Combination form analysis and experimental study of mechanical properties on steel sheet glass fiber reinforced polymer composite bar

Abstract

The concept of steel sheet glass fiber reinforced polymer (GFRP) composite bar (SSGCB) was put forward. An optimization plan was proposed in the combined form of SSGCB. The composite principle, material selection, and SSGCB preparation technology have been described in detail. Three-dimensional finite element analysis was adopted to perform the combination form optimization of different steel core structures and different steel core contents based on the mechanical properties. Mechanical tests such as uniaxial tensile, shear, and compressive tests were carried out on SSGCB. Parametric analysis was conducted to investigate the influence of steel content on the mechanical properties of SSGCB. The results revealed that the elastic modulus of SSGCB had improvements and increased with the rise of steel content. Shear strength was also increased with the addition of steel content. Furthermore, the yield state of SSGCB was similar to the steel bar, both of which indicated a multi-stage yield phenomenon. The compressive strength of SSGCB was lower than that of GFRP bars and increased with the increase of the steel core content. Stress-strain curves of SSGCB demonstrated that the nonlinear-stage characteristics of SSGCB-8 were much more obvious than other bars.

Graphical abstract

Keywords

steel sheet GFRP composite bar / combination form / numerical modeling / mechanical properties test / strength

1 Introduction

Fiber-reinforced polymer (FRP) bar is a hot research topic that has drawn growing attention from scholars worldwide. FRP is wildly used in the industry because of its advantages of superior mechanical, thermal, and chemical properties such as lightweight, high strength, and high corrosion resistance. Owing to its anticorrosive characteristics, the FRP bar is a promising substitute for conventional steel bars in reinforcing concrete structures [ 1– 9]. The recent investigation of glass fiber reinforced polymer (GFRP) bars in manufacturing slabs [ 10], columns [ 11], railway sleepers [ 12], and durability study [ 13] should be attached to vital significance. However, FRP bars displayed a linear elastic behavior up for failure with low elastic modulus and no ductility, which adversely affected the ductility of the concrete structures [ 7, 14]. Bakis et al. [ 15] indicated that the most useful application of FRP was as a tension-only member in structures, rather than typical reinforcement in concrete structures because of the relatively low modulus and high strength compared to steel. Sonmez [ 16] reviewed the optimum design of structures according to the type of the composite structure optimized by the loading conditions, the objective function, the structural analysis method, the design variables, the constraints, the failure criteria, and the search algorithm.

Despite the significance and concerns about steel sheet GFRP composite bar (SSGCB), the current number of studies on this topic is still not enough, namely about the different combination forms on the mechanical properties of SSGCB. The most relevant issues of previous investigations in this specific research field are illustrated next, together with the aspects that still require additional research efforts and motivated the present investigation. Therefore, it is vital to optimize the structural design to improve the elastic modulus and ductility of FRP bars. Considering the deficiencies of FRP bars, some scholars proposed the concept of hybrid fiber-reinforced polymer (HFRP) to improve FRP ductility [ 17– 21]. Carbon and E-glass fibers were used to develop a CFRP/GFRP reinforcement, and the tensile strength of hybrid rods was tested [ 22]. The results of the tests indicated that the ultimate strain of hybrid bars with dispersed type increased by 33% compared with that of FRP bars. The ductile hybrid FRP (DHFRP) was studied and proved that the elastic modulus of HFRP bars was much lower than that of steel, and the deformation of concrete structures with the composite bar was higher than that of steel [ 23]. Based on HFRP bars, some scholars further propose the concept of steel fiber reinforced polymer (SFRP). According to the hybrid theory, the problem of low modulus and ductility of FRP bars was solved by combining the advantages of fiber and steel [ 24– 29]. Cui et al. [ 30] summarized the development of ductile composite reinforcement bars in concrete structures. Behnam and Eamon [ 31] have analyzed alternative ductile fiber-reinforced polymer reinforcing bar concepts. Some researchers concentrated on the innovative hybrid reinforcement for flexural elements [ 32– 39]. Zhao et al. [ 40] investigated new types of the steel-FRP composite bar with round steel bar inner core on the mechanical properties and bonding performances in concrete.

