Experimental Investigation on Structural Shear Behaviour and Mechanical Characteristics of Steel Fiber Reinforced Concrete Beams Without Stirrups

Reinforced concrete (RC) is the most common construction material from which most structures are built, such as viaducts, buildings, retaining walls, tanks, conduits, tunnels, high-rise buildings, etc. The RC structures have different elements, such as slabs, columns, and beams [1-5]. The reinforced concrete beam (RCB) is a structural element used in constructing bridges and buildings, and it transfers the load from the load-carrying member to the supporting structure, such as walls or columns. Traditional steel bar reinforcement grids are used in RCBs at different locations to overcome the fragility nature of concrete and significantly improve flexural and shear forces. For instance, the increases in shear reinforcements improve, the shear resistance capacity of the RC sections, however, contributes to increased dead weight as well as material and construction costs. So many previous studies tested and analysed specimens of simply supported RCBs to obtain the characteristics of RCB with different test parameters, Liu et al. [6] examined the flexural capacity of simply supported steel-concrete composite beam and box beams under positive bending moment through combined experimental and finite element modelling. The results of this study suggest that flexural capacity increased with the increased of transverse reinforcement ratio, and the box-shaped composite beams have a high capacity than I-shaped composite beam. Sadati et al. [7], investigated the shear capacity of full-scale RCBs fabricated with high-volume fly ash and coarse recycled concrete aggregate. The performance of the RCB improved by reducing the dead load of the structure, and it is possible to improve the overall performance of the construction system. This may include increasing the load capacity of the structure, improving the flexibility of the system, or enhancing the durability of the materials.

