Issue 78

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

Investigating the effects of steel fibers and notch depth on the fracture behavior of reinforced concrete (RC) beams using both matrix-cracked and through-thickness cracked approaches

Manar Adel, Seleem S. E. Ahmad, Ahmed A. Elakhras Engineering Materials Department, Zagazig University, Zagazig 44519, Egypt m.adel22@eng.zu.edu.eg, https://orcid.org/0009-0001-4798-3812 ssdawod@eng.zu.edu.eg, http://orcid.org/0000-0001-9894-0209 ahmedali.elakhras@gmail.com,http://orcid.org/0000-0002-3821-1327

Citation: Adel, M., Ahmad, S.S.E., Elakhras, A.A., Experimental study on the role of steel fibers and notch depth in RC beam fracture behavior using matrix-cracked and through thickness cracked approaches, Fracture and Structural Integrity, 78 (2026) 168-185.

A BSTRACT . Steel fibers are commonly used to compensate for the tensile weakness of concrete. However, the interaction of steel fibers with conventional reinforcing bars after a crack has been formed is poorly understood. Most fracture studies use a cut through the thickness of the material that cuts through any fiber in its path, whereas a real crack would maintain the bridging action. In this study, the true mode I fracture toughness (K IC ) of steel-fiber-reinforced concrete beams with longitudinal steel bars is determined using a matrix-crack (MC) approach where short fibers are not broken through a natural pre-crack, and compared with companion beams prepared with a conventional through-thickness crack (TTC). Reinforced concrete (RC), normal strength concrete (NC), and fiber-reinforced concrete (FRC) beams with a steel fiber volume fraction of 1.0%, two ratios of notch depth to beam depth (a/d=0.25 and 0.5), and two reinforcement layouts were tested under four-point bending. Incorporation of steel fibers delayed crack propagation and improved ductility, increasing the ultimate load-carrying capacity by up to 50% and the modulus of toughness by nearly 94% relative to fiber-free beams. All cases showed higher first-cracking loads and better post-cracking load maintenance for MC specimens than for TTC specimens, confirming that the MC approach provides a more realistic representation of fiber bridging. K IC increased with a/d in plain reinforced beams, due to the growing moment arm of the steel closing force, but decreased with a/d in fiber-reinforced beams, indicating that fiber-bridging efficiency weakens as the crack deepens. The results support the MC method as a more reliable approach for assessing the intrinsic fracture toughness of fiber-reinforced RC members. KEYWORDS : Opening crack mode, Matrix crack, RC beam, Steel fiber, Crack depth ratio, Fracture toughness.

Received: 07.07.2026 Accepted: 11.08.2026 Published: 15.08.2026 Issue: 10.2026

Copyright: © 2026 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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