Optimizing crack control and ductility in sustainable ultra-high performance concrete by using hybrid steel and UHMW-PE fibers and mineral fillers

While the ductility of tensile strain-hardening UHPFRC is critical for load-bearing capacity at the ultimate limit state and crack control under service conditions, recent studies reveal that conventional UHPFRC with 2–4% fiber volume often fails to achieve the recommended 5‰ tensile strain for tension-critical concrete structures. This paradox arises because increased ductility is frequently linked to larger microcrack widths, necessitating a carefully optimized solution. Building on advances in UHPFRC with hybrid stiff steel and flexible Ultra-High Molecular Weight Polyethylene (UHMW-PE) fibers, this work uniquely addresses both crack width reduction under service conditions and enhanced ductility. Significant portions of cement were replaced with recycled waste granite and limestone powders to promote sustainability. Several UHPFRC mix designs were developed, incorporating variations in fiber type (steel and UHMW-PE), length (6 mm and 13 mm), dosage (0–2% by volume), and hybrid configurations. For steel fibers, two distinct casting methods were employed. Tensile properties were assessed through bending tests combined with inverse analysis in accordance with Swiss SIA 2052 standards, while post-peak behavior was modeled using non-linear finite element analysis (NLFEA). Crack widths were measured using digital image correlation (DIC). Results showed that hybrid UHPFRC, combining stiff steel and ductile UHMW-PE fibers, demonstrated superior strain-hardening, reduced microcrack widths, enhanced ductility, and increased fracture energy while lowering the embodied carbon footprint. Microcracks were kept below 30 μm to ensure impermeability, and hardening strains exceeded 5‰ in tensile-controlled sections. The developed UHPFRC with hybrid fibers and mineral fillers offers a promising solution for advanced structural applications, balancing durability, cost-efficiency, and sustainability.