The steel fibres are especially appealing for high-strength concrete mixtures, which can be somewhat fragile without them, when the traditional steel stirrups can be removed or reduced in RCBs, which lessens the congestion caused by reinforcements and conservatism of shear performance when utilized steel fibres. Steel fiber reinforcement is a construction technique that involves adding tiny, uniformly metallic fibers in concrete mixtures that have acquired popularity as reinforcement in concrete structures. Concrete reinforced with steel fiber is widely used in applications that require durability, high strength, and resistance to deformation and cracking [8]. Steel fibers can enhance the shear capacity of concrete elements, particularly under dead load conditions. It also provides additional reinforcement to the beam, enabling it to resist deformations and sagging under its weight [9, 10]. Kwak et al. [11] experimented with 12 steel fibers RCB specimens and demonstrated the impact of steel–fibers volume fractions $V_f$, a/d, as well as concrete compressive strength on the incipience of shear strength, shear cracking, ultimate deflection, and failure modes; they found that beams with a/d equal 2 were a failure in a combination of flexural and shear, and strength increased from 68% to 80%, while the beams with a large a/d were a failure in flexural with strength increased from 22% to 38%. Fiber-RCB increased shear strength and ductility stem from the post-cracking tensile strength of fiber-reinforced concrete. Also, these residual strengths tend to reduce crack widths and spacings. Altun and Aktaş [12] investigated whether adding steel fiber increased prismatic RCBs toughness capacity and flexibility. Through this experimental study, the addition of steel fibers increased the bearing strength and ductility of the RCBs and enhanced their shear strength performance. Steel fibers are available in different types depending on their physical properties and intended application. The most common steel fibers include hooked-end, straight steel, crimped, micro steel, and deformed steel fibers, which can be of different shapes, lengths, and aspect ratios and are typically added in small amounts to the concrete mixture. Bui et al. [13] presented the shear performance of steel fiber RCBs without traditional steel stirrups, by experimental investigation of the steel fibbers’ effect on the bending behaviour of RCB; the key purposes were first to realize whether the usage of steel fibers allows the total substitution of traditional transverse reinforcements (stirrups) and second to inspect the impact of a mixture of steel fibers and longitudinal bars. The reinforced concrete with steel fiber enhances concrete's flexibility, allowing it to deform without fracturing; this is important for structures that may experience significant deformation, such as earthquake-resistant structures or structures subject to thermal expansion and contraction. Furthermore, it increases the concrete tensile strength and enhances cracking resistance and deformations under loading, particularly for structures subject to dynamic loads, such as pavements, bridge decks, and industrial floors. Therefore, the durability of concrete will be improved with steel fibers due to increased resistance to environmental factors such as abrasion, freeze-thaw cycles, and chemical attacks. The previous research recorded that adding steel fibers to concrete can significantly increase tensile strength and improve resistance to bending and shear forces [14-16]. The study conducted by Yazıcı and Arel [17] carried out the effect of using steel fibers on the bond between deformed steel bars and concrete. It showed that when the number of steel fibers and aspect ratio increased, the loads increased by 7-16% compared to concretes without steel fiber. Wang et al. [18] added volumes of 1%, 2%, and 3% of steel fiber for every cement mortar mixture with four levels of superplasticizer dosage of about 0.180, 0.200, 0.240 and 0.220, respectively. the conclusion is that the addition of steel fibers to concrete may cause fiber interlock, which can lead to issues such as balling or clustering of steel fibers, causing poor mixing and reduced workability of the concrete. The increase in superplasticizer dosage improved the workability of the concrete mixtures and reduced the water content. Improved flowability can contribute to more homogenous distribution of steel fibers throughout the concrete matrix by reducing the chance of steel fiber interlock in the concrete mix and optimizing the particle dispersion of steel fibers in the mixture [19, 20]. Aoude et al. [21] studied the effects of steel fibers on shear capacity, failure mechanisms, and crack control, and the test results demonstrate that adding fibers improves shear resistance in shear-deficient beams. Furthermore, their findings demonstrated improvements in ductility and crack control. Reinforced concrete parts, especially those in the support zones, are vulnerable to shear failure when bent. This failure mode, which is particularly risky in components lacking shear reinforcement, can reduce the load-bearing capacity to less than the full bending capacity due to the longitudinal reinforcements. Krassowska and Kosior-Kazberuk [22] looked into the fact that members made of fiber-reinforced concrete did not deform easily and maintained their shape when subjected to a load. The fiber-RCB exhibited a higher density of narrow diagonal cracks. Quick and easily broken, brittle cracking was the hallmark of RCBs devoid of fibers. Contrary to plain concrete, they found the steel fibers could transfer a substantial amount of shear stress following cracking. Numerous studies have documented the failure behaviours of steel fiber and fiber-RCBs with and without steel stirrups in shear [23-25]. However, there is a necessity for increased comprehensive knowledge about the effect of hooked-end steel fibers on the mechanical characteristics and structural performance of concrete, especially in shear strength behaviour. Zhao et al. [26] explain that by supplementing steel fibers to the concrete mixtures, the shear capacity in concrete elements can be improved, tensile cracks substantially inhibited, and so on, portion or all the steel shear reinforcements can be superseded to minimize the cost of hand fixing; their results revealed that counting steel fibers with a volume fraction of 0.75% increment the shear capacity by 154.8%, under the same concrete strength and stirrups reinforcement ratio, which means steel-fibers addition can effectively enhance the shear capacity of the RCBs.

In the current study, the specific issue of this work is to replace stirrup reinforcement in the concrete beam with steel fibre to lessen cost and time work. Hooked-end steel fibers were utilized in various volume fractions 0% (non-fiber reinforced), 0.50%, 1.00%, and 1.50% in concrete mixtures. The study's objective is to investigate a new approach to improving the shear and flexural strength of RCBs, specifically through hooked-end steel fibers reinforcement, as well as provide data on structural performance and mechanical characteristics. Experimental investigations are conducted on small-scale RCBs with square cross-sections of 120×120 mm and lengths of 1000 mm to investigate structural performance, besides standard samples (cubes, cylinders, and prisms) to determine the mechanical characteristics. The RC beam samples are tested under a four-point monotonic load. The aim of the investigation, carried out by considering the effect of several parameters, such as ultimate load capacity, fiber volume fraction (v_f), modulus of rupture (f_r), splitting tensile strength (f_t), and concrete compressive strength (f_c), consists of the evaluation of the improvement in the peak load behaviour due to the presence of hooked-end steel fibers and eliminate stirrups.

