Shear Behavior of Deep Beams Reinforced with Single- and Dual-Geometry Steel Fibers

2.1. Beam Specimens

In order to achieve the objectives of the research, eleven simply supported beams were evaluated. All beams were cast with constant cross-section of 200 mm in depth and 120 mm in width. The beam specimens had a constant shear span - to - effective depth ratio (a/d) which is 2. All beams were reinforced with 2Φ10 mm and 1Φ12 mm bars as flexural reinforcement, placed along the beam soft. Approximately, 25 mm concrete clear cover was maintained around the section of the beam. The beams were tested under four-points loading scheme up to failure. The geometry and testing setup of tested beams are shown in Fig. 1. Two control specimens were evaluated in this research; one (B_0) has been left without any kind of shear reinforcement and the other (BS_0) has been reinforced by using Φ6 mm steel links placed at a spacing of 60 mm throughout the shear spans and 100 mm throughout the middle third of the beam. This quantity of shear reinforcement is more than the minimum reinforcement calculated according to ACI 318-25 specification by approximately 10%. The remaining beams have been divided into three groups of three specimens each. The specimens of the first group were reinforced with hooked-ends steel fibers as shear reinforcement with three volume fractions (V_f) of 0.7%, 1.05%, and 1.4%. The second group is similar to the first one, except that crimped steel fiber has been implemented instead of hooked-ends fiber. Mixed (hooked-ends and crimped) fibers of the same percentages have been utilized in the third group. Each specimen has been reinforced against shear stress by 0.7%, 1.05% or 1.4% of the concrete weight. Details of all beam specimens are listed in Table 1.

Figure 1:

2.2. Materials and Procedures

The materials used in this study include crimped and hooked steel fibers, steel rebar, 19 mm maximum size coarse aggregate, fine aggregates, cement and superplasticizer. The control concrete mix was designed to achieve a target cube compressive strength of 45 MPa tested according to BS EN 12390 (European Committee for Standardization, 2019). Three concrete cube specimens of 150 mm ×150 mm ×150 mm for each beam were cast, cured in water basin for 28 days, and then tested. The cement and fine aggregate conformed to ASTM C150 (ASTM International, 2015b) and ASTM C33 (ASTM International, 2013) standards, respectively. The steel rebar was tested and conformed to ASTM A615 (ASTM International, 2009). High-range water reducer (Conplast SP2000) conformed to ASTM C494 (ASTM International, 2015a) was used in this study to control the loss of workability due to the presence of steel fiber. Hooked and crimped steel fibers have been used throughout this study. The material properties of steel fibers and water reducer were obtained directly from the manufacturer. A summary of material properties is shown in Table 2.

Reinforcement cages were constructed at the Civil Engineering Department of the University of ThiQar. Formwork were made from plywood board and they had been oiled prior to the placement of the reinforcement cages. It should be noted that the longitudinal reinforcements of Φ10 mm were hooked upwards behind the supports and enclosed by additional two Φ10 mm bars as stirrups at each end to prevent the possibility of failure at supports. Figure 2 shows examples of reinforcement cages

Figure 2:

2.3. Mixing, Casting and Curing

Table 3 shows the mix proportions for both mixes with and without steel fiber. The control concrete mix was designed to achieve a target cube compressive strength of 45 MPa tested according to BS EN 12390 (European Committee for Standardization, 2019). As stated previously, each beam was cast with three concrete cube specimens of 150 mm ×150 mm ×150 mm to estimate the concrete compressive strength (f_cu). Curing and storage of the test cubes were kept identical to the conditions of the test beams. All specimens, beams and concrete cubes were covered with wet material and plastic sheets on top to prevent moisture loss. The beams were kept moist at all times during the curing period, which lasted 3 days. The formwork was stripped at the end of the third day, and the beams were stored in the laboratory until tests. Figure 3 shows the types of steel fibers used and specimens’ preparation.

2.4. Test Setup and Instrumentations

As schematically shown in Figure 1 above, the beam specimens were tested under four-points loading configuration up to the failure state is reached. All beams were painted by white-color painting liquid prior to testing to facilitate the crack propagation monitoring. The load was applied with a constant rate by a testing machine of 2000 kN maximum capacity. The mid-span deflection was measured using a high-accuracy dial gauge located at the mid-span of the beam. During the tests, the applied load, and deflection were recorded and the propagation of cracks was monitored from both sides of the tested beams.

Figure 3:

4.1. Effect of V_f

Figures 4 to 6 and Table 4 show the results of maximum loads and corresponding deflections with variable volume fraction of SF from 0 to 0.7%, 1.05%, and 1.4%. It was observed that with an increase in steel fiber percentage, regardless of the type of steel fiber used, the shear strength and corresponding ductility (based on the ultimate recorded midspan deflection) have been significantly increased. This can be explained due to the contribution of steel fiber to “bridging” the developed cracks and therefore providing better ability to resist shear stresses (Le et al., 2019; Li et al., 1993). Notably, for the beams with mixed fibers (M-Series), the 1.05% SF volume fraction achieved better performance than the 1.4% dosage, indicating that peak shear enhancement does not necessarily require highest fiber content. This finding is practically advantageous, as a lower fiber dosage can significantly reduce mixing time and effort, improve fresh concrete workability, and indeed offer a more economical fiber-reinforcement solution without compromising structural performance.

The reduced performance at 1.4% SF relative to 1.05% is likely attributable to workability loss and increased fiber clustering at higher mixed fiber dosages, which can compromise uniform fiber distribution despite the higher nominal fiber content. This suggests that 1.05% represents a practical optimum, balancing shear performance gains against workability, mixing effort, and material cost. Generally, the load carrying capacity of the fiber-reinforced concrete beams, regardless of the fiber type, ranged from 165% to 211%, and from 105% to 140% of the load carrying capacity compared to beams B_0 and BS_0, respectively.

