basalt-glass FRP bars

Construction and Building Materials xxx (2015) xxx–xxx

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of loading rate and chemical corrosion on the mechanical properties of large diameter glass/basalt-glass FRP bars Guowei Li a,1, Jiantao Wu a,c,⇑, Wanming Ge b a

College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, China Shandong Provincial Water Resources Research Institute, Jinan 250013, China c State Key Laboratory for Geomechanics & Deep Underground Engineering, Xuzhou, 221116, China b

h i g h l i g h t s  A novel B-GFRP bar, which is resistant to aggressive environment, was manufactured.  The effects of loading rates on tensile mechanical properties of FRP bars were not negligible.  The rate-related viscoelastic property of polymer could be dominant in FRP bar tensile test.  A new end anchorage system was developed for tensile tests on large diameter FRP bars.

a r t i c l e

i n f o

Article history: Received 25 November 2014 Received in revised form 11 April 2015 Accepted 2 May 2015 Available online xxxx Keywords: GFRP B-GFRP Soil nail Large diameter Loading rate Corrosion

a b s t r a c t In this study, the end anchorage device for large diameter FRP bar was firstly developed, based on which the effect of loading rate on the direct tensile mechanical properties of 25 mm diameter GFRP bars was tested, and the anticorrosion properties of a newly developed type of FRP bar, the basalt-glass fiber reinforced polymer (B-GFRP) bars with different thicknesses of basalt FRP protecting layers (0, 1, 3 and 5 mm) were investigated. The results show that, with the increase of loading rate, the tensile strength and elongation ratio of GFRP bar increase considerably, while the elastic modulus remains roughly constant. The addition of BFRP protecting layer is also proved to be an effective way to enhance the resistance of GFRP bars to the chemical attacks. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The corrosion of steel bars has long been held as one of the main factors that can significantly affect the durability of reinforced civil infrastructures (e.g. concrete structures, slopes and excavations). A lot of efforts have been made by researchers all over the world to control the steel corrosion process, such as decreasing the permeability of concrete or mortar by additives or admixtures and coating the steel bars with epoxy [1]. These methods have been proved effective to a certain extent, however the manufacturing technologies they involved are generally complicated and their long-term maintenance costs are considerably high [2]. Therefore, a completely different approach has been developed in the past few ⇑ Corresponding author at: College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, China. Tel.: +86 13915975806; fax: +86 2583713073. E-mail addresses: [email protected] (G. Li), [email protected] (J. Wu), [email protected] (W. Ge). 1 Tel./fax: +86 (0)25 83787561.

decades, which is to replace the conventional steel bars with a corrosion resistant material: fiber reinforced polymer (FRP). Among all the types of FRP materials developed, the glass fiber reinforced polymer (GFRP) is more attractive because of its lower cost and therefore, the durability and mechanical properties of GFRP bars have been studied most extensively since 1990s [3– 11]. It is commonly recognized that, compared with steel bars, GFRP bars have advantages such as lighter self weight, higher ultimate tensile strength, much better resistance to chemical corrosion and complete resistance to magnetic effects. It is also widely observed that the Young’s modulus of GFRP bars is smaller (normally 25–30% of that of steel) which can lead to much larger elongation than that of the steel bars under the same tensile stress. However, the lower elastic modulus can also be its advantage. Roger et al. (2002) observed that, when temperature changes dramatically, the thermal stress induced within GFRP can be significantly decreased, due to its reduced Young’s modulus and similar coefficient of thermal expansion with steel [12]. In addition, as

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Please cite this article in press as: Li G et al. Effect of loading rate and chemical corrosion on the mechanical properties of large diameter glass/basalt-glass FRP bars. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.05.044