Steel fibers at small volumes fractions 0.50–1.00% delay crack initiation and growth through mechanical interlocking and frictional resistance, while larger fibers enhance energy dissipation in macrocrack zones, providing enhanced post-yield behavior. Kakooei et al. [36] indicated fibers at 0.7% $V_f$ enhanced toughness without significant loss of mixture workability, outperforming higher $V_f$ mixes. Mohammadi et al. [37] explained longer fibers 50.00-60.00 mm provided superior flexural toughness but were less effective for microcrack control compared to shorter fibers 25.00 mm.

3.1 Machinal characteristics

3.1.1 Shear and flexural strengths

The shear strength represented by splitting tensile strength and flexural strength as modules of rapture behaviours of steel fiber-reinforced concrete mixtures is indicated in Figure 5. The indirect shear tests are used to investigate splitting tensile strength. The splitting tensile strength was obtained by subjecting the cylindrical samples to a compressive load, and the average of three sample results are used, shown in Table 3, and calculated by Eq. (1) [32]. It is considered essential for structural applications as it can control or defer the outset of brittle failure modes, such as shear compression, cracking, and splitting.

f_t=$\frac{2P}{\pi DL}$                            (1)

where, P is applied load, D is diameter of sample, L is length, π is constant.

Table 3. Load capacity of cylinders and prisms

image030.png

(a)

image032.png

(b)

Figure 5. Shear and flexural strength behaviour

The cylindrical specimen without steel fiber achieved 3.70 MPa in splitting tensile strength; this result was decreased when adding 0.5% (39.25 kg) steel fiber to the concrete mix and having an increase in the 1.0% (78.50 kg) steel fiber, and getting maximum capacity when 1.50% (117.75 kg) of steel fiber ${{V}_{f}}$ used, this explains that the addition of steel fiber can improve the shear behaviour of the concrete mixtures. This advancement in shear strength was due to the improved mixtures tensile capacity, which occurs owed to the reinforcing action of the steel fibbers. Moreover, the steel fiber increased the energy absorption capacity and improved ductility characteristics. The 1.50% (117.75 kg) fiber content recorded the maximum shear strength of about 6.36 MPa, and it can contribute to shear in RCB by replacing the amount of the required stirrups for the RCB; furthermore, the fibers delayed the onset of cracks and promoted multiple cracks, leading to a more ductile failure mode than traditional concrete beams [38]. The flexural strength (modulus of rupture) was evaluated by testing steel fiber-reinforced concrete prism specimens and calculated from Eq. (2) [35].

fr =$~\frac{PL}{b{{d}^{2}}}$                  (2)

where, P is applied load, L is span length, b and d are width and depth.

The modulus of rapture obtained from loads shown in Table 3 declared that flexure strength gradually improved with increased steel fibre ${{V}_{f}}$ content and reached a maximum at prism with a 1.50% steel fibre ${{V}_{f}}$ of about 116.53% compared with prism without steel fibres.

3.1.2 Compressive strength

The effect of steel fibre content on compressive strength appeared in Figure 6, showing 40.50 MPa for cubes without steel fibre. It can be noticed in this work that the addition of steel fibres to normal-strength concrete mixtures has little outcome on compressive strength; 0.50% and 1.00% of steel fibre ${{V}_{f}}$ decreased the compressive strength of concrete mixtures, while 1.50% of$~{{V}_{f}}$ caused an increase, which achieved about 43.20 MPa. A similar behaviour was also observed in Miao et al. [39] and Ou et al. [40]. These results declared that the normal concrete mixture design with f_c 35 MPa required a percentage of 1.50% of steel fibre ${{V}_{f}}$ to improve the compressive strength, also, it was compatible with Behbahani et al. [38] and Tanoli et al. [41] test results. The addition of steel fibres ${{V}_{f}}~$to the concrete mixture may cause fibres to interlock, which can lead to issues such as balling or clustering of steel fibers, causing poor mixing and reduced workability of the concrete; for this issue, the superplasticizer content increased when the steel fiber ${{V}_{f}}$ ratio increased [42].

