Issue 78

M. Adel et.al, Frattura ed Integrità Strutturale, 78 (2026) 168-185; DOI: 10.3221/IGF-ESIS.78.10

resulting from the incorporation of fibers. Compared with a plain RC beam, an RC-FRC beam exhibited 13.3% higher stiffness, 69.7% higher fracture strength, and 94.4% higher modulus of toughness. Of particular interest is the near-doubling of the MOT value, confirming that steel fibers primarily enhance the beam's energy absorption capacity after cracking rather than its elastic stiffness, as reported in the literature for steel fiber-reinforced concrete [15,16]. The above results collectively indicate that the use of steel fibers is highly effective in improving the ultimate load-carrying capacity, ductility, and energy absorption of RC beams, leading to a more stable, less brittle failure behavior.

Figure 11: P- δ curves of uncracked beams ; deflection measurements were limited to a maximum of 13 mm.

% Relative RC /RC-FRC % Stiffness % Fracture Strength % MOT

Stiffness kN/mm

Fracture Strength MPa

MOT MPa 0.012 0.024

Beam ID

L/d

RC [0.0]

5 5

46.7 52.9

9.4

-

-

-

RC-FRC [0.0]

14.2

13.3

69.7

94.4

Table 6: Mechanical behavior of Smooth (uncracked) beams.

Impact of SF on the load–deflection response of cracked RC beam Fig. 12. Load-deflection curves of cracked reinforced concrete beams with and without steel fiber reinforcement. The RC beam reinforced with steel fibers and having a through-thickness crack of 0.25 (TTC-RC-FRC [0.25]), matrix-cracked approach (MC-RC-FRC [0.25]), and uncracked RC-FRC has a much higher maximum load capacity than the conventional RC beams without steel fibers. The maximum load was increased from about 71 kN for the control RC beam to about 88 123 kN for the fiber-reinforced specimens, representing about a 50% increase in the load-carrying capacity [14]. Most importantly, the steel fiber-reinforced beams showed significant improvement in post-cracking behavior with a longer plateau range and higher energy absorption before failure. This behavior indicates improved toughness and ductility, since the fibers effectively bridge growing cracks and inhibit their propagation [6,9]. We find large differences between the MC and TTC approaches. The MC-RC-FRC specimens exhibited higher first cracking loads and better post-cracking ductility than the TTC-RC-FRC specimens, indicating that the MC method is more effective in capturing the fiber-bridging mechanisms that control fracture toughness [11,12]. The failure patterns of the tested specimens are shown in Fig. 13. It can be seen that the failure behavior of the fiber-reinforced concrete beams differs from that of the conventional concrete beams. The control RC beam (a/d = 0.0) showed a number of cracking patterns such as shear, main tension, and secondary cracks throughout the section. In contrast, the MC-FRC specimens with steel fiber reinforcement exhibited more controlled cracking with narrower and shorter main cracks. The plain MC-NC beam (a/d = 0.25) without fiber reinforcement exhibited severe cracking with wide main cracks extending in the full depth of the section, typical of brittle concrete behavior [2]. These findings confirm the high efficiency of steel fibers in preventing crack widening and in controlling crack propagation paths, resulting in a significantly improved structural performance and durability of reinforced concrete members [21,22].

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