Experimental Study on Mechanical Properties of Novel FRP Bars with Hoop Winding Layer

Due to the fact that steel reinforcement is vulnerable to corrosion, FRP bars with light weight, high strength, and excellent durability have become a good substitute for ordinary steel bars. FRP bars have high tensile strength, but their compressive strength is relatively low and often neglected, so the application of FRP bars in compression members has been restricted. +is paper proposes a new pultrusion-winding-pultrusion method to improve the compressive ability of FRP bars. A hoop FRP layer is winded on the outer surface of the pultruded FRP core, and a longitudinal pultruded layer and ribs are also added on the outermost surface. In this paper, mechanical properties of this novel FRP bar with hoop winding layer are investigated. First, monotonic tensile and compressive tests on traditional and novel GFRP bars were conducted. +en, cyclic tension-compression loading tests were also carried out on the two types of GFRP bars. Test results showed that the compressive ultimate bearing capacities of GFRP bars with winding layers were 10∼20 kN greater than those of the traditional GFRP bars, and the compressive ductility of the novel GFRP bars was also improved. Furthermore, the tensile stress-strain behaviors of both GFRP bars were linear-elastic and the added winding layer did not greatly influence the tensile properties of the GFRP bars. Moreover, for the cyclic loading test, the compressive ultimate load of GFRP bars was 80%∼90% of that under monotonic compressive test, and the tensile ultimate load was 45%∼65% of that under monotonic tensile test. Compared with the GFRP bar without winding layer, the overall stiffness of the novel GFRP bar was greater than that of the traditional one and the ultimate load of the novel GFRP bar was also greater. In addition, seeing that the residual displacement of the novel GFRP bar was greater than that of the traditional GFRP bar, winding hoop fibers on the outer surface of the core is a useful way to improve the energy dissipation capacity of the GFRP bar.


