Issue 78

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

I NTRODUCTION

C

oncrete is one of the most widely used construction materials, valued for its strength, durability, and low cost. Its main drawback, however, is a well-known weakness in tension, and this has kept reinforcement research active for decades [1]. One of the more effective responses to this problem has been fiber-reinforced concrete (FRC), in which short discontinuous fibers are mixed directly into the cementitious matrix to improve tensile performance [2]. Steel fiber-reinforced concrete (SFRC) is arguably the most successful version of this idea: randomly distributed short steel fibers give the material mechanical properties well beyond those of plain reinforced concrete. The fibers restrain both microcracks and larger cracks as they form, which in turn raises tensile strength, flexural capacity, impact resistance, fatigue life, and ductility all at once [3]. This combination of properties is why SFRC now shows up in rigid pavements, bridge decks, tunnel linings, and various composite structural systems. Reinforced concrete structures rarely have an easy service life. Overloading, repeated cyclic loading, corrosion, and general material degradation all take their toll, and together they lead to structural damage, a gradual loss of performance, safety problems, and eventually failure before the intended design life is reached. Any rehabilitation or strengthening strategy therefore has to be both durable and economically sensible [4]. Fibers can be used in two broad ways here: as the main reinforcing mechanism in an FRC system on its own, or as a supplementary reinforcement alongside conventional steel bars in a hybrid arrangement [4]. Interestingly, the compressive strength gain from adding fibers is usually modest — the real payoff shows up in tensile strength, ductility, crack resistance, and general toughness [3]. There is also a long history of fracture-mechanics work on cracked reinforced concrete beams strengthened with steel or FRP plates. Superposition methods have been the standard tool for working out stress intensity factors in these systems, and the general finding is that high stress intensity factors push a member toward failure, while a stiffer, thicker plate lowers the stress intensity factor but raises the closure force it applies. Similar work on steel and carbon-FRP panels under three point bending has shown that the stress intensity factor depends mainly on the load-induced stress concentration and on the closure force from the embedded steel, with the geometry correction factor moving inversely with the amount of internal steel and bonded plate present. Steel fibers themselves have been shown, separately, to improve the flexural behavior and ductility of high-strength hollow beams [5], and adding conventional steel reinforcement on top of the fibers further improves residual flexural stiffness after cracking [6]. A parallel line of research has looked at fracture behavior in more advanced, nano-modified cementitious composites. A moving-mesh R-curve model based on linear elastic fracture mechanics has been used to follow crack propagation in nano filled ultra-high-performance fiber-reinforced concrete (UHPFRC) [7], and a separate study examined the age-dependent size effect in ultra-high-performance concrete (UHPC) [8]. Both point to the same conclusion: fracture behavior in these nano-modified systems does not follow classical scaling rules and depends heavily on specimen size and crack configuration. This is a useful reminder that fracture parameters in cement-based materials are sensitive to fairly small changes in matrix composition and geometry, which is directly relevant to the fiber-bridged notches studied here. Even with all this work, most fracture-mechanics studies on reinforced concrete beams still treat crack growth as a single crack running through continuous reinforcement or bonded FRP plates, both of which act, mechanically, like one long fiber. What has received far less attention is how short, randomly oriented fibers bridging an existing crack or notch actually affect the mechanical and fracture behavior of the beam. Part of the reason is practical: very few studies have tested truly matrix-cracked SFRC specimens under proper fracture assessment. Most work instead relies on beams with a conventional sawn through-thickness notch, as in a recent study on the tensile strength and fracture toughness of SFRC using small notched beams [9]. Griffith's classical fracture theory offers a useful way to think about this: crack growth happens through the matrix, and long fibers acting as tensile reinforcement stay intact. By the same logic, short fibers in an FRC system should remain unbroken within the pre-cracked zone of a naturally forming matrix crack, which is a fundamentally different situation from a notch that has been sawn straight through. Fiber content itself has a strong effect on how SFRC behaves. Even a fiber volume fraction as low as 0.5% noticeably reduces workability and usually calls for more superplasticizer. Tests on C20/C30 beams with fiber dosages between zero and 60 kg/m 3 suggest that around 30 kg/m 3 gives the best trade-off between strength gain and the loss of ultimate load capacity and elastic modulus that comes with higher dosages. The equivalent flexural strength ratio rises more or less linearly for fiber volume fractions between 0.25% and 0.5%, and falls as the concrete grade increases; beams with higher fiber contents, up to 2.0%, also show better toughness and post-peak behavior under both quasi-static and impact loading [10]. Increasing the steel fiber volume fraction from 0% up to 1.6% has been shown to markedly improve fracture behavior [11]. In UHPFRC beams specifically, fibers aligned with the tensile stress direction perform better in flexure than fibers aligned transversely, and notched UHPFRC beams show almost no size effect thanks to their high inherent ductility, a result that

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