In addition, according to test results presented in Table 4 and Figures 4 to 6, it can be seen that the beams involved a small volume fraction of steel fiber (i.e., 0.7% in this study regardless of the type) have provided higher strengths than the beam constructed with minimum traditional reinforcement (BS_0). Therefore, it can be concluded that the steel fiber, especially the mixed fiber, can be successfully used to resist shear stresses in RC beams instead of stirrup with minimum (or even more quantity) according to ACI 318-25. It should be mentioned that the present results corroborate those obtained by (Zhi et al., 2025), where it has been reported that incorporating 1.0% hooked-end steel fiber improved the load-carrying capacity by 63% compared to normal concrete and such a steel fiber dosage can be equivalent to D8@200mm conventional stirrups.

Furthermore, Table 4 and Figures 4 to 6 clearly illustrate that the deflection at maximum load increases consistently with the percentage of steel fibers. This indeed, will result in a better ductility as compared to the beam reinforced with minimum traditional shear reinforcement as in beam BS_0. This dual-geometry approach offers a novel strategy for optimizing fiber-reinforced shear design in deep beams, particularly where enhanced ductility and crack control are needed without relying solely on conventional shear reinforcement.

4.2. Effect of Fiber Type

Table 4 and Figures 7 to 9 present the load carrying capacity and load-deflection relationships for beams reinforced with different types of steel fibers (hooked, crimped and mix of hooked and crimped). As was previously discussed, all beams have been constructed with identical longitudinal reinforcement ratio, fiber volume fraction, effective depth and shear span – to – depth (a/d) ratio. It can be noted that the beams constructed with hooked fibers (H-Series) exhibited 5.4%, 6.8% and 6.3% higher max load compared to those with crimped fibers (C-Series) for the volume fraction of 0.7%, 1.05%, and 1.4% respectively. Similarly, the beams constructed with a mixed steel fiber (M-Series) exhibited 27%, 19.5%, and 9% higher max load compared to those with crimped fibers for a volume fraction of 0.7%, 1.05%, and 1.4%, respectively. It should be mentioned that the beams reinforced with mixed type presented higher max load as well as higher deflection at the maximum load with respect to hooked and crimped fibers. This may be attributed to the enhanced bond provided by the mixed fibers type.

Figure 10, on the other hand, illustrates the effect of steel fiber (SF) volume fraction on the shear strength of the beams with the three types of fibers tested. All three fiber types exhibited roughly a similar trend, with shear strength increasing sharply between 0% and 0.7% SF volume fraction, followed by a more gradual increase up to 1.4%. It has been noted also that as the volume fraction increases, the mixed-fiber specimens consistently outperform the single-fiber types, reaching a peak shear strength of about 80 kN at a volume fraction of approximately 1.05%, before slightly declining to around 77.5 kN at 1.4%. This suggests that combining fiber geometries produces a synergistic effect on crack-bridging and shear resistance, though a slight reduction beyond the optimum fraction (∼1.05%) may indicate diminishing returns from fiber balling or reduced workability at higher dosages.

The hooked-end fiber specimens showed the second-highest performance, achieving roughly 75.5 kN at 1.4% volume fraction, while the crimped fiber specimens exhibit the lowest shear strength gain, reaching about 71 kN at the same fraction. The consistently lower performance of crimped fibers compared to hooked-end fibers can likely be attributed to differences in mechanical anchorage; hooked-end fibers typically provide superior pullout resistance due to their end-hook geometry compared to the wave-like deformation of crimped fibers (Abdallah & Rees, 2019; Zīle & Zīle, 2013).

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4.3. Failure Mode and Crack Patterns

The last column in Table 4 summarizes the failure modes of the beams tested in this research. In general, three distinct failure modes were observed in this experiment, namely, a pure shear failure by diagonal shear cracks, a combination of flexure and shear failure, and a pure flexural failure.

In beam B_0, which has no steel fiber, few and small flexural cracks first observed in the middle third region of the beam and followed by one major shear crack formed between the applied load and nearest support; within the regions of constant shear. Finally, the beam failed abruptly by a diagonal shear crack. The same behavior was approximately demonstrated by beam BS_0, which had been strengthened by means of conventional shear reinforcement, but with larger load carrying capacity as compared to beam B_0.

The incorporation of steel fiber showed a significant effect on the crack patterns, where clear differences in the crack intensity and distribution of the steel fibrous beams were observed. Regardless of the type of steel investigated, as the steel fiber percentage increased from zero to 1.4%, the failure mode changed from pure diagonal shear failure at low dosage of SF to a pure flexural failure at higher SF content, passing through shear-flexural failure at intermediate (i.e., 1.05%) SF content. Rather than a single major crack, the transition from pure shear to pure flexural failures was accompanied by developing extensive small cracks distributed at constant shear and constant moment regions along the span of the tested beams. This transition, in addition, indicates the effectiveness of the steel fiber incorporation for the shear strengthening applications where the failure has been changed from sudden to a more gradual failure due to the improved ductility of the tested beams with SF. This behavior was observed in all tested groups. For instance, Figure 11 shows the crack patterns of beams B_0, BS_0, BH_1.05, BC_1.05 and BM_1.05.

Similarly, the failure modes of beams BM_0.7, BM_1.05 and BM_1.4 reinforced with mix of steel fibers were combination of shear and flexure failure for beam BM_0.7 and pure flexural failure for both beams BM_1.05 and BM_1.4. This indicates the extraordinary performance of mixed steel fiber as compared to a single-type fiber at each fiber content percentage.

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