Introduction
Reinforced concrete structure is one of the most widely used structures at present [1]. e steel reinforcements and concrete work together through the bonding force to give full play to their respective advantages, so the reinforced concrete structures can have good load-bearing capacity. Furthermore, their project cost is relatively low. However, steel reinforcements are susceptible to harsh environments, such as marine, humid, and deicing salt environments, which may cause the severe corrosion of steel bars and greatly reduce the service life of reinforced concrete structure, and even result in the structural failure. e corrosion of steel reinforcements has caused huge infrastructure maintenance cost. According to statistics, nearly 1/11 of reinforced concrete bridges in USA are in various degrees of structural damage states due to reinforcement corrosion, which cost the government more than $20 billion per year on maintenance [2]. In China, more than ¥200 billion are spent to deal with the structural damage caused by corrosion every year, which accounts for about 3% of Chinese GDP, and the cost of steel reinforcement corrosion accounts for a large proportion [3].
Fiber-reinforced polymer (FRP) is a high-performance composite material with advantages such as light weight [4], high strength [5], and excellent corrosion resistance [6]. Common FRPs can be mainly divided into four types according to the different types of fibers used, namely, glass fiber-reinforced polymer (GFRP), carbon fiber-reinforced polymer (CFRP), basalt fiber-reinforced polymer (BFRP), and aramid fiber-reinforced polymer (AFRP) [7]. Replacing ordinary steel bars with FRP bars in reinforced concrete structures is able to not only maintain the advantages of traditional reinforced concrete structure but also fundamentally solve the problem of steel corrosion and hence significantly improve the durability of structures. In addition, with FRP reinforcements, the self-weight of the structures can be also reduced, which is beneficial for fabricated structures. e research of FRP bars' application to reinforced concrete structures began in USA in the 1960s. At that time, due to the widespread use of deicing salts, many serious corrosion problems occurred to the steel bars in concrete highway bridges [8]. ereafter, studies on FRP-reinforced concrete structures were carried out in Japan, Canada, China, and many other countries [9][10][11]. Up to present, the related studies of FRP-reinforced concrete structures mainly includes the mechanical properties and durability of FRP bars [12][13][14][15], the bonding properties between FRP bars and concrete [16][17][18], the failure mode and design theory of FRP reinforced concrete beams [19][20][21][22], columns [23][24][25][26][27], joints [28][29][30][31], and fire resistance [32,33].
Deep research promoted the application of FRP bars, especially the relatively cheap GFRP bars in engineering structures, and many countries have published their codes and standards of FRP reinforced concrete structures [34][35][36]. FRP bars have been widely used in engineering structures for their excellent tensile properties [37,38]; however, they have some mechanical problems, including the following: (1) their elastic modulus is lower than that of steel bars, which leads to smaller stiffness, larger deformation, and more serious cracking of concrete structures under load; (2) the stress-strain curve of FRP bars is linear, there is no obvious yield point, and the ductility of FRP bars is poor; (3) the fire resistance of FRP bars is poor; and (4) the mechanical properties of FRP bars are different under compressive and tensile loads, and the compressive strength of FRP bars is much lower than the tensile strength. e compressive properties of FRP bars were studied by Maranan et al. [25], Khan et al. [39], Khorramian and Sadeghian [40], and other researchers [41][42][43][44][45]; their studies indicate that the compressive strength of FRP bars is only approximate 50%∼60% of the tensile strength. For the contribution of FRP bars in the compressive load-bearing capacity of reinforced concrete structural members, many codes choose to neglect it (e.g., ACI 440. 1R. 2015 [35], CAN/CSA S806-12 2012 [46], and Fib Bulletin 40 2007 [34]), because the compressive strength and ductility of FRP bars are regarded as too small.
So far, few scholars have explored the mechanical properties of FRP bars under cyclic loading. Kobayashi and Fujisaki [47] tested the ultimate strength of CFRP, GFRP, and AFRP bars under monotonic compressive and tensile loads and then studied the mechanical properties of the three types of bars under progressively increasing cyclic tension and compression load, respectively. It is concluded that under cyclic loading the compressive strengths of AFRP bars and GFRP bars are 20%∼50% of that under monotonic loading, whereas the cyclic loading has almost no effect on the mechanical properties of CFRP bars. Wu et al. [48] proposed a novel steel-FRP composite bar (SFCB) and applied monotonic and cyclic tensiletensile loads on it. It is found that the stress-strain curve of SFCB under monotonic loading presents bilinear characteristics, and the specimen has stable secondary stiffness after yielding. Under cyclic loading, SFCB has small residual deformation and good recoverable performance, and the cyclic tensile load does not significantly weaken its strength and stiffness.
In this paper, a novel method is proposed to improve the compressive load-bearing capacity and ductility of FRP bars, namely, winding hoop impregnated fibers around the outer surface of pultruded FRP bars. Firstly, the manufacture process of the FRP bars with winding layer was introduced. en, the tensile and compressive properties were studied through monotonic tensile and compressive tests, respectively. Finally, the mechanical properties of novel FRP bars under cyclic tension and compression loading were investigated and compared with the traditional FRP bars.

Manufacturing of FRP Bars with Hoop
Winding Layer e production process of the novel FRP bar can be divided into four steps, as depicted in Figure 1. First, the core of the FRP bar is manufactured by the pultrusion process. e direction of the fibers in the core is longitudinal, which is consistent with the traction direction of the pull rollers. In the process of the pultrusion, the pulling speed is set to 1.2 m/min. e pre-die former is divided into 8 drying tunnels, and the temperature is reduced from 220°C to 160°C gradually, so the resin can be set and hardened sufficiently. After the core is produced, the fibers are impregnated and winded on the outer surface of the core. is process is also called the filament winding. e speed of the rotating device is 1.2-1.6 times of the pultrusion speed. By controlling the winding time of the rotating device, the winding layer with certain thickness can be achieved. After the winding layer is manufactured, a thin layer with longitudinal fibers is added on the outermost surface through the pultrusion; this process can ensure a good surface condition of the FRP bar and protect the hoop winding layer.
For the traditional FRP bar, after the completion of the pultrusion, the ribs are winded on the surface of the core in the circumferential direction. However, for the novel FRP bar, the ribs are added on the outermost longitudinal pultruded layer by the filament winding (see Figure 1).

