1. Introduction
Concrete, as one of the most widely used building materials, consumes large amounts of raw materials and generates high levels of carbon emission. Therefore, the development of concrete with high mechanical performance and superior durability is of great necessity [
1]. The addition of steel fibers in reactive powder concrete (RPC) or ultra-high-performance concrete (UHPC) is a common method to improve its strength, toughness, energy absorption capacity and ductility after crack formation [
2,
3,
4,
5,
6,
7,
8,
9]. On the other hand, the adding of steel fibers may introduce a negative impact on the workability by increasing the air content of the concrete [
6]. RPC or UHPC is a type of construction material made of a high content of cementitious materials and fine aggregate with a low water/binder ratio [
2]. Steel fibers can not only improve the mechanical performance of RPC but can also effectively restrain the development of cracks and provide post-cracking strength and toughness, thus increasing the service life [
10,
11]. Al-Baghdadi et al. [
12] found that the steel fiber can advance the brittleness ratio of concrete. Furthermore, Lau et al. [
13] pointed out that the fatigue life of concrete can be improved by at least 135% with the addition of fibers at 0.5% volume content. Steel fibers present with varying geometrical forms, lengths, and sizes. The commonest shapes include round-straight, crimped, dumbbell, end-hooked, and twisted.
The addition of steel fiber greatly increases the properties of RPC, especially the ductility and energy dissipation [
14,
15,
16,
17]. The cracks in the concrete reinforced with steel fiber can be restrained due to the bridging effect at the crack locations [
18,
19]. Therefore, the residual bond strength of concrete is improved via the addition of steel fibers [
16]. Furthermore, the steel fibers can restrain shear cracks and delay or even prevent shear failure [
17]. Therefore, the specimens reinforced with enough steel fiber can have large deformation and large amount of energy dissipation even under a strong shear effect [
18]. The specimen reinforced with steel fiber can stay intact after failure, with only few and small cracks in the concrete [
14], thus greatly improving the ductility of RPC. Furthermore, the steel fibers help to dissipate energy during progressive damage, fracture, and steel fiber pullout [
20]. The work of both Kravchuk et al. [
20] and Landis et al. [
21] found that fiber pullout dissipates much more energy than concrete matrix cracking. Based on the above literature, the addition of steel fiber in the concrete is highly beneficial in improving the seismic performance of concrete structures.
It was reported that the properties were greatly influenced by the aspect ratio [
22,
23], tensile strength [
23], shape [
24,
25,
26,
27], and volume content [
28,
29,
30] of steel fibers, and the shape and volume content are the two most frequently studied factors. Generally, the strength of RPC or UHPC improves gradually with the increase of steel fiber content [
31,
32,
33,
34,
35,
36,
37,
38]. Some research reported that the strength of UHPC improved gradually in proportional to fiber content up to 3% [
39]. Wu et al. [
40] found that the compressive strength was improved by 8–32% by adding 1–3% straight steel fibers into the UHPC. However, a high steel fiber fraction of over 3% may impair the properties of UHPC [
41,
42]. High fiber content may bring too much air into the concrete, thus reducing the workability and mechanical performance of concrete. Therefore, the maximum steel fiber content is usually 3% by taking the cost, working performance, and mechanical performance into consideration [
39]. However, the maximum steel fiber content varies in different conditions. Lehner et al. [
43] pointed out that the threshold of the fiber fraction is 1.4% for steel-fiber-reinforced concrete in a chloride environment as high fiber content increases the diffusion coefficient.
In addition to the volume content, previous research has found out that the shape of the steel fibers has a significant influence on the properties of the concrete [
33,
44,
45,
46]. Deformed steel fiber is more effective than round-straight steel fiber in terms of improving the mechanical properties of concrete [
46], with a much higher pullout strength due to the bridging effect [
47,
48,
49]. Previous research has concluded that the tensile strength of UHPC varies from 5% decrease [
3,
50] up to 40% increase [
50,
51] compared with that for straight fibers. Kim et al. [
51] found that about 20–40% improvement in tensile strength of concrete can be obtained by using three different deformed steel fibers compared with only straight steel fibers. However, some other research has reported that deformed steel fiber can provide more than three times greater fiber–matrix bond strength than that of straight fibers [
44,
52]. Katzer et al. [
53] used crimped steel fiber to create high-performance concrete based on a mix of waste and natural aggregates, which was proved to be practicable and efficient. Wu et al. [
40] applied three different shaped steel fibers, i.e., straight, corrugate, and hooked, with volume content ranging from 0 to 3%. The results showed that the steel-fiber-reinforced UHPC presented the best performance with the hooked shape at a volume fraction of 2%. However, Zhang et al. [
26] found a different result. In the work of Zhang et al., the corrugated steel fiber had the best reinforced effect on the strength of concrete. Therefore, there is still a debate on the influence of steel fiber shape on the concrete.