However, experimental research on these bars has been limited due to bond performance. The tensile stress experimental results of composite reinforcement revealed a slip deformation phenomenon between the steel and the outer fiber when steel was yielding [ 41, 42]. Therefore, many investigations into steel-core-FRP composite bars (SCFCB) and steel-wire-FRP composite bars (SWFCB) were implemented [ 43]. Furthermore, it was difficult to locate steel wire in the pultrusion process, and the bonding effect between steel wire and basalt fiber would be worse when using too much steel wire. Considering steel content, construction technology, and mechanical properties of composite bars, three kinds of steel forms, including SSGCB-4, SSGCB-6, and SSGCB-8, were applied to simulate the characteristics of composite bars in this study. These results revealed that the steel and FRP bond strength was the limitation of the SFRP. The hybrid bars can be manufactured from resin-impregnated FRP to encase the steel core fully. And the interfacial properties between FRP and steel become the main factor that influences the yield and ductility performance of SFRP [ 29]. Only tensile properties of GFRP were researched in the numerical simulation, and the following assumptions were made, including the smooth surface of the steel, good adhesion between steel and GFRP, good bonding performance between steel anchor pipe, and GFRP, and no relative slip between anchor pipe and GFRP. Several experiments have been conducted to evaluate the mechanical properties of steel-FRP composite bars. The SFCB pultrusion process produced many composite bars, and the stress-strain curves of SFCB revealed a prominent double-linear property under monotonic loading [ 44].

The main differences between this study and those published in the literature are as follows. 1) The results show that the HFRP tendons can yield, and the ductility is improved. The main reason is that the elastic modulus of most fiber materials is relatively low. The hybrid FRP bars, after hybrid fiber composite, appear in the tensile process yield stage, but its deformation is greater than that of steel bars. High-performance fiber-reinforced composites (such as carbon fiber) are needed, and the economy of hybrid FRP bars is poor. 2) The effect of the composite reinforcement with fiber braiding or spiral wrapping outside the steel core is not ideal, resulting in the waste of the mechanical properties of the fiber. The steel wire and basalt fiber are combined with the pultrusion forming process to get composite reinforcement. The test results show that the composite reinforcement has a high elastic modulus, but the content of steel wire should not be too large, because there is unavoidable interference between longitudinal stress fiber and steel wire in the pultrusion forming process, the steel wire is difficult to locate, and the forming effect is not good. For certain glass fiber reinforced composite bar with steel strand as steel core, the research shows that the bar has a high elastic modulus and yield characteristics. However, after the outer fiber of the composite bar breaks, the steel strand slips due to the failure of effective bonding, and the composite bar stops working after the outer steel strand with greater stress breaks. Steel continuous fiber-reinforced bar (SFCB) has a higher elastic modulus and better ductility, but the surface treatment of ribbed bars is more complex, which is not conducive to industrial production. At the same time, although the cladding of ribbed steel bar can achieve a good combination with a steel bar and longitudinal fiber, it also leads to the bending of longitudinal fiber, which is not conducive to the exertion of fiber strength and leads the adjacent longitudinal fiber to premature failure.

To sum up, there are two technical bottlenecks in the construction principle and mechanical properties of GFRP composite bars. In this paper, the radial arrangement of 304 steel sheet GFRP composite reinforcement is proposed to increase the contact area between the steel and GFRP reinforcement. By optimizing the arrangement form of the steel sheet inner core and glass fiber and the improvement of corresponding mechanical properties, the above two breakthroughs are realized.

The key problem of the SFRP composite bar was how to improve the coordinated deformation between steel and GFRP. The steel and FRP bond strength can be improved by combing steel sheets with FRP. The idea that a steel sheet was used to replace the steel core and steel wire in SSGCB had two significant advantages. First, the contact area between GFRP and steel could be increased to improve the bonding property to achieve the purpose of coordinated deformation of the composite material; Secondly, steel sheet was much easier to be located than steel wire during the production process. Different steel-to-FRP cross-sectional area ratios could be used to allow for different mechanical performance. To investigate the mechanical properties of SSGCB, a three-dimensional finite element model (FEM) of SSGCB with different steel content was established. And the destructive uniaxial tensile and shear test was conducted on SSGCB to evaluate its mechanical properties. For researchers and end-users, using SSGCB as the reinforcing material can help achieve post-yield stiffness and good reparability of a structure. Based on this new composite reinforcing bar, new damage-controllable structures can be developed that implement performance-based seismic designs more easily. Meanwhile, in severe aggressive environments such as marine and coastal concrete structures, SSGCB has a promising and cost-effective advantage of durability over traditional steel bars that avoids corrosive environmental properties. It provides references for the research, and structural design of SSGCB reinforced concrete structures.