Materials.
e glass fiber was produced by Taian Glass Fiber Co., Ltd., and the resin was produced by Jiangsu Jinlong New Material Co., Ltd. e core, winding layer, and ribs were all manufactured by E-glass fibers with a monofilament diameter of 24 microns. Moreover, the E-glass fibers in the core were 4800TEX, and those in the winding layers and ribs were 1200TEX. e resins used in the core, winding layer, and ribs were all unsaturated polymer resin. e manufacturer provided the mechanical properties of the E-glass fiber and resin, which are listed in Table 1. In order to ensure the consistency of the experimental data, all the GFRP bars were produced in the same batch. e mass fractions of the fibers and resin are 80% and 20%, respectively.

Test Matrix.
e specimens of traditional and novel GFRP bars were prepared. e traditional GFRP bar with a diameter of 16 mm was used as the control group in the experiment. e novel GFRP bar's core diameter was 16 mm and total diameter was 21 mm, because a hoop winding layer with the thickness of 1.5 mm and a 1 mm pultrusion layer were added outside of the core. Table 2 shows the details and specifications of compressive and tensile specimens. According to GB/T 1448-2005 "Fiber-reinforced plastics composites-determination of compressive properties" [49] and GB/T 30022-2013 "Test method for basic mechanical properties of fiber reinforced polymer bar" [50], the compressive and tensile tests were conducted. e control GFRP bar was termed as C specimen, and the novel GFRP bar was termed as W specimen.
Two kinds of specimens with different heights of 40 mm and 60 mm were prepared for the compressive test, and the total length of the specimen under tensile test was 1000 mm.
ere were five specimens for each case. In the compressive test, taking specimen C-H40-1 as an example, where "C" stands for GFRP bars without winding layer, "H40" means that the height of the specimen is 40 mm, and "1" stands for the first specimen in the testing group; other specimens were Advances in Materials Science and Engineering labeled in the same way. In the tensile test, taking specimen T-C-1 as an example, which "T" stands for tensile test, "C" stands for GFRP bars without hoop winding layer, and "1" stands for the first specimen in the testing group. e photos of GFRP bars are shown in Figure 2.

Test Setup.
For the compressive test, the H40 and H60 specimens were loaded by a 1000 kN MTS universal testing machine. e loading system was established according to the Chinese code GB/T 1448-2005 "Fiiber-reinforced plastics composites determination of compressive properties" [49], as shown in Figure 3. e displacement loading method was adopted, and the loading rate was set to 2 mm/min. e actual strain during loading was recorded by axial strain gauge pasted on the GFRP bar surface in the middle. e bottom of the universal testing machine was a spherical hinge, which can automatically level the specimen when the compressive load was biased. At the end of the test, all the data were collected and recorded by DH3816N static data logger.
For the tensile test, it is noted that the transversely shear strength of GFRP bar is relatively low, in order to prevent the specimen from early failure at the loading end, steel pipes with internal threads were installed at both ends of GFRP bars according to the Chinese code GB/T 30022-2013 "Test method for basic mechanical properties of fiber reinforced polymer bar" [50], and the steel pipes and GFRP bars were bonded by high-strength grouting material. As shown in Figure 4, the length of anchorage part and free part of tensile specimens were 350 mm and 300 mm, respectively. e loading rate of the test was 2 mm/min, and the actual strain of the specimen during the loading was measured by the strain gauge pasted on the surface of the middle of the specimen as shown in Figure 5. Figure 6 shows the failure modes of GFRP bars; it can be found that the splitting failure occurred at both the C and W specimens. e ultimate loads borne by C specimens and W specimens are listed in Tables 3 and 4, respectively, and are compared in Figure 7.