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stated by Li et al. (2013), the lower Young’s modulus of GFRP bar also means less reduction in the pre-stressing force due to shrinkage, creep, and other structural deformations [13]. It should be noted that most previous studies on performance properties of GFRP bar reported in literature were conducted on smaller size (diameter smaller than 20 mm) specimens as concrete rebars. Recently, the great advantages observed in these studies have increasingly drawn interest in adopting this technique to the reinforcement of slopes and excavations as soil nails (normally 20–40 mm in diameter) [13–15]. However, it has been reported that the mechanical properties (particularly the ultimate tensile strength) of FRP bars are sensitive to their diameters [8,16], the conclusions achieved by studies based on smaller size FRP bars may not apply to the larger ones. In addition, it is found that, when doing mechanical tests on FRP bars, the loading rates chosen by different researchers [17–19] and/or specifications of different countries[20,21] differ from each other, and this makes their results mixed and inconclusive. Therefore, this study firstly tested the tensile mechanical properties of 25 mm diameter GFRP bars at 5 different loading rates, so that the effects of loading rate on the ultimate tensile strength, elastic modulus and elongation rate of large diameter GFRP bar could be investigated and an optimum loading rate be selected for further studies. Another objective of this study is to examine the resistance of a newly developed Basalt-GFRP bar to different aggressive chemical environments. The deterioration of mechanical properties of GFRP under alkaline environment has been widely observed [3–11,22,23]. As stated above, the large diameter FRP bar was developed in this study to work as soil nails, which are very possible to get in touch with alkaline environment due to the cement grouting. Therefore, a novel kind of FRP bar, which has the glass fibers on the periphery of GRFP bars replaced by basalt fibers, was manufactured. Based on this, the large diameter (>25 mm) basalt-glass fiber reinforced polymer (B-GFRP) bars with different thicknesses of basalt FRP protecting layers (0, 1, 3 and 5 mm) were immersed in H2SO4 solution (PH = 1), distilled water (PH = 7) and NaOH solution (PH = 14) at 20oC for 30, 70 and 120 days, respectively. The mass, ultimate tensile strength, elastic modulus and elongation ratio of different samples were tested both before and after the immersion treatment. 2. Materials and experimental programme 2.1. Materials In this study, four types of FRP bars, with 0, 1, 3 and 5 mm BFRP protecting layers respectively, were manufactured by Zhongshan Fumei Composites Co. Ltd., China. The pultruded bars were helically wrapped by glass fibers and sand-coated to prevent sliding between the bar and anchorage. The thermosetting epoxy resin NPEL128, obtained from Nanya Epoxy Resin Ltd., was selected as the polymer matrix. The E-glass fiber was purchased from Chongqing International Conposites Co. Ltd., China, with the fiber diameter of 0.023 mm and density of 2.49 g/cm3. The continuous basalt fiber was from Sichuan Hangtian Tuoxin Basalt Co. Ltd., China, with the fiber diameter of 0.013 mm, tensile strength of 2360 MPa, elastic modulus of 91.7 GPa. Fig. 1 shows the picture of the four types FRP bars in this study, and their physical properties are listed in Table 1.

2.3. Experimental programme In this study, the mechanical properties of different specimens were tested on an electrical-hydraulic UTM SHT4106, and the extensometer was used to measure the strain over 10 cm of the specimen. It should be noted that, due to the limit of the testing chamber of UTM, the total length of the specimens in this study was 90 cm, which means the tested part of FRP bar between the anchored ends was 30 cm. As stated above, the effects of loading rates on the mechanical properties of pure GFRP bars were firstly investigated. The ultimate tensile strength, elastic modulus and elongation of GFRP bars were tested under loading rates of 2, 4, 6, 10 and 15 mm/min. In order to quantitatively demonstrate the improvement of corrosion resistance of B-GFRP, the B-GFRP bars with 1, 3 and 5 mm BFRP protecting layers and pure GFRP bars were immersed in H2SO4 solution (PH = 1), distilled water (PH = 7) and NaOH solution (PH = 14) at 20 °C for 30, 70 and 120 days, respectively. The mass, ultimate tensile strength, elastic modulus and elongation ratio of different samples were tested both before and after the immersion treatment. The tests after treatment were conducted when the specimens had been dried in conditioning chamber for 20 days. 6 specimens were tested for each type of bars at every testing step, and a minimum of 4 valid results were requested, otherwise repeat tests on backup specimens would be carried out. Therefore, 300 specimens in total were prepared in this study, as listed in Table 2.

3. Results and discussion 3.1. Performance of GFRP bars at different loading rates In this study, the load and deformation applied to the specimens were recorded by the UTM during the tests, and the strains were checked using extensometer over a length of 10 cm of testing specimens. According to these testing results, the ultimate tensile strength and elongation rate of each specimen were calculated with Eqs. (1) and (2). In addition, it was observed that, for the FRP bars in this study, the relationship between the stress and strain was linear (excluding damage period). Fig. 3 shows the results for GFRP bars under loading rate of 2 mm/min (v2) as an example, together with results for smaller sizes of GFRP bars from literature [24], in which the loading rate was also 2 mm/min. It is believed that the higher fiber content in this study resulted in this linear results. Therefore, the elastic modulus of GFRP bars at each loading rate in this study was calculated using Eq. (3). The calculated results for all specimens with valid data, the average and coefficient of variation (CV) for each group are summarized in Table 3. In addition, the relationship between the mechanical properties of GFRP bars and the loading rates are also graphically shown in Fig. 4.

ru ¼

ð1Þ

where ru is the ultimate strength of GFRP bar (Pa), Pu is the peak load when sample completely damaged (N), and D is diameter of the sample (m).