2 Combination form analysis of SSGCB

Based on the formation and principle of steel-GFRP composite bar (SGCB), the concept of SSGCB was put forward to improve co-deformation of steel and GFRP by increasing the outer surface area of steel. In the layout of the steel sheet, if only the contact area between steel and fiber was considered, the evaluation index of n (ratio of perimeter and area of steel sheet section) was shown that the area between fiber and steel was larger when the steel sheet was thinner. However, it was difficult to produce a thin steel sheet and locate it into composite bars due to the limitations of fabrication technology, then the tensile and shear properties of SSGCB were reduced. Therefore, the size and number of steel sheets should be adequately controlled to improve the contact area by increasing the n value. On the overall layout, the surface of SSGCB was spiraled by GFRP fiber bundles and sandblasted to enhance the adhesion between composite bars and concrete. There are two kinds of interfaces in steel core GFRP composite bars, namely resin-glass fiber and resin-steel. When a silane coupling agent is used, the contribution of the silane coupling agent to the bond strength of the two interfaces should be considered. When the binder can be well wetted on the reinforced material, the surface roughness of the reinforced material should be increased, especially for the steel core with a relatively small contact area ratio, the surface sanding treatment should be carried out, and the interfacial transition zone should be increased to increase the interfacial adhesion and reduce the risk of debonding. To improve the coordinated deformation between the steel core and GFRP, the GFRP bars with radial steel sheets were proposed by increasing the contact area between the steel core and resin. The overall layout model of SSGCB is shown in Fig. 1. The composition of the composite reinforcement is as follows: 304 stainless steel sheet is used with a thickness of 1 mm and a width of 6 mm. The fiber is an E-glass fiber, and the surface coupling agent is a silane coupling agent. The adopted resin is thermosetting epoxy resin.

2.1 Composite principles

The thin steel sheets were used as the force materials in SSGCB to replace the fiber partially. A perfect bond between the steel sheet and GFRP was assumed by neglecting the change in the bond stress with slip. According to the composite law, the stress-strain relationship of the steel-GFRP composite bar (SGCB) was divided into three different stages: the elastic stage, the plastic yielding stage, and the failure stage. The period from the natural state to the steel yield state was defined as the elastic stage of the composite bar, which was shown in Eq. (1a). In this stage, the steel core and the fiber were working together, and the modulus of the composite bar was lower than that of steel and higher than GFRP. Equation (1b) expressed the period from steel yield to the outer fiber breakage, which meant the plastic yielding stage. The elastic modulus of the composite bar decreased due to the yield of steel. The period of the failure stage was defined as shown in Eq. (1c). The fiber was pulled out of the work and all of the force was bore by the steel until the increasing strain broke it.

where \epsilon = the strain of SGCB; E_{\mathrm{s}} = the elastic modulus of steel; A_{\mathrm{s}} = the cross-sectional area of steel; {\epsilon }_{\mathrm{s}\mathrm{y}} = the yield strain of steel; {\epsilon }_{\mathrm{s},\mathrm{m}\mathrm{a}\mathrm{x}} = the fracture strain of steel; f_{\mathrm{s}\mathrm{y}} = the yield stress of steel; E_{\mathrm{f}} = the elastic modulus of fiber; A_{\mathrm{f}} = the cross-sectional area of fiber; {\epsilon }_{\mathrm{f}\mathrm{u}} = the fracture strain of fiber; and A = the cross-sectional area of the composite bar, A=A_{\mathrm{s}}+A_{\mathrm{f}} .

2.2 Manufacturing method

Glass fiber is an organic fiber to be used as a reinforcement in advanced SSGCB due to its superior performance at a relatively competitive cost. The main justifications for applying the GFRP composite bar are high strength, lightweight, corrosion resistance, and anti-fatigue. Meanwhile, good bonding properties and plasticity of GFRP composite bars enabled the preparation technology of SSGCB much more manageable than others. Steel sheet is the primary bearing component in SSGCB. Different shape features of steel and steel grade and type can be used to allow for different mechanical performances of SSGCB. The thermosetting resin is the matrix that bonds glass fiber and steel sheets together to form SSGCB. Besides, the resin has two main effects, including transferring the load to each fiber and steel sheet; protecting SSGCB from exposure to environmental effects. Steel sheet surface has been usually smooth and coated with antirust grease, which would inevitably affect the bonding strength between resin and steel. Therefore, the steel sheet surface should be rough enough to enlarge bonding strength. The pretreatment process to increase steel sheet surface roughness is illustrated in Fig. 2.