Ultimate Load and Compressive Strength.
It can be seen that the ultimate load of C-H60 specimens was slightly less than that of C-H40 specimens, and the ultimate load of W-H60 specimens was also less than that of W-H40 specimens. e ultimate compressive load of the same type of GFRP bars decreased with the increase of height due to the influence of slenderness.
In addition, when the height of the GFRP bar is same, the ultimate load of the GFRP bars with winding layer (W) increased by 10∼20 kN compared with that of the GFRP bar without winding layer (C). e ultimate load of H40 GFRP bars increased by 10.9% with an additional winding layer. Similarly, the ultimate load of H60 GFRP bars increased by 19.5% with the winding layer. e trend in Figure 7 indicates that the winding layer can help increasing the GFRP bar's ultimate compressive load.
Compressive strengths of C specimens and W specimens are presented in Tables 5 and 6, respectively. e compressive strength was obtained by dividing the compressive load (P u ) with the nominal cross-sectional area (A) as specified by the Chinese code GB/T 1448-2005 "Fiberreinforced plastics composites-determination of compressive properties" [49]. e nominal diameter of C specimens and W specimens is 16 mm. erefore, the nominal cross-sectional area A is equal to 200.96 mm 2 . With one winding layer, the compressive strengths of the H40 and H60 specimens were increased by 10.9% and 19.5%, respectively.   Advances in Materials Science and Engineering Figure 8 shows the change trend of the compressive strength of GFRP bars; it can be seen that the winding layer has a positive effect on increasing the compressive strength for both heights of GFRP bars. Figures 9 and 10 show the stress-strain curves of C specimens and W specimens, respectively. It can be seen from Figures 9(a) and 9(b) that the stress-strain curves of C specimens changed linearly during the loading process, but Figures 10(a) and 10(b) indicate that the stress-strain curves of the W specimens appeared to be nonlinear to a certain extent; namely, many specimens have an approximate horizontal segment before failure.
is also indicates that the ductility of the GFRP bars has been improved by winding hoop fibers on the surface of the core. e compressive modulus of elasticity was also calculated apart from the ultimate load and compressive strength. According to the Chinese code GB/T 1448-2005 "Fiberreinforced plastics composites-determination of compressive properties" [49], the compressive modulus of elasticity was calculated by where ε 1 is the strain at 0.05 με during the loading process, ε 2 is the strain at 0.25 με during the loading process, and σ 1 and σ 2 are the stresses corresponding to ε 1 and ε 2 , respectively. e calculated compressive moduli of C specimens and W specimens are listed in Tables 7 and 8. As can be seen, the elastic modulus of the W specimen was significantly greater than that of the C specimen.

Tensile Test Results and Discussion.
In the initial stage of loading, the sound of fiber breakage could be heard sparsely.
As the load continued to increase, the sound of internal fibers being pulled to rupture became louder. When the load approached the ultimate load, a loud noise could be heard from the GFRP bar, and the specimen burst like a lantern.  Tables 9 and  10. e difference of average ultimate tensile load between the two types of GFRP bars was not obvious, which was due   Advances in Materials Science and Engineering    to the fact that the axial fibers mainly carried the external load, and the winding fibers had little effect on the bearing capacity of the GFRP bar. Figures 12 and 13 present the ultimate load and tensile stress of the GFRP bars. During the test, the outer surface of the GFRP bar first burst before the specimen failed, which caused the strain gauge failure in advance. Figures 14 and 15 show the measured stress-strain curve and load-displacement curve of T-C specimens and T-W specimens, respectively. Figure 16 shows the comparison of tensile elastic modulus. Taking the linearly elastic sections in Figure 14 for calculation, the values of each specimen's elastic modulus can be obtained. It can be found that the tensile modulus of elasticity of T-C specimens is smaller than the T-W specimens' tensile modulus of elasticity.