/¼

DL0u  100% L0

ð2Þ

where / is the elongation rate when sample completely damaged, L0 is the length covered by extensometer (100 mm), and DL0u is the increase of L0 when sample completely damaged.

2.2. End anchorage system As the resistance of FRP bars to the transversal compression is very low, it was worried that the gripping hands of the Universal Testing Machine (UTM) may easily cause damage to the bars due to stress concentration. Therefore, it was decided to use the anchorage system to protect the testing bars, and the expansive cement method was chosen for its ability of distributing the loading force uniformly along the anchored length. After several rounds of preliminary tests, a steel tube with outer diameter of 54 mm and thickness of 6.5 mm was chosen, and the anchorage length was determined to be 30 mm so that the anchorage system could survive from the tensile test. Fig. 2 shows the sketch and picture of the GFRP bar with anchorage.

4Pu

pD2

E¼

4DP 2

p D De

¼

DrL0 1000DL

ð3Þ

where E is the elastic modulus of GFRP bar (GPa), Dr is magnitude of stress increase during a certain period of loading (MPa), L0 is the length covered by extensometer (100 mm), and DL is the increase of L0 corresponding to Dr (mm). Fig. 4(a) clearly shows that the influence of loading rate on the ultimate tensile strength can be divided into three phases. The tensile strength firstly increases slowly with the increasing loading

Please cite this article in press as: Li G et al. Effect of loading rate and chemical corrosion on the mechanical properties of large diameter glass/basalt-glass FRP bars. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.05.044

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3

Fig. 1. The four types of FRP bars in this study.

Table 1 Physical properties of FRP bars in this study.

*

Specimen code

Density/ g/cm3

Mass ratio of resin/%

Ratio of basalt fiber mass to total fiber/%

Nominal diameter/ mm

bft0* bft1 bft3 bft5

2.00 2.07 2.11 2.17

17.3 19 18.8 18

0 13.16 36.32 53.86

25.5–26.5 27.0–28.5

bft0: basalt fiber thickness = 0 mm.

rate. After that, a sharp raise can be observed between the rates of 4 and 6 mm/min. Finally, when the loading rate is larger than 6 mm/min, the increase of ultimate tensile strength becomes slow again, and the increasing speed reduces gradually. This is considered to be reasonable for the nature of FRP composite materials. In the FRP composites, the polymer matrix not only protects the fibers from environment attacks, but also transfers the stresses among the fibers via shear force. It is well known that the epoxy resin is a viscoelastic material, and the modulus (either shear or tensile modulus) of viscoelastic material is highly time- (rate-) dependant. It displays elastic properties at high loading rates with a limiting maximum modulus value known as glassy modulus, and displays viscous-flow properties at low loading rates [25]. Therefore, when higher loading rate is applied, the higher shear modulus will enable the epoxy resin to transfer the stress from the periphery to the center of GFRP bar more efficiently, which means the inner fibers of the bar can share more loading force than

that under lower loading rates and therefore, the ultimate tensile strength of the GFRP bars increased with a limiting maximum value when the shear modulus of epoxy resin approaches its glassy modulus. A nonlinear regression was developed (Eq. (4)) to quantify the relationship between the tensile strength and loading rate in this study. According this regression, when v ? 0, ru = 922.32 MPa, and when v ? + 1, ru = 957.44 MPa. It can be seen that the difference between the two limits is only 4%, which is much lower than that of steel bars. The regression line and asymptote are also included in Fig. 4(a).

ru ¼ 957:44 

35:12 1 þ eð2:04v 10:83Þ

ð4Þ

where v is the loading rate (mm/min). The relationship between the elastic moduli of GFRP bars and loading rates is presented in Fig. 4(b). It should be noted that the elastic modulus was calculated based on the data obtained when the relationship between stress and strain was linear (excluding the data during damage period). This is different from tensile strength and elongation, which were calculated at the point of sample damage. From Fig. 4(b), it can be seen that the elastic modulus of GFRP bar generally keeps as a constant when the loading rate increases from 2 to 15 mm/min. This indicates that, as long as the GFRP bar did not start damaging, it is the elastic fiber that dominates the mechanical properties of GFRP bar. In contrast, the viscoelastic epoxy only plays the role of transferring stresses among the fibers, and its rate dependant modulus has very limited

Fig. 2. The sketch and picture the GFRP bar with anchorage.