Glass fibers should be distributed uniformly to transfer the load to each fiber effectively. Glass fibers were put on the shelf and pull the materials out according to design requirements and specifications, as shown in Fig. 3(a). Otherwise, the place where the spindle was in contact with fiber should be smooth to avoid damage to glass fiber. Glass fibers were introduced into the dipping tank with thermosetting resin through the guide roller, then the redundant resin was cleaned through the carding board. Finally, glass fibers were preformed by the pultrusion mold. During the process, engineers should make sure that the fibers and steel sheets in the dipping tank were dispersed to guarantee the strength of SSGCB. Figure 3(b) demonstrates the immersion process. It was critical for materials to infuse sufficiently with the resin mixture. If glass fibers were not impregnated with the resin, the strength of SSGCB would not attain the designed strength.

The main function of preforming was to model the resin-impregnated materials in the shape of the final product. This process facilitated the close connection between steel and fiber, and the excess resin was removed. The preform model was used to transit the material gradually from dispersion to centralization and eventually reached the designed shape, and the SSGCB mold is shown in Fig. 3(c). After yarned and sandblasted, the SSGCB was sent to the model for curing. Thermosetting was a chemical reaction process with high temperature and high pressure, and the temperature plays a decisive role in the process, as shown in Fig. 3(d). The curing rate was faster when the temperature was high, but the high temperature would damage SSGCB, however, if the curing rate was too quick, it would lead to more significant tension and influence the stress and appearance of SSGCB. At the same time, too slow a curing rate would lead to resin inhomogeneity and affected the quality and preparation efficiency. Therefore, the temperature of the three heating zones was 160°C, 180°C, and 170°C, respectively.

2.3 FE modeling

Due to the small cross-sectional size of the steel core GFRP bar, the bonding interface between the steel core and GFRP is thinner, and the amount of slip is small. The slip between the steel core and GFRP interface is not considered here, and the surface of the steel core is assumed to be smooth and good bonding performance with the GFRP bar. The integrated model is used for analysis. The steel core and GFRP elements are common nodes on the joint surface, and each material is regarded as continuous and uniform material. Through numerical analysis, the overall tensile performance of the composite bar under load and the effect of load sharing by the steel core were studied. The yield strength, tensile strength, elastic modulus, and Poisson’s ratio of glass fiber, 304 steel, and Q345 steel are presented in Table 1.

Three-dimensional (3D) solid elements (Solid 45) were adopted to simulate GFRP and steel to simplify the numerical calculation. As a crucial step in this modeling, a detailed meshing can ensure a relatively accurate result [ 45]. Structural mesh generation technology was applied in this modeling. Because the section of the steel sheet is 1 mm × 5 mm, the mesh size was 1 mm to have a good analysis of the structural performance. The finite element model of SSGCB is shown in Fig. 4.

2.4 Analysis of different steel core structures

This section only considers the numerical calculation results of three structural forms of composite reinforcement. The material properties of the composite reinforcement of each structural form are the same. Steel wire, the specific section form is shown in Fig. 5, and the structural parameters are shown in Table 2.

The composite bar has a stress concentration at the anchoring junction. The main consideration in this section is the tensile force performance of the composite bar, which is ignored here. For the stress of this part, select the distal end 150 mm section for force analysis. The axial stress distributions of the three composite bars under various working conditions are not much different. Figure 6 illustrates the SSGCB-6 axial tensile stress cloud diagram. The stress on the steel core is about 258 MPa when the tensile stress is 150 MPa. The stress is about 138 MPa when the tensile stress is 600 MPa, the stress on the steel core is about 358 MPa, and the stress on the GFRP is about 635 MPa. From the stress-strain relationship of the two materials as mentioned above, it can be seen that the elastic modulus of the steel core decreases rapidly after tensile yielding and is lower than the elastic modulus of GFRP, so the steel core is stressed during the elastic phase during the stretching process. It is greater than the external GFRP material, and the stress level of the steel core after yielding gradually decreases and be lower than that of the GFRP material. It can be seen that the high modulus steel core can increase the elastic modulus of the composite rib in the early stage of the stretching process, and in the later stage, the overall tensile modulus of the composite rib is reduced due to the rapid decrease of its modulus.

The steel core shape and layout of the three types of composite bars are different, inevitably affecting the stress distribution of the GFRP material in the composite bar during tension, thereby affecting the force transmission effect between the materials and the failure form of the composite bar materials. In the composite reinforcement, the force performance of the interface between the steel core and GFRP is not only related to the interface but also closely related to the force of the fiber and resin in the vicinity of the steel core. If fiber fracture or resin failure occurs in this area, the bonding performance of the interface is weakened and eventually fails. Figure 7 shows the stress cloud diagrams of GFRP in composite bars under 150 and 600 MPa tensile stress working conditions.