image034.png

Figure 6. Steel fibre content influences on compressive strength

ping_mu_jie_tu_2025-04-02_154618.png

Figure 7. Failure mode of prisms and cylinders samples

Table 4. Mechanical characteristics and strength effectiveness

Table 4 demonstrates the effectiveness of splitting tensile strength, modulus of rupture, and compressive strength results from steel fibre concrete mixtures included.

The failure modes in mechanical characteristics samples (prism and cylinder samples) are indicated in Figure 7. Prism specimen failure explains that a prism without steel fibre distributed and splits into two pieces because it is under reinforcement; the second with ${{V}_{f~}}$0.5% steel fibre has a wide crack failure, and the diameter of this crack decreased by using ${{V}_{f}}~$1.0% steel fibre in the third specimen. Meanwhile, the fourth one, which contains ${{V}_{f}}~$1.50% steel fibre, shows higher resistance against cracks and failure in the diagonal shear mode because the effective bond between concrete and hooked-end steel fibre assists against defects and cracks and absorbs energy from applied loads, which leads to improving toughness and making the RCB more resistant to failure [43].

The test specimen cylinder without steel fibres split the specimens into two parts through the testing process and the addition of steel fibres enhances the toughness of the concrete cylinder by increasing its resistance to cracking, particularly under shear loading conditions. The hooked-end steel fibres help to bridge the cracks formed in the concrete, thereby inhibiting their propagation and increasing the cylinder's shear strength and redistribution stresses in microstructure [44].

3.2 Structural performance

3.2.1 Load deflection response

The group SF0 the RCB-0 first crack appeared at 15.00 kN of load, with a mid-span deflection of 1.80 mm. The loading continued until the RCB-0 failed at a peak load of 22.10 kN and a deflection of 9.36 mm. The beam's tensile reinforcement was less than the reinforcement of the balanced section. As a result, the tensile reinforcement reached its yield strength (f_y) before the concrete reached its compressive strength 0.85 f_c, causing the strain in the tension reinforcement to reach the yield strain (ε_y). In contrast, the strain in concrete at its maximum (ε_c) reaches the value 0.003. This type of failure is preferable to other types because it gives warning indicators before collapse through the appearance of cracks and gradually increasing delamination, providing the opportunity or space to avoid sudden collapse. In this type of failure, the reinforcement steel yields before cracking occurs in the concrete. The RCB-0 section contains a small amount of flexural reinforcing steel. The reinforcement of the RCB-0 was improved by adding hooked-end steel fibre reinforcement to investigate the influences of the steel fibre on the shear and flexural performance of the RCB without stirrups, The ratio 0.5% ${{V}_{f}}$ of (39.25 kg) in group SF 0.5; the RCB-0.5 testing appears delayed in the first crack load, which is equal to 20.00 kN with 1.96 mm in mid-span deflection and increasing in the peak load of 32.50% (29.30 kN) with increasing of 5.10% (9.84 mm) in deflection. The second ratio of steel fibre (group SF1) was 1.0% of the ${{V}_{f}}$ (78.50 kg); the testing result of RCB-1 declared significant improvement in the first crack load, which reached 23.80 kN with 2.05 mm deflection, and the beam failure load happened at 36.00 kN with deflection of 7.30 mm, the load caring capacity of the RCB-1 in this ratio enhanced by 62.40% from the load in reference beam (RCB-0) and the deflection decrease by 22.0%. In comparison, the third ratio of ${{V}_{f}}$ steel fibre is 1.50% (117.75 kg) in group SF 1.5; the RCB-1.5, resulting in 28.00 kN for first crack loading corresponding to 2.61 mm deflection. The RCB-1.5 still caring load until failure at 37.60 kN, which represents the maximum peak load increasing by 70.13% from the load in RCB-0 as well as controlled the mid-span deflection to reach 5.65 mm, causing a reduction by 39.64%, as shown in load-deflection curves in Figure 8, which exhibited a comparison between the RCB-0 (without steel fibre) with the other specimens (RCB-0.5, RCB-1 and RCB-1.5) that contain a different ratio of steel fibres v_f. Also, the influences on the first crack loading are shown in Figure 9, These curves explain the optimum ratio of hooked-end steel fibre v_f that can control mid-span deflection of about 1.50% in this study, which exhibited the maximum ultimate load. This is due to getting solid bond interactions between steel fibre and concrete matrix decreases the number of cracks, and as a result, toughness improves. Also, the increasing steel fibre v_f increased ductility and improved shear strength in concrete were previously reported [45].