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Table 2 Number of specimens at different testing steps (Piece). Specimen code

Loading rate test

Corrosion resistance test Without treatment

bft0 bft1 bft3 bft5 Total

30 – – – 300

6 6 6 6

Backup PH = 1

PH = 7 70d

120d

30d

70d

120d

30d

70d

120d

6 6 6 6

6 6 6 6

6 6 6 6

6 6 6 6

6 6 6 6

6 6 6 6

6 6 6 6

6 6 6 6

6 6 6 6

1000 800

σ / MPa

600 bft0-v2-01 bft0-v2-02 bft0-v2-03 bft0-v2-04 φ10mm[24] φ17mm[24]

400 200 0 0

0.5

1

ε /10 -2

1.5

PH = 14

30d

2

2.5

Fig. 3. Stress versus strain curves for GFRP bars at loading rate of 2 mm/min.

effects to the modulus GFRP bar in this period. It is considered that the relatively low resin contents in GFRP bars of this study (<20% by mass as shown in Table 1) might be the cause of this phenomenon. Fig. 4(c) presents the effects of loading rates on elongation of the GFRP bars. It shows that the elongation increases rapidly with the increase of loading rate when the rate is smaller than 6 mm/min. After that, the elongation becomes relatively insensitive to the increasing loading rate. The elongation of GFRP bar is mainly caused by the tensile deformation of fibers and the shear deformation of epoxy resin. As stated above, the higher loading rate can result in higher shear modulus of epoxy resin due to its viscoelastic property, which will enable the epoxy resin to transfer

12 6 6 6

the stress from periphery to the center of GFRP bar more efficiently. Therefore, as the loading rate increases, the loading stress will distribute more evenly throughout the crossing area of GFRP bar, and the fibers and epoxy in the center will undertake higher proportion of the whole stress compared with those under lower loading rate. In such a way, the tensile deformation of the fibers and shear deformation of epoxy resin in the center of bar increased, which resulted in an increased elongation of the GFRP bar. In addition, because the shear modulus of all polymers including epoxy has a limiting maximum value known as glassy modulus, when the loading rate continued increasing, the increasing rate of epoxy shear modulus reduced and therefore, the elongation of the GFRP bar increased much more slowly at the loading rates ranging from 6 to 15 mm/min. According to the analysis above and the national specification [20], a loading rate of 6 mm/min is recommended for the 25 mm diameter GFRP bar.

3.2. The performance of B-GFRP bars under different chemical treatments 3.2.1. The effects of corrosion on appearance and mass of specimens According to the visual inspection, only the specimens immersed in NaOH solution for 120 days were observed to have color faded slightly, and no color changes were observed for other treated specimens. The diameter measurement showed no difference between the diameters of specimens before and after treatments. In this study, the masses of representative specimen from

Table 3 Mechanical properties of 25 mm diameter GFRP bars under different loading rates. Loading rate/ mm/min

Specimen code

2

bft0v2-01 bft0v2-02 bft0v2-03 bft0v2-04 bft0v4-01 bft0v4-02 bft0v4-03 bft0v4-04 bft0v4-05 bft0v6-01 bft0v6-02 bft0v6-03 bft0v6-04 bft0v10-01 bft0v10-02 bft0v10-03 bft0v10-04 bft0v15-01 bft0v15-02 bft0v15-03 bft0v15-04

4

6

10

15

Nominal diameter/mm

25.93 25.86 26.11 25.93 26.12 26.04 26.00 26.03 26.02 25.91 25.93 25.86 26.12 26.02 25.86 25.91 26.11 25.86 25.94 26.11 26.00

Ultimate tensile strength

Elastic modulus

Elongation rate

Results/MPa

Average/MPa

CV/%

Results/GPa

Average/GPa

CV/%

Results

Average

CV/%

956.40 915.05 904.50 913.51 942.81 910.33 925.21 922.70 921.49 940.96 985.56 939.92 932.21 955.27 963.79 960.34 941.75 947.87 960.28 944.69 975.34

922.36

2.5

52.916

2.3

4.3

1.3

51.327

1.8

1.80%

1.5

949.66

2.6

51.664

1.8

1.84%

2.8

955.29

1.0

51.92

1.5

1.84%

1.7

957.05

1.5

51.498

1.4

1.85% 1.70% 1.73% 1.69% 1.82% 1.81% 1.80% 1.82% 1.75% 1.83% 1.90% 1.78% 1.84% 1.85% 1.88% 1.81% 1.82% 1.87% 1.84% 1.82% 1.91%