In addition to fiber content and fiber shape, Wu et al. [
44] pointed out that the hybrid use of two types of deformed fibers is more effective compared to the utilization of only straight fibers. Meng and Khayat [
47] suggested that a hybrid combination of steel fibers is more effective in improving UHPC’s strength than increasing the fiber content. The combination of longer and hooked-end fibers is the most common hybrid combination of steel fibers [
47,
54]. The steel fiber hybrid can not only be created with fibers of different shapes but also with those of different types, lengths, and aspect ratios. Therefore, there are many possible schemes for fiber hybrids. Though many studies have approved that the fiber hybrid is a favorable method to improve the mechanical performance of concrete, controversial debates still exist, and other researchers demonstrated that hybrid steel fibers might lower the strength of UHPC [
55,
56].
So far, all the research was carried out on the open steel fibers, such as the straight, crimped, hooked, etc. The authors proposed a new type of steel fiber, i.e., the closed steel fiber, which is a potential fiber that can be applied in RPC and UHPC. However, the authors have searched and found that there is still a lack of study on the performance of RPC with closed steel fibers. The shape of closed steel fibers is totally different from that of the existing fibers, and the ring of closed steel fibers affects the workability and performance of RPC via not only confining but also friction. Therefore, the closed steel fiber may produce a different improvement in the RPC. The existing research does not provide references for the closed steel fibers. In order to realize the properties of RPC reinforced with closed steel fibers, experiments on this topic should be carried out. The objective of this research is to investigate the effects of three different shaped closed steel fibers (circular, triangular, and rectangular) with different fiber contents (0, 0.5%, 1%, 1.5%, and 2%) on the working and mechanical performance of RPC. The improvement mechanism of closed steel fiber was also developed. Furthermore, the properties of RPC reinforced with three different shaped open steel fibers (short-straight, long-straight, and semicircular) with different fiber contents (0, 1%, 1.5%, and 2%) were compared.
2. Theoretical Hypothesis
According to the literature review, the fiber content, geometrical difference (shape, length, and aspect ratio), distribution, and orientation of steel fibers has a significant influence on the mechanical properties of concrete. The more distorted and twisted the shape of the steel fibers, the more remarkable the enhancement of strength and crack suppression on concrete. In the literature review, the research was carried out mainly on the open and straight steel fiber, and great improvement was found. Therefore, the authors propose that if the straight steel fiber was replaced with closed steel fiber, a much greater improvement in the performance of RPC will be realized. In this study, a hypothesis that the increase of the closure degree of steel fiber improves the restrain effect of steel fibers on concrete was proposed, as shown in
Figure 1. When the shape of the steel fibers is closed, then the concrete enclosed by the steel ring will play an essential role in the RPC’s mechanical performance. Research on concrete filled in a steel tube reported that the performance of the concrete was enhanced remarkably [
57,
58]. The effect of rectangular and triangular fibers was also studied to compare with the circular ones in this research.
As aforementioned, the concrete enclosed by the closed steel fiber affects the performance of concrete. As presented in
Figure 1, the concrete enclosed by rectangular steel fiber and circular steel fiber is 1.71 and 1.69 times of the concrete enclosed by triangular steel fiber. Therefore, it can be confirmed that the influence of triangular steel fiber on the RPC will be obviously smaller than that of the rectangular and circular steel fibers.
