica of the aggregate are attacked and dissolved by alkali hydroxides (NaOH or KOH) from the pore solution of the cementing material [36]. The equation for this reaction can be written as follows: Na + + (K + ) + SiO2 + OH - ⟶ Na(K) - Si -H(gel)

(5)

Fig. 12. SEM micrographs of specimens: (a) bulge at interface of concrete-BFRP bar (location of ribs); (b) white gel at interface of concrete-BFRP bar; (c) concrete substrate.

Fig. 13. Chemical-element spectrograms of surface of specimens by EDS: (a) spectrogram 1; (b) spectrogram 2; (c) spectrogram 3. 15

Fig. 14. SEM micrographs of specimens: (a) interface of rib-basalt fibers; (b) under ribs; (c) bulge of BFRP bar. Table 5 EDS test results for conditioned areas and reference area. Specimen label

Percentage of chemical element by mass (%) C

O

Na

Mg

Al

Si

S

K

Ca

Fe

Au

Spectrogram 1

10.82

24.60

0.65

0.28

0.56

3.93

0.83

1.08

38.13

0.56

18.55

Spectrogram 2

3.46

10.57

0.16

0.26

1.36

8.53

-

1.49

63.20

2.45

8.53

Spectrogram 3

14.28

28.17

-

0.19

0.70

34.38

-

-

3.17

0.71

18.40

Note: “-” indicates that percentage is less than 0.1%. 3.3. Alkali-aggregate reaction of basalt fiber In order to confirm that the AAR caused by basalt fibers, a series of tests was performed. The elongation of a concrete prism with basalt fiber is shown in Table 6. As can be seen, as the immersion time increased, the elongation of the concrete prism increased. It is worth noting that the elongation of the three specimens exhibited very large differences: the elongation of specimen 2 increased to 0.386% after 14 days, and the test results confirmed than an alkaline aggregate reaction occurred (the threshold value was 0.2%) [27]. With regard to specimens 1 and 2, the elongation increased to 0.14% and 0.098%, respectively. Based on ASTM C1260 [27], specimen 1 only showed the possibility of an AAR, and specimen 3 indicated that no AAR occurred. The results can be explained in that the content of the basalt fiber was different even when the specimens were cast during the same stirring time. The content of the basalt fiber in the concrete prism was empirical.

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Table 6 Test results for elongation of specimens.

Age/Day 1 3 5 7 9 11 13 15

Reference specimen (mm) 249.680 249.640 247.700 249.660 249.630 249.620 249.675 249.600

Specimen 1 (mm)

Specimen 2 (mm)

Specimen 3 (mm)

248.975 249.045 249.225 249.130 249.225 249.340 249.345 249.250

249.780 250.690 250.650 250.620 250.675 250.660 250.730 250.665

249.435 249.460 249.490 249.495 249.505 249.615 249.640 249.600

Specimen 1 — 0.044 0.092 0.138 0.120 0.170 0.150 0.140

Elongation (%) Specimen Specimen 2 3 — — 0.380 0.026 0.340 0.014 0.344 0.032 0.378 0.048 0.376 0.096 0.382 0.084 0.386 0.098

To confirm the AAR in the concrete prism, EDS was employed to test the chemical composition of the interface between the basalt fiber and concrete substrate. The test results are shown in Fig. 15. Spectrogram 1, spectrogram 2, and spectrogram 3 are the basalt fiber, basalt fiber-concrete interface, and concrete substrate, respectively. Fig. 15a shows the reference specimen, which has no special features. Fig. 15b shows specimen 2; floc gel can be clearly found on the surface of the basalt fiber. The floc gel can be attributed to an AAR, which led to the elongation of the concrete prism. Compared with specimen 2, a clean morphology can be found for specimen 3 (see Fig. 15c). Therefore, the elongation of the concrete prism was only 0.098%.

Fig. 15. SEM micrographs of concrete prism with basalt fiber: (a) reference specimen; (b) specimen 2; (c) specimen 3. 17

The contents of specimen 3 are shown in Fig. 16 and summarized in Table 7. As shown in Fig. 16a, Si was the main element, and its content was 25.16% (see Table 7). A large amount of Si nay be caused by basalt fiber. Fig. 16b shows the interface of the concrete substrate and basalt fiber. Compared with specimen 1, the amount of Ca increased to 33.42% because of the concrete prism, and the amount of Si decreased to 13.47%. Fig. 16c shows the obvious concrete characteristics. The Si continued to decrease and the Ca continued to increase because the distance increased from the basalt fiber. With regard to the element Na, the content decreased from the basalt fiber to the concrete substrate. This phenomenon indicates that an AAR occurred at the interface between the basalt fiber and concrete substrate. Thus, an AAR occurred in specimen 3 even though the elongation was less than 2%. This is owing to less basalt fiber in the concrete prism.