image060.png

Figure 8. Load-deflection (beam with and without steel fibres)

image062.png

(a)

image064.png

(b)

Figure 9. Steel fibre content influences on first crack load and mid-span deflection

3.2.2 Failure mode

The failure mechanisms of SFRC beams are complex, involving the interaction between the concrete matrix and the steel fibers, particularly the hooked-end fibers. These mechanisms can be categorized based on the type of loading (e.g., flexural, shear, or combined) and the behaviour of fibers under stress. Hooked-end fibers improve the shear capacity of beams by bridging shear cracks and providing a mechanism for stress redistribution. In flexural loading, cracking initiates in the tension zone, and hooked-end fibers help resist crack opening. With higher fiber content ${{V}_{f}}$, the beam exhibits more ductile behaviour, characterized by multiple cracks before the ultimate failure. At ultimate load, failure may occur due to crushing of the concrete in the compression zone, particularly if the fiber content primarily enhances tensile resistance without significantly altering compressive behaviour. The hooked ends significantly enhance the bond strength, reducing the likelihood of slip and improving the overall load transfer efficiency [46, 47].

The failure modes of the RCB-0 to RCB-1.5 are shown in Figures 10(a) to 10(d), respectively. The crack pattern of the RCB-0 forms with the growth of vertical cracks (flexural cracks) at the base of the beam due to flexural tensile stress. Then, as the load on the RCB-0 rises, this crack extends both in width and length and curves in a diagonal direction as it transfers to the upper part of the beam toward the loading point; this failure mode is a shear failure as shown in Figure 10 (a), this is attributed to the lack of shear reinforcements, no stirrups reinforcement set to control the shear failure which causes shear force exceeds the shear capacity of different materials of the RCB resulting from shear resistance lower than flexural strength of the RCB [48, 49]. This failure shear classified as diagonal tension failure, occurred because it was under-reinforced; the reinforcement ratio in the RCB-0 is lower than the balanced reinforced ratio and its agreement with Abbass et al. [50].

The RCB-0.5 with ${{V}_{f}}$ 0.5% steel fibre in a concrete mixture resulted in a composite material that enhanced shear resistance effects and improved the behaviour of the RCB-0.5; the concrete tensile failure mode happened due to shear forces. As a result, the crack width is decreased, and the stress distribution becomes more uniform compared to RCB-0. It also increased ultimate load and shear strength, lessening dependence on stirrups, as shown in Figure 10(b). The increase of steel fibre ${{V}_{f}}$ from 0.5% to 1.0% and 1.5% in the RCB1, and RCB1.5 appeared to gradually improve in post-cracking behaviour and fewer cracks’ distributions in mid-span; it occurred because hooked-end steel fibres enhanced shear resistance properties and depended on shear resistance on the main reinforcements of the RCB. This behaviour allowed for higher loads to be carried before shear failure occurred, and then its failure was flexural as appeared in Figure 10(c), and Figure 10(d) agreement with Kwak et al. [11], Yuan et al. [51], and Biolzi and Cattaneo [52].

ping_mu_jie_tu_2025-04-02_154823.png

Figure 10. Failure mode of RCB