Issue 78

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

R ESULTS AND DISCUSSION

B

efore presenting the individual comparisons, it is worth reiterating that the tested specimens belong to two structurally different series, as described in the experimental work section: a reinforced series (RC, RC-FRC, MC RC-FRC, TTC-RC-FRC, TTC-RC; b × d = 150 × 300 mm) containing main longitudinal steel bars, and an unreinforced, fiber-only series (FRC, MC-NC,NC, MC-FRC; b × d = 150 × 180 mm). Because the two series differ in absolute cross-sectional dimensions, all comparisons of load-carrying capacity, stiffness, and toughness discussed below are made within each series only, i.e., between RC and RC-FRC, or between MC-NC and MC-FRC at the same a/d ratio so that the reported percentage differences can be attributed unambiguously to the steel fiber content or the notch-depth ratio rather than to a difference in specimen size. Impact of steel fiber on the load-deflection response of normal and unreinforced concrete It was found that the addition of steel fibers (SF) to normal concrete (NC) beams, normally not reinforced and therefore prone to brittle tensile failure, greatly enhanced the tensile strength and ductility of the beams, which is consistent with the crack-bridging mechanism widely reported for fiber-reinforced concrete (FRC) [6]. The addition of steel fibers in reinforced concrete beams under load also leads to a similar significant improvement in structural performance. The load–deflection response is shown in Fig. 9.

Figure 9: Effect of steel fiber on load-deflection behavior; deflection measurements were limited to a maximum of 13 mm.

Based on a direct comparison between the unreinforced, fiber-free NC beam and its FRC counterpart, both cast at the smaller 150 × 180 mm cross-section, the addition of steel fibers increased the maximum load capacity from about 25.5 kN to more than 37.7 kN, an increase of about 47%. This comparison, like all others in this subsection, is made strictly within the unreinforced (150 × 180 mm) series and should not be read against the reinforced (150 × 300 mm) RC/RC-FRC results discussed later in this section This result agrees with earlier studies that indicated that adding steel fiber in small amounts improves compressive strength and enhances flexural and tensile strength in steel fiber-reinforced concrete (SFRC) beams and reduces their cracking tendency [21,22]. Apart from improved ultimate load-carrying capacity, the load-deflection behavior indicates that SFRC members with steel fibers were able to support loads at higher post-peak deflection levels, meaning the members were more resistant to progressive cracking failure. This behavior is due to the fibers bridging cracks during this process, which has also been established to provide stability to the flexural rigidity of the members after the cracking of the concrete [23]. A similar behavior was noted for unreinforced matrix cracked (MC) containers. The peak load for the MC-NC [0.25] specimen was about 25 kN, while that of the MC-FRC [0.25] specimen was recorded at about 33.5 kN, implying an increase of about 32%. The fiber-reinforced mixtures also showed better ductility and crack resistance, as they exhibited greater ultimate load-carrying capacity and greater ability to perform under high loading [14,15]. Hence, the addition of steel fibers in the concrete matrix significantly improved flexural performance, as the MC-FRC [0.25] specimen clearly presented a better post-cracking load-carrying capacity compared to the plain MC-NC [0.25] specimen. This behavior is attributed to the steel fibers bridging the developing crack, slowing down the crack propagation and allowing the beam to carry higher loads at larger deflections, a mechanism which shows an increase in ductility and energy absorption capacity, and finally in fracture toughness, according to the matrix-crack fracture toughness framework used in this study [12].

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