1. Introduction
Steel-concrete composite structures combine the excellent material performance of steel and concrete and have the superiorities of high bearing capacities and a short construction period [
1,
2,
3]. Connectors are used to resist the sliding and lifting effect between steel girders and concrete decks [
4,
5]. Stud connectors are the most common shear connectors in bridge engineering because of their same shear performance in all directions [
6,
7]. Cracks often appear in ordinary concrete decks of traditional composite structures, especially in the hogging moment area of continuous composite structures; thus, studs get corroded and seriously affect the bearing capacity and durability of composite structures [
8,
9]. Steel fiber-reinforced cementitious composite (SFRCC) has high strength, excellent tensile hardening ability, and good ductility [
10,
11,
12,
13]. Replacing ordinary concrete bridge decks with SFRCC bridge decks can easily solve the problem of cracking in traditional composite beam bridge decks, decrease the dead weight of composite beams, and increase the span of bridge structures.
Static tests and numerical simulations have been extensively adopted to probe the mechanical performance of studs embedded in normal strength concrete (NC). Buttry [
14] performed a static experiment on steel-NC composite structures and demonstrated that the shear bearing resistance of studs in NC was mainly dominated by the material property of the concrete. Ding [
15] studied the static behavior of studs through static tests and FE simulations and noted that the shear bearing capacity of each stud in the bidirectional specimens improved when the size and tensile strength of the studs increased, whereas the stud’s aspect ratio had no significant impact on the ultimate shear capacities of the studs. In addition, push-out tests have been widely performed [
16]. It is suggested that the influence of steel beam type on the ultimate shear capacities of stud connectors cannot be ignored, and the quality of stud welding should be strictly controlled. Research on the mechanical behavior of stud connectors embedded in steel-NC composite beam is relatively mature, and some practical formulas for the ultimate shear resistance and load-slip curves of studs have been developed by considering stud geometry and concrete properties [
17,
18].
In recent years, steel fiber-reinforced cement-based composite materials have come to be used extensively in construction engineering. Researchers have proposed replacing NC with engineering cementitious composite (ECC) and ultra-high-performance concrete (UHPC) to enhance the longevity and traffic-carrying capacities of steel-concrete composite structure bridges [
19,
20,
21]. Zhu [
22] carried out two static tests on composite beams for different strengths of concrete and connectors and reported that ECC significantly limited the extension of cracks in the negative moment area of the composite structure and also enhanced the static performance of the composite structures. Guan [
23] employed an FE model, which was verified by experimental results, to explore the mechanical behavior of studs in ECC and indicated that the interfacial bonding force between concrete decks and steel girders was conducive to improving the original shearing stiffness of ECC, and the diameter and tensile strength of studs effectively improved the stud shear capacity; however, opposite trends were noticed for the stud length and the ECC strength. Wang [
24] experimentally investigated the mechanical properties of large-diameter studs embedded in UHPC and posited that UHPC was fitted properly with the large-diameter studs. An accurate formula was put forward by taking the stud diameter into account to calculate the load-slip relationships of studs in UHPC. In addition, the breaking of the composite structure mainly occurred at the boundary of studs and welds, and a simplified expression for the ultimate shearing resistance of studs in UHPC was proposed by taking the influence of the weld ring into account [
25].
Although ECC and UHPC can improve the bearing resistance and service life of steel-concrete composite structures, they still have some deficiencies. ECC has good strain hardening capacity and ultra-high ductility; however, its compressive strength is almost similar to that of NC; thus, the ultimate shear capacities of steel-concrete composite beams cannot be significantly improved [
26]. UHPC faces the problems of relatively high material preparation cost and complex structural design, and its low water-to-binder ratio causes large self-shrinkage [
27]. Therefore, SFRCC could be a better choice for steel-concrete composite beams. As SFRCC has better crack resistance and toughness, the dominant crack may not be apparent until it is very close to the final fracture, and it can greatly improve the shear resistance of different components [
28,
29]. The steel matrix mainly transmits the load through the tensile strain hardening effect and the fibers bridging effect. The shear stress on the interface between the steel fibers and the surrounding matrix (bond strength between steel fibers and matrix) is a key parameter in the bridging effect, and concrete fails once the steel fibers either break or are completely pulled out from the concrete [
30,
31]. The presence of steel fibers supports the redistribution of tensile stress, prevents the propagation and opening of diagonal cracks, and the total yield bearing capacity of a steel fiber-reinforced composite beam can exceed the applied load when the steel bar starts to yield; thus, the composite beam exists in a multi-crack development mode and experiences an improvement in ductility [
32,
33]. SFRCC is mainly used for crack control and reinforcement works. However, the stress mechanism of steel-SFRCC composite beams is still unclear. Therefore, it is necessary to deeply explore the working performance and different influencing factors of studs embedded in steel-SFRCC composite beams to improve the promotion and application of steel-SFRCC composite structures.
Overall, the current research on the shear performance of stud connectors mainly focused on the studs in traditional steel-concrete composite structures, and it seems that there are few reports on the stud shear connectors in SFRCC composite beams. In order to investigate the shear performance and failure mechanism of stud shear connectors embedded in steel-SFRCC composite beams, the shear performances of six steel-NC and six steel-SFRCC specimens were investigated by push-out test for different stud diameters and lengths. The failure mode, load-slip relationship, shear capacities and stiffness of the push-out specimens and stud strains were examined. In addition, the FE models of different specimens were developed and verified by push-out test results, and the effects of concrete properties, stud diameter, stud yield strength, and stud aspect ratio on the shearing behavior of stud shear connectors were explored. Finally, according to the as-obtained push-out experiment and FE analysis results, empirical formulas were presented to forecast the load-slip curves and ultimate shear resistance of stud shear connectors embedded in the steel-SFRCC beams. This study presented a new idea for improving the mechanical performance of steel-concrete composite beams. In addition, the shear mechanism of studs in steel SFRCC composite beams was studied in detail. The research results could provide a reference for the popularization of SFRCC in composite beams.
6. Conclusions
Push-out tests and FE simulations with different parameters were conducted to research the shear performance and failure mechanism of stud shear connectors embedded in steel-NC and steel-SFRCC composite beams. This study could promote the application of steel-SFRCC composite beams in civil engineering. The main findings of this research are listed below.
(1) All steel-NC and steel-SFRCC push-out specimens ruptured due to stud shearing failure with a smooth cross-section, and the concrete at the corresponding position of the stud root was also crushed at the same time. The failure modes of the steel-NC and steel-SFRCC push-out specimens were similar; however, oblique cracks propagated further in the NC slabs. Contrary to the steel-NC specimens, there were no obvious cracks in the SFRCC slabs.
(2) The mechanical performance of the stud shear connectors was mainly dominated by the concrete compressive strength and the stud diameter and tensile strength. The studs had larger strains at the root and smaller strains at the end, indicating that the stud root was under a high-stress state and prone to fracture.
(3) Based on the as-obtained push-out experiment and FE simulation results, empirical formulas to predict the load-slip curves and shear resistance of stud shear connectors in steel-SFRCC were presented. The stud connectors load-slip curve calculation formula considered the effect of the concrete strength factor, and the stud shear resistance calculation formula was suitable for both NC and SFRCC and had higher precision and a wider application range than previous formulas.