Overall, the GFRP stress distribution levels of the three types of reinforcements during the stretching process are equivalent and relatively uniform. The maximum stress of GFRP appears in the vicinity of the contact with the steel core. The stress in this area of the three composite bars under 150 MPa tensile stress is not much different, and the difference is more evident under 600 MPa tensile stress. For SSGCB-6, there is a specific stress concentration phenomenon at the end of the steel sheet, while the GFRP stress distribution near the side of the steel sheet is relatively uniform, and there is no stress concentration phenomenon. For SFCB-1 and SFCB-6 composite bars with circular cross-section steel cores, larger stresses in the circumferential area contacts the steel core. The maximum stress of GFRP in the composite bar under the action of average tensile stress of 600 MPa of SSGCB-6, SFCB-1, SFCB-6 were 693.3, 672.9, 675.1 MPa, respectively. The maximum stress value of SSGCB-6 is about 19 MPa larger than the other two. The stress cloud diagram shows that in the same kind of workconditions, the GFRP stress in the side area of the steel sheet in the SSGCB-6 composite bar is about 655 MPa, which is about 18 MPa lower than the other two. Due to the small stress concentration area at the end of the steel sheet, the overall stress of the GFRP in the contact area with the thin steel sheet is lower than that of the other two, indicating that the GFRP on the outer edge of the steel sheet has a better stress condition. Regardless of interface slip, the shear stress cloud diagrams of the interface between the steel core and GFRP in the three composite reinforcements under a load of 600 MPa are shown in Fig. 8.

The load application method in this chapter is to apply a load to the end of the composite bar anchorage. The anchorage transfers the load to the composite bar through shear force, so significant shear stress appears as the steel core-GFRP interface near the two ends of the anchorage. Among them, the maximum shear stress appears in the transition section of the composite reinforcement anchorage area, and the shear stress gradually reaches a lower level in a smaller range. The shear stress distribution law conforms to the law mentioned above of interfacial shear force transfer in composite materials. Due to the difference in the three types of structure of the steel core GFRP composite bars, the interface shear stress distribution is different, as shown in Fig. 8. The maximum shear stress at the interface of the SSGCB-6 composite bar is 98.0 MPa. At the stress concentration area, the maximum shear stress at the interface of the SFCB-6 composite bars is 74.8 MPa, and the maximum interface shear stress in the SFCB-1 is 34.8 MPa. The shear stress at the interface of the SFCB-1 bars is evenly distributed along the steel core circumferential direction. For SSGCB-6 and SFCB-6, the outer edge of the composite bar is closer to the surface of the composite bar than the steel core of the SFCB-1 bar, so the outer edge shear stress is more significant that gradually decreases from the outward axis direction, which conforms to the shear stress distribution law in material mechanics. The purpose of this paper is to find a steel core GFRP composite bar to improve the mechanical properties of GFRP so that it can replace the stressed steel bar and be used in concrete. The reinforced concrete structure is uniformly stressed, and there may not be such obvious stress at the concentrated end. However, the above analysis shows that while the thin steel sheet increases the shear capacity of the interface, it also has shortcomings; that is, there will be relatively large shear stress on the outer edge. The mechanical properties of composite bars need to be studied in conjunction with related tests.

According to ASTM A304-20 [ 46], the tensile strength of 304 steel as steel core is 540 MPa, and that of GFRP reinforcement is 744 MPa. The convergence criteria of different steel core cross-section bars are GFRP bars ultimate tensile strength and steel sheet ultimate tensile strength. It is considered that the ultimate bearing capacity of composite bars is the corresponding bearing capacity when the steel core and GFRP bars reach the tensile strength. It can be seen from Fig. 9. that the GFRP bar should reach an ultimate tensile strength before the steel core during loading. Therefore, it can be considered that the ultimate bearing capacity of the composite reinforcement is the ultimate bearing capacity when the GFRP Bars reach the yield limit. The ultimate bearing capacity of SSGCB-6, SFCB-1, and SFCB-6 composite bars is 168.53, 168.09, and 167.76 kN. The comparison of the three cases is shown in Table 3 and Fig. 9.

The ultimate bearing capacity of SSGCB-6, SFCB-1, and SFCB-6 composite bars is 168.53, 168.09, and 167.76 kN. The comparison of the three cases is shown in Table 3 and Fig. 10. The ultimate bearing capacity of SSGCB-6 is the largest, SFCB-6 is the smallest, and SFCB −1 is between them. The ultimate bearing capacity of SSGCB-6 is 0.77 kN higher than that of SFCB-6. The main reason is that the steel sheet increases the contact area between the steel and GFRP bars relative to the steel core and improves the bonding capacity.