1.74%

924.51

51.61 53.86 52.20 54.00 51.91 50.23 51.40 50.57 52.52 51.33 51.81 52.87 50.64 51.57 51.29 53.07 51.76 50.66 52.12 52.04 51.18

1.86%

2.1

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2.4

3.0

2.5 Tensile strength CV

940

2.0

930

1.5 922.32

920 4

8

12

16

Elastic modulus

2.0

CV

30

1.8

20

1.6

10

1.0 0

2.2

40

0

20

1.4 0

2

Loading rate /mm/min

(a) Tensile strength versus loading rate

CV/%

Elastic modulus/GPa

50 950

CV/%

Tensile strength/MPa

60

957.44

960

4 6 8 10 12 Loading rate /mm/min

14

16

(b) Elastic modulus versus loading rate 4.5

1.88

4.0

Elongation/%

1.84

3.0

CV

CV/%

3.5 Elongation

1.80

2.5

1.76 2.0 1.72

1.5 0

2

4

6

8

10

12

14

16

Loading rate /mm/min

(c) Elongation versus loading rate Fig. 4. The effects of loading rates on mechanical properties of GFRP bars.

each group were measured both before and after the chemical immersion, and the results are summarized in Table 4. From Table 4 it can be seen that all the specimens became heavier after the immersion, and their masses kept increasing with the immersion time, despite the differences of solution types. In general, the mass increasing rates of a certain type specimen immersed in NaOH (PH = 14) solution were higher than those immersed in H2SO4 (PH = 1) and distilled water (PH = 7), which indicates that alkaline environment interacts more severely with both the GFRP and B-GFRP bars in this study. However, it can also be seen that, as the thickness of BFRP protecting layer increases, the mass increasing rate of specimens immersed in NaOH solution decreases from 0.08% of bft0 to 0.05% of bft5. It should also be noted that, for the group of bft5, the mass increasing rates of specimens immersed in NaOH solution and H2SO4 solution are almost the same as those of specimens in distilled water with the same immersion time. To a certain extent, the above two results imply that the addition of BFRP protecting layer can effectively limit the interaction of FRP bars with alkaline and/or acidic chemicals.

Table 4 Masses of specimens before and after chemical immersion. Group

bft0

CIM ¼

Mb  Ma  100% Mb

ð5Þ

where CIM is the corrosion index expressed by mechanical property M, M can be tensile strength (r), elastic modulus (E) or elongation (EL), Mb and Ma are mechanical properties before and after corrosion.

1

7

14

bft1

1

7

14

bft3

3.2.2. The effects of corrosion on mechanical properties of specimens Table 5 lists the average mechanical properties of each group at different corrosion steps (0, 30, 70 and 120 days treatment in different solutions). The tests were conducted with a tensile loading rate of 6 mm/min. In order to show the effects of corrosion more clearly, the corrosion index (CI) is defined as Eq. (5), and the calculated results are summarized in Table 6.

PH value of solution

1

7

14

bft5

1

7

14

Immersion time/days

Mass before immersion/ g

Mass after immersion/ g

Increasing rate/%

30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120 30 70 120

1070.6 1063.6 1067.3 1068.6 1068.7 1072.1 1068.8 1072.5 1068.3 1230.1 1225.8 1227.5 1224.2 1225.1 1227.1 1226.9 1222.1 1226.4 1200.6 1195.6 1196.7 1202.5 1205.1 1198.1 1201.5 1195.6 1196.7 1241.1 1246.6 1240.0 1249.8 1242.7 1244.0 1241.1 1246.8 1247.6

1070.9 1063.9 1067.9 1068.7 1069.0 1072.6 1069.1 1072.9 1069.2 1230.3 1226.0 1228.1 1224.4 1225.3 1227.6 1227.2 1222.4 1227.1 1200.8 1195.9 1197.5 1202.6 1205.3 1198.8 1201.8 1196.0 1197.5 1241.2 1246.8 1240.7 1249.9 1242.9 1244.6 1241.2 1247.0 1248.2

0.03 0.03 0.06 0.01 0.03 0.05 0.03 0.04 0.08 0.02 0.02 0.05 0.01 0.02 0.04 0.02 0.02 0.06 0.02 0.03 0.07 0.01 0.02 0.06 0.02 0.03 0.07 0.01 0.02 0.06 0.01 0.02 0.05 0.01 0.02 0.05

