Issue 78

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

The corresponding failure modes shown in Fig. 10 further support the load–deflection observations for both the fiber reinforced and conventional concrete beams. The control FRC beam ( α = 0.0) presented a narrow and short main propagated tension crack with fine, hairline inclined cracking, which implies that the fibers were able to restrain the crack widening in the absence of a pre-formed notch. In contrast, the MC-FRC beam (a/d = 0.25) showed a wider and longer main propagated tension crack originating from the initial matrix crack and spanned by steel fibers, indicating additional stress concentration due to the pre-crack. The plain MC-NC beam (a/d = 0.25) without fiber reinforcement showed the most severe cracking, with a wide main crack propagating the full depth of the section, which is typical for the brittle, largely unrestrained crack growth in plain concrete after the introduction of a stress riser [1,2]. Overall, these observations confirm the role of steel fibers not only in delaying crack initiation but also in actively controlling crack width and propagation path after cracking, consistent with the crack-bridging mechanisms discussed in the literature [9].

(a) Control beam, FRC (a/d = 0.0): narrow and short main propagated tension crack, with fine hairline inclined cracking.

b) a/d = 0.25, MC-FRC: initial crack bridged by steel fibers, with a wider and longer main propagated tension crack.

a) a/d=0.0,NC.

b) a/d = 0.25, MC-NC: wide main crack propagating over the full depth of the section.

Figure 10: Failure pattern for normal concrete.

Impact of SF on the load–deflection response of uncracked RC beam The load–deflection responses of the uncracked RC and RC-FRC beams (a/d = 0.0) are presented in Fig. 11. The load deflection behavior of both beams was essentially linear during the early loading stage, indicating elastic behavior prior to the formation of the first crack. The initial slope of the RC-FRC beam was slightly higher than that of the plain RC beam, indicating a slight increase in stiffness due to the presence of steel fibers in the concrete matrix. The conventional RC beam showed a substantial reduction in stiffness after the first cracking, and further loading resulted in small increments of load accompanied by significant increases in deflection, signifying progressive cracking and crack propagation in the tensile region of the beam. Finally, the RC beam failed at an ultimate load of about 82 kN. On the other hand, the load of the RC-FRC beam increased after the first cracking stage due to the crack-bridging action of the steel fibers, which carried the tensile stresses across the crack faces and restricted crack widening and propagation [6,9]. The RC-FRC beam achieved an ultimate load of about 120 kN, which is about 46% higher than that of the plain RC beam, in accordance with previous studies where steel fiber reinforcement improves the post-cracking flexural resistance and delays the crack propagation in fiber-reinforced concrete members [14,21]. The RC-FRC beam demonstrated much higher toughness (the area under the load-deflection curve), indicating much higher energy absorption capacity than the RC beam. This agrees with the well-known crack-bridging and toughening mechanisms in steel fiber-reinforced concrete, where fibers inhibit crack opening and enable the composite to sustain load over a much broader range of deformation [15,16]. In conclusion, the addition of steel fibers enhanced the pre-cracking stiffness and, more importantly, the post-cracking ductility and toughness of the reinforced concrete beam. These qualitative observations are further validated by the quantitative results presented in Tab. 6, which tabulates the stiffness, fracture strength, and modulus of toughness (MOT) of the two beams, along with the percentage improvement

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