2.5 Analysis of SSGCB with different steel core content

Researchers just aimed at steel core and steel wire, while the optimum steel sheet content of SSGCB would not be the same as them. The tensile properties of SSGCB were studied to explore the influence of the steel content on them. Considering the content of steel, construction technology, and mechanical properties of SSGCB, three kinds of steel structure forms including SSGCB-4, SSGCB-6, and SSGCB-8, were applied in numerical simulation to analyze its characteristics. In this section, the modeling of SSGCB and the structure optimization analysis of SSGCB based on mechanical properties were carried out. To study the improvement of elastic modulus and ductility of the SSGCB with different steel content effectively, a simplified model of SSGCB with three kinds of materials such as GFRP, inner steel, and the steel anchor pipe was adopted. Three types of SSGCB, namely SSCGC-4, SSGCB-6, and SSGCB-8 were chosen to analyze their mechanical properties, of which the structural parameters with different steel content are tabulated in Table 4.

The materials of GFRP and steel were regarded as continuous homogeneous materials to analyze the tensile properties of SSGCB. GFRP was assumed to behave as a linear-elastic material before the failure of stress and strain in longitudinal tension. Steel was anisotropic material, so the multilinear isotropic hardening plasticity model was used to simulate. The inner steel of SSGCB was 304 thin steel sheets in accordance with the experiment protocol. The yield strength was 257 MPa, and the stress-strain curve did not have an obvious yield platform. The tensile characteristics of SSGCB with different steel content were researched in the finite element models. The constant loads (150 and 600 MPa) were applied at the end of the anchor pipe to avoid stress concentration and the corresponding boundary condition was followed as the experimental test. Figure 11 illuestrates the von Mises stress of composite bars (including SSGCB-4, SSGCB-6, and SSGCB-8).

The results indicated that the stress of steel was greater than fiber and decreased slightly with the increase of steel content when SSGCB under the tensile stress of 150 MPa. The maximal stresses of the SSGCB were, respectively, 263.3, 260.6, and 257.6 MPa, which was because the modulus increased with the increase of steel content. Therefore, the stress was decreased with the increase of steel content under the same load. Under the stress of 600 MPa, the stress of GFRP was greater than steel due to the yield of steel. Also, there was a significant increase in stress with the increase of steel content, and they were 634.3, 653.3, and 672.2 MPa, respectively. Because the elastic modulus of SSGCB decreased faster when steel was yielded, and the stress of GFRP will increase. However, because the capacity of GFRP was limited, the strength of SSGCB would be lower with the higher steel content. To factually analyze the deformation of SSGCB, the parts of which were kept away from the steel anchor pipe were chosen as the effective part of the specimen in the paper. The displacement curves of SSGCB are shown in Fig. 12. The stress-displacement curves of SSGCB were presented with a convex linear in both the elastic stage and the plastic stage. From the stress-displacement curve, it can be observed that the elastic modulus of SSGCB was larger than that of GFRP under the elastic stage. The elastic modulus and ductility of SSGCB would increase with the increase of steel content. Then the elastic modulus of SSGCB was gradually decreased with the increase of the stress. And when the steel yielded, the modulus of the composite would be decreased and showed the ductility characteristics. Also, the nonlinear-stage characteristics of SSGCB-8 were more obvious than that of other composite bars.

3 Experimental optimization analysis of SSGCB of combination form

The slip phenomenon between steel and fiber appeared because the steel and glass fiber have different deformation ratios when the steel yields. Therefore, four groups of SSGCB specimens consisted of GFRP and steel sheet were tested experimentally to evaluate the tensile, shear, and compressive strengths of SSGCB with different steel contents, which the specimen identification is SSGCB-0, SSGCB-4, SSGCB-6, and SSGCB-8.

3.1 Materials design

304 stainless steel was selected as the test material, and the thickness of the steel sheet was 1 mm, the width was 5 mm; glass fiber type was E-glass fiber, and silane coupling agent was selected as a surface coupling agent; the bonding agent was epoxy resin. This bar was made up of unidirectional roving of E-glass and epoxy vinyl ester resin as manufactured through the pultrusion process. The appearance was milky white, and the surface of the spiral sandblasting, each screw length was 14 mm, and the height was 0.325 mm. SSGCB specimens are shown in Fig. 13.

Considering the size and layout of the steel sheet, the test was carried out for four kinds of steel content composite bars, including SSGCB-0, SSGCB-4, SSGCB-6, and SSGCB-8. The parameters of SSGCB specimens are shown in Table 5.