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Table 5 Average mechanical properties of different groups at different corrosion steps. Group

Corrosion time/days

bft0

0 30 70 120 0 30 70 120 0 30 70 120 0 30 70 120

bft1

bft3

bft5

Tensile strength/Mpa

Elastic modulus/Gpa

Elongation/%

PH = 1

PH = 7

PH = 14

PH = 1

PH = 7

PH = 14

PH = 1

PH = 7

PH = 14

888.32 873.43 862.92 853.92 906.40 894.56 884.98 874.49 912.21 901.93 889.16 881.29 914.31 904.34 895.57 889.98

888.32 883.13 878.26 869.43 906.40 895.34 888.73 877.12 912.21 898.31 893.96 885.71 914.31 903.38 901.60 894.00

888.32 855.81 838.65 821.68 906.40 879.55 861.57 840.25 912.21 890.95 872.08 851.45 914.31 903.16 881.24 871.76

52.47 51.71 52.00 51.13 52.26 51.67 51.93 51.48 51.27 51.02 50.64 50.69 50.63 49.34 49.15 50.14

52.47 51.41 50.31 51.01 52.26 52.69 51.28 51.36 51.27 49.87 50.48 50.67 50.63 50.71 49.36 51.35

52.47 50.59 50.58 51.50 52.26 52.00 51.84 52.09 51.27 49.43 50.11 50.39 50.63 50.32 50.60 49.39

1.69 1.69 1.66 1.69 1.73 1.73 1.71 1.70 1.78 1.77 1.76 1.74 1.81 1.83 1.82 1.77

1.69 1.72 1.75 1.70 1.73 1.70 1.73 1.71 1.78 1.80 1.77 1.79 1.81 1.78 1.83 1.74

1.69 1.69 1.67 1.60 1.73 1.69 1.66 1.61 1.78 1.80 1.74 1.69 1.81 1.78 1.75 1.77

Table 6 Corrosion indices of different groups at different corrosion steps. Group

Corrosion time/days

bft0

30 70 120 30 70 120 30 70 120 30 70 120

bft1

bft3

bft5

CIr/%

CIE/%

CIEl/%

PH = 1

PH = 7

PH = 14

PH = 1

PH = 7

PH = 14

PH = 1

PH = 7

PH = 14

1.68 2.86 3.87 1.31 2.36 3.52 1.13 2.53 3.39 1.09 2.05 2.66

0.58 1.13 2.13 1.22 1.95 3.23 1.52 2.00 2.91 1.20 1.39 2.22

3.66 5.59 7.50 2.96 4.95 7.30 2.33 4.40 6.66 1.22 3.62 4.65

1.44 0.89 2.55 1.14 0.63 1.49 0.48 1.24 1.14 2.55 2.93 0.97

2.02 4.11 2.79 0.82 1.88 1.72 2.73 1.55 1.18 0.16 2.50 1.42

3.59 3.59 1.84 0.50 0.80 0.32 3.60 2.27 1.72 0.61 0.06 2.44

0.02 1.76 0.10 0.11 1.06 1.77 0.69 1.29 2.32 1.30 0.71 2.06

1.66 3.33 0.84 1.75 0.17 1.25 1.18 0.42 0.78 1.52 0.96 3.82

0.13 1.13 5.54 2.20 3.86 6.74 1.27 2.20 5.05 1.90 3.31 2.21

The effects of BFRP thicknesses on the corrosion indices expressed by different mechanical properties (CIr, CIE and CIEl) are graphically presented in Fig. 5. From Fig. 5(a), it can be clearly seen that, for the specimens immersed in NaOH and H2SO4 solutions, the ones with thicker BFRP protecting layer have lower CIr values than those with thinner BFRP layer at the same corrosion step, which means that the addition of BFRP layer can effectively eliminate the influences of alkaline and acidic attacks to the ultimate tensile strength of GFRP bars. However, for the specimens treated in distilled water, the CIr values of all B-GFRP bars are slightly higher than those of pure GFRP bars at the same corrosion step, which means the distilled water has more impacts on the strength of B-GFRP. This is possibly because the basalt fiber is more hydrophilic due to its special mineral properties, and this will affect the adhesion between the fiber and epoxy resin. The effects of rock hydrophilicity on its interaction with polymers have been extensively studied in paving asphalt area, and it is widely accepted that the effect is only restricted on the surface. Therefore, when the thickness of BFRP layer continues increasing, the proportion of BFRP affected by hydrophilic decreases and the decrease of CIr values are observed from bft3 to bft5 group. In addition, Fig. 5(a) also illustrates that the alkaline liquid is the most harmful to all kinds of FRP bars in this study, and the distilled water is the least harmful one. This result coincides well with that of mass measurement results above. Fig. 5(b) and 5(c) show that most of the corrosion indices expressed by elastic modulus (CIE) and elongation (CIEl) are smaller than 4%, and some data are unexpectedly smaller than 0. In addition, it can be seen that the CIE and CIEl values for all types of FRP

