1. Introduction
In cases where the major tensile stresses in the shear region of a RC beam exceed the concrete tensile strength, diagonal cracks finally cause failure. Several experimental and theoretical studies have confirmed that the key parameters affecting the response of RC beams are the type and direction of external loads, the shape and dimensions of the specimen, the strength of the concrete and steel reinforcement, the shear span to depth ratio and the reinforcement arrangement [
1,
2,
3,
4,
5,
6,
7,
8]. Depending on the factors, the ultimate loads and failure modes can change. In the most favourable case, ductile flexural failure follows, and in the worst case sudden shear failure occurs a short time after the first diagonal crack forms due to the brittle concrete properties. Research emphasizes that the strength and ductility of RC beams are increased by using fibres as an additive to plain concrete [
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19]. This is thanks to the greater resistance to cracking of fibre-reinforced concrete (FRC). This post-cracking tensile strength also tends to reduce crack sizes and spacing [
20]. Generally, FRC contains a single type of fibre. The use of at least two types of fibres in a suitable combination can potentially not only improve the concrete properties, but also result in performance synergy. The combination of fibres is often referred to as hybrid fibres [
21,
22]. Positive interaction between different fibres in hybrid FRC exceeds the sum of the single fibre properties. The first type of fibre is smaller and therefore bridges microcracks as well as controls their growth, which results in a higher tensile strength of the composite. The second type of fibre is larger and arrests the propagation of macrocracks, thus leading to a significant improvement in fracture toughness [
21]. Furthermore, it has been revealed that it is possible to replace conventional shear reinforcement by steel fibres and achieve similar ductility and strength [
18,
23,
24,
25,
26,
27,
28,
29,
30,
31]. Numerous tests have also been carried out to select the type of fibre and optimum volume percentage, as well as to ensure the shear strength of RC beams without brittle failure [
13,
18,
19,
20,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42].
Nevertheless, the increase in shear strength depends not only on the fibre volume fraction, but also on the aspect ratio as well as the fibre anchorage conditions. From the workability point of view, the use of smooth and stocky fibres is effective. However, hook-ended or crimped steel fibres are most commonly used in proportions of 0.5 to 1.5 vol % as they effectively bridge cracks due to their high pull-out resistance and consequently FRC has higher tensile deformations than plain concrete. In addition, owing to the increased tensile strength of FRC, there are large tensile strains in the longitudinal rebars, which result in high ductility of the beams. The increase in shear strength varies considerably depending on the beam geometry and material properties (from 12% to >100%) [
33]. The ultimate shear strength of FRC beams diminishes as the shear span to depth ratio increases and rises with an increasing flexural reinforcement ratio and concrete compressive strength. The other tests suggested a scale effect linked to the height of the steel fibre RC beam without stirrups. For higher beams, wider cracks at failure were observed [
43]. Dinh et al. [
30] presented a model to estimate the shear strength of hook-ended steel FRC beams without stirrup reinforcement on the basis of large-scale beams experiments. They assumed that shear stress carried in the compression zone and tension transferred across diagonal cracks by steel fibres. The reduced diagonal crack spacing and width increase the aggregate interlock effect. Additionally, the presence of steel fibres indicates that the usage of fibre reinforcement could potentially lead to a reduction of the size effect in shear beams without stirrups within the beam depths range from 455 to 685 mm. Spinella [
44] developed a simple model for predicting the shear capacity of FRC beams without stirrups under transversal load taking into account the abilities of steel fibres to bridge tensile stress across crack as well as to contain shear crack slips. The ability of fibre for mitigating the shear size effect was taken into account by the geometrical characteristics of fibres.
Although a number of experimental studies have been carried out so far to assess the shear strength of steel FRC beams, there is little research on the shear strength of steel fibre-reinforced high-strength concrete members [
35,
37,
40,
45]. Therefore, further experiments are needed to evaluate the behaviour of fibre-reinforced beams made of high-strength concrete. The use of steel fibres in such concrete is particularly attractive as high-strength concrete brittleness may be limited by the fibre addition [
46,
47]. Despite the significant increase in the cost of a HPC mix, the application of this steel fibre combined with another fibre type, e.g., polypropylene fibre, may be interesting to provide a structural alternative to traditional shear reinforcement. The typical volume percent of polypropylene fibres ranges from 0.1% to 0.3% [
48], but even the lower percentage of polypropylene fibres can improve crack control in the early age of curing and early stages as well as increase fire resistance.
The influence of steel fibres in increasing the shear strength of RC structural members has been recognized in the MC2010 [
49] and is defined as shear strength extension recommended in Eurocode 2 [
50] by introducing a modified longitudinal reinforcement ratio. Researchers have developed empirical equations for predicting the ultimate average shear stress [
24,
33,
35,
39]. The empirical models of shear strength calculation are a function of several factors such as specimen shape and size, shear span to depth ratio, longitudinal reinforcement ratio, fibre aspect ratio, fibre volume content and the strength of the concrete and reinforcing steel. The first group of models predicts shear resistance by establishing a separate contribution of concrete and fibres, while the second group is based on global improvement of FRC shear strength [
31]. An altered method based on the strain approach was suggested in [
51]. With respect to the analytical response of plain or FRC beams under the impact of flexure and shear, some models have also been also developed, whose main purpose was to determine the moment-curvature diagrams [
52]. These models are based on the finite element method within the hypothesis of the plane section theory or of approximate models, taking into account the compressive and tensile behaviour of FRC. Spinella et al. [
53] reported the effectiveness of nonlinear finite element method to predict the structural behaviour of RC beams with steel fibres as well as beams with stirrups and steel fibres as transverse reinforcement. The experimental and numerical results highlighted that the transverse reinforcement provided by stirrups and steel fibres is an optimum solution in terms of the costs and structural performance. However, there is not a large number of test results to evaluate the existing design procedures for estimating shear strength.
This study reports mainly experimental investigations of the implications of using steel and polypropylene fibres as shear reinforcement in RC flexural beams. The tests described in this paper were carried out on six beams, with openings in the shear regions and without openings, made of HPC with/without stirrups under a four-point bending configuration. All the beams were reinforced with compressive and tensile longitudinal steel deformed rebars, while conventional RC beams were also prepared with transversal stirrups. The main purpose of this study is to investigate the mechanical behaviour at shearing of RC beams with/without openings, made of HPC (up to 120 MPa), containing long steel fibres and short polypropylene fibres in order to determine the possibility of using different fibre combinations to replace the stirrups in order to improve the shear and flexural strengths, crack control, failure modes as well as ductility. Moreover, based on the experimental results [
54], a small amount of polypropylene fibres significantly improved the resistance to high temperature (up to 1000 °C) of hybrid steel/polypropylene fibre-reinforced HPC compared with HPC and steel fibre-reinforced HPC. The response of the RC beams was evaluated based on the crack pattern results, load at the first cracking, ultimate shear capacity, failure modes, toughness, over-strength, ductility, initial stiffness, maximum strength as well as strains. The experimental evidence has confirmed that the addition of steel and polypropylene fibres improved the mechanical response, both in terms of flexural and shear strengths, as well as the flexural ductility of the beams.