3.2 Experiment methodology

Figure 14(a) depicts that the specimens for tensile testing were prepared by anchoring two ends of the SSGCB in steel plugs filled with epoxy resin. The interspace between SSGCB and the steel anchor pipe was filled with epoxy resin. To ensure the anchorage performance between SSGCB and steel anchor pipe, there were screw threads on the anchor pipe interior. The length of the steel pipe was 300 mm, the diameter was 35 mm, and the wall thickness was 6 mm. The epoxy resin was used as adhesive to fix the composite. The free length between steel plugs was about 300 mm to ensure the anchor bonding strength is higher than the tensile stress, according to the guidelines as specified in ACI 440.3R-04 [ 47]. A universal testing machine (SHT4106-G) was used in the tensile test, and an extensometer of 50 mm gauge length was mounted with clips at the center of the test specimen, as can be seen from Fig. 14(b). The displacement control mode was adopted with a loading rate of 2 mm/min, and an automatic data acquisition system was used to collect the test data with a data acquisition frequency of 1 Hz. The extensometer model (YYU-10/50) was used in the electronic extensometer measurement test.

The length of the shear specimen was 300 mm to ensure the anchor bonding strength according to the specification specified in JG/T 406−2013 [ 48]. The universal testing machine (SHT4106-G) was adopted in the shear test, and the shearing mechanism is shown in Fig. 15(a). And the force arrangement of SSGCB can be seen from Fig. 15(b). The test loading rate was 40 MPa/min, and an automatic data acquisition system was applied to collect the test data with the data acquisition frequency of 1 Hz.

The standard GB/T 1448−2005 [ 49] stipulates that the test piece with a diameter of 5–16 mm has a compression test height of 2.5 d. The “Metal Compression Test Method” stipulates that the length of metal material specimens is generally 2.5–3 times the diameter. Combining the specifications and the above tests, the height of the specimen used in the compression test is 50 mm. During the production process of the test piece, ensure that the material section is flat and intact, and ensure that the test is vertically compressed during the test.

3.3 Results and discussion

The failure modes of SSGCB in tensile, shear, and compression tests are depicted in Fig. 16. There was no specimen split off the anchor due to the sufficient bond strength between the specimen and anchor pipes during the test. The failure process showed an obvious phase and a stable post-yielded stiffness after the yield of the steel sheet. The tensile failure position of SSGCB was mainly concentrated on the middle of the specimens and a little close to the end of the anchor. The main failure mode of SSGCB was that the fiber was broken away from the steel sheet, after which was yielded. There would be a micro-crack when the load reached about 40% of the ultimate load, and part of fiber would be fractured when the load reached up to about 70% of the ultimate load, and then the breakage of fiber would increase sharply, which finally led to the damage of the specimens. No steel sheet was pulled out or slipped from the SSGCB, and the failure modes of glass fiber in SSGCB were not the same as GFRP. In the shear test, the sounds of steel brittle and fiber damage could be heard, and the frequency of sound would be quickened with the increase of load. Then the whole GFRP bar and SSGCB were cut off, and the sections of GFRP and steel sheet were trim. There were no apparent cracks and compression deformation of the section during the shear test. Figure 16 shows the shear test failure of the SSGCB. During the compression process, there will be multiple brittle noises. After the load reaches the peak, it will drop, and then there will be a smaller rising stage. Finally, the load will drop at a low level, and the specimen will be damaged. The main failure mode of GFRP tendons is split failure. The main failure modes of SSGCB composite bar specimens are as follows. 1) End failure. The end of the composite bar is fractured into a truncated cone shape, and part of the steel core end is unstable. 2) Split failure. The main split failure in the composite bar is concentrated at the end of the steel sheet, mainly due to the inconsistent deformation of the steel core with the GFRP deformation and splitting failure during the compression process.

The tensile test data of SSGCB are illustrated in Table 6. The elastic modulus and ultimate tensile strength of the SSGCB are shown in Fig. 17. The elastic modulus of SSGCB improved and increased with the increase of steel content. The elastic modulus of SSGCB-8 was 67.4 GPa, which was 49.8% bigger than that of GFRP. However, the tensile strength declined because of the low strength of steel, and the elastic modulus would decrease quickly after the steel was yielded. Therefore, the ultimate tensile strength of SSGCB was lower than that of GFRP and decreased with the increase of steel content. The ultimate tensile strength of SSGCB-8 was 663.4 MPa, which was 17.0% lower than that of GFRP.

