1. Introduction
Reactive powder concrete (RPC) is one of the most advanced cement-based composite materials in the group of ultra-high performance concretes (UHPC). Such composites can provide a compressive strength above 140 MPa, usually about 200 MPa, as already observed by Richard [
1], Aïtcin [
2], and Zdeb and Śliwiński [
3]. In these innovative concrete types, very high material strength is achieved by replacing coarse aggregate with finely ground quartz of grain sizes from 1 µm to 4 µm and sand with grains from 0.2 to 0.4 mm. Improved homogeneity and compactness can be achieved through the ideal grading by adding also silica fume. By using superplasticizers, it is possible to reduce water and cement ratio (w/c). Its low value of about 0.2 ensures that a significant part of the cement is not hydrated. Improved granular compactness through the use of powders with a complementary grain size distribution, including silica fume, eliminates the presence of a transition zone between sand and slurry particles and ensures very low porosity of the material.
A new group of RPC composites is reinforced with short, thin steel fibers, usually 0.2 mm in diameter and 12 mm long emerged, in a quantity of approximately 2% by volume. As a result of further research in the 1990s, one of the first composite, according to [
4], was called Ductal, which means a whole family of composites with excellent properties, with a compressive strength going from 180 MPa to 230 MPa, and flexural one is ranging between 30 MPa and 50 MPa. The composite is also characterized by high tensile strength, excellent impact, as well as abrasion resistance and can provide extraordinary durability [
5]. Potential Ductal modeling and testing were specified by Adeeb et al. in [
6]. Examples of construction applications of this bendable composite can be found also in [
7]. But soon, many other commercial products of this type were documented by Zych [
8]. In this study, especially unconventional and more environmentally resistant basalt fiber’s effects on RPC properties were examined and compared to steel admixtures.
The favorable properties of RPC composites permit to create thin profiles and to overspan considerable structural areas. Aïtcin [
9], Perry and all [
10], Li and Kanda [
11] illustrated in an exemplary way light and slender structures as well as innovative structural forms, simultaneously durable and resistant to corrosion. Moreover, precast lighter elements, compared to massive and heavy structural parts made of traditional materials, are preferred today. Blais and Couture [
12] illustrated the application of precast RPC elements of complex shapes, which are only 20 mm thick for the facade, roof, and coating of buildings. Mazzacane et al. [
13] gave details on the roof of Jean Bouin stadium in Paris, consisting of 3600 precast RPC panels, only 35 mm thick, covering an area of 23,000 m
2.
Buildings constructions are often in need of spanning greater distances between vertical supports. Composite steel beams cooperating with the concrete flooring deck, placed on the upper flange, represent a preferable solution. The interiors of the building can be better used without restrictive vertical columns. In particular, trusses may allow the placement of various actually required several installations into the free space between the cords. Besides, steel-concrete composite bridges are used as an alternative to ordinary structural systems because of their ability to adapt their geometry to design constraints and the possibility of using favorable properties of both structural materials [
14]. However, in the case of presently preferable truss main girders, the transmission of longitudinal forces takes place only discontinuously in the nodes, where the web members are connected to the compressed chord [
15]. But in actually available specifications, this design situation is treated only rather approximately. The phenomenon of slippage at the steel-concrete interface depends on many factors, including the type and size of the shear connectors, their spacing, the type of concrete slab, and the strength of the concrete at the location of the connectors. Combining shear connectors and RPC in these locations can be an effective solution to prevent the relative slip at the steel-concrete interface when particularly precast deck is used. As it may be uneconomical to make a concrete slab entirely of such concrete, the use of RPC to fill element joints with steel shear connectors seems greatly appropriate. By increasing joint stiffness, higher composite effectiveness in steel-concrete elements can be achieved, which has also been confirmed by other authors [
16,
17].
Thanks to its properties, RPC is increasingly being used in widely understood composite structures in which layers of concrete of different classes are joined together. This allows for the strengthening of existing structures [
18] but also for the production of composite structural elements with less weight and dimensions and interesting mechanical properties [
19].