bars are extremely discrete, and the relationships either between the thicknesses of BFRP layer and the CIE and CIEl values or between the corrosion times and the CIE and CIEl values are not regular. As shown in Fig. 5(b) and 5(c), for all types of FRP bars immersed in different solutions, their CIE values just randomly oscillate around 2%, and CIEl values around 1%. The only exception is that, after 120 days immersion in alkaline solution, the corrosion indices expressed by elongation (CIEl) of B-GFRP bars decrease with the increasing thickness of BFRP layer. Therefore, with consideration of the unavoidable data variation during the tests (for instance, a maximum of 4.3% CV value can be seen in Table 3), it is reasonable to deduce that, for all types of FRB bars in this study, the effects of different corrosion treatments on their elastic modulus and elongation were restricted within a limited level, and the corrosion influences mainly happened at the beginning stage of the treatments. Tannous and Saadatmanesh (1999) stated that the corrosion of aggressive environments to the FRP bars only happens within a very thin layer on the surface, the materials inside the FRP remains their initial mechanical properties [26]. This is considered as the main reason that most of the CIE and CIEl values remained in a relative low level during the testing period. 4. Conclusions and recommendations This study investigated the effects of loading rate on the mechanical properties of large diameter GFRP bars. In addition, a newly developed novel method for improving the corrosion resistance of FRP bars was introduced. Based on this study, several conclusions can be drawn as follows:

Please cite this article in press as: Li G et al. Effect of loading rate and chemical corrosion on the mechanical properties of large diameter glass/basalt-glass FRP bars. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.05.044

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(1) Based on several rounds of preliminary tests, the expansive cement method was established as the optimum end anchorage for the large diameter FRP bars, and the

anchoring length on each end of the bar was 30 cm. The testing results showed that this method can distribute the loading force uniformly along the anchored

Please cite this article in press as: Li G et al. Effect of loading rate and chemical corrosion on the mechanical properties of large diameter glass/basalt-glass FRP bars. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.05.044

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length and therefore, effectively prevent premature damage to the specimens. The loading rate can significantly influence the ultimate tensile strength and elongation rate of large diameter GFRP bars, especially when the loading rate is lower than 6 mm/min. When the loading rate is higher than 6 mm/min, these two mechanical properties became insensitive to the increase of loading rate. It is believed that this phenomenon is mainly caused by the special rate-related viscoelastic property of epoxy polymer. In addition, as the elastic modulus of the each GFRP bar was calculated based on the testing data during the linear range of stress–strain relationship (not including the damage period), it was found the loading rate has limited effects on elastic modulus of GFRP bar. Based on the testing results of this study and the national specification, a loading rate of 6 mm/min is recommended for the 25 mm diameter GFRP bar. The novel B-GFRP technique developed in this study was proved an effective way to improve the resistance of GFRP bar to the aggressive environment. Particularly, the results showed that the effects of alkaline treatment on the ultimate tensile strength of FRP bars were significantly limited when 5 mm of BFRP protecting layer was added to the GFRP bars. The B-GFRP technique introduced in this study was just an initial attempt, same technique can also be applied as carbon-GFRP (C-GFRP), aramid-GFRP (A-GFRP), etc., depending on the environment where the bars are used.

Acknowledgement The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (Project No. 41472240), the government of Guangdong province and Ministry of education of China (Project No. 2009B09060011), and the State Key Laboratory for Geomechanics & Deep Underground Engineering (Project No. SKLGDUEK1211). References [1] Ehsani MR, Saadatmanesh H, Tao S. Design Recommendations for Bond of GFRP Rebars to Concrete. J Struct Eng 1996;122(3):247–54. [2] Bedard C. Composite reinforcing bars: assessing their use in construction. Concr Int ACI 1992;14(1):55–9. [3] Steckel GL, Rawkins GF, Bauer JL. Environmental durability of composites for seismic retrofit of bridge column. In: Proceeding of the Second International Conference on Fiber Composites in Infrastructure, Tucson, 1998, p. 460–475.