Figure 18 demonstrates the stress-strain curves of SSGCB. In the nonlinear stages, characteristics of steel-sheet-GFRP composite bars’ stress-strain curves became more obvious with the increase of steel content. The results suggested that the deformation effect of SSGCB was better, and the mechanical properties were more stable. Besides, the test results were in good agreement with the numerical simulation results, which indicated that the adhesion between the steel sheet and fiber was pretty good. The GFRP stress-strain curve was linear, and there was no yield phenomenon. However, the stress-strain curves of the SSGCB were nonlinear. These curves were presented a ‘convex’ trend, and the trend was increased with the increase of steel content. The uptrend of curves could be slow, which showed a yield phenomenon that was more obvious with the increase of steel content. And the SSGCB-8 had a distinct plastic stage under the uniaxial tensile load. The elastic modulus of steel and SSGCB declined sharply when the steel entered the plastic stage. After the fracture of glass fiber, the load was transferred to the steel, and then the steel was destroyed rapidly.

As the shear test results of the SSGCB are tabulated in Table 6. The average compressive strength of the SSGCB is shown in Fig. 18. There was no brittle failure from the failure mode of the specimen, and no steel sheet was pulled out of SSGCB. This phenomenon indicated that the bonding property between the steel sheet and fiber was good. And the shear strength increased with the increase of steel content, mainly owing to the shear capacity of steel is better than that of GFRP. The shear capacity of SSGCB improved when steel was added to SSGCB, and the shear strength was 22.2% greater than that of GFRP. The measured data shows that the compressive strength of the steel core added to the composite bar has decreased slightly, with a maximum decrease of 11.5%, but the compressive strength will increase as the steel core content increases. Combining the failure morphology, due to the uncoordinated lateral deformation of the steel core and fiber during the compression process, the resin’s bonding performance is not enough to provide the uncoordinated lateral force, which leads to the separation of the materials under the action of large compressive stress. The fiber and the steel sheet are destabilized and damaged. It is the existence of this uncoordinated deformation that causes the overall compressive strength of the composite bar to decrease, but at the same time, the steel core itself has better compressive performance, which will contribute to the compressive strength of the SSGCB composite bar. The compressive strength will increase with the increase of the steel core content.

To sum up, according to the combination form analysis from mechanical evaluations and experimental test, the optimum combination form was SSGCB-8, which had the best comprehensive performance of elastic modulus, displacement, stress-strain, tensile strength, and shear strength.

4 Conclusions

The combination of steel sheet and GFRP for improving the tensile modulus of the composite bars has proven to be a viable technology for applications in concrete structures. Steel sheet was used to replace steel core/wire, which would increase the contact area between GFRP and steel to improve the bonding property so as to achieve the purpose of coordinated deformation for composite materials. Three-dimensional finite element models and mechanical tests were conducted on SSGCB to evaluate its mechanical properties, and the following conclusions were drawn based on the results presented and discussed.

1) In a combination form analysis of SSGCB, based on the interface characteristics of GFRP and the bond mechanism of the composites, the concept of SSGCB was put forward to improve the tensile modulus and ductility of the composite bar. The composite principle, materials selection, and preparation technology of SSGCB were described in detail.

2) The selection and comparison of SSGCB based on mechanical property analysis were carried out. The elastic modulus of SSGCB was greater than that of GFRP at the elastic stage, which gradually decreased with the increase of stress. After the yield of steel, the elastic modulus of the SSGCB decreased and presented ductility characteristics.

3) The elastic modulus and ductility of SSGCB increased with the increase of steel content. When the steel was yielded, the modulus of the composite decreased and illustrated the ductility characteristics. Furthermore, the nonlinear stage characteristics of SSGCB-8 were much more evident than that of others. At the elastic stage, the main force component of SSGCB was the steel sheet. With the enlargement of load, the steel sheet was yielded, and fiber became the main forcing component.

4) According to the selection and comparison of SSGCB based on the mechanical properties experiment, the tensile strength decreased because of the low steel strength, and the elastic modulus would decrease quickly after the yield of steel. The ultimate tensile strength of the SSGCB was lower than that of GFRP and decreased with the increase of steel content. The shear capacity of SSGCB would be improved when steel was added into the composite bar, which indicated that the bonding performance between the steel sheet and fiber was good.

5) The tensile failure process illustrated an obvious phase and a stable post-yielded stiffness occurred after the steel sheet was yielded. The primary failure mode of SSGCB was that fiber was broken away from the steel sheet when steel yielded, and the outer glass fiber was scattered.

6) In the compression process, due to the inconsistent lateral deformation of the steel core and fibers, the compressive strength of SSGCB is lower than that of GFRP bars, but it will increase with the increase of the steel core content.

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