Long-Term Mechanical and Fracture Properties of Steel and Polypropylene Fibre-Reinforced Concrete with Partial Cement Replacement by Silica Fume

The sustainable development of high-performance concrete requires reducing cement consumption while maintaining adequate mechanical and fracture performance. Silica fume is a widely used supplementary cementitious material; however, the combined long-term effects of high replacement levels and fibre reinforcement on fracture behaviour have not been systematically investigated. In this study, concretes containing 0-50 % silica fume reinforced with 1 % steel or polypropylene fibres were tested after curing periods of 28, 90, and 365 days. A total of 105 specimens were evaluated using three-point bending and compression tests to determine compressive strength, flexural strength, modulus of elasticity, fracture toughness, and fracture energy. The results showed that the optimum silica fume content depended on the investigated property. Approximately 10 % silica fume generally provided favourable stiffness and strength characteristics, whereas fracture-related parameters exhibited optimum behaviour mainly within the range of 10-20 %. Steel fibre-reinforced concretes achieved compressive strengths up to 137 MPa and fracture energies exceeding 17 kN/m. Statistical analysis demonstrated that both Normal and Weibull distributions adequately described the measured variability, with the Normal distribution providing slightly better overall agreement. Non-linear regression models describing the effects of curing age and silica fume content showed satisfactory predictive capability within the investigated range. The findings improve understanding of fibre-reinforced concretes incorporating silica fume and provide guidance for mixture design in engineering applications.