Fig. 16. Chemical-element spectrograms of surface of specimens by EDS: (a) spectrogram 1; (b) spectrogram 2; (c) spectrogram 3.

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Table 7 Percentage of chemical elements of EDS test results. Percentage of chemical element by mass (%)

Specimen label

C

O

Na

Mg

Al

Si

S

K

Ca

Fe

Ti

Spectrogram 1

0

46.46

2.89

4.07

8.32

25.16

0.77

Spectrogram 2

1.91

32.92

2.33

2.56

5.20

13.47

2.37

0.45

5.49

5.64

0.75

0.06

33.42

4.63

1.14

Spectrogram 3

0

43.19

1.64

0.39

0.95

4.60

1.75

0.02

46.60

0.92

0.13

4. Conclusions The present work studied the durability of concrete-covered and uncovered BFRP bars in ocean water and laboratory-accelerated marine environments. The following conclusions can be drawn based on the test results: 1. The durability of BFRP bars is insensitive to chloride ions but very sensitive to the alkalinity of the surrounding environment. The higher the alkalinity, the more severe the degradation of BFRP. 2. Studying the durability of uncovered BFRP bars by using an accelerated-simulation marine environment in a laboratory is feasible, but it is conservative for concrete-covered BFRP bars members due to the higher alkalinity in the standing solution. 3. An alkaline aggregate reaction induced by SiO2 in basalt fiber was certified for concrete-covered BFRP bars. The alkaline solution led to a degradation of the resin matrix, and fiber etching provided the essential condition for AAR.

Data availability Apart from the analytical results presented in the manuscript, there are none other data appropriate for sharing.

Acknowledgments The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China, No. 51708132, the Science Found of Guangdong Province, No. 2017A030310491, and Science and Technology Planning Project of Guangzhou City, No. 201804010470.

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Figure Captions Fig. 1. Location of specimens in ocean water and laboratory: (a) overview map; (b) details. Fig. 2. Immersion environment for specimens. Fig. 3. Flexure test setup: (a) sketch map; (b) physical map. Fig. 4. BFRP specimens for flexure test. Fig. 5. Transverse shear test device. Fig. 6. Photograph of specimens immersed in NaOH solution. Fig. 7. Tensile properties of BFRP specimens: (a) tensile strength; (b) tensile modulus. Fig. 8. SEM micrographs of tensile fracture surfaces of BFRP bars after 180 days’ immersion: (a) uncovered bars in 23 °C simulated seawater; (b) uncovered bars in 40 °C simulated seawater; (c) uncovered bars in 60 °C simulated seawater; (d) concrete-covered bars in 23 °C simulated seawater; (e) concrete-covered bars in ocean water. Fig. 9. Flexure properties degradation of BFRP specimens: (a) flexural strength; (b) flexural modulus. Fig. 10. Transverse shear strength as a function of immersion time. Fig. 11. White gel at interface of BFRP bars and concrete. Fig. 12. SEM micrographs of specimens: (a) bulge at interface of concrete-BFRP bar (location of ribs); (b) white gel at interface of concrete-BFRP bar; (c) concrete substrate. Fig. 13. Chemical-element spectrograms of surface of specimens by EDS: (a) spectrogram 1; (b) spectrogram 2; (c) spectrogram 3. Fig. 14. SEM micrographs of specimens: (a) interface of rib-basalt fibers; (b) under ribs; (c) bulge of BFRP bar. Fig. 15. SEM micrographs of concrete prism with basalt fiber: (a) reference specimen; (b) specimen 2; (c) specimen 3. Fig. 16. Chemical-element spectrograms of surface of specimens by EDS: (a) spectrogram 1; (b) spectrogram 2; (c) spectrogram 3.

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Tables Table 1 Components of basalt fibers. Table 2 Components of ocean water. Table 3 Chemical composition of simulated seawater. Table 4 Summary of all specimens used in this study. Table 5 EDS test results for conditioned areas and reference area. Table 6 Test results for elongation of specimens. Table 7 Percentage of chemical elements of EDS test results.

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