The paper investigates the formation and propagation of flexural cracks in beams made of ultra-high performance steel fiber reinforced concrete (UHPFRC). A comprehensive series of laboratory tests was carried out on seventeen beams of rectangular and T-shaped cross-sections, varying in longitudinal reinforcement ratios, fiber volume contents, and fiber types (straight and wave-shaped). The results demonstrate that the inclusion of steel fibers significantly enhances the crack resistance of the beams, promotes a more uniform crack distribution, and improves their load-bearing capacity. In under-reinforced UHPFRC beams, failure typically occurs due to fiber pull-out localized within one or more dominant cracks. Prior to the onset of deformation localization in the tensile zone – which coincides with the yielding of the longitudinal reinforcement – the crack widths in fiber-reinforced specimens remain below 0.25 mm. This behavior ensures that even under service-level loads (65–70% of ultimate), the crack openings stay within the allowable design limits (0.3–0.4 mm). The experimental findings contribute to a better understanding of the cracking mechanisms in UHPFRC beams and provide a valuable foundation for refining numerical models and optimizing design approaches for flexural members made of advanced high-performance cementitious composites.

Keywords: ultra-high performance concrete, beams, bending moment, steel fibers, flexural cracks

The development of methods for calculating structures made of ultra high-performance steel fiber-reinforced concrete (UHPC) in terms of strength and deformations is an important step towards the introduction of this innovative material into the practice of design and construction. This paper presents the results of finite element modeling of the operation of UHPC I-beam in the area of action of shear forces. The SMM model integrated in the OpenSees framework application is used for the walls of beams under conditions of flat stress. The authors have added new classes of materials that implement the averaged deformation laws of UHPC (with and without fiber) and the reinforcement placed in it. The calculations showed good convergence with the authors' own test results of the UHPC I-beams, as well as with the results of experiments by other researchers.

Keywords: shear strength, shear force, ultra high-performance steel fiber-reinforced concrete, I-shaped beams, OpenSees