4. Discussion of Results
The effects of processing additions were analyzed based on the work conducted. The composition of the RPC mix was analyzed using X-ray fluorescence spectroscopy. The data shown in
Table 1 complied with the standard chemical requirements. The cement, silica fume, quartz powder, and sand were analyzed by laser. Examination of
Figure 1 shows that there was some increase in quartz sand particles in the samples. Relatively significant effects of steel fibers appeared to be involved in the affected systems slightly more than the basalt ones. The mix parameters that seemed to be most sensitive to changing cementitious systems were compressive and flexural strengths, as shown in
Table 5. The results observed in changing flowability in
Table 3 indicated only a few important influences of admixtures. Similarly, there did not appear to be a substantial difference between mixed frost resistances summarized in
Table 6. None of the systems exhibited significant damage under testing. But the concrete density variation of RPC was larger, especially in those containing steel fibers, as shown in
Table 4. It is evident from
Table 7 that total concrete porosity was generally greater in systems with steel fibers and lower when made with basalt ones.
The data generated from compressive tests of ordinary and high-strength concrete reported in
Table 8 were inside reasonable performance parameters with acceptable scatters. The plots of Poisson’s ratio in
Figure 3 did not indicate significant variability from the presence of fibers in the concrete samples. But Young’s modulus plots in
Figure 4 were more separated and did illustrate the effect of steel fibers on the RPC performance.
The connector and the concrete strength are the main factors affecting the behavior of shear connections at the steel-concrete interface in composite member. As actual design methods are based on the test results of studs embedded in normal strength concrete, the load-slip behavior and the shear capacity in higher strength fiber-reinforced concrete were necessary to evaluate the appropriate design. In this study, the tests were conducted by employing six specimens, which differed in material properties of the concrete and stud, as shown in
Figure 6,
Figure 7 and
Figure 8. Experimental push-out tests were used to evaluate both the shear capacity and the load-slip curves of the connection, given in
Figure 9 and
Figure 10. In the ordinary concrete samples, the cracks were appeared on the surface slightly, concentrated mainly around the studs, because connectors failed by shear, as shown in
Figure 9. The samples with connectors cast with RPC in the square holes were damaged by earlier cracking of the normal concrete region, as shown in
Figure 10, at a significantly higher ultimate load.
The results of the finite element models were compared with push-out tests and values provided by testing. The graphical overviews in
Figure 9 and
Figure 10 allowed observing negligible differences of test and theoretically stated outcomes. Besides, the failure modes were appeared pretty similar.
5. Conclusions
Two reactive powders concrete reinforced with the steel or basalt fibers and control normal concrete specimens were investigated for structural applications in this study. High performance fiber-reinforced reactive powder concrete revealed its exceptional mechanical and cracking resistances. Thus, the RPC using fine steel or basalt fiber aggregates could meet the mechanical properties requirements for practice in structural applications after being cured in the ordinary curing condition.
A maximum mean compressive strength of 198.3 MPa at 28 days curing age was produced using finely dispersed steel fibers. Compared to the control mix of normal concrete, a 33% larger compressive strength was achieved. Nevertheless, basalt fiber reinforcement of concrete with correspondingly mean values of 135.6 MPa had a less significant impact on the compressive RPC strengths, but with better resistance in an aggressive environment. The flexural strengths of the RPC were, in all cases, greater than those of normal concrete.
Fiber-reinforced concrete is characterized by a dense microstructure and thus very high strength as well as excellent durability properties. This enables a material saving and weight-reduced, slim construction method that opens up completely new possibilities and areas of application predominantly in precast constructions. But RPC is today still relatively expensive. Besides, a combination with normal concrete may minimize the construction cost. Especially, the shear connection of composite systems can be designed using two types of concrete. The deck slabs could be cast as commonly of normal strength concrete, while the adjacent space of connector location could use higher performance concrete. Executed push-out specimen testing proved to describe suitably shear connection behavior as well as the resistance of shear studs and concrete slab. Besides, post-cracking tensile capacity and pseudo-ductility with work-hardening behavior, accompanied by multiple micros-cracks, were observed in experimental and theoretical studies. This new hybrid connection provided outstanding strength and resistance to transfer shear at the steel-concrete interface. This type of innovative shear connection at the steel-concrete interface could be attractive, especially in the case of precast decks of building or bridge composite structures. The mutual assembly of two different structural parts was simplified and executed faster.
Numerical models making use of the finite element method were developed to simulate the behavior of shear connection in composite structures of steel and concrete of different forms. The comparison with experiments confirmed that these analytical tools were able to describe the real behavior of composite structures, including concrete cracking, crushing, and reinforcement yielding. Therefore, the above procedures may potentially provide more powerful means in practical design.