Figure 2 shows the improvement mechanism on the mechanical performance of concrete with different shapes of steel fibers. The figure clearly illustrates that the straight steel fiber restrains the development of cracks of RPC mainly through friction. The steel fiber can be easily pulled out under low tensile force. When the steel fiber is bent into a semicircular shape, the arc fiber not only produces friction with the concrete when it is pulled out but also produces an extrusion effect on the concrete. Therefore, the combined action of friction and extrusion produces a greater pullout force. When the steel fiber is totally closed, the circular steel fibers will provide more significant extrusion and restriction. The concrete enclosed by the closed steel fiber will present a higher performance. Therefore, the mechanical performance of RPC will be improved remarkably. To prove this hypothesis, experimental tests on RPC reinforced with different shaped steel fibers with different volume contents were carried out.
5. The Utilization of Hybrid Fibers of Open and Closed Steel Fiber
Rambo et al. [
38] found that the fiber hybridization of straight and hooked steel fibers can effectively limit the initiation and propagation of microcracks. In this research, the short-straight steel fiber can improve the strength of RPC by its support and crack resistance in the concrete matrix, and the closed steel fiber can improve the concrete strength by the protection of the closed section.
Table 6 shows that the closed and open steel fibers work differently at improving the compressive and flexural strength of RPC. In addition,
Figure 16 displays that the short-straight steel fiber uses its anchoring performance to connect the concrete enclosed by the circular steel fibers, thus improving the total performance of RPC. At the same time, the concrete enclosed by the circular steel fiber can also provide more stable support and anchorage for the short-straight steel fiber–reinforced RPC. Therefore, the compressive and flexural strength of the RPC can be improved by making full use of the advantages of straight and circular steel fibers, which is verified in
Figure 17. The figure shows that the flexural and compressive strength of the RPC is greater when using hybrid fibers rather than when using only a single type of fiber. The figure also shows that the hybrid utilization of 0.5% short-straight fibers and 1% circular fibers works better than the hybrid application of 1% short-straight fibers and 0.5% circular fibers.
The working mechanism of RPC reinforced with hybrid steel fibers is illustrated in
Figure 18. The circular steel fiber protects the concrete enclosed by the steel ring while the straight steel fiber connects the concrete protected by the circular steel fiber, thus forming a dumbbell shape. The dumbbell works like a huge hooked steel fiber, which has been proved effectively in enhancing the flexural and compressive strength of concrete [
47]. The failure mode of the dumbbell shape can be proved by
Figure 16. The circular steel fiber constrains the concrete and forms the dumbbell end, while the straight fiber works as the middle part of the dumbbell. Compared with the failure patterns of only the single steel fiber shown in
Figure 18, the hybrid fibers provide more bridging effect. As displayed in
Figure 19, the single type of steel fibers only protects and restrain the cracks around the fiber.
6. Conclusions
In this study, an experimental investigation on the geometrical shape of steel fibers, including three different open and three different closed steel fibers, was carried out in terms of fluidity, flexural strength, pullout strength, and compressive strength. The conclusions are summarized below:
(1) The results show that the geometrical difference and the enclosed area formed by closed fibers have a significant influence on the fluidity, flexural strength, and compressive strength of RPC. The fluidity of triangular steel fiber–reinforced RPC is the best, followed by that of the rectangular fibers, and then the circular ones. However, the shape of the circular fibers works the most efficiently at improving the flexural and compressive strength of RPC. The smaller the enclosed area formed by the closed steel fiber, the better the fluidity while the worse the mechanical performance.
(2) Among the three types of open steel fibers, the short-straight steel fiber works the most efficiently in improving the compressive strength of RPC while the semicircular steel fiber works more efficiently in improving the flexural strength of RPC.
(3) Under the same volume content, the compressive strength or RPC is improved more than its flexural strength.
(4) At the same volume content of steel fibers, the open steel fibers work more effectively than the closed steel fibers at improving the compressive strength of RPC while the closed steel fibers are good at enhancing the flexural strength of RPC.
(5) The open steel fibers enhance the mechanical performance of RPC via its anchoring performance as a result of friction, while the closed steel fibers work both by confining the RPC and providing friction to improve its strength.
(6) The hybrid of steel fibers improves RPC’s mechanical performance to a higher level, and the compressive and flexural strength of RPC with 0.5% short-straight and 1.0% circular steel fiber reached 147.2 and 17.3 MPa, respectively.