Test Setup.
e premise of the test was to ensure that the GFRP bars did not buckle during the loading process. Considering the limitation of the fixture length of the universal testing machine; meanwhile, in order to prevent the slippage between the GFRP bar and the anchorage, the length of anchored section was set to be 200 mm, and the free section of the GFRP bar was 100 mm long. Table 11 shows the specifications of the specimens. A 300 kN MTS low-cycle fatigue testing machine was used in the test. In order to ensure that both ends of the specimens were well fixed to the testing machine, the clamping load of the fixture on the anchorage was set to 15 MPa. e deformation of GFRP bar during the loading process was measured by LVDT. e test setup of cyclic loading is depicted in Figure 17. As shown in Figure 18, a displacement-controlled loading process was adopted in the test. Furthermore, the loading rate was set to 0.05 mm/sec, and the loading frequency was 20 Hz. . Figures 19(a) and 19(b) show the failure modes of TC-C specimens and TC-W specimens, respectively. All the failure modes of specimens were burst split, and cracks can be obviously observed in the free section of GFRP bars, which was the result of matrix crushing under the cyclic loading and the separation between fiber and resin.       All the GFRP bars were failed during bearing the compressive load in the cyclic loading test. Compared with TC-C specimens, the compressive failure of TC-W specimens was delayed due to the existence of the winding layer, which played a vital role in the compression stage during the loading process and further improved the compressive ultimate bearing capacity.

Test Results and Discussion
Due to the increase of the residual displacement and the ultimate load, the energy dissipation capacity of GFRP bars with winding layers was improved to a certain extent. In addition, the overall stiffness of TC-C specimens was less   Advances in Materials Science and Engineering 13 than that of TC-W specimens, and the energy dissipation capacity of TC-W specimens was better than that of TC-C specimens. It can be also found from Figures 19 and 20 that the compressive ultimate bearing capacities of GFRP bars were greater than the tensile ultimate bearing capacities, which can explain that the compressive failure of GFRP bars occurred before the tensile failure. Under cyclic loading, the ultimate load of TC-C specimens and TC-W specimens is just approximate 80%∼90% of that under monotonic compressive loading and is approximate 45%∼65% of that under monotonic tensile loading,   Advances in Materials Science and Engineering which indicates that the GFRP bars first reached the compressive ultimate bearing capacities and then failed.

Conclusion
In this paper, a novel GFRP bar with hoop winding layer is studied through the monotonic compression, monotonic tension, and cyclic tension-compression tests. e mechanical responses were investigated and compared with the tradition GFRP bar. e following key conclusions can be drawn from the work: (1) e ultimate compressive bearing capacity of the GFRP bars with winding layer is significantly greater than that of the traditional GFRP bars. e stressstrain relationship of the tradition GFRP bar is linear, while that of the novel GFRP bar is nonlinear, and the winding layer can also help improving the compressive ductility of the GFRP bar. (2) e tensile test results indicate that the stress-strain curves of both GFRP bars are linear, and the winding layer does not greatly influence the tensile strength of the GFRP bar but can slightly increase the tensile elastic modulus. (3) Under the cyclic tension-compression loading, the compressive ultimate load of the GFRP bar is 80%∼ 90% of that under monotonic compressive test, and the tensile ultimate load is 45%∼65% of that under monotonic tensile test. Compared with the traditional GFRP bar without winding layer, the deformation of the novel GFRP bar with winding layer is reduced when the same load is applied. erefore, the overall stiffness of the novel GFRP bar is greater than that of the traditional GFRP bar. Moreover, the ultimate load of the GFRP bar is also increased by the winding layer. In addition, because the residual displacement of the novel GFRP bar is greater than that of the traditional GFRP bar, winding hoop fibers on the outer surface of the core is a useful way to improve the energy dissipation capacity of the GFRP bar.

Data Availability
e experimental data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.