[4] Micelli F, Nanni A. Durability of FRP Rods for concrete structures. Construction Building Mater 2004;18(7):491–503. [5] Abbassi A, Hogg PJ. Temperature and environmental effects on glass fiber rebar’s modulus, strength and interfacial bond strength with concrete. Compos Part B: Eng 2005;36(5):394–404. [6] Kim HY, Park YH, You YJ, Moon CK. Short-term durability test for GFRP rods under various environment conditions. Compos Struct 2008;83(1):37–47. [7] Chen Y, Davalos JF, Ray I, Kim HY. Accelerated aging tests for evaluation of durability performance of FRP reinforcing bar for concrete structures. Compos Struct 2007;78(1):101–11. [8] Wang YC, Wong PMH, Kodur V. An experimental study of the mechanical properties of fiber reinforced polymer (FRP) and steel reinforcing bars at elevated temperatures. Compos Struct 2007;80(1):131–40. [9] Davalos JF, Chen J, Ray I. Long-term durability prediction models for GFRP bars in concrete environment. J Compos Mater 2012;46(16):1899–914. [10] Robert M, Benmokrane B. Physical, mechanical and durability characterization of preloaded GFRP reinforcing bars. J Compos Construction 2010;14(4):368–75. [11] Al-Salloum YA, El-Gamal S, Almusallam TH, Alsayed SH, Aqel M. Effect of harsh environmental conditions on the tensile properties of GFRP bars. Compos Part B: Eng 2013;45(1):835–44. [12] Roger H, Chen L, Choi JH. Effects of GFRP reinforcing rebars on shrinkage and thermal stresses in concrete. In: Proceeding of 15th ASCE Engineering Mechanics Conference, New York, 2002, p. 1–8. [13] Li GW, Ni C, Pei HF, Ge WM, Ng WW. Stress relaxation of grouted entirely large diameter B-GFRP soil nail. China Ocean Eng 2013;27(4):495–508. [14] Zhu HH, Yin JH, Yeung AT, Jin W. Field pullout testing and performance evaluation of GFRP soil nails. J Geotech Geoenviron Eng 2011;137(7):633–41. [15] Benmokrane B, Xu H, Bellavance E. Bond strength of cement grouted glass fibre reinforced plastic (GFRP) Anchor Bolts. Int J Rock Mech Min Sci Geomech Abstr 1996;33(5):455–65. [16] Wu WP, GangaRao H, Prucz JC. Mechanical properties of fiber reinforced plastics, Res. Rep., Constructed Fac. Ctr., College of Eng., West Virginia Univ., Morgantown, W. Va., 1991. [17] Robert M, Cousin P, Benmokrane B. Durability of GFRP reinforcing bars embedded in moist concrete. J Compos Construction 2009;13(2):66–73. [18] Cain JJ. Long term durability of glass reinforced composites [Ph.D. thesis]. Blacksburg, Virginia: the Virginia Polytechnic Institute and State University; 2008. [19] Ravindaran N, Cho EH. Durability of glass-fiber-reinforced polymer nanocomposites in an alkaline environment. J. Vinyl Addit. Technol. 2006;12(1):25–32. [20] The national standards compilation group of People’s Republic of China. Test method for mechanical properties of pultruded glass fiber reinforced plastic rods, GB/T 13096–2008, Beijing; 2008. (In Chinese). [21] ACI committee 440. Guide test methods for fiber-reinforced polymers (FRPs) for reinforcing or strengthening concrete structures. American Concrete Institute; 2004. [22] Chu W, Wu LX, Karbhari VM. Durability evaluation of moderate temperature cured E-Glass/vinylester system. Compos Struct 2004;66(4):367–76. [23] Chen Y, Davalos JF, Ray I. Durability prediction for GFRP bars using short-term data of accelerated aging tests. J Compos Construction 2006;10(4):279–86. [24] Zhou JK, Du QQ, Chen LH. Experimental study on size effect in tensile mechanical properties of GFRP rebars. J Hohai Univ: Nat Sci 2008;36(2):242–7 (in Chinese). [25] Ferry JD. Viscoelastic properties of polymers. 3rd ed. New York: John Wiley and Sons; 1980. [26] Tannous FE, Saadatmanesh H. Durability of AR glass fiber reinforced plastic bars. J Compos Construction 1999;3(1):12–9.

Please cite this article in press as: Li G et al. Effect of loading rate and chemical corrosion on the mechanical properties of large diameter glass/basalt-glass FRP bars. Constr Build Mater (2015), http://dx.doi.org/10.1016/j.conbuildmat.2015.05.044